structure and reactivity of dehydroxylated …
TRANSCRIPT
STRUCTURE AND REACTIVITY OF DEHYDROXYLATED BRONSTED ACID
SITES IN H-ZSM-5 ZEOLITE:
GENERATION OF STABLE ORGANIC RADICAL CATION AND CATALYTIC
ACTIVITY FOR ISOBUTANE CONVERSION
by
Jang Ho Yun
A thesis submitted to the Faculty of the University of Delaware in partial
fulfillment of the requirements for the degree of Master of Chemical Engineering
Summer 2011
Copyright 2011 Jang Ho Yun
All Rights Reserved
STRUCTURE AND REACTIVITY OF DEHYDROXYLATED BRONSTED ACID
SITES IN H-ZSM-5 ZEOLITE:
GENERATION OF STABLE ORGANIC RADICAL CATION AND CATALYTIC
ACTIVITY FOR ISOBUTANE CONVERSION
by
Jang Ho Yun
Approved: __________________________________________________________
Raul F. Lobo, Ph.D.
Professor in charge of thesis on behalf of the Advisory Committee
Approved: __________________________________________________________
Norman J. Wagner, Ph.D.
Chair of the Department of Chemical Engineering
Approved: __________________________________________________________
Babatunde A. Ogunnaike, Ph.D.
Interim Dean of the College of Engineering
Approved: __________________________________________________________
Charles G. Riordan, Ph.D.
Vice Provost for Graduate and Professional Education
iii
TABLE OF CONTENTS
LIST OF TABLES .............................................................................................................. v LIST OF FIGURES ........................................................................................................... vi LIST OF SCHEMES........................................................................................................ viii ABSTRACT ...................................................................................................................... ix
Chapter
1 INTRODUCTION .................................................................................................. 1
1.1 Introduction to Zeolites ..................................................................................... 1
1.2 Activation Mechanisms of Zeolites as Acid Catalysts ..................................... 3
1.2.1 Activation of Hydrocarbons on Bronsted Acid Sites............................ 3 1.2.2 Heterolytic Pathway of Dehydroxylation of Bronsted Acid Sites ........ 6
1.2.3 Homolytic Pathway of Bronsted Acid Sites Dehydroxylation ............. 8 1.2.4 Properties of the Sites Formed by Dehydroxylation of Bronsted
Acid Sites ......................................................................................... 10
1.3 The FCC Process and Effect of Redox Chemistry .......................................... 11
1.3.1 FCC Process and Zeolite..................................................................... 11
1.3.2 Effect of Redox Chemistry on Hydrocarbon Chemistry .................... 14
1.4 Thesis Outline ................................................................................................. 15 1.5 References ....................................................................................................... 17
2 EXPERIMENTAL METHOD .............................................................................. 20
2.1 Introduction ..................................................................................................... 20 2.2 Zeolite ZSM-5 Synthesis ................................................................................ 21
2.3 Sample Pre-treatment ...................................................................................... 22 2.4 Sample Characterization ................................................................................. 25
2.4.1 X-ray Powder Diffraction ................................................................... 25
2.4.2 Scanning Electron Microscopy (SEM) ............................................... 26 2.4.3 N2 Adsorption Isotherm ...................................................................... 27
2.5 UV/vis Spectroscopy ...................................................................................... 29
2.6 Temperature Programmed Desorption (TPD) of Ammonia ........................... 32
iv
2.7 Fourier Transform Infrared Spectroscopy (FTIR) .......................................... 35 2.8 Gas Chromatography (GC) ............................................................................. 37 2.9 Reactor Setup for GC ...................................................................................... 38 2.10 Summary ....................................................................................................... 39
2.11 References ..................................................................................................... 41
3 GENERATION OF STABLE ORGANIC RADICAL CATIONS IN
THERMALLY TREATED ZSM-5 ZEOLITES ......................................... 43
3.1 Introduction ..................................................................................................... 43
3.2 Generation of Naphthalene Radical Cations ................................................... 44
3.3 Migration of Electrons and Holes within the Zeolite Framework .................. 53
3.4 Structure of the Dehydroxylated Sites ............................................................ 60
3.4.1 Ammonia TPD .................................................................................... 60
3.4.2 IR Spectroscopy and the Thermal Decomposition of Bronsted
Acid Sites ......................................................................................... 63 3.4.3 Effect of Si/Al Ratio ........................................................................... 65
3.5 Conclusions ..................................................................................................... 71 3.6 References ....................................................................................................... 73
4 EFFECT OF HIGH TEMPERATURE ON THE CATALYTIC ACTIVITY
OF ZEOLITE H-ZSM-5 FOR ISOBUTANE CONVERSION ................... 75
4.1 Introduction ..................................................................................................... 75 4.2 Kinetic Analysis of Isobutane Cracking on Zeolites ...................................... 78
4.3 Effect of Pre-treatment of the Sample on Isobutane Conversion and
Selectivity .................................................................................................... 81 4.4 Conclusions ..................................................................................................... 92
4.5 References ....................................................................................................... 94
5 CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS .......................... 96
5.1 Summary ......................................................................................................... 96
5.1.1 Generation of Organic Radical Cations in Thermally Treated
ZSM-5 Zeolites ................................................................................ 97
5.1.2 Catalytic Activity of ZSM-5 Zeolite for Isobutane Conversion ......... 97
5.2 Future Research Directions ............................................................................. 98
5.2.1 Determination of the Structure of Lewis Acid Sites and Redox
Sites .................................................................................................. 98 5.2.2 Low Temperature CO Oxidation ...................................................... 100
5.3 References ..................................................................................................... 102
v
LIST OF TABLES
Table 2.1 Temperature-programmed desorption protocol corresponding to the
high-temperature treatment of the zeolite samples. ..................................... 33
Table 2.2 Temperature-programmed desorption protocol corresponding to the
oxygen treated zeolites................................................................................. 33
Table 3.1 The desorption areas and the maximum temperatures of TPD signal. ........ 68
Table 4.1 Thermodynamic properties for cracking and dehydrogenation
reaction of isobutane. ................................................................................... 78
Table 4.2 Conversion of isobutane over ZSM5 for three different treatments. ........... 82
Table 4.3 TOF per aluminum for ZSM5-12 and for ZSM5-18 after treatment 1. ....... 83
Table 4.4 Contribution of newly generated sites and remaining Bronsted acid
sites after treatment 2. .................................................................................. 84
Table 4.5 Measured activation energies for isobutane cracking and
dehydrogenation after each treatment. ......................................................... 87
Table 4.6 Ionization potentials of light hydrocarbons ................................................. 88
vi
LIST OF FIGURES
Figure 1.1 Poresize of (a) chabazite, (b) ZSM-5, and (c) beta zeolite ............................ 2
Figure 1.2 Bronsted acid site ........................................................................................... 4
Figure 1.4 Schematic diagram of FCC process ............................................................. 13
Figure 2.1 Diagram of the reactor used for sample pre-treatment ................................ 24
Figure 2.2 Temperature protocol for the sample pre-treatment .................................... 24
Figure 2.3 Experimental protocol for detecting the generation of radical cations ....... 25
Figure 2.4 Sample XRD pattern for ZSM-5 .................................................................. 26
Figure 2.5 (a) SEM image for ZSM-5 and (b) EDAX spectrum analysis ..................... 27
Figure 2.6 (a) The schematic layout of UV/vis spectroscopy, (b) optical
geometry of the integrating sphere. ............................................................. 30
Figure 2.7 UV/vis spectrum of naphthalene radical cation in ZSM-5 (Zeolyst,
SiO2/Al2O3 = 15) .......................................................................................... 31
Figure 2.8 Temperature profile with time and the gases during TPD
corresponding oxygen treatment. ................................................................. 34
Figure 2.9 The IR cell – a heater, a reactor, and a gas line ........................................... 37
Figure 2.10 GC system setup used to study isobutane cracking process ........................ 39
Figure 3.1 UV/vis spectra of pure zeolite and naphthalene adsorbed on ZSM-5
zeolite as control experiments: Pure ZSM-5, ZSM-5 in Ar at 500 °C,
Pure Silicalite-1, Silicalite-1 in Ar at 500 °C, and 780 °C .......................... 46
Figure 3.2 UV/visible spectra for control experiments (ZSM-5 in Ar at 500 °C,
Silicalite-1 in Ar at 500 °C and 780 °C) and for naphthalene@ZSM-5
heated to 780 °C in Ar and 500 °C in O2 ..................................................... 48
Figure 3.3 Rescaled UV/vis spectra for naphthalene adsorbed into ZSM-5 heated
to 780 °C in Ar and to 500 °C in O2 ............................................................ 49
vii
Figure 3.4 (a) The UV/visible spectra after mixing with the naphthalene and (b)
intensity change with time of the 695 nm peak of the samples treated
at different temperatures. ............................................................................. 51
Figure 3.5 Mechanism of the single-electron migration in ZSM-5 framework. ........... 53
Figure 3.6 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after
treatment 2 during (a) increasing and (b) decreasing absorption
intensity. ....................................................................................................... 55
Figure 3.7 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after
treatment 3 during (a) increasing and (b) decreasing absorption
intensity. ....................................................................................................... 56
Figure 3.8 (a) Pure spectral components and (b) concentration change with time
for NPT radical cation and electron-hole pair on the sample after
treatment 2 ................................................................................................... 58
Figure 3.9 (a) Pure spectral components and (b) concentration change with time
for NPT radical cation and electron-hole pair on the sample after
treatment 3 ................................................................................................... 59
Figure 3.10 TPD corresponding for (a) treatment under Ar at 780 °C and (b)
treatment under O2 at 500 °C. ...................................................................... 62
Figure 3.11 FTIR spectra in the OH vibration region of ZSM-5 measured (a) after
heated to 500 and 800 °C, and (b) after heated to 500 and 500 °C in
an oxygen flow............................................................................................. 64
Figure 3.12 TPD of ZSM-5 having Si/Al of 12 compared with TPD of ZSM-5
having Si/Al of 18. ....................................................................................... 67
Figure 3.13 UV/visible spectra of ZSM-5 sample with different Si/Al ratios for
(a) treatment 2 and (b) treatment 3. ............................................................. 70
Figure 4.1 The cracking-to-dehydrogenation ratios with temperature for (a)
ZSM5-18 and (b) ZSM5-12. ........................................................................ 86
Figure 4.2 Arrhenius plots for isobutane cracking and dehydrogenation (a)
ZSM5-18 and (b) ZSM5-12. ........................................................................ 90
Figure 5.1 CO2 production from naphthalene-ZSM-5 catalyst .................................. 101
viii
LIST OF SCHEMES
Scheme 1.1 Initial protonation of C-H or C-C bonds of isobutane on Bronsted
acid sites in zeolites ....................................................................................... 5
Scheme 1.2 Heterolytic Bronsted acid site decomposition pathway ................................. 8
Scheme 1.3 Bronsted acid sites Homolytic Dehydroxylation ......................................... 10
Scheme 4.1 Protolytic mechanism of isobutane (cracking and dehydrogenation) .......... 79
Scheme 4.2 Suggested pathway of monomolecular reaction of isobutane over
redox sites on ZSM-5. .................................................................................. 92
ix
ABSTRACT
Zeolites are crystalline aluminosilicate materials that have wide application in
industry as solid acid catalyst. Since zeolites have high acidity, high surface area as well
as the ability to do shape selectivity, they are used primarily as a solid catalyst in oil
refining and petrochemical industries, in processes such as hydrocracking, fluid catalytic
cracking (FCC). Bronsted acid sites are described as a hydroxyl group bridged between
Al and Si (Al-OH-Si). It is well known that Bronsted acid sites are significant in a
number of hydrocarbon processes, such as alkane cracking and isomerization and many
other.
The Bronsted acid sites of zeolites are decomposed at high temperatures, usually
above 600 °C. This high temperature condition is commonly found in the fluidized
catalytic cracking where catalyst is recycled forth and back between the riser and the
regenerator under an oxidative atmosphere. The process of decomposition of hydroxyl
group from the initial structure is called dehydroxylation. The dehydroxylation is
believed to proceed via a dehydration mechanism of the acid sites. This heterolytic
pathway of Bronsted acid site decomposition has been the accepted dehydroxylation path
for low-silica zeolites for decades, although the molecular details of the structure
remaining inside the zeolites are still unknown. However, our group reported that
hydrogen is also formed during the dehydroxylation process. Our group has also
x
proposed a new pathway to explain the decomposition of Bronsted acid sites of high-
silica zeolites and the formation of [AlO4]0 sties in zeolites.
Oxidized zeolites are known to extract electrons from molecules having small
ionization potential. Since oxidation of zeolites is considered to lead to the
dehydroxylation of Bronsted acid sites, we suggest that the dehydroxylated Bronsted acid
sites are responsible for the electron-transfer process. Using naphthalene as a probe
molecule, it can be shown that the new sites have the ability to extract an electron from
naphthalene and form stable radical cations. We investigated the formation of these new
sites by thermal treatment and oxidation treatment. A series of UV/vis spectra showed
that after naphthalene radical cations were generated, single-electron transfers back into
the ZSM-5 framework to form a stable electron-hole pair and reform the naphthalene
neutral molecule. Using ammonia TPD, IR spectra, and UV/vis spectra of the sample
with different Si/Al ratios, the structure of the new generated sites was characterized.
These observations suggest that the most common site generated is different depending
on each treatment.
The activation of small alkanes over acid sites has been investigated extensively
because of its relevance to technologically important processes such as fluidized catalytic
cracking in petroleum refineries, but also because C-H and C-C bond activation is of
fundamental scientific interest. The reactivity and selectivity of newly generated sites is
investigated using isobutane conversion. The conversions of the samples, which were
treated by high temperature treatment and oxygen treatment for dehydroxylation, are
xi
greater than the conversions of the acid catalyst. When conversion is low, the product
distribution is limited to the monomolecular cracking of the C-C bond and
dehydrogenation of the C-H bond. The cracking-to-dehydrogenation ratio significantly
increases after dehydroxylation treatments. Several groups have proposed that carbonium
or carbenium ion intermediates on Bronsted acid sites in zeolites play the key role in the
activation of isobutane. However, in this thesis, we proposed that the presence of redox
sites resulted in radical cation chemistry instead of protolytic chemistry in the propane
and isobutane cracking process.
1
Chapter 1
INTRODUCTION
1.1 Introduction to Zeolites
Zeolites are generally referred to as crystalline aluminosilicate materials with
unique three dimensional framework structures and pore size (3-13Å in diameter) [1]. All
zeolites have unique framework structures constituted by combining oxide tetrahedra
such as SiO4 or AlO4-. Framework compositions can be extended by substitution of metal
atoms like B, Fe, Ga, Mg, Mn, Ti, and Zn into the tetrahedral positions within the
framework.
Zeolites properties such as adsorption capacity, molecular sieving, and catalytic
activity are directly related to zeolite structure. The primary building unit of zeolites is
the three-dimensional TO4 tetrahedron: By combination of primary building units, other
structures, such as squares, pentagons, and octagons, called secondary building units
(SBU) can be formed [2]. The SBU consist of n-ring structures, in which n is commonly
4, 5, 8, 10, or 12. The linkage of SBUs can form cages and channels that are essential
elements of all zeolite structures. Due to the large diversity of zeolites reported to data,
the zeolite frameworks have been codified into framework type-codes describing only the
framework topology, but not framework composition, distribution of the tetrahedral
2
atoms, or cell dimension. The approved number of the framework types is over 190
species according to the three-letter structure codes established by the International
Zeolite Association [3].
Different types of zeolites have different channel dimensions (Figure 1.1). For
example, chabazite (CHA) has small pores formed by 8-membered rings, ZSM-5 (MFI)
has medium pores (10-membered rings), and beta zeolite (*BEA) has large pores (12-
membered rings). The shape selectivity of different zeolites is due, to a large extent, to
differences in pore size. The different pore sizes allow for the selective adsorption of
certain reactants, or the selective desorption of certain products and can inhibit or
promote different reaction intermediates in catalytic reactions.
(a) (b) (c)
Figure 1.1 Poresize of (a) chabazite, (b) ZSM-5, and (c) beta zeolite
3
The chemical and physical properties and the application of zeolites are also
determined to a great extent by the amount of aluminum in the framework of zeolite. The
amount of aluminum in the zeolites framework is typically represented by the (atomic)
Si/Al ratio. The minimum Si/Al ratio is 1 due to the Loewenstein rule, which establishes
that no Al-O-Al bond exists in a zeolite [4]. Since an alumina tetrahedron (AlO4-) has a
negative charge while a silica tetrahedron (SiO4) is neutral, a counter ion, such as H+ or
alkali-metal ion, must be present to balance the negative charge. The sites compensated
by H+ form bridging hydroxyl group (Si-OH-Al) that are chemically and functionally
Bronsted acid sites. The Si/Al ratio affects the acidity of the zeolites: the total number of
acid sites increases as the Si/Al ratio decreases but at the same time the acidity becomes
weaker. Acid-base reactions, the most common class of industrial chemical reaction, and
acid base catalysis can be applied to every area of the chemical industries, including the
oil refining industry. Since zeolites have high acidity, high surface area as well as the
ability to do shape selectivity, they are used primarily as a solid catalyst in oil refining
and petrochemical industries, in processes such as hydrocracking, fluid catalytic cracking
(FCC), aromatization and isomerization [5-7].
1.2 Activation Mechanisms of Zeolites as Acid Catalysts
1.2.1 Activation of Hydrocarbons on Bronsted Acid Sites
The actual details of the hydrocarbon activation mechanism of zeolites in the
processes mentioned above have not been completely established, although zeolites are
4
used as the main catalyst component and as additive in petrochemical processes such as
FCC. Bronsted acid sites are distorted tetrahedral structures of alumina-substituted
zeolites (Figure 1.2) with a longer Al-O(H)-Si bond than the three Al-O-Si. Bronsted acid
sites are described as a hydroxyl group bridged between Al and Si (Al-OH-Si). The
Bronsted acid form of a zeolite is obtained by exchanging the framework cations with an
ammonium solution such as aqueous ammonium nitrate (NH4NO3). The acid site is
formed by desorption of ammonia from such materials at elevated temperatures leaving a
proton on the zeolite surface.
Figure 1.2 Bronsted acid site
It is well known that Bronsted acid sites are significant in a number of
hydrocarbon processes, such as in the synthesis of ethylbenzene from ethylene and
benzene, disproportionation of toluene to form xylenes and benzene, alkane cracking and
isomerization and many others. In hydrocarbon conversion processes, Bronsted acid sites
donate a proton to an absorbed species forming carbonium or carbenium ions, which are
considered as transition states in alkane cracking reactions. The carbonium ion
5
intermediates on Bronsted acid sites in zeolites play an important role in the activation of
alkanes: this is called protolytic activation of hydrocarbon. To be specific, on Bronsted
acid sites in zeolites, the reaction of isobutane is initiated by protonation of C-H or C-C
bonds [8]. First, a pentacoordinated carbonium ion is formed on the Bronsted acid site in
zeolite, and then it is decomposed into carbenium ion, such as the t-butyl and propyl
carbenium ion. Hydrogen is generated by the t-butyl pathway (dehydrogenation pathway)
and methane is formed by the propyl pathway (cracking pathway).
Scheme 1.1 Initial protonation of C-H or C-C bonds of isobutane on Bronsted
acid sites in zeolites
As the number of hydrogen atoms attached to the carbon atom from which the
hydride ion is abstracted increases, the energy required for the formation of carbonium
ion also increases. The high energy of formation decreases the stability of carbonium ions.
For example, the tertiary carbenium ion is so stable that the formation of carbonium ion is
easy and prevalent, while there is no formation of carbonium ion since the methyl
carbenium ion is the least stable. In a zeolite, the charge separation, which exists when a
carbonium ion is formed, may occur over the oxygen atoms so that the micropore in the
6
zeolite surrounds the carbonium ion. In addition, cracking of heavy hydrocarbon is
known to be faster than cracking of light hydrocarbon. For example, cracking of n-C18H38
is 20 times faster than cracking of n-C8H18 [9]. A longer chain length provides more
chance to contact the hydrocarbon to the surface of a catalyst. Adsorption equilibrium
data for hydrocarbons on zeolite also support this explanation.
1.2.2 Heterolytic Pathway of Dehydroxylation of Bronsted Acid Sites
The acid sites of zeolites are decomposed at high temperatures, usually above
600 °C [10, 11]. The process of decomposition of hydroxyl group from the initial
structure is called dehydroxylation. For instance, in a FCC regenerator, the catalysts are
treated at temperatures in the range of 670 °C - 720 °C, and dehydroxylation occurs
under these conditions. As a result of dehydroxylation, Lewis acid sites are also
generated. Since it is difficult to characterize and indentify the molecular structures of
Lewis acid sites, the research about Lewis acid sites is still on-going and there is no
consensus as to the structure of the Lewis acid sites. Three-coordinated aluminum units
have been typically proposed as the source of Lewis acidity [12, 13]. Other non-
framework aluminium moieties such as AlO+, Al(OH)
+2, Al(OH)
+2, Al(OH)3, Al2O3 have
also been suggested to be the true Lewis acids [14, 15]. The effect of Lewis acid sites on
the catalytic activity and selectivity has been investigated and it has been revealed that
the turnover rates of cracking and dehydrogenation are not related to Lewis acid sites
7
concentration [16]. Instead, Lewis acid sites increase the reaction rate by enhancing the
adsorption of reactants [17].
Dehydroxylation has been investigated by various spectroscopy techniques
including Al nuclear magnetic spectroscopy (NMR), Fourier transform infrared
spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption
spectroscopy (XAS) [18-23]. The trigonal structure has not been detected by NMR while
octahedral and pentacoordinated aluminum have been observed [22, 23]. For instance,
bridged Si-OH-Al hydroxyl groups have an absorption band around the 3600 cm-1
region
of IR spectrum, and the intensity of this region decreases after zeolites are heated at
elevated temperature above 600 °C [24]. An IR peak at 3720-3750 cm-1
is assigned to
silanol groups [25]. The peak at 3666 cm-1
is assigned to aluminum in partially
extraframework positions. The aluminum in a three-coordination environment is often
proposed as the origin of peak at 3666 cm-1
[25]. CO adsorption studies have revealed
that the peak at 3666 cm-1
is less acidic than the normal Bronsted acid sites and more
acidic than the silanol group [25]. Extraframework aluminum [26], extra-lattice
amorphous materials [12, 27], or the silanol group [28] has been considered to the
assignment of the peak at around 3700 cm-1
. Bugaev et al. found that by XANES only 5 -
10 % of aluminum has trigonal structure after dehydroxylation of mordenite [13].
The heterolytic dehydroxylation of Bronsted acid sites is illustrated in scheme 1.2
[11, 29], which in this case proceeds by dehydration [29, 30]. In the left side of this figure,
the Bronsted acid sites are described as OH-groups. In this reaction two moles of
8
Bronsted acid sites react to give acid-base and positive-negative site pairs: one mole of
aluminum in a trigonal structure (Lewis acid sites) and one mole of aluminum with a
symmetric tetrahedral structure. The reverse of dehydroxylation can occur by inducing
water from at the highest temperature of dehydroxylation to the temperature which water
vapor can be present [31]. This heterolytic pathway has provided the idea that Lewis acid
sites are important for the hydrocarbon cracking process at high temperature condition
above 600 °C.
Scheme 1.2 Heterolytic Bronsted acid site decomposition pathway
1.2.3 Homolytic Pathway of Bronsted Acid Sites Dehydroxylation
The heterolytic pathway of Bronsted acid site decomposition has been the
accepted dehydroxylation path for low-silica zeolites for decades [11, 29], although the
molecular details of the structure remaining inside the zeolites are still unknown. In high-
silica zeoites, the high energy is needed to decompose the Bronsted acid sites through the
heterolytic pathway since the Bronsted acid sites are sparsely placed.
9
Our group recently examined the dehydroxylation of Bronsted acid sites of high-
silica zeolites using mass spectrometry-temperature programmed desorption (MS-TPD)
[11]. We have found that the Bronsted acid sites of high-silica zeolites are decomposed to
produce hydrogen and a small amount of water. The MS-TPD of two samples of ZSM-5
heated stepwise to 250 °C, 525 °C and 750 °C was carried out and only until the
temperature reaches about 750 °C, a large amount of hydrogen is found. The amount of
hydrogen is also related to the Si/Al ratio. With a small Si/Al ratio (high aluminum), less
hydrogen gas is produced. An electron hole pair generation for H-ZSM-5 calculated by
hybrid quantum mechanics and a shell-model ion-pair potential approach also support our
group‟s observation [32]. These results of the MS-TPD experiment and electronic
structure calculation show that Bronsted acid sites of high-silica zeolites are decomposed
by a redox process, not by dehydration.
Also, it has been known that the acid form of zeolites treated at high temperature
(under dehydroxylation conditions) react with molecules having small ionization
potentials to form stable radical cations [29, 33, 34]. These radical cations have been
studied frequently by the electron-spin-resonance (ESR) because they give excellent
high-resolution ESR spectra [35]. Alkenes, polyaromatics, nitrogen-, oxygen-, and sulfur-
containing organic molecules and others have shown to form radical cations in zeolite
after heating at high temperatures.
Our group has proposed a new pathway to explain the decomposition of Bronsted
acid sites of high-silica zeolites (scheme 1. 3) and the formation of [AlO4]0 sties in
10
zeolites [32, 33]. A single electron hole is generated on one of the oxygen atom
surrounding aluminum atom by dehydroxylation. The active sites which are formed under
the condition of dehydroxylation are considered nonacidic single-electron redox sites [36].
Scheme 1.3 Bronsted acid sites Homolytic Dehydroxylation
1.2.4 Properties of the Sites Formed by Dehydroxylation of Bronsted Acid Sites
Moissette and coworkers reported that oxidized zeolites can extract electrons from
molecules having small ionization potential [26]. Since oxidation of zeolites is considered
to lead to the dehydroxylation of Bronsted acid sites, we suggest that the dehydroxylated
Bronsted acid sites are responsible for the electron-transfer process. Using naphthalene as
a probe molecule, it can be shown that the new sites have the ability to extract an electron
from naphthalene and form stable radical cations. This hypothesis is the basis for the
experiments described in Chapter 3 and 4.
The sites generated by dehydroxylation of Bronsted acid sites can also activate
hydrocarbons and show a different selectivity with respect to the case when Bronsted
acid sites activate hydrocarbons, that is, with the pristine zeolite. In our group‟s previous
reports, the propane cracking process was investigated using zeolite catalysts, such as
11
ZSM-5, beta zeolites, and mordenite [37]. In this case, the monomolecular initiation step
proceeds by two parallel pathways: cracking of the C-C bond and dehydrogenation of the
C-H bond. Before dehydroxylation, the distribution of products produced by the propane
reaction is selective towards cracking by factor of about two. After dehydroxylation at the
high temperature, the dehydrogenation rate increased significantly compared to the rate
before dehydroxylation. Investigation of the distributions of products and the conversion
of isobutane can clarify the structure of the reaction sites after Bronsted acid sites
decomposition and the structure-activity relationships in hydrocarbon conversions. To
this end, in Chapter 4 we investigate the isobutane reaction over pristine and activated
zeolites.
1.3 The FCC Process and Effect of Redox Chemistry
1.3.1 FCC Process and Zeolite
Fluidized catalytic cracking (FCC) is an important process in oil refining and
petrochemical industry. The FCC process currently accounts for 47% of the refinery
catalyst market value estimated to be a $2.9 billion [38]. In addition, among the catalytic
applications of zeolites, the FCC process accounts for over 95% of synthetic zeolites
consumption on a per mass basis [39]. The FCC units were in operation at 400 petroleum
refineries worldwide in 2006 [40, 41]. During 2009, the FCC units processed at total of
5,000,000 barrels per day of feedstock worldwide [42].
12
The FCC process converts vacuum distillation oils to high value products such as
gasoline, olefinic gases, and other products. Specifically, carbon bonds are broken to
convert preheated heavy hydrocarbons to light hydrocarbons, which are used as reactants
for alkylation [43]. The cracking process is catalyzed using very short contact times (3 -
5s) over solid catalysts at the lower portion of a FCC riser, a reactor in the FCC process.
Cracked products are separated from the coke and deactivated catalysts at the top of riser.
Solid catalysts are then treated to burn coke on the surface of the catalysts and to be
activated again in a regenerator at high temperature around 670 - 720 °C under oxidizing
conditions. The heated regenerated catalysts retain the heat balance providing the energy
needed for endothermic cracking process. A simplified schematic diagram is depicted in
Figure 1.4.
Catalysts of the FCC process have the following composition: an ultrastable Y
zeolite (USY) is a main component (40%) in the FCC process (since 1964), and ZSM-5 is
used as an additive to significantly enhance catalytic activity (since 1984). FCC catalysts
typically contain a filler (e.g., kaolin clay (Al2Si2O5(OH)4)), a binder, catalytically active
and acidic matrix (e.g., -Al2O3, SiO2), and other kinds of additive (such as a CO
oxidation promoter) besides the ZSM-5.
13
Figure 1.4 Schematic diagram of FCC process
Zeolite satisfies the complex requirements of a FCC unit: good thermal stability,
high activity, high selectivity to gasoline vs. coke, low coke production, resistance to
poisons, low cost, etc [43, 44]. Before zeolite was applied to FCC process, the FCC units
were needed to reduce the formation of hydrocarbon on catalysts and to shorten the
residence time for control of coke generation. The introduction of zeolites to the FCC
units not only solved these issues, but also highly improved both catalytic activity and
selectivity. The productivity (yield of gasoline) was increased significantly by 60%. This
is related to a tremendous cost savings and a reduction in crude oil consumption [45].
14
After zeolites (Ultra Stable Y zeolites, USY) were introduced to the FCC process as
catalysts in 1962 (by Plank and Rosinski working for Mobil Oil) instead of clay-based
synthetic catalysts [44], they have been used as the main catalysts in the FCC process.
1.3.2 Effect of Redox Chemistry on Hydrocarbon Chemistry
From the description of FCC process in the previous section, it can be seen that a
large fraction of the Bronsted acid sites of zeolites would be dehydroxylated at the point
of contact of catalysts and hydrocarbons. The catalytic chemistry can be greatly affected
by radical cation intermediates [46] formed by interaction with redox sites [47]. The
potential role of redox chemistry has been suggested by McVicker et al. [48] where the
selectivity patterns revealed a „radical-like‟ pattern rather than an acid-like pattern. In
addition, our previous observation leads a necessity for new plausible mechanism that
can explain formation and role of redox sites in the hydrocarbon chemistry.
The main catalysts of FCC process are not high-silica zeolites. However, USY are
dealuminated during contacting high temperature steam in the modern FCC process, and
then act as high-silica zeolites. Steam at high temperature causes the dealumination of
zeolites and the increase of Si/Al ratio [49]. Thus, understanding the mechanisms of high-
silica zeolites is considered relevant a very important issue in the FCC process.
The motivation of this research starts from the observation of the unexpected
thermal decomposition of Bronsted acid sites of high-silica zeolites. If a redox process is
15
revealed as contributing significantly to the chemistry of a FCC unit, this opens new
possibilities to improve hydrocarbon chemistry including the cracking process. The aim
of this thesis is to assess the chemistry and structure of these redox sites and determine if
indeed, they can play a substantive role in the catalytic cracking of hydrocarbons.
1.4 Thesis Outline
The main objective of this thesis is to investigate the structure, composition, and
properties of the sites generated by dehydroxylation of Bronsted acid sites in ZSM-5 and
ZSM-5 like materials and their roles in electron-transfer and reactivity on hydrocarbon
chemistry.
Chapter 2 describes the experimental methods and techniques used in this thesis.
This chapter includes ultraviolet/visible light spectroscopy (UV/vis), temperature
programmed desorption (TPD) of ammonia, gas chromatograph (GC), and the reactors
set up.
Chapter 3 discusses that the formation of the sites can extract an electron from
naphthalene and form stable radical cations on differently treated ZSM-5 samples. The
formation of naphthalene radical cations is detected by UV/vis spectroscopy and time-
series spectra are recorded to study various electron-transfer processes that occur within
the zeolite over time.
16
Chapter 4 examines the catalytic activity of ZSM-5 samples pretreated at different
conditions adopting isobutane as a model reactant. The distributions of products, the
conversions of isobutane, and the kinetic parameters including activation energy are
investigated using GC.
Chapter 5 summarizes the main findings in this thesis, describes the main
conclusions from our investigation and outlines possible directions for the future studies.
17
1.5 References
[1] Lobo, R.F., Introduction to the structural chemistry of zeolite, in Handbooks of
Zeolite Science and Technology, K.A.C. Scott M. Auerbach, Prabir K. Dutta,
Editor. 2003, Marcel Dekker.
[2] Meier, W.M.a.U., J.B, Molecular Sieves. 1973, Washington, D.C.: American
Chemical Society.
[3] Database of Zeolite Structure. Available from: http://www.iza-
structure.org/databases/.
[4] Loewenstein, W., Am. Miner., 1954. 39(1-2): p. 92-96.
[5] Corma, A., Chem. Rev., 1995. 95(3): p. 559-614.
[6] Garcia, H. and H.D. Roth, Chem. Rev., 2002. 102(11): p. 3947-4007.
[7] Flanigen, E.M., Pure Appl. Chem., 1980. 52(9): p. 2191-2211.
[8] Engelhardt, J., J. Catal., 1996. 164(2): p. 449-458.
[9] Nace, D.M., Product R&D, 1969. 8(1): p. 24-31.
[10] Szostak, R., Secondary Synthesis Methods, in Introduction to Zeolite Science and
Practice. 2001, Elsevier: New York.
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[12] Borade, R., et al., J. Phys. Chem., 1990. 94(15): p. 5989-5994.
[13] Bugaev, L.A., et al., J. Phys. Chem. B, 2005. 109(21): p. 10771-10778.
[14] Catana, G., et al., J. Phys. Chem. B, 2001. 105(21): p. 4904-4911.
[15] Jacobs, P.A. and H.K. Beyer, J. Phys. Chem., 1979. 83(9): p. 1174-1177.
[16] Gounder, R. and E. Iglesia, J. Am. Chem. Soc., 2009. 131(5): p. 1958-1971.
[17] van Bokhoven, J.A., et al., J. Catal., 2004. 224(1): p. 50-59.
[18] Inaki, Y., et al., J. Phys. Chem. B, 2002. 106(35): p. 9098-9106.
[19] Datka, J., B. Gil, and A. Kubacka, Zeolites. 17(5-6): p. 428-433.
18
[20] Sonnemans, M.H.W., C. Den Heijer, and M. Crocker, J. Phys. Chem., 1993.
97(2): p. 440-445.
[21] Al-majnouni, K.A., et al., J. Phys. Chem. C, 2010. 114(45): p. 19395-19405.
[22] Kentgens, A.P.M., et al., J. Am. Chem. Soc., 2001. 123(12): p. 2925-2926.
[23] Omegna, A., J.A. van Bokhoven, and R. Prins, J. Phys. Chem. B, 2003. 107(34): p.
8854-8860.
[24] Szanyi, J. and M.T. Paffett, Microporous Mater., 1996. 7(4): p. 201-218.
[25] Zecchina, A., et al., J. Chem. Soc. Faraday Trans., 1992. 88(19): p. 2959-2969.
[26] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.
[27] Jacobs, P.A. and R. Von Ballmoos, J. Phys. Chem., 1982. 86(15): p. 3050-3052.
[28] Védrine, J.C., et al., J. Catal., 1979. 59(2): p. 248-262.
[29] Stamires, D.N. and J. Turkevich, J. Am. Chem. Soc., 1964. 86(5): p. 749-&.
[30] Uytterhoeven, J.B., L.G. Christner, and W.K. Hall, J. Phys. Chem., 1965. 69(6): p.
2117-2126.
[31] Gates, B.C., J.R. Katzer, and G.C.A. Schuit, Chemistry of catalytic processes.
1979, New York: McGraw-Hill.
[32] Solans-Monfort, X., et al., J. Chem. Phys., 2004. 121(12): p. 6034-6041.
[33] Shih, S., J. Catal., 1983. 79(2): p. 390-395.
[34] Cano, M.L., et al., J. Phys. Chem., 1995. 99(12): p. 4241-4246.
[35] Marquis, S., et al., C. R. Chim., 2005. 8(3-4): p. 419-440.
[36] Leu, T.M. and E. Roduner, J. Catal., 2004. 228(2): p. 397-404.
[37] Al-majnouni, K.A., HIGH TEMPERATURE DECOMPOSITION OF BRONSTED
ACID SITES: STRUCTURES FORMED AND THEIR CATALYTIC ACTIVITY
TOWARD SMALL ALKANES ACTIVATION, in Chemical Engineering. 2011,
University of Delaware: Newark.
[38] Stell, J., Oil Gas J., 2005. 103(39): p. 50-+.
[39] Vermeiren, W. and J.P. Gilson, Top. Catal., 2009. 52(9): p. 1131-1161.
19
[40] Speight, J.G., The Chemistry and Technology of Petroleum. 4th ed. 2006: CRC
Press.
[41] Jones, D.S.J., P.R. Pujadó, and SpringerLink. Handbook of petroleum processing.
2006; Available from: http://dx.doi.org/10.1007/1-4020-2820-2.
[42] U.S. Downstream Processing of Fresh Feed Input by Catalytic Cracking Units.
Available from:
http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRCCUS2
&f=A.
[43] Cheng, W.C., et al., Catal. Rev.-Sci. Eng., 1998. 40(1-2): p. 39-79.
[44] Fletcher, R.P., The history of fluidized catalytic cracking: A history of innovation:
1942-2008, in Innovations in Industrial and Engineering Chemistry, M.A.A.
William H. Flank, and Michael A. Matthews, Editor. 2009, Oxford University
Press.
[45] Yan, Z.-F. and F.-S. Xiao, J. Porous Mater., 2008. 15(2): p. 115-117.
[46] Corma, A. and H. Garcia, Top. Catal., 1998. 6(1-4): p. 127-140.
[47] Boronat, M., P.M. Viruela, and A. Corma, J. Am. Chem. Soc., 2004. 126(10): p.
3300-3309.
[48] McVicker, G.B., G.M. Kramer, and J.J. Ziemiak, J. Catal., 1983. 83(2): p. 286-
300.
[49] Kerr, G.T., J. Phys. Chem., 1969. 73(8): p. 2780-&.
20
Chapter 2
EXPERIMENTAL METHOD
2.1 Introduction
The experimental methods and techniques used in this thesis are described in this
chapter. The synthesis of the zeolites ZSM-5 is described first, followed by a section
describing the treatment protocols used for dehydroxylation of Bronsted acid sites of this
zeolite. The key characterization techniques, such as X-ray diffraction (XRD), N2
adsorption, scanning electron microscopy (SEM), and Fourier transform infrared
spectroscopy (FTIR) are described to the extent needed for understanding their
application in this thesis. Ultraviolet/visible light spectroscopy (UV/vis) is used
extensively and the set up as well as the diffusive-reflectance cell and its application are
described in detail. The technique of temperature programmed desorption (TPD) of
ammonia is described in relation to its application in zeolites. The final section describes
the reactor setup, the analysis of the product gas composition by gas chromatography
(GC) and the experimental protocols used for the iso-butane cracking and
dehydrogenation reaction (Chapter 4).
21
2.2 Zeolite ZSM-5 Synthesis
ZSM-5 is most often prepared using an organic structure-directing agent (SDA)
such as tetrapropylammonium hydroxide. Here, however, we use a completely inorganic
synthesis gel to avoid the calcination step needed to remove the organic SDA in the
typical ZSM-5 synthesis. The all-inorganic synthesis of zeolites ZSM-5 is the method
used to prepare this zeolites in the industry mainly because it is inexpensive. This
synthesis thus has the added advantage that the results can be directly applicable to ZSM-
5 samples used in industrial catalytic reactors.
ZSM-5 with different framework composition (Si/Al ratios) is synthesized from
the following molar batch compositions: x Na2O: y Al2O3: 100 SiO2: z H2O. Based on
100 mol of SiO2, x has a range from 7 to 14, y varies from 1.5 to 5, and z from 2250 to
4200 [1]. The compositions are calculated according to the described Si/Al ratios. Two
samples with high and low Si/Al ratios are used throughout the thesis. The sample ZSM-
5-18 with a Si/Al ratio of ~18 is synthesized using a gel of composition 12 Na2O: 2.86
Al2O3: 100 SiO2: 3000 H2O. The sample with a Si/Al ratio of 12.5 (ZSM-5-12, higher
alumina content) is synthesized with a synthesis gel of composition 9 Na2O: 4 Al2O3: 100
SiO2: 3000 H2O. Colloidal silica (Ludox AS-40, Sigma-Aldrich) and sodium aluminate
(NaAlO2, EM Science) are used as the silica and alumina sources, respectively.
The reactant solution is prepared in two containers (polypropylene beakers).
Colloidal silica with 5M NaOH solution and deionized water are dissolved in container 1.
Sodium aluminate with 5M NaOH solution and deionized water are dissolved in
22
container 2. After mixing for 1 hour separately, the solutions are combined. After one
additional hour of continuous stirring, the final solution is loaded into Teflon-lined Parr
autoclaves and heated at 190 °C for 2 days under rotation. The Parr autoclaves are cooled
to room temperature in air and the zeolites samples are separated from the solution by
using vacuum filtration. The separated samples are washed using deionized water, and
dried at 80 °C overnight. Ammonium exchange of these samples is performed twice in
0.1 M NH4NO3 solution at 80 °C for 1 day to convert the sodium form of the zeolites to
the ammonium form.
2.3 Sample Pre-treatment
A total of ~0.25g of the ammonium-exchanged form of zeolite is put in a quartz
vertical tube reactor (ID = 19 mm), designed to flow gas through the sample space. The
reactor has a porous (4-15 m) fritted disc in the middle where the sample is placed
(Figure 2.1). The reactor is heated by using a ceramic radiant heater (Omega Engineering,
OMEGALUX® CRFC). A gas manifold system allows an inert (Ar) and/or oxygen gas
to flow through the reactor. The sample is first dehydrated at 200 °C in an Ar gas for 2
hours, and then the ammonium ions are decomposed to form acid zeolites by heating the
sample at 450 °C in Ar for 4 hours. Three different treatments are carried out on three
different zeolite samples. In the first treatment the sample is heated at 500 oC in Ar, in the
second treatment the sample is heated at 780 oC in Ar, and in the third treatment the
sample is heated at 500 oC in an oxygen (99.999%) flow. The purpose of the second
23
treatment is to dehydroxylate the BAS of the zeolite and the purpose of the third
treatment is to generate redox sites by exposing the sample to a highly oxidizing
environment as it has been shown before that oxygen treatment forms redox sites in H-
ZSM-5 [2]. The first treatment using an inert gas is the base-case scenario where the
zeolite is expected to be in a „pristine‟ state without any changes induced by the heat
treatment (See Figure 2.2).
After the sample treatment process, the samples are cooled down to room
temperature in an inert atmosphere. A weighed amount of naphthalene (~0.005g, Sigma
Aldrich, ≥ 99.7%), corresponding to ~1 molecule per unit cell of zeolites, is mixed with
the treated sample using a mortar and pestle in a glove bag (Glove bagTM
Inflatable Glove
Chambers) filled with dry argon. To create an inert atmosphere in the glove bag, the inert
gas is repeatedly filled and purged into and from the glove bag. After this step, time-
series UV/visible spectra are measured at 0.5 nm of resolution over a wavelength range
of 220 - 850 nm. The time-series spectra are recorded for 2 days right after mixing the
pre-treated sample with naphthalene. These experiment steps are depicted in Figure 2.3.
24
Figure 2.1 Diagram of the reactor used for sample pre-treatment
Figure 2.2 Temperature protocol for the sample pre-treatment
25
Figure 2.3 Experimental protocol for detecting the generation of radical cations
2.4 Sample Characterization
2.4.1 X-ray Powder Diffraction
Synthesized samples were characterized using X-ray powder diffraction (XRD),
which is a widely used characterization technique for polycrystalline materials. XRD
provides information about the atomic structure of the crystal and the dimensions and
symmetry of the periodic three dimensional lattice structure of the material. XRD
patterns are recorded on a Phillips X‟Pert X-ray diffractometer using a Cu K radiation.
The patterns are collected from 5 ° to 50 ° 2 using a step size of 0.02 ° and 2s per step.
Figure 2.4 is an example of an XRD pattern of the ZSM-5 used in this study. By
26
comparison to published patterns we establish that the samples are pure ZSM-5 and that
they do not contain any detectable impurities or amorphous material.
Figure 2.4 Sample XRD pattern for ZSM-5
2.4.2 Scanning Electron Microscopy (SEM)
A scanning electron microscope (SEM, JEOL JSM 7400F) is used to obtain
electron microscopy images of the ZSM-5 samples. The detector collects emitted
electrons and photons by hitting the sample with an electron beam, and thus obtaining an
image. A sample example for ZSM-5 is provided in Figure. 2.5. EDAX spectrum analysis
27
shows that the ZSM-5 structure contained Si and Al in approximately 94.89:5.11 (Si/Al
ratio is 18.57).
(a) (b)
Figure 2.5 (a) SEM image for ZSM-5 and (b) EDAX spectrum analysis
2.4.3 N2 Adsorption Isotherm
Physisorption of nitrogen is frequently used to determine surface area, pore
volume, pore diameter, and pore size distributions of catalysts. Nitrogen adsorption
isotherms are measured using Micrometrics ASAP 2010 instrument at 77K. The surface
area of a material is commonly found using the Brunauer, Emmett, and Teller (BET)
equation. The BET equation assumes the adsorption potential from one wall and the
subsequent layers of adsorption are controlled by condensation [3, 4]. Since adsorption
28
potential from both walls affect the adsorption in materials with micropores, the BET
cannot be used for materials such as zeolites.
Micropore volume, surface area, and mesopore volume of zeolites are determined
by the deBoer t-plot method. In the t-plot method, the statistical thickness of the
adsorption layer is plotted against the adsorbed volume measured from the N2 adsorption
isotherm. The statistical thickness is estimated from a semi-empirical formula, such as
Harkins-Jura equation, which is commonly used for the analysis of zeolites. This
equation is based on adsorption on nonporous Al2O3. The intercept is related to the
micropore volume and the slope is related to the external surface area using following
equations;
3 / intercept 0.001547mpV cm g
2 / slope 15.47extS m g
where, Vmp represents the micropore volume and Sext indicates the external surface area.
The ZSM-5 micropore volume is reported in the range 0.13 - 0.23 cm3/g [5-7]. The
micropore volumes of the samples used in this thesis are consistent with the range (~ 0.15
cm3/g).
29
2.5 UV/vis Spectroscopy
UV/vis spectroscopy is used to study molecular structure and dynamics through
electronic transitions and vibrations in the ultra-violet and visible range (200 – 800nm
wavelength) of the electromagnetic spectrum. Molecules in the ground state absorb a
specified range of UV/vis light to induce electronic transitions or vibrations and show an
absorption spectrum [8]. The wavelength of the absorption bands depends on the atomic
or molecular structure and composition, and the intensity of absorbance determines the
concentration of the molecule or absorbing species. A schematic layout of UV/vis used in
this study is illustrated in Figure 2.6(a) where the traditional transmission mode is
presented. While UV/vis spectroscopy is a powerful tool to detect and identify organic
species that absorb radiation in UV/vis energy range in the liquid phase, it is not
applicable to directly obtain the spectrum of a powdered sample since transmission of the
light through the powdered sample can be very low and limited due to light scattering.
Instead of transmission, a diffuse-reflective method is used for powdered samples. To this
end, an integrating sphere is used as shown in Figure 2.7(b). The most widely used theory
of diffuse-reflectance is the Kubelka-Munk theory which assumes the radiation is
composed of two oppositely directed radiation flux through a continuous medium. Using
this theory, diffusive reflectance UV/vis spectra were translated by the Kubelka-Munk
function:
21
2
R KF R
R S
30
where, R indicates the ratio of the diffuse reflectance of the sample to reference material
(spectralon made of polytetrafluoroethylene (PTFE) or barium sulfate), K an absorption
coefficient, S the scattering coefficient of the powder.
Figure 2.6 (a) The schematic layout of UV/vis spectroscopy, (b) optical
geometry of the integrating sphere.
Figure 2.7 shows a UV/vis spectrum of naphthalene radical cations adsorbed in
ZSM-5. The spectra show that there are peaks in the range between 550 and 680 nm and
in the range between 350 and 400 nm. The UV/vis spectrum of the naphthalene radical
cation is well known [2, 9-13]. It is characterized by strong vibronic bands at 675 nm (8
peaks at 675, 653, 635.5, 616.5, 598, 586, 567, and 551 nm) and weak visible absorptions
at 380 nm (2 peaks at 382 and 366.5 nm) in Ar [12, 13].
31
Figure 2.7 UV/vis spectrum of naphthalene radical cation in ZSM-5 (Zeolyst,
SiO2/Al2O3 = 15)
The time-series spectra recorded for 2 days are analyzed to extract to the spectra
of pure species and the concentration change of each species by using an interactive self-
modeling mixture analysis program (SIMPLISMA). This approach extracts pure
component spectra without any basic information about components while a conventional
principal component analysis needs some basic initial information to deconvolute the
multivariable spectra [2, 14, 15]. The SIMPLISMA approach uses the average value of
multivariable spectra and the difference between average value and mixed value to
calculate the pure spectra for the first component [14, 15]. The contribution of the first
component to the multivariable spectra is removed using the difference. The pure spectra
32
for the next component can then be calculated by repeating this procedure. The number
of components and the noise level are chosen as the parameters [14, 15].
2.6 Temperature Programmed Desorption (TPD) of Ammonia
TPD is one of the most frequently used techniques to characterize the acidity of
materials. Here this technique is used to obtain information about the initial state (number
and strength) of the acidic sites of the sample. The measurement is repeated then for both
the high temperature and oxygen treated samples and by difference, we can quantify the
effect of the treatment on the concentration of BAS. A 6.35mm diameter U-shape quartz
flow reactor (Quartz Plus) is connected to a piping network integrated in a catalyst
characterization system (Altamira Instruments, AMI-200i). The reactor is installed in a
clam-shell style furnace to control temperature. Temperature is measured using a K-type
thermocouple and automatically controlled by the control software (Altamira Instruments,
AMI-5200). A bed of quartz wool and quartz chips (Quartz Plus) are placed in the reactor,
and 30 mg of ammonium-exchanged ZSM-5 is put on the bed. The corresponding TPD
protocol for the high temperature treatment and the oxygen treatment is presented in
Table 1 and 2. Figure 2.8 shows the temperature profile with time during the
corresponding TPD experiment to the oxygen treatment.
33
Table 2.1 Temperature-programmed desorption protocol corresponding to the
high-temperature treatment of the zeolite samples.
No. Ramp to Rate Treatment gas Flow rate Procedure
1 200 oC +10 oC/min Inert (He)
Dehydrate
2 550 oC +20 oC/min Inert (He)
Desorption of ammonium
3 100 oC -30 oC/min Inert (He)
Cool down
4 100 oC
Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3
5 200 oC +10 oC/min Inert (He)
6 780 oC +20 oC/min Inert (He)
Desorption of ammonium and dehydroxylate
7 100 oC -30 oC/min Inert (He)
Cool down
8 100 oC
Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3
9 200 oC +10 oC/min Inert (He)
10 550 oC +20 oC/min Inert (He)
Desorption of ammonium
Table 2.2 Temperature-programmed desorption protocol corresponding to the
oxygen treated zeolites.
No. Ramp to Rate Treatment gas Flow rate Procedure
1 200 oC +10 oC/min Inert (He)
Dehydrate
2 550 oC +20 oC/min Inert (He)
Desorption of ammonium
3 100 oC -30 oC/min Inert (He)
Cool down
4 100 oC
Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3
5 200 oC +10 oC/min Inert (He)
6 550 oC +20 oC/min Inert (He)
Desorption of ammonium
7 550 oC
Oxygen (O2) 30cc/min Oxygen treatment
8 100 oC -30 oC/min Inert (He)
Cool down
9 100 oC
Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3
10 200 oC +10 oC/min Inert (He)
11 550 oC +20 oC/min Inert (He)
Desorption of ammonium
34
Figure 2.8 Temperature profile with time and the gases during TPD
corresponding oxygen treatment.
In ammonia TPD, both BAS and LAS can be observed. BAS interact with
ammonia to form ammonium ion, and LAS interact with the unpaired electrons on the
nitrogen of ammonia. The ammonia in LAS is desorbed at lower temperatures than that
in BAS. Different peaks can usually be observed in the TPD trace [16-18].
Ammonia TPD can provide useful information about the initial state of the acidic
site of the sample. However, it is recognized that some limitations of ammonia TPD
technique hinder a quantitative analysis of Bronsted acid sites densities [19, 20]. It was
35
reported that ammonia adsorbs more strongly on CaO than on a USY zeolite [21]. In this
sense, zeolites contain non-framework alumina or other species, for instance Lewis acid
sites, and ammonia can be adsorbed on such non-BAS. In addition, the temperature
observed the peak maxima can be strongly affected by the conditions used for the
measurement. The heat of adsorption can be estimated from the temperature of the
desorption peak using a simple kinetic model [19]. However, the desorption kinetics of
crystalline materials are much complicated because of molecular interactions [22]. The
application of TPD is assumed that adsorption and desorption are in local equilibrium
where diffusion limits the desorption process. However, this assumption is not valid since
desorption and adsorption occur simultaneously with diffusion in these kind of
microporous materials [19]. Therefore, using the TPD of ammonia experiment, strict
quantitative analysis of the obtained profile is not possible.
2.7 Fourier Transform Infrared Spectroscopy (FTIR)
Infrared (IR) spectroscopy has been frequently used to identify and study
molecules and materials, especially to investigate the functional groups present in the
samples. Specific energies are absorbed at specific frequencies matching the normal
mode of vibrating bonds or functional groups. When the dipole moment changes due to
the vibration, the bonds or groups of molecules are observed by IR spectroscopy.
Different molecules vibrate in different ways. The types of vibrational modes are
categorized as symmetrical and asymmetrical stretching, scissoring, rocking, wagging,
36
and twisting. When the size of a molecule is large, the observed peaks in an IR spectrum
are so many that the analysis becomes complicated. An IR spectrum gives exact
information of the position, shape, and height of the abdorption bands. From the spectral
information, we can infer the nature and the concentration of the functional group or
bond and the conditions (environment) where the molecules are placed.
IR spectra are recorded by Fourier Transform Infrared (FTIR) spectroscopy. FTIR
guarantees a fast measurement and good repeatability because all wavelengths
simultaneously pass through the sample. In FTIR, a Michaelson interferometer, which
consists of a beamsplitter, fixed mirror, and moving mirror, is adopted as the optical
component. The interfered beams are transformed by using the Fourier transform, and
then the IR spectrum can be recorded.
In this thesis, the hydroxyl groups in zeolites are observed to establish the effect
of dehydroxylation by the FTIR. Figure 2.9 shows the in-house-build IR cell which is
designed to heat the sample, to keep the sample under low pressure by evacuation, and to
introduce gases to the IR chamber for pre-treatment. Using this IR cell, the sample is
dehydrated first, and then, dehydroxylated at around 800 °C in vacuum and at 500 °C in
an oxygen atmosphere. The sample is then cooled to room temperature and placed across
the beam path. The measurements are recorded in the transmission mode.
37
Figure 2.9 The IR cell – a heater, a reactor, and a gas line
2.8 Gas Chromatography (GC)
Gas Chromatography (GC) is used for identifying chemical compounds in
mixture of unknown compositions. GC passes a sample containing a mixture of
compounds through a column, which is a thin tube, and electronically detects each
component as it reaches, at different times, at the end of a column. Two detectors, a flame
38
ionization detector (FID) and a thermal conductivity detector (TCD) are used in this work.
These are the two most common detectors for GC. FID is more sensitive to hydrocarbons
than TCD, thus is generally used for identifying hydrocarbons. TCD is more frequently
used for detecting hydrogen and inert gases. The GC was calibrated using two mixture
calibration gases (two point calibration).
The GC instrument used for this research is GC model 2014 (Shimadzu) with two
columns. One column is a molesieve connected to a TCD detector and another column is
a RT-alumina connected to a FID detector. The catalytic reaction rate were determined
using a quartz tube plug flow reactor (ID = 5mm). Differential reaction conditions were
used whenever possible to measure reaction rates without the assumption of a reaction
rate expression model.
2.9 Reactor Setup for GC
The reactor, gas connection, and GC (MS) are connected as depicted in Figure
2.10. The reactant gases, including inert gases, oxygen and alkanes, flow through the
quartz tube reactor heated at the specified temperature. The temperature inside the reactor
is monitored by a K-type thermocouple and controlled by a temperature controller
(NC74000, Omega Engineering). Ammonium form of catalysts on the quartz bed inside
the reactor is heated to 200 °C for dehydration, and then it is heated to 450 °C to convert
the ammonium form to the acid form of the zeolite. The samples are treated at 500 °C
(treatment 1) and 800 °C in an inert gas flow (treatment 2) and 500 °C in oxygen flow
39
(treatment 3), respectively. After each treatment, the temperature is lowered to 450 °C for
the reaction with isobutane. The products, produced by contacting the reactant gas to the
pre-treated catalyst, are separated and recorded by GC.
Figure 2.10 GC system setup used to study isobutane cracking process
2.10 Summary
We have described briefly the experimental methods and techniques used in this
thesis. First, the synthesis of zeolite was described followed by the three different sample
treatment protocols. The use of some important characterization techniques was briefly
illustrated. As main experimental techniques, UV/vis spectroscopy for detecting organic
radical cations, ammonia TPD for determining structures of acid sites, and GC with the
reactor setup for measuring catalytic activity and selectivity were explained. The
C4H10
40
generation of sites which have an ability to extract an electron from naphthalene will be
addressed in chapter 3, and catalytic activity and selectivity related to the newly
generated sites will be examined in chapter 4.
41
2.11 References
[1] Kim, S.D., et al., Microporous Mesoporous Mat., 2004. 72(1-3): p. 185-192.
[2] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.
[3] Lowell S., S.J.E., Thomas M.A., Thommes M., Characterization of Porous Solids
and Powders: Surface area, Pore Size and Density. 2004, AA Dordrecht, The
Netherlands: Kluwer Academic Publishers.
[4] Brunauer, S., P.H. Emmett, and E. Teller, J. Am. Chem. Soc., 1938. 60: p. 309-
319.
[5] Sayari, A., et al., Langmuir, 1991. 7(2): p. 314-317.
[6] Carrott, P.J.M. and K.S.W. Sing, Chem. Ind., 1986(22): p. 786-787.
[7] Handreck, G.P. and T.D. Smith, J. Chem. Soc. Faraday Trans., 1989. 85: p. 645-
654.
[8] Skoog, D.A. and J.J. Leary, Principles of Instrumental Analysis. 4th ed. 1992:
Saunders College Publishing.
[9] Andrews, L. and T.A. Blankenship, J. Am. Chem. Soc., 1981. 103(19): p. 5977-
5979.
[10] Andrews, L., B.J. Kelsall, and T.A. Blankenship, J. Phys. Chem., 1982. 86(15): p.
2916-2926.
[11] Kelsall, B.J. and L. Andrews, J. Chem. Phys., 1982. 76(10): p. 5005-5013.
[12] Szczepanski, J., et al., J. Phys. Chem., 1992. 96(20): p. 7876-7881.
[13] Salama, F. and L.J. Allamandola, J. Chem. Phys., 1991. 94(11): p. 6964-6977.
[14] Windig, W. and J. Guilment, Anal. Chem., 1991. 63(14): p. 1425-1432.
[15] Bu, D.S. and C.W. Brown, Appl. Spectrosc., 2000. 54(8): p. 1214-1221.
[16] Woolery, G.L., et al., Zeolites, 1997. 19(4): p. 288-296.
[17] Hunger, B., et al., J. Therm. Anal., 1990. 36(4): p. 1379-1391.
[18] Hunger, B., et al., J. Phys. Chem. B, 2002. 106(15): p. 3882-3889.
[19] Gorte, R.J., Catal. Today, 1996. 28(4): p. 405-414.
42
[20] Gorte, R.J., Catal. Lett., 1999. 62(1): p. 1-13.
[21] Juskelis, M.V., et al., J. Catal., 1992. 138(1): p. 391-394.
[22] Falconer, J.L. and R.J. Madix, Surf. Sci., 1975. 48(2): p. 393-405.
43
Chapter 3
GENERATION OF STABLE ORGANIC RADICAL CATIONS IN THERMALLY
TREATED ZSM-5 ZEOLITES
3.1 Introduction
It has been shown before that when zeolite ZSM-5 is activated by thermal
treatment in an inert gas flow as well as in oxygen, adsorption of organic molecules with
relatively low ionization energy spontaneously give an electron to the zeolite framework
forming stable occluded radical species. These observations point to the generation of
new reaction sites upon thermal treatment. The classical conception of a zeolite catalyst is
based on the presence of Bronsted acid sites in the zeolite pores, sites that play a key role
as catalytically active sites. We observe, as many others have in the past, catalytic
activation of hydrocarbons by Bronsted acid sites when using the first (mild) thermal
treatment described in Chapter 2 (500 °C in Ar). The zeolite activation by the second and
the third thermal treatments leads to important differences in selectivity from the typical
reactions catalyzed by the Bronsted acid sites (the subject of Chapter 4 of this thesis). It is
clear that new catalytic sites are formed by dehydroxylation of Bronsted acid sites in
ZSM-5 by these two treatments. Using naphthalene as a probe molecule, it can be shown
that the new sites have the ability to extract an electron from naphthalene and form stable
44
radical cations. This observation suggests that the new sites can also activate
hydrocarbons by a redox mechanism.
In this chapter, we investigated the formation of these new sites, sites that can
extract an electron from naphthalene and form stable radical cations on the differently
treated ZSM-5 samples. Differences between the sites generated by treatments 2 and 3
can be revealed by comparing the UV/vis spectra of each sample upon adsorbing
naphthalene. Surprisingly, naphthalene radical cations can be generated on samples
treated at temperatures as low as 200 °C. We also observe that after naphthalene radical
cations were generated, single-electron transfers back into the ZSM-5 framework to form
a stable electron-hole pair and reform the naphthalene neutral molecule. Using ammonia
TPD, IR spectra, and UV/vis spectra of the sample with different Si/Al ratios, the
structure of the new generated sites was characterized.
3.2 Generation of Naphthalene Radical Cations
We treated the H-ZSM-5 samples at different conditions as described in chapter 2,
and then in a glove bag flushed with dry argon, mixed the zeolite with the solid
naphthalene using a pestle and mortar. The amount of naphthalene is added enough to
adsorb one molecule of naphthalene per unit cell of the MFI framework. In a very short
time a change of color of the naphthalene-zeolite mixture is evident. At this point, a
portion of the sample is loaded into a UV/vis diffuse reflectance sample cell and the time
evolution of the sample was followed using UV/vis spectroscopy. The first treatment was
45
heating the sample at 500 °C in Ar, the second treatment was heating the sample at
780 °C in Ar to dehydroxylate the Bronsted acid sites of the zeolite. Since earlier research
has shown that oxygen treatment also leads to the formation of the naphthalene radical
cation in H-ZSM-5[1], the third treatment was heating the sample at 500 °C in oxygen to
generate electron-abstracting sites.
Figure 3.1 shows that no reaction occurred between naphthalene and all-silica
ZSM-5 (silicalite-1) treated in Ar at both 780 °C and 500 °C. The samples mixed with
naphthalene commonly have a very large peak at 270 nm (data not shown), which is
attributed to sorption of naphthalene in the ground state within the pore or channel of the
zeolites. The spectra of silicalite-1 treated in Ar at both 780 °C and 500 °C are flat just
like the spectrum of pure silicalite-1 at wavelengths above 270 nm. This result shows that
aluminum is a key component of the sample needed to form electron-abstracting sites,
that is framework aluminum and Bronsted acid sites must be present in the sample before
treatment in order for the initial electron transfer to occur [1]. Also, Figure 3.1 shows that
no reaction occurred between naphthalene and the pre-treated H-ZSM-5 in Ar at 500 °C
(treatment 1). The spectrum is also entirely flat like the spectra of pure silicalite-1 and
pure ZSM-5. The lack of reactivity means that the naphthalene radical cation does not
form on ZSM-5 by reaction with the classical Bronsted acid sites (Si-OH-Al), external
silanol groups, or internal silanol groups. This temperature is deemed too low to start
decomposing the Bronsted acid sites [2-4].
46
Figure 3.1 UV/vis spectra of pure zeolite and naphthalene adsorbed on ZSM-5
zeolite as control experiments: Pure ZSM-5, ZSM-5 in Ar at 500 °C,
Pure Silicalite-1, Silicalite-1 in Ar at 500 °C, and 780 °C
The acid form of ZSM-5 treated in Ar at 780 °C (treatment 2) and treated in O2 at
500 °C (treatment 3) reacted with the naphthalene to form naphthalene radical cations.
This is clearly indicated by the absorption peaks observed in Figure 3.2. The spectra
show that there are peaks in the 550 and 680 nm range and in the 350 and 400 nm range.
The UV/vis spectrum of the naphthalene radical cation is well known [1, 5-9]. It is
characterized by strong vibronic bands at 675 nm (8 peaks at 675, 653, 635.5, 616.5, 598,
586, 567, and 551 nm) and weak visible absorptions at 380 nm (2 peaks at 382 and 366.5
nm) in Ar matrix [8, 9]. According to Hückel molecular orbital energy levels of
47
naphthalene cation [10, 11], the vibronic transition corresponding to 670 nm wavelength
is 2 2
3 2 0g uB D X A D and that corresponding to 380 nm wavelength is
2 2
2 4 0g uB D X A D [8, 9]. Our observed peaks are consistent with the UV/visible
spectra of naphthalene radical cations in an Ar matrix.
The intensity is directly related to the molar concentration of the component based
on Lambert-Beer Law [12];
A dc , 0log /A I I
where, A represents the absorbance, the molar extinction coefficient, d pathlength in cm,
c molar concentration, I intensity of the transmitted light, and I0 intensity of the incident
light. The concentration of the radical cations on the sample treated in O2 at 500 °C is
clearly higher than the concentration of the radical cations on the sample treated in Ar at
780 °C, although the shapes and positions of the characteristic peaks of both samples are
similar.
48
Figure 3.2 UV/visible spectra for control experiments (ZSM-5 in Ar at 500 °C,
Silicalite-1 in Ar at 500 °C and 780 °C) and for naphthalene@ZSM-5
heated to 780 °C in Ar and 500 °C in O2
Besides the intensity, there are other differences between the two spectra. For ease
of comparison, the spectra of the sample treated in O2 at 500 °C and the sample treated in
Ar at 780 °C were rescaled in Figure 3.3. The broad band from 400 to 550 nm is different
between treatments 2 and 3. The sample treated in O2 at 500 °C has a prominent peak at
460 nm in the range of the broad band while the sample treated in Ar at 780 °C does not.
Since this band is related to electronic transitions associated to the electron-hole pair and
electron traps in the zeolite [1], the difference can be related to the structural difference
between sites generated by treatments 2 and 3.
49
Figure 3.3 Rescaled UV/vis spectra for naphthalene adsorbed into ZSM-5 heated
to 780 °C in Ar and to 500 °C in O2
A number of possible models can explain the differences between spectra after the
two treatments: 1) two different sites were generated by the different treatments
(treatments 2 and 3); 2) the sites generated in the sample treated in O2 were much more
reactive than the sites in the sample treated in Ar.
To understand better the differences between the samples, the naphthalene
adsorption experiment was repeated at temperatures lower than 500 °C (always in the
presence of oxygen). As shown in Figure 3.4(a), when the sample was treated at 200 °C,
300 °C, and 400 °C, naphthalene radical cations were readily observed immediately after
50
the sample was mixed with the naphthalene. Even though the treatment temperatures
were low, the intensity levels seemed to be similar for all the temperatures above 200 °C.
When the samples were treated at temperatures less than 100 °C, some small peaks
around 650 and 675 nm were observed, but the characteristic peaks of the radical cation
did not appear over the entire spectral range. It is surprising that naphthalene radical
cations can be generated on the ZSM-5 samples treated at temperatures as low as 200 °C.
The intensity at 675.5 nm – the wavelength of the adsorption peak with the highest
intensity for the naphthalene radical cation – was tracked as a function of time to
investigate the effect of the temperature during the oxygen treatment on the appearance
and disappearance of the radical cations. Since it is assumed that the oxygen-treated
ZSM-5 powder and the naphthalene were completely and evenly mixed, the intensity at
each of the different temperatures was normalized to the intensity of the first spectrum
obtained five minutes after mixing. Initially, the intensities of all the samples increased,
and then after about 100 min the intensity began to decrease. This observation is thought
to be due to the migration of the electrons of the naphthalene radical cations into the
ZSM-5 framework [1].
51
Figure 3.4 (a) The UV/visible spectra after mixing with the naphthalene and (b)
intensity change with time of the 695 nm peak of the samples treated
at different temperatures.
(a)
(b)
52
There were some differences between samples treated at different temperatures,
but the intensity changes with time were essentially the same. Therefore, we can
conclude that the active sites and the number of active sites resulting from the oxygen
treatment were nearly the same by heating at temperatures above 200 °C. The reactivity
of the active sites or the migration mechanism of electrons was not very different,
regardless of the temperature.
Naphthalene radical cations with a single-electron were generated on new sites in
pre-treated ZSM-5 (treatments 2 and 3). It is reported that the electron-transfer from
neutral naphthalene is responsible to the formation of trapped electrons, not the zeolite
framework [13]. It can be considered that the trapped electron is generated by electron-
transfer from neutral naphthalene to a site that can extract a single-electron. During the
time that naphthalene radical cations are formed by electron-transfer, an increase in the
intensity of the absorption spectrum is detected by UV/vis spectroscopy. Then, as the
unpaired electrons migrate to the ZSM-5 framework, the concentration of electron-hole
pairs increases. As a result, the concentration of the naphthalene radical cations begins to
decrease, and concurrently the intensity of the absorption spectrum also starts to decrease.
After the electron-hole pairs are stabilized, the concentrations of both components remain
stable.
53
3.3 Migration of Electrons and Holes within the Zeolite Framework
It is known that the generation and migration of electrons and holes within
zeolites occur after n-acene adsorption, including naphthalene (i.e., n=2) [1, 14].
Moissette et al. have reported that the naphthalene radical cation was generated by
spontaneous ionization after mixing of solid naphthalene with H-ZSM-5 [1, 15]. Then,
electron-hole pairs were generated by electron transfer between the naphthalene radical
cation and the zeolite framework [1, 15]. Finally, after gentle warming at 400K for
several days, the electron-hole pairs disappeared and naphthalene was reformed as the
form of neutral naphthalene adsorbed into the zeolite framework by the charge
recombination from the framework different from the one that was initially lost [1, 15].
The mechanism of single-electron migration is illustrated in Figure 3.5 [1].
Figure 3.5 Mechanism of the single-electron migration in ZSM-5 framework.
54
A series of UV/visible spectra were recorded to trace single-electron migration
from the ZSM-5 structure to the naphthalene radical cations during a 48-hour period after
the sample was mixed with the naphthalene. Figure 3.6 and 3.7 show the series of UV/vis
spectra after treatments 2 and 3, respectively. The characteristic peaks of the naphthalene
radical cation increase with time, and then they begin to decrease at some point. It is
difficult to determine the precise time at which the intensity begins to decrease, because
the cusp of the curves is relatively flat (Figure 3.4(b)) and the times at which the intensity
decrease began were different for different samples and conditions. For instance, the
times at which the intensity begins to decrease were 50 min and 12 hr for the same ZSM-
5 treated at 500 °C in O2 (data not shown). Note that the change of intensity can be
observed for samples treated in both Ar and O2.
There are, however, some important differences between the samples treated in
Ar and O2. First, the times when we start to observe a decrease in the absorption intensity,
was around one hour for the oxygen treated samples (Figure 3.7), but was approximately
15 hours (Figure 3.6) for the sample treated in Ar at 780 °C. While the times when we
start to observe a decrease in the absorption intensity for the oxygen treated samples vary
greatly (50 min - 12 hr) for different samples or different conditions, the times when we
start to observe a decrease in the intensity for the samples after treatment 2 are between
12 - 15 hr. In addition, the broad band from 450 nm to 550 nm of the sample treated at
780 °C in Ar, which is related to the formation electron-hole pairs in the zeolite
framework, still has the peak at 460 nm 2 days after mixing. This band for the sample
treated in O2 relatively becomes much broader and less defined.
55
Figure 3.6 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after
treatment 2 during (a) increasing and (b) decreasing absorption
intensity.
(b)
(a)
56
Figure 3.7 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after
treatment 3 during (a) increasing and (b) decreasing absorption
intensity.
(a)
(b)
57
To investigate the contribution of specific absorption features to the changes in
the spectra with time, the Simple-to-use Interactive Self-modeling Mixture Analysis
(SIMPLISMA) program was used. To start, the spectrum with peaks in the range of 350 -
400 nm and in the range of 550 - 680 nm was resolved by the SIMPLISMA program, as
shown in Figure 3.8(a) for treatment 2 and in Figure 3.9(a) for treatment 3. The peaks in
this spectrum are the ones assigned to the naphthalene radical cations [1, 5-9]. The
second spectrum, with broad peaks around 500 nm, has been explained by assuming that
there is a trapped electron associated with the hole (electron-hole pair) formed upon
recapture of an electron by the naphthalene radical cation, as was concluded in previous
works with biphenyl [1, 16-18]. As we pointed out above, the difference between the
spectra of the samples after treatments 2 and 3, the second pure component, is related to
the electron-hole pair. The sample after treatment 2 has much clear band than the second
pure component of the sample after treatment 3. This difference in the second component
between two samples is related to the structural difference between sites generated by
treatments 2 and 3. The SIMPLISMA methodology provides the changes in the relative
concentration of species with time. The concentration of the naphthalene radical cations
and the electron-hole pairs are illustrated in Figure 3.8(b) for treatment 2 and in Figure
3.9(b) for treatment 3. The concentrations in Figure 3.8(b) clearly reflect the delay in the
onset of the neutral naphthalene regeneration.
58
Figure 3.8 (a) Pure spectral components and (b) concentration change with time
for NPT radical cation and electron-hole pair on the sample after
treatment 2
(a)
(b)
59
Figure 3.9 (a) Pure spectral components and (b) concentration change with time
for NPT radical cation and electron-hole pair on the sample after
treatment 3
(a)
(b)
60
3.4 Structure of the Dehydroxylated Sites
3.4.1 Ammonia TPD
The temperature-programmed desorption (TPD) of ammonia experiment can
provide information about the initial state of the acidic properties of zeolites. An
ammonia TPD experiment for ZSM-5 was performed on a sample after the three thermal
treatments (see Figure 3.10). In what follows, we use the term “Before” to indicate that
the ammonia desorption is performed after subjecting the sample to treatment 1
(increasing temperature to 500 °C in an inert gas). The term “After” means that the
ammonia is adsorbed on and desorbed from the samples after treatment 2 or 3.
In all cases TCD signal profiles with one peak maximum were recorded, as shown
in Figure 3.10. This peak is consistent with the typical profile of ammonia TPD for NH4+
form of ZSM-5 prepared by ammonium exchange [19]. Typical ammonia TPD for ZSM-
5 shows two chemisorptions peaks at a low and at a high temperature. The peak at low
temperature is attributed to non framework aluminum (e.g., Lewis acid sites). The peak at
high temperature is known to be due to Bronsted acid sites in the zeolite [19-21]. Since
aqueous ammonium nitrate, which is the fluid used for the ammonium exchange of ZSM-
5, fills the Lewis acid sites with water rather than ammonia, we observe only one peak at
high temperature for the sample used in this thesis. However, it still cannot be excluded
that some desorption of ammonium from strong Lewis acid sites can be part of (but not
been resolved) in the high temperature peak [21].
61
We have found that treatment 1 does not change the number or character of the
acid sites in either of the two samples (data not shown). That is this treatment has no
effect on the structure or composition of the sample. However, after dehydroxylation at
780 °C, the amount of ammonia desorbed decreases by approximately 50% (see Figure
3.10). This decrease indicates that some of the acid sites initially involved in the
adsorption of ammonia were destroyed by treatment 2. In the other words, the Bronsted
acid sites, which are the site for ammonia chemisorption, were eliminated at high
temperature. In addition, the position of the peak maximum was shifted to a slightly
lower temperature. The temperature of the peak maximum is known to be affected by the
number of acid sites in the zeolite, the change in the zeolite structure, the carrier gas flow
rate, or the amount of the sample [21]. Since the carrier gas flow rate and the amount of
the sample are the same, it can be considered that the decrease of the temperature of the
peak maximum reflects the reduced number of acid sites after dehydroxylation at 780 °C.
In contrast, oxygen treatment at 500 °C did not affect the adsorption of ammonia
on the ZSM-5 sample. Dehydroxylation of Bronsted acid sites can proceed by
dehydration and dehydrogenation channels [2]. We consider that treatment 2 is attributed
to the dehydroxylation of Bronsted acid sites through the dehydrogenation channel [2],
while the dehydration channel is preferred for the dehydroxylation by treatment 3. The
dehydration channel generates Lewis acid sites and an Al tetrahedron in the zeolite
framework containing a negative charge. The Lewis acid sites, can adsorb ammonia,
while the latter sites, being basic, can not. However, the same amount of desorption of
ammonia indicates that certain sites, which can react with ammonia, are generated by
62
decomposition of the Bronsted acid sites. Thus, the combination of the remaining
Bronsted acid sites and the newly generated sites by treatment 3 (oxygen treatment)
makes the amount of desorbed ammonia the same. With this information, we can
understand better the UV/vis spectra of naphthalene@ZSM-5 after treatments 2 and 3.
The intensity observed after treatment 3 (oxygen treatment) was higher than that of
treatment 2. This means that the reactivity of the sites generated by treatment 3 with the
naphthalene to form the naphthalene radical cation is stronger than the reactivity of the
redox sites, which are formed by dehydrogenation channel (treatment 2) [2].
Figure 3.10 TPD corresponding for (a) treatment under Ar at 780 °C and (b)
treatment under O2 at 500 °C.
(a)
Tmax=414 °
C
Tmax=393 °
C
63
Figure 3.10 Continued
3.4.2 IR Spectroscopy and the Thermal Decomposition of Bronsted Acid Sites
Figure 3.11(a) shows the FTIR spectra of dehydrated ZSM5-18 sample. The
samples have Bronsted acid sites and silanol groups after heating at 500 °C. Bronsted
acid sites have an absorption band around the 3600 cm-1
region of IR spectrum [22]. The
IR peak at 3720-3750 cm-1
is assigned to silanol groups [23]. The peak at 3666 cm-1
is
assigned to hydroxyl groups attached to extraframework aluminum [23, 24]. Low
temperature CO adsorption studies using FTIR have revealed that the peak at 3666 cm-1
is less acidic than the normal Bronsted acid sites but more acidic than the silanol group
[23]. Extraframework aluminum [1], extra-lattice amorphous materials [25, 26], or the
silanol group [27] has been considered to the assignment of the peak at around 3700 cm-1
.
(b)
Tmax=410 °
C
64
The intensity of the Bronsted acid sites stretching vibration decreases after
heating to 780 °C (Figure 3.11(a)). This is evidence of the dehydroxylation of the
Bronsted acid sites during high-temperature treatment. However, the spectrum of the
sample after heating to 500 °C in oxygen flow does not show the decrease of the intensity.
The intensity after treatment 3 looks lower than the intensity before treatment 3. However,
all peaks still remain with relatively small intensities. The IR spectra and the TPD signals
in the previous section suggest that certain unidentified sites, which behave like Bronsted
acid sites in TPD experiment and IR spectroscopy, are generated after treatment 3.
Figure 3.11 FTIR spectra in the OH vibration region of ZSM-5 measured (a) after
heated to 500 and 800 °C, and (b) after heated to 500 and 500 °C in
an oxygen flow.
(a)
65
Figure 3.11 Continued.
3.4.3 Effect of Si/Al Ratio
The Si/Al ratio affects the acidity of the zeolites. Since Al is directly associated
with the formation of Bronsted acid sites in zeolites, the number of acid sites per unit cell
increases, as the Si/Al ratio decreases. At the same time the sites become weaker
although this does not occur until Si/Al ratios below 10 [28, 29]. The effect of the Si/Al
ratio on the amount of ammonia desorption and naphthalene adsorption was investigated.
In the previous sections, the Si/Al ratio of the sample investigated was 18. Here we
compare these results to a zeolite with a lower Si/Al ratio of 12 (higher alumina
concentration).
(b)
66
Figure 3.12 compares the TCD signals of ammonia TPD of ZSM-5 with Si/Al
ratio of 18 and ZSM-5 with Si/Al ratio of 12. The TCD signals were rescaled to show the
differences more clearly. The amount of ammonia desorption of ZSM-5 having Si/Al
ratio of 12 is larger than that of ZSM-5 with Si/Al ratio of 18 because the sample with
low Si/Al ratio has more acid sites. The areas below the desorption peak were calculated
between 220 and 560 °C of the temperature range for quantitative comparison and
tabulated in Table 3.1. The areas of ZSM5-12 before treatments 2 or 3 are 1.5 and 1.2
times larger than the areas of ZSM5-18. Since ZSM5-12 has more acid sites, the
maximum peak temperatures (420, 401, 411 °C) are also higher than the maximum
temperatures of ZSM5-18 (414, 393, 410 °C). The desorption areas reduced by 40.7 %
and 36.5% after treatment 2 while the areas reduced by 4.7% and 3.4% after treatment 3.
But generally the effect of dehydroxylation is qualitatively the same: the intensity of the
second TPD traces decreases and the position of the peak maximum shifts to lower
temperatures.
The most remarkable difference between the UV/vis spectra of the sample with a
high Si/Al ratio and the sample with low Si/Al ratio is the level of the naphthalene cation
absorption intensity. For both treatments 2 and 3, the sample with Si/Al ratio of 18 has
higher intensities than the sample with Si/Al ratio of 12. This is counterintuitive because
the sample with a low Si/Al ratio definitely has more acid sites, as confirmed by the TPD
experiments. However, the reaction by which the naphthalene radical cations are formed
occurs less frequently when the total acid sites increase as the aluminum content
67
increases. We suggest that this is likely due to differences in the Bronsted acid sites
decomposition mechanism.
ZSM-5 (Si/Al=18) ZSM-5 (Si/Al=12)
Tre
atm
ent
2
Tre
atm
ent
3
Figure 3.12 TPD of ZSM-5 having Si/Al of 12 compared with TPD of ZSM-5
having Si/Al of 18.
68
Table 3.1 The desorption areas and the maximum temperatures of TPD signal.
Sample Process Area Max. Temp., °C
ZSM5-18
Treatment 2
Before 554.5 414
After 328.9 393
Reduction, % 40.7
Treatment 3
Before 476.7 410
After 454.4 410
Reduction, % 4.7
ZSM5-12
Treatment 2
Before 849.1 420
After 539.2 401
Reduction, % 36.5
Treatment 3
Before 588.1 411
After 567.9 411
Reduction, % 3.4
From the TPD experiments, we concluded that Lewis acid sites were generated
in ZSM-5 after treatment 3 (oxygen treatment) while redox sites were generated in ZSM-
5 treated at 780 °C in Ar (treatment 2). However, this does not mean that only one type of
sites is generated by each treatment. We can consider that the sites generated by
treatments 2 and 3 or that the third and fourth type of site is formed. From the UV/vis
spectra of naphthalene adsorbed in ZSM-5 with Si/Al ratio of 18, we know the sites
generated by treatment 3 are much more reactive with naphthalene than the sites formed
by treatment 2. In this sense, at high Si/Al ratio of 18, the decomposition pathway leads
predominantly to the same type of sites generated by treatment 3, even at the high
temperature treatment (treatment 2). At low Si/Al ratio of 12, it seems that the formation
69
of the same type of sites formed by treatment 2 in ZSM-5-18 becomes dominant. The
adsorption of aromatic molecules can be affected by the concentration of guest molecules
or surface diffusion based on structures [13, 14]. Since the sample with a low Si/Al ratio
has more acid sites, their density in zeolite sample is considered to be high. The
interaction between the generated sites by decomposition of Bronsted acid sites or
between naphthalene molecules (e.g, form naphthalene anion [13]) can be also
considered to be a possible explanation. In addition, we know that the more Bronsted acid
sites remain in ZSM-5-12 than in ZSM-5-18 from the TPD experiments. The remaining
Bronsted acid sites could have some effect on the reaction with naphthalene. Further
research, however, is needed to decide what the exact chemical identities of these sites
are. This is further discussed in the next chapter using the hydrocarbon cracking reaction.
70
Figure 3.13 UV/visible spectra of ZSM-5 sample with different Si/Al ratios for
(a) treatment 2 and (b) treatment 3.
(b)
(a)
71
3.5 Conclusions
Now sites, that can extract a single-electron from a neutral organic molecule, are
generated by dehydroxylation of Bronsted acid sites in ZSM-5 framework. Two
treatments, which are the high temperature treatment in an inert gas (Ar) and the oxygen
treatment, are used for dehydroxylation of Bronsted acid sites. Naphthalene radical
cations are formed as a result of the electron transfer from neutral naphthalene molecule
to the newly generated sites into the ZSM-5 framework after mixing naphthalene with the
treated ZSM-5.
The characteristic peaks of the naphthalene radical cation were detected by
UV/visible spectroscopy on the samples of the ZSM-5 zeolite after two different thermal
treatments. The spectra of the sample after treatment 3 show higher intensity than the
spectra of the sample after treatment 2. The broad band from 400 to 550 nm is different
between treatments 2 and 3. For treatment 3, naphthalene radical cations can be generated
on the ZSM-5 samples treated at temperatures as low as 200 °C. These observations
suggest that different sites are generated by different treatments for dehydroxylation.
A series of UV/visible spectra were recorded to trace single-electron migration
from the ZSM-5 structure to the naphthalene radical cations during a 48-hour period after
the sample was mixed with the naphthalene. The characteristic peaks of the naphthalene
radical cation increase with time, and then they begin to decrease. This observation and
deconvoluted component spectra by SIMPLISMA algorithm support the generation and
migration of electrons and holes within zeolites.
72
Ammonia TPD experiment shows that the newly generated sites in ZSM-5 after
treatments 2 and 3 are different sites. The sites generated in the sample treated in O2 are
much more reactive than the sites in the sample treated in Ar. At high Si/Al ratio of 18,
the decomposition pathway leads dominantly to the same type of sites generated by
treatment 3, even in treatment 2. At low Si/Al ratio of 12, it seems that the formation of
the same type of sites formed by treatment 2 in ZSM-5-18 becomes dominant.
These observations suggest that the most common site generated is different
depending on each treatment. We cannot establish the identity of newly generated sites
by decomposition of Bronsted acid sites. In the next chapter, this conclusion is validated
using a different approach, i.e., isobutane conversion.
73
3.6 References
[1] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.
[2] Nash, M.J., et al., J. Am. Chem. Soc., 2008. 130(8): p. 2460-2462.
[3] Stamires, D.N. and J. Turkevich, J. Am. Chem. Soc., 1964. 86(5): p. 749-&.
[4] Uytterhoeven, J.B., L.G. Christner, and W.K. Hall, J. Phys. Chem., 1965. 69(6): p.
2117-2126.
[5] Andrews, L. and T.A. Blankenship, J. Am. Chem. Soc., 1981. 103(19): p. 5977-
5979.
[6] Andrews, L., B.J. Kelsall, and T.A. Blankenship, J. Phys. Chem., 1982. 86(15): p.
2916-2926.
[7] Kelsall, B.J. and L. Andrews, J. Chem. Phys., 1982. 76(10): p. 5005-5013.
[8] Szczepanski, J., et al., J. Phys. Chem., 1992. 96(20): p. 7876-7881.
[9] Salama, F. and L.J. Allamandola, J. Chem. Phys., 1991. 94(11): p. 6964-6977.
[10] Clark, P.A., F. Brogli, and E. Heilbronner, Helv. Chim. Acta, 1972. 55(5): p.
1415-1428.
[11] Obenland, S. and W. Schmidt, J. Am. Chem. Soc., 1975. 97(23): p. 6633-6638.
[12] Ingle, J.D.J.a.C., S.R., Spectrochemical Analysis. 1988, New Jersey: Prentice Hall.
[13] Hashimoto, S., et al., J. Chem. Soc. Faraday Trans., 1996. 92(19): p. 3653-3660.
[14] Turro, N.J., et al., J. Am. Chem. Soc., 2000. 122(47): p. 11649-11659.
[15] Marquis, S., et al., C. R. Chim., 2005. 8(3-4): p. 419-440.
[16] Gener, I., A. Moissette, and C. Bremard, Chem. Comm., 2000(17): p. 1563-1564.
[17] Moissette, A., et al., Angew. Chem. Int. Ed. Engl., 2002. 41(7): p. 1241-1244.
[18] Gener, I., G. Buntinx, and C. Brémard, Angew. Chem. Int. Ed. Engl., 1999.
38(12): p. 1819-1822.
[19] Woolery, G.L., et al., Zeolites, 1997. 19(4): p. 288-296.
[20] Topsøe, N.-Y., K. Pedersen, and E.G. Derouane, J. Catal., 1981. 70(1): p. 41-52.
74
[21] Kapustin, G.I., et al., Appl. Catal., 1988. 42(2): p. 239-246.
[22] Szanyi, J. and M.T. Paffett, Microporous Mater., 1996. 7(4): p. 201-218.
[23] Zecchina, A., et al., J. Chem. Soc. Faraday Trans., 1992. 88(19): p. 2959-2969.
[24] Kustov, L.M., et al., J. Phys. Chem., 1987. 91(20): p. 5247-5251.
[25] Borade, R., et al., J. Phys. Chem., 1990. 94(15): p. 5989-5994.
[26] Jacobs, P.A. and R. Von Ballmoos, J. Phys. Chem., 1982. 86(15): p. 3050-3052.
[27] Védrine, J.C., et al., J. Catal., 1979. 59(2): p. 248-262.
[28] Shirazi, L., E. Jamshidi, and M.R. Ghasemi, Cryst. Res. Technol., 2008. 43(12): p.
1300-1306.
[29] Gayubo, A.G., et al., J. Chem. Technol. Biotechnol., 1996. 65(2): p. 186-192.
75
Chapter 4
EFFECT OF HIGH TEMPERATURE ON THE CATALYTIC ACTIVITY OF
ZEOLITE H-ZSM-5 FOR ISOBUTANE CONVERSION
4.1 Introduction
We have shown in the previous chapters that Bronsted acid sites in zeolites are
decomposed by thermal treatment into redox sites and Lewis acid sites. Since zeolite acid
catalysts are known to catalyze a number of hydrocarbon reactions, it is of scientific
interest and of potential technological value to investigate the impact of the newly-
generated sites on hydrocarbon reactions. To be specific, the effects of these new sites on
hydrocarbon cracking processes will be investigated by determining the distributions of
product formed in the cracking process, the cracking-to-dehydrogenation ratio, and the
activation energy of the reactions.
The activation of small alkanes over acid sites has been investigated extensively
because of its relevance to technologically important processes such as fluidized catalytic
cracking in petroleum refineries, but also because C-H and C-C bond activation is of
fundamental scientific interest [1, 2]. In our group‟s previous work, the propane cracking
process was scrutinized using various zeolite catalysts, such as ZSM-5, beta zeolites, and
mordenite [3]. The distribution of products produced by propane is controlled by
76
cracking on the classical Bronsted acid sites (after mild activation such as treatment 1) [1,
3]. After dehydroxylation was performed on the acid zeolite by a high temperature
treatment (treatment 2), the dehydrogenation rate increased significantly compared to the
rate before dehydroxylation [3]. When the sample was treated in an oxygen flow
(treatment 3), the distribution of products changed slightly; only a small decreased
cracking-to-dehydrogenation ratio is observed.
The isobutane cracking process is the topic of investigation in this chapter. There
are several reasons to look beyond the propane cracking process, discussed previously,
into the isobutane cracking process. An important aspect of our investigation is to
examine the initial step for isobutane cracking. The protonation of C-H or C-C bonds,
which is referred to as a protolytic reaction mechanism, has been considered by many
researchers to be the initial step of hydrocarbon activation in acid zeolites. Several groups
have proposed that carbonium or carbenium ion intermediates on Bronsted acid sites in
zeolites play the key role in the activation of isobutane [1, 4-7]. The effect of Lewis acid
sites on activity and selectivity has also been investigated. While Lewis acid sites
enhance the adsorption of the reactant and increase the strength of Bronsted acid sites by
withdrawing electron density [2], turnover frequency (TOF) does not correlate with
Lewis acid sites concentration [1]. Thus, we can consider that Lewis acid sites, by
themselves, are not responsible for the activation of alkanes. In contrast to this view,
McVicker et al. have suggested that radical cation intermediates play an important role in
the conversion of isobutane [8]. They also suggested that highly acidic catalysts crack
isobutane by a combination of radical cation and carbonium ion routes [8].
77
Different reactivity has been observed in the cracking of isoalkanes and the
cracking of normal alkanes (n-alkanes) [1, 9]. For n-alkanes, monomolecular
dehydrogenation is preferred rather than the cracking process on acid catalysts [1].
However, the cracking process, rather than dehydrogenation, is the preferred reaction
channel for isobutane [9]. Moreover, the relationship between the cracking-to-
dehydrogenation ratio and temperature is different for the two classes of compounds,
with a direct relationship for one class (n-alkanes) and an inverse relationship for the
other class (iso-alkanes). Based on our observation for propane activation on thermally
treated zeolites, the study of the behavior of isobutane is a clear next step. According to
several reports on light hydrocarbon conversions [1, 9-11], the rate of the reaction of
isobutane is higher than the rate of the reaction of propane. Thus, small amounts of
catalysts (5 - 10 mg in this thesis) are used for isobutane conversion rather than propane
reaction (70 mg in our previous work).
In this chapter, we examine the catalytic activity of the ZSM-5 samples treated
under different conditions by using isobutane as a reactant. The samples were treated 1)
at 500 °C in He, 2) 800 °C in He, and 3) 500 °C in O2 before contacting the catalyst with
the isobutane stream. The products were separated and quantified using gas
chromatography (GC). The isobutane reaction on Bronsted acid sites and on
dehydroxylated sites can be discussed in relation to our group‟s previous work on
propane. Investigation of the distributions of products and the conversion of isobutane
may help to clarify the structures after Bronsted acid sites after decomposition and the
structure-activity relationships in hydrocarbon conversions.
78
The cracking and the dehydrogenation of isobutane can be analyzed using
thermodynamics including the enthalpy of reaction, Gibbs free energy of reaction,
equilibrium constants, and conversions. Table 4.1 shows the relevant thermodynamic
information of the isobutane reaction [12, 13]. The enthalpies of reaction change slightly
as the temperature increases from 480 to 560 °C: the enthalpies of cracking reaction
decrease ~1 kJ/mol, while those of dehydrogenation reaction increase ~0.2 kJ/mol. The
Gibbs free energies of both cracking and dehydrogenation reactions decrease ~12 kJ/mol.
The equilibrium conversions of both cracking and dehydrogenation process increase with
temperature because the reactions are endothermic. The cracking of isobutane shows
higher equilibrium conversion than the dehydrogenation of isobutane. The information
can provide us an insight into the further analysis in this chapter.
Table 4.1 Thermodynamic properties for cracking and dehydrogenation reaction of
isobutane.
Temp.
(°C)
Cracking Dehydrogenation
4 10 3 6 4iC H iC H CH 4 10 4 8 2iC H iC H H
rxnH rxnG eqK Conversion rxnH rxnG eqK Conversion
kJ/mol % kJ/mol %
480 76.3 -38.1 436.7 99.989 122.1 16.8 0.0678 67.502
560 75.3 -50.2 1397.7 99.997 122.3 5.7 0.4420 91.034
4.2 Kinetic Analysis of Isobutane Cracking on Zeolites
Several groups have studied the isobutane cracking reaction [4, 8, 9, 14, 15].
These studies conclude that the initial step of the reaction associated with isobutane
79
conversion, which proceeds by monomolecular and bimolecular channels, occurs
primarily on Bronsted acid sites. At low conversion, the monomolecular reaction
dominates the product distribution. Scheme 4.1 illustrates the rate-limiting step, which is
the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. The
reaction rate expressions for cracking and dehydrogenation are similar and are expressed
by equation 1. At low conversion and high temperatures, the rate expression can be
simplified to a first order rate equation (equation 2) since most of the sites are vacant.
The intrinsic activation energy and entropy are represented by equations 3 and 4. From
transition state theory, the intrinsic rate constant can be expressed in term of the transition
state enthalpy and entropy as shown in equation 5 [1, 9]. The measured entropy is scaled
by the number of C-C bonds for cracking and H-C bonds for dehydrogenation (equation
6).
Scheme 4.1 Protolytic mechanism of isobutane (cracking and dehydrogenation)
80
4 10
4 10 3 6 4 8
1
3
1 4 41
C H
C H c C H d C H
K Pr k
K P K P K P
1
4 10 4 103 1 C H app C Hr k K P k P 2
int meas adsE E H 3
int meas adsS S S 4
5
ln ln /meas meas BS R A k T h 6
The product distribution for the protolytic monomolecular reaction mechanism is
represented as the ratio of the rate constants (k3c/k3d). The measured activation energy
and entropy are estimated from the Arrhenius plot of the rate data obtained at 450 -
560 °C. The weight hour space velocity (WHSV) is set at ~30 h-1
for a short residence
time because low conversion is needed to limit the reaction only to the monomolecular
mechanism. Equimolar ratios of methane to propylene, which confirm that only the
monomolecular reaction occurs, were observed for all experiments.
81
4.3 Effect of Pre-treatment of the Sample on Isobutane Conversion and Selectivity
Isobutane conversions and product distributions were measured in a plug flow
quartz reactor (ID = 4 mm) with a bed supported by fine quartz chips. The ammonium
form of catalyst (5 - 10mg) was loaded in the reactor and activated at 500 °C under He
(99.999%) flow of 100 sccm (treatment 1); at 800 °C under He flow of 100 sccm
(treatment 2); and at 500 °C under O2 (99.999%) flow of 70 sccm (treatment 3). Small
amount of catalysts was used for the reaction because of higher rate of the isobutane
reaction than the propane reaction. Different experiments were performed on separate
samples for each treatment. The flow rate of isobutane (Matheson tri-gas, research grade,
99.995%) was fixed so that WHSV was ~30 h-1
in order to minimize the residence time
and decrease conversion. The WHSV is calculated using the following relationship;
WHSV = mass flow / catalyst mass
When WHSV is low, the reaction intermediates have enough time to react through
bimolecular reaction mechanisms. However, when WHSV is high, the contact time of the
reaction intermediates with the catalysts is decreased. In order to obtain high WHSV, the
mass flow rate of the reactant needs to increase and the catalyst mass needs to be reduced
as much as possible. The temperature measured at the catalyst bed was controlled
between 450 and 560 °C and the products were separated and recorded by the GC. The
GC instrument used is GC model 2014 (Shimadzu) with two columns. One column is a
molesieve connected to a TCD detector for H2 and N2 detection and another column is a
RT-alumina connected to a FID detector for hydrocarbon analysis.
82
Table 4.2 Conversion of isobutane over ZSM5 for three different treatments.
Conversion, %
Temp., °C
ZSM5-18 ZSM5-12
Treatment 1 Treatment 2 Treatment 3 Treatment 1 Treatment 2 Treatment 3
450 0.199 0.069 0.171 0.076
480 0.028 0.718 0.116 0.081 0.408 0.124
500 0.065 1.275 0.174 0.183 0.868 0.205
530 0.192 3.333 0.445 0.557 2.300 0.538
560 0.577 7.299 0.911 1.525 5.670 1.504
Table 4.2 shows the conversions for each treatment. For both samples (ZSM5-18
and ZSM5-12), the conversions for treatment 2 were significantly greater than the
conversions for treatment 1. For treatment 3, the conversions were slightly different from
the conversions of treatment 1, but the differences were not significant. As discussed in
chapter 3, dehydroxylation at high temperatures leads predominantly to the formation of
redox sites, and these sites produce significantly increases in the conversions of the
isobutane reaction. The dehydroxylation obviously leads to the loss of Bronsted acid sites.
When we compared sample ZSM5-18 with sample ZSM5-12 for treatment 1, the
conversions of ZSM5-12 were greater than the conversions of ZSM5-18 because ZSM5-
12 has more acid sites. Table 4.3 also shows turn-over frequencies (TOF) per aluminum
for ZSM5-12 are higher than those for ZSM5-18 after treatment 1.
83
Table 4.3 TOF per aluminum for ZSM5-12 and for ZSM5-18 after treatment 1.
Temp.
°C
TOF (mol/s Al mol) × 10-5
ZSM5-18 ZSM5-12
Cracking Dehydrogenation Cracking Dehydrogenation
450 1.89 3.24
480 1.71 1.87 3.68 3.30
500 3.43 4.10 7.22 8.36
530 10.78 11.61 24.00 22.90
560 35.17 30.95 73.07 54.24
For treatment 2, the observed reaction rate is the sum of the reaction rate of the
dehydroxylated Bronsted acid sites (new sites) and the remaining Bronsted acid sites. The
contribution of newly generated sites can be separated from the contribution of remaining
Bronsted acid sites using the information from the TPD experiments in Chapter 3. The
remaining Bronsted acid sites are still responsible to the protolytic process while the
newly generated sites are responsible to the redox chemistry process. The reaction rates at
reaction temperature 500 °C were calculated for both ZSM-5 samples in Table 4.4. The
reaction rate contributed by the newly generated sites was significantly larger than the
reaction rate on the remaining Bronsted acid sites. It is assumed that there is a certain
effect to enhance the isobutane conversion after treatment 2 contributed by the newly
generated sites.
84
Table 4.4 Contribution of newly generated sites and remaining Bronsted acid sites
after treatment 2.
Reaction rate at 500 °C, r [mol/s∙gcat] × 108
Cracking Dehydrogenation
Remaining BAS New sites Remaining BAS New sites
ZSM5-18
Treatment 1 3.01 3.60
Treatment 2
133.03 28.28
1.78 131.25 2.13 26.25
59.3% 59.3%
ZSM5-12
Treatment 1 8.96 10.37
Treatment 2
91.45 25.63
5.69 85.76 6.58 19.05
63.5% 63.5%
The conversions of ZSM5-18 after treatment 2, however, exceeded those of
ZSM5-12. As was noted in Chapter 3, at high Si/Al ratio, the decomposition of the
Bronsted acid sites pathway produces more redox sites that the low Si/Al ratio sample.
Therefore, the conversion of ZSM5-12 after treatment 2 was lower than the conversion of
ZSM5-18. From the Chapter 3 and literature [16, 17], it is expected that Lewis acid sites
were formed mainly by the decomposition of the Bronsted acid sites in treatment 3. Even
if the Lewis acid sites enhance the adsorption of reactant and increase Bronsted acid sites
strength by withdrawing electron density [2], the effect on conversion was small
compared to the redox sites generated by treatment 2.
85
The product distributions are shown in Figure 4.1. For treatment 1, the cracking-
to-dehydrogenation ratios were approximately 1. However, for treatment 2, the cracking-
to-dehydrogenation ratios showed a significant increase. This observation is opposite to
the change in product distribution in the propane activation process, for which the
cracking-to-dehydrogenation ratios decrease by treatment 2 [1, 3]. This opposite tendency
also was reported in a recent report by Gounder and Iglesia [1, 9]. It was observed by
Gounder and Iglesia that the selectivity of the isoalkane and n-alkane cracking reactions
on acid catalysts change in opposition to each other [9]. For treatment 3, even if the
cracking-to-dehydrogenation ratios increase with temperature, it was considered that the
results are due to the small number of redox sites generated by treatment 3, rather than
the effect of the Lewis acid sites.
86
Figure 4.1 The cracking-to-dehydrogenation ratios with temperature for (a)
ZSM5-18 and (b) ZSM5-12.
(a)
(b)
87
Measured activation energies (Table 4.5) were calculated assuming first-order
kinetics. For treatment 1, the measured activation energies were consistent with values
found in literature [9]. The activation energy for cracking was ~ 197 kJ/mol, and that for
dehydrogenation was ~ 182 kJ/mol. Measured activation energies determined in this
work were 15 - 16 kJ/mol larger for cracking than those determined for the
dehydrogenation of isobutane for both ZSM-5 samples. These differences were also
consistent with values found in literature [7, 9, 14]. These differences are related to the
high cracking-to-dehydrogenation ratios. It is explained that the transition states for
cracking is less stable than the transition states for dehydrogenation since the activation
energies for cracking is higher than the activation energies for dehydrogenation [1, 9].
Table 4.5 Measured activation energies for isobutane cracking and
dehydrogenation after each treatment.
Sample Cracking
Ea (kJ/mol)
Dehydrogenation
Ea (kJ/mol)
Difference
Ea,c – Ea,d
ZSM-5-18 – treatment 1 197.8 182.4 15.4
ZSM-5-18 – treatment 2 163.5 142.2 21.3
ZSM-5-18 – treatment 3 183.2 92.4 90.8
ZSM-5-12 – treatment 1 196.8 180.8 16
ZSM-5-12 – treatment 2 164.3 142.7 21.6
ZSM-5-12 – treatment 3 192.6 124.3 68.3
88
After treatment 2, the measured activation energies for both ZSM-5 samples
decreased significantly to ~ 164 kJ/mol for cracking and to ~142 kJ/mol for
dehydrogenation. We observed that the redox sites in zeolites have the ability to extract a
single-electron from neutral organic molecules in Chapter 3. Likewise, the redox sites can
also oxidize molecules that have high ionization potential such as ethylene (IP = 10.52
eV) [18]. The ionization potentials of isobutane, isobutylene, propane, and propylene are
tabulated as Table 4.6 [19]. The IP of isobutane is 10.57 eV and therefore, the reaction of
isobutane over the dehydroxylated sample after treatment 2 could proceed by radical
cation chemistry [20]. The IP of propylene is 9.73 eV and the IP of isobutylene is 9.23 eV.
The isobutylene is less stable than the propylene. Therefore, the selectivity to the
formation of propylene is much higher than the formation of isobutylene when the radical
cation chemistry plays a kinetically controlling role. The combination effect of the
stability of transition state and the difference of ionization potential between the
isobutylene and the propylene significantly increase the selectivity on cracking rather
than dehydrogenation.
Table 4.6 Ionization potentials of light hydrocarbons
Isobutane Isobutylene Propane Propylene
Ionization potential (eV) 10.57 9.23 11.07 9.73
89
The measured activation energies for cracking after treatment 3 were slightly less
than those after treatment 1. However, the measured activation energies for
dehydrogenation decreased very significantly to 92.4 kJ/mol for ZSM5-18 and 124.3
kJ/mol for ZSM5-12. From Chapter 3, the generated sites by treatment 3 also have the
ability to extract a single-electron from neutral organic molecules. In addition, it is
expected that the transition state for cracking is less stable. However, the selectivity is
slightly different from the selectivity of the sample after treatment 1. Even if there is a
combination effect like the case of treatment 2, the activation barriers for
dehydrogenation are so small that the selectivity on cracking cannot significantly increase
as was observed on the sample after treatment 2.
The rate constants as a function of reciprocal temperature after treatments 1 and 2
are shown in Figure 4.2. When the sample was treated at 500 °C in He, the rate constants
for cracking and for dehydrogenation were of similar magnitude. After treatment 2, the
rate constants for cracking became larger than those for dehydrogenation, while the rate
constant for both processes were greater than those for treatment 1. After treatment 3, the
rate constants for dehydrogenation were greater than those for cracking at low
temperatures. At high temperatures, the rate constants for cracking became larger than
those for dehydrogenation. This observation is basically the same to the rate constants
after treatment 1, while the effect of treatment 3 magnifies the difference between the rate
constants for cracking and dehydrogenation. The changes that were observed for the
activation energies and the rate constants suggest that the generated sites have a different
nature altogether from Bronsted acid sites.
90
Figure 4.2 Arrhenius plots for isobutane cracking and dehydrogenation (a)
ZSM5-18 and (b) ZSM5-12.
(b)
(a)
91
We have proposed that a single electron deficient site (a redox site) was generated
by the high temperature treatment (treatment 2) of the acid zeolites. The electron
deficient site leads to catalytic chemistry and is responsible for the variations in the
kinetic parameters and for variations in the distribution of products observed above. In
addition, they suggest a different reaction mechanism from the classical protolytic
mechanism. In our group‟s previous work [3], it was suggested that the presence of redox
sites resulted in radical cation chemistry instead of protolytic chemistry in the propane
cracking process [21]. Other researchers also have proposed this radical chemistry as the
mechanism for the cracking of isobutane and n-butane [8, 22]. Therefore, we believe that
the reaction of isobutane over the zeolite sample that had redox sites proceeded through
the radical cation chemistry mechanism. Scheme 4.2 represents a feasible mechanism for
the unimolecular reaction of isobutane when redox sites are available.
92
Scheme 4.2 Suggested pathway of monomolecular reaction of isobutane over
redox sites on ZSM-5.
4.4 Conclusions
In addition to the catalytic effect of typical acid catalyst, Lewis acid sites and
redox sites, which are generated by high temperature treatment and oxygen treatment for
dehydroxylation, also promote alkane cracking processes. The conversions of the samples
after treatments 2 and 3 are greater than the conversions of the samples after treatment 1
(acid catalyst). For treatment 1, the conversions of ZSM5-12 are greater than the
conversions of ZSM5-18 because ZSM5-12 has more acid sites. However, the
conversions of ZSM5-18 become greater than the conversion of ZSM5-12 after
treatments 2 and 3.
93
When conversion is low, the product distribution is limited to the monomolecular
cracking of the C-C bond and dehydrogenation of the C-H bond. In the propane
conversion reaction, the dehydrogenation process is preferred over the cracking process
on the zeolite samples after treatments for the dehydroxylation. However, in the
isobutane conversion in this thesis, the cracking-to-dehydrogenation ratio significantly
increases after dehydroxylation treatments.
The measured activation energies are larger for cracking than those for the
dehydrogenation of isobutane for both ZSM-5 samples. The transition states for the
cracking process are less stable due to the higher measured activation energies. The sites
generated by dehydroxylation have the ability to form organic radical cations. The effect
of the stability of transition state and the radical cation chemistry affect the selectivity on
cracking process. The change of the rate constants after treatments 2 is much larger that
the change of the rate constants after treatment 1.
While Lewis acid sites enhance the reactivity of Bronsted acid sites and rarely
affect the conversions and selectivity of the isobutane reaction, the redox sites result in a
reaction that is based on radical cation chemistry. We proposed that the presence of redox
sites resulted in radical cation chemistry instead of protolytic chemistry in the propane
and isobutane cracking process. In the industrial hydrocarbon cracking processes, such as
the FCC process, the existence of redox sites can affect the distribution of products in the
way that have not been considered by the zeolite community to this point.
94
4.5 References
[1] Gounder, R. and E. Iglesia, J. Am. Chem. Soc., 2009. 131(5): p. 1958-1971.
[2] van Bokhoven, J.A., et al., J. Catal., 2004. 224(1): p. 50-59.
[3] Al-majnouni, K.A., HIGH TEMPERATURE DECOMPOSITION OF BRONSTED
ACID SITES: STRUCTURES FORMED AND THEIR CATALYTIC ACTIVITY
TOWARD SMALL ALKANES ACTIVATION, in Chemical Engineering. 2011,
University of Delaware: Newark.
[4] Engelhardt, J., J. Catal., 1996. 164(2): p. 449-458.
[5] Xu, B., et al., J. Catal., 2006. 244(2): p. 163-168.
[6] Sendoda, Y. and Y. Ono, Zeolites, 1988. 8(2): p. 101-105.
[7] Corma, A., P.J. Miguel, and A.V. Orchilles, J. Catal., 1994. 145(1): p. 171-180.
[8] McVicker, G.B., G.M. Kramer, and J.J. Ziemiak, J. Catal., 1983. 83(2): p. 286-
300.
[9] Gounder, R. and E. Iglesia, Angew. Chem. Int. Ed. Engl., 2010. 49(4): p. 808-811.
[10] Hoobler, R.J., B.J. Opansky, and S.R. Leone, The Journal of Physical Chemistry
A, 1997. 101(7): p. 1338-1342.
[11] Copeland, L.R., et al., J. Chem. Phys., 1992. 96(8): p. 5817-5826.
[12] Sandler, S.I., Chemical, Biochemical, and Engineering Thermodynamics. 4 ed.
2006: John Wiley & Sons, Inc.
[13] NIST Chemistry Webbook. Available from: http://webbook.nist.gov/chemistry/.
[14] Yaluris, G., et al., J. Catal., 1995. 153(1): p. 65-75.
[15] Sanchez-Castillo, M.A., et al., J. Catal., 2002. 205(1): p. 67-85.
[16] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.
[17] Marquis, S., et al., C. R. Chim., 2005. 8(3-4): p. 419-440.
[18] Yoon, K.B., Chem. Rev., 1993. 93(1): p. 321-339.
95
[19] Occupational Training Inc.; Available from:
http://www.otrain.com/OTI_MSDS(IP)800.html.
[20] McAdoo, D.J., S. Olivella, and A. Solé, The Journal of Physical Chemistry A,
1998. 102(52): p. 10798-10804.
[21] Narbeshuber, T.F., et al., J. Catal., 1997. 172(1): p. 127-136.
[22] Bizreh, Y.W. and B.C. Gates, J. Catal., 1984. 88(1): p. 240-243.
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Chapter 5
CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS
5.1 Summary
Bronsted acid sites are known to be the active sites in many hydrocarbon
conversions processes by forming carbonium or carbenium ions as reaction intermediates.
In some industrial hydrocarbon processes, such as fluidized catalytic cracking (FCC),
Bronsted acid sites are decomposed by dehydroxylation at high temperatures under an
oxidizing environment. After Bronsted acid sites are decomposed, new sites having a
different „chemical‟ nature from Bronsted acid sites are generated. The catalytic
chemistry of the zeolites must be affected by the newly generated sites. In this thesis, the
Bronsted acid sites are decomposed by a high temperature pretreatment and an oxygen-
rich pretreatment. The structures and the reactivity of the newly generated sites by
decomposition of Bronsted acid sites were investigated and compared with the Bronsted
acid sites.
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5.1.1 Generation of Organic Radical Cations in Thermally Treated ZSM-5 Zeolites
The newly generated sites can extract single-electrons from neutral organic
molecules having small ionization potential, such as naphthalene, to form radical cations.
The adsorbed naphthalene, the naphthalene radical cations, electron-hole pairs, and
reformed naphthalene were observed by time-series UV/vis spectroscopy inside the
ZSM-5 framework. Ammonium TPD experiment provided information about how many
Bronsted acid sites are left, and some information about the structure of the newly
generated sites by the different treatments. The sites generated in the sample treated in O2
are much more reactive than the sites in the sample treated in Ar towards naphthalene.
The amount of ammonia desorbed from ZSM-5 decreased after the high temperature
treatment while there is no significant change after the oxygen treatment. At high Si/Al
ratio of 18, the decomposition pathway leads dominantly to the same type of sites
generated by treatment 3, even in treatment 2. At low Si/Al ratio of 12, it seems that the
formation of the same type of sites formed by treatment 2 in ZSM-5-18 becomes
dominant. These observations suggest that the most frequently generated site is different
depending on each treatment.
5.1.2 Catalytic Activity of ZSM-5 Zeolite for Isobutane Conversion
The reactivity and selectivity of isobutane conversion on these sites are also
revealed to be different depending on treatment. The conversion of isobutane
significantly increased after treatment 2 while the conversion of isobutane only slightly
98
increased after treatment 3. The product distribution is categorized into the
monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. Before
dehydroxylation (acid catalyst), the cracking-to-dehydrogenation ratio is around 1.
However, the cracking-to-dehydrogenation ratio significantly increased after the high
temperature treatment while the ratios are slightly affected by the oxygen treatment. The
measured activation energies are larger for cracking than those for dehydrogenation. The
higher activation energies are related to less thermodynamically stable transition states.
Thus, the cracking process is preferred to the dehydrogenation process for isobutane on
the new reaction sites. The kinetic parameters suggest that the newly generated sites after
treatments 2 and 3 have a different nature from Bronsted acid sites and each other. We
have proposed that the presence of redox sites resulted in radical cation chemistry instead
of protolytic chemistry in the propane and isobutane cracking process.
5.2 Future Research Directions
5.2.1 Determination of the Structure of Lewis Acid Sites and Redox Sites
To elucidate the structure of newly generated sites (assuming Lewis acid sites and
redox sites) after the high temperature treatment in an inert gas (treatment 2) and the
oxygen treatment (treatment 3), further experiments are necessary.
We have collected structural information using ammonia TPD experiment and
FTIR spectroscopy in this thesis. To reveal the actual Lewis acid sites, other TPD
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experiment, such as TPD of isopropylamine should be explored. To elucidate the
structure of the acid sites after dehydroxylation, spectroscopic techniques, such as X-ray
absorption spectroscopy, would also be helpful. Al K edge XANES has been applied to
investigate the local coordination environment of aluminum in several zeolites [1-4].
Bugaev et al. reported that the aluminum atoms remain in the zeolite framework at high
temperatures [5]. We expect that the structure of acid sites in zeolite after
dehydroxylation and the pathway for dehydroxylation of high-silica and low-silica
zeolites can be revealed by the X-ray spectroscopy.
In this thesis we have used ZSM-5 zeolite as the main catalyst. ZSM-5 has a high
silicon aluminum ratio and it is one of most versatile heterogeneous catalysts known. In
addition, due to the pore size (~5.6 Å ) of ZSM-5, it shows a good confinement for
naphthalene (~5 Å along its short dimension), our probe molecule for the formation of
radical cation. However, ZSM-5 is known to have 24 crystallographically different
tetrahedron sites [6, 7] in the monoclinic setting. This complexity of the structure of
ZSM-5 makes the analysis of dehydroxylation of Bronsted acid sites very difficult
although all the sites retain their original coordination sequences and the distortion is
subtle [6, 7]. We need to examine other kinds of zeolites with a simpler structure than
ZSM-5. Chabazite (CHA) and Ferrierite (FER) are possible alternatives for further
experiments. Chabazite has only one topologically distinct T-site and only 4 non-
equivalent oxygen atoms that upon protonation from four non-equivalent Bronsted acid
sites [8, 9]. Ferrierite has 4 non-equivalent tetrahedron atoms [10]. As simpler structures
are investigated, in addition to ZSM-5, we can identify the structural characteristics that
100
lead to Lewis acid sites and redox sites. Since Chabazite is a small-size pore zeolite, other
probe molecules other than naphthalene will be needed to observe the generation of the
sites that can extract a single-electron from a neutral organic molecule. Since Ferrierite is
a medium pore size zeolite, newly generated sites by decomposition of Bronsted acid
sites can be observed using naphthalene in the same way used in this thesis. For catalyst
activation, Chabazite and Ferrierite are also expected to provide much information about
the Lewis acid sites and redox sites present in the sample. Based on our previous report
[3, 11] and this thesis, propane and isobutane reactions can be examined using both
catalysts and analyzed to clarify the reaction pathway and the local structures involved in
the catalytic activity.
5.2.2 Low Temperature CO Oxidation
Low temperature oxidation is an important academic and industrial issue because
the high temperatures are required for oxidation by current catalysts, such as platinum,
limit its applicability for purification processes such as CO removal from hydrogen gas.
For example, currently catalysis by Au is of high interest since it was discovered that
novel gold catalysts are highly active for H2 and CO oxidation at low temperature as low
as 70 °C [12]. It would be interesting if we could oxidize CO at low temperatures using
catalysts without the need for expensive transition metals.
In a few preliminary experiments, using the generated electron-hole pair from the
interactions of naphthalene with ZSM-5 after treatments for dehydroxylation, CO
101
oxidation was observed at low temperatures (close to the room temperature). CO2 was
generated from the oxidation of CO with O2 (300% excess) at temperature between 40 to
80 °C even though the generation of CO2 was not sustainable over long periods of time.
Above 80 °C, no further oxidation was observed, probably because the naphthalene was
destroyed. We still need further experiments to prove that the generated electron-hole
pair is involved to the formation of CO2, and to clarify chemistry behind this process.
However, there is the potential using other molecules besides naphthalene, to carry out
this reaction under conditions in which long-term catalytic stability is obtained.
Figure 5.1 CO2 production from naphthalene-ZSM-5 catalyst
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5.3 References
[1] van Bokhoven, J.A., A.M.J. van der Eerden, and D.C. Koningsberger, J. Am.
Chem. Soc., 2003. 125(24): p. 7435-7442.
[2] Xu, B., et al., Journal of Catalysis, 2006. 241(1): p. 66-73.
[3] Nash, M.J., et al., J. Am. Chem. Soc., 2008. 130(8): p. 2460-2462.
[4] Joyner, R.W., et al., A soft X-ray EXAFS study of the local structure of tetrahedral
aluminium in zeolites, in Recent Advances in the Science and Technology of
Zeolites and Related Materials, Pts a - C. 2004. p. 1406-1410.
[5] Bugaev, L.A., et al., J. Phys. Chem. B, 2005. 109(21): p. 10771-10778.
[6] Cejka, J., A. Corma, and S. Zones, Zeolites and catalysis : synthesis, reactions
and applications. 2010, Weinheim; Chichester: Wiley-VCH ; John Wiley,
distributor].
[7] Fyfe, C.A., et al., Nature, 1982. 296(5857): p. 530-533.
[8] Stoyanov, S.R., et al., J. Phys. Chem. C, 2008. 112(17): p. 6794-6810.
[9] Lo, C. and B.L. Trout, J. Catal., 2004. 227(1): p. 77-89.
[10] Pinar, A.B., et al., J. Catal., 2009. 263(2): p. 258-265.
[11] Al-majnouni, K.A., HIGH TEMPERATURE DECOMPOSITION OF BRONSTED
ACID SITES: STRUCTURES FORMED AND THEIR CATALYTIC ACTIVITY
TOWARD SMALL ALKANES ACTIVATION, in Chemical Engineering. 2011,
University of Delaware: Newark.
[12] Haruta, M., et al., J. Catal., 1989. 115(2): p. 301-309.