the effect of an electric field on the hydrogenation of ......subhas kumar sikdar a thesis submitted...
TRANSCRIPT
-
The effect of an electric field on thehydrogenation of ethylene on zinc oxide
Item Type text; Thesis-Reproduction (electronic)
Authors Sikdar, Subhas K.
Publisher The University of Arizona.
Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.
Download date 02/06/2021 03:19:31
Link to Item http://hdl.handle.net/10150/318141
http://hdl.handle.net/10150/318141
-
THE EFFECT OF AN ELECTRIC FIELD ON
THE HYDROGENATION OF ETHYLENE ON ZINC OXIDE
by
Subhas Kumar Sikdar
A Thesis Submitted to the Faculty of the
DEPARTMENT OF CHEMICAL ENGINEERING
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
-
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfilment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
Richard D. Williams /DateAsst. Prof. of Chemical Engineering
-
'ACKNOWLEDGMENTS
I express my gratitude to the Department of Chemical
Engineering for the financial support which enabled me to pursue
higher education in this country. I am thankful to Professor R. D.
Williams, my adviser, for his guidance and useful suggestions during
the course of this research.
I appreciate the cooperation, obtained from the faculty
members and fellow .graduate students of this department. Special
mention should be made of Mr. S. A. Shinde, who did the drawings of
the reactor and its various parts, and of Dr. N. R„ Schott for his
overall helpful attitudes. It is my pleasure to thank Mrs. I. A, .
Shafiqulla for typing the final copy of this thesis.
Finally, I like to express my indebtedness to my parents
who from several thousand miles offered me a constant source of
encouragement.
iii
-
TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS ........ vi
LIST OF TABLES , . ............ ix" -A B S T R A C T ................. . . . . . . . , . . . . . . . . xi
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 1
THEORY AND-PREVIOUS WORK .............. 12
APPARATUS AND EXPERIMENTAL PROCEDURE.......... 29
A Flow Reactor . . . . . . . . . .Reactor.............. . . .A s s e m b l y ........ ..Sand Bath ..................Chromatograph ‘........ . .The High Voltage Source . . .
Experimental Procedure . . . . . .Calibration of Flow Meters Activation of the Catalyst Analysis by the Chromatograph Kinetic Runs . . . . . . . .
RESULTS AND DISCUSSIONS . . . . . . . . . . . . . . . . . . . 50
Method of Preparation and Activation . . .......... 51Catalyst Effectiveness .............. 53Kinetic Runs, No Field.......... . 53
Ethylene Dependence . ................. 56Hydrogen Dependence . . . . . . . . . ......... 63
Effect of the F i e l d .......... 71Kinetic Runs, with Field............. 75
Ethylene Dependence 75Hydrogen Dependence............ 75
Activation Energy .......... . . . . . . 83Mechanism............... '.......... 87
Temperatures greater than 150°C................. 88Temperatures less than 150 C ............ 89
CONCLUSIONS AND RECOMMENDATIONS . . ...................... .91
2930 35 38 40 40 42 42 424445
iv
-
V
TABLE OF CONTENTS— Continued
Page
APPENDIX A: DERIVATION OF EQUATION 18. . . . ................92
APPENDIX B: CALIBRATION CURVES . . 93
APPENDIX C: KINETIC DATA WITHOUT THE F I E L D ................ 98
APPENDIX D: KINETIC DATA IN PRESENCE OF A FIELD........... 109
APPENDIX E: ARRHENIUS ACTIVATION DATA . . . . .............114
APPENDIX F: NOMENCLATURE ............ . .......... . . . 1 1 7
LIST OF REFERENCES ..............................118
-
LIST OF ILLUSTRATIONS
Figure Page
.1 o Lattice Structure of Non-stoichiometric Zinc Oxide, . . . 6
. 2« Energy Band Diagram of Intrinsic Semiconductors ........ 7
3. Energy Band Diagrams Showing Situations for (a) Donor: Type and (b) Acceptor Type Semiconductors . . . . . . . 7
4. Effect of Temperature on the Fermi Level of Semiconductors,(a) n-Type (b) p-Type . . . . . . ........ . . . . . . . 10
5. Reactor Assembly ................ 32
6. Exploded View of the Reactor................ 33
7. Electrodes and Transite Rings . . . . . . . 34
8. Schematic of the Apparatus............... . 39
9. A Typical Chromatogram Showing Separation of (1) Hydrogen(2) Ethylene and (3) Ethane in a Porapak Q Column at 50°C. 46
10. Order with Respect to Ethylene, 195°C, 20-28 MeshCatalyst . * .......... 58
11. Ethylene Order at 1950C, Run la Replotted onCartesian Coordinates ........................ . . . . . 59
12. Order with Respect to Ethylene, 146°C, 20-28 MeshCatalyst * . ................................ 60
13. Order with Respect to Ethylene, 146°C, 35-48 MeshCatalyst ......... 61
14. Order with Respect to Ethylene, 100°C, 20-28 MeshCatalyst . . . . . . . . . . . . . 61
15. Order with Respect to Ethylene, 56.5°C, 35-48 MeshCatalyst .......... 62
16. Order with Respect to Hydrogen, 195°C, 20-28 MeshCatalyst * » ............ 64 *
17. Hydrogen Order as Shown in Cartesian Plots * .......... 6.5
vi
-
vii
LIST OF ILLUSTRATIONS— Continued'
Figure Page
18o Order with Respect to Hydrogen, 146°C, 20-28 MeshCatalyst.......... .......................... 66
19. Order with Respect to Hydrogen, 146°C, 35-48. MeshCatalyst . .............. . . . . . . . . . . . . . . 67
20. Order with Respect to Hydrogen, 100°C, 20-28 MeshCatalyst . 68
21. Order with Respect to Hydrogen, 56.5°C, 35-48 MeshCatalyst ................ . ........ . . . . . . . . 69
22. Hydrogen Order as Shown in Cartesian Plots 70
23. Ethylene Order in Presence of a Field of 3,800 Volts,146°C, 35-48 Mesh Catalyst . . . . . . . . 76
24. Ethylene Order in Presence of a Field as ShownIn Cartesian Coordinates, 146°C, V = 3,800 . . . . . . 77
25. Ethylene Order in Presence of a Field of 4,180 Volts,86.5°C, 35-48 Mesh Catalyst, , ....................... 78
26. Hydrogen Order in Presence of a Field of 3,800 Volts,1460C, 35-48 Mesh Catalyst . . . . . . 79
27. Hydrogen Order in Presence of a Field as Shown in,Cartesian Coordinates, 146°C, V = 3,800. . . . . . . . . 80
28. Hydrogen Order in Presence of a Field of 4,180 Volts,86c50C, 35-48 Mesh Catalyst , * ........................ 81
29. Hydrogen Order in Presence of a Field as Shown inCartesian Coordinates, 86,5°C, V = 4,180 * ............. 82
30. Activation Energy Plots of Zinc Oxide Catalyst „ . . . 85
31. Activation Energy Plots of Zinc Oxide Catalyst,with and without a Field . . . . 86
32. Calibration Curve for Hydrogen Flow Meter , . . . . . . 94
-
viii
LIST OF ILLUSTRATIONS™ Continued'
Figure Page
33. Calibration Curve for Ethylene Flow Meter . . . . . . 95
34. Calibration Curve for Helium Flow M e t e r ............ 96
35. Calibration Curve for Ethylene Estimation 97
-
LIST OF TABLES
Table ' Page
1.. Experiment on Catalyst Activity . 54I .
2 c Experiment on Catalytic Activity . . . . . . . . . . . 55
3. Observed Orders of Reaction „ . . . . . . . . . . • . 57
4. Effect of Field Strength on the Rate of Reaction . . . 72
5c Effect of Field Duration on the Rate of Reaction » . . 72
6, Effect of Field Strength on Reaction Rate atLow Temperature» . 73
7* Effect of Field Duration on Rate . ........ . . . . . 73
8e Activation Energies Under Different Conditions » , ♦ . 84
9. Data of Ethylene Order Study at 195°C (Run lb) • . . . 99
10c Data of Hydrogen Order Study at 195°C.(Run la) » . « . 10U
11c Data of Hydrogen Order Study at 146°C (Run 2b) « . . . 101
12. Data of Ethylene Order Study at 146°C (Run 2a) . . . . 102
13. Data of Hydrogen Order Study at 146°C (Run 3b) . . . . 103
14. Data of Ethylene Order Study at 146°C (Run 3a) . . . . 104
15. Data of Hydrogen Order Study at 100°C (Run 4b) . . . . 105
16. Data of Ethylene Order Study at 100°C (Run 4a) ...... 106
17. Data of Ethylene Order Study at 56.5°C (Run 5a) . . . 107
18. Data of Hydrogen Order Study at 56.5°C (Run 5b) . . . 108
19. Data of Ethylene Order Study with Field at 146°C~Run,15f(a) . . , . ... ................ .. . . . . . 110
20. Data of Hydrogen Order Study with Field at 146°C-R m 155 (b) ............................. Ill
-
X
LIST' OF TABLES— Continued
Table Page
21. Data of ethylene order study with field at 86.5°C~Run 16f (a). . . . '.............. 112
22. Data of Hydrogen Order Study with Field at 86-. 5°C-.Run 16.f (b ) * ’ . ............ . . .113
23. Data for Activation Energy Plots . . . . . . . . . . . . 115
24. Data for Activation Energy Plots with andwithout Field ........................................ 116
-
ABSTRACT
Semiconducting oxide catalysts are characterized by non
stoichiometry in their lattice structures, A correlation between the
catalytic activity and non-stoichiometry has been a major question for
such oxidesc Since application of an electric field modifies the semi
conductivity of an oxide catalyst, a study of a catalytic reaction in
presence of a field would lead to a changed reaction pattern. A study
of this kind has been made here on the hydrogenation of ethylene on
zinc oxide (an n-type semiconductor) with an A.C. electric field up to
about 5,000 volts. The field appeared to decelerate the reaction rate
at a higher temperature, e.g., 150°C whereas at a"lower temperature,
e.g., 85°C, it appeared to accelerate it. The reaction kinetics have
been found to be too complicated to be explained by the existing mecha
nistic models.
The reaction was studied in the temperature range of 560C to
195°C without the field and a mechanism has been suggested on the basis
of the Langmuir-Hinshelwood model.
-
CHAPTER I
INTRODUCTION
Catalytic hydrogenation of ethylene was discovered and first
studied by the French chemists9 Sabatier and Senderens (in 1) who used
a finely divided Ni catalyst. Their discovery then triggered off a
whole series of detailed studies of this reaction in particular and
the hydrogenation of olefins and aromatics in general in the following
years. Solid catalysts for hydrogenation of unsaturated hydrocarbons
can be conveniently classified into three groups (2) : (a) metallic
catalystss mostly studied are platinum, nickel, palladium, cobalt etc.
(b) insulator oxides like alumina, silica and silica-alumina(c) semi
conductor oxides like zinc oxide and nickel oxide. Among the classes
mentioned, semiconductor oxide catalysts are the least studied and the
least understood catalysts in relation to hydrogenation of olefins.
However, some papers have recently reported kinetic studies of the
hydrogenation of ethylene catalized by zinc oxide in batch reactors.
In the present work, a study of the hydrogenation of ethylene
on zinc oxide catalyst (which is an n-type semiconductor) in a dif
ferential flow reactor was undertaken, and the effect of an A.C.
electric field on the reaction rate, activation energy and mechanism
was analyzed. The reason for choosing this reaction is that this is
by far the simplest reaction known among olefin hydrogenations and is
also relatively free,of undesirable side reactions. Despite the large
-
volume of work which has been done on ethylene hydrogenation, the
mechanism of the reaction is still debated (3) and apparently depends
on the particular catalyst and the conditions used = An attempt was
made in the present work to indicate a possible mechanism in relation
to the particular catalyst used on the basis of the result obtained
from the application of the electric field on the catalyst particles.
The reason an effect of the field on the kinetics is expected and how
it should related to a mechanism will be given shortly.
For an element or a compound to exhibit semiconductivity, it
must have a defect of some sort in its crystal lattice. Perfect crys
tals seldom, if ever, exist (4). As a matter of fact, at any tempera
ture greater than absolute zero, one should expect from thermodynamics
to find seme form of lattic imperfections : the increased disord^L
brings about an increased entropy and ̂ hence a decreased free energy
F = U - TS
where F, U and S are, respectively, the Helmholtz free energy, the
internal energy and the entropy, T being the absolute temperature.
The semiconducting oxides important as catalysts have molecular
non-stoichiometry as their defect in the lattice. That is, they have
either metal or oxygen atoms in excess of a stoichiometric proportion
in the lattice. Easy ionization of the metal or the oxygen atom gives
rise to, respectively, free electrons, in which case the substance is
called an n-type semiconductor, and free holes (or free positive char
ges) in which case it is called a p-type semiconductor. The holes, like
the electrons, are capable of conducting electric current. Obviously,
this kind of behavior would not be expected from an insulating oxide.
-
3
Attempts have been made in recent times to prove that the
catalytic property of semiconducting oxides is due to their semi
conductivity but without much success. According to electronic
theory of catalysis on semiconductors (5), since the free electrons or
the holes of the semiconductor lattice can be accelerated by the appli
cation of an external electric field on the substance, the rate of a
catalytic reaction can be modified. Almost no work has been initiated
in this direction.
Pure metallic zinc and pure stoichiometric zinc oxide have
been found to be inactive to hydrogen chemisorption (6). Zinc oxide,
after a treatment in vacuo or hydrogen at an elevated temperature,
however, shows activity towards hydrogen. This contrast in behavior
suggests that the active species are neither zinc nor zinc oxide, buu
probably non-stoichiometric zinc oxide. According to Wagner's thermo
dynamic theory of defective oxides, at sufficiently high temperature,
an equilibrium exists between solid zinc oxide and gaseous oxygen
whereby excess of zinc atoms (Zn^, interstitial) can be accomodated in
interstitial positions of the lattice :
Zn‘h2 + 0~2 = Zni + 1/2 Og
The interstitial Zn can then be thermally ionized :
Zn^ = Zn+ + e
Parravano and Boudart (6) have shown that a pure single crystal of
-
4zinc oxide will become a non-stoichiometric semiconductor through chemi-
sorption of hydrogen at high temperatures. Given the following repre
sentation of ZnO crystal
0 2 - Zn+2 - 0-2 - Zn+2 - 0 2
chemisorption of hydrogen will at first lead to
— —— 0 2 — (ZnH)^" — (OH) — Zn+2 — 0 2 -----
The adsorption will not stop at this stage. The (ZnH)+ complex may
dissociate, the proton moving to the next oxygen ion to form another
stable hydroxyl ion
Zn — (OH) — Zn — (OH) — Zn~̂ 2 — 0 2
That gives an interstitial Zn atom in the lattice. The ionization
energy of interstitial Zn in the crystal is much lower than for an
isolated zinc atom, as a consequence of the high dielectric constant,
k, of the crystal. For an isolated zinc atom, the ionization energy
is 9.4 eV. In the crystal it is equal to 9.4/k^; k for zinc oxide is
about 10, thus the ionization energy is only 0.1 eV. This suggests
that at moderate temperatures the interstitial zinc atom will ionize
freely, releasing one electron which can be accelerated by an external
electric field. The lattice structure of non-stoichiometric zinc oxide
is shown in Figure 1.
-
The band theory of solids can be utilized to explain the
development of semiconductivity in substances in general♦ Two bands of
interest in this case are the so called valence band and the conduction
band, which in all crystals are separated by what is called the for
bidden energy gap (Figure 2). Figure 2(a) depicts the case of the
semiconductor at absolute zero at which temperature no free electrons
exist in the conduction band and no free holes in the valence band. At
high temperatures, however9 as shown in Figure 2(b), thermal ionization
will cause electrons to cross the energy barrier to get to the conduc
tion band,
Figure 3 depicts the energetics of donor or acceptor type of
semiconductors (7). Here only the region between the top of the
valence band and the bottom of the conduction band is of interest.
Figure 3(a) would describe the energetics of, say, n-type ZnO and
Figure 3(b) would describe that of, for example, p-type NiO*
The free electrons from interstitial zinc atoms lie in a
higher energy state than the normal valence electrons of the ZnO
molecule. Hence a localized extra level of electrons must lie above
the top of the filled valence band. But since there is some binding
energy for an electron in this state to remain on the ZnO, the level
must lie below the lowest free electron state in the conduction band.
An analogous argument would explain why the acceptor levels, arising
from NiO must lie as shown in Figure 3(b).
A great deal of chemisorption studies on semiconducting
oxides have been made to date. The result has been to throw some
light on the electronic mechanism that is involved in such a process.
-
6
Z n 2 o2 Z n 2 o2 Z n 2'
0 Z n 2-20 Zn2
-20
7 4- ® In
+ 2 Zn 0
+ 2 In 0
12n
042Zn
-20
-20
-20
-20
42Zn
Figure 1. Lattice Structure of Non-stoichiometric Zinc Oxide.
-
7
conduction band
forbidden energy gap
valence band
energy
electrons
T production
recombination
+ “h f 4" 'h h + 4 -f-holes
(a) Low Temperature (b) High Temperature
Figure 2. Energy Band Diagram of Intrinsic Semiconductors.
conduction band conduction band
donor level energy
acceptor level -♦»
valence band
4- 4 4 4 4 4 4-
(a) (b)
Figure 3. Energy Band Diagrams Showing Situation for (a) Donor Type and (b) Acceptor Type Semiconductors.
-
The theory that has evolved is called the boundary layer theory of
chemisorption (8).
Chemisorption on the surface of a semiconductor is followed
by the rise or fall of the electrical conductivity, A fall in the
conductivity of a p-type semiconductor signifies the transfer of elec
trons from the chemisorbed molecules to the solid. As is obvious3 this
is due to the neutralization of positive charge carriers (holes) by the
electrons. Similarly3 a fall in the conductivity of an n-type semi
conductor indicates an electron transfer from the conduction band of
the semiconductor to the adsorbed species, ■ To bring the charge carriers
from the donor or acceptor levels to the conduction band, one has to
supply the necessary energy, thermal or otherwise. It is to be noted
that in the above mentioned two cases, although the net result is the
same, the mechanism of the depletion of carriers is the opposite. In
contrast to the above phenomenon, known as depletive chemisorption, the
other type, known as cumulative chemisorption, is characterised by a
rise in conductivity in both n- and p-type semiconductors. Clearly,
cumulative 'chemisorption involves cationic chemisorption on an n-type
(e.g., H2 on ZnO) and anionic chemisorption on a p-type (e.g., on Nib) semiconductor. Both of these adsorption phenomena are restricted
to the surface. The concentration of electrons in the bulk will remain
unchanged. The direction of electron transfer depends on the relative
position of the. Fermi potentials in the semiconductor and the adsorbing
gas, and electron flow will take place until these are identical.
Fermi potential is defined as the potential at which the probability
of being occupied by electrons is equal to one half, that is, on the
-
average-50% of available valence electrons have energy equal to the
Fermi level. This flow of electrons will result in a space charge
appearing between the surface of the semiconductor and its interior (2).
Application of Fermi statistics shows that the number- of particles in
a particular state depends upon the energy gap between the acceptor or
donor level and the.Fermi level. This means that the adjustment of the
Fermi level in a catalyst will alter the concentration of the adsorbed
species in a system; in catalysis it can influence the selectivity of
a reaction if more than one reaction route exists. These predictions
have been experimentally verified, although a completely satisfactory
explanation of what actually happens is yet to be found. One method
of altering the Fermi level is to increase.the temperature. Figure
4(a) shows the Fermi level of an n^type semiconductor being depressed
with temperature as more electrons are being drawn into the conduction
band. The reverse occurs with a p-type semiconductor. Another method
of accomplishing the same thing is the application of an electric
field normal to the surface where chemisorption is occuring, thereby
changing the Fermi level of the crystal. One difficulty in the inter
pretation must, for example, arise from the alteration of the Fermi
level by the adsorbed species themselves. Such changes will be a
function of the extent of the adsorption of the reactants and the
products (2), An experiment on the chemisorption of oxygen on ZnO (9)
coated on a pyrex plate across which a D.C. electric field was applied
showed that the chemisorption could be hastened or decelerated accord
ing to whether the positive electrode was on the top or at the bottom
-
10
conduction band
T
valence band
conduction band
acceptors
valence band
(b)
Figure 4. Effect of Temperature on the Fermi Level of Semiconductors, (a) n-Type (b) p-Type.
-
11
of the pyrex plate. Another very interesting aspect of the modifi
cation of the Fermi level of a semiconductor by the application of a
field of 229000 volts A.C. across NiO catalyst and the subsequent
acceleration in the rate of carbon monoxide oxidation has revealed
that the increase in rate is a function of the frequency of the field
(10), This experiment showed that in this particular reaction, other
things remaining constant, rate increased first with frequency, reached
a maximum and then decreased with further increase in frequency.
The present work was an attempt to examine the effect on the
hydrogenation of ethylene of applying 60 cycle A.C. fields up to
5,000 volts across a bed of zinc oxide catalyst particles. The
reactions were carried out in the temperature range of 70 to 250° C.
No attempt was made to examine the effect of frequency.
-
CHAPTER 2
THEORY AND PREVIOUS WORK
Being the simplest of all olefin hydrogenation reactions,
ethylene hydrogenation has caught the attention of a multitude of
investigators. Most of the data has been obtained with nickel and
platinum. Other noble and transition metals have also been studied.
Earlier works were done on reduced powders, more recently they were
done on wires activated by oxidation and reduction and on the extremely
active deposits formed by evaporation of a film in vacuo (11).
A bimolecular surface reaction like the hydrogenation of
ethylene may in general proceed by either of two mechanisms : (a)
adsorption of the two reactants on adjacent active sites, followed by
their interaction and desorption of the products or (b) the inter
action between an adsorbed molecule of one species with a molecule of
the other from the gas phase. The former is called a Langmuir-
Hinshelwood mechanism and the latter a Rideal-Eley mechanism.
The rate of reaction between two species (say A and B) in a
bimolecular surface catalyzed reaction, is proportional to the probabi
lity that A and B are adsorbed on adjacent sites to effect a reaction.
This probability in turn, is proportional to the product of surface
coverages of A and B, and the rate expression, therefore, can be
written as v^ = k 9^ 9^
where 9^ and 9g represent the surface coverages of A and B; k is the
12
-
reaction rate constant. For the case where both A and B compete for
the same surface adsorption sites and are not dissociated on adsorp
tion :
bA p aeA =
( 1 + bA Pa + bB PB >and
bB pBeB -------------------------
( 1 + bA PA + bB PB ^where b^, bg are the adsorption coefficients and p^, Pg, the partial
pressures of A and B respectively. Thus the rate is given by
k bA b B pA pBv h = : (i)
̂ i + da PA + uB PB )2
It can be inferred here, a fact that has been borne out by
experiments in some hydrogenation reactions, that if p^ or p^ is kept
constant and the other varied, the rate passes through a maximum when
p^b^ = Pgbg* The decrease in rate at high pressures may be explained
by assuming that the more strongly adsorbed species displaces the
other species.from the surface as its pressure is increased.
Further simplifications in specific cases can be made from
equation 1: ' 1
(a) If both A and B are weakly adsorbed giving a surface which is
sparsely covered -
-
14The hydrogenation of ethylene over copper catalysts satisfies this
second order rate equation under certain conditions.
(b) In the case where one of the reactants, say A, is weakly adsorbed
so that
(c) When the reactant A is very weakly adsorbed and B is adsorbed with
Low temperature hydrogenation of ethylene over copper follows this
kind of behavior, i.e., the rate is proportional to hydrogen partial
pressure but varies inversely as the pressure of ethylene. In all
these cases it is assumed that the product of hydrogenation is not
adsorbed nor does it inhibit the reaction.
acting species compete for the same kind of surface active sites. If,
however, the species adsorb on different kinds of surface sites, the
surface coverages will be given by the following expressions:
bA PA « (1 + bB PB)» one gets
k b A bn Pa PA B A fBvH (3)
(1 + bB PB)2
Here the rate is proportional to p for constant pB, and, for constantAp^, the rate passes through a maximum as p^ increases.
sufficient strength fui. b^ p^ >> 1 to be satisfied, equation 3 simplifies
to
vH kpB
(4)
In the Langmuir-Hinshelwood models described above, the re-
-
and the rate of reaction will be
- — L V i J i l i -------< 1 ♦ b 1 P j ) ( i t b i p B )
Simplifications of equation 5 can be made assuming strong or weak
adsorption of one species compared to the other as before.
When the molecules of one species, say A, dissociate on
adsorption, for example, into two atoms and the species compete for
same surface sites, the surface coverages will assume the following#
expressions:
(i+ hi pA2 + bB pb)and
^ —
(1 + b A PA + bB PB)
and the reaction rate in this case is
-
If, however, the species are adsorbed on different kinds of sites, the
reaction rate will be given by
b% b pvH = k -----4... (7)
(1 + ) (1 + bB PB)
While a Langmuir-Hinshelwood mechanism requires the adsorption
of both the reactants on the surface of the catalyst to effect any
reaction, the Rideal-Eley treatment applies to the case where one of
the gases reacts from the gas phase with the other component adsorbed
on the surface. Thus in the case of B not adsorbed
VH = k eA PB
If B is not adsorbed at all
bA pA6a =
(1 + bA Pa )
and the rate expression becomes
v = k ^ Pa P3-- (8 )(1 + bA Pa)
If, however, B is adsorbed but adsorbed B does not enter into reaction,
then
-
17
so that
(1 + h PA + b pB)
In contrast to a Langmuir-Hinshelwood mechanism, a Rideal-
Eley mechanism implies that the rate does not pass through a maximum
as the pressure of either component is increased keeping the other
constant. Until now very few hydrogenation reactions have been found
to follow a Rideal-Eley mechanism. The only cases for which there is
good evidence for such a mechanism are certain atom and radical-recombi-
nation reactions (1).
Reference to the equilibrium constant as a function of tempera
ture £itvwo ttie iulwetjlu reaction
H2 + C2H4 “ C2H6
is favored for ordinary pressures up to 600°C, at which temperature at
1 atmosphere hydrogen pressure the yield of ethane is 97 per cent. With
larger olefin concentration, however, the reverse reaction becomes impor
tant at lower temperatures (11).
Nickel catalyzed hydrogenation of ethylene has been studied by
many workers. It has been found that a Langmuir-Hinshelwood mechanism
satisfies the data of most of the workers. It has been postulated that
hydrogen and ethylene are reversibly adsorbed on two separate parts of
the surface and interaction occurs at the borderline. Thus equation 5
represents the rate equation. With A as hydrogen and B as ethylene,
equation 5 can be rewritten
-
18
v.H k (5)a + bH p h) (i + bE p e)
In the case when hydrogen adsorption is weaker than ethylene adsorption
so that
With excess ethylene and especially with a pretreatment of the catalyst
with ethylene, the initial rate goes down. Schwab (12) has pointed out
that this behavior is associated with poisoning of catalyst surfaces by
'acetylenic complexes’ formed by ethylene.
The Arrhenius plot of log versus 1/T starts to decrease
passes through a maxumum; the corresponding temperature is called the
optimum temperature. Zur Strassen (13) postulated that a saturated
layer of ethylene forms below the optimum temperature. Above this
temperature! ethylene starts desorbing. He wrote the following rela
tionships between the true and apparent activation energies with res
pect to the optimum temperature:
and respectively, representing the heats of adsorption of hydro
gen and ethylene on the catalyst.
with increasing temperatures around 100°C, and the reaction velocity
T < ToptT > Topt
-
19
With other catalysts9 various different kinetic behaviors were
obtained. Nd general pattern comes out of these conflicting reports.
Thus a review of the literature reveals that the mechanism of this
reaction depends on the type of the catalyst (i.e., whether foil, wire
or particles), temperature range of study and the method of preparation
and activation of the catalyst. A summary of the rate laws applicable
to ethylene hydrogenation catalyzed by various catalysts under various
conditions is tabulated in a review article by Laidler (14).
Many investigators have tried to formaulate a quantitative
expression for the rate of catalytic ethylene hydrogenation from some
theoretical mechanistic models. Laidler (15) has tried the Langmuir
kinetics and derived an expression for the maximum rate of reaction in
terms of some parameters of the transition state theory. Beeck has
taken the Rideal-Eley kinetics into consideration to explain his experi
mental data on the hydrogenation of ethylene with a number of different
metal catalysts. He postulated that the metal surface is largely
covered, to a fraction 0, by adsorbed acetylenic complexes only slowly
removed by reaction with chemisorbed hydrogen which is adsorbed on the
surface not covered by ethylene. Hydrogenation occurs by collision
between ethylene from the gas phase and chemisorbed hydrogen. After
some simplifying assumptions, the rate expression is reducible to one
which is dependent on the first power of hydrogen partial pressure. It
is quite evident that no mechanism is entirely satisfactory, although
some have done quite a remarkable job in shedding light on this rather
obscure reaction.
-
20
The other scheme that invites attention now is the so called
half hydrogenated state, also known as associative theory. The asso
ciative theory links hydrogenation and the hydrogen-deuterium exchange
reaction via the half hydrogenated state, the ethylene being adsorbed
by opening of the double bond as
H* + H* + *C2H^* t H* + *C2H5 J C2H6 (Horiuti-Polanyi)
or
H2 + *C2H4* ^ H* + *C H5 i C2H (Twigg-Rideal)
The dissociative theory of Farkas regards hydrogenation as not related
in any way to the H-D exchange reaction. Hydrogenation is effected by
simultaneous addition of 2H* to a (presumably physically) adsorbed
ethylene.
C2H4 + 1)4 + H* 1 C2H6
This mechanism, however, has not attained much support from experimental
results.
Oxide catalysts for hydrogenation of ethylene have attracted
attention rather recently and since much more has to be done to eluci
date the behavior of these catalysts. As mentioned earlier, oxide
catalysts are of two types, insulators and semiconductors. Detailed
kinetic studies of ethylene hydrogenation on alumina in the temperature
-
21
range of 120°C to 430oC and a suggested mechanism are available . (16)«
These results have been obtained from a flow, reactor, Catalysts such
as platinum on silica have also been studied (17).
Zinc oxide was reported to be a catalyst for hydrogenation
about thirty years ago but only recently have papers been published
concerning kinetic and infra-red studies. All of these works were
carried out in batch reactors and reaction was monitored by noting the
pressure changes. Zinc oxide, being a semiconductor, is much less
active than the metals as a hydrogenation catalyst. It is therefore a
suitable catalyst for observaing the effect of an electric field since
a small change in catalytic activity is detectable. The reason why it
is a favored catalyst in hydrogenation of ethylene lies in the fact
that its electronic structure determines its activity. Thus a stoi
chiometric zinc oxide is inactive whereas non-stoichiometry is a
necessary condition for its catalytic behavior. Despite this, however,
many workers have expressed doubt as to whether there is any correlation
.at all between the activity of the semiconducting oxides and their elec
tronic structures (18, 19, 20). Aigueperse and Teichner (18) noted that
doping with lithium or gallium does not change the activity of zinc
oxide catalyst although they do modify its electronic .property because
of their being altervalent ions.
To understand the'complex behavior of ethylene hydrogenation,
conclusions have to be based on all of the kinetic, exchange, chemi-
sorption and infra-red studies taken together. Adsorption studies of
-
22
hydrogen and ethylene on pure zinc oxide and exchange reaction of
deuterium have been extremely useful in giving some insight into the
mechanisme
In studies on the hydrogenation of ethylene on zinc oxide and
chromia using hydrogen-deuterium mixtures (21), the product at low.
conversions consisted of a mixture of C^H^D, and the
relative amounts of which corresponded closely with the amounts of
HD and in the reactant hydrogen. When pure deuterium is used
C^H^Dg only is obtained, From this experimental information it was
proposed that the ethylene hydrogenation reaction occurs by irreversible
two step addition to adsorbed ethylene molecules of hydrogen atoms
adsorbed in pairs on isolated sites. The hydrogen involved in the
hydrogenation of ethylene is identified primarily with hydrogen respon
sible for the ZnH and OH bands obtained in the infra red spectra.. The
kinetics of the reaction were observed to be half order with respect to
the hydrogen pressure at room temperature (22). Dent and Kokes (19)
have divided hydrogen adsorption into two types and have experimentally
demonstrated the role of hydrogen in the hydrogenation reaction on zinc
oxide. Thus type 1 hydrogen adsorption is rapid and reversible; type
2 is irreversible and occurs rapidly initially but slowly in the
latter stages. Type 1 hydrogen gives rise to ZnH and OH bands in the
infra red spectra and is the one responsible for hydrogenation of
ethylene. In contrast, type 2 hydrogen does not take part in the
hydrogenation of ethylene at room temperature but modifies the catalyst
and enhances its activity. In the presence of ethylene, however,, type
2 hydrogen is reduced by a factor of 3. Type 1 hydrogen can be totally
-
removed from the surface by evacuation but type 2 hydrogen cannot be
so removed. On a fresh catalyst, the very first reaction experiment
gave an unusually high rate of reaction suggesting that slow chemi-
sorption modified the activity of the catalyst. The effect, however,
did not last after the first run. This phenomenon has been termed
1 hydrogen promotion’. The rather striking conclusion by these authors
is that non-stoichiometry is not responsible for ethylene hydrogena
tion. Instead, it is proposed that the strained sites, perhaps formed
by dehydration, are the active sites. According to their model chemi-
sorption of hydrogen can be represented by the following sets of
equations :
H Hi I |
(g) + - Zn - 0 - ^ - Zn - 6 — (10)
H H H HI I , . I i I— Zn — 0 — 4* — 0 — -
-
24
CHo ~ CHoI I + I IH2C = CH2 + - Zn - 0 - % - Zn - 0 (14b)
H H H0C = CH.I I I I— Zn — 0 — + H^C — CH^ ~ Zn — 0 — (14c)
and hydrogenation is represented by
H H0C = CH0 CH0-CHqI I I 2 3— Zn — 0 — ->■ — Zn — 0 — (15)I
CHo-CH-3 HI I I I I— Zn — 0 — + “ 0 — + — Zn — 0 — 4- — 0 —i 2 (16)
From these equations, they deduced a. i-aLc c^p^uaaiuu that satisfied tiie half order hydrogen dependence
v H = k ’e' (H2 (g))^ (17)
where 9 1 represents the adsorption of ethylene on an oxide site adjacent
to the exposed zinc. It is evident therefore, from the rate equation,
that the ethylene dependence is similar to that found for ethylene
chemisorption.
ZnO has been found by Aigueperse and Teichner (18) to be excep
tionally prone to oxygen poisoning. For the catalyst activated in a
hydrogen stream, if exposed to oxygen or air for a short time, the acti
vity was found to be reduced to less than 1% of its initial value.
Dent and Kokes (19,22) worked exhaustively on oxygen poisoning. Their
-
25
finding is very interesting in the fact that they suggested that oxy
gen by itself is not a poison. If there was even a minute amount of
chemisorbed hydrogen, exposure to air drastically reduced the activity
revealed in the rate of reaction. On the contrary, if the catalyst
was preheated with dry oxygen at 400°C, it still had the same activity
as a catalyst conventionally treated in a hydrogen atmosphere. This
result led to the suggestion that water or its precursor rather than
oxygen itself acted as the poison. An oxygen activated sample had no
chemisorbed hydrogen on its surface and so no poisoning was observed.
Oxygen treatment made zinc oxide more stoichiometric and since it
still exhibited the same catalytic activity at room temperture, the
non-stoichiometry as a reason of activity was ruled out by these authors.
It is deeply suspected, though, that the correlation between semi
conductivity and catalytic activity will eventually be established at
high temperatures (23).
Teichner (24) showed that doping with altervalent ions changed
the character of surface coverage, the behavior being also dependent on
temperature. In other words, as a result of doping, the activation
energy for a doped catalyst was different from the one with an undoped
catalyst. However, this difference in activation energy and for that
matter in the different coverage characteristics by the reactants, may
result not only from the modification of the Fermi level but also from
the fact that in a given temperature range the modification by the dope
of the nature of the surface of the catalyst may be such that two reac
tants are adsorbed in a different manner.
-
26
Ethylene acts as a strong poison for hydrogenation of ethylene
on zinc oxide (23). This kind of behavior has been reported with
metals and alumina catalysts also. The retarding effect on the rate
due to poisoning is very critical in an ethylene rich hydrogenating
mixture. A pretreatment of the catalyst with ethylene before introduc
tion of hydrogen also reduces the rate of reaction considerably. Only
in mixtures rich in hydrogen is this poisoning effect negligible. It
has been proposed that the poisoning effect of ethylene is due to a
reaction involving dissociation of C-H bonds to form a hydrogen defi
cient species similar to the 'acetylenic complex’ proposed by Schwab,
Beeck and Rideal in studies of ethylene hydrogenation over nickel and
other metals.
High temperature hydrogenation of ethylene was reported by
Bozon-Verduraz and Teichner (80°C- 400°C). Their kinetic data showed
that the mechanism of the reaction was indeed strongly dependent on
the range of temperature while the activation energy approached zero
at higher temperatures from a value of 22 Kcals/g. mole at lower
temperatures. Their rate expressions and the corresponding tempera
ture ranges are as follows:
(a) 80° - 125°C vH = kpH
(b) 140° - 175°C vH = k (PH)°'3 (PE)°-7
for (P%)/(Pg) >> 1
-
27
(c) 210°C vR = k pE
(d) Above 210°C the overall reaction order was still unity but the
initial rate and the rate constant were lower than those for 210°C.
In summary, it can be said that the hydrogenation of ethylene
on semiconducting zinc oxide is an extremely complicated reaction.
From the theoretical standpoint, it was widely believed that non-stoi
chiometry is the reason for the catalytic activity of zinc oxide. Dent
and Kokes, however, from their study at room temperature concluded that
strained sites rather than non-stoichiometry gives rise to the activity
of the catalyst. Bozon-Verduraz and Teichner came to the same conclu
sion from their observation that doping with altervalent ions did not
change rhe react j on rptf*, In opposirinn tr. this, Teichner 1 s earlier observation showed that the activation energy of chemisorption was
changed due to doping. Whether this constancy in rate on doped cata
lysts, despite the variation in the surface coverages and activation
energy, is a manifestation of the compensation effect is yet to be deter
mined. The question of a possible correlation between the catalytic
activity and the non-stoichiometry is, however, far from settled.
Bozon-Verduraz and Teichner have themselves suggested that a lot more
work has to be done on the doping and that non-stoichiometry may become
important at higher temperatures.
The present study is the first ethylene hydrogenation study on
pure zinc oxide at temperatures higher than room temperature in the
presence of an electric field. From theory it can be argued that the
-
28
field may only bring about a change in the Fermi level and cannot
influence the surface irregularities in any way and probably also
cannot influence the way the gases are adsorbed on them. The effect
of the field is, therefore, to be thought of as a consequence of the
change in electronic properties of the catalyst.
-
CHAPTER 3
APPARATUS AND EXPERIMENTAL PROCEDURE
A Flow Reactor
In the present .studyj the hydrogenation of ethylene on zinc
oxide with and without the electric field was conducted in a differen
tial flow reactor under isothermal conditions. In such a system the
reactants are simply passed through a bed of catalyst at flow rates
chosen to give the desired low conversion, A flow reactor has the
following advantages over a static or batch reactor:
(a) Control of the temperature of the reactor as the reaction progresses
is relatively easier,
(b) The change in catalytic activity can be easily followed.
At low conversion in the flow reactor9 the rate of reaction can
be written as the product of the reactants flow rates and the conversion
and this rate is representative of the initial rate of reaction since
the composition of the gases do not change over the catalyst appreciably
and heats of reaction or adsorption are sufficiently low that no appre
ciable temperature variations occur. Thus rate of reaction based on
mass of catalyst may be expressed by
,v = - . x (18)H m
29
-
30
where F represents the feed rate of ethylene to the reactor In gram
moles per hour, m represents weight in grams of zinc oxide, and x,
the fraction of ethylene converted to ethane. The reaction rate
is then expressed as gram moles of ethylene hydrogenated per hour
per gram of zinc oxide. The conversion per pass was in most cases
kept below 10%, although in some cases it exceeded that limit. The
derivation of equation 18 is given in Appendix A.
Sinfelt (25) has described a simple experimental method for
catalytic kinetic studies of this sort. His method was largely followed
here, although special attention had to be given to the design of the
reactor for the application of an electric field. In short, the
metered gases (viz., hydrogen, ethylene and helium as a diluent) passed
through the catalyst bed and the conversion was measured by a gas
chromatograph. In the runs with the electric field, the field was
applied for a short time, typically two minutes, across the bed between
two electrodes insulated from the wall of the reactor while the gases
went in and out through the electrodes which were perforated. A
detailed description of the reactor will now be given, followed by the
description of the whole system.
Reactor
Figure 5 shows the reactor assembly and Figure 6 and Figure 7,
its various parts with the pertinent dimensions. Essentially the
reactor consisted of a cylindrical body of carbon steel, 2" long and
3^n in outside diameter threaded at both ends to accommodate two cover
-
31
nuts. The cover nuts are identical having two ports on top of each.
One port was used for gas inlet or outlet and the other port for
electrical connection. For the runs without the field, the ports for
electrical connection were plugged.
The body is a cylindrical section with circular cut-out depre
ssions at both ends about 5/8" deep. The internal diameter at the
depressions is 2 3/16". The middle portion of the body, 3/4" in length,
is of internal diameter 3/4" having a thermocouple well on the wall.
An iron-constantan thermocouple in an 1/8" steel tube went through the
well as far as the center of the body. The tubing was held at the
outside wall by an 1/8" swagelok male connector. The other end of the
steel tubing was sealed with silicone rubber cement so that the reactant
gases could not leak out through the tubing. This thermocouple was
connected to a calibrated temperature indicator (West Instrument Co.)
which showed the temperature of the reactor.
Two identical transite rings, %" thick with outside diameter
the same as the inside diameter of the depressions, and whose inter
nal diameter was the same as the internal diameter of the body at the
central portion sat in the depressions, one at each end, thus forming
a cylindrical section lh" in length and 13/4" in diameter. This
cylindrical section was filled with the catalyst. An alundum filter
disc of 2" in diameter, 3/32" in thickness and of porosity 40-60
microns was placed on top of the transite ring. A steel disc with
perforations as shown in Figure 7 covered the alundum disc completely.
The steel disc acted as one electrode with an 1/8" hole at the center
-
32
/
3 / 3 2
Figure 5. Reactor Assembly
8 B OD Y MILD S T E E L 1
7 RING- T R A N S I T S 2
6 RING TRANSITS 25 F RI T TE D D/ SC ALUNDUM 2
4 COVER MILD S TE E L I
3 DISC MILD S T E E L 2
2 R I N G ' T R A N S I T S 2
1 C O V E R MILD S T E E L 1
NO. D E S C R I P T I O N M A T E R I A L OFF
-
33
15!G
z- PIPE THREAD
/ .8>
COVER
77 t :tX\o' PIPE _
° T H R 'P —
Figure 6. Exploded View of the Reactor
-
34
T
TRANSIT E. RING-T R A N S I T S RING
(fj -THROUGH ■ ™ - - T Y P / C A L , ON
° SQ. PITCH
ii
C O U N T E R S U N K
T R A N S I T S RI NG
Figure 7. Electrodes and Transite Rings
-
35
for attaching the electrical wire with a nut and screw arrangement«
Another transite ring was placed between the protruding wall of the .
body and the outer diameter of the steel disc, thus insulating electri
cally the steel electrode from the wall of the reactor. On top of
this another transite ring Sh11 in outside diameter and l%If in inside
diameter, %n thick rested. Its purpose was to eliminate or reduce the
chance of arcing across the space between the electrode and the cover
nut. The nut was then screwed down on top of the body. The tolerances
of these parts were such that in the assembled configuration no move
ment of the internal pieces was possible. The bottom part of the body
had exactly the same arrangement as the top. Thus in the assembled
position gases entered the reactor through the top of the vertically .
mounted reactor, passed through the perforated electrode, the alundum
disc and the catalyst bed and through the disc the electrode on the
other end out of the bottom of the reactor. The reactor had a volume
of 49.1 cc. Inlet and outlet ports of the reactor were fitted with
swagelok male connectors which held V ! O.D. steel tubing. The
system was found to be leak proof by applying vacuum to the outlet,
closing all other ports, and using a soap solution.
Assembly
Cylinders for the two reactant gases, hydrogen and ethylene,
and the diluent helium, fitted with pressure regulators, were connected
in parallel to three flow meters. Hydrogen and helium were of reactor
grade purity (99.998%) and ethylene was Matheson C.P. grade (99.5%).
Hydrogen and ethylene each had a deoxo unit (Engelhard Industries,
-
36
model D-'10-2500) in the line. These versatile catalyticvpurifiers ope-
rate at room temperature and remove oxygen, carbon monxide, carbon
dioxide, nitric oxide and nitrogen peroxide from hydrogen in the form
of nonreactive water, methane, carbon dioxide and nitrogen by the pro
cess of deoxidation, methanation, selective oxidation and reduction*
Traces of acetylene, carbon monoxide and hydrogen in ethylene are
removed by processes of selective hydrogenation and oxidation in the
form of ethylene, ethane and carbon dioxide.
The gases out of the flow meters were led to a cross through a
1/8" stainless tube and swagelok fittings. The flow meters for hydro
gen and ethylene were of the thermal conductivity variety and were
manufactured by Hastings & Raydists Corporation, Hampton, Virginia.
An LF-300 monitored the flow of hydrogen and an LF-50,' the ethylene.
An F-300 flow transducer was connected to the hydrogen flow meter and
an F-50, to the ethylene flow meter. These consisted of a heated tube
and an arrangement of thermocouples to measure the differential cooling
caused by the passing gas. A direct current voltage proportional to
the rate of mass flow through the tube was generated by a thermoelectric
element. The transducer outputs were practically insensitive to inlet
pressure and temperature changes since the operation depended only on
the mass flow and specific heats of the gases. A Hoke no. 2231 micron
filter of type 316 stainless steel inserted before each transducer
prevented any dust from entering the transducer and damaging its
characteristics. No particle greater than 5 micron in size could pass
through the filter. The helium flow meter was a Brooks rotameter with
-
37
steel and glass balls with a tube size R-2-15AA. Each feed'line had
a Whitey needle valve for control of flow through the flow meters.
The fourth outlet of the cross brought the gaseous mixture out ■
to a horizontal drier which was a tube, one foot in length and in
diameter which was filled with anhydrous calcium chloride, guarded by
glass wool on both sides. Connected to the outlet of the drier was a
preheat coil made out of a 12T long 1/8" O.D. stainless steel tubing
immersed in a constant temperature sand bath. The sand bath was heated
and controlled by a proportional controller which could work both _
manually and automatically. For further details about this instrument
reference is made to Cise (26). The reactor support equipment with
flow meters and sand bath was designed according to Hall, et*al. (27).
■ The preheated gaseous mixture entered the reactor through a
tee fitted to the inlet port of the reactor. A pressure gauge fixed
on the tee measured the gauge pressure in the reactor which was found
to read zero for the kind of flow rates used (106 to 190 standard cc
per minute). The product gases were air cooled after emerging from
the reactor and passed through a micron filter similar to the ones used
for the transducers to prevent any catalyst fines that might have been
carried in the stream to move further downstream. The outgoing gases
were then dried further in a calcium chloride guard tube immersed in an ■
ice bath before entering the sample loop of the chromatograph. Except
for a small volume of sample taken for analysis, all effluent product
gases went to the vent.
-
38
A tee was placed downstream of the drier and preceded the reac
tor t° form a bypass that went directly to the air cooler via a Whitey
needle valve» This bypass was used to measure the initial concentra
tion of ethylene in the feed gas, When the bypass was used, another
Whitey needle valve on the tubing that went to the preheat coil in the
sand bath was closed so that no gas mixture could enter the preheater
and for that matter in the reactor.
The reactor was placed on a transite board vertically. The
board had a hole in the middle sufficiently big to accommodate the
outlet channel and the port for electrical cable. The board was in
turn placed on a firm stand.
Sand Bath
The preheating sand bath (26) as shown in figure 8 was a bed
of 60 mesh sand contained in a 13" long section of 3%n O.D. stainless
steel pipe. A 100 mesh stainless steel screen was fixed at the bottom
to hold the sand and fluidizing air was blown through the sand bath.
Heating of the sand bath was done by two sets of eight electrical coils
wrapped around notched strips of transite boards„ One set of coils was
connected to a 20 amp, 130 volt, type W 20 N variac (General Radio, Con
cord, Mass). The other set of coils was connected to a Leeds and Nor-
thrup model MA-800 magnetic amplifier that supplied a variable A.C.
voltage. The regulation of the magnetic amplifier's output was achieved
through a direct current signal not exceeding five milliamps from a
Leeds and Northrup series 60 three mode control unit joined with a model
-
Ho 2
0
Figure 8. Schematic of the Apparatus
6 © I
Thermocouple
Legends :
\ (1) Deoxo Purifier! (2) Flow meter
(3) CaCl2 drier
(4) Sand bath
(5) Preheating coil
(6) Pressure gauge
(7) Reactor
(8) Voltage source
(9) Micron filteru>
(10) Chromatograph ^
-
40
S speedomax ?H r recorder equipped with an adjustable zero and range
package and an adjustable set point.
Two iron-constantan thermocouples placed in a lance were
immersed in the sand bath. One of these was connected to the tempera
ture controller which measured the temperature with reference to a
cold junction temperature of 0°C in an ice bath. The other thermo
couple was connected to a calibrated temperature meter (West Instru
ment Co.). The setting in the air flow rotameter was made by visual
inspection of the bed. Depending on the temperature^ a higher tempera
ture required less flow of air through the sand bath for fluidization.
Chromatograph
A Perkin-Elmer model 154D vapor fractometer equipped with a
Speedomax type G recorder (Leeds and Northrup Co.) was used to analyze
the product stream. A precision gas sampling valve of volume 5 cc was
used for sampling the product mixture. Helium was the carrier gas and
the components were detected using a thermal conductivity cell. The
column packing chosen was Porapak Q porous polymer beads which have
been reported (28, 29) to give good resolution of light hydrocarbon
gases in a single column. The column installed was a coil of 10T copper
tubing, 3/16" in internal diameter filled with Porapak Q, 80-100 mesh.V ,
The High Voltage Source
The power supply used in this work has been described previously
in full detail (30). The power supply.was primarily built to supply
10,000 volts D.C. output although it could be changed to an A.C. source
-
by changing several wire connections. In order to change from a B.C.
supply to A.C., the wires connecting the' transformers to the rectifier
circuit were removed. Then the high voltage connector was wired
directly to the transformers. This arrangement eliminated the recti
fying system from the circuit. The maximum A.C. voltage output avail
able was 7 «> 980 volts. Since the voltmeter mounted on the chassis of
the voltage source was a B.C. voltmeter, it was necessary to use a
voltage divider in the ratio of 19:1 to measure the voltage with a
Simpson V.O.H. connected across the output terminals.
For the runs with the electric field the cable used connecting
the voltage to the reactor was rated for 10,000 volts. For the portion
that went into the reactor from both ends of the reactor, the rubber
sheath was peeled off and the bare strands were insulated with a layer
of silicone rubber cement. The insulated cable was passed through Zytel
male connectors fitted at the ports at each end and was connected to
the electrodes. After the reactor was thus assembled, the Zytel swage-
lok male connectors were sealed with silicone rubber cement and kept at
room temperature for 48 hours. This cement could very easily withstand
150°C indefinitely.
-
42
Experimental Procedure
Calibration of Flow Meters
The principle of flow meter calibration in all cases was to
measure the time taken by a soap film to travel up a. specified length
of a calibrated burette. Thus the time was measured for a known volume
of gas to flow up the burette. The temperature and pressure of the
room were recorded and the volume was converted to 760 mm and 20°C.
The flow rate that was thus obtained was expressed as standard cc per
minute. This method of flow meter calibration has been both accurate
and easy to do (26). The flow meters calibrated were those for hydro
gen s ehtylene, diluent helium and for the carrier gas in the chromato
graph.
Flow rates were varied by use of the needle valves in each gas
line. Five readings were taken at each setting and the mean of the
readings were noted. The values of the flow rates in standard cc per
minute were plotted against the flow meter settings. These calibration
plots can be found in the Appendix B.
Activation of the Catalyst
Zinc oxide catalyst was obtained from the Harshaw Chemical Co.
in the form of 3/16M extrudates (Zn-0401 E 3/16"). To avoid the effect
of pore diffusion, these extrudates were crushed to two different mesh
sizes, namely, 20-28 mesh and 35-48 mesh. Both of these sizes were used
in the hydrogenation experiments without the field whereas only the
latter was used when the effect of the field was being examined.
-
The properties supplied by Ears haw on the.' extrudates' were as
follows:
ZnO 100% pure
ABD '75 lbs/eft
SA 3 m2/g
Several different methods of activation of zinc oxide .catalyst
have been cited in the literature (18, 19), but the method followed in
the present work was different for reasons to be discussed later (see
Results and Discussions).
The usual method of activation used in this study was to give
the catalyst particles a pretreatment in a glass tube heated by a
surrounding cylindrical oven at 450°C for one hour under vacuum (28
inches). This treatment would remove adsorbed water and other gaseous
impurities from the surface. After this treatment, the catalyst was
brought to room temperature and was placed in the reactor. Vacuum was
then applied to the reactor itself from the gas outlet channel by
temporarily disengaging a reducing union that was connected to the air
cooler♦ Tht reactor was then heated to 300°C for two hours. The heat
ing was done by resistance heating band wrapped around the reactor.
Power to this heater was adjusted by a variac. The reactor with its
heating band was surrounded by cylindrical glass fibre insulation. A
satisfactory temperature control was obtained by manual operation (with
in ±1°F).
Heating in vacuum at a higher temperature (e.g., 450°C) helps
in removing poisons, especially water, from the surface of the catalyst
Strength 8 lb.
PV 0.26 cc/g
-
44
The reactor could not be heated to that high a temperature because the
thread lubricant hardened at the threads of the cover nuts and the
reactor could then be opened only with great difficulty. To avoid damag
ing the threads and the reactor itself, a less rigorous in situ activa
tion like the one described was carried out in the reactor itsdlf pre
ceded by a pretreatment at 450°G in vacuum in the glass tube.
After the two hour vacuum treatment at 300°C5 hydrogen was
passed through the catalyst at that temperature for half an hour.oVacuum was again applied at 300 C for one hour. This treatment was
directed at removing any oxygen from the surface in the form of water
and result in a reactive surface. The catalyst was then cooled to the
reaction temperature.
Analysis by the Chromatograph
The column was installed in the chromatograph and as. prescribed
by the manufacturer was conditioned at 230°C for 2 hours. The carrier
helium flow was 40 ml/min. For analysis of reaction runs, the tempera
ture of the column was maintained at 50°C with a carrier flow rate of
40 ml/min. The bridge voltage was 8.0 volts and the chart speed was
0.75 in/min.
A calibration run of ethylene was made by varying the ethylene
flow rate while keeping the total flow constant and measuring the peak
area under the ethylene peak. Ethylene peak area was plotted against the
concentration (mole fraction) of ethylene and a linear plot was obtained
(See Appendix B). As a check, during an actual series of runs, the by
pass was frequently used to measure the initial concentration of ethylene.
-
, 45
An ethane lecture bottle (Matheson Gas) fitted with„a precision
regulator was used in the feed line to "give a similar -calibration curve
of ethane and another linear plot was obtained«, The ethane calibration
matched the ethylene calibration pretty well so that in an actual
conversion measurement the conversion was simply obtained by dividing
the ethane peak area by the initial ethylene peak area.
With the 5 cc sample loop and the flow rates used, the
hydrogen peak was small and M-shaped. For this reason all conversion
measurements were referred to peak areas of ethylene and ethane. A
typical chromatogram is shown in Figure 9.
Kinetic Runs
Kinetic data were taken at three different temperatures, 194°C5
146°C and 100°C for 20-28 mesh catalyst particles. With 35-48 mesh
particles similar studies were made at 146°C and 56.5°C.
Hydrogen dependence of the reaction rate was determined in the
following manner. Hydrogen flow rates were varied while ethylene flow
rates were kept constant. The diluent helium flow rates were adjusted
to keep the total flow rates constant for all readings in a particular
run. Conversions were calculated by method mentioned before and since
the ethylene flow rate and hence ethylene concentration was constant,
the conversion was proportional to the rate reaction as can be seen
from equation 18. Conversions corresponding to different hydrogen
mole fractions were therefore plotted on a log-log graph, the slope,of
which gave the order with respect to hydrogen. Reactant concentrations
-
X32
X 3212.0 10.0
Figure 9. A Typical Chromatogram Showing Separation of (1) Hydrogen (2) Ethylene and (3) Ethane in a Porapak Q Column at 50°C.
-
were taken as the initial values even though in some cases, since
conversion was greater than 10%, the average concentrations would be
somewhat less.
Ethylene dependence determinations were similarly carried out
by keeping the hydrogen flow rate constant and varying the ethylene
flow rate while adjusting the helium flow rate so that the total flow
rate was constant for all runs in this series» In this case, however,
as suggested by equation 18, the reaction rate is proportional to the
product Fx, where F is the ethylene flow rate and x, the fraction of
ethylene converted to ethane; Fx values were therefore plotted versus
concentration of ethylene on a log-log graph, the slope of which gave
the ethylene partial.order. This was done since all conversions were
calculated from ethylene measurements, hydrogen being difficult to
determine accurately.
A typical run could be described as followsThe reactor was
brought to the reaction temperature and the sand bath temperature was
controlled by adjusting the current input from the temperature control"
ler manually. Prior to the first run hydrogen was passed through the
catalyst for half an hour. Then the needle valve on the inlet to the
reactor was closed and the one on the bypass was opened. Ethylene and
helium flows were adjusted by turning on the needle valves on their feed
lines and then the inlet needle valve was opened, the one on the bypass
was closed so that the gas mixture entered the reactor. The flow was
continued for three minutes at the end of which sampling was done in
the chromatograph and the ethylene flow was stopped but hydrogen
-
48
and helium flows were continued. It took about 15-minutes'for the
analysis to be done by the chromatograph. The next run was conducted
by repeating the same procedure.
During the first few experiments3 attempts were made to dupli
cate the initial run after every other different run to check the
activity of the catalyst, as suggested by Sinfelt (25). This was
discontinued as the activity did not change appreciably so as to
vitiate the kinetics. •
Each day before taking runs, the catalyst was given activation
treatment in situ. At the later stages of experiments, however, either
helium was passed continuously overnight and runs were taken on that
catalyst the next day or vacuum was applied overnight.
The runs for the Arrhenius activation plots were taken in a
similar manner except that in this case all flow rates were kept cons
tant and the temperature was varied. The log of fractional conversion
(being representative of the reaction rate) was plotted against 1000/T
to give the activation plot. The slope of the straight line thus
obtained was equal to -E/R where E is the activation energy.
For the runs with the field, the catalyst was first given a opretreatment at 450 C for one hour in vacuum in the glass tube, tempera
ture was then brought down to 300°C and the catalyst was treated with
hydrogen for % hour after which vacuum was applied again at 300°C for
one hour. After cooling the catalyst in vacuum to room temperature it
was put into the reactor and the reactor was assembled and sealed in
the manner described. The reactor.was grounded. The catalyst was then
-
- 49.'
heated for 2 hours at 150°C in vacuum in the reactor and then was cooled
to. the reaction temperature» The reactor was then ready for operation.
Voltage was applied for two minutes typically, the last two minutes of
the three minute reaction periodBetween two consecutive days of data
taking the reactor was given vacuum treatment at 150°C overnight.
-
CHAPTER 4
RESULTS AND DISCUSSION
Heterogeneous catalysis by nature is an extremely complicated
phenomenon. Activity of the catalyst depends, among other things, on
the method of preparation of the catalyst, method of its activation and
in some cases also on the composition of the reactant gases. It is
obvious therefore that classical kinetic studies alone in terms of some
empirical mechanistic model is not a very satisfactory approach to try
ing to elucidate the "facts" about any particular heterogeneous chemical
reaction. More attention should therefore be paid to knowing more about
the surface that effects the catalysis. There exists no general approach
either to explain why a particular method of preparation and activation
of a catalyst is more effective than another or to control it. There
has been, nevertheless, an encouraging trend in applying infra red
spectra to study the physical and chemical nature of the adsorbed species
on the surface. The present work has had its obvious limitations because
of the fact that no attempt was. made to follow the surface condition as
hydrogenation occured on zinc oxide surface with or without an electric
field. This study was made in large excess of hydrogen in order to avoid
the frustrating reaction pattern at high ethylene concentration, pre
sumably due to poisoning.
50
-
51
Method of Preparation and Activation-
Catalytic activity of zinc oxide depends to a considerable
extent on the mode of preparation of the catalyst. Thus a catalyti-
cally active zinc oxide is obtained by careful thermal decomposition
of zinc hydroxide or carbonate, but the oxide from the nitrate has a
low activity. The lowere the temperature of preparation of the cata
lyst from hydroxide or carbonate, the greater is the activity. Vari
ous preparations lead to the same crystal structure, but differ in the
extent of lattice imperfections (31). Bozon-Verduraz and Teichner (23)
have shown that rate of reaction increases with the temperature of
activation until about 500°C above which sintering brings about a reduc
tion in rate* They have expressed the activity of zinc oxide in terms
of the half life periods of ;the hydrogenation of ehtylene and have
also shown that the surface area suffers a progressive diminution with
the increasing temperature of activation.
As mentioned in Chapter 2, the activation procedure followed in
the present work was designed to give a measurable conversion. During
the earlier stage of experiments, activation was carried out in the
glass tube at 450°C and the catalyst, while still warm, was transferred
to the reactor. But no hydrogenation activity was noted. Similar
results.were obtained even when the catalyst after activation was brought
to room temperature while still under vacuum before transferring to the
reactor. This presumably was due to reaction of oxygen with type 2
chemisorbed hydrogen (slow chemisorption) described by Dent and Kokes
-
. 52
that could not be removed by vacuum, to form water which acted as poi
son for the catalyst, Another attempt was made to activate the catalyst
by heating in vacuum at 190°C for 48 hours*» This catalyst also did not
work. It is probable that the temperature was too low to drive out
moisture already adsorbed on the catalyst supplied by Harshaw Chemical
Co. Oxygen activation as described by Dent and Kokes was also tried
but again no conversion could be detected. In this case no vacuum was
applied to the reactor itself before exposing the catalyst for the first
time to hydrogenation. As is well known, zinc oxide being an n-type
semiconductor adsorbs oxygen at room temperature with a consequent fall
in its conductivity and, therefore, the adsorbed oxygen most likely
reacted with hydrogen to form the poisoning wo ter-
In contrast, the preactivation at 450°C under vacuum given to
the catalyst helped in driving out moisture and other impurities from
the surface and when exposed to air at room temperature for a short
while, only oxygen could have been adsorbed. But in situ activation
which involved heating in vacuum at 300°C drove out the adsorbed oxy
gen once again. Introduction of hydrogen presumably removed the last
traces of oxygen in the form of water.vapor. During the time the acti
vation and all following experimental runs were taken, the catalyst
was never exposed to oxygen or air. During the first few days of
experiments, the catalyst had to be given the same in situ treatment
before taking data since the vent was connected to the reactor through
the outlet tubing. This could be avoided by fixing a needle valve on
the exit line. During the latter part, however, either helium was •
-
53
passed through the bed overnight or the vacuum applied prior to next
days experiment.
Catalyst Effectiveness
Catalytic activity was found to change from day to day. Even,
for two different batches of fresh catalysts5 the activity differed
somewhat. This is presumably because of the problems involved in .
strictly controlling the activation procedures (temperature and time
of heating). Nevertheless as shown in Table 1 and Table 2, the cata
lyst activity remained reasonably constant over a period of five hours
of data taking. The variation of activity, however, inhibited any
formal determination of the effectiveness factor of the catalyst.
Nevertheless activation energy plots in Figures 30 and 31 show that
the logarithm of the fractional conversions versus 1/T was a straight
line over a certain temperature range, changing in slope drastically
at some transition temperature. This change in slope is expected as
will be shown shortly and has been reported in the literature for most
metal catalysts. Since diffusion is not an activated process, a dif
fusion controlled process would not give rise to this kind of behavior
with varying temperature. The straight line plot of Arrhenius activa
tion is therefore a proof that the process was essentially surface
reaction controlled and the effectiveness factor of the catalysts was
essentially unity (2, 32).
Kinetic Runs, No Field
Table 3 summarizes the observed dependencies of the reaction
rate on the concentrations of hydrogen and ethylene under five
-
54
Table 1
Experiment on Catalyst Activity
Conditions :Temperature 146°C
Hydrogen flow 68.0 std. cc/min
Ethylene flow 10.9 11 11
Helium flow 93.0 11 11
Total flow 171.9 11 It
Time from run 1 to run 8 = 5 hours
observations A C°2H4 frac. conv. (x)0.3995 V o V v/ ̂ V 0.107 .
2 ii M 0.1055
3 ir 0.1045
4 H 11 0.1005
.5 11 11 0.1040
6 11 11 0.1032
7 It 11 0.1075'
8 11 11 0.1000*
* After this series of runs5 helium was flown thrown through thecatalyst bed overnight at room temperature prior to the next series of similar runs the next'day (See Table 2),
-
55
Table 2
Experiment on Catalytic Activity
Conditions : Same as in Table 1
Time from run. 1 to run 10 - 6 hours
observationC2H4
frac. conv.(x).
1
2
3
4
5
6
7
8 9
10
0.3995 0.0630 • 0.1125
0.0985
0.0980
0.0912
0.0872
0.0935
0.0887
0.084
0.0815
0.0860
t
-
56
different temperature conditions» The effect of the field on the
partial orders is also shown.
Ethylene Dependence
As stated earlier, (Fx)/m was plotted against the concentration
of ethylene of log-log graph paper, the slope of which was the ethylene
order. Hydrogen rich mixtures were always used. Ethylene concentration
was varied from 0.04 to 0.35 mole fraction, while the total pressure was
kept at 1 atmosphere.
At 195°C with 20-28 mesh catalyst, the ethylene order appeared
to be one half (Figure 10). Figure 11 represents the same data plotted
on cartesian coordinates to check that they satisfy the initial condi
tion i.e., zero conversion at zero ethylene concentration. The ethyl
ene order determination at other temperatures, e.g., 146°C, 100°C and 56.5°C, was characterized by marked scattering of the data points.
Figures 12 and 13 show ethylene order at 1460C for 20-28 mesh and 35-48
mesh catalysts, respectively. The apparent ethylene order at 146°C is
zero. The same type of behavior is noted at 100°C and 56.5°C (Figures
14 and 15, respectively). Figures 13 through 15 would probably suggest
a negative dependence on ethylene but this cannot be said with certainty
because of scattering. Similarly Figure 12 might appear to suggest a
positive dependence less than one half but since it differs from Figure
13 only in mesh size, a suggested zero order would be conceptually more
realisticb
-
Table 3 .
Observed Orders of Reaction
Reaction Temperature ( °C )
56.5 86.5 . 100 146 195
Voltage 0 4,180 0 0 3,800 0
order 1 % 1 1 . % ' %
C2H4order
. o* 0* . 0**0 ■ % 1
* uncertain due to scatter
Ln
-
001 X
58
10 T 1---1---1— | | TI T
1.0
Run la
0.5.01
.1 I ! 1 1 1 1.10
J L .40
Figure 10. Order with Respect to Ethylene, 1950C, 20-28 Mesh Catalyst.
-
001 X
59
dE
3,0
Run la
1.0
00.20.10
'C2H4
Figure 11. Ethylene Order at 195°C, Run la Replotted, on Cartesian Coordinates.
-
001 X
60
10
E
Run 2a
.01 .10 .40c2H4
Figure 12. Order with Respect to Ethylene, 146°C, 20-28 Mesh Catalyst.
-
61
Run 3a
0.3
Figure 13. Order with Respect to Ethylene, 146°C, 35-48 Mesh Catalyst.
8.0 O i— ] j f r T T
oo O O Ox O
O O O
Run 4a
i.O_______ I_I I___ L_J__I._L_l_l____________ I---------.02 0.1 0.3
cc2h4Figure 14. Order with Respect to Ethylene, 100°C, 20-28 Mesh
Catalyst.
-
X 100
62
1.0
Run 5a
— O'
.10.4.03 0.1
Figure 15. Order with Respect to Ethylene, 56.5°C, 35-48 Mesh Catalyst.
-
63
Hydrogen Dependence
For hydrogen order determinations5 fractional conversion, x,
was plotted against the concentration of'hydrogen on log-log graph
paper 9 the slope of the straight line being the order with respect to
hydrogen. One half hydrogen order was observed at 195°C (Figure 16).
Figure 17 (Run lb) shows the same data on a cartesian plot which
verifies the initial condition at zero hydrogen concentration. Runs
2b and 3b were taken at 146°C for 20-28 mesh and 35-48 mesh catalyst
respectively. Figures 18 and 19 show the approximate first order
hydrogen dependence for these runs. These runs were replotted on carte
sian coordinates (Figure 17) to check the validity of the observed
hydrogen order at this temperature. The data for Run 2b and 3b fall
on two different straight lines which can be explained on the assump
tion that the activity (e.g., available surface area) of the two diffe
rent batches of catalyst was different such that no effective reproduci
bility could be achieved.
At 100°C and 56.5°C, the observed hydrogen order was one (Figure
20, Run 4b for 100°C and Figure 21, Run 5b for 56.5°C). These data
plotted on cartesian coordinates are shown in Figure 22.
-
0.2
Run
0.1.2 .5 1.0
Figure 16. Order with Respect to Hydrogen, 195 C, 20-28 Mesh catalyst.
ON
-
65
0H
0.4 2 0.8
p o,1 6
Run lb195 C
XRun 3b, 146 C
.08
.04
Run 2b, 146 C
0.80.40
Figure 17. Hydrogen Order as Shown in Cartesian Blots
-
Run 2b
01
0051.00.1
Figure 18. Order with Respect to Hydrogen, 146° 20-28 Mesh Catalyst.
-
67
0.3
.1
Run 3b
.0 21.00.1
Figure 19. Order with Respect to Hydrogen, 146°c, 35-48 Mesh Catalyst.
-
68
03
Run 4b
0010.80.|
Figure 20. Order with Respect to Hydrogen, 100°C, 20-28 Mesh Catalyst.
-
Run 5b
.0030.2 0.5 1-0
Figure 21. Order with Respect to Hydrogen, 56 35-48 Mesh Catalyst.
-
70
.016
.012
Run 4b, 100 C.004Run 5b, 56.5 C
00 0.4 0.8 1.0
Figure 22. Hydrogen Order as Shown in Cartesian Plots.
-
Effect of the Field
Maximum voltage that could be applied’was 3,800 volts at 146°C
and 5,700 volts at 86,5°C. At still higher voltages the indicator of
the Simpson V.O.M. was fluctuating wildly indicating some instability
arising presumably out of the failure of the transite ring insulators
in the reactor, There was a very definite reduction in rate in the
presence of the field observed at 146°C whereas the rate was accelera
ted at 86.5°C. The reduction was as much as three times greater with
longer duration of the field and higher field strength. Table 4 shows
how with increasing voltages for the same field duration, the conversion
under otherwise identical conditions was progressively reduced. Table
5 shows that with larger duration of the field before sampling, the rate
is decelerated, longer duration giving rise to higher reduction in the
rate of reaction, •
In contrast to this deceleratory effect of the field on the
reaction rate, at lower temperatures like 86.5°C the field enhanced the
reaction; a five hundred per cent increase over the zero field rate was
observed with 5,700 volts. The effect of the field was exactly opposite
that at higher temperatures, higher field strength progressively accele
rated the rate. Longer duration of the field yielded higher accelera