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Catalysis over solid acids and bases
S. Sivasanker
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CATALYSIS OVER SOLID ACIDS
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CONTENTS
1. Solid acid catalysis - Introduction- Examples of solid acids- Acidity characterization - Acidity measurement- Intermediates in acid catalysis2. Catalysis over zeolites3. Acid catalyzed reactions
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Solid acid catalysis - Introduction
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SOLID ACID CATALYSTSExamples:• Zeolites• SAPOs • Clays; pillared clays• Ion-exchange resins• Oxides; X, SO4-oxides • Mixed oxides; amorphous • Heteropoly acids
ACID CATALYSIS Two types of acid sites are recognized
- Brönsted - Lewis
Mineral acids such as H2SO4, HF and AlCl3 are widely used in the industryThe US petroleum refining industry alone uses ~ 2.5 M tons of H2SO4 and ~ 5000 tons of anhydrous HF annually
Catalytic cracking is the Largest user of any solidCatalyst
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Name of reaction
Description Solid-acid catalystused
Cracking /hydrocracking
Crack large molecules in petroleum oils FCC additives for more C3 and octane
Silica-alumina; ZeoliteY ZSM-5
Dewaxing Crak n-paraffins (waxes) in petroleum oils ZSM-5
Isodewaxing Isomerization of waxy molecules. SAPO-11
Xylene isomerisation
p- and o-xylenes from m-xylene. ZSM-5; Mordenite
Naphtha reforming
Isomerization reactions for aromatization of paraffins.
Chlorided alumina
Hydrotreating Remove N and S from petroleum oils Alumina support
Hydration Hydrate olefins to alcohols. Ion-exchange resin; ZSM-5; Heteropolyacids
Reactions / processes based on acid catalysis
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Strength of acidity required for different reactions is different:
It is important to know the strength of the acid catalyst to achieve maximum selectivity for the desired reaction
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Acidity of solids is measured experimentally by many methods:
1. Titration with organic bases2. Adsorption – desorption of bases (TPD)3. NMR methods4. IR Spectroscopy – on neat sample 5. IR Spectroscopy of adsorbed bases 6. Sanderson’s intermediate electronegativity
Strength, type and the number of acid sites in a solid catalyst are important
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In the case of dilute acids, we can use pH to characterize the strength of the acid
In the case of strong acids, pH is not valid as a ≠ ca = c . f, where f is the activity coefficient.
In the case of solid acids, it is even more difficult to quantify acid strength
Acid strength definition
1. Titration with organic bases
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Hammett acidity function, H0
Ho (Hammett acidity function) is used to define acidity of concentrated solutions (or strong acids)
This function can be conveniently estimated with reference to known bases (indicators).
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Hammett acidity function
For the reaction, B + H+ BH+ ; KB = [BH+] / [B] [H] (in dilute solutions);
Hammett acidity, ho = [H] = (1/ KB )[BH+]/[B]
Ho (Hammett acidity function) = - log ho = log KB - log [BH+]/[B]Ho = - pKB + log [B]/[BH+]
pKB and [B]/[BH+] are obtained experimentally and Ho calculated
In dilute solutions, Ho = pH; in conc. solutions, it is Ho = pH - log (fB/fBH+)
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Acid Hoa
Conc. H2SO4 ~ -12
Anhydrous HF ~ -10
SiO2-Al2O3 - 8.2 - 10
SiO2-MgO < + 1.5
SbF5- Al2O3 < -13.2
Zeolite, H-ZSM-5 -8.2 - 13
Zeolite, RE-H-Y -8.2 - 13
a : Denotes the strength of the strongest acid sites in solid acids
Typical Hammett acidity (Ho)
of some strong acidsused in catalysis
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1. Titration with organic bases
HR = H0 + log aH2O
HR indicators
H0 indicators
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Ho
Ho
HR
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Today, characterization of acidity by H0 or HR
functions is not considered sound because of the inapplicability of the concept to solids
So other methods have to be developed
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a) Adsorption of bases
Heat of ads. of NH3 on two acid catalysts
2. Adsorption – desorption of bases (TPD)
Difficult to relatereaction requirement to heat of adsorption
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Heat of adsorption:Clausius-Clapeyron equation:
ln(p2/p1) = (Qst/R) [(T2- T1) / T1 T2]
Used for chemisorption: H2, CO on metals;NH3 /acidic solids; CO2 /basic solids
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H. A. Benesi, J. Catal., 28 (1973)176
How toestimateBronstedand Lewis sites?
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100 200 300 400 500 600
108
110
112
114
116
118 H-Beta D-Beta 34 D-Beta 46 D-Beta 175
Des
orpt
ion
Temperature(oC)
b) Temeperature programmed desorption
Basic compounds from acidic solids Acidic compounds from basic solids
Sample is adsorbed and then desorbed by raising temp.
Effect of Si/Al Ratio of zeolite –Acidity decreases with decrease in Al-content
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Strongly acidic
Effect of zeolite type on acidity
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100 200 300 400 500 6000
2
4
6 H-Beta
Des
orpt
ion
Temperature(o C)
Plots are deconvoluted to derive WEAK and STRONG acidity
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Pinto et al. Appl. Catal. 284 (2005) 39
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Pinto et al. Appl. Catal. 284 (2005) 39
3. NMR methods
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IR spectra of –OH groups (zeolites)
(Defect site)
(external surface of crystallites)
4. IR Spectroscopy – on neat sample
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Acidity in zeolitesAcid site inside 10 MR pore
Strength of acid sites depends on T-O-T angle T-O-T angle depends on framework structure, Al-content, nature of T-ion etc.
Stron
g acidity
3610cm-1
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Lewis site (weak)
Bronsted site (strong)
(pyridinium ~ 1545cm-1 ; coordinated Py ~ 1451cm-1)
IR of adsorbed pyridine
5. IR Spectroscopy of adsorbed bases
Eg. Phosphotungstic acid
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O
Si Al
O
H
O
Bronsted acid sites
Si
O
Si Al
O
H
O
Si
O
[A]
- H2O
O
Si Al
O O
Si
O
Si Al
O
Si
O
- -
++
+
Lewis acid site
-
Basic site
[B]
In zeolites and silica-alumina Brönsted acid sites
Transform into Lewis acid sites on heating
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J.W. Ward, J. Catal. 9 (1967) 225
H-Y
IR spectra of adsorbed bases
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[JPC 96 (1992) 8480]
Composition (average electronegativity) and acidity
For a compound PpQqRr,
Sint = [Spp Sq
q Srr]1/(p+q+r)
Sanderson’s intermediateelectronegativity
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CATALYSIS BY ACIDS
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Acid catalyzed reactions of hydrocarbons are mediated by carbocations
Tri-coordinated
Penta-corodinated
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Relative stability of the carbocations
Reaction velocity and
product yield are generally
determined by the stability of the carbocation
intermediates:
Tert-C+ > Sec-C+ > Prim-C+
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Some examples of carbocations
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The different ways of forming carbocations
1. Addition of H+ to olefins; Easy with olefins, alcohols
2. Addition of H+ to paraffins; Requires very strongAcids
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3. Bimolecular hydride transfer reaction
4. Condensation
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A metallic component helps in generating olefins making C+ formation easy (bifunctional catalysts)
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Examples ofACID CATALYZED REACTIONS
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Alkylation is the introduction of an alkyl group into a molecule It may involve a new C-C, O-C, N-C bond formationAlkylation is catalyzed by acidic or basic catalysts
ALKYLATION REACTIONS
Acid catalysts are used mainly in aromatics alkylation at ring-C
Basic catalysts are used in alkylation at side-chain-C
CH3
+ MeOH
CH3
CH2CH3
CH3
Acid Catalyst
Basic Catalyst
(p-Xylene)
(Ethylbenzene)
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Typical acid catalysts: Friedel-Crafts catalysts: HF, H2SO4, HCl-AlCl3 and (ZEOLITES)
Mechanism of alkylation over Friedel-Crafts catalysts:
MECHANISTIC ASPECTS
Typical alkylating agents: Olefins, alcohols, ethers, alkyl halides, dialkyl carbonates (DMC), etc
ALKYLATION REACTIONS
R Cl + AlCl3 R Cl AlCl3-
R Cl AlCl3+ -
R
HCl AlCl3
-
+
RAlCl3
H-Cl
+
+
+
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HY = HCl, HF etc;MXn = AlCl3, SbF5, BF3
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Reactants Product Catalyst Status
Benzene + ethylene
Ethylbenzene Zeolite (ZSM- 5) Commercial
Toluene + methanol
p- Xylene ZSM- 5 (proprietory)
(Commercial)
Benzene + propylene
Cumene Solid phosphoric acid / zeolite
Commercial
Benzene + C10 - C13
olefins
LAB Proprietory Commercial
Solid-acid based alkylation reactions in commercial practice
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Importance of alkylation Processes
Green Chem. 6 (2004) 274
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Examples of alkylation mechanisms
Because the Sec-C+ is more stable, mostly cumene is (> 99.9 %)is produced and not n-propyl benzene (requires the Prim-C+)
Mechanism 1; Sec-C+ is formed
1. Cumene production:
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2. Alkylate production: ( Global production = ~ 80 million tpa)
The first step is the formation of isobutyl carbenium ion The important step is the hydride transfer between adsorbed C+ and i-C4 : this ensures supply of isobutyl C+ for the reactionIsobutane / butene ratio is 10 - 15 to prevent oligomerizationMany solid acid catalysts are being developed to replace HF / H2SO4
The reactionis alkylationof i-butane with butene
[Butene]
[Isobutane]Hydride transfer
+
++
Trimethylpentane
Superacid needed
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Important isomerization reactions:1. Petroleum refining: Wax isomerization for lubes; isomerization of light naphtha (C5 – C6)
2. Petrochemicals: Xylene isomerization
Catalysts are usually bifunctional:-Metal/support typeTypical examples:-Pt-SAPO-11 for wax isomerization-Pt-Mordenite /acidic-alumina for C5 – C6 hydrocarbons-Pt-ZSM-5 /mordenite/(silica)-alumina for xylene isomerization
ISOMERIZATION
Mostly acid catalyzed
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1. Isomerization of xylenes
CH3
CH3
CH3
CH3+
CH3
CH3
+
CH3
CH3
Zeolite
Equilibration of thecarbocation occurs on The acid catalyst
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CATALYST: Bifunctional Metal: Pt Acid: Alumina (F / Cl); SiO2-Al2O3;
Zeolites (mordenite; Y) Mechanism: + C-C-C-C-C-C C-C=C-C-C-C [ C-C-C-C-C-C (n-hexane) metal acid + C-C-C-C-C ] C (Carbenium ions)
C-C-C=C-C C-C-C-C-C C C metal (i-hexene) (3-methyl pentane)
2. Isomerization of alkanes: For octane improvement and pour point reduction (petroleum refining)
Bifunctional mechanism – acid and metal catalyzed
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Cracking reactions
Catalytic cracking mechanism – occurs via carbocations
A. Carbenium ions are produced mainly by:1) Addition of H+ to an olefin:CH3-CH2-CH2-CH=CH2 + H+ CH3-CH2-CH2-CH+-CH3
2) Addition of H+ to a paraffin and subsequent loss of H2:
R-CH2-CH2-CH3 + H+ R-CH2-CH3+-CH3 (carbonium ion)
R-CH3-CH+-CH3 + H2
B. Beta-fission of the carbenium ion produces the products:
R-CH2-CH2-CH2-CH+-CH3 R-CH2-CH2+ + CH2=CH-CH3
(or) R-CH=CH2 + CH2
+-CH-CH3
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CH3
CH3
CH3
Catalyst
Toluene disproportionation
C9+ aromaticstransalkylationCH3
CH3
CH3
+
CH3
CH3 CH3
Catalyst
Disproportionation reactions (cracking + alkylation)
• Disproportionation reactions are used in petrochem. industry• Catalysts are usually Pt-mordenite, Pt-silica-alumina etc
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CH3
+i-Pr
i-Pr
i-PrCatalyst
Diisopropyl benzene transalkylation
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Hydration of olefins:
+ OH2
OH
ZSM-5Asahi Chem
+ OH2MFI
CH3
CH3
CHOH DIPE-H2O
Mobil
Dehydration of alcohol
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CH3CHO + HCHO + NH3Solid Acid
N
+
N
Condensation reactions
NH2
NH2 OH
OH
R N
N
+
N
N
N
NSolid Acid
(R= H) (R= Me) (R= Et)
Catalysts are silica-alumina & zeolites like ZSM-5, MOR These are commercial processes.
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Ph
O
NO2
+O Ph
O
NO2
O
1 2 3
HeterogeneousCatalyst
r.t., 22 h
Michael addition
Condensation of --unsaturated ketones (2) with nitro compounds (1)
R. Ballini, D. Florini, M. V. Gil, A. Palmieri, Green Chem., 5 (2003) 475
Catalysts: silica, alumina, clays and zeolites
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Molecular rearrangements
O OH O
solid acid solid acid
zeolite zeolite
Allyl phenolAllyl phenyl ether 2-methyl, 2,3 dihydrobenzofuran
Claisen rearrangement
ONH3 + H2O2
Ti-silicate
NOH
~ 90% yieldcyclohexanone oxime
molecular sieves
O
NH
caprolactam
Beckmann rearrangement
Catalyst:BEA; Y
Catalyst: MFI
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CATALYSIS SOLID BASES CATALYSIS SOLID BASES CATALYSIS SOLID BASES CATALYSIS SOLID BASES
1. Introduction2. Characterization of basicity3. Examples of reactions over solid bases
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Though solid acid catalysts have found numerous applications, solid base catalysts have not found as many commercial uses.
Out of 127 acid and base catalyzed commercial processes listed in 1999 (Tanabe & Hölderich, Appl. Catal. A, 181 (1999) 399) 10 were based on basic catalysts & 14 based on acid-base catalysts
1. Introduction
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# Activity depends on concentration and strength of basic sites# Basicity may be measured by adsorption of acids# Often involve carbanion intermediates# Acid-base pairs may also be involved
# Activity depends on concentration and strength of basic sites# Basicity may be measured by adsorption of acids# Often involve carbanion intermediates# Acid-base pairs may also be involved
Solid bases:•Alkali and alkaline earth oxides; •RE-oxides; ThO2; •Alkaline-zeolites; •Alkali metals or oxides on Al2O3 and SiO2; •Hydrotalcite; Sepiolite
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Estimation of Basicity - By adsorption of organic acids - titration- By TPD of gases – CO2
- FTIR of adsorbed species: CO2, pyrrole etc- Dehydrogenation reactions- Calculate intermediate electronegativity
2. Characterization of basicity
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H- = pKBH – log [BH]/[B-]
1. By adsorption of organic acids - titration
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TEMPERATURE PROGRAMMED DESORPTION OF CO2
TPD plots of CO2 adsorbed on different Cs loaded samples: a, b, c, d and e refer to samples with Cs loading of 0.075, 0.375, 0.75, 1.5 and 2.25 mmole/g silica, respectively.
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FTIR spectra of CO2 a,b: Li/SiO2; c,d: Na/SiO2;e,f: K/SiO2 and g,h: Cs/SiO2
at 0.4 and 5 torr.
Bal et al. J. Catal. 204 (2001) 358.
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Basicity from FTIR spectraBasicity from FTIR spectra
Sample Antisymmetric Symmetric
cm-1 cm-1 cm-1 cm-1 3
Li(1.5) SiO2 1679 1421 258 1652 1498 154
Na(1.5) SiO2 1683 1365 318 1643 1462 181
K(1.5) SiO2 1663 1347 316 1633 1407 226
Cs(1.5) SiO2 1648 1329 319 1617 1383 234
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Catalytic activity in isopropanol dehydrogenation
Catalyst Conversion
(mole %)
Selectivity (Acetone)
Acetone - TOF x 10-3 (w.r.t. alkali metal)
SiO2 4.0 1.3 -
Li(1.5)SiO2 4.4 65.2 0.77
Na(1.5)SiO2 5.6 76.3 1.16
K(1.5)SiO2 6.8 83.7 1.35
Cs(1.5)SiO2 9.5 90.8 1.93
BASICITY from alcohol dehydrogenation
Conditions: 723K and WHSV(h-1) = 3.14
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a: Numbers in brackets are mmole of alkali oxide / g of silica; b: Relative band intensity of adsorbed CO2 (1200 – 1750 cm-1); c: Acetone formation in dehydrogenation of i-PrOH
Catalysta S. Area(m2/g)
Relative basicity
TPD(mmole CO2 /g)
FTIRb (De-H2)c
TOF x 10-3
SiO2 166 - - -
Li(1.5)SiO2 104 0.062 92 0.77
Na(1.5)SiO2 99 0.071 132 1.16
K(1.5)SiO2 91 0.078 153 1.35
Cs(0.075)SiO2 149 0.031 19 -
Cs(0.375)SiO2 121 0.049 88 -
Cs(0.75)SiO2 102 0.061 120 -
Cs(1.5)SiO2 70 0.079 216 1.93
Comparison of basicity of a series of catalysts
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3. Examples of reactions over solid bases
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In the case of phenols (or anilines), acid and base catalysts cause both ring and hetero-atom alkylation, the latter increasing with basicity.
CH3 CH2CH3MeOH
Base
OHOMe
MeOH
Base
NH2NH(Me)
MeOH
Base
ALKYLATION
Acid catalysts cause ring alkylation of alkyl aromatics and basic catalysts lead to side-chain alkylation
There are only a few commercial applications of basic catalysts inalkylation of hydrocarbons
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Important Industrial alkylation reactions using basic catalysts
Reaction
Catalyst
MgO Fe-V-O/ SiO2 Na/ K2CO3 K/ KOH/Al2O3
OH
+ MeOH
OH
OH
+ MeOH
OH
+
OH
+
+
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Table 1. Properties of ion exchanged zeolites
Catalyst Si / Al % Na % K % Cs BET area (m2/ g)
Sint
NaX 1.34 100 - - 712 3.28
KX(Cl) 1.34 18 82 - 624 3.1
KX(OH) 1.34 12 88 - 600 3.09
CsX(Cl) 1.34 49 - 51 572 3.08
CsX(OH) 1.34 48 - 52 550 3.07
Side-chain alkylation of toluene over alkaline-X zeolite
Sint (intermediate electronegativity) = geometric mean of the electro-negativity of constituent atoms (Mortier, J. Catal. 55 (1978) 138)
[Bal et al. Stud. in Surf. Sci. Catal. 130 (2000) 2645]
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Alkylation of toluene with dimethylcarbonate
Activity increases with basicity: CsX>KX>NaX
Styrene is absent in the product
Conditions: W/F (g.h.mole-1) = 30; Tol/DMC (mole) = 5; 400°C
Side-chain alkylation more predominant
Cumene directly from toluene
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Mode of adsorption determines product selectivity:(I) favours more C-alkylation in o-position than (II)
C- & O- alkylation occur over acid catalystsO-alk. Increases with basicity of catalyst
O
Mg O MgMg O
(I)
O
(II)
Si Si AlO
H O
( H)
Alkylation of phenol with methanol
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Reactivity of different aromatic hydroxy compounds
METHYLATION OF HYDROXY AROMATICSMETHYLATION OF HYDROXY AROMATICS
•Activity increaseswith basicity
•All compounds equally activeover most basiccatalyst
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ALKYLATION OF 2-NAPHTHOLWITH METHANOL
OH
OMe
CH3
OH
CH3
OMe
2-NaphtholII; 1-Methyl-2-hydroxy naphthalene
I; 2-Methoxy naphthlene
III; 1-Methyl-2-methoxy naphthalene
Scheme 2. Products of methylation of 2-hydroxynaphthalene (2-naphthol)
Catalyst Conv. % O-/-C
Methylation
SiO2 9 Only II
Li/SiO2 45 1.1
K/SiO2 57 2.7
Cs/SiO2 100 ~10
Basicity increases conv.Basicity increases O-Me selectivity
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Catalyst SBET
(m2/g) Rel basicity (FTIR)
Conversion (%)
NMA / NNDMA (selectivity)
Cs-X 550 182 65.3 4.8 Cs-silicalite 379 83 38.0 2.1 Cs-MCM-41 625 49 48.9 2.3 Cs-SiO2 130 40 17.0 2.5
Catalyst SBET
(m2/g) Rel basicity (FTIR)
Conversion (%)
NMA / NNDMA (selectivity)
Cs-X 550 182 65.3 4.8 Cs-silicalite 379 83 38.0 2.1 Cs-MCM-41 625 49 48.9 2.3 Cs-SiO2 130 40 17.0 2.5
Activities of catalysts in aniline alkylation
NH2
MeOH / catalyst
k1
NHCH3
k2
MeOH / catalyst
N(CH3)2
ALKYLATION OF ANILINE
NMA NNDMA
Activity increases with basicityMM/DM ratio is not dependent on support or measured basicity(Conditions: 548K, 1/WHSV (h) = 0.58, methanol/aniline (mole) = 5)
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CH2=CH-CH2
O
O
a)CH2=CH-CH
O
OSafrol isosafrol
Na/NaOH/Al2O3
b) CH2=C=CH2 K2O/Al2O3CH3-C=CH3
Base catalyzed isomerization reactions
Commercial processes:
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Knovenagel condensation
O
R1
OHOEt
O
+R3
R2
R1
R3
O O
Mg-Al HT
Toluene, heat
(Coumarins)R1 = H, MeR2
R3= H, OMe= H, CN,COMe,CO2Et
Heck Reaction
Ar-X +R
M / HT Ar
R
R = CO2Et, Ph etc.
Other reactions
1.
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OH
OH
OH
OH
+ (C2H5O)3Si(CH2)3NH2
OH
O
O
O
Si(CH2)3NH2
MCM-48 (A)
H
C6H5
O +COOEt
H2C
CN
Base Catal.C C
COOEt
CN
H
C6H5
(B)
Aldol condensation also takes place on solid bases, like hydrotalcites
Knoevenagel condensation
Shu-Guyo Wang, Catal. Commun., 4 (2003) 469
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OH
+ NH3
NH2
MgO, Al2O3
2. Amination of alcohols
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X
O
+ R-S-HHAP
MeOH/r.t.
X
OSR
1 2 3
Catalyst: Synthetic hydroxyapatite [Ca10(PO4)6(OH)2 –HAP]
Condensation of an -unsatured ketone (1) and a mercaptan (2)
S.J. Miller, Microporous Materials, 2 (1994) 439
3. Michael addition
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+ 3 H2 ( Hr = 63.6 kcal/mol)
1. Monofunctional catalytic reforming
Catalyst:Pt-(Ba)-K-L(benzene yield ~ 80%)
Carbon No. of alkane
Pt-Ba-KL
Pt-Re-Al2O3
Basic
Acidic
AROMAX Process (Chevron)
Benefits of basic supports
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Reasons attributed for the superiority of Pt-KL are:1. The basic support donates electrons to Pt making it electron rich - electron rich Pt desorbs easily the aromatic product 2. Steric effects of the pores and cage-system ensure cyclization of olefinic hydrocarbons and subsequent dehydrogenation occurs to produce aromatics3. Extremely good dispersion of Pt4. Low coke deposition on the catalyst
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2. Heck reaction
X
+R
Base, Pd-catalyst
solvent DMF, 403 K
R
+ HX
X = I, Br, Cl R = Ph, -COOEt Coupling product
(Et or any alkyl group)Catalyst = Pd-ETS-10
ETS-10 is a basic molecular sieve. It is a titanosilicate with Si/Ti = 5 and Ti in Oh cordination.
As each Ti exchanges with two alkali ions (Na and K), it is a highly basic material
S.B. Waghmode, S.G. Wagholikar, S. Sivasanker, Bull. Chem. Soc.. Japan, 76 (2003).