Catalysis over solid acids and bases

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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

• Solid acid catalysis - Introduction
• - Examples of solid acids
• - Acidity characterization
• - Acidity measurement
• - Intermediates in acid catalysis
• 2. Catalysis over zeolites
• 3. Acid catalyzed reactions

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Solid acid catalysis - Introduction
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Two types of acid sites are recognized -
Brönsted - Lewis
ACID CATALYSIS

• SOLID ACID CATALYSTS
• Examples
• Zeolites
• SAPOs
• Clays pillared clays
• Ion-exchange resins
• Oxides X, SO4-oxides
• Mixed oxides amorphous
• Heteropoly acids

Catalytic cracking is the Largest user of any
solid Catalyst

• ? Mineral acids such as H2SO4, HF and AlCl3 are
  widely
• used in the industry
• The US petroleum refining industry alone uses
  2.5 M tons
• of H2SO4 and 5000 tons of anhydrous HF
  annually

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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 bases 2.
Adsorption desorption of bases (TPD) 3. NMR
methods 4. IR Spectroscopy on neat sample 5.
IR Spectroscopy of adsorbed bases 6. Sandersons
intermediate electronegativity

• Strength, type and the number of acid sites in a
  
• solid catalyst are important

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1. Titration with organic bases
Acid strength definition
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
? c a c . f, where f is the activity
coefficient.
In the case of solid acids, it is even more
difficult to quantify acid strength
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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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Typical Hammett acidity (Ho) of some strong
acids used in catalysis
a Denotes the strength of the strongest acid
sites in solid acids
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1. Titration with organic bases
H0 indicators
HR indicators
HR H0 log aH2O
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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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2. Adsorption desorption of bases (TPD)
a) Adsorption of bases
Heat of ads. of NH3 on two acid catalysts
Difficult to relate reaction 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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How to estimate Bronsted and Lewis sites?
H. A. Benesi, J. Catal., 28 (1973)176
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b) Temeperature programmed desorption
Sample is adsorbed and then desorbed by raising
temp.

• Basic compounds from acidic solids
• Acidic compounds from basic solids

Effect of Si/Al Ratio of zeolite Acidity
decreases with decrease in Al-content
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Effect of zeolite type on acidity
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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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3. NMR methods
Pinto et al. Appl. Catal. 284 (2005) 39
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4. IR Spectroscopy on neat sample
IR spectra of OH groups (zeolites)
(Defect site)
(external surface of crystallites)
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Acidity in zeolites
Acid site inside 10 MR pore
Strong acidity
3610cm-1

• 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.

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5. IR Spectroscopy of adsorbed bases
IR of adsorbed pyridine
Eg. Phosphotungstic acid
(pyridinium 1545cm-1 coordinated Py
1451cm-1)
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In zeolites and silica-alumina Brönsted acid
sites Transform into Lewis acid sites on heating
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H-Y
IR spectra of adsorbed bases
J.W. Ward, J. Catal. 9 (1967) 225
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Composition (average electronegativity) and
acidity
Sandersons intermediate electronegativity
For a compound PpQqRr, Sint Spp Sqq
Srr1/(pqr)
JPC 96 (1992) 8480
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CATALYSIS BY ACIDS
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?Acid catalyzed reactions of hydrocarbons are
mediated by carbocations
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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 gt Sec-C gt 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 of ACID CATALYZED REACTIONS
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ALKYLATION REACTIONS
?Alkylation is the introduction of an alkyl group
into a molecule ?It may involve a new C-C, O-C,
N-C bond formation ?Alkylation is catalyzed by
acidic or basic catalysts
?Acid catalysts are used mainly in aromatics
alkylation at ring-C ?Basic catalysts are used in
alkylation at side-chain-C
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ALKYLATION REACTIONS
MECHANISTIC ASPECTS
?Typical acid catalysts Friedel-Crafts
catalysts HF, H2SO4, HCl-AlCl3 and (ZEOLITES)

• Typical alkylating agents Olefins, alcohols,
  ethers, alkyl halides, dialkyl carbonates (DMC),
  etc

Mechanism of alkylation over Friedel-Crafts
catalysts

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HY HCl, HF etcMXn AlCl3, SbF5, BF3
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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
1. Cumene production
Mechanism 1 Sec-C is formed
Because the Sec-C is more stable, mostly cumene
is (gt 99.9 ) is produced and not n-propyl
benzene (requires the Prim-C)
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2. Alkylate production ( Global production
80 million tpa)
The reaction is alkylation of i-butane with
butene
Superacid needed

• ?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 reaction
• Isobutane / butene ratio is 10 - 15 to prevent
  oligomerization
• Many solid acid catalysts are being developed to
  replace HF / H2SO4

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ISOMERIZATION

• Mostly acid catalyzed

• Important isomerization reactions
• Petroleum refining
• Wax isomerization for lubes
• isomerization of light naphtha (C5 C6)
• Petrochemicals
• Xylene isomerization

• Catalysts are usually bifunctional
• Metal/support type
• Typical examples
• Pt-SAPO-11 for wax isomerization
• Pt-Mordenite /acidic-alumina for C5 C6
  hydrocarbons
• Pt-ZSM-5 /mordenite/(silica)-alumina for xylene
  isomerization

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1. Isomerization of xylenes
Equilibration of the carbocation occurs on The
acid catalyst
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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
• Addition of H to an olefin
• CH3-CH2-CH2-CHCH2 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
  CH2CH-CH3
• ? ?
  (or)
• R-CHCH2 CH2-CH-CH3

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Disproportionation reactions (cracking
alkylation)
Toluene disproportionation
Catalyst
C9 aromatics transalkylation
Catalyst

• Disproportionation reactions are used in
  petrochem. industry
• Catalysts are usually Pt-mordenite,
  Pt-silica-alumina etc

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Diisopropyl benzene transalkylation
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Hydration of olefins
Asahi Chem
Mobil
Dehydration of alcohol
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Condensation reactions
Catalysts are silica-alumina zeolites like
ZSM-5, MOR These are commercial processes.
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Michael addition
Condensation of ?-?-unsaturated ketones (2) with
nitro compounds (1)
Catalysts silica, alumina, clays and zeolites
R. Ballini, D. Florini, M. V. Gil, A. Palmieri,
Green Chem., 5 (2003) 475
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Molecular rearrangements
Claisen rearrangement
Catalyst BEA Y
Beckmann rearrangement
Catalyst MFI
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CATALYSIS SOLID BASES
1. Introduction 2. Characterization of
basicity 3. Examples of reactions over solid bases
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1. Introduction
? 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
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Solid bases Alkali and alkaline earth oxides
RE-oxides ThO2 Alkaline-zeolites Alkali
metals or oxides on Al2O3 and SiO2
Hydrotalcite Sepiolite
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
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2. Characterization of basicity

• 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

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1. By adsorption of organic acids - titration
H- pKBH log BH/B-
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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 spectra
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BASICITY from alcohol dehydrogenation
Conditions 723K and WHSV(h-1) 3.14
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Comparison of basicity of a series of catalysts
  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

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3. Examples of reactions over solid bases
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ALKYLATION
?Acid catalysts cause ring alkylation of alkyl
aromatics and basic catalysts lead to side-chain
alkylation
?In the case of phenols (or anilines), acid and
base catalysts cause both ring and hetero-atom
alkylation, the latter increasing with basicity.
There are only a few commercial applications of
basic catalysts in alkylation of hydrocarbons
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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
?Side-chain alkylation more predominant
Conditions W/F (g.h.mole-1) 30 Tol/DMC
(mole) 5 400C
?Cumene directly from toluene
?Activity increases with basicity CsXgtKXgtNaX
?Styrene is absent in the product
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• C- O- alkylation
• occur over acid catalysts
• O-alk. Increases
• with basicity of catalyst

Alkylation of phenol with methanol
Mode of adsorption determines product
selectivity (I) favours more C-alkylation in
o-position than (II)
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METHYLATION OF HYDROXY AROMATICS
Reactivity of different aromatic hydroxy
compounds

• Activity increases
• with basicity
• All compounds
• equally active
• over most basic
• catalyst

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ALKYLATION OF 2-NAPHTHOL WITH METHANOL
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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ALKYLATION OF ANILINE
NMA
NNDMA
Activities of catalysts in aniline alkylation
?Activity increases with basicity

• MM/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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Base catalyzed isomerization reactions
Commercial processes
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Other reactions
1.
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Knoevenagel condensation
Shu-Guyo Wang, Catal. Commun., 4 (2003) 469
Aldol condensation also takes place on solid
bases, like hydrotalcites
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2. Amination of alcohols
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3. Michael addition
Condensation of an -unsatured ketone (1) and a
mercaptan (2)
Catalyst Synthetic hydroxyapatite
Ca10(PO4)6(OH)2 HAP
S.J. Miller, Microporous Materials, 2 (1994) 439
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Benefits of basic supports
1. Monofunctional catalytic reforming
AROMAX Process (Chevron)
Pt-Ba-KL
Basic
Pt-Re-Al2O3
Acidic
Catalyst Pt-(Ba)-K-L (benzene yield 80)
Carbon No. of alkane
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Reasons attributed for the superiority of Pt-KL
are1. 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
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).