1. Catalysis over solid acids and bases
S. Sivasanker

2. CATALYSIS OVER SOLID ACIDS

3. 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

4. Solid acid catalysis - Introduction

5. Catalytic cracking is the Largest user of any solid Catalyst
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

6. Reactions / processes based on acid catalysis
Name of reaction
Description
Solid-acid catalyst
used
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

7. for different reactions is different:
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

8. 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. Sanderson’s intermediate electronegativity
Strength, type and the number of acid sites in a
solid catalyst are important

9. In the case of solid acids, it is even more
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

10. 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).

11. 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+)

12. Typical Hammett acidity (Ho) of some strong acids used in catalysis
Hoa
Conc. H2SO4
~ -12
Anhydrous HF
~ -10
SiO2-Al2O3
SiO2-MgO
< + 1.5
SbF5- Al2O3
< -13.2
Zeolite, H-ZSM-5
Zeolite, RE-H-Y
Typical Hammett
acidity (Ho)
of some strong acids
used in catalysis
a : Denotes the strength of the strongest acid sites in solid acids

13. HR = H0 + log aH2O 1. Titration with organic bases H0 indicators
HR indicators
HR = H0 + log aH2O

14. Ho Ho HR

15. 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

16. 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

17. Used for chemisorption: H2, CO on metals;
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

18. How to estimate Bronsted and Lewis sites?
H. A. Benesi, J. Catal., 28 (1973)176

19. 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

20. Effect of zeolite type on acidity
Strongly acidic

21. Plots are deconvoluted to derive WEAK and STRONG acidity

22. Pinto et al. Appl. Catal. 284 (2005) 39

23. 3. NMR methods
Pinto et al. Appl. Catal. 284 (2005) 39

24. 4. IR Spectroscopy – on neat sample
IR spectra of –OH groups (zeolites)
(Defect site)
(external surface of crystallites)

25. 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.

26. 5. IR Spectroscopy of adsorbed bases
IR of adsorbed pyridine
Lewis site (weak)
Bronsted site (strong)
Eg. Phosphotungstic acid
(pyridinium ~ 1545cm-1 ; coordinated Py ~ 1451cm-1)

27. In zeolites and silica-alumina Brönsted acid sites
Transform into Lewis acid sites on heating

28. H-Y
IR spectra of
adsorbed bases
J.W. Ward, J. Catal. 9 (1967) 225

29. Sanderson’s intermediate electronegativity
Composition (average
electronegativity) and acidity
Sanderson’s intermediate
electronegativity
For a compound PpQqRr,
Sint = [Spp Sqq Srr]1/(p+q+r)
[JPC 96 (1992) 8480]

30. CATALYSIS BY ACIDS

31. Tri-coordinated Penta-corodinated
Acid catalyzed reactions of hydrocarbons are mediated by carbocations
Tri-coordinated
Penta-corodinated

32. product yield are generally
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+

33. Some examples
of carbocations

34. 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

35. 3. Bimolecular hydride transfer reaction
4. Condensation

36. A metallic component helps in generating olefins
making C+ formation easy (bifunctional catalysts)

37. ACID CATALYZED REACTIONS
Examples of
ACID CATALYZED REACTIONS

38. 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

39. Mechanism of alkylation over Friedel-Crafts catalysts:
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: +

40. HY = HCl, HF etc;MXn = AlCl3, SbF5, BF3

41. Solid-acid based alkylation reactions in commercial practice
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 b.
Benzene + C10 - C13 olefins
LAB
Proprietory

42. Importance of alkylation Processes
Green Chem. 6 (2004) 274

43. Examples of alkylation mechanisms
1. Cumene production:
Mechanism 1;
Sec-C+ is formed
Because the Sec-C+ is more stable, mostly cumene is (> 99.9 %)
is produced and not n-propyl benzene (requires the Prim-C+)

44. 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 to prevent oligomerization
Many solid acid catalysts are being developed to replace HF / H2SO4

45. 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

46. 1. Isomerization of xylenes
Equilibration of the
carbocation occurs on
The acid catalyst

47. 2. Isomerization of alkanes:
For octane improvement and pour point reduction (petroleum refining)
Bifunctional mechanism – acid and metal catalyzed

48. Cracking reactions
Catalytic cracking mechanism – occurs via carbocations
A. Carbenium ions are produced mainly by:
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-CH CH2=CH-CH3
  (or)
R-CH=CH CH2+-CH-CH3

49. 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

50. Diisopropyl benzene transalkylation
Catalyst

51. Hydration of olefins:
Asahi Chem
Mobil
Dehydration of alcohol

52. Condensation reactions
Catalysts are silica-alumina & zeolites like ZSM-5, MOR
These are commercial processes.

53. Condensation of --unsaturated ketones (2) with nitro compounds (1)
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

54. Molecular rearrangements
Claisen rearrangement
Catalyst:
BEA; Y
Beckmann rearrangement
Catalyst: MFI

55. CATALYSIS SOLID BASES
1. Introduction
2. Characterization of basicity
3. Examples of reactions over solid bases

56. 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

57. 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

58. 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

59. 1. By adsorption of organic acids - titration
H- = pKBH – log [BH]/[B-]

61. 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.

62. 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.

63. Basicity from FTIR spectra
Sample
Antisymmetric
Symmetric
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

64. BASICITY from alcohol dehydrogenation
Conditions: 723K and WHSV(h-1) = 3.14

65. Comparison of basicity of a series of catalysts
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
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

66. 3. Examples of reactions over solid bases

67. ALKYLATION and basic catalysts lead to side-chain 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

69. 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 (2000) 2645]

70. Alkylation of toluene with dimethylcarbonate
Side-chain alkylation more
predominant
Conditions: W/F (g.h.mole-1) = 30;
Tol/DMC (mole) = 5; 400°C
Cumene directly from toluene
Activity increases with
basicity: CsX>KX>NaX
Styrene is absent in the product

71. Alkylation of phenol with methanol
C- & O- alkylation
occur over acid catalysts
O-alk. Increases
with basicity of catalyst
Mode of adsorption determines product selectivity:
(I) favours more C-alkylation in o-position than (II)

72. Reactivity of different aromatic hydroxy compounds
METHYLATION OF HYDROXY AROMATICS
Reactivity of different aromatic hydroxy compounds
Activity increases
with basicity
All compounds
equally active
over most basic
catalyst

73. Basicity increases conv. Basicity increases O-Me selectivity
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

74. Activities of catalysts in aniline alkylation
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)

75. Base catalyzed isomerization reactions
Commercial processes:

76. Other reactions 1.

77. Knoevenagel condensation
Shu-Guyo Wang, Catal. Commun., 4 (2003) 469
Aldol condensation also takes place on solid bases, like hydrotalcites

78. 2. Amination of alcohols

79. 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

80. AROMAX Process (Chevron)
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

81. Reasons attributed for the superiority of Pt-KL are: 1
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 aromatics 3. Extremely good dispersion of Pt 4. Low coke deposition on the catalyst

82. 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).