Transcript MS PowerPoint - Catalysis Eprints database

Catalysis over solid acids and bases
S. Sivasanker
CATALYSIS OVER SOLID ACIDS
CONTENTS
1. Solid acid catalysis - Introduction
- Examples of solid acids
- Acidity characterization
- Acidity measurement
- Intermediates in acid catalysis
2. Catalysis over zeolites
3. Acid catalyzed reactions
Solid acid catalysis - Introduction
ACID CATALYSIS
Two types of acid sites
are recognized
SOLID ACID CATALYSTS
Examples:
• Zeolites
• SAPOs
• Clays; pillared clays
• Ion-exchange resins
• Oxides; X, SO4-oxides
• Mixed oxides; amorphous
• Heteropoly acids
- Brönsted
- Lewis
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
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
Remove N and S from petroleum oils
Alumina support
Hydrate olefins to alcohols.
Ion-exchange resin; ZSM5; Heteropolyacids
Hydrotreating
Hydration
 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
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
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
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).
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+)
Typical Hammett
acidity (Ho)
of some strong acids
used in catalysis
a
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
: Denotes the strength of the strongest acid sites in solid acids
1. Titration with organic bases
H0 indicators
HR indicators
HR = H0 + log aH2O
Ho
Ho
HR
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
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
Heat of adsorption:
Clausius-Clapeyron equation:
ln(p2/p1) = (Qst/R) [(T2- T1) / T1 T2]
Used for chemisorption:
H2, CO on metals;
NH /acidic solids; CO /basic solids
How to
estimate
Bronsted
and
Lewis
sites?
H. A. Benesi, J. Catal., 28 (1973)176
b) Temeperature programmed desorption
Sample is adsorbed and then desorbed by raising temp.
 Basic compounds from acidic solids
 Acidic compounds from basic solids
H-Beta
D-Beta 34
D-Beta 46
D-Beta 175
118
Effect of Si/Al
Ratio of zeolite –
Acidity decreases
with decrease in
Al-content
Desorption
116
114
112
110
108
100
200
300
400
Temperature(oC)
500
600
Effect of zeolite type on acidity
Strongly acidic
Plots are deconvoluted to derive
WEAK and STRONG acidity
H-Beta
Desorption
6
4
2
0
100
200
300
400
Temperature(o C)
500
600
Pinto et al. Appl. Catal. 284 (2005) 39
3. NMR methods
Pinto et al. Appl. Catal. 284 (2005) 39
4. IR Spectroscopy – on neat sample
IR spectra of –OH groups (zeolites)
(Defect site)
(external surface of crystallites)
Acid site inside
10 MR pore
Acidity in zeolites
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.
5. IR Spectroscopy of adsorbed bases
IR of adsorbed pyridine
PW
Eg. Phosphotungstic acid
573 K
Bronsted site (strong)
1445
1486
1542
1534
373 K
1606
473 K
1631
Absorbance
673 K
Lewis site (weak)
1700 1650 1600 1550 1500 1450 1400
-1
Wavenumber (cm )
(pyridinium ~ 1545cm-1 ; coordinated Py ~ 1451cm-1)
In zeolites and silica-alumina Brönsted acid sites
Transform into Lewis acid sites on heating
Bronsted acid sites
H+
H+
O
O
Si
Al
O
O
O
Al
Si
Si
O
O
[A]
Si
- H2O
Basic site
O
O
Si
Al
Lewis acid site
O
O
Si
O
O
Si
Al
+
Si
[B]
H-Y
IR spectra of
adsorbed bases
J.W. Ward, J. Catal. 9 (1967) 225
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]
CATALYSIS BY ACIDS
Acid
catalyzed reactions of hydrocarbons are
mediated by carbocations
Tri-coordinated
Penta-corodinated
Relative stability of the carbocations
Tert-C+ > Sec-C+ > Prim-C+
Reaction
velocity and
product yield
are
generally
determined by
the stability
of the
carbocation
intermediates:
Some examples
of carbocations
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
3. Bimolecular hydride transfer reaction
4. Condensation
A metallic component helps in generating olefins
making C+ formation easy (bifunctional catalysts)
Examples of
ACID CATALYZED REACTIONS
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
CH3
(p-Xylene)
CH3
Acid Catalyst
CH3
+ MeOH
Basic Catalyst
CH2CH3
(Ethylbenzene)
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:
R Cl
+
R
AlCl3
+
R
-
Cl
AlCl3
-
+
H
R
AlCl3
Cl
+ Cl
AlCl3
R
+ AlCl3
+ H-Cl
HY = HCl, HF etc;MXn = AlCl3, SbF5, BF3
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.
Commercial
Benzene +
C10 - C13
olefins
LAB
Proprietory
Commercial
Importance of alkylation Processes
Green Chem. 6 (2004) 274
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+)
2. Alkylate production: ( Global production = ~ 80 million tpa)
The reaction
is alkylation
of i-butane
with butene
+
+
+
[Butene]
Superacid needed
Hydride transfer
[Isobutane]
Trimethylpentane
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
ISOMERIZATION
 Mostly acid catalyzed
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 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
1. Isomerization of xylenes
CH3
CH3
Zeolite
CH3
CH3
CH3
CH3
+
+
CH3
CH3
Equilibration of the
carbocation occurs on
The acid catalyst
2. Isomerization of alkanes:
For octane improvement and pour point reduction
(petroleum refining)
 Bifunctional mechanism – acid and metal catalyzed
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
metal
(3-methyl pentane)
C-C-C=C-C
C
(i-hexene)
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
b

(or)
R-CH=CH2 + CH2+-CH-CH3
Disproportionation reactions (cracking + alkylation)
CH3
Toluene
disproportionation
CH3
Catalyst
CH3
CH3
CH3
+
CH3
C9+ aromatics
transalkylation
CH3
CH3
Catalyst
CH3
• Disproportionation reactions are used in petrochem. industry
• Catalysts are usually Pt-mordenite, Pt-silica-alumina etc
Diisopropyl benzene transalkylation
i-Pr
CH3
+
i-Pr
i-Pr
Catalyst
Hydration of olefins:
OH
+
+
OH2
OH2
Asahi Chem
ZSM-5
CH3
MFI
CH3
CHOH
-H2O
DIPE
Dehydration of alcohol
Mobil
Condensation reactions
CH3CHO + HCHO + NH3
Solid Acid
+
N
NH2
HO
N
N
N
N
N
N
Solid Acid
+
NH2
N
HO
R
(R= H)
(R= Me)
(R= Et)
Catalysts are silica-alumina & zeolites like ZSM-5, MOR
These are commercial processes.
Michael addition
Heterogeneous
Catalyst
O
+
Ph
O
r.t., 22 h
NO2
1
O
O
Ph
NO2
2
3
Condensation of -b-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
Molecular rearrangements
2O2
Claisen rearrangement
O
OH
O
solid acid
solid acid
zeolite
zeolite
Allyl phenyl ether
Allyl phenol
Catalyst:
BEA; Y
2-methyl, 2,3 dihydro
benzofuran
Beckmann rearrangement
O
NOH
molecular sieves
te
cyclohexanone oxime
~ 90% yield
NH
caprolactam
Catalyst: MFI
CATALYSIS SOLID BASES
1. Introduction
2. Characterization of basicity
3. Examples of reactions over solid bases
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
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
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
1. By adsorption of organic acids - titration
H- = pKBH – log [BH]/[B-]
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.
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.
Basicity 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
K(1.5) SiO2
1683
1365
318
1643
1462
181
1663
1347
316
1633
1407
226
Cs(1.5) SiO2
1648
1329
319
1617
1383
234
BASICITY from alcohol dehydrogenation
Catalytic activity in isopropanol dehydrogenation
Conversion
(mole %)
4.0
Selectivity
(Acetone)
Acetone - TOF x 10-3
(w.r.t. alkali metal)
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
Catalyst
SiO2
Conditions: 723K and WHSV(h-1) = 3.14
Comparison of basicity of a series of catalysts
Catalysta
a:
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
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
3. Examples of reactions over solid bases
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.
CH3
MeOH
CH2CH3
Base
OH
MeOH
OMe
Base
NH2
NH(Me)
MeOH
Base
There are only a few commercial
applications of basic catalysts in
alkylation of hydrocarbons
Important Industrial alkylation reactions using basic catalysts
Reaction
OH
Catalyst
OH
+
MgO
MeOH
O
H
O
H
+
+
+
MeOH
OH
+
Fe-V-O/ SiO2
Na/ K2CO3
K/ KOH/Al2O3
Side-chain alkylation of toluene over alkaline-X zeolite
Table 1. Properties of ion exchanged zeolites
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
Catalyst
Sint (intermediate electronegativity) = geometric mean of the electronegativity of constituent atoms (Mortier, J. Catal. 55 (1978) 138)
[Bal et al. Stud. in Surf. Sci. Catal. 130 (2000) 2645]
Alkylation of
toluene with
dimethylcarbonate
Side-chain alkylation more
predominant
Cumene directly from toluene
Styrene is absent in the product
Conditions: W/F (g.h.mole-1) = 30;
Tol/DMC (mole) = 5; 400°C
Activity increases with
basicity: CsX>KX>NaX
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)
H
M
g
O
O
M
g
(I)
O
M
g
Si
O
O
Si
O
(II)
Al ( H)
METHYLATION OF HYDROXY AROMATICS
Reactivity of different aromatic hydroxy compounds
Phenol
p-Cresol
Hydroquinone
2-Naphthol
Conversion (%)
100
80
60
•Activity increases
with basicity
•All compounds
equally active
over most basic
catalyst
40
20
0
Si
Li
Na
K
Catalysts
Cs
OMe
I; 2-Methoxy naphthlene
CH3
ALKYLATION OF
2-NAPHTHOL
WITH METHANOL
OH
HO
2-Naphthol
II; 1-Methyl-2-hydroxy
naphthalene
CH3
OMe
III; 1-Methyl-2-methoxy
naphthalene
Catalyst Conv. % O-/-C
Methylation
Scheme 2. Products of methylation of 2-hydroxynaphthalene (2-naphthol)
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
ALKYLATION OF ANILINE
NH2
NHCH3
N(CH3)2
MeOH / catalyst
k2
MeOH / catalyst
k1
NMA
NNDMA
Activities of catalysts in aniline alkylation
Catalyst
Cs-X
Cs-silicalite
Cs-MCM-41
Cs-SiO2
SBET
(m2/g)
550
379
625
130
Rel basicity
(FTIR)
182
83
49
40
Conversion
(%)
65.3
38.0
48.9
17.0
NMA / NNDMA
(selectivity)
4.8
2.1
2.3
2.5
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)
Base catalyzed isomerization reactions
Commercial processes:
a)
CH2=CH-CH
CH2=CH-CH2
Na/NaOH/Al2O3
O
O
b)
CH2=C=CH2
O
isosafrol
O
Safrol
K2O/Al2O3
CH3-C=CH3
Other reactions
1. Knovenagel condensation
R1
O
R3
O
R1
+
Mg-Al HT
R3
OEt
Toluene, heat
OH
R1 = H, Me
R2 = H, OMe
R3 = H, CN,COMe,CO2Et
Heck Reaction
M / HT
Ar-X +
R
Ar
R
R2
O
O
(Coumarins)
Knoevenagel condensation
OH
OH
OH
OH
OH
MCM-48
(A)
COOEt
H
COOEt
H
Base Catal.
O + H2C
C6H5
O
O Si(CH 2)3NH2
O
+ (C2H5O)3Si(CH 2)3NH2
C
CN
C6H5
C
CN
(B)
Shu-Guyo Wang, Catal. Commun., 4 (2003) 469
Aldol condensation also takes place on solid bases, like hydrotalcites
2. Amination of alcohols
OH
NH2
+
NH3
MgO, Al2O3
3. Michael addition
R
O
S
+ R-S-H
X
1
2
O
HAP
MeOH/r.t.
X
3
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
Benefits of basic supports
1. Monofunctional
catalytic reforming
+ 3 H2
(
Hr = 63.6 kcal/mol)
AROMAX Process (Chevron)
Pt-Ba-KL
Basic
Pt-Re-Al2O3 Acidic
Catalyst:
Pt-(Ba)-K-L
(benzene yield ~ 80%)
Carbon No. of alkane
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
2. Heck reaction
R
X
Base, Pd-catalyst
+
X = I, Br, Cl
+
R solvent DMF, 403 K
R = Ph, -COOEt
(Et or any alkyl group)
Coupling product
HX
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).