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 catalysis2. Catalysis over zeolites3. Acid catalyzed reactions

Solid acid catalysis - Introduction

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

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

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 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

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

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+)

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

1. Titration with organic bases

HR = H0 + log aH2O

HR indicators

H0 indicators

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

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

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

H. A. Benesi, J. Catal., 28 (1973)176

How toestimateBronstedand Lewis sites?

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

Strongly acidic

Effect of zeolite type on acidity

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

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

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

3. NMR methods

IR spectra of –OH groups (zeolites)

(Defect site)

(external surface of crystallites)

4. IR Spectroscopy – on neat sample

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

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

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

J.W. Ward, J. Catal. 9 (1967) 225

H-Y

IR spectra of adsorbed bases

[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

CATALYSIS BY ACIDS

Acid catalyzed reactions of hydrocarbons are mediated by carbocations

Tri-coordinated

Penta-corodinated

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+

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 ofACID CATALYZED REACTIONS

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)

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

+

+

+

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

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

Importance of alkylation Processes

Green Chem. 6 (2004) 274

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:

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

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

1. Isomerization of xylenes

CH3

CH3

CH3

CH3+

CH3

CH3

+

CH3

CH3

Zeolite

Equilibration of thecarbocation occurs on The acid catalyst

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

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

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

CH3

+i-Pr

i-Pr

i-PrCatalyst

Diisopropyl benzene transalkylation

Hydration of olefins:

+ OH2

OH

ZSM-5Asahi Chem

+ OH2MFI

CH3

CH3

CHOH DIPE-H2O

Mobil

Dehydration of alcohol

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.

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

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

CATALYSIS SOLID BASES CATALYSIS SOLID BASES CATALYSIS SOLID BASES CATALYSIS SOLID BASES

1. Introduction2. Characterization of basicity3. Examples of reactions over solid bases

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

# 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

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

H- = pKBH – log [BH]/[B-]

1. By adsorption of organic acids - titration

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 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

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

 

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

3. Examples of reactions over solid bases

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

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

+

+

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]

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

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

Reactivity of different aromatic hydroxy compounds

METHYLATION OF HYDROXY AROMATICSMETHYLATION OF HYDROXY AROMATICS

•Activity increaseswith basicity

•All compounds equally activeover most basiccatalyst

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

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)

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:

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.

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

OH

+ NH3

NH2

MgO, Al2O3

2. Amination of alcohols

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

+ 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

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

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).