ZEOLITE CATALYSTS ZEOLITE CATALYSTS IN GREEN IN GREEN

CHEMISTRYCHEMISTRY

Dipak Kumar ChakrabartyDipak Kumar ChakrabartyProfessor EmeritusProfessor Emeritus

INDIAN INSTUTUTE OF TECHNOLOGY, BOMBAYINDIAN INSTUTUTE OF TECHNOLOGY, BOMBAYMUMBAI 400076MUMBAI 400076

In the year 1987, the World Environment Commission publisheda report Our Environment that emphasized the need for sustainabledevelopment. Sustainable development means development to fulfillthe needs of the present generation without endangering the needs of the future generation. The concept includes many aspects of which utilization of renewable resources and protection of the environment is primary.

The traditional concept of process efficiency that considered product yield

as the main criteria is being replaced by including such considerations as elimination of waste and protection of environment.

A positive development of the twenty first century is that we have woken up to

the danger to our future - the danger to our environment created my mindless

industrial expansion of the last century.

This has brought the concept of GREEN CHEMISTRY – producing chemicals

With minimum damage to the environment .

Green ChemistryGreen Chemistry

Chemical Industry has its share in polluting the environment and today

it has realized that the time has come to make the industry more and more

efficient not only in terms of profit and yield, but more accountable in terms

of pollution abatement and eco-friendliness.

This has led to the term GREEN CHEMISTRY.

Traditionally, organic synthesis is highly logical, but highly inefficient. Thissituation was allowed to continue mainly because 1. The scale of production was not very large,2. Organic chemists were not much concerned with catalysis.3. Short life cycle of the products and the processes.

Situation has changed since then and more and more organic chemistsare taking the catalytic route. Our concern here will be zeolite catalysts.

Briefly, Green Chemistry is Briefly, Green Chemistry is

Efficient use of (preferably Efficient use of (preferably renewable) raw materials,renewable) raw materials,

Elimination of wasteful byproducts,Elimination of wasteful byproducts, Avoiding use of toxic/hazardous Avoiding use of toxic/hazardous

reagents and solvents,reagents and solvents, Use of safer final (biodegradable) Use of safer final (biodegradable)

products, andproducts, and Increasing energy efficiency.Increasing energy efficiency.

Two considerations that dominatedgreen chemistry are:

A.Maximum atom utilization,

B.The minimum waste produced (E Factor). The waste includes byproducts, reagents,solvent loss and even fuel.

In simple form, it defined as:(Chemicals in (kg) - Desired product (kg) ) / Total product (kg)

The enormous waste in different segments of industry areshown in the table below.

Industry Industry segmentsegment

Product, Product, tonn/anumtonn/anum

kg waste/ kg kg waste/ kg productproduct

Oil Oil refiningrefining

101066-10-1088 <0.1<0.1

Bulk Bulk chemicalschemicals

101044-10-1066 <1-5<1-5

Fine Fine chemicalschemicals

101022-10-1044 5-505-50

PharmaceuPharmaceuticalsticals

10 - 1010 - 1033 25-10025-100

E FACTOR

M. Lancaster, “Green Chemistry : An Introductory Text, Roy.Soc.Chem., cambridge, 2002.

Another important parameter is the extent of harmfulness of the waste. For example, sodium sulphate as a waste is certainly far less harmful than a cyanide waste. A new term environment quotient (Q) has been coined to emphasize this difference and some number has been arbitrarily assigned to different wastes according to their extent of their harmful effect.

Q FACTOR

R. A. Sheldon, Chemtech., 38, (1994).

Some examples of atom utilization 3PhCH(OH)CH3 + 2CrO3 + 3H2SO4 3PhCOCH3 + Cr2(SO4)3 + 6H2O

atom efficiency = 360/860 or 42% catalyst

3PhCH(OH)CH3 + ½ O2 PhCOCH3 + H2O atom efficiency = 120/138 or 87%

C2H2 + 1/2O2 C2H2Oatom efficiency = 100%

The idea was first introduced by Trost. See B. M. Trost, Science, 254, 147 (19910.

Zeolite StructureZeolite Structure Zeolites are crsytalline alumino-silicates with Zeolites are crsytalline alumino-silicates with

exchangeable cationsexchangeable cations The cations can be exchanged with protons that The cations can be exchanged with protons that

make them acidic solids.make them acidic solids. Si:Al ratio can vary in zeolites from 1 to infinity.Si:Al ratio can vary in zeolites from 1 to infinity. Number of acid sites decreases with increase in the Number of acid sites decreases with increase in the

Si:Al ratioSi:Al ratio Strength of the acid sites increases with the Si:Al Strength of the acid sites increases with the Si:Al

ratio.ratio. There are also Lewis acid sites (tri-coordinated Al).There are also Lewis acid sites (tri-coordinated Al). They have micro-pores of entry port size varying from They have micro-pores of entry port size varying from

about 3 to 12 A.about 3 to 12 A.

Zeolite Pore StructureZeolite Pore Structure

Zeolites are formed by oxygen-sharing T (Si or Al) Zeolites are formed by oxygen-sharing T (Si or Al) atoms, each T atom linked to four oxygen making atoms, each T atom linked to four oxygen making tetrahedra.tetrahedra.

These tetrahedra are linked to form rings These tetrahedra are linked to form rings containing equal number of oxygen and silicon containing equal number of oxygen and silicon atoms.atoms.

The rings share common oxygen (or silicon) The rings share common oxygen (or silicon) forming different polyhedra.forming different polyhedra.

The polyhedra then join by sharing common The polyhedra then join by sharing common oxygen (or silicon) forming the three dimensional oxygen (or silicon) forming the three dimensional network structure giving rise to pores.network structure giving rise to pores.

The pores may be all in one direction or may run The pores may be all in one direction or may run in several directions.in several directions.

Each corner is a Si atom and oxygen atoms are at the middle of each edge.a: sodalite polyhedron; b: zeolite sodalite; c: zeolite A; d: faujasite

Five-member silicon-oxygen ring. Oxygen sharing between rings formation chain.

a: Chains joining to form 10-Oxygen pores in ZSM-5; b: three-dimensional pore structure of ZSM-5 showing zigzag structure

Pore openings in a: Ferrierite and b: ZSM-23

8-Oxygen, 10-Oxygen and 12-Oxygen rings found in erionite, ZSM-5and faujasite.

Table 2. Pore size of zeolites.

Pore typePore typeNo. of O No. of O atoms in the atoms in the ringring

Framework Framework structurestructure

Pore size, APore size, A DimensionalitDimensionalityy

SmallSmall 88 ZEOLITE AZEOLITE A 4.14.1 33

mediummedium 1010 ZSM-5ZSM-5 ZSM-11ZSM-11 SAPO-11SAPO-11

1.1.x 5.4x 5.45.4 x 5.35.4 x 5.36.3 x 3.96.3 x 3.9

33 33 11

LargeLarge 1212 X, YX, YMORDENITEMORDENITE BETA BETA SAPO-5SAPO-5 LL

7.47.4 7.0 x 6.57.0 x 6.5 7.5 x 5.77.5 x 5.7 7.37.3 7.17.1

33 22 33 11 11

ExtralargeExtralarge 1414 1818 2020

SAPO-8SAPO-8 VPI-5VPI-5 CLOVERITECLOVERITE

8.7 x 7.98.7 x 7.9 12.112.1 13.2 x 6.013.2 x 6.0

11 33 33

A major application of the zeolites in catalysisis in acid catalyzed reactions such as alkylation,acylation, electrophilic aromatic substitution, cyclization, isomerization and condensation. We shall take some examples here.

+ C2H2 HZSM-5

Catalysis with acidic zeolites1. ALKYLATION Mobil-Badger Process (Polyalkylation is suppressed)

Similarly, propylbenzene could be manufacturedusing a 3-dimensional dealuminated mordenite(3-DDM) catalyst

+

dealuminatedmordenite

Si/Al = 100-1000

Dealumination enabled to obtain very high Si:Al ratio (up to 1000). In these form,the micropores of mordenite were connectedthrough mesopores (5-10 nm).

K.Tanabe and W.F. Holderich, Appl.Catal.A General, 181,399 (1999).

Naphthalene is dialkylated with propene over 3-DDM.The product is used in making the carboxylic acid thatis an important monomer for making plastics.

+

Acylation Acylation with heterogeneous catalysis ismuch more difficult because of the polarity difference of the substrate and the acylating reagent that makes it difficult to achieve Favourable adsorption ratio of the two. This could be achieved by the use of H-Beta.

+ (CH3CO)2O

O

G.R. Meima, G.S. Lee and J.M. Garces, in “Fine Chemicals Through heterogeneousCatalysis (Ed. R.A. Sheldon, Wile-VCH, Weinheim, 2001.

A. Vogt and A. Pfenninger, EP0701987A1, 1996 to Uetikon AG.

.

Hydroxyalkylaton using zeolites is difficult because of unfavourable adsorption ratio of the reagent and the substrate. This difficulty is avoided by having the aromaticand the epoxide functions in the same molecule

Ce3+ exchanged Y zeolite could catalyze toluene and xylenes using with higher carboxylic acids showing that free carboxylic acids can be used in acylation.

+ RCO2H

O

R

O

H-ZSM-5

or H-Beta

J.A. Elings, R.S. Downing and R.A. Sheldon, Stud.Surf.Sci.Catal, 105, 1125 (1997).

B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch,J. Org. Chem., 51, 2128 (1986).

Formation of N-heterocycles by intermolecular cyclization

is catalyzed by acidic zeolites. Synthesis of pyridine and

picoline from a mixture of acetaldehyde, formaldehyde and

ammonia in presence of H-ZSM-5 is an example.

N N+

NH2H2N N

H-ZSM-5 Pd

NH

CH 3CHO + HCHO + NH 3

NOHsulfuric acid

H-ZSM-5Si : Al > 1000, 350oC

vap. phase

O

NH

produces 2 kg 0f ammon. sulfateper kg of product

conventional

Sumitomo

Beckmann rearrangement

W. F. Holderich et al. in “Fine Chemicals through Catalysis”, pp.217-231, Wiley-VCH, (2001)

M.J.Burk et. Al., J. Org. Chem., 64, 3290 (1999)

O

TS-1 orZSM-5 O

H TS-1 ZSM-5conv. 100% 99%select. 98% 97-100%

Epoxide reaarangement ( key step in the manufacture of many intermediates inthe fine chemical industry. Traditional method used stoichiometric Lewis acids or bases.

G. P. Heitmann, G. Dahlhoff and W.F.Holderich, J.Catal, 186,, 12 (1999)

Traditional method for preparing 2,6-dichlorobenzonitrile uses stoichiometric

Amonts of chlorine, HCN and POCl3 with atom efficiency 31%. The new process

Was developed uses zeollite catalysts Eur.Pat.Appl. EP948988 (1999)

+ Cl 2Ag-H-Mor350 o

Cl Cl

Cl

Cl

Cl

Cl Cl ClCl ClCl

CN

adsorption infaujasite

NH 3 + O 2catalyst

350-450 oC

C

K. Iwayama, S. Yamakawa, M. Kato and H. Okino, Eur. Pat. Appl. EP948988 (1999) to Toray

Oxidation

Traditional methods of oxidation in organic chemistry uses stoichometric reagents (salts of manganese

and chromium. The new chemistry tries to use molecular oxygen or hydrogen peroxide. TiO2 supported

on silica was not effective with hydrogen peroxide because the water produced gets strongly adsorbedo

on silica. TS-1 has been used successfully because this titanium substituted silicalite-1 is hydrophobic.

Phenol is converted with hydrogen peroxide to a mixture of hydroquinone and catechol. Rhone-Poulenc

Process uses perchloric acid and phosphoric acid whereas Enichem process uses TS-1. A comparision

Is given here

Table 3. Comparision of phenol conversion processes

ProcessProcess (catalyst)(catalyst)

Rhone-Poulenc Rhone-Poulenc (H3PO4,HClO4)(H3PO4,HClO4)

EnichemEnichem (TS-1)(TS-1)

Phenol conversion (%)Phenol conversion (%) 55 2525

Selectivity on phenolSelectivity on phenol 8080 9090

Catechol/hydroquinoneCatechol/hydroquinone 2.32.3 11

TS-1 is also called a redox molecular sieve and can be used as a catalyst for many oxidations with hydrogen peroxide. In presence of TS-1, ammonia and hydrogen peroxide forms in-situ hydroxylamine which reacts with a ketone. An example is:

+ NH3 + H2O2

HO

NOH

TS-1

H+HO

O

NHCOCH3

HO

Beckmann rearrangement

paracetamol

A. Corma, L.T. Nemeth,M. Rench and S. Valencia,Nature, 412, 423 (2001).

Bayer-Villiger Oxidation (cinversion of ketone or an aldehyde to the ester

The peracid undergoes a nucleophylic attack on the carbonyl group givimg an inetrmediate.

In the next step, a concerted migration of one of the alkyl groups takes place releasing the

Carboxylate anion. The reaction is widely used in organic chemistry.

Zeolite beta containing 1.4 wt% of tin is a good catalyst using hydrogen peroxide.

O

O O

1 eq. H2O2

0.66 mol% Sn-Betadioxane, 90 oC

A lrge number of bulk chemicals are produced by using molecular oxygen either in the liquidor in the vapour phase reaction. S0me of these are:Benzene/ethene to styrene, p-xylene to terephthalic acid, formaldehyde to methanol, Ethene ot ethene oxide, n-butane to acetic acid, propene to acrylonitrile, n-butane toMalic anhydride, o-xylene to phthalic anhydride, isobutene to methyl methacrylate etc.

Molecular oxuygen is a spin triplet and its direct reaction to a organic singlet compoundis spin forbidden. To overcome this, the triplet is allowed to react with paramegneticMetal ions forming a superoxo-metal complex that forms a variety of metal-oxygen species.Use of catalytic route to selective oxidation in presence of several functional groups isA big challenge.

Basic zeolitesComarattively much less attention has been paid to basic zeolites. Zeolites can be made basic by1.Exchange of protons with alkali or rare earth ions or 2. by depositing nano-particles of alkali oralkali earth oxides in the pores. The basic sites are weak. They can be used to generate C-C bond in the side chains of substituted aromatics.

CHO

+ NCCH2CO2Et

COOEt

CN

Na-X

NH2

+ (MeO)2CO

NHCH3

+ MeOH + CO2

CN+ MeOH

CN+ H2O

K-Y

Cs-X

K.R. Kloestra, H. van Bekkum, Chem.Soc.Chem.Commun., 1005 (1995).

Chemicals through Hetero. Catal.,Cormma and S. Iborra in “Fine 309 (2001).

SIDE CHAIN ALKYLATIONS