ZEOLITE CATALYSTS IN GREEN CHEMISTRY

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ZEOLITE CATALYSTS IN GREEN CHEMISTRY

• Dipak Kumar Chakrabarty
• Professor Emeritus
• INDIAN INSTUTUTE OF TECHNOLOGY, BOMBAY
• MUMBAI 400076

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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.
In the year 1987, the World Environment
Commission published a report Our Environment
that emphasized the need for sustainable developme
nt. Sustainable development means development to
fulfill the 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.
This has brought the concept of GREEN CHEMISTRY
producing chemicals With minimum damage to the
environment .
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Green 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. This situation
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 chemists are
taking the catalytic route. Our concern here will
be zeolite catalysts.
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Briefly, Green Chemistry is

• Efficient use of (preferably renewable) raw
  materials,
• Elimination of wasteful byproducts,
• Avoiding use of toxic/hazardous reagents and
  solvents,
• Use of safer final (biodegradable) products, and
• Increasing energy efficiency.

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• Two considerations that dominated
• green chemistry are
• Maximum atom utilization,
• B.The minimum waste produced (E Factor).
• The waste includes byproducts, reagents,
• solvent loss and even fuel.

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E FACTOR
In simple form, it defined as (Chemicals in (kg)
- Desired product (kg) ) / Total product
(kg) The enormous waste in different segments of
industry are shown in the table below.
Industry segment Product, tonn/anum kg waste/ kg product
Oil refining 106-108 lt0.1
Bulk chemicals 104-106 lt1-5
Fine chemicals 102-104 5-50
Pharmaceuticals 10 - 103 25-100
M. Lancaster, Green Chemistry An Introductory
Text, Roy.Soc.Chem., cambridge, 2002.
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Q FACTOR
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.
R. A. Sheldon, Chemtech., 38, (1994).
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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 ?
C2H2O atom efficiency 100
The idea was first introduced by Trost. See B. M.
Trost, Science, 254, 147 (19910.
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Zeolite Structure

• Zeolites are crsytalline alumino-silicates with
  exchangeable cations
• The cations can be exchanged with protons that
  make them acidic solids.
• SiAl ratio can vary in zeolites from 1 to
  infinity.
• Number of acid sites decreases with increase in
  the SiAl ratio
• Strength of the acid sites increases with the
  SiAl ratio.
• There are also Lewis acid sites (tri-coordinated
  Al).
• They have micro-pores of entry port size varying
  from about 3 to 12 A.

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Zeolite Pore Structure

• Zeolites are formed by oxygen-sharing T (Si or
  Al) atoms, each T atom linked to four oxygen
  making tetrahedra.
• These tetrahedra are linked to form rings
  containing equal number of oxygen and silicon
  atoms.
• The rings share common oxygen (or silicon)
  forming different polyhedra.
• The polyhedra then join by sharing common oxygen
  (or silicon) forming the three dimensional
  network structure giving rise to pores.
• The pores may be all in one direction or may run
  in several directions.

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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
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Five-member silicon-oxygen ring. Oxygen sharing
between rings formation chain.
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a Chains joining to form 10-Oxygen pores in
ZSM-5 b three-dimensional pore structure of
ZSM-5 showing zigzag structure
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Pore openings in a Ferrierite and b ZSM-23
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8-Oxygen, 10-Oxygen and 12-Oxygen rings found in
erionite, ZSM-5 and faujasite.
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Table 2. Pore size of zeolites.
Pore type No. of O atoms in the ring Framework structure Pore size, A Dimensionality
Small 8 ZEOLITE A 4.1 3
medium 10 ZSM-5 ZSM-11 SAPO-11 x 5.4 5.4 x 5.3 6.3 x 3.9 3 3 1
Large 12 X, Y MORDENITE BETA SAPO-5 L 7.4 7.0 x 6.5 7.5 x 5.7 7.3 7.1 3 2 3 1 1
Extralarge 14 18 20 SAPO-8 VPI-5 CLOVERITE 8.7 x 7.9 12.1 13.2 x 6.0 1 3 3
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A major application of the zeolites in
catalysis is in acid catalyzed reactions such as
alkylation, acylation, electrophilic aromatic
substitution, cyclization, isomerization and
condensation. We shall take some examples here.
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Catalysis with acidic zeolites 1. ALKYLATION
Mobil-Badger Process (Polyalkylation is
suppressed)
Similarly, propylbenzene could be
manufactured using a 3-dimensional dealuminated
mordenite (3-DDM) catalyst
Dealumination enabled to obtain very high SiAl
ratio (up to 1000). In these form, the micropores
of mordenite were connected through mesopores
(5-10 nm).
K.Tanabe and W.F. Holderich, Appl.Catal.A
General, 181,399 (1999).
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Naphthalene is dialkylated with propene over
3-DDM. The product is used in making the
carboxylic acid that is an important monomer for
making plastics.
G.R. Meima, G.S. Lee and J.M. Garces, in Fine
Chemicals Through heterogeneous Catalysis (Ed.
R.A. Sheldon, Wile-VCH, Weinheim, 2001.
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.

A. Vogt and A. Pfenninger, EP0701987A1, 1996 to
Uetikon AG.
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Hydroxyalkylaton using zeolites is difficult
because of unfavourable adsorption ratio of the
reagent and the substrate. This difficulty is
avoided by having the aromatic and the epoxide
functions in the same molecule
.

J.A. Elings, R.S. Downing and R.A. Sheldon,
Stud.Surf.Sci.Catal, 105, 1125 (1997).
Ce3 exchanged Y zeolite could catalyze
toluene and xylenes using with higher
carboxylic acids showing that free
carboxylic acids can be used in acylation.
B. Chiche, A. Finiels, C. Gauthier, P. Geneste,
J. Graille and D. Pioch, J. Org. Chem., 51, 2128
(1986).
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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.

M.J.Burk et. Al., J. Org. Chem., 64, 3290 (1999)
W. F. Holderich et al. in Fine Chemicals through
Catalysis, pp.217-231, Wiley-VCH, (2001)
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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)
K. Iwayama, S. Yamakawa, M. Kato and H. Okino,
Eur. Pat. Appl. EP948988 (1999) to Toray
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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
Process (catalyst) Rhone-Poulenc (H3PO4,HClO4) Enichem (TS-1)
Phenol conversion () 5 25
Selectivity on phenol 80 90
Catechol/hydroquinone 2.3 1
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

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

paracetamol
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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.
A lrge number of bulk chemicals are produced by
using molecular oxygen either in the liquid or 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 to Malic
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 compound is spin forbidden.
To overcome this, the triplet is allowed to react
with paramegnetic Metal 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 is A big challenge.
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Basic zeolites Comarattively much less attention
has been paid to basic zeolites. Zeolites can be
made basic by 1.Exchange of protons with alkali
or rare earth ions or 2. by depositing
nano-particles of alkali or alkali 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.
Chemicals through Hetero. Catal., Cormma and S.
Iborra in Fine 309 (2001).
K.R. Kloestra, H. van Bekkum, Chem. Soc.Chem.Commu
n., 1005 (1995).
SIDE CHAIN ALKYLATIONS