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8/14/2019 Greeen Chemistry
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ZEOLITECATALYSTS IN
GREEN CHEMISTRYDipak Kumar Chakrabarty
Professor EmeritusINDIAN INSTUTUTE OF TECHNOLOGY,
BOMBAY
MUMBAI 400076
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In the year 1987, the World Environment Commission publisheda report Our Environmentthat emphasized the need for sustainable
development. 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 isprimary.
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 CHEMISTRYproducing chemicals
With minimumdamage 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 (preferablyrenewable) raw materials,
Elimination of wasteful byproducts,
Avoiding use of toxic/hazardousreagents and solvents,
Use of safer final (biodegradable)products, and
Increasing energy efficiency.
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Two considerations that dominated
green chemistry are:
A.Maximum atom utilization,
B.The minimum waste produced (E Factor).
The waste includes byproducts, reagents,
solvent loss and even fuel.
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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
productOil
refining106-108
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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).
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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 + 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.
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Zeolite Structure
Zeolites are crsytalline alumino-silicates withexchangeable cations
The cations can be exchanged with protons thatmake them acidic solids.
Si:Al ratio can vary in zeolites from 1 to infinity. Number of acid sites decreases with increase in
the Si:Al ratio
Strength of the acid sites increases with the Si:Al
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 orAl) atoms, each T atom linked to four oxygenmaking tetrahedra.
These tetrahedra are linked to form ringscontaining 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 typeNo. of O
atoms in the
ring
Framework
structurePore size, A Dimensionalit
y
Small 8 ZEOLITE A 4.1 3
medium 10 ZSM-5ZSM-11
SAPO-11
1.x 5.4
5.4 x 5.3
6.3 x 3.9
3
3
1
Large 12 X, YMORDENITE
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 1418
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. Weshall take some examples here.
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+ C2H2HZSM-5
Catalysis with acidic zeolites1. ALKYLATION
Mobil-Badger Process
(Polyalkylation is suppressed)
Similarly, propylbenzene could be manufactured
using a 3-dimensional dealuminated mordenite
(3-DDM) catalyst
+
dealuminated
mordenite
Si/Al = 100-1000
Dealumination enabled to obtain veryhigh Si:Al 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.
+
AcylationAcylation 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.
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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
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).
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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.
N N+
NH2H2N N
H-ZSM-5 Pd
NH
CH 3CHO + HCHO + NH 3
NOH
sulfuric acid
H-ZSM-5
Si : Al > 1000, 350oCv ap. 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)
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O
TS-1 or
ZSM-5 O
H TS-1 ZSM-5
conv. 100% 99%select. 98% 97-100%
Epoxide reaarangement ( key step in the manufacture of many intermediates ithe fine chemical industry. Traditional method used stoichiometric Lewis acidsor bases.
G. P. Heitmann, G. Dahlhoff and W.F.Holderich, J.Catal, 186,, 12 (1999)
Traditional method for preparing 2,6-dichlorobenzonitrile uses stoichiometricAmonts 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-M or
350 o
Cl Cl
Cl
Cl
Cl
Cl Cl Cl
Cl ClCl
CN
adsorption in
faujasite
NH 3+ O 2
catalyst
350-450 oC
C
K. Iwayama, S. Yamakawa, M. Kato and H. Okino, Eur. Pat. Appl. EP948988 (1999) to Toray
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Bayer-Villiger Oxidation (cinversion of ketone or an aldehyde to the esterThe 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-Beta
dioxane, 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.
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Basic zeolitesComarattively 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.
CHO
+ NCCH2CO2Et
COOE
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 Fine309 (2001).
SIDE CHAIN ALKYLATIONS