366
imparts the properties of sieving and selective catalysis. Moreover, the presence of
compensating cations in its structure adds the properties of ion exchangeability and
catalysis.
Zeolites have established themselves as efficient catalysts for various industrial
and laboratory-scale reactions. This catalytic activity is expressed by the Brønsted
as well as Lewis acid sites present in their framework [164]. The pores (of varied
ring sizes) of zeolites can always act as encapsulating agents for guest species of
adequate sizes. These species can be treated and reacted inside these pores, given
the catalytically active sites present in their structure. They also endow the advantages of high thermal stability and easy availability of the precursors for production
[165]. Zeolites contain ambient number of Brønsted acid sites. The number of these
sites will be equivalently proportional to the number of aluminum atoms present in
the zeolitic framework. These sites can be highly active as proton donors to a number of catalytic reactions. Substitution of silicon atoms with high valence metals
like tin, titanium, and zirconium can help produce Lewis acid sites into the zeolitic
framework. Reactions which require Lewis acidity can be run inside the pores of
these catalysts. A relevant example that requires Lewis acidity is the aldol condensation of keto esters generating diacid esters as products. Also, zeolites can be modified to introduce both Lewis and Brønsted acidities to their overall framework.
These come handy in reactions which require both of these catalytic sites in different steps.
The most fundamental and conventional catalytic activity shown by a zeolite is
that of ZSM-5 for the selective formation of the para-isomer of xylene [166]. The
catalytic activity of zeolites owes to the presence of both Lewis and Brønsted acidic
sites in their three-dimensional framework. The cavities in these porous materials
can effectively embed small molecules and can henceforth act as reactors. Moreover,
the heterogeneous nature of the catalyst adds the benefits of easy separation and
reusability. The refining and petrochemical industry depends heavily on zeolites as
catalysts for carrying out several important chemical processes [167, 168]. To name
a few, cracking, cyclization, isomerization, dehydrogenation, and transalkylation
are some of the reactions applying zeolites as catalysts [165]. The channels present
in their structures imply properties of proton and ion conduction to these materials.
Although microporous zeolites have several advantages attributed to their name,
a pore size of 2 nm is considerably small. Also, the accessibility to these pores is
tough and is limited only to small molecules. To address these problems in virtue,
the design of materials having bigger cavities is critically being looked upon.
The strategies commonly applied for the design of these materials having “mesopores” may be one of the following:
1. Pre-synthesis strategy—Supramolecular aggregates like surfactants and block
copolymers have sizes in the dimensions of a few nanometers. Their use as a
template for the structural development of large pored zeolitic materials is obvious. A research group at Mobil Oil Corporation was the first one to adopt this
strategy [169], and henceforth, they did prepare the mesoporous silica which is
today famously known as the MCM.
K. C. Maheria et al.
imparts the properties of sieving and selective catalysis. Moreover, the presence of
compensating cations in its structure adds the properties of ion exchangeability and
catalysis.
Zeolites have established themselves as efficient catalysts for various industrial
and laboratory-scale reactions. This catalytic activity is expressed by the Brønsted
as well as Lewis acid sites present in their framework [164]. The pores (of varied
ring sizes) of zeolites can always act as encapsulating agents for guest species of
adequate sizes. These species can be treated and reacted inside these pores, given
the catalytically active sites present in their structure. They also endow the advantages of high thermal stability and easy availability of the precursors for production
[165]. Zeolites contain ambient number of Brønsted acid sites. The number of these
sites will be equivalently proportional to the number of aluminum atoms present in
the zeolitic framework. These sites can be highly active as proton donors to a number of catalytic reactions. Substitution of silicon atoms with high valence metals
like tin, titanium, and zirconium can help produce Lewis acid sites into the zeolitic
framework. Reactions which require Lewis acidity can be run inside the pores of
these catalysts. A relevant example that requires Lewis acidity is the aldol condensation of keto esters generating diacid esters as products. Also, zeolites can be modified to introduce both Lewis and Brønsted acidities to their overall framework.
These come handy in reactions which require both of these catalytic sites in different steps.
The most fundamental and conventional catalytic activity shown by a zeolite is
that of ZSM-5 for the selective formation of the para-isomer of xylene [166]. The
catalytic activity of zeolites owes to the presence of both Lewis and Brønsted acidic
sites in their three-dimensional framework. The cavities in these porous materials
can effectively embed small molecules and can henceforth act as reactors. Moreover,
the heterogeneous nature of the catalyst adds the benefits of easy separation and
reusability. The refining and petrochemical industry depends heavily on zeolites as
catalysts for carrying out several important chemical processes [167, 168]. To name
a few, cracking, cyclization, isomerization, dehydrogenation, and transalkylation
are some of the reactions applying zeolites as catalysts [165]. The channels present
in their structures imply properties of proton and ion conduction to these materials.
Although microporous zeolites have several advantages attributed to their name,
a pore size of 2 nm is considerably small. Also, the accessibility to these pores is
tough and is limited only to small molecules. To address these problems in virtue,
the design of materials having bigger cavities is critically being looked upon.
The strategies commonly applied for the design of these materials having “mesopores” may be one of the following:
1. Pre-synthesis strategy—Supramolecular aggregates like surfactants and block
copolymers have sizes in the dimensions of a few nanometers. Their use as a
template for the structural development of large pored zeolitic materials is obvious. A research group at Mobil Oil Corporation was the first one to adopt this
strategy [169], and henceforth, they did prepare the mesoporous silica which is
today famously known as the MCM.
K. C. Maheria et al.