392
elongated temperature ranges. Basically the new zeolitic catalysts are an evolutionized form of novel bifunctional shape selective catalysts. These highly engineered
zeolites have been successful in replacing the conventional catalyst, i.e., ZSM-5
used in MLDW.
Zeolite catalyst advances continue to improve remarkably the critical petrochemical processes including lighter alkenes production, aromatics isomerization, and
alkylation processes. In order to meet new industrial challenges, bureaucratic ordinance, and environmental concern, the development of new eco-friendly materials
which can perform better and faster is being focused recently [4].
The classical definition of zeolite is “a crystalline, porous aluminosilicate built of
adjacent tetrahedral metal atoms (called T atoms) surrounded by four oxygen anions
to form an approximate tetrahedron.”[15] Zeolites are microporous in nature.
According to The International Union of Pure and Applied Chemistry (IUPAC),
porous solids are categorized into following categories: microporous (pore size
<2 nm), mesoporous (pore size 2–50 nm), and macroporous (pore size 50–1000 nm).
Usually these T atoms are of either Si or Al. The Si/Al ratio varies 1–5 in natural
zeolites. This Si/Al ratio was further worked upon and led to the synthetic zeolite
formation with greater Silica content in it. The first high silica zeolite was manufactured in the 1960s, and called “Beta.” The continuous quest for porous materials
resulted in the formation of the all-silica zeolite in the 1970s. Likewise, the dynamic
nature of zeolites attracted many researchers’ interest which resulted in periodic
development of the zeolites. In the 1980s, aluminophosphate molecular sieves were
discovered followed by ordered mesoporous materials (OMM). However, all these
forms of zeolite came with their own disadvantages and limitations. The major limitations observed with zeolites were their relatively small pore diameter and low acid
strength. These limitations could not fulfil the required expectations of refineries to
convert the crude oil into desired lighter fractions. Therefore, the investigation of
pristine materials in having improved structure variety, morphology and composition of framework is an area to be looked upon.
2.2 Preparation of Zeolites
Zeolites are formed from a hydrogel rich with alkaline aluminosilicate under hydrothermal conditions. In this process, water acts as a reactant as well as reaction
medium. The alkaline solution helps in dispersion and transportation of material
from the aluminosilicate gel to growing crystals. This dissolution and transportation
of aluminosilicate precursors leads to super saturation, causing nucleation in particular zones of the system (Fig. 11, Table 5).
Zeolite nucleation is a spontaneous process. There are various controlling parameters of the zeolite nucleation process, such as: gel composition, reactants purity,
aging time, rate of formation of number of nuclei, crystal size, and crystallization
temperature. The unit cell formula of zeolites is usually written as:
Subhashini and T. Mondal
elongated temperature ranges. Basically the new zeolitic catalysts are an evolutionized form of novel bifunctional shape selective catalysts. These highly engineered
zeolites have been successful in replacing the conventional catalyst, i.e., ZSM-5
used in MLDW.
Zeolite catalyst advances continue to improve remarkably the critical petrochemical processes including lighter alkenes production, aromatics isomerization, and
alkylation processes. In order to meet new industrial challenges, bureaucratic ordinance, and environmental concern, the development of new eco-friendly materials
which can perform better and faster is being focused recently [4].
The classical definition of zeolite is “a crystalline, porous aluminosilicate built of
adjacent tetrahedral metal atoms (called T atoms) surrounded by four oxygen anions
to form an approximate tetrahedron.”[15] Zeolites are microporous in nature.
According to The International Union of Pure and Applied Chemistry (IUPAC),
porous solids are categorized into following categories: microporous (pore size
<2 nm), mesoporous (pore size 2–50 nm), and macroporous (pore size 50–1000 nm).
Usually these T atoms are of either Si or Al. The Si/Al ratio varies 1–5 in natural
zeolites. This Si/Al ratio was further worked upon and led to the synthetic zeolite
formation with greater Silica content in it. The first high silica zeolite was manufactured in the 1960s, and called “Beta.” The continuous quest for porous materials
resulted in the formation of the all-silica zeolite in the 1970s. Likewise, the dynamic
nature of zeolites attracted many researchers’ interest which resulted in periodic
development of the zeolites. In the 1980s, aluminophosphate molecular sieves were
discovered followed by ordered mesoporous materials (OMM). However, all these
forms of zeolite came with their own disadvantages and limitations. The major limitations observed with zeolites were their relatively small pore diameter and low acid
strength. These limitations could not fulfil the required expectations of refineries to
convert the crude oil into desired lighter fractions. Therefore, the investigation of
pristine materials in having improved structure variety, morphology and composition of framework is an area to be looked upon.
2.2 Preparation of Zeolites
Zeolites are formed from a hydrogel rich with alkaline aluminosilicate under hydrothermal conditions. In this process, water acts as a reactant as well as reaction
medium. The alkaline solution helps in dispersion and transportation of material
from the aluminosilicate gel to growing crystals. This dissolution and transportation
of aluminosilicate precursors leads to super saturation, causing nucleation in particular zones of the system (Fig. 11, Table 5).
Zeolite nucleation is a spontaneous process. There are various controlling parameters of the zeolite nucleation process, such as: gel composition, reactants purity,
aging time, rate of formation of number of nuclei, crystal size, and crystallization
temperature. The unit cell formula of zeolites is usually written as:
Subhashini and T. Mondal