507
gases. However, their crude nature, complex structure, and associated impurities
make them less suitable for direct use as fuel sources. The industrial refining and
processing makes them ideal fuel with excellent calorific values. In this regard, in
the industries several solid acid catalysts with desired pore apertures have been
utilized to enhance the fuel quality. The rampant increasing crude oil cost and their
depleting resources in coming future compel the researchers to adopt alternative
pathways to produce light olefins. Methanol-to-olefin (MTO) reaction established
by Mobil researchers in 1977 is the best example in this course [32]. It is the most
important process technology to produce light olefins, especially C 2 –C 4 olefins from
nonpetroleum feedstock of coal, natural gas, and biomass [33].
3.1.1 Studies over SAPO-34
Small–pore-sized solid acid catalysts in their H form, i.e., HSAPO-34 and HZSM-5,
have been the most explored catalysts in MTO reaction [34]. However, the product
selectivity in both differs remarkably. SAPO-34 has exclusive selectivity (>80%)
for C 2 –C 3 olefins on the other hand in ZSM-5, C 4 olefins have higher selectivity [35].
The products are often accompanied with various side products like, H 2 O, H 2 , CO X ,
higher paraffins, aromatics cokes, etc. [34, 35]. The occurrence of these side products imposes barrier in understanding the exact underlying reaction mechanism and
rational design of catalyst synthesis for product selectivity [36–38]. The major steps
involved in MTO reaction can be divided into following steps:
1. Dehydration of methanol to give equilibrium mixture of methanol and dimethyl
ether (DME). This step also involves the formation of electron deficient free
radical and carbonation intermediate.
2. Dehydration of resultant DME to give lighter olefin particularly ethylene, the C 2
product.
3. The addition reaction between the carbocation intermediate formed in the first
step with the C 2 olefin of the second step to give C 3 olefin.
4. The oligomerization of olefinic products to give C 4 and higher homologues.
5. Dimerization of various cationic species and olefin intermediates to give different paraffin, aromatic products, which ultimately with time on stream get oligomerized, leading to coke formation.
Collectively these steps have been termed as hydrocarbon pool mechanism [33] as
shown in Fig. 2. The circular cavity in the figure is analogous to three-dimensional
cavity of catalyst where various steps (1–5) involved in MTO reaction take place.
More than 20 reaction mechanism pathways have been proposed to describe various
possible product formations in the process [33]. Among several mechanistic routes,
the oxonium ylide mechanism proposed by Van den Berg et al. [39] is widely
accepted. They proposed that dimethyl ether (DME) species obtained by the dehydration of methanol reacts with the methanol attached to the Brӧnsted acid sites of
catalyst. It results in the formation of trimethyl oxonium ion (TMO) intermediate.
This TMO species forms trimethyloxonium ylide, which undergoes intramolecular
Recent Developments on Clean Fuels over SAPO-Type Catalysts
gases. However, their crude nature, complex structure, and associated impurities
make them less suitable for direct use as fuel sources. The industrial refining and
processing makes them ideal fuel with excellent calorific values. In this regard, in
the industries several solid acid catalysts with desired pore apertures have been
utilized to enhance the fuel quality. The rampant increasing crude oil cost and their
depleting resources in coming future compel the researchers to adopt alternative
pathways to produce light olefins. Methanol-to-olefin (MTO) reaction established
by Mobil researchers in 1977 is the best example in this course [32]. It is the most
important process technology to produce light olefins, especially C 2 –C 4 olefins from
nonpetroleum feedstock of coal, natural gas, and biomass [33].
3.1.1 Studies over SAPO-34
Small–pore-sized solid acid catalysts in their H form, i.e., HSAPO-34 and HZSM-5,
have been the most explored catalysts in MTO reaction [34]. However, the product
selectivity in both differs remarkably. SAPO-34 has exclusive selectivity (>80%)
for C 2 –C 3 olefins on the other hand in ZSM-5, C 4 olefins have higher selectivity [35].
The products are often accompanied with various side products like, H 2 O, H 2 , CO X ,
higher paraffins, aromatics cokes, etc. [34, 35]. The occurrence of these side products imposes barrier in understanding the exact underlying reaction mechanism and
rational design of catalyst synthesis for product selectivity [36–38]. The major steps
involved in MTO reaction can be divided into following steps:
1. Dehydration of methanol to give equilibrium mixture of methanol and dimethyl
ether (DME). This step also involves the formation of electron deficient free
radical and carbonation intermediate.
2. Dehydration of resultant DME to give lighter olefin particularly ethylene, the C 2
product.
3. The addition reaction between the carbocation intermediate formed in the first
step with the C 2 olefin of the second step to give C 3 olefin.
4. The oligomerization of olefinic products to give C 4 and higher homologues.
5. Dimerization of various cationic species and olefin intermediates to give different paraffin, aromatic products, which ultimately with time on stream get oligomerized, leading to coke formation.
Collectively these steps have been termed as hydrocarbon pool mechanism [33] as
shown in Fig. 2. The circular cavity in the figure is analogous to three-dimensional
cavity of catalyst where various steps (1–5) involved in MTO reaction take place.
More than 20 reaction mechanism pathways have been proposed to describe various
possible product formations in the process [33]. Among several mechanistic routes,
the oxonium ylide mechanism proposed by Van den Berg et al. [39] is widely
accepted. They proposed that dimethyl ether (DME) species obtained by the dehydration of methanol reacts with the methanol attached to the Brӧnsted acid sites of
catalyst. It results in the formation of trimethyl oxonium ion (TMO) intermediate.
This TMO species forms trimethyloxonium ylide, which undergoes intramolecular
Recent Developments on Clean Fuels over SAPO-Type Catalysts