366
the focus toward olefins such as ethene, propene, and butene. While ethene occupies
the highest market share, there is thrust to increase the selectivity towards propene
[11, 38, 113] and butene placed at the bottom of pyramid. Here, we will refer lighter
olefins for the combined amount of ethene and propene as MTO products. Selectivity
toward propene compared to ethene is represented by the P/E ratio. Product distribution significantly depends on the physicochemical properties of zeolite discussed
earlier. Briefly, small-pore zeolites (8MR) with suitable cage size promote the light
olefins. CHA framework zeolite such as SAPO-34 has been very successful in
obtaining high C2 and C3 yield along with better hydrothermal stability. Reported
yield has varied from 85 to 90%. It suffers from the problem of higher coking rate
compared to ZSM-5; but lots of efforts have been made to increase the lifetime by
modulating acidity, mesoporosity, crystal size, and crystal defects. Several smallpore zeolites with varied cage sizes have shown promise for olefinic products with
different product distribution [18, 114]. Relatively meek acid site density and lowto- medium range acid strength of this framework have been key contributing parameters. Still, there is a need to develop a multifaceted approach for optimal catalyst
synthesis which is facile and economical. As we move toward medium-pore zeolites
such as ZSM-5 and ZSM-11, we still obtain a higher yield of olefins with products
rich in propene compared to ethylene. These catalyst starts forming some aromatics
and other higher alkene such as butenes. It should be noted that the overall yield of
lighter olefins is less compared to small-pore zeolites. However, mesoporosity
development in the medium-pore zeolite, isolating Brӧnsted acidity, and decreasing
crystal size have shown improved selectivity toward commercially attractive propylene [115–118]. Bigger pore size and cage size stabilizes larger intermediates and
accelerate reaction pathways via side chain and olefinic reaction pathways leading
to aromatics and higher olefins [119]. Moving further to large pore zeolites such as
MOR and BEA, they produce more aromatics and higher alkenes. Mechanism pathways show evidence of different intermediates and products. Each framework can
further be optimized to tune the selectivity toward given products by promoting
desired reaction pathways.
Another critical dimension of performance analysis is the lifetime of the catalyst.
It is assessed by studying the mechanism of deactivation and measuring the deactivation rate which plays the key role for industrial application [120, 121]. These
investigations in turn determine the reactor types and regeneration process depending upon the nature of deactivation. These processes can be reversible and irreversible. Here, MTO presents coking as a major challenge that can be recovered by
high-temperature treatment. The molecular structure of these hydrocarbons resembles polycyclic aromatic rings that get captured within the cage. It is the desired
reaction intermediates that transform into larger aromatic structures under suitable
conditions, thus blocking the access of active sites. These challenges are alleviated
by providing an exit strategy from the cage or modulating the acid strength to control the metamorphosis of the desired intermediate. The comparative study clearly
shows that the larger pore size of ZSM-5 sustains for a longer time compared to
small pore SAPO-34 [122]. These inherent challenges are being tackled by introducing mesopore suitably to reduce diffusion barriers and thereby increasing the
M. Kumar
the focus toward olefins such as ethene, propene, and butene. While ethene occupies
the highest market share, there is thrust to increase the selectivity towards propene
[11, 38, 113] and butene placed at the bottom of pyramid. Here, we will refer lighter
olefins for the combined amount of ethene and propene as MTO products. Selectivity
toward propene compared to ethene is represented by the P/E ratio. Product distribution significantly depends on the physicochemical properties of zeolite discussed
earlier. Briefly, small-pore zeolites (8MR) with suitable cage size promote the light
olefins. CHA framework zeolite such as SAPO-34 has been very successful in
obtaining high C2 and C3 yield along with better hydrothermal stability. Reported
yield has varied from 85 to 90%. It suffers from the problem of higher coking rate
compared to ZSM-5; but lots of efforts have been made to increase the lifetime by
modulating acidity, mesoporosity, crystal size, and crystal defects. Several smallpore zeolites with varied cage sizes have shown promise for olefinic products with
different product distribution [18, 114]. Relatively meek acid site density and lowto- medium range acid strength of this framework have been key contributing parameters. Still, there is a need to develop a multifaceted approach for optimal catalyst
synthesis which is facile and economical. As we move toward medium-pore zeolites
such as ZSM-5 and ZSM-11, we still obtain a higher yield of olefins with products
rich in propene compared to ethylene. These catalyst starts forming some aromatics
and other higher alkene such as butenes. It should be noted that the overall yield of
lighter olefins is less compared to small-pore zeolites. However, mesoporosity
development in the medium-pore zeolite, isolating Brӧnsted acidity, and decreasing
crystal size have shown improved selectivity toward commercially attractive propylene [115–118]. Bigger pore size and cage size stabilizes larger intermediates and
accelerate reaction pathways via side chain and olefinic reaction pathways leading
to aromatics and higher olefins [119]. Moving further to large pore zeolites such as
MOR and BEA, they produce more aromatics and higher alkenes. Mechanism pathways show evidence of different intermediates and products. Each framework can
further be optimized to tune the selectivity toward given products by promoting
desired reaction pathways.
Another critical dimension of performance analysis is the lifetime of the catalyst.
It is assessed by studying the mechanism of deactivation and measuring the deactivation rate which plays the key role for industrial application [120, 121]. These
investigations in turn determine the reactor types and regeneration process depending upon the nature of deactivation. These processes can be reversible and irreversible. Here, MTO presents coking as a major challenge that can be recovered by
high-temperature treatment. The molecular structure of these hydrocarbons resembles polycyclic aromatic rings that get captured within the cage. It is the desired
reaction intermediates that transform into larger aromatic structures under suitable
conditions, thus blocking the access of active sites. These challenges are alleviated
by providing an exit strategy from the cage or modulating the acid strength to control the metamorphosis of the desired intermediate. The comparative study clearly
shows that the larger pore size of ZSM-5 sustains for a longer time compared to
small pore SAPO-34 [122]. These inherent challenges are being tackled by introducing mesopore suitably to reduce diffusion barriers and thereby increasing the
M. Kumar