367
lifetime. Another approach includes modulating the acid site strength along the diffusing path of intermediates, reactants, and products. Strong acid sites promote
cracking and aromatization, leading to the formation of deactivating species. To
alleviate this problem, modification procedures such as incorporation, ion-exchange,
and impregnation have been proposed [123–125]. Boron incorporation results in
smaller [B]-H-ZSM-5 crystal size, reduced strong/mild acid sites ratio, and shows
decreased coke formation rates, thus ensuring longer catalytic lifetime [125]. At
times, it requires controlling the distribution [7, 126–128] and location [78] of acid
density as well. Typically, there is lower acid density on the periphery of crystals
which makes stronger acid sites. This results in the higher coking on the external
surface and underutilization of catalyst and smaller catalyst lifetime. This can be
reduced by avoiding aluminum zoning by making smaller crystallites or controlling
the release of aluminum species in the manner that uniformity is maintained.
Overall, a quick exit strategy and controlled active sites are the prime drivers of a
catalyst lifetime. Recently more mechanistic studies are available that predict the
role of formaldehyde in deactivation by Bhan Group [129, 130]. The group [131]
has suggested an approach to enhance lifetime by scavenging the coke promoting
reagents such as formaldehyde. Overall, control over the structure and physicochemical properties of zeolite in conjunction with the understanding of reaction
mechanism help optimize the performance parameters with higher efficacy.
3 Reaction Mechanism and Intermediates
There is a significant body of work done that explores the underpinnings of MTO
reactions. These mechanistic know-hows truly help us to design catalysts and reactions conditions for optimum performance. A detailed review can be found in elsewhere [8, 20, 132–137] but Table 3 shows the major steps and typical chemical
species involved during the reaction. For completeness purposes, we will discuss
the major understanding of the MTO reaction mechanism. There are two important
questions that all researchers are chasing: (I) How does the first C–C bond form?
and (II) How does the reaction proceed resulting in different products and deactivation? While the mechanism of C–C bond formation is still intriguing, there is a good
understanding of reaction mechanism progression post-induction period. Consensus
gravitates toward the hydrogen pool mechanism which encapsulates the dual-aromatic cycle and olefinic cycle. The latter cycle predominantly generates propylene
and butenes. It propagates via the multiple methylations of olefins and subsequent
cracking. On the other hand, the hydrocarbon pool mechanism has a complex product distribution. It can proceed via either paring mechanism which allows contraction and expansion of ring or side chain mechanism with a fixed ring structure.
Major precursors for these pathways can be polymethyl cyclopentadiene or polymethyl benzene which gets protonated to generate cation and start the autocatalytic
cycle for hydrogen pool mechanism. Sparsely methylated species generate ethene
while the higher density of methyl group promotes propene. The nature and
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts
lifetime. Another approach includes modulating the acid site strength along the diffusing path of intermediates, reactants, and products. Strong acid sites promote
cracking and aromatization, leading to the formation of deactivating species. To
alleviate this problem, modification procedures such as incorporation, ion-exchange,
and impregnation have been proposed [123–125]. Boron incorporation results in
smaller [B]-H-ZSM-5 crystal size, reduced strong/mild acid sites ratio, and shows
decreased coke formation rates, thus ensuring longer catalytic lifetime [125]. At
times, it requires controlling the distribution [7, 126–128] and location [78] of acid
density as well. Typically, there is lower acid density on the periphery of crystals
which makes stronger acid sites. This results in the higher coking on the external
surface and underutilization of catalyst and smaller catalyst lifetime. This can be
reduced by avoiding aluminum zoning by making smaller crystallites or controlling
the release of aluminum species in the manner that uniformity is maintained.
Overall, a quick exit strategy and controlled active sites are the prime drivers of a
catalyst lifetime. Recently more mechanistic studies are available that predict the
role of formaldehyde in deactivation by Bhan Group [129, 130]. The group [131]
has suggested an approach to enhance lifetime by scavenging the coke promoting
reagents such as formaldehyde. Overall, control over the structure and physicochemical properties of zeolite in conjunction with the understanding of reaction
mechanism help optimize the performance parameters with higher efficacy.
3 Reaction Mechanism and Intermediates
There is a significant body of work done that explores the underpinnings of MTO
reactions. These mechanistic know-hows truly help us to design catalysts and reactions conditions for optimum performance. A detailed review can be found in elsewhere [8, 20, 132–137] but Table 3 shows the major steps and typical chemical
species involved during the reaction. For completeness purposes, we will discuss
the major understanding of the MTO reaction mechanism. There are two important
questions that all researchers are chasing: (I) How does the first C–C bond form?
and (II) How does the reaction proceed resulting in different products and deactivation? While the mechanism of C–C bond formation is still intriguing, there is a good
understanding of reaction mechanism progression post-induction period. Consensus
gravitates toward the hydrogen pool mechanism which encapsulates the dual-aromatic cycle and olefinic cycle. The latter cycle predominantly generates propylene
and butenes. It propagates via the multiple methylations of olefins and subsequent
cracking. On the other hand, the hydrocarbon pool mechanism has a complex product distribution. It can proceed via either paring mechanism which allows contraction and expansion of ring or side chain mechanism with a fixed ring structure.
Major precursors for these pathways can be polymethyl cyclopentadiene or polymethyl benzene which gets protonated to generate cation and start the autocatalytic
cycle for hydrogen pool mechanism. Sparsely methylated species generate ethene
while the higher density of methyl group promotes propene. The nature and
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts