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these features compared to other physicochemical properties on the nature of intermediate and correspondingly the selectivity of products and lifetime of the catalyst.
Elementary steps in reaction pathways such as alkylation, deprotonation, dealkylation, cracking, etc., depend significantly on the topological arrangement of confined spaces during the initiation, propagation, and termination of typical
double-cycle hydrogen pool mechanism. While the reactivity and intermediate will
depend on the molecular arrangement of acidic sites as well, however, frameworks
with similar acid strength and distribution show varied intermediate depending
upon cavity shape and size [28]. To this end, comparative review [20] of product
speciation was shown using H-SAPO-34, H-ZSM-5, and H-FER having different
cages and topology. Similarly, a study [29] on SSZ-13 and H-UTM-1 shows that
MTO reaction cannot activate because of smaller cavity not promoting intermediate
such as polymethylbenzene. Supercages in MCM-22 (10-MR) with 2D sinusoidal
channels and delaminated ITQ-2 were evaluated for MTO reactions [30]. It was
shown that supercage helps with slow deactivation, but sinusoidal channels help
with higher propene selectivity compared to ethene. While SAPO-34 has established dominance for high light olefins selectivity, ZSM-58 (DDR Topology, 8-MR
pore) [31] has proved to be another candidate with similar selectivity but better
thermal stability. Separate studies by Liu et al. have explored disparate 8-MR-SAPO
zeolite to assess the effect of pore cavities. Using SAPO-34, SAPO-18, and
SAPO-35, group [32] has shown higher selectivity for propene and butenes for former two frameworks while SAPO-35 generated ethene predominantly along with
some propene. To ascertain the role of intermediates in product selectivity, they further investigated the SAPO-35, SAPO-34, and DNL-6 [33] with different cavities
but the same pore size. These frameworks showed the highest selectivity toward
ethene, propene, and butene in order respectively. Commensurate with product distribution, cages of varied size stabilized different intermediates and transition states
such as DNL-6 supported carbenium cation with bulkier (butyl) side chain contrary
to SAPO-34 with carbenium ion with shorter (methyl and ethyl) side chain due to
steric hindrance. SAPO-35 prefers even smaller intermediates such as methylbenzenium (1,2,2,3,5-pentaMB
+
) and methylcyclopentadienium cations producing ethene. Another interesting study [34] considered three different cavities such as cha,
lta, and lev, and simulated side chain mechanism to estimate energy barriers.
Observations complied with prior understanding of depicting minima with cha cavities of suitable cage size compared to lta and lev of being either too small or too big.
These observations got further strengthened by experimental studies on an extended
list of small pore zeolite (SSZ-13, SAPO-34, SAPO-39, MCM-35, ERS-7, and
RUB-37) by Davis group [19, 35]. Framework with CHA topology showed the
MTO activation while rest failed to activate due to small-size cavities. Some studies
are looking at the preferential diversion of reaction pathways to generate ethene or
propene selectively. To this end, two small-pore zeolites, H-RUB-50 [36], and
SAPO-14 [37] (AFN topology) have shown higher yield of ethene and propene,
respectively. These observations can be reverse engineered where catalysts can be
crystallized using organic mimicking reaction intermediates which can promote
desired pathways leading to selective products. Recent work from the Corma group
M. Kumar
these features compared to other physicochemical properties on the nature of intermediate and correspondingly the selectivity of products and lifetime of the catalyst.
Elementary steps in reaction pathways such as alkylation, deprotonation, dealkylation, cracking, etc., depend significantly on the topological arrangement of confined spaces during the initiation, propagation, and termination of typical
double-cycle hydrogen pool mechanism. While the reactivity and intermediate will
depend on the molecular arrangement of acidic sites as well, however, frameworks
with similar acid strength and distribution show varied intermediate depending
upon cavity shape and size [28]. To this end, comparative review [20] of product
speciation was shown using H-SAPO-34, H-ZSM-5, and H-FER having different
cages and topology. Similarly, a study [29] on SSZ-13 and H-UTM-1 shows that
MTO reaction cannot activate because of smaller cavity not promoting intermediate
such as polymethylbenzene. Supercages in MCM-22 (10-MR) with 2D sinusoidal
channels and delaminated ITQ-2 were evaluated for MTO reactions [30]. It was
shown that supercage helps with slow deactivation, but sinusoidal channels help
with higher propene selectivity compared to ethene. While SAPO-34 has established dominance for high light olefins selectivity, ZSM-58 (DDR Topology, 8-MR
pore) [31] has proved to be another candidate with similar selectivity but better
thermal stability. Separate studies by Liu et al. have explored disparate 8-MR-SAPO
zeolite to assess the effect of pore cavities. Using SAPO-34, SAPO-18, and
SAPO-35, group [32] has shown higher selectivity for propene and butenes for former two frameworks while SAPO-35 generated ethene predominantly along with
some propene. To ascertain the role of intermediates in product selectivity, they further investigated the SAPO-35, SAPO-34, and DNL-6 [33] with different cavities
but the same pore size. These frameworks showed the highest selectivity toward
ethene, propene, and butene in order respectively. Commensurate with product distribution, cages of varied size stabilized different intermediates and transition states
such as DNL-6 supported carbenium cation with bulkier (butyl) side chain contrary
to SAPO-34 with carbenium ion with shorter (methyl and ethyl) side chain due to
steric hindrance. SAPO-35 prefers even smaller intermediates such as methylbenzenium (1,2,2,3,5-pentaMB
+
) and methylcyclopentadienium cations producing ethene. Another interesting study [34] considered three different cavities such as cha,
lta, and lev, and simulated side chain mechanism to estimate energy barriers.
Observations complied with prior understanding of depicting minima with cha cavities of suitable cage size compared to lta and lev of being either too small or too big.
These observations got further strengthened by experimental studies on an extended
list of small pore zeolite (SSZ-13, SAPO-34, SAPO-39, MCM-35, ERS-7, and
RUB-37) by Davis group [19, 35]. Framework with CHA topology showed the
MTO activation while rest failed to activate due to small-size cavities. Some studies
are looking at the preferential diversion of reaction pathways to generate ethene or
propene selectively. To this end, two small-pore zeolites, H-RUB-50 [36], and
SAPO-14 [37] (AFN topology) have shown higher yield of ethene and propene,
respectively. These observations can be reverse engineered where catalysts can be
crystallized using organic mimicking reaction intermediates which can promote
desired pathways leading to selective products. Recent work from the Corma group
M. Kumar