359
[38] established this principle by synthesizing RTH which favors the paring route of
hydrogen pool mechanism, thereby increasing the yield of propylene over conventional CHA framework. Cage size not only impacts the activity of catalysts and
selectivity of the product but the deactivation rate as well. Olsbye and coworkers
[39] show that the topological maneuvering emerging out of channel intersection
for 10 MR-ringed zeolites (i.e., IM-5, TNU-9, ZSM-11, and ZSM-5) has significantly different lifetime owing to complex polyaromatic naphthene intermediates
promoted inside the cage. Diffusion pathways were investigated with multi-level
and multi-dimensional zeolite frameworks and established the stability order (1L,
2D H-SAPO-35 < 2L, 2D H-SAPO-57 < 3L, 2D H-SAPO-59 < 3D H-SAPO-34)
[40] depicting the role of pore dimensionality and cage size. Another study investigated small-pore zeolites frameworks (CHA, DDR, LEV) [14] with large cages
interconnected by 8-MR channels for deactivation mechanism during MTO using
the nature of reaction intermediates. Observations were interesting not only because
there were different aromatic species causing deactivation but their location varied
with the framework. In this end, it is quite evident that the cage shape and size play
a primary role in activating MTO reaction, generating suitable intermediates, producing desired products, and finally sustaining catalyst activation.
2.1.3 Crystallinity and Defects
Catalyst synthesis is the first portal where major intervention can be introduced to
modulate the course of MTO reaction. The understanding of zeolite crystallization
is a holy grail to MTO catalysis, but it is still elusive. Limitations emerge from the
lack of in situ analytic tools, enough computing power, and siloes in the research
community. The overall goal of zeolite synthesis is to tune the physicochemical
properties of catalyst amenable to MTO reaction. It involves controlling the crystallinity, external/internal defects, elemental distribution, phase uniformity, surface
architecture, and crystal shape and size. Here, we will explore the crystallinity of the
zeolite structure and internal/external silanol defects (as shown in Table 2). The
presence of a disordered phase in the catalyst particle compromises the stability and
integrity of structure which promotes coking and leaching of the ordered phase.
Reaction species face extra mass transfer limitations due to the loss of micropore
connectivity [41] and intra-crystalline structural defects. A study by Feng group
[42] shows that lower crystallinity causes a lower life of the catalyst. A superior
catalyst with higher efficiency was synthesized by Yu group [43] using seeds with
differing crystallinity. Lower acidity and improved ordered phase ensured a better
lifetime compared to convention SAPO-34. Another study used a combination of
three organic templates [44] to generate high purity and crystalline particles which
showed enhanced light olefin yield. A similar study by Wu group [45] showed high
conversion and olefin selectivity (100% and 87.9% respectively) because of small
catalyst size and highly crystalline structure. Loss of crystallinity and large pore
architecture (such as meso- and macropore) promotes the severe presence of internal isolated and nested silanol (T-OH where T = P, Si, and Al) defects which reduces
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts
[38] established this principle by synthesizing RTH which favors the paring route of
hydrogen pool mechanism, thereby increasing the yield of propylene over conventional CHA framework. Cage size not only impacts the activity of catalysts and
selectivity of the product but the deactivation rate as well. Olsbye and coworkers
[39] show that the topological maneuvering emerging out of channel intersection
for 10 MR-ringed zeolites (i.e., IM-5, TNU-9, ZSM-11, and ZSM-5) has significantly different lifetime owing to complex polyaromatic naphthene intermediates
promoted inside the cage. Diffusion pathways were investigated with multi-level
and multi-dimensional zeolite frameworks and established the stability order (1L,
2D H-SAPO-35 < 2L, 2D H-SAPO-57 < 3L, 2D H-SAPO-59 < 3D H-SAPO-34)
[40] depicting the role of pore dimensionality and cage size. Another study investigated small-pore zeolites frameworks (CHA, DDR, LEV) [14] with large cages
interconnected by 8-MR channels for deactivation mechanism during MTO using
the nature of reaction intermediates. Observations were interesting not only because
there were different aromatic species causing deactivation but their location varied
with the framework. In this end, it is quite evident that the cage shape and size play
a primary role in activating MTO reaction, generating suitable intermediates, producing desired products, and finally sustaining catalyst activation.
2.1.3 Crystallinity and Defects
Catalyst synthesis is the first portal where major intervention can be introduced to
modulate the course of MTO reaction. The understanding of zeolite crystallization
is a holy grail to MTO catalysis, but it is still elusive. Limitations emerge from the
lack of in situ analytic tools, enough computing power, and siloes in the research
community. The overall goal of zeolite synthesis is to tune the physicochemical
properties of catalyst amenable to MTO reaction. It involves controlling the crystallinity, external/internal defects, elemental distribution, phase uniformity, surface
architecture, and crystal shape and size. Here, we will explore the crystallinity of the
zeolite structure and internal/external silanol defects (as shown in Table 2). The
presence of a disordered phase in the catalyst particle compromises the stability and
integrity of structure which promotes coking and leaching of the ordered phase.
Reaction species face extra mass transfer limitations due to the loss of micropore
connectivity [41] and intra-crystalline structural defects. A study by Feng group
[42] shows that lower crystallinity causes a lower life of the catalyst. A superior
catalyst with higher efficiency was synthesized by Yu group [43] using seeds with
differing crystallinity. Lower acidity and improved ordered phase ensured a better
lifetime compared to convention SAPO-34. Another study used a combination of
three organic templates [44] to generate high purity and crystalline particles which
showed enhanced light olefin yield. A similar study by Wu group [45] showed high
conversion and olefin selectivity (100% and 87.9% respectively) because of small
catalyst size and highly crystalline structure. Loss of crystallinity and large pore
architecture (such as meso- and macropore) promotes the severe presence of internal isolated and nested silanol (T-OH where T = P, Si, and Al) defects which reduces
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts