363
produces low coke content. To get lower Brӧnsted acid sites and defects, Louis
group [50] synthesized fluoride-mediated ZSM-5 to selectively increase propene
yield. The role of paired Al sites was explored by Davis group [77]. Like earlier
observation, increasing SAR value for SSZ-13 helped in obtaining lighter olefins
with better lifetime; however, decreasing in SAR led to the pairing of Al sites which
promoted selectivity toward propane. Analysis of deactivating species was interesting as the nature varied with Al content. Low SAR resulted in less branched aromatic like naphthalene while high SAR caused highly alkylated species. Framework
analogue of SSZ-13, SAPO-34 showed pyrenes as occluded species. Another interesting perspective on acid sites is their locations in the cage and channels. A study
by Wang group [78] shows that these locations impact the intermediate species and
hence the product selectivity. Al siting in channels promotes olefinic cycle; however, the same species in cages promote aromatic cycle. Accordingly, propene is
promoted over ethene by Al in channels compared to cage, resulting in product
speciation. More aromatic species are clogged in crystallite having acid sites in a
cage compared to channels. They extended the work further to preferentially shift
the acid sites by introducing alkali metal in synthesis [79]. By biasing acid sites,
they drifted reaction mechanism to yield either ethene or propene. Acid sites are
further differentiated in Lewis and Brӧnsted acid sites. Lercher group [80] studied
their mechanistic impact where they ascertained that Lewis sites supported hydride
transfer and Brӧnsted sites led to aromatization. High SAR values also ensure less
diffusion limitation due to a lack of active sites. Chemical species can traverse multiple channels without interacting with acid sites and hence creating product selectivities [81]. The synergistic effect of both types of acid sites (Lewis and Brӧnsted)
[82] was illustrated which gives details about first C–C bond formation which is still
elusive. However, these attributes are highly dependent on the synthesis mixture
composition and protocol. For instance, the silicon-to-aluminum ratio of mother
liquor and the presence of stabilizing structure-directing agent among others are
some critical parameters which ascertain the final incorporation of elemental species in the framework, thereby controlling the density, strength, type, and distribution. Other invasive techniques [83–87] have been developed using steam and/or
alkali solutions to facilitate the altercation in these physicochemical properties of
catalysts.
2.2.2 Hydrothermal Stability
Structural integrity of the zeolite structure determine the lifetime of catalyst and
usage viability. Deactivated coked catalysts can be regenerated but the loss in the
ordered crystalline phase is difficult to recover in the continuous process. The presence of water at high temperatures plays a detrimental role in collapsing the zeolite
framework. The interaction of water with the surfaces can be modulated by changing hydrophobicity or hydrophilicity of crystallite. Pure siliceous zeolites are hydrophobic in nature and hence present higher stability. Hydrophilic moieties introduced
by the presence of framework or extra-framework aluminum and silanol defects
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts
produces low coke content. To get lower Brӧnsted acid sites and defects, Louis
group [50] synthesized fluoride-mediated ZSM-5 to selectively increase propene
yield. The role of paired Al sites was explored by Davis group [77]. Like earlier
observation, increasing SAR value for SSZ-13 helped in obtaining lighter olefins
with better lifetime; however, decreasing in SAR led to the pairing of Al sites which
promoted selectivity toward propane. Analysis of deactivating species was interesting as the nature varied with Al content. Low SAR resulted in less branched aromatic like naphthalene while high SAR caused highly alkylated species. Framework
analogue of SSZ-13, SAPO-34 showed pyrenes as occluded species. Another interesting perspective on acid sites is their locations in the cage and channels. A study
by Wang group [78] shows that these locations impact the intermediate species and
hence the product selectivity. Al siting in channels promotes olefinic cycle; however, the same species in cages promote aromatic cycle. Accordingly, propene is
promoted over ethene by Al in channels compared to cage, resulting in product
speciation. More aromatic species are clogged in crystallite having acid sites in a
cage compared to channels. They extended the work further to preferentially shift
the acid sites by introducing alkali metal in synthesis [79]. By biasing acid sites,
they drifted reaction mechanism to yield either ethene or propene. Acid sites are
further differentiated in Lewis and Brӧnsted acid sites. Lercher group [80] studied
their mechanistic impact where they ascertained that Lewis sites supported hydride
transfer and Brӧnsted sites led to aromatization. High SAR values also ensure less
diffusion limitation due to a lack of active sites. Chemical species can traverse multiple channels without interacting with acid sites and hence creating product selectivities [81]. The synergistic effect of both types of acid sites (Lewis and Brӧnsted)
[82] was illustrated which gives details about first C–C bond formation which is still
elusive. However, these attributes are highly dependent on the synthesis mixture
composition and protocol. For instance, the silicon-to-aluminum ratio of mother
liquor and the presence of stabilizing structure-directing agent among others are
some critical parameters which ascertain the final incorporation of elemental species in the framework, thereby controlling the density, strength, type, and distribution. Other invasive techniques [83–87] have been developed using steam and/or
alkali solutions to facilitate the altercation in these physicochemical properties of
catalysts.
2.2.2 Hydrothermal Stability
Structural integrity of the zeolite structure determine the lifetime of catalyst and
usage viability. Deactivated coked catalysts can be regenerated but the loss in the
ordered crystalline phase is difficult to recover in the continuous process. The presence of water at high temperatures plays a detrimental role in collapsing the zeolite
framework. The interaction of water with the surfaces can be modulated by changing hydrophobicity or hydrophilicity of crystallite. Pure siliceous zeolites are hydrophobic in nature and hence present higher stability. Hydrophilic moieties introduced
by the presence of framework or extra-framework aluminum and silanol defects
Shifting Trend of Rational Design Heuristics for Methanol-to-Olefins (MTO) Catalysts