364
make them susceptible to attack by water molecules. To this end, SAR and silanol
density are used as quantifiable parameters to ascertain the hydrothermal stability of
the structure. Higher SAR values ensure a higher resistance to decay under harsh
operating conditions. As discussed earlier, usage of OSDA, growth mixture composition, synthesis protocol, and photosynthesis treatment can be used to improve the
SAR values [88, 89] and hence the thermal stability. Resasco group [90] and others
[91] have performed extensive body of research work studying the impact above
mentioned parameter to improve and understand the mechanistic principle of structure loss. In his seminal work, he showed higher resistance to water attack by surface modification of external sites with silanes. Incorporation of metal such as Ni
[92] has shown to increase the stability in the harsh condition of higher water content. Corma group [93] improved the resistance to water exposure by steaming the
catalyst at 550 °C because of Si migration from isolated place to Si islands without
generating any defects. Another interesting synthesis path was developed by Liu
group [94] where they synthesized mesopores maintaining hydrothermal stability
by the introduction of seed and mesoporogen by reconstructing the disordered channel wall. This attribute makes SAPO-34 a desirable framework for MTO and critical
to avoid irreversible damage in general.
2.2.3 Mesoporosity
Pore size and cage/cavity sizes are the inherent property of the crystalline zeolite
structure as discussed earlier. However, these features do not facilitate enough
mobility to the reaction intermediates or products which lead to final metamorphosis to the coking agent. Broadly, there are two major barriers that restrict the free
movement of chemical species through these catalyst particles. Firstly, reactants
face challenges at the entry of pore mouth either because of size or blockage due to
dangling hydroxyl groups. Later, bulkier intermediates and products find diffusion
resistance through the channels. These resistances can be alleviated by introducing
mesopores (2–50 nm) within the crystallite. Channel interconnectivity and diffusive
transport phenomenon depend on the quality and quantity of mesopores. This helps
in increasing the lifetime of catalyst by reducing the coke build-up on the external
surface and inside the channel and cage. It contributes to product speciation by
allowing better and quick expulsion of desired species. There are detailed reviews
[95, 96] elsewhere that have discussed protocol for generating mesopores and their
advantages. The research groups have explored various in situ or post-synthesis
approaches to introduce mesopores. The general protocol includes the use of soft
template [97], hard template [98, 99], additives [100–105], composition [43, 106,
107], chemical treatment [108–110], or synthetic method [111]. While there exist
several techniques to generate the mesopores, the quality of the new catalyst varies
based on the methodology adopted. One must be mindful of the fact that the generation of mesopores impacts the nature of acid sites, internal/external defects, and
orderliness of pore and channel. To this end, assessment pertaining to the
M. Kumar
make them susceptible to attack by water molecules. To this end, SAR and silanol
density are used as quantifiable parameters to ascertain the hydrothermal stability of
the structure. Higher SAR values ensure a higher resistance to decay under harsh
operating conditions. As discussed earlier, usage of OSDA, growth mixture composition, synthesis protocol, and photosynthesis treatment can be used to improve the
SAR values [88, 89] and hence the thermal stability. Resasco group [90] and others
[91] have performed extensive body of research work studying the impact above
mentioned parameter to improve and understand the mechanistic principle of structure loss. In his seminal work, he showed higher resistance to water attack by surface modification of external sites with silanes. Incorporation of metal such as Ni
[92] has shown to increase the stability in the harsh condition of higher water content. Corma group [93] improved the resistance to water exposure by steaming the
catalyst at 550 °C because of Si migration from isolated place to Si islands without
generating any defects. Another interesting synthesis path was developed by Liu
group [94] where they synthesized mesopores maintaining hydrothermal stability
by the introduction of seed and mesoporogen by reconstructing the disordered channel wall. This attribute makes SAPO-34 a desirable framework for MTO and critical
to avoid irreversible damage in general.
2.2.3 Mesoporosity
Pore size and cage/cavity sizes are the inherent property of the crystalline zeolite
structure as discussed earlier. However, these features do not facilitate enough
mobility to the reaction intermediates or products which lead to final metamorphosis to the coking agent. Broadly, there are two major barriers that restrict the free
movement of chemical species through these catalyst particles. Firstly, reactants
face challenges at the entry of pore mouth either because of size or blockage due to
dangling hydroxyl groups. Later, bulkier intermediates and products find diffusion
resistance through the channels. These resistances can be alleviated by introducing
mesopores (2–50 nm) within the crystallite. Channel interconnectivity and diffusive
transport phenomenon depend on the quality and quantity of mesopores. This helps
in increasing the lifetime of catalyst by reducing the coke build-up on the external
surface and inside the channel and cage. It contributes to product speciation by
allowing better and quick expulsion of desired species. There are detailed reviews
[95, 96] elsewhere that have discussed protocol for generating mesopores and their
advantages. The research groups have explored various in situ or post-synthesis
approaches to introduce mesopores. The general protocol includes the use of soft
template [97], hard template [98, 99], additives [100–105], composition [43, 106,
107], chemical treatment [108–110], or synthetic method [111]. While there exist
several techniques to generate the mesopores, the quality of the new catalyst varies
based on the methodology adopted. One must be mindful of the fact that the generation of mesopores impacts the nature of acid sites, internal/external defects, and
orderliness of pore and channel. To this end, assessment pertaining to the
M. Kumar