362
Varied size shows non-uniform spatial distribution [70] of coke leaving the core of
catalyst under-utilized. Collectively, reduced crystallite size is an important determinant for the lifetime of the catalyst.
2.2 Property
The inherent structural features discussed in the earlier section imparts the physicochemical properties to make an efficient catalyst. These properties control the whole
catalytic lifecycle from the entry of reactant to the exit of the product. Physical
transport of reactant/intermediate/product species is controlled by pore and cage
size distribution and arrangement. Micropore arrangement is determined by the
crystalline phase which is inherent in nature. On the other hand, a new set of larger
pore systems can be introduced through the invasive method alleviating challenges
associated with mass transport limitation. Another critical paradigm is the chemical
attributes that are dictated by the elemental arrangement of constituent species.
These properties are dependent on the topological arrangement within the crystallite
such as confinement effect and spatial distance owing to cage/cavity shape and size.
These factors play a significant role in determining the reaction pathways which we
will discuss below.
2.2.1 Acid Site Density, Strength, Type, and Distribution
These parameters have been studied to significantly impact the methanol-to-olefin
reaction pathways, but details and clear-cut understanding are still elusive. Here, we
will explore how they impact the performance and discuss the pathways to tune
these properties. Acid density is typically quantified as silicon-to-aluminum ratio
(SAR) which is an average value depicting the activity of the catalyst. The density
can vary across the crystallite, leading to the phenomenon of “Al zoning.” Depending
upon the density, the acid strength varies. The isolated bridged hydroxyl group (–
Si–OH–Al–) presents the strongest acidity compared to Si(0Al), Si (2Al), Si(3Al),
and Si(4Al) [71] as shown in Table 2. Higher acid density gives rise to weaker acid
sites. It is well known that a decrease in acid density and site strength reduces the
chances of polyaromatics generation and hence slower deactivation [72]. A study by
Tatsumi group [73] looks at the lifetime of catalyst as a function of Al content.
These acid sites’ strength was further manipulated by inserting gallium [74] in the
framework resulting in higher stability. Li group [75] looked at four different aluminosilicate frameworks and showed that higher acid sites ensure the higher adsorption of methanol and activation of hydrocarbon pool mechanism. At the same time,
high density starts to behave antagonistically by condensing the species from the
pool to produce polyaromatic hydrocarbon. Hunger group [76] synthesized lowdensity isolated acid sites responsible for activating MTO, olefins selectivity, and
higher lifetime in AlPO-34. Their study confirms that a low silica framework
M. Kumar
Varied size shows non-uniform spatial distribution [70] of coke leaving the core of
catalyst under-utilized. Collectively, reduced crystallite size is an important determinant for the lifetime of the catalyst.
2.2 Property
The inherent structural features discussed in the earlier section imparts the physicochemical properties to make an efficient catalyst. These properties control the whole
catalytic lifecycle from the entry of reactant to the exit of the product. Physical
transport of reactant/intermediate/product species is controlled by pore and cage
size distribution and arrangement. Micropore arrangement is determined by the
crystalline phase which is inherent in nature. On the other hand, a new set of larger
pore systems can be introduced through the invasive method alleviating challenges
associated with mass transport limitation. Another critical paradigm is the chemical
attributes that are dictated by the elemental arrangement of constituent species.
These properties are dependent on the topological arrangement within the crystallite
such as confinement effect and spatial distance owing to cage/cavity shape and size.
These factors play a significant role in determining the reaction pathways which we
will discuss below.
2.2.1 Acid Site Density, Strength, Type, and Distribution
These parameters have been studied to significantly impact the methanol-to-olefin
reaction pathways, but details and clear-cut understanding are still elusive. Here, we
will explore how they impact the performance and discuss the pathways to tune
these properties. Acid density is typically quantified as silicon-to-aluminum ratio
(SAR) which is an average value depicting the activity of the catalyst. The density
can vary across the crystallite, leading to the phenomenon of “Al zoning.” Depending
upon the density, the acid strength varies. The isolated bridged hydroxyl group (–
Si–OH–Al–) presents the strongest acidity compared to Si(0Al), Si (2Al), Si(3Al),
and Si(4Al) [71] as shown in Table 2. Higher acid density gives rise to weaker acid
sites. It is well known that a decrease in acid density and site strength reduces the
chances of polyaromatics generation and hence slower deactivation [72]. A study by
Tatsumi group [73] looks at the lifetime of catalyst as a function of Al content.
These acid sites’ strength was further manipulated by inserting gallium [74] in the
framework resulting in higher stability. Li group [75] looked at four different aluminosilicate frameworks and showed that higher acid sites ensure the higher adsorption of methanol and activation of hydrocarbon pool mechanism. At the same time,
high density starts to behave antagonistically by condensing the species from the
pool to produce polyaromatic hydrocarbon. Hunger group [76] synthesized lowdensity isolated acid sites responsible for activating MTO, olefins selectivity, and
higher lifetime in AlPO-34. Their study confirms that a low silica framework
M. Kumar