356
these materials. Typical frameworks such as SAPO-34 [3, 4] and ZSM-5 [5, 6] have
been extensively explored due to the inherent advantages of crystalline structure and
desired product selectivity. Dalian Institute of Chemical Physics has reached a new
height by establishing a commercial MTO plant of 600 kiloton /annum using
SAPO-34 [7]. We will explore the challenges and possibilities of MTO catalyst
from the prism of catalyst triad.
2.1 Structure
Physical attributes of zeolite crystallites such as pores and channels act as a gateway
to the reactants (i.e., methanol), intermediates, and products for MTO reactions during their complete lifecycle. These dynamic species interact differently with different sites of these rigid architectures of catalyst particles. It begins with external
surfaces of crystals and pore-mouth of channels within these particles. Subsequently,
the path is more tortuous with changing diffusing diameter and chemical environment along the maze of channels. Reactants are screened early on at the crystallite
surface. After reaction activates, the shapes and sizes of intermediates and products
along the course of diffusion depend strongly on cage topology along with the
molecular arrangement of atomic species within these cavities. In this section, we
will discuss attributes pertaining to MTO and way forward for new potentials pathways for rational design.
2.1.1 Pore Size
It refers to the size of openings on the terminating crystalline planes or diffusing
diameter of channels running through the crystallites. These aluminosilicate frameworks are classified into the small-pore (8-MR), medium-pore (10-MR), and largepore (12-MR) zeolite that acts as a screening sieve for reactants and products.
Methanol has a kinetic diameter of 3.6 Å and faces little barrier while diffusing
inside the channel, but the product speciation is significantly impacted by the pore
size. Stocker et al [8]. summarized the evolution of different zeolite structures for
MTO reaction, promoting different conversion and selectivity of products. The
review outlines the preferable use of small-pore zeolite for higher yield of light
olefins and medium- and large-pore zeolite for aromatic, and larger alkenes and
alkanes. It should be noted that larger pore zeolites have correspondingly larger
cages/cavities. Hence, it will be difficult to deconvolute the exact effect of pore size
and cage on intermediates and hence products. However, small-pore zeolites suppress the transfer of branched aliphatic and heavy aromatic hydrocarbons, thus
impeding the exit of aromatic species and higher alkenes in the product stream and
thereby promoting the selectivity of lighter olefins [9]. A dramatic effect of pore
size was illustrated by song group [10] where a small variation in pore size by 0.3 Å
resulted in no activity for MTO reactions in H-ZSM-22 compared to H-ZSM-12.
M. Kumar
these materials. Typical frameworks such as SAPO-34 [3, 4] and ZSM-5 [5, 6] have
been extensively explored due to the inherent advantages of crystalline structure and
desired product selectivity. Dalian Institute of Chemical Physics has reached a new
height by establishing a commercial MTO plant of 600 kiloton /annum using
SAPO-34 [7]. We will explore the challenges and possibilities of MTO catalyst
from the prism of catalyst triad.
2.1 Structure
Physical attributes of zeolite crystallites such as pores and channels act as a gateway
to the reactants (i.e., methanol), intermediates, and products for MTO reactions during their complete lifecycle. These dynamic species interact differently with different sites of these rigid architectures of catalyst particles. It begins with external
surfaces of crystals and pore-mouth of channels within these particles. Subsequently,
the path is more tortuous with changing diffusing diameter and chemical environment along the maze of channels. Reactants are screened early on at the crystallite
surface. After reaction activates, the shapes and sizes of intermediates and products
along the course of diffusion depend strongly on cage topology along with the
molecular arrangement of atomic species within these cavities. In this section, we
will discuss attributes pertaining to MTO and way forward for new potentials pathways for rational design.
2.1.1 Pore Size
It refers to the size of openings on the terminating crystalline planes or diffusing
diameter of channels running through the crystallites. These aluminosilicate frameworks are classified into the small-pore (8-MR), medium-pore (10-MR), and largepore (12-MR) zeolite that acts as a screening sieve for reactants and products.
Methanol has a kinetic diameter of 3.6 Å and faces little barrier while diffusing
inside the channel, but the product speciation is significantly impacted by the pore
size. Stocker et al [8]. summarized the evolution of different zeolite structures for
MTO reaction, promoting different conversion and selectivity of products. The
review outlines the preferable use of small-pore zeolite for higher yield of light
olefins and medium- and large-pore zeolite for aromatic, and larger alkenes and
alkanes. It should be noted that larger pore zeolites have correspondingly larger
cages/cavities. Hence, it will be difficult to deconvolute the exact effect of pore size
and cage on intermediates and hence products. However, small-pore zeolites suppress the transfer of branched aliphatic and heavy aromatic hydrocarbons, thus
impeding the exit of aromatic species and higher alkenes in the product stream and
thereby promoting the selectivity of lighter olefins [9]. A dramatic effect of pore
size was illustrated by song group [10] where a small variation in pore size by 0.3 Å
resulted in no activity for MTO reactions in H-ZSM-22 compared to H-ZSM-12.
M. Kumar