360
the activity and accelerates the catalyst deactivation in MTO reaction. A case in
point was presented by the Beato group [46] and others [42, 47, 48] who correlated
defects with the lower activity and higher deactivation of the catalyst. A systematic
study was presented on ZSM-5 by Gonsiorová [49] and Louis [50] where product
selectivity and deactivation were studied. The defective framework results in more
of paraffins and C6+ species with shorter lifetime while pure phase promotes propene, ethene, and butenes with slower deactivation. There is an extensive body of
work where the defects were healed using secondary growth [51], F
−
ion treatment
[7, 52], NaOH/CTAB treatment [47], resulting in a longer lifetime of the catalyst.
At times, these defects were used as precursors for generating the mesopores and
thereby creating a better performing catalyst [4].
2.1.4 Crystal Size
Zeolite crystallization is a complex phenomenon. The most pertinent question
toward rational design has been aimed at controlling the crystal size. Reduction in
the size of catalyst particles results in the increase of effective active sites and a
reduction in mass transfer limitation. It has a direct impact on the efficacy by
increasing the conversion, selectivity, and lifetime of the catalyst. While this is commonly true, but there is a need to have a relook. A common perception that smaller
particles are always better catalysts can be challenged depending upon the reaction
system of interest. At times, higher selectivity toward particular product requires
longer residence time such as MTH [53], and hence redefinition of the rational
design is required which should include developing deterministic pathways of modulating catalyst size based on reaction under consideration. These concepts have
been visualized in Fig. 2. Even the catalyst particle produced from the same batch
behaves differently [54]. Continuing on the same thought, Holmen group [55]
Table 2 Nature and molecular arrangement of defect sites and acid sites within the crystallite
M. Kumar
the activity and accelerates the catalyst deactivation in MTO reaction. A case in
point was presented by the Beato group [46] and others [42, 47, 48] who correlated
defects with the lower activity and higher deactivation of the catalyst. A systematic
study was presented on ZSM-5 by Gonsiorová [49] and Louis [50] where product
selectivity and deactivation were studied. The defective framework results in more
of paraffins and C6+ species with shorter lifetime while pure phase promotes propene, ethene, and butenes with slower deactivation. There is an extensive body of
work where the defects were healed using secondary growth [51], F
−
ion treatment
[7, 52], NaOH/CTAB treatment [47], resulting in a longer lifetime of the catalyst.
At times, these defects were used as precursors for generating the mesopores and
thereby creating a better performing catalyst [4].
2.1.4 Crystal Size
Zeolite crystallization is a complex phenomenon. The most pertinent question
toward rational design has been aimed at controlling the crystal size. Reduction in
the size of catalyst particles results in the increase of effective active sites and a
reduction in mass transfer limitation. It has a direct impact on the efficacy by
increasing the conversion, selectivity, and lifetime of the catalyst. While this is commonly true, but there is a need to have a relook. A common perception that smaller
particles are always better catalysts can be challenged depending upon the reaction
system of interest. At times, higher selectivity toward particular product requires
longer residence time such as MTH [53], and hence redefinition of the rational
design is required which should include developing deterministic pathways of modulating catalyst size based on reaction under consideration. These concepts have
been visualized in Fig. 2. Even the catalyst particle produced from the same batch
behaves differently [54]. Continuing on the same thought, Holmen group [55]
Table 2 Nature and molecular arrangement of defect sites and acid sites within the crystallite
M. Kumar