(521dl) Modulating the Physicochemical Attributes of ZSM-5 Using Multivalent Ions-Assisted Synthesis

ZSM-5 is a medium-pore silica-rich zeolite requiring high crystallization temperatures (423-473 K) and alkaline pH (>11) for synthesis. Acid sites in zeolite ZSM-5 are responsible for numerous reactions including conversion of methanol to gasoline/olefins and paraffin isomerization, among others. Also, shape-selective catalysis in zeolites is achievable via modulation of crystal size and morphology to enhance exposure of certain crystal facets using zeolite growth modifiers. Thus, tuning zeolite acidity along with crystal morphology is an interesting proposition for catalytic efficiency. Here, we present a novel approach to synthesizing ZSM-5 with controllable morphology and acidity. ZSM-5 was synthesized using a hydrothermal method in which the role of effective charge density of modifiers in tuning the morphology and acidity of zeolite was investigated in detail.

Electron microscopy revealed that the use of modifiers has a limited impact on the size of crystals, but has a significant effect on crystal morphology. Conventional ZSM-5 crystals have twinned boat morphology with an average size of ~1.5 μm. ZSM-5 synthesized in presence of charged modifiers resulted in varying crystal sizes (1-2 µm) and morphological attributes depicting enhanced basal plane (higher c-axis growth), appearance of facets relevant to reaction studies, and higher b-axis growth. AFM studies on the as-synthesized zeolite helped explore the underlying growth mechanisms in presence of different modifiers. Further, modifiers were able to significantly increase Bronsted acid sites (IR band at 1548 cm^-1) from 104 μmol gm^-1 (control) to 198 μmol gm^-1 (modified) in ZSM-5. A careful selection of modifiers also helped us to modulate extra-framework Aluminum sites, increase Lewis acidic character and reduce the H-bonded acid sites by half.

Overall, these observations provided unprecedented insights into the critical role of growth modifiers on crystal morphology and acid characteristics (effected by Aluminum speciation in growth solution) and thereby the catalytic potential of these as-synthesized zeolite.