New insights into hierarchical metal-containing zeolites; synthesis and kinetic modelling of mesoporous gallium-containing ZSM-5 for propane aromatization

Abstract Hierarchical (mesoporous) gallium-containing HZSM-5 zeolites were synthesized via incipient wetness impregnation, followed by top-down post-synthetic treatments; alkaline treatment, CTAB-mediated assembly of Ga-containing zeolite seed into MCM-41 mesostructure, or CTAB-mediated coating of Ga-containing zeolite with MCM-41 layer. It was observed that the resulting Ga-containing HZSM-5 with microporous/mesoporous hierarchical structure exhibited improved propane aromatization, as compared to the corresponding microporous sample. Mesoporous Ga-containing HZSM-5 displayed large external surface area (S_meso = 265-350 m² g^-1), relatively preserved microporosity (V_micro = 0.08 cm³ g^-1), and random intracrystalline mesopores of ∼7-8 nm in size or ordered intercrystalline MCM-41 mesopores of ∼2-4 nm in size, compared to S_meso of 194 m² g^-1 and V_micro of 0.10 cm³ g^-1 for the corresponding microporous sample. C₃ conversion increased to 33.9%, 47.4% and 33.4% upon alkaline treatment, CTAB-mediated hydrolysis, CTAB-mediated coating, respectively, up from 30.3% (microporous Ga-containing ZSM-5). At comparable conversion level, mesoporous samples were more selective to BTX aromatics, with ∼56% compared to 48.9% for microporous sample. The apparent activation energy (E_i) for the cracking of propane over mesoporous Ga-containing HZSM-5 was 130 kJ mol^-1, compared to 110 kJ mol^-1 for microporous sample. Whereas, the E_i (dehydrogenation) over mesoporous and microporous Ga-containing HZSM-5 was 12.65 kJ mol^-1 and 70 kJ mol^-1, respectively. The E_i (propene to aromatics) over mesoporous and microporous Ga-containing HZSM-5 was 140 kJmol^-1 and 220 kJ mol^-1, respectively. The improved C₃ aromatization over mesoporous Ga-containing ZSM-5 was tentatively attributed to the hierarchical topology (mesoporosity), which resulted in substantial improvements in the mass-transport properties and (probably) facilitated the migration of extracrystalline Ga₂O₃ formed during impregnation into the micropores.