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 (Smeso=265–350m2g−1), relatively preserved microporosity (Vmicro=0.08cm3g−1), and random intracrystalline mesopores of ∼7–8nm in size or ordered intercrystalline MCM-41 mesopores of ∼2–4nm in size, compared to Smeso of 194m2g−1 and Vmicro of 0.10cm3g−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 (Ei) for the cracking of propane over mesoporous Ga-containing HZSM-5 was 130kJmol−1, compared to 110kJmol−1 for microporous sample. Whereas, the Ei (dehydrogenation) over mesoporous and microporous Ga-containing HZSM-5 was 12.65kJmol−1 and 70kJmol−1, respectively. The Ei (propene to aromatics) over mesoporous and microporous Ga-containing HZSM-5 was 140kJmol−1 and 220kJmol−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.