Hollow ZSM-5 encapsulated with single Ga-atoms for the catalytic fast pyrolysis of biomass waste

The development of efficient metal-zeolite bifunctional catalysts for catalytic fast pyrolysis (CFP) of biomass waste is highly desirable for bioenergy and renewable biofuel production. However, conventional metal-loaded zeolites often suffer from metal sintering during pyrolysis and are thus inactivated. In this study, single-site Ga-functionalized hollow ZSM-5 (GaOx@HS-Z5) was synthesized via an impregnation-dissolution-recrystallization strategy without H2 reduction. The Ga atom was coordinated to four oxygen atoms in HS-Z5 frameworks. Benefitting from the highly dispersed single-Ga atoms and hollow zeolite framework, 3GaOx@HS-Z5 performed the best in producing hydrocarbon-rich bio-oil compared to impregnated 3GaOx/HS-Z5 and H2-reduced 3Ga@HS-Z5 in the maize straw CFP. In particular, 3GaOx@HS-Z5 delivered the highest bio-oil yield (23.6 wt%) and hydrocarbon selectivity (49.4 area%). 3GaOx@HS-Z5 also retained its structural integrity and catalytic activity after five pyrolysis-regeneration cycles, demonstrating its advantage in practical biomass CFP. The elimination of H2 reduction during the synthesis of catalyst provides an additional advantage for simplifying the CFP process and reducing operating costs. The retained Ga micro-environment and anti-sintering properties were unique for 3GaOx@HS-Z5, as severe metal sintering occurred during pyrolysis for other metals (e.g., NiOx, ZnOx, FeOx, and CoOx) that encapsulated HS-Z5.