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 (GaO_x@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, 3GaO_x@HS-Z5 performed the best in producing hydrocarbon-rich bio-oil compared to impregnated 3GaO_x/HS-Z5 and H₂-reduced 3Ga@HS-Z5 in the maize straw CFP. In particular, 3GaO_x@HS-Z5 delivered the highest bio-oil yield (23.6 wt%) and hydrocarbon selectivity (49.4 area%). 3GaO_x@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 3GaO_x@HS-Z5, as severe metal sintering occurred during pyrolysis for other metals (e.g., NiO_x, ZnO_x, FeO_x, and CoO_x) that encapsulated HS-Z5.