Gallium-Containing ZSM-5 Zeolites Improve Hydrocracking Selectivity to Propane

Polyolefin hydrocracking over Brønsted acidic ZSM-5 produces light hydrocarbons with narrow product distributions, but further increases in propane and butane selectivity are required to improve process economics and sustainability. Here, we examined the effect of various metal and metal oxide dopants, namely Pt, Pt-Sn, SnOx, ZnOx, and GaOx, on ZSM-5-mediated polyolefin hydrocracking and identified GaOx as a selective promoter for light gas formation. Incorporation of gallium oxide (3 wt % Ga) was shown to increase propane selectivity from 40 c-wt %, achieved over ZSM-5, to near 60 c-wt %, reaching a combined propane and butane selectivity of 85 c-wt %, while maintaining solids conversion. To elucidate the role of gallium oxide, we used ex situ and operando characterization techniques to probe acidity, local coordination environment, and the spatial distribution of GaOx species. Extra-framework gallium oxide introduced via incipient wetness impregnation reduced the measured acid site density from 1275 to 911 μmol g–1, decreased the Brønsted/Lewis acid site ratio, and formed a high-coverage GaOx layer on the external surface of the zeolite. Operando experiments show that Ga is present in both Ga3+ and Ga1+ under reaction conditions. Attempts to disperse gallium oxide through reductive pretreatments led to an increase in the Lewis acid site concentration and a 10 c-wt % decrease in propane selectivity, highlighting the importance of the specific location, structural, and acidic configurations. Systematic variation of impregnation methods to control location and dispersion revealed a synergistic interaction between gallium oxide and zeolite acid sites, with propane selectivity strongly influenced by spatial proximity of GaOx to acid sites. Furthermore, vapor-phase flow reactions with a model compound demonstrated a decrease in intermediate product formation compared to the zeolite alone. The optimized catalyst was validated across diverse polyolefin substrates and resulted in similar increases to propane selectivity, yielding >80 c-wt % combined C3–C4 selectivity across the range of polyolefin substrates.