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(229c) Selective Hydrogenolysis of Polyolefin Waste to Liquid Hydrocarbons over Bifunctional Ru/Acid Catalysts

Rorrer, J. E. - Presenter, Massachusetts Institute of Technology
Ebrahim, A., Stony Brook University
Questell-Santiago, Y. M., Ecole polytechnique fédérale de Lausanne
Asundi, A., Stanford University
Bare, S., SLAC National Accelerator Laboratory
Beckham, G. T., National Renewable Energy Laboratory
Roman, Y., MIT
Polyolefins, including polyethylene (PE) and polypropylene (PP), are among the most common single-use plastics consumed worldwide, and are among the most pervasive in landfills and the environment. Chemical recycling of polyolefins via hydrogenolysis is an emerging method of cleaving C-C bonds in polyolefins to produce processible liquid alkanes. In a previous study, we identified Ru-based catalysts as highly active for the hydrogenolysis of C-C bonds in PE and PP under relatively mild conditions (200-250ºC, 20-40 bar H2, 2-16 h). Despite their high activity, one of the remaining challenges with Ru-catalyzed hydrogenolysis over supports such as carbon is the production of methane, resulting from cleavage of terminal C-C bonds. In this study, we found that supporting Ru nanoparticles on a Brønsted-acidic support could improve selectivity by promoting a bifunctional acid/metal-catalyzed hydrocracking mechanism that suppresses methane formation. A series of Ru-based catalysts were synthesized on supports of varying acidity and structure and characterized with a variety of ex situ and operando techniques including XRD, TEM, H2-TPR, NH3-TPD, XANES, and EXAFS to relate the Ru and support structure to activity and selectivity. For both PE and PP hydrogenolysis, strongly Brønsted-acidic supports improved liquid yields by over 30% at comparable plastic conversions. Operando EXAFS with model polyolefin hydrogenolysis revealed that the oxidation state of the Ru is unchanged across acidic versus inert supports, but the support affects the reducibility of the nanoparticles, consistent with ex situ H2-TPR measurements. The characterization tools, combined with reactivity data on various supports and reaction conditions, demonstrate the importance of the acid-noble metal cooperativity in promoting selective C-C bond scission towards liquid alkanes.