Dual-Pathway Polyolefin Upcycling over an Unexpectedly Bifunctional Low-Loading Ru/TiO2-Anatase Catalyst

The large-scale production of non-degradable plastic waste calls for effective catalytic upcycling processes. In this work, we discovered that an anatase TiO2 synthesized by a sol-gel method (TiO2-A-SG) exhibits drastically better activity and selectivity as the support for low-loading Ru than other oxides in polypropylene (PP) upcycling under H2. Detailed analysis and control experiments with Ni and Pt revealed that this is because Ru/TiO2-A-SG also catalyzes hydrocracking besides hydrogenolysis. In addition to favoring high-valued liquid products, this increases branched alkane fraction from low-density polyethylene (LDPE), and allows efficient PP upcycling without noble metals. The unexpected Brønsted acidity of TiO2-A-SG is linked to the carboxylate layers TiO2 surfaces adsorb from air. Kinetic studies revealed that under testing conditions, the two pathways compete for dehydrogenated intermediates on metal surfaces, with low hydrogen pressure (PH2) and high branching level of the substrate favoring hydrocracking. Hydrogenolysis is the main pathway at PH2 = 30 bar for all tested substrates less branched than PP, in which the cleavage of 2C−2C bonds is strongly favored over that of 3C−xC bonds. This work reveals and compares the two co-occurring polyolefin upcycling pathways on a metal-acid bifunctional catalyst, offers insights to the complex reaction network, and how it is affected by catalysts and conditions. In addition, it uncovers a new type of Brønsted acid sites on TiO2 with deep implications in the broader scope of acid catalysis.