Rapid Polyolefin Plastics Hydrogenolysis Mediated by Single-Site Heterogeneous Electrophilic/Cationic Organo-Group IV Catalysts

: A homologous series of cationic electrophilic group IV metal hydrides (M = Ti, Zr, Hf) created by chemisorption of the corresponding MNp4 precursors on highly Brønsted acidic sulfated alumina (AlS) to yield loosely coordinated surface AlS/MNp2 (Np = neopentyl) species are systematically characterized by ICP, EXAFS/XANES, DRIFTS, and solid-state high-resolution multi-dimensional NMR spectroscopy (SSNMR), as well as by energy span DFT computation. With effective stirring, these complexes readily undergo reaction with H2 to yield AlS/M(alkyl)H species which are highly active for the hydrogenolysis of diverse commercial polyethylenes, α-olefin-ethylene copolymers, isotactic polypropylene, and post-consumer polyolefins including high-density polyethylenes, yielding medium and small linear and branched hydrocar-bons at turnover frequencies as high as 36,300 h-1 at 200 °C/17 atm H2 for M = Zr. For a given polyolefin and reaction conditions, turnover frequencies scale approximately as M = Zr > Hf > Ti, while catalyst thermal stability scales approxi-mately as M = Hf ≈ Zr > Ti, and these trends are qualitatively understandable from the DFT analysis. These catalytic re-sults reveal that the AlS/Hf(R)H- mediated hydrogenolysis favors wax-like and liquid products while the AlS/Zr(R)H-mediated hydrogenolysis can be tuned between gases and liquids. DFT analysis identifies β-alkyl elimination as the turn-over-limiting C-C scission process, which is particularly facile in these cationic d0 complexes, but not so in the neutrally charged analogues