A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h(-1) at 150 degrees C. Both catalysts exhibit approximately linear relationships between rate and [H-2] at substoichiometric [H-2] with rate law orders of 0.66 +/- 0.09 and 0.48 +/- 0.07 for Hf and Zr, respectively; at higher [H-2], the rates approach zero-order in [H-2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H-2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular beta-alkyl elimination. The Zr and Hf catalyst activation parameters, Delta H double dagger = 16.8 +/- 0.2 kcal mol(-1) and 18.2 +/- 0.6 kcal mol(-1), respectively, track the relative turnover frequencies, while Delta S double dagger = -19.1 +/- 0.8 and -16.7 +/- 1.4 cal mol(-1) K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 degrees C, while time-on-stream data indicate multiday activities with an extrapolated turnover number similar to 92,000 at 150 degrees C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.

The novel electrophilic organo-tantalum catalyst AlS/TaNpx (1) (Np=neopentyl) is prepared by chemisorption of the alkylidene Np3 Ta=CHt Bu onto highly Brønsted acidic sulfated alumina (AlS). The proposed catalyst structure is supported by EXAFS, XANES, ICP, DRIFTS, elemental analysis, and SSNMR measurements and is in good agreement with DFT analysis. Catalyst 1 is highly effective for the hydrogenolysis of diverse linear and branched hydrocarbons, ranging from C2 to polyolefins. To the best of our knowledge, 1 exhibits one of the highest polyolefin hydrogenolysis activities (9,800 (CH2 units) ⋅ mol(Ta)-1  ⋅ h-1 at 200 °C/17 atm H2 ) reported to date in the peer-reviewed literature. Unlike the AlS/ZrNp2 analog, the Ta catalyst is more thermally stable and offers multiple potential C-C bond activation pathways. For hydrogenolysis, AlS/TaNpx is effective for a wide variety of pre- and post-consumer polyolefin plastics and is not significantly deactivated by standard polyolefin additives at typical industrial concentrations.

: 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

The novel electrophilic organo-tantalum catalyst AlS/TaNp () (Np = neopentyl) is prepared by chemisorption of the alkylidene Ta(CHtBu)Np onto highly Brønsted acidic sulfated alumina (AlS). The proposed catalyst structure is supported by EXAFS, XANES, ICP, DRIFTS, and SSNMR measurements and is in good agreement with DFT analysis. Adsorbate is a highly effective catalyst for the hydrogenolysis of linear and branched hydrocarbons, ranging from C2 to polyolefins. To the best of our knowledge, 𝟏 exhibits the highest polyolefin hydrogenolysis activity (9,200 (CH₂ units)·mol(Ta)⁻¹·h⁻¹ at 200 °C/17 atm H₂) reported to date in the peer-reviewed literature. Unlike the AlS/ZrNp₂ analog, the Ta catalyst is more thermally stable and has multiple potential reaction pathways for C-C bond activation. For hydrogenolysis, AlS/TaNp₃ is effective for a wide variety of pre- and post-consumer polyolefin plastics and is not significantly deactivated in the presence of standard polyolefin additives.

Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H 2 with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH 2 units)·mol(Zr) −1 ·h −1 at 200 °C/2 atm H 2 pressure. Under similar solventless conditions, polyethylene- co −1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß -alkyl transfer rather than the more common σ-bond metathesis.

Surface-bound organometallic molecules have recently enabled the development of single-site heterogeneous catalysts, advancing the atomic scale understanding and diversity of heterogeneous catalysis. Here we report that supporting Cp*ZrMe3 (Cat1) on acidic sulfated-alumina (AlS) affords the surface catalyst Cat1/AlS, which was characterized by multi-dimensional solid-state NMR spectroscopies, and is active in ethylene homo- and copolymerizations, as well as propylene and 1-hexene homopolymerizations. In contrast to propylene (or 1-hexene) polymerization by homogeneous Cp*ZrMe2+ B(C6F5)(4)(-) which yields atactic polyolefins, Cat1/AlS promotes remarkable isotacticity with mmmm >95 %. Complementary DFT analysis argues that the restrictive local Cat1/AlS C-1-symmetry favors activation and enchainment at the propylene re enantioface, promoting isotactic polymerization via a "back-skip-like" mechanism.

Polyolefins comprise a major fraction of single-use plastics and yet their catalytic deconstruction/recycling has proven challenging due to their inert hydrocarbon connectivities. Here an electrophilic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions. For n-hexadecane, hydrogenolysis to light hydrocarbons proceeds with an activity of 690 mol n-hexadecane · mol Zr-1 · h-1 at 150°C/2.5 atm H2 pressure. Under similar solventless conditions, polyethylene, polyethylene-co- 1-octene, isotactic polypropylene, and a post-consumer sandwich bag are rapidly hydrogenolyzed to low molecular mass hydrocarbons via a turnover-limiting C-C scission pathway involving ßalkyl transfer rather than more common σ-bond metathesis.

Heterogeneous catalysts have long dominated polyethylene and polypropylene production, but understanding their catalysis is challenged by uncertainties in active site structures and percentages. Su...