(388a) Polyolefin Melt-Phase Effects on Alkane Hydrogenolysis over Pt Catalysts

Chemical recycling of plastic waste to value-added products through processes that are technically feasible, economically viable, and environmentally sustainable can complement mechanical recycling to reduce plastic waste. Hydrogenolysis of polyethylene (PE) over Pt-based catalysts to lubricant-range products has drawn much attention recently due to the high market value and volume. The PE is in melt-phase at reaction conditions, T~300 ºC. Hence, the catalytic reaction on the surface is under direct impact of the melt-phase, thereby potentially changing the reaction energetics compared to that in gas-phase environments. As a result, fundamental understanding of melt-phase effects on the hydrogenolysis of the PE over the catalyst surfaces is necessary to direct the chemistry toward increased lubricant production. In this context, we adopted a hybrid Quantum Mechanical/Molecular Mechanical Free Energy Perturbation (QM/MM-FEP) scheme, that was developed in our group recently for adsorption processes in liquid phase, to quantify the melt-phase effect on adsorption of 12 intermediates and 4 transition states with 1 to 4 carbons in their backbone over a Pt(111) surface model at 300 ºC using C₃₆H₇₄ PE surrogate melt that predicted essentially the same melt-phase effects as C₁₄₄H₂₉₀. We found significant melt-phase effects on adsorption ranging from 0.05 eV endergonic for adsorbed H to 1.37 eV endergonic for adsorbed butane. Next, we successfully correlated the melt-phase effects with the cavity volume of the adsorbate and the surface corrugation upon adsorption using a simple linear model. The descriptors can readily be obtained from gas-phase calculations and no liquid-phase calculations are needed. Finally, the correlation is employed to predict the melt-phase effect on the adsorption of other intermediates and transition states to build a microkinetic model that predicts reaction observables such as hydrogenolysis rates and product selectivities.