Hydrogenolysis of Polyethylene over Earth-Abundant Cobalt Catalysts

In recent times, plastics have become indispensable to human life. The availability of raw materials, ease of production, and versatility have made plastics ubiquitous. The pollution, however, caused by improper disposal has become a major global concern. Most plastics used today are single-use and end up in landfills and water bodies. Only about 9% of them are recycled. To remedy plastic pollution's environmental consequences and move toward creating a circular plastic economy, effectively upcycling and recycling plastic waste has gathered significant attention. One avenue toward chemically repurposing polyolefins is via hydrogenolysis. Hydrogenolysis is a depolymerization reaction wherein hydrogen molecules break the relatively inert C-C backbone of the polyolefin. Ru is regarded as the most active hydrogenolysis catalyst. This work studied the hydrogenolysis of polyethylene using earth-abundant cobalt immobilized on a neutral support, silica (Co/SiO2). Co showed ~30% n-octadecane (n-C18) conversion into the full range of C1-C17 alkanes, compared to Ru (100% conversion to methane), Pt, Pd, and Fe (~3%, 0%, and 0% conversion, respectively). It was shown that Co/SiO2 is an excellent catalyst that selectively converts polyethylene into liquid range hydrocarbons (C5-C30) at mild conditions (250-275 °C, 20-30 bar H2, and 4-8 h). At optimized conditions (275 °C, 30 bar H2, 8 h), ~19.5% gaseous and ~58.2% liquid products were yielded with an average liquid carbon of 22 (diesel-motor oil range). The catalyst further successfully converted end-of-use polyethylenes despite additives and impurities. The catalyst's active phase was CoO, and it showed exceptional regeneration between runs, giving nearly identical product distributions. Mechanistically, it was identified that Co favors the non-terminal C-C bond cleavage route, selectivity producing oligomers over gases at low conversions, which subsequently hydrogenolyze into lower hydrocarbons, eventually forming methane in secondary and tertiary events

Elucidating the Role of Strain in Catalysis toward Modulating Surface-Adsorbate Interactions and Tuning Catalytic Activity

Strain has been shown to modulate adsorption and reactions on metal surfaces. While its effect on surface-adsorbate interactions has been rationalized, an understanding of the electronic factors that drive these interactions and their consequences on catalytic activity is lacking. In this work, we use ab initio density functional theory (DFT) and microkinetic modeling (MKM) to develop electronic descriptors that govern the effect of biaxial strain in the modulation of interactions between adsorbate and transition states with catalyst surface and report its significance in enhancing the activity of fcc Pd(111) in the synthesis of ammonia (NH3), an important renewable-energy and hydrogen (H2) vector. We established the p-band center (pcenter) of the adsorbates and transition states (TS) and the hybridized d-band center (dcenter) of the surface metal as key electronic descriptors for adsorbate and TS energy variations with strain. Specifically, the pcenter of the adsorbates is lowest for the sites with the strongest adsorption, and the upshift of the dcenter of the surface metal atoms is greatest for the adsorption site with the highest strain susceptibility (i.e., the change in adsorption energy per unit applied strain). Importantly, we showed significant deviations in scaling relations with strain compared to periodic scaling relationships, both for adsorption and reaction. Over a net 4% tensile strain (±2%), the dcenter of Pd(111) moved upward by 0.21 eV, enhancing N2 dissociation, the rate-determining step in NH3 synthesis by ~37×, and the pcenter in N bound to the catalyst surface moved downward in the adsorbed state and upward in the TS (i.e., electron density shifted toward the bonding and anti-bonding states, respectively). Thus, tensile strain played a dual role in enhancing N2 dissociation, strengthening the adsorption of atomic N and weakening the N-N bond in the TS. We then evaluated N2 dissociation at 3/4 ML H-coverage under industrial conditions (150 atm H¬2, 50 atm N2, and 723 K), revealing the effect of tensile strain on the rate enhancement to be nearly two orders of magnitude greater (~3273× vs. ~37×) at high surface coverages. Overall, this study highlights strain as a useful design tool to improve catalytic activity

Enabling the Circular Economy through Chemical Recycling and Upcycling of End-of-Use Plastics

Widespread plastic pollution has led to an environmental crisis, motivating new and effective methods for recycling and upcycling “end-of-use” plastics. In this review, we highlight recent advances in chemical recycling and upcycling pathways, namely, hydroconversion, pyrolysis, and solvent treatment for the deconstruction and valorization of post-consumer plastics. We highlight the advances in the design of supported metal catalysts (Pt, Ru, Zr), for the hydroconversion of plastics, especially polyolefins (PO) and polyesters. We deduce mechanistic insights by comparing and contrasting small alkane and PO hydroconversion reactions. We also review the two types of solvent treatments: chemical solvent treatment (solvolysis) for condensation polymers and solvent extraction for composite polymers. Further, we discuss advances in pyrolysis and cross alkane metathesis to deconstruct POs into liquid hydrocarbons, and finally, the functionalization of POs into vitrimers and adhesives. We highlight the challenges and envision the path forward in optimal catalyst and process design that will enable the development of chemical upcycling technologies for building a circular plastic economy