38 Search Results

Catalytic hydrogenolysis and hydrocracking are promising technologies for the conversion of polyolefin waste into hydrocarbons. Here, to explore the economic and environmental viability of these approaches, we conducted techno-economic analysis and life cycle assessment of polyethylene hydrogenolysis and hydrocracking processes that produce fuel-range products or olefins, respectively. Relative to primary production, the minimum selling price (MSP) of alkanes for linear-chain naphtha was estimated to be 1.8-fold higher, whereas branched-chain naphtha and propylene were shown to be cost competitive. Environmental impacts showed similar trends across scenarios, with propylene production by hydrocracking exhibiting greenhouse gas emissions comparable with conventional propylene due tomore » relatively low hydrogen demand and high yield. Together, these results suggest that innovations in catalysis and reaction engineering will be required to make viable products that are scale matched with today's plastics, such as light olefins for closed-loop recycling.« less

Supported platinum nanoparticles are known to convert polyolefins to high-quality liquid hydrocarbons with hydrogen under relatively mild conditions. However, no systematic study has been undertaken using bimetallic catalysts for polyethylene upcycling. Specifically, a total of 98 monometallic and bimetallic combinations (Ag, Cr, Co, Cu, Fe, Ga, In, Mn, Ni, Pd, Pt, Rh, Ru, Zr) on alumina were synthesized utilizing surface organometallic chemistry (SOMC) technique via robotic platform. These were investigated at a small scale (10 mg of catalyst and 50 mg of polyethylene) for their activity for the hydrogenolysis of polyethylene in a high-throughput batch reactor. Combinations of Ni andmore » Co were selected as candidates with high activity toward conversion into paraffin oils. Reaction conditions were optimized with Ni/Co/Al₂O₃ catalyst at a larger scale (300 mg catalyst and 3 g polyethylene) to obtain a high yield (93.1%) of paraffin wax with desired properties (M_n = 380 Da) and low polydispersity (Đ = 1.2). Ni/Co/Al₂O₃ was compared against Co/Ni/Al₂O₃ to understand the role of the deposition sequence. When Co is deposited before Ni, a layer of cobalt aluminate is formed upon reduction, stabilizing the deposition of 5 nm metallic Ni particles. When nickel is deposited before Co, particles are larger (average >20 nm) and more oxidized (Ni^δ+ in NiAl₂O₄), decreasing the availability of the catalytically active metallic Ni. In conclusion, the difference in electronic environments was also described by DFT calculations, which revealed that smaller 3D clusters of Ni are preferred on CoAl2O4 over the 3D clusters on NiAl₂O₄ and that these smaller clusters are more reducible, as confirmed experimentally.« less

The hydrogenolysis of plastics is limited by active-site inaccessibility and inefficient mass transport of bulky polymer chains. To overcome these challenges, this work developed two-dimensional MXene-supported Ru (Ru@MXene) catalysts. Lyophilization of a solution containing dispersed MXene sheets and Ru precursors enabled the confinement of Ru species within the MXene interlayers, which act as pillars to expand the interlayer spacing. Building on this, a silica-pillared MXene-supported Ru (Ru@P-MXene) with even larger interlayer spacing exhibited a reaction rate of 914.9 g_C5–C35 g_Ru ^–1 h^–1 for the hydrogenolysis of low-density polyethylene (LDPE) into valuable liquid chemicals (e.g., C₅–C₃₅). A comparison of product yieldsmore » between Ru@P-MXene and Ru@MXene suggests that elongated Ru particles confined within the MXene support expose their side facets for the reaction. In conclusion, this work demonstrates a new application of MXene in thermochemical catalysis, offering a solution to the challenges of active-site accessibility, mass transport, and reaction confinement in chemical plastic upcycling.« less

CuH-catalyzed olefin hydrogenation is rare compared to those of carbonyl-derived substrates. Olefin insertion into Cu–H to form Cu-alkyl is ubiquitous; however, subsequent H₂ activation remains unknown to our knowledge. Herein, we investigated the transformations of β-H elimination, H₂ cleavage, and catalytic olefin hydrogenation in a series of linear and trigonal planar Cu(I)-alkyl complexes supported by monodentate N-heterocyclic carbene and bidentate naphthyridine-bis(carbene) ligands, respectively. Contrary to unreactive linear species, trigonal planar variants promote β-H elimination, hydrogenolysis, and catalytic hydrogenation of unactivated alkenes at mild temperatures and H₂ pressure. The rare isolation of a naphthyridine-bis(carbene)CuH monomer further affirms two predominant competing pathwaysmore » for H₂ cleavage of metal–ligand cooperativity at Cu(I)-alkyl or internal electrophilic substitution at Cu(I)-OH. Employing either isolated or in situ generated Cu(I)-OH complex, via protonolysis of alkyl precatalyst by adventitious water, significantly accelerated catalysis compared to that operating primarily by the metal–ligand cooperativity pathway. DFT calculations and energy decomposition analysis on the disparate β-H elimination reactivity between linear and trigonal planar tert-butyl complexes and the mechanism of H₂ activation at a hydroxide complex, indicate that coordination geometry at Cu(I) and properties of the naphthyridine-bis(carbene) ligand are integral to the transformations reported here.« less

Ru/C-catalyzed polyethylene (PE) and hydrocarbon hydrogenolysis under quasi-supercritical fluid of isopentane was kinetically and mechanistically investigated. PE hydrogenolysis with C–C and C–H cleavage showed zeroth order, suggesting strong adsorption of hydrocarbons. PE yielded broad product distribution of heavy (C_21–40) and diesel-range (C_11–20) hydrocarbons in the primary step of hydrogenolysis due to stochastic C–C cleavage over Ru surface. Catalytic hydrogenolysis of n-hexadecane, squalane, and light hydrocarbons such as n-pentane, iso-pentane, and n-hexane further described C–C cleavage reactivity between primary and secondary carbons, i.e., ¹C–²C and ²C–²C, which has an order of magnitude higher hydrogenolysis rate than that involving a tertiary carbon.more » The PE saturated Ru surface and lower C–C cleavage reactivity of tertiary carbon in iso-pentane, therefore, imited sovlent conversion during hydrogenolysis, whereas leading to selective PE conversion. Using hexadecane, we observed comparable hydrogenolysis rates between H₂ and D₂ (k_H/k_D ~ 1), indicating the kinetically relevant step of C–C cleavage with facilitating C–H cleavage and rehydrogenation. However, the normal kinetic isotope effect between hexadecane and deuterated hexadecane (k_C16H34/k_C16D34 ~ 5) revealed that the dehydrogenation, i.e., C–H cleavage, can be kinetically involved in the hydrogenolysis kinetic. By considering the 8-fold lower H-D exchange rate with deuterated hexadecane compared to n-hexadecane, the lower rate for hydrogenolysis and H-D exchange with deuterated hexadecane can be attributed to the C–D bond dissociation energy being 3 kJ/mol higher than that of the C–H bond. Increasing H₂ pressure favors internal C–C bond cleavage over terminal one. This minimizes the formation of lower hydrocarbons, particularly methane. However, the increase in H₂ pressure increases the coverage of adsorbed hydrogen on the Ru particles due to competitive adsorption of H₂ and polyethylene, which, in turn, reduces the polyethylene conversion rates.« less

Kinetic and isotopic assessments combined with theoretical calculations shed light on the mechanistic differences in terms of the number of hydrogen addition steps and the electronic charges of reactive hydrogen in the catalytic sequence leading to the Ar−OCH₃ scission of 2-methoxyphenol occurring at various interfaces of vapor−Ru, cyclohexane−Ru, and water−Ru. At vapor−Ru and cyclohexane−Ru interfaces, the initial hydrogen adatom (H*) additions on the phenyl ring disrupt its aromaticity, as a step required to occur before kinetically relevant Ar−OCH₃ scission on an ensemble of Ru sites. The number of H* addition events prerequisite to the formation of the kinetically relevant Ar−OCH₃more » scission transition state, however, is larger at the cyclohexane−Ru interface than those at the vapor−Ru and water−Ru interfaces. The preferential solvation of 2-methoxyphenol derived reaction precursors and H* weakens their adsorption on Ru sites, which leads to a decrease in the activation barrier for the third H* addition step and in turn a larger number of H* additions. At the water−Ru interface, water as a polar protic solvent promotes the Ar−OCH₃ scission by enabling a mechanistically distinct catalytic route an adjacent water molecule from the solvent layer assists with the intramolecular proton shuttling from the hydroxyl group (−OH) to the vicinal methoxy group (−OCH₃) of 2-methoxyphenol, evolving a highly charged transition state that is further stabilized by the water solvent layer, thereby leading to Ar−OCH₃ scission rate constants that are much higher than those at the vapor−Ru interface and, when comparing at the same H₂ chemical potential, higher Ar−OCH₃ scission turnovers. Furthermore, these distinct mechanistic details of the Ar−OCH₃ scission at various reaction interfaces, inferred from experiment and theory, illustrate how a solvent either solvates reaction intermediates and transition states or directly participates in the formation of kinetically relevant transition states, leading to changes in the free energy landscape, thus rewriting the catalytic fates in hydrodeoxygenation reactions.« less

Supported transition metal catalyzed, chemical upcycling of polyolefins by hydrogenolysis typically occurs in a polymer melt phase at elevated temperatures (T > 200 ºC). Currently, the impact of the melt phase on the catalytic activity and selectivity of the transition metal is largely unknown. Herein, we use a hybrid quantum mechanical/molecular mechanical (QM/MM) approach to investigate the melt-phase effects on the adsorption free energy (∆∆G$$^{gas→liq}_{Adsorbate}$$) of atomic hydrogen, 12 hydrocarbon molecules, and 4 transition states in the hydrogenolysis mechanism of butane on a Pt(111) catalyst surface at 573 K in the presence of a polyethylene surrogate melt consisting of C₃₆H₇₄more » chains. The smallest and largest endergonic melt phase effects, (∆∆G$$^{gas→liq}_{Adsorbate}$$), belong to hydrogen (0.045 eV) and butane (1.357 eV). Altogether, we find melt-phase effects are significant and change the activity of transition metal catalysts. Beyond an overall reduced adsorption strength, elementary surface reactions are also affected by the melt phase.« less

Esters are often produced as unwanted byproducts during the catalytic upgrading of ethanol to diesel fuel precursors through Guerbet coupling. Removal of esters from the product stream is important to prevent the loss of downstream catalyst activity from ester-derived carboxylic acids. In this work, we studied ester hydrogenolysis to the parent alcohols as a viable route for enhanced diesel fuel production. Specifically, we investigated the reduction of hexyl acetate in butanol over ZrO₂-supported Ni, Co, Cu, Rh, Pd, and Pt catalysts, where Cu/ZrO₂ was the most selective catalyst for the hydrogenolysis of hexyl acetate into hexanol and ethanol. Thermodynamic analysismore » reveals that a 90% alcohol yield can be obtained at 200 °C, 30 bar, and a relatively high H₂:hexyl acetate molar ratio of 480:1. Experimentally, an alcohol yield of 88% yield was obtained with a 10 wt % Cu/ZrO₂ catalyst at these conditions with a residence time of 5.4 h kg_cat kmol_gas^–1. Catalytic tests on the support revealed that ZrO₂ catalyzes the transesterification reaction between hexyl acetate and butanol. However, only the Cu sites can catalyze the hydrogenolysis of the esters into the final alcohols. We developed a kinetic model for our experimental results, which shows that the transesterification and hydrogenolysis reactions run at two different timescales, the former being 10 times faster than the latter. Data regression has been used to develop a model to predict the mole fraction distribution of ester hydrogenolysis products over a wide range of contact times. Cu/ZrO₂ loses half its catalytic activity after 80 h of time on stream. Modeling of deactivation data reveals that the ZrO₂ support conserves a residual activity due to external active sites, while active sites over the Cu surface deactivate at different rates. Furthermore, the catalytic conversion of esters into their parent alcohols is relevant to the production of surrogate liquid fuels since alcohols can be bimolecularly dehydrated to produce a blend of ethers with diesel fuel-like properties.« less

We explore hydrogenolysis over ruthenium supported on zirconia (Ru/ZrO₂) and hydrocracking over platinum (Pt) supported on zeolites as an effective end-of-life strategy for ethylene vinyl acetate (EVA)–a widely used performance heat sealant in hard-to-recycle multilayer packaging. For Ru/ZrO₂ hydrogenolysis, EVA reacts slower than low-density polyethylene (LDPE) and the catalyst deactivates due to carbonaceous deposits originating from polyenes generated in situ during EVA thermal degradation. High H₂ pressures and temperatures can overcome catalyst deactivation; however, CH₄ yields are excessive due to cascade hydrogenolysis stemming from strong C=C/metal interactions. Polyene hydrogenation allows chains anchored by C=C to desorb from Ru, shifting productmore » selectivity from CH₄ to higher-value liquids. Hydrogenolysis of mixed EVA and linear low-density polyethylene (LLDPE), mirroring typical frozen food packaging formulations, results in comparable catalyst activity and CH₄ yield as the pure EVA resin. For Pt/zeolite hydrocracking, pure EVA and EVA:LLDPE mixtures are deconstructed to propane or light naphtha with minimal CH₄ production. Among catalysts tested, Pt/HY gives the highest liquid productivity (g_C5+products/g_cat·h). Furthermore, these findings showcase the recalcitrant nature of EVA and its associated mixtures for Ru/ZrO₂ hydrogenolysis, highlighting that hydrocracking catalysts may be superior for complex packaging waste.« less

The hydrogenolysis of polymers is emerging as a promising approach to deconstruct plastic waste into valuable chemicals. Yet, the complexity of plastic waste, including multilayer packaging, is a significant barrier to handling realistic waste streams. Herein, we reveal fundamental insights into a new chemical route for transforming a previously unaddressed fraction of plastic waste – poly(ethylene–co­­–vinyl alcohol) (EVOH) and related polymer blends – into alkane products. Additionally, we report that Ru/ZrO2 is active for the concurrent hydrogenolysis, hydrogenation, and hydrodeoxygenation of EVOH and its thermal degradation products into alkanes (C1–C35) and water. Detailed reaction data, product analysis, and catalyst characterizationmore » reveal that the in–situ thermal degradation of EVOH forms aromatic intermediates that are detrimental to catalytic activity. Increased hydrogen pressure promotes hydrogenation of these aromatics, preventing catalyst deactivation and improving alkane product yields. Calculated apparent rates of C–C scission reveal that the hydrogenolysis of EVOH is slower than low–density polyethylene. We apply these findings to achieve hydrogenolysis of EVOH/polyethylene blends and elucidate the sensitivity of hydrogenolysis catalysts to such blends. Overall, we demonstrate progress towards efficient catalytic processes for the hydroconversion of waste multilayer film plastic packaging into valuable products.« less