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Open access publications by faculty, staff, postdocs, and graduate students from the Center for Plastics Innovation.
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Item Effect of Dynamic and Preferential Decoration of Pt Catalyst Surfaces by WOx on Hydrodeoxygenation Reactions(Journal of the American Chemical Society, 2024-05-22) Marlowe, Justin; Deshpande, Siddharth; Vlachos, Dionisios G.; Abu-Omar, Mahdi M.; Christopher, PhillipCatalysts containing Pt nanoparticles and reducible transition-metal oxides (WOx, NbOx, TiOx) exhibit remarkable selectivity to aromatic products in hydrodeoxygenation (HDO) reactions for biomass valorization, contrasting the undesired aromatic hydrogenation typically observed for metal catalysts. However, the active site(s) responsible for the high selectivity remains elusive. Here, theoretical and experimental analyses are combined to explain the observed HDO reactivity by interrogating the organization of reduced WOx domains on Pt surfaces at sub-monolayer coverage. The SurfGraph algorithm is used to develop model structures that capture the configurational space (∼1000 configurations) for density functional theory (DFT) calculations of a W3O7 trimer on stepped Pt surfaces. Machine-learning models trained on the DFT calculations identify the preferential occupation of well-coordinated Pt sites (≥8 Pt coordination number) by WOx and structural features governing WOx–Pt stability. WOx/Pt/SiO2 catalysts are synthesized with varying W loadings to test the theoretical predictions and relate them to HDO reactivity. Spectroscopy- and microscopy-based catalyst characterizations identify the dynamic and preferential decoration of well-coordinated sites on Pt nanoparticles by reduced WOx species, consistent with theoretical predictions. The catalytic consequences of this preferential decoration on the HDO of a lignin model compound, dihydroeugenol, are clarified. The effect of WOx decoration on Pt nanoparticles for HDO involves WOx inhibition of aromatic ring hydrogenation by preferentially blocking well-coordinated Pt sites. The identification of preferential decoration on specific sites of late-transition-metal surfaces by reducible metal oxides provides a new perspective for understanding and controlling metal–support interactions in heterogeneous catalysis.Item Biphasic Plasma Microreactor for Oxyfunctionalization of Liquid Hydrocarbons(Industrial & Engineering Chemistry Research, 2024-05-22) Nguyen, Darien K.; Cameli, Fabio; Dimitrakellis, Panagiotis; Vlachos, Dionisios G.Oxyfunctionalization of linear alkanes is important but challenging to achieve. Herein, we demonstrate a biphasic gas–liquid modular plasma microreactor utilizing Ar/O2 gas to selectively oxidize liquid n-dodecane (C12) in an electrified, catalyst-free fashion. C12 secondary alcohols and ketones are the major products, with selectivities of 45–60% and a maximum yield of 23%. Fine-tuning gas and liquid flow rates enhance the plasma–liquid interfacial area, leading to a conversion of >50%. Difunctional and oligomerized oxygenates, alongside lighter hydrocarbons stemming from carbon–carbon cleavage, form at higher conversions. The energy efficiency (0.189 μmol/J) of the modular microreactor is the highest reported among plasma systems. Alkane conversion can be further improved by increasing the length of the plasma region while maintaining excellent energy efficiencies. Similarly, sequential processing/recirculation can enhance the extent of the reaction. This system is also amenable to treating mixtures of liquid n-alkanes, where smaller hydrocarbons are oxidized preferentially to a certain extent. The vapor pressure and liquid temperature are the key parameters. The chemistry occurs primarily in the gas phase for the lighter hydrocarbons and switches to interfacial reactions for the larger ones.Item Lignin-derivable, thermoplastic, non-isocyanate polyurethanes with increased hydrogen-bonding content and toughness vs. petroleum-derived analogues(Materials Advances, 2024-04-02) Mahajan, Jignesh S.; Hinton, Zachary R.; Nombera Bueno, Eduardo; Epps, Thomas H. III; Korley, LaShanda T. J.The functionality inherent in lignin-derivable bisguaiacols/bissyringols can improve the processability and performance of the resulting polymers. Herein, non-isocyanate polyurethanes (NIPUs) were synthesized from bisguaiacols/bissyringols with varying degrees of methoxy substitution and differing bridging groups. Notably, the presence of increasing numbers of methoxy groups (0, 2, and 4) in bisphenol F (BPF)-, bisguaiacol F (BGF)-, and bissyringol F (BSF)-NIPUs led to higher percentages of hydrogen-bonded –OH/–NH groups (i.e., ∼65%, ∼85%, ∼95%, respectively). Increased hydrogen bonding between chains improved the elongation-at-break (εbreak) and toughness of lignin-derivable NIPUs over their petroleum counterparts without a reduction in Young's moduli and tensile strengths. For example, BSF-NIPU exhibited the highest εbreak ∼210% and toughness ∼62 MJ m−3, followed by BGF-NIPU (εbreak ∼185% and toughness ∼58 MJ m−3), and then BPF-NIPU (εbreak ∼140% and toughness ∼42 MJ m−3). Similar trends were found in the dimethyl-substituted analogues, particularly for the bisphenol A-NIPU and bisguaiacol A-NIPU. Importantly, the melt rheology of the lignin-derivable NIPUs was comparable to that of the petroleum-derived analogues, with a slightly lower viscosity (i.e., improved melt flow) for the bio-derivable NIPUs. These findings suggested that the added functionalities (methoxy groups) derived from lignin precursors improved thermomechanical stability while also offering increased processability. Altogether, the structure–property-processing relationships described in this work can help facilitate the development of sustainable, performance-advantaged polymers.Item Conformations of polyolefins on platinum catalysts control product distribution in plastics recycling(Chemical Science, 2023-01-03) Zare, Mehdi; Kots, Pavel A.; Caratzoulas, Stavros; Vlachos, Dionisios G.The design of catalysts for the chemical recycling of plastic waste will benefit greatly from an intimate knowledge of the interfacial polymer–catalyst interactions that determine reactant and product distributions. Here, we investigate backbone chain length, side chain length, and concentration effects on the density and conformation of polyethylene surrogates at the interface with Pt(111) and relate them to experimental product distributions resulting from carbon–carbon bond cleavage. Using replica-exchange molecular dynamics simulations, we characterize the polymer conformations at the interface by the distributions of trains, loops, and tails and their first moments. We find that the preponderance of short chains, in the range of 20 carbon atoms, lies entirely on the Pt surface, whereas longer chains exhibit much broader distributions of conformational features. Remarkably, the average length of trains is independent of the chain length but can be tuned via the polymer–surface interaction. Branching profoundly impacts the conformations of long chains at the interface as the distributions of trains become less dispersed and more structured, localized around short trains, with the immediate implication of a wider carbon product distribution upon C–C bond cleavage. The degree of localization increases with the number and size of the side chains. Long chains can adsorb from the melt onto the Pt surface even in melt mixtures containing shorter polymer chains at high concentrations. We confirm experimentally key computational findings and demonstrate that blends may provide a strategy to reduce the selectivity for undesired light gases.Item Tuning High-Density Polyethylene Hydrocracking through Mordenite Zeolite Crystal Engineering(ACS Sustainable Chemistry and Engineering, 2023-06-19) Kots, Pavel A.; Doika, Panagiota A.; Vance, Brandon C.; Najmi, Sean; Vlachos, Dionisios G.We investigate the hydrocracking of high-density polyethylene using a bifunctional Pt/Al2O3 and modified mordenite acid catalyst. Mass transport limitations impact polymer diffusion into the mordenite pore complex. Initial reaction intermediates are formed on the zeolite’s outer surface. Intercrystallite open-end mesopores improve the diffusion of reaction intermediates deeper into the crystal. Recrystallization and desilication of mordenite lead to a higher polymer conversion and shift the product distribution maximum from pentanes to hexanes and heptanes. The nature of mesopores (occluded or open) and total Brønsted acidity significantly impact zeolite activity and selectivity.Item Techno-Economic and Life Cycle Analyses of Thermochemical Upcycling Technologies of Low-Density Polyethylene Waste(ACS Sustainable Chemistry and Engineering, 2023-05-08) Hernández, Borja; Kots, Pavel; Selvam, Esun; Vlachos, Dionisios G.; Ierapetritou, Marianthi G.In this study we compare techno-economics and life cycle assessment of thermochemical depolymerization technologies, including pyrolysis, gasification, hydrocracking, hydrothermal liquefaction, and hydrogenolysis, to generate various products from low-density polyethylene (LDPE) waste. We elucidate the effects of production scale, collection cost, and concentration of LDPE in plastic waste. Pyrolysis of LDPE to olefins followed by their conversion to lubricant oils is the most profitable technology. Hydrogenolysis, producing a small fraction of lubricant oils, becomes profitable at plant sizes above 25 kt/y and produces the lowest CO2 emissions. Hydrocracking is the second most environmentally friendly technology but becomes economically competitive at sufficiently large scales, and the supply chain for collecting plastics is optimized. Gasification of LDPE to H2 produces high emissions, and the price of H2 of ∼3 $/kg is higher than current markets and recently announced goals. Similarly, hydrothermal liquefaction also gives high emissions, making carbon capture systems imperative for both technologies. Our results demonstrate that lowering the cost of sorting LDPE from plastic waste, collecting waste near big cities, building sufficiently large plants, and achieving high selectivity to value-added products are critical to successful plastic waste management.Item Reductive Enzyme Cascades for Valorization of Polyethylene Terephthalate Deconstruction Products(ACS Catalysis, 2023-04-07) Gopal, Madan R.; Dickey, Roman M.; Butler, Neil D.; Talley, Michael R.; Nakamura, Daniel T.; Mohapatra, Ashlesha; Watson, Mary P.; Chen, Wilfred; Kunjapur, Aditya M.To better incentivize the collection of plastic wastes, chemical transformations must be developed that add value to plastic deconstruction products. Polyethylene terephthalate (PET) is a common plastic whose deconstruction through chemical or biological means has received much attention. However, a limited number of alternative products have been formed from PET deconstruction, and only a small share could serve as building blocks for alternative materials or therapeutics. Here, we demonstrate the production of useful monoamine and diamine building blocks from known PET deconstruction products. We achieve this by designing one-pot biocatalytic transformations that are informed by the substrate specificity of an ω-transaminase and diverse carboxylic acid reductases (CAR) toward PET deconstruction products. We first establish that an ω-transaminase from Chromobacterium violaceum (cvTA) can efficiently catalyze amine transfer to potential PET-derived aldehydes to form monoamine para-(aminomethyl)benzoic acid (pAMBA) or diamine para-xylylenediamine (pXYL). We then identified CAR orthologs that could perform the bifunctional reduction of terephthalic acid (TPA) to terephthalaldehyde or the reduction of mono-(2-hydroxyethyl) terephthalic acid (MHET) to its corresponding aldehyde. After characterizing 17 CARs in vitro, we show that the CAR from Segniliparus rotundus (srCAR) had the highest observed activity on TPA. Given these elucidated substrate specificity results, we designed modular enzyme cascades based on coupling srCAR and cvTA in one pot with enzymatic cofactor regeneration. When we supply TPA, we achieve a 69 ± 1% yield of pXYL, which is useful as a building block for polymeric materials. When we instead supply MHET and subsequently perform base-catalyzed ester hydrolysis, we achieve 70 ± 8% yield of pAMBA, which is useful for therapeutic applications and as a pharmaceutical building block. This work expands the breadth of products derived from PET deconstruction and lays the groundwork for eventual valorization of waste PET to higher-value chemicals and materials.Item Oxidative Functionalization of Long-Chain Liquid Alkanes by Pulsed Plasma Discharges at Atmospheric Pressure(ACS Sustainable Chemistry and Engineering, 2022-11-17) Nguyen, Darien K.; Dimitrakellis, Panagiotis; Talley, Michael R.; O'Dea, Robert M.; Epps, Thomas H. III; Watson, Mary P.; Vlachos, Dionisios G.We introduce the oxidation of long aliphatic alkanes using non-thermal, atmospheric plasma processing as an eco-friendly route for organic synthesis. A pulsed dielectric barrier discharge in He/O2 gas mixtures was employed to functionalize n-octadecane. C18 secondary alcohols and ketones were the main products, with an optimal molar yield of ∼29.2%. Prolonged treatment resulted in the formation of dialcohols, diketones, and higher molecular weight oxygenates. Lighter hydrocarbon products and decarboxylation to CO2 were also observed at longer treatment times and higher power inputs. A maximum energy yield of 5.48 × 10–8 mol/J was achieved at short treatment times and high powers, associated with higher selectivity to primary oxygenates. Direct hydroxylation of alkyl radicals, as well as disproportionation reactions, are proposed as the main pathways to alcohols and ketones. The results hold promise for functionalizing long hydrocarbon molecules at ambient conditions using catalyst-free plasma discharges.Item Polyolefin plastic waste hydroconversion to fuels, lubricants, and waxes: a comparative study(Reaction Chemistry and Engineering, 2021-12-01) Kots, Pavel A.; Vance, Brandon C.; Vlachos, Dionisios G.Hydroconversion technologies have surged to the forefront of deconstructing plastic waste. Recent studies have been performed over several catalysts with varying conditions and plastics that make comparisons difficult. We compile and compare data from the literature by introducing various metrics and perform a simple energy analysis. We draw mechanistic similarities to and differences from the past literature on small alkane hydroconversion and leverage the former to propose standard approaches to tune product selectivity. We exemplify the plastics materials gap and the challenges it creates. Finally, we discuss the current limitations and suggest future work.