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Open access publications by faculty, staff, postdocs, and graduate students from the Catalysis Center for Energy Innovation.

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    Unraveling Multiscale Kinetics over Subnanometer Cluster Catalysts: H2 Desorption from Pt3(-H)2/γ-Al2O3(110)
    (ACS Catalysis, 2023-08-18) Yan, George; Vlachos, Dionisios G.
    Despite the attractiveness of highly dispersed supported metal catalysts due to the efficient usage of the active metal component, the structural complexity of subnanometer metal cluster active sites and the interconnectedness of reaction networks over many active site configurations elude detailed understanding. Here, we perform density functional theory (DFT) calculations and state-based kinetic simulations of the desorption of H2 from Pt3(-H)2 clusters supported on dehydroxylated γ-Al2O3(110), serving as a prototype of such coupled reaction networks. Different from ideal low Miller index metal surfaces and highly symmetric gas-phase clusters, we find many unique H binding sites on the supported Pt3 clusters, resulting in an ensemble of metastable Pt3(-H)2 cluster configurations interwoven within a network of H diffusion, active site restructuring, and H2 desorption elementary steps. Simulations and spectral analysis show that the catalyst and chemistry expose three principal time scales, corresponding to the diffusion of H, restructuring of Pt3(-H)2, and desorption of H2. Free energy span-based interpretations of the reaction pathways and sensitivity analysis of the eigenvalues uncover favorable Pt3(-H)2 restructuring and H2 desorption processes as being kinetically relevant at intermediate and long times. Interestingly, H2 desorption implicates catalyst restructuring as a prerequisite for forming more favorable desorption channels. We introduce simplified ensemble-based models and effective rate constants for the modeling of such multiscale reaction processes.
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    Direct Conversion of Ethane to Oxygenates, Ethylene, and Hydrogen in a Noncatalytic Biphasic Plasma Microreactor
    (ACS Sustainable Chemistry and Engineering, 2023-05-29) Cameli, Fabio; Dimitrakellis, Panagiotis; Vlachos, Dionisios G.
    We selectively upgrade ethane (C2H6) to ethanol (C2H5OH), methanol (CH3OH), and acetic acid (CH3COOH) in a catalyst-free, continuous, argon/water biphasic plasma microreactor. The water (H2O) evaporates and electron- dissociates into OH· radicals. OH· recombines with alkyl radicals, produced via electron dissociation of ethane, to generate the oxygenates that absorb into H2O. A plasma-assisted path, reminiscent of the low-temperature thermocatalytic ethane steam reforming, leads to significant H2 coproduction. The gaseous stream also comprises CO2 and C2H4. Up to 1.3 and 1 μmol min–1 of liquid C2H5OH and CH3OH are attained, respectively. Compared to CO2-assisted ethane plasma conversion, which produces many oxygenates with low selectivity, the carbon selectivity can range from >70% C2H5OH, CH3OH, and CH3COOH to 60% C2H4. The low carbon footprint, electrified, modular, intensified process using a reactive evaporation and separation plasma could pave the way for the valorization of underutilized shale gas resources in remote areas.
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    Deducing subnanometer cluster size and shape distributions of heterogeneous supported catalysts
    (Nature Communications, 2023-04-08) Liao, Vinson; Cohen, Maximilian; Wang, Yifan; Vlachos, Dionisios G.
    Infrared (IR) spectra of adsorbate vibrational modes are sensitive to adsorbate/metal interactions, accurate, and easily obtainable in-situ or operando. While they are the gold standards for characterizing single-crystals and large nanoparticles, analogous spectra for highly dispersed heterogeneous catalysts consisting of single-atoms and ultra-small clusters are lacking. Here, we combine data-based approaches with physics-driven surrogate models to generate synthetic IR spectra from first-principles. We bypass the vast combinatorial space of clusters by determining viable, low-energy structures using machine-learned Hamiltonians, genetic algorithm optimization, and grand canonical Monte Carlo calculations. We obtain first-principles vibrations on this tractable ensemble and generate single-cluster primary spectra analogous to pure component gas-phase IR spectra. With such spectra as standards, we predict cluster size distributions from computational and experimental data, demonstrated in the case of CO adsorption on Pd/CeO2(111) catalysts, and quantify uncertainty using Bayesian Inference. We discuss extensions for characterizing complex materials towards closing the materials gap.
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    Dynamic Electrification of Dry Reforming of Methane with In Situ Catalyst Regeneration
    (ACS Energy Letters, 2023-02-10) Yu, Kewei; Wang, Cong; Zheng, Weiqing; Vlachos, Dionisios G.
    We report the design and performance of a rapid pulse Joule heating (RPH) reactor with an in situ Raman spectrometer for highly endothermic, reversible reactions. We demonstrate it for methane dry reforming over a bimetallic PtNi/SiO2 catalyst that shows better performance than its monometallic counterparts. The catalyst temperature ramp rate can reach ∼14000 °C/s, mainly owing to the low thermal mass and resistivity of the heating element. Joule heating elements afford temperatures unachievable by conventional technology to enhance performance and more than double the energy efficiency. Dynamic electrification can increase syngas productivity and rate. Extensive characterizations suggest that pulse heating creates an in situ catalyst regeneration strategy that suppresses coke formation, sintering, and phase segregation, resulting in improved catalyst stability, under many conditions. Potentially driven by renewable electricity, the RPH can provide superb process advantages for high-temperature endothermic reactions and lead to negative carbon emissions.
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    A Life Cycle Greenhouse Gas Model of a Yellow Poplar Forest Residue Reductive Catalytic Fractionation Biorefinery
    (Environmental Engineering Science, 2022-09-13) Luo, Yuqing; O’Dea, Robert M.; Gupta, Yagya; Chang, Jeffrey; Sadula, Sunitha; Soh, Li Pei; Robbins, Allison M.; Levia, Delphis F.; Vlachos, Dionisios G.; Epps, Thomas H. III; Ierapetritou, Marianthi
    The incentive to reduce greenhouse gas (GHG) emissions has motivated the development of lignocellulosic biomass conversion technologies, especially those associated with the carbohydrate fraction. However, improving the overall biomass valorization necessitates using lignin and understanding the impact of different tree parts (leaves, bark, twigs/branchlets) on the deconstruction of lignin, cellulose, and hemicellulose toward value-added products. In this work, we explore the production of chemicals from a yellow poplar-based integrated biorefinery. Yellow poplar (Liriodendron tulipifera L.) is an ideal candidate as a second-generation biomass feedstock, given that it is relatively widespread in the eastern United States. Herein, we evaluate and compare how the different proportions of cellulose, hemicellulose (xylan), and lignin among leaves, bark, and twigs/branchlets of yellow poplar, both individually and as a composite mix, influence the life-cycle GHG model of a yellow poplar biorefinery. For example, the processing GHG emissions were reduced by 1,110 kg carbon dioxide (CO2)-eq, 654 kg CO2-eq, and 849 kg CO2-eq per metric ton of twigs/branchlets, leaves, and bark, respectively. Finally, a sensitivity analysis illustrates the robustness of this biorefinery to uncertainties of the feedstock xylan/glucan ratio and carbon content.
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    A review of thermal and thermocatalytic valorization of food waste
    (Green Chemistry, 2021-04-08) Ebikade, Elvis Osamudiamhen; Sadula, Sunitha; Gupta, Yagya; Vlachos, Dionisios G.
    Food waste (FW) remains a global challenge due to the increasing demand for food production to support a growing global population and the lack of effective waste management technologies for recycling and upcycling. Unique compounds in FW – such as carbohydrates, proteins, lignin, fats, and extractives – can be repurposed to produce important biobased fuels, bulk chemicals, dietary supplements, adsorbents, and antibacterial products, among many others. We review the thermal and thermocatalytic FW valorization strategies and the fundamental pathways. We discuss the potential integration of various valorization processes, their economic viability, the technical and marketing challenges, and the need for further developments. By overcoming several technical hurdles, repurposing FW into modular plants can create exciting economic and environmental prospects.
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    Accelerating manufacturing for biomass conversion via integrated process and bench digitalization: a perspective
    (Reaction Chemistry and Engineering, 2022-01-25) Batchu, Sai Praneet; Hernandez, Borja; Malhotra, Abhinav; Fang, Hui; Ierapetritou, Marianthi; Vlachos, Dionisios G.
    We present a perspective for accelerating biomass manufacturing via digitalization. We summarize the challenges for manufacturing and identify areas where digitalization can help. A profound potential in using lignocellulosic biomass and renewable feedstocks, in general, is to produce new molecules and products with unmatched properties that have no analog in traditional refineries. Discovering such performance-advantaged molecules and the paths and processes to make them rapidly and systematically can transform manufacturing practices. We discuss retrosynthetic approaches, text mining, natural language processing, and modern machine learning methods to enable digitalization. Laboratory and multiscale computation automation via active learning are crucial to complement existing literature and expedite discovery and valuable data collection without a human in the loop. Such data can help process simulation and optimization select the most promising processes and molecules according to economic, environmental, and societal metrics. We propose the close integration between bench and process scale models and data to exploit the low dimensionality of the data and transform the manufacturing for renewable feedstocks.
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    Improved slit-shaped microseparator and its integration with a microreactor for modular biomanufacturing
    (Green Chemistry, 2021-04-30) Bhattacharyya, Souryadeep; Desir, Pierre; Prodinger, Sebastian; Lobo, Raul F.; Vlachos, Dionisios G.
    Modular and distributed biomanufacturing requires continuous flow microreactors integrated with efficient separation units operating at comparable time scales: biphasic reactive extraction of 5-hydroxymethyl furfural (HMF) by fructose dehydration is an excellent example. The liquid–liquid extraction (LLE) and fast reaction kinetics in biphasic microchannels can immensely benefit from a downstream microseparator enabling separation of an HMF-rich organic extract and an aqueous raffinate. Here we demonstrate the successful implementation of an effective slit-shaped microseparator for eleven organic-water biphasic systems. The microseparator successfully separates six of these over reasonable flow rates. The ratio of capillary and hydraulic pressures qualitatively rationalizes the separation performance, while a transition to non-segmented flow patterns correlates with performance deterioration. Acids and salts, integral parts of the chemistry, significantly expand the flow rates for efficient separation enabling a broader slate of organic solvents. For the MIBK/water biphasic system, we demonstrate perfect separation performance over a 16-fold variation in the organic to aqueous flow ratio. Here we also integrate the microseparator and extractive microreactor into a modular system and achieve an HMF yield of up to 93% – the highest reported fractional HMF productivity of 27.9 min−1 – at an ultrashort residence time of 2 s. This unprecedented performance is maintained over a 50-fold fructose concentration range and is stable with time-on-stream. This microseparator exhibits a ten-fold reduction in separation time and substantial energy savings over conventional decanters. As such, it holds promise for continuous process intensification and modular biomanufacturing.
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    Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel Ranged Alkanes over a Rhenium Modified Iridium Catalyst
    (Wiley-Blackwell, 2017-07-07) Saha, Basudeb; Liu, Sibao; Dutta, Saikat; Zheng, Weiqing; Gould, Nicholas S.; Cheng, Ziwei; Xu, Bingjun; Vlachos, Dionisios G.; Sibao Liu, Saikat Dutta, Weiqing Zheng, Nicholas S. Gould, Ziwei Cheng, Bingjun Xu, Basudeb Saha, and Dionisios G. Vlachos; Saha, Basudeb; Liu, Sibao; Dutta, Saikat; Zheng, Weiqing; Gould, Nicholas S.; Cheng, Ziwei; Xu, Bingjun; Vlachos, Dionisios G.
    Renewable jet-fuel ranged alkanes are synthesized by hydrodeoxygenation of lignocellulose derived high carbon furylmethanes over ReOx modified Ir/SiO2 catalysts under mild reaction conditions. Ir-ReOx/SiO2 with a Re/Ir molar ratio of 2 exhibits the best performance, achieving a combined alkanes yield of 82-99% from C12-C15 furylmethanes. Catalyst can be regenerated in three consecutive cycles with only ~12% loss in the combined alkanes yield. Mechanistically, the furan moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high yield of Ir-ReOx/SiO2 arises from a synergy between Ir and ReOx. The acidic sites of partially reduced ReOx activate the C-O bonds of the saturated furans and alcoholic groups, while the Ir sites are responsible for hydrogenation with H2.
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    Poisoning of Ru/C by Homogeneous Brønsted Acids in Hydrodeoxygenation of 2,5-Dimethylfuran via Catalytic Transfer Hydrogenation
    (Elsevier, 2017) Gilkey, Matthew J.; Vlachos, Dionisios G.; Xu, Bingjun; Matthew J. Gilkey, Dionisios G. Vlachos, Bingjun Xu; Gilkey, Matthew J.; Vlachos, Dionisios G; Xu, Bingjun
    It has been proposed that the combination of metal and acid sites is critical for effective ring opening of biomass-derived furans to linear molecules, a reaction that holds promise for the production of renewable polymer precursors and alkanes. In this work, we use 2,5-dimethylfuran (DMF) as a model compound to investigate hydrogenolysis and hydrogenation pathways using a combination of H2SO4 and Ru-mediated catalytic transfer hydrogenation in 2-propanol. Acid-catalyzed hydrolytic ring opening of DMF to 2,5-hexanedione (HDN) occurs readily at 80 °C with a selectivity of 89% in 2-propanol. Over Ru/C, HDN is fully converted after only 2 h at 80 °C, forming a mixture of both ring-closed products (~68% total yield), i.e., 2,5-dimethyltetrahydrofuran (DMTHF) and 2,5-dimethyl-2,3-dihydrofuran (DMDHF), as well as ring opened products (~28% total yield), i.e., 2,5-hexanediol (2,5-HDL) and 2-hexanol (HOL). Rather than observing sequential hydrolysis/hydrogenation reactions, we observe severe suppression of metal chemistry when having both Ru/C and H2SO4 in the reaction system. While minor leaching of Ru occurs in the presence of mineral acids, X-ray photoelectron spectroscopy coupled with CO chemisorption studies suggest that the primary cause of the lack of Ru-mediated chemistry is poisoning by strongly adsorbed sulfate species. This hypothesis is supported by the observation of Ru-catalyzed chemistry when replacing H2SO4 with Nafion, a solid Brønsted acid, as sulfonic acid groups tethered to the polymer backbone cannot adsorb on the metal sites.
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