For much of the past century, carbon dioxide (CO2) has received little attention scientifically outside of its role as a byproduct in the industrialization of the global economy. This trend has recently been upended where, due to mounting environmental concerns, CO2 has been brought squarely into the public consciousness. This surge in activity has contributed to a once unimaginable idea now pervading the scientific community: could CO2, a highly stable byproduct of hydrocarbon combustion, be recycled and converted back into useful chemicals and fuels? Owing to its ubiquitous nature and availability at truly massive quantities, it is thought that CO2-based products could offer a meaningful pathway toward lowering the environmental impact of many of the top industrial products while also enhancing supply chain diversification and resilience. In this manuscript we provide a holistic review of the pathways for CO2 conversion, the underlying chemistry and challenges involved in the transformation to products, and considerations for commercialization.

Dual function materials (DFMs) enable reactive carbon capture (RCkeC), an intensified approach to carbon dioxide capture and utilization for cost and energy input reductions. Yet, there is a fundamental lack of understanding of mechanisms around CO2 adsorption and subsequent conversion on these materials, hindering further development. Herein, we investigated several supported alkaline metal oxides for their CO2 adsorption characteristics to find that Na/Al2O3 had the highest CO2 adsorption capacity, accompanied by a variety of CO2 adsorption geometries as identified by in situ DRIFTS and computational modeling. The addition of catalytic metals (Ru, Pd) increased the adsorption capacity of Na/Al2O3 without altering binding modes. In the subsequent reactive desorption step, acetate and formate intermediates were observed. Notably, this mechanistic investigation identified that the formation of acetate species was unique to RCC on a DFM, as these species were not observed in co-fed hydrogenation over the DFM or RCC over a Na-free catalyst.

Converting high-performing powder catalysts from the laboratory reactor scale into effective extruded catalysts at the industrial scale remains a hurdle for advancing sustainable catalytic processes, such as the conversion of biogenic syngas into high octane gasoline. Recently, a process-intensified syngas-to-hydrocarbons (STH) reaction in a single reactor under relatively mild conditions (220-250 degrees C, 0.75-2.0 MPa) was reported, enabled by the development of a dimethyl ether (DME) homologation catalyst, Cu-modified H-BEA (Cu/BEA) zeolite. In this study, we explore approaches for synthesizing engineered Cu/BEA catalysts for use in the STH reaction to retain the high performance observed with the powder catalyst. We demonstrate that changes to the order of manufacturing steps (i.e., Cu deposition, alumina binder addition, and extrusion) result in observable changes to key active sites (Br & oslash;nsted acid sites and zeolitic Cu+ species), and ultimately, catalyst performance. When the Cu precursor was added directly to BEA before extrusion, both types of active sites were stabilized, preserving the activity of the powder catalyst. However, when the Cu precursor was added after extrusion, the resulting Cu species were mobile, destabilizing Br & oslash;nsted acid sites and leading to near-zero activity.

Reactive carbon capture (RCC) is a promising nascent approach to harness the untapped resource of waste CO2 to produce fuels and platform chemicals. However, process economics relies on the design of dual-function materials (DFMs) that exhibit maximal conversion of captured CO2 and high selectivity toward valuable carbon products (e.g., CO). Herein, we report the use of low-concentration (ca. 0.5 wt %) metal promotors (Pt and Au) to improve the conversion of captured CO2 on a Zn-Al mixed-oxide DFM by >30%, while also more than doubling per-cycle CO yield due to enhanced retention of captured CO2 and H-2 activation. In situ DRIFTS of bound CO2 on the DFMs showed distinctions in surface carbonate formation for the Pt- and Au-modified materials that were correlated with product evolution during the reactive desorption step of RCC. Particularly, Pt afforded superior H-2 activation but formed irreversibly-bound bicarbonates, compromising overall productivity, while the Na/Au/ZA DFM predominantly adsorbed CO2 on Na sites, resulting in polydentate carbonates that are retained until higher temperatures before conversion to CO. These results indicate that adding a low concentration of oxidation-resistant noble metals, especially Au, is an effective strategy for improving key performance metrics of Zn-Al mixed-oxide DFMs to further advance the development of scalable RCC technologies.

CO2 hydrogenation to methanol, a key reaction for decarbonizing the fuel and chemical industries, requires catalyst formulations that hydrogenate CO2 selectively to methanol at temperatures where methanol conversion is not significantly equilibrium limited (<423 K). Herein we report continuous CO2 hydrogenation at low temperatures (348-408 K, H-2/CO2 = 0.1-50, 5-35 bar) with high selectivity to methanol (up to ca. 80%) over unsupported beta-Mo2C catalysts. Active site density quantification via titration with trifluoroacetic acid at reaction temperatures enables an assessment of site-specific rates. Methanation and reverse water gas shift (RWGS) occur concurrently with methanol synthesis during CO2 hydrogenation over Mo2C. Reaction pathway analysis, product cofeeds, and reversibility formalisms show that all products form through primary reaction pathways from CO2, but secondary reactions of CO contribute significantly to rates of methanation. Dependences of forward rates on reactant and product concentration determined by independently varying the CO2, H-2, CO, H2O, CH3OH, and CH4 pressure in conjunction with reversibility formalisms reveal that all products form through H-assisted CO2 activation and involve partially hydrogenated CO2-derived intermediates. These inferences were verified by quantitative agreement between measured site-time yields and site-time yields predicted by closed form kinetic rate expressions in an integral reactor model over widely varying conditions (85-2000 kPa H-2, 80-1500 kPa CO2, 0-45 kPa H2O, 0-21 kPa CO, 0-25 kPa CH3OH, 0-75 kPa CH4, 5-87 mol Mo-s s mol CO2-1). Coverages calculated based on the kinetic model reveal that the Mo2C surface is covered with bidentate CO- and CO2-derived intermediates of the stoichiometry H2CO2 and H2CO, indicating that H-2 and COx do not compete for surface occupancy but instead adsorb cooperatively to form partially hydrogenated intermediates. Hydrogenation of the CO-derived H2CO** intermediate favors methanation, while hydrogenation of CO2-derived H2CO2** favors methanol synthesis. Together, these findings demonstrate the ability of unsupported Mo2C to catalyze the hydrogenation of CO2 to methanol at low temperatures and provide insight into the reaction network and mechanisms involved in its formation.

Biomass-derived energy sources represent a promising domestic route for fuel and chemical production, taking advantage of largely underutilized biological and waste resources. Heterogeneous catalysis plays a key role in these biomass conversion processes, as reflected by all American Society for Testing and Materials-approved pathways for producing sustainable aviation fuel proceeding through a catalytic step. This concise review seeks to establish the state of the art in thermal catalytic process development for various biomass-derived feedstocks and the current enabling capabilities that aid this development. Research needs are identified and described throughout the article, as further advancements in heterogeneous catalysis are required to improve the affordability and realize the full potential of biomass-derived products.

Reactive carbon capture (RCC), whereby a dual function material (DFM) captures dilute CO2 and subsequently converts it within the same reactor, can address persistent cost concerns in carbon capture and utilization technology advancement. Many DFMs utilize costly precious metals (e.g., Pt or Ru) or oxidation-sensitive transition metals (e.g., Ni and Cu), the latter of which can hinder utilization of CO2 sources containing O-2 and/or increase the H-2 demand for oxide-metal reduction cycles. We report a precious metal-free, K-modified Zn-Al mixed oxide (ZnAlOx) DFM that exhibited 99 % selectivity to CO at temperatures more than 250 degrees C lower than conventional RCC-to-CO routes. While unmodified ZnAlOx catalysts were effective for RCC, K-modification was found to significantly increase CO2 adsorption capacity, improve retention of adsorbed CO2 through the reactive desorption step, and suppress the formation of unreactive spectator species during reactive desorption. These effects combined to improve CO yield (53 %) far beyond equilibrium-limited predictions for CO2-to-CO under conventional steady state reaction conditions (<1 %). The DFM also exhibited remarkable oxidative stability, with negligible performance loss over 25 RCC cycles using simulated flue gas with 5 % O-2, demonstrating its promise as a robust, low-cost material to enable highly selective conversion of CO2 to CO in a single unit operation.

Our group recently developed dual-function materials (DFMs) and reactive carbon capture (RCC) processes for the selective production of methanol (MeOH) or CO, offering two novel and unique pathways for MeOH production. This study conducted a comparative techno-economic analysis (TEA) of the two RCC pathways from exhaust CO2: 1) a "Direct RCC-to-MeOH" pathway and 2) an "Indirect RCC-to-CO" pathway followed by MeOH synthesis. The "Direct RCC-to-MeOH" pathway produced a lower levelized cost of MeOH (LCOM) at $0.78/kg, compared to $0.84/kg for the "Indirect RCC-to-CO" pathway. The key difference is the need to recompress the syngas from RCC before MeOH synthesis in "Indirect RCC-to-CO." Nonetheless, with reduced catalyst costs and hydrogen requirements for "RCC-to-CO," this pathway merits further study to produce syngas rather than MeOH. Both pathways are comparable in LCOM to baseline e-MeOH production from CO2 hydrogenation ($0.72/kg) while having lower carbon intensities (0.45 and 0.51 kg-CO2e/kg vs 0.54 kg-CO2e/kg).

Active site occlusion by carbon species, often referred to as coke, is a common deactivation mechanism for heterogeneous catalysts. While it is known that transition metals can lower the temperature required for oxidative coke removal, the roles of ionic species and metal nanoparticles in coke removal are not well understood. This work aims to differentiate how ionic and nanoparticle Cu sites catalyze oxidative regeneration of a coked beta (BEA) zeolite catalyst. This was accomplished by synthesizing catalysts containing exclusively CuOx nanoparticles (NPs) or Cu2+ ions supported on BEA, physically mixing Cu/BEA with a coked BEA catalyst, and monitoring coke removal using in situ spectroscopy. Our results point to an improved combustion activity for CuOx NPs relative to Cu2+ ions, especially in the combustion of graphitic-type coke species. This improved understanding of coke combustion in transition metal containing catalysts informs strategies to improve catalyst regeneration, thereby increasing operational lifetimes.

The hydrogenation of CO2 holds promise for transforming the production of renewable fuels and chemicals. However, the challenge lies in developing robust and selective catalysts for this process. Transition metal oxide catalysts, particularly cobalt oxide, have shown potential for CO2 hydrogenation, with performance heavily reliant on crystal phase and morphology. Achieving precise control over these catalyst attributes through colloidal nanoparticle synthesis could pave the way for catalyst and process advancement. Yet, navigating the complexities of colloidal nanoparticle syntheses, governed by numerous input variables, poses a significant challenge in systematically controlling resultant catalyst features. We present a multivariate Bayesian optimization, coupled with a data-driven classifier, to map the synthetic design space for colloidal CoO nanoparticles and simultaneously optimize them for multiple catalytically relevant features within a target crystalline phase. The optimized experimental conditions yielded small, phase-pure rock salt CoO nanoparticles of uniform size and shape. These optimized nanoparticles were then supported on SiO2 and assessed for thermocatalytic CO2 hydrogenation against larger, polydisperse CoO nanoparticles on SiO2 and a conventionally prepared catalyst. The optimized CoO/SiO2 catalyst consistently exhibited higher activity and CH4 selectivity (ca. 98%) across various pretreatment reduction temperatures as compared to the other catalysts. This remarkable performance was attributed to particle stability and consistent H* surface coverage, even after undergoing the highest temperature reduction, achieving a more stable catalytic species that resists sintering and carbon occlusion.

Polyoxymethylene ethers (POMEs) are a class of low-soot and high-cetane oxygenate oligomers of structure RO(CH2O-)n-R, with different chain lengths (n) and end-groups (R) that determine their diesel-like fuel properties. Commercial POMEs with methyl end-groups (MM-POME3-6) exhibit undesirably low energy density and high water solubility. A previous computational assessment indicated that the lower heating value (LHV) and water solubility for MM-POME3-6 both improve upon end-group exchange with larger butyl, iso-butyl and iso-pentyl end-groups. Here, we expanded upon our initial trans-acetalization reaction that employed 1-butanol to install butyl end-groups to also include branched, higher carbon-number end-groups using iso-butanol and fusel oil as reagents. These new products are termed iB*POME1-6, and FOil*POME1-5, respectively, and collectively referred to as R*POMEs. They possess the advantaged properties of the parent MM-POME3-6 while exhibiting higher LHV (31 MJ kg-1 and 28 MJ kg-1 for iB*POME1-6, and FOil*POME1-5, respectively) and much reduced water solubility (2.7 g L-1 and 1 g L-1 for iB*POME1-6, and FOil*POME1-5, respectively). Additional fuel property analyses were performed using 20 vol% blends of the R*POMEs with a base diesel fuel. Overall, the greater energy density and decreased water solubility of the R*POMEs, as well as their synergistic blending with diesel at moderate blend levels, provide the greatest benefits to consumers and position this group of products as an environmentally friendlier blendstock alternative to the commercially-available MM-POME3-6.

A novel RCC process using modified CZA DFMs to produce renewable MeOH is presented. K/CZA provides exceptionally high productivity of MeOH compared to previously reported attempts of RCC to MeOH.

Closed-loop recycling via an efficient chemical process can help alleviate the global plastic waste crisis. However, conventional depolymerization methods for polyolefins, which compose more than 50% of plastics, demand high temperatures and pressures, employ precious noble metals, and/or yield complex mixtures of products limited to single-use fuels or oils. Superacidic forms of sulfated zirconia (SZrO) with Hammet Acidity Functions (H-0) <= - 12 (i.e., stronger than 100% H2SO4) are industrially deployed heterogeneous catalysts capable of activating hydrocarbons under mild conditions and are shown to decompose polyolefins at temperatures near 200 degrees C and ambient pressure. Additionally, confinement of active sites in porous supports is known to radically increase selectivity, coking and sintering resistance, and acid site activity, presenting a possible approach to low-energy polyolefin depolymerization. However, a critical examination of the literature on SZrO led us to a surprising conclusion: despite 40 years of catalytic study, engineering, and industrial use, the surface chemistry of SZrO is poorly understood. Ostensibly spurred by SZrO's impressive catalytic activity, the application-driven study of SZrO has resulted in deleterious ambiguity in requisite synthetic conditions for superacidity and insufficient characterization of acidity, porosity, and active site structure. This ambiguity has produced significant knowledge gaps surrounding the synthesis, structure, and mechanisms of hydrocarbon activation for optimized SZrO, stunting the potential of this catalyst in olefin cracking and other industrially relevant reactions, such as isomerization, esterification, and alkylation. Toward mitigating these long extant issues, we herein identify and highlight these current shortcomings and knowledge gaps, propose explicit guidelines for characterization of and reporting on characterization of solid acidity, and discuss the potential of pore-confined superacids in the efficient and selective depolymerization of polyolefins.

A model was developed to conduct techno-economic analysis (TEA) and life cycle assessment (LCA) for reactive carbon capture (RCC) and conversion of carbon dioxide (CO2) to methanol. This RCC process is compared to a baseline commercialized flue gas CO2 hydrogenation process. An ASPEN model was combined with existing TEA and LCA models into a larger TEA/LCA framework in Python. From preliminary experimental data, the model found a levelized cost of $0.79/kg methanol for the baseline process and $0.99/kg for the RCC process. The cradle-to-gate carbon intensity of the baseline process was 0.50 kg-CO2e/kg-methanol, compared to 0.55 kg-CO2e/kg-methanol for the RCC process. However, water consumption for RCC (10.21 kg-H2O/kg-methanol) is greatly reduced compared to the baseline (12.89 kg-H2O/kg-methanol). Future improvements in hydrogen electrolysis costs will benefit the RCC. A target H2/methanol mass ratio of 0.26 was developed for RCC laboratory experiments to reduce methanol cost below the baseline. If a ratio of 0.24 can be achieved, a levelized cost of $0.76/kg methanol is projected, with a carbon intensity of 0.42 kg-CO2e/kg-methanol.

Controlled synthesis of Ni 2− x Rh x P nanoparticle catalysts enables an understanding of composition-dependent selectivity for the hydrodeoxygenation reaction of phenolic molecules.

In this work, wereport the simultaneous dehydration of glucoseand xylose present in a process-relevant biorefinery hydrolysate tofurfural and 5-hydroxymethylfurfural (HMF) using heterogeneous solidacid catalysts in a microwave reactor. Initially, several solid acidcatalysts with varied Bronsted and Lewis acidity were screenedto evaluate their activity and selectivity in dehydration of pureglucose to HMF. A noticeable improvement in HMF yield from dehydrationof 8 wt % glucose was obtained by combining an acidic ion-exchangeresin (Purolite CT-275DR) with an amorphous silica-alumina catalyst(Davicat-3115) resulting in HMF yields of 27-33% using a homogeneoussolvent system of aqueous dioxane (dioxane/water, 2:1 v/v) at 195 & DEG;C in 5 min. Under the same reaction conditions, catalysts, andsolvent system but with the addition of NaCl in catalytic amounts(33-100 mM), a more than 2-fold increase in HMF yields (66-70%)was achieved for the dehydration of 8 wt % glucose, whereas furfuralyields approaching 95% were achieved for the dehydration of 6 wt %xylose, when conducted separately. Notably, using the same catalystand solvent system while slightly modifying the reaction conditionsto 197 & DEG;C and 5 min, simultaneous dehydration of 4 wt % xyloseand 9 wt % glucose present in a process-relevant corn stover hydrolysateresulted in furfural and HMF yields of 96 and 74%, respectively, resultingin a combined furfural yield of 80%. The results further showed thatthe pH of the reaction solution played an important role in maximizingproduct yields. A pH < 2 resulted in low HMF yields due to theincreased formation of HMF degradation products, whereas a pH 2-3gave high HMF yields by possibly stabilizing the reaction intermediatesand product, suppressing the occurrence of side reactions.