Advanced Catalysis for Renewable Carbon Conversion: Part 1

Graphite is identified by the U.S. Department of Energy as a critical material due to its essential role in energy storage technologies. With its widespread use as the anode material in lithium-ion batteries, the demand for high-purity graphite for battery applications is expected to grow steadily in the coming years. Synthetic graphite is generally preferred for these applications because of its high purity and controlled properties. However, the conventional synthesis of graphite relies on the Acheson process, which is highly energy-intensive and requires heat treatment of carbon precursors at temperatures up to 3300 K to achieve full graphitization. In this presentation, we introduce a new graphitization method that operates at significantly lower temperatures through an electrochemical catalytic approach. Cathodic polarization of biomass-derived amorphous carbon in molten CaCl₂ at approximately 1100 K for a few hours transforms the amorphous carbon into a highly graphitic structure. This process has the potential to substantially reduce energy consumption compared to conventional graphite production methods.

The catalytic recycling of polymers has gathered significant attention in the past years, as an approach to circulate the petroleum-based carbon flow in the plastic sector and alleviate its environmental damage. Compared to conventional small-molecule catalytic reactions, polymer reactions often exhibit distinct kinetic behaviors and active-site structure requirement for effective catalysis, due to an additional layer of complexity – the macromolecular behavior of polymer chains. Thus, in addition to the characterization of active sites and surface intermediate at the bonding scale, the holistic understanding of these reaction systems also requires characterizing polymeric configuration and dynamics at nm-/μm-scale, to allow accurate prediction of reaction outcomes. For the last task, which is foreign to the catalysis community but not so to the polymer community, neutron-based methods provide H-sensitive measurements that selectively probe polymers in the presence of high-Z atoms in catalysts, yielding information that complements X-ray spectroscopy that probes surface active sites. Here I will present progress from our group in understanding multi-scale phenomena in polymer recycling catalysis combining these techniques. Ru active sites, the most widely studied metal for polyolefin hydrogenolysis, were characterized by careful analysis of X-ray absorption spectroscopy (XAS), to understand how the size, shape, and density of Ru particles regulate kinetics via elementary steps at chemical-bonding scale and longer-range polymer interactions. Meanwhile, reacting polyolefin chains of varying structures were characterized by quasi-elastic neutron scattering (QENS), to reveal their dynamics around active sites. This is complemented by X-ray photoemission spectroscopy (XPS) and electron microscopy probing their chemisorbed states. The effort of multi-scale characterization combining neutron and X-ray methods can potentially offer holistic understanding regarding structure and interactions at the polymer-catalyst interface, which guides steering the reactivity and selectivity in polymer recycling catalysis.

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 H2 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 H2 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.

An important processing aspect that has received scant attention in the literature is the effect of mineral content in biomass on organosolv/acetosolv fractionation. Biomass feedstocks contain significant amounts of inorganic minerals which manifest as ash and adversely affect their downstream conversion via processes such as pyrolysis and gasification. Biomass demineralization has the potential to not only enhance downstream thermal and catalytic processes but also promote sustainable agriculture by returning the removed mineral nutrients back to the soil. In this talk, we shall present results on the isolation of cellulose-rich pulp and lignin from non-demineralized and chemically demineralized beetle-killed pine (BKP) using the acetosolv process. Specifically, various extents of BKP demineralization, characterized by different ash contents, and their effects on the yields, structure and composition of the cellulose and lignin fractions (via Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, gel permeation chromatography, elemental analysis, and thermogravimetric analysis) will be presented. We shall also present results on how demineralization significantly improves bio-oil yields during reductive catalytic fractionation of the demineralized BKP. Process aspects, including mass transfer limitations and scalability, along with the fundamental and practical significance of our findings will be discussed.

CO₂ hydrogenation can yield a wide range of products, including CO, CH₄, methanol, long chain hydrocarbons and alcohols. However, achieving high selectivity for a single product with high activity remains a significant challenge. In this talk, I will share our recent two studies of converting CO₂ at two temperature regimes: high temperature reverse water-gas shift (RWGS) reaction to CO and intermediate temperature methanol synthesis. For RWGS, we show that 2-dimensional Mo₂C MXene nanosheets can 100% selectively convert CO₂ into CO at record high reaction rate at 873 K and extended reaction stability over 200hr. Kinetics and mechanistic investigations revealed a distinct entropy-favored RWGS reaction over the 2D carbides through a likely associative pathway. The results highlight the potential of MXene materials as metal-free catalysts for CO₂ hydrogenation to value-added products.
In the other study, SiO₂-supported Ni–In intermetallic compound (IMC) nanoparticles were investigated for methanol synthesis from CO₂ hydrogenation, with emphasis on the role of Ni:In composition in determining bulk and surface structures and catalytic performance. Ni₂In₃/SiO₂ achieved highest methanol selectivity (~70%) and a space–time yield (STY) a few times higher than the benchmark CuZnAl industry catalyst. Structural and surface analyses of fresh, operating, and spent catalysts revealed that Ni–In interactions critically tune the nature and density of active sites. The high density of surface InO domains together with the electron-rich NiIn IMC phase in Ni₂In₃/SiO₂ accounts for its superior methanol productivity compared with other IMCs and monometallic catalysts. This work provides new insights into the rational design of Ni-based IMCs as efficient catalysts for methanol synthesis from CO2.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science program.

Pyrolysis of waste plastics produces pyrolysis oils rich in alkenes (>50 wt.%). It has been demonstrated that the plastic pyrolysis oils can be converted into aldehydes, alcohols, carboxylic acids and amines using hydroformylation chemistry [1]. The distribution and rates of aldehyde formation are governed by isomerization-hydroformylation tandem reactions which influence the product selectivity. A quantitative predictions of the hydroformylation reaction is required to tune the product selectivity. In this study, we investigate the hydroformylation of plastic pyrolysis oil model compounds under mild conditions (<80 bar syngas, <453 K), offering insights into the reaction kinetics and the factors influencing formation rates of different aldehydes. We then outline how this insight can be used to improve the selectivity of hydroformylation of plastic pyrolysis oils.
[1] Li, H., et al., and Huber, G. W. Science, 381 (6658), 660-666 (2023).

Polyolefins are ubiquitous in consumer products but are notoriously difficult to recycle due to the inherent incompatibility of their common varieties. Current approaches to addressing this challenge often involve relatively complex syntheses or may compromise the properties of the parent materials. Here, a method is developed to compatibilize mixed polyolefins via acid–base interactions. With a single-step photocatalytic process, acid or base functionality can be readily installed onto polyolefins. The combination of acid- and base-modified polyolefins functions as compatibilizers. Incorporating them into polyolefin blends results in excellent mechanical strength, with up to a 96-fold increase in ductility (from 22% to 2110%). Importantly, compatibilization can be readily achieved on post-consumer polyolefin mixtures. Furthermore, direct functionalization and compatibilization of polyolefin blends is achieved.