The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High-resolution solid-state NMR spectroscopy shows water interacting with zeolite Brønsted acid sites, converting them to hydrated hydronium ions over a wide range of temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2–9 water molecules per Brønsted acid site and is assigned to hydrated hydronium ions on the basis of the evolution of the signal with increasing water content, chemical shift calculations, and the direct comparison with HClO4 in water. The intensity of 1H–29Si cross-polarization signal first increased and then decreased with increasing water chemical potential. This indicates that hydrogen bonds between water molecules and the tetrahedrally coordinated aluminum in the zeolite lattice weaken with the formation of hydronium ion–water clusters and increase the mobility of protons. DFT-based ab initio molecular dynamics studies at multiple temperatures and water concentrations agree well with this interpretation. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occurs even in the presence of only one water molecule per acid site.

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Sustainable energy generation calls for a shift away from centralized, high-temperature, energy-intensive processes to decentralized, low-temperature conversions that can be powered by electricity produced from renewable sources. Electrocatalytic conversion of biomass-derived feedstocks would allow carbon recycling of distributed, energy-poor resources in the absence of sinks and sources of high-grade heat. Selective, efficient electrocatalysts that operate at low temperatures are needed for electrocatalytic hydrogenation (ECH) to upgrade the feedstocks. For effective generation of energy-dense chemicals and fuels, two design criteria must be met: (i) a high H:C ratio via ECH to allow for high-quality fuels and blends and (ii) a lower O:C ratio in the target molecules via electrochemical decarboxylation/deoxygenation to improve the stability of fuels and chemicals. The goal of this review is to determine whether the following questions have been sufficiently answered in the open literature, and if not, what additional information is required: What organic functionalities are accessible for electrocatalytic hydrogenation under a set of reaction conditions? How do substitutions and functionalities impact the activity and selectivity of ECH? What material properties cause an electrocatalyst to be active for ECH? Can general trends in ECH be formulated based on the type of electrocatalyst? What are the impacts of reaction conditions (electrolyte concentration, pH, operating potential) and reactor types?

Electrocatalytic hydrogenation is increasingly studied as an alternative to integrate the use of recycled carbon feedstocks with renewable energy sources. However, the abundant empiric observations available have not been correlated with fundamental properties of substrates and catalysts. In this study, we investigated electrocatalytic hydrogenation of a homologues series of carboxylic acids, ketones, phenolics, and aldehydes on a variety of metals (Pd, Rh, Ru, Cu, Ni, Zn, and Co). We found that the rates of carbonyl reduction in aldehydes correlate with the corresponding binding energies between the aldehydes and the metals according to the Sabatier principle. That is, the highest rates are obtained at intermediate binding energies. The rates of H2 evolution that occur in parallel to hydrogenation also correlate with the H-metal binding energies, following the same volcano-type behavior. Within the boundaries of this model (e.g., compounds reactive at room temperature and without important steric effects over the carbonyl group), the reported correlations help to explain the complex trends derived from the experimental observations, allowing for the correlation of rates with binding energies and the differentiation of mechanistic routes.

Electrocatalytic hydrogenation and catalytic thermal hydrogenation of substituted phenols and diaryl ethers were studied on carbon-supported Rh. The rates of electrocatalytic hydrogenation increase with increasingly negative potentials, which have been related with the coverage of adsorbed hydrogen. The lowest and highest negative potentials in electrocatalytic hydrogenation correspond to the onset of H 2 evolution and to the onset of reactions involving the electrolyte, respectively. For electrocatalytic and catalytic thermal hydrogen addition reactions, the dominant reaction pathway is hydrogenation to cyclic alcohols and cycloalkyl ethers. The presence of substituting methyl or methoxy groups led to lower rates compared to unsubstituted phenol or diphenyl ether. Methoxy or benzyloxy groups, however, undergo CO bond cleavage via hydrogenolysis and hydrolysis (minor pathway). The surface chemical potential of hydrogen can be increased also by generating a H 2 atmosphere above the reaction media, supporting the conclusion that thermal and electrochemical routes share the same reaction pathways.

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Mononuclear and dinuclear copper species were synthesized at the nodes of an NU-1000 metal–organic framework (MOF) via cation exchange and subsequent oxidation at 200 °C in oxygen. Copper-exchanged MOFs are active for selectively converting methane to methanol at 150–200 °C. At 150 °C and 1 bar methane, approximately a third of the copper centers are involved in converting methane to methanol. Methanol productivity increased by 3–4-fold and selectivity increased from 70% to 90% by increasing the methane pressure from 1 to 40 bar. Density functional theory showed that reaction pathways on various copper sites are able to convert methane to methanol, the copper oxyl sites with much lower free energies of activation. Combining studies of the stoichiometric activity with characterization by in situ X-ray absorption spectroscopy and density functional theory, we conclude that dehydrated dinuclear copper oxyl sites formed after activation at 200 °C are responsible for the activity.

Electrocatalytic reduction of benzaldehyde to benzyl alcohol on Pd supported on carbon felt was conducted in the aqueous phase using a continuous flow fixed-bed reactor at room temperature and atmospheric pressure. Methanol, ethanol, or isopropanol was added to the electrolyte to study the impact of alcohol type and concentration on the rates of benzaldehyde electrocatalytic hydrogenation (ECH) and H2 evolution, which is the prevalent side reaction. Whereas the ECH rates and Faradaic efficiency decreased with increasing alcohol concentrations, H2 evolution rates remained constant. The impact of the alcohol on hydrogenation was greater as the length of the alcohol’s hydrocarbon chain increased. Increasing the benzaldehyde concentration allows for high ECH rates and high Faradaic efficiency. The reaction order increased from ∼0.13 to ∼0.66 with half-cell potential increasing from −650 to −1150 mV (vs Ag/AgCl). Kinetic analysis reveals that the changes in reaction order are due to changes in benzaldehyde (and H) surface coverages as a function of half-cell cathodic potential. Thus, the results shown here reveal how the performance of the continuous electrocatalytic operation is affected by the electrolyte composition and half-cell cathodic potential.