Brian’s recent article published in the Journal of Catalysis elucidates the reaction mechanism for methyl lactate dehydration to acrylic acid (and esters) over zeolite NaY. Expanding on our previous work covering ion exchange between adsorbed molecules and zeolites (Topics in Catalysis 2015), we show that Brønsted acid sites can be generated in-situ on the catalyst surface via interactions with water, and are subsequently the primary active sites for the desired dehydration pathway. The dissociative adsorption of methyl lactate to form adsorbed sodium lactate and a surface methyl group is the key branching point between the dehydration (desired) and decarbonylation (undesired) pathways, as undissociated methyl lactate rapidly reacts to acetaldehyde over Brønsted acid sites. The mechanistic level understanding of the active sites and species involved in this key reaction will allow for the rational design of new catalytic systems.
In a recent article published in Science Advances, we revealed a universal correlation between HOR/HER activity and underpotential deposited hydrogen (Hupd) desorption peak potential Epeak or hydrogen binding energy (HBE) for four supported platinum-group-metal catalysts (Pt/C, Ir/C, Pd/C and Rh/C) over a broad pH range (0-13), indicating that the HBE is the dominant descriptor for HOR/HER activities. The results were covered by phys.org.
Matt’s article in ACS Catalysis reviews the mechanisms and applications of catalytic transfer hydrogenation in heterogeneously catalyzed biomass upgrade processes.
Marco’s recent work investigates the behavior of water at the water-Au interface as a function of potential in acid, neutral, and alkaline electrolytes using attenuated total reflectance surface enhanced infrared spectroscopy. In acid, sharp νOH peaks appear at 3583 cm-1 at high potentials attributed to non-H-bonded water coadsorbed in the hydration sphere of perchlorate near the electrode surface, as shown previously in the literature. An adsorbed hydronium bending mode at near 1680 cm-1 is observed at low potentials in low pH solutions (1.4, 4.0, 6.8). At high pH (10.0, 12.3), a potential-dependent OH stretching band assigned to adsorbed hydroxide emerges from 3400-3506 cm-1. The observation of adsorbed hydroxide, even on a weakly oxophilic metal such as Au, provides the framework for further studies of hydroxide adsorption on other electrodes to determine the role of adsorbed hydroxide on important reactions such as the hydrogen oxidation reaction.
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In Junhua’s recent work, a series of poly(2,4-dimethyl-1,4-phenylene oxide) hydroxide-exchange membranes (HEMs) with cation strings of a well-defined number of cations (1 to 6 denoted as CS-1 to CS-6) and similar, high ion-exchange capacities are synthesized to investigate the effect of cation distribution on key hydroxide exchange membrane properties. As the number of cations on each string grows, the size of ionic clusters increases from 10 to 55 nm. Well-connected ion pathways and hydrophobic framework are observed for n = 4 and above. The enhanced phase segregation increases the hydroxide conductivity from CS-1 to CS-6 (30 to 65 mS cm-1) and suppresses the water uptake (from 143% to 62%). Moreover, molar hydroxide conductivities for CS-n membranes show two distinctive stages as n increases: ~23 S cm2 mol-1 for n = 3 or below; and ~34 cm2 mol-1 for n greater than or equal to 4.
Fundamental understanding of the active sites mediating the hydrogen oxidation/evolution reactions (HOR/HER) is critical to the design of an efficient HOR/HER electrocatalysts for affordable hydrogen exchange membrane fuel cells and electrolyzers. HOR/HER activities in terms of exchange current density were systematically investigated on carbon supported Ir nanoparticles with size from 3 to 12 nm in alkaline electrolyte. The portion of the sites with the lowest hydrogen binding energy (HBE) increases with the increase of the particle size or the decrease of the total electrochemical active surface area (t-ECSA). The HOR/HER activity normalized to t-ECSA decreases as t-ECSA increases while remaining constant when normalized to the surface area of the sites with an average HBE of – 0.33 eV, indicating that those sites, accounting for only about 15 to 30 % of the total sites, shoulder the great majority of the HOR/HER activity.
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Hydrodeoxygenation processes via catalytic transfer hydrogenation (CTH) have provided a sustainable and selective method in converting biomass-derived furfural to 2-methylfuran, a potential drop-in fuel additive. Although high yields have been attained (roughly 80%) over a Ru/RuOx/C catalyst using 2-propanol as the hydrogen source, mechanistic understanding of the cascade reaction has remained elusive. Through a series of isotopic labeling and kinetic studies using perdeuterated 2-propanol (2-propanol-d8), we have shown key insights into the synergistic effect of both the Ru and RuOx phases. Hydrogenation of furfural proceeds primarily through a Lewis acid-catalyzed Meerwein-Pondoorf-Verley (MPV) hydrogenation over the RuOx sites, where a hydrogen atom is transferred in one, concerted step. Meanwhile, we have shown the existence of ring activation as a major, if not dominant, pathway in the hydrogenolysis of furfuryl alcohol to 2-methylfuran.
For more information, visit: http://pubs.acs.org/doi/abs/10.1021/acscatal.5b00586
Biomass-derived furanics offer a unique pathway toward a variety of drop-in fuel additives and commodity chemicals. However, furanics derived from biomass tend to be highly oxygenated and must be selectively reduced for use in commercial processes. We have recently shown that ring activation might play a key role in selectively upgrading these furanics in pathways such as ring opening, ring saturation, or substituent reduction.
For more information, visit: http://pubs.acs.org/doi/full/10.1021/jp512649b
This month, Brian had a paper accepted for publication by Topics in Catalysis. In this work, we employed in-situ transmission Fourier transform infrared (FTIR) spectroscopy is to investigate the ion exchange between acidic hydrogen in organic molecules adsorbed on a surface from the gas phase and sodium cations in zeolites. We demonstrated that this process produces Brønsted acid sites and the corresponding adsorbed salt species by identifying signature spectroscopic bands. Furthermore, the gas-phase deprotonation enthalpy (GPDE) of the organic compounds is identified as a key descriptor in determining the feasibility and extent of the exchange process. FTIR data for acetic acid is shown above, and the bands at 3645 wavenumbers (Brønsted acid sites) and 1585 wavenumbers (sodium acetate) are of particular interest. These data indicate that the degree of dissociation of the acidic hydrogen is a key element in the ion exchange. The generality of this process in zeolites was established by the observation of similar results on zeolites with differing topologies (FAU, MFI, *BEA, and MOR).