Bingjun Xu

Vapour-phase gold-surface-mediated coupling of aldehydes with methanol

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

Selectivity Control in Gold-Mediated Esterification of Methanol†

The Midas touch: The low-temperature transformation of methanol to methyl formate, formaldehyde, and formic acid is promoted by atomic oxygen adsorbed on metallic gold (see picture). The reactions occur with O-containing Au nanoparticles formed on Au(111) upon oxidation with ozone at 200 K; the facile esterification to methyl formate occurs well below room temperature.

Surface-Mediated Self-Coupling of Ethanol on Gold

The transformation of ethanol to its carbonyl compounds, namely acetaldehyde, ethyl acetate, acetic acid, and ketene, occurs on Au(111) with O-containing Au nanoparticles formed as a result of Au atom release upon ozone exposure. The product distribution strongly depends on the surface oxygen coverage. Ethoxy and acetate are identified as two key reaction intermediates during the oxidation of ethanol. The formation of acetaldehyde is due to the deprotonation of ethoxy, which can be further oxidized into acetate. The low-temperature formation of the ester, ethyl acetate, proceeds via the coupling of acetaldehyde with excess surface ethoxy. These reaction pathways appear relevant to heterogeneous processes catalyzed by supported gold nanoparticles, thus providing further insight into the mechanistic origin of gold-mediated oxidation of alcohols.

Achieving optimum selectivity in oxygen assisted alcohol cross-coupling on gold.

Oxidative coupling of alcohols is of great practical importance due to the wide range of synthetic applications using esters. Controlling the selectivity toward production of specific esters in cross-coupling is a long-sought goal in designing efficient synthetic routes. We report a quantitative study of the factors that determine the esterification selectivity in the oxidative cross-coupling of alcohols mediated by oxygen-covered Au(111). The high reactivity of gold is attributed to the activity of atomic oxygen bound to Au particles in forming surface-bound alkoxy species. The relative surface concentrations of alkoxys and the ease of the β-H elimination play critical roles in determining the product distribution. For a given reactant composition the surface concentration of alkoxys is skewed toward the higher molecular weight alcohol in the mixture exposed to the surface. Vibrational spectroscopic studies reveal that the relative stabilities of alkoxys parallel the gas phase acidities of the corresponding alcohols, in agreement with other coinage metal surfaces. Further, the activation energies for β-H cleavage of alkoxys are found to follow the descending order: E(methoxy) > E(ethoxy) > E(butoxy). The combination of these factors optimizes cross-coupling in a reactant mixture containing an excess of the lower molecular weight alcohol. The excellent match of product distribution between our low-pressure, single-crystal study and that observed for atmospheric pressure, liquid-phase reactions over supported gold catalysts provides strong evidence that the mechanistic insights gained from fundamental surface science study can serve as guiding principles in controlling selectivity under realistic catalytic conditions.

Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy

A universal correlation is established between HOR/HER activity and hydrogen binding energy on platinum-group metals. Understanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.

Highly Selective Acylation of Dimethylamine Mediated by Oxygen Atoms on Metallic Gold Surfaces

Throughout the ages, gold has been highly valued because of its seeming chemical inertness, its luster and beauty resulting from its resistance to bulk tarnishing reactions. However, the surface of gold is not completely inert, particularly in the presence of adsorbed oxygen. Indeed, there has been a resurgence of research on heterogeneous catalysis by gold recently due to its potential for developing environmentally benign processes, since Haruta s breakthrough observation of low-temperature CO oxidation on gold nanoparticles supported on reducible metal oxides. Gold particles supported on oxide surfaces selectively promote a wide range of reactions under various conditions, including aerobic oxidation of alcohols and amines, as well as acylation of amines as does unsupported gold powder. Herein, we report for the first time the vapor-phase, surface mediated acylation of an amine to an amide on metallic gold, and we establish a molecular-level mechanism for this process based on a specific characteristic of the adsorbed amide intermediate that provides a general basis for predicting such reactions. Amides are widely used in chemical synthesis, in pharmaceutical production, and in the synthesis of polymers, including nylon. Conventional methods for synthesizing amides use either activated acid derivatives, such as acid chlorides or anhydrides, or rearrangement reactions induced by an acid or base, which often produce toxic chemical waste. Amine acylation reactions catalyzed by homogeneous transitionmetal complexes in solution have been reported along with those on supported Au. Ideally, direct synthesis of amides through heterogeneous catalytic processes with high selectivity under environmentally benign conditions would be possible. The performance of catalytic processes can be improved through understanding the reaction mechanism at a molecular level so that the kinetics and selectivity of the overall process can be accurately predicted. A first step is to deconvolute the roles of gold and the oxide support. Our approach is to investigate O-covered Au(111), since without oxygen, Au is inert towards many reactions, including those of alcohols, aldehydes, and amines. 16–20] The general concept for amine acylation on O/Au(111) originates in the chemical nature of adsorbed oxygen and other nucleophilic adsorbates formed by selective deprotonation of their conjugate acids. For example, adsorbed O on Au surfaces activates alcohols, ammonia, 20] and amines through Brønsted acid–base reactions:

The Central Role of Bicarbonate in the Electrochemical Reduction of Carbon Dioxide on Gold

Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO2. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO2 reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO2, rather than the supplied CO2, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO2 near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO2.

Oxygen-assisted cross-coupling of methanol with alkyl alcohols on metallic gold

We demonstrate for the first time that selective cross-coupling of methanol with either ethanol or n-butanol occurs below room temperature on metallic gold with no metal oxide support in a reaction sequence that occurs entirely on the surface. The esterification proceeds via activation of the alcohols by adsorbed oxygen and a sequence of reactions that involve both surface-bound alkoxys and hemiacetals as intermediates. The reaction selectivity is dictated by competing β-hydride elimination from the alkoxys. Due to the higher activation energy for β-hydride elimination from methoxy, no formate esters are formed. A molecular-scale mechanism constructed using our results is in excellent agreement with studies of heterogeneous catalysts, providing insight into selectivity control under a broad range of conditions.

Structure–Property Relationships in Hydroxide‐Exchange Membranes with Cation Strings and High Ion‐Exchange Capacity

A series of poly(2,4-dimethyl-1,4-phenylene oxide) hydroxide-exchange membranes (HEMs) with cation strings containing a well-defined number of cations (CS-n) and similar, high ion-exchange capacities are synthesized to investigate the effect of cation distribution on key HEM properties. As the number of cations on each string grows, the size of the ionic clusters increases from 10 to 55 nm. Well-connected ion pathways and a hydrophobic framework are observed for n≥4. 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 cm(2)  mol(-1) for n≤3; and ∼34 cm(2)  mol(-1) for n≥4.

Heterogeneous Catalysis: A Central Science for a Sustainable Future

Developing active, selective, and energy efficient heterogeneous catalytic processes is key to a sustainable future because heterogeneous catalysis is at the center of the chemicals and energy industries. The design, testing, and implementation of robust and selective heterogeneous catalytic processes based on insights from fundamental studies could have a tremendous positive impact on the world.