(481h) New Insights into Oxidative Methanol Conversion through Operando Interrogation of the Near-Surface Gas Phase Above a Catalyst

Oxidative conversions of alcohols are critical transformations involved in the production of commodity chemicals, and methanol specifically serves as an important C1 feedstock for C2+ organic synthesis. Although not commonly experimentally investigated in the context of alcohol oxidations, it has been established that chemical events in the near-surface gas phase influence reaction outcomes, including yields of C2+ products.

Despite this influence, there has been a distinct lack of experimental data reported on the speciation of the near-surface gas phase for oxidative upgrading reactions of interest (here, near-surface refers to a region within a few millimeters of a catalyst particle surface). Here, we investigate the oxidative conversion of methanol over metal and metal alloy surfaces (eg, Ag, Pd, AuPd) with emphasis on interpreting the speciation of the near-surface gas phase and its relationship to stable product distributions measured in the effluent. In the presented studies, the near-surface region is interrogated through a novel combination of optical spectroscopy, Raman spectroscopy, and molecular-beam mass spectrometry with universal species detection capability, which has recently been developed by the authors for new investigations of heterogeneous catalytic processes involving both gas-phase and surface-mediated steps [ACS Catal. 2021, 11, 155−168].

The included figure shows representative results contrasting molecular-beam mass spectrometry signals obtained in the near-surface with high resolution (a) with that of a flow reactor effluent (b) for the same catalytic conditions. It is found (data not shown) that the near-surface region contains all stable products commonly observed in reactor studies of methanol oxidation over these metal catalysts (eg, formaldehyde, dimethyl ether, methyl formate, and dimethoxymethane), but additionally contains unstable or otherwise highly reactive species, including methyl radicals and methoxymethanol. All results are additionally interpreted in the context of density functional theory-derived microkinetic models, which are uniquely informed by measurements of product yields directly above surface sites.