A Reusable Unsupported Rhenium Nanocrystalline Catalyst for Acceptorless Dehydrogenation of Alcohols through γ‐C–H Activation

Abstract

Rhenium nanocrystalline particles (Re NPs), of 2 nm size, were prepared from NH 4 ReO 4 under mild conditions in neat alcohol. The unsupported Re NPs convert secondary and benzylic alcohols to ketones and aldehydes, respectively, through catalytic acceptorless dehydrogenation (AD). The oxidant-and acceptor-free neat dehydrogenation of alcohols to obtain dihydrogen gas is a green and atomeconomical process for making carbonyl compounds. Secondary aliphatic alcohols give quantitative conversion and yield. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Re K-edge X-ray absorption nearedge structure (XANES), and X-ray absorption fine structure (EXAFS) data confirmed the characterization of the Re NPs as metallic rhenium with surface oxidation to rhenium(IV) oxide (ReO 2 ). Isotope labeling experiments revealed a novel g-CH activation mechanism for AD of alcohols.

Deoxydehydration of Biomass-Derived Polyols with a Reusable Unsupported Rhenium Nanoparticles Catalyst

“…Primary alcohols and diols were not as effective. This is in agreement with ReO x NPs acting as a dehydrogenation catalyst for alcohols and having the highest activity with secondary alcohols as has been noted previously . The effect of 3-octanol versus glycerol concentrations on the DODH reaction was investigated in the range of [glycerol] = 0.6–4.2 M (Figure S2b).…”

“…For the homogeneous methyltrioxorhenium (MTO) DODH catalyst with glycerol as both substrate and reductant, the KIE of d 5 -glycerol–(OH) 3 was reported to be 2.4, indicating involvement of the C–H/D bond of glycerol in the rate-determining step . Reduction of Re species in DODH reaction in the presence of 3-octanol is associated with dehydrogenation of 3-octanol by ReO x NPs to give 3-octanone . In this study, glycerol–(OD) 3 in the presence of either 3-octanol or 3-(OD)-octanol showed no kinetic isotope effect (Figure S14).…”

“…The EXAFS spectrum of the as-prepared ReO x NPs revealed three different scattering paths which include two Re–O bond lengths of 1.73 ± 0.01 Å (consistent with ReO double bond) and 2.02 ± 0.01 Å (consistent with Re–O single bond) and a Re–Re scattering at 2.57 ± 0.01 Å. Based on the reported range of rhenium standards, the longer Re–O path and Re–Re scattering might represent Re 5+ oxorhenium cluster, while the shorter Re–O bond length is close to Re 7+ standards (i.e., 1.73 Å). , It is reported that EXAFS analysis of oxorhenium­(V) clusters in Re–Pd/C catalysts showed Re–O path and Re–Re path with bond lengths of 2.03 and 2.57 Å . As the catalyst goes through more reaction cycles, the shorter Re–O path (1.73–1.76 Å) decreases in intensity while the longer Re–O scattering (2.02 Å) increases in amplitude.…”

“…We have found that unsupported ReO x nanoparticles reported earlier by our group for alcohol dehydrogenation afford heterogeneous DODH of polyols with high recyclability and provide a good system to study for mechanistic insights without the influence of a support. Investigations of ReO x NPs in DODH via in situ and ex situ X-ray spectroscopic analysis, kinetics studies, and kinetic isotope effect measurements have been conducted to elucidate heterogeneous DODH mechanism driven by alcohol reductants.…”

“…All other reagents were purchased from Sigma-Aldrich or Alfa Aesar. Unsupported rhenium oxide nanoparticles (ReO x NPs) were prepared in neat alcohol as reported previously . In order to simplify synthesis process and avoid formation of metallic rhenium, the nanoparticles were prepared under ambient atmosphere instead of under nitrogen.…”