Intermediate stabilization for tuning photocatalytic selective oxidation of CH4 to CH3OH over Co3O4/ZnO

Selective catalytic oxidation plays a crucial role in both fine chemical and petrochemical industries for the production of significant intermediates, however, it remains challenging to obtain the target product selectively when multiple reaction paths coexist. To overcome this challenge, the recently developed single atom catalysts provide opportunities for highly selective catalytic oxidation, whose catalytic behaviors are distinct from those of nanoclusters and nanoparticles. This review is devoted to summarizing the recent advances in the exploitation of novel catalytic materials (single atoms, nanoclusters and nanoparticles) for four challenging selective oxidation reactions in terms of adsorption, activation and reaction, including selective oxidation of methane, aerobic oxidation of alcohols, epoxidation of alkenes, and preferential oxidation of carbon monoxide in hydrogen. The key factors affecting the catalytic performance, especially the electronic and geometric structures of the single atoms, nanoclusters, and nanoparticles will be discussed. A deep understanding of the active species, active structures, activity-structure relationship and mechanisms for these catalytic systems are highlighted, and the current challenges and future developments are also provided, with the aim to give guidance for the design of efficient and highly selective catalysts for heterogeneous oxidation reactions, and to better understand their catalytic behaviors in a unified way. 

Heterogeneous selective oxidation over supported metal catalysts: from nanoparticles to single atoms

Elucidation of single atom catalysts for energy and sustainable chemical production: Synthesis, characterization and frontier science

: Developing highly efficient catalysts for chemoselective oxidation of methane to methanol under mild conditions is a grand challenge. We report the successful design and synthesis of a heterogeneous single-site cobalt hydroxide catalyst [Ce-UiO-Co(OH)] supported by the nodes of a cerium metal − organic framework (Ce-UiO-66 MOF), which is efficient in partial methane oxidation using hydrogen peroxide at 80 ° C, giving an extraordinarily high methanol yield of 2166 mmol g cat − 1 in 99% selectivity with a turnover number of 3250. The Ce-UiO-Co catalyst is significantly more active and selective than its iso-structural zirconium analogue Zr-UiO-Co in methane to methanol conversion. Experimental and computational studies suggest the formation of the Co III ( η 2 -hydroperoxide) intermediate coordinating with one μ 4 -O − and two neutral carboxylate oxygens of Ce 4+ oxo nodes within the pores of Ce-UiO-66, which undergoes σ -bond metathesis with the methane C − H bond in the turnover limiting step of the catalytic cycle. The significantly lower activation energy of Ce-UiO-Co than Zr-UiO-Co is due to the highly electron-deficient nature of the cobalt ion of the Co( η 2 -O 2 H) species supported by the Ce-UiO nodes, which promotes facile C − H activation of methane via σ -bond metathesis. This MOF-based catalyst design holds promise in developing molecular electrophilic abundant metal catalysts for chemoselective functionalization of saturated hydrocarbons.

Metal–Organic Framework-Encaged Monomeric Cobalt(III) Hydroperoxides Enable Chemoselective Methane Oxidation to Methanol

Methane is one of the promising alternatives to non-renewable petroleum resources since it can be transformed into added-value hydrocarbon feedstocks through suitable reactions. The conversion of methane to methanol with a higher chemical value has recently attracted much attention. The selective oxidation of methane to methanol is often considered a “holy grail” reaction in catalysis. However, methanol production through the thermal catalytic process is thermodynamically and economically unfavorable due to its high energy consumption, low catalyst stability, and complex reactor maintenance. Photocatalytic technology offers great potential to carry out unfavorable reactions under mild conditions. Many in-depth studies have been carried out on the photocatalytic conversion of methane to methanol. This review will comprehensively provide recent progress in the photocatalytic oxidation of methane to methanol based on materials and engineering perspectives. Several aspects are considered, such as the type of semiconductor-based photocatalyst (tungsten, titania, zinc, etc.), structure modification of photocatalyst (doping, heterojunction, surface modification, crystal facet re-arrangement, and electron scavenger), factors affecting the reaction process (physiochemical characteristic of photocatalyst, operational condition, and reactor configuration), and briefly proposed reaction mechanism. Analysis of existing challenges and recommendations for the future development of photocatalytic technology for methane to methanol conversion is also highlighted.

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Recent Advances in Photocatalytic Oxidation of Methane to Methanol

Atomic-scale metals as active center have been widely investigated for efficient photocatalysis. Understanding the specific electronic structure of atomic-scale center is of frofound fundamental importance for superior catalytic performance. Here, we report an atomically dispersed gold on tungsten trioxide (Au1/WO3) catalyst for photocatalytic oxidation of methane toward value-added methanol. The Au1 species reveal a specific tip-enhanced local electrons field which favors the C-H dehydrogenation of methane and thus form methanol (up to 589 µmol g-1 h-1). Both experimental and theoretical results demonstrate such tip-enhanced effect-dependent activity of methane oxidation. The theoretical calculations further reveal a lower adsorption energy of product methanol on Au1, in contrast to Au particles, which suppresses the overoxidation of methanol, and thus promotes its selectivity. Establishing the electric structure-activity may create a platform for designing efficient atomic-scale catalysts for C1 catalysis and green chemistry.

Zeng Yi

Papers

Photocatalytic Oxidation of Methane to Methanol by Tungsten Trioxide-supported Atomic Gold at Room Temperature