Oxygen Vacancy Promoted O₂ Activation over Mesoporous Ni–Co Mixed Oxides for Aromatic Hydrocarbon Oxidation

Abstract

Whether the oxygen vacancies of heterogeneous catalysts improve their catalytic activity or not has recently been the topic of intense debate in the oxidation of hydrocarbons. We designed an effective strategy to construct mesoporous Ni–Co mixed oxides via a ligand-assisted self-assembly approach. The surface oxygen vacancy concentrations of the mesoporous Ni–Co mixed oxide catalysts were regulated by changing the doping amount of Ni or the reduction method, and the relationship between oxygen vacancies and catalytic activity was studied. Controlled experiments and DFT calculations revealed that oxygen molecules were more favorably adsorbed and activated on oxygen vacancies to form active oxygen species. Increasing the oxygen vacancy concentration within a certain range can effectively enrich the active oxygen species, therefore improving the oxidation rate of ethylbenzene. The optimized mCo3O4-0.1NiO catalyst exhibited a remarkable catalytic activity for the solvent-free oxidation of ethylbenzene to acetophenone, typically including 68.0% conversion and 95.4% selectivity (20 mg mCo3O4-0.1NiO, 10 mL ethylbenzene, and 0.6 MPa O2).

Expediting Ethylbenzene Oxidation via a Bimetallic Cobalt–Manganese Spinel Structure with a Modulated Electronic Environment

“…To sum up, a high AP yield of 69.7% (conversion: 77.9%; selectivity: 89.5%) can be obtained in the presence of 0.07 g of CMO catalyst at 130 °C for 6 h in 20 mL of EB using 0.7 MPa of O 2 . This AP yield value of 69.7% is higher than that of the recent reports using Co- or Mn-based catalysts for the selective oxidation of EB under similar reaction conditions, such as mesoporous Co 3 O 4 (37.8%), CeO 2 /Co 3 O 4 (57.6%), mesoporous Ni–Co mixed oxides (64.9%), MnO x /HTS (25.5%), Mn/N–C/Al 2 O 3 (27.5%), and tremella-like δ-MnO 2 (67.0%) …”

Manipulation of Electronic Effect and Assembly Architecture to Invoke Oxidation of Ethylbenzene by Hierarchical Co3O4 Wreaths

“…Based on these results, the optimal reaction condition for proceeding with the EB oxidation reaction catalyzed by the CCW can be summarized as the following: 0.08 g of CCW dispersed in 20 mL of EB with an oxygen pressure of 0.8 MPa at 130 °C for 6 h. Under this screened optimal reaction condition, the CCW catalyst can offer a 79.1% conversion of EB accompanied by a selectivity of 92.3% to AP. Compared with the literature reports, the CCW catalyst presents higher AP yield (73.0%) than the typical Co-based catalysts under similar reaction conditions, such as Co–N–C/CeO 2 (24.8%), Co/SBA-15 (53.4%), Ce 0.9 Co 0.1 O 2 (52.2%), Co–N–C/CNTs (14.5%), and Co 3 O 4 –NiO (64.9%) …”

Bridging Effect of Carbon Nitride with More Negative Conduction Potential and Halogens Promotes the Liquid-Phase Oxidation of Aromatic C–H Bonds

“…Based on these results, the EB oxidation mechanism is shown in Figure 10c: (1) Initially, EB was adsorbed onto the Br-CN catalyst surface and was oxidized to • R (A) via h + ; (2) then the Br-CN catalyst adsorbed O 2 , which was reduced to • O 2 − via e − ; (3) leading to the formation of intermediate C with A; (4) A could react directly with oxygen to form peroxyl radical B, (5) while C reacting with H + produced ethylbenzene hydroperoxide D, which was the most important intermediate in the EB oxidation process; (6) through the HAT process, B also generated D; (7) D dehydrated in the presence of the catalyst to produce the final product AP; (8) D was unstable and decomposed, and reacted with A to produce the byproduct PEA, along with radical E; (9) E reacted with EB to also produce the byproduct PEA. Under visible light conditions, due to the mild reaction conditions, the ethylbenzene hydroperoxide decomposition rate was low, so the reaction rates of steps (8) and (9) were much lower than that of step (7), and therefore the selectivity of the byproducts was lower. However, when • OH and 1 O 2 were inhibited, the conversion of EB was not affected, indicating that the entire reaction system proceeded through free radicals and that • O 2 − was the most important reactive oxygen species in the reaction system.…”

“…In contrast, heterogeneous catalysts, owing to their easy recovery and separation, 9 have been widely used, 7 but their reaction mechanisms are often relatively complex. 8 Despite the remarkable progress in photocatalysis in recent years, 10 the design of heterogeneous catalysts capable of selectively producing AP from EB under mild conditions remains a significant challenge. As illustrated in Scheme 1, previous researchers have conducted various studies on the solvent-free oxidation of EB.…”

“…Although homogeneous catalysts such as hydrobromic acid and Lewis acid have been employed for the solvent-free oxidation of EB, these catalysts suffer from disadvantages such as complex reactions, equipment corrosion, and difficult separation. In contrast, heterogeneous catalysts, owing to their easy recovery and separation, have been widely used, but their reaction mechanisms are often relatively complex . Despite the remarkable progress in photocatalysis in recent years, the design of heterogeneous catalysts capable of selectively producing AP from EB under mild conditions remains a significant challenge.…”