Room-temperature tandem conversion of cyclic alkenes into 1,2-diols using molecular oxygen and β-MnO2 heterogeneous catalyst

Abstract

In the cyclic alkene epoxidation reactions using molecular oxygen as oxidant, aldehyde is generally added as co-oxidant to assist transition metal catalyst to activate the oxygen molecule, oxidizing cyclic alkene and producing the corresponding epoxide; however, the added aldehyde would be converted into byproduct carboxylic acid. Herein, we report the room-temperature tandem conversion of cyclic alkenes into 1,2-diols through the application of byproduct carboxylic acid as a catalyst for epoxide hydration reaction, realizing effective utilization of the byproduct and saving energy consumption. Four kinds of nanosized MnO₂ specimens, including α-, β-, δ-, and γ-MnO₂, were hydrothermally synthesized and applied as heterogeneous catalysts for the room-temperature tandem conversion of cyclohexene into 1,2-cyclohexanediol using molecular oxygen as oxidant and isobutyraldehyde as co-oxidant. The characterization results demonstrated that the obtained β-MnO₂ sample had the maximum Mn⁴⁺/Mn³⁺ ratio of 1.42 and the minimum O_ads/O_Latt ratio of 0.34, consistent with its least oxygen vacancies evidenced by EPR measurement. Accordingly, the β-MnO₂ sample revealed the quickest alkene epoxidation reaction rate and could achieve a 99.2% cyclohexene conversion within 1 h, and meanwhile, a 73.6% 1,2-cycloheanediol yield could be attained within 24 h. A plausible mechanism for the tandem conversion of cyclohexene into 1,2-cyclohexanediol over the β-MnO₂ heterogeneous catalyst was proposed and further verified by quenching experiments and DFT calculations. Furthermore, the synthesized β-MnO₂ catalyst sample could be reused more than ten times, and meanwhile, the tandem conversion way could be expanded to most of the cyclic alkene substrates, which demonstrated that the β-MnO₂ catalyst was an active and stable heterogeneous catalyst for tandem conversion of cyclic alkenes into 1,2-diols.