Selective Regulation of Oxygen Transfer for Green Oxidation of N-Butane Under Mild Conditions: Performance and Mechanism

Abstract

Developing a green and efficient catalytic process for inert n-butane to produce Methyl-Ethyl-Ketone (MEK) is critical for the C4 chemical industry, however, realizing efficient activation of molecular oxygen under mild conditions to produce reactive oxygen intermediates and selectively transfer them to the target product still faces challenges. Herein, a synergistic catalytic strategy based on Proton-Coupled Electron Transfer (PCET) is developed, and the oxidation performance and mechanism of our Cu(TBBD) catalyst for air oxidation of n-butane in liquid-phase is evaluated and analyzed. Through meticulous optimization of process conditions, and comprehensive evaluation of reusability and scalability, this catalytic system shows excellent oxidation performance under mild reaction temperature. Besides, the reaction system is clean, and the products have high-value application prospects (MEK selectivity of 74%, and acetic acid selectivity of 26%). Thermodynamic and kinetic studies exhibit that the reaction system is endothermic and disordered, and can be well described by the zero-order dynamic with an apparent activation energy of 68.83 kJ/mol. The mechanism is revealed by some advanced instruments (in-situ IR, ESR, ESI-HRMS, etc.) and experimental derivations to be that oxygen molecular is efficiently synergistic activated by Cu(TBBD) and NHPI to generate alkoxy and peroxide free radical intermediates through the PCET process, which directly oxidize the substrate to produce the corresponding high-value chemicals. Moreover, DFT calculations further prove the rationality of the reaction mechanism. This work provides a reliable basis for reactor design and technology iteration of industrial oxidation of n-butane.

Keywords: Green chemistry, air oxidation, mild conditions, Methyl-Ethyl-Ketone, free radical mechanism