Copper-based deep eutectic solvents (Cu-DES) assisted the VPO catalyst as a structural and electronic promoter for n-butane selective oxidation

Currently, deep eutectic solvents (DESs), as an exclusive class of ionic liquid analogs, are extensively employed in various fields. Herein, vanadium phosphorus oxide (VPO) catalysts assisted by copper-based deep eutectic solvent (Cu-DES) as an electronic promoter and structure directing agent were ingeniously synthesized and exploited in the selective oxidation of n-butane to maleic anhydride (MA). The DES is efficient, cost effective and highly green compared to conventionally use metal inorganic modifiers. Cu-DES was prepared from a eutectic mixture of an organic salt (ChCl) and a metal salt hydrate/metal salt of cupric (Cu(NO₃)₂.3H₂O, CuCl₂) and cuprous (CuCl) salt with a molar ratio of 1:1–1:3, which notably improved the catalytic performance compared to that of an unpromoted VPO catalyst. The MA selectivity and n-butane conversion of Cu–DES–VPO increased significantly, while the CO_x (CO and CO₂) selectivity decreased. FTIR, DSC, XRD, SEM, EDS, TEM, Raman, TGA, XPS, and NH₃-TPD characterization techniques were utilized to fully analyze the DES, precursor and catalyst to explore MA selectivity. This work shifts the significant increase of 13% n-butane conversion, 11% MA selectivity and 16% MA mass yield, which greatly influences low carbon alkane utilization. Importantly, the CO: CO₂ ratio is reduced from 2.02 to 1.62, which is more beneficial for chemical plants as well as green environments.