Effect of promoter in the oxidative coupling of methane over synthesized Mn/SiO2 nanocatalysts via incipient wetness impregnation

The M–Na–Mn/SiO2 nanocatalysts (M = W, Mo, Nb, V, Cr) were synthesized with the size of 12–92 nm by incipient wetness impregnation method to study the effect of different promoters on the catalytic performance in the oxidative coupling of methane. The results at 1 atm, 1048 K, 2500 ml h−1 g−1, and CH4/O2/N2 = 2/2/1 revealed that C2 selectivity was significantly increased (31.6%) in the order of W > Mo > Nb > Cr > V whereas moderate enhancement (12.6%) was observed in the CH4 conversion in the order of W > Cr > Nb > Mo > V. The results of the characterization techniques (Raman, FT-IR, BET, TGA/DTA and XRD) demonstrated that Mn2O3 and α-cristobalite were the predominant species and active sites in the nanocatalyst surface and Na2MoO4, Na2WO4 and Mn2O3 crystalline phases contributed to achieving high selectivity of C2 products. The redox mechanism involving two metal sites such as Mn3+/2+ and W6+/5+ or Mn3+/2+ and Mo6+/5+ was found to be the most compatible route with the OCM reaction path in which CH4 and O2 adsorption was the controlling step.