Non-oxidative Methane Activation over Molybdenum and Tungsten Nitride Catalysts

In this study, molybdenum (Mo), tungsten (W), and mixed metal oxide and -nitride catalysts are synthesized via incipient wetness impregnation on silica support (SBA-15) with a total target metal loading of 4 wt %. The effect of the Mo : W ratio on the non-oxidative methane conversion, product selectivity, and coke formation is evaluated. The reduced Mo/SBA-15 and MoW/SBA-15 catalysts show higher CH4 conversion activity to benzene (selectivity of similar to 75.5 %) than for ethylene (selectivity of similar to 9.5 %) with a rather fast coke formation rate of 19-24 mg(Coke) g(Cat)(-1) h(-1) without a clear influence of the Mo : W ratio. In contrast, the metal nitrides have a much smaller coke formation rate. The smallest rate of 0.45 mg(Coke) g(Cat)(-1) h(-1) is achieved for the WN/SBA-15 sample, which increases with the molybdenum content to 4.7 and 8.6 mg(Coke) g(Cat)(-1) h(-1) for the MoWN(5 : 1) and MoN/SBA-15 catalysts, respectively. There seems to be a yet unknown but negative correlation between the coke formation rate and the number of silanamine functional groups formed during the catalyst nitridation (Si-OH ->equivalent to Si-NH2 ->equivalent to Si-NH-Si equivalent to). The C2H4 selectivity is significantly increased with the relative amount of tungsten, with the highest C2H4 selectivity of 63 % for mixed-metal nitride catalyst containing Mo : W ratio of 1 : 5. While the Mo/SBA-15 the MoN/SBA-15 achieve a C2H4 selectivity of less than 10 %.

Automated summary

In this study, catalysts consisting of molybdenum, tungsten, and mixed metal oxide and nitride were synthesized and tested on a silica support. The results showed that the nitride catalysts had a lower coke formation rate and higher selectivity towards ethylene, while the Mo/SBA-15 and MoW/SBA-15 catalysts had higher selectivity towards benzene and a faster coke formation rate.