Preparation Of Rh-based Catalysts And Application On Conversion Of CH₄ Into CH₃Br Through Oxy-Bromination Path

Methane which is the main component of natural gas is mainly used for providing heat through direct burning.In addition,venting CH ₄ during crude oil extraction is not only wasteful but also a cause of greenhouse effect.To maximize effective use of CH₄,efforts were put in for its chemical transformation.However,it is difficult to activate the inert C-H bonds of CH₄.In general,the conversion of methane is carried out at high temperature and high pressure which is energy consuming.Among the known strategies,the halogen-based ones are attractive because the activation of methane C-H bonds can be achieved under relatively mild conditions?T<600°C,P?1 bar?.Herein,the paper focused on the effect of supportson catalytic performance of Rh-based catalysts in methane oxy-bromination?OBM?,especially on the CH₃Br selectivity.The reaction conditions were also optimized.In addition,the apparent activation energy was calculated.Firstly,the influence of SiO₂/Al₂O₃ ratio of HZSM-5 on methane oxy-bromination?OBM?over Rh/HZSM-5 catalysts was studied.It was found that a decrease of SiO₂/Al₂O₃ ratio would result in enhancement of catalytic activity,especially the selectivity to CH₃Br.At low SiO₂/Al₂O₃ ratio,the enrichment of acid sites on HZSM-5 is beneficial for Rh dispersion.The high CH₃Br selectivity can be attributed to the promoted HBr oxidation activity as a result of improved Rh dispersion on HZSM-5.The best catalytic performance was achieved over the0.06Rh/HZSM-5-50 catalyst,displaying a CH₄ conversion of 14.9%and a CH₃Br selectivity of 73.3%.After a short induction period,the catalyst exhibited stable performance in a test of over 20 h.Based on the obtained results,a possible reaction mechanism of OBM reaction over Rh/HZSM-5 catalysts is proposed.Secondly,to obtain a higher OBM reactivity,a series of Rh-based catalysts were synthesized and tested,and the 0.03Rh/SiC catalyst was found to be superior.The flow rate of reactant and the reaction temperature were investigated,and the best performance displaying a CH₄ conversion of 34.0%and a CH₃Br selectivity of 76.6%was achieved at 600°C when the flow rate of CH₄,O₂ and 40%HBr solution were 6.8cm³/min,4 cm³/min and 7.5 cm³/h,respectively.The reaction apparent activation energy was also calculated.