A Novel Method For Methane Dehydroaromatization Reaction Over Mo/HZSM-5 Catalysts

Methane conversion to aromatization under non-oxidative conditions (MDA Reaction) is a promising utilization route of natural gas. Mo/HZSM-5 is a bifuctional catalyst and performs well in MDA reaction, molybdenum carbides and/or oxycarbides and Bronsted acid sites are active sites. CH4 is reducted into C2Hx species over Mo species, while Bronsted acid sites are responsible for the oligomerization of C2Hx to aromatics.Preparation conditions such as Mo loadings, calcination temperature, Si/Al ratio of the zeolites have significant influence on Mo species location, Bronsted acid sites and the interaction between Mo and HZSM-5. We prepared a series of Mo/HZSM-5 and investigated their catalytic performance for MDA reaction, then find that 6Mo/HZSM-5 catalyst roasted in 773K and Si/Al raito of which is 38, performed best. The high temperature peak in TG-DSC profiles was attributed to polyaromatic-type carbon, while the low temperature peak included the graphite-like C as a component of Mo2C and/or MoOxCy and the filamentous carbon deposition on Mo2C and/or MoOxCy. The amout of polyaromatic-type carbon increased with time-on-stream, leading to the decrease of Bronsted acid sites. That was the main reason for catalyst deactivation.We selected methanol as a co-reactant with methane, thus introducted Fridel-Crafts Alkylation reaction into MDA reaction. After investigating the effect of methanol on MDA reaction performance under selected conditions, we found that CH4 conversion was increased from 11.8% to 22.7%, and the catalytic activity remained stable after 40h which meant that the stability of the catalysts was enhanced significantly. The distribution of aromatics changed significantly, the selectivity of benzene and naphthalene decreased sharply, while the amount of toluene, dimethylbenzene and trimethybenzene increased. The analysis of 1H MAS NMR、NH3-TPD、TG-DSC、TEM revealed that the amount of polyaromatic-type carbon reduced, while the amout of Bronsted acid sites kept stable during reaction process. Benzene reacted with methanol to produce C7-C9 aromatics, and then the oligomerization of benzene to naphthalene and polyaromatic-type aromatics was suppressed. The consumption of intermediate and decrease of carbon deposition which caused catalyst deactivation leaded to a high CH4 converson and better catalyst stability.We also used NaOH solution to treat HZSM-5 and then obtained hierarchical HZSM-5. Alkali-treament could creat mesoporous structure by removing several Si in zeolite frameworks. CH4 conversion and catalytic stability of hierarchical HZSM-5 was improved, the improvement was attributed to the fully acess of CH4 with active sites and better diffusion of intermediate in mesoporous structure. A combined hierarchical HZSM-5 and methanol addition has also showed a better improvement effect on the MDA performance.