Methane Aromatization Over MCM-49 Zeolites

Methane is the main component of natural gas. The activation and conversion of methane is the key of natual gas chemical engineering.Since the first non-oxidative aromatization catalyst Mo/HZSM-5 was found in 1993, methane aromatization in the absence of oxidant has become a valuable and challengeable research in both academic and applied significance on methane conversion. Mo/HZSM-5 is the most important catalyst that has been under intensive scrutiny. About 9-11% of methane conversion, 50-70% of benzene selectivity was obtained over Mo/HZSM-5 and the derived catalysts. Although much attention has been paid to the metal supported HZSM-5 catalysts, further improvement of the catalytic performance was imperatively desirable. Recently, as a new aromatization catalyst, Mo/HMCM-49 was much accounted of the researchers due to high stability. In this paper, Mo/HMCM-49 catalysts were prepared and tested as the catalyst of methane aromatization in a fixed bed reactor under systematic reaction conditions. Nanosize MCM-49 zeolite was synthesized under the conditions of high temperature and low content of Na and water.MCM-49 zeolite support with Si/Al2O3=12.5 was synthesized by a hydro-thermal procedure using hexamethyleneimine (HMI) as the template and silica gel as the silica source. The original slurry was obtained by stirring the mixture of certain amounts of sodium aluminate, sodium hydroxide, HMI, silica gel and water. After being crystallized at 443 K with rotating rate of 60 rpm for 3 days. The MCM-49 zeolite synthesized here has highly crystallinity and purity. On transmission electron micrographs, the crystals are uniform, thin regular hexagonal sheets, about 300-500 nm in diameter and 20-50 nm in thickness. Catalytic performance of methane aromatization over Mo/HMCM-49 with different Mo contents was investigated in detail under various reaction conditions. The optimal methane conversion could reach 12% with benzene selectivity of 90% under following typical conditions: wt.6% Mo content catalyst, the reaction temperature is 973 K and feed space velocity is 1500 mL/g·h. The optimal catalysts of Mo/HMCM-49 can maintain a lifetime over 150 hours that has not been reported before. The particular performance of Mo/HMCM-49 on the improvement of methane aromatization reaction compared with HZSM-5 support metal, has shown more promising application value. Mo/HMCM-49 has a long lifetime not only in aromatization reaction of methane but also in aromatization of actual natual gas with high yield of benzene. Ion-exechange on MCM-49 can't improve the catalytic performance of aromatization. The reaction aromatization in mixed gas of propane and methane was carried out and the conversion of methane was not observed. MCM-22 has conformable structure of MWW type but different aluminum content and distribution with MCM-49.It also has a excellent performance of methane aromatization reaction.But MCM-56,which has low acid intensity and has no supercage,is not a good choice of methane aromatization. We consider that the difference of pore structure between HMCM-49 and...