The Study Of Non-oxidative Aromatization Of Methane With Propane

Conversion of methane to aromatics by low-temperature activation in the presence of higher hydrocarbon is a new way of utilization of methane and natural gas. In this paper, the non-oxidative activation and aromatization of methane with propane as co-reactant over metal-loaded HZSM-5 catalysts, especially Mo/HZSM-5 and Pd-Ga/HZSM-5 were studied. The activity, carbon deposition, CH₄ adsorption, each effect of alkanes were investigated. Several conclusions were drawn from these investigations.Methane non-oxidative aromatization using propane as co-reactant was achieved over Mo/HZSM-5 and Pd-Ga/HZSM-5. The conversion of methane and propane were 7.9% and 87.8%, respectively, and the selectivity of aromatics was as high as 48.9% over Mo loaded HZSM-5 and the above values were 15.4%,48.7% and 68.6% over Pd-Ga/HZSM-5 at 823 K and GHSV (C1+C3+N₂)=410 cm³g^-1h^-1 with C1/C3 molar ratio= 5:1, C1/N₂ molar ratio=10:1.CH₄ incorporation has been conclusively conformed by mass spectral analysis of aromatics produced in a broadened pulse of ¹3CH₄ and C₃H₈ under identical condition, which show significant ¹3C enrichment in the aromatics fragments. And propane dehydroaromatization was affected by methane.The induction period of methane aromatization was studied by TPSR, DTA and CH₄ pulse. It was found the catalysts were reduced and carburized by alkanes and formed not only the active centre, but also the hydrogen-rich carbonaceous (C_xH_y) over Mo/HZSM-5. Thus, the factors required for methane aromatization are complete. Propane promoted the reduction of catalyst and the formation of hydrogen-rich carbonaceous (C_xH_y) thus shortened the induction periods. Over Pd-Ga/HZSM-5 catalyst, the propane cracking maintain the needed hydrogen-rich carbonaceous (C_xH_y) to aromatization.CH₄ pulse, TPO, TPH and TPCO₂ results indicated that in the process of aromatization propane would transform to the fragments (C_xH_y), then C_xH_y would attack and activate CH₄. The activated CH₄ would dehydrogenate and form intermediates. The intermediates which deposited over catalyst surface were called active carbon deposits. Active carbon deposits could react with hydrogen to form all kinds of products. C₃H₈ was much easier than CH₄ to form enough active carbon deposits needed for aromatization. UV-Ramen and CH₄ pulse results indicated that this kinds of coke were identical in both propane alone system and in propane with methane system.Both TPH and TPCO₂ could eliminate coke at certain temperature which showed in TPO. TPO results conformed that the alone reactant system would be effected by adding methane or propane.The CH₄-TPD-MS experiments gave rich information about adsorption, desorption, and decomposition of CH₄. It was found that the CH₄ absorption was related closely to reaction performance of co-aromatization of methane with propane. At 823 K and /or lower temperature, methane could adsorb or even adsorb dissociatively over Mo, Pd-Ga, La-Zn, Ag, Cr and La-Zn loaded zeolites. And the adequate key intermediate ethene was needed for CH₄ directional convert to aromatics. The absorption of hydrogen affected the stability of catalysts and propane conversion. And we hope that the method can provide a new way for the study of the activation and reaction mechanisms of methane.