Methane Aromatization: Effect Of Different Additives And Study Of Novel Zeolite Supporters

Methane dehydro-aromatization (MDA) over zeolite supported molybdenumcatalysts is a novel way for direct catalytic conversion of methane into valuablechemicals.Mo/HZSM-5 catalysts show good catalytic performance in MDA. However,the formation of carbonaceous deposits during the reaction results in thedeactivation of the catalyst. Modifying the catalyst and introducing anoxygen-containing reactant additive are two approaches to enhance the stability ofthe catalyst. In this work, we selected dimethyl ether (DME) as anoxygen-containing additive and investigated the effect of DME co-feed on thereaction performance of MDA under different reaction conditions. The resultsshowed that an appropriate amount of DME added to the CH₄ feed was effectivein increasing the benzene yield and the catalyst's stability. It was found that highertemperatures, higher Mo loadings and a second assistant metal such asRe/Fe/Co/Rh were in favor of the aromatization of CH4/DME co-feed. TG andTPO results revealed that the amount of the coke on the used catalyst decreasedand TEM images showed that the fiber carbon deposits disappeared when DMEwas added to the CH4 feed. It was suggested that the coke on the B acid sites wassuppressed. The effect of H₂O co-feed on the reaction performance of MDA wasinvestigated in a stacked two-bed micro-reactor. It was found that H2O was able toplay a remarkable promotional role as well as CO2 or even better in enhancing thestability of Mo/HZSM-5 catalyst, and did not impaired the catalytic activity of thecatalyst, on condition that H2O in the feed-gas was controlled in a little amountlimit. Zeolite NU-87 has a pore system built of parallel 10-member ring (MR)channels with a diameter of 4.8×5.7 ?2 linked together by 12-MR cavities whichare accessible only via the 10-MR windows of the channels. Herein, we firstlyinvestigated the catalytic performance of MDA over 6Mo/HNU-87 catalyst. Thereaction results revealed that 6Mo/HNU-87 was not a good catalyst for MDAalthough the structure of zeolite NU-87 was very analogous to that of thesupercages of zeolite MCM-22 as well as zeolite HNU-87 had an acidity similarto zeolite HMCM-22. One-dimensional micro-and meso-porous molecular sieveHZSM-22, HZSM-23 and Al-SBA-15 (post synthesized) were also investigated.The results showed that they all were not the good supporters of catalyst for MDA.But interestingly, the initial CH4 conversion over 6Mo/Al-SBA-15 was veryremarkable. This implicated that the molybdenum supported meso-porousmolecular sieve with weak acidity was also effective in activating CH4,transforming into coke. Here, we further studied the catalytic characteristic of theexcellent catalyst 6Mo/HMCM-22, demonstrating that MDA mainly carried out inthe large 12-MR supercages with 10-MR windows or the 10-MR sinusoidalchannels, whereas that the external 12-MR hemicages maintained the benzeneyield at a plateau value.