Aromatization Of Methyl Bromide On Modified HZSM-5 Catalyst

Aromatic compounds are important chemical feed stock and solvent, in which light aromatic hydrocarbons such as benzene, toluene and xylene (BTX) are most useful chemicals. They are widely used in dyeing industry, agricultural chemicals manufacture, spice production, paint fabrication, pharmacy and so on. The manufacture of aromatics in traditional industry mostly comes from catalytic reforming of straight run naphtha. Based on the investigations of our laboratory, the synthesis of CH3Br from oxidative bromination of methane, we developed a new technology on the aromatization of CH3Br to product aromatics (especially BTX).Above all, molecular zeolites HZSM-5 with various SiO2/Al2O3 ratios, the loading of different active compounds, the content of active compounds and many other factors were tested to evaluate the catalytic performances in terms of methyl bromide conversion and product distribution. As the results shown, HZSM-5 with SiO2/Al2O3 ratios of 100 was found to have the highest BTX selectivity. The primary products were C7, Cg, C9 aromatics and C3, C4 aliphatic compounds.And then Cr, Mo and Zn were confirmed to be the best three modifiers. Among these three modifiers, modifier Cr was the most excellent one on which we could get the highest aromatics selectivity and BTX selectivity and the best content of Cr was 2wt%. Then, we investigated the influence of the reaction temperature and the space velocity of feed stock on the reaction. From the results of the investigation, we chose 350℃and 350mL·g-1·h-1 as the excellent condition for the successive experiments. On the aspect of stability of catalyst, we had contrasted between modifier Cr and modifier Mo, Zn and blank HZSM-5, and the catalyst modified by Cr was also the best one. At the optimal reaction condition, i.e. at 350℃, the space velocity of stock was 350mL·g-1·h-1 and the catalyst was 2wt%Cr/HZSM-5 (SiO2/Al2O3=100), the methyl bromide conversion, the BTX and aromatics selectivity could reach 98.7%,38.5% and 57.2% respectively.What’s more, we found the regenerated catalyst presented a longer lifetime but lower BTX selectivity than the fresh one. We speculated that the strange phenomenon attributed to residual CrBr3 which had been tested to be existence in the catalyst by EDS didn’t completely convert to Cr2O3 during the process of regeneration. To confirm our assumption, we designed some experiments using CrBr3 as modifier on the catalyst, and we found that CrBr3 gave a better methyl bromide conversion and a better performance of resistance to coke than Cr2O3, although the latter presented a higher BTX selectivity. These ideally confirmed our assumption. In addition, we performed many characterizations to investigate the specific surface area, the surface acidity, and the reasons of catalytic deactivation and many other factors of catalysts. From the results, the deactivation of the catalyst might be ascribed to the reason that the deposited coke blocked the channels of the catalyst and covered the activate sites.