Synthesis And Aromatization Performance Of Hierarchical HZSM-5 Zeolite

With the intensification of environment pollution and energy crisis, the traditional path of BTX production by catalytic cracking of crude oil is a great challenge. Due to our country is rich in coal and a large amount of methanol can be produced from coal chemical industry, aromatics production from methanol is a cost effective route. In addition, the strong development of biodiesel industry in recent years has resulted in the accumulation of by-product glycerol. Therefore, the transformation from glycerol to BTX aromatics with high economic value can not only improve the economic value of biodiesel industry, but also ensure the carbon balance in the entire process and reduce the production of greenhouse gases. HZSM-5 zeolite molecular sieve with unique properties and pore structure has been regarded as the most effective catalyst for selective preparation of BTX aromatics. Traditional micro-porous HZSM-5 zeolite with very narrow pore often deactivate in aromatization reaction due to carbon deposition. Hierarchical HZSM-5 zeolite can not only provide acid properties, hydrothermal stability and shape-selective pore structure of micro-porous zeolite but also introduce meso-pore, which can provide necessary aromatization activity center and reduce the diffusional resistance in aromatization or similar diffusion-controlled reaction. Therefore, the research and development of hierarchical HZSM-5 zeolite have an important significant for aromatization.In this paper, a series of hierarchical HZSM-5 zeolites were firstly prepared by desilication of traditional micro-porous HZSM-5 zeolite with different concentration of NaOH solution. The aromatization activity of the prepared zeolites was investigated in the aromatization of glycerol under the condition of atmospheric pressure, reaction temperature of 400℃ and WHSV of 0.95 h-1. The results showed that the modified HZSM-5 zeolite through alkaline treatment with 0.3 M NaOH had relatively appropriate pore size and acid properties, which also obtained twofold increase in the total yields of BTX aromatics and threefold improvement in lifetime than the zeolite without modification.Another series of hierarchical HZSM-5 zeolites were synthesized by hydrothermal method with five different mesoporous templates (PTEOS, and CTAB, and DDAC, and PDD-AM and CNT) respectively. The aromatization activity of the samples was investigated in the aromatization of methanol. The study found that all the prepared hierarchical HZSM-5 zeolites had better crystallinity and a certain amount of mesopore. Among the samples the one based on carbon nanotube (CNT) as the mesoporous template had distinct micro-mesoporous structure. In the aromatization of methanol, this sample also exhibited high catalytic activity and stability and the slowest carbon deposition rate. Therefore, the CNT was chosen as the optimal mesoporous template and the influence of crystallization time and temperature, pH of synthetic system, Si/Al ratio, the amount of CNT for the synthesis of hierarchical HZSM-5 zeolite were investigated. The results showed that the hierarchical HZSM-5 zeolite had relatively good crystallization, moderate acidic properties, larger pore volume and surface area, higher catalytic activity and stability in methanol aromatization under the crystallization time of 5 days, crystallization temperature of 170℃, pH range of 10-11, Si/Al of 80 and the quality of CNT takes up the total quality of the raw materials of 0.67%.The hierarchical HZSM-5 zeolite prepared under the optimal synthesis conditions was chosen as the optimal catalyst, the effects of reaction temperature and WHSV on the aromatization of methanol were investigated under the condition of atmospheric pressure and methanol feed alone. The results showed that under the reaction temperature of 428℃ and WHSV of 2.67 h-1, the higher BTX aromatics yields and catalytic stability can be obtained.