Conversion Of Bio-ethanol To Propene Catalyzed By Fluorine Modified Nanoscale HZSM-5 Zeolites

In this paper, the catalytic performances of the nanoscale HZSM-5 zeolite modified by NH4F, HF and NH4F-HF for conversion of bio-ethanol to propene were investigated. Effects of fluorine modification on the physico-chemical properties and catalytic performances were studied by the characterizations of XRD, TEM, N2 adsorption, ICP-OES, 27Al NMR,NH3-TPD, Py-IR, and TG.The external surface area, pore volume and pore size of the modified catalysts enlarged obviously after NH4F treatment. Meanwhile, the influences of various amounts of NH4F on acidic properties were quite different. When NH4F content was < 20%, both the amount and strength of the acid sites on the surface of modified HZSM-5 zeolites decreased with the increase in fluorine contents; while the extortionate fluorine content (25%) could lead to a slightly stronger acidity on account of the inductive effect of fluoride ions. Moderate NH4F(20%) make both the channel and acid properties be optimized, thus the propene selectivity and catalytic stability were improved significantly. In addition, calcination temperature had a remarkable impact on the catalytic performances of NH4F modified nanoscale HZSM-5 zeolites. When the calcination temperature was 500?, the catalytic performance was the best.However, when the calcination temperature further incresed, the too weak acidity resulted in the higher ethene selectivity, while the propene selectivity plunged and the catalysts deactivated rapidly.Also, HF treatment can modulate the acid properties of HZSM-5 zeolites. When HF content was < 6%, the acidity of the HF modified samples weakened gradually with increasing HF concentration. When HF contents increased to 8%,the acidity strengthened somewhat.Nevertheless, the improvement for pore structure of HF treatment was less than that of NH4F treatment. Although HF modification enlarged the external surface area of HZSM-5 zeolites,it had little effect on pore volume and pore size. Due to the appropriate acidity and pore structure, HF-6 showed the best catalytic performances, maintaining the propene selectivity above 10% for 95 h.The effects of NH4F-HF on the acidity of HZSM-5 zeolites are similar to that of NH4F and HF. When NH4F(s) was 2 g, HF content was < 6%, the acidity of modified catalysts weakened gradually with increasing concentrations of HF. When HF content increased to 8%, the acidity of NH4F-HF-8 strengthened to some extent. Compared to HF, the influences of NH4F-HF on the channel were more comprehensive. It not only increased the external surface area of modified samples, but also enlarged the pore volume and pore size. With the optimized acidity and pore structure, NH4F-HF-6 exhibited the optimal catalytic performances,maintaining the propene selectivity above 10% for 105 h, which is better than HF-6.