氟改性对纳米 HZSM-5 分子筛催化甲醇制丙烯的影响

 在比较了纳米和微米 HZSM-5 分子筛催化甲醇制丙烯反应性能的基础上, 对纳米 HZSM-5 分子筛进行了氟改性. 利用透射电镜、N2 吸附、X 射线衍射、氨程序升温脱附和吡啶吸附-红外光谱技术对改性前后的样品进行了表征, 并在常压、500 oC 和甲醇空速 (WHSV) 为 1.0 h–1 的反应条件下, 在连续流动固定床微型反应器上考察了其催化甲醇制丙烯的性能. 结果表明, 当氟含量<10% 时, 随氟含量的增加, 改性纳米 HZSM-5 分子筛的酸量减少, 酸强度降低, 从而使丙烯选择性和催化剂稳定性不断提高. 但过量 (15%) 氟的改性使纳米 HZSM-5 分子筛的酸量、比表面积和孔容均明显减小, 致使其稳定性反而降低. 在适量 (10%) 氟改性的纳米 HZSM-5 分子筛上, 丙烯选择性和维持甲醇完全转化的反应时间分别由原来的 30.1% 和 75 h 增加到 46.7% 和 145 h.

The Effect of Fluorine Modification on Catalytic Performance of Nanosized HZSM-5 Zeolite for Conversion of Methanol to Propene

Based on the comparison of catalytic performance of nanosized and microsized HZSM-5 zeolites for the conversion of methanol to propene, the fluorine-modified nanosized HZSM-5 zeolites were prepared by impregnation with NH₄F aqueous solution. The parent and modified samples were characterized by transmission electron microscopy, N₂ adsorption, X-ray diffraction, temperature-programmed desorption of ammonia, and infrared spectroscopy of adsorbed pyridine. Their performance for conversion of methanol to propene was tested in a continuous flow fixed-bed micro-reactor at atmospheric pressure, 500 ^oC, and methanol space velocity (WHSV) of 1.0 h^–1. The results showed that when the content of fluorine was lower than 10%, the amount and intensity of acidic sites on the fluorinated nanosized HZSM-5 zeolites decreased continuously with fluorine content increasing, which resulted in the enhanced selectivity for propene and catalyst stability. However, the modification with too higher content (15%) of fluorine decreased the catalyst stability due to the distinctly reduced amount of acid sites, surface area, and pore volume. It was concluded that after modification with appropriate amount (10%) of fluorine, the selectivity for propene and the reaction time that can maintain total conversion of methanol was increased significantly to 46.7% and 145 h from 30.1% and 75 h over the parent nanosized HZSM-5 zeolite, respectively.