超声辅助制备酸碱双功能CaO/HMCM-22分子筛催化剂

Preparation of CaO/HMCM-22 zeolite catalyst with acid-base bifunction with ultrasonic assistance

采用超声浸渍法制备了一系列CaO改性的HMCM-22分子筛催化剂,使用XRD、N2物理吸附-脱附、SEM、FT-IR、NH3-TPD及CO2-TPD等技术对催化剂物化性质进行了表征。结果发现在超声作用下经CaO改性后MCM-22分子筛的结构没有发生变化;超声空化作用降低了催化剂晶粒之间的团聚,促进了CaO在分子筛表面的分散,增加了可接触的催化活性位;随着CaO负载量的增加,催化剂上碱强度和碱量显著增加,而强酸含量明显减少,弱酸酸位有所增加。对催化剂在Knoevenagel缩合反应中的催化性能研究表明:与传统浸渍法相比,超声浸渍法所制备催化剂的催化活性明显较高,在优化的超声条件下反应2 h苯甲醛转化率即可达88.3%。催化剂CaO/HMCM-22的催化性能优于HMCM-22和CaO/NaMCM-22,对Knoevenagel缩合反应体现出明显的酸碱协同催化作用。超声法制备的催化剂重复使用性能显著提升,经过5次重复使用后苯甲醛转化率仍保持在65%以上。

CaO/HMCM-22 zeolite catalysts were prepared by ultrasonic impregnation and characterized by XRD, N2 physical adsorption-desorption, SEM, FT-IR, NH3-TPD and CO2-TPD. The structure of MCM-22 zeolite still remained after CaO modification with ultrasonic assistance. Ultrasonic cavitation could reduce the agglomeration between particles, improve the dispersion of CaO on the surface of zeolite and increase the accessible catalytic active sites. By increasing CaO loading, the strength and content of base increased, while the strength of strong acid decreased significantly and the amount of weak acidic sites increased slightly. Knoevenagel condensation reactions were conducted over the synthesized catalysts. The strategy developed here showed excellent catalytic performance for this reaction compared with the conventional impregnation method and the conversion of benzaldehyde reached 88.3% under optimal ultrasonic irradiation within 2 h reaction. The catalytic performance of CaO/HMCM-22 was better than that of HMCM-22 and CaO/NaMCM-22, resulting in good catalytic activity for Knoevenagel condensation reactions and obvious acid-base synergetic effects. The reusability of the synthesized catalysts with ultrasonic assistance was also investigated. Obvious improvment was achieved and conversion of benzaldehyde was above 65% after being reused 5 times.