甲醇催化转化制丙烯中HZSM-5分子筛的尺寸效应

甲醇催化制丙烯(MTP)是一个具有重要工业应用的研究课题, 目前普遍采用的催化剂是HZSM-5 分子筛. 通过调节分子筛合成原料的配比、晶化温度和晶化时间等参数, 对所制备的不同晶粒尺寸的HZSM-5 分子筛, 综合利用X射线衍射(XRD)、扫描电镜(SEM)、N₂吸附和氨气程序升温脱附(NH₃-TPD)等手段表征了其晶格结构、表观形貌、孔结构以及酸性质. 利用固定床反应装置对HZSM-5 分子筛甲醇催化制丙烯的活性和稳定性进行了评价, 并采用热重(TG)分析技术对催化剂的积炭性能进行了考察. 实验结果表明, HZSM-5 分子筛粒度的减小可以增加分子筛比表面积、孔体积, 同时有更多开放的孔口及短的孔道长度, 有利于反应物分子的吸附和传质,并降低了产物分子在孔道中的扩散距离及发生二次反应的几率, 提高了催化剂的抗积炭能力和容炭能力以及稳定性; 而且所合成的小尺寸分子筛单位质量的总酸量及强酸量均有不同程度的下降, 有利于提高目标产物丙烯的选择性.

Size Effect of HZSM-5 Zeolite on Catalytic Conversion of Methanol to Propylene

Catalytic conversion of methanol to propylene (MTP) by HZSM-5 zeolite is of great importance in industrial applications. In this paper, a series of HZSM-5 zeolites with different crystal sizes were synthesized by adjusting the initial gel composition, crystallization temperature, and crystallization time. The crystal structure, size, morphology, pore structure, and acidity of HZSM-5 were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, and temperature-programmed desorption of ammonia (NH₃-TPD). The catalytic activity and stability of HZSM-5 with different crystal sizes for MTP were evaluated on a continuous flowing fixed-bed reactor. Coke deposited on HZSM-5 was analyzed by thermogravimetric (TG) analyzer. Results indicated that with smaller crystal size, HZSM-5 zeolite had larger surface area and pore volume, higher density of pore openings, and shorter path length of micropore channels that prevent side reactions. For MTP reaction, smaller crystal sizes of HZSM-5 showed a higher resistance and better tolerance to coke, and longer catalytic lifetime. The lowering of both the total and strong acidity on HZSM-5 with smaller crystal size favored a higher selectivity of target product, propylene.