Sn改性b轴取向ZSM-5分子筛催化剂催化甲醇制丙烯反应性能研究

Catalytic performance of Sn modified b-axis oriented ZSM-5 zeolite catalysts in methanol to propylene reaction

以葡萄糖为第二模板剂,采用两步水热晶化法合成了b轴取向ZSM-5分子筛,并采用浸渍法负载Sn对其进行了改性处理,以提高ZSM-5分子筛的寿命和丙烯选择性。采用粉末X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、N_(2)吸/脱附、吡啶红外光谱(Py-FTIR)和氨气程序升温脱附(NH3-TPD)等分析手段对Sn改性b轴取向ZSM-5分子筛催化剂进行了表征,并在固定床微反评价装置上考察了催化剂的表面酸性质(酸量和酸强度)对甲醇制丙烯反应性能的影响。结果表明,Sn改性b轴取向ZSM-5分子筛催化剂呈现出相似的孔结构性质,随Sn负载量(质量分数,下同)增加,Lewis酸量和Lewis酸强度逐渐升高,强酸量先降低后升高。表面酸性质对甲醇反应的产物分布和催化剂寿命有较大影响,当Sn负载量增加引起Lewis酸量和强酸量过高时,丙烯选择性和催化剂寿命均出现大幅降低。具有适宜Lewis酸量和强酸量的Z5-S2催化剂表现出较高的丙烯选择性(53.53%)和丙烯与乙烯质量比(6)、最高的低碳烯烃(C_(2)=~C_(4)=)选择性(77.43%)和催化剂寿命(40 h)。

b-Axis oriented ZSM-5 zeolites were synthesized by a two-stage hydrothermal crystallization with using glucose as the second template,and modified by impregnation with Sn to improve the service life of ZSM-5 zeolites and the selectivity of propylene.The Sn modified b-axis oriented ZSM-5 zeolite catalysts were characterized by powder X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),N_(2) adsorption/desorption,pyridine adsorption Fouriertransform infrared(Py FT-IR)and NH_(3) temperature-programmed desorption(NH_(3)-TPD)methods.Thereafter,the prepared catalysts were evaluated in a fixed bed micro reactor to investigate the effects of their surface acidity(acid quantity and acid strength)on the performances of methanol to propylene(MTP)reaction.The results show that Sn modified b-axis oriented ZSM-5 zeolite catalysts exhibits similar pore structure properties.With the increase of Sn loads(mass fraction,the same below),the Lewis acid amount and its acid strength gradually increase,and the strong acid amount first decrease and then increase.Anyway,acidity exhibits a great influence on the product distribution and catalyst lifetime in the MTP reaction.Excessive amounts of Lewis acid and strong acid caused by high loads of Sn,significantly decrease propylene selectivity and catalyst lifetime.Z5-S2,which possesses appropriate Lewis acid amount and strong acid amount,exhibits high propylene selectivity(53.53%)and mass ratio of propylene to ethylene(6),the highest light olefin(C_(2)=~C_(4)=)selectivity(77.43%)and longest catalyst lifetime(40 h).