不同硅铝比和晶体粒径的ZSM-5分子筛的二甲苯异构化反应研究

XYLENE ISOMERIZATION PERFORMANCE OF ZSM-5 ZEOLITES WITH DIFFERENT n(SiO_(2))/n(Al_(2)O_(3))RATIOS AND CRYSTAL SIZES

采用水热晶化法合成了不同硅铝比和晶体粒径的ZSM-5分子筛,采用X射线衍射(XRD)、X射线荧光光谱(XRF)、扫描电子显微镜(SEM)、氨气程序升温脱附(NH_(3)-TPD)、氮气物理吸附等进行物化性质表征,在固定床微反装置上进行二甲苯异构化反应性能评价。结果表明,纳米ZSM-5分子筛呈聚集体形貌,晶体粒径为50~100 nm,聚集体尺寸随着硅铝比增加由约500 nm增大至约1μm。ZSM-5分子筛的硅铝比由32增加至127,其酸量由0.55 mmol/g逐渐降低至0.19 mmol/g,乙苯脱烷基及二甲苯分子间、乙苯与二甲苯的烷基转移反应活性均显著降低,二甲苯收率呈升高趋势;酸量为0.24~0.55 mmol/g时,二甲苯异构化活性稳定在24.08%~24.17%。减小ZSM-5分子筛晶体粒径有利于显著增加外表面积,从而获得更高的二甲苯异构化活性,但也会使降低二甲苯收率;不同晶体粒径的ZSM-5分子筛乙苯脱烷基反应性能变化不大。

ZSM-5 zeolites with different n(SiO_(2))/n(Al_(2)O_(3))ratios and crystal sizes were hydrothermally synthesized.The physical and chemical properties of the samples were investigated by X-ray diffraction(XRD),X-ray fluorescence spectrometer(XRF),scanning electron microscope(SEM),NH_(3)-temperature programmed desorption(NH_(3)-TPD)and N 2 physical adsorption.The xylene isomerization performance of the samples was evaluated on a fixed bed reactor.The results indicate that the nanoscale ZSM-5 zeolite exhibits an aggregated morphology,with individual nanocrystal sizes ranging from approximately 70 nm to 100 nm.When the n(SiO_(2))/n(Al_(2)O_(3))ratio increases from 32 to 127,the size of the aggregates increases from about 500 nm to about 1μm,while the amount of the acid sites decrease from 0.55 to 0.19 mmol/g,resulting in a significantly decrease in the activity of ethylbenzene dealkylation and trans-alkylation reactions between the xylene isomers,as well as between the xylene isomers and ethylbenzene,but an increase in the yield of xylenes.The xylene isomerization activity maintained between 24.08%-24.17%when the amount of the acid sites ranged from 0.24-0.55 mmol/g.Reducing the crystal size of ZSM-5 molecular sieve is beneficial for significantly increasing the surface area,thereby obtaining higher isomerization activity of xylene,but it also reduces the yield of xylene.On the contrary,the use of ZSM-5 zeolites with different crystal sizes results in almost no change in ethylbenzene dealkylation activity.