物理化学学报 >> 1989, Vol. 5 >> Issue (02): 145-150.doi: 10.3866/PKU.WHXB19890204

研究论文 上一篇    下一篇

二甲苯在ZSM-5分子筛中的吸附与扩散

曾昭槐; 潘贵生   

  1. 中山大学化学系
  • 收稿日期:1987-08-04 修回日期:1988-07-26 发布日期:1989-04-15
  • 通讯作者: 曾昭槐

SORPTION AND DIFFUSION OF XYLENES IN ZSM-5 ZEOLITES

Zeng Zhaohuai; Pan Guisheng   

  1. Department of Chemistry; Zhongshan University; Guangzhon
  • Received:1987-08-04 Revised:1988-07-26 Published:1989-04-15
  • Contact: Zeng Zhaohuai

摘要: 本文在298K-493K温度范围内,测定了二甲苯在ZSM-5分子筛中的吸附等温线和扩散系数。间二甲苯在HZSM-5分子筛中的吸附是需要较高活化能而为扩散控制的物理吸附,只有在较高温度下(>393K),才能进入分子筛的敛孔,其扩散系数比对二甲苯小三个数量级。对、间二甲苯在HZSM-5分子筛中的吸附量随温度升高而渐趋接近。但HZSM-5经镁改性后,间二甲苯吸附量大幅度下降,而对二甲苯吸附量变化不大,说明孔道结构因素对提高烷基苯转化的对位选择性起着相当重要的作用。增大分子筛的晶粒尺寸,二甲苯的扩散系数变化不大,表明大晶粒ZSM-5之所以能提高对位选择性,原因在于扩散路线的增长导致扩散较慢的间二甲苯异构化为扩散较快的对二甲苯。

关键词:

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Abstract:

The equilibrium isotherms have been determined gravimetrically for xylenes in HZSM-5 and MgHZSM-5 zeolites at the temperature range of 298K—493K.The sorption of m-xylene in HZSM-5 is a physical adsorption which is diffusion-controlled and the activation energy is high.Its adsorption isobar shows two descending portions with an ascending portion between them.It was not found before for this system. Only at the temperature higher than 393K, can m-xylene molecules overcome the energy barrier and enter into the micropores of HZSM-5.
The amounts of sorption for p-xylene and m-xylene in HZSM-5 become closer each other when temperature rises. This may be the cause of that only equilibrium composition well be obtained if zeolite HZSM-5 is used as catalyst for the alkylation of toluene with alcohol or olefin. But for MgHZSM-5, the amount of sorption for m-xylene decreases sharply and p-xylene doesn't. This implies that the zeolite HZSM-5 must be modified in order to enhance the para-selectivity for the shape-selective conversion of single-ring aklyl aromatics.
The diffusivities of xylenes were measured by constant pressure gravimetric sorption methode at the temperature range of 313 K-433 K. The diffusivity of p-xylene is three orders higher than that of m-xylene in HZSM-5. The diffusive activation energies for px/HZSM-5, pX/MgHZSM-5 and mX/HZSM-5 are 16 kJ.mol~(-1), 17 kJ.mol~(-1) and 23 kJ.mol~(-1) respectively. The value of diffusivity for different crystal size of zeolite HZSM-5 is not substantially different.