Acta Phys. -Chim. Sin. ›› 1989, Vol. 5 ›› Issue (02): 145-150.doi: 10.3866/PKU.WHXB19890204

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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

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.