Effect of the acid properties on the diffusion of C 7 hydrocarbons in UL-ZSM-5 materials Hoang Vinh-Thang a , Qinglin Huang b , Adrian Ungureanu a , Mladen Eic ´ b , Do Trong-On a , Serge Kaliaguine a, * a Department of Chemical Engineering, Universite ´ Laval, Ste-Foy, Que ´., Canada G1K 7P4 b Department of Chemical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3 Received 8 July 2005; received in revised form 22 August 2005; accepted 26 August 2005 Available online 20 February 2006 Abstract In this study, a series of UL-MFI materials with different Si/Al ratios has been synthesized following a solid-state crystallization start- ing from amorphous wormhole-like mesostructured aluminosilicates. The acid properties of UL-MFI materials are varied by changing the content of aluminum as observed by FTIR spectra of adsorbed pyridine. The effect of the acid properties on the diffusion of C 7 hydrocarbons n-heptane and toluene in UL-MFI materials was further studied using the ZLC technique. As expected, the interaction with the acid sites causes a decrease in the diffusivities of both sorbates. The values of the activation energy of diffusion for both C 7 hydrocarbons in H-form UL-MFI materials are higher than those in Na-forms. The effect of the acid properties on the decrease of these values is more pronounced at high Si/Al ratio. Ó 2005 Published by Elsevier Inc. Keywords: UL-ZSM-5; Bro ¨ nsted/Lewis-acid; Diffusion of n-heptane and toluene; Micro/mesoporosity; ZLC technique 1. Introduction In our previous papers [1,2], we reported the diffusion of linear and aromatic C 7 hydrocarbons in steam-stable mesostructured zeolitic UL-ZSM-5 materials using the zero length column (ZLC) method. UL-ZSM-5 is a dual micro/mesostructured pore UL-zeolite materials, which has been developed by our group since 1999 [3,4]. Materials with a micro/mesostructure are of particular interest because they combine the advantages of both microporous crystalline zeolites such as strong acidity and good thermal/ hydrothermal stability as well as those of mesoporous materials, i.e., highly ordered large pores and high specific surface areas [5–8]. In these UL-ZSM-5 materials, the dif- fusion of hydrocarbons was found to be controlled by a combination of mesopore diffusion, which is due to a sur- face slip in the main pores, and surface/micropore diffusion in the intrawalls of the bi-porous structure. The activation energies of diffusion for smaller critical diameter n-heptane are higher than for toluene which has however a larger crit- ical diameter [2]. Recently, the acid properties of these UL-ZSM-5 mate- rials have been thoroughly investigated using several tech- niques such as FTIR of adsorbed pyridine, heat flow microcalorimetry of adsorbed ammonia, TPD of ammonia and FTIR of adsorbed DTBPy (2,6-diterbutylpyridine) [9– 11]. The results indicated that overcoming the low acid strength of the amorphous mesostructured precursors, UL-ZSM-5 materials show medium-strong acid sites com- parable to those of ZSM-5 zeolites. Up to now, several studies related to the influence of acid sites on diffusivities of porous materials have been reported [12–15]. For example, the diffusion of benzene in ZSM-5 was found to be twice slower than that in silicalite 1387-1811/$ - see front matter Ó 2005 Published by Elsevier Inc. doi:10.1016/j.micromeso.2005.08.030 * Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810. E-mail addresses: kaliagui@gch.ulaval.ca, Serge.Kaliaguine@gch. ulaval.ca (S. Kaliaguine). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 92 (2006) 117–128