Effect of water on toluene adsorption over high silica zeolites

Breakthrough studies on toluene adsorption in the absence and presence of water were performed over high silica MFI and FAU zeolites. Toluene adsorption was found to be insensitive to the presence of water only on the MFI-100 zeolite. According to FTIR spectroscopy of adsorbed probe molecules (CO, quinoline), this is caused by different spatial location of the water and toluene adsorption sites. In particular, Lewis acid sites, hydroxyls and Na cations, which are able to adsorb water, reside on the external surface, while toluene adsorption sites in the zeolite pores (channels), so water does not hinder the adsorption of toluene. In the case of FAU zeolite, acid-base sites are located both in the zeolite cavities and on its external surface. Due to a stronger binding of water to these sites, the adsorption characteristics of toluene substantially deteriorate in the presence of water. In addition, large water clusters are formed on the nests of silanol groups due to hydrogen bonding; such clusters can significantly decrease (or even block) the toluene mass transfer into zeolite pores.