A new charge alternating model of localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms in the framework

Abstract

The present communication reports an unexpected result on quantitative substitution in HZSM-5 of all acidic protons by bivalent zinc cations via chemical reaction with zinc vapor at 800 K. Based on this result we suggest a new model for localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms in the framework. According to this model some of the bivalent cations can be localized at the isolated singly negatively charged aluminum occupied oxygen tetrahedra. In parallel the equivalent number of the isolated framework aluminum atoms remains without compensating protons or zinc cations. Their negative charges are compensated indirectly by coulomb interaction with the surrounding excessively charged cationic sites. The resulting Lewis sites with only partially compensated positive electric charges possess unusually high chemical activity. They very strongly perturb molecular hydrogen at 77 K and dissociatively adsorb it at temperatures only slightly above room temperature. Application of low temperature H 2 adsorption as a molecular probe enables us to distinguish such unusual sites from Zn 2+ cations at the conventional ion exchange positions. The similar unusual Lewis sites distantly separated from basic oxygen have also been previously detected by low temperature hydrogen adsorption in the samples prepared by conventional incipient wetness impregnation of HZSM-5 with aqueous solution of zinc nitrate though in smaller amount.