Adsorption studies of the acid sites in high-silica zeolites
The adsorption of simple organic bases on high-silica zeolites was studied using simultaneous temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) in order to understand more about the nature of the acid-sites in these materials. Specifically, the effect of the adsorbate type and the zeolite structure and composition on the interaction between adsorbate and the acid-site was examined. The adsorption of toluene on H-ZSM-5 was examined using TPD-TGA, infrared spectroscopy, and proton NMR. The isotope exchange between toluene and D-ZSM-5 was found to occur through arenium-ion-like intermediates which were formed on H-ZSM-5 at a stoichiometry of one per Al-site at temperatures as low as 273K. To examine the effect of Al concentration on the acid sites, the adsorption of methanol, 2-propanol, and 2-methyl-2-propanol was investigated on H-ZSM-5 samples with silica-alumina ratios of 38, 70, and 520. Adsorption complexes of one molecule/Al were identified on each of the samples. Furthermore, the unimolecular reactivities of this 1:1 state were unaffected by the Al concentration, implying that the acid-sites are identical and are formed one per Al. Adsorption complexes were also observed for 2-propanamine and 2-propanone on H-ZSM-5 samples. To examine the effect of zeolite structure on the acid-sites, the adsorption of 2-propanol and 2-propanamine of H-ZSM-12 and H-mordenite was examined. The 1:1 adsorption complexes which were formed were identical in reactivity to those formed in H-ZSM-5, implying that the acid-sites formed by isolated Al atoms in a zeolite lattice are chemically equivalent, independent of the structure of the lattice. The adsorption of ammonia, 2-propanamine, and 2-propanol on H-(B) -ZSM-5, H-(Fe) -ZSM-5, and H-(Ga) -ZSM-5 was examined to investigate the effect of the incorporation of trivalent ions into the zeolite framework. Adsorption complexes corresponding to one molecule per substituent atom were identified. The reactivity of the complexes on H-(B) -ZSM-5 indicates that the acid-sites formed by B-incorporation are much weaker than those associated with Fe, Ga, or Al. TPD-TGA techniques were found to be valuable in characterizing the materials created by the incorporation of trivalent cations into the zeolite framework. To investigate the oligomerization chemistry which can occur in H-ZSM-5, TPD-TGA experiments examining the low-temperature reaction of ethene, propene, and 2-methyl-2-propanol were carried out using gas chromatographic analysis of the products. The similarity in the temperature range of desorption and in the products obtained for these adsorbates indicates a similarity in the intermediates formed in the zeolite following adsorption.
Kofke, Tamara Gricus, "Adsorption studies of the acid sites in high-silica zeolites" (1989). Dissertations available from ProQuest. AAI9004797.