Temperature-programmed desorption and carbon-13 nuclear magnetic resonance studies of adsorption complexes in acidic zeolites

Temperature programmed desorption (TPD) studies were used to show a linear correlation between the cracking activity of faujasites and faujasite-based FCC catalysts and the concentration of Bronsted acid sites in these materials. TPD of isopropylamine is particularly useful for characterizing acid site concentrations. In these studies, we obtained a constant turnover frequency for both n-hexane and gas oil cracking rates as a function of the site concentration determined with TPD of isopropylamine. TPD techniques were also applied to the study of silica-aluminophosphate molecular sieves. In these studies, we were able to show that AlPO-5 and SAPO-5 could be readily distinguished using our techniques. By measuring acid site concentrations as a function of silicon content, we were able to elucidate details of the mechanism of silicon incorporation into the aluminophosphate lattice. Solid state NMR techniques were used to study adsorption complexes of acetone in zeolite H-ZSM-5. Using $\sp{13}$C NMR, we were able to identify a concentration dependent order-disorder transition that occurs at a coverage of one acetone molecule per Bronsted acid site. This transition is due to an exchange between acetone molecules that are chemisorbed at the acid sites and a more mobile, physisorbed species. Bimolecular chemical reaction between acetone molecules to form mesityl oxide occurs at coverages above one molecule per site at room temperature, while no reaction is observed at coverages below one per site until the temperature is raised to 150$\sp\circ$C. The reduced exchange rates for surface-adsorbed species that were observed at low coverages allowed us to study reactive intermediates that are not stable at higher coverages. In particular, $\sp{13}\rm C$ NMR was used to study the intermediates in the acid-catalyzed decomposition of allyl alcohol localized at the acid sites in H-ZSM-5. We observed either the formation of a stable allylic carbenium ion, of the formation of reaction products that are consistent with the formation of a transient, secondary carbenium ion. Both of these products are shown to be consistent with the thermodynamics of gas-phase proton transfer chemistry