Hua Huo

From Grey Lab Page

My study is focused on the characterization of the acidity and local structures of zeolites and zeolitic materials. Zeolites are microporous aluminosilicates with widespread industrial applications, which include their use as catalysis, in separations, ion exchange, and even in medical applications. The catalytic activity and selectivity of the protonic forms of zeolites mainly depend on their unique porous structures and the acidity of the bridging hydroxyl groups, Si-OH-Al, which function as Brønsted acid sites. The factors that control the Brønsted acidity of these materials continue to be debated but are generally believed to be influenced by a number of often interrelated factors. These include the local (Si-O(H)-Al and T-O-T bond angles, OH distances), and intermediate (shapes and sizes of the rings and pores that form the zeolite) structures and also the proximity between acid sites. The different factors manifest themselves in very different acidities, proton mobility and catalytic activities. We have been using a wide range of NMR techniques in order to understand how local structure varies between structures, in order to help assess the relative importance of the different structural features in controlling zeolite function. Our recent studies have focused on a detailed characterization of the ¹⁷O parameters (chemical shift, quadrupolar coupling constant, ¹⁷O-¹H dipolar couplings) that characterize of the local environment of the oxygen atom bound to the Brønsted acid site, in order to explore the sensitivity of this nucleus to structure and acidity. We explore the use of ¹⁷O-¹H double resonance methods to determine structure and examine the effect of mobility on the NMR spectra.