UC Santa Barbara

Zeolites are a class of crystalline, nanoporous aluminosilicates used for many applications, including water softening, gas separations, hydrocarbon conversions, and a wide range of other reaction chemistries. The diverse applications of zeolites arise from local negative charges introduced by four-coordinate aluminum heteroatoms in the zeolite framework, which are charge-balanced by exchangeable cations in the zeolite pores. Exchanging in different cations enables the many different industrial applications of zeolites. Thus, understanding and controlling the complicated distributions of aluminum heteroatoms in the zeolite framework enables the catalytic reaction properties to be modified and improved through manipulation of cation locations and distributions. However, characterization of such distributions and their direct influences on catalysis is challenging. In this dissertation, I demonstrate how advanced characterization techniques provide unprecedented insights on aluminum distributions, in both zeolite nanopores and hierarchical zeolites with multiple pore sizes.Local aluminum configurations in zeolite nanopores are hypothesized to affect the catalytic performance of the small-pore zeolite chabazite, which is used in its Cu2+-exchanged form industrially as a deNOx catalyst in diesel exhaust systems. It has been proposed that closely paired aluminum configurations (separated by 1-2 bridging Si-O-Si) are desirable. However, the non-stoichiometric and non-periodic ordering of aluminum in the zeolite framework makes characterizing these aluminum pairing distributions in zeolites intractable by many characterization techniques. By using advanced solid-state nuclear magnetic resonance spectroscopy (NMR) analyses, two industrially relevant chabazite catalysts are shown to have different relative amounts of paired-framework aluminum atoms that are not detected by conventional methods. The differences in aluminum pairing correlate with differences in catalytic activity and help to guide industrial zeolite syntheses for improved Cu-chabazite deNOx catalysts. In addition, effects of hydrothermal aging on Cu and Al distributions in a Cu-CHA zeolite are investigated by advanced electron microscopy and NMR techniques. Advanced NMR techniques show that the distributions of and interactions between adsorbed species under deNOx reaction conditions evolve due to hydrothermal treatment. In addition, it is shown that distributions of paired aluminum species in the zeolite framework evolve as a result of hydrothermal treatment. In addition to local aluminum configurations within nanopores, catalytic activity can be influenced by aluminum distributions between different pore sizes. Commercial H+-exchanged zeolite Y (H+-Y), a large-pore catalyst, has been used as hydrocarbon cracking catalysts for decades, and is a classic example of a zeolite catalyst with complicated distributions of aluminum. The industrial form of the H+-Y catalyst is modified to create mesoporous defects in the zeolite crystallites, which improve the mass transport of large hydrocarbon reactants to active sites and directly expose some framework aluminum, and thus active sites, to the mesopores. To better understand the role of mesopore active sites in cracking chemistry, the mesopore sites on a H+-Y catalyst were selectively passivated and compared with an otherwise identical unpassivated catalyst for hexane cracking activity. Passivation results in a decrease in initial activity but overall reduction in amount of coke formation on the catalyst. Solid-state NMR measurements show that the passivation results in a modification of mesopore defect acid sites, and additionally show that bulkier polyaromatic hydrocarbon species form on the unmodified catalyst. Overall, the results discussed in this dissertation will aid in the design of zeolite catalysts with more desirable distributions of aluminum heteroatoms and reaction properties.