Tailoring the hierarchical architecture of beta zeolites using base leaching and pore-directing agents

The tailoring of hierarchical structures in beta zeolites of differing Al contents (Si/Al: 14-250) has been systematically investigated through a top-down base leaching process with various cationic tetraalkylammonium (TAA) salts and organic amines as pore-directing agents (PDAs) to examine their effectiveness in protecting beta zeolite framework while directing the formation of mesopores. The structure-property relationship was established between PDA structure and average mesopore sizes in resulting hierarchical zeolites, especially for high Si beta zeolites when the presence of PDA is necessary to avoid framework amorphization during NaOH desilication. All PDAs studied are effective in directing the mesopore formation in alkaline solutions without experiencing significant decrease in microporosity and crystallinity of parent beta zeolites. For high Si beta zeolites, hierarchical structures with non-ionic amines possess larger mesopores (average mesopore sizes, d(alpha): 5-8 nm) than those treated with TAA cationic PDAs (d(alpha): 2-6 nm), which should be due to better accommodation of TAA-type cationic PDAs within the inner channel of beta framework and more favorable interaction and protection of parent framework as compared to organic amines. For similarly structured TAA cations, the resulting mesopore sizes and surface areas are correlated with the molecular weights of different cations, i.e. higher molecular weight cations result in smaller da and correspondingly larger surface areas. Therefore, it is feasible to tailor the mesopore sizes by a top-down base leaching with an appropriate selection of PDAs for beta zeolites. The successful introduction of mesopores in beta zeolites also led to significantly improved catalytic conversion for the acid-catalyzed alpha-pinene isomerization, as compared to the parent beta catalysts.