UC Santa Barbara

Site-specific heteroatom incorporation imparts catalytic and adsorption properties to silicate-based catalysts (e.g., zeolites and clays), leading to diverse technologically important industrial processes. In such applications, reactions are typically catalyzed at Brønsted acid sites associated with H+ cations that balance the anionic charges near framework heteroatom species (generally aluminum) in heteroatom-containing silicate frameworks. Despite the fact that the catalytic and adsorption properties of these catalysts have been observed to be greatly influenced by several factors, such as heteroatom type, contents, and distributions, the associated fundamental understanding of heteroatom site distributions is still very limited and has been elusive. This is partially due to lack of experimental methods that can probe the complicated order and disorder around heteroatom sites even in well-ordered crystalline silicate frameworks. Among various characterization methods, solid-state nuclear magnetic resonance (NMR) spectroscopy can be considered as a promising technique because of its high sensitivity to electronic environments of NMR-active nuclei, which allows their molecular proximities and connectivities to be established.

Herein, newly modified two-dimensional (2D) NMR techniques were applied to probe specific covalent site connectivities between framework boron or aluminum heteroatoms and their nearby silicate sites in zeolite and layered boro- and alumino-silicate materials. The established site connectivities, in conjunction with crystallography, enable the determination of boron or aluminum heteroatom site distributions. We applied the new 2D NMR methods to boron- or aluminum-containing surfactant-directed silicates and zeolite SSZ-70, aiming for understanding similarities and differences between boron and aluminum incorporation. Interestingly, boron atoms are shown to be preferentially incorporated into specific silicate sites in most cases, whereas aluminum atoms appear to be non-selectively distributed among different silicate sites in all cases. Such observations may suggest that boron and aluminum species participate differently in the formation of boro- and aluminosilicates, respectively.

Based on the molecular understanding of boron and aluminum siting learned from these materials, a synthesis protocol to control heteroatom siting in zeolite catalysts is proposed using borosilicate zeolite SSZ-70 (B-SSZ-70) as an example. The protocol begins with the preparation of B-SSZ-70 that was delaminated to increase its external surface area for reactions involving bulky molecules, during which boron siting was monitored ex-situ at different synthesis stages. The results show that boron site distributions were retained during the course of post-synthetic treatments. Subsequently, the framework boron species in the delaminated B-SSZ-70 were post-synthetically exchanged with aluminum atoms. Notably, as expected, the reinserted aluminum species are concluded to be located at certain silicate sites on external surface sites. This is very different from the aluminum siting in Al-SSZ-70 (i.e., Al is everywhere ). Overall, these analyses, methods, and synthesis protocols are expected to provide insights into the local environments of heteroatoms and their distributions in zeolite catalysts, which would enable rational zeolite synthesis with engineered heteroatom site distributions.