Selective active site placement in Lewis acid zeolites and implications for catalysis of oxygenated compounds

• Published in: Chemical science (Cambridge) Vol. 11; no. 37; pp. 1225 - 1235
• Main Authors: Rodríguez-Fernández, Aída, Di Iorio, John R, Paris, Cecilia, Boronat, Mercedes, Corma, Avelino, Román-Leshkov, Yuriy, Moliner, Manuel
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 30.09.2020; The Royal Society of Chemistry
• Subjects: Acetonitrile; Catalysis; Chemical synthesis; Chemistry; Crystallography; Deuteration; Germanium; Grafting; Infrared spectroscopy; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Lewis acid; Low concentrations; NMR; Nuclear magnetic resonance; Silicon dioxide; Tin; Zeolites
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc03809a

The selective incorporation of isolated framework Lewis acid sites at specific crystallographic positions in high-silica zeolites was achieved by applying a rationalized post-synthetic grafting methodology. The removal of framework Ge atoms from a Ge-BEC zeolite with low concentrations of Ge in the framework (Si/Ge ∼ 150) followed by grafting allows the synthesis of Sn-BEC zeolites with Sn atoms positionally biased into the double-4-ring (D4R) crystallographic positions of the BEC framework. Spectroscopic characterization using solid-state nuclear magnetic resonance (NMR) coupled with theoretical calculations revealed that Sn atoms preferentially form open Sn sites in the D4R of Sn-BEC. This observation was supported by IR spectra of adsorbed deuterated acetonitrile (CD 3 CN), a known titrant of Sn sites in zeolites. The catalytic implications of selective incorporation of open Sn sites in Sn-BEC were probed using the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction. Although the MPVO turnover rates normalized by the total number of open Sn sites were comparable on Sn-BEC and a conventional Sn-Beta catalyst synthesized in fluoride media (Sn-Beta(F)), Sn-BEC demonstrated higher per gram reaction rates because of its larger fraction of open sites compared to Sn-Beta(F). These results highlight the advantage of placing active sites in targeted locations within a zeolite structure. The methodology presented here to selectively place catalytic active sites via sacrificial heteroatoms, such as Ge, can be generalized for the design of many other tetrahedrally-coordinated metal-containing zeolites. The selective incorporation of isolated framework Lewis acid sites in specific crystallographic positions in high-silica zeolites was achieved successfully by applying a rationalized post-synthetic grafting methodology.