Solid-State MAS NMR Studies of Brønsted Acid Sites in Zeolite H-Mordenite

Abstract

(17)O-(1)H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O-H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of (17)O chemical shift correlations to help assign sites is discussed.

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid‐State NMR

“…For example in zeolites, the Brønsted acid site of solid acid catalysts plays essential roles in the heterogeneous catalytic process in the petrochemical industry . Besides the frequently utilized 1 H and 27 Al NMR spectroscopy, Grey and co‐workers introduced a series of 17 O NMR experiments to explore the detailed structure of Brønsted acid sites in various zeolites . In their study of H‐Mordenite (MOR), a 17 O{ 1 H} rotational‐echo double‐resonance (REDOR) experiment was employed to quantitatively determine the OH distances for the different Brønsted acid sites in the MOR zeolite.…”

Solid-State NMR Studies of Zeolites

“…The Brønsted acid sites in other zeolites such as H-ZSM-5 and H-mordenite [ 61 ] were also studied by 17 O MAS NMR spectroscopy, which provides insights into the active center in acid-mediated zeolite catalysts. 129 …”

“…7.14a , the autocorrelation peak at 27 Al MAS and DQ MAS NMR spectra of ( a ) parent HY, ( b ) HY-500, ( c ) HY-600, and ( d ) HY-700 zeolites (Reproduced from Ref. [ 40 ] by permission of John Wiley & Sons Ltd) (61,122) ppm in the 27 Al- 27 Al DQ MAS NMR spectrum indicates that fourcoordinated framework Al (FAL) species are in close proximity to one another. For the HY-500 zeolite, the cross peak pair at (61, 61) and (0, 61) ppm results from the spatial proximity between the four-coordinated FAL and the six-coordinated EFAL (see Fig.…”

Zeolites and Other Structurally Microporous Solids as Acid–Base Materials

“…By carefully examining the structure, it has been concluded that there are four most plausible locations for Brønsted acid sites in zeolite H-MOR: (a) O1/O9, pointing into the center of the 8-ring channel and inaccessible to TMP probe molecules; (b) an adjacent pair of O2 atoms sharing one proton through H-bonding; (c) O5, which points slightly toward the side pocket; and (d) O10, pointing into the center of the 12-ring. 168 The adsorption of different probes shows that monosubstituted aromatic compounds and compounds having the tert-butyl group diffuse in the main channels, but are not allowed to enter the side pockets. Even the access of n-hexane in the side pockets is hindered.…”