Structure and Framework Association of Lewis Acid Sites in MOR Zeolite

Abstract

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D 1H, 15N, and 27Al MAS NMR spectroscopy, carried out at different temperatures (from 298 down to 107 K), we were able to obtain the NMR spectroscopic signatures of LAS and Brønsted acid sites (BAS) in mordenite zeolite in the presence and the absence of adsorbed pyridine (Py). Combined with DFT modeling, this information enabled the structure of LAS to be revealed, namely (SiO)3Al sites interacting with pyridine, thus indicating that the corresponding base-free framework-associated sites are pseudo tricoordinated Al sites, namely tricoordinated Al sites interacting with an additional coordinated adjacent siloxane bridge. With this information in hand, we propose a molecular-level understanding on how the AlIV and AlVI framework and framework-associated sites evolve upon dehydration and exposure to Py into BAS and LAS, and their associated Py adducts. By measuring and analyzing the changes in quadrupolar coupling constants (CQ) that reflect electrical charge distribution around the nuclei, we further show that the lower C Q values observed at 298 K are due to residual dynamics that makes the electric field around aluminum nuclei more symmetric. Thus, NMR spectroscopic signatures of 27Al greatly vary with temperature; this information illustrates the importance of accounting for the temperature effect when confronting experimental and calculated C Q values of the corresponding aluminum sites in zeolites in order to obtain accurate structural assessment.

Solid-state NMR spectra of protons and quadrupolar nuclei at 28.2 T: resolving signatures of surface sites with fast magic angle spinning

“…3b) as previously discussed. 40 The 2D 27 Al triple-quantum MAS (TQMAS) spectrum 41 of the zeolite (Fig. 3c) separates the two signals further and enables their spectroscopic parameters to be extracted, yielding CQ values consistent with the previous assignments.…”

“…3c) separates the two signals further and enables their spectroscopic parameters to be extracted, yielding CQ values consistent with the previous assignments. 40 Interested in the 27 Al spectroscopic signature of a true tricoordinate aluminum species with oxygen atoms in the first coordination sphere, we measured the tris(aryloxide) Al(OAr*)3 (Ar* = 2,6-di-tert-butyl-4methyl-phenyl) 42 as a model compound (Fig. 3d).…”

Classifying and understanding the reactivities of Mo based alkyne metathesis catalysts from 95Mo NMR chemical shift descriptors

“…The 95 Mo quadrupolar coupling constants (C Q ) were also estimated to be between 2.0 and 4.5 MHz, typically significantly lower that the values calculated from first principles (SI Table S4.1), likely due to dynamic effects, present even under the low temperature measurement conditions. 35 The 95 Mo chemical shift tensors are nearly axially symmetric for all alkylidyne complexes (η CSA between 0.0 and 0.3), in relation to their almost C 3v symmetry. Density functional theory (DFT) was then employed to understand the origin and orientations of the tensor components.…”

Solid-state NMR spectra of protons and quadrupolar nuclei at 28.2 T: resolv-ing signatures of surface sites with fast magic angle spinning

“…Recently, our group provided evidence that the Lewis acid sites in mordenite zeolite are pseudo-tricoordinate framework Al interacting with a coordinated siloxane bridge. 40 The 1D 27 Al NMR spectrum of dehydrated mordenite is substantially narrowed at 28.2 T compared to 16.4 T (Fig. 3a), and two different signals are resolved that can be associated by analysis of the 2D 27 Al{ 1 H} D-HMQC spectra with Brønsted and Lewis acid sites (Fig.…”

“…Comparison of the 27 Al spectra of mordenite zeolite and Al(OAr*)3 show no evidence for large-CQ species consistent with tri-coordinate Al, corroborating our recent conclusion that the Lewis acid sites in mordenite zeolite under these conditions are predominantly pseudo-tri-coordinate Al sites having a labile siloxane moiety coordinated. 40 Overall, the adoption of high field (28.2 T) and fast MAS (>50 kHz) provide substantial advantage for measurement of highly resolved solid-state NMR spectra of surfaces and materials. Though demonstrated only for select cases here, the methods will be extensible to diverse other inorganic, organometallic, and organic-inorganic hybrid materials.…”