Solid-State NMR Spectra of Protons and Quadrupolar Nuclei at 28.2 T: Resolving Signatures of Surface Sites with Fast Magic Angle Spinning

Advances in solid-state nuclear magnetic resonance (NMR) methods and hardware offer expanding opportunities for analysis of materials, interfaces, and surfaces. Here, we demonstrate the application of a very high magnetic field strength of 28.2 T and fast magic-angle-spinning rates (MAS, >40 kHz) to surface species relevant to catalysis. Specifically, we present as case studies the 1D and 2D solid-state NMR spectra of important catalyst and support materials, ranging from a well-defined silica-supported organometallic catalyst to dehydroxylated γ-alumina and zeolite solid acids. The high field and fast-MAS measurement conditions substantially improve spectral resolution and narrow NMR signals, which is particularly beneficial for solid-state 1D and 2D NMR analysis of 1H and quadrupolar nuclei such as 27Al at surfaces.

S olid-state nuclear magnetic resonance (NMR) is a powerful tool for materials characterization, with applications spanning biomolecules, 1 polymers, 2 battery materials, 3 semiconductors, 4 and catalysts. 5 It can provide precise element-specific information on the local structure, interactions, and dynamics of NMR active nuclei. 6 However, it is limited by its intrinsically low sensitivity due to low nuclear spin polarization and by signal broadening due largely to strong internuclear and/or quadrupolar interactions and inhomogeneous distributions of chemical species that yield corresponding distributions of chemical shifts.
Measurements at increasingly high magnetic field strengths improve both signal sensitivity and spectral resolution. The ongoing development of NMR instrumentation including stable high magnetic fields >20 T and fast-spinning NMR probeheads capable of MAS rates up to 150 kHz has enabled new opportunities for understanding biomolecules 7−9 and determining their 3D structures in the solid-state 10 including challenging cases such as metalloproteins 11 and membrane proteins in native environments. 12 However, the application of these capabilities to functional inorganic materials and their surfaces, including catalysts, has been so far more limited. In fact, very high magnetic fields and fast MAS rates would be especially powerful to characterize such materials, 13,14 in particular for the analysis of surface sites that are associated with highly unsymmetrical inhomogeneous environments with broad and complex spectroscopic signatures that are challenging to measure and interpret under typical conditions. NMR analysis of quadrupolar nuclei, for example, greatly benefits from very high magnetic fields, 15−18 as demonstrated by recent studies of 27 Al, 19−21 17 O, 22,23 67 Zn, 24 and 95 Mo nuclei 25 in materials like aluminosilicate zeolites, alumina, and metal organic frameworks at magnetic fields up to 36 T and MAS rates up to 30 kHz. Despite the inhomogeneously broadened lineshapes, NMR of such materials can also benefit from very fast MAS rates (>40 kHz), though applications have been limited to selected cases such as organic−inorganic hybrid materials 26 and resolving paramagnetic shifts in inorganic oxides. 27,28 With commercial NMR spectrometers now operating at 28.2 T (1200 MHz for 1 H), we became interested in exploiting these high stable magnetic fields combined with fast-spinning solid-state NMR probes for analysis of inorganic oxides, particularly focusing on the structures and dynamics of surface species relevant to catalysis. Here, we highlight several representative case studies showing the dramatic improvements in resolution that can be obtained for inorganic systems. This is demonstrated for a well-defined silica-supported organometallic catalyst, dehydroxylated γ-alumina and a zeolite as illustrative examples. The improved signal resolution, particularly for proton NMR and quadrupolar nuclei, yields highly resolved 1D and 2D MAS NMR spectra that contain key information on surface structures, validating the approach.
We focus first on assessing the resolution obtained in 1 H MAS NMR spectra of heterogeneous materials, which can be significantly broadened due to inhomogeneous effects like chemical shift dispersion. 18 Nevertheless, the resolution benefits of fast MAS and 28.2 T acquisition conditions are still remarkable. This is illustrated by the 1D 1 H MAS NMR spectra of a well-defined silica-supported organometallic species, 2 9 , 3 0 the W alkylidene (ArN)W(Me 2 Pyr) 2 (CHCMe 2 Ph) (Ar = 3,5-dimethyl-phenyl; Me 2 Pyr = 1,4dimethylpyrrolide) grafted on partially dehydroxylated silica (SI Figure S2.1). Signals from different methyl, pyrrolidine, aromatic, and alkylidene 1 H species are broad and unresolved under conventional measurement conditions (Figure 1, red).
Resolution improves considerably at 28.2 T and 65 kHz MAS (Figure 1, blue, black; SI Section S2). Notably, the 1 H NMR signal of the alkylidene proton is well-separated from the aromatic resonances. A shoulder at 10.5 ppm is also partially resolved and assigned based on literature reports to the anti alkylidene rotamer, 31 typically present in lower amounts compared to the syn rotamer, which shows a more intense signal at 9.4 ppm. The resolution of different alkylidene species by 1 H NMR in the solid state is noteworthy, as the alkylidene moiety is responsible for their olefin metathesis catalytic activity and is typically impossible to observe by 13 C MAS NMR without isotopic enrichment. 32,33 Such highly resolved 1 H MAS NMR spectra could support and improve NMR-based tools for three-dimensional structural determination of surface species. 34,35 As a second case study, we observe substantially improved resolution in the solid-state 1 H MAS NMR spectra of needleshaped γ-alumina as a function of magnetic field and MAS rate. Recently, we reported the synthesis and solid-state NMR characterization of needle-shaped γ-alumina crystallites with a larger proportion of edge and surface sites. 36 The 1 H MAS NMR spectra of the γ-alumina needles resolve surprisingly narrow 1 H signals from different OH sites, with resolution improving with both higher field and faster MAS (Figure 2a   to different extents in a network of interacting and dipole− dipole coupled surface OH groups. Only the signals at 1.1 and 1.7 ppm narrow substantially with increasing MAS rate ( Figure  2a,b, Figure S3.1). The different MAS-dependencies of the 1 H signals suggest the influence of substantial 1 H dipole−dipole couplings for specific surface 1 H species and appear related to their very different measured nuclear spin relaxation time behavior (Table S3.1). The mutual interactions of these specific sites are corroborated by 2D 1 H{ 1 H} nuclear Overhauser effect spectra (NOESY, Figure S3.2), which show that the bridging μ 2 −OH species associated with the 1 H signals at 1.1 and 1.7 ppm are highly dynamic/fluctional and are in very close mutual spatial proximity compared to those with 1 H signals at 2.2 and 2.5 ppm. These physiochemical insights can provide valuable constraints on models of the γ-alumina surface, the structure of which is still a matter of considerable investigation.
The second-order quadrupolar contribution to the NMR lineshapes of quadrupolar nuclei like 27 Al depends inversely on magnetic field strength, 15 36 However, this value is lower than expected for tetrahedrally coordinated 27 Al sites on the surface of highly dehydroxylated γ-alumina based on firstprinciples calculations. 39 It has been suggested that such 1 H− 27 Al double-resonance experiments might enhance relatively narrow signals from subsurface 27 Al sites, rather than broader signals from surface species. 39 Correspondingly, unambiguous insights into the nature of γ-alumina surfaces have been elusive. Lineshape analyses of 1D slices of the 2D 1 H{ 27 Al} D-HMQC spectrum indicate the presence of correlated 27 Al signals with C Q values of at least 15.5 MHz ( Figure S3.4, Table S3.2), within the 15−20 MHz range calculated for tetrahedrally coordinated surface 27 Al sites. The combination of very high magnetic fields and fast spinning thus appears a promising route to detect surface species.
The extensibility of the high field and fast MAS conditions to diverse material systems is illustrated by analysis of a prototypical aluminosilicate catalyst, dehydrated microporous mordenite zeolite. Elucidating the local structures of aluminum heteroatoms in zeolites is of great importance in understanding their reactivities, though the nature and distributions of framework and extra-framework Al sites have long been elusive. 40 This is particularly true after dehydration of the framework, which leads to significant broadening of 27 Al NMR signals. Recently, our group provided evidence that the Lewis acid sites in mordenite zeolite are pseudo-tricoordinate framework Al interacting with a coordinated siloxane bridge. 41 The 1D 27 Al NMR spectrum of dehydrated mordenite is substantially narrowed at 28.2 T compared to 16.4 T (Figure  3a), and two different signals are resolved that can be associated by analysis of the 2D 27 Al{ 1 H} D-HMQC spectra with Bro̷ nsted and Lewis acid sites (Figure 3b) as previously discussed. 41 The 2D 27 Al triple-quantum MAS (TQMAS) spectrum 42 of the zeolite (Figure 3c) separates the two signals further and enables their spectroscopic parameters to be estimated, yielding C Q values consistent with those previously reported. 41 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-4-methyl-phenyl) 43 as a model compound (Figure 3d). The compound exhibits a 27 Al isotropic shift of 44 ppm and a quadrupolar coupling constant of 29.6 MHz, in very good agreement with values predicted from first-principles calculations (Table S4.1). The 27 Al chemical shift of this compound is significantly shielded compared to what is expected for tricoordinate Al in aluminosilicates (δ iso = 87 ppm and C Q = 35 MHz), 44 due to the differences of aryloxide vs (surface) siloxide ligands and associated σ/π effects, 45 but shows similar quadrupolar coupling constants as expected from their similar trigonal planar geometry. 44 At lower magnetic field strengths or without adequate MAS rates, static wide-line excitation and detection methods are necessary to extract the 27 Al parameters of such Al sites, 46 which would limit spectral resolution. Though such tricoordinate Al species have been proposed to exist under some conditions in aluminosilicate zeolites, 47 their spectroscopic signatures have never been observed before and would be basically impossible to resolve at lower magnetic fields due to overlap of signals from other Al sites ( Figure  S4.1). Comparison of the 27 Al spectra of mordenite zeolite and Al(OAr*) 3 shows no evidence for large-C Q species consistent with tricoordinate Al, corroborating our recent conclusion that the Lewis acid sites in mordenite zeolite under these conditions are predominantly pseudo-tricoordinate Al sites having a labile siloxane moiety coordinated. 41 Overall, the adoption of high field (28.2 T) and fast MAS (>50 kHz) provides a 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. We anticipate that the advent and broader adoption of very high field NMR spectrometers and fast-spinning probe-heads including probes capable of MAS rates >100 kHz will additionally spur the development of new solid-state NMR pulse sequences, instrumentation, and methods to optimize sensitivity and resolution. In particular, these measurement conditions provide exceptional promise for high-resolution spectra of quadrupolar nuclei.