Probing local atomic configurations around hydrogen in kaolinite, gibbsite, and bayerite: A high-resolution 1H fast magic-angle spinning NMR study

다국어 초록 (Multilingual Abstract)

Detailed knowledge of the atomic configurations around hydrogen of crystalline materials in the Earth’s surface environment (e.g., clay and hydroxide minerals) is essential to understand the dehydration process during diagenesis which are related to water storage capacity of minerals in sedimentary basins. While the refinement of crystal structure of hydrogen-bearing minerals was performed by several experimental probes such as neutron diffraction, it is difficult to resolve the crystallographically distinct hydrogen site with quantitative estimation. ¹H solid-state NMR is a powerful tool to provide the detailed structural information around hydrogen atoms, making it possible to quantify the hydrogen-related species in the minerals. However, the conventional MAS (magic-angle spinning) NMR techniques have been limited to resolve the individual hydrogen sites in the minerals due to the peak overlap. The lack of resolution among hydrogen sites arises from the strong dipolar interaction among proton spins. Recent advances in fast-to-ultrafast MAS NMR techniques with > 60 kHz of sample spinning speed allow us to effectively resolve the peaks of each hydrogen sites. In this study, we report the ¹H MAS NMR spectra at 14.1 T for diverse hydroxide and clay minerals (i.e., kaolinite, gibbsite, and bayerite) with varying sample spinning speeds up to 65 kHz, permitting the direct observation of the crystallographically distinct hydrogen sites in those crystalline materials. With increasing sample spinning speed, overall peak width decreases and peak intensity increases, indicating that the magnitude of ¹H-¹H dipolar interaction is reduced. Diverse peaks corresponding to varying hydroxyl groups become separated above 30 kHz, and each individual peak is more clearly resolved as the spinning speed increases. For example, the ¹H NMR spectra for kaolinite under 65 kHz show the distinct peaks corresponding to the inner surface hydroxyl (bonded to the outer part of Al octahedron) and inner hydroxyl (located between Al octahedron and Si tetrahedron). In addition, the fraction of ch crystallographically inequivalent hydrogen site can be directly obtained from the quantification of resolved peak area. The current results show that the ¹H MAS NMR with fast spinning speed allows us to directly observe the varying proton sites in the minerals and open a way to explore the site-specific dehydration paths of diverse Earth materials including crystalline and amorphous hydroxides and oxides.