Abstract
A solid-state H-1 magic-angle spinning (MAS) NMR method is described to characterize the internal pore spaces and the external void spaces of powdered mesoporous solids containing surface OH groups. This method is based on fast proton exchange between homogeneously distributed surface OH groups of the porous hosts and added guests containing exchangeable NH protons. The position of the coalesced ...
Abstract
A solid-state H-1 magic-angle spinning (MAS) NMR method is described to characterize the internal pore spaces and the external void spaces of powdered mesoporous solids containing surface OH groups. This method is based on fast proton exchange between homogeneously distributed surface OH groups of the porous hosts and added guests containing exchangeable NH protons. The position of the coalesced NH/OH signal in a given local space depends on its surface/volume ratio (S/V). In the case of slow guest exchange between spaces with different S/V ratios, coalesced signals are observed for each space where the relative signal intensities reflect the mole fractions of the guests in the different spaces. This method was tested by performing H-1 MAS NMR experiments on the samples of liquid 4-methyl-1H-pyrazole (MPz) embedded in mesoporous silica of the MCM-41 type (2.9 nm pore diameter) and the SBA-15 type (8.9 nm pore diameter). To guarantee fast NH/OH proton exchange, the experiments were performed at 398 K well below the boiling point. Three distinct signals were observed for MPz assigned to (i) the internal cylindrical pores, (ii) the external interstitial void space between the packed particles, and (iii) the space outside the powdered solid containing neat liquid MPz. From the NMR data analysis, surface/ volume ratios are derived for the internal pores, which agree well with those obtained with a simple geometrical model after applying a surface-roughness correction. In addition, S/V ratios for the external space, as well as internal surface/external surface (S/S) and internal volume/external volume (V/V) ratios, are derived. It is shown that at low filling fractions, MPz preferentially enters the internal pores, but at larger fractions, MPz enters the external spaces. Moreover, it is shown that the final internal pore NH/OH ratio is achieved before the pores are filled. A scenario that rationalizes these findings in connection with the hydrogen-bonded states of MPz is presented where the number and the hydrogen-bonded state of guest molecules in a given pore are discussed.
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