TY - JOUR

T1 - NMR-Detected Host-Guest Proton Exchange as a Tool to Explore Surface/Volume Ratios and Fluid Filling of Internal and External Spaces of Porous Solids Containing Surface OH Groups

AU - Torres-Barthelemy, Veronica

AU - Perez-Hernandez, Natalia

AU - Shenderovich, Ilya G.

AU - Tolstoy , Peter M.

AU - Denisov, Gleb S.

AU - Limbach, Hans-Heinrich

N1 - Publisher Copyright: © 2020 American Chemical Society.

PY - 2020/10/8

Y1 - 2020/10/8

N2 - A solid-state 1H 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 1H 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.

AB - A solid-state 1H 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 1H 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.

KW - DIFFUSION

KW - DRUG-DELIVERY

KW - ISOBUTYRIC ACID

KW - MCM-41

KW - MESOPOROUS SILICA NANOPARTICLES

KW - N-15 NMR

KW - SBA-15

KW - STATE NMR

KW - WATER

KW - X-RAY

UR - http://www.scopus.com/inward/record.url?scp=85096087423&partnerID=8YFLogxK

UR - https://www.mendeley.com/catalogue/fe2d02c3-ff73-3aa5-86d4-adcd1aec7410/

U2 - 10.1021/acs.jpcc.0c04889

DO - 10.1021/acs.jpcc.0c04889

M3 - Article

VL - 124

SP - 22082

EP - 22095

JO - Journal of Physical Chemistry

JF - Journal of Physical Chemistry

SN - 0022-3654

IS - 40

ER -