15N-pyridine—a mobile NMR sensor for surface acidity and surface defects of mesoporous silica

Abstract

experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 15 N CPMAS and MAS experiments (CP cross-polarization) performed on pyridine- 15 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be nOH 3n m -2 for MCM-41 and 3.7 nm -2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers nOH. Using a correlation established previously between the 15 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about rHN ) 1.68 A,rOH ) 1.01 A, andrON ) 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pKa of about 4. Roomtemperature 15 N experiments on static samples of pyridine- 15 N in MCM-41 at low coverage show a residual 15 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d1. In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 29 Si CPMAS of a substantial amount of Si(OH)2 groups in SBA-15, in contrast to the MCM-41 materials. The Si(OH)2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.