On the silica edge, an NMR point of view (original) (raw)
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Journal of the American Chemical Society, 2011
We demonstrate fast characterization of the distribution of surface bonding modes and interactions in a series of functionalized materials via surface-enhanced nuclear magnetic resonance spectroscopy using dynamic nuclear polarization (DNP). Surface-enhanced silicon-29 DNP NMR spectra were obtained by using incipient wetness impregnation of the sample with a solution containing a polarizing radical (TOTAPOL). We identify and compare the bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions. Furthermore, the remarkable gain in time provided by surface-enhanced silicon-29 DNP NMR spectroscopy (typically on the order of a factor 400) allows the facile acquisition of two-dimensional correlation spectra.
Effects of proton-proton spin exchange in the silicon-29 CP-MAS NMR spectra of the silica surface
The Journal of Physical Chemistry, 1992
Four 29Si CP-MAS NMR pulse sequences have been used to study the effects of IH spin exchange among various 'H spin reservoirs on a silica gel surface. Both 1H-29Si dipolar-dephasing experiments and proton-coupled 29Si CP-MAS results indicate that various hydroxyl groups of silanols in silica gel undergo rapid 'H spin exchange and that the most strongly coupled protons provide the dominant source of cross polarization to geminal-silanol silicons. IH-IH dipolar dephasing prior to IH-29Si cross polarization shows a rapid 'H spin exchange between 'H reservoirs of single-silanol groups and of geminal-silanol groups; however, the 'H spin exchange rate is too fast to be measured by this strategy. A slower 'H-'H dipolar-dephasing decay due to 'H spin exchanges is found for the 'H spin reservoir of single silanol groups. A pulse sequence that utilizes a 'H-IH dipolar-dephasing period followed by an evolution period for z-axis spin exchange prior to 'H-29Si cross polarization provides a measure of a proton (chemical) exchange rate. The results are discussed in terms of 8-cristobalite models of the silica surface.
Water Adsorption on Pyrogenic Silica Followed by1H MAS NMR
Journal of Colloid and Interface Science, 1997
physisorbed water), and their accessibility to water (internal On the surface of two commercial pyrogenic silicas (Degussa or external silanols). Study of the state of physisorbed water and Cabot), five resonances were identified on the basis of the itself led to the distinction between structured monolayers, chemical shift, homonuclear coupling ( T 2 ), and spin-lattice relaxclusters, and weakly bound liquid-like water molecules (14). ation behavior (T 1 ). In accordance with previous studies we ob-Nevertheless, several difficulties remain when studying served three different types of silanol groups: (i) weakly coupled silica surfaces. First, the surface is not structured. Therefore, (long T 2 ), water inaccessible, isolated ''internal'' silanols at 1.8 although models based on small crystalline domains have ppm; (ii) weakly coupled, external ''free'' silanols revealed upon been proposed (7, 10, 15), surface irregularity leads to a dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between dispersion of the physicochemical properties of the hydroxyl 3 and 7 ppm. The resonance of water, whose position between 2.6 sites. Second, thermogravimetric dosing of structural and and 4.6 ppm depended on water content, corresponded to two adsorbed water is difficult as (de)hydration overlaps with unresolved species of slightly different T 1 . By equating this reso-(de)hydroxylation. Third, amorphous silica is a metastable nance to the weighted average of two distinct populations of water, phase, and its surface is heavily reconstructed during the we were able to distinguish the first layer of strongly hydrogen conditioning of the samples. bound water at 2.7 ppm from liquid-like water at 5 ppm. The first Among the different techniques mobilized to scrutinize layer is complete for water relative humidity as low as 3.6% and protons on the surface of silica in particular, and oxides in corresponds to a surface coverage of 4.75 H 2 O/nm 2 . If we assumed general, 1 H NMR is certainly among the most promising a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. because of its great sensitivity and dependency on hydrogen This ratio was the same for the two silicas regardless of surface bonds. Despite severe line broadening due to a combination area. ᭧ 1997 Academic Press of strong homodipolar coupling and chemical shift disper-Key Words: 1 H MAS-NMR; silica.
Analytical Chemistry, 1988
Nuclear magnetic resonance (NMR) studles have been carried out on a series of samples prepared by derlvatlzatlon of silica gels with (3-aminopropyl)triethoxysilane (APTS) under systematically varled sillca pretreatment temperatures, APTS reaction conditions, and APTS-modifled sillca posttreatment temperatures. %i and I3C technlques employlng cross polarization (CP) and magic-angle spinning (MAS) were used. 13C CP/MAS NMR studies of the structures of the APTSmodlfied slllcas reveal that the amino groups in samples prepared In dry toluene can be either hydrogen bonded or protonated by acidic slianols at the silica gel surface; the relative amount of the protonated form Increases wlth the amount of water present at the dllca surface. Silica hydrath EXPERIMENTAL SECTION NMR Measurements. Solid-state 13C and %i NMR spectra were obtained in natural abundance at frequencies of 50.3 and 39.7 MHz, respectively, on a modified Nicolet NT-200 spectrometer equipped with a home-built CP/MAS unit. All cross polarization spectra were obtained with spin-temperature alternation (12). The CP/MAS probes were based on a doubly tuned, single-solenoid arrangement. Magic-angle spinning was routinely carried out at 1.5-2.0 kHz (13C) and 3.5-4.0 kHz (%i) with rotors machined from Kel-F and Delrin, respectively. Modified silica samples prepared under anhydrous conditions were used to determine if moisture from the laboratory air was altering samples during CP/MAS experiments. In test experiments, no 13C or =Si spectral differences were found between samples spun with dry nitrogen and compressed air, so the latter was used for all samples on which data are reported in this paper. For 13C spectra, the magic angle was adjusted to within 0.lo using the 79Br spectrum of a sample of KBr placed in the spinner (13). 13C spectra were obtained with a 1-ms contact time and a recycle of 1 s. 29Si spectra were acquired with a 5-ms contact time and a recycle time of 1 s. Solid-state W i CP/MAS NMR spectra were obtained with 1K data tables and a spectrum width of 20 kHz. A total of 10000-40000 accumulations were coadded for each spectrum. 29Si NMR spectra were obtained by using the CP sequence with the flip-back feature (14). 13C and %i spectra were externally referenced to liquid tetramethylsilane (TMS), based on substitution of hexamethylbenzene and tetrakis(trimethy1silyl)methane, respectively. All chemical shifts in this paper are reported in parts per million, with lower values corresponding to higher shielding. Solution-state 13C and 29Si NMR spectra were acquired on a Bruker WP-2OOSY spectrometer at 50.3 and 39.7 MHz, respectively. Spectra were obtained in the pulse mode with simultaneous broad-band decoupling. Spectral widths of 20 kHz (W) or 10 kHz (29Si) with 16K data tables were used. NMR spectra were recorded on samples that were 15-25% (v/v) in deuteriated chloroform, used as solvent and for deuterium lock, with TMS as an internal chemical shift reference.
Pyridine- 15 N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica
The Journal of Physical Chemistry B, 2003
The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1 H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 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 n OH ≈ 3 nm -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 n OH . 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 r HN ) 1.68 Å, r OH ) 1.01 Å, and r ON ) 2.69 Å. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a 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 C 2 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-d 1 . 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.