Quantitative Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by combining Adsorption, Liquid Intrusion and solid-state NMR spectroscopy (original) (raw)
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Zeitschrift für Physikalische Chemie, 2018
A series of novel functionalized mesoporous silica-based materials with well-defined pore diameters, surface functionalization and surface morphology is synthesized by co-condensation or grafting techniques and characterized by solid-state NMR spectroscopy, DNP enhanced solid state-NMR and thermodynamic techniques. These materials are employed as host-systems for small-guest molecules like water, small alcohols, carbonic acids, small aromatic molecules, binary mixtures and others. The phase-behavior of these confined guests is studied by combinations of one dimensional solid-state NMR techniques (1H MAS, 2H-line shape analysis, 13C CPMAS) and two-dimensional correlation experiments like 1H-29Si- solid-state HETCOR.
Investigation of the surfactants in CTAB-templated mesoporous silica by 1H HRMAS NMR
Microporous and Mesoporous Materials, 2003
High resolution magic angle spinning (HRMAS) leads to nearly liquid-state quality NMR spectra of molecules with restrained mobility. We show here how 1 H HRMAS can be applied to organic molecules encapsulated inside mesoporous materials. We investigated an uncalcined surfactant-templated mesoporous silica synthesized from a mixture of cationic and anionic surfactants, CTAB and HPMSP. The pyrazolone HPMSP is adding cation-extracting properties to the silica, which contains 60% of organic compounds in weight. MAS NMR at moderate spinning speeds allows to resolve proton spectra on samples where a small amount of methanol is added to the dried as-synthesized silica. NMR experiments allow to distinguish between solvated surfactants involved in ion pairs and less mobile templating surfactants. Liquid state NMR experiments like 2D NOESY can be performed in these conditions, but suffer from spin diffusion. 1D and 2D solidstate NMR experiments, like Rotational Resonance, which take advantage of the partly solid-state behavior of the surfactant system, are proposed as alternative experiments to get information about spatial connectivity.
Evaluation of silica-water surface chemistry using NMR spectroscopy
Geochimica et Cosmochimica Acta, 2002
We have combined traditional batch and flow-through dissolution experiments, multinuclear nuclear magnetic resonance (NMR) spectroscopy, and surface complexation modeling to re-evaluate amorphous silica reactivity as a function of solution pH and reaction affinity in NaCl and CsCl solutions. The NMR data suggest that changes in surface speciation are driven by solution pH and to a lesser extent alkali concentrations, and not by reaction time or saturation state. The 29 Si cross-polarization NMR results show that the concentration of silanol surface complexes decreases with increasing pH, suggesting that silanol sites polymerize to form siloxane bonds with increasing pH. Increases in silica surface charge are offset by sorption of alkali cations to ionized sites with increasing pH. It is the increase in these ionized sites that appears to control silica polymorph dissolution rates as a function of pH. The 23 Na and 133 Cs NMR results show that the alkali cations form outersphere surface complexes and that the concentration of these complexes increases with increasing pH. Changes in surface chemistry cannot explain decreases in dissolution rates as amorphous silica saturation is approached. We find no evidence for repolymerization of the silanol surface complexes to siloxane complexes at longer reaction times and constant pH.
Aqueous-sensitive reaction sites in sulfonic acid-functionalized mesoporous silicas
Journal of Catalysis, 2008
Local differences in surface hydrophilicities/hydrophobicities of propyl-and arene-sulfonic-acid modified mesoporous silica and organosilica catalysts have been compared and correlated with their bulk catalytic properties for aqueous-sensitive organic reactions. Syntheses of propyland arene-SO 3 H-modified mesoporous silicas and organosilicas yield materials with different hydrophilicities, especially when ethylsiloxane moieties are incorporated into the silica frameworks. Solid-state two-dimensional (2D) 13 C{ 1 H} and 29 Si{ 1 H} heteronuclear correlation (HET-COR) NMR spectra prove that the incorporation of hydrophobic ethylsiloxane groups into functionalized mesoporous silica frameworks result in reduced interactions of adsorbed water with the silica framework in general and, importantly, in the immediate vicinities of the SO 3 H active sites. The hydrophilic/hydrophobic character of the surface, as well as the active site properties depend on the functional species attached. Propylsulfonic acid moieties are less acidic but more hydrophobic than arene-SO 3 H species, leading to superior overall activities for water-mediated acid-catalyzed organic reactions. The etherification of vanillyl alcohol (4-hydroxy-3-methoxybenzylalcohol) with 1-hexanol to yield 4-hydroxy-3-methoxybenzyl-1-hexyl ether is shown to proceed significantly more effectively on SO 3 H-modified mesoporous organosilicas, compared to wholly siliceous mesoporous supports. The correlation of macroscopic adsorption and reaction results with 2D NMR measurements allows the hydrophilic/hydrophobic surface properties of the mesoporous support to be optimized with respect to water-retention capacities and activities for water-sensitive organic reactions.
Journal of Porous Materials, 2020
Ordered mesoporous silica (OMS) is an important and useful material for a variety of applications, including catalysis, adsorption, sensing and controlled drug delivery. The surface chemistry and the silanol groups on OMS pores are key properties for the potential modification and application of this material. This research aimed to synthesize (using standard protocols) and differentiate the accessibility and strength of the H-acceptor Si-OH from FDU-12, SBA-16, MCM-41 and SBA-15 by pyridine (Py) donor, where the first two have cubic pore structures and the last two have hexagonal pore structures. Donor-acceptor properties were assessed by calculation of the surface Si-OH densities by thermogravimetry (TG), H 2 O-TPD/MS, and 29 Si MAS and CP/MAS NMR. The nature of the Si-OH groups on these materials was determined to be hydrogen-bonding sites using FT-IR spectroscopy of Py adsorption. The reactivity of these silanol groups was probed by Py-TG and slurry microcalorimetry of Py adsorption in cyclohexane. Differences in accessibility and reactivity were discussed considering the total potential sites on the surface (n OH) versus the actual sites that can react with the Py molecule (n Py). By using microcalorimetry, it was possible to quantitatively distinguish the strength of the sites: The acidity order was approximately the same as the relative amount of silanol groups (Si-OH) and Py on the surface of the OMS materials (α Py): FDU-12 > MCM-41 ≥ SBA-16 > SBA-15.
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.
Analytical Chemistry, 2008
cross-polarization magic angle spinning (CPMAS) NMR spectroscopy is demonstrated to be a valuable characterization tool for quantitative measurements of Hg 2+ adsorption capacity in thiol-functionalized mesoporous silica SBA-1 (Santa Barbara Amorphous-1). This is the first report on the investigation of the spectral change in the 13 C CP signals for mercaptopropyl-functionalized mesoporous materials doped with different Hg 2+ concentrations. The chemical shift of the carbon atom adjacent to the thiol group is sensitive to the binding of the mercury ion, and its peak intensity can be used as a quantitative sensor for the amount of the mercury ion adsorbed. The 13 C CPMAS NMR results are in good agreement with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis.
Chemistry (Weinheim an der Bergstrasse, Germany), 2004
The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), po...