Correlation between deuterium NMR spectral components and MCM-41 pore surface hydration sites (original) (raw)
Related papers
NMR study of exchange and hydration site identification in MCM-41
2006
Deuteron 1D and 2D NMR spectroscopy was used to study the dynamics of water molecules within the mesoporous material MCM-41. The deuteron spectra show three magnetization components for a sample hydrated to a 0.2 monolayer level. One component was assigned to the pore surface silanol group deuterons that exhibit a broad Gaussian line of 32.6 kHz FWHM and the other components were assigned to the water deuterons. At room temperature one water deuteron component has a powder-pattern line shape (splitting of about 4.2 kHz and population of about 61%) and the other has a Lorentzian line shape (about 388 Hz FWHM and population of 39%). Magnetization exchange occurs between these components. An exchange model, based on multi-site exchange, was constructed and used to analyse the results for exchange. For the 0.2 monolayer sample the rate of magnetization exchange out of the hydration site, where the water deuterons exhibit a Lorentzian line in the 2 H spectra is, 1.3 ms −1. 2D measurements at 233 K and room temperature confirmed the magnetization exchange scenario for the two water deuteron sites. By combining the deuteron results with proton-silicon cross polarization magic angle spinning experiments, together with heat treatment of the sample, definitive hydration site identification for MCM-41 was achieved. This study has shown that the water molecules bound to the hydrogen-bonded silanol groups produce the powder pattern while water molecules bound to the single silanol groups produce the Lorentzian line. This represents a necessary first step toward a meaningful modeling of NMR observables in terms of site-specific water molecule coordination and dynamics in MCM-41. iii 5-22 Experimental 2D contour maps at room temperature for Cos Cos, Sin Sin along with their differences at different mixing times.. .
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...
Water-Saturated Mesoporous MCM-41 Systems Characterized by 1H NMR
The Journal of Physical …, 1994
lH N M R measurements on a new family of mesoporous molecular sieves, designated MCM-41, saturated with water show abrupt changes in the signal intensity of the water signal at specific temperatures, denoted as transition temperatures. A model equation is presented describing the intensity vs temperature behavior. Similar transition phenomena are also observed in line width vs temperature. A linear relationship between the spinlattice relaxation rate (at -10 "C) and the first transition temperature, as determined from the intensity measurements, is found. This behavior is expected if it is assumed that the transition temperature can be predicted by Kelvin's equation and that the spin-lattice relaxation rate is proportional to the inverse of the pore radius, as previously reported in the literature.
Water-saturated mesoporous MCM-41 systems characterized by 1H NMR spin-lattice relaxation times
The Journal of Physical Chemistry, 1995
lH N M R measurements on a new family of mesoporous molecular sieves, designated MCM-41, saturated with water show abrupt changes in the signal intensity of the water signal at specific temperatures, denoted as transition temperatures. A model equation is presented describing the intensity vs temperature behavior. Similar transition phenomena are also observed in line width vs temperature. A linear relationship between the spinlattice relaxation rate (at -10 "C) and the first transition temperature, as determined from the intensity measurements, is found. This behavior is expected if it is assumed that the transition temperature can be predicted by Kelvin's equation and that the spin-lattice relaxation rate is proportional to the inverse of the pore radius, as previously reported in the literature.
Probing Pore Size Distribution by Cryogenic- and Relaxation 2 H-NMR
The Journal of Physical Chemistry B, 2002
Cryogenic NMR and NMR spin-lattice relaxation time (NMRT) measurements of pore-confined water (D 2 O) have been performed using deuterium NMR to probe the pore size distribution (PSD) of silica materials and porous membranes. NMRT measurements were performed at a temperature slightly below the normal freezing point (277 K) of bulk water (D 2 O) to ensure that all interparticle water was frozen out. PSD derived from cryogenic NMR was in excellent agreement with PSD obtained from N 2 -adsorption measurements. Also, PSD obtained by NMRT revealed approximately the same average pore dimension as obtained by N 2 adsorption. However, the former experimental technique resulted in somewhat narrower PSD than obtained by cryogenic NMR and N 2 adsorption and is discussed in the text. An attempt to determine the PSD of a TiO 2 membrane on a silica support by NMRT will also be discussed. The main results obtained in this work suggest that a combined use of cryogenic NMR and NMRT may give information on both PSD and pore-connectivity.
Multiscale Approach to the Structural Study of Water Confined in MCM41
The Journal of Physical Chemistry B, 2009
We present a protocol for simultaneous structural characterization of a confined fluid and the confining substrate, along with the extraction of site-site pair correlation functions of the liquid of interest. This is based on neutron diffraction experiments, exploiting where feasible the isotopic substitution technique, analyzed through numerical coarse graining calculations and atomistic simulations. All of the subtleties of the experimental procedure, the needed ancillary measurements, and the recipe for tailoring the numerical codes to the real experiment and sample are described in the case of water confined in MCM41-S-15. In particular the excluded volume effects and the relevance of liquid-substrate cross-correlation terms in the neutron cross section are quantitatively discussed. The results obtained for the microscopic structure of water evidence a nonhomogeneous distribution of molecules within the pore, with the presence of water-substrate hydrogen bonds, and a strong distortion of the water-water radial distribution functions with respect to those of bulk water extending at least up to three hydration layers.
Applied Surface Science, 2019
The temperature dependent morphologies of the co-existing solid ice and the interfacial water within two classic mesoporous materials, MCM-41 and SBA-15, are investigated below the Gibbs-Thomson transition temperature using Cryo-NMR, NMR spin-diffusion and NMR spectral analysis (second moment calculations). By using the combined NMR approaches, the difference of the ice cores and interfacial water layers in MCM-41 and SBA-15 is unveiled. Based on the different thickness of the interfacial water layers, the surface features in the two porous materials are