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A General Correlation for the 129Xe NMR Chemical Shift-Pore Size Relationship in Porous Silica-Based Materials Victor V. Terskikh,† Igor L. Moudrakovski,† Steven R. Breeze,† Stephen Lang,† Christopher I. Ratcliffe,*,† John A. Ripmeester,† and Abdelhamid Sayari‡ Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada K1A 0R6, and Center for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 Received March 8, 2002. In Final Form: May 14, 2002 A general correlation for the 129Xe NMR chemical shift-pore size relationship (δ versus D) in porous silica-based materials over the range 0.5-40 nm has been demonstrated: δ ) δs/(1 + D/b), with δs ) 116 ( 3 ppm and b ) 117 ( 8 Å for the 34 materials studied. The correlation may be used in the characterization of silica samples with unknown pore structure. Even within this general correlation, subsets of materials of similar origin display finer correlations that indicate an acute sensitivity to details of the pore surfaces.
The NMR spectroscopy of adsorbed 129Xe has evolved into a sophisticated scientific tool for the study of different aspects of the structure and topology of internal voids in porous substances.1 Although some disadvantages of the technique have been noted,2 it remains attractive and popular, as judged by the fact that in 2001 about 50 Xe NMR-related papers were published. The latest improvements include the production of hyperpolarized xenon (HP Xe),3 which gives a dramatic increase in the sensitivity for a variety of applications.4 Besides such practical novelties, important theoretical developments directed at a much needed understanding of chemical shielding phenomena of confined 129Xe are in progress.5 For example, through empirical correlations were proposed between isotropic Xe chemical shifts and the pore size (δ-D correlation) in zeolites6a as well as clathrates and solid Xe,6b there is still no clear quantitative appreciation of the origin of this behavior. Attempts to extend it beyond zeolites have failed, and it has been pointed out that different correlations must exist for small pores, with a diameter less than about twice the diameter of a Xe atom, and large pores, as in the latter case account must be taken of Xe not adsorbed on the pore walls.7 * Author for correspondence. E-mail:
[email protected]. Telephone: (613) 991-1240. Fax: (613) 998-6775. † National Research Council. ‡ Center for Catalysis Research and Innovation. (1) (a) Raftery, D.; Chmelka, B. F. Nucl. Magn. Reson. 1994, 30, 111. (b) Ratcliffe, C. I. Annu. Rep. NMR Spectrosc. 1998, 36, 123. (c) SpringuelHuet, M. A.; Bonardet, J. L.; Gedeon, A.; Fraissard, J. Magn. Res. Chem. 1999, 37, 1. (2) Bonardet, J.-L.; Fraissard, J.; Gedeon, A.; Springuel-Huet, M.-A. Catal. Rev.sSci. Eng. 1999, 41, 115. (3) (a) Grover, B. C. Phys. Rev. Lett. 1978, 40, 391. (b) Happer, W.; Miron, E.; Schreiber, D.; van Wingaarden, W. A.; Zeng, X. Phys. Rev. A 1984, 29, 3092. (4) (a) Moudrakovski, I. L.; Nossov, A.V.; Lang, S.; Breeze, S. R.; Ratcliffe, C. I.; Simard, B.; Santyr, G.; Ripmeester, J. A. Chem. Mater. 2000, 12, 1181. (b) Terskikh, V. V.; Moudrakovski, I. L.; Du, H.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 2001, 123, 10399. (c) Demco, D. E.; Blumich, B. Curr. Opin. Solid State Mater. Sci. 2001, 5, 195. (d) Rubin, S. M.; Spence, M. M.; Dimitrov, I. E.; Ruiz, E. J.; Pines, A.; Wemmer, D. E. J. Am. Chem. Soc. 2001, 123, 8616. (5) (a) Jameson, C. J.; Jameson, A. K.; Lim, H.-M. J. Chem. Phys. 1996, 104, 1709. (b) Jameson, C. J.; deDios, A. C. J. Chem. Phys. 2002, 116, 3805. (6) (a) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136, 314. (b) Ripmeester, J. A.; Ratcliffe, C. I.; Tse, J. S. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3731. (7) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 7652.
We have shown previously that a simple fast exchange model explains a qualitatively similar, yet distinct, δ-D correlation found for mesoporous amorphous silica gels with a wide range of mean pore diameters from 2 to 40 nm.8 Assuming that the 129Xe NMR chemical shift of xenon adsorbed in mesoporous silica is a dynamic average between the gas and adsorbed states, it is straightforward to derive a parabolic dependence of the 129Xe NMR chemical shift, δ (ppm), on the mean pore diameter, D (Å),
δ ) δs/(1 + D/b)
(1)
where δs is the chemical shift characteristic of interactions of Xe with the silica surface, and the parameter b depends on the pore geometry (η), the adsorption constant (K), and the temperature (T), as8
b ) ηKRT
(2)
The mean pore diameter (D) is usually given through the volume-to-surface ratio as D ) ηV/S, where the geometry factor η is dependent on the model adopted for the pores. It can vary from 2.8 in a model of randomly packed globular particles (D ) 2.8V/S), to 4 for cylindrical pores (D ) 4V/ S) or 6 for unconnected spherical pores (D ) 6V/S). In this work we show that the model is more generally applicable by extending the range of silica-based porous materials, and we illustrate the correlation with previous work8,9 and new results10 on porous glasses and porous organosilicates. The δ-D dependence, which now extends over the 0.5-40 nm range, is shown in Figure 1 with the nonlinear least-squares fitting parameters summarized in Table 1.11 Considering first the silica gel samples, 1-18, it should be emphasized that these were of diverse origins, ranging (8) Terskikh, V. V.; Moudrakovski, I. L.; Mastikhin, V. M. J. Chem. Soc., Faraday Trans. 1993, 89, 4239. (9) Terskikh, V. V. Ph.D. Thesis, Novosibirsk State University, 1997. (10) Texture characteristics, pore sizes, and 129Xe NMR chemical shifts for the three groups of materials are given in Tables 1s, 2s, and 3s of the Supporting Information, together with details of the characterization of the pore structure. (11) Experimental data were fitted with a nonlinear least-squares fitting (NLSF) procedure based on the Levenberg-Marquardt (LM) algorithm (Origin 5.0, Microcal Software, Inc.). Detailed NLSF parameters, together with χ2 (sums of squares of deviations), are given in the Supporting Information, Table 4s.
10.1021/la025714x CCC: $22.00 © 2002 American Chemical Society Published on Web 06/22/2002
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Figure 1. 129Xe NMR chemical shifts vs mean pore diameters for porous silica-based materials: b, silica gels; O, Vycor/CPG; 2, POS-I; 4, POS-II. The solid curve is the nonlinear least-squares fit for samples 1-18, with prediction bands given at a confidence level of 95% shown as the dotted curves. The dashed curve is the fit for all 34 results. Inset: Fits for samples 26-29 (lower curve) and 30-34 (upper curve).
from specially prepared laboratory samples to bulk commercial sorbents;10 therefore, one would not expect a perfect correlation. Nevertheless, the correlation obtained is characterized by a reasonably narrow data distribution, thus providing strong support for the proposed model. The adequacy of this approach for studying amorphous mesoporous silicas has recently been confirmed.12 Moreover, analysis of the correlation obtained shows that the model is self-consistent: the physical validity of the δs and b parameters can be confirmed in independent experiments. The parametrized chemical shift δs ) 118 ( 5 ppm characterizing interaction of xenon with the SiO2 surface is essentially the same as has been found for xenon adsorbed on silicas at low temperatures of 170-200 K.8,12 Analysis of the parameter b ) 122 ( 12 Å according to eq 2 gives the xenon/silica adsorption constant K ∼ 1 × 1014
Torr-1 m-2, which is close to the results found in our roomtemperature adsorption experiments and in the literature.8,12b We note, however, that the parabolic dependence, eq 1, is somewhat insensitive to variations in b: in our case even (50% changes in b would still give experimental results falling within the derived confidence limits. In this sense, the correlation is not particularly sensitive to the pore geometry. The second series of materials examined, namely the controlled pore glass (CPG) samples, 20-25, and the Vycor porous glass sample, 19, are almost pure vitreous silica.13 CPG pore sizes can be controlled over a wide range by appropriate preparative procedures, with the pore size distribution remaining quite narrow. It makes them an ideal model with which to examine the general applicability of 129Xe NMR in studying pore size. The new results14a displayed in Figure 1 (129Xe NMR spectra are shown in Figure 2) are within the 95% confidence limit of the correlation derived for the amorphous silica gels, for almost all the samples. Nevertheless, a slightly different δ-D correlation curve can be found for these samples if they are treated separately (Table 1). This is not unexpected, since amorphous silica gels and porous glasses have entirely different arrangements of internal voids. However, both seem to obey a common set of 129Xe
(12) (a) Julbe, A.; De Menorval, L. C.; Balzer, C.; David, P.; Palmeri, J.; Guizard, C. J. Porous Mater. 1999, 6, 41. (b) Cros, F.; Korb, J.-P.; Malier, L. Langmuir 2000, 16, 10193.
(13) (a) Elmer, T. H. In ASM Engineered Materials Handbook; Schnieder, S. J., Jr., Ed.; ASM: Materials Park, OH, 1992; Vol. 4, p 427. (b) Gelb, L. D.; Gubbins, K. E. Langmuir 1998, 14, 2097.
Table 1. Nonlinear Least Squares Fitting Parameters sample
δs, ppm
b, Å
1-18 19-25 26-29 30-34
silica gels Vycor & CPGs POS-I POS-II
118 ( 5 97 ( 6 112 ( 2 118 ( 2
122 ( 12 130 ( 18 115 ( 16 143 ( 17
1-34
all samples
116 ( 3
117 ( 8
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Figure 3. Hyperpolarized (HP) continuous-flow (CF) NMR spectra for the samples of the POS-I series.
Figure 2. 129Xe NMR spectra for Vycor and controlled pore glass (CPG) samples.
NMR rules, where the determining factors are the interaction of xenon with the SiO2 surface and the fast xenon exchange between adsorbed and free gas states within the same particle. Contrary to this, however, it has been reported that some of the 129Xe NMR data for ordered mesoporous silicas (MCM-41 family) do not seem to fit the model proposed for amorphous silicas.2,16 We suspect that the bulk properties of the porous materials, which can be determinative in obtaining meaningful 129Xe NMR data,17 play (14) (a) The Vycor (Corning) and Controlled Pore Glass (CPG Inc.) samples (19-25) were dehydrated in a vacuum at 400-450 °C and then loaded with ∼800 Torr of Xe at room temperature and flame-sealed in the NMR tubes. 129Xe NMR spectra were measured at room temperature at 110.6 MHz on a Bruker DSX-400 NMR spectrometer (9.4 T), and referenced to Xe gas at low pressure. Short pulses were used with delays of several seconds to ensure complete relaxation. Chemical shifts are essentially independent of the Xe loading. (b) Porous organosilicate (POS) samples (26-34) were prepared as described in ref 19. Immediately prior to 129Xe NMR measurements, the POS samples (which are essentially hydrophobic) were dehydrated in a flow of helium at 100-110 °C. 129Xe NMR spectra were measured using a home-built continuous flow (CF) hyperpolarized (HP) Xe static probe. The CF system15 delivers HP Xe (1% in a mixture of 1% N2 and 98% He) directly to the sample in the NMR coil. (15) Moudrakovski, I. L.; Lang, S.; Ratcliffe, C. I.; Simard, B.; Santyr, G.; Ripmeester, J. A. J. Magn. Res. 2000, 144, 372. (16) (a) Springuel-Huet, M.-A.; Sun, K.; Fraissard, J. Microporous Mesoporous Mater. 1999, 33, 89. (b) Springuel-Huet, M.-A.; Bonardet, J.-L.; Gedeon, A.; Yue, Y.; Romannikov, V. N.; Fraissard, J. Microporous Mesoporous Mater. 2001, 44-45, 775. (c) Pietrass, T.; Kneller, J. M.; Assink, R. A.; Anderson, M. T. J. Phys. Chem. B 1999, 103, 8837. (17) Ripmeester, J. A.; Ratcliffe, C. I. Anal. Chim. Acta 1993, 283, 1103.
129
Xe
a role in these apparent discrepancies. For example, in powdered solids fast exchange of the xenon between interand intraparticle space can result in a particular sensitivity of 129Xe NMR experiments to the size and packing of the particles.17,18 If the xenon sampling distance (several micrometers in common solids at room temperature) exceeds the average size of the particles, the 129Xe NMR line will be broad and shifted upfield from its expected position toward the free gas line. This is especially the case for ordered mesoporous silicas, which often consist of aggregates and loose agglomerates of submicron particles. Such effects can be minimized by either (a) using the largest particles available or (b) compressing the material, provided that the integrity of the sample and the pore structure remain intact under the pressure applied. Indeed, it has been found that 129Xe NMR chemical shifts in certain compressed periodic mesoporous powders16b attain the values predicted for amorphous silicas with the same pore size. We have exploited approach (a), viz using samples with particles which are known to be large, in studying a third series of materials, namely porous organosilicates (POS) which are akin to the ordered mesoporous silica materials. These were prepared using 1,2-bis(trimethoxysilyl)ethane (BTME) as silica precursor and alkyltrimethylammonium chloride (CnTMACl) surfactants with different chain lengths (n ) 8, 10, 12, 14, 16, 18), as described elsewhere.19 The pore sizes of such materials depend on the surfactant carbon chain length, and can range from microporous for shorter chains to mesoporous for longer chains. Synthesis at room temperature (POS-I, samples 26-29) and in an autoclave at 95 °C (POS-II, samples 30-34) produced two slightly different strains of materials.19 Both types were comprised of fairly large particles ranging in size from 2 to 10 µm. Nevertheless, in the 129Xe NMR spectra14b of some of these samples, shown in Figures 3 and 4, broad (18) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Appl. Magn. Reson. 1995, 8, 385. (19) Hamoudi, S.; Yang, Y.; Moudrakovski, I. L.; Lang, S.; Sayari, A. J. Phys. Chem. B 2001, 105, 9118.
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Figure 4. Hyperpolarized (HP) continuous-flow (CF) NMR spectra for the samples of the POS-II series.
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129
Xe
lines from interparticle xenon can be seen along with the narrow line from xenon inside the large particles. Similar broad lines can be seen in the spectra of Vycor and CPG, Figure 2. The use of HP Xe in this work with smaller pore materials allowed us to work at very low concentrations of xenon, where the contribution of the Xe-Xe interactions is negligible and the observed 129Xe chemical shift reflects mainly interactions between the xenon atoms and the surface. The δ versus D results for these POS samples, Figure 1, once again fall within the 95% confidence limit of the fit derived for the amorphous silica gels, indicating that this correlation suits ordered silicas as well. Of even more significance, it works for both mesoporous and microporous types. It is also interesting that if the two data sets for POS-I and POS-II samples are treated individually, they can be fitted with two slightly different
NLSF curves characterized by very narrow data distributions, which do not deviate from the fits by more than 3 ppm (Figure 1 inset). From our previous 29Si MAS NMR studies, we are aware that the relative amount of fully condensed CSi(OSi)3 species for samples made by method II (POS-II) is systematically higher than that for samples prepared by method I (POS-I), where larger amounts of partially hydrolyzed C(OH)Si(OSi)2 and C(OH)2Si(OSi) were found.19 The higher degree of condensation in POS-II was attributed to the higher synthesis temperature. We believe that the slightly different δ-D dependencies can be related to the degree of condensation, since the interactions of Xe with the POS surface and, therefore, the different wall densities will alter its 129Xe NMR response. Even though these data sets show such fine discrimination in their correlations, nonetheless, they are both still well within the 95% confidence limits found for amorphous silica samples. Currently, we are studying the possible influence of microdefects, or surface roughness,16a in the internal surface of the ordered mesoporous silicas on 129Xe NMR spectra. In conclusion, we have demonstrated the applicability of 129Xe NMR of adsorbed xenon to study porous silicabased materials ranging from microporous to mesoporous (pore sizes 0.5-40 nm). The derived δ-D correlations can be used in the characterization of samples with unknown pore structure. It has also been made clear that textural factors such as size and packing of the particles (bulk properties), as well as surface factors, including variations in the chemical composition (chemical properties) and structure of the surface, all factor into the observed Xe shift. An intriguing example is provided by the porous organosilicates, which demonstrate an acute sensitivity of their 129Xe NMR shifts to the intricate features of the surface structure. This sensitivity can be used to one’s benefit, provided that the samples studied are of similar origin and preparation. In a broader sense, one may cautiously predict that other oxide-based materials will give a very similar dependence as the one shown here as long as the xenon atoms contact only the surface oxygens rather than the other types of atom buried within the oxide framework. Supporting Information Available: Tables of materials characterization and properties, and detailed nonlinear least-squares fitting parameters. This material is available free of charge via the Internet at http://pubs.acs.org. LA025714X
