Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 04-NMR of Physisorbed 129Xe Used as a Probe to Investigate Molecular Sieves

NMR of Physisorbed 120XE Used as a Probe to Investigate Molecular Sieves

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3.2

Heteropolyoxometalate Salts (HPOM)

Another class of catalysts, heteropolyoxometalate salts (HPOM), which have been much used in recent years as selective oxidation catalysts, has been also investigated by 129Xe NMR spectroscopy. The first study on these compounds was performed by Bonardet et al. [230]. Such compounds are ionic solids with a discrete arrangement of anions and cations forming a variety of cage-like structures similar to that of zeolites. The most common are those whose anions have the Keggin structure: the basic unit consists of 12 MO6 octahedra (M = W or Mo) which, by sharing edges, form four M3O13 groups assembled through common oxygen atoms, leading to a tetrahedral cavity at the center of which a nonmetallic atom (P, Si, As) is located. This anion has a charge of

3- or 4- compensated by cations. The general formulas can be written (X+)3 [P (W or Mo)12 O40]3– or (X+)4 Si[W or MO)12 O40]4–with X+ = NH4+, Cs+ or K+ for the samples studied by the authors. Generally a nonpolar gas cannot

penetrate either into the Keggin anion or into the interionic region. Nevertheless, analysis of porosity by nitrogen adsorption-desorption isotherms by Moffat [231] has shown that certain of the monovalent cation salts have relatively high surface areas and a microporous structure. However, the average pore sizes (1.9 < d < 2.9 nm) obtained by porosimetry seemed too large in comparison with the lattice parameter of the salts (1.17 nm); for this reason the 129 Xe NMR technique was tested on these samples. Surprisingly and despite the small amount of xenon adsorbed, the NMR signal is easy to detect and the resonance lines are narrow (2 < ∆H < 7 ppm at a resonance frequency of 24.9 MHz).

As found with zeolite Y, the δ = f (N) plots are linear over the entire range of xenon concentrations studied (1019 < N < 1021 atoms/g) for all the samples. These results strongly support a narrow micropore size distribution with micropores isotropically distributed in the solid. The similarities of the xenon NMR results for zeolites and HPOM allowed the authors to apply the Demarquay–Fraissard model. The δN→0 for three salts with the same cation (Fig. 35) are identical (72 ± 2 ppm) and lead to a mean free path of 0.46 nm. Assuming that NH4+ has no electrical effect, as in zeolites, and that the microporous structure consists of infinitely long cylindrical or spherical cavities, the pore diameter is 0.9 or 1.36 nm, respectively. In comparison with the lattice parameters of ammonium salts (1.1–1.3 nm), these pore sizes appear to be more reasonable than those obtained from porosimetry measurements. This microporosity could result from a translation and/or a rotation of the Keggin anion in the crystal when the size of the cation increases. This motion could create interconnections between the interstitial void volumes, with the appearance of channels into which small molecules can enter.

Later, Terskikh et al. [232] studied the structure of 12-tungstophosphoric heteropolyacids (HPA) (H3 PW12 O40 ) supported on silica. At low HPA load-

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Fig. 35 δ = f (N) curves at 300 K for NH4 + salts: PW12O40 ; PMO12O40 ; SiW12O40 •; dehydrated NaY zeolite

Fig. 36 Schematic representation of the sorbed and interporous xenon atoms in the pore structure of HPA/SiO2 catalysts

ing (< 20 wt %) the surface HPA species are mainly isolated molecules, denoted HPA-I (Fig. 36). The volume and pore distribution do not change considerably compared to pure silica, therefore, the difference observed in chemical shift (≈ 20 ppm) is due to interactions of xenon with a limited number of HPA-I species acting as strong adsorption sites.

Under conditions of fast exchange, the chemical shift can be written:

δ = δSiO2 pSiO2 + δHPA-IpHPA-I

(36)

where pi represents the relative xenon population adsorbed on the surface i. The contributions of the silica and HPA-I (measured at low xenon pres-

sure) are 69 and 95 ppm, respectively. At higher loading (40 wt %), c.a. 5 nm HPA clusters appears (denoted HPA-II in Fig. 36). The chemical shift is then

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written:

δ = δSiO2 pSiO2 + δHPA-IIpHPA-II

(37)

where δHPA-II denotes the chemical shift of xenon adsorbed in the open microporosity of the clusters. This microporosity has been confirmed by N2 adsorption measurements. At the same time a broad downfield line of unknown origin appears at about 140 ppm. At the highest HPA loading (60 wt %) the spectrum consists of three overlapping lines at 106 (A), 117 (B) and 140 (C) ppm. The chemical shift of line A decreases when xenon pressure P increases, as for the 20 and 40% loaded samples, whereas the chemical shift of line B increases with P, as in zeolites: it is assigned to xenon adsorbed in the more closed porosity of larger (≈ 50 nm) HPA clusters.

This work shows that the high sensitivity of the 129Xe NMR technique makes it possible to study silica-supported HPA and to follow the clustering of HPA molecules “step-by-step”.

3.3

Activated Carbon

Porous carbons and activated charcoal are widely used both as catalysts and in separation techniques. They have a microporous structure with more or less extended graphitic zones. Whereas the adsorption of xenon on graphite has been studied extensively, there has been little work with 129 Xe NMR. Generally the signals are very broad, as is shown in [233–236] (revue 1999).

Contrary to previous authors, Suh et al. [237] obtained better resolved and relatively narrow signals for activated carbon at ambient temperature. For different samples with high specific area (500 < S < 1000 m2/g) they obtain at relatively low xenon pressure (P < 1000 torrs) a linear relationship between the chemical shift and the surface xenon density ρ, δ = δ0 + ρδ1, analogous to that previously proposed by Ito and Fraissard [1]. Like these authors, they attribute the δ0 term to the Xe-surface interaction which is sensitive to the nature of the surface. After nitric acid treatment to introduce acid sites on the surface of activated carbons, the values of δ0 are shifted 20 ppm downfield without change in the slope (Fig. 37) despite significant variations in the specific area and the mean pore size. This could be due to the fact that the Xe – Xe interactions in the gas phase inside the pores have little effect on the value of δ1 , as compared to those at the internal surface of the pores. The latest study published on this subject was by Bansal et al. [238] in 1992. By combining microporosity measurements treated by the method of Horvath–Kawazoe [239] and extrapolating Eq. 20 to activated carbons, they obtain the dimensions of the graphitic microzones. The values (1–3 nm) are quite consistent with those measured by small angle X-rays (1.5–6 nm).

There was a recent 129Xe NMR study of a series of catalytic filamentous carbons (CFC) obtained from the gas-phase reaction on iron subgroup metal

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Fig. 37 Dependence of the 129Xe NMR chemical shift on the concentration of adsorbed xenon for different amorphous carbons (SA0N and DA0N are samples SA and DA activated with nitric acid)

catalysts.A connection between 129 Xe NMR and structural, textural and paramagnetic properties of CFC was found [240]. The chemical shift δ was shown to depend on the structure of the CFC surface formed by edge, basal or both (edge and basal) graphite faces. This dependence follows a general trend of the chemical shift to increase with adsorption potential of a surface. The term describing Xe – Xe interaction (δ(Xe–Xe)) in confined space decreased with the average pore size of CFC granule. For hollow multi-carbon nanotubes two 129 Xe NMR signals were attributed to voids inside nanotubes and to interstices between the interlaced nanotubes.

4

Mesoporous Solids

4.1

Amorphous Oxides

Amorphous solids such as silica, silica-alumina, etc. have not been studied extensively by the 129 Xe NMR technique because their amorphous structure and widely opened porosity lead generally to broad signals. Nevertheless, we can report some studies of such materials. However, it is useful to consider first some specifics of Xe NMR spectroscopy on mesoporous systems: Very large and easily accessible pores, plus fast diffusion of xenon, cause exchange to have pronounced effects on the observed spectra.

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The first study was performed in 1989 by Conner et al. [241] who studied the pore structure created by compressing, at 17.5 tons/m2, nonporous aerosil silica spheres 12 and 7 nm in diameter. Compression of these materials created interparticle voids. Although nitrogen adsorption measurements give no evidence for micro or small mesopores, the chemical shift of adsorbed xenon, which depends on the compression (70–90 ppm) but which is not pressure-dependent, was much higher than the authors expected (Fig. 38). However, there appears to be a difference in the nature of the chemical shift of xenon adsorbed in aerosils as compared to that in the micropores of zeolites.

The absence of xenon pressure dependence, the large increase in the chemical shift with decreasing adsorption temperature and the narrowness of the NMR signal must be interpreted as the result of a fast exchange between adsorbed xenon atoms and free xenon located in the interporous space created by compressing the silica spheres. In this case, adsorbed xenon can be located in a defect of the silica surface or in small spaces at the point of contact between particles, the dimension of which must be very small (i.e. < 1 nm).

The authors showed that the experimental chemical shift is given by the equation:

	δexp =	KPδads + (NV pP/RT)δpores	,	(38)
		KP + NVpP/R

where K is the Henry’s law coefficient for xenon adsorption, P and T are the pressure and the temperature of the experiment, N is the Avogadro number, Vp the pore volume of the silica and R the gas constant. If the concentration

Fig. 38 Effect of compression and Xe pressure on the chemical shift of xenon adsorbed on aerosil samples

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of both the adsorbate and interporous xenon have first-order dependence on the xenon pressure, the chemical shift is then not pressure dependent.

In 1990 Cheung [242] studied xenon adsorption at low temperature (144 K) on a wide range of amorphous oxides. He found a nonlinear dependence on xenon concentration; a parabolic-like curvature is observed for alumina and silica-alumina, similar to that observed in Y zeolites with 2 + cations (Ca2+ , Mg2+, Ni2+, Co2+ etc.). His explanation is based on the distribution of micropore sizes: at very low xenon concentration, the chemical shift is due to xenon atoms adsorbed in the smallest micropores. As the loading increases, xenon is adsorbed in larger micropores and the chemical shift, which is the exchange average of the shifts in the large and small micropores, is therefore smaller. At higher xenon pressure, Xe – Xe interactions become preponderant and the chemical shift increases again. More recently, Terskikh et al. [243] characterized the porous structure of a number of silica gels by 129 Xe NMR. On the basis of experimental data for samples with well defined structure they proposed an empirical correlation between the chemical shift and the parameters of the pore structure.

δ =	δS	=	115	,	(39a)
	1 + D/ηKRT		1 + 0.0076D

where δS is the chemical shift extrapolated at zero pressure, η a factor depending on the more or less interconnecting pore system and its geometry (η = 4 for an interconnecting cylinder, 2 for slit-like non interconnecting pores), K is Henry’s constant of the adsorption isotherm and D the mean pore size. According to the authors this relationship should be valid in the range 2 < D < 40 nm. These authors proposed also this equation in another form:

δ = δS/(1 + Vg/KSRT),

(39b)

where S is the specific surface area and Vg the free volume inside the adsorbent. This equation shows that the observed shift is expected to be independent of xenon pressure, which is commonly observed, at least at low pressures when Xe – Xe interactions are insignificant and the adsorption follows Henry’s law. Equation 39b shows that δ depends strongly on S/Vg, the surface-to-volume ratio of the pores occupied by Xe atoms involved in fast exchange. It is therefore expected that the spectra of materials with a distribution of S/Vg will reflect this feature either by the appearance of more than one line, or by the presence of a broad line presenting a minimum in the variation of δ against Xe concentration, as shown by Cheung [242].

The presence of small particles (less then about 10 µm) in the material studied will also result in broad lines [244, 245]. Specifically, the effects of xenon exchange and bulk properties of porous materials on the spectra have been demonstrated and discussed in terms of xenon diffusion path length

using 40 ˚ Vycor controlled-pore glass of different particle sizes [245]. Ac-

A

cording to Eq. 39b the mean pore size D related to the Vg/S as D = ηVg/S

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Fig. 39 129Xe chemical shifts vs. mean pore diameters for porous silica-based materials:

• silica gels; ◦ Vycor/CPG; and porous organo-silicates of two different origins. 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 samples. Inset: Fits for two subsets of porous organo-silicates

(η is a parameter dependent on the pore shape), can be found from the NMR experiment provided that K is known from adsorption experiments. The correlation of the observed chemical shift with D (Fig. 39) as obtained by conventional adsorption methods has uncovered a general correlation between δ and D of the form δ = δS/(1 + D/b) [244, 246] that is similar to those found for zeolites [56].

For 34 silica-based materials with pores in the range of 0.5–40 nm, the parameters of the equation obtained from the fit are δS = 116 ± 3 ppm and

± ˚ [246]. With some caution, the correlation can be used for the b = 117 8 A

characterization of silica samples with unknown pore structure. One needs to note that even within this general correlation, subsets of materials of similar origin display yet finer correlations that indicate an accurate sensitivity of the method to details of the pore structure.

Temperature-dependent chemical shift data can be used to extract the physical parameters related to the adsorption properties of materials. In the fast exchange approximation with weak adsorption, as described by Henry’s law, the temperature dependence of the observed xenon chemical shift δ for arbitrary pores can be expressed as [244]:

δ = δs 1 +	Vg	1/2 e–		RT		–1	(40)
						.
				∆Hads

K0SRT

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The heat of adsorption ∆Hads can be found by fitting the experimental temperature dependence of the chemical shift and, for Xe physically adsorbed on silica-based surfaces, can range from just a few kJ/mol to about 20 kJ/mol [244]. If the pre-exponent of the Henry’s constant is known from independent adsorption measurements, then the Vg/S ratio can be found as well. Application of Eq. 41 necessitates working at very low xenon concentrations to reduce the effects of Xe – Xe interactions and to prevent Xe condensation in the pores at low temperature.

Supported V2O5 is used as a catalyst for the selective oxidation of hydrocarbons. 129Xe NMR was used to investigate structural features of V2O5/TiO2 (anatase) and V2O5/TiO2 catalysts [247]. The chemical shift of xenon adsorbed either on the support TiO2 or on the oxidation catalyst V2O5 varies linearly with the xenon coverage, showing a lack of strong adsorption sites. Extrapolated values at low adsorption temperature, characteristic of Xesurface interactions, are 109 ± 3 ppm for TiO2 and 93 ± 5 ppm for V2O5. On the contrary, for V2O5/TiO2-supported catalysts and particularly for those with a high proportion of monomeric vanadyl sites (low % of V2O5) a curvature in the δ-plots appears at low coverage. The authors concluded that monomeric vanadyl sites act as strong adsorption sites, excluding the existence of large vanadia domains; then, for low V2O5 loading, the vanadia units are well dispersed on the support. As the V2O5 loading increases the curvature becomes less pronounced, due to a decrease in the fraction of monomeric species and an increase in the fraction of polymeric species; this result can be compared to that for MFI zeolites with extra-framework alumina species [183]. Two-dimensional 129 Xe spectroscopy (Fig. 40) reveals that xenon diffuses between two distinct environments: the exchange rate shows that these environments are in close proximity.

In 1994 Mansfeld and Veeman [248] studied the sintering of Al2O3 and Al2O3 – ZrO2 fibers using high xenon pressure (4 < P < 10 bar). For a nonsintered sample, the spectrum at 200 K exhibits two peaks, one at 228 ppm assigned to liquid xenon (considering the temperature and the pressure) and the other, at 231 ppm, which disappears after sintering, corresponding to xenon adsorbed in the pores of the fibers. Nearly the same observations can be made for the Al2O3 – ZrO2 sample. To explain the temperature dependence of the chemical shift the authors proposed a simple theoretical model: for the low temperature region, xenon atoms can move in a potential well, as already described by Ripmeester et al. [53] and Cheung [54], due to interaction with an oxygen atom of the surface; for the high temperature region, the temperature and pressure dependence of δ are described by a Langmuir model. Finally, Oepen and Gunther [249] studied the condensation of surface hydroxyl groups of silica after high temperature treatment. For the samples pretreated at 773 and 973 K, cross-peaks between signals of interand intraparticle xenon are observed in 2D EXSY experiments, showing an increased rate of exchange. This is not the case for silica with a high concentration of su-

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Fig. 40 129Xe 2D exchange NMR spectrum, at 11.7 T (138 MHz), of xenon adsorbed on V2O5 at 290 K and at xenon pressure of approximately 2 atm. The spectrum was acquired using a mixing time of 2 ms and a recycle delay of 300 ms. The contour lines represent 1–20% of the maximum intensity

perficial hydroxyl groups which increase the polarity of the silica surface and hinder exchange.

Different types of aerogels represent an important class of open-pore mesoporous materials with extremely low framework density, reaching values as low as 0.05 g/cm3 and possessing great potential for various industrial applications. A recent 129 Xe NMR study of aerogels [250], performed as a combination of spectroscopic and spatially resolved NMR spectroscopy, has proved to be a powerful approach for characterizing the average pore structure and steady-state spatial distribution of xenon atoms in different physicochemical environments. The method offers unique information and insights into the microscopic morphology of aerogels, the dynamical behavior of adsorbates, and provides spatially resolved information on the nature of the defect regions found in these materials [250]. The extremely low density of aerogels, however, required very long accumulations. The problem of long experimental data acquisition times was addressed in another study by employing the Xe-131 isotope and a very high density of xenon [251].

A more practical approach to obtaining the Xe NMR spectra of aerogels is to employ the high sensitivity of HP Xe. Application of NMR microimaging using continuous flow HP Xe resulted in a visual picture of the dynamics

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of gases inside the particles of aerogels [252]. The “polarization-weighted” images of gas transport in aerogel fragments are correlated to the diffusion coefficient of xenon obtained from NMR pulsed-field gradient experiments. In another diffusion-related study [253] the ingression of HP Xe in Vycor porous glass was followed by 1D NMR imaging and by observing the intensity of the NMR signal. The resulting diffusion coefficients compared well with the results obtained from the pulsed-field gradient measurements with thermally polarized Xe.

We can also mention the study of some other mesoporous solids, such as soils [254] or of wood cellular structures [255].

4.2

New Mesoporous Solids (MCM, SBA, etc.)

The development of a novel group of mesoporous materials, such as MCM-41, MCM-48, SBA-15, etc., and related periodic porous oxides [256] has generated a great deal of interest due to their highly uniform pore sizes. They have great potential as catalysts, chromatographic supports, separation materials, photonic crystals, and in electronic devices [257]. The large pore size (diameter of

20 500 ˚), surface area, and easy functionalization of the silica wall provide

– A

further applications as supports for chemical and biological reactions [258]. Often such materials are also either amorphous or poorly crystalline,

which limits the applications of diffraction-based methods. After many successful applications of Xe NMR to microporous materials (zeolites, pore

diameters ≤ 20 ˚), as has been shown in previous paragraphs, naturally there

A

is a growing interest in the application of this technique to such porous materials, especially with the use of hyperpolarized xenon [259–267]. Indeed these silica-based materials can be prepared with pores in a very broad range of sizes and are a good testing-ground for the chemical shift-pore size correlations.

The first studies demonstrated [259, 260], however, that the observed shifts could fall well outside the range predicted by the general correlations obtained for silicates [261, 262]. In most cases the chemical shift of adsorbed xenon is practically independent of the xenon pressure, and the observed values are almost always below those estimated from the empirical correlations. Since the materials are prepared as very fine powders with the particle size rarely exceeding 10 µm, the exchange between adsorbed and gas phases is expected to contribute heavily to the observed shifts. Indeed, compression of the samples [260] produced significant downfield shifts similar to those in compressed aerosils [261] or in Vycor porous glass with different particle sizes [262].

In a recent study [263] the NMR of continuously circulating hyperpolarized Xe has been used to characterize purely siliceous and ordered Alcontaining mesoporous MCM-41 and SBA-15. The effect of compression on