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

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the mesopore structure has also been studied. The NMR spectra obtained can be interpreted in terms of exchange between adsorbed xenon in the pores and gaseous Xe atoms in the interparticle spaces. The greatly increased sensitivity arising from hyperpolarized Xe has also been used to detect the spectra of xenon adsorbed on very small quantities of mesoporous silica thin films [264].

Additional information about the internal surface of the pores can be obtained from the data on co-adsorption of Xe with some other small molecules [259–261, 265]. The co-adsorption studies can be particularly useful for screening intra-wall micropores, surface defects and inhomogeneities in porosity. In certain cases, using guest molecules of different sizes, it is possible to distinguish and estimate the sizes of micropores inside the mesopore walls [265].

The assessment of the distribution and accessibility of moieties attached to the walls of mesoporous silicates is another area of 129Xe NMR applications.

The effects of hydrocarbon chains attached to the silica walls has been the subject of an extensive hyperpolarized 129 Xe NMR study [266] The variable temperature measurements revealed a nonuniform porosity and irregular pore structure, and allowed one to follow the changes in the adsorption properties of xenon due to modification of the mesopore voids [266]]. In another study [267] the presence of Pt in the ordered mesoporous silicates results in a notable chemical shift of adsorbed xenon. It was concluded that the Pt clusters are situated inside the pores of the mesoporous molecular sieve rather than on the external surface, in a similar way as shown in paragraph 2.5 with zeolites.

4.3

Xe NMR Cryoporometry

Although there are still few applications we will mention this technique. Xenon porometry is a novel method for the determination of pore dimensions. It has been developed on the basis of NMR cryoporometry [268]. In this method, a porous material is immersed in an organic substance, and the freezing and melting behaviors of the substance are explored by means of the 129 Xe NMR spectroscopy of xenon dissolved in the sample [269]. An example of the spectra measured at different temperatures is shown in Fig. 41. The signals have been labeled in the figure.

The origins of the components are the following: Signal A arises from the inner xenon thermometer, which is a capillary tube in the middle of a 10 mm sample tube containing ethylbromide and xenon gas [270]. Signals B and C originate from xenon dissolved in the liquid substance. Signal B arises from bulk substance located in the spaces between particles of porous material and on the top of porous material, and vanishes below the freezing temperature of the substance (227 K in the case of acetonitrile), as the transition from li-

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Fig. 41 The 129Xe NMR spectra of the sample containing Silica gel 100, acetonitrile and xenon at different temperatures. The measurement temperatures are shown beside the spectra. The chemical shift range of the signals B and C at 230 K has been expanded in the inset of the figure

quid to solid is accompanied by an abrupt reduction of gas solubility. Signal C originates from liquid inside the pores, and gradually vanishes at lower temperatures, as the confined substance freezes. If the density of the substance increases substantially (as occurs in the case of acetonitrile), signal D appears at lower temperatures. This signal arises from xenon in very small gas bubbles appearing inside the pores. The bubbles build up during the freezing of confined substance, as this contracts. The signal vanishes, as the confined substance melts. As bulk substance freezes, bubbles also build up in between the particles of porous materials due to the density change, and signal G originates from xenon in this environment. Since the bubbles are much bigger than those inside the pores the chemical shift of signal G is close to that of bulk gas.

The method provides three novel possibilities for determining pore sizes:

1.As, on one hand, the melting point of the bulk substance can be deduced from the emergence of signal B and from the disappearance of signal G, and, on the other hand, the disappearance of signal D and the changes of the intensity of signal C reveal the lowered melting point of the confined substance, the melting point depression can be determined by measuring the NMR spectra at variable temperature, and the average pore size of the material can be calculated by the Gibbs–Thompson equation in the same way as in the usual NMR cryoporometry.

2.The chemical shift difference of signals C and B is dependent on pore size. By determining the correlation for a certain substance at a certain

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temperature using known mesoporous materials, the pore size of an unknown sample material can be measured. This is very easy to do, because the signals can be recorded by a single scan measurement at any temperature above the bulk melting point. On the other hand, the correlation is quite inaccurate because the line-width of the signals is large compared to the distance between them. The correlation is best suited for pore size determination of smaller pore sizes in the mesoporous range.

3.As the size of the gas bubbles formed inside the pores during the freezing of the confined substance depends on the size of the pores, the chemical shift of xenon atoms inside the bubbles also depends on the pore size. These studies prove that the large majority of bubbles are isolated (i.e. they are not connected with each other) giving resonance signals characteristic of each pore size. As signal D is made up of these components, the shape of the signal represents the pore size distribution. The correlation between the chemical shift of signal D and the pore size can be determined using known reference samples. After that, the pore size distribution of an unknown material can be determined by measuring its NMR spectrum [269].

5 Conclusions

Since the first applications on Xe-NMR of adsorbed xenon used as a probe, this technique has shown its interest in many applications.

In the case of zeolites it is possible:

•to determine the dimensions and the form of their internal free volumes without knowing their structures;

•to reveal structure defects, produced for example by dealumination, and to determine their characteristics;

•to calculate the short-range crystallinity, as opposed to that determined by X-rays;

•to follow the mechanism of the synthesis and crystallization of zeolites during their preparation;

•to locate cations in the zeolite structure and to follow their migration or change in their environment depending on various factors;

•to locate any “encumbering” species. For example, adsorbed molecules, extra-framework species, coke formed during catalytic cracking reactions, etc.;

•to determine the dispersion of metals (particularly when the particles are too small to be seen by electron microscopy) and the distribution of the molecules adsorbed on the particles (as opposed to the mean coverage);

•to compare the diffusion of species as a function of the size of the zeolite “windows”.

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The porosity of new mesoporous solids (MCM, SBA, etc) is relatively well defined by means of nitrogen adsorption-desorption techniques. Xe-NMR is however an excellent complement, in particular to demonstrate the roughness of the pore surface or, more recently, to study systems with a zeolite monolayer on the surface. It is also useful for various applications similar to the previous ones for zeolites. This list of applications is certainly not exhaustive. This technique can also be applied to any microporous or mesoporous solids such as clays, amorphous oxides, activated carbons, solid polymers, etc. provided one work at low temperature.

This wide field of applications has been considerably extended by the advent of xenon hyperpolarization which increases the sensitivity of NMR detection by 3 or 4 orders of magnitude.

6

Appendix: Hyperpolarized Xenon

Optical pumping is a procedure whereby the population of the spin states of the valence electron of an alkali metal atom can be changed. In this way a population distribution very different from that of thermodynamic equilibrium is achieved [271]. By irradiating an alkali metal vapor with circularly polarized monochromatic light whose frequency corresponds to an absorption line of the atom at the ground state, a given excited level can be populated and the distribution between the Zeeman levels of the ground state changed. This electronic polarization can be transmitted by contact with nuclear spins of a monoatomic gas like xenon [8–10, 14–17]. One then speaks of “optical pumping of xenon”, somewhat incorrectly, since it is the valence electron of the alkali metal which is optically pumped and not the nuclear spin of the xenon.

The optical pumping of rubidium is displayed schematically in Fig. 42. The electronic ground state 52S1/2 is split into two states, spin-down and spin-up. Only the spin-down state can absorb right circularly polarized light σ +. Usually, the gas pressure applied in the experiments is so high that the population of the substrates of the excited state 52P1/2 is randomized by collisional mixing. Then, relaxation occurs to either of the ground states with the same probability. Since only the spin-down state, can be depleted upon irradiation, a net polarization builds up in the spin-up ground state. The radiative decay of the excited state generates photons without preferential spin orientation. To avoid depumping of the rubidium by reabsorption of these photons, a quenching gas such as nitrogen is often added.

In Fig. 43, the mechanism of spin exchange is illustrated schematically. A rubidium and a xenon atom collide in the presence of a buffer gas molecule (nitrogen) and form a van der Waals molecule, whose lifetime τ is limited by collision with another buffer gas molecule. During the lifetime of the van der

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Fig. 42 Scheme of the optical pumping process, shown for σ + polarized light. Photons can only be absorbed by the – 1/2 ground state 52S1/2. Collisional mixing randomizes the population of the excited states 52P1/2 so that relaxation occurs to either ground state with the same probability. Due to depletion of only the – 1/2 state, a net polarization results in the 1/2 state

Fig. 43 Spin exchange between optically polarized Rb and Xe during the lifetime τ of the van der Waals pair

Waals pair, spin exchange occurs. The rubidium electronic spin S flips down, whereas the xenon nuclear spin I flips only partly up.

The Hamiltonian to be considered is:

ˆ ˆ	ˆ ˆ	ˆˆ	ˆ	(41)
H = HHyperfin + γ NS + αJS + HZeeman

Sˆ = spin of the valence electron of the rubidium atom

Nˆ = angular moment of rotation of the van der Waals complex

ˆ

J = nuclear spin of the xenon atom.

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In the right hand side the first term represents the hyperfine coupling between the electron spin and that of the nucleus of the Rb atom. The second term represents the spin-rotation interaction between the spin of the valence electron of Rb and the rotational angular moment of the complex. This interaction dominates the relaxation of rubidium and is, therefore, the main reason for the loss of polarization. The Fermi contact term, αˆJSˆ, is responsible for polarization transfer between Rb and Xe. By means of the optical pumping of rubidium the polarization of 129 Xe can reach values as high as 10–2 or 10–1, that is, a polarization 4 orders of magnitude higher than that obtained at thermodynamic equilibrium.

The main applications of this technique to the study of solid surfaces are by A. Pines et al. In [11] there is a description of the apparatus used and, in the reviews of Raftery and Chmelka [7] and Pietras and Gaede [272], the presentation of much work.

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