Mesoporous materials, relaxation times

Contrary to V modified zeolites [180-184], as-synthesized mesoporous vanadosilicates did not exhibit any EPR signals at 77 K [173,174], This was attributed to the presence of V in highly symmetrical lattice positions. Because of the electronic degeneracy and the associated very short relaxation times, lower temperatures maybe needed for the observation of such species. The presence of or clustered in the as-synthesized materials was excluded based on Nl data [173]. 

Some information can be obtained on porous media from conventional NMR spectroscopy, and this is discussed in Section 2. Relaxation time measurements have been widely used to characterize porous solids, and this technique is discussed in Section 3. Pulsed field gradient (PFG) methods may be used to probe the local structure of the pore space and to characterize transport within it, and these are discussed in Section 4. Magnetic resonance imaging (MRI) techniques can also be used to characterize the pore space and to measure transport, and applications are discussed in Section 5. The bulk of this review will be concerned with mesoporous and macroporous materials, as it is for these systems that NMR is particularly useful in characterizing the pore space. However, some applications of NMR techniques to probe the pore space and transport within microporous materials will be mentioned in Section 6. Finally, some general conclusions are given in Section 7. 

Relaxation times are commonly measured for porous media that have been saturated with a fluid such as water or an aqueous brine solution. The observed relaxation times are strongly dependent on the pore size, the distribution of pore sizes, the type of material (e.g. content of paramagnetic ions) and the water content. While relaxation times in porous media have been modelled using random walk methods and finite-element methods, simplified models are usually needed to obtain information on pore space. Section 3.2 reviews the standard model used to analyse relaxation behaviour of fluid in macroporous samples such as rocks. Mesoporous materials such as porous silica will be discussed in Section 3.3. 

Porous silicas are usually mesoporous materials and they can be made with a variety of pore dimensions. In particular, silica glasses can be made with well-defined pore diameters, typically in the range 30-250 A, using sol-gel methods. Such a system provides a good model for testing the models of relaxation behaviour of fluids in porous solids. It is normally found that the two-site fast-exchange model for relaxation described above for macroporous systems is still valid. For instance, H and relaxation times have been measured during both adsorption and desorption of water in a porous silica. Despite hysteresis in the observed adsorption isotherms, it was found that the relaxation times depended solely on water content.For deuterated water in some porous silicas, multicomponent relaxation behaviour for T2 and Tip has been observed, and this has been attributed to the fractal nature of the pore structure. 

Relaxation time studies of the filling process of porous silica with water and cyclohexane have been used to establish whether the adsorption is homogeneous. It was found that water initially collected in small puddles at interstices in the structure, and then formed a surface layer over the silica surface before the remaining pore volume was filled. On the other hand, cyclohexane appeared to fill the smaller pores completely before spreading to the larger pores. A similar effect was observed for water adsorbing in a silica that had been chemically treated to make the surface hydrophobic. Thus, the fluid location in mesoporous materials at low loadings depends critically on the wettability of the surface. 

We begin with the work where the studied fluid was different than water. Some papers have dealt with gases contained in the porous materials. Yager and co-workers reported relaxation time measurements for He adsorbed in the pores of mesoporous molecular sieve MCM-41 at low temperatures (down to 1.7 K) and at a range of frequencies. They observed the behaviour, characteristic for one-dimensional... 

Every sorption set-up also allows for the extraction of information about sorption dynamics and gas transport provided that the resolution in time of the sensor applied is adjusted to the process under investigation. In an earlier publication we studied the transport of gases into the mesopores of monolithic materials by analyzing the pressure relaxation after the dosing of a defined amount of gas onto the sample [3]. An improvement of the resolution in time allows to investigate also small monoliths or samples with larger pores. 

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