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Black plate (238,1) There are two recent developments of the PFG NMR technique that have been proposed for the measurement of the diffusion of molecules in zeolites as absorbed in the form of multi-component liquids. This approach is of potential practical importance since in almost all real applications, the zeolite interacts with a mixture of components rather than with a single substance. The methods include options for enhancing the sensitivity with respect to displacements over reduced dimensions by a novel principle of field gradient pulse matching and with respect to selectivity between different components by combining PFG NMR with magic angle spinning (MAS) NMR and with a micro-imaging gradient system. The diffusion coefficients were determined for ethane, ethene, water and benzene adsorbed in zeolite NaX, and the results are compared with those measured previously for these materials as present in simpler adsorbed systems32. The energies and entropies which control the adsorption of benzene on acidic (HyZSM-5) and non-acidic (Silicalite-1) MFI (ZSM-5) type materials have been studied using both gravimetric and calorimetric methods along with in situ IR spectroscopy in order to make both a qualitative and quantitative evaluation of the way benzene interacts with the pores and the functional Si(OH) and Si(OH)Al groups within the structure. According to an interpreta- tion of the adsorption isotherms, there are adsorption sites for benzene present in MFI type materials that are sterically constrained. It is the interaction between benzene molecules and the pore walls which dominates the adsorption energetics, while the more localized interaction with the bridging hydroxyl groups is of little significance. When a benzene molecule is close to an Si(OH)Al group O2H stretching frequency shifts are observed, in reflection of the local adsorption geometry of the benzene molecules in addition to the acid strength of the OH group and the base strength of benzene. Two kinds of OH perturbation were observed which are accounted for in terms of their being two orientations of the benzene molecules inside the pores. One orientation has the ring parallel to the pore wall and in the other the ring is oriented towards the bridging OH groups. As the surface coverage increases, benzene molecules become adsorbed addition- ally at Si(OH)Al groups whose environment is unconstrained, it is thought most likely at the openings of the pores33. Transport diffusivities have been determined for perdeuterated benzene adsorbed in NaX zeolite using neutron spin-echo measure- ments. In order to aid comparison with recently reported molecular dynamics simulations, corrected diffusivities were employed. The 238 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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