NMR diffusion measurements in heterogeneous media using pulsed radio-frequency field gradients

The measurement of self-diffusion coefficients D by the NMR spin echo pulsed-field gradient G technique can be corrupted for systems having a background magnetic field gradient G(o). The corrupting cross-term G(o)G can be eliminated by the use of alternating pulsed-field gradient sequences. Measurement of D can also be performed by a rather new technique using pulsed radio-frequency field gradients and rotary echoes. This technique is basically insensitive to background gradients and does not generate any adverse effects such as eddy currents or sample vibration. These methods have been applied to mesoporous solids and heteropolyacids. (C) 1998 Elsevier Science B.V. All rights reserved

Effect of the evacuation mode of the solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium

This work studies the evacuation mode of the solvent to optimise the catalytic properties of sulfated zirconia doped with cerium prepared by the sol-gel method. The xerogel solid obtained by ordinary gel drying and calcined at different temperatures exhibits a very low surface area. On the contrary, the aerogel obtained by solvent evacuation under supercritical conditions has a more developed surface. Both aerogel and xerogel exhibit the tetragonal phase of zirconia and/or the zirconium-cerium solid solution phase. Aerogel exhibits more developed superficial Ce4+ and higher acidity. The latter confers it a good reactivity in n-hexane isomerization in the whole temperature range investigated

Deactivation of Pt/mordenite (MOR) during the hydroisomerisation of n-pentane

The deactivation of Pt/MOR catalysts during the isomerisation of n-pentane was studied measuring the catalytic performances under favourable deactivation conditions and using several physicochemical techniques for characterisation: ICP, XRD, XPS, IR-pryridine, NMR, NH3-TPD and CO chemisorption. Results showed that deactivation is favoured by a high density and strength of acid sites, small pore diameters and probably by a small amount of octahedrally co-ordinated aluminium. Deactivation is more limited by larger pore diameters and higher Si/AI ratios. In this case, coke is formed in the deepest part of the pores. Catalysts with higher Si/AI atomic ratios are more difficult to regenerate. The nature of coke is similar in all cases and independent of the Si/Al ratio values