The interaction of known numbers of water molecules with the acidic OH groups of H-form commercial mordenites, giving H30+ and H,O. . .HO, is studied using 'H broad-line NMR at 4 K and MAS NMR at 300 K. The H,O+ concentration for one adsorbed water molecule per Brsnsted-acid site is related to t h e Si/AI ratio. The differences between the H,O+ concentrations for the two samples at a higher water concentration is associated, semi-quantitatively, with synergy between Brsnsted-and Lewis-acid sites.Mordenites in the H form are used to catalyse industrial reactions that require a high acidity. For this reason, their properties, as those of many other zeolites, have been extensively studied. Titrations, acid-catalysed test reactions, calorimetric methods and various spectroscopic techniques have been used in these studies. The efficiency of zeolite catalysts depends on the preparation conditions, in a way which is not really understood. As early as 1968, Hopkins2 and Lunsford3 mentioned that the presence of Lewis sites is either necessary, or has a synergistic effect on the catalytic activity. Klyachko and c o -w o r k e r ~~-~ investigated the acidity using calorimetric measurements on NH, interacting with Brarnsted and Lewis sites. These authors described two ranges for the heats of ammonia adsorption: 120-150 and 100-170 kJ mol-'. On the basis of IR data,8 the value of 170 kJ mol-' was attributed to the dissociative adsorption of NH, on Lewis-acid sites and that of 100 kJ mol-' to adsorption on Lewis-acid sites without dissociation. Such attributions show how difficult it is to interpret the results. Kapustin et al." compared the results obtained using calorimetry and temperatureprogrammed desorption (TPD). In order to avoid readsorption of the desorbed gas, Karge and Dondur" performed ammonia and pyridine TPD in an ultrahigh vacuum apparatus, with mass spectral monitoring. Stach et a1.12713 used calorimetric techniques, associated with proton magic-angle spinning NMR ('H MAS NMR) to follow how the various properties depend on the A1 content of the samples. They related their results to Barthomeufs studyI4 of the influence of the A1 topological density.Using diffuse reflectance IR, Zholobenko et a1." discovered a new type of mordenite acidic OH group in samples free from any adsorbed phase. IR is often used for basesorption studies: ammonia by Lok et a1.;l6 ammonia, benzene and cyclohexane by Zholobenko et a1.;I7 pyridine by Kojima et ~1 . ; ' ~ pyridine and ammonia by Ghosh and Curthoys" and also Xue Zhiyuan and Zhu Leiming," these two authors using improved IR-TPD techniques. Ghosh and Curthoys studied the adsorption of methylamine and nbutylamine;21 they related their results" to the structural acidity parameter22 calculated from Sanderson's electronegativity model introduced by M ~r t i e r . ~, S ~l t a n o v ~~ studied the interaction between acid sites and CO; Gruver and Fripiat2' did the same, characterizing and quantifying the Lewis acid sites. Ammonia TPD, IR, cyclohexane adsorption and 27AI NMR were used...
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