Influence of Na exchange on the acidic and catalytic properties of an HMOR zeolite

IR spectroscopy was used to characterize the hydroxyl groups of a series of NaHMOR samples resulting from sodium exchange of an HMOR sample with an Si/Al ratio of 10. Na exchange causes a significant decrease in the intensity of the band corresponding to bridged OH groups. This band presents two components, the first one at 36083611 cm(-1) corresponding to bridged OH groups in the large channels, the second one at 3583-3585 cm(-1) corresponding to bridged OH groups in the side pockets. At low exchange rate, this latter is more affected than the former demonstrating a preferential exchange of the protonic sites of side pockets, Pyridine adsorption shows that part of the protonic sites of the side pockets of HMOR are accessible to pyridine molecules; sodium exchange causes the preferential disappearance of the non-accessible sites of the side pockets. Furthermore, part of the hydroxyl groups of the large channels do not interact with pyridine, suggesting a blockage of their access. Sodium exchange of HMOR causes a decrease in the activity for in-xylene transformation at 623 K, this decrease being initially more significant. Sodium exchange also affects the product distribution, causing an increase in the selectivity to isomers at the expense of disproportionation products. This effect is proposed to be due to the decrease in the proximity of the protonic acid sites, The substitution of nitrogen as carrier gas by hydrogen does neither affect the activity nor the selectivity of the parent HMOR sample, but decreases significantly the disproportionation activity of a sodium-exchanged sample. (C) 2002 Elsevier Science B.V. All rights reserved.