SURFACE-ACIDITY OF AL-SILICATE, GA-SILICATE AND FE-SILICATE ANALOGS OF ZEOLITE NCL-1 CHARACTERIZED BY FTIR, TPD (NH3) AND CATALYTIC METHODS

The surface acidity of the high-silica, large-pore zeolite NCL-1 has b een studied using infrared spectroscopy and the temperature-programmed desorption of ammonia (TPD NH3). The Al-NCL-1 samples (Si/Al=41, 83 a nd 153) exhibit two infrared signals at 3628 and 3598 cm(-1) correspon ding to bridged hydroxyl Si-(OH)-Al groups (Bronsted acid sites). The acidic properties of isomorphously substituted Ga- and Fe-NCL-1 have a lso been measured and compared with that of Al-NCL-1 at similar Si/M ( M=Al, Ga, Fe) molar ratios. The FTIR spectra of chemisorbed pyridine a nd ammonia as well as TPD NH, measurements show that (1) all the sampl es have Bronsted and Lewis acid sites, and (2) the strength and therma l stability of Bronsted acid sites follow the order Al->Ga->Fe-NCL-1. The isomerization and disproportionation of 1,3,5-trimethylbenzene (1, 3,5-TMB) over Al-, Ga- and Fe-NCL-1 samples show that the activity (TO F) also follows the same trend. However, the selectivity towards isome rization (vis-a-vis disproportionation) and 1,2,4-TMB selectivity comp ared at similar conversion levels, reaction temperatures and feed rate s (WHSV was varied by changing the weight of the catalysts) follow the reverse order (i.e. Fe- > Ga- > Al-NCL-l). (C) 1998 Elsevier Science B.V. All rights reserved.