Structural and acidic characteristics of Cu-Ni-modified acid-leached mordenites

Two series of dealuminated Na-mordenite zeolites (DML and DMH) were impregnated, in comparison with the parent nondealuminated NaMSP, in aqueous nitrate solutions of Cu and Ni to achieve varying loadings for both of the cations. These samples were characterized by N-2 adsorption, XRD, DSC of ammonia desorption, ammonia volumetric sorption, IR of ammonia adsorption, and FTIR-photoacoustic (FTIR-PAS) techniques. The FTIR-PAS spectrum of CuNi-loaded NaMSP shows a band at 935 cm(-1) ascribed to O3Si-O-SiO3 linkages produced as a result of dealumination caused by the synergistic effect of Cu and Ni cations under the preparation conditions. As a confirmation, this band was intensified upon acid dealumination (DML) where, at the extent of dealumination (DMH), collapsing of the zeolite structure was obtained subsequent to cation modification. In addition, the dealumination effect was markedly enhanced upon increasing the load of Cu in proportion to Ni. A total erosion of OH group characteristics of Si-(OH)-AL at 3610 cm(-1) was depicted when the Ni content exceed that of Cu where it did not show any change when the Cu content surpasses that of Ni. The amount of adsorbed ammonia measured volumetrically was enlarged after dealumination as well as after increasing the contents of the modificating cations. The IR study of ammonia adsorption revealed a band at 1428 cm(-1), in either nondealuminated or dealuminated-modified samples, assigned to stronger Bronsted acid sites than those at 1455 cm(-1). The band at 1428 cm(-1) was markedly enhanced in the latter samples than in the former. This was due in part to the replacement of the protons by cations, producing sufficiently mobile protons. In conformity, DeltaH values obtained for DSC effects via ammonia desorption were enhanced after dealumination. Other correlations with XRD and surface texturing on one hand and the structural variations following cations incorporation on the other hand are evaluated and discussed. (C) 2003 Elsevier Inc. All rights reserved.