Nature of cadmium cationic sites in cadmium-modified ZSM-5 zeolite according to the DRIFT studies of molecular hydrogen adsorption

The samples of CdZSM-5 prepared by ion-exchange, incipient wetness impregnation, and the interaction of HZSM-5 with cadmium vapor were studied by DRIFT spectroscopy using molecular hydrogen as a probe adsorbed at 77 K. Three different cadmium cationic species were detected. Dicadmium Cd22+ ions most weakly perturb adsorbed hydrogen molecules. They are characterized by a band at 4102 cm−1. Two other kinds of isolated Cd2+ sites are characterized by bands at 3982 and 3940 cm−1. The former band was ascribed to isolated Cd2+ cations which compensated the negative charges of neighboring aluminum-oxygen tetrahedra. The latter band corresponds to isolated Cd2+ cations which compensate the negative charges of distantly separated aluminum-oxygen tetrahedra. The calcination of samples containing Cd22+ species in oxygen converts these species into cadmium oxocations [Cd2Ox]2+. Hydrogen molecules dissociate at room temperature on the isolated Cd2+ cations which most strongly perturb adsorbed hydrogen molecules at 77 K.