HEU-type zeolites modified by transition elements and lead

Research on pure samples of HEU-type zeolites modified by d- and f-block transition elements and Pb is reviewed. The interest on such modified HEU-type zeolites mainly arises from their use in pollution abatement, and for their potential catalytic properties. In addition, new composite materials composed of transition-metal loaded crystals with organic complexes stabilised on the zeolite substrate are gaining importance for versatile applications. If sorption is governed by ion-exchange, the structural distribution of the transition elements can be determined by single-crystal XRD. In more complicated cases, where an intense metal accumulation is observed on the surface of the crystals, the structural characteristics can only be defined using a combination of microscopic, spectroscopic, and thermal techniques. Detailed crystal-structure information is available for HEU-type crystals completely cation exchanged by Ag+, Pb2+, Cd2+, Mn2+, and Cu2+. Cd2+, Cu2+ and Mn2+ mainly occupy two extra-framework sites: one in the centre of the ten-membered ring, octahedrally coordinated by six H2O molecules, and one in the eight-membered ring, coordinated to framework oxygen and additional H2O. Ag+ and Ph2+ do not occupy the centre of the ten-membered ring but are shifted towards the framework walls. Complementary microscopic, spectroscopic, and thermal data (e.g. SEM-EDS, IR, EPR, NMR, EXAFS, XPS, RBS, DTA, TPD) on heulandite and clinoptilolite interacted with Co, Ni, Cu, Pd and Hg indicated non-homoionic and non-stoichiometric metal-loading. Excessive accumulation on the crystal surface, due to adsorption and surface precipitation phenomena, was commonly observed. Only very low incorporation of trivalent ions of lanthanides/rare-earth elements into the heulandite channel system was experimentally achieved. Th4+- and UO22+-ions interact significantly with heulandite but the metal sorption is mainly attributed to adsorption and surface precipitation processes. (C) 2003 Elsevier Inc. All rights reserved.