Acidic porous clay heterostructures: study of their cation exchange capacity

The cation exchange capacities (CECs) of two porous clay heterostructures ( PCHs), derived from natural montmorillonite (PMH) and synthetic saponite (P SH), have been studied. Both materials are highly porous, with surface area s of 997 m(2)/g (PMH) and 1118 m(2)/g (PSH). In order to obtain exchangeabl e ammonium cations in the pore structures of the PCHs, different modificati ons are performed on calcined and extracted PMH and PSH. Three methods for the formation of NH4+-exchanged PCH forms are described and evaluated: (1) adsorption of ammonia under a gas flow on calcined and extracted PCHs in ac idified methanol; (2) direct exchange in NH4Cl solution; (3) solvent extrac tion with NH4Ac/EtOH/H2O. The obtained ammonium containing materials are su bsequently exchanged for KI cations in aqueous solution in order to determi ne the CEC of the PCH solids. With Kj-N analysis and infrared-DRIFT techniq ue, evidence is found for the presence of ammonium cations on the surface o f PCHs. The stability (crystallinity and porosity) of the porous solids und er the different treatments has been investigated using X-ray diffraction a nd N-2-adsorption isotherms. The adsorption under NH3 gas flow proves to be the best method for maintaining the porosity of the studied PCHs. Maximal K+ loadings of 0.55 mmol/g (PMH) and 0.37 mmol/g (PSH) are obtained for amm onia-loaded and extracted PCH samples in MeOH/HCl. (C) 2001 Elsevier Scienc e B.V. All rights reserved.