Generation of ion-exchange capacity by silicon incorporation into the aluminophosphate VPI-5/AIPO4-8 molecular sieve system
Abstract
CuII ions have been ion-exchanged into SiVPI-5 and SAPO-8, which were synthesized by introducing SiO2 into the VPI-5 aluminosilicate synthesis mixture. The coordination of CuII during various stages of dehydration and reoxidation and its interaction with polar and non-polar adsorbate molecules were studied by EPR and electron spin–echo modulation spectroscopy. In comparison to CuII impregnated into VPI-5, a substantially higher ion-exchange capacity and better stabilization of the CuII ions by the framework was found in CuH-SiVPI-5, where CuII has been introduced by conventional aqueous solution ion exchange. Copper(II) ions coordinate directly to three molecules of water and two molecules of ammonia. However, CuII is also still coordinated to framework oxygens in these complexes, indicating a strong interaction of CuII with the framework. Non-polar bulky adsorbate molecules such as benzene do not coordinate directly to the CuII ions, but polar adsorbates such as pyridine are able to induce CuII migration from less accessible sites to the more accessible main channel, where direct coordination occurs. The differences between impregnated Cu/VPI-5 and ion-exchanged CuH-SiVPI-5 or SAPO-8 for dehydration behaviour and adsorbate interactions indicate successful incorporation of a small amount of silicon into the VPI-5 framework, which results in a material with enhanced ion-exchange capacity.