Generation of ion-exchange capacity by silicon incorporation into the aluminophosphate VPI-5/AIPO4-8 molecular sieve system

Abstract

Cu^II 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 Cu^II 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 Cu^II impregnated into VPI-5, a substantially higher ion-exchange capacity and better stabilization of the Cu^II ions by the framework was found in CuH-SiVPI-5, where Cu^II 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, Cu^II is also still coordinated to framework oxygens in these complexes, indicating a strong interaction of Cu^II with the framework. Non-polar bulky adsorbate molecules such as benzene do not coordinate directly to the Cu^II ions, but polar adsorbates such as pyridine are able to induce Cu^II 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.