The nature of ‘overexchanged’ copper and platinum on zeolites

Abstract

The uptake of platinum and copper tetra-ammine (PTA and CTA,[(NH3)4Pt]2+ and [(NH3)4Cu]2+) into zeolites was compared over silica and three zeolites (Y, MOR and MFI) withdifferent points of zero charge and aluminium content. Adsorption was determined as afunction of pH at several metal concentrations, and pH shifts relative to metal free controlexperiments were carefully monitored.The uptake of both metal ammine complexes onto silica is well described by electrostaticadsorption. We suggest that the metal cations interact with zeolites by two mechanisms,ion exchange at the Al exchange sites and electrostatic adsorption at silanol groups. Theformer is the dominant mechanism at low to mid pH, and the latter at high pH.This effect is most clearly manifested in zeolites with low aluminium contentsuch as ZSM5; electrostatic adsorption at high pH in ZSM5 yields metal loadingsmuch in excess of the ion exchange capacity and so gives rise to ‘overexchange’.Differences between PTA and CTA can be explained by the weaker stabilityof the CTA complex and its response to the decrease in local pH near theadsorption plane of low PZC zeolites. This change in local pH near oxidesurfaces is characteristic of electrostatic adsorption. As the local pH decreases,the CTA ion is probably converted to a dimerized copper complex, perhapsCu2(OH)22+. A portion of the released ammonia is protonated, increasing the solution pH. In highPZC, high aluminium zeolites with high ion exchange capacity, there is relatively littlecontribution from electrostatic adsorption.