Proton spin relaxation of molecules adsorbed in NaY and NaPtY zeolite; The influence of adsorbed water on the distribution of atomic platinum

Abstract

The longitudinal proton spin relaxation time T 1 of molecules adsorbed in commercial zeolites is predominantly caused by paramagnetic impurities, typically of the order of 0.1 wt% of iron. If only 1 wt% of platinum is introduced into the zeolite (as [Pt(NH 3) 4] 2+), T 1 of water is enhanced up to one order of magnitude, as was shown in a preceding paper. The reason for this effect is that, under suitable thermal pretreatment of the Pt-exchanged zeolite, the Pt 2+ ions are reduced by the ammonia to atomic platinum and part of the Pt atoms migrate preferentially to Fe 3+ ions. Thus, Fe 3+ ions, controlling T 1, are covered and up to 90% of paramagnetic sites for water is occupied by Pt atoms. In this paper, it is shown that Pt atoms preferentially migrate to Fe 3+ ions only under the influence of water and that, therefore, various adsorbed organic molecules do not show the enhancement of T 1, unless the NaPtY zeolite is suitably pretreated with water before the organic molecules are adsorbed.