No abstract

Cu-Zn hydroxycarbonates have been studied as precursors of Cu-ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution ( C u / Z n molar ratio 2 : 1 ) at 333 K and pH 7.0 consisted of zincian malachite ( C u / Z n = 8.5: 1.5) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times. Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched ( C u / Z n = 2 : 1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis cs. reverse shift in the CO,+ H, reaction at normal pressure.

Theoretical methods are used to study the adsorption of linear hydrocarbons, C1-C8, in silicalite, H-ZSM-5, and siliceous faujasite. The aim is to model the behavior of sorbed species in zeolites and evaluate the influence of the zeolite structure and composition on the sorption process. The method of calculation is energy minimization using the Born model of solids with specified interatomic potentials. Atom-atom potentials are used to model the zeolite and zeolite-hydrocarbon interactions. The effect of relaxation both of the zeolite framework and of the molecule is investigated. The location of adsorption sites and heats of adsorption for the molecules are presented. The sorption process is controlled by the shape and volume of the available cavity and the packing ability of the molecule concerned. In silicalite and H-ZSM-5, adsorption at the straight channel is preferred to that of sinusoidal channel. The simulations show the extent to which hydrocarbons sorb more strongly in ZSM-5 zeolites than in siliceous faujasites.

No abstract

No abstract