Ni, Co, Fe and Zn-containing silicalites-1 in propane conversion
Three series of metal-containing silicalites-1 (M=Zn, Ni, Co or Fe) were synthesized using different procedures. According to the first approach Me salts were introduced into the silicalite synthesis gels along with sodium sulfide to form insoluble/poorly soluble sulfides; then silicalites were crystallized around of metal-containing particles (M(S)/sil samples). The reference samples were prepared by impregnation of silicalite-1 from methanolic and aqueous metal salt solutions (M(CH3OH)/sil and M(H2O)/sil samples). The samples were characterized by SEM, XAFS, XRD and TPR; the catalytic properties were tested in propane conversion process. It was found out that Ni and Co form oxihydrates and Ni(Co) silicate-like structures. Oxihydrates are further converted into NiO (Co3O4) upon calcination. In the presence of Fe, extremely disperse oxide and sulfide species are formed, the latter being converted into oxide during calcination. In the presence of Zn, ZnS particles are formed which are only superficially oxidized upon calcination.Under reaction conditions metal oxide particles can be reduced to metallic clusters or remain in an oxidized state depending on the metal and its localization in the silicalite structure. Metallic clusters were more active in propane conversion, mainly for destructive reactions, and less stable in comparison with metal oxide active phase. Higher selectivities to aromatics over Ni(CH3OH)/sil and Co(CH3OH)/sil as compared to the aqueous impregnation catalysts may be attributed to reaction on metal silicate species. For Zn-containing samples, the highest selectivity to aromatics was observed for Zn(S)/sil, which can be ascribed to a better dispersion of active phase inside silicalite crystals.