Search

A comparative study of the activation of n-hexane over 12-molybdophopshoric acid (H3PMo12O40 or HPA), Fe3+ doped HPA (Fe0.69H0.93PMo12O40) and an iron phosphate catalyst (P/Fe = 1.22) was carried out. The Fe doped HPA catalyst is thermally more stable and less acidic than the unmodified HPA catalyst. The HPA and the Fe doped HPA catalysts both catalytically produced 2,5-dimethyltetrahydrofuran and 2,5-hexadione (oxygenates), with the Fe doped HPA catalyst selectively producing more oxygenates than the HPA catalyst. The iron phosphate catalyst produced cis-2-hexene and 1-hexene but no oxygenates, which implies that the iron phosphate catalyst promotes dehydrogenation, but not oxygen insertion. At 360 °C, the HPA catalyst initiated benzene formation reactions. At isoconversion of ca. 8% under isothermal conditions, the iron phosphate catalyst gave the highest selectivity (72%) towards benzene, which may have formed through both catalytic and non-catalytic reactions. In contrast, the Fe doped HPA catalyst showed a lower benzene selectivity than the HPA catalyst with a corresponding increase in cyclic oxygenates.

An iron phosphate catalyst prepared using the ammonia gel method was supported on silica (CAT 1) and a commercial iron phosphate catalyst was supported on silica using the wet impregnation method (CAT 2). The XRD patterns of both the catalysts showed the presence of quartz-like iron phosphate and a tridymite like phase. In situ XRD under a reducing environment and TPR showed the formation of iron pyrophosphate for both the catalysts at 500 °C. The Mossbauer spectra of the catalysts were similar and showed the presence of the ferric ion only. NH3-TPD revealed the presence of Lewis and Bronsted acidic sites on both the catalysts. Oxidative dehydrogenation reactions of methyl iso-burate (MIB) were carried out using a continuous flow fixed bed reactor at contact times of 0.4 and 0.8 s. The conversion of MIB was marginally higher at a contact time of 0.8 s. CAT 2 gave an 11 % yield of methyl methacryalate (MMA) compared to 21 % over CAT 1 at a contact time of 0.8 s. The conversion of MIB increased with co-feeding water. A maximum conversion of MIB (82 %) was obtained at 400 °C, giving a yield of 20 % MMA at a contact time of 0.8 s over CAT 1. However, co-feeding water increased the hydrolysis reaction also, which increased the yield towards iso-butyric acid and methacrylic acid.

Three kinds of bulk iron phosphates were prepared via different methods and employed as model catalysts in the oxy-bromination of methane (OBM). Na3Fe2(PO4)3 was obtained via a fluoride route for the first time that showed even superior catalytic performance than FePO4. Phase evolution from Na3Fe2(PO4)3 to Na2Fe3(PO4)3 after the OBM was confirmed both by X-ray diffraction and 57Fe Mossbauer spectroscopy. Temperature-programmed reduction of hydrogen and desorption of CH3Br revealed that the redox capacity of iron phosphates were responsible for the generation of bromine radicals on catalyst surfaces while CH3Br-involved reactions likely belonged to gas-phase reactions. Model iron phosphate catalysts were synthesized by three different methods. It was found that the Na3Fe2(PO4)3 catalyst prepared by a fluoride route showed superior OBM performance than the other two catalysts both with the phase of FePO4, especially for producing higher CH3Br selectivity.

The effects of water vapor on the oxidative dehydrogenation of isobutyric acid to methacrylic acid over an iron phosphate catalyst were studied. The presence of water vapor suppresses both the carbon deposit and the formation of phosphorus-rich species on the surface of the catalyst and, as a result, it prevents the degradation in both the catalytic activity and the selectivity which occurs during the reaction, though it has no clear influence on both the redox properties and the structure of the catalyst. The pretreatment of the catalyst with a gas containing water vapor or the reaction in the presence of water vapor restores the activity and selectivity of the degraded catalyst. It is proposed that water plays a role in removing the carbon and oxygenated products from the surface, in suppressing the side reaction, that is, the decomposition of isobutyric acid to propylene and CO, and also in modifying the surface species.

Oxidative dehydrogenation of isobutyric acid to methacrylic acid was studied over an iron phosphate catalyst with a P/Fe atomic ratio of 1.2. The freshly prepared catalyst consisting of Fe3+ is reduced with isobutyric acid to iron(II) pyrophosphate [Fe2P2O7] via a bluish black intermediate. Under the reaction conditions, the fresh catalyst is reduced gradually to the intermediate and the Fe2P2O7 is also reoxidized to the intermediate. However, both the catalytic activity and the selectivity were scarcely affected with a large variation in the valence states of iron in the bulk of iron phosphate. On the other hand, the studies on the reaction in the absence of oxygen revealed that the reaction is promoted by the redox cycle of iron phosphate.