MANGANESE OXIDE - SUPPORTED IRON FISCHER-TROPSCH SYNTHESIS CATALYSTS: PHYSICAL AND CATALYTIC CHARACTERIZATION.

摘要

It has been claimed that catalysts containing iron and manganese are especially selective for production of low molecular weight olefins in the Fischer-Tropsch Synthesis (FTS). In this study, manganese oxide-supported iron (Fe/MnO) was prepared by impregnation of powdered manganese(II) oxide with aqueous iron(III) nitrate and subjected to various calcination and reduction treatments. It was then employed as a catalyst for FTS and its steady-state activity and selectivity behavior was observed. The FTS reaction studies were run with nearly equimolal carbon monoxide/hydrogen feed at 515 and 540 K, 7.9 and 14.8 bar pressure. Feed conversion level was kept low in order to avoid transport limitations, and was varied by adjusting space velocity. The FTS reaction rate decreased strongly with increasing conversion. Compared to unpromoted iron catalysts, the Fe/MnO catalysts were more active for the water-gas shift reaction and less selective for methane and alcohols, especially at higher conversion, lower temperature and higher pressure. The olefin selectivity was high and secondary hydrogenation was not apparent. Catalysts calcined at higher temperature exhibited stronger effects of promotion, and yielded unusually high selectivity for C-2 to C-4 hydrocarbons at low temperature and high pressure. The general conclusion is that manganese promotion of iron can promote FTS selectivity towards low molecular weight olefins, but at the expense of high carbon dioxide formation.; The Fe/MnO was also physically examined using Moessbauer spectroscopy and X-ray diffraction. Iron and manganese were found to interact strongly in the calcined catalyst, though extensive phase separation occurred during reduction. The promoting effects are apparently due to partial surface coverage of iron with MnO, the extent of which is enhanced by "wetter" reducing conditions.