| The synthesis of low molecular weight olefins (C(,2)-C(,4)) from hydrogen and carbon monoxide has been investigated over unsupported iron-manganese catalysts. Nine iron-manganese catalysts with different atomic Mn/Fe ratios were prepared by coprecipitation of iron and manganese nitrates. The atomic Mn/Fe ratios of the catalysts ranged from 0/100 to 11.8/100. The technique was capable of preparing catalysts with reproducible atomic Mn/Fe ratios. The major bulk phase of the calcined iron-manganese catalysts was hematite ((alpha)-Fe(,2)O(,3)). The average particle size of hematite in the catalysts decreased as the atomic ratio of Mn/Fe increased. In addition, the range of the (alpha)-Fe(,2)O(,3) crystallite size for these catalysts was 15-24 nm.; The catalysts were evaluated in a conventional bench-scale fixed-bed reactor system. A period of 12 hours was generally required to stabilize activity and selectivity at the operating conditions investigated: 473-483 K, 1465 kPa, 2/1:H(,2)/CO, and 1.08 cm('3)g('-1)s('-1).; A catalyst reduction study in a thermogravimetric analysis (TGA) system showed that hydrogen reduction at 673 K and ambient pressure for 8 hours was sufficient to activate the catalyst. The results from an evaluation of the effect of catalyst bed dilution on activity and selectivity indicated that a volumetric dilution ratio of Denstone 57/catalyst of 4/1 was effective in removing the global heat of reaction in the catalyst bed and in modifying the C(,2)-C(,4) olefin selectivity. Experimental data on the influence of the catalyst pretreatment on activity and selectivity with hydrogen, carbon monoxide, and synthesis gas (H(,2)/CO ratios 1, 2, and 3) indicated that the hydrogen pretreatment resulted in a more stable catalyst activity and higher olefin selectivity (i.e., 70 mole percent).; The experimental results from carbon monoxide hydrogenation at the standard operating conditions (i.e., 2/1:H(,2)/CO, 1465 kPa, 1.0 cm('3)g('-1)s('-1) and 503 K) indicated that the catalyst activity generally decreased as the manganese concentration of the catalyst increased; however, the C(,2)-C(,4) olefin selectivity was significantly enhanced with a small amount of manganese in the catalyst (i.e., 2.5 Mn/100 Fe). Data on selective chemisorption of hydrogen and carbon monoxide at 292 K for these iron-manganese catalysts indicated that the carbon monoxide uptake was much higher than the hydrogen uptake. |
