Effect Of Alkali-earth Metals On Iron-based Catalyst For Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis is the key process for converting syngas (H₂+CO) into hydrocarbons and other chemicals. The production of liquid fuel from coal via syngas through Fischer-Tropsch synthesis is of great significance based on the energy framework of abundance in coal and scarce in crude oil and natural gas reserve of our country. Owing to high F-T activity, low cost as well as high water-gas-shift (WGS) activity, the iron-based catalyst is more preferable for F-T process of coal-derived syngas with a low H₂/CO ratio.Many studies suggest the addition of promoters and supports can optimize F-T performances of iron-based catalyst to some extent. K, Cu and Mn are the common promoters and there are many studies about them, however the studies about alkali-earth metal promoters are very little and not systematical. The present study is carried out to investigate the effect of alkali-earth metals especial for Mg and Ca on the F-T performace of spray-dried iron-based catalyst. Two series of catalysts: Fe/Cu/K/Mg/SiO₂ and Fe/Cu/K/Ca/SiO₂ were prepared by the combination of co-precipitation and spray-drying technology. The FTS performance of these catalysts were tested in a fixed bed reactor under the conditions of 250℃, 2.0 MPa, and 2000 h^-1 with syngas of H₂/CO = 2 (molar ratio) prepared by methanol decomposition. The catalysts were characterized by using N₂ physisorption, H₂ or CO temperature-programmed reduction (TPR), X-ray diffraction (XRD), and Mossbauer spectroscopy (MES) methods. The effects of Mg and Ca promoter on the physic-chemical properties of Fe/Cu/K/SiO₂ catalyst were systematically investigated and discussed, and the effect of calcination temperature on Mg-promoted iron-based catalyst was also systematically investigated and discussed. 1. Effect of Mg promoter on iron-based catalyst for Fischer-Tropsch synthesisThe addition of the magnesium promoter significantly improves the BET surfacearea of the fresh catalysts. However, in the experimental range of Mg/Fe mass ratio (0.02-0.11), the BET surface area of the fresh catalysts monotonously declines with the increase of the magnesium content. Mg promoter can decrease the crystallite size of a-Fe2O3, and promote the dispersion of the catalysts. Optimal magnesium content can promote the reduction and carburization. After reduction, the catalyst without magnesium promoter has a-Fe2()3 and FeBCU, but no iron carbide in bulk phase; the Mg-promoted catalysts have no a-Fe2C>3 and Fe30/t. Except for the catalyst M-l(Mg/Fe=O.02), the other Mg-promoted catalysts have iron carbide, and the content of iron carbide increases with the increase of the magnesium content and reaches a maximum at the ratio of Mg/Fe=0.07, but the content of iron carbide decreases with further increase of magnesium content. All the catalysts were further reduced and carburized during FTS reaction process. The appropriate amount of magnesium can improve the activity and stability of the catalysts, enhance the product selectivity of C5-C11 and increase the space time yield of Cs+ and the selectivity of total hydrocarbon. The magnesium promoter can also slightly inhibit the activity of WGS, resulting in the decline of CO2 selectivity and increase of the carbon utilization. However, excessive addition of the magnesium promoter will lead to a rapid deactivation of the catalyst. The effect of magnesium promoter on the selectivity of oxygenate in water phase is not significant, but the selectivity of oxygenate in oil phase increase with the increase of magnesium content. 2. Effect of calcination temperature on magnesium promoted iron-based catalyst forFischer-Tropsch synthesis.Increasing calcination temperature decreased the BET surface area and increased the average pore diameter and the size of a-Fe2O3 crystallite of the catalyst, leading to a difficult reduction of the active phase. The Fischer-Tropsch performance of the catalysts after calcination was tested in a fixed bed reactor under the condition of 250°C, 2.0 MPa and 2000 h"1. With the increase of the calcination temperature, the initialactivity of the catalyst decreased, and the activity gradually approached to a stablevalue with time on stream, depending on the induction period which prolonged withincreasing calcination temperature. Additionally, the increase of the calcinationtemperature also favored the formation of heavy hydrocarbons and declined the CH4selectivity.3. Effect of calcium promoter on iron-based catalyst for Fischer-Tropsch synthesisThe addition of calcium promoter had no significant effect on the BET surface area and pore volume of the catalysts. The FTS results showed that the change of activity was not significant, but the addition of calcium promoter markedly increased the stability of the catalysts. The selectivity of CH4 decreased and the selectivity of C5"1" increased after the addition of calcium, but the selectivity of CH4 increased and the selectivity of Cs+ decreased with the increase of time on stream. The effect of calcium promoter on the selectivity of CO2 was not significant. The selectivity of alcohol in water phase increased with the increase of calcium promoter, but the other oxygenates in water phase were not obvious.