Study On Improving Light Olefins Of Fischer-Tropsch Synthesis

Direct conversion of synthesis gas to light olefins has been considered as a possible solution to meet the growing demand for C2H4and C3H6. Improving the content of light olefins is one of the most important research directions of Fischer-Tropsch synthesis (FTS). The iron-based catalysts added with structure promoter (Mn) and electron donors (K, Na) were prepared based on coprecipitation method. The effects of promoters and content on the textural properties, surface composition, reduction behavior, chemisorption, surface basicity and bulk composition are investigated by N2-physisorption, X-ray photoelectron spectrometer (XPS), H2temperature programmed reduction (H2-TPR), H2temperature programmed reduction(H2-TPD), CO temperature programmed desorption (CO-TPD), CO2temperature programmed desorption (CO2-TPD), X-ray diffraction (XRD) and Mossbauer spectroscopy (MES). At the high temperature (300～350℃) for Fischer-Tropsch synthesis, the effects of Mn, K and Na promoted catalysts on CO conversion and light olefins production are studied. For100Fe33Mn2.8Na catalyst, the influence of reaction temperature, pressure and space velocity on the improvement of light olefins content are investigated. Finally, the hydrocarbon product distribution over FeMnK and FeMnNa catalysts are discussed by combining in situ diffused reflection Fourier transform infrared spectroscopy (in situ DRIFTS).The FeMn and FeMnNa catalysts with different Mn contents were prepared respectively by coprecipitation and coprecipitation-calcination-impregnation-calcination methods. The N2adsorption results show that with the increase of Mn content, the surface area and pore volume of catalysts increased, but the crystal size of catalysts diminished. The H2-TPR and H2-TPD results show that the content of Mn in the catalysts has no obvious effect on the reduction in H2and H2adsorption when the content of Mn is in the range of10～33%. As Mn content is40%, excessive Mn inhibits the reduction of the catalysts in H2and H2adsorption. The CO-TPD and CO2-TPD results show that the surface basicity and CO adsorption of the catalysts are enhanced with Mn content increasing. The XRD results of the catalysts after reaction indicate that the diffraction peak intensity of FeCx reaches a maximum at Mn content of33%. Under the reactions conditions of325℃,1.0MPa,2000h-1and H2/CO=3.5, the FTS tests of FeMn and FeMnNa with different Mn loadings show that the catalyst has the highest CO conversion and content to light olefins with the ratio of Fe:Mn=100:33.The FeMnK catalysts with different K loadings were prepared by coprecipitation-calcination-impregnation-calcination method. As revealed by N2physisorption, the increasing of K results in the increase of crystal size, the low surface area and pore volume. XPS results indicate that the contents of K and Mn on catalysts surfaces are more than those in the bulk. H2-TPR results show that with the increase of K content, the reduction of the catalysts in H2is suppressed. H2-TPD and CO2-TPD results indicate that increase of K content inhibits the H2adsorption and strengthens the basicity of the FeMnK catalysts; CO-TPD results show that the CO adsorption of the catalysts is enhanced with increasing K content when the content of K is in the range of0.7～2.8%, but the CO adsorbability declines when the content of K is more than2.8%. The MES results of catalysts after reduction reveal that after reduction with syngas (H2/CO=20) for48h, the iron oxides are converted to magnetite and few iron carbides are in the bulk. XRD and MES results of catalysts after reaction show that with the increase of the K content, FeCx in the bulk of the catalysts increases and Fe3+decreases gradually. The FTS tests show that the CO conversion and light olefins content are highest when the K content reaches2.8%.The FeMnNa catalysts with different Na loadings were prepared by coprecipitation-calcination-impregnation-calcination method. The N2adsorption results show that the increase of Na content results in low surface area and pore volume, and has no obvious effect on the crystal size of catalysts. XPS results indicate that contents of Na and Mn on the catalysts surfaces are more than those in the bulks. H2-TPR results indicate that with the increase of Na content, the H2consumption of H2-TPR decreases and the reduction of the catalysts in H2is suppressed slightly. H2-TPD results show that Na inhibits the H2adsorption of the catalysts and the content of Na has no obvious effect on the H2adsorption. According to the CO2-TPD and CO-TPD, the CO2and CO adsorptions of the catalysts are enhanced with Na content increasing, but excessive Na inhibits the CO adsorption. XRD and MES results of catalysts after reaction indicate that the FeCx of FeMnNa catalysts increases with Na increasing and reaches a maximum at Na loading of2.8%, following by a decline with further increasing in Na content. The FTS test results show that the increase in Na loadings improves the CO conversion and light olefins content when the content of Na is in the range of0.7～2.8%, and that the CO conversion and light olefins content are highest when the Na content reaches2.8%, following by a decline with further adding of Na. Furthermore, the effects of reaction conditions (temperature, pressure and space velocity) on100Fe33Mn2.8Na catalyst were also studied for producing light olefins. The temperature is in the range of315～355℃has no obvious effect on light olefins and the light olefins content reaches the highest at a temperature of325℃. When the pressure is in the range of0.5～2.5MPa, increasing pressure is beneficial to CO conversion. The light olefins content reaches a maximum at the pressure of1.0MPa. At the space velocity of500～2500h"1, the CO conversion increases firstly and then decreases slightly with the increase of space velocity. The increasing space velocity promotes the production of light olefins. Based on an overall analysis of CO conversion and light olefins, the optimal conditions over100Fe33Mn2.8Na catalyst for light olefins production are as follows:reaction temperature is325℃, pressure is1.0MPa and space velocity is2000h-1, the corresponding CO conversion and light olefins content are96.2%and30.6%, respectively.The olefins and paraffins distributions over FeMnK with different K loadings and FeMnNa catalysts with different Mn and Na loadings are investigated. The results show that the increasing of Mn improves the chain growth probability of olefins and reduces the chain growth probability of paraffins when the content of Mn is in the range of10～33%. The increasing of K and Na content promotes the formation of light olefins and heavy hydrocarbons. For the100Fe33Mn2.8Na catalyst, under the condition of temperature of315～355℃, pressure of0.5～2.5MPa and space velocity of500～2500h-1,the effects of reaction conditions (temperature, pressure, and space velocity) on olefins and paraffins distributions are studied. The results indicate that increasing temperature is beneficial to the formation of light olefins when the temperature is in the range of315～325℃. Rising pressure increases the chain growth probability of heavy hydrocarbons and decreases the light olefins content. Increasing space velocity improves the content of light olefins.