Fe/Mn catalyst for Fischer-Tropsch synthesis and its preparation method

Field of the invention The present invention relates to Fe/Mn catalysts for Fischer-Tropsch synthesis and to methods for preparing such Fe/Mn catalysts.

Background of the invention

Fischer-Tropsch (F-T) synthesis relates to a process for converting synthesis gas (mixtures of CO and H ₂ ) to hydrocarbons over catalysts. Recently, during industrialization of F-T synthesis, Fe/Mn catalysts are widely used in F-T synthesis due to their high activity and wide temperature range.

As known, even a small change of catalyst composition may cause a noticeable change of catalyst characteristics. In order to improve some catalyst characteristics, such as selectivity for a certain product, activity, lifetime and etc, research on F-T catalysts has never ceased.

Summary of the invention

Embodiments of the present invention provide a catalyst for F-T synthesis. The catalyst includes iron (Fe), manganese (Mn), potassium (K) and copper (Cu) with a Fe/Mn molar ratio between 1 :1 and 2:1. The catalyst when applied in an F-T-synthesis reaction under certain conditions is able to keep a CO conversion of the reaction above 80% for a time close to or above 100 hours.

Further, the catalyst when applied in an F-T-synthesis reaction under conditions that pressure is 2.0 MPa, temperature is 24O ⁰ C, H ₂ /CO molar ratio is 1.7, and gaseous hourly space velocity (GHSV) is 200Oh ^"1 is able to keep a CO conversion of the reaction above 80% for a time close to or above 100 hours.

Embodiments of the present invention further provide a method for making Fe/Mn catalysts. According to the method, a solution containing precursors of Fe and Mn is prepared according to a predetermined Fe/Mn molar ratio, and then a first dry material containing Fe and Mn is obtained from the solution. Then K and Cu are added into the first dry material containing Fe and Mn by impregnation to obtain a second dry material containing Fe, Mn, K and Cu wherein K loading is not more than 1% by weight and Cu loading is about 0.5% by weight. When the method is used to make a set of catalysts with Fe/Mn molar ratios of 9:1, 7:3, 6:4, 5:5, 4:6, i

3:7, and 2:8, respectively, and the set of catalysts are applied in a set of F-T-synthesis reactions, respectively, under same or approximately same F-T reaction conditions, a CO conversion of the F-T-synthesis reaction preformed over the catalyst with Fe/Mn molar ratio of 6:4 is consistently higher than CO conversions of others of the set of the F-T-synthesis reactions over a time period close to or longer than about 100 hours.

In another aspect, embodiments of the present invention provide a method for making Fe/Mn catalyst comprising steps of preparing a solution containing precursors of Fe and Mn according to a Fe/Mn molar ratio of about 6:4 and adding a precipitator to the solution until a PH of the solution reaches a predetermined value. Then a material of substantially power form is obtained from the solution. K and Cu are added into the material of substantially power form by impregnation so as to obtain a dry material containing Fe, Mn, K and Cu wherein K loading is not more than 1% by weight and Cu loading is about 0.5% by weight. The dry material containing Fe, Mn, K and Cu is calcined.

In yet another aspect, embodiments of the present invention provide a method for making Fe/Mn catalyst comprising steps of preparing a solution containing precursors of Fe and Mn according to a Fe/Mn molar ratio of about 6:4 and adding a complexing agent to the solution to obtain a gel-like material. The gel-like material is dried to obtain a dried material and then the dried material is decomposed to obtain a decomposed material. The decomposed material is calcined to obtain a calcined material. Then K and Cu are added into the calcined material by impregnation so as to obtain a dry material containing Fe, Mn, K and Cu wherein K loading is not more than 1% by weight and Cu loading is about 0.5% by weight. The dry material containing Fe, Mn, K and Cu is calcined.

Brief description of the drawings

FIG. 1 is a flowchart illustrating a method of making Fe/Mn catalyst for F-T synthesis according to one embodiment.

FIG. 2 is a block diagram of a parallel reaction system that can be used to evaluate F-T catalysts.

FIG. 3 is a collection of plots of CO conversion vs. reaction time for respective F-T-synthesis reactions performed over respective catalysts with different Fe/Mn molar ratios that are prepared using the method shown in FIG. 1 and using NH ₄ HCO ₃ solution as a precipitator.

FIG. 4 is a collection of plots of CO conversion vs. reaction time for respective F-T-synthesis reactions performed over respective catalysts with different Fe/Mn molar ratios that are prepared using the method shown in FIG. 1 and using ammonium hydroxide (NH ₃ -H ₂ O) as a precipitator. FIG. 5 is a flowchart illustrating a method of making Fe/Mn catalyst for F-T synthesis according to an alternative embodiment.

FIG. 6 is a collection of plots of CO conversion vs. reaction time for respective F-T-synthesis reactions performed over respective catalysts with different Fe/Mn molar ratios that are prepared using the method shown in FIG. 5 and using glycolic acid-NH ₃ -H ₂ O as a complexing agent.

FIG. 7 is a collection of plots of CO conversion vs. reaction time for respective F-T-synthesis reactions performed over respective catalysts with different Fe/Mn molar ratios that are prepared using the method shown in FIG. 5 and using citric acid-NH ₃ -H ₂ O as a complexing agent. Detailed description of embodiments

Example 1

Referring to FIG. 1, a Fe/Mn catalyst preparation method 100 includes a solution preparation step 101, a co-precipitation step 103, a static-aging step 105, a separating and washing step 107, a drying and calcining step 109, an impregnation step 111 , a second calcining step 113, a pressing and shaping step 115 and etc.

In the solution preparation step 101, appropriate amounts of Fe(NO ₃ ) ₃ -9H ₂ O, 50 wt% Mn(NO ₃ ) ₂ solution and water are weighed with respect to a desired Fe/Mn molar ratio (called as Fe/Mn hereafter)and are mixed together to form a solution (a mixed solution of Fe and Mn nitrate). Appropriate amounts of Fe(NO ₃ ) ₃ -9H ₂ O, 50 wt% Mn(NO ₃ ) ₂ solution and water corresponding to different desired Fe/Mn can be calculated with reference to table 1 below. In the co-precipitation step 103, a precipitator may be added into the mixed solution of Fe and Mn nitrates with stirring to cause co-precipitation until a PH value of the solution reaches a predetermined value. In one embodiment, NH ₄ HCO ₃ solution of 1.3 mol/L is prepared as a precipitator and added to the mixed solution of Fe and Mn nitrates at a dripping rate of around 900 ml/min while the solution is stirred at a speed between 200-400 rpm until the solution's PH is about 7.5. In the static-aging step 105, the solution after co-precipitation is kept without stirring and aged at room temperature for 1-4 hours. In the separating and washing step 107, the solution

after aging may be centrifuged and washed with deionized water for several times (e.g. 5 times) to obtain a centrifuged cake. In the drying and calcining step 109, the centrifuged cake may be dried at HO ⁰ C, then ground into a material of substantially powder form (powder of Fe and Mn oxides), and then calcined for about 1 hour at 35O ⁰ C in flowing air. In the impregnation step 111, K and Cu may be added into the calcined powder by wet impregnation to obtain a material containing Fe, Mn, K, and Cu wherein K loading is not more than 1% by weight and Cu loading is about 0.5% by weight. For example, 9 grams of the calcined powder can be added into 18 ml solution containing 0.16 g of K ₄ CO ₃ compound. The resulting material is dried at HO ⁰ C, and then added into 18 ml solution containing 0.283 g of Cu(NO ₃ ) ₂ -3H ₂ O compound, followed by drying at HO ⁰ C, and a material containing Fe, Mn, K, and Cu wherein K and Cu loading are 1 wt% and 0.5 wt%, respectively, in substantially power form is obtained. In the second calcining step 113, the dried powder containing Fe, Mn, K, and Cu may be recalcined for about 4 hours at 400°C in flowing air. In the pressing and shaping step 115, the calcined powder is pressed to obtain pellets, the pellets are crushed to form catalyst particles, and the catalyst particles are sieved to collect 20-40 mesh particles.

Table 1 : Amounts of Fe, Mn source materials and water for preparing catalysts with different Fe/Mn

The Catalysts may be reduced under certain conditions before being applied in real F-T-synthesis reactions or simulative F-T-synthesis experiments for catalyst evaluation. Both the catalyst reduction and experiments may be carried out in a reaction system, such as a parallel reaction system developed by Accelergy (Shanghai) R&D center. By using the parallel reaction system, two or more catalysts of different properties can be evaluated in parallel and under same or approximately same F-T synthesis conditions. In this way, time is saved by running the experiments in parallel, the experiment results are more comparable, and the

evaluation can be more reliable.

Referring to FIG. 2, a parallel reaction system 200 used to evaluate the catalysts in this example includes a parallel reaction module 20 having a plurality of reactors (e.g. reaction vessels) 20-1, 20-2, ..., 20-n (n is an integer), which can be used to carry out a plurality of separate reactions. The parallel reaction system 200 further includes a material feeding module 21 adapted to feed reactants into the plurality of reactors 20-1, 20-2, ..., 20-n, a real-time control module 23 adapted to monitor and control reaction conditions in the plurality of reactors, and an analysis module 25 adapted to analyze reaction products from the plurality of reactors. In order to evaluate characteristics of the catalysts, the catalysts with different Fe/Mn molar ratio prepared in this example are loaded in the respective reactors 20-1, 20-2, ..., 20-n, and F-T-synthesis reactions under same or approximately same conditions are carried out in the reactors. Conditions for the F-T-synthesis reactions, such as temperatures and pressures in the reactors, flow rates of H ₂ and CO, and etc., can be monitored and controlled by the control module 23 such that the catalysts in different reactors can be reduced under same or approximately same reduction conditions and then evaluated under the same or approximately same F-T reaction conditions. In one example, catalysts with different Fe/Mn ratios are loaded in different reactors and respectively reduced under the conditions that pressure is normal pressure, temperature is 27O ⁰ C, H ₂ /CO molar ratio is 1.7, and gaseous hourly space velocity (GHSV) is 200Oh ^"1 . Then the reduced catalysts are respectively applied in F-T-synthesis experiments under the conditions that pressure is 2.0MPa, temperature is 24O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 . FIG. 3 shows the CO conversions vs. reaction time corresponding to these F-T-synthesis experiments, which are performed over respective ones of a set of catalysts with Fe/Mn of 9:1, 7:3, 6:4, 5:5, 4:6, 3:7, and 2:8 prepared using the method discussed above in this example.

As shown in FIG. 3, CO conversion of the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 6:4 is consistently higher than the experiments performed over any other of the set of catalysts in this example when the F-T-synthesis experiments are stable (e.g., 15-20 hours after the experiments have started). As can be seen from FIG. 3, within 15-160 hours after the experiments have started, the CO conversion curve 301 the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 6:4 is not only higher but also flatter and more stable than the CO conversion curves of the F-T-synthesis experiments performed

over the other catalysts. In this example, the CO conversion curve 301 extends above a level of 80% and is relatively flat and stable within 10-160 hours after the experiments have started. The difference between a peak value (about 90%) and a bottom value (about 80%) for the CO conversion curve 301 is less than or about 10%. In other words, within 16-128 hours after the experiments have started (i.e., for more than 100 hours of reaction time), the bottom value of curve 301 is not lower than 90% of the peak value of the curve. Within 16-128 hours after the experiments have started (i.e., for more than 100 hours of reaction time), curve 301 never drops below 90% of its peak value during the period. Also, within 16-160 hours after the experiments have started (i.e., for more than 140 hours of reaction time), curve 301 never drops below 80% of its peak value during the period. Therefore, the catalyst with Fe/Mn of 6:4 in this example has a stable activity for F-T synthesis and is able to keep the CO conversion of the F-T-synthesis reaction stable for a long period of time. Compared to the catalyst with Fe/Mn of 6:4, catalysts with much lower Mn loading can not maintain a high activity for a long period of time. For example, a catalyst with Fe/Mn of 7:3 or 9:1 has a high initial activity but their activities diminish after a period of short time. CO conversion of the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 7:3 or 9:1 reaches 90% 20 hours after the experiment is started but reduces to 40% 10 hours later, i.e. 20-30 hours after the experiment is started. On the other hand, catalysts with much higher Mn loading have improved stability but lower activity. For example, the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 2:8 has a CO conversion always below 60%. Therefore, it is demonstrated that an optimum value of Fe/Mn falls between 5:5 (i.e., 1 :1) and 7:3 (approximately 2: 1). In one embodiment, the optimum value of Fe/Mn is around 6:4.

Example 2

In example 2, the preparation method as show in FIG. 1 is used. In step 103, ammonium hydroxide (NH ₃ -H ₂ O) of 5.3 mol/L is prepared. Then the ammonium hydroxide is slowly (100 ml/min) added into a mixed solution of Fe and Mn nitrate prepared in step 101 while stirring at 200-400 rpm, until the solution PH reaches 9.0. The other steps can be carried out in the same way as disclosed in example 1 , and therefore are not repeated here.

A set of catalysts prepared in this example can be reduced and evaluated either alone or together with a set of catalysts in example 1 through the parallel reaction

system 200. In one embodiment, a set of catalysts prepared in this example are reduced under the same condition that pressure is normal pressure, temperature is 27O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 , and then evaluated in F-T-synthesis experiments under a same condition that pressure is 2.0MPa, temperature is 24O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 . An evaluation result can be referred to FIG. 4, which shows CO conversion vs. reaction time for the F-T-synthesis experiments using the catalysts respectively with Fe/Mn of 9:1, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8 in this example. As shown in FIG. 4, after the experiments become stable (e.g. 10-20 hours after the experiments are started), CO conversion of the experiment performed over the catalyst with Fe/Mn of 6:4 is consistently higher than the experiments performed over other catalysts. As can be seen in FIG. 4, a CO conversion curve 401 corresponding to the experiment performed over the catalyst with Fe/Mn of 6:4 stably extends at a high level within 10-160 hours after the experiment is started. The CO conversion curve 401 stays above a level of 80% within 10-100 hours after the experiment is started and stays above a level slightly lower than 80% within 100-160 hours after the experiment is started. Moreover, the CO conversion curve 401 is consistently higher above the CO conversion curves for the experiments performed over catalysts with Fe/Mn of other values within 10-160 hours after the experiments are started. Furthermore, the CO conversion curve 401 is flatter and more stable than the CO conversion curves of the F-T-synthesis experiments performed over the other catalysts. The difference between a peak value (about 90%) and a bottom value (about 80%) for the CO conversion curve 401 is less than or about 10%. Especially, corresponding to a period within 40-160 hours after the experiment is started (more than 100 hours of reaction time), a section of the CO conversion curve 401 is almost horizontally flat, and never drops below 90% of its peak value within this section of the CO conversion curve 401. That is to say, when applied in a F-T-synthesis reaction, the catalyst with Fe/Mn of 6:4 in this example is able to keep the CO conversion of the reaction stable and high for a long period of time. Compared to the catalyst with Fe/Mn of 6:4, the catalysts with much lower Mn loading can not maintain a high activity for a long period time. For example, the catalyst with Fe/Mn of 9: 1 initially has a high activity but its activity decays after a short period of time. As shown, the CO conversion of the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 9: 1 reaches about 80% 15 hours after the experiment is started but drops to lower than 40% 25 hours later, i.e. 40 hours after the experiment is started. On the other hand, catalysts with much higher Mn

loading, have improved stability but lower activity. For example, the CO conversion of the F-T-synthesis experiment performed over the catalyst with Fe/Mn of 2:8 is always below 60%. Therefore, it is demonstrated that an optimum value of Fe/Mn falls between 5:5 (i.e., 1 :1) and 7:3 (approximately 2:1). In one embodiment, the optimum value of Fe/Mn is around 6:4.

Example 3

Referring to FIG. 5, another Fe/Mn catalyst preparation method 500 includes a solution-preparation step 501, a sol-gel-processing step 503, a drying and decomposition step 505, a calcining step 507, an impregnation step 509, a second calcining step 511 , a pressing and shaping step 513 and etc.

In the solution preparation step 501, appropriate amounts of Fe(NO ₃ ) ₃ -9H ₂ O, 50 wt% Mn(NO ₃ ) ₂ solution and water, depending on the Fe/Mn value wanted, may be mixed together to form a nitrate solution. Appropriate amounts of Fe(NO ₃ ) ₃ -9H ₂ O, 50 wt% Mn(NO ₃ ) ₂ solution and water corresponding to different Fe/Mn can be calculated by referring to Table 1. In the sol-gel-processing step 503, a complexing agent may be added into the nitrate solution under stirring in order to get a gel-like material. In one embodiment, 34.89 g of glycolic acid and 5.34 g water are added into 40.13 ml of 25 wt% NH ₃ -H ₂ O to obtain a glycolate solution having a pH value of 6.5. Then, the glycolate solution was added to the nitrate solution under stirring to obtain a gel-like material. In the drying and decomposition step 505, the gel-like material obtained in the step 503 may be dried in air at 100°C and decomposed in air at 130-180 ⁰ C to obtain a material of substantially powder form. In the calcining step 507, the material of substantially powder form obtained from the decomposition step may be calcined for about 1 hour at 300-450°C in flowing air to obtain a calcined material. In the impregnation step 509, K and Cu may be added into the calcined material by wet impregnation so as to obtain a material containing Fe, Mn, K, and Cu wherein K loading is not more than 1% by weight and Cu loading is about 0.5% by weight. In one embodiment, 9 g of the calcined material is added into 18 ml solution containing 0.16 g Of K ₂ CO ₃ compound, followed by drying at HO ⁰ C, and then added into 18 ml solution containing 0.283 g of Cu(NO ₃ ) ₂ -3H ₂ O compound, followed by drying at HO ⁰ C, so as to obtain powder containing Fe, Mn, K, and Cu wherein K and Cu loading are 1 wt% and 0.5 wt%, respectively. In the second calcining step 511, the dried Fe-Mn-K-Cu powder may be re-calcined for about 4 hours at 400°C in flowing air to obtain a re-calcined material. In the pressing and shaping step 513,

the re-calcined material is pressed to obtain pellets and then the pellets are crushed to catalyst particles and the catalyst particles are sieved to collect 20-40 mesh particles.

A set of catalysts prepared in this example can be reduced and evaluated either alone or together with the sets of catalysts in example 1 and/or example 2 through the parallel reaction system 200. In one embodiment, the set of catalysts prepared in this example are reduced under the same condition that pressure is normal pressure, temperature is 27O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 , and then evaluated in F-T-synthesis experiments under the same condition that pressure is 2.0 MPa, temperature is 24O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 . An evaluation result can be referred to FIG. 6, which shows CO conversion vs. reaction time for the F-T-synthesis experiments using a set of catalysts with Fe/Mn of 9:1, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8, respectively, in this example. As shown in FIG. 6, after the experiments become stable (e.g. 10-20 hours after the experiments are started), the CO conversion of the experiment performed over the catalyst with Fe/Mn of 6:4 is consistently higher than those of the experiments performed over other catalysts. As can be seen from FIG. 6, a CO conversion curve 601 corresponding to the experiment using the catalyst with Fe/Mn of 6:4 stably extends at a level above 80% and lies above the CO conversion curves corresponding to the experiments using other catalysts, during 10-160 hours after the experiment starts. Further, within the 10-160 hours after the experiment is started, the CO conversion curve 601 remains relatively stably, with a difference between its peak value (about 90%) and its bottom value (about 85%) less than or about 5% and never dropping below 90% of the peak value. That is to say, the catalyst with Fe/Mn of 6:4 in this example, when applied in F-T-synthesis reactions, is able to make the CO conversion of the reaction stable and high for a long period of time. Relatively, both the catalysts with higher or lower Mn loading in the set of catalysts prepared for this evaluation can not either cannot achieve such high activity or cannot maintain such high activity for a such a long period of time. Therefore, it is demonstrated that an optimum value of Fe/Mn falls between 5:5 (i.e., 1 :1) and 7:3 (approximately 2: 1). In one embodiment, the optimum value of Fe/Mn is around 6:4.

Example 4 In example 4, the preparation method as show in FIG. 5 is used. In step 503,

29.4 g of citric acid was added to 40.13 ml of 25 wt% NH ₃ -H ₂ O to obtain a citrate solution having a pH value of 6.5. Then, the citrate solution is added to the nitrate solution prepared in step 501 under stirring to get a gel-like material. The other steps can be carried out in the same way as disclosed in example 3, and therefore are not repeated here.

A set of catalysts prepared in this example can be reduced and evaluated either alone or together with the sets of catalysts in example 1 , example 2, and example 3 through the parallel reaction system 200. In one embodiment, the set of catalysts prepared in this example are reduced under the same condition that pressure is normal pressure, temperature is 27O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 , and then evaluated in F-T-synthesis experiments under the same condition that pressure is 2.0MPa, temperature is 24O ⁰ C, H ₂ /CO molar ratio is 1.7, and GHSV is 200Oh ^"1 . An evaluation result can be referred to FIG. 7, which shows CO conversion vs. reaction time for the F-T-synthesis experiments using the catalysts respectively with Fe/Mn of 9:1, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8 in this example.

As shown in FIG. 7, after the experiments become stable (e.g. 10-20 hours after the experiments started), the CO conversion of the experiment performed over the catalyst with Fe/Mn of 6:4 is always higher than those of the experiments performed over other catalysts. As can be seen from FIG. 7, during 10-160 hours after the experiments started, a CO conversion curve 701 corresponding to the experiment performed over the catalyst with Fe/Mn of 6:4 stably extends at a level above 80% and lies above the CO conversion curves corresponding to the experiments using other catalysts. Further, within 10-160 hours after the experiment started, the CO conversion curve 701 remains stable, with a difference between its peak value (about 88%) and its bottom value (about 82%) less than or about 6% and never dropping below 90% of the peak value. That is to say, the catalyst with Fe/Mn of 6:4 in this example, when applied in F-T-synthesis reactions, is able to make the CO conversion of the reaction stable and high for a long period of time. Relatively, both the catalysts with higher or lower Mn loading in the set of catalysts prepared for this evaluation can not either cannot achieve such high activity or cannot maintain such high activity for a such a long period of time. Therefore, it is demonstrated that an optimum value of Fe/Mn falls between 5:5 (i.e., 1 :1) and 7:3 (approximately 2:1). In one embodiment, the optimum value of Fe/Mn is around 6:4.