Iron-based catalyst and method for preparing the same and use thereof

Abstract

The present invention relates to a method for preparing liquid or solid hydrocarbons from syngas via the Fischer-Tropsch synthesis in the presence of iron-based catalysts, the iron-based catalysts for the use thereof, and a method for preparing the iron-based catalysts; more specifically, in the Fischer-Tropsch reaction, liquid or solid hydrocarbons may be prepared specifically with superior productivity and selectivity for C.sub.5+ hydrocarbons using the iron-based catalysts comprising iron hydroxide, iron oxide, and iron carbide wherein the number of iron atoms contained in the iron hydroxide is 30% or higher, and the number of iron atoms contained in the iron carbide is 50% or lower, relative to 100% of the number of iron atoms contained in the iron-based catalysts.

Claims

1. A method of making a catalyst, comprising: preparing iron-based catalysts from a first precursor comprising iron hydroxide and iron oxide, wherein the number of iron atoms contained in the iron hydroxide ranges from 65% to 85%, and the number of iron atoms contained in the iron oxide ranges from 15% to 35%, relative to 100% of the number of iron atoms contained in the first precursor.

2. The method of claim 1, wherein the first precursor further comprises iron carbide and the number of iron atoms in a phase fraction of the iron hydroxide ranges from 30% to 60%, the number of iron atoms in a phase fraction of the iron oxide ranges from 10% to 30%, and the number of iron atoms in a phase fraction of the iron carbide ranges from 10% to 50%, relative to 100% of the number of iron atoms contained in the iron-based catalysts.

3. A method of making a catalyst, comprising: preparing iron-based catalysts from a first precursor comprising iron hydroxide and iron oxide, wherein the iron hydroxide is ferrihydrite, and the iron oxide is selected from the group consisting of magnetite, hematite, maghemite, and a combination thereof.

4. The method of claim 3, wherein the first precursor further comprises iron carbide and the number of iron atoms in a phase fraction of the iron hydroxide ranges from 30% to 60%, the number of iron atoms in a phase fraction of the iron oxide ranges from 10% to 30%, and the number of iron atoms in a phase fraction of the iron carbide ranges from 10% to 50%, relative to 100% of the number of iron atoms contained in the iron-based catalysts.

5. A method of making a catalyst, comprising: preparing iron-based catalysts from a first precursor comprising iron hydroxide and iron oxide, wherein the preparing iron-based catalysts comprises heating the first precursor under a gas atmosphere comprising carbon dioxide (CO.sub.2), hydrogen (H.sub.2), and carbon monoxide (CO).

6. The method of claim 5, wherein the volume of carbon dioxide is from 25% to 60%, relative to 100% volume of the gas atmosphere, and the volume ratio between hydrogen and carbon monoxide is from 0.7:1 to 1.3:1.

7. The method of claim 5, wherein the first precursor further comprises iron carbide and the number of iron atoms in a phase fraction of the iron hydroxide ranges from 30% to 60%, the number of iron atoms in a phase fraction of the iron oxide ranges from 10% to 30%, and the number of iron atoms in a phase fraction of the iron carbide ranges from 10% to 50%, relative to 100% of the number of iron atoms contained in the iron-based catalysts.

8. A method for preparing iron-based catalysts comprising: preparing a first precipitation slurry by mixing optionally an aqueous solution containing salt of metal selected from copper, cobalt, manganese, and a combination thereof, an aqueous solution containing acidic salt of iron, and a basic aqueous solution; preparing a second precipitation slurry by adding at least one oxide selected from silicon oxide, aluminum oxide, zirconium oxide, or chromium oxide, and optionally at least one aqueous solution containing an alkali metal or an alkaline earth metal, to the first precipitation slurry; preparing a first precursor by drying the second precipitation slurry; preparing a second precursor comprising iron hydroxide and iron oxide by calcining the second precursor; and preparing iron-based catalysts comprising iron hydroxide, iron oxide, and iron carbide by heating the second precursor under the gas atmosphere comprising carbon dioxide (CO.sub.2), hydrogen (H.sub.2), and carbon monoxide (CO).

9. The method of claim 8, wherein, for the second precipitation slurry, a weight ratio of (i) iron (Fe) contained in the aqueous solution containing acidic salt of iron added during the formation of the first precipitation slurry is 100:3, a weight ratio of (ii) a metal contained in the aqueous solution of the metal salt added during the formation of the first precipitation slurry is 7:3, a weight ratio of (iii) a metal contained in the aqueous solution of an alkali metal or an alkaline earth metal added during the formation of the second precipitation slurry, and a weight ratio of (iv) an oxide added during the formation of the second precipitation slurry is 30:1.

10. The method of claim 8, wherein the first precursor comprises iron hydroxide and iron oxide-hydroxide.

11. The method of claim 10, wherein the iron hydroxide is ferrihydrite, and the iron oxide-hydroxide is goethite.

12. The method of claim 8, wherein the number of iron atoms in a phase fraction of the iron hydroxide ranges from 30% to 60%, the number of iron atoms in a phase fraction of the iron oxide ranges from 10% to 30%, and the number of iron atoms in a phase fraction of the iron carbide ranges from 10% to 50%, relative to 100% of the number of iron atoms contained in the iron-based catalysts.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is the H.sub.2-TPR profiles of the as-prepared catalysts (a) in the absence of CO.sub.2 and (b) in the presence of CO.sub.2.

(2) FIG. 2 is the CO-TPR profiles of the as-prepared catalysts (a) in the absence of CO.sub.2 and (b) in the presence of CO.sub.2.

(3) FIG. 3 is a graph showing the syngas consumption for the activation of catalysts during the activation pre-treatment using syngas with different amounts of CO.sub.2 as a function of activation time.

(4) FIG. 4 is the XRD patterns of the catalysts activated by syngas with different amounts of CO.sub.2: (a) 0% CO.sub.2, (b) 20% CO.sub.2, (c) 33% CO.sub.2, and (d) 50% CO.sub.2.

(5) FIG. 5 is the Mössbauer spectra of the catalysts activated by syngas with different amounts of CO.sub.2: (a) 0% CO.sub.2, (b) 20% CO.sub.2, (c) 33% CO.sub.2, and (d) 50% CO.sub.2.

(6) FIG. 6 is the TPH spectra of the catalysts activated by syngas with different amounts of CO.sub.2: (a) 0% CO.sub.2, (b) 20% CO.sub.2, (c) 33% CO.sub.2, and (d) 50% CO.sub.2.

(7) FIG. 7 is a graph showing the overall CO conversion during 66 hours to 114 hours of reaction in the Fischer-Tropsch synthesis as a function of inlet CO.sub.2 content during activation.

(8) FIG. 8 is a graph showing the productivity of hydrocarbons during 66 hours to 114 hours of reaction in the Fischer-Tropsch synthesis as a function of inlet CO.sub.2 content during activation.

(9) FIG. 9 represents (a) the hydrocarbon distribution and the 1-olefin selectivity in C.sub.2 to C.sub.4 hydrocarbons during 66 hours to 114 hours of reaction in the Fischer-Tropsch synthesis as a function of inlet CO.sub.2 content during activation, and (b) the carbon number distribution of C.sub.5+ hydrocarbons and chain growth probability per each inlet CO.sub.2 content during activation.

(10) FIG. 10 shows the phase conversion as a function of the inlet CO.sub.2 content during activation.

(11) FIG. 11 shows the productivity of hydrocarbons per unit mass of iron carbides during 66 hours to 114 hours of reaction as a function of the inlet CO.sub.2 content during activation, which is calculated from the results of XRF, Mössbauer analyses (FIG. 5 and Table 1), and catalytic performance (FIG. 8).

(12) FIG. 12 shows analytical results of catalytic performance according to reaction temperature during the Fischer-Tropsch synthesis.

(13) FIG. 13 shows analytical results of catalytic performance according to reaction pressure during the Fischer-Tropsch synthesis.

(14) FIG. 14 shows analytical results of catalytic performance according to space velocity during the Fischer-Tropsch synthesis.

(15) FIG. 15 shows analytical results of catalytic performance according to the ratio between H.sub.2 and CO of syngas during the Fischer-Tropsch synthesis.

(16) FIG. 16 shows analytical results of catalytic performance according to CO.sub.2 content in syngas during the Fischer-Tropsch synthesis.

(17) FIG. 17 shows a graph illustrating analytical results of phase fractions of the first precursor prepared in the iron-based catalyst preparation method of Example 1 via Mössbauer spectroscopy. It was confirmed that the first precursor consisted of a phase fraction consisting of 81.6% ferrihydrite and 18.4% hematite, based on the number of iron atoms.

BEST MODEL

(18) Hereinafter, the present invention will be described in more detail with reference to the following examples, comparative examples, and experimental examples. However, the following examples, comparative examples, and experimental examples are provided for illustrative purposes only, and the scope of the present invention should not be limited thereto in any manner.

Example 1

Preparation of Iron-based FTS Catalysts

(19) In the present invention, the iron-based FTS catalysts were prepared through a combination of a co-precipitation technique and a spray-drying method.

(20) Specifically, a 2 M aqueous solution of Fe(NO.sub.3).sub.3 and a 2 M aqueous solution of Cu(NO.sub.3).sub.2 were mixed to prepare a mixed solution, and then a 2 M aqueous solution of Na.sub.2CO.sub.3 was added to the mixed solution at 80±1° C. until the pH reached 8.0±0.1.

(21) The resultant precipitate slurry was filtered, washed with distilled water to sufficiently remove residual sodium, and subsequently re-slurried in distilled water. After completing the washing process, an aqueous K.sub.2CO.sub.3 solution and a colloidal suspension of SiO.sub.2 were added to the precipitate slurry by controlling the weight ratio of Fe:Cu:K:SiO.sub.2 to be 100:5:5:18, and the final mixture was spray-dried (inlet: 200° C.; outlet: 95° C.). Then, the spray-dried sample was calcined at 400° C. for 8 hours. The calcined catalysts were pressed into pellets and then crushed and sieved to obtain 300 μm to 600 μm particles.

Experimental Example 1

Characterization of as-prepared Catalysts

(22) The chemical composition of the as-prepared catalysts in Example 1 was analyzed by X-ray fluorescence spectroscopy (XRF) using a Rigaku model ZSX Primus II. Further, the Brunauer-Emmett-Teller (BET) surface area, the single point pore volume, and the average pore diameter of the as-prepared catalysts were analyzed by means of N.sub.2 physisorption using a Micrometrics model Tristar II 3020. The crystal structure of the as-prepared catalysts was characterized by X-ray diffraction (XRD) with a Rigaku DMAX-2500 using a Cu Kα source. The quantitative analysis on the phase structure was carried out by Mössbauer spectroscopy using a 50 mCi .sup.57Co source in a rhodium matrix. The spectrometer was operated in the constant acceleration mode, and the spectra were recorded at −268.8° C. (=4.2 K) with a fixed absorber and a moving source. The spectra were deconvoluted based on the value of magnetic hyperfine field (H.sub.hf), isomer shift (δ), and quadruple splitting (E.sub.Q) for each iron-based species. The value of H.sub.hf was calibrated using metallic iron (α-Fe) foils.

(23) The chemical composition of the as-prepared catalysts analyzed by XRF was 100 Fe/5.26 Cu/4.76 K/18.2 SiO.sub.2 in parts by weight. The BET surface area, the single point pore volume, and the average pore diameter of the as-prepared catalysts were 183 m.sup.2/g, 0.458 cm.sup.3/g, and 9.98 nm, respectively. The crystal structure of the as-prepared catalysts was identified as a combination of hematite (Fe.sub.2O.sub.3) and ferrihydrite (Fe.sub.5O.sub.7(OH).4H.sub.2O), as characterized by XRD and Mössbauer spectroscopy. The fractions of hematite and ferrihydrite analyzed by Mössbauer spectroscopy were 18.4% and 81.6%, respectively.

Experimental Example 2

Investigation of the Influence of CO2 on the Reduction and Carburization Behavior of the Catalysts

(24) The influence of CO.sub.2 on the reduction and carburization behavior of the catalysts was analyzed by means of temperature-programmed reduction using H.sub.2 (H.sub.2-TPR) or CO (CO-TPR) as a reducing agent. Two different carrier gases were used for each reducing agent: 5% H.sub.2/Ar and 5% H.sub.2/5% CO.sub.2/Ar for the H.sub.2-TPR and 5% CO/He and 5% CO/5% CO.sub.2/He for the CO-TPR. The TPR was performed at up to 800° C. at a heating rate of 6° C./min, and the temperature was then held at the maximum temperature for 60 min. During the H.sub.2-TPR process, the amount of H.sub.2 or CO.sub.2 consumption was analyzed with a quadruple mass spectrometer (MS) with a capillary inlet system. For the CO-TPR, the amount of CO consumption was measured by a thermal conductivity detector (TCD). Ascarite (223921, Aldrich) was used as a CO.sub.2 removal trap to measure the true CO consumption profiles without considering the influence of CO.sub.2 prepared during the CO-TPR.

(25) The influence of CO.sub.2 on the reduction behavior of the catalysts in the H.sub.2 atmosphere was investigated by H.sub.2-TPR, as shown in FIG. 1. In the absence of CO.sub.2, the catalysts showed two distinct stages of reduction in the H.sub.2 atmosphere. The first stage of reduction at 200° C. to 350° C. mainly indicates the reduction of hematite and a part of ferrihydrite to magnetite (Fe.sub.3O.sub.4), and the second stage of reduction at 350° C. to 800° C. mainly indicates the reduction of magnetite and residual ferrihydrite to metallic iron. In the case of H.sub.2-TPR with CO.sub.2, the H.sub.2 was consumed via the reverse water-gas shift (RWGS) reaction in addition to the reduction of iron oxides:
CO.sub.2+H.sub.2CO+H.sub.2O

(26) Therefore, the H.sub.2 consumption for the reduction of catalysts (C.sub.H2-Red.(TPR)) was calculated as:
C.sub.H.sub.2.sub.- Red(TPR)=C.sub.H.sub.2.sub.-Total(TPR)−C.sub.CO.sub.2.sub.(TPR)

(27) wherein C.sub.H2-TOTAL(TPR) and C.sub.CO2(TPR) are the total H.sub.2 consumption and the total CO.sub.2 consumption, respectively. The signal of CO was also detected by MS. The total CO preparation was almost identical to the total CO.sub.2 consumption, which confirms the occurrence of RWGS during the H.sub.2-TPR with CO.sub.2.

(28) As shown in FIG. 1b, the first stage of reduction showed no considerable difference with and without the presence of CO.sub.2. In contrast, the second stage of reduction nearly disappeared in the presence of CO.sub.2 compared to the case in the absence of CO.sub.2. This means that the reduction of magnetite and ferrihydrite to metallic iron in the H.sub.2 atmosphere was significantly suppressed by the CO.sub.2. The suppression in the second stage of reduction by CO.sub.2 can be attributed to the preferential consumption of H.sub.2 via RWGS over the magnetite surface. Magnetite is a well-known active phase for (R)WGS in the temperature range of 310° C. to 450° C.

(29) The reduction and carburization behavior in the CO atmosphere either with or without CO.sub.2 was analyzed by CO-TPR, as shown in FIG. 2. In the absence of CO.sub.2, two major peaks were observed in the CO-TPR profiles: one is a small and sharp peak at about 212° C. and the other is a large and broad peak at about 349° C. The first peak indicates the reduction of hematite and a part of ferrihydrite to magnetite, and the second peak indicates the carburization of magnetite and residual ferrihydrite to iron carbides. Similar to the results of H.sub.2-TPR, the first peak of the CO-TPR was barely affected by the CO.sub.2, but the second peak considerably shrank when the CO-TPR was performed in the presence of CO.sub.2. This implies that the CO.sub.2 significantly suppresses the carburization of iron oxides without influencing the reduction of hematite to magnetite.

(30) The in-situ activation behavior in the syngas atmosphere with different amounts of CO.sub.2 was investigated by measuring the flow rates and composition of the outlet gases during the activation pre-treatment. FIG. 3 shows the syngas consumption for the activation of catalysts as a function of activation time. The syngas consumption for the activation of catalysts (C.sub.Syngas-Acti.) was calculated as:
C.sub.Syngas-Acti.=C.sub.H.sub.2.sub.-Total+C.sub.CO-Total−(C.sub.H.sub.2.sub.-FTS+C.sub.CO-FTS)

(31) wherein C.sub.H2-Total and C.sub.CO-Total are the total H.sub.2 consumption and the total CO consumption, respectively, and C.sub.H2-FTS and C.sub.CO-FTS are the H.sub.2 consumption via the FTS and the CO consumption via the FTS, respectively. The lower value of C.sub.Syngas-Acti. was observed at the higher inlet CO.sub.2 content during activation, which implies that the iron-based catalysts can be mildly reduced and carburized in the syngas atmosphere with CO.sub.2, compared to the case without CO.sub.2. This corresponds well to the results of H.sub.2-TPR and CO-TPR (FIGS. 1 and 2).

Example 2

Preparation of Activated Catalysts

(32) The activated catalysts for characterization were prepared by exposing the as-prepared catalysts in Example 1 to the various activation environments. After exposing the catalysts to the activation environment for 20 hours, the reactor was cooled to room temperature and pressurized to 0.3 MPa. The reactor containing the activated catalysts was unloaded from the main reactor system using quick connectors (Swagelok. SS-QC4-D-400 and SS-QC4-B-200 for inlet and SS-QC6-D-600 and SS-QC6-B-600 for outlet) and transferred to a glove box in a N.sub.2 atmosphere (purity: 99.999%). After carefully withdrawing the activated catalysts from the reactor, the catalysts were washed with hexane to remove residual liquid/solid hydrocarbons from the catalysts. The catalysts were passivated with a gas mixture of 1% O.sub.2 in He at room temperature.

Experimental Example 3

Characterization of Activated Catalysts

(33) The crystal structure of the activated catalysts was characterized by XRD and Mössbauer spectroscopy. The carbon content of the activated catalysts was analyzed by an ASTM E1019 method with an ELTRA GmbH model ONH2000. The BET surface area, the single point pore volume, and the average pore diameter of the activated catalysts were analyzed by means of N.sub.2 physisorption. The carbonaceous species of the activated catalysts were characterized by temperature-programmed hydrogenation (TPH). The catalysts (about 50 mg) were loaded into the sample cell and purged with He at 40° C. for 30 min. The TPH was carried out at up to 900° C. in the flow of H.sub.2 at a heating rate of 10° C./min, and the temperature was then held at the maximum temperature for 2 hours. The amount of CH.sub.4 formation was measured by MS. A mass signal of 15 (CH.sub.3 fragments of CH.sub.4) was used instead of 16 to avoid the potential interference of water vapor and CO.sub.2 cracking.

(34) FIG. 4 shows the XRD patterns of the activated catalysts. The crystal structure of the activated catalysts can be identified as a combination of ferrihydrite, magnetite, χ-carbide (Fe.sub.2.5C), and ε′-carbide (Fe.sub.2.2C). This implies that hematite and a part of ferrihydrite in the as-prepared catalysts are reduced and carburized into magnetite and iron carbides during the activation pre-treatment. The catalysts had similar XRD patterns in terms of phase constitution regardless of the CO.sub.2 content during activation. However, they displayed considerable difference in terms of the relative intensity of each phase depending on the CO.sub.2 content during activation. Stronger peaks of ferrihydrite and magnetite and weaker peaks of iron carbides were observed at the higher CO.sub.2 content during activation.

(35) Detailed quantitative analyses on the phase structure were performed by Mössbauer spectroscopy, as shown in FIG. 5. The Mössbauer spectra at −268.8° C. were fitted with six sextets for all samples, which reflects ε′-carbide, χ-carbide, magnetite, and ferrihydrite with different hyperfine parameters. The results of Mössbauer deconvolution are summarized in Table 1 below.

(36) TABLE-US-00001 TABLE 1 Site Sample ε′- χ-Carbide Mag- name Carbide 8f 8f 4e netite Ferrihydrite 0CO.sub.2 H.sub.hf (kOe) 185.7 258.4 211.6 112.2 510.6 475.7 δ (mm/s) 0.23 0.29 0.24 0.12 0.39 0.32 E.sub.Q (mm/s) 0.04 0.08 −0.02 0.02 −0.02 −0.00 Area (%) 6.31 16.6 16.6 9.03 11.4 40.0 20CO.sub.2 H.sub.hf (kOe) 187.8 257.1 208.7 103.2 510.1 485.4 δ (mm/s) 0.26 0.31 0.20 0.16 0.38 0.31 E.sub.Q (mm/s) 0.04 0.06 0.01 0.05 −0.02 −0.01 Area (%) 9.04 9.83 10.1 5.03 17.3 48.7 33CO.sub.2 H.sub.hf (kOe) 188.0 265.9 213.3 100.6 510.1 475.6 δ (mm/s) 0.26 0.31 0.21 0.17 0.38 0.32 E.sub.Q (mm/s) 0.04 0.06 0.01 0.04 −0.02 −0.01 Area (%) 13.1 6.02 5.94 2.99 22.9 49.1 50CO.sub.2 H.sub.hf (kOe) 189.6 262.4 210.5 103.8 508.8 478.8 δ (mm/s) 0.26 0.30 0.21 0.16 0.39 0.32 E.sub.Q (mm/s) 0.03 0.06 0.02 0.03 −0.02 −0.01 Area (%) 8.28 2.59 2.59 1.68 28.1 56.8

(37) In the 0% CO.sub.2, the content of iron carbides was about 49%, which means that about 49% of iron atoms in hematite and ferrihydrite in the as-prepared catalysts was reduced and carburized to χ-carbide and ε′-carbide after the activation pre-treatment. The content of iron carbides decreased with an increased CO.sub.2 content during activation, and the content of magnetite and ferrihydrite showed the opposite tendency, which confirms the XRD results in FIG. 4. This also corresponds well to the TPR results (FIGS. 1 and 2) which show mild reduction and carburization of the catalysts when the TPR was carried out in the presence of CO.sub.2. The evidence of hematite was detected in neither the XRD patterns nor the Mössbauer spectra while a considerable amount of ferrihydrite was observed, which suggests that the hematite is more reducible than the ferrihydrite. The total carbon content of the activated catalysts analyzed by ASTM E1019 is summarized in Table 2 below.

(38) TABLE-US-00002 TABLE 2 CO.sub.2 content Total Textural properties during carbon BET Pore Average Sample activation content surface volume pore name (%) (wt %) area (m.sup.2/g) (cm.sup.3/g) size (nm) 0CO.sub.2 0 12.1 88.9 0.302 13.6 20CO.sub.2 20 4.94 101 0.358 14.2 33CO.sub.2 33 3.98 104 0.360 13.9 50CO.sub.2 50 2.92 101 0.367 14.5 As-prepared 183 0.458 9.98

(39) The carbon content showed a steep decrease with increased CO.sub.2 content during activation. This indicates that the formation of carbonaceous species was significantly suppressed by the CO.sub.2 during activation. This corresponds well to the result of CO-TPR (FIG. 2), which shows a considerable shrinkage of the second peak when the CO-TPR was performed in the presence of CO.sub.2. Detailed analyses on the carbonaceous species were performed by TPH, as shown in FIG. 6. The smaller TPH profiles were observed at the higher CO.sub.2 content during activation. This indicates that a smaller amount of carbonaceous species formed on the catalysts with an increased CO.sub.2 content during activation. This is consistent with the result of ASTM E1019 in Table 2. The suppressed formation of carbon or carbonaceous species may be attributed to the decreased carbon chemical potential induced by the presence of CO.sub.2. The TPH profiles can be deconvoluted by five peaks: (i) reactive surface carbon below 450° C., (ii) ε′-carbide at 480° C. to 530° C., (iii) χ-carbide at 600° C. to 650° C., and (iv and v) inactive bulk carbon above 690° C. The peak temperatures and corresponding fractional areas are summarized in Table 3 below.

(40) TABLE-US-00003 TABLE 3 Sample Surface Iron carbides Bulk name carbons ε′-Carbide χ-Carbide carbons 0CO.sub.2 Peak (° C.) 400 488 636 709 839 Area (%) 7.77 6.32 19.6 48.0 18.3 20CO.sub.2 Peak (° C.) 379 496 611 720 825 Area (%) 21.5 15.3 37.3 17.6 8.24 33CO.sub.2 Peak (° C.) 385 497 613 707 798 Area (%) 33.4 18.2 26.8 12.2 9.43 50CO.sub.2 Peak (° C.) 403 527 618 720 824 Area (%) 40.1 21.1 16.0 15.4 7.32

(41) In the case of 0% CO.sub.2, the fraction of bulk carbon was about 66%, which indicates that the activation using CO.sub.2-free syngas involves the considerable formation of inactive bulk carbon in addition to the formation of active iron carbides. In contrast, in the 20% to 50% CO.sub.2, surface carbon, ε′-carbide, and χ-carbide were observed as major carbonaceous species. Specifically, the fraction of surface carbon, ε′-carbide, and χ-carbide was higher than 70% in total. This suggests that the activation using CO.sub.2-containing syngas is beneficial to selective formation of active species for the iron-based FTS catalysts.

(42) The textural properties of the activated catalysts analyzed by N.sub.2 physisorption are summarized in Table 2. For easy comparison, the textural properties of the as-prepared catalysts are also inserted into Table 2. All the activated catalysts had a lower BET surface area and smaller pore volume than the as-prepared catalysts. This indicates that the initial pore structures of the as-prepared catalysts are inevitably degraded during the activation pre-treatment. Among the activated catalysts, the catalysts activated using CO.sub.2-containing syngas (20% to 50% CO.sub.2) had a higher BET surface area and larger pore volume than the catalysts activated using CO.sub.2-free syngas (0% CO.sub.2). As revealed in the results of XRD and Mössbauer spectroscopy, the mild reduction and carburization of the catalysts in the CO.sub.2-containing syngas may reduce the degradation of pore structures during the activation pre-treatment.

Example 3

Conducting Fischer-Tropsch Synthesis by Using Syngas with Different CO2 Content During the Activation and Analysis of Catalyst Performance

(43) The FTS was carried out in a fixed-bed reactor composed of stainless steel (5 mm i.d. and 180 mm length). The catalysts (0.8 g) were diluted with glass beads (1.6 g; 425 μm to 600 μm) and then charged into the fixed-bed reactor. The catalysts were activated in-situ using syngas (H.sub.2/CO=1.0) with different amounts of CO.sub.2 (0%, 20%, 30%, and 50%) at 280° C. and ambient pressure for 20 hours. In the activation process, a flow rate of H.sub.2+CO at 2.8 NL/g.sub.(cat) h was maintained and the flow rate of CO.sub.2 was increased for different levels of inlet CO.sub.2 content. After the activation treatment, the FTS was performed at 275° C. and 1.5 MPa using CO.sub.2-free syngas (H.sub.2/CO=1.0, 2.8 NL/g.sub.(cat) h). The composition of the outlet gases was analyzed using an online gas chromatograph (GC; Agilent, 3000A Micro-GC) equipped with a molecular sieve and Plot Q columns. The flow rates of the outlet gases were measured by a wet-gas flow meter. The composition of wax and liquid products was analyzed with an offline GC (Agilent, 6890N) with a simulated distillation method (ASTM D2887).

(44) The catalytic performance was evaluated in terms of CO conversion, and productivity and selectivity of hydrocarbons. The total CO conversion (X.sub.CO(Total)) was calculated as:
X.sub.CO(Total) (%)=(F.sub.CO(In)−F.sub.CO(Out))/F.sub.CO(In)×100
wherein F.sub.CO(In) and F.sub.CO(Out) are the inlet flow rate of CO and the outlet flow rate of CO, respectively. The total CO conversion can be divided into the CO conversion to hydrocarbons and the CO conversion to CO.sub.2. The CO conversion to CO.sub.2 (X.sub.CO to CO2) was calculated as:
X.sub.CO to CO.sub.2 (%)=F.sub.CO.sub.2.sub.(Out)/F.sub.CO(In)×100

(45) wherein F.sub.CO2(Out) is the outlet flow rate of CO.sub.2. The CO conversion to hydrocarbons (X.sub.CO to HC) was calculated as:
X.sub.CO to HC (%)=X.sub.CO(Total)−X.sub.CO to CO.sub.2

(46) The productivity of hydrocarbons from carbon number n to carbon number n+k (P.sub.Cn−Cn+k) was calculated as:

(47) $P_{C_n-C_{n+k}}\left(g/g_{\left(\mathrm{cat}\right)}/h\right)=\underset{i=n}{\overset{n+k}{.Math.}}{m}_{C_i}/\left(m_{\left(\mathrm{cat}\right)}\times \Delta t\right)$

(48) wherein m.sub.Ci, m.sub.(cat), and Δt are the mass of hydrocarbons with carbon number i prepared during the mass balance period, the mass of catalysts, and the time interval of mass balance period, respectively. The selectivity of hydrocarbons from carbon number n to carbon number n+k (S.sub.Cn−Cn+k) was calculated as:

(49) $S_{C_n-C_{n+k}}\left(\mathrm{wt}\%\right)=\underset{i=n}{\overset{n+k}{.Math.}}{m}_{C_i}/m_{\mathrm{HC}\left(\mathrm{Total}\right)}\times 100$

(50) wherein m.sub.HC(Total) is the total mass of hydrocarbons prepared during the mass balance period.

(51) As above, the influence of the activation using CO.sub.2-containing syngas on the catalytic performance was evaluated in the FTS condition at 2750° C. In addition to the unreacted CO and H.sub.2, gaseous hydrocarbons (CH.sub.4 and C.sub.2 to C.sub.4 hydrocarbons) and CO.sub.2 were detected in the outlet gases. Liquid hydrocarbons and H.sub.2O were obtained in the cold trap (1° C.), and solid hydrocarbons were obtained in the hot trap (240° C.). This indicates that the formation of CO.sub.2 via WGS accompanies the formation of hydrocarbons via the FTS as below:
nCO+(2n+1)H.sub.2C.sub.nH.sub.2n+2+nH.sub.2O(n≧1)
nCO+2nH.sub.2C.sub.nH.sub.2n+nH.sub.2O(n≧2)

(52) The CO and H.sub.2 conversion showed a slight increasing trend with an increased reaction time. It was assumed that the overall catalytic performance during 66 hours to 114 hours of reaction is representative of the performance of the catalysts activated by syngas with different amounts of CO.sub.2. FIG. 7 shows the overall CO conversion during 66 hours to 114 hours of reaction as a function of inlet CO.sub.2 content during activation. The total CO conversion (X.sub.CO(Total)) gradually decreased as the inlet CO.sub.2 content during activation increased. The X.sub.CO(Total) can be divided into the CO conversion to hydrocarbons (X.sub.CO to HC) and the CO conversion to CO2 (X.sub.CO to CO2). The X.sub.CO HC and the X.sub.CO to CO2 reflect the rate of hydrocarbon formation and the rate of CO.sub.2 formation, respectively. The X.sub.CO to HC decreased with increased inlet CO.sub.2 content during activation.

(53) This is attributed to the suppressed preparation of undesired products, CH.sub.4 and C.sub.2 to C.sub.4 hydrocarbons, as described in FIG. 8 below. The X.sub.CO to CO2 also decreased with an increased CO.sub.2 content during activation. This is considered to result from the decreased X.sub.CO to HC. Since the formation of CO.sub.2 via WGS occurs as a secondary reaction of FTS, the decreased formation of H.sub.2O via the FTS may decrease the formation of CO.sub.2 via WGS.

(54) FIG. 8 shows the productivity of hydrocarbons during 66 hours to 114 hours of reaction as a function of inlet CO.sub.2 content during activation. The productivity of hydrocarbons can be used as a critical performance index of the FTS catalysts, which directly shows a combined value for the catalytic activity and selectivity. While the productivity of undesired products, CH.sub.4 and C.sub.2 to C.sub.4 hydrocarbons, significantly decreased with an increased inlet CO.sub.2 content during activation, the productivity of valuable products, C.sub.5+ hydrocarbons, showed an even or slight increasing trend. In particular, the productivity of C.sub.19+ hydrocarbons showed a dramatic increase as the inlet CO.sub.2 content during activation increased. Specifically, the productivity of C.sub.19+ hydrocarbons at 50% CO.sub.2 (0.160 g/g.sub.(cat)h) was about twice as high as the value at 0% CO.sub.2 (0.0797 g/g.sub.(cat)h). However, when the inlet CO.sub.2 content during activation was higher than 50%, the productivity of C.sub.5+ hydrocarbons and C.sub.19+ hydrocarbons showed no further enhancement with an increased inlet CO.sub.2 content during activation. Specifically, the productivity of C.sub.5+ hydrocarbons and the productivity of C.sub.19+ hydrocarbons at 67% CO.sub.2 were 0.266 g/g.sub.(cat) h and 0.128 g/g.sub.(cat) h, respectively.

(55) The effects of the activation using CO.sub.2-containing syngas on the hydrocarbon selectivity are shown in FIG. 9. As shown in FIG. 9a, there was a considerable decrease in the selectivity of CH.sub.4 and C.sub.2 to C.sub.4 hydrocarbons as the inlet CO.sub.2 content during activation increased. In other words, the selectivity of C.sub.5+ hydrocarbons remarkably increased with increased inlet CO.sub.2 content during activation.

(56) This suggests that the chain growth occurs more favorably over the catalysts activated by CO.sub.2-containing syngas than by the case using CO.sub.2-free syngas. In addition, the selectivity of I-olefins in C.sub.2 to C.sub.4 hydrocarbons showed a gradual increase with increased inlet CO.sub.2 content during activation. This indicates that the chain termination as paraffin by secondary hydrogenation was relatively suppressed. The carbon number distribution of C.sub.5+ hydrocarbons is also displayed in FIG. 9b. The carbon number distribution of C.sub.5+ hydrocarbons corresponded well to the Anderson-Schulz-Flory (ASF) distribution as below:
log(W.sub.n/n)=log χ.Math.n+log(In.sup.2α)

(57) wherein W.sub.n is the weight fraction of hydrocarbons with carbon number n, and α is the chain growth probability of the hydrocarbons. Two values of α (α1 from C.sub.7 to C.sub.16 and α2 from C.sub.16 to C.sub.44) can be obtained from two linear regressions. Both α1 and α2 values showed a significant increase with an increased inlet CO.sub.2 content during activation, which confirms the favorable chain growth of hydrocarbon monomers over the catalysts activated by CO.sub.2-containing syngas. Therefore, the activation of iron-based FTS catalysts using CO.sub.2-containing syngas is considered highly beneficial to selective preparation of C.sub.5+ hydrocarbons, in particular C.sub.19+ hydrocarbons, in the low-temperature FTS process, without sacrificing the overall productivity of C.sub.5+ hydrocarbons.

(58) However, when the inlet CO.sub.2 content during activation was higher than 50%, the selectivity of C.sub.5+ hydrocarbons experienced no further improvement with an increased inlet CO.sub.2 content during activation. Specifically, the selectivity of C.sub.5+ hydrocarbons at 67% CO.sub.2 was 77.2 wt %. This suggests that the beneficial effects of the activation using CO.sub.2-containing syngas on the selectivity are significant below 50% CO.sub.2.

Example 4

Conducting Fischer-Tropsch Synthesis Under the Different Condition and Analysis of Catalyst Performance

(59) As reaction conditions, each one of the reaction temperature, reaction pressure, space velocity, H.sub.2/CO ratio of syngas, and the CO.sub.2 content was subject to change, while maintaining the rest of the reaction conditions. The Fischer-Tropsch synthesis was then performed and the catalytic performance was analyzed, as shown in Example 3.

(60) The analytical results are shown in FIGS. 12 to 16.

(61) It was confirmed through FIG. 12 that the desirability of the catalytic performance was achieved when the reaction temperature was adjusted between 240° C. to 275° C. during the Fischer-Tropsch synthesis. Specifically, if the temperature was below 240° C., C.sub.5+ productivity decreased, whereas if the temperature exceeded 275° C., C.sub.5+ selectivity decreased.

(62) It was confirmed through FIG. 13 that the desirability of the catalytic performance was achieved when the reaction pressure was adjusted between 1.5 MPa to 2.25 MPa during the Fischer-Tropsch synthesis. Specifically, if the reaction pressure deviated from the above range, both C.sub.5+ productivity and C.sub.5+ selectivity decreased.

(63) It was confirmed through FIG. 14 that the desirability of the catalytic performance was achieved when the space velocity was adjusted between 2.8 NL/g.sub.(cat)/h to 5.6 NL/g.sub.(cat)/h during the Fischer-Tropsch synthesis. Specifically, if the space velocity was below 2.8 NL/g.sub.(cat)/h, C.sub.5+ productivity decreased, whereas if the temperature exceeded 5.6 NL/g.sub.(cat)/h, C.sub.5+ selectivity decreased.

(64) It was confirmed through FIG. 15 that the desirability of the catalytic performance was achieved when the H.sub.2/CO ratio of syngas was adjusted between 0.7 to 1.0 during the Fischer-Tropsch synthesis. Specifically, if the H.sub.2/CO ratio of syngas was below 0.7, C.sub.5+ productivity decreased, whereas if the H.sub.2/CO ratio of syngas exceeded 1.0, C.sub.5+ selectivity decreased.

(65) It was confirmed through FIG. 16 that the desirability of the catalytic performance was achieved when the volume of syngas further contained 0.1% to 10% of CO.sub.2 relative to the overall volume of syngas during the Fischer-Tropsch synthesis. Specifically, the use of syngas containing more than 10% CO.sub.2 decreased C.sub.5+ productivity.

Comparative Example 1

(66) The iron-based catalyst having a composition weight ratio between Fe, Cu, K, SiO.sub.2, and Na of 100:5.96:4.53:33.2:2.01 was charged into the Fischer-Tropsch synthesis reactor and reduced with a first gas containing H.sub.2 and CO having a volume ratio of 1:1. The first gas was supplied at velocity of 0.6NL.sub.(H2+CO)/g.sub.(cat)−h, and the reduction step was performed at 280° C. for 20 hours.

(67) The preparation step for preparing a hydrocarbon compound was performed by injecting a second gas containing H.sub.2 and CO having a volume ratio of 1:1 into the Fischer-Tropsch synthesis reactor where the reduction took place. The second gas was supplied at velocity of 3.0NL.sub.(H2+CO)/g.sub.(cat)−h, and the preparation step was performed at 275° C. and 1.5 MPa to prepare a hydrocarbon compound using the Fischer-Tropsch synthesis.

Example 5

(68) The hydrocarbon compound was prepared by the same method as in Comparative Example 1, except that the first gas containing CO.sub.2, H.sub.2, and CO having a volume ratio of 1:1:1 was used in the preparation method of the hydrocarbon compound using the Fischer-Tropsch synthesis. The results are shown in Table 4 below.

(69) TABLE-US-00004 TABLE 4 Comparative Example 1 Example 5 Type of reducing gases H.sub.2 + CO H.sub.2 + CO + CO.sub.2 (H.sub.2/CO = 1/1) (H.sub.2/CO/CO.sub.2 = 1/1/1) Time Interval (h) 12 to 54 12 to 54 CO Conversion (%) 56.8 53.2 Hydrocarbon Distribution 41.9 36.5 (C-mol %) CH.sub.4 4.97 2.96 C.sub.2 to C.sub.4 19.2 12.2 C.sub.5+ 75.9 84.9 C.sub.5+ productivity (g/g(cat-h)) 0.246 0.285

(70) As shown in Table 4, in the case of Example 5, the selectivity for CO.sub.2, CH.sub.4, and C.sub.2 to C.sub.4 hydrocarbons were low, whereas the selectivity for C.sub.5+ hydrocarbons was high, compared to Comparative Example 1. Further, although CO conversion slightly decreased in case of the Example 5, the increase in selectivity of C.sub.5+ hydrocarbons was largely attributed to the overall increase in the selectivity of C.sub.5+ hydrocarbon per each g of catalyst.

(71) Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.