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Catalysis Communications 8 (2007) 1820–1824

Preparation of a new precipitated iron catalyst forFischer–Tropsch synthesis

Hiroshi Hayakawa a,b,*, Hisanori Tanaka c, Kaoru Fujimoto a

a Department of Chemical Processes and Environment, The University of Kitakyushu, 1-1 Hibikino Wakamatsu-ku Kitakyushu-shi Fukuoka 808-0135, Japanb Technology Development Center, Electric Power Development Co. Ltd., 6-15-1, Ginza, Chuo-ku, Tokyo 104-8165, Japan

c Wakamatsu Research Institute, Electric Power Development Co. Ltd., 1 Yanagisakimachi Wakamatsu-ku Kitakyushu-shi Fukuoka 808-0111, Japan

Received 2 September 2006; received in revised form 4 January 2007; accepted 13 February 2007Available online 20 February 2007

Abstract

It is well known that a typical precipitated iron catalyst was prepared by iron(III) nitrate (Fe(NO3)3). The modified iron catalyst wasprepared by iron(II) sulfate in our laboratory. The results from catalytic performance tests showed that the iron catalyst prepared fromiron sulfate (cat-S) has the higher activity for Fischer–Tropsch synthesis (FTS) than the catalyst prepared from iron nitrate (cat-N). Theresults from XRD of catalyst before use showed the difference definitely. The XRD patterns of these catalysts indicate that the mainphase of cat-N is Fe2O3, whereas the phases of cat-S are a mixture of Fe2O3 and Fe3O4. It is considered that magnetite was directlyformed during precipitation and contained in the catalyst before use enhances the activity for FTS.� 2007 Elsevier B.V. All rights reserved.

Keywords: Fischer–Tropsch synthesis; Iron catalyst; Iron(II) sulfate; Magnetite

1. Introduction

Iron catalysts have been used for the commercialFischer–Tropsch Synthesis (FTS) process, especially forsynthesis of the coal-derived syngas. Many kinds of precip-itated iron catalysts which contain various promoters andsupports have been investigated to improve the activity,selectivity, and attrition strength. Also, many studies oniron catalysts have been reported in which efforts werefocused on the change in phase of a precipitated catalystduring activation and reaction.

Bukur studied that the commercial promoted precipi-tated iron catalyst was characterized before and after usefor FTS. It was found that a hematite (Fe2O3) existed inthe catalyst before pretreatment, and that after pretreat-ment by syngas, hematite was transformed to magnetite

1566-7367/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2007.02.017

* Corresponding author. Address: Technology Development Center,Electric Power Development Co. Ltd., 6-15-1, Ginza, Chuo-ku, Tokyo104-8165, Japan. Tel.: +81 3 3546 2211; fax: +81 3 3546 1685.

E-mail address: hiroshi_hayakawa@jpower.co.jp (H. Hayakawa).

and iron carbide [1]. Bian et al. studied the reductionbehavior with precipitated iron catalysts. the catalyst whichshowed Fe2O3 from XRD analysis before use, transformedto metallic iron and iron carbide after reduction by CO [2].

Sirimanothan et al. studied the changes in phase andactivity during reaction in a stirred tank reactor. Potassiumand/or silicon promoted catalyst were prepared and usedas samples for tests. All samples were indicated to Fe2O3

before use and transformed to Fe3O4 and iron carbidesduring FT reaction [3]. All catalysts studied so far were pre-pared from iron nitrate.

On the other hand, several groups reported that ironcatalysts prepared from iron sulfate showed the positiveeffects on FT reactions. Bromfield and Coville have pro-posed that low-level sulfided iron catalyst exhibitedenhanced FTS activity and WGS activity [4]. Wu et al.studied on the iron catalyst which was prepared byspray-drying using iron sulfate. It was found that this cat-alyst contained the sulfate and had the higher activity thana sulfate-free catalyst. However, its structure was notclaimed particularly [5].

H. Hayakawa et al. / Catalysis Communications 8 (2007) 1820–1824 1821

In our previous studies, the precipitated iron catalystswere prepared by iron nitrate, and were tested in a stirredtank reactor of laboratory scale. From the results of char-acterization study, the silica-free catalyst (Fe/Cu/K) andthe silica-containing catalyst (Fe/Cu/K/SiO2) showed dif-ferent reduction behavior [6].

The purpose of the present study is to investigate thecatalytic performance and the changes in phase of the sil-ica-free precipitated iron catalysts which was prepared ironsulfate, instead of iron nitrate to be clear its effect.

2. Experimental

2.1. Catalyst preparation

Two silica-free catalyst were prepared, and one (cat-N)was prepared by precipitation from an aqueous nitratesolution by adding an aqueous solution of (NH4)2CO3

which was similar to that of Kunugi et al. [7]. And secondwas prepared by precipitation from an aqueous sulfatesolution by adding an aqueous solution of Na2CO3. Theprecipitates were filtered and washed with distilled waterseveral times. The washed filter cakes were dried in air atthe range of 373–393 K and subsequently calcined in airat the range of 653–693 K.

Both catalysts were promoted by copper and potassium.Cat-N was prepared by mixing aqueous solutions of

Table 1Physical properties of precipitated iron catalysts

Catalyst Component (on mass basis)analyzed

BET results

Fe Cu K S Surface area(m2/g)

Pore volume(cm3/g)

Cat-N 100 0.7 2.2 – 55 0.330Cat-S 100 0.9 1.9 0.09 33 0.326

Fig. 1. XRD patterns of the

Cu(NO3)2 Æ 3H2O and Fe(NO3)3 Æ 9H2O, prior to precipita-tion, whereas cat-S was prepared by CuSO4 Æ 5H2O andFeSO4 Æ 7H2O, instead of copper nitrate and iron nitrate.The concentrations of aqueous solutions were 0.5–1.0 mol/L, and the volumes of solutions corresponded to20–30 g of the obtained catalysts. Both cat-N and cat-Swere impregnated heated samples with an aqueous solutionof K2CO3.

2.2. Activation and reaction procedure

Activation on the iron catalysts in the slurry phase wasperformed in a 100 cc continuous stirred tank reactor(CSTR). Three grams of the catalyst precursor were sus-pended in 50 ml of n-C16H34 in the 100 cc reactor.

The system was purged with 150 cc/min in the 100 ccreactor of N2, then the pressure and temperature wereraised to 0.5 Mpa and 523 K respectively, and these condi-tions were retained for 0.5 h, after which time the temper-ature was raised to 573 K.

Subsequently, catalyst activation was conducted byswitching the feed gas from N2 to syngas. Activation wascarried out at 0.5 MPa, 573 K, H2/CO ratio of 1.0, andspace velocity (W/F) was 7.5 (g-cat h/mol) for a certainperiod. The stirring speed in the reactor was 1800 rpm.

Following the activation, the reactor was subjected tothe reaction conditions. Reaction was carried out at2.0 MPa, 533 K, H2/CO ratio of 1.0, and apace velocity(W/F) was 7.5 (g-cat h/mol) for a certain period. Lighthydrocarbons and water were collected in a separator (coldtrap) connected to the down stream of the reactor, andheavy hydrocarbons were collected in the reactor with theinitial solvent. These were analyzed by an off-line FIDgas chromatograph (GC) (column packing: Silicone SE-30). Effluent gas from the separator was analyzed with anon-line TCD GC (column packing: Molecular Sieve 5A

fresh cat-N and cat-S.

1822 H. Hayakawa et al. / Catalysis Communications 8 (2007) 1820–1824

and Porapak N) and FID (column packing: PLOT FusedSilica).

2.3. Characterization of the catalysts

The surface area of the catalysts were measured by theBET method with QUANTA CHROME AUTOSORB-1MP.

XRD patterns of catalyst samples were measured by aRIGAKU X-ray diffractometer. The samples after reduc-tion and reaction for XRD measurements were obtainedfrom the reactor. The reduced and reacted samples were

Table 2Catalytic performance of precipitated iron catalysts

Catalyst Cat-N Cat-S

CO conversion (%) 85.9 90.3CO2 selectivity (%) 43.9 45.9Kp 5.9 16.7STY (mg/g-cat/h) 297 349

Hydrocarbon selectivityCH4 (%) 4.9 4.4 ± 0.3C2-4 (%) 27.5 16.8 ± 3.6C5+ (%) 67.6 78.5 ± 3.2

STY: C5+ hydrocarbon productivity.Kp: PCO2

PH2/PCOPH2O.

Fig. 2. Carbon number product hydrocarbon distribution and ASF plots of catmol). Catalyst activation at 300 �C, 0.5 MPa, H2/CO = 1 and W/F = 7.5 (g-ca

separated from the wax product and solvent by filtrationin a glove box before being transferred to the XRDinstrument.

The sulfur composition of the catalyst was determinedby the high temperature oxidized method of JIS M 8813.

3. Results and discussion

Cat-N and cat-S were subjected to the activity tests andthe characterization studies. Their physical properties andthe structures of the catalysts before use are shown in Table1 and Fig. 1.

It can be seen from Table 1 that the two catalysts havesimilar compositions and surface area. The surface area ofcat-S was smaller than that of silica-containing iron cata-lyst which was prepared from iron nitrate in our previousstudies [6]. It is considered that pore structure of cat-Swas similar to cat-N.

The XRD patterns of cat-N indicate that the mainphases are all hematite (Fe2O3), while the XRD patternsof cat-S indicate that the phases are a mixture of hematiteand magnetite (Fe3O4). Both catalysts present sharp andstrong intensity.

Fe3+ from iron nitrate formed to hematite (Fe2O3),whereas Fe2+ from iron sulfate formed to hematite(Fe2O3) and magnetite (Fe3O4) whose structure consist

-N.Catalyst activity at 260 �C, 2 MPa, H2/CO = 1 and W/F = 7.5 (g-cat h/t h/mol).

H. Hayakawa et al. / Catalysis Communications 8 (2007) 1820–1824 1823

of a mixture of FeO and Fe2O3. It seems that some ofFe2+ from iron sulfate might be oxidized to Fe3+, whichformed to Fe2O3 during the air calcination. The reasonwhy the above phenomenon occurs is not clear. The for-

Fig. 3. Carbon number product hydrocarbon distribution and ASF plots of catmol) Catalyst activation at 300 �C, 0.5 MPa, H2/CO = 1 and W/F = 7.5 (g-ca

Fig. 4. XRD patterns of fresh cat-N and cat-N reduced in the slurry phase for 3

mation of magnetite must have strong relations to thefact that the raw material of iron is ferrous ions(Fe2+). The results of reaction tests are shown inTable 2.

-S Catalyst activity at 260 �C, 2 MPa, H2/CO = 1 and W/F = 7.5 (g-cat h/t h/mol).

h: reduction at 300 �C, 0.5 MPa, H2/CO = 1 and W/F = 7.5 (g-cat h/mol).

Fig. 5. XRD patterns of fresh cat-S and cat-S after reaction in the slurry phase: reduction at 300 �C, 0.5 MPa, H2/CO = 1 and W/F = 7.5 (g-cat h/mol).Reaction at 260 �C, 2.0 MPa, H2/CO = 1 and W/F = 7.5 (g-cat h/mol).

1824 H. Hayakawa et al. / Catalysis Communications 8 (2007) 1820–1824

The results of the reaction tests showed that the activityof the cat-S was higher than that of the cat-N. Cat-S wasprepared three times and each sample was tested. TheCO conversion data were 90.3%, 92.3% and 87.0%. Thereproducibility was defined. The methane and C2-4 selec-tivity of cat-S was smaller than that of cat-N. So, theC5+ hydrocarbon productivity of cat-S was higher thanthat of cat-N.

It has been reported that the activity of the catalystwhich was prepared by iron sulfate, was higher than thatof the catalyst iron nitrate. And, the sulfate speciesenhances the activity of FT catalyst from the literature[4]. In the case of our catalyst, sulfur content was only0.06 wt%.

The structures of cat-N and cat-S catalysts after reduc-tion/reaction are shown in Figs. 2 and 3.

The XRD results of the reduced catalysts showed thatiron carbide (Fe2.5C) was present in both cat-N after reduc-tion and cat-S after reaction for 3 h, as shown in Figs. 4and 5. As both catalysts have high activity after reduction,the activated phase can be observed as iron carbide(Fe2.5C). All of the hematite (Fe2O3) in cat-N was reducedand rapidly transformed into active sites. As the reductionby syngas (hydrogen and carbon monoxide) proceeded,iron oxide was transformed from Fe2O3) Fe3O4) a-Feor iron carbide. a-Fe could not be observed in our reducedcatalyst, because the metallic iron was fairly reactive to car-bon dissociated from carbon monoxide.

On the other hand, the behavior of hematite in cat-S wassimilar to cat-N above. The magnetite in cat-S was trans-

formed to the active sites without the transformation fromhematite to magnetite. It suggests that the existence ofmagnetite in cat-S prevents the coagulation of iron oxideand keep the pore structure during reduction.

4. Conclusions

The catalyst prepared from iron(II) sulfate has higheractivity and C5+ hydrocarbon productivity than that ofthe catalyst prepared from iron nitrate. The XRD resultsof catalysts before use indicate that hematite and magnetiteexisted in the catalyst prepared from iron sulfate, differentfrom the catalyst prepared from iron nitrate. The reasonwhy the activity of the catalyst prepared from iron sulfatewas higher was considered to prevent iron oxide from coag-ulation during reduction from Fe2O3 to Fe3O4 by the exis-tence of magnetite.

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