283444 A Highly Active Supported Fecuk Fischer-Tropsch Catalyst

Monday, October 29, 2012: 9:30 AM
315 (Convention Center )
Kamyar Keyvanloo, Calvin H. Bartholomew and William C Hecker, Chemical Engineering, Brigham Young University, Provo, UT

A highly active supported FeCuK Fischer-Tropsch catalyst

Kamyar Keyvanloo, Calvin H. Bartholomew, and William C. Hecker

Chemical Engineering Department, Brigham Young University, Provo, UT, 84602

Introduction:

Precipitated FeCuK catalysts are effective for production of high molecular hydrocarbons in fixed bed reactors [1]. Unfortunately, despite their high activity and selectivity, they lack sufficient mechanical strength to be used in slurry bubble column reactors (SBCRs) [2]. These catalysts undergo attrition to fine particles leading to loss of the catalyst due to difficulty in catalyst/wax separation. To alleviate this particle breakage problem, it is proposed that a support material be used to create a cost effective supported Fe catalyst. Supported iron catalysts produced in the past have usually suffered from lower activity and higher methane selectivity compared to precipitated iron catalysts. To overcome this problem, an intensive study on preparation variables such as type of support, preparation method, iron loading, and promoter type and loading is needed.

In this study we report our on-going effort to develop an active and selective supported iron catalyst. To date, an active alumina-supported catalyst has been prepared, which, compared to others reported in the literature [4-6], is one of the most active supported iron catalysts ever made.  The activity and selectivity are also compared to those of carbon nanotube (CNT) supported catalysts and unsupported iron catalysts previously synthesized in the BYU Catalysis lab [3]. To accomplish our work on the preparation of an active and selective supported iron catalyst, silica and SiC supports, different pretreatments, and different preparation variables/ methods will be explored.

Materials and method:

For the alumina supported catalysts, alumina was sieved to 30-60 mesh and calcined at 700 C in air for 4h prior to incipient wetness impregnation. Catalysts were prepared by co-impregnation with aqueous solutions containing desired amounts of ferric nitrate and copper nitrate in successive steps. The sample was dried overnight at 80C and calcined at 300C for about 16 h. Potassium was then added by impregnating with potassium bicarbonate. Nominal compositions (on a relative mass basis) of synthesized catalysts were 100Fe/ 7.5Cu/4 K/400 Al2O3 (Al/Fe/Cu/4K) and 100 Fe/7.5 Cu/8 K/400 Al2O3 (Al/Fe/Cu/8K).

The unsupported iron catalyst (Fe/Cu/K/SiO2) was prepared from iron and copper nitrate salts by a simple, proprietary, co-precipitation method developed by Cosmas, Inc. Potassium (KHCO3) and silica (Cab-O-Sil) promoters were added to the wet precursor before the catalyst was dried. Nominal catalyst composition was 100 Fe/5 Cu/4 K/16 SiO2 by mass. This method was modified to prepare a supported iron catalyst on CNT (CNT/Fe/K/Cu) that contained 30% iron loading.

Activity studies were performed in a 3/8 inch ID fixed-bed reactor. 0.25 g of calcined catalyst (30 60 mesh) was mixed with 1 g of quartz sand (50 70 mesh) and charged to the reactor. Catalysts were reduced in situ at 280-340C with GHSV > 2000 h-1 and 10% H2/He for 10 hours followed by 100% H2 for 6 hours. Activation and reaction conditions were 300 psig, H2:CO=1. Activation was at 280C for 24-48 hours in syngas. Reaction temperatures were varied from 230C to 260C. The reactor effluent was analyzed online by an HP 6890 GC with a 15 ft x 1/8 inch SS Supelco column packed with 60/80 carboxen 1000 phase.

Results and discussion:

Table 1 shows BET results and extent of reduction on four catalysts prepared to date. The carbon nanotube-supported (CNT) catalyst had the highest BET surface area after calcination followed by alumina-supported and unsupported catalyst. The alumina-supported catalysts were fairly reducible with extents of reduction of 43-45% after reduction in H2 at 300 C for 16 h, but only half as reduced as the unsupported catalyst.

Table 1.  Textural properties of catalysts

Catalyst

BET, m2/g

Pore volume, cm3/g

EOR, %

Fe/Cu/K/SiO2

120

0.11

90

CNT/Fe/Cu/K

163

0.31

-

Al/Fe/Cu/4K

145

0.43

43

Al/Fe/Cu/8K

139

0.41

45

Reaction rates at 250C for the 4 catalysts are given in Figure 1. The highest activity was obtained on alumina-supported catalyst with 4 parts K with a CO depletion rate of 38 mmol/gcat/h. On the other hand, this catalyst also had the highest methane selectivity (12-16 mol% - CO2-free basis) followed by unsupported iron, Fe/Cu/K/SiO2 (7 mol%) and CNT-supported (5.1 mol%). Higher selectivity of methane on alumina can be explained by acidic sites on alumina compared to the inert surface of CNT, which provide cracking sites for production of methane. To our surprise, alumina-supported catalyst with the higher amount of potassium (8 vs. 4%) didn't change methane selectivity significantly, while the activity decreased slightly.

Fig. 1. CO conversion rate for different supports and unsupported catalyst.

The alumina supported iron catalyst prepared with the impregnation method compares favorably with catalysts described in the literature. Table 2 compares our catalyst with one unsupported and two supported catalysts from the literature. In order to obtain a quantitative comparison of catalyst activities to account for different gas space velocities a first-order FTS reaction is assumed. Bukur et al. [4-6] compared the performance of their best most active precipitated catalysts (TAMU1) with silica-supported iron catalyst (TAMU2) and alumina-supported catalyst (TAMU3). They reported that the silica-supported catalyst was two-fold less active than precipitated catalyst if they compared per gram catalyst (100 vs. 221 mmol(CO+H2)/g cat/MPa/h). They also showed that the alumina-supported catalyst had nearly 50% less activity than silica-supported catalyst. Alumina-supported catalyst prepared in the current work is nearly two-fold more active than the silica-supported catalyst reported by Bukur. Catalyst activity at 260 C was 183 mmol (H2+CO)/g cat/MPa/h compared with 221 mmol (H2+CO)/g cat/MPa/h for Bukur's most active unsupported catalyst. In addition, catalyst productivity was 0.49 gHC/g cat/h compared with 0.51 gHC/g cat/h (TAMU1). The catalyst activity comparison was even more favorable if they are compared per g Fe (915 vs. 370 mmol (H2+CO)/g cat/MPa/h). On the other hand, the methane selectivity (CO2-free basis) was higher than those in the literature (16 vs. 3-7 mol%). This catalyst shows great promise and work continues on optimizing catalyst preparation, performance, and stability.

Table 2. Comparison of catalyst performance

Property

Al/Fe/Cu/4K

TAMU11

TAMU22

TAMU33

Time on stream, h

145

120

100

100

Temp, C

260

260

260

260

Pressure, MPa

2.15

2.2

1.5

1.5

H2/CO in feed

1

0.67

0.67

0.67

mmol (H2+CO)/g cat/MPa/h

189

221

100

40

mmol (H2+CO)/g Fe/MPa/h

946

370

300

120

Hydrocarbon selectivity, mol%

 

 

 

 

CH4

16

3

6-7

3-5

CO2

29

48.4

-

-

C2+

55

50.2

-

-

Catalyst productivity, gHC/ g Fe/h

2.55

0.86

-

-

Catalyst productivity, gHC/ g cat/h

0.51

0.51

-

-

1100Fe/3Cu/4K/16SiO2

2100Fe/5Cu/6K/139SiO2

3100Fe/5Cu/9K/139Al2O3

References:

[1] D.B. Bukur, X. Lang, D. Mukesh, W.H. Zimmerman, M.P. Rosynek, and C. Li, Binder/support effects on the activity and selectivity of iron catalysts in the Fischer-Tropsch synthesis, Ind. Eng. Chem. Res. 29 (1990) 1588.

[2] D.S. Kalakkad, M.D. Shroff, S. Kohler, N. Jackson, and A.K. Datye, Attrition of precipitated iron Fischer-Tropsch catalysts, Appl. Catal. A: Gen. 133 (1995) 335.

[3] K.M. Brunner, K. Keyvanloo, C.H. Bartholomew, W.C. Hecker, A Simple and Novel Preparation Method For Iron FT Catalysts, ACS Meeting, San Diego, March 25-29, 2012.

[4] D.B. Bukur and X. Lang, Highly active and stable iron Fischer-Tropsch catalyst for synthesis gas conversion to liquid fuels, Ind. Eng. Chem. Res. 38 (1999) 3270.

[5] D.B. Bukur and C. Sivaraj, Supported iron catalysts for slurry phase Fischer-Tropsch synthesis, Appl. Catal. A: Gen. 231 (2002) 201.

[6] D.B. Bukur, X. Lang, and Y. Ding, Pretreatment effect studies with a precipitated iron Fischer-Tropsch catalyst in a slurry reactor, Appl. Catal. A: Gen. 186 (1999) 275.

 


Extended Abstract: File Not Uploaded
See more of this Session: CO Hydrogenation I
See more of this Group/Topical: Catalysis and Reaction Engineering Division