Effect of silica on iron-based Fischer-Tropsch catalysts

NATURALGAS CONVERSIONV Studies in Surface Science and Catalysis,Vol. 119 A. Parmalianaet al. (Editors) o 1998Elsevier Science B.V. All rights reserved.

215

Effect of silica on iron-based Fischer-Tropsch catalysts K. J o t h i m u r u g e s a n , a J a m e s J. Spivey, b Santosh K. Gangwal, b and J a m e s G. Goodwin, Jr. c aDepartment of Chemical Engineering, Hampton University, Hampton, VA 23668, USA bResearch Triangle Institute, Research Triangle Park, NC 27709, USA CDepartment of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15260, USA ABSTRACT The effect of silica addition via coprecipitation and as a binder to a doubly promoted Fischer-Tropsch (F-T) synthesis iron catalyst (100 Fe/5 Cu/4.2 K) was studied. The catalysts (nominally 50 ]~n particles) were prepared by coprecipitation, followed by binder addition and spray drying at 250 ~ in a 1 m diameter, 2 m tall spray dryer. The binder silica content was varied from 0 to 20 wt%. A catalyst with 12 wt% binder silica was found to have the highest attrition resistance. F-T reaction studies over 100 hours in a fixed-bed reactor showed that this catalyst maintained around 95% CO conversion with a methane selectivity of less t h a n 7 wt% and a C5 + selectivity of greater t h a n 73 wt%. The effect of adding precipitated silica from 0 to 20 parts by weight (pbw) to this catalyst (containing 12 wt% binder silica) was also studied. Addition of precipitated silica was found to be detrimental to attrition resistance and resulted in increased methane and reduced wax formation. 1. I N T R O D U C T I O N

The conversion of natural gas into liquid fuels is an increasingly important means of meeting the energy needs of developing regions and transporting energy from remote gas fields to the existing refining and distribution infrastructure [1]. These plants rely on F-T synthesis to convert synthesis gas into liquid fuels. Although F-T plants based on natural gas-derived syngas (e.g., Shell's Bitulu plant) use, or plan to use, cobalt-based catalysts because of their generally higher yield of linear, saturated paraffins, iron-based catalysts have also been used for F-T synthesis of fuels from gas-derived syngas due to their lower cost and lower methane selectivity [2-4]. The development of attrition-resistant iron catalysts for commercial slurry bubble column reactors (SBCRs) is a critical challenge. A comparison of silica with other

216 supports for iron-based F-T catalysts showed t h a t silica-supported materials have generally higher activity and wax selectivity [5]. Dry [5] and Yang and Goodwin [6] report improved mechanical stability with silica supports, but no systematic variation of the silica content has been undertaken. The literature on the effect of silica on these catalysts is limited, but increasing levels of precipitated silica have been shown to decrease the F-T activity of 100 Fe/5 Cu/4.2 K catalysts, although the stability increased [7]. Water-gas shift activity decreased with silica content in this study, and product selectivity also changedm less branched hydrocarbons and total olefins (but more internal olefins) were produced, consistent with the results of Egiebor and Cooper [8]. These effects were ascribed to a lower degree of reduction of the iron and a decrease in the surface basicity. Studies on spray-dried iron F-T catalysts are also limited. Shroff et al. [9] describe in detail the effect of activation procedures on spray-dried Fe/Cu/K/Si catalysts, but they did not study the effect of silica content on attrition resistance or activity. The objective of the work reported here is to develop 50 to 90 tim attritionresistant iron-based F-T catalysts for SBCRs and to compare the effects of precipitated silica and binder silica.

2. EXPERIMENTAL 2.1. Catalyst preparation Two types of silica-containing iron catalysts were prepared. The first series of catalysts contained binder silica but no precipitated silica and had a composition of 100 Fe/5 Cu/4.2 K (plus binder silica). These catalysts were prepared by coprecipitation using an aqueous solution containing Fe(NO3)3 9 9H20 and Cu(NO3) 2 9 2.5 H20 in the desired Fe/Cu atomic ratio, which was precipitated by adding ammonium hydroxide. The resulting precipitate was then filtered and washed three times with deionized water. The potassium promoter was added as aqueous KHCO 3 solution to the undried, reslurried Fe/Cu coprecipitate. To this catalyst, five different levels of binder silica were added: 4, 8, 12, 16, and 20 wt%. These catalysts were then spray dried at 250 ~ using a large bench-scale spray drier and calcined at 300 ~ for 5 h in a muffle furnace. These catalysts are designated Fe-bSi(x), denoting that they contain x wt% binder silica. A standard Ruhrchemie catalyst (identified as Batch 52119) was obtained from the United States Department of Energy (DOE), Pittsburgh, PA, as a benchmark catalyst. The second series of catalysts contained both precipitated and binder silica. Four such catalysts were prepared containing 5, 10, 15, and 20 pbw precipitated silica (yielding catalysts of the composition 100 Fe/5 Cu/4.2 K/y SiO2, where y is 5, 10, 15, or 20). The precipitated silica was added as a 1.0 M solution of Si(OC2H5) 4 to the nitrate solution described above. To each of these catalysts, 12 wt% binder silica was added. These catalysts are designated Fe-pSi(y). These catalysts were then spray dried and calcined in the same way as those above.

217

2.2. C a t a l y s t c h a r a c t e r i z a t i o n The BET surface areas of the catalysts were determined by N 2 physisorption using a Micromeritics Gemini 2360 system. The hydrogen reduction behavior was measured in a Micromeritics 2705 system. X-ray powder diffraction patterns were obtained using a Phillips PWl800 X-ray unit using a CuKa radiation. The attrition of the iron-based catalysts was tested using method ASTM-D-5757-95 in a 3-hole attrition tester. [The ASTM test is more severe t h a n what would be experienced in an SBCR.] The SEM micrographs were taken using a Cambridge Stereoscan 100. 2.3. A c t i v i t y t e s t i n g The F-T activity of the catalysts was measured in a fixed-bed microreactor system at 250 ~ 1.48 MPa, and 2.0 NL/g cat-h, at an H2/CO ratio of 0.67. The catalyst charge was 1.3 g, and the catalyst was reduced in-situ in CO at 0.1 MPa, 280 ~ 35 cc/min for 16 h. After reduction, the reactor temperature was lowered to 50 ~ The system was then pressurized to 1.48 MPa, the carbon monoxide flow was cut off, and synthesis gas (H2/CO=0.67) was introduced at a gas space velocity of 2 NL/(g cat.h). The reactor temperature was then increased gradually to 250 ~ The catalysts were then tested over a period of 100 h. 3. R E S U L T S 3.1. C h a r a c t e r i z a t i o n Table 1 shows the BET surface areas of the fresh and reduced catalysts, the hydrogen uptake, and the attrition resistance of all the catalysts synthesized. The BET surface area decreased as the catalyst was reduced, but decreased less after reduction as the silica content (either precipitated or binder) increased, as expected. This trend is consistent with the results of Bukur et al. [7], as are the absolute values of the surface areas of comparable catalysts reduced in CO. The hydrogen uptake generally decreased with silica content, though the effect of the precipitated silica is much less t h a n the effect of the binder silica. Figure 1 shows the hydrogen uptake pattern of one of the catalysts. There were slight variations among the catalysts, with all showing peaks at 320 and 750 ~ The peak at 320 ~ corresponds to the reduction of Fe203 -* Fe304, and the peak at 750 ~ corresponds to the reduction ofFe304 to metallic iron [10]. The small shoulder peak at roughly 250 ~ is due to the reduction of CuO ~ Cu [10]. The higher H 2 consumption by the Fe-bSi catalysts compared to the Fe-pSi catalysts indicates a greater extent of reduction for catalysts containing binder silica. SEM micrographs of the Fe-bSi(4) catalyst show t h a t the catalyst particles are roughly spherical and approximately 50 to 90 ~m in diameter (Figure 2). The attrition resistance is perhaps the most interesting result. There is a m i n i m u m in the weight loss as a function of binder silica content, with the most attrition-resistant material being the Fe-bSi(12) catalyst, containing 12 wt% binder silica. For this reason, this material was used as the basis for preparing the Fe-

218 Table 1 Chemical and physical properties of iron-silica catalysts Binder

BET surface area (m2/g) H2 TPR Attrition loss b (wt%)

Catalyst designation

silica Precipitated (wt%) silica (pbw) Fresh

Fe-bSi(4) Fe-bSi(8) Fe-bSi(12) Fe-bSi(16) Fe-bSi(20) Fe-pSi(5) Fe-pSi(10) Fe-pSi(15) Fe-pSi(20)

4 8 12 16 20 12 12 12 12

-0-0-0-0-05 10 15 20

80.3 95.7 121 151 172 163 168 189 218

Reduced a 35.6 50.8 68.7 103 98.9 116 144 163 181

(mmol H2/g cat) 1 h 24.3 23.0 20.6 19.0 18.4 18.8 17.9 17.7 17.6

24.4 25.7 12.8 22.0 34.9 24.2 31.0 42.1 39.1

5h 32.6 35.4 22.7 30.1 35.0 37.3 39.6 c c

a Reduced in CO, 27 cc/g cat.min, 0.1 MPa, 280 ~ 16 h. b Measured using ASTM-D-5757-95. The higher the weight loss, the poorer the attrition resistance. For comparison, a commercial FCC catalyst was tested; wt% loss was <1% (1 h) and 2% (5 h). c Attrition was severe enough to plug the tester; wt% loss was >40 wt%.

pSi(y) series of catalysts with various levels of precipitated silica. The catalysts containing precipitated silica were 12 wt% SiO 2 J .Q less a t t r i t i o n - r e s i s t a n t t h a n those conL_ [Fe-bS taining only binder silica. None of the C" ._o catalysts, as expected, were as attritionQ. E resistant as a commercial FCC material, thowever. O 0 XRD of all the fresh catalysts showed "1" I I I I I I I I only Fe203, while all the reduced cata0 100 200 300 400 500 600 700 800 900 lysts showed only the xFe2.5C and Fe304 Temperature (~ phases. This result is similar to t h a t of Figure 1. Hydrogen TPR of catalyst B u k u r et al. [10], who also observed containing 12 wt% binder silica but no xFe2.5C after a similar reduction proprecipitated silica. cedure. Shroff et al. [9] also show the formation of an unspecified carbide phase after CO activation at 270 ~ but their short (2 h) activation left a s u b s t a n t i a l proportion of unreduced Fe304. u}

t--

I

3.2. F i s c h e r - T r o p s c h activity tests Table 2 shows the CO conversion and hydrocarbon selectivity for the various catalysts, along with d a t a obtained on a Ruhrchemie catalyst for comparison. [Data

219

Figure 2. SEM micrograph of reduced Fe-bSi(4), containing 4 wt% binder silica.

t a k e n from 0 to 90 h showed little v a r i a t i o n in CO conversion after an initial induction period, during which the CO conversion reached a steady value. Hydrocarbon selectivities were not m e a s u r e d during this period. There was no significant change with time in the CO conversions or hydrocarbon selectivities reported in Table 2 over the 100 h test d u r a t i o n for either the Fe-bSi or Fe-pSi catalysts.] All catalysts tested were more active t h a n the R u h r c h e m i e catalyst. Except for the Fe-pSi(16) and Fe-pSi(20) catalysts, the CO conversion was essentially the same, 94%. However, the selectivity varied with the silica content. There was a beneficial effect of binder silica up to 8 to 12 wt% on selectivity (reduced m e t h a n e , nearly c o n s t a n t C12+). However, as the binder silica content increased above 12 wt%, the C1, C2-C4, and C5-Cll selectivities

Table 2 Catalyst activity and selectivity Catalysts with binder silica Fe-bSi Catalyst Time-onstream (h) CO conversion (%)b

Ruhrchemie (4)

Catalysts with precipitated silica a Fe-pSi

(8)

(12) (16)

(20) (4)

65

67

90

68

(8)

(12)

(16)

66

66

66

42.7

52

42

86

94.3

94.1 94.3 95.5 94.5 95.5

94.4

90.1

88.2

7.4 6.8 6.8 9.9 9.6 8.8 18.1 1 7 . 6 19.6 25.0 23.5 23.2 12.7 1 3 . 0 12.8 17.3 17.6 22.0

10.2 23.5

10.2 22.4

9.5 20.1

26.5

30.5

32.8

61.8

39.8

36.9

37.7

Hydrocarbon selectivity (wt%) C1 C2-C4

8.3 21.3

C5-C 11 C12+

14.3 56.1

62.5 60.8 47.8 49.3 46.0

a These catalysts also contain 12 wt% binder silica. b Measured at 250 ~ 1.48 MPa, 2 NL/g cat'h, H2/CO=0.67.

220 increased, while the C12 + selectivity decreased. As the precipitated silica content increased (at 12 wt% binder silica), the selectivities of all the CvC11 products increased. However, the C5-Cll selectivity for the catalysts containing precipitated silica was generally higher than the selectivity for those catalysts containing only binder silica. The chain growth parameter (a, from C30-C40 yields) was around 0.9 for all catalysts. The relatively low methane selectivity of the most attritionresistant catalyst, Fe-bSi(12), at 94% CO conversion and the high selectivity of C12+ suggest the addition of a binder silica that not only improves the attrition resistance but also results in a catalyst with higher F-T activity and C5+ selectivity than the Ruhrchemie material. 4. C O N C L U S I O N The addition of binder silica to a precipitated 100 Fe/5 Cu/4.2 K F-T catalyst, followed by spray drying, increases the attrition resistance significantly. Within the range of catalysts tested here, the optimum binder silica content is approximately 12 wt%. The F-T activity of this catalyst is higher than a Ruhrchemie catalyst at 250 ~ 1.48 MPa. The addition of precipitated silica to a catalyst containing 12 wt% binder silica decreases its attrition resistance and increases its methane selectivity. ACKNOWLEDGMENT

The authors are grateful for U.S. DOE/FETC support of this research under grant DE-FG22-96PC96217 and for the technical guidance of Project Officer Dr. Richard E. Tischer. REFERENCES

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