ZrO2 catalyst in Fischer-Tropsch synthesis

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 02004 Elsevier B.V. All rights reserved.

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Effect of impregnation pH on the catalytic performance of Co/ZrO catalyst in Fischer-Tropsch synthesis H.-X. Zhao, J.-G. Chen, Y.-H. Sun* State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, China [email protected] ABSTRACT A series of Co/ZrO2 catalysts were prepared by controlling the pH value of the cobalt nitrate solution. The interaction of cobalt precursor and ZrO2 support showed to be dependent on the impregnation pH and then influenced the catalytic activity of the Co/ZrO2 catalysts in Fischer-Tropsch synthesis. 1. INTRODUCTION Fischer-Tropsch synthesis (FTS) is one of major routes for converting coal-based and/or natural gas-derived syngas into chemicals and fuels [1]. Cobalt based catalysts are widely used for Fischer-Tropsch synthesis [2], especially when high chain growth probability and a low branching probability are required [3]. The focus in the development of this process is the improvement of the catalyst activity with low CH4 production. The usual supports, such as silica, titania and alumina, are easily to form surface compound with cobalt precursor, which decrease the reduction degree of the catalysts. Hence, the activity and selectivity for heavier hydrocarbons are suppressed. Zirconium oxide has attracted considerable attention recently as a catalyst support and a promoter for its high selectivity for heavier hydrocarbons in Fischer-Tropsch synthesis [2, 4]. The purpose of this paper is to study the impregnation solution pH on the catalytic performance of Co/ZrO2 catalysts in Fischer-Tropsch synthesis.

2. EXPERIMENTLE 2.1 Catalyst preparation The zirconia support was prepared by co-precipitation of zirconyl nitrate aqueous solution with ammonia hydroxide solution. Catalysts were prepared by

356 incipient wetness impregnation using cobalt nitrate as the precursor. Nitric acid and urea were employed to regulate the solution pH. The pH value was measured by a pH meter, which was calibrated at pH values of 4 and 10 prior to each run. The catalysts were designated as Z1, Z2, Z3 and Z4 with the solution pH 1.3, 2.7, 4.4 and 6.1, respectively. The relatively higher pH of 6.1 of the cobalt nitrate solution could just keep the solution from body precipitation of cobalt species. The catalyst precursors were aged for a certain time at room temperature and then dried at 393K and calcined at 673K for 6h. The cobalt content of all the catalysts was 10 wt %. The texture properties of the catalysts are listed in Table 1.

2.2 Catalyst Characterization The textural properties of support and catalysts were measured with ASAP-2000 Micromeritics instrument at 77 K. XRD was recorded in Rigaku D/max-TA with Cu target at 40 kV and 100 m A. Diffuse reflectance spectra (DRS) were recorded over a wavelength range from 850 to 250 nm on a double-beam Shimadzu spectrophotometer model UV-2501PC equipped with a diffuse reflectance attachment. Specpure BaSO4 (Shimadzu) was the reference material for the catalysts. Temperature programmed reduction (TPR) was carried out in a quartz reactor with a mixture of 5% H2/N2 as the reductive gas. The samples (0.1 g) were flushed with a N2 flow of 40 mL/min at 393 K to remove adsorbed water and then reduced in a flow of H2/N2 at a rate of 10 K/min. The effluent gas was monitored by TCD. The TPR procedure was calibrated using the reduction of CuO. The measurement of reduction degree was described elsewhere [5]. Oxygen titration was performed on a TG-151 analyzer. The catalyst (0.2 g) was reduced at 673 K, further purged with a flow of argon for 1 h to remove physically adsorbed H2, and then maintained at 673 K for oxidation. The reducibility of metal cobalt was measured based on the weight gain after re-oxidation of the reduced sample.

Table 1 Characteristics of samples Catalysts Solution pH

BET/mZ/g Pore size/nm Reduction a/% 28.0 15.2 --Z1 1.3 27.1 15.6 100 Z2 2.7 27.5 15.4 100 Z3 4.4 28.1 16.0 100 Z4 6.1 27.9 16.1 40 aExtent of reduction was obtained from the weight gain during reoxidation at 673 K ZrO2

---

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DRIFTS spectra were recorded with Nicolet Magna 550 spectrometer using a spectral resolution of 8cm -~. The catalyst was reduced in-situ for 6 h under atmospheric pressure by a stream of H2 at 673 K. After introduction of CO for 1 h, the catalyst surface was purged with argon to remove gaseous CO and then IR spectra were recorded.

2.3 Catalytic Test CO hydrogenation was carried out at 2 MPa, 1000 h~and a H2/CO ratio of 2 in a fixed bed reactor of i.d. 10 mm. The samples were reduced in a flow of hydrogen at 673 K for 6 h at 1000 h -~ and then cooled down and switched to syngas. Mass balances were commenced after the reaction was on-line for 24 h. The CO, H2, CO2, and CH4 products were analyzed on the TCD, and the gas hydrocarbons were detected on the FID. Liquid products and wax were collected in a cold trap and a hot trap respectively and then were offline analyzed on the FID which was equipped with a 35 m OV-101 capillary column. 3. RESULTS AND DISCUSSION

3.1 Surface properties of the catalysts N2 adsorption at 77 K showed that the texture hardly changed before and after the impregnation (see Table 1), and cobalt existed in the Co304 phase (see Fig. 1). Fig. 2 showed UV-Vis diffuse reflectance spectra of supported Co/ZrO2 catalysts. All the catalysts showed the similar spectra, and two broad bands at about 400 and 700 nm corresponding to octahedral Co 3+ confirmed the existence of Co304 [6]. The increase in the band intensity with the rise of the impregnation pH was due to the increase in the surface density of cobalt species [7], suggesting cobalt oxides were well dispersed on the zirconium surface in this case. 1.7 //~/~"f~"K\

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Fig. 1. XRD profiles of Co/ZrO2 catalysts Fig.2. DRS profiles of Co/ZrO2 catalysts

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3.2 Reduction behavior of Co/ZrOz catalysts The TPR profiles of Co/ZrO2 catalysts are shown in Fig. 3. Two kinds of reduction behavior were observed. The TPR plots of samples Z1, Z2 and Z3 were almost identical to each other. Double peaks observed in samples Z1, Z2 and Z3 illustrated the step reduction of Co304. This indicated that larger Co304 particles existed on the surface of ZrO2. The reduction of sample Z4 showed three peaks below 600~ suggesting that different cobalt species [8] existed over ZrO2. The zero point charge (ZPC) of ZrO2 is reported to be about 5 [9], thus the surface of ZrO2 was positively charged at lower pH than 5 and the adsorption of cobalt was suppressed, and consequently the interaction between cobalt and zirconium decreased. The similarity of reduction behavior of samples Z1, Z2, and Z3 implied that the same cobalt species existed on the zirconia surface. On the contrary, at higher pH than 5, the zirconia surface was negatively charged and the adsorption of positively charged cobalt ions was improved, and then the interaction between cobalt and zirconia increased. Such a different interaction directly led to the different reduction behavior of the samples. 3.3 In-situ CO adsorption on reduced catalysts Infrared spectra of CO adsorbed on reduced catalysts are shown in Fig. 4. A group of distinct bands at 2048, 1974, 1936, 1886cmlwere observed after CO adsorption on sample Z1. For Z2, the adsorption bands appeared at the same position as that of Z1 except for the relatively weaker adsorption intensity. The adsorption behavior on Z3 and Z4 was quite different from that of Z 1 and Z2.

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Fig. 4. FTIR spectra of CO adsorption on reduced catalysts

359 The adsorption intensity at 2048, 1974, and 1936cm -~ became weaker and the adsorption band at 1886cm ~ disappeared, and at the same time, a new adsorption band appeared at 203 l cm -j. In addition, the adsorption intensity of sample Z4 was weaker than that of Z3. The bands above 2000 cm -~ are assigned to the linearly adsorbed CO, and those below 2000 cm -~ are attributed to the bridged-form species on Co o particles [5, 10]. Therefore, the adsorption bands at 2048 and 2031 cm -~ should be attributed to linearly adsorbed CO species, and the bands at 1974, 1936, 1886 cm -~ should be attributed to bridged adsorbed CO species. The strong adsorption of bridged-form CO on Z1 surface indicated that cobalt particle was larger. These facts clearly indicated that the impregnation solution pH had a significant influence upon the formation and then the adsorption properties of the surface cobalt species.

3.4 Catalytic performance for Fischer-Tropsch synthesis CO conversion and product selectivity strongly depend on the impregnating solution pH (see Table 2). CO conversion and C5+ hydrocarbons selectivity increased and CH4 selectivity decreased with the decrease of the pH. At 483 K, sample Z1 showed the highest CO conversion and C5+ hydrocarbons selectivity of 82.4% and 86.5%, respectively. Sample Z4 had the lowest catalytic performance, and the CO conversion and C5+ hydrocarbons selectivity were 33.2% and 44.4%, respectively. Such a pH dependent performance appeared to be closely related to the CO adsorption over the catalysts, especially the bridged-adsorbed CO. That might be the reason that Z1 had a better activity and selectivity.

Table 2 The performance of Co/ZrO2 catalysts for Fischer-Tropsch synthesis Catalysts

Reactiontemperature /K

Z1

483 493 483 493 483 493 483 493

Z2 Z3 Z4

CO C o n v e r s i o n / % CH4/wt% 82.4 94.1 82.6 95.5 82.7 92.5 33.2 40.2

5.5 5.9 6.3 6.1 8.3 8.5 33.1 17.6

Cs+/wt% 86.5 82.3 84.8 81.3 83.3 82.0 44.4 71.9

360 4. C O N C L U S I O N S The interaction of cobalt precursor and ZrO2 support showed to be dependent on the impregnation pH and then influenced the cobalt dispersion on the ZrO2 surface. Consequently, the adsorption of CO on the catalyst surface was greatly influenced and the content of adsorbed CO increased with the increase of cobalt particle size. Meanwhile, the activity for Fischer-Tropsch synthesis increased monotonously with the increase of CO adsorption. Therefore, the impregnation solution pH should be selected lower than the ZPC of the support to weaken the interaction between cobalt and ZrO2 and thus improve the catalytic performance of the catalysts.

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