Purification Influence of Synthesis Gas Derived from Methanol Cracking on the Performance of Cobalt Catalyst in Fischer-Tropsch Synthesis

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1 Journal of Natural Gas Chemistry 14(2005) Purification Influence of Synthesis Gas Derived from Methanol Cracking on the Performance of Cobalt Catalyst in Fischer-Tropsch Synthesis Wei Zhou 1,2, Shengying Liu 1, Yulan Wang 1, Kegong Fang 1, Jiangang Chen 1, Yuhan Sun 1 1. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan , China 2. Graduate School of Chinese Academy of Sciences, Beijing , China [Manuscript received September 29, 2005; revised November 17, 2005] Abstract: Synthesis gas derived from methanol cracking (SGMC) was applied as simulating feedstock of Fischer-Tropsch synthesis (FTS) in laboratory. With MS and GC detector, a trifle of sulfur compounds, a small amount of oxygenates including H 2O, CH 3OH, DME and CO 2 as well as a few of low carbon alkanes were found in the SGMC. After purification, the sulfur compounds, H 2O, CH 3OH and DME could be eliminated efficiently from the SGMC while CO 2 and the low carbon alkanes were partly removed. When the unpurified SGMC, the desufurized SGMC and the totally purified SGMC were sequentially applied in cobalt-based FTS, the catalytic performance of Co/ZrO 2/SiO 2 catalyst was gradually improved corresponding to the degree of purification. The untreated SGMC led to the serious deactivation of the cobalt catalyst, the partially treated SGMC slowed down the deactivation rate and the totally purified SGMC resulted in little deactivation of the catalyst, which was similar to what the pure synthesis gas (the mixture of pure H 2 and CO) did. The results indicated that the SGMC should be purified and the purification course used in this paper was effective for the SGMC. Furthermore, the totally purified SGMC could substitute for the pure synthesis gas in cobalt FTS. Key words: Fischer-Tropsch synthesis, synthesis gas, methanol cracking, purification 1. Introduction Fischer-Tropsch synthesis (FTS) has gained industrial attention for converting synthesis gas to highboiling points waxes that can be further converted to sulfur-free motor fuels by hydrogenation or hydrocracking as alternative resources to the limited crude oil. In the Fischer-Tropsch synthesis processes, synthesis gas is first generated, then the F-T reaction forms in the second step, in which synthesis gas is converted into the hydrocarbons over the catalysts. In the final step, the hydrocarbons undergo further hydrogenation or hydrocracking to yield clean middle distillates as products [1,2]. Among the above three stages, the manufacture of synthesis gas is by far the most capital-intensive stage [2]. Generally, synthesis gas is manufactured by partial oxidation of natural gas or gasification of coal in industry, while it is produced by mixing the pure CO and H 2 or decomposing methanol in laboratory [3 8]. Thereof, synthesis gas derived from methanol cracking (SGMC) has particular advantages such as higher contents of effective components (CO+H 2 >95%) relative to coal-based synthesis gas (CO+H 2 >85%), more suitable stoichiometric ratio of H 2 to CO (2:1) than coal-based synthesis gas (H 2 /CO= ) and nat- Corresponding author. Tel: (0351) ; Fax: (0351) ; yhsun@sxicc.ac.cn. Financial supported from National Natural Foundation of China ( and ) and State Key Foundation Program for Development and Research of China (2005cb221402).

2 194 Wei Zhou et al./ Journal of Natural Gas Chemistry Vol. 14 No ural gas-based synthesis gas (H 2 /CO>2) [3] and much cheaper than the mixture of pure CO and H 2. Therefore, the SGMC is applied widely in many laboratories [6 8] and even was used in AMSTG course, which was one modified FT technology in Japan several years ago [3]. With respect to the catalyst for FTS, supported cobalt catalysts are highly attractive due to their high activity, high yields of long-chain paraffins and low activity for the water-gas shift reaction [9 11]. However, when the SGMC is applied in cobalt-based FTS, the feed gas must be kept very pure for the active metallic cobalt was inclined to be poisoned. Though the SGMC is used widely, the purifying course has not been reported so far. Therefore, this paper mainly investigated the purifying course of the SGMC on the reaction behavior of cobalt-based FTS. 2. Experimental 2.1. Analysis system The analysis system consisted of one mass spectrometer (MS, HPR-20) and two sets of gas chromatograph (GC). One GC was equipped with thermal conductivity detector (TCD) and a carbon molecular sieve column, while the other was set with flame ionization detector (FID) and a Porapak-Q column. Ar was the carrier gas for GC. The sulfur contents were monitored by the HC-2 Sulfur Analyzer. The SGMC was routed on-line into the MS, GC and Sulfur Analyzer, separately Catalyst preparation The Co/ZrO 2 /SiO 2 catalyst was applied in this paper for its good catalytic performance in FTS [1,12 14]. A 15% Co/10% ZrO 2 /SiO 2 catalyst was prepared by sequentially incipient wetness impregnation method with drying at 303 K after each impregnation. Sol-gel derived SiO 2 (278 m 2 /g) was used as the support. The zirconium and cobalt precursors were zirconium (IV) nitrate and cobalt (II) nitrate, respectively [15]. The catalyst impregnated with zirconium was first calcined at 773 K for 4 h in the air and then impregnated with the cobalt solution, redried and then calcined with the same procedure as mentioned above. The cobalt loading was verified by using an inductively coupled plasma atomic emission spectrometer (ICP-AES) analysis method F-T reaction A series of experiments were carried out on the SGMC, the partially purified SGMC, the totally purified SGMC and the pure synthesis gas (the mixture of pure H 2 and CO), respectively. 1 ml catalyst was diluted with 4 ml SiO 2 to minimize the temperature gradients. Then the catalyst activation was conducted in pure hydrogen at 673 K, 0.5 MPa and 2000 h 1 for 6 h in a fixed bed reactor of i.d. 10 mm. After the activation period, the reactor temperature was decreased to the room temperature and the synthesis gas (H 2 /CO=2.0) was introduced. The F-T reaction proceeded at 493 K, 2.0 MPa and 1500 h 1. Liquid products and wax were obtained through a cold trap keeping at 273 K and a hot trap keeping at 413 K, respectively. The products analysis was just the same as previous work [16]. All reported data were measured after 72 h time-on-stream in order to ensure steady-state behavior of the reaction. The mass and carbon balance of all the reactions both maintained between 95% and 105%. 3. Results and discussion 3.1. Analysis of the SGMC The SGMC was admitted into mass spectrometer and Sulfur Analyzer (see Figure 1 and Table 1). The result suggested that there were a trifle of sulfur compounds including H 2 S and COS and a small amount of oxygenates including H 2 O, CO 2, CH 3 OH, and DME in the SGMC (see Figure 1). Table 1. MS analyses results of the synthesis gas by cracking of methanol before and after purifying Compound in Before After the SGMC purifying (%) purifying (%) H CO CO CH H 2 S COS CH 3 OH DME H 2 O * The contents were monitored by the sulfur analyzer

3 Journal of Natural Gas Chemistry Vol. 14 No Figure 1. MS detection results of the SGMC (a) Unpurified SGMC, (b) Purified SGMC after desulfurizing, (c) Purified SGMC after desulfurizing and oxygenates-removing GC results (see Figure 2 and Table 2) proved CO 2 and DME existed in the SGMC. Moreover, a trace of low carbon alkanes was also detected in the SGMC through MS and GC. Figure 2. GC detection results of the SGMC (1) Unpurified synthesis gas, (2) Purified synthesis gas

4 196 Wei Zhou et al./ Journal of Natural Gas Chemistry Vol. 14 No Table 2. GC analyses results of the synthesis gas by cracking of methanol before and after purifying Compound in Before After the SGMC purifying (%) purifying (%) H CO CO CH C 2 H C 3 H C 4 H DME Purification of SGMC It was well known that the sulfur compounds could poison metallic cobalt and lead to the deactivation of the catalyst [17]. So the removal of sulfur compounds from the SGMC was crucial. H 2 O was also one species that could make cobalt catalyst deactivate in FTS [4,5,18 30]. It was found that water could result in the reoxidation of cobalt [18 23], formation of interacting species [24 26] and sintering [27 30]. Schanke et al. [18] thought that the oxidation of surface cobalt atoms or highly dispersed cobalt phases by water on alumina supported cobalt catalysts was responsible for the catalyst deactivation. Krishnamoorthy et al. [23] proved that water could reoxidize the surface cobalt of Co/SiO 2 catalyst at H 2 O/H 2 ratio greater than 0.8. In addition, the formation of interaction species between cobalt and Al 2 O 3 or SiO 2 supports under hydrothermal conditions had been reported[24 26]. As for the sintering, Bartholomew [27] pointed that water vapor would increase the sintering rate of supported metals. The sintering had been proved on cobalt catalysts at high water partial pressure, which was thought from the oxidation-reduction cycles induced by water in F-T reaction [28 30]. Therefore, the synthesis gas must be kept dry. As for other oxygenates, such as, CH 3 OH and DME, no report is published whether they were harmful to cobalt catalyst. Moreover, CO 2 was found to incorrelate with the deactivation of cobalt in FTS if CO 2 content was lower than 19% [31]. Considering the oxidation effects, the oxygenates should be cleaned up. However, a small amount of low carbon alkanes, which would be produced in FTS, could be ignored for having no report about the negative effects of the alkanes on the cobalt catalyst. The purification courses should be simple and could not induce side-reactions. ZnO catalyst (T305) was a good choice for removing sulfur compounds for it was widely used in the purification of the synthesis gas when the content of the sulfur compounds was small [27,32]. In addition, blue silica gel was good at absorbing water while 5A molecular sieve was a good oxide adsorbent for its uniform reticulate structure [33]. Therefore, T305, blue silica gel and 5A molecular sieve were applied in the SGMC purification courses. T305 catalyst was put into a desulfurizing container that was set in a heating furnace. The mixture of the blue silica gel and 5A molecular sieve (mixing ratio was 1/1) was sent into the other vessel. The desulfurizing course was carried out at 500 h 1 and 230 and the second course was at 500 h 1 and room temperature. The purification effects for the SGMC are shown in Figures 1, 2 and Tables 1, 2. The MS and Sulfur Analyzer results exhibited that the amount of the sulfur compounds decreased by one order of magnitude after desulfurizing and the amount of the CH 3 OH and DME decreased by two orders of magnitude after total purification. However, the amount of H 2 O and CO 2 decreased not significantly according to the MS results. Furthermore, GC results showed that the DME could not be detected any more and the amount of CO 2 as well as the low carbon alkanes decreased in part after the purification courses. It demonstrated that the removal of the sulfur compounds, CH 3 OH and DME from the SGMC were efficient, while CO 2 and the low carbon alkanes could be partly absorbed in the purification course. To assess the water absorption in the mixture of blue silica gel and 5A molecular sieve, the SGMC was introduced at 500 h 1 and ambient temperature. After one month, about 1/5 blue silica gel turned pink. It proved that the existence of water in the SGMC and the validity of the water removal in the purification course. Thus it could be seen that a part of water and carbon dioxide detected by MS might come from the residual atmosphere in the ultra-high vacuum house of MS. In brief, the sulfur compounds, methanol, DME and water in the SGMC could be eliminated efficiently and CO 2 and the low carbon alkanes could be removed partly through the purified courses F-T synthesis performance of SGMC over the Co catalyst The reaction results of F-T synthesis were graphed in Figure 3. For the experiment that was

5 Journal of Natural Gas Chemistry Vol. 14 No run in the SGMC, the catalyst activity and the selectivities for CH 4 and C 5+ decreased rapidly, implying that the impurities in the SGMC were harmful to the cobalt catalyst in the F-T reaction. When the SGMC was desulfurized, the catalyst activity and the selectivities for CH 4 and C 5+ were improved evidently. Still, after the SGMC was purified by T305 and the mixture of blue silica and molecular sieves, sequentially, the catalyst didn t deactivate for at least 550 h, indicating that the SGMC should be purified and the purification method for the SGMC in this paper was efficient. In addition, the catalyst showed an excellent performance though a few of carbon dioxide and low carbon alkanes existed in the totally purified SGMC, which implied that the existence of a few of carbon dioxide and low carbon alkanes did not affect the performance of the cobalt catalyst in F-T synthesis. Figure 3 also illuminated that the catalyst showed nearly the same reaction performance whether in the pure synthesis gas or in the purified SGMC, implying that the purified SGMC could substitute for the pure synthesis gas in the evaluation of the cobalt catalyst. 4. Conclusions A trifle of sulfur compounds, a small amount of oxygenates consisting of water, methanol, DME and CO 2 as well as a trace of low carbon alkanes were detected in the synthesis gas derived from methanol cracking. After two step purification, the sulfur compounds, water, methanol and DME were eliminated efficiently, while CO 2 and low carbon alkanes were removed partly. When the unpurified synthesis gas was applied in the cobalt F-T reaction, the Co/ZrO 2 /SiO 2 catalyst deactivated severely. Next when the desulfurized synthesis gas was used, the cobalt catalyst showed an amendatory performance. At last, when the totally purified synthesis gas through the desulfurizing and oxygenates-removing courses was employed in the F-T reaction, the cobalt catalyst exhibited good performance and stability, which was nearly the same as in the pure synthesis gas. It proved that: (1) the SGMC should be purified; (2) the purification method for the SGMC in this paper was efficient; (3) the purified SGMC could substitute for the pure synthesis gas in cobalt-based FTS. Acknowledgements The authors acknowledge the financial support from National Natural Foundation of China ( and ) and State Key Foundation Program for Development and Research of China (2005cb221402). References Figure 3. F-T synthesis performance in different synthesis gases over Co/ZrO 2 /SiO 2 catalyst (1) Unpurified SGMS, (2) Purified SGMS after desulfurizing, (3) Purified SGMS after desulfurizing and oxygenatesremoving, (4) Pure synthesis gas [1] Dry M E. Catal Today, 2002, 71: 227 [2] Geerlings J J C, Wilson J H, Kramer G J et al. Appl Catal A, 1999, 186: 27 [3] Zhou J L, Zhang Z X, Zhang B J. J Fuel Chem Tech, 1999, 27(suppl): 58 [4] Li J L, Jacobs G, Das T K et al. Appl Catal A, 2002, 236: 67 [5] Li J L, Zhan X D, Zhang Y Q et al. Appl Catal A, 2002, 228: 203

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