An active and selective production of gasoline-range hydrocarbons over bifunctional Co-based catalysts

Fuel 86 (2007) 50–59 www.fuelfirst.com

An active and selective production of gasoline-range hydrocarbons over bifunctional Co-based catalysts Chawalit Ngamcharussrivichai a, Xiaohao Liu b, Xiaohong Li b, Tharapong Vitidsant a, Kaoru Fujimoto b,* a

Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Phyathai Road, Patumwan, Bangkok 10330, Thailand b Department of Chemical Processes and Environments, Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Fukuoka 808-0135, Japan Received 20 April 2006; received in revised form 19 June 2006; accepted 21 June 2006 Available online 10 August 2006

Abstract Catalytic performance of several bifunctional cobalt-based catalysts in slurry-phase Fischer–Tropsch synthesis has been investigated at 1 MPa and 230 °C. The catalysts were prepared according to the conventional impregnation method using SiO2, Al2O3, montmorillonite (K10Ò), and various zeolites (USY, ZSM-5 and MCM-22) as a support. When an acidic support was applied, the formation of heavier hydrocarbons was retarded and the carbon number distribution was shifted to gasoline range (C4–C12). Montmorillonite loaded with Co (Co/MONT) was a good catalyst, showing higher the CO conversion than Co/SiO2 and Co/Al2O3. MCM-22 exhibited a superior performance to other zeolitic supports. With decreasing aluminum content, the activity of Co/MCM-22 catalysts was remarkably enhanced and the selectivity to methane and isoparaffins decreased. Besides the high CO conversion (55–60%), Cþ 4 isoparaffins were produced more selectively over Co/MCM-22 (43–53%) than Co/USY (43.5%) and Co/ZSM-5 (36%). Compared to the results under supercritical conditions, the slurry-phase conditions are more suitable for an active and selective production of gasoline-range hydrocarbons through the Fischer–Tropsch synthesis over MCM-22 supported Co catalysts. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fischer–Tropsch synthesis; Zeolite; Slurry phase

1. Introduction As a result of limited petroleum reservoirs and environmental constraints these days, there is a significantly increasing interest in the production of ultra clean fuels for transportation, e.g. diesel and gasoline [1–3]. Recently, the Fischer–Tropsch (FT) synthesis has received more attentions than ever since it is considered as an effective process to produce wide-range liquid hydrocarbon fuels and high-value added chemicals from relatively abundant resources, such as natural gas, coal and biomass, via synthetic gas [4]. However, the FT products are controlled *

Corresponding author. E-mail address: [email protected] (K. Fujimoto).

0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.06.021

by the Anderson–Schulz–Flory (ASF) polymerization kinetics, resulting in a non-selective formation of any hydrocarbons. Many attempts have been made to circumvent the ASF distribution and to selectively produce high-octane gasoline. Accordingly, several combinations of active FT metals, i.e. Ni, Fe, Co and Ru, and acidic function containing materials, especially zeolites, have been extensively investigated [5–13]. Due to suitable shape-selective properties and high acidity of the support, HZSM-5 supported FT catalysts show a good selectivity to aromatic and isoparaffin products but the CO conversion is low [12–14]. This is attributed to a strong interaction between the FT metal cations and negative charge of zeolitic framework, lowering reducibility to the active metal form [15].

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Although many catalyst preparation and modification procedures have been applied, the resulting catalysts do not exhibit an obvious improvement of the activity [8,9,12]. Moreover, catalytic deactivation, especially by carbon deposition, is a serious subject affecting the performance. These may be controlled by optimization of operating conditions and/or engineering design of reactor systems [7,10,11,16,17]. In the present study, we have investigated the performance of various acidic material-supported Co catalysts in a slurry-phase FT synthesis. MWW zeolites, so-called MCM-22, with very unique structure composing of two different pore channel systems, sinusoidal 10-membered ring channels and large supercages with 10-membered ring pore entrance existing between sheet layers, have been applied as a support for the FT catalyst for the first time since it may be selective for gasoline synthesis in a similar manner to ZSM-5. Furthermore, its high external surface area derived from terminal 12-membered ring cup-like pores is expected to provide an alternative result. In addition, we applied our synthesized Co/MCM-22 catalysts to a FT synthesis under supercritical conditions. It has been expected that, under these conditions, a quick diffusion of reactants and products, and an effective removal of reaction heat will result in an enhancement of the activity, selectivity and stability of the catalysts [18,19]. Interestingly, we found that MCM-22 supported Co catalysts exhibited superior performance to others, and gasolinerange products can be actively and selectively produced under the slurry-phase conditions. 2. Experimental 2.1. Catalyst preparation Co catalyst (20 wt.% Co) was prepared by incipient-wetness impregnation of aqueous solution of cobalt nitrate (Co(NO3)2 Æ 6H2O) on various supports. The supports used include silica gel (Fujisilycia Q-15), alumina (Sumitomo Chemical Co.), montmorillonite K10Ò (Aldrich), and acid-form zeolites, i.e., USY and ZSM-5 with a SiO2/ Al2O3 ratio of 33 and 38, respectively (Tosoh), and our synthesized MCM-22 with a SiO2/Al2O3 ratio of 30 and 50. After well mixing the support and the nitrate solution, the sample was dried in a rotary evaporator for 30 min. It was then calcined in a muffle furnace at 400 °C for 2 h. Hereafter, the catalyst samples were designated as Co/ XXX (YY), where XXX denotes the types of support and YY indicates the SiO2/Al2O3 ratio. MCM-22 was hydrothermally synthesized by modifying the procedures reported by Corma et al. [20]. Typically, hexamethyleneimine (HMI) was used as a structure-directing and fumed silica (Aerosil 200) and NaAlO2 were used as a silica and an alumina source. The final gel mixture had a molar composition of SiO2:0.033Al2O3:0.3NaOH: 0.9HMI:40H2O. After the hydrothermal treatment at 140 °C for 7 days under rotating conditions, the solid prod-

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uct was recovered by filtration, followed by washing and drying at 120 °C overnight. To obtain MCM-22, the dry solid was calcined at 540 °C for 8 h in a muffle furnace. Before being used as a support, the calcined MCM-22 was treated with 1 M NH4NO3 solution three times at 80 °C for 3 h. A proton-form MCM-22 was attained by subsequent calcination at 500 °C for 5 h. DeMCM-22 with the SiO2/Al2O3 ratio of 80 (DeMCM22 (80)) was prepared by dealumination of MCM-22 (30) in an acidic solution. Typical procedure is dispersion of the parent MCM-22 into 0.5 M HNO3 solution under vigorous stirring at 80 °C for 2 h. Solid-to-liquid ratio was 1:20. The dealuminated product was recovered by filtration, followed by washing. The procedure was repeated twice before subsequently drying at 120 °C overnight and calcining at 500 °C for 4 h in a muffle furnace. 2.2. Experiment apparatus and reaction procedure 2.2.1. Slurry-phase reactor system Schematic diagram of the experiment apparatus used in the present study is shown in Fig. 1. The FT synthesis was carried out in a 250-cm3 stainless steel autoclave. Mixture of H2 and CO with a H2/CO molar ratio of 2 (Sumitomo Chemical Co.) was used as syngas reactant. For a typical procedure, 1 g of Co catalyst was activated ex situ at 400 °C for 10 h in H2 gas with a flow rate of 100 ml min1. The reduced catalyst was then transferred to a mortar filled with 20-ml n-hexadecane solvent, where it was ground to fine particles. After transferring the slurry mixture into the autoclave followed by fixing it to a cover equipped with a motor-driven flat blade impeller, the reaction system was purged with the syngas. The flow rate of syngas was controlled and monitored using a Brooks 5850E mass flow controller. The operating conditions were T = 230 °C, P = 1.0 MPa, W/F = 5 gcat h mol1, and stirring speed was maintained at 800 rpm. Effluent gas passed through two consecutive traps to condense remaining liquid products. The effluent gas flow rate was measured with a wet gas flow meter. Products in the effluent gas were analyzed by three consecutive online gas chromatographs (GC). CO, CO2, CH4 were analyzed with GC equipped with a packed column and a thermal conductivity detector (TCD) while amount of other gaseous hydrocarbon products, C1–C5 and C6– C10 were determined with two different GCs equipped with a packed column and a flame ionization detector (FID). After the course of reaction, liquid products in the traps were mixed into the reaction mixture, where an internal standard was added. Subsequently, the product distribution was analyzed with a FID GC equipped with a capillary column (DB-2881). The yield of hydrocarbons was calculated on the basis of carbon number. 2.2.2. Supercritical phase reactor system Experiment apparatus and setup of the system were described thoroughly elsewhere [17]. Typically, in the present

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3. Results and discussion

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study, the reaction was carried out in one-stage fixed bed reactor using n-hexane (Kanto Kaguku) as a supercritical solvent. Before being loaded into the reactor, catalyst powder was pelletized and sieved to obtain 20–40 mesh particles. Reduction of the catalyst was performed in situ under H2 flow at 400 °C for 3 h. Then, n-hexane solvent and syngas were introduced at desired conditions: flow rate of nhexane = 1.42 ml/min, Ptotal = 4.5 MPa, Psyngas/Pn-hexane = 1.0/3.5, CO/H2 = 1/2, Tvaporizer = 240 °C. Effluent gaseous products were analyzed online with two consecutive GCs. The first one was equipped with a TCD detector for analysis of CO, CH4 and CO2. The other one was a FID GC equipped with an Al2O3-KCl capillary column for analysis of C1–C5 products. After the course of reaction, the liquid products were collected and analyzed with a FID GC in the same manner as the experiment under slurry-phase conditions.

CO Conversion/ %

Fig. 1. Schematic flow diagram of slurry-phase FT synthesis apparatuses.

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Reaction time/ h Fig. 2. Dependence of CO conversion (close symbols) and methane selectivity (open symbols) on reaction time in slurry-phase FT synthesis catalyzed by Co/SiO2 (r), Co/Al2O3 (j), Co/MONT (m), and Co/USY (d). Reaction conditions: temperature, 230 °C; pressure, 1.0 MPa; W/F, 5.0 g h mol1; catalyst, 1.0 g.

3.1. Influence of support types Fig. 2 shows change in the CO conversion and the methane selectivity with time over various Co catalysts under the slurry-phase conditions. The low initial CO conversion and methane selectivity were due to a low temperature of reaction slurry at the beginning. Co/SiO2 was an active catalyst, giving the CO conversion as high as 72% at the first hour. However, the conversion declined very fast with increasing reaction time. The methane selectivity over Co/SiO2 catalyst was relatively low, ca. 7.2%. After 5 h, the CO conversion and the methane selectivity became con-

stant at 52% and 7.3%, respectively. Ten-hour test indicated that the performance of Co/SiO2 is stable under the studied conditions. A similar stability was also observed for other catalysts used in the present work. Therefore, the experimental data up to 5 h of the reaction course is reported hereafter. Compared to the results from Co/SiO2, it can be seen that using acidic materials as support, Al2O3, montmorillonite (MONT), and USY, resulted in a relatively inactive catalyst as indicated by lower CO conversion. Although a similar severe drop of the conversion to Co/SiO2 catalyst

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Moreover, it was found that the final reaction mixture was colorless and not viscous. The control of hydrocarbon products in the gasoline range should be due to shape-selective properties of the supports [21]. As summarized in Table 1, USY and montmorillonite have mainly micropores with an average diam˚ , which are much smaller than the pore sizes eter of ca. 7 A of SiO2 and Al2O3 supports. However, both pore structure and acidity of zeolitic materials contributed to narrower product distribution in the consecutive dual fixed-bed reactor where the cracking reaction was conducted at relatively high temperature (300 °C) [16]. After we investigated the cracking performance of the zeolites under our studied conditions in the absence of syngas, it was found that a small amount of cracked products, mainly C4–C12, derived from n-hexadecane solvent was detected, corresponding to 5–10% product yields. These results ensured that, under the slurry-phase synthesis conditions, the carbon number distribution of products was essentially controlled by pore structure of the zeolite while its acidity should have a minor contribution. 3.2. Influence of topology of zeolitic supports Catalytic activity of cobalt on different zeolitic supports was revealed in Fig. 4. It can be seen that there was no

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was found over Co on zeolitic support (Co/USY), it was not the case for Co/Al2O3 and Co/MONT. At 5 h, the CO conversion attained from Co/MONT (56%) was higher than that from Co/Al2O3 (50%) and even Co/SiO2 (52%), indicating a high activity and stability of acid-form montmorillonite as a support of FT catalyst. The selectivity to methane over Co/Al2O3 was similar to that over Co/SiO2 whereas Co/MONT showed the selectivity 2% higher. A high selectivity to methane was observed when Co/USY was used, 14.7%. As a known effect, its low activity and high methane selectivity should be due to low reducibility of cobalt species formed on zeolitic framework with a large negative electrochemical potential [15]. Even though the present catalyst preparation method was based on the incipient-wetness impregnation, ion exchanging inside the zeolite channels may occur at a significant extent. Carbon number distribution in Fig. 3 indicates that Co/ SiO2 (Fig. 3a) and Co/Al2O3 (Fig. 3b) exhibited a broad spectrum of hydrocarbon products ranging from gas to light wax (C23–C30). Wax-like reaction mixture after the course of catalytic test implied a significant amount of heavier molecular weight hydrocarbons (>C30) formed. In contrast, the products with carbon number > 15 were significantly reduced over Co/MONT (Fig. 3c). This is very obvious when Co/USY catalyst was used (Fig. 3d); it showed a particular range of products from C5 to C12.

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Fig. 3. Carbon number distribution in slurry-phase FT synthesis catalyzed by Co/SiO2 (a), Co/Al2O3 (b), Co/MONT (c), and Co/USY (d). Reaction conditions: see Fig. 2.

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Table 1 Physicochemical properties of Co-based catalysts (20 wt.% Co) used in the present study Catalyst

SiO2/Al2O3a

Langmuir surfaceb area (m2 g1)

Average pore diameterb ˚) (A

Total pore volumeb (cm3 g1)

Co/SiO2 Co/Al2O3 Co/MONT Co/USY Co/ZSM-5 Co/MCM-22 (30) Co/MCM-22 (50) Co/DeMCM-22 (80) Co/NaMCM-22

– – n.d. 33 38 30 51 84 30

178 149 282 696 330 468 461 458 471

42 37 7.0 7.6 5.1 5.5 5.4 5.5 5.5

0.85 0.76 0.23 0.35 0.16 0.38 0.37 0.33 0.40

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n.d. means not determined. a Determined by ICP elemental analysis. b Determined by N2 adsorption measurement.

nounced. This result implied a fast deactivation of the active Co species. It was possibly due to carbon deposition on the acid sites of MCM-22 support as well as adjacent Co metals, rendering the adsorption of CO and H2 reactants on the active sites difficult. It is worth noting that black carbon deposit layer was found in the final reaction mixture. Compared to Co/SiO2, the product distribution was shifted to lower hydrocarbons over all of the zeolite-supported Co catalysts (Fig. 5b–d). This should be also attributed to the shape-selective control of the zeolitic pores. Although MCM-22 has a similar 10-membered ring zigzag channel structure to ZSM-5, Co/MCM-22 showed relatively sharp cut of products between C4 and C12. Concepcio´n et al. reported that using ITQ-2 and ITQ-6, sheet-like siliceous delaminated MWW and FER zeolites, respectively, as a support for Co catalyst enhanced the selectivity to heavier hydrocarbons, reflected by the chain growth probability (a) from the Anderson–Schulz–Flory (ASF) plots in C1–C20 range; a was 0.83 for the former and 0.85 for the latter [22]. These results were particularly derived from high surface area of delaminated zeolites which their structures are terminated by large pore opening. However, it seemed that another type of channel systems in MCM-22, cup-like 12-membered ring pore opening at the external surface, did not significantly promote the formation of heavier hydrocarbons. Therefore, MCM-22 zeolite has a suitable structure for using as a Co-catalyst support for the active and selective production of gasoline-range hydrocarbons through the slurry-phase FT synthesis.

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3.3. Influence of aluminum content in Co/MCM-22 catalysts

Fig. 4. Dependence of CO conversion (close symbols) and methane selectivity (open symbols) on reaction time in slurry-phase FT synthesis catalyzed by Co/SiO2 (r), Co/USY (j), Co/ZSM-5 (m), and Co/MCM22 (30) (d). Reaction conditions: see Fig. 2.

Since acidity of zeolite can be tuned by altering the SiO2/ Al2O3 ratio, the effect of aluminum content in MCM-22 support on the catalytic performance of the resulting cobalt catalyst was investigated. As shown in Fig. 6, with decreasing aluminum content (increasing the SiO2/Al2O3 ratio), the CO conversion increased obviously whereas the selectivity to methane decreased. These results revealed that Co species on the low aluminum-content MCM-22 support is more active than those on the high aluminum-content one. Moreover, it can be seen that Co/MCM-22 (50) and Co/DeMCM-22 (80) exhibited higher conversion than Co/SiO2 at longer reaction time, indicating the higher performance of MCM-22 supported Co catalysts. Fig. 7 illustrates carbon number distribution over different Co/ MCM-22 catalysts. A similar sharp cut in the gasoline product range was observed from all Co/MCM-22 catalysts (Fig. 7a–c), although there was a slight enhancement of heavier hydrocarbon formation when the FT reaction was catalyzed by Co/MCM-22 (50) and Co/DeMCM-22 (80). Many literatures suggested the strong effects of support on the size, reducibility and activity of Co species in the FT synthesis [23–26]. With increasing basicity or negative

correlation between size of pore opening (Table 1) and the CO conversion or the selectivity to methane (Fig. 4). Co/ ZSM-5 had low activity; the conversion was only 16% at 5 h. This is much lower than the conversion attained from Co/USY and Co/MCM-22. It may be ascribed to inactive Co species formed on ion-exchanged sites of the ZSM-5 zeolite [15]. On the other hand, Co/MCM-22 exhibited the most active catalyst, giving the conversion as high as 67% at the first hour. The CO conversion dropped to 45.5% within 5 h and maintained at 43% up to 10 h, which is similar to the conversion changed with time observed over Co/SiO2 catalyst. Among the zeolitic catalysts used, Co/MCM-22 showed the lowest selectivity to methane (10.5%), indicating that Co species on MCM-22 are more active than those on USY and ZSM-5 towards CO hydrogenation to higher hydrocarbon products. However, an increase in the methane selectivity with time over Co/MCM-22 was more pro-

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Fig. 5. Carbon number distribution in slurry-phase FT synthesis catalyzed by Co/SiO2 (a), Co/USY (b), Co/ZSM-5 (c), and Co/MCM-22 (30) (d). Reaction conditions: see Fig. 2.

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Reaction time/ h Fig. 6. Dependence of CO conversion (close symbols) and methane selectivity (open symbols) on reaction time in slurry-phase FT synthesis catalyzed by Co/SiO2 (r), Co/MCM-22 (30) (j), Co/MCM-22 (50) (m), Co/DeMCM-22 (80) (d), and Co/NaMCM-22 (30) ( ). Reaction conditions: see Fig. 2.



charge of the support framework, the size, reducibility and CO hydrogenation activity decreased. Correspondingly, one could expect that the Co oxide particle size and reducibility decreased in the order: Co/SiO2 > Co/DeMCM-22 (80) > Co/MCM-22 (50) > Co/MCM-22 (30). The large Co oxide particle is reduced to active Co metal more easily

than the small one, resulting in an active and selective production of long-chain hydrocarbons [27,28]. In other words, the smaller the Co particles, the shorter the hydrocarbon chain and the larger the amount of methane is produced. Therefore, our observed trends of the CO conversion and the selectivity to methane towards the SiO2/ Al2O3 ratio; the CO conversion decreased in the order: Co/DeMCM-22 (80) > Co/MCM-22 (50) > Co/MCM-22 (30) and the methane selectivity decreased in the order: Co/MCM-22 (30) > Co/MCM-22 (50) > Co/DeMCM-22 (80) > Co/SiO2, are in accordance with the previous reports. From our results, however, one may question the role of acid sites in the reaction since dealuminated MCM-22 supported Co catalyst showed the best performance. To clarify this, we applied a Co catalyst using Na-form MCM-22 with the SiO2/Al2O3 ratio of 30 as a support to the slurry-phase reaction, based on hypothesis that this support has no acidity and no activity in an acid-catalyzed reaction. As revealed in Fig. 6, the CO conversion over Co/NaMCM22 catalyst was lower than the catalysts using proton-form MCM-22 as support. Furthermore, the selectivity to methane increased around 2% compared to that from Co/ DeMCM-22 (80). The less active Co species in NaMCM22 may be attributed to higher negative charge of this support in the Na form than the H-form one, resulting in a strong Co–support interaction and low reducibility Co

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Fig. 7. Carbon number distribution in slurry-phase FT synthesis catalyzed by Co/MCM-22 (30) (a), Co/MCM-22 (50) (b), Co/DeMCM-22 (80) (c), and Co/NaMCM-22 (30) (d). Reaction conditions: see Fig. 2.

oxide [15]. However, Fig. 7d indicated a similar hydrocarbon distribution to other Co/MCM-22 catalysts. This result implied that the sharp cut of gasoline-range products was derived mainly from the pore channel structure of the support and the active Co species present. Here, we can see that a good Co/MCM-22 catalyst for the production of gasoline-range products in the slurryphase FT synthesis can be prepared through the incipient-wetness impregnation of cobalt nitrate precursor on to dealuminated MCM-22 support with the SiO2/Al2O3 ratio of 80. 3.4. Cþ 4 hydrocarbon species distribution The distribution of Cþ 4 hydrocarbon species is summarized in Fig. 8. It is well known that the main hydrocarbon species produced over Co/SiO2 and Co/Al2O3 catalysts are straight chain paraffins. However, some acidity in the Al2O3 support resulted in a slight increase in the selectivity to isoparaffin products. As shown in the literatures, the isoparaffins and olefins selectivity was improved remarkably when zeolites were applied as support, due to their defined pore structure and high acidity promoting the dehydrogenation and isomerization reactions [7,13,14,21,29], whereas Co/MONT exhibited a high selectivity to n-paraffins, and its selectivity to isoparaffins and olefins. This is not surpris-

ing since much lower acidity of H-form montmorillonite, compared to that of the zeolites, is known. Surprisingly, Co/ZSM-5 gave relatively low selectivity to isoparaffins and olefins compared to Co/USY although the former support is widely used in commercial bifunctional catalysts for isomerization reaction. This result may be related to an alternation of amount of acid sites by ion exchanging upon the impregnation of Co onto the zeolite supports. This does not only decrease the reducibility as well as catalytic activity of the resulting catalyst as mentioned above, but also reduce the number of acid sites in the support. Consequently, the yields of isomerized products are decreased. The Co catalysts using MCM-22 supports exhibited higher selectivity to isoparaffins and olefins than those using other zeolites (Fig. 8); the isoparaffins selectivity over Co/MCM-22 catalysts was 43–53% whereas Co/USY and Co/ZSM-5 showed the selectivity around 45.3% and 36%, respectively. With decreasing aluminum content, the selectivity to isoparaffins decreased while the olefin selectivity slightly increased. This result should be due to the decrease in the number of acid sites as well as a deactivation of Co metal by carbon deposition. Although Co/DeMCM-22 (80) showed 10% lower the selectivity to isoparaffins than other MCM-22 supported catalysts, it is considered as the best catalyst in our study due to its high activity in

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Fig. 8. Comparison of hydrocarbons species distribution in slurry-phase FT synthesis over various Co catalysts. Reaction conditions: see Fig. 2.

the CO hydrogenation (Fig. 6). Thus, we can see that MCM-22 is the most suitable zeolitic support among others for the preparation of Co-based FT catalyst for the production of isoparaffins. 3.5. Influence of reactor systems The CO conversion and the selectivity to methane over Co/DeMCM-22 (80) catalyst under supercritical phase conditions are revealed in Fig. 9. The conversion attained

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reached 100% up to 2 h after which decreased with time on stream to 51% at 5 h of the reaction course (Fig. 9). It is worth to note that the initial conversion in this case (100%) was much higher than that obtained in the presence of Co/SiO2 catalyst (60%) under the similar conditions [17], ensuring very high CO hydrogenation activity of Co/ DeMCM-22. When compared to the slurry-phase system (Fig. 6), the initial conversion in the case of supercritical conditions was much higher. This should be due to the supercritical regime with a relative high temperature, fast removal of hydrocarbon products from catalyst particles,

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Time on stream/ h Fig. 9. Dependence of CO conversion (close symbols) and methane selectivity (open symbols) on reaction time in FT synthesis catalyzed by Co/DeMCM-22 (80) under slurry-phase (r) and supercritical phase conditions (d). Reaction conditions: for slurry phase see Fig. 2, and for supercritical phase see Fig. 10.

0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Carbon number Fig. 10. Carbon number distribution in supercritical phase FT synthesis catalyzed by Co/DeMCM-22 (80). Reaction conditions: temperature, 240 °C; pressure, 4.5 MPa; W/F, 5.0 g h mol1; catalyst, 1.0 g.

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100% 90%

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Supercritical phase

Fig. 11. Comparison of hydrocarbons species distribution in slurry-phase and supercritical phase FT syntheses over Co/DeMCM-22 (80) catalysts. Reaction conditions: for slurry phase see Fig. 2, and for supercritical phase see Fig. 10.

and using of relatively small molecule solvent, promoting the reaction rate and diffusion of reactant gases and hydrocarbons produced [19]. The carbon number distribution shown in Fig. 10 revealed that, compared to the slurry phase (Fig. 7c), the distribution was shifted to light gaseous products, C2–C6 hydrocarbons, implying an enhancement of cracking activity in the supercritical system. Moreover, the investigation of the selectivity to Cþ 4 hydrocarbon species indicated that isoparaffins, olefins and n-paraffins produced under supercritical conditions were 22.7%, 25.6% and 51.7%, respectively (Fig. 11). These results are in contrast to the previous observation over hybrid Co/SiO2 and Pd/b catalyst; the main products were in the gasoline range and the Cþ 4 isoparaffin selectivity was 60–80% [17]. It is likely that, when Co is directly loaded on the zeolite, hydrocarbons produced are more easily cracked to small ones with high olefin content than in the case of using the hybrid catalysts [17], because the sites for hydrocarbon formation and the acid sites are present in a closer environment. Although relatively high selectivity to olefins under supercritical conditions can be expected [19], with the presence of zeolite the isoparaffins selectivity should be more enhanced than under slurry-phase conditions. It seems that cracking in the supercritical system is more severe than that in the slurry-phase system due to the aforementioned advantages of supercritical conditions which probably maintain the life of strong acid sites longer. Fernandes et al. suggested that the acid strength required for isomerization is lower than that for cracking [30]. Consequently, the methane and small olefin product selectivity was high as described above. The olefins themselves could also deactivate the active Co metal, rendering the hydrogenation activity and the CO conversion in the supercritical system

(Fig. 9) dropped very fast. To mitigate the carbon deposition and enhance the isoparaffin selectivity, the hydrogenation performance of the catalyst has to be improved, for example, by an introduction of noble metals. Therefore, we can see that the slurry-phase conditions are more suitable for the active and selective production of gasoline-range hydrocarbons over MCM-22 supported Co-based catalysts. 4. Conclusions The catalytic performance of Co-based catalysts, using various aluminosilicate materials as support, in the Fischer–Tropsch synthesis has been studied under slurryphase conditions at 1.0 MPa and 230 °C. Using the microporous acidic supports retarded the formation of heavier hydrocarbons and the product distribution was shifted to lower carbon numbers, as a result of their micropore confinement limiting the extension of hydrocarbon chain. Co/ MONT was a good catalyst, showing relatively high activity compared to Co/SiO2 and Co/Al2O3. However, the selectivity to methane was increased, possibly as a result of the Co species modified by negative charge of the oxide support framework. The zeolitic supports rendered the hydrocarbon distribution shifted to gasoline range. Moreover, the selectivity to isoparaffins increased remarkably due to the known effect of micropore structures and acidity which are suitable for isomerization reaction. MCM-22 with a unique channel structure was the most appropriate zeolitic support, giving very active and selective bifunctional Co catalyst for the CO hydrogenation to gasoline products. With reducing aluminum content, its activity increased while the selectivity to methane and isoparaffins decreased

C. Ngamcharussrivichai et al. / Fuel 86 (2007) 50–59

gradually. Co/DeMCM-22 with the SiO2/Al2O3 ratio of 80 was the most active and selective catalyst, yielding the CO conversion as high as 60% at 5 h with the isoparaffin selectivity of 43%. Although the supercritical conditions promote the high initial CO hydrogenation, more isoparaffins and gasolinerange products were attained in the slurry-phase system. It is likely that cracking was more pronounced under the supercritical conditions. Therefore, our study showed that the active and selective production of gasoline-range hydrocarbons with high isoparaffin selectivity can be achieved over MCM-22 supported Co-based catalysts under the slurry-phase conditions. Acknowledgements The authors are grateful for the financial support from OECF-JBIC under Thailand–Japan Technology Transfer Project (TJTTP) and Grants for Development of New Faculty Staff, Chulalongkorn University. Cooperation from Dr. Wensheng Linghu and Dr. Zhong-Wen Liu is acknowledged. References [1] Weyda H, Kohler E. Catal Today 2003;81:51–5. [2] Tsubaki N, Yoneyama Y, Michiki K, Fujimoto K. Catal Commun 2003;4:108–11. [3] Matsuda T, Sakagami H, Takahashi N. Catal Today 2003;81:31–42. [4] Vannice ME. Cat Rev Sci Eng 1976;14:153–61.

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