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Ind. Eng. Chem. Res. 2010, 49, 9004–9011
Co/Pillared Clay Bifunctional Catalyst for Controlling the Product Distribution of Fischer-Tropsch Synthesis Qing-Qing Hao,†,‡ Guang-Wei Wang,†,‡ Zhao-Tie Liu,†,‡ Jian Lu,‡ and Zhong-Wen Liu*,†,‡ Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal UniVersity), MOE, Xi’an 710062, China, and School of Chemistry & Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, China
Pillared interlayer clays (PILCs) were synthesized by pillaring montmorillonite with SiO2, Al2O3, and ZrO2, and Co-supported PILCs were prepared by the impregnation method. For comparison, Co/SiO2 and Cosupported montmorillonite treated with dilute HNO3 (acid-clay) were also prepared. The materials were characterized by XRD, H2-TPR, NH3-TPD, O2 titration, and N2 adsorption-desorption. The Co-supported catalysts were comparatively investigated for Fischer-Tropsch (FT) synthesis in a fixed-bed reactor under the conditions of 1 MPa, 508 K, H2/CO ) 2, and W/F ) 5.02 g · h · mol-1. The results indicate that the activities of the catalysts (i.e., CO conversions) increase in the order of Co/Na-Clay , Co/Si-PILC ≈ Co/ acid-clay ≈ Co/Al-PILC < Co/Zr-PILC ≈ Co/SiO2. Moreover, Co/PILCs can effectively narrow the product distribution of FT synthesis showing significantly increased C5-C12 selectivity and much decreased C21+ selectivity. The results are well explained based on the acidic properties of the PILCs together with the textural properties, pore size distributions, and reduction behaviors of the catalysts. 1. Introduction Fischer-Tropsch (FT) synthesis is an effective process for producing clean fuels and high-value-added fine chemicals from syngas originating from coal, natural gas, and biomass. Presently, the FT synthesis process is receiving increasing worldwide attention because of the high demand for a decreased dependence on nonrenewable petroleum and increased public concern about the environment.1,2 However, because the reaction is limited by Anderson-Schulz-Flory (ASF) polymerization kinetics, the product of FT synthesis is a very complicated mixture, composed mainly of normal paraffins with varied carbon numbers and nonselective to any specific product.1–3 Thus, controlling the product distribution is crucial for the selective synthesis of clean fuels through the FT route. On the basis of the commonly accepted polymerization mechanism of FT synthesis, three strategies have been investigated for selectively controlling the FT product distribution. The first is to end the chain growth of the adsorbed surface hydrocarbon species by controlling the size of the FT active metals4 or developing new processes such as in a supercritical media.5 However, the product distribution cannot be controlled efficiently even though this method really works. Moreover, the isomer composition of the hydrocarbons cannot be changed because the catalytic component for isomerization is not present. The second strategy is to conduct FT synthesis in one reactor, followed by a separate unit for cracking the long-chain FT hydrocarbons. Indeed, by loading zeolite or metal/zeolite in the downstream reactor, both the product distribution and the isomer composition of the hydrocarbons can be significantly changed.6,7 Moreover, the operating conditions for each reactor can be independently regulated. For a detailed analysis of the merits of this technology, see ref 6. However, the main shortcomings of this second strategy are the complex reactor system and the difficult delivery of the FT wax to the cracking reactor. Thus, * To whom correspondence should be addressed. Tel.: +86-29-85303200. Fax: +86-29-8530-7774. E-mail:
[email protected]. † Key Laboratory of Applied Surface and Colloid Chemistry. ‡ School of Chemistry & Materials Science.
it is more desirable if the FT catalyst and the cracking/ isomerization component are integrated into one reactor, as done in the third strategy. In this case, bifunctional catalysts, namely, combinations of FT-active metals with solid acids for cracking and isomerizing the FT hydrocarbons, have been extensively studied.6–13 The solid acids investigated have mainly been zeolites,6–9,11–13 although use of superacids such as SO42-/ZrO2 has also been reported.10 Moreover, combinations of FT-active metals with zeolites have been obtained in different ways, such as direct deposition of Co on or in zeolite,8,9 physical mixing of FT catalyst and zeolite,6,7,11 and application of a zeolite membrane coating on Co/Al2O3.12 This third strategy has proven to be very effective, and the traditional FT product distribution can be significantly changed with much increased selectivity to liquid fuels. However, in addition to problems with the individual methods of combining the zeolite and FT catalyst discussed in refs 13 and 14, the main challenges for the further progress of this approach are (1) the mismatch between the optimal operating temperatures for FT synthesis and cracking/ isomerization reactions and (2) the instability of the composite catalyst under reaction conditions as a result of the migration of different species, as observed in our previous works.13,15 For FT synthesis over Ru/CNT (carbon nanotube), the spatial confinement effect on the nanoscale and acidic functional groups on the CNTs are reported to be the main factors for a high selectivity to liquid hydrocarbons.16 Thus, a solid acid with an adjustable acidity and an easily tailored structure is desirable for selectively synthesizing liquid fuels via FT synthesis. As one of the most important classes of layered materials, pillared interlayer clays (PILCs) have potentially wide applications in the areas of adsorption and catalysis.17–20 The introduction of oxide pillars into the interlayer space results in a significant increase in surface area, thermal stability, and microporosity. More importantly, by changing the pillar oxide and raw clay, the acidity, layer space, and pore size distribution of the PILCs can be regulated over relatively wide ranges.20 These properties make PILCs promising catalyst components for FT synthesis, provided that the clays are purposefully tailored. Indeed, several preliminary investigations using PILCs
10.1021/ie101163w 2010 American Chemical Society Published on Web 08/30/2010
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 21,22
as supports show promising results for FT synthesis. Sapag et al. investigated the FT performance over Co supported on a Spanish montmorillonite pillared with Al2O3, in which a clear effect of the pillaring procedure on the activity and product selectivity was observed.21 Results from Su et al. indicated that the addition of ceria as a promoter or copillar can change the activities and selectivities for methane and C5+ hydrocarbons over Co-supported montmorillonite pillared with Al2O3.22 Based on the possible nanospatial confinement effect and the adjustable acidity of PILCs, this work aims at studying the behavior of PILCs as a catalytic component for controlling the product distribution of FT synthesis. Both cobalt and iron are industrially important catalysts for FT synthesis.1,2,23 Considering the higher productivity and lower oxygenate byproduct generation indicated in refs 1 and 2, in the present work, cobalt was used as the FT-active metal. The results indicate that the Co-supported clays give clearly increased selectivity to liquid fuels (C5-C20), which depends strongly on the treatment methods including pillaring oxides, namely, SiO2, Al2O3, and ZrO2. The experimental results are well explained based on characterization by X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), and NH3 temperatureprogrammed desorption (NH3-TPD). 2. Experimental Section 2.1. Preparation of Catalysts. 2.1.1. Synthesis of PILCs. Purified Na-type clay (Na-clay) with the montmorillonite structure (provided by Zhejiang Fenghong Clay Chemicals Co., Ltd., Zhejiang, China) was used directly without further purification. To increase its acidity, 2 wt % Na-clay was dispersed in 1 wt % nitric acid solution and aged at 323 K for 4 h. After centrifugation, washing until neutral pH, and drying at 333 K, acid-clay was obtained. To prepare the pillaring solution of zirconia, a procedure similar to that used in ref 24 was employed. Specifically, ZrOCl2 aqueous solution (0.1 mol/L) was hydrolyzed at 333 K for 12 h and aged for 12 h at the same temperature. Subsequently, Zrpillaring agent with a Zr4+/clay ratio of 10 mmol/g was added dropwise into a 2 wt % suspension of the Na-clay. The final suspension was stirred at 333 K for 6 h and further aged for 24 h. After this, the suspension was centrifuged, washed with deionized water until no chloride could be detected, and dried at 333 K. After calcination at 773 K for 4 h, zirconia-pillared clay (Zr-PILC) was obtained. The pillaring solution of alumina was prepared by using AlCl3 as the pillar precursor. To a 0.20 mol/L AlCl3 aqueous solution was added a 0.11 mol/L Na2CO3 solution, slowly and under vigorous stirring at 333 K, until an OH-/Al3+ molar ratio of 2.2 was reached (pH 4.1). The alumina polycation was obtained after aging at 333 K for 24 h. The silica pillaring solution was formed by slowly hydrolyzing (3-aminopropyl)trimethoxysilane (APTMS, 10 mol/L ethanol solution) with 5 mol/L HCl solution at pH 1 and room temperature. Following the same procedure as used for preparing Zr-PILC, Al-PILC and Si-PILC were obtained. To completely remove the organic compounds, a higher calcination temperature of 873 K was used for Si-PILC. 2.1.2. Catalyst Preparation. Co/PILCs catalysts with a metallic cobalt loading of 20 wt % were prepared by the incipient impregnation method. Cobalt nitrate [Co(NO3)2 · 6H2O, 99.0%] was used as the cobalt precursor. For comparison purposes, 20 wt % Co/SiO2 was prepared using silica gel (Fujisilicia Q-15) as the support. The catalysts were dried at 393 K for 12 h and calcined in air at 473 K for 2 h.
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2.2. Characterization Techniques. N2 adsorption-desorption isotherms for the PILCs were measured with a Micromeritics ASAP 2020 instrument at 77 K. Prior to analysis, the samples (ca. 150 mg) were degassed at 623 K for 8 h. The size distribution of micropores was determined using the HorvarthKawazoe (H-K) equation. The t-plot method was applied to calculate the surface areas and volumes of micropores. XRD patterns were obtained at room temperature on a Rigaku D/Max-3c X-ray diffractometer using monochromatized Cu/KR radiation (40 kV, 40 mA). The crystal size of the Co3O4 over the calcined catalysts was estimated from the Scherrer formula and the 440 diffraction (2θ ) 65.2°). The crystal size of the metallic cobalt in the reduced catalysts was estimated according to the relation d(Co0) ) 0.75d(Co3O4), as explained in ref 25. The reduction behavior of the supported cobalt phases was studied by H2-TPR using a Micromeritics Autochem 2920 instrument. First, 50.0 ( 0.5 mg of sample was loaded and flushed with an Ar flow at room temperature for 30 min. After the flow had been switched to 10 vol % H2 in Ar, TPR was started at a heating rate of 10 K/min up to 1173 K. A downstream 2-propanol/liquid-N2 trap was used to retain the water generated during the reduction. The H2 consumption rate was monitored with a thermal conductivity detector (TCD) previously calibrated using the reduction of CuO as a reference. The extent of reduction for cobalt oxide over fresh catalyst was determined by O2 titration method. First, about 10 mg of catalyst was reduced at 673 K for 4 h under a flow of highpurity hydrogen. After the system had been flushed with highpurity Ar at 673 K, a series of pulses of 3 vol. % O2 in Ar were injected to oxidize the reduced catalyst. Based on the oxygen consumed, the extent of reduction of the catalyst was estimated by assuming that metallic Co was converted to Co3O4 during the oxygen pulses. The NH3-TPD of different PILCs was measured with a Micromeritics Autochem 2920 instrument. For each test, 100.0 ( 4.0 mg of sample was used. After the system had been flushed with a He flow at 773 K for 60 min, NH3 adsorption was started at 393 K for 30 min. Subsequently, the system was purged for 2 h in a flow of helium at the same temperature. Finally, NH3TPD was carried out from 393 to 773 K at a heating rate of 10 K/min under a helium flow of 30 mL/min. 2.3. FT Reaction. The catalytic tests were carried out in a stainless steel fixed-bed reactor (i.d. ) 10 mm). Before reaction, 0.5 g of each catalyst (40-60 mesh) diluted with quartz sands was reduced at atmospheric pressure in a flow of pure H2 (50 cm3/min) at 673 K for 4 h. After reduction, syngas (H2/CO ) 2, 4% Ar as an internal standard) was applied, and the FT reaction was started with the desired reaction parameters. To prevent condensation of the products, the line between the outlet of the reactor and the inlet of the gas chromatography (GC) column was heated at or above 453 K. The effluent hydrocarbons were analyzed by online GC (GC-9560, Shanghai Huaai Chromatographic Analysis Co., Ltd., Shanghai, China) equipped with an HP-PONA capillary column (0.20 mm ×50 m, 0.5 µm) and a flame ionization detector (FID). Then, the CO, CH4, and CO2 in the effluent after cooling in an ice-water trap were online analyzed with a packed activated-carbon column and a TCD detector. The selectivity for hydrocarbons was calculated on the basis of carbon number. 3. Results and Discussion 3.1. Textural and Structural Properties. The XRD patterns of the clay materials are shown in Figure 1. The d001 spacings of Na-clay and acid-clay are exactly the same (0.96 nm),
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Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Table 1. Crystal Sizes and Extents of Reduction of Cobalt over Different Catalysts XRD
oxygen titration
O2 uptake extent of d(Co3O4) (nm) d(Co0) (nm) (mmol · g-1) reduction (%) Co/SiO2 Co/Na-clay Co/acid-clay Co/Al-PILC Co/Si-PILC Co/Zr-PILC
Figure 1. XRD patterns of (a) Na-clay, (b) acid-clay, (c) Si-PILC, (d) AlPILC, and (e) Zr-PILC.
22.50 23.04 21.46 21.66 23.33 21.84
16.88 17.28 16.10 16.25 17.49 16.38
1.59 1.22 1.41 1.38 1.39 1.49
70.29 53.77 62.64 60.96 61.63 65.79
crystal size of Co3O4 over the catalysts was quite similar (about 22 nm), indicating that part of the Co3O4 is outside the clay interlayer. The N2 adsorption-desorption isotherms of the clays are displayed in Figure 3. Irrespective of treatment method, each of the clays shows a type IV isotherm, together with an H4type hysteresis loop based on the IUPAC classification,26 which characterizes the slit-shaped open-pore structure. The clear increase in the amount of N2 adsorbed with increasing relative pressure (P/P0) at about 0.5 indicates the presence of mesopores. With the increase of P/P0 in lower ranges ( ZrPILC > Al-PILC > acid-clay > SiO2 > Na-clay. Thus, the amount of acidic sites of Na-clay can be largely adjusted by acidic treatment or pillaring with different oxides, even though the parent material contains essentially no acidic sites. Furthermore, the very broad peak for acid-clay indicates a uniform distribution of acidic sites with different strengths. Importantly, the similar TPD profiles and the identical peak temperatures (about 483 K) for the PILCs suggest that the distributions of acid strength are quite similar. 3.3. Reduction Behavior of the Catalysts. The H2-TPR profiles of the Co-supported catalysts are shown in Figure 7. The typical two-step reduction of Co3O4 to metallic Co via CoO is observed for Co/SiO2 catalyst, as indicated by the two poorly separated peaks at 573 and 613 K. However, different reduction behaviors are seen for the clay-based catalysts. In the cases of Co/Na-clay and Co/acid-clay, the two low-temperature peaks
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Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Table 3. Product Distributions of FT Synthesis over Different Catalysts* hydrocarbon distribution (%)
catalyst
CO2 selectivity (%)
C1
C2-C4
C5-C12
C13-C20
C21+
Co/SiO2 Co/Na-clay Co/acid-clay Co/Al-PILC Co/Si-PILC Co/Zr-PILC
2.50 3.22 2.30 2.56 1.99
12.30 23.53 25.17 16.41 17.24 19.20
6.60 18.96 12.80 10.05 12.01 12.30
29.70 36.97 36.86 39.50 42.00
11.70 13.96 14.26 15.14 14.40
39.70 11.10 22.42 16.11 12.10
* Operating conditions: W/F ) 5.05 g · h · mol-1, P ) 1 MPa, T ) 508 K, TOS ) 10 h.
Figure 8. Time-on-stream CO conversions over (a) Co/Na-clay, (b) Co/ Si-PILC, (c) Co/Al-PILC, (d) Co/acid-clay, (e) Co/SiO2, and (f) Co/ZrPILC.
can be assigned to the reduction to CoO of Co3O4 with larger and smaller particle sizes, whereas the broad peak at about 673 K is for the reduction of CoO to metallic Co. This indicates that the reduction of part of the CoO is inhibited due to the layer structure of the clay. Comparing the reduction behaviors of Co/Si-PILC and Co/SiO2, the most important observation is the long tail of the high-temperature peak over Co/Si-PILC. From these results, we believe that the high-temperature reduction part of the TPR pattern might be from the reduction of the cobalt oxides located between the clay layers. In this case, the electric field imposed by the clay interlayer retards the reduction of the cobalt oxides, which is similar to the difficult reduction of cobalt entrapped in zeolite cages.27 The reduction behavior of Co/Zr-PILC is quite similar to that of Co/Al-PILC. In addition to the electric field effect induced by the clay layers, the interaction of cobalt with zirconia or alumina might also contribute partially to the high-temperature reduction peak. To determine the extent of reduction of cobalt, oxygen titration was performed, and the results are summarized in Table 1. The extent of reduction was calculated assuming that metallic Co was reoxidized to Co3O4 during the oxygen pulses. It is clear that the extent of reduction of cobalt for clay-based catalysts is lower than that for Co/SiO2. This is in agreement with the TPR results and can be explained as resulting from the electric field effect induced from the clay interlayer. It should be noted that the extent of reduction of cobalt for Co/Na-clay is obviously lower than the extents for the remaining clay-based catalysts, which can be attributed to the inhibiting effect of sodium ions on the reduction of cobalt as reported in refs 28 and 29. The slightly higher extent of reduction of cobalt for Co/Zr-PILC might be because zirconia promotes the reduction of cobalt.30,31 Based on the calculation of d(Co0) ) 0.75d(Co3O4),25 as shown in Table 1, the estimated crystal sizes of metallic Co over all of the catalysts are quite similar (about 17 nm). 3.4. FT Performance. From the time-on-stream (TOS) catalytic activity results shown in Figure 8, the steady state can be approached at a TOS of about 6 h. The CO conversion over Co/SiO2 for a typical FT synthesis is used as a basis for comparison. The steady CO conversion over Co/Na-clay is very low. However, the CO conversion over Co/acid-clay is significantly increased. This is understandable based on the characterization results. Because the surface area and pore volume of Na-clay are very low, large particles of cobalt are formed after
reduction, as indicated by the TPR and oxygen titration results. The very low CO conversion over Co/Na-clay can thus be explained by the fact that larger Co particles are less active for FT reaction. Although the reduction behavior of Co/acid-clay is quite similar to that of Co/Na-clay, the significantly increased surface area and number of micropores improve the dispersion of cobalt over acid-clay and inhibit the formation of very large cobalt particles. Thus, CO conversion over Co/acid-clay is significantly increased. One more aspect that should be considered is the sodium ion contained in all of the clays because sodium is a poison of Co for FT reaction.28,29 Results reported by Borg et al. indicate that the FT site-time yield over γ-alumina-supported cobalt catalysts decreased obviously with increasing sodium content in the range of 20-113 ppm.28 Because acid-clay contains less Na+ as a result of the substitution of H+ for Na+ during acid treatment, the much higher CO conversion over Co/acid-clay than over Co/Na-clay can also be attributed to this phenomenon, namely, the poisoning effect of Na+ on Co for FT reaction. In the case of Co/PILC catalysts, Co/Si-PILC and Co/Al-PILC show almost the same CO conversions, similar to that obtained over Co/acid-clay. This can be explained provided that the poisoning effect of Na+ on Co for the FT reaction is taken into account, in which Na+ content is significantly decreased as a result of the ionexchanging of Na+ with the precursor of the oxide pillar. The clearly higher CO conversion over Co/Zr-PILC can be ascribed to the promotional effect of ZrO2, as reported in many works.30,31 The steady results of the product distribution are summarized in Table 3. Quite similar CO2 selectivities of less than 4% were obtained over all of the catalysts investigated because of the low catalytic activity of cobalt for the water-gas shift reaction. It is worth noting that the products over Co/Na-clay cannot be accurately analyzed because of the very low CO conversion. In the case of methane, all of the clay-based catalysts give higher methane selectivities than the Co/SiO2 catalyst. Moreover, Co/ acid-clay shows the highest methane selectivity, whereas the Co/PILC catalysts give quite similar methane selectivities. Based on the commonly recognized carbonium mechanism,32 methane formation is not favored for the catalytic cracking of long-chain hydrocarbons over solid acids. Moreover, negligible changes in methane selectivity have been observed over physical mixture of Co/SiO2 and Pd/β zeolite,33 in which case FT hydrocarbons are significantly cracked. Considering the lower reaction temperature of 508 K, thus, methane contributed from the cracking of the long-chain FT hydrocarbons should be insignificant. It has been reported that methane selectivity during FT synthesis clearly decreases with increasing extent of reduction of cobalt.34 The very high methane selectivity over Co/Na-clay might be due to this factor because this catalyst has the lowest extent of reduction of cobalt. Moreover, the similar methane selectivities over Co/PILC catalysts are in agreement with their equivalent extents of reduction of cobalt. The slightly different methane
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Figure 9. Carbon number distributions of C2-C12 hydrocarbons obtained from FT synthesis over (a) Co/SiO2, (b) Co/acid-clay, (c) Co/Al-PILC, (d) Co/ Si-PILC, and (e) Co/Zr-PILC.
selectivities over the specific Co/PILC catalysts (i.e., Co/SiPILC, Co/Al-PILC, and Co/Zr-PILC) might be caused by the promotional effect of the pillaring oxides of silica, alumina, and zirconia. Because the clay-based catalysts generally show higher methane selectivities than Co/SiO2, one additional aspect that should be examined is the textural properties of the clays. Higher methane selectivity is generally observed over the FT catalyst with smaller pores,34 which can be explained as the increased
effective H2/CO ratio as a result of the higher diffusivity of H2 than CO.35 Indeed, after acid treatment or pillaring with oxides, significantly rich micropores are created over acid-clay and PILCs, as indicated by the greatly increased micropore surface area (Table 2). Thus, the higher methane selectivities over claybased catalysts can also be explained as the confinement effect of micropores, that is, the diffusion limitation of CO. It should be emphasized that the two factors, namely, the reduction degree
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of cobalt and the confinement effect of micropores, might contribute differently for the different clay-based catalysts because no single factor can explain all of the results for methane selectivity (Table 3) well. After acid treatment or pillaring, acidic sites are created over the clay as discussed in section 3.2. It is commonly known that acidic sites can catalyze the cracking and isomerization reactions of hydrocarbons. Moreover, clays pillared with various oxides have been investigated as solid acids for the cracking and/or isomerization of various feedstocks.18,19,24,36–38 To determine the extents of cracking and isomerization reactions, detailed carbon number distributions of C2-C12 hydrocarbons were determined, and the results are given in Figure 9. Following the characteristics of the FT products over cobalt-based catalysts, Co/SiO2 produces mainly straight hydrocarbons, and the selectivities for isoparaffins are very low irrespective of the carbon number (Figure 9a). In comparison with the results over Co/ SiO2, the prominent observation over Co/acid-clay is a shift of the product distribution to light ends, that is, significantly increased selectivities for C2-C8 hydrocarbons. Together with the moderate increase of C13-C20 selectivities and the significant decrease of C21+ selectivity (Table 3), these results indicate that cracking of the long-chain FT hydrocarbons actually occurs over the acidic sites of acid-clay. If the isomers are examined (Figure 9b), a significant increase in the selectivities for C4-C10 normal alkanes is found, accompanied by a minor increase in the selectivities for C4-C10 isoparaffins and C4-C8 alkenes. This indicates that, over acid-clay, the cracking of long-chain FT hydrocarbons (i.e., C21+) is much more favorable than the isomerization of the normal alkanes produced by FT synthesis. Moreover, as a result of the asymmetric increase in the selectivities for C4-C10 normal alkanes and C4-C8 alkenes, the cracking of long-chain FT hydrocarbons mainly occurs through hydrocracking, although catalytic cracking cannot be totally excluded. In the case of Co/PILCs (Figure 9c-e), the results are very similar to those obtained over Co/acid-clay. Again, hydrocracking of the long-chain FT hydrocarbons is more significant than isomerization of the alkanes. These observations are consistent with the hydroconversion results of normal heptane over 1 wt % Pt supported on montmorillonite pillared with alumina and silica,39 in which isomerization is significant only above 550 K. Thus, the low degrees of isomerization over clay-based catalysts can be explained as a result of the low reaction temperature (508 K) and lower acidity of the clay in comparison with zeolites. However, a noticeable increase in the selectivities for C9-C12 and a clear decrease for C2 and C3 hydrocarbons are found. Additionally, a more even distribution of C4-C9 hydrocarbons can also be observed. Considering the clear differences in acidic properties between acid-clay and PILCs (Figure 6), these differences for the distributions of C2-C12 hydrocarbons over Co/acid-clay and Co/PILCs cannot originate solely from the variation of the acidic properties of the clays. By examining the pore properties of acid-clay and PILCs such as Al-PILC (Table 2), one can see that the micropore volume for Al-PILC is much higher than that for acid-clay. Thus, the confinement of micropores for the migration or diffusion of the FT hydrocarbons to the acidic sites over Co/ PILCs might be more severe than that over Co/acid-clay, leading to the similar C2-C12 patterns over Co-supported acid-clay and PILCs. It should be emphasized that the decrease of the intrinsic chain-growth probability of FT synthesis induced by the pore structure of the clays might also be questioned as one of the reasons for the observed phenomena. However, the much lower micropore volume over acid-clay than over PILCs but the similar
C2-C12 patterns over Co-supported acid-clay and PILCs are contradictory to this hypothesis. This indicates that the decrease of the chain-growth probability of FT synthesis induced by the pore structure of the clays plays a less important role for shifting the product distribution of the FT synthesis if it happens. Thus, the hydrocracking of the long-chain FT hydrocarbons is mainly responsible for the significantly increased selectivities for liquid fuels over clay-based catalysts. In summary, based on three factors, namely, the acidic properties, the textural properties, and the extent of reduction of cobalt, the catalytic activity and product distribution over the investigated catalyst for FT synthesis can be well explained. 4. Conclusions Co-supported PILCs are promising catalysts for FischerTropsch synthesis with narrowed product distributions. Moreover, both the CO conversion and product distribution can be adjusted by changing the pillar oxide. Based on the correlation of the reaction results and the characterization results of XRD, H2-TPR, NH3-TPD, O2 titration, and N2 adsorption-desorption, the acidic properties, the microporous structure of the PILCs, and the extent of reduction of cobalt were determined to be main factors influencing the CO conversion and product distribution over the Co/PILC catalysts. Because of the effect of the electric field in the interlayer of the clay, the reduction of the cobalt species located between the clay interlayer is inhibited. Among the catalysts investigated, 20 wt % Co/Zr-PILC shows CO conversion comparable to that of 20 wt % Co/SiO2. However, the selectivity for liquid fuels (C5-C21) is significantly increased, whereas the selectivity for C21+ wax is largely inhibited over clay-based catalysts as a result of the hydrocracking of long-chain FT hydrocarbons. The slightly increased methane selectivity over the Co/PILC catalysts is mainly caused by the lower degree of reduction of cobalt and the diffusional limitation of CO induced from the microporous properties of the PILCs. Acknowledgment The authors acknowledge the financial support of this work provided by the National Natural Science Foundation of China (20876095). Partial financial support from the Natural Science Foundation of Shaanxi Province (2007B12) and the SRF for Returned Overseas Chinese Scholars by MOE is also acknowledged. Literature Cited (1) Dry, M. E. The Fischer-Tropsch Process: 1950-2000. Catal. Today 2002, 71, 227. (2) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer-Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. ReV. 2007, 107, 1692. (3) Sethuraman, R.; Bakhshi, N. N.; Katikaneni, S. P.; Idem, R. O. Production of C4 Hydrocarbons from Fischer-Tropsch Synthesis in a Follow Bed Reactor Consisting of Co-Ni-ZrO2 and Sulfated-ZrO2 Catalyst Beds. Fuel Process. Technol. 2001, 73, 197. (4) Yang, Y.; Pen, S.; Zhong, B. A New Product Distribution Formulation for Fischer-Tropsch Synthesis. Effect of Metal Crystallite Size Distribution. Catal. Lett. 1992, 16, 351. (5) Tsubaki, N.; Fujimoto, K. Product Control in Fischer-Tropsch Synthesis. Fuel Process. Technol. 2000, 62, 173. (6) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. High performance pd/ beta catalyst for the production of gasoline-range iso-paraffins via a modified Fischer-Tropsch reaction. Appl. Catal. A: Gen. 2006, 300, 162. (7) Li, X.; Luo, M.; Asami, K. Direct synthesis of middle iso-paraffins from synthesis gas on hybrid catalysts. Catal. Today 2004, 89, 439.
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ReceiVed for reView February 5, 2010 ReVised manuscript receiVed August 9, 2010 Accepted August 18, 2010 IE101163W
