Energy Fuels 2010, 24, 4099–4110 Published on Web 07/01/2010
: DOI:10.1021/ef1003107
Effect of Properties of Various Activated-Carbon Supports and Supported Fe-Mo-Cu-K Catalysts on Metal Precursor Distribution, Metal Reduction, and Fischer-Tropsch Synthesis Wenping Ma, Edwin L. Kugler, and Dady B. Dadyburjor* Department of Chemical Engineering, West Virginia University (WVU), Morgantown, West Virginia 26506 Received March 18, 2010. Revised Manuscript Received June 11, 2010
The physical and surface chemistry properties of four activated carbons (ACs) have been studied, and the effects of these properties on the distribution and reduction of metal precursors of Fischer-Tropsch (FT) catalysts have been noted. The four ACs used have been derived from peat, generic wood, pecan, and walnut. The catalysts used are Fe with Mo-Cu-K additives, supported on the various ACs, successfully used earlier for selective production of C1-C34 hydrocarbons. Characterization techniques used include Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), surface pH, temperature-programmed desorption-mass spectrometry (TPD-MS), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). The BET and SEM results show that the four ACs have similar overall pore-size distributions qualitatively. All ACs contain 75-94% micropores. The four ACs differ considerably in their surface morphology, surface area, and amounts of micro-, meso-, and macropores. The AC derived from peat is the least porous. ACs derived from pecan and walnut show ridges containing small spherical agglomerates. These ACs contain relatively more meso- and macropores (wide pores) than the wood AC, which has a wide distribution of irregular agglomerates. The EDS result indicates that all four ACs contain 5.6-7.5% oxygen; the wood- and peat-based varieties include smaller amounts of other impurities as well. The results of pH and TPD indicate that the surface of all four ACs are primarily covered by neutral and/or basic oxygen-containing groups, along with small amounts of acidic oxygen groups. Analyzing the BET, TPD, and TEM results of the four carbonsupported Fe-Mo-Cu-K catalysts shows that metal precursors are uniformly distributed on each AC surface, but the distribution of metal precursors on the carbon surface is likely related to pore types and amounts and not the total surface area of supports. Wide pores play a more important role to distribute metal precursors than micropores. Metal precursors tend to be present predominantly inside the pores of the peat-, pecan-, and walnut-based ACs, which contain greater amounts of wide pores. However, for the wood-based AC, which contains greater amounts of micropores, more metal precursors tend to be present on the exterior surface and the average metal particle size is also larger. The TPR and BET results indicate that the interaction between metallic precursors and the AC surface appears to be associated with both the chemical nature of the AC support and size of metal precursors. TPR and TPD indicate that stronger interactions between metal precursors and supports are observed on the peat-based ACs, with more neutral or basic oxygen-containing groups and smaller metal sizes, and the weakest interactions are observed on the wood-based AC support. The large particles on the exterior of the wood-based AC surface likely weaken the interaction. All four catalysts were used in FT reactions. Estimated values of the turnover frequency and activation energy are consistent with values reported in the literature for similar catalysts. The relationships between the performance of the catalyst in the FT reaction and AC type are discussed in this paper as well. The Fischer-Tropsch synthesis (FTS) activity can be related directly to the BET parameters associated with the relative ease of mass transfer and the relative fraction of metal crystallites present in the wide pores and inversely to the metal crystallite size. The selectivity toward C5þ products is related to the amount of species evolved during the TPR of the AC support, primarily the number of basic and neutral species on the surface. High selectivity toward C1 correlates well to the low pH of the external surface of the support.
texture, and surface properties of the ACs. Reactions include hydrogenation,1-4 oxidation,5-8 hydrodenitrogenation (HDN),
1. Introduction In recent years, there has been growing research interest in developing catalysts with activated-carbon (AC) supports for several types of reactions, because of the low cost, unique
(3) Coloma, F.; Sepulveda-Escribano, A.; Rodriguez-Reinoso, F. Appl. Catal. 1995, 123, L1–L5. (4) Fuente, A. M.; Pulgar, G.; Gonzalez, F.; Pesquera, C.; Blanco, C. Appl. Catal. 2001, 208, 35–46. (5) Choi, J. S.; Kim, T. H.; Choo, K. Y.; Sung, J. S.; Saidutta, M. B.; Ryu, S. O.; Song, S. D.; Ramachandra, B.; Rhee, Y. W. Appl. Catal. 2005, 290, 1–8. (6) Castro, C. S.; Guerreiro, M. C.; Oliveira, L. C. A.; Goncalves, M.; Anastacio, A. S.; Nazzarro, M. Appl. Catal. 2009, 367, 53–58.
*To whom correspondence should be addressed. Telephone: (304) 293-9337. Fax: (304) 293-4139. E-mail:
[email protected]. (1) Halttunen, M. E.; Niemel€a, M. K.; Krause, A. O. I.; Vaara, T.; Vuori, A. I. Appl. Catal. 2001, 205, 37–49. (2) Vilella, I. M. J.; de Miguel, S. R.; Scelza, O. A. Chem. Eng. J. 2005, 114, 33–38. r 2010 American Chemical Society
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21,26-31
hydrodesulfurization (HDS), hydroformylation, and methanol decomposition.14 Transition metals (Fe, Co, Mo, and Cu) and noble metals (Pt, Ru, and Rh) have been found to be effective for these reactions.1,2,4,7,8,11,14-20 Because AC materials are relatively inexpensive and are known to have high surface areas, broad ranges of pore size distribution (PSD), and complex surface chemistry, many studies have focused on the effects of the physical and chemical properties of carbons on the performance of the carbon-supported catalysts.1,4,19,20 It was reported that the physical texture of AC (e.g., properties such as surface area and porosity) remarkably impacts reduction and dispersion of active metals on the AC surface, especially when the conventional incipientwetness impregnation (IWI) technique is used. The higher surface area of AC and its larger pores have been reported to lead to higher dispersion and higher reduction of the metal precursor on the support surface, in turn leading to better catalytic performance.1,4 The surface chemistry properties of AC (e.g., amount and types of oxygen groups) are another important factor in the catalytic behavior of AC-supported metal catalysts. This influence is assumed to take place by modifying the metal dispersion.1,19,21,25 The oxygen-containing groups on the AC surface are postulated to improve Pt dispersion, by acting as nucleation anchors and/or being easily accessed by metal solution because of the decrease in the hydrophobicity of the carbon.19,21-24 Amounts and types of oxygen-containing groups on the AC surface could change surface acid/base properties, which would influence the interaction between metals and the AC support. This might result in different catalytic
1,19
Furthermore, it has been reported that behavior. the dispersion of Rh and Pt on carbon can be improved by the decomposition of the oxygen-containing groups during thermal treatment of the catalyst, which can relocate active sites on the carbon surface. Iron and cobalt catalysts supported on AC have attracted significant attention for the Fischer-Tropsch synthesis (FTS) reaction in the past.32-42 Besides the advantages noted above for AC supports, the pores enable mainly gasoline and diesel hydrocarbons to be formed by the FTS, with little need for further hydrocracking or isomerization treatments. Early work with transition metals supported on AC for CO hydrogenation was reported by Vannice and co-workers32-34 and Bartholomew and co-workers.35 However, these studies primarily focused on understanding the interaction between the metal and carbon surface, and only C1-C6 light hydrocarbons at low conversions were reported. When Ma et al.39,40 and Liang and co-workers41,42 studied almond-based AC supports for Fe and Co catalysts for FTS, hydrocarbons with less than 20 carbon atoms were shown to be formed in the CO conversion range of 30-80%. Recent studies in our laboratory36-38 made progress on the selective synthesis of gasoline and diesel-range hydrocarbons through the FTS reaction using Fe-Mo-Cu-K catalysts supported on AC. The catalyst displays high conversion, and hydrocarbon chain growth is restricted to C34 under the conditions of 260320 °C, 2.0 MPa, 3 nL (g of catalyst)-1 h-1, and H2/CO = 0.9. A preliminary study38 showed that catalytic behavior varies significantly with carbon support types, indicating that catalyst properties are closely associated with physical and chemical properties of the support as well. Efforts have been made to understand the relationship between the textural properties of mesopore materials as supports and the performance of supported metal catalysts. Supports include other types of carbons (carbon black, graphite, etc.), molecular sieves (FSM-16, MCM-41, NaY, NaZSM-5, and SBA-15) and simple oxides (Al2O3, MgO, SiO2, and TiO2). Metals used include Fe as well as Co, Mo, Pt, Pd, and Cu, with or without promoters (Cu, K, and others), and reactions include FTS and others. It has been reported43,44 that the size of the pores directs the crystal size of metals, with
(7) Cao, Y.; Yang, P.; Yao, C. Z.; Yi, N.; Feng, W. L.; Dai, W. L.; Fan, K. N. Appl. Catal. 2004, 272, 15–22. (8) Zhao, J. H.; Liu, Z. Y.; Sun, D. K. J. Catal. 2004, 227, 297–303. (9) Escalona, N.; Yates, M.; Avila, P.; L opez Agudo, A.; Garcı´ a Fierro, J. L.; Ojeda, J.; Gil-Llambı´ as, F. J. Appl. Catal. 2003, 240, 151–160. (10) Puello-Polo, E.; Brito, J. L. J. Mol. Catal. 2008, 281, 85–92. (11) Sayag, C.; Benkhaled, M.; Suppan, S.; Trawczynski, J.; DjegaMariadassou, G. Appl. Catal. 2004, 275, 15–24. (12) Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430–445. (13) Kainulainen, T. A.; Niemela, M. K.; Krause, A. O. I. J. Mol. Catal. 1999, 140, 173–184. (14) Ubago-Perez, R.; Carrasco-Marı´ n, F.; Moreno-Castilla, C. Appl. Catal. 2004, 275, 119–126. (15) Feng, L. J.; Li, X. G.; Dadyburjor, D. B.; Kugler, E. L. J. Catal. 2000, 190, 1–13. (16) Liang, C. H.; Ma, W. P.; Feng, Z. C.; Li, C. Carbon 2003, 41, 1833–1839. (17) Liang, C. H.; Ying, P. L.; Li, C. Chem. Mater. 2002, 14, 3148– 3151. (18) Dhandapani, B.; Ramanathan, S.; Yu, C. C.; Fr€ uhberger, B.; Chen, J. G.; Oyama, S. T. J. Catal. 1998, 176, 61–67. (19) Fraga, M. A.; Jord~ao, E.; Mendes, M. J.; Freitas, M. M. A.; Faria, J. L.; Figueiredo, J. L. J. Catal. 2002, 209, 355–364. (20) Arenz, M.; Stamenkovic, V.; Blizanac, B. B.; Mayrhofer, K. J.; Markovic, N. M.; Ross, P. N. J. Catal. 2005, 232, 402–410. (21) Aksoylu, A. E.; Faria, J. L.; Pereira, M. F. R.; Figueiredo, J. L.; Serp, P.; Hierso, J. C.; Feurer, R.; Kihn, Y.; Kalck, P. Appl. Catal. 2003, 243, 357–365. (22) Rodrı´ guez-Reinoso, F. Carbon 1998, 36, 159–175. (23) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Appl. Catal. 1998, 173, 259–271. (24) Prado-Burguete, C.; Linares-Solano, A.; Rodrı´ guez-Reinoso, F.; Salinas-Martı´ nez de Lecea, C. J. Catal. 1989, 115, 98–106. (25) Serp, J. C. Ph.; Hierso, R. F.; Kihn, Y. Carbon 1999, 37, 527–530. (26) Rom an-Martı´ nez, M. C.; Cazorla-Amor os, D.; Linares-Solano, A.; Salinas-Martı´ nez de Lecea, C. Carbon 1993, 31, 895–902. (27) Rom an-Martı´ nez, M. C.; Cazorla-Amor os, D.; Linares-Solano, A.; Salinas-Martı´ nez de Lecea, C.; Yamashita, H.; Anpo, M. Carbon 1995, 33, 3–13. (28) Van Dam, H. E.; Van Bekkum, H. J. Catal. 1991, 131, 335–349. ao, (29) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orf~ J. J. M. Carbon 1999, 37, 1379–1389.
(30) Prado-Burguete, C.; Linares-Solano, A.; Rodriguez-Reinoso, F.; Salinas-Martinez de Lecea, C. J. Catal. 1991, 128, 397–404. (31) Boehm, H. P. Carbon 1994, 32, 759–769. (32) Venter, J. J.; Kaminsky, M.; Geoffroy, G. L.; Vannice, M. A. J. Catal. 1987, 103, 450–465. (33) Venter, J. J.; Vannice, M. A. Catal. Lett. 1990, 7, 219–240. (34) Jung, H. J.; Vannice, M. A.; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N. J. Catal. 1982, 76, 208–224. (35) Jones, V. K.; Neubauer, L. R.; Bartholomew, C. H. J. Phys. Chem. 1986, 90, 4832–4839. (36) Ma, W. P.; Kugler, E. L.; Dadyburjor, D. B. Energy Fuels 2007, 21, 1832–1842. (37) Ma, W. P.; Kugler, E. L.; Wright, J.; Dadyburjor, D. B. Energy Fuels 2006, 20, 2299–2307. (38) Ma, W. P.; Kugler, E. L.; Dadyburjor, D. B. Stud. Surf. Sci. Catal. 2007, 163, 125–140. (39) Ma, W. P.; Ding, Y. J.; Lin, L. W. Ind. Eng. Chem. Res. 2004, 43, 2391–2398. (40) Ma, W. P.; Ding, Y. J.; Yang, J.; Liu, X.; Lin, L. W. React. Kinet. Catal. Lett. 2005, 84, 11–19. (41) Lin, L. W.; Ding, Y. J.; Liang, D. B. Proc.;Annu. Int. Pittsburgh Coal Conf. 1989, 6th, 729–733. (42) Shen, J. Y.; Lin, L. W.; Zhang, S.; Liang, D. B. J. Fuel Chem. Technol. 1992, 20, 1–7 (in Chinese). (43) Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.; Zholobenko, V. L. J. Catal. 2002, 206, 230–241. (44) Dunn, B. C.; Covington, D. J.; Cole, P.; Pugmire, R. J.; Meuzelaar, H. L. C.; Ernst, R. D.; Heider, E. C.; Eyring, E. M.; Shah, N.; Huffman, G. P. Energy Fuels 2004, 18, 1519–1521.
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larger pores resulting in larger metal crystallites inside the pores. The larger the metal size, the higher the metal reduction and the lower the metal dispersion.45-48 The ability of the metal to be reduced and dispersed is a vital parameter for FTS. Some previous characterization studies of Co on the supports mentioned above demonstrate high rates of FTS on the supports with large-sized pores.43,46 Ruel and Bartholomew47 reported strong support effects in carbon monoxide hydrogenation on cobalt catalysts; however, Iglesia et al.48 showed that FT activity is proportional to metal dispersion and almost independent of the support. Vannice and co-workers49,50 showed that carbon porosity and preparation techniques influence the Fe crystal size and FTS behavior, with an increase in the turnover frequency (TOF) with increasing crystal size. TOF values obtained by Holmen et al.51 for 812% Fe on oxide and zeolite supports are lower than those of other studies, perhaps because of lower Fe dispersions. Bartholomew and co-workers52-55 showed that Fe with the K promoter has a high degree of reduction, with a corresponding change in the TOF. Mass-transfer limitations for syngas and hydrocarbons formed by the FTS have also been used to explain different FTS behaviors on the metal catalysts supported on these supports.43 Zimmerman et al.56 reported that internal mass transfer may be neglected when the particle is below 230 mesh. It is clear that the impact of the AC surface and texture on the FTS behavior of Fe/AC catalysts is not well-understood. Furthermore, studies of the effect of the metal particle size on FTS remain inconsistent. Accordingly, it is necessary to study the physical and chemical nature of the AC supports and their impact on the catalytic properties of Fe-Mo-Cu-K catalysts for FTS. AC supports from four different origins are used in this study. The textural properties of the ACs and catalysts are characterized by surface pH measurements, nitrogen physisorption, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM), and the surface chemistry is characterized by temperature-programmed reduction (TPR) and temperature-programmed desorption with mass spectrometry (TPD-MS). The textural and chemical nature of the materials is correlated with the metal distribution on the catalysts, the reducibility of the catalysts, and the catalyst performance in FT. Some data have been reported earlier38 but are repeated here for completeness.
supplied by Sigma-Aldrich; AC from generic “wood” was supplied by Norit; and ACs from specific woods, pecan and walnut, were obtained from the U.S. Department of Agriculture. The four ACs were washed in hot distilled water, calcined at 500 °C for 2 h in flowing N2, and sieved to 20-40 mesh before impregnation and characterization. For convenience of discussion, peat-, wood-, pecan-, and walnut-derived ACs are abbreviated as PTAC, WDAC, PNAC, and WTAC, respectively. 2.2. Catalyst Preparation. AC-supported Fe-Mo-Cu-K catalysts were prepared using sequential IWI. Aqueous ammonium molybdate solution containing 6% Mo was first impregnated on the AC supports. The materials were then dried in air at 90-100 °C overnight. An aqueous solution containing ferric nitrate and cupric nitrate, corresponding to final iron and copper contents on the catalyst of 15.7 and 0.8 wt %, respectively, was impregnated on the prepared Mo/C samples, again followed by drying in air at the same temperature overnight. Potassium nitrate solution (corresponding to 0.9 wt % K) was the last to be put onto the sample, again followed by drying in air at the same temperature overnight. 2.3. Catalyst Characterization. 2.3.1. pH. About 0.5 g of sieved AC (120-140 mesh) was accurately weighed and added to 15 mL of deionized water previously poured into a small glass bottle (d = 25 mm, and L = 97 mm). The slurry was thoroughly mixed by manually shaking the bottle for 1 h and then left overnight to allow the AC particles to settle. The solid AC was removed the next day, and the clear liquid was transferred to a new bottle for pH measurement. The pH was determined using a digital pH meter (Orion Research 611). The meter was calibrated by buffer solutions with known pH values of 4.0 and 10.0. The pH was measured several times until a constant value was obtained. 2.3.2. N2 Physisorption. The Brunauer-Emmett-Teller (BET) surface area and pore volume of the four AC supports and the fresh catalysts were determined by isothermal adsorption of N2 at 77 K in a Coulter Omnisorp 360 system. Prior to the adsorption measurements, the samples (100-150 mg) were degassed under vacuum at 300 °C for 10 h. 2.3.3. Electron Microscopy. The surface morphology of the supports was determined by a Hitachi S-4700 scanning electronic microscope at an acceleration voltage of 10 kV. The samples were prepared on a double-stick adhesive carbon tape mounted previously on specimen stubs and then sent into the SEM sample chamber. The supports were also analyzed by EDS in SEM to confirm the elements. The morphology and size of supported catalysts after a small amount of time on stream (1.5 h) were measured using a transmission electron microscope. The instrument was a JEOL JEM 4000EX, with structural resolution of 0.16 nm and Cs = 1.0 mm, and operated at 100 kV. The sample for TEM analysis was prepared by mixing ground catalyst in epoxy, followed by 24 h of drying in a oven at 60 °C. Each image was analyzed by a GATAN digital microscope to obtain a PSD. The PSD allows the mean particle size for each catalyst to be calculated. We used the Sauter mean diameter, defined as57 P 3 di Ni d3, 2 ¼ P 2 ð1aÞ di Ni
2. Experimental Section 2. 1. AC Supports and Treatment. As mentioned above, four types of ACs were used as catalyst supports. AC from peat was (45) Jacobs, G.; Das, T. K.; Zhang, Y. Q.; Li, J. L.; Racoillet, G.; Davis, B. H. Appl. Catal. 2002, 233, 263–281. (46) Agustı´ n, M.; Carlos, L.; Francisco, M.; Isabel, D. J. Catal. 2003, 220, 486–499. (47) Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984, 85, 78–88. (48) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. Adv. Catal. 1993, 39, 221–302. (49) Jung, H. G.; Walker, P. L., Jr.; Vannice, M. A. J. Catal. 1982, 75, 416–422. (50) Martin-Martinez, J. M.; Vannice, M. A. Ind. Eng. Chem. Res. 1991, 30, 2263–2275. (51) Holmen, A.; Schanke, D.; Sundmark, G. Appl. Catal. 1989, 50, 211–221. (52) Zennaro, R.; Pederzani, G.; Morselli, S.; Cheng, S. Y.; Bartholomew, C. H. Stud. Surf. Sci. Catal. 2001, 136, 513–518. (53) Rankin, J. L.; Bartholomew, C. H. J. Catal. 1986, 100, 526–532. (54) Rankin, J. L.; Bartholomew, C. H. J. Catal. 1986, 100, 533–540. (55) Rameswaran, M.; Bartholomew, C. H. J. Catal. 1989, 117, 218– 236.
where di is the particle diameter and Ni is the number of particles of size di. The dispersion D, the percentage of Fe atoms on the surface of the particles, was estimated as D ð%Þ ¼ 75=d3, 2 ðnmÞ ð1bÞ This relation assumes that the average area of a surface Fe atom is 0.94 nm.2,49 The role of Mo and other metals and their oxides (56) Zimmerman, W. H.; Rossin, J. A.; Bukur, D. B. Ind. Eng. Chem. Res. 1989, 28, 406–413. (57) Allen, T. Particle Size Measurement; Chapman and Hall: London, U.K., 1997, Vol. 1, p 44.
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is neglected here. These species are undetected by earlier X-ray diffraction (XRD) measurements37 and may be assumed to be highly dispersed for these purposes. 2.3.4. TPR. TPR of the catalysts was conducted in 5% hydrogen (balance argon) at a total flow rate of 50 cm3/min. About 50 mg of the catalyst was accurately weighed and placed in a quartz U-tube (5 300 mm). The tube and contents were heated from room temperature (RT) to 915 °C at a constant rate of 10 °C/min and then held at 915 °C for 30 min. The H2 consumption was monitored in a thermal conductivity detector (TCD), calibrated previously by reduction of 14 mg of ultrahigh-purity CuO (99.9999%) between RT and 500 °C at the same heating rate. Additional details for the TPR procedures can be found elsewhere.37 2.3.5. TPD-MS. TPD of the ACs and the catalysts was conducted in the same apparatus used for TPR. The solids were first reduced in the unit using a mixture containing 5% H2 (balance argon) at 400 °C for 10 h. The TPD was carried out in a pure argon environment between RT and 915 °C at 10 °C/min. The desorbed products were detected by a HP5890 gas chromatograph (GC) and identified by a Balzers quadrupole mass spectrometer (MS), which was connected to the quartz U-tube reactor. The sensitivity of the MS was set to 10-9 for CO and CO2. 2.4. FTS Reactions. Catalysts were tested in a computercontrolled fixed-bed reactor system. Details have been provided earlier.37 The feed mixture contained 5% He as an internal standard and H2 and CO in the ratio of 0.9. The flow rate was adjusted by a mass-flow controller and sent to a stainless-steel reactor (8 630 mm). The exit gas stream passed through a product trap to condense liquids. A liquid sample was withdrawn approximately every 24 h. Incondensible gases passed through a back-pressure regulator and then went either to a bubble-flow meter or to a GC for online product analysis. Typically, about 1 g of catalyst (20-40 mesh) was used in the reaction study. The catalyst was diluted 1:4 (v/v) with quartz chips of the same size before being loaded into the reactor. The catalyst was reduced in situ by H2 at 400 °C, 0.5 MPa, and 3 nL (g of catalyst)-1 h-1 for 12 h. After the pretreatment, the reactor was cooled to 290 °C in flowing He. The system was then pressurized to 300 psig, the He flow was stopped, and the syngas feed was introduced. The reactor temperature was finally increased to the required value. Times on stream of up to 396 h have been recorded,38 but only results up to 175 h are shown in this work.
Table 1. pH Values for AC Supports type
pH
PTAC WDAC PNAC WTAC
9.9 9.6 8.5 8.4
and PTAC but only ∼6% for WDAC. This trend is also observed for the total pore volume. These results suggest that the four AC supports are highly microporous (75-94% surface area), even though PNAC, WTAC, and PTAC contain relatively more of the wide pores. After impregnation of the metals (6 wt % Mo, 15.7 wt % Fe, 0.8 wt % Cu, and 0.9 wt % K) on each carbon support, the total BET specific surface area and total specific pore volume of each material are decreased by about the same extent, about 46% for the case of the surface area and about 48% for the case of the pore volume. However, taking into account only the change for the wide pores, the catalysts supported on PNAC and WTAC are subjected to the largest decrease in specific surface area (∼73%) and specific pore volume (∼61%) after impregnation, while the WDAC-supported catalyst exhibits the smallest decrease (9% in surface area and 39% in volume), with intermediate decreases in surface area and volume for the PTAC-supported catalyst (31 and 49%, respectively). For the micropore surface area, the decrease for the WTAC-supported catalyst is less than that for the other three catalysts. Accordingly, it is reasonable to suppose that relatively more active metals are present in the wide pores of PNAC and WTAC than in the micropores of these supports. However, the loss of the micropore surface of the ACs upon impregnation may not be directly proportion to the amount of metal in the micropores, because micropores could be part of wider pores58 and/or some metals could block the micropores. It is noted that the surface area of micropores in the WDAC is at least 22% higher than in the other three supports; also, WDAC has the smallest loss of the surface area of the wide pores after impregnation. These results suggest that, for WDAC, a significant fraction of the metal is present on the exterior surface. Finally, note that the average pore diameter of the fresh catalysts is 3.7-5.4 nm, still in the mesopore range. The isotherms of N2 adsorption for supports and catalysts are shown in panels a and b of Figure 1, respectively. The shape of isotherms for all supports and catalysts is similar. The nearly horizontal adsorption curves stop sharply at P/ P0 = 1, which implies a type-I isotherm for all samples according to the classification of Gregg et al.59 and Brunauer et al.60 This type of adsorption indicates that micropores contribute the primary N2 adsorption capacity for all supports and catalysts. Finally, the isotherms of PTAC and PNAC and their supported catalysts are steeper after the relative pressure P/P0 of 0.4 compared to the other two supports and catalysts, suggesting that PTAC and PNAC have a more-developed mesoporous structure. Differential pore volume curves for the AC supports and the corresponding catalysts are shown in panels a and b of
3. Results 3.1. Textural Properties of Supports and Supported Catalysts. 3.1.1. pH. The pH values are shown in Table 1. It is clear that the four AC supports have an overall basic surface. Of these, PTAC is the most basic, while PNAC and WTAC are somewhat less basic. 3.1.2. EDS. EDS analysis38 shows that the PTAC, WDAC, PNAC, and WTAC supports contain 6.1, 7.5, 5.6, and 5.9% oxygen, respectively, with almost all of the balance being carbon. No other elements are detected on the PNAC and WTAC supports. Traces of Ca are detected on WDAC. In addition to C, O, and Ca, the PTAC sample contains Mg, Al, Si, and Fe. 3.1.3. BET. The BET specific surface area, specific pore volume, and average pore diameter of the PTAC, WDAC, PNAC, and WTAC supports and the corresponding catalysts are shown in Table 2. The total BET specific surface area (SA) of the supports varies with the AC type. The highest value (∼1000 m2/g) is on the PNAC, and the lowest value (∼600 m2/g) is on the PTAC. Wide pores (meso- and macropores, i.e., those with a diameter greater than 2 nm) contribute 25-28% of the surface area for PNAC, WTAC,
(58) Kawabuchi, Y. J.; Oka, H.; Kawano, S.; Mochida, I.; Yoshizawa, N. Carbon 1998, 36, 377–382. (59) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, U.K., 1982; pp 2-4. (60) Brunauer, S.; Emmett, P. H. J. Am. Chem. Soc. 1940, 62, 1732– 1746.
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Table 2. BET Results of AC Supports38 and Catalystsa specific surface area (m2/g)
specific pore volume (cm3/g)
percentage of pores > 2 nm
supports and catalystsa
total
pore > 2 nm
total
pore > 2 nm
mean pore size (nm)
surface area
pore volume
PTAC WDAC PNAC WTAC PTAC-supported catalysta WDAC-supported catalysta PNAC-supported catalysta WTAC-supported catalysta
606 970 1016 892 347 534 499 476
151 56 280 233 104 52 89 54
0.48 0.58 0.72 0.50 0.25 0.32 0.32 0.29
0.255 0.138 0.358 0.182 0.131 0.084 0.122 0.081
5.9 6.6 5.1 3.9 4.1 5.4 4.0 3.7
25 6 28 26 30 10 18 11
53 24 50 37 52 25 38 28
a
Catalysts are AC-impregnated with 6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K.
Figure 2. Pore-size distribution curves for (a) ACs and (b) ACsupported catalysts as in Figure 1. Note the change in the scale between panels a and b.
Figure 1. Nitrogen adsorption (BET) isotherms for (a) ACs and (b) AC-supported catalysts (6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K). Note the change in the scale between panels a and b.
and WTAC (panels c and d of Figure 3) are similar. They have creases and ridges filled with small spherical agglomerates. 3.1.5. TEM. Two TEM images were obtained from each of four AC-supported catalysts removed after 1.5 h of FTS reactions. The images are shown in Figure 4. The iron-active species is evidenced by darker contrasts as well as lattice fringes in iron carbide or oxide crystals. The active metals are widely distributed on the four AC supports. It can be seen that the metal particles on each AC have different shapes: spherical, oblong, rod-shaped, etc. PSDs obtained for the catalysts are shown in Figure 5. Note that the two TEM images for each sample were taken from different spots, so that the measured metal PSDs on the carbon supports represent the real cases as much as possible. From Figure 5, the particles of the PTAC, PNAC, and WTAC catalysts mainly fall in the range of 3-100 nm but WDAC particles have a wider range of 3-200 nm. Furthermore,
Figure 2, respectively. Overall, the pore size distribution of all ACs is rather wide, about 1-100 nm. All have the same V shape, and the maximum differential pore volume for both supports and catalysts is located within a narrow range of 3.5-3.7 nm, which is further evidence that meso- and macropores are present on all AC supports and supported catalysts. The PTAC support appears to show another maximum at a somewhat lower pore size. 3.1.4. SEM. The PTAC, PNAC, and WTAC are irregularly shaped, while the WDAC has been extruded to a cylindrical shape. However, the external surface of all of the supports is quite rough. Panels a-d of Figure 3 show the surface morphologies of the four AC supports revealed by SEM. The PTAC (Figure 3a) demonstrates a cracked and sheet-like surface morphology and is the least porous, while the WDAC (Figure 3b) has a wide distribution of irregularly shaped agglomerates. The surface morphologies of PNAC 4103
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Figure 3. SEM images of (a) PTAC, (b) WDAC, (c) PNAC, and (d) WTAC from ref 38.
metallic Fe. Also, the first and second reduction peaks for these last three catalysts shift down 40-90 °C relative to that of the PTAC-supported catalyst, suggesting a greater metalsupport interaction on the PTAC support. This is also quantitatively demonstrated in Table 4. The peak areas in Figure 6 were deconvoluted and quantified using Peakfit software (Jandel Scientific, Inc.), and the corresponding consumption of hydrogen was obtained from the calibration with CuO. The theoretical consumption of hydrogen was calculated by modeling the initial states as Fe2O3 and MoO3 and the final states as Fe and Mo. Then, the degree of reduction, DR in Table 4 is defined as the ratio of the experimental consumption of hydrogen to the theoretical consumption. Other details can be found in earlier work.37 In Table 4, the lowest magnitude of the degree of reduction (70%) is found for the PTAC-supported catalyst, consistent with the greater metal-support interaction noted above. Further, the WDAC catalyst has the highest degree of reduction (82%). This large value may be due to the presence of greater numbers of larger metal crystallites (over 60 nm) on the outer surface of the WDAC support, as noted in the BET and TEM results above. The outside particles are more likely to be easily reduced during TPR.43,45 3.3. TPD. 3.3.1. AC Supports Alone. TPD was carried out to determine the surface chemistry of the four AC supports. MS was used to identify the species emitted during the desorption. The amount of species evolved is quantified through deconvolution of TPD peaks, again using Peakfit software. The results are shown in Figure 7a and Table 5. All four AC supports present a small peak at 500-750 °C and a distinctive peak at 750-915 °C. The lower and higher temperature peaks can be attributed to CO2 and CO species,
on PTAC, PNAC, and WTAC catalysts, “small” crystallites, i.e., less than 25 nm, make up about 75% of the total but, on the WDAC support, the PSD shifts to larger particles and only about 34% of the crystallites are “small”. Note that all supports contain some “big” particles, i.e., greater than 60 nm, suggesting that these particles could be present on the outside surface of the AC support. This is consistent with observations from the BET data discussed above. The values of the Sauter mean particle diameter, d3,2, and dispersion, D, as defined by eqs 1a and 1b, are shown in Table 3 for the four catalysts. The values of d3,2 follow the order: WDAC > WTAC > PNAC > PTAC. 3.2. TPR. TPR patterns of the four fresh AC-supported catalysts are shown in Figure 6, and quantitative results are tabulated in Table 4. The TPR pattern of the PTAC-supported catalyst basically shows a three-step reduction of iron oxide and a partial reduction of MoO3 to an oxide of Mo with low chemical valence, i.e., MoO2, or to Mo.15,37 The three steps of reduction of iron oxide are ð2Þ Fe2 O3 f Fe3 O4 ðat 200-300 °CÞ Fe3 O4 f FeO ðat 300-400 °CÞ
ð3Þ
and FeO f Fe ðat 400-800 °CÞ
ð4Þ
The reduction of MoO3 occurs between 400 and 800 °C. For the other three catalysts, the TPR patterns are similar but are characterized by one reduction peak between 300 and 700 °C, instead of the two seen for the PTAC. This result may suggest that reduction of Fe3O4 to FeO on these three catalysts is too fast to be noted or that Fe3O4 may be directly reduced to 4104
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tive peaks occur at the same temperature ranges as those of carbon supports, and thus are again identified to be CO2 and CO species. However, the two peaks in Figure 7b are significantly larger than those of the corresponding AC support (Figure 7a). This is particularly true for the lower-temperature (CO2 desorption) peak. The numbers in Table 5 confirm the qualitative results of Figure 7b. Apparently, the metals significantly enhance the desorption, particularly of CO2. Such an increase of CO and CO2 species during the TPD of the catalysts in the present study is in good agreement with several studies with Pt/C catalysts24,26,62 but disagrees with other results.19 It might be that some studies using liquid acids to treat the AC before TPD wind up decreasing or damaging the oxygen groups on the ACs. 3.4. Reaction Studies. A partial summary of the results is shown in Table 6. The parameters used are the conversion of CO and the selectivity. The conversion of CO is defined as XCO ¼ 100 ðflow rate of CO at the inlet flow rate of CO at the outletÞ=ðflow rate of CO at the inletÞ
ð5Þ The selectivity of product i is defined as selectivityi ¼ 100 ðproduction rate of product i at the outletÞ= ðproduction rates of all productsÞ
ð6Þ
The selectivity values shown in Table 6 are lumped as methane, C2-C4, and C5 and higher. Selectivities toward other product lumps and details at other times on stream are reported elsewhere.38
Figure 4. TEM images of AC-supported catalysts with composition as in Figure 1 after 1.5 h on stream: (a and b) PTAC-supported catalyst, (c and d) WDAC-supported catalyst, (e and f) PNACsupported catalyst, and (g and h) WTAC-supported catalyst.
4. Discussion 4.1. Role of Metal in TPR and TPD. The TPD patterns for the catalysts in Figure 7b display significant increases of emitted CO and CO2 species relative to the AC supports. Two reasons may be responsible for the increased emission. First, redox interactions may exist between Fe, Mo, Cu, K, and the AC surface, which may create new (weakly) acidic oxygen-containing groups, which cause the larger emissions of CO and/or CO2. This explanation is consistent with the redox observed during the impregnation of Pt on carbon.19 A second possible path involves partial oxides MoOx and FeOx present after H2 reduction. These oxides could react with active carbon sites on the AC to produce incremental CO2 and/or CO when the temperature is high enough during TPD. This explanation is indirectly supported by studies15,16 that report that Mo carbide can be formed at high temperatures (>750 °C) by heating MoO3/C in Ar. If this latter hypothesis is correct, we expect the specific incremental amount of TPD area (i.e., [CO þ CO2] area for the catalyst less than that for the AC support, per unit BET total specific surface area of the AC) to be related to the fraction of unreduced metal on the support. Accordingly, Figure 9 shows the specific incremental area (as obtained from Table 5) versus the fraction of unreduced metal (100 - RD, as obtained from Table 4). The good linear correlation suggests that the unreduced metal oxides on the catalyst surface are mainly responsible for the additional emission after impregnation. Because the BET surface area of the catalysts varies with the
respectively, released from the AC surface during the TPD, as confirmed by a typical MS shown in Figure 8. Two types of oxygen-containing groups on the ACs are responsible for these emissions:1,21,27,61 acidic groups (carboxylic groups, anhydrides, lactone groups, carbonxyl groups, etc.) are responsible for giving off CO2 at 400-750 °C, and neutral or basic groups (such as quinonic, phenolic, carbonyl, or other ether groups) are responsible for giving off CO at 8001000 °C. This is consistent with our results. Quantitative amounts of CO and CO2 from the curves of Figure 7a are tabulated in Table 5. All supports show much larger emissions of CO than CO2. This implies an overall basic surface, which is consistent with the pH values in Table 1. The ratio of the emission peak area of CO to CO2 (CO/CO2) listed in Table 5 can indicate the relative properties of the four AC supports. However, the trend of CO/CO2 with the AC type in Table 5 is not consistent with the order of pH results of the four ACs in Table 1. One possible reason is that the highest temperature of 915 °C used for TPD is not high enough to decompose all of the basic groups on the ACs. Also, some acid oxygen-containing groups, such as carboxyls, could simultaneously give off both CO and CO2 during TPD.29 3.3.2. Reduced Fe-Mo-Cu-K/AC Catalysts. Figure 7b shows the TPD profiles of the four AC-supported catalysts, and the quantitative results are given in Table 5 for these materials as well. In Figure 7b, the profiles show two distinc(61) Bansal, R. C.; Donnet, J. S.; Stoeckli, F. Z. M. D. Active Carbon; Marcel Dekker: New York, 1988: pp 27-45.
(62) Okhlopkova, L. B.; Lisitsyn, A. S.; Likholobov, V. A.; Gurrath, M.; Boehm, H. P. Appl. Catal. 2000, 204, 229–240.
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Figure 5. Metal crystallite size distributions for AC-supported catalysts as in Figure 1: (a) PTAC-supported catalyst, (b) WDAC-supported catalyst, (c) PNAC-supported catalyst, and (d) WTAC-supported catalyst. Table 3. Sauter Mean Diameter and Dispersion Results for Catalystsa on Different Supports AC support PTAC WDAC PNAC WTAC
d3,2 (nm) 29.8 98.3 36.5 49.6
Table 4. H2 Consumption during TPR and Degree of Reduction (DR) of AC-Supported Catalystsa 38 AC support
measured H2 consumption (mmol/g of catalyst)
theoretical H2 consumption (mmol/g of catalyst)
DR (%)
PTAC WDAC PNAC WTAC
4.34 5.08 4.91 4.67
6.22 6.22 6.22 6.22
69.8 81.8 79.0 75.1
D (%) 2.52 0.76 2.05 1.51
a Catalysts are AC-impregnated with 6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K.
a
Catalysts are AC-impregnated with 6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K; nominal sample weight = 50 mg.
result again demonstrates that the metals impregnated are uniformly distributed, in good agreement with the TEM observations. 4.2. Values of Activation Energy and Turnover Frequency. Rate constants, activation energies, and turnover frequencies are generally computed for well-defined catalysts and surfaces. This is not the case here nor, in fact, for FTS catalysts in general, especially because the active site for the complex catalysts used in the present work is not clearly established. However, with the appropriate caveats, it may be worthwhile to compare TOF values to some of those in the literature. Table 7 summarizes the values of the apparent reaction rate constant, k, the activation energies, Eact, and the TOFs from the data of Table 6. The values of k are obtained by assuming that the rate of loss of CO is first-order in hydrogen partial pressure. Note that all rate measurements were made at significant times on stream. Hence, the values of k obtained
Figure 6. TPR pattern of AC-supported catalysts as in Figure 1.
support types and the incremental TPD area per surface area is linearly related to the fraction of unreduced metal, this 4106
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are expected to be smaller than those for fresh catalysts at low conversions.
The values of Eact are obtained from the usual Arrhenius relationship ð7Þ k1 =k2 ¼ exp½ðEact =RÞð1=T2 - 1=T1 Þ Note that only two values of Eact are given in Table 7. This is because measurements at two different temperatures were carried out for only these two catalysts. Because the k values at 310 °C correspond to longer times on stream than those at 320 °C, catalyst deactivation is expected to be more of a factor at the lower temperature. Hence, the values of Eact shown in Table 7 are expected to be somewhat larger than the actual values. The large difference between the two values of Eact in Table 7 indicate that, for at least one of them, if not both, the overall catalytic process is characterized by strong pore diffusion. The results of Zimmerman et al.56 mentioned earlier are consistent with this, because the mesh size used in the present work is 20-40 mesh, leading to much larger particles than the limit56 of 170-230 mesh. Zimmerman et al. further estimated an effectiveness factor of around 0.4 at a temperature of around 250 °C; the value of the effectiveness factor would probably be even lower at the higher temperatures used in the present work. Again, this is consistent with strong pore diffusion in the present work. The strong resistance to pore diffusion for the PTAC support (at least) can perhaps be related to the smaller value of the effective diffusion coefficient, Deff. Accordingly, following the relationship Deff ¼ Di ε=τ
where Di is the “conventional” diffusivity, we would expect that the PTAC catalyst would have a smaller porosity, ε, and/or a larger tortuosity, τ, than PNAC. Table 2 indicates that this is in fact the case for ε. It is not clear if the slit-shaped morphology of PTAC would correspond to a larger value of τ than the honeycomb-shaped PNAC. Further details of the effect of the pore structure on FTS activity are discussed in the next section. Finally, values of TOF in Table 7 are calculated from
Figure 7. TPD patterns of (a) AC supports and (b) AC-supported catalysts as in Figure 1. Table 5. Areas of Various Species Evolved during Ar-TPD of AC Supports and Catalysts a
supports and catalystsa
CO2 peak (ACO2)
CO peak (ACO)
total area
ACO/ ACO2
PTAC WDAC PNAC WTAC PTAC-supported catalysta WDAC-supported catalysta PNAC-supported catalysta WTAC-supported catalysta
518 1948 760 966 31703 43520 49766 43022
25382 16520 39438 31927 72489 76578 99655 94328
25900 18468 40198 32893 104192 120098 149421 137350
49.0 8.5 51.9 33.1 2.3 1.8 2.0 2.2
ð8Þ
TOF ðs- 1 Þ ¼ SVyCO, in XCO, out MFe =fwFe RDDð22:4Þð0:36Þg ð9Þ where SV is the space velocity [=3 nL (g of catalyst)-1 h-1], yCO,in is the mole fraction of CO in the inlet stream, XCO,out is the fractional conversion of CO, MFe is the molecular weight of Fe (=55.8 g/gmol), wFe is the weight percent of Fe in the catalyst (=15.7 wt %), RD is the percentage degree of reduction of the catalyst, and D is the percentage dispersion
a Catalysts are AC-impregnated with 6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K; nominal sample weight = 200 mg.
Figure 8. Mass spectra versus time for (top) CO and (bottom) CO2 during Ar-TPD of the PTAC.
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Table 6. FT Reaction Results over AC-Supported Catalystsa 38 T = 320 °C T = 310 °C
hydrocarbon selectivity (wt %)b c
AC support PTAC WDAC PNAC WTAC
time on stream (h)
CO conversion, XCO (%)
C1
C2-C4
C5þ
time on stream (h)
CO conversion, XCO (%)
49-73 49-72 47-70 47-70
87.1 37.9 90.1 58.5
10.9 13.9 15.3 14.9
40.9 54.8 32.4 34.6
48.2 31.3 52.3 50.5
173-175
81.7
173-175
71.6
a Reaction conditions: 3 nL (g of catalyst)-1 h-1, 300 psig, and H2/CO = 0.9. b Mean values over time range shown. c Catalysts are AC-impregnated with 6% Mo, 15.7% Fe, 0.8% Cu, and 0.9% K.
Table 7. Estimates of Kinetic Parameters of AC-Supported Catalysts for FTSa reaction rate constant, k (mol (g of catalyst)-1 h-1 MPa-1) AC support
T = 320 °C, t = 49-70 h
T = 310 °C, t = 173-175 h
PTAC WDAC PNAC WTAC
0.110 0.029 0.109 0.052
0.083
81
0.067
143
a
TOF at Eact (kJ/mol) 320 °C (s-1) 0.33 0.40 0.36 0.34
Catalysts and reaction conditions are the same as in Table 6.
supported on the molecular sieves SBA-15 and MCM-4143,63,64 report that cobalt in larger pores is more active and undergoes higher degrees of reduction. As indicated earlier, impregnation of the same amounts of Fe-Mo-Cu-K precursors on the different ACs leads to a decrease of the surface area of wide pores (i.e., those with diameters greater than 2.5 nm) by approximately 77% for WTAC, 68% for PNAC, 31% for PTAC, but only 9% for the WDAC. The decrease of the wide-pore surface should reflect the amount of metal residing inside the wide pores of the ACs. Hence, it is reasonable to assume that approximately 77% of wide pores on WTAC but only 9% of wide pores on WDAC are filled by metal or their precursors. According to the studies cited above, one would expect that WTAC-supported catalysts would be the most active, with WDAC far behind. The activity data in Table 6 do in fact show the WDAC-supported catalyst as the least active, but the others do not follow the order expected when only the amount of metal in the large pores is considered. Therefore, other factors are also at work. It may be reasonable to assume that FTS activity should also depend upon the fraction of surface area in the catalyst that is associated with wide pores, by allowing more rapid mass transfer. Table 2 indicates that approximately 11% of the surface area on the WTAC-supported catalyst is associated with wide pores, 18% of the surface area on the PNAC-supported catalyst is associated with wide pores, 10% of the surface area on the WDAC-supported catalyst is associated with wide pores, and 30% of the surface area on the PTAC-supported catalyst is associated with wide pores. This ordering also accounts for some but not all of the conversion data of Table 6. If all three factors noted above are in play, then it is reasonable to expect that the product of parameter R
Figure 9. Specific incremental TPD area for AC-supported catalysts versus the percentage of unreduced metal.
for the catalyst. Because the values of XCO have been obtained after some time on stream, the TOF values represent a lower bound to the values expected for the fresh catalyst. An approximate comparison to TOF values for related catalysts from the literature is shown in Table 8. Because many of the literature values correspond to lower temperatures than those used in the present work, Table 8 contains estimates of TOF values for the present catalysts in the temperature range of 215-275 °C. These estimates were obtained using the values of TOF and Eact from Table 7, again assuming an Arrhenius relationship. From Table 8, it can be seen that the TOF and Eact values from the present work are comparable to the literature values. 4.3. Effect of the Catalyst and Support Properties on FTS Activity. Metal crystallite size is inversely linked with the dispersion of metal on the support,45-48 and this may be an important parameter affecting FTS activity for our ACsupported catalysts. With this hypothesis, the activity of the catalyst should decrease in the order: PTAC > PNAC > WTAC > WDAC. Comparing this to the results of Table 6 indicates that the correlation is valid for the two least active catalysts but not for the other two. Clearly, other parameters are also at work, and the pore structure is a likely candidate. The BET results imply that Fe-Mo-Cu-K metal precursors are present in micropores, wide pores, and the external surface of all ACs. However, the distribution of the metal particles among the pores is different on the different supports. Several FTS studies over cobalt catalysts
R ¼ ðSAw, S - SAw, C Þ=SAw, S
ð10Þ
where SA denotes the specific surface area, subscript w indicates wide pores, and subscripts S and C indicate the
(63) Wang, Y.; Noguchi, M.; Takahashi, Y.; Ohtsuka, Y. Catal. Today 2001, 68, 3–9.
(64) Panpranot, J.; Goodwin, J. G.; Sayari, A. J. Catal. 2002, 211, 530–539.
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Table 8. Comparison to Literature Values reference
catalysts
T (°C)
P (atm)
H2/ CO
this work this work Jung et al.49 Martin-Martinez and Vannice50 Holmen et al.51 Zennaro et al.52 Rankin and Bartholomew53,54 Rameswaran and Bartholomew55
Fe-Mo-Cu-K/PTAC Fe-Mo-Cu-K/PNAC Fe/AC Fe/AC Fe/oxides, zeolites Fe-K-Cu/SiO2 Fe-K/SiO2 Fe/Al2O3
215-275 215-275 275 275 300 221-247 250 215-240
20 20 1 1 10.8 20 1 1
0.9 0.9 3 3 3 2 3 3
a
RD (%)
D (%)
XCO (%)
70 79
2.5 2.1
50 65-90 55
2-35 0.6-2.0 3.5 2.6-4.7 32
TOF (s-1)
87 0.009-0.085a 89 0.0007-0.033a 2.8-4.0 0.006-0.554 2-38 0.004-0.095 6-21 0.51-1.67 50-69 0.013-0.017 1-9 0.0001-0.006 0.97-1.71 0.00035-0.0006
Eact (kJ/mol) 81 143 70-133 75-110 72-124 85
Values estimated from TOF and Eact values of Table 7.
4.4. Effect of the Catalyst and Support Properties on FTS Selectivity. Selectivity of higher hydrocarbons (C5þ) is an important criterion for a successful FTS catalyst. It is of interest to note which, if any, of the properties measured above correlate with C5þ selectivity. For the catalysts considered in this work, the C5þ selectivity increases with AC support in the order: WDAC < PTAC < WTAC e PNAC. This correlates fairly well, but not completely, with the fraction of wide pores filled after impregnation (parameter R defined above). A better ordering is obtained with the amount of CO evolved during TPR of the AC support before impregnation (the amount of CO þ CO2 evolved also correlates well, but this is expected, because the amount of CO generally overwhelms the amount of CO2). The neutral or basic groups clearly play a role in the higher hydrocarbon selectivity. It is likely that these groups assist in hydrocarbon chain propagation and, therefore, cause more of the higher hydrocarbons to be produced. Methane is not a product of value; hence, C1 selectivity is a parameter to be minimized. C1 selectivity decreases with support AC in the order: PNAC g WTAC > WDAC > PTAC. This correlates well with the pH of the AC surface, as shown in Table 1. The catalyst with the PTAC support has the lowest C1 selectivity (preferred), which corresponds to the largest pH, and the remaining selectivities and pH values follow suit.
Figure 10. Conversion of CO versus parameter δ defined in the text.
support and catalyst, respectively; parameter β β ¼ SAw, C =SAtot, C
ð11Þ
where SAtot,C indicates the total surface area of the catalyst; and parameter γ, the reciprocal of the mean crystallite size γ ¼ 1=d3, 2
ð12Þ
should be proportional to the activity of the catalyst. Figure 10 shows the relationship between the CO conversion and parameter δ δ ¼ Rβγ
5. Conclusions The pore texture and surface chemistry properties of four ACs based on peat (PTAC), generic wood (WDAC), pecan (PNAC), and walnut (WTAC) are studied in this paper using BET, EDS, SEM, TPR, TPD-MS, and TEM. The four ACs show considerable differences in surface area, pore structure, and surface chemistry. Results from BET and TEM indicate that the pore structure of the AC supports primarily affects the distribution of metal crystallites and the particle size on the AC surface. Larger numbers of wide pores on the ACs (e.g., for PNAC, WTAC, and PTAC) lead to a better overall distribution of metal on the AC surface and smaller metal crystallites. During the impregnation, micropores likely do not distribute metal on ACs as effectively as the wide pores, because of mass-transfer limitation. More micropores on the WDAC support induce more aggregation of metals and more large crystallites on the external surface. Irrespective of the difference of metal distribution on the ACs, the results from TPD, TEM, and BET demonstrate that the metals are uniformly dispersed on all ACs. TEM shows different metal dispersions on the four AC-supported catalysts, resulting from their different surface and pore structure. The TPD study reveals that the species (primarily CO) emitted from the support ACs are due to neutral or basic groups on the
ð13Þ
Figure 10 shows that the conversion increases monotonically with parameter δ. The straight line through the points is a remarkably good fit, considering the errors associated with the measurements. Besides the wide pores, BET results indicate that micropores of the supports also lose surface area after impregnation and, therefore, are likely to have been filled with active metals. However, it is likely that the FTS reaction in the micropores is not significant, for various reasons. First, it is likely that mass-transport limitations are severe in the micropores.1,65 Also, the degree of reduction of the metal precursors in the micropores is lower.43 Finally, the actual amounts of metals in the micropores of ACs may be much less than that reflected by the loss of the surface area in the micropores after impregnation, because at least some of that loss may be due to metal precursors in large pores blocking some of the associated micropores.58 (65) Iglesia, E.; Reyes, S. C.; Madon, R. J. J. Catal. 1991, 129, 238– 256.
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support. The increase of species emitted from the catalyst after 500 °C primarily stems from the reaction of active carbon with partially oxidized metals. When these AC-supported catalysts are used for FTS, the estimates of TOF and activation energy agree with literature values. The relationships between the physical and chemical properties of the support ACs and the FTS performance (activity and selectivities) over catalysts impregnated with the same metals (15.7% Fe, 6% Mo, 0.8% Cu, and 0.9% K) but over different AC supports are discussed. The different values of the FTS activity can be related to the differences in the metal particle size and the pore structure of the ACs. The metal particle size reflects metal dispersion. With respect to the pore structure, metal crystallites present in the wide pores can be related to the fractional decrease of the surface area attributed to these pores after impregnation. The ease of mass
transfer can be related to the fraction of total surface area associated with the wide pores after impregnation. The product of these three parameters is seen to be well related to the activity of the catalysts. The selectivity toward C5þ products is related to the amount of species evolved during the TPR of the AC support, i.e., the number of basic and neutral species on the surface. High selectivity toward C1 correlates well to low pH of the external surface of the support. Acknowledgment. This study was supported by the U.S. Department of Energy under Cooperative Agreement DEAC22-99FT40540 with the Consortium for Fossil Fuel Science (CFFS). We are grateful to Dr. Wayne Marshall, U.S. Department of Agriculture, New Orleans, LA, for the AC from pecan and walnut. At WVU, Steve Carpenter and Dr. Liviu Magean helped with the EDS measurements and Vinod Berry carried out the TEM experiments.
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