Ind. Eng. Chem. Res. 1998, 37, 1181-1188
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Titania-Supported Bimetallic Catalysts Combined with HZSM-5 for Fischer-Tropsch Synthesis K. Jothimurugesan*,† and S. K. Gangwal‡ Department of Chemical Engineering, Hampton University, Hampton, Virginia 23668, and Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709
Fischer-Tropsch synthesis was studied in a fixed-bed reactor over single-metal and bimetallic alloy catalysts, selected from Co, Ni, and Fe, supported on TiO2 at a total metal loading of 10 wt %. The catalysts, prepared by incipient wetness impregnation using nitrate precursors, were tested as is and in combination with a HZSM-5 zeolite. The test conditions were 1 MPa, 250 °C, H2/CO ) 1, and weight hourly space velocity (WHSV) ) 0.77 h-1. Alloying of metals resulted in a significant enhancement in CO conversion without an increase in methane selectivity. A 50:50 weight ratio Co-Ni catalyst physically mixed with HZSM-5 (5% Co-5% Ni/TiO2 + HZSM5) gave the highest CO conversion (45.2%) at the conditions tested. This compares to conversion of 8.9% and 10.5% with Co-only and Ni-only catalysts, respectively. Mixing the Co-Ni catalyst with HZSM-5 resulted in a significant reduction in methane selectivity and a significant increase in C4+ selectivity. The aromatic fraction increased from 1.5 to 8.1 wt %, the C2+ olefins were nearly eliminated, and i-C4H10 increased from 2.3 to 58.5 wt % in the C4 fraction. Introduction The Fischer-Tropsch synthesis (FTS) can convert coal or natural gas derived synthesis gas (CO + H2) to liquid fuels and high-value chemicals. It represents one of the best current alternatives to the use of crude oil, which is limited in resource. FTS involves a polymerization reaction beginning with a methylene intermediate derived from CO and H2 to produce a wide distribution of hydrocarbons ranging from methane to wax (C1 to C60+). The Fischer-Tropsch (F-T) reaction has been extensively studied and reviewed (Adesina, 1996; Anderson, 1984; Bell, 1981; Biolen and Sachtler, 1981; Dry, 1996; Mills and Steffgen, 1973; Vannice, 1976). F-T catalysts are typically based on group VIII metalssFe, Co, Ni, and Ruswith Fe and Co being the most popular. The product distribution over these catalysts is nonselective and is generally governed by the Anderson-Schulz-Flory (ASF) polymerization kinetics (Dry, 1981). There is recent significant industrial interest in FTS (Kogelbauer et al., 1996), particularly to produce middle distillates (C10-C20 hydrocarbons) from CO and H2 synthesis gas derived from natural gas in remote locations around the world (Parkinson, 1997). The key to improving the economics of FTS is increasing the selectivity to desired products such as diesel and high-octane gasoline. Bifunctional catalysts composed of a single F-T active metal and a zeolite (e.g., ZSM-5) have been the focus of attention for 20 years in order to overcome the nonselective ASF distribution and produce high-octane gasoline-range hydrocarbons (Bessell, * Corresponding author. Phone: (757) 727-5817. Fax: (757) 727-5189. E-mail:
[email protected]. † Hampton University. ‡ Research Triangle Institute.
1992a,b, 1993, 1994, 1995; Brennan et al., 1981; Caesar et al., 1979; Calleja et al., 1991; Chang, 1983; Chang et al., 1979; Desmond and Pepera, 1986; Fujimoto et al., 1985; Gormley et al., 1988; Guan and Wang, 1982; Haag et al., 1990; Jong and Cheng, 1995; Koh et al., 1995; Lapidus et al., 1977; Marczewski et al., 1991; Pennline et al., 1984; Rabo and Coughlin, 1987; Rao and Gormley, 1987; Stencel et al., 1983; Varma et al., 1985, 1986). Both the admixing of F-T metal oxide with the medium pore (∼6 Å) zeolite ZSM-5 (Bruce et al., 1984; Dwyer and Garwood, 1984; Pennline et al., 1984; Varma et al., 1985) and impregnation of an F-T metal nitrate or organometallic onto a ZSM-5 support (Rao and Gormley, 1980; Shamsi et al., 1984, 1986) have been shown to produce gasoline-range hydrocarbons from synthesis gas containing a high percentage of aromatics. ZSM-5 is a preferred support because (i) its pore structure provides shape selectivity; (ii) its high acidity promotes oligomerization, isomerization, cracking, and aromatization to restructure the primary F-T products; and (iii) it is stable under F-T conditions and resistant to coking. The bifunctional F-T and HZSM-5 catalyst provides a means of circumventing ASF kinetics and produces a gasoline-range product in excess of 48 wt % of the total hydrocarbon product. Catalysts prepared by supporting alloys of two or more group VIII F-T metals over conventional supports such as SiO2 and TiO2 have been shown to reduce methane, possess increased selectivity for C5+ hydrocarbons, and have higher F-T activity versus the corresponding monometallic catalysts (Arai et al., 1984; Horiuchi et al., 1988; Ishihara et al., 1987, 1988, 1990, 1991, 1992a,b; Lin et al., 1986). The mixing of metal components has a great effect on the activity and selectivity because of the possible electronic interaction
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1182 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998
between metal species. However, studies that report on the combination of F-T alloy catalysts containing two group VIII metals with zeolites are rare in the literature. Baeurle et al. (1993) studied the combination of a group VIII metal, Co, with a group VII metal, Mn, and ZSM-5 zeolite. Mn suppressed CH4 and increased olefins, whereas ZSM-5 transformed oxygenates and olefins to aromatics and cracked heavy hydrocarbons to lower alkanes. Koikeda et al. (1986) studied the combination of iron with ruthenium or cobalt and a zeolite and showed that these catalysts gave high CO conversion and enriched C5+ and gasoline fractions. This work was carried out to evaluate the combination of the effect of alloying (reduced CH4, increased CO conversion, increased C5+ yield) with the shape-selective properties of zeolite. In this paper, we report the performance of bimetallic F-T catalysts supported on TiO2 containing two metals selected from Fe, Ni, and Co, combined with HZSM-5. Experimental Section Catalysts. The TiO2-supported Co, Ni, and Fe catalysts (the F-T catalyst) were prepared by aqueous incipient wetness impregnation. The titania support (P25 Degussa) was used as received in the impregnation. The surface area of the support, as determined by N2 physisorption at -196 °C, was 48 m2/g. Ultrapure cobalt nitrate hexahydrate (Aldrich), nickel nitrate hexahydrate, and iron (III) nitrate nonahydrate (Aldrich) were used as the metal salts in the catalyst preparation. The incipient wetness impregnation was carried out in a single step followed by drying (110 °C for 12 h) and calcination (450 °C for 5 h). All the catalysts studied had a total metal loading of 10 wt %. The weight percent loading of Co, Ni, and Fe was verified using atomic absorption spectroscopy. ZSM-5 zeolite with a SiO2/Al2O3 ratio of 30 (CBV 3062E) was obtained from PQ Corp. The ammonium form of the catalyst (NH4-ZSM-5) was obtained by exchanging twice with a 1.0 M NH4NO3 solution at 80 °C for 3 h. HZSM-5 catalyst was prepared by calcining NH4-ZSM-5 at 550 °C for 4 h. The BET surface area of HZSM-5 was 290 m2/g. In addition, a catalyst containing 5% Co and 5% Ni supported on HZSM-5 (rather than TiO2) was also prepared. Catalyst Characterization. The BET surface area of the fresh and used catalysts was measured by N2 physisorption using a Micromeritics Gemini 2360 system. The samples were degassed in a Micromeritics Flow Prep 060 at 120 °C for 1 h prior to each measurement. A scanning electron microscope (SEM) micrograph of selected catalysts was taken using a Cambridge Stereoscan 100. X-ray diffraction (XRD) patterns were obtained using a Philips PW1800 X-ray unit using Cu KR radiation. Analyses were conducted using a continuous scan mode at a scan rate of 2θ ) 0.05°/s. For determination of the reduction behavior and the reducibility of the catalysts, temperature-programmed reaction (TPR) experiments were carried out using a Micromeritics 2705 system. A sample of ∼0.2 g was dried and degassed under high-purity Ar at 400 °C for 1 h followed by cooling to ambient temperature. Reduction was achieved using an H2/Ar gas mixture (volume ratio 5/95). Total gas flow was 40 cm3/min, and the temperature program was from 25 to 700 °C at a heating rate of 10 °C/min. Hydrogen consumed by the
catalyst was detected using a thermal conductivity detector (TCD) and recorded as a function of temperature. Hydrogen and carbon monoxide temperature-programmed desorption (TPD) measurements were also carried out using the Micromeritics 2705 system. In the case of H2-TPD, H2 was passed over the catalyst at 400 °C for 1 h. In the case of CO-TPD, CO was passed over the catalyst at 400 °C for 1 h. The sample was then cooled to 25 °C in the H2 or CO stream. The TPD spectrum was obtained using a heating rate of 10 °C/ min to 700 °C in a 40 cm3/min flow of high-purity Ar for H2-TPD or He for CO-TPD. During the programmed heating, desorption of H2 or CO from the sample was monitored by the TCD and recorded as a function of temperature. Activity and Selectivity Measurements. Study of the catalyst activity and selectivity was carried out in a 1-cm-i.d. high-pressure stainless steel downflow, fixedbed reactor. The reactor system shown in Figure 1 was similar to the system used by Bukur et al. (1989). The F-T catalyst (1.0 g) was pretreated in situ under flowing hydrogen at 450 °C for 16 h before reaction. The catalyst temperature was lowered to the reaction temperature before introducing the feed gas. For the combined FT + HZSM-5 experiments, the two catalysts were physically mixed before loading into the reactor. In the follow-bed arrangement, the synthesis gas mixture passed through the bed of F-T catalyst followed by that of HZSM-5. The two beds were separated by quartz wool. The weight ratio of HZSM-5 to F-T catalyst in either of the bed arrangements was fixed at 4.0. Unless otherwise stated, all reaction data were taken at 250 °C. The feed was a premixed gas of CO and H2 (ratio 1:1) containing 5% Ar as an internal standard for product analysis. The gas was fed to the catalysts bed at a weight hourly space velocity (WHSV) of 0.77 h-1. All the reactant gases were of high purity (99.9%) and were further purified by passing them through a Matheson model 6411 oxygen-absorbing purifier. This is a high-efficiency purification device which removes trace amounts of both oxygen and water vapor. The flow rate of the feed gas was automatically controlled by a mass flow controller (MFC). The feed was preheated before entering the reactor. The reactor temperature was controlled to (1 °C. The product was analyzed by an online Hewlett-Packard (HP) 5890 Series II plus gas chromatograph (GC), with advanced ChemStation control and capabilities. The HP 3365 Series II Chemstation offered automatic flow and split ratio of GC pneumatics. Three valves were used in the system: a 6-port gas sampling valve, a 10-port gas sampling valve with backflush to vent, and a 6-port column isolation valve. The system was configured for an extended analysis of hydrocarbons. The hydrocarbons C1-C15 and the oxygenates were analyzed using an HP-1 100 m × 0.25 mm × 0.5 µm capillary column and detected by a flame ionization detector (FID). The CO, CO2, and Ar were separated by a 2.6 ft × 1/8 in. Haysep Q column and 3.15 ft × 1/8 in. molecular sieve 13X columns and detected by TCD. The calibration was carried out using various calibration mixtures and pure compounds obtained from Supelco and HP. Results and Discussion BET, XRD, and SEM. The BET surface areas and the lattice constants of the metal crystals on the
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1183
Figure 1. F-T reaction fixed-bed test apparatus. Table 1. BET Surface Area and Lattice Constants catalyst
BET surface area (m2/g)
lattice constant (nm)
TiO2 support 10% Co/TiO2 7% Co-3% Ni/TiO2 5% Co-5% Ni/TiO2 3% Co-7% Ni/TiO2 10% Ni/TiO2
48 39.9 41.9 40 40.8 41.8
0.3521 0.3518 0.3513 0.3511 0.3505
catalyst
BET surface area (m2/g)
lattice constant (nm)
10% Fe/TiO2 5% Fe-5% Co/TiO2 5%Fe-5% Ni/TiO2 HZSM-5 5% Co-5% Ni/HZSM-5
42.7 40.6 38.9 290 241
0.3990 0.3885 0.3626
Table 2. Results of TPR/TPD Measurements TPR measurement
H2-TPD measurement
CO-TPD measurement
catalyst
peak max temp (°C)
H2 consumed (µmol/g‚cat)
peak max temp (°C)
H2 desorbed (µmol/g‚cat)
peak max temp (°C)
CO desorbed (µmol/g‚cat)
10% Ni/TiO2 10% Co/TiO2 10% Fe/TiO2 5% Co-5% Ni/TiO2 5% Fe-5% Co/TiO2 5% Fe-5% Ni/TiO2
570 430, 550 630 360, 530 480, 630 500, 580
1346 1700 587 1629 1445 1350
240 270, 510 480 270, 480 420, 540 180, 420
180 161 128 280 172 241
480, 570 480, 570 630 450, 570 660 690
654 500 436 621 873 950
catalysts calculated from XRD data are summarized in Table 1. XRD was performed after reduction at 450 °C to find the phases and lattice constants of the F-T catalysts. Only the diffraction peaks from the facecentered cubic (fcc) structured Co, Ni, and Fe metals were detected in the catalysts except for peaks from the TiO2 support. XRD results indicate that samples consist of TiO2 of both anatase and rutile forms, with anatase being more abundant. No metal titanate peaks were found. The lattice constant of bimetallic samples slightly deviated from that of the single metal, suggesting the formation of alloy. Alloy formation was further indicated by a systematic shift in the (111) line in the bimetallic sample with a change in the relative amounts of the two metals. The values and trend of the lattice constant from this work agree closely with the values reported in the literature (Ishihara et al., 1987). The values in Table 1 show a somewhat more linear relationship with metal content than those reported by Ishihara et al. (1987).
The SEM micrographs of the blank titania and Co and Ni catalysts are shown in Figure 2a-d. The bimetallic Co-Ni catalyst showed uniform fine crystals at the surface in contrast to the blank titania. These crystals have a uniform size of
