Ind. Eng. Chem. Res. 1998, 37, 4657-4668
4657
Higher Alcohol Synthesis over Double Bed Cs-Cu/ZnO/Cr2O3 Catalysts: Optimizing the Yields of 2-Methyl-1-propanol (Isobutanol) Maria M. Burcham, Richard G. Herman, and Kamil Klier* Zettlemoyer Center for Surface Studies and Department of Chemistry, Lehigh University, Seeley G. Mudd Building, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015
Higher alcohol synthesis from H2/CO has been carried out over two tandem beds of a cesiumpromoted Cu/ZnO/Cr2O3 catalyst with a temperature difference between the beds. Lower alcohols produced in the low temperature top bed are supplied as oxygenated precursors to the higher temperature bottom bed to react further in the chain growth process leading to the formation of higher alcohols. In this configuration, a high space time yield of 202 g/kg of catalyst/h of 2-methyl1-propanol (isobutanol) was obtained. This presently observed yield of isobutanol in a double bed configuration with the copper-based catalyst in both beds represents a very significant enhancement over the earlier reported isobutanol yields in a double bed confiuration with a copper-free catalyst in the second bed (Beretta et al., 1996). A kinetic reaction network (Smith et al., 1991) was modified to model this double bed catalyst configuration. This model was then used to obtain kinetic parameters and predict product yields for the reaction system in the current study. It was also demonstrated that the kinetic model could be used as a predictive tool to optimize experimental parameters. In particular, the model was used to predict the optimum mass ratio of the two catalyst beds to produce the maximum yield of isobutanol. Introduction Recent work in the area of oxygenate synthesis from H2/CO has focused on higher alcohol synthesis over alkali-metal-promoted copper-based catalysts, namely for the production of 2-methyl-1-propanol (isobutanol) (Smith and Anderson, 1984; Calverley and Anderson, 1987; Elliott and Pennella, 1989; Nunan et al., 1989ac; Forzatti et al., 1991; Calverley and Smith, 1993; Hindermann et al., 1993; Boz et al., 1994; Beretta et al., 1995 and 1996). Much interest in the production of isobutanol is in response to reports that methyl tertiary butyl ether (MTBE), a gasoline octane enhancer, could be produced via the one- or two-step dehydrative coupling of methanol and isobutanol (Keim and Falter, 1989; Klier et al., 1993a,b; Sanfilippo, 1993; Bremen et al., 1994 and 1995; Wang et al., 1995; Calafat and Laine, 1994; Chaumette et al., 1994; Herman and Lietti, 1994; Herman et al., 1994a,b; Mouaddib et al., 1994; Sun et al., 1995; Campos-Martin et al., 1995 and 1996; Finkeldei at al., 1996; Lietti et al., 1996; Moser and Connolly, 1996; Apesteguia et al., 1997; Cai et al., 1997). The catalytic production of methanol is already a wellestablished process (Kung 1980; Wade et al., 1981; Klier, 1982; Marchner and Moeller, 1983; Chinchen et al., 1988; Bridger and Spencer, 1989; Herman, 1991). However, new catalytic routes for the synthesis of isobutanol could help to make the alcohol-coupling pathway to methyl tertiary butyl ether an attractive, nonpetroleum-based alternative industrial process for the synthesis of this high octane ether. Alkali-metal-promoted copper-based catalysts such as those used in the current study are bifunctional basehydrogenation catalysts. The higher alcohol forming aldol reactions are thought to occur within the coordina* To whom correspondence should be addressed. Telephone: (610) 758-3577. Fax: (610) 974-6469. E-mail: kkØ4@ lehigh.edu.
tion sphere of the heavy alkali cation (Klier et al., 1993a), the base function of the catalyst. Among the alkali-metal dopants, it has been shown that the order of promotion is Cs > Rb > K > Na, Li (no promotion) at submonolayer molecular dispersion of the alkali-metal salt such as formate (Nunan et al., 1988) or hydroxide (Vedage et al., 1985). The Cu/ZnO/(Cr2O3) or ZnO/Cr2O3 portion of the catalyst provides a mild hydrogenation function, along with a moderate basicity of ZnO. Reaction intermediates over these alkali-metal-doped catalysts are thought to be 1,3-ketoalkoxides, and the calculated transition state is consistent with the order of promotion of the alkali-metal dopants (Klier et al., 1993a) in that the larger alkali-metal cations provide for easier opening of the proposed 1,3-ketoalkoxide intermediates. Furthermore, the aldol reaction requires a low hydrogen to carbon monoxide ratio because alcohols must first be dehydrogenated to aldehydes which in turn are converted to enolates that are the reactive species (see Figure 1). The doping levels of the molecularly dispersed cesium salts on these binary and ternary copper-containing catalysts have been optimized (Nunan et al., 1988; Nunan et al., 1989c). Maximum activity for methanol synthesis over Cs-Cu/ZnO catalysts was observed at a cesium doping level corresponding to 1.5 × 1018 Cs atoms per m2 of catalyst surface area (Nunan et al., 1988). The product formation rates and selectivities show that cesium plays the same role in the alcohol synthesis mechanism over both the Cu/ ZnO and Cu/ZnO/Cr2O3 catalysts (Nunan et al., 1989c). In addition to promoting alcohol synthesis, the presence of cesium also suppressed the production of the side product dimethyl ether (DME) by blocking a portion of the acid chromia sites in the ternary catalyst (Nunan et al., 1989c). The approach demonstrated previously by Beretta et al. (1995 and 1996) for increasing isobutanol yield was the use of a double bed reactor, where the first bed was
10.1021/ie9705620 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/31/1998
4658 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 1. Schematic representation of the β-addition and linear growth mechanisms showing the possible routes to primary and secondary alcohols. The alkali cation is represented by a circled plus, and Cu/ZnO is the hydrogenation component.
a 3 mol % cesium-doped Cu/ZnO/Cr2O3 catalyst at 598 K and the second bed consisted of a copper-free 4 mol % cesium-doped ZnO/Cr2O3 catalyst at 678 K. In the double bed configuration, the top bed produced predominantly C1 - C3 oxygenated intermediates that were then supplied to the bottom bed where they were converted into higher alcohols. This approach showed that two beds could be used to advantage, in that the double bed reactor produced a greater yield of isobutanol (1.88 mol/kg of catalyst/h) than either catalyst in a single bed configuration. In the current study, the double bed reactor was employed with the same 3 mol % Cs-Cu/ZnO/Cr2O3 catalysts in both beds, but with the top and bottom bed temperatures of 598 and 613 K, respectively. The double bed reactor with copper-based catalyst in both beds used in the present study produced a greater yield of isobutanol (2.73 mol/kg of catalyst/h) than the previous experiment by Beretta et al. (1995 and 1996). This is due to the fact that the copper-based catalyst used in place of the copper-free catalyst in the bottom bed is more active; therefore, the bottom bed can operate at a lower temperature, which favors higher equilibrium amounts of the methanol precursor that can subsequently be converted to higher alcohols. The current study also examined the stability of the copper-based catalyst at high temperatures of 613-703 K. The advantage of the double bed design is supported herein by an integrated kinetic model that uses the output from the first catalyst bed as a feed into the second bed. A kinetic model for methanol synthesis from H2/CO mixtures over copper/zinc oxide-based catalysts was developed by Smith and co-workers (1990 and 1991) based, in part, on experiments performed by Young (1987) and Nunan et al. (1988 and 1989a). In the current work, this model was used in a modified form to predict the activity over Cs-Cu/ZnO/Cr2O3 catalysts utilized in a double catalyst bed configuration. The successful double bed model was then shown to be
a useful predictive tool for optimizing the relative amounts of the catalyst in each bed to maximize isobutanol production. Experimental Section Catalyst Preparation. The 30/45/25 mol % Cu/ZnO/ Cr2O3 catalysts were prepared by the coprecipitation method described elsewhere (Nunan et al., 1989d). The initially precipitated single-phase hydrotalcite-like precursor was rinsed with distilled water, dried, and calcined in air at 623 K for 3 h. It has been shown that the optimum loading of cesium for promoting higher alcohol synthesis over these catalysts is 3 mol % (Nunan et al., 1989c), and the catalysts were doped accordingly. The procedure for cesium doping consisted of adding the calcined catalysts to a N2-purged aqueous solution of cesium formate that was evaporated to dryness, with constant stirring under flowing N2. The 3 mol % cesium-doped Cu/ZnO/Cr2O3 catalysts were calcined again in air at 623 K for 3 h. The surface areas and particle sizes of the tested catalysts are compared with the previous binary catalysts in Table 1, in which also the individual surface areas of the Cu, ZnO, and Cr2O3 components are shown as calculated from particle sizes determined from X-ray diffraction patterns (Figure 2). Catalyst Characterization. X-ray powder diffraction (XRD) patterns of the tested catalysts were obtained using a Philips APD1700 automated powder diffractometer. Cu KR radiation (λ ) 0.154 nm) was used as the X-ray source. Patterns were obtained for 2θ values from 20 to 70° at a rate of 1.0 deg/min. Catalyst BET surface areas were measured using a Micromeritics Gemini 2360 instrument. Nitrogen was used as the adsorbate at 77 K. Catalyst surface areas were computed by built-in software. Surface areas of copper and zinc oxide were calculated on the basis of crystallite sizes determined from XRD peak broadening. The Scherrer equation in the form
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4659 Table 1.
Comparison of Physical Properties of the Cs-Doped Binary Cu/ZnO and Ternary Cu/ZnO/Cr2O3 Catalysts Cs-Cu/ZnO (0.4/30/70; tested at 598 K)
particle sizes from XRD line broadening surface areas (SA)
Cs-Cu/ZnO/Cr2O3 (3/30/45/25; tested at 703 K)
Cu, 10 nm; ZnO, 20 nm (Bogdan et al., 1988)
Cu, 6 nm; ZnO, 12 nm
tot. 34 m2/g of catalyst;a Cu, 10 m2/g of catalyst;b ZnO, 24 m2/g of catalyst (Bulko, 1980)
tot., 86 m2/g of catalyst;a Cu, 11 m2/g of catalyst;b ZnO, 48 m2/g of catalyst;b Cr2O3, 27 m2/g of catalystc
a BET surface area. b Calculated from XRD particle sizes (see Figure 2) by assuming copper particles are hemispheres on the ZnO and ZnO particles are rectangular prisms (see discussion in Mehta et al., 1979). c Surface area (SA) (Cr2O3) ) total SA - SA(Cu) - SA(ZnO).
Figure 2. X-ray powder diffraction (XRD) pattern of the 3 mol % Cs-Cu/ZnO/Cr2O3 catalyst tested as a single bed at temperatures up to 703 K.
d)
Kλ Bd cos θ
(1)
was used, where d is the crystallite size in nm, K is the Scherrer constant (0.98) and λ is the wavelength of the X-ray radiation (0.154 nm). Bd is the full width at halfmaximum of the XRD peak in radians, and θ is the Bragg angle in radians. In the calculation of surface areas, ZnO particles were assumed to be rectangular prisms with hemispheres of copper on the surface. For this reason, the area of contact between the copper and zinc oxide was excluded, as was the area of the zinc oxide basal plane (Bulko, 1980) in the calculated surface areas. Catalytic Testing. Catalysts were reduced in situ with flowing 2% (vol) H2 in N2 at 523 K and ambient pressure prior to the start of testing. The reduction was monitored by using a gas chromatograph to observe the production of water and was considered complete when the peak for water no longer appeared on a chromatogram. Testing was carried out in a continuous downflow copper-lined stainless steel reactor system that has been described previously (Nunan et al., 1989a; Beretta et al., 1996). The total reaction pressure was held constant at 7.6 MPa, and the reactant consisted of H2/CO ) 0.75 with gas hourly space velocity (GHSV) ) 18 375 L(STP)/ kg of catalyst/h. For the double bed experiment, 0.5 g of 20-35 mesh size particles of the Cs-Cu/ZnO/Cr2O3 catalyst, diluted with 0.5 mm Pyrex beads, was used in each bed. In addition, the catalyst was examined in a single bed configuration. In the single bed test, the GHSV was 4990 L (STP)/kg of catalyst/h over 1.5 g of catalyst with H2/CO ) 0.45 and temperatures of 613703 K. The reactor exit stream was sampled every 2060 min using an automated heated sampling valve and analyzed by a gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Chromatographic data were transferred to a personal computer for data analysis using ChromPerfect software (Justice Innovations).
Figure 3. Schematic representation of the double bed reactor used in Beretta et al. (1996) and the current study.
Results Figure 3 shows a schematic of the double bed reactor, which is also shown in greater detail in Beretta et al. (1996). It should be noted that the two catalyst beds were well-separated, with approximately 20 cm of space filled with Pyrex beads between them. Two individual thermocouples measured and controlled the temperature in each bed separately. The results of the double bed experiment in the current study with the copper-based catalyst in each bed are shown in Table 2 compared with the results of the double bed experiment with the Cs-Cu/ZnO/Cr2O3 catalyst in the first bed and Cs-ZnO/Cr2O3 in the second bed (Beretta et al., 1996). The current experiment with the copper-based bottom bed produced a yield of 2.73 mol of isobutanol/kg of catalyst/h, which is approximately 42% more than with the copper-free bottom bed. The other major products formed in the current experiment were methanol, ethanol, and 1-propanol. Only small amounts of other minor products were observed, showing a high selectivity to the desired product, isobutanol. In addition to the fact that more isobutanol was produced in the current experiment, the data in Table 2 also show that the selectivity to isobutanol is higher than in Beretta et al. (1996), where the selectivity to isobutanol was 26 C atom %, whereas in the current study isobutanol selectivity was 31 C atom %. The stability of the copper-based catalyst at high temperature was also of interest in this study. Figure 4 gives the productivities for methanol and isobutanol observed in a single bed test of the Cs-Cu/ZnO/Cr2O3 catalyst. As shown in Figure 4, the catalyst performance was relatively stable at temperatures of 613703 K over a period of nearly 40 h. Although fluctuations were evident, there was no dramatic deactivation observed at these high temperatures over the duration of the testing period.
4660 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 Table 2. Comparison of the Productivities of the Double Bed Experiment Reported in Beretta et al. (1996) with the Double Bed Experiment in the Current Studya
a
product (m ) methyl)
Beretta et al. (1996): 3% Cs-Cu/ZnO/Cr2O3 (598 K); 4% Cs-ZnO/Cr2O3 (678 K) space time yield (mol/kg of catalyst/h)
Burcham et al. (current): 3% Cs-Cu/ZnO/Cr2O3 (598 K); 3% Cs-Cu/ZnO/Cr2O3 (613 K) space time yield (mol/kg of catalyst/h)
methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol 2-m-1-propanol 2-m-1-butanol 2-m-1-pentanol 2-m-1-hexanol methyl formate methyl acetate 2-butanol 3-m-2-pentanol 3-pentanol 2-m-3-pentanol dimethyl ether methane
5.59 0.15 0.39 0.06 0.04 0.02 1.88 0.37 0.21 0.21 0.19 0.02 0.07 0.08 0.09 0.23 0.10 0.88
17.94 0.35 1.10
