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Ind. Eng. Chem. Res. 1998, 37, 1786-1792
n-Hexane Conversion Catalyzed by Sulfated Zirconia and by Ironand Manganese-Promoted Sulfated Zirconia: Catalytic Activities and Reaction Network S. G. Ryu† and B. C. Gates* Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616
The conversion of n-hexane catalyzed by unpromoted sulfated zirconia and Fe- and Mn-promoted sulfated zirconia was investigated with a flow reactor at atmospheric pressure, temperatures of 0-25 °C, and an n-hexane partial pressure of 0.001 atm. The n-hexane conversion initially increased as a function of time on stream, followed by a rapid decline. The major primary reaction was isomerization, giving monobranched isomers (predominantly 2-methylpentane). The secondary isomerization reactions gave dibranched products (predominantly 2,3-dimethylbutane). Disproportionation reactions were apparently secondary, giving isobutane, isopentane, heptanes, and octanes from the isomerization products. Small amounts of cracking products (methane, ethane, and propane) were also observed at the shortest times on stream, before the catalyst deactivated significantly. On the basis of the different induction times observed for the various reaction products, it is inferred that the disproportionation is bimolecular and the isomerization (predominantly) monomolecular. Introduction Low temperatures are preferred for alkane isomerization processes because the equilibria favor branched (high-octane-number) products. Alkane isomerization catalysts are typically acids, and the only solid acids that have significant activity at low temperatures are extremely strong acids, e.g., AlCl3 supported on alumina. This catalyst is applied in industrial isomerization processes, but it is a candidate for replacement because of its corrosiveness and potential for pollution. Active, stable solid acid catalysts that might replace supported AlCl3 have been made by sulfation of metal oxides. Sulfated zirconia is so active that it catalyzes the isomerization of n-butane at room temperature (Hino and Arata, 1980), and the addition of Fe and Mn promoters increases its activity by 2 or 3 orders of magnitude (Hsu et al., 1992). The high activity of the promoted catalyst has been confirmed by several groups (Adeeva et al., 1994; Cheung et al., 1995a; Coelho et al., 1995; Jatia et al., 1994; Tabora et al., 1995). The results obtained with butane suggest that the catalyst might be useful for isomerization of other alkanes, such as n-hexane. Our goal was to characterize the performance of unpromoted and Fe- and Mn-promoted sulfated zirconia catalysts for conversion of n-hexane at subambient temperatures. This work, in combination with results characterizing the performance of these catalysts for conversion of n-pentane with the same catalysts (Rezgui and Gates, 1996), provides a basis for comparison of the reactivities of the various light alkanes and for elucidation of the reaction mechanisms and how they vary with the number of carbon atoms in the reactant alkane. The results reported here were obtained with a low partial pressure of n-hexane reactant (0.001 atm) to allow * To whom correspondence should be addressed. † Present address: Agency for Defense Development, P.O. Box 35, Yuseong, Taejon 305-600, Korea.
measurements of relatively simple product distributions and to facilitate the determination of an approximate reaction network. Experimental Methods Materials and Catalyst Preparation. The gaseous reactant mixture was fed from a cylinder containing 1.0 mol % n-hexane in N2 (Puritan Bennett), and this was diluted with a stream of N2 (Puritan Bennett, 99.997%). The mixture was found by gas chromatography to contain 0.0169 mol % 3-methylpentane and 0.0067 mol % 2,2-dimethylpentane impurities (which were taken into account in the data analysis). The catalysts were prepared from sulfated zirconium hydroxide (4.0 wt % sulfate; Magnesium Elektron). Sulfated zirconia was prepared by calcining sulfated zirconium hydroxide in static air as the temperature was raised at a rate of 2.5 °C/min from room temperature to 600 ˚C and then held for 3 h. The sample had a surface area of approximately 100 m2/g. The promoted catalyst, containing 1.0 wt % Fe, 0.5 wt % Mn, and 1.8 wt % sulfur (FMSZ), was made by incipient wetness impregnation of sulfated zirconium hydroxide with aqueous solutions of Fe(NO3)3 and Mn(NO3)2; 37.55 g of sulfated zirconium hydroxide was mixed with a solution consisting of 6 mL of distilled water, 3.256 g of Fe(NO3)3‚9H2O (Aldrich, 98%; the solution was 0.62 M), and 0.784 g of Mn(NO3)2‚6H2O (Aldrich, 98%). The resultant promoted material was dried at 110 °C for 5 h, calcined in static air as the temperature was raised at a rate of 2.5 °C/min from room temperature to 600 °C, and then held for 3 h. Catalytic Reaction Experiments. Before each catalytic reaction experiment, the catalyst in flowing N2 [30 mL (NTP)/min] was heated from room temperature to 450 °C at a rate of 7.1 °C/min, and the temperature was then held at 450 °C for 1.5 h. After this pretreatment, the catalyst was cooled to the desired reaction temperature in N2 flowing at 30 mL (NTP)/min. Cata-
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lytic reactions were carried out in either of two oncethrough flow reactors, a thermostated stainless steel U-tube or a straight quartz tube. Typically, 0.5 g of catalyst was used for each experiment, unless stated otherwise. The finely ground catalyst particles were placed on a porous frit or on quartz wool. Experiments were done with the feed gas diluted with N2 so that the n-hexane inlet partial pressure was 0.001 atm. The reaction experiments were carried out at 1 atm and a temperature of 0 °C (with the U-tube reactor in an icewater bath) or 25 ( 1 °C with the quartz reactor. The total volumetric flow rate of feed was 80 mL (NTP)/min. Products were analyzed with an on-line gas chromatograph (Hewlett-Packard model 5890II) with a J&W PLOT fused silica capillary column (30 m in length and 0.53 mm in diameter) and a flame ionization detector. Products were identified by gas chromatography/mass spectrometry. Results Definitions. n-Hexane conversion and selectivity are defined as follows: Normalized conversion of n-hexane to each of the individual gas-phase products (containing n carbon atoms) is defined as (n × number of moles of products)/(6 × number of moles of n-hexane fed). Normalized selectivity for the formation of an individual product is defined as (normalized conversion to gasphase product)/(n-hexane conversion). n-Hexane conversion is defined as the sum of the individual gas-phase product conversions. Catalyst Performance. When the reactants flowed through the reactor in the absence of any catalyst, conversion of n-hexane was negligible. Both the unpromoted sulfated zirconia (SZ) and the FMSZ were found to be active for n-hexane conversion at 0 and 25 °C. The products formed in the presence of either catalyst were methylpentanes (mostly 2-methylpentane), dimethylbutanes (mostly 2,3-dimethylbutane), isobutane, isopentane, propane, and higher-molecularweight alkanes (heptanes and traces of octanes). Traces of n-butane and n-pentane were observed when the reaction was catalyzed by either the promoted or unpromoted sulfated zirconia, and traces of methane and ethane were observed with the promoted catalyst when the conversion of n-hexane was high and the temperature was 25 °C. Carbon balances determined from the flow rates and gas chromatographic analyses of products closed within a few percent. In the reported experiments, the number of molecules of n-hexane converted in the flow reactor was typically only about 0.3 per sulfate group. As the number of sulfate groups is likely greater than the number of catalytic sites, the results suggest that the reactions were catalytic but are not in themselves sufficient to demonstrate that they were; however, results for other alkanes demonstrate catalysis (e.g., Zarkalis et al., 1994). The unpromoted catalyst was white and the promoted catalyst rust colored. The colors of the catalysts did not change perceptibly during operation in the flow reactors for periods of up to 400 min. We infer that any carbonaceous deposits were minimal. However, small amounts of dark-colored deposits were observed on the porous frit after reaction at 25 °C in the presence of the promoted sulfated zirconia. Conversions were e55% at the higher temperature; most data were obtained at low conversions. Conversions changed with time on stream (e.g., Figure 1). The
Figure 1. Conversion of n-hexane to products in the presence of iron- and manganese-promoted sulfated zirconia and unpromoted sulfated zirconia in nitrogen in a flow reactor. Reaction conditions: mass of catalyst, 0.5 g; feed n-hexane partial pressure, 0.001 atm; total feed flow rate, 80 mL (NTP)/min.
data indicate an initial (induction) period followed by a period of declining conversion whether the conversion was high or low. The conversions of n-hexane to all products at 0 and 25 °C are shown in Figure 1 as a function of time on stream; these data provide a comparison of the performances of the promoted and unpromoted catalyst. The length of the induction period (the time to maximum conversion) increased with decreasing temperature for each catalyst, and the maximum conversion increased with increasing temperature. The induction period characterizing FMSZ was shorter than that characterizing SZ, and the maximum conversion characterizing FMSZ was higher than that characterizing SZ (Figure 1), indicating that the promoters increased the activity of the sulfated zirconia catalyst. The deactivation rates (during the periods following the maximum in conversion) were higher with FMSZ than with SZ, and the deactivation rates increased with temperature. In attempts to determine the effects of the promoters in n-hexane conversion, experiments were done with SZ and FMSZ under the same conditions. The conversion data at 0 and 25 °C (Table 1) show that the promoting effect is strongly temperature dependent: The conversion characterizing the promoted catalyst at 25 °C was found to be about 3 times that characterizing the unpromoted catalyst, but at 0 °C the promoted catalyst was 2 orders of magnitude more active than the unpromoted catalyst by this measure. In attempts to determine whether conversions were differential, plots of conversion vs inverse space velocity were prepared (Figure 2), but because of the complexity of the conversion vs on-stream time plots, it was not obvious how to determine a single representative conversion for each catalytic reaction experiment. As a first approximation, we used the conversion at the maximum as a function of time on stream. This choice is arbitrary but consistent with the choices of earlier authors who investigated n-butane conversion (Cheung et al., 1995a) and n-pentane (Rezgui et al. 1998, in press) using nearly the same catalyst as ours. In contrast to the differential
1788 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 1. n-Hexane Conversion Catalyzed by Fe- and Mn-Promoted Sulfated Zirconia and by Unpromoted Sulfated Zirconia: Comparison of Activities and Selectivities of the Catalystsa Fe- and Mn-promoted sulfated zirconia 107 × space velocityb reaction temperature (°C) conversion (%) product selectivitiesc (mol %) methane ethane propane n-butane isobutane n-pentane isopentane methylpentanes dimethylbutanes C6+ species
25 55.5 trace trace 1.0 trace 76.6 trace 16.3 3.9 1.8 0.4
1.19 0 19.8 0 0 0.3 trace 11.6 trace 15.0 68.3 4.3 0.5
sulfated zirconia 1.19 25 0 16.4 0.2 0 0 0.2 trace 19.5 trace 16.2 53.7 9.3 1.1
0 0 0 0 0 0 0 100 0 0
a Data taken at the maximum conversions as a function of time on stream. b Space velocity in units of mol/(g of catalyst‚s). c Selectivity for product i defined as the molar ratio of the gasphase product i to the sum of the gas-phase products.
Figure 2. Conversion of n-hexane in the presence of iron- and manganese-promoted sulfated zirconia at 0 °C as a function of time on stream. Values are from the maximum conversion for each product. Reaction conditions: mass of catalyst, 0.15-0.7 g; feed n-hexane partial pressure, 0.001 atm; total feed flow rate, 80 mL (NTP)/min.
conversion plots obtained for n-butane and for npentane, the plots for n-hexane are not straight lines through the origin. Thus, we cannot accurately estimate rates of n-hexane conversion. Product Distributions. Conversions to each product as a function of time on stream are shown in Figures 3 and 4 for FMSZ at 0 °C and SZ at 25 °C, respectively. The initial products formed in the presence of each catalyst were mainly monobranched hexane isomers (predominantly 2-methylpentane). The conversion to each product initially increased toward a maximum as a function of time on stream. At times beyond the maximum conversion to each product, the conversion decreased rapidly. The times to maximum conversion were different for the different products, with the values increasing in the order methylpentanes ≈ dimethylbutanes < isopentane ≈ isobutane < C6+ compounds
Figure 3. Conversion profiles for each product of n-hexane conversion in the presence of iron- and manganese-promoted sulfated zirconia at 0 °C. Reaction conditions as stated in the caption of Figure 1.
Figure 4. Conversion profiles for each product of n-hexane conversion in the presence of unpromoted sulfated zirconia at 25 °C. Reaction conditions as stated in the caption of Figure 1.
(Figures 3 and 4). The same qualitative pattern was observed for each catalyst at each reaction temperature. However, some products (methane, ethane, and propane) were not characterized by the same trend of the maximum conversion as a function of time on stream. Only small amounts of these products were detected, at the shortest times on stream (data not shown). With either the promoted or the unpromoted catalyst, the conversion of n-hexane into isobutane and isopentane was characterized by nearly the same molar ratio of the two products as a function of time on stream at conversions < 25%. However, at 25 °C, the conversion of n-hexane catalyzed by FMSZ (55.5%, Table 1) was characterized by a maximum selectivity to isobutane (76.6 mol %) that was much higher than the maximum selectivity to isopentane (16.3%). Dibranched hexane isomers (dimethylbutanes, mostly 2,3-dimethylbutane) and C6+ species were also formed at each reaction temperature, but only small amounts
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Figure 5. Normalized selectivities to products of n-hexane conversion in the presence of iron- and manganese-promoted sulfated zirconia at 0 °C. Reaction conditions as stated in the caption of Figure 1.
Figure 6. Normalized selectivities to products of n-hexane conversion in the presence of unpromoted sulfated zirconia at 25 °C. Reaction conditions as stated in the caption of Figure 1.
of dimethylbutanes were detected at times on stream < 80 min when the catalyst was FMSZ. At the shortest times on stream, no C6+ products were detected with FMSZ or SZ. Only small amounts of C6+ were detected with SZ at 25 °C (data not shown). The time to reach maximum conversion to C6+ products was always greater than that to reach maximum conversion to isopentane or isobutane (Figure 3). The normalized selectivities to products versus time on stream are shown in Figure 5 for FMSZ at 0 °C and in Figure 6 for SZ at 25 °C. The selectivities to methylpentanes initially decreased with increasing time on stream and then increased to approximately the initial selectivities. The trends in the selectivity curves characterizing isobutane and isopentane are opposite to the trend characterizing the methylpentane isomerization products. The selectivities to methylpentanes extrapolated to zero time on stream are nearly 100% for both FMSZ and
Figure 7. Selectivity of n-hexane conversion in the presence of iron- and manganese-promoted sulfated zirconia at 0 °C. Reaction conditions: feed n-hexane partial pressure, 0.001 atm; total flow rate, 80 mL (NTP)/min; mass of catalyst, 0.15-0.7 g.
SZ (Figures 5 and 6). The initial selectivity for formation of methylpentanes (which was close to 100%) did not change substantially when the conversion of nhexane changed as a result of changes in the space velocity (data not shown). Correspondingly, the selectivities for formation of the other products, namely, dimethylbutanes (dibranched isomers), C6+ species, isobutane, and isopentane, were found to be zero or nearly zero when the data were extrapolated to zero time on stream (Figures 5 and 6). Furthermore, extrapolation of the plots of the maximum selectivity for formation of each of the products dimethylbutanes, C6+ alkanes, isobutane, and isopentane vs n-hexane conversion (which was changed by variation of the space velocity) showed that the selectivity approached zero as the conversion approached zero (Figure 7). The selectivities for formation of propane decreased with time on stream. The normalized propane selectivity observed with 0.3 g of FMSZ catalyst at 0 °C vs time on stream is shown in Figure 8; propane was detected only during the first 100 min on stream. This pattern was qualitatively the same whether the conversion was high or low and whether the catalyst was FMSZ or SZ. The data are not sufficient to determine whether propane was a primary product. Traces of n-pentane, n-butane, methane, and ethane were also detected, but only under some of the reaction conditions (e.g., high conversions of n-hexane) and at short times on stream. Discussion Activities of FMSZ and SZ Catalysts for nHexane Conversion. The data demonstrate that FMSZ catalyzes the conversion of n-hexane into isohexanes even at temperatures as low as 0 °C. Thus the promoted catalyst is highly active for isomerization of n-hexane, just as it is highly active for isomerization of n-butane (Cheung et al., 1995a) and n-pentane (Rezgui and Gates, 1996). The pattern of an induction period followed by rapid deactivation observed in this work for n-hexane conversion is qualitatively the same as that
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Figure 8. Normalized propane selectivity in n-hexane conversion in the presence of iron- and manganese-promoted sulfated zirconia at 0 °C. Reaction conditions: feed n-hexane partial pressure, 0.001 atm; total flow rate, 80 mL (NTP)/min; mass of catalyst, 0.3 g.
observed for n-butane and n-pentane conversion in the presence of the same catalyst (Cheung et al., 1995a; Rezgui and Gates, 1996). The induction period was observed in this work for n-hexane conversion in the presence of the unpromoted SZ also. The Fe and Mn promoters were reported to increase the activity of sulfated zirconia by 2 or 3 orders of magnitude for n-butane isomerization (Hsu et al., 1992) and to increase the activity substantially for n-pentane isomerization at low reaction temperatures; the statement for n-pentane is qualitative because the activity of SZ was too low to measure (Rezgui et al., 1998). However, no significant promoting effect was observed for conversion of these alkanes at high reaction temperatures (e.g., 450 °C), at which mainly cracking and disproportionation reactions took place (Cheung et al., 1995b). Similarly, the effects of the promoters are different at different temperatures for n-hexane conversion. The data (Table 1) show that FMSZ is approximately 2 orders of magnitude more active than SZ at 0 °C but only 3 times more active at 25 °C. Because the isomerization was the predominant reaction at 0 °C and the disproportionation and cracking were predominant at 25 °C, we infer that Fe and Mn promote the isomerization reactions more effectively than the disproportionation and cracking reactions. Comparison of Reactivities of n-Alkanes. Rezgui and Gates (1996) reported that n-pentane is about 3 times more reactive than n-butane in the presence of FMSZ at 40 °C. The data (Table 2) show that the reactivity of n-hexane is only slightly greater than that of n-pentane in the presence of the same catalyst at 0 °C. Reaction Network. The product distribution data provide a basis for a statement of an approximate reaction network for n-hexane conversion catalyzed by Fe- and Mn-promoted sulfated zirconia or unpromoted sulfated zirconia. The normalized selectivity vs time on stream plots (Figures 5 and 6) and the normalized selectivity vs n-hexane conversion plot (Figure 7) lead to the identification of methylpentanes as primary products; thus, a primary reaction was isomerization. The data identify isobutane, isopentane, dimethyl-
butanes, and C6+ alkanes as nonprimary products, as the selectivities for formation of each of these, in contrast to that of methylpentanes, was nearly zero in the limit of zero conversion. Thus, we identify the reactions giving these products (dibranched isomers, dimethylbutanes) as nonprimary (and presumably secondary) reactions. The formation of C6+ products indicates the occurrence of carbon-carbon bond forming reactions, which we postulate to be disproportionation on the basis of the chemistry determined earlier for n-butane (Cheung et al., 1995A) and n-pentane (Rezgui and Gates, 1996). Other products included methane, ethane, and propane; these are cracking products. As these were formed in only trace amounts, it is not possible to determine the reactants from which they were formed. Thus, the reaction network is represented in Scheme 1 (with the primary reactions highlighted in boldface), as this is perhaps the simplest network accounting for all the observations. The cracking reactions represented here could involve any of the higher-molecular-weight alkanes as reactants, but the data are not sufficient to determine which of these were converted into cracking products in significant amounts. Thus, the statements of the cracking reactions in the network are largely arbitrary; we emphasize that these reactions were not of much significance kinetically. We also emphasize that the reaction network is no doubt simplifiedsthere are too many products and too few data to determine the network fully. At the shorter times on stream (e.g.,
