Ind. Eng. Chem. Prod. Res. Dev. 1980, 25, 11-14
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CATALYST SECTION Deactivation and Regeneration of a Promoted Transition-Metal-Zeolite Catalyst Henry W. Pennline' and Sidney S. Pollack Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236
The regeneration of a promoted transition-metal-zeolite catalyst that had been used to convert synthesis gas to gasoline-range hydrocarbons was investigated in a mixed reactor system. Carbonaceous deposits were found on the catalyst-a coprecipitated cobalt-thoria admixed with ZSM-5 zeolite-after it had been subjected to low hydrogen to carbon monoxide ratio synthesis gas at the processing conditions. Oxidative regenerations did not succeed in returning the catalyst to its initiil synthesis activity. X-ray diffraction analysis of the used and regenerated catalyst samples suggested that sintering of the cobalt occurred. The crystallite size of the Co,O, formed after regeneration was inversely related to the synthesis gas conversion after reduction of the Co,O, to the metal.
Introduction
A particularly interesting scheme for converting mixtures of hydrogen and carbon monoxide to high-quality liquid fuels is to directly react these synthesis gas mixtures over a bifunctional catalyst. The catalyst would be a combination of a Fischer-Tropsch catalyst and a shapeselective zeolite. The use of synthesis gas with low ratios of H 2 / C 0 is of interest, since these low ratios would be produced by more efficient, state of the art gasification processes (Kuo, 1984). Commercially, a process of this type would be most efficient if the hydrogen to carbon monoxide usage ratio approached the feed gas ratio. The catalyst, in this case, would need to possess good water-gas shift activity. Past work with iron and/or cobalt with ZSM-5 zeolite has revealed interesting results. The cobalt-based catalysts, however, exhibit low water-gas shift activity. A low-ratio H 2 / C 0 feed gas at elevated temperatures exposes the catalyst to carbonforming regions. In a study with cobalt-based catalyst in a backmixed reactor, Pennline et al. (1984) explained that for a 1H2/1C0 gas the catalyst sees a ratio even lower, owing not only to the lack of water-gas shift but also to the mixing characteristics of that particular reactor system. For a cobalt-thoria ZSM-5 catalyst tested with a lH,/lCO gas feed and for another batch of the same catalyst at identical process conditions but tested with a 2H2/ 1CO feed gas, the actual reactor H 2 / C 0 ratios were 0.3 and 14, respectively. The drop in conversion, or deactivation, was 0.067% (H, + CO) conversion per hour for the former case, whereas it was 0.003% for the latter, approaching a near-constant conversion throughout this latter test. This corresponded to a carbon deposition rate of 0.0086 g of carbon/g of catalyst per hour for the former case and 0.0006 for the latter case. Although it has been claimed that cobalt carbide is the active Fischer-Tropsch species (Craxford, 1946), most investigators agree that the metallic cobalt is responsible for the synthesis activity. The X-ray diffraction analysis on the above used cobalt-thoria ZSM-5 bifunctional catalyst
indicated some carbide formation that is thought to be related to the deactivation of the catalyst. High liquid yields are obtained with the thoria-promoted cobaltZSM-5 catalyst by using synthesis gas with low H 2 / C 0 ratios. However, carbon deposition, as a carbide and/or amorphous carbon, on the catalyst accompanies this synthesis activity. A method of catalyst regeneration would be required to enhance the commercial application of this catalyst. This paper reports the results of an investigation of the deactivation/regeneration of this catalyst and the effect of the regeneration on catalyst activity, stability, and product selectivity. E x p e r i m e n t a l Section
The catalyst consisted of a cobalt-thoria coprecipitate that was physically admixed with zeolite ZSM-5 and then extruded with uncalcined Catapal SB alumina. To prepare the coprecipitate, an aqueous solution of sodium carbonate was slowly added to a stirred aqueous solution of cobalt nitrate and thorium nitrate. This mixture was filtered and washed with hot water until no nitrate was present in the filtrate. The coprecipitate was dried, crushed, and sieved through 200 mesh (74 pm). The H-ZSM-5 was fabricated by following the patent of Argauer and Landolt (1972). The synthesized zeolite was washed and dried and then calcined at 538 "C in air. The sodium form of the ZSM-5 was ion-exchanged with hydrogen chloride, washed, dried, crushed, and sieved through 200 mesh (74 pm). The sodium content in the ZSM-5 was kept below 0.1 wt %, and the final silica/alumina molar ratio was about 40/1. Scanning electron microscopy results indicate the granule particle size of the ZSM-5 was about 1 pm. The coprecipitate and zeolite were mixed and then extruded with the alumina binder into 1/8-in.-diameter cylinders with random lengths averaging in. The final catalyst extrudate composition was about 12.5 wt % cobalt, 2.0 wt % thoria, 10-15 w t % alumina, and 57.1 wt % ZSM-5. Other tests were conducted with the above catalyst without the alumina binder and with the catalyst with calcined Catapal SB y-alumina substituted for the zeolite. In all
This article not subject to U S . Copyright. Published 1986 by the American Chemical Society
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cases, the amount of cobalt-thoria in the catalyst was the same. The experiments were conducted in a gas-phase stirred-tank reactor, as described by Berty (1974). A lH,/lCO synthesis gas flowed into the reactor, which typically contained an 18-g batch of catalyst pellets. Products and unreacted synthesis gas exited the reactor and flowed through a series of traps, where the heavier hydrocarbons were condensed. The product gas flow was metered by a wet test meter and was periodically analyzed by an on-line gas chromatograph. The backmixed reactor system, which provides uniform exposure to the catalyst and is an excellent method to study deactivation, was modified so that in situ oxidative regenerations of the used catalyst could be performed after synthesis. Cylinders of air and nitrogen were connected to the system and arranged such that the feed gas composition during oxidation could be varied to that desired. Note that after a particular catalyst processing, the gas sources used in the process were physically disconnected from the reactor system to avoid a possible hazardous situation. The catalyst was brought to synthesis conditions in an identical manner for each test. Initially, the reactor was pressurized to 2.17 MPa with hydrogen. The activation procedure began by flowing hydrogen over the catalyst at a weight hourly space velocity (WHSV) of 0.12 while the catalyst was rapidly heated to 200 "C. (WHSV is defined as grams of gas per hour per gram of total catalyst.) After this temperature was maintained for 2 h, the catalyst was heated to 350 "C under hydrogen flow and maintained under these conditions for 21 h. For subsequent testing after an oxidative regeneration, the catalyst was directly heated to 350 "C for a 21-h period under hydrogen. In either case, afterward the catalyst temperature was reduced to 250 "C, and then the pressure was decreased to 0.79 MPa. A t these conditions, the synthesis gas flow rate was incrementally increased over an hour until the design space velocity for the test was reached. Care was taken during this hour to prevent temperature runaway. After this induction step, the pressure was increased to operating conditions, and then the temperature was increased (10 "C/h) to synthesis conditions. Trap drainings, flow, and gas analyses were done on a 24-h basis for the material balance determination. After conducting several trial experiments to determine an acceptable temperature for carbon burn-off, a standard regeneration procedure was established and used throughout this work. The catalyst temperature was increased to 350 " C under a flow (12.4 WHSV) of 1%oxygen in nitrogen a t 1.48 MPa. These conditions were then maintained constant for 1h, after which air was introduced at 12.7 WHSV for an additional hour. The temperature was then increased to 400 " C and maintained for 1 h. At the end of the hour, the system was shut down and cooled under the flow of air. An infrared analyzer monitored the carbon dioxide concentration in the exit gas during the oxidative regeneration, and typically the carbon dioxide concentration reached a maximum before asymptotically approaching zero. The gaseous and liquid synthesis products were characterized by various analytical techniques. Gas exiting the reactor system was analyzed for hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons up to CBby gas chromatography. The liquid condensate in the trapping system was collected and separated into an aqueous fraction and an oil fraction. The aqueous phase was analyzed by mass spectroscopy to detect oxygenates, and the water content was determined by the Karl Fischer reagent
technique. In all cases with the cobalt-based catalyst, the aqueous fraction contained less than 170oxygenates. The liquid hydrocarbon samples were characterized by simulated distillation ASTM D-2887 to determine boiling-range distributions, by fluorescent indicator adsorption ASTM D-1319 to determine the functionality of the liquid oil, and by bromine number ASTM D-1159 to corroborate the olefin content. Infrared studies were also performed on the oil fractions. Relative amounts of terminal, transinternal, and @branched terminal olefins were determined by infrared absorption. Catalyst samples were withdrawn from the initial catalyst charge after a synthesis conditions test and after an oxidative regeneration. Catalyst sample was easily removed after purging and then opening the reactor. The reactor was then sealed and readied for the next step. The several withdrawn extrudates were then crushed and examined by bulk chemical analysis to determine the extent of the carbon removal. X-ray diffraction studies were also performed to determine carbide formation and crystallite growth size after oxidation.
Results and Discussion Before the oxidation procedure described above became standard, various other attempts were made to regenerate the catalyst. In one case, the used catalyst batch was washed with hot toluene in a Soxhlet extractor with the intention of removing carbon or any possible waxes plugging the catalyst pores. Upon synthesis startup, the catalyst activity was approximately equal to the catalyst activity at the end of the previous test. A used catalyst after direct hydrogen treatment gave similar results upon restarting of the synthesis conditions. The 400 "C oxidative regeneration, which is similar to regeneration of hydrocarbon processing catalysts (Satterfield, 1980),was adopted after a test and successive regeneration indicated a decrease in carbon content on the catalyst from 6.75 to 0.29 wt %. Oxidations were performed at higher temperatures (>400 "C), but synthesis gas conversion results were poor. Regeneration tests were conducted with three catalysts: a Co-Th-ZSM-5 with alumina binder, test 1-64; the same catalyst without binder, test 2-20; and Co-Th-alumina, test 1-63. Results of these tests are compiled in Table I. Except for the initial synthesis test, only the first periods are listed after each regeneration. In all cases during constant synthesis conditions, the conversion decreased with time on stream, the hydrocarbon distribution shifted to a lighter fraction, and the functionality of the liquid oil was approximately constant. During the 200 h on stream, the catalyst in the initial run of test 1-64 deactivated, as evidenced by a decrease in the (H2 + CO) conversion. This can also be seen in Figure 1. The product shifts to a lighter fraction during this time, as noted by the increase in percent methane (19.3% to 22.6%) and the decrease in the percent of C,, (69.4% to 63.8%). The liquid oil in the gasoline range averaged about 80%, and the functionality of the liquid oil was nearly constant throughout the test, with a high olefin content (81%)and low content of paraffins (17%) and aromatics (2%). This particular catalyst system at these processing conditions did not produce significant quantities of aromatics, but the olefinic product as characterized by infrared studies contained much internal bonding and was highly branched. The catalyst in test 1-64 was regenerated oxidatively five times. The initial activity, which is directly related to the conversion, decreased after each synthesis run, as seen in Figure 1. By the slopes of these least-squares-fit lines, it is determined that the rate of deactivation also increased
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986 70
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after each successive regeneration and synthesis testing. After each regeneration and the ensuing synthesis, the hydrocarbon distribution shifted to a lighter fraction, as noted by the increase in the Cz-C4 fraction and decrease in C5+ (see Table I). The functionality of the product oil also changed after each regeneration, with the olefin content decreasing and the paraffin content increasing. Over the course of five regeneration attempts, the olefins dropped from 81% to 5%. Although the olefin content decreased drastically over the five regenerations, the olefin can still be characterized as internally bonded and highly branched. Bulk chemical analysis of the catalyst before and after the first and fifth regenerations indicated a substantial decrease in carbon content after each oxidative regeneration and no change in the percent of cobalt in the catalyst during the entire test. Results with the bifunctional catalyst without an alumina binder in test 2-20 are also tabulated in Table I. Although only two regenerations were performed, the synthesis results are very similar to those of test 1-64. This would indicate that the amorphous alumina binder has negligible effects on the bifunctional catalyst performance. Test 1-63, cobalt-thoria admixed with calcined alumina and extruded with amorphous alumina binder, also showed the same trend in catalyst activity that the ZSM-5-based catalyst showed-a decrease in activity with time on stream and a decrease in initial activity after each regeneration (see Table I). However, a definite difference did exist between the hydrocarbon distribution and functionality synthesis results of the two catalysts. The alumina-based catalyst product contained much heavier hydrocarbons, as indicated by the simulated distillation results of the product oil; only 55% fell in the gasoline range vs. 85% for the zeolite-based catalyst. Also, much wax, which is solid a t room temperature, was formed in the alumina-based test, while a negligible amount was formed with the ZSM-5-based catalyst. The functionality of the liquid oil product in the initial run of test 1-63 was highly paraffinic in contrast to the ZSM-5-based catalyst product, which was highly olefinic. Infrared studies of the olefinic fraction in the oils indicate that the ratio of internal olefins to terminal olefins is much greater with the zeolite-based catalyst. Also, branching is negligible with the aluminabased catalyst as compared to the ZSM-5-based catalyst.
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Figure 2. Synthesis gas conversion after regeneration and reduction vs. the crystallite size of Co304after regeneration from test 1-64.
These isomerization reactions seem to be related to the acidic function of the zeolite. A similarity did exist between tests 1-63 and 1-64 pertaining to synthesis activity after regeneration. The decreased conversions after each regeneration are almost identical and would indicate that the promoted transition metal is responsible for the synthesis activity and that this function is affected by each successive oxidation. Also, the product oil analyses in tests 1-63 and 1-64 both showed a decrease in the olefin functionality after oxidative regenerations. It appears that after the oxidation treatments, the hydrogenation activity of the promoted transition metal increased and thus was responsible for this decrease in oil olefin functionality. Catalyst samples withdrawn from the reactor after synthesis gas processings and after the oxidative regenerations were subjected to X-ray diffraction analysis. The following relationships were revealed from the results of test 1-64: the crystallite size of the Co304formed after regeneration was inversely related to the initial conversions after reduction of the Co304to the metal, and the amount of measurable hexagonal cobalt present at the end of a run was inversely related to the conversion at the end of the run. The crystallite size of the Co304increased after each regeneration, while the (H, + CO) conversion decreased, as shown in Figure 2. Assuming that after reduction the metallic cobalt had a crystallite size proportional to that of the precursor Co304,these data suggest that the decrease in conversion as the number of oxidative cycles increased is due to the decrease in surface area as the crystallite size increased, thus to sintering. Additional evidence for the increase in the size of the cobalt crystallites is shown by the intensities in the hexagonal cobalt diffraction peak at 1.91 A. At the end of each of the first four runs, there was no intensity in this peak above the background. However, the peak was easily seen after the last two runs. Probably in the initial run there was a large amount of cobalt with a crystallite size too small to be detected. With each successive run and regeneration, the crystallite size of the hexagonal cobalt grew
until in the next to the last run, the 1.91-A peak became detectable. After the last run, the peak became 1.4 times as intense as in the previous run. Summary Deactivation of cobalt-thoria-ZSM-5 catalyst occurs when low-ratio synthesis gas at an elevated temperature is reacted over this bifunctional catalyst. Analytical results of used catalyst indicate carbon deposition, either as a carbide and/or amorphous carbon, occurs on the catalyst. When synthesis gas conversion results with the zeolitebased catalyst are compared with those of an aluminabased cobalt-thoria catalyst, the hydrocarbon products are vastly different, but the conversions are nearly the same. This similarity in catalyst activity indicates that the promoted transition metal is the component responsible for the synthesis activity. Likewise, it is this promoted transition-metal component that deactivates with synthesis time on stream a t the particular processing parameters. An oxidative regeneration process was adopted with the intent of removing carbon deposition on the catalyst. Although this technique may not have been optimum, most of the carbon was removed from the catalyst after the oxidative treatment. However, as determined by X-ray diffraction analysis, sintering of the cobalt component of the catalyst occurred with each successive oxidative regeneration, and this caused a decline in the catalyst activity and possibly a change in the hydrogenation activity of the cobalt-thoria component. A general consideration of this type of bifunctional catalyst for low H2/C0 ratio synthesis gas conversion is that deactivation will ultimately occur-the rate depending on the severity of the process conditions. A regenerative technique would need to be developed, if possible, to remove the carbon deposit from the transition-metal component and possibly from the zeolite component without sintering the crystallites of the transition metal. Acknowledgment We wish to thank R. R. Schehl, R. J. Gormley, R. E. Tischer, R. D. H. Chi, E. R. Bauer, D. H. Finseth, and F. W. Harke for contributions to this project. Registry No. Thoz, 1314-20-1; CO, 630-08-0; Co, 7440-48-4;
C, 7440-44-0. Literature Cited Argauer, R. J.; Landolt, G. R. U S . Patent 3702886, 1972. Berty, J. M. Chem. Eng. Prog. 1974, 70, 68. Craxford, S . R. Trans. Faraday SOC. 1946, 4 2 , 580. Kuo, J. C. W. "Gasification and Indirect Liquefaction", in "The Science and Technology of Coal and Coal Utilization"; Cooper, B. R.; Ellinson, W. A,, Ed.; Plenum Press: New York, 1984. Pennline. H. W.; Gormley, R. J.; Schehl, R. R. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 388. Satterfield, C. N. "Heterogeneous Catalysis in Practice"; McGraw-Hili, New York, 1980.
Received for review May 23, 1985 Accepted September 6, 1985 Reference in this report t o any specific commercial product, process, or service is t o facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy.
