Ind. Eng. Chem. Res. 2001, 40, 1589-1590
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Regeneration of Poisoned Nickel Catalyst by Supercritical CO2 Extraction L. Vradman,†,‡ M. Herskowitz,*,†,‡ E. Korin,‡ and J. Wisniak‡ Blechner Center for Industrial Catalysis and Process Development and Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Regeneration of a thiophene-poisoned Ni-supported catalyst was carried out by supercritical CO2 extraction. The catalyst activity was measured in the hydrogenation of 2-butanone to 2-butanol at 373 K and 1.7 MPa. The supercritical extraction was tested over a range of operating conditions. Regeneration at 313 K and 41 MPa for 16 h recovered completely the catalyst activity. Other methods cited in the literature displayed a lower performance in regeneration of Ni catalysts. Introduction Supported Ni catalysts are widely used in hydrogenation reactions.1 One of the major problems in the application of Ni catalysts is the severe poisoning by sulfur-containing organic compounds.2 At the relatively low reaction temperatures (473 K), thiophene was hydrogenolized to hydrogen sulfide. It actually produced surface nickel sulfide (Ni3S2), a much more stable compound than bulk sulfides.2 Several regeneration processes have been studied, including reduction in hydrogen,2 oxidation using water,6 or a sequence of oxidation (0.005-25 vol % oxygen in inert gas) and reduction (hydrogen) treatments.7 None of them was capable of recovering the initial activity of the catalyst. Even at 90% sulfur removal from the catalyst, the catalyst activity was damaged significantly because of active-phase sintering.7 In the last years much effort has been devoted to studying the applications of supercritical fluids in heterogeneous catalysis. A comprehensive review is given by Baiker.8 Supercritical CO2 exhibits special properties such as high solubility of heavy organic compounds and enhanced diffusivity. No data on regeneration by supercritical CO2 of catalysts poisoned by thiophene have been reported. The purpose of this work was to investigate the supercritical CO2 regeneration of Ni catalyst poisoned †
Blechner Center for Industrial Catalysis and Process Development. ‡ Chemical Engineering Department.
with thiophene. Hydrogenation of 2-butanone to 2-butanol at 373 K and 1.7 MPa was employed as a test reaction. Experimental Section The hydrogenation experiments were carried out in a 300 cm3 autoclave manufactured by Autoclave Engineers. Supported Ni powder (1-30 µm) catalyst was purchased from SU ¨ D-Chemie (G-69). The catalyst contains 50 wt % Ni and has a bulk density of 0.45 g/cm3, a surface area of 170 m2/g, and a pore volume of 1.2 cm3/g (as given by the manufacturer). The catalyst was activated for 2 h at 2.1 MPa of hydrogen, 373 K, and 1000 rpm, prior to the hydrogenation tests. About 0.5 g of catalyst mixed with 100 mL of 2-propanol was activated in the autoclave. An additional 30 mL of 2-butanone in 20 mL of 2-propanol was fed at 303 K. In deactivation experiments, 2 g of thiophene/g of catalyst was also fed. All experiments were run at 1.7 MPa, 373 K, and 1200 rpm. The reaction rate was calculated from the measured rate of the pressure decrease. The pressure was allowed to drop by less than 0.1 MPa, so as to maintain constant-pressure conditions. The stoichiometric consumption of hydrogen with respect to 2-butanol was confirmed in preliminary experiments by the analysis of liquid samples in a gas chromatograph. No byproducts were identified. During deactivation experiments, the hydrogen consumption stopped completely after less than 10 min, corresponding to less than 3% conversion. The poisoned catalyst was filtered and regenerated. The regeneration was carried out by removal of thiophene from the Ni catalyst using CO2 supercritical fluid (SF). The extraction was performed using an SF extraction system model SFX 220, manufactured by ISCO, U.K. The system can operate in static mode (holding the extraction cell closed for a specific period of time), dynamic mode (operating at a constant flow rate of SF), and combined modes (static mode followed by dynamic mode).
10.1021/ie000805f CCC: $20.00 © 2001 American Chemical Society Published on Web 02/23/2001
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Ind. Eng. Chem. Res., Vol. 40, No. 7, 2001
Figure 1. Pseudo-first-order kinetics of 2-butanone hydrogenation with 0.57 g of fresh catalyst and 0.5 g of catalyst regenerated during 16 h.
A total 3 g of poisoned Ni catalyst was loaded into a 10 mL cell. A multistep continuous extraction process was carried out at 313 K, 41 MPa, and 40 mL of CO2/h. The regeneration was stopped periodically to take catalyst samples for testing the regenerated catalyst activity as a function of the extraction time. Results and Discussion The hydrogenation reaction rate constant was calculated assuming pseudo-first-order kinetics with respect to 2-butanone. Typical results of 2-butanone conversion are plotted as a function of time in Figure 1 for fresh and regenerated catalysts. Poisoned catalyst with thiophene displayed no hydrogenation activity. Activation conditions (2.1 MPa and 373 K) did not recover catalyst activity, while supercritical CO2 extraction had a significant effect. The catalyst activity recovery ratio (RR) was determined from the ratio of kr and kf, the pseudo-first-order rate constant of regenerated and fresh catalysts, respectively. RR increased with regeneration time, as shown in Figure 2. Furthermore, after 16 h, RR reached unity, meaning that the catalyst initial activity was completely recovered. These results prove the extremely high efficiency of the regeneration process with supercritical CO2 compared with other processes. It seems also that, at low-temperature supercritical extraction, no sintering of the active phase occurred. Conclusions A new process for regeneration of the Ni catalyst completely poisoned by thiophene has been presented. Low-temperature hydrogenation of 2-butanone to 2-butanol was employed as a test reaction, having pseudofirst-order kinetics with respect to 2-butanone. The new
Figure 2. Activity recovery as a function of the catalyst regeneration time.
regeneration process is based on supercritical CO2 extraction of the adsorbed thiophene. It was found that the catalyst activity recovery during regeneration at 313 K, 41 MPa, and CO2 flow rate of 40 mL/h increased with regeneration time. A total of 16 h of regeneration was sufficient for full recovery of fresh catalyst activity. A high efficiency of extraction with supercritical CO2 was demonstrated. Acknowledgment Mrs. Liat Katzav, Keren Machlof, and Mary Mamana carried out the experimental work. Literature Cited (1) Augustine, R. B. Catalytic Hydrogenation; Marcel Dekker: New York, 1965. (2) Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Sulfur Poisoning of Metals. Adv. Catal. 1982, 31, 135. (3) Ahmed, K.; Chadwick, D.; Kershenbaum, L. S. Mechanisms for Thiophene Poisoning of Nickel Catalysts: Effect of Crystallite Size. Stud. Surf. Sci. Catal. 1987, 34, 513. (4) L’Argentiere, P. C.; Liprandi, D. A.; Figoli, N. S. Regeneration of Ni/Al2O3 Poisoned by Thiophene during the Selective Hydrogenation of Styrene. Ind. Eng. Chem. Res. 1995, 34, 3713. (5) L’Argentiere, P. C.; Figoli, N. S. Poisoning and regeneration of an Ni/SiO2 Catalyst. J. Chem. Technol. Biotechnol. 1997, 69, 261. (6) Rostrup-Nielsen, J. R. Chemisorption of Hydrogen Sulfide on a Supported Nickel Catalyst. J. Catal. 1968, 11, 220. (7) Aguinaga, A.; Montes, M. Regeneration of a Nickel/Silica Catalyst Poisoned by Thiophene. Appl. Catal. A 1992, 90, 131. (8) Baiker, A. Supercritical Fluids in Heterogeneous Catalysis. Chem. Rev. 1999, 99, 453.
Received for review September 12, 2000 Revised manuscript received December 27, 2000 Accepted January 10, 2001 IE000805F
