Energy Fuels 2011, 25, 580–590 Published on Web 01/11/2011
: DOI:10.1021/ef1015445
Deactivation Studies of Alkali-Promoted Trimetallic Co-Rh-Mo Sulfide Catalysts for Higher Alcohols Synthesis from Synthesis Gas Venkateswara Rao Surisetty,† Ajay Kumar Dalai,*,† and Janusz Kozinski‡ †
Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK, S7N 5A9, Canada, and ‡Faculty of Science & Engineering, York University, 4700 Keele Street, Toronto, ON, M3J 1P3, Canada Received August 21, 2010
Multiwalled carbon nanotubes (MWCNTs) and activated carbon (AC)-supported K (9 wt %)-promoted trimetallic Co (4.5 wt %)-Rh (1.5 wt %)-Mo(15 wt %) catalysts are used to study the long-term deactivation for continuous 720 h of higher alcohols synthesis (HAS). The fresh catalysts are extensively characterized in both oxide and sulfide phases, together with the spent catalysts. The catalysts are tested for the synthesis of higher alcohols from synthesis gas under similar conditions of 330 °C, 9.1 MPa (1320 psig), and 3.8 m3 (STP)/((kg of cat.)/h), using a H2:CO molar ratio of 1.25. The alkali-promoted trimetallic Co-Rh-Mo catalyst supported on MWCNTs has shown two different deactivation steps: loss of sulfur from the surface and sintering of the catalyst species located on the outer surfaces of the carbon nanotubes. After regeneration, the total activity recovery (∼10%) over this catalyst is close to the total activity loss during the first deactivation step. The percentage CO conversion decreased from 58% to 51% and from 43% to 30% over the catalysts supported on MWCNTs and AC, respectively, over 720 h of timeon-stream; indicating a loss of activity of 12% and 30% over the catalysts supported on MWCNT and AC, respectively. Characterizations of the spent catalyst supported on activated carbon revealed that deactivation occurs because of the sintering of metal sulfides, which causes low metal dispersions and high pore blockage of the support. The total hydrocarbon formation decreased rapidly during the first deactivation step and then slowly leveled off. The initial decrease in the total hydrocarbons space time yield (STY) may be explained from the loss of sulfur from unstable metal sulfide crystallites on the surface of the catalyst to the products and sintering of the catalyst species is responsible for the later steps. The total alcohols STY of 0.281 and 0.202 g/(g of cat./h) are observed during the reaction period of 24 h over the catalysts supported on MWCNTs and activated carbon, respectively. The sulfided MWCNT-supported catalyst suffers a 1% decrease while the AC-supported catalysts exhibits a 4% decrease in normalized higher alcohols yield between 24 and 720 h on-stream.
promoter that shifts the product distribution toward higher alcohols. The products obtained over these catalysts yield a series of linear primary alcohols and gaseous hydrocarbons with Anderson-Schulz-Flory (ASF) carbon number distributions.6,7 The activity and selectivity of higher alcohols over these catalysts were greatly improved by the promotion of Rh, because of the synergistic effect of Rh and Mo, leading to the formation of an Rh-Mo mixed phase.8 Vit et al.9 defined
1. Introduction Catalytic conversion of synthesis gas to produce higher alcohols is one of the most promising processes for reducing greenhouse gas effects and utilizing a natural source of carbon.1,2 A higher alcohol mixture can replace tetraethyl lead and methyl tertiary butyl ether (MTBE) as an octane booster in gasoline.3 Alkali-modified MoS2-based catalysts are of special interest among different higher alcohols synthesis (HAS) catalysts, because of their excellent sulfur resistance and high activity for water-gas shift (WGS) reactions, which saves the cost of ultradesulfurization for feed gas and water separation.4,5 In this catalytic system, potassium is a key
(4) Surisetty, V. R.; Tavasoli, A.; Dalai, A. K. Synthesis of Higher Alcohols from Syngas Over Alkali Promoted MoS2 Catalysts Supported on Multi-Walled Carbon Nanotubes. Appl. Catal., A 2009, 365, 243–51. (5) Woo, H. C.; Park, K. Y.; Kim, Y. G.; Namau, I.-S.; ShikChung, J.; Lee, J. S. Mixed alcohol synthesis from carbon monoxide and dihydrogen over potassium-promoted molybdenum carbide catalysts. Appl. Catal. 1991, 75, 267–280. (6) Tatsumi, T.; Muramatsu, A.; Yokota, K.; Tominga, H. Mechanistic study on the alcohol synthesis over molybdenum catalysts: Addition of probe molecules to CO-H2. J. Catal. 1989, 115, 388–398. (7) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Intrinsic reaction kinetics of higher alcohols synthesis from synthesis gas over sulfided alkalipromoted Co-Rh-Mo trimetallic catalyst supported on MWCNTs. Energy Fuels 2010, 24, 4130–4137. (8) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Effect of Rh promoter on MWCNT-supported alkali-modified MoS2 catalysts for higher alcohols synthesis from CO hydrogenation. Appl. Catal., A 2010, 381, 282–288.
*Author to whom correspondence should be addressed. Tel.: þ1-306966-4771/4768. Fax: þ1-306-966-4777. E-mail:
[email protected]. (1) Ma, L.; Wainwright, M. S. Development of skeletal copperchromia catalysts. I. Structure and activity promotion of chromia on skeletal copper catalysts for methanol synthesis. Appl. Catal., A 1999, 187, 89–98. (2) Smith, K. J.; Anderson, R. B. The higher alcohol synthesis over promoted copper/zinc oxide catalysts. Can. J. Chem. Eng. 1983, 61, 40–45. (3) Breman, B. B.; Beenackers, A. A. C. M.; Schuurman, H. A.; Oesterholt, E. Kinetics of the gas-slurry methanol-higher alcohol synthesis from CO/CO2/H2 over a Cs-Cu/ZnO/Al2O3 catalyst, including simultaneous formation of methyl esters and hydrocarbons. Catal. Today 1995, 24, 5–14. r 2011 American Chemical Society
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: DOI:10.1021/ef1015445
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the magnitude of the synergetic effect (SE) as the ratio of the rate constants over the promoted catalyst and the sum of the individual rate constants over Mo and metal catalysts. Lamber et al.10 observed the formation of bimetallic Rh-Mo particles and concluded that these particles were responsible for the improvement in the catalytic behavior in the CO hydrogenation. The interaction of rhodium with molybdenum in sulfide state changes the status of the rhodium species by favoring the formation of electron-deficient sites (Rh-Mo-S) on the catalytic sites.11 The electron-deficient sites decrease the heat of CO chemisorption and increase the concentration of surface molecular CO, thus favoring the formation of higher alcohols.12 The hydrocarbon products are formed via CO dissociation and hydrogenation of surface carbon over the electron-rich metallic sulfide species.13 The high rates of higher alcohols were observed and compared to the rate of methanol formation over the alkalipromoted Co-Rh-Mo trimetallic catalyst, because of the increased availability of electron-poor sites on the catalyst system.14 The Co incorporation to the K-Rh-MoS2 catalyst supported on multiwalled carbon nanotubes (MWCNTs) resulted in substantial changes in both structure properties, thus improving the catalytic performance toward higher alcohols formation from synthesis gas.15 According to the literature,11,16 the catalytic species mainly exist in the form of K-Mo-S, Co-Mo-S, Rh-Mo-S, MoS2, and Co9S8 over the alkali-, Co-, and Rh-promoted MoS2 bimetallic catalysts. Sun et al.17 explained that one Mo atom of every two is substituted by transition-metal promoters (Co or Rh) and the promoter atoms located at edge surfaces generate partially promoted Co (Rh)-Mo-S surface species. Thus, the presence of metal promoters reduces the formation of metallic sulfide species that are responsible for the formation of hydrocarbons. The formation of the metallic sulfide species (MoS2 and Co9S8) was greatly reduced using the alkali-promoted trimetallic Co-Rh-Mo catalysts for higher alcohols synthesis from CO hydrogenation.15 Catalyst deactivation is an industrial problem related to the use of heterogeneous catalysts that results in loss of catalytic
activity and/or selectivity over time. The stability of the catalyst greatly impacts on the research, development, design, and operation of commercial processes, as well as the costs for catalyst replacement and process shutdown.19 The coke or carbon formation rate in a given reaction depends on the reaction conditions, such as temperature and reactant composition, and on catalyst structure, including metal type, metal crystallite size, promoter, and catalyst support.20 For coke-insensitive reactions, such as higher alcohols synthesis, methanol synthesis, or Fischer-Tropsch synthesis on metals, carbon/coke formation is avoided in regions of temperature in which the precursor gasification rate exceeds the deposition rate.19 It is also known that supported metals such as cobalt, iron, and nickel are active for coke formation at temperatures above 350-400 °C from CO and hydrocarbons. The filamentous carbon formation can be reduced by adding noble metals such as platinum, ruthenium, and rhodium to base metals.21 Bitter et al.22 demonstrated that catalyst deactivation occurs rapidly on larger metal crystallites, compared to those containing small crystallites. The coke formation rate is higher on acidic metal oxide supports, such as Al2O3, SiO2, and ZrO2, because of their surface acidity.23 The deactivation of alkali-promoted MoS2 catalysts in a synthesis gas atmosphere is mainly due to the loss of sulfur (sulfur leaching) and coke deposition.24 Marafi et al.25 compared the deactivation behavior of Mo/Al2O3 and transitionmetal (nickel)-promoted Mo/Al2O3 for hydrotreating reaction and found that transition-metal promotion to Mo/Al2O3 reduced the tendency of coke formation. The presence of a Co (Rh)-Mo-S phase on the transition-metal-promoted MoS2 catalysts has the ability for better hydrogenation of CO, reducing the deposition of coke.26 Courty et al.27 mentioned that the catalyst deactivation can be reduced using 50-100 ppmv H2S in the synthesis gas feed. Christensen et al.28 concluded that the presulfided alkali-promoted Co-Mo catalyst supported on activated carbon stabilizes rapidly in the presence of H2S in the synthesis gas feed, but the presence of H2S reduces the alcohol selectivity, by enhancing hydrocarbon formation. The sintering of MoS2 crystallites during the reaction is another possibility for catalyst deactivation,
(9) Vit, Z.; Cinibulk, J.; Gulkova, D. Promotion of Mo/Al2O3 sulfide catalyst by noble metals in simultaneous hydrodesulfurization of thiophene and hydrodenitrogenation of pyridine: A comparative study. Appl. Catal., A 2004, 272, 99–107. (10) Lamber, R.; Jaeger, N. I. Formation of bimetallic Rh-Mo crystallites supported on SiO2 by decomposition of a heteronuclear cluster precursor. Catal. Lett. 1991, 11, 1–10. (11) Li, Z.-R.; Fu, Y.-L.; Jiang, M. Structures and performance of Rh-Mo-K/Al2O3 catalysts used for mixed alcohol synthesis from synthesis gas. Appl. Catal., A 1999, 187, 187–198. (12) Sudhakar, C.; Bhore, N. A.; Bischoff, K. B.; Manogue, W. H.; Mills, G. A. In Proceedings of the 10th Meeting of the Catalysis Society of North America, San Diego, CA, 1987. (13) Van Den Berg, F. G. A.; Glezer, J. H. E.; Sachtler, W. M. H. The role of promoters in CO/H2 reactions: Effects of MnO and MoO2 in silica-supported rhodium catalysts. J. Catal. 1985, 93, 340–352. (14) Storm, D. A. The production of higher alcohols from syngas using potassium promoted Co/Mo/Al2O3 and Rh/Co/Mo/Al2O3. Top. Catal. 1995, 2, 91–101. (15) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Alkali-promoted trimetallic Co-Rh-Mo sulfide catalysts for higher alcohols synthesis from synthesis gas: Comparison of MWCNT and activated carbon supports. Ind. Eng. Chem. Res. 2010, 49, 6956–6963. (16) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Synthesis of higher alcohols from synthesis gas over Co-promoted alkali modified-MoS2 catalysts supported on MWCNTs. Appl. Catal., A 2010, 385, 153–162. (17) Sun, M.; Nelsona, A. E.; Adjaye, J. On the incorporation of nickel and cobalt into MoS2-edge structures. J. Catal. 2004, 226, 32–40. (18) Petrov, L.; Kumbilieva, K. Kinetic studies of catalyst deactivation in heterogeneous catalysis (Review). J. Univ. Chem. Technol. Metall. 2006, 41, 5–14.
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: DOI:10.1021/ef1015445
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resulting in a decrease in surface area that leads to a collapse of the internal porous structure.29 The advantages of activated carbon-supported catalysts are a large support surface area, limited interaction between the support and the active material, resistance to acidic or basic media, and stability at high temperatures and pressures.30 However, the microporous structure (pore size
