Energy & Fuels 2009, 23, 3645–3651
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Hydro-treating of Asphaltenes in Supercritical Toluene with MgO-Supported Fe, Ni, NiMo, and CoMo Catalysts Chunbao (Charles) Xu,*,† Huasu Su,† and Mainak Ghosh‡ Department of Chemical Engineering, Lakehead UniVersity, Thunder Bay, ON, Canada P7B 5E1, and Imperial Oil Resources, 3535 Research Road NW, Calgary, AB, Canada T2L 2K8 ReceiVed February 12, 2009. ReVised Manuscript ReceiVed May 26, 2009
For the first time, the AVTB-derived asphaltenes (containing very high concentrations of sulfur (7.70 wt % S) and nitrogen (1.08 wt % N)) were hydrotreated over novel MgO-supported Fe, Ni, NiMo, and CoMo catalysts in supercritical toluene at 380 °C under hydrogen of 5 MPa (cold pressure) in a batch reactor. The supercritical toluene alone without any catalyst was found to be effective for conversion of the highly aromatic asphaltenes (AS) to less aromatic maltenes (MA) through a thermal cracking mechanism. All the MgO-supported catalysts proved to be effective for hydro-conversion of AS to MA in supercritical toluene. The activities of these catalysts for AS conversion showed the order of sequence of Fe/MgO < Ni/MgO < CoMo/MgO < NiMo/ MgO. Among all the MgO-supported catalysts tested, NiMo/MgO was found to be the most active one for promoting the removal of sulfur and AS. The sulfur/nitrogen/AS removal efficiencies for the NiMo/MgO catalyst increased with the treatment time, and the efficiencies at 120 min of treatment attained as high as 43%, 30%, and 70%, respectively. The MgO-supported catalysts showed excellent performance for preventing and retarding the formation of highly condensed aromatics or coke in the hydro-treatment of AS in supercritical toluene. With the presence of NiMo/MgO catalyst, hydro-treating AS in the supercritical toluene for a reaction time up to 120 min produced a strikingly low yield (85% (w/w) of pitch (+524 °C residue), was recently investigated by us employing supercritical fluids of lower boiling-point hydrocarbon solvents (pentane, heptane and toluene) at 350-410 °C and 10 MPa H2 with NiMo catalysts supported on either activated charcoal or γ-Al2O3.13 Supercritical toluene was found to be an effective reaction medium for hydro-conversion of the AVTB into lighter distillates and removal of sulfur/nitrogen. Asphaltenes as the “heaviest” component in VR are condensed polyaromatic rings associated with each other as micelles and aggregates.19,20 They are very thermally unstable and would (10) Scott, D. S.; Radlein, D.; Piskorz, J.; Majerski, P.; deBruijn, Th. J. W. Upgrading bitumen in supercritical fluids. Fuel 2001, 80, 1087–1099. (11) Li, C.; Shi, B.; Cui, M.; Shang, H.; Que, G. Application of CoMo/CNT catalyst in hydro-cracking of Gudao cacuum residue. J. Fuel. Chem. Technol. 2007, 35, 407–411. (12) Fukuyama, H.; Terai, S.; Uchida, M.; Cano, J. L.; Ancheyta, J. Activated carbon catalyst for heavy oil upgrading. Catal. Today 2004, 98, 207–215. (13) Xu, C.; Hamilton, S.; Mallik, A.; Ghosh, M. Upgrading of Athabasca vacuum tower bottoms (VTB) in supercritical hydrocarbon solvents with activated carbon-supported metallic catalysts. Energy Fuels 2007, 21, 3490–3498. (14) Rana, M. S.; Huidobro, M. L.; Ancheyta, J.; Gomez, M. T. Effect of support composition on hydrogenolysis of thiophene and Maya crude. Catal. Today 2005, 107/108, 346–354. (15) Trejo, F.; Rana, M. S.; Ancheyta, J. CoMo/MgO-Al2O3 supported catalysts: An alternative approach to prepare HDS catalysts. Catal. Today 2008, 130, 327–336. (16) Breysse, M.; Afanasiev, P.; Geantet, C.; Vrinat, M. Overview of support effects in hydrotreating catalysts. Catal. Today 2003, 86, 5–16. (17) Zdrazil, M. MgO-supported Mo, CoMo, and NiMo sulfide hydrotreating catalysts. Catal. Today 2003, 86, 151–171. (18) Klicpera, T.; Zdrazil, M. Preparation of high-activity MgOsupported Co-Mo and Ni-Mo sulfide hydrodesulfurization catalysts. J. Catal. 2002, 206, 314–320. (19) Yen, T. F. In Chemistry of Asphaltenes; Burger, J. W., Ed.; Advances in Chemistry Series, Vol. 195; American Chemical Society; Washington, DC, 1981; p. 39.
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crack down readily to form coke.5,9 On the other hand, most contaminants (sulfur, nitrogen, and metals) are bound to asphaltenes. The asphaltenes in the heavy oils or VR are usually responsible for deactivation of catalysts in the hydro-treating processes, and the efficiency of a vacuum residue upgrading process (either a carbon-rejection process or a hydrogen-addition process) is strongly dependent on how the asphaltenes are treated.21,22 It is thus of significance and interest to investigate on upgrading petroleum asphaltenes into light distillates. Journal literature on upgrading asphaltenes is very limited so far except for a few reports described below. Hydro-cracking of petroleum asphaltenes was conducted by Ohtsuka’s group at Tohoku University in Japan using an autoclave reactor at 300 °C in 5 MPa H2 for 1 h with SBA-15-supported Fe/Ni catalysts.23 A high asphaltene conversion of 40-70 wt % was obtained, but the yields of maltenes were relatively low at about 20-40 wt %, and the process was associated with significant formation of coke over the catalysts (10-30 wt %).23 Sato et al.24 reported upgrading of asphalt with supercritical water under oxidative or inert atmosphere at 340-400 °C for 1 h, and achieved 25-45 wt % conversion of asphaltenes and 16-26% sulfur removal efficiency. In the present work, asphaltenes separated from Athabasca vacuum tower bottoms (AVTB) was treated for the first time in supercritical toluene over MgO-supported Fe, Ni, NiMo, and CoMo catalysts at 380 °C (the optimum hydrotreatment temperature for AVTB in supercritical toluene, proven by a previous work of the authors13). The focus of the present work will be on the performance of supercritical toluene and MgO-supported catalysts in the conversion of asphaltenes, removal of sulfur and nitrogen in asphaltenes, and on the extraordinary properties of the MgO-supported catalysts for inhibiting coke formation. Experimental Section Materials. The asphaltenes feedstock used in this work was separated from Athabasca vacuum tower bottoms (AVTB), supplied by Syncrude Canada Limited. The separation for asphaltene was as per the Soxhlet method using n-heptane and toluene as solvents for the extraction of maltenes (denoted as MA, heptane-soluble) and asphaltenes (denoted as AS, heptane insoluble but toluenesoluble), respectively. Analyses of the AVTB-derived asphaltenes (AS) are provided in Table 1, where analyses of AVTB are also given for comparison. As shown in the Table, the AS feedstock contains very much higher concentrations of sulfur and nitrogen than the AVTB: 7.70 wt % S (vs 6.84 wt % for AVTB) and 1.08 wt % N (vs 0.68 wt % for AVTB). In addition, the AS contains an H/C molar ratio of 1.19, much lower than that of AVTB (H/C ) 1.32), suggesting an extremely high aromaticity of the AS feedstock to be used for the tests. The n-heptane and toluene used in the tests were HPLC-grade solvents obtained from Sigma-Aldrich and they were used as received. Toluene has a critical temperature and pressure of 318.7 °C and 4.1 MPa, respectively. Preparation and Characterization of MgO-Supported Catalysts. As stated in the Introduction, one of the objectives of the present work is to investigate MgO-supported catalysts for upgrad(20) Artok, L.; Su, Y.; Hirose, Y.; Hosokowa, M.; Murata, S.; Nomura, M. Structure and reactivity of petroleum-derived asphaltene. Energy Fuels 1999, 13, 287–296. (21) Sheu, E. Y. Petroleum asphaltene-properties, characterization, and Issues. Energy Fuels 2002, 16 (1), 74–82. (22) Yen, T. F., Chilingarian, G. V., Eds. Asphaltenes and Asphalts; Elsevier: Amsterdam, 1994; Vol. 1, p. 1. (23) Byambajav, E.; Ohtsuka, Y. Hydrocracking of asphaltene with metal catalysts supported on SBA-15. Applied Catal. A: Gen. 2003, 252, 193– 204. (24) Sato, T.; Adschiri, T.; Arai, K.; Rempel, G. L.; Ng, T. T. Upgrading of asphalt with and without partial oxidation in supercritical water. Fuel 2003, 82, 1231–1239.
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Table 1. Properties of the AVTB-Derived Asphaltenes (AS) in Comparison with AVTB toluene insoluble, wt % maltenes (MA), wt % asphaltenes (AS), wt % carbon, wt % hydrogen, wt % sulfur, wt % nitrogen, wt % H/C (-) MCR, wt % pitch (+524 °C), wt % IBP, °C a
AS
AVTB
0 0 100 80.3 7.99 7.70 1.08 1.19 n.a.a n.a.a n.a.a
1.4 76 23 81.7 9.01 6.84 0.68 1.32 28 >85 430
Not analyzed.
ing of asphaltenes by hydro-treating in supercritical toluene. Nanopowder of MgO (with average particle size of 30 nm and a BET specific surface area of about 60 m2/g) supplied by Inframat Advanced Materials was used as the catalyst support material. The supported metallic catalysts: 13% Fe/MgO (Fe/MgO in short), 13% Ni/MgO (Ni/MgO in short), 3% Ni-10% Mo/MgO (NiMo/MgO in short), and 3% Co-10% Mo/Al2O3 (CoMo/MgO in short) were synthesized by successive incipient wetness impregnation method with ACS reagent-grade iron(III) nitrate 9-hydrate (Fe(NO3)3 · 9H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2 · 6H2O), ammonium molybdate tetrahydrate ((NH4)6Mo7O24 · 4H2O), and cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O), all received from Sigma-Aldrich. The as-synthesized supported metallic catalysts were calcinated in air at 500 °C for 4 h, followed by sulfidation in a flow of 5% H2S/ H2 at 400 °C for 8 h, and the resulting catalysts were crushed into fine particles of a particle size
