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Toward Less Dependence on Platinum Group Metal Catalysts: The Merits of Utilizing Tin Richard D. Adams,*,† Douglas A. Blom,† Burjor Captain,‡ Robert Raja,*,§ John Meurig Thomas,*,| and Eszter Trufan† Department of Chemistry and Biochemistry at the USC Nanocenter, UniVersity of South Carolina, Columbia, South Carolina 29208, Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida 33146-0431, Department of Chemistry, UniVersity of Southampton, Highfield, Southampton SO17 1BJ, U.K., and Department of Materials Science, UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. ReceiVed June 6, 2008. ReVised Manuscript ReceiVed July 24, 2008 Minute stoichiometric bimetallic clusters rich in tin (PtSn2, RhSn2, and RuSn2) are powerful selective hydrogenation catalysts: these “molecular metallic” entities, supported on mesoporous silica and characterized by aberration-corrected electron microscopy, yield high percentages of cyclododecene (CDE) at fractional conversions ranging from 0.45 to 0.70 of the parent cyclododecatriene (CDT) at modest temperatures and under solvent-free conditions.
Platinum group metals (PGM) are so extensively used for a wide variety of catalytic processes that there is now an exigent need to seek effective, cheaper, and more readily available substitutes. Whereas there are few indications, at present, that total substitution will shortly prove feasible, what is clear is the reality of using bimetallic catalytic species in which a second constituent is bound to a member of the PGMs. Previously we described how FePt nanocluster catalysts function efficiently in the so-called “Prox” (preferential oxidation) process for the selective oxidation of carbon monoxide in the presence of hydrogen, a reaction of considerable industrial interest, in both ammonia synthesis and the Fischer-Tropsch reaction, where traces of CO poison the active catalyst. Here, we focus on a selective hydrogenation process that is also of importance commercially (Scheme 1), and we demonstrate that the incorporation of tin into (triatomic) clusters of nanocatalysts confers a remarkable degree of catalytic performance that substantially surpasses that of the “bare” (monatomic) PGM catalystssPt, Rh, and Ru. Cyclododecene (CDE), which is of pivotal importance in many industrial processes (Scheme 1), is normally prepared by the selective hydrogenation of 1,5,9-cyclododecatriene (CDT), with metallic Pd being the catalyst of choice. But Pd suffers somewhat from its lack of selectivity, and considerable quantities of the cycloalkane (CDA) appear as a coproduct. Having recently found1 that the selectivity of this reaction is greatly improved by using small bimetallic clusters in which Ru is combined with stoichiometric amounts of Sn and that the selectivity toward CDE is a function of Sn content, increasing smoothly from 85 to nearly 100% in proceeding from Ru4Sn2 to Ru4Sn4 and finally to Ru4Sn6, we argued that if this is generally valid for other members of the platinum group metals (PGM) then it is prudent to examine whether two other popular * Corresponding authors. (R.D.A.) Fax: +1-803-777-6781. E-mail:
[email protected]. (R.R.) Fax: +44-2380-593781. E-mail: r.raja@ soton.ac.uk. (J.M.T.) Fax: +44-1223-740360. E-mail:
[email protected]. † University of South Carolina. ‡ University of Miami. § University of Southampton. | University of Cambridge. (1) Adams, R. D.; Boswell, E. M.; Captain, B.; Hungria, A. B.; Midgley, P. A.; Raja, R.; Thomas, J. M. Angew. Chem., Int. Ed. 2007, 46, 8182–8185.
Scheme 1. Range of Commodity and Specialty Chemicals That Can Be Derived from the Selective Partial Hydrogenation of Cyclododecatriene (CDT)
hydrogenation catalysts, Pt and Rh, are also improved when they, too, are stoichiometrically combined with Sn. It is known2 that the homogeneity of the bimetallic cluster catalysts is improved by combining the metals within discrete bimetallic organometallic complexes prior to deposition on a silica support and then eliminating the ligands by mild thermal treatment. Accordingly, we have prepared and tested three new tin-containing nanoscale catalysts PtSn2, RhSn2, and RuSn2 and compared them with their non-tin-containing homologues for the hydrogenation of CDT. For these studies, the catalyst (2) (a) Hungria, A. B.; Raja, R.; Adams, R. D.; Captain, B.; Thomas, J. M.; Midgley, P. A.; Golovko, V.; Johnson, B. F. G. Angew. Chem., Int. Ed. 2006, 45, 4782–4785. (b) Hermans, S.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G.; Gleeson, D. Angew. Chem., Int. Ed. 2001, 40, 1211–1215.
10.1021/la801759d CCC: $40.75 2008 American Chemical Society Published on Web 08/09/2008
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Figure 1. Diagrams of the molecular structures of catalyst precursor complexes 1 (left), 2 (right), and 3 (bottom).
Figure 2. (a) STEM of PtSn2 (left) and Pt (right) nanoclusters on mesoporous silica.
precursors [(COD)Pt(SnPh3)2 (1)3 (COD ) 1,5-cyclooctadiene) and Ru(CO)4(SnPh3)2 (2)] have been prepared.4 The structures of 1 and 2 are shown in Figure 1. For the RhSn2 catalyst, we have used the precursor Rh3(CO)6(SnPh3)2(µ-SnPh)2 (3) (Figure 1), the preparation and structure of which were recently reported.5 (3) Pt(COD)2 (100.00 mg, 0.243 mmol) was dissolved in 20 mL of hexane. Ph3SnH (171.2 mg, 0.488 mmol) was added and allowed to stir at rt for 1 h. The product was purified by chromatography (silica gel column) to yield a lightyellow band (108.0 mg, 44% yield) of Pt(COD)(SnPh3)2 (1). Spectral data for 1: 1H NMR (CDCl3, rt): δ ) 7.06-7.33 (m, 30 H, Ph), 5.57 (s, 4H, CH, 2JPt-H ) 47 Hz), 2.21 (broad, 8H, CH2). 119Sn{1H} NMR (CD2Cl2, rt): δ ) -84.3 (s, 2 Sn, 1J195Pt-119Sn ) 12 737 Hz, 2J119Sn-117Sn ) 711 Hz). Crystal data for 1: PtSn2C44H42, Mr ) 1003.25, monoclinic, space group P21/n, a ) 9.9731(2) Å, b ) 16.5529(3) Å, c ) 22.9675(5) Å, β ) 95.500(1)°, V ) 3774.1(2) Å3, Z ) 4, T ) 294 K, Mo KR ) 0.71073 Å. The final residual R1(F2) was 0.0313 for 7013 reflections I > 2σ(I).
Each of the complexes was deposited from solution onto mesoporous silica (Davison type 911 having a pore size of ca. 38 Å diameter6 and activated by heating to 473 K in vacuo for 1 h to remove the ligands). The nanoscale character of the catalysts and (4) To a solution of 30 mg of Ru3(CO)10(NCCH3)2 in CH2Cl2 was added 50 mg of HSnPh3 (0.142 mmol). The solution was stirred for 30 min at rt. The product was separated by TLC on silica gel to yield 16.1 mg (13%) of colorless Ru(CO)4(SnPh3)2 (2). Spectral data for 2: IR νCO (cm-1 in CH2Cl2): 2031(vs). Mass spectrometry for Ru(CO)4(SnPh3)2: EI/MS m/z: 914, M+; 830, M+ - 3CO. Crystal data for 2: RuSn2O4C46H36, Mr ) 991.20, monoclinic, space group P-1, a ) 11.7425(5) Å, b ) 14.1211(6) Å, c ) 14.4268(6) Å, a ) 72.460(1), β ) 66.603(1)°, γ ) 75.254(1),V ) 2068.43(15) Å3, Z ) 2, T ) 294 K, Mo KR ) 0.71073 Å. The final residual R1(F2) was 0.0338 for 8240 reflections I > 2σ(I). For alternative syntheses for 2, see Cotton, J. D.; Knox, S. A. R.; Stone, F. G. A. Chem. Commun. 1967, 965–966. (5) Adams, R. D.; Captain, B.; Smith, J. L., Jr; Hall, M. B.; Beddie, C. L.; Webster, C. E. Inorg. Chem. 2004, 43, 7576–7578.
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Figure 3. Effect of tin in the selective hydrogenation of 1,5,9cyclododecatriene (CDT) using anchored monometallic and bimetallic cluster catalysts.
their composition was confirmed by aberration-corrected scanning transmission electron microscopy (STEM) as described previously.7 For comparison, catalysts derived from pure forms of the PMG catalysts (Pt, Rh, and Ru) were also prepared by using supported nanocatalysts thermally derived from precursors Pt(COD)2, Rh4(CO)12, and Ru3(CO)12, respectively. Representative images of the PtSn2 and Pt catalysts on the silica support are shown in Figure 2. The bimetallic clusters were shown to be very similar in composition to that of the precursor complexes by electroninduced energy-dispersive X-ray (EDX) emission spectroscopy.
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The catalytic reactions, using the same conditions as those described previously (substrate = 50 g; catalysts = 25 mg; H2 pressure = 30 bar, T = 373 K, and t = 8 h),1,2b were carried out in a robotically controlled catalytic test reactor.2a The results of the catalytic performance of the seven related, supported cluster catalysts are shown in Figure 3. The selectivity toward the desired cyclododecene (CDE) is remarkable. Whereas the monometallic cluster catalysts produce almost complete hydrogenation of the CDT to the corresponding alkane, all three bimetallic clusters RuSn2, RhSn2, and PtSn2 exhibit high selectivities toward CDE at approximately the same level of total conversion (50 to 75%) of the CDT. Pure tin clusters derived from Ph2SnH2 are poor catalysts both in terms of activity and selectivity (conversion less than 10%). The kinetic plots (Figure 4) for the hydrogenation of 1,5,9cyclododecatriene using PtSn2 and Pt clusters are also quite revealing. Whereas large amounts of cyclododecane (the product of complete hydrogenation) are produced right at the onset using the Pt cluster, the introduction of Sn (in PtSn2) suppresses the formation of cyclododecane until ca. 5 h of reaction time has elapsed. Moreover, with PtSn2, the selectivity for the partially hydrogenated (cyclododecene) product predominates during the entire course of reaction as opposed to cyclododecane being the major product beyond ca. 10 h using Pt. Although more information pertaining to the electronic and atomic structures of the bare and silica-anchored bimetallic clusters is required and DFT calculations8 and X-ray absorption spectroscopy9 studies are underway, it is striking that the
Figure 4. Kinetic profiles for the selective hydrogenation of 1,5,9-cyclododecatriene using PtSn2 (A) and Pt (B) clusters anchored on mesoporous silica.
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performance of the small bimetallic clusters exceeds that of the pure Pd, now the preferred agent in the industrial process.10 Moreover, it is especially noteworthy that monometallic PMG cluster catalysts exhibit poor selectivity and activity and that the presence of Sn as a stoichiometric component of these minute bimetallic clusters confers such a powerful influence on the minority (PGM) component. From a practical standpoint, this bodes well for the conservative use of PGMs now in such prominence as selective hydrogenation catalysts.11 These particles are among the smallest that we have observed.12 From a theoretical and conceptual standpoint, it is appropriate to consider their catalytic performance to be more like that of a multinuclear single-site catalyst (such as that found in certain metallo-enzymes such as hydrogenases13 and bimetallic cluster complexes14) than that of the much larger nanoparticle catalysts (of Au15 or Au-Pd16) that fall in the range of 2 to 5 nm in diameter containing more than 100 times as many atoms. (6) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G.; Vaughan, D. E. W. J. Am. Chem. Soc. 2003, 125, 14982–14983. (7) (a) Ward, E. P. W.; Arslan, I.; Midgley, P. A.; Bleloch, A.; Thomas, J. M. Chem. Commun. 2005, 5805–5807. (b) Pyrz, W. D.; Blom, D. A.; Vogt, T.; Buttrey, D. J. Angew. Chem., Int. Ed. 2008, 47, 2788–2791. (8) Gronbeck, H.; Thomas, J. M. Chem. Phys. Lett. 2007, 443, 337–341. (9) (a) Bromley, S. T.; Sankar, G.; Catlow, C. R. A.; Maschmeyer, T.; Thomas, J. M. Chem. Phy. Lett. 2001, 340, 524–530. (b) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 3588–3628. (10) (a) Cabiac, A.; Delahay, G.; Duran, R.; Trens, P.; Ple´e, D.; Medevielle, A.; Coq, B. Appl. Catal. A 2007, 318, 17–21. (b) Stuber, F.; Delmas, H. Ind. Eng. Chem. Res. 2003, 42, 6–13. (c) Julcour, C.; Jaganathan, R.; Chaudhari, R. V.; Wilhelm, A. M.; Delmas, H. Chem. Eng. Sci. 2001, 556, 557–564. (11) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. Acc. Chem. Res. 2003, 36, 20–30. (12) Thomas, J. M.; Adams, R. D.; Boswell, E. M.; Captain, B.; Gronbeck, H.; Raja, R. Faraday Discuss. 2008, 138, 301–315. (13) Mealli, C.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2007, 46, 8942– 8944. (14) Adams, R. D. J. Organomet. Chem. 2000, 600, 1–6. (15) (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (b) Edwards, P. P.; Thomas, J. M. Angew. Chem., Int. Ed. 2007, 46, 5480–5486. (16) Mejia-Rosales, S. J.; Fernandez-Navarro, C.; Perez-Tijerina, E.; Blom, D. A.; Allard, L. F.; Jose-Yacaman, M. J. J. Phys. Chem C. 2007, 111, 1256– 1260.
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It is well known that the addition of Sn to bulk Pt greatly improves the latter’s ability to function as a dehydrogenation and petroleum-reforming catalyst.17 It is also acknowledged that Sn-containing ligands significantly boost the catalytic activity of metal complexes in homogeneous systems.18 However, the benefits of using Sn as a modifier in heterogeneous catalytic hydrogenation is of relatively recent lineage.2,19 It is probable that the use of Sn-containing complexes as precursors will, as herein described, continue to provide superior bimetallic cluster catalysts. Acknowledgment. This research was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under grant no. DE-FG02-00ER14980 and by National Science Foundation (CHE-0743190). Supporting Information Available: Crystallographic data are available at the Cambridge Crystallographic Data Centre via deposition numbers CCDC 680093 and 680094. Details on the synthesis, elemental composition, spectroscopic characterization, and crystallographic analyses of compounds 1 and 2 and data on the STEM and EDX measurements. This material is available free of charge via the Internet at http://pubs.acs.org. LA801759D
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