Bimetallic Catalysts and Their Relevance to the Hydrogen Economy

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Bimetallic Catalysts and Their Relevance to the Hydrogen Economy John Meurig Thomas*,† Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K., and Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, U.K.

Robert Raja,*,‡ Brian F. G. Johnson,*,§ Sophie Hermans, Matthew D. Jones, and Tetyana Khimyak Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.

The need to develop powerful new catalysts capable of effecting reductions with molecular hydrogen under mild conditions is a pressing one, particularly with the looming importance of the hydrogen economy. We show that nanoparticle (1-1.5-nm diameter) supported bimetallic catalysts, such as Ru-Pd, Ru-Pt, and Ru-Sn, prepared by decarbonylation without skeletal disintegration of precursor, mixed-metal carbonylates, exhibit high performances in a number of reactions likely to be of considerable future industrial relevance. We also show that certain types of anchored, chiral organometallic moieties, constrained within the cavities of mesoporous silica, function as good enantioselective hydrogenation catalysts for the generation of industrially important organic products such as adipic acid and cyclohexane carboxylic acid. Preamble We welcome the opportunity of paying homage to Dr. John Sinfelt on the approach of his 70th birthday. His contributions to catalysis have been major, and his pioneering investigations of bimetallic nanoparticles from the standpoint of their structural and kinetic behavior has been seminal. It was in his group at the Exxon Central Research laboratory in New Jersey that the foundations of modern aspects of the chemical physics of bimetallic solids were laid. Following his fruitful collaborations with Lytle, with Via, and (rather later) with Meitzner, the community of experts in catalysis has benefited enormously in regard to the various applications of X-ray absorption spectroscopy to the elucidation of the structure of bimetallics in general. No less significant was his seminal work (with C. P. Slichter) on novel aspects of NMR spectroscopy for the study of finely divided metal catalysts. Our own recent endeavors have been much influenced by the studies of John Sinfelt and his collaborations and some of what we describe herein constitutes an extension of the kind of work that one of us (J.M.T.) discussed with J.H.S. on a number of occasions during his visits to New Jersey. What Do We Mean by Bimetallic Clusters? Sinfelt argued1,2 that, “since the ability to form bulk alloys was not a necessary condition for a system to be of interest as a catalyst, it was decided not to use the term alloy in referring to bimetallic catalysts in general”. Instead, bimetallic cluster was the term adopted in preference to alloy. “In particular, bimetallic clusters * To whom correspondence should be addressed. † Tel.: +44-207-670-2928. Fax: +44-207-670-2988. ‡ Tel.: +44-1223-336335. Fax: +44-1223-336017. E-mail: [email protected]. § Tel.: +44-1223-336339. Fax: +44-1223-336017. E-mail: [email protected].

refer to bimetallic entities which are highly dispersed on the surface carrier.” We agreed with this definition: all of the bimetallic systems described herein conform to it. There are key differences, however, between our type of bimetallic clusters and those described by Sinfelt. Whereas, in his examples, surface atoms constitute only a relatively small fraction (a few percent) of the total number of metal atoms in the catalysts, in ours, they are a very high (greater than 90%) fraction. This is chiefly because the particle sizes of our monomodal bimetallic clusters are considerably smaller (the degree of dispersion is higher) than the sizes in the Sinfelt samples. Our bimetallic clusters are just 1-1.5 nm in diameter. They are also prepared in a fundamentally different way3-5 from the Sinfelt clusters, which are generated by the so-called coprecipitation2 or sequential precipitation2 methods. In ours, we invariably start from a mixed-metal carbonylate, such as any one of those shown in Figure 1 (or analogue carbonylates). These are anchored to the surface silanol groups situated at the interiors of mesoporous silica (via -M-C-O‚‚‚H-OSi- hydrogen bonding) in such a way that a high concentration of mixed-metal carbonylate anions [together with the charge-neutralizing cations, such as Et4N+ or PPN+, where PPN stands for bis(triphenylphosphane)iminium] are strewn uniformly along the higharea (500-900 m2 g-1) surfaces of the siliceous support. Gentle thermolysis (heating to ca. 195 °C in vacuo for 2 h) serves to drive away the carbonyl moieties as well as the organic cationic material, thereby producing an array of activated bimetallic nanoparticles, as seen electron microscopically (and depicted color-graphically) in Figure 2. Earlier3,4 in situ FTIR spectroscopic studies and X-ray absorption experiments3 (XANES and EXAFS) showed that no residual peaks corresponding to the carbonyl frequencies remained after this treatment and that the chlorine was carried away as phosgene during the thermolysis. The techniques that we have employed to characterize our bimetallic catalysts rely heavily on X-ray absorption

10.1021/ie0206610 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/03/2003

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Figure 1. Typical parent anionic carbonylates from which naked nanoparticle (10-15 Å in diameter, depending on the constituents of the bimetallic core) catalysts are generated. (A) [Ru6C(CO)16SnCl3]-, (B) [Ru12C2(CO)32Cu4Cl2]2-, (C) [Ru5PtC(CO)15]2-, (D) [Ru10Pt2C2(CO)28]2-.

spectroscopy (as in Sinfelt’s early work), and especially on various kinds of scanning transmission electron microscopy, where we have used both high-angleannular-dark-field(HAADF)5-7 andelectrontomography8-10 (see Figure 3). Also, by electron microscopy, elemental mapping (through electron-simulated X-ray emission) has been used to establishsfrom the fact that the X-ray emission image of one element matches rather well with that of the other element present in the bimetallic nanoparticlesthat the two metal constituents of the parent mixed-metal carbonylate anion remain intact in the final, denuded nanoparticle. (It must be remembered that the X-ray emission response factor from electronirradiated specimens is element-dependent. Hence, exact intensity correspondence is not to be expected for Pd and Ru, even when the ratio of these elements is exactly 1:1.) Synergy and Durability of Catalytic Activity of Bimetallic Nanoparticles Soon after we began our studies of bimetallic catalysts,3-5 we discovered that, in all of the hydrogenation reactions we had investigated, the activity of monometallic nanoparticles of either of the two constituents was substantially less than that of the corresponding bimetallic nanoparticles. This is well illustrated (see Table 1) in the hydrogenation of 1-hexene, where we show the results for Pd6Ru6, Ru6, and Pd nanoparticles anchored within the same kind of mesoporous silica (of the MCM-41 type). It is noteworthy that turnover frequencies (TOFs) for the bimetallic nanoparticles for Pd6Ru6 exceed by more than a factor of 10 the TOF values for Ru6 and Pd nanoparticles. Another noteworthy feature of our bimetallic nanoparticles, anchored to silica supports, is their very low tendency to sinter and coalesce during use. In addition,

such is the disparity in diameter between the bimetallic nanoparticles and the pore dimensions of the high-area siliceous support that there is essentially free diffusional access of reactants to, and migration of the products away from, the nanoparticle catalysts (as is evident in Figure 2b). Relevance to the Hydrogen Economy and Sustainable Development Partly because of legislative demands (that require diminution of global-warming practices and increased development of more environmentally benign industrial processes),11,12 and partly also because of the related general desirability of generating, storing, and using molecular hydrogen as a chemical reagent, there is now a pressing need for the discovery and development of single-step (and preferably solvent-free), highly active, and highly selective catalysts for the hydrogenation of a growing range of key organic compounds. With the predicted future decline of chemical feedstocks from fossil fuels and the parallel growth of those extracted from readily sustainable plant sources, the fraction of organic molecular products that are manufactured industrially by direct hydrogenation is inevitably going to rise. Already there are clear indications (see, for example, the work of Draths and Frost13) where a conscious decision has been made to reject (or at least minimize) petroleum-based feedstocks, toxic starting materials, and the environmentally incompatible byproducts and to concentrate instead on evolving biosynthetic pathways that, for example, microbially covert D-glucose into cis,cis-muconate, which is then catalytically hydrogenated to yield the highly desirable adipate (a precursor to nylon). It is clear that there is an urgent need for new, high-performance catalysts that are capable of operating under mild, environmentally benign condi-

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Figure 2. (A) Electron micrographs (HAADF) of the denuded (i.e., the native cluster stripped off its ligands) (a) Ru6Sn/MCM-41 and (b-f) Ru10Pt2/MCM-41 after catalysis (postreaction). (B) Computer graphics model of Pd6Ru6 nanocatalyst clusters anchored (originally in their carbonylated form via pendant silanol groups) to the inner walls of mesoporous silica. A typical reactant polyene (2,5-norbornadiene) and H2 are also shown in the mesopore channel, which has a diameter of ca. 30 Å.

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Figure 3. BF and HAADF images of the Pd6Ru6/MCM-41 catalyst. Corresponding X-ray elemental maps of Pd, Ru, O, and Si are also shown. (Images and maps acquired on a VG HB 603 dedicated STEM. X-ray emission images obtained from the Pd LR line, the Ru LR line, the O K line, and the Si K line.) Table 1. Hydrogenation of 1-Hexene with Pd6Ru6/MCM-41 Catalysta substrate (mass)

catalyst

1-hexene (∼50 g)

Pd6Ru6/MCM-41 Ru6/MCM-41 Pd/MCM-41 no catalyst

reaction time (h)

conversion (%)

TOF (h-1)

A

product (mol %) B

C

4 4 24 4 24 24

99 13 19 6 14 7

4954 325 277 250 196 -

68 14 10 6 5 -

22 42 36 45 33 32

9 45 53 48 63 67

Reaction conditions: catalyst ) 20 mg, temperature ) 373 K, starting H2 pressure ) 20 bar, no solvent. A ) n-hexane, B ) cis-2hexene, C ) trans-2-hexene. a

tions. We believe that the large family of mixed-metal carbonylates, which one of us (B.F.G.J.), in particular, has investigated for many years, offer a rich variety of powerful bimetallic nanocatalysts that are well-suited to cope with the demands of the hydrogen economy. In addition to the bimetallic nanoparticles, isolated atoms of noble metals (Pt, Pd, Rh) in low oxidation states, when they are appropriately complexed (as organometallic species) and then tethered to the internal surfaces of mesoporous silica, are also, as we outline below, promising hydrogenation catalysts exhibiting the extra desirable attribute of enantioselectivity. Aspects of Synthesis of Catalyst Precursors To arrive at desirable bimetallic nanoparticle catalysts, we have evolved a simple strategy that entails the formation of active cationic multimetal fragments that react selectively and nearly quantitatively with anionic monometallic clusters of another metal. An example of this approach is outlined in Scheme 1, where a platinum dichloro complex and a chloride scavenger react with a pentaruthenium carbido cluster carbonylate {[Ru5C(CO)14][PPN]2]}. The sole product of this reaction is

Scheme 1. Synthesis of [Ru5C(co)14Pt(COD)]a

a

Carbonyl ligands omitted for clarity.

the desired mixed-metal bimetallic catalyst precursor: [Ru5C(CO)14Pt(COD)].14 A related reaction, involving the complex [Pt(MeCN)2Cl2], yields the dianionic species [Ru10C2Pt2(CO)28]2-, which is, effectively, the dimer of the pentaruthenium species depicted in Scheme 1. Using a similar synthetic strategy, a negatively charged Ru-Sn mixed-metal cluster, [Ru6C(CO)16SnCl3]-, suitable as a catalyst precursor, was also prepared (see Scheme 2) and fully characterized. Regarding the tethered organometallic catalysts situated within the mesopores of silica (as depicted in Figure 4), the strategy for their assembly proceeds as follows:

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Figure 4. Depiction of the catalytically active center [Rh(1,5-cyclooctadiene)(S)-(-)-2-aminomethyl-1-ethylpyrrolidine]BF4, anchored (via a tether) in a constrained manner inside the inner walls of mesoporous silica (left). The homogeneous analogue of the heterogeneous catalyst is shown on the right. Note that the BF4- counterion has been omitted for clarity.

First, the kind of ligand (see Table 2), which, on chemical grounds, is thought to be promising as an enantioselective catalyst when bound to an isolated Pt, Pd, or Rh atom, is identified. Second, the X-ray crystal structure of the complex is determined so as to yield the precise nature of the organometallic complex (see, for example, Figure 5). Catalytic Performance We first deal with the activities and selctivities of some bimetallic catalysts for the following reactions: (a) the single-step, solvent-free hydrogenation of 1,5,9cyclododecatriene, (b) the conversion of trans,transmuconic acid to adipic acid, and (c) the hydrogenation of benzoic acid to cyclohexane carboxylic acid. All three types of reactions are of considerable importance industrially in the context of the hydrogen economy. The selective hydrogenation of 1,5,9-cyclododecatriene to cyclododecane and cyclododecene is industrially important in the synthesis of valuable organic and polymer intermediates such as 12-laurolactam and dodecanedioic acid,6,12 which are important monomers for nylon 12, nylon 612, copolyamides, polyesters, and coating applications and in the synthesis of bicarboxylic aliphatic acids. Processes b and c above, unlike process a, which is solvent-free, necessarily require a solvent to deliver the

Figure 5. Fragments of the crystal structures of (A) [Rh(1,5cyclooctadiene)(S)-(-)-2-aminomethyl-1-ethylpyrrolidine]BF4, (B) [Rh(norbornadiene)(S)-(-)-2-amino methyl-1-ethylpyrrolidine]BF4, and (C) [Rh(1,5-cyclooctadiene)(1R,2R)-(+)-1,2-diphenylethylenediamine]BF4. Note that the BF4- counterion has been omitted for clarity. Color code: gray represents carbon atoms, blue nitrogen, and purple rhodium.

reactant to the catalytic sites at the anchored bimetallics. Fortunately, an environmentally benign solvent (ethanol) suffices for this purpose. The results of the catalytic tests for all three reactions are presented in Figures 6-8.

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Figure 6. Effect of temperature on the activity and selectivity in the single-step, solvent-free hydrogenation of 1,5,9-cyclododecatriene using the Ru6Sn and Ru6Pd6 bimetallic catalysts anchored on mesoporous silica. Reaction conditions: 1,5,9-cyclododecatriene ≈ 50 g; catalyst ) 25 mg; H2 pressure ) 30 bar; t ) 24 h.

Scheme 2. Synthesis of [Ru6C(co)16SnCl3]- a

a

Carbonyl ligands omitted for clarity.

Scheme 3. Schematic Drawing Illustrating the Different Products Arising from the Hydrogenation of E-r-Phenylcinnamic Acid

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Figure 7. Efficacy (activity and selectivity) of the various anchored bimetallic nanocatalysts in the single-step hydrogenation of trans,transmuconic acid to adipic acid. Commercially available Pd on silica and Rh on alumina were used for relative comparisons.

Figure 8. Bar chart summarizing the relative performances and selectivities of the Ru5Pt and Ru10Pt2 catalysts when compared with other bimetallic nanocatalysts (Ru12Cu4, Ru6Sn, and Ru6Pd6) for the hydrogenation of benzoic acid. Note that the Ru5Pt and Ru10Pt2 catalysts display a high degree of selectivity for the cyclohexane carboxylic acid in marked contrast to the other bimetallic analogues. Reaction conditions: benzoic acid ≈ 2.5 g (dissolved in 75 mL of ethanol), catalyst ) 50 mg, H2 pressure ) 20 bar, temperature ) 373 K, t ) 24 h.

It is to be noted that our bimetallic catalysts exhibit exceptionally high activities and very good selectivities for all three reactions. Turning to the second category of catalyst, where we focus on enantioselective hydrogenation, we have restricted our tests to just one representative reaction: the conversion of E-R-phenylcinnamic acid (Scheme 3).

The important feature of the results shown in Table 2 is that we can compare the performance of the homogeneous catalyst with that of its heterogeneous analogue. The key point here, as our earlier work has shown,15-21 is that we gain in enantioselective performance by constraining the active site and its associated organic

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Table 2. Hydrogenation of r-Phenylcinnamic Acid Using the Rhodium Amine Complexesa amineb

homogeneous/ heterogeneous

time (h)

conversion (%)

selectivity (%)

ee (%)

(S)-(-)-2-aminomethyl-1-ethylpyrrolidine (S)-(-)-2-aminomethyl-1-ethylpyrrolidine (S)-(-)-2-aminomethyl-1-ethylpyrrolidine (S)-(-)-2-aminomethyl-1-ethylpyrrolidine (1R,2R)-(+)-1,2-diphenylethylenediamine (1R,2R)-(+)-1,2-diphenylethylenediamine

homogeneousc heterogeneousc homogeneousd heterogeneousd homogeneousc heterogeneousc

24 24 24 24 24 24

74 70 88 99 57 98

73 77 76 66 84 80

93 96 64 91 81 93

a Reaction conditions: reactions carried out at 20 bar of H and a temperature of 40 °C, using 10 mg of homogeneous catalyst, 50 mg 2 of heterogeneous catalyst, and 500 mg of substrate and using methanol as the solvent. b Amines are shown in the crystal structures (Figure 5). c Cyclooctadiene. d Norbornadiene.

environment, as a result of the spatial restriction imposed upon the reactant as it approaches the locus of catalytic conversion. The catalysts were synthesized as previously described,22 and both the heterogeneous and homogeneous systems were extensively characterized by a vast range of techniques, including NMR spectroscopy and X-ray crystallography.23 It is noteworthy that, in the examples given in Table 2, both the catalytic activity and the degree of enantioselectivity are significantly enhanced in the constrained, anchored catalysts compared with their homogeneous counterparts. This augurs well for the future use of such catalysts in the production of desirable chiral molecules. Acknowledgment We thank EPSRC (U.K.) for a rolling grant to Sir John Meurig Thomas and an award to Professor Brian F. G. Johnson. We also thank the Royal Commission for the Exhibition of 1851 and Bayer AG, Leverkusen, Germany, for their support to Dr. Robert Raja; the EC and Newnham College, Cambridge, U.K., for a Research Fellowship for Dr. Sophie Hermans; EPSRC (U.K.), ICI, and the Newton Trust for a case award to Matthew Jones; and the Cambridge Overseas Trust (Schlumberger research) and ICI for supporting Tetyana Khimyak. Literature Cited (1) Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts and Applications (Exxon Monograph); Wiley: New York, 1983. (2) Sinfelt, J. H. Ruthenium coppersModel bimetallic system for studies of surface chemistry and catalysis. Int. Rev. Phys. Chem. 1988, 7, 281-315. (3) Shephard, D. S.; Maschmeyer, T.; Sankar, G.; Thomas, J. M.; Ozkaya, D.; Johnson, B. F. G.; Raja, R.; Oldroyd, R. D.; Bell, R. G. Preparation, characterization and performance of encapsulated copper-ruthenium bimetallic catalysts derived from molecular cluster carbonyl precursors. Chem. Eur. J. 1998, 4, 12141224. (4) Raja, R.; Sankar, G.; Hermans, S.; Shephard, D. S.; Bromley, S. T.; Thomas, J. M.; Maschmeyer, T.; Johnson, B. F. G. Preparation and characterisation of a highly active bimetallic (Pd-Ru) nanoparticle heterogeneous catalyst. Chem. Commun. 1999, 15711572. (5) Raja, R.; Khimyak, T.; Thomas, J. M.; Hermans, S.; Johnson, B. F. G. Single-step, highly active and highly selective nanoparticle catalysts for the hydrogenation of key organic compounds. Angew. Chem., Int. Ed. Engl. 2001, 40, 4638-4642. (6) Hermans, S.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G.; Gleeson, D. Solvent-free, low-temperature, selective hydrogenation of polyenes using a bimetallic nanoparticle RuSn catalyst. Angew. Chem., Int. Ed. Engl. 2001, 40, 1211-1215. (7) (a) Ozkaya, D.; Zhou, W. Z.; Thomas, J. M.; Midgley, P. A.; Keast, V. J.; Hermans, S. High-resolution imaging of nanoparticle bimetallic catalysts supported on mesoporous silica. Catal. Lett. 1999, 60, 113-120. (b) Thomas, P. J.; Midgley, P. A. An introduction to energy-filtered transmission electron microscopy. Curr. Top. Catal. 2002, 21, in press and references therein.

(8) Thomas, J. M.; Terasaki, O.; Gai, P. L.; Zhou, W. Z.; Gonzalez-Calbet, J. Structural elucidation of microporous and mesoporous catalysts and molecular sieves by high-resolution electron microscopy. Acc. Chem. Res. 2001, 34, 583-594. (9) Midgley, P. A.; Weyland, M.; Thomas, J. M.; Johnson, B. F. G. Z-contrast tomography: A technique in 3-dimensional nanostructural analysis based on Rutherford scattering. Chem. Commun. 2001, 907-908. (10) Weyland, M.; Midgley, P. A.; Thomas, J. M. Electron tomography of nanoparticle catalysts on porous supports: A new technique based on Rutherford scattering. J. Phys. Chem. B 2001, 105, 7882-7886. (11) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G.; Lewis, D. W. Benign by design. New catalysts for an environmentally conscious age. Pure Appl. Chem. 2001, 73, 1087-1101. (12) Thomas, J. M.; Raja, R.; Sankar, G.; Johnson, B. F. G.; Lewis, D. W. Solvent-free routes to clean technology. Chem. Eur. J. 2001, 7, 2973-2978. (13) Draths, K. M.; Frost, J. W. Environmentally compatible synthesis of adipic acid from D-glucose. J. Am. Chem. Soc. 1994, 116, 399-400. (14) Hermans, S.; Khimyak, T.; Johnson, B. F. G. High yield synthesis of Ru-Pt mixed-metal cluster compounds. J. Chem. Soc., Dalton Trans. 2001, 3295-3302. (15) Thomas, J. M. Tales of tortured ecstasy: Probing the secrets of solid catalysts. Faraday Discuss. 1995, 100, C9-C27. (16) Thomas, J. M.; Maschmeyer, T.; Johnson, B. F. G.; Shephard, D. S. Constrained chiral catalysts. J. Mol. Catal. A 1999, 141, 139-144. (17) Johnson, B. F. G.; Raynor, S. A.; Shephard, D. S.; Maschmeyer, T.; Thomas, J. M.; Sankar, G.; Bromley, S. T.; Oldroyd, R. D.; Gladden, L. G.; Mantle, M. D. Superior performance of a chiral catalyst confined within mesoporous silica. Chem. Commun. 1999, 1167-1168. (18) Thomas, J. M.; Raja, R. The materials chemistry of inorganic catalysts. Aust. J. Chem. 2001, 54, 551-560. (19) Raynor, S. A.; Thomas, J. M.; Raja, R.; Johnson, B. F. G.; Bell, R. G.; Mantle, M. D. A one-step, enantioselective reduction of ethyl nicotinate to ethyl nipecotinate using a constrained, chiral, heterogeneous catalyst. Chem. Commun. 2000, 1925-1926. (20) Thomas, J. M.; Raja, R. Catalytically active centres in porous oxides: Design and performance of highly selective new catalysts. Chem. Commun. 2001, 675-687. (21) Thomas, J. M. Design, synthesis and in situ characterization of new solid catalysts. Angew. Chem., Int. Ed. Engl. 1999, 38, 3588-3628. (22) Pertici, P.; D’Arata, F.; Rosini, C. Synthesis, chirooptical properties and catalytic activity of diene-rhodium(I) and -iridium(I) cationic complexes containing binaphthyl, C2-symmetric diamine ligands. J. Organomet. Chem. 1996, 515, 163-171. (23) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Lewis, D. W.; Rouzard, J.; Harris, K. D. M. Enhancing the enantioselectivity of novel homogeneous organometallic hydrogenation catalysts, manuscript submitted.

Received for review August 26, 2002 Revised manuscript received October 31, 2002 Accepted November 1, 2002 IE0206610