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Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41. Part I.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
789
Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41. Part I. Douglas S. Shephard The University Chemical Laboratories, Lensfield Road, Cambridge, CB2 lEW, U.K. These papers describe the production and study of several high-performance materials based on MCM-41. First, bimetallic nanoparticle catalysts derived from precursor metal-cluster carbonylates anchored inside the mesoporous channels of silica. In-situ X-ray absorption and FT-IR spectroscopies as well as ex-situ high-resolution scanning transmission electron microscopy have been used to chart the progressive conversion, by gentle thermolysis, of the parent carbonylates to the denuded, bimetallic nanoparticle catalysts. These bimetallic catalysts exhibit no tendency to sinter, aggregate or fragment into their separate component metals during use and display good performance in the catalytic hydrogenation of hex-1-ene a detailed kinetic study. Secondly, the ordering of these particles within the channels at high loading and their conductivity on denuding. Thirdly, how the internal surface of mesoporous silica may be selectively fiinctionalised with propyl ammonium groups, and their presence and position may be directly imaged by high resolution electron microscopy (HRTEM) after 'staining' with the cluster-cluster crown compound [Ru6C(CO)i4(h6-C6H4CioH2006] • Fourthly, it is demonstrated that a chiral ligand (derived from 1,1'-bis-diphenylphosphinoferrocene (dppf)), bound to an active metal (Pd) and anchored via a molecular tether of appropriate length to the inner walls of a mesoporous silica support {ca. 30A diameter) yields a degree of catalytic regioselectivity as well as an enantiomeric excess that is far superior to those of the same free (i.e. homogeneous) catalyst in the synthetically useful process of ally lie amination. Fifthly, how a new reuseable heterogeneous oxidation catalyst immobilised within MCM-41 has been prepared and used in clean organic synthesis, with molecular oxygen as cooxidant. 1. INTRODUCTION In recent years, with the advent of mesoporous silicate materials, the science of oxide supports (especially siliceous ones) has undergone a revolution. These silicates have a highly regular structure composed of channels (diameter 20 to lOOA) which provide huge porosity and surface areas (>lOOOm^/g). The large pore sizes of these materials (c./ microporous zeolites) offer the possibility of creating catalytically active sites within the silicate framework, and the topological restraints produced by the confinement of solvent, substrate and reactant may be expected to give rise to greater efficiency and selectivity in the reaction process. 2 . BIMETALLIC CATALYSTS Allied to this, current interest in the use of bi-metallic catalysts is largely due to the enhanced activity and selectivity that may be achieved.'"^ The break-through came when a
790 catalytically active metal was used in conjunction with another that was significantly less active, but worked in a complementary manner. At Exxon Sinfelt' et al studied the Pt-Re and Ru-Cu systems for catalytic reforming and showed highly encouraging results for these systems. However, despite considerable effort, the preparation of these systems is limited by the rather crude method of metal salt deposition (metal salts are deposited onto an amorphous support material {e.g. alumina, carbon, silica) and then calcined under O2 before reduction to the active low valent species with H2). This method has many drawbacks, of which, the greatest being precise control over size, morphology and homogeneity of the bi-metallic particles cannot be achieved; these factors and the random nature of the support lead to loss of selectivity/activity of the catalyst - a wasteful and costly problem in industry. Several mixed-metal molecular cluster carbonyls that have recently been used as precursors for heterogeneous catalysts (e.g. Nasher et al? and Shephard et al, ^ In the former -i
^
2-
mstance, where the cluster anion was [Re6C(CO)i8{m -Re(C0)3}{m -Ir(C0)2}] , it was found that upon thermolysis the metals segregated into separate entities, possibly because of the nature of the oxide support used. In the latter, where the cluster anions were [Ag3RuioC2(CO)28Cl]^" and [Ru6C(CO)i6Cu2Cl]22-, there was no evidence for segregation and sintering, and we believe this to arise for two reasons. First, the mesoporous silica support, onto which the cluster carbonylate is initially anchored, is replete with silanol groups that interact strongly with the carbonylate anion. Second, the relative oxophilicity of the silver/copper atoms makes them ideal bonding centres securing the bimetallic cluster to the oxide support. Active Catalyst protective sheati capable of physisorption to^ support
and M^ + M^ known exactly
MVM^
and M U M^ is known exactly -highly disperse -MVM^
cluster deposition MCM41 (mesoporous silicate)
>
-highly regular -huge surface area sheath removal (activation + anchoring)
bimetallic particle
Scheme 1. Cluster deposition and activation.
2.1. Preparation and Characterisation When considering the more precise route of depositing and thermolysing carbonyl clusters for the production of supported nanoparticles, several important criteria concerning the choice of bimetallic cluster precursor have to be borne in mind."^ First, the protective sheath surrounding the organometallic precursor must be readily removable (we find that mild thermolysis suffices). Second, interactions with the surface must be stronger than those involved in solvation or between the precursor species so that aggregation into small molecular crystallites and subsequent sintering on the surface is suppressed upon removal of the CO sheath. Anionic cluster carbonyl species,^ typified by [Ru6C(CO)i6Cu2Cl]2[PPN]2
791 and [Ag3RuioC2(CO)28Cl][PPN]2, fulfil these criteria as their interaction with the MCM41 surface is of the Si-OH^"'"--d-0-C-M type^ and intermolecular Coulombic repulsion prevents their aggregation prior to thermolysis. A representation of this approach is given in scheme 1.
Figure 1. STEM images of the RuI2Cu4 catalyst in MCM-41 a) before and b) after vitrification by the electron beam. Inset labelling of particles a,b,c,d shows how they remain anchored in place during this process. Apart from gaining deep insights into the nanostructures and morphology of the resulting catalysts using a combination of annular dark-field (ADF) and bright field (BF) high resolution scanning transmission electron microscopy (see figure 1)7 we have tracked the precise structural details of the conversion of this precursor material into its active catalytic state principally by using in-situ X-ray absorption techniques. We were able to gain accurate local structural information for the catalyst by the application of these element specific (e.g. Cu and Ru) techniques. The X-ray absorption near edge structure (XANES) provides the electronic state and qualitative local structural information, whereas the extended X-ray absorption fine structure (EXAFS) establishes the precise local structural details, thus, revealing the internal structure of the nanoparticle (see figure 2 for example). Confirmation of the loss of the protective carbonyl sheath during thermolysis was found by in-situ FT-IR and ftirther corroborative evidence was established by thermogravimetric analysis. 2.2 Catalytic Evaluation The catalytic performance of the supported bimetallic nano-particles in the hydrogenation of unsaturated molecules was tested on a wide variety of unsaturated species: hex-1-ene, phenyl acetylene, diphenyl acetylene, trans-stilbene, cis-cyclooctene and D-limonene. The highly efficient hydrogenation of hex-1-ene was accompanied by the isomerisation reaction to cisand trans-hex-2-ene, which were subsequently hydrogenated (albeit at a much slower rate) as reaction ensued. Phenylacetylene is completely converted to ethylbenzene under the reaction conditions used. No hydrogenation of the phenyl group was detected. This shows a considerable degree of selectivity of the catalyst. This selectivity was ftirther illustrated in the hydrogenation of diphenylacetylene which gave both stilbene (predominently trans-) and bibenzyl.^ Careful kinetic studies at 20 bar hydrogen and 373 ^C show an induction time of 60 minutes and an
792 overall turnover frequency of 25,700 mol[Hex]mol[Cu4Rul2]-lh-l. The kinetic details for this catalyst and for a new Ag4Rul2 system are summarised in Fig. 3.
Figure 2. The formation of a Pd-Ru bimetallic nanoparticle catalyst from a single source bimetallic molecular carbonylate anion [Pd6Ru6(CO)24]2-. •H
95 Con.AgRu
-•
H2 (bar) AgRu
••
n-Hex«ne AgRu
^-
Cis-Hex-2-en* AgRu
•
Trans-Hex-2-#r>* AgRu
—
% Con.CuRu H2 (bar) CuRu
n-Hexane CuRu
Cis-H*x-2-*r>e CuRu
200
300
Time (mim)
Figure 3. A comparrison of the hex-1-ene hydrogenation behaviour of the two nanoparticle catalysts Ag4Rul2 and Cu4Rul2. Note how the silver containing catalyst is significantly superior in terms of TOF in the early stages of the reaction (i.e. no induction time).
793 Together these results demonstrate that our initial strategy of removing the stabilising CO sheath from a mixed metal cluster to produce a well-defmed metal nanoparticle and anchoring the more oxophilic second metal to the MCM-41 surface has met with success. This work also reveals that there is abundant scope for further exploitation of bimetallic metal-cluster carbonylates as precursors for other supported nanoparticle catalysts. Moreover, a wide range of catalytic reactions besides hydrogenation awaits study. 3. SUPRAMOLECULAR ORDERING In the course of our work on the exploitation of mesoporous silicas for the production of novel supported metallic catalysts we have discovered a method of producing ordered arrays of nanoparticles, in this case anionic ruthenium cluster carbonylates, [Ru6C(CO)i6]^' and [H2Ruio(CO)25]^" interspersed with bis(triphenylphosphino)iminium (PPN"^) counterions. Thus providing a new methodology for the production of ordered platinum group metal nanoparticles. The nature of these materials has been probed by high resolution electron microscopy (HREM both real and reciprocal space),^''^ FT-IR and other techniques. We have found that, in the case of the Ru6 and Ruio cluster carbonyls accommodated inside MCM-41 mesoporous silica, the metal cluster anions are repeated at ca. 17.0A and 26.6A respectively, along the axis of the mesopores (30+2A internal diameter), the inner surfaces of which are essentially structurally disordered. The formation process and structure of these one dimensional crystals and their inter-relation in the three dimensional framework will be discussed. In view of the intense current interest in the properties of nanoelectronic materials, ranging in dimentionality from zero (quantum dot) to two (i.e. ID and 2D), much effort has recently been expended,^' in developing novel ways of producing ordered arrays, including linked cluster networks of metallic nanoparticles in a size range down to ca. 15A. The only prior comparable preparation of linear arrays of metallic nanoparticles (apart from the special instances of "decorated" atomic steps on graphite'^ and molybdenite'^ surfaces with nanoparticles of the coinage metals and the insertion of materials inside carbon nanotubes)'"* is that of Schmid and Homyak, who recently reported that the 500A diameter pores of alumina membranes could be packed with 130A diameter, ligand stabilised Au55 gold colloids by vacuum induction. Figure 4 reveals the regular nature of the mesopores of the MCM-41 silica (inset Fourier shows spots in 100 direction); absence of 001 spots in the Fourier transform shows that there is no crystallographic order in the direction of the pore axis. Figures 5 and 6 show typical HREM and STEM bright field images (together with their Fourier transforms) of the MCM-41 loaded with the carbonylate salts. These images show regular repeats along the pore axis [001] in both Ru6.1 MCM-41 with d-spacings, derived from their Fourier transforms, of c^. 17 and 27A, respectively. An interpretation of the structural features contained in figures 5 and 6 may be afforded by examination of the individual cluster carbonylate salts and the possible packing motifs available to them. Bearing in mind charge compensation and CO stretching frequencies the individual hexa-ruthenate clusters may be viewed as being hydrogen bonded to the pore wall {via several interactions of the Si-0-H-O-C-Ru type), flanked by two PPN counterions. Given an internal pore diameter of 30±2A, the centre to centre distance between two hexaruthenate clusters along the pore axis is 17A at an angle (q) of 48 deg. (figure 7) and 1=25.5A.
794 This suggests that the clusters are most likely to pack such that they sit alternately on opposite sides of the MCM-41 channel, forming a zig-zag arrangement in the pore axis direction. In the case of MCM-41/[H2Ruio(CO)25][PPN]2 the clusters may also be assumed to be bound to the silica surface and separated by two PPN moieties with an ideal inter-cluster distance of ca. 21.ik (as found in the crystal structure). The average d-spacing calculated from the well defined diffraction spots [001] is about 26.6A which is very close to the linear model distance (figure 7). Consequently we may describe the packing of the deca-ruthenate clusters as approximating to linearity with 0° < 0 < 5°. This essentially linear arrangement of the deca-ruthenate clusters may be ascribed to their larger Van der Waals radii over that of the hexa-ruthenate anion and possibly their differing spatial linkages with the PPN counterions (as notedfi-omtheir crystal structures). A model detailing the packing of the [H2Ruio(CO)25]^" anion together with its [PPN]"^ cations inside a single mesopore which is consistent with all the facts retrievedfi-omFTIR and HREM (and FT) is given in figure 7.
.'jjii^&ii^.
Figure 4. HRTEM image of pure silica MCM-41, with its Fourier transform (inset), viewed perpendicular to the pore axis ([001] direction, indicated by the arrow). Scale bar, 10 nm.
Figure 5. HRTEM image of MCM-41 loaded with [Ru6C(CO)16 ][PPN]2, with its Fourier transform (inset). The average repeat distance derived from the reciprocal space (002) spots of the Fourier transform is 2.1 nm, corresponding to a projection of the average repeat distance on the [001] axis for the clusters. Scale bar, 10 nm.
Although much work remains to be done on these novel materials, many possible applications may be envisaged for these 'constrained' arrays of mono-disperse nanoparticles. It is already clear from additional experiments that it is possible to denude the clusters of their carbonyls by gentle heat treatment in vacuo thus producing nanoparticles of ruthenium metal. Further work, in this highly promising area, involves using clusters and counterions of different sizes to tailor the intercluster distance and examining the electronic, magnetic and optical properties. Indeed we have very recently examined the conductivity of an MCM-41 strand ca. 1 pores packed with denuded Cu4Rul2 clustrers. The \N graph thus produced shows 'electron hopping' behaviour with a bias voltage of ca. 2.5V which is perfectly consistent with the calculated capacitance of these small particles.
795
*»:nnp»]ainp»ininp' ,•
30A±2
A^=^niax-^ii
B
L2
^*i?>ait5i^t'i.+.
•30A±2
• dmin ' ^location of arc El = cation (PPN) ( ^ = cluster dianion (Ru6C(CO)i6)^- or (H2Ruio(CO)25)^Figure 6. STEM bright-field image (24)ofMCM-41 loaded with [H 2 Ru 10 (CO)25 ][PPN]2 (II) show-ing highly regular features along the pore axis, with its Fourier transform (inset). The repeat distance derived from the reciprocal space (001) spots of the Fourier transform is 2.95 nm. Scale bar, 20 nm.
Figure 7. A schematic diagram showing how the maximum and minimum d spacings can be derived from geometrical packing considerations. (A) Packing that gives the maximum d spacing dmax ; (B) packing that gives the minimum d spacing for a cluster carbonyl of given van der Waals radius. Because the intercluster spacing , is constant, the relative position of B to A is determined by angle q. Therefore, B must lie somewhere on an ellipse (in a cylindrical channel), and the d spacing dmin is determined by dmin =l(cos q). The observed packing may be expected to lie within the limits 0
References 1. 2.
J.H. Sinfelt, Int. Rev. Phys. Chem., 1988, 7 281. J.M. Thomas and W.J. Thomas, 'Principles and Practice of Heterogeneous Catalysis', VCH, Weinheim, 1996. 3. M. Ichikawa, Adv. Catalysis, 1992, 38, 283. 4. B.C. Gates, Chem. Rev., 1995, 95, 511. 5. J.M. Basset, J.P. Candy, et al., in 'Perspectives in Catalysis', (ed. J.M. Thomas, K.I. Zamaraev) Blackwells/IUPAC, Oxford, 1992. 6. M.S. Nasher, D.M. Sommerville, P.D. Lane, D.L. Adler, J.R. Shapley and R.G. Nuzzo, J. Amer. Chem. Sec., 1996, 118,12964. 7. J.H. Sinfelt, ''Bimetallic Catalysts", J. Wiley, New York, 1983.
796 8. 9.
10. 11. 12. 13. 14.
15. 16. 17.
18. 19. 20. 21.
M.S. Nasher, D.M. Sommerville, P.D. Lane, D.L. Adler, J.R. Shapley and R.G. Nuzzo, J. Amer. Chem. Soc, 1996, 118, 12964. D.S. Shephard, T. Maschmeyer, B. F. G. Johnson, J.M.Thomas, G.Sankar, D.Ozkaya and Wuzong Zhou, Richard D. Oldroyd. Ang.Chem., Int.Ed.Eng., 1997, 36, 2242; D.S. Shephard, T. Maschmeyer, G. Sankar, J.M. Thomas, D. Ozkaya, B.F.G. Johnson, R. Raja, R.D. Oldroyd, R.G. Bell, Chemistry Eu. J., 1998,4, 1127. D.S. Shephard, T. Maschmeyer, G. Sankar, J.M. Thomas, D. Ozkaya, B.F.G. Johnson, R.Raja, R.D. Oldroyd, R.G. Bell, Chemistry Eu. J., 1998,4,1127. M.A. Beswick, Ph.D Thesis, University of Cambridge. 1992 and references therein; M.A. Beswick et. al., Angew. Chem., Int. Ed. Engl., 1997, 36, 291. S.J. Tavemer, J.H. Clark, G.W. Gray, P.A. Heath, D.J. Macquarrie, J. Chem. Soc, Chem. Commun., 1997, 1073. D. Ozkaya, J.M.Thomas, D.S.Shephard, T.Maschemeyer, B.F.G.Johnson, G. Sankar, R.Oldroyd, Institute of Physics Conference Series, 1997, 153, 403. The overall TOF of this reaction was unexpectedly low and may be due to deactivation by long lived intermediates blocking reactive sites on the bimetallic particles. We are currently attempting to determine the cause of this loss of activity. Jefferson, D.A., Thomas, J.M., et al. . Nature, 1986, 323, 428. Millward, G.R., and Thomas, J.M., Proc. of Carbon and Graphite Conf. (Soc. Chem. Ind., London.) 1982, 492. Brust, M., Bethell, D., et al.„ Adv. Mater., 1995, 7, 795; Whetten, R.L., et al.. Adv. Mater., 1996, 8, 428; Andres, R.P., et al.. Science, 1996, 273, 1690; Alvisatos, A.P. et al.. Nature, 1996,382,609. Thomas, J.M., Evans, E.L. and Williams, J.O., Proc. Roy. Soc.A 1972,331, 417; Francis, G.M., Goldby, I.M., et al., J. Chem. Soc, Dalton Trans. 1996, 665. Bahl, O.P., Evans, E.L., and Thomas, J.M., Surf.Sci., 1967, 8,473. Sloan, J., Cook, J., Green, M.L.H., Hutchison, J.L., Tenne, R., J. Mater. Chem., 1997, 7, 1089. M. Thomas, D. Shephard, et al. Unpublished results. 1999.
789
Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41. Part I. Douglas S. Shephard The University Chemical Laboratories, Lensfield Road, Cambridge, CB2 lEW, U.K. These papers describe the production and study of several high-performance materials based on MCM-41. First, bimetallic nanoparticle catalysts derived from precursor metal-cluster carbonylates anchored inside the mesoporous channels of silica. In-situ X-ray absorption and FT-IR spectroscopies as well as ex-situ high-resolution scanning transmission electron microscopy have been used to chart the progressive conversion, by gentle thermolysis, of the parent carbonylates to the denuded, bimetallic nanoparticle catalysts. These bimetallic catalysts exhibit no tendency to sinter, aggregate or fragment into their separate component metals during use and display good performance in the catalytic hydrogenation of hex-1-ene a detailed kinetic study. Secondly, the ordering of these particles within the channels at high loading and their conductivity on denuding. Thirdly, how the internal surface of mesoporous silica may be selectively fiinctionalised with propyl ammonium groups, and their presence and position may be directly imaged by high resolution electron microscopy (HRTEM) after 'staining' with the cluster-cluster crown compound [Ru6C(CO)i4(h6-C6H4CioH2006] • Fourthly, it is demonstrated that a chiral ligand (derived from 1,1'-bis-diphenylphosphinoferrocene (dppf)), bound to an active metal (Pd) and anchored via a molecular tether of appropriate length to the inner walls of a mesoporous silica support {ca. 30A diameter) yields a degree of catalytic regioselectivity as well as an enantiomeric excess that is far superior to those of the same free (i.e. homogeneous) catalyst in the synthetically useful process of ally lie amination. Fifthly, how a new reuseable heterogeneous oxidation catalyst immobilised within MCM-41 has been prepared and used in clean organic synthesis, with molecular oxygen as cooxidant. 1. INTRODUCTION In recent years, with the advent of mesoporous silicate materials, the science of oxide supports (especially siliceous ones) has undergone a revolution. These silicates have a highly regular structure composed of channels (diameter 20 to lOOA) which provide huge porosity and surface areas (>lOOOm^/g). The large pore sizes of these materials (c./ microporous zeolites) offer the possibility of creating catalytically active sites within the silicate framework, and the topological restraints produced by the confinement of solvent, substrate and reactant may be expected to give rise to greater efficiency and selectivity in the reaction process. 2 . BIMETALLIC CATALYSTS Allied to this, current interest in the use of bi-metallic catalysts is largely due to the enhanced activity and selectivity that may be achieved.'"^ The break-through came when a
790 catalytically active metal was used in conjunction with another that was significantly less active, but worked in a complementary manner. At Exxon Sinfelt' et al studied the Pt-Re and Ru-Cu systems for catalytic reforming and showed highly encouraging results for these systems. However, despite considerable effort, the preparation of these systems is limited by the rather crude method of metal salt deposition (metal salts are deposited onto an amorphous support material {e.g. alumina, carbon, silica) and then calcined under O2 before reduction to the active low valent species with H2). This method has many drawbacks, of which, the greatest being precise control over size, morphology and homogeneity of the bi-metallic particles cannot be achieved; these factors and the random nature of the support lead to loss of selectivity/activity of the catalyst - a wasteful and costly problem in industry. Several mixed-metal molecular cluster carbonyls that have recently been used as precursors for heterogeneous catalysts (e.g. Nasher et al? and Shephard et al, ^ In the former -i
^
2-
mstance, where the cluster anion was [Re6C(CO)i8{m -Re(C0)3}{m -Ir(C0)2}] , it was found that upon thermolysis the metals segregated into separate entities, possibly because of the nature of the oxide support used. In the latter, where the cluster anions were [Ag3RuioC2(CO)28Cl]^" and [Ru6C(CO)i6Cu2Cl]22-, there was no evidence for segregation and sintering, and we believe this to arise for two reasons. First, the mesoporous silica support, onto which the cluster carbonylate is initially anchored, is replete with silanol groups that interact strongly with the carbonylate anion. Second, the relative oxophilicity of the silver/copper atoms makes them ideal bonding centres securing the bimetallic cluster to the oxide support. Active Catalyst protective sheati capable of physisorption to^ support
and M^ + M^ known exactly
MVM^
and M U M^ is known exactly -highly disperse -MVM^
cluster deposition MCM41 (mesoporous silicate)
>
-highly regular -huge surface area sheath removal (activation + anchoring)
bimetallic particle
Scheme 1. Cluster deposition and activation.
2.1. Preparation and Characterisation When considering the more precise route of depositing and thermolysing carbonyl clusters for the production of supported nanoparticles, several important criteria concerning the choice of bimetallic cluster precursor have to be borne in mind."^ First, the protective sheath surrounding the organometallic precursor must be readily removable (we find that mild thermolysis suffices). Second, interactions with the surface must be stronger than those involved in solvation or between the precursor species so that aggregation into small molecular crystallites and subsequent sintering on the surface is suppressed upon removal of the CO sheath. Anionic cluster carbonyl species,^ typified by [Ru6C(CO)i6Cu2Cl]2[PPN]2
791 and [Ag3RuioC2(CO)28Cl][PPN]2, fulfil these criteria as their interaction with the MCM41 surface is of the Si-OH^"'"--d-0-C-M type^ and intermolecular Coulombic repulsion prevents their aggregation prior to thermolysis. A representation of this approach is given in scheme 1.
Figure 1. STEM images of the RuI2Cu4 catalyst in MCM-41 a) before and b) after vitrification by the electron beam. Inset labelling of particles a,b,c,d shows how they remain anchored in place during this process. Apart from gaining deep insights into the nanostructures and morphology of the resulting catalysts using a combination of annular dark-field (ADF) and bright field (BF) high resolution scanning transmission electron microscopy (see figure 1)7 we have tracked the precise structural details of the conversion of this precursor material into its active catalytic state principally by using in-situ X-ray absorption techniques. We were able to gain accurate local structural information for the catalyst by the application of these element specific (e.g. Cu and Ru) techniques. The X-ray absorption near edge structure (XANES) provides the electronic state and qualitative local structural information, whereas the extended X-ray absorption fine structure (EXAFS) establishes the precise local structural details, thus, revealing the internal structure of the nanoparticle (see figure 2 for example). Confirmation of the loss of the protective carbonyl sheath during thermolysis was found by in-situ FT-IR and ftirther corroborative evidence was established by thermogravimetric analysis. 2.2 Catalytic Evaluation The catalytic performance of the supported bimetallic nano-particles in the hydrogenation of unsaturated molecules was tested on a wide variety of unsaturated species: hex-1-ene, phenyl acetylene, diphenyl acetylene, trans-stilbene, cis-cyclooctene and D-limonene. The highly efficient hydrogenation of hex-1-ene was accompanied by the isomerisation reaction to cisand trans-hex-2-ene, which were subsequently hydrogenated (albeit at a much slower rate) as reaction ensued. Phenylacetylene is completely converted to ethylbenzene under the reaction conditions used. No hydrogenation of the phenyl group was detected. This shows a considerable degree of selectivity of the catalyst. This selectivity was ftirther illustrated in the hydrogenation of diphenylacetylene which gave both stilbene (predominently trans-) and bibenzyl.^ Careful kinetic studies at 20 bar hydrogen and 373 ^C show an induction time of 60 minutes and an
792 overall turnover frequency of 25,700 mol[Hex]mol[Cu4Rul2]-lh-l. The kinetic details for this catalyst and for a new Ag4Rul2 system are summarised in Fig. 3.
Figure 2. The formation of a Pd-Ru bimetallic nanoparticle catalyst from a single source bimetallic molecular carbonylate anion [Pd6Ru6(CO)24]2-. •H
95 Con.AgRu
-•
H2 (bar) AgRu
••
n-Hex«ne AgRu
^-
Cis-Hex-2-en* AgRu
•
Trans-Hex-2-#r>* AgRu
—
% Con.CuRu H2 (bar) CuRu
n-Hexane CuRu
Cis-H*x-2-*r>e CuRu
200
300
Time (mim)
Figure 3. A comparrison of the hex-1-ene hydrogenation behaviour of the two nanoparticle catalysts Ag4Rul2 and Cu4Rul2. Note how the silver containing catalyst is significantly superior in terms of TOF in the early stages of the reaction (i.e. no induction time).
793 Together these results demonstrate that our initial strategy of removing the stabilising CO sheath from a mixed metal cluster to produce a well-defmed metal nanoparticle and anchoring the more oxophilic second metal to the MCM-41 surface has met with success. This work also reveals that there is abundant scope for further exploitation of bimetallic metal-cluster carbonylates as precursors for other supported nanoparticle catalysts. Moreover, a wide range of catalytic reactions besides hydrogenation awaits study. 3. SUPRAMOLECULAR ORDERING In the course of our work on the exploitation of mesoporous silicas for the production of novel supported metallic catalysts we have discovered a method of producing ordered arrays of nanoparticles, in this case anionic ruthenium cluster carbonylates, [Ru6C(CO)i6]^' and [H2Ruio(CO)25]^" interspersed with bis(triphenylphosphino)iminium (PPN"^) counterions. Thus providing a new methodology for the production of ordered platinum group metal nanoparticles. The nature of these materials has been probed by high resolution electron microscopy (HREM both real and reciprocal space),^''^ FT-IR and other techniques. We have found that, in the case of the Ru6 and Ruio cluster carbonyls accommodated inside MCM-41 mesoporous silica, the metal cluster anions are repeated at ca. 17.0A and 26.6A respectively, along the axis of the mesopores (30+2A internal diameter), the inner surfaces of which are essentially structurally disordered. The formation process and structure of these one dimensional crystals and their inter-relation in the three dimensional framework will be discussed. In view of the intense current interest in the properties of nanoelectronic materials, ranging in dimentionality from zero (quantum dot) to two (i.e. ID and 2D), much effort has recently been expended,^' in developing novel ways of producing ordered arrays, including linked cluster networks of metallic nanoparticles in a size range down to ca. 15A. The only prior comparable preparation of linear arrays of metallic nanoparticles (apart from the special instances of "decorated" atomic steps on graphite'^ and molybdenite'^ surfaces with nanoparticles of the coinage metals and the insertion of materials inside carbon nanotubes)'"* is that of Schmid and Homyak, who recently reported that the 500A diameter pores of alumina membranes could be packed with 130A diameter, ligand stabilised Au55 gold colloids by vacuum induction. Figure 4 reveals the regular nature of the mesopores of the MCM-41 silica (inset Fourier shows spots in 100 direction); absence of 001 spots in the Fourier transform shows that there is no crystallographic order in the direction of the pore axis. Figures 5 and 6 show typical HREM and STEM bright field images (together with their Fourier transforms) of the MCM-41 loaded with the carbonylate salts. These images show regular repeats along the pore axis [001] in both Ru6.1 MCM-41 with d-spacings, derived from their Fourier transforms, of c^. 17 and 27A, respectively. An interpretation of the structural features contained in figures 5 and 6 may be afforded by examination of the individual cluster carbonylate salts and the possible packing motifs available to them. Bearing in mind charge compensation and CO stretching frequencies the individual hexa-ruthenate clusters may be viewed as being hydrogen bonded to the pore wall {via several interactions of the Si-0-H-O-C-Ru type), flanked by two PPN counterions. Given an internal pore diameter of 30±2A, the centre to centre distance between two hexaruthenate clusters along the pore axis is 17A at an angle (q) of 48 deg. (figure 7) and 1=25.5A.
794 This suggests that the clusters are most likely to pack such that they sit alternately on opposite sides of the MCM-41 channel, forming a zig-zag arrangement in the pore axis direction. In the case of MCM-41/[H2Ruio(CO)25][PPN]2 the clusters may also be assumed to be bound to the silica surface and separated by two PPN moieties with an ideal inter-cluster distance of ca. 21.ik (as found in the crystal structure). The average d-spacing calculated from the well defined diffraction spots [001] is about 26.6A which is very close to the linear model distance (figure 7). Consequently we may describe the packing of the deca-ruthenate clusters as approximating to linearity with 0° < 0 < 5°. This essentially linear arrangement of the deca-ruthenate clusters may be ascribed to their larger Van der Waals radii over that of the hexa-ruthenate anion and possibly their differing spatial linkages with the PPN counterions (as notedfi-omtheir crystal structures). A model detailing the packing of the [H2Ruio(CO)25]^" anion together with its [PPN]"^ cations inside a single mesopore which is consistent with all the facts retrievedfi-omFTIR and HREM (and FT) is given in figure 7.
.'jjii^&ii^.
Figure 4. HRTEM image of pure silica MCM-41, with its Fourier transform (inset), viewed perpendicular to the pore axis ([001] direction, indicated by the arrow). Scale bar, 10 nm.
Figure 5. HRTEM image of MCM-41 loaded with [Ru6C(CO)16 ][PPN]2, with its Fourier transform (inset). The average repeat distance derived from the reciprocal space (002) spots of the Fourier transform is 2.1 nm, corresponding to a projection of the average repeat distance on the [001] axis for the clusters. Scale bar, 10 nm.
Although much work remains to be done on these novel materials, many possible applications may be envisaged for these 'constrained' arrays of mono-disperse nanoparticles. It is already clear from additional experiments that it is possible to denude the clusters of their carbonyls by gentle heat treatment in vacuo thus producing nanoparticles of ruthenium metal. Further work, in this highly promising area, involves using clusters and counterions of different sizes to tailor the intercluster distance and examining the electronic, magnetic and optical properties. Indeed we have very recently examined the conductivity of an MCM-41 strand ca. 1 pores packed with denuded Cu4Rul2 clustrers. The \N graph thus produced shows 'electron hopping' behaviour with a bias voltage of ca. 2.5V which is perfectly consistent with the calculated capacitance of these small particles.
795
*»:nnp»]ainp»ininp' ,•
30A±2
A^=^niax-^ii
B
L2
^*i?>ait5i^t'i.+.
•30A±2
• dmin ' ^location of arc El = cation (PPN) ( ^ = cluster dianion (Ru6C(CO)i6)^- or (H2Ruio(CO)25)^Figure 6. STEM bright-field image (24)ofMCM-41 loaded with [H 2 Ru 10 (CO)25 ][PPN]2 (II) show-ing highly regular features along the pore axis, with its Fourier transform (inset). The repeat distance derived from the reciprocal space (001) spots of the Fourier transform is 2.95 nm. Scale bar, 20 nm.
Figure 7. A schematic diagram showing how the maximum and minimum d spacings can be derived from geometrical packing considerations. (A) Packing that gives the maximum d spacing dmax ; (B) packing that gives the minimum d spacing for a cluster carbonyl of given van der Waals radius. Because the intercluster spacing , is constant, the relative position of B to A is determined by angle q. Therefore, B must lie somewhere on an ellipse (in a cylindrical channel), and the d spacing dmin is determined by dmin =l(cos q). The observed packing may be expected to lie within the limits 0
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