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Surface organometallic chemistry: a molecular approach to heterogeneous catalysis and a new perspective for the synthesis of metal carbonyl clusters
Mater&s
Chemistry
and Physics,
29 (1991)
191
191-199
Surface organometallic chemistry: a molecular approach to heterogeneous catalysis and a new perspective for the synthesis of metal carbonyl clusters C. Dossi, A. Fusi, R. Psaro, D. Roberto
and R. Ugo
Dipartimento di Chimica Inorganica e Metallorganica C.N.H., Via Venezian, 21, 20133 Milan (Italy]
dell’lJniversita’
and Centro
Abstract The chemical behavior of supported metal fragments obtained from metal carbonyl clusters has been shown to be dependent upon the acid-base properties of the surface of the inorganic oxide and the chemical reactivity of the metal cluster precursor. This ‘molecular approach’ to material science, heterogeneous catalysis in particular, via supported clusters has allowed the identification of new synthetic pathways to molecular metal carbonyl complexes as well as new heterogeneous catalysts.
Introduction In the last few years, the chemical reactivity of molecular metal fragments supported on the surface of high surface area inorganic oxides has been recently rationalized in a very elegant manner through the concept of ‘surface organometallic chemistry’, with the surface sites of the support being regarded as a rather rigid multidentate ligand [ 11. In the meantime, the original idea of investigating supported molecular clusters as ‘heterogenized homogeneous catalysts’ has evolved into the more generalized concept of supported clusters as precursors of unconventional materials. Highly dispersed metal crystallites, as well w surface molecular complexes of specific nuclearity can be obtained by a chemical approach based on the reactivity of metal clusters and the surface properties (acidity/basicity, topology) of the inorganic supports. In addition, the organometallic species formed by surface reactions can be re-extracted from the solid support [ 2, 31; a new route to the selective synthesis of organometallic complexes is thus offered. Finally, this molecular approach to supported molecular clusters is also providing a new and deeper insight into heterogeneous catalysis. Activationdeactivation processes, as well as promotion and poisoning effects can in fact be rationalized on the basis of molecular reactions and cluster transformations. In general, a molecular picture of cluster-surface interactions and reactions can be given from the point of view of the organometallic chemist. In this
0254-0584/91/$3.50
0 1991 - Elsevier Sequoia, Lausanne
192
respect, much attention is devoted here to basic aspects of catalysis and material science, viz. the reactivity of metal-oxide surfaces, the preparation and properties of specific mono- and bime~lic particles, some elementary steps of heterogeneous processes and, finally, the surface-mediated synthesis of molecular carbonyl complexes. Experimental
Supported clusters have been obtained by solvent impregnation or by sublimation [ 3, 41. In situ FTIR and TPDE studies have been carried out on specially-designed equipments, as described elsewhere [3]. Catalytic hydroformylation of a CzH4+ CO + Ha (1: 1: 1) mixture was carried out under flow conditions at 180 “C and atmospheric pressure. Product analysis was obtained by on-line go-chromato~aphy. High-pressure heptane conversion was studied on a computer-controlled, automatized catalytic reactor (Max II system, Xytel Corporation) at 500 “C and 5 atm total pressure, with a hydrogen/hydrocarbonratio of 5 and a weight hourly space velocity (WHSV) of 10 f5]. Results
and discussion
Su?ffaces as solid nmdia for the synthesis of molecular complexm
carbonyl
The peculiar properties of ‘solid ligand’ shown by the surfaces of inorganic oxides [ 1 f can be usefully exploited for the selective preparation of molecular metal carbonyls. This aspect does not simply imply surface transformations of previously supported clusters into higher nuclearity species, as in the first reported example of the selective synthesis under mild conditions of tetranuclear H4M4(C0)i2 (M = Ru, OS) hydrides from silica-supported M3(C0)12 and hydrogen [Sl. The aggregation of mononu~le~ surface complexes, e.g. the surface grafted [Os(CO),(OSif),], (x= 2,3) species 171,or metal ions, e.g. Pd2+ in the NaY structure [81 or Ru3+ and 0s3+ ions on MgO [9 1, to polynuclear structures is observed under suitable conditions, thanks to the unexpected high surface mobility of metal fragments, even if strongly bound to the surface. The presence of surface modifiers, such as chlorine compounds, has also a strong influence in addressing the nature of the final products [ 101. Recently, we performed careful studies on SiO,-supported IrC133H20.Initially, b-Cl3simply deposits on the surface of silica, but upon CO treatment at 150 “C, the impregnated material progressively discolors, and a dark sublimate with a metallic appearance is formed on the cold walls of the glass reactor. MS, IR and elemental analyses indicate that pure Ir(CO)&l is selectively and almost quantitatively formed. By treatment with a CO + Hz0 mixture at 70 “C, the silica support assumes a pale yellow color, and we could not observe any sublimation of
193
Ir(CO)&l. The IR spectrum of the SiOz powder in nyjol mull showed carbonyl bands at 2073 and 2033 cm- ‘, suggesting the presence of physisorbed was Ir, (CO) 12 cluster. By subsequent solvent extraction, pure Ir,(C0),2 recovered and characterized by IR spectroscopy [ 111. Further investigations confirmed that the complete removal of chloride ions from the surface is the key factor for ensuring the complete transformation of the Ir3+ salt into the tetranuclear cluster. Surface chloride ions are likely to favor a low surface mobility of the Ir atoms, preventing their nucleation. A similar effect of surface (CH,COO)ions was noted in the case of SiO,-grafted Os(I1) carbonyls prepared from the model [Os(CO),(CH,COO)], complex. If acetate ions are not carefully removed from the surface, significant amounts of [Os(CO)3(CH&OO)]2 form upon recarbonylation between 150 and 200 “C.
Cluster reactivity at the metal-oxide interface A completely different approach was used on the grafted surface species, such as the HOs3(CO)io(OSi<) cluster, covalently attached to the surface via OS-0-Si bonds. The relatively high stability of the chemisorbed clusters thus makes possible specific chemical reactions to be conducted on the surface complex. Valuable information on the reactivity of the bonds between the metal and the surface, i.e. of the metal-oxide interface, which is a major concern in areas such as catalysis and material science, can be inferred from the reaction products. As a further consequence, the specificity of such surface reactions, generally conducted under mild conditions for preserving the metal framework intact, often offers high-yield routes to the synthesis of metal carbonyl clusters. By treating the grafted HOs3(CO)io(OSi$ cluster with aqueous HF, the Si-0 bonds are broken, and the OS-O -0s bridges are preserved. After dissolution of silica and protonation, pure HOs3(CO),,(OH) is then recovered in high yield by extraction with CH2C12 (56% after recrystallization) [ 121. When using aqueous HCl, on the contrary, HOS~(CO)~~CI is formed by Clinsertion in the OS-O bond and extracted in dichloromethane if working under biphasic (aqueous + organic) conditions (Fig. 1). Accordingly to data in the literature [ 131, its IR spectrum in CHzC12 showed carbonyl bands at 2116(w), 2097(w), 2077(s), 2068(s), 2027(vs), 2015(sh) and 1987(m) cm-i, while a single hydride resonance at - 14.23 ppm from TMS was observed in the ‘H-NMR spectrum. This surface reaction indicates how coordinating ligands, such as Cl-, are likely to be inserted at the metal-oxide interface, with the parallel formation of new surface species. The effect of chlorine is, therefore, completely different from that of a simple ‘acidity enhancer’, as often suggested in heterogeneous catalysis. Consequently, amore complex role of surface chlorine (and other surface modi6ers), involving modifications not only of the acid properties of the surface [lo], but also of the surface metal species, is now proposed.
194
HOs,(CO),, (OH) + H,SiF, ____--_--
I
Si-
0 H
/\ + W),O&~s(co),
I
‘4
Fig. 1. Surface reactions at the metaf-oxide interface on the grafted
HOs~(CO),*(OS~~) cluster.
Cluster-de-rived heterogeneous catalysts A) Monmmtallic s@stems The low thermal stability of chemisorbed molecular clustersunder reaction conditions has severely limited their utihzation as intact cluster catalysts to low-temperature hydrocarbon reactions [ 14, 151 or to the hydrogen transfer reduction of saturated ketones [ 161. In parallel, a new concept of supported metal clusters as precursors of ‘cluster-derived’ heterogeneous catalysts has subsequently evolved. New materials are thus obtained, whose properties can be ‘tuned’ to some extent on the basis of the chemical reactivity of the original cluster precursor and of the physico-chemical properties of the surface. In particular, metal particles are best obtained by mild thermal decarbonylation of the supported molecular cluster in the absence of strong chemical interactions with the support. Such chemical interactions lead, on the contrary, to oxidized surface-grafted metal complexes, often showing high stability and low reducibili~. SiO,-supported I&(CO),, and Osaka are typical examples of the two different behaviors, as well-evidenced by the Temperature Programmed Decomposition (TPDE) technique [ 171,a specially-developed dynamic technique for cluster-support interaction studies. In the case of Ir,(CO),a supported on SiOa, a single decomposition peak around 180 “C is observed (Pig. Z), corresponding to the complete evolution of CO and formation of Ir metal particles: Ir,(CO)Iz/SiOz -
4/x Irz/Si02+ 1X0
195
-
co
-e-
co,
4
CH4
12.00 A
80
0
240
160
Temperature Fig. 2. Temperature
Programmed
320
4
(deg C) Decomposition
(TPDE)
in flowing
He of SiO+upported
in flowing
He of SiO+upported
Ir,(CO),C?.
-
-
co
-a- CH4
co2
9
72
4.3
24
0 ! 0
80
160
Temperature Fig. 3. Temperature oss(co),2.
Programmed
240
320
400
(deg C) Decomposition
(TPDE)
The high dispersion of the iridium particles has been demonstrated by chemisorption studies [ 18 ] and TEM investigations. The more reactive Os,(CO)ia cluster reacts with the Si-OH groups of the silica surface above 80 “C (Pig. 3). HOsa(CO)&OSi<) is formed in a first step below 150 “C, and subsequently decomposes to mononuclear surfacegrafted Os(I1) carbonyls, [ Os(CO),(OSi+], (x = 2,3), of high thermal stability [ 191. Osmium metal particles start to be formed at a much higher temperature, above 330 “C, as evidenced by the methane formation in TPDE (Pig. 3). These SiOa-grafted OS(U) carbonyls are effective catalysts in a wide range of reactions such as hydrogen transfer to ketones [ 161 and olefin isomerization 1291. Ethylene hydroformylation was then tested as a model reaction of CO activation under flow conditions at 1 atm and 180 “C. No formation of
196
oxygenated products (aldehyde and/or alcohols) was observed, but only a very moderate activity for hydrogenation to ethane, failing off to zero within 1-2 hours. At the same time, a yellow material appeared as long needles on the cold walls of the reactor, and was further characterized as a mixture of OSCAR and H,OS,(CO),~ by IR and MS analyses. The transformation of the starting surface-grafted Os(I1) carbonyls into the two volatile clusters was quantitative, since no osmium was left on the catalyst. The practical impossibility of using cluster-derived OS catalysts in a reaction involving CO under flow conditions is, once more, demonstrated [ 71. The reported CO hydrogenation in batch conditions, giving mainly methane [ 211, is, on the contrary, probably related to a stoichiometric reaction with parallel formation of metallic osmium in the presence of excess surface water. The use of basic surfaces, such as magnesium oxide, is able to effectively prevent removal of metal as volatile carbonyls, since anionic carbonyl clusters, of very low volatility, are formed and stabilized by the basic nature of the surface 19, 221. It is, however, essential to have careful control on the presence of surface modifiers, e.g. chloride ions, since they accelerate metal losses, via preferential labilization of the grafting bonds at the metal-oxide interface, as we have previously shown. Some deactivation processes typical of heterogeneous catalysis can therefore be studied and rationalized through the use of molecular surface model reactions. For instance, in I-butene isomerization catalyzed by the SiO,-grafted Os(I1) carbonyls, carbon monoxide acts as a reversible poison, as well as deactivating agent. After CO is injected into the feed at reaction temperature, the original catalytic activity under steady state conditions cannot be completely restored [ZO] (Fig. 4).
l
CONVERSION
0
CIS/TRANS
ratio
1.0 .g t; L 0.8 9 d 0.6 I\
0
10
20
0 TIME
10
20
(h)
Fig. 4. I-butene isomerization catalyzed by [Os(CO),(Si+], addition at 115 “C.
species. Poisoning effect of CO
197
Careful IR studies under reaction conditions show in fact that CO addition causes some reaggregation of the mononuclear OS(H) species to volatile OS clusters with loss of metal. Bimetallic systems We previously suggested that formation of small metallic particles from supported clusters is best achieved in the absence of strong cluster-support interactions; high-temperature reduction treatments would be otherwise needed, consequently leading to very large particles. Therefore, if highly unreactive metal precursors, such as Irq(C0)r2, are not available, the complete removal of all the reactive surface sites of the support should be achieved. This is easily accomplished with large-pore zeolites, since all reactive framework protons can be completely ion-exchanged against alkaline (Naf or K+) cations. Low-nuclearity (up to 4-atom) carbonyl clusters can be conveniently introduced by sublimation into the supercages of Na+ zeolites, where simple physisorption occurs. Metal particles are subsequently generated by mild thermal treatment [ 231. This new approach has thus opened tailor-made pathways to the preparation of bi- or poly-metallic catalysts by ‘step-by-step depositions’. Small particles of a second metal, previously prepared inside zeolite cages, serve as nucleation sites for the anchoring and the decarbonylation to metal of the molecular carbonyl precursor [4], mimicking the reactivity observed in solution [ 241. Unusual alloys, as well as cherry crystallites, can be obtained via this route. For examples, zeolite-supported PtRe catalysts are not conveniently prepared by the conventional technique because of the difficulty of introducing the anionic Re precursor into the zeolite framework. Bimetallic PtRe particles are instead generated by sublimation and further decarbonylation of Re2(CO),, into pre-reduced Pt particles inside the cages of NaY zeolite. TPRD and catalytic studies have confirmed the specific and high efficient formation of metal alloy via this route [ 41. PtRe alloy particles are in fact unambiguously characterized by a high selectivity for deep hydrogenolysis of alkanes to methane in the absence of sulfur [ 251. Heptane conversion at 500 “C and 5 atm was thus chosen as model reaction [ 51. The selectivity for CH, formation (> 90%) is accordingly much higher than that of the two pure metals (Fig. 5). The bimetallic PtRez(CO)rz cluster has also been used as a catalyst precursor. This complex is, however, too big to easily enter the windows interconnecting the supercages. Most is left at the external surface of the zeolite crystal, where phase separation and Re segregation occur, due to the low strength of metal-metal bonds and to the different reactivity of the external surface compared to the Na-exchanged cages. The reactivity is therefore completely different from that of a bimetallic PtRe catalyst (Fig. 5), but is typical of conventional Pt catalysts supported on amorphous alumina, as reported by Nogueira and Pines [ 261. Phase separation and preferential segregation are in fact major concerns when supporting bimetallic clusters on neutral or acidic surfaces [ 271. A
198 120 -
g
loo-
{
60 -
0 3w
60-
6a
40 -
E
20 oPt/NaY
Re/NoY
Em
Es
AROMATICS
C7
PtRs/NaY
PtReZ(CO)12/NaY
Ea Isomer*
METHANE
C2-C6
Fig. 5. Product selectivity in n-heptane conversion supported We catalysts.
Isomers
at 500 “C and 5 atm for a series of NaY-
viable alternative is, again, offered by basic supports. The formation of highly stable anionic structures is likely to prevent cluster break-up to a large extent [28, 291. Conclusions An unifying approach to hybrid materials derived from the combination of metal carbonyl clusters and oxide surfaces as closely interacting entities is proposed and supported by the well-known concept of ‘surface organometallic chemistry’. A better understanding of some elementary steps in heterogeneouslycatalyzed processes is reached, as well as the discovery of new, high efficiency synthetic pathways to molecular organometallic complexes and tailor-made materials, such as very dispersed metals on oxides or unstable bimetallic alloys. Acknowledgements The work was supported by National Research Council (grant Progetto Pinalizzato Chimica Pine II) and by the Ministry of University and Scientific and Technological Research. One of us, C.D., is deeply indebted to Prof. W. H. M. Sachtler, Northwestern University, for a postdoctoral position and fruitful discussions. References 1 R. Psaro and R. Ugo in B. C. Gates, H. Kniizinger and L. Guczi (eds.), Metal Clusters in Cutm!ysis, Elsevier, Amsterdam 1986, p. 427; H. H. Lamb, B. C. Gates and H. Knijzinger, Angew. Chem. Int. Edn. En&, 27 (1989) 1127; J.-M. Basset, J. P. Candy, A. Choplin, C. Santini and A. Theolier, Catal. Today, 6 (1989) 1.
199 2 3 4 5
R. C. C. C.
Psaro, Dossi, Dossi, Dossi,
C. Dossi and R. Ugo, J. Mol. Cutwl., 21 (1983) 331. R. Psaro, R. Zanoni and F. S. Stone, Spectroch. Acta (A), 43A (1987) 1507. J. Schaefer and W. M. H. Sachtler, J. Mol. Catal., 52 (1989) 193. C. M. Tsang, W. M. H. Sachtler, R. Psaro and R. Ugo, Energy and F’wl, 3
(1989) 468. 6 C. Dossi, R. Psaro, D. Roberto, R. Ugo and G. M. Zanderighi, Inorg. Chem., 29 (1990) 4368. 7 C. Dossi, A. Fusi, E. Grilli, R. Psaro, R. Ugo and R. Zanoni, Cutal. Today, 2 (1988) 585. 8 L. L. Sheu, H. KnGzinger and W. M. H. Sachtler, Cutal. L&t., 2 (1989) 129. 9 H. H. Lamb, A. S. Fung, P. A. Tooley, J. Puga, T. R. Krause, M. J. Kelley and B. C. Gates, J. Am. Chem. Sot., III (1989) 8367. 10 C. Dossi, R. Psaro and R. Ugo, J. Organomet. Chem., 359 (1989) 105. 11 R. Della Pergola, L. Garlaschelli and S. Martinengo, J. Orgunomet. Chem., 331 (1987) 271. 12 C. Dossi, A. Fusi, M. Pizzotti and R. Psaro, Orgar~ornetallics, 9 (1990) 1994. 13 A. .J. Deeming and S. Hasso, J. Organomet. Chem., 114 (1976) 313. 14 B. Besson, A. Choplin, L. D’Omelas and J. M. Basset, J. Chem. Sot. Chem. Comm. (1982) 843. 15 R. Barth, B. C. Gates, Y. Zhao, H. Kniizinger and J. Hulse, J. CutaL., 82 (1983) 147. 16 J. Kaspar, A. Trovarelli, M. Graziani, C. Dossi, R. Psaro, R. Ugo, G. M. Zanderighi, M. Lenarda and R. Ganzerla, J. Mol. Catal., 44 (1988) 183; J. Kaspar, A. Trovarelli, M. Graziani, C. Dossi, A. Fusi, R. Psaro, R. Ugo, M. Lenarda and R. Ganzerla, J. Mol. Catal., 51 (1989) 181. 17 A. Brenner, in M. Moskovitz (ed.), Metal Clusters, Wiley, New York, 1986, p. 249. 18 J. R. Anderson and R. F. Howe, Nature, 268 (1977) 129. 19 C. Dossi, A. Fusi, R. Psaro and G. M. Zanderighi, Appl. Catal., 46 (1989) 145. 20 C. Dossi, A. Fusi, E. Grilli, R. Psaro, R. Ugo and R. Zanoni, J. Cutal., 123 (1990) 181. 21 R. Psaro, R. Ugo, G. M. Zanderighi, B. Besson, A. K. Smith and J. M. Basset, J. Organorrzet. Cha., 213 (1981) 215. 22 H. H. Lamb and B. C. Gates, J. Am. Chem. Sot., 108 (1986) 81. 23 C. M. Tsang, S. M. Augustine, J. B. Butt and W. M. H. Sachtler, Appl. Catul., 46 (1989) 45. 24 J. R. Shapley, G. A. Pearson, M. Tachikawa, G. E. Schmidt, M. R. Churchill and F. J. Hollander, J. Am. Chem. Sot., 99 (1977) 8064. 25 1. H. B. Haining, C. Kemball and D. A. Whan, J. Chem. Res. (S’, (1977) 2056. 26 L. Nogueira and H. Pines, J. Cutal., 70 (198 1) 391. 27 A. Chaplin, M. Leconte, J. M. Basset, S. Shore and W. L. Hsu, J. Mol. Cutal., 21 (1983) 389. 28 A. Choplin, L. Huang, A. Theolier, P. Gallezot, J. M. Basset, U. Siriwardane, S. G. Shore and R. Mathieu, J. Am. Chem. Sot., IO8 (1986) 4224. 29 J. P. Scott, J. R. Budge, A. L. Rheingold and B. C. Gates, J. Am. Chem. SW., 109 (1987) 7736.
Chemistry
and Physics,
29 (1991)
191
191-199
Surface organometallic chemistry: a molecular approach to heterogeneous catalysis and a new perspective for the synthesis of metal carbonyl clusters C. Dossi, A. Fusi, R. Psaro, D. Roberto
and R. Ugo
Dipartimento di Chimica Inorganica e Metallorganica C.N.H., Via Venezian, 21, 20133 Milan (Italy]
dell’lJniversita’
and Centro
Abstract The chemical behavior of supported metal fragments obtained from metal carbonyl clusters has been shown to be dependent upon the acid-base properties of the surface of the inorganic oxide and the chemical reactivity of the metal cluster precursor. This ‘molecular approach’ to material science, heterogeneous catalysis in particular, via supported clusters has allowed the identification of new synthetic pathways to molecular metal carbonyl complexes as well as new heterogeneous catalysts.
Introduction In the last few years, the chemical reactivity of molecular metal fragments supported on the surface of high surface area inorganic oxides has been recently rationalized in a very elegant manner through the concept of ‘surface organometallic chemistry’, with the surface sites of the support being regarded as a rather rigid multidentate ligand [ 11. In the meantime, the original idea of investigating supported molecular clusters as ‘heterogenized homogeneous catalysts’ has evolved into the more generalized concept of supported clusters as precursors of unconventional materials. Highly dispersed metal crystallites, as well w surface molecular complexes of specific nuclearity can be obtained by a chemical approach based on the reactivity of metal clusters and the surface properties (acidity/basicity, topology) of the inorganic supports. In addition, the organometallic species formed by surface reactions can be re-extracted from the solid support [ 2, 31; a new route to the selective synthesis of organometallic complexes is thus offered. Finally, this molecular approach to supported molecular clusters is also providing a new and deeper insight into heterogeneous catalysis. Activationdeactivation processes, as well as promotion and poisoning effects can in fact be rationalized on the basis of molecular reactions and cluster transformations. In general, a molecular picture of cluster-surface interactions and reactions can be given from the point of view of the organometallic chemist. In this
0254-0584/91/$3.50
0 1991 - Elsevier Sequoia, Lausanne
192
respect, much attention is devoted here to basic aspects of catalysis and material science, viz. the reactivity of metal-oxide surfaces, the preparation and properties of specific mono- and bime~lic particles, some elementary steps of heterogeneous processes and, finally, the surface-mediated synthesis of molecular carbonyl complexes. Experimental
Supported clusters have been obtained by solvent impregnation or by sublimation [ 3, 41. In situ FTIR and TPDE studies have been carried out on specially-designed equipments, as described elsewhere [3]. Catalytic hydroformylation of a CzH4+ CO + Ha (1: 1: 1) mixture was carried out under flow conditions at 180 “C and atmospheric pressure. Product analysis was obtained by on-line go-chromato~aphy. High-pressure heptane conversion was studied on a computer-controlled, automatized catalytic reactor (Max II system, Xytel Corporation) at 500 “C and 5 atm total pressure, with a hydrogen/hydrocarbonratio of 5 and a weight hourly space velocity (WHSV) of 10 f5]. Results
and discussion
Su?ffaces as solid nmdia for the synthesis of molecular complexm
carbonyl
The peculiar properties of ‘solid ligand’ shown by the surfaces of inorganic oxides [ 1 f can be usefully exploited for the selective preparation of molecular metal carbonyls. This aspect does not simply imply surface transformations of previously supported clusters into higher nuclearity species, as in the first reported example of the selective synthesis under mild conditions of tetranuclear H4M4(C0)i2 (M = Ru, OS) hydrides from silica-supported M3(C0)12 and hydrogen [Sl. The aggregation of mononu~le~ surface complexes, e.g. the surface grafted [Os(CO),(OSif),], (x= 2,3) species 171,or metal ions, e.g. Pd2+ in the NaY structure [81 or Ru3+ and 0s3+ ions on MgO [9 1, to polynuclear structures is observed under suitable conditions, thanks to the unexpected high surface mobility of metal fragments, even if strongly bound to the surface. The presence of surface modifiers, such as chlorine compounds, has also a strong influence in addressing the nature of the final products [ 101. Recently, we performed careful studies on SiO,-supported IrC133H20.Initially, b-Cl3simply deposits on the surface of silica, but upon CO treatment at 150 “C, the impregnated material progressively discolors, and a dark sublimate with a metallic appearance is formed on the cold walls of the glass reactor. MS, IR and elemental analyses indicate that pure Ir(CO)&l is selectively and almost quantitatively formed. By treatment with a CO + Hz0 mixture at 70 “C, the silica support assumes a pale yellow color, and we could not observe any sublimation of
193
Ir(CO)&l. The IR spectrum of the SiOz powder in nyjol mull showed carbonyl bands at 2073 and 2033 cm- ‘, suggesting the presence of physisorbed was Ir, (CO) 12 cluster. By subsequent solvent extraction, pure Ir,(C0),2 recovered and characterized by IR spectroscopy [ 111. Further investigations confirmed that the complete removal of chloride ions from the surface is the key factor for ensuring the complete transformation of the Ir3+ salt into the tetranuclear cluster. Surface chloride ions are likely to favor a low surface mobility of the Ir atoms, preventing their nucleation. A similar effect of surface (CH,COO)ions was noted in the case of SiO,-grafted Os(I1) carbonyls prepared from the model [Os(CO),(CH,COO)], complex. If acetate ions are not carefully removed from the surface, significant amounts of [Os(CO)3(CH&OO)]2 form upon recarbonylation between 150 and 200 “C.
Cluster reactivity at the metal-oxide interface A completely different approach was used on the grafted surface species, such as the HOs3(CO)io(OSi<) cluster, covalently attached to the surface via OS-0-Si bonds. The relatively high stability of the chemisorbed clusters thus makes possible specific chemical reactions to be conducted on the surface complex. Valuable information on the reactivity of the bonds between the metal and the surface, i.e. of the metal-oxide interface, which is a major concern in areas such as catalysis and material science, can be inferred from the reaction products. As a further consequence, the specificity of such surface reactions, generally conducted under mild conditions for preserving the metal framework intact, often offers high-yield routes to the synthesis of metal carbonyl clusters. By treating the grafted HOs3(CO)io(OSi$ cluster with aqueous HF, the Si-0 bonds are broken, and the OS-O -0s bridges are preserved. After dissolution of silica and protonation, pure HOs3(CO),,(OH) is then recovered in high yield by extraction with CH2C12 (56% after recrystallization) [ 121. When using aqueous HCl, on the contrary, HOS~(CO)~~CI is formed by Clinsertion in the OS-O bond and extracted in dichloromethane if working under biphasic (aqueous + organic) conditions (Fig. 1). Accordingly to data in the literature [ 131, its IR spectrum in CHzC12 showed carbonyl bands at 2116(w), 2097(w), 2077(s), 2068(s), 2027(vs), 2015(sh) and 1987(m) cm-i, while a single hydride resonance at - 14.23 ppm from TMS was observed in the ‘H-NMR spectrum. This surface reaction indicates how coordinating ligands, such as Cl-, are likely to be inserted at the metal-oxide interface, with the parallel formation of new surface species. The effect of chlorine is, therefore, completely different from that of a simple ‘acidity enhancer’, as often suggested in heterogeneous catalysis. Consequently, amore complex role of surface chlorine (and other surface modi6ers), involving modifications not only of the acid properties of the surface [lo], but also of the surface metal species, is now proposed.
194
HOs,(CO),, (OH) + H,SiF, ____--_--
I
Si-
0 H
/\ + W),O&~s(co),
I
‘4
Fig. 1. Surface reactions at the metaf-oxide interface on the grafted
HOs~(CO),*(OS~~) cluster.
Cluster-de-rived heterogeneous catalysts A) Monmmtallic s@stems The low thermal stability of chemisorbed molecular clustersunder reaction conditions has severely limited their utihzation as intact cluster catalysts to low-temperature hydrocarbon reactions [ 14, 151 or to the hydrogen transfer reduction of saturated ketones [ 161. In parallel, a new concept of supported metal clusters as precursors of ‘cluster-derived’ heterogeneous catalysts has subsequently evolved. New materials are thus obtained, whose properties can be ‘tuned’ to some extent on the basis of the chemical reactivity of the original cluster precursor and of the physico-chemical properties of the surface. In particular, metal particles are best obtained by mild thermal decarbonylation of the supported molecular cluster in the absence of strong chemical interactions with the support. Such chemical interactions lead, on the contrary, to oxidized surface-grafted metal complexes, often showing high stability and low reducibili~. SiO,-supported I&(CO),, and Osaka are typical examples of the two different behaviors, as well-evidenced by the Temperature Programmed Decomposition (TPDE) technique [ 171,a specially-developed dynamic technique for cluster-support interaction studies. In the case of Ir,(CO),a supported on SiOa, a single decomposition peak around 180 “C is observed (Pig. Z), corresponding to the complete evolution of CO and formation of Ir metal particles: Ir,(CO)Iz/SiOz -
4/x Irz/Si02+ 1X0
195
-
co
-e-
co,
4
CH4
12.00 A
80
0
240
160
Temperature Fig. 2. Temperature
Programmed
320
4
(deg C) Decomposition
(TPDE)
in flowing
He of SiO+upported
in flowing
He of SiO+upported
Ir,(CO),C?.
-
-
co
-a- CH4
co2
9
72
4.3
24
0 ! 0
80
160
Temperature Fig. 3. Temperature oss(co),2.
Programmed
240
320
400
(deg C) Decomposition
(TPDE)
The high dispersion of the iridium particles has been demonstrated by chemisorption studies [ 18 ] and TEM investigations. The more reactive Os,(CO)ia cluster reacts with the Si-OH groups of the silica surface above 80 “C (Pig. 3). HOsa(CO)&OSi<) is formed in a first step below 150 “C, and subsequently decomposes to mononuclear surfacegrafted Os(I1) carbonyls, [ Os(CO),(OSi+], (x = 2,3), of high thermal stability [ 191. Osmium metal particles start to be formed at a much higher temperature, above 330 “C, as evidenced by the methane formation in TPDE (Pig. 3). These SiOa-grafted OS(U) carbonyls are effective catalysts in a wide range of reactions such as hydrogen transfer to ketones [ 161 and olefin isomerization 1291. Ethylene hydroformylation was then tested as a model reaction of CO activation under flow conditions at 1 atm and 180 “C. No formation of
196
oxygenated products (aldehyde and/or alcohols) was observed, but only a very moderate activity for hydrogenation to ethane, failing off to zero within 1-2 hours. At the same time, a yellow material appeared as long needles on the cold walls of the reactor, and was further characterized as a mixture of OSCAR and H,OS,(CO),~ by IR and MS analyses. The transformation of the starting surface-grafted Os(I1) carbonyls into the two volatile clusters was quantitative, since no osmium was left on the catalyst. The practical impossibility of using cluster-derived OS catalysts in a reaction involving CO under flow conditions is, once more, demonstrated [ 71. The reported CO hydrogenation in batch conditions, giving mainly methane [ 211, is, on the contrary, probably related to a stoichiometric reaction with parallel formation of metallic osmium in the presence of excess surface water. The use of basic surfaces, such as magnesium oxide, is able to effectively prevent removal of metal as volatile carbonyls, since anionic carbonyl clusters, of very low volatility, are formed and stabilized by the basic nature of the surface 19, 221. It is, however, essential to have careful control on the presence of surface modifiers, e.g. chloride ions, since they accelerate metal losses, via preferential labilization of the grafting bonds at the metal-oxide interface, as we have previously shown. Some deactivation processes typical of heterogeneous catalysis can therefore be studied and rationalized through the use of molecular surface model reactions. For instance, in I-butene isomerization catalyzed by the SiO,-grafted Os(I1) carbonyls, carbon monoxide acts as a reversible poison, as well as deactivating agent. After CO is injected into the feed at reaction temperature, the original catalytic activity under steady state conditions cannot be completely restored [ZO] (Fig. 4).
l
CONVERSION
0
CIS/TRANS
ratio
1.0 .g t; L 0.8 9 d 0.6 I\
0
10
20
0 TIME
10
20
(h)
Fig. 4. I-butene isomerization catalyzed by [Os(CO),(Si+], addition at 115 “C.
species. Poisoning effect of CO
197
Careful IR studies under reaction conditions show in fact that CO addition causes some reaggregation of the mononuclear OS(H) species to volatile OS clusters with loss of metal. Bimetallic systems We previously suggested that formation of small metallic particles from supported clusters is best achieved in the absence of strong cluster-support interactions; high-temperature reduction treatments would be otherwise needed, consequently leading to very large particles. Therefore, if highly unreactive metal precursors, such as Irq(C0)r2, are not available, the complete removal of all the reactive surface sites of the support should be achieved. This is easily accomplished with large-pore zeolites, since all reactive framework protons can be completely ion-exchanged against alkaline (Naf or K+) cations. Low-nuclearity (up to 4-atom) carbonyl clusters can be conveniently introduced by sublimation into the supercages of Na+ zeolites, where simple physisorption occurs. Metal particles are subsequently generated by mild thermal treatment [ 231. This new approach has thus opened tailor-made pathways to the preparation of bi- or poly-metallic catalysts by ‘step-by-step depositions’. Small particles of a second metal, previously prepared inside zeolite cages, serve as nucleation sites for the anchoring and the decarbonylation to metal of the molecular carbonyl precursor [4], mimicking the reactivity observed in solution [ 241. Unusual alloys, as well as cherry crystallites, can be obtained via this route. For examples, zeolite-supported PtRe catalysts are not conveniently prepared by the conventional technique because of the difficulty of introducing the anionic Re precursor into the zeolite framework. Bimetallic PtRe particles are instead generated by sublimation and further decarbonylation of Re2(CO),, into pre-reduced Pt particles inside the cages of NaY zeolite. TPRD and catalytic studies have confirmed the specific and high efficient formation of metal alloy via this route [ 41. PtRe alloy particles are in fact unambiguously characterized by a high selectivity for deep hydrogenolysis of alkanes to methane in the absence of sulfur [ 251. Heptane conversion at 500 “C and 5 atm was thus chosen as model reaction [ 51. The selectivity for CH, formation (> 90%) is accordingly much higher than that of the two pure metals (Fig. 5). The bimetallic PtRez(CO)rz cluster has also been used as a catalyst precursor. This complex is, however, too big to easily enter the windows interconnecting the supercages. Most is left at the external surface of the zeolite crystal, where phase separation and Re segregation occur, due to the low strength of metal-metal bonds and to the different reactivity of the external surface compared to the Na-exchanged cages. The reactivity is therefore completely different from that of a bimetallic PtRe catalyst (Fig. 5), but is typical of conventional Pt catalysts supported on amorphous alumina, as reported by Nogueira and Pines [ 261. Phase separation and preferential segregation are in fact major concerns when supporting bimetallic clusters on neutral or acidic surfaces [ 271. A
198 120 -
g
loo-
{
60 -
0 3w
60-
6a
40 -
E
20 oPt/NaY
Re/NoY
Em
Es
AROMATICS
C7
PtRs/NaY
PtReZ(CO)12/NaY
Ea Isomer*
METHANE
C2-C6
Fig. 5. Product selectivity in n-heptane conversion supported We catalysts.
Isomers
at 500 “C and 5 atm for a series of NaY-
viable alternative is, again, offered by basic supports. The formation of highly stable anionic structures is likely to prevent cluster break-up to a large extent [28, 291. Conclusions An unifying approach to hybrid materials derived from the combination of metal carbonyl clusters and oxide surfaces as closely interacting entities is proposed and supported by the well-known concept of ‘surface organometallic chemistry’. A better understanding of some elementary steps in heterogeneouslycatalyzed processes is reached, as well as the discovery of new, high efficiency synthetic pathways to molecular organometallic complexes and tailor-made materials, such as very dispersed metals on oxides or unstable bimetallic alloys. Acknowledgements The work was supported by National Research Council (grant Progetto Pinalizzato Chimica Pine II) and by the Ministry of University and Scientific and Technological Research. One of us, C.D., is deeply indebted to Prof. W. H. M. Sachtler, Northwestern University, for a postdoctoral position and fruitful discussions. References 1 R. Psaro and R. Ugo in B. C. Gates, H. Kniizinger and L. Guczi (eds.), Metal Clusters in Cutm!ysis, Elsevier, Amsterdam 1986, p. 427; H. H. Lamb, B. C. Gates and H. Knijzinger, Angew. Chem. Int. Edn. En&, 27 (1989) 1127; J.-M. Basset, J. P. Candy, A. Choplin, C. Santini and A. Theolier, Catal. Today, 6 (1989) 1.
199 2 3 4 5
R. C. C. C.
Psaro, Dossi, Dossi, Dossi,
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(1989) 468. 6 C. Dossi, R. Psaro, D. Roberto, R. Ugo and G. M. Zanderighi, Inorg. Chem., 29 (1990) 4368. 7 C. Dossi, A. Fusi, E. Grilli, R. Psaro, R. Ugo and R. Zanoni, Cutal. Today, 2 (1988) 585. 8 L. L. Sheu, H. KnGzinger and W. M. H. Sachtler, Cutal. L&t., 2 (1989) 129. 9 H. H. Lamb, A. S. Fung, P. A. Tooley, J. Puga, T. R. Krause, M. J. Kelley and B. C. Gates, J. Am. Chem. Sot., III (1989) 8367. 10 C. Dossi, R. Psaro and R. Ugo, J. Organomet. Chem., 359 (1989) 105. 11 R. Della Pergola, L. Garlaschelli and S. Martinengo, J. Orgunomet. Chem., 331 (1987) 271. 12 C. Dossi, A. Fusi, M. Pizzotti and R. Psaro, Orgar~ornetallics, 9 (1990) 1994. 13 A. .J. Deeming and S. Hasso, J. Organomet. Chem., 114 (1976) 313. 14 B. Besson, A. Choplin, L. D’Omelas and J. M. Basset, J. Chem. Sot. Chem. Comm. (1982) 843. 15 R. Barth, B. C. Gates, Y. Zhao, H. Kniizinger and J. Hulse, J. CutaL., 82 (1983) 147. 16 J. Kaspar, A. Trovarelli, M. Graziani, C. Dossi, R. Psaro, R. Ugo, G. M. Zanderighi, M. Lenarda and R. Ganzerla, J. Mol. Catal., 44 (1988) 183; J. Kaspar, A. Trovarelli, M. Graziani, C. Dossi, A. Fusi, R. Psaro, R. Ugo, M. Lenarda and R. Ganzerla, J. Mol. Catal., 51 (1989) 181. 17 A. Brenner, in M. Moskovitz (ed.), Metal Clusters, Wiley, New York, 1986, p. 249. 18 J. R. Anderson and R. F. Howe, Nature, 268 (1977) 129. 19 C. Dossi, A. Fusi, R. Psaro and G. M. Zanderighi, Appl. Catal., 46 (1989) 145. 20 C. Dossi, A. Fusi, E. Grilli, R. Psaro, R. Ugo and R. Zanoni, J. Cutal., 123 (1990) 181. 21 R. Psaro, R. Ugo, G. M. Zanderighi, B. Besson, A. K. Smith and J. M. Basset, J. Organorrzet. Cha., 213 (1981) 215. 22 H. H. Lamb and B. C. Gates, J. Am. Chem. Sot., 108 (1986) 81. 23 C. M. Tsang, S. M. Augustine, J. B. Butt and W. M. H. Sachtler, Appl. Catul., 46 (1989) 45. 24 J. R. Shapley, G. A. Pearson, M. Tachikawa, G. E. Schmidt, M. R. Churchill and F. J. Hollander, J. Am. Chem. Sot., 99 (1977) 8064. 25 1. H. B. Haining, C. Kemball and D. A. Whan, J. Chem. Res. (S’, (1977) 2056. 26 L. Nogueira and H. Pines, J. Cutal., 70 (198 1) 391. 27 A. Chaplin, M. Leconte, J. M. Basset, S. Shore and W. L. Hsu, J. Mol. Cutal., 21 (1983) 389. 28 A. Choplin, L. Huang, A. Theolier, P. Gallezot, J. M. Basset, U. Siriwardane, S. G. Shore and R. Mathieu, J. Am. Chem. Sot., IO8 (1986) 4224. 29 J. P. Scott, J. R. Budge, A. L. Rheingold and B. C. Gates, J. Am. Chem. SW., 109 (1987) 7736.