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SiO2 catalysts prepared by the sol–gel method from Ru3(CO)12
Applied Catalysis A: General 182 (1999) 257±265
Ru/SiO2 catalysts prepared by the sol±gel method from Ru3(CO)12 Pietro Moggia,*, Giovanni Predierib, Fabio Di Silvestria, Andrea Ferrettia a
Dipartimento di Chimica Organica e Industriale, UniversitaÁ di Parma, Parco Area delle Scienze 17/A, 43100, Parma, Italy b Dipartimento di Chimica Generale e Inorganica, Chimica Analitica, Chimica Fisica, UniversitaÁ di Parma, Parco Area delle Scienze 17/A, 43100, Parma, Italy Received 15 September 1998; received in revised form 13 January 1999; accepted 13 January 1999
Abstract Ru/SiO2 catalysts were prepared by sol±gel methods from Ru3(CO)12, or [Ru3H(CO)11]ÿ, Si(OMe)4 (TMOS) and H2O, followed by thermal activation in helium up to 573 K and reduction with hydrogen. They were compared with Ru/SiO2 corresponding material prepared via sol±gel from RuCl3. The catalysts were characterized by FT-IR spectra, surface area, metal dispersion and activity tests in the Fischer±Tropsch reaction at atmospheric pressure and CO/H2 1:1, in the 473±573 K temperature range. The use of ruthenium clusters as precursors afforded higher metal dispersions than RuCl3, as well as higher catalytic activities in the hydrogenation of CO (particularly in the absence of alkali or halogen ions retained in the gel structure). TEM investigations on the Ru/SiO2 material, prepared from Ru3(CO)12 by sol±gel, without promoters, showed the presence of very ®ne metal particles homogeneously embedded in the silica matrix, with dimensions ranging from 1 to 4 nm. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium catalysts; Ruthenium clusters; Sol±gel; Silica support; Metal dispersion; CO hydrogenation
1. Introduction Among the group VIII metals Ru is the most active and the most selective for the production of long-chain hydrocarbons from CO±H2 mixtures [1]. Metal/support interactions, metal loading and dispersion notably affect the activity and selectivity of Ru supported catalysts [2±4]. As a consequence, the performance of the catalysts strongly depends on the choice of metal precursor and support and on the preparation method [5,6]. Silica-supported Ru is generally indicated as the most active and stable catalyst in the CO *Corresponding author. Fax: +39-521-905-472; e-mail: [email protected]
hydrogenation, while other systems are less active and more sensitive to deactivation [7]. It has also been proved that thermal decomposition of metal carbonyl clusters, such as Ru3(CO)12, used as Ru precursors on inorganic oxides as supports, provides high metal dispersions, then resulting in higher speci®c activities [8±12]. Several methods for supporting ruthenium carbonyl clusters on silica are reported in literature; these include: 1. Impregnation of dichloromethane [13,14], trichloromethane [15], pentane [16], hexane [17± 19] or toluene [20] solutions of Ru3(CO)12, at room temperature, and evaporation to dryness in an inert atmosphere.
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00014-9
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2. Impregnation by refluxing benzene [21] or cyclohexane [22,23] solutions of Ru3(CO)12. 3. Impregnation of Ru3(CO)12 via vapor phase in an evacuated sealed pyrex cell, held at a temperature of 353 K over a period of two weeks [24]. 4. Impregnation of methanol solution of Na[Ru3H(CO)11] (synthesized by reaction of Ru3(CO)12 with Na[BH4], at room temperature in tetrahydrofuran (THF) [25]), followed by evaporation of the solvent at 333 K [26]. 5. Anchoring Ru3(CO)12 to silica via a pendant thiol, by reaction of Ru3(CO)12 with HS(CH2)3Si(OMe)3 in refluxing benzene and by grafting the resulting product [Ru3H(CO)10{S(CH2)3Si(OMe)3}] to silica in hexane solution; alternatively, Ru3(CO)12 is reacted in refluxing benzene with the thiol-prefunctionalized silica HS(CH2)3SiO3/2nSiO2 (obtained from silica refluxed in a xylene solution of HS(CH2)3Si(OMe)3) [21]. 6. Anchoring Ru3(CO)12 to silica via a pendant amino (or phosphino) group, by reaction of Ru3(CO)12 with H2N(CH2)3Si(OEt)3 (or Ph2P(CH2)3Si(OEt)3), in toluene (or hexane) under nitrogen at 333 K. The resulting amino- or phosphino-substituted carbonyl clusters are tethered to silica suspended in toluene, at room temperature; alternatively, Ru3(CO)12 is reacted in toluene at room temperature with ligand-functionalized SiO2 (obtained from silica refluxed in a toluene solution of H2N(CH2)3Si(OEt)3 or Ph2P(CH2)3Si(OEt)3] [27]. 7. Anchoring the cluster Na[Ru3H(CO)11] to silica by reaction with ammonium (±NMe3 and ±NEt3)or pyridinium (NC5H5)-functionalized silicas suspended in methanol at room temperature [26]. Recently, sol±gel methods have been proposed for the preparation of Ru/SiO2 catalysts, starting from RuCl33H2O and tetraethoxysilane (TEOS), as Ru and SiO2 precursors, respectively; these methods consist in re¯uxing ethanol±water solutions [28,29], eventually using ammonium hydroxide [30,31] or concentrated hydrogen chloride [32,33] as catalysts to promote the hydrolysis and condensation reactions. By comparing Ru/SiO2 catalysts prepared by impregnation and by sol±gel methods, it was concluded that the sol±gel catalysts showed a much higher resistance to deactivation and substantially enhanced selectivity for the o-xylene hydrogenation [31]. Ru/SiO2 pre-
pared by the sol±gel method also revealed high selectivity in the hydrogenation of benzene to cyclohexene [34] and toluene to methylcyclohexene [35], due to the small particle size. However, the presence of residual chlorine in silicasupported Ru catalysts acts to decrease the catalytic activity in the hydrogenation of CO, probably inducing structural rearrangements [36]. This work deals with the preparation of chlorinefree Ru/SiO2 catalysts by a new sol±gel method, starting from Ru3(CO)12 or related anionic clusters, in order to obtain highly disperse metal particles, exhibiting high speci®c activities and selectivities in the CO hydrogenation reaction, also preventing the thermal deactivation by sintering, which generally occurs on impregnated catalysts. 2. Experimental 2.1. Catalysts preparation Ru/SiO2 sol±gel catalysts, all containing 2 wt% of the metal, were prepared starting from Ru3(CO)12 as Ru precursor and tetramethoxysilane (TMOS) as SiO2 precursor, following the synthetic routes described below. 1. Ru3(CO)12 (Aldrich-Chemie, >99%) (0.05 g) was completely dissolved in dry THF (40 ml) at room temperature, then TMOS (Aldrich-Chemie, >99%) (2.88 ml) and, after 1 h, distilled water (2.43 ml) were added dropwise, under continuous stirring, under a nitrogen stream. In the ®rst preparation, no catalyst was added and the initial yellow solution, transferred in a vessel for the evaporation of the solvent, progressively turned to a grey gel in 8 days, which was then crushed and screened to 40±60 mesh, washed with THF, and ®nally dried in vacuo at room temperature. In another preparation NH4F (0.19 mmol), as nucleophilic catalyst, was dissolved in distilled water (2.43 ml) and added to the homogeneous sol, prepared as above, which after stirring (2 h) under nitrogen, was transferred in a vessel suitable for the evaporation of the solvent and the gelation. A homogeneous gel was obtained in 4 h, which was successively treated as indicated above. The same procedures, without addition of Ru3(CO)12, were used to prepare pure silica gels.
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2. Ru3(CO)12 (0.05 g) was dissolved in dry methanol (50 ml), then Na[BH4] (0.015 g), or KOH (0.010 g), was added under a nitrogen stream and the initial orange solution was constantly stirred until it turned deep red, owing to the formation of the anionic cluster [Ru3H(CO)11]ÿ. TMOS (2.85 ml) was then added dropwise under stirring, and after 1 h, distilled water (2.43 ml) was added to the solution, which was transferred in a vessel suitable for easy evaporation of the solvent and gelation. In three days, dark yellow, highly homogeneous gels were obtained, which were crushed and screened to 40±60 mesh, washed with methanol, and dried in vacuo at room temperature. 3. Ru3(CO)12 (0.05 g) and H2N(CH2)3Si(OEt)3 (Janssen Chimica, >99%) (0.051 g) were refluxed in dry hexane (200 ml) in a nitrogen stream for 60 min, then TMOS (2.91 g) and distilled water (3.48 ml) were added and the mixture was refluxed until gelation. The catalysts prepared from Ru3(CO)12 or [Ru3H(CO)11]ÿ were activated as follows: a weighed amount of the composite material was charged into a ¯ow microreactor and gradually heated (2 K minÿ1) up to 573 K in a helium stream (50 ml minÿ1), then the reduction to Ru metal was performed by feeding hydrogen (75 ml minÿ1) at 573 K for 2 h. Three solutions were prepared separately by dissolving RuCl33H2O (Carlo Erba) (0.06 g) in absolute ethanol (6 ml), and then adding TEOS (Aldrich-Chemie, >98%) (4.1 ml) drop by drop under vigorous stirring. The three solutions were differently added, according to the literature methods previously described: (i) only with water (1.30 ml), (ii) with concentrated ammonium hydroxide (0.06 mol) in water (1.30 ml), and (iii) with concentrated hydrogen chloride (0.07 mol) in water (1.30 ml). In all cases, the solutions were then re¯uxed until gelation occurred, i.e. after 24, 8 and 4 h, respectively. The gels were dried in vacuo at 353 K for 16 h, and ®nally calcined in an oven at 573 K for 5 h, under a nitrogen stream. 2.2. Catalysts characterization The Ru/SiO2 catalysts were characterized by FT-IR spectroscopy, surface area determinations, dispersion measurements, TEM analysis, and catalytic activity tests.
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FT-IR spectra of the supported Ru catalysts before and after activation were recorded in the range 4000± 400 cmÿ1 by using a Nicolet 5 PC spectrophotometer. The diffuse re¯ectance (DRIFTS) spectra were obtained with an HTHP cell (Spectratech) equipped with two ZnSe windows by using an MCT detector. Surface area of the activated Ru catalysts were measured by a Micromeritics 2200 analyser, based on nitrogen adsorption at the temperature of liquid nitrogen, after degassing the samples at 573 K for 1 h under a nitrogen stream. The dispersion measurements of the metal Ru particles on the surface of the activated catalysts were performed by the gas chromatographic pulse method described elsewhere [11], based on the chemisorption of CO at 423 K under a helium stream (60 ml minÿ1). TEM analyses were performed at 200 keV with the Jeol JEM 2000EX instrument of the MASPEC Institute, CNR, Parma. Catalytic activity in the Fischer±Tropsch (FT) synthesis reaction, in the temperature range 473± 573 K, under atmospheric pressure was tested by a ¯ow microreactor containing about 0.5 g of activated catalyst by feeding a CO±H2 1:1 gas mixture (10 ml minÿ1). The ef¯uent products were periodically analysed by a sampling device connected to a Dani 3400 GC apparatus. 3. Results and discussion The sol±gel method via hydrolysis and condensation of silicon alkoxides has been successfully employed to prepare silica xerogels containing ®nely dispersed ruthenium particles derived from carbonyl clusters. In particular the proposed method consists in performing the gelation process with silicon alkoxides in the presence of a dissolved ruthenium carbonyl cluster as metal precursor in such a way that growing of solid support and anchoring of metal species occur at the same time. This procedure combines the main advantages of using metal carbonyl clusters as metal catalyst precursors (low activation temperature, size control of the metal particles, high metal dispersion) with those of the sol±gel process (homogeneity, texture and porosity control). The adopted synthetic routes allow the preparation of Ru/SiO2 dispersed systems in only two steps: gelation and thermal treatment.
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Fig. 1. FT-IR spectra in the 4000±1800 cmÿ1 region: (a) reference silica gel thermally activated at 573 K; (b) Ru/SiO2 sample after gelation without catalysts not activated.
The FT-IR spectrum of the reference silica gel prepared without addition of catalyst before gelation and thermally pretreated in air at 573 K (Fig. 1(a)) shows in the region from 4000 to 1800 cmÿ1 a broad band with three maxima at about 3550, 3480 and 3415 cmÿ1 assignable to terminal silanol groups and to O±H bonds of water and ethanol entrapped molecules. Two weak bands around 2840±2910 cmÿ1 are due to C±H stretching vibrations from the alkoxide groups which partially remain in the gel. In the FT-IR spectrum of the gel similarly obtained from Ru3(CO)12 and TMOS, recorded before the activation treatment (Fig. 1(b)), a large band remains in the silanol region centred at about 3450 cmÿ1, while two strong bands appear in the carbonyl stretching region, whose locations at 2065 and 2000 cmÿ1 are typical of Ru(II) dicarbonylated species anchored to the oxygen atoms of the support [37]. In the region from 1400 to 400 cmÿ1 the FT-IR spectrum of the reference silica gel (Fig. 2(a)) shows four bands, due to Si±O stretching vibration (1080 cmÿ1), Si±Oÿ bending vibration (795 and 620 cmÿ1), and ÿ OÿSiÿOÿ bending vibration (470 cmÿ1) [31]. In the ruthenium containing gel
(Fig. 2(b)) a new band appears near the Si±O stretching (1080 cmÿ1), at 955 cmÿ1, which could be assigned to the Si±O(±Ru) linkage [31]. During the gelation important modi®cations occur involving both the silicon alkoxides (hydrolysis and condensation) and the ruthenium carbonyl species (anchoring to the gel structure via formation of Ru± O±Si bonds). The evolution of the anchoring process can be conveniently monitored by IR spectroscopy in the carbonyl region. Fig. 3 compares the FT-IR spectra recorded at different times during the gelation process: Ru3(CO)12 dissolved in THF (Fig. 3(a)) shows three carbonyl bands at 2060vs, 2029vs and 2015w cmÿ1, according to the literature data for different Ru3(CO)12 solutions [38], which are not in¯uenced by the addition of TMOS and water. After gelation (Fig. 3(b)), with or without catalyst, two very strong IR carbonyl bands at 2065 and 2000 cmÿ1 appear, which are due to the formation of dicarbonyl Ru(II) species Ru±O-bonded to silica as stated above. This suggests that strong interactions between the ruthenium cluster and the incoming support take place during the hydrolysis and condensation steps of the gelation process.
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Fig. 2. FT-IR spectra in the 1400±400 cmÿ1 region: (a) reference silica gel thermally activated at 573 K; (b) Ru/SiO2 sample after gelation without catalysts not activated.
Fig. 3. FT-IR spectra in the carbonyl (2150±1900 cmÿ1) region, during the sol±gel process from Ru3(CO)12 (in THF) and TMOS, without catalyst added: (a) solution immediately analysed after addition of water; (b) sample analysed after completion of the gelation process.
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The FT-IR spectra recorded for both preparations where Ru3(CO)12 reacted with Na[BH4] or KOH show two strong bands in the carbonyl stretching region, at 2039±2041 and 1972±1973 cmÿ1, respectively, before addition of TMOS and water, quite good according to the literature data for the anionic cluster [Ru3H(CO)11]ÿ [26], but those bands are shifted after gelation to about 2055 and 1990 cmÿ1 due to the oxidative interaction on the silica. is reacted with When Ru3(CO)12 H2N(CH2)3Si(OEt)3 in hexane solution, the FT-IR spectrum shows three bands at 2062, 2032 and 2012 cmÿ1 according to literature [27]; in this case again only two bands are present in the spectra recorded after gelation shifted to even lower frequencies at 2053 and 1981 cmÿ1. Twin carbonyl bands have been reported by many authors [39], being attributable to monomeric species [Ru(CO)2]2 or polymeric Ru CO2 m n grafted to the silica matrix, the latter containing Ru±Ru bonds. The oxidation state of the metal atoms is comprised between 2 and m/n, and accordingly, the frequency values of the CO stretching bands exhibit different values. Lower frequencies are indicative of less oxidized dicarbonyl species, whose formation appears to be favoured when [Ru3H(CO)11]ÿ is used as precursor or in the presence of the anchored aminopropyl group; it is possible that this nitrogen donor ligand protects to a certain extent the ruthenium clusters from the oxidative fragmentation. The occurrence of different ruthenium species during the gelation process is also indicated by the observed colour changes from orange to greenish grey
in the case of Ru3(CO)12 or from red to dark yellow in the case of [Ru3H(CO)11]ÿ. The results of the measurements of surface area and metal dispersion on the activated Ru/SiO2 catalysts prepared by the different sol±gel techniques are given in Table 1. The content of metal Ru is always 2.0% by weight on the silica. The contents (wt% on silica) of Na or K are speci®cally indicated in the table. The Ru and SiO2 precursors and the type of catalyst eventually added to favour the gel formation are also speci®ed. As stated elsewhere [11,40], the use of Ru3(CO)12, rather than RuCl3, as Ru precursor affords higher metal dispersions corresponding in some cases to the chemisorption of more than 2 mol of CO per mole of Ru. Moreover, an excess of chlorine, when hydrogen chloride is used as gelation catalyst with RuCl3 as precursor, gives the lowest dispersion. The Ru/SiO2 catalyst, prepared via sol±gel from Ru3(CO)12 and TMOS (without NH4F), activated under He up to 573 K and reduced with H2 has been subjected to pulse CO chemisorption in a DRIFTS cell at 473 K. Fig. 4 shows that the catalyst binds CO giving again twin carbonyl stretching bands (2058 and 1983 cmÿ1) and suggesting that ruthenium is adequately dispersed being able to readsorb CO after activation, affording dicarbonylated species. The same catalyst has been analysed by transmission electron microscopy (TEM). The micrographs of the sample (see Fig. 5) evidence a lot of spheroidal metal particles homogeneously dispersed into the silica matrix, whose diameter ranges from 1 to 4 nm. The average values given in literature [32]
Table 1 Characterization data for the sol±gel catalysts Catalyst
Precursors
Surface area (m2 gÿ1)
Dispersion (CO/Ru moles)
SiO2 SiO2 Ru/SiO2 Ru/SiO2 RuNa(0.68%)/SiO2 RuK(0.82%)/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2
TMOS TMOS/NH4F Ru3(CO)12, TMOS Ru3(CO)12, TMOS/NH4F [Ru3H(CO)11]ÿ, TMOS [Ru3H(CO)11]ÿ, TMOS Ru3(CO)12, aminosilane, TMOS RuCl3, TEOS RuCl3, TEOS/NH4OH RuCl3, TEOS/HCl
335 640 656 409 660 750 550 600 660 495
± ± 2.21 2.07 1.36 2.37 1.04 0.61 0.60 0.15
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Fig. 4. DRIFT spectra in the carbonyl stretching region of a Ru(2.0 wt%)/SiO2 sample prepared by the sol±gel method from Ru3(CO)12 dissolved in THF, TMOS and H2O, without NH4F, after activation in helium stream up to 573 K and reduction with H2: (a) before CO exposure; (b) after CO chemisorption at 473 K.
prevailing reaction product is always CH4 (85±90% at 523 K, 90±95% at 573 K) accompanied by C2H6 (10± 15% at 523 K, 3±5% at 573 K) and minor amounts of C3H8. CO2 is formed only in traces up to 548 K and reaches 2±3% at 573 K. The highest catalytic activity is obtained with the Ru/SiO2 sample sol±gel prepared from Ru3(CO)12 and TMOS, without addition of catalyst for the hydrolysis±condensation steps, prob-
for the metal particles of a Ru(1.0%)/SiO2 catalyst prepared via sol±gel from RuCl3 are about 3±5 nm. The results of the catalytic activity tests in the FT synthesis at atmospheric pressure in the temperature range between 473 and 573 K are summarized in Table 2, where the activities are compared referring to the total hydrocarbon production rate in ml hÿ1 (room temperature, 1 atm) per gram of catalyst. The
Table 2 Catalytic activity of Ru/SiO2 catalysts (FT synthesis reaction) expressed as total hydrocarbon (HC) production rate at different temperatures Catalyst
Precursors
Ru/SiO2 Ru/SiO2 RuNa(0.68%)/SiO2 RuK(0.82%)/SiO2 Ru/SiO2 Ru/SiO2
Ru3(CO)12, TMOS Ru3(CO)12, TMOS/NH4F [Ru3H(CO)11]ÿ, TMOS [Ru3H(CO)11]ÿ, TMOS Ru3(CO)12, aminosilane, TMOS RuCl3, TEOS/NH4OH
HC production rate (ml hÿ1 gÿ1) 473 K
498 K
523 K
548 K
573 K
22 6 ± ± 3 3
50 19 ± ± 17 7
170 78 4 4 29 13
462 204 16 19 96 38
879 424 65 29 282 152
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Fig. 5. Bright-field TEM images (X 400 K) of a Ru(2.0 wt%)/SiO2 sample prepared by the sol±gel method from Ru3(CO)12 dissolved in THF, TMOS and H2O, without NH4F, after activation in helium stream up to 573 K and reduction with H2: (a) on the border of a thin silica grain; (b) in the bulk.
ably because the presence of Fÿ ions in¯uences negatively the catalytic activity, as even more clearly occurs in the presence of Na or K. In the other cases activities are generally well correlated with the measured dispersion values. 4. Conclusions The following conclusions can be drawn out from this work: 1. Very high dispersed metal Ru particles on silica can be obtained by using the sol±gel method starting from a cluster compound such as Ru 3 (CO) 12 or an anionic cluster such as [Ru3H(CO)11]ÿ rather than from a Ru salt. 2. Ru/SiO2 sol±gel prepared from Ru3(CO)12, free of halogen or alkali ions, is a very active catalyst for the Fischer±Tropsch synthesis reaction at relatively low temperatures (473±573 K) and at atmospheric pressure.
Acknowledgements The facilities of the Istituto MASPEC, CNR, Parma have been used for the TEM investigations: the authors are grateful to Dr. G. Salviati for his skillful assistance. Mr. Pier Antonio Bonaldi is acknowledged for technical laboratory support. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Ru/SiO2 catalysts prepared by the sol±gel method from Ru3(CO)12 Pietro Moggia,*, Giovanni Predierib, Fabio Di Silvestria, Andrea Ferrettia a
Dipartimento di Chimica Organica e Industriale, UniversitaÁ di Parma, Parco Area delle Scienze 17/A, 43100, Parma, Italy b Dipartimento di Chimica Generale e Inorganica, Chimica Analitica, Chimica Fisica, UniversitaÁ di Parma, Parco Area delle Scienze 17/A, 43100, Parma, Italy Received 15 September 1998; received in revised form 13 January 1999; accepted 13 January 1999
Abstract Ru/SiO2 catalysts were prepared by sol±gel methods from Ru3(CO)12, or [Ru3H(CO)11]ÿ, Si(OMe)4 (TMOS) and H2O, followed by thermal activation in helium up to 573 K and reduction with hydrogen. They were compared with Ru/SiO2 corresponding material prepared via sol±gel from RuCl3. The catalysts were characterized by FT-IR spectra, surface area, metal dispersion and activity tests in the Fischer±Tropsch reaction at atmospheric pressure and CO/H2 1:1, in the 473±573 K temperature range. The use of ruthenium clusters as precursors afforded higher metal dispersions than RuCl3, as well as higher catalytic activities in the hydrogenation of CO (particularly in the absence of alkali or halogen ions retained in the gel structure). TEM investigations on the Ru/SiO2 material, prepared from Ru3(CO)12 by sol±gel, without promoters, showed the presence of very ®ne metal particles homogeneously embedded in the silica matrix, with dimensions ranging from 1 to 4 nm. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium catalysts; Ruthenium clusters; Sol±gel; Silica support; Metal dispersion; CO hydrogenation
1. Introduction Among the group VIII metals Ru is the most active and the most selective for the production of long-chain hydrocarbons from CO±H2 mixtures [1]. Metal/support interactions, metal loading and dispersion notably affect the activity and selectivity of Ru supported catalysts [2±4]. As a consequence, the performance of the catalysts strongly depends on the choice of metal precursor and support and on the preparation method [5,6]. Silica-supported Ru is generally indicated as the most active and stable catalyst in the CO *Corresponding author. Fax: +39-521-905-472; e-mail: [email protected]
hydrogenation, while other systems are less active and more sensitive to deactivation [7]. It has also been proved that thermal decomposition of metal carbonyl clusters, such as Ru3(CO)12, used as Ru precursors on inorganic oxides as supports, provides high metal dispersions, then resulting in higher speci®c activities [8±12]. Several methods for supporting ruthenium carbonyl clusters on silica are reported in literature; these include: 1. Impregnation of dichloromethane [13,14], trichloromethane [15], pentane [16], hexane [17± 19] or toluene [20] solutions of Ru3(CO)12, at room temperature, and evaporation to dryness in an inert atmosphere.
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00014-9
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2. Impregnation by refluxing benzene [21] or cyclohexane [22,23] solutions of Ru3(CO)12. 3. Impregnation of Ru3(CO)12 via vapor phase in an evacuated sealed pyrex cell, held at a temperature of 353 K over a period of two weeks [24]. 4. Impregnation of methanol solution of Na[Ru3H(CO)11] (synthesized by reaction of Ru3(CO)12 with Na[BH4], at room temperature in tetrahydrofuran (THF) [25]), followed by evaporation of the solvent at 333 K [26]. 5. Anchoring Ru3(CO)12 to silica via a pendant thiol, by reaction of Ru3(CO)12 with HS(CH2)3Si(OMe)3 in refluxing benzene and by grafting the resulting product [Ru3H(CO)10{S(CH2)3Si(OMe)3}] to silica in hexane solution; alternatively, Ru3(CO)12 is reacted in refluxing benzene with the thiol-prefunctionalized silica HS(CH2)3SiO3/2nSiO2 (obtained from silica refluxed in a xylene solution of HS(CH2)3Si(OMe)3) [21]. 6. Anchoring Ru3(CO)12 to silica via a pendant amino (or phosphino) group, by reaction of Ru3(CO)12 with H2N(CH2)3Si(OEt)3 (or Ph2P(CH2)3Si(OEt)3), in toluene (or hexane) under nitrogen at 333 K. The resulting amino- or phosphino-substituted carbonyl clusters are tethered to silica suspended in toluene, at room temperature; alternatively, Ru3(CO)12 is reacted in toluene at room temperature with ligand-functionalized SiO2 (obtained from silica refluxed in a toluene solution of H2N(CH2)3Si(OEt)3 or Ph2P(CH2)3Si(OEt)3] [27]. 7. Anchoring the cluster Na[Ru3H(CO)11] to silica by reaction with ammonium (±NMe3 and ±NEt3)or pyridinium (NC5H5)-functionalized silicas suspended in methanol at room temperature [26]. Recently, sol±gel methods have been proposed for the preparation of Ru/SiO2 catalysts, starting from RuCl33H2O and tetraethoxysilane (TEOS), as Ru and SiO2 precursors, respectively; these methods consist in re¯uxing ethanol±water solutions [28,29], eventually using ammonium hydroxide [30,31] or concentrated hydrogen chloride [32,33] as catalysts to promote the hydrolysis and condensation reactions. By comparing Ru/SiO2 catalysts prepared by impregnation and by sol±gel methods, it was concluded that the sol±gel catalysts showed a much higher resistance to deactivation and substantially enhanced selectivity for the o-xylene hydrogenation [31]. Ru/SiO2 pre-
pared by the sol±gel method also revealed high selectivity in the hydrogenation of benzene to cyclohexene [34] and toluene to methylcyclohexene [35], due to the small particle size. However, the presence of residual chlorine in silicasupported Ru catalysts acts to decrease the catalytic activity in the hydrogenation of CO, probably inducing structural rearrangements [36]. This work deals with the preparation of chlorinefree Ru/SiO2 catalysts by a new sol±gel method, starting from Ru3(CO)12 or related anionic clusters, in order to obtain highly disperse metal particles, exhibiting high speci®c activities and selectivities in the CO hydrogenation reaction, also preventing the thermal deactivation by sintering, which generally occurs on impregnated catalysts. 2. Experimental 2.1. Catalysts preparation Ru/SiO2 sol±gel catalysts, all containing 2 wt% of the metal, were prepared starting from Ru3(CO)12 as Ru precursor and tetramethoxysilane (TMOS) as SiO2 precursor, following the synthetic routes described below. 1. Ru3(CO)12 (Aldrich-Chemie, >99%) (0.05 g) was completely dissolved in dry THF (40 ml) at room temperature, then TMOS (Aldrich-Chemie, >99%) (2.88 ml) and, after 1 h, distilled water (2.43 ml) were added dropwise, under continuous stirring, under a nitrogen stream. In the ®rst preparation, no catalyst was added and the initial yellow solution, transferred in a vessel for the evaporation of the solvent, progressively turned to a grey gel in 8 days, which was then crushed and screened to 40±60 mesh, washed with THF, and ®nally dried in vacuo at room temperature. In another preparation NH4F (0.19 mmol), as nucleophilic catalyst, was dissolved in distilled water (2.43 ml) and added to the homogeneous sol, prepared as above, which after stirring (2 h) under nitrogen, was transferred in a vessel suitable for the evaporation of the solvent and the gelation. A homogeneous gel was obtained in 4 h, which was successively treated as indicated above. The same procedures, without addition of Ru3(CO)12, were used to prepare pure silica gels.
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2. Ru3(CO)12 (0.05 g) was dissolved in dry methanol (50 ml), then Na[BH4] (0.015 g), or KOH (0.010 g), was added under a nitrogen stream and the initial orange solution was constantly stirred until it turned deep red, owing to the formation of the anionic cluster [Ru3H(CO)11]ÿ. TMOS (2.85 ml) was then added dropwise under stirring, and after 1 h, distilled water (2.43 ml) was added to the solution, which was transferred in a vessel suitable for easy evaporation of the solvent and gelation. In three days, dark yellow, highly homogeneous gels were obtained, which were crushed and screened to 40±60 mesh, washed with methanol, and dried in vacuo at room temperature. 3. Ru3(CO)12 (0.05 g) and H2N(CH2)3Si(OEt)3 (Janssen Chimica, >99%) (0.051 g) were refluxed in dry hexane (200 ml) in a nitrogen stream for 60 min, then TMOS (2.91 g) and distilled water (3.48 ml) were added and the mixture was refluxed until gelation. The catalysts prepared from Ru3(CO)12 or [Ru3H(CO)11]ÿ were activated as follows: a weighed amount of the composite material was charged into a ¯ow microreactor and gradually heated (2 K minÿ1) up to 573 K in a helium stream (50 ml minÿ1), then the reduction to Ru metal was performed by feeding hydrogen (75 ml minÿ1) at 573 K for 2 h. Three solutions were prepared separately by dissolving RuCl33H2O (Carlo Erba) (0.06 g) in absolute ethanol (6 ml), and then adding TEOS (Aldrich-Chemie, >98%) (4.1 ml) drop by drop under vigorous stirring. The three solutions were differently added, according to the literature methods previously described: (i) only with water (1.30 ml), (ii) with concentrated ammonium hydroxide (0.06 mol) in water (1.30 ml), and (iii) with concentrated hydrogen chloride (0.07 mol) in water (1.30 ml). In all cases, the solutions were then re¯uxed until gelation occurred, i.e. after 24, 8 and 4 h, respectively. The gels were dried in vacuo at 353 K for 16 h, and ®nally calcined in an oven at 573 K for 5 h, under a nitrogen stream. 2.2. Catalysts characterization The Ru/SiO2 catalysts were characterized by FT-IR spectroscopy, surface area determinations, dispersion measurements, TEM analysis, and catalytic activity tests.
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FT-IR spectra of the supported Ru catalysts before and after activation were recorded in the range 4000± 400 cmÿ1 by using a Nicolet 5 PC spectrophotometer. The diffuse re¯ectance (DRIFTS) spectra were obtained with an HTHP cell (Spectratech) equipped with two ZnSe windows by using an MCT detector. Surface area of the activated Ru catalysts were measured by a Micromeritics 2200 analyser, based on nitrogen adsorption at the temperature of liquid nitrogen, after degassing the samples at 573 K for 1 h under a nitrogen stream. The dispersion measurements of the metal Ru particles on the surface of the activated catalysts were performed by the gas chromatographic pulse method described elsewhere [11], based on the chemisorption of CO at 423 K under a helium stream (60 ml minÿ1). TEM analyses were performed at 200 keV with the Jeol JEM 2000EX instrument of the MASPEC Institute, CNR, Parma. Catalytic activity in the Fischer±Tropsch (FT) synthesis reaction, in the temperature range 473± 573 K, under atmospheric pressure was tested by a ¯ow microreactor containing about 0.5 g of activated catalyst by feeding a CO±H2 1:1 gas mixture (10 ml minÿ1). The ef¯uent products were periodically analysed by a sampling device connected to a Dani 3400 GC apparatus. 3. Results and discussion The sol±gel method via hydrolysis and condensation of silicon alkoxides has been successfully employed to prepare silica xerogels containing ®nely dispersed ruthenium particles derived from carbonyl clusters. In particular the proposed method consists in performing the gelation process with silicon alkoxides in the presence of a dissolved ruthenium carbonyl cluster as metal precursor in such a way that growing of solid support and anchoring of metal species occur at the same time. This procedure combines the main advantages of using metal carbonyl clusters as metal catalyst precursors (low activation temperature, size control of the metal particles, high metal dispersion) with those of the sol±gel process (homogeneity, texture and porosity control). The adopted synthetic routes allow the preparation of Ru/SiO2 dispersed systems in only two steps: gelation and thermal treatment.
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Fig. 1. FT-IR spectra in the 4000±1800 cmÿ1 region: (a) reference silica gel thermally activated at 573 K; (b) Ru/SiO2 sample after gelation without catalysts not activated.
The FT-IR spectrum of the reference silica gel prepared without addition of catalyst before gelation and thermally pretreated in air at 573 K (Fig. 1(a)) shows in the region from 4000 to 1800 cmÿ1 a broad band with three maxima at about 3550, 3480 and 3415 cmÿ1 assignable to terminal silanol groups and to O±H bonds of water and ethanol entrapped molecules. Two weak bands around 2840±2910 cmÿ1 are due to C±H stretching vibrations from the alkoxide groups which partially remain in the gel. In the FT-IR spectrum of the gel similarly obtained from Ru3(CO)12 and TMOS, recorded before the activation treatment (Fig. 1(b)), a large band remains in the silanol region centred at about 3450 cmÿ1, while two strong bands appear in the carbonyl stretching region, whose locations at 2065 and 2000 cmÿ1 are typical of Ru(II) dicarbonylated species anchored to the oxygen atoms of the support [37]. In the region from 1400 to 400 cmÿ1 the FT-IR spectrum of the reference silica gel (Fig. 2(a)) shows four bands, due to Si±O stretching vibration (1080 cmÿ1), Si±Oÿ bending vibration (795 and 620 cmÿ1), and ÿ OÿSiÿOÿ bending vibration (470 cmÿ1) [31]. In the ruthenium containing gel
(Fig. 2(b)) a new band appears near the Si±O stretching (1080 cmÿ1), at 955 cmÿ1, which could be assigned to the Si±O(±Ru) linkage [31]. During the gelation important modi®cations occur involving both the silicon alkoxides (hydrolysis and condensation) and the ruthenium carbonyl species (anchoring to the gel structure via formation of Ru± O±Si bonds). The evolution of the anchoring process can be conveniently monitored by IR spectroscopy in the carbonyl region. Fig. 3 compares the FT-IR spectra recorded at different times during the gelation process: Ru3(CO)12 dissolved in THF (Fig. 3(a)) shows three carbonyl bands at 2060vs, 2029vs and 2015w cmÿ1, according to the literature data for different Ru3(CO)12 solutions [38], which are not in¯uenced by the addition of TMOS and water. After gelation (Fig. 3(b)), with or without catalyst, two very strong IR carbonyl bands at 2065 and 2000 cmÿ1 appear, which are due to the formation of dicarbonyl Ru(II) species Ru±O-bonded to silica as stated above. This suggests that strong interactions between the ruthenium cluster and the incoming support take place during the hydrolysis and condensation steps of the gelation process.
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Fig. 2. FT-IR spectra in the 1400±400 cmÿ1 region: (a) reference silica gel thermally activated at 573 K; (b) Ru/SiO2 sample after gelation without catalysts not activated.
Fig. 3. FT-IR spectra in the carbonyl (2150±1900 cmÿ1) region, during the sol±gel process from Ru3(CO)12 (in THF) and TMOS, without catalyst added: (a) solution immediately analysed after addition of water; (b) sample analysed after completion of the gelation process.
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The FT-IR spectra recorded for both preparations where Ru3(CO)12 reacted with Na[BH4] or KOH show two strong bands in the carbonyl stretching region, at 2039±2041 and 1972±1973 cmÿ1, respectively, before addition of TMOS and water, quite good according to the literature data for the anionic cluster [Ru3H(CO)11]ÿ [26], but those bands are shifted after gelation to about 2055 and 1990 cmÿ1 due to the oxidative interaction on the silica. is reacted with When Ru3(CO)12 H2N(CH2)3Si(OEt)3 in hexane solution, the FT-IR spectrum shows three bands at 2062, 2032 and 2012 cmÿ1 according to literature [27]; in this case again only two bands are present in the spectra recorded after gelation shifted to even lower frequencies at 2053 and 1981 cmÿ1. Twin carbonyl bands have been reported by many authors [39], being attributable to monomeric species [Ru(CO)2]2 or polymeric Ru CO2 m n grafted to the silica matrix, the latter containing Ru±Ru bonds. The oxidation state of the metal atoms is comprised between 2 and m/n, and accordingly, the frequency values of the CO stretching bands exhibit different values. Lower frequencies are indicative of less oxidized dicarbonyl species, whose formation appears to be favoured when [Ru3H(CO)11]ÿ is used as precursor or in the presence of the anchored aminopropyl group; it is possible that this nitrogen donor ligand protects to a certain extent the ruthenium clusters from the oxidative fragmentation. The occurrence of different ruthenium species during the gelation process is also indicated by the observed colour changes from orange to greenish grey
in the case of Ru3(CO)12 or from red to dark yellow in the case of [Ru3H(CO)11]ÿ. The results of the measurements of surface area and metal dispersion on the activated Ru/SiO2 catalysts prepared by the different sol±gel techniques are given in Table 1. The content of metal Ru is always 2.0% by weight on the silica. The contents (wt% on silica) of Na or K are speci®cally indicated in the table. The Ru and SiO2 precursors and the type of catalyst eventually added to favour the gel formation are also speci®ed. As stated elsewhere [11,40], the use of Ru3(CO)12, rather than RuCl3, as Ru precursor affords higher metal dispersions corresponding in some cases to the chemisorption of more than 2 mol of CO per mole of Ru. Moreover, an excess of chlorine, when hydrogen chloride is used as gelation catalyst with RuCl3 as precursor, gives the lowest dispersion. The Ru/SiO2 catalyst, prepared via sol±gel from Ru3(CO)12 and TMOS (without NH4F), activated under He up to 573 K and reduced with H2 has been subjected to pulse CO chemisorption in a DRIFTS cell at 473 K. Fig. 4 shows that the catalyst binds CO giving again twin carbonyl stretching bands (2058 and 1983 cmÿ1) and suggesting that ruthenium is adequately dispersed being able to readsorb CO after activation, affording dicarbonylated species. The same catalyst has been analysed by transmission electron microscopy (TEM). The micrographs of the sample (see Fig. 5) evidence a lot of spheroidal metal particles homogeneously dispersed into the silica matrix, whose diameter ranges from 1 to 4 nm. The average values given in literature [32]
Table 1 Characterization data for the sol±gel catalysts Catalyst
Precursors
Surface area (m2 gÿ1)
Dispersion (CO/Ru moles)
SiO2 SiO2 Ru/SiO2 Ru/SiO2 RuNa(0.68%)/SiO2 RuK(0.82%)/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2
TMOS TMOS/NH4F Ru3(CO)12, TMOS Ru3(CO)12, TMOS/NH4F [Ru3H(CO)11]ÿ, TMOS [Ru3H(CO)11]ÿ, TMOS Ru3(CO)12, aminosilane, TMOS RuCl3, TEOS RuCl3, TEOS/NH4OH RuCl3, TEOS/HCl
335 640 656 409 660 750 550 600 660 495
± ± 2.21 2.07 1.36 2.37 1.04 0.61 0.60 0.15
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263
Fig. 4. DRIFT spectra in the carbonyl stretching region of a Ru(2.0 wt%)/SiO2 sample prepared by the sol±gel method from Ru3(CO)12 dissolved in THF, TMOS and H2O, without NH4F, after activation in helium stream up to 573 K and reduction with H2: (a) before CO exposure; (b) after CO chemisorption at 473 K.
prevailing reaction product is always CH4 (85±90% at 523 K, 90±95% at 573 K) accompanied by C2H6 (10± 15% at 523 K, 3±5% at 573 K) and minor amounts of C3H8. CO2 is formed only in traces up to 548 K and reaches 2±3% at 573 K. The highest catalytic activity is obtained with the Ru/SiO2 sample sol±gel prepared from Ru3(CO)12 and TMOS, without addition of catalyst for the hydrolysis±condensation steps, prob-
for the metal particles of a Ru(1.0%)/SiO2 catalyst prepared via sol±gel from RuCl3 are about 3±5 nm. The results of the catalytic activity tests in the FT synthesis at atmospheric pressure in the temperature range between 473 and 573 K are summarized in Table 2, where the activities are compared referring to the total hydrocarbon production rate in ml hÿ1 (room temperature, 1 atm) per gram of catalyst. The
Table 2 Catalytic activity of Ru/SiO2 catalysts (FT synthesis reaction) expressed as total hydrocarbon (HC) production rate at different temperatures Catalyst
Precursors
Ru/SiO2 Ru/SiO2 RuNa(0.68%)/SiO2 RuK(0.82%)/SiO2 Ru/SiO2 Ru/SiO2
Ru3(CO)12, TMOS Ru3(CO)12, TMOS/NH4F [Ru3H(CO)11]ÿ, TMOS [Ru3H(CO)11]ÿ, TMOS Ru3(CO)12, aminosilane, TMOS RuCl3, TEOS/NH4OH
HC production rate (ml hÿ1 gÿ1) 473 K
498 K
523 K
548 K
573 K
22 6 ± ± 3 3
50 19 ± ± 17 7
170 78 4 4 29 13
462 204 16 19 96 38
879 424 65 29 282 152
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Fig. 5. Bright-field TEM images (X 400 K) of a Ru(2.0 wt%)/SiO2 sample prepared by the sol±gel method from Ru3(CO)12 dissolved in THF, TMOS and H2O, without NH4F, after activation in helium stream up to 573 K and reduction with H2: (a) on the border of a thin silica grain; (b) in the bulk.
ably because the presence of Fÿ ions in¯uences negatively the catalytic activity, as even more clearly occurs in the presence of Na or K. In the other cases activities are generally well correlated with the measured dispersion values. 4. Conclusions The following conclusions can be drawn out from this work: 1. Very high dispersed metal Ru particles on silica can be obtained by using the sol±gel method starting from a cluster compound such as Ru 3 (CO) 12 or an anionic cluster such as [Ru3H(CO)11]ÿ rather than from a Ru salt. 2. Ru/SiO2 sol±gel prepared from Ru3(CO)12, free of halogen or alkali ions, is a very active catalyst for the Fischer±Tropsch synthesis reaction at relatively low temperatures (473±573 K) and at atmospheric pressure.
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