Preparation of highly dispersed Ru-Sn bimetallic supported catalysts from the single source precursors Cp(PPh3)2Ru-SnX3 (X = Cl or Br)

Preparation of highly dispersed Ru-Sn bimetallic supported catalysts from the single source precursors Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br)

Ana Cláudia Bernardes Silva^I; Ana Paula Guimarães de Sousa^I; José Domingos Ardisson^II; Helmuth Guido Luna Siebald^I; Edmilson Moura^I; Eduardo Nicolau dos Santos^I; Nelcy Della Santina Mohallem^I; Rochel Montero LagoI, ^*

^IDepartamento de Química, ICEx - UFMG, 31270-901 Belo Horizonte - MG, Brazil

^IICDTN, Centro de Desenvolvimento de Tecnologia Nuclear, Belo Horizonte - MG, Brazil

ABSTRACT

In this work highly dispersed Ru-Sn bimetallic catalysts have been prepared from organobimetallic Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br) complexes. These single source precursors can be easily impregnated in high surface area supports, such as activated carbon and sol-gel SiO₂, and upon controlled thermal treatment the ligands are released as volatile products resulting in the formation of the bimetallic system Ru-Sn. Catalytic reactions, such as hydrodechlorination of CCl₄ and chlorobenzene and TPR (Temperature Programmed Reduction) experiments carried out with these RuSn catalysts suggested a strong interaction between Ruthenium and Tin. Mössbauer measurements showed that these materials when exposed to air are immediately oxidized to form Sn (IV). It was shown that upon controlled reduction conditions with H₂ it is possible to reduce selectively Sn to different oxidation states and different phases. The Sn oxidation state showed significant effect on the catalytic hydrogenation of 1,5-cyclooctadiene. The use of these single source precursors with a controlled decomposition/reduction procedure allows the preparation of unique catalysts with an intimate interaction between the components ruthenium and tin and the possibility of varying the Sn oxidation state around the Ru metal.

Keywords: bimetallic catalyst, rutheniun, tin, hydrogenation

1. Introduction

Bimetallic catalysts have been used in industrial process for many years due to their unique catalytic properties¹. However, only recently an understanding of the structure dependence on the preparation method and treatment has been developed^2-5. An important application of bimetallic catalysts, which has been extensively investigated in the last years is the selective hydrogenation of C=O group in a,b-unsaturated aldehydes, such as crotonaldehyde, acrylaldehyde and citral⁶. These reactions should produce the respective unsaturated alcohols which are fine chemicals of great importance to the cosmetic and pharmaceutical industry^7,8. These catalysts are generally based on a noble metal, e.g. Pt, Rh or Ru, which catalyzes the reaction, and a second metal usually Sn, Fe, Pb, Ni, Co or Ge which is mainly responsible for the selectivity in the hydrogenation^9-16. Among these promoters, Sn has shown the best results in selectivity for the carbonyl hydrogenation, what has been explained in terms of a strong interaction between the C=O group with the tin oxide¹⁵. Therefore, two factors are essential to produce an active and selective catalyst: (i) the proximity of Sn to the noble metal and (ii) the Sn oxidation state. The bimetallic system Ru and Sn has been pointed out recently as one of the most active and selective catalyst for these hydrogenation¹⁷. These catalysts have been prepared by the co-impregnation of two different ruthenium and tin precursors, typically RuCl₃ and SnCl₄. However, the co-impregnation method produces mostly the segregation of the metals resulting in a poor interaction of Ru and Sn.

In this work, Ru-Sn catalysts have been prepared from a single source precursor containing both metals Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br). The Ru-Sn chemical bond in the precursor should result in a close proximity of the two metals throughout the catalyst preparation steps leading to a much better interaction compared to materials prepared by the traditional co-impregnation method.

2. Experimental

The preparation of the bimetallic complexes Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br) is described in detail elsewhere¹⁸.

The Ru-Sn supported catalysts have been prepared by impregnation on activated carbon (100 mg, Aldrich Norit, S_BET = 950 m²/g) with a dichloromethane solution (100 mg in 2 ml) of the complexes Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br). The CH₂Cl₂ was removed by vacuum at room temperature. The amount of the precursor was adjusted in order to produce a metallic content in the catalysts of 5 wt% ruthenium and 6 wt% of tin. The material was heated at 10 °C/min under hydrogen flow (15 ml/min) and kept at 500 °C for 60 min.

Ru-Sn/SiO₂ catalysts have been prepared by mixing TEOS (tetraethoxysilane), ethanol and water in a 1/3/10 molar ratio, with HCl and HF as catalysts. Cp(PPh₃)₂Ru-SnCl₃ (1 mol%) was added to the starting solution during preparation. The solution was kept under stirring at room temperature for two hours for homogenization and left still for gelation. The wet gels, prepared in monolithic shape, were dried at 110 °C for 24 h.

Some of the catalyst after reduction were embedded with liquid benzene, before they were exposed to the atmosphere, in order to hinder metal oxidation by air.

Thermogravimetric analyses were carried out in a Mettler TA 4000. The samples were heated from room temperature to 750 °C at 10 °C/min under air or nitrogen flow. Scanning electron microscopy analyses were obtained in a Jeol JXA-8900RL.

Mössbauer analyses were obtained at 23 K in a transmission geometry equipment with a ¹¹⁹Sn source in CaSnO₃. A least squares fitting program was used to analyze the spectra and to calculate the chemical shifts, quadrupole splitting and area of each component.

The TPR analyses were carried out in a Micromeritics TPR/TPD 2900 equipment. The samples were heated from room temperature to 800 °C under 50 ml/min H₂/N₂ (10% v/v H₂) flow. The samples were pre-treated from room temperature to 500 °C (10 °C/min) under He flow.

The hydrogendechlorination reactions have been carried in a fix bed reactor using 30 mg of Ru-Sn catalysts under H₂ flow (30 ml/min). CCl₄ or chlorobenzene were introduced in the H₂ stream by a controlled temperature saturator at 0 °C, producing vapor pressures of 29 mmHg and concentration of 30 mmol/l for CCl₄ and 2.5 mmHg and concentration of 3 mmol/l for chlorobenzene. The diolefin hydrogenation reactions were carried in an autoclave using 40 mg of supported Ru-Sn catalysts and 40 ml of 1,5-ciclooctadiene (COD)/benzene solution (0,4 mol/l), under hydrogen pressure (20 atm). The reaction products were analyzed by gas chromatography (Shimadzu/GC 17 A) with a FID detector and capillary column Carbowax 20M (25 m × 0.32 mm × 0.25 mm ).

3. Results And Discussion

3.1 Thermal Decomposition of the Complexes

The thermogravimetric (TG) profiles of the precursor RuCp(PPh₃)₂SnCl₃, pure and supported on activated carbon, in a nitrogen atmosphere, are shown in Fig. 1.

It can be observed a weight loss of ca. 74%, in the temperature range 250-650 °C. This weight loss suggests the decomposition of the complex with the formation of the pure metals and elimination of the ligands as volatile products:

Similar results were obtained for the RuCp(PPh₃)₂SnBr₃ complex with a weight loss of 76% in the same temperature range.

The expected weight losses for the decomposition described in Eq. 1 are 76 and 79% for the Cl and Br derivatives, respectively, which are very similar to the experimental value of 74 and 76%.

TG profiles were also obtained for the organometallic precursors supported on activated carbon (Fig. 1). It was observed an initial weight decrease probably related to the presence of residual water and CH₂Cl₂ adsorbed on the carbon. From 250 up to 450 °C, the complex supported on the carbon surface seems to decompose with the expected weight loss of approximately 22%.

3.2 Activated Carbon Supported RuSn Catalysts

The activated carbon supported catalysts have been obtained by impregnation methods resulting in a final ruthenium and tin content of 5 and 6 wt%, respectively. The following materials have been prepared: (i) Ru/AC, impregnation of RuCl₃ followed by reduction in H₂ at 500 °C, (ii) coimp-Ru+Sn/AC, prepared by co-impregnation of RuCl₃ and SnCl₂ followed by reduction in H₂ at 500 °C, and (iii) RuSn/AC prepared by the impregnation of the complex RuCp(PPh₃)₂SnX₃ followed by reduction in H₂ at 500 °C. For comparison, SnCl₂ was also impregnated on activated carbon and treated in H₂ at 500 °C (Sn/AC). An advantage the precursors RuCp(PPh₃)₂SnX₃ should offer over RuCl₃ and SnCl₃, is related to their hydrophobic character which will allow a much better interaction with the hydrophobic carbon support and consequently a better dispersion over the carbon surface.

3.3 SiO₂ sol-gel Supported RuSn Catalysts

The SiO₂ sol-gel supported catalysts have been obtained by dispersion of the different precursors during the sol-gel preparation, to obtain ruthenium and tin content of 1 and 1.2 wt%, respectively. The following materials have been prepared: (i) Ru/SiO₂, addition of RuCl₃ during the sol-gel preparation followed by reduction in H₂ at 500 °C, (ii) coadd-Ru+Sn/SiO₂, prepared by co-addition of RuCl₃ and SnCl₂ followed by reduction in H₂ at 500 °C, and (iii) RuSn/SiO₂ prepared by the addition of the complex RuCp(PPh₃)₂SnCl₃ followed by reduction in H₂ at 500 °C. For comparison, SnCl₂ was also added during sol-gel preparation and treated in H₂ at 500 °C (Sn/SiO₂).

3.4 ¹¹⁹Sn Mössbauer Spectroscopic Studies

The Mössbauer data obtained for the activated carbon supported RuSn catalysts are shown in Table 1 and Fig. 2. The RuSn/AC sample was treated with H₂ at 500 °C and after cooling to room temperature it was exposed to air. Mössbauer analysis of this sample showed that Sn is completely oxidized to form Sn(IV). Similar results have been obtained by Stievano et al.¹⁹ who carefully treated carbon supported Ru 70% Sn 30% with H₂ at 400 °C and also observed the tin oxidation after exposure to air or moisture. The Mössbauer spectrum fitting suggests the presence of two different species with isomer shift (IS) of - 0.12 and 0.50 mm/s and quadrupole splitting (QS) of 0.48 and 0.4 mm/s, respectively, both related to Sn(IV). If the sample RuSn/AC treated with H₂ at 500 °C is protected with liquid benzene to hinder the oxidation by air, the spectrum can be fit with 4 Lorentzians indicating the presence of mainly Sn(II) and Sn(IV) with small amounts of Sn(0). These oxidized species are probably produced by the oxidation by traces of water in benzene and also to some oxygen diffusion through the liquid protection.

For the sample reduced at 900 °C it can be observed the presence of mainly Sn(IV) 54% and Sn(0) 16%. However, the Mössbauer spectrum seems to suggest the presence of three new phases: oxidic Ru-Sn, Ru₃Sn₁₅O₁₄ and Ru₃Sn₇. The oxidic Ru-Sn and Ru₃Sn₁₅O₁₄ prepared by Söhel et al²⁰. under similar experimental conditions (900 °C) and have been described as RuSn alloys containing some oxygen into their structure. The formation of the Ru₃Sn₇ alloy has been reported previously^19,21 in RuSn catalysts. These results suggested that the treatment at higher temperatures (900 °C) favors the formation of alloys containing Ru and Sn.

The activated carbon catalyst obtained by co-impregnation, coimp-Ru+Sn/AC, after treatment at 500 °C with H₂ and exposed to air at RT, showed the presence of the fully oxidized Sn(IV) (45%) and the reduced Sn(0) (4%). The Mössbauer fitting also suggests the presence of the phases Ru₃Sn₁₅O₁₄ 15% and Ru₃Sn₇ 12% and a Sn(II) species. The formation of these Sn rich phases, i.e. Ru₃Sn₁₅O₁₄ and Ru₃Sn₇, in the co-impregnated catalyst treated at temperature as low as 500 °C, might suggest that the impregnation method was not efficient in dispersing the tin throughout the carbon surface. This poor dispersion would originate Sn enriched regions which would favor the formation of the Sn rich phases observed.

It is interesting to note the different behavior of the RuSn/AC catalysts where Sn is completely oxidized whereas for the coimp-Ru+Sn/AC the Sn is only partially oxidized after reduction and exposure to air.

Mössbauer analyses have also been carried out for the RuSn catalysts supported on SiO₂ sol-gel matrix. The results are displayed in Table 2 and Fig. 3.

It can be observed similar spectra for both samples, RuSn/SiO₂ and coadd-Ru+Sn/SiO₂, with the presence of Sn(IV), Sn(II) and Sn(0) species. On the other hand, the material RuSn/SiO₂ showed a major contribution of Sn(II) species whereas coadd-Ru+Sn/SiO₂ produced mainly Sn(IV).

3.5 Temperature Programmed Reduction (TPR) Studies

The TPR profiles for the activated carbon supported catalysts were obtained after reduction with H₂ at 500 °C and exposure to air at room temperature to reoxidize the metals. Figure 4 shows the TPR profiles obtained for co-imp-Ru+Sn/AC, RuSn/AC and Sn/AC.

It can be observed for Sn/AC two peaks at the temperature 270 and 350-400 °C. According to the literature the first peak is probably related to the reduction of Sn⁺⁴® Sn⁺² which typically takes place in the temperature range 200300 ^oC, whereas the second peak might be related to the reduction Sn⁺²® Sn^o22.

The co-imp-Ru+Sn/AC catalyst showed a broad peak centered at 160 °C related to the reduction of ruthenium Ru⁺⁴ or Ru⁺³® Ru^o.²³ It can also be observed peaks for the reduction of Sn species similar to those observed for the Sn/AC sample, indicating that most of the Sn species present in the co-imp-Ru+Sn/AC catalyst are similar to those formed in the Sn pure sample Sn/AC. On the other hand, the TPR profile for the RuSn/AC showed significant differences. The Ru reduction peak is shifted to lower temperatures, 130 °C, whereas the reduction of Sn seems to occur in a broad peak in the temperature range 250-350 °C. Although the detailed explanation for these differences is not clear, they suggest that Ru and Sn species formed from the precursor Cp(PPh₃)₂Ru-SnX₃ are significantly different from those obtained by the impregnation methods with RuCl₃ and SnCl₂. These TPR differences could be produced by a different dispersion or by a chemical interaction of the phases present on the surface of the activated carbon.

For the reduction experiments of the sol-gel SiO₂ supported catalysts (Fig. 5), all the samples were pre-treated in O₂ (air) flow at 600 °C for 30 min. TPR experiments with the sample Ru/SiO₂ showed only one intense peak at ca. 110 °C, related to the reduction of Ru⁺⁴ dispersed in the sol-gel matrix. On the other hand, it can be observed in the TPR profile of the catalyst RuSn/SiO₂ three well defined reduction peaks at 100, 150 and 210 °C and two broad peaks centered at 380 and 480 °C. The peak at 100 °C is similar to that observed in the sample Ru/SiO₂. The intense peak at 150 °C, is related to a less reactive ruthenium species, which can be reduced only at higher temperatures. This species is only observed when the organometallic complex is used as precursor. As discussed above the broad peaks at 380 and 480 °C are likely related to the reduction of Sn species dispersed in the matrix²⁴. However, their peak areas are too small to account for the reduction of all tin present in the sample. Therefore, the peak at 210 °C is probably also related to the reduction of tin. These two shifted peaks at 150 °C (related to the reduction of Ru species) and at 210 °C (related to the reduction of Sn species) suggest a strong interaction of the Ru and Sn species. The increase in the ruthenium reduction temperature can be caused by the close proximity of the tin oxide. On the other hand, the significant decrease in the reduction temperature of the Sn species should occur via a spillover effect,²⁵ where the Ru^o metal activates H₂ and transfer to the surrounding Sn oxides species.

3.6 Catalytic Reactions

The catalytic activity of the activated carbon supported materials were investigated for the following hydrodechlorination reactions:

The activities presented by the different catalysts at 200 °C are shown in Fig. 6.

It can be observed that Ru/AC and Ru+Sn/AC show very similar results with low CCl₄ conversions of approximately 0.1 mmol/g_cat.min. These results reinforces that in the catalyst prepared by co-impregnation Ru+Sn/AC the Ru is not interacting significantly with the Sn species, since it showed similar performance when compared to the pure Ru/AC catalyst. On the other hand, the catalysts RuSn/AC obtained from the organometallic precursors shows a much stronger effect of tin resulting in an activity of 0.3-0.6 mmol/g_cat.min, indicating a promoting effect of this metal. The results obtained for the hydrodechlorination of chlorobenzene also suggest a significant effect of tin on the catalytic activity of the catalysts RuSn/AC, but with a negative effect on the conversion.

3.7 The Effect of the Sn Oxidation State on the Catalytic Activity

To investigate the effect of the Sn oxidation state on the catalytic properties of the catalyst it was studied the hydrogenation 1,5-cyclooctadiene in the presence of the catalyst RuSn/AC submitted to two different treatments:

• after thermal treatment the catalyst was exposed to air leading to the complete oxidation of tin to Sn(IV) as shown by the Mössbauer spectrum and treated with H₂ at 200 °C in order to reduce only the ruthenium to Ru^º. This catalyst was named Ru^o/Sn(IV)/AC;

• after thermal treatment, the catalyst was reduced with H₂ at 500 °C and not exposed to air. As suggested by Mössbauer experiments, this catalyst contains mainly Sn(II) with some Sn(0) and Sn(IV). This catalyst was named Ru^o/Sn(II/IV)/AC.

The results are displayed in Fig. 7a and 7b.

The 1,5-COD hydrogenation affords mainly two products: the selective hydrogenation compound cyclooctene (COE) and the total hydrogenation molecule cyclooctane (COA) (Scheme 1).

Very small amounts of the isomerization products 1,3COD and 1,4-COD were also detected (yields < 1%).

From Fig. 7a and 7b it can be observed similar conversions for both catalysts Ru^o/Sn(IV)/AC and Ru^o/Sn(II/IV)/AC. However, it is interesting to note that these catalysts showed significantly different product distribution during the reaction. Ru^o/Sn(IV)/AC showed much better selectivity for the production of COE, with ca. 60% yield in the beginning of the reaction. For longer reaction times this yield will decrease due to hydrogenation of the COE to COA. On the other hand, the catalyst Ru^o/Sn(II)/AC produces mainly the total hydrogenation compound COA.

These results show that the oxidation state of tin does not affect the catalyst activity but it has a significant effect on the product selectivity. The effects of Sn species around the Ru metal catalytic center are likely related to electronic and steric effects⁷.

4. Conclusion

Supported RuSn bimetallic catalysts can be prepared by impregnation and thermal decomposition of the organometallic precursors Cp(PPh₃)₂Ru-SnX₃ (X = Cl or Br). Catalysts prepared by these precursors and by the co-impregnation method showed significantly different properties, as revealed by the catalytic hydrogendechlorination of CCl₄ and chlorobenzene, TPR experiments and Mössbauer spectroscopy. These differences could be related to a better metal dispersion and a stronger interaction of ruthenium with tin. The interaction Ru-Sn in the prepared catalysts is favored by the chemical bond Ru-Sn existent in the organometallic precursor which will keep the two metals in a close proximity throughout the catalyst preparation. With these catalysts it is also possible to have some control of the Sn oxidation state which showed a significant effect on the catalytic hydrogenation of 1,5-cyclooctadiene.

Received: November 11, 2001

Revised: March 24, 2003

Trabalho apresentado no I Simpósio Mineiro de Ciências dos Materiais, Ouro Preto, Novembro de 2001

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Publication Dates

• Publication in this collection
  03 July 2003
• Date of issue
  June 2003

History

• Received
  11 Nov 2001
• Reviewed
  24 Mar 2003