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Selective hydrogenation of dimethyl adipate on titania-supported RuSn catalysts
Applied Catalysis A: General 353 (2009) 101–106
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective hydrogenation of dimethyl adipate on titania-supported RuSn catalysts Adriana M. Silva a,b, Marco A. Morales c, Elisa M. Baggio-Saitovitch c, Elizabete Jorda˜o b, Marco A. Fraga a,* a
Instituto Nacional de Tecnologia/MCT, Laborato´rio de Cata´lise, Av. Venezuela 82/518, Centro, 20081-312, Rio de Janeiro, RJ, Brazil Laborato´rio de Desenvolvimento de Processos Catalı´ticos, Departamento de Engenharia de Sistemas Quı´micos e Informa´tica, UNICAMP, Cidade Universita´ria Zeferino Vaz, C.P. 6066, 13083-970, Campinas, SP Brazil c Centro Brasileiro de Pesquisas Fı´sicas, R. Dr. Xavier Sigaud, 150, Urca, 22290-180 Rio de Janeiro, RJ, Brazil b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2008 Received in revised form 13 October 2008 Accepted 20 October 2008 Available online 28 October 2008
The hydrogenation of dimethyl adipate over RuSn/TiO2 catalysts was studied to examine the effect of tin species and titania on selective production of 1,6-hexanediol. The bimetallic catalysts were prepared by co-impregnation and the reaction was carried out in a liquid phase semi-batch reactor at moderate conditions (255 8C and 50 bar). The optimal Sn/Ru ratio to reach maximum selectivity changed according to the catalyst reduction temperature. A remarkable selectivity of 70% of 1,6-hexanediol was accomplished over the catalyst with Sn/Ru ratio of 2 and reduced at 400 8C. The characterization of the bimetallic catalysts reduced at such a high temperature was performed by Mo¨ssbauer spectroscopy and it was found that all systems presented a similar distribution of tin species and no entity revealed to be preferentially formed. The establishment of strong-metal–support interaction and thus a synergistic effect between TiOx moieties and Snn+ species was suggested to determine the reaction pathway. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Dimethyl adipate 1,6-Hexanediol Titania Metal–support interaction RuSn catalysts Selective hydrogenation
1. Introduction Dimethyl adipate (DMA) is a six-carbon dicarboxylic acid derived ester which may lead to a wide variety of other chemicals such as monoesters, cyclic ethers, lactones, alcohols and hydrocarbons. A simplified reaction scheme presenting some of the possible products for the hydrogenation of DMA is displayed in Fig. 1. Amongst those compounds, 1,6-hexanediol deserves special attention as it is a highly valued linear diol featuring two hydroxyl groups terminally located. 1,6-Hexanediol is recognized as a valuable intermediate for the synthesis of several di-substituted products due to its structural configuration. It is industrially important as it is a raw material for several chemicals, pharmaceuticals and polymers. Polyurethanes, specialty acrylates, surface coatings, polymeric plasticizers and adhesives may be listed as the most important of its end uses. Production of 1,6-hexanediol may be performed through a two-step process; the esterification of adipic acid is first promoted, followed by the ester hydrogenation. This second step is a catalyst-assisted reaction, on which copper-based systems are conventionally used [1]; however, quite severe
* Corresponding author. Tel.: +55 21 21231152; fax: +55 21 21231051. E-mail address: [email protected] (M.A. Fraga). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.10.025
reaction conditions are demanded, especially high hydrogen pressures (300 bar). Our group has been reporting alternative and more active catalysts based on noble metals [2–4]. Such systems are able to operate under considerably lower hydrogen pressure (50 bar). Monometallic catalysts based on group VIII noble metals are totally unselective to 1,6-hexanediol producing mostly highly hydrogenated compounds, basically alcohols and hydrocarbons [2]. Bimetallic ruthenium catalyst, in turn, may lead the reaction towards diol. Amongst the elements investigated in our previous work [2], cobalt, zinc and tin were the only few on which some selectivity towards diol could be accomplished under mild conditions. Nevertheless, upon the addition of cobalt and zinc the selectivity to 1,6-hexanediol was relatively low; the introduction of tin, on the other hand, provided a rather significant formation of diol, coming out thus as a potential promoter [2]. The benefits of choosing a suitable methodology to prepare such catalysts have been investigated as well [4]. A detailed study concerning the preparation of alumina supported ruthenium–tin indicated that more selective catalysts are obtained through coimpregnation, whereby the ionic tin moieties are more suitably dispersed on the catalyst surface. More recently, we have also highlighted the effects brought about by the support on chemoselectivity, comparing the catalytic performance of RuSn bimetallic systems supported on Al2O3, SiO2,
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Fig. 1. A simplified reaction scheme for hydrogenation of dimethyl adipate.
TiO2, CeO2 and Nb2O5 [3]. Even though 1,6-hexanediol could be produced over all studied samples, the selectivity achieved over titania-supported catalyst was remarkable. Indeed, selectivity improvements have often been reported when titania is used as support in selective hydrogenation reactions [5,6]. Such effect is believed to be a consequence of the formation of special sites at the metal–support interface, which is generally accepted to be consisted of an ensemble formed by at least one atom of metal adjacent to a defect site on the support (Ti3+ cation or oxygen vacancy). These exposed sites, generated through the partial reduction of titania, would interact with the oxygen atom in the carbonyl group thereby activating the C O bond. Nonetheless, only few reports concerning the possible combined effects of tin used as promoter and that reducible support are found for such chemoselective reactions [5]. In the present contribution further experimental results are reported on the chemoselective hydrogenation of dimethyl adipate over RuSn catalysts. The combined effect of tin species and titania on 1,6-hexanediol production is examined over several bimetallic catalysts with different Sn/Ru atomic ratios; the benefits of applying different activation temperatures, controlling thus the support–metal strong interaction, are investigated. 2. Experimental The monometallic and bimetallic catalysts were prepared by impregnation and coimpregnation, respectively. A commercial titania (anatase >99%, 21 m2 g1) was used as support. The metal precursor salts were hydrated ruthenium chloride (RuCl3, Aldrich) and tin(II) chloride (SnCl2, Aldrich). All catalysts were prepared so as to obtain a ruthenium content of 2 wt.%, while the tin content was varied in order to achieve different Sn/Ru atomic ratios. The catalysts nominal compositions are summarized in Table 1. The support was slurried with ethanolic solution of the necessary quantities of the precursors in a rotary evaporator at room temperature for 15 h. Afterwards, the solvent was slowly evaporated under vacuum. The powder obtained was first dried overnight at 120 8C and then further dried under argon at the same temperature for 15 h. The samples were calcined under an air stream of 50 mL min1 for 4 h at 400 8C. After calcination, the
catalysts were reduced with hydrogen flow of 40 mL min1 for 2 h. All catalysts were reduced at two different temperatures, 250 8C (low temperature reduction – LTR) and 400 8C (high temperature reduction – HTR), always following a heating rate of 10 8C min1. The hydrogenation of dimethyl adipate (Aldrich, 99.9%) was carried out in the liquid phase in a PARR semi-batch reactor at 255 8C and hydrogen pressure of 50 bar. The catalyst (1.4 g) was mixed with dioxane (80 mL) used as solvent and preconditioned in situ at reaction conditions for 1 h. The reaction started by injecting 7.0 g of dimethyl adipate in the reactor. The experiments were conducted with stirring speed of 1500 rpm to favour hydrogen diffusion and ensure kinetic control. The hydrogen pressure in the reactor was controlled during the whole experiment. Samples were taken periodically and analyzed by gas chromatography. A metallic frit was used in order to get clear liquid samples. A Thermosquest/Trace GC equipped with a 30 m 0.25 mm capillary column (100% dimethyl-siloxane) was used. The products identification was carried out in a HP GC–MS 5988A. The reaction results were analyzed in terms of selectivity: cj S j ¼ selectivity to product j ¼ 100 P cj The cj is the concentration of the main identified hydrogenation products of DMA, namely e-caprolactone, methyl caproate, monomethyl ester of adipic acid and 1,6-hexanediol. Mo¨ssbauer spectra were obtained at temperature of liquid helium using a constant acceleration spectrometer with a BaSnO3 source. Prior to the analyses, the samples were reduced following the same protocol applied before the catalytic tests. After reduction, they were handled in inert atmosphere to avoid Table 1 The RuSn studied catalysts – nominal composition and Sn//Ru ratio. Catalysts
Sn/Ru
2 wt.% Ru/TiO2 2 wt.% Ru–0.3 wt.% Sn/TiO2 2 wt.% Ru–0.6 wt.% Sn/TiO2 2 wt.% Ru–1.2 wt.% Sn/TiO2 2 wt.% Ru–2.4 wt.% Sn/TiO2 2 wt.%Ru–4.7 wt.% Sn/TiO2
0.0 0.1 0.2 0.4 1.0 2.0
A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
oxidation. The 119Sn Mo¨ssbauer parameters were determined by Lorentzian lines computer fitted by the least squares method, which yielded the mean electric quadrupole splitting (QS), the mean isomer shift (IS) and the relative resonance areas (area) of the different components of the spectra. 3. Results and discussion The role of Sn/Ru ratio on the liquid phase hydrogenation was initially investigated using catalysts reduced at low temperature (LTR 250 8C) in order to prevent the well known strong-metal– support interaction (SMSI) effect. The DMA time-conversion patterns were taken to evaluate the global activity of the RuSn bimetallic catalysts under this reduction condition (Fig. 2). It can be observed that DMA conversion is quite low at the beginning but increases in time for all catalysts. This behavior has been observed by other authors and is characteristic of weakly reactive esters [7]. DMA conversion also revealed to be strongly dependent on the presence of tin and Sn/Ru ratio. The monometallic catalyst Ru/TiO2 reached 65% of DMA conversion after 15 h. However, with the addition of quite small amount of tin in the catalyst composition (Sn/Ru = 0.1), the DMA conversion dropped strongly to 25% within the same reaction interval. Similar behavior was also showed by those samples containing a Sn/Ru ratio of 0.2, 0.4 and 1.0. This trend is similar to those previously observed for the hydrogenation of a,b-unsaturated aldehydes [5,8]. In those cases, the results published so far have showed that the activity decays gradually as Sn/metal ratio increases, rendering the catalytic system totally inactive. This fashion is currently explained by a blockage of the metal sites by tin oxides moietes. These species are not able to dissociatively chemisorb hydrogen or adsorb the hydrogen eventually spilled over from adjacent ruthenium atoms [8–10]. Nonetheless, a turning point can be observed in the present work. The bimetallic catalyst exhibited the highest activity at a Sn/ Ru ratio of 2.0, which might denote the creation of novel ruthenium–tin sites specific to hydrogenolysis/hydrogenation of the diester. Interestingly, a similar activity increase at high Sn content was also verified in the hydrogenation of oleic acid [11] and DMA [4] on alumina-supported RuSn catalysts. The selectivity values obtained over all RuSn/TiO2 catalysts after 15 h reaction are collected in Table 2. Monomethyl ester of adipic acid is the main product detected over monometallic Ru sites; however,
Fig. 2. DMA time-conversion profiles for all LTR catalysts supported on TiO2.
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Table 2 Hydrogenation of DMAa over Ru/TiO2 and RuSn/TiO2 catalysts reduced at 250 8C. Sn/Ru fb (%) Selectivity (%)
e-Caprolactone 1,6-Hexanediol Methyl
Others Adipic acid caproate monomethyl ester
0 0.1 0.2 0.4 1.0 2.0
65 25 22 20 41 81
– 5 20 18 11 6
– 2 26 37 50 45
– 9 40 40 39 28
47 22 14 5 – –
53 62 – – – 21
a Experimental conditions – temperature: 255 8C; pressure 50 bar; reaction time: 15 h. b f: conversion.
some by-products originated from the excessive hydrogenation of the reactant molecule were detected as well. These products were all grouped and named as ‘‘others’’, for the sake of clarity and simplicity as they cover a wide range of chemical compounds and each were detected at a rather low concentration. Similar results were also observed over Ru/Al2O3 [2], denoting that Ru0 sites are responsible for splitting the O–CH3 bond in the ester group. Additionally this result agrees well with those reported for other dicarboxylic acid/esters, which suggests that monometallic catalysts are not able to activate the C O group and lead to diol production [3,4,12]. The catalysts containing Sn exhibited a distinct performance (Table 2); the presence of the additive changed drastically the product distribution leading to the formation of methyl caproate, e-caprolactone and 1,6-hexanediol. It should be noted that monomethyl ester of adipic acid was also produced but at significantly lower levels. These results provide clear evidence that tin presence is indeed responsible for the diol production. Moreover, the selectivity to diol was found to be totally related to the amount of Sn in the catalysts (Sn/Ru) as depicted in Fig. 3. The sample with Sn/Ru of 1
Fig. 3. Evolution of selectivity to 1,6-hexanediol for Ru/TiO2 and RuSn/TiO2 LTR catalysts along with Sn/Ru ratio at isoconversion and after a reaction period of 15 h.
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Fig. 4. DMA time-conversion profiles: (a) monometallic Ru/TiO2 catalysts (LTR and HTR); (b) all HTR catalysts supported on TiO2.
exhibited the best performance toward diol production. It is worth mentioning that this conclusion can be reached by either analyzing the reaction data collected after a reaction period of 15 h or at isoconversion level. The role of tin oxides on the hydrogenation of unsaturated aldehydes and acids/esters has been extensively discussed over the last years [5,8–11,13,14]. The enhanced selectivity towards the hydrogenation of C O bond by introducing tin is generally attributed to an interaction established between the C O bond and the promoter cations acting as Lewis acid sites on the metal surface. As reported in our previous contribution [5], the optimal Sn/Ru ratio to alcohol production changes according to the substrate. It is now interesting to note, however, that this ratio also varies according to the support used for RuSn catalysts, considering the same chemical structure/substrate; an optimal ratio of 2 was found for producing diol from dimethyl adipate over RuSn/Al2O3 [4] whereas a lower value (Sn/Ru = 1) showed to be good enough for titania-supported bimetallic system (Fig. 3). This difference could be attributed to the different chemical affinity between tin and these supports. As a matter of fact, the strong interaction between tin and alumina favours the generation of inactive Sn-aluminate species [4,15]. The influence of the carrier over the redox properties of supported tin oxide systems is also reported in the literature [16,17]. It seems to be a consensus that metallic tin as well as metal–tin alloys are present whenever an inert support, such as SiO2, is used; on the other hand, stabilized Sn2+ species is mostly detected on alumina supported samples. By using titania such Sn–support interaction is inhibited making it possible to detect metallic b-Sn and RuSnO species [3]. In order to gain some insight on the effects brought about by non-stoichiometric titania oxides, these catalysts were also tested after reduction at high temperature (HTR), 400 8C. The DMA conversions versus time are represented in Fig. 4(a) for Ru/TiO2, reduced at both low and high temperature, and in Fig. 4(b) for all RuSn/TiO2 samples reduced at HTR conditions. As can be seen, the increase in reduction temperature led to a significant activity drop since the DMA conversion on Ru/TiO2 (HTR) reached only 35% after 15 h while an expressive value of 65% could be achieved at the same reaction conditions after reduction at 250 8C. This behavior is generally attributed to the migration of reduced TiOx species created at high temperature to the metal surface, diminishing the number of active sites. As for the bimetallic catalysts (Fig. 4(b)), reduction at 400 8C seems also to diminish the activity. Though the
bimetallic catalysts may be essentially distinguished by the reduction temperature applied, and thus by the establishment or not of SMSI, it was found that the evolution of DMA conversion as a function of Sn/Ru ratio follows a similar trend irrespective of the activation treatment (Fig. 5). This result therefore supports the conclusion that the role played by Sn species would be more relevant than that from TiOx species. The selectivities of the products obtained over RuSn/TiO2 catalysts reduced at 400 8C after 15 h reaction are summarized in Table 3. The increase in reduction temperature led to considerable changes in the catalysts performance. The monomethyl ester of adipic acid was the main product, reaching 91% of selectivity over monometallic Ru/ TiO2 sample. It is worth noting, however, the timid formation of byproducts verified so far only on LTR catalysts containing tin, particularly e-caprolactone (5%) and methyl caproate (4%). Besides,
Fig. 5. DMA conversion as a function of Sn/Ru ratio.
A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
105
Table 3 Hydrogenation of DMAa over Ru/TiO2 and RuSn/TiO2 catalysts reduced at 400 8C. Sn/Ru
0 0.1 0.2 0.4 1.0 2.0
fb (%)
35 16 24 18 28 57
Selectivity (%)
e-Caprolactone
1,6-Hexanediol
Methyl caproate
Adipic acid monomethyl ester
5 21 22 21 17 4
– 15 31 36 44 70
4 40 40 41 38 26
91 28 7 2 1 –
a Experimental conditions – temperature: 255 8C; pressure 50 bar; reaction time: 15 h. b f: conversion.
it is also observed the total suppression of products resulting from excessive hydrogenation of the substrate molecule and thus the group of compounds grouped as ‘‘others’’ is no longer detected. These results suggest that the migration of TiOx oxides to the catalyst surface cover non-selective sites. Moreover, as TiOx moieties may also act as Lewis acid sites, the formation of e-caprolactone and methyl caproate may be tentatively related to these centers. Nevertheless, it is conceivable that the strength of Lewis acid sites on TiOx is not enough to activate the C O bond and promote the formation of diol. Once again this behavior contrasts with those found for a,bunsaturated aldehydes [5,8] suggesting that the promoter sites, more specifically its acidity character, must be designed taking into account the molecular configuration of the substrate. Contrasting the selectivity results after a period of 15 h, one can easily note that the lactone formation is fairly constant for catalysts with Sn/Ru up to 1.0. On the other hand, this value decreased drastically with further increase in Sn/Ru atomic ratio. In a similar way, methyl caproate kept constant values (around 40%), decreasing only for Sn/Ru of 2 (26%). The production of monomethyl ester of
Fig. 7.
119
Sn Mo¨ssbauer spectra of 4.7 wt.%Sn/TiO2 reference sample.
adipic acid decreased gradually as the Sn/Ru increased, vanishing completely at Sn/Ru = 2. It may be considered that the monomethyl ester of adipic acid formed on bimetallic samples could be instantaneous hydrogenated towards other products. In Fig. 6, differently from what was observed over LTR samples (Fig. 3), 1,6-hexanediol selectivity raised as Sn/Ru increased, achieving a maximum at Sn/Ru = 2; over such sample a striking selectivity level of 70% of diol could be accomplished. It is interesting to note that again the evolution of diol selectivity along with Sn/Ru presented the same pattern either after a long reaction time interval (15 h) or at isoconversion (20%). The maximum selectivity reached at higher Sn/Ru ratio under HTR conditions provides evidence that a synergistic effect of TiOx and SnOx arises and plays an important role in the reaction selectivity. Based on the outstanding catalytic performance accomplished with the bimetallic catalysts reduced at 400 8C, they were characterized by Mo¨ssbauer spectroscopy in order to access information about tin species present in the samples. However,
Table 4 Mo¨ssbauer hyperfine parameters for 4.7%Sn/TiO2 reference sample and RuSn/TiO2 catalysts reduced at 400 8C. Catalyst
Sn species
IS (mm s1)
QS (mm s1)
Area (%)
4.7% Sn/TiO2
Sn4+ Sn2+ b-Sn
0.00 2.90 2.56
0.51 2.20 –
11 20 69
2% Ru–1.2% Sn/TiO2 (Sn/Ru = 0.4)
Sn4+
0.08
0.70
6
Sn2+ RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
2.90 1.50 1.50 2.65 2.20
1.80 1.50 0.00 0.70 1.40
13 31 11 8 31
Sn4+
0.05
0.70
11
2+
Sn RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
2.90 1.75 1.45 2.74 2.20
1.90 1.75 0.00 0.67 1.45
17 27 9 6 30
Sn4+
0.06
0.70
13
2+
2.90 1.70 1.40 2.70 2.15
1.80 1.70 0.00 0.70 1.59
9 36 6 7 29
2% Ru–2.4% Sn/TiO2 (Sn/Ru = 1.0)
2% Ru 4.7% Sn/TiO2 (Sn/Ru = 2.0)
Fig. 6. Evolution of selectivity to 1,6-hexanediol for Ru/TiO2 and RuSn/TiO2 HTR catalysts along with Sn/Ru ratio at isoconversion and after a reaction period of 15 h.
Sn RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
106
A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
irrespective of their tin content (Fig. 8). As could be observed for the reference Sn/TiO2 sample, in all cases two spectral components can be attributed to oxidic Sn4+ and Sn2+ species. A third strong component presented hyperfine parameters similar to those first reported by Stievano et al. for RuSn/C systems [18] and was accordingly associated with an binary oxidic Ru–Sn species hereinafter designated as RuSnO. These species have been described as an oxygen-bonded tin species with intermetallic bonds between ruthenium and tin; however, the accurate composition of this compound could not be established so far. Two more intermetallic compounds could also be fitted, namely Ru3Sn7-I and Ru3Sn7-II. These compounds are bimetallic alloy phases, which differ basically in the position that tin atoms occupy in the crystal lattice [19]. Finally, the presence of Sn surface species was identified and is usually interpreted as a monolayer of metallic tin deposited onto the metallic surface of Ru [18,20]. The hyperfine parameters of all different tin sites and their corresponding relative areas are summarized in Table 4. It is seen that all systems presented a similar distribution of tin species when supported on titania and no entity seemed to be preferentially generated. Nevertheless, the reactive experiments described initially in the present work showed a gradual increase in diol selectivity along with Sn/Ru ratio. These results allow the suggestion that a synergy between SnOx and TiOx species might indeed be taking place and would be responsible for the improved behavior of bimetallic HTR catalysts. 4. Conclusions The results obtained over RuSn/TiO2 catalysts reduced at high temperature showed that the production of 1,6-hexanediol is more effective at high Sn/Ru ratio, evidencing the main role played by the tin oxide species on the activation of ester carbonyl bond. The establishment of strong-metal–support interaction under this condition also showed to have a striking promoting effect over selectivity, suggesting a synergistic effect between reduced TiOx moieties and Snn+ species. Acknowledgment A.M.S. acknowledges the financial support from CAPES. References Fig. 8. 119Sn Mo¨ssbauer spectra of RuSn/TiO2: (A) Sn/Ru = 0.4; (B) Sn/Ru = 1; (C) Sn/ Ru = 2.
it should be mentioned that the low tin loading in samples with Sn/ Ru of 0.1 and 0.2 did not allow getting a satisfactory spectral resolution and thus any deconvolution attempt of Mo¨ssbauer spectra to differentiate the possible tin species would be highly speculative. These two samples were therefore not considered. Fig. 7 presents the spectra collected for the reduced reference sample 4.7 wt.% Sn/TiO2. It exhibited three spectral components, whose hyperfine parameters and relative resonance areas are collected in Table 4. Two quadrupole doublets are related to the Sn4+ and Sn2+ ionic species while a third single line present as the main component (69%) can be associated with a b-metallic Sn phase. The appearance of metallic tin would denote a weak interaction between the metal phase and titania in contrast with what has been observed for similar alumina-supported powders [4]. The experimental Mo¨ssbauer data obtained from the bimetallic RuSn catalysts required six components to fit the spectra
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
T. Turek, D.L. Trimm, Catal. Rev. Sci. Eng. 36 (4) (1994) 645. S.M. Santos, A.M. Silva, E. Jorda˜o, M.A. Fraga, Catal. Commun. 5 (2004) 377. S.M. Santos, A.M. Silva, E. Jorda˜o, M.A. Fraga, Catal. Today 107–108 (2005) 250. A.M. Silva, O.A.A. Santos, E.M. Baggio-Saitovitch, E. Jorda˜o, M.A. Fraga, J. Mol. Catal. A 253 (2006) 62. A.M. Silva, O.A.A. Santos, M.J. Mendes, E. Jorda˜o, M.A. Fraga, Appl. Catal. A 217 (2003) 155. T. Ekou, A. Vicente, G. Lafaye, C. Especel, P. Marecot, Appl. Catal. A 314 (2006) 73. A. Piccirili, Y. Pouilloux, Y. Pronier, S. Barrault, Bull Soc Chim Fr. 132 (1995) 1109. P. Gallezot, D. Richard, Catal. Rev. Sci. 40 (1&2) (1998) 81. P. Kluson, L. Cerveny, Appl. Catal. A 128 (1995) 13. V. Ponec, Appl. Catal. A 149 (1997) 27. M.J. Mendes, O.A.A. Santos, E. Jorda˜o, A.M. Silva, Appl. Catal. A 217 (2001) 253. M. Toba, S.-I. Tanaka, S.I. Niwa, F. Mizukami, Z. Koppa´ny, L. Guczi, K.-Y. Cheah, T.-S. Tang, Appl. Catal. A 189 (1999) 243. P. Ma¨ki-Arvela, J. Ha´jek, T. Salmi, D.Yu. Murzin, Appl. Catal. A 292 (2005) 1. Y. Pouilloux, F. Autin, C. Guimon, J. Barrault, J. Catal. 176 (1998) 215. E.A. Sales, J. Jove, M.J. Mendes, F. Bozon-Verduraz, J. Catal. 195 (2000) 88. H. Lieske, J. Vo¨lter, J. Catal. 90 (1994) 96. O.A. Barias, A. Holmen, E.A. Blekkan, J. Catal. 158 (1996) 1. L. Stievano, S. Calogero, F.E. Wagner, S. Galvagno, C. Milone, J. Phys. Chem. B 103 (1999) 9545. L. Ericksson, J. Lanner, Acta Crystallogr. E57 (2001) i85. B. Coq, A. Goursot, T. Tazi, F. Figueras, D.R. Salahub, J. Am. Chem. Soc. 113 (1991) 1485.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective hydrogenation of dimethyl adipate on titania-supported RuSn catalysts Adriana M. Silva a,b, Marco A. Morales c, Elisa M. Baggio-Saitovitch c, Elizabete Jorda˜o b, Marco A. Fraga a,* a
Instituto Nacional de Tecnologia/MCT, Laborato´rio de Cata´lise, Av. Venezuela 82/518, Centro, 20081-312, Rio de Janeiro, RJ, Brazil Laborato´rio de Desenvolvimento de Processos Catalı´ticos, Departamento de Engenharia de Sistemas Quı´micos e Informa´tica, UNICAMP, Cidade Universita´ria Zeferino Vaz, C.P. 6066, 13083-970, Campinas, SP Brazil c Centro Brasileiro de Pesquisas Fı´sicas, R. Dr. Xavier Sigaud, 150, Urca, 22290-180 Rio de Janeiro, RJ, Brazil b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2008 Received in revised form 13 October 2008 Accepted 20 October 2008 Available online 28 October 2008
The hydrogenation of dimethyl adipate over RuSn/TiO2 catalysts was studied to examine the effect of tin species and titania on selective production of 1,6-hexanediol. The bimetallic catalysts were prepared by co-impregnation and the reaction was carried out in a liquid phase semi-batch reactor at moderate conditions (255 8C and 50 bar). The optimal Sn/Ru ratio to reach maximum selectivity changed according to the catalyst reduction temperature. A remarkable selectivity of 70% of 1,6-hexanediol was accomplished over the catalyst with Sn/Ru ratio of 2 and reduced at 400 8C. The characterization of the bimetallic catalysts reduced at such a high temperature was performed by Mo¨ssbauer spectroscopy and it was found that all systems presented a similar distribution of tin species and no entity revealed to be preferentially formed. The establishment of strong-metal–support interaction and thus a synergistic effect between TiOx moieties and Snn+ species was suggested to determine the reaction pathway. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Dimethyl adipate 1,6-Hexanediol Titania Metal–support interaction RuSn catalysts Selective hydrogenation
1. Introduction Dimethyl adipate (DMA) is a six-carbon dicarboxylic acid derived ester which may lead to a wide variety of other chemicals such as monoesters, cyclic ethers, lactones, alcohols and hydrocarbons. A simplified reaction scheme presenting some of the possible products for the hydrogenation of DMA is displayed in Fig. 1. Amongst those compounds, 1,6-hexanediol deserves special attention as it is a highly valued linear diol featuring two hydroxyl groups terminally located. 1,6-Hexanediol is recognized as a valuable intermediate for the synthesis of several di-substituted products due to its structural configuration. It is industrially important as it is a raw material for several chemicals, pharmaceuticals and polymers. Polyurethanes, specialty acrylates, surface coatings, polymeric plasticizers and adhesives may be listed as the most important of its end uses. Production of 1,6-hexanediol may be performed through a two-step process; the esterification of adipic acid is first promoted, followed by the ester hydrogenation. This second step is a catalyst-assisted reaction, on which copper-based systems are conventionally used [1]; however, quite severe
* Corresponding author. Tel.: +55 21 21231152; fax: +55 21 21231051. E-mail address: [email protected] (M.A. Fraga). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.10.025
reaction conditions are demanded, especially high hydrogen pressures (300 bar). Our group has been reporting alternative and more active catalysts based on noble metals [2–4]. Such systems are able to operate under considerably lower hydrogen pressure (50 bar). Monometallic catalysts based on group VIII noble metals are totally unselective to 1,6-hexanediol producing mostly highly hydrogenated compounds, basically alcohols and hydrocarbons [2]. Bimetallic ruthenium catalyst, in turn, may lead the reaction towards diol. Amongst the elements investigated in our previous work [2], cobalt, zinc and tin were the only few on which some selectivity towards diol could be accomplished under mild conditions. Nevertheless, upon the addition of cobalt and zinc the selectivity to 1,6-hexanediol was relatively low; the introduction of tin, on the other hand, provided a rather significant formation of diol, coming out thus as a potential promoter [2]. The benefits of choosing a suitable methodology to prepare such catalysts have been investigated as well [4]. A detailed study concerning the preparation of alumina supported ruthenium–tin indicated that more selective catalysts are obtained through coimpregnation, whereby the ionic tin moieties are more suitably dispersed on the catalyst surface. More recently, we have also highlighted the effects brought about by the support on chemoselectivity, comparing the catalytic performance of RuSn bimetallic systems supported on Al2O3, SiO2,
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A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
Fig. 1. A simplified reaction scheme for hydrogenation of dimethyl adipate.
TiO2, CeO2 and Nb2O5 [3]. Even though 1,6-hexanediol could be produced over all studied samples, the selectivity achieved over titania-supported catalyst was remarkable. Indeed, selectivity improvements have often been reported when titania is used as support in selective hydrogenation reactions [5,6]. Such effect is believed to be a consequence of the formation of special sites at the metal–support interface, which is generally accepted to be consisted of an ensemble formed by at least one atom of metal adjacent to a defect site on the support (Ti3+ cation or oxygen vacancy). These exposed sites, generated through the partial reduction of titania, would interact with the oxygen atom in the carbonyl group thereby activating the C O bond. Nonetheless, only few reports concerning the possible combined effects of tin used as promoter and that reducible support are found for such chemoselective reactions [5]. In the present contribution further experimental results are reported on the chemoselective hydrogenation of dimethyl adipate over RuSn catalysts. The combined effect of tin species and titania on 1,6-hexanediol production is examined over several bimetallic catalysts with different Sn/Ru atomic ratios; the benefits of applying different activation temperatures, controlling thus the support–metal strong interaction, are investigated. 2. Experimental The monometallic and bimetallic catalysts were prepared by impregnation and coimpregnation, respectively. A commercial titania (anatase >99%, 21 m2 g1) was used as support. The metal precursor salts were hydrated ruthenium chloride (RuCl3, Aldrich) and tin(II) chloride (SnCl2, Aldrich). All catalysts were prepared so as to obtain a ruthenium content of 2 wt.%, while the tin content was varied in order to achieve different Sn/Ru atomic ratios. The catalysts nominal compositions are summarized in Table 1. The support was slurried with ethanolic solution of the necessary quantities of the precursors in a rotary evaporator at room temperature for 15 h. Afterwards, the solvent was slowly evaporated under vacuum. The powder obtained was first dried overnight at 120 8C and then further dried under argon at the same temperature for 15 h. The samples were calcined under an air stream of 50 mL min1 for 4 h at 400 8C. After calcination, the
catalysts were reduced with hydrogen flow of 40 mL min1 for 2 h. All catalysts were reduced at two different temperatures, 250 8C (low temperature reduction – LTR) and 400 8C (high temperature reduction – HTR), always following a heating rate of 10 8C min1. The hydrogenation of dimethyl adipate (Aldrich, 99.9%) was carried out in the liquid phase in a PARR semi-batch reactor at 255 8C and hydrogen pressure of 50 bar. The catalyst (1.4 g) was mixed with dioxane (80 mL) used as solvent and preconditioned in situ at reaction conditions for 1 h. The reaction started by injecting 7.0 g of dimethyl adipate in the reactor. The experiments were conducted with stirring speed of 1500 rpm to favour hydrogen diffusion and ensure kinetic control. The hydrogen pressure in the reactor was controlled during the whole experiment. Samples were taken periodically and analyzed by gas chromatography. A metallic frit was used in order to get clear liquid samples. A Thermosquest/Trace GC equipped with a 30 m 0.25 mm capillary column (100% dimethyl-siloxane) was used. The products identification was carried out in a HP GC–MS 5988A. The reaction results were analyzed in terms of selectivity: cj S j ¼ selectivity to product j ¼ 100 P cj The cj is the concentration of the main identified hydrogenation products of DMA, namely e-caprolactone, methyl caproate, monomethyl ester of adipic acid and 1,6-hexanediol. Mo¨ssbauer spectra were obtained at temperature of liquid helium using a constant acceleration spectrometer with a BaSnO3 source. Prior to the analyses, the samples were reduced following the same protocol applied before the catalytic tests. After reduction, they were handled in inert atmosphere to avoid Table 1 The RuSn studied catalysts – nominal composition and Sn//Ru ratio. Catalysts
Sn/Ru
2 wt.% Ru/TiO2 2 wt.% Ru–0.3 wt.% Sn/TiO2 2 wt.% Ru–0.6 wt.% Sn/TiO2 2 wt.% Ru–1.2 wt.% Sn/TiO2 2 wt.% Ru–2.4 wt.% Sn/TiO2 2 wt.%Ru–4.7 wt.% Sn/TiO2
0.0 0.1 0.2 0.4 1.0 2.0
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oxidation. The 119Sn Mo¨ssbauer parameters were determined by Lorentzian lines computer fitted by the least squares method, which yielded the mean electric quadrupole splitting (QS), the mean isomer shift (IS) and the relative resonance areas (area) of the different components of the spectra. 3. Results and discussion The role of Sn/Ru ratio on the liquid phase hydrogenation was initially investigated using catalysts reduced at low temperature (LTR 250 8C) in order to prevent the well known strong-metal– support interaction (SMSI) effect. The DMA time-conversion patterns were taken to evaluate the global activity of the RuSn bimetallic catalysts under this reduction condition (Fig. 2). It can be observed that DMA conversion is quite low at the beginning but increases in time for all catalysts. This behavior has been observed by other authors and is characteristic of weakly reactive esters [7]. DMA conversion also revealed to be strongly dependent on the presence of tin and Sn/Ru ratio. The monometallic catalyst Ru/TiO2 reached 65% of DMA conversion after 15 h. However, with the addition of quite small amount of tin in the catalyst composition (Sn/Ru = 0.1), the DMA conversion dropped strongly to 25% within the same reaction interval. Similar behavior was also showed by those samples containing a Sn/Ru ratio of 0.2, 0.4 and 1.0. This trend is similar to those previously observed for the hydrogenation of a,b-unsaturated aldehydes [5,8]. In those cases, the results published so far have showed that the activity decays gradually as Sn/metal ratio increases, rendering the catalytic system totally inactive. This fashion is currently explained by a blockage of the metal sites by tin oxides moietes. These species are not able to dissociatively chemisorb hydrogen or adsorb the hydrogen eventually spilled over from adjacent ruthenium atoms [8–10]. Nonetheless, a turning point can be observed in the present work. The bimetallic catalyst exhibited the highest activity at a Sn/ Ru ratio of 2.0, which might denote the creation of novel ruthenium–tin sites specific to hydrogenolysis/hydrogenation of the diester. Interestingly, a similar activity increase at high Sn content was also verified in the hydrogenation of oleic acid [11] and DMA [4] on alumina-supported RuSn catalysts. The selectivity values obtained over all RuSn/TiO2 catalysts after 15 h reaction are collected in Table 2. Monomethyl ester of adipic acid is the main product detected over monometallic Ru sites; however,
Fig. 2. DMA time-conversion profiles for all LTR catalysts supported on TiO2.
103
Table 2 Hydrogenation of DMAa over Ru/TiO2 and RuSn/TiO2 catalysts reduced at 250 8C. Sn/Ru fb (%) Selectivity (%)
e-Caprolactone 1,6-Hexanediol Methyl
Others Adipic acid caproate monomethyl ester
0 0.1 0.2 0.4 1.0 2.0
65 25 22 20 41 81
– 5 20 18 11 6
– 2 26 37 50 45
– 9 40 40 39 28
47 22 14 5 – –
53 62 – – – 21
a Experimental conditions – temperature: 255 8C; pressure 50 bar; reaction time: 15 h. b f: conversion.
some by-products originated from the excessive hydrogenation of the reactant molecule were detected as well. These products were all grouped and named as ‘‘others’’, for the sake of clarity and simplicity as they cover a wide range of chemical compounds and each were detected at a rather low concentration. Similar results were also observed over Ru/Al2O3 [2], denoting that Ru0 sites are responsible for splitting the O–CH3 bond in the ester group. Additionally this result agrees well with those reported for other dicarboxylic acid/esters, which suggests that monometallic catalysts are not able to activate the C O group and lead to diol production [3,4,12]. The catalysts containing Sn exhibited a distinct performance (Table 2); the presence of the additive changed drastically the product distribution leading to the formation of methyl caproate, e-caprolactone and 1,6-hexanediol. It should be noted that monomethyl ester of adipic acid was also produced but at significantly lower levels. These results provide clear evidence that tin presence is indeed responsible for the diol production. Moreover, the selectivity to diol was found to be totally related to the amount of Sn in the catalysts (Sn/Ru) as depicted in Fig. 3. The sample with Sn/Ru of 1
Fig. 3. Evolution of selectivity to 1,6-hexanediol for Ru/TiO2 and RuSn/TiO2 LTR catalysts along with Sn/Ru ratio at isoconversion and after a reaction period of 15 h.
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Fig. 4. DMA time-conversion profiles: (a) monometallic Ru/TiO2 catalysts (LTR and HTR); (b) all HTR catalysts supported on TiO2.
exhibited the best performance toward diol production. It is worth mentioning that this conclusion can be reached by either analyzing the reaction data collected after a reaction period of 15 h or at isoconversion level. The role of tin oxides on the hydrogenation of unsaturated aldehydes and acids/esters has been extensively discussed over the last years [5,8–11,13,14]. The enhanced selectivity towards the hydrogenation of C O bond by introducing tin is generally attributed to an interaction established between the C O bond and the promoter cations acting as Lewis acid sites on the metal surface. As reported in our previous contribution [5], the optimal Sn/Ru ratio to alcohol production changes according to the substrate. It is now interesting to note, however, that this ratio also varies according to the support used for RuSn catalysts, considering the same chemical structure/substrate; an optimal ratio of 2 was found for producing diol from dimethyl adipate over RuSn/Al2O3 [4] whereas a lower value (Sn/Ru = 1) showed to be good enough for titania-supported bimetallic system (Fig. 3). This difference could be attributed to the different chemical affinity between tin and these supports. As a matter of fact, the strong interaction between tin and alumina favours the generation of inactive Sn-aluminate species [4,15]. The influence of the carrier over the redox properties of supported tin oxide systems is also reported in the literature [16,17]. It seems to be a consensus that metallic tin as well as metal–tin alloys are present whenever an inert support, such as SiO2, is used; on the other hand, stabilized Sn2+ species is mostly detected on alumina supported samples. By using titania such Sn–support interaction is inhibited making it possible to detect metallic b-Sn and RuSnO species [3]. In order to gain some insight on the effects brought about by non-stoichiometric titania oxides, these catalysts were also tested after reduction at high temperature (HTR), 400 8C. The DMA conversions versus time are represented in Fig. 4(a) for Ru/TiO2, reduced at both low and high temperature, and in Fig. 4(b) for all RuSn/TiO2 samples reduced at HTR conditions. As can be seen, the increase in reduction temperature led to a significant activity drop since the DMA conversion on Ru/TiO2 (HTR) reached only 35% after 15 h while an expressive value of 65% could be achieved at the same reaction conditions after reduction at 250 8C. This behavior is generally attributed to the migration of reduced TiOx species created at high temperature to the metal surface, diminishing the number of active sites. As for the bimetallic catalysts (Fig. 4(b)), reduction at 400 8C seems also to diminish the activity. Though the
bimetallic catalysts may be essentially distinguished by the reduction temperature applied, and thus by the establishment or not of SMSI, it was found that the evolution of DMA conversion as a function of Sn/Ru ratio follows a similar trend irrespective of the activation treatment (Fig. 5). This result therefore supports the conclusion that the role played by Sn species would be more relevant than that from TiOx species. The selectivities of the products obtained over RuSn/TiO2 catalysts reduced at 400 8C after 15 h reaction are summarized in Table 3. The increase in reduction temperature led to considerable changes in the catalysts performance. The monomethyl ester of adipic acid was the main product, reaching 91% of selectivity over monometallic Ru/ TiO2 sample. It is worth noting, however, the timid formation of byproducts verified so far only on LTR catalysts containing tin, particularly e-caprolactone (5%) and methyl caproate (4%). Besides,
Fig. 5. DMA conversion as a function of Sn/Ru ratio.
A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
105
Table 3 Hydrogenation of DMAa over Ru/TiO2 and RuSn/TiO2 catalysts reduced at 400 8C. Sn/Ru
0 0.1 0.2 0.4 1.0 2.0
fb (%)
35 16 24 18 28 57
Selectivity (%)
e-Caprolactone
1,6-Hexanediol
Methyl caproate
Adipic acid monomethyl ester
5 21 22 21 17 4
– 15 31 36 44 70
4 40 40 41 38 26
91 28 7 2 1 –
a Experimental conditions – temperature: 255 8C; pressure 50 bar; reaction time: 15 h. b f: conversion.
it is also observed the total suppression of products resulting from excessive hydrogenation of the substrate molecule and thus the group of compounds grouped as ‘‘others’’ is no longer detected. These results suggest that the migration of TiOx oxides to the catalyst surface cover non-selective sites. Moreover, as TiOx moieties may also act as Lewis acid sites, the formation of e-caprolactone and methyl caproate may be tentatively related to these centers. Nevertheless, it is conceivable that the strength of Lewis acid sites on TiOx is not enough to activate the C O bond and promote the formation of diol. Once again this behavior contrasts with those found for a,bunsaturated aldehydes [5,8] suggesting that the promoter sites, more specifically its acidity character, must be designed taking into account the molecular configuration of the substrate. Contrasting the selectivity results after a period of 15 h, one can easily note that the lactone formation is fairly constant for catalysts with Sn/Ru up to 1.0. On the other hand, this value decreased drastically with further increase in Sn/Ru atomic ratio. In a similar way, methyl caproate kept constant values (around 40%), decreasing only for Sn/Ru of 2 (26%). The production of monomethyl ester of
Fig. 7.
119
Sn Mo¨ssbauer spectra of 4.7 wt.%Sn/TiO2 reference sample.
adipic acid decreased gradually as the Sn/Ru increased, vanishing completely at Sn/Ru = 2. It may be considered that the monomethyl ester of adipic acid formed on bimetallic samples could be instantaneous hydrogenated towards other products. In Fig. 6, differently from what was observed over LTR samples (Fig. 3), 1,6-hexanediol selectivity raised as Sn/Ru increased, achieving a maximum at Sn/Ru = 2; over such sample a striking selectivity level of 70% of diol could be accomplished. It is interesting to note that again the evolution of diol selectivity along with Sn/Ru presented the same pattern either after a long reaction time interval (15 h) or at isoconversion (20%). The maximum selectivity reached at higher Sn/Ru ratio under HTR conditions provides evidence that a synergistic effect of TiOx and SnOx arises and plays an important role in the reaction selectivity. Based on the outstanding catalytic performance accomplished with the bimetallic catalysts reduced at 400 8C, they were characterized by Mo¨ssbauer spectroscopy in order to access information about tin species present in the samples. However,
Table 4 Mo¨ssbauer hyperfine parameters for 4.7%Sn/TiO2 reference sample and RuSn/TiO2 catalysts reduced at 400 8C. Catalyst
Sn species
IS (mm s1)
QS (mm s1)
Area (%)
4.7% Sn/TiO2
Sn4+ Sn2+ b-Sn
0.00 2.90 2.56
0.51 2.20 –
11 20 69
2% Ru–1.2% Sn/TiO2 (Sn/Ru = 0.4)
Sn4+
0.08
0.70
6
Sn2+ RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
2.90 1.50 1.50 2.65 2.20
1.80 1.50 0.00 0.70 1.40
13 31 11 8 31
Sn4+
0.05
0.70
11
2+
Sn RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
2.90 1.75 1.45 2.74 2.20
1.90 1.75 0.00 0.67 1.45
17 27 9 6 30
Sn4+
0.06
0.70
13
2+
2.90 1.70 1.40 2.70 2.15
1.80 1.70 0.00 0.70 1.59
9 36 6 7 29
2% Ru–2.4% Sn/TiO2 (Sn/Ru = 1.0)
2% Ru 4.7% Sn/TiO2 (Sn/Ru = 2.0)
Fig. 6. Evolution of selectivity to 1,6-hexanediol for Ru/TiO2 and RuSn/TiO2 HTR catalysts along with Sn/Ru ratio at isoconversion and after a reaction period of 15 h.
Sn RuSnO Ru3Sn7-1 Ru3Sn7-2 Sn surface
106
A.M. Silva et al. / Applied Catalysis A: General 353 (2009) 101–106
irrespective of their tin content (Fig. 8). As could be observed for the reference Sn/TiO2 sample, in all cases two spectral components can be attributed to oxidic Sn4+ and Sn2+ species. A third strong component presented hyperfine parameters similar to those first reported by Stievano et al. for RuSn/C systems [18] and was accordingly associated with an binary oxidic Ru–Sn species hereinafter designated as RuSnO. These species have been described as an oxygen-bonded tin species with intermetallic bonds between ruthenium and tin; however, the accurate composition of this compound could not be established so far. Two more intermetallic compounds could also be fitted, namely Ru3Sn7-I and Ru3Sn7-II. These compounds are bimetallic alloy phases, which differ basically in the position that tin atoms occupy in the crystal lattice [19]. Finally, the presence of Sn surface species was identified and is usually interpreted as a monolayer of metallic tin deposited onto the metallic surface of Ru [18,20]. The hyperfine parameters of all different tin sites and their corresponding relative areas are summarized in Table 4. It is seen that all systems presented a similar distribution of tin species when supported on titania and no entity seemed to be preferentially generated. Nevertheless, the reactive experiments described initially in the present work showed a gradual increase in diol selectivity along with Sn/Ru ratio. These results allow the suggestion that a synergy between SnOx and TiOx species might indeed be taking place and would be responsible for the improved behavior of bimetallic HTR catalysts. 4. Conclusions The results obtained over RuSn/TiO2 catalysts reduced at high temperature showed that the production of 1,6-hexanediol is more effective at high Sn/Ru ratio, evidencing the main role played by the tin oxide species on the activation of ester carbonyl bond. The establishment of strong-metal–support interaction under this condition also showed to have a striking promoting effect over selectivity, suggesting a synergistic effect between reduced TiOx moieties and Snn+ species. Acknowledgment A.M.S. acknowledges the financial support from CAPES. References Fig. 8. 119Sn Mo¨ssbauer spectra of RuSn/TiO2: (A) Sn/Ru = 0.4; (B) Sn/Ru = 1; (C) Sn/ Ru = 2.
it should be mentioned that the low tin loading in samples with Sn/ Ru of 0.1 and 0.2 did not allow getting a satisfactory spectral resolution and thus any deconvolution attempt of Mo¨ssbauer spectra to differentiate the possible tin species would be highly speculative. These two samples were therefore not considered. Fig. 7 presents the spectra collected for the reduced reference sample 4.7 wt.% Sn/TiO2. It exhibited three spectral components, whose hyperfine parameters and relative resonance areas are collected in Table 4. Two quadrupole doublets are related to the Sn4+ and Sn2+ ionic species while a third single line present as the main component (69%) can be associated with a b-metallic Sn phase. The appearance of metallic tin would denote a weak interaction between the metal phase and titania in contrast with what has been observed for similar alumina-supported powders [4]. The experimental Mo¨ssbauer data obtained from the bimetallic RuSn catalysts required six components to fit the spectra
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
T. Turek, D.L. Trimm, Catal. Rev. Sci. Eng. 36 (4) (1994) 645. S.M. Santos, A.M. Silva, E. Jorda˜o, M.A. Fraga, Catal. Commun. 5 (2004) 377. S.M. Santos, A.M. Silva, E. Jorda˜o, M.A. Fraga, Catal. Today 107–108 (2005) 250. A.M. Silva, O.A.A. Santos, E.M. Baggio-Saitovitch, E. Jorda˜o, M.A. Fraga, J. Mol. Catal. A 253 (2006) 62. A.M. Silva, O.A.A. Santos, M.J. Mendes, E. Jorda˜o, M.A. Fraga, Appl. Catal. A 217 (2003) 155. T. Ekou, A. Vicente, G. Lafaye, C. Especel, P. Marecot, Appl. Catal. A 314 (2006) 73. A. Piccirili, Y. Pouilloux, Y. Pronier, S. Barrault, Bull Soc Chim Fr. 132 (1995) 1109. P. Gallezot, D. Richard, Catal. Rev. Sci. 40 (1&2) (1998) 81. P. Kluson, L. Cerveny, Appl. Catal. A 128 (1995) 13. V. Ponec, Appl. Catal. A 149 (1997) 27. M.J. Mendes, O.A.A. Santos, E. Jorda˜o, A.M. Silva, Appl. Catal. A 217 (2001) 253. M. Toba, S.-I. Tanaka, S.I. Niwa, F. Mizukami, Z. Koppa´ny, L. Guczi, K.-Y. Cheah, T.-S. Tang, Appl. Catal. A 189 (1999) 243. P. Ma¨ki-Arvela, J. Ha´jek, T. Salmi, D.Yu. Murzin, Appl. Catal. A 292 (2005) 1. Y. Pouilloux, F. Autin, C. Guimon, J. Barrault, J. Catal. 176 (1998) 215. E.A. Sales, J. Jove, M.J. Mendes, F. Bozon-Verduraz, J. Catal. 195 (2000) 88. H. Lieske, J. Vo¨lter, J. Catal. 90 (1994) 96. O.A. Barias, A. Holmen, E.A. Blekkan, J. Catal. 158 (1996) 1. L. Stievano, S. Calogero, F.E. Wagner, S. Galvagno, C. Milone, J. Phys. Chem. B 103 (1999) 9545. L. Ericksson, J. Lanner, Acta Crystallogr. E57 (2001) i85. B. Coq, A. Goursot, T. Tazi, F. Figueras, D.R. Salahub, J. Am. Chem. Soc. 113 (1991) 1485.