Aerobic oxidation of alcohols over carbon nanotube-supported Ru ...

134 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137Table 1Effects of Ru species, catalyst support, and Ru loading on the activity of the supportedruthenium catalysts for the selective oxidation of benzyl alcohol. a,b .Entry Catalyst Substrate/Ru Conversion (%)1 6.0 wt% RuCl 3/CNTs 11 102 RuO 2 17 153 6.0 wt% Ru/CNTs 17 984 6.0 wt% Ru/TiO 2 17 415 6.0 wt% Ru/AC 17 806 4.0 wt% Ru/CNTs 26 777 2.0 wt% Ru/CNTs 50 41a Reaction conditions: substrate (2 mmol), solvent (toluene: 10 mL, H 2O: 5 mL),85 ◦ C, p(O 2) = 0.1 MPa, 3 h.b Selectivity of benzaldehyde is 100%.supported one, indicating that CNT is a suitable support for alcoholoxidation reactions. As the Ru loading increases from 2 wt% to 6 wt%(entries 7, 6, and 4), the conversion of benzyl alcohol increases correspondingly,while the benzaldehyde selectivity remains at 100%,which is rationalized by the fact that even at higher catalyst loading,small clusters are still dominant.The effect of solvents on the activity of Ru/CNTs catalysts isshown in Table 2. Under identical reaction conditions, the catalyticactivity of Ru/CNTs in water is much higher than in typical polarand non-polar organic solvents such as ethanol and toluene. Whenwater is used as solvent, the main product is benzoic acid, implyingthat water promotes carboxylic acid formation. With tolueneas solvent, a higher conversion is achieved in comparison to thatobtained with the ethanol solvent, and at the same time, a muchhigher selectivity to benzaldehyde is also obtained. With ethanolas solvent, the main product is ethyl benzoate. It is known thatthe solubility of O 2 in the organic solvents is much higher than inwater. Obviously, the concentration of O 2 dissolved in a solvent isnot the governing factor for the remarkable changes in the oxidationof benzyl alcohol in water. In fact, it is water, a weak base,which plays an important role during the reaction [31]. Water canfacilitate H abstraction from alcohol, thus promoting the reactivity.Table 3Effect of bi-solvents on the catalytic activity of Ru/CNTs for the selective oxidationof benzyl alcohol. a .Toluene (mL) Water (mL) Conversion (%) Selectivity ofbenzaldehyde (%)10 1 67 10010 3 92 10010 5 98 10010 7 96 10010 10 67 100a Reaction conditions: benzyl alcohol (2 mmol), Ru/CNTs (Ru: 5.9 mol%), 85 ◦ C,p(O 2) = 0.1 MPa, 3 h.In addition, water can react with the aldehyde to form an aldehydehydrate, which further reacts with oxygen to form carboxylic acid.The effect of co-solvents on the catalytic activity of Ru/CNTscatalysts is shown in Table 3. With toluene as solvent, the conversionof benzyl alcohol is 50% and the selectivity to benzaldehyde is100%. When water is added, a substantial increase in the conversionof benzyl alcohol is obtained. When 5 mL of water and 10 mL oftoluene are used as solvents, the highest conversion of benzyl alcoholof 98% is achieved, and the TOF is 178 h −1 after the first 0.5 hof reaction. Nevertheless, more water is not favored, which is evidencedby the fact that the conversion of benzyl alcohol decreaseswhen more water is added to the system.In the reaction system adopted here, four phases are involved:vapor, organic phase, aqueous phase, and solid catalyst. The aqueousphase and the organic phase are mixed homogeneously byvigorous agitation during the reaction. The aqueous phase isdispersed homogeneously in the organic phase in the form of emulsiondroplets under stirring. Optical microscopy showed that theRu/CNTs catalysts assemble at the surface of the emulsion droplets(Fig. 5). It is known that lipophilic powder-like solids, such as carbonblack and graphite powder, can be used as emulsifying agentsfor a W/O emulsion, where the size of the solid particulates ismuch smaller than that of the dispersed phase. Furthermore, theTable 2Effect of solvents on the catalytic activity of Ru/CNTs for the selective oxidation of benzyl alcohol. a .Solvent Conversion (%) Selectivity (%)Benzaldehyde Benzoic acid EsterEthanol 7 31 0 69Toluene 50 100 0 0Water 100 30 70 0a Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (15 mL), 85 ◦ C, p(O 2) = 0.1 MPa, 3 h.Fig. 5. Optical micrographs of emulsion system a) during the process of reaction and b) after reaction formed with 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mLof toluene, and 5 mL of water. In order to clearly identify the interphases of the water phase and oil phase, we marked water and oil with methylene blue and methyl orange,respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)