Catalytic activity of bimetallic AuPd alloys supported MgO and MnO₂ nanostructures and their role in selective aerobic oxidation of alcohols

2.1 Preparation of MgO and MnO₂

The synthesis of MgO is described in an earlier paper (Ding et al., 2001). Briefly, 1 M of MgCl₂.6H₂O (Aldrich) solution was prepared in 100 ml of de-ionized water. Then 1 M of NH₄OH solution was added at 100 °C and followed by stirring at 25 °C for 2 h. After that, the solution was kept for ageing for 24 h. The resulting white precipitates were filtered and washed with de-ionized water and then ethanol (Aldrich) for several times to remove the by-products and/or impurities. The filtered cake was dried in air at 100 °C for 4 h. The as synthesized samples were calcined at 400 °C in static air for 3 h to obtain MgO nanoparticles.

Well-stable MnO₂ nanoparticles were prepared by directly stirring 5.68 g of hydrated manganese sulphate MnSO₄.H₂O (Aldrich) in 66 ml of water at room temperature until dissolved. Then, 7.67 g of ammonium persulfate (NH4)₂S₂O₈ (Aldrich) were added to the solution while stirring until a clear solution was formed. The solution was transferred to a sealed autoclave (Baskerville, 125 ml) and heated at 120 °C for 72 h. Following the reaction, the resulting black material was recovered by filtration then washed by water (0.5 L) and dried at 60 °C for 24 h.

2.2 Catalyst preparation

The aim of this study is to prepare and test a new Au-Pd catalyst immobilized onto MgO and MnO₂. To understand the motivation and progression of this work, highlights of the important preparation parameters of catalyst are mentioned here. Full details of catalyst preparation protocols can be found in literatures (Pritchard et al., 2010). 1% Au-Pd catalysts were synthesised using the sol-immobilisation method. Typically, a freshly prepared solution of PVA (1 wt%, Aldrich, Mw = 10,000, 80% hydrolysed) was added to an aqueous solutions of PdCl₂ (Johnson Matthey, 6 mg in 1 ml) mixed with HAuCl₄.3H₂O (Johnson Matthey, 12.25 g in 1000 ml) by stirring for 15 min (PVA/metal 0.65 w/w). 0.1 M of NaBH₄ [Aldrich, NaBH₄/(Au + Pd) (mol/mol = 5)] was prepared freshly and then added to form a dark brown solution. Then, it was stirred for 30 min and the pH value was adjusted to 1 by adding H₂SO₄. Then, 1.98 g of support (MgO or MnO₂) was added to the mixture and stirred for 1 h. The catalyst was then recovered by filtration, washed with distilled water (2 l) and dried at 110 °C for 24 h. Based on the calculation, the weight percentage of Au and Pd in the metal alloy are 0.65 and 0.35 respectively .

2.3 Catalyst testing

The Radleys carousel reactor (50 ml glass stirred reactor) was employed for the liquid phase benzyl alcohol and 1-octanoloxidation. The supported Au-Pd catalyst (200 mg) was suspended in the substrate (2 g) under solvent-free conditions. The experimental method used here is similar to the standard reaction protocols that are used for solvent-free oxidation of benzyl alcohol which was reported earlier (Morad et al., 2014). The reaction mixture was stirred at 1000 rpm and the oxygen pressure was kept at 1 bar. The pressure was maintained during the reaction by using pressure gauge to avoid any pressure fluctuations. After a specific reaction period (Morad et al., 2014), a sample was taken for analysis by using gas chromatography (GC - Varian star CP-3800) with CP-wax 52 column and a flame ionization detector (FID). The catalyst loading (mol%) has been calculated and found be 0.356% and 0.429% for benzyl alcohol and 1-octanol respectively.

2.4 Catalyst characterization

Powder X-ray diffraction was conducted using a PANalyticalX’Pert Pro with a CuK_α X-ray source operated at 40 kV and 40 mA fitted with an X’Celerator detector. The XRD patterns of MgO and MnO₂ are shown in Fig. 1. BET surface area was measured using nitrogen adsorption and a micrometrics surface area analyser. Samples were prepared for Transmission electron microscopy (TEM) examination by dispersing the catalyst powder. TEM was performed using a Jeol 2100 with a filament operating at 200 kV. The TEM images of the recovered catalyst are reported in Fig. 2. The morphology of the catalysts was observed by scanning electron microscopy (SEM) using a Carl Zeiss EVO 40 SEM fitted with a BSD, an Everhart-Thornley detector, and a variable pressure chamber with a tungsten source. SEM images of the catalyst are presented in Fig. 3.

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Fig. 1

The XRD pattern of MgO and MnO₂.

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Fig. 2

SEM image of A) MgO and B) MnO₂.

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Fig. 3

TEM image of A) MgO and B) MnO₂.

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3.1 Synthesis of MgO and MnO₂

X-ray diffraction patterns of the synthesised MgO and MnO₂ materials are illustrated in Fig. 1. For MgO sample, the figure shows that the sample essentially consists of MgO phase. The peaks at 2θ = 36.86°, 42.82° and 62.09° are related to (1 1 1), (2 0 0) and (2 2 2) planes of MgO, respectively (Boo et al., 1999). For MnO₂ sample, the presence of the diffraction peaks at 28.88°, 37.32°, 42.62°, 56.65°, 59.10°, 64.96°, 67.15° and 72.30° confirmed the formation of β-MnO₂ phase (Liang et al., 2008). It was noticed after the electrochemical experiments that the β-MnO₂ nanomaterials showed different specific capacitance (Cs) values which allows the β-MnO₂ to exhibit an excellent long-term stability (Ede et al., 2014).

The morphology of the prepared MnO₂ and MgO samples is shown in Fig. 2. The SEM images of MgO in Fig. 2A show that the sample contains agglomerated nanoparticles with the particle size of about 100 nm. The obtained MnO₂ sample contains nanowires with an average length of 1 µm and average diameter of 50 nm as shown in Fig. 2B. The TEM images in Fig. 3 illustrate the size of each material and it is obvious from the figure that both synthesised materials are in nano scale.

The surface areas of the synthesised MgO and MnO₂ samples and their average pore diameters are shown in Table 1. MgO nanoparticles sample exhibits higher surface area than MnO₂ nanowire sample. The BET surface area and pore diameter distribution are shown in Fig. 4. It can be observed from the figure that MnO₂ sample has narrower pore diameter distribution than MnO₂.

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Fig. 4

BET surface area and pore diameter plots. Upper panel for MgO. Lower panel for MnO₂.

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3.2 Oxidation of benzyl alcohol using 1% Au-Pd/MgO and 1%Au-Pd/MnO₂

Synthesised sol-immobilised Au-Pd/MgO and Au-Pd/MnO₂ catalysts were evaluated for the oxidation of benzyl alcohol (Table 2). Both catalysts exhibited good catalytic activity towards benzyl alcohol oxidation. Under the optimised reaction conditions, there was a stable increase in the conversion with reaction time for both catalysts. However, Au-Pd/MnO₂ catalyst showed better activity than Au-Pd/MgO catalyst. This can be attributed to the Au-Pd particle size distribution. Fig. 5 demonstrates that although Au-Pd/MgO catalyst has smaller mean particle size (mean = 4.43 nm), Au-Pd/MnO₂ catalyst has narrower particle size distribution (the majority of particle size was between 2 and 5 nm) unlike the Au-Pd/MgO catalyst. The size of the gold particles is the major factor that determines the catalyst performance in addition to other factors such as oxidation state of gold, synthesis method and the choice of catalyst support. These factors are, in the case of supported gold catalysts, strongly influenced by the morphology of the support and as a result the catalyst activity is influenced by the support properties as well (Alhumaimess et al., 2014). The main observation was that both MgO and MnO₂ supports almost switched off the disproportionation reaction and hence the toluene production. It is important to note that it is well known that the disproportionation of benzyl alcohol has been identified as the source of toluene formation in the solvent-free oxidation of benzyl alcohol using supported gold palladium catalysts. There is a slight increase in the disproportionation reaction, and hence the toluene selectivity, when this reaction is performed in a continuous mode (Cao et al., 2013).This is in agreement with Hutchings' group work (Enache et al., 2006; Sankar et al., 2012; Bartley et al., 2012) which illustrated that basic supports completely switch-off this disproportionation reaction, whereas acidic supports like activated C, Nb₂O₅ and Al₂O₃ promote this disproportionation reaction. The effect of the reaction time on selectivity is shown in Table 2. It can be seen from the Table 2 that the reaction with both catalysts resulted in >98% selectivity for benzaldehyde with increasing conversion. It should be mentioned that Hutching's group did not investigate MnO₂ support for benzyl alcohol oxidation in their work (Hutchings and Kiely, 2013; Sankar et al., 2011, 2012; Enache et al., 2006; Bartley et al., 2012).

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Fig. 5

Particle size distributions and TEM images of A) Au-Pd/MgO and B)Au-Pd/MnO₂ prepared by sol-immobilisation method.

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3.3 Oxidation of 1-octanol using 1% Au-Pd/MgO and 1% Au-Pd/MnO₂

Having evaluated the catalysts performance for the oxidation of aromatic alcohol, the obtained catalysts were then evaluated for the oxidation of the aliphatic alcohol for 1-octanol oxidation (Table 3). Under the optimised conditions showed in Table 2 both catalysts illustrated a steady increase in conversion as the reaction time increased. Moreover, the catalyst prepared by using MnO₂ as support showed enhanced activity for 1-octanol oxidation compared to MgO catalyst, which could be due to the narrower gold and palladium nanoparticle distribution shown in Fig. 5 as explained above in the case of benzyl alcohol oxidation.

Table 3 also shows the reaction time effect on the selectivity for 1-octanol oxidation using Au-Pd/MnO₂ and Au-Pd/MgO catalysts. The Au-Pd/MgO achieved the highest selectivity toward 1-octanal (100% selectivity to 1-octanal) whereas Au-Pd/MnO₂ catalyst achieved significant selectivity toward 1-octanoic (15%) acid after 2 h of the reaction time. Furthermore, the selectivity toward 1-octanal remained constant with the reaction time (100%) when using Au-Pd/MgO catalyst unlike the Au-Pd/MnO₂ catalyst, which showed a significant decrease in selectivity toward 1-octanal and hence increase the production of 1-octanoic acid. For the Au-Pd/MnO₂ catalyst, at short reaction times (2 h) the selectivity towards the 1-Octanal is around 80%, however, the selectivity towards 1-Octanal decreases dramatically as the reaction proceeded suggesting that this may oxidised to 1-octanoic acid. The selectivity of 1-octanoic acid increased with reaction time, which is likely to be a result of the oxidation of 1-Octanal, which remains quite constant throughout the reaction time, suggesting that it is oxidised at a similar rate to which it is formed.