Plant-mediated synthesis of Au–Pd alloy nanoparticles supported on MnO₂ nanostructures and their application toward oxidation of 5-(hydroxymethyl)furfural

Abstract

A facile and eco-friendly preparation method for Au–Pd bimetallic nanoparticles (NPs) supported on MnO₂ with two different morphologies was adopted with Cacumen platycladi leaf extract. The biosynthetic NPs, MnO₂ nano-structures and the resulting catalysts were characterized. The effects of Au–Pd ratio, catalyst size and reaction conditions were examined.

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1. Introduction

The rising cost, diminishing supply and environmental impact of fossil fuels have stimulated recent research activities for the utilization of renewable biomass resources for sustainable alternative energies and chemical feed stocks. It has been presented that one of the foremost natural sources of potentially relevant chemicals are sugars. 5-Hydroxymethyl-2-furfural (HMF), produced by fructose or glucose dehydration,¹ is regarded as a versatile key platform, which is potential to serve as the intermediate of fine chemicals, plastics, pharmaceuticals, and liquid fuels.^2–4 Therefore, there is overwhelming research on the oxidation of HMF in various catalytic systems in recent years.

Oxidation of HMF can generate some important chemicals such as 5-hydroxymethyl-2-furancarboxylic acid (HFCA), 2,5-furandicarboxylic acid (FDCA), and 2,5-diformylfuran (DFF). DFF, as one of the important oxidation products of HMF, is also considered as a potential chemical intermediate. It can serve as a precursor for the production of furan polymers,⁵ pharmaceuticals,⁶ antifungal agents,⁷ and renewable furan–urea resin.⁸ In this context, growing attention has also been paid to the synthesis of DFF from HMF. However, as the oxidation of HMF can result in several furan compounds, the selective oxidation of HMF to DFF is still needful. Therefore, many oxidants like BaMnO₄, pyridinium chlorochromate (PCC), NaOCl, and 2,2,6,6-tetramethylpiperidine-1-oxide (TEMPO) free radicals^9–11 have been employed. It has been reported that homogeneous metal bromide catalysts (MBr₂, M = Co(II), Mn(II), Zr(II)) at 70 bar oxygen pressure can gave a 99.7% HMF conversion with 61% DFF selectivity¹² in the oxidation of HMF. And Sá daba et al. recently reported that the aerobic oxidation of HMF catalyzed by zeolite supported vanadium could give rise to high DFF selectivity to 99% and HMF conversion to 84% under 10 bar oxygen pressure.¹³ However, many of these methods mentioned above have distinct drawbacks such as harsh conditions, releases of toxic waste, high costs of oxidants and recycling problems of homogeneous catalysts. Therefore, now molecular oxygen is really accepted as terminal oxidants and use of heterogeneous catalysts is strongly demanded for recycling purpose. Consequently, study for stable heterogeneous catalysts with high catalytic activities for aerobic oxidation of HMF under mild reaction conditions is still challenging.

Recently, our group has reported that Au/MnO₂ in DMF can smoothly oxidize HMF to DFF.¹⁴ However, during the oxidation process, we found some disadvantages, including (1) the activity of Au may be influenced by the formed products or intermediates such as carboxylic acid without a base promoter,¹⁵ (2) the loading method we adopted utilized not only chemical reductants but auxiliary stabilizers, and often require specialized and expensive equipment, (3) the oxidation process involved the use of DMF which is not environmental. Thus, to address problem (1), alloying of another metal with Au (e.g., Pd and Pt, but Nikolaos Dimitratos et al. showed that Au–Pt has a negative synergistic effect, so Pd was selected¹⁵) may serve as a solution as the alloying effect has been found in many aerobic oxidation reactions.^15–26 It can combine the functions of each component in the atomic level to enhance the activity and stability. For instance, Nikolaos Dimitratos et al.¹⁵ revealed that Au/Pd catalyst showed a positive synergistic effect. The alloy Au–Pd nanoparticles have lattice parameters between those of bare Au and of Pd. Such modifications also cause changes in surface interatomic distances and thus in the electronic structure. Then modifications in chemical properties of bimetallic systems are caused by electronic interaction between the components as well as geometric effects due to changes in lattice constants. It is very difficult to disentangle these two effects experimentally, but, depending on the direction of these two effects, two different types of bimetallic systems can be obtained. In Au–Pd bimetallic system, the qualitative effects were in the same direction, we would expect superior properties compared with monometallic systems. Moreover, Hutchings et al. also have demonstrated the supported Au–Pd alloy catalysts showed excellent performances for many oxidation reactions such as the direct synthesis of H₂O₂ and the selective oxidation of alcohols or hydrocarbons.^16,19–26 Accordingly, to solve problem (2), a biological approach that utilizes the “nature factory” (namely, plants and microorganisms) which emerges as a novel method for the synthesis of various metal nanostructures^27–31 was adopted. As for problem (3), DMF was replaced by water. And two other MnO₂ morphologies were synthesized here as the support.

Herein, we report a much more environmentally benign catalyst for the oxidation of HMF. To the best of our knowledge, it's the first time that alloy Au–Pd nanoparticles were supported on MnO₂ and act as the catalyst (Au–Pd/MnO₂) for HMF oxidation to DFF. The whole reaction is totally green, free of chemical reductant, organic solvent or toxic oxidant under mild conditions with oxygen.

2. Experimental section

2.1 Preparation of Cacumen platycladi leaf extract³²

The Cacumen platycladi leaves were milled, and 1.0 g of the milled powder was dispersed in a 250 mL conical flask with 100 mL of deionized water and kept in a water bath shaker at 30 °C. After 2 h, the mixture was filtered to obtain the extract.

2.2 Preparation of MnO₂ microspheres and microcubes³³

To realize the synthesis of MnO₂ microspheres and microcubes, MnCO₃ microspheres and microcubes were prepared first. MnCO₃ microspheres were produced by a simple mixing method. Typically, manganese sulphate (1 mmol) and ammonium hydrogen carbonate (10 mmol) were separately dissolved in water (70 mL). Ethanol (7 mL) was then added to the MnSO₄ solution with stirring, and after its complete dispersion, the NH₄HCO₃ solution was added to the mixture mentioned above at room temperature. After about 3 min the solution turned milky white, which indicated the initial formation of MnCO₃ microspheres. The mixture was maintained at room temperature for 3 h and the color of the reaction solution became milky white mixed with light purple. MnCO₃ microspheres obtained were separated from the reaction mixture by centrifugation, and washed several times with ultrapure water and ethanol to remove impurities. Finally, MnCO₃ microspheres were dried at room temperature in a vacuum oven (ca. 0.1 MPa) for 6 h prior to being characterized. MnCO₃ microcubes were synthesized by adding the (NH₄)₂SO₄ (10 mmol) into the initial mixture and aged for 7 h in water bath at 50 °C. MnO₂ microspheres and microcubes were prepared by mixing different quantities of aqueous 0.032 M solution of KMnO₄ (Beijing Chemical Reagent Ltd, China) and the solid MnCO₃ crystals with different morphologies.

2.3 Loading of Au–Pd NPs on MnO₂ microspheres and microcubes

10 mL of the aqueous solution mixture of HAuCl₄ and Pd(OAc)₂ (the concentration of the total metal is 5 mM and the amount of HAuCl₄ and Pd(OAc)₂ could be adjusted depending on the desired mole ratio) was heated to 90 °C in an oil bath with a constant temperature rate and a stirring rate of 600 rpm. Subsequently, 0.99 g MnO₂ and 10 mL of the Cacumen platycladi leaves extract was added to this precursor solution with vigorous stirring for 2 h, and the final products were filtered, washed thoroughly with distilled water and dried (110 °C, 16 h) to obtain the final catalyst accordingly (calculated loading amount is between 1% wt and 2% wt). The composition of Au–Pd NPs could be controlled by changing the ratio of HAuCl₄/Pd(OAc)₂. For comparison, the monometallic NPs were prepared in the same manner by substituting a mixture of HAuCl₄/Pd(OAc)₂ solution with HAuCl₄ (5 mM) or Pd(OAc)₂ (5 mM) solutions. The catalyst was characterized by X-ray diffraction analyser (Bruker D8), X-ray photoelectron spectrometer (D8 Bruker Avance), surface area and particle size distribution analyzer (ASAP2020), energy dispersive X-ray spectroscopy (EDX).

2.4 Aerobic oxidation of HMF

The oxidation of HMF was performed on a laboratory scale in a 25 mL round-bottomed glass flask equipped with a reflux condenser, a magnetic stirrer, and a gas inlet, allowing a flow rate of oxygen (20 mL min⁻¹) to bubble through the reaction mixture. HMF (1 mmol, 126 mg) was added to 20 mL water. Then catalyst (148 mg) was added to the reaction system and flushed with pure oxygen for 5 min to dispel the remaining air in the system. Thereafter, the reactor was placed in a preheated oil bath (set to a fixed temperature) to start the reaction. After the reaction, the products in the reaction mixture were analyzed by high performance liquid chromatography instrument (HPLC) (Bruker HPLC-450). And the residual Au, Pd and Mn contents after centrifugation for recycling were determined with Inductively Coupled Plasma Mass Spectrometer (ICP-AES) (OPTIMA5300DV).

3. Results and discussion

3.1 Characterization

In order to characterize Au–Pd nanoparticles and Au–Pd/MnO₂ catalyst, TEM and SEM were both employed. As shown in Fig. 1, TEM images of Au–Pd NPs with different Au/Pd molar ratio revealed that Au–Pd NPs are well dispersed with different size distributions. And the particle size of Au–Pd NPs decreased along with the decreasing Au/Pd molar ratio. In other words, the higher concentration of Pd precursor resulted in smaller bimetallic particles formed. This phenomenon was identical with the previous work done by Hutchings et al.³⁴ Then scanning electron microscopic (SEM) images showed the morphology of the as-synthesized MnO₂ products in Fig. 2 (see (a), (b) and (d), (e)). Apparently, they have a microsphere/cubic morphology. The average size of the support is about 2 μm. After loading of Au–Pd NPs, TEM images were taken (see Fig. 2c and f). It can be observed that Au–Pd NPs have sticked to the supports successfully. XRD patterns of as-prepared products before and after the loading of Au–Pd nanoparticles were also collected. As shown in Fig. 3, intense peaks at 38.3°, 44.5°, 64.7° and 77.8° of Au–Pd/MnO₂ were found, which indicates successful loading of Au–Pd nanoparticles. Diffraction peaks at 44.5°, 64.7° and 77.8° are attributed to crystalline gold. Peaks in 38.3° located between the values of pure Au crystal (111) at 38° and pure Pd crystal at 40.0° can be assigned to the Au–Pd alloy phase. Such results further verified the alloy structure of the as-synthesized NPs. Additionally, it can be also noted that the XRD pattern of the recycled catalyst is the same with the original catalyst, revealing that characteristics of the catalysts are stable. Then, the composition of the Au–Pd bimetallic NPs was measured by scanning TEM (STEM)-EDX mapping (shown in Fig. 4) to investigate the element distribution of the NPs. The colored elemental mapping images show that Au atoms (yellow) and Pd atoms (green) were homogeneously distributed over the entire NP structure. Therefore, the Au–Pd bimetallic structure was alloyed in nature. Again, EDX elemental line scanning on a single particle (Fig. 4D) confirmed that the bimetallic NP was composed of two alloyed metal elements. Afterwards, the BET surface areas and average pore sizes of the samples were determined by the adsorption isotherms of N₂ at −196 °C using a Micromeritics ASAP 2020 instrument. The samples were outgassed under vacuum at 150 °C for 10 h, prior to the adsorption measurements. As summarized in Table 1, MnO₂ microsphere structure has greater pore volume, which may be beneficial to the loading of Au–Pd nanoparticles. And after the loading of Au–Pd NPs, the surface area of these structures increased because of the Au–Pd NPs in the exterior. On the contrary, pore diameters decreased due to Au–Pd NPs entering into the carrier and sticking to pores.

Fig. 1 TEM images of Au–Pd NPs with different Au/Pd molar ratio (Au : Pd = 7 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 7, respectively).

Fig. 2 SEM images of MnO₂ supports with different morphologies: (a), (b), (d) and (e); TEM images of Au–Pd/MnO₂: (c) microsphere MnO₂; (f) cubic MnO₂.

Fig. 3 XRD patterns of pure MnO₂ [1], related catalyst [2] and recycled catalyst [3]. Mole ratio of Au : Pd = 2 : 1.

Fig. 4 STEM image of the Au–Pd bimetallic NPs (A); EDX elemental maps of Au (B) and Pd (C) concentrations in the NPs; cross-sectional compositional line profiles of a single Au–Pd NP (D).

Table 1 BET results of MnO₂ supports with different morphologies

Material	Support		Catalyst
	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
Microsphere MnO2	20	0.6	23	0.2
Cubic MnO2	15	0.3	19	0.1

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3.2 Oxidation of HMF with different Au–Pd mole ratios and MnO₂ morphologies

As illustrated in Fig. 5, the catalytic activity can be affected by different Au–Pd mole ratios and morphologies of MnO₂. For comparison, Au/MnO₂ and Pd/MnO₂ were also tested. In all the cases, catalysts with microsphere MnO₂ as the support contributes to higher conversions, which may be due to the larger surface areas and pore volumes. And Au/MnO₂ (microsphere or cubic) with Au as the active component leads to a low conversion of HMF while Pd/MnO₂ exhibits good performance. The conversions of HMF are between 63–76% and the selectivities for DFF are over 90% in all the cases. When Au/Pd molar ratio was 2 : 1, the conversion of HMF is better than the monometallic Pd/MnO₂. The synergistic interaction between Au and Pd might be responsible for the enhanced HMF conversion as addition of small amounts of gold to palladium markedly enhanced the activity of supported Au–Pd NPs because of electronic effect.³⁴ However, no linear correlation between the Au ratio and the conversion of HMF was found. In other words, it's not the situation that the more the Au ratio, the better. As shown in Fig. 1, the higher concentration of Pd precursor resulted in smaller bimetallic particles formed. Therefore, when the mole ratio of Au–Pd gets to 1 : 7, the particle size of the alloy NPs restricts the activity of the catalyst, leading to lower conversions. Therefore, it can be seen that the HMF conversion was associated with both the Au/Pd molar ratio and the alloy size.

Fig. 5 Catalytic performance of catalysts with different Au–Pd ratios supported on microsphere/cubic MnO₂. Reaction conditions: 90 °C, 6 h, 1 atm.

3.3 Oxidation of HMF under different reaction temperatures and bio-reduction temperatures

As shown in Fig. 6, the conversion of HMF increased along with the increase of the reaction temperature and bio-reduction temperature while the selectivity for DFF remains high all the time. Poor conversions (below 10%) were obtained when the reaction temperature is below 70 °C. With the elevating temperature, conversions of HMF were remarkably enhanced to 76% when the reaction temperature reached 90 °C due to the endothermic nature of the reaction. However, when temperature kept increasing to 100 °C, the conversion stopped increasing. Obviously, higher reaction temperature promoted the reaction process and the moderate reaction temperature of 90 °C provided the highest conversion. As to the effect of the bio-reduction temperature, it can be noted that the conversion increased but the selectivity to DFF remained almost the same with the increasing preparation temperature. Therefore, high preparation temperature favored the reaction as the conversion increased from 14% to 76% when the temperature was switched from 40 °C to 90 °C.

Fig. 6 Catalytic performance of catalysts under different reaction temperatures and bio-reduction temperatures. Reaction conditions under different reaction temperatures: 90 °C (bio-reduction temperature), 6 h, 1 atm, microsphere support, Au : Pd = 2 : 1. Reaction conditions under different bio-reduction temperatures: 90 °C (reaction temperature), 6 h, 1 atm, microsphere support, Au : Pd = 2 : 1.

3.4 Oxidation of HMF using analogous catalysts

The comparison of analogous Au–Pd based catalysts were shown in Table 2. Au–Pd/MnO₂ we reported herein is superior to other conventional catalysts using chemical reduction methods (like NaBH₄, etc.), and showed higher conversions than bare Au–Pd (2 : 1) catalyst. This may be because the support MnO₂ itself can be a little useful to catalyze HMF with a 3% conversion and MnO₂ provides a large surface area for grafting Au–Pd nanoparticles that enhances the activity. On the other hand, all the Au–Pd alloy catalysts resulted in better conversions of HMF, but catalyst in this work is the most mild and environmental. Therefore, from the aspects of catalytic performance and environmental issues, our catalyst here is more efficient compared to others in the literature.

Table 2 Comparison of different catalysts for the oxidation of HMF^a

Catalyst	Conversion (%)	Reaction temperature (°C)	Reaction time (h)	Reference
a Catalysts were prepared according to the given references. Reaction conditions: HMF (1 mmol, 126 mg), catalyst (148 mg). Determined by HPLC analysis.
Au–Pd/MnO2	76	90	6	This work
Au/MnO2	29	90	6	This work
Au/MnO2	65	120	8	Our previous work^14
Pd/MnO2	70	90	6	This work
MnO2	3	90	6	This work
Au–Pd (2 : 1)	63	90	6	This work
Au–Pd/TiO2	68	90	6	35
Au/TiO2	3	90	8	36
Au/SBA	5	110	8	37
Au–Pd/SBA	71	110	6	38

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3.5 Durability of the bioreduction catalysts

The recyclability of the catalyst was examined first and results were summarized in Table 3 (entry 1 and 2). From the first run to the fifth run, the activity and selectivity had no obvious decreases. All five cycles achieved about 76% of the conversion and 98% of the selectivity. Throughout five cycles, the catalysts could be recycled through centrifugation. The whole separation and recycling process is suitable for all the synthesized catalysts.

Table 3 The residue contents of Au, Pd, Mn in the mixture after centrifugation^a

Entry	Item	Cycle 1^st	Cycle 2^nd	Cycle 3^rd	Cycle 4^th	Cycle 5^th
a Reaction conditions: HMF (1 mmol, 126 mg), catalyst (Au : Pd = 2 : 1) (148 mg), 90 °C, under O2 atmosphere, 6 h. Determined by HPLC analysis.
1	Conversion of DMF (%)	76	76	76	75	76
2	Selectivity for DFF (%)	98	98	98	98	98
3	Au in the mixture (μg mL^−1)	0.012	0.017	0.017	0.016	0.016
4	Pd in the mixture (μg mL^−1)	0.003	0.005	0.007	0.006	0.006
5	Mn in the mixture (μg mL^−1)	0.031	0.034	0.035	0.034	0.034

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Next, in order to test the stability of the catalysts, the residual Au, Pd and Mn contents after centrifugation were determined with ICP-AES. As summarized in Table 3, the Au, Pd concentration was only 0.012 and 0.003 μg mL⁻¹ respectively in the first run (Table 3, Cycle 1^st). In the next four runs, the contents of Au, Pd slightly increased and then remained constant (Table 3, entry 3 and 4). The results indicate that Au–Pd nanoparticles are stable for at least five cycles. Likewise, contents of Mn (Table 3, entry 5) in the mixture are almost constant through five cycles, which demonstrate the stability of the support. Therefore, the stability of the whole catalyst is excellent.

4. Conclusions

In summary, the new bio-reduction method applied in the loading process achieved great efficiency without any harsh conditions or pollutions. And bimetallic Au–Pd NPs supported on MnO₂ afford high conversions and selectivities under milder conditions. The best reaction condition was obtained here, including bio-reduction temperature of 90 °C, reaction temperature of 90 °C, Au : Pd mole ratio of 2 : 1. And the catalyst is proved to have great stability and recyclability for at least five runs. The whole process is totally clean, efficient and environmental.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13157j

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