Preparing acid-resistant Ru-based catalysts by carbothermal reduction for hydrogenation of itaconic acid

Abstract

Catalytic conversion and application of bio-based platform chemicals is of great significance. Itaconic acid, one of the abundant and renewable bio-based platform chemicals, could be hydrogenated to produce methylsuccinic acid, which has important applications in pharmaceutical synthesis. Traditional catalysts can be corroded, leached and deactivated in an acid reaction environment, and the stability of the catalysts is a great challenge. Carbothermal reduction was employed to prepare stable ruthenium-based catalysts, and the results confirmed that CO generated in situ functioned as an efficient reducing species in the carbothermal reduction process. The catalysts were applied for the hydrogenation of itaconic acid, and showed good stability and resistance to acid with no obvious loss of activity and less leaching in recycling tests compared to hydrogen reduced samples which exhibited an apparent decrease of 22% for the conversion of itaconic acid. The carbothermal reduced catalysts could maintain 99% conversion and 99% selectivity for methylsuccinic acid, and maintained good stability and acid-resistance under a higher temperature and longer reaction time.

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Introduction

Itaconic acid, one of the abundant bio-based platform chemicals, is an important renewable unsaturated organic acid that can be easily obtained from fermentation of biomass-derived carbohydrates.^1,2 It has great potential as a key substrate to produce methylsuccinic acid through hydrogenation, which has chirality and extensive applications in organic synthesis especially for pharmaceutical synthesis.^3,4

Direct hydrogenation of unsaturated organic acids to produce saturated carboxylic acids is an essential and significant reaction, which has been studied extensively.^1,5 Homogeneous metal catalysts are widely applied affording good catalytic performance.⁶ However, the costs of metal complexes and the challenges in recovering and reusing them, make it difficult to meet the needs of the application. Using heterogeneous metal catalysts could be an alternative for avoiding these problems.^7–9 Nevertheless, there is an important issue that the stability of the catalyst is hard to guarantee in the acid reaction environment. This lack of stability may result from acid corrosion and leaching of the active component, which leads to deactivation of the catalyst.^10,11 Developing stable heterogeneous catalytic systems with good resistance to acid has gained lots of attention and has become a key issue for the direct hydrogenation of organic acids. Liu et al. prepared a silica-supported Pt complex for the hydrogenation of itaconic acid, which showed about 90% product yield. The yields were affected by the number of times the catalyst was used, and decreased by 9% after fourth time.¹² Catalysts of Rh-BPPM and Ru-BINAP heterogenised on hexagonal mesoporous silica were prepared for the hydrogenation of itaconic acid in Jamis’s work. The catalysts showed more than 10% loss of activity after being recycled three times.^13,14 This trend of loss of the catalyst activity during recovery and reuse is consistent with that observed in a number of reports.^15–17 It is important and essential to develop heterogeneous catalysts with good acid-resistance for hydrogenation of organic acids.

Carbothermal reduction has provided an opportunity to develop novel potential methods for preparing highly efficient and stable catalysts in recent reports.^18–20 Catalysts prepared by this method have a uniform dispersion of nanoparticles, which results in higher catalytic activity.^21–23 More importantly, the stability of these catalysts was studied with remarkable results from the recycling tests such as negligible loss of activity, unnoticeable leaching and a lack of aggregation.²⁴ Su et al. prepared Ru catalysts supported on carbon substrates using a thermal reduction method, and these highly active catalysts showed good stabilities for the hydrogenation of monoaromatic compounds.^25,26 The carbothermal reduction process is complex and has much importance for the reduction of the metal species, the interactions between the metal and support, the stability and so on. Further research of this process could help to obtain a better understanding of carbothermal reduction and its influence. In the work of Su et al., it was observed that the carbon species played a vital role in the reduction and formation of Ru–carbon contacts in the reduction process, which may anchor and immobilize the Ru particles.²⁶ Yang et al. prepared cobalt-based catalysts via carbothermal reduction, and they proposed that the diffused oxygen atoms could react with the carbon support yielding CO.^27,28 We have prepared Ni/AC catalysts with a carbothermal reduction method and applied them in the hydrogenolysis of a lignin model compound. We considered that nickel nitrate was decomposed and then reduced to metallic Ni with oxidization of carbon to CO_x in the carbothermal reduction process.^29,30 It is noteworthy that there is a common phenomenon that CO was discovered in the reduction process, but the function of CO has been rarely researched. Also, this reduction process has not been monitored in situ, and the participating or dominant reducing species in the carbothermal reduction along with its specific function remain unclear.

In the reported study, carbothermal reduction was employed to prepare stable ruthenium based catalysts for hydrogenation of itaconic acid. The catalytic performance and acid-resistance of the carbothermal reduced Ru-based catalysts were tested and compared with hydrogen reduced ones. The intermediate carbon containing species – CO released in situ – was detected in the carbothermal reduction process, and the function of this efficient reducing agent was verified as well. The stability and resistance to acid of the carbothermal reduced Ru-based catalysts are discussed with a possible postulation associated with the function of the CO generated in situ proposed.

Experimental

Catalyst preparation

The 5 wt% Ru based catalysts were prepared using an incipient wetness impregnation method. RuCl₃ (Shenyang Nonferrous Metal Research Institute) was dissolved in deionised water, and then activated carbon (AC, coconut shell, 80–100 mesh, Aladdin Industrial Inc.) was added into the above solution under stirring. The above sample was treated with ultrasound for 3 minutes before impregnation for 24 h, and after that the slurry was dried in an oven at 110 °C overnight under an air atmosphere. This sample was named as Ru(Cl)/AC. The sample was then treated at a desired temperature of 250 °C, 350 °C or 450 °C under an inert atmosphere such as N₂, He or Ar, and then kept under these conditions for 3 h, which was the carbothermal reduction process. Such samples were named as Ru/AC-C-250, Ru/AC-C-350, and Ru/AC-C-450 for example. In general, Ru(Cl)/AC carbothermally reduced at 450 °C under N₂ was named simply as Ru/AC-C, and that reduced under H₂ via the same temperature program was named Ru/AC-H. AC calcined at 900 °C under N₂ for 3 h was named AC900. AC physically mixed with fumed SiO₂ (Aladdin Industrial Inc.) at a molar ratio of 1 : 9 was named AC + SiO₂. Ru(Cl)/AC900, Ru(Cl)/SiO₂ and Ru(Cl)/(AC + SiO₂) were prepared the same as for Ru(Cl)/AC.

Catalyst characterization

The texture properties were obtained from nitrogen adsorption–desorption experiments conducted at −196 °C on a Quantachrome Autosorb-1. The samples were outgassed at 300 °C for over 6 h before measurement. The surface area was obtained using the Brunauer–Emmett–Teller (BET) model for adsorption data in a relative pressure range of 0.05 to 0.30. The total pore volume was obtained at P/P_o = 0.99, while the micropore volume and mesopore volume were calculated using the HK and BJH method respectively. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/Max 2500PC diffractometer with Cu-Kα radiation (λ = 1.5418 Å) at a scan rate of 5° min⁻¹ from 10° to 85° (2θ) at 40 kV/200 mA. The surface morphology of catalysts was determined using transmission electron microscopy (TEM) with a JEM-2000EX microscope operated at 120 kV. Surface chemical analysis was performed using X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 250Xi spectrometer using Al-Kα radiation (E_hν = 1486.6 eV) operated at 15 kV and 10.8 mA as the excitation source. The content of Ru was measured with a Perkin-Elmer ICP-OES 7300DV and calculated from the average of two parallel samples.

The reduction process and property profiles of the catalysts were investigated using a temperature-programmed reduction (TPR) with hydrogen, carbon monoxide and inert gas performed on a Micromeritics Auto Chem II 2920 Chemisorption Analyzer with both a thermal conductivity detector (TCD) and mass spectrometry (MS) detector. The catalyst samples were first pretreated at 200 °C under an argon flow to drive away the moisture and impurities. Afterwards, a H₂/Ar, CO/He or He gas stream (25 cm³ min⁻¹) was switched on and the temperature was raised from 50 to 800 °C. A method for in situ chemical absorbing and analyzing of the gas released in the reduction process was designed using a chemical absorption reagent and titration methods (shown in Scheme S1†).

Catalytic hydrogenation of itaconic acid

The catalytic reaction was performed in a 50 mL autoclave (T316 Stainless Steel, Parr Instrument) equipped with an electronic temperature controller and a magnetic stirrer. In a typical reaction, a 6 wt% itaconic acid aqueous solution and the reduced catalyst in a desired molar ratio were introduced into the reactor, which then was purged several times with highly pure H₂ and subsequently pressurized up to 10–30 bar with H₂ after sealing. It was then heated to 50–100 °C and kept at this temperature for 0.5–1 h with vigorous stirring. As soon as the reaction stopped, the autoclave was cooled to room temperature using a cold water bath.

The products were collected and analyzed using a HPLC system (Waters e2695) equipped with an UV/visible detector (Waters 2489) and a refractive index detector (Waters 2414). Itaconic acid and methylsuccinic acid were analyzed using a Waters Atlantis T3 column at 35 °C, and a 0.013 mol L⁻¹ H₃PO₄ aqueous solution with 15% methanol was used as the mobile phase with a flow rate of 0.5 mL min⁻¹. The RID temperature was 30 °C and the UV wavelength was 210 nm. The itaconic acid conversion and the selectivity for methylsuccinic acid were calculated with a formula using the internal standard calibration curves (shown in S4†).

Results and discussion

Characteristics and properties of the prepared catalysts

The textural characteristics of the supports and catalysts were evaluated using nitrogen adsorption–desorption measurements and the results are listed in Table 1. A high BET surface area of about 1130 m² g⁻¹ accompanied with a total volume of 0.60 cm³ g⁻¹ were obtained for the AC sample. While the sample Ru(Cl)/AC had a much lower BET surface area and a smaller total pore volume, due to the blockage of some pores by the metal precursor ruthenium trichloride hydrate solution being impregnated on the AC. However, the Ru/AC-C and Ru/AC-H samples had similarly high BET surface areas of around 1080 m² g⁻¹, which were a little less than that of the support but much better than Ru(Cl)/AC as a consequence of the metal precursor being reduced and the blockage removed. In addition, it is notable that Ru/AC-C had a total pore volume of 0.70 cm³ g⁻¹, which was larger than the others. That could be proposed to be as a result of reduction of the metal species in or on the pores combined with a loss of carbon species during the carbothermal reduction process. As a consequence, a number of micropores were changed to large pores which made the total pore volume higher than that of AC and Ru/AC-H (shown in Table S1†).

Table 1 Comparison of the physical properties for different samples

Sample	SBET (m^2 g^−1)	Total pore volume (cm^3 g^−1)
AC	1130	0.60
Ru(Cl)/AC	842	0.54
Ru/AC-C	1080	0.70
Ru/AC-H	1090	0.62

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The carbothermal reduced and hydrogen reduced samples were also investigated using TEM as shown in Fig. 1. A phenomenon was obviously noted that the particles were homogeneously distributed and the average size was around 1.0 nm in both samples. The very small average particle size for the samples was probably due to the large surface area and abundant micropores of the support, which conforms to the results in Table 1. Therefore it was supposed that carbothermal reduction has little influence on the particle distribution and the average size at this temperature compared to the hydrogen reduced results with the same support.

Fig. 1 TEM images of (a) Ru/AC-C and (b) Ru/AC-H.

The XRD patterns of the catalysts reduced using different methods are shown in Fig. 2. The wide peak around a 2θ value of 25° was associated with the amorphous structure of the activated carbon. The diffraction peaks at 2θ values of 38.4° and 44.0° were attributed to Ru(100) and Ru(101) crystal planes (ICDD-JCPDS card no. 06-0663), while no peaks characteristic of ruthenium oxide appeared in the curves. Both the Ru/AC-C and Ru/AC-H profiles showed the emergence of broad and poor diffraction peaks around 44°, suggesting that the Ru species supported on the AC were reduced to metallic Ru under an inert non-reducible atmosphere such as N₂ or He as well as a reducible atmosphere of H₂ at 450 °C. This fact is consistent with the results from the MS profiles of the H₂-TPR tests (Fig. S2†). Besides, the broad diffraction peak of Ru(101) was perhaps due to uniformly distributed small Ru particles, from association with the TEM images (Fig. 1).

Fig. 2 Powder XRD patterns of Ru/AC-H and Ru/AC-C.

In situ generation and function of CO in the carbothermal reduction process

The carbothermal reduction of Ru/AC at different temperatures was studied using H₂-TPR with both a TCD and MS detector, and the obtained profiles are shown in Fig. 3. There were two peaks around 180 °C and 230 °C in the TPR curve for the Ru(Cl)/AC sample, which were attributed to reduction peaks of the Ru species.^31–33 For the Ru/AC-C-250 sample, the reduction peaks still appeared in the TPR curve, which meant that the Ru species had not reduced to metallic Ru at that treatment temperature. Whereas for the Ru/AC-C-350 and Ru/AC-C-450 samples, there were no apparent peaks, indicating that the Ru species had been carbothermally reduced completely. All the curves began to increase slowly at high temperature, which was related to gasification of the carbon under the H₂ atmosphere, catalysed by the Ru species.³⁴ Thus, it is reasonable to conclude that the carbothermal reduction process could not occur entirely below 250 °C, and the Ru species were completely reduced to Ru⁰ when the carbothermal reduction temperature was above 350 °C.

Fig. 3 H₂-TPR profiles of the Ru(Cl)/AC sample and the Ru/AC-C samples which were carbothermally reduced at 250, 350 and 450 °C respectively.

It is known that carbon species play a vital role in the reduction of metal species at high temperature,²⁶ however, the major reducing agent and its specific function were not confirmed yet. Hence, to better understand the carbothermal reduction, experiments for in situ monitoring and analysis were developed. (1) Carbothermal reduction of Ru/AC was carried out under an inert atmosphere of He on the chemical adsorption instrument with a MS detector; after that a H₂-TPR was subsequently conducted to investigate the degree of reduction, and the results are shown in Fig. 4. (2) A method for in situ chemical absorbing and analyzing of the gas released in the reduction process was designed using a chemical absorption reagent and titration methods (shown in Scheme S1 and Table S2†), and the results of the CO₂ produced are presented in Table 2.

Fig. 4 MS profiles of the carbothermal reduction process of Ru(Cl)/AC under He, with a subsequent H₂-TPR conducted on the chemical adsorption instrument with a MS detector. The m/z values of 28, 44 and 2 represent CO, CO₂ and H₂ respectively.

Table 2 Amounts of CO₂ produced in the carbothermal reduction

Sample	Theoretical value of CO2^a (mmol g^−1)		Measured value of CO2^b (mmol g^−1)
	C as the reductant	CO as the reductant
a Amounts of CO2 calculated from the reduction reaction equations of Ru(III) to metallic Ru with C or CO as the reducing species respectively.b Amounts of CO2 produced from the reduction calculated from the chemical absorption experiments of the carbothermal reduction process aside from the amount of CO2 from the support (details in S2†).
Ru(Cl)/AC	0.32	0.63	0.56
Ru(Cl)/AC900	0.32	0.64	0.54

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In experiment 1 of the carbothermal reduction process, the MS detector monitored two kinds of gases, whose m/z were 28 and 44, attributed to CO and CO₂ respectively (as shown in Fig. 4). In the subsequent H₂-TPR test, no apparent peak appeared for each signal curve, including for the m/z values of 28, 44 and 2 (which represents H₂). This result indicates that the Ru species were totally reduced into metallic Ru during the former carbothermal reduction process. The released CO₂ and CO were probably produced from the oxidized Ru precursors and the surface oxygen-containing groups of the activated carbon via REDOX reactions. RuCl₃ could be oxidized and decomposed easily, and transformed into ruthenium oxides, ruthenium oxychlorides or other Ru species when exposed or heated in air.^31–33 Ru(Cl)/AC and dried RuCl₃ were investigated with XPS to obtain the chemical states of the Ru species. The spectra in the vicinity of the Ru 3d and Ru 3p peaks of these samples confirmed that there were RuCl₃·xH₂O and RuO_x formed in Ru(Cl)/AC (details in S2.4, XPS spectra are shown in Fig. S3†). Also, it has been reported that surface oxygen-containing groups existed on the activated carbon, and amounts of CO and CO₂ could be released during high temperature treatments.^35–37 In conclusion, it can be inferred that the oxidized Ru precursors and the surface oxygen-containing groups of the activated carbon were both a kind of oxygen resource in the formation of CO for the reduction of Ru species.

It could be learned from the profiles (Fig. 4) that CO₂ was detected from 150 °C with a gradual trend of first increasing and then decreasing, while CO appeared after 350 °C. Combining with the H₂-TPR profiles (Fig. 3), the Ru species couldn’t be carbothermally reduced completely below 350 °C. This means that only when the extra released CO starts to be detected (around 350 °C), the reduction reactions appeared to be completed. According to the previous literature, CO is an efficient reducing agent for metals.^38,39 Moreover, the amount of released CO was much more than the CO₂ during the temperature programmed desorption of activated carbon and carbon supported catalysts.^37,40 It could be speculated that CO released below 350 °C participated in the reduction of the Ru species, which helped to produce CO₂ at low temperatures. As the reduction process progressed, the extra released CO was detected while the amount of CO₂ decreased.

The results of experiment 2 also showed that there were amounts of CO₂ released from both the support and the catalysts, as displayed in Table 2 and S2.† When calculated from the reduction reaction equations of Ru(III) to Ru⁰, double the amount of CO₂ was produced when CO was the reducing species compared to carbon reducing the Ru(III) (see the theoretical values of CO₂ in Table 2). The amounts of CO₂ calculated from the absorption experiments were much more than that from the support (shown in Table S2†), indicating that additional molecules of CO₂ were produced by the carbothermal reduction reaction. Aside from the amount of CO₂ from the support, the remaining amounts of CO₂ (that is the measured value of CO₂) were close to the amount of CO₂ produced by CO reducing trivalent ruthenium. From the results of the above experiment, we proposed that CO released in situ functioned as a reducing agent in the carbothermal reduction reaction of Ru species to metallic Ru. The extra CO molecules may act as a ligand adsorbed on the support with the remaining unsaturated carbon atoms in contact with metallic Ru, which probably helps to form stable particles.

To further understand the function of the CO participating in the reduction reaction, another experiment was designed. Non-carbon catalyst Ru(Cl)/SiO₂ reduced using CO and carbon mixed catalyst Ru(Cl)/(AC + SiO₂) thermally reduced under an inert atmosphere were explored and tested in the TPR experiment. The profiles are shown in Fig. 5. For Ru(Cl)/SiO₂, there was an obvious peak around 370 °C (Fig. 5a), which could be attributed to the CO reduction peak. For Ru(Cl)/(AC + SiO₂), a similar reduction peak emerged as well below 450 °C (Fig. 5c). The results were similar to the carbothermal reduction process for Ru(Cl)/AC (Fig. 5b). This phenomenon can result in it being inferred that new CO molecules produced in situ reduced the Ru species into metallic Ru.

Fig. 5 In situ TPR profiles of the reduction processes: (a) Ru(Cl)/SiO₂ reduced in a 10% CO–He gas mixture; (b) Ru(Cl)/AC carbothermally reduced in He; (c) Ru(Cl)/(AC + SiO₂) reduced in He.

According to the above results, a possible mechanism for the carbothermal reduction of Ru(Cl)/AC was proposed and is shown in Scheme 1. The Ru precursor was first impregnated on the AC support, and after drying in air at 110 °C, the precursor changed into different Ru species including ruthenium oxides and chloride oxides.^31–33 Then, under the inert atmosphere, the carbothermal reduction of the sample took place as the temperature increased. At the beginning of the reduction, amounts of CO₂ and CO were produced from the Ru precursor and surface oxygen-containing groups of the activated carbon. Meanwhile, the Ru species were reduced in situ by the released CO gradually resulting in the formation of flat and uniform particles. Afterwards, more CO molecules were released which completely reduced the Ru species to fresh metallic Ru. During the carbothermal reduction, Ru atoms may come into contact with the adsorbed CO and the unsaturated carbon atoms, and then rearrange into a novel, tight and stable configuration which maintains the uniform morphology of a homogeneous dispersion.

Scheme 1 The proposed schematic diagram for the carbothermal reduction process of the Ru(Cl)/AC catalyst.

Catalytic performance in the hydrogenation of itaconic acid

Hydrogenation of itaconic acid with the carbothermally reduced Ru based catalysts was carried out in a stainless autoclave at 50 °C for 0.5 h with a 10 bar initial pressure of H₂, and the catalyst was recycled nine times. As a contrast, the analogues reduced using hydrogen were also tested. For the results discussed below, both catalysts were reduced at 450 °C. The results are presented in Fig. 6. Ru/AC-H showed a higher catalytic activity as the conversion reached 75.4% the first time. However, the conversion dramatically dropped down to 64.5% for the fourth recycle time and then decreased to 53.0% the ninth time. The selectivity for methylsuccinic acid was maintained above 95.0%. Comparatively, the fresh Ru/AC-C sample provided 51.2% conversion in the first reaction, and Ru/AC-C showed a little fluctuation around an average value of 51% during the nine recycle times and reached 47.8% for the last recycle time. It also had an excellent selectivity for methylsuccinic acid with an average value of 97.6%. Obviously, for the selectivity for methylsuccinic acid, there was no apparent difference in both samples. Ru/AC-C showed much less change in the itaconic acid conversion with a difference of 3.4%, while Ru/AC-H turned out to show a precipitous decline with a loss of 22.4% during the nine recycles. It could be concluded that the carbothermally reduced catalyst exhibited much better stability than the hydrogen reduced one.

Fig. 6 Comparison of catalyst resistance to acid for Ru/AC-C and Ru/AC-H in the hydrogenation of itaconic acid. Reaction conditions: 20 g of 6 wt% itaconic acid aqueous solution, 0.1 g of 5 wt% Ru catalyst, 10 bar initial pressure of H₂, 50 °C, 0.5 h.

It is known that catalytic activity generally is associated with the particle size related to the available amounts of active metal atoms. Ru/AC-H showed a little higher catalytic activity perhaps because of the smaller particle size.²⁶ The results of the catalyst recyclability tests showed that Ru/AC-C is much more stable. In addition, the Ru content in Ru/AC-C and Ru/AC-H after the nine cycle tests was measured using ICP-OES, and was 3.8 wt% and 2.8 wt% respectively. That means the carbothermally reduced sample had less leaching compared to the hydrogen reduced sample, which had a much lower Ru content after the nine cycle tests. This is in accordance with the activity changes. It was inferred that the carbothermal reduction could result in efficient resistance to acid, and could probably prevent the leaching of active sites during the catalytic process. The reason may be that CO in situ reduced the Ru species during the carbothermal reduction process, leaving unsaturated carbon atoms coordinated to the new fresh Ru atoms forming more stable Ru particles embedded on the support.²⁶ Therefore, such a process could influence both the stability and activity.

Though Ru/AC-C had a little lower catalytic activity under the above conditions, it showed much better resistance to acid. Hence, to get a higher conversion of itaconic acid and to further investigate the stability, we carried out the hydrogenation reaction under another set of conditions (100 °C for 1 h with 30 bar initial pressure of H₂). The results of six recycle reactions are shown in Fig. 7 and the HPLC profiles of the products analyzed are presented in Fig. S5.† It was apparent that the catalytic activity had increased substantially to 99% conversion of itaconic acid with 99% selectivity for methylsuccinic acid as shown in Fig. 7. More strikingly, there was no apparent difference in the conversion and selectivity, with no trend to decrease, during the recycle test of six reactions. In conclusion, the catalytic activity of Ru/AC-C for the hydrogenation of itaconic acid can be increased to more than 99% by modulating the reaction conditions. Moreover, the resistance to acid of the carbothermally reduced Ru/AC was maintained well under a higher temperature and longer reaction time. Above all, it could be derived that the carbothermally reduced catalyst has good stability.

Fig. 7 The catalyst performance and stability of Ru/AC-C in the hydrogenation of itaconic acid at a high reaction temperature with a long reaction time. Reaction conditions: 20 g of 5 wt% itaconic acid aqueous solution, 0.2 g of 5 wt% Ru catalyst, 30 bar initial pressure of H₂, 100 °C, 1 h.

Conclusions

A stable ruthenium based catalyst was prepared via carbothermal reduction for the hydrogenation of itaconic acid. It showed much better resistance to acid with no obvious loss of activity and less leaching in recycling tests than a hydrogen-reduced sample, which exhibited an apparent decrease of 22% for the conversion of itaconic acid. The carbothermally reduced catalysts could maintain 99% conversion and 99% selectivity for methylsuccinic acid, and also maintained good stability and acid-resistance under a higher temperature and longer reaction time. The results confirmed that the CO generated in situ on the surface of the support functioned as an efficient reducing agent in the carbothermal reduction process. Thus the reduction reaction results in the formation of uniformly dispersed stable Ru nanoparticles. The carbothermal reduction method provides an alternative method for preparing catalysts with good resistance to acid, which would be applied in a variety of acid hydrogenation reactions.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant no. 21203183, 21233008, 21473188).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the synthesis and characterization of the materials and the calculation methods for the experiments. See DOI: 10.1039/c5ra16239d

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