Facile synthesis of supported RuO₂·xH₂O nanoparticles on Co–Al hydrotalcite for the catalytic oxidation of alcohol: effect of temperature pretreatment

Abstract

RuO₂·xH₂O supported on a CoAl-LDH catalyst was synthesized by the co-precipitation (CP) method and the deposition–precipitation (DP) method for the selective oxidation of alcohols. The catalyst prepared by the CP method exhibited higher activity compared with that obtained by the DP method due to stronger interaction between RuO₂ and the CoAl-LDH support as well as the slightly smaller particle size of the RuO₂ nanoparticles. The influence of the temperature pretreatment on catalytic performance was then investigated. Among the catalysts pretreated at different temperature, RuO₂/CoAl-LDH treated at 200 °C showed the highest activity with a TOF of 142 h⁻¹, which was nearly 55% higher than that of the untreated catalyst. It could be related to not only the suitable amount of RuO₂·xH₂O for β-H cleavage, but also the presence of Co³⁺ species for the activation of O₂ molecules and storage of the resulting active O* species. Furthermore, the strong interaction between RuO₂ and the support was revealed to promote the adsorption and activation of benzyl alcohol and thus enhance the catalytic performance. Significantly, RuO₂/CoAl-LDH treated at 200 °C was found to selectively oxidize various alcohols to the corresponding aldehydes and ketones with respectable activity and had greater advantage comparable to that of some Ru catalysts.

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1. Introduction

Catalytic oxidation of alcohols to their corresponding aldehydes or ketones using heterogeneous catalysts with molecular oxygen is essential for the preparation of food additives, fragrances, and many organic intermediates.^1–3 Among various noble metal catalysts, Ru-based catalysts are more significantly in accordance with the main tendency of green chemistry because of their relatively low-expense and high selectivity.^4–7 Matsumoto et al.⁸ first demonstrated that RuO₂ could catalyze allylic alcohols to form unsaturated carbonyl compounds under mild conditions. Madhavaram et al.⁹ revealed that the good activity and selectivity of RuO₂ for the aerobic oxidation of alcohols was derived from the strong bonding of the reactants over the under-coordinated Ru atoms together with the presence of weakly bound undercoordinated oxygen species, serving as the oxidizing agent. Except for the intrinsic characteristics of RuO₂, structural water along with the oxide boundaries also plays an important role in improving activity and selectivity.^10–13 Peng et al.¹⁴ utilized carbon supported RuO₂·xH₂O as a catalyst for the alcohol oxidation reaction. In the catalytic cycle of hydrous RuO₂, the transportation of protons and electrons between the reaction substrate and Ru⁴⁺ active sites could be facilitated by structural water to accelerate the β-hydrogen elimination as the rate-determining step.^14,15 Normally, RuO₂·xH₂O catalysts are prepared by traditional methods such as impregnation and deposition–precipitation which usually have weak interaction with the support and undergo aggregation leading to deactivation in the oxidation of alcohols. Thus, adopting a novel synthesis strategy for RuO₂·xH₂O immobilized on a suitable support is potentially the most interesting and fruitful way to solve this problem.

Layered double hydroxides (LDHs), a family of synthetic anionic clays, are a class of two-dimensional (2D) brucite Mg(OH)₂-like layered inorganic materials.^16,17 They can be represented by the general formula of [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n·mH₂O, where M²⁺ and M³⁺ cations are homogeneously located on the layers. Owing to the cation-tunability of the brucite-like layers and the special structure, LDHs have been shown to be promising supports and actual catalysts. The advantages of LDH materials as support for catalyst derive from excellent adsorption capacity and the confinement effect, which ensures high dispersion of metal particles.¹⁸ Kaneda and co-workers¹⁹ reported the catalytic activity of Au/LDH was significantly higher than that loaded on other supports in the oxidation of neat 1-phenylethanol and revealed the strong interaction effect of Au nanoparticles (NPs) and LDH by electron transfer. Furthermore, it has been proven that the catalytically active metals can be introduced into the layers of LDHs such as Mn,²⁰ Ga,²¹ Cu,²² Ti²³ and Co.²⁴ Lately, our group²² prepared bimetallic PdCu catalysts and found that PdCu nanoalloys could be inlaid in MgAl-LDH with partial utilizing the lattice orientation effect and the net trap structure of LDH to suppress the aggregation of PdCu NPs. Therefore, the material of LDH with flexibility in composition and the special structure is a good choice for the dispersion and immobilization of active components.

Exploiting suitable promotor to the catalysts is another way to improve the catalytic performance. Kozhevnikov et al.²⁵ synthesized a series of hydrous binary Pd^II–M oxide (M = Co^III, Fe^III, Mn^III and Cu^II) catalysts for oxidation of benzyl alcohol (BA). Compared with the other Pd–M oxides, the modification of Pd^II oxide with Co cations exhibited better catalytic performance due to the synergistic effect between Pd and reducible Co species. Similarly, our group²⁶ reported that the catalytic activity of Au catalysts for aerobic oxidation of alcohols can be significantly improved by using Ni-containing LDH as the support. Accidentally, it was found that in the reaction cycle process, the Au⁺–Ni²⁺ hybrid sites were partially oxidized and transformed to Au³⁺–Ni³⁺ to promote the rate-determining step of alcohol oxidations. Therefore, except for the variety of catalysts, oxidation states of active components also have significant effect on the catalytic performance.

In this work, we designed a novel RuO₂·xH₂O/CoAl-LDH catalyst by the co-precipitation (CP) method in which Co species serving as the promotor of RuO₂·xH₂O were introduced into the layer of LDH. The obtained RuO₂·xH₂O/CoAl-LDH catalyst was then treated under different temperature. Particular attention was paid to the influence of temperature pretreatments on the structure and oxidation states of the catalyst, and therefore the catalytic performance. As a comparison, RuO₂·xH₂O/CoAl-LDH catalyst was also prepared by the conventional deposition–precipitation (DP) method. The structure and properties of these two catalysts, including their catalytic performance in the oxidation of BA, were also investigated.

2. Experimental

2.1 Materials

Co(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaOH, Na₂CO₃ and RuCl₃·xH₂O were all of A.R. grade, and were used without further purification. The deionized water used in all experiments had a conductivity less than 10⁻⁶ S cm⁻¹.

2.2 Synthesis of RuO₂·xH₂O/CoAl-LDH catalysts by CP method

RuO₂·xH₂O/CoAl-LDH catalyst was synthesized by CP method. RuCl₃·xH₂O was employed as the Ru³⁺ precursor. RuCl₃ solution was prepared by dissolving 1 g of RuCl₃·xH₂O in 100 mL of deionized water and stirred until it was completely dissolved. In a typical synthesis, solution A was a mixture of Co(NO₃)₂·6H₂O (15 mmol), Al(NO₃)₃·9H₂O (5 mmol) and RuCl₃·xH₂O (0.2 mmol) dissolved in 100 mL of distilled water. Solution B was obtained by dissolving Na₂CO₃ (10.4 mmol) and NaOH (32.3 mmol) in 100 mL of deionized water. Solution A and B were simultaneously added into a glass reactor under vigorous stirring at room temperature with adding ramp rate of 1.0 mL min⁻¹ and the pH was maintained at 9–10. The slurry was aged under stirring at 85 °C for 24 h, filtered off, and washed thoroughly with distilled water until the filtrate was free from Cl⁻ ions as tested by silver nitrate solution and pH = 7. The obtained filter cake was dried in an oven at 60 °C for 12 h. The solid material named as RuO₂/CoAl-LDH_CP, was ground and stored under vacuum. RuO₂/CoAl-LDH_CP was then undergone pretreatment by different temperature (110 °C, 200 °C, 300 °C, 500 °C) and gained a series of catalysts named RuO₂/CoAl-LDH_CP-110 °C, RuO₂/CoAl-LDH_CP-200 °C, RuO₂/CoAl-LDH_CP-300 °C and RuO₂/CoAl-LDH_CP-500 °C, respectively.

2.3 Synthesis of RuO₂·xH₂O/CoAl-LDH catalysts by DP method

The pristine CoAl-LDH support was synthesized by the same method for RuO₂/CoAl-LDH_CP without RuCl₃·xH₂O, with the name CoAl-LDH_CP. The support (0.1 g) was first suspended in 100 mL aqueous solution. The pH value of the suspension was controlled by the addition of NaOH (0.1 M). Until the pH reached 9–10, 2.4 mL RuCl₃ (0.0346 M) was poured into the suspension at 85 °C for 24 h with vigorous stirring. After that, NaOH solution was used to maintain the overall pH at 9–10. The precipitate was centrifuged and thoroughly washed with deionized water until the pH value of the filtrate reached to 7. The washed solid was dried at 60 °C for 12 h and the obtained catalyst was denoted as RuO₂/CoAl-LDH_DP.

2.4 Catalyst characterization

The morphology and structure of the samples were examined using a Zeiss Supra 55 scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of the samples were recorded on a Shimadzu XRD-600 X-ray powder diffractometer (Cu K_α radiation, λ = 0.15406 nm) in the range from 3° to 70° with a scan step of 10° min⁻¹. Elemental analysis for Ru was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-AES). The morphology and size distribution of the samples were examined using JEOL JEM-2100F high-resolution transmission electron microscopy (HRTEM). The X-ray photoelectron spectroscopy (XPS) spectra of the samples were collected using a Thermo VG ESCALAB 250 spectrometer equipped with an Al K_α anode. The calibration peak is the C 1s peak at 284.6 eV. XPS spectra of Co 2p and Ru 3p were deconvoluted by using a XPS peak 4.1 software. Temperature-programmed reduction (TPR) was conducted on a Micrometric ChemiSorb 2720 chemisorption instrument with a thermal conductivity detector (TCD). About 100 mg of samples was loaded in a quartz reactor. TPR was carried out with a heating ramp rate of 2 °C min⁻¹ in a stream of 10% H₂ in Ar to a sample temperature 800 °C, with a total flow rate of 25 mL min⁻¹.

2.5 Benzyl alcohol oxidation

The liquid-phase aerobic oxidation of BA with solvent was carried out in a 50 mL glass stirred reactor. In a typical reaction, BA (1 mmol), toluene (5 mL) and RuO₂·xH₂O/CoAl-LDH catalyst (alcohol : Ru = 100 : 1 mol mol⁻¹) were charged into the reactor, which was then purged with O₂ three times before closing, and the relative pressure was maintained at 1 bar. The reactor was then kept in a heating block, which was preheated to the reaction temperature. The reaction mixture was stirred at 1000 rpm using a magnetic bar inside the reactor. After a specific time, the reactor was rapidly cooled in a cold water bath, and the content was centrifuged to remove the solid catalyst. The solid catalyst was sequentially washed with acetone and deionized water. After drying at 60 °C for 6 h, the recycled catalyst could be reused in the next run under the same conditions. The reaction products were analyzed by an Agilent J&W GC-FID (DB-Wax, 30 m × 0.320 mm, d_f = 0.25 μm) using an external standard technique. Mesitylene was used as external standard for GC analysis. The products were identified by comparison with known standards. In all cases, the carbon balances were 100 ± 5%.

3. Results and discussion

3.1 Characterization and catalytic performance of catalysts with different preparation methods

To investigate the structure of the catalysts prepared by different methods, we have characterized RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP catalysts with SEM and XRD. SEM images of the two catalysts are shown in Fig. S1.† Fig. S1(a) and (b)† display that RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP have a similar flower-like structure with the mean size of 400–500 nm. It can be seen that the structure of catalysts is composed of dozens of plate-like crystals intercrossed with each other. The XRD patterns of RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP, shown in Fig. 1(A) and (B), exhibit the characteristic (003), (006), (009) and (110) reflections of LDH. It is worth noted that only the diffraction peaks of LDH are observed in these catalysts. Theoretically, Ru cation could be partially substituted Al cation and introduced to the brucite-like layers using the advantage of the cation tenability. However, according to the partially enlarged views (Fig. 1(B)) at around 2θ 11° and 60°, no obvious shift of the (003) and (110) diffraction peaks could be observed, indicating that Ru species were supported on the surface of LDH rather than introduced into the brucite-like layers due to larger ionic radii of Ru (0.68 Å) than that of Al (0.53 Å). The diffraction peak at 28.1° is the characteristic (110) reflections of RuO₂ (PDF # 88-0322), where no information about Ru species can be obtained. Moreover, the content of Ru is increased to 5% and 7% separately (Fig. S4†), we still could not observe any diffraction peaks about Ru species. As indicated above, it suggests the as-prepared RuO₂ existed in the amorphous state or RuO₂ NPs highly disperse in the surface of support.^27,28

Fig. 1 XRD patterns (A) of RuO₂/CoAl-LDH_CP (a), RuO₂/CoAl-LDH_DP (b) and CoAl-LDH_CP (c) and their partially enlarged views (B).

The HRTEM images of the as-synthesized RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP are shown in Fig. 2(a) and (b). RuO₂ NPs of RuO₂/CoAl-LDH_CP catalyst with uniform size disperse homogeneously without obvious aggregation, while some particles in the RuO₂/CoAl-LDH_DP catalyst aggregate. Size distributions are obtained by counting approximately 200 particles from several HRTEM images. The histograms (Fig. 2(a) and (b) insets) exhibit that the mean particle size of RuO₂ in RuO₂/CoAl-LDH_CP is 1.69 nm, which is slightly smaller than that in RuO₂/CoAl-LDH_DP (2.14 nm).

Fig. 2 HRTEM images and RuO₂ NPs size distributions of RuO₂/CoAl-LDH_CP (a) and RuO₂/CoAl-LDH_DP (b).

To investigate the interaction between RuO₂ NPs and the support, H₂-TPR measurements are performed for the RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP. From Fig. 3, the reduction profiles of RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP display three hydrogen consumption peaks. The hydrogen consumption peak under 150 °C is the reduction of RuO₂ to metallic Ru.^2,27 The hydrogen consumption peak of RuO₂/CoAl-LDH_DP at 356 °C is attributed to the reduction of Co³⁺ species to Co²⁺ species,²⁹ and the broad peak in the range of 400–700 °C is attributed to the reduction of Co²⁺ species.^24,30,31 Compared with RuO₂/CoAl-LDH_DP, the reduction peaks of Co species do not obviously shift. However, the peak center for the reduction of RuO₂ over RuO₂/CoAl-LDH_CP shifts from 101 °C to 127 °C. Therefore, the results of H₂-TPR clearly show that stronger interaction between RuO₂ NPs and support occurs over RuO₂/CoAl-LDH_CP which leads to the difficult reduction of RuO₂ NPs.

Fig. 3 H₂-TPR profiles: RuO₂/CoAl-LDH_CP (a) and RuO₂/CoAl-LDH_DP (b).

In order to further investigate the interaction between RuO₂ NPs and the support, XPS spectra of RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP catalysts are recorded. Due to the low Ru loading and the overlap with the C 1s region of carbon (284.6 eV), it is very difficult to gain detailed information of the XPS signal of Ru 4d. Thus, the region of Ru 3p_3/2 (458.0–468.0 eV) is used to determine the oxidation states of ruthenium species on the RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP catalysts (see Fig. 4(a) and (b)) and the relative fractions of the Ru species are listed in Table S1.† The FWHM is constrained to be equal in each sample for all deconvoluted peaks from the photoemission spectra of the same species using an 80%/20% Gaussian/Lorentzian sum and a Shirley background. The Ru 3p_3/2 region is deconvoluted into two contributions with different intensities for RuO₂/CoAl-LDH_DP. A main peak centred at 462.9 eV can be assigned to the photoemission from anhydrous Ru(IV) oxide (RuO₂),³² and the peak centred at 464.9 eV is related to RuO₂·xH₂O owing to the presence of OH functional.^33,34 By comparison, it is worth noting that the binding energy (BE) values of RuO₂ and RuO₂·xH₂O in RuO₂/CoAl-LDH_CP are slightly lower than those in RuO₂/CoAl-LDH_DP, suggesting that distinct interaction between Ru species and the CoAl-LDH support is induced by the CP method. Therefore, a strong electron interaction between Ru and Co occurs on RuO₂/CoAl-LDH_CP catalyst surface, wherein electrons likely transfer from Co species to Ru species because of higher electro-negativity of Ru species. Ru–OH is generally considered as the active sites for the aerobic oxidation of alcohols over the RuO₂·xH₂O species, the electron-enriched Ru(IV)–OH^δ− sites can promote O₂ activation and adsorb alcohol.^2,14,35 Moreover, we also find that the different preparation methods have little influence on the amount of RuO₂·xH₂O in the catalysts according to the result of XPS.

Fig. 4 XPS spectra in Ru 3p_3/2 of RuO₂/CoAl-LDH_CP (a) and RuO₂/CoAl-LDH_DP (b).

The catalytic performance of the as-synthesized RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP catalysts for the alcohol oxidation was investigated by determining their activity in the oxidation of BA with toluene as solvent under 0.1 MPa O₂ at 80 °C. As shown in Fig. 5, the conversion of BA over RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP catalysts increase with increasing time. Meanwhile, the activity of the RuO₂/CoAl-LDH_CP catalyst is always higher than that of RuO₂/CoAl-LDH_DP at identical time. When the reaction time prolonging to 3 h, almost all the BA is consumed over RuO₂/CoAl-LDH_CP catalyst, while the conversion is only 90% over RuO₂/CoAl-LDH_DP catalyst. As for the selectivity, benzaldehyde is the predominant product and the selectivity is over 99% in all cases. In general, the RuO₂·xH₂O catalyst exhibits much higher activity compared with the anhydrous RuO₂ catalyst.¹⁴ However, based on the XPS results, there is no obvious change of the RuO₂·xH₂O amount in catalysts prepared by the different preparation methods. Therefore, the enhanced activity of RuO₂/CoAl-LDH_CP could be attributed to stronger interaction between RuO₂ NPs and support as well as slightly higher dispersion of RuO₂ NPs.

Fig. 5 Catalytic performance of RuO₂/CoAl-LDH_CP and RuO₂/CoAl-LDH_DP.

3.2 Catalytic performance of catalysts with different pretreated temperature

Based on the above results, the RuO₂ NPs supported on CoAl-LDH prepared by CP method exhibits higher activity than that prepared by DP method. To further investigate the influence of the RuO₂·xH₂O amount and the state of active species on catalytic performance, RuO₂/CoAl-LDH_CP catalyst was treated under different temperature from 110 °C to 500 °C. The catalytic performance of the catalysts was tested in the oxidation of BA and the resulting data are listed in Table 1. From Table 1, the turnover frequency (TOF) and BD yield of catalyst increase in the order RuO₂/CoAl-LDH_CP-200 °C > RuO₂/CoAl-LDH_CP-300 °C > RuO₂/CoAl-LDH_CP > RuO₂/CoAl-LDH_CP-110 °C > RuO₂/CoAl-LDH_CP-500 °C. The selectivity of all catalysts is over 99%. From the results, it is interesting to note that RuO₂/CoAl-LDH treated at 200 °C shows the highest activity with the TOF of 142 h⁻¹, which is nearly 55% higher than that of the untreated catalyst. However, with pretreated temperature further increasing, the activity decreases significantly and the TOF of RuO₂/CoAl-LDH_CP-500 °C is only 23 h⁻¹. In order to investigate the factors responsible for the activity, average size of RuO₂ NPs over the catalysts with different temperature pretreatment is collected in Table 1. When the pretreated temperature increases from 60 °C to 200 °C, the particle size of RuO₂ increases slightly from 1.67 nm to 2.07 nm. While, with the pretreated temperature above 200 °C, RuO₂ particle size dramatically increases and reaches to 4.98 nm at 500 °C, which can be related to the aggregation of the initially small RuO₂ NPs during the pretreatment.¹⁴ However, the superior performance of RuO₂/CoAl-LDH_CP-200 °C catalyst still cannot be explained by average size of RuO₂ NPs in the catalysts. In our previous work, we found that the oxidation states of cations in catalysts could directly affect the catalytic performance due to the difference in the adsorption and activation of reactants.^20,26 In addition, the amount of RuO₂·xH₂O also plays important roles in the activity of the catalyst. Thus, XRD, XPS and TPR measurements were carried out to explain the relationship between the oxidation states of cations in catalysts and the nonlinear tendencies of activity with different pretreated temperature.

Table 1 Effect of pretreated temperature in air on the activity of RuO₂/CoAl-LDH_CP

Catalyst^a	Ru^b (wt%)	RuO2 size (nm)	Yield^c (%)	TOF^d (h^−1)
a A mixture of alcohol (1 mmol), RuO2 catalyst (alcohol : Ru = 100 : 1 mol mol^−1) and toluene (5 mL) is treated with O2 bubbling (0.1 MPa), and 80 °C for 1 hour.b Determined by ICP.c Yields of BD are determined by GC-FID.d TOF based on BD yield and Ru loading for 0.5 h, and given in molBD moltotal Ru^−1 h^−1.
RuO2/CoAl-LDHCP	0.64	1.69	64.45	79
RuO2/CoAl-LDHCP-110 °C	0.66	1.85	41.80	51
RuO2/CoAl-LDHCP-200 °C	0.74	2.07	98.48	142
RuO2/CoAl-LDHCP-300 °C	0.72	3.20	68.40	100
RuO2/CoAl-LDHCP-500 °C	0.89	4.98	24.52	23
CoAl-LDHCP	—	—	3.31	—
CoAl-LDHCP-200 °C	—	—	6.75	—

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The XRD patterns for the as synthesized RuO₂/CoAl-LDH_CP catalyst after different temperature pretreatments from 110 °C to 500 °C are shown in Fig. S2.† Compared with the untreated catalyst (Fig. S2(a)†), the diffraction patterns of the RuO₂/CoAl-LDH_CP-110 °C can still keep the characteristic diffraction peaks of CoAl-LDH material, which indicates pretreating at 110 °C has little influence on the nature of CoAl-LDH. However, after treated at 200 °C, the characteristic reflections for the LDH phase disappear and broad reflections of (220), (311), (400), (511) and (440) planes for the Co₃O₄ phase (PDF # 73-1701) are present, suggesting the transformation from LDH phase to the mixed oxides. The aluminum compounds are thought to be well-dispersed or in an amorphous phase.³⁶ With pretreated temperature continuing to increase, Co₃O₄ exhibits a higher degree of crystallinity (Fig. S2(d) and (e)†). Based on the above result, we speculate that the phase transformation occurs when the pretreatment temperature reaches to 200 °C.

To explain the influence of temperature pretreatment on the oxidation states of cations in catalysts and the synergy between Ru and Co species, the detailed chemical and electronic states of the catalysts were determined by XPS. The Co 2p_3/2 and Ru 3p_3/2 regions of catalysts with different temperature pretreatments are recorded, as shown in Fig. 6, and the relative fractions of the species are listed in Table S2.† In the region of Co 2p_3/2, it can be well established that the peak of Co 2p_3/2 centers at 780.4 eV with the strong satellite feature for RuO₂/CoAl-LDH_CP, indicating the presence of predominant surface Co²⁺ species.³⁷ After pretreated at 200 °C, a new wide band is observed at 779.2 eV which is attributable to Co³⁺ species.²⁹ Furthermore, the amount of Co³⁺ gradually increases with the pretreated temperature continuing to increase from 200 °C to 500 °C, suggesting that partial Co²⁺ species is oxidized to Co³⁺ species when the pretreated temperature is beyond 200 °C. The result is in accordance with the XRD results. In the region of Ru 3p_3/2, it is deconvoluted into two contributions with different intensities for RuO₂/CoAl-LDH_CP sample without temperature pretreatment. It should be noted that, with pretreated temperature increasing to 500 °C, the peaks of both RuO₂·xH₂O and anhydrous RuO₂ shift toward a lower BE by approximately 0.9 eV and 0.3 eV compared with untreated catalyst. This result implies that the pretreatment can facilitate the interaction between Ru species and the CoAl-LDH support resulting in the increase of electron density of Ru⁴⁺ species and therefore easy absorption of alcohol reactants.^2,14,35 In addition, the amount of RuO₂·xH₂O slightly decreases from 34.84% to 31.08% when the pretreated temperature increases from 60 °C to 200 °C. However, when the pretreated temperature is over 200 °C, the amount of RuO₂·xH₂O obviously decreases and is only 20.86% at 500 °C, which suggests that the decreased content of structure water in RuO₂·xH₂O occurs in the temperature range of 200–500 °C and is in accordance with the result in the literatures.¹⁴ Based on the above discussion, when the catalyst is pretreated at 200 °C, Co²⁺ species can be oxidized mainly to Co³⁺ species which promote the formation of surface oxygen vacancies. It has been proposed that these vacancies can adsorb and activate O₂, which can enhance the oxidation performance of RuO₂ catalyst.^2,38 Moreover, the amount of RuO₂·xH₂O keeps fairly static at 200 °C to accelerate the β-hydrogen elimination as the rate-determining step.

Fig. 6 XPS spectra in Co 2p_3/2 and Ru 3p_3/2 of RuO₂/CoAl-LDH_CP (a), RuO₂/CoAl-LDH_CP-110 °C (b), RuO₂/CoAl-LDH_CP-200 °C (c), RuO₂/CoAl-LDH_CP-300 °C (d) and RuO₂/CoAl-LDH_CP-500 °C (e).

H₂-TPR analysis was conducted to investigate the influence of pretreated temperature on the interaction between active component and support. The corresponding profiles are shown in Fig. 7. As the previous results shown, RuO₂/CoAl-LDH_CP catalyst with pretreatment displays three hydrogen consumption peaks. The one under 150 °C is the reduction of RuO₂ to metallic Ru.^2,27 The peak between 200 °C and 400 °C is attributed to the reduction of Co³⁺ species to Co²⁺ species,²⁹ and the broad peak in the temperature range of 400–700 °C is attributed to the reduction of Co²⁺ species.^24,30,31 For curve (a) and (b), no obvious change can be observed, which indicates that the pretreatment at 110 °C has no obvious influence on the interaction between RuO₂ and support. It is noted that, in curve (c), the reduction peak of RuO₂ slightly shifts to higher temperature, while the reduction peak of Co³⁺ species significantly shifts to lower temperature (221.2 °C) compared with curve (a). The result suggests that pretreating at 200 °C can enhance the interaction between RuO₂ and support which promotes the reduction of Co³⁺ species. However, with the pretreated temperature over 200 °C, the reduction peak related to Co³⁺ species shifts gradually to higher reduction temperature, demonstrating that the reducibility of Co³⁺ species is weak. It has been reported that hydrous RuO₂ is much preferable to reduce Co³⁺ species than anhydrous RuO₂.²⁷ Combined with the information from XPS measurements described above, the amount of RuO₂·xH₂O decreases gradually with the pretreated temperature increasing. Thus, the reduction peak of Co³⁺ species shifting to higher temperature can be mainly attributed to the decrease of amount of RuO₂·xH₂O. Similarly, the reduction peak of RuO₂ also shifts to higher temperature with the pretreated temperature above 200 °C, which is attributable to the obvious increase of RuO₂ particle size resulting in the difficult reduction of RuO₂. Thus, based on the above results, it can be seen that pretreating at 200 °C is the optimum temperature for RuO₂/CoAl-LDH_CP catalyst. The enhancement of the activity of RuO₂/CoAl-LDH_CP-200 °C catalyst compared with other catalysts can be attributed to suitable amount of RuO₂·xH₂O, the presence of Co³⁺ species, small particle size of RuO₂ NPs, and applicable interaction between RuO₂ and support.

Fig. 7 H₂-TPR profiles of RuO₂/CoAl-LDH_CP (a), RuO₂/CoAl-LDH_CP-110 °C (b), RuO₂/CoAl-LDH_CP-200 °C (c), RuO₂/CoAl-LDH_CP-300 °C (d) and RuO₂/CoAl-LDH_CP-500 °C (e).

3.3 Mechanistic investigation

In order to further explore mechanism of oxidation of BA over RuO₂/CoAl-LDH_CP-200 °C catalyst, we also examined the fresh and used RuO₂/CoAl-LDH_CP-200 °C catalyst by XPS. The result is shown in Fig. 8 and the relative fractions of the Co species are listed in Table S3.† After the reaction, the percentage of Co³⁺ species decreased from 64.83% to 57.77%, while that of Co²⁺ species increased from 15% to 28.42%, indicating that part of surface Co³⁺ species were reduced to Co²⁺ species by alcohols during the reaction. Thus, based on our experimental results, we proposed a possible mechanism in the oxidation of BA over RuO₂/CoAl-LDH_CP-200 °C catalyst (Scheme 1). After pretreated at 200 °C, a large proportion of Co²⁺ species were oxidized to Co³⁺ species. Meanwhile, oxygen molecules were adsorbed and activated to active O* species which were further stored in the O vacancies of the support. At the beginning of the reaction, BA molecules primarily attacked basic Ru–OH^δ− active sites, which passed through the abstraction of protons from BA, to form an alkoxide intermediate by ligand exchange between surface Ru–OH species. Subsequently, the intermediate underwent coordination to afford an unsteady metal–alcoholate species, which was considered as the rate-determining step and was promoted by the induction of both Ru–OH^δ− species to hydroxyl, leading to the elimination of β-H. The final step was the rapid oxidation of the Ru–H species by active O* adsorbed Co³⁺ sites. With desorption of BD and water molecules, the catalytic cycle was thereby completed. The redox property of Co²⁺ ↔ Co³⁺ plays a vital role in promoting the cycle process of catalyst. According to this reaction mechanism, both basic Ru–OH^δ− sites and the redox property of Co²⁺ ↔ Co³⁺ are the key factors responsible for the enhancement of activity.

Fig. 8 XPS spectra in Co 2p_3/2 of the fresh RuO₂/CoAl-LDH_CP-200 °C (a) and the used RuO₂/CoAl-LDH_CP-200 °C (b).

Scheme 1 Possible reaction pathway of the oxidation of BA over the RuO₂/CoAl-LDH_CP-200 °C catalyst.

3.4 Recyclability of RuO₂/CoAl-LDH_CP-200 °C catalyst

The recyclability of the RuO₂/CoAl-LDH_CP-200 °C catalyst was investigated for three successive reactions as shown in Fig. 9. In repeated runs, the selectivity remained almost 100%, but the conversion of BA had a considerable decrease in the 2nd and 3rd run compared with the 1st. The Ru and Co content of the catalyst after the 3rd cycle were also detected by ICP and the values were approaching to those of the fresh catalyst, which suggested that neither Ru species nor Co species had leached from the catalyst. Furthermore, A. Alvarez³⁰ reported that formed water molecules could adsorb on the surface oxygen vacancies at Co species and thereafter retard the adsorption of O₂ molecules, which is considered as a reason for the loss of activity.^38–40 In order to prove this speculation, XPS was used to analyze the transformation of O species in the fresh and used RuO₂/CoAl-LDH_CP-200 °C catalysts during the reaction. The result was showed in Fig. S4† and the relevant fractions of O 1s were listed in Table S4.† Both samples showed three distinct peaks, which could be ascribed to lattice oxygen (denoted as “O_β”, 529.7 eV), surface labile oxygen such as chemisorbed active O* (designated as “O_α”, 530.9 eV), and adsorbed molecular water (labeled as “O_γ”, 532.2 eV).²⁰ Compared to fresh and used RuO₂/CoAl-LDH_CP-200 °C catalysts, there was an obvious increase in the content of O_γ (from 23.02% to 36.79%), which was attributed to the adsorption of creating water molecules during the reaction. Moreover, considering the influence valence transition Co species during the reaction on catalytic activity, XPS spectra in Co 2p regions of fresh and used RuO₂/CoAl-LDH_CP-200 °C catalysts were recorded. The result was shown in Fig. 8 and the relative fractions of the Co species were listed in Table S3.† According to the results, the amount of Co²⁺ species increased from 15% to 28.42% after the reaction, while the amount of Co³⁺ species decreased from 64.83% to 57.77%, which showed that part of surface Co³⁺ species were reduced to Co²⁺ species after oxidation reaction of BA. Thus, the recyclability of RuO₂/CoAl-LDH_CP-200 °C catalyst is improved to a large extent by reheating. The used catalyst at 200 °C for 3 h in order to remove the water molecules from dehydration of oxidation of alcohol and oxidize the reduced Co²⁺ species. The conversion of BA has significantly improved in the 2nd and 3rd.

Fig. 9 Reusability of RuO₂/CoAl-LDH_CP-200 °C (untreated) and RuO₂/CoAl-LDH_CP-200 °C (treated).

3.5 Various catalytic aerobic oxidations of alcohols over RuO₂/CoAl-LDH_CP-200 °C catalyst

To obtain universality information about the RuO₂/CoAl-LDH_CP-200 °C catalyst, the oxidation of various typical alcohols is evaluated, and the results are listed in Table 2. We note that RuO₂/CoAl-LDH_CP-200 °C can oxidize a wide range of alcohols to the corresponding carbonyl compounds with high activity especially benzylic and unsaturated alcohols. Moreover, it is worth noticing that the TOF (142 h⁻¹) of RuO₂/CoAl-LDH_CP-200 °C is much higher than those previously reported such as 8.5 h⁻¹ of RuO₂/FAU,⁴ 27 h⁻¹ of Ru/Al₂O₃,⁴¹ and even higher than 38 h⁻¹ of Ru–Co oxide⁵ and 78 h⁻¹ of Ru/HAP⁴² which are regarded as two of the most efficient catalysts for oxidation at present. This high catalytic activity can be attributed to the suitable amount of RuO₂·xH₂O, the appropriate size of particles, the cooperative interaction between RuO₂ and support, and the presence of Co³⁺ species.

Table 2 Aerobic oxidation of various alcohols using RuO₂/CoAl-LDH_CP-200 °C

Entry^a	Substrate	Main product^b	Catalyst	Temperature (°C)	TOF (h^−1)	Ref.
a The reaction conditions are the same as those in Table 1.b Determined by GC-FID.
1			RuO2/CoAl-LDHCP-200 °C	80	18	This work
2			RuO2/CoAl-LDHCP-200 °C	80	25	This work
3			RuO2/CoAl-LDHCP-200 °C	80	31	This work
4			RuO2/CoAl-LDHCP-200 °C	80	124	This work
5			RuO2/CoAl-LDHCP-200 °C	80	147	This work
6			RuO2/CoAl-LDHCP-200 °C	80	94	This work
7			RuO2/CoAl-LDHCP-200 °C	80	142	This work
			RuO2/FAU	80	8.5	4
			Ru/Al2O3	83	27	41
			Ru–Co oxide	110	38	5
			Ru/HAP	90	78	42

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4. Conclusions

In this work, we designed and synthesized RuO₂·xH₂O/CoAl-LDH catalysts by the CP method and the DP method. Aerobic oxidation of BA was then performed as a probe reaction to differentiate the catalytic performances. The RuO₂/CoAl-LDH_CP catalyst exhibited higher activity compared with RuO₂/CoAl-LDH_DP catalyst. The BA almost completely conversed over RuO₂/CoAl-LDH_CP catalyst after 3 h, while the conversion of BA was only 90% over RuO₂/CoAl-LDH_DP catalyst. The enhancement of catalytic activity was ascribed to stronger interaction between RuO₂ and CoAl-LDH support as well as slightly smaller particle size of RuO₂ NPs according to the results of HRTEM, XPS and TPR. In addition, the pretreatment of catalyst under different temperature also plays an important role in the enhancement of the catalytic performance. RuO₂/CoAl-LDH_CP catalyst was then treated at different temperature. Among catalysts pretreated by different temperature, the highest activity was shown over RuO₂/CoAl-LDH_CP-200 °C catalyst. The TOF of catalyst treated at 200 °C was nearly 55% higher than that of the untreated catalyst. The selectivity of all catalysts towards benzaldehyde could reach to over 99%. By XPS analysis, it was proven that the oxidation states of catalyst had no obvious change below 200 °C, while a large proportion of Co²⁺ species from CoAl-LDH support was oxidized to Co³⁺ species when pretreated at 200 °C. Meanwhile, the amount of RuO₂·xH₂O only slightly decreased, when pretreated temperature was no more than 200 °C. However, with the pretreated temperature over 200 °C, the amount of RuO₂·xH₂O obviously decreased. Moreover, the influence of temperature pretreatment on the interaction between RuO₂ NPs and the CoAl-LDH support was revealed by TPR. RuO₂/CoAl-LDH_CP-200 °C catalyst exhibited the strong interaction between RuO₂ NPs and the CoAl-LDH support due to the right amount of RuO₂·xH₂O and the appropriate size of particles. Consequently, pretreating at 200 °C was the optimum temperature for RuO₂/CoAl-LDH_CP catalyst. Furthermore, universality of RuO₂/CoAl-LDH_CP-200 °C for the oxidation of other alcohols of various types was measured. The catalyst was found to selectively oxidize various alcohols to the corresponding aldehydes and ketones with respectable activity and had greater advantage comparable to that of some Ru catalysts. In addition, we also found that the water molecules from dehydration of oxidation of alcohol and the percentage of Co³⁺ species had a great influence on the catalytic activity of RuO₂/CoAl-LDH_CP-200 °C catalyst. One feasible way to get good reusability of catalyst was used by heating the used catalyst at 200 °C for 3 h.

Acknowledgements

This work was supported by the National Natural Science Foundation, the National Basic Research Program of China (973 program, 2014CB932104), Beijing Engineering Center for Hierarchical Catalysts and Fundamental Research Funds for the Central Universities (YS1406). Specialized Research Fund for the Doctoral Program of High Education (20110010120012).

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Footnote

† Electronic supplementary information (ESI) available: Analysis on morphology and structure of catalyst; the relative fractions of the Co 2p_3/2 and Ru 3p_3/2 of XPS. See DOI: 10.1039/c6ra11167j

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