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Keywords:

  • embedding degree;
  • hydrogenation;
  • porous materials;
  • ruthenium;
  • nanoparticles

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Uniform ruthenium nanoparticles (1–2 nm) confined in ordered mesoporous carbon (Ru-OMC) with various embedding degrees have been fabricated by using a boric acid-assisted hard template method. The catalytic performance of Ru-OMC catalysts was determined through the hydrogenation of toluene at 110 °C and 4.0 MPa. The effects of pore size and embedding degree on the catalytic performance were studied and compared with those of OMC-supported ruthenium (Ru/OMC) catalysts with various pore sizes. The catalytic activities of embedding Ru-OMC catalysts are much higher than those of supported Ru/OMC catalysts, which can be attributed to the strong interaction between ruthenium nanoparticles and the carbon support. Furthermore, the activities of Ru-OMC catalysts are closely related to the embedding degree of ruthenium nanoparticles in the carbon matrix. The Ru-OMC catalysts with an appropriate embedding degree affords a turnover frequency of up to 4.69 s−1 in toluene hydrogenation.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Metal particles with nano- and subnanometer sizes are attractive as high-performance catalysts, owing to their high surface energy, large surface curvature, large surface-to-volume ratio, and large surface area exposed to the reactant as compared to bulk materials.14 However, the small size of the metal nanoparticles (NPs) not only makes the surface atoms dynamically active, but also makes the surface of metal NPs unstable. With respect to thermodynamics, “naked” NPs tend to aggregate to form larger particles, which results in a remarkable decrease in catalytic activity and selectivity.5

The unique and unexpected properties have been observed by confining/embedding metal NPs in inorganic channels or cavities, which offer new opportunities to design advanced catalytic systems.6 An improvement in the catalytic performances can be attributed to the confinement of the substrate within the pores of the support, which leads to an improved interaction between the active component and the substrate. In this sense, controlling the pore sizes of porous carbon materials accurately is important to study the confinement of metal NPs because the pore size in nanoscale decides the physiochemical properties of the noble metal and the diffusion of the guest molecules in a carbon matrix.7, 8 Another important issue regarding the confining/embedding metal NPs is the strength of the interaction between metal NPs and supports, which may be controlled by the embedding degree (or connectivity) of NPs in the framework of supports. Su et al.7, 9, 10 reported that RuNPs can be semi-embedded in a porous carbon matrix by using a chemical vapor deposition method, and they suggested that the strong interaction between RuNPs and the carbon support increased the catalytic activities. Liu et al. reported that a controlled synthesis of highly dispersed Pt and stable PtRuNPs in ordered mesoporous carbon (OMC) materials by dispersing platinum acetylacetonate in furfural alcohol and trimethylbenzene as the cofeeding carbon and Pt precursor.11, 12 In addition, Xiong et al. reported a method using furfural alcohol solution containing Ru(NO)(NO3)3 for the preparation of stable embedding RuNPs in the OMC material.13 Both the aforementioned catalysts with embedding structures demonstrate improved catalytic activities than do classical supported catalysts in various reactions. We reported a controlled synthesis of highly dispersed semi-embedding RuNPs in the porous carbon framework with more exposed active sites by using RuCl3/SBA-15 as hard templates.14 This Ru-containing OMC (Ru-OMC) catalyst demonstrates remarkably high activity and stability in the heterogeneous hydrogenation reactions owing to its large exposed Ru surface area and semi-embedding degree of RuNPs. However, a few reports have investigated the embedding degree of metal NPs in the carbon matrix, although a few reports have described the strong interaction between metal NPs and the carbon support.15, 16 The embedding of metal NPs results in a strong interaction that favors high stability of supported metal NPs, but may cover most part of the metal surface; however, the superficial embedding of the metal NPs results in a weak interaction that favors the intrinsic properties of metal NPs, but may result in leaching and agglomeration of metal NPs during the catalytic process.17

Kim et al. reported an easy pathway for the precise control of pore sizes in the range of 3–10 nm by using boric acid as the pore-expanding agent.18, 19 This method has been adapted for the synthesis of iron-doped OMC materials with precisely controlled pore sizes by using Fe-SBA-15 as hard templates.20, 21 Notably, this direct boric acid-assisted hard template method not only can expand the pore size of the carbon support, but also can reduce the wall thickness. Herein, these two strategies were adapted for the controlled synthesis of Ru-containing ordered mesoporous materials with a highly dispersed semi-embedding RuNPs in the porous carbon framework, in which boric acid is used as a pore-expanding agent and a framework whittle agent. On the basis of the characterization results of TEM, CO chemisorption, and N2 adsorption–desorption analysis, the amount of exposed active sites and pore sizes have been adjusted simultaneously. To our knowledge, no report exists on the precisely controlled synthesis of embedding metal NPs confined in mesopores. To evaluate the catalytic performance of Ru-OMC catalysts with various embedding degrees, toluene hydrogenation was used as a model reaction. To study the effect of pore size and embedding degree of RuNPs on the catalytic performance, a series of OMC-supported Ru (Ru/OMC) catalysts with various pore sizes were also prepared and compared with various Ru-OMC catalysts.

Results and Discussions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Textural properties of Ru-OMC catalysts

The isotherms and pore size distributions of Ru-OMC catalysts prepared with different amounts of boric acid are shown in Figure 1 a and b. The isotherm of Ru-OMC-B-x is a typical type IV isotherm with H2-type hysteresis loop, which is a typical adsorption isotherm for OMC materials.20 The physical and chemical properties of all materials are listed in Table 1. The specific surface areas of all Ru-OMC-B-x catalysts are in the range of 1000–1100 m2 g−1. The pore size increases and the wall thickness decreases with the increase in the amount of boric acid. The pore size of the desorption branch increases with the increase in the amount of boric acid from 4.2 to 8.4 nm. The wall thickness of various Ru-OMC-B-x samples decreases from 5.2 to 2.3 nm. In the low-angle XRD patterns of various Ru-OMC-B-x catalysts shown in Figure 2 a, a remarkable diffraction peak at 2 θ=0.9–1.1° is observed for Ru-OMC-B-x, which indicates the ordered mesoporous structures of these samples.22

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Figure 1. a)  N2 adsorption–desorption isotherms and b) pore size distribution of SBA-15 and Ru-OMC catalysts prepared with different amounts of boric acid.

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Table 1. Textural properties of Ru-OMC catalysts.
CatalystBoric acidSBETVpPore diameter[a]2 θa0[b]Wall thickness
[g][m2 g−1][cm3 g−1][nm][°][nm][nm]
  1. [a] Calculated from the desorption branch of the isotherm by using the BJH method; [b] a0=2 d100/equation image.

SBA-159041.286.40.8611.85.4
Ru-OMC-B-0none10351.054.21.089.45.2
Ru-OMC-B-0.110.1110941.305.21.079.54.3
Ru-OMC-B-0.220.2210141.345.61.059.74.1
Ru-OMC-B-0.330.3310511.507.71.039.92.2
Ru-OMC-B-0.440.4410411.528.40.9510.72.3
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Figure 2. a) Low-angle and b) wide-angle XRD patterns of Ru-OMC catalysts prepared with different amounts of boric acid. Graphitic carbon (*).

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Status of RuNPs

The status of RuNPs was characterized in detail by using wide-angle XRD, TEM, scanning TEM (STEM), and CO chemisorption techniques. The wide-angle XRD patterns of all catalysts are shown in Figure 2 b. No diffraction peak assigned to Ru is observed, which indicates the absence of large RuNPs in all catalysts. Two broad peaks are observed at 26 and 44°, which are assigned to carbon peaks with semi-graphitic structures. The TEM and STEM images of Ru-OMC-B-0.22 are shown in Figure 3 a and b, respectively. Highly dispersed RuNPs with particle sizes of 1–2 nm are uniformly distributed on the support, and no particle aggregation is observed. All samples possess similar particle size distributions. The particle size distributions and STEM images of other samples are shown in Figure 4. Most of the RuNPs are semi-embedded in the mesoporous carbon framework obtained from thermal reduction, in which carbon consumption during the reduction of Ru3+ ions to Ru enables RuNPs to sit in or become semi-embedding in the carbon matrix.

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Figure 3. a) TEM and b) SEM images and particle size distribution of the Ru-OMC-B-0.22 catalyst.

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Figure 4. Size distribution of Ru-OMC catalysts with different amounts of boric acid: a) Ru-OMC-B-0; b) Ru-OMC-B-0.11; c) Ru-OMC-B-0.33; d) Ru-OMC-B-0.44.

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To identify the strong interaction between RuNPs and the carbon framework, a series of temperature-programmed treatments of Ru-OMC and Ru/OMC samples were performed under 5 % H2/Ar. The H2-temperature programmed reduction patterns detected with a thermal conductivity detector and a mass spectrometer are shown in Figure 5. The consumption of H2 peaks is totally different for Ru-OMC and Ru/OMC samples (Figure 5). Two main H2 consumption peaks are observed at 235 and 525 °C for Ru/OMC samples and three H2 consumption peaks at 235, 370, and 490 °C for Ru/OMC samples according to the TCD signals. The first peak at 235 °C for the Ru/OMC sample can be assigned to the reduction of Ru3+, because water molecules are detected by a mass spectrometer in this temperature range; the second peak at 525 °C is attributed to the methanation of carbon by H2 in the presence of the Ru catalyst because Ru is a good catalyst for the methanation of carbon.23 This observation is confirmed by the methanation signal detected with a mass spectrometer and shown in Figure 5 c. For the Ru-OMC sample, a small peak is observed at 235 °C, which can be due to the reduction of partially oxidized RuNPs with sample in air. The main H2 consumption peaks at 370 and 490 °C are attributed to the methanation of carbon by H2. Compared to the methanation of the carbon support peaks for Ru/OMC samples, the methanation peaks for Ru-OMC samples occur at relatively lower temperatures. This observation can be explained by an easier methanation of the carbon support for embedding Ru-OMC catalysts than for supported Ru/OMC catalysts owing to the strong interaction between RuNPs and the carbon support.

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Figure 5. Temperature-programmed reduction profiles of the embedding Ru-OMC catalyst and the supported Ru/OMC catalyst under 5 % H2/Ar atmosphere detected with a a)  thermal conductivity detector and b, c) mass spectrometer: m/e=18 (panel b); m/e=15 (panel c).

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The characterization results for Ru dispersion measured by using CO chemisorption and TEM techniques are summarized in Table 2. The CO monolayer uptake of all catalysts increases from 174.3 to 231.5 μmol g−1 with the increase in the amount of boric acid in the precursor solution (Table 2). On the basis of the data of CO chemisorption and assuming an adsorption stoichiometry of one CO molecule on one metal surface atom,24 one can estimate the exposed metal surface area of metal catalysts. Moreover, the exposed Ru surface area increases from 6.4 to 8.6 m2 g−1 with the increase in the amount of boric acid, but the size of RuNPs in all catalysts remains similar. The estimated embedding degree is obtained by using the equation Dembedding=1−Ns/Nc, in which Ns is the number of metal atoms present on the surface measured by using the CO chemisorption technique and Nc is the number of metal atoms present on the surface based on the average crystallite size determined by using the TEM technique. The estimated embedding degrees of various RuNPs are listed in Table 2, and the relationship of embedding degree and boric acid content are drawn in Figure 6, which shows that the embedding degree decreases linearly with the increase in the amount of boric acid.

Table 2. Textural properties of various Ru-OMC and Ru/OMC catalysts.
CatalystRu amountCO chemisorptionTEMEstimated
[wt %]CO uptake (Nm)Ru amountDispersiondd[b]Dispersion[c]embedding
[μmol gcat−1][m2 gcat−1][%][nm][nm][%]degree[a] [%]
  1. [a] Estimated embedding degree was obtained by using the equation Dembedding=(1−Ns)/Nc, in which Ns is the number of metal atoms present on the surface measured by using the CO chemisorption technique and Nc is the number of metal atoms present on the surface based on the average crystallite size determined by using the TEM technique; [b] The average particle size was estimated on the basis of TEM data; [c] Ru dispersity was obtained by using the equation DRu=1.33/dRu.

Ru-OMC-03.3174.36.453.32.51.588.436.7
Ru-OMC-0.112.6181.06.770.31.91.588.718.2
Ru-OMC-0.222.5191.07.177.21.71.588.912.6
Ru-OMC-0.332.6201.67.478.31.71.588.78.9
Ru-OMC-0.442.5231.58.693.51.41.588.7−4.7
Ru/OMC-04.0280.610.471.41.91.870.5−1.2
Ru/OMC-0.114.0255.29.464.92.1
Ru/OMC-0.224.0237.18.860.32.22.163.32.0
Ru/OMC-0.334.0218.88.155.62.4
Ru/OMC-0.444.0196.77.350.02.72.551.93.7
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Figure 6. Effect of the amount of boric acid on the estimated embedding degree and turnover frequency of Ru-OMC-B-x catalysts for toluene hydrogenation.

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In approaches used herein, RuCl3/SBA-15 was used as a hard template so that the pore of SBA-15 is filled with Ru3+ before the infiltration of the carbon precursor (sucrose). The first step in carbonization is the treatment of the composite with sulfuric acid at 160 °C for 6 h, which helps in cross-linking of the carbon structure before the reduction of Ru3+.25 The second step in carbonization was operated at 850 °C for 3 h under inert gas atmosphere. The reduction of Ru3+ and further carbonization occur in this step. Boric acid is hydrophilic and has an affinity for silica, whereas the carbon precursor gradually becomes hydrophobic during the carbonization process, which results in a spontaneous phase separation of boron and carbon species. Boron species in the mixture of the semicarbonized composite of sucrose and boric acid are first transformed to boron oxide in the mesoporous silica template and then move from the boron–carbon mixture to the silica surface with the increase in temperature during the carbonization process, and finally reacts with the silica to produce a borosilicate layer between the silica and carbon frameworks. The detailed mechanism is given in Scheme 1. The formation of borosilicate was evaluated by using IR spectroscopy (Figure 7). The absorbance at 677 and 914 cm−1 assigned to borosilicate was observed. The existence of Ru3+ does not affect the formation of borosilicate, and the existence of Ru does not affect the spontaneous phase separation process. Various embedding degrees of RuNPs in the carbon matrix can be adjusted simultaneously owing to the different thickness of the borosilicate layer.

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Scheme 1. Adjustment mechanism for the embedding of RuNPs in the carbon framework of Ru-OMC catalysts.

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Figure 7. FTIR spectra of SBA-15, H3BO3/SBA-15, and H3BO3-RuCl3/SBA-15 after thermal treatment at 850 °C for 3 h under Ar flow.

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Catalytic performances of Ru-OMC-B-x catalysts

The catalytic performances of Ru-OMC-B-x catalysts were determined through the hydrogenation of toluene (Figure 6). Methylcyclohexane is the only product. The catalytic activity of Ru-OMC-B-x catalysts increases slightly with the increase in the amount of boric acid up to 0.22 g and then decreases with a further increase in the amount of boric acid.

Several factors affect the catalytic performance of the hydrogenation of monoaromatic molecules: the surface areas and the pore sizes of all catalysts, the size of the active metal particle, and the interaction between the active metal and the support. The surface areas of all catalysts are similar (data in Table 1), and all catalysts have the same particle size of RuNPs in the range of 1–2 nm. The catalysts differ in the pore size and the embedding degree of RuNPs in the porous carbon framework. Although a large pore is beneficial for the diffusion of the reactant and a large exposed metal surface area is beneficial for the interaction between the reactant and active sites, the confinement effect between RuNPs and the pore size of the carbon support could be too weak when the pore size is too large compared with the size of RuNPs. For example, the pore size of the Ru-OMC-B-0.44 sample is 8.4 nm and the particle size of RuNPs is 1.5 nm, which is more than five times that of the former; thus, the confinement effect of pore size on RuNPs plays a minor role.

To prove this assumption, OMCs with various pore sizes have been prepared according to our previous report20 and a series of supported Ru/OMC catalysts were also prepared. The catalytic performances of supported Ru/OMC catalysts were determined through the hydrogenation of toluene (Figure 8). Although the catalytic activity of supported Ru/OMC catalysts increases with the increase in pore size, the increase is small compared with the increase in the catalytic activity of embedding Ru-OMC catalysts. Notably, the turnover frequency of embedding Ru-OMC catalysts is higher than that of supported Ru/OMC catalysts, which is due to the strong interaction between RuNPs and the carbon support for Ru-OMC catalysts. A detailed comparison of the catalytic performance and characterization results of Ru-OMC and Ru/OMC catalysts has been reported in our previous report.14

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Figure 8. Effect of the amount of boric acid on the pore diameter and turnover frequency of Ru/OMC catalysts for toluene hydrogenation.

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The hydrogenation of toluene typically involves H2 spillover mechanisms;7, 26, 27 H2 may be adsorbed dissociatively on the Ru surface and spilled over from RuNPs onto the support and the aromatic compounds are often found to be adsorbed on the surface of the support.28 It has been reported that the hydrogenation of monoaromatic molecules occurs partly through the coadsorption of H atoms and monoaromatic molecules on active metals and partly through the coadsorption of H atoms and monoaromatic molecules on the carbon-support sites, as a result of the migration of some H atoms from the metal to the support, which thus increases the specific activity of the catalyst.27 The interaction (or connectivity) between the active metal and the support increases the spillover.16, 17 Because of the existence of physical and energy barriers for the transfer of H atoms from one site to another, the strong interaction should facilitate the spillover and acts as a bridge between the active metal and the carbon support. Therefore, the strong interaction between RuNPs and the carbon support could facilitate the transport of H2 spillover species from the Ru particles to the support for H2 spillover and hydrogenation reactions. As discussed in our previous report,14 embedding RuNPs in the carbon framework are more active than supported RuNPs on the carbon support owing to the strong interaction between RuNPs and the carbon support.

Here, Ru-OMCs with various degrees of Ru metal embedded in the carbon matrix has been adjusted simultaneously. Thus, the interaction between the metal and carbon can be adjusted by the embedding degrees of RuNPs. The embedding of RuNPs in the carbon framework leads to many interactions between RuNPs and the carbon support and thus forms a “surface contact” on the carbon support that results in a concave curvature in the carbon surface, which plays an important role in increasing metal–carbon interactions. The pz orbital of graphene (π-bonded states) hybridizes strongly with the d orbitals of Ru, which results in electron transfers from Ru to the graphene of the carbon substrate and hindrance of Ru oxidation as compared with those NPs lying on the planar surfaces.7 The strong interaction between RuNPs and the carbon support facilitates the transport of H2 spillover species from RuNPs to the support for H2 spillover29, 30 and thus improve the performance of the hydrogenation reactions. Furthermore, the substitution of boron species into the carbon support can significantly increase the interactions between hydrogen (both H and H2) and carbon and thus affect the performances of all catalysts.16 However, the residual boron species are undetectable in embedding catalysts by using energy-dispersive X-ray spectroscopy and inductively coupled plasma emission spectrometry mass spectrometry because boron species in the current system can form borosilicate easily than be doped into the carbon structure and thus can be removed with sodium hydroxide. Therefore, the embedding degree may play a significant role in the catalytic performance of embedding Ru–carbon catalysts: the higher the embedding degree, the easier the H2 spillover and thus the higher the catalytic activity of hydrogenation.

However, the exposed metal surface area accounts for the catalytic performance as well. The large exposed surface area of RuNPs could make the active sites more exposed to reactants during catalytic reactions. Although a higher embedding degree of RuNPs implies a higher activity of hydrogenation, the deep embedding of metal NPs caused problems such as less exposure between the metal surface and reactants during the reaction. In summary, the sample with a pore size of 5.6 nm has an appropriate embedding degree of approximately 10 %, which demonstrates the best performance of toluene hydrogenation. Moreover, the catalytic activity of toluene hydrogenation on the active sites is favored more for Ru-OMC catalysts with suitable strength of the interaction between RuNPs and the carbon support.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

We report an easy method for the preparation of Ru-containing mesoporous carbon materials with a controlled embedding degree of Ru nanoparticles (NPs). The catalytic performance of toluene hydrogenation was closely related with the strength of the interaction between RuNPs and the carbon support. Therefore, an optimized embedding degree of RuNPs in the porous carbon framework is necessary for the best catalytic performance. In addition, this approach can be extended to the preparation of other noble metal-doped mesoporous carbon materials, such as Pt, Pd, and other metal NPs, and thus provides a new approach for the preparation of stable metal-embedded carbon catalysts with variable embedding degrees and suitable strength of the interaction between metal NPs and carbon supports.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Chemicals and reagents

A triblock copolymer EO20PO70EO20 (Pluronic P-123, molecular mass=5800), tetramethoxysilane (99 %), and other reagents were obtained from Shanghai Chemical Reagent Inc. of Chinese Medicine Group. All materials were analytical grade and used without any further purification.

Synthesis of SBA-15 silica

The synthesis of ordered mesoporous SBA-15 template was performed as follows:31 The Pluronic P-123 surfactant (2 g, Aldrich) was completely dissolved in HCl solution (75 mL, 1.6 mol L−1) to obtain solution A. Then, tetramethoxysilane (3.2 mL, Hangzhou Guibao Chemical Co., Ltd.) was added dropwise to solution A. The solution was then stirred vigorously for 2 h at 40 °C and was maintained at the same temperature overnight. Afterward, the solution was transferred to an autoclave and heated at 100 °C for 24 h. The resultant solid was filtered, washed, and dried at 60 °C for 15 h. Finally, the product was calcined at 500 °C for 10 h and SBA-15 silica was obtained as the final product.

Synthesis of RuCl3/SBA-15

SBA-15 silica (0.5 g) was impregnated with the aqueous solution (2.3 mL) containing ruthenium chloride hydrate (0.05 g, Sino-Platinum Metals Co. Ltd). Then, the resulting mixture was dried at 110 °C overnight, and finally RuCl3/SBA-15 was obtained.

Synthesis of Ru-OMC with different pore sizes

The method for the synthesis of Ru-OMC materials was the same as the nanoreplication method described elsewhere,14 except for the use of boric acid as a pore-expanding agent and a framework whittle agent. The amount of boric acid in the precursor solutions was in the range of 0–0.44 g. The precursor solutions were prepared by adding different amounts of boric acid to a sucrose solution while keeping the sucrose concentration constant. A typical method for the preparation of, for example, Ru-OMC-B-0.22 can be described as follows: a precursor solution containing boric acid (0.22 g), sucrose (1.25 g), and distilled water (5.0 mL) was divided into two equal solutions, and one was allowed to infiltrate the mesopores of RuCl3/SBA-15. The composite was mixed with six drops of sulfuric acid before it was dried at 100 °C for 6 h and at 160 °C for 6 h thereafter. Notably, sulfuric acid was necessary to obtain Ru-OMC catalysts with high performance, and the amount of sulfuric acid needed to be controlled strictly.25 The infiltration and drying process was repeated once more with an additional precursor solution (66 % of the first infiltration) and one drop of sulfuric acid before the composite was carbonized at 850 °C for 3 h under N2 flow. Then, the resulting Ru-OMC-B-0.22 material was washed twice at 70 °C for 1 h with sodium hydroxide solution (NaOH/CH3CH2OH/H2O=1:22:44 in molar ratio) to dissolve the silica template completely. Finally, the Ru-OMC-B-0.22 material was obtained after filtering and drying in air at 110 °C overnight. All samples obtained above are labeled as Ru-OMC-B-x, in which x indicates the amount of boric acid used for pore expanding.

Synthesis of Ru/OMC with different pore sizes

The method for the synthesis of OMC materials was the same as the nanoreplication method described elsewhere.20 The Ru/OMC samples with different pore sizes were prepared through incipient impregnation using OMC materials with different pore sizes prepared above. The Ru/OMC samples were reduced under pure H2 (flow rate: 60 mL min−1) at 400 °C for 3 h before the catalytic reaction.

Measurement of catalytic activities

The toluene hydrogenation with Ru catalysts was performed in a 100 mL stainless steel stirred pressure reactor. A given amount of the Ru catalyst (0.085 g) and toluene (30 mL, >99.5 %, Quzhou Juhua Reagent Co., Ltd.) were placed in the reactor. The reaction pressure was generated by using H2 at the reaction temperature and kept constant. The reaction was stopped after no uptake of H2 was observed. The product was analyzed with an Agilent 7890A gas chromatograph equipped with a DB-1 capillary column and a flame ionization detector.

Characterization

The powder XRD patterns were recorded on a Rigaku D/MAX 2500 PC diffractometer by using CuKα radiation (40 kV and 100 mA) over the range 0.5°≤2 θ≤10° (low angle) and 10°≤2 θ≤80° (high angle). N2 adsorption–desorption isotherms were determined at −196 °C on a Micromeritics ASAP 2020 system in the static measurement mode. Samples were outgassed at 350 °C for 10 h before the adsorption measurement was performed. The BET method was used to calculate the specific surface area. The HRTEM images of samples were obtained with an FEI Tecnai G20 instrument. Energy-dispersive X-ray spectroscopy was performed for the analysis of sample composition. The dispersion of Ru was obtained by using the CO chemisorption technique, which was performed at 40 °C on a Quantachrome Autosorb-1/C chemisorb apparatus. Before measurements, pre-reduced catalysts were reduced in situ for 2 h at 450 °C under pure H2. The metal dispersion and particle size were estimated on the basis of the assumption of a spherical geometry of the particles and an adsorption stoichiometry of one CO molecule on one Ru surface atom. The average Ru particle size was calculated by using the equation d D=6 M ρsite/(ρmetalN), in which d is the Ru particle size, D is dispersion, M is the molecular mass (for Ru: 101 g mol−1), ρsite is the Ru surface site density (for Ru: 16.3 atoms nm−2), ρmetal is the metal density (for Ru: 12.3 g cm−3), and N is the Avogadro number (6.022×1023 mol−1), giving d=1.33/D (nm).24 The methanation of the carbon support was characterized by using H2-temperature programmed reduction and detected with a thermal conductivity detector and mass spectrometer. The catalyst (200 mg) was first treated with 5 %H2/Ar (flow rate: 30 mL min−1) at 100 °C for 1 h. The temperature was then increased from 100 to 800 °C (heating rate: 10 °C min−1) under 5 % H2/Ar. The m/e signals were analyzed with a quadrupole mass spectrometer (Hiden, HPR-20 QIC). The transmission IR spectra were recorded on a Nicolet Nexus 470 FTIR spectrometer equipped with an MCT-A detector, cooled by liquid N2, with a spectral resolution of 2 cm−1, by using the KBr pellet technique.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussions
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

We thank Prof. Qi-Hua Yang and Prof. Can Li in State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, for their useful contribution to this work. The financial support from the Natural Science Foundation of China (grant no. 20803064) and the Natural Science Foundation of Zhejiang Province (grant no. Y4090348) is gratefully acknowledged.