Article pubs.acs.org/IECR
Dimethyl Terephthalate Hydrogenation to Dimethyl Cyclohexanedicarboxylates over Bimetallic Catalysts on Carbon Nanotubes Yangqiang Huang, Yao Ma, Youwei Cheng,* Lijun Wang, and Xi Li Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ABSTRACT: The synthesis and utilization of bimetallic catalysts for the hydrogenation of dimethyl terephthalate (DMT) to dimethyl cyclohexanedicarboxylates (DMCD) is described in this Article. A variety of techniques, such as low temperature N2 adsorption−desorption, transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and H2 temperature-programmed desorption were employed to characterize both the supports and the catalysts. The influence of various operational parameters, for example reaction temperature, pressure, and time, on the catalytic performance for the hydrogenation of DMT was systematically analyzed. Under optimized conditions, a DMT conversion of 80% along with DMCD selectivity of 95% were achieved. Furthermore, efforts were also made to probe the catalyst stability. The enhanced catalytic performance of optimized Ru−Ni/CNT catalysts can be attributed to the tight immobilization of the metal particles on the carbon nanotubes and the unique properties of the nitric acid-treated carbon nanotubes.
1. INTRODUCTION Dimethyl cyclohexanedicarboxylates (DMCD) are valuable chemicals used in the manufacture of coating reins as well as in the production of 1,4-cyclohexanedimethanol (CHDM), a linker molecular in the polymer industry, which is a highly valued and significantly used reagent.1 It is, for example, preferred over ethylene glycol as a stepping stone in the production of polyester fibers for extensive use in photography, in antifogging agents, and in other applications involving polycarbonates and polyurethanes.2,3 Nowadays, it is prepared industrially by a two-step process using two reactors. The first step is the highly exothermic conversion of dimethyl terephthalate (DMT) into DMCD by using a supported Pd catalyst in the temperature range of 433−453 K and an H2 pressure of 30−48 MPa. The intermediate DMCD is then converted into CHDM by using a copper chromite catalyst at temperatures of about 473 K and a H2 pressure of about 4 MPa (Figure 1).1 We can figure out that the reported feasible catalytic route to CHDM is a two-step process, under harsh circumstances of high pressure and relatively high temperature. It is relevant to note that the DMT to DMCD conversion using a rhodium complex tethered to silica-supported palladium was carried out using Angelici’s special catalyst.4 Recently, Thomas and co-workers reported that bimetallic nanoparticle catalysts,5 such as (silica-supported) Ru5Pt, Ru10Pt2, Ru6Pd6, and Ru12Cu4, and a trimetallic cluster Ru5PtSn,6 can promote the single-step conversion of DMT into CHDM under mild reaction conditions. However, both the conversion and selectivity were not that satisfactory. Also, Fazhi Zhang et al. prepared Pd nanoparticles supported on modified porous θ-alumina by a wet impregnating method, which achieved quite high activity and selectivity.7 Carbon nanotubes (CNTs), since being first reported by Iijima,8,9 have received much attention, and these materials are attractive supports in heterogeneous catalytic processes and are being applied to fields such as composite elaboration and © 2014 American Chemical Society
directly introduced to electrodes. In summary, these special one-dimensional carbon nanomaterials possess specific adsorption properties, high resistance to abrasion, and excellent electronic properties, characteristics that make them promising for the near future.10,11 Hung Vu et al. developed a bimetallic catalyst supported on CNTs for selective hydrogenation of cinnamaldehyde under mild conditions. They found that CNTs as mainly comprising mesoporous structures will effectively avoid the mass transfer limitation and lead to more active and selective performance.12 More recently, Inusa Abdullahi et al. demonstrated the use of surface-modified carbon nanotubes without metal loading in the partial oxidation of ethanol to acetaldehyde.13 They further concluded that carbon nanotubes truly had special properties and could be highly effective in the design of new carbon-based nanostructure catalysts. Among hydrogenation catalysts, Pd has long been commonly used as an effective active component.14−16 But owing to the high cost and low abundance of palladium, researchers are devoted to seeking highly efficient and stable but more economical catalysts to replace Pd-based catalysts. Ru-based catalysts are attractive and have been extensively investigated in hydrogenation processes.17,18 For example, Ö zkar19,20 has reported that colloidal nanozeolite framework-stabilized ruthenium(0) nanoclusters act as a superb catalyst toward aromatic substrates with record catalytic activity and lifetime even under mild conditions. Likewise, Jasra21 synthesized ruthenium-containing hydrotalcite (Ru-HT), which was used as a catalyst for hydrogenation of benzene to cyclohexane. It had already been established in other contexts that there is special merit in using multimetallic22−25 nanoparticles or clusters in that their catalytic performance is generally far Received: Revised: Accepted: Published: 4604
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Figure 1. Industrial synthesis of CHDM via two reactions.
Subsequently, hydrazine hydrate solution was added with the initial pH of the suspension adjusted to 10−11 by using aqueous NaOH solution, where spontaneous reduction results in the formation of Ni particles. After 12 h, the suspension was filtered, washed thoroughly with deionized water, and dried in a vacuum oven (at 333 K, 6 h). The Ni/CNT catalysts were obtained by calcinations (at 573 K, 3 h) in a muffle. Ru-supported catalysts with Ru loading of 0.4 wt % were obtained by subsequently impregnating an aqueous solution of RuCl3 precursor with the as-synthesized Ni/CNT (uncalcined), followed by magnetic stirring at room temperature, for the fabrication of the Ru−Ni/CNT. Then the bimetallic catalysts were obtained by filtration. The residue was washed with deionized water followed by vacuum drying at 333 K for 6 h. The catalysts were activated by calcinations under N2 at 573 K for 3 h. The Ru/CNT was prepared by excess solvent impregnation of the metal precursor on the supports at room temperature for 12 h, a technique called the “dipping method”. Then, after filtration, the catalysts were washed thoroughly with deionized water and dried in a vacuum oven at 333 K for 6 h. The catalysts were activated by calcinations (at 573 K, 3 h) under N2.32For comparison, the Ni/AC, Ru/AC and Ru−Ni/AC catalysts were prepared by the impregnating method as aforementioned. 2.3. Catalyst Characterization. Low temperature N2 adsorption−desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020 instrument. The surface area was calculated by the Brunauer−Emmet−Teller (BET) equation while the pore volume and the pore size distribution were analyzed by the Barrett−Joyner−Halenda (BJH) theory from the desorption branches of the adsorption isotherms. XRD measurements for the dried and powdered samples were conducted on an X-ray diffractometer (Shimadzu, Japan) equipped with Cu Kα radiation (40 kV, 30 mA) (λ = 1.5406 Å) and using a 2θ angle ranging from 10° to 90°, a scanning rate of 10°/min, and a step size of 0.02°/s. The morphology and EDX analysis of the catalysts were studied by scanning electron microscope (SEM) combined with energy-dispersive X-ray spectroscopy (EDX) (Hitachi S-3700N). Transmission electron microscopy (TEM) measurements of the catalyst were carried out using a Tecnai G2 F20 S-TWIN (FEI) with an accelerating voltage of 200 kV, where the samples were prepared by dipping the copper grid in the catalyst dispersed in ethanol and allowing water to evaporate. H2 temperatureprogrammed desorption (H2-TPD) was performed on a Micromeritics ASAP 2920 instrument. The process of H2 adsorption was performed in H2−Ar (5 vol% H2) gas mixture at 323 K for 1 h. H2 desorption was then performed in Ar by increasing the temperature to 1073 K at the heating rate of 10 K/min. A thermal conductivity detector (TCD) was used to determine the amount of hydrogen consumption during the
superior to that of the single metal alone. For example, Thomas et al.26 found that nanoparticles of Pd and Ru alone were each far less active and less selective as hydrogenation catalysts than their Pd6Ru6 nanoparticle counterparts. Also they discovered that bimetallic nanocatalyst systems were more resistant to sulfur poisoning. Likewise, Uffalussy et al.27 reported a clusterderived catalyst, where the majority of the metal particles present were trimetallic in nature, and has been applied in citral hydrogenation. They found that formation of these catalysts resulted in close proximity of the three metals and greatly improved both the activity and selectivity. As Ru−Ni bimetallic catalysts have been described in a few reports,28−30 herein we report the use of this catalytic system for DMT hydrogenation. Both the catalysts and the supports were characterized using different techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed desorption (H2-TPD). Optimization of the catalytic performance of the nanobimetallic Ru−Ni/CNT catalysts in hydrogenation of dimethyl terephthalate (DMT) to dimethyl cyclohexanedicarboxylates (DMCD) was conducted, including both the catalyst preparation factors (different surface-modified methods) and the reaction operating conditions (reaction temperature, pressure, and time). With these catalysts, excellent catalytic activity even under mild reaction conditions was obtained. This work aimed to develop the catalytic application of bimetallic Ru−Ni/CNT and determine the optimal technological parameters for hydrogenation of DMT to DMCD over bimetallic Ru−Ni/CNT catalysts, which can serve as a guiding concept for industrial application.
2. EXPERIMENTAL SECTION 2.1. Materials. NiCl2·6H2O, polyvinyl pyrrolidone (PVP), hydrazine hydrate, RuCl3, and dimethyl terephthalate (DMT) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The reagents were all used as received without further purification. The water used in all experiments was deionized water. All glassware and the stainless steel autoclave were cleaned with copious rinsing with distilled water before drying in an oven at 373 K. Carbon nanotubes were obtained from Bema Environmental Science and Technology Co. Ltd. (Hangzhou, China). Active carbon was purchased from the Cabot Corporation. 2.2. Preparation of the Catalysts. The carbon nanotubes (CNTs) were modified with concentrated nitric acid (65 wt %) solution at 383 K under reflux for 12 h before use. Ru−Ni/ CNT catalysts were prepared via the wet impregnation technique described as follows:30,31 The required amount of NiCl2·6H2O was dissolved in deionized water, and then the aqueous PVP solution was added dropwise, followed by the dried carbon nanotube powder addition and stirring for 0.5 h. 4605
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The nitrogen adsorption−desorption isotherms of the three different samples are presented in Figure 2. All the isotherms of
process. XPS tests were measured with the VG ESCALAB MARK II equipment using monochromatic Mg Kα (1253.6 eV) radiation. The accurate chemical composition of each sample was determined by inductively coupled plasma atomic mass spectrometry using a Thermo Fisher Scientific ICP-MS XSeries II. 2.4. Catalysts Evaluation. All the selective hydrogenation reactions were carried out in a 100 mL stainless steel high pressure reactor (Yanzheng Laboratory Instruments, Shanghai, China). In a typical procedure for DMT hydrogenation, a certain concentration of DMT was dissolved in ethyl acetate (for example 4 wt %), and 0.1 g of 0.4% Ru−2% Ni/CNT bimetallic-supported catalyst was placed in the reactor. The reactor was sequentially flushed with flowing N2 and H2 at 4 MPa pressure before being pressurized to the desired pressure of H2. A K-type thermocouple was immersed into the liquid solution to measure the reaction temperature, and a connected pressure gauge was used to detect the reaction pressure. After the reaction, the reactor was quickly cooled in a water bath, and then the autoclave was evacuated. Liquid samples were analyzed by gas chromatography on a GC 1690 instrument (Ke Xiao, Hangzhou, China) equipped with an FID (OV-1 packed column) detector. Dodecane was chosen as the internal standard substance. The products were identified by comparison of their mass spectra with those of authentic samples. The experimental results allowed calculations of the values of DMT conversion, DMCD selectivity, and turnover frequency (TOF) to evaluate the catalyst performance. TOF was calculated as moles of DMT transformed per mole of Ru per hour. To ensure reproducibility of the results, repeated experiments were carried out under identical conditions, and data were found to be reproducible within ±2% variation.
Figure 2. Nitrogen adsorption−desorption isotherms of (a) CNT (uncalcined), (b) 2% Ni/CNT (uncalcined), and (c) 0.4% Ru−2% Ni/CNT (calcined).
the samples were typical Type IV with an obvious hysteresis loop, revealing typical mesoporous material.7The shape of each isotherm was essentially similar, which indicated that the morphology of the supports was mainly preserved after the dipping treatments, and all of them showed the characteristic of mesoporous nanostructure. The pore size distributions of CNT (uncalcined), 2% Ni/CNT (uncalcined), and 0.4% Ru−2% Ni/ CNT (calcined) are presented in Figure 3. Upon loading the Ni
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. To acquire the unique properties of the catalyst, they were first characterized with low temperature N2 adsorption−desorption to elucidate the structural features. The surface area, pore volume, and the average pore diameter are listed in Table 1. Table 1. Physical Properties of Synthesized Catalysts sample 1 2 3 4 a
carbon nanotube (uncalcined) 2% Ni/CNT (uncalcined) 0.4% Ru−2% Ni/CNT (calcined) activated carbon
SBET (m2·g−1)a Vp (cm3·g−1)b
Dp (nm)c
231
0.92
14.4
162 140
0.79 0.89
18.4 23.8
1439
0.64
2.7
Specific surface area. bPore volume. cAverage pore diameter. Figure 3. Pore size distributions of (a) CNT (uncalcined), (b)2% Ni/ CNT (uncalcined), and (c) 0.4% Ru−2% Ni/CNT (calcined).
Evidently, the carbon nanotubes possess a mesoporous nanostructure while traditional carbon materials such as activated carbon have a mean pore size around that of the microporous distribution. The employment of carbon nanotubes will significantly reduce the mass transfer limitation, and such diffusion phenomena in the micropores usually define the diffusion rate of not only the reactants but also the products to get free exchange between the catalytic center or the active sites.33 As a result, novel carbon material such as carbon nanotubes will be an attractive and competitive support in comparison with activated carbon.
species, the specific surface area decreased from 231 m2·g−1 to 162 m2·g−1 and the pore volume decreased from 0.92 cm3·g−1 to 0.79 cm3·g−1. This indicated that the Ni particles were immobilized within the channels of the carbon nanotubes which thereby led to reduction of the BET surface and the pore volume. Further introduction of Ru species caused the surface area to continue decreasing to 140 m2·g−1; however, the pore volume showed a different tendency, increasing to 0.89 cm3·g−1, 4606
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determined by ICP-MS, suggesting that the metallic nanoparticles maintained the same state in the recycled catalysts. The representative SEM images directly gave an overall view of the catalysts. Figure 5 shows that the particles were present on the surface of the carbon nanotubes. The SEM-EDX results showed the presence of ruthenium and nickle elements, which implied the successful support of the bimetallic particles. These results are in accordance with the ICP-MS-derived data, further confirming the successful preparation of the Ru−Ni bimetallic catalysts. TEM images of the untreated carbon nanotubes and the nitric acid-treated carbon nanotubes are shown in Figure 6a and Figure 6b, respectively. These images clearly reveal the importance of nitric acid treatment which resulted in the main, significant structure modification, occurring on the nanotubes tips and resulting in their opening and the formation of edges.36 The separate cut pieces and the opening tips could act as anchor sites for the metal particles. Moreover, the acid treatments eliminate the metallic impurities.37 The representative TEM and HRTEM images of the 0.4% Ru−2% Ni/CNT (calcined) catalysts (Figure 6c and Figure 6d) clearly illustrate that the bimetallic particles were well dispersed on the CNT surfaces. The bimetallic nanoparticle sizes determined from the image analysis have a relatively narrow size distribution, as shown in a typical histogram in Figure 6c. Evidently, the nanoparticle has a well-defined spherical shape with a mean size (diameter) of 37.2 ± 14.4 nm. The d-spacing of the adjacent (002) lattice of the bimetallic NPs was 2.16 Å (shown in Figure 6d), while those of the individual RuNPs and NiNPs were 2.14 Å and 2.17 Å, respectively. As shown, the supported bimetallic particles were mostly loaded between the bundles or ropes of the aggregated multiwalled carbon nanotubes (MWNTs), which led to the tight immobilization of the metal particles on the carbon nanotube supports. XPS measurements were carried out to evaluate surface composition and valence state. From the XPS spectra of the 0.4% Ru−2% Ni/CNT (calcined) (see Figure 7a), the existence of C and Ni elements was determined, but the Ru species could not be distinguished, observations which were consistent with XRD-derived data. On one hand, this might be explained by the low concentration of ruthenium; on the other hand, the abundant carbon nanotubes would easily cover the outside surface of the ruthenium, or the core−shell structure of nickel and ruthenium was synthesized31which in turn precluded capture of the signal by conventional XPS because of a lack of sensitivity. Moreover, the main peak of the Ru 3d was located at a binding energy of 284 and 280 eV, which would be totally obscured by the strong C 1s signal. To obtain more detailed information about these catalysts, a narrow scan around the main peak of nickel and ruthenium was carried out. As illustrated in Figure 7b, two intensive photoelectron peaks were ascribed to the Ni 2p3/2 and 2p1/2 binding energy (BE). A fitting routine was carried for the Ni 2p high resolution data, and the spectra were deconvoluted into three peaks assigned to Ni(0), NiO, and Ni(OH)2, respectively.38,39 An interesting phenomenon shown in Figure 7c was that the C 1s signal split into two peaks at 284.6 and 278.2 eV. The appearance of the two peaks is probably due to electron transfer between the ruthenium, nickel, and carbon, which might indicate a positive synergetic effect. The H2-TPD curves of CNT and 0.4% Ru−2% Ni/CNT (calcined) are presented in Figure 8. The bimetallic samples display a broad peak structure from 573 K to 973 K, and at
which almost reached the level of the fresh supports. Considering these phenomena, proper explanation may lie in the decomposition of the residual protective agents or reducing agents which filled the pore of the nanotubes during the process of calcination. Analysis of the average pore size indicated that the role of the metal nanoparticles was of interest in the extension of tubes, as they caused a slight increase in the catalyst pore size. The X-ray powder diffraction patterns of the CNT (uncalcined), 2% Ni/CNT (calcined), and 0.4% Ru−2% Ni/ CNT (calcined) are shown in Figure 4. The three samples
Figure 4. XRD patterns for (a) CNT (uncalcined), (b) 2% Ni/CNT (calcined), and (c) 0.4% Ru−2% Ni/CNT (calcined).
exhibited characteristic peaks of carbon at 2θ of 26.4° and 42.2° (ICDD-PDF-#26-1079), which could be ascribed to the hexagonal graphite structure.11,34,35 The corresponding lattice planes are marked in Figure 4. Furthermore, the characteristic diffraction peaks of Ni gave three signal responses. The strongest peak at 2θ = 44.5° was indexed with the (111) plane, and the other two weak peaks at 2θ = 51.8° and 2θ = 76.4° corresponded to planes (200) and (220), respectively (ICDDPDF-#65-2865). XRD analysis of the 0.4% Ru−2% Ni/CNT (calcined) catalysts showed that there were no obvious characteristic diffraction peaks of Ru species with about 0.4 wt % Ru loading. This phenomenon could be mainly attributed to the low percentage of ruthenium in the sample. Low ruthenium loading played a critical role in the bimetallic nanocatalyst system, which was proved in the “Catalyst Evaluation and Optimization” parts. Accurate Ru and Ni chemical loadings on the two different supports were determined by ICP-MS, and the results are listed in Table 2. ICP analysis of the catalysts indicated that the Ru loading was approximate to the theoretical value and the loss of metal was within the range of the control. Furthermore, the accurate Ru and Ni chemical loadings of the aged samples were Table 2. Metal Loadings of Synthesized Catalysts theoretical value (wt %)
accurate value (wt %)
catalyst
Ru loading
Ni loading
Ru loading
Ni loading
Ru−Ni/CNT Ru−Ni/AC
0.40 0.40
2.00 2.00
0.35 0.38
1.92 1.81 4607
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Figure 5. (a) SEM image of 0.4% Ru−2% Ni/CNT (calcined). (b) The SEM image from which the presented EDX analysis was obtained. (c) SEMEDX spectrum of 0.4% Ru−2% Ni/CNT (calcined). (d) The mass fraction of the bimetallic nanoparticle.
around 1073 K, a high temperature TPD peak was produced which increased the asymmetry of the broad peak. This observation indicates that the spectrum could not return to the baseline at high temperature and might be ascribed to the formation of a new peak. It has been reported that a noble metal such as ruthenium was involved in the process which was able to produce atomic hydrogen coming from the spillover of molecular hydrogen on the carbon surface.40 So, the occurrence of such peculiar results might be explained by the combined influence of the adsorbed hydrogen species and the spillover hydrogen which might be generated at the metal nanoparticles during the high operating temperature.30 Certainly, the excellent electronic properties possessed by the carbon nanotubes would also play an important role in the temperature-programmed desorption process. 3.2. Catalyst Evaluation and Optimization. In a typical experiment, the reaction conditions were as follows: 10 mL (4 wt %) of DMT/ethyl acetate and 0.1 g of 0.4% Ru−2% Ni/ CNT (calcined) catalysts were placed in the reactor. Reaction pressure was 6 MPa, reaction temperature was 373 K, and reaction time was 1 h. Under these circumstances, the conversion of DMT was 68.37% (below 70% instead of coming near to 100%), and the selectivity of DMCD was 91.50%. Under these typical reaction conditions, the influence on the reaction performance caused by differential conditions could be easily inferred. 3.2.1. Influence of Different Carbon Supports on the Catalytic Activity, Conversion, and Selectivity. The selective hydrogenation of DMT to DMCD was carried out using differently prepared supports or catalysts to determine the most suitable candidate under the introduced diverse trials. Interesting results are presented in Figure 9. First of all, the two different supports could not catalyze the hydrogenation reaction alone without metal support. This inferior perform-
ance might be attributed to the absence of active metal. The monometallic catalysts showed activity to some extent but fairly low selectivity when compared with the bimetallic system. The relatively low selectivity might be owing to the reaction progressing to produce intermediate products as testified by the GC-MS results. Furthermore, the byproducts were mainly 4methyl-1-methyl benzenecarboxylate and methyl 4-methylbenzoate, consistent with a previous report.7 Finally, the bimetallic catalyst performance experiments were conducted. Not surprisingly, the performance of these two catalysts was quite superior to that of the monometallic catalysts or the supports in the separate series. Also, the carbon nanotubes obviously displayed high activity when compared with the activated carbon not only in the conversion (68.37% vs 28.49%) but also in the turnover frequency (297.43 h−1 vs 148.01 h−1). Although the activated carbon possessed higher surface area, the relatively large reactant molecules which were accompanied by diffusion limitation might be the principle impediment during the hydrogenation reaction. On the basis of our results, the Ru− Ni/CNT catalyst was more desirable. 3.2.2. Influence of Reaction Temperature on the Catalytic Activity, Conversion, and Selectivity. Reaction temperature was shown to be particularly important for the hydrogenation reaction.41,42 Figure 10 shows the effect of reaction temperature (298−473 K) on the catalytic performance. Both the DMT conversion and the turnover frequency increased monotonously, whereas the DMCD selectivity initially rose and then fell with the increasing reaction temperature (298−473 K). It is a well-known fact that the hydrogenation of DMT to DMCD is an exothermal reaction, and from the view of chemical equilibrium, the conversion of DMT and the turnover frequency will increase while the reaction temperature rises. The conversion of DMT reached almost 100% at a temperature of 473 K. However, as the temperature increased, the gradient 4608
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Figure 6. TEM images of (a) CNT (untreated), (b) CNT (treated with nitric acid), and (c) 0.4% Ru−2% Ni/CNT (calcined). (d) HRTEM micrograph of the 0.4% Ru−2% Ni/CNT (calcined) nanocomposite. The inset in panel c indicates the size distribution of the bimetallic nanoparticle.
of the conversion and the turnover frequency became less steep and the the reaction shifted to less favored pathways, thereby decreasing the DMCD selectivity (as depicted in Figure 10). When the temperature was raised to 473 K, the selectivity of DMCD dropped to 87%. This could be attributed to the hydrocracking of the reagents or products under such a harsh reaction temperature, and the concentration of the byproducts increased. A reaction temperature of 423 K was the best for the catalytic reaction, affording about 80% DMT conversion, 95% DMCD selectivity, and 346.02 h−1 TOF. 3.2.3. Influence of Reaction Pressure on the Catalytic Activity, Conversion, and Selectivity. The effect of reaction pressure on the hydrogenation of DMT to DMCD is shown in Figure 11. With increasing H2 initial pressure, the conversion of DMT and the TOF consistently increased while the selectivity of DMCD maintained a high value, approaching 90%. It is wellknown that the H2 initial pressure plays an important role in the hydrogenation reaction.43,44 High pressure of hydrogen will not only drive the equilibrium to move in the desired direction but will also prevent the catalysts from losing activity. In accordance with the characterization results, there was convenient hydrogen adsorption and desorption over the catalyst surface accompanied by spillover hydrogen over the ruthenium and nickel bimetallic nanoparticles. The increase in reaction pressure should result in more desired products, but is is important to adhere to the specifications of the hydro-
genation reactor. On the basis of the limitations of the apparatus, a reaction pressure of 5 MPa was recommended. 3.2.4. Influence of Reaction Time on the Catalytic Activity, Conversion, and Selectivity. Figure 12 shows the reaction performance based on different reaction times. The reaction time was a dominant parameter that affected the ultimate catalyst performance. The conversion of DMT increased with the increase in reaction time (0.33 to 2 h), whereas the DMCD selectivity slightly decreased when the reaction was prolonged to 2 h, which implied the possible further hydrogenation of DMCD to CHDM or cyclohexanol.6 Furthermore, the TOF was very sensitive to the reaction time. As the reaction time was raised to 2 h, the TOF decreased severely to 151.2 h−1. As a consequence, the preferred reaction time was about 1 h, which implied that the hydrogenation of DMT to DMCD was quite rapid. Further increase in reaction time would only be detrimental to turnover frequency and would not distinctly affect conversion. Finally, experiments to evaluate the reaction stability over the 0.4% Ru−2% Ni/CNT catalyst were conducted. After the reaction, the solid catalyst was separated from the liquid product by centrifugation at a speed of 3000 rpm. The asreceived catalyst was washed thoroughly with deionized water and dried in a vacuum oven at 333 K for 6 h. The regenerated catalysts were used for the hydrogenation of DMT under the same reaction conditions as those used for the fresh catalysts. 4609
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Figure 7. XPS spectrum of 0.4% Ru−2% Ni/CNT (calcined): (a) full spectrum, (b) Ni 2p spectrum, and (c) C 1s spectrum.
Figure 9. Influence of different kinds of supports or catalysts on the catalytic activity in terms of TOF, conversion of DMT, and selectivity for DMCD. Reaction conditions: 1.0 g catalyst, 10 mL (4 wt %) of DMT/ethyl acetate, reaction pressure 6 MPa, reaction temperature 373 K, reaction time 1 h.
Figure 8. H2-TDP profiles of (a) CNT (uncalcined) and (b) 0.4% Ru−2% Ni/CNT (calcined).
No significant decrease in both the conversion of DMT and the selectivity of DMCD was observed for up to five cycles (Figure 13), which suggested that the catalysts were able to retain reactivity under the employed reaction conditions. Furthermore, the liquid reaction products were tested by the
ICP-MS, which indicated that there was no evident leaching of Ru after reaction. It was reasonable to assume that the tight immobilization of the metal particles on the carbon nanotube 4610
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Figure 12. Influence of reaction time on the catalytic activity in terms of TOF, conversion of DMT, and selectivity of DMCD. Reaction conditions: 1.0 gof 0.4% Ru−2% Ni/CNT catalyst, 10 mL (4 wt %) of DMT/ethyl acetate, reaction pressure 6 MPa, reaction temperature 373 K.
Figure 10. Influence of reaction temperature on the catalytic activity in terms of TOF, conversion of DMT, and selectivity of DMCD. Reaction conditions: 1.0 g of 0.4% Ru−2% Ni/CNT catalyst, 10 mL (4 wt %) of DMT/ethyl acetate, reaction pressure 6 MPa, reaction time 1 h.
Figure 11. Influence of reaction pressure on the catalytic activity in terms of TOF, conversion of DMT, and selectivity of DMCD. Reaction conditions: 1.0 g of 0.4% Ru−2% Ni/CNT catalyst, 10 mL (4 wt %) of DMT/ethyl acetate, reaction temperature 373 K, reaction time 1 h.
Figure 13. The reaction stability over the 0.4% Ru−2% Ni/CNT catalysts for the conversion of DMT and selectivity of DMCD. Reaction conditions: 1.0 g of catalyst, 10 mL (4 wt %) of DMT/ethyl acetate, reaction pressure 6 MPa, reaction temperature 373 K.
supports benefited from the remarkable stability of the Ru−Ni/ CNT catalyst.
process for DMT hydrogenation over Ru−Ni/CNT catalysts with high DMCD selectivity and remarkable stability. The enhanced catalytic performance of these optimized Ru−Ni/ CNT catalysts can be attributed to the tight immobilization of the metal particles on the carbon nanotube supports and the unique properties of the nitric acid-treated carbon nanotubes.
4. CONCLUSION A novel bimetallic catalyst composed of ruthenium and nickel was successfully prepared by dipping methods using nitric acidtreated carbon nanotubes as the support. Ru−Ni/CNT catalysts were found to be an efficient heterogeneous catalysts for the hydrogenation of dimethyl terephthalate to dimethyl cyclohexanedicarboxylates and both exhibited good activity and stability. The optimum operating conditions were as follows: reaction temperature of 423 K, initial reaction pressure of 5 MPa, and reaction time of 1 h. In addition, there was no significant decrease in either the activity or the selectivity when the catalysts were cycled up to five times. Although it would be premature to extensively apply these Ru−Ni/CNT catalysts in industry applications, the present study provides an optimal
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We express our appreciation for financial support from the National Natural Science Foundation of China (no. U1361112) 4611
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and the Fundamental Research Funds for the Central Universities (2013QNA4035), and we thank Professor Binghui Chen (Xiamen University) for his direction in the preparation of the catalysts.
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