Aqueous-Phase Hydrogenation of Succinic Acid to γ-Butyrolactone

May 16, 2014 - Charles S. Spanjers , Deborah K. Schneiderman , Jay Z. Wang , Jingyu Wang , Marc A. Hillmyer , Kechun Zhang , Paul J. Dauenhauer...
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Aqueous-Phase Hydrogenation of Succinic Acid to γ‑Butyrolactone and Tetrahydrofuran over Pd/C, Re/C, and Pd−Re/C Catalysts Zhengfeng Shao, Chuang Li, Xin Di, Zihui Xiao, and Changhai Liang* Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China ABSTRACT: Monometallic Pd/C and Re/C and bimetallic Pd−Re/C catalysts with different Re/Pd molar ratios were prepared by incipient-wetness impregnation and characterized by temperature-programmed reduction, X-ray diffraction, CO chemisorption, and transmission electron microscopy. The results indicated that there is a strong interaction between Pd and Re species and that Pd can significantly promote the reduction of rhenium oxide. The hydrogenation of succinic acid to γbutyrolactone and tetrahydrofuran was investigated over the as-prepared Pd/C, Re/C, and Pd−Re/C catalysts. Pd/C showed a low conversion of succinic acid and a high selectivity to γ-butyrolactone. Adding a small amount of Re evidently enhanced the hydrogenation activity of succinic acid and improved the yield of γ-butyrolactone, whereas more Re increased the yield of tetrahydrofuran. The main reaction pathway for the conversion of succinic acid in aqueous solution on Pd−Re/C catalysts is proposed through hydrogenation of the intermediates, including γ-butyrolactone, 1,4-butanediol, and tetrahydrofuran as the substrates.

1. INTRODUCTION With the decline of available fossil fuels and in response to environmental needs, demand for effective renewable resources that can replace the dwindling fossil fuels has continuously increased. The transformation of biomass into fuel and chemicals is currently being developed.1,2 Succinic acid (SA) has attracted much attention as a bioderived platform chemical that can be converted into γ-butyrolactone (GBL), tetrahydrofuran (THF), and 1,4-butanediol (BDO) by hydrogenation.3−10 These derivatives of SA have been widely used in the fine-chemicals and various other industries.4,11−14 Generally, in the hydrogenation of SA, the product distribution is varied through the use of different catalysts and experimental conditions. Hong and co-workers reported that monometallic catalysts such as palladium, ruthenium, and rhenium can be used in the hydrogenation of SA.15−20 Pd catalyst is a potential candidate for the selective production of GBL, and rhenium catalyst is advantageous for the selective production of THF. Various bimetallic catalysts have been reported in patents.21,22 Mabry et al. demonstrated that Pd−Re catalysts are active in the hydrogenation of maleic acid.21 Deshpande et al. proposed a speculative reaction pathway for the conversion of SA in organic mixed solvents over Ru−Co catalysts.3 n-Butanol (NBA) and n-propanol (NPA) were produced as byproducts. Recently, Minh et al. and Ly et al. studied the effect of the addition of Re to 2% Pd catalysts supported on carbon or TiO2 in the hydrogenation of an aqueous SA solution at 160 °C and 150 bar.23,24 They found that the bimetallic catalysts became selective to BDO after addition of Re. It is likely that bimetallic catalysts with typical hydrogenation metals (e.g., Pd) promoted by oxophilic metals (e.g., Re) could be advantageous for the hydrogenation of SA. It is thus meaningful to study the effect of the addition of Re to bimetallic Pd−Re/C catalysts in the hydrogenation of SA, as 4 wt % Re−2 wt % Pd/C catalyst was reported previously in the literature.23 © XXXX American Chemical Society

In this work, we prepared a series of Pd−Re/C catalysts with different Re/Pd molar ratios and investigated the interaction of Pd and Re in the bimetallic Pd−Re/C catalysts by temperatureprogrammed reduction (TPR), X-ray diffraction (XRD), CO chemisorption, and transmission electron microscopy (TEM). The catalytic transformation of SA was evaluated over monometallic Pd/C and Re/C and bimetallic Pd−Re/C catalysts. The expected aim was to study the effects of the composition and structure of catalysts on the product distribution in the hydrogenation of SA. The reaction path was also studied through the hydrogenation of the intermediate products, which explained the low yield of BDO over the Pd− Re/C catalysts in the aqueous-phase catalytic hydrogenation of SA.

2. EXPERIMENTAL SECTION 2.1. Materials. Activated carbon (derived from coconut, surface area of 1462 m2/g and pore volume of 0.7 mL/g) was supplied by SenSen Activated Carbon Industry Science and Technology Co., Ltd., Fujian, China. NH4ReO4 (99%) was purchased from Aldrich. HNO3, Pd(NO3)2 (99%), SA, GBL, THF, and BDO were purchased from Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. All chemicals and reagents used in this work were of analytical grade. 2.2. Catalyst Preparation. First, activated carbon was milled and sieved to 20−40 mesh size. To remove impurities and modify surface functional groups, the activated carbon (16.0 g) was refluxed at 90 °C in 10 M HNO3 (160 mL) for 4 h. After that, the mixture was cooled to room temperature and then filtered and washed repeatedly using distilled water. Received: February 14, 2014 Revised: May 12, 2014 Accepted: May 16, 2014

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with a flame ionization detector (FFAP column, 30 m × 0.25 mm, N2 as the carrier gas).

Finally, the obtained activated carbon was dried at 120 °C for 12 h. The supported Pd and Re catalysts were prepared by incipient-wetness impregnation of activated carbon with aqueous solutions of Pd(NO3)2 and NH4ReO4, respectively. The supported bimetallic Pd−Re catalysts were prepared by impregnating the support with an aqueous solution of both Pd(NO3)2 and NH4ReO4. Exactly 1.0 g of solution was used for every gram of support. After impregnation, the catalysts were dried in air at 110 °C for 4 h and finally reduced in H2 at 300 °C for 3 h. Monometallic 2 wt % Pd/C and 4 wt % Re/C catalysts and bimetallic Pd−Re/C catalysts were prepared. In the bimetallic catalysts, the Re content was changed, but the Pd loading was kept constant at 2 wt %, with Re/Pd molar ratios of 0.3, 0.6, and 1.1, respectively. All catalysts were prepared at a gram scale to ensure that the catalysts used in the experiments were consistent. 2.3. Catalyst Characterization. XRD analysis of the samples was carried out using a Rigaku D/Max-RB diffractometer with a Cu Kα monochromatized radiation source (λ = 1.54178 Å), operated at 40 kV and 100 mA. Diffraction data were collected in the 2θ range of 5−90° at a scan rate of 10°/min. The morphology and structure of the samples were examined by transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin, 300 kV transmission electron microscope) operated at 200 kV. The samples for TEM were prepared by ultrasonically dispersing the powder samples in ethanol and dropping the dispersions on carbon-coated Cu grids. Static CO chemisorption measurements were performed using a Quantachrome Autosorb automated gas sorption system. Before the analyses, the catalysts were reduced in flowing H2 (50 mL/min) at a temperature of 300 °C ramped at 5 °C/min and held for 2 h. After reduction, the catalyst was degassed at 10−6 mbar and 300 °C for 2 h to eliminate chemisorbed hydrogen and water. The isotherms were measured at 20 °C. Calculations were made with the amount of irreversibly adsorbed CO, which was defined as the difference between total adsorption and reversible adsorption isotherms. TPR experiments were carried out using a Quantachrome ChemBET-3000 analyzer with a thermal conductivity detector (TCD). About 50 mg of sample was placed in a quartz tubular reactor and pretreated in a helium stream, heated to 200 °C at a rate of 10 °C/min, and held for 60 min. After the sample had cooled, a gaseous mixture of 10% hydrogen in argon was fed at a flow rate of 30 mL/min to the sample. The TPR heating rate was 10 °C/min. 2.4. Hydrogenation of SA and Its Hydrogenated Intermediates. Liquid-phase hydrogenation was performed in a 50 mL hastelloy autoclave with a mechanical stirrer and an electric temperature controller. In a typical reaction, the reactor was loaded with 0.20 g of reduced catalyst and 20.00 g of aqueous solution including 2.00 g of SA (or 1.46 g of GBL, 1.22 g of THF, or 1.53 g of BDO). The reactor was purged with hydrogen to remove air and then pressurized to 8 MPa using hydrogen. The reactor was heated to 240 °C. The catalytic reaction was carried out for 2−10 h. During the reaction, the reaction mixture was stirred at 800 rpm to avoid mass-transfer limitations. After the reaction, the aqueous samples taken from the reactor were analyzed using both a Waters high-performance liquid chromatograph equipped with UV and refractive index (RI) detectors (Symmetry C18 column, 3.5 μm, 4.6 × 75 mm, part no. WAT066224, H2O as the mobile phase at a flow rate of 0.5 mL/min) and a 7980F gas chromatograph equipped

3. RESULTS AND DISCUSSION The TPR profiles of Pd/C, PdRe0.3/C, PdRe0.6/C, PdRe1.1/C, and Re/C catalysts are shown in Figure 1. As the reduction of

Figure 1. Temperature-programmed reduction profiles of the asprepared catalysts: (a) Pd/C, (b) PdRe0.3/C, (c) PdRe0.6/C, (d) PdRe1.1/C, (e) Re/C.

Figure 2. XRD patterns of the as-prepared catalysts: (a) activated carbon as a reference, (b) Pd/C, (c) PdRe0.3/C, (d) PdRe0.6/C, (e) PdRe1.1/C, (f) Re/C.

Table 1. CO Uptakes of Monometallic Pd/C and Re/C and Bimetallic Pd−Re/C Catalysts metal content [μmol/(g of catalyst)] catalyst

Pd

Re

CO uptake (μmol/g)

metal dispersiona CO/(Pd + Re)

Pd/C PdRe0.3/C PdRe0.6/C PdRe1.1/C Re/C

187.9 187.9 187.9 187.9 0

0 53.7 107.4 214.8 214.8

10.4 17.1 39.3 61.0 32.1

0.055 0.071 0.133 0.151 0.149

a

Irreversible adsorption of CO calculated per each metal atom in bimetallic Pd−Re.

the catalysts was performed in H2 at 300 °C in the preparation process before TPR, the reduction profile of Pd/C consisted of B

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A similar phenomenon was observed for Pd−Re/A2O3 and Pt− Re/C catalysts.28,29 Simonetti et al. suggested that rhenium oxide species have high mobility on carbon and can migrate toward reduced Pt centers to form bimetallic particles.29 Thus, it is likely that, on Pd−Re/C catalysts, mobile rhenium oxide species can migrate toward reduced Pd particles, where Re reduction takes place. Figure 2 shows XRD patterns of the catalysts. Strong Pd peaks could be seen for the Pd/C and the bimetallic catalysts. The introduction of Re to Pd did not cause strong changes in the positions of the Pd(111) reflections. This can be explained by the fact that the covalent radii of Pd and Re are similar (0.128 nm).28 The Re(002) reflection at about 43° for the Re/ C catalyst showed a broad peak, which indicated a good dispersion of Re on the support. Table 1 reports the results of CO chemisorption on monometallic Pd/C and Re/C and bimetallic catalysts. It can be seen that the Pd/C catalyst had lower CO uptake than monometallic Re/C catalyst. The PdRe0.3/C catalyst showed a slight increase in CO uptake compared with Pd/C, whereas PdRe0.6/C catalyst exhibited an obvious increase. The results suggest a strong interaction between Pd and Re. With the addition of a small amount of Re to Pd/C, Re could enter into the lattice of Pd, and the catalyst exhibited a CO uptake similar to that of Pd/C. With the gradual introduction of Re, Re could become enriched on the bimetallic particles.28,30,31 Furthermore, the CO uptake for the bimetallic PdRe1.1/C catalyst was more than the total CO uptake of the monometallic Pd/C and Re/C catalysts, which also suggests a close interaction between Pd and Re. The TEM images of Pd/C and PdRe0.6/C in Figure 3 further indicate the interaction between Pd and Re. For the Pd/C catalyst, the Pd particles are spherical and homogeneously dispersed on the activated carbon, with no appearance of agglomeration in Figure 3a. The fringe spacing of 2.35 Å in Figure 3a′ deviates distinctly from the Pd(111) lattice plane spacing (2.25 Å) with a face-centered-cubic (fcc) structure, which can be ascribed to the 1/3(422) reflection that is normally forbidden in an fcc structure. This observation is in good agreement with previous publications.32−34 For the PdRe0.6/C catalyst, the particles are anomalous and asymmetrical, with some appearance of agglomeration in Figure 3b. Figure 3b′ shows the high-resolution TEM (HR-TEM) image

Figure 3. TEM images of the as-prepared catalysts: (a,a′) Pd/C, (b,b′) PdRe0.6/C.

a broad peak with a maximum at 740 °C, accompanied by methane emission and results from hydrogenation of surface functional groups on the activated carbon support.25−27 The results indicated that Pd(NO3)2 was fully reduced during the preparation process. The reduction profile of Re/C consisted of two peaks. The peak at approximately 370 °C was caused by the reduction of rhenium oxide,28,29 and the broader peak centered at approximately 600 °C was caused by gasification of the support. With a gradual introduction of Re to Pd/C, the peaks caused by gasification of the support shifted to lower temperature from approximately 710 to 624 °C. Interestingly, the bimetallic catalysts did not show peaks at approximately 370 °C, whereas some other peaks centered at 140−230 °C were more obvious. The results demonstrated that Pd could significantly promote the reduction of rhenium oxide and the catalyst could be reoxidized after exposure to oxygen in the air.

Figure 4. (a) Relative concentration and (b) product selectivity in the hydrogenation of SA over Pd/C as a function of reaction time. C

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Figure 5. (a) Relative concentration and (b) product selectivity in the hydrogenation of SA over 4% Re/C as a function of reaction time.

Figure 6. (a) Relative concentration and (b) product selectivity in the hydrogenation of SA over PdRe0.3/C as a function of reaction time.

Figure 7. (a) Relative concentration and (b) product selectivity in the hydrogenation of SA over PdRe0.6/C as a function of reaction time.

of agglomerated particles on the edge of the support. The particles with fringes separated by 2.44 and 2.33 Å can be ascribed to the 1/3(422) reflection. The fringe spacing of 1.98 Å is similar to the Pd(200) lattice plane spacing (1.95 Å). The results indicate the formation of Pd particles. The fringe spacing of 2.05 Å is between the Re(101) lattice plane spacing (2.11 Å)

and the Pd(200) lattice plane spacing (1.95 Å), which indicates the formation of an alloy in the bimetallic catalysts. According to the TPR results, the reduction of Re needs higher temperature than the reduction of Pd. The previously formed Pd catalyzes the reduction of rhenium oxide and forms D

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Figure 8. (a) Relative concentration and (b) product selectivity in the hydrogenation of SA over PdRe1.1/C as a function of reaction time.

asymmetrical and anomalous particles around it. The observations in Figure 3 demonstrate this interaction. Figure 4 shows the relative concentration and product selectivity in the hydrogenation of SA over the Pd/C catalyst as a function of reaction time. When the reaction time was 2 h, GBL and a small quantity of THF were detected in the products. With increasing reaction time, the relative concentration of SA decreased slowly, and GBL, as the main product, increased. Meanwhile, more products, such as BDO, NBA, and NPA, were also detected. When the reaction time reached 10 h, the conversion of SA was 48%, and the yield of GBL was 45%. The results indicate that the Pd/C catalyst has a low activity for the further reaction of GBL and a high selectivity to GBL in the hydrogenation of SA. For the Re/C catalyst in Figure 5, the relative concentration of SA decreased faster than for Pd/C in Figure 4a. It was also noticed that the relative concentration of GBL increased before decreasing as the reaction time prolonged. The yield of GBL reached 64% at the highest relative concentration of GBL, and the yield of THF was 11% at this point. The relative concentration of THF increased rapidly after the relative concentration of GBL decreased. After a reaction time of 10 h, the conversion of SA reached 87%, the yield of GBL decreased to 42%, and the yield of THF increased to 39%. The results showed that Re was advantageous for dehydration and hydrogenation and that the hydrogenation of SA was a consecutive reaction on Re/C, in which GBL obtained from SA was subsequently converted into THF. BDO, NBA, and a little NPA were detected in the products, which indicated that the Re/C catalyst was disadvantageous for the formation of NPA.

Figure 9. Conversion of SA over monometallic and bimetallic catalysts.

Table 2. Effect of Re Addition to Pd/C on Catalyst Activity

a

catalyst

conversion (%)

time (h)

TOFa (h−1)

Pd/C PdRe0.3/C PdRe0.6/C PdRe1.1/C Re/C

20 20 20 20 20

2.8 0.6 0.6 0.4 1.3

582 1650 718 694 405

Turnover frequency.

Table 3. Product Distributions Resulting from Hydrogenation of GBL, BDO, and THF catalyst

substrate

conversion (%)

selectivity to GBL (%)

selectivity to THF (%)

selectivity to BDO (%)

selectivity to NBA (%)

selectivity to NPA (%)

Pd/C PdRe0.6/C Re/C Pd/C PdRe0.6/C Re/C Pd/C PdRe0.6/C Re/C

GBL GBL GBL BDO BDO BDO THF THF THF

6.2 96.5 86.7 11.6 87.6 65.0 1.3 5.7 0.8

− − − 0.7 3.2 11.5 0.9 1.9 10.0

85.0 79.2 85.9 72.6 71.3 83.1 − − −

1.9 5.5 9.3 − − − 41.4 3.5 58.9

8.6 10.5 4.1 22.1 15.0 5.0 54.2 80.5 29.9

4.5 4.8 0.7 4.6 10.5 0.4 3.5 14.1 1.2

E

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Scheme 1. Reaction Pathway for the Hydrogenation of SA

and the effect of geometric structure of the particles became more prominent. For all of the catalysts, the yield of BDO was very low. Even for the PdRe1.1/C catalyst, the highest yield of BDO was only 4%. To clarify the reaction path for the conversion of SA on Pd−Re/C catalysts, experiments were performed with GBL, BDO, and THF as the substrates. The catalytic reaction was carried out for 10 h. The product distributions resulting from these experiments are reported in Table 3. As can be seen from the table, the bimetallic Pd−Re catalyst exhibited a higher activity than the Pd/C and Re/C catalysts. The Pd/C catalyst exhibited a low activity when GBL was charged as the substrate in the experiments. The results indicated that Pd/C could have a high selectivity to GBL in the hydrogenation of SA, which was consistent with Figure 4. Obviously, Re/C and the bimetallic Pd−Re/C catalyst exhibited higher activities than Pd/C when GBL was charged as the substrate, and the main product was THF. Both of these catalysts demonstrated that Re had a high oxidative-hydrogenation activity. There was little further conversion of THF after 10 h of reaction, which showed that none of the catalysts were active for the hydrogenation of THF. These results can be explained by the effect of thermodynamics, as THF is thermodynamically favored.35 These results were in agreement with the high yield of THF in the hydrogenation of SA over Pd−Re/C catalyst. When BDO was charged as the substrate, Re/C and the bimetallic Pd−Re/C catalyst exhibited high activities, and the main product was THF. The results further demonstrated that BDO was a highly reactive intermediate that was easily converted into THF and NBA. For this reason, it was helpful to explain that the yield of BDO was very low on our catalysts in the hydrogenation of SA. Deshpande et al. reported one possible reaction pathway starting from SA on the Ru−Co bimetallic catalysts.3 There are different pathways for the hydrogenation of BDO because of the different solvent we used. Thus, a modified main reaction pathway for the conversion of SA in aqueous solution on Pd− Re/C is shown in Scheme 1. SA was first subjected to simultaneous dehydration and hydrogenation to produce GBL. Through a further hydrogenation and dehydration route, GBL was subsequently converted into THF. Simultaneously, BDO could also be produced through a hydrogenation route, but it was not the main product and it could be converted into THF through dehydration. In the final step, some byproducts, such as NBA and NPA, could be produced from BDO or THF.

From the results in Figure 6, the introduction of Re into Pd/ C significantly improved its catalytic performance. GBL, THF, BDO, NBA, and NPA were detected in the products. After a reaction time of 2 h, the conversion of SA reached 69%. At the highest relative concentration of GBL, the yield of GBL was 74%. Furthermore, the selectivity to GBL decreased slowly over a wide range of reaction times, and it was advantageous to maintain a high yield of GBL. When the reaction time was 10 h, the conversion of SA reached 92%, and the yield of THF was 30%. The yield of byproducts (NBA + NPA) was less than 3%. The improvement in the conversion of SA can be explained by the interaction between Pd and Re. The high selectivity to GBL over a wide range of reaction times indicates that little Re was exposed on the surface of the bimetallic particles when a small amount of Re was added to Pd/C. For the PdRe0.6/C catalyst in Figure 7, the changes in the relative concentration of SA were similar to those for PdRe0.3/ C. However, the selectivity to GBL decreased obviously, and the selectivity to THF increased rapidly with the reaction time. At the highest relative concentration of GBL, the yield of GBL was 72%. When the reaction time was 10 h, the conversion of SA reached 94%, and the total yield of GBL and THF was 84%. A 55% yield of THF was attained, accompanied by a 7% yield of byproducts. The results indicate that more Re was exposed on the surface of the bimetallic particles, in good agreement with the TPR, CO chemisorption, and TEM results. For the PdRe1.1/C catalyst, more metal atoms on the support made the reaction go faster. In Figure 8, at a reaction time of 2 h, the conversion of SA reached 89%, and the yield of GBL was only 51%. After that, the relative concentration of THF increased, and more byproducts were produced. At the reaction time of 10 h, the yield of THF reached 65%, with a 17% yield of byproducts. Figure 9 shows the conversion of SA in the hydrogenation of SA over monometallic Pd/C and Re/C catalysts and three bimetallic Pd−Re/C catalysts. It can be seen that the bimetallic Pd−Re/C catalysts significantly increased the conversion of SA compared with the monometallic catalysts. Because of the different metal loadings on the catalysts, we calculated the turnover frequency (TOF) at 20% SA conversion on the basis of CO uptakes by CO chemisorption measurements, assuming that the number of active sites on the catalysts is equal to the number of adsorbed CO molecules. The results are reported in Table 2. It can be observed that the PdRe0.3/C catalyst was more active than the other catalysts. This can be explained by electronic and geometric effects. For the addition of a small amount of Re, Re could diffuse into the lattice of Pd, and the electronic effect played an important role in improving the activity of the catalyst. As the amount of Re increased, the surfaces of the particles became progressively enriched with Re,

4. CONCLUSIONS The hydrogenation of SA was carried out over monometallic Pd/C and Re/C and bimetallic Pd−Re/C catalysts with the same palladium content. Pd could significantly promote the reduction of rhenium oxide. The bimetallic Pd−Re/C catalysts F

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anhydride to tetrahydrofuran over palladium. Catal. Today 1999, 50, 589−601. (14) Jeong, H.; Kim, T. H.; Kim, K. I.; Cho, S. H. The hydrogenation of maleic anhydride to γ-butyrolactone using mixed metal oxide catalysts in a batch-type reactor. Fuel Process. Technol. 2006, 87, 497− 503. (15) Hong, U. G.; Hwang, S.; Seo, J. G.; Yi, J.; Song, I. K. Hydrogenation of succinic acid to γ-butyrolactone over palladium catalyst supported on mesoporous alumina xerogel. Catal. Lett. 2010, 138, 28−33. (16) Hong, U. G.; Lee, J.; Hwang, S.; Song, I. K. Hydrogenation of succinic acid to γ-butyrolactone (GBL) over palladium-alumina composite catalyst prepared by a single-step sol-gel method. Catal. Lett. 2011, 141, 332−338. (17) Hong, U. G.; Hwang, S.; Seo, J. G.; Lee, J.; Song, I. K. Hydrogenation of succinic acid to γ-butyrolactone (GBL) over palladium catalyst supported on alumina xerogel: Effect of acid density of the catalyst. J. Ind. Eng. Chem. 2011, 17, 316−320. (18) Hong, U. G.; Park, H. W.; Lee, J.; Hwang, S.; Song, I. K. Hydrogenation of succinic acid to γ-butyrolactone (GBL) over ruthenium catalyst supported on surfactant-templated mesoporous carbon. J. Ind. Eng. Chem. 2012, 18, 462−468. (19) Hong, U. G.; Park, H. W.; Lee, J.; Hwang, S.; Yi, J.; Song, I. K. Hydrogenation of succinic acid to tetrahydrofuran (THF) over rhenium catalyst supported on H2SO4-treated mesoporous carbon. Appl. Catal. A: Gen. 2012, 415−416, 141−148. (20) Hong, U. G.; Kim, J. K.; Lee, J.; Lee, J. K.; Song, J. H.; Yi, J.; Song, I. K. Hydrogenation of succinic acid to tetrahydrofuran (THF) over ruthenium−carbon composite (Ru−C) catalyst. Appl. Catal. A: Gen. 2014, 469, 466−471. (21) Mabry, M. A.; Prichard, W. W.; Ziemecki, S. B. Pd/Re hydrogenation catalyst for making tetrahydrofuran and 1,4-butanediol. U.S. Patent 4,609,636, Sep 2, 1986. (22) Schwartz, J. T. Ru,Re/carbon for hydrogenation in aqueous solution. U.S. Patent 5,478,952, Dec 26, 1995. (23) Minh, D. P.; Besson, M.; Pinel, C.; Fuertes, P.; Petitjean, C. Aqueous-phase hydrogenation of biomass-based succinic acid to 1,4butanediol over supported bimetallic catalysts. Top. Catal. 2010, 53, 1270−1273. (24) Ly, B. K.; Minh, D. P.; Pinel, C.; Besson, M.; Tapin, B.; Epron, F.; Especel, C. Effect of addition mode of Re in bimetallic Pd−Re/ TiO2 catalysts upon the selective aqueous-phase hydrogenation of succinic acid to 1,4-butanediol. Top. Catal. 2012, 55, 466−473. (25) Ramos, A. D.; Alves, P. S.; Aranda, D. A.; Schmal, M. Characterization of carbon supported palladium catalysts: Inference of electronic and particle size effects using reaction probes. Appl. Catal. A: Gen. 2004, 277, 71−81. (26) Sepúlveda-Escribano, A.; Coloma, F.; Rodríguez-Reinoso, F. Platinum catalysts supported on carbon blacks with different surface chemical properties. Appl. Catal. A: Gen. 1998, 173, 247−257. (27) Fraga, M. A.; Jordão, E.; Mendes, M. J.; Freitas, M. A.; Faria, J. L.; Figueiredo, J. L. Properties of carbon-supported platinum catalysts: Role of carbon surface sites. J. Catal. 2002, 209, 355−364. (28) Malinowski, A.; Juszczyk, W.; Bonarowska, M.; Pielaszek, J.; Karpiński, Z. Pd−Re/Al2O3: Characterization and catalytic activity in hydrodechlorination of CCl2F2. J. Catal. 1998, 177, 153−163. (29) Simonetti, D. A.; Kunkes, E. L.; Dumesic, J. A. Gas-phase conversion of glycerol to synthesis gas over carbon-supported platinum and platinum−rhenium catalysts. J. Catal. 2007, 247, 298− 306. (30) Bonarowska, M.; Malinowski, A.; Karpiński, Z. Hydrogenolysis of C−C and C−Cl bonds by Pd−Re/Al2O3 catalysts. Appl. Catal. A: Gen. 1999, 188, 145−154. (31) Juszczyk, W.; Karpiński, Z. Hydrocarbon reactions on Pd−Re/ Al2O3 catalysts. Appl. Catal. A: Gen. 2001, 206, 67−78. (32) Schlotterbeck, U.; Aymonier, C.; Thomann, R.; Hofmeister, H.; Tromp, M.; Richtering, W.; Mecking, S. Shape-selective synthesis of palladium nanoparticles stabilized by highly branched amphiphilic polymers. Adv. Funct. Mater. 2004, 14, 999−1004.

exhibited higher conversion of SA than monometallic catalysts because of the strong interaction between Pd and the Re species. It was found that the addition of a small amount of Re could enhance the hydrogenation activity of SA and improve the yield of GBL, whereas more Re could increase the yield of THF. Based on separate experiments with GBL, BDO, and THF as substrates, a main reaction pathway for the conversion of SA in aqueous solution on Pd−Re/C catalysts explained the low yield of BDO.



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*Tel.: +86-411-84986353. Fax: +86-411-84986353. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21073023 and 21373038) and the Fundamental Research Funds for the Central Universities (DUT12YQ03).



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