Aqueous-Phase Hydrogenation of Succinic Acid to γ‑Butyrolactoneand Tetrahydrofuran over Pd/C, Re/C, and Pd−Re/C CatalystsZhengfeng 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 preparedby incipient-wetness impregnation and characterized by temperature-programmed reduction, X-ray diffraction, COchemisorption, and transmission electron microscopy. The results indicated that there is a strong interaction between Pd andRe species and that Pd can significantly promote the reduction of rhenium oxide. The hydrogenation of succinic acid to γ-butyrolactone and tetrahydrofuran was investigated over the as-prepared Pd/C, Re/C, and Pd−Re/C catalysts. Pd/C showed alow conversion of succinic acid and a high selectivity to γ-butyrolactone. Adding a small amount of Re evidently enhanced thehydrogenation activity of succinic acid and improved the yield of γ-butyrolactone, whereas more Re increased the yield oftetrahydrofuran. The main reaction pathway for the conversion of succinic acid in aqueous solution on Pd−Re/C catalysts isproposed through hydrogenation of the intermediates, including γ-butyrolactone, 1,4-butanediol, and tetrahydrofuran as thesubstrates.

1. INTRODUCTIONWith the decline of available fossil fuels and in response toenvironmental needs, demand for effective renewable resourcesthat can replace the dwindling fossil fuels has continuouslyincreased. The transformation of biomass into fuel andchemicals is currently being developed.1,2 Succinic acid (SA)has attracted much attention as a bioderived platform chemicalthat can be converted into γ-butyrolactone (GBL), tetrahy-drofuran (THF), and 1,4-butanediol (BDO) by hydrogena-tion.3−10 These derivatives of SA have been widely used in thefine-chemicals and various other industries.4,11−14

Generally, in the hydrogenation of SA, the productdistribution is varied through the use of different catalystsand experimental conditions. Hong and co-workers reportedthat monometallic catalysts such as palladium, ruthenium, andrhenium can be used in the hydrogenation of SA.15−20 Pdcatalyst is a potential candidate for the selective production ofGBL, and rhenium catalyst is advantageous for the selectiveproduction of THF. Various bimetallic catalysts have beenreported in patents.21,22 Mabry et al. demonstrated that Pd−Recatalysts are active in the hydrogenation of maleic acid.21

Deshpande et al. proposed a speculative reaction pathway forthe conversion of SA in organic mixed solvents over Ru−Cocatalysts.3 n-Butanol (NBA) and n-propanol (NPA) wereproduced as byproducts. Recently, Minh et al. and Ly et al.studied the effect of the addition of Re to 2% Pd catalystssupported on carbon or TiO2 in the hydrogenation of anaqueous SA solution at 160 °C and 150 bar.23,24 They foundthat the bimetallic catalysts became selective to BDO afteraddition of Re. It is likely that bimetallic catalysts with typicalhydrogenation 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 tobimetallic Pd−Re/C catalysts in the hydrogenation of SA, as 4wt % Re−2 wt % Pd/C catalyst was reported previously in theliterature.23

In this work, we prepared a series of Pd−Re/C catalysts withdifferent Re/Pd molar ratios and investigated the interaction ofPd and Re in the bimetallic Pd−Re/C catalysts by temperature-programmed reduction (TPR), X-ray diffraction (XRD), COchemisorption, and transmission electron microscopy (TEM).The catalytic transformation of SA was evaluated overmonometallic Pd/C and Re/C and bimetallic Pd−Re/Ccatalysts. The expected aim was to study the effects of thecomposition and structure of catalysts on the productdistribution in the hydrogenation of SA. The reaction pathwas also studied through the hydrogenation of the intermediateproducts, which explained the low yield of BDO over the Pd−Re/C catalysts in the aqueous-phase catalytic hydrogenation ofSA.

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) wassupplied by SenSen Activated Carbon Industry Science andTechnology Co., Ltd., Fujian, China. NH4ReO4 (99%) waspurchased from Aldrich. HNO3, Pd(NO3)2 (99%), SA, GBL,THF, and BDO were purchased from Sinopharm GroupChemical Reagent Co., Ltd., Shanghai, China. All chemicals andreagents used in this work were of analytical grade.

2.2. Catalyst Preparation. First, activated carbon wasmilled and sieved to 20−40 mesh size. To remove impuritiesand modify surface functional groups, the activated carbon(16.0 g) was refluxed at 90 °C in 10 M HNO3 (160 mL) for 4h. After that, the mixture was cooled to room temperature andthen filtered and washed repeatedly using distilled water.

Received: February 14, 2014Revised: May 12, 2014Accepted: May 16, 2014

Article

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Finally, the obtained activated carbon was dried at 120 °C for12 h. The supported Pd and Re catalysts were prepared byincipient-wetness impregnation of activated carbon withaqueous solutions of Pd(NO3)2 and NH4ReO4, respectively.The supported bimetallic Pd−Re catalysts were prepared byimpregnating the support with an aqueous solution of bothPd(NO3)2 and NH4ReO4. Exactly 1.0 g of solution was used forevery gram of support. After impregnation, the catalysts weredried 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/Ccatalysts and bimetallic Pd−Re/C catalysts were prepared. Inthe bimetallic catalysts, the Re content was changed, but the Pdloading was kept constant at 2 wt %, with Re/Pd molar ratios of0.3, 0.6, and 1.1, respectively. All catalysts were prepared at agram scale to ensure that the catalysts used in the experimentswere consistent.2.3. Catalyst Characterization. XRD analysis of the

samples was carried out using a Rigaku D/Max-RBdiffractometer with a Cu Kα monochromatized radiationsource (λ = 1.54178 Å), operated at 40 kV and 100 mA.Diffraction data were collected in the 2θ range of 5−90° at ascan rate of 10°/min. The morphology and structure of thesamples were examined by transmission electron microscopy(TEM, Tecnai G2 F30 S-Twin, 300 kV transmission electronmicroscope) operated at 200 kV. The samples for TEM wereprepared by ultrasonically dispersing the powder samples inethanol and dropping the dispersions on carbon-coated Cugrids. Static CO chemisorption measurements were performedusing a Quantachrome Autosorb automated gas sorptionsystem. Before the analyses, the catalysts were reduced inflowing H2 (50 mL/min) at a temperature of 300 °C ramped at5 °C/min and held for 2 h. After reduction, the catalyst wasdegassed at 10−6 mbar and 300 °C for 2 h to eliminatechemisorbed hydrogen and water. The isotherms weremeasured at 20 °C. Calculations were made with the amountof irreversibly adsorbed CO, which was defined as thedifference between total adsorption and reversible adsorptionisotherms. TPR experiments were carried out using aQuantachrome ChemBET-3000 analyzer with a thermalconductivity detector (TCD). About 50 mg of sample wasplaced in a quartz tubular reactor and pretreated in a heliumstream, heated to 200 °C at a rate of 10 °C/min, and held for60 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 thesample. The TPR heating rate was 10 °C/min.2.4. Hydrogenation of SA and Its Hydrogenated

Intermediates. Liquid-phase hydrogenation was performedin a 50 mL hastelloy autoclave with a mechanical stirrer and anelectric temperature controller. In a typical reaction, the reactorwas loaded with 0.20 g of reduced catalyst and 20.00 g ofaqueous solution including 2.00 g of SA (or 1.46 g of GBL, 1.22g of THF, or 1.53 g of BDO). The reactor was purged withhydrogen to remove air and then pressurized to 8 MPa usinghydrogen. The reactor was heated to 240 °C. The catalyticreaction was carried out for 2−10 h. During the reaction, thereaction mixture was stirred at 800 rpm to avoid mass-transferlimitations. After the reaction, the aqueous samples taken fromthe reactor were analyzed using both a Waters high-perform-ance liquid chromatograph equipped with UV and refractiveindex (RI) detectors (Symmetry C18 column, 3.5 μm, 4.6 × 75mm, part no. WAT066224, H2O as the mobile phase at a flowrate of 0.5 mL/min) and a 7980F gas chromatograph equipped

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

3. RESULTS AND DISCUSSIONThe 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

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

Figure 1. Temperature-programmed reduction profiles of the as-prepared 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) activatedcarbon 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 andBimetallic Pd−Re/C Catalysts

metal content[μmol/(g of cata-

lyst)]

catalyst Pd ReCO uptake(μmol/g)

metal dispersiona

CO/(Pd + Re)

Pd/C 187.9 0 10.4 0.055PdRe0.3/C 187.9 53.7 17.1 0.071PdRe0.6/C 187.9 107.4 39.3 0.133PdRe1.1/C 187.9 214.8 61.0 0.151Re/C 0 214.8 32.1 0.149

aIrreversible adsorption of CO calculated per each metal atom inbimetallic Pd−Re.

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a broad peak with a maximum at 740 °C, accompanied bymethane emission and results from hydrogenation of surfacefunctional groups on the activated carbon support.25−27 Theresults indicated that Pd(NO3)2 was fully reduced during thepreparation process. The reduction profile of Re/C consisted oftwo peaks. The peak at approximately 370 °C was caused bythe reduction of rhenium oxide,28,29 and the broader peakcentered at approximately 600 °C was caused by gasification ofthe support. With a gradual introduction of Re to Pd/C, thepeaks caused by gasification of the support shifted to lowertemperature from approximately 710 to 624 °C. Interestingly,the bimetallic catalysts did not show peaks at approximately370 °C, whereas some other peaks centered at 140−230 °Cwere more obvious. The results demonstrated that Pd couldsignificantly promote the reduction of rhenium oxide and thecatalyst could be reoxidized after exposure to oxygen in the air.

A similar phenomenon was observed for Pd−Re/A2O3 and Pt−Re/C catalysts.28,29 Simonetti et al. suggested that rheniumoxide species have high mobility on carbon and can migratetoward reduced Pt centers to form bimetallic particles.29 Thus,it is likely that, on Pd−Re/C catalysts, mobile rhenium oxidespecies can migrate toward reduced Pd particles, where Rereduction 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 inthe positions of the Pd(111) reflections. This can be explainedby 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 gooddispersion of Re on the support.Table 1 reports the results of CO chemisorption on

monometallic Pd/C and Re/C and bimetallic catalysts. It canbe seen that the Pd/C catalyst had lower CO uptake thanmonometallic Re/C catalyst. The PdRe0.3/C catalyst showed aslight increase in CO uptake compared with Pd/C, whereasPdRe0.6/C catalyst exhibited an obvious increase. The resultssuggest a strong interaction between Pd and Re. With theaddition of a small amount of Re to Pd/C, Re could enter intothe lattice of Pd, and the catalyst exhibited a CO uptake similarto that of Pd/C. With the gradual introduction of Re, Re couldbecome enriched on the bimetallic particles.28,30,31 Further-more, the CO uptake for the bimetallic PdRe1.1/C catalyst wasmore than the total CO uptake of the monometallic Pd/C andRe/C catalysts, which also suggests a close interaction betweenPd 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/Ccatalyst, the Pd particles are spherical and homogeneouslydispersed on the activated carbon, with no appearance ofagglomeration in Figure 3a. The fringe spacing of 2.35 Å inFigure 3a′ deviates distinctly from the Pd(111) lattice planespacing (2.25 Å) with a face-centered-cubic (fcc) structure,which can be ascribed to the 1/3(422) reflection that isnormally forbidden in an fcc structure. This observation is ingood agreement with previous publications.32−34 For thePdRe0.6/C catalyst, the particles are anomalous and asym-metrical, 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.

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

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of agglomerated particles on the edge of the support. Theparticles with fringes separated by 2.44 and 2.33 Å can beascribed to the 1/3(422) reflection. The fringe spacing of 1.98Å is similar to the Pd(200) lattice plane spacing (1.95 Å). Theresults indicate the formation of Pd particles. The fringe spacingof 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

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.

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asymmetrical and anomalous particles around it. Theobservations 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 asa function of reaction time. When the reaction time was 2 h,GBL and a small quantity of THF were detected in theproducts. With increasing reaction time, the relative concen-tration of SA decreased slowly, and GBL, as the main product,increased. Meanwhile, more products, such as BDO, NBA, andNPA, 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 forthe further reaction of GBL and a high selectivity to GBL in thehydrogenation 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 alsonoticed that the relative concentration of GBL increased beforedecreasing as the reaction time prolonged. The yield of GBLreached 64% at the highest relative concentration of GBL, andthe yield of THF was 11% at this point. The relativeconcentration of THF increased rapidly after the relativeconcentration of GBL decreased. After a reaction time of 10 h,the conversion of SA reached 87%, the yield of GBL decreasedto 42%, and the yield of THF increased to 39%. The resultsshowed that Re was advantageous for dehydration andhydrogenation and that the hydrogenation of SA was aconsecutive reaction on Re/C, in which GBL obtained fromSA was subsequently converted into THF. BDO, NBA, and alittle NPA were detected in the products, which indicated thatthe Re/C catalyst was disadvantageous for the formation ofNPA.

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

Figure 9. Conversion of SA over monometallic and bimetalliccatalysts.

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

catalyst conversion (%) time (h) TOFa (h−1)

Pd/C 20 2.8 582PdRe0.3/C 20 0.6 1650PdRe0.6/C 20 0.6 718PdRe1.1/C 20 0.4 694Re/C 20 1.3 405

aTurnover frequency.

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

catalyst substrateconversion

(%)selectivity to GBL

(%)selectivity to THF

(%)selectivity to BDO

(%)selectivity to NBA

(%)selectivity to NPA

(%)

Pd/C GBL 6.2 − 85.0 1.9 8.6 4.5PdRe0.6/C GBL 96.5 − 79.2 5.5 10.5 4.8Re/C GBL 86.7 − 85.9 9.3 4.1 0.7Pd/C BDO 11.6 0.7 72.6 − 22.1 4.6PdRe0.6/C BDO 87.6 3.2 71.3 − 15.0 10.5Re/C BDO 65.0 11.5 83.1 − 5.0 0.4Pd/C THF 1.3 0.9 − 41.4 54.2 3.5PdRe0.6/C THF 5.7 1.9 − 3.5 80.5 14.1Re/C THF 0.8 10.0 − 58.9 29.9 1.2

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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 areaction time of 2 h, the conversion of SA reached 69%. At thehighest relative concentration of GBL, the yield of GBL was74%. Furthermore, the selectivity to GBL decreased slowly overa wide range of reaction times, and it was advantageous tomaintain a high yield of GBL. When the reaction time was 10 h,the conversion of SA reached 92%, and the yield of THF was30%. The yield of byproducts (NBA + NPA) was less than 3%.The improvement in the conversion of SA can be explained bythe interaction between Pd and Re. The high selectivity to GBLover a wide range of reaction times indicates that little Re wasexposed on the surface of the bimetallic particles when a smallamount 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, andthe selectivity to THF increased rapidly with the reaction time.At the highest relative concentration of GBL, the yield of GBLwas 72%. When the reaction time was 10 h, the conversion ofSA reached 94%, and the total yield of GBL and THF was 84%.A 55% yield of THF was attained, accompanied by a 7% yieldof byproducts. The results indicate that more Re was exposedon the surface of the bimetallic particles, in good agreementwith 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 2h, the conversion of SA reached 89%, and the yield of GBL wasonly 51%. After that, the relative concentration of THFincreased, and more byproducts were produced. At the reactiontime of 10 h, the yield of THF reached 65%, with a 17% yield ofbyproducts.Figure 9 shows the conversion of SA in the hydrogenation of

SA over monometallic Pd/C and Re/C catalysts and threebimetallic Pd−Re/C catalysts. It can be seen that the bimetallicPd−Re/C catalysts significantly increased the conversion of SAcompared with the monometallic catalysts. Because of thedifferent metal loadings on the catalysts, we calculated theturnover frequency (TOF) at 20% SA conversion on the basisof CO uptakes by CO chemisorption measurements, assumingthat the number of active sites on the catalysts is equal to thenumber of adsorbed CO molecules. The results are reported inTable 2. It can be observed that the PdRe0.3/C catalyst wasmore active than the other catalysts. This can be explained byelectronic and geometric effects. For the addition of a smallamount of Re, Re could diffuse into the lattice of Pd, and theelectronic effect played an important role in improving theactivity of the catalyst. As the amount of Re increased, thesurfaces of the particles became progressively enriched with Re,

and the effect of geometric structure of the particles becamemore 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 only4%. To clarify the reaction path for the conversion of SA onPd−Re/C catalysts, experiments were performed with GBL,BDO, and THF as the substrates. The catalytic reaction wascarried out for 10 h. The product distributions resulting fromthese experiments are reported in Table 3. As can be seen fromthe table, the bimetallic Pd−Re catalyst exhibited a higheractivity than the Pd/C and Re/C catalysts. The Pd/C catalystexhibited a low activity when GBL was charged as the substratein the experiments. The results indicated that Pd/C could havea high selectivity to GBL in the hydrogenation of SA, which wasconsistent with Figure 4. Obviously, Re/C and the bimetallicPd−Re/C catalyst exhibited higher activities than Pd/C whenGBL was charged as the substrate, and the main product wasTHF. Both of these catalysts demonstrated that Re had a highoxidative-hydrogenation activity. There was little furtherconversion of THF after 10 h of reaction, which showed thatnone 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 inagreement with the high yield of THF in the hydrogenation ofSA over Pd−Re/C catalyst. When BDO was charged as thesubstrate, Re/C and the bimetallic Pd−Re/C catalyst exhibitedhigh activities, and the main product was THF. The resultsfurther demonstrated that BDO was a highly reactiveintermediate that was easily converted into THF and NBA.For this reason, it was helpful to explain that the yield of BDOwas very low on our catalysts in the hydrogenation of SA.Deshpande et al. reported one possible reaction pathwaystarting from SA on the Ru−Co bimetallic catalysts.3 There aredifferent pathways for the hydrogenation of BDO because ofthe different solvent we used. Thus, a modified main reactionpathway for the conversion of SA in aqueous solution on Pd−Re/C is shown in Scheme 1. SA was first subjected tosimultaneous dehydration and hydrogenation to produce GBL.Through a further hydrogenation and dehydration route, GBLwas subsequently converted into THF. Simultaneously, BDOcould also be produced through a hydrogenation route, but itwas not the main product and it could be converted into THFthrough dehydration. In the final step, some byproducts, suchas NBA and NPA, could be produced from BDO or THF.

4. CONCLUSIONSThe hydrogenation of SA was carried out over monometallicPd/C and Re/C and bimetallic Pd−Re/C catalysts with thesame palladium content. Pd could significantly promote thereduction of rhenium oxide. The bimetallic Pd−Re/C catalysts

Scheme 1. Reaction Pathway for the Hydrogenation of SA

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

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +86-411-84986353. Fax: +86-411-84986353. E-mail:changhai@dlut.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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