Selective Hydrogenation of Maleic Anhydride to Tetrahydrofuran over

Ind. Eng. Chem. Res. , 2009, 48 (24), pp 11220–11224. DOI: 10.1021/ie9013875. Publication Date (Web): November 6, 2009. Copyright © 2009 American C...
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Ind. Eng. Chem. Res. 2009, 48, 11220–11224

RESEARCH NOTES Selective Hydrogenation of Maleic Anhydride to Tetrahydrofuran over Cu-Zn-M (M ) Al, Ti, Zr) Catalysts Using Ethanol As a Solvent Dongzhi Zhang, Hengbo Yin,* Jinjuan Xue, Chen Ge, Tingshun Jiang, Longbao Yu, and Yutang Shen Faculty of Chemistry and Chemical Engineering, Jiangsu UniVersity, Zhenjiang 212013, P.R. China

A series of Cu-Zn-M (M ) Al, Ti, Zr) catalysts were prepared by coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption. The catalytic activity of the Cu-Zn-M catalyst in maleic anhydride hydrogenation using ethanol as a solvent was studied at 220-280 °C and 1 MPa. The presence of MxOy in Cu-Zn-M (M ) Al, Ti, Zr) catalysts favored the deep hydrogenation of maleic anhydride to tetrahydrofuran. The catalysts using TiO2 as a support showed the highest selectivity of tetrahydrofuran. The catalytic activity of the catalysts in the hydrogenation of maleic anhydride to tetrahydrofuran followed a sequence of Cu-Zn-Ti > Cu-Zn-Zr ≈ Cu-Zn-Al. 1. Introduction Hydrogenation of maleic anhydride (MA) produces γ-butyrolactone (GBL) and tetrahydrofuran (THF), which are important industrial chemicals. GBL is an alternative to the environmentally harmful chlorinated solvent and also an intermediate widely used in the polymer industry, producing 2-pyrrolidone/ N-vinyl-2-pyrrolidone/polyvinylpyrrolidone.1,2 THF is widely used both as a versatile solvent and as a raw material for the manufacture of polytetramethylene ether glycol (PTMEG), spandex fibers, and polyurethane elastomers.3-5 Hydrogenation of MA can be catalyzed by various kinds of catalysts, such as noble metals (Pd, Re, and Ru) in the liquid phase at pressures of 1-5 MPa and temperatures between 190 and 240 °C,2,6-9 copper-based catalysts in the liquid phase at pressures between 5 and 9 MPa and temperatures between 200 and 240 °C,6,10 and copper-based catalysts in the gas phase at pressures from 0.1 to 1 MPa and temperatures between 210 and 280 °C.11-13 The inexpensive copper-based catalyst is an alternative to the noble metal catalyst. Mu¨ller et al.5 suggested that Cu0 catalyzed the hydrogenation and hydrogenolysis reactions yielding GBL and 1,4-butanediol and that Al2O3 provided weakly acidic sites to promote the subsequent dehydration of 1,4-butanediol to THF. Lu et al.14 reported that TiO2 with acidic site-modified Cu-Al2O3 catalyst was beneficial to the formation of GBL in the gas-phase hydrogenation of MA. Castiglioni et al.15 and Lancia et al.16 found that MA can be hydrogenated to GBL with a selectivity close to 99% by Cu-Zn-Zr catalysts at 0.1-8 MPa. In the above-mentioned MA hydrogenation processes, the main hydrogenation product is GBL. An investigation into improving the selectivity of THF is valuably deserved. Meanwhile, the role of the supports, such as Al2O3, TiO2, and ZrO2, present in copper-based catalyst for the selective hydrogenation of MA is still worthy of investigation. In the MA hydrogenation process, GBL, THF, and alcohols are usually used as solvents because MA is in the solid state at room temperature (mp 52.8 °C). Budge et al.17 suggested that C4 alcohols, such as n- or i-butanol, were most preferred for

the MA hydrogenation. We used n-butanol as a solvent in our previous research works and found that n-butanol was dehydrogenated to butyl aldehyde and butyl acetate.11-13 It will be a good alternative to replace n-butanol by ethanol in the MA hydrogenation process because the dehydrogenation of ethanol catalyzed by copper-based catalysts mainly produces ethyl acetate and hydrogen. Ethyl acetate is a valuable fine chemical,18,19 and hydrogen is a feedstock in MA hydrogenation. According to the relative publications,11-13,19 the hydrogenation process of MA in the presence of ethanol as a solvent was suggested as Scheme 1. In the MA hydrogenation process, producing 1 mol GBL requires 1 mol MA and 3 mol H2, releasing 210.0 kJ heat; producing 1 mol THF requires 1 mol MA and 5 mol H2, releasing 269.4 kJ heat.5,11 While in the dehydrogenation process of ethanol catalyzed by copper-based catalysts, ethanol is mainly dehydrogenated to ethyl acetate, releasing H2 and requiring heat. Producing 1 mol ethyl acetate requires 2 mol ethanol and 151 kJ heat, releasing 2 mol H2.11,18 Using ethanol as a solvent in MA, hydrogenation should favor the two reactions in view of the compensation of H2 and heat. But, catalytic hydrogenation of MA using ethanol as a solvent was seldom investigated. Scheme 1. Routes of MA Hydrogenation and Ethanol Dehydrogenation

* To whom correspondence should be addressed. Tel.: +86-(0)51188787591. Fax: +86-(0)511-88791800. E-mail: [email protected]. 10.1021/ie9013875  2009 American Chemical Society Published on Web 11/06/2009

Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009 Table 1. Compositions and Specific Surface Areas of the Calcined Cu-Zn-M (M ) Al, Ti, Zr) Catalysts and the Crystallite Sizes of Metallic Copper (111) of the Reduced Catalysts atomic ratios

samples

C1 C2 C3 C4 C5 C6

Cu

Zn

Al

2 2 2 2 2 2

2 2 2 2 2 2

1 2

Ti

Zr

1 2 1 2

specific surface areas (m2/g)

crystallite sizes of Cu0 (111) (nm)

78.7 76.2 148.9 186.3 77.9 75.7

20.0 14.6 17.8 20.8 17.8 19.3

In our present work, Cu-based catalysts with different supports were prepared by a coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption. The major objective of this work is to gain an insight into the effects of supports and the catalytic activity of Cu-based catalysts in the MA hydrogenation using ethanol as a solvent at 220-280 °C and 1 MPa. 2. Experimental Section Catalyst Preparation. Cu-Zn-M (M ) Al, Ti, Zr) catalysts were prepared by a continuous coprecipitation method. The starting materials were Cu(NO3)2 · 3H2O, Zn(NO3)2 · 6H2O, Al(NO3)3 · 9H2O, TiCl4 and Zr(NO3)4 · 5H2O. A mixed solution of the starting materials with given atomic ratios was used as a precursor solution, and a Na2CO3 solution (1 M) was used as a precipitating agent. Coprecipitation was performed at 75 °C in a water bath. The flow rates of the two solutions were adjusted to give a constant pH value of ca. 8. The resultant suspension was aged for 12 h at room temperature. The precipitate was filtrated and washed with distilled water until the conductivity of the filtrate was less than 2 mS/m. After drying in air at 120 °C for 12 h, the catalysts were calcined at 450 °C for 2 h. The calcined catalysts were pressed at 10 MPa to form pellets, then the pellets were crushed to form small-sized particles with particle sizes ranging from 0.45 to 0.9 mm. The compositions of the as-prepared copper-based catalysts according to those in their precursors are listed in Table 1. Characterization. X-ray diffraction (XRD) was used to examine the bulk chemical structures of the calcined and reduced catalysts. The XRD data were recorded on a diffractometer (D8 super speed Bruke-AEX Company, Germany) using Cu KR radiation (1.5418 Å) with a Ni filter, scanning from 20 to 80° (2θ). The crystallite sizes of the Cu0 (111) in the reduced catalysts were calculated by using Scherrer’s equation: D ) Kλ/(B cos θ), where K was taken as 0.9, and B was the full width of the diffraction line at half of the maximum intensity. The data are listed in Table 1. X-ray photoelectron spectra (XPS) and X-ray induced Auger electron spectra (XAES) of the calcined and reduced catalysts were recorded on an ESCALAB 250 spectrometer (Thermal Electron Corp.) using Al KR radiation (1486.6 eV). The binding energies were calculated with respect to C1s peak at 284.65 eV. The specific surface areas of the calcined catalysts were measured on a NOVA 2000e physical adsorption apparatus by the BET method. Catalytic Test. The catalytic test was carried out in a stainless steel tubular fixed-bed reactor with diameter and length of 8 and 200 mm, respectively, packed with 5 mL of catalyst with

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particle sizes ranging from 0.45 to 0.9 mm, operating at 220-280 °C and 1 MPa. The reactor was fed with a stream of MA and ethanol solution (1:15.3 mol/mol) in hydrogen, the liquid space velocity was 0.2 h-1, and the molar ratio of H2 to MA was 50:1. The MA/ethanol solution was evaporated in an evaporator at 250 °C. Before catalytic test, the catalyst was first reduced in a mixed H2/N2 (10:90 v/v) stream with a flow rate of 250 mL/min from 25 to 280 °C at a temperature ramp of 1.5 °C/min. Then, the catalyst was continuously reduced at 280 °C for 2 h in a mixed H2/N2 (30:70 v/v) stream with a flow rate of 250 mL/min. The reaction products were condensed in an ice water bath and collected at different reaction temperatures after reaction for 1 h. Acompanied by MA hydrogenation, ethanol dehydrogenation took place. All the collected reaction products were analyzed on a gas chromatograph, equipped with flame ionization detection (FID) and a poly(ethylene glycol) (PEG) packed capillary column (0.25 mm × 30 m). 3. Results and Discussion XRD Analysis. The compositions of the calcined and reduced Cu-Zn-M (M ) Al, Ti, Zr) catalysts were determined by XRD analysis (See the Supporting Information). The XRD patterns of the calcined Cu-Zn-M catalysts exhibited the characteristic peaks of CuO and ZnO, respectively. The XRD patterns of the reduced Cu-Zn-M catalysts showed that CuO was reduced to Cu0 and that zinc species were still present in the form of ZnO. There were no diffraction peaks of Al2O3, TiO2, and ZrO2 detected by XRD analysis, revealing that Al2O3, TiO2, and ZrO2 were in the amorphous phase present in both calcined and reduced Cu-Zn-M catalysts. As the reduced catalysts were concerned, the crystallite sizes of metallic copper (111) were estimated by Scherrer’s equation. For all of the Cu-Zn-M catalysts (C1-6), the crystallite sizes of metallic copper were between 14.6 and 20.8 nm (Table 1), revealing that small-sized metallic copper was formed in the reduced Cu-Zn-M catalysts. XPS Analysis. The chemical states of the representative calcined Cu-Zn-M (C1, C3, C5) and H2-reduced Cu-Zn-M (C1*, C3*, C5*) catalysts were evaluated by XPS. Figure 1a and b shows the Cu 2p3/2 and 2p1/2 peaks of the calcined and H2-reduced samples. The calcined samples C1, C3, and C5 showed that the Cu 2p3/2 and 2p1/2 peaks appeared at 934.5, 954.6; 932.4, 952.3; and 932.7, 952.8 eV, respectively, which were the characteristic peaks of Cu2+ species.20 Furthermore, the presence of a Cu2+ satellite peak appearing at ca. 943 eV evidenced the presence of Cu2+ ions in the form of cupric oxide, whose origin was complex and has been explained as being due to electron shakeup processes, final state effects, and charge transfer mechanisms.21 The binding energies of Cu 2p3/2 and 2p1/2 of Cu-Zn-M catalysts were different, revealing that the chemical state of CuO was influenced by the composition of the catalyst, i.e., the electron density surrounding Cu atom was changed with the catalyst composition. After reduction, the Cu 2p3/2 and 2p1/2 peaks of the Cu-Zn-M (C1*, C3*, C5*) catalysts shifted to lower binding energies appearing at 933.2, 953.4; 932, 952; and 932.6, 952.4 eV, respectively. The shift of Cu 2p revealed the disappearance of cupric oxide but did not allow us to reach a conclusion about the presence of solely cuprous oxide or solely metallic copper or both because the Cu 2p3/2 peaks for metallic copper and Cu2O appear at almost the same binding energy.20 In an attempt to understand the differences in the chemical states of the copper species present in the calcined catalysts (C1, C3, C5) and H2reduced catalysts (C1*, C3*, C5*), the Wagner plot, as reported

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Figure 3. Catalytic activity of the Cu-Zn-M catalysts (C1-6) in MA hydrogenation at 1 MPa and different temperatures. b, C1; O, C2; 2, C3; 4, C4; f, C5; g, C6.

Figure 1. X-ray photoelectron spectra of the calcined catalysts (C1, C3, and C5) and the reduced catalysts (C1*, C3*, and C5*).

Figure 2. Wagner plot for the calcined and reduced Cu-Zn-M catalysts (C1, C3, and C5) and Cu-containing reference samples.22,23 Solid lines are Cu-containing reference samples. [, C1; ], C1*; f, C3; g C3*; 2, C5; 4, C5*.

earlier,22 was drawn and shown in Figure 2. Here, the kinetic energy of the Auger transition was on the Y-axis, and the Cu 2p3/2 binding energy of the photoemission line was on the X-axis. The data for Cu, Cu2O, and CuO reference samples were taken from refs 22 and 23. From Figure 2, it was found that the Auger parameters of the calcined catalysts fell on the CuO line. The Auger parameters of H2-reduced catalysts fell on the line of metallic copper. From the Wagner plot analysis, it was found that the CuO species present in the calcined catalysts were converted to Cu0 after reduction, being consistent with the XRD analysis. The Al 2p3/2 binding energies for both of the calcined Cu-Zn-Al (C1) and H2-reduced Cu-Zn-Al (C1*) catalysts remained the same (75.8 eV, Figure 1c). The Ti 2p3/2 binding energies of the calcined Cu-Zn-Ti (C3) and H2-reduced Cu-Zn-Ti (C3*) catalysts had the same value (458.1 eV, Figure 1d). The Zr3d5/2 binding energies of the calcined Cu-Zn-Zr (C5) and H2-reduced Cu-Zn-Zr (C5*) catalysts

also had the same value (181.7 eV, Figure 1e). The results revealed that aluminum, titanium, and zirconium species existed in the states of Al2O3,20 TiO2,13 and ZrO2,24 respectively, and that the chemical states of the supports, Al2O3, TiO2, and ZrO2, were not influenced by the reduction. Specific Surface Area. The specific surface areas of the Cu-Zn-M (C1-6) catalysts are listed in Table 1. The Cu-Zn-M catalysts had large specific surface areas of more than 76.2 m2/g. The specific surface areas of Cu-Zn-M catalysts were in the order Cu-Zn-Ti > Cu-Zn-Zr ≈ Cu-Zn-Al. Therefore, the effects of supports on the increase in the specific surface areas were in the order TiO2 > ZrO2 ≈ Al2O3. Hydrogenation of MA. The catalytic hydrogenation of MA by copper-based catalysts proceeds via consecutive hydrogenation steps, in which succinic anhydride, GBL, and THF are formed subsequently.13 The resultant succinic anhydride can react with the solvent, ethanol, to produce diethyl succinate (DS). Figure 3 shows the results of MA hydrogenation using ethanol as a solvent catalyzed by Cu-Zn-M (M ) Al, Ti, Zr) catalysts at 220-280 °C and 1 MPa. The Cu-Zn-M (M ) Al, Ti, Zr) catalysts exhibited good activity for the hydrogenation of MA, with the conversion of MA of ca. 100%. When the Cu-Zn-Al catalysts (C1 2:2:1 and C2 2:2:2) were used in the hydrogenation of MA, with increasing the reaction temperatures from 220 to 280 °C, the selectivity of THF rapidly increased from 26.0% to 68.7% and from 26.1% to 61.6%; the selectivity of GBL rapidly decreased from 56.7% to 27.1% and from 49.1% to 23.9%; and the selectivity of DS gradually decreased from 17.3% to 4.2% and from 24.8% to 14.5%, respectively. The Cu-Zn-Al catalysts with different Al2O3 contents showed comparable catalytic activity in hydrogenation of MA to GBL. But the Cu-Zn-Al catalyst (C1) with a lower Al2O3 content showed higher catalytic activity than that with a higher Al2O3 content in hydrogenation of MA to THF at higher reaction temperatures. When the Cu-Zn-Zr catalysts (C5 2:2:1 and C6 2:2:2) were used in the hydrogenation of MA, with increasing the reaction temperatures from 220 to 280 °C, the selectivity of THF rapidly increased from 8.4% to 79.4% and from 17.7% to 87.8%; the selectivity of GBL rapidly decreased from 82.9% to 8.7% and from 69.4% to 6.9%; and the selectivity of DS gradually

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Figure 5. Ratios of the hydrogen produced in the ethanol dehydrogenation to the hydrogen consumed in the MA hydrogenation (H2%). b, C1; O, C2; 2, C3; 4, C4; f, C5; g, C6.

Figure 4. Catalytic activity of the Cu-Zn-M catalysts (C1-6) in ethanol dehydrogenation at 1 MPa and different temperatures. b, C1; O, C2; 2, C3; 4, C4; f, C5; g, C6.

changed from 8.7% to 11.9% and from 13.0% to 5.3%, respectively. The results revealed that the Cu-Zn-Zr catalysts had higher catalytic activity than Cu-Zn-Al catalysts in the hydrogenation of MA to THF at a high reaction temperature of 280 °C. However, when the reaction temperatures were lowered to 220-240 °C, Cu-Zn-Zr catalysts showed lower catalytic activity than Cu-Zn-Al catalysts in the hydrogenation of MA to THF. The Cu-Zn-Zr catalysts were of high deep hydrogenation activity at high reaction temperature. Furthermore, increasing the ZrO2 content in Cu-Zn-Zr catalysts did not obviously affected their specific surface areas but significantly enhanced the deep hydrogenation activity. When the Cu-Zn-Ti catalysts (C3 2:2:1 and C4 2:2:2) were used in the hydrogenation of MA, the selectivity of THF remained at 100% while the selectivities of GBL and DS stayed at 0% for both catalysts at the reaction temperatures ranging from 220 to 280 °C. From the results, it was found that the Cu-Zn-Ti catalysts showed higher catalytic activity in the hydrogenation of MA to THF than the Cu-Zn-Al and Cu-Zn-Zr catalysts. By comparing the experiment results, it was found that the order of the catalytic activity for the hydrogenation of MA to THF was Cu-Zn-Ti > Cu-Zn-Zr ≈ Cu-Zn-Al. Interestingly, the order of the specific surface areas of the catalysts was Cu-Zn-Ti > Cu-Zn-Zr ≈ Cu-Zn-Al, being consistent with that of their catalytic activities. Therefore, it is reasonable to conclude that catalysts with large specific surface areas favor the deep hydrogenation of MA to THF. Furthermore, the area ratio of the Cu 2p peaks for the reduced Cu-Zn-Ti, Cu-Zn-Zr, and Cu-Zn-Al catalysts was 5.0:1.6:1.0. The metallic copper was well dispersed on the surface of Cu-Zn-Ti catalyst, exhibiting higher catalytic activity for the deep hydrogenation of MA to THF. Dehydrogenation of Ethanol. Copper-based catalyst is used not only for hydrogenation but also for dehydrogenation. In the dehydrogenation process, ethanol is first dehydrogenated to form acetaldehyde, then to ethyl acetate via reactions, such as disproportionation and esterification. Acetaldehyde can also be converted to a byproduct n-butanol via a series of complicated reactions.19 Figure 4 shows that the conversion of ethanol catalyzed by Cu-Zn-M catalysts (C1-6) increased from 4.9% to 23.6% with the increase of temperatures from 220 to 280 °C at 1 MPa.

The catalysts (C1-6) had similar catalytic activity in the dehydrogenation of ethanol. For all of the Cu-Zn-M catalysts (C1-6) used in the dehydrogenation of ethanol at reaction temperatures ranging from 220 to 280 °C and 1 MPa, the selectivities of ethyl acetate, acetaldehyde, and n-butanol were around 85%, 10%, and 5%, respectively. The main product was ethyl acetate. The selectivities of the dehydrogenation products, acetaldehyde, ethyl acetate, and n-butanol, were not sensitive to the reaction temperature. The Cu-Zn-Zr and Cu-Zn-Ti catalysts showed slightly higher esterification activity than the Cu-Zn-Al catalysts. Compensation between Hydrogenation and Dehydrogenation. Hydrogen produced by the ethanol dehydrogenation can be used as a feedstock for the MA hydrogenation. The ratios of the hydrogen produced in the ethanol dehydrogenation to the hydrogen consumed in the MA hydrogenation, H2%, were calculated according to Scheme 1 and the results are showed in Figure 5. In MA hydrogenation using ethanol as a solvent catalyzed by Cu-Zn-M catalysts (C1-6), the H2 compensation values increased when raising the reaction temperature from 220 to 280 °C. For the Cu-Zn-Al catalysts (C1 2:2:1 and C2 2:2:2), the H2 compensation values ranged from 37.3% to 67.1% and from 23.5% to 81%, respectively. For the Cu-Zn-Ti catalysts (C3, 2:2:1 and C4, 2:2:2), the H2 compensation values ranged from 25.6% to 69.5% and from 15.8% to 69.0%, respectively. For the Cu-Zn-Zr catalysts (C5 2:2:1 and C6 2:2:2), the H2 compensation values ranged from 38.5% to 73.4% and from 34.0% to 80%, respectively. The copper-based catalysts showed high H2 compensation values in MA hydrogenation using ethanol as a solvent. 4. Conclusions CuO present in the calcined Cu-Zn-M catalysts was reduced to metallic copper, which is the active site in MA hydrogenation and ethanol dehydrogenation. The catalytic activity of Cu-Zn-M catalysts in MA hydrogenation to GBL and THF was significantly affected by their composition. The presence of MxOy favored the deep hydrogenation of MA to THF. The effects of the supports MxOy on the hydrogenation of MA to THF were in the order of TiO2 > ZrO2 ≈ Al2O3. The Cu-Zn-M catalysts exhibited good catalytic performance not only for MA hydrogenation but also for ethanol dehydrogenation to ethyl acetate. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in MA hydrogenation, H2%, increased with the increase in the reaction temperature.

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Acknowledgment This work was financially supported by Zhenjiang Science and Technology Bureau (CZ2006006). The authors sincerely thank Professor Jianxin Wu at University of Science and Technology, China, for XPS measurement and Dr. Bin Xu at Yangzhou University for XRD measurement. Supporting Information Available: XRD patterns of the calcined and reduced Cu-Zn-M (M ) Al, Ti, Zr) catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Hara, Y.; Takahashib, K. A novel production of γ-butyrolactone catalyzed by homogeneous ruthenium complexes. Catal. SurVeys Jpn. 2002, 6, 73. (2) Jung, S. M.; Godard, E.; Jung, S. Y.; Park, K.-C.; Choi, J. U. Liquidphase hydrogenation of maleic anhydride over Pd/SiO2: effect of tin on catalytic activity and deactivation. J. Mol. Catal. A: Chem. 2003, 198, 297. (3) Kanetaka, J.; Asano, T.; Masamune, S. New process for production of tetrahydrofuran. S. Ind. Eng. Chem. 1970, 62, 24. (4) Pallassana, V.; Neurock, M.; Coulston, G. Towards understanding the mechanism for the selective hydrogenation of maleic anhydride to tetrahydrofuran over palladium. Catal. Today 1999, 50, 589. (5) Mu¨ller, S. P.; Kucher, M.; Ohlinger, C.; Kraushaar-Czarnetzkyi, B. Extrusion of Cu/ZnO catalysts for the single-stage gas-phase processing of dimethyl maleate to tetrahydrofuran. J. Catal. 2003, 218, 419. (6) Herrmann, U.; Emig, G. Liquid phase hydrogenation of maleic anhydride and intermediates on copper-based and noble metal catalysts. Ind. Eng. Chem. Res. 1997, 36, 2885. (7) Jung, S. M.; Godard, E.; Jung, S. Y.; Park, K.-C.; Choi, J. U. Liquidphase hydrogenation of maleic anhydride over Pd-Sn/SiO2. Catal. Today 2003, 87, 171. (8) Liu, P.; Yan, K.; Liu, Y; Yin, Y. Selective hydrogenation of maleic anhydride to γ-butyrolactone using homogeneous Ru/PPh3 catalyst system. J. Nat. Gas Chem. 1999, 8, 157. (9) Hara, Y.; Kusaka, H.; Inagaki, H.; Takahashi, K.; Wada, K. A novel production of γ-butyrolactone catalyzed by ruthenium complexes. J. Catal. 2000, 194, 188. (10) Herrmann, U.; Emig, G. Liquid phase hydrogenation of maleic anhydride to 1,4-butanediol in a packed bubble column reactor. Ind. Eng. Chem. Res. 1998, 37, 759.

(11) Hu, T. J.; Yin, H. B.; Zhang, R. C.; Wu, H. X.; Jiang, T. S.; Wada, Y. Gas phase hydrogenation of maleic anhydride to γ-butyrolactone by CuZn-Ti catalysts. Catal. Commun. 2007, 8, 193. (12) Zhang, D. Z.; Yin, H. B.; Zhang, R. C.; Xue, J. J.; Jiang, T. S. Gas phase hydrogenation of maleic anhydride to γ-butyrolactone by Cu-Zn-Ce catalyst in the presence of n-butanol. Catal. Lett. 2008, 122, 176. (13) Zhang, R. C.; Yin, H. B.; Zhang, D. Z.; Qi, L.; Lu, H. H.; Shen, Y. T.; Jiang, T. S. Gas phase hydrogenation of maleic anhydride to tetrahydrofuran by Cu/ZnO/TiO2 catalysts in the presence of n-butanol. Chem. Eng. J. 2008, 140, 488. (14) Lu, W.; Lu, G.; Guo, Y.; Guo, Y.; Wang, Y. Gas-phase hydrogenation of maleic anhydride to butyric acid over Cu/TiO2/γ-Al2O3 catalyst promoted by Pd. Catal. Commun. 2003, 4, 177. (15) Castiglioni, G. L.; Fumagalli, C.; Armbruster, E.; Messori, M.; Vaccari, A. Catalysis of Organic Reactions; Dekker: New York, 1998. (16) Lancia, R.; Vaccari, A.; Fumagalli, C.; Armbruster, E. Process for the production of gamma-butyrolactone. W.O. Patent 95/22539, 1995. (17) Budge, J. R.; Pedersen, S. E.; Solon. Hydrogenation of maleic anhydride to tetrahydrofuran. U.S. Patent 4,810,807, 1989. (18) Elliott, D. J.; Pennella, F. The formation of ketones in the presence of carbon monoxide over CuO/ZnO/Al2O3. J. Catal. 1989, 119, 359. (19) Inui, K.; Kurabayashi, T.; Sato, S.; Ichikawa, N. Effective formation of ethyl acetate from ethanol over Cu-Zn-Zr-Al-O catalyst. J. Mol. Catal. A: Chem. 2004, 216, 147. (20) Figueiredo, R. T.; Martinez-Arias, A.; Granados, M. L.; Fierro, J. L. G. Spectroscopic evidence of Cu-Al interactions in Cu-Zn-Al mixed oxide catalysts used in CO hydrogenation. J. Catal. 1998, 178, 146. (21) Aravinda, C. L.; Bera, P.; Jayaram, V.; Sharma, A. K.; Mayanna, S. M. Characterization of electrochemically desposited Cu-Ni black coatings. Mater. Res. Bull. 2002, 37, 397. (22) Dai, W. L.; Sun, Q.; Deng, J. F.; Wu, D.; Sun, Y. H. XPS stduies of Cu/ZnO/Al2O3 ultra-fine catalysts derived by a novel gel oxalate coprecipitation for methanol synthesis by CO2+H2. Appl. Surf. Sci. 2001, 177, 172. (23) Poulston, S.; Parlett, P. M.; Stone, P.; Bowker, M. Surface oxidation and reduction of CuO and Cu2O studied using XPS and XAES. Surf. Interface Anal. 1996, 24, 811. (24) Velu, S.; Suzuki, K.; Gopinath, C. S.; Yoshida, H.; Hattori, T. XPS, XANES and EXAFS investigations of CuO/ZnO/Al2O3/ZrO2 mixed oxide catalysts. Phys. Chem. Chem. Phys. 2002, 4, 1990.

ReceiVed for reView September 4, 2009 ReVised manuscript receiVed October 14, 2009 Accepted November 2, 2009 IE9013875