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Selective hydrogenation of maleic anhydride to γ-butyrolactone and tetrahydrofuran by Cu–Zn–Zr catalyst in the presence of ethanol
Journal of Industrial and Engineering Chemistry 15 (2009) 537–543
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Selective hydrogenation of maleic anhydride to g-butyrolactone and tetrahydrofuran by Cu–Zn–Zr catalyst in the presence of ethanol Dongzhi Zhang, Hengbo Yin *, Chen Ge, Jinjuan Xue, Tingshun Jiang, Longbao Yu, Yutang Shen Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 October 2008 Accepted 13 January 2009
A series of Cu–Zn–Zr catalysts were prepared by a coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, and N2 adsorption. The catalytic activity of the Cu–Zn–Zr catalyst in the hydrogenation of maleic anhydride using ethanol as a solvent was studied at 220–280 8C and 1 MPa. Maleic anhydride was mainly hydrogenated to gbutyrolactone and tetrahydrofuran while ethanol dehydrogenated to ethyl acetate. After reduction, CuO species present in the calcined Cu–Zn–Zr catalysts were converted to metallic copper (Cu0). The presence of ZrO2 favored the deep hydrogenation of g-butyrolactone to tetrahydrofuran while the presence of ZnO was beneficial to the formation of the intermediate product g-butyrolactone. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in maleic anhydride hydrogenation increased with the increase of the reaction temperature. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Maleic anhydride Ethanol Hydrogenation Cu–Zn–Zr catalyst Tetrahydrofuran
1. Introduction Hydrogenation of maleic anhydride (MA) produces g-butyrolactone (GBL) and tetrahydrofuran (THF), which are very important industrial chemicals. GBL is an alternative to the environmentally harmful chlorinated solvent and also an intermediate widely used in the polymer industry, producing 2pyrrolidone, N-vinyl-2-pyrrolidone, and polyvinylpyrrolidone [1– 3]. THF is widely used both as a versatile solvent and as a raw material for the manufacture of polytetramethylene ether glycol (PTMEG), spendex fibers, and polyurethane elastomers [4–8]. GBL is mainly produced by the following two processes: Davy McKee process based on the hydrogenation of diethyl or dimethyl maleate and Reppe process based on acetylene and formaldehyde [9]. THF is mainly produced by the following two processes: furfural process based on furfural and Reppe process based on acetylene and formaldehyde [4,6]. The starting materials, such as acetylene, formaldehyde, and furfural, are explosive or carcinogenic. Furthermore, the severe reaction conditions and multi-step reaction pathways make the above-mentioned production processes unfavorable. To overcome the disadvantages of the established processes, eco-friendly and economically alternative process is worthy of investigation. A promising alternative to the established processes is the single stage hydrogenation of MA since
* Corresponding author. Tel.: +86 0511 88787591; fax: +86 0511 88791800. E-mail address: [email protected] (H. Yin).
MA can be produced at a low cost and on a large scale by partial oxidation of n-butane instead of benzene now [4,10–12]. Hydrogenation of MA can be catalyzed by various kinds of catalysts, such as noble metals (Pd, Re, and Ru) in liquid phase at pressures between 1 and 5 MPa and temperatures between 190 and 240 8C [2,13–16], copper-based catalysts in liquid phase at pressures between 5 and 9 MPa and temperatures between 200 and 240 8C [13,17], and copper-based catalysts in gas phase at pressures from 0.1 to 1 MPa and temperatures between 210 and 280 8C [12,18,19]. The gas phase hydrogenation of MA commonly used Cu–Zn–M (M = Al, Cr, Ce, Ti) catalysts. The type and composition of supports significantly affect the performance of copper-based catalysts. Although zirconia-supported copper catalysts exhibit high activity in hydrogenation process, such as methanol synthesis starting from CO and H2 [20,21], utilizing zirconia as a support in copper-based catalyst for MA hydrogenation is seldom investigated. Castiglioni et al. [22] and Lancia et al. [23] made researches in the hydrogenation of MA using Cu–Zn–Zr catalysts prepared from the corresponding chloride salts of copper, zinc, and zirconium. They found that MA was mainly hydrogenated to GBL with the selectivity close to 99% at 0.1–8 MPa. The relationship between the catalytic activity and the structure of the Cu–Zn–Zr catalysts is worth of investigation in detail. In the industrial MA hydrogenation process, toxic methanol is used as a solvent. It will be a good alternative to replace methanol by environmental benign ethanol in the MA hydrogenation process. Furthermore, dehydrogenation of ethanol catalyzed by copper-based catalysts mainly produces hydrogen and ethyl
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.01.010
D. Zhang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 537–543
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Scheme 1. Routes of MA hydrogenation and ethanol dehydrogenation.
acetate [24,25]. Hydrogen is a feedstock of MA hydrogenation and ethyl acetate is a valuable fine chemical. According to Scheme 1, in the hydrogenation process of MA, producing 1 mol GBL requires 3 mol H2; producing 1 mol THF requires 5 mol H2; producing 1 mol diethyl succinate (DS) requires 1 mol H2 [6,11,12]. While in the dehydrogenation process of ethanol catalyzed by copper-based catalysts, ethanol is dehydrogenated to aldehyde and ethyl acetate. Producing 1 mol aldehyde releases 1 mol H2; producing 1 mol ethyl acetate releases 2 mol H2 [12,24]. Using ethanol as a solvent in MA hydrogenation should favor the two reactions in view of the compensation of H2. But MA catalytic hydrogenation using ethanol as a solvent was seldom investigated. In our present work, we reported a study on the preparation and characterization of Cu–Zn–Zr catalyst. The major objective of this work is to gain an insight into the catalytic activity of Cu–Zn–Zr catalyst in MA hydrogenation using ethanol as a solvent at 220– 280 8C and 1 MPa. 2. Experimental 2.1. Catalyst preparation Cu–Zn–Zr catalysts were prepared by a continuous coprecipitation method. A mixed solution of Cu(NO3)23H2O, Zn(NO3)26H2O, and Zr(NO3)45H2O with a given atomic ratio was used as a precursor solution, and a Na2CO3 solution (1 M) was used as a precipitating agent. Coprecipitation was performed at 75 8C 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 Table 1 The compositions, specific surface areas, and average pore diameters of the calcined Cu–Zn–Zr catalysts. Samples
C1 C2 C3 C4 C5
Atomic ratios Cu
Zn
Zr
1 1 2 2 2
2 2 2 0 2
1 2 1 1 0
Average pore diameters (nm)
Specific surface areas (m2/g)
11.1 9.6 16.4 10.6 17.1
69.6 80.8 77.9 48.8 28.6
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 8C for 12 h, the dried catalysts were calcined at 450 8C 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 Cu–Zn–Zr catalysts according to those in their precursors are listed in Table 1. 2.2. 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 Ka radiation (1.5418 A˚) with Ni filter, scanning from 208 to 808 (2u). The crystallite sizes of the Cu0 (1 1 1) in the reduced catalysts were calculated by using Scherrer’s equation: D = Kl/(Bcos u), where K was taken as 0.9, and B was the full width of the diffraction line at half of the maximum intensity. 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 Ka radiation (1486.6 eV). The binding energies were calculated with respect to C1s peak at 284.6 eV. The reduction behaviors of the calcined catalysts were investigated by temperature programmed reduction (TPR) technique using a mixed H2/N2 flow (5:95, v/v) of 30 ml/min and 50 mg of the calcined catalysts at a temperature ramp of 10 8C/min from 25 to 300 8C. H2 consumption was determined by analyzing the effluent gas with a thermal conductivity detector. The calcined catalysts were preheated in air at 200 8C before the TPR measurement in order to eliminate impurities and adsorbed water. The specific surface areas and the average pore sizes of the calcined catalysts were measured on a NOVA 2000e physical adsorption apparatus by BET and BJH methods, respectively. 2.3. 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,
D. Zhang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 537–543
respectively, packed with 5 ml of catalyst with particle sizes ranging from 0.45 to 0.9 mm, operating at 220–280 8C and 1 MPa. The reactor was fed with a stream of MA and ethanol solution (12:88, w/w) in hydrogen, the liquid space velocity was 0.2 h1, and the molar ratio of H2 to MA was 50:1. 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 8C at a temperature ramp of 1.5 8C/min. Then the catalyst was continuously reduced at 280 8C 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. The collected reaction products were analyzed on a gas chromatograph, equipped with FID and a PEG packed capillary column (0.25 mm 30 m).
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Table 2 The crystallite sizes of metallic copper (1 1 1) of the reduced catalysts. Samples
C1
C2
C3
C4
C5
D (nm)
14.4
14.7
17.8
47.0
18.4
of metallic copper were between 14.4 and 18.4 nm (Table 2). For the Cu–Zr catalyst (C4), the crystallite size of metallic copper was 47 nm. The results showed that the presence of ZnO promoted the dispersion of metallic copper in the reduced catalysts.
3. Results and discussion 3.1. Characterization of catalyst 3.1.1. XRD analysis The XRD patterns of the calcined and reduced Cu–Zn–Zr catalysts are shown in Figs. 1 and 2, respectively. The XRD patterns of the calcined Cu–Zn–Zr catalysts exhibited the characteristic peaks of CuO (PDF#48-1548) and ZnO (PDF#36-1451), appearing at 2u = 35.58, 38.78, 48.78, 53.58, 58.38, 61.58, 66.28, 68.18, 72.48, 75.08; 31.88, 34.48, 36.38, 47.58, 56.68, 62.98, 68.08, respectively. The XRD patterns of the reduced Cu–Zn–Zr catalysts showed that CuO was reduced to Cu0 (PDF#04-0836) with the characteristic peaks appearing at 2u = 43.38, 50.48, and 74.18 and that zinc species were still present in the form of ZnO. There was no diffraction peak of ZrO2 detected by XRD analysis (Figs. 1 and 2), revealing that ZrO2 was in amorphous phase present in both calcined and reduced Cu– Zn–Zr catalysts. As the reduced catalysts were concerned, the crystallite sizes of metallic copper (1 1 1) were estimated by Scherrer’s equation. For the Cu–Zn–Zr and Cu–Zn catalysts (C1–3, C5), the crystallite sizes
Fig. 1. XRD patterns of the calcined Cu–Zn–Zr catalysts. (!) CuO; (*) ZnO.
Fig. 2. XRD patterns of the reduced Cu–Zn–Zr catalysts. (5) Cu0; (*) ZnO.
Fig. 3. X-ray photoelectron spectra of the calcined catalysts (C2, C4 and C5) and the reduced catalyst C2 (denoted as C2*).
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3.1.2. XPS analysis The chemical states of the representative calcined Cu–Zn–Zr (C2), calcined Cu–Zr (C4), calcined Cu–Zn (C5), and H2-reduced Cu– Zn–Zr (C2*) catalysts were evaluated by XPS. Fig. 3a shows the Cu2p3/2 and Cu2p1/2 peaks of the calcined and H2-reduced samples. The calcined samples C2, C4 and C5 displayed that the Cu2p3/2 and Cu2p1/2 peaks appeared at 934.9, 955; 934.6, 954.6; 933.7, 953.6 eV, respectively, which were the characteristic peaks of Cu2+ species [26]. Furthermore, the presence of Cu2+ satellite peak appearing at ca. 942.5 eV evidenced the presence of Cu2+ ions in the form of cupric oxide, although whose origin was complex and has been explained as due to electron shake-up processes, final state effects, and charge transfer mechanisms [27]. The binding energies of Cu2p3/2 and Cu2p1/2 of Cu–Zr (C4) and Cu–Zn (C5) catalysts shifted to lower values than that of Cu–Zn–Zr (C2), revealing that the chemical state of CuO was influenced by the composition of the catalyst. The electron density surrounding Cu atom was changed with the catalyst composition. After reduction, the Cu2p3/2 and Cu2p1/2 peaks of Cu–Zn–Zr catalyst (C2*) shifted to lower binding energies at 933 and 953 eV and the satellite peak disappeared, meaning that after reduction, the copper species were Cu+ or Cu0, rather than Cu2+ [26]. The shift of Cu 2p3/2 revealed the disappearance of cupric oxide, but did not allow to reach a conclusion about the presence of solely cuprous oxide or solely metallic copper or both since the Cu2p3/2 peaks for metallic copper and Cu2O appear at almost the same binding energy. In an attempt to understand the differences in the chemical states of the copper species present in C2 and C2* catalysts, the Wagner plot, as reported earlier [28], was drawn and shown in Fig. 4. Here, the kinetic energy of the Auger transition was on the Yaxis and the Cu 2p3/2 binding energy of the photoemission line on the X-axis. The data for Cu, Cu2O and CuO reference samples were taken from the literatures 28 and 29. From Fig. 4, it was found that the Auger parameter of C2 catalyst fell on the CuO line. The Auger parameter of C2* catalyst almost fell on the line of metallic copper, a little lower than the reported Auger parameter of metallic copper. This could be due to the poor crystallinity of the Cu0 phase, as observed from the XRD analysis. From the Wagner plot analysis, it was found that the CuO species present in the calcined Cu–Zn–Zr catalysts were converted to Cu0 after reduction, being consistent with the XRD analysis. The Zn2p3/2 binding energies of the calcined Cu–Zn (C5), calcined Cu–Zn–Zr (C2), and H2-reduced Cu–Zn–Zr (C2*) catalysts were 1022.25, 1022.9, and 1022.9 eV, respectively, pertaining to that of ZnO [30] (Fig. 3b). The Zr3d5/2 binding energies of the calcined Cu–Zr (C4), calcined Cu–Zn–Zr (C2), and H2-reduced Cu– Zn–Zr (C2*) catalysts were 181.45, 181.9, and 181.9 eV, respectively, revealing that zirconium species existed in the form of ZrO2 [30] (Fig. 3c). The results revealed that the chemical states of Zn and Zr were influenced by the chemical composition of the catalysts rather than the reduction. 3.1.3. TPR analysis From the TPR profiles of the calcined samples (Fig. 5), it was found that the temperatures at the maximum reduction peaks of the Cu–Zn–Zr catalysts (C1–3) were 158, 160, and 156 8C, respectively, and that the temperatures of Cu–Zr (C4) and Cu– Zn (C5) catalysts were 149 and 153 8C. The TPR results revealed that all of the catalysts were easy to be reduced. However, the TPR analysis showed that the reduction temperatures of the catalysts were in an order of Cu–Zn–Zr > Cu–Zn > Cu–Zr. Interestingly, as observed by XRD analysis (Table 2), the crystallite sizes of metallic copper present in the catalysts were in an order of Cu–Zn–Zr < Cu– Zn < Cu–Zr. The formation of large-sized CuO precursors in ZnO-, especially ZrO2-supported catalysts decreased the interaction between CuO precursors and ZnO or ZrO2 supports, giving lower
Fig. 4. Wagner plot for the calcined and reduced Cu–Zn–Zr catalysts and Cucontaining reference samples [28,29]. Solid lines are Cu-containing reference samples.
Fig. 5. TPR profiles of the Cu–Zn–Zr catalysts.
reduction temperatures. CuO precursors were well dispersed in Cu–Zn–Zr catalysts due to a synergetic effect of their supports, ZnO and ZrO2, giving a strong interaction between CuO precursors and ZnO–ZrO2 supports, subsequently, causing their reduction at higher temperatures. 3.1.4. Specific surface area and average pore size The specific surface areas and the average pore sizes of the Cu– Zn–Zr, Cu–Zn, and Cu–Zr catalysts were listed in Table 1. The specific surface areas of the Cu–Zn–Zr (C1–3) catalysts were more than 69.6 m2/g, revealing that the as-prepared Cu–Zn–Zr catalysts had large surface areas. For the Cu–Zr (C4) and Cu–Zn (C5) catalysts, the specific surface areas were 48.8 and 28.6 m2/g, respectively. The co-presence of ZnO and ZrO2 increased the specific surface areas of the resultant Cu–Zn–Zr catalysts. For all of the catalysts (C1–5), the average pore sizes were in a range of 9.6–17.1 nm, meaning that the as-prepared catalysts were mesoporous materials. 3.2. Catalytic test 3.2.1. Hydrogenation of MA The catalytic hydrogenation of MA by copper-based catalyst proceeds via consecutive hydrogenation steps, in which succinic anhydride, GBL, and THF are formed subsequently [19]. The resultant succinic anhydride can react with the solvent, ethanol, to produce diethyl succinate (DS).
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Fig. 6 shows the conversion of MA in the hydrogenation of MA catalyzed by the Cu–Zn–Zr catalysts (C1–5) at 220–280 8C and 1 MPa. The experimental results showed that the conversion of MA was more than 97%, revealing that the catalysts exhibited good activity for the hydrogenation of MA. When the catalyst was mainly composed of CuO and ZnO (C5), the reduced catalyst C5 exhibited stable selectivity of GBL, ca. 65%, in a wide reaction temperature range of 220–260 8C. The selectivities of THF and DS slowly increased from 9.7% to 27.7% and from 19.8% to 37.6%, respectively, with increasing the reaction temperatures from 220 to 280 8C. The results revealed that the presence of ZnO in Cu–Zn catalyst favored the selective hydrogenation of MA to GBL. When the Cu–Zr catalyst (C4) was used in the hydrogenation of MA, the selectivity of THF rapidly increased from 25.6% to 82.8% even when the reaction temperatures were slightly raised from 220 to 240 8C, and then increased from 82.8% to 92.8% with further raising the reaction temperatures from 240 to 280 8C. The selectivity of GBL rapidly decreased to ca. 3% while the reaction temperature was raised to more than 240 8C. The selectivity of DS gradually decreased from 23.0% to 5.9% as the reaction temperatures increased from 220 to 280 8C. The presence of ZrO2 in Cu–Zr catalyst favored the deep hydrogenation of GBL to THF. When the Cu–Zn–Zr catalyst, C1 (1:2:1), was used in the hydrogenation of MA at 1 MPa, the selectivity of GBL was around 75% at reaction temperatures ranging from 220 to 260 8C and then decreased to 42.9% with further increasing the reaction temperature to 280 8C. The THF selectivity gradually increased from 7.8% to 44.7% with increasing the reaction temperatures from 220 to 280 8C. The selectivity of DS was around 10% at reaction temperatures ranging from 220 to 280 8C. The catalyst C1 shows good activity in the formation of GBL. Furthermore, the Cu–Zn–Zr catalyst (C1) exhibited higher selectivity of GBL than Cu–Zn (C5) and Cu–Zr (C4) catalysts, which should be due to the synergetic effect of ZnO and ZrO2. When the Cu–Zn–Zr catalyst, C2 (1:2:2), was used in the hydrogenation of MA, the THF selectivity rapidly increased from 16.8% to 80.3% with increasing the reaction temperatures from 220 to 260 8C and kept almost constant with further increasing the reaction temperature to 280 8C. The selectivity of GBL decreased rapidly from 74.8% to 6.2% with increasing the reaction temperatures from 220 to 260 8C. The selectivity of DS was around 10% at reaction temperatures ranging from 220 to 280 8C. The catalyst C2 with a high ZrO2 content showed higher THF selectivity than the catalyst C1 with a low ZrO2 content, revealing that the presence of ZrO2 is beneficial to the further hydrogenation of GBL to THF, in consistent with what certified by the Cu–Zr catalyst C4. When the Cu–Zn–Zr catalyst, C3 (2:2:1), with a higher copper loading was used in the hydrogenation of MA, the selectivity of GBL was kept at ca. 80% at lower reaction temperatures of 220 and 240 8C. Further increasing the reaction temperature caused the increase of the selectivity of THF to 79.3% at 280 8C, accompanied with the decrease of the selectivity of GBL to 8.7%. As compared to the Cu–Zn–Zr catalyst C1 with a lower copper loading, it was found that increasing the copper loading enhanced the deep hydrogenation activity of the Cu–Zn–Zr catalysts. 3.2.2. 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 aldehyde, then to ethyl acetate via reactions, such as disproportionation and esterification. Aldehyde can also be converted to a by-product n-butanol via a series of complicated reactions [25]. In our blank experiment, the dehydrogenation of pure ethanol catalyzed by Cu–Zn–Zr catalyst produced ethyl acetate, aldehyde, n-butanol and hydrogen.
Fig. 6. Catalytic activity of the Cu–Zn–Zr catalysts (C1–5) in MA hydrogenation at 1 MPa and different temperatures. (&) C1; (*) C2; (~) C3; (&) C4; (*) C5.
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Fig. 8. The ratios of the hydrogen produced in the ethanol dehydrogenation to the hydrogen consumed in the MA hydrogenation (H2%). (&) C1; (*) C2; (~) C3; (&) C4; (*) C5.
Fig. 7 shows that the conversion of ethanol catalyzed by the Cu– Zn–Zr catalysts (C1–5) increased from 4.9% to 24.1% with the increase of temperatures from 220 to 280 8C at 1 MPa, meaning that the catalysts (C1–5) had similar catalytic activity in the dehydrogenation of ethanol. When Cu–Zn–Zr catalysts C1–3 were used in the dehydrogenation of ethanol at reaction temperatures ranging from 220 to 280 8C and 1 MPa, the selectivities of ethyl acetate, aldehyde, and butanol were around 90%, 5%, and 5%, respectively. For the Cu–Zn and Cu–Zr catalysts, the selectivities of ethyl acetate, aldehyde, and butanol were around 80%, 10%, and 10%, respectively. The main product was ethyl acetate in the dehydrogenation of ethanol catalyzed by the copper-based catalysts. The selectivity of the
Fig. 7. Catalytic activity of the Cu–Zn–Zr catalysts (C1–5) in ethanol dehydrogenation at 1 MPa and different temperatures. (&) C1; (*) C2; (~) C3; (&) C4; (*) C5. Fig. 9. Effect of pressure on the hydrogenation of MA catalyzed by the Cu–Zn–Zr catalyst C2. (a) 1 MPa and (b) 0.1 MPa. C, conversion and S, selectivity.
D. Zhang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 537–543
dehydrogenation products, aldehyde, ethyl acetate and butanol, was not sensitive to the reaction temperature. But the Cu–Zn–Zr catalysts showed higher esterification activity than the Cu–Zn and Cu–Zr catalysts. 3.2.3. Compensation between hydrogenation and dehydrogenation Hydrogen produced by ethanol dehydrogenation can be used as a feedstock for MA hydrogenation. The ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in MA hydrogenation, H2%, were calculated according to Scheme 1 and the results were showed in Fig. 8. In the hydrogenation and dehydrogenation process catalyzed by Cu–Zn–Zr catalysts (C1–5), the H2 compensation values increased with raising reaction temperature from 220 to 280 8C. For the Cu–Zn–Zr catalysts C1–3, the H2 compensation values ranged from 25.3% to 86.8%, from 34.5% to 65.7%, and from 38.5% to 80.4%, respectively. For the Cu–Zr and Cu–Zn catalysts, the H2 compensation values ranged from 27.8% to 68.9% and from 28.5% to 109%, respectively. The copper-based catalysts favored the H2 compensation in the MA hydrogenation and ethanol dehydrogenation. 3.2.4. Effect of pressure on the MA hydrogenation Fig. 9 shows the effect of pressure on the hydrogenation of MA over the Cu–Zn–Zr catalyst C2. The experimental results showed that the conversions of MA under different pressures of 1 and 0.1 MPa were more than 97%, revealing that the catalyst exhibited good activity for the hydrogenation of MA under both 1 and 0.1 MPa. When the pressure was 1 MPa, the selectivity of THF rapidly increased from 16.8% to 80.3%; the selectivity of GBL rapidly decreased from 74.8% to 6.2%; the selectivity of DS kept around 10%, with increasing the reaction temperatures from 220 to 280 8C. When the pressure was 0.1 MPa, the selectivity of GBL rapidly increased from 42.5% to 95.5%; the selectivity of DS rapidly decreased from 57.5% to 0.1%; the selectivity of THF slightly increased from 0 to 4.5%, with increasing the reaction temperatures from 220 to 280 8C. As compared the catalytic activity of the Cu–Zn–Zr catalyst at different pressures, it was found that increasing the reaction pressure significantly promoted the hydrogenation reaction toward the positive direction. 4. Conclusions CuO present in the calcined Cu–Zn–Zr catalysts was reduced to metallic copper, which is the active site in the MA hydrogenation and the ethanol dehydrogenation. The co-presence of ZnO and ZrO2 favored the formation of small-sized metallic copper in the reduced Cu–Zn–Zr catalysts. The catalytic activity of Cu–Zn–Zr catalysts in the MA hydrogenation to GBL and THF was significantly affected by their composition. The presence of ZnO was beneficial to the formation of the intermediate product GBL while the presence of ZrO2 favored
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the deep hydrogenation of GBL to THF. At a lower copper loading, the co-presence of ZnO and ZrO2 in Cu–Zn–Zr catalysts synergistically increased the selectivity of GBL. Increasing the copper loading favored the hydrogenation toward the positive direction in the consecutive hydrogenation process of MA.The Cu–Zn–Zr catalysts exhibited well catalytic performance not only for MA hydrogenation but also for ethanol dehydrogenation. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in MA hydrogenation, H2%, increased with raising the reaction temperature. Acknowledgements This work was financially supported by Zhenjiang Science and Technology Bureau (CZ2006006). Authors sincerely thank Professor Jianxin Wu at the University of Science and Technology, China, for XPS measurement and Dr. Bin Xu at Yangzhou University for XRD measurement. References [1] Y. Hara, K. Takahashib, Catal. Surv. Jpn. 6 (2002) 73. [2] S.M. Jung, E. Godard, S.Y. Jung, K.-C. Park, J.U. Choi, J. Mol. Catal. A: Chem. 198 (2003) 297. [3] S.H. Yoo, J.Y. Jho, J. Won, H.C. Park, Y.S. Kang, J. Ind. Eng. Chem. 6 (2000) 129. [4] (a) J. Kanetaka, T. Asano, S. Masamune, Ind. Eng. Chem. 62 (1970) 24; (b) S. Yoon, J. Son, W. Lee, H.Y. Lee, C.W. Lee, J. Ind. Eng. Chem. 15 (2009) 370. [5] V. Pallassana, M. Neurock, G. Coulston, Catal. Today 50 (1999) 589. [6] S.P. Mu¨ller, M. Kucher, C. Ohlinger, B. Kraushaar-Czarnetzkyi, J. Catal. 218 (2003) 419. [7] D.W. Chung, T.G. Kim, J. Ind. Eng. Chem. 13 (2007) 979. [8] Y.M. Lim, P.H. Kang, Y.M. Lee, Y.C. Nho, J. Ind. Eng. Chem. 10 (2004) 267. [9] U.R. Pillai, E. S-Demessie, D. Young, Appl. Catal. B: Environ. 43 (2003) 131. [10] A. Ku¨ksal, E. Klemm, G. Emig, Appl. Catal. A: Gen. 228 (2002), p. 237-. [11] S.G. Girol, T. Strunskus, M. Muhler, C. Wo¨ll, J. Phys. Chem. B 108 (2004) 13736. [12] T.J. Hu, H.B. Yin, R.C. Zhang, H.X. Wu, T.S. Jiang, Y. Wada, Catal. Commun. 8 (2007) 193. [13] U. Herrmann, G. Emig, Ind. Eng. Chem. Res. 36 (1997) 2885. [14] S.M. Jung, E. Godard, S.Y. Jung, K.C. Park, J.U. Choi, Catal. Today 87 (2003) 171. [15] P. Liu, K. Yan, Y. Liu, Y. Yin, J. Nat. Gas Chem. 8 (1999) 157. [16] Y. Hara, H. Kusaka, H. Inagaki, K. Takahashi, K. Wada, J. Catal. 194 (2000) 188. [17] U. Herrmann, G. Emig, Ind. Eng. Chem. Res. 37 (1998) 759. [18] D.Z. Zhang, H.B. Yin, R.C. Zhang, J.J. Xue, T.S. Jiang, Catal. Lett. 122 (2008) 176. [19] R.C. Zhang, H.B. Yin, D.Z. Zhang, L. Qi, H.H. Lu, Y.T. Shen, T.S. Jiang, Chem. Eng. J. 140 (2008) 488. [20] Y.-W. Suh, S.-H. Moon, H.-K. Rhee, Catal. Today 63 (2000) 447. [21] K. Jung, A.T. Bell, J. Catal. 193 (2000) 207. [22] G.L. Castiglioni, C. Fumagalli, E. Armbruster, M. Messori, A. Vaccari, in: F.E. Herkes (Ed.), Catalysis of Organic Reactions, Dekker, New York, 1998, p. 391. [23] R. Lancia, A. Vaccari, C. Fumagalli, E. Armbruster, Process for the production of gamma-butyrolactone, WO Patent 9522539, 1995. [24] D.J. Elliott, F. Pennella, J. Catal. 119 (1989) 359. [25] K. Inui, T. Kurabayashi, S. Sato, N. Ichikawa, J. Mol. Catal. A: Chem. 216 (2004) 147. [26] R.T. Figueiredo, A. Martinez-Arias, M.L. Granados, L.G. Fierro, J. Catal. 178 (1998) 146. [27] C.L. Aravinda, P. Bera, V. Jayaram, A.K. Sharma, S.M. Mayanna, Mater. Res. Bull. 37 (2002) 397. [28] G. Moretti, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 365. [29] S. Poulston, P.M. Parlett, P. Stone, M. Bowker, Surf. Interface. Anal. 24 (1996) 811. [30] S. Velu, K. Suzuki, C.S. Gopinath, H. Yoshida, T. Hattori, Phys. Chem. Chem. Phys. 4 (2002) 1990.
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Selective hydrogenation of maleic anhydride to g-butyrolactone and tetrahydrofuran by Cu–Zn–Zr catalyst in the presence of ethanol Dongzhi Zhang, Hengbo Yin *, Chen Ge, Jinjuan Xue, Tingshun Jiang, Longbao Yu, Yutang Shen Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 October 2008 Accepted 13 January 2009
A series of Cu–Zn–Zr catalysts were prepared by a coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, and N2 adsorption. The catalytic activity of the Cu–Zn–Zr catalyst in the hydrogenation of maleic anhydride using ethanol as a solvent was studied at 220–280 8C and 1 MPa. Maleic anhydride was mainly hydrogenated to gbutyrolactone and tetrahydrofuran while ethanol dehydrogenated to ethyl acetate. After reduction, CuO species present in the calcined Cu–Zn–Zr catalysts were converted to metallic copper (Cu0). The presence of ZrO2 favored the deep hydrogenation of g-butyrolactone to tetrahydrofuran while the presence of ZnO was beneficial to the formation of the intermediate product g-butyrolactone. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in maleic anhydride hydrogenation increased with the increase of the reaction temperature. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Maleic anhydride Ethanol Hydrogenation Cu–Zn–Zr catalyst Tetrahydrofuran
1. Introduction Hydrogenation of maleic anhydride (MA) produces g-butyrolactone (GBL) and tetrahydrofuran (THF), which are very important industrial chemicals. GBL is an alternative to the environmentally harmful chlorinated solvent and also an intermediate widely used in the polymer industry, producing 2pyrrolidone, N-vinyl-2-pyrrolidone, and polyvinylpyrrolidone [1– 3]. THF is widely used both as a versatile solvent and as a raw material for the manufacture of polytetramethylene ether glycol (PTMEG), spendex fibers, and polyurethane elastomers [4–8]. GBL is mainly produced by the following two processes: Davy McKee process based on the hydrogenation of diethyl or dimethyl maleate and Reppe process based on acetylene and formaldehyde [9]. THF is mainly produced by the following two processes: furfural process based on furfural and Reppe process based on acetylene and formaldehyde [4,6]. The starting materials, such as acetylene, formaldehyde, and furfural, are explosive or carcinogenic. Furthermore, the severe reaction conditions and multi-step reaction pathways make the above-mentioned production processes unfavorable. To overcome the disadvantages of the established processes, eco-friendly and economically alternative process is worthy of investigation. A promising alternative to the established processes is the single stage hydrogenation of MA since
* Corresponding author. Tel.: +86 0511 88787591; fax: +86 0511 88791800. E-mail address: [email protected] (H. Yin).
MA can be produced at a low cost and on a large scale by partial oxidation of n-butane instead of benzene now [4,10–12]. Hydrogenation of MA can be catalyzed by various kinds of catalysts, such as noble metals (Pd, Re, and Ru) in liquid phase at pressures between 1 and 5 MPa and temperatures between 190 and 240 8C [2,13–16], copper-based catalysts in liquid phase at pressures between 5 and 9 MPa and temperatures between 200 and 240 8C [13,17], and copper-based catalysts in gas phase at pressures from 0.1 to 1 MPa and temperatures between 210 and 280 8C [12,18,19]. The gas phase hydrogenation of MA commonly used Cu–Zn–M (M = Al, Cr, Ce, Ti) catalysts. The type and composition of supports significantly affect the performance of copper-based catalysts. Although zirconia-supported copper catalysts exhibit high activity in hydrogenation process, such as methanol synthesis starting from CO and H2 [20,21], utilizing zirconia as a support in copper-based catalyst for MA hydrogenation is seldom investigated. Castiglioni et al. [22] and Lancia et al. [23] made researches in the hydrogenation of MA using Cu–Zn–Zr catalysts prepared from the corresponding chloride salts of copper, zinc, and zirconium. They found that MA was mainly hydrogenated to GBL with the selectivity close to 99% at 0.1–8 MPa. The relationship between the catalytic activity and the structure of the Cu–Zn–Zr catalysts is worth of investigation in detail. In the industrial MA hydrogenation process, toxic methanol is used as a solvent. It will be a good alternative to replace methanol by environmental benign ethanol in the MA hydrogenation process. Furthermore, dehydrogenation of ethanol catalyzed by copper-based catalysts mainly produces hydrogen and ethyl
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.01.010
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Scheme 1. Routes of MA hydrogenation and ethanol dehydrogenation.
acetate [24,25]. Hydrogen is a feedstock of MA hydrogenation and ethyl acetate is a valuable fine chemical. According to Scheme 1, in the hydrogenation process of MA, producing 1 mol GBL requires 3 mol H2; producing 1 mol THF requires 5 mol H2; producing 1 mol diethyl succinate (DS) requires 1 mol H2 [6,11,12]. While in the dehydrogenation process of ethanol catalyzed by copper-based catalysts, ethanol is dehydrogenated to aldehyde and ethyl acetate. Producing 1 mol aldehyde releases 1 mol H2; producing 1 mol ethyl acetate releases 2 mol H2 [12,24]. Using ethanol as a solvent in MA hydrogenation should favor the two reactions in view of the compensation of H2. But MA catalytic hydrogenation using ethanol as a solvent was seldom investigated. In our present work, we reported a study on the preparation and characterization of Cu–Zn–Zr catalyst. The major objective of this work is to gain an insight into the catalytic activity of Cu–Zn–Zr catalyst in MA hydrogenation using ethanol as a solvent at 220– 280 8C and 1 MPa. 2. Experimental 2.1. Catalyst preparation Cu–Zn–Zr catalysts were prepared by a continuous coprecipitation method. A mixed solution of Cu(NO3)23H2O, Zn(NO3)26H2O, and Zr(NO3)45H2O with a given atomic ratio was used as a precursor solution, and a Na2CO3 solution (1 M) was used as a precipitating agent. Coprecipitation was performed at 75 8C 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 Table 1 The compositions, specific surface areas, and average pore diameters of the calcined Cu–Zn–Zr catalysts. Samples
C1 C2 C3 C4 C5
Atomic ratios Cu
Zn
Zr
1 1 2 2 2
2 2 2 0 2
1 2 1 1 0
Average pore diameters (nm)
Specific surface areas (m2/g)
11.1 9.6 16.4 10.6 17.1
69.6 80.8 77.9 48.8 28.6
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 8C for 12 h, the dried catalysts were calcined at 450 8C 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 Cu–Zn–Zr catalysts according to those in their precursors are listed in Table 1. 2.2. 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 Ka radiation (1.5418 A˚) with Ni filter, scanning from 208 to 808 (2u). The crystallite sizes of the Cu0 (1 1 1) in the reduced catalysts were calculated by using Scherrer’s equation: D = Kl/(Bcos u), where K was taken as 0.9, and B was the full width of the diffraction line at half of the maximum intensity. 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 Ka radiation (1486.6 eV). The binding energies were calculated with respect to C1s peak at 284.6 eV. The reduction behaviors of the calcined catalysts were investigated by temperature programmed reduction (TPR) technique using a mixed H2/N2 flow (5:95, v/v) of 30 ml/min and 50 mg of the calcined catalysts at a temperature ramp of 10 8C/min from 25 to 300 8C. H2 consumption was determined by analyzing the effluent gas with a thermal conductivity detector. The calcined catalysts were preheated in air at 200 8C before the TPR measurement in order to eliminate impurities and adsorbed water. The specific surface areas and the average pore sizes of the calcined catalysts were measured on a NOVA 2000e physical adsorption apparatus by BET and BJH methods, respectively. 2.3. 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,
D. Zhang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 537–543
respectively, packed with 5 ml of catalyst with particle sizes ranging from 0.45 to 0.9 mm, operating at 220–280 8C and 1 MPa. The reactor was fed with a stream of MA and ethanol solution (12:88, w/w) in hydrogen, the liquid space velocity was 0.2 h1, and the molar ratio of H2 to MA was 50:1. 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 8C at a temperature ramp of 1.5 8C/min. Then the catalyst was continuously reduced at 280 8C 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. The collected reaction products were analyzed on a gas chromatograph, equipped with FID and a PEG packed capillary column (0.25 mm 30 m).
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Table 2 The crystallite sizes of metallic copper (1 1 1) of the reduced catalysts. Samples
C1
C2
C3
C4
C5
D (nm)
14.4
14.7
17.8
47.0
18.4
of metallic copper were between 14.4 and 18.4 nm (Table 2). For the Cu–Zr catalyst (C4), the crystallite size of metallic copper was 47 nm. The results showed that the presence of ZnO promoted the dispersion of metallic copper in the reduced catalysts.
3. Results and discussion 3.1. Characterization of catalyst 3.1.1. XRD analysis The XRD patterns of the calcined and reduced Cu–Zn–Zr catalysts are shown in Figs. 1 and 2, respectively. The XRD patterns of the calcined Cu–Zn–Zr catalysts exhibited the characteristic peaks of CuO (PDF#48-1548) and ZnO (PDF#36-1451), appearing at 2u = 35.58, 38.78, 48.78, 53.58, 58.38, 61.58, 66.28, 68.18, 72.48, 75.08; 31.88, 34.48, 36.38, 47.58, 56.68, 62.98, 68.08, respectively. The XRD patterns of the reduced Cu–Zn–Zr catalysts showed that CuO was reduced to Cu0 (PDF#04-0836) with the characteristic peaks appearing at 2u = 43.38, 50.48, and 74.18 and that zinc species were still present in the form of ZnO. There was no diffraction peak of ZrO2 detected by XRD analysis (Figs. 1 and 2), revealing that ZrO2 was in amorphous phase present in both calcined and reduced Cu– Zn–Zr catalysts. As the reduced catalysts were concerned, the crystallite sizes of metallic copper (1 1 1) were estimated by Scherrer’s equation. For the Cu–Zn–Zr and Cu–Zn catalysts (C1–3, C5), the crystallite sizes
Fig. 1. XRD patterns of the calcined Cu–Zn–Zr catalysts. (!) CuO; (*) ZnO.
Fig. 2. XRD patterns of the reduced Cu–Zn–Zr catalysts. (5) Cu0; (*) ZnO.
Fig. 3. X-ray photoelectron spectra of the calcined catalysts (C2, C4 and C5) and the reduced catalyst C2 (denoted as C2*).
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3.1.2. XPS analysis The chemical states of the representative calcined Cu–Zn–Zr (C2), calcined Cu–Zr (C4), calcined Cu–Zn (C5), and H2-reduced Cu– Zn–Zr (C2*) catalysts were evaluated by XPS. Fig. 3a shows the Cu2p3/2 and Cu2p1/2 peaks of the calcined and H2-reduced samples. The calcined samples C2, C4 and C5 displayed that the Cu2p3/2 and Cu2p1/2 peaks appeared at 934.9, 955; 934.6, 954.6; 933.7, 953.6 eV, respectively, which were the characteristic peaks of Cu2+ species [26]. Furthermore, the presence of Cu2+ satellite peak appearing at ca. 942.5 eV evidenced the presence of Cu2+ ions in the form of cupric oxide, although whose origin was complex and has been explained as due to electron shake-up processes, final state effects, and charge transfer mechanisms [27]. The binding energies of Cu2p3/2 and Cu2p1/2 of Cu–Zr (C4) and Cu–Zn (C5) catalysts shifted to lower values than that of Cu–Zn–Zr (C2), revealing that the chemical state of CuO was influenced by the composition of the catalyst. The electron density surrounding Cu atom was changed with the catalyst composition. After reduction, the Cu2p3/2 and Cu2p1/2 peaks of Cu–Zn–Zr catalyst (C2*) shifted to lower binding energies at 933 and 953 eV and the satellite peak disappeared, meaning that after reduction, the copper species were Cu+ or Cu0, rather than Cu2+ [26]. The shift of Cu 2p3/2 revealed the disappearance of cupric oxide, but did not allow to reach a conclusion about the presence of solely cuprous oxide or solely metallic copper or both since the Cu2p3/2 peaks for metallic copper and Cu2O appear at almost the same binding energy. In an attempt to understand the differences in the chemical states of the copper species present in C2 and C2* catalysts, the Wagner plot, as reported earlier [28], was drawn and shown in Fig. 4. Here, the kinetic energy of the Auger transition was on the Yaxis and the Cu 2p3/2 binding energy of the photoemission line on the X-axis. The data for Cu, Cu2O and CuO reference samples were taken from the literatures 28 and 29. From Fig. 4, it was found that the Auger parameter of C2 catalyst fell on the CuO line. The Auger parameter of C2* catalyst almost fell on the line of metallic copper, a little lower than the reported Auger parameter of metallic copper. This could be due to the poor crystallinity of the Cu0 phase, as observed from the XRD analysis. From the Wagner plot analysis, it was found that the CuO species present in the calcined Cu–Zn–Zr catalysts were converted to Cu0 after reduction, being consistent with the XRD analysis. The Zn2p3/2 binding energies of the calcined Cu–Zn (C5), calcined Cu–Zn–Zr (C2), and H2-reduced Cu–Zn–Zr (C2*) catalysts were 1022.25, 1022.9, and 1022.9 eV, respectively, pertaining to that of ZnO [30] (Fig. 3b). The Zr3d5/2 binding energies of the calcined Cu–Zr (C4), calcined Cu–Zn–Zr (C2), and H2-reduced Cu– Zn–Zr (C2*) catalysts were 181.45, 181.9, and 181.9 eV, respectively, revealing that zirconium species existed in the form of ZrO2 [30] (Fig. 3c). The results revealed that the chemical states of Zn and Zr were influenced by the chemical composition of the catalysts rather than the reduction. 3.1.3. TPR analysis From the TPR profiles of the calcined samples (Fig. 5), it was found that the temperatures at the maximum reduction peaks of the Cu–Zn–Zr catalysts (C1–3) were 158, 160, and 156 8C, respectively, and that the temperatures of Cu–Zr (C4) and Cu– Zn (C5) catalysts were 149 and 153 8C. The TPR results revealed that all of the catalysts were easy to be reduced. However, the TPR analysis showed that the reduction temperatures of the catalysts were in an order of Cu–Zn–Zr > Cu–Zn > Cu–Zr. Interestingly, as observed by XRD analysis (Table 2), the crystallite sizes of metallic copper present in the catalysts were in an order of Cu–Zn–Zr < Cu– Zn < Cu–Zr. The formation of large-sized CuO precursors in ZnO-, especially ZrO2-supported catalysts decreased the interaction between CuO precursors and ZnO or ZrO2 supports, giving lower
Fig. 4. Wagner plot for the calcined and reduced Cu–Zn–Zr catalysts and Cucontaining reference samples [28,29]. Solid lines are Cu-containing reference samples.
Fig. 5. TPR profiles of the Cu–Zn–Zr catalysts.
reduction temperatures. CuO precursors were well dispersed in Cu–Zn–Zr catalysts due to a synergetic effect of their supports, ZnO and ZrO2, giving a strong interaction between CuO precursors and ZnO–ZrO2 supports, subsequently, causing their reduction at higher temperatures. 3.1.4. Specific surface area and average pore size The specific surface areas and the average pore sizes of the Cu– Zn–Zr, Cu–Zn, and Cu–Zr catalysts were listed in Table 1. The specific surface areas of the Cu–Zn–Zr (C1–3) catalysts were more than 69.6 m2/g, revealing that the as-prepared Cu–Zn–Zr catalysts had large surface areas. For the Cu–Zr (C4) and Cu–Zn (C5) catalysts, the specific surface areas were 48.8 and 28.6 m2/g, respectively. The co-presence of ZnO and ZrO2 increased the specific surface areas of the resultant Cu–Zn–Zr catalysts. For all of the catalysts (C1–5), the average pore sizes were in a range of 9.6–17.1 nm, meaning that the as-prepared catalysts were mesoporous materials. 3.2. Catalytic test 3.2.1. Hydrogenation of MA The catalytic hydrogenation of MA by copper-based catalyst proceeds via consecutive hydrogenation steps, in which succinic anhydride, GBL, and THF are formed subsequently [19]. The resultant succinic anhydride can react with the solvent, ethanol, to produce diethyl succinate (DS).
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Fig. 6 shows the conversion of MA in the hydrogenation of MA catalyzed by the Cu–Zn–Zr catalysts (C1–5) at 220–280 8C and 1 MPa. The experimental results showed that the conversion of MA was more than 97%, revealing that the catalysts exhibited good activity for the hydrogenation of MA. When the catalyst was mainly composed of CuO and ZnO (C5), the reduced catalyst C5 exhibited stable selectivity of GBL, ca. 65%, in a wide reaction temperature range of 220–260 8C. The selectivities of THF and DS slowly increased from 9.7% to 27.7% and from 19.8% to 37.6%, respectively, with increasing the reaction temperatures from 220 to 280 8C. The results revealed that the presence of ZnO in Cu–Zn catalyst favored the selective hydrogenation of MA to GBL. When the Cu–Zr catalyst (C4) was used in the hydrogenation of MA, the selectivity of THF rapidly increased from 25.6% to 82.8% even when the reaction temperatures were slightly raised from 220 to 240 8C, and then increased from 82.8% to 92.8% with further raising the reaction temperatures from 240 to 280 8C. The selectivity of GBL rapidly decreased to ca. 3% while the reaction temperature was raised to more than 240 8C. The selectivity of DS gradually decreased from 23.0% to 5.9% as the reaction temperatures increased from 220 to 280 8C. The presence of ZrO2 in Cu–Zr catalyst favored the deep hydrogenation of GBL to THF. When the Cu–Zn–Zr catalyst, C1 (1:2:1), was used in the hydrogenation of MA at 1 MPa, the selectivity of GBL was around 75% at reaction temperatures ranging from 220 to 260 8C and then decreased to 42.9% with further increasing the reaction temperature to 280 8C. The THF selectivity gradually increased from 7.8% to 44.7% with increasing the reaction temperatures from 220 to 280 8C. The selectivity of DS was around 10% at reaction temperatures ranging from 220 to 280 8C. The catalyst C1 shows good activity in the formation of GBL. Furthermore, the Cu–Zn–Zr catalyst (C1) exhibited higher selectivity of GBL than Cu–Zn (C5) and Cu–Zr (C4) catalysts, which should be due to the synergetic effect of ZnO and ZrO2. When the Cu–Zn–Zr catalyst, C2 (1:2:2), was used in the hydrogenation of MA, the THF selectivity rapidly increased from 16.8% to 80.3% with increasing the reaction temperatures from 220 to 260 8C and kept almost constant with further increasing the reaction temperature to 280 8C. The selectivity of GBL decreased rapidly from 74.8% to 6.2% with increasing the reaction temperatures from 220 to 260 8C. The selectivity of DS was around 10% at reaction temperatures ranging from 220 to 280 8C. The catalyst C2 with a high ZrO2 content showed higher THF selectivity than the catalyst C1 with a low ZrO2 content, revealing that the presence of ZrO2 is beneficial to the further hydrogenation of GBL to THF, in consistent with what certified by the Cu–Zr catalyst C4. When the Cu–Zn–Zr catalyst, C3 (2:2:1), with a higher copper loading was used in the hydrogenation of MA, the selectivity of GBL was kept at ca. 80% at lower reaction temperatures of 220 and 240 8C. Further increasing the reaction temperature caused the increase of the selectivity of THF to 79.3% at 280 8C, accompanied with the decrease of the selectivity of GBL to 8.7%. As compared to the Cu–Zn–Zr catalyst C1 with a lower copper loading, it was found that increasing the copper loading enhanced the deep hydrogenation activity of the Cu–Zn–Zr catalysts. 3.2.2. 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 aldehyde, then to ethyl acetate via reactions, such as disproportionation and esterification. Aldehyde can also be converted to a by-product n-butanol via a series of complicated reactions [25]. In our blank experiment, the dehydrogenation of pure ethanol catalyzed by Cu–Zn–Zr catalyst produced ethyl acetate, aldehyde, n-butanol and hydrogen.
Fig. 6. Catalytic activity of the Cu–Zn–Zr catalysts (C1–5) in MA hydrogenation at 1 MPa and different temperatures. (&) C1; (*) C2; (~) C3; (&) C4; (*) C5.
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Fig. 8. The ratios of the hydrogen produced in the ethanol dehydrogenation to the hydrogen consumed in the MA hydrogenation (H2%). (&) C1; (*) C2; (~) C3; (&) C4; (*) C5.
Fig. 7 shows that the conversion of ethanol catalyzed by the Cu– Zn–Zr catalysts (C1–5) increased from 4.9% to 24.1% with the increase of temperatures from 220 to 280 8C at 1 MPa, meaning that the catalysts (C1–5) had similar catalytic activity in the dehydrogenation of ethanol. When Cu–Zn–Zr catalysts C1–3 were used in the dehydrogenation of ethanol at reaction temperatures ranging from 220 to 280 8C and 1 MPa, the selectivities of ethyl acetate, aldehyde, and butanol were around 90%, 5%, and 5%, respectively. For the Cu–Zn and Cu–Zr catalysts, the selectivities of ethyl acetate, aldehyde, and butanol were around 80%, 10%, and 10%, respectively. The main product was ethyl acetate in the dehydrogenation of ethanol catalyzed by the copper-based catalysts. The selectivity of the
Fig. 7. Catalytic activity of the Cu–Zn–Zr catalysts (C1–5) in ethanol dehydrogenation at 1 MPa and different temperatures. (&) C1; (*) C2; (~) C3; (&) C4; (*) C5. Fig. 9. Effect of pressure on the hydrogenation of MA catalyzed by the Cu–Zn–Zr catalyst C2. (a) 1 MPa and (b) 0.1 MPa. C, conversion and S, selectivity.
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dehydrogenation products, aldehyde, ethyl acetate and butanol, was not sensitive to the reaction temperature. But the Cu–Zn–Zr catalysts showed higher esterification activity than the Cu–Zn and Cu–Zr catalysts. 3.2.3. Compensation between hydrogenation and dehydrogenation Hydrogen produced by ethanol dehydrogenation can be used as a feedstock for MA hydrogenation. The ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in MA hydrogenation, H2%, were calculated according to Scheme 1 and the results were showed in Fig. 8. In the hydrogenation and dehydrogenation process catalyzed by Cu–Zn–Zr catalysts (C1–5), the H2 compensation values increased with raising reaction temperature from 220 to 280 8C. For the Cu–Zn–Zr catalysts C1–3, the H2 compensation values ranged from 25.3% to 86.8%, from 34.5% to 65.7%, and from 38.5% to 80.4%, respectively. For the Cu–Zr and Cu–Zn catalysts, the H2 compensation values ranged from 27.8% to 68.9% and from 28.5% to 109%, respectively. The copper-based catalysts favored the H2 compensation in the MA hydrogenation and ethanol dehydrogenation. 3.2.4. Effect of pressure on the MA hydrogenation Fig. 9 shows the effect of pressure on the hydrogenation of MA over the Cu–Zn–Zr catalyst C2. The experimental results showed that the conversions of MA under different pressures of 1 and 0.1 MPa were more than 97%, revealing that the catalyst exhibited good activity for the hydrogenation of MA under both 1 and 0.1 MPa. When the pressure was 1 MPa, the selectivity of THF rapidly increased from 16.8% to 80.3%; the selectivity of GBL rapidly decreased from 74.8% to 6.2%; the selectivity of DS kept around 10%, with increasing the reaction temperatures from 220 to 280 8C. When the pressure was 0.1 MPa, the selectivity of GBL rapidly increased from 42.5% to 95.5%; the selectivity of DS rapidly decreased from 57.5% to 0.1%; the selectivity of THF slightly increased from 0 to 4.5%, with increasing the reaction temperatures from 220 to 280 8C. As compared the catalytic activity of the Cu–Zn–Zr catalyst at different pressures, it was found that increasing the reaction pressure significantly promoted the hydrogenation reaction toward the positive direction. 4. Conclusions CuO present in the calcined Cu–Zn–Zr catalysts was reduced to metallic copper, which is the active site in the MA hydrogenation and the ethanol dehydrogenation. The co-presence of ZnO and ZrO2 favored the formation of small-sized metallic copper in the reduced Cu–Zn–Zr catalysts. The catalytic activity of Cu–Zn–Zr catalysts in the MA hydrogenation to GBL and THF was significantly affected by their composition. The presence of ZnO was beneficial to the formation of the intermediate product GBL while the presence of ZrO2 favored
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the deep hydrogenation of GBL to THF. At a lower copper loading, the co-presence of ZnO and ZrO2 in Cu–Zn–Zr catalysts synergistically increased the selectivity of GBL. Increasing the copper loading favored the hydrogenation toward the positive direction in the consecutive hydrogenation process of MA.The Cu–Zn–Zr catalysts exhibited well catalytic performance not only for MA hydrogenation but also for ethanol dehydrogenation. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in MA hydrogenation, H2%, increased with raising the reaction temperature. Acknowledgements This work was financially supported by Zhenjiang Science and Technology Bureau (CZ2006006). Authors sincerely thank Professor Jianxin Wu at the University of Science and Technology, China, for XPS measurement and Dr. Bin Xu at Yangzhou University for XRD measurement. References [1] Y. Hara, K. Takahashib, Catal. Surv. Jpn. 6 (2002) 73. [2] S.M. Jung, E. Godard, S.Y. Jung, K.-C. Park, J.U. Choi, J. Mol. Catal. A: Chem. 198 (2003) 297. [3] S.H. Yoo, J.Y. Jho, J. Won, H.C. Park, Y.S. Kang, J. Ind. Eng. Chem. 6 (2000) 129. [4] (a) J. Kanetaka, T. Asano, S. Masamune, Ind. Eng. Chem. 62 (1970) 24; (b) S. Yoon, J. Son, W. Lee, H.Y. Lee, C.W. Lee, J. Ind. Eng. Chem. 15 (2009) 370. [5] V. Pallassana, M. Neurock, G. Coulston, Catal. Today 50 (1999) 589. [6] S.P. Mu¨ller, M. Kucher, C. Ohlinger, B. Kraushaar-Czarnetzkyi, J. Catal. 218 (2003) 419. [7] D.W. Chung, T.G. Kim, J. Ind. Eng. Chem. 13 (2007) 979. [8] Y.M. Lim, P.H. Kang, Y.M. Lee, Y.C. Nho, J. Ind. Eng. Chem. 10 (2004) 267. [9] U.R. Pillai, E. S-Demessie, D. Young, Appl. Catal. B: Environ. 43 (2003) 131. [10] A. Ku¨ksal, E. Klemm, G. Emig, Appl. Catal. A: Gen. 228 (2002), p. 237-. [11] S.G. Girol, T. Strunskus, M. Muhler, C. Wo¨ll, J. Phys. Chem. B 108 (2004) 13736. [12] T.J. Hu, H.B. Yin, R.C. Zhang, H.X. Wu, T.S. Jiang, Y. Wada, Catal. Commun. 8 (2007) 193. [13] U. Herrmann, G. Emig, Ind. Eng. Chem. Res. 36 (1997) 2885. [14] S.M. Jung, E. Godard, S.Y. Jung, K.C. Park, J.U. Choi, Catal. Today 87 (2003) 171. [15] P. Liu, K. Yan, Y. Liu, Y. Yin, J. Nat. Gas Chem. 8 (1999) 157. [16] Y. Hara, H. Kusaka, H. Inagaki, K. Takahashi, K. Wada, J. Catal. 194 (2000) 188. [17] U. Herrmann, G. Emig, Ind. Eng. Chem. Res. 37 (1998) 759. [18] D.Z. Zhang, H.B. Yin, R.C. Zhang, J.J. Xue, T.S. Jiang, Catal. Lett. 122 (2008) 176. [19] R.C. Zhang, H.B. Yin, D.Z. Zhang, L. Qi, H.H. Lu, Y.T. Shen, T.S. Jiang, Chem. Eng. J. 140 (2008) 488. [20] Y.-W. Suh, S.-H. Moon, H.-K. Rhee, Catal. Today 63 (2000) 447. [21] K. Jung, A.T. Bell, J. Catal. 193 (2000) 207. [22] G.L. Castiglioni, C. Fumagalli, E. Armbruster, M. Messori, A. Vaccari, in: F.E. Herkes (Ed.), Catalysis of Organic Reactions, Dekker, New York, 1998, p. 391. [23] R. Lancia, A. Vaccari, C. Fumagalli, E. Armbruster, Process for the production of gamma-butyrolactone, WO Patent 9522539, 1995. [24] D.J. Elliott, F. Pennella, J. Catal. 119 (1989) 359. [25] K. Inui, T. Kurabayashi, S. Sato, N. Ichikawa, J. Mol. Catal. A: Chem. 216 (2004) 147. [26] R.T. Figueiredo, A. Martinez-Arias, M.L. Granados, L.G. Fierro, J. Catal. 178 (1998) 146. [27] C.L. Aravinda, P. Bera, V. Jayaram, A.K. Sharma, S.M. Mayanna, Mater. Res. Bull. 37 (2002) 397. [28] G. Moretti, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 365. [29] S. Poulston, P.M. Parlett, P. Stone, M. Bowker, Surf. Interface. Anal. 24 (1996) 811. [30] S. Velu, K. Suzuki, C.S. Gopinath, H. Yoshida, T. Hattori, Phys. Chem. Chem. Phys. 4 (2002) 1990.