Top Catal (2010) 53:517–522 DOI 10.1007/s11244-010-9480-1
ORIGINAL PAPER
The Promotion Effect of Cr on Copper Catalyst in Hydrogenolysis of Glycerol to Propylene Glycol Nam Dong Kim • Seogil Oh • Ji Bong Joo Kwang Seop Jung • Jongheop Yi
•
Published online: 1 April 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Binary Cu/Cr catalysts, containing various molar ratios of copper to chromium, were synthesized and their catalytic activities were examined for the hydrogenolysis of glycerol to propylene glycol. When catalyst containing Cu and Cr ratio of 1:2, it was mainly composed of CuCr2O4 phase. And it was found to have the highest catalytic activity in this reaction, due to its favorable reduction properties. Keywords Hydrogenolysis Copper catalyst Promotion effect Glycerol Propylene glycol
1 Introduction In recent years, biodiesel has been considered as a renewable and environmentally friendly energy source [1, 2]. During the biodiesel production via transesterification of vegetable oils and animal fats, considerable amount of glycerol by-product are formed in a ratio of about 1 kg of glycerol out of every 9 kg biodiesel. As the amount of glycerol production is greatly increased due to the increasing production of biodiesel, necessity of new methods for the production of value-added chemical from glycerol is also increasing.
N. D. Kim S. Oh J. B. Joo J. Yi (&) School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-742, Republic of Korea e-mail:
[email protected] K. S. Jung GS Caltex Corporation, 104-4 Munji-dong, Yusung-ku, Daejeon 305-380, Republic of Korea
Lots of catalytic conversion processes have been reported to convert glycerol into other useful chemicals and broad overview are presented in a recent review [3, 4]. Among the value-added chemicals, propylene glycol is a major commodity that is widely used in the production of unsaturated polyester resins, functional fluids (antifreeze, de-icing, and heat transfer), pharmaceuticals, foods, cosmetics, liquid detergents, tobacco humectants, flavors and fragrances, personal care, paints and animal feed [5]. The majority of propylene glycol is produced via petroleum routes in industry at present [6]. Because of the cost of these process, production of propylene glycol from renewable sources, biodiesel by-product glycerol, has great potential for the cost effective processes. Several researches have been reported on the catalytic conversion of glycerol to propylene glycol. Hydrogenolysis of glycerol to propylene glycol requires high hydrogenation activity for C–O bonds and poor hydrogenolytic activity toward C–C bonds. For this reason, copper based catalyst, such as Cu/Cr and Cu/Zn catalyst [5, 7–11], showed high catalytic performance compared to the other transition metal catalyst [12], among the various heterogeneous catalysts previously reported. Copper based catalysts are also economical compared to noble metal such as Ru, Pt and Au, which are reported before to have moderate catalytic activity in this reaction [13–15]. Among the copper based catalysts, Cu/Cr catalysts showed exceptionally high catalytic activity in hydrogenolysis of glycerol to propylene glycol. Dasari et al. [5] reported that Cu/Cr catalyst exhibited high selectivity of 85.0% at ca. 55% conversion at 473 K and 1.4 MPa which is much mild condition than that of previously reported. Liang et al. [7] synthesized Cu/Cr catalyst with high surface area by template method using carbon template giving almost 100% of selectivity to yield propylene glycol. Although significant
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progress has been achieved in terms of the activities of copper catalysts in hydrogenolysis reactions, researches for active component and promotion effect in Cu/Cr catalyst are still required. In this study, binary Cu/Cr catalysts containing with various Cu/Cr ratios were prepared by a co-precipitation method and were then evaluated in terms of the hydrogenolysis of glycerol to propylene glycol. The effect of added chromium was also examined, from the standpoint of crystal structure, catalytic properties and performances.
2 Experimental Binary Cu/Cr catalysts containing various molar ratios were prepared by a co-precipitation method. A known amount of copper nitrate (Cu(NO3)22.5H2O, Riedel-de Hae¨n) was dissolved in distilled water. Calculated amounts of chromium nitrate (Cr(NO3)29H2O, Sigma-Aldrich) were then added to copper salt solution with vigorous stirring. Sodium hydroxide (3 M NaOH) was then added until the pH of the solution reached 12. The resulting precipitates were filtered and dried overnight. The dried precipitates were ground and calcined at 550 °C in air to obtain the final calcined Cu/Cr catalysts. The prepared catalysts were designated as follows (CuCr-X, X denotes the relative portion of copper species in the catalyst.):
Catalyst composition
Designation
Cu:Cr = 1:0
CuCr-1
Cu:Cr = 1:1 Cu:Cr = 1:2
CuCr-0.5 CuCr-0.33
Cu:Cr = 1:3
CuCr-0.25
Cu:Cr = 0:1
CuCr-0
The X-ray diffraction (XRD) patterns of the powdered catalysts were recorded using a X-ray diffractometer (D-MAX2500-PC, Rigaku Corp.) equipped with CuKa ˚ ). To examine the reducibility of radiation (k = 1.5405 A the synthesized catalysts, temperature-programmed reduction (TPR) measurements were performed using a conventional flow system with a moisture trap connected to a thermal conductivity detector (TCD). The flow rate of the mixed gas (10% H2 in N2) was fixed at 20 mL/min and the heating rate was 5 °C/min. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a KRATOS AXIS electron spectrometer equipped with MgKa radiation for exciting photoelectrons. All binding energies (BE’s) are referred to the adventitious C 1s line at 284.6 eV.
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Acidity measurement were performed by temperature programmed desorption of ammonia (NH3-TPD) with a conventional flow apparatus equipped with a TCD. A given amount of the sample, 0.1 g, was reduced with a mixed stream of 10% H2 in N2 balance at 320 °C for 2 h and then cooled to room temperature. Excess amount of NH3 were injected with syringe for several times and subsequently purged with He at the same temperature with reduced pressure condition for 1 h to remove the physisorbed NH3. The TPD measurements were conducted in flowing He from room temperature to 600 °C with a heating rate of 10 °C/min. The measurement of occluded hydrogen species was conducted by H2-TPD with the same appatus which was used for NH3-TPD. Reduction procedure of the catalysts is same with NH3-TPD method. Temperature of the reduced catalysts were cooled to room temperature and then purged with N2 stream at the same temperature with reduced pressure condition to remove the physisorbed H2. The TPD measurements were conducted in flowing N2 from room temperature to 600 °C at a heating rate of 10 °C/min. The hydrogenolysis of glycerol to propylene glycol was carried out in a hastelloy-C autoclave equipped with a magnetic stirrer and heater. The autoclave was filled with glycerol (50 g, SIGMA-Aldrich, Tech., 90%) and catalyst (1 g) added. The autoclave was pressurized with hydrogen and heated at a ramping rate of 5 °C/min. The reaction was conducted at 220 °C, 80 bar in H2 for 12 h. Prior to the reaction, all of the catalysts employed in this study were reduced by treatment with a mixed stream of H2 (2 mL/min) and N2 (20 mL/min) at 320 °C for 2 h. After the reduction process, all catalysts were undergone a passivation treatment with the N2 flow containing very small amount of O2 (\1% O2 in N2) for 1 h. Reduced and passivated catalysts were introduced to reaction test. The reaction products were analyzed by gas chromatography, using a DS 6200 (DONAM INSTRUMENT INC.) equipped with a flame ionization detector. A SGE BP20 (WAX) capillary column (25 m 9 0.53 mm 9 1 lm) was used for separation.
3 Results and Discussion The crystalline structures of the CuCr-X catalysts were investigated by XRD measurements. Figure 1 shows XRD patterns of calcined CuCr-X catalysts with various molar ratios. The CuCr-1 catalyst showed a single phase of CuO. As the chromium ratio was increased, the characteristic peaks corresponding to copper oxide phase decreased and new characteristic peaks of copper chromite (CuCr2O4) spinel phase appeared at ca. 18.6°, 29.6°, 31.1° and 37.7°, corresponding to the (111), (220), (022) and (113) planes,
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CuCr-0
Intensity (a.u.)
CuCr-0.25
CuCr-0.33
CuCr-0.5
CuCr-1
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30
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50
60
2 Theta (degree)
Fig. 1 XRD patterns of calcined copper chromite catalysts containing various metal ratios. open triangle Cr2O3, open circle CuCr2O4, open square CuO
respectively [16]. When the ratio of Cu and Cr was 1 to 2 (CuCr-0.33), only the CuCr2O4 spinel phase was present. This indicates that the formation of the CuCr2O4 spinel phase is energetically favorable in mixtures of Cu and Cr. Further addition of chromium induced the Cr2O3 phase (CuCr-0.25). It thus appears that a Cr2O3 catalyst containing a pure chromium component (CuCr-0) can be produced. In order to investigate the reducibility of the CuCr-X catalysts, TPR measurements were carried out (Fig. 2). The amount of reducible species was estimated by measuring the area under the TPR curves. The CuCr-1 catalyst begins to be reduced at temperatures below 200 °C, in good agreement with previously results [17]. In the case of CuCr-0.5, the excess copper species, which does not form a CuCr2O4 spinel structure, appears to be well dispersed on
the catalyst. As a result, these species can be easily reduced at a low temperature. The CuCr-0.33 catalyst, which is mainly composed of CuCr2O4 spinel phase, shows different reduction characteristics from the above two catalysts. The reduction temperature was significantly shifted to a higher temperature, centered at 320–340 °C and the amount of reducible species were decreased. The TPR pattern of the CuCr-0.33 catalyst can be divided into two regions. The first peak, in the low temperature region (250–370 °C) can be assigned to the reversible property of a partially reduced Cu species having a CuCr2O4 spinel structure and the second flat region, at higher temperature (above 370 °C) can be attributed to an irreversible reduction property [18]. In the case of CuCr-0.25, the maximum reduction peak was shifted to the high temperature region, due to the formation of the less reducible Cr2O3 species, as the result of the addition of a higher amount of chromium. The reduction peak for the CuCr-0 catalyst appeared at the highest temperature region above 450 °C. Before the reaction test, all of the catalysts were reduced and the structures of the reduced materials were characterized by XRD (Fig. 3). The CuCr-1 catalyst was completely reduced from the CuO phase to the metallic Cu0 phase. On the other hand, the CuCr-0 catalyst is not reduced significantly at these reduction conditions, and maintains its Cr2O3 phase characteristics. Binary Cu/Cr catalysts (CuCr-0.5, CuCr-0.33 and CuCr-0.25) showed a significantly enhanced (111) plane intensity at ca. 18.6°. In addition, the diffraction peaks at ca. 29.6° corresponding to the (220) plane and 53.4° to the (422) plane are no longer present. This is due to the fact that the metallic Cu0 phase and the cubic spinel phase are now present after the reduction process compared to the initial sample that has a tetragonal spinel phase structure [18].
CuCr-0
CuCr-0 CuCr-0.25
Intensity (a.u.)
Intensity (a.u.)
CuCr-0.25
CuCr-0.33
CuCr-0.33
CuCr-0.5
CuCr-0.5 CuCr-1
CuCr-1 100
200
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400
500
600
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800
Temperature (oC) Fig. 2 TPR profiles of calcined copper chromite catalysts containing various metal ratios
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2 Theta (degree)
Fig. 3 XRD patterns of reduced copper chromite catalysts containing various metal ratios. open triangle Cr2O3, open circle reduced phase of CuCr2O4, open square Cu0
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Cu2+
CuCr-0.25
Table 1 Ratios of Cu0 and Cu2? of all samples evaluated from the peak separation analyses of Cu 2p spectra
Cu0
Satellite peaks
Intensity (a.u.)
Catalyst
XPS peak area percentage (%) Cu0 (932.4 ± 0.2 eV)
Cu2? (934.0 ± 0.2 eV)
CuCr-1
84.4
15.6
CuCr-0.5
53.6
46.4
CuCr-0.33 CuCr-0.25
37.2 56.3
62.8 43.7
CuCr-0.33
CuCr-0.5
CuCr-1
945
940
935
CuCr-0
930
Binding Energy (eV)
For further investigation of chemical state of catalysts, XPS analysis was conducted to the synthesized catalysts (Fig. 4). XPS spectra for Cu 2p were consisted of 2 peaks. Their binding energies were centered at 932.4 ± 0.2 eV and 934.0 ± 0.2 eV which were correspondent to Cu0 and Cu2? species, respectively. Relative peak area percentage of the two peaks was calculated and summarized in Table 1. Peak for Cu0 species is dominant in XPS spectra of CuCr-1. This means that most copper species are reduced to Cu0 and only small fractions of surface copper species are oxidized in CuO state. As Cr was introduced to Cu catalyst, relative fractions of Cu2? are increased even after the reduction process. In this case, Cu2? species are originated from the CuCr2O4 spinel structure, which are different from CuO of CuCr-1. As copper and chromium are strongly bonded in spinel structure, it can maintain its chemical bonding even after the reduction process. Such structural transformation, from tetragonal spinel structure to cubic spinel structure, is previously confirmed by XRD analysis. Therefore, large amount of Cu2? species in reduced catalyst means that it contains large fraction of spinel component in its structure. Among the catalyst listed in Table 1, CuCr-0.33 has most large amount of Cu2? species which mean it contains the highest proportion of CuCr2O4 spinel phase. Hydrogenolysis of glycerol to propylene glycol reaction is consisted with two individual reactions, one is dehydration of glycerol to acetol and the other is hydrogenation of acetol to propylene glycol. In the first step reaction, dehydration of glycerol to acetol, acidic properties of catalysts have a major effect on their catalytic activity [15]. For this reason, NH3-TPD of the CuCr-X catalysts was conducted to evaluate their acidic properties and the results are exhibited in Fig. 5. In the case of desorption profile of CuCr-1 and CuCr-0 catalysts, negligible amount of
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Intensity (a.u.)
240 o C
Fig. 4 X-ray photoelectron spectra (Cu 2p region) of synthesized catalysts after reduction process
CuCr-0.25
252 o C
CuCr-0.33
247 oC
CuCr-0.5
CuCr-1 316 o C
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100
150
200
250
300
350
o
Temperature ( C) Fig. 5 NH3-TPD profiles of synthesized catalysts
desorbed ammonia were detected which means its acidic property was also negligible. However, as chromium is added to copper catalyst, significant differences were found in the desorption profiles. There are two desorption characteristics, low temperature desorption peak at ca. 100 °C and high temperature desorption peak at ca. 250 °C. Both desorption peak area and temperature are important indicators to determine the acidic amount and acidic strength. Among the binary Cu/Cr catalyst, CuCr-0.33 catalyst showed the largest amount and the highest temperature of desorbed ammonia. This means that it has strong acidic properties and, as a result, it is expected to show high catalytic activity in dehydration reaction. High catalytic activities of reduced copper chromite catalysts in hydrogenation reactions have been reported previously [18–21]. A number of research groups claimed that the origin of the high activities of reduced copper chromite catalyst in hydrogenation was based on the characteristics of the reduction mechanism as follows. It has been reported that hydrogen atoms on the surface of partially reduced chromite can be occluded within the spinel structure [9, 21]. During the reduction process of the CuCr2O4 spinel structure within a certain temperature range (180–370 °C), atomic hydrogen species are inserted
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CuCr-0
Intensity (a.u.)
CuCr-0.25
467 o C
CuCr-0.33 470 o C
CuCr-0.5
462 oC
CuCr-1
200
300
400
Temperature (oC)
Fig. 6 H2-TPD profiles of synthesized catalysts
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600
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80
Percentage (%)
into the tetrahedral interstices of the tetragonal spinel structure. Some of the partially reduced Cu0 species, in tetrahedral positions of the spinel structure can be released on the catalyst surface. As a result, the overall structure of the catalyst is simultaneously transformed from a tetragonal spinel structure to a cubic spinel structure. The hydrogen atoms, which are able to participate in the hydrogenation reaction, are occluded as bulk hydrogen in the CuCr2O4 spinel structure and the HB are released in the form of surface hydrogen on the catalyst surface during the reaction [21]. Such a partially reduced CuCr2O4 state which involves reversible hydrogen adsorption, exhibits high catalytic activity in hydrogenation reactions [18]. For the evaluation of the hydrogenation ability of reduced CuCr-X catalysts, H2-TPD was conducted to measure the properties of occluded hydrogen species and the results were showed in Fig. 6. Characteristic signal of TCD detector for evolving hydrogen is downward direction under the baseline which is opposite to the results of TPR experiment. In the case of CuCr-1 and CuCr-0 catalysts, no significant characteristics were found maintaining its baseline. In the binary Cu/Cr catalysts, downward signal under the baseline were measured indicating occluded hydrogen is desorbed with increasing temperature. In all catalysts, hydrogen species started to be desorbed at ca. 250 °C and completely desorbed over the temperature of 500 °C. It was found that the largest amount of hydrogen was desorbed from CuCr-0.33 catalysts. This means CuCr0.33 catalyst contains the largest amount of copper chromite spinel structure which has high catalytic activity in hydrogenation reaction. The catalytic performances of the reduced CuCr-X catalysts in the hydrogenolysis of glycerol to propylene glycol performed at 220 °C in 80 bar of H2 are shown in Fig. 7. The CuCr-0 catalyst, which is comprised of pure chromium
Conver sion Selectivity Yield
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0 CuCr-1
CuCr-0.5
CuCr-0.33 CuCr-0.25
CuCr-0
Cu-Cr ratio Fig. 7 Catalytic performance of copper chromite catalysts containing various metal ratios in the hydrogenolysis of glycerol to propylene glycol, performed at 220 °C, 80 bar in H2 for 12 h
oxide, showed the lowest catalytic activities in terms of selectivity. This suggests that chromium is not an active site for Cu/Cr catalysts in this hydrogenolysis reaction. Although CuCr-1 showed no significant results in both NH3-TPD and H2-TPD analyses, it exhibited relatively high catalytic activity in this reaction. This is because of the intrinsic properties of copper metal as explained in the introduction. The addition of chromium was found to generally confer a positive effect on catalytic activity, compared to the CuCr-1 catalyst in terms of selectivity. The CuCr-0.33 catalyst showed ca. an 80.3% of conversion, an 83.9% selectivity and gave a 67.4% total yield, respectively. The other Cu/Cr catalysts also show a high selectivity, in the range of 70–83%. However, because of their relatively low conversion, the total yield of propylene glycol from glycerol was found to less than that of CuCr0.33. Such high catalytic activity of binary Cu/Cr catalysts can be found from the results of NH3 and H2-TPD. As copper and chromium were mixed, copper chromite spinel structure is formed. Hence, acidic property and hydrogen occluding property of the catalysts were significantly increased. Since a hydrogenolysis reaction is known as bifunctional reaction, both enhanced catalytic properties are beneficial to the hydrogenolysis reaction. Among the binary Cu/Cr catalysts tested here, CuCr-0.33, which is mainly composed of copper chromite spinel phase, are found to have the strongest acidic property and the largest amount of occluded hydrogen species. This is because CuCr-0.33 catalyst showed the highest catalytic activity in this study. Figure 8 shows reaction results with time on stream, conducted by CuCr-0.33 catalyst at 220 °C, 80 bar in H2. It was observed that the reaction rate over CuCr-0.33 catalyst first rapidly increased until 10 h, and then slowly and steadily increased with time on stream. This result reveals
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showed the best catalytic performance with an 80.3% of conversion, an 83.9% selectivity and gave a 67.4% total yield.
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1,2-PDO yield (%)
80
Acknowledgements The authors wish to acknowledge support from the GS Caltex Corporation. This research was partially supported by WCU (World Class University) program through the Korea science and Engineering Foundation funded by the Ministry Of Education, Science and Technology (400-2008-0230) and Korea Ministry of Environment as ‘‘The Eco-technopia 21 project’’.
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Time (hr) Fig. 8 Results of time on stream in the reaction test over CuCr-0.33 catalyst for 24 h
that CuCr-0.33 catalyst maintains its high catalytic activity throughout the long term reactions, which means that it possibly can be utilized for the industrial production of propylene glycol from glycerol.
4 Conclusion Copper catalysts promoted by chromium, containing various molar ratios (1:0, 1:1, 1:2, 1:3 and 0:1) were synthesized and their catalytic activities were evaluated for the hydrogenolysis of glycerol to propylene glycol. Cr was found to have only a promotion effect and showed negligible catalytic activity itself. As Cr was introduced to Cu catalyst, the CuCr2O4 tetragonal spinel phase began to be produced. The resulting CuCr2O4 tetragonal spinel phase was transformed into the CuCr2O4 cubic spinel phase during the reduction process and hydrogen atoms were occluded inside its crystal structure. Such reduced binary Cu/Cr catalysts were found to have strong acidic property and contain large amount of occluded hydrogen species, which are beneficial for dehydration of glycerol to acetol and hydrogenation of acetol to propylene glycol, respectively. Among the catalysts tested, the CuCr-0.33 catalyst showed the best catalytic performance. It can be concluded that the CuCr-0.33 catalyst, which had the highest fraction of CuCr2O4 spinel phase among the catalysts studied here,
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