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Prevention of Catalyst Deactivation in the Hydrogenolysis of Glycerol by Ga2O3-Modified Copper/Zinc Oxide Catalysts† Arne Bienholz,‡ Raoul Blume,§ Axel Knop-Gericke,§ Frank Girgsdies,§ Malte Behrens,§ and Peter Claus*,‡ Ernst-Berl-Institut, Technische Chemie II, Technische UniVersita¨t Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany, and Abteilung Anorganische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: September 3, 2010
Copper/zinc oxide catalysts prepared by coprecipitation were proved to be highly active and selective in the hydrogenolysis of glycerol. However, they suffer from strong deactivation in the course of reaction. Modifying the CuO/ZnO catalyst with Ga2O3 extremely enhances the stability of the catalyst as even after four consecutive experiments over a Cu/ZnO/Ga2O3 catalyst no deactivation is observed. The catalysts were characterized by temperature-programmed reduction, powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray analysis, and inductively coupled plasma optical emission spectrometry. As the Cu/ZnO/Ga2O3 catalyst is stable even under harsh reaction conditions of 220 °C and in the presence of water, a space-time-yield as high as 22.1 gpropylene glycol/(gCu h) can be obtained. 1. Introduction The conversion of glycerol to chemicals that are produced from fossil oil nowadays gains more and more interest as glycerol is available at large amounts and, thus, at low costs due to its formation as a byproduct during the biodiesel production from vegetable oils. Hence, there is increasing research work dealing with the use of glycerol in such varying reactions, such as the oxidation to dihydroxyacetone, the conversion to hydrogen, or the hydrogenolysis of glycerol to 1,2-propanediol, which can be further polymerized to polyesters and polyurethanes.1 Referring to the literature, a large number of supported catalysts can be applied in the hydrogenolysis of glycerol, where the catalysts differ in their preparation method, the support material, and in the active metal. Such varying metals, such as Pt, Co, Ru, and Cu, are able to catalyze the conversion of glycerol to 1,2-propanediol.2-7 However, over copper catalysts, both high selectivities toward 1,2-propanediol and high conversions of glycerol can be obtained.8-12 Despite the high activity, there are several reports about the deactivation of copper catalysts in the course of reaction. Recently, we reported about the tremendous loss of copper surface area of CuO/ZnO catalysts during the hydrogenolysis of glycerol, leading to a strong decrease of catalyst activity if it is used in a second run.13 However, the loss of copper surface area is favored in the presence of water, which is either added to the reaction mixture as a solvent or formed as an unavoidable byproduct during the reaction. Moreover, glycerol is obtained from the production of biodiesel as an aqueous solution, making the development of copper catalysts, which are stable in the presence of water, desirable. As the surface area of the copper catalysts plays a significant role14 for the catalytic activity, the problem of the †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. E-mail: claus@ ct.chemie.tu-darmstadt.de. Tel: +49-(0)6151-16-5369. Fax: +49-(0)615116-4788. ‡ Technische Universita¨t Darmstadt. § Fritz-Haber-Institut der Max-Planck-Gesellschaft.
copper particle growing has to be solved. Recently, a process was described where water is removed from the reaction mixture by a hydrogen flow passed through the reaction solution in order to increase the stability of the catalyst.15 However, a reactor with several catalyst beds is needed, and moreover, the temperature and pressure have to be kept in a certain range. Because of these drawbacks, water-stable copper catalysts have to be developed for the industrial application of glycerol hydrogenolysis. According to the literature, the stability of copper catalysts in the formation of methanol from a mixture of H2/CO2 can be enhanced by promoting the catalysts, for example, with Ga2O3.16-18 Therefore, CuO/ZnO catalysts were promoted with Ga2O3, characterized by XRD, ICP, TPR, SEMEDX, and XPS methods, and applied in the hydrogenolysis of glycerol. A remarkable enhancement of the catalyst stability was observed even if an aqueous glycerol solution was used. 2. Experimental Section 2.1. Catalyst Preparation. The Cu/ZnO/Ga2O3 catalyst was prepared by coprecipitation. Initially, Cu(NO3)2 · 2.5 H2O (4.86 g, 20.9 mmol), Zn(NO3)2 · 6H2O (12.20 g, 41 mmol), and Ga(NO3)3 (2.68 g, 10.5 mmol) were solved in 300 mL of deionized water. Precipitation was carried out by the dropwise addition of Na2CO3 solution (0.39 mol/L) at 70 °C and at a constant pH value of 7. The precipitate was separated by filtration, washed with deionized water, and dried at 50 °C overnight. Finally, calcination was carried out at 400 °C for 3 h in a stream of air (100 mL/min). Prior to the reaction, the catalyst was fractionated. One part was used without further pretreatment, whereas the other part was reduced in a stream of hydrogen (100 mL/min) at a heating rate of 2 °C/min to a final temperature of 260 °C. This temperature was held for 2 h. The preparation of the CuO/ZnO catalyst was conducted in the same manner, but without the addition of Ga(NO3)3 to the metal nitrate solution. 2.2. Catalytic Reaction. For the hydrogenolysis of glycerol, the required amount of catalyst along with pure glycerol or an aqueous 50 or 90 wt % glycerol solution was loaded into a
10.1021/jp104925k 2011 American Chemical Society Published on Web 10/15/2010
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stainless steel reactor. The autoclave was pressurized with hydrogen to 2.5 MPa and finally heated to the desired reaction temperature. Afterward, the pressure was adjusted to 5 MPa and maintained at that value during the course of reaction. Samples were taken at required time intervals in order to be analyzed by means of an HPLC device equipped with an Aminex HP × 87 column and a refractive index and ultraviolet detector. The selectivities were calculated as the ratio between the number of carbon atoms in the particular product and the number of carbon atoms in the converted amount of glycerol. 2.3. Catalyst Characterization. Temperature-programmed reduction (TPR) was conducted by the means of a Porotec TPD/ R/O 1100 device. The sample was heated in a stream of 4.95% hydrogen in argon (20 mL/min) at a heating rate of 5 °C/min to a final temperature of 400 °C. The consumed amount of hydrogen was monitored with a thermal conductivity detector. The composition of the catalysts was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Prior to analysis, the samples were prepared under microwave heating in a mixture of aqua regia/HF (1:1 v/v). The X-ray diffraction (XRD) measurements were carried out on a STOE STADI P transmission diffractometer equipped with a primary focusing germanium monochromator (Cu KR1 radiation) and a linear position-sensitive detector. The samples were mounted in the form of small amounts of powder sandwiched between two layers of polyacetate film and fixed with a small amount of X-ray amorphous grease. The chemical composition was also determined by scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX). The X-ray photoelectron spectroscopy (XPS) experiments were performed at the ISISS beamline of the Fritz-Haber Institute located at the BESSY II synchrotron radiation facility in Berlin during low-R mode. All samples were measured at 180 °C in 5 × 10-4 mbar He. The setup is described elsewhere.19 The spectra were collected in normal emission with a probe size of ∼100 µm × 1 mm. The samples were heated from the back to 180 °C using an external laser to compensate for charging. The temperature was controlled via a thermocouple in direct contact with the sample surface. Contamination was checked by survey spectra at the beginning of each experiment. The photoelectron spectra were taken at photon energies of 1400 eV (Ga 2p), 1270 eV (Zn 2p), 1180 eV (Cu 2p), 780 eV (O 1s), and 530 eV (C 1s), respectively, with a spectral resolution of 0.3 eV. The kinetic energies of the electrons correspond to an electron mean free path of ∼8 Å. The total XPS information depth is ∼2 nm; that is, 95% of all detected electrons originate from 3λ.20 For XPS analysis, the photoelectron binding energy is referenced to the Fermi edge, and the spectra are normalized to the incident photon flux. Background correction was performed by using a Shirley background.21 The spectra were fitted following the Levenberg-Marquardt algorithm to minimize the χ2. Peak shapes were modeled by using asymmetric DoniachSunjic functions convoluted with Gaussian profiles.22 The accuracy of the fitted peak positions is ∼0.05 eV. 3. Results and Discussion 3.1. Catalyst Composition and Morphology. Both the CuO/ ZnO and the CuO/ZnO/Ga2O3 catalysts were characterized by ICP-OES before and after being used in the hydrogenolysis of glycerol. The Cu, Zn, and Ga contents are listed in Table 1, respectively. For both catalysts, there is a good consistence between the nominal and the experimentally determined composition before their usage in the hydrogenolysis of glycerol. Regarding the metal contents of the spent catalysts, only slight changes become obvious. Moreover, the nominal Cu/Zn ratio
Bienholz et al. TABLE 1: Composition of the CuO/ZnO and the CuO/ ZnO/Ga2O3 Catalysts before and after Being Used in the Hydrogenolysis of Glycerola CuO/ZnO element
nominal [wt %]
before reaction [wt %]
after reaction [wt %]
Cu Zn Cu/Zn
28.0 57.8 0.48
28.1 55.3 0.51 (0.47)
30.4 58.2 0.52 (0.45)
CuO/ZnO/Ga2O3 element
nominal [wt %]
before reaction [wt %]
after reaction [wt %]
Cu Zn Ga Cu/Zn
24.0 47.4 12.6 0.51
Cu/Ga
1.90
24.3 47.6 15.0 0.51 (0.48) 1.62 (6.20)
24.7 45.3 15.1 0.55 (0.52) 1.64 (8.30)
a Determined by ICP-OES analysis. Values in parentheses: EDX analysis.
Figure 1. SEM images of a fresh and spent CuO/ZnO/Ga2O3 catalyst.
is in good agreement with the value that was determined for both the fresh and the spent catalysts, indicating that no leaching of an active component had occurred during the reaction. From an SEM image of a fresh and a spent CuO/ZnO/Ga2O3 catalyst (Figure 1), no changes in the catalyst’s morphology can be observed. The mass ratio of copper to zinc of about 0.5 for the four catalysts (CuO/ZnO and CuO/ZnO/Ga2O3, before and after being used in the hydrogenolysis of glycerol) is in good agreement with the ratio determined via SEM-EDX analysis (Table 1), indicating a homogeneous distribution of these two elements in the catalysts. However, the amount of Cu relative to Ga determined by EDX is much higher than the overall value determined via ICP-OES analysis, suggesting a heterogeneous distribution of gallium in the catalyst, which is even more pronounced in the case of the spent CuO/ZnO/Ga2O3 catalyst. The results point to differences between bulk and surface composition. Thus, the surface composition of the catalysts was analyzed via XP spectroscopy, and the results are discussed in section 3.4. 3.2. Reduction Behavior. In Figure 2, a TPR of the CuO/ ZnO and the CuO/ZnO/Ga2O3 catalysts is shown. For both catalysts, one dominant reduction peak can be observed, whereas for the CuO/ZnO catalyst, a pronounced shoulder at lower reduction temperatures is visible. Fierro et al. investigated the reducibility of CuO/ZnO catalysts where the CuO content was varied.23 For a CuO/ZnO catalyst with a Cu/Zn ratio of 30/70, which is quite similar to the catalyst used in this study, there was a maximum reduction peak at around 200 °C attributed to the reduction of CuO to Cu0. Considering the different conditions applied for the reduction, this is in good agreement with
Ga2O3-Modified Cu/ZnO Catalysts
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1001 TABLE 2: Reduction Properties of the CuO/ZnO and the CuO/ZnO/Ga2O3 Catalysts catalyst
Tmax [°C]
n(H2)theo [mmol]
n(H2)exp [mmol]
reduction degree [%]
CuO/ZnO CuO/ZnOGa2O3
211 210
0.350 0.296
0.273 0.242
78 82
catalyst along with the pattern of a spent Cu/ZnO/Ga2O3 catalyst, which was reduced in a stream of hydrogen prior to the reaction, are shown in Figure 3. The diffraction peaks in the XRD pattern of the calcined CuO/ZnO/Ga2O3 catalyst (Figure 3a) can be assigned to ZnO, whereas no peaks related to Cu or Ga species were determined. The lack of diffraction lines of CuO is due to the presence of very small particles and is in accordance with EXAFS results.26 Concerning the Cu/ZnO/Ga2O3 catalyst, which was reduced in a stream of hydrogen, diffraction peaks for ZnO, CuO, and Cu were detected (Figure 3b). Because of contact
Figure 2. TPR of the CuO/ZnO (a) and CuO/ZnO/Ga2O3 (b) catalysts.
the maximum reduction peak at 210 °C, which is observed for both the CuO/ZnO and the CuO/ZnO/Ga2O3 catalysts in the present study. According to Fierro et al., the observed shoulder close to the most prominent peak can be related to the presence of two different CuO species in the CuO/ZnO catalyst. One is characterized by bulk CuO, the other is represented by welldispersed CuO in contact with the surface of ZnO particles, resulting in a lower temperature.23 However, the low-temperature shoulder peak can be assigned to the reduction of CuO to Cu+.24 This explanation is supported by XAFS measurements, indicating that the reduction of CuO/ZnO catalysts occurs in two steps, whereas Cu2O is formed as an intermediate.25 In the case of the CuO/ZnO/Ga2O3 catalyst, a less intense low-temperature shoulder is visible, suggesting that Ga2O3 inhibits the direct contact between ZnO and CuO particles. Moreover, this feature could possibly be assigned to an electronic interaction between Ga2O3 and CuO, which favors the direct reduction of CuO to metallic copper. However, a retarded reduction of CuO to Cu+ in the presence of Ga2O3, which leads to a superposition of the peaks for the reduction of CuO to Cu+ and Cu+ to Cu0, might be considered as well. The theoretical amount of hydrogen that is needed to reduce CuO to Cu0, n(H2)theo, was estimated via the product of the initial sample weight and its CuO content. The CuO content was calculated from the Cu content of the catalysts determined by ICP-OES, assuming that all Cu is present as CuO after calcination of the catalysts. The reduction degree can be calculated as the ratio of n(H2)theo and the amount of hydrogen that was consumed during TPR measurements, n(H2)exp. The obtained values along with the maximum of the main reduction peak Tmax are listed in Table 2. For both catalysts, a reduction degree of around 80% was obtained indicating either that CuO was not completely reduced to metallic copper or that not only Cu2+ but also copper in a lower oxidation state was present in the catalysts before the TPR was carried out. 3.3. X-ray Diffraction Analysis. The XRD patterns of a calcined CuO/ZnO/Ga2O3 catalyst and a reduced Cu/ZnO/Ga2O3
Figure 3. XRD patterns of a calcined (a) and reduced (b) Cu/ZnO/ Ga2O3 catalyst and of a prereduced Cu/ZnO/Ga2O3 catalyst after being used in the hydrogenolysis of glycerol (c). Reaction conditions: 2.8 g of catalyst, 177 g of glycerol, 220 °C, 420 min. Symbols: /, ZnO; 9, Cu; O, Zn-Ga-hydrotalcite; •, CuO; 0, ZnGa2O4.
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Figure 4. XPS spectra of the Cu 2p, Ga 2p, and Zn 2p binding energy regions of a fresh CuO/ZnO/Ga2O3 catalyst before (S1) and after (S2) its reduction as well as a spent Cu/ZnO/Ga2O3 catalyst (S3). Note that all spectra are normalized to 1.
with air after the reduction and before XRD analysis, metallic copper was obviously partially oxidized to CuO. Additionally, diffraction peaks for (Zn0.67Ga0.33)(OH)2(CO3)0.17(H2O)0.5, Zn-Gahydrotalcite, can be observed. This phase is probably formed during coprecipitation but is likely not to be detected in the XRD of the calcined CuO/ZnO/Ga2O3 catalyst. In the case of the spent Cu/ZnO/Ga2O3 catalyst (Figure 3c), besides the diffraction peaks for ZnO, peaks for Cu and ZnGa2O4 appear, which either is formed under the harsh reaction conditions or already exists as an X-ray amorphous phase in the fresh catalyst. However, the direct precipitation of small ZnGa2O4 crystallites with an average size of 4 nm from an aqueous solution of zinc sulfate and gallium sulfate with NH3 is known in the literature.27 Therefore, small X-ray amorphous ZnGa2O4 crystallites that sinter in the course of reaction might be present in the calcined and in the reduced catalyst. Like for the reduced Cu/ZnO/Ga2O3 catalyst, copper is expected to be oxidized due to the contact of the sample with air. However,
no diffraction peaks for CuO were detected. On the one hand, these peaks may be superimposed by the ones for ZnO and ZnGa2O4. On the other hand, the sharp diffraction peaks indicate that sintering of the copper particles had occurred during the reaction, which prevents the larger Cu crystallites from oxidation by contact with air. 3.4. Surface Composition. The CuO/ZnO/Ga2O3 catalyst was analyzed by XPS before (S1) and after (S2) its reduction in a stream of hydrogen as well as after its use in the hydrogenolysis of glycerol (S3). The results are shown in Figure 4. Besides copper, zinc, gallium, and oxygen, carbon residues were detected on the catalyst’s surface. A careful analysis revealed mostly oxidized and amorphous carbon as well as hydrocarbons and some graphite. The total amount of carbon is heavily depleted in the case of the spent catalyst (S3), in particular, the graphite species that already started to decrease after the hydrogen treatment (S2).
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TABLE 3: Cu/Zn and Cu/Ga Ratios on the Surfaces of CuO/ZnO/Ga2O3 before and after Being Used in the Hydrogenolysis of Glycerola catalyst
(Cu/Zn)s
(Cu/Ga)s
fresh CuO/ZnO/Ga2O3 spent CuO/ZnO/Ga2O3
0.13 0.20
0.33 0.50
a
Calculated on the basis of both the CuO and the Cu2O contents in the XP spectra.
As expected, the intensity of the component related to CuO at 933.8 eV28 in the Cu 2p spectrum decreases after applying the catalyst in the hydrogenolysis of glycerol. Nevertheless, there is another peak at a lower binding energy, which corresponds to either Cu2O or metallic copper. Because of the low Cu amount on the catalyst’s surface and the almost identical binding energies of metallic Cu and Cu2O, an assignment is challenging. Because the Cu-Auger L3VV region does not show the typical sharp metal peak at Ekin ) 918.4 eV,29 but only a weak peak at ≈916.5 eV, we tentatively assign the second Cu component to Cu2O. In the case of the spent catalyst, this component becomes the dominant feature. The binding energy positions of the Zn 2p3/2 and Ga 2p3/2 agree well with the values found in the literature for ZnO and Ga2O3, respectively.30 Neither the Cu 2p3/2 nor the Ga 2p3/2 region reveals an electronic interaction between the two metals. The relative surface composition of the CuO/ZnO/Ga2O3 catalyst, expressed as (Cu/Zn)s and (Cu/ Ga)s ratios (Table 3), was estimated to 0.13 and 0.33, respectively, for the catalyst before glycerol hydrogenolysis, and 0.20 and 0.50, respectively, after its use. These results point to a catalyst system in which (i) Cu metal is covered, in part, with ZnO and/or Ga2O3 (prior and after the hydrogenolysis reaction) and (ii) an enrichment of surface Cu is obtained in the course of the reaction. A depletion of copper at the catalyst surface and, thus, differences between the (Cu/Zn)s and (Cu/Zn)bulk were also observed in the case of copper-zinc-based catalysts.31,32 In the presence of H2, these catalyst systems often exhibit metallic copper covered, in part (∼50%), with ZnO.33 Furthermore, independent of the catalyst’s treatment, the O 1s region exhibits very similar peak shapes with slightly varying overall intensities. The main features of the spectrum point to contributions of ZnO (∼530.1 eV), CuO (∼529.5 eV), and Ga2O3 (∼530.5 eV), as can be expected. Yet, on the high binding energy side of the oxide features, further contributions are visible that most likely stem from adsorbed species on the catalyst’s surface, such as water and C-O-like bonds. 3.5. Catalytic Properties in Glycerol Hydrogenolysis. In Table 4, the results of the hydrogenolysis of glycerol in terms of the conversion and the selectivity to propylene glycol, ethylene glycol, and other side products over copper catalysts with and without Ga2O3 as a promoter are summarized. By the use of a 50 wt % aqueous glycerol solution, promoting the CuO/ ZnO catalyst with Ga2O3 leads to a strong increase in the conversion of glycerol from 12% to 36% (entry 1 vs entry 2 in Table 4), although the amount of the active metal copper in the catalyst is decreased in the case of the CuO/ZnO/Ga2O3 catalyst. If pure glycerol is used instead of a 50 wt % aqueous glycerol solution, the conversion over the CuO/ZnO/Ga2O3 catalyst can be further increased (entry 2 vs entry 3 in Table 4) even though the amount of catalyst was kept constant; and thus, the ratio of glycerol to catalyst was doubled. Concerning the selectivity to propylene glycol, no influence of the initial glycerol concentration nor of the conversion level of glycerol can be observed, as propylene glycol in both cases is produced with a selectivity around 80%. Decreasing the molar ratio of glycerol to catalyst
TABLE 4: Conversion and Selectivity Towards Propylene Glycol and Ethylene Glycol over CuO/ZnO and CuO/ZnO/ Ga2O3 Catalysts selectivity (%) entry
catalyst
1 2 3 4 5 6 7 8 9 10 11
CuO/ZnOb CuO/ZnO/Ga2O3b CuO/ZnO/Ga2O3c CuO/ZnO/Ga2O3d Cu/ZnO/Ga2O3e Cu/ZnO/Ga2O3f Cu/ZnOf Cu/ZnOg Cu/ZnOg,h Cu/ZnO/Ga2O3g Cu/ZnO/Ga2O3g,h
conversion propylene ethylene (%) glycol glycol othersa 12 36 60 99 78 96 84 55 38 43 41
71 85 81 80 80 82 81 82 79 88 84
3 3 2 2 2 2 2 1 2 2 2
26 12 17 18 18 16 17 17 19 10 14
a Not identified side products. b 3 g of catalyst, 177 g of 50 wt % aqueous glycerol solution, 220 °C, 420 min. c 3 g of catalyst, 177 g of glycerol, 220 °C, 420 min. d 7 g of catalyst, 177 g of glycerol, 220 °C, 330 min. e 2.8 g of prereduced catalyst, 177 g of glycerol, 220 °C, 420 min. f 2.8 g of prereduced catalyst, 177 g of 90 wt % aqueous glycerol solution, 220 °C, 420 min. g 2.8 g of prereduced catalyst, 177 g of 90 wt % aqueous glycerol solution, 200 °C, 420 min. h Recycling experiment.
again (Table 4, entry 4) leads to a remarkable enhancement of the conversion. Thus, after a reaction time of 330 min, a conversion of 99% is obtained. Besides the influence of the glycerol concentration and the molar ratio of glycerol to catalyst, the effect of the prereduction of the CuO/ZnO/Ga2O3 catalyst was examined. Reducing the catalyst before the reaction in a stream of hydrogen positively effects the conversion as it increases from 60% to 78% (entry 3 vs entry 5 in Table 4). The selectivity toward propylene glycol is not effected by the ex situ reduction of the catalyst. The conversion over the Cu/ZnO/Ga2O3 catalyst further increases to 96% (Table 4, entry 6) if a 90 wt % aqueous glycerol solution is used instead of pure glycerol. This is likely due to the decreased ratio of glycerol to catalyst mass. Along with a selectivity to propylene glycol as high as 82%, this corresponds to a space-time-yield of 22.1 gpropylene glycol/(gCu h). If the same experiment is conducted over a Cu/ZnO catalyst, the conversion only amounts to 84%, which is considerably lower than the value obtained over the Cu/ZnO/Ga2O3 catalyst. The stabilizing effect of Ga2O3 becomes even more obvious if the conversion of glycerol over both the Cu/ZnO/Ga2O3 and the Cu/ZnO catalysts is plotted against the reaction time (Figure 5). Initially, by the use of the Cu/ZnO catalyst, a slightly higher conversion is obtained, which can be ascribed to the higher copper content in the catalyst, leading to a lower glycerol-to-copper ratio. However, in the case of the Cu/ZnO catalyst, the reaction rate decreases faster compared with the Cu/ZnO/Ga2O3 catalyst, indicating its deactivation. After a reaction time of 120 min, the conversion over Cu/ZnO/Ga2O3 is higher and this difference in the conversion level further increases in the course of reaction. 3.6. Catalyst Recycling. In Figure 6a, a plot of the glycerol conversion against the reaction time over a fresh and a spent Cu/ZnO catalyst is shown. With the fresh catalyst, a conversion of 55% (Table 4, entry 8) is achieved after a reaction time of 7 h. However, if the catalyst is separated from the reaction mixture by filtration, washed with deionized water, and used in a second run, the conversion amounts to only 38% (Table 4, entry 9). From Figure 6a, it becomes obvious that, by the use of the spent catalyst, a lower conversion is obtained throughout the whole reaction time. Repeating the same experiments with
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Bienholz et al. TABLE 5: Long-Term Stability of the Cu/ZnO/Ga2O3 Catalystb selectivity (%) run
conversion (%)
propylene glycol
ethylene glycol
othersa
1 2 3 4
57 53 57 52
80 82 83 85
2 2 2 3
18 16 15 12
a Not identified side products. b Reaction conditions: 0.3 g of Cu/ ZnO/Ga2O3, 200 °C, 8 g of 90 wt % aqueous glycerol solution, 50 bar H2, 300 min.
Figure 5. Comparison of the glycerol conversion over a Cu/ZnOGa2O3 (0) and a Cu/ZnO (9) catalyst with reaction time. Reaction conditions: 2.8 g of catalyst, 177 g of 90 wt % aqueous glycerol solution, 220 °C, 50 bar H2, 420 min.
propanediol are not effected by the reuse of the Cu/ZnO/Ga2O3 catalyst, indicating its high stability in the hydrogenolysis of glycerol. 3.7. The Stabilizing Effect of Ga2O3. Recently, we could prove by N2O chemisorption that the copper surface area of a CuO/ZnO catalyst tremendously decreases during the reaction,13 which is in good agreement with the results of Montassier et al., who ascribed the loss of activity of a Cu/C catalyst to leaching of copper and to sintering of the copper particles.34 However, this process seems to be favored in the presence of water, which is either used as solvent or formed as an inevitable byproduct in the hydrogenolysis of glycerol.13 For CuO/ZnO catalysts used in methanol synthesis, a deactivation due to thermal sintering of the metallic copper particles is known as well.35 However, the addition of metal oxides, such as Al2O3, Cr2O3, or Ga2O3, leads to an enhancement in the catalyst stability.16,36,37 The promoting effect of metal oxides in terms of the catalyst stability is assigned to the presence of a component that isolates the individual metal particles, preventing their sintering.38 As there is no clear evidence for an electronic influence of Ga2O3 or ZnGa2O4 on Cu, we assume that the stabilizing effect is a result of the separation of the copper particles by Ga2O3 or ZnGa2O4 grains, leading to a highly stable catalyst during its operation in the hydrogenolysis of glycerol even in the presence of water. 4. Conclusions
Figure 6. Comparison of the glycerol conversion over a fresh (0) and spent (9) Cu/ZnO catalyst (a) and a fresh (0) and spent (9) Cu/ ZnO/Ga2O3 catalyst (b). Reaction conditions: 2.8 g of catalyst, 177 g of 90 wt % aqueous glycerol solution, 200 °C, 50 bar H2, 420 min.
a Cu/ZnOGa2O3 catalyst (Figure 6b) leads to a decrease in the conversion to 43% (Table 4, entry 10) in the first run compared to the Cu/ZnO catalyst. But it is important to note that, in contrast to the catalyst without Ga2O3, the Cu/ZnO/Ga2O3 catalyst can be reused in a second run without any deactivation. Thus, when the experiments over spent catalysts are compared, the conversion level after 7 h of reaction is even higher in the case of the Cu/ZnO/Ga2O3 catalyst, although it contains less copper than the Cu/ZnO catalyst. For both the Cu/ZnO and the Cu/ZnO/Ga2O3 catalysts, the selectivities to 1,2-propanediol and ethandiol are not effected by the recycling of the catalysts (Table 4). To determine the long-term stability of the Cu/ZnO/Ga2O3 catalyst, the recycling experiments were extended to a higher number. The results of four consecutive reactions are shown in Table 5. Both the conversion and the selectivity to 1,2-
The preparation of Cu/ZnO and Cu/ZnO/Ga2O3 catalysts was carried out by coprecipitation of the metal nitrates with Na2CO3 solution. In the case of the Cu/ZnO catalyst, a remarkable deactivation was observed as the conversion of glycerol strongly decreases if the catalyst is used in a second run. By ICP-OES, the loss of an active component during the reaction can be excluded. In contrast to the CuO/ZnO catalyst, no deactivation was observed in the case of a Cu/ZnO/Ga2O3 catalyst. As the Cu/ZnO/Ga2O3 catalyst is stable even under harsh reaction conditions of 220 °C and in the presence of water, a spacetime-yield as high as 22.1 gpropylene glycol/(gCu h) can be obtained. Therefore, this study is the first report of a successful prevention of catalyst deactivation in the hydrogenolysis of aqueous glycerol solutions over copper/zinc oxide based catalysts. Acknowledgment. The authors A.B. and P.C. are grateful to the FAUDI-Stiftung (Project 73) for financial support of this work and Prof. R. Schlo¨gl for helpful discussions. We acknowledge the Helmholtz-Zentrum Berlin-Electron storage ring BESSY II for the provision of synchrotron radiation at the ISISS beamline. References and Notes (1) Brandner, A.; Lehnert, K.; Bienholz, A.; Lucas, M.; Claus, P. Top. Catal. 2009, 52, 278–287.
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