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MgO catalysts for hydrogenolysis of glycerol to 1, 2-propanediol
Applied Catalysis A: General 371 (2009) 108–113
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Co/MgO catalysts for hydrogenolysis of glycerol to 1, 2-propanediol Xiaohui Guo, Yong Li, Ruijuan Shi, Qiying Liu, Ensheng Zhan, Wenjie Shen * State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 June 2009 Received in revised form 23 September 2009 Accepted 23 September 2009 Available online 1 October 2009
Hydrogenolysis of glycerol to 1, 2-propanediol was investigated on bi-functional Co/MgO catalysts where the interaction between cobalt species and MgO was adjusted by varying the temperature of calcination. Higher temperature treatment not only enhanced the interaction between Co3O4 and MgO, but also promoted the formation of MgCo2O4 spinel and Mg–Co–O solid solution. Although the reducibility of cobalt oxides was greatly decreased in the Co3O4/MgO precursor, this strong interaction prevented the aggregation of Co particles in the resulting Co/MgO catalyst under the harsh reaction conditions, giving a much higher activity and stability. Results revealed that MgO was hydrated to Mg(OH)2 during the course of reaction and that the sizes of Mg(OH)2 and Co particles increased considerably, especially during the initial stage of the reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Glycerol Hydrogenolysis 1, 2-Propanediol Co/MgO
1. Introduction Biodiesel produced by transesterification reactions of plant and fatty oils is one of the most important biofuels [1]. However, significant amounts (about 10 wt.%) of crude glycerol are simultaneously generated as a byproduct. In order to improve the overall efficiency of biodiesel production, sustained efforts have been made to convert the crude glycerol to value-added chemicals, such as dehydration to acrolein [2,3], etherification to ethers [4,5], oxidation to glyceric acid [6,7], and hydrogenolysis to propanediol [8–21]. In particular, hydrogenolysis of glycerol to 1, 2-propanediol (1, 2-PDO), which is an important feedstock for manufacturing polymers and pharmaceutics, has been extensively studied to achieve mechanistic and kinetic understandings [10– 12] and improvement of product yield [13–17]. It is generally recognized that both the catalyst and the pH value of the reaction media play essential roles in determining the reaction pathway and the yield of 1, 2-PDO [18,19]. When solid acids like Amberlyst [9,15,17] or a heteropoly salt (Cs2.5H0.5[PW12O40]) [20] and precious metals (mainly Ru) are jointly used as catalysts, dehydration of glycerol to acetol takes place on the acidic sites, and the acetol produced is then hydrogenated to 1, 2-PDO over the metal particles. In the cases of catalytic systems containing precious metals and base materials, such as Pt/C + CaO [10], PtRu/
* Corresponding author. Tel.: +86 411 84379085; fax: +86 411 84694447. E-mail address: [email protected] (W. Shen). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.037
C + NaOH [18], and Ru/TiO2 + LiOH [21], glycerol is initially dehydrogenated to glyceraldehyde and/or pyruvaldehyde catalyzed by the basic component, followed by hydrogenation of these reaction intermediates to 1, 2-PDO over the precious metals. Although this reaction is usually performed under hightemperature and hydrogen pressure, the conversion of glycerol and the selectivity of 1, 2-PDO is still quite low. Moreover, little attention has been paid to the durability and the structural changes of the catalysts under the harsh reaction conditions. In this work, we report highly efficient Co/MgO catalysts for hydrogenolysis of glycerol, where the solid MgO acts as the basic component and the support of cobalt nanoparticles. The structural changes of the catalysts during glycerol hydrogenolysis have been investigated. 2. Experimental 2.1. Catalyst preparation The Co/MgO catalysts were prepared by a deposition–precipitation method. Ten grams of commercially available MgO powders (previously calcined at 873 K for 4 h in air, surface area 37 m2/g) was dispersed in 750 ml aqueous solution containing 8.44 g cobalt acetate (Co(CH3COO)24H2O) and 37.5 g urea. The mixture was heated to 363 K and maintained at this temperature for 8 h under stirring. After filtering and washing with deionized water, the precipitate was dried at 353 K overnight and calcined at 673 or 873 K for 4 h in air, giving the Co3O4/MgO-T samples, where T refers to the temperature of calcination. These calcined samples
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(0.2 g) were then reduced with pure H2 (50 ml/min) in a fixed-bed quartz reactor at 623 K for 1 h, yielding the Co/MgO-T catalysts. 2.2. Catalyst characterization The actual loading of cobalt was measured by inductively coupled plasma-atomic emission spectroscopy (ICP, Plasma-SpecII spectrometer). N2 adsorption–desorption isotherms were recorded at 77 K on a Micromeritics ASAP 2000 instrument. Before the measurement, each sample was degassed at 573 K for 3 h. The specific surface area was calculated by the BET method using the nitrogen adsorption isotherms. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2500 diffractometer with Cu Ka radiation operated at 40 kV and 300 mA. In situ XRD measurements were carried out in a high-temperature chamber installed in the diffractometer. The powder Co3O4/MgO-T samples were pressed into flakes and mounted in the chamber; they were then reduced with pure hydrogen at 623 K for 1 h. The average crystallite size was calculated from the Scherrer equation. Transmission electron microscopy (TEM) images were recorded using a Philip Tecnai G2 spirit microscope operated at 120 kV. The specimen was prepared by ultrasonically suspending the powder sample in ethanol, and drops of the suspension were deposited on a carbon-coated copper grid and dried in air. Temperature programmed reduction (TPR) of hydrogen of the Co3O4/MgO-T samples was performed in a U-shape quartz reactor. For each run, 50 mg samples were loaded and pretreated at 623 K for 0.5 h under N2 flow (30 ml/min). After the sample was cooled to room temperature and after the reduction agent of a 5% H2/N2 mixture (30 ml/min) was introduced, the sample was heated to 1073 K at a rate of 10 K/min. The outlet gas was collected by passing it through a molecular sieve trap to remove water; the consumption of hydrogen was analyzed by a thermal conductivity detector (TCD). To estimate the reduction degree of cobalt species, we performed H2-TPR experiments for the Co/MgO-T catalyst obtained by pre-reducing the Co3O4/MgO-T sample with H2 at 623 K for 1 h. The amounts of hydrogen consumed between the two runs were used to estimate the reduction degree of cobalt oxide. The amounts of hydrogen consumption were calibrated using known amounts of CuO. Temperature programmed desorption (TPD) of CO2 was conducted in a U-shape quartz reactor connected to a mass spectrometer (Omnistar QMS 200). Fifty-milligram samples was loaded and pre-reduced with a 20% H2/N2 mixture (30 ml/min) at 623 K for 40 min and then purged with He for 1 h. After being cooled to room temperature, the sample was exposed to a 49.8% CO2/He mixture (20 ml/min) for 1 h, followed by purging with He for 2 h. The sample was then heated to 773 K at a rate of 10 K/min under He flow (50 ml/min) and the desorption of CO2 was monitored by the mass spectrometer. 2.3. Catalytic test Hydrogenolysis of glycerol was conducted in a stainless steel autoclave (100 ml). Forty grams of glycerol aqueous solution (10 wt.%) and 0.2 g Co/MgO catalyst were added to the reactor. After purging with hydrogen five times, the reactor was pressured to 1.0–3.0 MPa and then heated to a desired temperature (453–523 K) and maintained at that temperature for a given period. The products were analyzed by gas chromatography equipped with a flame ionization detector (FID) connected to a Carbowax 20 M capillary column (for the liquid products) and a TCD connected to a Porapak Q packed column (for the gas products).
Fig. 1. XRD patterns of the precipitate (a) and the Co3O4/MgO-673 (b) and Co3O4/ MgO-873 (c) samples.
3. Results and discussion 3.1. Chemical and physical properties of the catalysts Fig. 1 shows the XRD patterns of the precipitate and the Co3O4/ MgO-T samples. The precipitate exhibited characteristic diffraction peaks of hexagonal Mg(OH)2 (JCPDS 83-0114) and b-Co(OH)2 (JCPDS 30-0443), indicating that the raw MgO powder was hydrated to Mg(OH)2 during the deposition–precipitation process. After calcination, Mg(OH)2 was converted to MgO (JCPDS 89-7746) and b-Co(OH)2 was transformed mainly to Co3O4 (JCPDS 73-1701). Meanwhile, minor MgCo2O4 spinel (JCPDS 02-1073) and/or Mg– Co–O solid solution (JCPDS 02-1201) phases were also detected, although their diffraction peaks were partially overlapped with those of Co3O4 and MgO. Since the lattice parameters (a) of MgO and CoO are 0.4211 and 0.4253 nm, respectively, the a values of 0.4221 and 0.4222 nm for MgO in the Co3O4/MgO-673 and Co3O4/ MgO-873 samples, estimated from the diffraction line of (2 0 0) plane, demonstrated the formation of Mg–Co–O solid solution in both cases. Moreover, the lattice parameter of the (4 4 0) plane in the Co3O4/MgO-873 sample was 0.8102 nm; this value is in the range of the standard a values for Co3O4 (0.8084 nm) and MgCo2O4 (0.8123 nm) [22], confirming the presence of MgCo2O4 phase [22]. As the temperature of calcination was elevated from 673 to 873 K, the crystallite size of MgO increased from 4.6 to 14.0 nm while the crystallite size of Co3O4 enlarged from 4.5 to 11.9 nm. Because of this, the specific surface area decreased from 197 to 74 m2/g. Notably, the crystallite sizes of MgO in these samples were much smaller than that of the raw MgO (34.6 nm), and thus the specific surface areas were also significantly larger than that of the raw MgO (37 m2/g). Fig. 2 shows the TEM images of the precipitate and the Co3O4/ MgO-T samples. The precipitate consisted of lamella-like Mg(OH)2 layers with lengths of several hundred nanometers. This lamellalike layer structure was partially maintained in the Co3O4/MgO673 sample with the coexistence of a wormlike structure, but the lamella-like layer was entirely transferred to a porous structure in the Co3O4/MgO-873 sample. This indicates that the precipitation and the subsequent calcination caused the reconstruction of MgO. Presumably, the original MgO particles were dissolved in the basic solution during the desorption–precipitation process and then hydrated to Mg(OH)2 primary particles, which further aggregated
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Fig. 2. TEM images of the precipitate (a) and the Co3O4/MgO-673 (b) and Co3O4/MgO-873 (c) samples.
to form a lamella-like structure. Upon calcination, the lamella-like Mg(OH)2 decomposed to wormhole-like MgO. This process might be similar to the transformation of MgO crystals to Mg(OH)2 nanoplates under hydrothermal condition [23].
Fig. 3 shows the H2-TPR profiles of the Co3O4/MgO-T samples. Both of them exhibited typical sequential reduction peaks of Co3O4. The minor reduction at about 400 K is assigned to the removal of chemisorbed oxygen species on the surface, as previously reported for Co3O4/MgO [24]. The intense reduction peak at 500–600 K represented the stepwise reduction of Co3O4 to metallic cobalt [24,25], which is closely related to the size of Co3O4 particle. The main reduction peak shifted from 552 to 586 K when the size of Co3O4 particle increased from 4.5 to 11.9 nm. The amount of hydrogen consumed at this temperature range
Fig. 3. H2-TPR profiles of the Co3O4/MgO-673 (a) and Co3O4/MgO-873 (b) samples. The solid lines refer to the fresh samples just after calcination while the dashed lines represent the samples that have been pre-reduced with hydrogen at 623 K for 1 h.
Fig. 5. CO2-TPD profiles of the Co/MgO-673 (a) and Co/MgO-873 (b) catalysts. Table 1 Chemical properties of the Co/MgO catalysts. Sample
Co loading (wt.%)
MgO crystallite size (nm)
Basicity (mmol CO2/g) Weak
Medium
Strong
Total
Co/MgO-673 Co/MgO-873
14.6 15.3
6.1 14.1
0.34 0.22
0.33 0.18
0.20 0.06
0.87 0.46
Table 2 Reaction results of glycerol hydrogenolysis over the Co/MgO catalysts.
Fig. 4. XRD patterns of the Co/MgO-673 (a) and Co/MgO-873 (b) catalysts.
Catalyst
Glycerol conversion (%)
Selectivity (%) 1, 2-PDO
EG
Ethanol
Others
Co/MgO-673 Co/MgO-873
5.3 44.8
45.3 42.2
6.8 10.5
5.5 3.4
42.4 43.9
Reaction conditions: 473 K, H2 2.0 MPa, 40 g 10 wt.% glycerol solution, 0.2 g catalyst, 9 h.
X. Guo et al. / Applied Catalysis A: General 371 (2009) 108–113
decreased greatly, indicating that the reducible Co3O4 species decreased with elevating the temperature of calcination. The broad peak at 600–1000 K is ascribed to the reduction of MgCo2O4 spinel [24,26], formed through the reaction of Co3O4 and MgO during calcination. The minor reduction above 1000 K was assigned to the reduction of Mg–Co–O solid solution species, which are rather
Fig. 6. Influences of temperature (a), H2 pressure (b) and glycerol concentration (c) on the conversion of glycerol and the selectivity of 1, 2-PDO. Reaction conditions: 40 g 10 wt.% glycerol solution, 0.2 g catalyst, 9 h.
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difficult to be reduced by hydrogen [22]. As the size of Co3O4 particles increased from 4.5 to 11.9 nm, the total amount of H2 consumed decreased from 2.11 to 1.37 mmol/g. However, the amounts are still less than the stoichiometric amount of hydrogen (3.39 mmol/g) required for the reduction of Co3O4 to metallic Co, implying that part of Co3O4 was converted to Mg–Co–O solid solution by interacting with MgO, as evidenced by the XRD measurements. Fig. 3 also compares the H2-TPR profiles of the Co3O4/MgO samples that have been pre-reduced with H2 at 623 K for 1 h. Estimated from the hydrogen consumptions between two TPR runs, the reduction degree of Co3O4 were 58% for the Co3O4/MgO673 sample and 38% for the Co3O4/MgO-873 sample. This clearly demonstrates that the high calcination temperature tended to form MgCo2O4 spinel and Mg–Co–O solid solution that cannot be reduced by hydrogen below 623 K. Fig. 4 shows the XRD patterns of the Co/MgO-T catalysts obtained by reducing the Co3O4/MgO samples with hydrogen at 623 K. The diffraction peaks of Mg–Co–O solid solution and MgCo2O4 spinel disappeared and only the diffraction peaks of MgO were detected. This might be due to the re-dispersion of Mg–Co–O solid solution and MgCo2O4 spinel during hydrogen reduction process. However, the diffraction peak of MgO (2 0 0) shifted slightly to lower angles. The shifts were more pronounced in the Co/MgO-873 sample, suggesting the presence of Mg–Co–O solid solution and MgCo2O4 spinel. The crystallite sizes of MgO were 6.1 and 14.1 nm for the Co/MgO-673 and Co/MgO-873 catalysts, respectively. They are almost the same as those in the Co3O4/MgO samples, indicating the stable presence of MgO during the hydrogen treatment. However, there were no diffraction peaks of metallic Co species, probably because they are too small to be detected and/or highly dispersed on MgO. Fig. 5 shows the CO2-TPD profiles of the Co/MgO-T catalysts; the amounts of CO2 desorbed are listed in Table 1. There were three desorption peaks: these can be defined as weak (<423 K), medium (423–523 K) and strong (<523 K) basicities [27]. The weak desorption is related to lattice-bound and isolated hydroxyl groups exhibiting Bro¨nsted basicity, while the medium and strong desorption peaks are ascribed to three- and/or four-fold-coordinated O2 anions, showing Lewis basicity [27]. The amounts of CO2 decreased from 0.87 mmol CO2/g for the Co/MgO-673 catalyst to 0.46 mmol CO2/g for the Co/MgO-873 catalyst, mainly because of
Fig. 7. XRD patterns of the Co/MgO-673 catalyst after reaction for 9 h (a)and the Co/ MgO-873 catalyst after reaction for 1 h (b), 4 h (c) and 9 h (d).
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the increased size of MgO particle from 6.1 to 14.1 nm that lowered the available sites (such as corners and edges) for the adsorption of CO2 [28]. Furthermore, the significant formation of MgCo2O4 spinel and Mg–Co–O solid solution in the Co/MgO-873 catalyst also weakened the overall basicity. 3.2. Hydrogenolysis of glycerol Table 2 summarizes the reaction results of glycerol hydrogenolysis over the Co/MgO-T catalysts at 473 K. On the Co/MgO673 catalyst, the conversion of glycerol was 5.3% and the selectivity of 1, 2-PDO was 45.3%. However, the conversion of glycerol rapidly increased to 44.8% over the Co/MgO-873 catalyst, while the selectivity of 1, 2-PDO only slightly decreased to 42.2%, giving a 1, 2-PDO yield of 19.3%. The influences of reaction temperature, hydrogen pressure and glycerol concentration on the production of 1, 2-PDO were then investigated over the Co/MgO-873 catalyst. As shown in Fig. 6, the
conversion of glycerol increased greatly from 21.1% at 453 K to 67.3% at 523 K, but the selectivity of 1, 2-PDO decreased from 47.8% to 21.7%. Obviously, higher reaction temperature favors the conversion of glycerol but lowers the selectivity of 1, 2-PDO due to the formation of significant amounts of lower alcohols and hydrocarbons. On the other hand, both the conversion of glycerol and the selectivity of 1, 2-PDO increased gradually with increasing the pressure of hydrogen. The conversion of glycerol was 36.1% and the selectivity of 1, 2-PDO was 38.5% at 1.0 MPa, while these values were increased to 46.9% and 49.2%, respectively, as the pressure of hydrogen was increased to 3 MPa. This might be caused by the enhanced concentration of hydrogen in the aqueous solution with increasing the pressure of hydrogen [29]. The conversion of glycerol decreased with increasing the concentration of glycerol in the reaction media, but the selectivity of 1, 2-PDO was almost unchanged, implying that the conversion of glycerol is simply associated with the number of active sites, in accordance with previous findings over Ru/CsPW and Cu/ZnO catalysts [16,20].
Fig. 8. TEM images of the Co/MgO-873 catalysts: (a and b) the fresh sample, (c and d) after the first run and (e and f) after the second run.
X. Guo et al. / Applied Catalysis A: General 371 (2009) 108–113
Accordingly, the activation was estimated to be 16 kJ/mol, while the reaction orders with respect to glycerol and hydrogen were 0.10 and 0.25, respectively. 3.3. Structure changes under reaction conditions Fig. 7 shows the XRD patterns of the Co/MgO catalysts used. Diffraction peaks of Mg(OH)2 were clearly observed in both cases, indicating that MgO was rehydrated to Mg(OH)2 during the course of reaction. Although the initial crystallite sizes of MgO were 6.1 and 14.1 nm in the Co/MgO catalysts, the crystallite sizes of Mg(OH)2 in the used samples were very close, being 15–17 nm. Meanwhile, diffraction peaks of metallic Co with hcp and fcc structures appeared, but the crystallite size of Co in the Co/MgO873 catalyst (17.3 nm) was much smaller than that in the Co/MgO673 catalyst (30.5 nm). This is perhaps related to the relatively strong metal support interaction and the presence of relatively larger MgCo2O4 and Mg–Co–O species in the Co/MgO-873 catalyst, which prevented the aggregation of Co particles under the harsh reaction conditions. For the Co/MgO-873 catalyst, the size of Mg(OH)2 particle increased slowly from 8.6 nm at 1 h to 15.1 nm at 9 h, but the size of Co particle rapidly increased to 14.9 nm after reaction for 1 h, and then increased slowly to 17.3 nm after reaction for 9 h. The agglomeration of these particles is induced by the high content of water in the reaction media, as previously reported for supported Au and Ru catalysts [18,19,30,31]. Such an agglomeration was also evidenced by the TEM observations. As shown in Fig. 8, no obvious metallic cobalt particles can be found in the fresh sample, and the size of MgO particles was about 50 nm. However, after reaction for 9 h, porous MgO was rehydrated to disc-like Mg(OH)2. Meanwhile, Co particles with size of 20–100 nm were clearly. This further confirms the severe aggregation of cobalt particle under the harsh reaction conditions. It is generally accepted that the glycerol hydrogenolysis reaction is a bi-functional catalytic process. MgO, which was hydrated to Mg(OH)2, provided basic sites for dehydrogenation of glycerol to glyceraldehyde that might be further converted to pyruvaldehyde. These reaction intermediates were then hydrogenated to 1, 2-PDO over Co particles, where the size of Co particle is the decisive factor because of the structure sensitivity [18]. Since the Co/MgO-873 catalyst had relatively smaller and more stable cobalt particles, it exhibited a better activity for the hydrogenolysis of glycerol. When reused for the reaction, however, the conversion of glycerol decreased greatly from 45% to 14%, but the selectivity of 1, 2-PDO increased significantly from 42% to 68%. Again, this was mainly caused by the severe aggregation of Co particles under the harsh reaction conditions. As shown in Fig. 8, the Co particles enlarged from several tens nanometers after the first run to hundreds nanometers after the reused operation, clearly demonstrating the essential role of Co particle in the overall reaction.
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4. Conclusions Calcination of Co3O4/MgO at 873 K significantly enhanced the interactions between Co3O4 and MgO but simultaneously promoted the formation of MgCo2O4 spinel and Mg–Co–O solid solution, which decreased the reducibility of cobalt oxides. However, this strong interaction prevented the aggregation of Co particles in the resulting Co/MgO catalyst under the harsh reaction conditions of glycerol hydrogenolysis, showing a much higher activity and stability. MgO was transferred to Mg(OH)2 during the course of reaction and the size of Co particle increased considerably, especially at the initial stage. Most probably, basic Mg(OH)2 particles provided active sites for dehydrogenation of glycerol to glyceraldehyde, and/or pyruvaldehyde while Co particles catalyzed hydrogenation of these reaction intermediates to 1, 2-propanediol, showing a bi-functional mechanism. References [1] D.S. Martino, T. Riccardo, P.M. Lu, S. Elio, Energy Fuels 22 (2008) 207–217. [2] E. Tsukuda, S. Sato, R. Takahashi, T. Sodesawa, Catal. Commun. 8 (2007) 1349– 1353. [3] S.H. Chai, H.P. Wang, Y. Liang, B.Q. Xu, J. Catal. 250 (2007) 342–349. [4] R.S. Karinen, A.O.I. Krause, Appl. Catal. A: Gen. 306 (2006) 128–133. [5] J. Barrault, J.-M. Clacens, Y. Pouilloux, Top. Catal. 27 (2004) 137–142. [6] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, C.J. Kiely, G.A. Attard, G.J. Hutchings, Top. Catal. 27 (2004) 131–136. [7] F. Porta, L. Prati, J. Catal. 224 (2004) 397–403. [8] A. Perosa, P. Tundo, Ind. Eng. Chem. Res. 44 (2005) 8535–8537. [9] T. Miyazawa, Y. Kusunoki, K. Kunimori, K. Tomishige, J. Catal. 240 (2006) 213–221. [10] E.P. Maris, R.J. Davis, J. Catal. 249 (2007) 328–337. [11] D.G. Lahr, B.H. Shanks, Ind. Eng. Chem. Res. 42 (2003) 5467–5472. [12] D.G. Lahr, B.H. Shanks, J. Catal. 232 (2005) 386–394. [13] Z.W. Huang, F. Cui, H.X. Kang, J. Chen, X.Z. Zhang, C.G. Xia, Chem. Mater. 20 (2008) 5090–5099. [14] M.A. Dasari, P.P. Kiatsimkul, W.R. Sutterlin, G.J. Suppes, Appl. Catal. A: Gen. 281 (2005) 225–231. [15] I. Furikado, T. Miyazawa, S. Koso, A. Shimao, K. Kunimori, K. Tomishige, Green Chem. 9 (2007) 582–588. [16] M. Balaraju, V. Rekha, P.S. Sai Prasad, R.B.N. Prasad, N. Lingaiah, Catal. Lett. 126 (2008) 119–124. [17] T. Miyazawa, S. Koso, K. Kunimori, K. Tomishige, Appl. Catal. A: Gen. 318 (2007) 244–251. [18] E.P. Maris, W.C. Ketchie, M. Murayama, R.J. Davis, J. Catal. 251 (2007) 281–294. [19] S. Wang, H.C. Liu, Catal. Lett. 117 (2007) 62–67. [20] A. Alhanash, E.F. Kozhevnikova, I.V. Kozhevnikov, Catal. Lett. 120 (2008) 307–311. [21] J. Feng, J.B. Wang, Y.F. Zhou, H.Y. Fu, H. Chen, X.J. Li, Chem. Lett. 36 (2007) 1274– 1275. [22] E. Ruckenstein, H.Y. Wang, Catal. Lett. 70 (2000) 15–21. [23] J.C. Yu, A.W. Xu, L.Z. Zhang, R.Q. Song, L. Wu, J. Phys. Chem. B 108 (2004) 64–70. [24] S. Tuti, F. Pepe, Catal. Lett. 122 (2008) 196–203. [25] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Geus, J.W. Niemantsverdriet, J. Catal. 239 (2006) 326–339. [26] H.Y. Wang, E. Ruckenstein, Appl. Catal. A: Gen. 209 (2001) 207–215. [27] Z. Liu, J.A.C. Concepcio´n, M. Mustian, M.D. Amiridis, Appl. Catal. A: Gen. 302 (2006) 232–236. [28] R. Richards, W.F. Li, S. Decker, C. Davidson, O. Koper, V. Zaikovski, A. Volodin, T. Rieker, K.J. Klabunde, J. Am. Chem. Soc. 122 (2000) 4921–4925. [29] H. Pray, C. Schweikert, B. Minich, Ind. Eng. Chem. 44 (1952) 1146–1151. [30] R. Bouarab, O. Akdim, A. Auroux, O. Cherifi, C. Mirodatos, Appl. Catal. A: Gen. 264 (2004) 161–168. [31] E.P. Maris, W.C. Ketchie, V. Oleshko, R.J. Davis, J. Phys. Chem. B 110 (2006) 7869– 7876.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Co/MgO catalysts for hydrogenolysis of glycerol to 1, 2-propanediol Xiaohui Guo, Yong Li, Ruijuan Shi, Qiying Liu, Ensheng Zhan, Wenjie Shen * State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 June 2009 Received in revised form 23 September 2009 Accepted 23 September 2009 Available online 1 October 2009
Hydrogenolysis of glycerol to 1, 2-propanediol was investigated on bi-functional Co/MgO catalysts where the interaction between cobalt species and MgO was adjusted by varying the temperature of calcination. Higher temperature treatment not only enhanced the interaction between Co3O4 and MgO, but also promoted the formation of MgCo2O4 spinel and Mg–Co–O solid solution. Although the reducibility of cobalt oxides was greatly decreased in the Co3O4/MgO precursor, this strong interaction prevented the aggregation of Co particles in the resulting Co/MgO catalyst under the harsh reaction conditions, giving a much higher activity and stability. Results revealed that MgO was hydrated to Mg(OH)2 during the course of reaction and that the sizes of Mg(OH)2 and Co particles increased considerably, especially during the initial stage of the reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Glycerol Hydrogenolysis 1, 2-Propanediol Co/MgO
1. Introduction Biodiesel produced by transesterification reactions of plant and fatty oils is one of the most important biofuels [1]. However, significant amounts (about 10 wt.%) of crude glycerol are simultaneously generated as a byproduct. In order to improve the overall efficiency of biodiesel production, sustained efforts have been made to convert the crude glycerol to value-added chemicals, such as dehydration to acrolein [2,3], etherification to ethers [4,5], oxidation to glyceric acid [6,7], and hydrogenolysis to propanediol [8–21]. In particular, hydrogenolysis of glycerol to 1, 2-propanediol (1, 2-PDO), which is an important feedstock for manufacturing polymers and pharmaceutics, has been extensively studied to achieve mechanistic and kinetic understandings [10– 12] and improvement of product yield [13–17]. It is generally recognized that both the catalyst and the pH value of the reaction media play essential roles in determining the reaction pathway and the yield of 1, 2-PDO [18,19]. When solid acids like Amberlyst [9,15,17] or a heteropoly salt (Cs2.5H0.5[PW12O40]) [20] and precious metals (mainly Ru) are jointly used as catalysts, dehydration of glycerol to acetol takes place on the acidic sites, and the acetol produced is then hydrogenated to 1, 2-PDO over the metal particles. In the cases of catalytic systems containing precious metals and base materials, such as Pt/C + CaO [10], PtRu/
* Corresponding author. Tel.: +86 411 84379085; fax: +86 411 84694447. E-mail address: [email protected] (W. Shen). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.037
C + NaOH [18], and Ru/TiO2 + LiOH [21], glycerol is initially dehydrogenated to glyceraldehyde and/or pyruvaldehyde catalyzed by the basic component, followed by hydrogenation of these reaction intermediates to 1, 2-PDO over the precious metals. Although this reaction is usually performed under hightemperature and hydrogen pressure, the conversion of glycerol and the selectivity of 1, 2-PDO is still quite low. Moreover, little attention has been paid to the durability and the structural changes of the catalysts under the harsh reaction conditions. In this work, we report highly efficient Co/MgO catalysts for hydrogenolysis of glycerol, where the solid MgO acts as the basic component and the support of cobalt nanoparticles. The structural changes of the catalysts during glycerol hydrogenolysis have been investigated. 2. Experimental 2.1. Catalyst preparation The Co/MgO catalysts were prepared by a deposition–precipitation method. Ten grams of commercially available MgO powders (previously calcined at 873 K for 4 h in air, surface area 37 m2/g) was dispersed in 750 ml aqueous solution containing 8.44 g cobalt acetate (Co(CH3COO)24H2O) and 37.5 g urea. The mixture was heated to 363 K and maintained at this temperature for 8 h under stirring. After filtering and washing with deionized water, the precipitate was dried at 353 K overnight and calcined at 673 or 873 K for 4 h in air, giving the Co3O4/MgO-T samples, where T refers to the temperature of calcination. These calcined samples
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(0.2 g) were then reduced with pure H2 (50 ml/min) in a fixed-bed quartz reactor at 623 K for 1 h, yielding the Co/MgO-T catalysts. 2.2. Catalyst characterization The actual loading of cobalt was measured by inductively coupled plasma-atomic emission spectroscopy (ICP, Plasma-SpecII spectrometer). N2 adsorption–desorption isotherms were recorded at 77 K on a Micromeritics ASAP 2000 instrument. Before the measurement, each sample was degassed at 573 K for 3 h. The specific surface area was calculated by the BET method using the nitrogen adsorption isotherms. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2500 diffractometer with Cu Ka radiation operated at 40 kV and 300 mA. In situ XRD measurements were carried out in a high-temperature chamber installed in the diffractometer. The powder Co3O4/MgO-T samples were pressed into flakes and mounted in the chamber; they were then reduced with pure hydrogen at 623 K for 1 h. The average crystallite size was calculated from the Scherrer equation. Transmission electron microscopy (TEM) images were recorded using a Philip Tecnai G2 spirit microscope operated at 120 kV. The specimen was prepared by ultrasonically suspending the powder sample in ethanol, and drops of the suspension were deposited on a carbon-coated copper grid and dried in air. Temperature programmed reduction (TPR) of hydrogen of the Co3O4/MgO-T samples was performed in a U-shape quartz reactor. For each run, 50 mg samples were loaded and pretreated at 623 K for 0.5 h under N2 flow (30 ml/min). After the sample was cooled to room temperature and after the reduction agent of a 5% H2/N2 mixture (30 ml/min) was introduced, the sample was heated to 1073 K at a rate of 10 K/min. The outlet gas was collected by passing it through a molecular sieve trap to remove water; the consumption of hydrogen was analyzed by a thermal conductivity detector (TCD). To estimate the reduction degree of cobalt species, we performed H2-TPR experiments for the Co/MgO-T catalyst obtained by pre-reducing the Co3O4/MgO-T sample with H2 at 623 K for 1 h. The amounts of hydrogen consumed between the two runs were used to estimate the reduction degree of cobalt oxide. The amounts of hydrogen consumption were calibrated using known amounts of CuO. Temperature programmed desorption (TPD) of CO2 was conducted in a U-shape quartz reactor connected to a mass spectrometer (Omnistar QMS 200). Fifty-milligram samples was loaded and pre-reduced with a 20% H2/N2 mixture (30 ml/min) at 623 K for 40 min and then purged with He for 1 h. After being cooled to room temperature, the sample was exposed to a 49.8% CO2/He mixture (20 ml/min) for 1 h, followed by purging with He for 2 h. The sample was then heated to 773 K at a rate of 10 K/min under He flow (50 ml/min) and the desorption of CO2 was monitored by the mass spectrometer. 2.3. Catalytic test Hydrogenolysis of glycerol was conducted in a stainless steel autoclave (100 ml). Forty grams of glycerol aqueous solution (10 wt.%) and 0.2 g Co/MgO catalyst were added to the reactor. After purging with hydrogen five times, the reactor was pressured to 1.0–3.0 MPa and then heated to a desired temperature (453–523 K) and maintained at that temperature for a given period. The products were analyzed by gas chromatography equipped with a flame ionization detector (FID) connected to a Carbowax 20 M capillary column (for the liquid products) and a TCD connected to a Porapak Q packed column (for the gas products).
Fig. 1. XRD patterns of the precipitate (a) and the Co3O4/MgO-673 (b) and Co3O4/ MgO-873 (c) samples.
3. Results and discussion 3.1. Chemical and physical properties of the catalysts Fig. 1 shows the XRD patterns of the precipitate and the Co3O4/ MgO-T samples. The precipitate exhibited characteristic diffraction peaks of hexagonal Mg(OH)2 (JCPDS 83-0114) and b-Co(OH)2 (JCPDS 30-0443), indicating that the raw MgO powder was hydrated to Mg(OH)2 during the deposition–precipitation process. After calcination, Mg(OH)2 was converted to MgO (JCPDS 89-7746) and b-Co(OH)2 was transformed mainly to Co3O4 (JCPDS 73-1701). Meanwhile, minor MgCo2O4 spinel (JCPDS 02-1073) and/or Mg– Co–O solid solution (JCPDS 02-1201) phases were also detected, although their diffraction peaks were partially overlapped with those of Co3O4 and MgO. Since the lattice parameters (a) of MgO and CoO are 0.4211 and 0.4253 nm, respectively, the a values of 0.4221 and 0.4222 nm for MgO in the Co3O4/MgO-673 and Co3O4/ MgO-873 samples, estimated from the diffraction line of (2 0 0) plane, demonstrated the formation of Mg–Co–O solid solution in both cases. Moreover, the lattice parameter of the (4 4 0) plane in the Co3O4/MgO-873 sample was 0.8102 nm; this value is in the range of the standard a values for Co3O4 (0.8084 nm) and MgCo2O4 (0.8123 nm) [22], confirming the presence of MgCo2O4 phase [22]. As the temperature of calcination was elevated from 673 to 873 K, the crystallite size of MgO increased from 4.6 to 14.0 nm while the crystallite size of Co3O4 enlarged from 4.5 to 11.9 nm. Because of this, the specific surface area decreased from 197 to 74 m2/g. Notably, the crystallite sizes of MgO in these samples were much smaller than that of the raw MgO (34.6 nm), and thus the specific surface areas were also significantly larger than that of the raw MgO (37 m2/g). Fig. 2 shows the TEM images of the precipitate and the Co3O4/ MgO-T samples. The precipitate consisted of lamella-like Mg(OH)2 layers with lengths of several hundred nanometers. This lamellalike layer structure was partially maintained in the Co3O4/MgO673 sample with the coexistence of a wormlike structure, but the lamella-like layer was entirely transferred to a porous structure in the Co3O4/MgO-873 sample. This indicates that the precipitation and the subsequent calcination caused the reconstruction of MgO. Presumably, the original MgO particles were dissolved in the basic solution during the desorption–precipitation process and then hydrated to Mg(OH)2 primary particles, which further aggregated
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Fig. 2. TEM images of the precipitate (a) and the Co3O4/MgO-673 (b) and Co3O4/MgO-873 (c) samples.
to form a lamella-like structure. Upon calcination, the lamella-like Mg(OH)2 decomposed to wormhole-like MgO. This process might be similar to the transformation of MgO crystals to Mg(OH)2 nanoplates under hydrothermal condition [23].
Fig. 3 shows the H2-TPR profiles of the Co3O4/MgO-T samples. Both of them exhibited typical sequential reduction peaks of Co3O4. The minor reduction at about 400 K is assigned to the removal of chemisorbed oxygen species on the surface, as previously reported for Co3O4/MgO [24]. The intense reduction peak at 500–600 K represented the stepwise reduction of Co3O4 to metallic cobalt [24,25], which is closely related to the size of Co3O4 particle. The main reduction peak shifted from 552 to 586 K when the size of Co3O4 particle increased from 4.5 to 11.9 nm. The amount of hydrogen consumed at this temperature range
Fig. 3. H2-TPR profiles of the Co3O4/MgO-673 (a) and Co3O4/MgO-873 (b) samples. The solid lines refer to the fresh samples just after calcination while the dashed lines represent the samples that have been pre-reduced with hydrogen at 623 K for 1 h.
Fig. 5. CO2-TPD profiles of the Co/MgO-673 (a) and Co/MgO-873 (b) catalysts. Table 1 Chemical properties of the Co/MgO catalysts. Sample
Co loading (wt.%)
MgO crystallite size (nm)
Basicity (mmol CO2/g) Weak
Medium
Strong
Total
Co/MgO-673 Co/MgO-873
14.6 15.3
6.1 14.1
0.34 0.22
0.33 0.18
0.20 0.06
0.87 0.46
Table 2 Reaction results of glycerol hydrogenolysis over the Co/MgO catalysts.
Fig. 4. XRD patterns of the Co/MgO-673 (a) and Co/MgO-873 (b) catalysts.
Catalyst
Glycerol conversion (%)
Selectivity (%) 1, 2-PDO
EG
Ethanol
Others
Co/MgO-673 Co/MgO-873
5.3 44.8
45.3 42.2
6.8 10.5
5.5 3.4
42.4 43.9
Reaction conditions: 473 K, H2 2.0 MPa, 40 g 10 wt.% glycerol solution, 0.2 g catalyst, 9 h.
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decreased greatly, indicating that the reducible Co3O4 species decreased with elevating the temperature of calcination. The broad peak at 600–1000 K is ascribed to the reduction of MgCo2O4 spinel [24,26], formed through the reaction of Co3O4 and MgO during calcination. The minor reduction above 1000 K was assigned to the reduction of Mg–Co–O solid solution species, which are rather
Fig. 6. Influences of temperature (a), H2 pressure (b) and glycerol concentration (c) on the conversion of glycerol and the selectivity of 1, 2-PDO. Reaction conditions: 40 g 10 wt.% glycerol solution, 0.2 g catalyst, 9 h.
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difficult to be reduced by hydrogen [22]. As the size of Co3O4 particles increased from 4.5 to 11.9 nm, the total amount of H2 consumed decreased from 2.11 to 1.37 mmol/g. However, the amounts are still less than the stoichiometric amount of hydrogen (3.39 mmol/g) required for the reduction of Co3O4 to metallic Co, implying that part of Co3O4 was converted to Mg–Co–O solid solution by interacting with MgO, as evidenced by the XRD measurements. Fig. 3 also compares the H2-TPR profiles of the Co3O4/MgO samples that have been pre-reduced with H2 at 623 K for 1 h. Estimated from the hydrogen consumptions between two TPR runs, the reduction degree of Co3O4 were 58% for the Co3O4/MgO673 sample and 38% for the Co3O4/MgO-873 sample. This clearly demonstrates that the high calcination temperature tended to form MgCo2O4 spinel and Mg–Co–O solid solution that cannot be reduced by hydrogen below 623 K. Fig. 4 shows the XRD patterns of the Co/MgO-T catalysts obtained by reducing the Co3O4/MgO samples with hydrogen at 623 K. The diffraction peaks of Mg–Co–O solid solution and MgCo2O4 spinel disappeared and only the diffraction peaks of MgO were detected. This might be due to the re-dispersion of Mg–Co–O solid solution and MgCo2O4 spinel during hydrogen reduction process. However, the diffraction peak of MgO (2 0 0) shifted slightly to lower angles. The shifts were more pronounced in the Co/MgO-873 sample, suggesting the presence of Mg–Co–O solid solution and MgCo2O4 spinel. The crystallite sizes of MgO were 6.1 and 14.1 nm for the Co/MgO-673 and Co/MgO-873 catalysts, respectively. They are almost the same as those in the Co3O4/MgO samples, indicating the stable presence of MgO during the hydrogen treatment. However, there were no diffraction peaks of metallic Co species, probably because they are too small to be detected and/or highly dispersed on MgO. Fig. 5 shows the CO2-TPD profiles of the Co/MgO-T catalysts; the amounts of CO2 desorbed are listed in Table 1. There were three desorption peaks: these can be defined as weak (<423 K), medium (423–523 K) and strong (<523 K) basicities [27]. The weak desorption is related to lattice-bound and isolated hydroxyl groups exhibiting Bro¨nsted basicity, while the medium and strong desorption peaks are ascribed to three- and/or four-fold-coordinated O2 anions, showing Lewis basicity [27]. The amounts of CO2 decreased from 0.87 mmol CO2/g for the Co/MgO-673 catalyst to 0.46 mmol CO2/g for the Co/MgO-873 catalyst, mainly because of
Fig. 7. XRD patterns of the Co/MgO-673 catalyst after reaction for 9 h (a)and the Co/ MgO-873 catalyst after reaction for 1 h (b), 4 h (c) and 9 h (d).
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the increased size of MgO particle from 6.1 to 14.1 nm that lowered the available sites (such as corners and edges) for the adsorption of CO2 [28]. Furthermore, the significant formation of MgCo2O4 spinel and Mg–Co–O solid solution in the Co/MgO-873 catalyst also weakened the overall basicity. 3.2. Hydrogenolysis of glycerol Table 2 summarizes the reaction results of glycerol hydrogenolysis over the Co/MgO-T catalysts at 473 K. On the Co/MgO673 catalyst, the conversion of glycerol was 5.3% and the selectivity of 1, 2-PDO was 45.3%. However, the conversion of glycerol rapidly increased to 44.8% over the Co/MgO-873 catalyst, while the selectivity of 1, 2-PDO only slightly decreased to 42.2%, giving a 1, 2-PDO yield of 19.3%. The influences of reaction temperature, hydrogen pressure and glycerol concentration on the production of 1, 2-PDO were then investigated over the Co/MgO-873 catalyst. As shown in Fig. 6, the
conversion of glycerol increased greatly from 21.1% at 453 K to 67.3% at 523 K, but the selectivity of 1, 2-PDO decreased from 47.8% to 21.7%. Obviously, higher reaction temperature favors the conversion of glycerol but lowers the selectivity of 1, 2-PDO due to the formation of significant amounts of lower alcohols and hydrocarbons. On the other hand, both the conversion of glycerol and the selectivity of 1, 2-PDO increased gradually with increasing the pressure of hydrogen. The conversion of glycerol was 36.1% and the selectivity of 1, 2-PDO was 38.5% at 1.0 MPa, while these values were increased to 46.9% and 49.2%, respectively, as the pressure of hydrogen was increased to 3 MPa. This might be caused by the enhanced concentration of hydrogen in the aqueous solution with increasing the pressure of hydrogen [29]. The conversion of glycerol decreased with increasing the concentration of glycerol in the reaction media, but the selectivity of 1, 2-PDO was almost unchanged, implying that the conversion of glycerol is simply associated with the number of active sites, in accordance with previous findings over Ru/CsPW and Cu/ZnO catalysts [16,20].
Fig. 8. TEM images of the Co/MgO-873 catalysts: (a and b) the fresh sample, (c and d) after the first run and (e and f) after the second run.
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Accordingly, the activation was estimated to be 16 kJ/mol, while the reaction orders with respect to glycerol and hydrogen were 0.10 and 0.25, respectively. 3.3. Structure changes under reaction conditions Fig. 7 shows the XRD patterns of the Co/MgO catalysts used. Diffraction peaks of Mg(OH)2 were clearly observed in both cases, indicating that MgO was rehydrated to Mg(OH)2 during the course of reaction. Although the initial crystallite sizes of MgO were 6.1 and 14.1 nm in the Co/MgO catalysts, the crystallite sizes of Mg(OH)2 in the used samples were very close, being 15–17 nm. Meanwhile, diffraction peaks of metallic Co with hcp and fcc structures appeared, but the crystallite size of Co in the Co/MgO873 catalyst (17.3 nm) was much smaller than that in the Co/MgO673 catalyst (30.5 nm). This is perhaps related to the relatively strong metal support interaction and the presence of relatively larger MgCo2O4 and Mg–Co–O species in the Co/MgO-873 catalyst, which prevented the aggregation of Co particles under the harsh reaction conditions. For the Co/MgO-873 catalyst, the size of Mg(OH)2 particle increased slowly from 8.6 nm at 1 h to 15.1 nm at 9 h, but the size of Co particle rapidly increased to 14.9 nm after reaction for 1 h, and then increased slowly to 17.3 nm after reaction for 9 h. The agglomeration of these particles is induced by the high content of water in the reaction media, as previously reported for supported Au and Ru catalysts [18,19,30,31]. Such an agglomeration was also evidenced by the TEM observations. As shown in Fig. 8, no obvious metallic cobalt particles can be found in the fresh sample, and the size of MgO particles was about 50 nm. However, after reaction for 9 h, porous MgO was rehydrated to disc-like Mg(OH)2. Meanwhile, Co particles with size of 20–100 nm were clearly. This further confirms the severe aggregation of cobalt particle under the harsh reaction conditions. It is generally accepted that the glycerol hydrogenolysis reaction is a bi-functional catalytic process. MgO, which was hydrated to Mg(OH)2, provided basic sites for dehydrogenation of glycerol to glyceraldehyde that might be further converted to pyruvaldehyde. These reaction intermediates were then hydrogenated to 1, 2-PDO over Co particles, where the size of Co particle is the decisive factor because of the structure sensitivity [18]. Since the Co/MgO-873 catalyst had relatively smaller and more stable cobalt particles, it exhibited a better activity for the hydrogenolysis of glycerol. When reused for the reaction, however, the conversion of glycerol decreased greatly from 45% to 14%, but the selectivity of 1, 2-PDO increased significantly from 42% to 68%. Again, this was mainly caused by the severe aggregation of Co particles under the harsh reaction conditions. As shown in Fig. 8, the Co particles enlarged from several tens nanometers after the first run to hundreds nanometers after the reused operation, clearly demonstrating the essential role of Co particle in the overall reaction.
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