Energy Fuels 2010, 24, 3810–3816 Published on Web 06/18/2010
: DOI:10.1021/ef100419m
CaO Supported on Porous Carbon as Highly Efficient Heterogeneous Catalysts for Transesterification of Triacetin with Methanol Yanhong Zu, Gang Liu,* Zhenlv Wang, Jinghui Shi, Min Zhang, Wenxiang Zhang, and Mingjun Jia* State Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry, Jilin University, Changchun 130021, China Received March 2, 2010. Revised Manuscript Received June 9, 2010
A series of carbon-supported CaO materials, prepared using different porous carbon materials as supports, were tested as basic catalysts for the transesterification of triacetin with methanol and characterized by means of X-ray diffraction (XRD), N2 adsorption, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). It was found that all of the carbon-supported CaO catalysts are active for the transesterification reaction, and their catalytic performance can be influenced by a variety of factors, such as the types of carbon supports, the concentration of impregnated CaO, the heattreated temperatures of the catalysts, and the reaction temperatures. Particularly, the supported CaO catalyst, which is prepared using a kind of porous carbon (NC-2) as a support, exhibits very high activity, stability, and recyclability. We suppose that the main characteristics of this porous carbon, such as the presence of relatively abundant surface oxygen-containing functional groups and well-developed porosity, may be beneficial to build a suitable interaction between CaO particles and the carbon support, thus resulting in the formation of an active and stable catalyst system.
reported to be active for the transesterification reactions.8-23 Among them, CaO has been investigated widely because of its high activity for the transesterification of triglycerides with alcohols.17-23 However, a problem for the CaO catalyst is that the polar species (e.g., glycerol) present during biodiesel synthesis can form the CaO-glycerin complex, which ends up leaching from the solid catalyst and acting as a homogeneous catalyst in the reaction.24-27 This drawback considerably
1. Introduction Biodiesel fuel, which consists of simple alkyl esters of fatty acids [preferentially fatty acid methyl esters (FAMEs)], has become a growing interest as an alternative to diesel fuel made from renewable sources. Biodiesel is produced by transesterification of triglycerides with low-molecular-weight alcohols from oils and fats.1-3 Conventionally, this reaction is performed using homogeneous alkaline agents, such as K or Na alkoxides or hydroxides.4,5 However, removal of the soluble base after the reaction is a major problem, which leads to increased costs and a series of environmental concerns.6,7 The use of solid-base catalysts may offer many process advantages, such as being easier to separate from liquid products. Different solid-base catalysts, including various metal oxides, alumina-supported potassium, alkali-exchanged zeolite, alkali earth oxides, hydrotalcites, and organic quaternary ammonium functionalized silica or resin, have been
(10) Verziu, M.; Florea, M.; Simon, S.; Simon, V.; Filip, P.; Parvulescu, V. I. J. Catal. 2009, 263, 56–66. (11) Lukic, I.; Krstic, J.; Jovanovic, D.; Skala, D. Bioresour. Technol. 2009, 100, 4690–4696. (12) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Appl. Catal., A 2005, 283, 111–116. (13) Xie, W. L.; Huang, X. M.; Li, H. T. Bioresour. Technol. 2007, 98, 936–939. (14) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Appl. Catal., A 2004, 257, 213–223. (15) Corma, A.; Hamid, S. B. A.; Iborra, S.; Velty, A. J. Catal. 2005, 234, 340–347. (16) Liu, Y. J.; Lotero, E.; Goodwin, J. G., Jr.; Lu, C. Q. J. Catal. 2007, 246, 428–433. (17) Gryglewicz, S. Appl. Catal., A 2000, 192, 23–28. (18) Reddy, C.; Reddy, V.; Oshel, R.; Verkade, J. G. Energy Fuels 2006, 20, 1310–1314. (19) Demirbas, A. Energy Convers. Manage. 2007, 48, 937–941. (20) Granados, M. L.; Poves, M. D. Z.; Aloson, D. M.; Mariscal, R.; Galisteo, F. C.; Moreno-Tost, R.; Santamarı´ a, J.; Fierro, J. L. G. Appl. Catal., B 2007, 73, 317–326. (21) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel 2008, 87, 216–221. (22) Kawashima, A.; Matsubara, K.; Honda, K. Bioresour. Technol. 2009, 100, 696–700. (23) Kouzu, M.; Kasuno, T.; Tajika, M.; Sugimoto, Y.; Yamanaka, S.; Hidaka, J. Fuel 2008, 87, 2798–2806. (24) Granados, M. L.; Alonso, D. M.; Sadaba, I.; Mariscal, R.; Oc on, P. Appl. Catal., B 2009, 89, 265–272. (25) Kouzu, M.; Kasuno, T.; Tajika, M.; Yamanaka, S.; Hidaka, J. Appl. Catal., A 2008, 334, 357–365. (26) Kouzu, M.; Yamanaka, S.; Hidaka, J.; Tsunomori, M. Appl. Catal., A 2009, 355, 94–99. (27) Ruppert, A. M.; Meeldijk, J. D.; Kuipers, B. W. M.; Erne, B. H.; Weckhuysen, B. M. Chem.;Eur. J. 2008, 14, 2016–2024.
*To whom correspondence should be addressed. Telephone: (þ86) 431-85155390. Fax: (þ86) 431-88499140. E-mail:
[email protected] (G.L.);
[email protected] (M.J.). (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044– 4098. (2) L opez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A. J. Catal. 2007, 245, 381–391. (3) Wilson, K.; Hardacre, C.; Lee, A. F.; Montero, J. M.; Shellard, L. Green Chem. 2008, 10, 654–659. (4) Helwani, Z.; Othman, M. R.; Aziz, N.; Kim, J.; Fernando, W. J. N. Appl. Catal., A 2009, 363, 1–10. (5) Ma, F. R.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (6) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Appl. Catal., A 2005, 287, 183–190. (7) Babu, N. S.; Sree, R.; Prasad, P. S. S.; Lingaiah, N. Energy Fuels 2008, 22, 1965–1971. (8) Albuquerque, M. C. G.; Santamarı´ a-Gonzalez, J.; Merida-Robles, J. M.; Moreno-Tost, R.; Rodrı´ guez-Castell on, E.; Jimenez-L opez, A.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; Maireles-Torres, P. Appl. Catal., A 2008, 347, 162–168. (9) Yan, S. L.; Salley, S. O.; Simon Ng, K. Y. Appl. Catal., A 2009, 353, 203–212. r 2010 American Chemical Society
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(carbogenic molecular sieves)-supported Cs catalyst used for the isomerization of but-1-ene to but-2-ene.40 Particularly, Sun and co-workers reported that a kind of CaO/ carbon material, prepared by carbonizing a solidified mixture containing CaCO3, hexamethylenetetramine, and phenolic resin 217, was an active and reusable catalyst for the synthesis of dimethyl carbonate from methanol and propylene carbonate.41 In this work, a few kinds of porous carbon materials, including carbon molecular sieves (CMS), active carbon (AC), and nanoporous carbon (NC-2)42 as well as CMK-3, were chosen as supports for the preparation of supported CaO catalysts. The catalytic properties of these carbon-supported CaO materials were studied for the transesterification of triacetin with methanol, because triacetin is the simplest triglyceride and has often been used as a model compound for larger triglyceride molecules.2,16,43 Moreover, various characterization techniques were carried out to correlate the physicochemical properties of these carbon-supported CaO catalysts with their catalytic performances.
limited the utility of such a simple, cheap, and environmentally benign catalyst. In fact, leaching of active metal species or catalyst deterioration in the liquid-phase reaction is a particular problem for most solid-base catalysts.10,28 Hence, much recent effort has been devoted to develop novel CaO-based catalysts with high activity and stability, and some significant progresses on designing novel and efficient CaO-based catalysts have been made.29-35 For example, Kawashima et al.32 studied the transesterification properties of a variety of calcium-contained A-B-O-type oxide catalysts and found that CaZrO3 and CaO-CeO2 show relatively high activity and reusability. Ngamcharussrivichai et al.33 prepared a series of CaO-ZnO mixed oxides with different Ca/Zn ratios by the co-precipitation method. These catalysts can be reused several times after regeneration by calcination or treating with a mixture of methanol and NH4OH solution. Using various porous silica materials (SBA-15, MCM-41, and fumed silica) as catalyst supports, Albuquerque et al.34 prepared a few silica-supported CaO catalysts and found that SBA-15-supported CaO was active for the transesterification of ethyl butyrate with methanol. Very recently, Zabeti et al.35 reported that the supported CaO/Al2O3 catalyst with very high CaO loading showed good activity for the transesterification of palm oil with methanol. In both cases, no significant leaching of active species could be detected under their test conditions. These results provide a clear hint that the active phase of CaO species might be stabilized by choosing suitable catalyst supports. In general, silica and alumina are two of the most frequent catalyst supports. Additionally, carbon materials have also been widely used as catalyst supports in industry processes, because they are stable in acidic and basic media and offer the possibility of tailoring both their texture and surface chemistry according to the specific needs of the process.36-38 Recently, the rapid development of novel porous carbon materials bearing attractive characteristics (e.g., uniform mesoporous structure, high specific surface area, graphitic phase, etc.) provides new opportunity to prepare carbon-supported catalysts with desirable catalytic performance. For instance, a series of supported solid-base catalysts have been obtained using different porous carbon as supports for special application in catalysis, including mesoporous carbon (CMK-1)supported layered double hydroxides used for the ClaisenSchmidt condensation reaction39 and nanoporous carbon
2. Experimental Section 2.1. Catalyst Preparation. Chemicals such as carbon molecular sieves (denoted as CMS, from Dalian Haixin Chemical Industrial Co. Ltd.) and coal-based activated carbon (denoted as AC, from Ningxia Henghui Actived Carbon Co. Ltd.) were sourced commercially. Nanoporous carbon (denoted as NC-2) was prepared by direct carbonizing a composite containing citric acid and aluminum phosphate, as described previously.42 CMK-3 was prepared according to a literature procedure.44 The carbon-supported CaO catalysts were prepared by the wet impregnation method. Typically, a Ca-containing solution was obtained by dissolving Ca(NO3)2 in a certain amount of water. With this solution, a fully dried support was mixed and the resultant mixture was stirred for about 2 h at room temperature. Thereafter, water was evaporated away from the mixture at a temperature of 353 K under an atmosphere pressure. The resultant mixture was heat-treated at desired temperatures (1073 K, without special illumination) for 6 h under an argon flow. Catalysts were labeled as nCaO/NC-2 (CMS, AC, or CMK-3), where n is the weight percentage of supported calcium oxide. For comparison, SBA-15-supported CaO catalysts was prepared using the above procedure. 2.2. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer (40 kV and 30 mA) using Ni-filtered Cu KR radiation. N2-adsorption/desorption isotherms were measured at 77 K, using a Micromeritics ASAP 2010N analyzer. Samples were degassed at 523 K for 8 h before measurements. Specific surface areas were calculated using the BrunauerEmmett-Teller (BET) model. Pore volumes were estimated at a relative pressure of 0.94 (P/Po), assuming full surface saturation with nitrogen. Pore size distributions were evaluated from adsorption branches of nitrogen isotherms using the Barret-Joyner-Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VGESCALABMKII spectrophotometer using MgK radiation for X-ray excitation under pressures lower than 5 10-7 Pa. The electron binding energies were referenced to C1s (Eb = 284.6 eV)
(28) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. Catal. Today 2005, 106, 190–192. (29) Umdu, E. S.; Tuncer, M.; Seker, E. Bioresour. Technol. 2009, 100, 2828–2831. (30) Yan, S. L.; Kim, M.; Salley, S. O.; Simon Ng, K. Y. Appl. Catal., A 2009, 360, 163–170. (31) Albuquerque, M. C. G.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; Santamarı´ a-Gonz alez, J.; Merida-Robles, J. M.; Moreno-Tost, R.; Rodrı´ guez-Castell on, E.; Jimenez-L opez, A.; Maireles-Torres, P. J. Mol. Catal. A: Chem. 2009, 300, 19–24. (32) Kawashima, A.; Matsubara, K.; Honda, K. Bioresour. Technol. 2008, 99, 3439–3443. (33) Ngamcharussrivichai, C.; Totarat, P.; Bunyakiat, K. Appl. Catal., A 2008, 341, 77–85. (34) Albuquerque, M. C. G.; Jimenez-Urbistondo, I.; Santamarı´ aGonz alez, J.; Merida-Robles, J. M.; Moreno-Tost, R.; Rodrı´ guezCastell on, E.; Jimenez-L opez, A.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; Maireles-Torres, P. Appl. Catal., A 2008, 334, 35–43. (35) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Appl. Catal., A 2009, 366, 154–159. (36) Rodrı´ guze-Reinoso, F. Carbon 1998, 36, 159–175. (37) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schlogl, R.; Su, D. Science 2008, 322, 73–77. (38) Liu, G.; Liu, Y.; Zhang, X. Y.; Yuang, X. L.; Zhang, M.; Zhang, W. X.; Jia, M. J. J. Colloid Interface Sci. 2010, 342, 467–473. (39) Dubey, A. Green Chem. 2007, 9, 424–426.
(40) Stevens, M. G.; Foley, H. C. Chem. Commun. 1997, 519–520. (41) Wei, T.; Wang, M. H.; Wei, W.; Sun, Y. H.; Zhong, B. Green Chem. 2003, 5, 343–346. (42) Liu, G.; Liu, Y.; Wang, Z. L.; Liao, X. Z.; Wu, S. J.; Zhang, W. X.; Jia, M. J. Microporous Mesoporous Mater. 2008, 116, 439–444. (43) L opez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A.; Lotero, E. Appl. Catal., A 2005, 295, 97–105. (44) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712–10713.
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Figure 1. (A) Transesterification activities of various carbon-supported CaO catalysts. (B) Leaching experiments of various carbon-supported CaO catalysts by continuing the reaction after filtration of the catalysts at the reaction temperature. Dashed lines indicate the conversions after the removal of the catalysts. Reaction conditions: (A) catalyst concentration, 2.0 wt % (catalyst/triacetin); (B) catalyst concentration, 0.7 wt % (catalyst/triacetin); methanol/triacetin, 6:1 (mol ratio); catalyst heat-treated temperature, 1073 K; reaction temperature, 333 K.
achieved after 4 h of reaction on the CaO/SBA-15 catalyst (with 12 wt % loading of CaO), which is much lower than that of carbon-supported CaO catalysts. To ensure the heterogeneous nature of the catalysts, a hot filtration test (typically half way through every reaction in which the materials were tested) was performed (Figure 1B). For 12 wt % CaO/CMK-3, the transesterification reaction is continued at a very high rate after the solid catalyst is removed, which means that serious leaching of active species occurs during the reaction process. For 12 wt % CaO/CMS, it was found that triacetin can be converted at a low rate in the filtrate after filtering the catalyst, which means that a small part of the active species is leached from the support. For 12 wt % CaO/NC-2 and 12 wt % CaO/AC, there is almost no detectable subsequent conversion in the filtrate after removing the catalyst, which is evidence for real heterogeneous catalysis. Multiple transesterification reaction cycles were carried out to examine the recyclability of the CaO/NC-2 catalyst. In these experiments, the catalyst samples were recovered after a cycle by decanting the reaction solution and calcination at 1073 K for 1 h between cycles. It was found that the 12 wt % CaO/ NC-2 catalyst was able to largely maintain its overall catalytic activity for at least five consecutive reaction cycles, indicating that this catalyst possesses relatively high recyclability. 3.1.2. Effect of the CaO Loading on the Catalytic Performances of CaO/NC-2 Catalysts. To determine the optimum amount of CaO impregnation in NC-2, a series of CaO/NC-2 catalysts with different CaO loading were prepared and tested for the transesterification reaction (Figure 2). As shown in Figure 2A, the yield of methyl acetate increases gradually when the loading of CaO increases from 2 to 12 wt %. No obvious change in the methyl acetate yield could be observed after further increasing the CaO loading to 15 wt %. When comparing the specific activity by turnover frequency (TOF), it can be clearly seen that 12 wt % CaO/ NC-2 is the most active catalyst under test conditions (Figure 2B). These results suggest that the specific activity of CaO/NC-2 is dependent upon the CaO loading, which means that the nature of active sites (i.e., CaO species) are varied with the change of the loading of CaO. Under the test conditions, the 12 wt % loading of CaO is a suitable amount for achieving a high reaction rate. 3.1.3. Effect of the Catalyst Concentration. In this catalytic test, the concentration of 12 wt % CaO/NC-2 varied in the
peak. Temperature-programmed desorption (TPD) was carried out using CO2 as probe molecules. In a standard procedure, 50 mg of fresh sample was first calcined at 1073 K under Ar stream for 60 min and then cooled to 323 K. Carbon dioxide (99.99%) was injected into the stream until saturation was reached, and the system was maintained at 323 K for 30 min. After the system was purged with flowing Ar for 1 h at 323 K, the sample was heated at a rate of 10 K min-1 in He (30 mL/min), and the concentration change of the desorbed CO2 was monitored using an online thermal conductivity detector (TCD). 2.3. Transesterification. The catalytic performances of carbonsupported CaO catalysts were evaluated in the transesterification of triacetin with methanol. The transesterification reaction was performed in a three-neck round-bottom flask with a reflux condenser, controlled temperature, and inert atmosphere (N2). Typically, the 50 mL three-neck round-bottom flask was filled with approximately 30 mL of the reagent solution, which contained methanol and triacetin in a 6:1 molar ratio. The reactants were then heated to the desired temperature, and a certain amount of solid catalyst was added shortly after the reactor temperature reached steady state, to initiate the reaction. Sample aliquots were withdrawn periodically from the reactor and analyzed in a gas chromatograph (BEIFEN GC model 3420A) equipped with a flame ionization detector (FID) and a capillary Carbowax 20000 column. Toluene and 2-propanol were used as an internal standard and a solvent [for gas chromatography (GC) analysis], respectively.
3. Results and Discussion 3.1. Catalytic Performance. 3.1.1. Transesterification of Different Carbon-Supported CaO Catalysts and the Test of Stability. To test the effect of the carbon support on the activity of the catalyst, four kinds of carbon-supported CaO catalysts were prepared using the following carbon materials as supports, including CMK-3, NC-2, AC, and CMS. The catalytic properties of the four catalysts (12 wt % CaO/ CMK-3, 12 wt % CaO/NC-2, 12 wt % CaO/AC, and 12 wt % CaO/CMS) are shown in Figure 1A. It can be seen that all of the catalysts are active for the transesterification reaction. A blank transesterification reaction (without the addition of catalyst) was also carried out, and no detectable yield of methyl acetate could be observed under the identical reaction conditions. These results suggest that the resultant high activity should be mainly assigned to the catalytic role of the carbon-supported CaO materials. For comparison, the catalytic property of SBA-15-supported CaO (CaO/SBA-15) was also investigated. A 10% yield of methyl acetate is 3812
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Figure 2. Effect of the CaO loading on transesterification activities of CaO/NC-2 catalysts. Reaction conditions: catalyst concentration, 2.0 wt % (catalyst/triacetin); methanol/triacetin, 6:1 (mol ratio); reaction temperature, 333 K; catalyst heat-treated temperature, 1073 K; reaction time, 10 min. TOF was determined after 10 min of reaction and expressed in mmol of triacetin (mmol of CaO)-1 h-1.
Figure 3. Effect of the catalyst concentration on the transesterification activity of the 12 wt % CaO/NC-2 catalyst. Reaction conditions: methanol/triacetin, 6:1 (mol ratio); reaction temperature, 333 K; catalyst heat-treated temperature, 1073 K; reaction time, 10 min.
Figure 4. Effect of the heat-treated temperature of the 12 wt % CaO/ NC-2 catalyst on the transesterification activities. Reaction conditions: catalyst concentration, 2.0 wt % (catalyst/triacetin); methanol/ triacetin, 6:1 (mol ratio); reaction temperature, 333 K.
range between 0.5 and 3.0 wt % referred to the starting triacetin weight, the relationship between the yield of methyl acetate, and the concentration of catalyst, displayed in Figure 3. Clearly, increasing the concentration of the catalyst from 0.5 to 2.0 wt % could result in a progressive increase in the yield of methyl acetate, and an almost proportional dependence is observed. This indicates that no mass transfer effects influence the reaction of triacetin under present conditions. However, a high catalyst concentration (3.0 wt %) does not lead to an obvious increase in the yield of methyl acetate. This should be due to the fact that a relatively high catalyst concentration leads to a more viscous reaction mixture, which increases the mass-transfer resistance in the liquid-liquid-solid system.30 It should be mentioned here that the fitted straight line shown in Figure 3 does not pass through the origin, which might be due to the fact that the CaO species need a period to become converted into more active forms [i.e., Ca(OCH3)2 or the calcium-glycerin complex], as reported by related literature.22 3.1.4. Effect of the Heat-Treated Temperature on the Catalytic Properties of CaO/NC-2. Figure 4 shows the transesterification activities of 12 wt % CaO/NC-2 catalysts heat-treated at different temperatures. It can be seen that the catalyst treated at 973 K exhibits the lowest methyl acetate yield and the methyl acetate yield increases obviously when the heat-treating temperature increases to 1073 K. With further increasing the treating temperature to 1173 K,
a slight increment in methyl acetate yield could also be observed. Obviously, the heat-treated temperature is also an important factor upon influencing the catalytic property of the carbon-supported CaO catalyst. In the present case, 1073 K is considered as the optimum temperature for the transesterification reaction. 3.1.5. Effect of the Reaction Temperature. Effect of the reaction temperature on the catalytic property of the 12 wt % CaO/NC-2 catalyst was studied over a range of 290-333 K, and the yield of methyl acetate as a function of the temperature is shown in Figure 5. The results show that an increase in the temperature accelerates the reaction, favoring the methyl acetate formation. Thus, a reaction temperature of 333 K is considered as the optimum temperature for the transesterification of triacetin. Previously, the transesterification has been generally assumed to be a pseudo-first-order reaction in the presence of excess methanol.30 When the reaction is considered to follow first-order kinetics, a plot of -ln[1 - yieldmethyl acetate] as a function of time will be linear, with a slope equal to the reaction rate constant k.45 From Figure 6A, the plots are represented by straight lines, validating the first-order reaction model. The slopes of the lines provide the activation energy (E) using the Arrhenius equation. The activation (45) Shamshuddin, S. Z. M.; Nagaraju, N. J. Mol. Catal. A: Chem. 2007, 273, 55–63.
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energy calculated from the slope of the Arrhenius plot in Figure 6B is 44.9 kJ/mol, which is comparable to the reported values in related literature,46 and the corresponding pre-exponential factor is 2.8 105 min-1. 3.2. Characterization and Discussion. To understand the nature of the active sites on the carbon-supported CaO catalysts, some characterization means, including XRD, N2 adsorption, CO2-TPD, and XPS, were carried out. Figure 7 shows the XRD patterns of various carbonsupported CaO catalysts calcined at 1073 K and the corresponding carbon supports. The two broad diffraction peaks centered at 26° and 44° can be generally indexed to graphitic carbon (002) and (10) diffraction for the carbon supports.47 As for the carbon-supported CaO catalysts with 12 wt % CaO loading, several additional diffraction peaks centered at 32°, 37°, 54°, 64°, and 67° are observed, which can be generally indexed to (111), (200), (220), (311), and (222) diffraction of cubic CaO.29 The intensity of these diffraction peaks is not very strong, implying that CaO should exist in a well-dispersed state. Figure 8A shows the XRD patterns of CaO/NC-2 with different CaO loading (calcined at 1073 K). It can be seen that no crystalline phase of metal oxide appears until the loading of CaO reaches 8 wt %, and the peak intensities of the crystalline metal oxide, which can be assigned to the cubic CaO crystalline phase, increase with further enhancement of the CaO loading. These results should be ascribed to the value of CaO loading, which has exceeded the amount of the monolayer dispersed threshold. Figure 8B gives the XRD patterns of 12 wt % CaO/NC-2 treated at different temperatures. It is found that the crystalline level of CaO is strongly dependent upon the calcination temperature. No crystalline phase of CaO can be observed when calcination was performed at 973 K. When the calcination temperature rose to 1073 K, characteristic diffraction peaks of cubic CaO appear and the mean grain size of CaO calculated by Scherrer equation was 29.9 nm. Previously, it was reported that a thermal treatment at 1073 K is necessary to transform CaCO3 into CaO.34 In our case, the thermal treatment at such a high temperature (1073 K) is also required to obtain
active carbon-supported CaO catalysts with a well-dispersed crystalline CaO phase. The textural characteristics of the support (NC-2) and 12 wt % CaO/NC-2 have been evaluated from the corresponding N2 adsorption-desorption isotherms (Figure 9). Both samples exhibit type-IV curves with a H2-type hysteresis loop, indicating the presence of an “ink-bottle”-type pore.48 In comparison to NC-2, the amount of adsorbed N2 decreased considerably on 12 wt % CaO/NC-2, which implied the obstruction of some primary pores by the introduction CaO particles. As shown in Table 1, both the specific surface area and the pore volume of 12 wt % CaO/NC-2 decreased significantly, which should also originate from the introduction of CaO on the carbon material. The surface basic properties of the CaO/NC-2 catalysts were probed by TPD of CO2 (Figure 10). For comparison, the basic property of pure CaO was also investigated (Figure 10d). On the basis of related literature, the desorption peak at about 373-423 K could be attributed to the interaction of CO2 with weak basic sites present in the catalyst and the peak appearing at 573-773 K should be due to the interaction of CO2 with moderate basic sites of the catalyst, while the peak beyond 823 K could be due to the interaction of CO2 with strong basic sites.7 Clearly, pure CaO exhibits an intensive desorption band at 910 K, indicating the presence of strong basic sites. These basic sites should be associated with Ca2þ-O2- pairs, for which the basic strength is very high. As for the 12 wt % CaO/NC-2 catalyst calcined at a relatively low temperature (i.e., 973 K), only weak basic sites can be detected (Figure 10a), while increasing the calcination temperature to 1073 K could result in the formation of strong basic sites. The existence of strong basic sites on the high-temperature caclined sample may suggest that a great number of Ca2þ-O2- pairs are present on the surface of the catalyst (12 wt % CaO/NC-2). Notably, the sample with 5 wt % loading of CaO, which is calcined at 1073 K, shows quite a weak basic characteristic (Figure 10b). This might be due to the existence of a strong interaction between CaO and the surface functional groups of the carbon support, thus considerably decreasing the basic intensity of CaO/NC-2 containing relatively low CaO loading (i.e., below 5 wt %). The XPS data of the O element on the surface of CaO/NC2 catalysts (calcined at 1073 K) and pure CaO are shown in Figure 11. All of the CaO/NC-2 catalysts possess broad O 1s spectra in the range of 531.6-531.0 eV. The O 1s for CaO in Figure 11c exhibited a single state at 530.9 eV, which is characteristic of O2-.6 It is clearly seen that the binding energies of O in 12 wt % CaO/NC-2 calcined at 1073 K are similar to pure CaO but lower than that of 5 wt % CaO/ NC-2. It is known that the negative shift in the O 1s binding energy indicates a higher effective negative charge on surface oxygen atoms. This leads to an increase in the electrondonating ability of the surface oxygen atom. The increased electron pair donating ability is supposed to originate from the formation of strong basic sites, which means that the surface O2- ions are more basic.7 The results obtained from XPS measurements confirm further the presence of strong basic sites on the surface of 12 wt % CaO/NC-2, which is in good agreement with the results of CO2-TPD. According to
(46) Zieba, A.; Pacula, A.; Drelinkiewicz, A. Energy Fuels 2010, 24, 634–645. (47) Lu, A. H.; Li, W. C.; Salabas, E. L.; Spliethoff, B.; Sch€ uth, F. Chem. Mater. 2006, 18, 2086–2094.
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Figure 5. Effect of the reaction temperatures on the transesterification activities of the 12 wt % CaO/NC-2 catalyst. Reaction conditions: catalyst concentration, 1.5 wt % (catalyst/triacetin); methanol/triacetin, 6:1 (mol ratio); catalyst heat-treated temperature, 1073 K.
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Figure 6. (A) Plots of -ln(1 - yieldmethyl acetate) versus time at different temperatures. (B) Arrhenius plot of ln k versus 103/T for the reaction of triacetin with methanol. Reaction conditions: catalyst concentration, 1.5 wt % (catalyst/triacetin); methanol/triacetin, 6:1 (mol ratio); catalyst heat-treated temperature, 1073 K.
Figure 7. XRD patterns of (A) the various carbon-supported CaO catalysts calcined at 1073 K and (B) the corresponding carbon supports.
Figure 8. XRD patterns of (A) CaO/NC-2 catalysts with different CaO loading calcined at 1073 K and (B) 12 wt % CaO/NC-2 heat treated at different temperatures.
XRD results, it was known that well-dispersed crystalline CaO species are present in 12 wt % CaO/NC-2 and these species should functionalize as strong basic centers. Therefore, the relatively high catalytic activity of the carbonsupported CaO catalyst (e.g., 12 wt % CaO/NC-2) could be mainly assigned to the presence of a large number of strong basic sites. On the contrary, the results of XPS and CO2-TPD confirm that CaO/NC-2 samples with low CaO loading (i.e., 5 wt %) or calcined at low temperature (i.e., 973 K) show a relatively weak basic characteristic, which can be correlated well with their relatively low transesterification activities. It is well-known that the presence of functionalities on the surface of carbon supports may directly affect the catalytic
behavior of the active phase. For the sample of CMK-3, it was known that this material possesses a relatively inert surface, and only a trace amount of the oxygen-containing functional groups is present on the surface of this carbon material. As for the NC-2 support, our previous work has suggested that there are abundant oxygen-containing functional groups on the surface of this material.38 We suppose that these surface functionalities may act as anchoring centers for the active phase, thus resulting in the formation of a stable linkage between CaO species and the NC-2 support. Besides, other parameters of the NC-2 support, including the uniform mesoporous structure, high specific surface area, as well as the special electrostatic interaction, should also play a crucial role in the formation of highly 3815
Energy Fuels 2010, 24, 3810–3816
: DOI:10.1021/ef100419m
Zu et al.
Figure 11. O 1s XPS spectra for (a) 5 wt % CaO/NC-2, (b) 12 wt % CaO/NC-2, and (c) pure CaO.
On the basis of the above results, we may propose here that there are at least two types of “CaO” phases in the active and stable carbon-supported CaO catalysts. The type-I phase is less stacked, containing some linkages, with the carbon support related to the strong interaction between CaO and surface oxygen-containing functional groups of carbon supports, thus having weaker basicity and lower activity. The type-II phase is highly stacked, exhibiting a relatively weak interaction with supports, thus showing stronger basicity and higher activity. Both types of CaO species could be regarded as a whole unit, such as a small CaO cluster or particle, which is highly dispersed on the surface of the carbon support.
Figure 9. N2 adsorption-desorption isotherms and BJH pore size distribution (in set) of (a) NC-2 and (b) 12 wt % CaO/NC-2. Table 1. Texture Properties for NC-2 and 12 wt % CaO/NC-2 sample
SBET (m2/g)
pore volume (cm3/g)
pore sizea (nm)
NC-2 12 wt % CaO/NC-2
1517 596
0.93 0.56
3.2 4.0
a Average pore diameters calculated from adsorption branchs using the BJH model.
4. Conclusions Carbon-supported CaO materials prepared by the impregnation method are active solid-base catalysts for the transesterification of triacetin with methanol. Particularly, the catalyst prepared using NC-2 as a support exhibits very high activity, stability, and recyclability. The highly dispersed CaO particles should be the main active sites for the reaction. The presence of a suitable interaction between CaO particles and carbon supports is a key factor for the formation of active and stable catalysts. Figure 10. CO2-TPD profiles of (a) 12 wt % CaO/NC-2 calcined at 973 K, (b) 5 wt % CaO/NC-2 calcined at 1073 K, (c) 12 wt % CaO/ NC-2 calcined at 1073 K, and (d) pure CaO.
Acknowledgment. This work is supported by the Program for New Century Excellent Talents in University of Education Ministry, the Specialized Research Fund for the Doctoral Program of Higher Education (20090061120024), and the National Natural Science Foundation of China (20403006 and 20773050).
dispersed CaO species with relatively strong basicity, which can be easily accessible for the activation of reagents.
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