Energy & Fuels 2009, 23, 19–24
19
Activity and Stability of Perovskite-Type Oxide LaCoO3 Catalyst in Lignin Catalytic Wet Oxidation to Aromatic Aldehydes Process Haibo Deng,† Lu Lin,*,† Yong Sun,† Chunsheng Pang,† Junping Zhuang,† Pingkai Ouyang,‡ Jingjiang Li,‡ and Shijie Liu*,†,§ State Key Laboratory of Pulp and Paper Engineering, South China UniVersity of Technology, Guangzhou 510640, Guangdong ProVince, China, College of Medicine and Life Science, Nanjing UniVersity of Technology, Nanjing 210009, Jiangsu ProVince, China, and Department of Paper and Bioprocess Engineering, College of EnVironmental Science and Forestry, State UniVersity of New York, 1 Forestry DriVe, Syracuse, New York 13210 ReceiVed July 4, 2008. ReVised Manuscript ReceiVed October 29, 2008
The perovskite-type oxide catalyst LaCoO3, prepared by the sol-gel method, was tested for catalytic wet oxidation (CWAO) of lignin to aromatic aldehydes. The lignin conversion and yield of each aromatic aldehyde were significantly enhanced by the catalytic process, compared to the noncatalytic process. A mechanism involving the reaction of lignin molecules with adsorbed oxygen surface sites, Co(surf)3+O2-, was proposed on the basis of experimental observations, yielding the cycle of Co(surf)3+ f Co(surf)2+ f Co(surf)3+O2- f Co(surf)3+. The formation rates of the intermediates quinonemethide and hydroperoxide were the rate-determining steps. The activity and perovskite-type structure of the LaCoO3 catalyst did not undergo any obvious changes after the five successive reactions.
1. Introduction Because petroleum reserves are gradually depleting and exploitation costs are increasing rapidly, the exploration of feasible pathways to transform abundant and renewable biomass into clean fuel to supplement or gradually replace petroleumbased chemicals and/or energy is highly desirable.1,2 Lignocellulosic biomass is an important component of bioresources and consists of three main components: cellulose, hemicelluloses, and lignin. Cellulose and hemicelluloses are ideal substrates for biochemical conversion to bioethanol. The biggest barrier to biorefinery is how to economically and environmentally convert the whole biomass into biofuels and/or biochemicals.3 One way to add economic value to the biomass conversion process is to transform waste biomass components, such as lignin, into fine chemicals. Lignin is an extremely complex three-dimensional polymer with an irregular structure, a result of random dehydrogenated polymerization of phenyl propane building units (coniferylic, sinapylic, and p-coumarylic alcohols) in the presence of peroxidase enzymes.4 Aromatic aldehydes, such as vanillin, syringaldehyde, and p-hydroxybenzaldehyde, can be * To whom correspondence should be addressed. Telephone: +86-208711-1139. Fax: +86-20-2223-6078. E-mail:
[email protected] (L.L.); Telephone: (1) 315-470-6885. Fax: (1) 315-470-6945. E-mail:
[email protected] (S.L.). † South China University of Technology. ‡ Nanjing University of Technology. § State University of New York. (1) Amidon, T. E.; Wood, C. D.; Shupe, A. M.; Wang, Y.; Graves, M.; Liu, S. Biorefinery: Conversion of woody biomass to chemicals, energy and materials. J. Biobased Mater. Bioenergy 2008, 2, 100. (2) Liu, S.; Amidon, T. E.; Francis, R. C.; Ramarao, B. V.; Lai, Y.-Z.; Scott, G. M. From forest biomass to chemicals and energy: Biorefinery initiative in New York. Ind. Biotechnol. 2006, 2, 113. (3) Eckert, C.; Liotta, C.; Ragauskas, A.; Hallett, J.; Kitchens, C.; Hillac, E.; Draucker, L. Tunable solvents for fine chemicals from the biorefinery. Green Chem. 2007, 9, 545. (4) Gaspar, A.; Evtuguin, D. V.; Pascoal, N. C. Appl. Catal., A 2003, 239, 157.
obtained from the catalyzed alkaline wet oxidation (CWAO) process of lignin and have wide applications, such as flavoring, chemical intermediaries for pharmaceutical drugs, and agricultural defensives.5 A catalyst may be used to increase the yield of aldehydes in the oxidation process; such catalysts are mainly composed of noble metals6-8 and transition metals.9-12 However, noble metals are expensive, greatly affecting the economics of commercial applications. Transition-metal catalysts are usually homogeneous ions of transition metals, and cupric sulfates are being investigated widely because of their effectiveness.10-12 However, homogeneous cupric sulfate catalysts can cause secondary pollution, resulting in high recycling costs, thus tremendously restricting their use in industry. The search for effective environmentally benign and heterogeneous catalysts that can be reused and recycled is therefore of great significance to the conversion of lignin to high-value chemicals. Perovskite-type oxides are being actively studied for catalytic hydrocarbon oxidations because of their high activity and thermal stability in the oxidation process, making them promising substitutes for noble metals.13 Perovskite-type oxides have (5) Sridhar, P.; Araujo, J. D. A.; Rodrigues, E. Catal. Today 2005, 105, 574. (6) Sales, F. G.; Abreu, C. A. M.; Pereira, J. A. F. R. Braz. J. Chem. Eng. 2004, 21, 211. (7) Sales, F. G.; Maranha˜o, L. C. A.; Filho, N. M. L.; Abreu, C. A. M. Ind. Eng. Chem. Res. 2006, 45, 6627. (8) Sales, F. G.; Maranha˜o, L. C. A.; Filho, N. M. L.; Abreu, C. A. M. Chem. Eng. Sci. 2007, 62, 5386. (9) Mathias, A. L.; Rodrigues, A. E. Holzforschung 1995, 49, 273. (10) Wu, G. X.; Heitz, M.; Chornet, E. Ind. Eng. Chem. Res. 1994, 33, 718. (11) Wu, G. X.; Heitz, M. J. Wood Chem. Technol. 1995, 15, 189. (12) Tarabanko, V. E.; Petukhov, D. V.; Selyutin, G. E. Kinet. Catal. 2004, 45, 603. (13) Seiyama, T. In Properties and Applications of PeroVskite-Type Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker: New York, 1993; p 215.
10.1021/ef8005349 CCC: $40.75 2009 American Chemical Society Published on Web 12/05/2008
20
Energy & Fuels, Vol. 23, 2009
the general formula of ABO3, where A cations are rare earth, alkaline earth, alkali, or large ions and B cations are transition metals. However, to our knowledge, the activity of perovskitetype oxides in wet air oxidation reactions has rarely been studied. Yang et al.14 conducted the CWAO of phenol, benzoic acid, salicylic acid, and sulfonic salicylic acid and found high activity and high stability in LaFeO3 catalysts, especially for the CWAO of salicylic acid. Royer et al.15 studied the CWAO of stearic acid with La1-xA′xBO3 (A′ ) Sr, Ce; B ) Co, Mn) as catalysts and claimed that the LaCoO3 sample presented the highest activity for this particular reaction, while pure and substituted LaMnO3 samples showed largely lower activity despite similar specific surface areas. Royer et al.15 ascribed this phenomenon to the accessibility of low-temperature active oxygen sites on the surface of the LaCoO3 catalyst in the process, while claiming that LaFeO3 possesses very high activities to salicylic acid and sulfonic salicylic acid because of their identical intramolecular hydrogen-bonding structures, in contrast with its very poor activities to phenol and benzoic acid. The objectives of this study are to investigate the activity and stability of LaCoO3 as a catalyst in the CWAO of lignin to aromatic aldehydes. 2. Experimental Section 2.1. Lignin Preparation. Steam-exploded lignin from cornstalk was prepared according to ref 16. The cornstalk used as the source of lignin was mechanically reduced in pieces of about 1 cm in length and added with water to an initial content of 50%. Steam explosion runs were carried out in a 4 L apparatus, loading about 0.5 kg of material in each cycle. Treatment conditions were 220 °C and 2 min and then discharged into a receiving vessel. The discharged fibers were enzyme-hydrolyzed and washed twice with hot water under the following conditions: fiber consistency, 5-6%; temperature, 75-100 °C; time, 30 min. The resultant products were further exposed to enzymatic hydrolysis with cellulases purchased from Novezyme Co. During enzymatic hydrolysis, the pH value was kept at 3.5, the temperature was 45 °C, and the reaction time was 48 h, with a buffer solution of 0.05 mol/L dibastic sodium phosphate and disodium hydrogen phosphate. The resulted lignin was dried in vacuum under 50 °C, before being stored in a desiccator with P2O5 as a drying agent. Further extraction and purification was conducted with 1.5% NaOH solution (250 mL) at 90 °C in 15 min 2 times. Lignin was precipitated with 20% HCl at pH of 2 when the solution was still warm, dried in vacuum under 50 °C, and then kept in a desiccator for the CWAO procedure. 2.2. Catalyst Preparation. LaCoO3 was prepared according to the sol-gel method described in refs 17 and 18. La(NO3)3 · 6H2O, Co(NO3)3 · 6H2O, and citric acid monohydrate were used as starting materials. Aqueous solutions with a cation ratio (La/Co) of 1:1 were prepared, and citric acid was added in a molar ratio (citric acid/ metal ion) of 1:1.5. The resulting solution was maintained at 100 °C for several hours until dry. The resulting materials were then calcined in air at a temperature of 800 °C for 6 h, with the heating rate of 3 °C /min to yield the desired perovskite structures. 2.3. Catalyst Characterization. Micromeritics ASAP 2010 equipment was used to measure the N2 adsorption isotherms of the (14) Yang, M.; Xu, A. H.; Du, H. Z.; Sun, C. L.; Li, C. J. Hazard. Mater., B 2008, 139, 86. (15) Royer, S.; Levasseur, B.; Alamdari, H.; Barbier, J., Jr.; Duprez, D.; Kaliaguine, S. Appl. Catal., B 2008, 80, 51. (16) Bentivengaa, G.; Boninib, C.; Auriab, M. D.; Bonaa, A. D. Biomass Bioenergy 2003, 24, 233. (17) Makshina, E. V.; Sirotin, S. V.; Berg, M. W. E.; Klementiev, K. V.; Yushchenko, G. N.; Mazo, V. V.; Gru¨nert, W.; Romanovsky, B. V. Appl. Catal., A 2006, 312, 59. (18) Zaˇbkova´, M.; Borges, E. A. S.; Rodrigues, A. E. J. Membr. Sci. 2007, 301, 221. (19) Nguyen, T. T.; Houshang, A.; Serge, K. J. Solid State Chem. 2008, 181, 2006.
Deng et al. samples at liquid N2 temperature (77 K). The specific surface area was determined by the multilayer Brunauer-Emmett-Teller (BET) equation. X-ray diffraction (XRD) measurements were performed on a Rigaku powder diffractometer (XD-3A, Shimadzu, Japan) with Cu Ka radiation. The selected 2θ range was 20-80°, scanning at a step of 0.02°. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra system with 0.1 eV per step for detail scan, and the binding energies for each spectrum were calibrated with a C 1s spectrum of 284.6 eV. The core levels of Co 2p and O 1s species were recorded, and their relative intensities were determined on the basis of the binding energies. 2.4. Activity Test. The CWAO reaction was carried out in a 1 L SS-316 Parr stainless-steel autoclave reactor at 120 °C, with an accuracy of 1 °C, according the procedure described in ref 7 for heterogeneous CWAO of lignin. In a typical experiment, the 500 mL alkaline solutions (NaOH, 2 mol/L) of lignin dissolved at a concentration of 60 g/L were introduced into the reactor and a catalyst of concentration of 5.00 wt % (on lignin) was added to the solution if necessary. Nitrogen at a cold pressure of 10 bar was introduced to the reactor to remove the residual air. The stirrer was set at 600 rpm. The nitrogen was then discharged to atmosphere conditions. This process was repeated 3 times. The heating program was started under a slight nitrogen pressure (1 bar), with the heating rate of 4 °C/min. When the solution in the reactor reached the desired temperature, nitrogen was added until a total pressure of 15 bar was attained. The time was recorded from zero, corresponding to the admission of oxygen, to a total pressure of 20 bar. The total pressure in the reactor was kept at 20 bar by continuous flushing of oxygen for supplement because of its consumption during the reaction. During reactions, sampling was conducted from the reactor to detect changes of reactant lignin and any aromatic aldehydes produced. After filtration, the liquid samples, collected at different time intervals, were acidified to a pH of 2-3 with an HCl solution. Acidification led to the precipitation of high-molecular-weight components, including aromatic aldehydes and lignin that was not converted. The resulting products were extracted with chloroform until the chloroform layer appeared colorless. The residual lignin was obtained by centrifugation of the suspensions after the extraction of resulting products. A small amount of NaHCO3 solid was added to the chloroform solution containing the aromatic aldehydes to neutralize residual acid carryover from acidification. Anhydrous Na2SO4 was also added at the ratio of 0.1 g/mL to the chloroform solution to adsorb the residual water in the solution. After filtration, a maroon semisolid was obtained from the chloroform solution and the solvent was removed under reduced pressure. The semisolid, containing vanillin, syringaldehyde, and p-hydroxybenzaldehyde, was dissolved in acetonitrile and diluted to a known volume. Quantitative analysis of each aldehyde of the acetonitrile solution was conducted using a high-performance liquid chromatograph equipped with an ODS2 column (250 × 4.6 mm) and a UV detector at 280 nm using known standards. A mixture of acetonitrile (10%), deionized water (90%), and acetic acid (1.5%) was used as the mobile-phase liquid. The residual lignin was diluted in 2 mol/L NaOH to dissolve the lignin. The amount of lignin was measured by an UV spectrophotometer at a wavelength of 280 nm.20 The conversion of lignin (L) is defined as
L ) (C0 - Ct)/C0 where C0 is the initial concentration of lignin and Ct is the concentration of lignin (i.e., nonconverted lignin) at any reaction time.
3. Results and Discussion 3.1. Surface Oxygen Species and Ion Oxidation States. Figure 1 illustrates the O 1s and Co 2p XPS spectra of the catalyst. As can be seen from the O 1s spectra (Figure 1a), there
PeroVskite-Type Oxide LaCoO3 Catalyst
Energy & Fuels, Vol. 23, 2009 21
Figure 1. (a) O 1s and (b) Co 2p XPS spectra of LaCoO3.
Figure 2. (A) Variation of lignin conversion and yields of (B) p-hydroxybenzaldehyde, (C) vanillin, and (D) syringaldehyde with reaction time. The reaction conditions are as follows: NaOH (2 mol/L), 120 °C, 5 bar partial pressure of oxygen in 20 bar total pressure, CL0 ) 60.00 kg/m3, in comparison to the noncatalyzed process (9), 5% (w/w, on lignin) LaCl3 (2), CoSO4 (b), and LaCoO3 (O) as catalysts in the catalytic process, and the lignin conversion ) (C0 - Ct)/C0, where C0 is the initial concentration of lignin and Ct is the concentration of lignin (i.e., nonconverted lignin) at any reaction time.
were two signals at binding energies (BE) of 528.7 and 531.0 eV, corresponding to surface lattice oxygen (metal oxygen bond) and adsorbed oxygen species (such as O-, O2-, or O22-), respectively.21,22 From Figure 1b, one can observe two signals at BE ) 780.0 and 795.0 eV, with the former belonging to Co 2p3/2 and the latter belonging to Co 2p1/2.23 The asymmetric main peaks at BE ) 779.9 and 795.1 eV imply the presence of multicomponents that could be resolved into two components, assignable to the signals because of Co3+ and Co2+ ions, respectively.24 Therefore, there were Co3+ and Co2+ ions present in the catalysts. 3.2. Catalytic Activity. The aromatic aldehydes produced by the oxidation of the lignin CWAO process can be further oxidized to aromatic acids, even degraded products with low molecular weights and carbon dioxide. The catalytic activity was thus evaluated by lignin conversion and aldehyde yield. (20) Lin, H.; Mahbod, B.; Serge, K. Appl. Surf. Sci. 2005, 243, 360. (21) Tejuca, L. G.; Fierro, J. L. G.; Tasco´n, J. M. D. AdV. Catal. 1989, 36, 329. (22) Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.; Muzychuk, R. Appl. Catal., A 2001, 209, 345. (23) Fierro, G.; Lo Jacono, M.; Inversi, M.; Dragone, R.; Porta, P. Top. Catal. 2000, 10, 39.
Lignin conversion and the yield of each aromatic aldehyde were significantly enhanced in the catalytic process (Figure 2). In the presence of the catalyst, the lignin conversion after 3.0 h of reaction increased 46.7% (Figure 2A). The maximum yield of p-hydroxybenzaldehyde was 2.23% after 120 min of the LaCoO3 catalytic process, which is 1.44 times the maximum yield of the noncatalytic process, 1.55% in 150 min (Figure 2B). The maximum yields of vanillin and syringaldehyde in the LaCoO3 catalytic process were 4.55% (60 min) and 9.99% (50 min), respectively, and were 1.41 and 2.11 times those obtained in the noncatalytic process, which were 3.22% (at 60 min) and 4.74% (at 30 min) (parts C and D of Figure 2). In all cases, the maximum concentrations of aldehyde were attained at different times (parts C and D of Figure 2), and the following time order of the maximum aldehyde attained was found: syringaldehyde > vanillin > p-hydroxybenzaldehyde. Lignin is probably depolymerized during oxidation, forming aldehydes, acids, and other products of low molecular weights. At the same time, aromatic aldehydes: vanillin (V), syringaldehyde (S), and p-hydroxybenzaldehyde (P) were prone to subsequent oxidations, producing other substances, such as organic acids. These products can also be degraded to carbon
22
Energy & Fuels, Vol. 23, 2009
Deng et al.
Figure 3. Proposed mechanism of the oxidation of lignin in the (A) noncatalytic process, (B) Co2+ catalyst process, and (C) LaCoO3 catalyst process.
dioxide. In other words, the quantity of aldehydes accumulated during the reaction was balanced with the formation and consumption of the aldehydes in the catalytic process. Hydroxymethyls on the benzene ring were also susceptible to further oxidation;7,8 thus, syringaldehyde produced from the CWAO of lignin was more reactive than p-hydroxybenzaldehydes. There were no hydroxymethyl groups on the molecule of the latter. At the same time, an interesting phenomenon was found: the residence time at maximum yield of aldehydes was longer in the CWAO process than in the process without LaCoO3,
although the maximum yield in the former process was larger than that in the latter process (Figure 2). A possible reason is that the catalyst LaCoO3 could lead to faster oxidation of aldehydes during its promotion of depolymerization of lignin to produce aldehydes, leading to a higher yield but delay in the residence time to achieve a maximum yield of aldehydes. 3.3. Possible Reaction Mechanism. As reported in ref 14, two aspects could account for the activity of ABO3 perovskite oxides in the CWAO: redox couples of the ions and active sites (24) Gierer, J.; Nilvebrant, N. O. Holzforschung 1986, 40, 107.
PeroVskite-Type Oxide LaCoO3 Catalyst
Figure 4. Lignin conversion and yields of aromatic aldehydes when no catalyst was added (0) and when catalyst was reused five successive times for 50 min. All reaction conditions were the same as those in Figure 2.
on the surface, such as La3+ and Co2+ in LaCoO3. It has been found that the concentration of La3+ in solution equivalent to that of the noncatalytic process did not show identical activity under these conditions, whereas an equivalent concentration of Co2+ showed uniformly higher activity under these conditions compared to the noncatalyzed process (parts A-D of Figure 2). Thus, only the Co3+/Co2+ redox couple was active for the lignin oxidation reaction; no redox reaction could take place for the inert lanthanum, because it has only one stable valence, the +3 state. Nevertheless, the LaCoO3 was more active in the oxidation reaction than Co2+ in the catalytic process under the same conditions (parts A-D of Figure 2); their mechanisms are therefore quite different. According to the reaction mechanisms established by other researchers,24-26 the oxidation of lignin under wet alkaline conditions occurs through a pathway of free radicals, including a procedure with two consecutive one-electron oxidations as suggested in Figure 3A. This mechanism begins with the dehydration of lignin structural units I, followed by the abstraction of an electron from the phenolate anion II, yielding a quinine methide radical III T IIIa. The radical IIIa can be attacked by O2-, resulting in the formation of quinonemethide hydroperoxide IV, which can be rapidly converted to dioxethane V. The dioxethane V degrades by the synchronous cleavage of C-C and C-O bonds to form aromatic aldehyde ions VI. In the oxidation mechanism, quinonemethide intermediates III and IIIa and quinonemethide hydroperoxide IV are the key intermediates and rate-determining steps. To explain the high homogeneous activity of Co2+ in the CWAO process of lignin, a probable mechanism is presented in Figure 3B, with a mechanism of Cu2+ in the CWAO of lignin similar to that proposed in ref 11. In the process with Co2+ as the catalyst, the improvement of lignin conversion and aldehyde yield can be attributed to the Co3+/Co2+ redox turnover, which can increase the formation rate of intermediate IIIa, resulting in an increase in the overall rate of lignin oxidation. A high activity of the LaCoO3 catalyst in the CWAO process of stearic acid was found in ref 15. It was asserted that the reaction mechanism was related to the adsorbed oxygen surface active sites (Co(surf)3+O2-), resulting from the oxygen dissolved in the solution participating in the stearic acid oxidation process. Moreover, the existence of adsorbed oxygen species (such as O-, O2-, or O22-) was confirmed by O 1s XPS spectra, and there were Co3+ and Co2+ ions present in the catalysts, which (25) Gierer, J. Holzforschung 1982, 36, 55. (26) Dardelet, S.; Froment, P.; Lacoste, N.; Robert, A. ReVue ATIP 1985, 39, 267.
Energy & Fuels, Vol. 23, 2009 23
Figure 5. XRD patterns of fresh and spent catalyst (5 times).
was confirmed by Co 2p XPS spectra. On the basis of the reactive mechanism of the LaCoO3 catalyst in the CWAO process of stearic acid and the O 1s and Co 2p XPS spectra of the catalyst, a mechanism for the CWAO of lignin was proposed as shown in Figure 3C, involving adsorbed oxygen surface active site (Co(surf)3+O2-) species, resulting from the oxygen dissolved in the solution adsorbed on the perovskite surface. During the formation of quinonemethide intermediates III from phenolate anions II, one electron was abstracted and then accepted by the Co3+ of the LaCoO3 catalyst surface, forming a Co2+ ion. The Co(surf)2+ can then adsorb oxygen dissolved in the solution, leading to the formation of adsorbed oxygen surface active site (Co(surf)3+O2-). The IIIa intermediates formed (after adsorption on the surface of the catalyst) can further react with the surface active site (Co(surf)3+O2-), yielding the quinonemethide hydroperoxide IV and non-adsorbed Co3+. In the process of II f IV, the cycle of Co(surf)3+ f Co(surf)2+ f Co(surf)3+O2f Co(surf)3+ occurs. The cycle can easily occur because the catalyst possesses a good oxygen storage capacity, which can speed up the formation rate of IIIa intermediates and quinonemethide hydroperoxide IV, which were the rate-determining steps. While Co2+ only promoted the formation of intermediate IIIa, the activity of Co2+ was lower than that of the LaCoO3 catalyst. However, the details of mechanisms for LaCoO3 in the CWAO process of lignin remain to be determined in future investigations. 3.4. Catalyst Stability. The reusability or recyclability of the catalyst is an important parameter for a heterogeneous catalyst. In this paper, repetitive use of the catalyst LaCoO3 was performed to test its stability. On the basis of the analysis in section 3.2, the yield of syringaldehyde was the highest of all of the aldehydes, and its maximum yield was attained much earlier than the other two aldehydes. The details of the reusability test were as follows: fresh catalyst and lignin solution were added to the reactor, and the process was carried out for 50 min. After the reactor temperature was cooled to room temperature after the CWAO reaction, the resulting products were carefully poured out and filtered for the regeneration of the catalyst in the reactor. Fresh lignin solution was then added, and the process was repeated under the same conditions for 50 min. This procedure was repeated 4 times. Figure 4 shows the lignin conversion and yields of aromatic aldehydes without the addition of catalyst and with catalyst addition with five successive reuses. Lignin conversion and the yield of each aromatic aldehyde were promoted significantly in the catalyzed process as compared to the noncatalytic process. No obvious changes in lignin conversion or in yields of aromatic aldehydes were shown for the five successive catalyst reuses. The catalyst can thus be reused at least 5 times.
24
Energy & Fuels, Vol. 23, 2009
The crystalline phases of the catalysts before and after the reaction were also determined by XRD (Figure 5). The X-ray diffraction pattern of the fresh LaCoO3 catalyst and the samples after successive reactions all showed distinctive reflections for the perovskite-type oxide. No other phases were found. No obvious changes can be seen in the catalyst structure after the fifth run; it was observed that the specific surface area of the fresh LaCoO3 catalyst and the samples after successive reactions remained almost unchanged, which were 17.6 and 16.7 m2/g, respectively. The small differences were within instrumental error. The active phase itself was not deactivated under the test conditions and was responsible for the reusability of the LaCoO3 in the CWAO of the lignin. The activity, the specific surface area, and perovskite-type structure of the LaCoO3 catalyst did not change after the fifth reaction cycle, suggesting that the LaCoO3 catalyst in the CWAO process is stable. 4. Conclusion The perovskite-type catalyst LaCoO3, prepared by the sol-gel method, exhibited high activity in the CWAO of lignin. Lignin conversion and yields of each aromatic aldehyde were significantly enhanced in the LaCoO3 catalyzed process as compared to the noncatalytic process. A mechanism involving the reaction
Deng et al.
of lignin molecules with adsorbed oxygen surface sites (Co(surf)3+O2-) was proposed on the basis of the experimental observations and the O 1s and Co 2p XPS spectra of the catalyst, yielding the cycle Co(surf)3+ f Co(surf)2+ f Co(surf)3+O2- f Co(surf)3+, with the formation rate of intermediates IIIa quinonemethide and hydroperoxide IV as the rate-determining step. Moreover, the activity, specific surface area, and perovskitetype structure of the LaCoO3 catalyst remained nearly unchanged after five successive recycles of catalytic reactions, indicating that the catalyst also possesses superior stability of activity and structure in the CWAO of lignin. The perovskite-type oxide LaCoO3 therefore has strong potential for the CWAO of lignin to high-value chemicals. Acknowledgment. The authors are grateful for the financial support from the Natural Science Foundation of China (50776035 and U0733001), Foundation of Scientific Research for Universities (20070561038), Initiative Group Research Project (IRT0552) from the Ministry of Education of China, National High Technology Project (863 project) (2007AA05Z408), and National Key R&D Program (2007BAD34B01) from the Ministry of Science and Technology of China. EF8005349
