Ind. Eng. Chem. Res. 2006, 45, 6627-6631
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Evaluation and Modeling of Lignin Catalytic Wet Oxidation to Selective Production of Aromatic Aldehydes Fernando G. Sales,*,† Laı´sse C. A. Maranha˜ o,‡,§ Nelson M. Lima Filho,‡,| and Cesar A. M. Abreu‡,⊥ Departamento de Engenharia Quı´mica, UniVersidade Federal do Ceara´ , Fortaleza-CE, 60455-760, Brazil, and Departamento de Engenharia Quı´mica, UniVersidade Federal de Pernambuco, Recife-PE, 50740-520, Brazil
The catalytic wet air oxidation (CWAO) of lignin, obtained from sugar-cane bagasse, was evaluated through kinetic studies of the process of aromatic aldehyde production. The operations were carried out in a high pressure mechanically stirred slurry reactor, in a semibatch way, and in the presence of a palladium catalyst supported on γ-alumina in a temperature range of 373-413 K, at a total pressure of 20 bar, where the partial pressure of oxygen varied in a range of 2-10 bar. The lignin concentration was 60.00 kg/m3 in an alkaline medium of 2 mol/L of NaOH. The lignin degradation reaction and the aromatic aldehyde buildup were described by a kinetic model quantified by a complex series/parallel reaction network. Introduction The wet air oxidation (WAO) process has been used as an alternative technology for treatment of organic products that are converted into carbon dioxide and carboxylic acid. Intending to valorize the lignocellulosic materials, WAO has been the most promising process since it conducts to a mixture of aromatic aldehydes of great industrial interest. Severe operating conditions (400-600 K; 5-20 bar) limited the application of this technology. With the introduction of suitable catalysts, the WAO process can be upgraded to a CWO (catalyst wet oxidation) process that can be carried out at milder temperature and pressure conditions. The catalytic wet air oxidation (CWAO) process using air and supported catalysts was indicated to be an attractive method for effluents treatment and byproduct processing for the biomass industry.1-6 Many heterogeneous catalysts have been studied for oxidation of phenol, carboxylic acids, and effluents from pulp and paper mills.7 Metal oxide8,9 and noble metal catalyst10,11 supported on alumina were effective for lignin conversions of black liquor.12 In the present work, CWAO was applied to lignin obtained as a byproduct from sugar-cane bagasse by the DFH (Dedine fast hydrolysis) process where the sugar-cane bagasse is totally hydrolyzed. Under selected reaction conditions of temperature and partial pressure of oxygen, the kinetic evolution of lignin catalytic wet oxidation was investigated. The reaction conditions were selected in order to obtain intermediary oxidation products, such as vanillin, syringaldehyde, and p-hydroxybenzaldehyde. Moderated partial pressures of oxygen and short reaction times must be employed in order to avoid the situation where the produced adehydes are oxidized into organic acids such as * To whom correspondence should be addressed. Fax: 55 81 40089610. E-mail:
[email protected]. † Universidade Federal do Ceara ´. ‡ Universidade Federal de Pernambuco. § Fax: 55 81 21267289. E-mail:
[email protected]. | Fax: 55 81 21267289. E-mail:
[email protected]. ⊥ Fax: 55 81 21267289. E-mail:
[email protected].
formic, lactic, syringic, vanillic, and p-hydroxybenzoic. These aldehydes have wide applications, from galvanoplasty to chemical intermediaries for pharmaceutical drugs and agricultural defensives. Experimental Section Catalyst Preparation. The catalyst was prepared from palladium chloride (PdCl3‚3H2O Vetec) and γ-alumina (γ-Al2O3 Procatalyse) as the catalytic support, with a BET superficial area of 3.65 × 105 m2/kg, a porous volume of 0.47 m3/g, a particle porosity (p) of 0.59, and a mean diameter in the range of 90-200 µm. The preparation method was the wet impregnation. Alumina was impregnated via the wet procedure by the palladium chloride solution. The impregnated solution in the support was evaporated to dryness in a roto-evaporator. In a next step, the particles were dried in an oven at 393 K for 12 h and, then, reduced at 673 K, in a hydrogen-argon (10 wt %) atmosphere for 2 h. Kinetic Evaluation. The kinetic experiments were carried out with a catalyst concentration of 4.00 wt % in a high-pressure SS-316 Parr slurry reactor (model 4843) equipped with a sixbladed turbine-type impeller mixer and with an effective volume of 0.50 × 10-3 m3. A thermal sensor, an external heating element, and an internal cooling coil provided, in the reactor, a controlled temperature at 373, 393, and 413 K with an accuracy of (1 K. A total pressure of 20 bar resulted in a partial pressure of oxygen in a range of 2-10 bar in a nitrogen medium. The liquid samples of the reaction products were collected through a metal filter and, after the separation of the residual reagent from the products, were analyzed by a gas chromatograph (HP 5890 series II) equipped with a capillary column (HP-1) and a flame ionization detector (FID). Results and Discussion Catalyst Characterization and Activity. The palladium content on the alumina support was 2.85 wt % as quantified by atomic absorption spectrophotometry, and the superficial area
10.1021/ie0601697 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/06/2006
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Figure 1. Experimental kinetic evolutions of the lignin and aldehydes in wet oxidations, catalytic vs noncatalytic processes. The reaction conditions were as follows: catalyst Pd/γ-Al2O3, 4.00 wt %, NaOH (2 mol/L), 393 K, 5 bar partial pressure of oxygen, 20 bar total pressure, CL0 ) 60.00 kg/m3. (a) Lignin consumption; (b) vanillin yield; (c) syringaldehyde yield; (d) p-hydroxybenzaldehyde yield.
Figure 2. Reaction scheme of the catalytic wet oxidation of lignin (ij ) 11, ..., 42; i ) reaction, j ) reaction step).
Figure 3. Reaction scheme for homogeneous and heterogeneous steps of the wet oxidation of lignin.
and porous volume by the BET-N2 method were 3.29 × 102 m2/g and 0.42 m3/g, respectively. Preparation steps were followed by a characterization analysis of the reduced materials, where the presence of palladium crystals on the alumina was confirmed via X-ray diffraction (XRD; 2θ ) 40.047, 46.609, 68.085, and 82.104) and the IR spectra data provided identification of HO- (3470.3 cm-1), OH (2359.9 cm-1), H2Ohid (1634.4 cm-1), and Al2O3 (803.4-559.3 cm-1). Temperature-programmed reduction (TPR) analysis of the system from 293 to 1173 K presented reduction peaks at 337.7 K (Pd reduction), 348.8 K (H2 desorption), and at 527.2 K (hydrogen reaction with alumina oxygen). The oxidative processes of lignin were performed by a noncatalytic route (WAO) and in the presence of the palladium catalyst (CWAO), where the selectivity of the Pd/γ-alumina catalyst for aldehyde production was evaluated (Figure 1). The catalytic activity was indicated by the lignin oxidation steps into aldehydes. The two processes may be evaluated by comparing, in each case, the lignin consumption and the aldehyde productions. In the presence of the catalyst, lignin after 1.5 h of reaction presented a conversion increase of 50% (Figure 1a). Each test (noncatalytic and catalytic) was programmed for 2.5 h, and the conversion in the presence of catalyst, at 393 K and 5 bar of oxygen, was enhanced significantly (Figures 1bd). The maximum aldehyde concentrations, on the order of 1021 mmol/L, obtained in the catalytic process were, in average,
10 times higher than those that resulted from the noncatalytic process, which where in the range of 0.5-1.5 mmol/L. The evaluation of the catalytic oxidation of the lignin indicates that the main catalyst action occurs after the lignin hydrolysis to polifenate ions, which are the precursors of the aromatic aldehyde formation. The degradation rate does not suffer relevant alterations due to the catalyst presence, but the aldehyde formation reaches higher levels in the presence of the catalyst. The syringaldehyde suffers the action of the catalyst in both steps: hydrolysis and formation. Meanwhile, the p-hydroxybenzaldehyde has its selectivity enhanced, mainly, due to the relative difficulty of the catalyst to degrade it. The presence of the catalyst reduces the formation of undesirable products by approximately 40 times, indicating an improvement in the selectivity for aldehydes associated with the catalytic process. Mechanism and Kinetic Modeling. The reaction mechanism of the catalytic wet oxidation presents a high level of complexity, even considering simple structures such as phenol.4,5 This fact is accentuated in the lignin oxidation with the reaction leading to the production of a great variety of intermediary products. In a general way, it is considered that the wet oxidation proceeds in two or more steps, where lignin, a macromolecule of complex structure, is hydrolyzed breaking up into fragments producing aromatic aldehydes and other products of lower molecular weight and smaller molecules such as carbon dioxide and water. These aldehydes are then degraded into acids through a consecutive reaction.
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For the consumption of lignin to produce aldehydes or acids and carbon dioxide,
[(
khij ) KL
)
-1
1 1 + kIij k-Lad
+ kIIij
]
(1)
For the production of aldehydes from lignin,
{ [
′ + khij′ ) khIIij
kh′ij2kAad
](
k′Iij 1 + KA(1 + k′IIij) kLad KL
)} -1
(2)
For the consumption of aldehydes to produce acids and carbon dioxide,
[(
Figure 4. Kinetic evolutions of lignin catalytic wet oxidation according to a pseudo-first-order model: PO2 ) 5 bar, P ) 20 bar, CL0 ) 60.00 kg/ m3, Ccat ) 4.00 wt %.
Experimental evidences obtained in this work lead to a mechanism proposition, assuming that the lignin (L) is depolymerized in an oxidant medium with the productions of aldehydes, acids, and other products of low molecular weights. The aromatic aldehydes vanillin (V), syringaldehyde (S), and p-hydroxibenzaldehyde (P) were submitted to subsequent oxidations forming other products (R), such as organic acids that can degrade into carbon dioxide.5,6 Having in mind the kinetic evaluation of the lignin catalytic wet oxidation process, a global mechanism was proposed, and it is represented by the reaction scheme in Figure 2). In this process, it was assumed that lignin can be oxidized by two concurrent reactions either directly in the liquid phase or on the catalyst surface. Consequently, lignin (L) in the liquid phase (LL) and lignin adsorbed on the catalyst (LS) can produce aldehydes (A) in the liquid phase (AL) and adsorbed aldehydes (AS), respectively. The adsorbed compounds are selectively oxidized under the catalyst action. To describe this behavior, a detailed mechanism was formulated as shown in Figure 3, where I and II are the heterogeneous reaction steps and Lad, Aad, -Lad, and -Aad represent the liquid-solid adsorption and desorption steps, respectively. Assuming adsorption equilibrium and steady-state conditions for the adsorbed components (dCLs/dt ) dCAis/dt ≈ 0), the rate equations of lignin consumption (rL ) khijCL), aldehyde (Ai) production (rAi ) khij′ CL - khij′′CAi), and other product formation (rR ) khij′′CAi + khijCL) were obtained, where khij, khij′ , and khij′′ are the pseudo-first-order kinetic constants (eqs 1-3), including the oxidation rate kinetic constants of the reactions steps I, II, and III (khmij ′ ) kmijCO2; m ) I, II, III), the constants of the adsorption steps of lignin and aldehydes (kLad, k-Lad, kAad, and k-Aad), and their adsorption equilibrium constants (KL, KA). The indexes i and j represent the reaction steps (i ) 1, 2, 3, 4), the production (j ) 1), and consumption (j ) 2) of intermediate aldehydes.
khij′ ) kAad 1 +
) ]
k′IIIij k-Aad
-1
+1
(3)
To describe the experimental kinetic evolutions of the lignin and the aldehydes in the liquid phase of the slurry reactor, the mass balances of the components in the reaction medium are presented in eqs 4, 5, and 6:
dCL dt
3
ki1 + k42)CL ∑ i)1
) -(
(4)
dCAi ) k′i1CL - k′′i2CAi dt dCR dt
(5)
3
) k42CL +
k′′i2CAi ∑ i)1
(6)
where ki1 ) khi1mc/VL, k′i1 ) khi1 ′ mc/VL, k′′i2 ) khi2 ′′mc/VL, and k42 ) kh42 ′′ mc/VL. VL is the volume of the liquid phase, and mc is the mass of catalyst at t ) t0, CL ) CL0, CAi ) CAi(t0), and CR ) CR(t0). Estimation of Kinetic Parameters. The lignin processing in aqueous alkaline medium, in the presence of the Pd/γ-alumina catalyst, under the oxygen action, led to its depolymerization. Figure 4 shows the advancement of the reactions up to 2 h, indicating an increasing influence of the temperature on the lignin conversion. The experimental results of the lignin conversion can be represented by a pseudo-first-order reaction kinetic model, considering the occurrence of one step for a temperature of 373 K and two steps for temperatures of 393 and 413 K, indicating, clearly, a change in the lignin consumption rate for higher temperatures after 1 h of operation. It is possible that in this step the oxygen consumption has been directed to the aldehyde degradation in a process phase where there is little lignin to be depolimerizated. The same behavior was observed before3 for
Table 1. Kinetic Rate Constants and Activation Energies (E) for the Catalytic Reaction Steps of Lignin Consumption and Aromatic Aldehyde Productiona k 103 (min-1)
components key component lignin vanillin syringaldehyde p-hydroxybenzaldehyde a
parameter kL k′11 k′′12 k′′21 k′′22 k′′31 k′′32
373 K
393 K
413 K
E (kJ/mol)
5.39 ( 0.32 0.53 ( 0.02 23.28 ( 1.47 1.93 ( 0.09 70.30 ( 4.22 0.60 ( 0.01 4.67 ( 0.24
11.60 ( 0.64 2.08 ( 0.08 28.73 ( 1.64 7.93 ( 0.44 127.0 ( 6.99 1.45 ( 0.06 8.96 ( 0.43
23.50 ( 1.53 3.83 ( 0.15 31.93 ( 1.71 15.00 ( 0.83 216.7 ( 9.84 4.50 ( 0.20 18.33 ( 0.99
43.76 ( 2.63 63.64 ( 3.18 10.18 ( 0.59 66.04 ( 3.36 35.97 ( 1.87 64.11 ( 3.27 43.62 ( 2.48
PO2 ) 5 bar; Ccat ) 4.00 wt %; Ni (2.85 wt %)/γ-Al2O3.
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Figure 5. Kinetics evolutions of the aldehyde production from lignin wet oxidation, temperature effect. The reaction conditions were as follows: catalyst Pd/γ-Al2O3, 4.00 wt %, NaOH (2 mol/L), 5 bar partial pressure of oxygen, 20 bar total pressure, CL0 ) 60.00 kg/m3. (a) 373; (b) 393; (c) 413 K.
black liquor oxidation (CWAO) from the paper and cellulose industry which is an industrial effluent and has a high content of lignin. Since lignin was converted following the reaction steps of aldehyde production and other product formation, there is a 3 lignin consumption rate (rL ) kLCL) where kL (kL ) Σi)1 ki1 + k42) is the corresponding parameter. From the linear model fitted to the experimental data from the first step of the lignin consumption (Figure 4), the pseudo-first-order parameter values were estimated as the following: kL ) 5.85 × 10-3 min-1 (373 K), 13.98 × 10-3 min-1 (393 K), and 44.26 × 10-3 min-1(413 K). These values were considered initialization values to a subsequent optimization procedure of parameter estimation. To estimate the kinetic parameters of the system of differential equations (eqs 5-7) representing the slurry reactor operations including the kinetic reaction model, an optimization procedure was applied. The system was numerically solved using a fourth-
Figure 6. Kinetics evolutions of the aldehyde production from lignin wet oxidation, oxygen partial pressure effect. The reaction conditions were as follows: catalyst Pd/γ-Al2O3, 4.00 wt %, NaOH (2 mol/L), 393 K, 20 bar total pressure, CL0 ) 60.00 kg/m3. (a) 2; (b) 5; (c) 10 bar. Table 2. Kinetic Rate Constants for the Catalytic Reaction Steps of Lignin Consumption and Aromatic Aldehyde Productiona k 103 (min-1)
components key component lignin vanillin syringaldehyde p-hydroxybenzaldehyde
parameter
2 bar
kL k′11 k′′12 k′21
6.53 ( 0.26 1.37 ( 0.07 46.4 ( 2.29 3.83 ( 0.14
k′′22 k′31 k′′32
a
5 bar
10 bar
11.60 ( 0.58 16.40 ( 0.66 2.08 ( 0.08 2.24 ( 0.11 28.73 ( 1.64 49.10 ( 2.73 7.93 ( 0.44 2.97 ( 0.12
133.0 ( 4.47 127.0 ( 6.99 69.10 ( 2.46 1.33 ( 0.05 1.45 ( 0.06 3.33 ( 0.11 23.6 ( 1.13
8.96 ( 0.43 66.10 ( 2.39
T ) 393 K; Ccat ) 4.00 wt %; Ni (2.85 wt %)/γ-Al2O3.
order Runge-Kutta integration method, associated with an optimization method.13 The calculated values (CiTh), CLiTh of CL and CAiTh of CV, CS, and CP were compared with the experimental ones (CiExp), minimizing an objective function Fob
Ind. Eng. Chem. Res., Vol. 45, No. 20, 2006 6631 Table 3. Degradation/Production Ratios of Aromatic Aldehyde Productiona components
degradation/production ratios
key component
parameter
373 K
393 K
413 K
vanillin syringaldehyde p-hydroxybenzaldehyde
k′12/k′′11 k′21/k′′22 k′31/k′′32
43.86 36.36 7.78
13.81 16.03 6.18
8.34 14.45 4.07
a
PO2 ) 5 bar; Ccat ) 4.00 wt %; Pd (2.85 wt %)/γ-Al2O3.
productions occur with activation energies on the order of 65.00 kJ/mol, in equal proportion to the lignin content, based on the three precursors of the aldehydes: vanillin, syringaldehyde, and p-hydroxybenzaldehyde. Acknowledgment The authors acknowledge the financial support received from CAPES, Brazil, for this work. Nomenclature
n (Fob ) Σi)1 [CiExp - CiTh]2). From the optimized parameters of the model based on the model equations fitted to experimental data, the calculated evolutions of aldehydes were represented in Figure 5 at three temperatures of operation, under 5 bar of oxygen partial pressure. The calculated aldehyde yields were compared with the experimental results. The aldehydes yields from the lignin conversion, under the temperature effects, are showing occurrences of parallel reactions with posterior aldehyde degradations. In all the cases, the maximum aldehyde concentrations were attained faster as the reaction temperatures were increased. Considering that both the hydroxyl and the aldehyde group may be subjected to oxidation, the aldehyde compound with the highest number of metoxyls substituted in the benzene ring was the most susceptible to oxidation. In this sense, it was observed that syringaldehyde was the most reactive and p-hydroxybenzaldehyde which has no metoxyl group was the most stable one. The optimized kinetic rate constants of the lignin wet oxidation steps and the corresponding activation energies are presented in Table 1. In Figure 6, experimental results and model computations under pressures of 2, 5, and 10 bar at 393 K are presented. The influence of the partial pressure of oxygen on the yield evolutions of the aldehydes indicates changes of maximum yields as a function of the reaction time. In Table 2, the kinetic rate constants at the three partial pressures are presented. The kinetic evolutions of the aldehydes contained in the reacting medium based on the kinetic rate constants of the consecutive oxidation reactions (see Table 1) may be quantified by the aldehyde degradation/production ratios. So, for the vanillin (k′′12/k′11), syringaldehyde (k′′12/k′11), and p-hydroxybenzaldehyde (k′′32/k′31), the corresponding degradation/production ratios diminish as a function of the temperature increase (see Table 3).
Conclusions The wet air oxidation process of lignin, obtained from sugarcane bagasse, was carried out in a slurry reactor with a palladium catalyst, in a temperature range of 373-413 K, under an oxygen partial pressure range of 2-10 bar, providing aromatic aldehyde yields approximately 10-20 times higher than those obtained with the noncatalytic oxidation process. The lignin degradation reaction and the aromatic aldehyde buildup have been described by a kinetic model quantified by a complex series/parallel reaction network. The lignin consumption was characterized for a faster reaction step, with an activation energy of 44.26 kJ/mol. The aromatic aldehyde
C ) concentration (mol/L) dp ) particle diameter (m) HO2 ) Henry constant for oxygen K ) adsorption equilibrium constant (L/mol) k ) kinetic constant (min-1) mc ) mass of catalyst (g) RTD ) residence time distribution V ) volume (L) Greek Letters ) bed porosity Fcat ) catalyst specific mass Literature Cited (1) Mathias, A. L.; Rodrigues, A E. Production of vanillin by oxidation of pine kraft lignins with oxygen. Holzforschung 1994. (2) Harmsen, J. M. A.; Jelemensjy, L.; van Andel-Scheffer, P. J. M. Kinetic modeling for wet air oxidation on formic acid on carbon supported platinum catalyst. Appl. Catal. A: Gen. 1997, 165, 499-509. (3) Zhang, Q.; Chuang, K. T. Kinetics of wet oxidation of black liquor over a Pt-Pd-Ce/alumina catalyst. Appl. Catal. B: EnViron. 1998, 17, 321332. (4) Zhang, Q.; Chuang, K. T. Lumped kinetic model for catalytic wet oxidation of organic compounds in industrial wastewater. AIChE J. 1999, 45 (1), 145-150. (5) Hamoudi, S.; Belkacemi, K.; Larachi, F. Catalytic oxidation of aqueous phenolic solutions catalyst deactivation and kinetics. Chem. Eng. Sci. 1999, 54, 3568-3576. (6) Zhang, Q.; Chuang, K. T. Wet oxidation of bleach plant effluent: effects of pH on the oxidation with or without a Pd/Al2O3 catalyst. Can. J. Chem. Eng. 1999, 77, 399. (7) Fafelt, J. S.; Clark, J. H. Recent advances in the partial oxidation of molecules using heterogeneous catalysis. Catal. Today 2000, 57, 33-44. (8) Be´ziat, J. C.; Besson, M.; Gallezot, P.; Dure´cu, S. Catalytic wet air oxidation on a Ru/TiO2 catalyst in a trickle-bed reactor. Ind. Eng. Chem. Res. 1999, 38 (4), 1310-1315. (9) Miro´, C.; Alejandre, A.; Fortuny, A.; Bengoa, C.; Font, J.; Fabregat, A. Aqueous phase catalytic oxidation of phenol in a trickle bed reactor: effect of the pH. Water Res. 1999, 33 (4), 1005-1013. (10) Zhang, Q.; Chuang, K. T. Alumina-supported noble metal catalysts for destructive oxidation of organic pollutants in effluent from a softwood kraft pulp mill. Ind. Eng. Chem. Res. 1998, 37 (8), 3343-3349. (11) Alejandre, A.; Medina, F.; Salagre, P.; Fabregat, A.; Sueiras, J. E. Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions. Appl. Catal. B: EnViron. 1998, 18 (3-4), 307-315. (12) Mishra, V. S.; Mahajani, V. V.; Joshipp, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34 (1), 2-48. (13) Sales, F. G. Oxidac¸ a˜o u´mida catalı´tica da lignina em reatores trifa´sicos com produc¸ a˜o de aldeı´dos aroma´ticos. Ph.D. Thesis, UNICAMP, Brasil, 2001.
ReceiVed for reView February 10, 2006 ReVised manuscript receiVed June 22, 2006 Accepted June 26, 2006 IE0601697
