Kinetics of n-Butanol Partial Oxidation to Butyraldehyde over

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Kinetics of n‑Butanol Partial Oxidation to Butyraldehyde over Lanthanum−Transition Metal Perovskites Bing-Shiun Jiang, Ray Chang, Yi-Chen Hou, and Yu-Chuan Lin* Department of Chemical Engineering and Materials Science, Yuan Ze University, 135 Yuan-Tung Road, Chungli, Taoyuan 32003, Taiwan S Supporting Information *

ABSTRACT: Partial oxidation of butanol to butyraldehyde over a series of LaBO3 (B = Mn, Fe, and Co) perovskites was investigated in a continuous fixed-bed system under ambient pressure. Physicochemical properties of catalysts were characterized by X-ray diffraction, H2 temperature-programmed reduction, and temperature-programmed oxidation. LaMnO3 was more favorable to be reduced and reoxidized than LaFeO3 and LaCoO3. Catalytic results have indicated that all catalysts show similar butanol and oxygen conversions and over 90% butyraldehyde selectivities below 300 °C. Side reactions such as butanol or butyraldehyde combustion could be enhanced at high temperatures. To gain an in-depth understanding of perovskite’s chemistry involved, kinetic analysis has been carried out. Eight reaction pathways based on the Mars−van Krevelen redox cycle were proposed. These pathways have been lumped and associated with the Langmuir−Hinshelwood−Hougen−Watson formalism to derive a set of rate equations. Parameter estimation via nonlinear regression of derived rate equations has shown that surface reaction, evolving chemisorbed butanol and oxygen, is probably rate-determining. The estimated activation energy of LaMnO3 (15.0 kcal/mol) by assuming surface reaction as the rate-limiting step was the lowest among all perovskites. This can be ascribed to the better redox property of LaMnO3, thereby decreasing the energy barrier in butanol partial oxidation.

1. INTRODUCTION

redox cycle. Table 1 lists plausible elementary reactions of POB to butyraldehyde by the redox mechanism.

Butyraldehyde is a key intermediate in the chemical industry. For example, selective oxidation of butyraldehyde forms butyric acid, a fragrance compound in the food industry.1,2 The commercial method for butyraldehyde synthesis is hydroformylation (oxo synthesis) of syngas and propylene,3 a homogeneous reaction at 2−50 MPa and 363−453 K.4 However, hydroformylation has several drawbacks: (1) downstream separation is required due to the coproduction of isobutyraldehyde, (2) homogeneous catalysts need to be recycled, and (3) safety concerns exist for high pressure operations. Partial oxidation of n-butanol (POB) is another approach for butyraldehyde preparation. Unlike hydroformylation, POB can be conducted with heterogeneous catalysts under mild reaction conditions (0.1 MPa).4,5 Copper or ruthenium supported catalyst has already shown promising outcomes in a continuous fixed-bed system.4,5 Moreover, butanol can be derived from biomass. Using biobutanol instead of fossil fuel-derived syngas and propylene to prepare butyraldehyde should be renewable and environmental benign. Partial oxidation of alcohol to aldehyde follows a reduction− oxidation (redox) pathway, also called the Mars−van Kreleven mechanism.6 Methanol partial oxidation to formaldehyde7,8 and ethanol partial oxidation to acetaldehyde9,10 are the most frequently encountered redox processes. At the outset, an alcohol molecule chemisorbs on the catalyst surface and forms an alkoxyl and a hydroxyl. Subsequently, the alkoxyl dehydrogenates and desorbs from the surface as aldehyde; the hydroxyl reduces the surface by dehydration. Lastly, the reduced surface is reoxidized by gaseous oxygen to allow a © 2012 American Chemical Society

Table 1. Elementary Reactions of POB on the Basis of Redox Mechanism entry

elementary reaction

s1

C4 H 9OH(g) + S ↔ C4 H 9OHS

s2

C4 H 9OHS + OS ↔ C4 H 9OS + OHS

s3

C4 H 9OS + OS ↔ C4 H8OS + OHS

s4

C4 H8OS ↔ C4 H8O(g) + S

s5

2OHS ↔ H 2OS + OS

s6

H 2OS ↔ H 2O(g) + S

s7

O2 (g) + S ↔ O2 S

s8

O2 S + S ↔ 2OS

Perovskite (ABO3) has great thermal stability, reactive lattice oxygen, and is flexible in catalyst design (varying A- and/or Bsite cations).11 This enables perovskite to be an attracting catalyst in oxidative reactions. Our group has lately reported that lanthanum−transition metal perovskites have great potential in methanol12−14 and ethanol partial oxidation.15 Perovskite’s reducibility and oxygen’s mobility play key roles in controlling catalytic reactivity and aldehyde selectivity. This Received: Revised: Accepted: Published: 13993

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The effluent was analyzed by a GC (SRI-8610) equipped with a TCD, a methanizer, and a FID. Molecular sieves (5 Å) and Porapak Q were used for gas separation. Detected compounds included N 2, O 2, CO, CO2 , C 4 H8 O, and C4H9OH. The amount of water was calculated on the basis of the oxygen atom balance. Three trails were recorded and used to estimate 95% confidence intervals for conversions and product selectivities. The butanol conversion was calculated as moles of butanol reacted divided by moles of butanol fed in. The oxygen conversion was calculated in the same way as that of butanol. The selectivities of carbon products were calculated on the basis of carbon atoms. 2.4. Kinetic Analysis. Kinetic analysis was performed to gain the insight of POB chemistry. Three C4H9OH/O2 ratios (0.40, 0.50, and 0.66) and three space velocities (6.2 × 10−7, 7.3 × 10−7, and 9.2 × 10−7 g h/cm3) were recorded in the range from 250 to 350 °C. Note that the differential method (operating at low butanol conversions) was employed to suppress influences of side reactions. There are 42 different measurements for LaMnO3, 26 tests for LaFeO3, and 44 trials for LaCoO3. All data were recorded under steady-state conditions. These data were then treated by nonlinear regression of derived rate equations to gain the kinetic parameters. Parameter estimation was performed using Athena Visual Studio18 according to the criterion of the least residual sum of the squares (RSS).

inspired us to explore perovskite’s application in POB to butyraldehyde. This study investigates LaMnO3, LaFeO3, and LaCoO3 perovskites in POB. Physicochemical properties of these catalysts were surveyed. The kinetics of POB was investigated to provide mechanistic insight into the redox mechanism. The results can be corroborated with perovskite’s redox chemistry.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. LaMnO3, LaFeO3, and LaCoO3 perovskites were synthesized by a sol−gel method.16 Metal acetylacetonates [La-, Mn-, Fe-, and Co(acac)3] were used as the precursors. Approximately 10 mmol of A-site (La) and Bsite (Mn, Fe, or Co) metal ions were dissolved in a 260 mL of solution containing 25% ethylene glycol and 75% methanol. After vigorous stirring for 1 h, the solution was dried at 150 °C and calcined at 750 °C as described elsewhere.12,13 2.2. Catalyst Characterization. The powder X-ray diffraction (XRD) pattern was obtained by a Shimadzu Labx XRD-6000 with Cu Kα radiation (0.154 18 nm). The pattern was collected from a 2θ angle of 20°−80° at 4°/min scanning rate. The voltage and current were 40 kV and 30 mA, respectively. Crystallite sizes were estimated using the Scherrer relation. Hydrogen temperature-programmed reduction (H2-TPR) and temperature-programmed oxidation (TPO) were both performed in the same system.17 Approximately 0.05 g of sample was consumed per trial. Prior to the test, the sample was dehydrated at 120 °C in an Ar stream (25 mL/min) for 30 min. H2-TPR was carried out under a 25 mL/min flow of 7.4% H2 in Ar. For TPO experiments, the sample was prereduced in a 7.4% H2/Ar stream (25 mL/min) from room temperature to 450 °C with a 5 °C/min heating rate. After cooling and purging with Ar, the sample was reoxidized in a 2.5% O2/He stream (25 mL/ min). The criterion for prereduction condition of TPO was decided based on the following: (1) the reduced sample had similar crystalline structure as freshly prepared catalyst and (2) sufficient oxygen was removed from the treated sample to amplify the signal-to-noise TCD response. 2.3. Catalyst Tests. Catalytic performances were determined in a continuous fixed-bed reactor under atmospheric pressure. Butanol was fed in through a HPLC pump (JASCO PU-2080 plus) and vaporized at ∼150 °C. All tubing was wrapped with heating tape at 150 °C to avoid condensation. Nitrogen served as the carrier gas as well as the internal standard. The molar composition of the feed was C4H9OH/ O2/N2 = 2/4/94. The catalyst bed consisted of approximately 0.01 g of catalyst and 0.15 g of SiO2 particles in the range of 40−80 mesh size, sandwiched by quartz wool. SiO2 served as the diluent and was inert. The space velocity was kept at 9.1 × 10−7 g h/cm3. Internal and external mass transfer limitations were examined. Internal mass transfer resistance was evaluated by measuring kinetics of two particle sizes in the ranges of 40− 80 and 20−40 mesh. External mass transfer limitation was tested by two different catalyst loadings (0.01 g of catalyst mixed with 0.15 g of SiO2, and 0.008 g of catalyst mixed with 0.12 g of SiO2) at the same space velocity (9.1 × 10−7 g h/ cm3). Both internal and external mass transfer resistances were negligible (differences in butanol conversions were less than 5%). The system was operated isothermally at 200−350 °C by an electronically heated furnace. No deactivation was observed. Mass balances on carbon and hydrogen were within ±5%.

3. RESULTS AND DISCUSSION Figure 1 displays the XRD patterns of perovskites. The index reflections of LaMnO3 (JCPDS 35-1353), LaFeO3 (JCPDS 37-

Figure 1. XRD patterns of LaMnO3, LaFeO3, and LaCoO3.

1493), and LaCoO3 (JCPDS 48-0123) were observed. Estimated crystallite sizes are 16.0, 26.6, and 13.0 nm for LaMnO3, LaFeO3, and LaCoO3, respectively. The large crystallite size of LaFeO3 suggests its high crystallinity and the presence of agglomerated particles.19 The highly crystalline structure of LaFeO3 may affect its catalytic performances; for example, LaFeO3 was reported to be the least reactive in LaBO3 (B = Mn, Fe, and Co) for methane oxidation.20 Figure 2 shows the H2-TPR profiles of perovskites. LaFeO3 is hard to to be reduced: no appreciable signal was observed. Therefore, the response of LaFeO3 was exaggerated by a multiple of 5. The beginnings of H2-TPR for LaMnO3 (287 °C) and LaCoO3 (288 °C) were similar, while the initial 13994

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LaMnO3 is more favorable to be reoxidized than LaFeO3 and LaCoO3. Figure 4 shows the reactivities of LaMnO3, LaFeO3, and LaCoO3. Both butanol and oxygen conversions increased with

Figure 2. H2-TPR profiles of LaMnO3, LaFeO3, and LaCoO3.

reduction temperature of LaFeO3 could not be specified. Two peaks were identified for LaMnO3 and LaCoO3. The former had its first response at 469 °C and the second at 778 °C and the latter at 395 and 562 °C. For LaFeO3, a weak signal at 532 °C and an increasing trend at 800 °C were found. These separate peaks could be attributed to a consecutive reduction step: the B-site ions are initially reduced from 4+/3+ to 2+, followed by reduction of the 2+ state to metallic element.21,22 Note that a complete reduction of Mn ions to Mn0 is not possible; therefore, the final state of Mn ions should be Mn2+.23 Apparently, LaMnO3 and LaCoO3 are more reducible than LaFeO3. Figure 3 depicts the TPO profiles. Unsymmetrical and multiple responses were observed, suggesting complicated

Figure 4. Conversions of (a) butanol and (b) oxygen as functions of temperature over LaMnO3, LaFeO3, and LaCoO3.

elevating temperature. Butanol conversions were similar for all catalysts. Different oxygen conversions could be found at 350 °C, declining as LaMnO3 (41%) > LaCoO3 (27%) > LaFeO3 (9%). Figure 5 displays the selectivities of butyraldehyde and

Figure 5. Carbon selectivities of (a) C4H8O, (b) CO2, and (c) CO as functions of temperature over LaMnO3, LaFeO3, and LaCoO3.

carbon oxides. Over 90% butyraldehyde and trace carbon oxides were generated at 300 °C and below. At 350 °C, CO2 increased and butyraldehyde decreased. This implies that side reactions (e.g., butanol and butyraldehyde combustions) were enhanced at high temperatures. Among the tested perovskites, LaMnO3 yielded the greatest amount of CO2 with the highest oxygen conversion. This may be ascribed to its oxygen-excess nature caused by Mn4+ cations.24−26 The presence of surplus oxygen makes LaMnO3 an active catalyst in hydrocarbon combustion. For instance, LaMnO3 was reported to be more active than LaFeO3 and LaCoO3 in methanol combustion due to its surpassing oxygen availability and mobility.26 It seems that the same could be stated for POB.

Figure 3. TPO profiles of LaMnO3, LaFeO3, and LaCoO3.

oxidation processes. Although it is difficult to interpret TPO patterns with certainty, a comparative study can approximate the reoxidation behaviors of partially reduced perovskites. The TPO initial temperature increased as LaMnO3 (53 °C) < LaCoO3 (60 °C) < LaFeO3 (64 °C). The first oxidation peak increased following the trend of LaMnO3 (93 °C) < LaFeO3 (111 °C) < LaCoO3 (187 °C). This indicates that reduced 13995

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To gain in-depth understandings of POB chemistry, rate equations based on the Langmuir−Hinshelwood−Hougen− Watson (LHHW) formalism27 were derived. The LHHW formalism is commonly used in deriving a surface-reaction kinetic model for heterogeneous catalysis, e.g., methanol28 or ethanol partial oxidation.15 It is likely that the same paradigm can be applied in POB. Accordingly, the elementary reactions of POB (Table 1) can be lumped into five steps: butanol adsorption (ad,1), oxygen adsorption (ad,2), surface reaction (surf), butyraldehyde desorption (de,1), and water desorption (de,2). Table 2 summarizes these five steps. The rate expression Table 2. Lumped Reaction Steps of POB designation butanol adsorption s1 oxygen adsorption 1 /2(s7 + s8) surface reaction s2 + s3 + s5 butyraldehyde desorption s4 water desorption s6

ad,1 ad,2 surf de,1

de,2

reaction step Kad,1

C4 H 9OH(g) + S ←→ ⎯ C4 H 9OHS Kad,2 1 O2 + S ←→ ⎯⎯ OS 2

Figure 6. Parity plots of reaction rates for the most appropriate models.

K surf

C4 H 9OHS + OS ←→ ⎯ H 2OS + C4 H8OS Kde,1

C4 H8OS ←→ ⎯ C4 H8O(g) + S Kde,2

H 2OS ←→ ⎯⎯ H 2O(g) + S

was then derived by assuming one of these steps is the ratedetermining step (RDS) while the remaining steps are at their quasiequilibriums. Derivation of rate equations is similar to that of partial oxidation of ethanol.15 Appendix A of the Supporting Information lists the derived rate equations. Because substantial amounts of butyraldehyde and water were produced, their corresponding equilibrium constants, i.e., Kde,1 and Kde,2, should be extremely large. Therefore, the terms with Kde,1 and Kde,2 in the denominator are negligible. Appendix A (Supporting Information) also includes the simplified rate equations based on these hypotheses. The simplified rate equations were fitted to the data of kinetic analysis (see Appendix B, Supporting Information) through nonlinear regression subject to the minimization of RSS to recover the kinetic parameters. Appendix C (Supporting Information) presents the resultant RSS values. Among them, surface reaction as the RDS for all perovskites and oxygen adsorption as the RDS for LaMnO3 yielded relatively low RSS values compared to the remaining models. This suggests that these four expressions were adequate representations for POB. Figure 6 illustrates the parity plots of experimentally determined reaction rates and their corresponding calculated values. Appendix D (Supporting Information) lists the obtained parameters and RSS values of these rate equations. The experimental and calculated values are in fairly good agreement for selected rate expressions. Statistical discrimination of either oxygen adsorption or surface reaction as the RDS of POB over LaMnO3 is not possible in this work. However, since LaMnO3 is easier to be reoxidized than LaFeO3 and LaCoO3, surface reaction is more likely to be the RDS than oxygen adsorption. Figure 7 shows the Arrhenius plots of the rate constants kad,2 for LaMnO3 and ksurf for all catalysts listed in Tables D1−D4 of Appendix D (Supporting Information). The activation energies and pre-exponential factors derived from the slopes and intercepts are summarized in Table 3. The oxygen adsorption step as the RDS for LaMnO3 showed the lowest activation

Figure 7. Arrhenius plots of the best-fit models for LaMnO3, LaFeO3, and LaCoO3.

energy (10.1 kcal/mol). As regards to surface reaction as the RDS, the activation energies declined following the trend of LaCoO3 (27.7 kcal/mol) > LaFeO3 (27.0 kcal/mol) > LaMnO3 (15.0 kcal/mol). The comparatively low activation energy of LaMnO3 by either oxygen adsorption or surface reaction as the RDS suggests that it is an effective POB catalyst. The promising outcomes of LaMnO3 may be associated with its redox property. The H2-TPR and TPO results evidenced that LaMnO3 is more favorable to be reduced and further reoxidized than LaFeO3 and LaCoO3. Presumably, both reduction and reoxidation steps in the redox cycle of POB should be promoted to facilitate the reaction rate. Taking LaCoO3, for example, the greatest activation energy in POB should be related to its reoxidation threshold, which hampers the redox cycle even though LaCoO3 is highly reducible. Comparably, LaFeO3 is difficult to be reduced, resulting in a high energy barrier in POB. Due to a lack of POB kinetic studies, it is impossible to compare this work with earlier publications. However, a comparison of our kinetic results with different catalysts in partial oxidation of alcohol to aldehyde should elucidate 13996

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Table 3. Activation Energies (Ea) and Pre-Exponential Factors (A) Estimated by the Linear Fits in Figure 6 LaMnO3 RDS

LaFeO3

Ea (kcal/mol)

A

10.1 15.0

2.3 × 10 1.1 × 108

oxygen adsorption (ad,2) surface reaction (surf)

A

Ea (kcal/mol)

A

− 27.0

− 4.4 × 109

− 27.7

− 1.4 × 1012

4

perovskite’s potential in this field. The apparent activation energy of methanol partial oxidation to formaldehyde is approximately 20 kcal/mol for vanadium29,30 and molybdenum oxide31 catalysts. Ethanol partial oxidation to acetaldehyde yields activation energies in the range of 10−31 kcal/mol for vanadia catalysts.15 Schwartz et al.32 studied the partial oxidation of C1 to C3 aliphatic alcohols over Pt catalyst, and reported similar activation energies at around 13 kcal/mol. It is believed that the activation energy of POB is close to those of the aforementioned alcohols. Indeed, lanthanum−transition metal perovskites, especially LaMnO3, are effective catalysts in POB.

4. CONCLUSIONS This study characterized the redox properties of LaMnO3, LaFeO3, and LaCoO3 perovskites and investigated kinetics of POB. LaMnO3 was found to have the lowest activation energy of POB at 15.0 kcal/mol. This is because LaMnO3 is more reducible and is easier to be reoxidized than LaFeO3 and LaCoO3. Enhancing both reduction and reoxidation steps is proposed to play an important role to reduce the activation barrier. Surface reaction evolving chemisorbed butanol and oxygen is probably the RDS. Future investigations of plausible butanol derivatives combined with microkinetic analysis should provide a molecular-level understanding of the POB mechanism, which is the key for rational perovskite design.

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ASSOCIATED CONTENT

S Supporting Information *

Appendix A summarizes the derived rate equations, Appendix B lists the experimental data for kinetic analysis, Appendix C shows the resultant RSS value by nonlinear regression for each model, and Appendix D presents the parameters and RSS values of the best fitted models. This material is available free of charge via the Internet at http://pubs.acs.org

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LaCoO3

Ea (kcal/mol)

AUTHOR INFORMATION

Corresponding Author

*Tel: (886) 3 463 8800, ext. 3554. Fax: (886) 3 455 9373. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Council (Taiwan) under Contract Number NSC-100-2221-E155-035-MY2.

de,2 = water desorption step Ea = activation energy, kcal/mol FID = flame ionization detector GC = gas chromatography HPLC = high-performance liquid chromatography H2-TPR = temperature-programmed reduction Kad,1 = equilibrium constant of the butanol adsorption step, (mol/h)−1 Kad,2 = equilibrium constant of the oxygen adsorption step, (mol/h)−1/2 Kde,1 = equilibrium constant of the butyraldehyde desorption step, (mol/h) Kde,2 = equilibrium constant of the water desorption step, (mol/h) Ksurf = equilibrium constant of the surface reaction step, dimensionless kad,1 = rate constant of the butanol adsorption step, 1/g kad,2 = rate constant of the oxygen adsorption step, (mol/ h)1/2/g kde,1 = rate constant of the butyraldehyde desorption step, mol/g/h kde,2 = rate constant of the water desorption step, mol/g/h k surf = rate constant of the surface reaction step, dimensionless LHHW = Langmuir−Hinshelwood−Hougen−Watson POB = partial oxidation of n-butanol rad,1 = reaction rate by assuming butanol adsorption as the rate-determining step, mol/g/h rad,2 = reaction rate by assuming oxygen adsorption as the rate-determining step, mol/g/h rde,1 = reaction rate by assuming butyraldehyde desorption as the rate-determining step, mol/g/h rde,2 = reaction rate by assuming water desorption as the ratedetermining step, mol/g/h redox = reduction−oxidation rsurf = reaction rate by assuming surface reaction as the ratedetermining step, mol/g/h RDS = rate-determining step RSS = residual sum of the squares surf = surface reaction step TCD = thermal conductivity detector TPO = temperature-programmed oxidation XRD = X-ray diffraction

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NOMENCLATURE A = pre-exponential factor, (mol/h)1/2/g for rad,2 and dimensionless for rsurf acac = acetylacetonates ad,1 = butanol adsorption step ad,2 = oxygen adsorption step de,1 = butyraldehyde desorption step 13997

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