Partial Oxidation of Ethanol to Acetaldehyde over LaMnO3

Article pubs.acs.org/IECR

Partial Oxidation of Ethanol to Acetaldehyde over LaMnO3-Based Perovskites: A Kinetic Study Bing-Shiun Jiang, Ray Chang, and Yu-Chuan Lin* Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli, Taoyuan 32003, Taiwan S Supporting Information *

ABSTRACT: A set of experiments was carried out in a continuous fixed-bed reactor to investigate the relative catalytic activities of LaMnO3 and LaMn0.95Pd0.05O3 for the partial oxidation of ethanol to acetaldehyde. Both catalysts were prepared by the sol− gel method. The resultant data have indicated that LaMn0.95Pd0.05O3 is more active than LaMnO3 and that acetaldehyde selectivities of both are close at around 90%. To gain an in-depth understanding of perovskite’s chemistry involved, kinetic analysis of the data has been conducted with the differential method. Accordingly, eight elementary reactions have been proposed by resorting to the Mars−van Krevelen redox cycle. Subsequently, these elementary reactions have been lumped into five steps comprising ethanol adsorption, oxygen adsorption, surface reaction, acetaldehyde desorption, and water desorption. This has rendered it possible to derive a set of rate equations based on the Langmuir−Hinshelwood−Hougen−Watson formalism. The exploration of these rate equations has revealed that surface reaction, evolving chemisorbed ethanol and oxygen, is rate-limiting. The estimated activation energies of 11.72 kcal/mol for LaMnO3 and 12.44 kcal/mol for LaMn0.95Pd0.05O3 are nearly identical; however, the pre-exponential factor of the latter is about twice the value of the former. This can be attributed to the better reducibility of Pd-promoted LaMnO3, thereby leading to greater reactivity in ethanol partial oxidation.

1. INTRODUCTION Perovskite has an ABO3-type crystal structure and has been widely applied.1 Sometimes, for example, in automotive emission control, the catalytic activity of the perovskite is more pronounced than those of noble metal catalysts.2 We have recently reported that by substituting manganese with trace Pd cations in the B-site position of LaMnO3 perovskite, its catalytic activity and formaldehyde selectivity in methanol partial oxidation can be substantially improved.3,4 This is attributed to the enhanced reducibility and oxygen mobility by Pd substitution in LaMnO3. Similar to methanol partial oxidation, partial oxidation of ethanol (POE, eq 1) to acetaldehyde has been known to follow a redox route,5,6 also known as the Mars−van Krevelen mechanism.7 At the outset, ethanol chemisorbs on the catalyst surface to form an ethoxyl and a hydroxyl. Subsequently, the ethoxyl dehydrogenates as an acetyl and desorbs as an acetaldehyde; the hydroxyl reduces the surface by dehydration, thus leaving an oxygen vacancy. Finally, the reduced surface regains its oxidation state by dissociative chemisorption of O2. Figure 1 portrays the Mars−van Krevelen mechanism of POE. 1 C2H5OH + O2 → C2H4O + H 2O (1) 2 Table 1 lists earlier studies of POE. It is usually conducted in the mild temperature regime (100−300 °C) under atmospheric pressure. The table reveals that vanadium-based catalysts are most frequently deployed. This can probably be attributed to the unique active phase-support interaction of these catalysts,25,26 which enhances both activity and selectivity in POE. Compared to the vanadium catalysts, relatively little effort has been devoted to the exploration of perovskite as catalysts in © 2012 American Chemical Society

Figure 1. Mars−van Krevelen mechanism of POE.

POE although the redox chemistry of perovskite is highly likely to play a central role. The current work appears to be the first to deploy both pure and Pd-doped LaMnO3-based catalysts for POE to gain insight into redox chemistry. The rate equations have been derived from the resultant experimental data according to the Mars− van Krevelen redox mechanism. This has rendered it possible to identify the rate-determining step. Moreover, the values of parameters in the rate equations involving this rate-determining step can be corroborated with the performances of the aforementioned catalysts in POE.

2. EXPERIMENTAL SECTION Two catalysts, LaMnO3 and LaMn0.95Pd0.05O3, were prepared via the sol−gel method. Metal acetylacetonates (acac) (La-, Mn-, and Pd-(acac)3) served as the precursors. Specific Special Issue: L. T. Fan Festschrift Received: Revised: Accepted: Published: 37

February April 18, April 20, April 20,

8, 2012 2012 2012 2012

dx.doi.org/10.1021/ie300348u | Ind. Eng. Chem. Res. 2013, 52, 37−42

Industrial & Engineering Chemistry Research

Article

Table 1. Selected Studies of POE to Acetaldehyde catalyst

a

entry

active phase

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17a

Ta2O5 Cu(II) or Co(II) CrOx V2O5 MoO3 LaNiO3 V2O5 V2O5 Fe2O3, Fe2O3/CaO, Fe3O4, TiO2, CaO, or SiO2 V2O5 V2O5 V2O5 V2O5 V2O5 V2O5 V2O5 LaMnO3

support NaY zeolite SiO2 SiO2 SiO2, Al2O3, or TiO2 MgO TiO2/SiO2 TiO2/SiO2 or TiO2/ZrO2 TiO2/SiO2 MCM-41 TiO2, MCM-41, or Na-promoted supports SiO2, TiO2/SiO2, or ZrO2/SiO2 Al2O3 nanosized TiO2

temp. (°C)

C2H4O selectivity (%)

ref.

150−340 450−550 240 217−297 125−290 100−550 180−240 127−427 200−300 127−277 100−180 200−400 127−427 100−200 200−250 175−220 80−220

90 100 25−67 100−62 15−92 0−20 100−90 100−17 100−57 100−45 100−40 100−20 100−10 100−10 80 82−100 100−0

8 9 10 11 12 13 14 15 16 17 5,18 19 20,21 22 6 23 24

Operated in deep oxidation environments.

Friedler and Fan and their collaborators.27−30 Parameters were estimated by the nonlinear regression using Athena Visual Studio31 based on the criterion of the least residual sum of the squares (RSS).

amounts of precursors, containing about 10 mmol A- and B-site cations, were dissolved in a 1-to-4 volumetric ratio mixture of ethylene glycol and methanol (260 mL). The mixture was vigorously stirred for 30 min. Solvents were removed in a rotary evaporator at 80 °C. The remaining paste was calcined for 2 h at 700 °C. Catalyst characterizations, including X-ray diffraction (XRD), temperature-programmed reduction (H2-TPR), and temperature-programmed oxidation (TPO), are detailed elsewhere.3,4 All reactivity tests were performed with a continuous-flow fixed-bed reactor. The exhaust from the reactor was directly fed to a dual-column gas chromatograph (SRI 8610) equipped with a thermal conductivity detector, a methanizer, and a flame ionization detector. The analytes were separated with a 5 Å molecular sieve and a Porapak Q columns. Column response factors were calculated by injecting specific amounts of known species relative to N2. While N2, O2, CO, CO2, C2H4O, and C2H5OH were identified, ethyl acetate and acetic acid were undetectable. Water was estimated based on the oxygen atom balance. Mass balances on carbon and hydrogen were both closed within ±10%. Ethanol was injected into the system by a syringe pump (KDS-100) and vaporized at the inlet at 150 °C. All tubing was wrapped in heating tape maintained at 150 °C. The feed contained approximately 88 mol % of N2 balanced with O2/ C2H5OH ratios at 0.5. All catalysts were sieved in the range of 40−80 mesh. A sample of approximately 10 mg was blended with 150 mg of SiO2 in each trial; SiO2 served as an inert diluent. The space velocity was set at 2.2 × 10−6 g h/cm3. Internal and external mass transfer limitations were examined by varying particle sizes and space velocities of the feed; they were both negligible. A thermocouple of the chromel-alumel Ktype was inserted in the middle of the catalyst bed. All trials were carried out isothermally under ambient pressure. Kinetic analysis was conducted at 170, 200, 220, and 250 °C. Catalytic results for three residence times and three O2/ C2H5OH ratios were recorded at each temperature. All data were recorded under steady state conditions. The stoichiometrically feasible independent pathway of POE was identified by the graph-theoretic method based on P-graphs developed by

3. RESULTS AND DISCUSSION Figure 2 shows ethanol and oxygen conversions as functions of temperature. Both ethanol and oxygen conversions increased

Figure 2. Conversions of (a) ethanol and (b) oxygen as functions of temperature over LaMnO3 and LaMn0.95Pd0.05O3.

with increasing temperature. Note that the reactivity of LaMn0.95Pd0.05O3 was generally higher than that of LaMnO3. Figure 3 displays the selectivities of acetaldehyde and carbon oxides. Acetaldehyde generation exceeded 90% even at relatively low temperatures. Carbon oxides gradually increased with increasing temperature accompanied by declining acetaldehyde generation. This implies that in partial oxidation of ethanol, acetaldehyde formation is the primary reaction, followed by acetaldehyde conversion to carbon oxides.15,22 To explore the intrinsic chemistry of POE to acetaldehyde, kinetic 38



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Table 3. Lumped Reaction Steps of POE designation ethanol adsorption s1 oxygen adsorption (1/2)(s7 + s8) surface reaction s2 + s3 + s5 acetaldehyde desorption s4 water desorption s6

analysis was performed in the range of 170 to 250 °C. Supporting Information, Appendix A lists the experimental results obtained on LaMnO3 and LaMn0.95Pd0.05O3 for kinetic analysis. Table 2 summarizes plausible pathways derived from the Mars−van Krevelen redox cycle of POE. All the elementary Table 2. Elementary Reactions of POE elementary reaction

s1

C2H5OH(g) + S ↔ C2H5OHS

s2

C2H5OHS + O S ↔ C2H5OS + OHS

s3

C2H5OS + OS ↔ C2H4OS + OHS

s4

C2H4OS ↔ C2H4O(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

ad,2 surf de,1

de,2

reaction step Kad,1

C2H5OH(g) + S ←→ ⎯ C2H5OHS Kad,2 1 O2 + S ←→ ⎯⎯ OS 2 K surf

C2H5OHS + OS ←→ ⎯ H 2OS + C2H4OS Kde,1

C2H4OS ←→ ⎯ C2H4O(g) + S Kde,2

H 2OS ←→ ⎯⎯ H 2O(g) + S

or water desorption is rate-limiting. Since substantial amounts of acetaldehyde and water were generated in POE, their corresponding equilibrium constants, that is, Kde,1 and Kde,2, should be inordinately large. Thus, the terms containing Kde,1 and Kde,2 in the denominator are negligible small. Supporting Information, Table C1 also summarizes the simplified rate equations according to the aforementioned assumptions. The simplified rate equations have been fitted to the experimental results (see Supporting Information, Appendix A) via nonlinear regression subject to the minimization of RSS to recover the values of parameters (see Supporting Information, Appendix D). The RSS values of rad,2 and rsurf are very close, ranging from 1.0 × 10−3 to 3.1 × 10−3 for LaMnO3 and LaMn0.95Pd0.05O3 catalysts, respectively. Nevertheless, the rate equations for rad,1, rde,1, and rde,2 have failed to converge: Their RSSs are appreciable. That is, the models of rad,1, rde,1, and rde,2 are not capable of fitting the data. This indicates that the rate-determining step should involve either oxygen adsorption or surface reaction. Figures 4 show the Arrhenius plots of the rate constants, kad,2 and ksurf, for LaMnO3 and LaMn0.95Pd0.05O3 listed in the Supporting Information, Appendix D, Tables D1 and D2. The activation energies and pre-exponential factors have been estimated from the slopes and intercepts of linear trends, respectively; see Table 4. Note that the activation energies for rad,2 and rsurf differ significantly: The former is 0.73 and 5.08 kcal/mol, and the latter, 11.72 and 12.44 kcal/mol. Moreover, the pre-exponential factor for rsurf is orders of magnitude greater than that for rad,2. It is noteworthy that while the activation energy of ethanol partial oxidation to acetaldehyde has been estimated to be in the range of 9.5 to 30.7 kcal/mol,5,9,10,12,14,15 the pre-exponential factor of the adsorption of gaseous species is frequently greater than 103.34,35 Hence, the surface reaction between chemisorbed ethanol and oxygen is more likely to be the rate-determining step in POE to acetaldehyde than the oxygen adsorption step. It should be pointed out that the activation energies for LaMnO3 and LaMn0.95Pd0.05O3 closely approximate each other: The difference is less than 1 kcal/mol, corresponding to about 6%. The pre-exponential factor of LaMn0.95Pd0.05O3, however, is approximately 2-fold higher than that of LaMnO3. The reactivity of Pd-doped perovskite is superior to that of untainted LaMnO3, and thus, the difference in the preexponential factors should be related to the difference in POE reactivities. Oyama and co-workers12 have found a similar trend in selective oxidation of ethanol over molybdenum oxide catalysts. The reactivities of MoO3 on various supports follow the trend as follows: TiO2 > Al2O3 > SiO2; however, the

Figure 3. Carbon atom selectivities of (a) C2H4O, (b) CO2, and (c) CO as functions of temperature over LaMnO3 and LaMn0.95Pd0.05O3.

entry

ad,1

reactions are expressed as bimolecular reactions. A set of stoichiometric feasible pathway, s1+ s2+ s3+ s4+ s5+ s6+1/2 s7+1/ 2 s8, has been generated from these eight elementary reactions via the aforementioned graph-theoretic method based on Pgraphs. This independent pathway can be lumped as ethanol chemisorption (s1), oxygen chemisorption (s7 and s8), surface reaction (s2, s3, and s5), acetaldehyde desorption (s4), and water desorption (s6); see Table 3. Similar pathways have been proposed for vanadium-based catalysts5 as well as ethanol oxidation to acetic acid.32 The kinetic rate expression has been obtained via the Langmuir−Hinshelwood−Hougen−Watson (LHHW) formalism.33 It takes into account the concentrations of active sites on catalyst surface. Subsequently, the rate expression is derived by assuming one of the pathways is rate-limiting while the remaining pathways are all at their equilibrium states. A detailed derivation of rate equations based on the LHHW formalism is presented in Supporting Information, Appendix B. Supporting Information, Appendix C, Table C1 presents five rate equations derived by assuming that ethanol adsorption, oxygen adsorption, surface reaction, acetaldehyde desorption, 39



Article

Figure 4. Arrhenius plots of (a) LaMnO3 and (b) LaMn0.95Pd0.05O3 based on the assumption that oxygen adsorption and surface reaction are the rate-determining steps.

that Pd-doping results in higher reactivities of oxygen adsorption (redox); this is in accord with our earlier findings.3,4 Figure 5 illustrate the parity plots between the experimentally determined reaction rates and those obtained based on the simplified rate equation for surface reaction. The experimental and estimated values are in fairly good agreement for both catalysts, thus again implying that the surface reaction is ratelimiting in POE.

Table 4. Estimated Activation Energies (Ea) and PreExponential Factors (A) Based on the Assumption of Oxygen Adsorption and Surface Reaction As the RateDetermining Steps LaMnO3

LaMn0.95Pd0.05O3

r.d.s

rad,2

rsurf

rad,2

rsurf

Ea (kcal/mol) A

0.73 12.20

11.72 1.02 × 107

5.08 1.47 × 103

12.44 2.45 × 107

4. CONCLUSIONS The current study investigated the kinetics of POE over LaMnO3-based catalysts. Pd-promoted LaMnO3 has displayed greater apparent reactivity than pure LaMnO3. On the basis of the redox cycle, mechanistic rate equations have been derived by resorting to the LHHW formalism. It is envisioned that the surface reaction between chemisorbed ethanol and oxygen is rate-limiting. The difference between the activation energies of LaMn0.95Pd0.05O3 and LaMnO3 is insignificant. Nevertheless, the pre-exponential factor of LaMn0.95Pd0.05O3 is substantially greater than that of LaMnO3. This can be attributed to the superior redox properties of Pd-promoted LaMnO3, thereby enhancing its reactivity in POE.

apparent activation energies are almost identical (∼23.6 kcal/ mol). This is ascribable to the compensation effect of quasiequilibrated ethanol adsorption and rate-limiting decomposition of ethoxide.12 Oyama and coworkers12 have, therefore, proposed that the reducibility of catalysts, which is related to the availability of empty electronic energy states, magnifies the reactivity. It is plausible that the same could be stated for our sy stem: Relatively speakin g, the red ucibility o f LaMn 0.95 Pd 0.05 O 3 is superior to that of unpromoted LaMnO3.3,4 Furthermore, the fitted Kad,2 (see Supporting Information, Appendix D, Table D2) for LaMn0.95Pd0.05O3 are mostly greater than those of LaMnO3. This appears to indicate

Figure 5. Parity plots of the reaction rates for (a) LaMnO3 and (b) LaMn0.95Pd0.05O3. 40



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Article

ASSOCIATED CONTENT

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S Supporting Information *

Appendix A presents the experimental results for kinetic analysis, Appendix B presents the derivation of the rate equation according to LHHW formalism, Appendix C presents the derived rate equations, and Appendix D presents the fitted kinetic parameters of rad,2 and rsurf. This material is available free of charge via the Internet at http://pubs.acs.org.

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XRD = X-ray diffraction

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Phone: (886) 3 463 8800, ext. 3554. Fax: (886) 3 455 9373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.-C.L. extends his special gratitude to Professor L.T. Fan for his guidance and support. The authors also appreciate the technical assistance of Dr. Andres Argoti. This work was supported in part by the National Science Council (Taiwan) under Contract Number NSC-2221-E-155-035-MY2.

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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 = ethanol adsorption step ad,2 = oxygen adsorption step de,1 = acetaldehyde desorption step de,2 = water desorption step Ea = activation energy, kcal/mol H2-TPR = temperature-programmed reduction Kad,1 = equilibrium constant of ethanol adsorption step, (mol/h)−1 Kad,2 = equilibrium constant of oxygen adsorption step, (mol/h)−1/2 Kde,1 = equilibrium constant of acetaldehyde desorption step, (mol/h) Kde,2 = equilibrium constant of water desorption step, (mol/ h) Ksurf = equilibrium constant of surface reaction step, dimensionless kad,1 = rate constant of ethanol adsorption step, 1/g kad,2 = rate constant of oxygen adsorption step, (mol/h)1/2/g kde,1 = rate constant of acetaldehyde desorption step, mol/g/ h kde,2 = rate constant of water desorption step, mol/g/h ksurf = rate constant of surface reaction step, dimensionless rad,1 = reaction rate by assuming ethanol adsorption as ratedetermining, mol/g/h rad,2 = reaction rate by assuming oxygen adsorption as ratedetermining, mol/g/h rde,1 = reaction rate by assuming acetaldehyde desorption as rate-determining, mol/g/h rde,2 = reaction rate by assuming water desorption as ratedetermining, mol/g/h rsurf = reaction rate by assuming surface reaction as ratedetermining, mol/g/h r.d.s = rate-determining step surf = surface reaction step TPO = temperature-programmed oxidation 41



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(22) Lin, Y.-C.; Chang, C.-H.; Chen, C.-C.; Jehng, J.-M.; Shyu, S.-G. Supported vanadium oxide catalysts in selective oxidation of ethanol: Comparison of TiO2/SiO2 and ZrO2/SiO2 as supports. Catal. Commun. 2008, 9, 675. (23) Jørgensen, B.; Kristensen, S.; Kunov-Kruse, A.; Fehrmann, R.; Christensen, C.; Riisager, A. Gas-phase oxidation of aqueous ethanol by nanoparticle vanadia/anatase catalysts. Top. Catal. 2009, 52, 253. (24) Najjar, H.; Batis, H. La-Mn perovskite-type oxide prepared by combustion method: Catalytic activity in ethanol oxidation. Appl. Catal., A 2010, 383, 192. (25) Wachs, I. E.; Briand, L. E.; Jehng, J.-M.; Burcham, L.; Gao, X. Molecular structure and reactivity of the group V metal oxides. Catal. Today 2000, 57, 323. (26) Weckhuysen, B. M.; Keller, D. E. Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis. Catal. Today 2003, 78, 25. (27) Friedler, F.; Tarjan, K.; Huang, Y. W.; Fan, L. T. Graphtheoretic approach to process synthesis: axioms and theorems. Chem. Eng. Sci. 1992, 47, 1973. (28) Friedler, F.; Tarjan, K.; Huang, Y. W.; Fan, L. T. Graphtheoretic approach to process synthesis: Polynomial algorithm for maximal structure generation. Comput. Chem. Eng. 1993, 17, 929. (29) Fan, L. T.; Bertók, B.; Friedler, F.; Shafie, S. Mechanisms of ammonia-synthesis reaction revisited with the aid of a novel graphtheoretic method for determining candidate mechanisms in deriving the rate law of a catalytic reaction. Hung. J. Ind. Chem. 2001, 29, 71. (30) Fan, L. T.; Bertók, B.; Friedler, F. A graph-theoretic method to identify candidate mechanisms for deriving the rate law of a catalytic reaction. Comput. Chem. 2002, 26, 265. (31) Stewart, W. E.; Caracotsios, M.; Sørensen, J. P. Parameter estimation from multiresponse data. AIChE J. 1992, 38, 641. (32) Li, X.; Iglesia, E. Selective catalytic oxidation of ethanol to acetic acid on dispersed Mo-V-Nb mixed oxides. Chem.Eur. J. 2007, 13, 9324. (33) Hougen, O. A.; Watson, K. M. Solid catalysts and reaction rates. Ind. Eng. Chem. 1943, 35, 529. (34) Dumesic, J. A.; Trevino, A. A.; Milligan, B. A.; Greppi, L. A.; Balse, V. R.; Sarnowski, K. T.; Beall, C. E.; Kataoka, T.; Rudd, D. F. A kinetic modeling approach to the design of catalysts: formulation of a catalyst design advisory program. Ind. Eng. Chem. Res. 1987, 26, 1399. (35) Amiridis, M. D.; Rekoske, J. E.; Dumesic, J. A.; Rudd, D. F.; Spencer, N. D.; Pereira, C. J. Simulation of methane partial oxidation over silica-supported MoO3 and V2O5. AIChE J. 1991, 37, 87.

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Partial Oxidation of Ethanol to Acetaldehyde over LaMnO3

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