Bimolecular Decomposition Pathways for Carboxylic Acids of

Article pubs.acs.org/JPCA

Bimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels Jared M. Clark, Mark R. Nimlos, and David J. Robichaud* National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: The bimolecular thermal reactions of carboxylic acids were studied using quantum mechanical molecular modeling. Previous work1 investigated the unimolecular decomposition of a variety of organic acids, including saturated, α,β-unsaturated, and β,γunsaturated acids, and showed that the type and position of the unsaturation resulted in unique branching ratios between dehydration and decarboxylation, [H2O]/[CO2]. In this work, the effect of bimolecular chemistry (water−acid and acid−acid) is considered with a representative of each acid class. In both cases, the strained 4-centered, unimolecular transition state, typical of most organic acids, is opened up to 6- or 8-centered bimolecular geometries. These larger structures lead to a reduction in the barrier heights (20−45%) of the thermal decomposition pathways for organic acids and an increase in the decomposition kinetics. In some cases, they even cause a shift in the branching ratio of the corresponding product slates.

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INTRODUCTION

The thermal deoxygenation of carboxylic acids is an important step in the conversion of biomass into aliphatic hydrocarbons suitable for use in renewable biofuels and as petrochemical replacements.2,3 A fundamental understanding of the thermal conversion process is essential in determining reaction conditions that lead to a desired outcome. For example, it may be desirable to restrict the degradation of organic acids to paraffinic and olefinic hydrocarbons via decarboxylation, a primary decomposition pathway under pyrolysis conditions.2−6 Or the goal may be the formation of ketene and ketene derivatives (RR′CCO) via dehydration, a second major competing mechanism.7−10 Ketenes are used in the production of acid anhydrides,11,12 β-lactones and β-lactams, 13,14 and in the synthesis of a wide range of industrial chemicals. Judicious selection of reaction conditions may allow for the selection of one these pathways over the other. Under certain experimental conditions (i.e., low acid concentrations and/or high temperatures) the thermal breakdown of carboxylic acids precedes unimolecularly.1,15−18 In general, this can be represented by a small set of reactions that can be described as occurring through competing 4-centered, pericyclic mechanisms. Decarboxylation (reaction 1) and dehydration (concerted, reaction 2, stepwise reactions 3 and 4) represent the major competing unimolecular pathways (illustrated using acetic acid). © 2014 American Chemical Society

These reactions have been studied experimentally for a number of organic acids, including formic acid,19−22 acetic acid,16,18,23−25 and crotonic acid,26−30 and a comprehensive theoretical review of these unimolecular decomposition pathways has recently been performed by Clark et al.1 It was concluded that the location (e.g., α,β) and extent of unsaturation (double or triple) provided opportunities for larger, less strained hydrogen transfer transition states. Similar to reaction 1, Received: September 13, 2014 Revised: December 2, 2014 Published: December 16, 2014 501

DOI: 10.1021/jp509285n J. Phys. Chem. A 2015, 119, 501−516

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The Journal of Physical Chemistry A

thermal decomposition of carboxylic acids. Using quantum mechanical methods, we have investigated the bimolecular chemistry and gas phase kinetics of saturated acids, acetic and butyric acids, and unsaturated acids, crotonic (α,β), isocrotonic (α,β), and vinylacetic (β,γ) acids and have compared the results derived from unimolecular chemistry. The effect of water and acid on the degradation of formic acid is also presented in an effort to validate the model chemistry employed within the current work [M06-2X/6-311++G(2df,p)] against a body of published experimental data.

vinylacetic acid can decarboxylate via a 4-centered transition state, with a barrier height of 69.3 kcal mol−1. Because of the β,γposition of the double bond in vinylacetic acid, a 6-center transition state is also available with a barrier of 39.2 kcal mol−1, reaction 5.

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These results are consistent with the experimentally measured value of 37.0 kcal mol−1, as reported by Bigley et al.27 Similarly, α,β-unsaturated organic acids with eclipsed −CR1R2H and −OH groups, for example, isocrotonic acid, can dehydrate via a 6-centered transition state with barriers that are 20−30% reduced from the 4-centered transition states, reaction 6.

COMPUTATIONAL METHODS The calculations presented here were performed using the Gaussian09 suite of programs.39 The structures, energies, and kinetic parameters of all species were fully optimized through use of the hybrid meta-GGA functional M06-2X of Truhlar and Zhao40 with the 6-311++G(2df,p) basis set. Convergence for each calculation was verified by the use of tight convergence criteria and ultrafine pruned (99 590:99 radial shells and 590 angular points per shell) integration grids. The M06-2X method has been shown to perform exceptionally well against databases involving thermochemistry (TC177), diverse barrier heights (DBH76), and noncovalent interaction energies (NCIE53) with mean errors of 1.3, 1.2, and 0.37 kcal mol−1, respectively.41 All transition states were identified as first-order saddle points, attested to by the presence of a single imaginary frequency corresponding to the motion along the reaction coordinate. Further, intrinsic reaction coordinate (IRC) calculations with a step size of 0.1 amu0.5 bohr were carried out to establish a connection for each transition state with the appropriate reactants, intermediates, and/or products. Vibrational frequencies were modeled using the rigid rotor harmonic oscillator (RRHO) approximation. Charge density was calculated using natural population analysis. The bimolecular reactions represented in this work are assumed to follow a three-step mechanism: step 1, R1 + R2 ⇌ PreRC; step 2, PreRC → PostRC; step 3, PostRC ⇌ products This mechanism involves a rapid pre-equilibrium between the reactants and a prereactive complex (PreRC) and between products and a postreactive complex (PostRC). The prereactive complex proceeds through internal rearrangement leading to the formation of product or a postreactive complex that dissociates to products. Considering the forward reaction, if k1 and k−1 represent the forward and reverse rate constants for the first step and k2 corresponds to the second step, a steady-state analysis results in an overall rate constant which can be written as

Unimolecular reactions rely on unique structural motifs to provide larger, less strained transition state geometries. In the absence of such features, the size of transition state geometries can only be enlarged by the inclusion of a catalyst that can facilitate proton transfer, such as water or a second acid molecule. This process is illustrated in reactions 7a and b for acetic acid.

Reactions 7a and 7b have been studied experimentally and theoretically in the case of formic acid.19−21,31 In this instance, the presence of water not only alters the rate of decomposition of formic acid, but changes the dominant decomposition pathway from dehydration to decarboxylation. This effect of water on the rate and selectivity of acid decomposition is not without precedence. For example, water has been shown to enhance the rate of thermolysis of the HO2 radical32−34 and to affect the rate and product branching ratio of isoprene peroxy radicals formed during the atmospheric oxidation of isoprene.35−37 Water-assisted, thermal decomposition of organic acids could be of particular importance when considering the fast pyrolysis (rapid, thermal decomposition of organic compounds in the absence of oxygen) of biomass. Water can make upward of 15−20% of the mass of the produced bio-oil.38 This amount of water will undoubtedly have a significant effect on the rate of decomposition of organic acids, as well as alter the resulting landscape of the products formed. Similarly, large concentrations of organic acids or other possible hydrogen shuttles (e.g., alcohols) will also increase the rate of decomposition and alter the product branching ratio. The present work explores the effects of bimolecular chemistry in the thermal decomposition of carboxylic acids through acidassisted and water-assisted catalytic reactions. The effects of such interactions will have a large impact on the kinetics of gas-phase

k=

⎛AA ⎞ k1k 2 = ⎜ 1 2 ⎟exp[− (E1 + E2 − E−1)/RT ] k −1 ⎝ A −1 ⎠

(1)

where E1 and E−1 represent the energies for the formation and dissociation of the PreRC, respectively, and where E2 is the energy barrier separating the pre- and postreactive complexes. The quantities denoted as An (n = −1, 1, and 2) are the preexponential factors for steps 1 and 2. Since formation of the PreRC is a barrierless process, E1 is zero, and the overall activation energy for steps 1 and 2 may be represented as Ea = E 2 − E−1 = (E TS − EpreRC) − (∑ E R − EpreRC) = E TS −

∑ ER (2)

where ETS, EpreRC, and ER are the total energies, including zeropoint corrections, of the transition state, prereactive complex, and reactants, respectively. Under high pressure conditions, the 502


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The Journal of Physical Chemistry A activation energy may be found as a difference between energies of the transition state and the reactants, without need to identify the prereactive complex. Applying basis statistical thermodynamic principles, equilibrium constant of the rapid pre-equilibrium between reactants and the prereactive complex is given as Q preRC

Keq(T ) =

∑ QR

exp[−(EpreRC −

∑ ER )/RT ]

temperature in Kelvin, Erel represents the energy of the higher energy trans conformer to the minimum energy cis conformer, and Ek is the relative energy of the k-th conformer from the minimum energy cis conformer. For the cis isomer, Ek is equal to zero. All kinetic calculations are based on geometric and thermochemical quantities as established at the M06-2X/ 6-311++G(2df,p) level of theory. The performance of the employed computational methods in describing four-center and six-center proton-transfer transition states was established in a previous publication.1

the the the the

(3)

Under high-pressure conditions, an equilibrium distribution of reactants is maintained in a unimolecular process, and the classical TST formula can be applied to calculate k2 k 2(T ) = κ

kBT QTS exp[−(E TS − EpreRC)/RT ] h Q preRC

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RESULTS Energetics. Formic Acid. The pyrolysis of carboxylic acids can proceed catalytically in the presence of water or other hydrogen donor/acceptor groups (including other acids). In the case of formic acid, water not only lowers the barrier heights to the various routes of decomposition, but plays a key role in altering the product branching ratio, H2O/CO2 (dehydration/ decarboxylation). Under dry conditions, gas-phase experiments show that the yield of H2O (dehydration path) is substantially greater, ∼10 times, than the yield of CO2.19,20,22,31,52 Conversely, the thermal degradation of formic acid under aqueous conditions results primarily in decarboxylation,53−55 as shown by Savage et al., who measured the [H2O]/[CO2] ratio for the aqueous decomposition of formic acid to be 0.034 ± 0.018. The following discussion will show that the current model chemistry is capable of reproducing the effects of water on the branching ratio of formic acid thermolysis. Water catalyzed, concerted dehydration of formic acid, including the formation of a prereactive complex, is shown in reaction 8.

(4)

where kB is Boltzmann’s constant, h is Planck’s constant, R is the ideal gas constant, T is temperature in Kelvin, QTS is the molecular partition function of the transition state, QpreRC is the molecular partition function of the prereactive complex, and κ(T) is the asymmetric Eckart tunneling factor.42−45 Transition state theory corrected for tunneling effects has been successfully used to calculate the rate coefficients for hydrogen abstraction reactions.46−51 The kinetics of the reverse bimolecular reactions considered, those involving a postreactive complex, are determined via a similar treatment as applied to the forward reaction using eqs 1−4. In this case, the products are involved in a rapid preequilibrium with a postreactive complex. The overall rate constant, as determined from the product of eqs 3 and 4, was used to model all unimolecular reactions, shown in eq 5 κ (T ) = κ

kBT QTS exp[−(E TS − E R )/RT ] h QR

(5)

The calculated rate constants, k(T), were then fitted to a standard Arrhenius expression, eq 6, to obtain the kinetic rate parameters A and Ea κ(T ) = A exp( −Ea /RT )

Although prereactive complexes are considered for reaction 8 and all subsequent reactions, one should keep in mind that such complexes will become less important as the temperature rises. Step-wise dehydration (see reactions 3 and 4) is also affected by the presence of water, as illustrated in reactions 9 and 10 for the formation and dehydration of dihydroxycarbene, respectively.

(6)

The calculated rate constants for the different decomposition pathways are then used to simulate the overall decomposition of each organic acid. Product branching ratios are determined as a function of temperature and acid/water starting concentrations to ascertain the propensity of a given acid to undergo decarboxylation or dehydration, leading to the formation of specific products (i.e., ketones). This information becomes useful when developing specific strategies for the conversion of acids into hydrocarbon targets or into synthetically important ketene intermediates. Kinetic simulations (integration of the rate equations) were performed using Python. The population distribution of the cis/trans-RC(O)−OH rotational conformers used in the simulations follows a Boltzmann distribution, as defined below. Ntrans exp( −Erel /RT ) = N Ntotal ∑k =total1 exp( −Ek /RT )

Reaction 11 represents the water catalyzed decarboxylation of formic acid.

(7)

The temperature dependent ratio on the left-hand side of eq 7 is a measure of the equilibrium ratio of the trans conformer to the total concentration of the acid. The ratio of the cis conformer is found by difference. In eq 7, R is the ideal gas constant, T is the 503


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Figure 1. (A) Total loss of formic acid arising from unimolecular (dashed dot line) and bimolecular (solid line) thermal decomposition mechanisms as a function of acid concentration and temperature. (B) The effect of water concentration on the product branching ratio of formic acid at different acid concentrations at a temperature of 750 K. Simulation time is 5 s.

unimolecular and acid-catalyzed chemistry can be seen to happen in the range of 1019,20 molecules cm−3 of formic acid, as evidenced by an increase in the slope, consistent with the crosspoint shown in Figure 1A. At the lowest formic acid concentration modeled, 1.0 × 1018 molecules cm−3, and an initial water concentration of 1.0 × 1016 molecules cm−3, the [H2O]/[CO2] ratio is 2.8. It should be noted that any of the initial branching ratio values at this water concentration are the same as in the absence of water. When the concentration of formic acid is increase by over 2 orders of magnitude to 2.1 × 1020 molecules cm−3, the branching ratio increases to 9.6, consistent with that reported by Blake et al.19 ([H2O]/[CO2] ≈ 10) measured at the same formic acid concentration. As the concentration of water increases from 1 × 1016 to 1022 molecules cm−3, the branching ratio drops significantly to 0.4, regardless of the initial acid concentration used. This switch from dehydration to decarboxylation as the major decomposition pathway supports the outcome of aqueous phase formic acid decomposition experiments.53,54 The change from H2O to CO2 selectivity can be explained by comparing the activation energies and pre-exponential factors for dehydration and decarboxylation in the presence and absence of water. Under dry conditions, the activation energy for decarboxylation, 69.1 kcal mol−1, is 2.7 kcal mol−1 lower than that for dehydration, 71.8 kcal mol−1. At 750 K, this would result in a factor of 6 difference in the observed reaction rate constants, in favor of decarboxylation. However, the pre-exponential factor for dehydration, 3.43 × 1014 s−1, is ∼15× larger than that for decarboxylation, 2.28 × 1013 s−1. Taken together, it becomes clear that dehydration is the preferred decomposition pathway in the absence of water. When water is present, the activation energies for decarboxylation and dehydration are lowered to 46.5 and 50.3 kcal mol−1, respectively. The difference in activation energy between the two pathways increases to 3.8 kcal mol−1, resulting in a factor of 10 difference in the observed rate constants. In addition, the difference in the pre-exponential factors (4.11 × 10−12 cm3 molecule−1 s−1 and 8.09 × 10−13 cm3 molecule−1 s−1 for dehydration and decarboxylation, respectively) is reduced significantly to a factor of 5. As a result of these changes, the total effect of water is to preference the decomposition of formic acid toward decarboxylation. The results presented herein for the thermal decomposition of formic acid demonstrate the utility of the current model chemistry to accurately describe the various decomposition pathways of organic acids in the gas-phase under fast pyrolysis conditions.

The 4-centered barrier height for decarboxylation (no water) is calculated to be 70.1 kcal mol−1,1 at the M06-2X/6-311++G(2df,p) level of theory. With the addition of one water molecule, we calculate that the barrier to loss of CO2 is reduced to 44.0 kcal mol−1 (37%). In the case of dehydration, the barrier height is reduced from 70.61 to 46.6 kcal mol−1 (34%). Whereas the barrier height for both decomposition pathways appear to be equivalent in the absence of water, the presence of one molecule of H 2 O shifts the balance in favor of decarboxylation, which is 2.6 kcal mol−1 lower than that for dehydration. Considering the current model chemistry, this energy difference is small, although not insignificant (the uncertainty for the method is 1.5 kcal mol−1). To more fully understand the effect of water on formic acid decomposition, an analysis of the kinetics is required. Figure 1A describes the competition between uni- and bimolecular thermal decomposition mechanisms of formic acid at different temperatures. In Figure 1A, the dashed-dot lines denote unimolecular loss pathways and the solid lines represent the loss of formic acid due to bimolecular mechanisms. The data in Figure 1A reveals that under high temperature and/or low acid concentration, unimolecular reactions are the principle decomposition pathways for formic acid in the absence of any initial water. At 750 K, the unimolecular/bimolecular transition takes place at ∼6 × 1019 molecules cm−3. If this concentration represents 10% of the total gas concentration in a batch reactor setup, the resulting pressure would need to be >61 atm to achieve bimolecular decomposition. Concentrations below this would result in thermal decomposition dominated by unimolecular processes. This example is within the range of experimental pressures employed in the literature for the thermal decomposition of formic acid in batch systems (50−300 atm).53,56 Having said this, such high pressures may prove problematic in flow-type reaction setups where pressures are much lower.19,20 Figure 1B illustrates the effect of initial water concentrations on the decomposition of formic acid as a function of formic acid concentration at 750 K. The complete kinetic model used to simulate the thermal reactions of formic acid is given in Supporting Information Table 1. The simulation was run with acid concentrations ranging between 1.0 × 1018 and 2.1 × 1020 molecules cm−3 at 750 K and with water concentrations ranging from 1.0 × 1016−1.0 × 1022 molecules cm−3. The time scale used was that relevant to fast pyrolysis, 5 s. The results in Figure 1B show that the dehydration/decarboxylation branching ratio for formic acid is dependent on the initial concentration both of formic acid and water. The cross over point between 504


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The Journal of Physical Chemistry A Table 1. Activation Energies (kcal mol−1) for the Water-Catalyzed Decomposition of Various Organic Acidsa decarboxylation acetic butyric vinylacetic crotonic isocrotonic

acetic butyric vinylacetic crotonic isocrotonic

dehydration ΔEab

unimolecular

water

70.4 70.3 34.0 (69.4)c 66.3 65.1

57.1 13.3 58.6 11.7 56.4 −22.4 (13.0) 45.8 20.5 44.7 20.4 1,1-enediol formation

unimolecular

ΔEab

water

75.0 74.0 69.7 79.9 56.4 (79.4)d

48.6 49.4 43.5 60.1 42.7 (59.1)e 1,1-enediol dehydration

26.4 24.6 26.2 19.8 13.7 (20.3)

unimolecular

water

ΔEab

unimolecular

water

ΔEab

70.2 70.0 65.0 73.4 35.3 (74.2) f

36.2 37.1 32.1 44.8 48.1 (43.8)g

34.0 32.9 32.9 28.6 −12.8 (30.4)

70.4 69.8 63.9 87.5 85.9

44.7 44.3 37.9 62.7 61.1

25.7 25.5 26.0 24.8 24.8

a

Activation energies are relative to the lowest energy conformation of the parent acid. bRelative to the lowest energy unimolecular transition state. 6-Centered, unimolecular transition state; 4-centered, unimolecular Ea is 69.4 kcal mol−1. d6-Centered, unimolecular transition state; 4-centered, unimolecular Ea is 79.4 kcal mol−1. e8-Centered, water-catalyzed transition state; water + 4-centered, unimolecular Ea is 59.1 kcal mol−1. f6-Centered, unimolecular transition state; 4-centered, unimolecular Ea is 74.2 kcal mol−1. gWater +6-centered, unimolecular transition state; water + 4-centered, unimolecular Ea is 43.8 kcal mol−1. c

Bimolecular Catalyzed Reactions. The thermal degradation of organic acids can be catalyzed by a water molecule or another carboxylic molecule. The interaction of these catalytic species can have a pronounced effect not only on the rate of decomposition but on the resulting product branching ratios as well. The following sections will discuss the energetics of water- and acid-catalyzed reactions on a set of saturated, α,βunsaturated, and β,γ-unsaturated acids. Finally, the results of kinetic simulations derived from these calculations will be presented. Water Catalyzed Reactions. The thermal decomposition of organic acids in the presence of water is illustrated by reactions 12−15. Although these reactions illustrate the role of water in the thermolysis of acetic acid, it should be understood that they apply generally to all of the acids discussed herein. The presence of water increases the size of the transition state for concerted dehydration from a 4-center (unimolecular transition state, shown in reaction 2) to a 6-center configuration, reaction 12, where the water molecule acts as both a proton donor and acceptor.

Because of the presence of the α-carbon, decarboxylation of larger acids (those larger than formic acid) results in the formation of Cn−1 hydrocarbons, instead of H2, reaction 15.

The M06-2X/6-311++G(2df,p) activation energies for the water-assisted decomposition of various organic acids through decarboxylation and dehydration pathways are given in Table 1. The change in activation energy between the water-catalyzed

The process is similar for the first half of the stepwise dehydration pathway that leads to the formation of a 1,1-enediol intermediate, Reaction 13, with the proton being transferred to the carbonyl oxygen in lieu of the −OH group. Figure 2. Activation energies (kcal mol−1) for unimolecular and watercatalyzed decarboxylation (A), dehydration (B), 1,1-enediol formation (C), and 1,1-enediol dehydration (D) of acetic acid (red), butyric acid (blue), vinylacetic acid (green), crotonic acid (black), and isocrotonic acid (magenta). The dotted lines are meant to help guide the eye from the unimolecular to the water-catalyzed values. The yellow stars represent low energy, 6-centered unimolecular transitions states.

The dehydration of the 1,1-enediol intermediate results in C−O double bond formation, reaction 14. 505


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Table 2. Activation Energies (kcal mol−1) for the Acid-Catalyzed Bimolecular Decomposition of Various Organic Acids with 6- and 8-Center Transition State Geometriesa decarboxylation 6-center acetic butyric vinylacetic crotonic isocrotonic

acetic butyric vinylacetic crotonic isocrotonic

62.7 65.4 62.0 49.1 48.0

dehydration ΔEab

6-center

7.7/25.3 4.9/25.0 −28.0/−11.6 17.2/32.6 17.1/30.8

51.7 52.1 47.8 69.0 68.4

8-center 45.1 45.3 45.6 33.7 34.3 1,1-enediol formation

ΔEab

8-center 34.6 36.4 29.3 45.9 45.9 1,1-enediol dehydration

23.3/40.4 21.9/37.6 21.9/40.4 10.9/34.0 −12.0/10.5

6-center

8-center

ΔEab

6-center

8-center

ΔEab

38.7 38.7 34.7 46.5 45.7

17.3 18.3 13.2 30.2 30.2

31.5/52.9 31.3/51.7 30.3/51.8 26.9/43.2 −10.4/5.1

50.1 49.3 41.5 67.2 65.5

34.3 34.4 27.9 50.9 50.2

20.3/36.1 20.5/35.4 22.4/36.0 20.3/36.6 20.4/35.7

a Activation energies are relative to the lowest energy conformation of the parent acid. bRelative to the lowest energy unimolecular transition state (see Table 1) and reported as 6-center/8-center.

and stabilized by the β,γ-double bond. This results in a lower reaction barrier (69.7 kcal mol−1) and a reduction in the overall endothermicity of concerted dehydration to ∼30 kcal mol−1. In contrast to saturated and β,γ-unsaturated acids, the presence of α,β-unsaturation leads to greater instability in the dehydration transition state (∼80 kcal mol−1) and a larger endothermicity (∼49 kcal mol−1) of the ketene product. This results from the addition of an additional cumulated carbon center in line with that of the forming ketene product (>CβCαCO). The presence of water lowers the energy required for saturated and β,γ-unsaturated acids to cross the dehydration transitional saddle point by ∼26 kcal mol−1. These values indicate that the presence of a β,γ-double bond is of no additional advantage to concerted dehydration in the presence of water. Water acts to a lesser extent to lower the barrier to direct dehydration (∼20 kcal mol−1) for α,β-unsaturated acids. This is the result of

and noncatalyzed transitions states is shown graphically in Figure 2. The data indicates that the presence of the β,γ-double bond does not affect decarboxylation in the presence or absence of water. As published previously,1 the unimolecular (4-center) barrier heights for the decarboxylation of saturated and β,γunsaturated acids (vinylacetic acid) are similar, ∼70 kcal mol−1. Note that the vinylacetic acid has a 6-centered transition state with an energy of 34.0 kcal mol−1. In the presence of a water molecule, this relationship does not change. Water lowers the 4-centered barrier to CO2 loss by ∼13 kcal mol−1 for each molecule as shown in Table 1. The interaction of water molecules with α,β-unsaturated acids leads to an even larger lowering of the decarboxylation barriers. The unimolecular decarboxylation barrier heights of α,βunsaturated acids are ∼4 kcal mol−1 lower (∼65 kcal mol−1) than those for saturated and β,γ-unsaturated acids. Water acts to lower the decarboxylation barrier by ∼20 kcal mol−1 for these acids, which is ∼8 kcal mol−1 lower than acids with saturated α-carbons. In α,β-unsaturated acids, the α-carbon, which receives the proton, is sp2 hybridized. The greater “s” character of this carbon results in a larger electron density, drawn from the π-electrons of the double bond, that can facilitate the formation of a new C−H bond. Water enhances transfer of a proton to the α-carbon in two ways. First, involvement of a water molecule creates a larger, less-strained 6-center transition state. This larger transition state geometry allows for more efficient, linear overlap of the participating molecular orbitals, facilitating the transfer. The linear dependency of the α-C···H−O proton transfer is not surprising as it is very similar to that seen in hydrogen bonding. Second, when water is present, the negative charge on the α-C in the transition state is increased by 0.19e, facilitating C−H bond formation. Direct dehydration leads to the formation of ketene products (>CαCO). Ketenes can be characterized as a cumulated diene system, a situation in which two double bonds share a common carbon atom center (e.g., allene, CH2CCH2). For saturated acids in the absence of water, the formation of ketene products is endothermic by ∼36 kcal mol−1, with a barrier height of ∼75 kcal mol−1. For vinylacetic acid (β,γ-unsaturated, CH2CHCH2C(O)OH), the formation of cumulated double bonds in the ketene functionality become conjugated with

Figure 3. Activation energies (kcal mol−1) for unimolecular and acidcatalyzed decarboxylation (A), dehydration (B), 1,1-enediol formation (C), and 1,1-enediol dehydration (D) of acetic acid (red), butyric acid (blue), vinylacetic acid (green), crotonic acid (black), and isocrotonic acid (magenta). The dotted lines are meant to help guide the eye from the unimolecular to the 6-center and the 8-center acid-catalyzed values. The yellow stars represent low energy, 6-centered unimolecular transitions states. 506


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The Journal of Physical Chemistry A stronger α-C−H bonds in the α,β-unsaturated acids compared to saturated and β,γ-unsaturated acids. The two-step dehydration of organic acids first involves the formation of geminal enediol intermediates. Once formed, the intermediates can either reform the starting acid or move through a dehydration step to form ketene products. The activation energies shown in Table 1 indicate that the formation of the 1,1-enediol intermediate is lower in energy than the concerted dehydration pathway for all the acids studied. For the saturated acids, the activation energy for the second step, dehydration of the 1,1-enediol intermediate, is equal in magnitude to that of the first step, and both are ∼4 kcal mol−1 lower than that for concerted dehydration. In fact, the overall barrier height for the stepwise dehydration of saturated acids is equal to that of decarboxylation. For example, the activation energies for decarboxylation and the two-step dehydration pathways for butyric acids are 70.3 and 69.9 kcal mol−1, respectively. The two-step, unimolecular dehydration process for β,γ-unsaturated acids is lower in energy, ∼5 kcal mol−1, than either the concerted dehydration or decarboxylation mechanisms. However, since these acids have a low lying 6-centered, unimolecular decarboxylation transition state, the two-step dehydration pathway will not be competitive. For α,β-unsaturated acids, the two step dehydration process is limited by the dehydration of the 1,1-enediol intermediate. The 4-center barrier height for this process is ∼86 kcal mol−1. In the case of isocrotonic acid, Z-CH3CHCHC(O)OH, a 6-centered transition state exists for formation of the 1,1-enediol, which has a barrier height of 35.3 kcal mol−1. However, dehydration of the 1,1-enediol remains rate limiting, rendering this low-lying unimolecular pathway insignificant. The presence of water effects the activation energies toward the formation of the 1,1-enediol intermediate for saturated and β,γ-unsaturated acids in a similar manner. The difference between the unimolecular and water-assisted mechanisms is ∼33 kcal mol−1. Similar to the direct dehydration mechanism, water has a smaller effect on 1,1-enediol formation for α,βunsaturated acids, with a difference of ∼30 kcal mol−1, as a result of stronger α-C−H bonds. For the second dehydration step, the effect of water on the transition state for all acids is similar. The water-assisted barrier height for this step is ∼25 kcal mol−1 lower. This difference represents the difference in ring strain that exists between cyclobutane and cyclohexane,57 suggesting that the role of water, in this case, is to simply relieve ring strain in the transition state and act as a proton shuttle. It is of note that while the barriers to the formation of the 1,1-enediol from α,βunsaturated acids are lowered considerable in the presence of water, the barriers for the dehydration of the 1,1-enediol are still high, indicating that the two-step dehydration mechanism will be of little importance. Water-catalysis of the low lying 6-centered, unimolecular transition states that exist for isocrotonic acid and vinylacetic acid was also explored. These are illustrated in reactions 16 and 17 for the direct dehydration and the 1,1-enediol formation reactions of isocrotonic acid and reaction 18 for the decarboxylation of vinylacetic acid.

As can be seen in Table 1, the presence of water lowers the barrier to direct dehydration of isocrotonic acid by an additional 13.7 kcal mol−1. This low bimolecular dehydration pathway has an activation energy similar to that for the decarboxylation route (∼2 kcal mol−1 lower) indicating that the dehydration of isocrotonic acid will be competitive for the decomposition of this α,β-unsaturated acid. The effect of water on the formation of the 1,1-enediol, however, is the opposite of that for direct dehydration. The barrier height for this reaction is actually raised in the water-catalyzed transition state by ∼13 kcal mol−1. The geometry of the 6-centered unimolecular transition state forming the 1,1-enediol is planar, with the proton transfer that forms the 1,1-enediol taking place within that plane. The water-catalyzed transition state requires that the −C(O)OH function to rotate ∼130° out-of-plane to accommodate the water molecule, resulting in a higher barrier height. A water-catalyzed decarboxylation transition state originating from the 6-centered unimolecular transition state of vinylacetic acid could not be identified, although it is likely that one exists. To be a major contributor to the decomposition of vinylacetic acid, water would have to lower the 6-centered, unimolecular transition state by ∼20 kcal mol−1, relative to the water-catalyzed 4-centered, unimolecular decarboxylation route to be competitive. This, however, is a mute point as the ultimate fate of vinylacetic acid, that is to say decarboxylation, will remain unchanged. Acid Catalyzed Reactions. In addition to water and alcohol catalyzed reactions, the decomposition of organic acids can be 507


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As the concentration of the 1,1-enediol intermediate increase, the probability of acid catalyzed dehydration of the 1,1-enediol becomes a possibility, with both 6- and 8-center transition states, as shown in reactions 23 and 24, respectively.

achieved via the assistance of another acid molecule. Acid catalyzed decomposition of organic acids can take place through both 6- and 8-center transition. Reactions 19 and 20 represent the 6- and 8-centered dehydration transition states (illustrated with acetic acid).

The 6- and 8-center reactions for the acid-assisted decarboxylation of organic acids are shown in reactions 25 and 26. In reaction 19, the catalytic acid molecule assists with acid decomposition by facilitating a double proton exchange involving the −OH group of the helping acid, which both receives and donates a proton. These reactions are quite similar to those involving water as a catalyst, differing primarily in the acidity of the −OH proton. In reaction 20, the proton exchange takes place involving both the −OH and the >CO moieties of the catalytic molecule. In this instance, the catalytic molecule essentially undergoes a resonance isomerization. In a similar manner, reactions 21 and 22 represent the 6- and 8-center, acid assisted formation of the 1,1-enediol intermediate, respectively.

The M06-2X/6-311++G(2df,p) activation energies calculated for the decomposition of various organic acids through acidassisted decarboxylation and dehydration pathways are given in Table 2. The change in activation energy between the watercatalyzed and noncatalyzed transitions states is shown graphically in Figure 3. 6-Center Transition States. The 6-center, acid-assisted decomposition pathways of organic acids have transition state structures that are very similar to those determined for the waterassisted geometries. However, it is clear from a comparison of Tables 1 and 2 and Figures 2 and 3 that they do not produce the same magnitude of change in the activation energies toward thermal degradation. For 6-centered, acid-assisted 508


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This represents a lowering of the activation energy, vis-à-vis the 4-centered, unimolecular transition state, by ∼25 kcal mol−1. For α,β-unsaturated acids, the barrier height is reduced by ∼34 kcal mol−1, ∼32 kcal mol−1 lower than the unimolecular pathway. Compared to the 6-centered, acid-catalyzed transition states mentioned earlier, these reductions in the decarboxylation barrier heights represent a gain of ∼18 kcal mol−1 and ∼15 kcal mol−1 for saturated and β,γ-unsaturated acids and α,βunsaturated acids, respectively. These numbers indicate that when the concentration of acid is sufficiently high, acid-assisted decarboxylation will take place primarily through the 8-centered transition state geometry. In the case of β,γ-unsaturated acids, this point is trivial as the primary decarboxylation pathway is unimolecular with a low lying decarboxylation transition state (39.3 kcal mol−1). This is borne out experimentally for vinylacetic acid, which has a measured value of 39.4 ± 1.0 kcal mol−1.30,58 For crotonic acid (α,β-unsaturated), a barrier height of 33.5 kcal mol−1 has been measured,27 consistent with that predicted for the 8-center pathway, 33.7 kcal mol−1. The 8-center concerted dehydration transitions states have activation energies that are ∼14 kcal mol−1 smaller than those calculated for the water-assisted structures. Compared to the 4-center pathway, the reduction in activation energy for the saturated and β,γ-unsaturated acids, is ∼22 kcal mol−1, which is double that for the α,β-unsaturated acids that see only a reduction of ∼11 kcal mol−1. The data in Table 2 indicates that, for saturated and β,γ-unsaturated acids, the presence of larger acid-assisted transition states pushes the [H2O]/[CO2] branching ratio toward greater water production, compared to the 4-center mechanism. This is not the case for α,β-unsaturated acids, which favor decarboxylation in the presence and absence of the larger transition states. It is clear, from an examination of the data in Table 2 that the 8-center transition states are superior to the 6-centered ones by as much as 15−20 kcal mol−1. This indicates that acid-assisted decomposition will be dictated by 8-center transition state structures.

decarboxylation, the barrier height for loss of CO2 from a saturated or β,γ-unsaturated acid is ∼63.0 kcal mol−1. This value is ∼6 kcal mol−1 larger than that in the water-assisted scenario (∼57.0 kcal mol−1). For α,β-unsaturated acids, the difference between the water- and acid-catalyzed decarboxylation pathways is only 3 kcal mol−1. The explanation for this is similar to that given previously, referencing the larger “s” character of the α-carbon of α,β-unsaturated. Similar to decarboxylation, the 6-center, acid-assisted dehydration, both concerted and stepwise, of saturated and β,γ-unsaturated acids is weaker than water-assisted dehydration, different by ∼3 kcal mol−1. Like before, it does not appear that the presence of a β,γ-double bond affects the water- or acidcatalyzed decomposition of organic acids when compared with saturated acids. For α,β-unsaturated acids, the difference between water- and acid-catalyzed concerted dehydration is ∼9 kcal mol−1. For α,β-unsaturated acids, the barrier to concerted dehydration remains high owing to the energy required to form the cumulated ketene product. In relation to the one-step dehydration pathway, the overall barrier height to the two-step dehydration involving the 6-center transition state geometry is lower in energy, ∼2 kcal mol−1 for the saturated and α,βunsaturated acids and ∼6 kcal mol−1 for the β,γ-acids. In the latter case, the formation of the ketene product (step 2) is stabilized by the conjugated 1,1-enediol formed in the first step. Dehydration of the 1,1-enediol represents the rate limiting step of the stepwise dehydration process. This deviates from the 4-centered, unimolecular mechanisms. For saturated and β,γ-unsaturated acids, the barrier height relative to the formation and the dehydration of the 1,1-enediol are equivalent within the computational uncertainty of the current method. In the absence of bimolecular chemistry, the stepwise dehydration pathway is favored by ∼5 kcal mol−1. The 6-centered acid-assisted transition states favor the formation of the 1,1-enediol, but because the dehydration barrier is ∼10 kcal mol−1 larger, the reversion of the 1,1-enediol to the acid becomes a viable loss pathway. This reduces the significance of the stepwise dehydration mechanism compared to concerted loss of H2O. The relationship between the two steps of the stepwise dehydration mechanism does not change for α,β-unsaturated acids going from the 4-center to the 6-center transition state geometry. However, the reduction in barrier height for the second step is ∼10 kcal mol−1 larger than that calculated for the other acid types. It should be noted that some α,β-unsaturated acids can decompose through loss of water through a low-lying 4-center mechanisms. Isocrotonic acid, see Figure 3, has a 6-center, unimolecular transition state for concerted dehydration that lies lower in energy than the acid-catalyzed, bimolecular 6-center geometry. Although isocrotonic acid also has a low barrier transition state to the formation of the 1,1-enediol intermediate, the second step barrier height greatly restricts the importance of this pathway. 8-Center Transition States. Unlike water, which is limited to forming 6-centered transition state geometries, acids catalysts can form 8-centered structures. For decarboxylation, the relative reduction in barrier height arising from this larger transition state, when compared with the water-assisted transition states, is the same for all acids, ∼12 kcal mol−1. As the change is the same for each type of acid, the difference can be attributed entirely to the larger transition state structure. For the saturated and β,γunsaturated acids, the barrier height is reduced to ∼45 kcal mol−1.

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DISCUSSION Geometries. The M06-2X/6-311++G(2df,p) optimized geometries for transition states involved in the water catalyzed thermolysis of organic acids are shown in Figure 4. Geometries for the 6-center and 8-center acid catalyzed reactions are shown in Figures 5 and 6, respectively. The bond lengths (rn) and hydrogen bond length (r(αorβ)) and angles (α or β) for the various acids are given in Supporting Information Tables 2−4. Inspection of the molecular structures in Figures 4−6 indicates that an organic acid must be in the correct geometrical configuration for either decarboxylation or concerted dehydration to proceed, just as they are for the unimolecular decomposition processes.1,22 The different conformations of organic acids are shown below.

Dehydration requires the cis conformation, where the −OH proton eclipses the carbonyl oxygen. As the water- or acidcatalyzed dehydration transition state indicates (Figure 4-6), a hydrogen from the participating water or acid −OH group is passed to the −OH moiety of cis-formic acid, forming a molecule 509


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Figure 4. M06-2X/6-311++G(2df,p) optimized structures representing the 6-centered transition states (TS) for the H2O catalyzed decomposition of various organic acids. Dashed-dot lines represent hydrogen bonds, dotted lines represent bond scission, and hashed lines represent bond formation. Bond lengths are in Å and angles are in degrees. Values shown are for acetic acid. The symbols refer to bond and angle positions for reference to the Supporting Information tables. For the saturated and β,γ-unsaturated acids, the yellow R group spheres represent H− (acetic acid), CH3CH2− (butyric acid), or CH2CH− (vinylacetic acid). For α,β-unsaturated acids, the yellow A and B spheres represent (A = CH3; B = H) and (A = H; B = CH3) for crotonic acid and isocrotonic acid, respectively.

Figure 5. M06-2X/6-311++G(2df,p) optimized structures representing the 6-centered transition states (TS) for the acid catalyzed decomposition of various organic acids. Dashed-dot lines represent hydrogen bonds, dotted lines represent bond scission, and hashed lines represent bond formation. Bond lengths are in Å and angles are in degrees. Values shown are for acetic acid. The symbols refer to bond and angle positions for reference to the Supporting Information tables. For the saturated and β,γ-unsaturated acids, the yellow R group spheres represent H− (acetic acid), CH3CH2− (butyric acid), or CH2CH− (vinylacetic acid). For α,β-unsaturated acids, the yellow A and B spheres represent (A = CH3; B = H) and (A = H; B = CH3) for crotonic acid and isocrotonic acid, respectively. The purple spheres represent −C(O)R portion of the catalytic acid molecule (e.g., −C(O)CH3 for acetic acid).

one −OH moieties points toward the double bond (c) and the other away (t), whereas the trans isomer gives the tt-isomer. The different 1,1-enediol rotamers are shown below.

of water. Concurrently, a α(C−H) hydrogen of the cis acid is passed back to the catalytic water or acid −OH reforming the catalytic molecule. This process ensures that the catalytic molecule is reformed, but it does not preserve the identity of the atoms in the original catalytic molecule. On the other hand, decarboxylation requires the trans isomer to move forward, as this conformer leads to the correct positioning of the acidic −OH proton. In contrast to both decarboxylation and direct dehydration, either of the conformers will lead to the formation of the 1,1-enediol intermediate. The cis acid leads to the ct-1,1-enediol rotamer geometry shown in Figures 4-6, where

Before dehydration of the 1,1-enediol intermediate can occur, one of the t-OH groups must rotate into the ct-conformation. 510


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Figure 6. M06-2X/6-311++G(2df,p) optimized structures representing the 8-centered transition states (TS) for the acid catalyzed decomposition of various organic acids. Dashed-dot lines represent hydrogen bonds, dotted lines represent bond scission, and hashed lines represent bond formation. Bond lengths are in Å and angles are in degrees. Values shown are for acetic acid. The symbols refer to bond and angle positions for reference to the Supporting Information tables. For the saturated and β,γ-unsaturated acids, the yellow R group spheres represent H− (acetic acid), CH3CH2− (butyric acid), or CH2CH− (vinylacetic acid). For α,β-unsaturated acids, the yellow A and B spheres represent (A = CH3; B = H) and (A = H; B = CH3) for crotonic acid and isocrotonic acid, respectively. The purple spheres represent −R portion of the catalytic acid molecule (e.g., −CH3 for acetic acid).

The mechanism describing the dehydration of the 1,1-enediol is similar to that illustrated for concerted dehydration, except the hydrogen used to reform the catalyst molecule comes from one of the 1,1-enediol −OH groups. Kinetics. The pre-exponential factors and activation energies for all of the reactions considered were calculated and are shown in Table 3. In the absence of bimolecular reactions all of the preexponential terms associated with 4-center transition states are within the range 1012.6−14.8, consistent with that of 1012.5−14.5, generally observed experimentally for four-centered transition states.59 For bimolecular reactions, the pre-exponential factors are several orders of magnitude smaller, in the range 109.7−11.5, consistent with the increasing complexity of these reactions. These smaller pre-exponential factors are consistent with those found by Savage and Akiya, 1011.4−12.0 for the water-catalyzed decomposition of formic acid.56 The kinetic data shown in Table 3 was used to model both the uni- and bimolecular decomposition pathways of a variety of organic acids. This was done to ascertain the effects of acid and water concentrations on the kinetics and product branching ratios, specifically dehydration/decarboxylation, regarding the thermal decomposition of organic acids and to identify under what conditions uni- and bimolecular chemistry dominates. Formic Acid. The results of kinetic simulations regarding formic acid were discussed previously and effectively demonstrated to utility of the current model chemistry to describe the competition between several decomposition pathways for formic acid. Acetic Acid/Butyric Acid (saturated). For saturated organic acids any bimolecular chemistry (whether acid−acid or acid− water) results in a shift in a higher prevalence for dehydration. The kinetic model used to simulate the thermal decomposition of acetic and butyric acids is shown in Table 4.

The transition points between the uni- and bimolecular decomposition regimes for acetic acid and butyric acid, as a function of temperature, are shown in Figures 7A and 7C, respectively. In contrast to the uni/bimolecular transition of formic acid, the transition points for saturated acids occur at much lower acid concentrations. For acetic acid, the uni/bimolecular transition point at 750 K is ∼3 × 1015 molecules cm−3, while that for butyric acid lies earlier at ∼9 × 1014 molecules cm−3. To place these values into context, let us consider a 5 s fast pyrolysis experiment wherein 0.14 g of biomass is reacted at 750 K. If we run at an inert gas flow rate of 160 L hr−1 and assume a 65% conversion rate to bio-oil with acetic acid comprising ∼5% of that oil, then the concentration of acetic acid produced is ∼2 × 1017 molecules cm−3. According to the data in Figure 7A, this concentration of acetic acid is well above the uni/bimolecular threshold and one would expect acetic acid decomposition to follow bimolecular decomposition kinetics. It should be noted that at this concentration, the total rate of loss of acetic acid is only ∼8 × 1014 s−1. Over 5 s, this represents only a 2% loss of acetic acid. Figures 7B and 7D show that for both acids the branching ratio increases in the favor of dehydration as a function of initial acid concentration, opposite to that previously discussed for formic acid. This trend is more pronounced for butyric acid, which reaches much higher [H2O]/[CO2] values at lower acid concentrations. For acetic acid, the branching ratio is ∼0.05 at an acid concentration of 1 × 1015 molecules cm−1 and increases to ∼470 at 1 × 1020 molecules cm−1. The corresponding branching ratio values for butyric acid are ∼1 and ∼4700, respectively. For both acids, the presence of water has a larger impact on the product branching ratio when the concentration of acid is low. As the acid concentration is raised, the impact of water on the product branching ratio becomes less significant. For both acetic and butyric acids, water ceases to play a major role on the branching 511


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Table 3. Calculatedb Activation Energies (kcal mol−1) and Pre-exponential Factors for Decarboxylation and Dehydration Reactionsa acid

decarboxylation

acetic butyric crotonic isocrotonic isocrotonicd vinylacetic vinylaceticd

70.3 (13.5) 70.4 (13.2) 66.7 (13.8) 65.4 (13.2) 69.3 (13.5) 39.1 (12.3)

acetic butyric crotonic isocrotonic vinylacetic

57.6 (10.9) 59.4 (10.8) 45.3 (10.9) 44.7 (11.5) 56.7 (11.2)


67.5 (11.5) 69.4 (11.6) 48.5 (11.5) 46.9 (11.5) 60.8 (12.0)


49.9 (11.0) 49.1 (11.5) 36.6 (12.2) 36.7 (12.7) 48.6 (10.5)

dehydration unimolecular 75.7 (13.7) 74.8 (13.7) 81.7 (14.8) 80.8 (14.0) 57.0 (12.1) 70.3 (13.5) water catalyzede 50.7 (10.5) 51.8 (10.6) 63.4 (11.0) 61.9 (10.9) 45.7 (9.8) acid catalyzed (6-center)f 59.8 (12.4) 57.8 (12.9) 65.5 (11.7) 64.1 (12.0) 53.7 (11.2) acid catalyzed (8-center)f 40.0 (10.6) 41.4 (12.9) 52.8 (11.8) 52.0 (12.2) 34.9 (9.7)

1,1-enediol formation

1,1-enediol dehydrationc

68.9 (12.7) 69.1 (13.0) 74.1 (14.0) 74.4 (13.0) 34.6 (11.6) 64.0 (12.9)

45.1 (13.0) 45.0 (12.6) 44.9 (12.9) 45.0 (12.8) 45.5 (13.0)

39.2 (9.7) 40.2 (10.6) 49.0 10.5) 46.3 (10.2) 35.7 (10.7)

25.3 (11.1) 25.4 (10.9) 26.2 (11.2) 26.3 (11.5) 25.9 (11.2)

46.4 (11.6) 45.5 (10.9) 55.5 (12.6) 52.8 (12.2) 42.9 (11.4)

25.8 (11.7) 26.2 (11.2) 25.8 (11.5) 25.7 (11.3) 25.6 (10.9)

24.9 (10.6) 26.4 (11.3) 44.8 (11.5) 42.1 (11.0) 21.1 (11.3)

19.8 (12.0) 19.7 (11.3) 19.6 (11.9) 20.2 (11.7) 20.2 (12.3)

a

Values are expressed as Ea (log10(A)). For the water- and acid-catalyzed reactions, the Arrhenius parameters represent those relative to the pre-reactive intermediate. All Arrhenius parameters were determined relative to the appropriate isomer of the reacting acid. bCalculated using M06-2X/6-311++G(2df,p). c Barrier height relative to the 1,1-enediol intermediate. d6-Centered, unimolecular transition state. eUnits in s−1. fUnits in cm3 molecule−1 s−1.

Crotonic/Isocrotonic Acids (α,β-Unsaturated). The branching ratio (dehydration/decarboxylation) for α,β-unsaturated acids depends significantly on the orientation of the acid group relative to the double bond. In systems, such as crotonic acid, neither acid concentration or steam will significantly impact the branching ratio. However, in the case of isocrotonic acid, both acid−acid and acid−water chemistry leads to an increase in the dehydration process. The kinetic model used to simulate the thermal degradation of butyric acid was used to calculate the product branching ratio and kinetics of crotonic and isocrotonic acids. In the case of the latter, two additional unimolecular reactions were added, representing the 6-centered, unimolecular dehydration pathways available to it. 6-centered dehydration pathways

Table 4. Kinetic Model Describing the Thermal Decomposition of Acetic and Butyric Acida isomerization cACID ↔ tACID unimolecular tACID ↔ CO2 + HC cACID ↔ H2O + ketene cACID ↔ enediol enediol ↔ H2O + ketene water assisted tACID + H2O ↔ CO2 + HC + H2O cACID + H2O ↔ H2O + ketene + H2O cACID + H2O ↔ enediol + H2O enediol + H2O ↔ H2O + ketene + H2O acid catalyzed (6-center) cACID + tACID ↔ CO2 + HC + tACID cACID + cACID ↔ H2O + ketene + tACID cACID + cACID ↔ enediol + tACID enediol + tACID ↔ H2O + ketene + cACID acid catalyzed (8-center) cACID + tACID ↔ CO2 + HC + cACID cACID + tACID ↔ H2O + ketene + cACID cACID + cACID ↔ enediol + cACID enediol + cACID ↔ H2O + ketene + cACID

cACID ↔ H 2O + ketene cACID ↔ enediol

The model result indicates, as will be discussed, that the effect of acid concentration is dependent on the stereochemistry of the α,β-double bond. For crotonic acid, increasing acid concentrations lead to smaller branching ratios, while for isocrotonic, higher branching rations will be observed. This divergence among the α,β-acids results from the presence of the low lying 6-centered unimolecular dehydration pathway available to isocrotonic acid. Unlike acid concentration, the presence of water has a uniform effect on these acids, which is to lower the branching ratio, even under high-acid concentration conditions. From Figures 8A and 8C, it is clear that the concentration required to move from a unimolecular regime to bimolecular one is greater for acids with α,β-unsaturation. In the absence of initial

a

cACID represents the cis-conformation; tACID represents the transconformation; HC is the hydrocarbon product; ketene is the ketene product; enediol is the 1,1-enediol intermediate product.

ratio at an acid concentration of ∼1019 molecules cm−3, indicating the dominance of bimolecular acid-catalyzed chemistry. 512


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Figure 7. (A) Total loss of acetic/butyric acid arising from unimolecular (dashed dot line) and bimolecular (solid line) thermal decomposition mechanisms as a function of acid concentration and temperature. (B) The effect of water concentration on the product branching ratio of acetic/butyric acid at different acid concentrations at a temperature of 750 K. Simulation time is 5 s.

Figure 8. (A) Total loss of crotonic/isocrotonic acid arising from unimolecular (dashed dot line) and bimolecular (solid line) thermal decomposition mechanisms as a function of acid concentration and temperature. (B) The effect of water concentration on the product branching ratio of crotonic/ isocrotonic acid at different acid concentrations at a temperature of 750 K. Simulation time is 5 s.

crossover concentrations, relative to saturated acids, as a result of fewer intermolecular orientations that result in a reaction. For example, when considering the dehydration of crotonic acid,

water, the concentration required for crotonic acid and isocrotonic acid are 1.0 × 1018 molecules cm−3 and 5.5 × 1018 molecules cm−3, respectively. Generally, these acids have larger 513


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Figure 9. (A) Total loss of vinylacetic acid arising from unimolecular (dashed dot line) and bimolecular (solid line) thermal decomposition mechanisms as a function of acid concentration and temperature. (B) The effect of water concentration on the product branching ratio of vinylacetic acid at different acid concentrations at a temperature of 750 K. Simulation time is 5 s.

there is only one α-proton that can be removed; whereas, there are two for butyric acid. In the absence of water, the crossover concentration for isocrotonic acid is largely unaffected by the low lying 6-center, unimolecular transition state as it is still ∼20 kcal mol−1 higher than the 8-centered acid-catalyzed route. The effect of acid concentration of α,β-unsaturated acids is dependent on the stereochemistry of the double bond. As can be seen in Figure 8B, the effect of increasing the crotonic acid concentration is to reduce an already very low branching ratio, as noted by the magnitude of the y-axis. The opposite is true for isocrotonic acid, Figure 8D, which sees a marked increase in the branching ratio, as a result of competitive bimolecular dehydration pathways. The effect of water on the product branching ratio for α,β-unsaturated acids, regardless of the double bond orientation, is to reduce the amount of dehydration that occurs as a function of increasing water concentration, similar to formic acid. As a consequence of the effects of acid and water concentration, dehydration, for crotonic acid, is excluded as a reasonable thermal degradation pathway. For, isocrotonic acid, which possesses lower lying dehydration pathways, dehydration is a dominant degradation pathway at high acid and low water concentrations. Vinylacetic Acid (β,γ-Unsaturated). In the case of β,γunsaturated acids, acid−acid and acid−water chemistry tends to increase the amount of dehydration that occurs. However, under the conditions in Figure 9, the change in branching ratio is for all intents fully decarboxylation. The kinetic model used for the modeling of vinylacetic acid is identical to that used for butyric acid, save the addition of a 6-centered, unimolecular decarboxylation pathway. 6-centered decarboxylation pathway

similarly to that observed earlier by the saturated acids, only shifted toward lower branching ratio values. Over the concentration range sampled, 1.0 × 10151.0 × 1021 molecules cm−3, the ratio never gets greater than ∼1 × 10−2, indicating that decarboxylation will be the dominant decomposition pathway over a wide range of reaction conditions. Alcohols. In addition to water and carboxylic acids, alcohols (e.g., ethanol) can also act as catalysts in the gas phase thermolysis of carboxylic acids. The manner in which they interact with organic acids in the gas phase is analogous to water (reactions 12−15). The M06-2X/6-311++G(2df,p) activation energies for the alcohol-catalyzed decomposition of the organic acids presented in this work are ∼2−4 kcal mol−1 and ∼2−3 kcal mol−1 lower, relative to water-catalyzed activation energies, for decarboxylation and dehydration, respectively. These activation energies are tabulated in Supporting Information Table 1. While not explicitly treated in the kinetic studies already presented, it is expected that they will affect the decomposition of organic acids in a manner similar to that of water.

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CONCLUSIONS The current work demonstrates that the presence of proton exchange catalyst, like water or carboxylic acids, can alter the product branching ratio of organic acids undergoing thermal degradation. At the short time scale relevant to fast pyrolysis that is modeled in this study, 5 s, the presence of such bimolecular chemistry will push the product branching ratio toward the formation of more H2O, in the case of saturated and β,γunsaturated acids and isocrotonic acid. The branching ratio for crotonic acid is unaffected by the presence of water or acid catalysts. While, for the most part, the selectivity of carboxylic acid thermolysis is markedly skewed toward dehydration in the presence of such catalysts, the total conversion is not expected to change much under pyrolysis conditions. This would indicate that homogeneous catalysis (e.g., steam cofeeding) will not be particularly amenable toward the upgrading of the organic acid component of biomass pyrolysis vapors. However, this chemistry can be significant under other conditions (lower temperatures, higher concentrations, longer time scales).

tACID ↔ CO2 + propene

Model results for β,γ-unsaturated acids, as will be discussed, indicate an over similar behavior to that seen for saturated acids, with the exception that the branching ratios are always small, favoring loss of CO2, as a result of the low lying 6-centered decarboxylation transition state available to this group of acids. Similar to the α,β-unsaturated acids, the concentration of acid, 5.8 × 1018 molecules cm−3, at which bimolecular chemistry becomes the dominant decomposition pathway is larger than for the saturated acids, see Figure 9A. In this case, the larger crossover concentrations are the result of the low lying 6-centered, unimolecular decarboxylation transitions state. The product branching ratio, Figure 9B, as modeled at 750 K, behaves

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ASSOCIATED CONTENT

S Supporting Information *

Geometries of the transition states. This material is available free of charge via the Internet at http://pubs.acs.org. 514


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Corresponding Author

*Telephone: 303-384-7790. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was conducted as part of the Computational Pyrolysis Consortium supported by the Department of Energy (DOE) Bioenergy Technology Office under contract number DE-AC-08GO28308. This research was supported in part by the Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Awards under the EERE Biomass Program administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

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Bimolecular Decomposition Pathways for Carboxylic Acids of

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