Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study

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Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study on Dimerization of Hexanoic Acid Yesol Woo, Yunsu Lee, Jae-Wook Choi, Dong Jin Suh, Chang-Ha Lee, Jeong-Myeong Ha, and Myung-June Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04605 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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Industrial & Engineering Chemistry Research

Submitted to Industrial & Engineering Chemistry Research

Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study on Dimerization of Hexanoic Acid Yesol Woo1,†, Yunsu Lee2,3,†, Jae-Wook Choi2, Dong Jin Suh2, Chang-Ha Lee3, JeongMyeong Ha2,*, Myung-June Park1,4,* 1

Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea

2

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea

3

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea

4

Department of Chemical Engineering, Ajou University, Suwon 16499, Republic of Korea

────────────────── †

These authors equally contributed to this work.

*To whom all correspondence should be addressed. (J.-M. Ha) Phone: +82 (2) 958-5837. Fax: +82 (2) 958-5209. E-mail: [email protected] (M.-J. Park) Phone: +82 (31) 219-2383. Fax: +82 (31) 219-1612. E-mail: [email protected];

1 ACS Paragon Plus Environment


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ABSTRACT Ketonization

of

hexanoic

acid

(CH3(CH2)4COOH)

to

produce

6-undecanone

((CH3(CH2)4)2CO) was performed and the reaction pathway was investigated through a kinetic study. Unlike studies suggesting β-keto acid as an undetectable intermediate of ketonization, hexanoic anhydride ((CH3(CH2)4)COOCO(CH2)4CH3) was observed to form as a result of the condensation of two hexanoic acid molecules by the loss of a water molecule. In order to investigate the role of hexanoic anhydride on the ketonization reaction, this kinetic study compared the performances of the reaction rate equations under different models for the reaction mechanism. Results indicate that ketonization occurs by the condensation of two hexanoic acid molecules producing hexanoic anhydride, followed by decarboxylation to produce 6-undecanone. By contrast, the formation of a β-keto acid is not observed in any experimental attempt.

Keywords: ketonization, hexanoic acid, hexanoic anhydride, 6-undecanone, kinetics


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1. INTRODUCTION The production of carbon-neutral biomass-derived fuels and chemicals has been rigorously studied because of concerns over global warming and depletion of fossil fuel reserves. While abundant resources of lignocellulose and marine-biomass are attractive to achieve a sustainable world without traditional fossil fuels, the oxygen-rich hydrocarbons obtained from biomass cannot be directly used in current petroleum-based fuel systems; the modification of engines and fuel transportation systems is required. The low heating values of low carbon-number hydrocarbons derived from sugars and other small molecule precursors must also be overcome to use these biofuels as a substitute for petroleum-grade fuels. In order to commercialize the biofuels exhibiting these issues, the upgrading of biomass-derived feedstocks including thromolysis oils,1-4 sugar-derivatives,5-7 and biologically obtained hydrocarbons,8-10 is required. Among the fuel candidates obtained from biomass, there has been interest in using carboxylic acids for the production of n-alkane hydrocarbons. Carboxylic acids and their derivatives including acetic acid, propionic acid, butyric acid, hexanoic acid,11-13 and methyl ketones8 can be synthesized during biological processes while acetic acid is found in pyrolysis oils,14-17 and high carbon-number fatty acids (C16 and larger) are observed in biodiesel.8,18,19 Although the content of oxygen atoms in these carboxylic acids is not very high, deoxygenation to obtain deoxygenated hydrocarbons and the condensation of small molecules to improve the fuel properties are required before using these biofuels as petroleum-like

fuels.

Hydrodeoxygenation,

decarboxylation,

decarbonylation,

and

ketonization have been suggested for producing deoxygenated hydrocarbons from biomassderived feedstocks.20,21 Among these processes, ketonization is selected in this study because it can produce high carbon-number hydrocarbons from smaller molecules using mild reaction



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conditions. While ketonization has been studied previously, its reaction pathway is still not clear and a better understanding is required. The objective of this study is (i) to elucidate the reaction pathway of ketonization, (ii) to discover the descriptor for ketonization catalysts, and (iii) to suggest an improved process to obtain high carbon-number hydrocarbons from biomass-derived oxygenated feedstocks. The ketonization of hexanoic acid, which can be obtained by biological processes,12,13 is the focus of kinetic investigation toward these goals. There are several reports for the suggested pathways of ketonization, including (i) decomposition of metal carboxylate, (ii) via an acid anhydride intermediate, (iii) via a β–keto acid intermediate, and (iv) via a ketene intermediate route (see the reference22,23 for a recent review). Many recent works on the ketonization of acetic or propionic acid suggest that ketene intermediates react with a surface carboxylate to produce ketones over several types of catalysts including heteropoly acid (H3PW12O40), TiO2, and CeO2.24-27 A density functional theory (DFT) study suggested that β-keto acid was formed from two monodentate carboxylates via α-hydrogen abstraction over oxide catalysts (including bulk Zn-Cr mixed oxide, CeO2, and monoclinic zirconia), and the subsequent decarboxylation of β-keto acid occurs to produce ketones.22,23,28-30 The formation of ketene and ketones from carboxylic acids on TiO2 were also reported as parallel processes.31 The formation of acid anhydride intermediates is suggested only to explain the formation of cyclic ketones from dicarboxylic acids,31,32 and there are several reported mechanistic studies using acid anhydrides as reactants.33,34 Despite the many suggested mechanisms, kinetic studies of ketonization have been limited to one-step reactions where carboxylic acids are directly converted to ketones.35-39 Meanwhile, anhydride was identified as the common intermediate for both alkane and ketone when stearic acid was used as the starting material over Ni catalyst.40 In addition, hexanoic anhydride was experimentally observed in our previous work on the ketonization of hexanoic 4 ACS Paragon Plus Environment

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acid,21 while no related kinetic study has been suggested for the reaction. Therefore, in the present study, a kinetic model is developed and the governing mechanism is suggested using the model developed to evaluate the effects of the anhydride fraction in the feed on the productivity of ketone products.

2. EXPERIMENTAL 2.1. Preparation of catalyst Zirconia aerogel catalyst was prepared by a sol-gel method described in previous work.21 A mixture of zirconium butoxide (Aldrich, Zr(OC4H9)4, 80% in butanol, 22.8 mL), methanol (Daejung, 99%, 51.2 mL), aqueous nitric acid (Junsei, 61%, 3.6 mL), and deionized water (3.6 mL) was stirred at room temperature for 1 h. An aqueous ammonia solution (Junsei, 28%, 0.516 mL) was added to the mixture, which was further stirred for 1 h. The mixture turned into a gel during ageing for 6 h at 60 °C followed by drying under supercritical CO2. The prepared aerogel was calcined in an air flow at 500 °C for 6 h.

2.2. Fixed-bed reaction The catalyst powder (0.1 g) was placed on quartz wool in a stainless steel fixed-bed reactor and pretreated at 773 K under a N2 flow for 4 h before being supplied to the reactor (Figure 1). The weight-hourly space velocity (WHSV) was adjusted to 4–16 h-1. All feed mixtures were prepared using hexanoic acid (HA, Aldrich, 99%) and hexanoic anhydride (HAn, Alfa Aesar, 98%) with adjusted compositions (HA/HAn = 1/0, 2/1, 1/1, 1/2, 0/1 (mol/mol)). The feeds were converted using the zirconia aerogel catalyst at 573, 583, 593, and 603 K. The detailed experimental conditions are listed in Table S1. The gaseous products obtained were cooled to -2 °C and the liquid products were collected using a 20 cm-long 15 mL-separator.



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The liquid products were mixed with 2-propanol (internal standard) and with chloroform (diluent), which were quantified using a GC-FID (Hewlett-Packard 5890 series gas chromatograph equipped with 60 m, 0.25 mm HP-5 capillary column).

Figure 1. Schematic diagram of the reaction system.

The reaction results were described using the conversions of HA and HAn and the yield of 6undecanone (UDO), which are calculated by following equations: HA reacted [mol/h] HA fed [mol/h]

HA conversion [%] =

HAn conversion [%] =

UDO yield [%] =

HAn reacted [mol/h] HAn fed [mol/h]

UDO produced × 11 [C-mol/h] HA ( fed × 6 + HAn fed × 12 ) [C-mol/h]

3. RESULTS AND DISCUSSION 6 ACS Paragon Plus Environment

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3.1. Ketonization of hexanoic acid to 6-undecanone The ketonization of hexanoic acid (HA) and hexanoic anhydride (HAn) to 6-undecanone (UDO) was performed using a fixed-bed continuous flow reaction system, and the detailed experimental conditions and results are listed in Table S1 of Supporting Information. It must be noted that negative values of HA conversion (entries 21 – 44, corresponding to the cofeeding of HA and HAn) were observed. Water, which is required for the production of carboxylic acid from anhydride or ketone, is not available in the feed, hence negative HA conversions can be attributed to negligible conversions amplified by the calculation methods. Particularly in the entries 21 – 26, an extremely small amount of acid (see the values in the parentheses) was used. Therefore, although the amount of acid produced was very small, the values close to zero in the denominator for the calculation of conversion may produce large negative values. On the basis of the experimental results presented in Table S1, the production of HAn during ketonization was clearly observed. The formation of β-keto acid and 2-butyl-3-oxooctanoic acid, which are the possible intermediates suggested in the literature,21 was not observed in our experimental set-up although the production of the unstable β-keto acid and its fast conversion to ketone on the catalyst surface cannot be completely overruled. Because the production of anhydrides during the ketonization has not been well discussed in the literature, the formation of anhydrides must be reasoned to understand the entire ketonization reaction path. For this purpose, three plausible reaction pathways including HAn as an intermediate or a product, are suggested (Scheme 1); (A) competitive formation of anhydride and ketone from HA (parallel reactions), (B) formation of HAn as an intermediate, and (C) formation of ketone from HA followed by its conversion to HAn.



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Scheme 1. Schemes for the ketonization reaction: (A) Competitive production of anhydride and ketone from carboxylic acids, (B) Formation of HAn as an intermediate, and (C) Formation of ketone from HA, followed by its conversion to HAn.

(A) Competitive production of anhydride and ketone from carboxylic acids The acid sites existing on the catalysts (zirconia in the present study and other solid acids in the literature22,32,41) can dehydrate HA to HAn regardless of the ketonization of HA to UDO. In other words, both dehydration and ketonization reactions take place competitively in the present model (A). As described in Table S1, HA conversion was almost zero when both HA and HAn were co-fed (entries 21 – 44), indicating that UDO was mostly produced from HAn via the hydrolysis of HAn followed by the ketonization of HA. In order for the hydrolysis of


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HAn to take place in the beginning, H2O should be present in the feed. However, H2O was not included in the present study. This feature can only be explained if the ketonization of HA precedes the hydrolysis of HAn (it should be noted that HA was included as an impurity for HA/HAn = 0/1 in Table S1). H2O is produced by the ketonization of HA and then, the hydrolysis of HAn follows. Consequently, although the net reaction is the production of UDO from HAn, the reaction is triggered by the ketonization of HA.

(B) Formation of HAn as an intermediate This reaction pathway explains the conversion of HA to HAn and UDO by dehydration of HA and subsequent decarboxylation of HAn. The results for entries 1 – 20 suggest that a portion of HAn produced from HA is converted to UDO while the remainder stays unreacted as HAn. Meanwhile, when co-feeding HA and HAn (entries 21 – 44), only HAn is converted to UDO while HA is not involved in the reaction. This feature probably means that two HA molecules are converted to HAn and H2O, increasing the HAn/H2O ratio to high levels (HAn pre-exists in the feed). The excess HAn over H2O results in the reverse reaction of RB,1 (returning to HA).

(C) Formation of ketone from HA, followed by its conversion to HAn UDO is directly produced from HA via ketonization, and the anhydride is produced by the addition of CO2 to UDO. This pathway also explains the experimental observations of entries 1 – 20, while the formation of excess UDO over H2O (production from HAn) may explain the predominant reverse reaction of RC,1 (returning of the produced UDO to HA).

The formation of HAn as (A) a dehydration product, (B) an intermediate during the ketonization, and (C) a final product must be discussed based on further experiments and 9 ACS Paragon Plus Environment


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mathematical analysis. The model reactions of HA and HAn were designed to confirm the role of HAn during the ketonization of HA (Table 1).

Table 1. Detailed elementary steps considered in the present work Reactions

Parameters

 → HA(s) HA + s ←   → HAn(s) HAn + s ←  Adsorption

ad K HA ad KHAn

 → UDO(s) UDO + s ←   → H2O(s) H2O + s ← 

ad KUDO

 → CO2 (s) CO2 + s ← 

ad KCO 2

KHad2O

(A) Competitive reaction pathways (RA)

 → HAn(s) + H2O(s) RA,1: 2HA(s) ← 

kA,1, KA,1

 → UDO(s) + H2O(s) + CO2 (s) RA,2: 2HA(s) + s ← 

kA,2, KA,2

(B) Formation of HAn as an intermediate (RB) Surface reaction

 → HAn(s) + H2O(s) RB,1: 2HA(s) ← 

kB,1, KB,1

 → UDO(s) + CO2 (s) RB,2: HAn(s) + s ← 

kB,2, KB,2

(C) Formation of HAn from UDO (RC)

 → UDO(s) + H2O(s) + CO2 (s) RC,1: 2HA(s) + s ← 

kC,1, KC,1

 → HAn(s) + s RC,2: UDO(s) + CO2 (s) ← 

kC,2, KC,2

Elementary steps are provided in Table 1. On the basis of the Langmuir-Hinshelwood mechanism, the reaction rates were developed as follows (see Appendix for the detailed procedure):

Reaction model (A)

(

2 RA,1 = k A,1 PHA − PHAn PH 2 O K A,1

)

DEN

2


(1)

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(

2 RA,2 = kA,2 PHA − PUDO PCO2 PH 2O K A,2

)

DEN 3

(2)

Reaction model (B)

(

2 RB,1 = kB,1 PHA − PHAn PH2O K B,1

)

(

RB,2 = kB,2 PHAn − PUDO PCO2 K B,2

DEN 2

)

DEN 2

(3) (4)

Reaction model (C)

(

2 RC,1 = kC,1 PHA − PUDO PH2O PCO2 K C,1

(

RC,2 = kC,2 PUDO PCO2 − PHAn K C,2

)

)

DEN 3

DEN 2

(5) (6)

ad ad ad ad where DEN is defined to be 1 + K HA PHA + K HAn PHAn + K UDO PUDO + K Had2 O PH 2O + K CO P . Kad 2 CO 2

denotes the adsorption equilibrium constant, and kj and Kj represent the reaction rate constant and the reaction equilibrium constant for the reaction j (Rj), respectively. Units are referred to in the Nomenclature section. It should be noted that, all the catalytic active sites were assumed to be identical in the present study. However, in the case of different active sites, only the denominator of the rate equations (DEN) might be changed while the same experimental data be used to fit those models. Therefore, the analysis might reach at the same conclusion about the role of HAn. Because powder catalysts were used, the existence of external mass transfer and internal pore diffusion limitations were assumed to be negligible. The dimensionless Mears parameters were calculated under all the experimental conditions in the present study and the values were less than 10-5 (much less than the threshold value of 0.15), confirming negligible external mass diffusion.42 The occurrence of any internal pore diffusion limitation was evaluated by the Weisz-Prater criterion. Dimensionless Weisz-Prater parameters (CWP) were calculated to be much below 0.01,42,43 indicating no internal diffusion limitation; the



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dispersion coefficient (De/usL), which is defined as the ratio of the transport rate by dispersion to that by convection (the reciprocal of the dimensionless Péclet number),42 was calculated using the correlation for effective diffusivity (De)44 and linear velocity at the inlet, and replacing the reactor length (L) with packing depth. The values under all operating conditions showed that the contribution of dispersion was less than 0.001%; thus, a plug flow model without dispersion was used to simulate the reactor as follows:

Mass balance: −u s

NR dci + ρ B ∑ Ri , j = 0 dz j =1

Energy balance: us C p ρ g

NR dT 4U = ρ B ∑ ( −∆H ) j R j + (TW − T ) dz Dt j =1

Boundary conditions: ci = ci,0 , T = Tin at z = 0

(7)

(8)

(9)

3.2. Estimation of kinetic parameters Based on the experimental reaction results in Table S1, kinetic parameters were estimated by minimizing the objective function (Fobj), which is the sum of the residuals of square errors of the objective elements as follows:

2   X i ,calc − X i ,exp   Fobj = ∑ ∑ wi     X n  i i ,exp    n  NE

(10)

where NE and wi denote the number of experimental conditions and the weighting factor, respectively. The subscript i represents the element of the object function. When the estimation of the pre-exponential factor and activation energy in the Arrhenius equation of kj is performed, differences in the orders of magnitude between parameters may 12 ACS Paragon Plus Environment

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cause extremely high statistical correlation and ill-conditioning problems; thus, the following form was used in the estimation procedure:

 E k j = k j ,ref exp  − j  R

1 1  −  T Tref

 ∆H j K j = K j ,ref exp  −  R

    

(11)

1 1   −    T Tref  

(12)

where gas constant (R) is 8.314 J/(mol·K) and the subscript ‘ref’ denotes the reference condition whose temperature (Tref) was specified to be 593.15 K. The estimation was performed using the “lsqcurvefit” subroutine in MATLAB (MathWorks, Inc.), where the Levenberg-Marquardt method was applied. Since a limited number of parameters were to be estimated using an insufficient number of experimental data points, temperature effects on the adsorption equilibrium constants were assumed to be negligible. The weighting factors for conversions and UDO yield were determined to be 1 and 2 by trial and errors, under the criterion that UDO yields fit slightly better than the conversions of HA and HAn, while maintaining balanced estimation between objective elements. Figure 2 depicts parity plots for HA and HAn conversion and UDO yield for each reaction pathway suggested in the present study. Means of absolute relative residuals (MARR) and relative standard deviation of individual errors (RSDE) are listed in the upper left corner of each diagram. As seen from the plots, reaction rates based on the model B fit the results better than the other two models. Model A fails to explain the high HA conversion (first diagram of Figure 2a), while HAn conversion at high values exhibits large deviation when model C was used (second diagram of Figure 2c). In addition, model C suggests that both HA 13 ACS Paragon Plus Environment


and HAn may be converted to produce UDO in the case of co-feeding. Considering that the complexity of each reaction rate set (number of kinetic parameters) is same, the estimated results indicate that HAn is mostly produced as an intermediate during ketonization.

HA conversion [%]

a.

HAn conversion [%]

UDO yield [%] 50

30 20 10 0

MARR = 19.29% RSDE = 15.73%

80 60 40 20 0

0

10

20

30

40

50

20

40

60

20 10

0

Experimental data [%]

10

20

30

40

50

Experimental data [%] 50

40 30 20 10 0

MARR = 26.67% RSDE = 19.66%

80

Calculated data [%]

MARR = 16.96% RSDE = 17.88%

Calculated data [%]

Calculated data [%]

30

80

50

60 40 20 0

0

10

20

30

40

50

MARR = 20.14% RSDE = 14.61%

40 30 20 10 0

0


c.

MARR = 34.46% RSDE = 21.88%

40

0 0


b.

Calculated data [%]

MARR = 35.17% RSDE = 21.84%

40

Calculated data [%]

Calculated data [%]

50

20

40

60

80

0


10

20

30

40

50


50

50

40 30 20 10 0

MARR = 29.94% RSDE = 19.95%

80

Calculated data [%]

MARR = 125.92% RSDE = 148.18%

Calculated data [%]

Calculated data [%]

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60 40 20 0

0

10

20

30

40

50


MARR = 34.02% RSDE = 28.50%

40 30 20 10 0

0

20

40

60

80


0

10

20

30

40

50


Figure 2. Parity plots for HA conversion (first column), HAn conversion (second column), and UDO yield (third column) for reaction pathway (a) competitive production of anhydride and ketone from carboxylic acids (model A), (b) formation of HAn as an intermediate (model B), and (c) Formation of ketone from HA, followed by its conversion to HAn (model C). Dashed lines represent ±20% error from the experimental values.

The estimated parameters are listed below:


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 177, 057  1  1 k B,1 = 2.14 × 10−4 exp  −  −  R  T 593.15[K]     151,343  1  1 k B,2 = 8.16 × 101 exp  −  −  R  T 593.15[K]     78,806  1  1 K B,1 = 5.76 × 10−2 exp  −  −  R  T 593.15[K]     55,586  1  1 K B,2 = 8.61× 105 exp   −   R  T 593.15[K]  

[mmol/(gcat·h·Pa2)]

[mmol/(gcat·h·Pa)]

[-]

[Pa]

ad KHA = 2.33×10−3 [Pa–1] ad KHAn = 8.70×10−2 [Pa–1] ad KUDO = 2.74×10−2 [Pa–1]

KHad2O = 4.19 ×10−3

[Pa–1]

ad KCO = 2.85×10−2 2

[Pa–1]

Simulated results based on these parameters with respect to the effects of operating conditions are provided in Figure 3. Note that the conversions calculated for HA in Figure 3a correspond to the reaction results with no HAn in the feed (entries 1 – 20 in Table S1), while the others show conversion of HAn as the conversion of HA was close to zero.



a.

60 WHSV = 4 h-1

60 40

UDO yield [%]

HA conversion [%]

80 WHSV = 7 h-1 WHSV = 10 h-1 WHSV = 13 h-1 WHSV = 16 h

-1

20 0

e.

UDO yield [%]

60 40 20

40 20 0 60

UDO yield [%]

0 100 80 60 40 20

40 20

0

0

100

50 UDO yield [%]

HAn conversion [%]

20

60 WHSV = 7 h-1 WHSV = 13 h-1

80 60 40 20

40 30 20 10

0

0

100

50 UDO yield [%]

d.

HAn conversion [%]

c.

80

40

0

100

HAn conversion [%]

b.

HAn conversion [%]

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80 60 40 20 0 570

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580 590 600 Temperature [K]

610

40 30 20 10 0 570

580 590 600 Temperature [K]

610

Figure 3. Comparison between experimental data (circles) and simulated results (solid lines) when the HA/HAn ratio is (a) 1/0, (b) 2/1, (c) 1/1, (d) 1/2, and (e) 0/1. Left columns are conversions (HA conversion for (a), HAn conversion for (b)–(e), while right columns represent the UDO yield.

The conversions of HA and HAn increased with increasing reaction temperature. For pure feeding cases, pure HAn feed (actually, 96% of HAn) yields more UDO than pure HA feed case, especially when the temperature is high (Table S1 in the Supporting Information). For example, for the entries 7 and 21 (583 K and 7 h-1), the conversions and yield were 16 ACS Paragon Plus Environment

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calculated almost same. When the temperature was increased to 593 K, HAn conversion of the entry 22 was higher than HA conversion in the entry 12 under the same space velocity, leading to higher UDO yield. The degree of increase in HAn conversion (entry 23) became more severe than HA conversion (entry 17) with further increase in the temperature (603 K). Therefore, it seems that the conversion of HAn to UDO is high compared to the conversion of HA with increasing temperature. Fast conversion of HAn to UDO is also observed in the concentration profile in Figure S1 (Supporting Information), where the concentration of HAn was much lower than HA and UDO, indicating that HAn which was produced from HA as an intermediate converted to UDO very quickly. In addition, a smaller conversion of HAn was observed with larger fraction of HAn because the larger fraction of species increases the space velocity. Note that the yield of UDO increased to the maximum when increasing fraction of HAn from 0, then decreased with increasing fraction of HAn. This is seen in Figure 4, which depicts the effects of fraction of HAn on the catalytic activity (conversion and yield of UDO) as a function of reaction temperature. With increasing fraction of HAn, the conversion of HA significantly decreased (Figure 4a), which can be attributed to the excess HAn in the feed.



HA conversion [%]

a.

40 30

Temperature increasing

20 10 0

HAn conversion [%]

b.

c.

125 Temperature increasing

100 75 50 25 0 80

UDO yield [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60 40

Temperature increasing

20 0 0.0

0.2

0.4 0.6 0.8 HAn molar fraction

1.0

Figure 4. Effect of the HAn fraction on (a) HA conversion, (b) HAn conversion, and (c) –1

UDO yield, when the SV was specified to be 7 h . Temperature was increased from 573 K to 623 K by 10 K. Dashed line is the connected points for the maximum yield at each temperature.

As the HAn fraction increased, HA conversion (Figure 4a) decreased significantly and reached a value of zero. This feature is attributed to the excess amount of HAn in the feed, especially when the fraction was high. When only HA was included in the feed (no HAn), HA was converted to HAn and then converted to UDO. For further analysis, accumulated reaction rates were calculated when the temperature and SV were specified to be 573 K and SV = 7 h–1, respectively (case 1), in Figure 5. The results show that most of the HAn formed from HA was converted to UDO. However, when the HAn fraction was 0.1 (case 2 in Figure 5), the accumulated rate of RB,1 (RB,1,acc) was significantly reduced resulting in HA conversion of less than 3%. When two HA molecules combined and formed HAn and water, the amount 18 ACS Paragon Plus Environment

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of water is smaller than HAn (HAn is present in the feed), and thus, excess of HAn accelerated the reverse reaction of RB,1 instantly, leading to the formation of two HA molecules from HAn. As a result, a small amount of HA was converted to HAn and then, further converted to UDO. When the fraction was further increased to 0.5 (case 3 in Figure 5), no HA was converted and it was only HAn that was converted to UDO. Since only HAn was used for the formation of UDO and the increased HAn fraction (increased SV for HAn) decreased the HAn conversion, the UDO was decreased as observed in Figure 4c (the lowest line for T = 573 K).

Accumulated reaction rates [mmol/gcat/h]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 R1

3

R2

2 1 0 case1

case2

case3

case4

Figure 5. Accumulated reaction rates along the reactor axis when the HAn fraction (xHAn) –1

was 0 (case 1), 0.1 (case 2), and 0.5 (case 3) for T = 573 K, SV = 7 h , and 0.5 –1

(case 4) for T = 603 K, SV = 7 h .

The feature that HAn plays a key role in the production of UDO under co-feeding conditions is also observed in simulated results for different HAn fraction in Table S1. As shown in the entries 28, 34, and 40, the decreased HAn fraction (from 67 to 35%) increased HAn conversion from 32 to 63% due to the increased residence time and thus, the production of UDO was increased, resulting in the increase of UDO yield (from 23 to 30%). However, since absolute amount of HAn in the feed was also decreased (from 2.58 to 1.63 mmol/h), the degree of increase in UDO yield was not as high as that of HAn conversion. Similar behavior was observed for the entries 22 and 37. Since the feed molar flow rate of HAn in the entry 22 19 ACS Paragon Plus Environment


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(3.19 mmol/h) was lower than the entry 37 (4.02 mmol/h), both HAn conversion and UDO yield were increased in the entry 22. In this case, the degree of increase in HAn conversion was larger than the degree of decrease in the molar flow rate of HAn, and amount of total carbon in the feed (the denominator in the definition of UDO yield) was decreased from 71 (entry 37) to 39 (entry 22) mmol-C/h. For those two reasons, UDO yield was highly increased. When the temperature was increased, both HA and HAn conversions increased (Figures 4a and 4b) with the HAn conversion increasing faster than the HA conversion. As a result, the HAn fraction for the maximum UDO yield was increased with increasing temperature –1

(dashed line in Figure 4c). As depicted in the case 4 (xHAn = 0.5, T = 603 K, SV = 7 h ) of Figure 5, a small amount of HA was used for the formation of UDO via the formation of HAn, and the value of the yield was close to the maximum. Figure 4c indicates that the maximum UDO yield is a function of temperature and HAn fraction, suggesting that the optimal operating point should be determined by the feedstock price and energy cost. It is worth noting that, the analysis using the kinetic model might be limitedly applicable to zirconiabased catalyst in the present study. Different catalytic composition may involve with different reaction pathways with different intermediates; if β-keto acid, one of the reported intermediates for the ketonization, plays a significant role in the reaction network, different kinetic profile might be expected and further research works should be considered for the development of appropriate kinetic model.

4. CONCLUSIONS Among three possible models to elucidate the roles of hexanoic anhydride (HAn) during the ketonization of hexanoic acid (HA) to 6-undecanone (UDO), the reaction pathway of HA to


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HAn to UDO using HAn as an intermediate of ketonization is the most probable, based on the kinetic study. The formation of the stable HAn is confirmed unlike other intermediates suggested in literature.23,28 From the reaction pathway confirmed in this study, the ketonization of hexanoic acid is concluded to occur by the dehydration of HA followed by the decarboxylation of HAn. The two-step mechanism through HAn is not the exclusive reaction pathway for the ketonization of other carboxylic acids; although the formation of β-keto acid is not observed in any reaction result in this study, the possibility of β-keto acid or other complex intermediates is not discounted. However, the proposed pathway is supported by the experimental and kinetics results for the ketonization of hexanoic acid.



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ACKNOWLEDGEMENTS This research was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016M3D3A1A01913280), and the Human Resources Development of the KETEP grant funded by the Korea government Ministry of Trade, Industry & Energy (No. 20154010200820).


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NOMENCLATURE ci

concentration of the species i, mol/m3

Cp

heat capacity at constant pressure, J/(kg·K)

De

effective diffusivity, m2/s

Dt

tube diameter, m

DEN

denominator

E

activation energy, J/mol

Fobj

objective function

∆H

heat of reaction, J/mol

HA

hexanoic acid

HAn

hexanoic anhydride

kj

reaction rate constant for reaction j

Kad

adsorption equilibrium constant

Kj

reaction equilibrium constant for reaction j

L

length of reactor, m

NE

number of experimental conditions

NR

number of reaction

R

reaction rate, mmol/(gcat·h)

us

gas velocity, m/s

U

overall heat transfer coefficient, W/(m2·K)

UDO

6-undecanone

T

temperature, K

Tw

wall temperature, K



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Greek symbols θ

vacancy site fraction

ρb

bulk density, g/m3

Subscripts A

competitive reaction pathways

acc

accumulated

B

formation of HAn as an intermediate

C

formation of ketone from HA, followed by its conversion to HAn

calc

calculated data

exp

experimental data

i

species

in

inlet conditions

j

reactions

ref

reference


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Table of Contents (TOC) Graphic Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study on Dimerization of Hexanoic Acid Yesol Woo, Yunsu Lee, Jae-Wook Choi, Dong Jin Suh, Chang-Ha Lee, Jeong-Myeong Ha, Myung-June Park

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Supporting Information. (1) Table S1. Detailed experimental conditions and results, (2) detailed procedure to develop reaction rate equations for each pathway suggested in the present study, and (3) Figures S1 & S2. Concentration of profiles as a function of the inverse of space velocity as well as HA conversion.


Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study

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