Direct Synthesis of Linalyl Acetate from Linalool in Supercritical Carbon Dioxide: A Thermodynamic Study

Research Article

Direct Synthesis of Linalyl Acetate fromLinalool in Supercritical Carbon Dioxide:A Thermodynamic Study

The main components of Lavender and Lavandin essential oils are linalyl acetateand linalool. An enrichment of the product in linalyl acetate is of interest, sincethe market value of this component is considerably higher than that of linalool.This work presents a thermodynamic study of the synthesis of linalyl acetate fromlinalool by a catalytic transesterification with ethyl acetate in a supercritical CO2

medium. The study comprises the calculation of the equilibrium compositions ofthis reaction, and the correlation to the fluid phase equilibrium of this system,which is of interest for the determination of the maximum concentration ofreagents during the reaction, and for the study of the fractionation of the reactionproducts. The nonideal behavior of the system is accounted for by using theStryjek-Vera Peng-Robinson Equation of State (PRSV EoS). The results are usedto select optimum conditions for an experimental study of this reaction.

Keywords: Phase equilibrium, Plant cells, Supercritical fluids, Transesterification

Received: December 12, 2006; revised: February 6, 2007; accepted: February 14, 2007

DOI: 10.1002/ceat.200600407

1 Introduction

Terpene alcohols are usually employed in the fragrance and fla-vor industries and those obtained from aromatic herbs, suchas Lavender or Lavandin are widely used [1]. These compo-nents may be found in fragrances used in decorative cosmetics,fine perfumes, shampoos, toilet soaps and other toiletries, aswell as in non-cosmetic products such as household cleanersand detergents [2]. The main components of the essential oilof Lavender and Lavandin are linalyl acetate and linalool [3].The industrial uses of linalyl acetate and linalool are different.Linalyl acetate is highly appreciated as a food additive becauseof its flavor, while linalool is often used for non-food applica-tions, e.g., as a biocide. Therefore, the enrichment of the essen-tial oil in linalyl acetate is of interest, as it considerably in-creases the market value of the product. This enrichment canbe achieved by the conversion of the linalool present in theessential oil.

Linalyl acetate can be synthesized from linalool through dif-ferent reaction pathways. One of these comprises contact be-tween linalool and ketene at temperatures below 303 K in thepresence of para-toluene sulfonic acid. An alternative process

consists of the reaction of linalool and ketene in the presenceof a zinc salt of a carboxylic acid or alternatively in the pres-ence of zinc compounds [4, 5]. Finally, linalool and acetic an-hydrides produce linalyl acetate through an esterification reac-tion in the presence of a catalyst [6]. All of these preferredpathways involve the use of ketenes which are considered asenvironmentally hazardous and non-green compounds.

A cleaner pathway for the transformation of linalool into li-nalyl acetate is the catalytic transesterification using ethyl ace-tate (see Fig. 1). An interesting alternative is to perform thisreaction in supercritical CO2 media (SC-CO2) that results inthe improvement of the mass transfer of the reagents with thecatalyst [7, 8]. Additionally, the use of SC-CO2 as a solventeliminates the need for toxic organic solvents, which may con-taminate or degrade the product. Finally, a reaction carriedout in SC-CO2 can be easily coupled with a supercritical sepa-ration of the products. This type of separation also has impor-tant advantages over other conventional fractionation process-es for the food industry, i.e., achieving higher selectivities andproduct quality. Moreover, the use of CO2 has an additionalbenefit of using a non-toxic solvent under mild operating con-ditions [9].

In this work, a thermodynamic study of the synthesis oflinalyl acetate by transesterification of linalool with ethyl ace-tate is presented. The results comprise an analysis of the cal-culated equilibrium compositions and the correlation of thephase behavior of the reagents and products in supercriticalCO2. This work is required for the determination of the maxi-

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

Ángel Martín1

Verónica Silva1

Laura Pérez1

Juan García-Serna1

María José Cocero1

1 High Pressure ProcessResearch Group, Departmentof Chemical Engineering andEnvironmental Technology,University of Valladolid,Valladolid, Spain.

–Correspondence: Á. Martín ([email protected]), High PressureProcess Research Group, Department of Chemical Engineering andEnvironmental Technology, University of Valladolid, 47011 Valladolid,Spain.

726 Chem. Eng. Technol. 2007, 30, No. 6, 726–731

mum concentration of reagents in the supercritical solvent,and for the design of the subsequent separation process. Noni-deal behavior of the mixtures of the reagents and productswith SC-CO2 are described using the Peng-Robinson Equationof State as modified by Stryjek and Vera (PRSV EoS) [10]. Theresults are used for the estimation of the optimum conditionsfor the experimental study of this reaction.

2 Model Parameters

The calculations presented in this work are based on the Stry-jek-Vera modified version of the Peng-Robinson Equation ofState (PRSV-EoS) [10]. This modification improves the accu-racy of the calculation of vapor pressure and phase equilibriumin systems which include non-hydrocarbon fluids at low tem-peratures. The PRSV EoS can be written as Eq. (1)1):

P � RT

v � b� a � a�T�

v�v � b� � b�v � b� (1)

where:

a(T) = [1 + j(1 – (T/Tc)0.5)]2 (2)

Stryjek and Vera modified the temperature dependence ofthe a(T) term, as presented in Eqs. (3) and (4):

j � j0 � j1 1 � T0�5r

� �0�7 � Tr� � (3)

j0 = 0.378893 = 1.4897153x – 0.17131848x2 + 0.01196554x3

(4)

where j1 is a constant parameter characteristic of each sub-stance, used to improve the predictions of vapor pressure. Byusing this equation, each pure substance is characterized by itscritical properties, i.e., acentric factor and the j1 parameter.The values of these properties considered in this work are pre-sented in Tab. 1. The j1 parameter of linalyl acetate has been

correlated to experimental vapor pressure data in the tempera-ture range of 300–400 K [14].

For the characterization of mixtures, the classical quadraticmixing rules with two binary interaction parameters, as pre-sented in Eqs. (5)–(8), have been used:

a � �i

�j

xixjaij (5)

b � �i

�j

xixjbij (6)

aij � ��aiaj

�1 � kij

� �(7)

bij �bi � bj

21 � lij� �

(8)

The binary interaction parameters, kij and lij, between theCO2 and the other substances have been correlated to experi-mental phase equilibrium data. All the remaining binary inter-action parameters have been set to zero, due to the lack ofsuitable experimental information for their calculation. Thissimplification does not significantly affect the results in thereaction section since the fluid in this section is composed of a


Figure 1. Synthesis of linalyl acetate by transesterification of lina-lool with ethyl acetate.

Table 1. Pure component properties of PRSV-EoS.

Component Tc (K) Pc (MPa) x j1

CO2 [12] 304.2 7.37 0.225 0.04285

Linalool [12] 630.5 2.42 0.748 –0.3727

Linalyl Acetate [13] 677.6 1.91 0.646 –0.3652*

Ethanol [11] 513.9 6.15 0.644 –0.0337

Ethyl Acetate [11] 524.1 3.85 0.362 0.0228

* Value obtained in this work.

Table 2. Binary interaction coefficients of PRSV-EoS.

System Reference T (K) kij lij AD %

CO2-ethanol [15, 16] 303.1 0.060 0.015 2.50 %

313.4 0.109 –0.038 1.15 %

333.4 0.125 –0.066 2.14 %

CO2-ethyl acetate [17, 18] 313 0.0006 0.0330 4.28 %

333 –0.0006 0.0272 0.82 %

353 –0.0018 –0.0335 6.40 %

CO2-linalool [12] 313.2 0.1112 –0.1424 3.16 %

323.2 0.1062 –0.1138 1.09 %

333.2 0.1008 –0.0873 1.14 %

343.2 0.0940 –0.0616 1.13 %

CO2-linalyl acetate [13] 313 0.0693 –0.0319 0.82 %

323 0.0730 –0.0370 0.61 %

333 0.0640 –0.0138 0.64 %

343 0.0606 –0.0062 0.41 %–1) List of symbols at the end of the paper.

Chem. Eng. Technol. 2007, 30, No. 6, 726–731 Supercritical fluids 727

low concentration of the reagents and products in a SC-CO2

medium. It can affect the accuracy of the calculations in theseparation section, as the liquid phase in this section can havea high concentration of several components apart from CO2.

The interaction parameters obtained, along with the sourcesof data used to correlate these parameters, are listed in Tab. 2.This table also presents the deviations obtained in the correla-tion of each of the experimental data sets, calculated accordingto Eq. (9), where N is the number of points in the data set.Fig. 2 presents the calculated P-xy diagrams of each binary sys-tem at 313 K, as well as experimental phase equilibrium dataat this temperature. Tab. 3 presents the correlation functionsfor the temperature dependence of the binary interaction coef-ficients and the regression coefficients obtained in each case. Agood correlation with high regression coefficients are observedin all cases, except in the CO2-linalyl acetate binary system. In

this case, the reason for the lower value of the regressioncoefficient is the point at 313 K, which does not follow thetrend of the other temperatures. If this point is not consideredfor the calculation of the regression coefficient, the valuesobtained are R2 = 0.94 for kij and R2 = 0.93 for lij.

AD % = (100/N) · [R|(Pcal–Pexp)/Pexp|] (9)

The equilibrium constant of the transesterification reactionof linalool (see Fig. 1) is calculated at standard conditions as afunction of the standard Gibbs free energy for the reaction,Eq. (10). The variation of the equilibrium constant with tem-perature is calculated using the Van’t Hoff equation, Eq. (11).For the application of these equations, the enthalpies, Gibbsenergies of formation and ideal gas heat capacities of linalooland linalyl acetate have been estimated by the Joback group


Figure 2. Calculated and experimental P-xy diagrams at T = 313 K for the systems: a) ethyl acetate-CO2 [16], b) linalyl acetate-CO2 [13], c)ethanol-CO2 [15], and d) linalool-CO2 [12].

728 Á. Martín et al. Chem. Eng. Technol. 2007, 30, No. 6, 726–731

contribution method [19]. The results obtained for these com-ponents are presented in Tab. 4.

K0 ��

i

Pi � yi � �i

P0� exp

�DG0

RT

� �(10)

dln K�T�dT

� DH�T�RT2

(11)

When these properties are known, the variation of the equi-librium constant with temperature can be calculated as pre-sented in Eq. (12):

ln K � �18�99 � 22322

T� 0�165 � ln T (12)

3 Results and Discussion

The proposed phase equilibrium model can be used to selectappropriate conditions for the separation of mixtures of lina-lool and linalyl acetate. This can be performed by studying theselectivity of the fractionation, quantified by the separationfactor, a (Eq. (13)), and the yield of the extraction, quantifiedby the gas loading, L, Eq. (14):

a � Klinalool

Klinalyl� ylinalool�xlinalool

ylinalyl�xlinalyl(13)

L � g linalool � linalyl in extract

kgSC � CO2(14)

Fig. 3 presents the calculated separation factors and gasloadings for mixtures of linalool and linalyl acetate in SC-CO2,as a function of pressure and temperature in the pressure rangeof 4–12 MPa and temperature range of 313–353 K. Separationfactors of up to a = 2 are achieved with the lowest pressure

and temperature, showing that linalool is more soluble thanlinalyl acetate in SC-CO2. On the other hand, gas loadings ofup to 28 g/kg are obtained with the highest pressure and tem-perature. Therefore, the gas loading and separation factorsfollow opposite trends with pressure and temperature, and anintermediate combination of parameters with acceptable gasloading and separation factor must be selected. The calculatedseparation factors are almost independent of the compositionof the mixture of linalool and linalyl acetate, with variations ofless than 2 % with this composition. On the other hand, thepresence in the fluid of ethyl acetate and ethanol, reagent andproduct of the reaction respectively, can have a larger influ-ence.

Fig. 4 presents the separation factors and gas loadings for amixture of linalool and linalyl acetate with ethanol and ethylacetate, considering stoichiometric amounts of linalool andethyl acetate and 50 % conversion, thus leading to equalamounts of ethanol and ethyl acetate in the mixture. The pres-ence of these components in the mixture causes an increase inthe gas loading of linalool and linalyl acetate up to 31 g/kg atthe highest temperature and pressure, and a decrease in theseparation factor down to 1.7 at the lowest temperature andpressure. On the other hand, the trends of variation of theseparameters with pressure and temperature are the same as in


0

0.5

1

1.5

2

2.5

0 5 10 15

P (MPa)

se

pa

rati

on

fa

cto

r

313 K323 K333 K343 K353 K

0

5

10

15

20

25

30

0 5 10 15

P (MPa)

Ga

s L

oa

din

g (

g/k

g)

313 K323 K333 K343 K353 K

Figure 3. Variation of the separation factor and the gas loadingwith pressure and temperature for binary mixtures of linalooland linalyl acetate, calculated with the PRSV-EoS.

Table 3. Correlation functions of binary interaction coefficients ofPRSV-EoS.

System kij lij R2 (kij/lij)

CO2-ethanol 3.560 · 10–3 –4.049 · 10–3/T

–0.839 + 255.9/T 0.82/0.89

CO2-ethyl acetate 3.101 · 10–3 +1.508 · 10–1/T

–0.534 + 180.4/T 0.99/0.79

CO2-linalool 1.381 · 10–3 +1.620 · 10–2/T

0.7805 – 289.7/T 0.99/0.99

CO2-linalyl acetate 2.103 · 10–3 +1.369 · 10–2/T

0.4961 – 171.43/T 0.66/0.77

Table 4. Estimated thermodynamic properties of linalool and li-nalyl acetate.

Component DGf0

(kJ/mol)DHf

0

(kJ/mol)Cp

0

(J/mol K)

Linalool 58.85 –177.85 232.7

Linalyl acetate –97.86 –384.18 283.0


the case without ethanol and ethyl acetate. Since both ethanoland ethyl acetate are considerably more soluble than linaloolor linalyl acetate in SC-CO2, the fractionation could beperformed in two steps. In the first extractor, the ethanol by-product and unreacted ethyl acetate would be removed withSC-CO2, and in the second extractor the mixture of linalooland linalyl acetate would be separated. Considering thisscheme, the results for the binary mixtures of linalool andlinalyl acetate presented in Fig. 3 would be applicable forstudying the second fractionation.

The diagrams presented in Figs. 3 and 4 can be used toselect an adequate combination of process parameters for theexperimental study of the system. As an increase in tempera-ture causes a moderate increase in the gas loading with smallvariations of the separation factor, the highest feasible operat-ing temperature, 353 K, has been selected. On the other hand,an increase in pressure causes a noticeable reduction of theseparation factor and increases of the gas loading. Takingthis into account, an operating pressure of 7 MPa has beenselected, since this pressure allows an acceptable gas loading of5 g/kg to be achieved without an excessive reduction of theseparation factor down to 1.6. These conditions are suitablefor an experimental study of the system, which in turn, willallow a finer adjustment of the operating parameters. Fig. 5presents the ternary phase equilibrium diagram of the mix-tures of SC-CO2-linalool-linalyl acetate at the selected operat-ing conditions of 7 MPa and 353 K. This diagram can be usedto design the extractor used for the separation.

On the other hand, the maximum temperature acceptablefor the reaction in order to avoid thermal degradation of theproducts is 353 K. According to the phase equilibrium model,the minimum operating pressure required for complete misci-bility of all reagents and products at this temperature (i.e.,operation above the mixture critical point of all componentswith SC-CO2) is 13 MPa. Therefore, operating pressures in thereactor in the range of 15–20 MPa are high enough to ensure


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 5 10 15

P (MPa)

se

pa

ratio

n fa

cto

r

313 K323 K333 K343 K353 K

0

5

10

15

20

25

30

35

0 5 10 15

P (MPa)

Ga

s L

oa

din

g (

g/k

g)

313 K323 K333 K343 K353 K

Figure 4. Variation of the separation factor and the gas loadingwith pressure and temperature for binary mixtures of linalooland linalyl acetate with equimolar amounts of ethanol and ethylacetate, calculated with the PRSV-EoS.

Figure 5. Ternary phase equilibrium diagram of SC-CO2-Linalool-Linalyl acetate at 7 MPa and 353 K, calculated with the PR-EoS.

730 Á. Martín et al. Chem. Eng. Technol. 2007, 30, No. 6, 726–731

that there will not be solubility or interphase mass transferlimitations.

The application of Eq. (12) to calculate the equilibrium con-stant in this range of temperatures yields high values of theequilibrium constant, which decrease with temperature sincethe reaction is exothermic, i.e., 1 · 1025 at 298 K and 1 bar, and3 · 1018 at 353 K. The effect of pressure on the equilibrium issmall because the reaction does not cause a variation in thenumber of moles of gas. The nonideal behavior of the system,described with the fugacity coefficients, �i of Eq. (11), causesan increase of the equilibrium constant of ca. 2 % whenpressure is increased from 10 MPa to 20 MPa. The very highequilibrium constants obtained in the range of conditions ofinterest for the process indicate that the reaction will be practi-cally irreversible under these conditions. Thus, it will only beaffected or limited by kinetic factors or by side reactions.

4 Conclusions

A process for the valorization of Lavender and Lavandin essen-tial oils by the transformation of linalool to linalyl acetate bytransesterification with ethyl acetate in SC-CO2 has been pro-posed as an alternative to processes based on the use of toxicor contaminant substances. A thermodynamic study of this re-action, based on the Stryjek-Vera Peng-Robinson Equation ofState has been performed. The parameters of this model havebeen correlated to literature experimental phase equilibriumdata. The thermodynamic study comprises an analysis of thephase equilibrium in the system as well as of the equilibriumconstant of the reaction. It has been found that the reaction isalmost irreversible in all the conditions of interest for this pro-cess, and thus, can only be limited by kinetics or side reactions.From the study of the phase equilibrium, operating pressuresin the reactor of 15–20 MPa have been proposed in order toensure that the reaction proceeds in a single phase. An operat-ing pressure and temperature of 7 MPa and 353 K, respective-ly, have been proposed for the separation of the linalool-linalylacetate mixtures obtained from the reactor, as a trade-offbetween the separation factor between components and thegas loading.

Acknowledgement

The authors would like to thank the Spanish Ministry ofScience and Technology for funding under projects PPQ 2003-07209 and CTQ2006-02099/PPQ.

Symbols used

a [Pa m6/mol2] PRSV-EoS parameterb [m3/mol] PRSV-EoS parameterCp [J/mol K] heat capacityDG [J/mol] Gibbs free energy

DH [J/mol] enthalpykij [–] binary interaction parameterK [–] vapor-liquid equilibrium

distribution factor, K = y/xK0 [–] equilibrium constantL [g/kg] gas loadinglij [–] binary interaction parameterN [–] number of points in the data setP [MPa] pressurePc [MPa] critical pressureT [K] temperatureTc [K] critical temeperatureTr [–] reduced temperature, Tr = T/Tc

x [–] liquid mole fractioncomposition

y [–] gas mole fraction composition

Greek symbols

a [–] separation factor,a = Klinalool/Klinalyl

� [–] fugacity coefficientj1 [–] PRSV-EoS a(T) parameterx [–] acentric factor

References

[1] Ullmann’s Encyclopedia of Industrial Chemistry, 7th CD-ROMed., Wiley-VCH, Weinheim 2006.

[2] C. S. Letizia, J. Cocchiara, J. Lalko, A. M. Api, Food Chem.Toxicol. 2004, 41, 965.

[3] M. Cerpa, M. J. Cocero, 36th Int. Symp. of Essential Oils,Budapest, September 2005.

[4] W. Aquila, R. Pox, H. Etzrodt, European Patent 0949239, 1999.[5] W. Aquila, R. Pox, H. Etzrodt, US Patent 6156926, 2000.[6] J. Zhang et al., Chem. Ind. Forest Prod. 2005, 25 (2), 43.[7] B. Subramaniam, C. J. Lyon, V. Arunajatesan, Appl. Catal., B

2002, 37, 279.[8] S. Sabeder, M. Habulin, Z. Knez, Ind. Eng. Chem. Res. 2005,

44 (25), 9631.[9] G. Brunner, J. Food Eng. 2005, 67 (1–2), 21.

[10] R. Stryjek, J. H. Vera, Can. J. Chem. Eng. 1986, 64, 820.[11] M. Vázquez da Silva, D. Barbosa, J. Supercrit. Fluids 2004, 31,

9.[12] S. Raessi, C. J. Peters, J. Supercrit. Fluids 2001, 20, 221.[13] E. Franceschi et al., Fluid Phase Equilib. 2004, 226, 1.[14] D. R. Stull, Ind. Eng. Chem. 1947, 39, 517.[15] K. Suzuki et al., J. Chem. Eng. Data 1990, 35, 63.[16] C.-Y. Day, C. J. Chang, C.-Y. Chen, J. Chem. Eng. Data 1996,

41, 839.[17] Y.-L. Tian et al., J. Chem. Eng. Data 2004, 49, 1554.[18] A. Chrisochoou, K. Schaber, U. Bolz, Fluid Phase Equilib.

1995, 108, 1[19] E. B. Poling, J. M. Prausnitz, J. P. O’Connell, The Properties

of Gases and Liquids, 5th ed., Mc Graw-Hill, New York 2001.



Documents

Direct Synthesis of Linalyl Acetate from Linalool in Supercritical Carbon Dioxide: A Thermodynamic Study