Esterification of Sebacic Acid in Near-Critical and Supercritical Methanol

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Esterification of sebacic acid in near-critical and supercritical methanol Ram C. Narayan, and Giridhar Madras Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04769 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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Industrial & Engineering Chemistry Research

Esterification of sebacic acid in near-critical and supercritical methanol Ram C Narayan, Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India Abstract: Diesters of sebacic acid find various applications as plasticizers, solvents and lubricants. Non-catalytic esterification of dibasic acids under near – and supercritical alcohol conditions was conducted for the first time. Esterification of sebacic acid (decanedioic acid) and methanol was chosen as a candidate reaction. The effect of molar ratio of methanol to sebacic acid, temperature, addition of water and pressure was studied on the yields of the diesters and monoesters. A maximum molar yield of 87 % with a selectivity of 16 between mono and diester was obtained at 623 K with a molar ratio of 5:1 within a reaction time of 25 min. A kinetic model was developed to correlate the experimental data and activation energies were computed using the Arrhenius equation. The effects of addition of water and pressure on the yield and selectivity show that esterification is fairly water resistant and could be carried out at operating pressures much lower than the critical pressure of methanol. Keywords: supercritical fluids; esterification; sebacic acid; diesters; selectivity

*

Corresponding author. Tel. +91 80 22932321

Email: [email protected] (G.Madras)

ACS Paragon Plus Environment


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1. Introduction The raw materials for many chemicals manufactured today are perilously dependent on the fast depleting coal, petroleum and natural gas reserves. Fuels, lubricants and solvents are almost completely derived from petroleum based sources. Many biomass based pathways to manufacture these chemicals have been proposed in literature in lieu of conventional methods.1-2 There is a growing impetus in both industry and academia in the pursuit of green manufacturing processes. The aim is to minimize overall water and carbon footprint, improve sustainability and reduce process wastes without compromising on economy and productivity.3 Pathways using supercritical fluids satisfy many aspects of green manufacturing due to their inherent physical and chemical properties. The liquid-like density confers excellent solvation properties while gas-like diffusivities and viscosities help in enhanced mass transfer. Carbon dioxide, methanol and ethanol have been investigated as solvents/reactants for the synthesis of biodiesel or fatty acid esters from vegetable oils, fats or fatty acids.4-6 However, there are no reports on the synthesis of dicarboxylic acid esters using supercritical fluids. Dicarboxylic acid esters of succinic, adipic, maleic and sebacic acid are useful as lubricants, solvents and plasticizers.7 Dimethyl sebacate is used as a precursor to synthesize sebacic esters of higher alcohol using transesterification reaction that are useful as biodegradable lubricants.8 Conventional methods of synthesis involve use of acid, base or heterogeneous catalysts. These routes are fraught with complications of downstream processing, catalytic poisoning and inactivation in the presence of impurities or water. Further, these alkali catalyzed routes are also prone to disproportionation of diesters to monoesters in the presence of alcohols that reduce the yield of diesters and thus selectivity of the process.9 1


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Sebacic acid is obtained by alkaline pyrolysis of castor oil.10 It should be noted that the transesterification of castor oil (triacylglycerides) to fatty acid esters is well studied under both catalytic and supercritical (non-catalytic) conditions.11-12 However, sebacic acid (or any other dicarboxylic acid) has not been studied in the context of supercritical fluid reactions. In this study, dimethyl sebacate was synthesized by esterification of sebacic acid with supercritical methanol for the first time at isochoric reaction conditions without adding any external catalysts. The objectives of this work were threefold. Firstly, the effect of different variables like initial molar ratio of alcohol to sebacic acid, temperature, pressure and addition of water on the molar yield and selectivity was studied. Secondly, an attempt was made to understand the phase behavior and its effect on esterification. Thirdly, a kinetic model was developed and fitted to the experimental yield and selectivity data. 2. Materials, Methods and Analysis 2.1. Materials Sebacic acid (>98 % by GC) was procured from SRL chemicals. Methanol (>99.5 % by GC) and n-heptane (analytical grade) was purchased from Merck India Pvt Ltd. Butyl laurate (>99 % by GC) was obtained from Sigma Aldrich Ltd (USA). All chemicals were used as is without further purification. 2.2. Methods A constant volume batch reactor was fabricated using seamless steel tubes with a capacity of 10 cm3 (length 14 cm, with an O.D of 0.5 inch) by permanently sealing one of the ends and closing the other end using a 0.5 inch male plug. The latter is achieved by a compression 2



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fitting using a nut and a ferrule. This fitting ensures that the reactors can hold pressures up to 400 bar. The setup is so that the male plug can be loosened to introduce a known quantity of reactants and to collect products after a suitable reaction time for further chromatographic analysis. All the materials used in the fabrication of reactor were stainless steel (SS-316). The reaction procedure is that of isochoric reaction systems, where a predetermined amount of reactants are sealed into the batch reactor and heated to the required temperature (consequently reaching the desired pressure). This procedure is reported by many authors and has also been used in our previous studies.13-15 The reactant loading was calculated using a PR-EOS (Peng Robinson Equation of State) (with Lorentz-Berthelot (LB) mixing rules16). The pure component critical properties for methanol were taken from NIST fluid property database. The critical properties for sebacic acid were estimated using suitable group contribution methods.17 The reactor was loaded with the desired amount of sebacic acid and methanol and placed in the furnace that is set to the reaction temperature. The uncertainty in measuring the furnace temperature is about ± 1 ˚C. The system takes roughly ̴ 3 min to reach the operating temperature. This corresponds to about 1-3 % uncertainty in the global densities (the ratio of total amount of reactants to the total volume of the reactor (10 mL)), which is less than the experimental uncertainty in reactant loading. The batch system was isochorically heated at constant global density conditions to achieve the operating pressure at the particular temperature. The global density was determined as discussed in Section 2.3

After the desired temperature was reached, the reactors were quenched in ice causing the chemical reaction to drastically subside due to altered phase conditions, causing no further change in the composition of reactants and products. It should be noted that as there is no added catalyst and the reaction takes place at high temperatures and pressures, the reaction 3


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rates at ambient conditions are negligible to produce any appreciable change in composition of the reaction mixture. To further ensure this, the unreacted sebacic acid and the formed mono and diesters of sebacic acid (mono methyl sebacate and dimethyl sebacate) were concentrated by evaporating excess methanol and water from the reaction mixture. In order to prepare a sample for GC (Gas Chromatography) analysis, this mixture was diluted suitably in n-heptane and spiked with 20 µL of butyl laurate as internal standard (IS) (to minimize injection errors in GC) and centrifuged at 2000 rpm for 5 min to separate the unreacted solid sebacic acid (that is insoluble in n-heptane) and obtain a clear homogenous phase suitable for GC analysis. 2.3. Density calculation The global density of the reaction system calculated using PR-EOS with LB mixing rules could be different from the experimental densities reported for pure methanol as mentioned in the NIST fluid property database. The latter was not used for calculating reactor loading as experiments were done by varying reaction compositions as discussed in sections 3.1 and 3.3, for which the assumption of equality between global densities and pure methanol density may not hold true. The global densities of the reaction system at molar ratios of 40:1 and 5:1 for sebacic acid-methanol system were compared to the density of pure methanol in Fig. 1 at a pressure of 200 bar. It can be observed that the deviation from NIST data is most profound at lower temperatures (523 K – 573 K) at 40:1 molar ratio. At molar ratio 5:1, the densities were much higher than those reported for pure methanol by NIST and also those obtained by using PR-EOS with LB mixing rules at 40:1. The variation between global densities (or reactor densities) and those reported for pure methanol from NIST database was also observed for methanol-triacylglyceride systems in the perspective of transesterification leading to fatty acid methyl esters, even at higher molar ratios.18 This difference can be attributed to the high 4



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molecular weight of triacylglycerides/sebacic acid compared to methanol and wide variation in critical properties of both these components.

2.4. GC Analysis The samples of reaction prepared in the manner mentioned above were analyzed using gas chromatography (Varian CP 3800). Flame ionization detector (FID) was used as a detector for all components with high pure hydrogen and oxygen as fuel gases and UHP nitrogen as make up gas. A capillary column (VF-5ms, 30 m x 0.25mm with 0.25 µm film thickness of 5% phenyl-methylpolysiloxane column) was used to separate all the components of the sample. UHP Helium (99.9995 %) was used as a carrier gas at a flow rate of 0.5 mL.min-1. The FID oven and injector ports were maintained at a constant temperature 275 ˚C. An isothermal method at 250 ˚C was sufficient and ensured separation of all components. Different amounts of purified esters (derived by Fischer’s esterification using sulphuric acid as a catalyst) along with butyl laurate as internal standard (IS), was injected into the GC to obtain a linear calibration curve. The parameters of this curve were used to interpolate the concentrations (and hence the yields) of the esters in the reaction samples. The global experimental error in the data corresponds to about ±3 % of the yield based on triplicate experiments. 3. Results and Discussion The system chosen involves two reactants: a high melting solid sebacic acid and methanol that are inherently soluble in each other at ambient temperature and pressure. This is in contrast to popularly studied reactions of triacylglycerides/fatty acids and methanol, which are insoluble in each other at ambient conditions. The esterification reaction occurs in two 5


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stages: first, the formation of mono-methyl ester from sebacic acid and second, the formation of dimethyl sebacate from mono methyl sebacate, with the net release of two moles of water. The reaction steps and subsequent kinetic analysis are discussed in the succeeding section. The ester of interest is dimethyl sebacate and not mono methyl sebacate. Therefore, along with the yield of these esters, the variation of selectivity ( α =

CC ) of the reaction, that is the CE

ratio of molar concentration of diester and mono ester was also studied. The effect of molar ratio of methanol to sebacic acid, temperature, pressure and water was studied on the yield of mono and diester and on the selectivity of the reaction towards diester. 3.1. Effect of molar ratio of methanol to sebacic acid Reactions between carboxylic acids and alcohols are reversible i.e. the molar composition of various components of the reaction mixture determines whether esterification or hydrolysis takes place. In order to maximize the ester synthesis, an excess of alcohol must be used so that the forward reaction (esterification/transesterification) is favored. This is well known in the literature and is applicable for conventional catalytic synthesis.19 In supercritical alcohol reactions for the synthesis of biodiesel, excess alcohol also solvates the triacylglyceride precursor and enhances miscibility (due to decrease in the difference between the solubility parameters between alcohol and triacylglycerides at supercritical conditions).20 In this work, the effect of molar compositions was studied by varying the initial molar ratio (of methanol to sebacic acid) and also by varying the amount of added water. The effect of molar ratio was studied between 5 (2.5 times the stoichiometric ratio) and 40 (highly dilute with respect to sebacic acid) at temperatures 573 K and 623 K at a fixed time of 25 min. It should be noted that the corresponding variation in global density is in the range of 6



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300 – 550 kg. m-3 at 573 K and 210 – 460 kg.m-3 at 623 K between 40:1 and 5:1, respectively (corresponding to a pressure of 200 bar). The reaction time was chosen to show sufficient variation in the yield at both these temperatures. The pressure was fixed at 200 bar (effect of pressure is discussed later). The variation of yield and selectivity is reported in Fig. 2 and Fig. 3 respectively. From these figures, it is evident that the yield (molar yield, henceforth referred simply as yield) of dimethyl sebacate at higher molar ratio was lesser than those at other molar ratios. This phenomenon of increased conversion at lower molar ratios was observed for reactions between fatty acids and alcohols.15, 21 This can be explained on the basis of increase in acid catalytic activity, due to increased sebacic acid loading. Although, carboxylic acids are not as acidic as mineral acids, at higher loading (at lower molar ratios) they could contribute substantially to acid catalysis. Being a dibasic acid, the effect is much more pronounced than those observed for reaction of fatty acids with different alcohols.15 Thus sebacic acid and methoxide ion (from methanol) contribute towards esterification at different molar ratios. The yield of the diester was higher at higher temperature of 623 K than 573 K due to higher reaction rate as the formed monoester converts quickly into diester. At 573 K, there is a decrease in yield at a molar ratio of 5 due to the enhancement of hydrolysis reaction. The highest yield of diester of about 75 % was achieved at a molar ratio of 10:1 at this temperature. At 623 K, the yield increases from 58 % at molar ratio 40:1 to nearly 87 % at molar ratio of 5:1. This variation of yield with molar ratio was also observed for the reaction of palm fatty acid distillate (PFAD) with supercritical methanol.6 Fig. 3 is a plot between logarithm of selectivity (ln α) and molar ratio. The logarithm of selectivity (the logarithm is purely representative and holds no specific relevance to the process) was chosen as the ordinate as the selectivity was very different at both these temperatures. It can be seen that 7


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selectivity improves with a decrease in molar ratios. The selectivity was always greater than one at both 573 K and 623 K at 25 min. Thus, the yield of monoester decreases at lower molar ratio, with increasing yield of diesters for both the temperatures investigated. The phase behavior of mixtures of alcohol and carboxylic acid at subcritical or supercritical conditions is quite complex. The critical parameters for the reaction mixture are different from the critical parameters of pure methanol. The difference becomes more profound at lower molar ratios, where there is increased contribution of sebacic acid that has much higher critical temperature and lower critical pressure than methanol. The phase equilibria of fatty acid/alcohol and triacylglyceride/alcohol systems have been extensively studied for noncatalytic synthesis of fatty acid esters (in supercritical/subcritical alcohol systems).18, 22-23 The vapor- liquid equilibria of several dicarboxylic acid-alcohol-water systems up to the boiling point of the alcohol-water have been investigated in literature.24-25 However, there is lack of phase equilibria data for dicarboxylic acid-alcohol systems at the temperature and pressure conditions at which reactions are carried out in this study. Thus, LB mixing rules were used to calculate the critical properties of the mixture at different compositions of sebacic acid and methanol. The critical temperatures and pressures of initial reaction mixtures were computed and represented in Fig. 4. It can be observed that the critical temperature of mixture increases and the critical pressure of the mixture decreases with decreasing molar ratio. The operating pressures of 200 bar used in the study of effect of molar ratio are clearly greater than (twice) the critical pressure at all molar ratios. At 623 K, the mixture is supercritical at all the initial molar ratios that were studied. Hence, high yields were obtained at even a lower molar ratio of 5:1. It should be noted that the critical point of the reaction mixture can also vary with the progress of reaction due to varying compositions 8



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of the reaction mixture.10,

26

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However, in case of methanol-sebacic acid system that is

investigated in this work, the critical properties of the reaction mixture does not vary significantly with the progress of reaction as the critical properties between the acid and the formed ester are not very different and the volumetric contribution of formed water towards the critical point of the mixture is very small. The

requirement

of

lower

molar

ratios

at

higher

temperatures

for

esterification/transesterification was noted by other investigators in the synthesis of biodiesel.27-28 At 573 K, the mixture is supercritical for all molar ratios other than 5:1. Thus, at 5:1 molar ratio, a slight drop in yield is observed as compared to 10:1 and 20:1, but the yields were better than those at 40:1 molar ratio. The slight drop in the yields at 5:1 molar ratio can be explained on the basis of enhancement of hydrolysis reaction/backward reaction, in accordance with the Le Chatelier’s principle. The effect of temperature was studied at 573 K and 623 K at a constant reaction time of 25 min and molar ratios in a broad range between 5:1 and 40:1 molar ratio. Among these temperatures and molar ratios, the maximum yield and selectivity was obtained at 5:1 molar ratio and 25 min reaction time at 623 K. Beyond this reaction time, equilibrium was reached, as lower molar ratio was used (that enhances the backward/hydrolysis reaction). As esterification is associated with lower heat of reaction29, significant increase in equilibrium yields may not be expected at higher temperatures that are also more prone to thermal degradation as will be discussed in Section 3.2. 3.2. Effect of temperature The effect of temperature was studied at a molar ratio of 40:1 at temperatures 523 K, 573 K, 623 K and 673 K at a constant pressure of 200 bar. It should be noted that the reactor loading (as discussed in Section 2.2) varies at different temperatures. The corresponding variation in 9


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global density is in the range of 426 – 165 kg.m-3. Using LB mixing rules, the temperature of 523 K is around the critical temperature. At other temperatures investigated, the reaction mixture was in supercritical state. In general, the variation in composition of the reaction mixture reflects in the critical point of the reaction mixture. However, from LB mixing rule calculations, it was estimated that the variation in critical point at different reaction times was not substantially different from the assumption of initial molar ratio at the start of the reaction. Moreover, at operating pressures that is twice the critical pressure and high molar ratio of 40:1, the reaction is expected to be in homogenous supercritical state in the range of 523 K to 673 K. The variation of the yield of all esters with time for different temperatures is represented in Fig. 5 (a)-(d). The curves represent the kinetic model used to fit the experimental data that will be discussed in the succeeding section. In general, yields of diester and total ester monotonically increased with time and temperature (within the range of time and temperature studied). The yields of diester show an initially time lag, but rise steadily with time. The yield of the monoesters increases steadily to a maximum value and then decreases (theoretically to zero after a very long time). By the variation of yields, it can be discerned that the esterification reaction occurs in two steps, one in which the dibasic acid reacts to form a monoester and in the next step reacts with another molecule of alcohol to form the diester. This variation of yields of diesters and monoesters with time is observed even in a conventional catalytic route.30 The maximum yield of 85 % of diester was obtained at the end of 40 min at 623 K, where the highest yield of 94 % total esters was obtained. A maximum yield of 80 % of diester was achieved at the end of 30 min at 673 K and at the end of 55 min at 573 K. At 523 K, a 10



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maximum yield of about 60 % of diester was obtained. The maximum yield of mono methyl sebacate was about 25 % obtained at the end of 25 min at both 523 K and 573 K. At 623 K and 673 K, the mono methyl ester rapidly forms and converts to dimethyl sebacate. At 623 K and 673 K, degradation of both monoesters and diesters (though the reaction mixture is highly selective to diesters at these conditions) were observed beyond 45 min and 30 min, respectively. Thermal degradation of esters at higher temperatures and greater times is due to the onset of pyrolytic reactions. Several side reactions could occur leading to decarboxylation and formation of hydrocarbons, hydrolysis etc.27,

31

The analysis of decomposed products

may not be very useful in the synthesis of pure diesters that have specific applications. However, in case of biodiesel synthesis under supercritical conditions, these products add to the calorific value and total yield of the final product, making such analysis more useful. The variation of selectivity as a function of time at the investigated temperatures is represented in Fig. 6. Selectivity monotonically increases with temperature. The steepness of the selectivity curves, however, keeps increasing with both time as well as temperature, due to the rapidly increasing yield of diesters and diminishing yield of monoesters. This variation of selectivity is not uncommon in catalytic esterification of dicarboxylic acids.7 The highest selectivity of 9 is obtained at 623 K at the end of 40 min. Beyond 25 min of reaction, the reaction is more selective towards dimethyl sebacate at all temperatures. 3.3. Kinetics of reaction The esterification of dibasic acids to diesters occurs in two steps, as discussed in the previous sections. Unlike transesterification of triacylglycerides with high excess of methanol or esterification of mono carboxylic acids, the assumption of a single step pseudo first order kinetics does not hold good. This assumption of single step pseudo first order reaction 11


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overestimates the yields of diesters at lower reaction times especially at 523 K and 573 K. The experimental data obtained also seems to suggest this phenomenon. Usually, in such cases, a two step reversible second order homogenous reaction is considered. As the variation of yield with respect to time was investigated at high molar excess of methanol, both the steps can be considered to be a pseudo-first order reaction in series. The reaction scheme and equations describing the variation of yield of the esters (CE and CC) with respect to reaction time (t) are given below. CA denotes the concentration of sebacic acid. CAi is the initial concentration of sebacic acid. The rate constant for first and second steps are denoted as k1 and k2. k k A  → E  →C 1

(1)

2

The rate equations for the above reactions and their solutions are

dC A C = −k1C A ; A = e− k1t dt C Ai

dCE = k1C A − k2CE dt dCC = k2CE dt

;

;

k CE = 1  e− k1t − e−k2t  C Ai k2 − k1  

CC 1  = k (1 − e− k1t ) − k1(1 − e−k2t )  C Ai k2 − k1  2 

(2a)

(2b)

(2c)

The selectivity (α), is the ratio of moles of diester and monoester, and can be expressed as

CC k2 − k1 − k2e− k1t + k1e− k2t α= = CE k  e− k1t − e− k2t  1





12


(2d)


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The variation of yield of monoester with time goes through a maximum, where the rate of reaction drops to zero. The optimum time (topt), after which the yield of mono ester starts decreasing can be calculated by the following equations

 dCE   dt

  t =topt

= 0 ; topt =

1 ln k2 k2 − k1 k1

(3)

In Fig. 5, the lines represent the variation in the yield of mono ester, diester and total ester formation. The kinetic model described above fits the experimental data and explains the variation of yields with time. The rate constants have been tabulated in Table I at different temperatures for both the reaction steps. The rate constant for esterification of sebacic acid to mono methyl ester varies between 0.025 to 0.08 min-1. The rate constant for the esterification of mono methyl ester to the corresponding diester (dimethyl sebacate) varies between 0.05 to 0.13 min-1. The rate constant, k2 was always greater than k1, leading to a higher yield of diester and diminishing amount of mono ester with time. The rate constants at 573 K are similar to the transesterification reaction of triacylglycerides with supercritical methanol, especially those of linseed and sunflower oil.11, 32 Further, k1 and k2 are functions of temperature and can be described by Arrhenius equation that hypothesizes a linear relationship between ln (k1) (or ln k2) and inverse of temperature. The plots depicted in Fig. 7 show that the experimental rate constants follow Arrhenius equation. The activation energy and pre-exponential factor were estimated. The activation energies of 21 kJ mol-1 and 15 kJ mol-1 were obtained for first and second step of the esterification reaction respectively. The activation energies are comparable to those of esterification of fatty acids with supercritical methanol (22 kJ mol-1).33 The activation energies were however lower than those of heterogeneously catalysed esterification of

13


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dicarboxylic acids with lower alcohols. For example, the reaction with succinic acid (a four carbon dicarboxylic acid) with ethanol had activation energy of 66 kJ mol-1 and 70 kJ mol-1 for the first and second steps respectively, where the reaction was catalysed by Amberlyst15.30 In case of reaction of adipic acid (a six carbon dicarboxylic acid) with methanol catalysed by Amberlyst-35, an activation energy of about 34 kJ mol-1 (similar for both the steps) was observed.7 The pressure in the reactor is attained by isochoric increase of temperature. Thus, after the attainment of constant pressure of 200 bar, the density of the reaction system varies with temperature. Thus different operating temperatures, 523-673 K correspond to densities of methanol between 100-500 kg m-3. In addition to plotting ln (k) and 1/T, ln (k) was also plotted against the density of the reaction mixture. Interestingly, it was observed that this plot also follows a straight line as shown in Fig. 7. Table I also shows the reaction densities at these temperatures. Thus, for isochoric supercritical reaction systems such as those investigated in this study, the temperature and indirectly the density affects the rate constant and hence the yield of esters in the reaction. This was shown earlier for the reactions of sunflower oil with supercritical methanol.32 A similar variation of rate coefficient with density was also observed for non-catalytic degradation of polymers in supercritical fluids.3435

The optimum time for maximizing the yield of monoester can be calculated using Eq. (5). The optimum time (topt) differs with temperatures at the same molar ratio. From Fig. 8, at higher temperatures, topt occurs at lesser reaction times than at lower temperatures. The topt occurs within 5 min at temperatures 623 K and higher. This means that at higher temperatures, the mono ester formation (though fast) also diminishes rapidly after topt. The

14



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value of topt depends on the value rate constant and thus on the temperature and density of the reaction system.

3.4. Effect of addition of water As water is a product of esterification, addition of water/ presence of water should inhibit the esterification. The effect of water was studied at 623 K with an initial molar ratio of 5:1 at 20 MPa. Various amounts of distilled water (5- 18 %) were added to the initial reaction mixture of sebacic acid and methanol (as w/w % on the basis of sebacic acid). The basis for the choice of range is that, upon complete conversion of one mole of sebacic acid two moles of water are produced. On mass basis, this amounts to 18 % of initial mass of sebacic acid added (approximately 10 % by mass of the initial reaction mixture based on loading calculations at these operating conditions). Correspondingly, this also amounts nearly to 18 % (by mass) of the methanol that is taken under these conditions. Interestingly, it can be observed from Fig. 9 that the yield of diesters is nearly constant at these three levels of water addition. This phenomenon was also observed for other reactions in supercritical alcohols.36-38 However, the yield of monoesters slightly increases with water content, leading to decreased selectivity. The variation in the yields of the monoester and diester is not straightforward, as can be deciphered from the kinetic models in Section 3.3, where pseudo first order kinetics is assumed due to excess of methanol. The yield of mono ester goes through a maximum, before dropping nearly to zero (at higher molar ratio) or to a non-zero equilibrium yield (at lower molar ratios) during long reaction times. However, as the effect of water addition at 5:1 molar ratio (that is only 2.5 times excess), the possibility of hydrolysis of diester to monoester and monoester to sebacic acid increases, with increasing water content of the reaction mixture resulting in slightly higher yields of monoesters (and

15


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thus lower selectivity). The main purpose of studying this effect was to verify the suitability of using commercial alcohols (that contains water, but are relatively inexpensive), in lieu of pure alcohols (that are much more expensive) without potentially lowering the yields of diester, especially when lower molar ratios are used. The nearly constant yield with varying added volumes of water can be attributed to the catalytic activity of water (as water exists in the form of hydronium and hydroxide ions at the reaction conditions).39 Further elucidation of the competing effects between the reversibility of reaction by water and its catalytic activity, requires detailed kinetic investigation at lower molar excess of methanol with initial addition of water, which is modeled by multi-step concomitant hydrolysis-esterification reactions catalyzed by hydronium ion.40

3.5. Effect of pressure The effect of pressure on yield is usually minimal at operating pressures that are greater than twice the critical pressure of the alcohol (as there is no significant change in the density with pressure).15, 41-42However, to confirm the same for sebacic acid-methanol system, the effect of pressure was studied at 623 K at molar ratios, 5 and 40 over the pressure range (50-300 bar) and represented in Fig. 10. At constant temperature, in order to achieve different reactor pressures, the reactor loading was suitably manipulated, i.e. lower loading for lower pressures. It was found that the variation of yield of diester was almost constant at molar ratio 5:1. However, at a molar ratio of 40:1, the yield increased from 50 bar to 200 bar and then remained constant. From Fig. 4 (b), at operating pressures of 50 bar, the reaction mixture is below the critical point of the mixture and thus is in the gas phase. At 40:1 molar ratio, the yield decreases at 50 bar unlike 5:1 molar ratio. This could be due to the increased acid

16



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

catalysis due to higher loading of sebacic acid that is not much dependent on the phase of methanol (dependent more on temperature as can be inferred from Fig. 2 unlike at 40:1.

4. Conclusions Dimethyl sebacate was synthesized in high yields (>85 %) from sebacic acid and methanol. The reaction occurs in two steps: first the formation of monoester and then subsequent esterification to diesters. The reaction in series kinetic model fits the experimental yield data for monoester, diester and total esters. At a given temperature, the rate constants for the formation of monoester were always lesser than the rate constants for the formation of diester. The activation energy of the first step of esterification is higher than the second step. The yield of diesters and selectivity was higher at lower molar ratios at a constant reaction temperature. This has economic and environmental advantages due to lower heating energy costs for evaporating excess alcohol. The esterification was highly selective towards the diester formation at higher temperatures, greater reaction times and at lower molar ratio. At 40:1 molar ratio, the maximum yields of 25 % of monoester was obtained. The effect of pressure on yield was minimal at 5:1 in the pressure range of 50-300 bar, unlike at 40:1 molar ratio. The phase of the reaction mixture can be either subcritical or supercritical depending not only on the temperature and pressure of the system, but also on the molar ratio (of methanol: sebacic acid). At 200 bar, the reaction mixture was always in homogenous liquid phase or supercritical phase. The addition of up to 18 % mass of water (w/w % sebacic acid) does not affect the yield of diester, but the selectivity slightly decreases at higher water content. The proposed supercritical pathway satisfies many of the twelve guiding principles of green chemistry and unveils new avenues for neat type-I (non-catalytic, solvent free) synthesis of diesters with wide variety of applications.

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22



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Figure Captions Table I: Rate constants at 40:1 initial molar ratio and pressure of P = 200 bar at different temperatures.

Figure 1: Variation of density of pure methanol (NIST data) (■), global density at 40:1 (●) and 5:1 (▲) initial molar ratio, respectively, calculated using PR-EOS with LB-mixing rules with temperature at P = 200 bar.

Figure 2: Variation of yield of esters (●, dimethyl sebacate and

■, mono methyl sebacate)

with initial molar ratio at (a) 573 K and (b) 623 K and pressure P = 200 bar and reaction time of 25 min. The lines represent the B-spline interpolation of the experimental data.

Figure 3: Variation of logarithm of selectivity (ln α) with initial molar ratios at temperatures of 573 K (●) and 623 K (■) at pressure of P = 200 bar and reaction time of 25 min.

Figure 4: Variation of (a) critical temperature and (b) critical pressure of reaction mixture with initial molar ratio. The dotted lines show the operating (a) temperatures and (b) pressures for sebacic acid and methanol mixtures.

Figure 5: Variation of yield of total ester (■), mono methyl sebacate (▲) and dimethyl sebacate (●) with reaction time at temperature (a) 523 K (b) 573 K (c) 623 K and (d) 673 K at a molar ratio of 40:1 at pressure P = 200 bar. The lines represent the kinetic models derived in Section 3.3.

Figure 6: Variation of selectivity (α) with time at different temperatures, ■, 523 K; ●, 573 K;▲,623 K and ▼, 673 K at 40:1 molar ratio and pressure P = 200 bar. The lines represent the kinetic models derived in Section 3.3.

23


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Figure 7: Arrhenius plot for the variation of (a) k1 (■) and k2 (●) with temperature and (b) k1 (■) and k2 (●) with density at pressure of P = 200 bar.

Figure 8: Variation of topt with (a) temperature (■) and density (●) at pressure of P = 200 bar and 40:1 molar ratio.

Figure 9: Variation of yield of dimethyl sebacate (●) and mono methyl sebacate (■) with addition of distilled water at 623 K, 200 bar and 5:1 initial molar ratio of methanol to sebacic acid. Inset graph: Variation of selectivity with addition of distilled water at 623 K, 200 bar 5:1, initial molar ratio of methanol to sebacic acid. The reaction time is 25 min.

Figure 10: Variation of yield of diester at different pressures at molar ratio of 5:1 (■) and 40:1 (●) at 623 K and 25 min reaction time.

24



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Table I: Rate constants at 40:1 initial molar ratio at different temperatures at pressures of P = 200 bar. Temperature (T / K) 523 573 623 673 Activation energy (kJ mol-1)

k1x 103/ s-1 0.44 0.71 1.04 1.30 21

k2x 103 /s-1 0.95 1.35 1.71 2.15 15

ρ / (kg.m-3)

25


426 304 212 165

Page 26 of 37

Page 27 of 37

700 600 500

ρ / (kg.m-3)

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


400 300 200 100 500

550

600

650

700

750

T/K

Figure 1: Variation of density of pure methanol (NIST data) (■), global density at 40:1 (●) and 5:1 (▲) initial molar ratio, respectively, calculated using PR-EOS with LB-mixing rules with temperature at P = 200 bar.

26



1.0

Cester/CAi

0.8 0.6 0.4 0.2 0.0

(a) 0

5

10

15

20

25

30

35

40

45

40

45

Initial molar ratio

1.0 0.8

Cester/CAi

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

0.6 0.4

(b)

0.2 0.0

0

5

10

15

20

25

30

35

Initial molar ratio

Figure 2: Variation of yield of esters (●, dimethyl sebacate and

■, mono methyl sebacate)

with initial molar ratio at (a) 573 K and (b) 623 K and pressure P = 200 bar and reaction time of 25 min. The lines represent the B-spline interpolation of the experimental data.

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5 4 3

ln α

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


2 1 0

0

5

10

15

20

25

30

35

40

45

Initial molar ratio

Figure 3: Variation of logarithm of selectivity (ln α) with initial molar ratios at temperatures of 573 K (●) and 623 K (■) at pressure of P = 200 bar and reaction time of 25 min.

28



680

Tc,mix / K

640

T = 623 K

600 T = 573 K

560

(a) 520 20

40

60

80

100

Initial molar ratio

P = 300 bar

300 250

Pc,mix / bar

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

P = 200 bar

200

P = 100 bar

100 80

(b) 60 40

P = 50 bar 20

40

60

80

100

Initial molar ratio

Figure 4: Variation of (a) critical temperature (Tc,mix) and (b) critical pressure (Pc,mix) of reaction mixture with initial molar ratio. The dotted lines show the operating (a) temperatures and (b) pressures for sebacic acid and methanol mixtures.

29


Page 30 of 37

1.0

0.8

0.8

0.6

0.6

Cester /C Ai

1.0

0.4 (a)

0.2 0.0

0

10

20

30

40

50

0.4 0.2 0.0

60

(c)

0

10

20

t / min

1.0

1.0

0.8

0.8

0.6

0.6

0.4 (b)

0.2 0.0

0

10

20

30

30

40

50

60

t / min

Cester /C Ai

Cester /C Ai

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


Cester /C Ai

Page 31 of 37

40

50

0.4 (d)

0.2 0.0

60

0

10

t / min

20

30

40

50

t / min

Figure 5: Variation of yield of total ester (■), mono methyl sebacate (▲) and dimethyl sebacate (●) with reaction time at temperature (a) 523 K (b) 573 K (c) 623 K and (d) 673 K at a molar ratio of 40:1 at pressure P = 200 bar. The lines represent the kinetic models derived in Section 3.3.

30


60


14 12 10 8

α

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

6 4 2 0

0

10

20

30

40

50

60

t / min

Figure 6: Variation of selectivity (α) with time at different temperatures, ■, 523 K; ●, 573 K; ▲,623 K and ▼, 673 K at 40:1 molar ratio and pressure P = 200 bar. The lines represent the kinetic models derived in Section 3.3.

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-2.0 -2.2 -2.4

ln k

-2.6 -2.8 -3.0 -3.2

(a)

-3.4 -3.6 -3.8

1.5

1.6

1.7

1.8

1.9

-1

(1000/T) / K

-2.0 -2.2 -2.4 -2.6

ln k

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


-2.8 -3.0 -3.2

(b)

-3.4 -3.6 -3.8

150

200

250

300

350

400

450

ρ / kg.m-3

Figure 7: Arrhenius plot for the variation of (a) k1 (■) and k2 (●) with temperature and (b) k1 (■) and k2 (●) with density at pressure of P = 200 bar.

32



14 12

topt / min-1

10 8 6 4 (a)

2 0

1.5

1.6

1.7

1.8

(1000/T) / K

1.9

-1

14 12 10

topt / min-1

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

8 6 4

(b) 2 0 150

200

250

300

350

400

450

ρ / kg.m-3

Figure 8: Variation of topt with (a) temperature (■) and density (●) at pressure of P = 200 bar and 40:1 molar ratio.

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1.0 0.8 20

15

0.6 α

Cester/CAi

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


0.4

10

5

0

2

4

6

8

10

12

14

16

18

20

Water (w/w % of sebacic acid)

0.2 0.0

0

0

2

4

6

8

10 12 14 16 18 20

Water (w/w % of sebacic acid)

Figure 9: Variation of yield of dimethyl sebacate (●) and mono methyl sebacate (■) with addition of distilled water at 623 K, 200 bar and 5:1 initial molar ratio of methanol to sebacic acid at pressures P = 200 bar. Inset graph: Variation of selectivity with addition of distilled water at 623 K, 200 bar, 5:1 initial molar ratio of methanol to sebacic acid. The reaction time is 25 min

34



1.0 0.8

Cester/CAi

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

0.6 0.4 0.2 0.0

50

100

150

200

250

300

P / bar

Figure 10: Variation of yield of diester at different pressures at molar ratio of 5:1 (■) and 40:1 (●) at 623 K and 25 min reaction time.

35


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For use as Table of Contents (TOC) graphic:

O

O

OH

HO

O

OC H3

C H 3O

O

36


Esterification of Sebacic Acid in Near-Critical and Supercritical Methanol

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