Synthesis and Characterization of Diethyl Citrate ... - ACS Publications

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Synthesis and Characterization of Diethyl Citrate and Phase Equilibria in Mixtures with Ethanol and Water Claudia E. Berdugo, Andrea Suaza, Miguel A. Santaella, Orlando A. Sánchez, Gerardo Rodríguez, and Alvaro Orjuela* Department of Chemical and Environmental Engineering, Universidad Nacional de Colombia, 111321 Bogotá D.C., Colombia

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S Supporting Information *

ABSTRACT: In this work, diethyl citrate (DEC) was synthesized and isolated for further physicochemical characterization and phase equilibria study. The synthesis was performed via partial esterification of citric acid with ethanol, and the final product was separated from the resulting mixture using a pH-controlled solvent extraction method. Characterization using 1H NMR and 13C NMR spectroscopy confirmed the presence of both DEC isomers with few impurities. Results indicate that a 6:5 molar ratio of the symmetric to the nonsymmetric ester structures was attained during synthesis. Thermogravimetric analysis and differential scanning calorimetry allowed us to measure, for the first time, glass transition temperature (228 K) and decomposition temperature (493 K) of diethyl citrate. Additionally, P−x data for the vapor−liquid equilibrium of the binary mixtures diethyl citrate + ethanol and diethyl citrate + water were collected at 308 K using an accurate dynamic device. Data were regressed to generate binary interaction parameters for the UNIQUAC equation. The regressed model shows good agreement with experiments, and it can be used for further process design in the esterification of citric acid. esters, and water is the main byproduct of the process.11,12 Current citrate production is carried out in semibatch processes, with a large excess of alcohol that is required to dissolve the acid and to overcome chemical equilibrium limitations. The need for alcohol-excess removal and product purification makes the process energy intensive, inefficient, and costly. In this regard, intensification by using reactive distillation (RD) technologies is envisioned as an alternative to improve the process sustainability.13 The implementation of RD in a multireactant system is a major challenge, and it requires the tuning of the reaction selectivity for an optimal economic performance.14 Besides the main product (TEC), the commercial exploitation of the intermediate esters (MEC and DEC) has not been done, and it might be of major interest to improve the portfolio of products and the process profitability. Partially esterified citrates could possibly be obtained by taking side cuts in a RD system during

1. INTRODUCTION Reports on the potential harmful health effects caused by petrochemical plasticizers, specifically some phthalates, are of increasing concern for consumers and producers.1−3 This concern has impacted the plasticizer market which is expected to grow, mainly in the segments of nonphthalates and the biobased ones.4−6 However, higher prices of biobased plasticizers is a major limitation that prevents a greater market share. Among the variety of biobased plasticizers, citric acid esters are generally recognized as safe (GRAS) and preferred in the fabrication of polymers, resins, and films for highly sensitive applications (medical devices, food packaging, toys, cosmetics, etc.).7−9 Most widely used citric plasticizers are triethyl, tributyl, and trihexyl citrates with or without substitution of the hydroxyl group (acetylated or butyrated).10 The synthesis of citrates is generally accomplished by direct acid-catalyzed esterification of citric acid with the corresponding alcohol. In particular, the production of triethyl citrate (TEC) proceeds by reacting citric acid with ethanol through a series-parallel reaction scheme (Figure 1). Both monoethyl citrate (MEC) and diethyl citrate (DEC) are intermediate © XXXX American Chemical Society

Received: December 5, 2017 Accepted: May 31, 2018

A

DOI: 10.1021/acs.jced.7b01060 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Reaction scheme in the synthesis of ethyl citrates.

for NMR analysis. Sodium hydroxide (NaOH, >99.0%, Merck) was used for titration analysis. Purities and sources are summarized in Table 1.

TEC production. As other polyprotic acids, MEC might be valuable as a chelating agent, cross-linker for polymers, and antifungal component.15−17 On the other hand, DEC has the potential to become a major value-added citrate because it can be used as a pore-forming agent in biobased polymers,18 pHcontrolling agent in dialysis solutions,19 chelating agent for printing inks,20 skin aging agent,21 absorbent material in cigarette filters,22 and most importantly a potential anticoagulant that can replace citrate salts.23−25 In order to tweak the RD technology to produce a specific blend of citrate esters, a reliable model of the reactive system is needed. In this sense, vapor−liquid equilibrium (VLE) models along with the physicochemical properties of the involved components are required. The commercial unavailability of the intermediate citric acid esters partially explains the lack of such information.26 The few available studies dealing with the intermediate citrates were done on DEC, specifically trying to understand the coordination dynamics and coordination mechanism with calcium ions.25 Unfortunately, no information on the physicochemical properties of DEC (e.g., melting point, boiling point, heat of vaporization, etc.) has been published. In this regard, this study focuses on the synthesis, isolation, and characterization of diethyl citrate. Once obtained and purified, determination of thermal properties and experimental VLE data for two systems (DEC + ethanol and DEC + water) was carried out. VLE data were correlated to obtain binary interaction parameters of the UNIQUAC equation using a genetic algorithm optimization, and the regressed model can be used for computer simulation and conceptual design of RD processes.

Table 1. Sample Description Table chemical name

source

initial mass purity (%)

analysis method

citric acid ethanol sodium carbonate hydrochloric acid diethyl ether sodium sulfate diethyl citrate

Sucroal Panreac Merck Panreac J.T. Baker Merck Synthetized

99.9 99.5 99.5 37.0 99.0 99.0 97.0

HPLC GC N/A N/A N/A N/A NMR, HPLC

2.2. Experimental Procedures. 2.2.1. DEC Synthesis. Among the reported chemical routes and catalysts to produce citric esters,11,12,24,26,27 DEC was prepared by partial esterification of citric acid (CA) and anhydrous ethanol (EtOH) using Amberlyst 15 as catalyst.11 Citric acid was dissolved in EtOH in a 1:5 molar ratio, within a 2 L SS-316 stirred tank reactor coupled with a reflux condenser and an electrical heating jacket. The mixture was kept under mechanical agitation at room temperature during 1 h. Prewashed and vacuum-dried Amberlyst 15 catalyst was added in 5 wt % of the reactive mixture. In order to maximize DEC concentration in the reactive mixture, the temperature was set at 346 K, and the reaction was carried out for 5 h.11 After the reaction was completed, the catalyst was separated by using a Buchner filter, and the excess ethanol was removed by vacuum distillation. A yellowish liquid, mainly containing CA and partially esterified citrates, was obtained. 2.2.2. DEC Purification. Experimental procedure for DEC isolation and purification is schematically described in Figure 2. DEC separation was accomplished by pH-controlled solvent extraction, following a reported procedure.24 The first component to be removed from the vacuum-distilled solution was the triester. Initially, 100 g of the ethanol-free reactor effluent was dissolved in distilled water using a 1:1 mass− volume ratio, at room temperature and under agitation with a magnetic stirrer. Then, the pH of the solution was slowly adjusted to 8 by adding solid Na2CO3 under agitation, keeping track with a pH meter (Orion SA520). After reaching the desired pH, the mixture was extracted with 150 mL of diethyl ether in a glass separation funnel. At pH 8, all the acid groups of CA and those in the intermediate citrates are neutralized to form water-soluble sodium salts. Meanwhile TEC, that is mostly nonsoluble in the aqueous phase, was extracted in the organic layer. After liquid−liquid separation and vacuum

2. MATERIALS AND METHODS 2.1. Materials. Anhydrous citric acid (99.9%, Sucroal) and anhydrous ethanol (>99.5%, Panreac) were used for DEC synthesis. The concentration of all chemicals was verified by gas chromatography and high-performance liquid chromatography (HPLC), and the chemicals were used without further purification. Sodium carbonate (Na2CO3, >99.5%, Merck) and hydrochloric acid (HCl, 37%, Panreac) were used for pH adjustment. Diethyl ether (ultra resi-analyzed for organic residue analysis, J.T. Baker) and distilled water were used as solvents during the separation and purification process. Anhydrous sodium sulfate (Na2SO4, >99.0%, Merck) was used as desiccant. Acetonitrile (99.9% HPLC grade, Panreac) and deionized water were used for chromatographic analyses. Amberlyst 15 ion-exchange resin, purchased from the Dow Chemical Co., was used as acid catalyst in the esterification process. Deuterated chloroform (CDCl3, Cambridge Isotope Laboratories, Inc.) and deuterated dimethyl sulfoxide ((CD3)2SO, Cambridge Isotope Laboratories, Inc.) were used B



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equilibrium chamber was filled with the desired composition of the mixture. Once the vessel was loaded in, it was stirred with a magnetic bar and degassed under vacuum at the desired temperature. The system pressure was lowered below the expected vapor pressure (according to the vapor pressure of the pure more volatile component), and then it was allowed to stabilize for about to 20 min until no further change in the observed pressure. After stabilization, the cell was depressurized, and a liquid sample was withdrawn for analysis. Subsequent equilibrium points were obtained by adding a defined amount of the more volatile component. In order to test the reliability of the experimental setup, isothermal VLE data of the binary mixture TEC + EtOH were measured at 313 K and compared with previous literature reports.11 VLE of DEC + EtOH and DEC + H2O were measured at low temperature (308 K), to minimize DEC esterification or hydrolysis during experiments. Despite that VLE data were evaluated for the binary mixtures in a broad range of mass fractions, the large difference of molecular weights of DEC (248.23 g/mol), EtOH (46.06 g/mol), and H2O (18.01 g/mol) turned into a narrow range of data in a molar basis. 2.3. Analysis and Characterization. To assess the efficiency of the extraction process during experiments, the purity of DEC was constantly tracked by acid−base titration. It was continuously compared with the theoretical DEC acid value (226 mg KOH/g DEC) to establish the end point of the extraction procedure. For this, a 0.1 g sample of the obtained DEC was diluted in 25 mL of distilled water and titrated with a standardized (with potassium hydrogen phthalate) 0.07 M NaOH solution, using phenolphthalein as indicator. The composition of liquid samples collected during experiments was determined by liquid chromatography. Samples were dissolved in acetonitrile (∼2% w/w) before analysis. A DionexUltiMate 3000 HPLC system equipped with a reversed-phase C18 column (Acclaim 120, 3 × 150 mm, 3 μm, 120 Å) was used. The oven temperature was set to 313 K. The mobile phase operated with gradient, using microfiltrated and degasified acetonitrile and ∼2 mmol sulfuric acid solution in deionized water (pH ∼ 2.1). A diode array detector was used with a wavelength of 210 nm. The technique lasted 25 min, with a programmed gradient of acetonitrile (ACN)/water (pH 2.1) as follows: 0% ACN (t = 0 min), 60% ACN (t = 20 min), 0% ACN (t = 22 min), and 0% ACN (t = 25 min). Samples of known composition in the range of interest were used for previous calibration. This technique allows measuring citrate species only, so water and ethanol concentrations in the mixtures were obtained by difference. Nuclear magnetic resonance (NMR) was used to confirm the chemical structure of the obtained DEC. This analysis was done in an Agilent DDR2 500 MHz NMR spectrometer equipped with a 7600AS 96 auto sampler. 1H and 13C NMR were carried out using deuterated dimethyl sulfoxide ((CD3)2SO) and deuterated chloroform (CDCl3) as solvents. The chemical shifts corresponding to each solvent are 1H 7.26 and 13C 77.09 for (CD3)2SO and 1H 2.49 and 13C 39.83 for CDCl3. Thermogravimetric analysis (TGA) was performed to determine the onset decomposition temperature with a TGA 1 Stare System (Mettler Toledo). A sample of 48.6 mg was placed in the microbalance of the system, and it was heated from room temperature to 572 K at a rate of 10 K/min with a nitrogen flow rate of 50 mL/min.

Figure 2. Scheme of the DEC synthesis and purification process.

distillation of the ethereous extract, a dark-yellow TEC-rich liquid was obtained, and its nature was verified by HPLC analysis. The aqueous phase was extracted again with 150 mL of diethyl ether to remove the remaining TEC before proceeding with the next step. After TEC extraction, the pH of the obtained aqueous solution was carefully reduced to 4 by adding concentrated HCl dropwise, under continuous agitation. At this pH, mostly CA and MEC remained in the aqueous phase as sodium salts,24 allowing DEC removal by solvent extraction with diethyl ether. Once the pH was adjusted, the aqueous solution was extracted with 150 mL of diethyl ether in a glass separation funnel. After liquid−liquid separation, the organic layer was removed and then treated with anhydrous Na2SO4 as desiccant, followed by filtration in a Buchner funnel. After diethyl ether removal from the water-free organic phase by rotary evaporation, a lightyellow DEC-rich liquid was obtained. Finally, this liquid was distilled under high vacuum. The procedure previously described was conducted six times in order to obtain enough DEC for vapor−liquid experiments. The obtained liquid was filtrated and stored in a brown-glass hermetic container at room temperature before further use. 2.2.3. Vapor−Liquid Experiments. VLE data were obtained by using an isothermal glass cell connected to a high accuracy pressure gauge (MKS PDR 2000 Baratron - dual capacitance diaphragm) and a thermo-probe (Precision RTD Thermometer 407907 - EXTECH). Temperature measurements were within ±0.05 K, and pressure uncertainty was within ±0.02 kPa. The operating procedure for P−x measurements was described in detail in a previous work.28,29 In brief, after testing for leaks, the C



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Tests to measure glass transition and to verify thermal stability of DEC were carried out by differential scanning calorimetry (DSC). Analyses of samples were performed on a DSC 1 Stare System (Mettler Toledo). A sample of 15−20 mg was placed in an aluminum pan and heated from room temperature to 573 K at 10 K/min. For glass transition temperature analysis, 8 mg of sample was cooled from room temperature until 208 K and maintained at that temperature for 2 h. Finally, the sample was slowly heated from 208 to 273 K at 5 K/min. All the experiments were done under nitrogen atmosphere at a flow rate of 50 mL/min. Glass transition temperature was analyzed as an onset temperature, and delta cp was calculated by the equipment software.

3. RESULTS AND DISCUSSION Due to the low DEC recovery during each synthesis attempt, several reaction batches were necessary to obtain enough

Figure 4. 13C NMR spectra for synthesized diethyl citrate with CDCl3.

Figure 5. TGA curve of a DEC sample measured under N2 atmosphere at 101.3 kPa.

3), and the obtained spectra agree with previous reports.24,25 The presence of 1,3-diethyl citrate (symmetric) and 1,2-diethyl citrate (asymmetric) was indistinctly found in all samples. Also diethyl ether presence was confirmed by the triplet at δ = 1.04 ppm and the quadruplet at δ = 3.42 when (CD3)2SO was used as solvent. These peaks correspond to the protons in terminal methyl groups (CH3−) and the protons in methylene groups (−CH2‑) in the diethyl ether molecule, respectively. When CDCl3 was used as solvent, the triplet corresponding to diethyl ether appeared at the same chemical shift of methyl groups in the DEC molecule. The group of peaks in the chemical shift interval between δ = 1.15−1.26 ppm for CDCl3 and δ = 1.12−1.20 ppm for (CD3)2SO is attributed to the protons in methyl groups in the DEC molecule. The presence of several triplets within the same interval indicates that the methyl groups are surrounded by slightly different chemical environments. These shifts are

Figure 3. 1H NMR spectra for synthesized diethyl citrate with CDCl3.

sample for physicochemical characterization and for equilibrium experiments. By performing the pH-controlled separation, almost all TEC, MEC, and CA were removed from the reactive mixture, and the clear-yellowish liquid used for further characterization was a 97 wt % DEC product. According to the HPLC analysis, the major impurities corresponded to CA and MEC. The low concentration of CA and MEC is not expected to alter significantly the vapor−liquid equilibrium measurements due to their low volatility. Therefore, the obtained DEC was used without further purification. During storage, the product remained stable, and no further hydrolysis or esterification was detected. 3.1. Hydrogen NMR. 1H NMR analysis was carried out in order to verify the chemical structure of purified DEC (Figure D



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Figure 6. Calorimetric curves obtained by DSC of a DEC sample measured under N2 atmosphere at 101.3 kPa from 208 to 273 K (a) and from 298 to 580 K (b).

Table 2. Experimental Physical Properties of Pure DEC at 101.3 kPaa

a

property

units

value

glass transition temperature (Tg) delta cp at glass transition (J/gK) thermal decomposition temperature (TDT)

K J/gK K

228 0.36 493

Standard uncertainties u are u(T) = 1 K and u(P) = 0.3 kPa.

Table 3. Experimental Equilibrium Data (P−x) for the Binary Mixture EtOH + TECa at 313 K P (kPa)

xTEC

P (kPa)

xTEC

2.84 6.29 7.29 8.28 8.83 9.92 10.36 10.73 11.11 11.72 13.77 14.04

0.905 0.784 0.742 0.698 0.649 0.578 0.551 0.539 0.506 0.493 0.345 0.299

14.04 14.48 14.77 15.43 15.43 15.57 15.71 15.71 16.47 16.88 16.89

0.307 0.282 0.246 0.186 0.201 0.181 0.149 0.159 0.074 0.040 0.041

Figure 7. Binary P−x diagram for the binary TEC + EtOH at 313 K. (blue ●) This work. (○) Literature.11

Table 4. Experimental Equilibrium P−x Data for the DECRich Mixtures with EtOHa at 308 K

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.01 P, and u(x) = 0.005.

explained by the existence of the two isomers of DEC, in this case 1,3-DEC (symmetric) and 1,2-DEC (asymmetric). Similarly, the peaks corresponding to protons in the methylene groups, characteristic of the citric backbone (located at δ = 1.15−1.26 ppm and at δ = 1.12−1.20 ppm for CDCl3 and (CD3)2SO, respectively), and the peaks attributed to protons in the methylene groups from DEC (located at δ = 4.03−4.26 ppm and at δ = 3.96−4.12 ppm when CDCl3 and (CD3)2SO are used as solvents) show peak superposition due to the presence of the isomers. The relative area between the different groups of peaks attributed to DEC molecules are consistent, indicating that the two isomers are the principal components in the sample. Nevertheless, CA and MEC isomers that were also identified in the sample by HPLC were not clearly identified in 1H NMR

xDEC

xCA

xMEC

xTEC

xEtOH

P (kPa)

0 0.053 0.095 0.119 0.141 0.170 0.201 0.233 0.288

0 0.006 0.013 0.016 0.019 0.023 0.028 0.032 0.041

0 0.002 0.007 0.009 0.011 0.013 0.015 0.018 0.019

0 0.005 0.007 0.010 0.012 0.014 0.017 0.035 0.025

1.000 0.934 0.877 0.846 0.817 0.779 0.739 0.682 0.627

13.83 12.69 10.99 10.63 10.07 9.12 7.76 6.23 4.79

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.01 P, and u(x) = 0.005.

analysis mainly due to the low concentration and the structural similarities of the molecules. Based upon the relative areas of the isomer peaks in NMR spectra, the relative molar ratio30 between 1,3-DEC and 1,2-DEC was 1.17 when using CDCl3 as solvent and 1.23 with (CD3)2SO. This result indicates that the symmetric diester (1,3-DEC) is slightly more favorable in the synthesis, mainly because of the geometric hindrance and the steric effects of the hydroxyl group in the citric backbone. E



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attributed to carbons of the methyl (CH3−) groups at the extremes of the DEC molecule. The isomer presence was confirmed by the variety of revealed peaks in NMR; that is, when using CDCl3 as solvent the single peak at δ = 13.9 ppm corresponds to 1,3-DEC (symmetric), whereas the other peaks at δ = 13.85 ppm and δ = 13.95 ppm are related to 1,2-DEC (asymmetric). The peaks located within the interval from δ = 42 to δ = 43.4 when using CDCl3 as solvent and from δ = 43 to δ = 43.65 when using (CD3)2SO are attributed to carbons of the methylene groups in citric species. Besides, the peaks in the interval between δ = 60.05 and 62.6 ppm when using CDCl3 and between δ = 60.15 and 61.15 ppm with (CD3)2SO corresponded to the methylene (−CH2−) groups found in ethyl citrates. In both cases, several peaks were obtained due to the presence of isomers. The group of peaks located around δ = 73 ppm when either CDCl3 or (CD3)2SO was used is attributed to the central carbon in the DEC molecule. Given that this is a single carbon, one peak is expected to occur for each isomer, and both were observed for the two DEC structures. Smaller peaks were also found, those corresponding to other citric species (i.e., CA and MEC). The peaks corresponding to carbons in carboxyl groups (−COOH) and in ester groups (−COO−) are located in the interval from δ = 169.5 ppm to δ = 177 ppm for CDCl3 and from δ = 169.5 ppm to δ = 175 ppm for (CD3)2SO. Bigger signals corresponded to DEC isomers, whereas smaller peaks are related to other citric esters, mainly MEC. When comparing relative areas for each isomer, a ratio is observed similar to that obtained when analyzing 1H NMR spectra. The area ratio between 1,3-DEC and 1,2-DEC was about 1.28 in CDCl3 and 1.19 with (CD3)2SO. Similarly to the 1H NMR spectra, this indicates a molar ratio of about 1.24 between the symmetric and the asymmetric isomers. Hydrogen and carbon NMR results indicate that the extraction and purification methods were successful in obtaining mostly DEC. However, the final product is a mixture of isomers. The further separation of both symmetric and asymmetric isomers is out of the scope of this work and needs to be further studied. 3.3. Thermogravimetric Analysis. Results of the thermogravimetric analysis (TGA) on obtained DEC are shown in Figure 5. As observed, DEC slowly decomposes above 420 K, but the rate of weight loss increases rapidly above 440 K. According to the thermal analysis, the onset decomposition temperature was 493 K. This value is consistent with the decomposition temperatures of other citric species.31,32 This is a limiting condition that has to be considered during reactor design (or reactive distillation) and when establishing downstream processing. 3.4. Differential Scanning Calorimetry. The differential scanning calorimetry curves of DEC are presented in Figure 6. According to the results, DEC decomposes before or simultaneously with boiling, which was confirmed by the carbonaceous residue remaining after the analysis and the baseline shift due to weight loss. Similarly to citric acid thermal decomposition,31,32 DEC thermal degradation was endothermic, and the onset temperature of the process was around 490 K, confirming TGA results. Calorimetric data also indicate that DEC presents a glass transition around 228 K, and the delta cp at this point corresponded to 0.36 J/gK. This temperature is lower than the glass transition temperature reported for anhydrous and monohydrated amorphous citric acid.33 In this

Table 5. Experimental Equilibrium Data P−x for the DECRich Mixtures with H2Oa at 308 K xDEC

xCA

xMEC

xTEC

xH2O

P (kPa)

0 0.032 0.046 0.057 0.059 0.060 0.068 0.070 0.070 0.072 0.073 0.075

0 0.006 0.003 0.005 0.007 0.008 0.011 0.011 0.011 0.012 0.012 0.013

0 0.002 0.004 0.005 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003

0 0.000 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.003

1.000 0.960 0.945 0.931 0.929 0.928 0.916 0.914 0.914 0.911 0.909 0.907

5.63 5.71 5.64 5.40 5.36 5.51 5.28 5.32 5.27 5.28 5.28 5.28

a Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.01 P, and u(x) = 0.005.

Table 6. Calculated Activity Coefficients for the Binary Mixture DEC + EtOH at 308 K xDEC

γDEC

γCA

γMEC

γTEC

γEtOH

0 0.053 0.095 0.119 0.141 0.170 0.201 0.233 0.288

0.02 0.05 0.06 0.08 0.11 0.15 0.19 0.28

2.20 2.36 2.42 2.47 2.51 2.53 2.50 2.49

1.10 1.06 1.04 1.03 1.01 1.00 0.97 0.96

4.58 4.73 4.73 4.69 4.58 4.42 3.98 3.84

1.00 0.97 0.92 0.89 0.85 0.80 0.75 0.69 0.60

Table 7. Calculated Activity Coefficients for the Binary Mixture DEC + H2O at 308 K xDEC

γDEC

γCA

γMEC

γTEC

γEtOH

0 0.032 0.046 0.057 0.059 0.060 0.068 0.070 0.070 0.072 0.073 0.075

0.12 0.12 0.13 0.13 0.13 0.15 0.15 0.15 0.16 0.16 0.17

0.45 0.49 0.54 0.55 0.55 0.59 0.60 0.60 0.61 0.62 0.63

0.32 0.39 0.45 0.45 0.46 0.50 0.51 0.51 0.52 0.53 0.54

1.32 0.93 0.79 0.77 0.77 0.72 0.72 0.71 0.71 0.71 0.70

1.00 1.05 1.05 1.04 1.04 1.04 1.03 1.03 1.03 1.03 1.03 1.02

Table 8. Binary Parameters for the UNIQUAC Model i

DEC

j

EtOH

H2O

Bij Bji

4224.5 −3183.6

−3233.4 8235.4

DEC

3.2. Carbon NMR. 13C NMR analyses presented in Figure 4 agree with the results observed in the 1H NMR spectra, regarding the characteristic peaks of DEC isomers. Diethyl ether presence was also confirmed by this analysis. The obtained spectra agree with literature reports.24,25 DEC isomers were identified as follows: the peaks at δ = 13.8−14 ppm for CDCl3 and at δ = 14.25−14.55 ppm for (CD3)2SO are F



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Figure 8. Binary P−x diagram for the systems DEC + EtOH (a) and DEC + H2O (b) at 308 K. Solid curve: ideal model. Dotted gray line: Kolah et al. model.11 Dashed curve: regressed UNIQUAC model.

case, the glass transition might occurs for two reasons: first, because the solidification occurs without crystal formation and, second, due to a semicrystalline solid behavior. Taking into account that some crystals were observed in the capsules, the glass transition most probably occurred by a semicrystalline solid behavior of the isomeric mixture in the DEC sample. Table 2 summarizes calorimetric analysis results. 3.5. VLE Equilibria. To evaluate the phase equilibria for DEC−EtOH and DEC−H2O mixtures an isothermal cell was used. The experimental procedure and apparatus allowed measuring pressure, temperature, and composition of the liquid phase. These three variables are enough to determine the thermodynamic state of each mixture, and therefore data were used to regress an activity coefficient model. However, Gibbs− Duhem slope and area thermodynamic consistency tests were not possible to assess. Due to the equilibrium cell setup and the arduous procedure to obtain a high purity DEC, it was decided to collect data in the most efficient way regarding the economic resources available. This consisted of taking data along an isothermal path, making small changes in the liquid concentration by adding the more volatile component. This helped to minimize the amount of DEC required in experiments and to reduce the probability of DEC degradation due to esterification or hydrolysis with EtOH or H2O, respectively. The isothermal tests were carried out in single 10 h runs. Unfortunately, DEC started to degrade soon after the end of the first isothermal equilibrium tests for each component. Therefore, only one complete set of isothermal data was taken for each system. This also prevented us from doing a Pxy consistency test using the Gibbs−Helmholtz equation.34,35 Due to the small amount of pure DEC available, it was not possible to run a broader set of experiments. Prior to VLE experiments with DEC, the reliability of the isothermal cell was tested by evaluating P−x equilibrium data for the EtOH−TEC system. Isothermal VLE data for this system at 313 K are summarized in Table 3, and they are compared with previous reports11 in Figure 7. As observed, the experimental setup and methods used in this study can reproduce P−x equilibrium data in mixtures of similar nature. These data are also a good contribution for the modeling of EtOH + TEC mixtures. Isothermal VLE data for the DEC + EtOH system were collected at 308 K, and the results are listed in Table 4. In accordance with the HPLC analysis, and despite the intensive purification process, some impurities derived from the DEC synthesis and some others from DEC decomposition within the

equilibrium cell (i.e., CA, MEC, and TEC) were present in samples. Also, peak resolution between the symmetric and asymmetric isomers of both, MEC and DEC, was not achieved in the HPLC method. For this reason, each intermediate ester was treated as a single component in reported data. This approach has been found valuable when working with isomeric mixtures of biobased chemicals during VLE experiments, mainly when developing models for early stage process design.36,37 Isothermal VLE data for the DEC + H2O system at 308 K are presented in Table 5. Despite that the evaluated H2O mass fractions during experiments ranged between 0.45 and 1, the vapor pressure of the mixtures barely changed. A maximum vapor pressure change of 6% was observed in the whole concentration interval. Further evaluations at higher DEC concentrations were not carried out because the pressure gauge showed low sensitivity below 4 kPa mainly due to the vacuum pump limitations. 3.6. VLE Binary Parameter Regression. VLE data were fitted to the UNIQUAC equation (model included in Aspen plus38) using the ideal gas equation to model the vapor phase. The parameter regression was performed with a genetic algorithm optimization to minimize the error over the calculated pressure, by changing the binary parameters of the temperature-independent UNIQUAC term (Aij and Aji).38 The genetic algorithm had a population of 1000 individuals, 40 generations, Pc = 0.3, and Pm = 0.5. Taking into account the results from the sample characterization via HPLC, the equilibrium conditions were evaluated as a multicomponent mixture. Thus, the UNIQUAC parameters for EtOH + DEC and H2O + DEC systems were obtained as follows. The regression was conducted as a multicomponent system including the four citric species plus water and ethanol and adjusting only the binary UNIQUAC parameters for EtOH−DEC and H2O−DEC. Because, as already presented, the citric species tend to decompose before their normal boiling, there is no information on the boiling points of DEC and MEC. Therefore, these values were estimated using Gani’s group contribution approach39 since it was the method that better predicted the reported normal boiling point for TEC. The obtained boiling temperatures are listed in Table S1 of the Supporting Information. As both MEC and DEC are mixtures of symmetric and asymmetric structures, the estimated boiling temperatures were assumed as the arithmetic average of the calculated boiling points of the two isomers of each component (3.2% and 3.6% temperature difference between both values for MEC and DEC isomers, respectively). Then, the normal G



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citric species, experimental data were fitted with the UNIQUAC equation to obtain binary interaction parameters for phase equilibria calculation. The VLE model using the UNIQUAC equation agrees well with experimental observations, and this model can be used for further design and simulation of esterification processes of citric acid with ethanol.

boiling points of MEC and DEC were used to regress Antoine coefficients for vapor pressure estimation, using the Riedel equation included in Aspen Plus. Antoine coefficients are listed in Table S2 in the Supporting Information. In order to model the systems as multicomponent mixtures, the binary interaction parameters with other components were either obtained from the literature11 or predicted by UNIFAC. The final list of interaction parameters is included within Table S3 of the Supporting Information, together with the description of the used UNIQUAC equation. For comparison, the systems were also modeled with two alternative methods: first using Raoult’s law based upon the vapor pressure of the more volatile component only (EtOH or H2O). This can be assumed because most citrates were nearly nonvolatile in the mixtures under experimental conditions. The second method used the UNIQUAC parameters published by Kolah et al.11 The calculated activity coefficients for the equilibrium conditions reported in Tables 4 and 5, with the regressed UNIQUAC parameters of EtOH−DEC and H2O−DEC, are listed in Tables 6 and 7. As mentioned before, traditional thermodynamic consistency tests34,35,40,41 were not applied to the isothermal experimental data because the vapor-phase concentration was not measured. Despite that some attempts were done for its quantification, the concentration of citrates was well below the level of detection of the employed techniques. Additionally, it was not possible to evaluate equilibrium conditions at higher temperatures because of the observed decomposition of DEC. Nonetheless, the regressed models were able to reproduce experimental observations with better agreement than the ideal solution or than any previously published binary parameters. The obtained set of parameters for the UNIQUAC equation is listed in Table 8. In Figure 8, a comparison of experimental data with the regressed models is presented in a molar basis. Despite that most molar-based equilibrium data are concentrated in a small region, in a mass basis they cover a broader range of concentrations. This is caused by the large difference of molecular weights between the studied components. It can be verified from Figure S1 in the Supporting Information that the mass concentration of the more volatile component varied from 20 to 100 wt % in binary DEC + EtOH and from 40 to 100 wt % in the DEC + H2O mixture. While the DEC + EtOH mixture shows a negative deviation from ideality, the DEC + H2O mixture shows a slight positive deviation from ideality at high water concentrations. Clearly, the UNIQUAC models fit better the experimental observation, and the obtained parameters can be used for further process modeling and simulation, specifically for reactive distillation operations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01060. Table S.1. Normal boiling temperatures (Tb) calculated by the Gani Method. Table S.2. Antoine coefficients. Table S.3. Binary interaction parameters of the UNIQUAC equation. Figure. S1. Mass basis binary P− x diagram for binary mixtures of DEC + EtOH (a) and DEC + H2O (b) at 308 K. Solid curve: Ideal model. Dotted gray line: Kolah et al. model 11. Dashed curve: Regressed UNIQUAC model (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+571) 3165000 x 14303. E-mail: [email protected]. ORCID

Alvaro Orjuela: 0000-0003-0329-5601 Funding

This work was supported by “Departamento Administrativo de Ciencia, Tecnologiá e Innovación − Colciencias”, under the ́ Projects: “Producción de plastif icantes a partir de ácido citrico ́ usando procesos hibridos de reacción y separación simultánea”, code: 1101-569-33201; and “Obtención y purif icación de citrato de dietilo y determinación de los equilibrios de fase en mezclas con etanol y agua”. The last one corresponded to “Convocatoria nacional jóvenes investigadores e innovadores 2015”. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS Diethyl citrate was produced by partial esterification of citric acid and ethanol using Amberlyst 15 as catalyst. Several liquid− liquid extractions under controlled pH were used to obtain a high purity DEC for further characterization. After isolation, the nature of the product was confirmed by NMR analysis, and a mole ratio of 6:5 of the symmetric to the asymmetric isomer was observed. Thermal and calorimetric analyses allowed us to determine that DEC has a glass transition around 228 K with a change in the cp of 0.36 J/gK and starts decomposing before boiling around 490 K. Additionally, isothermal VLE equilibrium experiments were carried out for mixtures of DEC + EtOH and DEC + H2O at 308 K. Accounting for the presence of different H



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Synthesis and Characterization of Diethyl Citrate ... - ACS Publications

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