Dibutyl Citrate Synthesis, Physicochemical Characterization, and Px

Article pubs.acs.org/jced

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

Dibutyl Citrate Synthesis, Physicochemical Characterization, and Px Data in Mixtures with Butanol Andrea Suaza, Miguel A. Santaella, Luz A. Rincón, Á ngela L. Alarcón, and Alvaro Orjuela* Department of Chemical and Environmental Engineering, Universidad Nacional de Colombia, 111321 Bogotá D.C., Colombia S Supporting Information *

ABSTRACT: Dibutyl citrate (DBC) was synthesized via partial esterification of citric acid (CA) with n-butanol (BuOH) and characterized for the first time. Then, the monoacid diester was isolated from the reactive mixture by using a pH-controlled solvent extraction procedure. The identity of the obtained product was verified by NMR spectroscopy, finding that it corresponds to a mixture of asymmetric and symmetric isomers. The obtained product was characterized by thermogravimetric and differential scanning calorimetry analysis and by density and viscosity measurements. As per the calorimetric analysis, DBC exhibited a crystallization point below 213 K, a decomposition temperature of 541 K, and an average heat capacity of 2.461 kJ/kg K in the evaluated temperature range (320−375 K). The average liquid density and viscosity in the studied temperature interval (298−313 K) were 1.122 g/cm3 and 2.03 Pa·s, respectively. Additionally, isothermal vapor−liquid equilibrium data (P−x) in mixtures containing the obtained DBC and BuOH were measured at 313, 323, and 333 K. Experimental data were used to fit UNIQUAC interaction parameters for the binary DBC−BuOH. The regressed model showed good agreement with experimental results, making it suitable for further process design and simulation. The obtained data can be used in the development of processes for citrate plasticizer production and in the development of separation processes for DBC.

1. INTRODUCTION

highly sensitive applications such as pharmaceuticals and cosmetics, medical devices, and children’s products.3,4 In particular, acetylated (ATBC) and nonacetylated tributyl citrate (TBC) are commonly used as biobased plasticizers in food wrapping films and as solvents in cosmetic applications. TBC synthesis proceeds through direct esterification of citric acid with n-butanol, involving a set of series-parallel equilibrium reactions (Figure 1).5 As the citric acid contains three carboxylic groups, each intermediate ester, monobutyl citrate (MBC) and dibutyl citrate (DBC), can have two different structural isomers. In the case of MBC, the butyl-branched substituent can be located in a carboxyl group adjacent to a secondary carbon (2-hydroxy-1,2,3-propanetricarboxylic acid-1butyl ester; MBC Isomer 1) or to a quaternary carbon (2hydroxy-1,2,3-propanetricarboxylic acid-2-butyl ester; MBC Isomer 2). For a similar reason, in the case of DBC, there is

Plasticizers are one of the most important additives for polymers, providing flexibility and plasticity and facilitating their processing and transformation. The current global plasticizer market size is ca. 16 billion dollars and 8.5 million tons per year,1,2 and it is mostly dominated by petroleumderived chemicals. Among these, the phthalic acid esters of medium and high molecular weight alcohols represent nearly 80% of the global volume mainly due to their widespread use in PVC products.3 However, due to the growing environmental and health awareness, and the increasing number of legal provisions and regulations for the use of phthalates producers are being forced to switch to nonphthalate alternatives. In this regard, the development and production of biobased substitutes obtained from renewable resources has been boosted in recent years. Among these, citric acid esters have regained importance as “green” plasticizers because they are recognized as safe chemicals (GRAS), and they are approved by the FDA as food additives. For this reason, they are suitable ingredients for © XXXX American Chemical Society

Received: December 6, 2017 Accepted: May 18, 2018

A

DOI: 10.1021/acs.jced.7b01064 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

neous catalysts with large alcohol excess to allow citric acid dissolution and to overcome chemical equilibrium limitations.6 This generates a great impact on the purification steps as the excess alcohol removal and recovery is energy intensive and costly. As a processing alternative, simultaneous reaction and separation via reactive distillation (RD) has been proposed for citrate production, in order to reduce separation costs and to improve productivity.7 However, one of the major limitations for industrial implementation is the lack of information and models required to predict the behavior of intermediate esters in RD systems.8 In particular, for the modeling and exploration of TBC via RD, there are no data on the physicochemical properties of the intermediate esters (i.e., MBC and DBC) or phase equilibria data in mixtures with the other components of the reactive media. This information is fundamental in order to establish process constraints, to develop accurate energy balances, and to perform phase and chemical equilibrium calculations. In this regard, this study focuses on the synthesis and isolation of DBC and its further characterization via evaluation of physicochemical and thermal properties (density, viscosity, decomposition temperature, and heat capacity). Furthermore, vapor−liquid equilibrium (VLE) data in mixtures with nbutanol were collected under isothermal conditions. Finally, binary interaction parameters from the UNIQUAC model were regressed.

Figure 1. Reaction scheme on the synthesis of tributyl citrate. AC, citric acid; BuOH, n-butanol; MBC, monobutyl citrate; DBC, dibutyl citrate; TBC, tributyl citrate.5

2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis of DBC was carried out by partial esterification of anhydrous citric acid (CA) with nbutanol (BuOH) using Amberlyst 15 ion-exchange resin as catalyst (Dow Chemical Co.). The concentration of purchased reactants was verified by gas and liquid chromatography, and they were used without further purification. The list of chemicals and standards used in experiments and the corresponding purities is presented in Table 1.

a symmetric structure (2-hydroxy-1,2,3-propanetricarboxylic acid-1,3-dibutyl ester; DBC Isomer 1) and an asymmetric one (2-hydroxy-1,2,3-propanetricarboxylic acid-1,2-dibutyl ester; DBC Isomer 2). Since TBC is generally the product of interest, intermediate esters are seldom studied, and they are not commercially available. Industrial-scale esterification to TBC is carried out in semibatch processes using acidic homogeneous or heteroge-

Table 1. Characteristics of Chemicals and Standards Used during Experimentation chemical name

source

purity (wt %)

CAS number

citric acid n-butanol sodium carbonate diethyl ether sulfuric acid sodium sulfate acetonitrile amberlyst 15 deuterated dimethyl sulfoxide-d6 sodium hydroxide Karl Fischer solution (Aquametric composite 5RV) methanol indium calibration standard zinc calibration standard sapphire disc standard for ASTM E1269-05 ME29800-Isatherm curie point 149 °C−T0814 ME29799-Ni-Alloy curie point 355 °C−N0414 ME29798-Trafoperm 86 Ci point 749 °C−T0814 sodium chloride standard solution (1000 mg of Na) sodium tartrate dihydrate tetramethylsilane

Sucroal Panreac Panreac Panreac Merck Merck Panreac, HPLC grade Dow Chemical Co. Cambridge Isotope Laboratories Merck Panreac J.T. Baker, HPLC grade Mettler Mettler Toledo Mettler Toledo Mettler Toledo Mettler Toledo Mettler Toledo Merck Panreac, 15.66% water content Sigma

99.9 99.5 >99.5 99.7 95−97 99.0 99.9

>99.9 99.999 99.999

77-92-9 71-36-3 497-19-8 60-29-7 7664-93-9 7757-82-6 75-05-8 39389-20-3 2206-27-1 1310-73-2 2025-88-4 67-56-1 7440-74-6 7440-66-6

99.5

7647-14-5 6106-24-7 75-76-3

B

99.9 99.0



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Sodium carbonate (Na2CO3) and sulfuric acid (H2SO4) were used for pH adjustment during extraction, and diethyl ether (DEE) and distilled water were used as solvents. Anhydrous sodium sulfate (Na2SO4) was used as desiccant in DBC purification. Acetonitrile (ACN) and deionized water were used as a mobile phase for chromatographic analyses. Deuterated dimethyl sulfoxide ((CD3)2SO) was used for 1H NMR analysis with tetramethylsilane (Si(CH3)4) as calibration standard. Sodium hydroxide (NaOH) was used for acid titration analysis. Karl Fischer solution (Aquametric composite 5RV) and methanol were used for water content determination using standard titration, with previous calibration with sodium tartrate dihydrate solution. Indium, zinc, and sapphire standards were used for calibration of differential scanning calorimetry. Mettler Toledo alloys (ME29800-Isatherm, ME29799-Ni-Alloy, ME29798-Trafoperm 86) were used for TGA calibration. The sodium chloride standard solution was used for calibration of atomic absorption measurements. 2.2. Methods. 2.2.1. DBC Synthesis. Taking into account reported kinetic models for the production of TBC,5,9 the synthesis was performed under the experimental conditions that maximized DBC productivity. The process was carried out by partial esterification of CA and BuOH using Amberlyst 15 as heterogeneous catalyst. First, CA was dissolved at room temperature in BuOH, in a 1:15 molar ratio, within a 1 L stirred tank glass reactor coupled with a reflux condenser. Afterward, prewashed and vacuum-dried Amberlyst 15 catalyst was added in a 5% mass fraction of the reactive mixture. The temperature of the system was set to 393 K, and the reaction was carried out for 3 h. Finally, the catalyst was filtrated out, and the volatile components (BuOH and H2O) were removed by vacuum distillation in a rotary evaporator. The final product consisted of a mixture of CA, MBC, DBC, and TBC with minor impurities of water and BuOH. 2.2.2. DBC Purification. DBC separation was accomplished by following a pH-controlled solvent extraction method similar to that reported for diethyl citrate purification.10 The schematic of the experimental procedure is summarized in Figure 2. First, 100 g of the reactive mixture obtained from the rotary evaporator was mixed with distilled water using a 1:1 mass− volume ratio. The mixture was vigorously agitated within a glass separation funnel, and the pH was slowly adjusted to 8 by adding Na2CO3, keeping track with a pH meter (Orion SA520). During this step, the acid groups in CA, MBC, and DBC were neutralized to form water-soluble sodium salts. Then 100 mL of DEE was added to the mixture to form two immiscible liquid phases. As TBC is virtually nonsoluble in the aqueous phase, it was extracted in the organic layer. After liquid−liquid separation and vacuum distillation of the organic extract, a light-yellow TBC-rich liquid was obtained, and it was analyzed by HPLC. The TBC removal process was repeated twice. In a second stage, the pH of the obtained aqueous phase was carefully adjusted to 5 adding sulfuric acid. Under these acidic conditions, DBC sodium salt returned to the nondissociated carboxylic acid form, making it less water-soluble. Conversely, CA and MBC remained as sodium salts being preferentially soluble in the aqueous phase. Thereafter, the DBC was collected by solvent extraction with DEE. To ensure a higher purity, the obtained DBC was mixed again with distilled water using a 1:1 mass−volume ratio and adjusting the pH at 2. Under these conditions traces of sodium species were removed in the aqueous solution. Then, another solvent extraction with

Figure 2. Scheme of the DBC purification process (AL: aqueous layer; OL: organic layer).

DEE was made using a 1:1 volume−volume ratio, and most of the DBC was recovered in the ethereous organic layer, which was subjected to vacuum distillation. Finally, water traces were removed by adding anhydrous Na2SO4 as desiccant, and the solids were removed by vacuum filtration. A light-yellow DBCrich liquid was stored in a brown-glass hermetic container at room temperature for characterization and further experimentation. 2.2.3. DBC Characterization. High-performance liquid chromatography (HPLC) was used to analyze samples from reaction and from the phase equilibrium experiments. The samples were dissolved in acetonitrile (∼2% w/w) and separated in a reversed-phase C18 column (Acclaim 120, 3 × 150 mm, 3 μm, 120 Å) within a Dionex-UltiMate 3000 HPLC system. The oven temperature was set at 313 K, and a diode array detector was used with a wavelength of 210 nm. The mobile phase operated with gradient elution using microfiltrated and degassed acetonitrile and ∼2 mmol of sulfuric acid solution in deionized water (pH ∼ 2.1). The programmed gradient of ACN and water (pH 2.1) was run as follows: 0% ACN (t = 0 min), 60% ACN (t = 20 min), 90% ACN (t = 25 C



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at 298 K followed by a heating ramp from 298 to 398 K and finally an isothermal stabilization for 5 min at 398 K. The method requires previous calibration with a sapphire disc following the standard test method ASTM E1269-05.11 Deionized water was used to verify the reliability of the method, and the results are presented in Figure S2 in the Supporting Information. 2.3. Vapor−Liquid Equilibrium Determination. VLE data were obtained under isothermal conditions using a jacketed glass cell. The temperature (±0.05 K) was tracked with a precision RTD thermometer (407907, EXTECH), and the pressure (±0.02 kPa) was measured with a high accuracy gauge (MKS PDR 2000 Baratron - dual capacitance diaphragm). For each equilibrium run, the equipment was verified to be leak proof by performing a tightness test under vacuum conditions. The equilibrium cell was connected to the high vacuum system until achieving constant pressure, and then it was tightly closed and kept at the desired temperature of the equilibrium experiment. The system passed the air tightness test when the pressure was kept constant for 24 h or more. The operating procedure for P−x measurements was done as described in a previous work.12 In brief, the procedure started by filling the equilibrium cell with a liquid mixture of the desired composition. Then, the mixture was stirred with a magnetic bar and degassed under vacuum at the desired temperature to remove all the air molecules. The cell was then allowed to stabilize for about 15 min until the reading became constant. Once this was achieved, the pressure was recorded, the equilibrium cell depressurized, and a liquid sample withdrawn for characterization. As the amount of pure DBC obtained in the synthesis was limited, additional equilibrium points at a given temperature were obtained by evaporation of the liquid mixture under low vacuum. In order to verify the reliability of the VLE experimental setup and procedure, two previous tests were carried out. First, the vapor pressure for pure BuOH was measured between 293 and 328 K and compared with literature reports.13 Second, the VLE of a similar system (triethyl citrate−ethanol) was reproduced.14 Once verified, VLE experiments with mixtures of the obtained DBC and BuOH were measured at low temperature (313, 323, and 333 K) to avoid DBC esterification or hydrolysis. 2.4. VLE Binary Parameter Regression. Due to the low operating pressure during experiments, the vapor phase was considered ideal within the phase equilibrium modeling. In contrast, the UNIQUAC equation was used to model the nonidealities of the liquid phase. The binary interaction parameters (temperature-independent UNIQUAC parameter) were regressed with a differential evolution algorithm, by minimizing the difference between the calculated and experimental equilibrium conditions.15

min), and 0% ACN (t = 28 min) for 28 min. For quantification purposes, samples of known composition in the range of interest of the commercially available compounds (CA and TBC) were used for calibration. The response factor was assumed to have linear proportionality with respect to the molecular weight of the citrate species, as successfully implemented in a previous work.5 The developed technique allowed measuring citrate species only; therefore, the n-butanol concentration was obtained by difference. Acid−base titration was used to verify the purity of DBC during the extraction process. For that purpose, the experimental value was compared with the theoretical DBC acid value (184.3 mg KOH/g DBC). In the acid−base titration, a 0.1 g sample of extracted DBC was diluted in 25 mL of distilled water and then titrated with a standardized 0.07 M NaOH solution. Phenolphthalein was used as the final point indicator. The sodium content in the final product was evaluated by atomic absorption spectroscopy with a Thermo Fisher Scientific S2 AA system, using an acetylene/air flame. The calibration was performed by analyzing diluted solutions of a sodium chloride standard (Merck, Darmstadt, Germany) in deionized water, at different concentrations (0.50, 1.00, 1.50, and 2.25 ppm). The DBC sample was diluted to approximately 1 wt % with deionized water, and the atomic absorption spectrum was read at 589.0 nm. Nuclear magnetic resonance (NMR) was used to identify and confirm the chemical structure of the obtained DBC. The 1 H NMR characterization was evaluated using a Bruker of 400 MHz NMR spectrometer, with deuterated dimethyl sulfoxide as solvent ((CD3)2SO-d6) and tetramethylsilane (Si(CH3)4) as internal standard. The oven temperature was set to 293 K. Thermogravimetric analysis (TGA) was performed using a TGA 1 Stare System (Mettler Toledo) to determine DBC decomposition temperature. For characterization, a 48.6 ± 0.1 mg sample was placed in the microbalance of the TGA unit. The sample was heated from room temperature up to 670 K, at 10 K/min, under nitrogen atmosphere (at 50 mL/min). Karl Fischer titration was used to determine the water content in purified DBC and to verify water formation during vapor−liquid experiments. Titrations were carried out in a DL53 titrator (Mettler Toledo) equipped with DM142 electrode. Samples of ca. 1.0 g were previously diluted in 40 mL of HPLC grade methanol, and they were titrated with standarized Karl Fischer reactant (1 mL of Karl Fischer = 5 mg of water). Densities were measured by using a Mettler digital density meter - DMA 45 (Graz, Austria). Samples were evaluated by triplicate at different temperatures (298, 303, and 313 K). Viscosity tests were carried out in a CVO-R 150 NF Rheometer (Woecestershire, United Kingdom) at 298, 303, and 313 K. Approximately a 3 mL sample was inserted in a bob and cup system, and the experiments were run between 0.1 and 100 (1/ s) shear rates. Each point was evaluated by triplicate. Differential scanning calorimetry (DSC) tests were used to evaluate crystallization and boiling points of DBC. A DSC 1 Stare System (Mettler Toledo) was used, under a nitrogen atmosphere at a flow rate of 50 mL/min. For the crystallization point measurement, an 8 mg sample was heated from 213 to 405 K at 5 K/min, with a previous temperature stabilization at 213 K for 2 h. For boiling point evaluation, a 20 mg sample was heated from 298 to 730 K at 10 K/min. The evaluation of specific heat capacity was accomplished by differential scanning calorimetry (DSC) using an isothermal stabilization for 5 min

3. RESULTS AND DISCUSSION 3.1. DBC Results and Analysis. 3.1.1. Acid Value and Chromatography. The reactive mixture, free of volatile components, had an average weight composition of: 0.1% CA, 14.7% MBC, 38.1% DBC, and 46.7% TBC. Due to the low recovery percentage of the purification process described in Figure 2, several reaction batches and separations were required to collect a reasonable amount of DBC for physicochemical characterization and to operate the VLE cell. For DBC characterization, a sample from the batch with the highest purity detected by HPLC was used (97 wt % DBC with few D



Article

Figure 3. 1H NMR spectra for synthesized dibutyl citrate.

carbons of the butyl groups. For symmetric DBC a sixtuplet and a quintuplet were obtained, while as expected for the asymmetric DBC, two sixtuplets and two quintuplets were noticed. There are other small peaks in the range of 1.38 to 1.43 ppm, which correspond to MBC structures. Zone 3 in Figure 3, ranging from 2.60 to 2.90 ppm, corresponds to protons bonded to the carbons in the central part of the molecule, which are characteristic of citric moieties. Here, the expected two doublets for the asymmetric DBC were observed, together with other small peaks associated with MBC and CA (2.63, 2.64, 2.67, 2.68, 2.73, 2.77, 2.78, 2.83, 2.81, 2.88, and 2.89 ppm). Finally, zone 4 in Figure 3 corresponds to the protons bonded to the carbons in the butyl groups closest to the oxygen, with a relative area of 3.91. This zone was analyzed in the range of 3.94 to 4.08 ppm, where a big triplet corresponding to symmetric DBC can be observed.16 The complete 1H NMR spectrum is presented in Figure S1 of the Supporting Information. Considering the difficulties associated with the separation of these isomers, and because DBC could be processed as an isomeric mixture at the industrial scale, the obtained DBC was treated as a single compound for further analysis. This approach has been found to be valuable when working with isomeric mixtures of biobased chemicals, mainly when developing models for early stage process design.17,18 3.1.3. Density and Viscosity. Density and viscosity of DBC were evaluated at different temperatures, and the experimental results are presented in Figure 4 and Figure 5. The corresponding experimental data are included in the Tables S1 and S2 of the Supporting Information. As noticed in the liquid−liquid separation processes during synthesis, and as observed with most citrates, DBC density is also higher than water, with an average value of 1.122 g/mL (±0.007). This property has to be accounted for when designing liquid−liquid separation systems, mainly during the final purification of bottom products of an RD system for tributyl citrate

amounts of CA (0.1 wt %), MBC (1 wt %), and TBC (1.5 wt %)). The final products from all reaction−separation batches were blended, and ca. 40 g of this blend was obtained and used for phase equilibria experiments. At the end, the final mixture corresponded to a clear-yellowish liquid with an acid value of 190.1 mg KOH/g. Because this value was slightly higher than the theoretical acid value for DBC (184.3 mg KOH/g), it was clear that the product contained minor impurities of CA and MBC. According to HPLC analysis, the obtained DBC blend had a purity of 93.5 wt %, with some CA (0.1%), MBC (4%), and TBC (1%). Additionally, 0.26 wt % water content was measured by Karl Fischer titration. Some n-butanol was detected in the mixture by its characteristic smell, but it was not quantified. Some minor traces corresponded to sodium salts (presumably DBC sodium salt) as determined by atomic absorption spectroscopy. Despite that additional separations were attempted, no additional purification was achieved. Then, obtained DBC was stored and refrigerated at 273 K in a dark and dry environment for further use. The product remained stable without noticeable hydrolysis or esterification. 3.1.2. 1H NMR. In order to verify and confirm the chemical structure of the purified DBC, a hydrogen nuclear magnetic resonance (1H NMR) experiment was carried out. The obtained 1H NMR spectrum of the DBC sample is presented in Figure 3. The peaks at 2.5 and 3.4 ppm correspond to water and ether, respectively. The spectrum can be analyzed in four different zones. The first zone located between 0.8 and 0.95 ppm represents the protons at the end of the butyl groups of the DBC molecule. In this range, a triplet of symmetric DBC (Isomer 1) is shown in spite of overlapping peaks. Also, there are small peaks corresponding to asymmetric DBC (Isomer 2). The small peak on the right (0.85 ppm) most probably corresponds to monobutyl citrate (MBC). The second zone in Figure 3 ranges from 1.25 to 1.60 ppm. This zone corresponds to the protons at the intermediate E



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Figure 6. DSC curve of a dibutyl citrate sample measured under N2 atmosphere at 74.7 kPa from 213 to 405 K.

Figure 4. Dibutyl citrate liquid density at different temperatures.

production. On the other hand, viscosity and shear stress results indicate that DBC has the rheological behavior of a Newtonian fluid. The average viscosity in the studied temperature range (298−313 K) was 2.03 Pa·s (±0.87). 3.1.4. Differential Scanning Calorimetry and Thermogravimetric Analysis. The calorimetric profile of DBC obtained by DSC is shown in Figure 6. Unfortunately, the DBC crystallization point was impossible to observe because it was below the minimum operating temperature of the available equipment (213 K). However, it is important to know that DBC will not precipitate during storage or when present as a TBC impurity. On the other hand, DBC boiling point was not detected before the decomposition temperature (around 500 K) according to TGA analysis. The profiles from the thermogravimetric analysis (TGA) are presented in Figure 7. According to this analysis, the DBC sample exhibits a slow weight loss above 436 K. Then, the rate of weight loss increases rapidly above 462 K, reaching a 90% weight loss at 555 K. The decomposition was verified by visual inspection of the TGA crucible after the run finished. As a typical behavior of organic substances subjected to high temperatures, the color change into a dark brown indicated decomposition of the sample. The onset temperature was 541 K, indicating that DBC decomposition occurs at higher temperatures than that reported for citric acid (426 K).19 This constraint is important when defining the maximum operating temperatures in an RD column for TBC synthesis. In Figure 8, the liquid heat capacity of DBC is presented as a function of temperature in the range of 320 to 375 K. The whole set of data is summarized in Table S3 of the Supporting Information. The average heat capacity was 2.461 kJ/kg K (±749.11 J/mol K) in the studied temperature range, being similar to other organic compounds.20

Figure 7. TGA curve of dibutyl citrate measured under N2 atm at 74.7 kPa.

Figure 8. Specific heat capacity of liquid DBC at different temperatures. (○) Experimental data. (―) Third-order polynomial regression (CpL (J/kg K) = 0.0006T3 − 0.6158T2 + 222.99T + 24880; T in K; R2 = 0.999)

Figure 5. Dibutyl citrate viscosity (a) and shear stress (b) as a function of temperature and shear rate. (○) 298 K. (□) 303 K. (Δ) 313 K. F



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Table 3. Experimental Equilibrium P−x Data for the System DBC + BuOHa at Different Temperatures

The results from HPLC, acid value, H NMR, DSC, and TGA characterizations indicated that the obtained component had a high DBC purity, despite the simplicity of the used extraction methods. In general, the desired concentration of a component for physicochemical characterization has to be as high as possible (>99.5%). However, as DBC is not commercially available, it was considered that the obtained sample, with a 97 wt % purity, was pure enough to perform the reported physicochemical characterizations. Also, the DBC product corresponded to a mixture of the isomeric compounds that are hardly separable. As both isomers are produced during the tributyl citrate synthesis, and not separated in that process, the obtained properties and models for DBC and its mixtures would be useful in the modeling of the operation. This approach has been used before with success in the modeling of esterification processes that involve isomer formation.17,18 Also, the data here obtained for the first time can be valuable in the future development of DBC separation processes and in the isolation of its isomers. Additionally, as it was not possible to obtain a higher concentration of DBC than 93.5 wt % in the blend of the different purification batches, the VLE experiments were carried out with this product. All the impurities were measured during equilibrium experiments and considered during the multicomponent phase equilibria regression. 3.2. Vapor−Liquid Experiments and UNIQUAC Regression (VLE). Prior to VLE experiments with DBC, the reliability of the isothermal cell was tested by measuring the vapor pressure of pure BuOH at different temperatures. Experimental vapor pressures and reported data are compared in Table 2. The reported data were calculated with the

molar fraction temperature (K) 313.15

323.15

333.15

a

reported

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15

0.56 0.84 1.23 1.76 2.44 3.33 4.35 5.33

0.61 0.89 1.27 1.80 2.50 3.42 4.62 5.97

DBC

TBC

BuOH

pressure (kPa)

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.009 0.010 0.010 0.010 0.011 0.012 0.014 0.013 0.015 0.005 0.008 0.009 0.011 0.013 0.015 0.018 0.000 0.000 0.000 0.004 0.004 0.008 0.013

0.061 0.063 0.064 0.067 0.069 0.073 0.077 0.081 0.084 0.052 0.055 0.062 0.069 0.076 0.085 0.096 0.030 0.032 0.035 0.042 0.048 0.063 0.081

0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.002 0.002 0.000 0.000 0.001 0.001 0.002 0.003 0.005 0.000 0.000 0.000 0.000 0.000 0.001 0.004

0.929 0.927 0.926 0.922 0.918 0.913 0.907 0.903 0.898 0.943 0.938 0.928 0.919 0.909 0.897 0.881 0.969 0.968 0.965 0.954 0.948 0.927 0.902

2.42 2.39 2.36 2.28 2.22 2.16 2.12 2.09 2.03 4.36 4.27 4.18 4.01 3.88 3.80 3.67 8.15 7.29 6.99 6.68 6.59 6.59 5.97

Standard uncertainties u are u(T) = 0.05 K. u(P) = 0.05 P and u(X) = 0.005.

binary interaction parameters between BuOH and DBC. As the evaluated mixture was multicomponent, mostly containing other citrate species, they were also considered in the model regression. The binary interaction parameters with the other components were obtained from a previous report from the authors,21 and they are listed in Tables S4 and S5 of the Supporting Information. The correspondent binary parameters with MBC were estimated using UNIFAC predictions. The final set of regressed parameters are listed in Table 4.

vapor pressure (kPa) experimentala

MBC

a

Table 2. Comparison between Experimental and Reported Vapor Pressures of BuOH at Different Temperatures temperature (K)

CA

Table 4. Binary Parameters for the UNIQUAC Modelq

Standard uncertainties u are u(T) = 0.05 K and u(P) = 0.05 P.

regressed Antoine equation obtained with previously validated parameters.13 This preliminary experiment allowed us to obtain a correction expression for the systematic pressure offset of the equipment. Isothermal VLE data for mixtures of the obtained DBC (with minor amounts of CA, MBC, and TBC) and BuOH were collected at 313.15, 323.15, and 333.15 K, and the results are recorded in Table 3. For each experiment, the pressure measurements were expected to range between the vapor pressure of pure BuOH at the operating temperature and the limit of low sensitivity (0.5 kPa) of the experimental apparatus. This low sensitivity was due to the minimum absolute pressure achieved with the available vacuum pump. As the phase equilibrium is useful for computer-aided process design purposes, the VLE data were regressed to obtain parameters to model the phase equilibrium in mixtures with DBC. A genetic algorithm was used to adjust the UNIQUAC

q

i

DBC

j

BuOH

Aij Aji Bij Bji

6.5 −0.81 −1809.9 −77.7

ln τij = Aij + Bij/T.

A comparison of the experimental data with the regressed models at the evaluated temperatures is presented in Figure 9. Despite that the VLE data were measured for mixtures in a broader range of mass fractions, the large difference of molecular weights between DBC (304.34 g/mol) and BuOH (74.12 g/mol), turned into a narrow range of data in a molar basis. Then, most of the obtained data corresponded to BuOHrich mixtures in a molar basis. In addition, lower concentrations of BuOH in the mixture resulted in negligible changes of the vapor pressure, involving a higher uncertainty in the data. Results indicate that the system shows a slight positive G



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AUTHOR INFORMATION

Corresponding Author

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

Alvaro Orjuela: 0000-0003-0329-5601 Funding

This work was supported by “Departamento Administrativo de Ciencia. Tecnologiá e Innovación − Colciencias”, under the ́ Project “Producción de plastificantes a partir de ácido citrico ́ usando procesos hibridos de reacción y separación simultánea”, code: 1101-569-33201 and Obtención y purificación 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”.

Figure 9. Binary P−x diagram for the mixture of DBC + BuOH at 313.15 K (Δ), 323.15 K (□), and 333.15 (○). Lines correspond to calculated values.

Notes

The authors declare no competing financial interest.

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deviation from ideality at high n-butanol concentrations. As can be observed from Figure 9, the model has good agreement with the experimental observation. In the absence of any further phase equilibrium information for the evaluated mixtures, the obtained model can be used with confidence for further process modeling and simulation of reactive distillation processes for TBC production.

NOMENCLATURE AND UNITS ACN, Acetonitrile; Aij, Binary parameter for UNIQUAC equation for pair ij (K); CA, Citric acid; DBC, Dibutyl citrate; BuOH, n-Butanol; H2O, Water; MBC, Monobutyl citrate; NaDBC, Sodium citrate species (DBC); P, Pressure (kPa); T, Temperature (K); TBC, Tributyl citrate; VLE, Vapor−liquid equilibrium; xi, Mole fraction of component i

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4. CONCLUSIONS Dibutyl citrate was obtained by partial esterification of citric acid with n-butanol and further purified up to 97 wt % using a pH-controlled solvent extraction. To verify and confirm the chemical structure of the isolated product an 1H NMR analysis was carried out. Calorimetric analyses allowed us to determine that DBC crystallization occurs below 213 K, decomposing at 541 K, and its boiling temperature was not detected under decomposition temperature at working pressure (74.6 kPa). The product density is between 1.1297 and 1.1134 g/mL, and its viscosity ranges from 2.56 to 1.17 Pa·s in a temperature range of 298−313 K. Isothermal VLE equilibrium experiments were carried out for pseudobinary mixtures of n-butanol with 93 wt % pure DBC blend at 313.15, 323.15, and 333.15 K. The experimental data were fitted with the UNIQUAC equation to account for nonidealities of the liquid phase, and the regressed model can be confidently used for process design and simulation of reactive distillation units for tributyl citrate production. Also, the obtained data can be used in the development of separation processes for DBC production.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01064. Figure S1. Complete dibutyl citrate 1H-NMR spectrum. Table S1. Density at different temperatures for DBC. Table S2. Viscosity at different temperatures and Shear Stress for DBC. Table S3. Liquid heat capacity (Cp) for DBC as a function of temperature. Table S4. UNIQUAC R. Q and Q′ pure component parameters. Table S5. Binary interaction parameters for UNIQUAC equation. Figure S2. Water Cp confirmation by ASTM 1269-05 (PDF) H



Article

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Dibutyl Citrate Synthesis, Physicochemical Characterization, and Px

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