Experimental Study on Reactive Extraction of Malonic Acid with

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

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Experimental Study on Reactive Extraction of Malonic Acid with Validation by Fourier Transform Infrared Spectroscopy Vicky R. Dhongde,† Biswajit S. De,† and Kailas L. Wasewar*,# †

Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016 ,India Advanced Separation and Analytical Laboratory, Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra 440010 ,India

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ABSTRACT: Malonic acid is an essential carboxylic acid that is used as a backbone reactant in deriving multiple higher-order carboxylic compounds. The successful attempt of producing malonic acid from a biological route has attracted the scientific community to extract malonic acid from a fermentation broth or dilute aqueous stream. Experimental study on the recovery of malonic acid by reactive extraction was performed, and the effects of TBP concentration, diluents (alkane, ketone, and alcohol), and malonic acid concentration were investigated. The equilibrium models (relative basicity and mass action law models) and the number of theoretical units (NTU) were also evaluated. Fourier transform infrared spectroscopy provided evidence for the formation of significant bonds among extractant−diluent−acid complexation systems which support the experimental outcomes. The results of extraction equilibrium are discussed regarding dimerization constant (DMA), partition coefficient (PMA), extraction efficiency (EMA%), distribution coefficient (KD(MA)), overall extraction complexation equilibrium (KE(MA)), and loading factor (ZMA). The highest reactive extraction efficiency was 73.5% with 0.5962 mol·kg−1 TBP used in MIBK. The distribution coefficient (KD(MA)) of malonic acid in various diluents exhibited the trend MIBK > octanol > heptane. The comparison between values predicted by the equilibrium model and experimental outcomes shows that the relative basicity models predicted better results than the mass action law for reactive extraction of malonic acid. The NTU was calculated to be 2, which was estimated by the modified Kremser equation for designing a countercurrent reactive extraction column.

1. INTRODUCTION Malonic acid (MA), which is well-known as (1,3 propanedioic acid with (C3) dicarboxylic acid), has several applications as a precursor for polyesters and alkyd resins which are used in various coating applications for protection against corrosion, oxidation, and ultraviolet (UV) light. It is specifically preferred by the coating industry as a cross-linking agent for most valuable heat-sensitive substrates, speeding up the coating process. The most impressive use of malonic acid is as a crosslinking agent in the preparation of biodegradable thermoplastic from corn and potato starch. The malonic acid is among the top 30 chemicals produced from biomass as listed by the U.S. Department of Energy.1,2 Some well-known derivatives from malonic acid are barbituric acid, Meldrum’s acid, and cinnamic acid. Barbituric acid has been synthesized by the reaction of phosphorus oxychloride with malonic acid and urea; Meldrum’s acid is prepared by Knoevenagel condensation of malonic acid with acetone, and cinnamic acid is produced from Knoevenagel condensation of malonic acid with benzaldehyde.3 Esters derived from malonic acid can be used as an intermediate in the synthesis of various fragrances, flavors, vitamins, herbicides, and pharmaceuticals (valproate).4 Eastman Kodak Co. had used malonic acid and © XXXX American Chemical Society

its derived compounds as a surgical adhesive. It is also used as a potential reagent in decontamination for Ni-rich alloy surfaces, chelating agent, and indicator for orange fruit senescence.5,6 Malonic acid was first produced by oxidation of malic acid which was extracted from the rhubarb plant by the French chemist Victor Dessaignes in 1858. It occurs naturally in the pigment of fruits and flowers.7,8 Malonic acid produced from the chloroacetic acid route using sodium cyanide and sodium carbonate has a disadvantage because of the cyanide group associated with the process. Another route is hydrolysis of diethyl malonate or dimethyl malonate.9,10 Lygos, Inc. has developed the first biological route of malonic acid from a microbial process.11,12 This route has many advantages over the petroleum-based chemical synthesis route, liberating cyanide from the process. The biological route utilizes sugar and glucose via fermentation.13 Because of the limitation of conventional resources such as petroleum, a carboxylic acid derived from biological fermentation processes gained attention for large-scale production. The major Received: October 26, 2018 Accepted: January 30, 2019

A

DOI: 10.1021/acs.jced.8b00972 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Physiochemical Properties of Chemicals Used in Present Experimental Worka

a

Standard uncertainties u are u(T) = 1 K and u(P) = 1 kPa. M.W., molecular weight; S.G., specific gravity.

The present study provides a spotlight on the separation of malonic acid from the aqueous stream using diluents such as octanol, MIBK, and heptane in the presence of tri-n-butyl phosphate (TBP) as an extractant. The reactive extraction of malonic acid is discussed in terms of loading ratio, distribution coefficient, equilibrium complexation, and extraction efficiency. Fourier transfrom infrared (FT-IR) analysis helps to apprehend the interaction between the diluent−acid−extractant equilibrium complex. The mass action law and relative basicity models explain the behavior of TBP−diluent−malonic acid at the equilibria for the reactive extraction process.

challenge associated with the fermentation process is the separation of the dilute acid from the fermentation broth.13−15 Several techniques have been employed for the separation of dilute acids from fermentation broth, namely, distillation,16 ultrafiltration and membrane separation,17,18 ion exchange,19 adsorption,20−22 electrodialysis,23 and extraction.24 Most of these methods have limitations such as being time-consuming and/or energy intensive, producing toxic byproducts, exhibiting low selectivity, and being less effective for dilute solutions. For example, the conventional technique of precipitation by a calcium or sodium salt could be a possible route of malonate recovery from fermentation broth. However, calcium malonate solubility can be decreased by increasing temperature up to 100 °C, but on other hand, it could have a negative impact on the yield of malonate as the thermal decomposition temperature of malonic acid, which is around 100 °C.12 The recovery of dilute acid from fermentation broth by employing the reactive separation process has been preferred in many research studies.25−27 This separation technique is an amalgamation of reaction and separation simultaneously. The organic conventional diluents used for the recovery of carboxylic acids are categorized as (i) high molecular weight aliphatic amines, (ii) conventional oxygen-bearing and hydrocarbon extractants, and (iii) organophosphorus compounds. The reaction between the solute and extractant occurs in reactive extraction, unlike solvent extraction, by forming an acid−extractant complex. The reactive extraction technique was used for the recovery of different carboxylic acids, such as lactic acid,28−30 acrylic acid,31−33 levulinic acid,34 itaconic acid,35,36 picolinic acid,37−39 gallic acid,40,41 nicotinic acid,37,42 tartaric acid,43 benzoic acid,44 caproic acid,45 phenylacetic acid,46−48 and protocatechuic acid.49−52 However, few literature contributions have been reported on malonic acid recovery by reactive extraction. Dietrich et al. (2017) reported on reactive extraction of malonic acid using alcohol-based solvents along with the amine-based extractants.12 No work has been reported using a phosphate-based extractant, and it is important to perform an exhaustive study on the reactive extraction of malonic acid in order to develop the process further.

2. MATERIALS AND METHODS 2.1. Materials. All chemical purchased are listed in Table 1 and were used as received without further treatment unless otherwise mentioned. For all experiments, Millipore Milli-Q water was used. 2.2. Experiments. The concentration of malonic acid was 0.119−0.596 mol·kg−1 (20−100 mg L−1) in the aqueous phase. The range of malonic acid concentration (MA) was preferred by its concentration in the dilute fermentation broth.53 Various diluents were used to dilute TBP with concentrations between 0.379 and 2.273 mol·kg−1 for recovery of malonic acid by reactive extraction. The initial pH of malonic acid in aqueous solution was small (∼1.75) as compared to its pKaMA = 2.83, which is associated with the unionized acid molecules available for extraction in aqueous phase.12 Malonic acid (aqueous phase) and diluents with extractant (organic phase) were taken in an equal ratio in a 100 mL conical flask and provided orbital shaking for 3 h at atmospheric pressure and temperature of 300.15 ± 1 K using orbital shaker REMI S-24BL (India), followed by separation of mixed phases by centrifugation (REMI R-4C centrifuge) for 3 min at 5000 rpm. To analyze the aqueous concentration of malonic acid, a Shimadzu 1800 UV−vis spectrophotometer at λmax = 210 nm was used. The concentration of the raffinate phase (MAorg) was determined by subtracting the value in the aqueous phase at equilibrium from its initial value, and no change in volume, i.e., negligible water coexistence, was assumed. The pH before and after equilibrium was B



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solvents: octanol, heptane, and methyl isobutyl ketone (MIBK). The transfer of ions is caused by acid−diluent interaction allowing the extraction and formation of weak bonds between the species. In the aqueous phase, malonic acid exists in the form of a monomer, as the intermolecular hydrogen bonding between acid is weaker compared to bond formation due to the interaction of hydrogen molecules of acid and water.24 The existence of acid as a dimer in the organic phase of nonpolar diluents is due to intermolecular hydrogen bonding. The evaluation of the performance of the extraction process for malonic acid follows some important steps, such as dimerization of organic and acid phase, dissociation of acid in the aqueous phase, and separation of the undissociated molecules of malonic acid between organic and aqueous phase.56 The extraction mechanism of malonic acid is illustrated in Figure 1. The mechanism of dissociation of malonic acid molecules in the aqueous phase is as follows: The dissociation of malonic acid in aqueous phase:

determined using a Spectral Lab Digital pH meter. The reproducibility of the outcomes was determined by repeating all experiments twice and examined under similar conditions. The National Institute of Standards and Technology (NIST) has established standard guidelines for uncertainties, by which the results were evaluated.54 The following equation was used to evaluate the uncertainty to be within x ± 0.001: ÄÅ N É ÅÅ ∑ (x − x)2 ÑÑÑ1/2 ÅÅ i = 1 i ÑÑ ÑÑ μ(x) = ÅÅÅ ÅÅ (N − 1) ÑÑÑ ÅÇ ÑÖ where N is the number of experiments performed, x the mean experimental value, and xi an experimental value. The holdup time required to achieve equilibrium was determined by performing a few preliminary experiments. The organic phase and the aqueous phase were shaken for 3 h in the present study. Preliminary studies on the extraction of malonic acid revealed that 2 h is sufficient to achieve equilibrium, but to be on the safe side, initially it was taken as 5 h. This was used because there may be some power fluctuations while using the shaker which may reduce the speed of shaker and unavoidable breakdowns, such as power outage. In general, the shaking was vigorous; hence, in some cases, equilibrium may be reached in 1 h.50 The experiments were conducted for 3 h, which was considered sufficient time to achieve equilibrium from our preliminary studies.55 2.3. FT-IR Analysis. The nature of the TBP−malonic acid−diluent complex in the organic phase was understood by performing FT-IR analysis (Shimadzu Corporation IR Affinity-1). The analysis was performed using a 0.02 cm sodium chloride cell-equipped window with the resolution of 4 cm−1 in the range of 4000−400 cm−1 wavenumbers. The malonic acid with TBP diluted in octanol, MIBK, and heptanes forms an acid−organophosphorus complex, which was analyzed to understand interactions and bonding. The spectroscopic technique provides evidence of the interactions behind the extraction mechanism which was predicted in most of the previous studies. The peak intensity depicts the results concerning the formation of bonds between involved species. The IR ranges of specific functional groups are depicted in Table 2.

KMA

[HMA] ←→ [H+] + [MA−]

(1)

[H+][MA−] [HMA]

(2)

KMA =

where [HMA] is the malonic acid concentration (mol·kg−1). Partition of undissociated malonic acid molecules: The undissociated molecules of acid molecules are distributed in organic and aqueous phases as PMA

[HMA aq] ←→ [HMA org]

PMA =

carboxylic acid

hydrogen bonded (O−H stretch) C=O stretch hydrogen bonded (O−H stretch) C=O stretch C−H stretch −CH3 (bend) −CH2− (bend) P=O stretch O=P···OH P−O−C =PO4 stretch

3400−2400 1730−1650 3600−3100 1725−1700 3000−2840 1375 and 1450 1465 1320−1140 2725−1600 1088−920 961−1012

alcohols ketones alkanes

phosphates

(4)

DMA

DMA =

−1

IR range (cm )

[HMA]aq

2[HMA]aq ←→ ⎯ [HMA]2,org

Table 2. FT-IR Analysis of Extraction Equilibrium Complexation of Extractant−Diluents−Acid type of vibrations

[HMA]org

where [HMA]aq is aqueous phase concentration (mol·kg−1), [HMA]org organic phase concentration (mol·kg−1), and PMA malonic acid partition coefficient.24,57 3.2. Dimerization of Malonic Acid in Organic Phase. The extracted undissociated acid molecules initiate the development of dimers in raffinate phase as the hydrogen bond between solute−solvent is weaker than the solute− solute hydrogen bond and is illustrated as

3. RESULTS AND DISCUSSION 3.1. Physical Extraction of Malonic Acid with Pure Solvents. The physical extraction of malonic acid from the aqueous phase was carried out using three conventional

functional group

(3)

(5)

[HMA]2,org 2 [HMA]org

(6)

where DMA is the dimerization constant of malonic acid in the organic phase (kg·mol−1). The distribution coefficient involves the effect due to ionic strength and the nature of ion concentration of H+ of the solution constituents. The distribution for malonic acid can be written as the ratio of the sum of free acid molecules and the molecules of acid in dimer form in the organic phase to the sum of undissociated acid molecules and dissociated acid molecules in undissociated forms which exist in the raffinate phase as C



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Figure 1. Reaction mechanism for extraction of malonic acid in diluents.

Table 3. Dimerization Coefficients, Partition Coefficients, Degree of Extraction, Distribution Coefficient for Malonic Acid Extracted from Aqueous Streams into Organic Solvents at 300 ± 1 Ka and 101.325 kPa with [MA]initial = (0.1193 to 0.5963 mol·kg−1) diluent

[MA]Initial (mol·kg−1)

[MA]aq (mol·kg−1)

[MA] org (mol·kg−1)

KD(MA)

0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963

0.3013 0.3521 0.4522 0.5376 0.6055 0.3350 0.4559 0.6507 0.8036 0.9077 0.2511 0.4053 0.5546 0.6585 0.7594

0.0166 0.0403 0.0704 0.1006 0.1412 0.0201 0.0515 0.1006 0.1567 0.2128 0.0138 0.0410 0.0761 0.1160 0.1707

0.0550 0.1145 0.1558 0.1870 0.2332 0.0599 0.1131 0.1546 0.1950 0.2344 0.0548 0.1012 0.1371 0.1762 0.2248

octanol

MIBK

heptane

avg KD(MA) 0.1441

0.1472

0.1398

E(MA)% 5.2125 10.2771 13.4778 15.7570 18.9111 5.65473 10.1562 13.3921 16.3195 18.9964 5.1971 9.1902 12.0573 14.9771 18.3542

avg E(MA)%

partition coefficient (PMA)

dimerization constant (DMA)

R2

12.0618

0.00175

28340

0.9985

12.3232

0.00104

84740

0.9982

11.7743

0.00075

136200

0.9966

Standard uncertainties u are u(T) = 1 K, u(P) = 1 kPa, and u([MA]) = 0.001 mol·kg−1.

a

KD(MA) = =

total [HMA]org total [HMA]aq

PMA + 1

=

[HMA]org + 2[HMA]2,org

E(MA)% =

[HMA]aq + [MA−]

2 2PMA DMA [HMA]aq + KMA /[H+]

1 + KD(MA)

× 100 (9)

The diluents such as heptane, octanol, and MIBK were utilized for separation of malonic acid by extraction. The values of dimerization coefficient (DMA) and partition coefficient (PMA) were obtained by plotting of KD(MA) versus [HMA]aq, and the values of 2P2MADMA as a slope and PMA as an intercept were determined by fitting the data linearly by eq 8 in Origin 8.0 (software package). The goodness of fit for linear regression was analyzed by R2 > 0.99. The obtained values of PMA and DMA for various diluents are given in Table 3; the trend of MIBK > octanol > heptane was observed for KD(MA), which also resulted in weak interactions and poor solvation properties for dimerization of acid, and the distribution coefficient of MIBK and octanol was higher than heptane. This specifies that the interaction between

(7)

The malonic acid dissociation (pKA) becomes trivial, and the denominator of eq 7 can be safely approximated to 1. The dimerization and partition coefficient for the lower acid concentration in the aqueous phase is given as follows: 2 KD(MA) = PMA + 2PMA DMA [HMA]aq

KD(MA)

(8)

The overall distribution coefficient is determined for extraction efficiency (E(MA)%) of malonic acid as D



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Table 4. Loading Ratio, Distribution Coefficient, Equilibrium Complexation Constant, Extraction Efficiency, and Resultant Values for the Chemical Equilibrium of Malonic Acid with TBP in Octanol at 300 ± 1 Ka [CTBP]Initial org (mol·kg−1)



[MA]org (mol·kg−1)

KD(MA)

0.379

0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963 0.1193 0.2385 0.3578 0.4770 0.5963

0.1041 0.2025 0.2880 0.3679 0.4454 0.0831 0.1603 0.2326 0.3028 0.3705 0.0676 0.1294 0.1881 0.2387 0.2943 0.0620 0.1105 0.1596 0.1980 0.2345 0.0501 0.0915 0.1336 0.1757 0.1908 0.0445 0.0845 0.1201 0.1506 0.1600

0.0152 0.0360 0.0698 0.1091 0.1509 0.0362 0.0782 0.1252 0.1743 0.2257 0.0517 0.1091 0.1697 0.2383 0.3019 0.0573 0.1280 0.1982 0.2790 0.3618 0.0692 0.1470 0.2241 0.3013 0.4054 0.0748 0.1540 0.2377 0.3264 0.4362

0.1464 0.1778 0.2423 0.2966 0.3388 0.4359 0.4879 0.5382 0.5755 0.6093 0.7644 0.8435 0.9023 0.9980 1.0258 0.9232 1.1591 1.2417 1.4089 1.5426 1.3811 1.6062 1.6773 1.7144 2.1244 1.6816 1.8226 1.9789 2.1676 2.7261

0.758

1.137

1.516

1.895

2.273

avg KD(MA) 0.2425

0.5226

0.8951

1.2329

1.7527

2.2039

E(MA)% 12.764 15.098 19.508 22.874 25.354 30.357 32.793 34.990 36.530 37.859 43.323 45.755 47.433 49.950 50.636 48.003 53.685 55.391 58.488 60.670 58.003 61.629 62.650 63.160 67.994 62.708 64.572 66.431 68.430 73.163

avg E(MA)% 19.037

34.108

46.980

54.336

62.998

67.935

Z(MA)

KE(MA)EXP

KE(MA)RBM

KE(MA)MAL

0.0402 0.0949 0.1841 0.2875 0.3981 0.0478 0.1032 0.1652 0.2299 0.2978 0.0455 0.0960 0.1493 0.2096 0.2656 0.0378 0.0845 0.1307 0.1840 0.2386 0.0365 0.0776 0.1183 0.1590 0.2140 0.0329 0.0677 0.1045 0.1436 0.1919

0.8578

0.8603

1.1397

0.8606

0.8608

1.001

0.9594

0.9595

1.096

0.9772

0.9773

1.1405

1.0363

1.0351

1.1733

1.0385

1.0372

1.1820

Standard uncertainties u are u(T) = 1 K, u(P) = 1 kPa, u([MA]) = 0.001 mol·kg−1, and u([TBP]) = 0.001 mol·kg−1.

a

during physical extraction was in the range of 11−12% which enhanced to 65% to 69% by chemical extraction. The reactive extraction of malonic acid has demonstrated improvement in E(MA)% observed as MIBK 69.22%, octanol 67.93%, and heptane 65.46%. Figures 2, 3, and 4 depict the isotherm equilibrium of reactive extraction for malonic acid with TBP as an extractant with diluents (octanol, MIBK, and heptane, respectively). The straight-line relationship is obtained at a lower malonic acid concentration, which implies that the system pivoted toward Henry’s law at the equilibrium between aqueous and organic phases. The higher malonic acid concentration exhibited a parabolic relationship, which is indicative of nonideal behavior leading to divergence from Henry’s law.59 3.4. Fourier Transform Infrared Spectroscopic Analysis. Figure 5 portrays the spectra for (a) malonic acid, (b) tri-n-butyl phosphate (TBP), (c) octanol, (d) spectra of MIBK, (e) heptane, (f) malonic acid−TBP−octanol, (g) malonic acid−TBP−MIBK, and (h) malonic acid−TBP− heptane. (a) The FT-IR spectra of carboxylic acid were identified at 3000 and 1700 cm−1, which correspond to O−H stretch and C=O stretch, respectively. (b) The tri-n-butyl phosphate has typical characteristics, which are CH stretch (2966 cm−1), −CH3 (1465 cm−1),

solvent and solute is more prominent than the solute−solute interaction in the case of physical extraction by MIBK. Here, MIBK exhibited better solvation of malonic acid by limiting the formation of dimers in the organic phase and forming a hydrogen bond with the acid molecule. The obtained values of KD(MA) are used to estimate E(MA)%, which was observed as 12.06%, 12.32%, and 11.77% for octanol, MIBK, and heptane, respectively. The solvents have limited efficacy toward malonic acid extraction, and hence, to achieve higher extraction efficiency and distribution of the acid into the organic phase requires a reaction-enhancing agent. To improve the extraction efficiency of the diluents, an organic phosphorus compound was used as a reactive extractant, i.e., tri-n-butyl phosphate (TBP). 3.3. Reactive Extraction Using Tributyl Phosphate. The reactive extraction experimental studies on malonic acid at (0.119−0.596 mol·kg−1) using TBP in the concentration range (0.379−2.273 mol·kg−1) with various solvents were performed. The (=PO4 group) of TBP allows the formation of complex molecular bonds as well as interaction by selfassociation due to the exchange of electrons, which helps in the formation of bonds between diluents and solutes.46,58 The equilibrium data for chemical extraction are reported in Tables 4−6. The obtained results illustrated that the reactive extraction process has the edge over conventional physical extraction. It was observed that the extraction efficiency E



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Table 5. Loading Ratio, Distribution Coefficient, Equilibrium Complexation Constant, Extraction Efficiency, and Resultant Values for the Chemical Equilibrium of Malonic Acid with TBP in MIBK at 300 ± 1 Ka [CTBP]Initial org (mol·kg−1)




KD(MA)

0.379

0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962

0.1063 0.2024 0.2859 0.3571 0.4204 0.0803 0.1575 0.2251 0.2870 0.3407 0.0658 0.1289 0.1859 0.2359 0.2797 0.0550 0.1034 0.1525 0.1980 0.2349 0.0441 0.0851 0.1240 0.1620 0.1990 0.0414 0.0790 0.1107 0.1380 0.1580

0.0131 0.0361 0.0719 0.1199 0.1759 0.0390 0.0810 0.1327 0.1900 0.2556 0.0536 0.1096 0.1719 0.2411 0.3166 0.0642 0.1350 0.2051 0.2790 0.3613 0.0751 0.1534 0.2337 0.3150 0.3975 0.0779 0.1595 0.2471 0.3390 0.4383

0.1229 0.1784 0.2513 0.3359 0.4185 0.4861 0.5145 0.5894 0.6621 0.7502 0.8143 0.8503 0.9246 1.0220 1.1318 1.1685 1.3055 1.3448 1.4090 1.5379 1.7028 1.8026 1.8849 1.9443 1.9960 1.8827 2.0185 2.2325 2.4565 2.7734

0.758

1.137

1.516

1.895

2.273

avg KD(MA) 0.2707

0.6181

0.9730

1.3532

1.8434

2.3281

E(MA)% 10.947 15.137 20.085 25.143 29.503 32.710 33.970 37.084 39.836 42.863 44.882 45.955 48.041 50.543 53.091 53.885 56.627 57.352 58.490 60.598 63.002 64.318 65.336 66.036 66.622 65.311 66.871 69.064 71.069 73.500

avg E(MA)% 20.225

37.786

48.986

57.241

64.812

69.405

Z(MA)

KE(MA)EXP

KE(MA)RBM

KE(MA)MAL

0.0345 0.0953 0.1896 0.3165 0.4642 0.0515 0.1069 0.1750 0.2507 0.3372 0.0471 0.0964 0.1512 0.2121 0.2784 0.0424 0.0890 0.1353 0.1840 0.2383 0.0396 0.0809 0.1233 0.1662 0.2096 0.0343 0.0701 0.1087 0.1491 0.1927

1.0064

1.0161

1.4183

1.0076

1.0315

1.0073

1.0116

1.0112

1.1698

1.0496

1.0501

1.1767

1.1338

1.1340

1.2371

1.1388

1.1386

1.2898


a

−CH2− (1375 cm−1), P=O bend, and P−O−C significant peak corresponding to 1155, 1056, and 921 cm−1. (c) The IR spectra of octanol consist of characteristic alcohol peaks associated with OH stretch (3440 cm−1) and C−H stretch (2965 cm−1); H−C−H bends and free OH bands are depicted at 2925, 1465, and 1060 cm−1. (d) MIBK has typical characteristics of ketone: C−H (2967 cm−1), C=O stretch (1726 cm−1), CH3 (1465 cm−1), H−C− H (1375 cm−1) bend, and C−O stretch (1170 cm−1). (e) Heptane shows significant peaks of alkane C−H stretch at 2967 cm−1 and −CH3 and H−C−H bend at 1465 and 1375 cm−1. After the reactive extraction of malonic acid, the transport of molecules from the dilute aqueous phase to the organic phase can be encountered by analyzing the nature of the TBP−MA−diluents complex, which described the mechanism and validated our assumption of complexation formation. The IR spectra of complexation formed by TBP− MA−octanol, TBP−MA−MIBK, and TBP−MA−heptane have been analyzed, and the identical spectrum of 3500− 2500 cm−1 were observed because of carboxyl and hydroxyl groups.60 Another identical change was observed as the spectra of P−O−C stretch associated with TBP in the range of 1088−920 cm−1 shifted to 1026 cm−1 for TBP−MA− octanol, 1028 cm−1 for TBP−MA−MIBK, and 1030 cm−1 for TBP−MA−heptane. The shift in the P−O−C spectrum is mainly caused by the interactions between TBP, malonic acid,

and diluents.61 Moreover, the spectrum range of 2725−1600 cm−1 attracts the attention toward formation of specific interactions of the weak intermolecular hydrogen bonding of P=O···HO between TBP and malonic acid.62 (f) The IR spectrum of TBP−malonic acid−octanol was analyzed thoroughly, and some major peaks were observed in the range of 3000−2800 cm−1 associated with H−C−H symmetric stretch. The shift of peak after extraction occurred from 1267 to 1170 cm−1, which was assigned to P=O stretching vibrations due to the interaction between the phosphoryl group and acid molecules. The electron donor and acceptor group, i.e., =PO4, was detected at 990 cm−1, which promotes self-associations and interactions between extractant and acid molecules.63 (g) The IR spectra of TBP−MA−MIBK have also depicted a major shift from 1155 to 1234 cm−1 because of P=O stretching vibrations, and a =PO4 group was observed at 991 cm−1; both detected groups help in promoting interactions between acid and the phosphoryl group.64 (h) TBP−MA−heptane exhibited major peaks in the range of 3000−2800 cm−1, which depict asymmetric stretch and the H−C−H symmetric stretch, which indicate the existence of alkane chains in the system. The spectrum of 1320−1140 cm−1 exhibits P=O stretching vibrations, which were shifted from 1265 to 1270 cm−1, and the =PO4 group observed at 1012 cm−1, which gives evidence of interaction occurring F



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Table 6. Loading Ratio, Distribution Coefficient, Equilibrium Complexation Constant, Extraction Efficiency, and Resultant Values for the Chemical Equilibrium of Malonic Acid with TBP in Heptane at 300 ± 1 Ka [CTBP]Initial org (mol·kg−1)




KD(MA)

0.379

0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962

0.1090 0.2080 0.3027 0.3940 0.4887 0.0862 0.1624 0.2360 0.3062 0.3799 0.0704 0.1308 0.1876 0.2445 0.3013 0.0613 0.1164 0.1663 0.2010 0.2425 0.0505 0.0968 0.1350 0.1713 0.2091 0.0467 0.0880 0.1201 0.1502 0.1780

0.0102 0.0304 0.0549 0.0830 0.1075 0.0330 0.0761 0.1216 0.1707 0.2163 0.0488 0.1076 0.1700 0.2325 0.2949 0.0581 0.1221 0.1915 0.2761 0.3538 0.0688 0.1416 0.2226 0.3056 0.3871 0.0725 0.1504 0.2375 0.3268 0.4181

0.0940 0.1465 0.1816 0.2107 0.2200 0.3833 0.4686 0.5153 0.5574 0.5692 0.6933 0.8231 0.9063 0.9508 0.9785 0.9476 1.0485 1.1518 1.3738 1.4589 1.3620 1.4630 1.6488 1.7832 1.8515 1.5503 1.7096 1.9766 2.1758 2.3483

0.758

1.137

1.516

1.895

2.273

avg KD(MA) 0.157

0.4763

0.8360

1.2032

1.6071

2.0077

E(MA)% 8.593 12.783 15.372 17.403 18.032 27.710 31.909 34.008 35.794 36.276 40.944 45.151 47.544 48.741 49.459 48.656 51.183 53.528 57.874 59.331 57.673 59.400 62.241 64.071 64.931 60.790 63.094 66.405 68.512 70.134

avg E(MA)% 13.313

31.993

45.201

53.993

61.302

65.462

Z(MA)

KE(MA)EXP

KE(MA)RBM

KE(MA)MAL

0.0270 0.0804 0.1451 0.2190 0.2837 0.0436 0.1004 0.1605 0.2252 0.2853 0.0429 0.0947 0.1496 0.2045 0.2593 0.0383 0.0805 0.1263 0.1821 0.2334 0.0363 0.0747 0.1175 0.1612 0.2043 0.0318 0.0661 0.1044 0.1437 0.1839

0.5517

0.5521

0.6965

0.8053

0.8055

0.9398

0.9176

0.9175

1.0499

0.9271

0.9270

1.0771

0.9833

0.9834

1.1075

0.9861

0.9865

1.108


a

Figure 2. Reactive extraction of malonic acid equilibrium isotherm for using tri-n-butyl phosphate extractant in octanol.

Figure 3. Reactive extraction of malonic acid equilibrium isotherm for using tri-n-butyl phosphate extractant in MIBK.

between extractant and acid molecules. The FT-IR analysis revealed the formation of complexation of acid−diluent− extractant.46 The spectrum analysis of the reactive extraction system has illustrated the mechanism of the process and the role of interactions in the formation of complexations.

However, the evaluated analyses have also demonstrated the 1:1 interactions, as no significant shift in the peak is observed for the phosphoryl group and acid molecules.65 G



Article

with MIBK diluent and at 0.59 mol·kg−1 concentration of malonic acid. 3.6. Effect of Diluents. In the extraction process, diluents play a major role in improving complex formation between acid and extractant. The hydrogen bonding and dipole−dipole interactions are the roots for the formation of unimolecular or dimolecular bonds of acid−organophosphate due to the presence of the polar ion pair of an organic solvate. To examine the effect of various diluents on the reactive extraction of malonic acid, distribution coefficient values for different concentrations of tri-n-butyl phosphate with octanol, MIBK, and heptane were explored. The values of Avg KD(MA) in Tables 4−6 follow the trend of MIBK (0.270−2.328) > octanol (0.242−2.203) > heptane (0.157−2.007), and Avg E(MA)% exhibited a sequence identical to that of KD(MA). For TBP (0.379−2.273 mol·kg−1) in MIBK, the Avg E(MA)% was found to be 20.22%−69.40%, whereas the corresponding values of octanol are in the range of 19.03%−67.93%, and heptane in the range of 13.31%−65.46%. The malonic acid and tri-n-butyl phosphate can react with each other at certain polarity by generating an ion pair complex. Octanol, MIBK, and heptane exhibited higher polarity, which provides strength to attain solubility of complexes. Maximum solubility to complexes were provided in the trend MIBK > octanol > heptane, resulting in obtaining higher values of KD(MA). The TBP−heptane system illustrated lower values of KD(MA) and hence is considered as a poor diluent for malonic acid extraction. However, the MIBK−TBP system exhibited higher values of KD(MA) and extraction efficiency E(MA)%; therefore, the MIBK−TBP system is considered significant for reactive extraction of malonic acid. 3.7. Effect of Tri-n-butyl Phosphate Concentration. The distribution coefficient KD(MA) is influenced by the concentration of extractant in the diluent phase and type of diluents. The extractant (TBP) is viscous in nature, and the use of diluents enhances their physical and interfacial

Figure 4. Reactive extraction of malonic acid equilibrium isotherm for using tri-n-butyl phosphate extractant in heptane

3.5. Effect of Concentration of Malonic Acid. In reactive extraction of malonic acid, it was found that at constant TBP concentration and with increasing malonic acid concentration (0.119−0.596 mol·kg−1), E(MA)% and KD(MA) also increases for all conventional diluents. The average KD(MA) values were observed as (0.119−0.596 mol·kg−1) with TBP (0.37−1.51 mol·kg−1) in the range of 0.242−2.203 (octanol), 0.270−2.328 (MIBK), and 0.157−2.007 (heptane). Figures 2−4 and Tables 4−6 illustrate the variations of KD(MA) and E(MA)% with respect to the concentrations of malonic acid and TBP. The highest extraction efficiencies (E(MA)%) of 73.50% (MIBK), 73.16% (octanol), and 70.13% (heptane) with KD(MA) of 2.77 (MIBK), 2.72 (octanol), and 2.34 (heptane) were achieved using 2.273 mol·kg−1 of TBP diluted

Figure 5. FT-IR analysis of acid, diluents, extractant, and extraction complexation. H



Article

properties, which helps to improve the solvation to form an acid-extraction complex. The amount of acid recovered significantly depends on the extractant and diluent phase concentrations. The effects of TBP concentration on extraction efficiency and distribution coefficient are demonstrated in Figures 6−8. The plots depict that increasing

Figure 8. Effect of tri-n-butyl phosphate concentration on average distribution coefficient and average extraction efficiency for reactive extraction of malonic acid in heptane

the organic phase must be considered as they play a significant role. 3.8. Comparison of Model and Experimental Parameters. Loading capacity is the extent to which the organic phase can be loaded with a carboxylic acid. The solvation number depends on the presence of the carboxylic group unit in the malonic acid−TBP system. Solvation number can be predicted by loading ratio, which is the ratio of total acid to extractant, denoted by Z(MA).66,67

Figure 6. Effect of tri-n-butyl phosphate concentration on average distribution coefficient and average extraction efficiency for reactive extraction of malonic acid in octanol.

Z(MA) =

[HMA]org initial [TBP]org

(10)

1:1 malonic acid−TBP complex: Z(MA) 1 − Z(MA)

= KE(MA)[HMA]aq Z(MA) < 0.5

(11)

2:1 malonic acid−TBP complex: Z(MA) 2 − Z(MA)

2 = KE(MA)[HMA]aq Z(MA) > 0.5

(12)

3:1 malonic acid−TBP complex: Z(MA)

Figure 7. Effect of tri-n-butyl phosphate concentration on average distribution coefficient and average extraction efficiency for reactive extraction of malonic acid in MIBK.

3 − Z(MA)

= KE(MA)[HMA]3aq Z(MA) ≫ 0.5

(13)

The structure of the malonic acid−TBP complex is depicted in Figure 9. A common 1:1 complex formation between acid and extractant is shown in Figure 9I.68,69 With a higher concentration of acid, loading of acid over the extractant increases and leads to the formation of a 2:1 complex, i.e., partial overloading as shown in Figure 9II. The 2:1 complexation forms because of interaction caused by the hydrogen bond of malonic acid over the 1:1 malonic acid− extractant complexation.46,70 Figure 9III shows 3:1 complexation initiates when the association is formed by the malonic acid by hydrogen bonding with the 2:1 acid−extractant complexation.71 The FTIR analyses have also depicted 1:1 complexation between malonic acid and extractant, as no major shift in peaks is observed. Thus, FTIR analysis validates our assertion of the formation of 1:1 malonic acid−TBP

extractant concentration in diluent leads to an increase in E(MA)% and KD(MA). It was observed that the KD(MA) and E(MA)% could get reduced for a system with higher viscosity due to higher tri-n-butyl phosphate and malonic acid concentration, and diluents are rendered incapable of solvating and forming complexes. The TBP at concentration of 2.273 mol·kg−1 gives higher E(MA)%, with a maximum acid concentration of 0.596 mol·kg−1 showing 73.49% (MIBK), 73.16% (octanol), and 70.13% (heptane). Similarly, the values of KD(MA) were obtained as 2.773 for MIBK, 2.726 for octanol, and 2.348 for heptane, which are related to transportation toward equilibrium extraction for complex formation. Although the obtained results could be improved, other surface properties, density, loading capacity, and viscosity of I



Article

Figure 9. Different complexes of malonic acid and tri-n-butyl phosphate in diluents: (I) 1:1, (II) 2:1, and (III) 3:1.

complex in the reactive extraction of malonic acid using MIBK, octanol, and heptane. In addition, the Z(MA) < 0.5 supports our claim, and hence, eq 11 was used to calculate KE(MA). The self-association factor and partition coefficient constants are strongly dependent on the nature of diluents; that is, when one diluent is replaced by another, it affects the thermodynamic properties of organic phase equilibrium.66 The effect of a particular diluent on the self-association factor and partition coefficient can be illustrated by specific interactions between the extractant and a particular diluent. Attempts have been made to establish correlations between diluent properties such as dipolar moment (μ), ET parameter, dielectric constant, refractive index, boiling point, density, and molecular mass with extraction efficiency with KS(MA). However, no practical correlation with dielectric constant, specific gravity, molar mass, refractive index, and density could be obtained. The diluent parameters, such as ET parameter and dipolar moment (μ), can be used to correlate with KS(MA), which was demonstrated on the absorption spectrum of pyridinium-N-phenolbetaine.46,63 The Dimroth−Reichardt ET parameter gives information about the ionizing power of the solvent, which is measured by the higher wavenumber of the longer wavelength of the absorption band of pyridinium-Nphenolbetaine in a given solvent, and it is also associated with an indication of a higher value of the ET parameter corresponding to the higher KD value.72 The solvating power of a diluent is greater for the diluents with higher values of ET parameter and dipolar moment (μ). For MIBK (μ = 2.79, ET = 39.4), octanol (μ = 1.70, ET = 48.3), and heptanes (μ = 0, ET = 45.7), the following equation predicts the KS(MA) value from μ and ET values.70,73,74 KS(MA) = 1.2 × 10−6μ−0.0266 ET4.14

The behavior at extraction equilibrium was predicted by mass action law and relative basicity model for reactive separation of malonic acid using TBP with various diluents, and results are presented in Tables 4−6. The relation between relative basicity56 and the 1:1 equilibrium complexation constant were expressed by the relative basicity model. A relative basicity model equation depicted is log(KE(MA)RBM) = [C MA1(pK BMA − pK aMA ) + log(C MA2PMA )] (15)

From eq 15, the behavior of extraction equilibrium for malonic acid with diluents and an extractant system can be predicted. CMA1 and CMA2 are constants. The solvating power of the complex, association forming with H-bonds, and formation of complex ion pairings which represent the extraction capacity of malonic acid is termed as KMA. The major factors which influence the behavior of extraction equilibrium for malonic acid are the apparent basicity of extractant to HCl (pKBMA), the dissociation equilibrium constant for malonic acid (pKaMA), and the hydrophobicity of malonic acid (log PMA). The nature of the system depends on the solute, extractant, and diluents involved, whereas solvating power denotes complex H-bonding between the complex and diluents. The relative basicity of the extractant can represent specific association, such as solvating power and the nature of extractant, solute, and diluents, if the basicity of the mixture of the extractant is relative to the solute phase. The KE(MA)RBM values evaluated using the relative basicity model are almost identical to the KE(MA)EXP experimental values presented in Tables 4−6. The KE(MA)RBM values were observed in the range of 0.860−1.038 for octanol, 1.016− 1.138 for MIBK, and 0.551−0.986 for heptane at the varying concentration range from 0.379 to 2.273 mol·kg−1 using TBP as extractant.76 The mass action law equilibria assume that activities of the organic phase and the aqueous phase are proportional to their respective concentrations, whereas the constants of proportionality or nonidealities are associated with a reactive system that takes care of the equilibrium constant.38 The separation of malonic acid by TBP in various diluents can be illustrated as

(14)

From the analysis, it was observed that the extraction efficiency was dependent on μ, and results are identical to those reported in studies of amine extraction of citric acid in which complex formation of solvation occurred because of dipole−dipole interactions.46,75 Various diluents have a distinctive dependency on particular properties such as molar mass, specific gravity, and boiling point. A diluent with a higher boiling point, molar mass, and specific gravity can hinder dissolution of TBP, which results in lower interaction and lower extraction equilibrium coefficients.63

KE(MA)

[HMA]aq + pSorg ←⎯⎯⎯→ [HMA. Sp]org J

(16)


ÅÄÅ lnÅÅÅÅ Å NTU = ÅÇ

Journal of Chemical & Engineering Data KE =

[HMA. Sp]org [HMA]aq [Sorg ] p

(

(17)

where p stands for the solvation number of TBP extractant. At the interface, rapid transfer of the malonic acid−TBP equilibrium complex takes place. The equilibrium complexation constant of tri-n-butyl phosphate−malonic acid KE(MA) is illustrated in terms of amount of extractant molecules involved in reaction and concentration of equilibrium product species. KE(MA) depends on the properties of malonic acid and the capacity of the diluent for solvation. KD is evaluated as a function of the number of reacting species and extraction constant as follows: KD =

=

=

[HMA. Sp]org [HMA]aq [MA−]aq P KS[HMA]aq [S ]org

[HMA]aq + K a[HMA]aq /[H +]aq

1

P KS[S ]org + K a /[H +]aq

EX = KD(MA)

(18)

)

1 Ñ ÑÑ Ñ EX Ñ Ñ

ÑÖ

ln EX S F

(25)

Table 7. Number of Theoretical Stages and Minimum S/F Ratio (Solvent to Feed) for Recovery of Malonic Acid from Aqueous Streama

(19)

diluent

X(MA)in

X(MA)out

KD(MA)

(S/F)min

(S/F)act

NTU

octanol

0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3577 0.4770 0.5962 0.1193 0.2385 0.3578 0.4770 0.5963

0.0444 0.0844 0.1200 0.1505 0.1600 0.0413 0.0790 0.1106 0.1380 0.1602 0.0468 0.0880 0.1202 0.1502 0.1721

1.6815 1.8226 1.9789 2.1675 2.7261 1.8827 2.0184 2.2324 2.4565 2.7218 1.5504 1.7097 1.9767 2.1759 2.4651

0.3729 0.3542 0.3356 0.3156 0.2683 0.3468 0.3312 0.3093 0.2893 0.2686 0.3921 0.3691 0.3359 0.3149 0.2886

0.5593 0.5314 0.5035 0.4735 0.4025 0.5203 0.4969 0.4640 0.4339 0.4030 0.5881 0.5536 0.5039 0.4723 0.4329

1.8332 1.9090 1.9894 2.0819 2.3312 1.9403 2.0092 2.1129 2.2150 2.3294 1.7596 1.8486 1.9883 2.0859 2.2189

(20)

MIBK

(21) heptane

(22)

The solvation number p and log KE(MA)MAL can be obtained from the slope and intercept of a graph between log KD(MA) versus log (S)org. The predicted values from the model depicted less accordance with the experimental values as shown in Tables 4−6. The KE(MA)MAL values were observed in the range of 1.139−1.182 for octanol, 1.418−1.289 for MIBK, and 0.6965−1.108 for heptane at concentration ranges of 0.379−2.273 mol·kg−1 using TBP as the extractant in diluents.51,77 3.9. Evaluation of NTU and (S/F) minimum. The feasibility of the separation of malonic acid was performed by estimating the number of theoretical units (NTU) and S/F ratio for countercurrent extraction: X(MA)in − X(MA)out ij S yz jj zz = KD(MA)X(MA)in − Y(MA)out k F {min

X(MA)out − Y(MA)in / KD(MA)

(1 − 1/EX ) +

To evaluate NTU and the S/F ratio, the experimental values were obtained at 2.273 mol·kg−1 of TBP in diluents for 300 K, at which it gives higher recovery and maximum distribution coefficient. With the evaluation of NTU,49,59 it was observed that two theoretical stages are suggested to be sufficient to achieve the desired extraction efficiency in a countercurrent reactive separation column (Table 7).

where [S]org can be expressed as Initial [S]org = [S]org − p[HMA]org

X(MA)in − Y(MA)in / KD(MA)

(24)

Because the acid dissolution is not significant for considerably lower acid concentration, the above equation could be simplified as log KD(MA) = log KE(MA)MAL + p log(S)org

ÑÉ

Article

a


4. CONCLUSION The transfer of malonic acid at equilibria using TBP as an extractant in the aqueous−organic system was elucidated with respect to the effect of physical−chemical interactions which relate to the nature of the diluents used. The various diluents used in reactive extraction of malonic acid were octanol (alcohol), heptane (alkane), and MIBK (ketone) in combination with tri-n-butyl phosphate. The various parameters, such as equilibrium complexation constants, KD(MA), Z(MA), and E(MA)%, were estimated. With Z(MA) < 0.5, no significant overloading was observed; hence, the 1:1 complex of TBP−malonic acid was considered and validated by FT-IR analysis. The FT-IR analysis of equilibrium complexation, diluents, extractant, and acid supported the understanding of the extraction mechanism and provided evidence of various interactions and bond formations. The values of KD(MA) and E(MA)% are follow the sequence MIBK > octanol > heptane. The values of the distribution coefficient in the range of 2.34− 2.77 and the reactive extraction efficiencies in the range of 70.13%−73.50% for malonic acid were observed with MIBK− TBP. The relative basicity model was best suited for predicting the behavior and expressing the malonic acid reactive extraction. The design of a countercurrent reactive

(23)

where X(MA)in is the initial malonic acid concentration, X(MA)out the concentration at raffinate, and Y(MA)out the concentration at the extracted phase. The (S/F)min for reactive separation of malonic acid using extractant (TBP) with diluents was evaluated. The value of (S/F)min depends on the initial malonic acid concentration, and according to a wellknown rule in extraction processes, (S/F)act is proportional to 1.5 times (S/F)min.52 The number of theoretical stages required for separation of malonic acid in countercurrent fashion was evaluated using a modified Kremser equation: K



Article

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extraction system for malonic acid was evaluated using a modified Kremser equation, which revealed that two theoretical units are required for the process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: k_wasewar@rediffmail.com, [email protected]. in. Phone: +91-712-2801561. Fax: +91-712-2801565. ORCID

Kailas L. Wasewar: 0000-0001-7453-6308 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank VNIT Nagpur for providing all required facilities and extend special thanks to Mr. Rohit Lade, Mr. Pratitya Sontakke, and Mr. Rupal Shelke for their consistent support during the entire experimentation process.

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