(682a) Reactive Extraction of Nicotinic Acid with Tri-n-Octylamine Dissolved in Different Diluents

Nicotinic acid (3-pyridine carboxylic acid) widely used in food, pharmaceutical and biochemical industries is an important chemical. Due to ecological problems and complicate the synthesis methods, the chemical route for nicotinic acid production will become unattractive in the future. In recent years, the application of enzymes to organic chemical processing has attracted the attention of researchers. Nitrilases enzymes are gaining popularity as biocatalysts for the mild and selective hydrolysis of nitriles. The production of nicotinic acid and nicotinamide can be intensified by enzymatic conversion of 3-cyanopyridine or biosynthesis (Kumar and Babu, 2009a). This fermentation process, because of various impurities and very low concentration of product in the fermentation broth, requires an economic separation method to compete with the synthetic process. Among various available alternate processes for simultaneous removal of the product, extraction is often the most suitable one. So a reactive extraction method has been proposed to be an effective primary separation step for the recovery of bio-products from a dilute fermentation process (Kumar et al, 2008).

Long-chain aliphatic amines are found to effective extractants for the separation of carboxylic acids from dilute aqueous solution (Kertes and King, 1986). Generally, these extractants are dissolved in a diluent such as ketone, alcohol, hydrocarbon, etc. to provide appropriate physical properties for use in the extraction process. Since the presence of hydroxyl and carboxylic groups increases the solubility of acids in the aqueous phase, the strong interactions of solvent with solutes are necessary to extract carboxylic acids from dilute aqueous solutions. The extraction constant and the number of reacting molecules of extractant are estimated from the mass action law but the obtained values are different according to the composition of the solvent for the same extractant and the organic acid. The distribution of nicotinic acid (constant concentration only) between water and Alamine 300 dissolved in diluents is studied at 298 K using a phase ratio of 1:1 (v/v) by Senol (2002). The comparative study of the reactive extraction of nicotinic acid with Amberlite LA-2 (lauryl-trialkyl-methylamine) and di-(2-ethylhexyl)-phosphoric acid (D2EHPA) has been presented by Cascaval et al, 2007. Kumar et al (2008) studied reactive extraction of nicotinic acid with TBP and TOPO at a fixed initial acid concentration to intensify the recovery from fermentation broth. Very recently Kumar and group (Kumar and Baba, 2009b) also performed the extraction of nicotinic acid by TOPO in different diluents. The aim of the present work is to study the reactive extraction of nicotinic acid from aqueous solutions using TOA in 4 different diluents covering wide range of categories (MIBK, toluene, butyl acetate and n-dodecane) to provide the extraction equilibrium data for intensification of nicotinic acid production via enzymatic route. The effects of initial acid concentration and composition of extractant (TOA) are also studied. Population based search algorithm, differential evolution (Babu, 2004) as optimization algorithm is used to determine the equilibrium extraction constants (KE) and the stoichiometry of reactive extraction through a proposed equilibrium model.

The extraction equilibrium experiments are carried out at constant temperature (298 K) with equal volumes (16 mL of each phase) of the aqueous and organic solutions shaken at 100 rpm for 8 hours in conical flasks of 100 mL on a temperature controlled reciprocal shaker bath. The initial concentration of nicotinic acid in aqueous solutions is varied between 0.02 - 0.12 mol/L. TOA concentration in organic phase is kept in the range of 0.23 ? 1.38 mol/L. The concentration of acid in the aqueous phase is determined using an UV spectrophotometer (Systronics, 119 model, 262 nm). The acid concentration in the organic phase is calculated by mass balance. The equilibrium pH values of aqueous solutions are measured using a digital pH-meter of Arm-Field Instruments (PCT 40, Basic Process Module).

Equilibrium data are presented for the extraction of nicotinic acid using TOA in MIBK, n-decane, toluene and butyl acetate and n-dodecane. The isotherms for nicotinic acid are determined from four aqueous solution concentrations, four concentrations of TOA dissolved in different diluents. The highest strength of the complex solvation is found for MIBK with a maximum loading ratio (Z = 0.42) followed by toluene with a maximum loading ratio (Z = 0.154) and butyl acetate (Z = 0.121) promoting probably (1,1) acid-TOA complex formation. In all the tested diluents, MIBK (oxygen bonded carbon based compound) with TOA is found to be good solvating agents for nicotinic acid-amine complexation. The distribution coefficients (KD) and degree of extraction (E) are found to initially increase, and then decrease with an increase in TOA concentration (0.229 ? 1.376 mol/L) at different concentrations of nicotinic acid (0.02 ? 0.12 mol/L) in case of MIBK and toluene (as a diluent). Degree of extraction (E) is found to decrease with an increase in TOA concentration (0.229 ? 1.376 mol/L) at different concentrations of nicotinic acid (0.02 ? 0.12 mol/L) for butyl acetate (as a diluent). The concentration of acid may be the limiting factor for this trend in degree of extraction. These trends in degree of extraction with extractant concentration are also observed for reactive extraction of propionic acid using TBP in different diluents by Keshav et al., (2008). Initial concentration of acid also affects the extraction efficiency. Since, low concentrations of nicotinic acid (0.02 to 0.12 mol/L) are used with respect to extractant, TOA (0.229 ? 1.376 mol/L), no exact trends are found for the variation in extraction efficiency with initial acid concentration.

For the estimation of the strength of the complex salvation of acid:TOA, the theoretical study as described by model equations is employed. The estimated values of equilibrium extraction constant (KE) and the number of nicotinic acid molecules (m) per extractant molecule with different diluents are used to find the effect of diluent on the extraction efficiency of extractant (TOA). The values of KE, m and n are determined by solving model equations using optimization procedure (DE algorithm). An objective function based on least square error between experimental data and predicted value of KD is minimized. With the low concentration range of nicotinic acid (0.02 to 0.12 mol/L) and high concentration range of TOA (0.229 to 1.376 mol/L), the loading ratio is found to be very low (Z < 0.42) with all type diluents. 1:1 complexes of acid and TOA are formed and obtained the values of equilibrium complexation (1:1) constant (KE1). In all the tested diluents, MIBK (aprotic diluent) containing the oxygen bonded carbon in the structure is the best solvating agents for acid-TOA complexation giving (KE = 143.81) due to a high solvation ability. The synergistic extraction power of TOA/MIBK system is noticeably larger than that of other systems yielding a maximum value of KD (5.78). Therefore, the dielectric constant and the dipole moments of the diluents, control the extraction constant through its influence on separation efficiency and mechanism. The values of KE are also correlated well with solvent dipole moment ì and the parameter, ET which is based on the absorption spectrum of pyridinium-N-phenolbetaine. The model based on dipole moment (ì) and the parameter, ET can be used to predict the values of KE.

References Babu, B.V. (2004) Process plant simulation, Oxford University Press, India. Cascaval, D., Galaction, A.I., Blaga, A.C., (2007) Camarut, M. Comparative Study on Reactive Extraction of Nicotinic Acid with Amberlite LA-2 and D2EHPA. Sep. Sci. Technol. 42, 1-13. Kertes, A.S., King, C. (1986) Extraction Chemistry of Fermentation product Carboxylic Acids. Biotechnol. Bioeng. 28, 269-282. Kumar, S., Babu, B.V. (2008) Process intensification for separation of carboxylic acids from fermentation broths using reactive extraction, J. Fut. Eng. Technol. 3, 19. Kumar, S.; Wasewar, K.L.; Babu, B.V. (2008) Intensification of Nicotinic Acid Separation using Organophosphorous Solvating Extractants: Reactive Extraction. Chem. Eng. Technol. 31, 1584-1590. Kumar, S.; Babu B.V. (2009a) Process Intensification of Nicotinic Acid Production via Enzymatic Conversion using Reactive Extraction. Chem. Biochem. Eng. Q. 23(3), 367-376. Kumar, S. and Babu, B.V. (2009b) Extraction of Pyridine-3-carboxylic Acid Using 1-Dioctylphosphoryloctane (TOPO) with Different Diluents: Equilibrium Studies, Journal of Chemical & Engineering Data, 54(9), 2669-2677. Senol, A. (2002) Extraction Equilibria of Nicotinic Acid using Alamine 300/Diluent and Conventional Solvent Systems. Turk. J. Chem. 26, 77.