Enzyme-catalyzed esterification of d,l-lactic acid in different SCF/IL media

Highlights

• Lipase-catalyzed synthesis of n-butyl lactate in SCF/IL media.

• Phase inversion appearance in d,l-LA/n-butanol/Novozym 435/CYPHOS IL-201/SC CHF3.

• Improvement of d,l-LA efficiency in SC CHF3/CYPHOS IL-201 and SC CO2/CYPHOS IL-201.

• The highest d,l-LA efficiency was achieved in SC CHF3/CYPHOS IL-201 at c(IL) = 44%.

Abstract

Esterification of n-butanol and d,l-lactic acid (d,l-LA), catalyzed by Candida antarctica lipase B (Novozym 435) in supercritical carbon dioxide (SC CO₂)/ionic liquid (CYPHOS IL-201) medium and supercritical trifluoromethane (SC CHF₃)/ionic liquid (CYPHOS IL-201) medium were studied. The experiments were successfully performed in a high-pressure batch stirred-tank reactor and were planned to elucidate the effect of CYPHOS IL-201 concentration, concentration of the enzyme, temperature and pressure on the efficiency of d,l-lactic acid and composition of d,l-LA. Concentration of CYPHOS IL-20,144%, 30 MPa, 55 °C and SC CHF₃ were predicted to be optimal conditions for synthesis of n-butyl lactate: a maximum d,l-LA efficiency of 169% was attained. CYPHOS IL-201 showed to be a potential medium for the n-butyl lactate biosynthesis. In addition, the phase behavior for d,l-LA/n-butanol/Novozym 435/CYPHOS IL-201 system in SC CO₂ at 65 °C and in SC CHF₃ at 55 °C at different pressures were also carried out.

Introduction

Volatile organic solvents are still often used in many chemical processes, but they have a detrimental impact on the environment and human health, therefore the need for alternative clean manufacturing processes and environmentally benign technologies drastically increased [1], [2]. In recent years supercritical fluids (SCFs) and ionic liquids (ILs) have increasingly attracted attention as emerging “green” and high-tech alternatives to classical organic solvents to carry out enzymatic reactions for the preparation of valuable and active organic materials [1], [3]. Strategy to use both, enzyme-catalyzed processes and environmental friendly media, presents the key area for future innovation in Green Chemistry. Clean chemical processes providing directly pure products results by using appropriate enzyme-ILs systems in combination with SCFs contain an exciting potential for developing the upcoming green chemical industry [1]. The combination of these two solvents owns advantages expressed by improvement in the enzyme stability and by the production of less or no pollutants. The technological purposes, for which enzymes can be used, have greatly increased in scope, since they can operate also under nearly anhydrous conditions in organic solvents as well as in ILs and SCFs [4]. Therefore, lipase catalysis in IL/SCF is green and benign to the environment [5].

The different miscibilities of SCFs and ILs lead to two-phase systems that have found application in several areas. The success of this two-phase system is based on solubility of SCF in the IL, which is controlled by pressure, but insolubility of the IL in SCF. The solubility of SCF in IL also facilities mass transfer processes by decreasing their viscosity [6].

Catalyzed reactions, based on IL/SC CO₂ biphasic systems, present promising alternatives for developing green chemical processes because of their physical and chemical characteristics [7].

ILs can have very low vapor pressure, high thermal stability, and widely tunable properties (by changing the cation/anion combination) such as polarity, hydrophobicity, and solvent miscibility [8], [9]. ILs are attractive reaction media for enzyme catalyzed reactions, because their use can enhance enzyme stability [8], [10], [11], [12], [13], selectivity [14], and reaction rates [8], [11], [15]. ILs are not only green solvents, they can have catalytic effect [16]. Due to international regulations to reduce the use of solvents with strong environmental impact, new alternative to organic solvents with characteristics more similar to the traditional ones may be considered, including the members of the lactate esters family [17]. Lactic acid esters are biodegradable and can be used as powerful high-boiling solvents for varnishes, paints, nitro and ethyl cellulose, gums, oils, dyes, etc. They are also used as food additives, in biochemicals, pharmaceuticals, cosmetics, detergents, etc. [18]. Since esters are much more volatile than lactic acid, they play also an important role in the purification process of lactic acid manufacture: highly purified lactic acid is yielded by subsequent hydrolysis of the rectified esters [19].

Lipases are used for various biotechnological applications as catalysts in biotransformations for the production of pharmaceuticals, agrochemicals, pesticides, insecticides, etc. and they can catalyze the reactions in various non-conventional solvents, such as SCFs and ILs [20].

Non-conventional solvents are receiving more and more attention in biocatalysis, since they can influence on the conformation of enzyme and hence altering its catalytic efficiency or specifity, which may increase regio- and enantioselectivity of reactions and also increase its stability. Supercritical carbon dioxide (SC CO₂) possesses many useful characteristics, such as nontoxic, non-flammable, inexpensive, it is available in large quantities, has tunable solvent and good solvation properties, and moderate critical temperature and pressure (31.1 °C and 7.38 MPa) [21]. Supercritical trifluoromethane (SC CHF₃) is chemically inert, non-flammable, has low toxicity and a low critical temperature (26.14 °C) and pressure (4.83 MPa) [22]. CHF₃ has no chloride or bromide atoms, it is not included in ozone-depletion fluorocarbons and can be practically reused by recovering and recycling [23].

N-Butyl lactate production via esterification by Candida antarctica lipase was studied. Novozym 435 (immobilized C. antarctica lipase B) was used as a biocatalyst. Till now, esterification of lactic acid (LA) was studied only in organic solvents, in toluene [24], [25], under microwave heating [25] and recently in some hydrophobic ethers and ketones[19]. Previous research on esterification of d,l-lactic acid (d,l-LA) with n-butanol was also successfully carried out in IL – CYPHOS IL-201 [26]. Synthesis of n-butyl lactate in phosphonium-based IL and two different SCFs, one without dipole moment (SC CO₂) and the other one with a strong dipole moment (SC CHF₃) [27] were performed. Carrying out this synthesis in SCF/IL system is a novel process. The informations, available in the literature, on the phase behavior of previous mentioned system are limited only to the binary systems. No experimental data for the phase behavior of the d,l-LA/n-butanol/Novozym 435/CYPHOS IL-201 system in SC CO₂ and SC CHF₃ were found in the literature published to date. The aim of the process is to dissolve the substrates in the IL to reduce the mass transfer limitations. SC CO₂ is added mainly to reduce the viscosity. Two different fluids with different hydrophobicity and dipolar moment were tested.

In the present work influence of different parameters (e.g. reaction media, temperature, pressure, etc.) on the d,l-LA efficiency and on the composition of d,l-LA monomer and lactoyllactic acid (dimer of d,l-LA) during the production of n-butyl lactate in SCF/IL system is presented. The performance of d,l-LA esterification with n-butanol in SCF/IL is a quite new research area and there is no literature data on the d,l-LA efficiency in such system. The objective of the research was to determine optimal operation parameters to achieve the maximum d,l-LA efficiency.