Isopropyl myristate continuous synthesis in a packed-bed reactor using lipase immobilized on magnetic polymer matrix
Abstract
The aim of this study was to synthesize isopropyl myristate, an emollient ester, in a continuous-flow packed-bed reactor using Candida antarctica lipase immobilized on poly(styrene-co-divinylbenzene) matrix prepared by suspension polymerization and magnetized by co-precipitation of Fe2+ and Fe3+ in alkaline medium. To determine the best esterification conditions, we investigated the effects of acid/alcohol molar ratio (1:5, 1:10, and 1:15) on reaction yield in shake flasks. The three tested conditions provided similar results, esterification yields of approximately 80%. An acid/alcohol molar ratio of 1:15 was chosen for further experiments because it allowed for better operability of the bioreactor. Subsequently, we compared the reactor performance in up flow and down flow modes. This experiment showed that greater ease of operation was achieved with down flow operation. We also evaluated the influence of space time (8 and 20 h) on reaction yield and productivity. A space time of 8 h provided better results. An experimental system consisting of two bioreactors and a molecular sieve packed column was used to remove the water formed during esterification and thus increase the yield of isopropyl myristate. There was a significant improvement in performance with the use of the two-stage system, which resulted in almost complete conversion of reagents, an increase of about 150% in biocatalyst half-life, and an isopropyl myristate productivity of 25 g L−1 h −1 , confirming the beneficial effect of adding a water extraction column to the experimental system.
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Introduction
Isopropyl myristate is an emollient ester widely used in cosmetic preparations, especially in skin care products, because of its excellent spreading properties, non-toxicity, great biocompatibility, and high skin permeation ability [1-2]. In the pharmaceutical industry, isopropyl myristate is used as a skin penetration enhancer in topical formulations for transdermal drug delivery[3].
Currently, most industrial processes for the synthesis of isopropyl myristate use conventional chemical catalysis at elevated temperatures, which affords a low-quality product with residual color and odor, demanding expensive purification steps before the product can be marketed [1]. Efforts have been intensified to replace industrial chemical processes with ecofriendly methods. A major problem in chemical industries is the use of chemical catalysts, as these compounds generate waste, have high environmental impact, and increase purification costs [4]. Bioprocesses can be a sustainable alternative to a wide variety of conventional chemical processes.
Enzymes have advantages over chemical catalysts, not only in terms of environmental impact but also in terms of productivity, specificity, toxicity, and temperature and pressure reaction conditions. Among the enzymes that are used industrially, lipases (EC 3.1.1.3) are notable for catalyzing reactions in aqueous and organic media, such as esterification and hydrolysis reactions. This class of enzymes has applications in the manufacture of pharmaceutical products, surfactants, and cosmetics [5]. Industrial biocatalysis can benefit from enzyme immobilization techniques to increase the biocatalyst’s thermal stability and pH stability and allow its recovery and reuse both in batch and continuous reactors [6].
Several materials have been researched as enzyme supports. Magnetic materials are outstanding for this application because they can be easily recovered from the reaction medium, which obviates the need for centrifugation, filtration, or column separation steps [7]. When lipase is immobilized on a highly hydrophobic support, such as a poly (styrene-codivinylbenzene) matrix, the hydrophobic lid, which controls access to the catalytic site, interacts with the support, exposing the enzyme’s active site and increasing its affinity for the substrate, a mechanism known as interfacial activation. Another advantageous characteristic of hydrophobic supports is that they absorb less water from the reaction medium, which is desirable in esterification reactions [8].
Conclusion
No significant differences in esterification yields were observed among the three tested acid/alcohol molar ratios (1:5; 1:10, and 1:15). Therefore, an acid/alcohol ratio of 1:15 was used for the synthesis of isopropyl myristate in a continuous packed - bed reactor, as it resulted in lower substrate viscosity and, consequently, greater operability. The down flow mode was selected because it caused a low bed pressure drop as compared with the upflow mode.
In the single-stage reactor, the highest yield and productivity were obtained with a space-time of 8 h. The extraction column used in the two-stage reactor was effective in removing the water formed during esterification and in increasing the stability of the bioprocess. Almost complete conversion of myristic acid to isopropyl myristate was obtained, and the half-life of the biocatalyst increased by approximately 150%.
The results were satisfactory, and reaction and operational conditions were established for the continuous enzyme-catalyzed synthesis of isopropyl myristate in solvent-free medium conducted in a fixed-bed bioreactor. The bioprocess is an interesting option for large-scale industrial application