Elsevier

Enzyme and Microbial Technology

Volume 41, Issue 4, 3 September 2007, Pages 533-538
Enzyme and Microbial Technology

Synthesis of a green biosolvent: Isopropyl esters: A statistical approach

https://doi.org/10.1016/j.enzmictec.2007.04.008Get rights and content

Abstract

The synthesis of a green clean biosolvent, isopropyl esters, using high-oleic sunflower oil (HOSO) as raw material over an enzymatic catalyst, has been developed and optimized following a factorial design and response surface methodology. A full three-factorial design has proved effective in the study of the influence of the variables (temperature, enzyme concentration, and alcohol/oil molar ratio) on the process. The process was studied in the range of 68–82 °C, 1.8–8.4% wt. catalyst concentration, and initial alcohol/oil molar ratio 1:1–11:1. The molar ratio of alcohol/oil was been found to be the most significant factor on the transesterification process and its influence is negative. The response surface model obtained, representing the yield of ester, was found to describe adequately the experimental results. The best conditions for the process are a catalyst concentration of 7%, an operation temperature of 71 °C and with low alcohol/oil molar ratio (3:1); with these conditions the maximum conversion obtained was 84%. The preparation of the product is a green engineering process, clean, solvent-free, with a very selective catalyst that minimizes water and energy consumption and the downstream processing of the integrated process.

Introduction

In recent years, concern over the potential impact of petroleum-based solvents on the environment has created an opportunity to promote environmentally acceptable alternatives. Biosolvents have been developed as one type of environment-friendly product.

Sustainable development has become the ideal key of the 21st century in the search for green engineering process. Considerable importance is attached for the use of renewable raw materials like vegetables oils.

Fatty acids alkyl esters such as isopropyl esters show an increasingly growing demand in Europe and the USA due to their numerous applications in cosmetic, pharmaceutical, and food industries [1], [2], as bio-lubricants for high precision machinery [3], and as biosolvents. Some of these esters can be extracted from natural sources (usually plant seeds or animals), but the cost of the production process is high due to the limited availability of vegetable and animal sources and the associated environmental and sustainable development problems.

Nowadays, new synthesis methods are being developed to produce esters in large quantities and low cost. Most synthesis processes are based on an esterification reaction where the ester is formed from the corresponding acid and alcohol in the presence of a catalyst [4]. Traditionally, strong mineral acids have been used as catalysts for esterification reactions, but the quality of the products is low due to side reactions [5].

Alcoholysis of fats and oils is simpler than direct esterification and the starting material is cheaper [6]. This process has been shown to be of interest for the production of wax esters of high commercial value whose applications vary from lubricants to cosmetics. Among the vegetable oils, high oleic sunflower oil is a very interesting substrate for the synthesis of esters. New hybrid varieties produce oil that can contain about 80% of oleic acid.

The potential industrial-scale application of enzymes as catalysts is being studied for a number of reactions [7]. Lipases have been successfully applied to the production of esters [8], [9]; hydrolysis of oils [10], interesterification of fats [11], polymers [12] and additives has also been suggested [13]. This is understandable since the trend towards ecologically acceptable processes is steadily growing.

For transesterification reactions immobilized lipases show many advantages over traditional catalysts: they allow working under mild operating conditions, show high selectivity and no significant side reactions, and lead to products of high purity. In addition they are easily recoverable and there is no contamination of the final product, saving time and cost in the purification stage.

Chemical processes which takes into account environmental considerations in the selection of reactants and reactions conditions is growing in importance as both industrial and academic researchers become aware of the environmentally benign or “green” approach. The principles of green chemistry [15] focus on reducing, recycling, or eliminating the use of toxic chemicals in chemistry by finding creative ways to minimize the human and environmental impact without stifling scientific progress.

In the present work, an immobilized lipase has been used as selective catalyst for the transesterification of high oleic sunflower oil and isopropanol to produce a biosolvent, isopropyl esters, based upon the principles of “green chemistry”, using techniques and methodologies that reduce or eliminate the use or generation of feed-stocks, products, by-products, solvents, reagents, etc., that are hazardous to human health or the environment.

Optimization is one of the most important problems when it comes to developing any chemical process. Trying to achieve the maximum conversion in the most adequate catalyst concentration, temperature, and the initial alcohol/oil molar ratio can be an exhausting task when changing each of the separate operating variables at a time, and most of the time only an apparent optimum is obtained, where the interactions among the variables involved frequently are ignored. To achieve this goal, the process has been developed and optimized following the response surface methodology [14]. This technique is a powerful tool to determine the optimum operating conditions (catalyst concentration, temperature, and the initial alcohol/oil molar ratio) necessary for the scale-up of the process, minimising the consumption of water and energy, and using moderate operations conditions: temperature, low organic solvent, and short reaction time.

Section snippets

Equipment

Experiments were carried out in a stirred tank reactor of 1000 cm3 volume provided with a reflux condenser and immersed in a thermostatic bath (HETO-HOLTEN A/S, Allerod, Denmark), capable of maintaining the reaction temperature at a constant value. The impeller speed was set at 0.02PN (700 rpm) to avoid mass transfer limitations.

Materials

Sunflower oil with a high content of oleic acid (75%) was supplied by Capicua (Coreysa) (Spain). Isopropanol of 99.8% C-plus from Alcoholes de Aroca S.A. (Spain). The

Linear stage

The experimental design applied in this study was a full 23 factorial design. Table 1 shows the standard experimentation matrix for the design, the results, and yield of ester, after 60 and 120 min of reaction. A statistical analysis was performed on these experimental values and the main effects and interaction effects for two and three variables were calculated. The analysis of the main effects and interaction for the chosen response, yield of ester together with the test of statistical

Discussion

The influence of variables, reaction temperature, catalyst concentration, and alcohol/oil molar ratio on the ester yield will now be discussed. The influence of the main factors and interactions will be discussed from statistical models.

Conclusions

In the present work, design of experiments has been applied to optimize the enzymatic synthesis process of isopropyl ester. A full three-factorial design has proved effective in the study of the influence of the variables on the process. Central Composite Design procedure has been followed to optimize the variables that determine the yield of ester. A response equation has been obtained for the yield of ester. From this equation, it is possible to predict adequately the operating conditions

Acknowledgment

Financial support from the Comunidad Autónoma de Madrid, Spain, Spanish project CAM 07M/0045/98 is gratefully acknowledged.

References (22)

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  • Cited by (20)

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      Fatty acid isopropyl esters (IPEs) have been in increasing demand because of their numerous applications in the cosmetic, pharmaceutical, food, and other industries [1–4].

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