Influence of the Free Fatty Acids, Water, Temperature, and Reaction Time on the Catalyst-Free Microwave-Assisted Transesterification of Triglycerides with 1‑Butanol Jeroen Geuens, †,‡ Sergey Sergeyev, † Bert U. W. Maes, † and Serge M. F. Tavernier* ,‡ † Department of Chemistry, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ‡ Department of Applied Engineering, Karel de Grote University College, Salesianenlaan 30, B-2660 Hoboken, Belgium * S Supporting Information ABSTRACT: The influence of the free fatty acids, water, temperature, and reaction time on the catalyst-free microwave-assisted transesterification of triglycerides with 1-butanol was studied using the response surface modeling (RSM) approach. Stearic acid and/or water were added to rapeseed oil to construct the model. Afterward, the general applicability of the model was verified using other triglyceride sources (refined oils as well as waste fats and oils), other free fatty acids, and other alcohols. In most cases, the model gives a fair approximation of the actual yield of fatty acid alkyl esters; however, the model tends to underestimate the yield when waste fats and oils are used. Next to that, a significant increase in prediction error was observed when using branched alcohols. For reactions of 2 h at 234-264 °C, yields over 90% could be reached when using waste fat streams. 1. INTRODUCTION Whereas fatty acid methyl esters are commonly used as biofuel, fatty acid alkyl esters of higher alcohols have a wide variety of applications, such as lubricants, cleaning agents, plasticizers in the production of plastics, cold flow improvers, or reactive diluents in the paint industry. 1 Moreover, higher alcohols are completely miscible with fats and oils, thus leading to a homogeneous reaction system in which phase-transfer issues do not occur. Transesterification of triglycerides is typically performed using homogeneous alkali catalysts. In this process, large wastewater streams are created because of the necessity to wash the ester layer. 2 Next to that, the glycerol, which is a byproduct of the reaction, has to be purified because it will contain salts from the neutralization of the catalyst. Former research by our group indicated that the catalyst-free microwave-assisted transesterification of rapeseed oil with 1- butanol is possible when high temperature and pressure are applied, thus eluding washing steps and producing pure glycerol. 3 However, because of the elevated cost of high- temperature and high-pressure equipment when working on a pilot scale or an industrial scale, it is desirable to perform the transesterification reaction at a lower temperature and pressure. Moreover, current pilot-scale and industrial-scale microwave reactors are not capable of reaching 80 bar and 310 °C needed for the catalyst-free transesterification of rapeseed oil with 1- butanol. 4 It is known from the literature that the presence of water and free fatty acids negatively affects the conversion of triglycerides to fatty acid esters when the reaction is performed using alkali catalysts. 5-7 However, water and free fatty acids have a positive influence on the conversion when the transesterification reaction is performed without a catalyst in supercritical conditions. 2,8-10 Thus, adding water and free fatty acids or using low-grade (i.e., low-cost) oils, which typically have a higher water content and a higher acid value, could create possibilities to lower the reaction temperature and pressure while still maintaining a high yield of fatty acid esters. In this paper, the influence of the reaction temperature, reaction time, water content, and acid value of the oil was studied. To study the influence of multiple, possibly non- independent parameters on the outcome of the trans- esterification reaction, the response surface modeling (RSM) approach was used. For the construction of the model, the water content and acid value of the oil were changed by adding water and/or stearic acid to rapeseed oil. The more general applicability of the model was tested by performing reactions with other alcohols, other oils and fats, and other fatty acids. 2. MATERIALS AND METHODS 2.1. Materials. Different oils and fats were used during the experiments, and the water content and acid value of these oils and fats were determined by means of Karl Fischer titration using a Mettler Toledo V20 Volumetric KF titrator and acid titration using a Mettler Toledo DL53 titrator equipped with a DG113-SC electrode. The mono-, di-, and triglyceride contents of the oils and fats were determined by means of size-exclusion chromatography (SEC) on an Agilent 1100 series system, using isocratic elution with tetrahydrofuran (THF) and refractive index detection. 11 The results of these analyses are shown in Table 1. All oils were used as such, except from the waste frying oil, which was filtered over a S&S 598 2 white ribbon filter before use. The fatty acid profiles of all oils and fats were analyzed by means of gas chromatography (GC). The results of the GC measurements are mentioned in Table 2. Different fatty acids, ranging from C 6 to C 18 and both saturated and unsaturated, were used to acidify the rapeseed oil. All of them were purchased from either Merck or Acros. 1-Butanol was obtained from Received: January 12, 2013 Revised: April 8, 2013 Published: April 8, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 2637 dx.doi.org/10.1021/ef400069x | Energy Fuels 2013, 27, 2637-2642