Transesterification of vegetable oils on basic large mesoporous alumina supported alkaline fluorides—Evidences of the nature of the active site and catalytic performances

Abstract

KF, LiF and CsF/Al₂O₃ catalysts with different loadings from 1 to 20 wt% were prepared using aqueous solutions of the alkaline fluoride compounds by wet impregnation of basic mesoporous MSU-type alumina. The catalysts were activated under Ar at 400 °C for 2 h and monitored by in situ XRD measurements. The catalysts were also characterized using several techniques: N₂ adsorption/desorption isotherms at −196 °C, FTIR, DR-UV–vis, CO₂-TPD, XRD, ²⁷Al CP/MAS NMR. These characterizations led to the conclusion that the deposition of alkaline fluorides on the alumina surface generates fluoroaluminates and aluminate species. The process is definitivated at 400 °C. The fluorine in these structures is less basic than in the parent fluorides, but the oxygen becomes more basic. The catalysts were tested for the transesterification of fatty esters under different experimental conditions using conventional heating, microwave and ultrasound irradiation. Recycling experiments showed that these catalysts are stable for a limited number of cycles.

Introduction

The transesterification of biomass feed stocks such as vegetable oil and/or animal fat with the production of “biodiesel” is one of the routes expected to lead to “green” fuels [1], [2], [3], [4]. The reaction requires 3 mol of methanol (or a higher alcohol) and 1 mol of triglyceride to give 3 mol of fatty acid methyl (or alkyl) ester and 1 mol of glycerol (Scheme 1). Under basic conditions exchanging glycerol with the desired alkanol occurs according to the following steps.

The “green” properties of these fuels are the consequence of the fact they are nature derived compounds that means a significant reduction in greenhouse emissions, and from their composition, fuel which is essentially free of sulfur, aromatics, metals or crude oil residues. Currently, the production of fatty acid methyl esters (FAME) in Europe is encouraged due to European legislation concerning the substitution of mineral diesel by biodiesel. In this respect, the European Union aims for 12% of the total energy output to consist of biodiesel, i.e. by renewable energy.

The transesterification reaction of fatty esters triglycerides is a relatively slow reaction that requires either strong acid or base catalysts. Different catalytic systems have been used to promote these reactions and to enable lower working temperatures. Acidic catalysts were found to be more effective for low quality oils. However, under homogeneous acid catalysis (H₂SO₄, H₃PO₄, HCl, p-toluenesulfuric acid, BuSn(OH)₃, Al(OR)₃ etc.) the requirement of high temperature, high molar alcohol/oil ratios, separation of the catalyst, generates serious environmental and corrosion related problems that make their use non-practical for biodiesel production [3]. Conversely, using heterogeneous acid catalysts such as pure and Cs-doped heteropolyacids as unsupported materials or impregnated on various supports, such as hydrous zirconia, silica, alumina or activated charcoal, have resulted in improved acid catalyzed reactions but need either high temperatures or high molar alcohol/oil ratios [5], [6], [7], [8]. Fe–Zn double-metal cyanide (DMC) complexes with zeolite-like cage structures were indicated to have similar activity to most of the basic oxides. These catalysts are Lewis acidic, hydrophobic (at reaction temperatures of about 443 K) structures [9].

The majority of conventional produced biodiesel uses homogeneous basic catalysts such as sodium or potassium hydroxides, alkoxides, or methoxides which are very active, but present also problems of corrosion, separation and waste streams. The substitution of homogeneous catalysts with heterogeneous analogues is crucial to eliminate problems associated with homogeneous catalysts [3], [10], [11]. Moreover, solid catalysts can potentially be used for long periods of time allowing a technology which is capable of continuous processing thus improving the economics of biodiesel production [12]. In this scope, a large variety of solid bases have been reported including basic zeolites, metal carbonates, supported alkali metal ions and alkali earth oxides, metal oxides (PbO, PbO₂, Pb₃O₄, Tl₂O₃, and ZnO) as well as hydrotalcite derived catalysts that have been investigated in transesterification of fats to esters [12], [13], [14], [15], [16], [17], [18]. In addition, alkali nitrate and alkali carbonate-loaded Al₂O₃, polymer resins, sulfated-tin and zirconia oxides and tungstated-zirconia have also been reported [19], [20]. Leaching of metal ions was encountered in the case of basic zeolite X and ETS-10 catalysts [11]. The use of these solid catalysts has, however, some drawbacks. They operate at high temperatures with high methanol–fat ratios and require protection from CO₂.

Concerning the nature of the reactor, glycerolysis of FAME has been investigated in both batch and continuous reactors [21], [22]. The transesterification of fatty esters has been carried out under different activation conditions. Several studies have been reported on the production of biodiesel under microwave irradiation, using both methanol and higher alcohols. The use of this technique has the advantage of lowering oil/methanol ratio to six [23], [24], [25], [26], [27]. Ultrasonication was reported as well. Ultrasonication is considered to have a general accelerating effect on heterogeneous reactions. Using this technique, high biodiesel yields were shown to be achieved in a remarkably short time [28], [29], [30], [31].

Transesterification reactions without any catalyst in near-critical or supercritical conditions using microwave irradiation have also been reported [32]. However, working under supercritical conditions requires high temperatures and pressure, resulting in higher energy consumption and the need for special equipment and safety conditions [33], [34], [35], [36], [37].

In this paper we report the use of various alumina supported alkaline fluoride compounds (KF/Al₂O₃, LiF/Al₂O₃ and CsF/Al₂O₃) for the transesterification of different vegetable derived fatty esters (sunflower, soybean, and rapeseed oil).

KF/Al₂O₃ is a solid base catalysts which has been extensively studied in various organic syntheses [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49] as well as for the transesterification of palm oil with methanol to biodiesel [50]. In contrast, CsF/Al₂O₃ has only been reported for the condensation of phenols with aryl halides [51] and no reports have been found regarding the catalytic properties of LiF/Al₂O₃. However, lithium-doped ZnO and ZrO₂ catalysts were recently reported as being effective for biodiesel production [52]. Studies on using KF–Al₂O₃ under both microwave activation and ultrasound irradiation for different other reactions have also been examined [53], [54], [55].

Another aim of this study was to provide evidences about the nature of the active sites in this reaction. For such a purpose several techniques have been used including in situ XRD measurements, ²⁷Al MAS NMR, in situ DR-UV–vis measurements and DRIFTS.