Heterogeneous Catalysts Based on H 3 PW 12 O 40 Heteropolyacid for Free Fatty Acids Esterification

The book "Biodiesel: Feedstocks and Processing Technologies" is intended to provide a professional look on the recent achievements and emerging trends in biodiesel production. It includes 22 chapters, organized in two sections. The first book section: "Feedstocks for Biodiesel Production" covers issues associated with the utilization of cost effective non-edible raw materials and wastes, and the development of biomass feedstock with physical and chemical properties that facilitate it processing to biodiesel. These include Brassicaceae spp., cooking oils, animal fat wastes, oleaginous fungi, and algae. The second book section: "Biodiesel Production Methods" is devoted to the advanced techniques for biodiesel synthesis: supercritical transesterification, microwaves, radio frequency and ultrasound techniques, reactive distillation, and optimized transesterification processes making use of solid catalysts and immobilized enzymes. The adequate and up-to-date information provided in this book should be of interest for research scientist, students, and technologists, involved in biodiesel production. work, following:

A plethora of works have described the development of heterogeneous catalysts based on acids solids, which appear to offer an attractive perspective to turn the biodiesel production more environment friendly (Kiss et al., 2006;Jothiramalingam & Wang, 2009;Refaat, 2011). These solid catalysts, which normally present Lewis acidity, are easily separated from the reaction medium and are potentially less corrosive for the reactors. Normally, these processes focus on transesterification reactions of the triglycerides presents in the vegetable oils, which after react with methanol are converted into biodiesel. However, serious technological drawbacks such as drastic conditions reaction, the strict control of raw material quality in relation to water content, beyond of the leaching catalyst provoked by presence of alcohol besides water generated into reaction medium seems suggest that those process yet are hard to become effective (Kozhevnikov, 2009). Particularly, the authors have concentrating efforts in developing alternative processes of esterification based on two recyclable catalysts linked to both acid types: i. heteropolyacids, with a special highlighted for the dodecatungstophosphoric acid (H 3 PW 12 O 40 12H 2 O) (Silva et al., 2010;Cardoso et al., 2008); ii. tin chloride, an simple, easily handling, water tolerant and inexpensive Lewis acid ( Cardoso et al., 2009;da Silva et al., 2010). On the hand, catalysis by heteropolyacids of the Keggin's structure such as H 3 PW 12 O 40 is one of the most important and growing areas of research in recent years (Timofeeva, 2003). They have been extensively used in both homogeneous and heterogeneous catalysis (Misono et al, 2000;Sharma et al., 2011). On the other hand, the use SnCl 2 catalyst is also most attractive, because it is solid, commercially available, and easy to handle. Moreover, its display remarkably tolerance to water, has an economically cost effective, and can be used in recyclable processes (Cardoso et al., 2008). Herein, the authors investigate the catalytic activity of heterogeneous catalysts based on acid solids composites (e.g. H 3 PW 12 O 40 supported on silicon, niobium and zirconium oxides) towards the esterification of oleic acid with ethanol.

Keggin heteropolyacid catalysts: a brief introduction
Tungtstophosphoric acid (H 3 PW 12 O 40 ) is a heteropolyacid largely used, in special under heterogeneous catalysis conditions. As a homogeneous catalyst the H 3 PW 12 O 40 has showed higher activity, selectivity and safety in handling in comparison to conventional mineral acids (Cardoso et al., 2008). Recent works have shown that the Keggin-type H 3 PW 12 O 40 , for which the physicochemical and catalytic properties have been fully described, is an efficient super-acid that can be used in homogeneous or heterogeneous phase (Kozhevnikov, 1998). Moreover, in the heterogeneous phase, supported on several solid matrixes, heteropolyacid composites also have showed highly efficient as catalysts in several types of reactions (Pizzio et al., 1998;Timofeeva et al., 2003;Sepulveda et al., 2005). The activity of H 3 PW 12 O 40 catalyst supported on zirconia was assessed in transesterification reactions with methanol (Sunita et al., 2008); high yields FAMEs were achieved in reactions performed at temperatures of 200 C. On the other hand, impregnated H 3 PW 12 O 40 heteropolyacid on four different supports (i.e. hydrous zirconia, silica, alumina, and activated carbon) also were investigated and converting low quality canola oil containing to biodiesel at 200 C temperature (Kulkarni et al., 2006). Recently, the use of an impregnation route to support H 3 PW 12 O 40 on zirconia in acidic aqueous solution and further applied in the oleic acid esterification with ethanol was described (Oliveira et al., 2010). Those authors verified that 20% w/w H 3 PW 12 O 40 /ZrO 2 was the most active catalyst (ca. 88% conversion, 4 h reaction, with 1:6 FA:ethanol molar ratio and 10% w/w of the catalyst in relation to FA. However, a minor leaching of catalyst (ca. 8% w/w related to the initial loading), affected drastically its efficiency, resulting in decreases yielding obtained from its reuse.

General aspects
Herein the H 3 PW 12 O 40 catalyst were supported on three different solid matrixes (i.e. silicon, niobium, and zirconium oxides) by impregnation in ethanol solutions under different loads (ca. 10, 30 and 50% w/w). The solids were characterized by FTIR spectroscopy and the H 3 PW 12 O 40 catalyst content was determined by UV-Vis and AAS spectroscopy analysis.

Syntheses of the H 3 PW 12 O 40 catalysts
Differently than others supports, which were used as received, zirconium oxide was obtained from thermal treatment of ZrOCl 2 .8H 2 O salt at 300 °C during 4 hours. Composites of H 3 PW 12 O 40 supported on silicon, niobium and zirconium oxides were prepared via impregnation method (Pizzio et al., 1998). During preparation, ethanol solutions of H 3 PW 12 O 40 in hydrochloric acid 0.01 mol L −1 were used to avoid any hydrolysis. All composites were prepared with concentrations depending upon the loading required to the support (e.g. 10, 30 and 50% w/w H 3 PW 12 O 40 ) using 10 ml of the solution per gram of support. The addition of the support to the solution formed a suspension, which after stirred, was evaporated at 80 °C until dryness. All samples of supported heteropolyacid were dried at 100 C for 12 h and then thermally treated for 4 h at 200 or 300 C in air.  (Pope, 1983). These groups are bonded each other by cornersharing oxygens. This structure gives rise to four types of oxygen atoms, being responsible for the fingerprint bands of the PW 12 O 40 3− Keggin ion (ca. 1200 -700 cm − 1 ). FTIR spectra were obtained from all samples with different content of HPW (ca. 10, 30 and 50% w/w). However, the typical bands of the Keggin ions were more evident for samples with HPW contents of 30 and 50 % w/w. Herein, only the FTIR spectra of the composites with 30 % w/w H 3 PW 12 O 40 , which were thermally treated at temperature of 100, 200 and 300 C are shown. Figures 1-3 shows the characteristic bands for absorptions of  (P-O) and  (W-O) bonds existent on H 3 PW 12 O 40 composites. All FTIR spectra of both supported H 3 PW 12 O 40 catalyst or pure are displayed in Figures 1-3. When niobium oxide was the support, only a stronger band at 1080 cm − 1 relative to  (P-O) bond was easily observed (Figure 1). All others bands were overlapping by support bands. Conversely, when the support employed was the SiO 2 , all the bands related to others oxygen atoms were observed  (W = O tethraedric ) bond at 985 cm   In Figure 3, where FTIR spectra obtained from HPW composites supported on ZrO 2 are shown, all characteristics bands of the Keggin anion are present. In general, FTIR spectra of the HPW composites on different supports were not affected by temperature of thermal treatment. On the temperature range studied herein, all they have shown similar characteristics. However, a measured of interaction strength of HPW with support may be obtained from shift of more well defined bands to a region of lower wave number in comparison with the same band present on HPW pure (Figures 1-3).

UV-Vis spectra of the supported heteropolyacid catalysts: H 3 PW 12 O 40 /SiO 2 , H 3 PW 12 O 40 /Nb 2 O 5 and H 3 PW 12 O 40 /ZrO 2
Beckman DU-650 UV-Vis spectrophotometer and quartz cells of 1.0 and 0.1 cm pathlength were employed for the adsorption experiment and measurements of H 3 PW 12 O 40 spectra, respectively (Oliveira et al., 2010). The concentration of H 3 PW 12 O 40 on catalysts was measured by UV-Vis spectroscopy before and after 6 hours of adsorption. The content of HPW in the solid was determined by AAS. In all composites yielding upper of 95% of impregnation were achieved.

Catalytic tests 2.5.1 Reaction conditions
The reactions conditions used were based on typical heterogeneous process ( Figure 4). The catalyst is recovered from solution from simples filtration; the ethanol used in excess is www.intechopen.com dried and reused in other catalytic run, similarly to solid catalyst. As will show on next section, ethanol in excess not favors the ester formation under these reaction conditions. The load catalyst used when the composites have 50% w/w HPW is corresponding to ca. 1 mol % in relation to oleic acid; in all reactions 1 mmol of oleic acid is used against 0.0087 mmols of HPW present in 50 mg of catalyst.

The effect of support on catalytic activity of HPW composites
The low surface area of solid H 3 PW 12 O 40 , which implies a small amount of H + ions available on the surface; for circumvent these problems, three supports with a higher surface area were selected on this study. When solid supported heterogeneous catalyst are prepared, important aspects such as temperature of the thermal treatment, method of synthesis, type and precursor nature and also of the support, besides catalyst loading can affect drastically the efficiency of catalyst (Hattori, 2010).
Herein the temperature of thermal treatment was the parameter selected for an adequate comparison between the catalytic activities of different HPW composites. High temperatures may favor the reduction of support surface area (300 C) and lower temperatures (100 C) may favor catalyst leaching when impregnation is synthesis method; for these reasons, the authors selected results obtained with catalyst treated at 200 C as displayed in Figure 5. However, another important aspect that can be affected by thermal treatment is the water content on both support and HPW catalyst. All solid supports were completely dried (ca. 120 C) before of the HPW composite synthesis. Conversely, termogravimetry analysis results described in literature (Essayem et al., 1999) et al., 2007). The thermal treatment herein employed was the same for all supported-composite; so is reasonable conclude that although not quantitatively determined, water effect act equally onto both composites. The HPW 50% w/w/niobium composite is strongest Lewis acid support; nevertheless, Figure 5 reveals the all catalyst have a very close behavior. The HPW/Nb 2 O 5 composite was the catalyst selected to assess the effects of others reaction parameters because there are scarce data on literature; moreover, as will showed it was the catalyst more efficient and less leached in reactions. All results obtained on HPW/niobium-catalyzed oleic acid esterification with ethanol are highlighted in next sections.

Temperature effects of the thermal treatment on catalytic activity of the HPW/niobium composites
The esterification of oleic acid with ethanol conducted in the absence of the acidic catalysts (HPW) produced no significance yields of the corresponding ethyl oleate in spite of the high molar ratio of ethanol/oleic acid used. For instance, only a very low oleic acid to ethyl oleate conversion (ca. 10%) was achieved even after a reaction time as long as 8 h ( Figure 6). Moreover, despite Lewis acidity of the support, when in presence only of niobium, a poor conversion of oleic acid into ethyl oleate was also reached ( Figure 6).

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Conversely, in the presence of H 3 PW 12 O 40 pure or niobium-supported and after a reaction time of 8 h much greater yields (ca. 86%) were attained, as concisely displayed in Figure 5. In all reactions, a high selectivity for the ethyl oleate greater than 90 % (analysis) was achieved, determined by GC analyses (no showed herein). Investigating the performance of supported HPW can be observed that the best and worst results were obtained when the HPW/niobium composites were treated at 100 and 300 C temperatures. A possible leaching of catalyst (see next section) and the reduction of surface area provoked by high temperature of thermal treatment may be reasonable explanations.
On the other hand, the highest conversion was obtained when a mechanic mixture of niobium and H 3 PW 12 O 40 was used, probably due the simultaneous presence of the first and second catalyst; this later soluble and consequently more reactive (Lewis and Brønsted acids respectively).

The effect of HPW loading on catalytic activity of the HPW/niobium composites
In many cases, there are obvious approaches to improving and optimizing the yielding of catalytic reactions. Among the mains, is highlighted an increase on amount of reactants and of the catalyst. Recognized, the catalyst load can affect remarkably the efficiency of catalyst. Kinetic curves obtained from HPW/niobium-catalyzed esterification reactions with loads of HPW equal to 10, 30 and 50 % w/w respectively are shown in Figures 7-9. Because the temperature used on the thermal treatment may also affect both stability and activity of catalyst, three results obtained at three different temperatures are reported.   Although literature data described that occur a significance decreases on surface area with an increase of acid content, which may then reduce its catalytic activity (Dias et al., 2003), results displayed in Figures 6 to 8 suggest that a higher HPW load increases the efficiency of HPW/Nb 2 O 5 catalyst. Interestingly, it's occurred independently of the thermal treatment employed on synthesis of these catalysts (Figures 7-9).

Evaluating catalyst leaching
Leaching affects the industrial application as extensive leaching may threaten the reusability and the environmental sustainability of catalyst (Di Serio et al., 2010). Conceptually, catalyst leaching is usually associated with a phase boundary. For example, the active component of an insoluble acid solid catalyst might slowly leach into solution by some mechanism, perhaps involving bond breaking. When the catalyst has leached into a product phase, the sample should exhibit some catalytic activity. Thus, an efficient procedure that allows evaluates if there is any leaching is remove the catalyst out of the reaction and continue to run in your absence. Figures 10 to 12 displayed kinetic curves of reactions catalyzed by HPW/niobium composite before and after its remove. It was found that the composites obtained at temperatures of 200 or 300 °C, seems be more stable under reactions conditions; noticeably, after catalyst remove the conversion of oleic acid into ethyl oleate remains constant. However, when the catalyst was synthesized at 100 °C, there was an increase in the conversion of oleic acid, suggesting that possibly a part of HPW can has been lixiviated to reaction solution. Interesting, the same occurred for the catalyst supported on zirconium and silicon (Figures 13 and 14).    Various measures of catalyst leaching must be interpreted based in others contexts. For example, atomic absorption spectroscopy and ICP-MS are very sensitive analytical methods. However, a simple qualitative procedure can be used based only on visual observation; the addition of ascorbic acid to a solution containing HPW soluble assume blue color. Herein, its procedure allows easily confirm the catalyst leaching treated at 100 C temperature; contrarily, in the runs with HPW/niobium-200 C catalyst the solution remained with color unaltered (pale yellow).

Recovery and reuse of catalyst
The greatest advantage of the heterogeneous goal of this study over the homogeneous catalyst is the prolonged lifetime of the solid catalyst for ethyl esters production. However, leaching of catalyst components can cause its deactivation quickly. Herein, the stability of HPW 50 % w/w/niobium-200 C after successive protocols of recovery/reuse was assessed ( Figure 15). The recovery yields of solid catalyst isolated from procedure of filtration are most commonly determined gravimetrically. A remarkable result was observed as the HPW/niobium catalytic activity stayed almost unaltered even after three recovery/reutilization cycles. However, it should be noted that a weights of catalyst fresh (ca. 20% in relation to started weight). It was found that although recovery rate has been kept constant (ca. 72-75 %) in all catalytic runs, its suggest that the catalyst leach to solution; however, in Figures 10 to 12 it was demonstrated that oleic acid conversion remains unaltered after catalyst remove. This observation suggests an absence of leaching of catalyst. Probably, the procedure used is not efficient as desired.
www.intechopen.com The procedure employed for catalyst recovery involves its separation from reaction by filtration, washed with ethyl ether and drying at 100 °C; then the catalyst has its mass determined. Losses of mass through of these several steps may be occurring. A more detailed treatment of the recovery procedure of catalyst may lead to efficient methods, which can reaches higher recovery rates. The authors are developing studies on this direction.

Mechanistic insights
Tunstophosphoric acid (H 3 PW 12 O 40 ) is strongest heteropolyacid of Keggin series being completely ionizable in water. Measurements of pKa in organic solvents showed that it is 100 units of pka more acid than sulfuric acid (Kozhevnikov, 1998); therefore is almost probably that its ionization in ethanol occur in greater extension. Thus, is possible that HPW/niobium catalyst undergoes at least a partial ionization along oleic acid esterification reaction in ethanol as described on equilibrium displayed in Figure 16. Consequently, if this part of the reaction pathway is similar to homogeneous systems, the others steps commonly involved in esterification reactions (e.g. protonation carbonyl group FA, attack of the alcohol molecule on protonated FA, water elimination, etc) may then proceed as described in Figure 16.  Conversely, is also possible that other FA molecules can be activated via protonation on surface of supported-catalyst. Thus, an alternative proposal is displayed in Figure 18.  In according with this mechanism (Figure 18), all steps of oleic acid esterification reaction with ethanol occur on surface of HPW/niobium catalyst. Nevertheless, is also possible that ethyl oleate formation may occur by both pathways of reaction Although both proposal are plausible, is important to note that studies in situ are require for a better and more detailed description of these mechanism of this reaction.

Conclusion
The efficiency of tungstophosphoric acid (H 3 PW 12 O 40 ) immobilized by impregnation method on silicon, zirconium and niobium oxides was assessed in the esterification of oleic acid with ethanol, at 60 C temperature. As a general tendency, it was observed that the catalytic activity decreases in the series HPW/Nb 2 O 5 > HPW/ZrO 2 > HPW/SiO 2 with all catalyst being treated on temperature range 100 to 300 C. Moreover, good yielding of recovery of HPW 50% w/w/Nb 2 O 5 catalyst (ca. 75 %) and high conversions of acid oleic were obtained in recycle experiments. From leaching tests and of the rates of recovery may be concluded that the HPW/Nb 2 O 5 catalysts are stable under reaction conditions used; however the recovery procedure employed it should be enhanced. Thus, it can be concluded that although yet non-finished, present methodology offers several advantages such as high yields, simple procedure for recovery and reuse of catalyst and mild reaction conditions.