The use of LDH as catalysts for transesterification reactions is less common if compared to the use of LDOs derived from LDH by calcination. However, the t-butoxide intercalated Mg/Al LDH (LDH/t-BU) was shown to be catalytically active for production of b-ketoesters by transesterification with primary, second­ary and tertiary alcohols (Choudary et al., 2000).

Serio et al. (2006) synthesized Mg/Al LDHs by copre­cipitation at pH 10. After washing and drying, the LDHs were calcined at 500 °C for 14 h to produce the corre­sponding oxides. Besides, two samples of oxides identi­fied as MgO-1 and MgO~2 were obtained by calcination of Mg(OH)2 and (MgCO3)4 Mg(OH)2 at 400 °C. All these oxides were tested as catalysts for soybean oil methanol — ysis. Reactions carried out with 10 wt% catalyst at 100 °C yielded about 80% of products using the LDO solids and less than 20% with both MgO-1 and MgO~2. The higher activity of LDO, with respect to other catalysts, was justi­fied by the presence of a higher concentration of very strong base sites and large pores that favored the reaction by rendering the active sites more accessible to the bulky triglyceride molecules. In another study (Serio et al.,

2007) , an LDO obtained in the same way was used in the methanolysis of soybean oil with and without the addition of 10 wt% of its weight in oleic acid. The reaction was carried out at 180 °C for 1 h with 5 wt% catalyst using commercial MgO as a reference material. The methanol — ysis of neutral soybean oil was catalyzed with LDO and MgO and the yields were 92% and 75%, whereas the cor­responding values for the acidified soybean oil were 80.3% and 76.6%, respectively.

Unlike the direct use of LDOs, Xi and Davis (2008) rehydrated the LDO and tested the resulting material as catalyst for transesterification. The experiments started with the coprecipitation of an Mg/Al LDH with an Mg:Al molar ratio of four. The material was calcined at 500 °C under nitrogen atmosphere to form the LDO and then rehydrated with vapor under nitro­gen atmosphere. The crystallinity of the resulting rehy­drated LDH was lower than that of the initial LDH. The absence of CO2 in the rehydration process avoided formation of carbonate ions. Hence, the counterion in the LDH structure was the hydroxyl ion. For this reason, the hydrated LDH had more Bransted sites than a typical LDH. This material was subsequently used in the methanolysis of tributyrine and the yield of monoes­ters was around 80% when the reaction conditions involved 136.5 g of methanol, 43.0 g of tributyrine and 0.25 g of catalyst at 60 °C for 400 min.

Zeng et al. (2008) synthesized various LDHs with different Mg:Al molar ratios by coprecipitation and ripened them at 65 °C. The solid LDHs were washed and dried at 90 °C to be subsequently calcined in a muffle at 673—1073 °C for 7 h, with the resulting oxides being tested in the transesterification of refined colza oil. The catalytic activity was correlated with the tempera­ture and time of calcination as well as with the Mg:Al molar ratio. The best yield (90.5%) was obtained from the oxide with Mg:Al molar ratio of three that was calcined at 500 ° C for 12 h. In this case, the transesterifi­cation was carried out with 1.5% of catalyst in relation to the oil mass, a methanol:oil molar ratio of 6:1 and stir­ring at 300 rpm for 4 h at 65 °C. In addition, the reuse assays showed that the catalytic activity was kept for six cycles with a slight decrease in ester yield after each cycle.

Mg/Al LDOs were also tested by Xie et al. (2006) in the transesterification of soybean oil with methanol. The precursor was synthesized by coprecipitation at pH 7. The material was calcined for 8 h at different tem­peratures and the obtained LDO was tested in the trans­esterification of soybean oil with a methanol:oil molar ratio of 15:1, 7.5% of catalyst and heating under reflux. The Mg:Al molar ratio of three yielded 67% of ester, which was the best result if compared to other molar ratios of 2.0, 2.5,3.5 and 4.0. The calcination temperature also influenced the catalytic activity. Actually, the calci­nation temperature affected the basic strength of the oxides as determined by the Hammett method. When the calcination temperature was increased from 300 °C to 500 °C, the methyl ester yield reached a maximum of 66%. The highest yield was attributed to the achieve­ment of the highest basicity after calcination. According to XRD, this oxide corresponded to the MgO periclase phase. Temperatures above 500 °C transformed the crystalline phase into spinel with less basicity and also less catalytic activity. Calcination below 500 °C led Al3+ to replace Mg2+ sites and the basicity Al bonded to O2~ is lower than that of Mg bonded to O2~. For the LDH with an Mg:Al molar ratio of three, calcination at 500 °C led to the optimal basicity for catalytic applica­tions in the methanolysis of soybean oil.

Cantrell et al. (2005) reported the use of layered materials for the catalytic transesterification of glyc­erin tributyrate. For this purpose, a series of [Mg^l xj

Alx(OH)2]x+ (COs)^ compounds with the x value rang­ing from 0.25 to 0.55 were calcined at 500 °C for 3 h under wet N2 flux (95% humidity). Also, pure Al2O3 and samples of magnesium-impregnated calcined hydrotalcite were used as reference materials and no catalytic activity was detected in any of these com­pounds. On the other hand, the LDOs improved their catalytic efficiency with an increase in their magnesium content, achieving a maximum ester yield of 74.8% with 25% of magnesium in the LDO structure. The reactions were always performed at the same experimental condi­tions (60 °C for 3 h), in which pure MgO yielded only 11% of esters.

In another study, heterogeneous catalytic processes were developed for the alcoholysis of triglycerides using LDOs that were impregnated with alkaline metals (Trakarnpruk and Porntangjitlikit, 2008). Mg/Al-NO3 LDHs were synthesized by coprecipitation and calcined at 450 °C for 35 h. The resulting oxide was added to a potassium acetate solution in order to impregnate the oxide with potassium ions. The material was recovered from the solution, dried at 100 °C for 12 h and calcined again at 500 °C for 2 h. The potassium content of the resulting powder was 1.5%. FAMEs with a 96.9% ester content and methyl ester yields of 86.6% were obtained with these solids in reactions carried out at 100 °C for 6 h, using 7% of catalyst and a methanol:oil molar ratio of 30:1.

Liu et al. (2007) carried out the catalytic conversion of chicken fat to methyl esters using oxides that were derived from the Mg6Al2(CO3)(OH)16’4H2O hydrotalcite by calcination at different temperatures (400—800 °C) for 8 h. As a result, the effect of the calcination temperature on the catalytic performance of the oxide was confirmed, as already described by Xie et al. (2006). High yields of 94 wt% were obtained when the LDH was calcined at 550 ° C and the reaction was carried out at 120 °C for 6 h with a catalyst loading of 0.04 mg/l. The catalyst activity decreased slightly in the first recycling stage but dropped to only 60% of the original value after the fourth consec­utive reaction cycle. However, the original activity could be totally recovered by calcination of the spent catalyst in air.

Antunes et al. (2008) catalyzed the methanolysis of soybean oil with Mg/Al and Zn/Al oxides that were obtained by calcination of the corresponding LDH at 450 °C for 12 h. Transesterification was performed for

7 h at 70,100 and 130 °C with a methanol:oil molar ratio of 55:1. The highest activity was detected at 130 °C and the yield at this temperature was 80% with MgO, 70% with Mg/Al LDO, 63% with Zn/Al LDH, 30% with ZnO, and 11% with A^O3.

Ilgen et al. (2007) used LDOs derived from Mgg — Al2(OH)16CO3-4H2O for the catalytic conversion of canola oil into methyl esters. The LDH was prepared by coprecipitation of magnesium and aluminum carbon­ate salts at pH 10 and ripened for 18 h. After separation, the solid compound was dried at 80 °C and then calcined at 500 °C for 16 h. The LDO with particle diam­eter of 150—177 mm gave a 63% ester yield when the re­action was carried out at 60 °C with methanol:canola oil molar ratio of 6:1. Higher molar ratios of 9:1, 12:1 and 16:1 decreased the ester yields to values lower than 60%, and when LDOs with other particle sizes (125, 125—150 and 150—177 mm) were used, the best perfor­mance was obtained in the range of 125—150 mm. In the same report, the use of n-hexane as a cosolvent was shown to be detrimental to methanolysis. Also, methanol resulted in better ester yields than ethanol.

Barakos et al. (2008) calcined Mg/Al-CO3 at 350 °C for 6 h and tested it for methanolysis of cotton oil. Sam­ples with 95% esters were obtained for reactions carried out at 180 °C, using methanol:oil molar ratios of 6:1 wt% and 1 wt% of catalyst at 2200 kPa.

Albuquerque et al. (2008) prepared calcium and mag­nesium mixed layered hydroxides by coprecipitation from which LDO catalysts were generated. The LDH was calcined at 800 °C and the resulting oxides were tested in the catalytic methanolysis of sunflower oil at 60 °C. Higher yields of 92.4% were obtained in methanol­ysis after 3 h using a methanol:oil molar ratio of 12:1 and a 2.5 wt.% of the solid catalyst with a 3.8 Mg:Ca ratio.

Macedo et al. (2006) prepared (Al2O3)4(SnO) and (Al2O3)4(ZnO) LDOs from the corresponding Sn/Al and Zn/Al LDHs and both present catalytic activity in the alcoholysis of soybean oil, even when branched alco­hols were used. Yields higher than 80% were obtained with methanol after 4 h at 60 °C and the recycling tests indicated that these materials did not lose their catalytic activity.

Shumaker et al. (2008) used LDO catalysts to convert soybean oil in methyl esters. The LDH precursors (Mg/Al, Fe/Al and Li/Al) were obtained by coprecipi­tation and subsequently calcined at 450 ° C for 2 h. The best catalytic performance was obtained with the oxide derived from [LiAl2(OH)6](CO3)0,5.nH2O, reaching a conversion of 83.1% in 2 h with a methanol:oil molar ratio of 15:1. Under the same conditions, the LDO derived from the Mg/Al precursor yielded only 13.6% of products. The same catalysts were also tested in the methanolysis of glyceryl tributyrate. The reactions were carried out at 65 °C under reflux for 1 h with 20 mmol of glyceryl tributyrate, 600 mmol of methanol and 0.1 g of catalyst. The Li/Al LDO gave yields higher than 98% while the LDOs with Mg/Al and Mg/Fe yielded only 32% and 23.9%, respectively. These results were close to the 37.1% yield that was achieved with MgO under the same conditions. These authors also observed the influence of the calcination temperature on the catalytic performance and concluded that the optimal temperature to obtain the best synthetic LDOs is between 450 °C and 500 °C.

Ngamcharussrivichai et al. (2007) used CaO. ZnO mixed oxides as heterogeneous catalysts for the metha — nolysis of palm kernel oil. A layered hydroxide formed by a mixture of the divalent cations (Ca2+ and Zn2+) was coprecipitated in alkaline media. The mixed hydro­xide was then subjected to calcination between 600 °C and 900 °C for 2—6 h. Ester yields higher than 94% were obtained with this catalyst after 1 h at 60 °C using a methanol:oil molar ratio of 30:1 and a catalyst loading of 10 wt%. Also, the mixed oxide was shown to have a Ca:Zn molar ratio of 0.25.

LDHs containing Zn/Al and Mg/Al with different counterions (nitrate, chlorite and carbonate) and M2+/M3+ ratios were synthesized by Cordeiro et al. (2012) and used as catalysts in the esterification of fatty acids with methanol. The best conversion of 97 wt% was obtained with Zn5AlCl for a reaction that was carried at 140 °C with a methanol:lauric acid molar ratio of 6:1 and 2 wt% of the solid catalyst. However, all the LDHs tested were converted in situ to layered carboxyl — ates, which preserved their catalytic activity even after several consecutive cycles of reuse.

LDH compounds containing Mg2+, Ni2+ and Al3+ were synthesized by Wang and Jehng (2011) and calcined at 500 °C for 10 h to produce heterogeneous LDO catalysts for biodiesel production. The best condi­tion for synthesis involved the use of a methanol:soy — bean oil molar ratio of 21, 0.3 wt% of catalyst, 105 °C and 1200 rpm for 4 h, when an 87% conversion of soy­bean oil to methyl esters was obtained. The observed catalytic efficiency was related to the basicity and Mg content of the Mg/Al/Ni catalysts.

Corma et al. (2005) also applied LDOs in the transes­terification of monoesters with glycerol. LDHs were initially calcined at 450 ° C under nitrogen flux for 8 h to produce LDOs that were immediately rehydrated in N2 atmosphere to avoid the presence of CO2. MgO was also synthesized from magnesium oxalate by calci­nation at 500 °C for 6 h and used as a control. The LDOs containing Li/Al had a performance better than those containing Mg /Al or MgO due to formation of stronger Lewis basic sites, since Li+ ions, which are more electro­positive than magnesium, increase the charge density of the oxygen. Based on this, alumina was impregnated with KF and the resulting material revealed basicity even higher than that of the Li/Al LDO. The catalytic conversion of glycerol to glycerin oleate with KF/ Al2O3 was 98% with monoester selectivity of 69%.

Cordeiro et al. (2008) showed that the LHS zinc hy­droxide nitrate [ZHN, Zn5(OH)8(NO3)2-2H2O] can be used as a heterogeneous catalyst for the esterification of fatty acids and for the transesterification of vegetable oils. For the transesterification reaction carried out at 150 °C for 2 h with 5 wt% of ZHN and a methanol:palm oil molar ratio of 48:1, the resulting ester layer contained

95.7 wt% of methyl esters and the purity of the glycerin layer was as high as 93 wt%. Also, when esterification was carried at 140 °C for 2 h with a methanol:lauric acid molar ratio of 4:1, the final ester layer contained

97.4 wt% of methyl laurate. In addition, these authors were able to demonstrate that ZHN turned into zinc laurate—(C12H23O2)2Zn—during the reaction course and this new layered material was held responsible for the observed catalytic activity.

LDHs containing Zn/Al and Mg/Al with different counterions and M2+/M3+ ratios were used as cat­alysts in the esterification of fatty acids with methanol. All LDHs were synthesized by coprecipitation and high conversion rates were obtained depending on the reaction condition. For instance, a 97 wt% conversion of lauric acid to methyl laurate was obtained using a methanol:fatty acid molar ratio of 6:1 and 2 wt% of cata­lyst at 140 °C for 2 h. However, all LDHs were also converted in situ into layered carboxylates and this new material was responsible for the observed catalytic activity, which was preserved even after several consec­utive cycles of reuse (Cordeiro et al., 2012).

Reinoso et al. (2012) used zinc carboxylates (acetate, laurate, palmitate, stearate and oleate) as catalysts for the methanolysis of soybean oil. Methyl ester conver­sions as high as 98 wt% were obtained for yields in the range of 84 wt% when the reaction was carried out for 2 h with a 30:1 methanol:oil molar ratio and a catalyst loading of 3 wt% in relation to the oil mass.

Jacobson et al. (2008) developed a solid catalyst by immobilizing zinc stearate in silica using the sol—gel method. The resulting solids contained 6 wt% of zinc and a total available surface area of 35 m2/g. These solids were shown to be catalytically active in the meth — anolysis of used frying oil with high acid number (~15%). High yields of 98wt% were obtained at 200 °C for 10 h using a methanol:oil molar ratio of 18:1 and 3 wt% of catalyst in relation to the mass of the start­ing material.

Lisboa et al. (2012) described the synthesis and char­acterization of layered copper(II), manganese(II), lantha — num(III) and nickel(II) laurates as well as their catalytic activity in the methyl and ethyl esterification of lauric acid. Conversions between 80 wt% and 90 wt% were observed for all catalysts when methanol was used for esterification, whereas only manganese laurate gave a reasonable catalytic activity of about 75 wt% with the use of ethanol. In general, the best results were obtained at temperatures around 140 °C. Also, the structure of copper(II) and lanthanum(III) laurates was shown to be preserved after three consecutive reaction cycles.