In the article Advances in Heterogeneous Catalysis for Biodiesel Synthesis, Top Catal. 53, 721-736, 2010, Yan et al. describe both the limitations of homogeneous or first generation catalysts which act in the same reaction phase as the advantages of heterogeneous or second generation catalysts for the synthesis of biodiesel.
The limitations of the homogenous catalysts or so called “first generation” are:
a) their use is normally restricted to batch processing, and are primarily highly corrosive acids or bases.
b) the stages of the homogeneous biodiesel production process require long times and the processing is costly because of needed the steps such as: oil pretreatment, catalytic transesterification, fatty acid methyl ester (FAME) separation from glycerin, neutralization of the residues from the homogeneous catalyst, methanol distillation, washing of the FAME phase with water, and vacuum drying of the desired products.c) it is impossible to reuse the homogeneous catalyst in reaction in new cycles because of its loss in the waste streams.d) separation of the products requires a post-treatment with large volumes of water to neutralize the catalyst residues which generates waste water that must be treated before its disposal into the environment; ande) the homogeneous catalyst is sensitive to free fatty acids (FFA) and water present in vegetable oils.
The advantages of heterogeneous catalysts or so-called “second-generation” are:
a) the catalyst is not lost during the reaction and can be recovered from the reaction medium and reused in several reaction cycles;
b) the performance in fixed bed reactors for continuous flow processes;
c) the post-treatment of product is not required, greatly reducing the ecological impact by avoiding the liquid wastes generated during the purification of the products; and
d) the microcrystalline structure in the catalyst surface is stable, which extends its useful life
Some investigations have been focused on the study of transesterification basic catalysts in low-acid vegetable oils such as Jatropha oil and sunflower with high yields of biodiesel at non-severe reaction conditions. On the other hand, acidic catalysts are strongly recommended for use in the esterification and/or transesterification of vegetable oils with a higher degree of acidity and of animal fats, together with alkaline catalysts under conditions of moderate to high severity. Acid catalysts may be defined as oxygen carbonyl activators of the ester to increase its reactivity to the attack of the alcohol, typically methanol. These acid catalysts can be classified in those with Brönsted acid sites because they have carbonyl oxygen interactions with catalytic proton (H+) sites and those with Lewis acid sites because of interactions of carbonyl oxygen with cationic (M+) sites in the catalyst.
FIG. 1 describes the above explained, emphasizing experimental evidence showing the type of acidity on the surface of the catalyst by pyridine adsorption measured on an infrared spectrum which describes the interaction of a Lewis base on an acid surface. The interaction of the acidic sites on the surface with the pyridine molecule generates different bands. The characteristics for Brönsted acid sites appear at 1,545 cm−1 and the characteristics for Lewis acid sites appear at 1,445 cm+1 and the intermediate band at 1,490 cm−1 for the two interactions:                Section (a) of FIG. 1 shows how the acid catalyst with Brönsted (H+) sites acts on the triglyceride molecule by activating it for the transesterification reaction;        in section (b) of FIG. 1 illustrates how the acid catalyst with Lewis (M+) sites acts on the triglyceride molecule by activating it for the transesterification reaction;        a catalyst having both types of sites is presented in section (c) of FIG. 1.        
These interactions are also present in diglycerides and monoglycerides by activating themselves for transesterfication reactions with an alcohol, such as methanol. Thus, we can say that the catalysts of acidic nature that activate either the triglyceride, diglyceride or monoglyceride molecule, act as the promoter of the C═O ester.
Catalysts with Brönsted acidity, such as heteropolyacids (HPAs), have shown high catalytic activity, yield and conversion, in combination with monovalent cations, such as Cs+. HPAs based on Nb and W supported on W—Nb, tungstated zirconia, tantalum and silver pentoxide have shown greater resistance to catalyst leaching, as described in the following bibliographic citations.
Katada N. et al., in Applied Catalysis A General, (2009), 363 (164: 168) studied solid acid catalysts derived from heteropolyacids (HPAs) with W and Nb. They found that under a calcination temperature of 773° K, W and Nb-based HPAs are supported on WO3-Niobia (WO3—Nb2O5) and are transformed into insoluble NPNbW/W—Nb formulations in the reaction mixture with high catalytic activity for the transesterification of triolein and ethanol to ethyloleate. The reaction rate is increased when methanol is used instead of ethanol. Due to the potential and catalytic stability during at least 4 days of reaction of these catalysts, the authors recommend carrying out the reaction in fixed bed and continuous flow reaction systems.
Shi et al., in Chemical Engineering & Technology (2012), 35 (2), 347-352 disclose that heteropolyacids (HPAs) were used as triglyceride transesterification catalysts, arguing both Brönsted and Lewis acidity properties. HPAs that are strong Brönsted acids, depending on their composition and the reaction medium, possess good thermal stability, high acidity and high oxidizing capacity and are water tolerant. Among HPAs, 12-tungstophosphoric acid (H3PW12O40) is chosen because of its high activity since it shows a Keggin structure which is composed of a coordinated tetrahedral heteroatom of oxygen (PO4), surrounded by 12 additions of atoms sharing coordinated octahedral oxygen atoms (WO6) according to Oliveira C F et al. in Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia. Appl Catal A-Gen 372: 153-161 (2010).
Heterogeneous catalysts of acidic nature such as those based on sulfated zirconia have been reported, in which it emphasized the fact that the carbonyl oxygen is activating by acid sites of Brönsted nature according to Rattanaphra et al., “Simultaneous Conversion of Triglyceride/Free Fatty Acid Mixtures into Biodiesel Using Sulfated Zirconia”, Top Catal. 53: 773-782, 2010.
On the other hand, the esterification of palmitic acid with HPA catalyst was also carried out by Caetano et al. “Esterification of free fatty acids with methanol using heteropolyacids immobilized on silica. Catal Commun 9:1996-1999 (2008), using heterogeneous catalysts: Tungstophosphoric acid (PW), molybdophosphoric acid (PMo), and immobilized tungstosilicic acid (SiW) on silica by the sol-gel technique; from these prototypes the PW proved to be the best catalyst so it was studied with different concentrations of silica, obtaining 100% conversion of palmitic acid with a concentration of 0.042 g PW/g silica.
Alternatively, in the patent document reported by Tian et al., CN 103801282, “Solid base catalyst, and preparation method and application thereof”, the use of an aluminum-Zn spinel catalyst (ZnAlxO1+1.5x in which x=1.5-2.5) doped with La is described. The basic solid catalyst is used in the transesterification reaction of fatty acid esters with an alcohol to produce biodiesel; it is highly active and stable during the utilization, the active components are not lost.
In the patent document CN 103,752,297 “Zirconium-oxide catalyst for producing biodiesel, as well as a preparation method and application of zirconium-oxide catalyst” (2014) a zirconium oxide catalyst is claimed to produce biodiesel in a tubular reactor at a reaction temperature of 250-300° C., reaction pressure of 7 to 14 MPa and volume ratio of alcohol-oil of 0.5:1 to 7:1. The catalyst is characterized by containing zirconium oxide of 80-95 weight %, aluminum oxide of 2-18 weight %, 1 to 17% titanium dioxide, 5 to 25% sodium bicarbonate and 10 to 50% of sodium chloride and in the patent document CN 103,706,384 “Preparation method of bio-diesel catalyst” (2014), there is provided a method of preparing a catalyst for the production of biodiesel in a continuous flow process in which the composition is PO43−/ZrO2 doped with rare earth metals such as La, Ce, Pr, Nd, etc.
The French Institute of Petroleum has developed an industrial technology called Esterfip-H™, which refers to a continuous process of transesterification where the reaction is promoted by a heterogeneous catalyst, which is a zinc aluminate (ZnAl2O4) spinel, which promotes the transesterification reaction, without loss of catalyst. The reaction is carried out at an operating temperature of 180-220° C. and pressure of 40-60 bar. The yields obtained are greater than 98%, with an excess of methanol. However, the raw material must have a free fatty acid content of less than 0.25% and a water content of less than 1,000 ppm. (Juan A. Melero et al., Critical Review, Heterogeneous acid catalysts for biodiesel production: current status and future challenges, Green Chem., 2009, 11, 1285-1308.)
In the patent document U.S. Pat. No. 5,908,946 (Stem R. et al., Inst. Français du Petrole, Process for the production of esters from vegetable oils or animal oils alcohols, 1999), for the production of esters of linear monocarboxylic acids with oils of 6 to 26 vegetable carbon atoms or oils of animal origin are reacted with monoalcohols having a low molecular weight, for example 1 to 5 carbon atoms, in the presence of a catalyst selected from zinc oxide, mixtures of zinc oxide and aluminum oxide, and the zinc aluminates corresponding to the formula: ZnAl2O4, x ZnO, and Al2O3 (with x and y being 0-2) and with a spinel-like structure, allowing the direct production in one or more steps, of an ester which can be used as fuel and pure glycerin. In order to process vegetable oil, severe operating conditions, temperatures of 170-250° C., pressures lower than 100 bar, with excess of the stoichiometric alcohol are considered, obtaining conversions of 80-85%, in the case of acid oil charges conditions are used operating conditions of 180-220° C., with pressures less than 1 bar.
Considering metal phosphates as metal catalysts for transesterification of biodiesel, Xie et al., in Bioresource Technology (2012), 119, 60-65, describe acid catalysts for transesterification of triglyceride esters based on 30 wt % WO3 supported in AlPO4 which were tested in batch reaction systems at 180° C. for 5 h and a methanol/oil ratio of 30:1 at a dose of 5% by weight catalyst.
Other transesterification catalysis systems consist of calcium phosphates from animal bone pyrolysis, which generates hydroxyapatite at 800° C. as described by Obadiah et al., in Bioresource Technology (2012), 116, 512-516.
Similarly, in the patent document CN 103,484,258 “Method for preparing biodiesel by using nano hydroxyapatite to catalyze triglyceride” (2014), a method is described for preparing biodiesel in the presence of a nanohydroxyapatite catalyst using from 0.5 to 3 weight % and operating at 800 to 300° C. for 2 to 10 h.
Yin et al. describe in Fuel (2012), 93, 284-287 the catalytic activity of K3PO4 at conditions of 220° C., a methanol-oil ratio of 24:1 and 1% of the catalyst resulting in a conversion of 95.6%.
Sodium phosphate has also been used as a transesterification catalyst for triglyceride esters in biodiesel production according to De Filippis et al., in Energy & Fuels (2005), 19 (6), 2225-2228
Hitherto, no lithium and aluminum phosphates and sulfates have been considered, in addition to their combinations with metallic cations, such as magnesium, titanium, zinc, zirconium and gallium, as heterogeneous phase catalysts with a Lewis nature both in reaction systems by batch and in continuous flow reaction systems. Neither has the synthesis of the same catalyst from hydrolysis products of lithium aluminum hydride and these hydrolysis products been considered as the source of lithium and aluminum for the formation of phosphates and sulfates from the reaction with the corresponding acid (Phosphoric acid or sulfuric acid), and other sources of metals such as magnesium, titanium, zinc, zirconium and gallium, from metal acetates.
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