Many heterogeneous alkaline catalysts are available but the most frequently used are alkali metal, alkaline earth and metal salts. An overview is given in Table 5.3 (Bacovsky et al., 2007).

Heterogeneous basic catalysts can be classified as Brpnsted or Lewis catalyst. As in the case of homogeneous Brpnsted basic catalyst such as basic zeolites, the formed catalytically active compound is a homogeneous alkoxide (Fig. 5.11) (Lotero et al., 2006).

In the case of heterogeneous Brpnsted basic catalysts (e. g. resins with quaternary ammonium functions, QN+OH), the positive organic ammonium groups being bonded to the support surface electronically retain the catalytic anion on the solid surface.

Table 5.3 Overview on heterogeneous catalysts (Bacovsky et al., 2007)

Catalyst type examples

0“Na + CH3OH——————— ► O-H + CH30“Na+

5.11 Reaction mechanism for heterogeneous catalysis (I).

The reaction occurs between the methanol adsorbed on the cation and the ester from the liquid (Fig. 5.12).

CTN+OH” + CH3OH —————— ► 0-N+0CH3- + H20

5.12 Reaction mechanism for heterogeneous catalysis (II).

The formation of alkoxide functions is the fundamental step for heterogeneous Lewis basic catalyzed reactions. In the case of MgO, the reaction occurs between the methanol molecules adsorbed on magnesium oxide free basic sites and the esters functions in the triglycerides in the liquid phase (Fig. 5.13).

In the review by Di Serio et al. (2008) many applications of heterogeneous alkaline catalyst have been described.

0“R H+

> —— Mg—— 6——-

5.13 Reaction mechanism for heterogeneous catalysis (III).

In comparison with homogeneous catalysts, in order to obtain similar conversion rates, more severe reaction conditions have to be used:

• Temperatures up to 300°C under supercritical conditions are a pre-requisite to achieve conversions higher than 90%.

• High molar rates methanol:oil have to be utilized: 15-40:1.

• Longer reaction times (except in methanol supercritical conditions or microwave irradiation).

• Higher amounts of catalyst.

• Leaching of the catalyst into the biodiesel.

Good results using alkaline-earth metal hydrides, oxides and alkoxides have been reported by Gryglewicz (1999) and Demirbas (2007). The order of reactivity Ca(OH)2 < CaO < Ca(OCH3)2 is in agreement with the Lewis theory stating that methoxides of alkaline-earth metals are stronger bases than the corresponding oxides and hydroxides. Good biodiesel yields were also obtained in the transesterification of soybean oil using ZnO, loaded Sr(NO3)2 followed by
calcination. The active catalyst is SrO. When the reaction is carried out at reflux, five per cent catalyst and 12:1 mol ratio of methanol:oil, a conversion of 95% can be reached (Lopez Granados et al, 2007).

The preparation of new materials obtained from the co-precipitation of aluminum, tin and zinc oxides and their use as catalytic systems for the alcoholysis of vegetable oils have been reported by Macedo et al., 2006. These (Al2O3)X (SnO)Y(ZnO)Z type of metal-oxides were found to be active for the alcoholysis of soybean oil, using several alcohols, including branched ones. Best results were achieved using methanol, with conversion yields up to 80% in 4 hours. It was also possible to recycle the catalysts without apparent loss of activity.

Sodium silicates can be used at 60-120°C but the use of microwave energy greatly increases the conversion (Portnoff et al., 2006).

Good results have also been obtained by MgO/Al2O3 hydrotalcites and industrial applications could be possible if the reaction was carried out at 180°C and 12:1 methanol:oil molar ratio (92% yield) (Leclercq et al., 2001; Di Serio et al., 2006).

Waste oil was converted in good yields using Mg-Al layered double hydroxide catalysts in 80-160°C temperature range with up to 48:1 molar methanol:oil ratio and high amounts of catalyst (up to 12%) (Brito et al., 2009).

In addition calcined Li-Al layered double hydroxides (Shumaker 2007), sodium zeolites, titanium containing zeolites (Xie et al., 2007), anion resins (Shibasaki — Kitakawa et al., 2006) and polystyrene supported guanidine and biguanidines (Gelbard and Vielfaure-Joly, 2001) have shown promising results.

The use of strong alkaline ion exchange resins is limited due to the loss of stability at temperatures higher than 40°C and the neutralization of the catalyst by the FFA present in the feedstock.

In addition, the glycerol formed is absorbed in the polymeric matrix causing deactivation of the active sites.

Very recently, an efficient laboratory procedure has been developed using CaO after appropriate treatment which allows reaching high conversions of triacylglyceride (TAG) into FAME in a one-stage operation, meeting the requirements of the EN 14214 (Kouzu et al., 2008; Lengyel et al., 2009). The catalyst was activated by drying for 24 hours at 105°C. Using 6:1-12:1 molar ratio of methanol:oil, reflux temperature and eight per cent catalyst, conversion rates of 99% were obtained. However, organosols are formed due to the presence of calcium soaps, leading to a yield of 70%.

An industrial applied heterogeneous catalysis process (Fig. 5.14) has been selected by Diester Industrie using Axens biodiesel technology, Esterfip-H™ for a new plant in Sete (France) with a capacity of 160 000 tonnes per year, followed by a plant in Sweden in 2007. The catalyst consists of a mixed oxide of Zn and Al coated on y-alumina, which promotes the transesterification without catalyst loss. The reaction is performed at higher temperatures and pressures compared to those of homogeneously catalyzed processes, also using an excess of methanol. This

methanol excess is removed by vaporization and reused in the reaction together with the fresh methanol. The conversion is reached in two successive stages and separation of the glycerol in order to shift the equilibrium to methanolysis.

The catalyst section includes two fixed bed reactors. Excess of methanol is removed after each reactor by partial evaporation and the esters and glycerol are separated in as settler. The residual methanol in the glycerol is evaporated. Biodiesel purification consists of methanol vaporization under vacuum and adsorption of the soluble glycerol (Bournay 2005).

The advantage of the process is a very high biodiesel quality, salt-free glycerol, no soaps formation and no handling of hazardous chemicals. This process can be considered as green technology.

On a semi-industrial scale a new type of heterogeneous catalyst has been introduced by Catalin using nanoparticles. The catalyst preparation involves the utilization of organotrialkoxysilanes with various anionic, hydrophobic or hydrophilic functional groups that could provide different noncovalent interactions, for example electrostatic attractions, hydrophobic interactions etc., with cationic cetyltrimethylammonium bromide (CTAB) surfactant micelles in a base catalyzed condensation reaction of tetraethoxysilane.

Catalin T300 catalyst differs from the most solid catalyst that requires a fixed bed and high temperature and pressure to operate. The T300 catalyst can be used in existing plants with minimal modification as it reacts at common operational temperatures and pressure. The reactor consists of a reactive vessel within a plate with a mesh. The catalyst is stocked on top of the mesh and the oil flows through. The T300 catalyst is a ‘drop-in catalyst’ that can be used as a direct replacement for the commonly used sodium methoxide. Therefore there is no need for a fixed bed and the catalyst in the form of a granular powder can be directly mixed with oil.

5.15 Catalin process flow diagram (Catalin, 2009).

A filter system is used to keep the catalyst in the reactor and there is no need for water washing. The Catalin process flow diagram is depicted in Fig. 5.15.