Zeolites are natural or synthetic materials, classically defined as crystalline aluminosilicate compounds (Cundy and Cox, 2003). Zeolites can be prepared by different synthetic routes with different Si/Al molar ratios, crystal structures, and proton exchange levels. These modifica­tions favor the rationalization of catalytic properties such as pore size, hydrophobicity, strength and distribu­tion of acid sites. When designed in a positive way, all these properties can be interesting and useful for applica­tions in heterogeneous catalysis (Corma et al., 1989).

The catalytic activity of zeolytes can derive from the properties of the cation that is present in its chemical composition. Moreover, the exchange of these cations by protons may generate different degrees of zeolite acidity, which is also an interesting property for various catalytic processes (Csicsery, 1984). In fact, acid zeolites are used in many industrial catalytic applications, mainly in the petrochemical industry.

Another interesting property is the organized and uniform pore distribution and the existence of a cav­ity system of regular molecular dimensions ranging from <1 nm to over 10 nm, depending on the solid material. This feature may bestow rather important catalytic properties to the resulting material such as selectivity.

In general, zeolites and other porous materials of similar composition and textural properties, named zeolite-like materials and zeotypes, have been prepared and used as catalysts in various chemical processes. These solid catalysts present a strong scientific appeal for green chemistry applications since they are consid­ered environmentally benign when one takes into account their chemical composition.

There are several reports regarding the use of zeolite — based catalysts for various chemical reactions. Such uses have been recently reviewed (Martinez and Corma, 2011; Rinaldi and Schuth, 2009). This versatility of uses is not only justified for their great variety in chemical composition but also because of the uniformity of their pore structure.

Due to the presence of pores and channels, catalysts based on zeolites can present size selectivity that is rarely seen in other solids. This selectivity can be observed for reactants, products and transition state in­termediates that are expected to control a given catalytic reaction (Csicsery, 1984). However, for the same reason, these solids not always perform well in catalytic processes that are dependent on one of their main chem­ical properties (e. g. acidity). This occurs mainly for pro­cesses in which the reactants have dimensions exceeding the catalytic channels and pores provided by zeolitic solid catalyst. Therefore, the structure of a zeolite cata­lyst must be idealized in order to have not only the appropriate chemical property but also the textural properties that would offer an array of pores and chan­nels that are adequate for the diffusion of the chemical reactants. The strategy to meet these two goals is a chal­lenge for the catalytic application of these solids.

The preparation of zeotype materials with mesopores (2—50 nm) appears to be the solution to avoid the mass transfer limitations of zeolites in many catalytic pro­cesses. In this sense, many efforts have been made in the scientific community to prepare zeolite-like mesopo — rous materials that are able to address this goal (Tao et al., 2006).

For applications in the esterification of fatty acids or in transesterification of oils and fats, in which large mol­ecules are directly involved in the production of bio­fuels, it is expected that, apart from their high acidity, the surface of the zeolitic and/or zeotype solids should be hydrophobic enough to promote the adsorption of the substrate on the catalyst surface. In this regard, the adsorption of polar molecules may cause deactivation of catalytic sites, such as in the case of water in esterifi­cation reactions (Helwani et al., 2009a). For example, faujasite is a highly hydrophilic zeolite that presents high levels of water adsorption. Hence, this material is barely adequate for esterification because water may

not only inactivate the catalytic sites on the solid surface but also compromise the reaction yields by interfering with the reaction mechanism (Nijhuis et al., 2002).

The MCM-41 molecular sieves have been used as an alternative to zeolite microporous materials. Since its discovery in the early 1990s, these molecular sieves have been used as catalyst in different chemical reac­tions (Beck et al., 1992; Climent et al., 1999), including in the preparation of biofuels (Twaiq et al., 2003).

Similar to the zeolites, these mesoporous compounds also have a regular and ordered distribution of pores (mesopores) across the solid catalyst, allowing their use for the conversion of larger molecules (Carmo et al., 2009). Moreover, the incorporation of metals in the structure of mesoporous solids may lead to acidic materials with special characteristics such as a higher hydrothermal stability. For example, the incorporation of aluminum ions in zeolitic materials can lead to a decrease in the Si/Al ratio and a subsequent increase in the amount of the solid acid sites, since it is well known that the lower the framework Si/Al ratio of the zeolite, the lower the strength of its acid sites, regardless of its higher density (Ma et al., 1996; Corma et al., 1989).

Furthermore, the catalyst hydrophobicity can also be changed by modifying the Si/Al ratio. This leads to an alteration in the ability of the solid to adsorb nonpolar molecules in the catalytic reactions such as esterification and transesterification, as well as in desorption of polar molecules such as water (Luque et al., 2007). In general, high Si/Al ratios (or low aluminum contents) leads to high solid hydrophobicity. Thus, since the Si/Al ratio modifies the acidity and hydrophobicity of the catalyst, its influence on the catalytic properties is subtle, mainly in esterification reactions.

The presence of water is an important factor in the conversion outcome of esterification reactions. The equi­librium constant for ester formation is very low (3.38 for the reaction of acetic acid with ethanol in nonpolar sol­vents) (Corma et al., 1989). So, to obtain high ester yields, the reaction must be displaced toward the products, for example, by the continuous removal of water from the system. Furthermore, the reaction can be shifted toward the products when working with a large excess of reagents.

In order to segregate the water from the reaction envi­ronment, it is necessary to work with high reaction rates and this can be achieved with homogeneous acid cata­lysts such as sulfuric acid. However, for solid catalysts such as the zeolitic materials and zeotypes, the proper balance between strength and density of the acid sites, suitable for a good catalytic performance and water segregation, is often difficult to achieve. The rate of reac­tions catalyzed by zeolite catalysts and other solid mate­rials is usually very low compared to that of sulfuric acid (Ma et al., 1996).

Ajaikumar and Pandurangan (2007) prepared Al — MCM-41 materials with different Si/Al ratios (29, 52, 74 and 110) and used these solids in the esterification of acetic and propionic acids with various alcohols (1-hexanol, 2-ethyl-1-isoamyl alcohol and cyclohexanol). With a small addition of aluminum, which translates into a high Si/Al ratio of 110, these authors observed a higher hydrophobicity and a higher catalyst hydrother­mal stability of the material concerning the amount of water formed during esterification. Furthermore, the use of more hydrophobic solid materials prevented the subsequent hydrolysis of the ester formed. On the other hand, solid catalysts with lower Si/Al ratio promoted lower levels of alcohol dehydration, which can also be favored at high temperature. As a result, the selectivity of the catalytic reaction is improved toward the ester pro­duction as the accumulation of possible by-products (etherified and dehydrated compounds) is decreased. Hence, the hydrophobicity achieved at higher Si/Al ratios was an important factor for the best catalytic performance (catalytic efficiency), whereas the use of low aluminum contents led to more selective catalytic systems.

Corma et al. (1989) reported that strong Bron — sted—Lowry acid sites are required for the catalytic esterification of acetic acid since they are able to proton — ate the acetic acid carbonyl group. Working with proton — ated faujazite zeolite after dealuminization, these authors observed that some dealuminized HY zeolites with Si/Al ratio less than or equal to 15 had better cata­lytic performance. The strong acid sites present in that solid, which correspond to those aluminum vacant sites or sites occupied by one aluminum atom and the respec­tive nearest neighbors, are more active for the catalytic esterification of fatty acids. Zeolites with high Si/Al ratio presented a more hydrophobic surface and this hydrophobicity became the predominant factor for the equilibrium shifting toward the production of alkyl esters. Also, the higher the aluminum content of the zeolite, the higher the observed adsorption effect.

Carmo et al. (2009) also prepared solids based on Al-MCM-41 to investigate the relationship between the high availability of acid sites, introduced by increasing the aluminum content in the mesoporous solid, and the degree of esterification of palmitic acid with meth­anol, ethanol and isopropanol. However, these authors restricted their work to solids with low Si/Al ratios (8, 16 and 32) whose hydrophobicity was much smaller than the solids described in the previous work. This and most of the data already available in the literature pro­pose the utilization of solid catalysts for esterification reactions. In general, these studies have been motivated by the technological challenge of developing a suitable catalytic system to convert vegetable oils or animal fats of high acid number in biodiesel. Hence, by the catalytic esterification of their fatty acid content, these materials would be neutralized and become suitable for transes­terification in alkaline media.

The high aluminum content solid catalysts (Si/Al molar ratio of 8) prepared by Carmo et al. (2009) showed relatively modest palmitic acid conversion values. The highest value achieved in this study was 79 wt% of methyl ester. The authors did not report the effect of hydrophobicity on reaction conversion but only the increased effect of aluminum incorporation in the cata­lytic activity of the resulting solids.

Ma et al. (1996) used different zeolitic solids (zeolite ZSM-5 and three HY zeolites with Si/Al molar ratios of 30, 5.1 and 9.3) to evaluate the relationship between the solid hydrophobicity and its aluminum content with the observed catalytic efficiency in the preparation of ethyl, n-butyl, isopentyl and benzyl acetates; ethyl and n-butyl benzoates and dioctyl phthalates. For all the solid catalysts used in this study, a high selectivity for the expected ester was observed without any forma­tion of ether derivatives. The increase in selectivity with decreasing aluminum content was also reported by Corma et al. (1989).

Insoluble inorganic salts and other inorganic solids based on transition metals can also be used as acid catalysts for transesterification. The application of so­dium molybdate (Na2MoO4) and sodium tungstate (Na2WO4) has been recently reported as efficient cata­lysts for biodiesel production under relatively mild experimental conditions (Nakagaki et al., 2008; Santos et al., 2011). In these studies, soybean oil (0.7 mg/g KOH of acid number), degummed soybean oil contain­ing 180 ppm of phosphatides (1.0 mg/g of KOH), and waste cooking oil (1.5 mg/g KOH) were transesterified with methanol (methanolysis). At 65—80 °C using a 54:1 methanol:oil molar ratio and 5 wt% catalyst for

3— 5 h, both catalysts reached conversions higher than 92 wt% regardless of the feedstock used for methanoly — sis. The catalytic activity of these compounds was attrib­uted to the presence of molybdenum(VI) or tungsten(VI) strong Lewis acid sites that are probably able to polarize the methanol O—H bond leading to intermediate species that possibly have high nucleophilic character.

The heterogeneous nature of the above-mentioned catalysts was investigated through their reuse in several consecutive reaction cycles. Both compounds could be reused for at least three catalytic cycles. However, part of the solid catalysts was lost during the recycling pro­cesses due to their reduced particle size and noticeable adherence to the walls of the reaction vessel. To circum­vent these problems, both molybdenum (Bail et al., 2013) and tungsten (Santos et al., 2011) compounds were het — erogenized in silica obtained by the sol—gel process and used in esterification of stearic and oleic acids. Im­provements were observed in the catalysts’ recovery and reuse and a good catalytic activity was obtained in the first and subsequent recycling stages. Similarly, zir — conia impregnated with tungsten oxide (ZrO2/WO2) was also investigated as an acid catalyst for both esteri­fication and transesterification reactions with methanol (Lopez et al., 2007).