This section describes the application of functional­ized polymers as catalysts for esterification and transes­terification reactions to produce biodiesel. Polymeric catalysts consist of functionalized polymeric matrixes or polymeric matrixes that can be used as solid supports for a variety of catalysts, constituting catalytic systems (Coutinho et al., 2004a, b; Guerreiro et al., 2010; Lee and Saka, 2010; Zieba et al., 2010). These materials have long been studied as heterogeneous catalysts in systems that traditionally employ acid or basic homoge­neous catalysts.

Biodiesel production can be carried out in the pres­ence of different types of catalysts. In the specific case of polymeric catalysts belonging to the class of function­alized polymers, their use in biodiesel synthesis has focused on acid catalysts, such as in the case of ion exchange resins (Ma and Hanna, 1999; Guerreiro et al., 2006; Knothe et al., 2006; Soldi et al., 2009; Rezende et al., 2008; Helwani et al., 2009b; Lee and Saka, 2010).

Acid-catalyzed triacylglycerol transesterification is not commercially applied as often as catalysis in basic medium because acid catalysis in homogeneous me­dium is around 4000 times slower than the base- catalyzed reaction. However, acid catalysts can perform esterification and transesterification simultaneously, producing biodiesel directly from oils with high acid number. These oils are not suitable for biodiesel produc­tion via alkaline catalysis, since the FFAs promptly react with the base, generating soaps that make the separation between the ester and glycerin difficult during the washing step (Bondioli et al., 1995; Schuchardt et al., 1998; Ma and Hanna, 1999; Vicente et al., 2004; Lotero et al., 2005; Meher et al., 2006; Rezende et al., 2008; Lee and Saka, 2010).

Several acid catalysts can be used in alcoholysis, espe­cially sulfonic and sulfuric acids (Hayyan et al., 2011). Although these catalysts afford high yields of mono alkyl esters, they require high temperatures and long reaction times to achieve a satisfactory conversion rate. Another disadvantage is that residual acid cata­lysts can contaminate the fuel and attack the metal parts of the engine, corroding it. To avoid this situation, acid catalysts must be completely eliminated from the final product, which demands many purification steps (Canakci and Gerpen, 1999).

Some types of organic polymers and ion exchange resins can be used as polymeric catalysts, behaving as heterogeneous catalysts for esterification and transester­ification reactions. Heterogeneous catalysts such as these reduce the number of biodiesel purification steps, facilitates catalyst reuse, and decreases production costs (Schuchardt et al., 1998; Choudary et al., 2000; Fukuda et al., 2001; Harmer and Sun, 2001; Ramos et al., 2003; Abreu et al., 2004; Suppes et al., 2004; Chouhan and Sarma, 2011).

The pioneering studies of Merrifield on the solid — phase synthesis of polypeptides and subsequent works have shown that polystyrene (PS) is a suitable polymeric support for catalysts and reagents (Merrifield, 1963; Frechet, 1981). Styrene and divinylbenzene (DVB) are among the monomers that are most often employed to prepare solid polymeric matrixes. Polystyrene — co-divinylbenzene (PS-DVB), an insoluble copolymer, results from styrene polymerization in the presence of varied amounts of DVB. The characteristics of this copolymer depend on the quantity of DVB present in the material (Kapura and Gates, 1973; Xia et al., 2012).

In many cases, the success of a heterogeneous catalyst relies on the features of the polymeric material. Bergbreiter (2002) proposed that some physicochemical properties should be considered when choosing the catalyst support, including the catalytic activity, surface area, porosity, and thermal and mechanical stability of the material in the conditions of the catalyzed reaction (Bergbreiter, 2002; Chouhan and Sarma, 2011).

In the field of polymer chemistry, the term resin is indistinctly employed to describe polymers with and without cross-links (Akelah and Sherrington, 1981; Sharma, 1995). Alternatively, in heterogeneous catalysis, the term resin refers to species consisting of long polymeric chains interconnected via cross-links, the so-called polymeric matrix. Polymeric matrixes are
tridimensional, insoluble, and porous; their ability to ex­change ions arises from the introduction of adequate functional groups into the polymeric support (Kunin et al., 1962; Akelah and Sherrington, 1981; Frechet, 1981; Harmer and Sun, 2001; Hart et al., 2002).

Polymeric matrix functionalization can be achieved in two ways: (1) monomers containing the desired func­tional groups (or precursors of this functional group) can be directly polymerized; (2) the polymeric support can be prepared first, and the functional group is intro­duced by chemical modification of the polymeric sup­port (Molinari et al., 1979; Kucera and Jancar, 1998; Harmer and Sun, 2001).

Coutinho and Rezende (2001) and Coutinho et al. (2004a) showed that supported species can be prepared by chemically modifying the copolymer base (polymeric support). These authors reported the sulfonation of a sup­port consisting of PS reticulated with DVB (Figure 16.7). The aromatic rings on the insoluble PS-DVB copolymer react with concentrated sulfuric acid in the presence of

1,2- dichloroethane; the latter compound expands the polymeric matrix and allows sulfonation of the internal surface as well. Most of the functional groups introduced into polymeric matrices concentrate inside the resin beads (Coutinho and Rezende, 2001).

Cationic resins can be used as an option for catalytic reactions involving mineral or sulfonic acids. In the presence of water, the cationic groups on the polymer display different acidity constants, as in the case of com­pounds with low molecular mass.

The catalytic performance of an ion exchange resin is associated with the concentration of functional groups and the physicochemical properties of the support. Therefore, compared with homogeneous catalysts, different factors affect the activity of resins.

The use of ion exchange resins as catalysts has many advantages: (1) despite being equivalent to strong

——— CH2-CH-CH2-CH-CH2——————-

——— CH2—CH—CH2-CH—CH2————

FIGURE 16.7 Sulfonation of a polymeric matrix consisting of polystyrene and divinylbenzene.

FIGURE 16.8 PS sulfonation with acetyl sulfate.

mineral acids, resins are less oxidizing and corrosive, since most of the catalytic sites are located inside the beads—therefore, they do not pose any hazards to the operator and are easy to store; (2) resins behave as selec­tive catalysts and enable reaction control; (3) catalysts with high purity are recovered at the end of the reaction by simple filtration; (4) resins do not require neutraliza­tion before being separated from the reaction medium, a step that usually reduces product yield; (5) resins elimi­nate the steps and equipment necessary to separate the catalyst and purify the product, simplifying continuous or batch procedures based on ion exchange resins; and

(6) if the resins undergo deactivation due to contamina­tion or prolonged use, they can be reactivated via a sim­ple procedure that does not release hazardous gases into the atmosphere (Saha and Sharma, 1996; Coutinho and Rezende, 2001; Harmer and Sun, 2001; Marquardt and Eifler-Lima, 2001; Mitsutani, 2002; Coutinho et al., 2003, 2004a, b; Kiss et al., 2006).

The main drawback of ion exchange resins is that their maximum operation temperature is relatively low. Literature suggests that they should be used below 125 °C to ensure long catalyst lifetime (John and Israel — stam, 1960; Akelah and Sherrington, 1981; Gimenez et al., 1987; Coutinho and Rezende, 2001; Rezende et al., 2008).

Aromatic compounds are easy to functionalize, espe­cially if they contain acid groups like sulfonic acids. The sulfonation of organic compounds containing benzene rings, including polymers, has been extensively re­ported (Ma and Hanna, 1999; Coutinho and Rezende, 2001; Coutinho et al., 2003, 2004a, b, 2006; Rezende et al., 2008; Soldi et al., 2009). Figure 16.8 represents the sulfonation PS (Soldi et al., 2009).

Sulfonation significantly modifies the physical prop­erties of linear PS, especially the polarity. Hence, sulfo — nated PS should remain insoluble during biodiesel production. Soldi et al. (2009) studied methods to sulfo­nate linear PS and applied the resulting sulfonated material as heterogeneous polymeric catalyst to produce soybean methyl esters. Raw materials with different moisture degrees and the effect of different variables on the conversion rate have been investigated; biodiesel production from soybean oil and beef tallow led to significantly improved yields.

Recently, much interest has been taken in utilizing low-cost plant oil and fat containing a large amount of

FFAs. However, oils with high FFA content are difficult to transesterify using the commercially available alka­line catalyst (Zhang et al., 2003; Tesser et al., 2005; Marchetti and Errazu, 2008; Sharma et al., 2008; Liu et al., 2009; Tesser et al., 2010; Chouhan and Sarma, 2011; Shahid and Jamal, 2011; Borges and Diaz, 2012). Canakci and Van Gerpen (1999, 2001) found the transes­terification would not occur if the FFA content in the oil was beyond 3%. According to the research paper by Kouzu et al. (2011), the promising approach is to esterify FFA into FAMEs with the help of the solid acid catalyst, and there were some research papers studying utili­zation of several types of heterogeneous catalysts including sulfonated cation exchange resin (Russnueldt and Hoelderich, 2009; Tesser et al., 2010; Kouzu et al.,

2011) . With respect to utilization of the sulfonated resin for the preesterification of FFA, some researchers focused their attention on the macroreticular type but the use of two types of resins (macroreticular and gelular types) were also studied by other authors (Ramadhas et al., 2005; Soldi et al., 2009; Lam et al., 2010; Melero et al., 2010; Kouzu et al., 2011; Semwal et al., 2011; Li et al., 2012; Xia et al., 2012; Zhang et al., 2012a, b).

Feng et al. (2011) investigated the continuous esterifi­cation of FFAs from acidified oil with methanol by cation exchange resin in a fixed bed reactor to prepare biodiesel and the operational stability of continuous esterification by resin in the fixed bed reactor was also conducted (McNeff et al., 2008; Shibasaki-Kitagawa et al., 2010; Feng et al., 2011; Cheng et al., 2012).

According to Feng et al. (2010), from the viewpoint of cost savings, the use of cation exchange resins in hetero­geneous catalytic processes may be advantageous over enzymes and supercritical methanol (Feng et al., 2010). These resins are composed of copolymers of DVB and styrene containing sulfonic acid groups attached to ben­zene rings and these are the active sites for esterification and transesterification (Marchetti and Errazu, 2008; Rezende et al., 2008; Russnueldt and Hoelderich, 2009; Feng et al., 2010; Kouzu et al., 2011). However, other sul­fonated polymeric backbones such as Amberlyst®, Dow — ex® and Nafion®, a perfluorinated ion exchange resin, all of them having a very strong Bransted acid character, have also been used in these type of reactions (Ozbay et al., 2008; Talukder et al., 2009; Feng et al., 2010; Park et al., 2010; Galia et al., 2011; Yin et al., 2012; Zhang et al., 2012a). In general, cation exchange resins are

preferable for esterification (Gimenez et al., 1987; Chen et al., 1999; Coutinho et al., 2004b; Coutinho et al., 2006; Grob and Hasse, 2006), while anionic resins may be applied for transesterification of oils and fats (Shiba — saki-Kitakawa et al., 2007; Ren et al., 2012).

CONCLUDING REMARKS

Truly heterogeneous catalytic processes are attractive for many practical applications due to their recyclability, structural stability, high selectivity and good catalytic performance. However, all these properties are hardly achievable in a single catalytic system. In most cases, leachable catalytic species migrate to the reaction envi­ronment, causing a partial contamination of the final product as well as a loss in catalytic activity when the solids are applied in several consecutive reaction cycles. Moreover, in many situations found in the literature, the heterogeneity of catalytic systems is not approached with proper analytical methods, resulting in wrong conclusions and/or classification of the proposed solid catalyst. These usually arise from poor data on catalyst recovery and reuse, poor characterization of the catalyst structure and high leaching levels of catalytic species. Also, in many cases, no attempt is made to fully characterize these properties and solids are classified as heterogeneous catalysts just because they are partially filterable after reaction completion. One of such flaws was nicely demonstrated by Silva et al. (2013) using bismuth-containing mixed oxides. Apart from these ob­servations, the lack of suitable reaction controls such as in the case of TC in esterification reactions reveal unrealistic catalytic performance in reactions that are known to be autocatalytic under appropriate experi­mental conditions. Nevertheless, a number of rather attractive heterogeneous catalytic systems have been discovered so far for biodiesel applications, probably due to the wide scope of catalytic properties that are influential in both esterification and transesterification. However, many of these will never be able to reach in­dustrial applications because their benchmarking was never strong enough to support further investments at large scale.