e) Taller “Gymkana alimenticia saludable”

Organic electrosynthesis is an environment-friendly method used to oxidize or reduce organic compounds; in this process, hazardous chemical reagents are substituted by electric current and the overall energy consumption is reduced (Frontana-Uribe et al., 2010; Schäfer Hans et al., 2007). During electrosynthesis, direct electrolysis occurs, in which electrons transfer on the electrode surface. However, the heterogeneous electron transfer between the electrode and substrate can be a kinetic inhibitor for this process. As such, the redox catalyst/mediator can be added into electro-organic synthesis to achieve indirect electrolysis, in which the redox catalyst initiates a pair of reversible redox reactions at the electrode, followed by catalytic chemical reaction between the substrate and redox catalyst (Francke et al., 2014). The basic principle of indirect electrolysis is shown in Scheme 2.4. Therefore, the redox catalyst can prevent over- oxidation or over-reduction of the substrate. In addition, electrode passivation commonly caused by direct electrolysis can be avoided (Francke et al., 2014).

Scheme 2.4: General principle of redox catalyst (Francke et al., 2014).

A redox catalyst is a key component that effectively influences indirect electrosynthesis; hence, appropriate selection of a redox catalyst is essential. Generally, the redox potential of an effective redox catalyst must be lower than the potential of the

Medred Medox Anode n e- Redox reaction Substrat Reactive Intermediate n+ Product Electrolyte Follow-up Reaction (s)

substrate being oxidized or higher than the reduction potential (Costentin et al., 2012; Steckhan, 1986). The redox catalyst should also be inert to all processes apart from the electron transfer at the electrode. Moreover, the oxidized and reduced forms of the catalyst must be highly soluble in the electrolyte to ensure a homogeneous system (Francke et al., 2014). However, homogeneous catalysts are difficult to recover from the reaction medium. The usual recovery method involves precipitation with continuous recovery processes, such as distillation of the reaction mixture, in which high energy is employed (Farnetti et al., 2009). For example, TEMPO is the most frequently studied redox catalyst for alcohol oxidation (Semmelhack et al., 1983). TEMPO is used for oxidation, and the expensive azeotropic distillation can be applied for recovery; separation is favorably achieved by selective adsorption onto hydrophobic resins, such as hydrophobized silica gel or amberlite (Thornton et al., 2002). Furthermore, homogeneous catalysts are relatively more expensive than heterogeneous catalysts (Farnetti et al., 2009). Thus, a cheap heterogeneous catalyst must be developed to overcome the complicated recovery process. The solid acid catalyst Amberlyst-15 can be used as redox catalyst and acidic medium for electrochemical conversion of glycerol.

2.5.1 Amberlyst-15

Amberlyst-15 is a macro-reticular polystyrene compound containing cation exchange resin and strong sulfonic acid group. The molecular structure of Amberlyst-15 is shown in Figure 2.1, and its physical properties are summarized in Table 2.12. With its mild and highly selective properties, Amberlyst-15 is widely used as a heterogeneous catalyst in various chemical organic reactions, such as acylation, alkylation, halogenation, esterification, and transesterification (Frija et al., 2012; Pal et al., 2012). The macro- reticular pore structure of Amberlyst-15 allows liquid or gaseous reactants to penetrate the pores, thereby permitting them to react with hydrogen ions located throughout the beads. In traditional organic synthesis, mineral acids, such as hydrofluoric acid, sulfuric

acid, and paratoluenesulfonic acid, were used as catalysts. However, these homogeneous acids are corrosive, toxic, and difficult to remove at the end of the reaction (Zhou et al., 2012). These limitations can be overcome by using the heterogeneous acid Amberlyst-15. This catalyst is safe to use because of its environmentally benign characteristic. Furthermore, Amberlyst-15 can be readily removed from the reaction medium and can be regenerated or reused several times (Pal et al., 2012).

Figure 2.1: Chemical structure of Amberlyst-15 (Pal et al., 2012).

Table 2.12: Physical properties of Amberlyst-15 (Pal et al., 2012). Physical properties

Ionic form as shipped Hydrogen

Colour Brown-grey

Concentration of active sites ≥ 1.7 eq/L; ≥ 4.7 eq/kg

Moisture holding capacity 52 to 57 % (H+ _form)

Shipping weight 770 g/L

Particle size 0.600 to 0.850 mm

Average pore diameter 300 Å

Total pore volume 0.40 mL/g

2.5.2 Amberlyst-15 as a solid acid catalyst in glycerol conversion

Amberlyst-15 has been extensively studied for chemical catalytic conversion of glycerol. Amberlyst-15 is an effective heterogeneous catalyst for glycerol hydrogenolysis, dehydration, esterification, and etherification. Kusunoki et al. (2005) reported that glycerol can be hydrogenolyzed into 1,2-propanediol under a mild reaction condition (393 K and 4 MPa H2) over metal catalyst (Ru) supported on activated carbon

in the presence of Amberlyst-15, compared with other metal-acid bifunctional catalysts, such as tungstic acid (H2WO4) and liquid acid (H2SO4). Ru/C + Amberlyst showed

higher activity in glycerol hydrogenolysis than Ru/C + H2SO4, indicating that the solid

acid catalyst was more effective for the reaction (Kusunoki et al., 2005).

Amberlyst-15 is also broadly used in glycerol esterification study. Rezayat et al. (2009) conducted continuous esterification of glycerol with acetic acid and supercritical carbon dioxide in the presence of Amberlyst-15. Up to 100% selectivity for triacetin was achieved after 2 h of reaction under the following conditions: 24 molar ratio of acetic acid to glycerol, 200 bar pressure, and 110 °C reaction temperature. Considering that high molar ratio of acetic acid to glycerol is inapplicable to industrial processes because of difficulty in separation, Zhou et al. (2012) attempted to investigate the influence of molar ratio of acetic acid to glycerol and the reaction temperature on the product distribution of glycerol esterification over Amberlyst-15. The optimum yield was achieved at acetic acid to glycerol molar ratio of 9:1 and temperature of 110 °C after 2 h of reaction, with glycerol conversion of 97% and total yield of diacetin and triacetin of 90% (Zhou et al., 2012).

Glycerol etherification with tert-butyl alcohol can be performed over Amberlyst-15. Few studies investigated the effects of the concentration and type of catalyst. Frusteri et al. (2009) reported that glycerol ether formation increased with increasing catalyst

dosage. Klepáčová et al. and Pico et al. studied the performance of different types of Amberlyst on product distribution and glycerol conversion during etherification. In both studies, Amberlyst-15 showed the optimal performance relative to its high acidity and improved textural properties with high apparent surface area (44 m2_g−1_{) and pore}

volume (0.34 cm2_g−1_{). Pico et al. also characterized the catalyst before and after the}

reaction. The results indicated that Amberlyst-15 can be reused, and this catalyst only exhibited slight decrease in acidity and textural properties (Klepáčová et al., 2005; Pico et al., 2013).

Amberlyst-15 has been comprehensively studied in catalytic glycerol conversion. With its acidity and porous structure properties, Amberlyst-15 can serve as redox catalyst in electrochemical conversion of glycerol. Once a redox catalyst has been initially assessed, other parameters that can significantly influence the product selectivity and yield must also be determined; these parameters include type of supporting electrolytes and working electrodes.