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Sustain. Chem. 2021, 2 292 has a low equilibrium constant; therefore, it is thermodynamically limited. To obtain high conversion yields, it is necessary to shift the reaction in favor of product formation by either removing water or by adding acetone in excess [48]. 4.1. History The first solketal synthesis dates back to 1895 when it was obtained by the reaction of glycerol with acetone under acidic conditions (hydrogen chloride catalyst) in a batch reactor [49]. Years after that, the process was repeated varying the reaction medium (addition of anhydrous sodium sulphate) and the operating conditions, using different equipment, solvents (petroleum ether), and catalysts (p-toluenesulfonic acid-pTSA) [48,50]. At that time, the reaction with pTSA resulted in high conversion (87–90%); however, acetone was used in great excess and a long reaction time was necessary, which made the production in large scale unviable. New routes for solketal synthesis were deeply investigated only when the massive glycerol generated by the biofuel industry became a problem and society turned its attention to the Green Chemistry concepts [48]. In the following years, new approaches were proposed to obtain higher productivity in less time. Initially, the reaction was mainly made with homogeneous catalysts, as the pTSA was also used by Newman and Renoll back in 1945, and strong acids such as sulfuric, hydrochloric, hydrofluoric, and phosphoric acids [51]. Some authors registered good solketal yields (88 and 90% with pTSA as the catalyst). Despite that, the long reaction times were still a problem that rendered the process discouraging; moreover, acetone in excess was still required, and often a strategy to remove water from the reaction medium was applied, as the use of desiccants, entrainers, or molecular sieves [21,50,52]. Furthermore, the problems related to the use of homogenous catalysts are well known: difficult separation from the products, equipment corrosion, and serious concerns about effluent disposal [8,53]. The use of heterogeneous catalysts, as was later proposed, conforms to some of the Green Chemistry principles: less hazardous chemical synthesis, safer solvents, and auxiliary, inherently safer chemistry to accident prevention [2]. There has been an extensive study of alternative heterogeneous catalysts, with empha- sis on zeolites, acidic resins, and montmorillonite [8,10,48,54]. The performance of several zeolites under different conditions revealed that beta zeolites can achieve the highest conversion rates [10,55–58]. The resins tested, mainly Amberlyst–15, 35, and 36, showed satisfactory activity as a catalyst in the conversion of glycerol to solketal. Moreover, they can be used as adsorbent, which may help to overcome the thermodynamic limitation of glycerol conversion when used in multifunctional reactors [10,55,58,59]. Since the hybrid solid must perform two tasks, catalysis and adsorption, besides considering the catalytic activity in the ketalization of glycerol with acetone, the adsorbent must have a strong affinity to water. Interesting results about the use of ion exchange resins in the synthesis of oxygenated compounds have been reported in several studies, especially because of their affinity with water [60]. Da Silva et al. compared the performance of the three types of heterogeneous catalysts mentioned above. The study was conducted in a batch reactor at 373 K and the authors stated that Amberlyst–15 achieved the highest conversion (95%) in only 15 min of reaction. Zeolite beta and Montmorillonite K–10 achieved conversions up to 90%, but only after 40 min. Other zeolites tested, namely ZSM5 and USY, presented poor catalytic activity due to the small pore diameter of the first that impair the reaction from occurring inside the pores and to the hydrophilic character of the second, responsible for retaining the water formed as by-product inside the pores and deactivating the acid sites [58]. From the three types of catalysts, despite the low cost of Montmorillonite K–10, zeolites and ion exchange resins have been preferred due to the higher conversion rates with the same catalyst load [58,59,61]. Nanda et al. stated that the catalyst acidity and acetone to glycerol molar ratio have a great influence on the reaction (kinetics and conversion), but the physical characteristics of the resin, such as pore volume and particle size, have a negligible effect on catalystsPDF Image | Continuous Valorization of Glycerol into Solketal
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