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Sustain. Chem. 2021, 2 300 94% glycerol conversion and 98% selectivity for solketal. These results are attributed to its acidity and to its pseudo liquid behavior: the polar reactants absorb into the polyanion space of the catalyst, contact the active sites, react, and then the products desorb [89]. Recently, studies comprise a variety of ion exchange resins, as this type of catalyst is particularly interesting for glycerol ketalization because of its affinity with water. From Table 1, one can conclude that the mass transfer hinders the use of this type of hybrid solid, once acetone in large excess is necessary to attain conversions as high as 90% when selected a solvent-free environment [70,71]. To overcome this issue, Moreira et al. opted for using a solvent to enhance the reactants miscibility and the result was promising, achieving conversions almost 20% higher than the industrial process used nowadays (i.e., 52.55% with pTSA), accordingly to Rossa et al. [10,76]. One of the main advantages that renders clays a relevant alternative for industrial application is their low cost. The latest studies focus on modifying its textural and acid properties to enhance its conversion and selectivity. Timofeeva et al. studied the modifi- cation of montmorillonite with nitric acid (HNO3) and Amri et al. the modification of a natural clay with hydrochloric acid (HCl). Both systems used solvents (acetonitrile and isopropanol, respectively) in the reaction medium and mild conditions (273 K and ambient pressure). The first study presents better conversion values, 94%, with a lower catalyst loading, probably due to its higher density of strong acidic sites, together with the fact that it was more stable in terms of catalyst deactivation [79,80]. The mild conditions used, the results attained, and the catalyst cost makes the research on modified clays promising; however, further studies on the reaction kinetics must be performed. The metal oxides’ catalysis is similar to the zeolite’s; therefore, the surface area, the size of the pores, and the strength of the acidic sites play major roles in the conversion and selectivity to solketal [83]. Zhang et al. investigated the effect of introducing a heteroatom in the structure of a metal oxide and concluded that Ni promoted the highest solketal yield (75.44%), a fact that he attributed to the larger surface area of M-NiAlPO4, which increased the number of surface-exposed Ni atoms responsible for the stronger acidity of the catalyst [81]. Gadamsetti et al. and Rodrigues et al. found that molybdenum and niobium have high activity to glycerol conversion to solketal in similar systems, but the first presented better results because, besides the total conversion of glycerol, the reaction was performed under milder conditions [82,83]. There are numerous similar studies in the literature, many of them with low selectivity to solketal which were not included as inputs to Table 1 [100,101]. A noteworthy new green catalyst was proposed by Gonçalves et al., a carbon-based solid prevenient from biodiesel waste acidified by sulfuric acid (H2SO4). The catalysts were prepared with different amounts of sulfuric acid and the one that performed better (Xgly = 82%) was the GC–1:2 (i.e., 1:2 mol carbon–based material:acid), besides being more stable at higher temperatures (373 K). Doping the catalyst with higher sulfur concentrations does not make difference in its structural properties [86]. Similarly, Ballotin et al. produced a sulfonated catalyst based on bio-oil with amphiphilic characteristics: hydrophobic carbon matrix with hydrophilic oxygen and sulfonic surface groups. The system was kept under mild conditions, without solvent, and the catalyst was in emulsion. The conversion was up to 98% with a large excess of acetone [88]. The sulfonation strategy was also used by Vannucci et al.; however, the matrix was zirconium oxide. Despite achieving good conversion yields of 80%, the previous materials are more appealing environmentally because they are obtained from renewable sources and are more resistant to recycle [91]. The growing number of studies investigating alternative catalysts evidence the poten- tial and the relevance of solketal on the market, but it also reveals how far the industrial process applied nowadays is from the optimum. Many research works disclose promising catalysts that attain high glycerol conversions and that can be recycled; however, almost none assess the viability of using them on larger than laboratory scale. Furthermore, only a few assess the impacts of the impurities of crude glycerol over the catalysts or suggest pre-treatments for this reactant. There is also a lack of thermodynamic and kinetic studyPDF Image | Continuous Valorization of Glycerol into Solketal
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