## 4 Solketal Synthesis
### Catalyst Study
#### 4.2.1 Recent Advances on Catalysts
the modification of a natural clay with hydrochloric acid (HCI). 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-NiAlPO\({}_{4}\), 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 Goncalves et al., a carbon-based solid prevenient from biodiesel waste acidified by sulfuric acid (H\({}_{2}\)SO\({}_{4}\)). 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 potential 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 study for several of the mentioned catalysts, fundamental data to deliberate whether the catalyst is appropriate for industrial application. Therefore, the development of new catalysts must be supported by data that disclose their capacity to be used on a large scale.