## 4 Solketal Synthesis
### Catalyst Study
#### 4.2.1 Recent Advances on Catalysts
One of the main reasons that make zeolites so deeply studied is their versatility, once the acidity, pore size distribution, and crystallite size can be relatively easily manipulated under adequate treatment [99]. Nowadays, many researchers compare the performance of the modified zeolites (usually called Hierarchical--H--due to the presence of more than one type of pore) with their respective Parent (P) zeolites to assess the effect of the changes on the catalytic activity. Manjunathan et al. studied the effects of dealuminating, increasing crystallite size, and preparing copper ion exchange zeolites and concluded that the highest conversion (86%) was attained by H-Beta zeolite with enlarged pores, small crystallite size (low diffusion path length), and higher amount of strong acidic sites. The dealumination led to a decrease in strong acidic sites that reduced glycerol conversion [75]. The findings of Rossa et al., also with H-Beta, agree with this previous study. Furthermore, Rossa et al. attributed the activity reduction along with the number of recycles to the blockage of the acidic sites by the water molecules formed inside the pores, but not to the deactivation of the surface acidic sites, once the hydrophobicity of the zeolite impaired water to approach to the surface of the catalyst [76]. This statement is contrary to what Silva et al. reported in a similar study with the same catalyst, where it was affirmed that the hydrophobic zeolite environment expels off from the pores the water formed, preserving the acid sites, besides preventing water diffusion from the medium to the interior of the pores [99]. The hydrophobicity is related to the silicon to aluminum ratio, which is also related to the porosity and to the acidity of the catalyst, thus selecting the parent zeolites and the adequate treatment is a convoluted process [58,69]. Nonetheless, the good conversion achieved with modified zeolites, with reactions performed under mild conditions and with reasonable amounts of acetone makes the use of zeolites favorable for industrial application [69,75,76]. Talebian-Kiakalaieh et al. synthesized a NaY zeolite supported over heteropoly acid to enhance its acid strength and active sites. The zeolite went over a dealumination process to enlarge its pores and create the mesoporous structure, the downside is the consequent reduction in the total acidity. Nonetheless, 98% glycerol conversion and 96.6% selectivity were achieved, and the authors affirmed that the mesoporous and the acidity are the main parameters that affect the catalytic activity and stability [77]. Sandesh et al. also explored the potential of heteropoly acids and reported the use of (C\({}_{3}\)H\({}_{2}\))\({}_{4}\)N\({}^{+}\)/PWA to achieved94% 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 modification of montmorillonite with nitric acid (HNO\({}_{3}\)) and Amri et al.