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Sustain. Chem. 2021, 2 299 Iron (III) complexes homogeneous catalysts were investigated recently due to their potential to be recovered and their low cost, desirable characteristics when considered on the industrial scale. Esposito et al. tested several types of iron (III) catalysts in an apparatus where water was removed by molecular sieves to shift the reaction equilibrium towards product formation and concluded that FeCl3(1–NO2) had the better performance, achieving almost total glycerol conversion and 100% selectivity to solketal [73,98]. Da Silva et al. compared different transition metal salts and found the best result for Fe(NO3)3·9H2O, also converting almost all of the glycerol and achieving selectivity of 95%. Then, the authors compared the conversions of Brønsted acid catalysts (pTSA and H2SO4) and confirmed that the iron (III) salt is more efficient probably because besides polarizing acetone’s carbonyl group, it releases H+ ions in the reaction medium, therefore acting like Lewis and Brønsted acid simultaneously [74]. Another common conclusion of the articles is the high catalyst efficiency, once a small catalyst load leads to great conversion values [73,74]. Ionic Liquids (ILs), as iron (III) catalysts, combine the advantages of homo- and het- erogeneous catalysts, although its major drawback is usually the high cost. Ji et al. avoided this problem by exploring the tributyl (tetradecyl)phosphonium p-toluenesulfonate (TPTT), an inexpensive IL. The researcher obtained high solketal yield (85.9%), also with a high reaction rate, because the reaction was not impaired by internal mass transfer resistance; however, he had to use excessive acetone, probably to diminish external mass transfer resistance. The results are in line with the ones obtained previously by Gui et al. (Xgly = 96%) with 1–(4–sulfonylbutyl) triphenylphosphonium methanesulfonate. The second study used almost half of the catalyst load, but more than double the acetone to glycerol ratio. The ILs of both studies were recovered and used in several cycles without losing the catalytic activity, an important characteristic for the industrial application [84,85]. 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 mesoporosity 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 (C3H7)4N+/PWA to achievedPDF Image | Continuous Valorization of Glycerol into Solketal
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