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Synthesis of gvl from la: evaluation of catalysts and catalytic system

The reduction which is carried out in a heterogeneous environment leading to the formation of GVL is usually selective; it occurs in an interesting manner as yield ranges from good to excellent [39]. A complete conversion of LA with >97% GVL selectivity, using 5% Ru/SiO2 or Ru/C catalyst was reported [40]. Ruthenium-based homogeneous catalytic technique that employs formic acid [42, 43] of molecular hydrogen [41] as the hydrogen source have shown a high level of activity. Joó et al., recorded the first application of water-soluble homogeneous HRuCl(TPPMS)3 for the reduction of the oxo- and keto- acids several years ago before other reports. [44] Though, the turnover frequency obtained (TOF = 13 h-1) at pH = 1, depicts a very low catalytic activity. According to reports, GVL can be gotten from LA with Ru/TPPTS catalyst in an aqueous solution with 95% yield GVL [45]. Application of TPPTS modified Ru catalyst in biphasic hydrogenation of LA in CH2Cl2 /H2O was reported by Heeres et al., an excellent yield was achieved under a relatively mild condition [46]. The activity of Ruthenium catalyst can be step up by using better electron-donating phosphine ligands such as PBu3 [45″,47]. Ru/1″,1″,1-tris-(diphenylphoshinomethyl) ethane (Ru/TriPhos), chlorine free active catalysts have been confirmed for the reduction of LA to give GVL, 1″,4-pentanediol, and methyl-tetrahydrofurans (MTHF) [48]. For environmental conservation purpose, it is necessary to note that oxygenates, like 2-Metetrahydrofuran easily lead to the formation of peroxide. The fact that peroxides are threats to the environment should be put into consideration [49]. Despite the beneficial applications of alkylphosphines in transition metal catalysis, in terms of high activity and stability, they have shortcomings in their toxicity, unpleasant smell, and are usually oxygen and water sensitive. Introduction of ionic moieties such as –SO3Na into ligand structure can lower their vapor pressure, and as well accelerate their solubility and stability in polar solutions. Tukacs, J. M. et al., [50] investigated the electronic and steric characterization of novel electron-donating sulfonated phosphines, and also their application in the Ru catalyzed reduction of LA including its IR and NMR studies. In their work, they showed that γ-valerolactone could be derived from levulinic acid gotten from biomass, through reduction, in the presence of a catalyst. The catalyst is generated from Ru(Acac)3 and various sulfonated phosphine RnP(C6H4 -m-SO3Na)3-n (n = 1″,2; R = Me, Pr, iPr, nBu, Cp) ligands in a reaction environment devoid of solvent and promoters. The introduction of a sulfonato group into ligand structure does not significantly affect the electronic and steric properties, this was established through the characterization of sulfonated and non-sulfonated ligands. It was confirmed that the activity of electron-donating sulfonated phosphine modified catalysts hugely surpassed the activity of Ru/TPPTS system attaining turnover number up to 6370 and maintained this activity after complete transformation of LA to GVL. They claimed that there was no sign of formation of by-product during the conversion in the IR and NMR studies. The catalyst recovery was successfully demonstrated. The volatile compound was removed at the end of the process through a vacuum, while the catalyst compounds left behind was dissolved in LA, and this showed similar activities like before distillation. A second method is acid-catalyzed dehydration of levulinic acid (LA) to angelica lactone, followed by hydrogenation (Scheme 7), though; GVL yield is lower since acidic media promote coke formation and angelica lactone polymerization [53]. Scheme 8 summarizes the pathways of LA hydrogenation to GVL. (1) hydrogenation followed by dehydration; (2) dehydration followed by hydrogenation.

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