Easy and Fast Production of Solketal from Glycerol Acetalization via Heteropolyacids

Abstract

This work presents an effective and fast procedure to valorize the main waste produced from the biodiesel industry, i.e., the glycerol. The acetalization of glycerol with acetone represents an effective strategy to produce the valuable solketal, a fuel additive component. In this work, the catalytic efficiency of different commercial heteropolyacids (HPAas) was compared under a solvent-free system. The HPAs used were H3[PW12O40] (PW12), H3[PMo12O40] (PMo12) and H4[SiW12O40] (SiW12). The influence of reactional parameters such as reactants stoichiometry, catalyst concentration and reaction temperature were investigated in order to optimize experimental conditions to increase cost-efficiency and sustainability. HPAs demonstrated to be highly efficient for this type of reaction, presenting a high and fast glycerol conversion, with high selectivity to solketal under sustainable conditions (solvent-free system and room temperature medium). The activity of HPAs using 3 % to glycerol weight and a glycerol/acetone ratio of 1:15 followed the order: PW12 (99.2%) > PMo12 (91.4%) > SiW12 (90.7%) as a result of the strong acidic sites after 5 min. In fact, only 5 min of reaction were needed to achieve 97% of solketal product in the presence of the PW12 as a catalyst. This last system presents an effective, selective and sustainable catalytic system to valorize glycerol.

The Role of the Heterogeneous Catalyst to Produce Solketal from Biodiesel Waste: The Key to Achieve Efficiency

“…Such as been previously speculated, and a proposed mechanism can be seen in Figure 7, in this case specifically for a Brönsted acid catalyst [18]. In general, the reaction is kickstarted when the catalyst interacts with the carbonyl of the substrate, either by protonation or coordination with a metal site (for Brönsted and Lewis acids, respectively) [35,41,42,55]. This interaction forms a protonated intermediate structure that, when interacting with the hydroxyl groups in glycerol, originates a hemiketal/hemicetal.…”

“…Many different substrates have been used in acetalization reactions before, with the most reported ones being butanal [36], furfural [37], citral [38], benzaldehyde [39], formaldehyde [40] and acetone. This review will be focused on acetone, as it is by far the most studied substrate, and its application in glycerol conversion has proved incredibly effectiveness [41][42][43]. Further, excess acetone has been reported to increase glycerol conversion to solketal, while also acting as an entrainer, helping the removal of water from the reactional system, and increasing its miscibility with the viscous glycerol [10,12].…”

“…The evolution of research in the last years allowed the development of highly efficient catalysts, that can assure a favourable reaction equilibrium by themselves. As such, acetalization reactions have evolved into solvent-free environments [12,41,[48][49][50].…”

“…Balula et al studied the influence of Keggin-type HPAs, with the use of phosphotungstic acid (PW12), phosphomolybdic acid (PMo12) and silicotungstic acid (SiW12), in the acetalization reaction of glycerol at room temperature (Figure 8). [41] The results reported a catalytic efficiency trend of PW12 (99.2%) > PMo12 (91.4%) > SiW12 (90.7%) after only 10 min, where PW12 is widely reported to be the most acidic out of the three HPAs. [41,59,60] Da Silva et al developed a cation-exchanged HPA, where the protons of silicotungstic acid were substitued by tin(II) cations.…”

Catalysis for Oleochemical Platforms

“…Two years later, Juliao et al developed three simpler polyoxometalates of formula H 3 PW 12 O 40 , H 3 PMo 12 O 40 , and H 4 SiW 12 O 40 obtaining glycerol conversions higher than 90 % in only 5 min, at room temperature, with a molar ratio acetone: glycerol of 15 : 1, and 3 % in weight of catalysts with respect to glycerol. [103] Another approach employs residues of industrial wastes or biomass-derived materials to produce innovative catalysts, most of them obtained upon sulfonation. Some examples are reported by Alsawalha, [98] da Silva, [104] Samoilov, [105] Pompeo [106] and Fernández, [107] Saikia, [108] Ravi [109] and Mota.…”