Parts-Per-Million (Salen)Fe(III) Homogeneous Catalysts for the Production of Biodiesel from Waste Cooking Oils

Abstract

This work describes the application of a library of iron(III)-salen catalysts in the production of biodiesel from vegetable oils. The conversion of neutral soybean oil is complete within two hours at 160–180 °C with low catalyst loading (0.10 mol%). A comparative screening reveals that the catalysts containing acetate as a fifth ligand are the most performing, and these have been conveniently used to convert acidic and waste cooking oils (WCO). WCOs were used as received without further purification to produce biodiesel in high yield (85–90%) under optimized conditions (2 h at 180 °C, catalyst loading 0.1 mol%, oil to alcohol molar ratio 1:20). The iron content in the lipophilic and hydrophilic phases of the crude mixture was investigated and the residual concentration in biodiesel was found to be in the order of 10–14 ppm, comparable to that contained in biodiesels from other sources. Graphical Abstract

Biodiesel from Waste Cooking Oil: Highly Efficient Homogeneous Iron(III) Molecular Catalysts

“…Six of them have different substituents on the salophen ligand (H, Me or OMe), and vary in the nature of the fifth anionic ligand that can be acetate or chloride. Other less coordinating anions (e.g., triflate, trifluoroacetate, tetrafluoroborate) were not considered since they have already returned less satisfactory results in a previous investigation [20]. Catalysts 2H-OAc, 2H-Cl and 2OMe-Cl are described in the literature [26][27][28], while the other three unpublished complexes, 2Me-OAc, 2Me-Cl and 2OMe-OAc, were analogously prepared by reacting iron(II) acetate or iron(III) chloride with the appropriate salophen ligand under air (synthesis details in the experimental section; melting points and FT-IR spectra are collected in the supporting materials, Figures S1-S5 and Table S1).…”

“…Although Brønsted's bases have undeniable good performance [11], their use is compromised or complicated by the presence of free fatty acids [12], whose concentration increases upon cooking [13]. For this reason, Lewis acid catalysts based on cheap metals have been proposed as an alternative, both homogeneous (e.g., complexes of Zn(II) [14][15][16][17], Sn(IV) [18,19] or Fe(III) [20]) or heterogeneous (metal oxides [21][22][23][24], their mixtures or salts) that simultaneously promote transesterification of triglycerides (Scheme 1) and esterification of free fatty acids (Scheme 2). cat.…”

“…Recently, we proposed salen iron(III) complexes (1 in Figure 1) as homogeneous catalysts for the production of biodiesel [20]. The product is a mixture of fatty acid methyl esters (FAME's or biodiesel) and glycerol.…”

“…Recently, we proposed salen iron(III) complexes (1 in Figure 1) as homogeneous catalysts for the production of biodiesel [20]. Pros of the former catalysts are high activity and tailored structure design, while their cons are the difficult separation from the products and recycling.…”

“…Recently, we proposed salen iron(III) complexes (1 in Figure 1) as homogeneous catalysts for the production of biodiesel [20]. At low iron loadings (0.1% mol(Fe)/mol(oil)), a 90% conversion of a waste cooking oil (WCO) was achieved in convenient conditions, and the percentage of iron found in biodiesel was in many cases a few ppm.…”