Eco-friendly synthesis of bio-additive fuels from renewable glycerol using nanocrystalline SnO2-based solid acids

Abstract

The present work has been undertaken with an aim to synthesize valuable bio-additive fuels from glycerol acetalization using SnO₂-based solid acids. Various promoters, namely SO₄²⁻, MoO₃ and WO₃ were incorporated to the SnO₂ using a wet-impregnation method. An extensive physicochemical characterization has been achieved by means of XRD, BET surface area, BJH analysis, FT-IR, pyridine adsorbed FT-IR, NH₃-TPD, ICP-OES and XPS techniques. The BET surface area of SnO₂ is significantly improved from 11 to 32, 56 and 41 m² g⁻¹ after the addition of the WO₃, MoO₃, and SO₄²⁻ promoters, respectively. The XPS studies revealed that Sn is present in the +4 oxidation state, whereas Mo, W and S are in the +6 oxidation state in the prepared samples. In addition, the SO₄²⁻/SnO₂ sample contained super acidic sites, along with strong- and medium-acidic sites. The amount of acidic sites was found to be 46.47, 61.81, 81.45 and 186.98 μmol g⁻¹ for the SnO₂, WO₃/SnO₂, MoO₃/SnO₂, and SO₄²⁻/SnO₂ samples, respectively. The pyridine adsorbed FT-IR studies revealed the existence of a superior quantity of Brønsted acidic sites than Lewis acidic sites in the synthesized catalysts. Promoted SnO₂ catalysts exhibited a promising catalytic performance for glycerol acetalization with acetone and furfural, and the activity of the catalysts was found to increase in the following order: SnO₂ < WO₃/SnO₂ < MoO₃/SnO₂ < SO₄²⁻/SnO₂. The outstanding performance of the SO₄²⁻/SnO₂ catalyst is mainly due to the existence of a large amount of acidic sites associated with the super acidic sites. The achieved optimum glycerol conversions with acetone and furfural were ~98 and 99% over the SO₄²⁻/SnO₂ catalyst, respectively.