Bimodal cesium hydrogen salts of 12-tungstosilicic acid, CsxH4−xSiW12O40, as highly active solid acid catalysts for transesterification of glycerol tributyrate with methanol

• CsxH4−x[SiW12O40] (Cs-x) with both micropores and mesopores are synthesized.
• Mesopores are formed by treatment of microporous Cs-x in refluxing ethanol.
• Mesopores are formed only from microporous Cs-x with low Cs content.
• Post-treatment of bimodal Cs-x in sulfuric acid enhances catalytic performance.
• Post-treated Cs-x exhibits high catalytic activity for biodiesel production reaction.

Bimodal CsxH4−x[SiW12O40] (Csx-bimodal) with mesopores interconnected with micropores were synthesized from microporous CsxH4−x[SiW12O40] (Csx-micro) with x = 1.0–2.5, which were prepared in advance by titrating an aqueous solution of H4[SiW12O40] with an aqueous solution of Cs2CO3, followed by treatment in refluxing ethanol to mainly dissolve the H4[SiW12O40] in the particles. Mesopore size distributions and their pore volumes changed depending on x in Csx-micro. Microporous Cs2.5-micro transformed into bimodal Cs2.5-bimodal with mesopores having average diameters of 3.7 nm and large mesopore volumes. Although Cs2.5-bimodal exhibited only low catalytic activity for the decomposition of isopropyl acetate, post-treatment in H2SO4 enhanced the catalytic activity due to substitution of the Cs+ ions on the surface with H+. H2SO4-treated Csx-bimodal showed high activity toward transesterification of glyceryl tributyrate with methanol due to its strong acid strength and mesoporosity.

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