Solvent-free semihydrogenation of acetylene alcohols in a capillary reactor coated with a Pd–Bi/TiO2 catalyst

• A novel capillary microreactor wall-coated with Pd–Bi/TiO2 catalyst was demonstrated in solvent-free semihydrogenation of an acetylene alcohol.
• Selectivity above 98% and stability for 100 h on stream was shown.
• Leaching of Bi from the catalyst via oxidation was prevented adding 1 vol.% acetic acid.
• The throughput of 50 g/day for a reactor operated at atmospheric pressure showed feasibility of hydrogenation in capillary reactors for small-scale syntheses

A solvent-free semihydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol was performed in a capillary reactor (10 m long, 0.53 mm i.d.) coated with a titania supported Pd–Bi catalyst. Several coatings with different Pd/Bi ratio have been prepared. The catalysts have been characterized with SEM, TEM, EDX, XRD analysis and N2 adsorption–desorption measurements. The maximum alkene yield of 90% was obtained at a molar Pd/Bi ratio of 11. The yield was increased to 95% in the presence of 10 mol.% pyridine in the reaction mixture. The alkene selectivity decreased with time due to leaching of Bi. The leaching was fully suppressed in the presence of 1 vol.% acetic acid in the reaction mixture. The catalyst remained stable for 100 h of continuous operation. The results demonstrate that capillary reactors provide alkene selectivity the same compared to ideal stirred tank batch reactors.

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