Highly porous boron nitride as a metal-free heterogeneous catalyst for cycloaddition of CO2 to epoxides†

Boron nitride has received much attention as an emerging heterogeneous catalyst. Porous boron nitride catalysts were synthesized using boric acid (B) and urea (U) at different molar ratios via a pyrolysis method and applied for cycloaddition of CO₂ to epoxides as metal-free catalysts. The synthesized boron nitride samples had a turbostratic structure, and their porous properties, such as surface area, pore size distribution, and pore volume, largely depended on the molar ratio of B : U precursors. The sample synthesized at a molar ratio of B : U = 1 : 5 with the highest pore volumes among the samples prepared from boric acid and urea exhibited the highest activity for cycloaddition of CO₂ to epoxides, epichlorohydrin and styrene oxide in the presence of tetrabutylammonium bromide (TBAB). There was a good correlation between the corresponding carbonate yield and the pore properties of the catalyst. The addition of melamine (M) during the synthesis greatly developed the porous structure, exceeding 1000 m² g⁻¹ surface area. The sample synthesized at a molar ratio of B : M : U = 1 : 1 : 5 having a large surface area of 1380 m² g⁻¹ with a high pore volume of 1.8 mL g⁻¹ afforded a remarkable yield of 96% for the reaction of epichlorohydrin. The catalyst could be reused at least three times without a significant loss of activity. A cooperative reaction mechanism was proposed in which the hydroxyl groups of porous boron nitride catalysts as weak Brønsted acid sites polarize the oxygen atom of the epoxide, and the bromide ions of TBAB as Lewis base sites activate the carbon atom of the epoxide by the nucleophilic attack.