Hierarchical porous Ca(BO2)2 microspheres: Hydrothermal–thermal conversion synthesis and their applications in heavy metal ions adsorption and solvent-free oxidation of benzyl alcohol

Abstract

Hierarchical three-dimensional (3D) porous structures are of great significance for their versatile applications in diverse fields. Herein, uniform Ca4B10O19·7H2O microspheres were successfully fabricated via a facile hydrothermal method, followed by a thermal conversion (700°C, 2.0h) without any additives, leading to the 3D hierarchical porous calcium metaborate (Ca(BO2)2) microspheres with well-preserved spherical morphology and high crystallinity. When employed as adsorbents, the adsorption isotherms of Pb2+ and Cd2+ onto porous Ca(BO2)2 microspheres were well fitted with the Langmuir adsorption model, giving rise to the maximum adsorption capacities for Pb2+ and Cd2+ as 140.2 and 110.6mgg−1, respectively. When utilized as catalyst support, the good dispersion and smaller size of metallic Pd nanoparticles (NPs) as well as more and stronger acid sites on the catalyst resulted in the 1.05wt% Pd/Ca(BO2)2 catalyst, by using which the conversion of benzyl alcohol and selectivity to benzaldehyde with O2 as oxidant reached 94.3% and 96.0%, respectively. The resultant supported catalyst also exhibited satisfactory high conversion and especially excellent selectivity during 10 cycles of catalysis, revealing the as-prepared porous Ca(BO2)2 microspheres as superior catalyst carrier. The present hierarchical porous Ca(BO2)2 microspheres afforded promising and competitive candidates for water treatment and heterogeneous catalysis.