Influence of calcium addition on catalytic urea synthesis from recovered ammonia and carbon dioxide in the nanospaces of porous hollow silica spheres

Abstract

This work investigated the influence of calcium addition to porous hollow silica spheres on urea synthesis from recovered ammonia and carbon dioxide onto the surfaces of the hollow spheres in the presence of catalytically active copper species. Calcium species were composited or encapsulated with porous hollow silica spheres, and we analyzed the effect of the calcium species on the recovery of ammonia and carbon dioxide and urea synthesis within the nanospaces of the hollow spheres. From the DRIFT spectra before and after treatment under a pressurized argon atmosphere, band intensities assigned to recovered ammonium and carbonate ion species significantly decreased after the treatment, and the degrees of intensity decrease for both species and urea yield were the highest in samples prepared with 0.019 and 0.038 g of calcium nitrate, indicating that the recovered species converted into urea with high efficiency in the samples prepared with 0.019 and 0.038 g of calcium nitrate with appropriate strength of calcium–carbonate interaction. We also investigated the influence of the encapsulation effect of calcium addition to hollow silica–calcium composite spheres. Addition of appropriate amount of encapsulated calcium species further improved recovery of ammonia and carbon dioxide and urea yield probably due to appropriate strength of the interaction of the composited and encapsulated calcium species with the carbonate species.