Fructose is a vital ketose in the production of 5-hydroxymethylfurfural platform molecules. Herein, Zr-doped hexagonal mesoporous silica was prepared over in situ co-assembly (Zr-x-HMS, x is the mole ratio of Si to Zr, x = 20, 50) and post-impregnation (Zr-Imx-HMS, x = 20, 50, 100) processes to regulate its catalytic performance against the isomerization of glucose to fructose in ethanol. The post-impregnation process did not significantly change the original pore structure and morphology of HMS, Zr-Im50-HMS with medium Zr loading possessed the most Lewis acid sites with medium strength among Zr-Imx-HMS catalysts. The in situ process changed the physical properties of HMS and enhanced the interaction of Zr sites and silica framework, facilitating the formation of Lewis acid sites with medium strength. Zr-Im50-HMS possessed the most suitable acidity for fructose production, and an optimal yield of 31.2% was achieved at 160 °C and 20 min. Moreover, the recyclability of Zr-Im50-HMS was acceptable.
The hierarchical pore structures of carbons are significant for their properties. The template strategy is a typical pore-making approach of carbons. However, few reports on the micropore templates of porous carbons have been found. Herein, a micropore template is incorporated into a carbon precursor by adopting a post-impregnation approach for the first time, thus preparing hierarchically porous carbons. Polystyrene-b-poly(ethylene oxide) (PS-b-PEO) micelles are used as a soft template. Poly(3-aminophenyl) boronic acid-encapsulated PS-b-PEO composite micelles are assembled on the surface of graphene oxide, forming sandwich aggregations. They are immersed in a zinc acetate solution and then directly pyrolyzed into porous carbons. During pyrolysis, the PS cores of micelles are decomposed to generate abundant mesopores, while the coordinated zinc ions are reduced to zero-valent zinc and then evaporated above its boiling temperature, creating numerous micropores. This in situ-reduced metallic zinc functions as a micropore template. The resulting carbons achieve hierarchical pore structures and a high specific surface area. As an application demonstration, they show one of the best catalytic performances for the oxygen reduction reaction among metal-free doped carbons. This post-impregnation approach can be extended to apply to numerous polar carbon precursors, thus preparing many hierarchically porous carbons with abundant micropores.
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