Microwave-assisted synthesis of highly dispersed ZrO2 on CNTs as an efficient catalyst for producing 5-hydroxymethylfurfural (5-HMF)

Abstract

This work presents an efficient method of supporting zirconia (ZrO2) on COOH functionalized multi-walled carbon nanotubes (ZrO2/MWCNTs(C)) via a microwave (MW)-assisted method, which can be used as an efficient catalyst for converting fructose to 5-HMF. The developed composite by the MW-assisted route shows highly dispersed ZrO2 with small sizes of 4–5 nm on the carbon support, which could not be achieved the conventional hydrothermal synthesis. Characterization shows a unique zirconia amorphous structure which was ascribed to the special interaction between the carbon support and MW irradiation. Specifically, MWCNTs were heated selectively due to their strong MW-absorbing ability, which led to the formation of microscopic “hot spots”, enabling rapid synthesis (8 min) of highly dispersed ZrO2 with smaller sizes on the hot surface of MWCNTs. Conversely, under conventional heating, the nucleation rate of ZrO2 was slow and prone to form agglomerated particles on the carbon support. The amorphous ZrO2 contributed to the excellent activity of ZrO2/MWCNTs(C) in fructose conversion to 5-HMF. In detail, fructose conversion and 5-HMF yield were achieved at ~72.8% and ~62.9%, respectively, for the catalyst prepared by the MW method, whilst they are only about 24.7% and 15.7% over the catalyst prepared by the conventional hydrothermal synthesis.