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

This work presents an efficient method of supporting zirconia (ZrO 2 ) on COOH functionalized multi-walled carbon nanotubes (ZrO 2 /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 ZrO 2 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 ZrO 2 with smaller sizes on the hot surface of MWCNTs. Conversely, under conventional heating, the nucleation rate of ZrO 2 was slow and prone to form agglomerated particles on the carbon support. The amorphous ZrO 2 contributed to the excellent activity of ZrO 2 /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.