Niobia-based magnetic nanocomposites: Design and application in direct glucose dehydration to HMF.

[Display omitted] • Niobia-based magnetic nanoparticles (MNP) coated with Nb 2 O 5 or Nb 2 O 5 -SiO 2 shells were prepared. • Prepared catalysts may produce HMF selectively by glucose dehydration. • High selectivity to HMF was correlated to pseudohexagonal niobium oxide (TT-Nb2O5) phase. • High catalytic activity was correlated to the small nanoparticles size. Niobia-based magnetic nanocomposites were prepared by covering magnetite nanoparticle cores (Fe 3 O 4 , MNP) with either Nb 2 O 5 or Nb 2 O 5 -SiO 2 shells using a two-step procedure. In the first step magnetite nanoparticles were prepared by the coprecipitation method. The second step involved their coverage with either Nb 2 O 5 shells, through a precipitation method, or with Nb 2 O 5 -SiO 2 shells, through a sol-gel protocol followed by precipitation in the presence of the CTAB surfactant. The obtained materials were exhaustively characterised by X-ray diffraction, Mössbauer spectroscopy, magnetic measurements, ICP-OES, DRIFT and Raman spectroscopy, and CO 2 - and NH 3 -TPD measurements, and investigated for glucose dehydration to HMF. The catalytic performances were directly correlated to the nature of the supported niobia phases, which, in turn, has been dictated by the niobia content and the preparation route. The high selectivity to HMF was correlated with to the large pseudohexagonal niobium oxide (TT-Nb 2 O 5) phase while the catalytic activity was directly correlated to the small nanoparticles size. A proper combination of these features led to an optimum catalytic system for the selective production of HMF through glucose dehydration. A third important feature making the developed catalyst promising is its magnetic property, ensured by the magnetite nanoparticles core. This allowed its easy separation from the reaction products. [ABSTRACT FROM AUTHOR]