Niobium-based heterogeneous catalysts synthesised in supercritical carbon dioxide for selective olefin epoxidation

In this work, two different protocols for the synthesis of Nb2O5-SiO2 with a sol-gel route in which supercritical carbon dioxide was used as solvent have been developed. The tailored design of the reactor allowed the reactants to come into contact only when supercritical CO2 is present, and the high-throughput experimentation scCO2 unit allowed the screening of synthetic parameters, that led to a Nb2O5 incorporation into the silica matrix of 2.5 wt%. N2-physisorption revealed high surface areas and the presence of meso- and micropores. XRD allowed to demonstrate the amorphous character of these materials. SEM-EDX proved the excellent dispersion of Nb2O5 into the silica matrix. These materials were tested in the epoxidation of cyclooctene with hydrogen peroxide, which is considered an environmentally friendly oxidant. The catalysts were virtually inactive in an organic, polar, aprotic solvent (1,4-dioxane). However, the most active scCO2 Nb2O5-SiO2 catalyst achieved a cyclooctene conversion of 44% with a selectivity of 88% towards the epoxide when tested in ethanol. Catalytic tests on cyclohexene revealed the presence of the epoxide, which is remarkable, considering that this substrate is easily oxidised to the diol. The behaviour in protic and aprotic solvents is compared to that of TS-1.