Remarkable epoxidation activity of neat and carbonized niobium silicates prepared by evaporation-induced self-assembly

Abstract

A series of mesoporous Nb-silicates with varying Nb loading from 2.2 to 22.0 wt% were synthesized by the Evaporation-Induced Self-Assembly (EISA) method using Pluronic P123 as mesopore forming agent. High dispersion of NbOx species was achieved up to a Nb loading of ∼22 wt% without any formation of bulk crystalline Nb2O5. Small angle X-ray scattering, TEM and N2 physisorption confirm mesoporosity in these materials with short-range ordering. The BET surface area and pore volume range from 779 to 978 m2/g and 0.76–1.21 cm3/g, decreasing at higher Nb loadings. Isolated NbO4 and oligomeric NbOx species were evident from diffuse reflectance UV-Vis spectra. For the epoxidation of cyclohexene with H2O2, the Nb-EISA catalysts generally perform better in terms of activity, epoxidation selectivity and H2O2 utilization compared to other mesoporous niobium silicates. Further, a significant improvement in the epoxide selectivity was achieved with Nb-EISA samples calcined in N2 during synthesis such that a thin C layer is left behind. The C layer may induce pore hydrophobicity that prevents the ring-opening reaction of the primary epoxide product.