*2.1. Synthesis of Different Molecular Sieves*

2.1.1. Nb2O5 Synthesis Supported on MCM-41 through the Sol-Gel Method (Nb-MCM-41-Solgel)

A total of 8.8 g of N-cetyl-N,N,N-trimethylammonium bromide (Merck, Darmstadt, Germany) were dissolved in a solution of 208 mL of deionized water and 96 mL of aqueous ammonia solution (25% w/w, Merck, Darmstadt, Germany) at 35 °C. A total of 40 mL of TEOS (98% purity, Aldrich®, San Luis, MO, USA) and an aqueous solution of ammonium tris(oxalate) complex of niobium(V) (CBMM, Araxá, Minas Gerais, Brazil) were slowly added under agitation to this clear solution, in 0.1 M of oxalic acid; the amounts of niobium oxalate aggregated were 0.4756, 0.9512, and 1.4268 g to obtain niobium percentages of 10%, 20%, and 30%, respectively, in the mesoporous silica material. Thereafter, the system was agitated for three hours and the gel aged at room temperature for 24 h in a sealed container. The precipitate was filtered, washed with 600 mL of deionized water, and air dried at room temperature. Finally, to eliminate the organic matter, it was calcined at 550 °C for eight hours. The procedure previously described was carried out taking into account its simplicity, reproducibility, and the infrastructure currently available in the laboratory. Certainly, the aim in future would be to incorporate further stages, used to good effect by other researchers, in order to optimize the characteristics of the end product.
