Green synthesis of nitrotoluene isomers over Nb2O5/SiO2 and MoO3/SiO2 nanocatalysts

Niobium pentoxide (Nb2O5) and molybdenum trioxide (MoO3) supported on silica (SiO2) nanocatalysts are synthesized and characterized by pyridine-adsorbed Fourier Transform Infrared, Scanning Electron Microscopy combined with Energy Dispersive X-ray, Transmission Electron Microscopy, X-ray Diffraction, and Brunauer-Emmett-Teller. They were applied for the nitration of toluene to nitrotoluene (NT) using N2O4 under O-2. The purpose was to decrease the yield of meta-NT isomer (Meta(Nb) and Meta(Mo)) and increase the para-NT/ortho-NT ratio ((Para/Ortho)(Nb) and (Para/Ortho)(Mo)). For this end, parameters including reaction temperature (T), reaction time (t), N2O4/toluene molar ratio (chi), and amount of nanocatalyst (m) were optimized with the response surface method based on the central composite design by Minitab software. Since, Meta(Nb), Meta(Mo), (Para/Ortho)(Nb), and (Para/Ortho)(Mo) responses are reported as four models. The validation of each model was confirmed using the analysis of variance test by the coefficient of multiple determination, adjusted multiple determined coefficient, standard error of estimate, Fisher statistic, t-value, and p-value. The GC analysis was used to determine the yield of ortho-NT, meta-NT, and para-NT isomers. This analysis also showed the conversion of toluene in optimized reactions over Nb2O5/SiO2 (97.5%) and MoO3/SiO2 (89.2%). The reusability of nanocatalysts demonstrates the stability of Nb2O5/SiO2 is higher than MoO3/SiO2. Optimum values for T, t, chi, and m were obtained over nanocatalysts. It was found that Nb2O5/SiO2 has better performance than MoO3/SiO2 due to its less selectivity towards meta-NT and more selectivity to para-NT. Its reason was attributed to the more acidic sites of the Nb2O5/SiO2 than the MoO3/SiO2 and higher electron affinity of Nb2O5 than the MoO3, in addition to the smaller size of the Nb2O5 particles and the higher dispersion of these particles on the SiO2.

Automated summary

Niobium pentoxide and molybdenum trioxide supported on silica nanocatalysts were synthesized and characterized. The performance of these catalysts in the nitration of toluene was investigated, and niobium pentoxide showed better selectivity and stability compared to molybdenum trioxide. This study is of significant importance for improving the yield of para-nitrotoluene.