ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

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

Sara Miralibozorg, Mohammad Nasiri, and Arash Shokrollahi

Department of Chemistry, Malek-Ashtar University of Technology, Shahin-Shahr, Iran

E-mail: nasiri@mut-es.ac.ir

Received: 23 February 2022  Accepted: 6 June 2022

Abstract:

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 O₂. 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 (χ), 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, χ, 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.

Keywords: Solid-acid nanocatalyst; Toluene nitration; Nb2O5/SiO2; MoO3/SiO2; Nitrotoluene

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-022-02325-7

Chemical Papers 76 (10) 6475–6485 (2022)