Effect of process parameters on mechanochemical nitration of toluene

Aromatic compounds such as toluene are commercially nitrated using a combination of nitric acid with other strong acids. The process generates waste, which is difficult to handle and dispose of. A solventless process for mechanochemically nitrating toluene has been proposed recently, producing mononitrotoluene (MNT) with sodium nitrate as a source of nitrogen and molybdenum oxide as a catalyst. Here, this mechanochemical method of nitrating toluene is explored further to establish the effect of process parameters on the product yield. MNT yields exceeding 60% were achieved in different tests. A parametric study addressed the effects of milling time, temperature, milling media, and catalyst additives on the MNT yield and on formation of various by-products. Recovered amounts of MNT showed a maximum with milling time, which occurred earlier for smaller milling balls. However, the yield was reduced and reaction was slowed down when steel balls were replaced with finer glass beads. Overall recovery rate, including all by-products and unreacted toluene decreased after the maximum MNT yield, was observed. Milling temperature had only a weak effect on MNT formation but affected the formation of other aromatic by-products. Replacing various fractions of MoO3 with fumed silica led to an increased yield of MNT for up to 30% of silica. The yield dropped when higher percentages of MoO3 were replaced. The degree of refinement of MoO3 by milling was quantified by measuring the surface area of the inorganic fraction of the milled material. The surface measurements were correlated with the product yield.