Aromatic compounds such as toluene are commercially nitrated using a combination of nitric acid with other strong acids. This study investigated mechanochemical nitration of aromatic compounds with a specific focus on the nitration of toluene because of the considerable industrial significance of nitrotoluene. This process relies on the use of highly corrosive chemicals and generates environmentally harmful waste, which is difficult to handle and dispose of. In this study, aromatic nitration using solvent-free mechanochemical processing of environmentally benign precursors has been achieved and investigated.
Mononitrotoluene (MNT) was synthesized by milling toluene with sodium nitrate and molybdenum trioxide as a catalyst. Several parameters affecting the desired product yield and selectivity were identified and varied. MNT yields in excess of 60% have been achieved in different tests. The desired product yield and selectivity were found to depend on the ratios of the reactants and the catalyst. A parametric study addressed the effects of milling time, temperature, milling media, and catalyst additives on the MNT yield and on the formation of various byproducts. Toluene conversion as a function of milling time exhibited a maximum, which occurred earlier for smaller milling balls. Milling temperature had only a weak effect on MNT formation, but affected the formation of other aromatic byproducts. 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 attained in the mill has been quantified by measuring the surface area of the inorganic fraction of the milled material. The surface measurements were correlated with the product yield. Feasibility of secondary nitration for toluene as well as of mechanochemical nitration of anisole and naphthalene has also been shown experimentally.
This study investigated mechanochemical nitration of aromatic compounds with a specific focus on the nitration of toluene because of the considerable industrial significance of nitrotoluene. The feasibility of this approach has been clearly demonstrated and practically significant product yields have been obtained. A number of important parameters affecting the desired product yield and selectivity have been identified and studied. These include the ratios of the reactants to each other and to the catalyst, milling time and temperature, milling media size and density. The enhancement of the MoO3 catalytic activity by adding silica has been explored and the optimum fraction of silica has been identified.
The mechanism of this process appears to be complex and still needs to be explored in the future studies, but this study did provide several clues which promise to be useful in that quest. It is interesting that the reaction appears to occur heterogeneously, at the surface of catalyst particles, suggesting a new nitration mechanism observed here.
Potentially, the explored method of nitration of organic compounds is readily scalable and does not require development of new equipment or facilities. Understanding the reaction mechanism would allow one designing process and selecting the process parameters best suited for practical nitration. It is expected varying the catalyst and combining it with a material, which could serve as a molecular sieve, like zeolite, could lead to a substantially better selectivity and thus greater value of the generated nitrated products. It is further necessary to consider possibility of recycling the catalyst, which would significantly improve the value of the new technology.