Russo, Danilo (2017) Safe production of nitrated intermediates of industrial interest using traditional batch reactors and innovative microdevices. [Tesi di dottorato]

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Abstract

2 and 3- nitrobenzaldehydes are two isomers involved in a wide range of fine chemicals production, from dyes to pesticides. They are also employed in highly priced materials production such as pharmaceuticals, photo-removable protecting groups, and non-linear optical materials. Direct fed-batch nitration of benzaldehyde for their production suffers from different drawbacks such as the difficulties in controlling the two isomers yields at different nitrating agent compositions and the possibility of thermal runaway of the system. Moreover, adopting the traditional processes of production of benzaldehyde, it is impossible to obtain chlorine-free benzaldehyde and high temperatures and pressures are required. In this thesis, the possibility of synthesizing 2 and 3-nitrobenzaldehyde, using both benzyl alcohol and benzaldehyde as a row material, is investigated in concentrated mixtures of nitric and sulfuric acids, i.e. mixed acid. In fact, despite the new interest for innovative and alternative nitrating agents, mixed acid still represents the most used and effective reaction media in nitration processes. The adoption of benzyl alcohol as an alternative organic substrate is attractive since it is a lower priced material obtainable in high purity. In this work a successful attempt of conducting tandem oxidation-nitration reaction in a one step process is shown, highlighting the advantages and the drawbacks of this process and proposing some possible alternative reactor design solutions. A first investigation was devoted to clarify some aspects concerning nitric acid speciation in concentrated ternary mixture of nitric acid, sulfuric acid and water and the relative influence of the acids concentrations on their dissociation. As a result, different semi-empirical and empirical correlations were proposed to predict nitronium ion concentrations at varying conditions, defining and overcoming the limitations of the conflicting theories proposed in the literature. Moreover, the study of the influence of sulfuric acid on nitric acid dissociation helped in developing more reliable kinetic models. A kinetic modeling of tandem oxidation-nitration of benzyl alcohol to nitrobenzaldehydes was developed in order to successfully predict the behavior of the reacting system at varying process conditions and clarify the mechanisms of reaction. Specifically, the influence of temperature and composition of the reaction media was clarified. The developed kinetic model elucidated the mechanism of formation of important intermediates such as the coordinated form of nitronium ion with the aldehydic group of benzaldehyde to form ortho-nitrobenzaldehyde. The intensification of nitrobenzaldehydes synthesis was studied using a commercially available microreactor, which allowed to carry out the process under harsher conditions. As a result, it was possible to produce the two isomers with the highest yields obtained so far. Finally an extension of the developed model to investigate benzaldehyde nitration kinetics under heterogeneous liquid-liquid conditions was provided.

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