Acid-Free Nitration of Benzene and Toluene in Zeolite NaZSM-5

The syntheses of nitrobenzene and p-nitrotoluene directly from benzene, toluene, and NO2 within the pore network of the initially acid-free zeolite NaZSM-5 are reported for the first time. The active species , formed by the interaction of NO2 with the Na+ cations present on the internal surface, results in the acid-free electrophilic substitution of the aromatic ring. There are two distinct reservoirs for the reagents: one associated with close proximity to the cation sites and the other associated with the siliceous areas of the pore network. Up to 34% of the hydrocarbon and 70% of the available NO2 are reacted at 50°C. Only the cation associated sites are reactive at low temperature, and there appears to be little mobility between the sites under the reaction conditions. There is no evidence of a second nitration occurring. This represents a novel route to the single nitration of benzene and toluene and for toluene, the generation of the para isomer exclusively. The pore network of the NaZSM-5 restricts the available reaction volume and transition state geometry allowing only the para-substituted product. 1. Introduction As part of an investigation into employing heterogeneous catalysis to selectively produce small industrial intermediates, nitrobenzene and para-nitrotoluene were synthesized directly from benzene and toluene and NO2 in the initially acid-free zeolite, NaZSM-5. Approximately 95% of the >1.5 106 tonnes of nitrobenzene produced annually is used in the production of aniline [1]. Most of the remainder is used for precursors in rubber, pesticides, dyes, and pharmaceuticals such as Acetaminophen [1, 2]. Para-nitrotoluene is used in the synthesis of p-toluidine, which in turn is used to manufacture dyes and as accelerators for cyanoacrylate adhesives [3]. ZSM-5 is a medium pore pentasil zeolite [4, 5] with two perpendicular channel systems (see Figure 1). The first is a straight channel of elliptical cross section of 0.55 0.51？nm, and the second is sinusoidal with dimensions of 0.56 0.53？nm [6]. Its most valuable industrial process is the isomerization of xylenes to enhance the fraction of para-xylene in the product stream [7–9]. This is thought to result from the aluminosilicate channel wall restricting the available transition state volume and enhancing the diffusion of the para-isomer down the pores [10]. The void space of the channel system easily accommodates para-substituted benzene rings, but is too restricted to allow easy movement of ortho- and meta-substituted species. ZSM-5 would thus be ideal for the selective formation of