Selective Nitration of Chlorobenzene by NO2 in the Alkali Zeolite NaZSM-5

Chlorobenzene was reacted with NO2, in the initially acid-free zeolite NaZSM-5, to yield para-chloronitrobenzene exclusively. The precursors were loaded sequentially into self-supporting pellets of the zeolite, contained within a stainless steel cell, from the gas phase. The reaction proceeds spontaneously at room temperature. It is, however, very temperature dependent and effectively ceases at zero degrees Celsius. The reaction was monitored in situ using FT-IR. The active nitrating agent is formed from the partial electron donation by the NO2 to the Na+ cations present in the zeolite lattice. Under the reaction conditions, chlorobenzene is not readily mobile through the pore system; thus, only the molecules adsorbed near a cation site react to form para-chloronitrobenzene. 1. Introduction As part of a continuing investigation into employing heterogeneous catalysis to selectively produce small industrial intermediates, para-chloronitrobenzene was synthesized from chlorobenzene and in the initially acid-free zeolite NaZSM-5. Para-chloronitrobenzene is an intermediate in the production of fine chemicals, principally dyes, pesticides, and herbicides [1–3], where it is hydrogenated to para-chloroanaline before further processing. Chloronitrobenzenes are, however, highly toxic, causing methemoglobinemia [4, 5]. Given this toxicity, and their value as intermediates, the chloronitrobenzenes would seem to be ideal candidates for a study into selective synthesis in order to diminish byproducts. ZSM-5 is a medium-pore pentasil zeolite [6, 7] with two perpendicular channel systems. One channel is straight with an elliptical cross-section of 0.55 × 0.51？nm, while the second forms a zigzag with dimensions of 0.56 × 0.53？nm [8]. Industrially, ZSM-5’s most important process is to enhance the fraction of paraxylene during xylene isomerization [9–11]. 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 [12]. The void space of the channel system is nearly perfectly sized to easily accommodate the adsorption and diffusion of para-substituted benzene rings, but is too restricted to allow easy movement of ortho- and metasubstituted species. ZSM-5 would thus be ideal for the selective formation of para-chloronitrobenzene. Economic considerations for industrial intermediates favor selection of the least expensive form, hence for nitrations. Traditional methods for the nitration of aromatic rings using involve strong acids to generate , which is the