Nitration of simple aromatics over zeolite catalysts

The aim of this study was to develop new nitration systems using zeolite catalysts for the nitration of a highly activated compound, phenol, and for highly deactivated aromatics. It was hoped that such systems would produce high yields of the nitro products and in high pora-selectivity. Chapter One Chapter one describes the important role of Green Chemistry in the chemical industry. For example, modern synthetic methods should ideally be safe, be atom efficient, generate minimal waste, not involve solvent, have minimal energy requirement, be based on renewable resources, and be environmentally benign. Advances have been made by the use of solid catalysts such as zeolites and employing green solvents such as water, supercritical carbon dioxide and ionic liquids. Chapter Two Chapter Two describes a literature survey for the use of zeolite catalysts in nitration reactions of various aromatic compounds. Zeolite catalysts offer approaches to para-nitro aromatics, with the advantage of easy separation of the catalyst from the reaction mixture by filtration. Zeolites can be recovered and reused several times to give almost the same yields and selectivities as those obtained when fresh zeolite is used, with the avoidance of toxic waste generated by traditional Lewis acids. Chapter Three Chapter Three describes the nitration of phenol over several types of zeolites using wo-propyl nitrate as the nitrating reagent. Zeolite Hp with a Si/Al ratio of 12.5 was the most active of the catalysts tried and also gave the highest proportion of para-nitrophenol, but the paralortho ratio was still only around 1:1. Mono nitrophenols are formed in high yields when reactions are carried out over two days in refluxing dichloroethane as a solvent. A somewhat higher yield of mononitration products and a slightly better paralortho ratio could be achieved with a larger quantity of HZSM-5 under otherwise similar conditions. The reactions also produced small quantities of z'so-propoxybenzene as the main by-product. Chapter Four Chapter Four describes the nitration of deactivated aromatics using a nitric acid/acid anhydride/zeolite system. Various types of zeolites were used including proton-exchanged and passivated Hp. For example, nitration of benzonitrile over Hp produces a quantitative yield of nitrobenzonitriles comprising only 3- and 4-nitrobenzonitriles, with no 2-nitrobenzonitrile produced in most cases under the conditions tried. Trifluoroacetic and chloroacetic anhydrides were found to be the most active of the anhydrides tried. Also, zeolite Hp with a Si/Al ratio of 12.5 was the most active of the catalysts tried and also gave high pora-selectivity. The highest yield of 4-nitrobenzonitrile (33%), with a paralmeta ratio of ca. 0.50, was obtained using passivated Hp. Furthermore, heating easily regenerates the zeolite, which can be reused up to six times to give results similar to those obtained with a fresh sample of the catalyst. The process was found to be general for the nitration of monosubstituted deactivated aromatic compounds and gave significantly increased proportions of para-substituted isomers compared with the results obtained from the traditional mixed acid method