SHORT COMMUNICATIONSNitrophenols are generally formed as by-products in the nitration of aromatic hydrocarbons [1]. This also applies to such well studied process as liquid-phase nitration of benzene with concentrated nitric acid or sulfuric acid-nitric acid mixtures, which underlies preparation of nitrobenzene as important intermediate product in organic synthesis [2]. Under the optimal conditions the yield of the target product attains 99.6%; however, the concentration of nitrophenols in crude nitrobenzene is no less than 2000 ppm. Therefore, additional purification of nitrobenzene becomes necessary, and hence economic and ecological parameters of the process are considerably impaired.In the recent years, a large number of studies were concerned with an alternative procedure for the nitration of benzene with 65-70% nitric acid in the gas phase over solid catalysts (see, e.g., review [3] and references therein). This procedure is somewhat advantageous, for it requires no sulfuric acid, and thus its regeneration for repeated use is excluded. Some authors [4-6] succeeded in obtaining nitrobenzene in 95-96% yield, which is undoubtedly excellent result for laboratory experiments but is still insufficient for large-scale application.Nevertheless, gas-phase nitration could be of higher competitive capacity provided that the amount of nitrophenols formed as by-products be considerably reduced and additional purification of nitrobenzene be avoided. On the other hand, side formation of nitrophenols in the gas-phase nitration of benzene has not received due attention.In the present work we examined at a quantitative level the formation of mono-, di-, and trinitrophenols in the gas-phase nitration of benzene with 65% nitric acid under different conditions. For this purpose, a large number of known catalysts were tested on a laboratory scale; in particular, natural and synthetic aluminosilicates, metal oxide systems, and silica gels were examined. The best results were obtained with the use of silica gel preliminarily subjected to hydrothermal treatment; it was modified with 10% sulfuric acid and calcined at 150°C.Experiments were performed on a laboratory flow setup with a fixed bed of catalyst (50 cm 3 ) using excess benzene (65% HNO 3 -benzene molar ratio 1 : 2). Condensed nitration products (both liquid and gaseous) were analyzed for unreacted benzene, nitric acid, nitrobenzene, and dinitrobenzene (by-product; its amount ranged from 0.1 to 0.6%) by standard methods. The formation of nitrophenols was monitored by highperformance liquid chromatography (HPLC) using specially developed procedures. Unexpectedly, raising the temperature to 150°C and higher resulted in increase in the conversion of nitric acid, whereas the concentration of nitrophenols in crude nitrobenzene decreased (see table). The observed tendency persisted over a fairly wide range of reaction time.Presumably, elevated temperature favors desorption of excess benzene and nitration products (which is the rate-determining stage of the process), so that they a...
