Improved Regioselective Di-nitration of Biphenyl over Reusable HBEA-500 Zeolite

Abstract

Nitration of aromatics is one of the most broadly studied and famous non-selective organic reaction which provides key organic intermediates or energetic materials. However, the industrial syntheses of nitro compounds suffer many disadvantages: employing large quantities of sulfuric acids as catalysis, which may generate acid waste that is expensive to dispose off, over nitration, low selectivity, oxidation byproducts and safety problems. Major efforts have therefore been made to dispense the use of sulfuric acid for the aromatic nitration and develop environmentally benign and clean processes to replace conventional nitration. The nitro derivatives of biphenyl are used as intermediates for the preparation of fragrance, dyes, plasticizer and polymer. The earlier attempts of nitration of biphenyl derivatives have been made by using nitric acid and sulfuric acid or acetic anhydride as the nitrating mixture gives a mixture of 4-nitrobiphenyl and 2-nitrobiphenyl. Recently, many new methods of the nitration of biphenyl were reported. However, few of them mentioned about the di-nitration of biphenyl. Thus, it is necessary to develop a clean and commercial process for the selective di-nitration of biphenyl in high conversion and yield. In pursuit of our former efforts on nitration of toluene, chlorobenzene, naphthalene and biphenyl using zeolite catalysts, we hope to improve the selectivity and yield in the di-nitration of biphenyl by the use of zeolite. Catalysts always play an important role in the kinetics of chemical reactions, thereby influencing the selectivity of a desired isomer directly. Zeolites have well-defined pore structures and channels that are derived from the networking of SiO2 and Al2O3 making them attractive candidates for shape selective catalysis. To optimize the reaction time for maximum di-nitration yield, samples were analyzed at varied time ranging from 1 to 24 h. The main data (Table 1) showed that the highest dinitration yield was obtained at 24 h with HBEA-500 (0.3 g) zeolite as a catalyst. Zeolites facilitated the reaction and gave a high di-nitration yield as compared with no catalyst (Entry 1 Table 1). With the Si/Al ratio increasing of HBEA zeolite catalyst, a high ratio of formation of 4,4'-dinitrobiphenyl 3c was favored (Entries 3-8, from Table 1). HBEA zeolite catalyst with a low Si/Al ratio showed a worse selectivity for 3c than that of high Si/Al ratio possibly because of the dealuminiation, which would modify the distribution of the size and shape of the pores in zeolites. The average particle sizes of the HBEA zeolites decrease with Si/Al ratio which was detected from the SEM images. High alumina containing zeolites show a very-hydrophilic character, whereas, high