Vapor phase nitration of toluene using dilute nitric acid and molecular modeling studies over beta zeolite

Abstract

The regio-selective nitration of toluene to para-nitrotoluene with dilute HNO3 has been studied over catalyst beta zeolite. The effect of reaction temperature, concentration of HNO3 and molar ratio of toluene to HNO3 is reported. The maximum conversion of 55% (toluene to HNO3 ratio, 1.7:1), selectivity to para-nitrotoluene of 70%, and catalyst life of 75 h (weight hourly space velocity (WHSV)=0.2) were obtained using 20% nitric acid at temperature of 120 °C. The catalyst deactivated faster when the concentration of acid and the temperature of reaction and WHSV was increased. The physicochemical characterization of the deactivated catalyst showed the structural stability of beta zeolite under the reaction conditions, but the para-nitrotoluene, and other oxidation products 4-nitrobenzoic acid, benzaldehyde, benzoic acid and anthraquinone were found deposited on the catalyst surface. Even then, the selectivity for para-nitrotoluene remains nearly constant. The molecular modeling study revealed that para-nitrotoluene encounters the least resistance for diffusion in the beta zeolite. These observations indicate that the shape selective nitration of toluene takes place inside the zeolite pores. The catalyst with alumina binder was found to be more active than the catalyst without binder because of higher acidity. The deactivated catalyst can almost be regenerated by washing the catalyst with organic solvents.