KINETICS OF TOLUENE DISPROPORTIONATION OVER UNMODIFIED AND MODIFIED ZSM-5 ZEOLITES

Abstract

The importance of disproportionation of toluene to produce benzene and xylenes is considerable due to the increasing demand of benzene and xylenes in chemical industries. The demand of toluene is found to be the lowest, whereas, the para-isomer of xylene is of greater significance due to its use in the manufacture of polyester fibres and many other petrochemicals. The commercial process for p-xylene production from toluene involves the vapour phase disproportionation which yields benzene and a mixture of xylenes as initial products inside the eolite pores. The products subsequently diffuse out of the zeolite pores. Owing to its smaller size and very high diffusivity, p-xylene diffuses out of the pores at a very high rate in comparison to o- and m-xylene. The ortho- and meta-xylenes undergo isomerization reaction within the zeolite. Several researchers have shown enhanced para selectivity of zeolites by the use. of modified agents such as Si, B, P and Mg. These modifiers have been reported to (i) partially block the pores, increase tortuosity and thereby delay the exit of larger molecules of o- and m-xylenes, and (ii) block the undesired (unselective) active sites and inactive at the external active sites for secondary iomerization. The modification of zeolite by ion exchange has been reported to increase the acid strength when pretreated by La' ion and increasing the initial toluene conversion and also the hydrothermal stability. Based on the fact that the ion — exchange of a zeolite may lead to (i) selectivity of the reactions based on the nature of the cation, (ii) blockage of the M zeolite pore mouth in proportion to the size of the exchange ion, leading to shape selectivity for the desired product (p-xylene) and also (iii) greater stability of the exchange zeolite than the modified form, the present work has been under taken using ion exchange from of ZSM-5. The present work deals with preparation , characterization activity and kinetics tests of the catalyst based on ZSM-5 zeolite for toluene disproportionation. The ion-exchange of the zeolite powder was performed by mixing the zeolite powder with respective metal nitrate solution having a desired metal content. The solution was heated at 60°C for about 2 h with constant stirring. The contents were further refluxed under a total condenser at 95±5°C for 72 h. The solid catalyst was then filtered, dried at 110 °C overnight and calcined at 550° for 5h. The dried powder,- in order to make it suitable for tests in the reactor, was pelletized at 10 tonne/cm2 pressure, and then broken and sieved to get particles in the specific size range of B.S.S. mesh -20 to +25. The catalyst activity and kinetic tests were carried out in a down flow vertical silica glass to bular reactor. The reactor tube having 1 cm inside diameter and 50 cm length was heated electrically and the temperature was controlled to within ±1 °C. The catalyst particles were mixed with quartz particles of similar size range and packed in the middle of the reactor tube. Above the catalyst bed, quartz particles (0.95— 1.14 mm size) were packed serving the dual purpose of preheating as well as mixing the reactants. Toluene feed was pumped into the reactor at a regulated rate using a peristalic pump. The nitrogen gas was simultaneously introduced at the top of the reactor tube trough a Y _junction and a specified weight hourly space velocity (WHSV) of the toluene feed was maintained. The nitrogen to toluene molar ratio in the reactor was maintained at 0,349 in all the runs. The product from the reactor were passed through a set of glass condensers fitted in series and cooled by ice-cold water. The liquid product at each reaction-temperature was collected separately in stoppered test-tubes for further analysis. The flow-rate of both the liquid and vapour was also recorded for overall mass-balance. The liquid product was analysed by a gas-liquid chromatograph using a 4 m long stainless steel column known as Celite 545 column. The oven temperature was kept at 100°C and the flow rate of nitrogen used as the carrier gas was maintained at 20 ml/min. The zeolite samples were characterized by FTIR and atomic absorption spectrometer for the strength of acid sites and percent ion exchange.