SELECTIVE .DISPROPORTIONATION OF TOLUENE TO PRODUCE BENZENE AND p-XYLENE

Abstract

Due to the increasing demand of benzene and xylenes in global market as intermediate raw material for the manufacture of polyester fibers and synthetic resins in recent years, newer techniques in converting toluene to benzene and xylenes is being explored. Among the xylenes, p-xylene is of considerable importance as raw material for terephthalic acid and dimethyl terephthalate manufacture required for polyester fibers, vitamin synthesis and other pharmaceuticals. Toluene disproportionation has the advantages for producing both xylenes as well as benzene. This reaction to produce equimolar amounts of benzene and xylenes is well known. Earlier studies on disproportionation has been done using Friedel Craft catalysts of metal halides; solid acid catalysts, such as silica-alumina and synthetic zeolites. Apparently none of these satisfied the need of industry. Zeolites are shown to be superior compared to amorphous solid acid catalysts for disproportionation reaction. Several studies have been performed using H-mordenite, X- and Y-zeolites and ZSM-5 zeolites in ion-exchanged forms. The ion-exchange was done using base metals/transition metals/rare earth cations. Among all the catalysts HZSM-5_ zeolites were found to be superior catalysts in giving better activity and selectivity towards the desired products. The present work is aimed at using modified and unmodified zeolites as prospective catalysts for the reactions. Modification has been done by cation exchange of rare earth/transition metal. The compound which has been used by previous workers for chemicaldeposition on HZSM-5 and other zeolites having much cost compare to the compound which has been used at present work for cation exchange. Previous workers have performed the disproportionation reaction at high pressure, whereas, in the present work, the reactions were fl performed at atmospheric pressure. By choosing the : conditions as mentioned ' '1 above operating cost as well as the material cost may be minimized. The parameters such as, effect of preparation method of catalysts, surface acidity, I shape selectivity, crystal size and operating condition on the activity and selectivity for p-xylene have been studied. In suceeding part of the work, the best catalyst from the above study was used to explore some of the kinetic aspects and the various reaction constants were evaluated. The thesis has been presented in five chapers, the summary of each is given below: Chapter I has been divided into two parts. (a) Part (a) deals with the introduction to the disproportionation process and the statistics consisting of present and future demands and supply of benzene, toluene and xylenes. _ _ _ __ (b) Review of the existing literature on toluene disproportionation catalyst as well as kinetics of the reaction. Chaper II describes the different methods employed for the preparation of catalysts. A discussion regarding zeolites structure and nature of acid sites has also been included in this chapter alongwith the methods of analysis of the catalyst samples. Chapter III gives the details of the experimental set-up and operational technique employed for taking runs. The studies conducted in different ranges of operating parameters for all the catalysts is given below. Chaper IV comprises of the calculation procedures and kinetic derivation. Homogeneous and heterogeneous model have been derived. The calculation procedure for OH, Kz etc. are also encorporated in this Chapter. Chapter V deals with the calculation procedures. of conversion and selectivity of the disproportionation reaction In this chapter results of disproportionation reaction is tabulated as well as some graphs have been presented. Chapter VI deals with the result of the present investigation and interpretation with pertinent discussion. The possible explanation to the variations in the activities of the catalysts related to their acidity, crystallinity, nature of cations and the extent of exchange have been given in detail which are in accordance with the findings of the previous workers. Kinetic study was carried out using over 1.756%Crl.923%Th HZSM-5 SAR(32), HZSM-5 SAR