Studies on transalkylation over modified zeolite catalysts

Abstract

Alkyl-transfer of alkyl aromatics like toluene and benzene were studied for the purpose of understanding the principles that governs them. Transalkylation of low valued alkyl aromatics was done so as to convert these aromatics to industrially important products like xylene, cumene and cymene. Transalkylation of aromatics ranging from mono to polyalkyl-benzenes was studied in a fixed bed reactor system on solid acid catalysts (zeolites). In order to replace highly toxic, corrosive and non renewable traditional catalysts with nontoxic as well as renewable catalysts zeolites were used as catalysts for transalkylation of aromatics. In present thesis two different types of large pore zeolites, namely, Hbeta and NaX zeolites were modified using various rare earth metals and were used as catalysts for different transalkylation reactions in heterogeneous mode. The catalysts were refluxed with rare earth nitrate solution at specific conditions to carry out the ion exchange. The chemical analysis of catalysts was done by EDS to find the metal ion incorporated into the zeolite while the catalysts structure and crystallinity was established by powder XRD characterization technique. The surface morphology and particle size was determined by SEM studies. The quantity and strength of acidic sites present in catalysts were found using TPD results. The rare earth metal ions chosen for the modification of Hbeta and NaX zeolites were lanthanum (La), cerium (Ce) and Praseodymium (Pr). The catalyst activity was found to be a function of its acidity which in turn depends on the rare earth metal ion type and its concentration in the catalyst. The modified zeolites were able to catalyze the transalkylation reaction more effectively than the parent zeolites. These rare earth metal ions modified zeolite catalyst have shown excellent stability on time-on-stream without significant loss in activity. However, the parent catalyst was found to be less active and stable to carry out the transalkylation of aromatics. Transalkylation reactions were studied at temperatures up to 500°C. Results showed that alkyl-transfer reactions seemed to be dominant at higher temperatures. In transalkylation reactions, the reactant conversion depends on the type of alkyl substituent/s, ring conjugation of the aromatic moiety and the number of alkyl groups on the aromatic ring/s and the chain length. The outlined mechanism showed that the catalyst pore sizes and the type of pores as xxi well as the feed composition of binary mixtures play important roles in the transfer of alkyl groups between aromatic molecules. The activity of the zeolites also depends on type of ions exchanged with the H+ or Na+ ions present in the framework of the zeolite. Besides evaluating the catalyt performance, kinetics and thermodynamic parameters of the reactions were also evaluated. A suitable kinetic model was developed following several mechanisms such as dual site, single site mechanism with surface reaction as the rate controlling step. Activation energy for various transalkylation reactions were found to be in range of 50-130 kJ/mol. Zeolitic catalysts are deactivated through molecular retention which is dominant at lower temperatures. Bulkier alkylaromatics readily available in petroleum liquids can be transformed to commercially valuable products through transalkylation reactions with the help of a suitable catalyst. Thus, the transalkylation reactions have promising future applications in petrochemical and related industries.