Mechanistic Kinetic Models for Catalytic Alkylation of Toluene with Methanol for Xylene Production

Abstract

Mechanistic kinetic models were developed for the catalytic alkylation of toluene with methanol over H-ZSM–5 coated silicon carbide (SiC) foam catalyst at atmospheric pressure in the temperature range of 623–723 K, molar methanol to toluene ratio of 2:1 and at different catalyst weight to the toluene molar flow rate in the range 0.72–5.5 kg catalyst h/kg mol toluene in a stainless-steel flow reactor fabricated to house the ceramic foam blocks coated with catalyst. The kinetic models developed for the transformation were Langmuir-Hinshelwood-Hougen-Watson (LHHW) rate expressions based on a reaction mechanism, which involved the adsorption of reactants species on the active catalyst sites, surface reaction of the adsorbed species to produce products and desorption of products from the catalyst surface, assuming same kind of active sites on the catalyst. The optimization routine of Nelder-Mead simplex method was used to estimate the inherent kinetic parameters in the proposed models. The selection of the best kinetic model amongst the rival kinetic models was based on physicochemical and thermodynamic tests and statistical analysis was employed to further validate the best model. The rate-determining step for the alkylation of toluene with methanol over H-ZSM-5 coated silicon carbide foam catalyst was found to be the surface reaction between adsorbed toluene and adsorbed methanol. Excellent agreement was obtained between the experimental rate of reaction and conversion of toluene and the model predictions, with absolute relative residuals being at most 3.8% for conversion and 3.9% for rate of reaction. The activation energies and enthalpies of adsorption were predicted, as well as, their corresponding pre-exponential factors. The results of this study can be used for sizing the alkylation reactor for xylene production and optimization studies.