Ethylbenzene disproportionation and ethylation over zeolite based catalysts: Effects of zeolite structure and acidity

Abstract

The disproportionation and ethylation of ethylbenzene has been studied over medium and large pore zeolite based catalysts in a fluidized-bed reactor. Focused is made on the effects of zeolites pore structure and acidity on activity, selectivity and reaction kinetics of EB disproportionation and ethylation. In this regard ZSM-5 with various Si/Al ratio and mordenite are selected as a medium and as large pore zeolites, respectively. The EB ethylation experiments are conducted using 1:1 EB to ethanol molar ratio and at various temperature levels and different contact times. The product analysis shows that diethylbenzene (DEB) is the main product. However, the large pore size mordenite catalyst also produces a small amount of triethylbenzene. Among the five HZSM-5 catalysts, the sample with SiO2/Al2O3= 80 gives highest EB conversion and is more selective to DEB although the p-DEB/m-DEB for the catalysts are comparable with other samples. Phenomenological based kinetics models are developed based on the experimental conversion and selectivity data for EB ethylation. The Langmuir-Hinshelwood mechanism with surface reaction control is considered for both the medium and large pore zeolites. The analysis of the developed model suggests that a Langmuir-Hinshelwood mechanism with weakly adsorbed EB and strongly adsorbed ethanol and product DEB fits the experimental data adequately. The kinetic analysis also shows that the modenite requires significantly low activation energy (45.2 kJ/mol) as compared to the ZSM-5 zeolite (112 kJ/mol). On the contrary, the estimated enthalpy of adsorption of product DEB on mordenite (85.3 kJ/mol) is significantly higher than the value on the ZSM-5 sample (67.7 kJ/mol). The enthalpy of adsorption of ethanol is found to be 32.4 and 52.4 kJ/mole on mordenite and ZSM-5 samples, respectively. In EB ethylation, the HZSM-5 catalyst with SiO2/Al 2O3= 80 requires lowest amount of activation energy compare to other ZSM-5 samples; which is reflected in higher DEB selectivity of this catalyst.