Shape-selective synthesis of para-diethylbenzene over pore-engineered ZSM-5: A kinetic study

• Detailed kinetic study of ethylbenzene alkylation with ethanol over P-modified ZSM-5.
• Good correlation for predicted and observed values for LHHW dual site mechanism.
• Good correlation of adsorption energetics with observed catalytic activity.
• Restricted transition state shape-selective mechanism for the formation of para isomer.

Alkylation of ethylbenzene (EB) with ethanol was carried out in a fixed bed, continuous, down-flow, tubular reactor at atmospheric pressure and hydrogen as a carrier gas in vapor phase. A detailed kinetic study was conducted over ZSM-5 zeolite extrudates pore-engineered by phosphate modification. The reaction was performed at EB: ethanol mole ratio of 4:1 and with varying contact time at temperatures of 613, 633 and 653 K. Due to the induced shape-selectivity in ZSM-5 by phosphate modification, para-diethylbenzene (PDEB) was formed as a major product and side products like benzene, meta-diethylbenzene (MDEB) and other isomers were formed in minor quantity. Langmuir–Hinselwood–Hougen–Watson (LHHW) method for dual site mechanism with an assumption of surface reaction controlling was applied to derive the rate equations. Activation energies for the ethylation of EB, dealkylation of EB and isomerization of PDEB were found to be 117.2, 186.0 and 298.6 kJ/mol respectively. Adsorption energetics showed that EB is weakly adsorbed, whereas ethanol is strongly adsorbed over the catalyst surface. Activation energy of PDEB formation was found to be lower than that of MDEB. Considering that the alkylation takes place mostly inside the zeolite channels, the results support restricted transition state selective mechanism for EB alkylation with ethanol over phosphate modified ZSM-5. Based on the proposed kinetic model, calculated values of EB conversion and PDEB formation were found to be in good agreement with the experimental results.

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