Friedel–Crafts acylation of 2-methylindole with acetic anhydride using mesoporous HZSM-5

Mesoporous HZSM-5 was compared with microporous HZSM-5 as catalyst for Friedel–Crafts acylation of 2-methylindole with acetic anhydride. Nitrogen sorption (BET and BJH methods) was used to determine the surface area, pore size and porosity of the materials. The adsorption isotherms showed presence of pores in the size range 6–7 nm for the mesoporous HZSM-5 and X-ray diffraction displayed a typical crystalline diffraction pattern corresponding to ZSM-5 for both the microporous and the mesoporous material. Dynamic light scattering was used to determine the particle size of the zeolites, which was shown to be approximately the same for the two materials. The mesoporous and the microporous ZSM-5 zeolites were prepared with the same Si/Al molar ratio and diffuse reflection infrared spectroscopy was used to confirm that the acidity of the materials was the same. Reaction over the microporous zeolite was regiospecific, giving acetylation only in the 3-position of 2-methylindole. Reaction over the mesoporous zeolite resulted in a mixture of two isomers, the N-acetylated product and the product with acetylation in 3-position, obtained in a 3:2 ratio. A kinetic model was constructed for the Friedel–Crafts acylation reaction, which correlated well with an Eley–Rideal reaction mechanism. The kinetic model allowed determination of rate constants, adsorption energy and activation energy for the reaction. It was found that the microporous HZSM-5 zeolite gave higher rate constants and lower activation energy compared to the mesoporous HZSM-5 zeolite. However, the mesoporous HZSM-5 zeolite gave lower adsorption energy and a considerably higher yield at the highest reaction temperature used, 89%, compared to 65% for the microporous HZSM-5 zeolite. Consequently, the catalytic material with the larger pore size gave a lower reaction rate but better resistance to poisoning and deactivation of the catalyst compared to its microporous counterpart.

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► Mesoporous HZSM-5 and microporous HZSM-5 identical in composition was prepared.
► The relationship between pore structure and catalytic activity was investigated.
► Microporous HZSM-5 had higher reaction rate compared to mesoporous HZSM-5.
► Mesoporous HZSM-5 gave C3- and N-acylation, microporous HZSM-5 gave only C3-acylation.
► Mesoporous and microporous HZSM-5 gave 89% and 65% acylation, respectively at 115 °C.