Alkylation of 3-methylthiophene by 2-methyl-1-pentene over HY, HBEA and HMCM-22 acidic zeolites
Résumé
The alkylation of 3-methylthiophene with 2-methyl-1-pentene was investigated over acidic zeolites at 85 degrees C under atmospheric pressure in a fixed bed reactor. Three zeolites were tested, HY, HBEA and HMCM-22, which presented different pore systems but the same Bronsted acidity. Over these zeolites, 3-methylthiophene was only transformed by alkylation whereas 2-methyl-1-pentene underwent isomerization, alkylation and dimerization. No effect of the zeolite pore architecture on the initial activity was noted, but differences in stability and selectivity were observed. Indeed, HBEA was the most stable whereas HMCM-22 was the most selective in alkylation compared to olefin dimerization. The addition of toluene led to a decrease of the initial activity and the stability for all zeolites, due to competitive adsorption and coke formation. An Eley-Rideal mechanism was proposed for the alkylation reaction, where the thiophenic molecule reacts with the protonated olefin. The product distribution obtained experimentally was explained on the basis of the calculated B3LYP/cc-pVDZ stabilities of the intermediate carbocations involved in the steps proposed as rate determining.