On the stability and nature of adsorbed pentene in Brønsted acid zeolite H-ZSM-5 at 323 K

Abstract

Adsorption of linear pentenes in H-ZSM-5 at 323K is investigated using contemporary static and molecular dynamics methods. A physisorbed complex corresponding to free pentene, a π-complex and a chemisorbed species may occur. The chemisorbed species can be either a covalently bonded alkoxide or an ion pair, the so-called carbenium ion. Without finite temperature effects, the π-complex is systematically slightly more bound than the chemisorbed alkoxide complex, whereas molecular dynamics calculations at 323K yield an almost equal stability of both species. The carbenium ion was not observed during simulations at 323K. The transformation from the π-complex to the chemisorbed complex is activated by a free energy in the range of 33–42kJ/mol. Our observations yield unprecedented insights into the stability of elusive intermediates in zeolite catalysis, for which experimental data are very hard to measure.