Role of Brønsted and Lewis acidity in the conversion of n-pentane on dealuminated H-Y, H-mordenite and HZSM-5

Abstract

Abstract n -pentane transformation has been used in order to study the respective role of Bronsted acid sites and, in one case, of Lewis acid sites. Pseudo-steady rates were used to evaluate the activity of four series of de aluminated acid zeolites, namely, HY (DHY), H-mordenite (DHM), H-ZSM-5 (DHZ) and fluorinated ultrastable Y (USYnF). These samples have been thoroughly characterized by 29 Si MAS NMR, low temperature CO adsorption FTIR, NH 3 chemisorption, XRD, adsorption isotherms of N 2 and pentane. For DHZ, DHM and USYnF, linear correlations are found between the total rate of conversion and Q 1 , which is the number of bridging OH groups linked to Si with one aluminum in its first coordination shell. The role of stronger Bronsted sites consisting of an OH bridging silicon to an aluminum with zero next-nearest-neighbor (O.n.n.n Al) has also been studied. In DHY a maximum in activity was observed when the Si/Al was about 5. Synergism between Lewis and Bronsted acid sites has been observed in the case of DHM. The selectivity to isomerization vs. cracking depends mainly on zeolite structure. Cracking is favored in DHZ while DHY and USYnF are more selective towards isomerization. In DHM the selectivity towards isomerization is almost 50%.