A Single-Event MicroKinetic assessment of n-alkane hydroconversion on ultrastable Y zeolites after Atomic Layer Deposition of alumina

Abstract

The acid strength of a commercial Y zeolite subjected to Atomic Layer Deposition (ALD) of alumina using alternating pulses of trimethylaluminium (TMA) and water was assessed by means of Single-Event MicroKinetic modelling of n-decane hydroconversion. Catalytic activity changes were rationalized in terms of changes in three catalyst descriptors: the physisorption saturation concentration, the total Brønsted acid site concentration, and the average acid strength of these sites. The former two descriptors were measured independently from N2 adsorption, and pyridine TPD and FT-IR, while the latter was determined as the reference alkene standard protonation enthalpy by model regression. Changes in aluminium content were characterized by means of 27Al MAS NMR. Physico-chemically reasonable differences in sorbate stabilization by Van der Waals interactions with the zeolite framework could not explain the observed differences in n-decane hydroconversion performance between the ALD-treated USY samples. Instead, an increase in catalytic activity by the ALD treatment could be assigned to the creation of new and stronger Brønsted acid sites in the zeolite micropores exhibiting alkene standard protonation enthalpies that were up to 7kJmol−1 more negative on the ALD-treated samples compared to the parent zeolite. The newly formed acid sites were associated with extraframework aluminium species in a distorted tetrahedral configuration and were predominantly obtained using long TMA pulse times and a large number of ALD cycles provided that water was effectively removed from the ALD reaction chamber prior to ALD.