Probing External Brønsted Acid Sites in Large Pore Zeolites with Infrared Spectroscopy of Adsorbed 2,4,6-Tri-tert-butylpyridine

The current work describes the development of a semiquantitative test to measure the density of Brønsted acid sites on the external surface of large pore zeolites. The method uses 2,4,6-tri-tert-butylpyridine (TTBP) as a probe molecule, which is protonated by Brønsted acid sites to form a TTBPH + ion. The N–H stretching vibration of the TTBPH + ion is detected by infrared (IR) spectroscopy. The TTBP molecule is excluded by size from entering the micropores of 12-membered ring zeolites and interacts only with the external surface of the zeolites. The IR-TTBP test is relevant for the optimization and better understanding of zeolite-based catalysts for applications such as hydrocracking, transalkylation, reforming, or ring-opening that use large pore zeolites and where external Brønsted acidity plays a role in catalyst activity and selectivity. The method was applied to zeolites Faujasite (FAU) and Beta (BEA) as well as to silica and alumina, which are relevant components in bound zeolite formulations. Unlike pyridine, TTBP does not interact with Lewis acid sites. A correlation was found between the amount of TTBPH + ions formed over the external Brønsted acid sites of zeolite FAU and Beta with various Si/Al ratios and the expected density of external Brønsted acid sites. The latter was determined from the total Brønsted acidity measured by IR-pyridine and the fraction of external to total surface area measured by N 2 physisorption. The use of triisobutylamine (TIBA) and perfluorotributylamine (PFTBA) to measure external Brønsted acidity in zeolite ZSM-5 and Faujasite was explored. These attempts were unsuccessful because of the decomposition of TIBA and the lack of a clear marker for protonated PFTBA.