Hexamethyleneimine and pivalonitrile as location probe molecules of Lewis acid sites on MWW-type zeolites

Abstract

Locations of Lewis acid sites (LASs) on three MWW-type zeolites, the conventional Al-containing MWW zeolite (Al-MWW), acid-treated Al-containing MWW zeolite (Al-MWW-AT) and interlayer-expanded Al-containing MWW zeolite (Al-IEZ-MWW), were clarified by IR method using hexamethyleneimine (HMI), pivalonitrile (PN) and CO as probe molecules. No significant difference was found for Brønsted acid sites (BASs) in the strength and positions, while the locations of LASs were distinguished. LASs existed only on the external surfaces of Al-MWW, and in the expanded interlayer micropores (12-membered ring, 12-MR) of Al-IEZ-MWW. The simple acid treatment with nitric acid was found to generate LASs in the interlayer 10-MR micropores of Al-MWW-AT. The reactivity of the three zeolites depending on the positions of the active LASs was further confirmed in the Meerwein–Ponndorf–Verley (MPV) reduction of different ketones in size; 4-methylcyclohexanone and 2-butanone. Al-IEZ-MWW was superior for the reduction of 4-methylcyclohexanone over LASs in the interlayer 12-MR micropores, and Al-MWW-AT also showed fair reactivity for the reduction of smaller 2-butanone over LASs in the interlayer 10-MR micropores.