Spatial distribution, strength, and dealumination behavior of acid sites in nanocrystalline MFI zeolites and their catalytic consequences

Abstract

The distribution, acid strength, and steam stability of internal and external acid sites were systematically investigated for nanocrystalline MFI zeolites with different crystallite size (2-300 nm), which were obtained using gemini-surfactants having different alkyl chain lengths. FT-IR spectroscopy using pyridine and 2,6-di-tert-butylpyridine as a base probe revealed that the concentration of external acid sites increased with a decreasing crystallite size. The external sites were weaker than the internal sites in acid strength, but could still catalyze n-octane cracking reactions that require the strongest acid strength among various acid-catalyzed reactions. Most notably, the external acid sites of zeolites exhibited remarkably higher steam stability at 873 K than the internal acid sites. Such an observation was supported by the catalytic results in methanol to dimethylether (DME) and gasoline conversion, and the Claisen-Schmidt condensation involving bulky molecules. (C) 2012 Elsevier Inc. All rights reserved.