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

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.

Spatial distribution, strength, and dealumination behavior of acid sites in nanocrystalline zeolite were studied using the MFI zeolites with various crystallite size templated by gemini-surfactants. External acid sites of zeolites exhibited slightly weaker acid strength, but remarkably higher steam stability than internal acid sites.Figure optionsDownload high-quality image (95 K)Download as PowerPoint slideHighlights
► MFI zeolites with different crystallite sizes were hydrothermally synthesized.
► Concentration of external acid sites increases with decreasing crystallite size.
► Steam stability of external acid site is remarkably higher than internal ones.
► External acid site is slightly weaker than internal acid site in acid strength.
► External acidity could catalyze acid-catalyzed reactions requiring strong acidity.