Control of the Al Distribution in the Framework of ZSM‑5 Zeolite
and Its Evaluation by Solid-State NMR Technique and Catalytic Properties
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Abstract
The
effects of the organic structure-directing agents (OSDAs) and
Na cations for the synthesis of ZSM-5 on the location of Al atom in
the framework as well as the acidic and catalytic properties were
investigated. To achieve these purposes, ZSM-5 zeolites were synthesized
by using four kinds of OSDAs including tetrapropylammonium hydroxide
cations, dipropylamine, cyclohexylamine, and hexamethylenimine with
or without Na cations. In situ FT-IR spectroscopy using CO as probe
molecule was applied to the evaluation of the acid property of the
ZSM-5 zeolites. The location of Al atoms was examined by high resolution <sup>27</sup>Al MAS and MQMAS NMR techniques. The constraint index (CI)
has also been used to estimate the distribution of acid sites in the
micropores. The location of acid sites was investigated based on the
difference in the transition-state shape-selectivity through the cracking
of <i>n</i>-hexane and 3-methylpentane. Furthermore, the
cracking of various types of paraffins and the conversion of aromatic
compounds were conducted to clarify the acid site distributions