Mechanistic study of zeolites recrystallization into micro-mesoporous materials

• The dissolution/re-assembling mechanism of zeolites recrystallization is proposed.
• 1 Step: ion exchange of protons with sodium cations and zeolite desilication.
• 2 Step: diffusion of CTAB in the zeolite pores and ion exchange of Na+ with CTA+.
• 3 Step: formation of micelles and condensation of siliceous species around micelles.
• The resulting material represents zeolite with ingrown and overgrown mesoporous phase.

The mechanism of mordenite recrystallization has been studied using multinuclear MAS NMR, X-ray diffraction, IR spectroscopy, transmission electron microscopy, thermogravimetric analysis and nitrogen adsorption–desorption. The recrystallization procedure involved mixing of zeolite with alkali solution containing cetyltrimethylammonium bromide (CTAB) surfactant, heating the mixture in the autoclave to 423 K and hydrothermal treatment at 423 K. The intermediate products were recovered during each reaction step and thoroughly investigated. The results pointed to dissolution/re-assembling mechanism involving (i) fast ion exchange of protons with sodium cations and zeolite desilication, (ii) the diffusion of CTAB inside the pores and the ion exchange of sodium with CTA-cations and (iii) the formation of micelles and the condensation of siliceous species around micelles both in the intracrystalline mesopores formed during mordenite desilication and on the external surface of zeolite crystallites.

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