Thierry Des Courieres

Study of the state of aluminium in zeolite-β

Abstract The modifications of the state of aluminium when introducing protons into zeolite-β have been investigated by combining several analytical techniques. Low temperature (350°C) thermolysis of ammonium cations or treatment with dilute nitric acid provokes the development of a27Al NMR signal at 0 ppm attributable to aluminium in octahedral symmetry. Calcination at 550°C generates NMR invisible aluminium and a strongly acidic hydroxyl group vibrating at 3780 cm−1 in the infra-red spectra. The neutralization of the protons by reaction with a solution of pyridine in ethanol at room temperature or with gaseous ammonia at 100°C allows for all the aluminium atoms of the solid to recover a tetrahedral symmetry. The AlIV → AlVI transformation can be also reversed by replacing the protons by sodium or potassium cations. It is then concluded that the octahedrally coordinated and NMR invisible aluminium must be regarded as inherent parts of the framework of the zeolite. Their formation is explained by a distortion of aluminic sites caused by the high electron affinity of the proton.

Single step dealumination of zeolite beta precursors for the preparation of hydrophobic adsorbents

Abstract Zeolite beta precursors containing the occluded organic template have been dealuminated by reaction with nitric acid solution at reflux temperature. Si/Al ratios greater than 1000 can be obtained in a single step, without significant loss of crystallinity, porous volume and thermal stability. The dealuminated zeolite contains mesopores and three types of silanol defect groups. The latter are readily annealed by calcination. Dealuminated zeolite beta exhibits a strong hydrophobic/organophilic character.

STABILITY OF THE TETRAHEDRAL ALUMINIUM SITES IN ZEOLITE BETA

Calcination of as-synthesized or ammonium-exchanged forms of zeolite BETA induces an extensive loss of framework aluminium evidenced by27Al MAS NMR, XRD and FTIR spectroscopies. Treatment of the calcined solids in ammonium nitrate restores the tetrahedral Al sites. Material balances show that the latter process is quantitative.

Metastability of Zeolites in Tetraethylammonium Media

Summary The influence of organic concentration, Si/Al ratio, stirring and nature of the silica source on zeolite synthesis in the presence of tetraethylammonium have been studied. Examples of metastability of zeolite beta and ZSM-5 towards ZSM-12 propose the last as the more stable zeolitic phase in tetraethylammonium media featuring highest Si/Al ratio. Evidences of heterogeneous nucleation of zeolites over amorphous or crystalline phases are discussed.