Effect of Al on Zeolite Beta Solid State Chemistry

Abstract

Zeolite beta free of alkali has been prepared over a wide range of Al concentrations in the presence of tetraethylammonium (TEA+). The as obtained materials have been characterized by a combination of magic angle spinning nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and thermal gravimetric analysis. The results show that zeolite beta materials exchange [[≡SiO]− TEA+] ion pairs for [[AlO2]− TEA+] ion pairs upon addition of more Al into the synthesis solutions. The term [[AlO2]− TEA+] accurately describes the 4-connected structure of Al in the zeolite beta framework. On the other hand, the [[≡SiO]− TEA+] ion pair descriptor hides the fact that these ion pairs are accompanied by the presence of high concentrations of Q3-Si(3Si, OH) and Q3-Si(3Si, O−) units. The isomorphous substitution of Al into the zeolite beta framework leads to Bronsted acid sites in samples which have longer coherence length and higher connectivity. Upon removal of the structure director from high Al zeolite beta samples the unit cell contracts along the c-axis but not along other axes presumably due to geometric flexibility provided by the parallel rows of oxygen atoms present along a and b directions.