Insight into the dry-gel synthesis of gallium-rich zeolite [Ga]Beta

Abstract

The conversion of a dry gel with an n Si/ n Ga ratio of 8.5 to zeolite [Ga]Beta (structure-type BEA) with a framework n Si/ n Ga ratio of 11.6 was studied by scanning electron microscopy, X-ray diffraction (XRD), and multi-nuclear solid-state NMR spectroscopy. XRD indicated a fast formation of the long-range order while, as revealed by NMR spectroscopy, the rearrangement of the local structure occurs during a period of up to 65 h. This rearrangement of the local structure during the dry-gel conversion (DGC) process consists of different concerted mechanisms: The dominating step at the beginning of the DGC is a breakage of chemical bonds leading to a strong increase in the concentration of defect SiOH groups as indicated by 1H MAS NMR spectroscopy. After a conversion time of 16 h, the intensity of the 1H MAS NMR signals of defect SiOH groups and of the 29Si MAS NMR signals of Q 1, Q 2, and Q 3 silicon species decreases significantly, indicating a condensation of terminal bonds. High-field ( B 0=17.6 T) 71Ga MAS NMR investigations evidenced that the silanol condensation is accompanied by an incorporation of gallium into the zeolite framework. This finding is corroborated by the increase of the 29Si MAS NMR signals of Si(1Ga) and Si(2Ga) species. Two-dimensional 71Ga MQMAS NMR spectroscopy shows the presence of two types of tetrahedrally coordinated gallium atoms at crystallographically non-equivalent framework positions, but also distorted tetrahedrally coordinated extra-framework gallium species. The existence of the latter species explains the difference between the bulk (determined by AES/ICP) and the framework (determined by 29Si MAS NMR spectroscopy) n Si/ n Ga ratios. 1H and 13C MAS NMR spectroscopy indicate that the template molecules (tetraethylammonium hydroxide) are not affected by the DGC process, excluding a loss of mobility in zeolite [Ga]Beta obtained after a DGC time of 65 h.