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

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 H-1 MAS NMR spectroscopy. After a conversion time of 16 h, the intensity of the H-1 MAS NMR signals of defect SiOH groups and of the Si-29 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) Ga-71 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 Si-29 MAS NMR signals of Si(1Ga) and Si(2Ga) species. Two-dimensional Ga-71 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 Si-29 MAS NMR spectroscopy) n(Si)/n(Ga), ratios. (1)Hand C-13 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. (C) 2003 Elsevier Inc. All rights reserved.