Cryst. Res. Technol. 45, No. 10, 1035 – 1040 (2010) / DOI 10.1002/crat.201000369 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Preparation and characterization of Si modified Ge 9 cluster Tiezhen Ren* 1 , Pengyan Xiao 1 , Xiang-Ying Xing 1 , and K. E. Christensen 2 1 School of Chemical Engineering and Technology, Hebei Univ. of Technology, Tianjin 300130, P. R. China 2 Diamond Light Source Ltd, Diamond House, Chilton, Didcot, Oxfordshire, OX11 0DE, United Kingdom Received 2 July 2010, revised 31 August 2010, accepted 2 September 2010 Published online 17 September 2010 Key words Ge 9 cluster, Si-modification, hydrothermal synthesis. Open-framework germanates show interesting pore structures due to their flexible building units. Germanates with different topology built from the Ge 9 cluster can be prepared in several amine systems. This paper reports, the Si modified germanate ICMM6 (denoted as HUT-2) prepared in ethylene diamine solution through a hydrothermal synthesis route. HUT-2 is a silicogermanate with space group C222 1 (a=12.1299(6) Å, b=13.9329(11) Å, c=14.2205(6) Å, V=2403.33(25) Å 3 ) formed by the Si modified Ge 9 cluster. A variation of unit cell parameters is observed that is in accordance with Si-content. Powder X-ray diffraction, ICP and TGA-DSC analysis were recorded to determine the Si-composition at the tetrahedral sites. Structure refinements against the X-ray powder diffraction (XRPD) pattern determined the occupancy of Si at the two tetrahedral sites. An approximate Si/Ge ratio of 0.13-0.18 is obtained based on these characterizations. TGA-DSC analysis showed obvious weight loss of amine at 315 °C, which is higher than the reported pure germanate ICMM6. This further confirms the existence of Si within the structure of HUT-2. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Research in open-framework germanates is important to construct new structures that can guide advanced application in many fields. A number of different clusters are found in open-framework germanates, such as the Ge 7 X 19 (Ge 7 ), Ge 8 X 20 (Ge 8 ), Ge 9 X 26-m (Ge 9 ; m = 0-1) and Ge 10 X 28 (Ge 10 ; X = O, OH, F) clusters showing 4-, 5- and/or 6-coordination for germanium. Several crystalline pore structures of germanates have been reported [1-6]. Chiral open-framework compounds are of great importance in applications of enantioselective separation and synthesis. Some chiral germanates have been reported in literature [3,7]. The chiral structure of ICMM6 [7] built from the Ge 9 cluster was tested in the reaction of acetalization and Michael addition and showed high activation and selectivity. The drawback of germanates is poor thermal stability, which limits their further applications. But the diversity of Ge-coordination to oxygen and the T-O-T angles (av. 130°) provide the possibility to incorporate Si, Al, Zn and B etc. into these frameworks [2-5,8]. Especially Si-incorporation can increase the thermal stability. Si-modifications have been reported for SU-15, SU-32, SU-57 [3,8] and more. The silicogermanate ITQ-21 with high amounts of Si at the tetrahedral sites have proved very effective for catalytic cracking of crude oil at 500 °C [2]. The incorporation of Si at the tetrahedral sites is possible if a reasonable reaction procedure can be designed. When comparing with pure germanates, the incorporation of Si contributes to the XRD pattern by changing the peak intensity and give slight shifts of peak positions. This is important information to judge whether Si has been incorporated or not. Here we combine several techniques to find the exact distribution of Si in the framework of ICMM6. ICMM6 has been prepared in several amine systems; both the homopiperazine and 1-methylpiperazine system produced ICMM6 and the impurity GeO 2 with the assistance of alcohol [7,9]. 1,4- diazabicyclo[2.2.2]octane (DABCO) used in an alcohol system within certain ratios produced the pure phase ICMM6. The charge balancing molecule is ethylenediamine, which is decomposed from DABCO [7]. In this paper we report a Si modified ICMM6 structure (HUT-2) with Si incorporated into the Ge 9 cluster (Spacegroup C222 1 , a=12.1299(6) Å b=13.9329(11) Å c=14.2205(6) Å). Another type of crystal with different Ge 9 cluster (HUT-5) can be prepared in a non-Si source system. The thermal stability of HUT-2 is improved to above ____________________ * Corresponding author: e-mail: rtz@hebut.edu.cn