Synthesis and characterization of the new cyclosilicate hydrate (hexamethyleneimine) 4 [Si 8 O 16 (OH) 4 ]12H 2 O Pieter L.H. Verlooy a , Koen Robeyns b , Luc Van Meervelt b , Oleg I. Lebedev c , Gustaaf Van Tendeloo c , Johan A. Martens a , Christine E.A. Kirschhock a, * a Centre for Surface Chemistry and Catalysis, Catholic University of Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium b Biomolecular Architecture, Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium c EMAT, Department of Physics, University of Antwerp, Groenenborglaan 171, B-2020 Antwerpen, Belgium article info Article history: Received 31 July 2009 Received in revised form 5 October 2009 Accepted 8 October 2009 Available online 13 October 2009 Keywords: Cyclosilicate hydrate Hydrogen bonds Cubic silicate octamer Hexamethyleneimine Microporous material abstract A new cyclosilicate hydrate with composition (C 6 H 14 N) 4 [Si 8 O 16 (OH) 4 ]12H 2 O was crystallized and the structure determined by single-crystal X-ray diffraction. The structure, described by the tetragonal space group I4 1 /a, with unit cell dimensions of a = 39.2150(2) Å and c = 14.1553(2) Å, contains columns of hydrogen-bonded cubic octamer silicate anions. The space between silicate columns holds hydrogen- bonded water and protonated hexamethyleneimine molecules compensating the negative charge of the silicate. The crystal water can be removed resulting in a rearrangement of the columns into ortho- rhombic symmetry. Removal of the organic moiety causes amorphisation. Flash evacuation results in a new microporous material with pore volumes typical of a zeolite. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Cyclosilicates are a class of crystalline silicates based on indepen- dent ring structures consisting of three or more corner sharing SiO 4 4 tetrahedra [1]. Most cyclosilicates consist of single rings built from 3, 4 or 6 silicate tetrahedra. But also structures where two identical rings are condensed into trigonal or hexagonal prisms, or cubes (double four ring, D4R) are classified as cyclosilicates. In naturally occurring cyclosilicate minerals all terminal oxygen atoms carry a negative charge which is compensated by metal cations. Several syn- thetic cyclosilicates have been reported [1–31]. These structures are usually hydrates and contain organic cations or charged transition metal complexes. Cyclosilicate hydrates have been obtained from synthesis media comprising organic molecules that often also can serve as templates for zeolite synthesis. Fig. 1 shows some examples of suitable organics for cyclosilicate synthesis. Cyclosilicates and zeolites have been crystallized from solutions with similar molecu- lar composition depending on conditions. While cyclosilicate hy- drates typically grow under mild reaction conditions in presence of large concentrations of organic cations, hydrothermal conditions and lower organic cation concentrations stimulate zeolite crystalli- zation. In view of this connection between both types of materials, investigation of cyclosilicate hydrate structures might provide in- sight into formation processes of the fully condensed silicate frame- works in zeolites [3,5–8,11,21,22]. Only few synthetic cyclosilicates containing the trigonal prism have been reported [18,23–25,27,31]. The only synthetic cyclosili- cate containing the hexagonal prism was realized using a-cyclo- dextrine in combination with potassium or sodium ions [31]. The majority of known cyclosilicate hydrate structures are based on the cubic octamer (D4R) [2–22,26,28–30]. In these the organic cat- ion often is a quaternary ammonium compound with one or more methyl substituents on the positively charged nitrogen atoms (Fig. 1A) [2–20]. This is reminiscent of silica oligomerization in solution, where it is well documented the tetramethylammonium cation stimulates the formation of D4R [14,25,29,32–34]. The use of alkylammonium cations with longer alkyl groups, such as ethyl in tetraethylammonium leads to trigonal prisms (D3R) in solution and can initiate the formation of cyclosilicates containing trigonal prisms (D3R) (Fig. 1B) [18,23–25]. A further common feature of many organic cations suitable for D4R cyclosilicate synthesis is the presence of a rigid aromatic or polycyclic moiety (Fig. 1A), which directs the spatial organization of the silicate by hydropho- bic/hydrophilic interactions. In presence of metal–ethylenedia- mine complexes either D3R or D4R cyclosilicate hydrates are obtained, depending on whether Ni(en) 3 , Cu(en) 2 or Co(en) 3 is used (Fig. 1E) [26–30]. Cyclosilicate hydrates containing ethylenedia- 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.10.006 * Corresponding author. Tel.: +32 16 32 15 97; fax: +32 16 32 10 62. E-mail address: Christine.Kirschhock@biw.kuleuven.be (C.E.A. Kirschhock). Microporous and Mesoporous Materials 130 (2010) 14–20 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso