The structure of PREFER, the layered precursor of silica-ferrierite, in its as-made, (C9H21N2)4[Si36O72(OH)4] * 8 H2O, and dehydrated form, (C9H21N2)4[Si36O72(OH)4

Abstract

Abstract “PREFER” (Precursor of Ferrierite) is a Hydrous Layer Silicate with chemical composition of (C 9 H 21 N 2 ) 4 [Si 36 O 72 (OH) 4 ] * 8H 2 O in its (hydrated) as-made form. The material was first synthesized by Schreyeck et al. in 1995. At that time, only a rough model of the crystal structure of the material was published due to ambiguous space group symmetry and a very complex arrangement of disordered guest molecules and water molecules in the inter-layer region. The crystal structures of as-made PREFER and dehydrated PREFER, (C 9 H 21 N 2 ) 4 [Si 36 O 72 (OH) 4 ], were now solved by simulated annealing. As-made PREFER crystallises in space group Im with a 0 = 13.9717(3) A, b 0 = 26.3466(7) A, c 0 = 7.4247(1) A, β = 90.279(2)° while dehydrated PREFER crystallises in space group Pnn 2 with a 0 = 14.0252(5) A, b 0 = 26.2850(17) A, c 0 = 7.4254(2) A. The volumes of the hydrated as well as the dehydrated structure are very similar, indicating that both structures resemble each other closely. In both structures the ferrierite-type silicate layers are stacked in an ABAB … -sequence along the b-axis with neighbouring layers being shifted relative to each other by ½ a 0 + ½ c 0 . This type of stacking creates two different types of voids, which are both occupied by one 4-amino-2,2,6,6-tetramethylpiperidinium cation, and, in the case of as-made PREFER, by additional water molecules. The silicate layers are linked via weak ionic bonds involving the nitrogen atoms of the cations and – in the hydrated form – also including hydrogen bonds with additional water molecules.