Abstract

Three linear diquats of different lengths (consisting of two quinuclidinium groups linked by a chain of 8, 6 or 4 methylene groups) have been used as structure directing agents in the synthesis of silica zeolites in the presence of fluoride anions. Phase selectivity changes induced by changes in the concentration of the crystallizing mixture or by the addition of Al have been investigated. For the longest diquat only Beta and ZSM-12 were detected by X ray diffraction in the product, with Beta being favoured at high concentrations, low temperatures and short crystallization times. As the length of the chain decreases other phases, in addition to Beta and ZSM-12, appear: ZSM-5 (for the diquat of intermediate length) and ITQ-8, ITQ-10 and ITQ-14 (for the shortest diquat). ITQ-8 is most likely a layered silicate, while ITQ-10 and ITQ-14 are stable zeolites with intergrown structures related to the zeolite Beta family. Notably, ITQ-14 was found to consist of crystallites with the so far hypothetical “structure C” overgrown on standard zeolite Beta crystals. The phase selectivity effect of introducing Al into the gel is generally similar to the effect of decreasing its water content. However, with the longest diquat a different zeolite, Nu-87, crystallized in the presence of Al in a wide range of Si/Al ratios (25–200) but not for pure silica compositions or for the shortest diquats.