The bulk material dissolution method with small amines for the synthesis of large crystals of the siliceous zeolites ZSM-22 and ZSM-48

The hydrothermal reaction between silica glass and aqueous solutions of amines and sodium hydroxide has been investigated for the preparation of large crystals of siliceous zeolites. In the presence of n-butylamine at 160 °C needle-shaped crystals of ZSM-22 (TON-type) are formed. By adjusting the amount of n-butylamine used in crystallisation, crystals of up to 400 μm in length may be formed after 5 weeks of reaction. Calcination in air at 800 °C produces crystals free of template and scanning electron microscopy shows the samples to be free of cracks, with no evidence of loss of integrity of the specimens. The quality and indentity of the ZSM-22 crystals, both before and after calcination, is verified by using single crystal X-ray diffraction and 29Si MAS NMR and the refined structures compared to those previously reported for the same material. If the amine used in synthesis is 1,6-diaminohexane, usually used to prepare TON-type zeolites, then samples of ZSM-48, a disordered zeolite, are produced. These have very different crystal morphology, the samples being made up of intergrown plate-shaped crystallites agglomerated into spherical objects. The results we present illustrate the scope for the general use of the ‘bulk material dissolution’ method in the synthesis of siliceous zeolites as large single crystals.