Location of 6-azonia-spiro- [5,5]-undecane molecules in ZSM-12 using X-ray synchrotron powder diffraction data

Abstract

High-silica ZSM-12 (MTW topology), synthesized with 6-azonia-spiro- (Rimer, 2018; Rimer, 2018) [5,5]-undecane as structure directing agent (SDA), was carefully characterized by using X-ray synchrotron powder diffraction. The structure refinement led to a monoclinic unit cell (space group Cc, Z = 4, a = 25.0814(13) Å, b = 15.0987(8) Å, c = 24.4419(17) Å, β = 107.844(6), V = 8810.8(8) Å3) with a three times enlargement of the unit cell along the b axis with respect to the calcined starting ZSM-12 (space group C2/c, Z = 1, a = 24.8633(3) Å, b = 5.01238(7) Å, c = 24.3275(7) Å, β = 107.7215(6)°). The SDAs conformation and position in the framework structure did not maintain the twofold axes coinciding with the crystallographic twofold axis thus changing the space group symmetry from C2/c to Cc. The structure refinement gave an extraframework content 6.13 SDA molecules per unit cell (corresponding to ~9.0 wt%) in good agreement with the weight loss given by the thermogravimetric analysis. The refined bond distances revealed the occurrence of non-bonding interactions of both azonia molecules with the MTW pore wall. AZO-1 and AZO-2 molecules are H-bonded between them thus forming clusters aligned along the b axis. This information is crucial to tailor the physicochemical properties of zeolites as well as to address their hydrophobicity, pore dimensions and cavities thus optimizing the selectivity of zeolite catalysts.