High-silica MTW (ZSM-12) was prepared by using a »two-silica« source strategy with tetraethylammonium hydroxyde (TEAOH), acting as a simple structure directing agent [A. Mitra et al., Micropor. Mesopor. Mater. 54, 2002, 175-186]. On the basis of conventional laboratory X-ray powder diffraction data (PANalytical X'Pert PRO MPD), a structure determination of this compound by using Topas Academic program suite was performed.

Due to the resemblance between powder patterns of calcined ZSM-12 (crystallizing in C2/c space group with unit cell parameters a=24.8633(3), b=5.01238(7), c=24.3725(7) Å, β=107.721(6)°) and that of ZSM-12 with TEA cations, the former was used as a starting model [ICSD Code 40137]. However, the powder pattern of the new compound possesses some additional diffraction peaks with k index not divisible by three revealing superstructure with three times longer edge b (15.1908(4) instead of 5.01238(7) Å). This is a consequence of ordered arrangement of TEA cations in silicate framework: in each channel traversing along b edge two of them (a repeating unit) are settled, what is also consistent with the results of TGA measurements. According to the results of Raman spectroscopy only TEA cations of tt.tt conformation are present. They were presumed to behave as rigid bodies and positioned by the use of simulated annealing approach. In space group C2/c, the obtained solution had some short contacts between the cations and the framework. Therefore, a decision to lower the symmetry and solve the structure in Cc space group with the absence of two-fold rotation axis in the channels (which allowed only rotations of TEA cations around b axis) was taken. It yielded a reasonable arrangement of TEA cations and acceptable agreement between calculated and observed powder patterns with R_wp 9.86%.

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