High-resolution electron microscopy study of ZSM-12 (MTW)

The effect of different structure-directing agents (SDAs) on the nucleation of MTW (the structure code for ZSM-12 recommended by the International Zeo lite Association Structure Commission) is shown by means of transmission el ectron microscopy (TEM). A loose-fit SDA interacting only weakly with the f ramework seems to have pore-filling character only and results in large dom ains with rather perfect ZSM-12 framework structure. Conversely, a more tig ht-fit SDA interacts markedly with the framework during the crystallization and causes simultaneous growth of small domains as well as frequent twinni ng. By comparison of simulated and experimental electron diffraction patter ns as well as high-resolution transmission electron microscopy (HRTEM) imag es, the proposed crystal structure of ZSM-12 is substantiated. The presence of an organic SDA confined in the one-dimensional channels is established in HRTEM images obtained from calcined as well as as-synthesized specimens. This result is corroborated not only by corresponding image contrast simul ations, but also by a significant reduction in intensity of low-angle refle ctions in quantitative electron diffraction patterns obtained from the as-s ynthesized material.