Atomic-Scale Insights into Complex Intergrowth in Pentasil Zeolites

Pentasil zeolites are important engineering materials that are commonly used as catalysts for energy conversion and in chemical processes. The two most common forms of pentasil zeolites are the MFI and MEL. They often exhibit intergrowth behavior in complex structures. Such intergrowth structures alter the channel structures and, thus, may have implications for the catalytic properties of these materials. Although commonly observed, the causes and crystallographic mechanisms of MFI/MEL intergrowth are much less clear. This limits our ability to design and control desired structures for applications. This study investigated the crystallographic mechanisms of MFI/MEL intergrowth by means of atomic-scale electron microscopy and crystallographic analysis. The structures examined include cogrowth of an MFI/MEL (100) nanolamellar structure, MFI antiphase structures, and orthogonal growth of MFI subcrystals. It reveals that (100) interfaces of cogrown MFI/MEL nanolamellae are crystallographically perfect. For MFI antiphase structures, the (100) mirror-interface antiphase boundaries are fully coherent, whereas the (010) and (001) interfaces are fully incoherent, resulting in the formation of a thin (100) plate morphology of the antiphase domains. For orthogonally grown MFI subcrystals, the (100)/(010) interfaces always contain 50% mismatch of pentasil chain units. Such structural variations alter the size and morphology of channels at every changeover of the mirror, inversion, and mismatched connections in these compounds.