Advanced scanning electron microscopy techniques for structural characterization of zeolites

In this work, low-voltage field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) analyses were applied for the investigation of zeolite structures. Low-voltage electron imaging was applied to visualize the surface structures of ZSM-5 zeolites with a mosaic surface morphology, and cross-sectional imaging techniques were used to show the sub-surface etching of SAPO-34 zeolites that cannot otherwise be directly revealed by SEM. In the case of SAPO-34 zeo-type crystals, the hidden macropores with a framed butterfly shape introduced by NH4F etching were revealed. In addition, the combined low-voltage imaging and EDS triple-detector analysis allowed recording of the near-surface-layer element distribution in the zeolite crystals; this was exemplified on SAPO-34 crystals during dissolution for the first time. The results showed that some inner parts of the SAPO-34 crystals are more vulnerable than the outermost shell of the crystals, which were preferentially removed during the NH4F etching.