Coating of zeolite nanosheets

Abstract

Zeolites are microporous crystalline aluminosilicates that are widely used as catalysts in petrochemistry and fine-chemical synthesis. While bulk zeolites can be used as catalysts and adsorption materials, thin zeolite films are suitable for applications such as catalytic membrane reactors, molecular sieve membranes and low-dielectric-constant materials. Zeolite nanosheets are silicate or aluminosilicate crystals with thicknesses on the order of one layer of the crystal structure (i.e., ~2 nm) and much larger lateral dimensions (i.e., ~10-100 nm). Nanosheets contain ordered molecular scale pores that are aligned through the sheet thickness. Compared with isotropic zeolites, synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets has more advantages with packing and processing. The overall goal of this research is to make coatings of zeolite nanosheets. To prepare the nanosheets, multilamellar MFI is synthesized as a precursor. Melt blending is applied to exfoliate the layered zeolite to achieve a polystyrene nanocomposite. A density gradient centrifugation process followed to purify the exfoliated zeolite nanosheets is able to remove both the polystyrene and the unexfoliated zeolite completely. After a suspension of zeolite nanosheets is produced, drop coating and spin coating are explored as the coating methods. Comparisons between these two coating methods are made after characterization of these films. For the drop coating method, drying temperature is varied and controlled to study its influence on the quality of zeolite films since it is a key factor for alignment of plate-like particles during sedimentation. For the spin coating method, spin rate is one of the most important operating parameters. Therefore, different spin rates accompanied with different dwell times are chosen for study when other parameters stayed the same. To ensure the removal of polystyrene and unexfoliated zeolite nanosheets, the purified MFI zeolite nanosheets are imaged by transmission electron microscopy. Surface information of the zeolite nanosheets films is characterized by scanning electron microscopy and optical microscope. The degree of particle orientation, close packing and surface coverage are determined from their images. Out-of-plane and in-plane X-ray diffraction data are recorded and analyzed to give more quantitative information about the orientation of the coatings.