Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane

Abstract

In the wake of the energy crisis, an efficient separation technology such as membrane is required to replace the energy intensive processes like distillation. High performance zeolite membrane can be fabricated by coating of a thin film of high-aspect-ratio zeolite nanosheets on a porous support. However, the synthesis of highly crystalline and morphologically intact zeolite nanosheets by the direct hydrothermal synthesis has been challenging. Successful reports on the synthesis of zeolite nanosheets by the exfoliation of their layered structure exist, but the synthesis routes provided in these reports often lead to significant damages to the structure and the morphology of nanosheets. This dissertation focuses on the development of a scalable method for the synthesis of zeolite nanosheets, while preserving their structure and the morphology. MWW and MFI nanosheets were prepared by polymer melt compounding of their layered precursors with polystyrene. Zeolite nanosheets were extracted out of the polymer matrix by solution processing of the zeolite-polymer nanocomposite. Exfoliated nanosheets and their coatings were then characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). A compact, oriented, 300-nm thick zeolite film was fabricated on a symmetric alumina support by a one-step filter coating method. This nanosheet film demonstrated molecular sieving capabilities after a mild hydrothermal treatment. Density gradient centrifugation was used to purify the zeolite nanosheets from the polymer matrix, and the large unexfoliated particles, resulting in a two-fold increase in the yield of nanosheets in the final coating suspension. Sub-100 nm thick films of these nanosheets were made on a symmetric alumina supports. Nanosheet films with thickness ranging from 10 nm to 100 nm were prepared on an asymmetric silica supports. In-plane secondary growth of these films by the impregnation growth method led to b-oriented, 100-150 nm thick zeolite film that separated xylene isomers with separation factors of 100-800, while providing a high permeance of p-xylene (4 x 10<super>-7</super> moles/m<super>2</super>-s-Pa).</