Synthesis of defect-free zeolite-alumina composite membranes by a vapor-phase transport method

The value of pH for gel preparation has a significant influence on the compactness of a dry gel. A compact gel was prepared at high pH of ca. 12. An amorphous dry gel on a porous alumina support was crystallized by a vapor-phase transport (VPT) method. Membranes of low silica zeolites, analcime (ANA) and mordenite (MOR), were formed because alumina was partly dissolved and incorporated into the framework of zeolite. Coating of the alumina support with colloidal silica depressed the dissolution of alumina. On the alumina support coated with colloidal silica, ferrierite (FER) and ZSM-5 (MFI) membranes were obtained. Compactness of the MOR membrane was examined by a permeation test of 1,3,5-triisopropylbenzene (TIPB) at room temperature. Since no permeation of TIPB was detected, it was concluded that there existed no pinholes and cracks in the MOR membrane. The pervaporation test of a benzene/p-xylene mixture of which the molar ratio was 0.860, was carried out. The separation factor ((benzene/p-xylene)permeate/(benzene/p-xylene)feed) exceeded 160. Thus some shape selectivity appeared at the pore mouths of MOR. The formation process of the MOR membrane was studied by using SEM and XRD. The gel which penetrated in the alumina pores crystallized to MOR and formed a defect-free MOR-alumina composite layer.