Application of zeolite membranes, films and coatings

Abstract

Abstract Preparation methods of zeolite molecular sieve membranes and films with and without support were reviewed. Unsupported films have been prepared by in situ synthesis, casting of zeolite nanoparticles and solid state transformation, and supported films have been obtained by in situ synthesis, vapor-phase synthesis, secondary growth, casting of nanoparticles and their combinations or modifications. It is favorable to use supports to give mechanical strength to the zeolite films. In the film synthesis of AFI and MFI type zeolites on the substrates with flat, smooth and polished surfaces, the crystals tend to be oriented, and among the preparation procedures, secondary growth method, especially, secondary growth procedure using a surface modifier is apt to produce oriented zeolite films. The application of pulsed laser ablation and microwave heating to the synthesis of zeolite films is also effective for the densification and orientation. In the separation of a mixture by the zeolite membranes, the selectivity is a function of the sorption and diffusion, and relative permeances can not be simply predicted on the basis of molecular size or shape alone. Generally the separation selectivity of a mixture does not reflect the single-component permeation ratio. In a mixture, the permeation of weakly adsorbing molecules often drop dramatically in the presence of strongly adsorbing molecules. Nature of zeolites, hydrophobicity or hydrophilicity, straightforwardly reflects the separation of aqueous mixtures, accordingly, in the pervaporation of the mixtures of an organic compound and water, an organic compound is selectively separated with hydrophobic MFI(Si) membranes, on the contrary, water is predominantly removed with hydrophilic LTA and FAU membranes. Permeation properties and separation performances of zeolite membranes are much affected by not only the properties of the membranes but also the operation conditions of the membranes. Zeolite membranes are also useful for the optical resolution of racemic isomers. Zeolite films can be applied to chemical sensors with high selectivity and sensitivity by making use of the properties such as molecular sieving, hydrophobicity or hydrophilicity. Using zeolite membranes, disks and coatings, esterification, alkylation, synthesis of ether, conversion of methanol, deNOx reaction, hydrogenation and dehydrogenation have been carried out. In those reactions, it has been pointed out that zeolite membranes, disks and coatings show high performance or potential as the catalysts compared with conventional catalysts.