(82d) From Zeolite Nanocrystals to Zeolite Films and Membrane

The idea of making zeolite into membrane for separation and films for molecular selective sensing starts about 15 years ago, with Prof. Y. H. Ma as one of the initiators. At about the same period, zeolites in the form of colloidal particles were first prepared, which brings additional momentum to the development of zeolites membrane and films. Over the last 15 years, substantial progress has been accumulated. Zeolite membrane process for alcohol and water separation has been commercialized, and zeolite films have been extended to various applications including corrosion resistance and hydrophilic coatings. At the same time, zeolite nanocrystals as small as 50 nm are now easily produced, leading to transparent film as well as self-assembly structure. Most of the zeolite membrane is prepared by the secondary hydrothermal growth on pre-coated, and hopefully aligned seed crystals. However, it will be difficult to control the thickness of the resulted zeolite separation layer when grow from a solution. For membrane application, it is desirable that the zeolite separation layer is as thin as possible, while remain pinhole-free. We had always believed that it might be better to coat a thin layer of zeolite precursors on support and convert them into zeolite by steam treatment. As we were developing this technique, it was found that the preparation of precursors was very critical. In the search of better preparation method, we tried to add surfactants to the zeolite precursor, and it turned out that the surfactant was able to orient the so prepared zeolite film into regular pattern after steamed. We then looked into the mechanism of zeolite nucleation during the steaming synthesis. We found that the control of humidity can be used a tool to limit the growth of crystal size. Recently, we also found that by concentrate the precursor sol, as happened after coating the precursor layer, nanocrystals as small as 50 nm can be very quickly produced. Zeolite nanocrystals of such size are transparent when coated onto substrate. The porosity and thus low refractive index of zeolite promise the application as anti-reflective coatings for plastic substrate, as long as we can remove the occluded templates without calcination. In this direction, there is a recent report that boiling in acetic acid may do the trick.