Effects of the synthesis hydrogel on the formation of zeolite LTA membranes

A zeolite/polymer composite hollow fiber (CHF) support is used to investigate the effects of three different silica sources (sodium metasilicate nonahydrate, silica sol and sodium silicate) of the synthesis hydrogel on the properties of zeolite LTA membrane prepared on the outer surface of the support by in situ hydrothermal synthesis at 100 °C. The as-synthesized membranes and powders from the synthesis hydrogel are characterized by X-ray diffraction. The morphology and thickness of the membranes are observed by scanning electron microscopy. The separation factor and flux are evaluated by pervaporation (PV) test at 75 °C using 90 wt.% ethanol aqueous solution. It is difficult to obtain high performance membranes on the CHF support using sodium metasilicate nonahydrate as silica source, while continuous zeolite membranes with high separation factor (>10,000) and flux (>6.4 kg/m2 h) can be well synthesized using silica sol or sodium silicate as silica source. The reason of these differences is further explored by changing the alkalinity of the synthesis hydrogel (the content of sodium hydroxide). It is found that the crystallization rate of the synthesis hydrogel is a crucial factor to obtain zeolite membranes with high PV performance by static in situ hydrothermal synthesis. The separation performance of the membrane synthesized even using sodium metasilicate nonahydrate as silica source can be greatly improved by dynamic hydrothermal synthesis using rotating autoclaves.

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► Zeolite/polymer composite hollow fibers are used as supports.
► Effects of three different silica sources on separation performance of zeolite membranes are investigated.
► Effects of the alkalinity of the synthesis hydrogel are also investigated.
► Uniform zeolite membranes with high separation performance are obtained with dynamic hydrothermal synthesis.