TOKYO Zeolites -- crystals composed of silicon, oxygen and aluminum -- are good filters. Their porous structure can be fashioned into membranes to separate small molecules like water and carbon dioxide from larger ones.
Only a handful of companies make zeolite membranes commercially, however, and applications are mostly limited to dehydration -- that is, the removal of water from a chemical mixture. For example, they are used to recycle solvents in the semiconductor industry. In recent years, the membranes have been used to make ethanol for fuel and alcoholic beverages.