Synthetic zeolite. Molecular graphic of part of the structure of the synthetic zeolite ZK-5. Zeolites are used as catalysts and molecular sieves. The ions within ZK-5 (chemical formula Na30. Al30. Si66. O192. 98H2O) are colour- coded: yellow (sodium), red (oxygen) and white (silicon). Aluminium ions are not shown. Zeolites consist of a porous, negatively-charged, crystalline, aluminosilicate framework. Positively-charged ions can fit into the pores, which are manufactured to be the ideal size and shape to filter a particular chemical or catalyse one type of reaction.