Inorganic crystalline membranes are of interest in many industries as functional layers in, for example, porous or protective films. Zeolites are a class of inorganic crystalline materials with desirable characteristics for such membranes. Zeolites are crystalline aluminosilicates, containing molecular sized pores. For example, ZSM-5 zeolites have an MFI framework with intersecting pores having a diameter of 5 to 6 Angstroms. Zeolites are an important material for catalyzing reactions, and are commonly used in the catalytic cracking of petroleum and other reactions of hydrocarbons. They are commonly provided in a powder form and are used to selectively crack straight chain molecules over branched chain molecules. Furthermore, zeolite particles have been used to selectively form para-xylene, which is used to make terephthalic acid for various polymerization reactions and in the synthesis or rearrangement of xylenes.
Zeolites are formed with a three-dimensional structure in which tetrahedra of primarily SiO4 and AlO4 are crosslinked by sharing oxygen atoms, whereby the ratio of Si to O atoms is 1:2. Depending on the manner of bonding, the crystal structures can have various ring sizes containing 8, 10, 12, or 14 metal/semi-metal atoms. These rings define pores and chambers that are of molecular size. The presence of these chambers promotes the separation of molecules according to their size and their affinity to the zeolitic surface. Because zeolites are commonly available in the form of fine powders, catalytic systems including zeolites frequently use packed or fluidized beds made of zeolite crystal powders or small pellets made up of multiple zeolite crystals.
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