Computer graphics space-filling representation of the m- (left) and p- (right) isomers of xylene in the zeolite catalyst ZSM-5. Zeolites are a group of materials, based on silicon, aluminium, phosphorus & oxygen, that have structures based on interconnecting cavities & channels which form an ideal environment for shape-selective catalysis. Industrial applications include the cracking of petroleum, isomerisation & hydrocarbon synthesis reactions. In this image, the proximity of methyl groups in m-xylene (left), prevent its free diffusion along ZSM-5's straight channel. In contrast p-xylene (left) is able to move freely.