Been learning about catalytic cracking of long chain hydrocarbons into shorter chain hydrocarbons, in which the catalyst zeolite is used so that the temperatures needed for the cracking are much lower (about $500\,^\circ\text{C}$ instead of $700\text{-}1000\,^\circ\text{C}$). The question is, how exactly does the zeolite help with the cracking process? Everywhere I look doesn't give a detailed answer, just that zeolite is porous, riddled with holes that can trap molecules. The only clue I have to the process I suppose is this video


which shows an animation of some hydrocarbons whacking against a tunnel-like zeolite into smaller chain carbons. But to me the video can't be that accurate, because the whole thing seems out of proportion, the hydrocarbons are made from hydrogen and carbon which are pretty small atoms, whereas the zeolite is made from aluminium or silicon which are much larger. Even if this is the actual way it works, why not just have a plane surface to hit the hydrocarbons off to break them then? (Perhaps the rate of collision is much smaller? I'm just pulling at straws here.) Any light shed will be helpful, thanks.


1 Answer 1


The video you reference to does not tell the whole story. Yes, zeolites are minerals with open pores, similar to a filter, occurring in nature and are produced commercially, too. Equally to their structure, their composition is interesting -- as both are used to tailor the reactivity and selectivity of this material. ZSM-5 is one example of designed remarkably high Brønsted acidity. Hydrocarbons in contact with these sites split into fragments and rearrange because of the intermediate generation of carbocations. It is advantageous, as these reactions are heterogeneously catalysed by these "solid state acids".


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