I read here (https://chemistry.stackexchange.com/a/34016/101483) that strongly acidic catalysts can facilitate dimerization of alkene. With catalysts on zeolite to allow only the isooctane (octane rating 100) to come out as the only product, can non-recyclable plastics be turned into useful petrochemicals.

In this particular case of isooctane, polystyrene (C8H8)n is broken down into styrene (C8H8) the monomer. Styrene is hydrogenated into octenes (C8H16). The hydrogenation on octenes at the crucial third stage with strongly acidic catalysts on zeolite only yields isooctane.

Is the whole process feasible?

  • $\begingroup$ The process is scientifically feasible. The problem is always the economics of doing this v the cost of preparing the product from raw ingredients $\endgroup$
    – Waylander
    Mar 12 '21 at 7:54
  • 1
    $\begingroup$ If you have a cost-effective process for depolymerizing polystyrene back to relatively clean styrene, it's more valuable as styrene than fuel. But so far, such processes,while known, are not economically feasible. $\endgroup$
    – Andrew
    Mar 12 '21 at 17:42
  • $\begingroup$ Well, the directions of ideas shall diverge then. Let us take polystyrene (PS) as an example. Firstly, find an alternative and/or replacement of PS. Secondly, pyrolyze PS to simple hydrocarbons and refine them in similar processes existent in petrochemical industries. Thirdly, potential eight carbons monomer -- styrene (S) -- is made into isooctane (IO) the fuel, instead of polymerization. $\endgroup$
    – Kav
    Mar 12 '21 at 17:48
  • $\begingroup$ Andrew, I think you are suggesting that the cost-effective process -- if exists --- makes S itself valuable than burning S or PS as fuel. Or in other words, the process counters the motives to find a plastic replacement of PS. $\endgroup$
    – Kav
    Mar 12 '21 at 17:57
  • $\begingroup$ And if you found a replacement for PS, then there would be no S. It is produced primarily for making polymers like PS and ABS etc $\endgroup$
    – Andrew
    Mar 12 '21 at 21:43

The broad question here is whether "non-recyclable" plastics can be turned into fuel. The short answer to that is yes. More on that below. However, the specific proposal to convert polystyrene to isooctane is not particularly feasible. I'll address that first.

The proposed process begins with depolymerization of polystyrene back to styrene. Although lab-scale processes have been reported that give as high as 80% yield of styrene monomer, there is not yet an economically feasible process for scale-up. In fact, if styrene could be depolymerized practically, it would not be considered "non-recyclable". Furthermore, if the styrene could be recovered, it would be much more valuable as styrene than as gasoline, so there would be no incentive to spend the money on the subsequent steps described by OP.

Nonetheless, if one wanted to make liquid fuel from styrene, there is no need to produce isooctane specifically. Liquid fuels are generally mixtures of a variety of hydrocarbons, and the exact chemical composition does not especially matter as long as the autocombustion temp, vapor pressure and other relevant properties fall within an acceptable range.

So rather than make isooctane, one could simply hydrogenate styrene to ethylbenzene, an already common anti-knock agent or all the way to ethylcyclohexane (a more likely final product than octene).

Going back to the original premise of getting liquid fuel from plastics, just as there is no need to produce a specific compound from the styrene, there is no need to recover styrene specifically from the polymer, nor even restrict one's self to a specific polymer type such as polystyrene. Instead, bulk plastic mixtures can be pyrolyzed to produce a mixture not all that different from some crude oils and gases. These liquids and gases can be used as fuels themselves or mixed with existing petroleum products and refined.

Many challenges remain to make such a process economically viable, however, not least of which is the cost of collecting and transporting plastic waste (generally quite low density) compared to recovering oil from the ground.

  • $\begingroup$ sasol.com/media-centre/media-releases/… So tetramerizing ethene to 1-octene and specialized hydrogenation of 1-octene to isooctane may be possible. Skip the plastics reusing altogether. Ethene are plenty. $\endgroup$
    – Kav
    Mar 13 '21 at 18:53

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