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The Fischer Tropsch reaction between $\ce{CO}$ and $\ce{H2}$ has the potential to give a wide range of products. However by judicious choice of catalytic metal it is possible to produce $\ce{CH4}$, $\ce{CH3OH}$, and other alkanes selectively. $\ce{Fe}$ and $\ce{Co}$ are most commonly used catalysts where $\ce{Fe}$ produces less $\ce{CH4}$, more oxygenated products and more alkenes and $\ce{Co}$ produces more $\ce{CH4}$, less oxygenated products and more alkanes. There are a number of other catalysts that can be used, for example $\ce{Rh}$ which can produce $\ce{C2}$ and higher oxygenates and hydrocarbons. This includes the production of $\ce{CH3OH}$. However, it is not selectively produced but is produced in a mixture of other products and in small amounts. My question is why can $\ce{CH3OH}$ not be produced selectively?

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  • $\begingroup$ Wikipedia says : <<The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons.>> However, you can obtain alcohols in small quantities, but that won't be satisfying for a synthesis. Do you ask about ethanol or ethane? Give more details about your research, and how did you think about this question. $\endgroup$ – Saghe Oct 18 '14 at 12:11
  • $\begingroup$ The question is not about how you can produce ethanol, but why it cannot be done selectively. You can produce ethanol using a Rh catalyst I think but it can't be done selectively and that is what i want to know the reason for. I am not enquiring about ethane. $\endgroup$ – user5181 Oct 18 '14 at 12:24
  • $\begingroup$ The Fischer Tropsch reaction between carbon monoxide and hydrogen has the potential to give a wide range of products. However by judicious choice of catalytic metal it is possible to produce methane, methanol and alkanes selectively. Fe and Co are most commonly used catalysts where Fe produces less methane, more oxygenated products and more alkenes and Co produces more methane, less oxygenated products and more alkanes. There are a number of other catalysts that can be used, for example Rh which can produce C2 and higher oxygenates and hydrocarbons. This includes the production of ethanol. $\endgroup$ – user5181 Oct 18 '14 at 12:52
  • $\begingroup$ However, it is not selectively produced but is produced in a mixture of other products and in small amounts. My question is why can ethanol not be produced selectively? $\endgroup$ – user5181 Oct 18 '14 at 12:52
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in Fischer-Tropsch synthesis-like reaction there are (roughly) following processes:

  1. $\ce{CO \rightarrow Surf-CO}$

  2. $\ce{H2 \rightarrow 2 Surf-H}$

  3. $\ce{CO + Surf-(CH2)_{n}-H \rightarrow Surf-CO-(CH2)_{n}-H}$

    3'. $\ce{CO + Surf-H \rightarrow Surf-CO-H}$

  4. $\ce{Surf-CO-(CH2)_{n}-H + 4 Surf-H \rightarrow Surf-(CH2)_{n\,+1}-H + H2O }$

  5. $\ce{Surf-CO-(CH2)_{n}-H + 3 Surf-H \rightarrow HO-(CH2)_{n\,+1}-H }$

  6. $\ce{Surf-CH2-CH2-(CH2)_{n}-H \rightarrow Surf-H + CH2=CH-(CH2)_{n}-H}$

  7. $\ce{Surf-(CH2)_{n}-H + Surf-H \rightarrow H-(CH2)_{n}-H}$

All processes happen simultaneously, and tuning catalyst and reaction conditions it is possible to hinder some processes. However, as you can see, as long as you have chain growth at all (that is necessary to have $\ce{C2}$ products, like ethanol), you cannot stop $\ce{C2}$ surface particles to participate in further chain growth, producing $\ce{C_{3+}}$ products. So, if chain growth happens at all, a mixture of products with different chain length is always obtained, and there is no way around it. On the other hand, on proper catalyst in proper conditions it is possible to prevent chain growth, but still allow reduction of carbon monoxide, gaining either methane or methanol depending on catalyst. So, in practice it is possible to obtain one of the following

  1. Methane

  2. Mixture of terminal alkenes and alkanes

  3. Methanol

  4. Mixture of linear terminal alcohols (OK, this case is a bit more complicated, as other oxygenates may be produced as well, giving rather complex mixture, but that's beyond the point)

It is still possible to obtain pure ethanol indirectly (using hydration of isolated ethylene), or using other processes. It is also possible to obtain product mixture where some products, say C2-C4 alcohols, will be dominating.

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  • $\begingroup$ The mechanisms you provide do offer a good explanation why ethanol isn't a common product, but your answer doesn't really draw this out. $\endgroup$ – matt_black Oct 18 '14 at 14:57

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