Question

Fisher process with explanation?

Answer 1

Fischer–Tropsch process

The Fischer–Tropsch process is a collection of **chemical reactions** that converts a mixture of **carbon monoxide** and **hydrogen** into liquid **hydrocarbons**. These reactions occur in the presence of certain metal **catalysts**, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of

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It serves as an important reaction in both coal liquefaction and gas to liquids technology as well as many other chemical processes aimed at producing compounds based on hydrocarbon chains.[1] It works by combining carbon monoxide and hydrogen that are produced from coal, natural gas, or biomass in a process known as gasification, and the Fischer-Tropsch process then turns these

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gases into a synthetic lubrication oil and synthetic fuel.[2] The Fischer-Tropsch process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons.

A Fischer-Tropsch-type process has also been suggested to have produced a few of the building blocks of DNA and RNA within asteroids.[3] Similarly, naturally occurring FT processes have also been described as important for the formation of abiogenic petroleum.

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The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula $(C_{n}H_{2n+2})$. The more useful reactions produce alkanes as follows:

$$(2n + 1) H_2 + n CO \rightarrow C_nH_{2n+2} + n H_2O$$

where $n$ is typically 10–20. The formation of methane ($n = 1$) is unwanted. Most of the alkanes produced tend to be straight-chain, suitable as diesel fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons.[4]

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Fischer-Tropsch intermediates and elemental reactions

Converting a mixture of $\mathrm{H}_2$ and CO into aliphatic products obviously should be a multi-step reaction with several sorts of intermediates. The growth of the hydrocarbon chain may be visualized as involving a repeated sequence in which hydrogen atoms are added to carbon and oxygen, the C–O bond is split and a new C–C bond is formed. For one –CH₂– group produced by CO + 2 H₂ → (CH₂) + H₂O, several reactions are necessary:

- Associative adsorption of CO

- Splitting of the C–O bond

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- Dissociative adsorption of $2 \mathrm{H}_{2}$

- Transfer of $2 \mathrm{H}$ to the oxygen to yield $\mathrm{H}_{2} \mathrm{O}$

- Desorption of $\mathrm{H}_{2} \mathrm{O}$

- Transfer of $2 \mathrm{H}$ to the carbon to yield $\mathrm{CH}_{2}$

The conversion of CO to alkanes involves **hydrogenation** of CO, the **hydrogenolysis** (cleavage with $\mathrm{H}_{2}$) of C–O bonds, and the formation of C–C bonds. Such reactions are assumed to proceed via initial formation of surface-bound **metal carbonyls**. The CO **ligand** is speculated to undergo dissociation, possibly into oxide and **carbide** ligands.[5] Other potential

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intermediates are various $C_1$ fragments including formyl (CHO), hydroxycarbene (HCOH), hydroxymethyl (CH₂OH), methyl (CH₃), methylene (CH₂), methylidyne (CH), and hydroxymethylidyne (COH).

Furthermore, and critical to the production of liquid fuels, are reactions that form C–C bonds, such as **migratory insertion**. Many related stoichiometric reactions have been simulated on discrete **metal clusters**, but homogeneous Fischer–Tropsch catalysts are poorly developed and of no commercial importance.

Addition of isotopically labelled alcohol to the feed stream results in incorporation of

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alcohols into product. This observation establishes the facility of C–O bond scission. Using $^{14}$C-labelled *ethylene* and *propene* over cobalt catalysts results in incorporation of these olefins into the growing chain. Chain growth reaction thus appears to involve both ‘olefin insertion’ as well as ‘CO-insertion’.[6]

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