25.8B: Fischer-Tropsch Carbon Chain Growth
- Page ID
- 34627
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The Fischer-Tropsch process is a catalytic chemical reaction in which carbon monoxide (\(\ce{CO}\)) and hydrogen (\(\ce{H2}\)) molecules (i.e., "syngas") are converted into hydrocarbons of various molecular weights according to the following equation:
\[\ce{(2n + 1) H2 + n CO -> C_{n}H_{2n + 2} + n H2O} \nonumber\]
where \(\ce{n}\) is an integer. Thus, for \(\ce{n}=1\), the reaction represents the formation of methane.
\[\ce{3 H2 + CO -> CH4 + H2O} \label{2}\]
The Fischer-Tropsch synthesis reaction, in theory, is a condensation polymerization reaction of \(\ce{CO}\). The Fischer-Tropsch process conditions are usually chosen to maximize the formation of higher molecular weight hydrocarbon liquid fuels which are higher value products.
There are other side reactions taking place in the process, among which the water-gas-shift reaction.
\[\ce{CO + H2O → H2 + CO2} \nonumber\]
is predominant. Depending on the catalyst, temperature, and type of process employed, hydrocarbons ranging from methane to higher molecular paraffins and olefins can be obtained. Small amounts of low molecular weight oxygenates (e.g., alcohol and organic acids) are also formed.
Mechanism
The Fischer-Tropsch process produces a wide range of hydrocarbon products with different chain lengths. The process involves several steps including:
- Initiation: Adsorption of \(\ce{CO}\) onto the catalyst surface and its dissociation into carbon and oxygen atoms.
- Propagation: Repeated addition of methylene units (\(\ce{-CH2-}\)) to the growing carbon chain.
- Termination: The chain growth stops when a hydrogen atom is added to the end of the chain, forming a final hydrocarbon product.
- Chain growth probability (\(α\)):
This parameter describes the likelihood of a growing chain adding another carbon atom in each propagation step. A higher \(α\) value indicates a greater tendency to form longer hydrocarbon chains.
Catalysts
The type of catalyst used significantly affects the chain growth probability and product distribution. For example, cobalt catalysts tend to produce shorter chains compared to iron catalysts. Factors like temperature, pressure, and hydrogen-to-carbon monoxide ratio can also influence chain growth by impacting the rate of various reaction steps. Catalysts considered for Fischer-Tropsch synthesis are based on transition metals of iron, cobalt, nickel and ruthenium. FT catalyst development has largely been focused on the preference for high molecular weight linear alkanes and diesel fuels production. Among these catalysts, it is generally known that:
- Nickel tends to promote methane formation as in a methanation process in Equation \ref{2}.
- Iron is relatively low cost and has a higher water-gas-shift activity, and is therefore more suitable for a lower hydrogen/carbon monoxide ratio (H2/CO) syngas such as those derived from coal gasification
- Cobalt is more active, and generally preferred over ruthenium (Ru) because of the prohibitively high cost of Ru. In comparison to iron, Co has much less water-gas-shift activity, and is much more costly.
Given these constraints, commercially available Fischer-Tropsch catalysts are either cobalt or iron based. In addition to the active metal, the Fe catalysts at least typically contain a number of promoters, including potassium and copper, as well as high surface area binders/supports such as silica and/or alumina. Only iron-based FT catalysts are currently used commercially for converting coal-derived syngas into FT liquids, given Fe catalyst's inherent water gas shift capability to increase the H2/CO ratio of coal-derived syngas, thereby improving hydrocarbon product yields in the FT synthesis. Fe catalysts may be operated in both high-temperature regime (300-350°C) and low-temperature regime (220-270°C), whereas Co catalysts are only used in the low-temperature range. This is a consequence of higher temperatures causing more methane formation, which is worse for Co compared to Fe.