Kinetic aspects of chain growth in Fischer–Tropsch synthesis

TLDR: During the FT reaction chain growth is much faster than chain depolymerization, which ensures high chain- growth probability, and the thermodynamic degree of chain-growth probability control emphasizes the critical role of the H and free-site coverage.

Abstract: Microkinetics simulations are used to investigate the elementary reaction steps that control chain growth in the Fischer-Tropsch reaction. Chain growth in the FT reaction on stepped Ru surfaces proceeds via coupling of CH and CR surface intermediates. Essential to the growth mechanism are C-H dehydrogenation and C hydrogenation steps, whose kinetic consequences have been examined by formulating two novel kinetic concepts, the degree of chain-growth probability control and the thermodynamic degree of chain-growth probability control. For Ru the CO conversion rate is controlled by the removal of O atoms from the catalytic surface. The temperature of maximum CO conversion rate is higher than the temperature to obtain maximum chain-growth probability. Both maxima are determined by Sabatier behavior, but the steps that control chain-growth probability are different from those that control the overall rate. Below the optimum for obtaining long hydrocarbon chains, the reaction is limited by the high total surface coverage: in the absence of sufficient vacancies the CHCHR → CCHR + H reaction is slowed down. Beyond the optimum in chain-growth probability, CHCR + H → CHCHR and OH + H → H2O limit the chain-growth process. The thermodynamic degree of chain-growth probability control emphasizes the critical role of the H and free-site coverage and shows that at high temperature, chain depolymerization contributes to the decreased chain-growth probability. That is to say, during the FT reaction chain growth is much faster than chain depolymerization, which ensures high chain-growth probability. The chain-growth rate is also fast compared to chain-growth termination and the steps that control the overall CO conversion rate, which are O removal steps for Ru.

Figures

Figure 5: Thermodynamic degree of chain growth control (TDCGC) as a function of temperature. A positive value indicates that stabilizing this species on the surface results in a higher chain-propagation probability and vice versa. Data for the FT reaction on stepped Ru at p = 20 atm and a H2/CO ratio of 2.

Figure 1: Microkinetics simulations of the FT reaction on stepped Ru at p = 20 atm and a H2/CO ratio of 2 of Ru(11231) surface: (a) CO and H2 consumption rates and CH4, H2O and C2+ production rates as a function of temperature. Note that the C2+ trace overlaps the H2O trace. (b) The chain-growth probability as a function of temperature, displaying a maximum at ~500 K.

Figure 2: Hydrocarbon product selectivity for C1-C50 as a function of temperature. The selectivity to C40+ hydrocarbons is highest at 500 K, consistent with the highest chain-growth probability determined at this temperature. Data for the FT reaction on stepped Ru at p = 20 atm and a H2/CO ratio of 2.

Figure 3: (a) Degree of chain-growth control (DCGC) as a function of temperature for relevant elementary reaction steps, whose DCGC value is larger than 0.01. Positive values imply that decreasing barrier for an elementary reaction steps increases the chain-propagation probability. (b) Reaction network diagram of the relevant elementary reactions steps and the direction of chemical conversion. Data for the FT reaction on stepped Ru at p = 20 atm and a H2/CO ratio of 2.

Figure 4: Surface coverage as a function of temperature for the FT reaction on stepped Ru at p = 20 atm and a H2/CO ratio of 2. The total coverage of growing chains on the surface is around 3%. The surface is predominantly covered with O.

Full text

Kinetic aspects of chain growth in Fischer-Tropsch synthesis
Citation for published version (APA):
Filot, I. A. W., Zijlstra, B., Broos, R. J. P., Chen, W., Pestman, R., & Hensen, E. J. M. (2017). Kinetic aspects of
chain growth in Fischer-Tropsch synthesis.
Faraday Discussions
,
197
, 153-164.
https://doi.org/10.1039/C6FD00205F
DOI:
10.1039/C6FD00205F
Document status and date:
Published: 01/04/2017
Document Version:
Accepted manuscript including changes made at the peer-review stage
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Download date: 09. Aug. 2022

Kinetic aspects of chain growth in Fischer
-
Tropsch synthesis
Journal:
Faraday Discussions
Manuscript ID
Draft
Article Type:
Paper
Date Submitted by the Author:
n/a
Complete List of Authors:
Filot, Ivo; Eindhoven University of Techynology, Department of Chemical
Engineering and Chemistry;
Zijlstra, Bart; Eindhoven University of Techynology, Department of
Chemical Engineering and Chemistry
Broos, Robin; Eindhoven University of Techynology, Department of
Chemical Engineering and Chemistry
Chen, Wei; Eindhoven University of Techynology, Department of Chemical
Engineering and Chemistry
Pestman, Robert; Eindhoven University of Techynology, Department of
Chemical Engineering and Chemistry
Hensen, Emiel; Eindhoven University of Techynology, Department of
Chemical Engineering and Chemistry
Faraday Discussions

Kinetic aspects of chain growth in Fischer-Tropsch synthesis
Ivo A.W. Filot, Bart Zijlstra, Robin J.P. Broos, Wei Chen, Robert Pestman, and Emiel J.M.
Hensen*
Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of
Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513,
5600 MB, Eindhoven, The Netherlands
E-mail: e.j.m.hensen@tue.nl
Page 1 of 27 Faraday Discussions

Abstract
Microkinetics simulations are used to investigate the elementary reaction steps that control
chain growth in the Fischer-Tropsch reaction. Chain growth in the FT reaction on stepped Ru
surfaces proceeds via coupling of CH and CR surface intermediates. Essential to the growth
mechanism are C-H dehydrogenation and C hydrogenation steps, whose kinetic
consequences have been examined by formulating two novel kinetic concepts, the degree of
chain-growth probability control and the thermodynamic degree of chain-growth probability
control. For Ru the CO conversion rate is controlled by the removal of O atoms from the
catalytic surface. The temperature of maximum CO conversion rate is higher than the
temperature to obtain maximum chain-growth probability. Both maxima are determined by
Sabatier behavior, but the steps that control chain-growth probability are different from those
that control the overall rate. Below the optimum for obtaining long hydrocarbon chains, the
reaction is limited by the high total surface coverage: in the absence of sufficient vacancies
the CHCHR → CCHR + H reaction is slowed down. Beyond the optimum in chain-growth
probability, CHCR + H → CHCHR and OH + H → H
2
O limit the chain-growth process. The
thermodynamic degree of chain-growth probability control emphasizes the critical role of the
H and free-site coverage and shows that at high temperature chain depolymerization
contributes to the decreased chain-growth probability. That is to say, during the FT reaction
chain growth is much faster than chain depolymerization, which ensures high chain-growth
probability. The chain-growth rate is also fast compared to chain-growth termination and
compared to the steps that control the overall CO conversion rate, which are O removal steps
for Ru.
Keywords: Fischer-Tropsch, Ruthenium, chain-growth, sensitivity analysis, degree of rate
control.
Page 2 of 27Faraday Discussions

Introduction
The widespread availability of cheap natural gas resources in the coming decades in
combination with the dwindling supplies of readily available crude oil has led to
commercialization of gas-to-liquids (GTL) processes in the fuels and chemicals industry. The
Fischer-Tropsch (FT) reaction plays a central role in these GTL efforts: it converts synthesis
gas into clean transportation fuels and chemicals. Despite significant research efforts in the
last decade, many aspects of the underlying mechanism need to be better understood. Such
insight will facilitate the guided design of improved catalysts. Modern approaches such as
computational advances and development of model systems are tools that may provide such
insights. In this work, we will discuss our recent findings regarding the dominant factors
governing the chain growth, one of the essential reaction steps in the FT reaction, for Ru
model catalysts.
The FT reaction is a polymerization reaction of C
1
monomer species, which are in situ
generated on the catalytic surface by CO dissociation.
1-2
Chain growth proceeds through
association reactions of C
1
monomers with growing hydrocarbon chains. Products leave the
surface as olefins (β-H elimination), paraffins (hydrogenation) or aldehydes (CO insertion).
1,
3
Termination of C
1
monomers by hydrogenation yields methane, which is an undesired
reaction product as it is the source of synthesis gas. In commercial practice, the aim is usually
to minimize methane yield and maximize the formation of liquid hydrocarbons that can be
converted to transportation fuels in downstream hydrocracking processes.
4
This can be
achieved by increasing the chain-growth probability (α), defined as the rate of propagation
(r
p
) over the sum of the rates of propagation and termination (r
t
).







(1)
Page 3 of 27 Faraday Discussions

Frequently asked questions

Q1. What is the role of the Fischer-Tropsch reaction in the GTL process?

The Fischer-Tropsch (FT) reaction plays a central role in these GTL efforts: it converts synthesis gas into clean transportation fuels and chemicals. 

Q2. What is the effect of the OH + H reaction on chain growth?

As chain growth is limited by only one hydrogenation step in this regime, whereas it is inhibited by two hydrogenation steps (vide infra), a lower H coverage results in increased chain-growth probability. 

Q3. What is the degree of chain-growth probability control?

Essential to the growth mechanism are C-H dehydrogenation and C hydrogenation steps, whose kinetic consequences have been examined by formulating two novel kinetic concepts, the degree of chain-growth probability control and the thermodynamic degree of chain-growth probability control. 

Q4. What was the method used to determine the steadystate coverages?

The in-house developed C++ program MKMCXX 19 was employed to determine the steadystate coverages by integrating this set of ordinary differential equations with respect to timeusing the backward differentiation formula method. 

Q5. What is the reaction that needs to be dehydrogenated?

For this reaction to propagate after the CH insertion reaction, the α-carbon atom in CHCR needs to be dehydrogenated followed by hydrogenation of the β-carbon to CH2. 

Q6. What is the effect of adsorption on the chain-growth probability?

The interesting consequence is that, when CO dissociation is not rate-controlling, an increase in the number of free surface sites does not necessarily increase the overall CO consumption rate and the chain-growth probability. 

Q7. What is the key selectivity parameter in commercial FT?

Microkinetics simulations of the complex FT reaction provide detailed insight into the elementary reaction steps that control the CO conversion rate and the chain-growth probability, a key selectivity parameter in commercial FT technology. 

Q8. How did Van Santen and his colleagues define the FT selectivity?

Van Santen and coworkers have discussed several aspects about FT selectivity by constructing theoretical models based on the steady-state solutions of coupled rate expressions, Monte Carlo simulations and density functional theory calculations. 

Q9. What is the stoichiometric coefficient of species i in elementary reaction step?

Given a system of N elementary reaction steps, 2N rate expressions (i.e., both forward and backward reactions) were obtained with the form: %& = &'()*+, ) (5) where ci is the concentration of species i in the elementary reaction step j on the surface, and -) is the stoichiometric coefficient of species i in elementary reaction step j. 

Q10. What was the effect of the elementary steps on the chain-growth probability?

Microkinetics simulations were performed to investigate the influence of the elementary reaction steps on the chain-growth probability α.