Direct catalytic conversion of methane to olefins and aromatics - A detailed analysis of the reaction characteristics of the Fe©SiO2 catalyst
Due to the COVID-19 crisis the PhD defence of Rolf Postma will take place (partly) online.
The PhD defence can be followed by a live stream.
Rolf Postma is a PhD student in the research group Catalytic Processes and Materials (CPM). His supervisor is prof.dr.ir. L. Lefferts from the Faculty of Science and Technology (S&T).
This thesis presents a detailed investigation into the reaction performance characteristics of non-oxidative coupling of methane using the Fe©SiO2 catalyst. Natural gas, consisting for 75-99 vol% of methane (CH4), is seen as a high potential substitute for crude oil in the synthesis of light olefins (ethylene and propylene) and aromatics (benzene, toluene and xylene). Current industrial processes converting natural gas to olefins and aromatics consist of multiple conversion steps starting with methane steam reforming to obtain syngas. For this reason, these processes tend to be energy intensive and require large installed capacity to become economically viable.
Direct conversion of methane to higher hydrocarbons has been investigated for decades as a potential competitive alternative to the indirect routes. The challenges for the direct routes stem from the high chemical stability of methane compared to the intended products (i.e. light olefins and aromatics) and the high endothermicity of the coupling reactions, requiring high reaction temperatures (>800 °C) and the undesired formation of carbonaceous deposits, leading to yield loss and system fouling. There are two main methods for tackling this challenge, i.e. oxidative coupling of methane, making the reaction exothermic and significantly lowering the required reaction temperature, and methane dehydroaromatization, which uses a shape selective zeolite catalyst to prevent deposit formation and steer the reaction towards aromatics. Although these two reactions do show high potential, they still operate at too low conversion and yield to become economically interesting. More details concerning these underlying motivations and background to this thesis are given in chapter 1.
A recent publication by Guo et al. proposes a high temperature system based around an atomically disperse iron on silica (denoted Fe©SiO2) catalyst, which is capable of converting methane at high conversion levels to a mixture of olefins and aromatic, without deposit formation, at temperature in excess of 950 °C. It is proposed that the catalyst activates methane to methyl radicals, which in turn undergo free radical coupling reactions in the gas-phase to obtain the final products. The system is still poorly understood, both due to the novelty of the system as well as the highly complex gas-phase chemistry that dictates the system performance.
This thesis aims to further the understanding of the catalytic system involving Fe©SiO2 and to uncover the main parameters with govern the performance of the system. It will furthermore look at the potential challenges for industrial application of the system and propose new and versatile methods for synthesizing the Fe©SiO2 catalyst. The interaction between the catalytic reaction and the gas-phase chemistry is poorly understood, leading to a reproducibility issue with regards to the performance achieved in the original publication by Guo1. A custom 3-zone oven, thermally insulated between each zone, was especially designed to achieve a high level of control over the temperature profile inside as well as up- and downstream of the catalyst bed. Chapter 2 of this thesis deals with the effect of this temperature profile. The position and amount of catalyst where systematically varied, as well as the space velocity and the temperature upstream and downstream of the catalyst bed. The results show that residence time at reaction temperature (1000 °C) downstream of the catalyst bed can significantly increase conversion, without negatively impacting the total carbon yield. In contrast, residence time at 1000 °C upstream of the catalyst causes a significant increase in deposit formation on the catalyst. In addition, achieving higher conversion purely via the catalyst is found to reduce the total hydrocarbon product selectivity when compared to achieving the same conversion increase by using the post-catalytic volume. It is concluded that that the catalyst is required solely for free-radical initiation, after which the reaction propagates at high activity in the gas phase. Any contact of the catalyst with the activated gas-phase leads to deposit formation.
Ethane is the primary product of the methane coupling reaction with ethylene as secondary product, as shown in the results of chapter 2 as well as literature. The commonly accepted mechanism for methane activation in the gas-phase concerns an auto-catalytic cycle involving both ethane and ethylene. The addition of these two C2 hydrocarbons is tested, chapter 3 shows that ethane and ethylene are highly potent free-radical initiators which significantly enhance methane conversion rate. There is no discernable difference between the effect of ethane or ethylene. Furthermore, C2 addition does not negatively impact the product selectivity distribution or carbon deposit formation. It is concluded that the C2 hydrocarbons are solely involved in accelerating the auto-catalytic cycle, analogues to the catalyst (chapter 2), after which gas-phase chemistry fully determines the product distribution.
Reproduction of the absence of deposit formation during methane coupling over the Fe©SiO2 catalyst proved impossible, as also shown by other publications concerning this system. It is proposed that the addition of hydrogen can largely prevent the formation of deposits on the catalyst. Many of the reactions involved in non-oxidative coupling of methane involve the formation of hydrogen. Furthermore, the high temperature nature of the reaction studied in this thesis causes most reactions to be reversible, this means that many coupling reactions will be slowed down by hydrogen addition. Chapter 4 shows that up to 10% H2 addition can indeed decrease the formation of carbonaceous deposits on the catalyst by an order of magnitude. The methane conversion is simultaneously decreased by a factor of two, while the hydrocarbon product distribution is shown to be mainly determined by the methane conversion level, independent of hydrogen addition.
The system is modelled using a microkinetic gas-phase model in combination with the catalytic cycle as published in the first publication1 concerning the Fe©SiO2 catalyst. The results of this model give more quantitative insight into the interaction between the catalyst and the gas-phase and are discussed in chapter 5. It is shown that the formation of methyl radicals on the catalyst, followed by gas-phase coupling can accurately predict the performance of the system. Pure gas-phase methane conversion goes through a induction period before the auto-catalytic cycle is fast enough to ensure a high gas-phase methane conversion rate. The catalyst should mainly be used to overcome the induction period, thus significantly reducing the residence time required to achieve decent levels of conversion, as experimentally shown in chapter 2. The addition of C2 hydrocarbons, as discussed in chapter 3 is also modeled, the model demonstrates that the addition of C2 can indeed also overcome the induction period, to directly achieve high methane conversion rates. Lastly the model demonstrates that gas-phase chemistry is the sole determinant of the product distribution, both radical and molecule concentration quickly achieve the gas-phase levels downstream of the catalyst.
The high temperatures and low pressures used in the methane coupling reaction with the Fe©SiO2 catalyst pose significant challenges for industrial implementation. Chapter 6 in this thesis presents a design concerning industrial implementation of this reaction assuming the performance recorded in the work of Guo1. The scope of the design focusses on maximizing the potential profit of the reaction. It is calculated that recovery of ethylene is economically unattractive, due to the low ethylene concentration in the product stream and the cryogenic methods required. The process thus results in a methane to aromatics process, with naphthalene as major product on carbon basis. Although the process shows economic potential as it is, it is advised to upgrade the naphthalene to value added products through a cracking reaction. The minimum pressure in the reactor is calculated to be 5 bars and both the reactor as well as the heat-exchanger over the reactor are determined to be the most significant investment costs. Lastly, hydrogen recovery is costly, due to the low temperatures and high pressures required, clashing with the requirements for the reactor, novel types of hydrogen recovery systems can significantly decrease costs.
The synthesis of the Fe©SiO2 catalyst is intensive difficult to control, due to the required 16h in a ball-mill as well as a fusion at 1700 °C for 6h. Chapter 7 proposes an alternative synthesis method via grafting the atomically disperse iron sites on the silica surface using a metal organic complex. bis(2,4-dimethyl-1,3-pentadienide)Fe(II) is determined to be a suitable candidate for this graft reaction, with performance matching that of the synthesis method1. It is proposed that the grafting method can be used to give a better control over site density and position as well as catalyst shape (i.e. monolithic structures), to optimize mass and heat transport as well as catalyst/free-volume ratio. An alternative Ru/SiO2 catalyst is proposed to increase the productivity of the catalyst, although this catalyst unfortunately performed worse than the Fe©SiO2 catalyst.
One of the main omittances in research into the Fe©SiO2 catalytic system concern the performance at higher pressures, 5 bars and above as discussed before, it is thus proposed in chapter 8 to measure the performance of the system at elevated pressured. Another proposed improvement is the tuning of the reactor shape to maximize both conversion and yield, the proposed design involves a capillary tube to feed the catalyst bed, ensuring rapid heating, while extending the diameter downstream of the catalyst to maximize residence time, as discussed in chapter 2. More catalyst formulations can tested, following a patent an Fe©SiC catalyst should significantly increase the activity. It is proposed that direct naphthalene upgrading in dual-purpose reactor can be possible, using the unreacted methane with the produced hydrogen as cracking agents. Lastly a proton conductive membrane is suggested as active hydrogen pump in a potential process, alleviating the need for low temperatures, high pressures and low aromatics concentration for hydrogen recovery.
Guo, X., G. Fang, G. Li, H. Ma, H. Fan, L. Yu, C. Ma, X. Wu, D. Deng, M. Wei, D. Tan, R. Si, S. Zhang, J. Li, L. Sun, Z. Tang, X. Pan, and X. Bao, Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen. Science, 2014. 344(6184): p. 616-619.