Gas-Phase Mechanism Study of Methane Nonoxidative Conversion by ReaxFF Method

Yuan Liu , Zenghui Duan , Jun Li ,3, Chunran Chang

1 Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China.

2 Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry,Tsinghua University, Beijing 100084, China.

3 Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, Guangdong Province, China.

Abstract:With the rapid consumption of petrochemical resources and massive exploitation of shale gas, the use of natural gas instead of petroleum to produce chemical raw materials has attracted significant attention. While converting methane to chemicals, it has long seemed impossible to avoid its oxidation into O-containing species, followed by de-oxygenation. A breakthrough in the nonoxidative conversion of methane was reported by Guo et al.(Science 2014, 344, 616), who found that Fe?SiO2 catalysts exhibited an outstanding performance in the conversion of methane to ethylene and aromatics. However, the reaction mechanism is still not clear owing to the complex experimental reaction conditions. One view of the reaction mechanism is that methane molecules are first activated on the Fe?SiC2 active center to form methyl radicals, which then desorb into the gas phase to form the ethylene and aromatics. In this study, ReaxFF methods are applied to five model systems to study the gas-phase reaction mechanism under nearexperimental conditions. For the pure gas-phase methyl radical system, the main simulation product is ethane after 10 ns simulation, which is produced by the combination of methyl radicals. Although a small amount of ethylene produced by C2H6 dehydrogenation can be detected, it is difficult to explain the high selectivity for ethylene in the experiment.When the methyl radicals are mixed with hydrogen and methane molecules, ethane remains the main product, together with some methane produced by the collision of hydrogen with methyl radicals, while ethylene is still difficult to produce.With the addition of hydrogen radicals to the methane atmosphere, methane activation can be enhanced by hydrogen radical collisions, which produce some methyl radicals and hydrogen molecules, but the methyl radicals eventually combine with the hydrogen species to produce methane molecules again. If some hydrogen molecules and methyl radicals are added to the CH4/H· system, the activation of methane molecules by hydrogen radicals will be weakened.Hydrogen radicals are more likely to combine with themselves or with methyl radicals to form hydrogen and methane molecules, and the high selectivity for ethylene remains difficult to achieve. Thermal cracking of C10H12 at high temperature can produce hydrogen radicals and ethylene at the same time, which can partially explain the enhanced methane conversion and ethylene selectivity in the experiment of Hao et al. (ACS Catal. 2019, 9, 9045). Overall, the selective production of ethylene by nonoxidative conversion of methane over Fe?SiO2 catalyst appears hard to achieve via a gas-phase mechanism. The catalyst surface may play a key role in the entire process of methane transformation.

Key Words:Methane activation;Nonoxidative conversion;Gas-phase mechanism;Reactive force field;Computational simulation;Thermal cracking reaction

1 Introduction

With the constant depletion of oil resources, using natural gas/shale gas to produce chemical raw materials has been receiving more and more attention. As the main component of natural gas and shale gas, the activation and conversion of methane is a challenging task due to its inherent inertia, such as zero dipole moment1, low electron affinity2, and high C―H bond energy (439 kJ·mol-1)3.

A number of methods have been developed to activate and convert methane, which are mainly classified into indirect routes and direct routes4–12. Indirect conversion of methane involves first converting methane into syngas and then converting syngas into other products13–17, the efficiency of which is relatively low because of the complexity of the transformation process. Direct conversion of methane includes oxidative and nonoxidative pathways. Oxidative conversion of methane such as selective oxidation of methane (SOM)and oxidative coupling of methane(OCM)cannot avoid the overoxidation of methane to carbon dioxide and thus leads to low carbon-atom efficiency and CO2 emission18–29. In contrast, the nonoxidative conversion of methane because of short process, high carbon-atom efficiency and less CO2emission is considered to be a more promising way30–47.

Since 1993, Wanget al. have firstly reported that Mo doped ZSM-5 catalysts can transform methane to aromatic products30,but the catalyst was easily deactivated due to carbon deposition.Until 2014, a breakthrough work was reported by Guo and colleagues that methane can be transformed to olefins,aromatics, and hydrogen (MTOAH)under nonoxidative conditions48. The conversion of methane reached at 48.1% and the selectivity of ethylene in the products peaked at 48.4% at 1363 K. Importantly, no coke deposition and no catalyst deactivation were observed during a 60-hour test. However, the mechanism of MTOAH is still under discussion. Through preliminary theoretical calculations and VUV-SPI-MBMS experimental analysis48, it was proposed that methane first dissociated at the Fe1?SiC2active center with the methyl and hydrogen atom adsorbed at Fe and C site, respectively. Then the methyl released to the gas phase and collided with each other to form the final product (ethylene and aromatics). In order to further demonstrate the possibility of gas-phase mechanism, in 2019, Haoet al.reported that methane conversion to olefins and aromatics over an iron-coated catalytic quartz reactor could be enhanced by hydrogen radicals which were produced by 1,2,3,4-tetrahydronaphthalene (THN)and benzene49. Kim’s group studied the mechanism of nonoxidative methane coupling on a single-atom iron catalyst by density functional theory (DFT)and microkinetic analyses50. They concluded that ethylene was formed by joint surface and gas-phase mechanisms, where ethane molecules generated in the gas phase were activated by surface active center to form ethylene molecules. Recently, our group also studied the mechanism of this system by utilizingab initiomolecular dynamics (AIMD)simulations and DFT methods51, we found that ethylene can be easily formed on the surface active center through the quasi-MvK mechanism. From these mechanism studies, the main argument is that whether the ethylene is formed in the gas phase or on the catalyst surface during MTOAH reaction.

In this work, the reactive force field (ReaxFF)simulation method was used to study the gas-phase mechanism of methane nonoxidative conversion under near experimental conditions. It is found that ethylene can be hardly formed under sole gas-phase reaction conditions. The main simulated productviagas-phase mechanism is ethane. The C2H4 can be produced by thermal cracking of THN molecules at high temperature, which may influence the conclusion of experiment that THN could promote the conversion of methane to olefins and aromatics.

2 Models and computational methods

Five different models were built to study the gas-phase reaction mechanism based on the experimental speculation. (A)CH4/CH3·/H2mixture with the ratio of 2 : 2 : 1 (i.e., 80 methane molecules, 80 methyl radicals and 40 hydrogen molecules); (B)200 methyl radicals; (C)CH4/CH3·/H2/H· mixture with the ratio of 4 : 4 : 1 : 2 (i.e., 80 methane molecules, 80 methyl radicals,20 hydrogen molecules and 40 hydrogen radicals); (D)CH4/H·mixture with the ratio of 1 : 1 (i.e., 100 methane molecules, 100 hydrogen radicals); (E)30 C10H12(THN)molecules. The periodic cubic box size for all the five models were set as 50 ? × 50 ? × 50 ? (1 ? = 0.1 nm), as shown in Fig. 1. Models A and B were used to study the mechanism of methyl radical collision reaction in the gas phase. Models C and D were used to study the effect of hydrogen radical in the gas-phase reaction. Model E was used to study the thermal cracking behavior of THN at high temperature.Three parallel simulations were performed for each model, one typical simulation result of products distribution is shown in the text, the others are shown in Figs. S1–S24 (see Supporting Information).

Fig. 1 The model C of CH4/CH3·/H2/H· mixture for ReaxFF simulation, i.e., 80 methane molecules, 80 methyl radicals, 20 hydrogen molecules and 40 hydrogen radicals in a 50 ? × 50 ? × 50 ? box.

Canonical NVT ensemble molecular dynamics simulations were performed using ReaxFF package in Amsterdam Density Functional (ADF)software suite52. The time step was set as 0.25 fs. The simulation started from a pre-equilibrium process at the first 2.5 ps and the total simulation time was set as 10 ns. The simulation and data collection temperature was 1363 K for model A/B/C/D, in agreement with the experimental temperature. The temperature 1500 and 2000 K were set for model E to accelerate THN cracking. The temperature was controlled using a Nosé-Hoover chains53. The initial force field parameters for C and H were obtained from the literature54. The identification and quantification of elementary reactions of ReaxFF trajectories were based on ChemTraYzer scripts55.

In order to verify the reliability of reactive force field parameters in our model systems, the reaction energies (ΔE)and reaction barriers (Ea)of the representative elementary reactions were calculated by ReaxFF method in ADF software suite and high-level quantum chemistry method (CCSD(T)/ccpVTZ//M06-2x/cc-pVTZ)56–58in Gaussian-09 software59,which is shown in Table 1. The maximum difference in energy values is within 0.40 eV, therefore ReaxFF method could describe the reaction trends of our model systems qualitatively and quantitatively.

Table 1 Comparison of energetics calculated by ReaxFF and quantum chemistry methods of typical elementary reactions.

3 Results and discussion

3.1 The transformation of methyl radicals

To examine the possibility of the gas-phase reaction mechanism proposed by the original literature48. ReaxFF simulations were performed from the CH4(80)/CH3·(80)/H2(40)mixture in a 50 ? × 50 ? × 50 ? box, in agreement with the theoretical products distribution upon methane conversion at 48.1% in the experiment. The population of the products obtained from 10 ns simulation is presented in Fig. 2a. It shows that the number of methyl radicals drop rapidly from 80 to 2 mainly due to the collision between methyl radicals to produce ethane. Hydrogen molecules decrease from 40 to 25 in the first 3 ns and methane as one of the original reactants increase from 80 to 115, which owes to the reactions CH3· + H2→ CH4+ H·and CH3· + H· → CH4. After 3 ns, the number of species and the potential energy curve (Fig. S1)become steady. The main products obtained are ethane and methane with the number of 18 and 34, respectively. In this simulation, C2H5· radicals, C3H8and C2H4molecules are also detected, but the number of each species is all less than 5. The reaction mechanism of the simulated trajectories is revealed in detail in the inset of Fig. 2a. Other two parallel simulations with the same reactants composition were also performed, the results of which are similar to those of the simulation above (Figs. S2 and S3).

Fig. 2 (a)Products distribution of ReaxFF simulations beginning from gas-phase CH4(80)/CH3·(80)/H2(40)mixture as a function of time,(b)Products distribution of ReaxFF simulations beginning from 200 gas-phase CH3· radicals as a function of time.

Another gas-phase reaction system with 200 pure methyl radicals in a 50 ? × 50 ? × 50 ? box was also simulated by ReaxFF. As shown in Fig. 2b, ethane and methane increase rapidly as methyl radical consumes at the beginning of the simulation. But at the end of the simulation, ethane is the only dominant product with the number of 78 due to the intensive collision between methyl radicals. When the reaction system reaches to the equilibrium at 10 ns, only four ethylene molecules are detected due to the dehydrogenation of ethyl radicals, similar to the reaction mechanism revealed by DFT calculations48. Guoet al.48proposed that the carbon chain might grow through C2H3·radicals that producedviathe reaction of C2H4with CH3· or H·.However, at the end of the simulation the number of C2H4, CH3·and H· is only 4, 2 and 0, respectively, indicating the formation of large amount of C2H3· radicals seems unlikely.

The ReaxFF simulations of the two sets of reaction systems demonstrate that the gas-phase reactions between CH3· radicals or CH4/CH3·/H2mixture cannot yield the main products(ethylene and aromatics)reported in the experiment48. Instead,the main products only stop at ethane/methane and a tiny proportion of ethylene/propane, suggesting that the gas-phase reaction mechanism between desorbed CH3· radical and others is not the only mechanism for the methane conversion on singleatom iron catalyst.

3.2 The role of hydrogen radical

Hydrogen radical is considered to be a critical intermediate to activate C―H bond and finally form C2H4products in the gasphase mechanism. In this section, two models containing some hydrogen radicals as the initial reactants were used to explore the role of hydrogen radicals in the reaction. The first model is CH4(80)/CH3·(80)/H2(20)/H·(40)mixture in a 50 ? × 50 ? × 50 ? box. As shown in Fig. 3a, the number of hydrogen radicals falls steeply to zero in the first 16 ps. By exploring the elementary reaction, the H· radicals mainly combine with CH3·and H· to form CH4 and H2 molecules respectively, which is demonstrated by the blue and celeste lines in Fig. 3a that the methane and hydrogen molecules increased rapidly in the first 16 ps. Specifically, 20 methane and 9 hydrogen molecules are produced. Compared with the role of H· radicals in activating C―H bond, H· radicals are more likely to combine with other radicals to form stable molecules, which can also be concluded from Table 1. The combination of CH3· and H· is about -2.45 eV exothermally by quantum chemistry calculation at 1363 K.While the activation of C―H bond of ethane molecule by H·needs to overcome a barrier of 1.54 eV. At the same time, methyl radicals can combine with themselves to form ethane molecules,as shown by the green line in Fig. 3a. After 75 ps of the simulation, the number of hydrogen molecules has a downward trend because methyl radicals can react with hydrogen molecules to form methane. The whole reaction reach balance after 3 ns simulation, as shown in Fig. S4. In the whole simulation process,only one ethylene molecule is detected, demonstrating H radicals do not play a key role to produce ethylene molecules with high selectivity.

Fig. 3 (a)Products distribution of ReaxFF simulations beginning from gas-phase CH4(80)/CH3·(80)/H2(20)/H·(40)mixture as a function of time,(b)Products distribution of ReaxFF simulations beginning from CH4(100)/H·(100)atoms as a function of time.

In order to study the role of hydrogen radicals in activating methane, a second model with CH4(100)/H·(100)mixture in a 50 ? × 50 ? × 50 ? box was constructed, as shown in Fig. 3b. At the first 70 ps of the simulation, the number of hydrogen radicals decrease rapidly and consume completely (Fig. 3b). Fifty-five hydrogen molecules are produced at the same time, about forty of them come from the H· + H· → H2reaction and the others are produced from the CH4+ H· → CH3· + H2reaction. The hydrogen radical does play an important role to activate methane and produce methyl radical because of the increased collision probability of H· and CH4 in this model system. However, the methyl radicals can hardly combine with themselves to form ethane molecules because of their relatively small quantity,whereas they are easier to collide with H2to form CH4again due to the large number of H2molecules around, which is shown by the blue line in Fig. 3b that methane molecules start to increase after 100 ps simulation. After 2 ns simulation, the reaction reach balance as shown in Fig. S5. In the whole simulation process, we only detect one ethylene molecule, which is formed by the dehydrogenation of ethane molecules. From this simulation, the large amount of H radicals could also hardly to promote the formation of ethylene molecules.

Fig. 4 (a)Products distribution of ReaxFF simulations beginning from gas-phase C10H12(30)as a function of time at 1500 K,(b)Products distribution of ReaxFF simulations beginning from C10H12(30)as a function of time at 2000 K.

From these two simulations, the H· radicals indeed help to activate methane to some extent. But compared with the activation of methane, the H· radicals are more likely to combine with themselves and methyl radicals to form the H2and methane.Therefore, the presence of hydrogen radical has very limited effect on the selective production of ethylene.

3.3 Thermal cracking of C10H12

According to the experimental report that hydrogen radicals produced by THN thermal decomposition can promote methane conversion to olefins and aromatics, the model with 30 THN molecules in a 50 ? × 50 ? × 50 ? box was built. In order to speed up the thermal cracking process, two high temperatures,1500 and 2000 K, were adopted. The simulation results are shown in Fig. 4a and Fig. 4b, respectively. After 10 ns simulation, the number of C10H12molecules decreased from 30 to 6 due to the thermal cracking in Fig. 4a. The maximum number of H radical is only 3, which is not as many as expected.Interestingly, ethylene molecules can be formed through thermal cracking (C10H12 → C8H8 + C2H4)from the simulation, as shown by the orange line in Fig. 4a. The maximum number of ethylene is 8, which may be one of the reasons for the promoted olefins production by adding THN in the experiment. The acetylene and hydrogen molecules can also be detected in the THN thermal cracking process from the simulation (Fig. 4a).

The other simulation at 2000 K shows similar tendency and phenomenon with those at 1500 K. Due to the higher simulating temperature, thirty C10H12molecules completely cracked in the first 5 ns. The maximum number of hydrogen radicals is also below 5. The maximum number of C2H4, C2H2and H2reach 20,48 and 33 respectively, which are the main cracking products.From these two simulations, the C10H12thermal cracking could produce C2H4at high temperature, which thus promotes the production of ethylene.

4 Conclusions

Five models were used to study the gas-phase mechanism of methane nonoxidative conversion by ReaxFF force field methods. It is found that only gas-phase methyl radicals released from the surface after methane dissociation can hardly produce ethylene. Instead, they are more inclined to combine with themselves to mainly produce ethane. The hydrogen radicals in the gas phase do play a role in activating methane to some extent,but they are also more likely to combine with themselves to form H2and difficult to promote ethylene production. The C2H4could be formed at high temperature by thermal cracking of C10H12molecules. Overall, the highly selective production of ethylene products can hardly be achievedviasole gas-phase mechanism,thus the catalyst surface is not only important in the activation of methane but also critical in the following transformation steps.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.