Direct, nonoxidative conversion of methane to ethylene, aromatics, and hydrogen
Authors: Hongjun Fan
Zichao Tang
Chao Ma
Guangzong Fang
Xiulian Pan
Jianqi Li
Dehui Deng
Shuo Zhang
Gang Li
Dali Tan
Xing Wu
Xinhe Bao
Liang Yu
Xiaoguang Guo
Hao Ma
Litao Sun
Rui Si
Mingming Wei
Journal: Science (2014)
Discipline: Multidisciplinary
Keywords: natural gas; catalysts; methane; iron sites; silica matrix
Affiliations: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China.

Abstract:  The efficient use of natural gas will require catalysts that can activate the first C-H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test. 

Retracted: No

PubPeer comments:
 The paper presents breakthrough results but contains fundamental flaws. It is even possible to prove the paper wrong, using its own data (vide infra). I will focus just on their characterization part, which is dealing with probing the character of iron sites, and the authors conclude that they are in the form of single iron sites. Their support for such a statement originates from electron microscopy (A) and XAS (B), but both of these investigations are flawed. Specifically, the authors show only ONE STEM micrograph showing presumably single sites (Figure 2A). Not two, not five, not ten. One. This is groundbreaking observation, yet only ONE micrograph is published? This amount accounts for probing way less than 0.1 % of the material. They did not even include this micrograph in a high-resolution in SI. They did not even carry out EDX, making it impossible to link these bright dots to iron atoms. In other words, this cannot be taken this as a relevant proof. These bright dots could be anything. Second, their XAS study contains serious flaws. Figure 2B shows XANES spectra of the fresh catalyst Fe@SiO2 and the same catalyst after a reaction with methane. Noteworthy, the Fe K-edge edge position shifts during reaction with methane at 950+ u00b0C from what can be expected for oxidized iron to metallic iron, i.e. iron(0). This reduction is not surprising (yet the authors do not comment on it). However, this does not at all fit with their proposed site in Figure 2A. They show single iron center, which is bound to two two-valent (!) carbons and one silicon. Such a structure does not chemically make sense. Furthermore, this complete reduction of iron implies that methane can reach all iron centers. How is that possible? You prepared your catalyst by melting all the components together. There is no way you can avoid having iron centers in the bulk. However, let s assume it is right and all of the iron is on the surface. The determined surface area of the catalyst is less than 1 m2 g and metal loading is 0.5wt%. This translates to more than 50 iron atoms per nm2 and with such a density, there is no chance of having iron single sites. Lastly, EXAFS part of XAS spectrum is treated exceptionally poorly, which is unexpected, given by the fact that other papers from the same group have EXAFS treatment of sound level. Figure 2C shows the EXAFS region of fresh catalyst Fe@SiO2 and spent catalyst. The spectra were never adequately fitted; there is only some kind of pseudo-fit in ESI (Table S2). However, there is no information on their k-range, no information on fitted R-range, no amplitude reduction factor, no energy shift, no statistics (u03c72, R-factor), errors on most of the values are absent, no mean square relative displacement, no information on the smaple environment during measurements, no information on the possible beam-damage. This is no way to publish EXAFS. Any reviewer of catalytic paper should ask for these details immediately. The authors use this experiment illustrate absence of Fe-Fe scattering (ie.e presence of single sites and not nanoparticles), yet they completely ignore possible convolution of Fe-Fe and Fe-Si scattering, or shell at R = 3 u00c5. Without proper fitting and analyses, this EXAFS experiment proves literally nothing, expect for the fact there have been some changes in the structure upon reaction with methane. To sum up, I have shown that the authors results are incomplete and incoherent. Their own data shows that their material contains iron nanoparticles and not single iron sites. This also renders their later considerations related to the speciation of the surface site completely void. Furthermore, the presence of nanoparticles agrees well with observations of Han et al. (2019) and Sot et al. (2020). Unfortunately, the mistakes from the 2014 paper now keep getting repeated by newer studies (Seok Ki Kim et al. 2020, Teng-Hao Li 2020 et al.), which motivated me to write this down. , 
 https: twitter.com GiannakakisGio status 1281421547085852672 , 
 Follow-up to my first comment: The authors claim that u201cNo aggregation was observed, even after prolonged reaction for 60 hours.u201d (page 618). However, they do not publish any characterization after 60 hours! In Figure 2, the samples were exposed to methane only for 2 hours. There s also no characterization after 60 hours in the supporting information. In other words, their claim is completely baseless. The authors themselves are not consistent in reporting their experimental procedure. The same group of authors in the later follow-up paper (10.1021 acscatal.9b01771) claims that the catalysis was in the case of 2014 Science paper carried out with a pelletized catalyst:  In the previous report4, the reaction takes place over pellet catalysts at high temperature ( 1223 K). (SI, page 7) On the other hand, the original Science paper says:  In all reaction tests, 0.75 g catalyst was packed in the reactor, unless otherwise stated. For the reaction at 1363 K, 0.375 g catalyst was used. (SI, page 2) There is literally nothing about pelletization. Which is true then? Pelletization would have a great influence on both heat and mass transfer, and the radical process itself. The authors claim that their XAS experiment was carried out in situ. However, any details regarding this are totally absent from the paper and its SI. Any normal catalytic paper would report how they were heating, how did the experiment look like, what sample container they used (capillaries), what detector they used, what resolution they operated in ... all of which is very important. In other words, based on the presented data, their experiment cannot be called in situ and cannot be repeated by an independent party. ,