Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

Due to the surge of natural gas production, feedstocks for chemicals shift towards lighter hydrocarbons, particularly methane. The success of a Gas-to-Chemicals process via synthesis gas (CO and H2) depends on the ability of catalysts to suppress methane and carbon dioxide formation. We designed a Co/Mn/Na/S catalyst, which gives rise to negligible Water-Gas-Shift activity and a hydrocarbon product spectrum deviating from the Anderson–Schulz–Flory distribution. At 240 °C and 1 bar, it shows a C2-C4 olefins selectivity of 54%. At 10 bar, it displays 30% and 59% selectivities towards lower olefins and fuels, respectively. The spent catalyst consists of 10 nm Co nanoparticles with hcp Co metal phase. We propose a synergistic effect of Na plus S, which act as electronic promoters on the Co surface, thus improving selectivities towards lower olefins and fuels while largely reducing methane and carbon dioxide formation.

(2) The loading amount of Na2S was very low for the studied catalyst. In order to further reveal the promoted effect of Na2S, a series catalyst with different loading amount of Na2S should be investigated. What is the best loading amount?
(3) The authors show an interesting FTO catalyst but do not provide enough evidence to explain the underlying mechanism. Why the addition of S and Na can hinder the secondary olefin hydrogenation or methanation formation? The only evidence from DFT calculations offers little insight and cannot convince me. The authors also supposed that higher charge donation to coincide with lower hydrogen coverages lead to lower methane selectivity (in similar to what they have reported for iron carbide). However, it is just a speculation. The authors should provide specific experimental data.
(3) At mild conditions of 240 °C, 1 bar, H2/CO=2, 1% CO conversion, the authors said the olefins selectivity was 54%C with a C2-C4 olefin/paraffin ratio of 17. However, at such a low CO conversion, it is difficult to ensure the accuracy of the reaction data, including the CO conversion and products selectivity. I noticed that a Varian CP3800 GC was used to analyze the hydrocarbons (C1-C16). Was the offgas kept warm from the reactor to the GC? Why did the authors neglect the hydrocarbons with carbon number larger than 16? I noticed there existed diffraction peaks for wax phase on the spent catalysts (in XRD analysis).
(4) I suggest the authors provide the detailed reaction conditions in the legend for Figure 1a (pressure), 1b (temperature) and 1c (temperature).
Reviewer #2: Remarks to the Author: I have read the authors responds to the reviewers requests and the changes made and propose publication of the revised manuscript. Perhaps, some of the newly added information could have been presented in the main article (instead among further information in the Internet) as thinking them actually relevant.
Reviewer #3: Remarks to the Author: Thank the author for their time to reply to my comments. Despite this, I remain unsatisfied with the theoretical DFT calculations in this work and cannot recommend it for publication in its current state. Another reason that I hesitate to suggest its publication is that the FT process is not of significant interest to a general scientific audience unless a very major breakthrough is made.  As shown in Figure 2b, the addition of Mn increased activity for Co-based catalysts, which is in agreement 59 with literature. 33,34 Catalysts with Co/Mn ≈0.3 showed highest activity per gram Co (CTY), and the addition 60 of Na2O, Na2S2O3 or Na2S decreased activity. Nonetheless, the activity of Co1Mn3-Na2S was still higher 61 than the remaining Co-based catalysts. In terms of stability, Co1Mn3, Co1Mn3-Na2O, Co1Mn3-Na2S2O3 and 62 Co3Mn1 showed deactivation but all other catalysts remained stable over 70 h. 63   were simplified models on their catalysts. DFT calculations at this stage would require too many assumptions. This discussion is now added to the 134 manuscript as follows: 135 To gain further understanding of the difference in Na2S and Na2O, DFT calculations of both species on 136 metallic Co (0001) surface were performed. Please note that these calculations are of a preliminary nature 137 and further work is needed to arrive at full reaction pathway analysis which is outside the scope of this 138 work. such low CO conversions, CO conversion is determined by C concentrations of all products measured by 158 FID detector and the FID detector is highly accurate. Secondly, the offgas was kept warm from the reactor 159 to the GC, as the line from reactor to GC was heated to at least 150 °C to prevent hydrocarbon condensation. 160 Thirdly, while the FID detector was calibrated for C1-C16 hydrocarbons, no hydrocarbon peaks were 161 observed after C13 and there were very small amounts of C10 -C13. Thus, we do not think that 162 hydrocarbons with carbon number larger than 16 would be produced at mild conditions (240 °C and 1 bar). 163 Lastly, the XRD analysis of spent catalysts (Figure 3) refers to spent catalysts after 240 240 -280 °C, 10 164 bar and H2/CO = 2, whereby wax was observed in both XRD and TEM analysis. This discussion is now 165 added to the manuscript as follows. 166  Another reason that I hesitate to suggest its publication is that the FT process is not of significant interest 194 to a general scientific audience unless a very major breakthrough is made. 195