Review of the Application of Hydrotalcite as CO₂ Sinks for Climate Change Mitigation

Round 1

Reviewer 1 Report

The reviewed manuscript studied the “Review of application of hydrotalcites as CO₂ sinks and climate change mitigation”, it addresses the recent a review of the application of hydrotalcites as CO₂ sinks. Different aspects were analysed: pH of the synthesis, the molar ratio, the specific area, pressure, temperature and time in CO₂ absorption. The whole review is detailed and comprehensive.Therefore, I think that this work is appropriate for publication after minor revision as follows:
1. The keywords at the beginning of the article are too many and illogical, so I suggest the author simplify and revise it again.

2. Chapter 2.1 summarizes and discusses the influence of calcination temperature on CO₂ adsorption capacity. In this part, the author needs to summarize and supplement specific experimental data to further verify the influence of calcination temperature on adsorption.

3. In line 259，“The relationship between the pH of the synthesis and the specific surface area obtained is very important,” whether this statement is a descriptive error, since this section discusses the effect of mole ratio versus surface area.

4. As a review article, the paper discussed in detail the influence of pH, mole ratio, calcination temperature, pressure and other factors on adsorption. The author needs to make a ranking of these summarized influencing factors. What has the greatest influence on adsorption, so that it can be used as a good reference for subsequent research.

5. The grammar check and usage of this manuscript need to be modified, it is also suggested that the authors check the similar grammar problems in the whole paper.

 

 

 

Author Response

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Reviewer 2 Report

A relatively brief review of the literature reporting thermal treatment of hydrotalcites from layered double hydroxides to calcined layered double hydroxides and their characterisation in CO2 uptake. The natural effect of calcination, as known in the use of CaCO3-CaO in calcium looping, in the production of basic metal oxides able to react and form carbonates in the presence of CO2, a mechanism used for CO2 capture. The manuscript structure and content are acceptable. Please address the following points.   Line 111: Please specify the "molar ratio" of what, otherwise it can be of anything.
  Lines 116-118: The sentence construction " Calcined or uncalcined hydrotalcite? The answer is immediate: calcined hydrotalcite or its use under high temperature(around 400 ºC)." is awkward since it does not state for which purpose. Please rewrite this part of the text. Also the content of section 2 does not fit with this title, please change it to a more pertinent one.   Line 130: Change "avoiding" to "hindering".   Line 145: "ocurrs" please correct.   Line 167: Chemical equation (4) is not balanced, please check. 2Al to 6Al and 19O to 16O.   Lines 192=199: " For LDH the layered structure can be observed" where? The SEM images in Figure 5 (A) and (B) do not show details of the layered structure, please amend this sentence. The same issue with the sentence right after "the structure collapses, as can be observed" where? The magnification and detail of these images do not show the layered structures and correspondingly their collapse. Also, FIgure 5 (C) is not mentioned in the text, please amend it, and the difference between Figure 5 (c) and (d) is really minor, very difficult to correlate any change from a layered to a disordered structure. This section of the manuscript needs major revision.   LInes 225-230: Please add numerical values of CO2 capture performance of the different LDH prepared at various pH values.   Line 226: " In this line" of the manuscript?   Line 243: Again, please specify the "molar ratio" of what.   Lines 316-318: Please review the statement " Therefore, researchers have to pool their efforts to study CO2 capture with calcined hydrotalcites at high pressures and low application temperatures." since compressing air is an unpracticable solution from an energy requirement point of view of the carbon capture process.   Lines 319-324: Please check the literature reporting the known issue of limited recyclability (calcination-carbonation cycles) of CaO in calcium looping and report the issue in this section.   Line 383: Again, please specify the "molar ratio" of what.   Please change all "et al" to "et al.".    

Author Response

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Reviewer 3 Report

The paper summarizes research to date on the applicability of hydrotalcite as a sink for carbon emissions. The topic of the development of sorbents for carbon emissions is very relevant, but from the summarized results the applicability of hydrotalcite in CCS or CCU technologies is controversial. The adsorption capacity is rather low, high-temperature activation is required (around 400 deg C), which increases energy losses and makes the process of carbon capture more expensive. BET surface area of hydrotalcite is low (below 200 m2/g).  In this sense, although the publication summarizes a number of studies on hydrotalcite, from the point of view of low-carbon technologies it is not of interest. In this regard, I recommend the authors to emphasize the advantages of the adsorbent, if any, over other widely studied adsorbents. Research on the applicability of hydrotalcite in building materials that absorb CO2 looks more promising, and I recommend expanding this part. It is necessary to improve the quality of  Fig. 3. To expand the discussion on the influence of pH on the properties and structure of the material, why the minimum is observed in the BET dependence and what is the reason for the increase of the surface with increasing pH. Discussions on morphological changes in calcination also need to be clarified. E.g. a decrease in particle size is observed, but this is not obvious from Fig. 5B. 30 of the referenced sources are addressed too generally to low carbon technologies and this part can be shortened.

Author Response

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Reviewer 4 Report

 

Dear Editor and Authors.

It has been my pleasure to review the paper “Review of applications of hydrotalcites as CO₂ sinks and climate change mitigation”. The author of the manuscript has provided an interesting review of the application of hydrotalcite as a CO₂ sink analysing some experimental aspects in the process, such as pH of the synthesis process, the Mg/Al moral ratio, specific surface area, temperature and pressure, and others.

The topic of this paper could be appropriate to the scope of Chemengineering. However, some comments and recommendations should be taken into consideration before.

Starting by listing a few typos:

Line 25: Missing full stop.

Lines 51, 78, 82 and others: Unnecessary quotation marks.

Line 237: Extra full stop.

Line 252: Missing a space.

Line 261: Write m²*g^-1, however, lines 252 and 256 white mg/g. Normalise the format.

Line 281: Mg₃Al-stearate instead of Mg₃Al-stereate?

Line 315: Extra full stop after atm.

 

And some comments I would like do:

First, I agree with lines 180-182 about the cost of calcination. In addition, I would add the synthesis, in the event of needing to do so. Of course, this is not the aim of this review, but it must have been the aim of some of the papers referred to. However, I would recommend the authors of this review refer not only to the economic costs but also, even more importantly, to the energy costs derived from the processes, since the ratio between the CO₂ produced (indirectly) from these processes and the CO₂ captured can be negative.

Second. Consider modifying the title of the paper, specifically, Review of the application of hydrotalcite as CO₂ sinks for climate change mitigation (for example).

Third. Delete from line 16: more and less.

I insist on a pleasure review of the manuscript submitted by the authors. I look forward to hearing from you.

Regards,

Reviewer.

Author Response

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Round 2

Reviewer 3 Report

Thanks to the authors for the answers and for the detailed economic evaluation of the calcination process. As the publication is entitled ""Review of application of hydrotalcites as CO2 sinks and climate change mitigation" and the introduction addresses carbon capture technologies, it cannot bypass the question of the  suitability and effectiveness of hydrotalcite as a CO2 adsorbent. According to the conceptualization of the article, it is expected to be reported a negative CO2 emissions as a result of synthesis/calcination process that not a zero emission effect.  This misleads the reader that it is a matter of using hydrotalcite to capture carbon emissions, and not of zero-emission hydrotalcite production. Please, in this case, change the title and conceptualization.

Author Response

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Round 3

Reviewer 3 Report

Some of the remarks made are clarified in the text of the publication.