Moderate-Temperature Carbon Capture Using Thermally Pre-Treated Dolomite: A Novel Approach

Round 1

Reviewer 1 Report

Dear authors,
The topic of the paper is interesting and current. You have presented a lot of results, however, I would like to make some remarks and suggestions.

Shorten the introductory part of the paper and be concise.
Paragraph 2.1. Why did you do TGA calcination? and you did not use an oven?
What amount of gas did you use and why was the heating rate 10°C/min and not less than that?

Improve the resolution of Figure 1.
Why did you use the method: Surface Area and Porosity Analyses? when your specific surface area is almost negligible?
Must show adsorption and desorption curves as well as pore distribution?!

In Figure 4, mark important wavelengths and functional groups.
Connect the analysis of the specific surface area with functional groups
Technically arrange the paper.
Expand the conclusion
Fix the English
Add some more references 5 years old

Dear authors,
The topic of the paper is interesting and current. You have presented a lot of results, however, I would like to make some remarks and suggestions.

Shorten the introductory part of the paper and be concise.
Paragraph 2.1. Why did you do TGA calcination? and you did not use an oven?
What amount of gas did you use and why was the heating rate 10°C/min and not less than that?

Improve the resolution of Figure 1.
Why did you use the method: Surface Area and Porosity Analyses? when your specific surface area is almost negligible?
Must show adsorption and desorption curves as well as pore distribution?!

In Figure 4, mark important wavelengths and functional groups.
Connect the analysis of the specific surface area with functional groups
Technically arrange the paper.
Expand the conclusion
Fix the English
Add some more references 5 years old.

Author Response

ANSWERS TO REVIEWERS:

Moderate Temperature Carbon Capture Using Thermally Pre-treated Dolomite: A Novel Approach

 

Iyiade G. Alalade^a, Javier E. Morales-Mendoza^a, Alma B. Jasso-Salcedo^a, Jorge L. Domínguez-Arvizu^a, Blanca C. Hernández-Majalca^a, Hammed A. Salami^a, José L. Bueno-Escobedo^a, Luz I. Ibarra-Rodriguez^a, Alejandro López-Ortiz^a* and Virginia Collins-Martínez^a

C – Journal of Carbon Research, Manuscript ID: carbon-3643170

Reviewer #1:

1. Shorten the introductory part of the paper and be concise.

Author`s answer: Thank you for your suggestion.

Action: According to the reviewer suggestion the introduction has been re-written in a shorten, clearer and concise manner as shown below:

“Since the mid-20th century, anthropogenic CO₂ emissions have raised global concerns, with amine-based chemical absorption dominating industrial capture methods despite drawbacks such as high energy demands, corrosion, poor solvent regeneration, and environmental issues [1]. As alternatives, solid sorbents in carbon capture and utilization (CCU) technologies have gained attention due to their lower energy requirements [2,3]. Among those, CaO-based sorbents are promising for their high theoretical CO₂ capacity (~0.78 g_CO₂/g_sorbent), fast sorption kinetics, abundance, low cost, and operability at flue gas temperatures (>500°C) [4–6] positioning them as viable solutions to emission challenges [7].

However, CaO-based sorbents suffer from fast performance degradation due to sintering during repeated high-temperature (>800°C) cycles [8–11], driven by the low Tammann temperature (~529°C) of CaCO₃ [12]. Sintering causes pore blockage and inhibit CO₂ diffusion due to surface CaCO₃ buildup [13].

To mitigate this, several strategies have been explored, such as inert additives [14–16], organic acid treatments [17,18], hydration reactivation [19,20], thermal pre-treatment [21], sintering-resistant precursors [22,23], and alkali molten salt promoters [24–26]. Notably, combining CaO with inert supports and modifying precursors with salts like NaNO₃ enhances sorbent performance by promoting CO₂ diffusion, while Al-based supports such as Ca₁₂Al₁₄O₃₃, Al₂O₃, and Ca₉Al₆O₁₈ have shown promising results [27–34]. Also, MgO has also emerged as an effective support due to its high Tammann temperature (1276°C), acting as a structural stabilizer in CaO–MgO systems [25,35–39]. Furthermore, thermal pre-treatment of calcite yields CaO-based sorbents with increased oxygen vacancies and crystalline defects, thus improving sorption-regeneration kinetics [40–43].

Despite these advances, gaps remain. The influence of MgO on calcined dolomite sorbent stability at moderate sorption and regeneration temperatures (≤650°C) has not been explored [36,37]. Most studies emphasize high-temperature calcination and sorption (>700°C), neglecting energy-saving alternatives [27–31]. Additionally, conflicting results regarding porosity and structural crystalline defects (i.e., oxygen vacancies, crystallite size, etc.) suggest unresolved challenges in sorbent design [38,44–51].

Therefore, this study addresses these gaps by developing a simple, cost-effective approach to prepare and optimize a dolomite-based sorbent prepared from air-calcined natural dolomite and further modified by an inert-atmosphere thermal pre-treatment. The work focuses on establishing the relationship between microstructural defects (e.g., oxygen vacancies, pore volume, crystallite size, etc.) and MgO fixed content on calcined dolomite thermal stabilization and enhanced CO₂ sorption performance at moderate temperatures (≤650°C). Additionally, it targets dolomite regeneration at temperatures lower than those typically required for CaO-based sorbents (>700°C), improving energy efficiency. The goal is to design a dolomite-based sorbent compatible with dual-function materials (DFMs) for integrated carbon capture and conversion (ICCU) technologies, particularly emerging hydrogen processes like dry reforming of methane (DRM), which demand cost-effective sorbents with high thermal stability under low regeneration temperatures. Our approach builds on prior work by Hu et al. [36] and Li et al. [37] but diverges in its focus on natural dolomite precursors and lower-temperature optimization. To our knowledge, this represents the first demonstration of dolomite-based sorbent regeneration at T < 700°C.”

2. Paragraph 2.1. Why did you do TGA calcination? and you did not use an oven?

Author`s answer: Thank you for your feedback, and we apologize for any confusion in our original submission. We appreciate the opportunity to clarify our methodology and rationale in greater detail.

The study aimed to optimize CO₂ sorption performance in dolomite-derived sorbents through calcination and structural defect engineering. Natural dolomite was first calcined in open-air atmosphere at varying durations (30, 60, 120, and 240 minutes) to produce samples labeled PCD30, PCD60, PCD120, and PCD240. To determine the CO₂ capture capacity at moderate temperature of these PCD samples, one sorption cycle at 450°C for 2 h and 90% CO₂/Ar was performed resulting in no difference among all tested samples (Figure 3).

Fig. 3 CO₂ uptake performance of calcined (900°C) dolomite at 450 °C for different isothermal times

Comprehensive characterization (ICP, XRD, BET) revealed that PCD60 exhibited superior crystallinity, surface area, and pore volume compared to other calcination times. These structural advantages positioned PCD60 as the optimal candidate for subsequent modifications.

To further enhance CO₂ interaction, PCD60 underwent thermal pretreatment in argon for 30 minutes. This inert-atmosphere activation intentionally introduced crystalline defects—such as oxygen vacancies and lattice dislocations—to destabilize the CaO/MgO lattice. Such defects are theorized to lower energy barriers for CO₂ chemisorption and regeneration by increasing reactive sites and improving interfacial diffusion due to unbalance of electric charge. The resulting sample, PCD60Act, was then screened across a temperature range (300–800°C) to identify the optimal sorption conditions. At 650°C, PCD60Act achieved its peak capacity of 0.477 gCO₂/g sorbent (Figure 4), a result attributed to balanced kinetics (enhanced CO₂ diffusion) and limited sintering (preserved surface area).

Fig. 4 CO₂ uptake performance of PCD60Act at different sorption temperatures after 60 min.

 

When compared to other activated samples (PCD30Act, PCD120Act, PCD240Act) at 650°C, PCD60Act maintained its superiority, demonstrating the highest capacity and sorption-regeneration kinetics (Figure 5). This performance features the critical role of the 60-minute calcination step in stabilizing pore structures and maximizing defect density.

Fig. 5 CO₂ uptake performance of activated PCD samples at 650°C and (b) isothermal sorption-regeneration kinetics of activated PCD at 650°C.

 

Reproducibility was rigorously validated through triplicate tests, where PCD60Act showed minimal variability (mean uptake: 47.5%; standard deviation: 0.23%; RSD: 0.48%). To mitigate confounding effects from ambient moisture, all activated samples were stored in desiccators, given the hygroscopic nature of CaO-MgO oxides.

Cyclic stability tests over 15 sorption-regeneration cycles (Figure 7) highlighted PCD60Act’s resilience, outperforming both unmodified dolomite (UD) and pristine CaO. Post-cycling characterization (e.g., BET, XRD, SEM) confirmed retained structural integrity, with minimal sintering observed. The UD sample, calcined in air via TGA without argon activation, served solely as a baseline reference.

Fig. 7 Isothermal multicyclic tests of PCD60Act, UD, and Pristine CaO at 650°C.

 

To elucidate the exceptional performance of PCD60Act, we conducted a comprehensive suite of crystalline-structural and spectroscopic analyses. X-ray diffraction (XRD) paired with Rietveld refinement provided critical insights into lattice distortions and phase purity, while spectroscopic techniques—including FTIR, Raman, XPS, and UV-vis—probed the atomic-scale effects of argon-mediated thermal pretreatment. Collectively, these analyses revealed that the inert-atmosphere treatment introduced a high density of oxygen vacancies and other crystalline defects (e.g., lattice dislocations, strained bonding networks), which directly enhanced CO₂ chemisorption kinetics and sorption capacity. Raman and XPS spectra, for instance, confirmed the presence of undercoordinated Ca²⁺ sites arising from oxygen vacancy clusters, while UV-vis data correlated defect states and oxygen vacancies with improved electronic interactions resulting in a reduction of the Bandgap Energy after thermal pre-treatment. These structural modifications, induced by controlled argon activation, underpinned PCD60Act’s superior performance compared to non-activated counterparts, as they facilitated faster CO₂ diffusion and stabilized reactive intermediates during sorption-regeneration cycles.

Finally, our methodology prioritized data-driven decisions, with PCD60Act selected for its structural and kinetic advantages. We hope this clarifies our approach and are happy to provide additional details if needed.

Action: Corrections were made within the Materials and Methods section and throughout the different sections of the manuscript as described above. Also, the methodology section is presented below for your perusal:

2.1 Sorbents Preparation

“The naturally occurring dolomite (CaMg(CO₃)₂) used in this investigation was procured from Vitromex®, Mexico (a subsidiary of Mohawk® Industries Inc.). Calcination of dolomite at 900°C in a quartz cylindrical furnace at a temperature ramp of 10°C/min [52,53] in an open-air atmosphere was performed first, to compare the performance of calcined dolomite at different calcination times (30, 60, 120, and 240 min) and labeled PCD30, PCD60, PCD120, and PCD240, respectively. These samples were evaluated for CO₂ sorption at 450°C and 90% CO₂/Ar. Additionally, a reagent grade calcium carbonate (CaCO₃, Acros®, 98.5%) was employed as reference material and underwent the same heat treatment as previous PCD samples and was labeled Pristine CaO, while a fresh dolomite sample was prepared via TGA under an air atmosphere at 900°C and labeled UD.”.

 

2.1.2 Sorbent Activation

“In a second study, the effect of thermal pre-treatment (activation) on the material properties as well as on the CO₂ capture performance of all PCD samples was examined. This consisted of exposing the calcined dolomite to an inert atmosphere (Argon) for 30 min at 650 °C to promote crystalline defects. Upon this activation, the PCD samples were labelled PCD30Act, PCD60Act, PCD120Act, and PCD240Act. Thereafter, these samples obtained were desiccator-stored to consider the hygroscopicity of the mixed oxides (CaO–MgO) [54,55]. Sorption evaluations began by changing the gas feed from argon to CO₂ at the target sorption temperatures (300 – 800°C) for 60 min unless otherwise specified, with sample masses recorded every second by the data acquisition system of a thermogravimetric analyzer (TA Instruments TGA-Q500). Resulting from this evaluation, the best sorbent was selected from all PCD samples, and this was based on its CO₂ sorption performance, capacity, favorable kinetics, and optimal sorption temperature.”.

3. What amount of gas did you use and why was the heating rate 10°C/min and not less than that?

Author`s answer: Thank you for your observation. Based on previous analyses and experience in our laboratory, 10°C/min was considered an optimum heating ramp to prevent sintering of the end-sorbent [1]. Furthermore, Section 2.1 has been re-written to carefully define the confusing terminologies (Calcination and Thermal pre-treatment). Also, ``in open air`` has been included in the methodology to reflect the atmosphere of the quartz cylindrical furnace without the use of a mass flow controller or flowmeter.

Action: Citations supporting the choice of 10°C/min heating ramp have been added to the manuscript.

4. Improve the resolution of Figure 1.

Author`s answer: Thank you for your observation. In response to the reviewer’s suggestion, we have enhanced the resolution of Figure 1 to ensure optimal clarity.

Action: The figure was re-exported in a high-resolution format (5000 DPI), with careful attention to preserving sharpness and detail. All labels and graphical elements were reviewed to guarantee readability, and the updated figure complies with the journal’s formatting requirements. The Figure 1 revised version has been incorporated into the manuscript and the caption has been expanded to provide a more detailed description of the key elements, improving interpretability (Reviewer #2 Q1).

Fig. 1 SEM micrographs of (a) Fresh dolomite, (b) PCD240, (c) PCD60, (d) PCD60Act after 15 sorption-regeneration cycles x5,000, (e) PCD60Act after 15 sorption-regeneration cycles x25,000, (f) EDS of PCD240, and (g) EDS of PCD60.

 

 

5. Why did you use the method: Surface Area and Porosity Analyses? when your specific surface area is almost negligible?

Author`s answer: Thank you for your insightful observation. We acknowledge that surface area and porosity are small. However, particularly in our investigation, these morphological properties critically influence the sorption capacity and regeneration efficiency of the materials. While the absolute values may appear small, the statistically significant variations between samples provided essential insights into their structural characteristics.

For instance, the surface area of PCD60 (26 m²/g) is fivefold greater than that of PCD240 (5 m²/g) and nearly double that of pristine CaO (16 m²/g) (Table 1). Similarly, the pore volume of PCD60 (0.06 cm³/g) is six times higher than PCD240 (0.01 cm³/g) and three times that of pristine CaO (0.02 cm³/g). These pronounced differences show how synthesis parameters (e.g., calcination time) directly modulate morphology, which in turn influence sorption performance. Thus, even subtle morphological variations provided a robust framework to rationalize performance trends at the particle size level.

We appreciate your attention to this aspect and hope this clarification reinforces the validity of our approach in linking morphology to functional behavior.

Action: N/A

6. Must show adsorption and desorption curves as well as pore distribution?

Author`s answer: Thank you for your observation.

Action: Pore size distribution and N₂ adsorption-desorption isotherm curves of PCD240 and PCD60 are presented in the supplementary material as Figures S3 and S4. Each copy of Figure S2 and S3 are shown below with their caption:

Fig. S3 Pore size distributions of PCD240 and PCD60.

Fig. S3 Adsorption-desorption isotherm curves of PCD240 and PCD60.

 

7. In Figure 4, mark important wavelengths and functional groups (missing).

Author`s answer: Thank you for your suggestion. In the materials under review, only the carbonate group (CO₃^2-) with different vibrational modes were identified.

Action: The prominent peaks corresponding to the C-O bonds of the carbonate group (CO₃^2-) in Figure 8 have been highlighted with their respective wavenumbers as seen below.

Fig. 8 FTIR spectra of carbonated samples (a) PCD60Act and (b) Pristine CaO.

8. Connect the analysis of the specific surface area with functional groups.

Author`s answer: Thank you for your observation. There was neither chemical nor solvothermal treatment of the sorbents under study. Therefore, the only functional group in the thermally pre-treated carbonated samples is the carbonate group (CO₃^2-) in four (4) vibrational modes. Specific surface area as well as pore sizes were used for comparison purposes among different samples. We think that while the techniques are complementary, they measure different properties. FTIR provides qualitative or semi-quantitative data on surface chemistry, while BET gives a quantitative measure of accessible surface area. A direct numerical correlation is often not possible without further modeling, and then qualitative trends and indirect relationships can be established. However, these are out of the scope of the present research.

Action: NONE

9. Technically arrange the paper.

Author`s answer: Thank you for your observation. Methodology and other sections of the manuscript have been rearranged and re-worded to provide more clarity to the body of the work. Please see answer to question 2 Reviewer 1.

Action:  Improvement of the methodology section with information provided on all prepared samples is shown in the response to question 2 reviewer 1.

 

 

10. Expand the conclusion.

Author`s answer: Thank you for your observation. The conclusions have been expanded and rewritten in a clear and concise fashion.

Action: Conclusions were rewritten as suggested by the reviewer. Thei section is included below for the reviewer’s perusal.

Conclusions

“This study demonstrates the successful development of a thermally pre-treated dolomite-based sorbent (PCD60Act) for efficient CO₂ capture at moderate temperatures. Calcination of natural dolomite at 900°C for 60 minutes, followed by activation under an inert argon atmosphere at 650°C, generated a defect-rich CaO–MgO composite with exceptional CO₂ uptake and cyclic stability. The PCD60Act sorbent achieved an initial CO₂ capacity of 0.477 gCO₂/gsorbent at 650°C and retained 84% of its capacity (0.38 gCO₂/gsorbent) after 15 cycles under isothermal sorption-regeneration conditions. Notably, regeneration at 650°C—significantly lower than conventional CaO-based systems (>800°C)—highlights the energy efficiency and practical viability of this approach.

The enhanced performance stems from the synergistic effects of MgO stabilization and defect engineering. Thermal pre-treatment introduced oxygen vacancies and lattice strain, as confirmed by Raman, XPS, and UV-vis analyses, which lowered the energy barrier for CO₂ chemisorption and facilitated rapid sorption-regeneration kinetics. The inert MgO framework mitigated sintering by spatially isolating CaO crystallites, preserving textural properties (23 m²/g post-cycling) and dislocation density (60 × 10¹⁰ m⁻²). These structural advantages were further evidenced by XRD and SEM, which revealed retained porosity and suppressed agglomeration after cycling.

A comparative analysis with current literature shows the superiority of PCD60Act, particularly under high CO₂ concentrations (90%), where it outperformed most dolomite-derived sorbents in capacity retention. The optimal CaO:MgO ratio (1.64) in the Mexican dolomite source aligned with theoretical thresholds for stability, enabling durable performance without costly synthetic modifications.

This work advances the integration of natural mineral sorbents into carbon capture systems, offering a scalable, cost-effective solution for applications such as integrated carbon capture and conversion (ICCC). The moderate operational temperatures and robust cyclic stability position PCD60Act as a promising candidate for hydrogen production processes (e.g., dry reforming) and industrial flue gas treatment, bridging the gap between energy efficiency and environmental sustainability. Future studies should explore pilot-scale validation and long-term stability under realistic gas compositions to accelerate industrial adoption.”

11. Fix the English

Author`s answer: Thank you for your observation.

Action: A comprehensive review of written English was performed, and corrections were made to the entire manuscript, and these were highlighted in a separate file.

12. Add some more references 5 years old.

Author`s answer: Thank you for your observation.

Action: Recent references (< 5 years) have been included in the body of the work

[1] Hu J, Jiang Y, Gao Q, Zhao Y, Dai S, Li X, et al. Material engineering of porous calcium oxide for boosting CO2 capture. Chemical Engineering Journal 2025;505:159237. https://doi.org/10.1016/J.CEJ.2025.159237.

[2] Yan X, Duan C, Yu S, Dai B, Sun C, Chu H. Revealing the mechanism of oxygen vacancy defect for CO2 adsorption and diffusion on CaO: DFT and experimental study. Journal of CO2 Utilization 2024;79:102648. https://doi.org/https://doi.org/10.1016/j.jcou.2023.102648.

[3] Oliveira CC, Hori CE. Hydrogen production from sorption enhanced steam reforming of ethanol using bifunctional Ni and Ca-based catalysts doped with Mg and Al. Int J Hydrogen Energy 2023;48:30263–81. https://doi.org/10.1016/J.IJHYDENE.2023.04.178.

[4] Morales-Mendoza JE, Jasso-Salcedo AB, Domínguez-Arvizu JL, Ibarra-Rodriguez LI, Hernández-Majalca BC, Bueno-Escobedo JL, et al. CO2 sorption and multicycle stability of dolomite promoted with Ba and Sr metals for sorption enhanced hydrogen production. Int J Hydrogen Energy 2025. https://doi.org/10.1016/J.IJHYDENE.2025.02.418.

[5] Motelica L, Oprea OC, Vasile BS, Ficai A, Ficai D, Andronescu E, et al. Antibacterial Activity of Solvothermal Obtained ZnO Nanoparticles with Different Morphology and Photocatalytic Activity against a Dye Mixture: Methylene Blue, Rhodamine B and Methyl Orange. Int J Mol Sci 2023;24:5677. https://doi.org/10.3390/IJMS24065677/S1.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

carbon-3643170 – Reviewer’s observations

In the paper “Moderate Temperature Carbon Capture Using Thermally Pre-2 treated Dolomite: A Novel Approach”, the authors present the synthesis, characterization, and sorption performance toward carbon capture for thermally treated dolomite (Batch A and B). The study is well organized; the introduction provides sufficient information on the topic.  The sections are well structured and nicely detailed. The samples have been characterized by multiple techniques (ICP-OES; SEM; XRD; SSA; FT-IR; XPS) and the sorption experiments have been done using a TGA. The stability has been checked as well by multicycle tests.   

Below are a few questions/suggestions:

1. For the SEM images (Figure 1), it is not clear what 1d and 1e refer to? Meaning 1d – SEM of dolomite after 15 cycles of absorption? And 1e after desorption?
2. In the first part the authors discuss Batch A (900C, 240 min) and Batch B (900C, 60 min) Table 1 and lines 316-320 – they say the CO₂ capture performance is better for Batch B and they link this performance with some textural features – OK, but later Section 3.7 the authors show the influence of time on dolomite and apparently there is no change in the CO₂ capture performance. Can you maybe explain this? It is hard to follow and understand the discussion.
3. Additionally, prior Section 3.7 there were 4 samples to follow: PCD60, PCD240, Pristine Cao and Fresh dolomite calcined in the TGA (900C) – but here 2 more emerge – PCD30 and PCD 120 – not mentioned before and not considered for the characterizations and previous discussion – Can you explain the reasoning of this, and also maybe incorporate the samples somewhere earlier? At first, I was inclined to ask what was the reason of this large gap 60min to 240min, only to learn later that there are 2 others in the series.
4. Section 3.8.2., line 424-427 – authors say that to improve the sorption capacity of PCD60 at 650C the sample preparation and operating conditions were optimized and a fresh batch was synthesized at 900C in cylindrical furnace – but in materials and methods Batch B it was synthesized exactly the same, so what is the optimization referring to? And why a new batch has been done? Seems that PCD60 was just synthesized again. If this is the case, I believe that the discussion here is a bit confusing and need to be addressed.
5. But I guess it is not the same sample, as the sorption capacity increases. It is confusing. What caused the improvement of CO₂ uptake from 0.36 to 0.45, since the sample is still PCD60?
6. As a general observation, the text seems crowded, the initial information refers to only 2 samples and 2 blank samples, but later on, additional materials emerge, making the results and discussion hard to follow and understand.

carbon-3643170 – Reviewer’s observations

In the paper “Moderate Temperature Carbon Capture Using Thermally Pre-2 treated Dolomite: A Novel Approach”, the authors present the synthesis, characterization, and sorption performance toward carbon capture for thermally treated dolomite (Batch A and B). The study is well organized; the introduction provides sufficient information on the topic.  The sections are well structured and nicely detailed. The samples have been characterized by multiple techniques (ICP-OES; SEM; XRD; SSA; FT-IR; XPS) and the sorption experiments have been done using a TGA. The stability has been checked as well by multicycle tests.   

Below are a few questions/suggestions:

1. For the SEM images (Figure 1), it is not clear what 1d and 1e refer to? Meaning 1d – SEM of dolomite after 15 cycles of absorption? And 1e after desorption?
2. In the first part the authors discuss Batch A (900C, 240 min) and Batch B (900C, 60 min) Table 1 and lines 316-320 – they say the CO₂ capture performance is better for Batch B and they link this performance with some textural features – OK, but later Section 3.7 the authors show the influence of time on dolomite and apparently there is no change in the CO₂ capture performance. Can you maybe explain this? It is hard to follow and understand the discussion.
3. Additionally, prior Section 3.7 there were 4 samples to follow: PCD60, PCD240, Pristine Cao and Fresh dolomite calcined in the TGA (900C) – but here 2 more emerge – PCD30 and PCD 120 – not mentioned before and not considered for the characterizations and previous discussion – Can you explain the reasoning of this, and also maybe incorporate the samples somewhere earlier? At first, I was inclined to ask what was the reason of this large gap 60min to 240min, only to learn later that there are 2 others in the series.
4. Section 3.8.2., line 424-427 – authors say that to improve the sorption capacity of PCD60 at 650C the sample preparation and operating conditions were optimized and a fresh batch was synthesized at 900C in cylindrical furnace – but in materials and methods Batch B it was synthesized exactly the same, so what is the optimization referring to? And why a new batch has been done? Seems that PCD60 was just synthesized again. If this is the case, I believe that the discussion here is a bit confusing and need to be addressed.
5. But I guess it is not the same sample, as the sorption capacity increases. It is confusing. What caused the improvement of CO₂ uptake from 0.36 to 0.45, since the sample is still PCD60?
6. As a general observation, the text seems crowded, the initial information refers to only 2 samples and 2 blank samples, but later on, additional materials emerge, making the results and discussion hard to follow and understand.

Author Response

ANSWERS TO REVIEWERS:

Moderate Temperature Carbon Capture Using Thermally Pre-treated Dolomite: A Novel Approach

 

Iyiade G. Alalade^a, Javier E. Morales-Mendoza^a, Alma B. Jasso-Salcedo^a, Jorge L. Domínguez-Arvizu^a, Blanca C. Hernández-Majalca^a, Hammed A. Salami^a, José L. Bueno-Escobedo^a, Luz I. Ibarra-Rodriguez^a, Alejandro López-Ortiz^a* and Virginia Collins-Martínez^a

C – Journal of Carbon Research, Manuscript ID: carbon-3643170

Reviewer #2:

1. For the SEM images (Figure 1), it is not clear what 1d and 1e refer to? Meaning 1d – SEM of dolomite after 15 cycles of absorption? And 1e after desorption?

Author`s answer: Thank you for your observation.

Action: The figure 1 was re-exported in a high-resolution format (5000 DPI). The Figure 1 revised version has been incorporated into the manuscript and the figure caption has been expanded to provide a more detailed description of the key elements, improving interpretability. The caption of the image was also re-written to reflect the change. Please see answer to reviewer 1, question 4. Here is the caption of this Figure for your perusal:

Fig. 1 SEM micrographs of (a) Fresh dolomite, (b) PCD240, (c) PCD60, (d) PCD60Act after 15 sorption-regeneration cycles x5,000, (e) PCD60Act after 15 sorption-regeneration cycles x25,000, (f) EDS of PCD240, and (g) EDS of PCD60.

 

2. In the first part the authors discuss Batch A (900C, 240 min) and Batch B (900C, 60 min) Table 1 and lines 316-320 – they say the CO₂ capture performance is better for Batch B and they link this performance with some textural features – OK, but later Section 3.7 the authors show the influence of time on dolomite and apparently there is no change in the CO₂ capture performance. Can you maybe explain this? It is hard to follow and understand the discussion.

Author`s answer: Thank you for your observation.  Firstly, section 2.1 has been re-written to carefully define the confusing terminologies (Calcination and Thermal pre-treatment). Secondly, section 3.7 has been re-written to reflect that the figure in this section shows CO₂ sorption capacity of dolomite samples calcined at 900°C in an open-air quartz cylindrical for different times but without thermal pre-treatment in an inert atmosphere. Please see the answer to question 2 of reviewer 1.

Action:  Both sections (2.1 & 3.5) have been rewritten to provide clarity on the sample labelling, procedures, and the results obtained from the prepared samples.

2.1 Sorbents Preparation

“The naturally occurring dolomite (CaMg(CO₃)₂) used in this investigation was procured from Vitromex®, Mexico (a subsidiary of Mohawk® Industries Inc.). Calcination of dolomite at 900°C in a quartz cylindrical furnace at a temperature ramp of 10°C/min [52,53] in an open-air atmosphere was performed first, to compare the performance of calcined dolomite at different calcination times (30, 60, 120, and 240 min) and labeled PCD30, PCD60, PCD120, and PCD240, respectively. These samples were evaluated for CO₂ sorption at 450°C and 90% CO₂/Ar. Additionally, a reagent grade calcium carbonate (CaCO₃, Acros®, 98.5%) was employed as reference material and underwent the same heat treatment as previous PCD samples and was labeled Pristine CaO, while a fresh dolomite sample was prepared via TGA under an air atmosphere at 900°C and labeled UD”.

3.5 Effect of Time on the Calcination of Dolomite

“Figure 3 shows the CO₂ sorption profile of PCD 30, 60, 120, and 240. The graph depicts no apparent difference in the CO₂ uptakes, as all PCD samples achieved almost the same CO₂ sorption performance (0.34 g_CO₂/g_sorbent) after 120 min of sorption at 450°C. The calcination conditions have been identified as crucial during the preparation of a sorbent. [73]. These conditions are connected to developing adequate surface area, pore volume, and carbon residue, thereby modifying the sorbent's uptake performance [62]”.

3. Additionally, prior Section 3.7 there were 4 samples to follow: PCD60, PCD240, Pristine CaO and Fresh dolomite calcined in the TGA (900C) – but here 2 more emerge – PCD30 and PCD 120 – not mentioned before and not considered for the characterizations and previous discussion – Can you explain the reasoning of this, and also maybe incorporate the samples somewhere earlier? At first, I was inclined to ask what was the reason of this large gap 60min to 240min, only to learn later that there are 2 others in the series.

Author`s answer: Thank you for your observation. Analysis of the PCD series have been made with PCD60 showing the maximum CO₂ sorption capacity and as such was the reason it was chosen as the best result for further tests. Please see answer to question 2 of reviewer 1. 

Action: Change of section title and technical rearrangement of the manuscript.

Old title “3.8.2 Sorption Performance of the Thermally Pre-treated Dolomite (PCD60)”
New title “3.6.1 Sorption Performance of the Thermally Pre-treated Dolomite”

Fig. 5a CO₂ uptake performance of activated PCD samples at 650°C

4. Section 3.8.2., line 424-427 – authors say that to improve the sorption capacity of PCD60 at 650°C the sample preparation and operating conditions were optimized and a fresh batch was synthesized at 900°C in cylindrical furnace – but in materials and methods Batch B it was synthesized exactly the same, so what is the optimization referring to? And why a new batch has been done? Seems that PCD60 was just synthesized again. If this is the case, I believe that the discussion here is a bit confusing and need to be addressed.

Author`s answer: Thank you for your observation. Please see answer to question 2 of reviewer 1.

Action: Our apologies on the labelling, which have been adjusted in all sections of the manuscript.

5. But I guess it is not the same sample, as the sorption capacity increases. It is confusing. What caused the improvement of CO2 uptake from 0.36 to 0.45, since the sample is still PCD60?

Author`s answer: Thank you for your careful revision.  The difference between the 0.35 and 0.45 consisted in the fact that this last sample (0.45) was desiccator-stored to avoid the humidity contact with the sample, which has been noticed to affect the CO₂ absorption performance, since the mixed calcined oxides (CaO-MgO) are known to be highly hygroscopic.

Action: Figure 4 have been updated to include only desiccator-stored samples. The new updated Figure is presented below for your perusal.

 

Fig. 4 CO₂ uptake performance of PCD60Act at different sorption temperatures after 30 min

6. As a general observation, the text seems crowded, the initial information refers to only 2 samples and 2 blank samples, but later on, additional materials emerge, making the results and discussion hard to follow and understand.

Author`s answer: Thank you for your observation. Please see answer to question 2 of reviewer 1.

Action: The methodology has been reviewed and modified to better reflect the materials and methods.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors provide valuable insights into the potential of using thermally pre-treated dolomite as a cost-effective, thermally stable, and moderately temperature-regenerable CaO-based sorbent for CO2 removal in integrated carbon capture and conversion (ICCC) processes aimed at producing high-purity hydrogen. This study holds significant guiding value for the implementation of CO2 capture technology; thus, it is recommended that this manuscript be accepted following the suggested revisions.

1. The text mentions achieving high CO2 capture at 650°C, but the comparison with similar research results shown in Table 3 fails to clearly highlight the uniqueness of this work. It is recommended that the authors provide a detailed description of the advantages of this study in terms of capture (e.g., thermal pretreatment time, the influence of Ca and Mg content in dolomite, etc.).
2. The CO₂ adsorption capacity data in the text lacks repeated experiments or error analysis. It is suggested to supplement at least three parallel experiments to ensure the accuracy of the results.
3. The authors mention a 240-minute heat treatment at 900°C for modified dolomite, which consumes high energy. It is recommended that the authors supplement an analysis of heat treatment energy consumption.
4. Further research is needed on the mechanism of cyclic stability. The 84% capture capacity retention is attributed to the MgO framework, but no explanation is provided for why PCD60 outperforms UD.
5. Figure 7 shows no significant difference in pretreatment effects between 30 and 240 minutes, yet subsequent experiments only use the 60-minute sample. The rationale for selecting 60 minutes (e.g., energy consumption, crystallinity, etc.) should be explained.
6. It is recommended that the authors supplement additional experimental conditions, such as the influence of other CO2 concentrations (100% CO2, 80% CO2, 70% CO2) on capture performance.
7. The study mentions the positive effect of MgO content in natural dolomite on performance but does not systematically explore the quantitative relationship between MgO ratio and CO2 adsorption/regeneration performance. It is suggested to design control experiments (e.g., adjusting MgO ratio) to verify its optimal range.
8. The authors state that this adsorbent is suitable for ICC technology (e.g., dry methane reforming), but its performance in mixed gases (containing SO2, H₂O, H₂, etc.) is not demonstrated.

Author Response

ANSWERS TO REVIEWERS:

Moderate Temperature Carbon Capture Using Thermally Pre-treated Dolomite: A Novel Approach

 

Iyiade G. Alalade^a, Javier E. Morales-Mendoza^a, Alma B. Jasso-Salcedo^a, Jorge L. Domínguez-Arvizu^a, Blanca C. Hernández-Majalca^a, Hammed A. Salami^a, José L. Bueno-Escobedo^a, Luz I. Ibarra-Rodriguez^a, Alejandro López-Ortiz^a* and Virginia Collins-Martínez^a

C – Journal of Carbon Research, Manuscript ID: carbon-3643170

Reviewer #3:

1. The text mentions achieving high CO₂ capture at 650°C, but the comparison with similar research results shown in Table 3 fails to clearly highlight the uniqueness of this work. It is recommended that the authors provide a detailed description of the advantages of this study in terms of capture (e.g., thermal pretreatment time, the influence of Ca and Mg content in dolomite, etc.).

Author`s answer: Thank you for your recommendation. Adjustments have been made to Table 3 to compare between our work and those reported in literature highlighting its uniqueness therein.

Action: Below are the adjustments made to Table 3 with further discussions included in the body of the article.

Table 3: Comparison of CO₂ capture capacities of some CaO-based dolomite sorbents

Sorbent	CaO/MgO ratio	Calcination conditions	Sorption conditions	Regeneration conditions	Cycles	First - Last Cycle Uptake (gCO₂/gsorbent)	References
Ball-milled Dolomite	Not mentioned	RT-900°C 300°C /min, 70% CO2/30%air	650°C, 15% CO2, 85% Air	900°C, 70% CO2, 30% Air	20	0.40 – 0.27	[99]
Acetic acid-treated Dolomite	Not mentioned	900°C, air, 2h during preparation.  In TGA RT-900°C 300°C /min, 70% CO2/30%air	650°C, 15% CO2, 85% Air	900°C, 70% CO2, 30% Air	20	0.29 – 0.12	[103]
Dolomite	1.64	800°C, 3h	690°C, 15% CO2	800°C, 100% N2	30	0.41 – 0.17	[109]
Citric acid-treated Dolomite	1.64	600°C, 100% N2, 2h and switch 800°C, air, 3h	690°C, 15% CO2	950°C, 100% CO2	20	0.44 – 0.25	[101]
Dolomite	2.03	800°C, 100% N2	700°C, 15% CO2	800°C, 100% N2	20	0.41 – 0.27	[39]
Gluconic acid-treated Dolomite	1.64	800°C, air, 2h	700°C, 15% CO2	950°C, 100% CO2	10	0.45 – 0.40	[102]
Dolomite	1.45	Not mentioned	650°C, 30% CO2, 10% H2O	850°C, 100% N2	50	0.45 – 0.26	[25]
Dolomite	55.61 wt% CaCO3, 44.2 wt% MgCO3	RT- 900°C, 50%CO2/50% N2 and switch 900°C, 100%N2,5 min	600°C, 50% CO2	900°C, 100% N2	15	0.21 – 0.17	[53]
Dolomite	23.63 wt% Ca, 9.63 wt% Mg	850°C, N2, 1h during Preparation.  In TGA RT-725°C 300°C /min, N2, 10 min	850°C, 100% CO2	725°C, 100% N2	20	0.45 – 0.40	[100]
Dolomite	3.46	RT- 800°C, 100% N2, 6°C/min	700°C, 100% CO2	750°C, 100% N2	8	0.23 – 0.18	[107]
Dolomite	1.93	800°C, vacuum	400°C, 100% CO2	800°C, vacuum	3	0.51 – 0.30	[108]
PCD60Act	1.64	900°C, Air, 1h during preparation.	450°C, 90% CO2	650°C, 100% Argon	15	0.25 – 0.17	This work
		In TGA RT- 650°C, 100% Ar, 30 min
PCD60Act	1.64	900°C, Air, 1h during preparation.	650°C, 90% CO2	650°C, 100% Argon	15	0.44 – 0.37	This work
		In TGA RT- 650°C, 100% Ar, 30 min

 

“Table 3 compares the CO₂ sorption capacities of CaO-based dolomite sorbents from literature with the results of this work. The long-term performance patterns of sorbents can be studied by simulation with realistic CO₂ concentrations. While, 15% CO₂ aligns with post-combustion gas streams [75], tests utilizing elevated CO₂ partial pressures (90 to 100% CO₂ concentration) have been reported to accelerate the deactivation in CaO-based dolomite sorbents (sintering and pore collapse) over prolonged cycles, thus allowing for rapid assessment of sorbent’s long-term stability under such conditions.

For instance, the CO₂ uptake capacity using 15% CO₂ reach 0.45 g_CO₂/g_sorbent in the first cycle with a rapid capacity loss at the 20th cycle [39,119–121]. Furthermore, the performance in studies using 100% CO₂ varies, stressing out that the sintering mechanisms observed under high CO₂ concentrations correlate strongly with real-world performance degradation, justifying accelerated testing protocols [46]. As expected, there are sorbents that deactivate within few cycles, demonstrating their poor stability [124,125]. But in our study, using 90% CO₂ retained a CO₂ uptake capacity of 0.38 g_CO2/g_sorbent after 15 cycles for the PCD60Act compared to 0.40 gCO₂/gs_orbent reported by Han`s team [123] after 20 cycles. It is noteworthy to highlight that Han et al. [123] study was carried out at high-temperatures cyclic sorption-regeneration with a low Mg content dolomite (high CaO:MgO ratio).

Furthermore, the optimal MgO content in synthetic sorbents for CO₂ capture has been a subject of significant research. However, in natural sorbents, there is limited literature. For instance, Yang et al. [57] identified that a 38.7 wt% MgO content in calcined dolomite (corresponding to 17.16 wt% in natural dolomite) yields the best results for CO₂ uptake capacity and cyclic stability. This optimal composition, found in commercially available dolomite from China, demonstrated a CO₂ uptake capacity of 0.45 g_CO₂/g_sorbent in the initial cycle and maintained stability over 50 cycles, with a CaO:MgO ratio of 1.45 [57].

Comparative studies using natural dolomites from various sources and commercial dolomite with lower MgO content have shown less favorable results in terms of stability. Japanese natural dolomite (CaO:MgO ratio of 1.93) [125], Chinese natural dolomite (CaO:MgO ratio of 2.45) [123], and Alfa Aesar commercial dolomite (CaO:MgO ratio of 3.4) [124] all exhibited poor stability similar to limestone-based sorbents. In contrast, the calcined Mexican natural dolomite used in the current study contained 38 wt% MgO, resulting in a CaO:MgO ratio of 1.63. This composition aligns with the optimal ratio identified by Yang and co-workers [57], suggesting that the Mexican natural dolomite may be more suitable for CO₂ capture applications, offering improved cyclic stability compared to the other dolomite sources mentioned”.

2. The CO₂ adsorption capacity data in the text lacks repeated experiments or error analysis. It is suggested to supplement at least three parallel experiments to ensure the accuracy of the results.

Author`s answer: Thank you for your valuable feedback regarding the reproducibility of our CO₂ sorption capacity measurements. We appreciate the opportunity to strengthen the statistical rigor of our work. In response to your suggestion, we conducted three consecutive experimental replicates of the PCD60Act sample under identical conditions (650°C heat pretreatment in an inert atmosphere) to assess variability and ensure the reliability of our results.

The CO₂ sorption capacities observed across the three replicates were 47.3%, 47.7%, and 47.5%, yielding a mean value of 47.5% with a remarkably small standard deviation of 0.23%. This corresponds to a relative standard deviation (RSD) of just 0.48%, showing the minimal scatter between measurements. Such low variability highlights the precision of our experimental setup, particularly the stability of the inert atmosphere and temperature control during testing.

This level of reproducibility aligns with established industrial standards for sorbent testing, where triplicate measurements are widely accepted for benchmarking materials like CaO/MgO-based systems.

While our focus on triplicate measurements balances practicality with statistical robustness, we acknowledge that expanding the sample size could marginally refine error estimates. Nevertheless, the observed consistency—coupled with the negligible RSD (<0.5%)—provides high confidence in the reported mean capacity of 47.5% for comparative analyses.

Action: In revising the manuscript, we have incorporated these details to enhance transparency:

• A supplementary table (Table S7) now includes the raw replicate data.
• Error bars reflecting the standard deviation have been added to Figure 3.
• The Methods section explicitly outlines our replicate testing protocol.

Your insightful comment has prompted a more nuanced presentation of our data’s reliability, and we are grateful for the opportunity to improve our work.

Fig. S5 Statistical analysis of three consecutive replicates of PCD60Act.

Here is the text of a new added section for your perusal:

2.1.4 CO₂ Sorption Repeatability Analysis

“The best-performing activated PCD sample was analyzed for repeatability with CO₂ sorption performed in triplicate, and data are presented as mean ± standard deviation (SD). Y-error bars in all graphs represent the standard deviation of three independent measurements for each sample. Statistical analysis was carried out using OriginPro, and standard deviations were computed based on the spread of replicate data points.”

3. The authors mention a 240-minute heat treatment at 900°C for modified dolomite, which consumes high energy. It is recommended that the authors supplement an analysis of heat treatment energy consumption.

Author`s answer: Thank you for your observation. Energy consumption analysis has been made, and necessary information has been included in both supplementary materials and the body of the work.

Action: Below is the energy consumption analysis alongside the tabulated data.

``The Thermolyne Tube Furnace 21100 used has a maximum power rating of 1.8 kW and consumes energy proportionally to heating time and temperature. In our case, the heating rate used was 10°C/min for calcination experiments. The ramp-up time to reach 900°C (from room temperature, ~25°C) is 87.5 min, consuming approximately 2.63 kWh for ramp-heating. For 30 min hold at 900°C, the total runtime is 87.5 min (ramp) + 30 min (hold) = 117.5 min, resulting in ~3.53 kWh of energy use. A 60 min hold increases total runtime to 147.6 min resulting in ~4.43 kWh, while for 120 min and 240 min calcinations, 207.6 min is equivalent to ~6.23 kWh and 327.6 min requires ~9.83 kWh, respectively (please see Table S2). These estimates assume continuous power draw at 1.8 kW, though actual consumption may vary slightly due to thermal inertia and insulation efficiency (open-air atmosphere) ``.

Table S2: Quartz cylindrical furnace calcination energy consumption

Hold Time at 900°C	Total Runtime (h)	Energy Consumed (kWh)
30 min. (PCD30)	1.96 (1.46 + 0.5)	~3.53
60 min. (PCD60)	2.46 (1.46 + 1.0)	~4.43
120 min. (PCD120)	3.46 (1.46 + 2.0)	~6.23
240 min. (PCD240)	5.46 (1.46 + 4.0)	~9.83

 

Even though the lowest energy consumption corresponds to sample PCD30, the tradeoff of the best performance of sample PCD60Act, which consist in the highest CO₂ capture and kinetics, as well as thermal stability of this sorbent justifies the slightly higher energy consumption.

4. Further research is needed on the mechanism of cyclic stability. The 84% capture capacity retention is attributed to the MgO framework, but no explanation is provided for why PCD60 outperforms UD.

Author`s answer: Thank you for your suggestion. The article highlights that the thermal pre-treatment in argon played a key role in obtaining optimal CO₂ uptake performance in PCD60 in comparison with UD 

Action:  Clarifications have been made in the ``Materials and Method`` section to provide more justification for the sample's preparations and operating conditions. Moreover, a brief explanation has been provided in section 5 to further emphasize the role of MgO as structural framework with adequate references.

“Furthermore, the calcination of the dolomite sample in an open-air quartz cylindrical furnace followed by thermal pre-treatment in argon induces the formation of a special CaO–MgO composite, with the MgO phase imparting structural stability on the material [39,117,118]. During multicyclic CO₂ capture tests, the thermally stable and chemically inert MgO phase acts as a structural stabilizer by spatially separating CaO grains, thereby limiting sintering-induced grain boundary contact. This mitigates deactivation and preserves the cyclic sorption-regeneration performance of the CaO phase as evidenced in Figure 7”.

5.Figure 7 shows no significant difference in pretreatment effects between 30 and 240 minutes, yet subsequent experiments only use the 60-minute sample. The rationale for selecting 60 minutes (e.g., energy consumption, crystallinity, etc.) should be explained.

Author`s answer: Thank you for your observation. Please see answer to question 2 from reviewer 1 and answer to question 3 from reviewer 2.

Action: Corrections have been made to the methodology in section 2.1 as well as section 3.5. Further tests obtained on the PCD series have been added to the body of the work with a corresponding discussion.

6. It is recommended that the authors supplement additional experimental conditions, such as the influence of other CO₂ concentrations (100% CO₂, 80% CO₂, 70% CO₂) on capture performance.

Author`s answer: Thank you for your suggestion. Due to technical limitations of the TGA in our laboratory, 100% CO<sub>2</sub> can`t be undertaken, but tests at 80% and 70% CO₂ concentrations have been performed.

Action: PCD60Act has undergone single cycle capture studies under 90%, 80% and 70% CO₂ concentrations and results obtained have been included in the ``Supplementary Materials``. A small discussion was added to the manuscript as follows:

“Also, CO₂ sorption capacities at different CO₂ concentrations (70, 80, and 90%) were studied and presented in Figure S6. As expected, the CO₂ uptake performance at 90% CO₂/Ar had the best capacity, while the uptakes of 70% and 80% CO₂ concentrations remained consistent (≈0.43 g_CO₂/g_sorbent), highlighting the outstanding capture capacity of the PCD60Act regardless of the CO₂ concentrations”.

Furthermore, Figure S6 is included below for your perusal.

Fig. S6 CO₂ uptake performance of PCD60Act at different CO₂ concentrations.

7. The study mentions the positive effect of MgO content in natural dolomite on performance but does not systematically explore the quantitative relationship between MgO ratio and CO₂ adsorption/regeneration performance. It is suggested to design control experiments (e.g., adjusting MgO ratio) to verify its optimal range.

Author`s answer: Thank you for your suggestion. The dolomite precursor is a mineral source and as such, the MgO content can`t be adjusted. Doping with MgO-based salts will only increase the cost of the material, hindering the cost-effective advantage of the mineral.

Action: NONE

8. The authors state that this adsorbent is suitable for ICC technology (e.g dry methane reforming), but its performance in mixed gases (containing SO₂, H₂O, H₂, etc.) is not demonstrated.

Author`s answer: Thank you for your observation. We acknowledge that the material is promising for this application. However, further analysis is needed to take realistic flue gases into consideration. These studies shall be undertaken in the nearest future.

Action: Future investigation.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors adequately answered the questions and suggestions, therefore I give a positive opinion for publication in the journal

The authors adequately answered the questions and suggestions, therefore I give a positive opinion for publication in the journal

Reviewer 2 Report

The authors worked on the paper and corrected all the observations.

Thanks for the replies and amendments on the manuscript. 

 

Reviewer 3 Report

The revised manuscript can be accepted.

The revised manuscript can be accepted.