Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper systematically evaluates the application of porous adsorbent materials (zeolites, metal-organic frameworks(MOFs), and activated carbons) in CO₂ capture and geological sequestration. The main research contents are:

(1) Zeolites offer robust thermal stability, high selectivity, and low cost, but exhibit performance limitations under humid conditions. MOFs have superior tunability and CO₂ adsorption capacity, but challenges in hydrothermal stability and scale economy limit their industrial application. Activated carbons, widely used and moisture-tolerant, provide a practical middle ground, especially when derived from sustainable biomass feedstocks.

(2) The operational advantages and limitations of five major underground delivery mechanisms are discussed: direct suspension injection, polymer-assisted transport, foam-assisted delivery, controlled-release encapsulation, and preformed particle gels (PPGs).

(3) The innovation of hybrid systems and the use of artificial intelligence to customize CO₂ capture materials according to specific geological and operational conditions.

But there are some problems:

1.Although the abstract outlines the research direction, it does not clearly highlight data results or quantitative findings.

2.The Introduction lacks performance comparison with traditional materials. It is recommended to add a comparison between traditional adsorbents and novel porous materials in terms of performance and scalability, to highlight the engineering value of this study.

3.It is suggested that the authors add more literature in the Introduction section, especially recent literature from the past two years, to support the advantages of solid porous adsorbents.

4.The paper lacks quantitative data in its analysis. It is recommended to supplement quantitative data to enhance persuasiveness.

5.The paper emphasizes the importance of pore size, but does not conduct an in-depth analysis. It is recommended to revise or supplement the correlation between pore size and particle dimensions.

6.Regarding the radar chart: how were the scores determined, and what are the data sources?

7.For the composite materials introduced by the authors, there is a lack of specific performance improvement data and practical examples.

8.There are broken reference errors during text composition “ (as seen in Error! Refer- 92

ence source not found. and Error! Reference source not found.) ”

Author Response

Reviewer 1 – Comments and Author Responses

Comment 1: The abstract does not clearly highlight data results or quantitative findings.
Response: Thank you for the suggestion. We have revised the abstract to explicitly include key quantitative findings, including CO₂ adsorption capacity ranges (e.g., 3.5–8.0 mmol/g), surface areas (up to 7000 m²/g), and normalized scoring ranges from the radar chart. This enhances the impact of the abstract and provides concrete insights into our comparative framework.

Comment 2: The Introduction lacks performance comparison with traditional materials.
Response: We agree and have added a comparative baseline in the Introduction, contrasting solid porous adsorbents with traditional amine-based solvents, highlighting differences in regeneration energy, moisture stability, and lifecycle impact.

Comment 3: Add more recent literature from the past two years to support the advantages of solid porous adsorbents.
Response: The manuscript has been updated with 15 new references from 2022–2024 (e.g., Jedli et al., 2024; Adegoke et al., 2024), covering recent developments in MOF synthesis, activated carbon from biomass, and CO₂ capture case studies.

Comment 4: The paper lacks quantitative data in its analysis.
Response: Quantitative values have been added throughout, particularly in the revised radar chart table, Tables 2–4, and delivery mechanism sections. Each material type now includes adsorption capacity, surface area, thermal stability, and cost figures supported by references.

Comment 5: In-depth correlation between pore size and particle dimensions is missing.
Response: We have included an additional subsection in Section 3.1 that analyses how microporous vs. mesoporous structures affect CO₂ diffusion and retention. This discussion integrates findings on mesostructured zeolites and MOFs with pore-engineered carbons.

Comment 6: Clarify how radar chart scores were determined.
Response: We added a clear description in the methodology, including the min-max normalization formula (Equation 1), source references for the values, and rationale for score assignment. Each parameter now links to specific citations.

Comment 7: Composite materials lack specific performance improvement data.
Response: Section 5 has been revised to include examples of MOF-carbon composites showing ~20% improved moisture stability and comparable CO₂ uptake (6.5–7.5 mmol/g) to pristine MOFs (references added: Ullah et al., 2024).

Comment 8: Broken reference errors.
Response: All instances of “Error! Reference source not found.” have been corrected. We verified cross-references to ensure integrity throughout the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a comparative review of zeolites, MOFs, and activated carbons for CO₂ capture and geological sequestration. Authors develop a quantitative framework using min-max normalization to evaluate five criteria: surface area, moisture resistance, regeneration, cost, and adsorption capacity. The work integrates material properties with five subsurface delivery methods, bridging the gap between laboratory research and industrial implementation.

1. Multiple "Error! Reference source not found." on pages 3, 11, and 12 compromise scientific credibility.
2. No discussion of pilot studies or field trials under real reservoir conditions.
3. Laboratory-to-field extrapolation is not addressed.
4. Missing data on effects of variable pressure, temperature, and salinity on material performance.
5. Superficial economic analysis, Only cost ranges without detailed techno-economic modeling. Missing complete lifecycle analysis, costs per ton CO₂ captured, and economies of scale for MOFs. Absence of quantitative cost-benefit comparison.
6. Missing optimization criteria, the five delivery methods lack decision matrices for selection based on reservoir characteristics. No algorithms or flowcharts to guide practical field selection.
7. Hybrid systems without comparative data in section 5 where mentions MOF-carbon composites without quantitative performance data versus pure materials. Absence of experimental evidence for claimed synergistic advantages.
8. Section 7 lacks quantitative LCA data. Missing specific values for carbon footprint, energy requirements, and synthesis emissions. Sustainability discussion without concrete metrics.

The suggested improvements will strengthen the manuscript's impact and utility for the scientific community.

Author Response

Reviewer 2 – Comments and Author Responses

Comment 1: Multiple "Error! Reference source not found."
Response: Corrected. All erroneous cross-references have been replaced with appropriate figure/table numbers or citations.

Comment 2: No discussion of pilot studies or field trials.
Response: We have added several examples of pilot deployments, such as Zeolite 13X injections (Chen et al., 2014) and MOF-polymer tests in synthetic cores (Demir et al., 2022), to bridge lab-to-field knowledge gaps.

Comment 3: Laboratory-to-field extrapolation is not addressed.
Response: A new paragraph in Section 6 discusses how adsorption behavior can shift due to pressure/salinity and outlines challenges in scaling (e.g., particle agglomeration, hydrothermal cycling). We cite Tavagh Mohammadi et al. (2025) to support this.

Comment 4: Missing performance data under variable reservoir conditions.
Response: Table 2–4 and the revised delivery mechanisms section now detail performance under high salinity, thermal cycles, and pressure gradients. Specific examples are added for UiO-66 and activated carbon stability under humid CO₂ injection.

Comment 5: Economic analysis is superficial.
Response: We included lifecycle cost ranges (e.g., $1–5/kg for activated carbon vs. $100–500/kg for MOFs), cost per ton of CO₂ captured, and synthesis costs. TEA results from Pan et al. (2020) are referenced.

Comment 6: Delivery method selection lacks decision-making tools.
Response: A new Table (now Table 5) compares delivery strategies across technical and geological factors. A flowchart was also added to guide selection based on formation properties.

Comment 7: Lack of performance data for hybrid systems.
Response: Section 5 was updated with studies showing improved stability for MOF-zeolite and MOF-carbon hybrids (Choi et al., 2024; Ullah et al., 2024), including comparative CO₂ capacity and regenerability.

Comment 8: No LCA data.
Response: Section 7 now includes a simplified LCA discussion, with environmental impact data (e.g., energy input per kg MOF) and carbon footprints of MOF vs. activated carbon synthesis.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents a comparative study on the application of Zeolites, MOFs and activated carbon in CO2 capture and geological sequestration applications, exploring five underground transport mechanisms. The topic aligns with the forefront of carbon neutrality technologies, and some experimental designs and data analyses hold certain reference value. However, the manuscript suffers from issues including insufficient rigor, logical disorganization, and a weak methodology. Therefore, this reviewer recommends rejection. The specific issues are detailed below:
1. Some figures and tables are not explicitly referenced in the main text. They should be closely integrated with the textual analysis, not presented in isolation.
2. The last paragraphs of 3.1-3.3 are repetitive of the content in the preceding paragraphs. It is suggested to restructure the logical expression, first separately describe the characteristics of the three types of materials, and then add a subsection to discuss the performance comparison among them, and integrate the data from the existing radar chart (Figure 1) and Table 4.
3. The number of references is insufficient. Only 38 references are cited in the main text, with less than 30% being from the last five years. Relevant literature from recent years should be supplemented to enrich the content. Furthermore, none of the figures in the manuscript cite their sources. Key figures should be properly cited using relevant literature.
4. Sections 3.1-3.3 introduce the CO2 capture performance of Zeolites, MOFs and activated carbon, respectively. However, this section relies on too few references, with the majority sourced solely from references [10] and [11].
5. Numerous formatting issues exist. Equation 1 has abnormally enlarged font; some figures/tables are cut off (Table 2, Figure 2); colors in some figures are unclear (Figure 5, Figure 6); Figure 6 appears twice in the manuscript; Figure 3 contains erroneous characters; Table 3 should be placed in section 3.2, not 3.1.

If the authors are willing to undertake revisions, the following suggestions may be considered:
1. The manuscript primarily introduces the effectiveness of three broad material classes in CO2 capture and storage. These classes can be further subdivided. For example, MOFs can comprise different metal elements or organic linkers and exhibit various morphologies (e.g., multi-layered, porous). Select at least one material class and provide a mechanistic analysis explaining the fundamental reasons for performance differences arising from different elements/structures within that class.
2. In 2.2.5 presents, the "normalized score equation to scale the performance value" and radar charts are provided. However, these methods only evaluate each material parameter separately and fail to capture the interrelationships between parameters. Propose a comprehensive evaluation method or formula that assesses multiple polymer properties simultaneously. For instance:
1) Use the normalized scoring equation to calculate a score for each property, then compute a weighted sum to obtain an overall score, enabling other researchers to use this formula to compare against peer materials.
2) Develop a parametric formula with adjustable weights that incorporates engineering considerations for CO2 storage (e.g., storage space size), allowing practical assessment of polymer suitability in real-world applications.
3. Figures 4 and 5 present the technical flowchart for the selection and evaluation of adsorbents. However, this is not sufficient. It is necessary to combine the calculation formulas in Chapter 2 and provide specific evaluation methods to make the article more complete.

Author Response

Reviewer 3 – Comments and Author Responses

Comment 1: Figures/tables not referenced in text.
Response: We revised the manuscript to ensure all figures and tables are explicitly referenced and discussed within the text (e.g., Figure 1 and Tables 2–4 are now integrated into Sections 3.1–3.3).

Comment 2: Repetition in Sections 3.1–3.3.
Response: The repetitive conclusions were condensed, and a new Section 3.4 (“Comparative Summary of Adsorbent Performance”) was created to consolidate the comparisons with reference to Figure 1 and Table 4.

Comment 3: Increase reference count and recency.
Response: The reference list now includes over 60 entries, with ~50% published between 2022–2024. Each material section was enriched with recent studies.

Comment 4: Over-reliance on two references in key sections.
Response: We expanded the literature base across all material sections, including new sources on zeolite modifications, MOF-water resistance, and activated carbon hybridization.

Comment 5: Formatting issues.
Response: All figures have been rechecked for resolution and placement. Duplicate figures were removed, erroneous characters corrected, and font sizes standardized.

Suggested Improvement 1: Subdivide material classes and provide mechanistic insight.
Response: We added a subsection analyzing MOFs by metal type (Zr vs. Cu-based) and discussed how structural morphology (e.g., ZIFs vs. UiO) affects stability.

Suggested Improvement 2: Add weighted scoring model.
Response: A new Equation 2 has been added to enable weighted multi-criteria scoring. Default weights and an example are provided for CCS prioritization scenarios.

Suggested Improvement 3: Combine flowcharts with formulas.
Response: Figure 4 has been updated to integrate evaluation equations with decision logic for material and delivery selection.

Reviewer 4 Report

Comments and Suggestions for Authors

Comparative Assessment and Deployment of Zeolites, MOFs, and Activated Carbons for CO₂ Capture and Geological Sequestration Applications

The manuscript presents a comparative review of porous materials for CO₂ capture and subsurface sequestration, with the stated goal of providing a practical framework for material selection and delivery strategies. Although the review is ambitious in scope and touches on many relevant topics, there is a lack of quantitative depth, methodological clarity, and integration of the current literature. Below is a detailed critique, incorporating both major and minor comments.

Major comments:

Introduction section:

• Literature support is weak: A reference to recent statistical sources (e.g., the latest IPCC Assessment Report, IEA CO₂ Emissions Report, or Global CCS Institute data) would help contextualize emission volumes by sector and quantify the projected contribution of CCS.
• Superficial treatment of solvent-based methods: The discussion of amine-based post-combustion capture is brief and lacks engagement with the state-of-the-art, such as fourth-generation solvents, blended amines, or phase-change systems. A comparative baseline is needed to contextualize the relevance of porous materials.

Methodology section:

• The study is claimed to propose a “comparative review” and “a practical framework for material selection and delivery strategies.” However, the authors should explain (a) the criteria for comparison, (b) why they were selected, and (c) how the technical literature is integrated with actual geological requirements.
• The approach of using five comparative parameters (surface area, adsorption capacity, ease of regeneration, cost, and moisture resistance) is reasonable and useful as a visual tool, but no theoretical justification or references are provided to explain why these five factors were selected as priorities.
• Add missing parameters as Adsorption/desorption kinetics, CO₂/N₂ and CO₂/ H₂O selectivity or explain why they were discarded.
• The authors should clarify whether the comparative metrics are weighted equally, or prioritized by application (e.g., post-combustion vs. geological injection).
• Figure 1 lacks source attribution for the numerical values presented.
• Section 2.2.2. Reference pressure and temperature are omitted. Do you mean adsorption at 1 atm and 25 °C? Or process conditions (e.g., 0.15 atm CO₂ post-combustion)?. No references or actual examples of specific materials are included.
• Section 2.2.3. Disadvantages of each method (e.g., thermal degradation of MOFs under TSA, high compression in PSA) are not discussed.
• In Moisture resistance section: No quantitative context is provided: how is moisture resistance measured? What capacity loss is tolerable?

Material performance section:

Zeolites

• Important limitations such as acid instability (e.g., in presence of SO₂, NOₓ) are not discussed.
• Quantitative moisture sensitivity data is missing (e.g., percentage capacity loss).
• No experimental validation or citations are provided for adsorption values.

MOFs

• Not all MOFs have high adsorption capacity; some have low affinity for CO₂ at pressures of interest. This complexity must be discussed.
• Mechanical and thermal stability under geologic conditions (pressure, temperature, salinity) is not analyzed, despite its critical relevance.
• The claim of “high regenerability” is overly general; many MOFs degrade under moist TSA/PSA cycles.
• Production cost is labeled as “high,” but no cost range or synthesis routes are discussed (e.g., green synthesis, automation).
• Toxicity risks (Cr, Zn, etc.) are not acknowledged, nor are recent advances like amine-functionalized MOFs for improved water tolerance.

Activated carbon

• No data on gas selectivity or adsorption kinetics is provided.
• A concrete case study (e.g., coconut shell-derived activated carbon) should be included with regeneration and stability metrics.
• Important variables such as bulk density and packing limitations affecting injection are overlooked.
• Absence of performance data under subsurface conditions (pressure, salinity, temperature) is a major omission.

Tables 2 and 3

• No references are provided for the data shown.
• Critical comparison metrics like CO₂/N₂ and CO₂/H₂O selectivity, kinetics, or energy for regeneration are excluded.

Delivery Mechanisms:

Direct suspension injection:

• Provide quantitative examples (e.g., injection rate, optimum concentration, particle size) and consider extended dispersion tests or oscillatory flows.
• It is assumed that adding surfactants solves the problem, without evaluation of chemical compatibility with real formations.

Polymer assisted transport:

• No mention is made of irreversible adsorption of the polymer in rocks (a well-documented phenomenon that reduces permeability).

Foam delivery:

• Discussion of foam stability under high salinity and temperature conditions is missing, which can be a strong limiting factor.

Encapsulation techniques:

• It is presented as a versatile solution, but without discussing industrial scalability and cost of capsule synthesis, which limits its applicability.
• A discussion of capsule degradation by-products (chemically inert? biodegradable?) is missing.

In PPGs:

• No data provided on CO₂ retention capacity within gels.
• Lack of discussion on injection pressure requirements and gel behavior under formation stress.

Add table with columns such as: type of delivery, compatible material, target training, advantages, limitations, technological maturity.

Hybrid Systems and Advanced Materials section

Hybrid Materials

• No quantitative data is presented: how much stability is improved? What is the loss of capacity compared to pure MOFs?.
• Absence of discussion on chemical stability in subsurface environments.
• Cite studies with cyclic performance (>50 cycles) and provide XRD/SEM data demonstrating structural retention.

Smart Adsorbent

• Incorporate values of volume change vs. temperature, functional reversibility cycles, and comparisons with non-intelligent systems in terms of delivery efficiency.
• It is important to discuss what happens in the presence of Ca²⁺, Mg²⁺ or sulfates?

Machine Learning

• Experimental validation of AI predictions is not discussed: how accurate has this approach been, and has it led to any actual functional material?

Environmental and Economic Impacts

• No quantitative life cycle analysis (LCA), net emissions or energy consumption estimates are included.

System-Level Challenges section:

Infrastructure and Field Compatibility

• The chemical or mechanical interaction between the materials and the rock is not discussed: is there clogging? secondary precipitation?
• Include a compatibility matrix between adsorbent type and reservoir type, based on real physical-chemical properties.

Monitoring and Verification

• Emerging techniques (e.g., tracer particles, reactive monitoring) are mentioned, but no case studies or resolution/cost analysis is provided.

Economic Viability

• No specific numerical data on costs ($/kg, $/ton CO₂ captured) are presented for any of the materials.

Policy Incentives

• Include at least one case study of successful implementation of incentives for CCS in real projects.

Integration

• Overly idealized vision of integrated CCS systems.
• Omit critical barriers: co-processing logistics, reactive byproducts, catalytic fouling, etc.

Sustainability considerations

• The risk of rebound effects - that the use of advanced materials will have a larger environmental footprint than the benefit per capture - is not analyzed. Present a case study with experimental data or cite recent studies quantifying adsorption performance by biomass type.
• The analysis of the final destination of the adsorbent after its useful life is omitted: is it recyclable, inertizable, pollutant?
• Incorporate an adsorbent life cycle flow diagram, from extraction/synthesis to final disposal, indicating inputs, outputs and critical points.

Limitations, recommendations and conclusions:

• Critical long-term limitations are overlooked:

- Disposal of spent adsorbents

- Leaching of metals in MOFs

- Geo-reactivity (e.g., acidification, mineral scaling)

• No gap analysis is conducted to identify missing data fields or oversaturated research areas.
• The conclusion section misses visual synthesis (e.g., comparative radar plot, decision matrix) to consolidate findings and guide practitioners.

Minor comments.

• Replace all cases of "Error! Reference source not found" with the correct citation or reference label. These errors appear throughout the manuscript and disturb the reading flow.
• Check all tables for completeness. Several tables are missing columns.

Author Response

Reviewer 4 – Comments and Author Responses

Major Comments – Introduction
Response: We revised the introduction to cite updated data from the IPCC (2023) and IEA (2024), and included a more detailed comparison with solvent-based CCS, including fourth-generation amines.

Major Comments – Methodology
Response:

• Selection of criteria (surface area, etc.) is now justified with references and explained in Section 2.
• Additional parameters such as CO₂/N₂ selectivity and kinetics are discussed with rationale for exclusion or integration.
• Weighting assumptions are clarified; equal weighting is used unless otherwise specified (see Equation 2).
• Adsorption conditions are specified (1 atm, 25 °C unless stated).
• Disadvantages of TSA/PSA and MOF degradation under regeneration added.

Major Comments – Material Performance
Response:

• Added discussion on acid gas instability (SO₂, NOx) in zeolites.
• MOF toxicity and green synthesis routes discussed.
• Activated carbon case study (coconut shell) added with specific metrics (3.5 mmol/g, 900°C thermal stability).

Major Comments – Delivery Mechanisms
Response:

• Added quantitative metrics for suspension injection (e.g., particle size 100–300 µm, injection rate ~0.1 m³/min).
• Scalability and degradation of encapsulated gels discussed.
• Table 5 added to summarize delivery method, compatible material, and technological maturity.

Major Comments – Hybrid and AI Approaches
Response:

• Section 5 updated with comparative figures on hybrid material improvement (e.g., 10–25% moisture resistance gain).
• Smart adsorbents tested under saline conditions discussed.
• AI-assisted material discovery (e.g., MOF-ML models) with reference to Park et al., 2024.

Major Comments – System-Level Considerations
Response:

• Geo-reactivity (e.g., scaling, acidification) and fouling risks discussed.
• A life cycle flow diagram for adsorbents is included (new Figure 7).
• Compatibility matrix of material vs. reservoir type is added to guide real-world implementation.

Minor Comments
Response: All broken references were corrected. Tables have been reviewed and expanded where necessary.

 

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript has been significantly improved and is suitable for publication. Thank you for the substantial revisions.