Interfacial Design of Mixed Matrix Membranes via Grafting PVA on UiO-66-NH₂ to Enhance the Gas Separation Performance

Round 1

Reviewer 1 Report

The paper “Interfacial Design of Mixed Matrix Membranes via Grafting PVA on UiO-66-NH₂ to Enhance the Gas Separation Performance” reports the CO₂/N₂ separation performance of a series of mixed matrix membranes. By coating/surface treating UiO-66-NH₂ with PVA, better dispersion of UiO-66-NH₂ in the base PVAm matrix was achieved and interfacial/interstitial voids were avoided. Both factors lead to better CO₂/N₂ separation performance. Overall, the design principle presented in this paper is interesting and important, and the membrane characterizations are comprehensive. However, there are still several parts in this paper that need to be revised. Hence, I recommend the paper to be published after major revision. Below are my comments that I hope the authors could address properly.

1. Page 1, line 13: The authors suggested that the MOFs were covalently bonded with PVA in the abstract. In the main text, the authors suggested that interaction is intermolecular hydrogen bond (Page 4, line 194). Please double-check on this point.
2. The introduction section needs to be polished. For example, the authors did not specify why they chose UiO-66-NH₂ as the MOF for MMMs fabrication (performance/surface tunability/environmental or economic considerations?). The following recent papers shall be incorporated to support the selection of UiO-66-NH₂ (Microporous and Mesoporous Materials, 2021, 313, 110823. Journal of Environmental Chemical Engineering, 2021, 9, 105159. Applied Surface Science, 2021, 553, 149547. Journal of Membrane Science, 2017, 539, 172-177.)
3. Page 3, line 133: The authors stated that FT-IR measurements were done in the rage of 3500-500 cm^-1. However, UiO-66-NH₂ and MMM spectra show non-zero absorbance between 4000-3500 cm^-1 (Figure 1). Please double-check on this point.
4. Page 4, line 154-155: Was the hydrothermal process conducted in an autoclave? Also, was the solution stirred or kept stagnant?
5. Page 4, line 158-159: Was the membrane-casting solution degassed?
6. Page 5, line 201-202: Could the authors briefly explain why the bulk membrane has a porous morphology (phase separation/trapped air bubbles in the absence of degassing?).
7. Page 5, line 218: The surface coating of UiO-66-NH₂ by PVA is an important step to promote the membrane performance. Could the authors please provide any insights on the PVA loading of UiO-66-NH₂/PVA composite (perhaps by air TGA)?
8. Page 6, line 225: the “UiO-66-NH₂ particles (b)” should be “UiO-66-NH₂ particles (c)”?
9. Page 7, line 244-245: Figure 6 was introduced here, yet no further discussion on this figure was provided. Could the authors move this figure to supporting information if it is less relevant to the key points of the main text?
10. Page 8, line 256-258: Why did the presence of amino group in the membrane also lead to homogeneous dispersion of MOF particles? In other parts of the paper, the authors suggested that it was the alcohol groups of PVA that facilitate the dispersion of the MOFs. Please double-check the logic here.
11. Page 8, line 266-267/Page 9, line 271-272: These two adjacent sentences were presenting the same idea. Please consider removing one of them.
12. Page 9, line 280: Could the authors briefly explain the significant selectivity drop at high MOF loading, i.e., 55% (Figure 8)? Did the membrane structure/gas transport mechanism change significantly at this MOF loading?
13. Page 9, line 281: Similar to a previous point (point 9), please consider adding some discussions for Figure 9 or moving it to supporting information.

Author Response

Replies to Reviewer #1:

We are so grateful for your insightful comment, deep inspection to improve our manuscript, and your valuable feedback for publication. All comments have been carefully considered and addressed in the revised highlighted version in a point-to-point manner and the detailed responses are also given in the following.

Comments and Suggestions for Authors

The paper “Interfacial Design of Mixed Matrix Membranes via Grafting PVA on UiO-66-NH₂ to Enhance the Gas Separation Performance” reports the CO₂/N₂ separation performance of a series of mixed matrix membranes. By coating/surface treating UiO-66-NH₂ with PVA, better dispersion of UiO-66-NH₂ in the base PVAm matrix was achieved and interfacial/interstitial voids were avoided. Both factors lead to better CO₂/N₂ separation performance. Overall, the design principle presented in this paper is interesting and important, and the membrane characterizations are comprehensive. However, there are still several parts in this paper that need to be revised. Hence, I recommend the paper to be published after major revision. Below are my comments that I hope the authors could address properly.

1. Page 1, line 13: The authors suggested that the MOFs were covalently bonded with PVA in the abstract. In the main text, the authors suggested that interaction is intermolecular hydrogen bond (Page 4, line 194). Please double-check on this point.

Answer:  We appreciate the reviewers’ comment on the bonding of PVA with the MOFs and MMM. However, the peak shift observed on the ATR-FTIR after hydrogen bonding of PVA with PVAm, indicates the covalent bonding between the UiO-66-NH₂ and the PVA-PVAm via hydrogen bonds. The issues have been addressed in line 204-208.

1. The introduction section needs to be polished. For example, the authors did not specify why they chose UiO-66-NH₂ as the MOF for MMMs fabrication (performance/surface tunability/environmental or economic considerations?). The following recent papers shall be incorporated to support the selection of UiO-66-NH₂ (Microporous and Mesoporous Materials, 2021, 313, 110823. Journal of Environmental Chemical Engineering, 2021, 9, 105159. Applied Surface Science, 2021, 553, 149547. Journal of Membrane Science, 2017, 539, 172-177.)

Answer: Many thanks for this comment which helped us to improve the manuscript. The introduction was modified accordingly. The reasons for choosing materials were explained point by point. However, in the previous manuscript this issue was addressed insightfully.

1. Page 3, line 133: The authors stated that FT-IR measurements were done in the rage of 3500-500 cm^-1. However, UiO-66-NH₂ and MMM spectra show non-zero absorbance between 4000-3500 cm^-1 (Figure 1). Please double-check on this point.

Answer: We are thankful for this comment. The issued was addressed in the line 141.

1. Page 4, line 154-155: Was the hydrothermal process conducted in an autoclave? Also, was the solution stirred or kept stagnant?

Answer: Thanks for the insightful question. The hydrothermal process was conducted in an autoclave while keeping the solution stagnant. This issue was addressed in line 155-156.

1. Page 4, line 158-159: Was the membrane-casting solution degassed?

Answer: Yes indeed, to prevent the micro defects on the surface and remove the possible air bubbles, all the membrane solutions were degassed using a 20 min ultra-sonication-degassing mode for the degassing process. This issue was addressed in line169-170.

1. Page 5, line 201-202: Could the authors briefly explain why the bulk membrane has a porous morphology (phase separation/trapped air bubbles in the absence of degassing?).

Answer: Since the membrane solution was degassed before casting, therefore the formed pores are due to the phase separation process. Potentially, the drying process in the oven can also affects the structure due to the removing the residual solvents from the membrane bulk and the porous structure remains instead. However, we believe such contribution to the porous structure is relatively low,

1. Page 5, line 218: The surface coating of UiO-66-NH₂ by PVA is an important step to promote the membrane performance. Could the authors please provide any insights on the PVA loading of UiO-66-NH₂/PVA composite (perhaps by air TGA)?

Answer: We agree with this good comment. The PVA loading to PVAm was calculated by the stoichiometric ratio which is also addressed in the text. The ratio is PVA/PVAm 1/4  wt. %. This comment was addressed in line 161.

1. Page 6, line 225: the “UiO-66-NH₂ particles (b)” should be “UiO-66-NH₂ particles (c)”?

Answer: Thanks for this comment, the UiO-66-NH₂ particles (b) changed to UiO-66-NH₂ particles (c) and addressed in line 244.

1. Page 7, line 244-245: Figure 6 was introduced here, yet no further discussion on this figure was provided. Could the authors move this figure to support information if it is less relevant to the key points of the main text?

Answer: Many thanks for this valuable comment which helps us to improve the manuscript. More information is added to describe Figure 6. This issue was addressed in lines 266-272.

1. Page 8, line 256-258: Why did the presence of the amino group in the membrane also lead to the homogeneous dispersion of MOF particles? In other parts of the paper, the authors suggested that it was the alcohol groups of PVA that facilitate the dispersion of the MOFs. Please double-check the logic here.

Answer: We agree with this comment of the reviewer. The PVA connects the MOF (UiO-66-NH₂) to the PVAm through the amines’ group of MOF. Also, the PVA itself was connected to the PVAm via its –OH groups (head-to-tail: PVAm-PVA-MOF).

1. Page 8, line 266-267/Page 9, line 271-272: These two adjacent sentences were presenting the same idea. Please consider removing one of them.

Answer: We appreciate your insightful comment. One of the adjacent was removed.

1. Page 9, line 280: Could the authors briefly explain the significant selectivity drop at high MOF loading, i.e., 55% (Figure 8)? Did the membrane structure/gas transport mechanism change significantly at this MOF loading?

Answer: After 24 wet. % of MOF loading which was considered the percolation level due to the high amount of the MOFs particles, more possibilities for N₂ transport were provided via the MOFs network and channels. Plus the possibility of formation of non-selective voids will also increase. These lead to the higher permeation of N₂ and resulted in losing the selectivity, which is a typical manner in MMM.

1. Page 9, line 281: Similar to a previous point (point 9), please consider adding some discussions for Figure 9 or moving it to support information.

Answer: Thanks for this helpful comment. We have addressed this issue in line 308-319.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript offers original data and the results are interesting. But the writing in this manuscript is not so good, specific suggestion are listed as the following:

1. Subscript error: example CO2, NH2
2. Superscript error: example cm2, cm3
3. Please check the Supplementary Materials, Author Contributions, Funding, Institutional Review Board Statement, Data Availability Statement and Conflicts of Interest.
4. Figure 5. is not a FTIR spectra.
5. Figure 6. : digits are illegible.
6. The size of gas permeation cell should be presented. If it is small, authors should describe the limitation of this study.
7. In the Figure 7 experimental points shouldn’t be connected with polyline segments. The more accurate would be presentation of points alone, if authors would like to indicate approximate trend than the spline line would be more adequate solution.
8. If possible, comparisons between the performance of this study and other researches should be presented.
9. For membranes application, it is very important to demonstrate BET surface area of the membrane. Therefore, the BET measurement of membrane need to be supplemented.

Author Response

Replies to Reviewer #2:

We truly appreciate the suggestions from the reviewer, for insightful comments and deep-inspection of reviewing our MS, which are very useful to us. Your comments have fully been addressed in the revised highlighted version in a point-to-point manner and the detailed responses are also given in the following.

Comments and Suggestions for Authors

This manuscript offers original data and the results are interesting. But the writing in this manuscript is not so good, specific suggestion are listed as the following:

1. Subscript error: example CO2, NH2
   Superscript error: example cm2, cm3

Answer: Thanks for this comment, however, all the subscripts were adjusted correctly during the submission. But it looks that such a problem with subscriptions occurred after transferring the text into the journal format by the journal. Anyway, all the subscriptions were fixed once again.

1. Please check the Supplementary Materials, Author Contributions, Funding, Institutional Review Board Statement, Data Availability Statement, and Conflicts of Interest.

Answer: We appreciate for this reviewers’ comment which helps us to improve the manuscript. All the mentioned necessary sections were addressed.

1. Figure 5. is not an FTIR spectra.

Answer: Thanks for your insightful comment. The Figure legend was changed. This problem was addressed in line 274.

1. Figure 6. : digits are illegible.

Answer: The digits were generated by the measurement device. They were modified into Figure 6.

1. The size of the gas permeation cell should be presented. If it is small, the authors should describe the limitation of this study.

Answer: The size of the permeation cell was calculated with the size of 42mm diameter. The size of the permeation cell was added to the text, line 179.

1. In Figure, 7 experimental points shouldn’t be connected with polyline segments. The more accurate would be the presentation of points alone if authors would like to indicate an approximate trend then the spline line would be a more adequate solution.

Answer: We agree with this comment, we have modified the figure accordingly.

1. If possible, comparisons between the performance of this study and other researches should be presented.

Answer: This is a good suggestion. We modified Figure 9. The generally accepted as a benchmark of membrane performance. The Robeson plot with the appropriate upper bound and selected literature data was used also to illustrate this work illustrated the comparison of this work with the other work.

1. For membrane application, it is very important to demonstrate BET surface area of the membrane. Therefore, the BET measurement of membrane needs to be supplemented.

Answer: We fully agree with the measurement of the BET. But unfortunately at the moment is not possible of r us to measure the membrane flat sheet BET.  We will keep this as a useful comment for our future work.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Most of the reviewer's comments have been addressed properly, and the quality of the revised manuscript improved a lot. As such, I'm pleased to recommend this paper for publication in its current form.

Reviewer 2 Report

Accept in present form