Interaction of SBA-15 with Dilute Acetic Acid and Ammonia

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study presented in this article is an important scientific contribution, because this analyzes the surface effects that mesoporous silica of the SBA-15 type present when subjected to weak acidic and alkaline environments; as for the effects on its textural properties (mainly on surface area). And, hence its importance, when these porous materials are subjected to potentiometric titrations, when both the Z potential and zero charge point are determined, where conventionally, the surface area of the material is considered constant; when this study shows that, in a weak alkaline environment, SBA-15 shows a significant decrease (almost half) in its surface area. This is very relevant in important applications, such as surface phenomena, for example, adsorption, catalysis, and assembly of hybrid systems, when mesoporous silica is present.

However, I have a question: what was the method used to determine the concentration of Si, [Si], in the weakly acid and alkaline solutions in wich SBA-15 was treated?.

And finally, a comment: it would be interesting to see the possibility of analyzing the nature of Si in both the acid solution end the alkaline solution. Based on this, an analysis could be initiated on the mode of interaction between the acid or the alkaline with SiO2 (SBA-15).  

Comments for author File: Comments.pdf

Author Response

The study presented in this article is an important scientific contribution, because this analyzes the surface effects that mesoporous silica of the SBA-15 type present when subjected to weak acidic and alkaline environments; as for the effects on its textural properties (mainly on surface area). And, hence its importance, when these porous materials are subjected to potentiometric titrations, when both the Z potential and zero charge point are determined, where conventionally, the surface area of the material is considered constant; when this study shows that, in a weak alkaline environment, SBA-15 shows a significant decrease (almost half) in its surface area. This is very relevant in important applications, such as surface phenomena, for example, adsorption, catalysis, and assembly of hybrid systems, when mesoporous silica is present.

However, I have a question: what was the method used to determine the concentration of Si, [Si], in the weakly acid and alkaline solutions in wich SBA-15 was treated?.

the following was added:

Molybdenum blue method was used to study Si concentration in solution. Ammonium molybdate produces a yellow silicomolybdate complex when added to an acidified solution containing silicic acid monomers. The yellow complex was reduced to molybdenum blue, and its concentration was determined spectrophotometrically. The chemistry of  silicomolybdate complexes is complicated, and it was discussed in detail in [Iler].

 

And finally, a comment: it would be interesting to see the possibility of analyzing the nature of Si in both the acid solution end the alkaline solution. Based on this, an analysis could be initiated on the mode of interaction between the acid or the alkaline with SiO2 (SBA-15).

the following was added:

Silica occurs mainly as orthosilicic acid in its dilute aqueous solutions over a broad pH range, and anionic monomeric species prevail at pH>9. Monomeric species at various degree of hydration were discussed in the literature. The aqueous chemistry of silica at concentrations >10^-3 M is more complicated and it involves species with various degrees of polymerization, hydration, and deprotonation. The aqueous chemistry of silica was discussed in detail in [Iler].

Reviewer 2 Report

Comments and Suggestions for Authors

This paper studies the changes in the porous structure, specific surface area, surface charge and other characteristics of SBA-15 after aging in water, dilute acetic acid and ammonia, and corrects and discusses the effects of these changes in potentiometric titration analysis. However, there are still many details in the text, including insufficient in-depth data interpretation, lack of sufficient explanation of some charts and formulas, language expression needs to be polished, and some citations need to be improved. Overall, I suggest that the manuscript can be accepted, but it needs to be further evaluated after careful revision.

• The description of the preparation conditions of the four batches of SBA-15 is somewhat scattered. It is recommended to tabulate and clearly list the preparation differences and main parameters of each batch.
• The data in the results and discussion section are important, large, and worth discussing. However, the legends for "white symbols" and "black symbols" in Figures 1-5 do not clearly correspond to the difference between the reference [5] and this study. In particular, the author has pointed out in the text that Figures 1-5 show the changes in various indicators of SBA-15, samples 1 and 2 after aging in water (pH 5.5 and 6), dilute acetic acid (pH 3, 3.3 and 3.4), and dilute ammonia (pH 7.8, 8 and 9). However, the data groups and quantities in the figures are inconsistent with the description in the text, which is very confusing. It is recommended that the author redraw the figure. First, it should correspond to the text description one by one; second, pay attention to the way the picture data is displayed. It is recommended to connect the points into lines to show the trend of change over time, and use different colors and symbols to distinguish each group of data.
• The assumption that "SSA decreases linearly during titration" (Equation 2) is mentioned in the Discussion, but its rationality is not verified (e.g., whether there is actually a nonlinear change). It is recommended to add experimental evidence (e.g., SSA measurement at intermediate time points) or cite similar studies to support this assumption. For example, a brief mention of the method and significance of "correcting the surface charge curve" in the Materials and Methods section would make the abstract more complete.
• The constant peak position in Figure 10 (mislabeled as Figure 9) indicates that the main pore diameter has not changed, but it does not discuss whether the change in peak intensity or half-peak width reflects the microscopic information of pore wall collapse. It is recommended to provide the complete SAXS data.s
• Figure 10 shows that the SAXS peak position has not changed, but it does not explain the relationship between "unchanged pore diameter" and "pore wall collapse leading to SSA decrease". It needs to be made clear in the discussion that SAXS only reflects the main pore diameter, and the collapsed area does not produce new peaks due to disorder.

 

Recommended:Major Revision

Comments for author File: Comments.pdf

Author Response

This paper studies the changes in the porous structure, specific surface area, surface charge and other characteristics of SBA-15 after aging in water, dilute acetic acid and ammonia, and corrects and discusses the effects of these changes in potentiometric titration analysis. However, there are still many details in the text, including insufficient in-depth data interpretation, lack of sufficient explanation of some charts and formulas, language expression needs to be polished

we introduced several linguistic corrections (marked in red)

, and some citations need to be improved. Overall, I suggest that the manuscript can be accepted, but it needs to be further evaluated after careful revision.

• The description of the preparation conditions of the four batches of SBA-15 is somewhat scattered. It is recommended to tabulate and clearly list the preparation differences and main parameters of each batch.

The new Table 1 contains details of preparation.

Table 1. The  preparation and SSA of SBA-15

Specimen	mass of  P123	aging after heating at 35oC	aging after heating at 80oC	SSA / m2/g
	g	days	days	before titration	after titration
1	40	0	0	968	-
2	40	0	0	1052	-
3	2	14	14	957	620
4	2	14	14	1151	485

and the text was modified, accordingly.

In all four lots of SBA-15 a solution composed of P123, HCl and water was slowly titrated with TEOS at 35^oC, and the dispersion was kept at 35^oC for 1 day. Then the dispersion was kept at 80^oC for another 1 day. Then the particles were washed with water, and dried at 60^oC in air for 1 day. Finally they were calcined in air at 500^oC. In lots 1 and 2 heating at 35^oC was followed by  heating at 80^oC and then by washing with water w/o delay. In lots 3 and 4 heating at 35^oC was followed by aging at room temperature for 2 weeks, and then heating at 80^oC was followed by aging at room temperature for next 2 weeks before washing with water as described in more detail in [9]. Four separate lots were obtained, but only 2 different recipes were used. The details of these recipes are summarized in Table 1.

 

• The data in the results and discussion section are important, large, and worth discussing. However, the legends for "white symbols" and "black symbols" in Figures 1-5 do not clearly correspond to the difference between the reference [5] and this study. In particular, the author has pointed out in the text that Figures 1-5 show the changes in various indicators of SBA-15, samples 1 and 2 after aging in water (pH 5.5 and 6), dilute acetic acid (pH 3, 3.3 and 3.4), and dilute ammonia (pH 7.8, 8 and 9). However, the data groups and quantities in the figures are inconsistent with the description in the text, which is very confusing. It is recommended that the author redraw the figure. First, it should correspond to the text description one by one; second, pay attention to the way the picture data is displayed. It is recommended to connect the points into lines to show the trend of change over time, and use different colors and symbols to distinguish each group of data.

 

New figures use many colors and they have descriptions according to the suggestions of the Referee, including the lines connecting data points. 

 

 

 

• The assumption that "SSA decreases linearly during titration" (Equation 2) is mentioned in the Discussion, but its rationality is not verified (e.g., whether there is actually a nonlinear change). It is recommended to add experimental evidence (e.g., SSA measurement at intermediate time points) or cite similar studies to support this assumption. For example, a brief mention of the method and significance of "correcting the surface charge curve" in the Materials and Methods section would make the abstract more complete.

The following was added to the abstract.

Especially a standard procedure, in which the SSA measured before the titration  is used to calculate the σ₀ is compared with a modified procedure, in which the SSA measured before and after the titration are used to estimate the SSA at certain pH.

we also added the following after eq. 2.

We do realize that eq.(2) is only a rough estimation, but direct  measurement of SSA for all data points in course of a single titration is impossible. Perhaps verification of Eq. (2) can be an object of a future study.

 

We explain that in principle a series of experiments can be carried out in which the titration is terminated at different pH and after different times, and then the final SSA is measured, but this would require the amount of powder higher by an order of magnitude than the amount required for a single titration. Such a series of experiments has also several disadvantages with respect to single titration, but we do not like to discuss this problem here.

 

 

• The constant peak position in Figure 10 (mislabeled as Figure 9) indicates that the main pore diameter has not changed, but it does not discuss whether the change in peak intensity or half-peak width reflects the microscopic information of pore wall collapse. It is recommended to provide the complete SAXS data.s

Thank you for this remark., we corrected the number of figure.

Complete SAXS data are added as an appendix.

the following sentence was added in the main text.

Complete SAXS data can be found in the appendix.

 

• Figure 10 shows that the SAXS peak position has not changed, but it does not explain the relationship between "unchanged pore diameter" and "pore wall collapse leading to SSA decrease". It needs to be made clear in the discussion that SAXS only reflects the main pore diameter, and the collapsed area does not produce new peaks due to disorder.

 the following was added.

In other words, locally the structure of aged SBA-15 is unchanged, and this leads to unchanged SAXS pattern, and in the cavities the distance between the walls is too high to produce reflexes in the accessible range of 2θ (the intensities below 0.5^o are very high and very irreproducible).

Reviewer 3 Report

Comments and Suggestions for Authors

In this manuscript authors have studied the effect of SBA-15 structure in the presence of dilute acetic acid and ammonia. But the mansucript is not written properly. Thorough revision is necessary.

1) Figure 9, SAXS figure should be Figure 10. But this is not the way of presenting the small angle X-ray data of SBA-15. Authors should provide the powder data in the form of 2theta vs. intensity for all the samples, follow this paper: J. Solid State Chem. 2010, 183, 1326-1333.

2) Authors should provide the BET surface areas of the SBA-15 and acid/ammonia treated samples for understanding any change in mesoporosity.

3) HRTEM images of the samples to be provided for understanding the 2D-hexagonal mesophases.

Comments on the Quality of English Language

Language editing is necessary

Author Response

In this manuscript authors have studied the effect of SBA-15 structure in the presence of dilute acetic acid and ammonia. But the mansucript is not written properly. Thorough revision is necessary.

1) Figure 9, SAXS figure should be Figure 10. But this is not the way of presenting the small angle X-ray data of SBA-15. Authors should provide the powder data in the form of 2theta vs. intensity for all the samples, follow this paper: J. Solid State Chem. 2010, 183, 1326-1333.

Complete SAXS data are added as an appendix.

the following sentence was added in the main text.

Complete SAXS data can be found in the appendix.

2) Authors should provide the BET surface areas of the SBA-15 and acid/ammonia treated samples for understanding any change in mesoporosity.

those are already presented in Fig. 2.

3) HRTEM images of the samples to be provided for understanding the 2D-hexagonal mesophases.

Unfortunately we used all the material and we cannot make more experiments with specimens 1-4. However we provide an additional SEM image in a higher magnification instead (Fig.A1)

The following was added.

An additional SEM image in a higher magnification is presented in the appendix (Fig.A1).

Reviewer 4 Report

Comments and Suggestions for Authors

The authors report on the effect of ageing of SBA at different pH. They find that only in basic solution there is a decrease of specific surface area (SSA) and a reduction of the volume of micropores (but not of the overall pore volume). If the change in SSA is taken into account there is no difference in the pH dependence of the surface charge for different specimens. These are interesting results worth to be published.

The results are very clearly presented and the conclusions are well supported. Unfortunately, SEM images before and after ageing are shown only for the basic case; it would be even more convincing to see pictures of the acidic case for comparison.

One correction should be implented: SAXS does not provide the pore diameter but the distance between the pore centers.

Finally, Figure 10 is mislabeled: The caption contains "Figure 9".

 

Author Response

The authors report on the effect of ageing of SBA at different pH. They find that only in basic solution there is a decrease of specific surface area (SSA) and a reduction of the volume of micropores (but not of the overall pore volume). If the change in SSA is taken into account there is no difference in the pH dependence of the surface charge for different specimens. These are interesting results worth to be published.

The results are very clearly presented and the conclusions are well supported. Unfortunately, SEM images before and after ageing are shown only for the basic case; it would be even more convincing to see pictures of the acidic case for comparison.

Actually there is no much difference between the original material and that aged in dilute acetic acid, so the image looks more or less .

the following was added.

The image in Fig. 8 is representative for the original specimen and for the samples aged  in acetic acid solutions, and  Fig. 9 is representative for the samples aged  in ammonia solutions (only a few selected samples were studied by SEM).

 

One correction should be implented: SAXS does not provide the pore diameter but the distance between the pore centers.

Thank you for this remark. Actually the distance is between the walls (interplanar spacings) rather than between the centers (empty space), but we replaced

 

This result indicates that the diameters of parallel channels in the hexagonal structure of SBA-15 are not affected by aging in the solutions studied in this paper (unless a wall between neighboring channels is destroyed).

by

This result indicates that the interplanar spacings are not affected by aging in the solutions studied in this paper (unless a wall between neighboring channels is destroyed).

Finally, Figure 10 is mislabeled: The caption contains "Figure 9".

Thank you for this remark., we corrected the number of figure.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have answered a series of questions well. The research content of this work is relatively systematic. It is recommended to accept it.

Author Response

no corrections were suggested in this review

Reviewer 3 Report

Comments and Suggestions for Authors

Although authors have revised the manuscript, revisions are not done properly. Thus, I don't recommend this manuscript.

Author Response

no specific corrections were suggested in this review.

the Referee does not like the Ms. and that's it.