Wasteless Synthesis and Properties of Highly Dispersed MgAl₂O₄ Based on Product of Thermal Activation of Gibbsite

Round 1

Reviewer 1 Report

Few minor edits/additions are suggested.

Introduction first paragraph – please reword this sentence. Reference cannot be a subject

Throughout the text the authors claim the low surface acidity, was this referring to CO adsorption performance or the acidity based on point of zero charge (PZC)? This needs to be clarified

Figure 1 – provide a legend for G and S

Figure 3 – Please be consistent with the scale bars

Table 1 is redundant and the information on this table can be easily incorporated into the text

Can the authors include Debye-Scherrer calculations using XRD data for crystallite size and compare the with the sizes displayed in SEM micrographs?

Please include chemical equations for the important steps of formation of double hydroxides

Author Response

Dear Reviewer! Thank you for your questions and comments, indeed, which are aimed at improving the article! Our responses to questions and comments are below. Text changes are highlighted in yellow.

1. Reviewer: Introduction first paragraph – please reword this sentence. Reference cannot be a subject.

Authors' comments: We reformulated the sentence: “For example, it was found that an increase in the Mg fraction in the MgAl-O oxide system synthesized by precipitation (according to the sol-gel technology) decreases both the strength and concentration of acid sites on the surface, thus changing the properties of supported platinum [4].”

2. Reviewer: Throughout the text the authors claim the low surface acidity, was this referring to CO adsorption performance or the acidity based on point of zero charge (PZC)? This needs to be clarified.

Authors' comments: By surface acidity, we mean the characteristics of Lewis acid sites (LAS) by IR spectra of adsorbed carbon monoxide on magnesium and aluminum cations. These characteristics are both the concentration and the strength of the LAS. The LAS force is determined on the scale of heats of CO adsorption by the equation: Qco [kJ/mol] = 0.5∙(νco-2143) + 10.5. This is the amount of thermal energy released during the formation of donor-acceptor complexes between cations and the base molecule - carbon monoxide. Note that according to the Lewis electron theory, LAS are atomic groups capable of accepting an unshared electron pair of an adsorbed base molecule (in our case, CO). The lower the absorption band of adsorbed CO on magnesium and/or aluminum cations (in our systems), the weaker in strength (type) LAS.

3. Reviewer: Figure 1 – provide a legend for G and S.

Authors' comments: The legends for G and S are provided in the figure caption: S – Mg(NO₃)₂·6H₂O, G – gibbsite.

4. Reviewer: Figure 3 – Please be consistent with the scale bars.

Authors' comments: The scale bars were corrected in Figure 3.

5. Reviewer: Table 1 is redundant and the information on this table can be easily incorporated into the text.

Authors' comments: In general, we agree with the remark. The same parameters of the samples summarized in table 1 are given on page 4-5 in the text, therefore, table 1 is excluded from the context.

6. Reviewer: Can the authors include Debye-Scherrer calculations using XRD data for crystallite size and compare the with the sizes displayed in SEM micrographs?

Authors' comments: The text on page 4 has been corrected. The X-ray size for MgAl spinels is 7 nm instead of the erroneously indicated value of 4 nm. For MgO in the sample MgAl(1)-550 X-ray size is 90Ǻ (9 нм, without correction).

Comparison of the crystallite sizes determined from the data on the broadening of X-ray diffraction lines (~7 nm) with the particle sizes calculated from the data on the specific surface area of the samples (~9–10 nm) allows us to conclude that, within the error of these methods, this is monocrystals. Since the particle size determined from the SEM data - for the MgAl(1)-550 sample 2-40 μm and for MgAl(2)-550 10-15 μm – is much larger than the crystallite size determined from the X-ray analysis data, it is possible to make the conclusion is that the micron particles observed by the SEM method contain open pores, and the high specific surface area of the prepared samples is due to their inner surface. Thus, mainly the surface of the single crystallites of which they are composed.

7. Reviewer: Please include chemical equations for the important steps of formation of double hydroxides.

Authors' comments: We believe that in order to write the basic chemical equations for the formation of hydroxides in those reaction media in which the synthesis of aluminum-magnesium systems is carried out, nevertheless, it is necessary to separately set the task of studying the mechanism of hydroxide formation, which was not the purpose of this work. In turn, this will require additional research in terms of involving physicochemical methods of analysis, as well as varying synthesis conditions. The formal equation for the formation of double hydroxide from the reagents used can be represented by the following scheme:

        Al₂O₃·nH₂O + Mg(NO₃)₂ + H₂O à [Mg_1-xAl_x(OH)₂]^x+(NO₃)_x·mH₂O 

Changes have been made to the text on page 6.

 Sincerely, Authors!  

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript review report "Wasteless Synthesis and Properties of Higly Dispersed MgAl₂O₄ Based on Product of Thermal Activation of Gibbsite " by A.V. Zhuzhgov et al.

This work is devoted to the description of the wasteless synthesis of magnesium aluminate (MgAl₂O₄) with spinel structure, which is currently one of the promising materials for catalysis. It is also very interesting that the material is obtained from natural raw materials.
Therefore, the manuscript is quite promising and it can be recommended for publication after clarification of several issues that will undoubtedly only improve this manuscript.

1. Page 1 - Please correct the affiliation as it has moved up. In addition, according to Authors Guide, Authors should be listed in the following order - First Name (second name initial) Last Name.

2. Page 1 - Regarding keywords defined by the Authors: I suggest to specify what type of synthesis was implemented in this study- "wasteless synthesis"

3. Page 2 – please correct CO2, to CO₂

4. Page 2 - no period needed before citations - . [22-24] i . [8,25,27,28] - the period should be after the parentheses.

5. Page 4 – Did the Authors want to write FTIR ? (Acidic properties of the surface were studied by low-temperature IR Fourier spectroscopy of adsorbed carbon monoxide.)

6. Page 4 equation with no boldface and no comma at the end. Each equation should be numbered (1)...(2) etc. When citing an equation, its number should be given in the text of the manuscript. When describing the symbols in the equation, put " : " after the word „where” (under the equation).

7. In the text of the manuscript, we write the designations of figures and tables: (Figure 1...2...etc) and (Table 1...2...etc) - if the reference to them is at the end of the sentence. At the beginning of a sentence, write Figure ... Table.

8. Page 5 - I would suggest to make XRD diagrams more readable, e.g.: standardize the font (bold - it will be more readable) and mark the phases in color.

9. Page 6 - I would suggest making the graphs in color.

10. Page 7 - in the SEM photo caption there is one dot too many (at the end). I would suggest making the A, B, C and D markings a little larger.

11. Page 7 - spacing between text and SEM images.

12. Page 8 - In Table 1, the number 17 in the last column needs to be centered.

13. Page 8 - graphs (Figure 4) should be standarized regarding scale, font (bold - it will be more readable) and I would suggest making the curves in color.

14. Page 9 - Adjust the header of the last column (Pore Volume, ml/g)

15. Page 10 - I would ask that the scale, font be standardized (make it bold - it will be more readable).

16. Page 10-11 - Table 3 - why are some of the results in bold ? Does it make a difference?

17. Page 11 - References - item [6] should be changed to black.

18. Pay attention to spaces.

Questions for Authors

1. What is the density of the obtained materials ?

2. How many specific surface area, porosity and other measurements were made ? Are the results presented in the tables an average calculated from several series of measurements ? If so, it would be useful to show how the results are distributed. Please discuss the results regarding their statistical sigificance.

3. What is the process yield ?

4. How much material do you get from each process ?

5. Why did the authors use two different scanning microscopes ?

Author Response

Dear Reviewer! Thank you for your questions and comments, indeed, which are aimed at improving the article! Our responses to questions and comments are below. Text changes are highlighted in yellow.

Comments and Suggestions for Authors:

1. Reviewer: Page 1 - Please correct the affiliation as it has moved up. In addition, according to Authors Guide, Authors should be listed in the following order - First Name (second name initial) Last Name.

Authors' comments:  This comment has been completely corrected.

2. Reviewer: Page 1 - Regarding keywords defined by the Authors: I suggest to specify what type of synthesis was implemented in this study- "wasteless synthesis"

Authors' comments: We agreed with this proposal and added it to the keywords – "wasteless synthesis".

3. Reviewer: Page 2 – please correct CO2, to CO₂

Authors' comments: Formula fixed.

4. Reviewer: Page 2 - no period needed before citations - . [22-24] i . [8,25,27,28] - the period should be after the parentheses.

Authors' comments: Appropriate corrections have been made to the text.

5. Reviewer: Page 4 – Did the Authors want to write FTIR ? (Acidic properties of the surface were studied by low-temperature IR Fourierspectroscopy of adsorbed carbon monoxide.)

Authors' comments: We have corrected in text – Fourier transform infrared spectroscopy (FTIR).

6. Reviewer: Page 4 equation with no boldface and no comma at the end. Each equation should be numbered (1)...(2) etc. When citing an equation, its number should be given in the text of the manuscript. When describing the symbols in the equation, put " : " after the word „where” (under the equation).

Authors' comments: The comments taken into account in the text.

7. Reviewer: In the text of the manuscript, we write the designations of figures and tables: (Figure 1...2...etc) and (Table 1...2...etc) - if the reference to them is at the end of the sentence. At the beginning of a sentence, write Figure ... Table.

Authors' comments: The comments have been taken into account and the text has been amended.

8. Reviewer: Page 5 - I would suggest to make XRD diagrams more readable, e.g.: standardize the font (bold - it will be more readable) and mark the phases in color.

Authors' comments: I also agree with the proposal. Changes were made in the manuscript.

9. Reviewer: Page 6 - I would suggest making the graphs in color.

Authors' comments: The graphs have been changed in color.

10. Reviewer: Page 7 - in the SEM photo caption there is one dot too many (at the end). I would suggest making the A, B, C and D markings a little larger.

Authors' comments: We agreed with the comments on this point, the dot was removed in the signature, the labeling of the images was made a little larger.

1. Reviewer: Page 7 - spacing between text and SEM images.

Authors' comments: The distance between text and SEM images has been increased.

1. Reviewer: Page 8 - In Table 1, the number 17 in the last column needs to be centered.

Authors' comments: We have removed table 1 from the text. The data in this table is shown above in the text on page 4-5.

1. Reviewer: Page 8 - graphs (Figure 4) should be standardized regarding scale, font (bold - it will be more readable) and I would suggest making the curves in color.

Authors' comments: The curves are in color.

1. Reviewer: Page 9 - Adjust the header of the last column (Pore Volume, ml/g).

Authors' comments: It is done.

15. Reviewer: Page 10 - I would ask that the scale, font be standardized (make it bold - it will be more readable).

Authors' comments: We agreed with this comment and it was corrected.

16. Reviewer: Page 10-11 - Table 3 - why are some of the results in bold? Does it make a difference?

Authors' comments: In table 3, the entire font was made in the same style.

1. Reviewer: Page 11 - References - item [6] should be changed to black.

Authors' comments: Link color changed to black.

1. Reviewer: Pay attention to spaces.

Authors' comments: It is done.

Questions for Authors:

1. Reviewer: What is the density of the obtained materials?

Authors' comments: The bulk density of the powders was 0.55-0.6 g/cm³.

1. Reviewer: How many specific surface area, porosity and other measurements were made? Are the results presented in the tables an average calculated from several series of measurements? If so, it would be useful to show how the results are distributed. Please discuss the results regarding their statistical sigificance.  

Authors' comments: The data of physicochemical methods of analysis presented in the tables are not average values from a series of measurements. However, experiments separated in time for the synthesis of the same samples subject to the preparation conditions show good reproducibility, both according to nitrogen porosimetry and phase composition.             

1. Reviewer: What is the process yield ?

Authors' comments: If the reviewer has in mind the yield of the target product – spinels of the “normal” composition MgAl2O4, then based on the data of X-ray phase analysis (Figure 1), the MgAl(1)-550 sample (obtained from the product of a mild interaction of reagents) consists of about 10 wt.% MgO and the rest is spinel (yield 90%), and MgAl(1)-550 (obtained from the product of hydrothermal interaction of reagents consists only of MgAl₂O₄ (i.e. 100%). Note that the overall chemical analysis for the samples showed a total magnesium content of 17 mass .%, which corresponds to the magnesium content in the formula MgAl₂O₄.

Thus, for MgAl(1)-550, part of magnesium does not react with aluminum; therefore, we observe magnesium oxide by XRD, and for MgAl(2)-550, all magnesium interacts with aluminum, as a result of which MgAl₂O₄ is observed, which is confirmed by a certain the value of the lattice parameter for MgAl(2)-550 is a = 8.093 Ǻ.

1. Reviewer: How much material do you get from each process?

Authors' comments: In accordance with the above procedure for preparing samples after hydrochemical treatment within the autoclave reactor, both at room temperature and after 150⁰C, the amount of product after drying at 110⁰C (xerogel) is 100 g. It should be noted that, according to the conditions of the procedure in this work, after hydration, the precipitates were not washed off, the mother liquor was not decanted from the gel, but was immediately subjected to drying to constant weight, followed by heat treatment at 550⁰C. Taking into account the weight loss on the thermal analysis curves of dried samples (Figure 2), out of 100 g of the intermediate after heat treatment, there will be 40 g of the final compound of the composition MgAl₂O₄ or MgO + MgAlO_x.        

1. Reviewer: Why did the authors use two different scanning microscopes ?

Authors' comments: One Regulus SU8230 microscope (Hitachi) was left in the experimental part. In this work, the images of the samples were obtained with Regulus SU8230.

Sincerely, Authors!