Controlling Magnesium Silicates Coprecipitation Conditions: A Tool to Tune Their Surface Acid–Base Reactivity

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a nice manuscript and I recommend to accepting it after the authors have made revisions based on my questions/comments below: 

1) In the experiment session, the authors mentioned SEM used in their study. However, there is no SEM data shown in the manuscript. Is it a useful techniques for their study?

 

2) The key findings in their research is: They proposed the formation of two different types of MSH phases. The authors only compared their XRD data with known MSH phases. Is it possible that some other phases can exist in the system like SiOx or MgO oxides?

 

 

 

Author Response

Comment 1: “In the experiment session, the authors mentioned SEM used in their study. However, there is no SEM data shown in the manuscript. Is it a useful technique for their study?”

Response 1: Data concerning sample characterization by SEM microscopy are discussed in section 2.3.2 of the manuscript (from line 255). The SEM images were included in the Supplementary information document. However, following the suggestion of another reviewer we choose four of them to incorporate into the revised manuscript (Figures 3 and 4). This technique is indeed important as it confirms the data obtained with nitrogen physisorption and highlights the presence of 2 families of materials with and without measurable S_BET.

 

Comment 2: “The key findings in their research is: They proposed the formation of two different types of MSH phases. The authors only compared their XRD data with known MSH phases. Is it possible that some other phases can exist in the system like SiOx or MgO oxides?”

Response 2: The presence or absence of a SiO₂-type crystalline phase had already been discussed in the submitted manuscript in the paragraph dealing with XRD results. Furthermore, in the revised manuscript, we have added a comment on the absence of diffraction peaks that might indicate the presence of a MgO-type crystalline phase (from line 357): “It is also noteworthy that the XRD patterns do not reveal the presence of an MgO-type crystalline phase. Indeed, based on JCPDS card 89-7746, the diffraction planes of magnesium oxide (111), (200), (220) and (311) correspond to 2θ = 37.4°, 43.6°, 63.2° and 78.4° respectively.”

Reviewer 2 Report

Comments and Suggestions for Authors

The Manuscript is well presented. However, A few issues need to be resolved before the manuscript can be published. 

1) Many important figures are shifted to the supplementary information section. The authors should bring some of the figures to the main manuscript. 

2) The authors could provide clarity on the MBOH reaction by using a diagram or reaction equation to present the conversion into various products

Author Response

Comment 1: “Many important figures are shifted to the supplementary information section. The authors should bring some of the figures to the main manuscript.”

Response 1: In the revised manuscript, we have already transferred some SEM images that were originally included in the Supplementary information document. As the revised manuscript already includes 9 figures and 3 tables, we feel that no further figures should be added, and that those originally included in the Supplementary information document can remain there. However, we are ready to incorporate other figures if you consider it necessary.

 

Comment 2: “The authors could provide clarity on the MBOH reaction by using a diagram or reaction equation to present the conversion into various products.”

Response 2: In the revised manuscript, in the section dealing with the catalytic results of MBOH conversion tests (section 2.5), we have added a scheme representing the reactivity of MBOH on acidic and basic sites (line 575).

Reviewer 3 Report

Comments and Suggestions for Authors

Catalyst preparation is the most critical step in catalyst production due to its practical complexity and the large number of details that must be known and understood by the catalyst manufacturer. These details undoubtedly influence the final properties of the catalyst, especially the selectivity and catalytic efficiency. Although the exact details of the preparation procedure vary, there are many methods that produce the same catalysts but with characteristic properties associated with the specific preparation procedure.

 The present study investigates the possibility of regulating the surface acid-base activity of Mg silicates by adjusting the co-precipitation conditions using a special micromixer. The authors investigated the influence of various synthesis parameters such as the relative proportion of magnesium and silicon precursors in solution and pH on the chemical and structural properties of the resulting material. In addition, based on the content of magnesium and silicon in solutions after precipitation the authors determined the solubility constants of the materials using a thermodynamic approach. All obtained catalysts were characterized by a set of methods, including nitrogen physisorption, SEM, thermogravimetric analysis, X-ray diffraction, Raman and 29Si NMR spectroscopies.

 Catalytic properties were examined using MBOH conversion as a model reaction.

 The paper is well written and structured and will be of interest to a wide range of readers since provided some valuable information and the content is very significant in this field.

 I recommended a minor revision of the article before it can be published in Catalysts.

 Some specific comments are as follows:

 1) line 817: only the results in terms of selectivity and yield in acidic route at a 2 min time on stream will be discussed in the following part.

 Authors should carefully justify the choice of a time range. Why 2 minutes and not 3 or 5?

 2) lines 841-844: In the case of the 3 samples with a ratio (Mg/Si)theo = 1.0, the same tendencies in terms of conversion, selectivity and yield in the acidic route emerge. It should be noted that the samples with a (Mg/Si)theo ratio of 1.0 present a much higher selectivity in Mbyne, and therefore a much higher surface acidity, than the silicates with a lower (Mg/Si)theo ratio.

 As I understand, we are talking about samples under the gray line in table 3? Authors need to include the necessary note in the caption to Table 3. The same applies to Table 1.

 3) Did the authors compare the results of catalytic tests for samples obtained using a mixer with those for traditional catalysts?

 

Author Response

Comment 1: “line 817: only the results in terms of selectivity and yield in acidic route at a 2 min time on stream will be discussed in the following part. Authors should carefully justify the choice of a time range. Why 2 minutes and not 3 or 5?”

Response 1: We have added a few lines to the revised manuscript from line 614, which we hope will clearly clarify the choice of the 2 min time on stream: “It should be noted that for the comparison of the catalytic performances of the Mg silicates, the shorter measurable time on stream (2 minutes) was chosen in order to compare, as much as possible, the materials as prepared i.e., free of potential heavy products linked to deactivation.”

 

Comment 2: “lines 841-844: In the case of the 3 samples with a ratio (Mg/Si)theo = 1.0, the same tendencies in terms of conversion, selectivity and yield in the acidic route emerge. It should be noted that the samples with a (Mg/Si)theo ratio of 1.0 present a much higher selectivity in Mbyne, and therefore a much higher surface acidity, than the silicates with a lower (Mg/Si)theo ratio. As I understand, we are talking about samples under the gray line in table 3? Authors need to include the necessary note in the caption to Table 3. The same applies to Table 1.”

Response 2: To make Tables 1 and 3 more user-friendly, a reminder of the sample nomenclature has been included in the legend to each table: “As described in "Materials and methods" section, samples are labelled rMgSiHAKB, with r: (Mg/Si)_theo molar ratio; A: [HNO₃]_in/[Si]_in ratio; B: [KOH]_in/[Si]_in ratio.”

 

Comment 3: “Did the authors compare the results of catalytic tests for samples obtained using a mixer with those for traditional catalysts?”

Response 3: We didn't compare the catalytic activity of the materials to other magnesium silicates. In fact, the aim of the study was not to obtain "the best" catalyst for MBOH conversion, that is a model reaction, but to investigate the link between the preparation conditions and the surface acid-base reactivity probed by MBOH catalytic conversion.