Controllable Acid/Base Propriety of Sulfate Modified Mixed Metal Oxide Derived from Hydrotalcite for Synthesis of Propylene Carbonate
Round 1
Reviewer 1 Report
This manuscript describes acidity/basicity and catalytic performance changes in hydrotalcite-derived metal oxides modified with various amount of sulfate (originally developed by the authors in ref. 28). The performance-property relationship has been well discussed. I support the publication of this manuscript, although several issues should be solved.
(1) Terminology. The authors used several phrases for the developed material: layered metal oxides, layered dioxides, layered mixed oxide. The term should be unified, and indeed I do not agree with all of these expressions. The mixed oxide prepared by calcination of layered double hydroxide such as hydrotalcite does not have layered structure in similar meaning to layered double hydroxide...the layer which is evident in the low angle peak in XRD has almost disappeared. "mixed metal oxide derived from hydrotalcite" is the most accurate expression.
In addition, I do not also think that "sulfur modification" is appropriate; "sulfate modification" is better.
(2) Calculation method for yield should be explicitly described in Experimental section.
(3) Mention the material balance.
(4) Describe the detailed procedure for reuse experiments. Whether the slight loss in reuse test can be explained by catalyst loss is an important point.
(5) Tables 4 and 8: The conversion level is too high to compare the activity. Because of the good linearity of the yield/time dependence, the activity should be compared by the reaction rate determined by the slope.
(6) Figure 3: Describe the electronic state more clearly: e.g. 2p1/2.
(7) Figure 5: Do not use "intensity" for IR. Use "transmittance" or "absorbance".
(8) Figure 6:
Set the origin of axes at zero for (a) and (d).
Put the result of blank run at amount of catalyst = 0%.
reaction time 5 h for (b), (c), (d); temperature 433 K for (a), (c), (d)
Use Kelvin for (b)
(9) Figure S1 is not readable.
Author Response
Dear Reviewer:
I am gratitude to receive your reviewing comments and thanks for your sparing time to read my manuscript. In the following part, I am gladly to respond and answer your comments and suggestions.
Comment 1:Terminology. The authors used several phrases for the developed material: layered metal oxides, layered dioxides, layered mixed oxide. The term should be unified, and indeed I do not agree with all of these expressions. The mixed oxide prepared by calcination of layered double hydroxide such as hydrotalcite does not have layered structure in similar meaning to layered double hydroxide...the layer which is evident in the low angle peak in XRD has almost disappeared. "mixed metal oxide derived from hydrotalcite" is the most accurate expression.
In addition, I do not also think that "sulfur modification" is appropriate; "sulfate modification" is better.
Response 1: I agree with the comments of the reviewers and have already corrected it in the corresponding sections of the manuscript.
Comment 2: Calculation method for yield should be explicitly described in Experimental section.
Response 2: Thanks for your impressive suggestion and the calculation method for the yield has been supplemented in the Catalyst Test section. See l. 152 – 154.
Comment 3: Mention the material balance
Response 3: Since the reaction process of material preparation is complicated, the reaction process, especially the modification process, inevitably has a certain loss of materials. Thus, the calculation of material balance is difficult to describe and their influence to our experiment is minimum. For efficiency, we think it is judiciously to ignore it.
Comment 4: Describe the detailed procedure for reuse experiments. Whether the slight loss in reuse test can be explained by catalyst loss is an important Comment.
Response 4: In the manuscript Catalyst Test section, the detailed process of reusing the catalyst was supplemented. Considering for having been washed for three times, there could be inevitably a little amount of catalyst loss in the process, which resulted in a slight decrease in product yield. See l. 141 – 142.
Comment 5: Tables 4 and 8: The conversion level is too high to compare the activity. Because of the good linearity of the yield/time dependence, the activity should be compared by the reaction rate determined by the slope.
Response 5: According to the reviewer's suggestion, the slop curve of yield depending on time under different catalysts was plotted by supplementary experiments. It can be aquired from the figure that the slope of the catalyst SMgAlO-5 is the largest, indicating that the catalytic rate is the fastest, and the catalytic activity also indicated the highest among all the catalysts.
Figure Supplementary. Time-dependent studies of yield for the benchmark reaction in the presence of the different catalysts. Reaction condition: n(PG)/n(urea) = 3; mass fraction of the catalyst = 1%; reaction time = 5 h; reaction temperature = 433 K.
Comment 6: Figure 3: Describe the electronic state more clearly: e.g. 2p1/2.
Response 6: Because the splitting of the Al 2p doublet is lower than the resolution of the Al Ka X-ray source. Then, Al 2p appears as a unique peak where are the unresolved Al 2p 3/2 and Al 2p 1/2 peaks. From literatures we can confirm that there are only the description of Al 2p and Mg 2p (refs: Applied Catalysis B: Environmental 243 (2019) 415–427 and Journal of Hazardous Materials 179 (2010) 818–827) which could be observed from the high resolution XPS characterization.
Comment 7: Figure 5: Do not use "intensity" for IR. Use "transmittance" or "absorbance".
Response 7: We have corrected the corresponding suggestion in the manuscript. See l. 284 or Figure 5.
Figure 5. Py-IR analysis of MgAlO and SMgAlO-y.
Comment 8: Figure 6:Set the origin of axes at zero for (a) and (d).Put the result of blank run at amount of catalyst = 0%.reaction time 5 h for (b), (c), (d); temperature 433 K for (a), (c), (d)
Use Kelvin for (b)
Response 8: We thanks for your helpful advice and we have corrected the corresponding contents in the manuscript. See l. 346 – 349 or Figure 6.
Figure 6. Process optimization research of the PC synthesis with effect of duration time(a), temperature(b), mole ratio of PG to urea (n(PG) : n(urea))(c), and amount of SMgAlO-5 catalyst in reaction. Reaction conditions: SMgAlO-5 amount [1% for (a), (b), (c)], reaction time [5 h for (a), (b), (c)], n(PG : n(urea) [3 : 1 for (a), (b), (d)] and temperature [433 K for (b), (c), (d)].
Comment 9: Figure S1 is not readable.
Response 9: We thanks for your helpful advice and we have corrected the corresponding contents in the supplementary information and the readable Figure should be the Figure S2.
Figure S2 N2 adsorption and desorption isotherms (77K) for MgAlO (blue), SMgAlO-1 (black), SMgAlO-3 (green), SMgAlO-5 (red), and SMgAlO-7 (magenta).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors prepared mixed Mg-Al oxide with surface modified by persulfate, and studied its catalytic activity for synthesis of propylene carbonate. The paper is straightforwardly written, however, interpretation of some key experimental data arouses some doubts which should be clarified before publication.
The major imperfections are:
Characterization of the material is rather poor and some results are somewhat contradictory. In XRD, only 2 diffractions found (one corresponds to MgO and other to spinel MgAl2O4). Morphology studied by SEM shown uniform shape of crystals. Why? Are MgO/MgAl2O4 phases too small crystallites formed in material, which keeps the morphology of starting hydrotalcite?
What is size distribution of the crystalline particles calculated from diffraction broadening?
Why so intense OH vibrations are present in the IR after long calcination at 823 K? If starting from hydrotalcite, some water stays? Or is re-adsorbed? What about carbonate?
Idealized formula of hydrotalcite is Mg6Al2(CO3)(OH)16∙4(H2O), why Mg:Al ratio 4:1 was used?
IR vibration at 852 cm-1 is attributed to MgO and Al2O3 – it is again misleading. Why content of sulphur is different from starting amount (nominal loading)? All material was just mixes, evaporated, and calcinated. Was some sulphur removed by calcination? Seems be improbable as sulphur content makes straight line with starting amount. See e.g. l. 284-285.
Sometimes, authors call their catalyst as Mg-Al dioxide, which implies mixture of MgO and Al2O3, but is inconsistent with XRD.
XPS spectra shows very high content of phases as Mg(OH)2, Al(OH)3, MgSO4 and Al2(SO4)3 on the surface, but intensity of sulphates does not correlate with amount of sulphur (SMgAlO-5 has obviously higher content than SMgAlO-7, Fig. 3). Why such inconsistency occurs? More details about modification of the materials should be given (concrete weights etc.).
How was quantifier intensity of temperature programmed desorption? Experimental details about TPD should be expanded.
Experimental part describing acidic/alkaline sites quantification should be expanded to bring detailed procedure for titrations.
Use of Kelvin and degree of Celsius (one in text, second in figures) brings inconveniences for readers.
Discussion about nature of sulphur is needless (l. 204) – from chemical point, sulphur cannot be reduced under conditions used, so obviously stays in form of sulphate.
What was reproducibility of catalytic experiments? Errorbars should be shown. Discussion about the best effectivity – when comparing yields 97.2 and 96.3 – are meaningless. Especially, conclusion that higher content of sulphur leads to collapse of crystalline framework is based on rather weak data.
l. 310 – what is by-product?
l. 325-326 – very speculative when comparing just tiny differences in yield.
l. 353-355 – very speculative – if sulphur serve as electron supplier, it should not increase electropositivity of neighbouring metal ions
Some formal errors (typos etc.):
l. 36 – “other” is improper word as simple carbonates are not macromolecules
l. 103 – power instead of powder
l. 172 – sulphur was introduced on surface, not into the material
Table 1 – column “Entry” is superfluous
l. 230 – acid sites are not due to presence of electrons, but due presence of suitable free orbitals
Table 3 – "Total" obviously corresponds to a number of basis sites determined in titration using indicator with lower pH of change, so column is superfluous... Instead, some explanation should be added into text.
l. 265-266 – Attraction of electrons by S=O bonds is speculative, sulphate is rather coordinating anion. Cannot S-OH groups be formed during calcination?
l. 276 – volcanic?
l. 364 – big “S”
Author Response
Dear Reviewer:
I am gratitude to receive your reviewing comments and thanks for your sparing time to read my manuscript. In the following part, I am gladly to respond and answer your comments and suggestions.
Comment 1: Characterization of the material is rather poor and some results are somewhat contradictory. In XRD, only 2 diffractions found (one corresponds to MgO and other to spinel MgAl2O4). Morphology studied by SEM shown uniform shape of crystals. Why? Are MgO/MgAl2O4 phases too small crystallites formed in material, which keeps the morphology of starting hydrotalcite?
What is size distribution of the crystalline particles calculated from diffraction broadening?
Response 1: Good question! Actually there are other slight peaks which can also be observed in the figure. However, the intensity of these peaks is too weak. This can be explained as that the main compositions in the mixed metal oxide derived from hydrotalcite are MgO and MgAl2O4 due to the different hydrothermal synthesis conditions, and these results are not hydrotalcite but the materials derived from it. Therefore, the XRD results are different from those in hydrotalcite.
Moreover, according to the Scherrer analysis on the basis of the XRD results, the mean sizes of MgO nanoparticles increased after modified with sulphur addition, while the result of MgAl2O4 nanoparticles almost keep the same. All the data are listed in the follow table.
Sample | MgO mean size (nm) | MgAl2O4 mean size (nm) |
MgAlO | 9.91 | 23.35 |
SMgAlO-1 | 17.73 | 22.84 |
SMgAlO-3 | 19.38 | 22.98 |
SMgAlO-5 | 18.15 | 25.63 |
SMgAlO-7 | 19.34 | 27.05 |
Comment 2: Why so intense OH vibrations are present in the IR after long calcination at 823 K? If starting from hydrotalcite, some water stays? Or is re-adsorbed? What about carbonate?
Response 2: Regarding the -OH peak appearing in the FT-IR, it may be due to the re-adsorption of a part of the water after calcination, and the explanation is given in the manuscript :“FT-IR spectra presented that all catalysts exhibited a broad band between 3750 – 2750 cm-1 (centered at 3504 cm-1), demonstrating the presence of hydrogen-bonded perturbed hydroxyl groups, such as Al-OH and re-adsorbed H2O (stretching vibration).”
Comment 3: Idealized formula of hydrotalcite is Mg6Al2(CO3)(OH)16∙4(H2O), why Mg:Al ratio 4:1 was used?
Response 3: Our work was carried out with MgAlO as the precursor. We have investigated the influence of Mg/Al ratio to the MgAlO samples’ crystallinity in another work. The result presented that it presented the best crystallinity when the Mg/Al = 4:1. Therefore we choose Mg/Al = 4:1 used in this work.
Comment 4: IR vibration at 852 cm-1 is attributed to MgO and Al2O3 – it is again misleading. Why content of sulphur is different from starting amount (nominal loading)? All material was just mixes, evaporated, and calcinated. Was some sulphur removed by calcination? Seems be improbable as sulphur content makes straight line with starting amount. See e.g. l. 284-285.
Response 4:
This question can be explained in Figure 2 of the manuscript, where 852 cm-1 corresponds to Al-O and 600 cm-1 corresponds to Mg-O. This has been corrected in the corresponding part of the manuscript.
The nominal amount in the sample refers to the adding percentage of ammonium persulfate to the precursor is defined in 1 g, which is different from the actual sulfur content in the total amount of the catalyst. However, partial amount of sulphur species can be inevitable lost during the hydration process and the calcination process afterwards. The sulfur content in the manuscript is based on the actual content.
Comment 5: Sometimes, authors call their catalyst as Mg-Al dioxide, which implies mixture of MgO and Al2O3, but is inconsistent with XRD.
Response 5: Regarding the nomination as Mg-Al dioxide, I agree with the reviewer's suggestion. We have corrected it to “mixed metal oxide derived from hydrotalcite” in the corresponding part of the manuscript.
Comment 6: XPS spectra shows very high content of phases as Mg(OH)2, Al(OH)3, MgSO4 and Al2(SO4)3 on the surface, but intensity of sulphates does not correlate with amount of sulphur (SMgAlO-5 has obviously higher content than SMgAlO-7, Fig. 3). Why such inconsistency occurs? More details about modification of the materials should be given (concrete weights etc.).
Response 6: There is a lapsus calami in the manuscript regarding to the description in Figure 3-d, which the sulfates of SMgAlO-7 is higher than SMgAlO-5. And its intensity is related to the sulphur content. We have corrected this expression to “Thus, the further addition of sulfate species (from 5 to 7) could increase the intensity of S 2p signal in the XPS figure gradually (Figure 3-d)”
Comment 7: How was quantifier intensity of temperature programmed desorption? Experimental details about TPD should be expanded.
Response 7: Thanks for your suggestion. We have expanded the details in the following sentences “Temperature-programmed desorption of CO2 (CO2−TPD) and NH3 (NH3−TPD) were carried out on an AutoChem II 2920 Chemisorption equipment with a TCD and an on-line MS. The sample (100 mg) was reduced at 573 K for 2 h in Ar atmosphere (1/9, v/v; 50 mL min−1), followed by purging with a high purity He flow (50 mL min−1) for 2.0 h at 583 K. When the temperature decreased to 323 K, CO2 (or NH3) was introduced until saturation, followed by purging He (50 mL min−1) for 90 min to remove physisorbed CO2 (or NH3). Finally, the sample was heated from 323 K to 1023 K at a rate of 10 K/min, and the released CO2 (or NH3) was monitored by a mass spectrometer.”
Meanwhile, in this manuscript, TPD is mainly used to observe the change trend of acid and alkali amount during the test and the quantification of acid and alkali amount is determined by IGC in the corresponding descriptions.
Comment 8: Experimental part describing acidic/alkaline sites quantification should be expanded to bring detailed procedure for titrations.
Response 8: We have supplemented the details in the manuscript.
Comment 9: Use of Kelvin and degree of Celsius (one in text, second in figures) brings inconveniences for readers.
Response 9: Thanks for your helpful suggestions. We have corrected all the unit of temperature in the manuscript to Kelvin degree.
Comment 10: Discussion about nature of sulphur is needless (l. 204) – from chemical point, sulphur cannot be reduced under conditions used, so obviously stays in form of sulphate.
Response 10: Thanks for your reflectively comment and we have corrected "suggesting the existence of S6+(SO42-) rather than the simple adsorption of sulphur molecules" to "suggesting the existence of SO42- rather than the simple adsorption of sulphur molecules"
Comment 11: What was reproducibility of catalytic experiments? Errorbars should be shown. Discussion about the best effectivity – when comparing yields 97.2 and 96.3 – are meaningless. Especially, conclusion that higher content of sulphur leads to collapse of crystalline framework is based on rather weak data.
Response 11: We have carried out the reproducibility of all the reactions, and the errorbars have been attached to the column of the yield in Table 4.
Comment 12: what is by-product?
Response 12: The main by-product of the preparation of PC by urea method is γ-Valerolactone.
Comment 13: 325-326 – very speculative when comparing just tiny differences in yield.
Response 13: Thanks for your comments. In order to demonstrate the details in reaction investigations and prove our reliability. The details of reactions with 3 runs under the catalysts are listed in the following table.
Yield of PC (%)
Times | MgAlO | SMgAlO-1 | SMgAlO-3 | SMgAlO-5 | SMgAlO-7 |
1 | 82.9 | 86.3 | 89.7 | 96.9 | 96.1 |
2 | 80.5 | 83.1 | 86.2 | 94.5 | 92.1 |
3 | 74.2 | 77.4 | 81.8 | 90.3 | 87.5 |
Reaction condition: n(PG)/n(urea) = 3; mass fraction of catalysts = 1%; reaction time = 5 h; reaction temperature = 433 K.
Comment 14: 353-355 – very speculative – if sulphur serve as electron supplier, it should not increase electropositivity of neighbouring metal ions
Response 14: Thanks for your comments, and the corresponding description have been appended in the manuscript as follows: The sulphur atom’s relatively lower electronegativity as well as the electron-rich outer shell could continuously transfer electrons to the adjacent O atoms, which results in the electron accumulation on the surface of Oxygen atoms. In order to keep the charge balance on the whole structure, it could further improve the electropositivity of the neighboring metal atoms connected with coordination bonds on the mixed metal oxide derived from hydrotalcite.
Comment 15: Some formal errors (typos etc.):
36 – “other” is improper word as simple carbonates are not macromolecules
Response: We have corrected it on line 36 of the manuscript.
103 – power instead of powder
Response: It is a misspelling and we have corrected it.
172 – sulphur was introduced on surface, not into the material
Table 1 – column “Entry” is superfluous
Response: We have corrected the sentence of "introduced into the catalytic material" on line 172 to "introduced on the catalytic surface", and the "Entry" column in Table 1 has been deleted.
230 – acid sites are not due to presence of electrons, but due presence of suitable free orbitals
Response: The reason for the formation of Lewis acid has been corrected in the manuscript.
Table 3 – "Total" obviously corresponds to a number of basis sites determined in titration using indicator with lower pH of change, so column is superfluous... Instead, some explanation should be added into text.
Response: The "Total" column in Table 3 should be the sum of the amount of strong base and weak base, which has been corrected in the manuscript.
265-266 – Attraction of electrons by S=O bonds is speculative, sulphate is rather coordinating anion. Cannot S-OH groups be formed during calcination?
Response: Since the S-OH groups could form a protonic acid─Brönsted acid site while there is no absorption peak corresponding to Brönsted acid is found in the Py-IR.
276 – volcanic
Response: “The volcanic tendency” refers to a trend of rising first and then falling, and we have corrected as the standard expression “The volcano curve”.
364 – big “S”
Response: We have changed the “S” to “s” in the manuscript.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
l 286-290 - "The sulphur atom’s relatively lower electronegativity as well as the electron-rich outer shell could continuously transfer electrons to the adjacent O atoms, which results in the electron accumulation on the surface of Oxygen atoms. In order to keep the charge balance on the whole structure, it could further improve the electropositivity of the neighboring metal atoms connected with coordination bonds on the mixed metal oxide derived from hydrotalcite."
The sentence is very speculative - charge balance is kept by oxidation states (Mg2+/ Al3+ vs. O2-). If presence of neighbouring SO4(2-) groups have some influence to electropositivity of metal ions, it should be due to reverse process - SO4(2-) should be better electron attractor than oxide anion itself (mezomeric effect), as oxygen atoms of sulphate are not more negative than oxide anions. Thus, sulphate polarizes Al-O bond and increases partial positive charge on Al atom. If authors original suggestion would be right, the "more basic" oxygen of sulphate should better compensate the positive charge of Al and in fact decrease Lewis acidity of Al. Please, remove this speculative part.
l 402 - a typo, selectivity
Author Response
Dear Reviewer:
I am gratitude to receive your reviewing comments and thanks for your sparing time to read my manuscript. In the following part, I am gladly to respond and answer your comments and suggestions.
Comment 1: l 286-290 - "The sulphur atom’s relatively lower electronegativity as well as the electron-rich outer shell could continuously transfer electrons to the adjacent O atoms, which results in the electron accumulation on the surface of Oxygen atoms. In order to keep the charge balance on the whole structure, it could further improve the electropositivity of the neighboring metal atoms connected with coordination bonds on the mixed metal oxide derived from hydrotalcite."
The sentence is very speculative - charge balance is kept by oxidation states (Mg2+/ Al3+ vs. O2-). If presence of neighbouring SO4(2-) groups have some influence to electropositivity of metal ions, it should be due to reverse process - SO4(2-) should be better electron attractor than oxide anion itself (mezomeric effect), as oxygen atoms of sulphate are not more negative than oxide anions. Thus, sulphate polarizes Al-O bond and increases partial positive charge on Al atom. If authors original suggestion would be right, the "more basic" oxygen of sulphate should better compensate the positive charge of Al and in fact decrease Lewis acidity of Al. Please, remove this speculative part.
Response 1: I agree with the comments of the reviewers and have already removed it in the corresponding sections of the manuscript.
Comment 2: l 402 - a typo, selectivity
Response 2: We have changed the “selectiviy” to “selectivity” in the manuscript.
Author Response File: Author Response.pdf
