Synthesis of Brominated Alkanes via Heterogeneous Catalytic Distillation over Al₂O₃/SO₄²⁻/ZrO₂

Round 1

Reviewer 1 Report

The manuscript by Xiao and co-workers reported conversion of n-propanol to bromopropane using alumina-modified sulfated zirconia as heterogenous catalyst with good reusability and recyclability. Furthermore, the authors characterize the catalyst using PXRD, FTIR, BET surface area, SEM, EDX via SEM, XPS and investigate the optimization of reaction parameters. This work is of interest to the readers and could be suitable for Catalysts, MDPI after consideration of the following points:   

1. The authors should highlight the substrate scope of the conversion, does the catalyst exhibit similar efficacy for sterically hindered alcohol?
2. In addition to percentage yield, the authors should compare the TON, TOF with other reported catalysts including homogeneous catalysts to provide an in-sight comparison. 
3. The plausible mechanism of the conversion in presence of Al₂O₃/SO₄^2-/ZrO₂ should be highlighted. 
4. The authors should elaborate on the comparison between surface area, pore size, and active sites of the catalyst. Which is more important, the accessible pore size or active metal sites.  
5. The plausible reason of decrease in percentage yield after 4th cycle should be discussed. Is this leaching? Furthermore, the physiochemical analysis of recycled catalyst should be studied to affirm the stability.  

Author Response

The manuscript by Xiao and co-workers reported conversion of n-propanol to bromopropane using alumina-modified sulfated zirconia as heterogenous catalyst with good reusability and recyclability. Furthermore, the authors characterize the catalyst using PXRD, FTIR, BET surface area, SEM, EDX via SEM, XPS and investigate the optimization of reaction parameters. This work is of interest to the readers and could be suitable for Catalysts, MDPI after consideration of the following points:

Response: We highly appreciate the reviewer’s very positive comments on our work.

1. The authors should highlight the substrate scope of the conversion, does the catalyst exhibit similar efficacy for sterically hindered alcohol?

Response: We appreciate the reviewer for bringing up this valuable comment. The catalyst was applied to other reactions for preparing brominated alkanes, as shown in Table S2. It is found that the selected catalyst has a good catalytic effect on the bromination of other short-chain alcohols to prepare brominated alkanes. Experiments have found that the catalyst has the same catalytic effect on hindered alcohols. In the experiment of preparing isobromopropane from isopropanol, the use of a catalyst can make the yield of the reaction reach more than 95%.

2. In addition to percentage yield, the authors should compare the TON, TOF with other reported catalysts including homogeneous catalysts to provide an in-sight comparison.

Response: We appreciate the reviewer for bringing up this valuable comment. Compared with other types of catalysts and homogeneous catalysts, as shown in Table S1. When Al₂O₃/SO₄^2-/ZrO₂ is used as the catalyst, the TOF at this time is larger and the reaction yield is better. In a homogeneous reaction, when ionic liquid and concentrated sulfuric acid are used as catalysts, the reaction temperature is relatively high.

3. The plausible mechanism of the conversion in presence of Al₂O₃/SO₄^2-/ZrO₂should be highlighted.

Response: We appreciate the reviewer for this thoughtful comment, and completely agree with the reviewer’s opinion. Under normal circumstances, metal oxides and SO₄^2- have three coordination forms: single coordination, chelating double coordination, and bridge double coordination. In the SO₄^2-/M_xO_y type solid super acid, the latter two coordination effects between SO₄^2- and metal oxide dominate. The acidic center of the catalyst super acid is due to the strong induction effect between the metal oxide and SO₄^2-. During the calcination process, the ionic S=O bond is transformed into a covalent S=O bond, and the S=O group is strong electron withdrawing group. Due to the electron-withdrawing induction effect of the covalent bond, the electron cloud density on the M-O bond is reduced, resulting in a strong L acid center, which then exhibits super acidity, and water molecules or OH- adsorb and dissociate at the L acid center. The B acid center is produced. The synergistic effect of B and L acid centers makes the acidity of the catalyst higher than that of 100% sulfuric acid. The reaction process is shown in the Figure S1.

4. The authors should elaborate on the comparison between surface area, pore size, and active sites of the catalyst. Which is more important, the accessible pore size or active metal sites.

Response: We appreciate the reviewer for this thoughtful comment. It can be seen from the BET test that with the continuous increase of the Zr/Al ratio, the specific surface area of the catalyst first increases and then decreases, while the pore diameter of the catalyst becomes smaller and smaller. By comparing with the catalytic performance, it is found that when the ratio of Zr/Al is 4:1, the catalytic performance at this time is the best and the specific surface area is the largest. Therefore, it is considered that the specific surface area of the catalyst has a greater impact on the acidic sites. The larger specific surface area provides more acidic sites.

5. The plausible reason of decrease in percentage yield after 4th cycle should be discussed. Is this leaching? Furthermore, the physiochemical analysis of recycled catalyst should be studied to affirm the stability.

Response: We appreciate the reviewer for bringing up this valuable comment. FT-IR characterization of the recycled catalyst was carried out and compare with fresh catalyst, as shown in figure 4(B). It was found that the peaks at 1240 and 1120 cm^-1 which attributed to the asymmetric O=S=O of the used catalyst were weakened, so it is considered that the loss of sulfate radicals leads to the reduction of catalytic performance.

Reviewer 2 Report

Attached file. 

Comments for author File: Comments.pdf

Author Response

Article “Synthesis of Brominated Alkanes via Heterogeneous Catalytic Distillation over Al₂O₃/SO₄^2-/ZrO₂” is interesting. However, there are many errors and questions which needs to be addressed before to publish the work.

Response: We highly appreciate the reviewer’s positive comments on our work.

1. In line 95 and 96 authors wrote as “Figure 2 displays the XRD spectra of Al₂O₃/SO₄^2-/ZrO₂ catalysts with different Al₂O₃ loadings, at a calcination temperature of 600 °C’’ I think it should be at different temperatures since the fig shows from 450–650 °C.

Response: Thanks for your valuable suggestion, I have made the modification according to your suggestion. The article should be Figure1(B).

2. Figure 2B has different diffraction peaks from 30 to 60.

Response: We appreciate the reviewer for bringing up this valuable comment. I have made the modification according to your suggestion. The legend has been corrected to Figure1(B).

3. Figure 4 FT-IR spectra of Al₂O₃/SO₄^2- /ZrO₂ catalysts with different loadings can be improved. Authors stated the peaks at 1120, 1240 and 1640 cm-1 with lines but seems the peaks are not so clear. They can redo the IR or zoom the specific area to show the peaks clearly

Response: We appreciate the reviewer for this thoughtful comment, and completely agree with the reviewer’s opinion. I have made the modification according to your suggestion. A partially enlarged image has been added to the figure.

4. Lines 207-208 are not clear.

Response: We appreciate the reviewer for bringing up this valuable comment. This section is for screening better catalyst preparation conditions, so screening needs to be done under the same reaction conditions. After certain experiments, the reaction conditions were selected as the benchmark reaction conditions.

5. Line 208-211. Authors stated ‘The higher the calcination temperature, the higher the conversion rate of n-propanol. When the calcination temperature was higher than 600 °C, crystals were formed in the catalyst, and the catalytic performance was highest” But after 600 °C, catalyst performance was decreased may be due to crystals formation.

Response: We appreciate the reviewer for bringing up this valuable comment. It is written in the article that through the XRD test, it is found that the catalyst has crystal formation at 600°C. At the same time, the performance test found that the conversion rate has been significantly improved at 550°C to 600°C. The conversion rate drops slightly after 600°C. High temperature conditions could convert sulfate ions into sulfur dioxide. Therefore, it is considered that the formation of crystals improves the performance of the catalyst to a greater extent. The reason for the performance degradation after 600 °C is the loss of sulfate ions.

6. Authors stated excessive acid concentration did not significantly improve the catalyst’s performance. What really happens when the acid concentration is beyond 1 mol/L.

Response: We appreciate the reviewer for this thoughtful comment. When screening the optimal acidification concentration, an experiment was conducted with an acid concentration higher than 1mol/L, and the data is in the text. When the acid concentration exceeds 1mol/L, the performance of the catalyst is not greatly improved. From the perspective of energy saving and environmental protection, 1 mol/L is selected as the acidification concentration.

7. As stated, ‘‘too high Al content reduced the number of acidic sites in the catalyst, and its performance declined’’ What if running with high Al content with high acid concentration? So there will be enough acidic sites in the catalyst for its activity

Response: We appreciate the reviewer for this thoughtful comment. When selecting the catalyst preparation conditions, the experiment is carried out through the controlled variable method. Too high aluminum content would not greatly improve the performance of the catalyst, but can block the pores. Moreover, it was found through experiments that high aluminum content does not provide more acidic sites, but reduces acidic sites. Therefore, it is not necessary to increase the acid concentration under high aluminum content, but it will cause more costs and waste acid.

8. For catalytic stability and reusability, it was stated 80% can be achieved even in 4^th cycle. Adding a little % of fresh catalyst to the next cycle may increase the conversion and yield which further helps for the in depth study of catalytic activity

Response: We appreciate the reviewer for this thoughtful comment. In this cycle experiment, it is ensured that the amount of catalyst and the reaction raw materials are carried out in a certain ratio, and no fresh catalyst is added during each cycle.

9. Reference format inconsistent, ref 12, 25, 6 and many other. Check all the references formats.

Response: We appreciate the reviewer for this thoughtful comment, and completely agree with the reviewer’s opinion. I have made the modification according to your suggestion.