Cu-Catalyzed Hydrodehalogenation of Brominated Aromatic Pollutants in Aqueous Solution

Round 1

Reviewer 1 Report

The manuscript deals with the hydrodebromination (HDB) of brominated phenolic compounds in aqueous alkaline solution over Cu-based catalysts. Authors have tested different Cu-based catalysts (i.e. metallic copper, Devarda´s Al-Cu-Zn alloy (Dev. alloy), Cu oxides, among others) by using both NaBH₄ and NaOH aqueous basic systems under different reaction conditions. They have also studied the structural and compositional changes in solid materials after reaction. Authors concluded that Cu species present in the catalytic systems are relevant for the HDB of Bromoxynil (BRX). In general, the research work is interesting from both academia and industrial point of views, although its quality needs further improvement. Thus, the efficiency of Dev. alloy is quite similar to Cu₂O/NaBH₄ system, and even lower when comparing to CuSO₄/NaBH₄ (see Fig. 13). In addition, it is not clear if the Cu-based systems work in homogeneous or heterogeneous manner, or via a combination of both catalytic systems. In this sense, further experiments are needed to confirm active species involved in HDB reaction, the effect of Al and Cu species in solution and the remaining solid system (see main comments and remarks below).

 

Main comments and remarks:

• In the Abstract text, authors mention Cu-based catalysts as reagents. In my opinion, this is not clear for the reader. Please, revise. In addition, the Abstract text needs further enhancement by the authors.
• Table 1 should be re-organized in more columns to provide the information in a more clear manner, as follows:

Catalysts / Type – Amount (in g) // NaOH conc. (in mmol) // Temp. (ºC) // Reaction time (in min.) // Products or Comp. (%) / BRX – HBN – HBA - others

• Reaction conditions: the amount of catalyst (Dev. alloy) used in the experiments is too high with respect to the BRX in solution (0.25 g of catalyst per mol of C-Br reduced). As the authors say in the text, maybe further optimisation is necessary. Please, comment on that.
• What about homogeneous vs heterogeneous catalysts in the reported system? Authors mention that Al and Cu hydroxides are recovered from the solution after reaction, and some amount of the remaining solid catalyst. At this point the questions arising are: Which are the catalytic active species in these Cu-based systems? Are they the Cu sites in the solid catalyst or Cu species dissolved in the liquid? Authors must answer these very relevant questions, for example by making experiments to rule out the action of homogeneous active species.
• In line with the latter point, have the authors essayed as catalysts the mentioned Al and/or Cu hydroxides? These data are essential to elucidate reaction mechanism.
• What is the amount of undissolved solid catalysts recovered after reaction? Have the authors measured that? Authors must assess this important issue.
• In Fig. 3, the DRX patterns of the original Dev. alloy must also be included, and the corresponding signals commented in the main text by the authors.
• From data of Fig. 13, both Cu₂O and CuSO₄ appears to be better catalysts for the HDB of BRX than the Dev. alloy. Authors should include the results of both catalytic systems in Table 1 and compare with those attained with the Dev. alloy used. In this sense, and from data of Fig. 13, what is the advantage of using Dev. alloy instead of the other Cu-based catalytic systems? Authors must argue on that.
• From kinetics data of Table 2, comparison between Dev. alloy and Cu₂O catalysts appears to be (at least) partial. No data are provided for Dev. alloy/NaBH₄ system to adequately compare to Cu₂O/NaBH₄ analogous system. In addition, and only considering these incomplete data, Cu₂O and Dev. alloy possessed very similar performance. Therefore, the question is: What happens when Cu₂O/NaOH system is used? Authors must address this important concern.
• As suggestion, the manuscript appears to be a little bit large in extension. Authors should reduce the general length of the paper, for example by eliminating (transferring to SI) some of the Figures containing catalytic data.

Author Response

Responses to the reviewer 1:

Main comments and remarks:

• In the Abstract text, authors mention Cu-based catalysts as reagents. In my opinion, this is not clear for the reader. Please, revise. In addition, the Abstract text needs further enhancement by the authors.

Answer: We have proved that Cu-based compounds work as catalysts in most cases. However, at least in the case of the Dev. alloy, the CuAl₂ seems to be active reagent (using alkali metal hydroxide without addition of other reductant for BRX or TBBPA hydrodehalogenation, the CuAl₂ works both as the source of reductant and hydrodebromination catalyst).

The abstract has been rewritten accordingly to be more clear for all readers.

 

• Table 1 should be re-organized in more columns to provide the information in a more clear manner, as follows:

Catalysts / Type – Amount (in g) // NaOH conc. (in mmol) // Temp. (ºC) // Reaction time (in min.) // Products or Comp. (%) / BRX – HBN – HBA – others

Answer: The Table 1 has been completed according to the above-mentioned suggestion and the quantities of dissolved Al and undissolved rest of Dev. alloy have been included.

• Reaction conditions: the amount of catalyst (Dev. alloy) used in the experiments is too high with respect to the BRX in solution (0.25 g of catalyst per mol of C-Br reduced). As the authors say in the text, maybe further optimisation is necessary. Please, comment on that.

Answer: The amount of the Devarda´s alloy seems to be high only in the case if the Al-Cu-Zn alloy is perceived or considered as a catalyst. However, copper in the form of CuAl₂ seems to be the catalytically active part of this alloy (content of Cu is only 45 wt. % of the Devarda´s alloy) with respect to other performed experiments (described e.g. in Tables 1 and Figures 2-4, 6-8, 11 and Table 2). The overall HDB activity of the Dev. alloy is dependent on the content of Al (ca. 50 wt. %) in this alloy (see Figure 1 for example). Reuse of the Dev. alloy is accompanied by the gradual loss of Al (which is gradually dissolved in alkaline aqueous solution as Al(OH)₄^-) and significant decrease of both HDB rate and conversion of BRX to 4-HBN. From this point of view, the Dev. alloy is not a catalyst but a HDB agent containing both reductant and catalytically active species (most probably CuAl₂).

• What about homogeneous vs heterogeneous catalysts in the reported system? Authors mention that Al and Cu hydroxides are recovered from the solution after reaction, and some amount of the remaining solid catalyst. At this point the questions arising are: Which are the catalytic active species in these Cu-based systems? Are they the Cu sites in the solid catalyst or Cu species dissolved in the liquid? Authors must answer these very relevant questions, for example by making experiments to rule out the action of homogeneous active species.

Answer: We are sorry for our unclear description dealing with the fate of metals from the used Devarda´s alloy. Metallic copper and copper oxides are generally not soluble in diluted alkaline aqueous solution. Due to this reason, we have paid no attention to the role of the dissolved copper because each of the tested soluble copper-based compounds are either very rapidly converted to the insoluble (nano)metallic Cu or Cu₂O by action of NaBH₄ (see Figures 12-13) or they are completely inactive in the HDB process (in case of CuBr₂, see results given in the Table S1). However, presumably low amount of copper nanoparticles is not removed by decantation and/or filtration (the measured content of Cu in aqueous filtrate is below 10 mg Cu/L), especially in the case where some complexing (for example NH₄OH) or chelating (glucose) agent was applied in case of in-situ generated copper catalyst by action of CuSO₄/NaBH₄.

For the better verification of the role of the „soluble“, in-situ produced copper-based nanoparticles (Cu and/or Cu₂O, see Table S6, entries 11 and 12), we changed the order of the addition of reagents. Firstly, nano-Cu was formed by the reduction of CuSO₄ using excess of NaBH₄ and, subsequently, aqueous BRX solution was added in additional experiments. However, conversion of BRX to the HDB products (3-Br-4-HBN, 4-HBN and 4-HBA) was always lower compared with direct addition of CuSO₄ to the dissolved mixture of BRX and NaBH₄ in aqueous solution. Moreover, the addition of complexing agents (NH₄OH and glucose) made the HDB conversion even much worse (compare with Table S6, entries 1-4).

We interpret these observations as negligible role within the homogeneous Cu-based agents in the studied hydrodehalogenation reaction.

• In line with the latter point, have the authors essayed as catalysts the mentioned Al and/or Cu hydroxides? These data are essential to elucidate reaction mechanism.

Answer: The effect of Al (hydr)oxides (Al(OH)₃ or AlO(OH)) on the HDB was tested and the results are presented in Table S1 and Table 1 (Entries 12-14). Both Cu(OH)_n (or hydrated Cu_nO) and are definitively not effective agents within the studied HDB processes (see Table S1).

• What is the amount of undissolved solid catalysts recovered after reaction? Have the authors measured that? Authors must assess this important issue.

Answer: We measured both amount of dissolved Al (using ICP-OES) and gravimetrically the amount of the isolated undissolved part of the used Dev. alloy (see Table 1 and Figures 1 and S3). The time dependence of quantity of dissolved Al has been added within Figures 1 and S3.

• In Fig. 3, the DRX patterns of the original Dev. alloy must also be included, and the corresponding signals commented in the main text by the authors.

Answer: The diffractogram of the Dev. alloy has been added within the Fig. 3.

• From data of Fig. 13, both Cu₂O and CuSO₄appears to be better catalysts for the HDB of BRX than the Dev. alloy. Authors should include the results of both catalytic systems in Table 1 and compare with those attained with the Dev. alloy used. In this sense, and from data of Fig. 13, what is the advantage of using Dev. alloy instead of the other Cu-based catalytic systems? Authors must argue on that.

Answer: In the new version of the manuscript Fig. 10 (formerly Fig. 13) compares action of tested Cu-agents under the coaction of NaBH₄. However, the Dev. alloy works well in HDB using excess of alkali metal hydroxide, as discussed in the manuscript. The addition of NaBH₄ significantly decrease corrosion of Al from the Dev. alloy which is accompanied by a decrease of the HDB efficiency (compare Fig. 1 and Fig. S1 and text in lines 190-200 within the revised manuscript (Word version).

• From kinetics data of Table 2, comparison between Dev. alloy and Cu₂O catalysts appears to be (at least) partial. No data are provided for Dev. alloy/NaBH₄system to adequately compare to Cu₂O/NaBH₄ analogous system. In addition, and only considering these incomplete data, Cu₂O and Dev. alloy possessed very similar performance. Therefore, the question is: What happens when Cu₂O/NaOH system is used? Authors must address this important concern.

Answer: We have not place higher importance to the Dev. alloy/NaBH₄ reaction system due to the low conversion of the HDB process in this case. The kinetic of the HDB using combination of the Dev. alloy/NaBH₄ reagents is added in this revised version of our manuscript. Surprisingly, applying kinetic model of subsequent reaction for the description of this mentioned inefficient reagents system proved that only the first reaction step (debromination of BRX to 3-Br-4-HBN) is significantly slower using the Dev. alloy/NaBH₄ reagents in comparison with the Dev. alloy/NaOH reagents. The value of rate constant k₂ (calculated for the system using the Dev.alloy/NaBH₄) is comparable with the rate constant k₂ (calculated for the system using the Dev. alloy/NaOH) of the subsequent HDB of 3-Br-4-HBN to 4-HBN (see Table 2). As one can see in Table 2, in the case of the Dev. alloy/NaBH₄ reagents, the values of rate constant of both reaction steps are comparable (with regard to the standard deviations). This fact causes that the reaction is close to an equilibrium state and therefore it is not possible to reach 100% conversion of BRX to 4-HBN (in real time).

The performance of the Dev. alloy/NaOH, Dev. alloy/NaBH₄ and Cu₂O catalysts is clear from Fig. 11. Using the Dev. alloy/NaOH system, entire amount of the BRX was converted to the 4-HBN during 120 minutes of the reaction. In the case of the Dev. alloy/NaBH₄ catalysts, it is about 63 % and for Cu₂O/NaBH₄ system, the conversion (in 120 min) is only 11 %.

Employment of the Cu₂O/NaOH system provides no HDB reaction(s) and only starting BRX is observed in the reaction mixture even after 2 hours of heating at 70 ^oC as described in Table S1, Entry 3.

• As suggestion, the manuscript appears to be a little bit large in extension. Authors should reduce the general length of the paper, for example by eliminating (transferring to SI) some of the Figures containing catalytic data.

Answer: Some Figures were moved into Supplementary Materials Chapter.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript ID: catalysts-1203123 reports ‘’Cu-catalyzed hydrodebromination of brominated aromatic pollutants in aqueous solution’’ Dehydrohalogenation of bromoxynil was investigated by a series of Cu-based catalysts in alkaline solution. Dev-Alloy shows superior activity. Authors have demonstrated the coaction of NaBH4 and Dev-Alloy on dehydrohalogenation of bromoxynil. The optimized conditions are applied to tetrabromobisphenol A. There is a lot of data in the manuscript which is hard for the reader to digest in a single read, yet the data is properly presented. This is competently conducted research. Authors honestly commented pros and cons of the catalyst. The work is potentially interesting to the catalysts community; therefore, I recommend the article for publication in the journal catalysts. 

I have questions/comments on this work: 

1)     the term hydrodebromonation is confusing. Authors should consider using the universal term.

2)      Lines 32-33: Copper and its salts exhibit broad catalytic activity mainly due to the easily accessible 32 and reasonable stability of Cu(0), Cu(I) and Cu(II) oxidation states REF??

3)     Lines 65-66: However, especially ortho-halogenated phenols tend to be transformed to the significantly more toxic dibenzo-p-dioxines using some oxidation processes REF??

4)     Lines 83-85: ‘’A set of experiments involving the HDB of 3,5-dibromo-4-hydroxybenzonitrile (BRX) dissolved in alkaline aqueous solution and the comparison of HDB efficiency of electropositive metals, different copper alloys containing electropositive metals and NaBH4 used as common reductant was performed.’’ This sentence is confusing.

5)     Line 193: ….(moreover, an extension of the reaction time 193 causes subsequent reduction of the cyano group instead of the desired HDB process)…I see no experimental proof for reduction of cyano group such as GCMS? Or NMR? 

6)     Line 236: ‘’The application of Cu nanoparticles instead of Cu powder increase the reaction rate of NaBH4 mediated HDB.’’ Did the authors characterize the size of the nanoparticles?

7)     Grammar check required. For example, Line 25: ‘’agentshas’’ should be ‘’agents have’’ 

Author Response

Responses to reviewer 2:

The manuscript ID: catalysts-1203123 reports ‘’Cu-catalyzed hydrodebromination of brominated aromatic pollutants in aqueous solution’’ Dehydrohalogenation of bromoxynil was investigated by a series of Cu-based catalysts in alkaline solution. Dev-Alloy shows superior activity. Authors have demonstrated the coaction of NaBH4 and Dev-Alloy on dehydrohalogenation of bromoxynil. The optimized conditions are applied to tetrabromobisphenol A. There is a lot of data in the manuscript which is hard for the reader to digest in a single read, yet the data is properly presented. This is competently conducted research. Authors honestly commented pros and cons of the catalyst. The work is potentially interesting to the catalysts community; therefore, I recommend the article for publication in the journal catalysts. 

I have questions/comments on this work: 

1)     the term hydrodebromination is confusing. Authors should consider using the universal term.

Answer: With all due respect, we used the hydrodebromination term to distinguish the studied Cu-catalyzed reductive displacement of bromine bound on aromatic rings of studied BRX or TBBPA by hydrogen from other known hydrodehalogenations. The main reason is the quite specific activity of studied Cu-based catalysts on aromatic debromination (which are generally much less or even inactive for dechlorination). The term “hydrodebromination” is commonly used by other authors for the reductive displacement of bromine bound in organic compounds by hydrogen (using the keyword “hydrodebromination”, one could find 95 articles using Web of Science database, where the described hydrodebromination is usually catalyzed by platinum group metals).

Anyway, the term “hydrodebromination” in the manuscript title has been replaced by a more general term “hydrodehalogenation” and it has been used within the Abstract as well.

2)      Lines 32-33: Copper and its salts exhibit broad catalytic activity mainly due to the easily accessible 32 and reasonable stability of Cu(0), Cu(I) and Cu(II) oxidation states REF??

Answer: The references are cited correctly now.

3)     Lines 65-66: However, especially ortho-halogenated phenols tend to be transformed to the significantly more toxic dibenzo-p-dioxines using some oxidation processes REF??

Answer: The references are cited correctly now.

4)     Lines 83-85: ‘’A set of experiments involving the HDB of 3,5-dibromo-4-hydroxybenzonitrile (BRX) dissolved in alkaline aqueous solution and the comparison of HDB efficiency of electropositive metals, different copper alloys containing electropositive metals and NaBH4 used as common reductant was performed.’’ This sentence is confusing.

Answer:  The sentence has been reformulated as follows:

A set of experiments involving chemically diverse reductants within the HDB of 3,5-dibromo-4-hydroxybenzonitrile (BRX) dissolved in alkaline aqueous solution was performed. Electropositive metals, various copper alloys containing electropositive metals and NaBH₄ were employed as potential HDB agents in these experiments.

5)     Line 193: ….(moreover, an extension of the reaction time 193 causes subsequent reduction of the cyano group instead of the desired HDB process)…I see no experimental proof for reduction of cyano group such as GC-MS? Or NMR? 

Answer: The discussed reduction of the cyano group is now confirmed by both ¹H NMR spectra (Figure S7-S9) and GC-MS data (Figure S10-11) in the revised manuscript.

6)     Line 236: ‘’The application of Cu nanoparticles instead of Cu powder increase the reaction rate of NaBH4 mediated HDB.’’ Did the authors characterize the size of the nanoparticles?

Answer: The used Cu nanoparticles are commercially available (Sigma-Aldrich Co. supplier, Cat. No.: 774103-5G) and are characterized in catalogue as nanopowder, 60-80 nm particle size (SAXS). The obtained results were not satisfactory enough from the effective HDB point of view (please, see Table S2, Entry 6). Due to this reason we have not continued in the detailed research focused on effect of nano Cu particles size.

7)     Grammar check required. For example, Line 25: ‘’agentshas’’ should be ‘’agents have’’ 

Answer: The entire manuscript has been check from the grammar and/or typing errors point of view thoroughly.

 

Author Response File: Author Response.pdf