Study of Deactivation in Suzuki Reaction of Polymer-Stabilized Pd Nanocatalysts

Round 1

Reviewer 1 Report

The submitted manuscript presents an interesting addition to the development of improved recyclable catalysts for the Suzuki reaction. The objectives of the work are set out relatively clearly and the methodology is quite thorough and sound. The conclusions are convincingly presented and the results have the potential to assist in the development of improved procedures for the recycling of catalysts or in the design of improved supports for Pd-nanoparticles that could avoid the need for washing of the catalyst between cycles. In this context, there are two points where this manuscript could be enhanced so as to have a greater impact:

• It would be interesting if the authors could expand further on the the potential of the MN100 support, which appears to retain its activity without washing but which has other disadvantages such as the need for a higher excess of PBA, the reasons for which are not clearly explained. The text would therefore be improved if the authors expressed some ideas why this support is apparently less favoured for this reaction.

 

• The authors have chosen to use water/ethanol as the solvent in order to maintain a homogeneous system. It could be argued that this system is not the most favourable for use with a hydrophobic support and for a reaction in which the products are also lipophilic and likely to be attracted to the support rather than remain in the partially aqueous solvent. There is therefore a need for a stronger justification for the use of the chosen solvent system, since it can be envisioned that the use of other homogeneous but more lipophilic systems (using higher alcohols, for example) might offer some improvement by avoiding the need for washing between successive cycles. It would be interesting to know if such systems were considered.

The text is, in general, quite well written although it would benefit from proofreading by a fluent or native speaker of English since there are a number of linguistic errors (too many to list in detail here). Most of these do not affect a proper understanding of the text but there are some, listed below, for which corrections are recommended:

1. Lines 36-38. The sentence "Metallic palladium ........ reaction products" could be improved. For example, combining it with the previous sentence to give: "It is known that this is a palladium-catalyzed condensation reaction which proceeds via a Pd(0)/Pd(II) cycle involving the oxidative-addition of the starting components and the reductive elimination of the resulting reaction products"

2. Line 40. Delete "and" from "and etc"

3. Line 42. Change "on contrary" to "on the contrary"

4. Line 54. The word "several" implies more than two so this sentence could be modified to "There are two main ways to study ....."

5. Line 61. Change "We carried out it ...." to "We carried it out ....."

6. Line 65. Correct "mater" to "matter"

7. Line 119. Correct "hydrophobisity" to "hydrophobicity"

8. Line 150 delete the full stop after "conditions"

Author Response

Point 1: It would be interesting if the authors could expand further on the the potential of the MN100 support, which appears to retain its activity without washing but which has other disadvantages such as the need for a higher excess of PBA, the reasons for which are not clearly explained. The text would therefore be improved if the authors expressed some ideas why this support is apparently less favoured for this reaction.

 

Response 1: We not yet understand well this phenomenon (the necessity of higher excess of PBA) in case of MN100, but we can propose that depending on the polymer hydrophobicity the ratio between components of complex solvent (EtOH and water) inside the pores of HPS may differ from the reaction volume . It can result to some side reactions of PBA, i.e. deboronation.

Action taken: We added this assumption in the manuscript text.

 

Point 2: The authors have chosen to use water/ethanol as the solvent in order to maintain a homogeneous system. It could be argued that this system is not the most favourable for use with a hydrophobic support and for a reaction in which the products are also lipophilic and likely to be attracted to the support rather than remain in the partially aqueous solvent. There is therefore a need for a stronger justification for the use of the chosen solvent system, since it can be envisioned that the use of other homogeneous but more lipophilic systems (using higher alcohols, for example) might offer some improvement by avoiding the need for washing between successive cycles. It would be interesting to know if such systems were considered.

 

Response 2: We agree that the use of hydrophobic solvent would be useful, but it would require the addition of phase-transfer agents. Indeed we tried to use other solvents. It was found that in pure EtOH the reaction proceeded much slower. The replacement of EtOH with isopropanol in alcohol-water mixtures also resulted in the decrease of catalytic activity.

Since at chosen concentrations the reaction product does not precipitate by the reaction end, and the catalyst is nicely suspended in the reactor, we think that the catalyst deactivation happens during the filtration process.

 

Point 3: The text is, in general, quite well written although it would benefit from proofreading by a fluent or native speaker of English since there are a number of linguistic errors (too many to list in detail here). Most of these do not affect a proper understanding of the text but there are some, listed below, for which corrections are recommended:

1. Lines 36-38. The sentence "Metallic palladium ........ reaction products" could be improved. For example, combining it with the previous sentence to give: "It is known that this is a palladium-catalyzed condensation reaction which proceeds via a Pd(0)/Pd(II) cycle involving the oxidative-addition of the starting components and the reductive elimination of the resulting reaction products"
2. Line 40. Delete "and" from "and etc"
3. Line 42. Change "on contrary" to "on the contrary"
4. Line 54. The word "several" implies more than two so this sentence could be modified to "There are two main ways to study ....."
5. Line 61. Change "We carried out it ...." to "We carried it out ....."
6. Line 65. Correct "mater" to "matter"
7. Line 119. Correct "hydrophobisity" to "hydrophobicity"
8. Line 150 delete the full stop after "conditions"

 

Response 3: Corresponding changes were made.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript contains many characterization results; however, the reviewer felt that the results should be more precisely documented as a scientific paper. Prior to publication, the following issues should be addressed.

• In abstract, the authors mentioned aromatic matrix of HPS allows stabilization of Pd nanoparticles during repeated reaction runs. Then deactivation occurs. What does author trying to claim?
• Proton and carbon NMR data should be included to prove cross coupling product was the culprit for the deactivation of catalyst. XPS proof cannot be accepted.
• Why only cross coupling product alone does occupies matrix, why not other side product (due to homocoupling) and reactants?
• What author meant by cocktail type catalyst?

Author Response

Point 1: In abstract, the authors mentioned aromatic matrix of HPS allows stabilization of Pd nanoparticles during repeated reaction runs. Then deactivation occurs. What does author trying to claim?

 

Response 1: We claim that HPS support stabilized effectively the catalytically active palladium since we observed no Pd leaching and no agglomerating of Pd(0) NPs. Nevertheless, the catalytic system (active metal and polymeric support) undergoes deactivation confirmed by a lower activity observed in the consecutive runs. This deactivation caused the support nature, which is able to retain MBP (we think that this happened during the catalyst filtration).

Action taken: Abstract was rewritten.

 

Point 2: Proton and carbon NMR data should be included to prove cross coupling product was the culprit for the deactivation of catalyst. XPS proof cannot be accepted.

 

Response 2: We agree that this would be useful. However, NMR cannot be used for our system due to the following: Hydrogen of substituted benzene rings of polymeric support will give a lot of signals in the range of 6.8-8 m.d. that will make impossible to identify the signal of hydrogen of benzene rings of the product (MBP). The signal of the hydrogen of OCH3 group will be lost among the signals of oxygen-substituted alkane chains of the polymer (HPS contains about 5% of oxygen remaining after the polymer synthesis). 13C-NMR will face the same problem.

 

Point 3: Why only cross coupling product alone does occupies matrix, why not other side product (due to homo-coupling) and reactants?

 

Response 3: We agree that different reactants canpotentially adsorb on the polymeric matrix (among them are 4-bromoanisole, phenylboronic acid, sodium hydroxide, side-products of the reaction (including biphenyl)).Although different reactants couldadsorb on the catalyst surface during the S-M reaction, we supposed that MBP is responsible for the observed deactivation due to the following reasons:

- bromine and boron were not detected on the surface of Pd/MN270 neither after the first run nor after the second run, which exclude strong adsorption of BrAn and PBA;

- sodium was found on the Pd/MN270 surface in trace amounts (less than 0.2 at.%), so, sodium-containing compounds can not be considered as the main adsorbed species;

- high selectivity of the catalyst with respect to MBP (share of MBP among the reaction products is higher than 96%) excludes the strong influence of BP;

- sharp increase of oxygen content was found after the repeated reaction runs, and this oxygen does not belong to the oxygen of polymeric matrix itself

Action taken: We added this argumentation in the manuscript text to make more clear the decision making process.

 

Point 4: What author meant by cocktail type catalyst?

 

Response 4: The term “cocktail” catalyst was proposed by Ananikov and co-workers for cross-coupling processes and describes catalytic systems containing more than one form of palladium including Pd(0) NPs, where the latter serve as pre-catalysts generating active species during the reaction. For such catalysts it was established that virtually any form of Pd (small clusters, NPs, inorganic salts, complexes containing palladium ions or atoms) might start catalytic cycle.

Action taken: More explanations were added to the manuscript text and general scheme of the reaction mechanism was added to illustrate the transformations of Pd during the reaction course.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors reported a detailed study of catalytic activity and deactivation of HPS Pd nanocatalysts in classical Suzuki cross-coupling reaction. The work is a logical extension of author's previous research on Pd/HPS catalysts. Special attention was given to origins of deactivation. Catalytic properties of Pd/MN270 and Pd/MN100 were analyzed and compared. The results suggest that hydrophobic nature of Pd/MN270 was the origin of catalyst deactivation, which was confirmed by increased activity of hydrophilic Pd/MN100 or washing Pd/MN270 by a hydrophobic solvent. I think the work is experimentally well-done with the use of modern methods for material characterization (TEM, STEM, XPS, XFA, FTIR, etc.). Although the manuscript is somewhat overfilled with technical details, I believe it fits to journal "Processes". 

Several suggestions to the authors:

1) Abstract must not contain sentences from the introduction and conclusion. The authors just combined them without even word changing and made an abstract. The abstract must be rewritten.  

2) Lines 35-38 - Add references to introduce Suzuki coupling as the reaction to make it more clear for engineers or non-experts.

3) Also need some references when mentioning Pd/MN270 and Pd/MN100 for the first time.

Author Response

Point 1: Abstract must not contain sentences from the introduction and conclusion. The authors just combined them without even word changing and made an abstract. The abstract must be rewritten.

 

Response 1: Abstract was rewritten.

 

Point 2: Lines 35-38 - Add references to introduce Suzuki coupling as the reaction to make it more clear for engineers or non-experts.

 

Response 2: The references were added.

 

Point 3: Also need some references when mentioning Pd/MN270 and Pd/MN100 for the first time.

 

Response 3: The references were added.

Author Response File: Author Response.docx