Hydrolytic Oxidation of Cellobiose Using Catalysts Containing Noble Metals
, Olga Kislitsa 1, Ekaterina Ratkevich 1, Yuriy Kosivtsov 1, Valentin Sapunov 2 and Valentina Matveeva 1,3Round 1
Reviewer 1 Report
This manuscript deals with the oxidation of cellobiose to produce gluconic and glucaric acids, by using noble metals supported on hypercrosslinked polystyrene. Although the catalytic results are interesting and the kinetic study is properly carried out, there are several key issues requiring to be addressed to clarify the information provided in the present work.
1. Why is a mixture of solvents (tetrahydrofuran, methanol and water) used for the impregnation of the support with the metal precursors?
2. The expressions used for the calculation of conversion and yields should be indicated.
3. The characterization of metal particles requires additional techniques such as H2 chemisorption, or XRD for the determination of average particle sizes. SEM could only provide information of a reduced section of the studied solid. Metallic properties, such as metal surface area, reduction degree and metal particle sizes should be evaluated.
4. The decrease in the specific surface area (Table 2), with respect to that of the support (HPS), is explained by the blockage of the pores by the metallic particles. Is it possible that the support has suffered some deterioration with the thermal treatment at 300ºC with hydrogen, used for the preparation of the metallic catalysts? In this sense, it is necessary to demonstrate the thermal stability of the support.
5. How do the authors explain the large diameter of the gold nanoparticles (32.1 nm) compared to the other metal catalysts, when they are quite often very small?
6. The different catalytic behavior (cellobiose conversion and selectivity to the different reaction products) depending on the type of metal must be explained based on the characteristics of the different metal catalysts.
7. Which are the active sites responsible of the hydrolysis of cellobiose?
8. Is it possible to reuse catalysts? This is an important advantage of heterogeneous catalysts.
9. It would be very useful to include information on the physical-chemical characteristics of used catalysts, to identify possible modifications with respect to fresh catalysts.
10. The results obtained should be compared with those published in the literature on oxidation of cellobiose and glucose
Therefore, all these issues must be clarified before recommending this manuscript for publication.
Author Response
We are grateful to the reviewers for their active participation in improving the quality of our manuscript.
- Why is a mixture of solvents (tetrahydrofuran, methanol and water) used for the impregnation of the support with the metal precursors?
Answer: Thanks for the clarifying question. The composition of this mixture was selected by us experimentally in previous studies. The use of a mixture of tetrahydrofuran-methanol-water is necessary to improve the process of impregnation with a solution of a precursor of hypercrosslinked polystyrene, which has a hydrophobic character.
Necessary clarifications are added to the text of the manuscript.
- The expressions used for the calculation of conversion and yields should be indicated.
Answer: Thank you for your comment. The formulas used to calculate the conversion and yield of products have been added to the text of the manuscript.
- The characterization of metal particles requires additional techniques such as H2 chemisorption, or XRD for the determination of average particle sizes. SEM could only provide information of a reduced section of the studied solid. Metallic properties, such as metal surface area, reduction degree and metal particle sizes should be evaluated.
Answer: Thank you for your comment. We have added some new data to the text of the manuscript, including the results of hydrogen chemisorption. Unfortunately, we do not have the opportunity to carry out additional studies of catalysts in the near future.
- The decrease in the specific surface area (Table 2), with respect to that of the support (HPS), is explained by the blockage of the pores by the metallic particles. Is it possible that the support has suffered some deterioration with the thermal treatment at 300 ºC with hydrogen, used for the preparation of the metallic catalysts? In this sense, it is necessary to demonstrate the thermal stability of the support.
Answer: Thank you for your comment. The heat resistance of HPS MN 270 was evaluated by us by thermogravimetry. It is shown that the polymer in an oxygen-free atmosphere is stable and does not undergo any changes up to a temperature of 400 °C.
These data have been added to the text of the manuscript.
- How do the authors explain the large diameter of the gold nanoparticles (32.1 nm) compared to the other metal catalysts, when they are quite often very small?
Answer: Thanks for the question. We assume that the larger size of gold nanoparticles is probably due to the nature of the precursor, interaction with the hydrophobic polymer matrix of the carrier and the tendency of the particles to aggregation. It should be noted that, using a similar procedure for the synthesis of the 0.5%-PdAu/HPS-R catalyst, our colleagues also obtained large gold particles (19.3 ± 8.7 nm) (Nikoshvili, L.Z.; Shkerina, K.N.; Bykov, A.V.; Sidorov, A.I.; Vasiliev, A.L. ; Sulman, M. G.; Kiwi-Minsker, L. Mono- and Bimetallic Nanoparticles Stabilized by an Aromatic Polymeric Network for a Suzuki Cross-Coupling Reaction. Nanomaterials 2022, 12, 94. https://doi.org/10.3390/nano12010094.) In the study [26], the size of gold particles was also relatively large (10.9 nm). Since our catalyst contains much more gold (3%), it seems that the particles have acquired a larger size.
We indicated in the text of the manuscript the tendency of gold nanoparticles to aggregate.
- The different catalytic behavior (cellobiose conversion and selectivity to the different reaction products) depending on the type of metal must be explained based on the characteristics of the different metal catalysts.
Answer: Thank you for your comment. According to hydrogen chemisorption data, the platinum catalyst is characterized by a significant amount of active sites on the surface. With this fact, we attribute the higher activity of this catalyst in the hydrolytic oxidation of cellobiose.
New data added to the text of the manuscript.
- Which are the active sites responsible of the hydrolysis of cellobiose?
Answer: Thanks for the question. Experience without a catalyst has shown that the reaction does not go. The catalyst plays the main role in the cellobiose hydrolysis reaction, and the degree of hydrolysis obviously depends on the nature of the metal that is part of the catalyst.
We have added this information to the text of the manuscript.
- Is it possible to reuse catalysts? This is an important advantage of heterogeneous catalysts.
Answer: Thanks for the question. The stability of the catalyst was examined in four successive experiments. The findings are reported in the Supplementary Materials (Figure S1) and discussed in the text of the manuscript at the end of the Results section.
- It would be very useful to include information on the physical-chemical characteristics of used catalysts, to identify possible modifications with respect to fresh catalysts.
Answer: Thanks for the question. In the Supplementary Materials section (Table S1, Figure S2), we present the results of spent catalyst studies. A discussion of these results is provided in the text of the manuscript at the end of the Results section.
- The results obtained should be compared with those published in the literature on oxidation of cellobiose and glucose.
Answer: Thank you for your comment. A literature review showed that there are few studies on the process of hydrolytic oxidation of cellobiose. In the works that we found (references 26 and 27 in the Introduction section), the main product is gluconic acid, while in our case it is glucaric acid, which the authors of these studies did not find in the mixture of reaction products. Therefore, it is somewhat difficult to compare the results objectively. However, we present the exact data of other authors, as well as the conditions under which they were obtained, in order to be able to correctly compare the results.
Therefore, all these issues must be clarified before recommending this manuscript for publication.
We have made every effort to answer questions.
Thank you!
Reviewer 2 Report
The revisions suggested for the publication of the article are as follows:
- The abstract should be rewritten in more detail with more emphasis on the results obtained. It is also necessary to define the abbreviation MN270 being a separate text from the article
- - Line 32: "However, the use of these methods in large-scale production is limited due to the harmful effects on the environment". What are these methods? Which kind of catalyst are involved? In necessary to develop this point more in detail, by introducing the several critical aspects of each technology proposed.
- Line 102: "Further, the catalyst was dried at 70 °С, consecutively treated with hot (80 °С) solutions of NaOH and Н2О2." Why the authors used this reagent mixture to wash the catalyst respect to the use of distilled water?
- Line 183: Figure 2. It it necessary to convert this figure in a Table in order to highlight the conversion and the product obtained. How much is the selectivity of the processes? If less than 100%, what are the secondary products which are formed?
- Line 183: What is the reason of the different reactivity of the metal tested? In which matter are involved in the mechanism of reaction proposed in Figure 6? In which matter the metal are connected on the surface of the support?
- Conclusion: A critical point in the use of this catalyst is the cost incurred for their synthesis and the possibilty of recovery and reuse. Have the authors assessed these aspects?
Author Response
We are grateful to the reviewers for their active participation in improving the quality of our manuscript.
The revisions suggested for the publication of the article are as follows:
- The abstract should be rewritten in more detail with more emphasis on the results obtained. It is also necessary to define the abbreviation MN270 being a separate text from the article
Answer: Thank you for your comment. We have completed the abstract and added the corresponding description of the HPS MN270 to the Materials section.
- Line 32: "However, the use of these methods in large-scale production is limited due to the harmful effects on the environment". What are these methods? Which kind of catalyst are involved? In necessary to develop this point more in detail, by introducing the several critical aspects of each technology proposed.
Answer: Thank you for your comment. We have updated the Introduction section with more detailed information on methods for obtaining gluconic acid.
- Line 102: "Further, the catalyst was dried at 70 °С, consecutively treated with hot (80 °С) solutions of NaOH and Н2О2." Why the authors used this reagent mixture to wash the catalyst respect to the use of distilled water?
Answer: Thanks for the question. We have added a correct description of the methodology for the synthesis of catalysts. Treatment with hot solutions of NaOH and H2O2 is used only in the synthesis of Ru-containing catalyst for the precipitation of ruthenium (IV) in the form of ruthenium oxide on the polymer surface.
- Line 183: Figure 2. It it necessary to convert this figure in a Table in order to highlight the conversion and the product obtained. How much is the selectivity of the processes? If less than 100%, what are the secondary products which are formed?
Answer: Thank you for your comment. We converted the diagram into a table (Table 4), where we showed the dependence of cellobiose conversion and selectivity for the main products on the nature of the metal of the active phase. Unfortunately, the quantitative and qualitative determination of by-products by the analysis method used by us is difficult. Therefore, we can only quantitatively evaluate the selectivity for by-products, and qualitatively - on the basis of studies in which such a reaction was studied [Ref. 26].
- Line 183: What is the reason of the different reactivity of the metal tested? In which matter are involved in the mechanism of reaction proposed in Figure 6? In which matter the metal are connected on the surface of the support?
Answer: Thank you for your comment. We tried to explain the different reactivity of metals by relating it to the results of characterization of catalysts (results of hydrogen chemisorption, particle size) and included appropriate discussions in the text of the manuscript. The figure containing the reaction scheme shows only the main products that we have characterized qualitatively and quantitatively. It is quite difficult to estimate the composition and amount of secondary products, as well as their participation in the proposed scheme of transformations.
In the work of our colleagues (Bykov, A.V.; Demidenko, G.N.; Nikoshvili, L.Z.; Kiwi-Minsker, L. Hyper-Cross-Linked Polystyrene as a Stabilizing Medium for Small Metal Clusters. Molecules 2021, 26, 5294. https://doi .org/10.3390/molecules26175294) it was shown that for very small clusters of Pt and Pd containing 4 and 9 atoms) a sufficiently strong interaction with benzene rings of HPS is possible, which can even deform the cluster. For much larger particles, which are discussed in our work, such interactions can probably also take place at the points of metal-polymer contact, however, we believe that the nanocluster-carrier interaction is largely mechanical in nature. Nanoclusters are held on the surface due to the corresponding pore sizes of the polymer.
- Conclusion: A critical point in the use of this catalyst is the cost incurred for their synthesis and the possibility of recovery and reuse. Have the authors assessed these aspects?
Answer: Thanks for the question. We have not evaluated the economic aspects of the hydrolytic oxidation of cellobiose using catalysts based on hypercrosslinked polystyrene. We present data from a 2020 study in the Introduction section, which states that both homogeneous oxidation of glucose with nitric acid and oxidation with oxygen in the presence of a catalyst can be economically viable for industrial applications - the cost of 1 kg of product is 2.91 and 2.53 $ for homogeneous and heterogeneous oxidation, respectively. But the process using heterogeneous catalysts has a 22% lower environmental impact.
The possibility of repeated use of the catalyst was evaluated by us, the data obtained are given at the end of the Results and Supplementary Materials section.
We have made every effort to answer questions.
Thank you!
Round 2
Reviewer 1 Report
The authors have made an important effort to adequately answer the questions raised in the revision process. In most of cases, this has allowed to improve and clarify the information provided in the manuscript, and accordingly with the other reviewers, I could recommend its publication
Reviewer 2 Report
The authors responded exhaustively to reviewers' comments therefore publication of the article is recommended
