Selective Hydrogenation of 5-Acetoxymethylfurfural over Cu-Based Catalysts in a Flow Reactor: Effect of Cu-Al Layered Double Hydroxides Synthesis Conditions on Catalytic Properties

Round 1

Reviewer 1 Report

Reviewer Report     

The current manuscript entitled “Selective hydrogenation of 5-acetoxymethylfurfural over Cu-based catalysts in a flow reactor: Effect of Cu-Al layered double hydroxides synthesis conditions on catalytic properties studied the effect of synthesis conditions such as pH, temperature, ageing time and precipitation rate of the Cu-Al layered double hydroxides on the formation of Cu-based materials as well as catalytic properties of the Cu-Al mixed oxides in flow hydrogenation of 5-acetoxymethylfurfural (AMF) to 5-(acetoxymethyl)-2-furanmethanol (AMFM). It was shown that the synthesis conditions influence the dispersion of the copper particles, which is preferential for hydrogenation reactions. The subject is of particular importance from the practical point of view and toward the rational design of catalysts. However, the following questions need to be addressed before being accepted for publication.   

The detailed revisions are as follows.

 

1. Since the manuscript is about optimising the synthesis conditions, the authors should briefly write the synthesis procedure of the materials with appropriate references. 
2. There is not enough information on copper dispersion. The XPS analysis alone cannot explain the copper dispersion, and Cu particle size distribution is an important parameter to discuss. 
3. What is the role of support Al₂O₃ for this reaction? Why do authors choose Alumina over other supports? 
4. The Cu 2p appeared at 934.8 and 932.6 eV, assigned to the CuO and Cu. However, CuO binding energy usually appears at 933.5 +/0.2. The 934.8 eV might be due to Cu(OH)2 phase, and the author must correct it. 
5. Optimal synthesis conditions favoured a higher dispersion of Cu particles. What were the dispersion patterns of CuOx on these supports? Was it atomically dispersed or clusters or particles? Microscopic images would be better to know the status of Cu.
6. The obtained catalytic results provide an insight into the conversion performance of as-prepared samples. No accurate conclusions were obtained about the intrinsic activity of the samples, and additional characterizations (like TEM) are required to support the conclusions. 
7. Are there any leftover Na impurities in the catalysts after washing? Sometimes these impurities can influence the dispersion of the copper.
8. How did the authors calculate the flow rate of the AMF? The authors should give the details in the catalytic test section.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

 

The topic of the reviewed paper entitled “Selective hydrogenation of 5-acetoxymethylfurfural over Cu-based catalysts in a flow reactor: Effect of Cu-Al layered double hydroxides synthesis conditions on catalytic properties” (Manuscript ID catalysts-1814769) by Marina V. Bukhtiyarova, Olga A. Bulavchenko, Andrey V. Bukhtiyarov, Alexey L. Nuzhdin, and Galina A. Bukhtiyarova is related to the optimized synthesis of Cu-Al layered double hydroxides and their structural, textural and functional characterization emphasizing the role of main synthetic variables such as: pH, precipitation temperature, aging time and precipitation rate. Authors’ conclusions inferred from the results of characterization are confronted with the results of catalytic tests in selective hydrogenation of 5-acetoxymethylfurfural to 5-(acetoxymethyl)-2-furanmethanol as the main product. The article is generally well suited for Catalysts and after major correction may appear in this journal. The manuscript is clear and relatively well written, there are however some points, listed below, which should be considered by the Authors prior to publication.

 

General remarks:

1. Influence of synthetic variables on the nature and speciation of surface sites, which might be active in the selective hydrogenation of AMF is strongly recommended. Authors’ considerations on textural properties and particle dispersion in relation to catalytic activity are simply insufficient to elucidate the effect of synthetic variables.

2. It is unclear why calcination temperature is not taken into account as one more parameter influencing catalytic properties of the investigated LDH materials.

3. Some concluding remarks are simply obvious and can be proposed even without any research done, e.g. “(…) several parameters determine catalytic activity of materials on the base of layered double hydroxides” (p. 11, lines 341-342); “(…) synthesis conditions influence on the particle size of the hydrotalcite phase (p. 12, lines 415-416), “(…) the synthesis conditions have influence on reducibility and dispersion of Cu metal particles” (p. 13, lines 419-420).

4. Discussion of the subsequent paragraphs describing the obtaind results should directly follow them. Some results are not interpreted at all.

5. Aspect of novelty should be substantially strengthened. It is difficult to say basing on the text, what’s the original contribution of the Authors to elucidate the role of synthetic parameters of LDH, which potentially may determine catalytic properties of these materials.

6. As the Authors remarked that sometimes they have difficulties with identification of some phases in the XRD patterns, Raman spectra could be helpful in that respect.

Minor remarks:

1. Rietveld refinement parameters should also be reported in the text;

2. Authors’ choice of synthetic parameters for a reference sample should be justified somehow;

3. the term layered binary hydroxides would perhaps be more adequate than layered double hydroxides used in the text ;

4. the term theoretical in Table 2 should be replaced with nominal and calculated with determined by AAS;

5. the Authors inform a reader about possible phase transformation of LDH without any specific information explaining what sort of transformation they mean (p. 3, line 119);

6. XRD cannot be considered as a technique confirming TG and DSC data;

7. it may potentially be not entirely clear for a reader what for stands T in Table 1 and what kind of temperature is meant in p. 2, line 89;

8. the curve for the sample aged for 4 hours is missing in Fig. 2;

9. XRD pattern collected for the sample calcined at 650°C is missing;

 

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This article applies LDHs for hydrogenation of 5-acetoxymethylfurfural to 5-(acetoxymethyl)-2-furanmethanol. The effects of preparation conditions on the dispersion and catalytic activity of copper were investigated. Solutions and related suggestions are provided for the preparation of high-quality CuAl-LDHs. Some suggestions are as follows:

1)     The background section should be simplified.

2)     The grain size calculated by XRD should preferably be accompanied by the corresponding electron microscope images.

3)     Orthogonal experimental design is recommended when exploring the effects of multiple factors on catalytic performance.

4)     Catalyst labeling for different preparation conditions needs to be clearer;

5)     The advantages of layered double hydroxides precursors versus other precursors are not stated;

6)     Obviously, the active site of the catalyst is Cu⁰ species, and the goal of exploration is to obtain better dispersed Cu⁰ species. Therefore, the focus of characterization and analysis should be on the dispersion of Cu⁰. It is recommended to use TEM-Mapping or STEM-HAADF to analyze the dispersion of Cu.

The main problem of the article is that the focus of analysis and characterization is not focused enough. It is suggested to add discussion on Cu dispersibility. Considering the above factors, this paper needs major revisions.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors’ responses and revisions have satisfactorily addressed my comments on the earlier version of the manuscript.  I support the publication of this version and look forward to reading the follow-up studies that utilize this work.

Author Response

Authors are thankful for the high evaluation of the work done