Anchoring Cu Species over SiO2 for Hydrogenation of Dimethyl Oxalate to Ethylene Glycol
, Yiming Lu 1, Juhua Xu 2, Liming He 2, Dan Meng 1,*, Guosheng Wang 1, Jian Qi 3,4,* and Quan Jin 2,*Round 1
Reviewer 1 Report
The authors of the presented manuscript have studied the preparation of Cu/SiO2 catalysts
through pyrolysis of Cu-containing MOFs, supported on as-prepared and surface modified silica microspheres. Then, the obtained materials were characterized by means of several methods (XRD, TEM, XPS, FT-IR, H2-TPR and NH3-TPD) and catalytic properties of materials were compared in the hydrogenation of dimethyl oxalate to ethylene glycol. The presented results may be of interest to researchers dealing with the preparation and investigation of Cu-based catalysts, as the stability of copper particles on the surface of supports is indeed very topical challenge. The proposed approach looks promising, but the article contains a number of contradictions and unproven statements, therefore, it can be accepted for publication after significant changes and clarifications.
Critical comments are listed below.
General comments.
1). Authors claimed in abstract and further in the text, that “the developed method opened another avenue to synthesize highly dispersed and stable Cu-based catalysts”. However, the article does not present the results of stability test in catalytic reaction and the results of catalysts investigation after the reaction, which could confirm this statement.
2). Designations “MOF-CMS”, “MOF-CS” and “CS” are assigned to catalysts calcined at 450°C under air (see 2.2-2.4). The same designations are used in the titles of figures 1-4. Were the samples reduced before the study by various methods, and if reduced, then under what conditions? It is known that copper is easily oxidized in air under ambient conditions. Were any measures taken to prevent oxidation of the reduced samples prior to their examination?
3). The conclusion of the authors about the interaction “between the copper species and the carrier” is not confirmed by experimental data.
4). The XPS data contradict the results of the study of catalysts by XRD methods. According to XPES, copper is present on the catalyst surface in the state, while XPA data indicate the presence of a copper oxide phase (CO). Please resolve this conflict.
The above comments refer to several sections, starting from the abstract and ending with the conclusions, so these comments should be taken into account when correcting the entire article.
Besides, several critical comments, related to specific sections, are listed below.
Abstract. The phrase “While a traditional Cu/SiO2 catalyst exhibited serious agglomeration of Cu active sites and low catalytic activity (DMO conversion of 86.9 % and EG selectivity of 46.6 %)” was not supported with the experimental results, since no data on agglomeration are provided. And why do authors consider CS catalyst as traditional one?
2. Materials and methods.
2.6.
- The flow rate of hydrogen is not indicated.
- What does the set of letters WLHSV mean? May be, WHSV or LHSV should be used?
- It would be better to explain, whether the liquid flow is normalized by the weight of the catalyst or by the weight of copper.
3. Results
- In SI data are given on the content and dispersion of copper in various samples (table S3), but these data are not discussed in any way in the article.
- The use of H2-TPR and NH3-TPD was claimed in 2.5, but the results were not shown.
3.1.
- Why TEM images on 1d, 1e and 1f pictures are shown at different scales?
- The authors claimed, that “the MOF-CS shows a sharper characteristic peak of CuO than MOF-CmS, which may be caused by CuO existing in two ways, i.e., free state and fixed state with silicon”. This statement needs in more detail explanation of difference between “free” and “fixed” states of CuO.
- The XPS data should be revised and discussed in comparison with the XRD results.
3.2.
- The LHSV value (0.82 h-1) should be clarified (0.82 g DO/g of cat in hour, 0.82 g DO/g of Сu in hour, or something else), and the “relationship between the catalytic performance and the internal structure of the catalyst” should be discussed with the data on Cu content and dispersion.
- The data on material balance and stability of catalytic properties were not provided.
4. Discussion
The authors said in introduction, that “Cu/SiO2 catalysts show excellent catalytic activity and selectivity in DMO-to-EG reaction owing to the active for the hydro-genation of the C-O bonds and relatively inactive for the hydrogenation of the C-C bonds [8-12]”; “Ye et al.[22]reported the preparation of Cu/SiO2-MOF catalyst by the sol-gel method, the highly dispersed Cu nanoparticles formed after calcination have good activity for DMO-to-EG reaction”. Please compare the performance of the proposed catalyst with the known literature data and reconsider the text of Discussion taking in mind my “General comments”.
English language needs improvement, below are some failed examples:
Page 4: No characteristic peaks of CuO were found in the CS sample, which indicated that the low concentration of CuO or existed in amorphous Cu species.
Page 5: The other is CuO, which can be seen from the bands at 580, 500, and 460 cm-1 cannot be observed due to the existence of a broad band at 471 cm-1.
Page 6: Sentence “And a shakeup satellite peak at around 938.7~945.8 eV” does not contain a predicate (verb).
Comments for author File: 
Comments.doc
Author Response
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Author Response.pdf
Reviewer 2 Report
In this work, the authors studied the HKUST-1 MOF with Cu/SiO2 catalysts for the hydrogenation of dimethyl oxalate to ethylene glycol. The catalysts were synthesized and characterized with several techniques. The catalysts provided high yield conversion and selectivity. The article has been well performed in a thorough scientific data. I have some comments:
- The N2 adsorption-desorption isotherms and total specific surface area and pore volume with BET measurement should be reported.
- The authors should illustrate the reaction mechanism with MOF-CmS in discussion part.
- How about the catalytic performance for the hydrogenation reaction of pure HKUST-1 or SiO2?
- It might be useful if the recyclability test of this catalysts are reported.
-The related publication should be added:
“Cu/SiO2 hybrid catalysts containing HZSM-5 with enhanced activity and stability for selective hydrogenation of dimethyl oxalate to ethylene glycol” Applied Catalysis A: General, 2012, Pages 287-296.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I am completely satisfied with the answers of the authors
