Effects of Basic Promoters on the Catalytic Performance of Cu/SiO₂ in the Hydrogenation of Dimethyl Maleate

Round 1

Reviewer 1 Report

The authors have made significant efforts on improving the manuscript during the last two revision so I recommend this article for publication.

Author Response

Dear Reviewer:

Thank you for your advice concerning our manuscript entitled “Effects of basic promoters on the catalytic performance of Cu/SiO₂ in the hydrogenation of dimethyl maleate”. Those comments are extremely invaluable and very helpful for revising and improving our paper.

Best wishes.

Reviewer 2 Report

This manuscript describes the hydrogenation of dimethyl maleate over base-modified Cu catalysts. The catalysts were characterized; however, the reaction results are too few to discuss the selectivity. I do not feel that the data are sufficient to support the main conclusions. Extensive revision is necessary...and I think that the revision requires more time to the standard revision.

(1) The reaction data with different contact times are necessary to discuss the difference of selectivity. Selectivity must be compared at similar conversion level.

(2) Deactivation by sintering is a frequently encountered problem in Cu catalysts for hydrogenation. The change of performance during time on stream should be provided for representative catalyst.

(3) "Dehydrogenation of BDO to GBL during the hydrogenation" has no evidence. Rather, GBL is an intermediate of BDO, as shown in Scheme 1. Is the equilibrium level of GBL/BDO ratio indeed so high at this H2 rich condition? The presence of GBL is simply explained by the lower level of reaction progress.

(4) The reduction degree of catalysts should be confirmed by H2 consumption amount in TPR (and XPS/XAES without exposure to air, if possible). If the Cu species are not totally reduced, the absence of XRD peak for Cu does not necessarily mean an increase of the active Cu species. Titration studies using N2O are usually carried out.

(5) The peak position in XRD is sometimes too fluctuated. Does this mean the formation of alloy?? If this means the experimental error, the quality is not acceptable.

(6) The material balance in the reaction results should be commented. The fate of methoxy group (methanol or methane) should be also commented.

Author Response

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Reviewer 3 Report

 

In this study, Cu-M/SiO2 catalysts modified by basic promoters (M = Mg, Ca, Sr, Ba, La) were prepared and their physical properties were characterized. Some experimental results are interesting to other researchers in related fields. I recommend this paper can be accepted after revisions. The following comments, however, should be addressed before publication.

 

1.     Page 1, line 27-35. Some latest research results should be mentioned and cited in the section of introduction instead of outdated or earlier papers.

2.     Page 2, line 80-88. (a) Do you evaluate the size of catalysts? (b) I suggest the micrographs of those catalysts should be presented in this work.

3.     Page 3, line 93-99. The crystallite size is related to the full width at half maximum (FWHM) of the diffraction peak not intensity or shape of the diffraction signal.

4.     Page 3, line 101. The authors report that suggesting the Cu particles were too small to detect by in situ XRD. Please cite references or provide experimental results to improve the readability.

5.     Figure 2. It was noted that the reduction peak of Cu-Mg/SiO2 (spectrum (b)) was broader that other peaks. What are the reasons or mechanisms?

6.     (a) Caption of the Table 2 (page 5, line 153). “Cu/SiO2 and Cu/SiO2 catalysts” should be modified to “Cu/SiO2 and Cu-M/SiO2 catalysts”. (b) The performance of catalysts does not compare with other works.

Author Response

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Round 2

Reviewer 2 Report

Many of my comments are poorly resolved. Further revision is necessary.

Comment/reply (1): The data for entry 3, Table R1 (LHSV 0.9 h-1, Conv. 83.65%) should be added to Table 2 in the main text. Explanation such as "The higher selectivity of Cu/SiO2 to THF than other catalysts was confirmed by the reaction test at shorter contact time to set similar conversion level to other catalysts." should be added to the main text.

Comment/reply (2): The stability of Cu-La/SiO2 is a very attractive result. The data (Table R2) should be added to this manuscript.

Comment/reply (3): The reply does not resolve the problem. In the conclusion section, the authors claimed that "the formation of GBL from BDO under the effect of the basic additives is one of the reasons for the reduce of the selectivity of THF". The formation of BDO from GBL is a reversible reaction, and the catalyst that can promote the formation of BDO should also have activity in the reverse reaction. As described in Scheme 1, GBL and BDO are intermediates of THF. Therefore, catalysts that have high selectivity to THF should have also high activity in GBL to BDO reversible reaction. The claim is not valid in physical chemistry. Rather, low activity in GBL to BDO reaction (which also means low activity in BDO to GBL reaction) can be a reason for the higher selectivity to GBL.

Comment/reply (4): Show the total H2 consumption amounts in TPR studies. It should be confirmed that the H2 consumption amount actually corresponded to the Cu amount (CuO + H2 -> Cu + H2O).

Comment/reply (5): I realize that the fluctuation in the peak position existed in the in situ XRD. This point should be mentioned in the experimental section such as "Because of the in situ measurement system, the peak positions had larger errors than the standard XRD patterns."
I advise that addition of internal standard such as silicon powder is beneficial to correct the errors induced by the in situ system.

Comment/reply (6): The authors did not answer my comment. As described in Scheme 1, two equivalent of methanol should be formed with formation of GBL, BDO or THF. Was the methanol actually detected, or detected as methane by over-hydrogenolysis?

 

Author Response

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Reviewer 3 Report

The authors addressed and answered the reviewers’ comments.

Thus, I recommend this revised manuscript can be accepted for publication.

Author Response

Dear Reviewer:

Thank you for your advice concerning our manuscript entitled “Effects of basic promoters on the catalytic performance of Cu/SiO₂ in the hydrogenation of dimethyl maleate”. Those comments are extremely invaluable and very helpful for revising and improving our paper.

Best wishes.

Round 3

Reviewer 2 Report

Comment/reply (3):
As I commented in the previous version, the explanation that the formation of GBL from BDO affected the high GBL selectivity cannot be accepted. The authors admitted that details in the reasons have not yet been clarified. I suggest revising the lines 171-176 into simple pointing out the possible effect of base of acid site on dehydration of BDO:

"The dehydrogenation of BDO to GBL on the basic active site after adding basic promoter to Cu/SiO2, and the similar result was also reported in the study of literature[34]. There were two main reasons for the reduce the selectivity of THF: the first was the formation of GBL from BDO under the effect of the basic additives, and the second was the covering of the part of the acid sites on the surface of SiO2 by the basic additives leading to the weakening of the dehydration of BDO to THF under acidic conditions."

into

"One possible reason for the low selectivity of THF over modified catalysts was the covering of the part of the acid sites on the surface of SiO2 by the basic additives leading to the weakening of the dehydration of BDO to THF under acidic conditions."

Comment/reply (4)
The authors' reply does not justify the ignorance of total H2 consumption amount. Add the H2 consumption amount to Fig. 2.

Comment/reply (6)
Surely methanol product could not be quantified; however, detection of methane is easy and reliable. I recommend the comment on the absence or presence of methane, for example, "Methane was not detected in the reaction products, and therefore the methoxy groups of DMM were probably converted to methanol. However, quantification of methanol was not possible because methanol was used as solvent."

 

Author Response

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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

For Editor and Authors

This paper described that the hydrogenation of dimethyl maleate using Cu/SiO2 catalyst at 300 ˚C to give the hydrogenated product (THF) with high selectivity.  The authors also reported the preparation method of catalyst and determination of structure of it. However, in this manuscript was described that the just selectivity of the hydrogenated products (GBL, BDO, THF) for the hydrogenation in this presented method. The authors have to show the % yield of product, mol% of catalyst, leaching of Cu to organic layer (contamination). 

Therefore, I think that the presented results can not support the authors idea and hypothesis enough. Therefore, I can recommend publication in catalysts in this time and this manuscript needs some modification before submitting. 

In line 39, 59 , 61, 193; Please revise "P^t", "stabilit^y", "He et al", "SiO2"  to "Pt^", "stability^", "They", "SiO₂".

That it.

Author Response

Dear Reviewer:

Thank you for your advice concerning our manuscript entitled “Effects of basic promoters on the catalytic performance of Cu/SiO₂ in the hydrogenation of dimethyl maleate”(ID: catalysts-469773). Those comments are extremely invaluable and very helpful for revising and improving our paper. Comments have been studied carefully and corresponding corrections have been made and we hope that it could meet with approval.We are so sorry for our negligence about "Pt", "stability" et al. Revised portion are marked in red in the paper. Moreover, by gas chromatograph analyzing, the feedstock conversion, products selectivity, and the yield of GBL, BDO, THF were obtained. The amount of catalyst used in fixed-bed reactor was expressed by LSV 0.3h^-1. The collected liquid-phase organic products were analyzed by ICP-mass, and no Cu element was detected, indicating that there was no significant loss of Cu on the catalyst.

Finally, special thanks to you for your good comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript (Effects of basic promoters on the catalytic performance of Cu/SiO2 in the hydrogenation of dimethyl maleate) reports the influence of basic promoters (Mg, Ca, Sr, Ba, La) of Cu active phase supported on SiO₂ on the hydrogenation of dimethyl maleate. However the paper is not well written, many sentences are not well explained. The authors should consider the following points:

1.       I see important problems concerning the textural characterization of the samples. The presented data show that textural properties of catalysts (X-ray diffraction measurements, BET, CO2-TPD) was examined for catalysts after calcination in air at 723K, 4h - Experimental Section. In the text “Hydrogenation reaction” the authors write "The catalyst … was loaded into the middle of the constant temperature zone, and activated at 573K with H2 (30 ml/min) for 240 min”. Why textural characterization of the catalysts were not carried out for reduced samples? Active in hydrogenation reaction is reduced catalyst.

2.       Experimental Section – what does it mean – “followed with drying at 393 K for 8 480 min” – line 84?

3.       What is the real metal loading? Any ICP or XRF results concerning the practical composition of the prepared samples?

4.       Did the authors calculate the degree of reduction of catalysts – H₂-TPR?

5.       How was the diffusion effect excluded during the catalytic test - hydrogenation reaction?

6.      Did the authors calculate the carbon balance between feed and product? Any carbon deposition was observed on catalysts after the catalytic test?

7.     How about the stability of the catalytic performance of the catalysts? How long was the reaction conducted?

8.       In conclusions the authors write “On the contrary, adding La promoter reduced the CuO particle for a Cu-O-La bond was formed in the interface between LaOx and Cu to improve the dispersion of CuO on SiO2.” – line 223 - what is the evidence justifying this conclusion? How did authors calculate the dispersion/particle size of Cu?

9.     Have the authors compare the performance of catalysts in this work with the catalytic activity of other catalysts reported in literatures?

10.   Some mistakes/errors can be found along the text. For example:

- lines 85 and 89 it should be "… calcination…" instead of "… baking…"

Author Response

Dear Reviewer:

Thank you for your advice concerning our manuscript entitled “Effects of basic promoters on the catalytic performance of Cu/SiO2 in the hydrogenation of dimethyl maleate”(ID: catalysts-469773). Those comments are extremely invaluable and very helpful for revising and improving our paper. Comments have been studied carefully and corresponding corrections have been made and we hope that it could meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as follows:

 

Point 1: I see important problems concerning the textural characterization of the samples. The presented data show that textural properties of catalysts (X-ray diffraction measurements, BET, CO₂-TPD) was examined for catalysts after calcination in air at 723K, 4h - Experimental Section. In the text “Hydrogenation reaction” the authors write "The catalyst … was loaded into the middle of the constant temperature zone, and activated at 573K with H₂ (30 ml/min) for 240 min”. Why textural characterization of the catalysts were not carried out for reduced samples? Active in hydrogenation reaction is reduced catalyst. 


 

Response 1: Cu in reduced samples is easily oxidized to CuO when exposed to air. The samples were calcined at 723K and reduced at 573K. The reduction temperature is lower than the calcination temperature. XRD is used to detect the dispersion of CuO. As the reduction temperature is lower than the calcination temperature, it was found that the dispersion of CuO and Cu in the Cu/SiO₂ catalyst before and after reduction is similar in our previous work^[1]. BET is used to detect the pore structure of the catalyst. Whether it has been reduced or not has little effect on the results. The purpose of CO₂-TPD is to explore the alkaline change of catalyst surface after adding alkaline promoter, and the effect of reduction process is very small. Therefore, in this paper, we can draw this conclusion by using pre-reduced catalysts.

 

Point 2: Experimental Section – what does it mean – “followed with drying at 393 K for 8 480 min” – line 84?

 

Response 2: We are very sorry for our incorrect writing"8 480 min". Correction is made as"480min".

 

Point 3: What is the real metal loading? Any ICP or XRF results concerning the practical composition of the prepared samples?

 

Response 3: Considering your suggestion, we have add the results about metal loading by using XRF in the table1 of the article.

 

Point 4: Did the authors calculate the degree of reduction of catalysts – H₂-TPR?

 

Response 4: Because the areas of consumption peaks of samples are close in H₂-TPR results, we consider that the reducibility of six samples is also close. According to the author's previous results, there is no Cu²⁺, only Cu⁺ and Cu⁰ in the reduction of Cu/SiO₂ catalyst, and the Cu⁺/Cu⁰=1.53 can be determined by the Cu LMM XAES spectra.^[1]

 

Point 5: How was the diffusion effect excluded during the catalytic test - hydrogenation reaction?

 

Response 5: The effects of particle size of Cu/SiO₂ Catalyst on the reaction rate were investigated by selecting Catalysts with particle size of 10-20 mesh, 20-40 mesh and 40-100 mesh respectively. The results of the catalytic reaction are shown in the table1. When the catalyst particle size is 10-20 mesh, the raw material can not be completely converted to THF; when the catalyst particle size is less than 20-40 mesh, the conversion of raw material has reached 100%, indicating that the effect of internal diffusion on the reaction performance can be eliminated. Moreover the effect of H₂/DMM when the liquid velocity is 0.3h^-1 and liquid velocity space are investigated by table2 and table3 respectively.

Table 1. The influence of catalyst particle size on catalytic performance

Catalyst particle size (mesh)	DMS conversion (%)	Selectivity (%)
		GBL	BDO	THF
10-20	81.24	0	0	100
20-40	100	0	0	100
40-100	100	0	0	100

Reaction conditions：T= 513 K, p= 5 MPa, LHSV= 0.3 h^-1 and H₂/ester = 50

 

Table 2. The influence of H₂/DMM (molar ratio) on catalytic performance

H2/DMM	DMS Conversion (%)	Selectivity (%)
		GBL	BDO	THF
25	75.78	0	0	100
50	100	0	0	100
100	100	0	0	100

Reaction conditions：T= 513 K, p= 5 MPa, LHSV= 0.3 h^-1

 

Table 3. The influence of space velocity on the catalytic performance

LHSV (h-1)	DMS Conversion (%)	Selectivity (%)
		GBL	BDO	THF
0.3	100	0	0	100
0.6	100	0	0	100
0.9	83.65	0	0	100

Reaction conditions：T= 513 K, p= 5 MPa, H₂/DMM = 50

Therefore, we choose particle size 20-40mesh, liquid velocity space 0.3h^-1 and H₂/DMM 50 as our reaction condition.

 

Point 6: Did the authors calculate the carbon balance between feed and product? Any carbon deposition was observed on catalysts after the catalytic test?

 

Response 6: There were some complex inlet and outlet pipelines in the fixed bed reactor, so some raw materials and products remained in the pipelines after the reaction. As a result, the carbon balance between feed and product was not easy to calculate accurately. The activity of the catalyst did not change significantly after 100h stability test. On this basis, we believe that there is no obvious phenomenon of carbon deposition. Therefore, we did not calculate the carbon balance.

 

Point 7: How about the stability of the catalytic performance of the catalysts? How long was the reaction conducted?

 

Response 7: We tested Cu-La/SiO₂ catalyst for 100h. During stability  experiment, our catalyst remained stable. The longer-term stability evalution will be examined in the follow-up work.

Table 4. The influence of space velocity on the catalytic performance

Reaction time （h）	DMS Conversion (%)	Selectivity (%)
		GBL	BDO	THF
0	100	24.54	4.29	71.17
5	100	25.12	4.04	70.84
10	100	24.98	4.16	70.86
15	100	24.67	4.28	71.05
20	100	24.38	4.35	71.27
25	100	24.87	4.31	70.82
30	100	25.22	4.17	70.61
35	100	25.16	4.42	70.42
40	100	24.45	4.55	71
45	100	24.23	4.26	71.51
50	100	24.79	4.13	71.08
55	100	25.72	4.59	69.69
60	100	25.48	4.26	70.26
65	100	25.56	4.37	70.07
70	100	24.94	4.14	70.92
75	100	24.42	4.3	71.28
80	100	24.1	4.17	71.73
85	100	23.87	4.46	71.67
90	100	24.71	4.58	70.71
95	100	25.66	4.27	70.07
100	100	25.34	4.33	70.33

 

Point 8: In conclusions the authors write “On the contrary, adding La promoter reduced the CuO particle for a Cu-O-La bond was formed in the interface between LaOx and Cu to improve the dispersion of CuO on SiO2.” – line 223 - what is the evidence justifying this conclusion? How did authors calculate the dispersion/particle size of Cu?

 

Response 8: The part about “a Cu-O-La bond was formed in the interface between LaOx and Cu to improve the dispersion of CuO on SiO2.”is concluded by reference[33] in the article^[2]. We have replaced this part by “On the contrary, adding La promoter reduced the CuO particle and improved the dispersion of CuO on SiO₂.”

What’s more, there was no obvious CuO diffraction peaks at 2θ = 35.5° and 38.7° in the XRD patterns of Cu-La/SiO₂ modified by La, suggesting the CuO particles were too small to detect by XRD. By Scherrer equation, we can calculating particle size through half-peak width. When the diffraction peak is sharper, we think that the larger the particle size is.

 

Point 9: Have the authors compare the performance of catalysts in this work with the catalytic activity of other catalysts reported in literatures?

 

Response 9: Guo et al used Cu–B/γ-Al₂O₃ catalysts to obatain 100% conversion at 533 K, 5 Mpa, H₂/DMM 120 and LHSV 0.36 h^-1. However, a part of DMS still exists in the products.^[3]

 

Point 10: Some mistakes/errors can be found along the text. For example:- lines 85 and 89 it should be "… calcination…" instead of "… baking…"

 

Response 10: We are very sorry for our negligence of these mistakes. And we have corrected.

 

Finally, special thanks to you for your good comments.

 

 

Reference

1.Han, X.-Q.; Zhang, Q.-F.; Feng, F.; Lu, C.-S.; Ma, L.; Li, X.-N. Selective hydrogenation of dimethyl maleate to tetrahydrofuran over Cu/SiO2 catalyst: Effect of Cu+ on the catalytic performance. Chinese Chemical Letters 2015, 26, 1150-1154, doi:https://doi.org/10.1016/j.cclet.2015.04.031.

2.Zheng, X.; Lin, H.; Zheng, J.; Duan, X.; Yuan, Y. Lanthanum Oxide-Modified Cu/SiO2 as a High-Performance Catalyst for Chemoselective Hydrogenation of Dimethyl Oxalate to Ethylene Glycol. ACS Catalysis 2013, 3, 2738-2749, doi:10.1021/cs400574v.

3.Guo, P.J.; Chen, L.F.; Yan, S.R.; Dai, W.L.; Qiao, M.H.; Xu, H.L.; Fan, K.N. One-step hydrogenolysis of dimethyl maleate to tetrahydrofuran over chromium-modified Cu–B/γ-Al2O3 catalysts. Journal of Molecular Catalysis A: Chemical 2006, 256, 164-170, doi:https://doi.org/10.1016/j.molcata.2006.04.047.

 

Round 2

Reviewer 1 Report

For Authors

Thank you for sending  the revised manuscript. However, unfortunately, the % yield was not described. High feedstock conversion and high selectivity but often low yields. For example, if large amount of polymers was produced, it will not detect in GC chromatography. In that case, the reaction efficiency can not be evaluated. Please describe the yield using the calibration curve for GC analysis. That it.

 

Reviewer 2 Report

My concern on this manuscript has not been properly addressed.

 

1. Response 1 - “Whether it has been reduced or not has little effect on the results.”, however, Authors do not provide any evidence.

2. Response 1 - “it was found that the dispersion of CuO and Cu in the Cu/SiO₂ catalyst before and after reduction is similar in our previous work”, but what with Cu-M (M=Mg, Ca, Sr, Ba, La)/SiO2 catalysts?

Active catalyst in hydrogenation reaction is reduced catalyst, so structural characterization should be done for reduced catalyst.

3. Response 2: „393 K for 8 480 min. Correction is made as"480min"” – in revision of manuscript still is 8 480 – p.6 lines 177 and 188. Where are corrections?

4. “Revised portion are marked in red in the paper.” – in the manuscript there are no selections in red. Authors introduced only Elemental composition of catalysts (Table 1) and they moved Experimental Section at the end of the manuscript.

5. Response 7, Table 4 – How the Authors calculated the space velocity? Is it “The influence of space velocity”? I think it is “The influence of time”.

6. In the revised manuscript, there is the lack compare the performance of catalysts with the catalytic activity of other catalysts reported in literature