The Support Effects on the Direct Conversion of Syngas to Higher Alcohol Synthesis over Copper-Based Catalysts

Round 1

Reviewer 1 Report

This manuscript deals with the synthesis and characterisation of heterogeneous copper catalysts supported on alumina or silica to investigate the support effects on the heterogeneous catalytic conversion of syngas to higher alcohols. Since the selective catalytic reactions have high impact on the production of chemicals and the metal–support interactions are key problems in heterogeneous catalysis, this investigation deserves attention.

In this form, however, this manuscript cannot be published for the following reasons:

1) ps. 1 lines 98–99 The authors should mark the two parts of Figure 1 as Fig. 1a and Fig. 1b, because it is relatively difficult to follow your explanations about the XRD patterns.

2) p. 5–6 lines 147–148 and 184–185  It would be more advantageous if the authors indicated the precise temperatures of the reduction peaks in the H₂-TPR figure (Fig. 3). Similarly, you should also mark the characteristic temperature values of the NH₃-TPD curves (Fig. 4).

3) p. 8 lines 243–246  The authors state that “… the percentages of ethanol and C₂₊ alcohols were 42.5 wt% and 56.0 wt% respectively ...” but, according to the data of Table 2, the sum amount of PrOH, BuOH and PenOH (no C₅OH!) obtained over Cu/Al₂O₃ is 13.5% and that of MeOH is 44.0%! Similarly, “… lower percentages of ethanol (34.6 wt%) and C₂₊ alcohols (40.3 wt%).” but, according to the data of Table 2, the sum amount of PrOH, BuOH and PenOH (no C₅OH!) obtained over Cu/SiO₂ is 5.7% and that of MeOH is 59.8%! Please, clarify these contradictions.

4) p. 9 lines 264–265 You should increase the size of Figure 9, because it is very difficult to read it. Furthermore, you should mark its four parts as Fig. 9a–Fig. 9d.

5) p. 11 line 368 The authors describe that “Al₂O₃ was synthesized by our laboratory.”, but without any details. If it is a known method, you should insert a relevant reference.

6) p. 12  line 421 You should also give the details of the GDX-403, GDX-401 and GC-7AG columns the following way: producer GDX-403 (X × Y mm, Z µm) column.

7) There are some typing or grammatical errors in the text:

p. 1 line 36 „… nonrenewable …”  instead of  non-renewable

p. 3 line 123 „… species might be reacts …”  instead of  species might react

p. 5 line 179 and elsewhere „As it is known, …”  instead of  As known

p. 6 line 202 and elsewhere „… XPS patterns …”  instead of  XP spectra

p. 8 line 237 „… Evalution”  instead of  Evaluation

       line 240 „Presented in Table 2 were the performances of the representative catalysts for higher alcohols synthesis from syngas.”  ??? (it is very difficult to understand)

p. 15 line 535 „… J CO₂ Util. …”  instead of  J. CO₂ Util.

Author Response

Dear Ms. Ivana Pesic,

Thank you for providing us with the referees’ comments, which helped us to improve the clarity of the manuscript. All questions raised were taken into account, and in the following please find our detailed answers.

Yours sincerely

Yisheng Tan

Response to Reviewer 1 Comments

Point 1: ps. 1 lines 98-99 The authors should mark the two parts of Fig. 1 as Fig. 1a and Fig. 1b, because it is relatively difficult to follow your explanations about the XRD patterns.

Response 1:  Thank you for your suggestion. We have marked the two parts of Fig. 1 as Fig. 1a and Fig. 1b in the revised version (highlighted in yellow).

Point 2: p. 5-6 lines 147-148 and 184-185  It would be more advantageous if the authors indicated the precise temperatures of the reduction peaks in the H₂-TPR figure (Fig. 3). Similarly, you should also mark the characteristic temperature values of the NH₃-TPD curves (Fig. 4).

Response 2:  Thank you for your suggestions.

In the revised version, we have marked the characteristic temperature values of the peaks in the H₂-TPR figure (Fig. 3) and NH₃-TPD figure (Fig. 4).

Point 3: p. 8 lines 243-246  The authors state that “… the percentages of ethanol and C₂₊ alcohols were 42.5 wt% and 56.0 wt% respectively ...” but, according to the data of Table 2, the sum amount of PrOH, BuOH and PenOH (no C₅OH!) obtained over Cu/Al₂O₃ is 13.5% and that of MeOH is 44.0%! Similarly, “… lower percentages of ethanol (34.6 wt%) and C₂₊ alcohols (40.3 wt%).” but, according to the data of Table 2, the sum amount of PrOH, BuOH and PenOH (no C₅OH!) obtained over Cu/SiO₂ is 5.7% and that of MeOH is 59.8%! Please, clarify these contradictions.

Response 3:  Thank you for your careful notions.

We are sorry to make you feel confused due to our error typewriting. Now, we have revised them in revised version, in which, ‘C₅OH’ has been replaced by 'C₅₊OH'. In addition, in this work, the value of C₂₊ alcohols is the sum amount of EtOH, PrOH, BuOH and C₅₊OH. As displayed in the Table 2, the sum amount of EtOH, PrOH, BuOH and C₅₊OH (C₂₊ alcohols) obtained over Cu/Al₂O₃ is 56.0 wt% and that of MeOH is 44.0 wt%. In the case of Cu/SiO₂ catalyst, the sum amount of EtOH, PrOH, BuOH and C₅₊OH (C₂₊ alcohols) is 40.3 wt% and that of MeOH is 59.8 wt%.

The related paragraph has been reworded in order for clear illustration (highlighted in yellow for easy reference).

 Table 2 The performances of Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts

Reaction conditions: 10 MPa, 400 ^oC, 5000 h^-1.

Point 4: p. 9 lines 264-265 You should increase the size of Fig. 9, because it is very difficult to read it. Furthermore, you should mark its four parts as Fig. 9a~Fig. 9d.

Response 4: Thank you for your suggestion. We have increased the size of Fig. 9 and marked its four parts as Fig. 9a~Fig. 9d in the revised version (highlighted in yellow).

Point 5: p. 11 line 368 The authors describe that “Al₂O₃ was synthesized by our laboratory.” but without any details. If it is a known method, you should insert a relevant reference.

Response 5: Thank you for your suggestion.

The γ-Al₂O₃ as a support was synthesized using a hydrothermal route, which was similar with the procedure described by Yang et al. [1]. Typically, ammonia solution (28 % NH₃), AlCl₃·6H₂O solution and NaOH solution were mixed under hydrothermal treatment. Then, the mixture was dried and calcined to obtain the γ-Al₂O₃.  As your suggestion, we have added the relevant description in the revised version on section of Materials and Methods (major changes highlighted in yellow for easy reference). 

References

1. Yang, Q. Synthesis of γ-Al₂O₃ nanowires through a boehmite precursor route. Bull. Mater. Sci. 2011, 34, 239. 

Point 6: p. 12  line 421 You should also give the details of the GDX-403, GDX-401 and GC-7AG columns the following way: producer GDX-403 (X × Y mm, Z µm) column.

Response 6: Thank you for your suggestion.

 In the new manuscript, the organic gas products, consisting of hydrocarbons and methanol, were detected online on EastWest GC4000A equipped with flame ionization detector and GDX-403 column (3 mm, 1 m). The inorganic gas products were detected online by thermal conductivity measurements using a GC4000A (carbon molecular sieves column, 3 mm, 3 m). The H₂O and methanol products in the liquid phase were detected by thermal conductivity measurements using a Shimazduo GC4000A (GDX-401 column, 3 mm, 3 m). The alcohol products in the liquid phase were detected by flame ionization measurements using a Shimazduo GC-7AG (GDX-201 column, 3mm, 4 m). As your suggestion, we have added the relevant description in the revised version on section of Materials and Methods (major changes highlighted in yellow for easy reference).

Point 7: There are some typing or grammatical errors in the text:

p. 1 line 36 „… nonrenewable …”  instead of  non-renewable

p. 3 line 123 „… species might be reacts …”  instead of  species might react

p. 5 line 179 and elsewhere „As it is known, …”  instead of  As known

p. 6 line 202 and elsewhere „… XPS patterns …”  instead of  XPS spectra

p. 8 line 237 „… Evalution”  instead of  Evaluation

        line 240 „Presented in Table 2 were the performances of the representative catalysts for higher alcohols synthesis from syngas.”  ??? (it is very difficult to understand)

p. 15 line 535 „… J CO₂ Util. …”  instead of J. CO₂ Util.

Response 7:  Thank you for your careful observation.

We have checked the paper carefully and revised the typing or grammatical errors. We have replaced the nonrenewable with non-renewable. The ‘… species might be reacts …’ was replayed by ‘… species might react …’ .  ‘As it is known, …’  instead of ‘As known’. The  ‘…XPS spectra…’ replaced ‘… XPS patterns …’. In addition, we have substituted ‘Catalyst  evaluation’  for ‘Catalyst evalution’. ‘Presented in Table 2 were the performances of the representative catalysts for higher alcohols synthesis from syngas’ was replaced by ‘The performances of Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts for higher alcohols synthesis from syngas were presented in Table 2’. ‘… J CO₂ Util. …’instead of ‘… J. CO₂ Util. …’ (major changes highlighted in yellow for easy reference).

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments are attached.

Comments for author File: Comments.pdf

Author Response

Dear Ms. Ivana Pesic,

Thank you for providing us with the referees’ comments, which helped us to improve the clarity of the manuscript. All questions raised were taken into account, and in the following please find our detailed answers.

Yours sincerely

Yisheng Tan

Response to Reviewer 2 Comments

Point 1: The English of the manuscript must be improved significantly. The grammar is
ok and the interested reader can understand the conclusions, but there are typos all
over the manuscript. Hence, if the paper is to be accepted, the revised version should
be free from such English errors.

Response 1: Thank you for your kind suggestion. 

We have checked the typewriting carefully. The typos have been amended completely and English language has been further polished(highlighted in yellow).

Point 2: I believe that section 3 (Materials and Methods) should be moved just after the introduction section and then, the authors should add the Results and Discussion as section 3 followed by the conclusions.

Response 2:  Thank you for your suggestions.

We have carefully considered the comments and have revised accordingly.

Point 3: On page 3, Fig. 1, please indicate the parameter on the y-axis. Is this the intensity or the normalized intensity? Also, in the same figure, I suggest you change the y-value of the intensity of the SiO2 support XRD results to make the small peaks of Cu/SiO2 and K-Cu/SiO2 visible.

Response 3: Thank you for your suggestion.

The parameter on the y-axis is normalized intensity. We have added the y-axis title and changed the y-value of the intensity of the SiO2 support XRD results in the revised version in order to make the small peaks of Cu/SiO2 and K-Cu/SiO2 visible. 

(a)                                                                                           (b)

Fig. 1 XRD patterns of Al₂O₃, Al₂O₃-900, Cu/Al₂O₃, K-Cu/Al₂O₃ catalysts (a) and SiO₂, SiO₂-900, Cu/SiO₂, K-Cu/SiO₂catalysts (b)

Point 4: On page 3, lines 120-125, the authors discuss the dramatic drop on the surface area of the studied catalysts when copper is added to the support. The authors justify this decrease in the surface area on the formation of additional phases. However, I believe copper can also act as a sintering agent, i.e. it is an element that facilitates sintering. Any chance that this drop in BET is related to copper acting as a sintering agent instead of the additional phases forming?

Response 4: Thank you for your considerations. 

We fully agree that copper can also act as a sintering agent, resulting in the decrease of the surface areas. Here in the presented work, the interfacial composite phases such as CuAlO₂, CuAl₂O₄ and copper phyllosilicate can be clearly identified (as revealed by XRD results). As reported [1-4], the formation of the interfacial composite phases via the thermal reaction was always accompanied by the loss of surface areas. Thus, in this work, copper species reacted with the support to generate interfacial composite phases during the calcination at high temperature, which was probably another key factor for the decrease in surface areas. Based on the above analysis, the drop in the surface areas could be ascribed to both the formation of additional phases and copper as a sintering agent. 

With your considerations, we have added the relevant description in the revised version on section of 3.1.2 N₂ absorption-desorption characterization (major changes highlighted in yellow for easy reference).

References

1. Kato, S.; Fujimaki, R.; Ogasawara, M.; Wakabayashi, T.; Nakahara, Y.; Nakata, S. Oxygen storage capacity of CuMO₂ (M= Al, Fe, Mn, Ga) with a delafossite-type structure. Appl. Catal. B Environ. 2009, 89, 183-188.

2. Choi, S. M.; Kang, Y. J.; Kim, S. W. Effect of γ-alumina nanorods on CO hydrogenation to higher alcohols over lithium-promoted CuZn-based catalysts. Appl. Catal. A Gen. 2018, 549, 188-196.

3. Xi, H. J.; Hou, X. N.; Liu, Y. J.; Qing, S. J.; Gao, Z. X.  Cu-Al spinel oxide as an efficient catalyst for methanol steam reforming. Angew. Chem. Int. Ed. 2014, 53, 11886-11889.

4. Li, X. L.; Xie, H. J.; Gao, X. F.; Wu, Y. Q.; Wang, P.; Tian, S. P.; Zhang, T.; Tan, Y. S. Effects of calcination temperature on structure-activity of K-ZrO₂/Cu/Al₂O₃ catalysts for ethanol and isobutanol synthesis from CO hydrogenation. Fuel 2018, 227,199-207.

Point 5: On page 4, lines 132-133, why did the average pore diameter increase for the SiO2 case when potassium was added?

Response 5: Thank you for your questions. 

Currently, the explanation on the change in value of average pore diameter was still unclear. The variations in average pore diameter were probably related to alkali amounts [1, 7], the kinds of alkalis [4, 6], supports types [8], and catalyst preparation parameters [2, 3]. Indeed, when alkali additives were added into the catalysts, the value of average pore diameter was found to decrease [1, 2, 7], increase [3-5, 7] or remain unchanged [5, 6]. In our results, the average pore diameter of the SiO₂-suported catalyst factually show a slight increase. We considered that the increase was caused by the interaction between potassium and SiO₂ support due to the strong corrosion of potassium to SiO_2. 

References

1.   Tian, S. P.; Wang, S. C.; Wu, Y. Q.; Gao, J. W.; Wang, P.; Xie, H. J.; Yang, G. H. Han, Y. Z.; Tan, Y. S. The role of potassium promoter in isobutanol synthesis over Zn-Cr based catalysts. Catal. Sci. Technol. 2016, 6, 4105-4115.

2.    Sun, J.; Wan, S. L.; Wang, F.; Lin, J. D.; Wang, Y. Selective synthesis of methanol and higher alcohols over Cs/Cu/ZnO/Al₂O₃ Catalysts. Ind. Eng. Chem. Res. 2015, 54, 7841-7851.

3.   Yang, Y.; Xiang, H. W.; Xu, Y. Y.; Bai, L.; Li, Y. W.  Effect of potassium promoter on precipitated iron-manganese catalyst for Fischer-Tropsch synthesis. Appl. Catal. A Gen.  2004, 266, 181-194.

4      Tan, L.; Yang, G. H.; Yoneyama, Y.; Kou, Y. L.; Tan, Y. S.; Vitidsant, T. Iso-butanol direct synthesis from syngas over the alkali metals modified Cr/ZnO catalysts. Appl. Catal. A Gen. 2015, 505, 141-149.

5     Li, T.; Virginie, M.; Khodakov, A. Y. Effect of potassium promotion on the structure and performance    of alumina supported carburized molybdenum catalysts for Fischer-Tropsch synthesis. Appl. Catal. A Gen. 2017, 542, 154-162

6     Li, J. F.; Cheng, X. F.; Zhang, C. H.; Chang, Q.; Wang, J.; Wang, X. P.; Lv, Z. G.; Dong, W. S.; Yang, Y.; Li, Y. W. Effect of alkalis on iron-based Fischer-Tropsch synthesis catalysts: Alkali-FeOx interaction, reduction, and catalytic performance. Appl. Catal. A Gen. 2016, 528, 131-141. 

7.    Amoyal, M.; Vidruk-Nehemya, R.; Landau, M. V. Herskowitz. M. Effect of potassium on the active phases of Fe catalysts for carbon dioxide conversion to liquid fuels through hydrogenation. J. Catal. 2017, 348, 29-39.

8. Jiang, F.; Zhang, M.; Liu, B.; Xu, Y. B.; Liu, X. H. Insights into the influence of support and potassium or sulfur promoter on iron-based Fischer-Tropsch synthesis: understanding the control of catalytic activity, selectivity to lower olefins, and catalyst deactivation. Catal. Sci. Technol. 2017, 7, 1245.   

Point 6: Regarding the H2-TPR and NH3-TPD measurements on pages 5 and 6, I would like the authors to supplement their results by conduction the same measurements on Al2O3, SiO2 and CuO alone to make sure that the peaks shown in the results can be ascribed to the specific events described in the manuscript with higher certainty. If such results are available in the literature, you can also use them but make sure you add them in the same plot.

Response 6: Thank you for your suggestions.

We have supplemented the H2-TPR and NH3-TPD measurements of Al2O3, SiO2 and CuO alone at the same conditions and the results were shown in Fig. 3 and Fig. 4. 

The reduction behaviors of Al₂O₃, SiO₂, CuO, Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts were studied by H₂-TPR and the results were presented in Fig. 3. No reduction peaks could be observed in the Al₂O₃ and SiO₂ and one reduction peak at 299 ^oC was clearly detected in the CuO phase. As displayed in Fig. 3, the H₂-TPR profile of Cu/Al₂O₃ catalyst showed three reduction peaks at around 280, 540 and 800 ^oC, which were corresponding to reduction of CuO [1], CuAl₂O₄ [2] and CuAlO₂ [3], respectively. From Fig. 3, four reduction peaks at 435, 540, 700 and 770 ^oC were clearly found in the Cu/SiO₂ catalyst, suggesting that four types of copper species formed on the catalyst [4]. These results indicted that the copper oxide interacted with Al₂O₃ or SiO₂ and different supports always led to the different interactions. 

The acidity of Al₂O₃, SiO₂, CuO, Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts were studied by NH₃-TPD measurements and the results were displayed in Fig. 4. No NH₃ desorption peak was found in CuO phase, revealing that the acid of CuO phase was very weak. It was clearly observed that the NH₃-TPD profiles of Al₂O₃ and Cu/Al₂O₃ catalysts were exactly the same. Specifically, two peaks at 270 and 500 ^oC, ascribed to the weak acidic sites and the strong acidic sites, respectively were obviously observed in the Al₂O₃ and Cu/Al₂O₃ catalysts. These results indicated that the acid were mainly stemmed from Al₂O₃ support. As also presented in Fig. 4, the SiO₂ support, Cu/SiO₂ and K-Cu/SiO₂ catalysts showed any NH₃ desorption peak hardly with or without potassium addition [5]. These findings indicated that the acid-base property of the prepared catalysts is closely related with the support employed such as SiO₂ and Al₂O₃.

As your suggestion, we have added the relevant description in the revised version on sections of  3.1.3. H₂-TPR and  3.1.4. NH₃-TPD (major changes highlighted in yellow for easy reference). 

Fig. 3 H₂-TPR profiles of Al₂O₃, SiO₂, CuO, Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts

Fig. 4 NH₃-TPD profiles of Al₂O₃, SiO₂, CuO, Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts

References

1. Li, M. M. J.; Zeng, Z. Y.; Liao, F. L.; Hong, X. L.; Tsang S. C. E. Enhanced CO₂ hydrogenation to methanol over CuZn nanoalloy in Ga modified Cu/ZnO catalysts. J. Catal. 2016, 343, 157-167.

2. Li, X. L.; Zhang, Q. D.; Xie, H. J.; Gao, X. F.; Wu, Y. Q.; Yang, G. H.; Wang, P.; Tian, S. P.; Tan, Y. S. Facile preparation of Cu-Al oxide catalysts and their application in the direct synthesis of ethanol from syngas. ChemistrySelect 2017, 2, 10365-10370.

3. Kato, S.; Fujimaki, R.; Ogasawara, M.; Wakabayashi, T.; Nakahara, Y.; Nakata, S. Oxygen storage capacity of CuMO₂ (M= Al, Fe, Mn, Ga) with a delafossite-type structure. Appl. Catal. B Environ. 2009, 89, 183-188.

4. Wang, B.; Cui, Y. Y.; Wen, C.; Chen, X.; Dong, Y.; Dai W. L. Role of copper content and calcination temperature in the structural evolution and catalytic performance of Cu/P25 catalysts in the selective hydrogenation of dimethyl oxalate. Appl. Catal. A Gen. 2016, 509, 66-74.

5. Soled, S. Silica-supported catalysts get a new breath of life. Science 2015, 350(6265), 1171-1172.

Point 7: Regarding the XPS results presented in figures 5-7, I would also like the authors to add XPS of CuO powder to see how much the spectra change due to a substrate when compared to the actual metal oxide. The current plots show that the substrate affects the surface chemistry, but I believe it will be interesting to see what differences exist when compared to the CuO powder. For figure 8, please add XPS of the SiO2 and Al2O3 alone.

Response 7: Thank you for your suggestions.

We are sorry that the XPS of CuO, SiO2 and Al2O3 failed to conduct because it is now the period of China's traditional Spring Festival holiday. In order to make a comparison, we consult Handbook of X-ray photoelectron spectroscopy: A reference book of standard spectra for identification and interpretation of XPS data [1], which has the reference value theoretical basis.

Fig. 1S showed the XPS spectra of Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts: (a) Cu2p_3/2, (b) Al2p and Si2p. The standard XPS spectra of Cu2p_3/2, Al2p and Si2p [1] were displayed in Fig. 2S, Fig. 3S and Fig. 4S, respectively.

 As displayed in Fig. 1S (a), two peaks at around 933.0 and 935.0 eV, ascribed to Cu²⁺ in CuO and Cu²⁺ in CuAl₂O₄, respectively [2, 3] were found Cu/Al₂O₃ catalyst. Two peaks at 933.3 eV and 935.0 eV, attributed to CuO and copper phyllosilicate [4-6], were clearly observed in the Cu/SiO₂ catalyst. From Fig. 2S, the standard XPS spectra of Cu2p_3/2 was 933.6 eV. Fig. 1S (b) presented the Al2p XPS spectra of Cu/Al₂O₃, K-Cu/Al₂O₃ and Si2p XPS spectra of Cu/SiO₂, K-Cu/SiO₂ catalysts. Three peaks at 74.4, 75.3 and 76.7 eV attributed to aluminum species were obviously observed in Cu/Al₂O₃ catalyst and two peaks centered at 103.1 and 103.9 eV were found on Cu/SiO₂ catalyst, indicating that the Si element possessed two chemical states in the catalyst [5, 6]. As shown in Fig. 3S and Fig. 4S, the standard XPS spectra of Al2p and Si2p were 103.3 and 74.4 eV, respectively. These results clearly shown that the values of Cu2p_3/2, Al2p and Si2p on the prepared catalysts obviously shifted compared to the standard XPS spectra of Cu2p_3/2, Al2p and Si2p. It revealed that the oxidation state or chemical environment in the Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts were different from that in Al2O3, SiO2 and CuO alone. As your suggestion, we have added the relevant description in the section of Supporting information.

                                   (a)                                                                        (b)

Fig. 1S XPS spectra of Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts: (a) Cu2p_3/2, (b) Al2p and Si2p

Fig. 2S XPS spectra of Cu2p_3/2 [1]

Fig. 3S XPS spectra of Al2p [1]

Fig. 4S XPS spectra of Al2p [1]

References

1. Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. E. Handbook of  X-ray photoelectron spectroscopy: A reference book of standard spectra for identification and interpretation of XPS data. Physcial Electronics. Inc. 6509 Flying Cloud Drive Eden Prairie, Minnesota 55344, United States of America. 

2. Li, X. L.; Zhang, Q. D.; Xie, H. J.; Gao, X. F.; Wu, Y. Q.; Yang, G. H.; Wang, P.; Tian, S. P.; Tan, Y. S. Facile preparation of Cu-Al oxide catalysts and their application in the direct synthesis of ethanol from syngas. ChemistrySelect 2017, 2, 10365-10370.

3. Li, X. L.; Xie, H. J.; Gao, X. F.; Wu, Y. Q.; Wang, P.; Tian, S. P.; Zhang, T.; Tan, Y. S. Effects of calcination temperature on structure-activity of K-ZrO₂/Cu/Al₂O₃ catalysts for ethanol and isobutanol synthesis from CO hydrogenation. Fuel 2018, 227,199-207.

4. Huang, X. M.; Ma, M.; Miao, S.; Zheng, Y. P.; Chen, M. S.; Shen W. J. Hydrogenation of methyl acetate to ethanol over a highly stable Cu/SiO₂ catalyst: Reaction mechanism and structural evolution. Appl. Catal. A Gen. 2017, 531, 79-88.

5. Ye, R. P.; Lin, L.; Yang, J. X.; Sun, M. L.; Li, F.; Li, B.; Yao, Y. G. A new low-cost and effective method for enhancing the catalytic performance of Cu-SiO₂ catalysts for the synthesis of ethylene glycol via the vapor-phase hydrogenation of dimethyl oxalate by coating the catalysts with dextrin. J. Catal. 2017, 350, 122-132.

6. Liu, Y. T.; Ding, J.; Bi, J. C.; Sun, Y. P.; Zhang, J.; Liu, K. F.; Kong, F. H.; Xiao, H. C.; Chen, J. G. Effect of Cu-doping on the structure and performance of molybdenum carbide catalyst for low-temperature hydrogenation of dimethyl oxalate to ethanol. Appl. Catal. A Gen. 2017, 529, 143-155.

Point 8: On page 9, please increase the size and fonts of Fig. 9.

Response 8: Thank you for your suggestion.

In the revised version, we have increased the size and fonts of Fig. 9.

Point 9: On page 11, line 368, please include the synthesis method for Al2O3 in detail.

Response 9: Thank you for your suggestion.

The γ-Al₂O₃ as a support was synthesized using a hydrothermal route, which was similar with the procedure described by Yang et al. [1]. Specifically, ammonia solution (28 % NH₃), AlCl₃·6H₂O solution and NaOH solution were mixed under hydrothermal treatment. Then, the mixture was dried and calcined to obtain the γ-Al₂O₃.  As your suggestion, we have added the relevant description in the revised version on section of Materials and Methods (highlighted in yellow). 

References

1. Yang, Q. Synthesis of γ-Al₂O₃ nanowires through a boehmite precursor route. Bull. Mater. Sci. 2011, 34, 239. 

Point 10: Line 343: typo in K-Cu/Al2O3 and K-Cu/SiO2

Response 10: Thank you for your observation.

We have revised the typing errors in the K-Cu/Al2O3 and K-Cu/SiO2. The manuscript has been thoroughly checked for typo errors (major changes highlighted in yellow for easy reference).

Author Response File: Author Response.pdf

Reviewer 3 Report

Paper provides a substantial structural characterization of Cu catalysis on two different substrates and data in the paper would be beneficial in catalysis society.  However, discussions are found to be weak and authors should provide better understanding to the fundamental difference in Cu catalytic behavior on Al₂O₃ and SiO₂ substrates.

1.     At the Table 1, authors presented textural properties of the representative samples for catalytic structures. However, authors did not clarify the consistency of these experimental results. Therefore, authors should provide error margin for data presented in the Table 1, as well as they should clarify the number of measurements done for each catalysis.

2.     Y-axis title is missing in Figure 2(b).

Author Response

Dear Ms. Ivana Pesic,

Thank you for providing us with the referees’ comments, which helped us to improve the clarity of the manuscript. All questions raised were taken into account, and in the following please find our detailed answers.

Yours sincerely

Yisheng Tan

 Response to Reviewer 3 Comments

Point 1: At the Table 1, authors presented textural properties of the representative samples for catalytic structures. However, authors did not clarify the consistency of these experimental results. Therefore, authors should provide error margin for data presented in the Table 1, as well as they should clarify the number of measurements done for each catalyst.

Response 1: Thank you for your comments.

The measurement of textural properties presented in Table 1 is accuracy (± 1%). The experiments were repeated three times and averaged the results.

As your suggestion, we have added the relevant description in the revised version on section of 2.3. Catalyst characterization (major changes highlighted in yellow for easy reference). 

Point 2:  Y-axis title is missing in Fig. 2(b).

Response 2:  Thank you for your careful notion. 

We have added the Y-axis title in Fig.2(b) in the new manuscript.

(b)

Fig.2(b) Pore size distribution of Cu/Al₂O₃, K-Cu/Al₂O₃ and Cu/SiO₂, K-Cu/SiO₂ catalysts

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have successfully addressed all my comments. I recommend the manuscript for publication.