Modifications in the Composition of CuO/ZnO/Al₂O₃ Catalyst for the Synthesis of Methanol by CO₂ Hydrogenation

Round 1

Reviewer 1 Report

The manuscript reports the studies of modified CuO/ZnO/Al₂O₃ catalysts for the synthesis of methanol by CO₂ hydrogenation.

The authors performed an accurate characterization of catalysts, and the data are well performed and presented. The manuscript could be accepted for publication after minor revision, and the following issues should be addressed:

• The authors should highlight the added value of their catalysts.
• Appropriate bibliographic references should be added in the description of the chemical reactions and thermodynamic mechanisms involved in the process of methanol synthesis.
• The Authors should compare the catalytic performance of their composites with similar ones reported in the literature.
• The yield of methanol produced should be reported, furthermore this result should be compared with those reported in the literature.

Author Response

The manuscript reports the studies of modified CuO/ZnO/Al₂O₃ catalysts for the synthesis of methanol by CO₂ hydrogenation.

The authors performed an accurate characterization of catalysts, and the data are well performed and presented. The manuscript could be accepted for publication after minor revision, and the following issues should be addressed:

Thank you very much for your valuable comments. I go on to describe the changes made to the manuscript based on your suggestions.

1. The authors should highlight the added value of their catalysts.

ANSWER:

The traditional catalyst for methanol synthesis is at high pressures using Cu/Zn/Al₂O₃. In order to work at lower pressures, some modifications have to be done, as using dopant agents, i.e. Pd. In our work, we compared the effect of Pd addition, we also studied the modification of the synthesis time to a larger one and we tested other kind of support as a zeolite (not shown). It allowed to conclude that the catalyst that was the most efficient in the process of methanol synthesis was the palladium-based catalyst, 5% Pd/Cu/Zn/Al₂O₃. This exhibited a selectivity of 64.2% at 210 ºC and 5 bar.

The following sentence has been added to the text: 

 “At 210 °C, CZA-30´selectivity had a value ca. 55 % and PdCZA-30’ of 63.4% with a similar conversion of 3.2 %. The addition of Pd represents an 18% increase in methanol yield”.

2. Appropriate bibliographic references should be added in the description of the chemical reactions and thermodynamic mechanisms involved in the process of methanol synthesis.

ANSWER:

The following references have been added to the manuscript:

“Although reaction (1) as well as reaction (3) both give methanol as a major product, it was confirmed through reagents that have isotopes, that with the industrial conditions the re-action that is more favoured is indeed the first one, and when reactions are enabled with-out carbon monoxide as a staring material, methanol is entirely formed through reaction (1) without passing through the third one [12].”

12. Riaz, A.; Zahedi, G.; Klemeš, J.J. J. Clean. Prod. 2013, 57, 19–37. [https://doi.org/10.1016/j.jclepro.2013.06.017]

“Methyl alcohol is exclusively achieved through the direct hydrogenation of carbon dioxide (reaction 1) [13].”

13. Chinchen, G.C.; Spencer, M.S.; Waugh, K.C.; Whan, D.A. J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases 1987, 83, 2193–2212. [https://doi.org/10.1039/F19878302193]

3. The Authors should compare the catalytic performance of their composites with similar ones reported in the literature.

ANSWER:

The following text have been added to the manuscript:

“At 210 °C, CZA-30´selectivity had a value ca. 55 % and PdCZA-30’of 63.4% with a similar conversion of 3.2 %. In other work,  Melian-Cabrera et al. [21], with similar catalysts at 200 °C, reported a selectivity value of 90 % with conversion was about 4 % for CZA. However, an important decrease of activity happened with the addition of Pd to 3% with 95% of selectivity. They suggest that Pd in effect may already have reached saturation and the addition of Pd to the CZA catalyst has a beneficial effect on the methanol synthesis but Pd does not act as an independent catalytic site for methanol synthesis, because the observed promotion is in-dependent of the metal loading.”

21. Melián-Cabrera, I.; López Granados, M.; Fierro, J.L.G. Reverse Topotactic Transformation of a Cu–Zn–Al Catalyst during Wet Pd Impregnation: Relevance for the Performance in Methanol Synthesis from CO2/H2 Mixtures, J. Catal. 2002, 210, 273–284. [https://doi:10.1006/jcat.2002.3676]

4. The yield of methanol produced should be reported, furthermore this result should be compared with those reported in the literature.

ANSWER:

The following text have been added to the manuscript:

“At 210 °C, CZA-30´selectivity had a value ca. 55 % and PdCZA-30’ of 63.4% with a similar conversion of 3.2 %. The addition of Pd represents an 18% increase in methanol yield. In other work, Melian-Cabrera et al. [21], with similar catalysts at 200 °C, reported a selectivity value of 90 % with conversion was about 4 % for CZA. However, an important decrease of activity happened with the addition of Pd to 3% with 95% of selectivity. In this case, PdCZA provides a 26% increase in methanol yield over CZA.”

21. Melián-Cabrera, I.; López Granados, M.; Fierro, J.L.G. Reverse Topotactic Transformation of a Cu–Zn–Al Catalyst during Wet Pd Impregnation: Relevance for the Performance in Methanol Synthesis from CO2/H2 Mixtures, J. Catal. 2002, 210, 273–284. [https://doi:10.1006/jcat.2002.3676]

Reviewer 2 Report

Dear Authors,

I am asking the authors to relate themselves to the below remarks:

1. Introduction

• Quote: „The purpose of this work is to prepare, characterize and test a series of different catalysts by modification of the traditional catalyst CuO/ZnO/Al₂O₃…”

Comment: Why didn't the Authors research the series of different catalysts by modification of the traditional catalyst,but only one modified catalyst Pd?

Why was only Pd selected for catalyst modification? As shown in the literature, good effects are obtained by using as promoters e.g. La, Ga, Ce, Cr.

2.2. Catalyst synthesis

• Why were the catalysts prepared with co-precipitation method? It is known from the literature that the gelation technique allows to obtain catalysts that are more active than their "classic" counterparts.
• Why is the ratio (mass basis) of copper to zinc 45:45?
• Why was no other carrier tested only Al₂O₃?
• Quote ”…. a period of 30 or 150 minutes….”                              Comment: Why is the time choice of 30 or 150 minutes?

• Quote„The catalyst impregnated with Pd was dried at 110℃ for 12 h…” Comment: What is the impregnation process here? What is the reason for choosing the temperature value - 110 °C and time - 12h? Does it just mean removing the moisture? If so, the choice of temperature is obvious, but the time should be specified by stating that it is dried to a solid mass and it is obligatory, not a specific time value.

3.1. Catalyst characterization

• Remark: The authors present as novelty the information, quote:. „The catalyst co-precipitated in 150 minutes forms larger crystals…”, yet this is a well known knowledge. Longer (slower) crystallization time favours formation of larger crystals.
• Fig. 3.

Not well visible Zn Ka1, Cu Ka1, Al Ka1

• Fig 4 and 5

What does it mean„CZZ-30 ́”? On the basis of the text, it should be „CZA-30’”.

• Fig 5.

How do the authors explain the sinusoidal course of the curve for temp.220℃?

4. Conclusions

• Fig. 8 (3.2. Reaction tests) doesn't reveal , that PdCZA-30’ shows clearly better selectivity than the conventional CuO/ZnO/Al₂O₃ catalysts. I propose to provide an analogical selectivity value for the CZA-30 '.

Best regards,

Rewiever

Author Response

Thank you very much for your valuable comments. I go on to describe the changes made to the manuscript based on your suggestions.

Dear Authors,

I am asking the authors to relate themselves to the below remarks:

1. Introduction

• Quote: „The purpose of this work is to prepare, characterize and test a series of different catalysts by modification of the traditional catalyst CuO/ZnO/Al₂O₃…”

Comment: Why didn't the Authors research the series of different catalysts by modification of the traditional catalyst, but only one modified catalyst Pd?

Why was only Pd selected for catalyst modification? As shown in the literature, good effects are obtained by using as promoters e.g. La, Ga, Ce, Cr.

ANSWER:
The catalyst screening carried out in this work aims to select a suitable catalyst for use in a membrane reactor that provides methanol yields higher than those obtained in a fixed bed. Therefore, if the addition of dopants does not entail a significant increase in performance, it is not worth doing an exhaustive study of a greater number of dopants.

2.2. Catalyst synthesis

• Why were the catalysts prepared with co-precipitation method? It is known from the literature that the gelation technique allows to obtain catalysts that are more active than their "classic" counterparts.

ANSWER: In order to make easier the student work, a more conventional catalyst synthesis method was used.

• Why is the ratio (mass basis) of copper to zinc 45:45?

ANSWER: We take this ratio from bibliography (Hong, Z. S et al. Catal. Letters 2002) and we did not try to optimize this parameter.

• Why was no other carrier tested only Al₂O₃?

ANSWER: We also tested a zeolite one (not shown). It presented an area around 340 m²/g which was very close to the ZSM-5 specific surface area 405 m²/g. This means that in the case of catalyst with no pH control, the synthesis was not quite successful, and the metal particles did not bind to the zeolite support.

• Quote” …. a period of 30 or 150 minutes….”      Comment: Why is the time choice of 30 or 150 minutes?

ANSWER: It was selected two time completely different in order to appreciate the effect of the synthesis variable.

• Quote„The catalyst impregnated with Pd was dried at 110℃ for 12 h…” Comment: What is the impregnation process here? What is the reason for choosing the temperature value - 110 °C and time - 12h? Does it just mean removing the moisture? If so, the choice of temperature is obvious, but the time should be specified by stating that it is dried to a solid mass and it is obligatory, not a specific time value.

ANSWER: The experience in other catalysts synthesis by impregnation method have given the knowledge that 12 hours at 110ºC are suitable conditions to carry out the dry step.

3.1. Catalyst characterization

• Remark: The authors present as novelty the information, quote: „The catalyst co-precipitated in 150 minutes forms larger crystals…”, yet this is a well-known knowledge. Longer (slower) crystallization time favours formation of larger crystals.

ANSWER: This sentence only tries to corroborate the well-known time favours formation of larger crystals by quantifying with the Scherrer equation obtained from XRD study.

• Fig. 3.

Not well visible Zn Ka1, Cu Ka1, Al Ka1

ANSWER: The image size has been increased in order to make visible the labels.

• Fig 4 and 5

What does it mean„CZZ-30 ́”? On the basis of the text, it should be „CZA-30’”.

ANSWER: Thank you for the observation. The mistake has been corrected.

• Fig 5.

How do the authors explain the sinusoidal course of the curve for temp.220℃?

ANSWER: We attribute this anomalous behavior to specific problems with the operation of the furnace in the middle of the experiment.

Conclusions

• Fig. 8 (3.2. Reaction tests) doesn't reveal, that PdCZA-30’ shows clearly better selectivity than the conventional CuO/ZnO/Al₂O₃ catalysts. I propose to provide an analogical selectivity value for the CZA-30 '.

ANSWER: We have change the sentence in the text:

“The best results were achieved for the palladium-based catalyst PdCZA-30’ which exhibit a selectivity to methanol with an average value of 63.4% for a conversion of 3.2%, better than the 55% with conventional CuO/ZnO/Al₂O₃ catalysts.”