MgO Dispersed on Activated Carbon as Water Tolerant Catalyst for the Conversion of Ethanol into Butanol
Round 1
Reviewer 1 Report
The topic of metal oxides dispersed on activated carbon to increase surface area and dispersion is not a novel concept in catalysis community. Some research with and without metals(Ni, Ru, etc) on MgO and MgO/AC has already been published. Especially for the purpose of supporting hypothesis with experimental data, it would be good to include SEM and XRD data for all three MgO loading. In particular, in line 175-177, authors have indicated that higher MgO content is likely due to formation of MgO aggregates. This can be investigated by XRD and SEM/TEM analysis so it would be good to include experimental/characterization evidence to support your hypothesis.
Author Response
The topic of metal oxides dispersed on activated carbon to increase surface area and dispersion is not a novel concept in catalysis community. Some research with and without metals (Ni, Ru, etc) on MgO and MgO/AC has already been published. Especially for the purpose of supporting hypothesis with experimental data, it would be good to include SEM and XRD data for all three MgO loading. In particular, in line 175-177, authors have indicated that higher MgO content is likely due to formation of MgO aggregates. This can be investigated by XRD and SEM/TEM analysis so it would be good to include experimental/characterization evidence to support your hypothesis.
The authors are grateful to the reviewer for his/her comments and suggestions: accordingly, we have included in the revised version of the manuscript the XRD patterns of all three catalysts. The analysis of the characteristic peaks of MgO and the evaluation of the average size of MgO crystallites according to the Scherrer equation (reported as a new column in Table 1) highlighted the formation of large cristallytes for 30%MgO/AC catalyst with an average dimension higher than the characteristic size of the micropores of the activated carbon support, thus confirming a limited dispersion of the active phase was achieved.
Concerning the general comment regarding the dispersion of metal oxides onto large surface area supports, this is of course not a novelty. However, the novelty in this work regards the development and use of a cheap, stable and water tolerant catalytic system for the Guerbet reaction in the presence of a feed containing water. As the authors reported in the Introduction, MgO/AC has been already used for several applications, mostly adsorption of small acid molecules; nevertheless, to the best of our knowledge this is the first report on the catalytic performance of such system for the conversion of ethanol into butanol, particularly in the presence of water. The low cost and the easy preparation of MgO/AC catalysts represent key features for a process requiring large amount of catalyst due to the low reaction rate.
Reviewer 2 Report
The paper is good, contains much of new data obtained by a broad number of different physical methods. The topic itself is more than important since it deals with the creation of a new catalyst with very promising characteristics as compared to more expensive, but sensitive to water catalysts. The presented review of corresponding literature is quite appropriate. The results are well illustrated by a reasonable number of figures. Conclusions are persuading enough and are adequately supported by the data. The length is OK, it is well structured and could be published as it is, but some minor drawbacks could be taken into account, which mostly can be considered as suggestions for the author that could be followed or not.
1) The authors explain the increased yield of butanol formation after water treatment exclusively by the increased surface area and porosity of the support, which was shown not active itself as a catalyst. Meanwhile, it was stated above about the key role of MgO dispersion in the process. The quantity of the oxide is not changed during the contact with water vapor, and it is written about the formation of Mg(OH)2 whose decomposition at the temperature of reaction can lead not only to modified porosity, but to higher dispersion of MgO. Hence, this could account for the increase of butanol yield. To our mind, it would be better if the authors, if agree, at least mention such possibility.
2) Line 228 Figure 6. Should be Figure 8.
Some minor points:
3) English is generally good and clear, however, although the reviewer is not a great expert in it, it seems that in a number of cases the authors overuse the indefinite article before uncountable objects, as (line 174) “which produced a 12.5 % butanol yield in correspondence of a 68% ethanol conversion”, “An aqueous solution of Mg(NO3)26H2O…” etc.
The style is generally clear and the text is easy to read, but sometimes it seems better to avoid repeating the same words in the same sentences or near. E.g. in the abstract the term ‘performance’ is used 4 times in the last 4 lines. Or (line 219) : “In order to highlight the key role of the activated carbon support in order to achieve…” , the same in lines 46-47.
4) In figures 5 and 6 conversion is marked white and the yield- black, in figure 8 (denoted by mistake in the legend as 6)- opposite colors without any reason, a bit uncomfortable for the reader.
5) Line 250: Waste2Fuels – correct?
6) Line 308 ref 25: H 2 S.
Author Response
Reviewer #2
The paper is good, contains much of new data obtained by a broad number of different physical methods. The topic itself is more than important since it deals with the creation of a new catalyst with very promising characteristics as compared to more expensive, but sensitive to water catalysts. The presented review of corresponding literature is quite appropriate. The results are well illustrated by a reasonable number of figures. Conclusions are persuading enough and are adequately supported by the data. The length is OK, it is well structured and could be published as it is, but some minor drawbacks could be taken into account, which mostly can be considered as suggestions for the author that could be followed or not.
1) The authors explain the increased yield of butanol formation after water treatment exclusively by the increased surface area and porosity of the support, which was shown not active itself as a catalyst. Meanwhile, it was stated above about the key role of MgO dispersion in the process. The quantity of the oxide is not changed during the contact with water vapor, and it is written about the formation of Mg(OH)2 whose decomposition at the temperature of reaction can lead not only to modified porosity, but to higher dispersion of MgO. Hence, this could account for the increase of butanol yield. To our mind, it would be better if the authors, if agree, at least mention such possibility.
The authors totally agree with the reviewer on the effect of MgO dispersion promoted by water and added this further hypothesis in the text.
2) Line 228 Figure 6. Should be Figure 8.
Corrected
Some minor points:
3) English is generally good and clear, however, although the reviewer is not a great expert in it, it seems that in a number of cases the authors overuse the indefinite article before uncountable objects, as (line 174) “which produced a 12.5 % butanol yield in correspondence of a 68% ethanol conversion”, “An aqueous solution of Mg(NO3)26H2O…” etc.
The style is generally clear and the text is easy to read, but sometimes it seems better to avoid repeating the same words in the same sentences or near. E.g. in the abstract the term ‘performance’ is used 4 times in the last 4 lines. Or (line 219) : “In order to highlight the key role of the activated carbon support in order to achieve…” , the same in lines 46-47.
The text has been revised according to the suggestions.
4) In figures 5 and 6 conversion is marked white and the yield- black, in figure 8 (denoted by mistake in the legend as 6)- opposite colors without any reason, a bit uncomfortable for the reader.
Bars colors of conversion and yield in Figure 8 have been inverted in order to improve the clarity.
5) Line 250: Waste2Fuels – correct?
Yes, this is the correct acronym of the EU project.
6) Line 308 ref 25: H 2 S.
Corrected
Reviewer 3 Report
This paper deals with the catalytic conversion of ethanol into butanol using MgO/activated carbon catalysts. The experimental method and the analytical methods are appropriate. Although the finding that their catalyst has water tolerant is worth reporting, there are some inquiries about the novelty of the research and the errors of the experiments.
Therefore, I recommend this paper for publication after the following points have been properly addressed by the authors.
1. The authors used activated carbon as a support having large surface area, and reported that larger yield was obtained than pure MgO. However, faster reaction when using highly dispersed catalysts is general knowledge in catalytic science. What is the novelty?
Also, the authors should compare the activity of their catalyst with that of previously reported catalysts.
2. Please add the data of experimental errors to each figure.
Also, did the authors checked carbon balance?
Author Response
Reviewer #3
This paper deals with the catalytic conversion of ethanol into butanol using MgO/activated carbon catalysts. The experimental method and the analytical methods are appropriate. Although the finding that their catalyst has water tolerant is worth reporting, there are some inquiries about the novelty of the research and the errors of the experiments.
Therefore, I recommend this paper for publication after the following points have been properly addressed by the authors.
1. The authors used activated carbon as a support having large surface area, and reported that larger yield was obtained than pure MgO. However, faster reaction when using highly dispersed catalysts is general knowledge in catalytic science. What is the novelty?
Also, the authors should compare the activity of their catalyst with that of previously reported catalysts
As the reviewer correctly asserts, dispersion of an active phase on large surface area supports leads to better activity, especially if a positive interaction occurs and this is not a novelty. Nevertheless, high performance using a cheap support with a very large surface area coupled to the absence of an expensive metal represents a significant advantage for a process (the conversion of bio-ethanol obtained from ABE fermentation into butanol) requiring limited costs. The authors themselves have previously reported on the attempt to increase the surface area of MgO by dispersion onto g-Al2O3 (ref. [18]): in that case the catalytic performance were better than with a reference bulk MgO catalyst but significantly lower than results presented in this work. Furthermore, alumina is more expensive than active carbon and a more complex preparation procedure was used. The novelty is the use of this very common support for this application.
In the revised version of the manuscript, the authors have added a more direct comparison of performance with previously reported catalysts. Nevertheless, it must be pointed out that such a comparison is not straightforward when experimental data were obtained under different conditions. For this reason, the authors have mainly compared the present results with those previously obtained with several different MgO-based catalysts tested in the same test rig under identical experimental conditions.
2. Please add the data of experimental errors to each figure.
Also, did the authors checked carbon balance?
As reported in the Experimental section, due to the overlapping of peaks of ethylene and diethylether, the carbon balance cannot be checked. Moreover, as also reported in the discussion, the formation/deposition of some C-containing species, and/or the formation of gasification products from the activated carbon support, especially during operation in the presence of water, cannot be excluded.
GC peaks were evaluated within 5% experimental error, as reported in the same section. This impacts consequently on conversion and yield. The authors did not report the experimental errors in the bar graphs for the sake of clarity.
Round 2
Reviewer 3 Report
The authors have satisfactorily addressed my questions and comments. I recommend this article for acceptance to publish in Applied Sciences.
