Search for the Microwave Nonthermal Effect in Microwave Chemistry: Synthesis of the Heptyl Butanoate Ester with Microwave Selective Heating of a Sulfonated Activated Carbon Catalyst

Round 1

Reviewer 1 Report

The manuscript submitted by Horikoshi and coworkers, titled "Search for the Microwave Effect in Microwave Chemistry: Synthesis of the Heptyl Butanoate Ester with Microwave Selective Heating of the Sulfonated Activated Carbon Catalyst" studied the effects of the presence of water in the reaction medium on the final yields, when the reaction was carried out under conventional heating or under microwave irradiation.

The model reaction was the esterification in the presence of an acid heterogeneous or homogeneous catalyst.

The reported results could be of interest for the readers of Catalysts, because it turned out that the presence of moisture was not a problem under microwave irradiation with the heterogeneous catalyst, while under conventional heating the eventual presence of water lower the final yield into the target ester, due to hydrolysis reaction.

Therefore, I suggest publication of the manuscript after the following major revision:

it would be interesting to extend this study to a more fascinating reaction, such as the transesterification of bio-oil to give FAMEs. Since waste cooking oils (wco) are generally plenty of water and free fatty acids (FFA), I suggest the authors to compare the same transesterification of a certain amount of wco under conventional heating and under MW irradiation with the same heterogeneous catalyst to see which system gives the highest yield towards FAMEs and the lowest final content of FFA.

Author Response

Thank you for your very important comment. As per the referee suggestion, the results obtained from this study suggest that esterification can be achieved by using microwaves even when the water content is high. This property is expected to be applied to valuable reactions such as the synthesis of FAMEs, we added that point to the "Concluding remarks" as a next strategy.

Reviewer 2 Report

This is a very good manuscript. It touches on important problems of microwave catalysis. This manuscript will be of interest to specialists in the field of organic chemistry, bioorganic chemistry, catalysis, physical chemistry and chemical synthesis. Hence, the article will be well cited. I recommend publishing this article in journal Catalysts.

Author Response

We would like to thank the reviewer for his/her kind comments on our research.

Reviewer 3 Report

Overall, the study demonstrates microwave heating effect on esterification yields in presence certain level of moisture content in the butanoic acid batches compared to conventional oil bath heating. The presentation of the all experimental table and results is clear, elaborate and easy to follow. Manuscript discussions and conclusions parts are well written and organised.

 

Demerits: In my view manuscript contains not enough novelty or research out comes to be useful for the readers of “Catalyst”.

1. Sulfonated activated carbon catalyst- well known catalyst several applications available in the literature including esterification of fatty acid under microwave irradiation.
2. Overall concept of this manuscript already published.
3. Only significant outcome from the investigations under microwave heating removal moisture during microwave reaction result in prevention of reversible hydrolysis and improve ester yields compared to conventional oil bath heating.
4. Sense of extension of previous work.

 

Therefore, I am unable to recommend this work for the publication in this journal.

Other minor comments:

Manuscript introduction is poorly written and did not describe what are objectives will be investigated in the study.

Author Response

Referee 3

Overall, the study demonstrates microwave heating effect on esterification yields in presence certain level of moisture content in the butanoic acid batches compared to conventional oil bath heating. The presentation of the all experimental table and results is clear, elaborate and easy to follow. Manuscript discussions and conclusions parts are well written and organised.

Demerits: In my view manuscript contains not enough novelty or research out comes to be useful for the readers of “Catalyst”.

• Sulfonated activated carbon catalyst - well known catalyst several applications available in the literature including esterification of fatty acid under microwave irradiation.

The purpose of this study has been clarified. There are several previous studies using sulfonated activated carbon, and it has been reported that the combined use of microwave irradiation increases the reaction yield. On the other hand, these cases are carried out under conditions such as the synthesis of FAMEs in which a highly polar substrate is excessively present.

Therefore, in this study, we set a model reaction in which microwaves can easily reach the catalyst using a substrate with low polarity, and thus the aim was to clarify the effect of the microwaves. This point has been clarified, and appropriate references have been added [16,17].

• Overall concept of this manuscript already published.

Our research did not find any research reports on the adsorption of water on the solid-catalyst in microwave chemistry. We wish the referee would have clarified for us what and which literature articles have already established the so-called “Overall concept”.

• Only significant outcome from the investigations under microwave heating removal moisture during microwave reaction result in prevention of reversible hydrolysis and improve ester yields compared to conventional oil bath heating.

This is already a plus when it comes to the yields of products.

• Sense of extension of previous work.

As part of our extensive studies in Microwave Chemistry, the present article attempted to find more evidence for the microwave non-thermal effect.

Therefore, I am unable to recommend this work for the publication in this journal.

We regret the reviewer feels that way.

Other minor comments:

Manuscript introduction is poorly written and did not describe what are objectives will be investigated in the study.

The objectives of the present study appear quite clearly now in revised text in the Introduction.

 

Reviewer 4 Report

The authors present an interesting paper describing the investigation of a potential special "microwave" effect in microwave heated chemical reactions.  Microwave chemistry is making large inroads in the field of synthetic organic chemistry and fundamental studies such as the one presented are important and welcome.  For the most part, this is a carefully prepared manuscript that clearly describes their studies on the microwave vs. conventional heating of an esterification reaction.  A few things to consider:

1. I'm not clear on exactly where the "cotton swab" is placed in the experimental setup and why a cotton swab would be expected to be an effective water trap.  Clarification of the positioning and experimental expectation of the swab would be welcomed.
2. I believe that the microwave and conventional heating times are identical to one another?  Is that so?  If so, indication of this would be helpful.
3. I believe that the first part of the results and discussion section describe experiments in which microwave heating is done but in the absence of any external air cooling (by, I assume, the air blower).  The temperature is maintained, therefore, by energy input from the microwaves at a level sufficient to maintain the ~130°C temperature.  However, described on page 10 are experiments in which the microwave energy is increased, but the bulk temperature is offset by introducing air cooling.  Is my reading of this correct?  If so, it should be stated more clearly at the beginning of the section that no air cooling was used since the experimental section seems to suggest that air cooling would be the norm.
4. Figure 7 shows max temperatures of carbon particulates.  Are these values purely from simulations or is their experimental evidence that the catalyst is undergoing heating to temperatures higher than the bulk solution?
5. I agree completely with the authors that it appears that the selective heating of the carbon particle catalyst is responsible for the increase in rate and yield of esterification.  This is certainly an advantage for microwave heating vs conventional heating.  However, I'm not convinced that it has to do with water being selectively absorbed onto the catalyst and therefore removed from potential reverse reaction.  First of all, generally, I believe, heating of a substrate would be expected to drive water OFF of a surface, and not onto the surface.  Second, if the water selectively absorbed to the surface, would not this facilitate the reverse reaction since it would put the water directly into contact with ester molecules in an activated (heated) environment? 

          I might suggest an alternative explanation:  it is obvious that the microwaves are selectively heating the catalyst to higher temperatures relative to the bulk solution (and I like that the authors included a homogeneous catalyst to test this theory) and those higher temperatures are promoting the reaction.  Combination of the polar hydrogen bonding alcohol and polar hydrogen bonding carboxylic acid at the superheated pocatalytic surface selectively forms the ester product.  As reaction begins to take place to form ester, the ester, being less polar and not as active a hydrogen bonding agent, is less likely to interact with the surface where the reverse reaction would be catalyzed.  Additionally, if water is rejected from the heated surface then it is incapable of reacting with the ester in bulk solution in the absence of the catalyst.  Hence, the equilibrium is shifted towards product as it is selectively concentrated in the bulk solution and away from the reactive surface.  For the conventional heating reaction, there is no temperature differential between the catalytic surface and the bulk solution and more water may reside on the hydrogen bonding surface relative to the bulk solution where the reverse reaction may be catalyzed.  So perhaps the answer is the exact opposite of what the authors propose:  the super heated catalytic surface under microwave radiation condition rejects water and therefore cannot effectively catalyze the reverse reaction.

Is there any experimental evidence for whether water is selectively absorbed or not onto the surfaces under microwave versus conventional heating conditions?  This would help to select between the two possible mechanistic interpretations.

 

I'd appreciate if the authors could give these comments some thought.  Clarification of some of the things discussed above will be important in a revised paper.  However, in my opinion, the paper should ultimately be published following such revisions.

Author Response

Referee 4

The authors present an interesting paper describing the investigation of a potential special "microwave" effect in microwave heated chemical reactions. Microwave chemistry is making large inroads in the field of synthetic organic chemistry and fundamental studies such as the one presented are important and welcome. For the most part, this is a carefully prepared manuscript that clearly describes their studies on the microwave vs. conventional heating of an esterification reaction. A few things to consider:

(1) I'm not clear on exactly where the "cotton swab" is placed in the experimental setup and why a cotton swab would be expected to be an effective water trap. Clarification of the positioning and experimental expectation of the swab would be welcomed.

The position where the cotton swab was set is shown in Figure 1 (b), and the shape of the cotton is indicated now in the text; its location is also described. In this experiment, the amount of reaction solution was small, so that the Dean-Stark apparatus normally used for dehydration reaction could not be used. Therefore, it was necessary to use a water-absorbing material, but if molecular sieves or magnesium sulfate were used, there was concern that the solution would be contaminated during the reaction, so that the use of a cotton swab was decided upon. Since no case was found in which cotton was used for dehydration in esterification, we confirmed in a preliminary stage that the cotton swab had sufficient water adsorption. The above points have now been added to the text.

(2) I believe that the microwave and conventional heating times are identical to one another?  Is that so?  If so, indication of this would be helpful.

The heating time for conventional heating and microwave heating is the same. The reaction was carried out for 30 minutes, with the time of immersion in the oil bath and the time of the start of microwave irradiation as the reaction start time. This is specified in the text.

(3) I believe that the first part of the results and discussion section describe experiments in which microwave heating is done but in the absence of any external air cooling (by, I assume, the air blower). The temperature is maintained, therefore, by energy input from the microwaves at a level sufficient to maintain the ~130°C temperature. However, described on page 10 are experiments in which the microwave energy is increased, but the bulk temperature is offset by introducing air cooling. Is my reading of this correct? If so, it should be stated more clearly at the beginning of the section that no air cooling was used since the experimental section seems to suggest that air cooling would be the norm.

The above recognition is correct. The text now explicitly states that unless otherwise specified the experiment was conducted without air cooling.

(4) Figure 7 shows max temperatures of carbon particulates. Are these values purely from simulations or is their experimental evidence that the catalyst is undergoing heating to temperatures higher than the bulk solution?

The temperature of the surface of activated carbon is a value obtained purely by calculation. The fact that the temperature of the catalyst is higher than the temperature of the bulk solution is indirectly inferred from the improvement in conversion rate, but no direct evidence has been obtained.

(5) I agree completely with the authors that it appears that the selective heating of the carbon particle catalyst is responsible for the increase in rate and yield of esterification. This is certainly an advantage for microwave heating vs conventional heating. However, I'm not convinced that it has to do with water being selectively absorbed onto the catalyst and therefore removed from potential reverse reaction. First of all, generally, I believe, heating of a substrate would be expected to drive water OFF of a surface, and not onto the surface. Second, if the water selectively absorbed to the surface, would not this facilitate the reverse reaction since it would put the water directly into contact with ester molecules in an activated (heated) environment?

I might suggest an alternative explanation: it is obvious that the microwaves are selectively heating the catalyst to higher temperatures relative to the bulk solution (and I like that the authors included a homogeneous catalyst to test this theory) and those higher temperatures are promoting the reaction. Combination of the polar hydrogen bonding alcohol and polar hydrogen bonding carboxylic acid at the superheated photocatalytic surface selectively forms the ester product. As reaction begins to take place to form ester, the ester, being less polar and not as active a hydrogen bonding agent, is less likely to interact with the surface where the reverse reaction would be catalyzed. Additionally, if water is rejected from the heated surface then it is incapable of reacting with the ester in bulk solution in the absence of the catalyst. Hence, the equilibrium is shifted towards product as it is selectively concentrated in the bulk solution and away from the reactive surface. For the conventional heating reaction, there is no temperature differential between the catalytic surface and the bulk solution and more water may reside on the hydrogen bonding surface relative to the bulk solution where the reverse reaction may be catalyzed. So perhaps the answer is the exact opposite of what the authors propose: the super-heated catalytic surface under microwave radiation condition rejects water and therefore cannot effectively catalyze the reverse reaction.

We would like to thank this reviewer for the alternative explanation that we find very interesting. It should be fully considered that the reverse reaction is inhibited as a result of the depolarized product ester moving away from the catalyst surface, and there are reports that support this [25], so the suggested alternative is now also described in the text, and related references have been added. On the other hand, regarding the question of whether water is adsorbed on the selectively heated catalyst, substrates such as heptanol and butyric acid are less polar and possess lower hydrogen bonding properties than water, but they are nonetheless also adsorbed on the catalyst for the reaction to proceed. Therefore, from our point of view, we should also consider the possibility that water will be adsorbed on the catalyst as well as the reacting substrates, but then is volatilized and discharged to the outside of the system. The above points have been added in the text.

(6) Is there any experimental evidence for whether water is selectively absorbed or not onto the surfaces under microwave versus conventional heating conditions? This would help to select between the two possible mechanistic interpretations.

I'd appreciate if the authors could give these comments some thought. Clarification of some of the things discussed above will be important in a revised paper. However, in my opinion, the paper should ultimately be published following such revisions.

 

Unfortunately, it has not been verified. However, it is a very interesting point of view as to whether or not the extent of water adsorption on a given solid catalyst changes when it is selectively exposed to adsorption of microwaves, a point well worth addressing in future studies.

Round 2

Reviewer 1 Report

The authors responded to all questions raised by the reviewers, therefore I judge the manuscript suitable for publication in Catalysts in the present form.

Author Response

Thanks for the referee's comment.

Reviewer 4 Report

I appreciate the authors taking my earlier comments into account and incorporating changes to the manuscript.  I am pleased to support publication at this time.

Author Response

Thanks for the referee's comment.