Tunable Production of Diesel Bio-Blendstock by Rhenium-Catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation
, Anna Maria Raspolli Galletti 1,*, Claudia Antonetti 1, Gonzalo A. Martinez 2, Emma Jones 2, Lorenzo Bertin 2,3, Nicola Di Fidio 1, Sara Fulignati 1, Gianluca Pasini 4 and Stefano Frigo 4Round 1
Reviewer 1 Report
In this work, the authors studied the hydrogenation reaction of hexanoic acid to 1-hexanol/hyxyl hexaoate with Re2O7, 5 wt% Re/C and 5 wt%Re/C+niobium phosphate. The application for the reaction on the crude graph pomace and dissel engine performance were studied. The article was well written and performed in a thorough scientific data. I have some comments:
- The authors should give the reason for using 5% weight of Re.
- The authors should give the reason for studing the reaction temperatures between 180 to 220 0C which higher than the previous literature ref 21 of 130 0C.
Author Response
Dear Editor,
enclosed you find our paper “Tunable Production of Diesel Bio-blendstock by Rhenium-catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation” (Manuscript ID: catalysts- -2004275 ) for publication in “Catalysts”. We have revised the paper according to all the suggestions of the referees and the new changes are red-highlighted in the manuscript. In particular:
Reviewer 1
In this work, the authors studied the hydrogenation reaction of hexanoic acid to 1-hexanol/hexylhexanoate with Re2O7, 5 wt% Re/C and 5 wt%Re/C+niobium phosphate. The application for the reaction on the crude graph pomace and diesel engine performance were studied. The article was well written and performed in a thorough scientific data.
We thank the reviewer for his/her comment.
I have some comments:
- The authors should give the reason for using 5% weight of Re.
The intermediate value of catalyst loading (5 wt%) has been chosen taking into account that for a monometallic system a sufficient amount of rhenium is necessary for the conversion of concentrated hexanoic acid without using an excessive amount of solid catalyst, as occurs for supported systems with lower metal loading.
- The authors should give the reason for studing the reaction temperatures between 180 to 220 0C which higher than the previous literature ref 21 of 130 0C.
Preliminary experiments, not reported in the text, evidenced that at 130 °C the catalytic activity of commercial monometallic Rhenium systems was too low at an acceptable duration (up to 6 h) under the adopted conditions (significantly different from those of the cited paper, where the reaction time was very long (23 h) and an expensive bimetallic Pd/Re catalyst was used).
Hoping that this revision is enough for its publication, I send you my best personal regards.
Sincerely yours,
Prof. Anna Maria Raspolli Galletti
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work presente interesting results about the hydrogenation of commercial hexanoic acid to give 1-hexanol/hexylhexanoate mixtures in the presence of a physical mixtures of area/C and niobium phosphate. The evaluation of the mixture of 1-hexanol/hexyl hexanoate as a diesel blend stock is very interesting.
Comments:
How the catalytic reation were performed?
Author Response
Dear Editor,
enclosed you find our paper “Tunable Production of Diesel Bio-blendstock by Rhenium-catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation” (Manuscript ID: catalysts- -2004275 ) for publication in “Catalysts”. We have revised the paper according to all the suggestions of the referees and the new changes are red-highlighted in the manuscript. In particular:
Reviewer 2
The work presente interesting results about the hydrogenation of commercial hexanoic acid to give 1-hexanol/hexylhexanoate mixtures in the presence of a physical mixtures of area/C and niobium phosphate. The evaluation of the mixture of 1-hexanol/hexyl hexanoate as a diesel blend stock is very interesting.
We thank the reviewer for his/her comment.
Comments:
How the catalytic reaction were performed?
We thank the referee for this remark and at pag. 13 the paragraph on hydrogenation procedure was added:
3.3. Hydrogenation procedure
Hydrogenation reactions were carried out in a 300 mL stainless steel Parr 4560 autoclave equipped with a P.I.D. controller (4848). 5 wt % Re/C was pre-reduced in methanol (180 °C, 90 bar of H2, 6 h) as previously reported [5], whilst Re2O7 was directly employed without any pre-reduction step. The reaction mixture was stirred using a mechanical overhead stirrer. The reactor was pressurized with hydrogen up to the selected value determined at a reaction temperature, pressure being held constant by automatically feeding more hydrogen. At the end of the reaction, the reactor was rapidly cooled at room temperature, depressurized and the reaction mixture was filtered to remove the catalyst and finally analysed through GC/FID and GC/MS chromatography. For the recycling tests, the spent catalyst was recovered by filtration under dry argon atmosphere, properly washed and recycled following to the same procedure adopted for the direct run.
Hoping that this revision is enough for its publication, I send you my best personal regards.
Sincerely yours,
Prof. Anna Maria Raspolli Galletti
Author Response File: Author Response.pdf
Reviewer 3 Report
The study “Tunable Production of Diesel Bio-blendstock by Rhenium-catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation” is devoted to conventional hydrogenation of hexanoic acid into 1-hexanol and hexyl hexanoate. Re/C + solid acids and Re2O7 were used as catalysts. The catalysts were treated under different conditions using pure and commercial hexanoic acid. Moreover, the products of hydrogenation were mixed up with diesel and used in as engine fuel demonstrating excellent results. In my opinion, the presented work was done very well and I recommend it for publication. Nevertheless, I have some questions for the authors.
1. Spaces are missed in many places in the text. I found 5 of these mistakes only in Abstract. The word “blendOstock” is also used in Abstract.
2. Page 1. “Remarkably, ethanol distillation is an energy intensive process, e.g. 22% of the ethanol contained energy is consumed in the recovery process.” I guess, it is better to add the reference.
3. I would recommend to include in Table 1 the results of the current study. It would be easier to compare.
4. Why was NbPO used? Why not Al2O3 or many other solid acids?
5. Page 3. “The potential of HexOH as additive to automotive diesel fuel has already been highlighted in many other scientific publications [28,29], while much less interest has been devoted up to now to the adoption of HexHex as blendstock for diesel[30].” I guess, the main reason for this is a price of bio-based fuel. It should be extremely expensive. In this regard, the positioning of this article is quite questionable. It is OK to use bio-based molecules in pharmacology or cosmetology but not in fuel production.
6. Re2O7 is not characterized, so, it is hardly to compare with other catalysts. Moreover, I did not understand well why this catalyst was used - the authors mention other studies showing very similar results (the only difference is loading?). I can assume that this part of the study was used as a reference point but in this case the catalyst should be characterized. Also, I guess, Re2O7, reduced Re2O7 and Re/C have different structures and active centers and their comparison is not representative. What is the logic in using the best reaction conditions for Re2O7 in the experiments with Re/C?
7. Page 6. “Remarkably, this temperature is higher than those generally reported for other noble metals, confirming that Re is hardly reducible to 0 state [27,32].” Is this only one sentence in the paragraph?
8. Page 8. “A moderate slowdown of the hexanoic acid conversion was ascertained by increasing the NbPO amount…”. I would recommend to add the explanation for this fact.
9. Purity if hexanoic acid was determined by GC-FID technique. I think, some heavy products will not be identified that may distort the final result.
10. Conversion of hexanoic acid is slightly decreases from one recycling experiment to other. I would recommend to add some discussion about this.
11. Page 12. “Crude hexanoic acid-rich liquor was produced from white grape pomace according to the previously reported procedure slightly modified.” Perhaps, a reference should be added.
12. Is a HP-5MS column stable under acidic conditions? Or maybe the acids were neutralized before analysis?
Author Response
Dear Editor,
enclosed you find our paper “Tunable Production of Diesel Bio-blendstock by Rhenium-catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation” (Manuscript ID: catalysts- -2004275 ) for publication in “Catalysts”. We have revised the paper according to all the suggestions of the referees and the new changes are red-highlighted in the manuscript. In particular:
Reviewer 3
The study “Tunable Production of Diesel Bio-blendstock by Rhenium-catalyzed Hydrogenation of Crude Hexanoic Acid from Grape Pomace Fermentation” is devoted to conventional hydrogenation of hexanoic acid into 1-hexanol and hexyl hexanoate. Re/C + solid acids and Re2O7 were used as catalysts. The catalysts were treated under different conditions using pure and commercial hexanoic acid. Moreover, the products of hydrogenation were mixed up with diesel and used in as engine fuel demonstrating excellent results. In my opinion, the presented work was done very well and I recommend it for publication.
We thank the Reviewer for this comment.
Nevertheless, I have some questions for the authors.
- Spaces are missed in many places in the text. I found 5 of these mistakes only in Abstract. The word “blendOstock” is also used in Abstract.
We thank the reviewer for this observation, sorry for the mistakes. The typing mistakes have been corrected.
- Page 1. “Remarkably, ethanol distillation is an energy intensive process, e.g. 22% of the ethanol contained energy is consumed in the recovery process.” I guess, it is better to add the reference.
The information analyzed in that paragraph was recently mentioned by our previous work, which was already cited [reference 5]. However, we have added another specific reference [reference 4]:
[4] Ingledew, W.M. The alcohol textbook: a reference for the beverage, fuel and industrial alcohol industries, Nottingham University Press, Nottingham, 5th ed, 2009.
- I would recommend to include in Table 1 the results of the current study. It would be easier to compare.
Table 1 has been corrected as requested inserting the best results obtained with the two different adopted systems (Re2O7and Re/C) as comparison. It must be underlined that the aim of this paper was not the maximization of HexOH yield, but the production of a mixture HexOH/HexHex of tunable composition. The new Table 1 is the following:
|
Hexanoic acid (g/L) |
Catalyst |
T (°C) |
P (bar) |
t (h) |
Conversion (mol%) |
HexOH yield (mol%) |
Ref. |
|
n.a. |
Re2O7 |
200 |
188 |
2 |
n.a. |
93 |
16 |
|
46.4 |
Re-Pt/TiO2 |
130 |
50 |
5 |
58 |
53 |
21 |
|
23.2 |
Re-Pd/C |
130 |
20 |
18 |
n.a. |
74 |
22 |
|
52.6 |
ReOx-Pd/SiO2 |
140 |
80 |
24 |
100 |
99 |
11 |
|
14.6 |
Re-Ni/TiO2 |
180 |
50 |
8 |
100 |
60 |
23 |
|
93 |
Re2O7 |
220 |
115 |
1 |
100 |
70 |
This work |
|
93 |
Re/C |
220 |
115 |
3 |
100 |
62 |
This work |
- Why was NbPO used? Why not Al2O3 or many other solid acids?
The aim of the work was to verify the effect of the addition of a solid acid co-catalyst with well-known acidity characteristics. NbPO4 acid sites (Lewis and Brønsted) are well characterized and their amount well known [references 37 and 38 of the manuscript]. Moreover it is a water-tolerant acid catalyst, a very important characteristic for this reaction where water is coproduced.
- Page 3. “The potential of HexOH as additive to automotive diesel fuel has already been highlighted in many other scientific publications [28,29], while much less interest has been devoted up to now to the adoption of HexHex as blendstock for diesel[30].” I guess, the main reason for this is a price of bio-based fuel. It should be extremely expensive. In this regard, the positioning of this article is quite questionable. It is OK to use bio-based molecules in pharmacology or cosmetology but not in fuel production.
Surely up to now the price of these pure ester biofuels is high and this works aims at a more sustainable economic approach, adopting as starting material a crude hexanoic acid obtained from fermentation of grape pomace, an abundant nonedible agro-industrial waste. Moreover a biorefinery approach has been proposed for the complete exploitation of grape pomace [5]. As underlined in the text at pag. 2“Such valorization represents an evolution of the carboxylate platform state-of-the-art, i.e. demonstrating the potential industrial technical feasibility through a proof-of-concept.”
- Re2O7 is not characterized, so, it is hardly to compare with other catalysts. Moreover, I did not understand well why this catalyst was used - the authors mention other studies showing very similar results (the only difference is loading?). I can assume that this part of the study was used as a reference point but in this case the catalyst should be characterized. Also, I guess, Re2O7, reduced Re2O7 and Re/C have different structures and active centers and their comparison is not representative. What is the logic in using the best reaction conditions for Re2O7 in the experiments with Re/C?
Re2O7 was adopted as a starting benchmark, it is an homogeneous precursor which is reduced in situ under hydrogen pressure. It is often reported in the literature for a preliminary evaluation of the possibility of adopting rhenium as hydrogenation catalyst. The reaction conditions optimized for Re2O7 where preliminarily adopted for heterogeneous Re/C and allowed to obtain good performances and high conversion but only after some hours of reactions, thus were selected for further studies with these systems, higher temperature and/or pressure being too harsh and milder conditions involving too long reaction times.
- Page 6. “Remarkably, this temperature is higher than those generally reported for other noble metals, confirming that Re is hardly reducible to 0 state [27,32].” Is this only one sentence in the paragraph?
The text has been revised by joining the two paragraphs.
- Page 8. “A moderate slowdown of the hexanoic acid conversion was ascertained by increasing the NbPO amount…”. I would recommend to add the explanation for this fact.
The addition of too high amount of acid co-catalyst to the slurry can make stirring less efficient causing diffusion limitations. Therefore the text has been revised as reported: A moderate slowdown of the hexanoic acid conversion was ascertained by increasing the NbPO amount, probably due to lower stirring efficiency and diffusion limitations in the slurry, but a much more relevant improvement of the HexHex selectivity was obtained.
- Purity if hexanoic acid was determined by GC-FID technique. I think, some heavy products will not be identified that may distort the final result.
Figure S1 (in the Supplementary Materials) evidences that hexanoic acid is the largely prevailing acid, higher ones being much less significant. On the other hand, it is possible that heavier different water-insoluble components are present in very low amount. In fact, hexanoic acid and, more in general, medium chain carboxylic acids were obtained by phase separation from aqueous medium, thus their contamination from water-soluble species should be low.
- Conversion of hexanoic acid is slightly decreases from one recycling experiment to other. I would recommend to add some discussion about this.
The conversion of hexanoic acid slightly decreased from one recycling experiment to other because small amount of catalytic systems could be lost during their separation, recovery and recycling. In fact, the chemical deactivation can be excluded thanks to the ICP analysis, already reported, which evidenced that leaching of Re was negligible for both catalytic systems being the Re content in the recovered reaction mixtures below the detection limit. On this basis, the following sentence has been added in the manuscript:
“In fact, it is possible to observe only a slight decrease of the hexanoic acid conversion with the progress of the recycling tests, mainly due to the partial loss of the catalyst during the separation, recovery and recycling steps rather than to the rhenium leaching from the catalysts.”
- Page 12. “Crude hexanoic acid-rich liquor was produced from white grape pomace according to the previously reported procedure slightly modified.” Perhaps, a reference should be added.
The reference [5] has been added:
- Is a HP-5MS column stable under acidic conditions? Or maybe the acids were neutralized before analysis?
As reported on the Agilent website (https://www.agilent.com/en/product/gc-columns/low-bleed-gc-ms-columns/hp-5ms-columns):
“HP-5MS is a (5%-phenyl)-methylpolysiloxane phase capillary column with very low bleed characteristics, ideal for GC/MS. HP-5MS capillary column is bonded, crosslinked, and solvent rinseable, and has excellent inertness for active compounds, including acidic and basic compounds, with improved signal-to-noise ratio for better sensitivity and mass spectral integrity.”
On this basis, the acids were not neutralized before the analysis.
Hoping that this revision is enough for its publication, I send you my best personal regards.
Sincerely yours,
Prof. Anna Maria Raspolli Galletti
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
I thank the authors for their comments. To be honest, I am not satisfied complitely with comment #6, however, the manuscript can be accepted.
