Biodiesel Production over Banana Peel Biochar as a Sustainable Catalyst

Round 1

Reviewer 1 Report

The introduction should include larger context for using banana peel biochar instead of other sources, any benefits, how does potassium content and other ash content compare to other biomass sources for biochar. 

In page 2 line 52-53 logical structure would follow mentioning which components (K, Na, Ca, Mg) would make the oxides for basic nature, and then talk about the other P, Si, and S elements oxides with acidic nature.... 

Suggest in p 3 experimental divided into subheadings for catalyst preparation, catalytic test, and characterization for easy reading 

Use same 20 wt %  for activation of all alkali carbonate salts, and then comment on atomic amount in EDS, though differences in molar mass of Li, Na, K would lead to large variation in alkali concentration between the three, seems strange design

It would be good to mention the composition of the ash for banana peel in Table 1 or in discussion of same.

Captions for Figures 3, 4 should be clearer, identify figures clearly that right panel is same sample but higher magnification

Page 8 and Figure 5 discussion mentioning that Na surface composition very low, and most of it must be in bulk, though no supporting evidence of how much Na, K, Li is in each sample, for example from ICP analysis

Mention page 8 225 first mention of sylvite can mention it is KCl not wait until line 235... Also label post-reaction as PR in text and in figure caption for clarity for Figure 7

XRD patterns in Fig 6 nicely overlayed so don't overlap, however in figures 7 and 8 they are not well overlayed so hard to look at

Page 9 lines 246-248 discussion out of place belongs with previous paragraph not related to FTIR discussion

Page 9 257-258 mention the alkali modifiers typically increase deoxygenation of biomass, however this contradicts the EDS results that you mention and also the FTIR...

For FTIR discussion, maybe add glycerin and methanol spectra to SI. it seems that there could also be some water related to the broad peak 3300 looking at the relative areas of other peaks

EDS post reaction would be good to confirm the leaching suspected post reaction

Surface area and pore volume analysis seems to be missing and may answer some open questions. Also quantitative measurements of surface acidity and basicity through titrations would aid the paper and strengthen conclusions specifically regarding the bifunctional nature .

how about the reuseability, cyclic behavior

Check tenses of words, spelling, typos etc. 

Example filtrated should be filtered p 3 line 117

page 5 line 189 powered should be powdered

page 8 line 219-221 prseents should be presents, and maening should be meaning, catalyse should be catalyze

page 10 lines 271-273 sentence grammar so confusing not sure what it is even saying

page 10 line 285 missing comma after "more complex"

page 8 line 241 sentence structure, confusing, missing words such as contact with methanol provided evidence that... 

Author Response

The introduction should include larger context for using banana peel biochar instead of other sources, any benefits, how does potassium content and other ash content compare to other biomass sources for biochar.

While banana peel biomass warrants further exploration, its detailed discussion is beyond the scope of this work. The current introduction provides sufficient background for interpreting the experimental results. Additionally, the chosen references offer a well-rounded perspective on the analysis and understanding of the reported findings.

In page 2 line 52-53 logical structure would follow mentioning which components (K, Na, Ca, Mg) would make the oxides for basic nature, and then talk about the other P, Si, and S elements oxides with acidic nature....

Amended taking into account the Referee's comment.

Suggest in p 3 experimental divided into subheadings for catalyst preparation, catalytic test, and characterization for easy reading

Taking into account the Referee's comments, the experimental section was divided into two subsections, with the first being the methodology for preparation and characterization of the chars.

Use same 20 wt %  for activation of all alkali carbonate salts, and then comment on atomic amount in EDS, though differences in molar mass of Li, Na, K would lead to large variation in alkali concentration between the three, seems strange design

The chosen alkaline carbonates, with their varying molar masses, contribute different concentrations of alkaline elements at the 20% by weight ratio used relative to the biomass. This value (20% wt.) is common in catalyzed pyrolysis tests and considers the already high potassium content in the biomass itself.

It would be good to mention the composition of the ash for banana peel in Table 1 or in discussion of same.

As suggested by the reviewer the ingorganics in banana peel ash were included in Table 1. Due to the inherent compositional variability of banana peel ash, the values presented represent average values reported in the literature. A new sentence was added in the EDS data analysis section:

Potassium is the main inorganic element detected for unmodified chars which agrees with banana peel ash composition in the Table 1.

Captions for Figures 3, 4 should be clearer, identify figures clearly that right panel is same sample but higher magnification

The magnification is presented for each micrography.

Page 8 and Figure 5 discussion mentioning that Na surface composition very low, and most of it must be in bulk, though no supporting evidence of how much Na, K, Li is in each sample, for example from ICP analysis

Heterogeneous catalysis occurs on the surface of the catalyst. Therefore, the surface composition is crucial for understanding its catalytic behavior. EDS provides a near-surface analysis, making it a suitable technique for characterizing catalysts. Conversely, Inductively ICP necessitates complete solubilization of the char, which is often challenging and expensive. Due to these limitations, EDS is the preferred method for analyzing the composition relevant to catalytic activity (as shown in Figure 5).

Mention page 8 225 first mention of sylvite can mention it is KCl not wait until line 235... Also label post-reaction as PR in text and in figure caption for clarity for Figure 7

Corrected.

XRD patterns in Fig 6 nicely overlayed so don't overlap, however in figures 7 and 8 they are not well overlayed so hard to look at

Each diffractogram displays two overlaid curves in distinct colors. This intentional overlap highlights the key differences between the diffractograms of the fresh and post-reaction samples. The authors favor this approach because it allows for a clearer visual comparison of the changes that occurred during the reaction.

Page 9 lines 246-248 discussion out of place belongs with previous paragraph not related to FTIR discussion

A paragraph has been added as suggested by the Reviewer.

Page 9 257-258 mention the alkali modifiers typically increase deoxygenation of biomass, however this contradicts the EDS results that you mention and also the FTIR...

EDS and FTIR–ATR are surface analysis techniques. The alkali elements improve biochar surface basicity thus improving CO₂, from the atmosphere, adsorption which can contribute to higher surface oxygen content. While the bulk oxygen content of biochars wasn't measured, hindering assessment of the deoxygenation effect from alkali during pyrolysis. The Raman spectra of the chars obtained in the presence of alkaline carbonates and the one obtained at 400 °C have a less intense valley band (band between the D and G bands), which implies fewer oxygen-containing species, proving the deoxygenating effect of the alkali and the carbonization temperature. A new sentence has been included in the section on the analysis of Raman spectra.

For FTIR discussion, maybe add glycerin and methanol spectra to SI. it seems that there could also be some water related to the broad peak 3300 looking at the relative areas of other peaks

To avoid cluttering the figures, well-established FTIR spectra of glycerin and methanol are omitted, as they are readily available in the literature. The broad band observed around 3500 cm⁻¹ (band a) originates from the O-H stretching vibrations within the -OH groups of both glycerin and methanol, not from water. The transesterification reaction itself does not produce water. Therefore, any minimal water signal likely stems from residual moisture due to exposure to ambient air.

EDS post reaction would be good to confirm the leaching suspected post reaction

Unfortunately, the EDS measurements for the post-reaction samples were not carried out. The leaching of KCl by solubilization in methanol was confirmed in the XRD characterization.

Surface area and pore volume analysis seems to be missing and may answer some open questions. Also quantitative measurements of surface acidity and basicity through titrations would aid the paper and strengthen conclusions specifically regarding the bifunctional nature.

During the work, it was not possible to obtain surface area and pore volume analysis data. As such, other techniques were used for the textural and functional characterization of the catalysts studied.

how about the reuseability, cyclic behavior

The data presented are preliminary data for the study of the catalytic activity of biomass-based catalysts in biodiesel production reactions. The work is still ongoing and the deactivation of the catalysts and their reactivation is being studied.

Example filtrated should be filtered p 3 line 117

Corrected

page 5 line 189 powered should be powdered

Corrected

page 8 line 219-221 prseents should be presents, and maening should be meaning, catalyse should be catalyze

Corrected

page 10 lines 271-273 sentence grammar so confusing not sure what it is even saying

Corrected

The part produced from virgin vegetable oils has questionable sustainability [50]. Was deleted because it didn't make sense.

page 10 line 285 missing comma after "more complex"

Corrected

page 8 line 241 sentence structure, confusing, missing words such as contact with methanol provided evidence that... 

As suggested the sentence was rewritten.

The diffractogram of biochar obtained by carbonization of raw banana peel at 350 °C, and subsequent methanol contact, confirms leaching of KCl by methanol. This aligns with published findings on silvite's solubility in methanol [42].

Reviewer 2 Report

The authors have reported methanolysis of waste frying oil using bio-char based catalysts, produced from banana peels. The manuscript is well written, and can be further improved considering following points: 

1) Page 1, paragraph 1, authors have highlighted the issues around higher cost of biodiesel compared to fossil diesel. The authors need to elaborate the recent literature covering more sustainable processes to improve the economics of biorefinery industry. For instance, authors should present the recent study by Pandit et al (Ind. Eng. Chem. Res. 2023, 62, 23, 9201–9210), where in detailed techno-economic analysis of esterification of biodiesel byproduct glycerol is shown to improve economic viability of biodiesel industry.

2) Page 2, paragraph starting line 62, the authors should quantify the acidity of biochar produced from different biomass feedstocks, which authors have reported such as seaweed, banana peels etc.

3) Fig.2, with the help of arrows (or different colours), highlight which data corresponds to primary axis, and secondary axis, to make it further easier for readers to distinguish.

4) Fig. 3. and Fig. 4, in SEM images, please provide scale bars.

5)  Fig. 5, can authors show Li (atomic %) content also?

6) Fig. 6, authors should label the peaks corresponding to Li, Na or K carbonate to distinguish the composition of materials.

7) Fig. 11, authors should also show FAME yield obtained using Na2CO2, Li2CO3 and K2CO3 as catalyst without biochar. 

8) Did authors investigate the leaching of alkali carbonates, stability and reusability of alkali carbonate doped biochar catalysts?    

N/A

Author Response

Reviewer 2:

The authors have reported methanolysis of waste frying oil using bio-char based catalysts, produced from banana peels. The manuscript is well written, and can be further improved considering following points:

1) Page 1, paragraph 1, authors have highlighted the issues around higher cost of biodiesel compared to fossil diesel. The authors need to elaborate the recent literature covering more sustainable processes to improve the economics of biorefinery industry. For instance, authors should present the recent study by Pandit et al (Ind. Eng. Chem. Res. 2023, 62, 23, 9201–9210), where in detailed techno-economic analysis of esterification of biodiesel byproduct glycerol is shown to improve economic viability of biodiesel industry.

The suggested paper is very interesting, and relevant, but according to the authors it focuses on the valorization of glycerin in diesel additives, so it falls outside the context of this work.

2) Page 2, paragraph starting line 62, the authors should quantify the acidity of biochar produced from different biomass feedstocks, which authors have reported such as seaweed, banana peels etc.

The acidity of biochars can be determined using Bohem's titration. This manuscript provides preliminary data on the catalytic activity of banana peel biochars. While the determination of acidity is planned for future work, the provided ATR-FTIR spectra can already indicate the presence of acid functional groups on the catalyst surface. A complete characterization of all catalyst parameters is beyond the scope of a single manuscript.

3) Fig.2, with the help of arrows (or different colours), highlight which data corresponds to primary axis, and secondary axis, to make it further easier for readers to distinguish.

The thermogram has been improved taking into account the Referee's comment. The TG and DTG curves and their axes now have different colors.

4) Fig. 3. and Fig. 4, in SEM images, please provide scale bars.

The scale bars were already on the SEM images. The contrast of the photos has been improved so that the bars are more visible.

5)  Fig. 5, can authors show Li (atomic %) content also?

The Li content was not quantified by the SEM-EDS detector used. For this reason, the value is not included in the graph.

6) Fig. 6, authors should label the peaks corresponding to Li, Na or K carbonate to distinguish the composition of materials.

Considering the Referee's comment, the sylvite diffraction lines were identified in all the diffractograms with a thin blue stripe. All the lines not identified as KCl correspond to the phases of the inserted dopant elements. A new sentence has been included in the manuscript. According to the authors, the insertion of markers for each crystalline phase of Na and Li would make the figure too complex and would not contribute to the interpretation of the results.

7) Fig. 11, authors should also show FAME yield obtained using Na₂CO₃, Li₂CO₃ and K₂CO₃ as catalyst without biochar.

The authors have recently released a manuscript (https://doi.org/10.1016/j.fuel.2022.124383) detailing the catalytic activity observed during oil methanolysis using sodium carbonate and hydrogen carbonate, showcasing their remarkable efficacy. Additionally, other alkali carbonates (such as Li and K) have been utilized by different researchers as catalysts in oil methanolysis, with corresponding data also being published. Due to the minimal quantities of alkaline carbonates present in the biochar catalysts, direct cssomparison with equivalent amounts of these salts for catalytic testing is not feasible. Additionally, the biomass carbonization process can alter the composition of these carbonates, potentially affecting their behavior compared to pristine salts. Therefore, the authors have opted not to include the catalytic activity of isolated alkaline carbonates in this work.

8) Did authors investigate the leaching of alkali carbonates, stability and reusability of alkali carbonate doped biochar catalysts?   

The leaching of KCl from the coals after reaction was observed and attributed to its dissolution in methanol. XRD data seems to show that sodium and lithium carbonates are more resistant to leaching by methanol.The data presented are preliminary data for the study of the catalytic activity of biomass-based catalysts in biodiesel production reactions. The work is still ongoing and the deactivation of the catalysts and their reactivation is being studied.

Reviewer 3 Report

The authors studied the synthesis of biodiesel from waste frying oil by methanolysis using biochar and modified biochar as a catalyst is of very much interest in recent times. The manuscript cannot be considered for publication at this stage as there is a lot flaws in the manuscript. I suggest the authors to revise the manuscript.

1.     The authors mentioned both acidic and basic sites are necessary for the methanolysis of WFO. Characterization of the biochar and modified biochar catalysts for acidity and/or basicity is not there in the manuscript.

2.     Surface area of catalysts also important factor. No data is available.

3.     The authors studied both modified and unmodified biochar for biodiesel synthesis. Why?

4.     After modification also, the authors did not notice any enhancement in activity. On what basis the authors are claiming both acid and basic sites are necessary for the biodiesel synthesis.

5.     Optimization of catalysts is not there.

6.     Recyclability and Scalability is not there.

Authors need to perform above mentioned reactions before being considered for publication.

Quality of English should be improved

Author Response

Reviewer 3:

The authors studied the synthesis of biodiesel from waste frying oil by methanolysis using biochar and modified biochar as a catalyst is of very much interest in recent times. The manuscript cannot be considered for publication at this stage as there is a lot flaws in the manuscript. I suggest the authors to revise the manuscript.

1. The authors mentioned both acidic and basic sites are necessary for the methanolysis of WFO. Characterization of the biochar and modified biochar catalysts for acidity and/or basicity is not there in the manuscript.

Measuring the acidity/basicity of functional groups in catalysts, using the Bohem titration with aqueous NaHCO₃ , Na₂CO₃, and HCl solutions, can be helpful. However, since oil methanolysis occurs in a non-aqueous environment, these measurements are only relevant for interpreting catalytic properties when obtained under similar conditions. Therefore, research is ongoing to find the best technique for characterizing the acidity/basicity of coals used in oil methanolysis.

2. Surface area of catalysts also important factor. No data is available.

During the work, it was not possible to obtain surface area data. As such, other techniques were used for the textural and functional characterization of the catalysts.

3. The authors studied both modified and unmodified biochar for biodiesel synthesis. Why?

To analyze the catalytic effect of a dopant, the base catalyst, without a dopant, must be evaluated to be used as a standard.

4. After modification also, the authors did not notice any enhancement in activity. On what basis the authors are claiming both acid and basic sites are necessary for the biodiesel synthesis.

The presence of alkali carbonates played a pivotal role in shaping the characteristics of the obtained char. To thoroughly evaluate the impact of introducing alkaline salts into char, it is crucial to impregnate the salts into the char post-production. Currently, this investigative study is in progress. Drawing insights from the outcomes observed with low-acid virgin oil and higher-acid waste frying oil (WFO), the researchers concluded: the tested chars exhibit bi-functionality, catalyzing both the esterification of free fatty acids and the transesterification of triglycerides. Consequently, the catalysts were categorized as bifunctional by the authors.

5. Optimization of catalysts is not there.

The aim of the work was to study the behavior of different types of biochars as catalysts in the production of biodiesel. Optimizing the banana peel catalyst for biodiesel production is an ongoing effort. This process requires a deep understanding of the reaction mechanism, including identifying the active sites and how they deactivate. Achieving this level of knowledge is beyond the scope of a single study.

6. Recyclability and Scalability is not there.

The production of biochars and testing them as catalysts in the production of biodiesel and adsorbents in the purification of the glycerin produced are the initial results obtained in this research. Further work will involve optimizing operating conditions and then studying the reuse of catalysts. Scale-up is a goal for the future.

Authors need to perform above mentioned reactions before being considered for publication.

Round 2

Reviewer 1 Report

Again, it is suggested to alter figure 7 and 8 to make it easier to see the observation made that the peaks for KCl are diminished post reaction, it is hard to see that the peaks for sylvite have disappeared or decreased since the overlay of the original is blocking those... perhaps add an inlay showing the intensity change with the intensity of each peak vs condition if you feel that strongly about keeping the figure the same...

Mentioned there is no access to surface area or porosity measurements... this seems important for the presented results... but fine. There was no comment on the other point about doing titrations to determine basic or acidic sites, these are relatively simple experiments that can be done...

Yes heterogeneous catalysis occurs at the surface, however you have made a conclusion that the Na and Li is predominantly in the bulk material... this is a scientific paper and any conclusion you draw needs to be well supported. Maybe the Na or Li didn't even make it into the biochar material. It is not clear without the ICP or other... perhaps a TGA of the material to get the ash and then analyze the ash... there are alternative ways to support this finding, otherwise it is just speculation that most of the Na is in the bulk. Which you can also say just make it clear that it is speculation and not fact. 

Author Response

The authors would like to thank the reviewers for their helpful comments, which have greatly improved the paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors addressed all the comments point by point in a detailed manner. Hence, the manuscript can be accepted in the present form

Author Response

Dear Editor,

We thank the reviewers for their comments, which we have implemented whenever possible in the manuscript. The changes for Round 2 are underlined in blue in the manuscript text.

Kind regards

Ana Paula Soares Dias