Review Reports

Round 1

Reviewer 1 Report

This is a brilliant study of the reaction engineering of esterification/transesterification systems, demonstrating the ability of ZnO / TBAI to catalyze these reactions with only 8-fold excess methanol achieving near completion of a notoriously "equilibrium" reaction at 65degC.  It is novel, original and important.   I recommend publication with only minor amendments:

The ordering of the sections.   The common format for scientific journal articles is the IMRaD style, which is often augmented with conclusions at the end.  This paper has Results and Discussions *before* experimental and the methods and materials section.   This ordering is just confusing, as the reader is asking the typical question about "how was that done?" all throughout the results and discussion, then the experimental sections.   I strongly recommend that the authors re-think their organization to a more conventional format that would logically put the results and discussion at the end!

The authors have neglected citing an important recent paper on a different heterogeneous catalysis advance with simultaneous separation and no recycle / separation steps downstream of the reactor:

Zimmerman WB, Kokoo R, Esterification for biodiesel production with a phantom catalyst: Bubble mediated reactive distillation Applied Energy, 221:28-40, 2018.   The authors have made clear that their catalyst is a Lewis acid / Lewis base and a phase transfer agent.   The above paper introduces free radical catalysis which also causes phase transfer using a dispersed catalyst.   

The authors should highlight that the recovery of the ZnO catalyst, using centrifugation, is to demonstrate that density difference segregation is effective.  It is, however, unlikely that centrifugation, which requires high power consumption, is a sensible processing step for the production of a fuel (low value added).  It is, however, potentially cost-effective for high value added ester production.   Personally, if centrifugation works, that suggests that much less power consumptive flotation separations are the target for unit operation based on density difference of either flocs or, with foam flotation, either attraction or repulsion from the bubble interface.

I would suggest that the authors consider what high value added esters they could target with this approach.   One point perhaps not made is that the glycerol produced by this approach should be of very high purity, is it is not contaminated by the acid or base catalyst, nor un-reacted free fatty acids.

Author Response

please, see attached pdf file

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments for catalysts-409383:

 

This manuscript reports zinc oxide/ionic liquids as heterogeneous bifunctional acid/base catalysts for production of biodiesel from waste lipids. Transesterification and esterification reactions were catalyzed simultaneously which made the process applicable in the case of common vegetable oils, animal fats, and acidic lipid wastes from oil industry. Although the main objectives of the manuscript is of hot interest in research community, there are several points of concerns that I would like to see addressed in a revised version before further consideration for publication. Here are my points of criticism on both material and stylistic nature:

1- In general, the English requires extensive corrections. Also there are lots of minor English issues in the current version of the manuscript. For example in “Introduction” line 33: They are an ideal substitute … ==> They are ideal substitutes ….

2- The introduction section must be improved. The present version of the manuscript does not provide broad information on other renewable hydrocarbon production researches (i.e. lignocellulosic biomass). Also the authors must cite the following recent publications to improve the manuscript:

-          Industrial and Engineering Chemistry Research, 57 (2018) 13257-13268.

-          Journal of Analytical and Applied Pyrolysis, 134 (2018) 150-161.

-          ChemCatChem, 10 (2018) 5201-5214.

-          Applied Energy 231 (2018) 997-1006.

3 - Throughout the manuscript, it has been repeated several times that ZnO/TBAI does not require “tedious preparation” in contrast to other catalysts!!! What is tedious preparation procedure? Please support your claims with proper literature.

4- Catalyst loading is a very important parameter that has to be deeply investigated. In this regard the effect of ZnO on biodiesel yield is only briefly explained. In section 2.1 line 80 please put “Fig. 1a” in parenthesis In the end of the sentence.

5- Section 2.1: Please provide data for the statement in line 83. To what level did the yield increase and explain why?

6- Fig. 1 must be improved. Some upper error bars fall outside the figure frames in the current version.

7- In the experimental results, the focus of this manuscript is only on biodiesel yield and the experimental parameters were optimized only based on the amount of yield. It would be nice if the authors include the chemical characterization of all samples in the supporting information document.

8- Did the authors test other order kinetic models such as zero order, second order, etc.? Please support the first-order kinetic with other published articles.

9- Although the main objective of this manuscript is to propose a new catalyst for biodiesel production, some very important catalytic parameters such as catalyst life and deactivation mechanisms were not investigated. This is the major omission of the present version of this manuscript.

10- In “Conclusion” section, please provide some of the most important kinetic study results.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Suggestions to authors:

1.-In a general view, this work is interesting in the line to looking for new pathways to produce alternative biofuels. Nevertheless, this niche is widely studied. Authors must increase the scientific value of their work by demonstrating the catalysis viability in reactor scale.

2.-Regarding the use of ionic liquids in the production of biodiesel, the authors could find interesting the following reference “New horizons in the enzymatic production of biodiesel using neoteric solvents, Renewable Energy, 98 (2016) 92-100”.

3.- Other corrections that must make in manuscript  are numbered below:

3.1.- Paragraph 205,  is repeated “and the”

3.2.- In figure 5, must corrected the axis colour to maintain the homogeneity with the rest of figures in the manuscript.

Author Response

please, see attached pdf file

 

Author Response File: Author Response.pdf

Reviewer 4 Report

This manuscript investigated synthesis of biodiesel using ZnO/ionic liqid catalytic system. Overall this manuscript was well organized and prepared. Therefore, I recommend the publication after following minor changes.

- The informaiton of ZnO such as surface area, particle size is needed.

- In fig. 1(a), 0,5 -> 0.5 on x axis.

- In fig.1 (a), on Y axis, biodisel yield % -> Biodiesel yield (%)

- In fig.5, on Y axis, biodisel yield (%) -> Biodiesel yield (%)

Author Response

please, see attached pdf file

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The revised version of the manuscript has been improved significantly and all shortcomings and concerns have been addressed properly. Therefore, I would like to recommend publication of the revised version in the present form.

Reviewer 3 Report

Reviewer considers acceptable the authors comments regarding the questions formulated in previous version of this manuscript.

Reviewer considers acceptable the authors comments regarding the questions formulated in previous version of this manuscript.