Preparation of Ca- and Na-Modified Activated Clay as a Promising Heterogeneous Catalyst for Biodiesel Production via Transesterification

Round 1

Reviewer 1 Report

In the present paper entitled “Preparation of Ca and Na modified activated clay as a promising heterogeneous catalyst for biodiesel production via transesterification”, Hanbing Zhang and co-workers reported the preparation of an acid-activated clay modified by calcium hydroxide and sodium hydroxide (called CaNa/AC) and its application as heterogeneous catalyst in the production of biodiesel via transesterification of Jatropha oil. Although the present study could become suitable for publication on Applied Sciences, I think that at this stage there are several issues that need to be carefully addressed, thus suggesting major revision.

1. First of all, as a general consideration, I found the present paper quite difficult to read and understand: in addition to some merely linguistic considerations (i.e., typos on line 6 “^1,3 and *” instead of “^1,3,*”; line 178: “spectrums” instead of “spectra”; line 189 “However, Due” instead of “However, due”), in my opinion the aim and the conclusion of the work are not clear enough, in comparison with the existing literature. On the one hand, what advantages should the preparation and catalytic study of CaNa/AC provide in comparison with other typical heterogeneous catalysts for biodiesel production? On the other hand, at the end of the paper, did the performance of CaNa/AC actually live up to authors' expectations? I believe the authors should better emphasize both of these aspects in the manuscript.

2. Considering in more detail some specific points of the paper, in the introduction the role and the typical benefits related to the use of heterogeneous and supported catalysts should be better emphasized (in particular, see DOI: 10.1016/j.cattod.2012.04.003; DOI: 10.3390/catal12010068; DOI 10.1039/C4CS00189C). At the same time, I also suggest a deeper literature overview on the use of acid-activated clay as a support for catalysts; in particular, are there any examples of acid-activated clay modified by calcium/sodium hydroxide as heterogeneous in the literature? These examples should be well emphasized in the manuscript.

3. In the “Base strength and basicity” section, I believe that the authors should partially change the discussion. If it appears evident that the catalytic activity is correlated to the base strength, it is not clear why the base strength of Na/AC was higher than that of Ca/AC; the highest base strength of CaNa/AC could be instead attributed to the synergystic effect of both sodium and calcium. All these point should be clarified in the text.

4. In the “CO₂-TPD analysis” section, why was the CO₂-TPD measurement on Na/AC sample not carried out? In order to verify the effect of Na on the basicity of CaNa/AC, I think that also the analysis on the Na/AC sample could be useful.

5. Concerning the BET analysis, the surface area of CaNa/AC was of 15 m²/g, about one order of magnitude lower than that of Ca/AC (that is, 110 m²/g). According to the authors’ interpretation, the smaller surface area of CaNa/AC indicated that the surface and pores of AC were covered by both sodium and calcium hydroxide, while in the case of Ca/AC both the surface and pores were less covered since only calcium hydroxide was present. According to this interpretation, why the surface area of Na/AC was of 13 m²/g, that is, lower than that of CaNa/AC?

6. Another critical point of the work is that most of the studies on transesterification (optimization of the reaction conditions and kinetic analysis) were carried out only on CaNa/AC. Honestly, this choice is not very clear, at least from my point of view: why both the Ca/AC and Na/AC samples were prepared and also characterized? I think that at least some representative tests of transesterification should also be performed on the other two heterogeneous systems synthesized in this work.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript is interesting. Authors investigated possibilities to use an acid-activated clay modified by calcium hydroxide and sodium hydroxide (CaNa / AC) as a catalyst for efficient biodiesel production. This manuscript can be considered for publication if a revision would be carried out focusing on the issues below:

• The properties of the AC-based clay should be added to the Materials and Reagents part.
• If acid and saponification values, fatty acids composition of the oil were determined, methodology has to be provided.
• The process is catalyzed by Ca and Na ions which are in the catalyst, so it is important to know the exact composition of the catalyst. It is also not justified why exact amount of CaO and NaOH are added.
• It is not specify which stirring speed was used and it is not clear why 2 hours and a temperature of 343 K were chosen in 2.2 part.
• It is not specify how much of n-hexane was used to wash the catalyst and why 2 hours and dried temperature 473 K was chosen in the methodological 2.6. Catalyst recycling?
• Which standard and what kind of chromatograph were used to determine the conversion of jatropha oil?
• The temperature of Kelvin used in the text, however the Celsius were used in Figure 3, therefore it has to be unified.
• The dimensions has to unified, there is mg KOH/g (line 84), mmol of HCl/g (Table 1), kJ/mol (line 328), etc., and m²g⁻¹ (Table 2), min^–1 (line 323).
• Incorrect spelling “catalyst/oil mass ratio of xx%”
• Lack of statistical data evaluation.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This is a manuscript in which a process of transesterification of triglycerides with heterogeneous catalysts is evaluated. This allows the reuse of the catalysts, with very good results. The catalysts used are acid-activated clay mineral, activated with calcium hydroxide and sodium hydroxide. Conventional processes for the characterization of the catalysts are used and the bibliography cited is very up-to-date. Therefore, its acceptance for publication is recommended. However, from the beginning of the manuscript, the meaning of AC must be clarified, which we assume will be "activated clay", but it is necessary to clear this doubt in the readers.

Author Response

Point: From the beginning of the manuscript, the meaning of AC must be clarified, which we assume will be "activated clay", but it is necessary to clear this doubt in the readers.

 

Response: Thank you for your suggestion. AC was prepared by a traditional wet method. 50 g bentonite was put into a beaker, mixed with 175 mL H₂SO₄, and heated at 363 K for 4 h in a water bath with a magnetic stirrer. The concentration of the H₂SO₄ solution was varied between 16% and 24% by mass. After acid treatment, each sample was filtered under vacuum and the precipitate was washed with distilled water until the pH of the filtrate was above 5.0. Then, AC was dried at 378 K for 4 h, ground to a powder, and stored in the desiccator.

We have added “acid-activated clay (AC)” in the “Introduction” and added related experimental procedures in yellow text in Line 17, Page 1 and Line 102–107, Page 3.

Round 2

Reviewer 1 Report

In the revised version of the manuscript “Preparation of Ca and Na modified activated clay as a promising heterogeneous catalyst for biodiesel production via transesterification”, Hanbing Zhang and co-workers well addressed all the issues listed in my previous report, thus allowing to improve the quality and clarity of the work. I have read this new version very carefully, and I want to thank the authors for their efforts to improve their paper. I am convinced that the manuscript now meets the standards for publication in Applied Sciences, suggesting its acceptance in the present form.

Reviewer 2 Report

Authors have adequately addressed the concerns previously raised. The manuscript may now be accepted for publication.