The Regeneration of Dolomite as a Heterogeneous Catalyst for Biodiesel Production

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer's Comment

The manuscript entitled "Regeneration of dolomite as a heterogeneous catalyst for bio- 2 diesel production". The authors report that dolomite as a heterogeneous catalyst can be used in biodiesel synthesis. Process material costs can be reduced by regenerating and reusing of catalyst. Catalytic efficiency and catalyst changes after 1-6 cycle of were evaluated. X-ray, FT–IR, SEM studies were performed. Though the results are interesting, however it lacks explanation, and the article lacks coherence. I believe that, given these issues, the current work is not suitable for publication at this stage. However, if it is necessary to accept publication, the following issues need to be addressed before considering publication:

1.     Lines 45-46, “This calcium was used as a solid catalyst for biodiesel production”. This sentence is abrupt here, it does not connect with the context.

2.     Lines 68-72, “catalytic activity of the porous dolomite pellets decreased linearly with increasing coke formation” and “Pore volume of the porous dolomite pellets increased to 0.55 cm³·g^-1 after calcination at 900 ℃ for 2 h, with mechanical strength remaining basically unchanged and acetic acid decomposition rate of 99.5%”. These two sentences are semantically contradictory.

3.     line 100, table 1: Calcination and Washing are used as a comparison group, with Washing having less data than Calcination. It is recommended to complete the data of Washing after 6 cycles.

4.     Lines 113-115, the authors write that “Because XRDA is the most widely employed technique for general crystalline material characterization and it allows to identification diffraction peak of crystalline phases in bulk materials”. XRDA has a writing error.

5.     Lines 118-119, “In this case, only the characteristic peaks of CaO and MgO are visible in the RSDA curves (Fig. 1 a)”. Unclear labeling of pictures.

6.     Lines 120-124, “structure of the catalyst do not change even after three use-regeneration cycles (Fig. 1, a, curves 2–4)” and “washing the remaining catalyst after the transesterification reaction with a mixture of methanol and hexane (ratio 1:10) does not remove all washing substances, because Figure 1, b curves 2 and 3 show many new peaks.”. Author's description does not match the actual image.

7.     line 132, figure 1 and line 165, figure 2: Calcined and washed as a comparison group, missing data for washing completed after 3 cycles and calcined prior to use. It is recommended that this be completed.

line 165, figure 2: Low clarity, inconsistent image size and labeling format.

Author Response

 Authors are very thankful for reviewer’s remarks and hope that quality of manuscript is increased taking into account of their suggestions.

 Reviewer: 1

Rewiever‘s comments

The manuscript entitled "Regeneration of dolomite as a heterogeneous catalyst for bio- 2 diesel production". The authors report that dolomite as a heterogeneous catalyst can be used in biodiesel synthesis. Process material costs can be reduced by regenerating and reusing of catalyst. Catalytic efficiency and catalyst changes after 1-6 cycle of were evaluated. X-ray, FT–IR, SEM studies were performed. Though the results are interesting, however it lacks explanation, and the article lacks coherence. I believe that, given these issues, the current work is not suitable for publication at this stage. However, if it is necessary to accept publication, the following issues need to be addressed before considering publication:

1. Lines 45-46, “This calcium was used as a solid catalyst for biodiesel production”. This sentence is abrupt here, it does not connect with the context.

Author‘s response

Authors agree with a comment, the sentence was removed.

Rewiever‘s comments

2. Lines 68-72, “catalytic activity of the porous dolomite pellets decreased linearly with increasing coke formation” and “Pore volume of the porous dolomite pellets increased to 0.55 cm³g^-1 after calcination at 900 ℃ for 2 h, with mechanical strength remaining basically unchanged and acetic acid decomposition rate of 99.5%”. These two sentences are semantically contradictory.

Author‘s response

Corrected based on the literature source [22].

Rewiever‘s comments

3. line 100, table 1: Calcination and Washing are used as a comparison group, with Washing having less data than Calcination. It is recommended to complete the data of Washing after 6 cycles.

Author‘s response

Clarification on the number of regeneration cycles using washing method has been added to the text:

The activity of dolomite was significantly reduced by washing with solvents when it was used for the synthesis of esters for the fourth time, when the yield of esters reached only 22.45%, i.e. more than 77% of the oil remained unreacted, so further regeneration studies by washing dolomite with solvents were not appropriate.

Rewiever‘s comments

4. Lines 113-115, the authors write that “Because XRDA is the most widely employed technique for general crystalline material characterization and it allows to identification diffraction peak of crystalline phases in bulk materials”. XRDA has a writing error.

Author‘s response

Thanks for the note, corrected

Rewiever‘s comments

5. Lines 118-119, “In this case, only the characteristic peaks of CaO and MgO are visible in the RSDA curves (Fig. 1 a)”. Unclear labeling of pictures.

Author‘s response

Corrected picture labeling in the  text.

Rewiever‘s comments

6. Lines 120-124, “structure of the catalyst do not change even after three use-regeneration cycles (Fig. 1, a, curves 2–4)” and “washing the remaining catalyst after the transesterification reaction with a mixture of methanol and hexane (ratio 1:10) does not remove all washing substances, because Figure 1, b curves 2 and 3 show many new peaks.”. Author's description does not match the actual image.

Author‘s response

Thanks for the note, the text description was corrected according with the picture data.

Rewiever‘s comments

7. line 132, figure 1 and line 165, figure 2: Calcined and washed as a comparison group, missing data for washing completed after 3 cycles and calcined prior to use. It is recommended that this be completed. line 165, figure 2: Low clarity, inconsistent image size and labeling format.

Author‘s response

 Thanks for the note, corrected.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The article “Regeneration of dolomite as a heterogeneous catalyst for biodiesel production” by R. Šlinkšienė, R. Paleckienė, I. Gaidė, V. Makarevičienė and E. Sendžikienė is devoted to regeneration of dolomite catalyst, which is widely used for processing of biomass, fast pyrolysis of waste cooking oil or animal fat into hydrocarbon fuel. Regeneration of the catalyst allows its application for a long time without any loss of activity that significantly decreases the process cost.  

 The authors clearly show that as-used dolomite catalyst regeneration through the calcination at high temperature is much more effective than washing with organic solvents. 

All the investigations are carried out on a good experimental level and described adequately.

            The main comment: the catalyst surface plays a key role in processing. The surface regeneration has to be the real aim of this work. However, no data about the surface specific surface area change, no data about the porosity change (size, concentration) were given. The only information about the surface composition on different use-regeneration stages follows from EDS analysis. However, it is difficult to read and to compare data depicted in Fig. 3. It seems that the most important positions have to be summarized in a special Table to show the dynamics of surface poisoning (by carbon, for example) and discussed in detail. The other question: how dolomite particle size agglomeration or diminishing affected the catalytic process?

 

Some other comments:

1. Line 31. “homogeneous or heterogeneous” (catalysis?).
2. Lines 43-50. There are a lot of repetitions in these sentences.
3. Line 84. “of organics substances surface of catalyst” (from surface?).
4. Lines 89 and 96. To explain abbreviations RME and FAME just here.
5. Line 119. To explain abbreviation RSDA.
6. Lines 125-131 as comment to Fig. 1. It is clear that if bulk phases were observed by XRDA, they also exist on the surface suppressing the catalytic activity.
7. Fig. 2. and depicted results description. Both washed and calcined samples have IR spectra different from this of original dolomite catalyst. It is not so easy to compare spectra on a base of authors’ explanations. Something has to be changed.
8. Lines 168,189 and 195. Why the silicon presence (and content?) is so important for catalytic process, as follows from the authors’ point of view?

Author Response

Authors are very thankful for reviewer’s remarks and hope that quality of manuscript is increased taking into account of their suggestions.

 Reviewer: 2

Rewiever‘s comments

The article “Regeneration of dolomite as a heterogeneous catalyst for biodiesel production” by R. Šlinkšienė, R. Paleckienė, I. Gaidė, V. Makarevičienė and E. Sendžikienė is devoted to regeneration of dolomite catalyst, which is widely used for processing of biomass, fast pyrolysis of waste cooking oil or animal fat into hydrocarbon fuel. Regeneration of the catalyst allows its application for a long time without any loss of activity that significantly decreases the process cost. 

 The authors clearly show that as-used dolomite catalyst regeneration through the calcination at high temperature is much more effective than washing with organic solvents. 

All the investigations are carried out on a good experimental level and described adequately.

            The main comment: the catalyst surface plays a key role in processing. The surface regeneration has to be the real aim of this work. However, no data about the surface specific surface area change, no data about the porosity change (size, concentration) were given. The only information about the surface composition on different use-regeneration stages follows from EDS analysis. However, it is difficult to read and to compare data depicted in Fig. 3. It seems that the most important positions have to be summarized in a special Table to show the dynamics of surface poisoning (by carbon, for example) and discussed in detail. The other question: how dolomite particle size agglomeration or diminishing affected the catalytic process?

Author‘s response

Thanks for the valuable insights. The text is supplemented.

Some other comments:

Rewiever‘s comments

1. Line 31. “homogeneous or heterogeneous” (catalysis?).

Author‘s response

Both homogenous and heterogeneous catalysis are used in various researches, however heterogeneous catalysis has an advantage because of easy separation of catalyst from final product and possibility to reuse it.

Rewiever‘s comments

1. Lines 43-50. There are a lot of repetitions in these sentences.

Author‘s response

Authors agree with a comment. It was corrected.

Rewiever‘s comments

1. Line 84. “of organics substances surface of catalyst” (from surface?).

Author‘s response

Authors agree with a comment, sentence is corrected.

Rewiever‘s comments

1. Lines 89 and 96. To explain abbreviations RME and FAME just here.

Author‘s response

Authors agree with a comment, explanations are added to the manuscript.

Rewiever‘s comments

1. Line 119. To explain abbreviation RSDA.

Author‘s response

Abbreviation RSDA is explained.

Rewiever‘s comments

6. Lines 125-131 as comment to Fig. 1. It is clear that if bulk phases were observed by XRDA, they also exist on the surface suppressing the catalytic activity.

Author‘s response
Authors agree with a comment, corrected.

Rewiever‘s comments

1. Fig. 2. and depicted results description. Both washed and calcined samples have IR spectra different from this of original dolomite catalyst. It is not so easy to compare spectra on a base of authors’ explanations. Something has to be changed.

Author‘s response

Thanks for the note, corrected.

Rewiever‘s comments

1. Lines 168,189 and 195. Why the silicon presence (and content?) is so important for catalytic process, as follows from the authors’ point of view?

Author‘s response

Since natural dolomite contains silicon impurities, the typical signals of this element are also detected in the instrumental research data. However, silicon is not suitable as a catalyst.