Calcium Hydroxyapatite: A Highly Stable and Selective Solid Catalyst for Glycerol Polymerization
Round 1
Reviewer 1 Report
This paper presents results for the use of calcium-hydroxyapatites as catalysts for the polymerization of glycerol. The calcium containing catalyst was used as either stoichiometric in calcium, deficient in calcium, or with excess calcium. The deficient and stoichiometric versions showed 100% selectivity at 15% conversion, while the excess calcium version showed 88% selectivity at 27% conversion. All versions deactivated in the presence of water and polymers.
It is an interesting result that more surface calcium species tend to improve the activity of the catalyst. This can guide catalyst synthesis that leads to the same result.
The paper concludes that the species identified on the surface lead to deactivation of the catalyst and also affect the surface concentration of calcium species.
The paper needs more details on the literature. There are only 4 papers referenced. From these few, it is clear that the authors catalyst holds potential, but are these the best in the field? What about non-calcium containing catalysts?
Author Response
We appreciate the reviewer’s comments. In fact, we have chosen the Ca-based catalysts as the best candidate for glycerol polymerization reaction due to their several advantages over other catalysts. This was discussed in details in our recent review paper (Ebadipour et al., 2020, Ref. 9 of this manuscript). However, more details have been added in the introduction of the present paper according to your comment.
Ebadipour, N., Paul, S., Katryniok, B., Dumeignil, F., 2020. Alkaline-Based Catalysts for Glycerol Polymerization Reaction: A Review. Catalysts 10, 1021. https://doi.org/10.3390/catal10091021
Reviewer 2 Report
In this manuscript authors have reported Calcium-hydroxyapatites (HAps) as solid acid catalyst for the polymerization of glycerol into selectivity triglycerol. Although results have merit, few areas need revisions before this manuscript can be considered for publication. See the comments below:
- For any solid acid catalysts, determination of total acid strength of the catalyst is necessary to understand the catalytic performance. Authors should provide total acidity of their Haps samples either by NH3-TPD or acid-base titration method.
- Introduction should be comprehensive covering wider area of acid catalysis. Authors should mention different phosphate based heterogeneous solid acid catalysts used. See and include these: ACS Sustain. Chem. Eng. 3 (2015) 1715-1723; Renew. Energy 101 (2017) 111-119; Energy Convers. Manag. 130 (2016) 156-164.
- Conclusion portion is little lengthy. Several paragraphs should be merged into one paragraph.
Author Response
- For any solid acid catalysts, determination of total acid strength of the catalyst is necessary to understand the catalytic performance. Authors should provide total acidity of their HAps samples either by NH3-TPD or acid-base titration method.
We thank the reviewer for the suggestion. However, HAps contain both basic and acidic sites. The basicity and acidity of these compounds are highly depending on the Ca/P ratio and these properties can significantly be tuned by the addition of Ca2+ ions (Silvester et al., 2014). Here, in this research, we have focused on basic HAps, i.e, stoichiometric HAp (noted HAp-S in the article) and Ca-rich HAp (noted HAp-R). Even the HAp deficient in Ca (noted HAp-D) tested in this work has shown a very close Ca/P ratio to HAp-S. Thus, we believe that the basic properties of HAps are predominant in these catalysts.
- Introduction should be comprehensive covering wider area of acid catalysis. Authors should mention different phosphate based heterogeneous solid acid catalysts used. See and include these: ACS Sustain. Chem. Eng. 3 (2015) 1715-1723; Renew. Energy 101 (2017) 111-119; Energy Convers. Manag. 130 (2016) 156-164.
In fact, we have chosen the alkaline catalysts as the best candidate for glycerol polymerization reaction due to their several advantages over acid catalysts as it was underlined in our recent review on the topic (Ebadipour et al., 2020, Ref 9 on this manuscript). Moreover, in the papers mentioned by the reviewer, hydroxyapatite-based catalysts have been also studied, but actually as solid basic catalysts not acid catalysts.
- Conclusion portion is little lengthy. Several paragraphs should be merged into one paragraph.
We thank the reviewer for the careful attention. We have merged the paragraphs in the conclusion.
References
Ebadipour, N., Paul, S., Katryniok, B., Dumeignil, F., 2020. Alkaline-Based Catalysts for Glycerol Polymerization Reaction: A Review. Catalysts 10, 1021. https://doi.org/10.3390/catal10091021
Silvester, L., Lamonier, J.-F., Vannier, R.-N., Lamonier, C., Capron, M., Mamede, A.-S., Pourpoint, F., Gervasini, A., Dumeignil, F., 2014. Structural, textural and acid–base properties of carbonate-containing hydroxyapatites. J. Mater. Chem. A 2, 11073–11090. https://doi.org/10.1039/C4TA01628A
