The manuscript investigated the biodiesel production catalyst based on CaO synthesized by gel-combustion. It is an interesting paper providing some useful results, and is suitable for short communication. However, the clarity of the paper needs to be improved before publication.
1) The authors used the term "FAME content" for several places. The term "FAME yield" is also employed as presented in Table 1. It seems that the authors used this term in the same meaning. I understood that the excess methanol was evaporated before analysis of FAMEs. However, I may disagree to use "FAME content" instead of "FAME yield"; otherwise the authors need to clarify.
In addition, according to the above mention, the following sentences "A typical process for preparing CaO catalysts has been known as a direct calcination of calcium carbonates or nitrates……….. From the literatures, it is difficult to achieve biodiesel with FAME content above 96.5%..." (Page 7) are need to reconsider. The authors need to ensure for the claimed sentences.
The authors need to clarify these points.
2) As known, the calcination temperature is one of the key factors to prepare active CaO catalyst; for the direct calcination method, the authors should explain why lower calcination temperature (600 C) was applied for calcium nitrate precursor.