Biodiesel: KOH vs Sulfuric Acid

Questions and Answers from Interns to Instructors:

Subject: Re: Question about catalysts in bio-diesel reaction
Date: Tuesday, February 1, 2011, 8:14 AM

I wanted to go back to my question I asked you Sunday evening. So if just a base (KOH) is added as a catalyst, then it might make soap with the water and free fatty acids. But if sulfuric acid is added before the base, then the acid can deprotonate the methanol so the methanol and free fatty acids react. I am still unclear about the role the acid plays. I don't think I stated the role of the sulfuric acid correctly because an acid wouldn't deprotonate the methanol, right? If the acid was not used, then the free fatty acids would be lost as soap and the yield wouldn't be as high.

How do the triglycerides and diglycerides break apart? I looked up the reaction mechanism and please correct me if I am wrong. So the base deprotonates the methanol and the deprotonated alcohol molecules now have a negative charge which is attracted to the positive charge of the esters of the triglyceride. If I stated that correctly, then I am clear on the KOH molecule's role, but what about the sulfuric acid's role. I understand it is a catalyst. But I am not sure of how it works with the free fatty acids and methanol. Thanks for your assistance.

Sincerely, Anura 

ANSWER

Hi Anura,

The sulfuric acid protonates the OH on the free fatty acid, which makes it a better leaving group (as H2O). The O of the methanol attacks the carbonyl carbon of the fatty acid and forms a bond as the protonated OH leaves as water, and the H of the methanol OH falls off to regenerate the acid catalyst.

When food is frying in the cooking oil, it has moisture( Think of the water content in meat , french fries, etc)The H2O molecule can add across the ester link(C=O)-O- in the triglyceride and make the (C=O-OH) of the acid, and the HO- on the glyceride. 

The KOH reacts with the methanol to make potassium methoxide (CH3O-   K+) and H2O.The methoxide anion attacks the triglyceride ester and knocks off a fatty chain as the methyl ester, which is biodiesel. After methoxide has knocked off 3 fatty chains as methyl esters, it leaves glycerin as a by-product. Hope this helps! 

Bill