Sciencemadness Discussion Board - Transesterification of fractionated coconut oil?

Transesterification of fractionated coconut oil?

blogfast25 - 13-5-2011 at 09:26

I wanted to make a mixture of ethyl esters from roughly C8 - C10 alkanoic acids and tried to transesterify some fractionated coconut oil (FCO) (ex eBay) today. According Wiki, FCO is a triglyceride mainly of C8 (caprylic acid) and C10 (capric acid) fatty acids, so transesterifying this with ethanol should give me something fairly close to target.

I got the transesterification recipe from a biodiesel site, which called for 800 ml virgin vegetable oil, 175 ml methanol and 7 gram of KOH per liter of veggie oil and adjusted the amount of methanol for the substitution by ethanol.

50 ml of the FCCO were warned up to about 50 C, meanwhile dissolving 0.35 g of KOH in 16 ml of hot ethanol. Then both liquids were then combined and stirred up. No noticeable reaction took place: the liquids mix perfectly to a clear solution. I took the temperature up 80 C and kept it there for a while but no change happened whatsoever. I even added some more KOH (maybe some free fatty acids had saponified the KOH?) but to no avail. I set the mixture aside.

Frustrated, I did three things:

1. Apply the exact same procedure to virgin sunflower oil. It worked perfectly: immediately the mixture becomes translucent and a glycerine holding phase separates out. I raised the temp. to about 80 and concluded the reaction was over after about 15 min. The phases were then separated and the ester bearing phase washed a few times in a separating funnel with water. The crude product will be worked up some more.

2. Transesterify the FCO ‘the long way around’, by first saponifying the oil, then neutralising the soap and acid catalysed esterifying the fatty acids with ethanol.

So 50 ml of FCO were combined with a solution of 15 g of NaOH in about 50 ml of deionised water and heated up. Saponification proceeded quickly and a perfectly white, quite stiff mass of mixed sodium soaps was obtained in about 15 mins. I drained off any excess liquid and added 40 g of 37 % HCl, slowly. The heat of neutralisation was considerable and caused the freed fatty acids to melt, separated neatly from what’s essentially a mixture of NaCl solution and glycerine. I was hoping the fatty acids would solidify on cooling (C8 and C10 have low melting points) but that didn’t happen. Odd. So I separated the phases by separation funnel and obtained 35 g of crude fatty acid mixture, which I didn’t work up any further, the smell is somewhat reminiscent of goat cheese.

Instead it was combined with 20 ml of ethanol and 5 ml of 95 % H<sub>2</sub>SO<sub>4</sub> (all into one phase) and refluxed on steam bath for 30 minutes. On cooling I found two distinct phases, the top one smelling very pleasantly and strongly of ester. After separation the organic phase was washed twice with water and once with 1 M NaOH. 30 ml of crude ester mix was obtained and will be worked up tomorrow.

3. The FCO/ethanol/KOH mixture that had failed to react was poured off into an clean pyrex receptacle and 15 g NaOH dissolved in 50 ml of water added to it and heated up. At first clearly some soap was formed in the shape of some ‘dross’ (it dissolves effortlessly in deionised water fells like and tastes of soap). But also a strong smell of ester developed and a small amount of organic phase settled underneath the soapy froth.

So why did the transesterification of the FCO failed at both attemps? Wrong quantities?

unionised - 13-5-2011 at 09:46

It's possible that the first reaction worked but all the products are miscible. Adding water might have got them to separate out.
Ethanol is quite a good solvent, so are most esters. If the ester mix you are after is a liquid (and I think it is) then there's every chance it will all mix together.
Capric, caprylic and caproic acids get their names from the same root as capricorn. They stink like goats.

Dr.Bob - 13-5-2011 at 10:08

The biodiesel transesterification with ethanol is very difficult, as ethoxide is MUCH more sterically hindered than methoxide, so it will usually require 10 times more time, double the heat and much more excess ethanol than the methanol reaction to go to completion. Also, with ethanol the phases do not separate as well.

That is why commercial biodiesel companies almost all use methanol, despite the "green-ness" that ethanol based fuel would have. It is likely that only by biochemical processes will ethyl ester based biodiesel be made practically. It would be awesome if you could just get some algae or bug to simply make fuel for you from sunlight or other wastes, but it is not working yet, as far as I know.

Bob

Ozone - 13-5-2011 at 11:33

I've had better results (soybean oil) proceeding through the acid-catalyzed (H2SO4) route (at 78°C). The ethanolysis is much more difficult than the methanolysis (which works well at rt). Strong-acid ion exchange resins, such as Amberlyst, also work. Phase separation is a pain, and stongly dependent on the amount of EtOH included in the mixture. Also, the fuels appear to be less stable over time, possibly due to residual catalyst (despite washing).

O3

blogfast25 - 13-5-2011 at 13:03

In the mean time, the product of 3. had in fact solidified out. The soap must indeed have remained molten or soluble in the mix. That soap has now also been converted to ‘C8/C10 ethyl ester’.

I didn’t realise the difference between methanol and ethanol was do dramatic in this biodiesel lark. Methanol does put a bit of a dampener on the ‘bio’ in biodiesel. But biodiesel isn't my purpose here.

So I’ll get some methanol ex eBay (probably ‘biodiesel grade’, LOL), the methyl esters are possibly more interesting anyway.

So, it’s likely the product obtained with ethanol and sunflower oil is low quality, with unreacted sunflower in it?

Thanks all…

fatty esters

Magpie - 28-10-2011 at 14:31

Introduction My purpose for doing this experiment was to prepare ethyl laurate (C12). I chose to do this at 1/6th scale using the procedure in Organic Syntheses, “Methyl Myristate and Methyl Palmitate and the Corresponding Acids,” Vol. 20, p.67 (1940) Coll. Vol. 3, p.605 (1955) by Sauer, Hain, & Boutwell. The synthesis is by transesterification of coconut oil with ethanol to produce (predominantly) three ethyl esters: ethyl caprylate (C8), ethyl laurate (C12), and ethyl myristate (C14). Org Syn gives the expected weight distribution of the three esters. I calculated this to be 8.4g:58.4g:10.0g for a 1/6th scale batch.

As a point of interest the saponification number for coconut oil is 256.5. Based on this number the average molecular weight was calculated to be 667. In comparison, using the expected ester weight distribution, a value of 625 was found.

For the 1/6th scale batch the total number of moles of ester is expected to be 0.34. The moles of ethanol charged will be 6.87. Therefore the mole ratio of ethanol:ester is 6.87/0.34 = 20.0. The synthesis occurs by transesterfication whereby one alcohol (ethanol) is used in large excess such that, in time, it replaces the existing alcohol (glycerol) of the triglycerides. Therefore, the products are ethyl esters.

Chemicals The coconut oil was Lou Ana brand, food grade, and the ethanol was Clear Spring 95% ethanol, food grade, as shown in the picture. Muriatic acid was used as catalyst. The barium carbonate was pottery grade. The sodium chloride was food grade.

Transesterification 166.7g of coconut oil (a solid), 422.5mL of 95% ethanol, and 8.3g of muriatic acid were charged to a 1000mL RBF. This was refluxed on a steam bath for 11hrs. (The Org Syn procedure specified 15-20hrs.)

Neutralization & Salting Out Following the reflux barium carbonate was added to bring the pH up to a methyl orange endpoint. The resulting pH was slightly above 3. The neutralized product was then mixed with an equal volume of saturated aqueous NaCl to salt out the esters. The separated products are shown in the separatory funnel in the picture below. The lower layer (NaCl, BaCl2, water, glycerol, ethanol) has a pink cast. The gold tinted upper layer contains the esters.

Water Wash After draining off the aqueous layer the ester layer was washed with 100mL of water. The separated layers are shown in the picture below with the clear water layer on the bottom. The upper ester layer is somewhat emulsified.

Fractional Vacuum Distillation Here’s where the fun begins: severe bumping, severe foaming, and column flooding. All of these must be overcome for successful distillation.

a. bumping At first I tried an uninsulated column with broken glass packing and an ebulliator tube to prevent bumping. The ebulliator tube was completely ineffective at preventing the severe bumping. I then switched from a 500mL pot to a 1000mL pot, removed the ebulliator tube, and added aggressive magnetic stirring. These measures were successful in preventing the bumping, but just barely.

b. foaming Severe foaming was also a problem. A couple drops of silicone oil were added to no avail. This was found to be controllable by bringing up the heat and the vacuum very slowly. The larger pot was also a great help here.

c. flooding The column also tended to flood. I then switched to a loosely packed stainless steel scrub pad for packing. It was also necessary to keep the distillate take off rate at no more than ~ 5drops/min. Even at this low rate the reflux was heavy, and the reflux ratio was estimated at around 20:1. The refluxing could be seen at the distillation head which acted as a reflux condenser.

By this time I was using a fully insulated column and pot. The condenser was air cooled throughout although the water cooling hoses are shown in the distillation setup. Water cooling was abandoned when condensate began solidifying in the condenser during the separation of ethyl myristate which has a mp of 11C.

d. vacuum pump This was my first use of a new Harbor Freight vacuum pump. I must say that despite my earlier reservations about buying it, it performed admirably. Since I had no experience with a vacuum pump I learned quickly that if you bleed in air a lot of oil will be exhausted at the air exhaust port. However, when the pump pulls against a closed system with no vapor flow no oil is lost! Since my product was esters of vegetable oil (essentially biodiesel) I saw no need for a cold trap. I planned to simply change out the oil when done with the numerous fractionations that this synthesis requires. Below is a picture of the vacuum pump. A second picture shows the vacuum it pulled when in use during a fractionation.

e. distillation cuts Separation of the three primary esters was accomplished by selecting 4 cuts based on boiling point ranges. Since most of the distillation was done at either 10mmHg or 25mmHg pressure boiling point ranges were chosen that centered around the boiling points corresponding to those pressures. For the 1st distillation: 1st cut bp range was 65-110C; 2nd cut bp range was 110-121C; 3rd cut bp range was 121-140C; and 4th cut bp range was 140-163C. Then for the 2nd distillation 3 cuts were made, each representing one of the three ethyl esters: caprylate, laurate, or myristate.

Following the 4th cut of the 1st distillation 25mL of coffee colored residue remained as shown in the 250mL RBF. This was discarded.

Results & Discussion Although ethyl myristate is very likely most of the 4th cut of the 1st distillation due to the evidence of freezing in the condenser the purity of the other two ester cuts is in question. This is evidenced by the weight distribution which was way too heavy in caprylate and way too light in laurate. Also the total weight of the esters is about 2 times as much as it should be. I attribute this principally to 2 errors: (1) the main error is my not knowing exactly what my distillation pressures are due to the rough indications possible with a cheap bourdon gage, and (2) my not distilling slowly enough to insure that there is no column flooding. Therefore, I plan to work on solving these two problems and then repurifying the 3 cuts I now have on hand.

I believe I can solve both the above problems. I have a Bennert manometer which will allow me to read the pressure accurately within +/- 2mmHg. It just needs to be filled with mercury at a deep vacuum. It is especially important to be able to read the vacuum accurately at pressures lower than 5mmHg as boiling point drops off rapidly here.

Since the distillation column is wrapped with insulation I can’t see if it is flooded except at the top. By distilling very slowly flooding will hopefully be prevented.

Comments, recommendations, and questions are welcomed.

precursors for transesterification.JPG - 97kB

neutalization & salting out the esters.JPG - 78kB

$fractional distillation of esters.JPG - 105kB$

[Edited on 28-10-2011 by Magpie]

[Edited on 29-10-2011 by Magpie]

bbartlog - 29-10-2011 at 06:23

Nice. I will have to try HCl catalyzed transesterification. My attempts using ethanol plus base have all turned into horrible emulsified glop.
Anyway...
- is ethyl laurate your final goal for this project, or do you then have something further in mind to do with it?
- why did you use BaCO3 for neutralization instead of something more common (cheaper) like CaCO3? Just had it handy?
- what are the actual weights (or volumes) of the fractions you obtained? You discuss them, but don't actually give them.

Also, while you may be right about the weight ratios being evidence of lack of purity, there is also a lot of variation in natural products like coconut oil. Finally, if a reflux ratio of 20:1 is not slow enough, I have to wonder if there isn't some other approach; do you have a taller column (or a wider one)? I realize that insulating it sufficiently could be a problem...

Ozone - 29-10-2011 at 07:14

That makes sense. We achieved good results with a 5:1 EtOH:soybean oil (which gave clear phase separaton) ratio using 2-5 % H2SO4 as a catalyst. The reaction was run just under reflux with a condenser to catch the incidental evaporate. At the end, most of the the remaining EtOH was distilled off and recovered. we left 5-10 % EtOH in it to improve the gel-point of the product.

Of interest, the acidified EtOH layer, prior to reflux, was found to have selectively extracted the tocopherol-complex and phytosterols.

BaCO3 makes sense for removing residual H2SO4 catalyst (e.g. filter or settle out the insoluble BaSO4). With HCl, either chloride is water soluble and ammenable to removal via wash.

In my experience, the various oils of commerce are remarkably reproducible, in terms of composition. The quality control on food-grade products is actually quite good.

cheers,

O3

[Edited on 29-10-2011 by Ozone]

Magpie - 29-10-2011 at 08:56

Thanks for your interest, bbartlog.

Quote: Originally posted by bbartlog

Nice. I will have to try HCl catalyzed transesterification. My attempts using ethanol plus base have all turned into horrible emulsified glop.

IIRC using a base like NaOH will saponify the triglycerides to soaps, some of which at least, are insoluble. I think that is specifically why OrgSyn specifies an acid catalyst.

Quote: Originally posted by bbartlog

- is ethyl laurate your final goal for this project, or do you then have something further in mind to do with it?

No, it will be a precursor for making lauryl alcohol per the Bouveault-Blanc reaction in OrgSyn.

Quote: Originally posted by bbartlog

- why did you use BaCO3 for neutralization instead of something more common (cheaper) like CaCO3? Just had it handy?

This was specified in the OrgSyn procedure, and I did have it handy.

Quote: Originally posted by bbartlog

- what are the actual weights (or volumes) of the fractions you obtained? You discuss them, but don't actually give them.

I didn't include them in the write-up as they are tentative, but will provide them here:

light fraction (caprylate): 35.2g........OrgSyn: 8.35g
middle fraction (laurate): 73.7g........OrgSyn: 58.4g
heavy fraction (myristate): 13.6g........OrgSyn: 10.0g
residue:.......................... ~21g
totals:............................... 143.5g...................: 76.8g

Quote: Originally posted by bbartlog

Also, while you may be right about the weight ratios being evidence of lack of purity, there is also a lot of variation in natural products like coconut oil. Finally, if a reflux ratio of 20:1 is not slow enough, I have to wonder if there isn't some other approach; do you have a taller column (or a wider one)? I realize that insulating it sufficiently could be a problem...

I only have the one 19/22 Hempel column. I'm sure a longer, wider column would be an improvement. Insulating it would not be a problem, just takes more fiberglass, aluminum foil, and duct tape.

-----------------------

edit: Here's another observation that confirms that cheap bourdon gages are not acceptable for close pressure measurement. Today I was running a leak test on my Bennert manometer (not yet filled with mercury) when I noticed that I could only get down to 25mmHg as indicated on my bourdon gage. The gage was laying horizontally. When I tipped it to a vertical position it read close to 5mmHg. So, gravity was bringing the needle down about 20mmHg. Not good.

[Edited on 30-10-2011 by Magpie]

Dr.Bob - 1-11-2011 at 13:08

Magpie,

If you can obtain anhydrous ethanol, it will work much better for the esterification. Most methanol sold is fairly dry (although it will absorb moisture from the air). But if you have 5% water in the ethanol, that will really impact the yield. That is another reason why ethanol is not used for biodiesel, as it holds water, which really kills the reaction, as it competes with the ethanol to hydrolyze the esters.

Worse yet, waters MW is lower than Ethanol, so 5% by weight, is actually a much larger molar percentage, plus you need 3-5 equivalents of ethanol, so the total amount of water present for 5% is actually more like 30-60 mole percent. The acid based methods are less sensitive to water than the basic ones, but water is still bad.

So for making esters, you can make the methyl fairly easy. Or if you want the ethyl, you really have to use 1) dry ethanol 2) pure reagents 3) a larger excess of alcohol than for methanol 4) more time 5) more temperature, and 6) good luck cleaning up the product, as glycerin and ethanol are quite miscible, so it is harder to separate the glycerin at the end.

That is why the only practical way to make ethanol based biodiesel will be an enzyme catalyzed or other similar biological pathway. If you could get algae to make the fatty acids/triglycerides as well as some ethanol, and then esterify them and somehow extrude the product from the algae, that would revolutionize the biofuels industry. Several groups are working on this, but it requires making yeast fermentation genes work in algae, plus creating a triglyceride enzyme catalyst that specifically works with ethanol, which may not be a trivial problem.

I do wish someone would solve this, as the biodiesel would be essentially non-toxic, very "green" and might be able to make an impact on the US fuel situation. But it is not an easy problem to solve.

Bob

Magpie - 1-11-2011 at 15:18

Quote: Originally posted by Dr.Bob

Magpie,

If you can obtain anhydrous ethanol, it will work much better for the esterification.... But if you have 5% water in the ethanol, that will really impact the yield. That is another reason why ethanol is not used for biodiesel, as it holds water, which really kills the reaction, as it competes with the ethanol to hydrolyze the esters.

Worse yet, waters MW is lower than Ethanol, so 5% by weight, is actually a much larger molar percentage, plus you need 3-5 equivalents of ethanol, so the total amount of water present for 5% is actually more like 30-60 mole percent. The acid based methods are less sensitive to water than the basic ones, but water is still bad.

....Or if you want the ethyl, you really have to use 1) dry ethanol 2) pure reagents 3) a larger excess of alcohol than for methanol 4) more time 5) more temperature....

Thanks for your useful comments, Dr Bob. I debated about whether to use my 95% ethanol or take the trouble to dewater it with 3A mol sieves. My reasoning was that with a 20:1 mole ratio of ethanol:ester I would have the luxury of ignoring the competition with water. It sounds like I made the wrong choice.

Quote: Originally posted by Dr.Bob

... and 6) good luck cleaning up the product, as glycerin and ethanol are quite miscible, so it is harder to separate the glycerin at the end.

Since this was to be a one-time experiment and I was just after the ethyl laurate, I was not concerned with salvaging the unused ethanol.

BTW, how do you read a pH for the methyl orange endpoint if the only water you have in the system is the little that comes in with the muriatic acid?

Dr.Bob - 2-11-2011 at 09:50

Quote: Originally posted by Magpie

BTW, how do you read a pH for the methyl orange endpoint if the only water you have in the system is the little that comes in with the muriatic acid?

I have found that if you prewet the pH paper with water, it will usually give a decently accuarate reading in non-aqueous systems, but you have to read it quickly. Also, if you can use conc. sulfuric acid instead of HCl, that might work better for precisely the reason that it avoids adding more water.

I am not saying that any trace of water will completely kill the reaction, only that less water will almost certainly improve it. I know it is sometimes hard to work in completely dry conditions. (Try doing moisture sensitive experiments in July in the South!)

Magpie - 5-12-2011 at 19:15

re: my 10-28-11 write-up above: ethyl esters of coconut oil

Here's the results after refractionation of the three cuts previously obtained:

.............................................................g predicted.....g actual
ethyl caprylate (C8)...................................8.35...............11.8
ethyl laurate(C12)...................................58.4.................62.8
ethyl myristate (C14)...............................10.0.................13.6
other ethyl esters (C9-C11?)....................0....................12.2

Of great help was the boiling point data at reduced pressures for the esters. This data was obtained from a search of Google Books. I plotted it on semi-log graph paper as shown in the photo below.

Questions and comments are welcomed.

[Edited on 6-12-2011 by Magpie]

bps of ethyl esters at reduced pressures2.JPG - 99kB