Optimized antioxidants for biodiesel

Richard A. Larsonand Karen A. Marley

Department of Natural Resources and Environmental

Sciences

Soybeans Rapeseed

Oilseeds = high in lipids (triglycerides)

Methyl linoleate

Methyl stearate

Methyl oleate

Glycerol

Hydrolysis of triglycerides with methanol and base affords fatty acid methyl esters (FAMEs) and the by-product, glycerol.

+

Unsaturated triglyceride

Biodiesel defined

B(n) = Fraction of biodiesel in fuel.

B100 = Pure biodiesel

B20 = 20% biodiesel, 80% petroleum biodiesel.

Biodiesel is susceptible to oxidative damage upon

storage

Unsaturated FAMEs are readily attacked by oxygen (linolenic [3 double bonds]> linoleic [2]> oleic [1])

Oxidized products are deleterious to engine performance

(N. Li, Biodiesel Magazine, June 2007)

Free radical chain

mechanism of lipid peroxidation

Free radical kinetics

Free radical chain reaction kinetics

(Peroxide concentration)

Types of antioxidants

Peroxyl radical quenchers (chain breakers)

Peroxide destroyers (reducing agents)

Metal chelating agents

Acid neutralizers

Chain-breaking antioxidants (radical quenchers)

Vitamin E (alpha-tocopherol)

Peroxide destroyers (reducing agents)

Phosphine Phosphine oxide

Peroxide analytical method

B. Mihaljevic et al., Free Radical Biol. Med. 21:53 (1996)

ROOH + Fe2++ H+ ROH + Fe3+

Fe3+ + 5 SCN- Fe(SCN)5-

= 510 nm𝜆𝑚𝑎𝑥

Propyl gallate was the most effective chain-breaking antioxidant (as shown by inhibition of peroxide formation) for biodiesel B100

0.0

100.0

200.0

300.0

400.0500.0

600.0

0 500 1000 1500 2000 2500

PV

(m

mo

les

/kg

)

Time (hours)

SME at 60 C

SME only

w/ BHT

w/trolox

w/ AP

w/PG

SME = soy methyl ester (B100); Trolox = a Vitamin E analog; AP = ascorbyl palmitate; PG = propyl gallate

Tests were run at 60C because propyl gallate rapidly decomposed at higher temperatures

-4-3.5

-3-2.5

-2-1.5

-1-0.5

0

0 200 400 600 800 1000

ln P

G t

/ PG

0

Time (hours)

2.5 mM PG in SME

100 C

80 C

60 C

Also observed by Dittmar et al., 2004 (Chem.-Ing.Technol. 1167-1170).

A peroxide destroyer (triphenylphosphine, TPP) alone was of limited effectiveness. However, note the great improvement when propyl gallate (PG)was added. This is a waste oil-derived B100

with a high initial peroxide value.

0 50 100 150 200 250 300 350 400 450 5000.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

ISTC BD at 60 C (after TPP tmnt)

No addns

w/ TPP

w/ TPP + 1 mM PG

Time (hours)

PV

(m

mo

les/

kg)

0 200 400 600 800 1000 1200 1400 1600 18000.0

0.1

0.2

0.3

0.4

0.5

0.6

Intsoy BD at 60 C

No addnsLTLT + PGHTHT + PG

Time (hours)

PV

(m

mo

les

/kg

)

A different B100 with a very high initial peroxide value was stable for more than 70 days (!) at 60C when treated with stoichiometric TPP and 1 mM PG

LT = low [TPP] (0.5 equivalent); HT = high [TPP] (1.0 equivalent)

Conclusions

Propyl gallate (PG) was the most effective chain-breaking antioxidant tested. This agrees with similar studies reported in the literature (Loh et al., 2006).

A peroxide destroyer (triphenylphosphine, TPP) was of limited effectiveness except when added in an equivalent amount to peroxide concentration.

Stoichiometric TPP plus a small amount of PG was very effective in preventing B100 oxidation over a long time at 60C.

Initial peroxide concentration is an important parameter for assessing biodiesel stability.

Acknowledgments

Illinois Sustainable Technology Center for financial support and for providing waste-oil derived B100.

Columbus Foods for purified B100.

The UIUC Intsoy program also provided B100.

W. Zhang, N. Holm, K. Rajagopalan and J. Scott for helpful discussions.