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The Chemistry of Biodiesel Oxidation
Presentation Overview
Chemical Properties and Environmental Factors affecting Biodiesel Stability
Mechanistic Pathways of Biodiesel Degradation
Focus on Oxidation
FAME conversion to volatile compounds
Diversity of Chemicals Produced
Summary
Instability Concerns
Food - Cooking Oils - Whole oils / Triglycerides
“Going Rancid” - Production of undesirable odors or flavors
Fuel - Biodiesel - FAME
Increase in Acidity
Corrosion
Increase in Viscosity
Deposit formation
Fuel filter plugging
Injector Deposits
Fuel economy
Excessive smoke emission
Factors Affecting Biodiesel Stability
Manufacturing Operation
conditions and processes used
Antioxidants
natural or added
Potential Impurities
Acids, glycerin/glycerides, metals
Storage Conditions
exposure to water, light or air
Composition
Whole Oil to Biodiesel
O
WHOLE OIL / TRI GLYCERIDES
7
O
O
O
O
O
7
MeOH
Catalyst
7
12
4
O
7
OH3CO
H3CO
H3CO
O
7 7
10
4
OH
OH
OH
Steryl
Linoleyl
Oleyl
PALM /TALLOW
SOY
RAPE SEED
FAME Composition
0
20
40
60
80
%
% Poly Unsaturated Fatty Acids Tallow
Coconut
Yellow
Palm
Rape
Canola
Peanut
Cotton
Soy
Sun
Linseed
Pathways to Biodiesel Instability
Thermal Instability
Associated with Polymer forming reactions
Hydrolytic Instability
Formation of acids via ester cleavage
Oxidative Instability
Mixture of degradation products
Mechanisms of Biodiesel Degradation
Thermal Degradation
Bio Diesel relatively thermally stable –in absence of oxygen and water
Increasing Temperature increases the rate of other Degradation Pathways
Mechanisms of Biodiesel Degradation
Hydrolytic Degradation
Breakdown of the Biodiesel by the reaction with water
Catalyzed by acids, bases and enzymes
Ester Hydrolysis
Water
Methanol
CH3OHFAME
+OCH3
O
76
OH
O
76
Mechanisms of Biodiesel Degradation
Oxidative Degradation Main mode of Biodiesel Instability
Two Step Process
Peroxide Formation
Peroxide Decomposition
Peroxide Formation
Air
Peroxide Decomposition
Complex Mixture
OCH3
O
76
OOH
OCH3
O
76
Two Stage Oxidation
Stage I
Peroxides - distinct step in the oxidation
Stage II
Rancimat measure decomposition products
Increased peroxide content directly impacts Induction period
Peroxide Content vs Temperature
0
10
20
30
40
50
20 30 40 50 60 70 80
Temperature C
Pe
rox
ide
co
nte
ng
mg
/kg
Rancimat Response to Stressing
22.5
33.5
44.5
55.5
20 30 40 50 60 70 80
Temperature C
Ra
nc
ima
t H
rs
Mechanisms of Biodiesel Degradation
Peroxide Formation
Peroxide Formation
OCH3R
OAirFAME
Peroxide Decomposition
Complex Mixture
OCH3
O
76
OOH Volatile Organics
Stage I - Peroxide Formation
Singlet Oxygen
Formed via a photochemical process
H3CO
O
7
OO
H3CO
O
7
OOH
6
6
H
Ene mechanism
Stage I - Peroxide Formation
Singlet Oxygen Oxidation
0
1
2
3
4
5
6
0 1 2 3
Weeks of Storage
Ra
nc
ima
t H
rs
Dark
Sun Light
Stage I - Peroxide Formation
Triplet Oxygen
Free radical oxidation
Common Initiation
Two pathways for propagation
ROOR + O2ROO· + ROO·
ROH + O2 + R=OROO· + ROO·
ROORR· + ROO·
RR or Olefin R· + R·
TERMINATION
RO2· + RO· + H2O2 ROOH
RO· + HOCH2C·HXROOH + CH2=CHX
RO· + R· + H2OROOH + RH
RO· + HO·ROOH
PROPAGATION [B] (OOH)
2 RO·ROO· + CH2=CHX
ROOH + R·ROO· + RH
RO2·R· + O2
PROPAGATION [A] (OO·)
R·I· + RH
I·I – I (Initiator)
INITIATION
Initiation Rate Factors
Hydrogen abstraction
Allylic hydrogen most easily removed
Main Component Oxygen Uptake Rates Rape (18:1) 1Soy (18:2) 41Linseed (18:3) 98
H3CO
O
7 7 7
O
H3CO4
Oleic Linoleic
Propagation - Peroxy Radical
Addition / Fragmentation
Rearangement
Hydrogen Abstraction
Peroxy Radical
H3CO
O
7
Oleic
OO
H3CO
O
7
OO
H3CO
O
7
O
H3CO
O
7
O
H3CO
O
7
O
O
7OOH
H3CO
O
7
Oleic
6
6 6
6
6
6
6
Propagation - Peroxy Radical
Addition / Fragmentation
Rearangement
Hydrogen Abstraction
Peroxy Radical
H3CO
O
7
Oleic
OO
H3CO
O
7
OO
H3CO
O
7
O
H3CO
O
7
O
H3CO
O
7
O
O
7OOH
H3CO
O
7
Oleic
66
6
6
6
6
6
Propagation - Peroxy Radical
Addition / Fragmentation
Rearangement
Hydrogen Abstraction
Peroxy Radical
H3CO
O
7
Oleic
OO
H3CO
O
7
OO
H3CO
O
7
O
H3CO
O
7
O
H3CO
O
7
O
O
7OOH
H3CO
O
7
Oleic
6
6
6
6
6 6
6
Peroxide Decomposition
Peroxide Formation
OCH3R
OAirFAME
Peroxide Decomposition
OCH3
O
76
OOH
HO
6
O
Peroxide Formation
Two alternative routes
Reasonable well defined
Peroxide Decomposition
Many reacting species involved
Alternative pathways
Less well defined
Hydroperoxide Rearrangement+
+
Hemiacetal / Hemiketal
HO R'
- H+
H+
Air
Ketone or Aldehyde
R'
O
H
OH2
OR'
H
OR'
OH
H2O
H
O
H
OR'
OH
H+
H +
- H+
HO6
O
OCH3
O
76
OOH
6
6
66
6
Many different decomposition pathways
Free radical
Metal mediated
Acid induced degradation
Many different decomposition products
Volatile polar compounds
Monomeric compounds
Dimers – trimers - polymers
Peroxide Formation
OCH3R
OAirFAME
Peroxide Decomposition
Volatile Acids
OCH3
O
76 Polymers
Complex OrganicsOOH
Peroxide Decomposition
Summary
Oxidation main mechanism of Biodiesel degradation
Oxidation occurs at or near sites of un-saturation
Oxidation two distinctive stages
Stage I Peroxide formation
Stage II Peroxide decomposition
Peroxide formation singlet or triplet oxygen
Peroxide decomposition follows multiple reaction pathways to yield a diversity products
The rate, amount and types of peroxides formed and the subsequent degradation products formed are dependant on a number of different factors
All chemical pathways must be considered when stabilizing Biodiesel
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