The possible impact of lewis acids on unsaturated fatty acid systems
It is well known from the literature that species of Lewis acids in the shape of various metal ions and their salts usually are present in e.g. raw materials for biodiesel production. The quantities and composition of these can vary significantly, in proportion to the refining step of the oils and fats respectively. Even used cooking oils usually have considerable amounts of these contaminants, mostly due to their function in food preparation. This paper wishes to discuss the oils and fats as carriers of Lewis bases due to their contents of double bonds and how the present metal ions and salts may affect the raw materials themselves, but also as a precursor for e.g. biodiesel production and their impact on final product quality.
Fats and oils
Naturally occurring fats and oils are usually in the shape of tri-glycerides and to various extents also free fatty acids. The triglyceride is built up by three fatty acids, attached to a backbone of glycerin. There is also almost always a content of free fatty acids. These are assumed to be a result of chemical, thermal or other stress to a triglyceride, which then has disassociated into its main components or parts thereof, since minor amounts of di- and mono-glycerides also can be detected in most such systems. The fatty acids are usually of the C18 length with various level of unsaturation, due to their respective origin. There is however significant amounts of C16 saturated fatty acids in e.g. palm oil. The most common unsaturated fatty acid chains are C18:1, C18:2 and C18:3 which stands for oleic, linoleic, and linolenic acid respectively, which have one, two and three double bonds on their chains. In this case this involves two carbon atoms that are bonded together with two electron pairs instead of the usual single pair. As functional groups, the reactivity of the fatty acids increases with the number of double bonds due to the electron-rich environment. This level of unsaturation is determined by analyzing the iodine number in a standardized manner. Some of the iodine used in the analysis attaches to the double bonds and the remaining iodine is then subtracted from the originally added.
Examples of common fats and oils and their typical iodine values : Fat / oil Iodine value Beef tallow 45-75 Herring oil 95-160 Castor oil 82-90 Lard 43-75 Chicken oil 80 Linseed oil 136-178 Coco nut oil 7-12 Olive oil 80-88 Corn oil 109-133 Palm oil 44-51 Cottonseed oil 100-117 Peanut oil 84-105 Rapeseed oil 98-120 Soya bean oil 120-136 Salmon oil 140 Sunflower oil 125-144 Sardine oil 141-212 Turkey oil 81
Common inorganic contaminants in fats and oils, in the shape of ions or salts, are typically Ca, Mg, P, Cl, S, Fe, Na and K, which levels can be diminished by pretreatment e.g. typical edible oil refining. Classical refining of edible oils often comprises the following steps : 1. Neutralization step, where the content of free acidity is turned into potassium soaps which then are removed from the batch. 2. Bleaching step, where most of the color of the oil is removed by bleaching with aid of chemicals such as carbon black and bleaching earth, which then is filtered off. 3. Deodorizing step, where any odors and typical oil smell characteristics are removed by steam. 4. Winterization step, where the oil is cooled to a temperature where crystallization of higher hydrocarbons and stearic acids is taking place. These crystals are then removed from the batch by filter pressing. These steps are standard procedure for the refining of crude triglyceride oils into edible oils ready for human consumption. Although none of the measures involved are directly pinpointed to remove any Lewis acids, it is well known that this becomes a consequence, finally meeting the common standards for edible oils .
Trans-esterification and esterification
Fats and oils are often subject to trans-esterification or esterification to form mono esters of e.g. methanol. Trans-esterification is commonly carried out by the aid of a catalyst, e.g. the methoxide ion, MeO- derived from sodium hydroxide, NaOH, potassium hydroxide, KOH, sodium methoxide, NaOMe or potassium methoxide, KOMe. Esterification, if so required, is also often used. Esterification is usually carried out in acidic environment by a catalyst e.g. derived from sulfuric acid, H2SO4. Note that glycerin is formed as a by-product during the trans-esterification process and that water is formed during the esterification process. It is a generally accepted fact, that the content of inorganic compounds of the batch is diminished to 10% of their original contents during these processes.
Lewis acids and bases
A Lewis acid is defined as an electron receiver, in contrast to the Brønsted-Lowry acids which are defined as proton donors. Since many metals appear in an ionized state, they can function as Lewis acids. Examples: Fe3+, Cu2+ +, Al3+ +, Mg2+ and Cr3+. A Lewis base, on the other hand, is defined as an electron donor. Examples of such organic bases suitable in this context are: ROH (alcohols), R2C=CR2 (unsaturated carbon chains), ROOH (carboxylic acids) , R1COOR2 (esters). Lewis bases and acids are often divided into three major groups; Hard, Borderline, and Soft. The particular virtue of these definitions is that the Lewis acid and Lewis base strengths are both estimates of the valence of the bond that links the cation with the anion. The most effective bonds will therefore occur between a cation whose Lewis acid strength (Sa) is close to the Lewis base strength (Sb) of the anion. This is known as valence matching principle . This carries in its simplest form that hard Lewis acids are most likely to form complexes with hard Lewis bases; soft Lewis acids are most likely to form complexes with soft Lewis acids, and so on. Examples of such categorizations , are displayed below :
Hard acids (metals) Borderline acids (metals) Soft acids (metals) Li+, Na+, K+,Mg2+, Ca2+, Al3+, Cr3+, Pb2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ Cu+, Ag+, Au+, Hg+ Fe3+, Co3+
Hard bases Borderline bases Soft bases H2O, OH-, ROH, RO-, ROOH, R1COOR2 N2, N3-, SO32-, C5H5N, R2C=NR C=CR2, CO, R2C=CR2, CN-, RNC, R2S
Importance in fuel applications
It is described by several authors in the literature that raw vegetable oils are unsuitable as diesel fuel, since the fuel is creating deposits inside the engines which surely will cause engine breakdown due to their formation on vital parts e.g. in combustion chambers, piston rings and exhaust outlets and valves. These problems seem less intense, however, when oils and fats with a low level of unsaturation is used; lard fat, palm oil etc. The composition of these deposits is practically unknown, but from the information given is there some signs suggesting that the unsaturation level of the fats and oils is of importance. It has never, however, been discussed what influence the present metal ions and their salts, have had on the combustion results. Since many of the common metal ions in fats and oils can form complexes with the unsaturated fatty acids, not in the least due to their electron-rich double bonds , is it likely to assume that they do and that these complexes are hardly disassociated into their compounds during combustion, whilst creating deposits. This assumption also have some support from the fact that during the trans-esterification and esterification processes of oils and fats, the released water and glycerin are able to act as a more preferable environment for the metal ions and their salts, than the produced mono ester environment is. Furthermore, the Lewis hard base spices OH-, ROH, RO-, are likely to react forming complexes with the spices of Lewis hard acids. The maximum levels of some metals contents for FAME (fatty acid methyl esters) are displayed in the European standard EN 14214. Trans-esterified and esterified fats and oils into methyl esters meeting the standard are not known to produce these deposits. This suggests that unsaturated fats and oils are more likely to form complexes with the present Lewis acids in shape of metal ions, due to their content of double bonds, which makes them less suitable as engine fuels, assuming that no proper pre treatment in order to diminish the contents of inorganic contaminants is carried out before use.