Dr B.B. Marvey
Fats and oils to the rescue
In recent years a lot of interest has developed around the industrial application of feedstock from renewable resources. It is for this reason that naturally occurring fats and oils could become one of the major players in the chemical industry for the near future. This might then result in a new economic order, placing the agricultural industry in the economic forefront as one of the largest wealth-generating sectors. These materials (fats and oils) are not chemically very different from petroleum, which explains why crude oil is a fossil fuel. There is, therefore, no reason why they cannot replace the gradually diminishing crude oil in the near future.
Fats and oils are obtained from vegetable and animal sources. Their main constituents are mixed triglycerides (esters of glycerol) having long-chain carboxylic acid moieties. A large proportion of the vegetable oils such as coconut, palm and palm kernel oil come from countries with tropical climates. Soybean, rape seed and sunflower oils come from moderate climates. Animal fat is obtained from the meat industry with beef tallow being the most abundant fat. Fish oil comes from the fishing industry.
In 1997, the total world production of fats and oils was estimated to at 100 million tons (Mt), of which 80 Mt was of vegetable origin and only 20 Mt of animal origin. Almost 75% of plant oils are derived from four major crops, viz. soybean, rapeseed, palm and sunflower oil. Of the total world production of fats and oils, more than 80% is used for human consumption. Half the remaining part is used in the animal feed industry and about 14% goes to the chemical industry.
Fats and oils in manufacturing
Although fats and oils do not presently command a wide industrial application like petroleum, they are nevertheless a significant feedstock, used in manufacturing a number of products including lubricants, surfactants, surface coatings, polymers, pharmaceuticals, cosmetics, to name a few. In fact, most scientists already see the potential in field crops as drivers for our economy in the next few decades. By developing more industrial processes that utilise fats and oils, it may be possible to significantly lessen global dependence on petroleum. In any case, before World War II, most products like paints, coatings and adhesives were made from vegetable oils or other plant products. Henry Ford, for example, made everything from clothing to automobile bumpers from vegetable oils. It is, therefore, not just wishful thinking that in the near future, some of these products will go back to using vegetable oils instead of petroleum. Already there are latex paints in the market, and Dow chemical Co. and Cargill Inc. are producing new plastics from corn.
"Green Chemistry "
Besides, fats and oils have a number of advantages over their petroleum counterpart, for example, they are produced from renewable resources, they are easily biodegradable, and their processing does not result in the production of large amounts of CO2, which is an environmental problem. Therefore, chemical processes involving natural fats and oils are largely "Green Chemistry" processes. On the other hand, the use of petrochemical feedstock has several disadvantages, namely, the resources are limited, they contribute to the net CO2 emissions on combustion, and they are poorly biodegradable.
In recent years biodiesel has also been gaining worldwide popularity as an alternative energy source. It comes from transesterification of vegetable oil, and it is basically a mixture of methyl esters. It is very light oil, less viscous, very lubricating, and can be used in any Diesel engine including trucks, generators, boats, trains, busses, and cars. With few or no real modifications it can just be poured straight into the fuel tank of Diesel engines. Tests done in the US and Europe have also shown that engines running on biodiesel have minor, if any differences, in torque, horse power, range, and top speed to those running on petroleum-based diesel. In fact, the engines running on biodiesel were generally found to idle smoother and accelerate more smoothly. Furthermore, biodiesel is biodegrable, non-toxic, and essentially free from sulphur and aromatics.
Overcoming its limitations
Although there is so much that can come out from fats and oils, there are still some limitations that need to be overcome and that is, most of them contain predominantly C16 and C18 fatty acids. This tends to make these oils suitable for edible use and only of limited value as oleochemicals. This limitation is serious since the synthesis of most other industrial products requires fatty acids and derivatives with shorter and longer chain-lengths.
To overcome this limitation scientists must exploit options for varying the chain-length of the available fatty acids to produce a wide variety of useful products. The possibilities include genetic modification of existing oil crops and chemical methods. For example, the seed oil of many Umbelliferae species including the spice plant coriander contains 70-80 per cent petroselinic acid, an isomer of oleic acid (C18) with the double bond in the C6 rather than the C9 position. This can be converted to hexane-1,6-dioic (adipic) acid through oxidative ozonolysis. Adipic acid, in turn, can be used for the manufacture of polymers: Since the coriander plant is not a high-yielding oilseed crop, attempts have been made to transfer its genes to the rapeseed plant for reasons of producing a high-petroselinic oil crop. When adipic acid is manufactured from petroleum, huge amounts of ozone depleting nitrous oxide, N2O, are produced. Therefore, the method for producing adipic acid through biosynthesis coupled with chemical synthesis has environmental advantages over the currently used method, which involves petroleum. Other chemical methods for transforming fatty acids and their derivatives include hydrogenation, isomerization, epoxidation, hydroformylation and dimerization.
Recently there has also been a growing interest in utilising what is known as the alkene metathesis reaction for altering chain-lengths of "oils" to form new compounds. Alkene metathesis is primarily a catalyst-driven reaction and involves the cleavage of carbon-carbon double bonds. Basically two alkene molecules come into contact with each other and swap partners (alkylidene moieties) as shown to the right.
This reaction has become synthetically useful since the discovery of various well-defined transition metal carbene complexes which can catalyse alkene metathesis. There are also several industrial processes based on alkene metathesis reaction. The Phillips Triolefin Process makes propene by reacting ethene and 2-butene metathetically. Propene is then used to make the polymer polypropylene, which in turn is used to make films, fibres, and plastic moulding materials. The Shell Higher Olefin Process (SHOP) also uses metathesis technology to convert ethene to detergent range molecules.
Good news for the continent
Fats and oils containing carbon-carbon double bonds can undergo transformation by metathesis reaction to form intermediates, which could then be used for the synthesis of a wide range of reaction products ranging from pharmaceuticals and cosmetics to polymers and fine chemicals. In South Africa, interest in alkene metathesis research is growing. The good news also is that most active metathesis catalysts are derived from platinum group metals PGMs (Platinum, Palladium, Rhodium, Ruthenium, Iridium, Osmium) of which South Africa has the privilege of being in the forefront as world producer of PGMs. Africa also has hectares of lands and favourable climate for the production of oil crops and for stock farming. This must be good news for the continent that is currently positioning itself to take a more proactive role in the global economy.