Glycerol is a by-product obtained during the production of biodiesel. As the biodiesel production is increasing rapidly, the crude glycerol generated from the transesterification of vegetables oils has been generated in a great quantity. About 10% of crude glycerol will be formed during the synthesis of biodiesel from triglycerides. Products obtained from glycerol can be used in food, pharmaceuticals, polymer, agricultural, cosmetics, resins functional fluid plastics etc. Increasing the production of biodiesel, excess of glycerol has been formed, causing market prices to fall; this would become a cheaper feedstock in the chemical synthesis. For this reason, it is essential a new technology for conversion of glycerol into valuable chemicals to make biodiesel production a cost effective process.
Glycerol can be converted into higher value products such as 1,2-propanediol (propylene glycol) and 1,3-propanediol from reduction of glycerol.
1,2-propanediol is an important product chemical derived from propylene oxide. Bio-routes enable reduction to 1,3-propanediol, as an important monomer which has potential effectiveness in a manufacture of cyclic compounds and polyurethanes, and production of polyester fibres.1,3-propanediol is the organic compound with the formula CH2(CH2OH)2 and its colourless viscous liquid that is miscible with water. They are also used in liquid detergents, wetter flavouring and fragrances, cosmetics, precursor in chemical and pharmaceutical industry, painting and animal food.
1,2-propanediol has an annual global demand estimated at between 1.18 and 1.58 billion tonnes1. By early 2007 it was selling at around US$1.8 per kg, with a 4% annual growth in market size2.
Either 1,2-propanediol or 1,3-propanediol can be produced by selective dehyroxylation of glycerol through chemical hydrogenolysis or by biocatalyst reduction. Researchers hope commercial production of 1,2propanediol is turning excess glycerol into an advantage for the biodiesel industry.
1. Hydration of acrolein
1,3-propanediol is currently produced by the hydration of acrolein to β-hydroxypropionaldehyde, which yields 1,3-Propanediol upon hydrogenation. In this process the yield is low and also acrolein is dangerous hazardous chemical. There is a low yield in the first step of the process is because acrolein has a large tendency to polymerize through self-condensation, the hydration reaction has to compete with acrolein self-condensation to produce the desired β-hydroxypropionaldehyde.
Due to the low efficiency and hazardous chemical nature of acrolein process, researchers have been interested for an alternative method to produce 1,3-propanediol. An alternative way is producing 1,3-propanediol from glycerol. Since glycerol has been derived from biomass it has been attractive process as it can be useful way of reduction of petroleum in the future.
2. Selective dehydroxylation
The production of 1,3-propanediol from glycerol through selective dehydroxylation the scheme is to selectively convert the middle hydroxyl group of glycerol into tosyloxyl group. Once it has been converted then to eliminate the transformed grouped by catalytic hydrogenolysis. The tosyloxyl group is a better leaving group than hydroxyl group and is easier to replace with a hydride ion. The conversion of glycerol to1,3-propanediol is done in three steps: acetalization, tosylation, and detosyloxylation. Glycerol dehydroxylation process attracts the attention of investigators for the fermentation process.
The first step (acetalization), in the conversion of glycerol to 1,3-propanediol is to acetalize the glycerol with benzaldehyde. The purpose of this step is to protect the first and third hydroxyl groups of glycerol. This is because that only the 2nd group can be tosylated in the second step and then removed in the third step.
The last step of the conversion is a detosyloxylation reaction or a hydrolysis reaction. The tosylated central hydroxyl group removes in the detosyloxylation reaction. The protection removes on first and final hydroxyl groups in the hydrolysis reaction. This last step converts to 1,3-Propanediol. It also regenerates the group protection reagent benzaldehyde, which can be recycled back to the acetalization reactor for reuse in the first-step conversion4.
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As it shown in figure 1 the detosyloxylation reaction is basically involves hydrogenolysis reaction. The reaction is done with hydrogen molecular in the presence of a transition metal catalyst. The tosylate has not been hydrogenolysed catalytically with hydrogen molecule as a reducing agent. Currently hydrogenolysis of tosylates is affected with lithium hydride. These reagents are too expensive to use industrial scale. The feasibility of catalytically hydrogenolysis the tosylate is focus of the current research.
3. Hydrogenolysis process:
The above diagram shows the conversion of glycerol to glycols. In the presence of hydrogen and metallic catalysts, glycerol can be hydrogenated to 1,2-propanediol, 1,3-propanediol, or ethylene glycol.
This glycol production by hydrogenolysis is a process used is economically and environmentally attractive compared to their production from petroleum derivatives.
Hydogenolysis of glycerol are used from supported metal catalysts from transition metals. For this reaction supported catalyst such as Ruthenium, Platinum, Rhodium, and Palladium are used. Addition of solid acid to metal catalysts enhances the conversion and selectivity of reaction5. Solid acid catalyst contributes the main role in conversion of glycerol hydrogenolysis. It is found that Ruthenium based catalysts exhibit better activity than other metals for this reaction5. However, Ruthenium gives excessive C-C bond cleavage which leads to degrative products.
In hydogenolysis of glycerol to get 1,2-propanediol it requires selective cleavages of C-O bond without cleavage of C-C bond. For this reason, copper based catalysts are better catalysts in comparison to other transition metal catalysts. The copper based catalyst is active under mild reaction conditions and does need a separate solid acid catalyst. Studies shows that copper chromite catalyst is a good selectivity and conversion for propylene glycol under mild reaction conditions particularly at low H2 pressures.
The figure below shows using copper chromite catalyst shows the highest selectivity for 1,2-propanediol with higher conversion compare to different catalyst at temperature 200oC and at pressure 13.8bar.
In a reactive distiallation glycerol can be hydrogenolysis over copper chromite catalyst at less than 10 bar and 200OC in a reactive distillation.
The aim for this process is to produce propylene glycol in pure condition. The reactive distillation process now achieves greater than 99.8% purity, which means the product can be used both as industrial feedstock and as antifreeze2.
The practical advantages of the reactive distillation approach are:
- Low water content of the feed
- low pressure (200psi)
- High selectivity (>90%)
- Low catalyst cost.
Using a two-step reaction process under mild reaction, the reaction pathway proceeds through acetol (hydoxyacetone) intermediate.
The first step: relatively pure acetol is produced from glycerol at 0.65bar pressure and 200oC in the presence of copper chromite catalyst.
The second step: using a copper catalyst again similar to the first step, the acetol is further hydrogenated to 1,2-propanediol at 200oC and 13.8 bar hydrogen pressure. This allows 1,2-propanediol in 90% yield and at considerably lower cost than starting from petroleum.
The selectivity to propylene glycol decreases if temperature is above 200oC due to excessive hydrogenolysis of the 1,2-propanediol.
The reaction is conducted in two step, because major problems can occur when the reaction is conducted in a single step are the catalyst becomes coated with oligomers and its difficult to achieve above 80% selectivity for 1,2-propanediol. However in two steps, by combining the reaction and separation steps, 1,2- propanediol yield is 99% and the catalyst life cycle is significantly extended. Water and acetol are simultaneously removed from the reaction mixture during the heating step as they are formed, the lower pressure used in the first of the two step prolongs catalyst life. Further reduction of acetol water feed with hydrogen over a similar copper chromite catalyst at 13.8bar and 185oC allows 1,2-propanediol selectivity greater than 95% and 99% conversion.
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Acetol formed as an intermediate is the advantage of this new process as it acetol is an important monomer used in industry to manufacture polyols. When this produced from petroleum it costs as little as $1 per kg, opening up even more potential applications and markets for glycerol8. The second advantage further purification is not required when using the copper chromite catalyst to convert crude glycerol, whereas supported noble metal catalysts are easily poisoned by contamination for example chlorides.
The disadvantage of this process is the use of high pressure and temperature as it is expensive to use high pressure equipment and also increases the capital cost of the process. An additional disadvantage is copper chromite based catalyst are undesirable for the environmental aspects as chromium is toxic. For this reason, research has studied using Cu-ZnO catalyst at high pressure instead of copper chromite catalysts. However the greatest selectivity (100%) for 1,2-propanediol obtained by hydrogenolyisis of an aqueous solution of glycerol in the presence of CuO-ZnO catalysts gives a low yield. Copper chromite catalyst has much better selectivity and conversion compare to CuO-ZnO catalysts.
There are a number of routes to produced propylene glycol from renewable feedstock. The most common is the hydrogenolysis process in presence of a metal catalyst. However this important reaction at the moment is limited in the laboratory scale.
New glycerol hydrogenolysis processes developed by Davy, soon to be commercialised indication suggest that the process will give high purity propylene glycol, suitable for all applications. In this process under relativity moderate conditions glycerol is reacted with hydrogen over a heterogeneous copper catalyst. The glycerol recycle stream is vaporised in a recalculating stream of hydrogen, typically from pressure-swing adsorption unit. High purity propylene glycol recovers from the refining scheme, where as the water produced in the reaction is to be passed to a biological treatment plant. High quality small by products stream can be used as functional fluids or as solvents. By products are removed by distillation and glycerol conversion is around 99%. High selectivity to the desired product is the advantage of the Davy process.
4. Fermentation process:
There are number different way to produce 1,3-propanediol. For example glycerol production by hydrogenolysis in presence of a metal catalyst and also by the hydration of acrolein to β-hydroxypropionaldehyde, which yields to 1,3-Propanediol. Even though it is possible to produce 1,3-propanediol by these methods, they are expensive and are environmental pollutants.
Glycerol can serve as a feedstock for the fermentative production of 1,3-propanediol and its production by fermentation appears to be a reasonable alternative to chemical synthesis. Bacterial strains are able to convert glycerol into 1,3-propanediol and are found in the species of Lactobacillus, Citrobacter, Klebsiella, and , Clostridum.
In a two-step enzyme-catalysed reaction sequence glycerol is converted to 1,3-propanediol(PDO). These equations are shown in figure 7.
In the first step: dehydrates the catalyses conversion of glycerol to 3-hydroxy-propionaldehyde (3-HPA) and water, equation 1.
In the second step: 3-HPA is reduced to 1,3-propanediol by a pyridine nucleotide: NAD+ oxidoreductase to yield 1,3-propanediol, a dead end cellular metabolite.
The overall reaction consumes a reducing equivalent in the form of a cofactor, reduced beta-nicotinamide adenine dinucleoride(NADH), which is oxidised to nicotinamide adenine dinucleotide (NAD+)9, equation 3.
The genes are responsible for the conversion of glycerol to 1,3-propanediol. Hetrologous genes in Escherichia coil for example from Cirobcater and klebsiella have shown to convert glycerol to 1,3-propanediol. From all these bacteria, Klebsiella pneumoniae is the most interesting because of their yield, productivity, efficient conversion to 1,3-propanediol and resistance to both reagents and products.
The technical and economic aspect of this process is attractive for this fermentation process. This technique uses immobilisation instead of freely suspended cells which causes an increase in productivity. One of the disadvantages of this process is its low theoretical yield. Another disadvantage is that the process is substrate-inhibited. The bacteria used in the fermentation are generally not able to stand a glycerol concentration above 17%10. Thus, both the productivity and product concentration are low.