Glycerol is a byproduct obtained during the production of biodiesel. As the biodiesel production is increasing quickly, the petroleum glycerin generated from the transesterification of veggies oils has been generated in a great measure. About 10 % of petroleum glycerin will be formed during the synthesis of biodiesel from triglycerides. Merchandises obtained from glycerin can be used in nutrient, pharmaceuticals, polymer, agricultural, cosmetics, resins functional fluid plastics etc. Increasing the production of biodiesel, surplus of glycerin has been formed, doing market monetary values to fall ; this would go a cheaper feedstock in the chemical synthesis. For this ground, it is indispensable a new engineering for transition of glycerin into valuable chemicals to do biodiesel production a cost effectual procedure.
1,2-propanediol is an of import merchandise chemical traditionally derived from propylene oxide. Bio-routes enable decrease to 1,3-propanediol, as an of import monomer which has possible public-service corporation in the production of polyester fibers and the industry of polyurethanes and cyclic compounds. 1,3-propanediol is the organic compound with the expression CH2 ( CH2OH ) 2 and its colourless syrupy liquid that is mixable with H2O. They are besides used in liquid detergents, wetting agent flavorer and aromas, cosmetics, precursor in chemical and pharmaceutical industry, picture and animate being nutrient.
1,2-propanediol has an one-year planetary demand estimated at between 1.18 and 1.58 billion tonnes24. By early 2007 it was selling at around US $ 1.8 per kilogram, with a 4 % one-year growing in market size.
Either 1,2-propanediol or 1,3-propanediol can be produced by selective dehyroxylation of glycerin through chemical hydrogenolysis or by biocatalyst decrease. Researchers hope commercial production of 1,2propanediol is turning extra glycerin into an advantage for the biodiesel industry.
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Procedures:
Hydration of propenal
1,3-propanediol is presently produced by the hydration of propenal to ?-hydroxypropionaldehyde, which yields 1,3-Propanediol upon hydrogenation. In this procedure the output is low and besides propenal is unsafe risky chemical. There is a low output in the first measure of the procedure is because propenal has a big inclination to polymerise through self-condensation, the hydration reaction has to vie with acrolein self-condensation to bring forth the coveted ?-hydroxypropionaldehyde.
Due to the low efficiency and risky chemical nature of propenal procedure, research workers have been interested for an alternate method to bring forth 1,3-propanediol. An alternate manner is bring forthing 1,3-propanediol from glycerin. Since glycerin has been derived from biomass it has been attractive procedure as it can be utile manner of decrease of crude oil in the hereafter.
Selective dehydoxylation
The production of 1,3-propanediol from glycerin through selective dehydroxylation the strategy is to selectively change over the in-between hydroxyl group of glycerin into tosyloxyl group. Once it has been converted so to extinguish the transformed grouped by catalytic hydrogenolysis. The tosyloxyl group is a better go forthing group than hydroxyl group and is easier to replace with a hydride ion. The transition of glycerin to1,3-propanediol is done in three stairss, viz. , acetalization, tosylation, and detosyloxylation. Glycerol dehydroxylation procedure attracts the attending of research workers for the agitation procedure.
Figure 1- The new attack from glycerin to 1,3-propanediol ( reference production of 1,3propanediol utilizing dehyodrxylation )
The first measure ( acetalization ) , in the transition of glycerin to 1,3-propanediol is to acetalize the glycerin with benzaldehyde. The intent of this measure is to protect the first and 3rd hydroxyl groups of glycerin. This is because that merely the 2nd group can be tosylated in the 2nd measure and so removed in the 3rd measure.
The condensation between glycerin and benzaldehyde is an equilibrium reaction, but it can be driven to completion by taking the H2O formed. The trouble with this measure is that, the desired 1,3-product ( 5-hydroxyl-2-phenyl-1,3-dioxane or HPD ) and the unsought 1,2-product ( 4-hydroxylmethy-2-phenyl-1,3-dioxolane or HMPD ) is besides formed in the reaction. These merchandises need to be separated. The detached 1,2-product can be returned to the acetalization reactor, where it can either be converted into the 1,3-product or assist switch the reaction toward the 1,3-product, to take advantage of the equilibrium nature of the acetalization reaction.
The 2nd measure of the transition ( tosylation ) is the unprotected hydroxyl group of the acetalized glycerin to change over it into a good departure group.
The concluding measure of the transition is a detosyloxylation reaction preceded or followed by a hydrolysis reaction. The detosyloxylation reaction removes the tosylated cardinal hydroxyl group, while the hydrolysis reaction removes the protection on the first and 3rd hydroxyl groups. This last measure yields the transition of 1,3-Propanediol. It besides regenerates the group protection reagent benzaldehyde, which can be recycled back to the acetalization reactor for reuse in the first-step transition.
As shown in Figure 1, there are two possible attacks to carry through the last-step
The detosyloxylation reaction shown in Figure 1 is fundamentally involves hydrogenolysis reaction. This reaction is to be done with molecular H in the presence of a passage metal accelerator. This reaction is expected to be the most hard of all of the reactions involved in the new transition attack because the tosylate has ne’er earlier been reported to hold been hydrogenolysed catalytically with H2 as the cut downing reagent. At the current clip, the hydrogenolysis of tosylates is by and large affected with a Li hydride, either LiAlH4 or LiHBEt3. However these reagents are excessively expensive to utilize on an industrial graduated table. As a consequence, the feasibleness of catalytically hydrogenolysing the tosylate is the focal point of the current research.
The find of this new dehydroxylation procedure is indispensable to the success of the hereafter of new glycerin transition attack.
Hydrogenolysis procedure:
Figure 2: Conversion of glycerin to ethylene ethanediol
The above diagram shows the transition of glycerin to ethanediols. In the presence of H and metallic accelerators, glycerin can be hydrogenated to 1,2-propanediol, 1,3-propanediol, or ethylene ethanediol.
This ethanediol production by hydrogenolysis is a procedure used is economically and environmentally attractive compared to their production from crude oil derived functions.
Hydogenolysis of glycerin are used from supported metal accelerators from passage metals. For this reaction supported accelerator such as Ruthenium, Platinum, Rhodium, and Palladium are used. Addition of solid acid to metal accelerators enhances the transition and selectivity of reaction [ 1, 5, 16 ] . Solid acerb accelerator contributes the chief function in transition of glycerin hydrogenolysis. It is found that Ruthenium based accelerators exhibit better activity than other metals for this reaction [ 15-18 ] . However, Ruthenium gives inordinate C-C bond cleavage which leads to degrative merchandises.
In hydogenolysis of glycerin to acquire 1,2 propylene glycol it requires selective cleavages of C-O bond without cleavage of C-C bond. For this ground, Cu based accelerators are better accelerators in comparing to passage metal accelerators. The Cu based accelerator is active under mild reaction conditions and does necessitate a separate solid acid accelerator. Surveies shows that Cu chromite accelerator is a good selectivity and transition for propene ethanediol under mild reaction conditions peculiarly at low H2 force per unit areas.
The figure below shows utilizing Cu chromite accelerator shows the highest selectivity for 1,2-propanediol with higher transition compare to different accelerator at temperature 200oC and at force per unit area 13.8bar.
Figure 3: Comparing different accelerators for transition and selectivity
The method is based on hygrogenolysis over a Cu chromite accelerator ( CuO.Cr2O3 ) at 200oC and less than 10 saloon, coupled with reactive distillment.
Figure 4 – reactive distillment
Using a two-step reaction procedure under mild reaction, the reaction tract returns through acetol ( hydoxyacetone ) intermediate.
The first measure: comparatively pure acetol is produced from glycerin at 0.65bar force per unit area and 200oC in the presence of Cu chromite accelerator.
The 2nd measure: utilizing a Cu accelerator once more similar to the first measure, the acetol is farther hydrogenated to 1,2-propanediol at 200oC and 13.8 saloon H force per unit area. This allows 1,2-propanediol in 90 % output and at well lower cost than get downing from crude oil.
The selectivity to propylene ethanediol lessenings if temperature is above 200oC due to inordinate hydrogenolysis of the 1,2-propanediol.
The purpose for production of propene ethanediol is in pure status. The reactive distillment procedure now achieves greater than 99.8 % pureness, which means the merchandise can be used both as industrial feedstock and as antifreeze.
The practical advantages of the reactive distillment attack are:
Low H2O content of the provender
low force per unit area ( 200psi )
High selectivity ( & A ; gt ; 90 % )
Low accelerator cost.
Figure 5- The two measure reaction procedure
The reaction is conducted in two measure, because major jobs can happen when the reaction is conducted in a individual measure are the accelerator becomes coated with oligomers and its hard to accomplish above 80 % selectivity for 1,2-propanediol. However in two stairss, by uniting the reaction and separation stairss, 1,2- propanediol output is 99 % and the accelerator life rhythm is significantly extended. Water and acetol are at the same time removed from the reaction mixture during the heating measure as they are formed, the lower force per unit area used in the first of the two measure prolongs catalyst life. Further decrease of acetol H2O provender with H over a similar Cu chromite accelerator at 13.8bar and 185oC allows 1,2-propanediol selectivity greater than 95 % and 99 % transition.
Advantages of this new procedure, is the acetol formed as an intermediate is an of import monomer used in industry in the industry of polyols. When this produced from crude oil it costs every bit small as $ 1 per kilogram, opening up even more possible applications and markets for glycerin. The 2nd advantage farther purification is non required when utilizing the Cu chromite accelerator to change over petroleum glycerin, whereas supported baronial metal accelerators are easy poisoned by taint for illustration chlorides.
The disadvantage of this procedure is the usage of high force per unit area and temperature as it is expensive to utilize high force per unit area equipment and besides increases the capital cost of the procedure. An extra disadvantage is copper chromite based accelerator are unwanted for the environmental facets as Cr is toxic. For this ground, research has studied utilizing Cu-ZnO accelerator at high force per unit area alternatively of Cu chromite accelerators. However the greatest selectivity ( 100 % ) for 1,2-propanediol obtained by hydrogenolyisis of an aqueous solution of glycerin in the presence of CuO-ZnO accelerators gives a low output. Copper chromite accelerator has much better selectivity and transition comparison to CuO-ZnO accelerators.
There are a figure of paths to bring forth propylene ethanediol from renewable feedstock. The most common is the hydrogenolysis procedure in presence of a metal accelerator. However this of import reaction at the minute is limited in the research lab graduated table.
New glycerin hydrogenolysis processes developed by Davy, shortly to be commercialised indicant suggest that the procedure will give high pureness propene ethanediol, suited for all applications. This procedure glycerin is reacted with H over a heterogenous Cu accelerator under relativity moderate conditions ( 20bar, 200oC ) . The glycerin, along with a recycle watercourse, is vaporised in a recalculating watercourse of H, typically from a pressure-swing surface assimilation unit. Glycerol transition is about 99 % and byproducts are removed by distillment. The advantage of the Davy procedure is its high selectivity to the coveted merchandise.
Agitation procedure:
There are figure different manner to bring forth 1,3-propanediol. For illustration glycerin production by hydrogenolysis in presence of a metal accelerator and besides by the hydration of propenal to ?-hydroxypropionaldehyde, which yields to 1,3-Propanediol. Even though it is possible to bring forth 1,3-propanediol by these methods, they are expensive and are environmental pollutants.
Glycerol can function as a feedstock for the fermentative production of 1,3-propanediol and its production by agitation appears to be a sensible option to chemical synthesis. Bacterial strains are able to change over glycerin into 1,3-propanediol and are found in the species of Lactobacillus, Citrobacter, Klebsiella, and, Clostridum. These bacteriums have been investigated due to its appreciable substrate tolerance, the output and productiveness of the procedure.
In a two-step enzyme-catalysed reaction sequence glycerin is converted to 1,3-propanediol ( PDO ) . These equations are shown in figure 6. In the first measure: dehydrates the contact actions transition of glycerin to 3-hydroxy-propionaldehyde ( 3-HPA ) and H2O, equation 1.
In the 2nd measure: 3-HPA is reduced to 1,3-propanediol by a pyridine base: NAD+ oxidoreductase to give 1,3-propanediol, a dead terminal cellular metabolite.
The 1,3-propanediol will non be metabolised farther and this it accumulates in the medium. This metabolic subdivision is indispensable for energy coevals and requires tierce of the available glycerin if lone ethanoate and 1,3-propanediol are produced.
The overall reaction consumes a cut downing equivalent in the signifier of a cofactor, reduced beta-nicotinamide A dinucleoride ( NADH ) , which is oxidised to nicotinamide adenine dinucleotide ( NAD+ ) , equation 3.
Figure 6 – The two measure catalysed reaction sequence
The cistrons are responsible for the transition of glycerin to 1,3-propanediol. Hetrologous cistrons in E.coil for illustration from Cirobcater and klebsiella have shown to change over glycerin to 1,3- propylene glycol. From all these bacteriums, Klebsiella pneumoniae in its wild signifier is the most interesting because of their output, productiveness, efficient transition to 1,3-propanediol and opposition to both reagents and merchandises.
The proficient and economic facet of this procedure is attractive for this agitation procedure. This technique uses immobilization alternatively of freely suspended cells which causes an addition in productiveness. This procedure has some disadvantages ; one of the disadvantages is its low theoretical output. Another chief drawback is that the procedure is substrate-inhibited. The bacteriums used in the agitation are by and large non able to stand a glycerin concentration above 17 % . As a consequence ; both the merchandise concentration and the productiveness are low.
This biological procedure for the production of 1,3-propanediol utilizations a expensive glycerin and has a low metabolic efficiency. An cheap method in China has been developed get downing from glucose instead than glycerin. It combines the tract from glucose to glycerol with the bacterial path signifier glycerin to 1,3-propanediol ( refrence 20 ) .
Figure 7- cheap method to bring forth 1,3-propanideiol
