Polyglycerol is a polymer composed of an inert polyether backbone and abundant highly reactive hydroxyl groups that are exposed and allow for further modification to produce a variety of related compounds [ 46 ],[ 52 ],[ 53 ].
The presence of a catalyst - either of homogeneous or heterogeneous type- which can be either acidic or basic- is required for glycerol polymerization. Din et al [ 54 ] polymerized the CG using microwave irradiation and the soap content as the catalyst. The authors found that this process results in the formation of glycerol oligomers, from diglyccerol to pentaglycerol. Polyglycerols of higher molecular weight and branched structure, directly synthesized from glycerol, could replace hyperbranched polymers obtained from glycidol, a toxic monomer [ 58 ] opening the possibilities of application at industrial levels for the production of complex polymeric structures derived from glycerol.
However, according to this study, the production of polyglycerol from CG as raw material does not occur. This research is aimed at understanding and identifying the factors that inhibit polymerization of CG using sulfuric acid as the catalyst.
Therefore, a sample of CG was characterized to identify impurities. Simulated-CG was prepared adding the impurities, previously identified as purified glycerol. The effect of each impurity on the polymerization reaction, as well as the interaction between them, was analyzed. An experimental design was conducted to study the effect of the most abundant impurities soap and sodium and the concentration of catalyst sulfuric acid in the polymerization of CG.
As a result, it was found that sulfuric acid in presence of soap produces side reactions that compete with polyglycerol synthesis. Furthermore, a statistical analysis to assess the effect of the presence and absence of impurities, soap and sodium, at a fixed concentration of catalyst, revealed that the presence of soap in CG is the main inhibitory factor of polyglycerol synthesis.
Pure glycerol, sodium hydroxide, and sulfuric acid were obtained from Merck. Santa Marta, Colombia , which uses palm oil as raw material for biodiesel production. Density of CG was determined using a 2 mL pycnometer. Glycerol content in the CG sample was determined using the iodometric-periodic acid method in accordance with the AACC 58 technique.
Soap content was determined according to a modified version of the AOC Cs method [ 59 ]. Ash content was determined following the ISO method. The presence of methanol was determined by gas chromatography with an Agilent Technologies N system equipped with a flame ionization detector. The method followed was the EN A thermogravimetric analysis TGA was performed to determined polymer degradation as a function of temperature.
Infrared spectrum was obtained in transmittance mode in a Shimadzu s spectrometer. All measurements were performed in duplicate.
The polymerization reaction was carried out on a 50 mL glass reactor equipped with nitrogen inlet, catalyst feeding, thermometer inlet, and a distillation trap for continuous water removal from the reaction mixture. A vacuum pump was attached to the reactor through the condenser; in the meantime, condensation reactions were carried out at pressures below The soap used to simulate CG was made from palm oil saponification [ 61 ], and the metal amount was obtained from the neutralization of sodium hydroxide with sulfuric acid.
The viscosity of CG is 0. The result can be explained by the presence of soaps in the CG sample, which catalyzes the oligomerization of glycerol increasing product viscosity [ 62 ]. Despite the increase of viscosity in the reaction products, there was not polyglycerol formation under any condition tested.
The fact that polymerization of CG did not occur, even though the reaction was performed under the same conditions for which successful polymerization of purified glycerol was achieved, encouraged us to study the factors that inhibit polymerization of CG. To find the polymerization inhibitory factors present in CG, simulated-CG samples were prepared according to the composition in the characterization of CG. The moisture content determined was As regards metals and FAMES composition, a considerable high amount of sodium with respect to other metals was observed; similarly, palmitic acid was found in a significantly higher proportion with respect to other free fatty acids.
This increase in sodium and palmitic acid could be explained by the use of sodium hydroxide or methoxide as a catalyst in the transesterification reaction; as well as African palm oil as raw material for biodiesel production. The spectra shows the presence of a broad OH stretching band from cm -1 to cm -1 , C-H stretching between to cm -1 and C-0 stretching from to cm -1 Figure la.
A carboxyl C peak only appears in the CG sample, see Figure lb. This peak indicates that soluble soaps present in CG were removed after the acidification stage [ 63 ],[ 64 ]. The thermo-gravimetric analysis reveals a 2. This weight loss is due to the presence of volatile materials such as methanol.
Based on the composition obtained from the characterization of CG, a simulated-CG sample was prepared by adding the impurities into purified glycerol. The soap used to simulate CG was made from palm oil saponification [ 61 ], and the metal was sodium because in the characterization of CG, there was a significantly higher amount as compared with other metals. To confirm if the simulated CG prepared resembles the characterized one, two polymerization reactions were conducted under the same conditions using the simulated and characterized crude glycerols as raw material.
The catalyst used was sulfuric acid at 4. Polymerization reactions were performed under conditions previously established in our previous work [ 60 ]. Hydroxyl number, mass spectroscopy, and thermogravimetry analysis were performed to compare reaction products obtained from polymerization reaction of the simulated and characterized crude glycerols. Results show a similar hydroxyl number for both glycerols used, simulated and the characterized one, which correspond to Mass spectroscopy peaks for both samples were comparable Figure 2 a-2b.
Related to ESI-MS results, it seems that the two reaction products exhibited similar mass distributions. Thus, the simulated-CG led to similar products than the original CG. This is also noticed in the TGA analysis, see Figure 3 a and 3b. However, the first weight loss region is related to the remaining water molecules in the final reaction product.
On the other hand, the second region is associated with glycerol biodegradation that could be catalyzed due to the presence of the impurities in the two samples [ 67 ]. Figure 2 Electrospray Ionization ESI mass spectrum comparison between the polymerization reaction products of characterized a and simulated b CG.
The concentration of sulfuric acid used as catalyst was 4. A similar mass distribution of the two samples was observed. Similar weight loss profile of the two samples was observed.
A 2 k full factorial experimental design was performed to determine the effect of impurities on the sulfuric acid- catalyzed polymerization reaction of simulated-CG.
Table 1 summarizes the factors and levels used in the experimental design. Samples for each treatment were prepared adding the impurities corresponding to the factor levels proposed in the factorial design into purified glycerol.
The reaction procedure was carried out as described in the experimental procedure section. Table 1 2 k experimental design layout. If in a treatment, an etherification or esterification reaction occurred, the hydroxyl number of the product should decrease with respect to the initial hydroxyl number of glycerol, as some of the initial OH groups have reacted. The response variable chosen may also suggest the formation of polyglycerol, as this polymerization reaction is an etherification.
This article is cited by 12 publications. Stephanie J. Merritt, Ian Houson, Joop H. Ter Horst. Jones, Gerard P. Organic Letters , 22 13 , Steendam, Patrick J.
Crystals , 11 11 , Chemical and Pharmaceutical Bulletin , 68 4 , Moynihan , Declan Armstrong. Stepwise dissolution and composition determination of samples of multiple crystals using a dissolution medium containing aqueous alcohol and fluorocarbon phases.
RSC Advances , 9 37 , Pure substances can be identified by comparing the melting point found in the experiment with published reference data of what the melting point should be. Which of the following substances A, B, C, D are impure?
Substances B and D are impure. This is because they show a broad melting point range. Substances A and C show sharp melting points and so are pure. In short, a reaction that converts a pure starting material completely to the desired product, without extensive purification steps, is perfect. Unfortunately, we seldom get this in the real world, because the real world has side reactions. These scupper your schemes by both decreasing the yield and creating the need for time-consuming and waste-producing purification steps.
Side-products are not to be confused with byproducts. Technically, or perhaps pedantically, a side-product is an unwanted alternative product formed under the reaction conditions, in addition to the one which is desired.
They are offshoots of the productive mechanism. Byproducts, on the other hand, are a necessary consequence of the mechanism. As an example, the Swern oxidation converts an alcohol into an aldehyde or a ketone Figure 1. The reagents, dimethyl sulfoxide and oxalyl chloride, react together to form the active electrophile, giving off carbon monoxide and carbon dioxide as necessary byproducts.
To obtain the desired product, the alcohol then reacts with the electrophile, and the intermediate then undergoes elimination to afford the desired carbonyl compound, along with another necessary byproduct, smelly dimethyl sulfide. And if this captures the alcohol, you have a side-product.
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