1 Introduction
Over the past few decades, the use of environmentally-friendly and green sources of energy, e.g. biomass-derived fuels, also known as biofuels, has gained prominence due to the imminent exhaustion of fossil fuels. Increased research in this field has had an effective impact on chemical and food markets and has turned it into a political-economic matter [1], [2], [3]. Among biofuels, biodiesel products are extensively produced and used in many developing and developed countries. When converting triglycerides into biodiesel, glycerin is generated at a considerable rate of around 10% v/v of the biodiesel produced [4], [5]. Given the huge amount of biodiesel produced worldwide, the quantity of this co-product, sometimes considered as waste, is extremely high. For instance, the 967 million gallons of biodiesel produced in the year 2011 throughout the US must have led to the generation of around 100 million gallons of glycerin [6]. This huge amount of glycerin, thanks to its inexpensive price and high availability, could be used as feedstock for economic production of different value-added products [1], [7], [8].
In line with this, numerous research groups have endeavored to develop new techniques to produce different valuable derivatives from glycerin regardless of its origin, e.g. acrolein [9], [10], a new class of solvo-surfactants [11], glycerol ethyl acetal [12], galactosyl glycerol [13], 1,3-dioleoyl-2-palmitoylglycerol (OPO) [14], organochalcogen compounds [15], glycerol-based hyperbranched polyesters [16], glycerol salicylate resin [17] and glycerol carbonate [10]. In one study, an attempt was made to produce hyperbranched aliphatic polyether from glycerin carbonate leading to the introduction of a novel technique for obtaining liquid alkanes from glycerin through Fischer-Tropsch synthesis [18]. Among other efforts being made to take advantage of this economical feedstock are the application of different catalysts and reaction conditions to synthesize 1,2-propanediol through the hydrogenolysis of an aqueous solution of glycerin [19] as well as the application of zeolite as a catalyst for the actualization of glycerin [20].
It is worth noting that in spite of the above-mentioned procedures proffered to use glycerin, the drastic development of the biodiesel industry globally has provided a considerable surplus of glycerin, for which there is not a sufficient demand in the market. Thus, the production of such chemical intermediates from biodiesel glycerin could be more economically feasible than the former production method from petroleum [3], [21]. However, the economics of the biodiesel production processes will be clearly improved as a result of the conversion of glycerin into value-added derivatives [22], [23]. Therefore, the present study mainly focuses on the conversion reaction of glycerin and formic acid to glycerin carbonate, which is a versatile component with a variety of applications. Owing to its high boiling point, biodegradability, low toxicity and the presence of two functional groups, glycerin carbonate has the potential to be used as a surfactant, a solvent, a monomer for polymers, or a valuable feedstock for other industries [10], [24]. Moreover, another crucial substance, i.e. allyl alcohol was also targeted through this reaction. Allyl alcohol has a wide range of potential uses including as a polymer cross-linking agent, a coupling agent, or a drying oil in applications dealing with coating, fire-resistant materials and plasticizers [25], [26], [27], [28].
During this esterification reaction, water is also produced as a byproduct which subsequently forms a stable azeotrope with formic acid (maximum boiling point of 107°C, with 77 wt.% formic acid [24]). The formation of the aforementioned azeotrope would make the removal of water, which is necessary to reach and stabilize the equilibrium, more sophisticated [29]; therefore, a series of dehydration catalysts as well as hydrogen bond (H-bond) breakers were also effectively tested in order to improve the yield of the production of allyl alcohol in a continuous flow reaction. The catalysts and H-bond breakers investigated included magnesium chloride (MgCl2·6H2O) [30], tin chloride (SnCl2·2H2O) [31], sodium sulfate (Na2SO4) [32], potassium bisulfate (KHSO4) [33], phosphotungstic acid {H3[P(W3O10)4]} [34] and urea [35].
2 Materials and methods
2.1 Materials and chemicals
All reagents and materials were purchased from Sigma Aldrich (São Paulo, Brazil) and were used without any further purification.
2.2 Experimental procedures
Each batch experiment was conducted using a fixed amount of glycerin (30 g~0.32 mol) and different quantities of formic acid in the presence or absence of different quantities of urea and catalysts (Table 1). In fact, the catalysts were used to accelerate the reaction progress and to break the azeotrope that was formed. In general, four different molar ratios of glycerin to formic acid were used, i.e. 1:1, 1:2, 2:1 and 1:0.85. The quantities of inorganic salts (catalysts) varied from 0.5 to 10 wt.% of the glycerin used. The effect of heating on the reaction progress was also studied at two different temperature values, i.e. room temperature (25±1°C) and an elevated temperature value of 120°C. Finally, the effect of the addition of urea was also investigated at two different temperature values, i.e. room temperature (25±1°C) and 120°C. Moreover, the influence of the reaction time was also taken into consideration and three different time durations, i.e. 6 h, 24 h and 168 h (1 week) were tested.
Overview of the different conditions under which the experiments were conducted.
| Trial no. | Molar ratio (glycerin:formic acid) | Catalyst | Thermal condition | Duration (h) | |
|---|---|---|---|---|---|
| Type | Amount (wt.%) | ||||
| 1 | 1:1 | – | – | Elevated temperature | 24 |
| 2 | 2:1 | – | – | Elevated temperature | 24 |
| 3 | 1:2 | – | – | Elevated temperature | 24 |
| 4 | 1:0.85 | – | – | Elevated temperature | 24 |
| 5 | 1:0.85 | KHSO4 | 0.5 | Elevated temperature | 24 |
| 6 | 1:0.85 | H3[P(W3O10)4] | 0.5 | Elevated temperature | 24 |
| 7 | 1:0.85 | – | – | Room temperature | 168 |
| 8 | 1:0.85 | KHSO4 | 0.5 | Room temperature | 168 |
| 9 | 1:0.85 | H3[P(W3O10)4] | 0.5 | Room temperature | 168 |
| 10 | 1:0.85 | KHSO4 | 1.0 | Room temperature | 168 |
| 11 | 1:0.85 | MgCl2·6H2O | 7.1 | Room temperature | 168 |
| 12 | 1:0.85 | SnCl2·2H2O | 2.0 | Room temperature | 168 |
| 13 | 1:1 (:1 [Urea]) | Na2SO4 | 10.0 | Room temperature | 6 |
| 14 | 1:1 (:1 [Urea]) | Na2SO4 | 10.0 | Elevated temperature | 6 |
2.3 Reactor set-up
The set-up used to accomplish the reaction consisted of a 250 ml round bottom glass flask, where the reactants were charged (Figure 1). The set-up was also equipped with a double layer condenser as well as a hot plate (IKA, Staufen, Germany) placed underneath the reactor flask. The temperature of the reactants mixture was regularly monitored using an infrared thermometer (Testo AG, Lenzkirch, Germany). To collect the water produced during the experiment and to help the completion of the equilibrium esterification reactions toward the products side, a Dean-Stark part was connected to the double layer condenser.
Mounted set-up consisting of (i) cold water inlet, (ii) cold water outlet, (iii) reactants, iv) toluene, (v) water and (vi) a conventional heater.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
2.4 Sampling and sample preparation
Sampling was performed regularly at 0, 1, 2 and 4 h after the initiation of the reaction and was then continued at 4-h intervals until the end of the first day (24 h). Subsequent samplings were conducted 48 h and 1 week after the reaction. A similar procedure for sample preparation was used in all the trials, i.e. a 1 ml sample was probed from the reactants mixture, weighed and further diluted in 100 ml of distilled water. In order to improve the accuracy and also the reliability of the results, each diluted solution was divided into four aliquots and each was titrated separately and the mean value of the four data was used for further analyses.
2.5 Instrumentation and analytical conditions
Except for the trials involving urea, the amount of formic acid in the samples was measured by chemical titration using a 0.1 m solution of sodium hydroxide (NaOH) with bromothymol blue as the pH indicator. It is worth noting that a 0.1 m solution of phthalic acid was used for calibration of the NaOH solution and the progression of the reactions was assessed based on the amount of formic acid consumed. To this end, the available amount of formic acid in each sample was considered for calculation of the reaction progress in an inverse manner. In other words, the optimum (ideal) reaction progress would, therefore, equal zero, while it is 100% at the beginning of the reaction. In the case of the experiments in which urea was included, the basic titration could not be performed because both urea and ammonia (NH3) (which is produced as a result of the decomposition of urea) are alkaline and impart base and buffer characteristics, thus falsifying the possible titration results with NaOH, which is also a base. Therefore, gas chromatography-mass spectrometry (GC-MS) was utilized to assess the progress of the reaction as well as to further analyze all the presumed products leading to exploring all the produced products and byproducts. According to the literature [36], [37], [38], despite the chemical titration method being less accurate than GC-MS, in the scope of the present study, particularly for the azeotropic mixtures, the analytical results of the chemical titration methods were expected to be in general agreement with the presumed results from a GC-MS method for the same samples.
The GC-MS analyses were performed using a GC-17A ver. 3 gas chromatograph (Shimadzu, Japan) combined with a GCMS-QP5050A quadrupole mass spectrometer (Shimadzu, Japan). A high polarity capillary column with a cross-linked and bonded polyethylene glycol (PEG) phase (DB-WAX, with a film thickness of 0.25 μm; diameter of 0.25 mm and length of 30 m, Agilent Technologies Inc., Glostrup, Denmark) was used for separation. This column has a polar character, so the elution of the apolar components was expected to occur prior to the polar ones. GC-MS conditions were as follows: positive electron impact mode; injector temperature, 240°C; initial temperature for MS, 260°C; carrier gas, helium; column flow rate, 1 ml/min; linear velocity, 36.5 cm/s; inlet pressure, 56.7 kPa; total flow rate, 7.5 ml/min; initial column temperature was 60°C and hold for 6 min, then increased at the rate of 10°C per min to 240°C and hold for 40 min; total run time, 45.9 min. The samples were diluted in methanol prior to measurement in 1:10 volumetric ratio and GC-MS analysis of samples was performed after 2, 4 and 6 h (see Supplementary Figures S1–S4). All the samples were analyzed in full scan mode with a mass-to-charge (m/z) range of 10–300.
3 Results and discussion
3.1 Kinetics and reactions
Allyl alcohol (C3H6O), a chemical intermediate used in the synthesis of various substances, is produced through the reaction between glycerin (C3H8O3) and formic acid (HCOOH) when heated above 200°C [39], [40], [41]. This reaction normally takes place through three steps as illustrated in Figure 2.
Multi-step reaction mechanism for the production of allyl alcohol from glycerin and formic acid.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
The first two steps of the mechanism are the equilibrium reactions and, to push the reactions toward the products side, the water produced should be removed. However, the close standard boiling points of water (100°C) and formic acid (101°C) as well as the desired product, i.e. allyl alcohol (97°C), makes the water removal process through simple distillation almost impossible [29]. As well as this, the formation of the maximum-boiling azeotrope between water and formic acid is considered to be another drawback. The common solution used to break the azeotropic mixture with water is to add inorganic salts. In fact, the addition of electrolytes could amend the equilibrium nature of the original mixture remarkably [42], [43]. Over the past few decades, the influence of electrolytes and inorganic salts on the equilibrium phase has been broadly investigated in different industries with the aim of separating azeotropic as well as close-boiling points mixtures [44], [45], [46]. In the design of the separation process, the investigation of the vapor-liquid equilibrium for these electrolytic mixtures is considered to be important and the effect of salts on the vapor-liquid equilibrium should be consequently evaluated based on their individual binary interactions [47], [48], [49]. In the case of a ternary system comprising two solvents (i.e. solvent 1 and solvent 2) and a salt, three pairs of interactions, i.e. solvent 1 – salt, solvent 2 – salt and solvent 1 – solvent 2, must be studied in parallel through precise thermodynamic models.
In addition to the known salts, urea, mainly due to its significant ability to break the H-bonds, has also been reported to be effective in breaking the azeotropic mixtures containing water [50], [51], [52]. However, in our ternary system (glycerin, formic acid and urea), adding urea could also lead to other chemical reactions; at least three. More specifically, apart from the reaction of glycerin and formic acid and the formation of glycerin monoformate (Figure 2), there is also reactivity between glycerin and urea. As shown in Figure 3, this reactivity at around 120°C could lead to a reaction producing glycerin carbonate (C4H6O4), also known as 4-(hydroxymethyl)-1,3-dioxolan-2-one, which is a cyclic compound mainly produced from other carbonate derivatives [53], [54]. Furthermore, formic acid and urea also react together and consequently, diformyl urea is simply synthesized as the product of this reaction. This synthesis reaction (Figure 3) has insignificant thermal sensitivity and could take place at different ranges of temperature and most effectively between 10°C and 140°C [55], [56].
Chemical reaction between glycerin and urea (upper reaction) and formic acid and urea (lower reaction).
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
3.2 Effect of molar ratio and reaction time
The results of the experiments with different molar ratios while keeping the rest of the reaction and heating conditions the same, proves the higher efficiency of a molar ratio of 1:0.85 (Figure 4A). In the same figure, the effect of the other parameters, e.g. heating and catalysts has been cancelled by keeping the same conditions for all the trials. It is apparent that the molar ratio of 1:0.85 has resulted in a significantly better progress of the reaction (~81%), whereas in the other ratios the consumption of formic acid did not exceed 35%. Therefore, the majority of the results presented have been accomplished at this molar ratio. To investigate the kinetics and behavior of the esterification reaction over time, some similar experiment runs using different catalysts were allowed to take place for an extended period of time while samples were taken at regular intervals (Figure 4B). The results demonstrate that the reaction is an equilibrium reaction and the reactants are reproduced after a few days. This is observed in the rising trends for the reaction progress of the trials after roughly 48 h. Consequently, the overall yield in these experiments is also reduced by heading the reaction toward the reactants. According to these results, one can say that the state of equilibrium is reached after a maximum of 48 h when approximately 50% of the formic acid has been consumed. After that, the values for the consumption of formic acid are around 50%–60%.
(A) Trials at different molar ratios and similar catalysis and heating conditions. (B) Effect of reaction time on the reaction progress.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
3.3 Effect of catalyst and heating conditions
As Figure 5A depicts, heating the system assists the progress of the reaction, which is clearly verified by a qualitative assessment of the trial pairs and quantitative analysis confirms the reaction progress of up to 17% of the formic acid (corresponding to the yield of 83%) in the experiments with elevated temperature, whereas that in trials at room temperature does not exceed 55% (i.e. a yield of 45%). In order to investigate the effect of different catalysis conditions, the results of five different trials at room temperature but with five different reaction conditions are compared in Figure 5B. However, they have all shown relatively similar trends, where the maximum overall progress for the reaction is not more than 52% and adding the catalysts has not demonstrated the expected effect of increasing the yield at room temperature. In the case where potassium bisulfate (KHSO4) was added and the temperature was elevated (Trial 5), the obtained amount of consumption of formic acid in the esterification reaction was insignificantly improved up to 83%, which is just 2% more than that in the similar experiment without adding any catalyst (Trial 4). Thus, the improvement of only 2% does not firmly justify the adding of a catalyst. Due to this fact and also the decomposition during heating that was observed from the carbonization of liquid on the walls of the reactor vessel, it was decided to continue studying the esterification reaction mainly at room temperature.
(A) The comparison of trials 4–6 and 7–9, at similar conditions but at elevated and room temperature, respectively. (B) Trials under similar reaction but different catalysis conditions.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
Moreover, the addition of catalysts showed a noticeable influence only within the first few hours of the reaction, while after a few hours the state of equilibrium is reached. In fact, the catalysts contribute to accelerating the reaction in the beginning and to reaching the equilibrium state after a few hours. Among the catalysts used in this set of experiments, MgCl2·6H2O showed the best results with the consumption of about 45% of formic acid after 1 week (i.e. reaction progress of 55% in the diagram). However, overall and despite the slight and unjustified improvement in the reaction yield of Trial 5, the most desired and justifiable result was obtained in Trial 4 after 24 h, which was 83% at the molar ratio of 1:0.85, with elevated temperature of 120°C–140°C and, surprisingly, adding no catalyst.
3.4 Effect of the addition of urea
In order to break the H-bonds which further cause the formation of an azeotrope of formic acid and water, we added urea (which is a well-known component for breaking H-bonds [35], [51]) to the reactants as a third component aiming to improve the reaction progress. However, we soon noticed that urea also reacts with glycerin and formic acid; therefore participating in the reaction in numerous ways, e.g. to produce glycerin carbonate and diformyl urea. Irrespective of the previous findings regarding molar ratio, given the one-to-one stoichiometric reactions of urea with the reagents (Figure 3), the equal molar ratio of 1:1:1 was used for glycerin, formic acid and urea, aiming to investigate the effect of urea for breaking H-bonds. We investigated this system at both elevated and room temperature through two different experiment trials. In these trials, as mentioned earlier, the basic titration could not be performed; therefore, a GC-MS approach was used for assessment of the reaction progress as well as analysis of all the expected products of the reactions previously illustrated in Figure 3. Analyses were further developed and the extracted quantitative results were incorporated into two expanded tables (results not shown here). These results demonstrated the presence of several compounds such as formamide [H(CO)NH2], methyl carbonate [H2N(CO)OCH3], glycidol (C3H6O2) and glycerin monoformate (C4H8O4) in addition to the glycerin carbonate, diformyl urea and the reactants.
Quantitative analyses on the obtained spectra by GC-MS revealed that glycerin carbonate and glycerin monoformate seemed to diminish, whereas the concentration of formamide initially rises up to a high level (19 wt.%) but remains constant after 4 h. Also, analyses demonstrated that glycerin carbonate has been produced only up to 3.7 wt.% and 12 wt.% at elevated and room temperature, respectively. Moreover, in both runs, urea seemed to be completely consumed; however, glycerin and formic acid were still present at high and low concentrations, respectively. Allyl alcohol was also detected in trace quantities in both runs. At the longest retention time, i.e. the most polar fraction, the amount of ally alcohol rises up to around 5.74 wt.% at room temperature, while it is only 3.75 wt.% at elevated temperature. Moreover, another byproduct that was found at both trials was glycidol, i.e. 3.40 wt.% and only 0.7 wt.% at elevated and room temperature, respectively. In both runs considerable amounts of glycerin and formic acid were detected as well, implying that the reaction progress was not as high as that of trials 4 and 5, where they showed the maximum reaction progress, i.e. above 80%. Also, in the experiment at room temperature, there was no evidence of the production of either glycerin monoformate or diformyl urea; while the results demonstrated that they both have been produced in the experiment at elevated temperature, though in a low concentration at the longest retention time. The absence of glycerin monoformate in the final analyzed solution is attributed to its intermediate role in the path of production of allyl alcohol, where it is expected to be consumed. As a matter of fact, the intensity is a function of ionization probability and the partial vapor pressure of each compound; thus, in some cases there is no definitive relationship between the peak area and the relative concentration of the analyzed compounds. Consequently, in the case of diformyl urea, which is the only expected product in solid state and has a substantially lower vapor pressure, finding a correlation for these parameters is difficult.
3.5 Mass spectrometry diffraction patterns
As mentioned earlier, several different compounds were detected through GC-MS analysis. Here we briefly report the possible fragmentation path for the main compounds detected in spectrometry analyses through different m/z ratios. For each compound, the reported m/z is sequenced by intensity. The available spectra for the reactants and some of the products are shown in Supplementary Figures S5–S10.
Glycerin showed m/z of 61, 43, 44, 60, 57, 45, 42, 55, 73 and 74; however, formic acid just showed that of 46, 45 and 44 and urea was not found in any of the two trials, suggesting that it has been completely consumed. However, there have been hints of its derivative with the methanol used to dilute the probes, e.g. carbamic acid methyl ester, to which the observed m/z data might correspond. They can all be explained by the reaction schemes illustrated in Figure 6. The best agreement with the experimental data was achieved for glycerin at a retention time of 23 min and for formic acid at 13 and 15 min.
Plausible fragmentation paths for radical cations of (A) glycerin, (B) formic acid and (C) urea.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
Also, some trace amounts of two undesired compounds, i.e. glycidol and formamide have been detected in both trials. Glycidol showed m/z of 44, 43, 55 and 73 (as shown in Figure 7), while the effective m/z data for formamide were 45, 44, 43 and 42 which might be associated with successive hydrogen loss. In fact, glycidol is formally the dehydration product of glycerin and it can also be a thermal reaction product from glycerin carbonate and glycerin monoformate. The best agreements with the experimental data were obtained at 28.9 and 17.2 min for glycidol and formamide, respectively.
Possible fragmentation path of glycidol radical cation.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
Furthermore, three other expected products were detected, i.e. glycerin carbonate, diformyl urea and glycerin monoformate. For glycerin carbonate three different fractions can be reasonable: (a) α-scission and abstraction of the CH2OH radical, (b) loss of CO and (c) loss of CO2 (by thermal reaction), which are represented in Figure 8. The experimentally verified masses that matched best are: 44, 43, 88, 87, 55, 56, 90, 72, 73, 101 which belong to the retention time of 6.5 min, observing low polarity of the compound when compared to reactants and other products. It is worth noting that glycerin carbonate was detected in the trial at elevated temperature only.
Most probable ions and their masses in the fragmentation of glycerin carbonate.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
For diformyl urea a simple spectrum is expected due to its symmetry. Evidentially, it does not matter if the electron was abstracted from the oxygen or the nitrogen. In any case the α-scission (always homolytically) would yield the same cation (see Figure 9).
Plausible α-scissions for diformyl urea.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
A second α-scission is not to be expected, due to the high energy involved in breaking off the cation M=72. A scission of the molecule ion, relative to the lateral oxygen, would yield a low mass fragment that is out of experimental reach (m/z=31). Another fragmentation to expect in diformyl urea is the abstraction of CO as shown in Figure 10.
Possible fragmentation path for diformyl urea.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
Abstraction of the central CO group in the molecule ion would give an intermediary, less stable hydrazine derivative whose breaking off would yield equally an m/z=44 fragment. Resuming, we can expect the masses 116, 88, 72 and 44 and best agreement with the experimental data shown at retention time of 45.8 min; therefore, the most polar component of the reaction mixture. Notably, there was no significant evidence for diformyl urea in the trial at elevated temperature.
Lastly, glycerin monoformate was detected in both trials. The ester group is known to fragmentize in a complex manner. Moreover, the two remaining free hydroxyl groups provide more places to accommodate the radical and/or the positive charge. Different fragmentation processes are possible as follows: (a) McLafferty rearrangements at the molecule ion and its dehydration product (Figure 11A); (b) α-scissions of the hydrocarbon skeleton in three positions (Figure 11B); (c) CO loss of the molecule ion or its dehydration product, where the fragments have the masses of glycerin and glycidol, respectively (Figure 12A); (d) oxonium reaction will give fragments of low masses (Figure 12B); and (e) H radical abstractions that are potential fragmentations of any of the former presented radical ions and will reduce the indicated masses by one or two units.
Two different fragmentation processes of glycerin monoformate: (A) McLafferty rearrangements at the molecule ion and its dehydration product and (B) α-scissions of the hydrocarbon skeleton in three positions.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
The second possibility to form the monoester that is on the central hydroxyl group of glycerin, would yield similar fragmentation reactions. However, there is no hydrogen in γ position, so the McLafferty rearrangement would be via a five-membered transition stadium. This process is considerably more difficult than the six-membered stadium as shown in Figure 11.
Returning to the masses, glycerin monoformate showed m/z of 120, 104, 92, 91, 74, 61, 59, 46 and 4, whereas every indicated mass can be reduced by 1 or 2 units. Best agreement was attained at retention time of 37.6 min. This assignment remains uncertain, for two reasons. First, in all runs without urea the ester was supposed to be formed, even under mildest conditions. Secondly, the ester is expected to have less polarity than free glycerin; therefore, it should be eluted before 23 min.
Two possible fragmentation paths of glycerin monoformate: (A) CO loss of the molecule ion or its dehydration product and (B) oxonium reaction that gives fragments of low masses.
Citation: Green Processing and Synthesis 7, 4; 10.1515/gps-2017-0028
4 Conclusion
In this work, an effort was made to optimize the esterification reaction of glycerin and formic acid. Different reactions parameters were altered such as the molar ratio of reactants, temperature, catalyst and duration of the reaction. Also, the addition of urea to the reactants, as a well-known compound for breaking H-bond and azeotrope mixtures, was experimentally investigated. In general, the production of various compounds such as allyl alcohol, glycerin monoformate, glycerin carbonate and diformyl urea were accomplished. The main analysis method used for evaluation of the reaction progress of the experiments was analytical titration. By this method, the yield of experiment was assessed based on the consumption rate of formic acid. Alternatively, two trials were evaluated using GC-MS.
Eventually, the optimum conditions for this particular esterification were developed among the altered parameters. The best feasible result was obtained at the molar ratio of 1:0.85 (glycerin to formic acid), after 24 h of reaction at elevated temperature up to 120°C–140°C and surprisingly without the addition of any catalyst. However, the addition of KHSO4 to the just specified conditions improved the yield only by about 2%, which obviously does not justify the feasibility of the addition of a catalyst. Also, the study of equilibrium in the esterification showed that equilibrium state is reached after consumption of about 55% of the formic acid in just 4 h. And finally, the results of GC-MS analysis confirmed the formation of glycerin carbonate up to 3.7 wt.% (at elevated temperature) and 12 wt.% (at room temperature) as well as the trace amount of diformyl urea and significant quantities of some unexpected but widely applicable products like formamide.
The authors would like to acknowledge the International Association for the Exchange of Students for Technical Experience (IAESTE) committee for providing the research fellowship under which this study was conducted. Also, the authors wish to thank Ms Anastasia Albrandt for her kind technical support and laboratory assistance through the course of the experiments and the analysis of the results.
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Footnotes
Supplementary Material:
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