applied sciences
Article Synthesis and Investigation of Thermal Properties of Highly Pure Carboxylic Fatty Esters to Be Used as PCM
Rebecca Ravotti, Oliver Fellmann, Nicolas Lardon, Ludger J. Fischer, Anastasia Stamatiou * and Jörg Worlitschek
Competence Centre Thermal Energy Storage (TES), Lucerne University of Applied Sciences and Arts, 6048 Horw, Switzerland; [email protected] (R.R.); [email protected] (O.F.); [email protected] (N.L.); ludger.fi[email protected] (L.J.F.); [email protected] (J.W.) * Correspondence: [email protected]; Tel.: +41-(0)41-349-3297
Received: 30 May 2018; Accepted: 27 June 2018; Published: 30 June 2018
Abstract: Latent heat storage systems are gaining the attention of researchers as possible substitutes to conventional sensible heat storage systems due to their compactness and their ability to absorb and release heat almost isothermally. Among the Phase Change Materials (PCM) for energy storage studied so far, esters are believed to show promising properties. In particular, a broad range of melting temperatures, little to no supercooling, low corrosivity, chemical and thermal stability, and high enthalpies of fusion are reported. Many esters have the advantage of being bio-based and biodegradable, making them more sustainable in comparison to other popular PCM. Still, a clear lack of experimental data exists in regards to this class. In the present study, esters derived from saturated fatty carboxylic acids (myristic, palmitic, stearic, behenic), coupled with primary linear alcohols of different length (methanol, 1-decanol) were synthesized through Fischer esterification and their properties were investigated. Purities higher than 89% were obtained for all cases as proven by gas chromatography coupled with mass spectroscopy and nuclear magnetic resonance analysis. Additionally, the esters’ formation and reaction kinetics were characterized by attenuated total reflectance infrared spectroscopy. The esters produced showed to possess relatively high enthalpies of fusion above 190 J/g and thermal stability over three repeated cycles with differential scanning calorimetry. The melting points measured ranged between 20 ◦C and 50 ◦C, therefore proving to be interesting candidates for low-medium temperature applications such as heating and cooling in buildings. A correlation could be observed between the chemical structure and melting point of the produced esters. Additionally, thermogravimetric analysis revealed a higher thermal resistance for esters with longer aliphatic chains in comparison to shorter-chained ones.
Keywords: latent heat storage; phase change material; esters; thermal energy storage; fatty esters; decyl esters; methyl esters
1. Introduction The recent trends in energy consumption and energy policies are shifting the focus to sustainable utilization of energy as a key issue [1,2]. Based on an International Energy Agency (IEA) report [1], a steep increase in global primary energy consumption has been observed over the last few years. Roughly 50% of this energy is being used to heat and cool applications in the industrial and domestic sectors. Thermal Energy Storage (TES) systems are arising as key technologies to help increase energetic efficiency and enable a high share of renewable energies for future energy systems [1–3]. TES can be divided into three main categories: Sensible Heat Storage (SHS), Latent Heat Storage (LHS) and
Appl. Sci. 2018, 8, 1069; doi:10.3390/app8071069 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1069 2 of 18
Thermo-Chemical Heat Storage (TCS). In LHS systems, heat is stored in the form of phase change enthalpy, which can be accumulated or released upon phase change of the energy-storing material [2,3]. As such, this application requires materials with high phase change enthalpies, which are referred to as Phase Change Materials (PCM). As reported by Sharma et al. (2009) [4], due to higher compactness and higher heat storage density, LHSs allow for greater flexibility in the location selection of the storage system and minimum space usage. PCMs accumulate energy while the phase change occurs at almost isothermal conditions and over narrower temperature ranges, therefore providing higher energy efficiency with a smaller temperature difference between charging and discharging in comparison to sensible storage [5–7]. However, in addition to higher investment costs compared to SHSs, PCMs often face issues with toxicity, flammability, and possible degradation of properties after several cycles [5]. Early studies focused on salt hydrates as inorganic PCMs due to their availability, low cost, and high latent heat storage capacities. However, because of their tendency to corrode, supercool and segregate, researchers have turned their attention towards organic PCMs as well [8–10]. Paraffins are the most commonly used organic PCMs as of today; however, they are oil derivatives and are characterized by high flammability and relatively limited melting point range. Therefore, carboxylic acids and their derivatives have been selected as a more sustainable alternative [10–12]. In particular, in order to reduce environmental impact, research is being conducted on organic PCMs based on readily available natural resources, such as carboxylic esters, which naturally occur in fats and oils from renewable feedstock [10–12]. Menoufli et al. [11] conducted preliminary studies on the environmental impact of salt hydrates and esters as PCM in house-shaped cubicles in Spain. The results showed that based on the Eco-indicator 99 method, an impact reduction of 10.5% for esters compared to paraffins was calculated. Carboxylic esters can be obtained via Fischer esterification of carboxylic acids with alcohols, and present comparable thermal and chemical characteristics to the original acids [12]. In fact, similarly to the corresponding carboxylic acids, they are reported to show low or no toxicity, no corrosivity, high enthalpies of fusion, high chemical and thermal stability, and are mainly inflammable [8,9]. Millions of possible combinations between carboxylic acids and alcohols exist, creating endless possibilities for esters as PCMs [8]. However, despite the advantages reported above, esters as PCM have not been investigated deeply so far. As reported by Stamatiou [12] and Aydin [13,14], a clear gap in the data concerning experimental thermal properties of esters derived from carboxylic esters exists. This is probably due to lack of easily available commercial products, which makes it challenging to collect experimental data. In order to allow for identification of potential esters to be used as future PCM in relevant temperature ranges, more experimental data on the thermal properties of esters is required. The interpretation of these data could also help gain a clearer understanding of the dependence between chemical structure and thermal properties. While little experimental data on enthalpies of fusion exists, previous studies on the correlation between molecular structure and melting temperature have been performed. Firstly, Carnelley [15] in 1882, and more recently Boese et al. [16] in 2001 both observed that symmetrical molecules with efficient crystal packing tend to have higher melting points due to stronger intermolecular bonding. This is true for molecules with higher molecular weights because of lower kinetic energy and therefore more tightly packed structures. For the same reason, branched molecules are reported to have lower melting points compared to linear homologues since they tend to form less packed structures [16]. As reported by Noël et al. [17] and Yang et al. [18], the melting points of alkanes increase with longer carbon chains, with two different trends for even and odd numbered carbon chains, where even-numbered ones present higher values. Stamatiou et al. [12] examined 11 commercially available linear fatty esters for use as PCM, and confirmed the trends reported above for melting points in relation to molar mass. Other studies have been performed on the subject. Aydin et al. [13,14] synthesized even-numbered fatty acid esters of myristyl alcohol and esters from dicarboxylic acids with promising enthalpies of fusion over 200 J/g. Sari et al. [19] synthesized esters derived from stearic acid with glycerol, isopropyl alcohol and n-butyl alcohol through Fischer esterification. Appl. Sci. 2018, 8, 1069 3 of 18
Appl.In Sci. this 2018, study, 8, x synthesis of linear fatty esters from fatty acids of different even-numbered 3 of 18chain lengths (Myristic Acid MY (C ), Palmitic Acid PA (C ), Stearic Acid SA (C ), Behenic Acid BE In this study, synthesis of 14linear fatty esters from fatty16 acids of different even-numbered18 chain (C22)), coupled with alcohols (methanol (C1) and 1-decanol (C10)) was performed. All IUPAC names, lengths (Myristic Acid MY (C14), Palmitic Acid PA (C16), Stearic Acid SA (C18), Behenic Acid BE (C22)), trivial names, chemical structures and shortenings from the esters produced are reported in Figure A1. coupled with alcohols (methanol (C1) and 1-decanol (C10)) was performed. All IUPAC names, trivial Thenames, thermal chemical properties structures of the and esters shortenings produced from were the esters analyzed produced to individuate are reported possible in Figure trends A1. with The the molecularthermal properties structure, of and the to esters obtain produced a broader were overview analyzed on the to possible individuate range possible of melting trends points. with Inthe order tomolecular optimize structure, and validate and to the obtain synthesis, a broader at first overview a commercially on the possible available range esterof melting (Methyl points. Palmitate) In wasorder reproduced to optimize in and the validate laboratory the synthesis, and the resulting at first a commercially properties compared available withester (Methyl the one Palmitate) purchased by manufacturers.was reproduced Followingin the laboratory optimization and the resulting of the synthesis properties procedure, compared new with esters the one of purchased fatty acids by with methanolmanufacturers. (MeOH) Following and 1-decanol optimization was produced. of the synthesis Methyl Palmitateprocedure, (MEPA), new esters Methyl of fatty Myristate acids with (MEMY) andmethanol Methyl (MeOH) Stearate and (MESA) 1-decanol have been was produced.previously Methyl produced Palmitate and are (MEPA), available Methyl at affordable Myristate prices, while(MEMY) Methyl and Methyl Behenate Stearate (MEBE) (MESA) is also have available been previously at higher produced prices. Little and are tono available thermal at dataaffordable has been foundprices, on while fatty Methyl esters Behenate derived from (MEBE) 1-Decanol, is also available and their at commercialhigher prices. availability Little to no is thermal limited. data has been found on fatty esters derived from 1-Decanol, and their commercial availability is limited. 2. Materials and Methods 2. Materials and Methods Carboxylic PA (≥90%) was purchased from Vopak (Rotterdam, Netherlands), MY (≥99%) from SigmaCarboxylic Aldrich (Saint PA (≥90%) Louis, was MO, purchased USA), SA from (≥90%) Vopak from (Rotterdam, Alfa Aesar Netherlands), (Haverhill, MA, MY USA), (≥99%) BE from (≥85%) fromSigma Alfa Aldrich Aesar (Saint (Haverhill, Louis, MO, MA, USA), USA), SA while (≥90%) alcohols from Alfa MeOH, Aesar (Haverhill, and 1-decanol MA, wereUSA), acquired BE (≥85%) from Sigmafrom Alfa Aldrich Aesar with (Haverhill, high purities MA, (USA),≥99%) while as synthesis alcohols precursors. MeOH, and MEPA 1-decanol from were Sigma acquired Aldrich from (≥99%) Sigma Aldrich with high purities (≥99%) as synthesis precursors. MEPA from Sigma Aldrich (≥99%) was used as commercial reference for synthesis optimization. Concentrated sulfuric acid (H2SO4) was used as commercial reference for synthesis optimization. Concentrated sulfuric acid (H2SO4) was was used as the acid catalyst, Sodium Sulfate anhydrous (Na2SO4) as a water-absorbing agent for used as the acid catalyst, Sodium Sulfate anhydrous (Na2SO4) as a water-absorbing agent for elimination of water, diethyl ether (Et2O) and ethyl acetate (EtOAc) as organic solvents for separation; elimination of water, diethyl ether (Et2O) and ethyl acetate (EtOAc) as organic solvents for separation; they were all purchased from Sigma Aldrich with purities ≥99%. All chemical materials were used they were all purchased from Sigma Aldrich with purities ≥99%. All chemical materials were used without any prior purification. without any prior purification. 2.1. Synthesis Development 2.1. Synthesis Development 2.1.1. Synthesis Mechanism and Instrumentation 2.1.1. Synthesis Mechanism and Instrumentation The esters were synthesized based on Fischer esterification process with H2SO4 as a catalyst. The esters were synthesized based on Fischer esterification process with H2SO4 as a catalyst. Under acidic conditions, the oxygen from the carbonyl group is protonated, thus creating a partial Under acidic conditions, the oxygen from the carbonyl group is protonated, thus creating a partial positive charge on the carbon, which is more prone to be attacked by the nucleophilic alcohol. Water is positive charge on the carbon, which is more prone to be attacked by the nucleophilic alcohol. Water then formed by proton transfer and eliminated. Upon deprotonation of the carbonyl oxygen, the double is then formed by proton transfer and eliminated. Upon deprotonation of the carbonyl oxygen, the bonddouble is formedbond is formed again to again create to thecreate resulting the resulting ester group.ester group. Since Since the reactionthe reaction is an is equilibriuman equilibrium based onbased Le-Ch onâ telierLe-Châtelier principle, principle, it can beit can shifted be shifted towards towards the products the products (ester) (ester) by working by working with anwith excess an of alcoholexcess of under alcohol refluxing under refluxing conditions conditions [20]. The [20]. reaction The reaction mechanism mechanism is shown is shown in Figure in Figure1. 1.
FigureFigure 1.1. Reaction mechanism mechanism of of Fischer Fischer esterification. esterification. Catalytic Catalytic activation activation of the of carboxylic the carboxylic group groupis isneeded needed to to favor favor the the nucleophilic nucleophilic attack attack from from the the alcohol. alcohol. The The red red circles circles highlight highlight the the different different attackingattacking andand leavingleaving groups. Appl. Sci. 2018, 8, 1069 4 of 18
The synthesis was performed in a two-necked round bottom flask with a magnetic stirrer and thermometer, equipped with a reflux condenser and immersed in an oil bath with a temperature-regulating sensor. In a 100 mL two-necked round bottom, the fatty acid (0.02 moL) and alcohol (0.1 moL, 5:1 alcohol:acid ratio) were added, and the oil bath was heated to 100 ◦C and the stirring set to 200 rpm. Upon complete melting of the acid, 1 mL of concentrated H2SO4 (concentrated at 95–98%) was added to the reaction mixture. The reagents were left to react for 5 h, after which the flask was allowed to cool down to room temperature. Afterwards, Et2O or EtOAc was added to the reaction mixture. The obtained solution was washed three times with deionized water, and the organic phase separated in a separating funnel. Na2SO4 (anhydrous) was then added to the organic phase to create an oversaturated solution in order to dry it from eventual residual water [20,21]. After 10 min, the solution was filtered using filtering paper and the excess organic solvent removed in the rotary evaporator (Rotavapor Büchi RII) at 50 ◦C bath temperature and 100 mbar for approximately 1 h. In order to ensure statistical relevance to the results obtained and to guarantee reproducibility, all synthesis described in the next chapters was repeated three times.
2.1.2. Purification The esters produced were purified according to the degree of impurity observed in the Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR) and Nuclear Magnetic Resonance (NMR). Esters derived from MeOH were not processed further following solvent removal with Rotavapor, since they presented degree of purity. Conversely, those derived from 1-Decanol presented relevant traces of residual alcohol following drying in Rotavapor. Therefore, they were first dissolved in MeOH at ratio 10:1 alcohol:ester to favor the migration of residual 1-Decanol in the MeOH hydrophilic phase, then crystallized at 2–5 ◦C overnight (1-Decanol melting point at 6 ◦C), and afterwards filtered through vacuum filtration by washing with MeOH. Upon observation of the resulting products, if 1-decanol was still detected in ATR-IR or any other analytical technique, the crystallization procedure was repeated until disappearance of the impurities. Generally, purification by one crystallization process was sufficient for most esters, but in some cases, it had to be repeated up to three times.
2.2. Characterization
2.2.1. Differential Scanning Calorimetry (DSC) Based on TA instruments application note [22], analysis of melting and crystallization temperatures was conducted via DSC 823e by METTLER TOLEDO between −25 ◦C and 100 ◦C at 10 ◦C/min heating rates. The DSC was calibrated with indium standard before the measurements, and the uncertainty of the instrument was ±0.1 K. The samples were measured under a constant flow of nitrogen at a rate of 100 mL/min, and samples masses were between 5 mg and 15 mg. In order to test the stability of the samples, the heating/cooling cycles were repeated three times with the same method for the same sample. Moreover, as stated previously, all syntheses were repeated three times to ensure reproducibility; therefore, each ester was measured three times, with three cycles per sample. All uncertainty values reported in the next pages are based on replicate measurements, therefore denoting the accuracy of the results. To evaluate the degree of supercooling, the difference between the onset melting peak and the onset crystallization peak reported from the instrument were considered.
2.2.2. Thermal Gravimetric Analysis (TGA) The samples’ thermal stability and degradation were analyzed by TGA on a STARe 2 System by METTLER TOLEDO in the temperature range between 25 ◦C and 600 ◦C with a heating rate of 10 ◦C/min and sample masses between 15–20 mg. The uncertainty of the instrument’s balance is Appl. Sci. 2018, 8, 1069 5 of 18 reported to be 0.1 µg. For each sample, a blank measurement of the empty crucible was performed with the same method as described above for correct baseline subtraction.
2.2.3. Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR) In order to gain structural information on the samples produced, as well as the degree of purity and the kinetics of the reaction, ATR-IR Cary 630 by Agilent Technologies (Santa Clara, CA, USA) in the wavenumber range between 4000 and 600 cm−1 with 4 cm−1 resolution was used. No prior sample preparation was needed, and a background spectra was registered every 30 min with 32 scans. Afterwards, a few mg of the samples were simply deposited on the diamond tip in either solid or liquid form and the spectrum was registered with 32 scans.
2.2.4. Gas Chromatography Coupled with Mass Spectroscopy (GC-MS) Gas chromatograms were measured on an Agilent Technologies GC 6890N connected to Agilent MSD 5973 with EI standard source, with Topaz 4.0 mm inlet liner with Wool in split mode 1:10 on a Supelco SLB-5 ms 30 m × 250 µm × 0.25 µm column and 1.3 mL/min flow rate. The oven program was 100 ◦C for 1 min, then 10 ◦C/min heating rate to 300 ◦C and at hold for 4 min afterwards. The temperature of the MSD transfer line was 250 ◦C. Mass spectra were measured with electron ionization (EI) at 70 eV, source temperature of 200 ◦C, acceleration voltage of 5 kV, and resolution of 2500. The instrument was scanned between m/z 100 und 600 at scan rate of 2 s/decade in the magnetic scan mode. Perfluorokerosene (PFK, Fluorochem, Hadfield, Derbyshire, UK) served for calibration. The samples were diluted in chloroform with a concentration of 0.1 mg/mL and injection volume was 1 µL.
2.2.5. Nuclear Magnetic Resonance (NMR) All 1H NMR spectra were recorded using a Bruker (Billerica, MA, USA) 400 MHz (1H) spectrometer at room temperature (unless otherwise stated). Chemical shifts (δ-values) are reported in ppm, spectra were calibrated relative to the residual proton chemical shifts (CDCl3, δ = 7.26) of the solvent.
3. Results and Discussion
3.1. Synthesis Development and Optimization In order to validate the suitability of the developed synthesis method, commercial MEPA was purchased and its main thermal properties were compared with those of the MEPA produced in the laboratory in-house. Based on the results obtained, the synthesis was adjusted accordingly to obtain the highest degree of purity and conversion possible. Once the synthesis was optimized, esters of methanol and 1-decanol were synthesized and studied.
3.1.1. ATR-IR Analysis At first, the raw materials were analyzed on ATR-IR for correct assignment of the main peaks and structure identification. The characteristic ester peak from the stretching of C=O group is observed at 1735 cm−1, while the stretching of C=O from the carboxylic acid occurs at 1705 cm−1, which allows for identification of residual acid in the ester product. At 1150 cm−1, a sharp peak for C–O–C stretching from esters is visible, while it is completely absent in the acid spectra. The peaks at 2850 and 2920 cm−1 belong to the alkane chain, and the ratio between them and the ester peak at 1735 cm−1 can be seen increasing for increasing carbon chain length [23]. A characteristic broad peak from the stretching of O–H in alcohols is observed between 3100–3600 cm−1. A shouldering broad peak from 2300 to 3000 cm−1 from the stretching of O–H in carboxylic acids is observed in the spectra of PA, but as expected is completely absent in MEPA spectra, which allows for further observation of impurities in produced esters (Figure2). Appl. Sci. 2018, 8, 1069 6 of 18 Appl. Sci. 2018, 8, x 6 of 18 Appl. Sci. 2018, 8, x 6 of 18
Figure 2. ATR-IR of commercial MEPA from Sigma Aldrich compared to raw materials PA and FigureFigure 2. 2.ATR-IRATR-IR of of commercial commercial MEPA MEPA from Sigma SigmaAldrich Aldrich compared compared to to raw raw materials materials PA andPA MeOH.and MeOH. The main peaks of interest for correct characterization of the esters produced and structure MeOH.The mainThe main peaks peaks of interest of interest for correct for correct characterization characterization of the estersof the produced esters produced and structure and structure control are control are highlighted; in particular, the stretch of C=O at 1705 and− 17351 cm−1 for carboxylic acids controlhighlighted; are highlighted; in particular, in particular, the stretch the of stretch C=O atof 1705 C=O and at 1705 1735 and cm 1735for cm carboxylic−1 for carboxylic acids and acids esters and esters respectively, the broad peaks between 3600–3100−1 cm−1 and 3000–2300−1 cm−1 from O–H andrespectively, esters respectively, the broad the peaks broad between peaks between 3600–3100 3600–3100 cm and cm 3000–2300−1 and 3000–2300 cm from cm−1 O–H from stretch O–H of stretch of alcohols and carboxylic acids, and finally the peak at 1150−1 cm−1 from C–O–C stretch in esters. stretchalcohols of alcohols and carboxylic and carboxylic acids, andacids, finally and finally the peak the at peak 1150 at cm 1150 cmfrom−1 from C–O–C C–O–C stretch stretch in esters. in esters. Following structural characterization of the raw materials, the synthesis was performed and the FollowingFollowing structural structural characterization characterization of the of raw the rawmaterials, materials, the synthesis the synthesis was performed was performed and the and product compared with commercial MEPA. In order to investigate the effect of heating time and productthe product compared compared with commercial with commercial MEPA. MEPA. In order In order to investigate to investigate the theeffect effect of heating of heating time time and and COOH:OH ratio on the product formation, synthesis with increasing heating times and with two COOH:OHCOOH:OH ratio ratio on onthe the product product formation, formation, synthesis synthesis with with increasing increasing heating heating times times and and with with two two different ratios (COOH:OH 1:4 and 1:5) have been performed. As Figure 3 shows, slight differentdifferent ratios ratios (COOH:OH (COOH:OH 1:4 1:4 and and 1:5) 1:5) have have been performed.been performed. As Figure As3 shows, Figure slight 3 shows, improvements slight improvements in the ester yield were observed when raising the COOH:OH ratio from 1:4 to 1:5 improvementsin the ester yield in the were ester observed yield were when observed raising the when COOH:OH raising ratiothe COOH:OH from 1:4 to 1:5ratio excess. from In1:4 fact, to 1:5 when excess. In fact, when comparing the absorbance of the ester peak (1735 cm−1) against the unreacted excess.comparing In fact, the when absorbance comparing of thethe esterabsorbance peak (1735 of the cm ester−1) against peak (1735 the unreacted cm−1) against PA peak the unreacted (1705 cm− 1), PA peak (1705 cm−1), a growth in the ester:acid ratio from 4.7 to 4.85 was observed for higher ratios PAa peak growth (1705 in thecm−1 ester:acid), a growth ratio in fromthe ester:acid 4.7 to 4.85 ratio was from observed 4.7 to for 4.85 higher was ratiosobserved COOH:OH for higher of 1:5.ratios As a COOH:OH of 1:5. As a result, such excess of alcohol was considered optimal for ester formation and COOH:OHresult, such of 1:5. excess As a of result, alcohol such was excess considered of alcohol optimal was considered for ester formation optimal for and ester used formation for all furtherand used for all further synthesis (Figure 3). usedsynthesis for all further (Figure synthesis3). (Figure 3).
Figure 3. Investigation on the influence of COOH:OH ratio on the ester formation. A gain in ester FigureFigure 3. Investigation 3. Investigation on on the the influence influence of of COOH:OH COOH:OH ratio ratio onon thethe esterester formation. formation. A A gain gain in in ester ester yield yield is observed for ratio of 1:5 COOH:OH compared to 1:4. yieldis observedis observed for for ratio ratio of 1:5of 1:5 COOH:OH COOH:OH compared compared to 1:4.to 1:4. Concerning the influence of heating time on the ester formation, as shown in Figure 4, the Concerning the influence of heating time on the ester formation, as shown in Figure 4, the unreacted acid can be seen decreasing over increasing heating times, while conversely the ester peak unreacted acid can be seen decreasing over increasing heating times, while conversely the ester peak is increased. No further change in the ester formation is observed following 5 h heating time, since is increased. No further change in the ester formation is observed following 5 h heating time, since Appl. Sci. 2018, 8, 1069 7 of 18
Concerning the influence of heating time on the ester formation, as shown in Figure4,
Appl.the Sci. unreacted 2018, 8, x acid can be seen decreasing over increasing heating times, while converselythe 7 of ester 18 peak is increased. No further change in the ester formation is observed following 5 h heating time, thesince curve the for curve 9 h and for 45 9 m h heating and 45 mtime heating equals time the one equals from the 5 h one heating. from Therefore, 5 h heating. 5 h Therefore, was considered 5 h was as consideredthe end point as of the the end reaction, point of and the followed reaction, andfor all followed subsequent for all syntheses. subsequent syntheses.
FigureFigure 4. Investigation 4. Investigation on onthe the influence influence of heating of heating time time on onthe the ester ester formation. formation. PA PAimpurities impurities are are seen seen decreasingdecreasing and and ester ester yield yield is increased is increased for forincreased increased heating heating time, time, with with a “plateau” a “plateau” reached reached after after 5 h 5 h heatingheating time. time.
3.1.2.3.1.2. GC-MS GC-MS and and NMR NMR Analysis Analysis In Inorder order to to confirm confirm the the ester ester structures structures andand thethe degree degree of of purity purity observed observed in thein the ATR-IR ATR-IR for MEPAfor MEPAproduced produced with 5:1with excess 5:1 excess of MeOH of MeOH and 5 h and heating 5 h time,heating GC-MS time, andGC-MS NMR and spectra NMR were spectra recorded were for recordedthe commercial for the MEPA commercial from Sigma MEPA Aldrich from and Sigma the Aldrichself-produced and the MEPA, self-produced and compared. MEPA, and compared.Regarding GC, both commercial MEPA and PA were analyzed, and retention times of 13.880 min Regarding GC, both commercial MEPA and PA were analyzed, and retention times of 13.880 and 14.226 min, respectively, were observed, confirming that PA impurities would be visible and the minpeak and well 14.226 separated min, respectively, in the self-produced were observed, MEPA confirming chromatogram. that PA This impurities would would allow for be identificationvisible and the peak well separated in the self-produced MEPA chromatogram. This would allow for of residual unreacted acid and impurities. Afterwards, MEPA produced in the laboratory with 5 h identificationheating and of 5:1 residual excess of unreacted MeOH was acid analyzed and impurities.on the same Afterwards, column, and theMEPA resulting produced chromatogram in the laboratory with 5 h heating and 5:1 excess of MeOH was analyzed on the same column, and the showed that the Ester produced had a retention time of 13.883 min as expected and purity estimated resulting chromatogram showed that the Ester produced had a retention time of 13.883 min as around 98%. expected and purity estimated around 98%. Following GC characterization, a mass spectra of the self-produced MEPA was registered to Following GC characterization, a mass spectra of the self-produced MEPA was registered to further confirm the molecular structure (Figure5). The peak from the molecular ion was observed at further confirm the molecular structure (Figure 5). The peak from the molecular ion was observed at 270.2 m/z with high abundance, as reported in literature for linear long-chained alkane systems [24]. 270.2 m/z with high abundance, as reported in literature for linear long-chained alkane systems [24]. From there, the cleavage of the methanol group was visible from the peak at 239 m/z, corresponding From there, the cleavage of the methanol group was visible from the peak at 239 m/z, corresponding to a loss of 31 of the methyl molecule (15 m/z) and alcohol atom (16 m/z). Side resonances at 241 and to a loss of 31 of the methyl molecule (15 m/z) and alcohol atom (16 m/z). Side resonances at 241 and 242 are thought to be due to resonance forms of the methoxy group. From 227 m/z to 101 m/z several 242 are thought to be due to resonance forms of the methoxy group. From 227 m/z to 101 m/z several peaks distanced 14 m/z each (227, 213, 199, 185, 171, 157, 143, 129, 115, 101) show the progressive peaks distanced 14 m/z each (227, 213, 199, 185, 171, 157, 143, 129, 115, 101) show the progressive breaking of the aliphatic chain by loss of -CH2 from the acid side. Hence, the overall structure of MEPA breaking of the aliphatic chain by loss of -CH2 from the acid side. Hence, the overall structure of was confirmed. MEPA was confirmed. Appl. Sci. 2018, 8, 1069 8 of 18 Appl. Sci. 2018, 8, x 8 of 18
FigureFigure 5. Mass 5. Mass chromatogram chromatogram of MEPA of MEPA produced produced in the in the laboratory. laboratory. The The main main peaks peaks are are highlighted, highlighted, andand particularly particularly the the molecular molecular ion ion at at270 270 m/mz,/ cleavagez, cleavage of of methanol methanol at at 239 239 m/mz,/ andz, and progressive progressive breakingbreaking of ofthe the aliphatic aliphatic chain chain from from 227 227 mm/z/ toz to101 101 m/mz /withz with peaks peaks distances distances 14 14m/mz (corresponding/z (corresponding to to
lossloss of -CH of -CH2). 2).
With regards to NMR results, the spectra of self-produced MEPA with 1:5 acid:alcohol ratio and With regards to NMR results, the spectra of self-produced MEPA with 1:5 acid:alcohol ratio and 5 h heating time was fully characterized, and the structure and purity confirmed. At 3.66 ppm (a), a 5 h heating time was fully characterized, and the structure and purity confirmed. At 3.66 ppm (a), singlet arising from the protons of the deshielded methyl group –CH3, which resonated as a singlet a singlet arising from the protons of the deshielded methyl group –CH3, which resonated as a singlet due to lack of neighbouring protons, was visible. The normalized area of the peak was assigned a due to lack of neighbouring protons, was visible. The normalized area of the peak was assigned a value value of 3 to correctly calculate the amount of protons in the spectra recorded. At 2.28–2.32 ppm (b), of 3 to correctly calculate the amount of protons in the spectra recorded. At 2.28–2.32 ppm (b), a triplet a triplet with area 2 from the –CH2 in α-position to the carbonyl, and a multiplet between 1.58–1.65 with area 2 from the –CH2 in α-position to the carbonyl, and a multiplet between 1.58–1.65 ppm (c) ppm (c) with area 2 from the –CH2 in β-position to the carbonyl was seen. The –CH2 in α-position with area 2 from the –CH2 in β-position to the carbonyl was seen. The –CH2 in α-position resonated resonated as a triplet due to proximity with the two protons from the the –CH2 in β-position, and as a triplet due to proximity with the two protons from the the –CH2 in β-position, and appeared appeared to be more de-shielded due to the neighboring carbonyl group. An intense peak to be more de-shielded due to the neighboring carbonyl group. An intense peak corresponding to corresponding to 24 hydrogen atoms resonating together was observed around 1.26 ppm (d), and 24 hydrogen atoms resonating together was observed around 1.26 ppm (d), and was assigned to was assigned to the long aliphatic chain from the fatty acid. Lastly, the –CH3 group at the end of the the long aliphatic chain from the fatty acid. Lastly, the –CH3 group at the end of the aliphatic chain aliphatic chain appeared as a triplet with integrated area of 3 at 0.86–0.90 ppm (e) due to strong appeared as a triplet with integrated area of 3 at 0.86–0.90 ppm (e) due to strong shielding effect and shielding effect and distance from the carbonyl group and electro-withdrawing elements. By distance from the carbonyl group and electro-withdrawing elements. By summing the integrated areas summing the integrated areas values of the peaks assigned, a total of 34 hydrogens were obtained, values of the peaks assigned, a total of 34 hydrogens were obtained, which matched the elemental which matched the elemental composition C17H34O2 of MEPA (Figure 6). No impurities were composition C17H34O2 of MEPA (Figure6). No impurities were observed, thus confirming the results observed, thus confirming the results obtained from GC-MS spectral data. Therefore, it could be obtained from GC-MS spectral data. Therefore, it could be concluded that commercial MEPA had been concluded that commercial MEPA had been successfully replicated in the laboratory with high successfully replicated in the laboratory with high purity, and new esters could be produced through purity, and new esters could be produced through the Fischer esterification procedure reported the Fischer esterification procedure reported above. above. Appl. Sci. 2018, 8, 1069 9 of 18 Appl. Sci. 2018, 8, x 9 of 18
Appl. Sci. 2018, 8, x 9 of 18
Figure 6. NMR spectra of MEPA produced in the laboratory. No impurities were detected, thus Figure 6. NMR spectra of MEPA produced in the laboratory. No impurities were detected, thus confirming confirming GC-MS results of purity over 98%. The main peaks have been assigned, in particular: the GC-MS results of purity over 98%. The main peaks have been assigned, in particular: the singlet at singlet at 3.66 ppm from –CH3, the triplet from α-CH2 and multiplet from β-CH2 at 2.28–2.32 and 1.58– α β 3.66 ppm1.65 ppm, from respectively, –CH3, the triplet the aliphatic from chain-CH at2 and1.26 ppm multiplet and finally from the-CH shielded2 at 2.28–2.32 –CH3 triplet and at 1.58–1.650.86– ppm, respectively,Figure0.90 ppm. 6. the NMRThe aliphatic green spectra lines of chain indicateMEPA at 1.26producedthe integral ppm in and areas,the finallylaboratory. while thethe Nonormalized shielded impurities –CH values were3 triplet are detected, indicated at 0.86–0.90 thus in ppm. The greenconfirmingblue. lines GC-MS indicate results the integralof purity over areas, 98%. while The themain normalized peaks have been values assigned, are indicated in particular: in blue.the singlet at 3.66 ppm from –CH3, the triplet from α-CH2 and multiplet from β-CH2 at 2.28–2.32 and 1.58– 1.65 ppm, respectively, the aliphatic chain at 1.26 ppm and finally the shielded –CH3 triplet at 0.86– 3.1.3.3.1.3. DSC DSC and and TGA TGA Analysis Analysis 0.90 ppm. The green lines indicate the integral areas, while the normalized values are indicated in Following structural and purity analysis, thermal characterization was performed by TGA and blue. FollowingDSC to determine structural thermal and stability purity (thermal analysis, degradation, thermal cycling), characterization melting and was crystallization performed points by TGA and DSC to3.1.3.Tm determine and DSC Tc ,and degree TGA thermal of Analysis supercooling stability (thermal and enthalpy degradation, of fusion cycling),ΔH of commercial melting and self-produced crystallization points Tm andMEPA. Tc, degree Commercial of supercooling MEPA presented and an enthalpy average ofonset fusion Tm of∆ 28.2H of °C commercial ± 0.2 °C, and andTc of self-produced25.3 °C ± 0.1 MEPA. °C inFollowing accordance structural to what and was purity stated analysis, by the provider, thermal characterization and a supercooling◦ was◦ performedof 2.9 °C for by 10 TGA °C/min◦ and ◦ Commercial MEPA presented an average onset Tm of 28.2 C ± 0.2 C, and Tc of 25.3 C ± 0.1 C in DSC to determine thermal stability (thermal degradation, cycling), melting and crystallizatione points accordanceheating torate. what The wasmelting stated and crystallization by the provider, peaks and were a calculated supercooling by the of STAR 2.9 ◦ Csoftware for 10 through◦C/min heating Tm and Tc, degree of supercooling and enthalpy of fusion ΔH of commercial and self-produced the tangent method, and the enthalpies were obtained by integration of the correspondinge peak. The rate. TheMEPA.standard melting Commercial deviations and crystallization MEPA were presented calculated peaks an from average were the calculatedresultsonset T m of of the 28.2 by cycling the °C ± STAR 0.2 tests °C, software and and T c repetitions. of 25.3 through °C ± For0.1 the tangent method,°Cdetermination in and accordance the enthalpiesof phaseto what change was were statedtemperatures, obtained by the byprovider, onset integration temperatures and a supercooling of the are correspondingreported, of 2.9 and °C supercooling for peak. 10 °C/min The is standard e deviationsheatingcalculated were rate. as calculatedThe the melting difference fromand crystallization the between results onset ofpeaks the melting were cycling calculated temperature tests andby the repetitions. and STAR onset software crystallization For determinationthrough of phasethetemperature, change tangent temperatures, method, unless andstated the onsetotherwise. enthalpies temperatures Awere high obtained ΔH areof by204 reported, integration J/g ± 2 J/g and of was the supercooling observedcorresponding for commercial is peak. calculated The as the standard deviations were calculated from the results of the cycling tests and repetitions. For differenceMEPA, between indicating onset its suitability melting temperatureas PCM for LHS and applications. onset crystallization As can be seen temperature, from Figure unless 7, stated determinationsynthesized MEPA of phase presents change phase temperatures, change temperatures onset temperatures in accordance are reported, with the andcommercial supercooling sample is otherwise. A high ∆H of 204 J/g ± 2 J/g was observed for commercial MEPA, indicating its suitability calculatedwith melting as temperature the difference of 26.3 between °C ± 0.0 °C onset and crystallization melting temperature temperature and of onset24.2 °C crystallization± 0.2 °C, same as PCMtemperature,degree for LHS of supercooling applications.unless stated and otherwise. As stability can be A over seen high threefrom ΔH heating/coolingof Figure 204 J/g7, ± synthesized 2 J/g cycles. was observed The MEPA enthalpy for presents commercial of fusion phase change ◦ ◦ temperaturesMEPA,observed indicating infrom accordance integration its suitability with of the the ascorresponding commercial PCM for LHS peak sample applications. amounted with melting to As 201 can ± temperature be1 J/g, seen indicating from of Figure 26.3a high 7,C ± 0.0 C and crystallizationsynthesizeddegree of purity MEPA temperature in thepresents MEPA phase ofproduced, 24.2 change◦C thus± temperatures0.2 confirming◦C, same inonce degree accordance again of the supercooling with results the previously commercial and stabilityreported. sample over three with melting temperature of 26.3 °C ± 0.0 °C and crystallization temperature of 24.2 °C ± 0.2 °C, same heating/cooling cycles. The enthalpy of fusion observed from integration of the corresponding peak degree of supercooling and stability over three heating/cooling cycles. The enthalpy of fusion ± amountedobserved to 201from integration1 J/g, indicating of the corresponding a high degree peak of amounted purity in to the 201 MEPA ± 1 J/g, produced, indicating thusa high confirming once againdegree theof purity results in the previously MEPA produced, reported. thus confirming once again the results previously reported.
Figure 7. DSC analysis of MEPA produced in the laboratory for comparison with commercial sample. The results found of Tm of 26.3 °C ± 0.0 °C and Tc of 24.2 °C ± 0.2 °C and ΔH of 201 ± 1 J/g are in
Figure 7. DSC analysis of MEPA produced in the laboratory for comparison with commercial sample. Figure 7. DSC analysis of MEPA produced in the laboratory for comparison with commercial sample. The results found of Tm of 26.3◦ °C ± 0.0 °C◦ and Tc of 24.2 °C ±◦ 0.2 °C and◦ ΔH of 201 ± 1 J/g are in The results found of Tm of 26.3 C ± 0.0 C and Tc of 24.2 C ± 0.2 C and ∆H of 201 ± 1 J/g are in accordance with the values expected. This indicates a high degree of purity for the sample produced, and its suitability as PCM. The STARe Software version (14.00) is reported on the bottom right. Appl. Sci. 2018, 8, x 10 of 18
accordance with the values expected. This indicates a high degree of purity for the sample produced, Appl.and its Sci. suitability2018, 8, 1069 as PCM. The STARe Software version (14.00) is reported on the bottom right. 10 of 18
ConcerningConcerning TGA TGA analysis, analysis, no residuals no residuals of methanol of methanol were were observed observed in in the the sample sample (boiling (boiling point point 64.7 °C),64.7 and◦C), degradation and degradation of the of theproduced produced sample sample was was complete complete by by 240 240 °C◦C in in accordance accordance with with what was registeredwas registered for forcommercial commercial MEPA. MEPA. The The starting starting temperature of of degradation degradation varied varied between between 100 and100 and 110110 °C◦C for laboratory samples, samples, while while it it was was around around 150 150◦C °C for for commercial commercial MEPA MEPA (Figure (Figure8). The 8). mass The massloss loss before before 150 150◦ C°C was was less less than than 5% 5% for for all all laboratory laboratory MEPA MEPA samples, samples, and and the remainingthe remaining 95% 95% was was lostlost in the rangerange betweenbetween 150–240 150–240◦ C°C as as for for commercial commercial MEPA. MEPA. Therefore, Therefore, it could it could be concluded be concluded that that no no significant significant differences differences in the in thermal the thermal stability stability between between the self-produced the self-produced sample and sample commercial and commercialone existed. one existed.
FigureFigure 8. TGA 8. TGA graph graph of mass of mass lossloss for forMEPA MEPA produced produced in inthe the laboratory. laboratory. No No significant significant differences differences ◦ with withcommercial commercial MEPA MEPA were were observed, observed, as both as both samples samples registered registered a amass mass loss loss ≤≤ 5%5% before before 150 150 °C,C, ◦ ◦ and theand remaining the remaining mass mass was was lost lostin between in between the the range range 150 150 °C–240C–240 °C Cas as expected. expected.
3.2. Synthesis3.2. Synthesis of Methyl of Methyl and andDecyl Decyl Esters Esters FollowingFollowing the optimization the optimization of the of synthesis the synthesis procedure, procedure, esters esters derived derived from fatty from carboxylic fatty carboxylic acids coupledacids with coupled MeOH with and MeOH fatty and alcohol fatty 1-decanol alcohol 1-decanol were produced, were produced, as reported as reported previously. previously. After ≥ confirmingAfter confirming an acceptable an acceptabledegree of purity degree (≥89%), of purity the (thermal89%), theproperties thermal of properties the different of the esters different were esters were determined and subsequently analyzed and compared, in order to identify possible trends determined and subsequently analyzed and compared, in order to identify possible trends between between chemical structure and thermal behavior, and to gain a broader overview of the melting chemical structure and thermal behavior, and to gain a broader overview of the melting points of points of different esters. different esters. Concerning ATR-IR characterization, the appearance of the ester characteristic peak at 1735 cm−1, Concerning ATR-IR characterization, the appearance of the ester characteristic peak at 1735 cm−1, the progressive disappearance of the broad alcohol peak at 3100–3600 cm−1, and of the acid at the progressive disappearance of the broad alcohol peak at 3100–3600 cm−1, and of the acid at 1705 1705 cm−1 over time proved the efficiency of the reaction and the formation of the desired product. −1 cm Forover all time 1-decanol provedesters, the efficiency however, of athe ratio reaction of about and 2:1 the between formation the of ester the desired peak and product. the acid For peak, all 1-decanolrespectively, esters, washowever, observed a ratio after of dryingabout 2:1 with between Rotavapor. the This,ester togetherpeak and with the theacid presence peak, respectively, of a peak in was observedthe alcohol after region, drying proved with that Rotavapor. not all reagents This, had together been consumed with the presence during the of reaction a peak time. in the Therefore, alcohol region,crystallization proved that in notMeOH all was reagents performed had beenuntil there consumed was no during visible theresidual reaction alcohol time. peak Therefore, and acid crystallizationpeak in the in final MeOH spectra. was Figure performed9 shows until the differencethere was betweenno visible unpurified residual decylalcohol palmitate peak and (DEPA) acid peakand in the DEPA final crystallized spectra. Figure twice 9 in shows MeOH the with difference a ratio of between 1:10 for DEPA:MeOH.unpurified decyl Generally, palmitate as reported(DEPA) and DEPApreviously, crystallized one or two twice crystallizations in MeOH with were a sufficientratio of 1:10 to obtain for DEPA:MeOH. degrees of purity Generally, over 89%. as reported previously, one or two crystallizations were sufficient to obtain degrees of purity over 89%. Appl. Sci. 2018, 8, 1069 11 of 18 Appl. Sci. 2018, 8, x 11 of 18
FigureFigure 9. 9. ATR-IRATR-IR spectra spectra of DEPAof DEPA without without purification purification and and with with two two crystallization crystallization in in MeOH MeOH purifications.purifications. A strong A strong increase increase in inthe the ester ester peak peak at at1735 1735 cm cm−1 for−1 esterfor ester purified purified twice twice can can be observed, be observed, togethertogether with with the the disappearance disappearance of ofthe the residual residual Alcohol Alcohol broad broad peak peak at at3100–3600 3100–3600 cm cm−1,− which1, which verifies verifies thethe degree degree of purity of purity of the of the ester ester produced produced and and its itssuitability suitability for for further further studies. studies.
A clear gain in the ester band was achieved, confirming the efficacy of the purification process. A clear gain in the ester band was achieved, confirming the efficacy of the purification process. However, besides purity control and synthesis control, the IR could not confirm the true identity of However, besides purity control and synthesis control, the IR could not confirm the true identity of the the esters created since no clear differences were visible in terms of the ester peak and alkane chain esters created since no clear differences were visible in terms of the ester peak and alkane chain peaks peaks at 2920 and 2850 cm−1. Therefore, the structure of the produced esters had to be recognized at 2920 and 2850 cm−1. Therefore, the structure of the produced esters had to be recognized through through NMR and GC-MS analysis. NMR and GC-MS analysis. The mass spectra recorded for methyl esters showed the same peaks reported for MEPA, The mass spectra recorded for methyl esters showed the same peaks reported for MEPA, confirming the cleavage of a methyl group with peak at 239 m/z. Esters derived from different fatty confirming the cleavage of a methyl group with peak at 239 m/z. Esters derived from different fatty acids could be recognized by the longer or shorter fragmentation of the alkane chain by progressive acids could be recognized by the longer or shorter fragmentation of the alkane chain by progressive loss of –CH2 (14 m/z) and by the molecular ion. On the other hand, spectra for decyl esters showed a loss of –CH2 (14 m/z) and by the molecular ion. On the other hand, spectra for decyl esters showed a different fragmentation pattern, especially regarding the cleavage of the alcohol chain. For instance, different fragmentation pattern, especially regarding the cleavage of the alcohol chain. For instance, for DEPA (molecular peak at 396 m/z), a strong signal was recorded due to the loss of the decyl alkane for DEPA (molecular peak at 396 m/z), a strong signal was recorded due to the loss of the decyl chain (loss of 141 m/z, peak at 255 m/z), thus forming a carboxylate group. The decyl aliphatic chain alkane chain (loss of 141 m/z, peak at 255 m/z), thus forming a carboxylate group. The decyl aliphatic is then thought to form a decene through resonance to give the observed mass loss of 139 m/z at chain is then thought to form a decene through resonance to give the observed mass loss of 139 m/z corresponding peak 257 m/z. The alcohol cleavage with loss of 1-decanol (loss of 157 m/z) was at corresponding peak 257 m/z. The alcohol cleavage with loss of 1-decanol (loss of 157 m/z) was observed at 239 m/z, which confirms that the ester analysed was indeed a decyl ester. Additionally, observed at 239 m/z, which confirms that the ester analysed was indeed a decyl ester. Additionally, the usual progressive fragmentation of the alkyl chain from the acid side is observed through steps the usual progressive fragmentation of the alkyl chain from the acid side is observed through steps of of 14 m/z loss each. 14 m/z loss each. Similarly, the structures of the different esters could be easily identified through 1H NMR Similarly, the structures of the different esters could be easily identified through 1H NMR analysis, analysis, as well as detection and quantification of impurities from residual alcohol or acid (Figure as well as detection and quantification of impurities from residual alcohol or acid (Figure 10). All the 10). All the 1H NMR spectra of methyl esters showed a high degree of purity and no residual alcohol, 1H NMR spectra of methyl esters showed a high degree of purity and no residual alcohol, therefore therefore their purity was estimated at ≥98%. The same main peaks as the ones observed in the spectra their purity was estimated at ≥98%. The same main peaks as the ones observed in the spectra of of replicated MEPA were observed, namely the singlet at 3.66 ppm from the deshielded methyl group replicated MEPA were observed, namely the singlet at 3.66 ppm from the deshielded methyl group –CH3 directly attached to the carbonyl group, the triplet at 2.28–2.32 ppm from the –CH2 in α-position –CH3 directly attached to the carbonyl group, the triplet at 2.28–2.32 ppm from the –CH2 in α-position to the carbonyl, the multiplet from the –CH2 in β-position at 1.58–1.65 ppm, the peak from the alkane to the carbonyl, the multiplet from the –CH2 in β-position at 1.58–1.65 ppm, the peak from the alkane chain around 1.26 ppm and the triplet at 0.86–0.90 ppm from the shielded –CH3 group at the end of chain around 1.26 ppm and the triplet at 0.86–0.90 ppm from the shielded –CH3 group at the end the aliphatic chain. On the other hand, some variances in the spectra for decyl esters allowed of the aliphatic chain. On the other hand, some variances in the spectra for decyl esters allowed differentiation between the diverse groups of esters. Here, a triplet with integral value of 2 instead of differentiation between the diverse groups of esters. Here, a triplet with integral value of 2 instead of a a singlet appeared at 4.05–4.07 ppm (a) from the –CH2 in α-position to the oxygen of the ester group. singlet appeared at 4.05–4.07 ppm (a) from the –CH2 in α-position to the oxygen of the ester group. This is due to the fact that in 1-decanol, unlike in methanol, the aliphatic chain continues, therefore This is due to the fact that in 1-decanol, unlike in methanol, the aliphatic chain continues, therefore the the –CH2 in α-position feels the influence of the neighboring –CH2. The triplet at 2.28–2.32 ppm (c) –CH2 in α-position feels the influence of the neighboring –CH2. The triplet at 2.28–2.32 ppm (c) from from the –CH2 in α-position to the carbonyl group remained the same, while the multiplet at 1.58– the –CH2 in α-position to the carbonyl group remained the same, while the multiplet at 1.58–1.65 ppm 1.65 ppm (d) from the –CH2 groups in β-position had an integral equal to 4 hydrogens instead of 2 (d) from the –CH2 groups in β-position had an integral equal to 4 hydrogens instead of 2 due to the due to the presence of the additional –CH2 in β-position to the oxygen from the alcohol side. Similarly, presence of the additional –CH2 in β-position to the oxygen from the alcohol side. Similarly, the peak the peak from the alkane chain at 1.26 ppm (e) had an integral value of about 38 hydrogens, and the triplet at 0.86–0.90 ppm (f) corresponded to the 6 hydrogens from the shielded –CH3 at the end of the Appl. Sci. 2018, 8, 1069 12 of 18
Appl.from Sci. the2018 alkane, 8, x chain at 1.26 ppm (e) had an integral value of about 38 hydrogens, and the 12 triplet of 18 at 0.86–0.90 ppm (f) corresponded to the 6 hydrogens from the shielded –CH3 at the end of the aliphatic aliphaticchains fromchains the from acid the and acid the alcoholand the sides. alcohol Small sides. alcohol Small impurities, alcohol impurities, when present, when were present, visible were in the visibleform in of the a triplet form of at a 3.62–3.66 triplet at ppm 3.62–3.66 (b) from ppm the (b) –CH from2 thein α –CH-position2 in α-position to the hydroxy to the hydroxy group of group the free of alcoholthe free molecules.alcohol molecules.
1 FigureFigure 10. 10. H1 NMRH NMR spectra spectra of of89% 89% pure pure DEPA. DEPA. Some Some differences differences arise arise compared compared to tothe the spectra spectra of of methyl esters, and in particular: the triplet instead of singlet at 4.05–4.07 ppm (a) from –CH2 in α to methyl esters, and in particular: the triplet instead of singlet at 4.05–4.07 ppm (a) from –CH2 in α the oxygen, the integral values of the peaks at 2.27–2.29 ppm (c) from –CH2 in α to the carbonyl, at to the oxygen, the integral values of the peaks at 2.27–2.29 ppm (c) from –CH2 in α to the carbonyl, 1.58–1.65 ppm (d) from –CH2 in β to the carbonyl, at 1.26 ppm (e) from the aliphatic chain of both the at 1.58–1.65 ppm (d) from –CH2 in β to the carbonyl, at 1.26 ppm (e) from the aliphatic chain of both 3 alcoholthe alcohol and the and acid, the acid,and at and 0.86–0.90 at 0.86–0.90 ppm ppm (f) from (f) from shielded shielded –CH –CH groups3 groups at the at theend end of the of the aliphatic aliphatic chainschains from from both both the the alcohol alcohol and and acid acid side. side. Impurities Impurities from from residual residual alcohol alcohol can can be beobserved observed and and
quantifiedquantified through through the the integral integral value value of of the the triplet triplet at at 3.62–3.66 ppm (b)(b) assignedassigned toto the the –CH –CH2 2in inα αto to the thehydroxyl hydroxyl groups groups in in the the free free unreacted unreacted alcohol alcohol molecules. molecules. Similarly Similarly as foras for previous previous pictures, pictures, the greenthe greenlines lines indicate indicate the integralthe integral areas, areas, while while the normalizedthe normalized values values are indicatedare indicated in blue. in blue.
The ratio between its integral value and the one from the ester signal at 4.05–4.07 ppm allowed The ratio between its integral value and the one from the ester signal at 4.05–4.07 ppm allowed to to quantify the degree of purity of the sample. As mentioned previously, in the absence of any quantify the degree of purity of the sample. As mentioned previously, in the absence of any additional additional peaks from unreacted reagents, the purity was estimated to be ≥98%, like in the case of peaks from unreacted reagents, the purity was estimated to be ≥98%, like in the case of methyl esters. methyl esters. Impurities still present in the decyl esters could also be observed through DSC analysis. In the Impurities still present in the decyl esters could also be observed through◦ DSC analysis. In the◦ presence of residual alcohol, a second broad peak appeared between 2–10 C (Tm of 1-Decanol: 6.4 C), presence of residual alcohol, a second broad peak appeared between 2–10 °C (Tm of 1-Decanol: 6.4 and lower ∆H of about 150–160 J/g were registered. Upon further crystallizations and purifications °C), and lower ΔH of about 150–160 J/g were registered. Upon further crystallizations and though, such peak decreased until it was barely visible over the baseline noise of the instrument, purifications though, such peak decreased until it was barely visible over the baseline noise of the and ∆H for the ester peak increased again to values over 190 J/g. instrument, and ΔH for the ester peak increased again to values over 190 J/g. In general, the results obtained concerning thermal properties of synthesized esters are In general, the results obtained concerning thermal properties of synthesized esters are summarized in Table1. As mentioned beforehand, the esters names have been abbreviated as follows: summarized in Table 1. As mentioned beforehand, the esters names have been abbreviated as methyl myristate (MEMY, C1–14), decy myristate (DEMY, C10-C14), methyl palmitate (MEPA, C1-C16), follows: methyl myristate (MEMY, C1–14), decy myristate (DEMY, C10-C14), methyl palmitate decyl palmitate (DEPA, C10-C16), methyl stearate (MESA, C1-C18), decyl stearate (DESA, C10-C18), (MEPA, C1-C16), decyl palmitate (DEPA, C10-C16), methyl stearate (MESA, C1-C18), decyl stearate methyl behenate (MEBE, C1-C22), decyl behenate (DEBE, C10-C22). (DESA, C10-C18), methyl behenate (MEBE, C1-C22), decyl behenate (DEBE, C10-C22). Appl. Sci. 2018, 8, 1069 13 of 18
Table 1. Summary of thermal properties of the ester produced. The colors indicate the temperature gradient from the lowest values (blue) to the highest (red). A trend between degradation temperatures and molecular structure is observed, with a progressive growth of the thermal stability together with carbon chain length. This is supposedly due to longer chains’ ability to create more compact crystal packing structures, thus stronger intermolecular forces. No clear correlation between ∆H (J/g) and the structure can be deduced, however all ∆H (J/g) values registered were over 190 J/g, confirming the fatty acid esters’ suitability as PCM. An increase in molar ∆H (KJ/mol) is observed for increasing molecular weights.
Carbon MW Supercooling ∆H T T Structure Purity T (onset, ◦C) T (onset, ◦C) ∆H (J/g) degradation degradation Number (g/mol) c m (◦C) (KJ/mol) (Start, ◦C) (End, ◦C) MEMY (C1-C14) C15H30O2 15 242.4 98% 13.5 ± 1.2 14.7 ± 1.9 1.2 190 ± 2 46.1 87 ± 12 233 ± 12 MEPA (C1-C16) C17H34O2 17 270.5 98% 24.2 ± 0.2 26.3 ± 0.0 2.1 201 ± 1 54.4 119 ± 10 272 ± 4 MESA (C1-C18) C19H38O2 19 298.5 98% 31.9 ± 1.1 35.6 ± 1.0 3.7 204 ± 1 60.9 117 ± 7 287 ± 6 MEBE (C1-C22) C23H46O2 23 354.6 98% 41.5 ± 1.0 47.9 ± 0.7 6.4 200 ± 5 70.9 155 ± 7 305 ±20 DEMY (C10-C14) C24H48O2 24 368.7 98% 22.9 ± 0.2 25.2 ± 0.4 2.3 196 ± 3 72.3 130 ± 5 318 ± 11 DEPA (C10-C16) C26H52O2 26 396.7 89% 28.9 ± 1.2 29.0 ± 1.9 0.1 193 ± 6 76.6 145 ± 7 310 ± 14 DESA (C10-C18) C28H56O2 28 424.8 98% 33.8 ± 1.3 36.2 ± 0.8 2.4 192 ± 6 81.6 205 ± 20 350 ± 14 DEBE (C10-C22) C32H64O2 32 480.9 95% 44.7 ± 0.6 44.8 ± 0.7 0.1 193 ± 9 92.8 200 ± 20 375 ± 21 Appl.Appl. Sci. Sci. 20182018, 8, ,x8 , 1069 14 of14 18 of 18
A trend can be observed in the degradation temperatures, with the longer chained esters being the mostA stable trend canand bestarting observed to degrade in the degradation at much higher temperatures, temperatures with compared the longer to chainedshorter ones. esters This being canthe be mostexplained stable using and literature starting to reports, degrade with at longer much higherchain esters temperatures being able compared to create tighter to shorter crystal ones. structuresThis can and be explainedtherefore stronger using literature intermolecular reports, bonding, with longer thus requiring chain esters higher being temperature able to create to break tighter thecrystal bonds structures [15–18]. and A similar therefore behavior stronger would intermolecular be expected bonding, regarding thus requiring melting higher temperatures temperature Tm. to However,break the as bonds can be [ 15clearly–18]. observed, A similar behaviorMEBE was would the ester be expected showing regarding higher melting melting temperature temperatures and T m. notHowever, DEBE, as aswould can be have clearly been observed, expected. MEBE was the ester showing higher melting temperature and notUpon DEBE, reversing as would the have order been of expected. the esters shown above, a trend is revealed between molecular structureUpon and reversingmelting points the order (Table of 2). the The esters melting shown points above, are seen a trend increasing is revealed based betweenon both the molecular fatty acidstructure carbon length and melting and the points alcohol (Table length,2). The but meltingthose from points decyl are esters seen follow increasing a parallel based trend on both to methyl the fatty ester,acid and carbon do not length show and the the same alcohol behavior length, (Figure but those 11). from What decyl reported esters followabove is a parallelfound to trend be true to methyl for bothester, Tm and T doc for not all show samples the. sameThis could behavior be due (Figure to the 11 odd-even). What reported numbered above effect is explained found to beby trueNoël for et bothal. [17] Tm andand Yang Tc for et all al. samples [18], according. This could to which be due different to the odd-even but parallel numbered trends effectcan be explained observed by betweenNoël et odd-numbered al. [17] and Yang and et even-numbered al. [18], according carbon to which chain different series. In but addition, parallel the trends length can of be the observed fatty acidbetween carbon odd-numberedchain seems to have and even-numbered a higher impact carbonon the resulting chain series. phase In addition,transition thetemperature length of theof the fatty ester,acid compared carbon chain to the seems alcohol. to have a higher impact on the resulting phase transition temperature of the ester, compared to the alcohol. Table 2. Rearrangement of the previous table to show the dependency of phase transition temperature on Table fatty 2. esterRearrangement type and carbon of the chainprevious length. table As to show for Table the dependency 1, the colors of phase indicate transition the temperature temperature gradienton fatty from ester the type lowest and carbontemperature chain values length. (blue) As for to Table the 1highest, the colors (red). indicate the temperature gradient from the lowest temperature values (blue) to the highest (red). Carbon Number Tc (Onset, °C) Tm (Onset, °C) ◦ ◦ MEMY (C1-C14) Carbon Number 15 Tc13.5(Onset, ± 1.2 C)14.7 Tm ± (Onset,1.9 C) MEMYDEMY (C1-C14) (C10-C14) 15 24 22.913.5 ±± 0.21.2 25.2 ±14.7 0.4 ± 1.9 DEMYMEPA (C10-C14) (C1-C16) 24 17 24.222.9 ±± 0.20.2 26.3 ±25.2 0.0 ± 0.4 MEPADEPA (C1-C16) (C10-C16) 17 26 28.924.2 ±± 1.20.2 29.0 ±26.3 1.9 ± 0.0 DEPA (C10-C16) 26 28.9 ± 1.2 29.0 ± 1.9 MESAMESA (C1-C18) (C1-C18) 19 19 31.931.9 ±± 1.11.1 35.6 ±35.6 1.0 ± 1.0 DESADESA (C10-C18) (C10-C18) 28 28 33.833.8 ±± 1.31.3 36.2 ±36.2 0.8 ± 0.8 MEBEMEBE (C1-C22) (C1-C22) 23 23 41.541.5 ±± 1.01.0 47.9 ±47.9 0.7 ± 0.7 DEBEDEBE (C10-C22) (C10-C22) 32 32 44.744.7 ±± 0.60.6 44.8 ±44.8 0.7 ± 0.7
Figure 11. Methyl esters and decyl esters both show a correlation between Tm and Tc and carbon chain Figure 11. Methyl esters and decyl esters both show a correlation between Tm and Tc and carbon chain length,length, but but following following different different behaviors; behaviors; this this is is believed believed to to be be due due to the to odd-even the odd-even effect effect [17–20 [17–,22,23]. 20,22,23].The acid The carbon acid carbon chain length chain seemslength toseems have to higher have impacthigher impact than the than alcohol. the alcohol.
The molar enthalpies of fusion (ΔH in KJ/moL) could be seen increasing with increasing chain The molar enthalpies of fusion (∆H in KJ/moL) could be seen increasing with increasing chain length and molecular weight; however, no clear trends could be observed for even and odd- length and molecular weight; however, no clear trends could be observed for even and odd-numbered numbered esters as for Tm. Nevertheless, all values measured were above 190 J/g with uncertainties around 1.5%, confirming the suitability of fatty esters as PCM. All of the samples proved to be stable Appl. Sci. 2018, 8, 1069 15 of 18
esters as for Tm. Nevertheless, all values measured were above 190 J/g with uncertainties around 1.5%, confirming the suitability of fatty esters as PCM. All of the samples proved to be stable to short cycles (three times), with variations lower than 0.5 ◦C. Additionally, supercooling lower than 7 ◦C were observed in all cases at a rate of 10 ◦C/min.
4. Conclusions In conclusion, fatty esters from methanol and 1-decanol and fatty acids of different carbon chain length (myristic acid, palmitic acid, stearic acid, behenic acid) to be used as possible PCM were successfully synthesized through Fischer esterification with purities over 89%. While methyl esters of myristic, palmitic and stearic acid have been synthesized before and their thermal properties partially investigated, to the best of the author’s knowledge, a clear lack in experimental thermal data exists in regards of methyl behenate and all decyl esters produced. The synthetic procedure was validated through comparison of properties of the self-produced methyl palmitate with the commercial homologue purchased. While simple drying with rotary evaporation was sufficient to purify the final methyl esters, all decyl esters had to be further purified through one or more consequent crystallizations in methanol. ATR-IR proved to be a valid technique for identification of residual unreacted acid or alcohol in a qualitative manner; however, it did not allow for identification of different esters. On the other hand, NMR and GC-MS analysis allowed for correct identification of structures and compound identity. Additionally, eventual residual impurities could be successfully detected and quantified through both methods. Identification of residual impurities was possible also through DSC analysis, where side peaks could be observed in case of impurities, together with lower ∆H. The phase change temperatures of the esters ranged between 14 ◦C and 45 ◦C circa. As reported by Du et al. [25], PCM in the range between 5 ◦C and 40 ◦C appear to be interesting for low-medium temperature applications, such as free cooling (19–24 ◦C), evaporative and radiative cooling systems (around 18 ◦C), solar absorption chillers (29 ◦C), and passive heating and cooling of buildings (19–29 ◦C). Preliminary works conducted on such setups with different PCM (salt hydrates, paraffins) showed promising results, with high cost saving and decrease in energy consumption (25–30%). The esters produced proved to be interesting PCM candidates thanks to ∆H over 190 J/g for 10 ◦C/min heating rates. Additionally, the DSC indicates rather moderate supercooling rates for all samples. A trend could be observed correlating both degradation temperatures and phase transition temperatures to the chemical structure of the molecules. In the first case, a higher thermal stability was observed for longer carbon chains, confirming the tendency of longer molecules to form more tightly packed structures and stronger intermolecular bonds. On 2424the other hand, an increase in the melting temperatures with increasing carbon chain length was observed for both methyl esters and decyl esters, but with parallel trends and a higher influence from the fatty acid carbon length compared to the alcohol. This is attributed to odd-even numbered carbon chain length effect described in previous works [17,18]. Given the lack of experimental thermal data on esters, future research on different ester classes (e.g. diesters, lactones, aromatic, benzylic esters) is planned, in order to collect further information about the thermal behavior and possible trends, and to individuate the best suitable candidates to be used as PCM for different scopes and applications.
Author Contributions: All authors listed conceived and designed the project and contributed to experiments’ development and in reviewing the paper; A.S., J.W, and L.J.F acquired the funding; R.R. and N.L. performed the experiments; R.R., N.L., O.F. and A.S. analyzed the data; O.F. contributed reagents, materials and analysis tools; R.R. wrote the paper. Funding: This research was funded by Swiss National Science Foundation (SNSF, project number PZENP2_173636). Acknowledgments: This work was developed within the framework of the project DENSE “Direct-contact ENergy StoragE” funded by the Swiss National Science Foundation (SNSF, project number PZENP2_173636) with the support of the Swiss Competence Center for Energy Research Storage of Heat and Electricity (SCCER). The authors wish to thank the University of Zürich (UZH, Zürich, Switzerland) and Fachhochschule Nordwestschweiz (FHNW, Basel, Switzerland) for the GC-MS and NMR measurements conducted. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2018, 8, 1069 16 of 18
Appl. Sci. 2018, 8, x 16 of 18 List of Abbreviations List of Abbreviations ATR-IR Attenuated Total Reflectance InfraRed Spectroscopy BEATR-IR Attenuated Behenic Total Acid Reflectance InfraRed Spectroscopy DEBEBE Behenic Decyl Acid Behenate (C10-C22) DEMYDEBE Decyl Decyl Behenate Myristate (C10-C22) (C10-C14) DEPADEMY Decyl Decyl Myristate Palmitate (C10-C14) (C10-C16) DESADEPA Decyl Decyl Palmitate Stearate (C10-C16) (C10-C18) DSCDESA Decyl Differential Stearate (C10-C18) Scanning Calorimetry EtDSC2O Differential Diethyl EtherScanning Calorimetry EtOAcEt2O Diethyl Ethyl Ether Acetate GC-MSEtOAc Ethyl Gas-Chromatography Acetate coupled with Mass Spectrometry HGC-MS2SO4 Gas-ChromatographySulfuric Acid coupled with Mass Spectrometry IEAH2SO4 Sulfuric International Acid Energy Agency LHSIEA International Latent Heat Energy Storage Agency MEBELHS Latent Methyl Heat Behenate Storage (C1-C22) MEMYMEBE Methyl Methyl Behenate Myristate (C1-C22) (C1-C14) MeOHMEMY Methyl Methanol Myristate (C1-C14) MEPAMeOH Methanol Methyl Palmitate (C1-C16) MESAMEPA Methyl Methyl Palmitate Stearate (C1-C16) (C1-C18) MYMESA Methyl Myristic Stearate Acid (C1-C18) NaMY2SO 4 MyristicSodium Acid Sulfate NMRNa2SO4 Sodium Nuclear Sulfate Magnetic Resonance PANMR Nuclear Palmitic Magnetic Acid Resonance PCMPA Palmitic Phase Acid Change Material SAPCM Phase Stearic Change Acid Material SHSSA Stearic Sensible Acid Heat Storage SHS Sensible Heat Storage Tc Crystallization Temperature TCSTc Crystallization Thermochemical Temperature Storage Material TGATCS Thermochemical Thermogravimetric Storage Analysis Material TGA Thermogravimetric Analysis Tm Melting Temperature ∆HTm Melting Enthalpy Temperature of fusion ΔH Enthalpy of fusion Appendix A Appendix A
FigureFigure A1.A1. StructuresStructures and trivialtrivial namesnames ofof estersesters produced,produced, withwith IUPACIUPAC nomenclaturenomenclature andand carboncarbon lengthlength shorteningshortening in between between brackets. brackets. The The carbon carbon length length shortening shortening indicates indicates first first the the number number of ofcarbons carbons in inthe the alcohol, alcohol, and and subsequently subsequently the thenumber number of carbons of carbons in the in thecarboxylic carboxylic acid acidchain. chain. For Forexample, example, methyl methyl myristate myristate derives derives from methanol from methanol (C1) and (C1) myristic and myristic acid (C14), acid therefore (C14), thereforeis indicated is indicatedas C1-C14. as C1-C14.
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