Water was also observed in reaction medium as well as diethyl ether when ethanol was used. Figure 1 Products of the glycerol and ethanol or t-butanol etherification reaction in the presence of acid catalyst. Identification of glycerol and ethanol etherification reaction products In the etherification of glycerol with ethanol, diethyl ether EE , ethanol ET and glycerol Gly were identified by co-injection of commercial reagents. Figure 2 illustrates a typical chromatogram of the reaction medium of glycerol and ethanol etherification.
Figure 2 Chromatogram of the reaction medium of the etherification reaction of glycerol with ethanol. Peak identification: diethyl ether EE ; ethanol ET ; 1,2,3-triethoxy-propane 3 ; 1,3-diethoxy-propanol 2a ; 2,3-diethoxy-propanol 2b ; 3-ethoxy-propane-1,2-diol 1a ; 2-ethoxy-propane-1,3-diol 1b ; glycerol Gly. The chromatographic separation is related to physicochemical properties such as polarity of the species.
Thus, the selective retention of sample components on stationary phase results in differentiated migrations of the ethers, due to polarity difference caused by the number of hydroxyl groups in their structures. To confirm the structures of these products, the comparison of the mass spectra of the etherification products with those found at mass spectroscopy database NIST Mass Spectral Search Program Version 2.
However, an additional difficulty was faced since not all mass spectra of the ethers produced are in the libraries. The problem worsens due to lack of information in the literature regarding the identification of the products formed in this etherification reaction. Just one chromatogram was found for the products of glycerol and ethanol etherification. According to Yuan et al.
The identification undergone in the present study differs from that found by Yuan et al. As already shown in Figure 2 , eight peaks were observed. The products 1,2,3-triethoxy-propane 3 , 3-ethoxy-propane-1,2-diol 1a and 1,3-diethoxy-propanol 2a were identified using mass spectra database. The other products were determined using their characteristic mass spectra and polarity, that is, monosubstituted ethers, which have two hydroxyl groups will have a higher retention time.
According to product polarity, 2-ethoxy-propane-1,3-diol 1b and 2,3-diethoxy-propanol 2b were identified. These identifications were confirmed by the analysis of the mass spectra of these compounds, as will be discussed in Figures 3 and 4. Figure 3 Proposed fragmentation pathway for 1a and 1b. Figure 4 Proposed fragmentation pathway for diethers 2a and 2b and triether 3. The non-identified product shown by Yuan et al. The trisubstituted ether was observed in a lower retention time due to the higher molecular weight and the polarity difference of the products.
The fragmentation proposals for products 1a and 1b are shown in Figure 3. The proposed fragmentations for the products 2a, 2b and 3 are shown in Figure 4. The same pattern was also observed in mono- and disubstituted products, as already mentioned.
This loss pattern of two carbon atoms belonging to glycerol structure is a characteristic of homologous series. Identification of glycerol and t-butanol etherification reaction products Figure 5 illustrates a typical chromatogram profile for the etherification reaction of glycerol with t-butanol. The identification of the products was also carried out by GC-MS using also the polarity of the products. Six peaks were observed on the chromatogram. Gly and t-butanol TA were identified by co-injection of these compounds, so that, their retention times were determined.
Under the experimental conditions used, the formation of 1,2,3-tri-tert-butoxy-propane was not observed. Only four products were verified. As well as ethyl ethers, the retention time for t-butyl ethers followed the same pattern. The mass spectra of these compounds are presented in Supplementary Information section Figures S6-S9.
Figure 5 Chromatogram profile of the reaction medium of the etherification of glycerol with t-butanol. Peak identification: t-butanol TA ; 1,3-di-tert-butoxy-propanol 5a ; 2,3-di-tert-butoxy-propanol 5b ; 3-tert-butoxy-propane-1,2-diol 4a ; 2-tert-butoxy-propane-1,3-diol 4b ; glycerol Gly. The patterns observed in mass spectra do not allow the distinction of glyceryl t-butyl ethers. The mass spectra for products 4a and 5a major products are identical to those observed by those authors and can be unequivocally identified.
From these results and based on retention time and polarity difference of hydroxyl groups, the isomers 4b and 5b were identified. These identifications were confirmed by the obtained mass spectra. As reported by Cavalcante et al. A proposed fragmentation for the monosubstituted ethers is shown in Figure 6.
Figure 6 Proposed fragmentation pathway for monosubstituted ethers 4a and 4b. In the spectra of disubstituted ethers Supplementary Information section, Figures S8 and S9 , the presence of molecular ion was not observed. So, it was not possible to distinguish the disubstituted ethers using only the spectra analysis. Thus, the higher relative abundance of peak 2 in Figure 5 combined with the spectra analysis allowed to identify compound 5a as the major isomer.
Introduction Cyclic ethers are well known as important species formed in the low temperature gas phase oxidation of different types of fuels [ 1 ], [ 2 ] and [ 3 ]. However while the emissions of carbonyl compounds and alcohols have been quantified in the exhaust gases of internal combustion engines [ 4 ] and [ 5 ], those of cyclic ethers have been very little investigated [ 6 ]. This is mostly due to the fact that these compounds are not easily available for calibration and few data are available about their mass spectra in electron impact mass spectrometry.
In order to illustrate the potential importance of cyclic ethers in the exhaust gases of engines, simulations were run under the conditions observed in HCCI Homogeneous Charge Compression Ignition engines [ 7 ]. The detailed kinetic model used for the simulations has been successfully validated against experimental ignition delay time data obtained in shock tubes and in a rapid compression machine [ 9 ].
It is clear that, when assuming a perfectly homogeneous mixture as in the present simulation, organic compounds are completely consumed during auto-ignition corresponding to the strong rise of the temperature profile shown in Fig. However experimental measurements in engines have demonstrated the presence of organic species in the exhaust gases. The persistence of these species would be due to the existence of cold zones in the combustion chamber e.
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