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. 2022 Jan 11;7(3):3064–3072. doi: 10.1021/acsomega.1c06452

Combination of Theoretical and Experimental Insights into the Oxygenated Fuel Poly(oxymethylene) Dibutyl Ether from n-Butanol and Paraformaldehyde

Gaojun An †, Yangfeng Xia †,*, Zhenzhen Xue †, Hongyan Shang ‡, Sainan Cui †, Changbo Lu †,*
PMCID: PMC8793044  PMID: 35097301

Abstract

graphic file with name ao1c06452_0010.jpg

Oxygenated fuel has the function of self-supplying oxygen during the combustion process, which can greatly improve emission performance and reduce diesel fuel soot production. In this paper, a novel oxygenated fuel poly(oxymethylene) dibutyl ether (PODBEn) is designed and synthesized through experiments in combination with density functional theory (DFT) calculation. The experimental results show that PODBEn has the advantages of high cetane number (73.6), moderate density (868 kg/m3), and low condensation point (−72 °C). According to the DFT calculation results, the molecular (PODBEn) polarity index of different polymerization degrees is similar to the value of diesel and has good mutual solubility with diesel. Moreover, the mechanism of the entire path of synthesis is calculated at the M06-2X/6-311G(d,p) level of theory. The energetic profile reveals that the rate-determining step is the nucleophilic addition step with the highest barrier energy (TS1 = 21.59 kcal/mol). This work provides a feasible method to synthesize high-performance oxygenated fuel PODBEn using NKC-9 ion-exchange resins.

1. Introduction

It is well known that oxygenated fuels such as methanol, dimethyl ether (DME), and poly(oxymethylene) DME (PODEn) can significantly reduce nitrogen oxides (NOx) and particulate matter (PM) emissions when used as diesel fuel additives.1−6 These oxygenated compounds have attracted much attention due to their clean energy properties. However, the physicochemical features of methanol and DME are considerably different from that of ordinary diesel fuels (such as high vapor pressure, low viscosity, and so on), requiring changes to the diesel engine infrastructure.7−10 Compared with methanol or DME, PODEn has physicochemical properties similar to that of standard diesel. When used as a substitute for diesel fuel or a mixed fuel with diesel, it will not have a significant impact on the infrastructure of the diesel engine.11,12 Furthermore, various pieces of literature have demonstrated that PODEn may effectively suppress NOx and PM emissions due to its high oxygen content (45–50%).13−17 As a result, PODEn is viewed as a viable oxygenated fuel for solving diesel pollution.

PODEn (molecular formula: CH3–O–(CH2O)n–CH3, where n is the degree of polymerization of the methoxy group) is a kind of polyether-oxidized fuel whose structure consists of a repeating unit −(O–CH2)n– and two ending groups (methoxy (−O–CH3) and methyl (CH3).5,18−20 Among PODEn with different chain lengths (n = 1–8), medium-chain PODE3–5 are considered to be more preferable for diesel additives due to their high oxygen content, moderate boiling range, and vapor pressure compared with diesel.11,21−24 When the chain length of PODEn is short (n < 2), its flash point is too low to meet the safety standards, and when the chain length is long (n > 5), solid precipitation may form when mixed with conventional fuels and block the fuel system. Although PODE3–5 have aroused widespread interest as diesel fuel additives, the industrial application of PODEn oxyfuel has not been carried out so far. The critical problem limiting PODEn’s large-scale research and application is its high density (≥1.02 g/mL) and molecular polarity, which result in poor solubility with diesel at low temperatures.25−27 Hence, it is very necessary and meaningful to develop a novel polyether-oxidized fuel with density and molecular polarity similar to that of diesel.

The manufacture of oxygen fuel poly(oxymethylene) dibutyl ether (PODBEn, molecular formula: C4H9–O–(CH2O)n–C4H9, where n is the degree of polymerization of the methoxy group) from n-butanol and paraformaldehyde (PF) as reactants is described in this study. The main catalytic reaction parameters such as molar ratio, temperature, and reaction time have been evaluated in an autoclave reactor, with NKC-9 ion-exchange resin as a catalyst. In addition, the molecular polarity of PODBEn was examined using the molecular polarity index (MPI).28,29 Electrostatic potential (ESP) was used to examine the charge distribution on the molecule surface.30 The transition states (TSs) were calculated at the M06-2X/6-311G(d,p) level of theory. The weak interactions between different atoms were analyzed by noncovalent interactions (NCIs).31

2. Results and Discussion

2.1. Synthesis of PODBEn (n = 1–6)

2.1.1. Influence of the Molar Ratio of n-Butanol and PF

The molar ratio of the reactants is a key factor affecting the distribution and yield of different PODBEn polymers. The effect of n-butanol and PF molar ratios (1:0.6, 1:0.8, 1:1.0, 1:1.2, and 1:1.4) on the yield and product distribution of PODBE1–6 was investigated using NKC-9 resin as a catalyst. Figure 1a shows that the distribution of the PODBEn (n = 1–4) polymer changes significantly as the molar ratio of n-butanol and PF increases. With the increase of PF molar ratio, the yield of PODBE1 gradually decreased, while that of PODBEn (n = 2–4) gradually increased, indicating that the increase of PF molar ratio was beneficial to the formation of products with higher degree of polymerization. When the degree of polymerization “n” up to 5 or 6, the yield of PODBEn (n = 5∼6) was small (0.42%) and hardly changed with the PF molar ratio.

Figure 1.

Figure 1

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Influence of molar ratio (n-butanol/PF) on the distribution and yield of PODBE1–6; (a) distribution of PODBEn (n = 1–6) and (b) yield of PODBE1–6. Reaction conditions: NKC-9 (2 wt %); temperature, 100 °C; and time, 6 h.

According to Figure 1b, as the PF molar ratio increases from 1:0.6 to 1:1.0, the overall yield of PODBE1–6 steadily increases (from 68 to 74%). When the PF molar ratio is increased to 1:1.2, the overall yield of PODBE1–6 nearly doubles (73%), indicating that the PF reaction concentration is saturated at this level. Therefore, further experiments were carried out under the condition that the molar ratio of n-butanol and PF was 1:1.0.

2.1.2. Influence of Reaction Temperature

Figure 2 illustrates the investigation of the effect of reaction temperature (from 70 to 110 °C) on the distribution and yield of PODBE1–6. The yield of PODBE1 gradually falls as the reaction temperature increases, but the yield of PODBE2 rises and subsequently decreases, reaching a maximum value (17%) at 100 °C, and the yield of PODBE3–6 does not change considerably with the reaction temperature. With the temperature increase from 70 to 100 °C, the total yield of PODBE1–6 increases from 67 to 74%. When the temperature rises to 110 °C, the overall yield of PODBE1–6 falls to 72%. The internal structure of the reactor may be damaged as a result of the maximum reaction temperature of 110 °C of the NKC-9 resin collapsing, thereby reducing its catalytic efficiency. The results show that the appropriate temperature is 100 °C.

Figure 2.

Figure 2

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Influence of reaction temperature on the distribution and yield of PODBE1–6; degree of polymerization T is the total yield of PODBE1–6. Reaction conditions: NKC-9 (2 wt %); the molar ratio of n-butanol and PF was 1:1.0; time, 6 h.

2.1.3. Influence of Reaction Time

The effect of reaction time (2, 4, 6, 8, and 10 h) on the yield and degree of polymerization of the product PODBE1–6 was investigated (Figure 3). In terms of polymer distribution, as the reaction time grows, the yield of PODBE1 decreases while the yield of PODBE2–6 increases, showing that the degree of polymerization of the product increases with reaction time. As the time increases from 2 to 6 h, the yield of PODBE1–6 increases from 68 to 74%. When the reaction time exceeds 6 h, the yield of PODBE1–6 hardly changes, indicating that the reaction has reached equilibrium at this time. Hence, 6 h was chosen as the optimized reaction time.

Figure 3.

Figure 3

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Influence of reaction time on the distribution and yield of PODBE1–6; degree of polymerization T is the total yield of PODBE1–6. Reaction conditions: NKC-9 (2 wt %); molar ratio of n-butanol and PF was 1:1.0; temperature, 100 °C.

2.2. Physical and Chemical Performance Analysis

To further assess PODBEn’s effectiveness as a diesel-blending component, the physical and chemical properties of PODBE1–6, PODE3–5, and diesel were examined (Table 1). The cetane number is the most important metric for measuring the diesel combustion performance. Table 1 shows that the cetane number is listed in the following order: PODBE1–6 (73.6) > PODE3–5 (55.2) > diesel (50.9) (entry 1), indicating that PODBE1–6 as a diesel-blending component can increase the diesel combustion performance significantly. Compared with the net heating value of PODE3–5 (24.7 MJ/kg), PODBE1–6 has a significant improvement (31.6 MJ/kg), which is beneficial to the fuel dynamic performance (entry 2). The density is listed in the order PODE3–5 (1046 kg/m3) > PODBE1–6 (868 kg/m3) > diesel (807 kg/m3), showing that PODBE1–6 is more compatible with diesel and is better suited as a diesel-blending component (entry 3). The condensation point of PODBE1–6 (−72 °C) is much lower than that of diesel (−40 °C) and PODE3–5 (−52 °C). This phenomenon shows that PODBE1–6 has excellent low-temperature performance and can be applied in high-altitude and low-temperature areas (entry 6). In addition, PODBE1–6 and PODE3–5 have comparable boiling range and flash point values, matching the diesel requirement (entries 4 and 5).

Table 1. Physical and Chemical Properties of PODBE1–6, PODE3–5, and Diesel.

entry property PODBE1–6a PODE3–5b dieselc
1 cetane number 73.6 55.2 50.9
2 net heating value (MJ/kg) 31.6 24.7 43.1
3 density 20 °C (kg/m3) 868 1046 807
4 boiling range (°C)d 186–239 159–201 208–267
5 flash point (°C) 69 58 77
6 condensation point (°C) –72 –52 –40
7 kinematic viscosity (mm2/s)e 1.557 1.416 2.673
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a

PODBE1 (67.9%), PODBE2 (23.0%), PODBE3 (6.5%), PODBE4 (1.9%), PODBE5 (0.57%), and PODBE6 (0.18%).

b

POBE3 (63.7%), POBE4 (31.5%), and POBE5 (3.7%).

c

–35# car diesel.

d

Distillation section (10 vol % to 90 vol %).

e

20 °C.

Above all, PODBEn (n = 1–6) are more suitable than PODEn (n = 3–5) as diesel-blending components, and when mixed with diesel, they can significantly improve the combustion performance and low-temperature performance of diesel.

2.3. MPI Analysis

The ESP and MPI studies of the simulation oil (n-heptane + toluene + cyclohexane),32 PODE1–6, and PODBE1–6 are shown in Figure 4 to compare the polarity of diesel, PODE1–6, and PODBE1–6 intuitively. The ESP distribution of n-heptane and cyclohexane in the simulation diesel is consistent (the molecular surface ESP distribution maps are entirely white), resulting in low molecular polarity (Figure 4a,b; MPI = 2.74 and 2.33 kcal/mol). The surface of the toluene molecule presents a negative ESP at the position of the benzene ring (the ESP distribution graph is blue), while the hydrogen atom presents a positive ESP (the ESP distribution graph is red), resulting in greater molecular polarity (Figure 4c; MPI = 7.90 kcal/mol).

Figure 4.

Figure 4

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MPI analysis of diesel (model oil), PODE1–6, and PODBE1–6; (a) n-heptane; (b) toluene; (c) cyclohexane; (d) PODE1; (e) PODE2; (f) PODE3; (g) PODE4; (h) PODE5; (i) PODE6; (j) PODBE1; (k) PODBE2; (l) PODBE3; (m) PODBE4; (n) PODBE5; and (o) PODBE6.

ESP and MPI investigations of PODE1–6 (Figure 4d–i) demonstrate that the oxygen atoms in the PODE1–6 molecules display negative ESP, while the alkyl group exhibits positive ESP, resulting in an unequal distribution of the molecular surface. The MPI value shows an increasing trend as the degree of molecular polymerization increases, and the order follows: PODE1 (13.55 kcal/mol) < PODE2 (15.71 kcal/mol) < PODE3 (17.12 kcal/mol) < PODE4 (18.09 kcal/mol) < PODE5 (18.82 kcal/mol) < PODE6 (19.38 kcal/mol).

When compared to PODE1–6, the MPI values of PODBE1–6 are all lower, indicating that the value of PODBE1–6 is closer to the molecular polarity of simulated diesel (Figure 4j–o). According to the rule of the likes dissolve each other, the compatibility of PODBE1–6 with diesel is better than that of PODE1–6 and is more suitable as a diesel-blending component.

2.4. Reaction Mechanism Analysis

2.4.1. Entire Reaction Path

Figure 5 shows a potential chemical pathway for the synthesis of PODBEn from n-butanol and PF. The reaction can be observed to be separated into two processes: (1) acetal reaction process; (2) chain growth process (Figure 5a). In the acetal reaction process, the condensation reaction between FA and n-butanol occurred to form the hemiacetal intermediate BuOCH2OH. Due to its unstable nature, the intermediate hemiacetal readily interacts with n-butanol to generate PODBE1. During the chain growth process, PODBE1 was protonated by an acidic NKC-9 catalyst to generate the intermediate BuOCH2CHO+Bu, which then undergoes a nucleophilic substitution reaction with the monomer FA to yield BuOCH2OCH2+ carbocations. Subsequently, the carbocation and n-butanol undergo a nucleophilic addition reaction to produce BuOCH2OCH2+, which is further deprotonated to produce PODBE2. Hence, PODBE3, PODBE4, PODBE5, and PODBE6 are similarly generated one by one.

Figure 5.

Figure 5

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Possible reaction mechanism for the synthesis of PODBE1–6 from PF and n-butanol; (a) reaction path and (b) mechanism detail.

2.4.2. Acetal Reaction Process Analysis

To deeply investigate the reaction mechanism, the reaction path of the acetal reaction process was first calculated by density functional theory (DFT) at the M06-2X/6-311G(d,p) level of theory, as shown in Figure 6. It can be seen from Figure 5b that the acetal reaction process consists of three main steps. The first step is a nucleophilic addition of FA to n-butanol, which results in the formation of the hemiacetal intermediate BuOCH2OH. The TS1 connects reactant A and intermediate B with a free barrier energy of 21.59 kcal/mol in this step. Further, the O14 atom of n-butanol attacks the C15 atom of FA, resulting in the formation of a new chemical bond between O14 and C15 (Figure 6a). The intrinsic reaction coordinate (IRC) analysis of step 1 proved the correctness of the transition-state TS1 structure (Figure 6b). The second step includes a dehydration process and the formation of carbocation BuOCH2+. According to the energy scanning analysis, as the distance between O18 and C15 atoms rises (from 1.264 to 2.264), the total energy of the system decreases. This occurrence suggests that this stage is a spontaneous process with no TS structure (Figure 6c). The third step is the nucleophilic reaction between BuOCH2+ and n-butanol to form BuOCH2OH+Bu and then across deprotonation to prepare PODBE1. The energy scan analysis of this step shows that when the distance between C15 and O31 gradually closes (from 2.509 to 1.504 Å), the total energy of the system gradually decreases, indicating that this step is also a spontaneous process (Figure 6d).

Figure 6.

Figure 6

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Energy studies of the acetal reaction process are as follows: (a) Gibbs free-energy barrier analysis, (b) IRC calculation, (c) energy analysis of the dehydration process, and (d) energy analysis of nucleophilic addition. The dashed lines represent the distances between two atoms in the structural development. The distance unit is Å.

2.4.3. Chain Growth Process Analysis

To further analyze the synthesis mechanism of PODBE1–6, the main TS structures forming the target product PODBEn (n = 1–6) are listed in Figure 7. From Figure 5b, it is observed that the chain growth process mainly includes nucleophilic substitution reaction and nucleophilic reaction. As previously stated, the nucleophilic addition reaction between carbocation and n-butanol was a spontaneous reaction with no TS structures (Figure 6d). As a result, a complete TS analysis of the nucleophilic substitution process is critical for understanding the mechanism of PODBEn synthesis.

Figure 7.

Figure 7

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Optimized geometries of TS structures for the chain growth process: (a) TS1, (b) TS2, (c) TS3, (d) TS4, (e) TS5, and (f) TS6.

From Figure 7, it is observed that the value of free Gibbs barrier energy of TS1 (21.59 kcal/mol) is the highest among all TS structures. This investigation has demonstrated that the acetal reaction process’s nucleophilic addition phase is the rate-determining step in the synthesis of PODBE1–6. The free Gibbs barrier energies for TS2–TS6 are as follows: TS2 (5.01 kcal/mol) ≈ TS3 (5.01 kcal/mol) > TS4 (5.01 kcal/mol), TS5 (5.01 kcal/mol), and TS6 (5.01 kcal/mol). According to the reaction mechanism of the chain growth process, the products from PODBE1 to PODBE6 are sequentially generated in the order of the degree of polymerization n from small to large (Figure 5b), so the contents of PODBE1–6 follow the sequence n = 1 > n = 2 > n = 3 > n = 4 > n = 5 > n = 6. Further, this sequence is consistent with the experimental results, verifying the reliability of the DFT calculation results.

Moreover, the TS structures are also analyzed by NCI, and the corresponding NCI maps are shown in Figure 8. In TS1 a blue circle isosurface is observed between O14 and C15, which means the O14 atom of n-butanol has a strong interaction with C15 of FA, thus resulting in the nucleophilic addition of n-butanol and FA (Figure 8a). The O atom of FA has a strong contact with the C atom of the carbocation in TS2–TS6, which facilitates the nucleophilic substitution process between FA and carbocation. The strong hydrogen bond between them is demonstrated by the blue circle isosurface distribution (Figure 8b–f).

Figure 8.

Figure 8

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NCI plots for the optimized TS structures; (a) NCI analysis of TS1, (b) NCI analysis of TS2, (c) NCI analysis of TS3, (d) NCI analysis of TS4, (e) NCI analysis of TS5, and (f) NCI analysis of TS6. On the left are the equivalent 3D representations; the blue area indicates strong electrostatic interaction, while the green area reflects a more dispersive attractive interaction.

3. Conclusions

In this research, a novel oxygen-containing fuel (PODBE1–6) was formulated to overcome the limitations of PODE, such as its high polarity, poor compatibility with diesel at low temperatures, and low heating value. The reaction parameters, physicochemical properties, and reaction mechanism of PODBEn were systematically investigated through experiments combined with DFT calculations. It was found that, under optimal conditions, the yield of PODBE1–6 can reach 74%. The MPI tests of PODE3–5, PODBE1–6, and diesel (model oil) revealed that the polarity of PODBEn is closer to that of diesel than PODEn, implying that it is theoretically more compatible with diesel. Furthermore, the physicochemical properties of PODBE1–6 such as cetane number, heating value, density, and low-temperature performance have obvious advantages compared with that of PODE3–5 and −35# diesel. DFT was used to calculate the mechanism of the complete path at the level of M06-2X/6-311G(d,p). The reaction consists of two processes, and the nucleophilic addition step with the highest Gibbs barrier energy (TS1 = 21.59 kcal/mol) is the rate-determining step.

4. Experimental Section

4.1. Materials

n-Butanol (99%), KBr (≥99.5%), and DMSO-d6 (99.9%) were purchased from Aladdin Reagent (Shanghai, China). PF (≥96%) was purchased from Sinopharm Chemical Reagent (Beijing, China). NKC-9 acidic ion-exchange resin (H+ type) was purchased from Tianjin Nankai Resin Technology Co., Ltd. (Tianjin, China). All reagents and chemicals were used directly without further purification and treatment.

4.2. Synthesis of PODBEn

n-Butanol, PF, and NKC-9 (2 wt %) were added in a 0.5 L stirred autoclave reactor with a magnetic stirrer. The air within the reactor was changed three times with N2 before the reaction began. The reaction conditions were optimized to be 1:0.6 to 1:1.2 n-butanol/HCHO mole ratio, 2–10 h reaction time, and 1.0 MPa reaction pressure. After the reaction, the autoclave was cooled to room temperature, the pressure in the autoclave was relieved, and then the reaction mixture was filtered to remove the catalyst NKC-9. To obtain pure products of PODBE1, PODBE2, and PODBE3, the filtrate was deacidified with a deacidification agent and distilled at 90 mbar. Unfortunately, the reaction mixture contains inadequate PODBE4, PODBE5, and PODBE6, and their boiling temperatures are too high. We cannot get pure PODBE4, PODBE5, and PODBE6, so only their GC–MS characterization data are available.

4.2.1. PODBE1

A colorless liquid. IR (KBr): ν = 2957, 2930, 2866, 1470, 1369, 1181, 1112, 1043, 1002, 933, 818, 736 cm–1. 1H NMR (600 MHz, d6-DMSO): δ = 4.58 (s, 2H, CH2), 3.45 (t, J = 6.5 Hz, 4H, CH2), 1.49 (dd, J = 14.7, 6.7 Hz, 4H, CH2), 1.34 (dd, J = 15.0, 7.4 Hz, 4H, CH2), 0.89 ppm (t, J = 7.4 Hz, 6H, CH3). 13C NMR (151 MHz, d6-DMSO): δ = 94.9 (O–C–O), 67.1 (C–O), 31.8 (CH2), 19.4 (CH2), 14.1 ppm (CH3).

4.2.2. PODBE2

A colorless liquid. IR (KBr): ν = 2962, 2935, 2862, 1456, 1383, 1195, 1108, 993, 938, 832, 736 cm–1. 1H NMR (600 MHz, d6-DMSO): δ = 4.68 (s, 4H, CH2), 3.48 (t, J = 6.6 Hz, 4H, CH2), 1.49 (dt, J = 14.5, 6.6 Hz, 4H, CH2), 1.33 (dd, J = 15.0, 7.5 Hz, 4H, CH2), 0.88 ppm (t, J = 7.4 Hz, 6H, CH3). 13C NMR (151 MHz, d6-DMSO): δ = 91.78 (O–C–O), 67.71(C–O), 31.72 (CH2), 19.29 (CH2), 14.09 ppm (CH3).

4.2.3. PODBE3

A colorless liquid. IR (KBr): ν = 2967, 2935, 2870, 1461, 1383, 1204, 1116, 970, 919, 823, 740 cm–1. 1H NMR (600 MHz, d6-DMSO) δ = 4.77 (s, 2H, CH2), 4.69 (s, 4H, CH2), 3.48 (t, J = 6.6 Hz, 4H, CH2), 1.53–1.43 (m, 4H, CH2), 1.32 (dd, J = 15.0, 7.5 Hz, 4H, CH2), 0.88 ppm (t, J = 7.4 Hz, 6H, CH3). 13C NMR (151 MHz, d6-DMSO) δ = 92.21(O–C–O), 88.32 (O–C–O), 67.79 (C–O), 31.69 (CH2), 19.27(CH2), 14.08 ppm (CH3).

4.3. Analysis Methods

The infrared (IR) spectra were obtained using an infrared spectrometer (Nicolet iS50, America), with KBr salt flakes serving as the test background. NMR spectra comprise 1H NMR and 13C NMR, which use a NMR spectrometer (Bruker AVANCE 600, Switzerland). The 1H NMR working frequency is 600 MHz, the 13C NMR working frequency is 151 MHz, and Me4Si is used as an internal standard. The cetane number of oxygenated fuels is measured by a cetane number machine (Waukesha, America). The flash point of oxygenated fuels is measured by a flash point machine (PM-93, England). The condensation point test adopts a freezing point meter (DSA-70X, Canada) for measurement. The net heating value is measured by an automatic bomb calorimeter (6300, America). The density of the petroleum products tester is used to determine the density (DSY-412, China). The reactants and products are determined using a gas chromatograph (Agilent 7820A, America), with the selected HP-5 capillary column as the chromatographic column (30 m × 0.32 mm × 0.25 μm, America), and an FID detector. The column temperature programming conditions are as follows: initial temperature, 50 °C for 5 min; heating rate, 15 °C/min; and final temperature, 250 °C for 10 min. The detector temperature is 300 °C.

4.4. Calculated Details

All calculations were performed by Gaussian 09 software.33 The geometries of PODE1–6, PODBE1–6, and diesel fuel surrogates were optimized at the M06-2X/6-311G(d,p) level of theory.34 There are no negative eigenvalues in any of the optimized structures. A linear transit approach was used to search the TS structures, which revealed only one negative eigenvalue at no uncertain saddle point. Furthermore, IRC computations verified that all of the discovered TSs connected the reactants and products. The ESP analysis is based on the M06-2X/6-311G(d,p) level of theory. The ESP distribution on the surface of PODBEn molecules with different degrees of polymerization is also analyzed. The ESP map’s blue–white–red colors represent the low, medium, and positive ESP levels in these regions, respectively. The polarity of different PODBEn molecules is quantitatively analyzed by comparing the values of different MPI (eq 1).28

4.4. 1

where A and V(r) refer to the specific surface area and ESP value of point r in space, respectively, and dS refers to the integration of the molecular surface. The higher value of MPI indicates the strong molecular polarity of the compound.

Acknowledgments

The authors would like to acknowledge the financial support from Beijing Institute of New Energy Technology.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c06452.

  • 1H NMR, 13C NMR, and IR spectra of PODBEn and the Cartesian coordinates of optimized geometries of stable points calculated at the M06-2X/6-311G(d,p) level of theory (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c06452_si_001.pdf (1.4MB, pdf)

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