CO2 Capture With Absorbents of Tertiary Amine Functionalized Nano–SiO2

To improve CO₂ adsorption performance of nanoparticle absorbents, a novel tertiary amine functionalized nano–SiO₂ (NS–NR₂) was synthesized based on the 3–aminopropyltrimethoxysilane (KH540) modified nano–SiO₂ (NS–NH₂) via methylation. The chemical structure and performances of the NS–NR₂ were characterized through a series of experiments, which revealed that NS–NR₂ can react with CO₂ in water and nanofluid with low viscosity revealed better CO₂ capture. The CO₂ capture mechanism of NS–NR₂ was studied by kinetic models. From the correlation coefficient, the pseudo second order model was found to fit well with the experiment data. The influencing factors were investigated, including temperature, dispersants, and cycling numbers. Results has shown the additional surfactant to greatly promote the CO₂ adsorption performance of NS–NR₂ because of the better dispersity of nanoparticles. This work proved that NS–NR₂ yields low viscosity, high capacity for CO₂ capture, and good regenerability in water. NS–NR₂ with high CO₂ capture will play a role in storing CO₂ to enhanced oil recovery in CO₂ flooding.

Introduction

In recent years, humans have been endangered by greenhouse effect leading to global warming. Carbon dioxide (CO₂) emission source from the burn of fossil fuels catches much attention because of it is a major factor to the greenhouse effect (Sarkodie and Strezov, 2019). One method to assuage the greenhouse effect is to capture CO₂ from emission sources and then save it in stratum or apply it for enhanced oil recovery in low permeability reservoirs. Therefore, a feasible approach called carbon capture and storage (CCS) technologies have developed, including membranes, cryogenic distillation, gas adsorption with liquids or solids, and others (Benson and Orr, 2011; Bui et al., 2018). However, membrane–based separation is not a suitable way for CO₂ capture because perfection wants to be made in the areas of CO₂ selectivity, permeability, cost, and performance depletion over time (all caused by a variety of factors). Moreover, because of the high energy costs involved, cryogenic distillation is not optimally suitable. Solvents and solid sorbents have been reported for CO₂ capture, such as basic solvents, supported amine, and ammonium materials, as the primary classes of chemical sorbents (Heydari-Gorji et al., 2011; Darunte et al., 2016; Hahn et al., 2016; Sanz-Perez et al., 2016; Kong et al., 2019). The adsorption of CO₂ by solvents is a commercially available method, but the regeneration process of the solvents is highly energy intensive and expensive (Rochelle, 2009). The adsorption of CO₂ by solid sorbents has recently attracted much attention in the study of CO₂ capture (Lee and Park, 2015).

Nano silica (SiO₂) solid sorbents have been investigated for CO₂ capture given their high pore volume, large surface, and ease of functionality (Liu et al., 2016; Lai et al., 2017, 2019; Wang et al., 2019). Jiao et al. (2015) prepared mesoporous silica (MSU–J) with a wormhole framework texture, the surface of MSU–J was modified with triethylenetetramine (TETA) for CO₂ capture. Hahn et al. (2016) reported the primary amine, secondary amine, and bibasic amine species functionalized porous SiO₂ and discussed the mechanism of CO₂ adsorption on the SiO₂. Bae (2017) showed that the 3–(2–aminoethylamino) propyldimethoxymetylsilane modified silica can be used as an adsorbent to improve CO₂ capture performance and obtained capture CO₂ capacity of 2.24 mmol/g. The surface of SiO₂ usually has been functionalized with primary and secondary amines for CO₂ capture. Amine modified SiO₂ reacted with CO₂ to form carbamate or bicarbonate species based on the acid–based chemical interaction for improved CO₂ adsorption (Huang et al., 2003). Without the presence of water, the amine groups reacted with CO₂ molecules to create the carbamates group. As another specific, the presence of water impairs this amino group adsorption (Ma et al., 2017). Therefore, the functionalized nano–SiO₂ with water impede is needed to further investigated in future development.

It is difficult to destroy the steady carbon–nitrogen bond of carbamate that is formed in primary amine and secondary amines reaction with CO₂. Also, fascinating tertiary amines as absorbents generate bicarbonates to replace carbamates when tertiary amines reacting with CO₂ (Crooks and Donnellan, 1990; Vaidya and Kenig, 2007) thereby leading to low energy for regeneration of absorbents. Therefore, tertiary amine as an energy saving absorbent is appropriate comparing with primary amines and secondary amines (Gao et al., 2017). Particularly, the rate of primary amines and secondary amines with CO₂ is faster than tertiary amines (Liu et al., 2019a). However, the solubility of CO₂ is higher in tertiary amines solution than one in primary amines and secondary amines solution due to different reaction mechanisms. For example, the reaction molar ratio of tertiary amine and CO₂ is 1:1 to formed bicarbonate structure, while the reaction molar ratio of primary amine or secondary amine and CO₂ is 0.5:1 to formed carbamate structure (Sartorl and Savage, 1983). Tertiary amine can be able to generate a bicarbonate due to no hydrogen on nitrogen when reacted with CO₂ and H₂O, resulting in a better CO₂ adsorption and lower energy depletion for regeneration (Xiao et al., 2016, 2019). Moreover, kinetics is important since it can explain the dynamic adsorption of the sorbent, a lot of kinetic models are applied to the CO₂ adsorption property of tertiary amine (Liu et al., 2019b).

Therefore, this study aimed to develop a sorbent to avert the limits of aqueous amine solutions and take advantage of tertiary amines for CO₂ capture. The tertiary amine loaded nano–SiO₂ was synthesized, and the CO₂ capture performance was studied in the presence of water. The CO₂ adsorption mechanism was investigated by kinetics, and the viscosity of the absorbent dispersion was measured before and after CO₂ adsorption. Finally, tertiary amine functionalized nano–SiO₂ (NS–NR₂) was investigated further in terms of temperature, dispersants, and cycling numbers.

Experimental Section

Materials

Methylbenzene(C₇H₈), 3–(trimethoxysilyl)−1–propanamine (KH−540), ethanol (C₂H₅OH), formic acid (HCOOH), formaldehyde (HCHO), hydrochloric acid (HCl), N, N–dimethylformamide (DMF), and sodium hydroxide (NaOH) were purchased from Chengdu Kelong Chemical Reagent Factory (Sichuan, China). Nano–SiO₂ (10–20 nm) was obtained from Aladdin Chemistry Co. (Shanghai, China). All chemical reagents were analytical–grade. CO₂ (g) and N₂ (g) were purchased from Jingli Gas Company (Chengdu, China). Water was double deionized with a Millipore Milli Q system to produce the 18 MM deionized water.

Synthesis and Characterization of Tertiary Amine Functionalized Nano–SiO₂

The nano–SiO₂ loaded with primary amines (NS–NH₂) was prepared first using 3–Aminopropyltrimethoxysilane (KH540) as modifiers, and then it was used as the matrix material to synthesize branched nanomaterials with a tertiary amine group on its surface (NS–NR₂) via methylation of primary amines based on formic acid and formaldehyde. The mechanism is shown in Figure 1.

FIGURE 1

Figure 1. Schematic of the reaction steps of NS–NR₂.

The specific reaction conditions of NS–NH₂, NS–NR₂, and the preparation methods of the nanofluid are shown in Supplementary Materials. It is worth noting that add anhydride to promote unreacted primary amine groups undergo acetylation. The optimum reaction conditions of NS–NR₂ are displayed in Table 1.

TABLE 1

Table 1. Conditions of NS–NR₂.

Fourier transform–Infrared (FT–IR) spectra were acquired by the KBr pellet method using a WQF520 spectrometer. Thermogravimetric analysis (TGA) was conducted on a synchronous comprehensive thermal analyzer (Netzsch Scientific Instruments). The microtopography of NS–NR₂ was characterized using an electron microscope (ZEISS Libra 200 FE). The carbon and nitrogen contents were detected by elemental analysis using a Var10EL III instrument. The hydrodynamic diameter and proportion of the nanoparticles were received by a BI 200SM wide-angle dynamic light scattering (DLS) instrument. The rheological property of NS–NR₂ dispersion was measured with a HAAKE MARS III rheometer at 25°C to assess the viscosity.

CO₂ Adsorption and Desorption

NS–NR₂ dispersion was introduced into a gas adsorption bottle. The gas adsorption bottle was put in a constant temperature water bath. The gas flow of CO₂ was controlled at 1 L/min by a gas flow controller, and CO₂ was bubbling into the high concentrated sulfuric acid to adsorb microscale water in CO₂ gas in a hermetic wild-mouth bottle. After that, the dry CO₂ was flowed through the gas adsorption bottle to reacted with absorbent in the water. The mass change of dispersion was confirmed by an accurate analytical balance (±0.1 mg) until the weight has no change. The amount of adsorption CO₂ on nanoparticles could be calculated under a control test (no nanoparticles). As shown in Figure S1, most of the tests were implemented to assure repeatability of the method. The CO₂ adsorption capacity of NS–NR₂ was measured using a gas adsorption system, as shown in Figure S2. The CO₂ desorption experiment was simply carried out by bubbling N₂ around 1 L/min instead of injecting CO₂ and keeping the system temperature at 25°C to avoid the huge energy depletion.

The mechanism studies of CO₂ adsorption into NS–NR₂ are often executed using kinetic models including pseudo first order, pseudo second order, and intraparticle diffusion model. The parameter R² and relative error (ε) were applied to evaluate the reliability of kinetic models in predicting adsorption capacity, as defined in Equation (1). Based on Equation (1), q_{e, cal} is the predicted date acquired from the model analysis, and q_{e, exp} is the experiment date.

$\begin{array}{ll}\epsilon (\%)=\frac{q_{ecal}-q_{eexp}}{q_{eexp}}\times 100\%& (1)\end{array}$

The pseudo first order model that introduced by Langergren in 1898 year (Langergren, 1898) is shown in Equation (2):

$\begin{array}{ll}log(q_e-q_t)=log(q_e)-(\frac{k_1}{2.303})t& (2)\end{array}$

where q_t is the adsorption capacity at a special time, q_e is the adsorption capacity at equilibrium, k₁ is the constant of pseudo first order with a unit of 1/min. The pseudo second order model (Ho and McKay, 1999) is represented by Equations (3) and (4):

$\begin{array}{ll}\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}& (3)\end{array}$

$\begin{array}{ll}h=k_2·q_e^2& (4)\end{array}$

where q_t is the adsorption capacity at a special time, q_e is the adsorption capacity at equilibrium, k₂ is the constant of pseudo second order with a unit of g/mmol min. The intraparticle diffusion model offers the diffusion mechanism of matter in adsorption process as defined in Equation (5):

$\begin{array}{ll}{q}_t=k{t}^{1/2}+C& (5)\end{array}$

Where q_t is the adsorption capacity at a special time (mmol/g), k is the rate constant of intraparticle diffusion (mmol/g min^1/2) and C (mmol/g) is as the thickness of the boundary layer; the intercept is positively correlated with the boundary layer. (Hameed et al., 2008; Yousef et al., 2011). Mass transfer of adsorbate to the adsorbent surface (bulk diffusion) and film diffusion into the internal sites (intraparticle diffusion) and other steps occur in the process of adsorption.

Results and Discussion

Characterization

The FT–IR spectrum of the nanoparticles is shown in Figure 2. Figure 2A shows the strong adsorption peaks at around 3,450 and 1,646 cm⁻¹, suggest the stretching vibration of the –O–H bonds on the surface of silica. The adsorption peaks near 1,106 and 812 cm⁻¹ are the adsorption peaks of the Si–O–Si group, which are characteristic adsorption peaks of SiO₂. In the FT–IR spectra of KH540, the adsorption peaks around 3,380 and 1,594 cm⁻¹ are the –N–H stretching and NH₂ deformation of hydrogen bonded amino groups (Jiao et al., 2015). The adsorption peak at 1,477 cm⁻¹ is C–N, and the peaks at 2,954 and 2,842 cm⁻¹ are feature adsorption peak of –CH₃ and –CH₂-, respectively. The peak at 694 cm⁻¹ is the adsorption peak of Si–C (Titinchi et al., 2014). In the FT–IR spectra of NS–NH₂, the 2,954 cm⁻¹ peak of –CH₃ disappeared, elucidating that the primary amine was grafted on the surface of SiO₂. In Figure 2D, the adsorption peak of –CH₃ is shown to appear, indicating that –(CH₂)₃NH₂ reacted to –(CH₂)₃N(CH₃)₂ on the surface of nano–SiO₂.

FIGURE 2

Figure 2. (A) FT–IR spectra of raw SiO₂, (B) KH540, (C) NS–NH₂, and (D) NS–NR₂.

The microscopic structure of the nanoparticles is shown in Figure 3 as observed from the TEM morphology. The diameter of NS–NR₂ was shown to be approximately 15 nm. The nanoparticles aggregated slightly because of the particle size being in the nanometer scale (Zhao et al., 2014). The evidence from DLS analysis (Figures 4A,B) shown the hydrodynamic diameter of NS–NR₂ to be approximately 123.6 nm with uniform size distribution. Moreover, it was found that the diameter distribution of nude particles was wider and the agglomeration was more serious than that of the modified nanoparticles.

FIGURE 3

Figure 3. TEM image of NS–NR₂ (the scale bars in a is 50 nm and b is 100 nm).

FIGURE 4

Figure 4. Diameter distribution of (A) NS–NR₂ and (B) nude nanoparticles in water; (C) Diameter distribution of NS–NR₂ in PEG−400 solution (1,000 mg/L).

The element contents of raw SiO₂, NS–NH₂, and NS–NR₂ are shown in Table 2. Carbon and nitrogen contents are present in raw SiO₂ because many SiO₂ materials are often synthesized in an aqueous organic solvent, resulting in remaining carbon and nitrogen in such materials. The contents of carbon and nitrogen for NS–NH₂ were shown to be 5.22 and 1.65 mmol/g, respectively, and for NS–NR₂ were shown to be 7.14 and 1.39 mmol/g, respectively. The contents of these elements were much higher than raw SiO₂. The nitrogen amount in NS–NR₂ was lower than that of NS–NH₂ because when the molar amount of carbon increases, the molar amount of nitrogen decreases in fixed mass products. The molar ratio of carbon to nitrogen is 3.16, 5.13 in NS–NH₂ (SiO₂-C₃H₈N) and NS–NR₂ (SiO₂-C₅H₁₀N), respectively. Here, the molar ratio of carbon to nitrogen was adopted to further confirm the grafting of tertiary amines on nano–SiO₂.

TABLE 2

Table 2. Element contents on nanoparticles.

The successful synthesis of NS–NR₂ could also be proved by thermos gravimetric analysis (TGA). Based on the TGA curves in Figure 5, the weight retention of raw nano-SiO₂, NS–NH₂ and NS–NR₂ at 900°C under the air atmosphere were 96.86, 89.38, and 87.06%, respectively. For raw nano-SiO₂, the mass depletion is attributed to the surface dihydroxylation. In terms of the structure of NS–NH₂ and NS–NR₂, when the temperature up to 900°C, the primary amine and tertiary amine chains were grafted on the nano-SiO₂ has decomposed, respectively. Therefore, compared with the TGA curve of raw nano-SiO₂, the surface of nano-SiO₂ was modified to primary amine. The different weight loss between in the NS–NH₂ and NS–NR₂ indicated that tertiary amine was synthesized from primary amine successfully.

FIGURE 5

Figure 5. TGA thermograms of raw nano–SiO₂, NS–NH₂, and NS–NR₂.

CO₂ Adsorption and Kinetic Studies

Any amine with a pKa value >5 can react with CO₂ in the presence of water (Field and Grolimund, 1988). According to Figure S3A, the pH value of NS–NR₂ dispersion decreased gradually with the addition of diluted hydrochloric acid (0.01 mol/L). The second derivative of the V_HCl-pH curve was obtained (Figure S3B), with the zero point of the second derivative corresponding to the extreme point of the curve in Figure S3A. As a result, the pKa value was 7.08. The pKa value of 7.08 indicates that NS–NR₂ dispersion can complete adsorption of CO₂. Along with the aeration of CO₂ at 1 L/min, the pH value of NS–NR₂ dispersion dropped rapidly in the first 5 min, after which there were no changes in pH value for 20 min, as shown in Figure S3C, which means that NS–NR₂ dispersion (0.1 wt.%) completely saturated CO₂ at 1 L/min CO₂ gas velocity in 20 min.

It is well-known that CO₂ capture is significant influenced by the viscosity of absorbent (Xiao et al., 2019). Figure 6 demonstrates the rheological property of NS–NR₂ dispersion before and after adsorption CO₂. A rheological plateau in the shear rate region was found for NS–NR₂ dispersion. Before CO₂ adsorption, the nanofluid viscosity was 2.23 mPa·s. After saturation with CO₂, there was a nanofluid viscosity increase to 2.96 mPa·s. This viscosity change is consistent with the ionic liquids in Xiao et al.'s work (Xiao et al., 2019). A possible conclusion is the increase of electrostatic interaction of chains on the NS–NR₂ surface due to CO₂ adsorption (Figure 7), resulting in the higher viscosity. However, unlike the high viscosity of ionic liquids, the nanofluid viscosity was very low. The CO₂ capture was not influenced by the increased viscosity.

FIGURE 6

Figure 6. Viscosity of NS–NR₂ dispersion before and after adsorption of CO₂ as a function of shear rate.

FIGURE 7

Figure 7. The process for NS–NR₂ reacting with CO₂.

The CO₂ adsorption capacity of NS–NR₂ was 25 mmol CO₂/g NS–NR₂ in water (0.1 wt.%) at 25°C, as shown in Table 3. Compared with other materials (with CO₂ adsorption capacity of 0.1–21.45 mmol/g) (Yu et al., 2012), NS–NR₂ in water has a better CO₂ adsorption capacity.

TABLE 3

Table 3. CO₂ adsorption of samples at 25°C.

Kinetic models, such as pseudo first order, pseudo second order, and intraparticle diffusion model, were applied to model the test data. The pseudo first order model is mostly appropriated to describe purely physisorption process without considering the any chemical reaction between CO₂ molecules and sorbent. The pseudo second order model is mostly appropriated to describe purely chemisorption processes with stable chemical bonds between CO₂ molecules and the sorbent. The comparison between the test curves and simulative curves are shown in Figure 8. The model parameters of kinetic at different temperatures and their corresponding coefficients are shown in Table 4. The R² parameters of pseudo second order and pseudo first order models with experimental data were 0.99 and 0.97, respectively. The relative error for the pseudo second order model, ε, was lower than pseudo first order model. Compared with the R², ε parameters, the pseudo second order model was found to fit well with the experiment data. Therefore, chemisorption of CO₂ on nanoparticles plays a dominant role in CO₂ capture.

FIGURE 8

Figure 8. Kinetic plot of CO₂ adsorption models at various temperatures.

TABLE 4

Table 4. Kinetic parameters of CO₂ adsorption on NS-NR₂.

The pseudo first order model and pseudo second order model provide interaction insight into the actual mechanism of CO₂ adsorption. The surface of the particles is grafted with functional groups to adsorb CO₂. This surface adsorption was further verified by the intraparticle diffusion. The model parameters of intraparticle diffusion are revealed in Table 4. It is notable that no linear curve can pass through the origin point that is thought to be caused by intraparticle diffusion, suggesting that intraparticle diffusion is not the only factor controlling CO₂ adsorption rate at all tested temperatures (Rashidi et al., 2013). Therefore, the adsorption process is not completely controlled by intraparticle diffusion, surface diffusion also plays a role in the whole CO₂ adsorption process.

Effect of Some Factors on CO₂ Adsorption With NS–NR₂

Effect of Temperature

A CO₂ capture test was implemented using 0.1 wt.% nanofluid at contrast temperatures of 25, 30, 40, 50, and 60°C, respectively. The CO₂ adsorption- temperature curve is revealed in Figure 9A. It is obvious that the maximum CO₂ loading on the nanoparticles decreased at higher temperatures. Higher temperatures go against CO₂ adsorption that is an exothermic reaction. The CO₂ adsorption of NS–NR₂ compared with that of MSU–J modified with TETA (Jiao et al., 2015) at higher temperatures is displayed in Figure 9B. The relative comparison presents a view of the advantages of the proposed nanoparticles although a slight difference in the experimental conditions. For the NS–NR₂ absorbent in this work, the CO₂ loading at 60°C decreased to 48% of the CO₂ loading at 25°C, while the CO₂ loading of MSU–J at 55°C decreased to 73% of the CO₂ loading at 25°C. This result can be ascribed to the different mechanisms of CO₂ adsorption. For modified MSU–J absorbent, huge energy was used to generate carbamate group with CO₂. But adsorption of CO₂ loaded nanoparticles enabled bicarbonate formation much easier because of the physical adsorption and chemical adsorption. This result indicates that the NS–NR₂ in this work can capture more CO₂ at higher temperatures.

FIGURE 9

Figure 9. (A) CO₂ adsorption of NS-NR₂ in water at different temperatures. (B) CO₂ adsorption of NS–NR₂ at (A) 25°C and (B) 60°C; CO₂ adsorption of MSU–J at (C) 25°C and (D)55°C.

Effect of Dispersants

CO₂ adsorption capacity in different dispersant agents were investigated. According to the previous test is shown in Figure S3C, it was not necessary to perform the experiment for longer than 20 min. Hence, CO₂ adsorption with NS–NR₂ was performed at a temperature of 30°C in different dispersants during this period of 15 min. CO₂ adsorption curves are drawn in Figure 10. The maximum CO₂ loading of NS–NR₂ was changed in water or PEG−400 dispersants. The maximum CO₂ loadings of NS–NR₂ in water and PEG−400 solution was 23.16 and 105.34 mmol/g, respectively. This result can be explained by the diameter distribution of nanoparticles, where the diameter distribution in water was 123.6 nm and in PEG−400 solution was 56.7 nm (Figure 4C). The tertiary amine groups as specific sites for CO₂ adsorption grafted on the surface of nano–SiO₂ to endow the adsorbents with CO₂ adsorption. Therefore, the more specific sites were exposed on the surface of nanoparticles because of better dispersibility. The dispersibility of nanoparticles is certified by the Derjaguin Landau Verwey Overbeek (DLVO) theory on interparticle interaction potential (Ilyas et al., 2014). The interaction among nanoparticles is caused by electrostatic repulsion and steric resistance in the PEG−400 solution. In addition, using PEG−400 as the dispersant is inexpensive and has lower surfactivity. Surfactants with similar properties can also be used to disperse the nanoparticles for CO₂ capture. Therefore, adding the surfactant improved CO₂ adsorption by enhancing the dispersibility of nanoparticles.

FIGURE 10

Figure 10. CO₂ adsorption of NS-NR₂ in different dispersants.

Effect of Cycling Numbers

It is crucial that an absorbent is reusable and retains an efficient CO₂ adsorption capacity (Ma et al., 2017). Therefore, the regeneration tests of the NS–NR₂ absorbent was carried out at 25°C. After CO₂ adsorption in the nanofluid for 20 min, the nanofluid was shifted to CO₂ desorption for regeneration. The CO₂ desorption test was executed for another 20 min by bubbling N₂ around 1 L/min before the absorbent was used for the next round of adsorption. As illustrated in Figure 11, five cycles of adsorption were implemented and the initial CO₂ adsorption of absorbent was set as the 100 % baseline. After five cycles, NS–NR₂ adsorbent shown favorable regeneration capacity with a slight decrease of 14.3% in water for NS–NR₂ compared to the initial capacity. The results shown regeneration and efficient CO₂ adsorption capacity of nanoparticles adsorbent. Considering cycling capacity, NS–NR₂ material shows intriguing regeneration ability for CO₂ adsorption.

FIGURE 11

Figure 11. Cyclicity of NS–NR₂ adsorbent in water.

Conclusion

This work synthesized tertiary amino functionalized nano–SiO₂ successfully. The measurements of pKa value and nanofluid viscosity change proved that NS-NR₂ can react with CO₂ in water and nanofluid has a low viscosity. NS–NR₂ shown better CO₂ adsorption capacity, and adsorption kinetics revealed the pseudo second order model was found to fit well with the experiment data. The influence of factors such as temperature, dispersants, and cycling numbers on CO₂ adsorption was investigated. Results indicated higher temperature to work against CO₂ adsorption of NS–NR₂. The CO₂ adsorption performance of NS–NR₂ was greatly promoted because of a better dispersity of nanoparticles with added surfactant. After recycling of absorbent, the NS–NR₂ maintained an efficient CO₂ capture and shown favorable regeneration capacity. The measurements of NS–NR₂ properties on the bases of viscosity, kinetic models, CO₂ capture, and regeneration manifests that NS–NR₂ exhibits satisfying performance to capture CO₂. NS–NR₂ with high CO₂ capture will play a role in storing CO₂ to enhanced oil recovery in CO₂ flooding.

Data Availability Statement

All datasets generated for this study are included in the article/Supplementary Material.

Author Contributions

NL, QZ, and LT conceived the idea. QZ, WH, TY, and YC implemented the preparation, characterization, and measurement of NS–NR₂. QZ wrote the manuscript. DQ and KC came up with ideas for the manuscript. QZ, LT, and DW discussed and analyzed the experiment data. LT and DW revised the manuscript. NL supervised the whole research work.

Funding

This work was supported by the National Natural Science Foundation of China (No. 51674208); Nanchong Science and Technology Project (NC17SY4017); State Key Laboratory of Molecular Engineering of Polymers (Fudan University) (No. K2017-25); PetroChina Innovation Foundation (No. 2018D−5007–0207); Open Foundation of State Key Laboratory of Oil and Gas Geology and Exploitation (Cheng Du University of Technology) (No. PLC20180103); Basic Research Programs of Sichuan Province (No. 2018JY0515); and the Undergraduate Extracurricular Open Experiment of Southwest Petroleum University (KSZ18440, KSZ18441, KSZ18442).

Conflict of Interest

DQ was employed by the Engineer Technology Research Institute, CNPC Xibu Drilling Engineering Company Limited company.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00146/full#supplementary-material

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