Effective Removal of Phenylamine, Quinoline, and Indole from Light

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Effective Removal of Phenylamine, Quinoline and Indol from Light Oil by #-Cyclodextrin Aqueous Solution through Molecular Inclusion Zunbin Duan, Tingting Bu, He Bian, Lijun Zhu, Yuzhi Xiang, and Daohong Xia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02086 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Effective Removal of Phenylamine, Quinoline and Indol from Light Oil by β-Cyclodextrin Aqueous Solution through Molecular Inclusion Zunbin Duan a, b, Tingting Bu a, b, He Bian a, b, Lijun Zhu a, Yuzhi Xiang a and Daohong Xia*a, b a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, People’s Republic of China. b College of Chemical Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China.

Abstract: The aqueous solution of β-cyclodextrin (β-CD) was reported for the first time being used as a molecular inclusion agent to simultaneously remove basic nitrogen (phenylamine and quinoline) and non-basic nitrogen (indol) compounds which are the main organic nitrogen compounds in light oil. The influence factors on the denitrification process, including the mass concentration of β-CD, denitrification time, the volume ratio of β-CD aqueous solution to oil, operation temperature and aromatic hydrocarbons and sulfides in light oil, and the performance of regeneration were investigated and the optimized denitrification conditions had been obtained. Both the basic nitrogen and non-basic nitrogen compounds were effectively removed by β-CD aqueous solution, which is obviously superior to the current nonhydrodenitrification methods. Furthermore, the structures of the inclusion complexes of β-CD with phenylamine, quinoline and indol were studied by ultraviolet absorption spectra, 1H nuclear magnetic resonance, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and computational simulations respectively. A possible mechanism model for denitrification and the

∗

Corresponding author. E-mail address: [email protected]. (D.H. Xia) Tel.: +86 532 86981869; fax: +86 532 86981787.

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structures of the inclusion complexes were proposed based on the analyses at the end. This work lays the foundation of a novel denitrification method by molecular inclusion with β-CD as a green bioresource material. Keywords: β-cyclodextrin; denitrification; molecular inclusion; inclusion mechanism.

1. Introduction Increasingly stringent regulation on environment and the development of automobile industry have been pressing a strong demand for cleaner and safer light oil. In reality the organic nitrogen compounds existing in fuel not only greatly affect the quality of oil, such as the stability and color change of fuel, but also cause the environmental problems. The main organic nitrogen compounds in light oil are phenylamine, quinoline and indol and their derivatives. Currently the main methods of denitrification for light oil are hydrogenation and nonhydrogenation. Hydrodenitrification (HDN) technology has been already mature and widely used, but it requires a large consumption of hydrogen and equipment investment. In addition, HDN would cause the saturation of olefin and aromatics, which reduces the octane number of oil. As an alternative, nonhydrodenitrification technologies have been rapidly developed in recent years. Its processing with gasoline are mainly extraction1,2 and adsorption3 methods. However, basic nitrogen (e.g. phenylamine, quinoline) and non-basic nitrogen (e.g. indol) compounds cannot be simultaneous removed by extraction and adsorption with high efficiency because of their different physicochemical properties. Wang et al.1 reported three dihydrogen phosphate-based ionic liquids and investigated their denitrification properties for quinoline in n-heptane by liquid-liquid


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extraction method. The acidity of the three ionic liquids plays an important role on denitrification for basic nitrogen (quinoline). But non-basic nitrogen compounds (indol) cannot be effectively removed by the reported ionic liquids. Hizaddin et al.2 evaluated the performance of deep eutectic solvents (DESs) for the extraction of denitrification of five non-basic and six basic nitrogen compounds. They found that the non-basic nitrogen compounds were much easier to be removed than the basic nitrogen compounds. Kim, et al.3 studied the adsorption denitrification for quinoline and indol using three adsorbents, including activated carbon, activated alumina and nickel-based adsorbent. The three reported adsorbents can quickly remove indol, but the performance of removing quinoline was poor. Another obstacle for nonhydrodenitrification is that the denitrification agents used in extraction and adsorption are not easy to regenerate due to the loss of the active components during the regeneration process or the strong interactions between denitrification agent and nitride. Hence the technology of highly effective nonhydrodenitrification still requires significant effort in research and development from both academic and industry. β-Cyclodextrin

(β-CD)

containing

peculiar

hydrophobic

cavum

is

a macrocyclic

oligosaccharide composed of 7 D-pyran glucose units and can form inclusion complexes with various guest organic4-6, inorganic7 and biologic8 molecules through molecular recognition. Krishnan Srinivasan et al studied the inclusion complexes of β-cyclodextrin with 2,6-dinitrobenzoic acid9, 2,4-dinitroaniline10 and 2,6-dinitroaniline11 by different analytical methods, and reported that all three nitrides formed 1:1 inclusion complex with β-CD. Sun et al studied the inclusion process of 1-propanethiol existing in light oil with β-CD and first reported


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the removal of mercaptan from light oil through inclusion interaction12. Molecular recognition with β-CD as a green bioresource material has been innovatively applied in many fields13-15, but so far we are aware that the concept has not been applied into the research for denitrification of light oil. It is always tantalizing to assume that when β-cyclodextrin aqueous solution mixed with light oil, partial organic nitrogen compounds existed in oil may be transferred to aqueous solution and selectively entered into the hydrophobic cavity of β-CD to achieve the denitrification through the inclusion interaction between β-CD and organic nitrogen compounds. Herein, we explored the idea of β-CD recognition with organic nitrogen compounds and applied it to remove phenylamine (P), quinoline (Q) and indol (I), which are the main organic nitrogen compounds in light oil, by using β-cyclodextrin aqueous solution. The inclusion process and complexes of β-CD with phenylamine, quinolone and indol were characterized and a possible denitrification mechanism through β-cyclodextrin aqueous solution was proposed. 2. Experimental Section 2.1. Materials and instruments All chemical reagents (β-CD, phenylamine, quinolone, indol, n-heptane, petroleum ether (60-90), toluene, thiophene, benzothiophene, activated carbon powder and ethanol) were A.R. grade and purchased from Sinopharm Chemical Reagent Co., Ltd.. β-CD was recrystallized with ultrapure water (>18.25 MΩ·cm) for three times before use. Phenylamine (P), quinolone (Q) and indol (I) were distilled before use. Ultraviolet absorption spectra of β-CD and P, Q, I in phosphate buffer solution were analyzed on a Hitachi U-3900H UV-Vis spectrophotometer at different temperature (25oC, 30oC, 35oC,


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40oC and 50oC) to study the inclusion process. FT-IR spectra of β-CD, nitrides and the three inclusion complexes were determined on a Nicolet 6700 FT-IR spectrophotometer using KBr (absorption peak range, 4000 cm-1 – 400 cm-1; scan number, 64; DTGS detector). 1H NMR spectra were recorded by a Bruker AVANCE III 600 MHz spectrometer (temperature, 25oC; number of scan, 16; spectrometer frequency, 600MHz) in DMSO-d6 and the chemical shifts (δ) were reported relative to Me4Si(TMS) as internal standard, which were to determine the structures of the inclusion complexes. XRD patterns of β-CD and the three inclusion complexes were recorded on a Panalytical X’Pert Pro MPD diffractometer (Cu Kα radiation; scan step size, 0.0167o; scan range (2θ), 5o – 75o; scan mode, continuous scan; sampling pitch, 0.04o; preset time, 0.3s). The morphologies of β-CD and the inclusion complexes were determined with Hitachi S-4800 scanning electron microscopy. The nitrogen content in simulated oil was measured by an ANTEK9000 NS analyzer. 2.2. Denitrification experiments Denitrification experiments by the molecular inclusion of β-CD were carried out as following. Typically, phenylamine, quinoline and indol were dissolved into n-heptane (simulated oil), respectively, and the initial nitrogen concentration was approximately 100µg·g-1. The aqueous solution of β-CD was mixed with simulated oil under mechanical stirring (500rpm) at different temperature (5oC, 15oC, 25oC, 30oC, 35oC and 40oC). The nitrogen content in simulated oil at different denitrification time was measured by an ANTEK 9000 NS analyzer and the removal percentage was calculated by the following equation, Denitrification efficiency (%) = [ (N0-Nt) / N0 ]×100


(1)

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where N0 is the initial concentration of nitrogen in the simulated oil (µg·g-1) and Nt is the concentration of nitrogen in the simulated oil at denitrification time t (µg·g-1). In order to evaluate the cyclic regeneration performance of β-CD aqueous solution, quinoline-loaded sample was used to investigate the regeneration performance. The used β-CD aqueous solution was separated, and then mixed with fresh petroleum ether (v: v =1: 3) to release the organic nitrogen compounds under mechanical stirring at 80oC for 30min. The experiment was repeated for three times, then the regenerated β-CD aqueous solution was further used in denitrification experiments. All the denitrification experiments were measured for three times to ensure the effectiveness of the results. 2.3. Preparation of inclusion complex between β-CD and organic nitrogen compound The inclusion complex of β-CD and organic nitrogen compound was prepared by saturated aqueous solution method16. The preparation process of inclusion complex between β-CD and phenylamine was as follows. Typically, 50mL of aqueous solution with 1.0g of β-CD was mixed at room temperature, then 0.2mL of phenylamine dissolving in 2mL of ethanol was added dropwise for 10min. The reaction solution was continuously stirred (500rpm) at 60oC for 12h and kept overnight at 0oC in a low temperature cycling box until a large white precipitate was formed. The white precipitate was filtered and washed with distilled water and ethanol respectively, then dried in an oven at 40oC for 6h. The final white solid powder was the inclusion complex of β-CD with P and characterized by 1H NMR, FT-IR, XRD and SEM. The white inclusion complex of β-CD with Q and I was prepared according to the aforementioned method.


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2.4 Inclusion complexation of β-CD with phenylamine, quinoline and indol The inclusion complexation of β-CD with phenylamine, quinoline and indol containing chromophores was researched by the direct optical spectrographic method17. Nitride with β-CD of different concentration in phosphate buffer solution (pH= 6.86) was firstly stored for a certain time (P, 40min; Q, 45min; I, 50min) at 25±0.1oC. Then, ultraviolet absorption spectra were measured at the same temperature. Ultraviolet absorption spectra at 30oC, 35oC, 40oC and 50oC were measured at the same condition. 2.5. Computational simulations for the inclusion complexes between β-CD and organic nitrogen compounds One β-CD molecule and one organic nitride molecule were randomly placed in the vacuum cube box with the edge of 9 nm. The exported coordinates of the molecular structures of the three inclusion complexes were simulated by molecular dynamics (MD) simulation. All the molecular configurations and snapshots were acquired by using Visual Molecular Dynamics software version 1.9.1. MD simulation was performed on the GROMACS 2016.1 program suite at 298.15K for 20ns with the time step of 2fs 18. Then the configurations of the three inclusion complexes were optimized by quantum mechanics (QM) calculation. QM calculation was performed on the MOPAC 2016 software with PM6-DH+ method 19. The intermolecular interaction between β-CD and nitride in the inclusion complex was studied by the independent gradient model (IGM) method

20

. The IGM analysis for the optimized

configuration of the inclusion complex was carried out with the Multiwfn program.


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3. Results and discussion 3.1. Denitrification experiments by molecular inclusion of β-CD The effect of the mass concentration of β-CD on the removal of P is shown in Fig.1(A). It could be seen that the removal efficiency of P firstly increased and then gradually decreased with the increase of the mass concentration of β-CD, and the optimum mass concentration of β-CD in aqueous solution was 1.8wt%. Fig. 1(B) shows the effect of denitrification time on the performance of β-CD aqueous solution. From Fig. 1(B), we can see that with the increase of removal time, the removal percentage of quinoline and indol increased initially and leveled after 40min. But for phenylamine, the removal efficiency rapidly reached the maximum value at 10 min. It shows that the inclusion interaction of phenylamine with β-CD was the strongest among the three types of organic nitrogen compounds. The removal efficiency for P, Q and I was in the order of P > I > Q. And the optimum removal time was 20min for phenylamine and 60min for quinoline and indol.


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Figure 1. Effect of (A) the mass concentration of β-CD (P; temperature, 30oC; time, 20min; volume ratio of β-CD aqueous solution to oil, 1:1), (B) time (concentration of β-CD, 1.8wt%; temperature, 30℃; volume ratio of β-CD aqueous solution to oil, 1:1), (C) the volume ratio of denitrifier to oil (concentration of β-CD, 1.8wt%; time, P, 20min, Q and I, 60min; temperature, 30oC) and (D) temperature (concentration of β-CD, 1.8wt%; time, P, 20min, Q and I, 60min; volume ratio of β-CD aqueous solution to oil, 1:1) on the removal of nitride by β-CD aqueous solution. We also investigated the effect of volume ratio of β-CD aqueous solution to oil on the removal of nitrides (see Fig.1(C)). β-CD aqueous solution has a better denitrification performance with increasing the ratio of denitrifier to oil. However, with further increasing the ratio of denitrifier to oil, phase interface could be emulsified and blistering when the ratio exceeds 1:1, so the optimum volume ratio of denitrifier to oil 1:1 was selected.


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Fig.1(D) shows the result of the effect of temperature on the removal of the three nitrogen compounds. With rising temperature from 5oC to 40oC, the removal percentage has a various degree of decrease. With the rise of temperature, molecular thermal motion aggravated, causing the decrease of denitrification efficiency. High temperature does not favor the inclusion process. Therefore, the denitrification process should be carried out at low temperature or room temperature. Considering the effectiveness and operability, the optimum denitrification temperature was suggested to be 25℃. After denitrification was finished, the β-CD aqueous solution containing nitrides was mixed with petroleum ether at stirring and heated to 80oC for 30min, the regeneration of β-CD could be easily realized since the included nitrogen compounds in the cavity of β-CD were released again at higher temperature and the released nitrogen compounds were transferred to organic solvent (petroleum ether). In this way, the regeneration of the denitrification agent was achieved. After 5 times recycle, β-CD aqueous solution still kept a relatively good function on the removal of the three nitrogen compounds (Fig. 2).

Figure 2. Reusability of β-CD aqueous solution for the removal of (A) P, (B) Q and (C) I


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The effects of other components in light oil, including aromatic hydrocarbons, thiophene and derivatives, on the denitrification performance of β-CD aqueous solution were investigated, respectively. Firstly, toluene was used as a model compound to investigate the effect of aromatic hydrocarbons on the removal of phenylamine by β-CD aqueous solution. The removal percentage of phenylamine in toluene was 49.8% (Fig. S1(A)), which was lower than that in n-heptane (63.4%). This indicates that the aromatic component in light oil interferes with the removal of nitrides by β-CD aqueous solution. This may be related to the competition of aromatic hydrocarbon (toluene) and n-heptane for the internal hydrophobic cavity of β-CD. Then the solutions of n-heptane containing both phenylamine and thiophene, indol and benzothiophene were prepared, respectively, and the removal of nitrogen and sulfur by β-CD aqueous solution were tested. As shown in Fig. S1(B, C), the removal efficiency of β-CD aqueous solution to phenylamine and indol was 48.6% and 44.1%, respectively. More interestingly, β-CD aqueous solution can also effectively remove thiophene and benzothiophene, which means that β-CD aqueous solution can be used to simultaneously remove nitrides and sulfides from light oil. It notices that β-CD aqueous solution preferential removes the nitride and then removes the sulfide. The β-CD in acetate buffer solution (pH=4.04) and carbonate buffer solution (pH=10.50) were used to investigate the effect of solvent on the removal of nitrides by the molecular inclusion of β-CD. The removal percentages of phenylamine by the β-CD in acetate buffer solution and carbonate buffer solution were 47.8% and 41.7% (Fig. S2), respectively, lower than that by the β-CD in aqueous solution (63.4%). This shows that β-CD is not suitable for removing nitride


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from n-heptane under both acidic and basic conditions. β-CD in a neutral environment (β-CD aqueous solution) is suitable for the removal of the nitrogen compounds from light oil. The activated carbon powder was used to compare with the removal efficiency of β-CD aqueous solution under the same denitrification conditions. As shown in Fig. S3, the phenylamine removal efficiency by the activated carbon powder and β-CD aqueous solution was 49.1% and 63.4%, respectively. The denitrification rate of the activated carbon was significantly slower than that of β-CD aqueous solution. It indicates that the β-CD aqueous solution has a prospect for the denitrification of light oil. The solution of n-heptane containing both basic nitrogen (phenylamine) and non-basic nitrogen (indol) was prepared, and the removals of phenylamine and indol by β-CD aqueous solution were investigated. It can be seen from Fig. S4 that β-CD aqueous solution can simultaneously remove the phenylamine and indol with high removal efficiency and rate, which is much better than the current nonhydrodenitrification methods 1-3. Studies on the inclusion process and the structure of the inclusion complexes between organic nitrogen compounds and β-CD can help us to understand the mechanism of denitrification process.

3.2. Inclusion process and inclusion complex of β-CD with phenylamine, quinoline and indol β-CD can include lots of inorganic and organic molecules and form inclusion complexes. UV-Vis or ultraviolet absorption spectra is a very important method to study the inclusion


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process21. Fig. S5(A1, B1 and C1) showed the change of absorbance of nitride aqueous solution adding different concentration of β-CD. It can be seen that the absorbance intensity of the inclusion system increases with the increase concentration of β-CD, and the maximum absorption wavelength does not change (P: 230 nm; Q: 280 nm; I: 270 nm). According to the equation of Benisi-Hildebrand22 (Eq. (2)), the plot with 1/∆A to 1/[β-CD]0 was plotted (Fig. S5(A2, B2 and C2)).

(2) where ΔA is the difference between the absorbance of organic nitrogen compound in the presence and absence of β-CD. Δεβ-CD·N is the difference between the molar absorption coefficient of organic nitrogen compound and the inclusion complex. [β-CD]0 and [N]0 are the initial concentration of β-CD and organic nitrogen compound in phosphate buffer solution, respectively. It shows a good linear relation between 1/∆A and 1/[β-CD]0, which means that the inclusion ratio of inclusion complex between β-CD and organic nitrogen compound is 1:1. The inclusion equilibrium constant (Ka) at different temperatures can be calculated by the slope and intercept of the linear equation. Table 1 Inclusion equilibrium constant (Ka) of inclusion complex at different temperatures T/ oC

25.0

30.0

P

2273.15

Q I

35.0

40.0

50.0

1618.08 1205.92

852.35

589.93

379.69

266.93

207.68

162.07

121.27

616.96

412.04

341.72

250.38

181.64


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Table 1 shows the change of the inclusion equilibrium constant (Ka) for the inclusion complexes at different temperatures. Ka decreases gradually with the increase of temperature, that means high temperature is not conducive to the inclusion process, consistenting with the result of denitrification. Furthermore, the inclusion equilibrium constants of the inclusion complex between phenylamine and β-CD in acetate buffer solution and carbonate buffer solution were obtained. The Ka in acetate buffer solution and carbonate buffer solution at 25oC were 1234.82 L·mol-1 and 548.51 L·mol-1, respectively, which was much lower than that in phosphate buffer solution (2273.15 L·mol-1). It indicates that β-CD does not easily form the inclusion complex with phenylamine under both alkaline and acidic conditions, which means that solvent has an important role in driving molecular inclusion. Meanwhile, the thermodynamic parameters of the inclusion process of β-CD and the three organic nitrogen compounds (Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS)) can be acquired from the Ka using van’t Hoff equation23 (Eq. (3), (4)). △G=-RTlnKa

lnKa = −

△H △S + RT R

(3)

(4)

where R is the universal gas constant (8.3145 J mol-1·K-1) and T is the absolute temperature (K).


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Figure 3. Relation graph of lnKa and 1000/T for the inclusion complex Table 2. Thermodynamic parameters of the inclusion process of β-CD and P, Q, I Thermodynamic parameters

∆G298.15K/ kJ·mol-1

∆H/ kJ·mol-1

∆S/ J mol-1·K-1

P

-19.16

-43.77

-82.92

Q

-14.72

-36.28

-72.98

I

-15.93

-38.54

-76.52

Fig. 3 displays the relationship between lnKa and 1000/T. A good linear correlation between lnKa and 1000/T was shown in Fig.3. The obtained thermodynamic parameters of the inclusion process (∆G, ∆H and ∆S) from the linear relation equation of lnKa and 1000/T are listed in Table 2. The values of ∆G, ∆H and ∆S are all negative, which means that the inclusion process for β-CD and organic nitrogen compound is a spontaneous, exothermic and controlled by enthalpy. The noncovalent interactions, including hydrogen bonding, dipole-dipole interaction, electrostatic interactions and van der Waals interactions 24,25, make a negative contribution to the enthalpy change, while hydrophobic interaction makes a positive contribution to ∆H. So we


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deduced that hydrogen bonding and van der Waals interactions between the organic nitrogen compounds and β-CD mainly affect the inclusion process studied.

Fig. 4. FT-IR spectra of β-CD, organic nitrogen compounds (P, Q and I) and inclusion complexes of β-CD with P, Q and I. The interaction between β-CD and nitride and structure of inclusion complex are usually analyzed by FT-IR. When organic nitrogen compounds entered into the inside cavity of β-CD,


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the absorption bands of organic nitrogen compound may be shifted, disappeared and covered up by β-CD. The FT-IR spectra of β-CD, organic nitrogen compounds (P, Q and I) and three inclusion complexes are shown in Fig. 4. From the spectra of the three inclusion complexes in Fig. 4, the characteristic peaks of P (3209cm-1, 2907cm-1 and 1275cm-1), Q (3070cm-1 - 2960cm-1 and 1620cm-1 – 1530cm-1) and I (3070cm-1, 2912cm-1, 1578cm-1 and 760cm-1 – 724cm-1) are disappeared or covered up by β-CD in the FT-IR, due to the inclusion of β-CD. Through the above analysis, it can be preliminarily concluded that P, Q and I molecule entered into the inside cavity of β-CD.

Scheme 1. Molecular structures, sizes and proton numberings of β-CD, P, Q and I.


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Fig. 5. 1H NMR spectra of β-CD, nitrides and the inclusion complexes. NMR spectroscopy is another effective tool for studying the inclusion interaction between β-CD and guest molecule. The molecular structures and sizes of β-CD, P, Q and I are presented in Scheme 1. The molecular size of the three organic nitrogen compounds is a little less than that of β-CD, which ensures the possibility of the formation of inclusion complex (size matching


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effects). The 1H NMR spectra and the corresponding chemical shifts (δ) of P, Q and I in the absence and presence of β-CD are shown in Fig. 5 and Table S1-S3, respectively. The chemical shifts of β-CD and nitride could be changed when nitride entered into the inside cavity of β-CD and they formed the inclusion complex26. It can be seen that a larger chemical shift was occurring for H-3 (∆δ= 0.0007) and H-5 (0.0018) protons located in the hydrophobic inside cavity of β-CD, for OH-2 (0.0012) and OH-3 (0.0009) protons situated in the wide end of β-CD in the inclusion complex of P and β-CD. Meanwhile, for P, the upfield shift of H-c (-0.0067) was relatively larger than those of other protons in inclusion complex. On the contrary, the chemical shifts of H-1, 2, 4, 6 and OH-6 which are on the outer surface and the narrow end of β-CD were slightly affected by P. It was assuredly illustrated that P molecule entered into the nano hydrophobic cavity of β-CD from the wide end of β-CD based on these chemical shifts phenomena of P and β-CD, which leaded the related protons moved large shift. Combining with the above discussion about the ultraviolet absorption spectra and FT-IR spectra, we may deduce that the entire P molecule is inserted into the cavity of β-CD. The most possible structure of the inclusion complex of P and β-CD is shown in Scheme 2. The possible structures of the other two inclusion complexes are also on display in Scheme 2 based on the similar analysis of 1H NMR spectra about Q and I.


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Fig. 6 (A) XRD patterns and (B) SEM photographs (A, β-CD; B, CD:P; C, CD:Q ; D, CD:I) of β-CD and the three inclusion complexes. The structure of the three inclusion complexes was further confirmed by XRD. The XRD patterns of β-CD and the three inclusion complexes are shown in Fig. 6(A). The XRD pattern of β-CD has seven broad peaks at 9.6o, 10.7o, 12.8o, 13.5o, 18.2o, 19.0o and 19.5o. The XRD patterns of the three inclusion complexes are different from that of β-CD, in which the peaks at 4.9o, 6.7o, 7.0o, 11.8o, 17.6o, 18.2o, 21.1o and 23.8o belong to CD:P, the peaks at 6.6o, 11.5o, 12.0o, 15.2o, 16.8o, 20.1o and 24.1o belong to CD:Q and the peaks at 11.7o and 17.7o belong to CD:I. The crystal strengths of β-CD and three inclusion complexes are significantly different. With the


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analysis of the XRD patterns, it could be concluded the formation of the inclusion complexes between β-CD and nitrides. The SEM analysis is very appropriate to observe the surface texture of the substance. Fig. 6(B) shows the scanning electron microscopy (SEM) images of β-CD, CD:P, CD:Q and CD:I. It illustrates that β-CD aggregates are the blocks of about 10µm in diameter, however the three inclusion complexes are the disordered block structures with the size significantly smaller than that of β-CD. These pictures clearly elucidated the different surface morphology of β-CD and the inclusion complexes of β-CD with nitrides.

Fig. 7. Possible configurations in vertical and side view of the inclusion complexes by MD simulation.


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Computational simulations were used to further demonstrate the structures and intermolecular interactions of the three inclusion complexes. It can be seen from MD simulation that two randomly placed β-CD and organic nitride molecules can form the inclusion complex, and two possible configurations of the inclusion complex were obtained. As shown in Fig. 7, one of the configurations (CD:X-1, where X=P, Q or I) is that the nitrogen atom of the nitride is close to the wide end of the cavity of β-CD, and the other (CD:X-2) is that the nitrogen atom is close to the narrow end. Then the energy of CD:X-1 and CD:X-2 was calculated by QM calculation and listed in Table 3. The lower energy configurations of the three inclusion complexes are CD:P-1, CD:Q-2 and CD:I-2, respectively. It means that CD:P-1, CD:Q-2 and CD:I-2 are the favorable configurations of the three inclusion complexes, consistenting with the experimental conclusion. Fig. 8 shows the intermolecular interactions between β-CD and nitride in the favorable inclusion complex configuration. It can be clearly seen that hydrogen bonding (blue electron cloud) and van der Waals interaction (green electron cloud) between organic nitrogen compound and β-CD were the intermolecular interactions existing in the favorable configuration of the three inclusion complexes, which is agree with the result of the ultraviolet absorption spectra.

Table 3. Energy of the two possible configurations of the inclusion complex calculated by QM calculation. Species

E-1 / hartree

E-2 / hartree

ΔE / kcal·mol-1

CD:P

-2.601509

-2.592774

5.48

CD:Q

-2.564949

-2.572851

-4.96

CD:I

-2.548459

-2.553734

-3.31

Notes: E-1 and E-2 are the energy of CD:X-1 and CD:X-2, respectively; ∆E = 627.5094(E-2 - E-1).


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Fig. 8. Intermolecular interactions between β-CD and nitride in the favorable inclusion complex configuration using the IGM method (the blue electron cloud represents the hydrogen bonding, and the green electron cloud represents the van der Waals interaction). Based on the above analysis, we can conclude that β-cyclodextrin has formed inclusion complexes with organic nitrogen compounds, which is the main driving force of the denitrification process.

3.3. Mechanism of denitrification process by molecular inclusion β-CD with a hydrophobic cavity can recognize small organic molecule with suitable molecular size. According to the experiment and computational simulation results and the molecular sizes of β-CD, P, I and Q, a possible denitrification mechanism was proposed (Scheme 2). The molecular size of the nitride is a little smaller than that of the inner cavity of β-CD, which ensures that the nitride can enter the hydrophobic cavity of β-CD (size matching effects). The nitrogen atom with lone pair electrons of the three nitrides easily formed hydrogen bonding with OH proton situated at the end of β-CD. Meanwhile, van der Waals interactions between β-CD and organic nitrogen compound play a role in the inclusion process. On account of the stability of inclusion complex in the neutral environment, organic nitrogen compounds existed in


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simulated oil preferentially diffused to aqueous solution, then quickly formed the inclusion complex with β-CD to achieve the denitrification.

Scheme 2. Mechanism of denitrification by β-cyclodextrin through molecular inclusion

4. Conclusions β-CD aqueous solution is used in the process of denitrification for light oil by molecular inclusion for the first time, and the factors impacting the process were researched. It has been found that β-CD aqueous solution can simultaneously remove the basic nitrogen and non-basic nitrogen compounds from light oil. Temperature was the most important influence factor in the process of denitrification. Among the three types of organic nitrogen compounds, phenylamine is the easiest to remove. Meanwhile, the denitrification mechanism and the structures of the inclusion complexes were studied in detail by ultraviolet absorption spectra, FT-IR, NMR, XRD, SEM and computational simulations, which evidenced the formation of β-CD inclusion complexes. The possible mechanism of denitrification by β-cyclodextrin through molecular inclusion was proposed.


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Acknowledgements The authors thank the National Natural Science Foundation of China (Grant No. 21376265, U1662115) and the Fundamental Research Funds for the Central Universities (Grant No. 17CX06023, 18CX02118A) for financial support.

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Effective Removal of Phenylamine, Quinoline, and Indole from Light

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