Jia Chaoyang; Liang Shuyuan; Liu Lei; Shao Xue;Zhang Longli; Wang Fangzhu; Jiang Cuiyu

(1. College of Chemical Engineering, China University of Petroleum, Qingdao 266580 2. College of Science, China University of Petroleum, Qingdao 266580)

Abstract: Seven kinds of Schiff base metal complexes (C1—C7) were synthesized by the reaction of substituted salicylaldehyde Schiff base with cobalt nitrate, nickel nitrate, and copper nitrate, respectively. The oxygen carrying performance, and the catalytic property of complexes for the oxidation of model sulfides 1-hexanethiol, dibutyl sulfide, and 2-methylthiophene along with their influencing factors were explored, while the oxidized products of the model sulfides were also analyzed and characterized. The results show that the catalytic oxidation property of the complexes is determined by their oxygen carrying performance and solubility in n-octane. The oxygen carrying performance of the complexes is mainly affected by the central ion species, the electronic effects, and the spatial effects of the substituents as well as the degree of conjugation. More specifically, the oxygen carrying performance can be improved by enhancing the oxygenation capacity of the central metal ions, increasing the electron donating ability of the ligand substituent, and diminishing the steric hindrance as well as extending the conjugated chain. Complexes C7 were found to be with high oxygen carrying capacity and high solubility in n-octane, which shows the best catalytic oxidation property, and the oxidation conversion rates for 1-hexylthiol, dibutyl sulfide, and 2-methylthiophene are 74.2%, 65.1%, and 22.7%, respectively. Upon using the oxidation catalyst of Schiff base metal complexes, three sulfides can be oxidized by oxygen to form sulfones and sulfoxides.1-Hexanethiol and dibutyl sulfide will continue to be oxidized to form sulfates and sulfites.

Key words: Schiff base metal complexes; 1-hexanethiol; dibutyl sulfide; 2-methylthiophene; catalytic oxidation; structureactivity relationship

1 Introduction

As an important desulfurization method, oxidative desulfurization features higher desulfurization efficiency,milder reaction conditions[1], lower cost, as well as stronger applicability. The key to the technology of oxidative desulfurization is to select an appropriate oxidant and catalytic system. Common oxidant sources are mainly hydrogen peroxide, alkyl peroxides, etc.However, the consumption of peroxide is huge and risky in storage and transportation[2]. With the development of catalysts and the need for environmental protection,the oxidants used in oxidative desulfurization are also developing in line with an environmentally friendly direction. Oxygen is a cheap, readily available, and environmentally friendly oxidant. The key to the use of oxygen for oxidative desulfurization is to select an appropriate catalyst.

As a biomimetic catalyst, the Schiff base metal complexes have been extensively investigated and applied in the catalytic oxidation of benzyl alcohol[3-5]and epoxidation of olefins[6-9]. However, little work has been reported to date on its application in the oxidative desulfurization of fuel oil using oxygen[10-11]. Therefore, the research on the catalytic oxidation of organic sulfides by oxygen with the Schiff base metal complexes catalyst is of theoretical significance and application value.

In this paper, the catalytic oxidation property of salicylaldehyde Schiff base metal complexes on model sulfides was studied. Firstly, salicylaldehydes with different electronic and steric effect substituents were used as raw materials to react with ethylenediamine ando-phenylenediamine for synthesizing five Schiff base ligands (L1—L5). The five ligands were characterized by NMR and IR spectroscopy. Then the synthesized ligands reacted with cobalt nitrate, nickel nitrate, and copper nitrate to prepare seven Schiff base metal complexes(C1—C7). Secondly, the molar oxygen carrying capacity and solubility of the complexes were investigated. The catalytic oxidation property of the complexes on the model sulfides system containing 1-hexanethiol, dibutyl sulfide, and 2-methylthiophene were further investigated by using oxygen as the oxidant, and the oxidation products of the model sulfides were also analyzed. Finally,the factors affecting the catalytic oxidative desulfurization activity of Schiff base metal complexes were discussed.The structure-activity relationship was investigated from the aspects of the types of metal ions, the electron and steric effects of ligands, and the solubility of complexes.The results can lay a foundation for further study and application of Schiff base metal complexes in catalytic oxidative desulfurization of fuel oil.

2 Experimental

2.1 Materials

All chemicals used herein belonged to analytical reagent(AR) grade. Ethanol, cobalt nitrate, copper nitrate,nickel nitrate,n-octane, and nitric acid were obtained from the Sinopharma Chemical Reagent Co., Ltd.Ethylenediamine,o-phenylenediamine, salicylaldehyde,o-vanillin, 5-chlorosalicylaldehyde, 3,5-di-tertbutyl salicylaldehyde, 1-hexanethiol, dibutyl sulfide,2-methylthiophene, andN,N-dimethylformamide were procured from the Aladdin Reagent Company.

2.2 Characterization techniques

The FT-IR spectra were recorded on a Spectrum one spectrometer (Perkin Elmer) operating in a frequency range of 400—4 000 cm-1. The1H-NMR spectra were recorded in an AMX-400 spectrometer (Bruker) using TMS as an internal standard, with chloroform serving as the solvent. The metal content of the complexes was measured using atomic absorption spectroscopy. The AAS spectra were recorded on a contrAA 700 High-Resolution Continuum Source Atomic Absorption Spectrometer(Analytik Jena AG) equipped with a high-focus short-arc xenon lamp.

The simulated oil samples oxidized at different times were analyzed by gas chromatography (GC) (Agilent 7820A,Shanghai). A HP-5 capillary column with a dimension of 30.0 m × 0.25 mm × 0.25 μm was used. The temperature of FID was set at 300 °C with a programmatic heating mode. Nitrogen was used as the carrier gas, with the flow rate being 20 mL/min, and the split ratio equating to 10:1. The oxidation products were analyzed using an ion chromatograph (IC) (ICS-90A, Dionex), equipped with an IonPacAG 22 capillary column (4 mm × 250 mm) and a DS5 conductance detector. The leachate was a mixture of 4.5 mmol/L of sodium carbonate and 1.4 mmol/L of sodium bicarbonate, and the regeneration solution used a 0.1% (φ) sulfuric acid solution. The injection volume was 10 μL and the carrier fluid flow rate was 1.20 mL/min.

2.3 Synthesis

Ligands L1—L5 were prepared by the reaction of salicylaldehyde and their derivatives with ethylenediamine ando-phenylenediamine at a molar ratio of 2:1 and were characterized by NMR and FTIR spectroscopy. Then the ligands reacted with cobalt nitrate,nickel nitrate, and copper nitrate at a molar ratio of 2:1 to synthesize the Schiff base metal complexes C1—C7, and the metal content of the complexes was determined by AAS. The synthetic steps were referred to Reference[12],with the reaction shown in Figure 1.

2.4 Determination of oxygen carrying performance and solubility of complexes

2.4.1 Determination of oxygen carrying performance of complexes

The oxygen carrying experimental device was designed according to Reference[13]. The solvent used in the oxygen carrying experiment wasN,N-dimethylformamide (DMF),and the molar oxygen carrying capacitynwas chosen to evaluate the oxygen carrying performance of complexes.

Figure 1 Synthesis of ligands and complexes

wherenis the molar oxygen carrying capacity, which is the amount of O2absorbed per mole of the complexes(mol/mol);Pis the atmospheric pressure during the experiment (Pa);Vis the oxygen absorption volume(mL);Mis the relative molecular mass of the complexes(g/mol);Ris the ideal gas constant (J/(mol·K));Tis the room temperature measured during the experiment (K),andmis the mass of the complexes (g).

2.4.2 Determination of solubility of complexes in DMF andn-octane

The static equilibrium method[14]was used to determine the solubility of C1—C8 in DMF andn-octane. The solubility of the complexes was calculated as follows.

whereSis the solubility of the complexes (g/mL);m2is the mass of the remaining solute after the evaporation of the solvent from the round-bottom flask (g);m1is the mass of the empty round-bottom flask (g), and a is the volume of supernatant added into the flask (mL).

2.5 Investigation on catalytic oxidation of model sulfides

2.5.1 Catalytic oxidation experiment of simulated oil 0.10 g of complexes, 100.0 mL of simulated oil (n-octane solution containing 0.5% of sulfide) and 30 mL of NaOH solution (30.0%) were added into the flask equipped with a magnetic stirrer and a condensing tube. The temperature was adjusted to 80 °C and oxygen was introduced into the system. The effects of the complexes on the catalytic oxidation of sulfide in simulated oil were investigated when the oxidation time was 15 min, 30 min, 45 min, 60 min, and 75 min, respectively.

2.5.2 Determination of oxidation conversion rate of model sulfides

The simulated oil samples treated at different oxidation times were analyzed by GC. To avoid measurement errors caused by volatilization of sulfur compounds in oil samples, a certain amount of standard materials such as 1-hexylthiol, dibutyl sulfide, and 2-methylthiophene were added to the oil samples. The mass of sulfides in the oil sample after oxidation can be described by the following equation.

wherematis the mass of sulfideaaftertminutes of oxidation (mg);masis the mass of standard materialaadded to the oil sample (mg);Ais the peak area of sulfideabefore the addition of standard materials, andA1is the peak area of sulfideaafter the addition of standard materials.

The oxidation conversion rate of sulfide is quantified as:

whereηatis the oxidation conversion of sulfideaaftertminutes of oxidation (%);mais the mass of sulfideain the oil sample before oxidation (mg), andmatis the mass of sulfideain the oil sample aftertminutes of oxidation(mg).

2.5.3 Analysis of model sulfides oxidation products

The simulated oil system is the same as that used in Section 2.5.1. The complex C1 was used to catalyze the oxidation of three simulated oil systems. The characteristic absorption peaks of S=O in the oil layers were analyzed by FTIR spectrometry before and after oxidation to determine whether sulfones and sulfoxides were formed. And the content of sulfate or sulfite in the water layer was analyzed by IC.

3 Results and Discussion

3.1 Synthesis and characterization

The ligands were synthesized via the nucleophilic addition reaction of substituted salicylaldehyde and diamine, the yield of which ranged from 75% to 90%with colors covering light yellow to orange hues. The key FTIR, and1H NMR data of the ligands and the metal content of the complexes data are shown in Table 1.It can be seen that the ligands were obtained successfully as evidenced by the peak at 1 640 cm-1(attributed to C=N vibration) and the peak of 8.3 (attributed to CH=N). The metal ion content of the complexes indicates that the molar ratio ligand to metal is 1:1, which corresponds to their chemical formula.

Table 1 The FTIR, 1H NMR data of the ligands and the metal content of the complexes

3.2 Oxygen carrying performance and solubility of complexes

3.2.1 Oxygen carrying performance

The molar oxygen carrying capacity (n) and the time (tmax)to reach the maximum oxygen carrying capacity of the seven complexes are shown in Table 2. The molar oxygen carrying capacity of the seven complexes decreases in the following order: C7> C4> C1> C2> C6> C5> C3. The molar oxygen carrying capacity of C7 is 0.376 mol, while that of C3 is only 0.047 mol. Further analysis shows that the oxygen carrying performance of the Schiff base complexes is determined by their structure.

Among the three complexes C1, C2, and C3 with the same Schiff base ligands and different central metal ions,C3 shows the worst oxygen carrying performance. The possible reason is that the uneven electron distribution on the two degenerate egorbits (dx2-y2and dz2) of Cu2+(3d9)leads to the Jahn-Teller effect[15], and C3 will be deformed into flat square complexes with less axial coordination.So, the axial oxygenation ability of C3 with oxygen is weak. Compared with C2, C1 shows a better oxygen carrying performance, because Co2+is easier to provide electrons to form superoxide (O2-) complexes[16]with oxygen.

Although the central metal ion and salicylaldehyde are the same, the oxygen carrying performance of C7 derived fromo-phenylenediamine is better than that of C1 synthesized from ethylenediamine. Because theo-phenylenediamine molecule is more conjugated, the intermediate formed by the combination of C7 with O2is more stable, and the oxygen carrying performance is better. As regards C1, C4, C5, and C6, the oxygen carrying performance varies with the substituents on the aromatic ring of the ligands. The aromatic ring in C4 contains a strong electron donating group -OCH3, which increases the density of the electron cloud of the aromatic ring and enhances the electron cloud density of the central metal ion through the conjugation effect. Therefore, it shows the best oxygen carrying performance. While C5 contains an electron withdrawing group -Cl, which reduces the electron cloud density of the central metal ion, so its oxygen carrying performance is poor. Although C6 contains the electron donating group -C(CH3)3, due to the large steric hindrance effect of the two -C(CH3)3groups, especially the group at position 3, the coplanarity of the complexes followed by its conjugation degree was affected, resulting in its poor oxygen carrying performance.

Table 2 The oxygen carrying performance of C1—C7

3.2.2 Solubility in different solvents

The solubility of complexes C1—C7 in DMF andn-octane at room temperature is shown in Table 3. C1—C7 can be well dissolved in DMF, while their solubility inn-octane is relatively small and differs remarkably.Their solubility inn-octane decreases in the following order: C6> C7> C2> C3> C1> C5> C4. It is necessary to note that the solubility of C1, C2, and C3 is similar,because they were synthesized by the same ligand, while C6 shows the highest solubility inn-octane owing to the two hydrophobic -C(CH3)3radicals on the aromatic ring.In addition, C7 contains more hydrophobic aromatic rings, thus showing high solubility inn-octane too. C5 contains -Cl species with high polarity, which shows a certain hydrophilicity, so its solubility is lower than C1. And C4 shows the lowest solubility inn-octane due to the strong hydrophilicity of -OCH3radical on the aromatic ring. The higher solubility of the complexes inn-octane implies that the complexes are dispersed more uniformly and their effective concentration is higher,which leads to a better mass transfer effect during the oxidation process.

Table 3 The solubility of C1—C7 in DMF/ n-octane

3.3 Investigation of catalytic oxidation property

The oxidation conversion rate of 1-hexylthiol, dibutyl sulphide, and 2-methylthiophene in connection with the oxidation time in the simulated oil is shown in Figures 2— 4.

Figure 2 Conversion rate of 1-hexanethiol catalyzed by different complexes

It can be seen from Figure 2 that with the extension of reaction time, the oxidation conversion rates of 1-hexanthiol obtained during the catalysis in the presence of different complexes show a trend of rising at the beginning,followed by a trend of levelling off. The catalytic oxidation performance of complexes C1—C7 decreases in the following order: C7> C6> C1> C4> C2> C5> C3. Among them, C7 and C6 show better catalytic oxidation reactivity(withηequating to 74.2% and 64.7%, respectively),followed by C1, while C4, C2, C5, and C3 show worse catalytic oxidation reactivity on 1-hexanethiol.

Figure 3 Conversion rate of dibutyl sulfide catalyzed by different complexes

It can be seen from Figure 3 that with the extension of the reaction time, the oxidation conversion rate of dibutyl sulfide obtained during the catalysis with different complexes basically increases at first and then stabilizes.The catalytic oxidation property of complexes C1—C7 decreases in the following order: C7> C6> C1> C4>C5> C3> C2. Among them, C7 and C6 show better catalytic oxidation performance (withηbeing 65.1% and 62.1%, respectively) followed by C1, C4, C5 and C3.Consequently, C2 shows worse performance for catalytic oxidation of dibutyl sulfide.

Figure 4 Conversion rate of 2-methylthiophene catalyzed by different complexes

It can be found from Figure 4 that the complexes show poor performance for catalytic oxidation of thiophene.With the extension of reaction time, the oxidation conversion rate of 2-methylthiophene obtained during the catalysis in the presence of different complexes increases too but rather slowly. Even for C1, which shows a better catalytic oxidation performance, the highest oxidation conversion rate of 2-methylthiophene is only 26.8%.

3.4 Analysis of sulfide oxidation products

Figure 5 (a) FTIR spectra of oil layer before and after oxidation and (b) IC spectrum of water layer after oxidation of simulated oil containing only 1-hexanethiol

The sulfide in the oil layer before and after oxidation was analyzed by FTIR spectroscopy. The FTIR spectra obtained thereby are shown in Figure 5 (a) — Figure 7 (a). The oxidized water layer was analyzed by ion chromatography,with the IC spectra shown in Figure 5 (b) — Figure 7 (b).It can be seen from Figure 5 (a) — Figure 7 (a) that the changes of the FTIR spectra before and after the oxidation of the three sulfide compounds are similar, and the S=O bending vibration peak appears at near 1040 cm-1. It shows that 1-hexanethiol, dibutyl sulfide, and 2-methylthiophene have been oxidized to form sulfones or sulfoxides.

The variation in IC spectra of the water layer after the oxidation of the three sulfides is not the same. In Figure 5 (b), the SO32-or SO42-signal appears at a retention time of 7.31 min (the retention time of SO32-and SO42-is quite close, and chromatographic peaks often overlap), and the mass concentration is 1.49 mg/L. In Figure 6 (b), the signal of SO32-or SO42-also appears at 7.54 min, with a mass concentration equating to 0.17 mg/L. It indicates that 1-hexanethiol and dibutyl sulfide are further oxidized to sulfates and sulfites. However, the concentration of oxidation products of dibutyl sulfide is lower than that of 1-hexanethiol, indicating that dibutyl sulfide is more difficult to be oxidized. In Figure 7 (b), there is no obvious SO32-or SO42-signal, indicating that 2-methylthiophene is only oxidized to sulfones or sulfoxides.

Figure 6 (a) FTIR spectra of oil layer before and after oxidation and (b) IC spectrum of water layer after oxidation of simulated oil containing only dibutyl sulfide

3.5 Factors affecting catalytic oxidation property

The structure of the complexes determines their catalytic oxidation performance. The differences in the structure of different complexes mainly include the central metal ions and the ligands. And the differences in ligand structure can be divided into two aspects, viz.: the substituents on the aromatic ring and the diamine structure.

3.5.1 Effect of central metal ions

Figure 7 (a) FTIR spectra of oil layer before and after oxidation and (b) IC spectrum of water layer after oxidation of simulated oil containing only 2-methylthiophene

Three complexes C1, C2, and C3 with the same ligand are investigated. It can be seen from Figure 8 that the catalytic oxidation performance of the complexes decreases in the following order: C1> C2> C3, which is consistent with the law of their oxygen carrying performance (see Section 3.2.1).Meanwhile, the solubility of the three complexes inn-octane is similar. So, it indicates that the catalytic oxidation activity is mainly affected by their oxygen carrying performance,showing the same results as Cater’s research[17]. Therefore,with the same ligand, the catalytic oxidation activity of the complexes with Co2+is better than that of the complexes with Ni2+. And the complexes with Cu2+show the worst catalytic oxidation performance due to the Jahn-Teller effect of Cu2+.

Figure 8 Effect of the central metal ions on catalytic property of complexes

3.5.2 Effect of substituents on aromatic rings of salicylaldehyde

Four complexes C1, C4, C5, and C6 with the same central metal ion but different ligands are investigated, and the difference of their ligand structure originates from the different substituents located on the aromatic ring of salicylaldehyde. It can be seen from Figure 9 that the catalytic oxidation property of the complexes decreases in the following order: C6> C1> C4> C5, which is not completely consistent with the oxygen carrying performance law (see Section 3.2.1). The reason is that the catalytic oxidation property of the complexes depends not only on the electronic effects of the substituents but also on their solubility in solvents. Although the -OCH3radical has a strongest electron donating capacity, making the oxygen carrying capacity of C4 maximized, its strong hydrophilicity results in a minimum solubility of C4 inn-octane, thus affecting the catalytic property of C4. In spite of the low oxygen carrying performance of C6, its solubility inn-octane is the highest. The higher concentration in the system favors the interaction between the complexes and sulfides, which makes up for the disadvantage of its poor oxygen carrying performance and shows better catalytic oxidation property. In summary, when selecting the raw material salicylaldehyde,both the electronic effects of the substituents and their solubility in the system should be considered.

Figure 9 Effect of substituent groups on catalytic property of complexes

3.5.3 Effect of amine structure

Two complexes C1 and C7, which have the same central metal ion but different structures of the synthetic material diamines, are investigated. The oxidation conversion rates of sulfides are shown in Figure 10. It can be seen that the catalytic oxidation performance of C7 is better than that of C1 due to the different structure of diamines. C7 is prepared fromo-phenylenediamine, and its conjugation degree is higher than that of C1 synthesized from ethylenediamine.The intermediate structure formed by the combination of complexes C7 with O2is more stable, which makes its oxygen carrying performance and solubility inn-octane much higher in comparison with C1, therefore the catalytic oxidation property of C7 is much better.

Figure 10 Effect of different diamines on catalytic property of complexes

4 Conclusions

In this paper, seven Schiff base metal complexes (C1—C7) were synthesized. The factors affecting the oxygen carrying performance and the catalytic activity for oxidation of the model sulfides were investigated. The oxidation products were also analyzed and the following conclusions were drawn:

(1) The oxygen carrying performance of the seven complexes decreases in the following order: C7> C4>C1> C2> C6> C5> C3. The molar oxygen carrying capacity of C7 is 0.376 mol. The oxygen carrying performance can be improved by enhancing the oxygenation capacity of the central metal ions, increasing the electron donating ability of the ligand substituent, and diminishing the steric hindrance as well as extending the conjugated chain.

(2) The catalytic oxidation property of the complexes C1—C7 is affected by their oxygen carrying performance and solubility inn-octane. The higher the oxygen carrying performance and the solubility inn-octane, the better the catalytic oxidation performance of complexes would be.The complexes show different catalytic oxidation property for oxidizing different sulfides, and C7 is the complexes with the best catalytic oxidation property for oxidizing the three sulfides, with the oxidation conversion rates of 1-hexylthiol and dibutyl sulfides equating to 74.2% and 65.1%, respectively, while the oxidation conversion rate of 2-methylthiophene is quite low.

(3) 1-Hexanethiol, dibutyl sulfide, and 2-methylthiophene are oxidized to form sulfones and sulfoxides, and 1-hexylthiol and dibutyl sulfide would continue to be oxidized to form sulfates and sulfites. It shows that 1-hexylthiol and dibutyl sulfide are easily oxidized, while 2-methylthiophene is difficult to be oxidized.

It can be seen that when the Schiff base metal complexes are used as the oxidation catalysts, the organic sulfides such as thiol and thioether can be oxidized effectively by oxygen under moderate reaction conditions, thereby achieving the purpose for the desulfurization of oil products. This research has certain theoretical and practical value.

Acknowledgements:This work was supported by the National Natural Science Foundation of China (No. 21576292).