Synthesis of Imid-pta

All chemicals and solvents were commercially available and used as received. Thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT), n-heptane, hydrogen peroxide (H2O2, 30 vol.%), acetic acid (CH3COOH), acetonitrile (MeCN), chitosan (low molecular weight with a degree of deacetylation of 75–85%), imidazole (C3H4N2), phosphotungstic acid (H3PW12O40.nH2O), and hydrochloric acid (HCl) were purchased from Sigma–Aldrich. Typical gasoline was used with the following specification: density of 0.7989 g mL-1 at 15 °C and total sulfur content of 0.4980 wt%.

Synthesis of IMID-PTA

The [C3H4N2]3[PW12O40] organic-inorganic hybrid was synthesized according to the method reported by Zonoz et al.24 0.03 g of PTA was dissolved in 1 mL of distilled water. Then, a solution of 0.08 g of IMID in 5 mL of HCl (1 M) was added to the stirred solution of PTA, slowly. The solution was stirred for 2 h at room temperature. The obtained product (referred as IMID-PTA) was filtered off, washed with hot distilled water and then dried in an oven at 80 °C for 2 h.

Immobilization of the IMID-PTA on CS

In a typical synthesis, 0.50 g of CS was dissolved in a solution of 2% CH3COOH to obtain a clear solution. Then, 0.10 g of dried IMID-PTA powder was added to CS solution and sonicated using the ultrasonic bath for 10 min at room temperature. The completely dispersed solution was precipitated by centrifugation (2000 rpm, 40 min). Finally, the formed precipitate (IMID-PTA@CS) was separated by filtration, washed several times with distilled water and dried in an oven at 80 °C for 2 h.

ODS Process of Model Fuel

In a typical experimental procedure, a certain amount of the heterocyclic sulfur compounds (HSCs) such as Th, BT, and DBT were dissolved in n-heptane as a model fuel to evaluate the catalytic performance of IMID-PTA@CS nanocatalyst in the ODS process. The sulfur concentration of each HSC was 500 ppmw. At first, the water bath was heated to the temperature range of 25 to 40 °C. Then, 50 mL of model fuel sample in a closed round-bottom flask equipped with a magnetic stirrer was heated to the reaction temperature. Afterward, 3 mL of CH3COOH:H2O2 in volume ratio of 1:2 and various quantities of prepared nanocatalyst from 0.02 to 0.12 g were added slowly to the reaction vessel. The ODS process was continued under stirring condition (500 rpm). After 1 h, the above mixture was cooled to room temperature and 10 mL of MeCN was added to extract the oxidized HSCs. The formed immiscible liquids (n-heptane and water phases) were separated by a separation funnel and decantation technique. The synthesized nanocatalyst (IMID-PTA@CS) was regenerated from the reaction system using simple filtration and reused in the next run. The total sulfur concentration after oxidation treatment was determined using the X-ray fluorescence spectrometer according to ASTM D4294 and D3227.

ODS Process of Gasoline Fuel

In the same manner as the ODS of the HSCs, after heating the water bath, 50 mL of gasoline fuel was added to the round-bottom flask and its temperature maintained at 35 °C during the experiment. Subsequently, 3 mL of CH3COOH:H2O2 and 0.1 g of IMID-PTA@CS were added into the vessel. The mixture was vigorously stirred by a magnetic stirrer for 1 h. When the oxidation process has been completed, the flask was cooled to room temperature and then 10 mL of polar organic extraction solvent MeCN was used to extract the polar oxidized sulfur compounds. In the separation step, the oil phase was separated by decantation. The total sulfur and mercaptan content in gasoline before and after ODS test were determined by using X-ray fluorescence. The ODS efficiency was expressed by the following eq. 1, where C0 and C correspond to the initial concentration and final concentration of total sulfur content in gasoline, respectively:

Characterization Methods

Fourier transform infrared spectroscopy (FTIR) studies were performed by means of Thermo-Nicolet-iS 10 spectrometer, using KBr disks in the range 400–4000 cm−1. Powder X-ray diffraction (XRD) analysis was collected between 2θ of 10 and 80° at room temperature on a Bruker D8 advance powder X-ray diffractometer with a Cu Kα (λ = 0.154 nm) radiation source. The surface morphologies were examined by scanning electron microscopy (SEM) by LEO 1455 VP. 31P nuclear magnetic resonance (NMR) spectra were recorded on Bruker Ultra Shield 250 MHz. The sonication process was accomplish by Bandelin Sonorex Digitec ultrasonic bath. The total sulfur and mercaptan content in gasoline before and after treatment were determined using X-ray fluorescence with a TANAKA X-ray fluorescence spectrometer RX-360 SH.

Results and discussion

Material Characterizations

In order to confirm the incorporation of the materials, FT-IR spectra were recorded on the pure PTA, IMID, CS, modified IMID-PTA, and IMID-PTA@CS hybrid nanocatalyst. As shown in Figure 1a, the unique characteristic peaks at 776, 895, 955, and 1080 cm-1 are caused by the stretching modes of Keggin-type [PW12O40]3- anions involving edge-sharing W–Oc–W, corner-sharing W–Ob–W, terminal W=Ot, and P−O bond, respectively.25 The absorption bands in the spectrum of IMID are present at 619 cm-1 and 657 cm-1 which assigned to the C2–N1–C5 bending vibrations (Figure 1b). In addition, the peaks around 1447 cm-1 is ascribed to the stretching vibrations of C–N, while the vibration modes of imidazole ring can be observed in the spectral areas of 1540-1573 cm-1.24 As shown in the spectrum of the synthesized IMID-PTA, intensive absorption bands from 767 cm−1 to 1081 cm−1 are clearly observed due to the characteristic vibrations of polyoxo anions (Figure 1c). According to Figure 2b, the appearance of the peaks at 987, 1592, and 1670 cm-1 indicated the vibrations of C–O–C, H–N–H, and C=O in the glycosidic structure of CS polymer, respectively.

The survey spectrum of the IMID-PTA@CS composite is depicted in Figure 2c. Compared to the bulk materials, the observed shifts of the peak locations of nanocatalyst are observed. The existence peaks in the region of 700-1100 cm-1 demonstrated the PTA retained its Keggin structure after immobilization on CS. The characteristic peaks of CS from 987 to 1670 cm-1 cannot be found in Figure 2c, which are overlapped by the absorption bands of IMID. Meanwhile, the peaks at 2850, 2926, and 3331 cm-1 are revealed to the stretching vibrations of alkyl, amino, and hydroxyl groups of CS. Besides, it should also be pointed out that the branched vibration of P–O at 1064 cm-1 confirmed the strong interactions between the anionic PTA and cationic IMID.

The materials were characterized by XRD technique in the scanning range 10° ≤ 2θ ≤ 80°. As shown in Figure 3a, the XRD pattern of bulk PTA is shown the unique sharp and narrow diffraction peaks at 2θ values of 17-23° and 26-30°. The CS with semi-crystalline nature is exhibited refraction at 19.9˚ (Figure 3c).26 The diffraction peaks of IMID-PTA are shown in Figure 3d, which displayed that the presence of characteristic peaks of pure PTA and IMID. From the spectra of IMID-PTA@CS, the appeared broad amorphous peaks in nanocomposite pattern can be suggested that the fine dispersion of IMID-PTA units on the CS surface (Figure 3e). The nanocrystallite size of organic-inorganic hybrid nanocatalyst is estimated to be about 61 nm by means of the Debye–Scherrer equation (eq 2), where the value of D is the size of the crystal, K is a constant equal to 0.89, λ is the wavelength of X-ray (1.5406 Ǻ), β is the full width at half maximum (fwhm), and θ is half of the diffraction angle.

The surface morphology of pure CS, IMI-PTA, and prepared IMID-PTA@CS catalyst was accurately determined using the SEM technique. The Figure 4a presented very smooth layers of CS polymer before immobilization.19 From the SEM image of IMID-PTA hybrid; it is revealed the thread-like structure (Figure 4b). However, after immobilization of modified IMID-PTA clusters on CS, the morphology of the organic-inorganic IMID-PTA@CS nanocomposite is demonstrated that entangled small flat particles (Figure 4c), which is in good agreement with the results of XRD analysis (Figure 3e).

The successful preparation of nanocatalyst was further investigated using 31P NMR spectroscopy. The PTA, IMID-PTA, and IMID-PTA@CS solids were dissolved in dimethyl sulfoxide (DMSO) solvent and then data collected. The spectrum of pure PTA is presented only one sharp peak at -15.567 ppm that is associated to the central phosphorus in the tetrahedral PO4 unit of Keggin-type PTA cluster (Figure 5a).28 As shown in the 31P NMR spectra of IMID-PTA hybrid, the resonance peak of [PW12O40]3- has been shifted to -11.408 ppm which may be resulted from the strong interaction between IMID ionic liquid and PTA (Figure 5b).29 Also, the observed slight shift of the peak position of phosphorus atom (-7.337 ppm) in IMID-PTA@CS spectrum is suggested the good composition of modified IMID-PTA units with CS support.