Phosphotungstic acid on zirconia-modified silica as catalyst for oxidative desulfurization

Abstract

H₃PW₁₂O₄₀/ZrO₂–SiO₂ catalyst was successfully prepared by immobilizing phosphotungstic acid (H₃PW₁₂O₄₀, HPW) on zirconia-modified silica and employed as a catalyst for oxidative desulfurization (ODS). Under the optimal reaction conditions, the sulfur removal rate from dibenzothiophene was nearly 100%. In addition, the catalyst can be recycled up to 19 times without notable reduction in catalytic activity for the ODS process. The HPW/ZrO₂–SiO₂ catalyst shows excellent catalytic activity for oxidative reactions, due to the strong interaction between the active HPW and ZrO₂–SiO₂ support.

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1. Introduction

With increasing environmental concerns and stringent legal requirements, many social communities are focusing on the desulfurization of fuel.^1–3 Oxidative desulfurization (ODS) processes have undergone major improvements, showing real potential for commercialization. In addition, the ODS technology can be applied at an atmospheric pressure and at low temperatures.

H₂O₂ is generally used as the oxidant in the ODS process, which can be catalyzed by formic acid,⁴ Ti–Si molecular sieves,⁵ ionic liquid catalyst,⁶ and heteropolyacids (HPAs). Liu et al.⁷ reported a novel strategy for the synthesis of a ODS catalyst by encapsulating the inorganic HPAs catalyst within the pores of a metal–organic framework (MOF) structure. The obtained catalyst exhibited an excellent catalytic performance evidenced by a desulfurization rate of up to 100%. Lal et al. reported the HPA/Al₂O₃ catalyzed oxidation of sulfur-containing compounds and showed that when the O/S molar ratio is 2, the desulfurization rate can reach 98.8% in 20 min. An organic hybrid of phosphotungstic acid catalyst for ODS was also reported. This heterogeneous catalyst can be easily recovered and reused by centrifugation, and showed excellent catalytic performance, with a 99.91% efficiency for the removal of dibenzothiophene (DBT).⁸

However, it is well known that the catalysis occurs at the interface between the aqueous and oil phases during the ODS process. As a homogeneous catalyst, HPAs are dissolved in the aqueous phase, which makes catalyst separation after the oxidation reaction difficult. Moreover, the prospect of commercial application is poor, because of its indigent recycling performance. In order to overcome this shortcoming, researches have attempted to load the active HPAs on various solid supports, and the solid catalyst can be easily separated by filtration after the ODS process. It is reported that mesoporous SiO₂,^9,10 TiO₂,¹¹ active carbon,¹² ZrO₂,¹³ Al₂O₃ ¹⁴ and carbon nanotube hybrid¹⁵ can act as the solid support for HPAs during the ODS process. However, it must be noted that these solid catalysts can only be recycled up to five times. The reason for the poor recyclability is that these active HPAs are immobilized on the surface of these solid supports and the interaction between the solid support and the active species is very weak. Therefore, the active HPAs can be easily leached from the support. We have attempted to overcome this problem by functionalizing the solid support by imparting a positive charge on the surface of MCM-41 ¹⁶ or C₃N₄ ¹⁷ support. The resulting phosphotungstic acid (HPW) immobilized on a C₃N₄ as support can be recycled for up to 15 times, and the activity of the regenerated catalyst is still over 98.7%, thereby suggesting the excellent regenerative capability of HPW/C₃N₄ catalyst.

Recently, it was reported that zirconia-modified silica support could provide more acidic sites on the surface of the solid support,¹⁸ which implies that more positive charges can be obtained on the surface of the silica support after the modification of zirconia. Therefore, we inferred that there may be an electrostatic force between the positive charge of the acidic site and negative charge of the active [PW₁₂O₄₀]³⁻, which may further enhance the recyclability of HPAs catalyst. In this study, a ZrO₂–SiO₂ support was synthesized by grafting SiO₂ on zirconium butoxide, and then immobilizing HPW on the support. The desulfurization rate and recyclability of the catalyst in ODS applications was evaluated.

2. Experimental section

2.1. Chemical

All chemicals were used as received without further purification. Dibenzothiophene (DBT, 99%) and 4,6-dimethyldibenzothiophene (4,6-DMDBT, 99%) were procured from J&K Chemical Technology. Benzothiophene (BT, 97%), thiophene (Th, 99%) and zirconium butoxide solution (80 wt%) were obtained from Aladdin. Silica was acquired from Jincheng HuaMeiRuiZe the new Material Co., Ltd. Phosphotungstic acid (HPW) was purchased from Chengdu KeLong Chemical Co., Ltd.

2.2. HPW/ZrO₂–SiO₂ catalyst preparation

About 22 g of commercial SiO₂ was dispersed in 180 mL of toluene and 10 mL of zirconium butoxide was added to the silica suspension with vigorous stirring under N₂ atmosphere. The solution was stirred for 16 h at 105 °C under reflux condensation. The resulting solution was washed with toluene four times at ambient temperature to obtain ZrO₂–SiO₂.

About 2.5 g of HPW was dissolved in 100 mL of acetonitrile. Then, 5 g of ZrO₂–SiO₂ was added to the aqueous HPW. The compound was stirred at 80 °C for 15 h. Finally, the products were washed with acetonitrile and dried overnight in air at 110 °C.

HPW/SiO₂ catalyst was synthesized following the method used for the synthesis of HPW/ZrO₂–SiO₂. SiO₂ alone was used as the support and zirconium butoxide was not added.

2.3. Material characterization

The surface area analysis of the samples was performed using nitrogen adsorption isotherms acquired using a Micromeritics ASAP 2020 system. All chemicals were degassed under vacuum for 6 h at 60 °C. The average pore volume and pore diameter were calculated according to the Barrett–Joyner–Halenda (BJH) method. XRD data were acquired using a Bruker D8 Advance X-ray diffractometer with Cu-Kα irradiation (λ = 1.5406 Å). Fourier transform infrared spectra (FTIR) were acquired using Nicolet AVATAR 360 FTIR spectrometer. The surface morphologies were evaluated using transmission electron microscopy (TEM, Tecnai G2 F20). Temperature-programmed desorption (TPD) was analyzed using AutoChem 2720 instrument. Energy dispersive spectra were acquired using an Energy Dispersive X-ray Detector (EDAX GENESIS system). The XPS analysis was carried out using a Kratos AXIS Ultra DLD spectrometer equipped with monochromatized Al-Kα X-ray source (225 W). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out using a Thermo Scientific TCAP 6000 Series ICP spectrometer.

2.4. Evaluation of catalytic activity of the catalyst

Model oils were obtained by dissolving BT, Th, DBT, and 4,6-DMDBT in n-octane with 100 ppm sulfur content. The ODS experiments were performed in a 100 mL three-necked batch reactor at different temperatures under stirring with reflux condensation. In addition, catalytic oxidation reaction was also carried out using 10 mL of model oil and 30% H₂O₂ solution at 60 °C. The model oil was collected after the reaction and the sulfur content in the model oil was detected using a micro coulometric detector (WK-2D). Finally, the catalysts were recovered through filtration.

3. Results and discussion

3.1. Characterization of catalysts

The N₂ adsorption/desorption isotherms were acquired to identify the configurational characteristics of SiO₂, ZrO₂–SiO₂, HPW/SiO₂, and HPW/ZrO₂–SiO₂ samples, and the results are shown in Fig. 1. It can be seen from Fig. 1, that all adsorption and desorption isotherms display a sharp rise at medium relative pressure P/P₀ of 0.2–0.8. All catalysts exhibited type IV adsorption isotherms.¹⁹ The physicochemical parameters (BET surface area, average pore size, and pore volume) of the samples are summarized in Table S1.† The pore volume and the BET surface area decreased due to the blockage of the pores by HPW.

Fig. 1 Nitrogen adsorption/desorption isotherms of catalysts.

Wide-angle XRD patterns of immobilized HPW on various supports were compared (Fig. 2). The XRD data for Zeolite Socony Mobil-5 (ZSM-5, Joint Committee on Powder Diffraction Standards, reference code 00-037-0359) was used as the reference. The index peaks for ZSM-5 are 2θ = 23.08, 23.48, 23.78, and 24.28 Å, which were observed in all the samples and verify ZSM-5 formation. It is revealed that the characteristic peaks of the crystalline phases for immobilized HPW are absent, which is in agreement with the results reported by Zhang et al.²⁰ who suggested that the HPW is homogeneously distributed onto the support in a noncrystalline manner and the crystalline component is very small.²¹ The low crystallinity is very important for the catalytic activity of the synthesized materials, as it increases the approachability of the active surface and active species.

Fig. 2 XRD patterns of (a) SiO₂, (b) ZrO₂–SiO₂, (c) HPW/SiO₂, (d) HPW/ZrO₂–SiO₂, and (e) HPW.

The presence of HPW was evaluated using FTIR and the results are shown in Fig. 3 (spectra in the range 3500–500 cm⁻¹). The results show that the most crucial vibrational mode changes when grafting occurs. The summary of the primary observed bands and their bond signatures are shown in Table S2.†^22,23 The peak at 967 cm⁻¹ can be assigned to the stretching band of Si–OH. After immobilization of HPW, the characteristic W–O_b–W and W–O_d stretching vibrations are attributed to the Keggin structure. Other vibrations of the Keggin structure, for instance asymmetric P–O stretching, were not observed in the IR spectra, which can be ascribed to the superimposition of the symmetrical stretching vibrations of silica Si–O–Si groups (∼1090 and ∼800 cm⁻¹). It is observed from the spectra of HPW/ZrO₂–SiO₂ samples that the stretching vibration of P–O bands are broadened, which can be attributed to the marginal neutralization of the original Keggin anions arising from contact with the support.

Fig. 3 FT-IR spectra of (a) SiO₂, (b) ZrO₂–SiO₂, (c) HPW/SiO₂ (d) HPW/ZrO₂–SiO₂, and (e) HPW.

TEM images of SiO₂, ZrO₂–SiO₂, HPW/SiO₂, and HPW/ZrO₂–SiO₂ samples are shown in Fig. 4. It can be seen that there are no significant differences in the surface profiles of these catalysts. This result indicates that HPW/SiO₂ and HPW/ZrO₂–SiO₂ catalysts retain the foam-like mesoporous structure of SiO₂ and ZrO₂–SiO₂, which is in agreement with the results from the N₂ adsorption/desorption isotherms and wide angle XRD.

Fig. 4 TEM images of (a) SiO₂, (b) ZrO₂–SiO₂, (c) HPW/SiO₂, and (d) HPW/ZrO₂–SiO₂.

To further investigate the acidity sites of SiO₂ and ZrO₂–SiO₂, we carried out the temperature-programmed desorption of NH₃ (NH₃-TPD) (Fig. 5a). It is obvious that the addition of ZrO₂ changes the peak area and position.²⁴ However, the peak in HPW/SiO₂ and HPW/ZrO₂–SiO₂ show no obvious change because HPW undergoes loss of water crystallization at 200 °C and the Keggin structures decompose at high temperatures (around 600 °C). This phenomenon was further verified by subsequent experiments. XPS spectrum of Zr 3d in HPW/ZrO₂–SiO₂ sample is shown in Fig. 5b. The reduced sample exhibits a splitting of the Zr 3d core level into 3d_5/2 and 3d_3/2 levels with an energy gap of 2.4 eV between them and a relative peak ratio of 1.5. This demonstrates the existence of ZrO₂-like species.²⁵ Decomposition of the spectra results in peaks attributed to the existence of two varieties of zirconium species, referred to as species I, with a low binding energy (BE) of 182.6 eV (Zr_I) and species II, with a higher BE of 184.3 eV (Zr_II). It should be noted that the fraction of Zr_I species in the HPW/ZrO₂–SiO₂ sample is larger compared with Zr_II. The BE of Zr_I is similar to that of Zr⁴⁺ ions in pure zirconia and is comparable to that of stoichiometric ZrO₂ (182.6 eV).²⁶ The BE of the higher energy component corresponds to the formation of a Zr species bound to a more electrophilic species and formation of partially reduced Zr^δ+ sites.²⁵

Fig. 5 (a) NH₃-TPD results of SiO₂, ZrO₂–SiO₂, HPW/SiO₂ and HPW/ZrO₂–SiO₂. (b) XPS deconvolution of Zr 3d of HPW/ZrO₂–SiO₂.

3.2. Catalytic reactivity

Reaction conditions, including the molar ratio of hydrogen peroxide to sulfur (O/S), temperature, reaction time, and catalyst quantity were investigated to determine the optimal ODS conditions for DBT using HPW/SiO₂ and HPW/ZrO₂–SiO₂ as the catalysts. The effect of O/S on the desulfurization rate is shown in Fig. 6a. For the HPW/SiO₂ sample, when the molar ratio of O/S increases up to 8, the ODS catalytic activity for DBT is maximum, and then shows no obvious change with increasing O/S. However, the stoichiometric ratio of O/S = 2 (molar ratio) is lower than the optimal value of O/S = 8.²⁷ This phenomenon can be ascribed to the thermal decomposition of hydrogen peroxide. However, for the HPW/ZrO₂–SiO₂ sample, when the molar ratio of O/S increases up to 2, the sulfur removal rate is nearly 100% and when the O/S molar ratio increases beyond this value, there is no further change. This optimal molar ratio of O/S is similar to the stoichiometric ratio of O/S = 2. A comparison of the catalytic activities of HPW/SiO₂ and HPW/ZrO₂–SiO₂ for ODS is shown in Fig. 6b. The results indicate that the conversion of DBT is affected by the temperature. When the temperature increases from 30 to 70 °C, the sulfur removal rate in case of the HPW/SiO₂ catalyst improves from 92% to 99%. In case of the HPW/ZrO₂–SiO₂ catalyst, the sulfur removal rate from DBT improves from 94% to 100%. Nevertheless, extremely high temperatures lead to the decomposition of H₂O₂ and decrease the concentration of the W(O₂)_n complex.²⁷ Therefore, the optimal temperature for the catalysts used in the ODS process is 60 °C.

Fig. 6 Effect of (a) O/S molar ratio, (b) reaction temperature, (c) reaction time, and (d) catalyst dosage on the sulfur removal of DBT. Experimental conditions: T = 60 °C, O/S = 8, catalyst dosage = 0.1 g/10 mL, t = 150 min.

As shown in Fig. 6c, the conversion of DBT increases along with the enhancement of the reaction time. In case of the HPW/SiO₂ sample, when the reaction time reaches 150 min, DBT is completely oxidized to the corresponding sulfones, whereas for the HPW/ZrO₂–SiO₂ sample the sulfur removal rate nearly reaches 100% after 30 min. The effect of the concentration of the catalysts on the conversion of DBT was also evaluated using 10 mL of model oil with a sulfur content of 100 ppm (Fig. 6d). Increasing the catalyst concentration increases the sulfur removal rate from DBT. Both the catalysts, although no significant change in the conversion of DBT was observed for both the catalysts when the curve saturates after the catalyst quantity reaches 80 mg. This results indicates that 80 mg is sufficient to provide enough active sites for the ODS process.

The desulfurization rate at different temperatures, for the two catalysts, were also evaluated (Fig. 7). The stirring speed was fixed at 1000 rpm, consequently, the effect of mass transfer resistance is maintained a minimum. The oxidation reaction of the sulfur components follows pseudo-first order kinetics, as shown in the following eqn (1)^28,29

ln(c_t/c₀) = −kt (2)

Fig. 7 (a) Pseudo-first-order rate model data at different temperatures of catalyst HPW/SiO₂. (b) Determination of activation energy for the reaction of catalyst HPW/SiO₂. (c) Pseudo-first-order rate model data at different temperatures of catalyst HPW/ZrO₂–SiO₂. (d) Determination of activation energy for the reaction of catalyst HPW/ZrO₂–SiO₂.

In eqn (2), k is the first-order reaction rate constant, which could be obtained from the slope of the linear plot of ln(c₀/c_t) as a function of reaction time, as shown in Fig. 7b and d. We calculated the activation energy of DBT oxidation using eqn (3). As shown in Fig. 7b, the activation energy for the ODS of DBT using the HPW/SiO₂ catalyst is 23.62 kJ mol⁻¹ and for the HPW/ZrO₂–SiO₂ sample is about 18.87 kJ mol⁻¹, which is much lower than the HPW/SiO₂ sample. This result demonstrates that the HPW/ZrO₂–SiO₂ catalyst has higher activity for the ODS process. However, these values are slightly lower than those reported in the literature.³⁰

The sulfur removal rate of various sulfur-containing compounds on HPW/ZrO₂–SiO₂ catalyst surfaces was also investigated, and the results are shown in Fig. 8. It can be seen that these sulfur-containing compounds follow the order: Th < BT < 4,6-DMDBT < DBT. This phenomenon is influenced by the electron density on the sulfur atom. The electron densities on the sulfur atoms of DBT, Th, 4,6-DMDBT and BT are 5.758, 5.696, 5.760, and 5.739, respectively.³¹ Th shows the lowest reactivity and the electron density of sulfur atom in Th is the lowest in these sulfides. The oxidation reaction of the 4,6-DMDBT is far lower than that of the DBT, though there is very minimal difference in the electron densities on the sulfur atoms of DBT and 4,6-DMDBT. This phenomenon may be due to the influence of steric hindrance. Therefore, both electron density and steric hindrance affect the catalytic performances of these sulfur compounds.

Fig. 8 Removal of S-compounds versus reaction time of catalyst HPW/ZrO₂–SiO₂. Experimental conditions: T = 60 °C, O/S = 8, catalyst dosage = 0.1 g/10 mL.

The effect of higher sulfur content on the sulfur removal efficiency from DBT was investigated using a sample with a sulfur content of 500 ppm (Fig. 9). The conversion of DBT increases along with reaction time. In case of the HPW/SiO₂ sample, the sulfur removal rate from DBT reaches 90% when the reaction time reaches 300 min and subsequently there was no further change in the ODS process. In case of the HPW/ZrO₂–SiO₂ sample, the sulfur removal rate reaches 100% after 240 min. This is because the loading of phosphotungstic acid on zirconia-modified silica improves the catalytic activity for the ODS process and imparts higher stability. This indicates that zirconia-modified silica can provide the adequate density of acid sites and can accelerate the ODS process.

Fig. 9 Effect of 500 ppm (a) HPW/SiO₂ and (b) HPW/ZrO₂–SiO₂ on the sulfur removal of DBT.

The recyclability of HPW/SiO₂ and HPW/ZrO₂–SiO₂ samples in the ODS process was compared. At the end of the experiment, the sample was recycled through filtration, washed with methanol, and dried at 80 °C. As shown in Fig. 10, the HPW/ZrO₂–SiO₂ sample can be reused 19 times and the desulfurization rate of the catalyst was still maintained at 100%. However, after 12 rounds of recycling, the desulfurization rate of the HPW/SiO₂ sample decreased from 99% to 79%. This indicates that the presence of ZrO₂ in the solid support can obviously increase the recyclability of the catalyst due to the immobilization method used, which promotes strong interaction with the HPW support.

Fig. 10 Effect of recycling times on DBT conversion. Experimental conditions: T = 60 °C, O/S = 8, catalyst dosage = 0.1 g/10 mL, t = 150 min.

The amount of active tungsten species in the HPW/SiO₂ catalyst after 17 cycles and HPW/ZrO₂–SiO₂ catalyst after 19 cycles was evaluated using ICP-AES. It is seen from Table 1 that the HPW content was 25.87 wt% in fresh HPW/ZrO₂–SiO₂ catalyst. The HPW content decreased from 25.87 to 20.03 wt% after 19 reaction cycles. However, the HPW content was 25.03 wt% in the fresh HPW/SiO₂ catalyst. In case of the HPW/SiO₂ catalyst, the HPW content decreased from 25.03 to 15.05 wt% after 17 cycles. A sharp decrease was observed especially between 13 and 17 cycles, which conform to the results obtained from the experiments on the effect of recycling times on DBT conversion. Therefore, these results demonstrate that the HPW/ZrO₂–SiO₂ catalyst shows better recycling performance in the ODS process.

Table 1 Reusability of HPW/SiO₂ and HPW/ZrO₂–SiO₂ catalysts

HPW/SiO2			HPW/ZrO2–SiO2
Run number	W^a (wt%)	HPW^b (wt%)	Run number	W^a (wt%)	HPW^b (wt%)
a As obtained by ICP-AES.b According to the ICP-AES results, calculating the molecular weight of HPW.
Fresh	19.56%	25.53%	Fresh	19.82%	25.87%
1 cycle	19.05%	24.87%	1 cycle	19.28%	25.17%
3 cycle	18.48%	24.13%	3 cycle	18.79%	24.53%
5 cycle	18.16%	23.71%	5 cycle	18.40%	24.02%
7 cycle	17.69%	23.09%	7 cycle	18.15%	23.69%
9 cycle	17.28%	22.56%	9 cycle	17.66%	23.05%
11 cycle	16.77%	21.89%	11 cycle	17.42%	22.74%
13 cycle	15.74%	20.55%	13 cycle	16.91%	22.08%
15 cycle	14.03%	18.32%	15 cycle	16.26%	21.23%
17 cycle	11.52%	15.05%	17 cycle	15.92%	20.78%
			19 cycle	15.34%	20.03%

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The fresh and spent catalysts were also characterized by FTIR (Fig. 11A). Both the fresh catalysts show almost identical peaks, before and after recovery, which indicates that the structure of the catalysts possess excellent stability. In addition, there is no significant difference in the surface morphology of catalysts (Fig. 11B), which indicates that there is no obvious change in the surface of the catalysts due to the ODS process and recycling experiments.

Fig. 11 (A) FT-IR spectra of (a) fresh HPW/ZrO₂–SiO₂, and (b) spent HPW/ZrO₂–SiO₂ after 19 recycles. (B) TEM images of (a) fresh HPW/ZrO₂–SiO₂, and (b) spent HPW/ZrO₂–SiO₂ after 19 recycles.

4. Conclusions

In summary, the synthesized HPW/ZrO₂–SiO₂ catalyst exhibited remarkable catalytic activity and recyclability in the ODS process. Under the optimal reaction conditions, the sulfur content of the model oil was reduced from 100 to 0 ppm. The HPW/ZrO₂–SiO₂ catalyst could be reused 19 times without a significant reduction in the catalytic activity. Considering its high catalytic activity and excellent recyclability, HPW/ZrO₂–SiO₂ is a promising heterogeneous catalyst for the ODS process.

Acknowledgements

This work was financially supported by young scientific and technological innovation leader of Bingtuan (2015BC001), Innovation project of graduate student in Xinjiang (XJGRI2015042), the Doctor Foundation of Bingtuan (No. 2013BB010), and the Foundation of Young Scientist in Shihezi University (No. 2013ZRKXJQ03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16622a

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