Zong Mengya; Zhao Yutong; Fan Cunzheng; Wang Danhong

(1. TKL of Metal and Molecule Based Material Chemistry, National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350;2. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023;3. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),College of Chemistry, Nankai University, Tianjin 300071)

Abstract: PTA/UiO-66 composites were successfully synthesized by the hydrothermal method. The results showed that the synergistic effect between phosphotungstic acid (PTA) and UiO-66 could enhance the oxidative desulfurization (ODS)activity. The XRD results proved that UiO-66 retained its structure in the PTA/UiO-66 composites. The SEM results showed that the PTA/UiO-66 catalysts exhibited regular octahedral shape. The Raman spectra revealed that PTA in the composites retained the Keggin structure. The XPS results showed that the electron transfer occurred from Zr-MOF to PTA. The ODS reaction mechanism was discussed. Electrons transfer from Zr-MOF to PTA can promote the generation of active species(·OH) and thus enhance the ODS activity. This explanation can be con firmed by the formation of oxygen vacancy and W0 as revealed by the XPS analysis.

Key words: phosphotungstic acid; Zr-MOF; oxidative desulfurization; synergistic effect; oxygen vacancy

1 Introduction

About 90 percent of the energy in the world is provided by fossil fuels, which often contain some impurities including sulfides such as dibenzothiophene,4,6-dimethyldibenzothiophene, and alkyl-substituted dibenzothiophenes[1]. In the most developed countries,diesel is generally treated to demove sulfur compounds,and the sulfur content in the liquid fuel is reduced to a lower level (＜10 μg/g) to be an ultra-clean diesel[2]. In order to achieve high desulfurization efficiency, people developed many desulfurization methods, such as oxidative desulfurization, extractive desulfurization[3],adsorptive desulfurization[4-5], hydrodesulfurization[6],etc. Among them, the oxidative desulfurization (ODS)[7]technology has attracted extensive interest of researchers because of its advantages in terms of low cost, mild reaction conditions, and high efficiency without using H2[8]. More importantly, the oxidative desulfurization is a green method to convert organic sulfur pollutants into oxidation products, which can then be removed from oils by adsorption or appropriate solvent extraction processes[9].

Metal-Organic Frameworks (MOFs) are a kind of new generation materials with an extended network structure,which are composed of metal ions or clusters connected by multiple organic ligands. MOFs have been developed as a special functional material for many years, and various new structures have been developed and widely used in drug delivery and biomedical applications[10-11], gas storage and absorption[12], sensing and microelectronics[13],as well as catalysis and photocatalysis[14-16]. However,there is one major disadvantage of MOFs which limits the scope of their applications owing to their weak stability[17].

In 2008, Cavka, et al. presented a new kind of zirconiumbased building brick UiO-66 that allows the synthesis of MOFs with unprecedented stability and high specific surface area (Figure 1a)[18]. An inner Zr6O4(OH)4core with eight-coordinated zirconium atoms is bridged by twelve carboxylates (-CO2) to form a Zr6O4(OH)4(CO2)12cluster[19-20]. UiO-66 has acid sites of Zr, and the functional groups of ligands are easy to be modified. Therefore, it is used as a catalyst for some chemical reactions[21-22].The catalytic activity of UiO-66 with Lewis acid can be improved by introducing defects[15].

Polyoxometallates (POMs) consist of heteropolyanions,counter cations, and water. According to research report,POMs show good catalytic activity in ODS[23]. Among all the types of POMs, the Keggin type POMs has the best oxidative desulfurization performance, and has been widely studied[24-25]. Wang designed and synthesized H3PWxMo12-xO40(x= 1, 3, 6) and H3PW12O40[26]. It will enhance the acidity of the Bronsted acid by substituting W for Mo. (Figure 1b).Nevertheless, it is still difficult and challenging for POMs to be applied in catalytic reaction. The small surface area and high solubility of POMs impede their recovery and reusability[27]. Moreover, under the condition of catalytic reaction, the stability of POMs remains low[28].

In this work, we innovatively compounded the Kegginstructured phosphotungstic acid (PTA) with UiO-66 by using the hydrothermal method to obtain a new catalyst for oxidative desulfurization. By using TBHP as the oxidant, the conversion of dibenzothiophene could reach 100% in half an hour and the conversion of 4,6-dimethyldibenzothiophene was more than 50%. The XRD, XPS, SEM, and Raman spectrometric analyses and other analytical methods were used to characterize the catalysts. The results indicate that the composite catalysts retained the basic structure of UiO-66 and the Keggin structure was retained in PTA/UiO-66 composite. Electrons produced during the formation of oxygen vacancy could lead to the reduction of W(VI), which would further enhance the oxidative desulfurization activity.

Figure 1 The Structure of UiO-66 (a) and PTA (b)●—P+5; ●—O-2; ●—W+6

2 Experimental

2.1 Catalyst preparation

ZrCl4(0.8415 g) and terephthalic acid (0.6 g) were dissolved in DMF (150 mL) and HAc (43.2 mL)for 30 min. Then the mixture was transferred into a stainless-steel autoclave equipped with an inner polytetrafluoroethylene tank. After being heated at 120 °C for 24 h, the product was cooled down to room temperature, and collected by centrifuging. The precipitate was washed with methanol and DMF, dried at 80 °C and the pure UiO-66 was obtained. UiO-66(1.008 g) and H3PW12O40·xH2O (0.186 g) were dissolved in ethanol (100 mL) and H2O (100 mL) for 30 min.Then the mixture was transferred into a stainless-steel autoclave equipped with an inner polytetra fluoroethylene tank. After being heated at 100 °C for 12 h, the product was cooled down to room temperature, and collected by centrifuging. The precursor was washed with ethanol and H2O, and dried at 80 °C. After the product was kept overnight, the solid was calcined for 2 h in a tubular furnace under nitrogen blanketing at 200 °C, 300 °C and 400 °C. Finally, PTA/UiO-66 (precursor, 200 °C, 300 °C and 400 °C) was prepared.

2.2 Catalyst characterization

The X-ray diffraction (XRD) spectrum of the catalysts was obtained at 298 K by a Rigaku Mini flex 600 diffratometer equipped with CuKα radiation (λ = 0.154178 nm),with a scanning rate of 10.0(°)/min and a scanning range of 3°—60°.

The X-ray photoelectron spectroscopy (XPS) spectra of catalysts were obtained by using an ESCALAB 250 X-ray instrument of Thermo Scienti fic.

Raman spectroscopic shifts of the catalysts were measured by a SR-500I-A Raman spectrometer at 273 K.The morphology of the catalysts was observed by a JSM-7 800 F scanning electron microscope (SEM).

2.3 Catalytic reaction

A solution containing 500 ppm of DBT and 4,6-DMDBT(simulated oil) was obtained by dissolving 0.0404 g of dibenzothiophene (DBT) and 0.0404 g of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in 80.0 g of n-octane, then 100 μL of tert-butyl peroxide (TBHP) was added to the solution with the O/S molar ratio equating to 3.0. The ODS process is in line with the literature reports.In an experiment of oxidative desulfurization reaction,10.0 g of simulated oil were placed in a 50-mL roundbottomed flask under stirring to keep the temperature at 80 °C, and 0.05 g of catalyst was also added. During the reaction process, the concentration of DBT was analyzed and detected by a GC 2060 chromatograph equipped with a FID every 6 minutes. 0.5 μL of the sample was injected for each single detection, and the activity test was conducted within 120 min after the catalyst was added.

3 Results and Discussion

3.1 Structure and morphology of the catalysts

Figure 2 (a) XRD spectra collected from PTA, UiO-66,precursor and PTA/UiO-66 heated at 200 °C, 300 °C, and 400 °C; (b) Raman spectra for PTA, UiO-66, precursor and PTA/UiO-66 heated at 200 °C—PTA;—UiO-66;—PTA/UiO-66-precursor;—PTA/UiO-66-200;—PTA/UiO-66-300;—PTA/UiO-66-400

Powder X-ray diffraction (XRD) patterns of the catalysts are shown in Figure 2(a), with samples tested in a 2θ range of 3°—60°.[29]The XRD spectra showed that the characteristic peaks of our catalysts were basically the same as that of UiO-66 referred to in literature[30],and the XRD patterns of the composites were almost indistinguishable from that of pure UiO-66. It can be concluded that, because of the low content of PTA, there is no obvious characteristic peak for PTA after loading,which may provide possible evidence for the high dispersion of PTA in Zr-MOF. The XRD data obtained at 200—400 °C showed that the structure of the catalysts was retained at 400 °C despite the decreased peak intensity. In summary, the XRD results showed that the crystal skeleton structure of UiO-66 was retained when we added PTA into the catalysts.As for Raman spectroscopic tests, pure PTA shows two strong signals at 1 006 cm-1and 990 cm-1, and a weak signal at 900 cm-1, respectively, corresponding to the tensile vibration of P-O, W=O and W-Oc-W bonds of the Keggin unit. The Raman spectra in Figure 2(b) clearly show that the PTA in the composite PTA/UiO-66 still retain the Keggin structure, and the characteristic peak for the Keggin structure at about 1 006 cm-1is clearly visible[31-32].It can be inferred from Figure 3(b, c) that the PTA/UiO-66 catalysts exhibited a regular octahedral shape in a size range of 100—200 nm, which was similar to the morphology of the UiO-66 crystal skeleton. This result was consistent with the XRD results, indicating that the crystal skeleton structure was not affected by the loading of PTA. The prepared crystals feature good morphology,uniform size, and high degree of crystallization.

Figure 3 SEM images of UiO-66 (a), PTA/UiO-66-precursor (b) and PTA/UiO-66-200 (c)

3.2 Oxidative desulfurization activity of the catalysts

The experimental details are shown in Section 2.3. As shown in Figure 4, we tested the ODS activity of PTA,UiO-66, and PTA/UiO-66 composites, which were treated at different temperatures (200 °C, 300 °C, and 400 °C) by calculating the conversion of DBT and 4,6-DMDBT. It can be seen from Figure 4(a) that the conversion of PTA and UiO-66 was quite low. Interestingly, the conversion of DBT was significantly improved after the compound treatment to reach 100% in half an hour, when using PTA/UiO-66-precursor, PTA/UiO-66-200, and PTA/UiO-66-300, respectively. In Figure 4(b), the conversion rate of 4,6-DMDBT shows similar results. The prepared composite catalyst has good catalytic activity for converting 4,6-DMDBT, and PTA/UiO-66-precursor shows the highest ODS activity. This result clearly indicates that synergistic effect occurs between PTA and UiO-66, as we have reported the case involving phosphomolybdic acid(PMA) and UiO-66[12]. The PTA/UiO-66 composites show a decreasing ODS activity with an increasing calcination temperature, which can be attributed to the collapse of UiO-66 structure as evidenced by the XRD results.

Figure 4 a) ODS activities of DBT, b) ODS activities of 4,6-DMDBT□—PTA; ○—UiO-66; △—PTA/UiO-66-precursor;▽—PTA/UiO-66-200; ◇—PTA/UiO-66-300; —PTA/UiO-66-400

3.3 Mechanism of increasing the ODS activity

XPS spectra of O1s and W4f of some specific samples are given in Figure 5, and the fitting spectra W4f are presented in Figure 6. As shown in Figure 5(a), compared with pure PTA and UiO-66, the O1s binding energy of the composite catalysts shifts towards high binding energy.Moreover, there is a significant peak at the position of 532 eV (Figure 5a) for the composites, which can be attributed to the existence of oxygen vacancies.

The W4f7/2and W4f5/2of PTA are 35.8 eV and 37.9 eV,respectively[33], and in the PTA/UiO-66 catalysts, a new peak signal appears in 30 eV-32 eV as shown in Figure 5b. It proves that W atoms with original W (Ⅵ) valence in PTA/UiO-66 composites have been partly converted into lower chemical valence. Figure 6 shows the fitting results of W4f, and the binding energy position is consistent with W0for PTA/UiO-66 composites, indicating that the chemical valence of W is partially reduced from W (Ⅵ) to W0in the composites. The XPS results also clearly indicate that the synergistic effect occurred between PTA and UiO-66 by electrons transferring from O atom to W atom.

Figure 5 XPS spectra for the PTA/UiO-66 composites obtained at different temperatures—PTA;—UiO-66;—PTA/UiO-66-precursor;—PTA/UiO-66-200

The above analysis shows that the electron transfer from O to W occurs during the formation of the composite catalyst, and the electron transfer process can be described by the following equation:

Figure 6 The fitting XPS spectra of W4f

We have investigated the reducibility of pure UiO-66 investigated by H2-TPR method.[9]UiO-66 is easy to lose lattice oxygen, resulting in the formation of oxygen vacancies as expressed by Eq. 1. The conversion of lattice oxygen to oxygen vacancy is accompanied by the creation of extra electrons that can be captured by W (Ⅵ) in PTA,which can be confirmed by the formation of W0. At the same time, the process of electron transfer can improve the ODS activity, which will be explained in detail below.The reaction mechanism of oxidative desulfurization process can be roughly expressed in Figure 7. Due to the electron transfer between O and W, the W atom in the catalyst has a high electron density. It can facilitate the break of O-O bonds in TBHP to make it readily generate hydroxyl radicals (·OH), when the oxidation reaction occurs (as shown in step 1). In our previous work[34], we have already proved that ·OH acts as the main active species in ODS reaction. Then, the S atom of DBT attacks the ·OH to produce sulfoxide (as shown in step 2). Judging from the above analysis, we can know that the high electron density of W in the composite is involved in the catalytic reaction, which can improve the catalytic activity of PTA/UiO-66 for DBT transformation. Clearly, PTA and UiO-66 have a synergistic catalytic effect on ODS.

Figure 7 The most probable ODS mechanism of the PTA/UiO-66 catalysts (in the case of DBT,□ represents oxygen vacancy)

4 Conclusions

In this paper, the catalyst PTA/UiO-66 was successfully prepared by hydrothermal synthesis for ODS. On the basis of the characterization results, we can speculate that PTA retains the Keggin structure in the composite catalysts and the composite of PTA and UiO-66 can make electrons transfer from Zr-MOF to W, while W(Ⅵ) atoms are converted into a lower valency state.Thus, the Keggin structure is retained in the PTA/UiO-66 composites and a synergistic effect occurs between PTA and UiO-66. Furthermore, the synergistic effect of W and O can promote the generation of active species (·OH), and then can enhance the oxidative desulfurization activity. This work finds a new method for improving the activity of ODS catalyst by using PTA/MOFs catalysts. More detailed work is needed to identify speci fic mechanisms.

Acknowlegments: This work was supported by the Natural Science Foundation of Tianjin (19JCTPJC46300).