Preparation of HPW@UiO-66 catalyst with defects and its application in oxidative desulfurization

https://doi.org/10.1016/j.cej.2020.127062Get rights and content

Highlights

  • The catalyst of HPW@UiO-66 with defects was prepared by hydrothermal synthesis.

  • The catalyst has high crystallinity, stable structure and dispersed HPW clusters.

  • The catalyst was applied to the oxidative desulfurization of oil-derived fuels.

  • The removal efficiency of DBT only was reduced to 94.70% after 12 cycles of use.

  • The incorporation of HPW and defects leads to the high catalytic activity.

Abstract

Phosphotungstic acid (H3PW12O40, HPW) exhibits a good desulfurization performance towards the oxidative desulfurization (ODS) of oil-derived fuels. However, it presents two drawbacks: a small specific surface area and poor recovery. In this work, HPW was loaded on the support UiO-66 with a stable structure and large specific surface area, and a family of HPW@UiO-66 catalysts was prepared. Based on the structure and property analyses of powder FT-IR, ICP-OES, XRD, SEM, TG and BET, 0.5-HPW@UiO-66 with defects possessed a high crystallinity, regular morphology, stable structure and dispersed HPW clusters. The removal efficiency of DBT reached 100% and was maintained at 94.70% after 12 times of repeated use under the optimal reaction conditions. The results showed that the catalytic performance of HPW@UiO-66 catalyst for model oil desulfurization was improved when the defects existed.

Introduction

The standards for the sulfur content of oil-derived fuels in various countries are becoming more and more stringent. Recently, the standard of sulfur content less than 10 µg/g has been suggested by United States Environmental Protection Agency [1], [2]. However, the oil-derived fuels still contain abundant sulfur compounds, including “highly reactive sulfur compounds”, such as hydrogen sulfide, mercaptans and thioethers, and “refractory sulfur compounds”, such as benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Therefore, deep desulfurization is extremely important to produce oil-derived fuels with ultralow sulfur content [3], [4], [5]. Oxidative desulfurization (ODS) is an environmentally friendly desulfurization technology that can effectively convert refractory sulfur compounds into sulfoxides or sulfones under mild conditions and then remove these species by extraction or adsorption due to their strong polarity [6], [7]. Hence, ODS has usually been employed for the ultradeep desulfurization of oil-derived fuels [8], [9], [10], and an active catalyst has been usually used to accelerate ODS process.

Keggin-type phosphotungstic acid (H3PW12O40, HPW) is a kind of heteropoly acid catalyst with the advantages of a stable structure, a simple preparation method, minimal environmental impact and a high catalytic activity for the desulfurization of oil-derived fuels [11]. However, the small specific surface area and low recovery rate of HPW limit its application [12]. To solve this problem, researchers prepared the catalysts with various heteropoly acids loading on molecular sieves and metal organic framework (MOF) materials, which possessed a large surface area [13], [14].

MOFs are three-dimensional network structures composed of metal atoms and organic ligands. Due to their large specific surface area and adjustable performance, MOFs are widely used in chemical sensing, catalysis and adsorption [15], [16], [17]. It was found that by introducing defects into the MOF, the material had a larger specific surface area and a higher pore volume [18], [19]. Meanwhile, the crystallinity of the material was increased and the crystal morphology was adjusted [20], [21]. Further, the properties of the material were optimized, such as catalytic properties, adsorption properties and electron conductivity [22], [23], [24]. Currently, the most common method for synthesizing MOF with defects is to add a small amount of monocarboxylic acid during the synthesis. The monocarboxylic acid and the ligand compete for the coordination with metal atom, which will affect the equilibrium reaction of the formation of the MOF skeleton, slow down the crystallization rate and form MOF with defects. UiO-66 is a typical MOF material and is composed of Zr ions and terephthalic acid [25], [26], [27]. With the advantage of a large specific surface area, UiO-66 is helpful to employ when loading with various heteropolyacids. Additionally, defects also can be introduced into UiO-66 to optimize its catalytic properties.

In this work, to improve the dispersion of HPW, UiO-66 was utilized as a carrier. A series of catalysts including defected and defect-free HPW@UiO-66 catalysts were prepared by hydrothermal synthesis. The morphology, structure and properties of the catalysts were analyzed by FT-IR, ICP-OES, XRD, SEM, TG and BET. The ODS performance and the recovery of the catalysts were further investigated.

Section snippets

Materials

Benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were obtained from McLean Technology Co., Ltd, Shanghai, China. Phosphotungstic acid (HPW), terephthalic acid, N,N-dimethylformamide (DMF), n-octane, acetonitrile, hydrochloric acid (HCl, 37 wt%), hydrogen peroxide (30 wt%), benzoic acid, methanol and anhydrous ethanol were provided by Kelon Chemical Reagent Company, Chengdu, China. Zirconium chloride (ZrCl4) was purchased from Aladdin Ltd., Shanghai, China.

Effect of HPW loading on the catalyst

To determine the effect of HPW, the structure and morphology of the catalysts were characterized by FT-IR, ICP-OES, XRD and SEM.

In Fig. 1, the peak in the range of 1500–1660 cm−1 belonged to Cdouble bondO in the carboxylate. The peak in the range of 1450–1500 cm−1 was ascribed to the aromatic Cdouble bondC of the organic linker. Meanwhile, the C-O peak in the range of 1250–1450 cm−1 corresponded to the C-OH group of the carboxylic acid in the molecule [28], [29], [30]. Evidently, all of the UiO-66-D and HPW@UiO-66-D

Effect of HPW loading on the removal of DBT

The oxidative desulfurization experiment was carried out under the same reaction conditions. The desulfurization efficiencies of HPW@UiO-66-D with different HPW loadings are shown in Fig. 9. The order of the ability of the catalysts to remove DBT was: 0.5-HPW@UiO-66-D > 1.0-HPW@UiO-66-D > 1.5-HPW@UiO-66-D > 2.0-HPW@UiO-66-D > UiO-66-D. The addition of HPW had a significant improvement in the desulfurization performance of the catalysts. As shown in Table 1, the actual HPW loading of

Conclusion

In summary, a series of HPW@UiO-66 catalysts with defects were successfully prepared by hydrothermal synthesis. From the analyses of the structure, morphology and properties of the catalysts, 0.5-HPW@UiO-66 with defects (0.5-HPW@UiO-66-D) presented the advantages of a regular morphology, good thermal stability, high dispersion of HPW, high specific surface area, and good recycling performance, which were beneficial for oxidative desulfurization to obtain ultraclean fuel. Under the optimal

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Applied Basic Research Programs of Sichuan Province’s Science and Technology Commission Foundation, China [grant numbers: 2016JY0176].

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