The oxidation of benzothiophene using the Keggin-type lacunary polytungstophosphate as catalysts in emulsion

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Abstract

A series of emulsion catalysts were successfully synthesized with quarternary ammonium cations and heteropolyanions, and they were characterized by TG/DTA, FTIR, 31P MAS NMR and EPR. The emulsion catalyst with intact Keggin-structure, [C18H37N(CH3)3]H2[PW12O40] (PW12), is inactive for benzothiophene (BT) oxidation with H2O2 as oxidant under atmospheric pressure at 30 °C. Moreover, the metal-substituted catalysts PW11M (M = Ti, Mn, Fe, Co, Ni and Cu) show rather low activity with the conversion less than 15% for BT oxidation. Whereas, the catalyst with mono-lacunary Keggin-structures, [C18H37N(CH3)3]5Na2[PW11O39] (PW11), could completely catalytic oxidize BT into the corresponding sulfone under the same conditions. After careful characterizations of the catalysts, it is found that only PW11 catalyst could effectively transform into the active polyperoxometalates species in the presence of hydrogen peroxide in non-polar solvent.

Graphical abstract

A catalyst [C18H37N(CH3)3]5Na2[PW11O39] (PW11), synthesized with quarternary ammonium cations and mono-lacunary Keggin-structures heteropolyanions, could completely oxidize BT into BT sulfone with H2O2 as oxidant under mild conditions in emulsion.

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Research highlights

▶ A series of emulsion catalysts were successfully synthesized. ▶ The catalyst with mono-lacunary structures shows high activity for BT oxidation. ▶ Metal-substituted catalysts show rather low activity for BT oxidation. ▶ PW11 catalyst could rapidly transform into the active polyperoxometalate.

Introduction

SOx produced from the burning of organic sulfur-containing compounds present in fuel oils not only can cause acid rain, air pollution, and harmfulness to human health, but also can poison irreversibly the three-way catalysts in the tail gas cleanup systems of engines. Therefore, ultra-deep desulfurization of fuel oils has been attracting researchers’ attention and becomes one of the most challenging subjects for petroleum refining industry. Conventional hydrodesulfurization (HDS) is highly efficient in removing thiols, sulfides, and disulfides [1], [2]. However, the capital investment and operating costs of HDS are both rather high due to more severe operating conditions to achieve the ultra-deep desulfurization [3], [4], [5], [6]. As an alternative to the other desulfurization processes, oxidative desulfurization (ODS) [7], [8], [9], [10] has stolen the limelight owing to its advantages like mild reaction conditions, being considered one of the most promising ultra-deep desulfurization processes. In this process, sulfur-containing compounds in fuels are oxidized with H2O2 using zeolitic titanosilicates [11], [12], [13], [14], ionic liquids [15], or metal oxide catalysts [16], [17]. Developing ultra-deep desulfurization catalysts with high activity has been one of the most challenging and important subjects.

Polyoxometalates (POMs) have been attracting much attention in the fields of acid and oxidation catalysis, because the acidic and redox properties of POMs can be designed and tuned at the molecular or atomic level [18], [19], [20], [21], [22], [23], [24], [25], [26]. POMs can be used as effective catalysts for benign oxidations of hydrocarbons with oxidants such as H2O2 or even O2 [19], [20], [21], [22], [23], [24]. For example, tungsten-based POMs have been demonstrated to be effective for the oxidation of olefins, alcohols, glycols, and phenols using H2O2 as oxidants [27], [28], [29], [30], [31], [32], [33], [34], [35], [36].

In recent years, POMs, such as a quaternary ammonium polytungstophosphate catalyst assembled at the interface of the emulsion droplets, have been used for the oxidation of sulfur-containing compounds presented in fuel oils [37], [38], [39], [40]. In our previous work [41], [42], we reported two catalysts, [(C18H37)2N(CH3)2]3[PW12O40] and [C18H37N(CH3)3]4[H2NaPW10O36], assembled in an emulsion system. The former catalyst could selectively catalyze oxidation reaction of dibenzothiophene (DBT) and its derivatives into their corresponding sulfones under mild conditions with H2O2 as oxidant. The latter catalyst could catalytically oxidize benzothiophene (BT), which has much lower reactivity than DBT, to the corresponding sulfone. This work developed another quarternary ammonium polytungstophosphate [C18H37N(CH3)3]5Na2[PW11O39] with lacunary Keggin-structures, which could be used for oxidative desulfurization in emulsion system. It was found that the catalyst exhibit high catalytic activity toward the oxidation of BT under mild conditions; however, the catalytic activity decreased considerably when its lacunary sites were coordinated with the transition metal cations. It is implied that the mono-lacunary POMs could be propitious to the oxidation of BT.

Section snippets

[C18H37N(CH3)3]5Na2[PW11O39] (abbreviated as: PW11)

This compound was prepared consulting the synthesis of [TBA]4H3[PW11O39] in the literature [43], [44], [45]. It is a direct synthesis process, which is starting from an aqueous solution of the required anion, to which was added a solution of octadecyltrimethylammonium chloride instead of TBA bromide. And the simple process is as following: sodium tungstate dehydrate (3.3 g), and disodium hydrogen phosphate (0.33 g) were dissolved in distilled water (80 mL), followed by adjusting pH to 4.8 with 1 M

Characterization of catalysts

In the curves of TG–DTA (Fig. 1), a distinct two-step weight loss process was observed for PW11, with an inflexion at 443 °C. The first weight loss of 34.9% observed in the temperature range from 161 to 443 °C, was due to the decomposition of the alkyl chains of quaternary ammonium cations. The molar ratio of quaternary ammonium cations to heteropolyanions is 4.6:1. And the molar ratio of Na/P is determined to be 1.8:1.1 by XRF. According to the results of elemental analysis, the amount of C, H,

Conclusion

Under mild conditions, mono-lacunary polyoxotungstates catalyst PW11 shows high catalytic activity on the oxidation of BT in emulsion. Once the lacunae are coordinated by the transition metal cations, the catalysts will lose the catalytic activity. It is proved that the well performance of these catalysts could be due to the mono-lacunary Keggin-structures of the tungstophosphoric compounds, which could rapidly transform into the active polyperoxometalate in the presence of hydrogen peroxide.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC Grant No. 20673114), and the State Key Project of China (No. 2006CB202506).

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