Magnetic mesoporous nanospheres supported phosphomolybdate-based ionic liquid for aerobic oxidative desulfurization of fuel

Abstract

Phosphomolybdate-based ionic liquid [(C₈H₁₇)₃NCH₃]₃PMo₁₂O₄₀ was prepared and supported on a magnetic mesoporous silica (γ-Fe₂O₃@SiO₂@mSiO₂) to obtain a magnetic mesoporous catalyst. The morphology and components of the catalyst were characterized by FT-IR, XRD, XPS, SEM, TEM, nitrogen adsorption-desorption isotherms, and VSM. With air as oxidant, the catalyst showed perfect desulfurization performance in oxidation of dibenzothiophene (DBT). The removal of DBT from model oil could reach 100% within 5 h at 120 °C. After reaction, the catalyst could be separated by a magnet and recycled at least four times without obvious decrease in the catalytic performance.

Introduction

Although liquid fuels derived from petroleum regarded as a secondary non-renewable energy, the efficient and clean utilization of them are still challenge issues in the following several decades. Sulfur compounds in diesel fuel make a negative effect on air due to the combustion product SO_x [1]. Most kinds of the sulfur compounds can be removed by hydrodesulfurization (HDS) but dibenzothiophenes (DBTs) are poor in HDS reactivity [2], [3], [4]. However, increasingly stringent regulations have been promulgated by government to limit the sulfur concentration of fuels to 10 mg kg⁻¹. Hence, harsh reaction conditions, such as higher temperature and pressure, have to be used to remove these refractory sulfides, leading to the loss of cetane number and increasing the cost of HDS process. Therefore, it is worthwhile to exploit supplementary or even alternative energy-efficient and high-efficiency desulfurization technologies to meet the demands of liquid fuels with ultra-low sulfur content [5], [6], [7], [8], [9], [10].

At present, many new desulfurization approaches including adsorption, extraction and oxidation have been developed to produce cleaner fuel [11], [12], [13], [14], [15]. Among them, oxidative desulfurization (ODS) may be one of the most feasible technologies because of its high reactivity to DBTs and mild operating conditions [16], [17], [18], [19], [20]. In the early beginning, the sulfur compounds are firstly oxidized to the sulfones which can be removed easily through extraction or adsorption [21]. Li and his group developed a method of oxidative desulfurization in emulsion system and the sulfur content could be reduced to 0.1 ppm after extraction with a polar solvent [22]. Afterward, a one-pot method, extraction combined with catalytic oxidative desulfurization (ECODS), was proposed by our group, where ionic liquids were used as extractants and polyoxometalates or Fe-based salts as catalysts [23], [24]. It was found that deep desulfurization of fuel can be obtained but the common drawback is that the ionic liquids (ILs) were very expensive to refinery. Heterogenization of the ionic liquids on suitable carriers is a promising solution which can reduce the dosage of ionic liquids and raise the catalytic activity [25], [26], [27].

Recently, solvent-free oxidative desulfurization system with heterogeneous catalysts has been emerged due to easy separation and recyclability of the catalysts. The reported catalysts included TS-1 [28], TiO₂ [29], boron nitride (BN) [30], Fe-based porphyrin complexes [31], polyoxometalate supported SiO₂ and other nanosized catalysts [32], [33], [34]. These catalysts all exhibited great catalytic activities on oxidation of aromatic sulfur compounds. And the most widely used oxidants are hydrogen peroxide and tert-butyl hydroperoxide. However, their high-cost and potential safety problems hinder further practical applications. Molecular oxygen may be a desirable oxidant due to its low cost and abundant air resource. Lü et al. has reported a kind of organic Anderson polyoxomolybdates for aerobic oxidative desulfurization of model diesel [35], [36]. The catalyst showed high catalytic activity in absence of any sacrificial agent. Nanocarbon materials, such as carbon nanotubes [34] and reduced graphene oxide [33], have been also applied to oxidative desulfurization with high performance. However, another major obstacle is the separation of nanoparticles from liquid fuels for recycling. Magnetic separation provides a very convenient approach to solve this problem by employing a magnetic catalyst [37], [38].

Core-shell mesoporous microspheres with different integrate functions have attracted an increasing research interest [38]. In this work, phosphomolybdate-based ionic liquid [(C₈H₁₇)₃NCH₃]₃PMo₁₂O₄₀ was synthesized and supported on the surface of magnetic mesoporous silica. The as-prepared catalyst showed extremely high performance for catalytic oxidative desulfurization with air as an oxidant. Under optimal conditions, DBT and 4-MDBT could be completely removed from model oil. After reaction, the catalyst could be separated by a magnet due to its superparamagnetism.