Degradation of benzothiophene in diesel oil by LaZnAl layered double hydroxide: photocatalytic performance and mechanism
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A new type of photocatalytic La3+–Zn2+–Al3+–MoO42− layered double hydroxide (LDH) material (molar ratio, La/Zn/Al = 1:7:2) was prepared by a complexing agent-assisted homogeneous precipitation technique. The structure of the prepared LDH material was systematically studied. Under UV irradiation, the desulfurization efficiency of the LDH material was 87% in 2 h. For La3+–Zn2+–Al3+–MoO42− LDH material, the introduction of MoO42− increased the interlayer space for promoting the adsorption of benzothiophene (BT), and MoO42− might provide active sites for the oxidation of BT, resulting in the high desulfurization efficiency.
KeywordsLDH Homogeneous precipitation Photocatalytic activities Catalytic oxidation mechanism
In recent years, sulfur compounds produced in the production and use of fuel have gradually become one of the main pollutants which affect air quality. Sulfur compounds mainly exist in diesel oil in the form of derivatives of thiophene and benzothiophene (BT). Therefore, it is an important issue to reduce the content of sulfur-containing compounds such as thiophene, benzothiophene, dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) to very low levels to prevent air pollution during the combustion of commercial diesel or gasoline (Fallah et al. 2014).
Various methods such as adsorption desulfurization (ADS) (Fallah and Azizian 2012; Wang et al. 2011; Miao et al. 2015), biodesulfurization (BDS) (Ismi et al. 2017; Paixão et al. 2016), extraction desulfurization (EDS) (Yu et al. 2017) and oxidative desulfurization (ODS) (Hao et al. 2017) have been reported. Currently, hydrodesulfurization (HDS) is a conventional method for removal of unwanted sulfur compounds in fuel oil (Muzic et al. 2008; Srivastava 2012). The mechanism of HDS reaction was explained by the destruction of the carbon to sulfur bond utilizing hydrogen with a catalyst to form a sulfur-free hydrocarbon and hydrogen sulfide (Song and Ma 2004; Chen et al. 2010; Qiu et al. 2009). However, HDS has a distinct disadvantage, its inefficiency toward heterocyclic sulfur compounds such as BT (Srivastava 2012; Xiao et al. 2008; Hasan et al. 2012).
In this paper, a new LDH material La3+–Zn2+–Al3+–MoO42− was prepared and used as a photocatalyst for oxidation decomposition of BT. Hydrotalcite is one of the new types of catalytic materials which have been studied in recent years (Taylor 1973; Zhang and Guo 2016; Ji and Wu 2017; Huang et al. 2017; Li and Wei 2015; Fan and Yang 2013). However, few people studied the desulfurization treatment of diesel with hydrotalcite catalysts. The desulfurization rate, after the oxidation decomposition of BT with the new LDH, reached 87% under UV light for 2 h. La3+ could bring more positive charges to the main layer, and more MoO42− was adsorbed on the surface of the layer, resulting in an increase in the interlayer space which was beneficial for the adsorption of BT on the surface of the layer. MoO42− might provide active sites for the oxidation of BT. So La3+–Zn2+–Al3+–MoO42− LDHs catalyst showed high desulfurization efficiency and stability.
2.1 Preparation of La3+–Zn2+–Al3+–MoO4 2− LDH
The preparation of the La3+–Zn2+–Al3+–MoO42− LDH was divided into three steps as follows: First, appropriate amounts of La(NO3)3·6H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O and urea were weighed and dissolved in 500 mL of deionized water. The La/Zn/Al molar ratio was 1:7:2. Once they were homogenized, the solution was added into a round flask at a constant temperature of 98 °C. After 8 h of reaction, the solution was filtered and washed 2–3 times with standard ethanol to get a white solid. The product was dried overnight in an oven at 60 °C and subsequently ground into powder to obtain a La3+–Zn2+–Al3+–CO32− LDH.
In the second step, La3+–Zn2+–Al3+–CO32− LDH, sodium chloride and concentrated hydrochloric acid were dissolved in the deionized water. After ultrasonic agitation, the volume was fixed at 500 mL and the solution was bubbled with nitrogen for 30 min. The solution was magnetically stirred for 8 h, then filtered, washed and dried at a room temperature to obtain a La3+–Zn2+–Al3+–Cl− LDH.
The prepared La3+–Zn2+–Al3+–Cl− LDH product was added to 200 ml of a 2.5 mol/L Na2MoO4 solution. The resulting solution was heated to 80 °C, treated with nitrogen for 30 min, and finally magnetically stirred for 12 h. The ion exchange products were washed, filtered and dried to obtain La3+–Zn2+–Al3+–MoO42− LDH material.
2.2 Characterization of catalyst and photocatalytic reaction
The crystal structure of the sample was determined on a SHIMADZU XRD-6000 X ray powder diffractometer with Cu-Kα (λ = 1.5406 Å) at an operating voltage and current of 40 kV and 40 mA, respectively. The SEM images were taken with a Quantum 200 environmental scanning electron microscope operating at an acceleration of 20 kV. The composition and structure of the sample were analyzed on a Bruker FTIR from Germany. The mass ratio of samples to KBr is 1:100. The BET surface area was determined by the multipoint BET method using the adsorption data over a relative pressure (P/P0) range of 0.65–0.95. The pore size distribution was calculated from the desorption branch of the isotherm following the Barrett–Joyner–Halenda (BJH) method, assuming a cylindrical pore model. UV–Vis diffuse reflectance spectra (DRS) were recorded with a UV-2600 Shimadzu spectrophotometer coupled with an integrating sphere (ISR-2600Plus) from 250 to 800 nm to determine the band gap of the materials.
The chemical composition was determined using atomic emission spectrometry in inductively coupled plasma (Agilent ICP-OES 720). In this method, 0.05 g LDH was added into volumetric plastic digestion tubes. Ten milliliters of ultrapure water was added to the tube along with 4 mL 6 mol/L hydrochloric acid. The digestion tubes were uncapped and heated in a hot block for 2 h. The samples were made up to 50 mL with ultrapure water for analysis.
3 Results and discussion
3.1 XRD pattern, FTIR, SEM images and EDS spectra of La3+–Zn2+–Al3+–MoO4 2− LDH
Figure 1b shows the XRD patterns of two materials. The diffraction peaks of the materials were the same as the characteristic of LDH materials, and no other diffraction peaks corresponding to complex phases were observed, which revealed the purity of the samples. The LDH layer spacing increased from 0.738 nm to 0.982 nm. The (003) diffraction peaks of the La3+–Zn2+–Al3+–MoO42− LDH material were symmetrical and very intense, indicating that the material with high crystallinity can be prepared by an ion exchange reaction. In addition, the characteristics of diffraction peaks of the crystal material were intense and sharp, which showed that the product had high crystallinity and crystal structure integrity.
Chemical composition of La3+–Zn2+–Al3+–MoO42− LDH
Molar ratio of La3+/Zn2+/Al3+
5.44/36.65/12.92 = 1.00/6.73/2.37
Theoretical molar ratio of La3+/Zn2+/Al3+
3.2 Comparison of activities of La3+–Zn2+–Al3+–MoO 4 2 and La3+–Zn2+–Al3+–CO3 2− LDHs
According to solid energy band theory and the light catalysis mechanism, the smaller the forbidden band width is, the more likely valence electrons are to be excited (Yang et al. 2005; Nakamura et al. 2004). Therefore, it is also suggested that La3+–Zn2+–Al3+–MoO42− LDHs had better light absorption activity than La3+–Zn2+–Al3+–CO32− LDHs.
3.3 Investigation of the reaction mechanism
A new complexing agent-assisted homogeneous precipitation technique was successfully developed to synthesize La3+–Zn2+–Al3+–MoO42− LDH. By comparing the desulfurization effect of two LDHs under UV irradiation, the La3+–Zn2+–Al3+–MoO42− LDH material had superior desulfurization performance to La3+–Zn2+–Al3+–CO32− LDH. The experimental result indicated that BT was stuck first to the layers of LDH, and then, it was photodecomposed to BTO or BTO2. In conclusion, it was expected that La3+–Zn2+–Al3+–MoO42− LDH has a potential in photocatalysis degradation of organic compounds such as thioether, thiophene, dibenzothiophene and its derivatives.
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