Full Length ArticleOxidative desulfurization of dibenzothiophene and simultaneous adsorption of products on BiOBr-C3N4/MCM-41 visible-light-driven core–shell nano photocatalyst
Graphical abstract
Introduction
Although fossil fuels have been playing a dominant role in developing modern life, the combustion of sulfur compounds such as disulfide, thiophene, dibenzothiophene, and their derivatives in fossil fuels releases sulfur oxides (SOx) into the atmosphere which are gradually causing acid rain, acid fog, haze, and other environmental issues [1], [2], [3]. Therefore, essential strategies should be taken to remove the sulfur compounds from fossil fuels.
Hydrodesulfurization (HDS) which is a conventional technique to reduce sulfur components from liquid fuels, operates under high temperature (320–380 °C) and high pressures (3–7 MPa) and needs large amounts of H2 [4], [5]. Nevertheless, this technique is less effective in removing sterically hindered compounds such as dibenzothiophene (DBT) and its derivatives [6]. To achieve ultra-deep desulfurization, other methods have been employed such as extractive desulfurization (EDS) [7], [8], biodesulfurization (BDS) [9], [10], and adsorptive desulfurization [11].
Oxidative desulfurization by a catalyst or photocatalyst is a promising strategy for degradation of sulfuric components in the liquid fuel. While catalytic desulfurization has been indicated promising results in oxidative desulfurization [12], [13], [14], [15], photocatalytic desulfurization because of milder operating conditions, keeping the octane number at high level, high efficiency, and using sustainable energy of sun is suitable particularly in the case of in situ desulfurization of marine fuels on board [16], [17]. In this process, in the presence of a photocatalyst and an oxidant, sulfur compounds are converted to the corresponding polar sulfones. Dipole-dipole interactions facilitate the separation of sulfones by solvent extraction [18], [19], however, the common solvents are expensive and harm the environment. Another alternative with outstanding economic and environmental benefits is adsorption of oxidation products on a porous solid. In this case, combining two stages of oxidation and adsorption in one vessel on the same material is a new development. As a result, the used photocatalyst should have dual functionality both in the oxidation of sulfur compounds and in the adsorption of oxidation products.
With regard to the photocatalytic phases, graphitic nitride carbon (g-C3N4), as a well-known polymer, has attracted much interest due to thermal stability, electronic characteristics, suitable band gap energy (2.7 eV), and low cost [7], [20]. g-C3N4 can be obtained through a simple one-step polymerization of urea, thiourea, melamine, or dicyandiamide [7], [21], [22]. However, this material has a high recombination rate of the electron-hole pairs [20]. Strategies such as morphological control [23], [24], element-doping [25], and creating heterojunction with other semiconductors [26], [27], [28], [29] have been developed so far to overcome this drawback.
Among different semiconductors, bismuth-based ones are widely used for building a heterojunction with g-C3N4 in order to resolve the problem of fast recombination rate. BiOBr as a bismuth oxyholide has a lamellar structure, visible light response, high chemical stability, and low cost. While the heterojunction between BiOBr and g-C3N4 has been used in the photocatalytic treatment of wastewater [30], [31], [32], [33], to the best of our knowledge, this heterojunction has not been used in photocatalytic desulfurization to date.
Porous silica, which has a good background in the adsorption of polar sulfur compounds produced from oxidation, is a suitable choice for our needed support of bifunctional photocatalyst-adsorbent [34].
Due to the proper performance of core–shell structures in bifunctional purposes, in this study, g-C3N4-BiOBr with different BiOBr percentages were loaded on MCM-41 as core. The synthesized core–shell photocatalysts were investigated in the simultaneous oxidative-adsorptive desulfurization of DBT. X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Brunauer–Emmett–Teller (BET), Fourier Transform Infrared (FTIR), Energy Dispersive X-Ray (EDX), UV–Vis Diffuse Reflectance Spectroscopy (UV–Vis DRS), Photoluminescence (PL), and Transmission Electron Microscopy (TEM) analyses were used to characterize the as-prepared samples.
In the following, the effect of operating parameters, such as reaction temperature, the content of H2O2, DBT concentration, and the amount of catalyst, were investigated on the highly active photocatalyst. The stability of this sample also was tested during successive runs.
Section snippets
Materials
Dibenzothiophene (Sigma–Aldrich), hydrogen peroxide (Fisher Scientific, 30% v/v.), n-hexane (Sigma–Aldrich), sodium hydroxide (Sigma–Aldrich), tetraethyl orthosilicate (Sigma–Aldrich), hexadecyltrimethylammonium bromide (CTAB, Merck), ethanol (Merck), melamine (Merck), bismuth (III) nitrate pentahydrate (Sigma–Aldrich), and potassium bromide (Merck) were used as received without any further purification.
Preparation of MCM-41
MCM-41 was synthesized by a sol–gel method [35]. For this, 0.28 g of NaOH and 1 g of CTAB in
Structural characteristics of BiOBr-C3N4/MCM-41 photocatalyst-adsorbents
Fig. 1 shows X-ray diffraction patterns of as-synthesized BiOBr-C3N4/MCM-41 photocatalyst-adsorbents with different percentages of BiOBr (0%, 5%, 10% and 15% wt.). The diffraction peaks at 2θ of 10.9˚, 21.9˚, 25.2˚, 30.0˚, 31.2˚, 37.5˚, 44.6˚, 47.6˚, 50.6˚ and 54.0˚ can be well-indexed for (0 0 1), (0 0 2), (0 1 1), (0 1 2), (1 1 0), (1 1 2), (0 0 4), (0 1 4) and (1 2 1) planes of tetragonal BiOBr (JCPDS 96–901-1784), respectively. A shift at 2θ = 30˚ toward higher 2θs could be attributed to the decrease of unit
CRediT authorship contribution statement
Fahime Abedini: Conceptualization, Methodology, Data curation, Validation, Writing – original draft. Somaiyeh Allahyari: Supervision, Writing – review & editing, Funding acquisition, Project administration. : . Nader Rahemi: Supervision, Funding acquisition.
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.
Acknowledgments
The authors gratefully acknowledge financial support from Sahand University of Technology.
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