Heterostructure of vanadium pentoxide and mesoporous SBA-15 derived from natural halloysite for highly efficient photocatalytic oxidative desulphurisation

Integration between conventional semiconductors and porous materials can enhance electron–hole separation, improving photocatalytic activity. Here, we introduce a heterostructure that was successfully constructed between vanadium pentoxide (V2O5) and mesoporous SBA-15 using inexpensive halloysite clay as the silica–aluminium source. The composite material with 40% doped V2O5 shows excellent catalytic performance in the oxidative desulphurisation of dibenzothiophene (conversion of 99% with only a minor change after four-cycle tests). These results suggest the development of new catalysts made from widely available natural minerals that show high stability and can operate in natural light to produce fuel oils with ultra-low sulphur content.


Characterisation techniques
The crystalline phase structure of the prepared materials was determined over the 2-theta range of 0.5-80 degrees (D8 ADVANCE, Bruker, Germany) using Cu K α radiation (λ = 0.154 nm) as the X-ray source at a scan rate of 2° min −1 . FT-IR spectra were recorded on a Bruker TENSOR37 instrument. SEM images were made on a JSM 740, operating at an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy analysis (EDS) was measured on a JED-2300 with gold coating. The Brunauer-Emmett-Teller (BET) surface areas of the samples were evaluated by the N 2 adsorption isotherm at 77 K using a BET Sorptometer (Automated Sorptometer BET 201-A, Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2021 USA). The UV-Vis diffuse reflectance spectra (DRS) were measured with a Shimadzu UV2550 spectrophotometer. The PL spectra of the photocatalysts were detected using a spectrofluorometer Fluorolog FL3-22 JobinYvon-Spex, USA, using a 450W xenon lamp as an excitation source with an excitation wavelength of 400 nm. The surface electronic state was identified through X-ray photoelectron spectroscopy (XPS) performed on an AXISULTRA DLD Shimadzu Kratos spectrometer (Japan) using monochromatic Al K a radiation (1486.6 eV). The Mott-Schottky measurement in dark condition was performed in the potential range from -0.5 to +3 V (V vs. Ag/AgCl). The intensity of sunlight on the surface of the reaction was measured by instrument LX1330B.   D p : Pore diameter and V p : Pore volume calculated from the N 2 desorption data based on the BJH method.

Bandgap energy calculation
The bandgap energy of the photocatalyst can be calculated using the following equation: where α can be determined according to the Kubelka-Munk theory; h is the Planck constant; ν is the frequency of light; A is absorption constant for direct transitions; E g is the value of band gap energy; n depends on the characteristics of the transition in a semiconductor, for example, n = 1, 4 for direct and indirect transitions, respectively [1]. The energy of the bandgap E g can be estimated from the x-axis interference of the tangent line of the graph (αhv) 2 compared to the photon energy (hv

Fig. S9. Plots of (a) pseudo-first-order and (b) pseudo-second-order kinetic models for the degradation of DBT by photocatalytic oxidative desulphurisation at different temperatures
According to the plot shown in Fig. S9a, the first-order kinetic equation of the desulphurisation reaction of 40%V 2 O 5 /Fe-Al-SBA-15 was determined at different temperatures. That is, y = 0.04209x with the value of correlation coefficient R 2 = 0.996, y = 0.05374x with the value of correlation coefficient R 2 = 0.998, y = 0.07123x with the value of correlation coefficient R 2 = 0.997, and y = 0.09892x with the value of correlation coefficient R 2 = 0.998 at temperatures of 40 °C, 50 °C, 60 °C, and 70 °C, respectively.
All the linear graphs agreed with the first-order reaction (R 2 > 0.99 According to the kinetics study, the half-life (t 1/2 ) of the DBT degradation reactions was determined to be 0.274, 0.215, 0.162, and 0.12 h at reaction temperatures of 40 °C, 50 °C, 60 °C, and 70 °C, respectively. These results indicate that as the relatively fast degradation rate of DBT increased, the reaction temperature increased (70 °C), which was consistent with the increases in the reaction temperature and the reaction rate according to the Arrhenius Equation 5 [2] and resulted in a higher conversion of DBT.
Thus, the oxidative photocatalytic desulphurisation for DBT in the n-octane solvent using 40%V 2 O 5 /Fe-Al-SBA-15 was the pseudo-first-order reaction.
In contrast, if the reaction followed the second-order kinetics, the kinetic equation could be expressed as follows: where C t and C o are the concentrations at time t and time zero, respectively, k is the second-order reaction rate constant, t is the irradiation time (h), and n is the reaction order (n = 2 for the secondorder equation). Plots were built to identify the relation between [(1/C) − (1/C o )] versus the irradiation time at different temperatures, as shown in Fig. S9b.
As calculated, the second-order reaction did not seem to be suitable for the sulphur removal reaction. The kinetic data collected at 70 °C yielded a considerably low correlation coefficient with the value of R 2 (0.57). In addition, the values obtained at 40 °C, 50 °C, and 60 °C did not converge, which showed an important fluctuation. These R 2 values were considerably lower in the pseudosecond-order reaction than in the pseudo-first-order kinetic reaction model (R 2 > 0.99), and the confidence value was insignificant for the former; hence, the pseudo-second-order reaction model was not considered for the photocatalytic degradation of DBT under sunlight irradiation.
The dependence of the rate constant k on the reaction temperature was expressed as an Arrhenius equation: where E a is the apparent activation energy, A is the pre-exponential factor, R is the gas constant, and T is the reaction temperature (K). The Arrhenius plot considering the first-order reaction is shown in Fig. S9a. The apparent activation energy (E a ) was calculated from the slope and the intercepts of the Arrhenius plot with a value of 30.52 kJ mol -1 . This value was similar to the previously reported results; the activation energy was 32.5 kJ mol -1 for DBT oxidation with C 5 H 9 NO.SnCl 2 coordinated ionic liquid [3]. Atsushi et al. reported that the E a value for the oxidative desulphurisation of DBT was 32±2 kJ mol -1 using MoO 3 /Al 2 O 3 catalyst [4]. Choi et al. reported that the E a value for the oxidative desulphurisation of DBT was 29 kJ/mol using polyoxometalate/H 2 O 2 catalyst [5]. Huang et al. reported that the E a value for the oxidative desulphurisation of DBT was 28.7 kJ mol -1 using a catalyst of quaternary ammonium bromide and phosphotungstic acid [6]. Alwan et al. reported that the E a value for the oxidative desulphurisation of DBT was 36.26 kJ mol -1 using Fe 2 O 3 /GO catalyst [7].