Synthesis of Al-MCM-41 @ Ag / TiO 2 Nanocomposite and Its Photocatalytic Activity for Degradation of Dibenzothiophene

Department of Chemical Engineering, Hanoi University of Mining and Geology, 18 Duc ang, Bac Tu Liem, Hanoit, Vietnam Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam Department of Chemical Engineering, Viet Tri University of Industry, 9 Tien Son Str., Viet Tri City, Vietnam Faculty of Chemistry, Hanoi University of Science, Vietnam National University, 19 Le anh Tong, Hoan Kiem, Hanoi, Vietnam


Introduction
Dibenzothiophene presenting in diesel is one of the main sulfur-containing organic pollutants in fuel oils and is difficult to be removed [1]. is organic pollutant is difficult to be reduced using the conventional hydrodesulfurization (HDS) due to its steric hindrance [2,3].Photocatalytic oxidation is one of the most promising pollution treatments to remove many organic pollutants in water [4] and air steam [5].Due to more stringent environmental regulations, it is critical to develop effective photocatalysts for the removal of sulfur-containing organic compounds from fuel oils through photocatalytic oxidation processes.Titanium dioxide (TiO 2 ) has been well known and was the most widely studied semiconductor photocatalyst due to its low cost, nontoxicity, and high chemical stability [6].However, the semiconductor TiO 2 has a wide band gap (3.0-3.2 eV) that strongly restricted its application because the material only absorbs a small fraction of the solar photons (the UV light occupies about 5% of the total solar energy).Besides, high recombination between electrons (e − ) and holes (h + ) results in reduced photocatalytic efficiency [7,8].
e incorporation and doping of noble metal nanoparticles (e.g., Ag, Au, and Pt) into the crystal lattice of TiO 2 (metal-TiO 2 ) to absorb the abundant visible light due to their surface plasmon resonance (SPR) have been investigated to overcome this limitation in the TiO 2 photocatalyst material [9][10][11][12].Silver has an advantage of having a lower cost compared with gold or platinum.However, the use of metal-TiO 2 powders in the treatment of aqueous organic pollutants has some drawbacks such as difficult recovery and poor adsorption capacity due to its low surface area and agglomeration in suspension [13].erefore, noble metal nanoparticles doping on titania with improved crystallinity, surface area, and surface properties to achieve higher adsorption capacity and photocatalytic activity [14] have been studied extensively such as metal-titania nanoparticles coated on the high surface area supports and thermally stable core materials [15,16].Nanoporous materials such as ordered mesoporous silica [17][18][19], activated carbon [20,21], microporous zeolites [22,23], and metal organic frameworks [24,25] have been widely used as TiO 2 carriers due to their versatile structures and high porosity to achieve more active sites per unit area, consequently, a higher photocatalytic reaction rate.Mesoporous aluminosilicate Al-MCM-41 is one of the most widely used support materials due to its remarkable acidic properties, high thermal and mechanical stability, highly ordered hexagonal structure, and high surface area [26][27][28][29][30]. Recent studies have focused on the use of low-cost raw materials, particularly in their natural forms such as bentonite clay to synthesize high value products.
e inorganic components consisting of silica and alumina were collected by annealing bentonite with sodium hydroxide at a temperature higher than 500 °C to use for the synthesis of Al-MCM-41 [31,32].Herein, the Al-MCM-41 mesoporous material was prepared by a hydrothermal method using the Vietnamese bentonite as Si and Al precursors.en, Ag/TiO 2 nanoparticles were deposited on the surface of the Al-MCM-41 material to improve the dispersion of TiO 2 , and the synthesized Al-MCM-41@Ag/TiO 2 nanocomposites were used as a photocatalyst for the oxidative desulfurization of dibenzothiophene under both UV and visible light irradiation.

Synthesis of Al-MCM-41 from Vietnamese Bentonite.
Firstly, Si and Al precursors were obtained following the protocol described by Ali-dahmane et al. [31].
e Vietnamese bentonite was mixed with sodium hydroxide (NaOH) in bentonite : NaOH weight ratio of 1 : 1.2 and heated at 550 °C for 3h in air.e fused bentonite obtained was cooled, milled, mixed with distilled water with a weight ratio of 1 : 4, and then stirred at room temperature for 24 h.e supernatant was separated by centrifugation to obtain the Si and Al precursors.
In a typical Al-MCM-41 synthesis, 1.2 g CTAB was added to 20 mL distilled water.en, 42 mL of the supernatant was added to the above solution and stirred at room temperature for 6 h at pH of about 9-10, which was adjusted by acetic acid.e crystallization step was carried out at 100 °C in a stainless steel autoclave for 24 hours.e white precipitate was then filtered and washed three times with distilled water and ethanol and dried overnight at 100 °C.Finally, the template CTAB was removed by calcining at 550 °C for 6 h in air.

Synthesis of
Al-MCM-41@nAg/TiO 2 Nanocomposite.Al-MCM-41@nAg/TiO 2 nanocomposite microspheres were synthesized by a sol-gel method.0.1 g of the synthesized Al-MCM-41 was dispersed in 100 mL ethanol via sonication for 1 h.en, 0.2 g F127 and 2 mL distilled water were added to the above solution.e mixture was stirred at 50 °C for 30 minutes.
A TiO 2 precursor (solution A) was prepared by dissolving titanium(IV) isopropoxide (TTIP) in a mixed solvent of ethanol and nitric acid to form a final composition of 1 TTIP : C 2 H 5 OH : 2 H 2 O : 0.2 HNO 3 .Solution B was prepared by adding n% (n � 5, 10, and 15) moles of AgNO 3 into 15 mL ethanol and stirred for 30 minutes.en, solution A was mixed with solution B and stirred vigorously for 30 minutes.e Ag/TiO 2 precursor was added dropwise to the Al-MCM-41 suspension to obtain Al-MCM-41@ nAg/TiO 2 .
e temperature was then increased to 80 °C under refluxing conditions for 6 h.e solid was collected by centrifugation and washed thoroughly with ethanol and dried at 80 °C.Finally, F127 was removed by calcining the obtained solid at 450 °C in air for 5 h.

Oxidative Photodesulfurization of Dibenzothiophene.
Dibenzothiophene was dissolved in n-octane to create a model fuel with known sulfur content in ppm.Next, 20 mL of DBT/n-octane solution was placed in a 500 ml three-neck round-bottom flask containing 0.05 g catalyst.e suspension was stirred for 30 minutes in the dark and then was illuminated with two 15 W UV lamps (UV light) or a 165 W tungsten lamp (visible light) at various temperatures (30 °C, 50 °C, and 70 °C) under refluxing conditions for 5 h.0.5 mL of H 2 O 2 as an oxidative agent was added to the mixture at the reaction temperatures.After certain reaction time, small amounts of the products were taken out, centrifuged, and analyzed by high-performance liquid chromatography (HPLC).en, peak areas were converted to their corresponding concentrations through the standard curves.e percentage of degradation of DBT (η) was calculated according to the initial, C 0 (ppm), and final, C (ppm), concentrations of DBT in the solution following this equation: η � 100 × (C 0 -C)/C 0 .

Characterization. Powder X-ray diffraction (XRD)
patterns were recorded on a D8-Advance Bruker with Cu-Kα radiation (λ � 0.15406 nm) as the X-ray source at a scan rate of 0.3 °-0.6 °min −1 .Transmission electron microscopy (TEM) images were taken by a TEM TECNAI G 2 20 with an accelerating voltage of 200 kV.Scanning electron microscopy (SEM) images were obtained on an S4800-Hitachi.Pore size distributions were calculated from N 2 isotherms using the 2 Journal of Chemistry Barrett-Joyner-Halenda (BJH) model.Energy dispersive X-ray (EDX) spectra were measured on a JED-2300 instrument.e surface electronic states identified through X-ray photoelectron spectroscopy (XPS) were taken on an AXIS ULTRA DLD Shimadzu-Kratos spectrometer using monochromatic X-rays Al-Kα radiation (1486.6 eV).UV-Vis diffuse reflection spectroscopy (UV-Vis/DRS) was performed on a Shimadzu UV2550 spectrophotometer with a BaSO 4 -coated integrating sphere in the wavelength range of 200-800 nm.200), ( 105), (211), and (204) planes of a typical TiO 2 anatase phase, respectively [33].Al-MCM-41@Ag/TiO 2 composites contain obvious diffraction peaks of TiO 2 , but no typical peaks of Ag were observed.is might be due to the low concentration of Ag, or the typical diffraction peaks of Ag were covered by a (004) diffraction peak of the TiO 2 anatase.e presence of Ag in the Al-MCM-41@Ag/TiO 2 composites will be discussed later using EDX and XPS methods.
SEM images of Al-MCM-41 and Al-MCM-41@ 0.1Ag/TiO 2 are shown in Figure 2. e SEM images of Al-MCM-41 revealed irregular spherical particles with a crosslinked network and a particle size of about 120 nm. e average particle size of Al-MCM-41@0 1Ag/TiO 2 (150 nm) was larger than that of Al-MCM-41 nanoparticles, which indicated a possible coverage of TiO 2 nanoparticles having a thickness of about 15 nm on the surface of the Al-MCM-41@0 1Ag/TiO 2 nanocomposite.
Figures 3(a) and 3(b) are the TEM images of Al-MCM-41@0 1Ag/TiO 2 catalyst.e TEM images showed that the nanocomposite possesses ordered mesoporous channels with a slight decrease in the orderly porous structure as compared to the parent Al-MCM-41 material, which is in agreement with the XRD result.e Ag/TiO 2 particles having the size of about 5-15 nm were well-dispersed on the surface and mesopore of the Al-MCM-41 support.
e UV-Vis diffuse reflectance spectra of Al-MCM-41, Al-MCM-41@TiO 2 , and Al-MCM-41@nAg/TiO 2 composites in the range of 250-800 nm are shown in Figure 4. e strong UV absorption spectrum of Al-MCM-41@TiO 2 observed at strong absorption band at wavelengths from 250 to 380 nm, with the sharp absorption edge of about 330 nm, is attributed to the intrinsic band gap absorption of anatase TiO 2 [34].Al-MCM-41@0 1Ag/TiO 2 and Al-MCM-41@0 15Ag/TiO 2 containing a high amount of Ag nanoparticles showed a light absorption band in the visible region with peaks at about 435 nm. is could be related to the localized surface plasmon resonance effect of silver nanoparticles [35], which varies with the Ag content of MCM-41@Ag/TiO 2 nanostructures.e band gaps of the composite samples were calculated from their UV-Vis/DRS spectra based on the method proposed by Kumar et al. [36] using the following equation: where E g is the band gap (eV), ] is the light frequency, A is the absorption constant, h is Planck's constant, and α is the absorption coefficient.e band gap E g values of Al-MCM-41@ TiO 2 , Al-MCM-41@0 05Ag/TiO 2 , Al-MCM-41@0 1Ag/TiO 2 , and Al-MCM-41@0 15Ag/TiO 2 were 3.20, 2.9, 2.83, and 2.81 eV, respectively.ese results indicated that the dispersion of silver nanoparticles on TiO 2 increases the absorption of light in the visible region and narrows their band gaps.In addition, the Ag nanoparticles acted as electron traps to inhibit recombination [37], which may be beneficial for improving the catalytic activity of the catalysts.e EDX analysis of the Al-MCM-41@0 1Ag/TiO 2 sample revealed a Si/Al ratio of about 12. is value confirmed that a fairly high amount of aluminum obtained from the Vietnamese bentonite was incorporated into the structure of silica MCM-41.
e N 2 adsorption-desorption isotherms of Al-MCM-41 and Al-MCM-41@0 1Ag/TiO 2 were typical for type IV, with a hysteresis loop characteristic of mesoporous materials (Figure 6).e calculated specific surface area (S BET ) of Al-MCM-41 was 633 m 2 •g −1 with a pore volume of 0.9 cm 3 •g −1 and of Al-MCM-41@0 1Ag/TiO 2 was 144 m 2 •g −1 with a pore volume of 0.3 cm 3 •g −1 .e specific surface area and mesoporous volume of Al-MCM-41 loading Ag/TiO 2 were lower than those of the pristine Al-MCM-41, which is due to the blocking of some pores by the doping of Ag/TiO 2 nanoparticles.
XPS analysis was carried out to analyze the surface composition and chemical states of Al-MCM-41@ 0.1Ag/TiO 2 (Figure 7(a)).
e spectrum confirmed the presence of O, Si, Al, Ti, and Ag elements without any other impurities.Figure 7 of Ag 3d of Al-MCM-41@0 1Ag/TiO 2 .Two observed energy bands at 367.51 eV and 373.51 eV correspond to Ag 3d 5/2 and Ag 3d 3/2 of metallic silver.ese bands are slightly lower than the bulk Ag metal at 368 eV and 374 eV indicating a strong interaction between Ag and TiO 2 .e splitting energy between Ag 3d 5/2 and Ag 3d 3/2 is 6 eV, which further confirmed the existence of metallic Ag nanoparticles in the synthesized Al-MCM-41@0 1Ag/TiO 2 material [35,38].It is also clearly observed that the presence of two different peaks for Ag 3d 5/2 binding energies at 368.1 eV and 367.6 eV were assigned to metallic silver (Ag 0 ) and silver ions in Ag 2 O (Ag + ), respectively [39].us, partially oxidized metallic Ag nanoparticles were deposited on the TiO 2 surface during the synthesis.Figure 7(c) is the high-resolution XPS spectrum of Ti 2p of Al-MCM-41@0 1Ag/TiO 2 .Ti 2p consists of two peaks at 458.48 eV and 464.18 eV (splitting energy � 5.7 eV) which are in accordance with Ti 2p 3/2 and Ti 2p 1/2 of Ti 4+ in Ag/TiO 2 nanostructures, respectively [35,40].

Photocatalytic Activity of Nanocomposite.
e photocatalytic performance of Al-MCM-41@Ag/TiO 2 with different Ag loadings was evaluated by the oxidative desulfurization of DBT in the model fuel under the visible light source in 30 minutes at 70 °C.e photocatalyst with low Ag loading (0.5wt % Ag/TiO 2 ) exhibited a weak performance (conversion of DBT∼48%) due to the low concentration of active catalytic sites, whereas the highest efficiency was obtained for Al-MCM-41@0 1Ag/TiO 2 (78% DBT conversion).Due to more silver decorated on the surface of TiO 2 for Al-MCM-41@ 0.15Ag/TiO 2 compared with Al-MCM-41@0 1Ag/TiO 2 , although they have almost the same band gaps, the overlapping of the plasmonic field region makes the photocatalytic activity of Al-MCM-41@0 15Ag/TiO 2 mesoporous structure decline (conversion of DBT is 65%).However, overloading of Ag will reduce the amount of available active sites due to the spatial charge repulsion, thereby affecting the photoactivity.Hence, Al-MCM-41@0 1Ag/TiO 2 as a photocatalyst has the best photocatalytic activity due to its suitable plasma resonance band, narrow band gap, and available active sites, which is also consistent with the literature [41][42][43].Al-MCM-41@ 0.1Ag/TiO 2 photocatalyst was then chosen to carry out the next catalytic tests.
For comparison, the degradation of DBT over Al-MCM-41@TiO 2 by the irradiation of UV and visible light was carried out.At 70 °C and after 2 hours, the Al-MCM-41@ TiO 2 photocatalyst degraded only about 40% of DBT under UV light (Figure 8), and almost negligible degradation was observed under visible light (not shown).Compared with Al-MCM-41@TiO 2 sample, Al-MCM-41@0 1Ag/TiO 2 exhibited remarkably enhanced photocatalytic activities under the same visible light and UV irradiation.e photodegradation efficiencies of DBT increased to almost 100% after 2 h, at 70 °C (Figure 8).e improved photocatalytic activity of nanocomposite is due to the strong UV and visible light absorption of Al-MCM-41@0 1Ag/TiO 2 material.e Al-MCM-41 support with a large surface area will increase the ability to disperse the catalytic activity center, which will increase the catalytic activity.
e surface plasmon resonance of silver nanoparticles was in the visible light region, leading to efficiently absorbing visible light irradiation.In addition, the excellent conductivity of silver nanoparticles may improve electron mobility to enhance the transfer of surface charge to the boundary and prevent the recombination of electrons and holes.Al-MCM-41 acted as an adsorbent in the reaction.By using the sol-gel method, the TiO 2 silver-modified silver nanoparticles were homogeneously distributed in ethanol, thereby well dispersed on the pristine material through an intermediate polymer layer of F-127.F-127 played a very important role in TiO 2 -coated denaturing with Ag because without F-127 TiO 2 , nanoparticles are difficult to bond with Al-MCM-41 formed.On the other hand, Al-MCM-41 covered with F-127 to help disperse the TiO 2 nanoparticles onto the surface.e F-127 also protected the hexagonal structure of Al-MCM-41 in the core without being broken down during synthesis.Teng et al. [44] showed that SiO 2 not only played a role in the dispersion of TiO 2 but also protected the Fe 3 O 4 core and prevented the core from dissolving into the solution in the formation of the sandwich structure of Fe 3 O 4 @SiO 2 @TiO 2 .
e effect of temperature on the oxidative desulfurization of DBT during UV and visible irradiations are shown in Figures 9 and 10.It can be seen that as the temperature increased from 30 °C to 70 °C under both UV and visible light, the conversion of DBT increased and reached 100% at 70 °C after 2 hours.e results are in good agreement with literature that increase in the reaction temperature improves the photodegradation efficiency [45].
After 2 h at the reaction temperature slightly above room temperature (30 °C), the deep desulfurization could be achieved with 89% and 81% conversions under UV and visible light irradiation, respectively.
is was attributed to the formation of conduction band (CB) electrons (e − ) and valence band (VB) holes (h + ) under irradiation.is indicates that the visible light absorption of TiO 2 samples was considerably improved by adding Ag and Al-MCM-41 to TiO 2 .
Al-MCM-41 has a uniform pore structure and high surface area, which facilitates the high adsorption of DBT. e Ag nanoparticles were photoexcited to enable the generation of electron and Ag + (h + ) due to the surface plasmon resonance effect, and the photoexcited electrons can be further introduced into the conduction band of TiO 2 (Equation ( 2)) [34].e next possible reaction steps could happen as follows: Ag + visible light ⟶ e + Ag + h +  (2) Species • OH and O 2 •− obtained in the presence of the photocatalyst and H 2 O 2 as the oxidant under irradiation could effectively oxidize DBT to its corresponding sulfone [46].Ag + (h + ) ions were reactive radical species, which were able to oxidize DBT and reduce to metallic silver.us, Ag could be rapidly regenerated, and the Al-MCM-41@ 0.1Ag/TiO 2 composites remained stable.

Conclusions
In summary, the Al-MCM-41@Ag/TiO 2 nanocomposites have been successfully synthesized using the Vietnamese bentonite as Si and Al sources and well characterized by various analytical techniques.Al-MCM-41@Ag/TiO 2 composites exhibited much higher photocatalytic activities for degrading DBT under visible light irradiation than Al-MCM-41@TiO 2 , and Al-MCM-41@0 1Ag/TiO 2 was found to have the best photocatalytic performance.Incorporating Ag and TiO 2 into Al-MCM-41 substrate had positive effects on the photocatalytic activity of the TiO 2 , under both visible light and UV irradiations.At a relatively mild condition of 30 °C, DBT degraded 90% under UV light irradiation and 81% under visible light after 2h.At higher temperature (70 °C), the DBT photooxidative desulfurization efficiency of 100% could be achieved after 2 hours under visible light.e Ag nanoparticle dispersed on Al-MCM-41@TiO 2 .nanocomposites has shown its superiority and the potential as a promising material for the removal of toxic organic pollutants either in the UV or visible light region.