Deep Oxidative Desulfurization of Model fuels by Prepared Nano TiO 2 with Phosphotungstic acid

*Corresponding author  Peer review under the responsibility of University of Baghdad.  https://doi.org/10.31026/j.eng.2018.11.04  2520-3339 © 2018 University of Baghdad. Production and hosting by Journal of Engineering.  ) /4.0/ nc http://creativecommons.org/licenses/by license (  NC This is an open access article under the CC BY  Article received: 3/6/2017  Article accepted: 17/12/2017  41  Chemical, Petroleum and Environmental Engineering


INTRODUCTION
The process of deep desulfurization of fuel has been known around the world.This is because of the air that is polluted by the oxides of sulfur.Strict regulations were recently put by governments around to protect the environment, with an increase to develop deep desulfurization technologies, Yazu, et al., 2001.Hydro-desulfurization (HDS) is used to get rid of the compounds of sulfur.It needs high values of temperature and pressure to be completed.Some sulfur compounds, like dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene (4,6-DMDBT), cannot be easily removed using this method.HDS needs long times of residence so the compounds of sulfur are removed, so the method is difficult to be accomplished and may cause a significant decrease in octane number and increased consumption of hydrogen.So, HDS may lead to high cost, Babich, and Moulijin, 2003.
To increase the activity and efficiency of desulfurization, suitable catalysts are needed, Lorencon, et al., 2014.Investigation and studied, catalysts of titanium and used in the process of oxidative desulfurization (ODS).These catalysts gave good performance in the system of ODS.This is because of producing active radical lead to interaction between the sites of Ti (IV) and H2O2.The materials that contain Ti were cheap and free of pollution, Jin, et al In the present work, the TiO2 catalyst has been synthesized and used in the system of ODS.The preparation of TiO2 is done using titanium isopropoxide.The effect of the temperature of calcination in addition to the various parameters of the reaction is to be evaluated in the model oil desulfurization.

Materials
Analytical grade purity reagents were used and did not need purification; tetra-isopropoxide (TTIP) with a purity of 97 %.Ti [OC3H7]4 is used as a starting material (Sigma-Aldrich company).Stearyl trimethylammonium bromide (STAB), hydrogen peroxide (H2O2) with 30% aqueous solution, n-octane (chemically pure) and phosphotungstic acid (PTA) were supplied by (High Media).Ethanol was supplied by Sigma-Aldrich Company.To prepare all standard solutions, De-ionized water was used.

Preparation of tio2 Nanoparticles
In this work, the solution of the precursor was a mixture of titanium tetra-isopropoxide TTIP (97%), and quaternary ammonium bromide was dissolved in the absolute ethanol.A weight ratio of 1/7 was used with stirring.The solution of PWA with the concentration of 0.15 M was added.Then, stirring of the resulted suspension was done for two hours.The mixing was done at room temperature.20 ml of deionized water was then added in a dropwise manner.An additional 1 hour stirring of the solution was done.Then, a rotatory evaporator was followed to accomplish the evaporation.The washing of the resulted solid product was done using deionized water and ethanol many times.Then it was dried in vacuum at 80 o C overnight.Then calcination was done at 500 o C for 2 hours, Huang, et al., 2006.

Characaterization of tio2 Nanoparticles
X-ray diffraction (XRD) with a Rigaku D/max-RB diffractometer in reflection mode using Cu-Ka radiation was used.A voltage of 40 kV was used to find the composition of the crystal phase and the size of the crystallite.(FTIR) was applied using (IR-Affinity, Shimadzu, Japan) with wave range between (400-4000) cm -1 .The specific surface area (BET) was determined by using Brunauer, Emmett, and Teller (BET) method with Thermo Analyzer/USA.The average particle size of each sample (different crystalline form) was tested using Atomic Force Microscope Device (type Angstrom, Scanning Probe Microscope, Advanced Inc, AA 3000, USA)

Oxidative Desulfurization
The model fuel was made by dissolving DBT in octane solution.A 150 mL glass-reactor with a stirrer was used.A water bath was used with the reactor to maintain a stable temperature of the reaction.A magnetic stirrer with 800 rpm was used.The model fuel and the prepared catalyst were added to the reactor, where preheating is accomplished for 5 min.Then, H2O2 was added to the reactor so that the reaction is initiated.At different times, the samples were collected.They are centrifuged and then analyzed by U-V spectroscopy.Studying the effects of different crystalline forms(amorphous, anatase, and anatase-rutile), stirring rate(200 to 1200 rpm), reaction temperature(40 to 70 o C), catalyst amount(0.1 to 0.5 gm), oxidant content(5:1 to 20:1mol ratio H2O2:DBT) and initial DBT concentration(300 to 3000ppm) on the removal of sulfur compounds from model fuels.

Characterization of the TiO2 Nanoparticles
The patterns of XRD show that different forms of nanoparticles of TiO2 can be produced by different calcination temperatures.Fig. 1 shows a sharp peak at 2Ѳ = 25.2.This is the main peak for the nanoparticles of TiO2 that are calcinated at 500 o C. Anatase form is obtained, and the peak of 2Ѳ is comparatively broad due to the nanosize of crystals.The pattern of XRD for nano-TiO2 that is calcined at 700 o C agrees with the pattern of anataserutile to a great degree.The XRD pattern of nano-TiO2 that is calcinated at 120 o C shows an amorphous structure.The results of BET for amorphous, anatase, and anatase-rutile TiO2 are given in Table 1.The result of (AFM) showed that the average particles size of the anatase phase was (77.92) nm and for anatase-rutile phase (96) nm.The value of the surface area of amorphous TiO2 is 1183 m 2 /g.It is much bigger than that of anatase form and anatase-rutile form of TiO2.Their surface areas are 423.5 and 284.2m 2 /g, respectively.The total pore volume of amorphous TiO2 is almost 0.51 cm 3 /g.The total pore volumes of the anatase form of TiO2 and anatase-rutile form of TiO2 are about 0.456 and 0.12 cm 3 /g, respectively.The contents of Ti, W, O, P, and others inside the calcined TiO2 were measured using XRF.The results are shown in Table 2.Both the tungsten and phosphorus were detected.Neither nitrogen (N) nor bromine (Br) was found.So, this proves that there I no STAB in the calcined TiO2.This suggests that a small quantity of STAB is found in the calcined material.STAB was removed during the process of calcination to a large extent.This led to pore size and volume of large value.
Comparing the structure of TiO2 forms with or without STAB was done.The results of surface area (BET), average pore size, and the total pore volume are given in Table 3. Different interactions between the surfactant and the precursor of titania exist.These include electrostatic, van der Waals interactions, and hydrogen bonding.This is in the case of STAB-paths and apart from the bonds of hydrogen that exist between both PTA and titanium tetra-isopropoxide.When these interactions are balanced to a good extent, there will be a self-assembly mechanism.This will give a wide range of forms and textural properties.STAB performs as the material that directs the structure.Calcination was used to successfully remove STAB.This will lead to a larger pore volume and size.The total pore volumes were from 0.278 to 0.456 cm 3 /g.The TiO2 forms using STAB have a greater surface area of 423.5 m 2 /g compared to 312 m 2 /g for the anatase phase.The FTIR spectrum confirmed the structure of TiO2.No special band of PTA was seen in the TiO2.This could mean that there is no sign of PTA crystals in TiO2 forms.The FTIR spectra recorded for the calcined TiO2 materials with STAB, given in Fig. 2, showing more information about the TiO2 structure.

Effect of the TiO2 Crystalline Form on DBT Oxidation
To ensure the removal of the sulfur compounds in the model fuels, three types of TiO2 were used as the catalysts oxidation.When adding H2O2 to the model fuels a faint yellow product is generated.This is adsorbed on the surface of the catalyst.This result is in agreement consistent with that of Yang, et al., 2007.The resultant TiO2 has different properties since the experiments showed that different TiO2 forms have different effects.In Fig. 3, the experimental results of the catalysts of nano-TiO2 anatase, nano-TiO2 anatase-rutile, and nano-TiO2 amorphous which were used as catalysts in oxidative desulfurization are shown.Nano-TiO2 of anatase form showed a high value of activity for the process of catalytic oxidation of DBT.DBT conversion of as high as 100% was reached at a time of reaction of 90 sec.The catalytic rates presented by anatase-rutile TiO2 and amorphous TiO2 were only 52% and 31.7%,respectively.

Effect of Stirring Rate on Conversion
Fig. 4 shows the effect of stirring rate on the DBT conversion.At 300 rpm rate of stirring, the rate of reaction was slow giving a DBT conversion of only about 44% at 120 sec.As the rate of stirring is increased to 500 rpm, the rate of reaction was enhanced.Under this condition, the rate of conversion of more than 70% was obtained at a time of 120 sec.The rate of the reaction showed no further increase beyond this stirring rate.In fact, higher rates of stirring will decrease the resistance to external diffusion.So, the rate of stirring is fixed at 800 rpm in other experiments.

Effect of Temperature on DBT Oxidation
Temperature is one of the important factors that affect any reaction.The effect of temperature on the oxidation of DBT was investigated and shown in Fig. 5.The conversion of DBT can be enhanced as the temperature of reaction increases from 40 to70 o C. DBT conversion was about 43.5% at the reaction temperature of 40 o C. DBT may be completely removed within a time of 90 sec as the temperature increased to 70 o C. Therefore, temperatures of high value are necessary for the reaction.

Effect of the Initial Concentration on the Conversion
The initial concentration of sulfur compounds is important.This concentration can define what is application range of catalyst.As shown in Fig. 6, the initial DBT concentration increased with decreasing the DBT conversion.It can be seen that when the initial DBT concentration was 300 ppm, the conversion of DBT was highest.When the sulfur concentration initially equals 600 ppm, the conversion is less than that at 300 ppm.The lowest rate of conversion was found at an initial concentration of 3000 ppm.As a result, TiO2 can be used in the deep oxidative desulfurization.

Effect of Catalyst Amount of Conversion
Fig. 7 shows the effect of the amount of catalyst on the conversion of DBT.The conversion was enhanced with the increase in the amount of catalyst from 0.1 g to 0.3 g.The rate of reaction could not be enhanced when the amount of catalyst increased from 0.3 g to 0.4 g.This could be because excess TiO2 causes an ease of agglomeration in the system of reaction.This reduces the active sites of TiO2.

Effect of Oxidant Amount on Conversion
The amount of oxidant largely affects the reaction.Fig. 8 shows the effect of the amount of oxidant on the rate of conversion of DBT.It could be seen that the reaction rate is enhanced at first and then decreased when the molar ratio of H2O2 to DBT increased from 5:1 to 20:1.As the ratio decreased to a value less than 10, the rate of reaction becomes low.As the H2O2 amount is increased, the reaction rate significantly increased.However, more increase in H2O2 reduced the rate of reaction.This may be attributed to: increases in H2O2 give more oxygen radicals and so oxidization of the DBT happens.So the reaction is promoted.In increasing the amount of H2O2 to a certain level, a liquid film on TiO2 may form by adding more H2O2.This affects the adsorption of DBT on active sites.By this and under high H2O2 content, the reaction is limited.

CONCLUSIONS
TiO2 nanomaterials with different forms were successfully prepared using a catalyst of PTA and octadecyl trimethyl ammonium bromide (STAB) as the agent that directs the structure.XRF results show that small amount of STAB existed in the calcined forms.During the calcination, STAB was removed to a great extent.This will lead to the larger value of pore volume and size.The results in the reaction system indicate that nano-TiO2 of anatase form is more selective than anatase-rutile nano-TiO2 form and of amorphous nano-TiO2.The complete DBT (300 ppm) conversion in experiments is obtained within 90 s with nano-TiO2 of anatase form as the catalyst.When the stirring rate was above 600 rpm, it has little effect on the reaction.As the temperature increases from 40 o C to 70 o C, the reaction rate is greatly increased.Different initial concentrations of DBT were studied.Their results showed that the rate of reaction increases with the increase in the initial concentration of DBT.An inadequate amount of the catalyst is not good for the reaction.The results showed that 0.3 g of TiO2 gives the best rate of reaction.The optimum molar ratio of H2O2 to sulfide is 10:1.As a conclusion, anatase nanoparticles of TiO2 are a favorable catalyst for the process of direct deep oxidative desulfurization.Tungeston element in TiO2 represents the active site for the oxidation reaction of DBT.In the presence of hydrogen peroxide, peroxide of W 6+ is formed.It moreover reacts with the compounds of sulfur.This is done by the insertion of oxygen into the S-atom of the molecule of DBT.

Figure 1 .
Figure 1.X-ray powder diffraction patterns of TiO2 calcined under different temperatures.

Figure 2 .Figure 3 .
Figure 2. FTIR-spectra of the nano titanium dioxide prepared using phosphotungstic acid and STAB.

Figure 6 .Figure 7 .
Figure 6.Effect of the initial concentration on the conversion of DBT.Conditions: 70 o C, molar ratio of H2O2 to DBT: 10:1, 0.3 g of TiO2.

Yan, et al., 2013. Fe
-TiO2 as a catalyst in desulfurization and found that 300 ppm of DBT can be removed at pH 0 and 10 wt% Fe by Fe-TiO2; the authors also revealed that Fe and Ti function as active sites in this reaction,

Table 1 .
BET results of different TiO2 crystalline forms

Table 2 .
The chemical composition of synthesized nano TiO2 by XRF