Facile Oxidative Desulfurisation of Benzothiophene Using Polyoxometalate H 4 [ α-SiW 12 O 40 ] / Zr Catalyst

A highly active catalyst H4[α-SiW12O40]/Zr based polyoxometalete Keggin type was prepared by wet impregnation method and was characterized by FTIR spectroscopy, X-ray diffractometer, surface textural property by SEM, and analysis of porosity by BET method. H4[α-SiW12O40]/Zr was successfully synthesized and showed uniform properties with block solid structure which was applied as heterogeneous stable catalyst for oxidative desulfurization of benzothiophene under simple and mild condition using H2O2 as oxidant. Facile conversion of benzothiophene to sulfone by using heterogeneus H4[α-SiW12O40]/Zr catalyst up to 99% was observed to show the active catalytically. Keggin H4[α-SiW12O40]/Zr cage structure after reaction was different from fresh catalyst which was indicated by the instability of H4[α-SiW12O40]/Zr under reaction condition. © 2015 BCREC UNDIP. All rights reserved


Introduction
Sulfur compound is well known being present in fuel and also contribute as the source of air pollution.Sulfur gas, SOx, is generated from combustion of various activities that use oil fuel such as vehicle engines and factories.Thus, the low content of sulfur in the oil fuel is needed, and the sulfur content has been agreed worldwide have to be less than 10 ppm [1].Tra-ditional method to decrease sulfur content in oil fuel is hydrodesulfurization process [2].In this method, high hydrogen pressure, high temperature reaction, and molybdenum based catalysts are used [3].These conditions are used in refinery petroleum companies until this time but reaction under mild condition is preferred due to safety conditions.Recently, oxidative desulfurization is still developed in order to decrease sulfur content in fuel.By using oxidative desulfurization process, reaction condition could easily be controlled not only in factory but also in the laboratory under mild condition with no hydrogen requirement [4].In the oxidative desulfurization, sulfur removed by two steps.First step is oxidation of sulfur to sulfone and the next step is removal of these compounds by extraction, adsorption, distillation, or decomposition [5].Catalyst in the oxidative desulfurization is playing a key role, studies in depth for exploring the roles still under investigation until now.
Polyoxometalates are metal-oxygen cluster with various structures.They have unique acid properties and redox activities.Catalytic using Keggin and Dawson types polyoxometalates as catalyst has been reported in various organic reactions including oxidative desulfurization [6].Keggin type of polyoxometalates H3PWxMo12-xO40 (x=1,3,6), Cs2.5H0.5PW12O40, and H3PW12O40 have high catalytic activity for oxidative desulfurization with desulfurization efficiency of 99.79% under hydrogen peroxide [7].In order to improve the catalytic activity in heterogeneous system and stability of the Keggin type polyoxometalatev H4[α-SiW12O40] which has indicated from specific surface area and porosity, we support H4[α-SiW12O40] with zirconium by wet impregnation, thus these material is applied for oxidative desulfurization of benzothiophene as sulfur compound model under mild conditions.Zirconium was selected as the support due to its acid properties and unique physico-chemical properties for organic reactions and transformations [8].Furthermore, this article presented the gradual characterization of H4[α-SiW12O40]/Zr catalyst, application of H4[α-SiW12O40]/Zr as desulfurization catalyst of benzothiophene.

Synthesis and Characterization of H4[α-SiW12O40]/Zr
Polyoxometalate H4[α-SiW12O40] was synthesized and characterized according to the litera-ture [9].Material H4[α-SiW12O40]/Zr was prepared according to Devassy et al. with a slight modification [10].Zirconium(IV) oxide chloride octahydrate was dissolved in ammonia solution and was mixed with H4[α-SiW12O40] solution, the solution was prepared in methanol.Solution mixtures were stirred overnight and then were followed by centrifugation at 15,000 rpm.White solid material of H4[α-SiW12O40]/Zr was collected and washed with acetone and dried at 100 o C for 1 d.Material H4[α-SiW12O40]/Zr was characterized using Shimadzu 8201 PC Shimadzu FTIR spectrophotometer on KBr disc.Xray diffraction was performed using Shimadzu Lab X Type 6000 diffractometer and data were collected over 2θ range of 0-90 o at scanning speed of 1 o min -1 .SEM surface photo was analyzed using JED-2300 SEM analysis station.

Catalytic Oxidative Desulfurization
The catalytic oxidative desulfurization of benzothiophene was carried out in 100 ml Schlenk flask equipped with magnetic bar as follow: 0.1 g of benzothiophene was dissolved in 5 ml of hexane and 0.1 g catalyst and 0.5 ml of H2O2 were added slowly in a flask.The mixture was stirred for 3 h at 40 o C using stirring speed at 100 rpm.The catalytic oxidative desulfurization was monitored using GC with RTX-1 column equipped with FID detector at initial and final reaction.Oxidative desulfurization of benzothiophene was studied deeply through investigation of reaction time, weight of catalyst, and temperature reaction.
Conversion of BT was calculated to evaluate the catalytic activity of H4[α-SiW12O40]/Zr by using Equation (1): in which Co is initial concentration of BT, and Ct is final concentration of BT.
Effect of reaction time for desulfurization of benzothiophene showed conversion up to 100% was reached at 3 h reaction time.Reaction at initial time converts benzothiophene to sulfone with low conversion.By increasing time reaction to 3 h, it can convert all the benzothiophene to sulfone.In relation to that, if the reaction time was extended for one or more hours, it will no further benefit for the process.This is because 3 h reaction has achieved 99 % conver- sion.If the reaction time was extended, H2O2 will be exhausted.
Long et al. [13] reported that oxidative desulfurization of dibenzothiophene achieved its optimum condition while using 0.3 g catalyst.In our research, we found 0.1 g catalyst of H4[α-SiW12O40]/Zr is effective for conversion of benzothiophene as shown in Figure 5.This results showed some similarity with C16PMo(O2)2/agarose catalyst, which use 0.1 g catalyst for desulfurization of dibenzothiophene [14].Catalyst weight of 0.1 g is already met the 1/10 mmol reactant standard which is sufficient for catalyzing desulfurization.Catalyst weight under that value (e.g.0.5 g) is not enough to activate H2O2 while above that value (e.g. more than 0.5 g) is useless (e.g.no longer efficient for the process).In the condition of H2O2 is not active, the catalyst will not facyle.
Thus catalyst weight should be on the precise value.Our result is similar to desulfuration of d i b e n z o t i o p h e n e u s i n g c a t a l y s t C16PMo(O2)2/agarose, this is due to the mol ratio of the catalyst which is at least 1/10 mol or mmol catalyst being used in the catalytic system was in accordance with condition of this experiment.
One of oxidative desulfurization advantages if it is compared to hydrodesulfurization is its reaction only require a mild operating condition, this was indicated by the reaction at low temperature.This research showed temperature at 40 o C is enough to convert benzothiophene to sulfone.Reaction at higher temperature can decrease the % conversion of benzothiophene.Figures 4 and 5 indicated a desulfurization reaction system where 3 h reaction time and 0.1 g catalyst H4[α-SiW12O40]/Zr weight resulted 99% conversion of dibenzotiophene in which H2O2 was optimum to activate the catalyst.
On the other hand, desulfurization of dibenzothiophene using anatase sandwich type polyoxometalate (Bu4N)7H3[P2W18Cd4(Br)2O68-TiO2 was successfully carried out at 60 o C with conversion up to 99% [15].This is probably related to the stability of H2O2 as oxidant.The effect of temperature desulfurization of benzothiophene is shown in Figure 6.
Catalyst of H4[α-SiW12O40]/Zr is catalytically active for oxidative desulfurization of benzothiophene due to the supporting of zirconium atom.In this study we observed that Keggin structure of H4[α-SiW12O40] was almost collapse after reaction.Afterwards, FTIR spectrum in Figure 7 show the unique and specific vibration of H4[α-SiW12O40]/Zr at wavelength 700-1100 cm -1 , it is altered and caused the appearance of an organic component at 2900 cm -1 of which was believed caused by reaction of benzothiophene.These results indicated that the reuse catalyst could not be performed.Although the h e t e r o g e n e o u s c a t a l y t i c u s i n g H4[α-SiW12O40]/Zr catalyst is a limited cycle but the facile desulfurization still can be obtained by using H4[α-SiW12O40]/Zr.

Conclusions
In this article we demonstrated that H4[α-SiW12O40]/Zr catalyst has a high catalytic activity for oxidative desulfurization of ben-zothiophene under mild condition.A complete benzothiophene conversion (99%) was achieved after 3 h reaction at 40 o C. Catalyst of H4[α-SiW12O40]/Zr could be recycled after catalysis cycle with a loss of polyoxometalate H4[α-SiW12O40]/Zr cage.Despite the catalyst lost its structure after the catalysis but it still have a better application compare to the industrial catalyst such as Co/Mo catalysts due to its mild catalyst reaction condition requirement.

Table 2 .
Porosity analysis of H4[α-SiW12O40] and H4[α-SiW12O40]/Zr other hand, material H4[α-SiW12O40]/Zr showed heterogeneous shape with small aggregate formation.Although size and shape are heterogeneous but analysis of textural properties as shown in Table2indicated material H4[α-SiW12O40]/Zr has higher specific surface and pore volume area about two times than that of H4[α-SiW12O40].Through this data,