The possibility of using an IL to enhance WO3 dispersion on an inorganic support, ensuring the formation of composites with a very large number of exposed catalytic sites, is reported. In addition, in some cases the IL was demonstrated to be able to direct the morphology of WO3 particles.
WO3 and Ionic Liquids in Fuel Desulfurization
One of the main problems in fuel combustion (beside CO2 formation) is the production of polluting and toxic gases, such as SO2 and SO3. In particular, naphtha shows the highest sulfur and olefine content, with a high octane number. The target of the desulfurization process is thus the elimination of the highest possible sulfur amount, while preserving a high olefine content (maintaining a high octane rating), in order to have good fuel performances during combustion, lowering to a minimum level the emissions of sulfurated compounds.
Of the organic sulfur compounds present in petroleum derivatives, thiols, sulfides, disulfides and thiophenes are among the most important ().
Regarding the available methods to carry out liquid fuel desulfurization, hydrodesulfurization (HDS) and oxidative desulfurization (ODS) are among the most used.
WO3 and Ionic Liquids in Oxidative Desulfurization (ODS)
As previously said, thiophene derivatives are less prone to HDS reaction. In order to improve thiophenes abatement, while maintaining acceptable process costs, oxidative desulfurization (ODS) can be considered a promising method, due to its simplicity and high efficiency [
[1],
[5],
[6]]. ODS is the chemical oxidation of sulfur compounds in liquid fuels (using as an example H
2O
2 as oxidant), yielding products which can be easily removed from the reaction mixture using a non-miscible solvent. In this regard, ILs can be efficiently used as extractive solvents of both starting and oxidized sulfur compounds and can be considered “greener” alternatives to conventional volatile organic compounds (VOCs).
As an example, Li and coworkers efficiently carried out the oxidative desulfurization of fuel using H
2O
2 as the oxidant agent in the presence of a WO
3/C composite catalyst [
[7]]. The extraction of sulfur compounds was carried out using an imidazolium IL (1-ethyl-3-methylimidazolium ethyl sulfate), added to the fuel as a non-miscible solvent (biphasic reaction medium). The WO
3/C composite was oxidized to the complex H
2[W
2O
3(O
2)
4(H
2O)
2]
2 in the IL phase, which also extracted from fuel the aromatic sulfur compounds; the complex then oxidized dibenzothiophene (DBT) to its sulfone (DBTO
2), which remained in the IL phase, allowing an easy separation. The same process could be carried out using 1-butyl-3-methylimidazolium tetrafluoroborate as DBTO
2 extraction solvent [
[8]].
In addition, Zhu, Li and coworkers reported the ability of an imidazolium IL (C
16MImBr) to direct the synthesis of a WO
3-SiO
2 composite towards a mesoporous material (W-SiO
2-20, ), which exhibited a high dispersion of tungsten throughout the structure (enhancing the catalytic activity) [
[9]]. The synthesis of the mesoporous catalyst was carried out starting from a polyoxometalate compound ([C
16mim]
3PW
12O
40) in a one-pot gel of tetraethyl orthosilicate, which was then calcinated at 550 °C.
The mesoporous catalyst was characterized using the usual techniques and efficiently used in ODS reactions () at the low temperature of 60 °C. The reaction times were quite short and very good yields were obtained after only 30 min. Moreover, the process did not require additional organic solvents as extractants.
The same authors reported a similar synthesis, utilizing a different support (in this case mesoporous ZrO
2) evidencing as both IL and calcination temperature, influenced the morphology and the dispersion of WO
3 [
[10]]. The best obtained catalyst (calcinated at 700 °C, using a C
16-ammonium IL, 700-C
16-WO
3/ZrO
2) performed very well in oxidation desulfurization. Dibenzothiophene (DBT) could be completely oxidized to DBT sulfone (DBTO2). Moreover, the catalyst could be recycled ten times with very low efficiency loss.
A functional IL ([(C
16H
33)
2N(CH
3)
2]
2W
2O
11) acted as WO
3 nanoparticle precursor and a large surface area (203 m
2/g) few-layer g-C
3N
4 support was used to disperse them, yielding a supported catalyst [
[11]]; this composite was characterized using SEM (), TEM, FT-IR, XRD and XPS, showing highly dispersed nanoparticles. The analysis showed that during the synthetic process of WO
3 dispersion on the support, the structure of few-layer g-C
3N
4 was not destroyed (D vs C), leading to a WO
3 catalyst with a very high surface, not obtainable using pure WO
3, whose structure showed agglomerates (A).
The enormous amount of exposed active sites rendered the composite an excellent catalyst in ODS processes, with the removal of 100% refractory sulfur-containing molecules at 50 °C in 1 h. Moreover, the catalyst was recycled up to six times without efficiency loss. A possible reaction mechanism is depicted in .
Last, tungsten trioxide-carbon nanotubes composite (WO
3/CNT) was demonstrated to be a very good catalyst in ODS of recalcitrant aromatic sulfur compounds, as reported by Li and coworkers [
[12]]. The catalyst synthesis was quite easy and was carried out in the presence of an imidazolium IL (C
16MImCl), using phosphotungstic acid (HPW) as tungsten source ().
The characterization of such a composite showed that the IL played a crucial role in determining the dispersion degree and the crystal phase of WO3 on the carrier (carbon nanotube, CNT). In fact, the IL improved the transformation of tungsten trioxide from monoclinic to tetragonal, inhibiting at the same time the growth of metal oxide grains. In this way, high WO3 dispersion was obtained, enhancing the catalytic activity. Moreover, a comparison of catalytic activity of different supported tungsten oxide forms was carried out, demonstrating the following activity order in sulfur oxidative removal: tetrahedral > tetragonal > monoclinic.