APT Applied in Tungsten Coated Catalysts for Thiophene Oxidation

The more stringent limitations on the content of sulfur in the petroleum raw materials stimulated in recent years new research dealing with catalytic desulfurization processes. The key trend in the purification of liquid hydrocarbon raw materials is hydrotreatment in which sulfur is removed as hydrogen sulfide, which is then subjected to the Claus process to be converted to elemental sulfur.

However, the achievable degree of sulfur removal is now insufficient; in addition, this brings about new environmental problems related to the storage of sulfur. One solution had been reported as catalysts being fixed on a surface of metal oxide substrate and dispersion of the catalytically active components. The properties could be enhanced. This method could be achieved by Plasma electrolytic oxidation (PEO).

PEO technique has been applied for modifying oxide coatings’ surfaces by tungsten compounds, and for the obtaining crystalline tungsten oxides attached to the surface of titanium and aluminum, and aluminum tungstate on aluminum surface

The synthesis process of tungsten oxide catalysts for thiophene oxidation is as below:

PEO-coatings were fabricated on flat samples of a size of 22 × 22 mm or 10 × 40 mm made from sheet technical aluminum (Al > 99.3%) of a thickness of 0.3 mm, aluminum alloy Al-Mn (Al + 1.6% Mn) of a thickness of 0.5 mm, and VT1-0 titanium (99.9% Ti) foil of a thickness of 0.1 mm. To standardize pre-coating sample surfaces, they were chemically polished to high luster (surface finish class 8–9) in a mixture of concentrated acids. A mixture of H3PO4:H2SO4:HNO3 = 4:2:1 (by volume) at 110–120 °C and a mixture of HF:HNO3 = 1:3 (by volume) at 70 °C were used for treating aluminum and titanium, respectively. Then the samples were washed with distilled water and dried by air at 70 °C.

Electrolytes for plasma electrolytic oxidation contained sodium tungstate, phosphotungstate, or ammonium paratungstate. To prepare the electrolytes, we used distilled water and reagents such as analytical grade ammonium paratungstate (APT), reagent-grade Na2WO4·2H2O and Na2H[PW12O40] xH2O, Na2SiO3·9H2O, H3BO3, Na3PO4·12H2O, and CH3COOH. Ammonium paratungstate was dissolved under heating to 70 °C. Electrolyte compositions.

Oxide layers were formed on anodically polarized aluminum and titanium samples immersed into electrolytes at effective current densities i = 0.03–0.2 A/cm2 during t = 5–15 min. A PC-controlled commercial TER-4/460N thyristor unit (Russia) was used as a current source. JSC Fleron (Vladivostok, Russia) developed the synchronizer unit and software. The electrochemical cell consisted of a thermal glass of a volume of 1 L and a cathode in the form of a coil pipe: a hollow tube (0.5 cm in diameter) made of the corrosion-resistant steel of the grade 12 Kh18N10T. Cold tap water was passed through the coil pipe for cooling. The solution was stirred using a magnetic stirrer. The solution temperature during the process did not exceed 35 °C. After PEO treatment, the samples were rinsed by the distilled water and air-dried at room temperature.

In summary, W coated oxide coatings formed on titanium and aluminum by Plasma electrolytic oxidation technique are active in the catalytic oxidation of thiophene. The coatings on aluminum and titanium including tungsten oxides into silica-titania or silica-alumina matrix are more stable than the layered coatings on aluminum containing crystal tungsten oxides and/or aluminum tungstate in outer layer. Among them, the Al-Mn aluminum alloy-supported coatings containing 0.7 at.% W have higher TOF value and are most stable. The formation of mixed oxides of tungsten and manganese of different valences in the pores of the coating may be a possible reason of their activity. These composites may be promising catalysts for the oxidative desulfurization of heavy fractions of petroleum products.

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