Preparation, characterization and mechanistic study of alumina supported polymolybdate based catalysts for catalytic oxidative desulfurization of diesel fuel

Abstract

Existing technique of hydrodesulfurization (HDS) is no longer applicable in achieving Euro IV standard diesel, as for it high operational cost, low efficiency and high operating temperature in the hydrogen gas atmosphere. Due to these drawbacks, the utilization of the oxidative desulfurization catalyst was introduced as an alternative or a complementary to the HDS process. In this study, the performance of the alumina supported polymolybdate based catalyst on the oxidative desulfurization (ODS) of the commercial diesel was investigated using tert-butyl hydroperoxide (TBHP) as an oxidizing agent and N,N-dimethylformamide as an extraction solvent. The commercial diesel with total of sulfur of 440 ppmw was employed to evaluate the elimination potential of the sulfur compounds. M/Al2O3 (M=Mo) and M1/M2/Al2O3 (M1 = Fe, Co, Cu, Ca, Ba, Sr, M2 = Mo) were prepared by the wet impregnation method and tested in this reaction. Further investigation on the doped molybdenum revealed that Fe/MoO3-PO4/Al2O3, calcined at 500ºC was the best catalyst in this study. Utilization of the catalyst was able to reduce the sulfur levels in the commercial diesel of 440 ppmw to Euro IV diesel of less than 18 ppmw, with 96% of total sulfur removal after the second extraction. X-ray diffraction analysis (XRD) results showed that the best catalyst is highly amorphous, while micrograph of the field emission scanning electron microscopy (FESEM) illustrated an inhomogeneous distribution of various particle sizes. The energy dispersive X-ray analysis (EDX) results have confirmed the presence of Mo, P and Fe in all of the prepared catalysts. X-ray photoelectron spectroscopy (XPS) analysis for the surface of Fe/MoO3-PO4/Al2O3 catalyst calcined at 500ºC showed the binding energy for Mo 3d5/2 231.7 eV and Mo 3d3/2 235.2 eV were corresponded to the formation of Mo6+. Temperature programme desorption of ammonia (TPD) analyses showed that the catalysts contribute a strong Lewis acid character. The phosphorus-31 nuclear magnetic resonance (31P NMR) analyses indicated the presence of mainly monomeric phosphates in catalyst calcined at 400ºC and 500ºC, whereas mixtures of monomeric and polyphosphates and AlPO4 are present in catalyst calcined at 600ºC. Statistical response surface methodology (RSM) by Box-Behnken design suggested that 0.10 g of Fe/MoO3-PO4 (10:90)/Al2O3 with Fe loading of 10 wt.% and calcination temperature of 530°C have achieved maximum sulfur elimination of 84.8%. Mechanistic study by Fourier transform infra-red (FTIR) showed an agreement with Langmuir-Hinshelwood mechanism with the adsorption of TBHP on the catalyst surface. The reaction mechanisms of peroxy oxygen was identified and proposed as the reaction between peroxide and dibenzothiophene to form dibenzothiophene sulfoxide. Further oxidation process with the presence of polymolybdate alumina supported catalyst has led to the formation of sulfone. The results obtained have proven that Fe/MoO3-PO4(10:90)/Al2O3 can be used as the potential catalyst for the removal of sulfur in the Malaysian diesel towards achieving the „green diesel‟ production.