불균일 촉매와 과산화수소에 의한 디젤연료유의 산화적 탈황 연구

Abstract

The aim of this study was to remove sulfur compounds in diesel using catalytic oxidative desulfurization. The International Maritime Organization (IMO) announced an international convention, the MARPOL ANNEX VI, to act against the pollution caused by ships. Further, heavy oil usage is prohibited in Emissions Control Areas (ECA). Moreover, following the advent of alarmingly high oil prices, ship owner and workers are exploring innovative technologies to generate power on board. Henceforth, fuel cells can be an alternative solution because of their higher efficiency and lower emissions. However, sulfur is known for its catalyst toxicity. Therefore, the desulfurization of diesel can remove toxic sulfur from the cathode in fuel cells and in the reforming process. In the following experiments, three type of heteropoly acids (H3PW12O40 (PW12), H3PMo12O40 (PMo12), H4SiW12O40 (SiW12)) supported on SiO2, MCM-41, Al2O3 and activated carbon(AC) as supports were prepared by incipient wetness impregnation (IWI) and impregnation method, respectively. Also, three kinds of metal oxides (WOx, MoOx, VOx) supported on SiO2 and SBA-15 as the support were prepared by impregnation method. The prepared catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy(SEM). Further, specific surface areas of catalysts were calcuated according to the Brunauer-Emmett-Teller (BET) equation. First, the effects of the reaction temperature (30 and 80 °C), time (1 and 3 h) and catalyst dosage (0.1-1.0 g) were investigated under the constant condition (oxidant = 30.0-35.5% H2O2, O/S molar ratio = 100). As a result, catalysts containing tungsten metal (PW12, WOx) were found to be effective for oxidative desulfurization in contrast to molybdenum and vanadium metal. The effect of catalyst dosage was found to be higher than the effect of temperature on the desulfurization performance; the reaction time was adequate for an hour. The sulfur removal efficiency increased in the order of SiW12 < PMo12 < 30 PW12. These results indicate that the active phase acidity can be attributed to oxidative desulfurization. Regarding the heteropoly acid catalyst supported PW12, the effects of PW12 loading (10-50 wt.%) on SiO2 and of supports (SiO2, MCM-41, γ-Al2O3, AC) over the PW12 catalysts were investigated for oxidative desulfurization. The sulfur removal efficiency was enhanced with increasing PW12 loading. The result, that sulfur removal efficiency was enhanced by increasing PW12 loading, was determined from the XPS analysis data. The XPS analysis indicated that over 34.5 wt.% of the PW12 loading took the form of multi-layers on silica. Thus, the sulfur removal efficiency of the 30-50 PW12/SiO2 catalysts increased slightly. And, PW12 supported on silica-based supports (SiO2, MCM-41) possesses higher sulfur removal efficiency compared to AC and Al2O3. The results show that the surface area of the catalyst is not important for performing oxidative desulfurization. Also, the support acidity is attributed to oxidative desulfurization. The recycling test for 40 PW12/SiO2 showed the stability of catalyst during regeneration; this stability is higher than the case ‘without’ regeneration. But, the PW12/SiO2 catalyst is not suitable for the oxidative desulfurization process using hydrogen peroxide. Regarding the metal oxide catalyst supported WOx, the effect of supports (SiO2, SBA-15) over the WOx catalysts and the effect of WOx loading on SBA-15 were investigated for oxidative desulfurization. WOx supported on SBA-15 possesses good desulfurization performance compared to that of SiO2. The results were such that the optimized loading of WOx on SBA-15 was about 10-15 wt.%. The XPS analysis results indicate that approximately 12 wt.% of the WOx loading was used to make the multi-layers on SBA-15. The influence of the catalyst dosage (0.05-0.4 g for 20 ml of marine diesel), reaction temperature (30-90 °C), reaction time (0.5-5 h) and molar ratio of the oxidant (hydrogen peroxide) and of sulfur (O/S = 10-200) were examined. The oxidative desulfurization performance was found to increase with the catalyst dosage, reaction temperature and reaction time. In addition, the optimal values of catalyst dosage, temperature and time for desulfurization were found to be 0.1 g, 90 °C and 3 h, respectively. However, the sulfur removal efficiency decreased as the O/S molar ratio increased because the excess of oxidant increases the sulfur removal efficiency, whereas the water produced from the oxidant can hinder the oxidative desulfurization process. In these experiments, the 20 WOx/SBA-15 catalyst exhibited a high catalytic activity, achieving sulfur removal efficiency of about 70% with the reaction conditions of catalyst dosage 0.1 g, reaction temperature 30 °C, reaction time 3 h and O/S molar ratio 10. These results suggest optimized reaction conditions of catalyst dosage of 0.1 g, reaction temperature of 90 °C, reaction time of 3 h and O/S molar ratio of 10 with 15 WOx/SBA-15 as the best catalyst.