465685 Nano-Sized Tungsten Carbide Catalyst for Upgrading Extra-Heavy Oil
Nowadays, extra-heavy oil has been received attention as alternative oil source as crude oil is being depleted. Extra-heavy oil is composed largely of resin and asphaltene which are macromolecules unable to use as fuel or chemical resources. These materials should be cracked into light oils to utilizing extra-heavy oil. The cracking process involves pyrolysis or hydrocracking. In these processes, some radicals can be produced and activate the formation of massive molecules like coke. Formation of coke reduces available carbon which means quantity of carbon atoms that compose liquid products. And also coke induces deactivation of catalysts and blocks pores of supports so it decreases catalytic activity. In this case, the provision of protons by hydrogenation process could suppress formation of coke. So it is obvious that hydrogenation process could perform important role in upgrading progress of extra-heavy oil.
Platinum, one of the most important element in catalysis, has outstanding performance in catalytic reaction involving hydrogen. For this reason, platinum had been used for hydrogenation process to saturate unsaturated hydrocarbons in spite of its high price. But, because of poisoning by sulfur, platinum could not be used with reactants such as petroleum which include sulfur. In 20th century, Levy and Boudart reported tungsten carbide, composed of tungsten and carbon atoms, could provide similar catalytic activity with platinum because of its electron structure. But unlike platinum, tungsten carbide has sulfur-resistance when it is exposed to sulfur in high temperature. In this respect, tungsten carbide has been considered as an alternative material of platinum for catalytic processes accompany with sulfuric reactants. In this case, tungsten carbides are used as hydrogenation catalysts and dispersed catalysts for the extra-heavy oil upgrading reaction.
The size of the catalyst particles should be small to use as dispersed catalysts, but obtaining small size tungsten carbide is difficult because tungsten carbide is formed in very high temperature2. In this temperature, sintering of tungsten particles becomes a serious problem. In traditional method, a ball-milling process is necessary to obtain small size tungsten carbide particles. But the ball-milling method is energy and time consuming process and its products contaminate milling machine. Recently, Sean T. Hunt reported the method which could restrict sintering while the carburization process. In this process, silica is used as impediment between the tungsten particles. As a result, nano-sized tungsten carbide could be produced in this method without any contamination. In this study, the nano-sized tungsten carbide catalyst, produced via tungsten oxide-silica core-shell based on the Hunt’s method, was applied to the extra-heavy oil upgrading process.
The reactivity of catalysts was measured in a 100-mL batch-type autoclave reactor. 30g of feedstock was charged into a reactor and the catalyst was added to the reactor. Vacuum residue was used as a feedstock, provided by the Energy Process Laboratory, SK Innovation. After charging the reactants, the reactor was purged three times with 10 bar of H2 gas to remove air. Then the reactor was pressurized to 70 bar with hydrogen at 27℃, followed by heating to 400℃ at a ramp rate of 12.5℃/min. Reaction was carried out at 400℃ for 4 hours.
For the bulk catalyst, tungsten carbide (2 μm, Aldrich, ≥99%) was used. In case of nano-sized tungsten carbide, tungsten isopropoxide in isopropanol (5 wt. %, Alfa) and tetraethyl orthosilicate (reagent grade, 98%, Aldrich) were used as tungsten and silicon precursor. These precursors were injected into reverse microemulsion and aged over 16.5 hours. Aged sample was separated by centrifuge at 10000 rpm for 10 min and washed once with acetone. After separation, the sample was dried over 12 hours at 50℃ and calcined for 1 hour at 450℃. Finally, calcined sample was carburized with 21% CH4/H2 gas at 835℃ and SiO2 shell was removed in ammonium bifluoride solution.
From XRD and TEM data, nano-sized tungsten carbide formed in SiO2 shell was observed. The thickness of SiO2 shell was 23 nm and diameter of nano-sized tungsten carbide was 5 nm which was 1/400 size of bulk tungsten carbide. In this study, the reaction were performed without catalyst, with bulk tungsten carbide and with nano-sized tungsten carbide to compare catalytic activities. The result was analyzed by using simulated distillation (SIMDIS) and SARA (Saturate-Aromatic-Resin-Asphaltene) analysis. Tungsten carbide catalyst showed high performance for upgrading extra-heavy oil to produce liquid products. In the reaction with tungsten carbide, decrease of pressure, which means hydrogen uptake, was observed. As expected, increment of liquid fraction and decrease of solid (coke) fraction was indicated when tungsten carbide catalyst was applied due to lower energy barrier for H2 dissociative adsorption.
 Levy and Boudart, Science, 181 (1973) 547-549
 Zhe Cheng et al., J. Electrochem. Soc., 153 (2006) A1302-A1309
 Zaoxue Yan et al., Sci. Rep., 3 (2013), 1646
 Sean T. Hunt et al., Angew. Chem. Int. Ed., 53 (2014) 5131-5136
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