Abstract
Cobalt- and cobalt–boron-loaded TiO 2 (anatase) catalysts were prepared and characterized before and after catalytic tests by XRD, HRTEM, IR, UV–visible, and laser Raman spectroscopy. Their activity was investigated in oxidative dehydrogenation (ODH) of ethane. In the absence of boron, the best performances were exhibited by the sample containing 7.6 wt% Co, which was selected for further investigations. At 550°C, it displayed a stationary state with a conversion of 22.2% and an ethylene selectivity around 60%. This catalyst also showed in the first 3 h on stream a 30% decay in activity that was attributed to a concomitant loss of specific surface area and the formation of CoTiO 3 and Co 2TiO 4 phases. The addition of 0.25 wt% boron to this Co(7.6)/TiO 2 sample improved the ethane conversion and the ethylene selectivity, which attained 28.4 and 67%, respectively. Boron concentrations superior to 0.25 wt% negatively affected the catalysts performances, probably because at high loadings it profoundly modified the acid–base properties of the surface. XRD and HRTEM analyses showed that at the same time the size of Co 3O 4 crystallites decreased. IR investigations confirmed the increase in acidity upon boron addition and the decrease in strength of the basic sites which were involved in the dehydrogenation processes. The catalytic behavior and the acid–base properties of Co(7.6)/TiO 2 loaded with different amounts of boron were also studied using butan-2-ol conversion. Boron addition enhanced the dehydration and the dehydrogenation reactions. However, above 0.25 wt% it decreased the dehydrogenation activity, confirming the modifications of the properties of the acid–base centers revealed by the IR studies. For this optimal concentration of boron, the activity and the selectivity in butan-2-ol dehydrogenation exhibited a maximum that coincided with the one observed in the ethane ODH, which suggests that both reactions possibly involved the same type of active centers.
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