Article pubs.acs.org/IECR
Effect of Calcination Temperature on the Structure and Catalytic Performance of the Ni/Al2O3 and Ni−Ce/Al2O3 Catalysts in Oxidative Dehydrogenation of Ethane Lucie Smoláková, Martin Kout,* Eva Koudelková, and Libor Č apek Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic S Supporting Information *
ABSTRACT: We studied the effect of calcination temperature of Ni/Al2O3 and Ni−Ce/Al2O3 catalysts on the specific surface area, acidity, ratio of pore volume and specific surface area, reducibility reflecting the changing population of Ni(Td) and Ni(Oh) species, and activity/selectivity in oxidative dehydrogenation of ethane. It should be stressed that the role of Ce as a promoter on the catalytic activity of Ni−Ce/Al2O3 catalysts decreased with an increasing value of the calcination temperature. At a reaction temperature of 500 °C, the highest productivity to ethene was observed for the Ni−Ce/Al2O3 catalyst calcined at 500 °C. Ni− Ce/Al2O3 catalysts calcined at 500−750 °C showed the same selectivity to ethene. The Ni−Ce/Al2O3 catalysts calcined at 900 and 1000 °C showed a sharp decrease in the selectivity and the activity, which was probably associated with the formation of NiAl2O4 spinel. consistent with the results of Heracleous et al.,21 which studied the interaction of nickel with alumina support. It was found that nickel strongly interacts with alumina support, and the nickel aluminate phase was formed. Many authors describe the effect of different promoters on the catalytic behavior of the Ni/Al2O3 catalyst in ODH of ethane. Heracleous et al.21 studied the effect of V, Co, Nb, and Ta promoters on the catalytic behavior of the Ni/Al2O3 catalyst in ODH of ethane. They reported that the promoters, which are accommodated between nickel and alumina, inhibit the incorporation of nickel into the alumina lattice, reduce the strong metal−support interaction, and increase the number of Ni reaction sites. From the selected promoters, Nb was reported as the most efficient. Besides Nb, cerium is another intensively studied promoter in ODH of ethane. Wang et al.22 published that cerium as a promoter has wide applications in catalysis due to the easy formation of oxygen vacancy sites at low temperature. Bortolozzi et al.23,24 suggested that the promoting effect of cerium is given by the synergetic effect of smaller NiO particles and the formation of Ni−Ce−O solid solution, which generates new active sites in Ni−Ce/Al2O3 catalysts in comparison with Ni/Al2O3 catalysts. They also reported that the structured systems show higher ethene productivity than the powder catalysts and that the catalytic features of the structured systems depend on the calcination temperature of the material.23 More recently, we reported that the distribution of Ni species and the catalytic behavior of Ni/ Al2O3 catalysts could also be affected by the type of nickel precursor (nickel nitrate vs nickel acetate)25 and by the Ce−Ni interaction.26 The results showed that Ni/Al2O3 catalysts
1. INTRODUCTION Oxidative dehydrogenation (ODH) of ethane belongs to the attractive method of ethene production. There is still an increasing demand for ethene. For example, more than 138 and 141 million tonnes of ethene were produced in 2011 and 2012, respectively.1 Ethene is mainly produced via steam cracking of gas oils or natural gas and fluid catalytic cracking. Steam cracking consumes high amounts of energy and produces a large amount of CO2 emission worldwide.2 Moreover, separation and purification make the process of steam cracking more expensive.3 Although steam cracking and fluid catalytic cracking processes are industrially used, there are intensively studied new alternatives, such as oxidative dehydrogenation (ODH). ODH is an exothermic reaction (steam cracking and nonoxidative dehydrogenation are endothermic reactions) and does not suffer from the coke deposition as the dehydrogenation reaction does.4 ODH can also be realized at lower temperature (
