47206-GB5
Large-Scale Quantum Mechanical Calculations for the Catalytic Dehydrogenation of Alkanes: Chromium Supported on Transition Aluminas
Chromium/transition-aluminas (Cr/transition-Al2O3) systems are widely used as catalysts in petrochemical industry for the production of alkenes (CnH2n) through the process of oxidative dehydrogenation of alkanes (CnH2n+2). While considerable progress has been made for the studies of the catalytic process and the pertinent catalytic materials, the question about the specific roles of chromium, its associated oxides, and the supporters (transition aluminas), in the catalytic process still remains open.
In the last year, we employed large-scale first-principles quantum mechanical calculations to investigate the catalytic chemical reaction of the dehydrogenation of a selected alkane, propane (C3H8), with the presence of Cr (in the form of dispersed chromium oxide species) supported on g-alumina (g-Al2O3). More specifically, calculations were carried out using density-functional theory, ultrasoft pseudopotentials, the generalized gradient-corrected exchange-correlation functionals, the plane-wave basis set, and periodic supercells. Two different polytypes of transition-aluminas, η- and γ-aluminas are often used for dehydrogenation of alkanes due to their stable porous structures with high specific surface areas that are important for catalytic activities. Experimental work has found that, while Cr/η-alumina and Cr/γ-alumina systems are significantly different in lifetime as catalysts, both η- and γ-aluminas support dispersed chromium oxide species which involve the catalytic reactions. We chose Cr/γ-alumina for our calculations.
Our calculations show that when a propane (C3H8) molecule reacts with to a CrO4 species supported on the γ-Al2O3 surface, dissociation of propane occurs - two hydrogen atoms of the C3H8 molecule are trapped by the oxygen sites of the chrominm oxide species, and a propene (C3H6) molecule is simultaneously released. This is similar to the previous conclusion of the PI and collaborators about catalytic dehydrogenation of ethane in the presence of CrO3-supported η-alumina. A key result here is that such dissociation is energetically favorable, with an energy gain of 0.2–1.6 eV (varying with the specific trap sites). All of the oxygen sites in the CrO4 species are active in capturing hydrogen from the C3H8 molecule, but the Cr atom itself does not trap any hydrogen atoms. Thus, the role of Cr is to bind active unsaturated oxygen atoms which then act as the trap sites of hydrogen atoms of an alkane molecule. We expect that a CrO3 species supported on the γ-Al2O3 surface shows a similar behavior to the CrO4 species, but the pertinent calculations have not yet been performed.
The Cr oxide species will be catalytically active as long as the oxygen atoms remain unsaturated. The hydrogen atoms that are trapped by active sites of the Cr oxide species can be removed from these sites by a flow of oxygen molecules. This process involves the chemical reaction CrO4H2 + 1/2O2 → CrO4 + H2O. Our calculations show that this process gains an energy (0.6–1.9 eV in the case of CrO4 species), indicating that the hydrogen atoms are removed in the form of water molecules. The importance of this process is to reactivate the oxygen sites of the CrO4 species.
