Catalytic dehydrogenation of ethane over mononuclear Cr(III) surface sites on silica. part I. C—H activation by σ‐bond metathesis

Abstract

Mononuclear Cr(III)-silica models have been studied by quantum chemical methods with respect to catalytic activity toward dehydrogenation of ethane. Both cluster and slab models have been developed and used to explore the conceptual model of mononuclear Cr(III) with three covalent ligands that coordinate through oxygen. The study focuses on a reaction mechanism consisting of three main reaction steps: (1) C-H activation of ethane according to -bond metathesis and accompanied by the formation of O-H and Cr-C bonds, (2) -H transfer to chromium with subsequent loss of ethene and (3) regeneration of the chromium site under evolution of H 2 . An alternative mechanism is also explored, in which C-H activation takes place at a reactive hydridochromium complex. Stationary points pertaining to these reactions have been optimized, and free energy calculations are used to identify the rate-determining steps. The influence of the local structure of the chromium surface sites is explored by means of a number of idealized surface models and electronic energy profiles for the reactions.

Mechanistic Insight in the Ethane Dehydrogenation Reaction over Cr/Al2O3 Catalysts

“…The mass fragments of ethane (figure 5a) and ethene (figure 5b) are both shifted one mass unit higher after introduction of 1) 13 C ethane compared to 12 C ethane. No change in the fragmentation pattern is observed with time (30-180 s after on-set of the 1) 13 …”

“…Mass fragmentation spectra from an experiment in which the catalyst was first exposed to 12 C-ethane under standard conditions, then flushed with He until no 28 peak was observed in the mass spectrometer, and finally exposed to mono-labelled ethane, 13 CH 3 12 CH 3 (C 2 H 6 :He=2:20 Nml/min, 580°C, 20% ethane conversion), are shown in figure 5. The mass fragments of ethane (figure 5a) and ethene (figure 5b) are both shifted one mass unit higher after introduction of 1) 13 C ethane compared to 12 C ethane.…”

“…In all these catalyst systems, a Cr-O moiety with chromium in its tri-or divalent state is envisaged as the active site for C-H activation. Figure 1 shows the dehydrogenation mechanism based on such mononuclear Cr 3+ -O site supported on a high-surface oxide, as recently discussed by Lillehaug et al [13]. The activation of the C-H bond starts with the reaction of the alkane with a coordinatively unsaturated Cr 3+ centre leading to the formation of a new O-H group and a Cr-alkyl bond.…”

“…The results of the transient experiment (figure 4) show that the residence time of any ethene-forming intermediate on the catalyst surface is very short. If an oligomer or aromatic compound were an intermediate in ethene formation, isotopic scrambling of ethene would be expected to occur within the order of seconds during the 1) 13 C ethane experiment (figure 5). No indication of isotopic scrambling is observed during the first 180 s after onset of the 1) 13 C ethane feed.…”

“…First of all, we would like to elucidate if carbon deposits are indeed active in the dehydrogenation of ethane. For this purpose, we have conducted isotopic labelling experiments for an ethane dehydrogenating Cr/Al 2 O 3 catalyst making use of labelled 13 C-ethane and deuterium. Secondly, the data obtained allow us to extend the work of Burwell [11,12] to alumina-supported chromium oxide catalysts and discuss the obtained insights in relation with the recently published theoretical study by Lillehaug et al on the same reaction system [13].…”