Dissociation of Acetaldehyde on β-Mo₂C To Yield Ethylidene and Oxo Surface Groups:  A Possible Pathway for Active Site Formation in Heterogeneous Olefin Metathesis

Abstract

The dissociative adsorption of acetaldehyde on beta-Mo2C was studied using reflection absorption infrared spectroscopy. In contrast to what is observed for all metals previously studied, acetaldehyde undergoes selective carbonyl bond scission on the carbide surface. By comparison to calculated spectra, the surface product is identified as an oxo-ethylidene species. The study thus provides the first extended-range infrared spectrum of a propene metathesis initiator or propagator alkylidene. Aldehydes may be formed in the presence of olefins during the induction period of supported metal oxide olefin metathesis catalysts. Hence, the observed dissociative chemisorption of acetaldehyde suggests a possible answer to the question of how initiator sites are formed in heterogeneous olefin metathesis. This question has never been satisfactorily answered. In the proposed mechanism, aldehydes formed during the induction period subsequently react with the catalyst surface to generate alkylidene sites.

Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst

“…In the specific case of alkene metathesis, it has been proposed that carbenes and metallacyclobutanes are key reaction intermediates (16). Although they have been unambiguously prepared and observed for homogeneous catalysts (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29), observing them as heterogeneous catalysts has proved to be a formidable challenge (30)(31)(32). Note that because the surface can play such a crucial role, one should never assume that the intermediates are identical in heterogeneous and homogeneous systems (31,32).…”

Preparation and olefin-metathesis activity of cyclopentylidene-oxo initiator sites on a molybdenum carbide surface

“…It is well known that alkylidene, alkylidyne and imido metal-carbon (nitrogen) stretching modes couple strongly with CC stretching and CH deformation modes [34][35][36]. The latter assignment may be compared with those reported for Mo@C 4 H 6 (1131 cm À1 ) [9,19] and Mo-ethylidene (1132/1120 cm À1 ) [18] formed through cyclobutanone and acetaldehyde dissociation on b-Mo 2 C, and for Tineopentylidene (1121 cm À1 ) formed during the chemical vapour deposition of TiC films [37].…”

“…This marks the saturation of the chemisorption layer and the formation of a condensed, multilayer, phase. The carbonyl band at 1690 cm À1 , observed at lower exposures is characteristic of an g 1 (O) chemisorption state [18,19]. This attribution is made on the basis of the $50 cm À1 redshift of the m(CO) frequency, coupled with the strong intensity of the band.…”

“…4 shows that the removal of the g 1 -state coincides with the development of new bands at 1324, 1043 and 978 cm À1 . The latter band is characteristic of a surface oxo group [9,18,19,33] indicating that carbonyl bond scission has taken place. In contrast, the absence of a new band in the 1600-2200 cm À1 region shows that decarbonylation leading to the formation of CO ads does not occur.…”

“…For example, work by Goodman and co-workers [17] on the acetoxylation of ethylene to vinyl acetate on gold-palladium shows that alloy formation is an effective method to isolate single Pd sites. Similarly, the selective formation of terminal alkylidenes on b-Mo 2 C shows that interstitial carbide formation leads to single Mo site surface chemistry [10,18,19]. The latter observation is especially interesting in that formation of carbide phases from molybdenum nanoparticles can occur in the presence of hydrocarbons at relatively low temperatures [20].…”