Abstract
A MoOx/Al2O3 catalyst was synthesised and tested for oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane in a reaction cycle of ODP followed by DP and a second ODP run. Characterisation results show that the fresh catalyst contains highly dispersed Mo oxide species in the +6 oxidation state with tetrahedral coordination as [MoVIO4]2− moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2− is present during the first ODP run of the reaction cycle and is reduced to MoIVO2 in the following DP run. The reduced species are partly re-oxidised in the subsequent second ODP run of the reaction cycle. The partly re-oxidised species exhibit oxidation and coordination states that are lower than 6 but higher than 4 and are referred to as MoxOy. These species significantly improved propene formation (relatively 27% higher) in the second ODP run at similar propane conversion activity. Accordingly, the initial tetrahedral [MoVIO4]2− present during the first ODP run of the reaction cycle is active for propane conversion; however, it is unselective for propene. The reduced MoIVO2 species are relatively less active and selective for DP. It is suggested that the MoxOy species generated by the reaction cycle are active and selective for ODP. The vibrational spectroscopic data indicate that the retained surface species are amorphous carbon deposits with a higher proportion of aromatic/olefinic like species.
Highlights
In order to bridge the propene supply and demand gap, the chemical industry has looked to highly selective on-purpose technologies like propane dehydrogenation (DP)
A novel reaction cycle consisting of oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane followed by the second ODP is tested over MoOx /Al2 O3
In situ X-ray Absorption Near Edge Structure (XANES) shows that the reaction cycle leads to formation of active and selective Mox Oy species for ODP as evident from the increased propene formation which is 27% relatively higher in the second ODP run
Summary
In order to bridge the propene supply and demand gap, the chemical industry has looked to highly selective on-purpose technologies like propane dehydrogenation (DP). Despite the selectivity benefits of DP, the process is highly endothermic and vulnerable to fast catalyst deactivation. Propane oxidative dehydrogenation (ODP) occurs in the presence of oxygen via an exothermic pathway. Catalysts 2020, 10, 1370 which has the potential to overcome the thermodynamic and catalyst deactivation constraints inherent in DP. The commercialisation of ODP is not yet feasible, due to propene selectivities being hampered by consecutive combustion reactions [1,2,3,4,5]. In order to overcome this problem, studies have been conducted into the use of supported molybdenum oxide to enhance selectivity for propylene during ODP and DP reactions [3,5,6]. It is widely accepted that the nature and distribution of Mo oxide species depend sensitively on several parameters such as Mo loading, catalyst support and calcination procedure, resulting in a variety of species spanning from monomeric, dimeric and oligomeric Mo oxide species to large Mo oxide clusters [8,11,15,16,17]
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