title: Chemical looping propane oxidative dehydrogenation studies as an alternative route for propene production
Creators: name: Moffat, Caitlin
Publisher: name: University of Glasgow
Year: 2023

description: Propene production has suffered drastically over the years since conventional steam cracking and fluidized catalytic cracking units have been refined to maximise ethene, diesel and gasoline yields. This has led to a significant gap between propene supply and demand, and the chemical industry has looked to alternative on-purpose propene production technologies, like propane dehydrogenation, to bridge this gap. Despite the selectivity benefits, the process is endothermic and requires high operational temperatures, where the catalyst is susceptible to fast deactivation. Alternatively, propane oxidative dehydrogenation is an exothermic process, which occurs in an oxidative atmosphere thus providing the potential to overcome these constraints. Unfortunately, the commercialisation of the oxidative route is not currently feasible due to undesired combustion reactions, which subsequently hamper propene yields. There also exists many drawbacks when considering the operational logistics due to the requirement of C3H8 and O2 co-feeds, since O2 production is costly and energy intensive and there is difficulty in controlling the mixtures to keep them under explosive limits. In order to combat these challenges, interest has shifted to systems which use alternative, softer N2O and CO2 oxidants as a co-feed, as well as adopting a chemical looping approach. Chemical looping propane oxidative dehydrogenation (CL-PODH) tests are carried out in an O2-free atmosphere, by utilising the lattice oxygen within metal oxide catalysts as the oxygen source, where a separate regeneration step is subsequently performed. Consequently, this work has investigated the efficacy of γ-Al2O3 supported VOx, MoOx and VOx-MoOx catalysts for propene production in a series of novel CL-PODH redox cycling tests. Benchmark CL-PODH tests were performed in a combined microreactor and mass spectrometer system and consisted of ten propane dehydrogenation-catalyst regeneration redox cycles, utilising O2 as the oxidant. Such tests provided valuable insight into the relationship between the activity of vanadia and molybdena species as a function of varying vanadium loading and increased molybdenum incorporation. The use of N2O and CO2 within the catalyst regeneration step was also explored to establish catalyst regenerability throughout the cycles when using alternative, softer oxidants. Fresh VOx/γ-Al2O3 and MoOx/γ-Al2O3 catalysts were synthesised via the incipient wetness impregnation technique with varying vanadium loadings of 5.0, 7.5 and 10.0 wt.% and a fixed molybdenum loading of 9.4 wt.%. VOx-MoOx/γ-Al2O3 catalysts were synthesised via the co-impregnation technique, where the vanadium loading was fixed to 10.0 wt.% and molybdenum loadings of 3.2, 4.7 and 9.4 wt.%, which gave V:Mo molar ratios of 6:1, 4:1 and 2:1, respectively. Various characterisation techniques were used to establish the catalysts properties, including MP-AES, ICP-OES, BET, H2-TPR, O2 chemisorption, Raman spectroscopy and XRD. Throughout the VOx/γ-Al2O3 catalyst series, the most promising results for propene production were obtained in tests using 10 wt.% V catalyst. Propene selectivities of up to 40.8 % were achieved, at 15.8 % propane conversion upon completion of the 10th PDH step. A long duration PDH step was also performed, revealing a substantial 30 % loss in catalyst activity after 3 h time on stream. Post-reaction Raman analysis revealed significant coke formation, thus highlighting the need for successive catalyst regeneration steps. Alternative N2O and CO2 oxidants were also employed in CL-PODH tests using the 10VAl catalyst. In comparison to using O2, an increase in propene selectivity was observed, reaching 47.6 % at a similar propane conversion of 15.4 %. A significant decline in catalyst activity is observed when using CO2 as the oxidant however, as propene selectivities of only 36.5 % were achieved, at 9.2 % propane conversion upon completion of the 10th PDH step. Post-reaction characterisation tests were conducted to probe the surface vanadia species present in the 10VAl catalyst removed at different stages throughout the ten cycles. Results from BET, Raman and XPS suggest the formation and removal of carbonaceous deposits with successive dehydrogenation and regeneration steps, thus reinforcing the regenerative ability of the O2 and N2O oxidant for coke removal. The inability of the CO2 oxidant to remove carbon laydown and regenerate activity is highlighted further in the post-reaction Raman with prominent features attributed to coke formation. A significant observation in CL-PODH redox cycling tests was the rise in propene production during the 6th PDH step, which was performed after holding the catalyst overnight under argon following the O2 regeneration step of cycle 5. The 10VAl catalyst was similarly removed and characterised after this overnight argon purge. XPS results reveal an increase in surface V4+ species at this stage, which agrees with findings from the thermodynamics and post-reaction Raman that surface vanadia sites are oxidising and removing residual low-level carbonaceous deposits, hence the surface vanadium is in a more reduced state. An increase in propene selectivity due to the presence of this reduced species establishes a potential for tuning specific V5+/V4+ ratios, which may be crucial in overcoming the selectivity limitations to enhance propene production. The use of 9.4 wt.% MoOx/γ-Al2O3 in the benchmark CL-PODH redox cycling test produced significantly lower propene selectivities in comparison to the 10VAl catalyst. The incorporation of molybdenum as a promotor in the 10VAl catalyst however, yielded an enhancement in catalyst activity, where an increase in propane conversion was observed with increasing molybdena loading across the VOx-MoOx/γ-Al2O3 series. The presence of V-O-Mo domains was implied in the pre-reaction Raman of all three VOx-MoOx/γ-Al2O3 catalysts, which showed a promotional effect on catalyst activity. In comparison to 10VAl and 9.4MoAl catalysts, a higher activity is observed in tests utilising the 10V9.4MoAl sample. Upon completion of the 10th PDH step, propene selectivities of 32.5 % were obtained at a propane conversion of 22.9 %. Despite the lower selectivity towards propene, this increase in activity results in an overall higher propene yield.