Synthesis of Olefins on Mixed Metal Oxide Catalysts at Sub-Monolayer and Monolayer Coverage Via Gaseous Alkanes Odh

CATEGORY: OLEFINS 
Synthesis of Olefins on Mixed Metal Oxide Catalysts at Sub-Monolayer and Monolayer Coverage Via Gaseous Alkanes Odh

Type	Conference Paper
Author	Carlos Carrero
URL	https://nam.confex.com/nam/2015/webprogram/Paper12723.html
Date	2015/06/16
Conference Name	24th North American Catalysis Society Meeting
Abstract	Introduction The traditional processes used nowadays for the production of gaseous olefins are untenable, supply-limited, and environmentally unfriendly. Therefore, studies on developing new routes for the production of olefins (such as alkane ODH and metathesis), have intensified. For ODH of light alkanes, highly dispersed vanadium oxide catalysts have shown the most attractive results in terms of activity and selectivity, although they still don’t meet the productivity criteria to be implemented industrially. Recently, mixed monolayer oxide-onoxide catalysts have been demonstrated to be the most productive catalysts toward propylene via propane ODH. Supported metal oxide catalysts can be prepared by a multitude of synthesis methods: metal precursors, dissolution media, and calcination treatments, all resulting in MOx species that range from metal oxide monomers and oligomers (twodimension) at sub-monolayer coverage, to three-dimensional MOx NPs (nanoparticles) above monolayer coverage (Figure 1A). Thus, the concentration of such surface species (monolayer being the concentration limit of 2D MOx species) is significantly affected by the precise preparation.5 Our work aims at mixing different metal oxides (VOx)m-(M2Oy)n/(SiO2)bulk catalyst (M2 = Al, Zr, Ta, Nb) in order to tune both the acidic and redox properties of such materials (preferentially at sub-monolayer and monolayer coverage) in order to reach industrially attractive olefin productivities. Materials and Methods Details about the catalysts preparation have been described elsewhere.6 Briefly; the supported M2Oy/SiO2 catalysts are prepared via the incipient wetness impregnation method. The synthesis is carried out inside the glove box to guarantee water-free conditions and avoid the hydrolysis of the M2-alkoxide, which leads to the undesired M2Oy NPs formation. Then, the samples are dried at 120 °C for 3h under N2 and right after calcined at 550 °C for 2 h. The impregnation of VOx on the calcined M2Oy/SiO2 support follows the same recipe described above. The characterization techniques used to characterize the obtained matrix of ternary catalysts are ICP, BET, Raman, and UV-vis spectroscopy. The samples showing promising results are going to be studied by solid state V51NMR, cw. and pulse EPR, and operando Raman spectroscopy. Results and Discussion: Currently we are preparing a matrix of different VOx/M2Oy/SiO2 catalysts bearing in mind the lessons learned on our previous work on VOx/TiOy/SBA-15 sytems.4,5 In summary, we are considering the next aspects: i) highly dispersed VOx and M2Oy surface species with the absence of VOx and MOx NPs ii) only close to monolayer coverage of both VOx and M2Oy are going to be tested since low coverage don´t show remarkable improvements in propylene productivities, and iii) the reaction needs to be performed at high temperature (480-520 °C) and avoiding total oxygen conversion in order to guarantee high propylene selectivity and catalyst stability, respectively. For this purpose, we have prepared a pool of catalysts keeping constant the amount of VOx (about 4 wt% V) and varying the M2 loading between 8 – 14 wt%. Raman spectroscopy has confirmed that both VOx and M2Oy nanoparticles are not present on the catalyst surface while UV-vis has evidenced the presence of oligomeric VOx species. Significance: In terms of applicability, reaching high olefin-productivity and at the same time overcoming the 8% propane conversion barrier obtained up to now is the goal of this study (Figure 1B). Fundamentally, ternary catalysts showing promising olefin productivities are going to be deeper characterized (e.g. in situ and operando Raman/Uv-vis spectroscopy, solid state NMR and EPR) and kinetically analyzed (e.g. applied kinetics, reaction orders and activation energy determination) to get new insights on the VOx and M2Oy surface species interaction and how they are coexisting on the catalysts surface.