Abstract
The increasing demand of ethylene and propylene, important building blocks in chemical industry, is nowadays hardly met by the conventional production via steam cracking of various hydrocarbon feedstocks. Therefore, alternative routes for producing light olefins are being explored. The availability of ethane in shale gas, as well as the interest in adding value to previously under-utilized refinery fractions, makes the activation of alkanes via oxidative dehydrogenation (ODH) an attractive alternative to the industrially established processes.
Complex Mo and V mixed oxides have been extensively studied as catalysts for selective oxidation. In particular, Mo-V-Te-Nb mixed-oxide is a promising system for the oxidative dehydrogenation, given its outstanding ability to activate alkanes [1, 2]. MoVTeNbOx can convert up to 40 % of ethane at moderate temperatures (350-400 °C) with very high selectivity to ethene (above 90%) [3]. The catalytic performance of Mo-V-Te-Nb mixed-oxide is mainly attributable to the crystalline phase "M1", in particular to V5+ species in specific positions of the crystal lattice. The role of the crystalline structure itself is to provide for the spatial configuration and redox properties necessary to create the active site.
Despite the knowledge gained over the last 10 years about M1 phase, it is still unclear a correlation between structure and reactivity for this catalyst. It has been reported that, when different synthetic methods are applied, the M1 crystals obtained can be chemically similar but they show different catalytic performance [4]. Further understanding is needed about how bulk size, surface structure and chemical composition of M1 particles affect the activity.
In this work, a series of MoVTeNb oxides was tested in ethane ODH and activity was found to correlate with the content of M1 phase. Kinetic studies led to similar energy of activation values for all the Ml-containing samples, however significant differences in the pre-exponential factor were noticed, even when the activity was normalized per M1 gram. This points to differences in the number of active sites that are available in each sample.
Atomic resolution aberration-corrected HAADF-STEM was used to directly image the termination of crystals of M1 phase in MoVTeNb complex oxides. Over 50 random particles were examined and it was possible not only to identify and quantify the most frequently exposed facets but also to obtain an average number of active sites exposed for each facet. It was observed that flattened large particles consist of extensive facets that expose almost no active sites. An immediate conclusion extracted from these observations is that samples with a predominant morphology of large flattened M1 particles must have a lower activity than other morphologies. This was experimentally validated by the results of catalytic tests obtained for Ml samples with different morphologies. Average size and shape of the M1 particle were calculated for each sample by statistical analysis of Helium Ion Microscopy images. In this way, it was possible to estimate the density of active sites that are exposed for a given morphology and quantitatively compare different MoVTeNbO samples.
In summary, STEM imaging provided an atomistic picture of the surface termination of the complex MoVTeNb oxide. By statistical analysis of STEM and HIM images we have quantitatively demonstrated that differences in reactivity among complex mixed oxide catalysts with apparent similar composition is primarily related to the morphology of the active phase crystalline particles. Based on the results shown here, new routes and approaches for the synthesis of oxide catalysts can be envisioned.
Original language | English |
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Pages (from-to) | 25-26 |
Number of pages | 2 |
Journal | DGMK Tagungsbericht |
Volume | 2014 |
Issue number | 3 |
State | Published - 2014 |