EPR enlightening some aspects of propane ODH over VOx–SiO2 and VOx–Al2O3

Abstract

In the oxidative dehydrogenation (ODH) of propane to propylene VOx-based catalysts prepared by flame pyrolysis (FP) showed more selective than those prepared by impregnation. Furthermore, samples prepared by the same method were less active, but more selective when VOx was supported on SiO2 than on Al2O3. In order to assess V local structure, V 4+ ions showed useful labels to characterise these catalysts by EPR spectroscopy. Indeed, the spectrum of a (10 wt%V2O5) FP-prepared VOx/SiO2 catalyst was typical of isolated, tetragonally distorted, paramagnetic complexes of V 4+ forming a monolayer on the sample surface with a strong out-of-plane V 4+ -O bond. In a sample with identical composition, but prepared by impregnation, this bond showed a bit weaker.Furthermore, ferromagnetic domains of clustered V ions formed in the latter sample, hindering at least in part the accessibility to the catalytically active V-based centres. This gives evidence of the higher dispersion of V in the sample bulk provided by the FP preparation method with respect to conventional ones. A by far weaker V 4+ -O bond was revealed by the EPR spectrum of a (10%V2O5) VOx/Al2O3 sample, accounting for its higher oxygen availability, leading to higher activity, but lower selectivity. However, the same catalyst, when prepared by impregnation, showed a ferromagnetic resonance pattern so intense that no EPR spectrum was detectable at all and no information on the V 4+ -O bond strength was available in that case.Such semi-quantitative index of the V-O bond strength can be used as an index of oxygen availability, as a tool to assess catalytic activity and selectivity to the desired olefin.

Spectroscopic Enlightening of the Local Structure Of VOX Active Sites in Catalysts for the Odh of Propane

“…Something similar has been already observed by means of EPR analysis [14][15][16] , though in such case referring to V(IV) ions, only.…”

“…However, with V10Al sample also bands attributed to VO x and isolated V=O species were detected. The latter were absent with sample V10Al-i, prepared by impregnation, which resulted to be much more similar to sample V50Al.Finally, electron paramagnetic resonance evidenced ferromagnetic domains constituted by agglomerated V(IV) sites in V10Al-i and V10Si-i samples prepared by impregnation and absent in the corresponding FP ones, further confirming the higher V dispersion in the latter catalysts [14][15][16] .All these evidences induced the conclusion of a much higher V dispersion of the FPprepared samples, of course decreasing with raising V loading, with respect to those prepared by impregnation, at similar V concentration. This reflected on catalyst performance [14][15][16] , showing a higher selectivity (at isoconversion) for sample V10Si with This intriguing statement has been here verified by comparing some FP-prepared samples with similar ones prepared by co-gelation, a method which should also lead to high V dispersion, and with other samples prepared by V impregnation of FP-prepared supports.…”

“…This reflected on catalyst performance [14][15][16] , showing a higher selectivity (at isoconversion) for sample V10Si with This intriguing statement has been here verified by comparing some FP-prepared samples with similar ones prepared by co-gelation, a method which should also lead to high V dispersion, and with other samples prepared by V impregnation of FP-prepared supports.…”

“…at 550°C) for 15 minutes. Propane conversion progressively decreased with time-on-stream due to oxygen depletion from the catalyst and contemporarily propylene selectivity increased [14][15][16] . However, the most interesting points to characterise the performance refer to activity of the fully oxidised catalysts, as reported in Fig.12.…”

“…This quenches the system in a metastable structure, where V remains finely incorporated into the support matrix as in a mixed oxide. Such situation is not easily allowed during the final calcination step of the traditional preparation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 routes, such as co-gelation, which leads to V segregation and aggregation on the catalyst surface even at relatively low V loading [14][15][16] .The local structures around each sample, as obtained by the EXAFS fittings, are summarised in Tables 3 and 4, with good agreement with literature data 38,39 . Numbers without errors have been kept constant to their crystallographic value during the fitting procedure.…”