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

Abstract

Site isolation of V active sites has been often correlated to catalytic performance for the oxidative dehydrogenation (ODH) of propane to propylene. In particular, catalyst selectivity seems favoured by high V dispersion. The latter property is hardly attainable by traditional preparation methods, especially by impregnation except at very low V loading, which however may lead to a too high surface exposure of the acidic sites of the support. In this paper, the effect of the preparation procedure on catalyst properties has been investigated, particularly considering catalysts prepared by flame pyrolysis, a synthesis method which induced a very high V dispersion also at relatively high vanadium loading.Transmission electron microscopy also allowed us to assess V oxide dispersion depending on both the support type and the preparation method. Furthermore, the local structure of the V active sites has been deeply investigated by X-ray Absorption Spectroscopy, allowing us to propose a possible structure of the active sites.The average oxidation state of surface V species was then studied by X-ray Photoelectron Spectroscopy (XPS), showing a role of V oxidation state on catalyst selectivity. The catalytic performance has been interpreted on the basis of V species and catalyst acidity (as measured by IR spectroscopy), another fundamental parameter that in turn results to be correlated with V dispersion on different supports. More selective catalysts were indeed characterized by the presence of weaker Brønsted acidic sites.Keywords: Oxidative dehydrogenation (ODH) of propane; Propylene production; V-based catalysts; X-ray absorption. Page 2 of 53 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry 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 3 -IntroductionLight olefins are important building blocks for polymers and intermediates industry 1 . The need for light olefins is progressively growing, with a higher rate for propene, leading to the necessity of alternative production routes 2 . Direct dehydrogenation can be an interesting alternative when low value feedstock is available. However, it is an endothermal reaction needing a high energy input, it is equilibrium controlled and the catalyst may be easily deactivated by coking. The oxidative dehydrogenation (ODH) of light paraffins to the corresponding olefins has been proposed as a means to overcome all these limitations. In addition, oxygen limits severe catalyst coking 3 . In spite of the considerable attention paid to this process and testified by a certain number of patents (see e.g. 4,5 and references therein), the ODH technology can be hardly considered mature and it has not yet reached industrial application due to selectivity issues. Indeed, oxygen should ideally consume H 2 , but paraffins and (most of all) olefins combustion is an unavoidable side reaction, w...

Propane Oxidative Dehydrogenation on Vanadium-Based Catalysts under Oxygen-Free Atmospheres

“…The molecular structures of surface vanadia species on metal oxide supports have been reported in the technical literature [85][86][87][88]. These studies suggest that, depending on the vanadium loading on the support, four kinds of VO x surface species can be present on the catalyst surface: (a) isolated VO 4 species (monovanadate); (b) polymeric VO 4 species (polyvanadates); (c) a mix of both isolated and polymeric VO 4 surface species; and, (d) V 2 O 5 crystals.…”

Oxidative dehydrogenation of propane on silica-supported vanadyl sites promoted with sodium metavanadate

“…45 This indicates that the activity and selectivity of the V-based catalysts for ODP does not depend solely on their reducibility 6,43 but is influenced by other factors, for instance, the interaction with the support, 20,43 dispersion of the active phase, 29,46,47 V-O binding energy, 45 and acid-base properties. 48 Considering that α-NaVO 3 /1 and β-NaVO 3 /1 contain similar species, but show distinct catalytic activity (vide infra), we were interested to compare H 2 -TPR profiles of these two catalysts. A slightly lower T max was observed for the more active catalyst, β-NaVO 3 /1, as compared to α-NaVO 3 /1 (555 and 580°C, respectively, Fig.…”

Structured Monolithic Catalysts vs. Fixed Bed for the Oxidative Dehydrogenation of Propane

“…An important parameter is the amount of vanadium oxide on the support, which determines the nature of the active species, their dispersion and consequently influences the reaction mechanism [17,18,19,20,21,22,23,24,25]. It was demonstrated that the selectivity to propene was higher in the presence of isolated VO x species.…”

“…It was demonstrated that the selectivity to propene was higher in the presence of isolated VO x species. By increasing the vanadium loading, the isolated VO x species tend to oligomerize, with the formation of V 2 O 5 species: the higher the vanadium load, the greater the conversion, but the lower the propene selectivity, as vanadia favours the complete combustion to CO and CO 2 [21,22,24,25,26,27].…”