Propane Oxidative Dehydrogenation Using Consecutive Feed Injections and Fluidizable VO_x/γAl₂O₃ and VO_x/ZrO₂–γAl₂O₃ Catalysts

Abstract

Propane oxidative dehydrogenation (PODH) was studied using VO x /γAl 2 O 3 and VO x /ZrO 2 −γAl 2 O 3 catalysts and consecutive propane injections. These catalysts were synthesized with 2.5, 5, and 7.5 wt % vanadium (V) loadings. Temperatureprogrammed reduction by hydrogen displayed one reduction peak for VO x /γAl 2 O 3 and two for VO x /ZrO 2 −γAl 2 O 3 (5 and 7.5 wt % vanadium). Temperature-programmed desorption of ammonia (NH 3 -TPD) and pyridine Fourier transform infrared spectroscopy showed that zirconia on γAl 2 O 3 reduces the catalyst acidity. NH 3 -TPD kinetics gave for VO x /ZrO 2 −γAl 2 O 3 higher desorption activation energies than those for VO x /γAl 2 O 3 . PODH runs in the Chemical Reactor Engineering Center Riser Simulator were developed under an oxygen-free atmosphere at 550 °C, close to 1 atm, 20 s, and a 42.0 catalyst/propane weight ratio (g/g). PODH runs for the 7.5% V/ZrO 2 −γAl 2 O 3 showed (a) 93% propylene selectivity and 25% propane conversion (based on propane converted into gaseous carbon-containing products), (b) 85% propylene selectivity at 28% propane conversion (based on propane converted including coke). The CO x selectivity remains at 2%. This makes the 7.5% V/ZrO 2 −γAl 2 O 3 catalyst a promising one for anticipated PODH industrial applications.

Vanadium Supported on Alumina and/or Zirconia Catalysts for the Selective Transformation of Ethane and Methanol

“…In general, the total H 2 -uptake increases when increasing the V-loading in good agreement with previous results reported by other authors [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. This suggests that all V-atoms are involved in the TPR experiments.…”

“…According to kinetic aspects of the ODH reactions [5][6][7]17,23,26,27,61,62] and the results obtained in the present work a simplified reaction network can be tentatively proposed. It must be mentioned that more catalytic data would be needed for a comprehensive kinetic analysis.…”

“…The first reduction peak can be tentatively assigned to isolated or polymeric V-species [15,56], in which the reducibility of V-species in ZrO 2 -supported catalysts are higher than those in Al 2 O 3 -supported catalysts [56]. The second peak can be related to V 2 O 5 crystallites on the surface of the support (mainly observed in catalysts with high V-loading), although probably with relatively low crystal size [51], and/or attributed to the formation of small ZrV 2 O 7 crystals [27]. In this way, the reduction of pure V 2 O 5 appears at temperatures higher than 600 • C ( Figure S1).…”

“…Supported vanadium oxide catalysts have been considered for many years the most promising catalysts in the ODH of C 2 -C 4 alkanes [4][5][6][7][8][9][10][11][12][13][14], although the nature of the optimal support is different depending on the nature of the alkane fed [5][6][7]14,15]. The catalytic behavior of supported vanadia catalysts can be related to [5][6][7][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]: (i) the nature of V n+ -O species, i.e., coordination, aggregation and oxidation state of V-species; (ii) the redox properties of the catalysts, i.e., the V 5+ /V 4+ ratio in reaction conditions, which are influenced by the presence of promoters and the nature of the metal oxide support; and (iii) the acid-base character of the catalysts (and support), which strongly influences the adsorption of reactants and the desorption of partial oxidation products and finally determines the selectivity to olefins. Depending on the support material and the VOx concentration, supported vanadium oxide can exist as isolated tetrahedral monovanadates, one or two-dimensional polyvanadate domains, or bulk V 2 O 5 crystallites [5][6][7]12,23].…”

“…Thus, zirconia has bifunctional properties as an acid and as a base, and high thermal stability. More interestingly ZrO 2 [25] and Al 2 O 3 -ZrO 2 [26,27] are appropriate supports for the synthesis of highly dispersed VOx-species. As highly-isolated vanadium species tend to overoxidize ethylene to a lower extent than more aggregated species [5][6][7][8][9][10][11] we have thought that by modifying the alumina support through the addition of zirconia, the ethylene decomposition could be mitigated.…”