Study of reaction parameters in the ODH of propane on VOx/HMS silica nanostructured catalysts
S.A. Karakoulia 1,2, K. S. Triantafyllidis 3, A. A. Lemonidou 1,2
1 Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2 Chemical Process Engineering Research Institute, CERTH, 57001 Thessaloniki, Greece
3 Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Catalytic oxidative dehydrogenation (ODH) of light alkanes provides a promising alternative route for the efficient production of C2 - C4 gaseous alkenes, which are widely used as raw materials in the chemical and petrochemical industry. Vanadia supported on oxides (ca. Al2O3, TiO2, SiO2) have been reported to be selective catalysts for the ODH of short-chain alkanes towards the production of the respective alkenes [1]. A crucial factor for catalyst effectiveness is the metal oxide dispersion and stability on the support materials. To this end, the use of high surface area mesoporous supports could provide better dispersion of metals compared to classical oxides even at relatively high loadings, resulting to the formation of sub-nanosized supported metal particles.
In a previous study we have shown that vanadia catalysts supported on mesoporous silicas (MCM-41, HMS, SBA-15, MCF) were far more active (up to 35-40% conversion of propane) than the respective catalysts supported on non-porous silica and were also remarkably selective (up to 50
% selectivity and 19% yield) towards the production of propene [2]. This performance was mainly attributed to the highly dispersed monomeric V5+ species (confirmed by Raman measurements), which could be succeeded even at relatively high V loadings (ca. 8 wt.% V). With regard to the type of mesoporous supports, the most promising catalytic results were achieved with the HMS mesoporous silica which possesses a 3-D relatively disordered (wormhole-like) pore structure with average framework pore diameter of ~3 nm and additional high textural porosity due to interparticle voids (formed between primary silica nanoparicles).
In an effort to further optimize the performance of the VOx/HMS nanostructured catalyst, we studied in this work the effect of various reaction parameters on catalyst’s activity and selectivity in the ODH of propane: a) W/F (T=550oC, pC3H8=90mbar, pO2=90mbar, W/F=0.02-0.2g·s·ml-1), b) partial pressure of C3H8
(T=550oC, W/F=0.06g·s·ml-1, pO2=90mbar, pC3H8=38-90mbar), c) partial pressure of O2
(T=550 oC, W/F=0.06g·s·ml-1, pC3H8=90mbar, pO2=20-90mbar) and d) type of oxidant (T=450- 600oC, F=105ml·min-1, C3H8/O2/CO2= 5/x/y with x=0,2,5, y=7,5,0).
By comparing the catalytic tests at varying reaction temperature with those at constant temperature but with varying W/F, it was shown that the selectivity of the ODH products is entirely controlled by the degree of the alkane conversion and is independent of the reaction temperature (see Figure). Increase in the partial pressure of propane (with pC3/pO2 ≤ 1) had a minor effect in activity and selectivity of the catalyst. On the contrary, increase in the partial pressure of O2 (with pC3/pO2 ≥ 1) resulted in linear increase in the propane conversion (following first order reaction kinetics) and decrease in propene selectivity. Finally, it was shown that the use of CO2 as oxidant, instead of O2, had a negative effect since the propane converion was remarkably decreased while propane selectivity remained at the same level values.
Co-funding of this work by the EU and the Greek Ministry of Development-GSRT throught the program PENED 2003 is gratefully acknowledged.
[1] T. Blasco, J.M. Lopez Nieto, Appl. Catal. A 157 (1997) 117.
[2] S.A. Karakoulia, K.S. Triantafyllidis, A.A. Lemonidou, Microp. Mes. Mater. 110 (2007) 157.
0 5 10 15 20 25 30 35
0 20 40 60 80 100
T=550oC, increasing W/F (0.02-0.08g s ml-1) W/F=0.06g s ml-1, increasing T (450-600oC)
Selectivity of propene, %
Conversion of propane, %
2 w.t.% V - HMS
PC-63
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