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Oxidative dehydrogenation of propane with CO2 over composite metal oxides

Tsokanis Christos

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Summary

Propylene (C3H6) is one of the most important petrochemical products with wide use in industry, and its demand has been growing intensively in recent decades. It is mainly derived through the steam cracking (SC) and fluid catalytic cracking (FCC) which are the main production processes. However, due to the growth in propylene demand, new innovative processes are being explored to address these needs. One of them is the oxidative dehydrogenation of propane (ODHP), using O2, CO2, N2O or SO2 as oxidant. The use of CO2 as an oxidant is of great interest, due to its mild oxidative action combined with the environmental benefits of its use. The main objective of the present study is the synthesis and evaluation of novel metal oxide-based catalytic systems for the oxidative dehydrogenation of propane with CO2 (ODHP-CO2). For this purpose, composite metal oxides 10% MxOy-TiO2, 10% MxOy- Al2O3 and 10% MxOy- SiO2 (MxOy: Ga2O3, Cr2O3, CaO, SnO2) were prepared by the wet impregnation method and characterized by N2 absorption (BET) and X-ray diffraction (XRD) techniques. The catalytic activity for the ODHP-CO2 reaction was investigated in the temperature range of 500-750 oC with a feed composition consisting of 5%C3H8+25%CO2/He. The results showed that the propane conversion and propylene yield are affected by the nature of the metal oxide (Ga2O3, Cr2O3, CaO, SnO2) dispersed on the support (TiO2, Al2O3 ή SiO2) with the catalysts containing Ga2O3 or Cr2O3 exhibiting optimal results. In particular, the addition of metal oxides on TiO2 carrier leads to a shift of the propane conversion curve towards lower temperatures, following the order 10%Cr2O3-TiO2 ≈ 10%Ga2O3-TiO2 >> 10%CaO-TiO2 > 10%SnO2-TiO2 ≈ TiO2. The optimal catalysts 10%Cr2O3-TiO2 and 10%Ga2O3-TiO2 leads to propane conversion (XC3H8) equal to 80% and propylene yield (YC3H6) equal to 18% at 750 oC. Similar results were found for metal oxides supported on Al2O3 or SiO2, with the activity following the order 10%Cr2O3-Al2O3 ≈ 10%Ga2O3-Al2O3 >> 10%SnO2-Al2O3 ≥ 10%CaO-Al2O3 ≥ Al2O3 and 10%Cr2O3-SiO2> 10%Ga2O3-SiO2 ≥ 10%SnO2-SiO2 > SiO2 > 10%CaO- SiO2, respectively. Optimum propane conversions and propylene yields were achieved for the 10%Cr2O3-Al2O3 (XC3H8=80%, YC3H6 =20% at 750 οC), 10%Ga2O3-Al2O3 (XC3H8=80%, YC3H6=20% at 750 οC) and 10%Cr2O3-SiO2 (XC3H8=80%, YC3H6 =30% at 750 οC) catalysts. Furthermore, the effect of CO2:C3H8 molar ratio in the feed for the 10%Ga2O3-Al2O3 and 10%Ga2O3-ΤιΟ2 catalysts was studied and the results showed that the catalytic activity is not significantly affected with increasing the CO2:C3H8 ratio from 1:1 to 10:1. Finally, the 10%Ga2O3- Al2O3, 10% Ga2O3-ΤιΟ2 and 10% Ga2O3-SiO2 catalysts were subjected to long-term stability experiments at 660 oC (600 oC for the 10%Ga2O3- Al2O3 catalyst) and 710 oC. With the exception of the 10%Ga2O3- Al2O3 catalyst which is deactivated only when the reaction is carried out at 600 oC, the other catalysts show excellent stability for 30-35 h on stream at both temperatures tested.

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