Oxygen-assisted conversion of propane over metal and metal oxide catalysts Leiv Låte Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

Outline Introduction and background Experimental Conventional routes to alkenes Oxidative systems, ODH/ADH/SHC Experimental Results and discussion Propane/O2 Propane/H2/O2 Propane/Propene/H2/O2 Conclusions

Light alkenes (propene) Light alkenes is a growing market Catalytic dehydrogenation of alkanes is commercially available e.g. UOP Oleflex, ABB/Catofin, Snamprogetti, Phillips STAR, Linde Processes use Pt-based or Cr-based catalysts The reaction is endothermic and requires high temperatures: Heat transfer and coke removal dominates the the reactor process design

Oxidative dehydrogenation C3H8 + ½O2 C3H6 + H2O H° = -118 kJ/mole An alternative is the oxidative dehydrogenation Exothermal, no equilibrium limitation Oxidic catalysts (e.g. V-Mg-O) No H2 produced Poor selectivity and propene yield (loss to COx)

New proposal: Catalytic dehydrogenation combined with selective H2 combustion Combine the selective catalytic dehydrogenation with in situ H2 combustion Provides heat in reactor Drives equilibrium towards products Oxygen/steam atmosphere reduces coking C3H8 C3H6 + H2 H° = +124 kJ/mole H2 + 0,5O2  H2O H° = -242 kJ/mole Heats of endo - and exothermic reaction approximately balanced if about 50% of the produced hydrogen is combusted Not so new, similar ideas were discussed some 50 years ago:*Calderbank et al., J. Appl. Chem Aug. 7, 425 (1957) and McCullogh and Walton, Ind. Eng. Chem., 41, 1455 (1949)

Autothermal catalytic dehydrogenation Oxidation part Main issues Find selective catalyst that combusts H2 without excessive combustion of hydrocarbons Selectivity Stability (steam environment, coking problems) Reactor and process design that allows the introduction of O2 (or air) to a hydrocarbon/hydrogen system (safety issues) Suitable catalyst candidates Some materials have been tried Oxides of Sb, Bi etc. (Grasselli et al.) Pt-based systems (UOP) Only a few studies available Catalytic combustion usually performed in lean conditions Selectivity issues in hydrocarbon - H2 mixtures are not studied

ODH of propane over a MgVO catalyst Part I ODH of propane over a MgVO catalyst Widely studied system Most promising ODH catalyst system High activity Investigation of propane and oxygen reactivity Investigation of product selectivity

SHC over metal catalyst Part II SHC over metal catalyst Obvious catalyst candidate: Pt Dehydrogenation catalyst Hydrogen combustion catalyst But also active for hydrocarbon combustion Sn often used as promoter (for dehydrogenation catalysts) Experimental results using SiO2-supported PtSn (and compared to Pt /SiO2 and Sn/SiO2) Investigation of oxygen reactivity and selectivity to CO and CO2 Propane and propene investigated

SHC over metal oxide-based catalysts Part III SHC over metal oxide-based catalysts Study of oxygen reactivity and selectivity to CO2 and CO Propane and propene investigated Propene more reactive towards oxygen than propane

Catalysts

Experimental Systems studied All experiments shown here Propane/O2 (ODH) Propane/H2/O2 (initial dehydrogenation conditions) Propane/Propene/H2/O2 (simulated high conversion of propane) All experiments shown here Total flow 100 ml/min (balance made up by He) H2/O2/propane/propene ratios varied Always starting with reducing conditions (no O2-flow) Temperature range: 500-550 °C In separate experiments the effect of pre-reduction of the catalysts was investigated. This effect was small, and mostly influenced the dehydrogenation activity. It is likely that the Pt-catalysts are reduced by the reacting mixture at start -up.

Apparatus Conventional continuous flow microreactor Reactor made from quartz Wall effects and homogeneous reaction ruled out Only dry system investigated (no additional water feed)

ODH of propane over a MgVO catalyst C3H8/Air/He = 5/30/65. W = 0.1g

ODH of propane over a MgVO catalyst

ODH of propane over a MgVO catalyst Selectivity to propene 60 % at 10 % conversion Reaction rate of propane: 1. order in propane partial pressure Close to zeroth order in oxygen partial pressure Mars Van Krevelen mechanism Activation of hydrocarbon slow step

SHC over Pt based catalysts Propane/H2 (10/2) feed with O2 addition C3H8 O2 conversion Over PtSn/SiO2 and Pt/SiO2 the conversion is high (>95%) Always a small fraction (<5%) of unconverted O2 - diffusion limitation? Over Sn the O2-conversion drops with increasing flow Propane conversion is the result of the sum of several reactions Dehydrogenation dominates at low O2 -flows Hydrocarbon combustion more important at higher O2-flows Conditions: 550 °C, propane/H2 10/2 ml/min, O2 shown in Figure, He balance to 100 ml/min.

SHC over Pt based catalysts Propane/H2 (10/2) feed with O2 addition PtSn (and also Sn) gives a high selectivity to water Oxygen reacts selectively with hydrogen up to the stoichiometric ratio As long as hydrogen is in excess the SHC principle can be applied At higher oxygen flows mostly CO2, but also some CO is formed (not shown) Conditions: 550 °C, propane/H2 10/2 ml/min, O2 shown in Figure, He balance to 100 ml/min.

SHC over Pt based catalysts Simulated high conversion with O2 addition Feed with a high concentration of propene: propene/propane/H2 = 28.5/5/2 ml/min O2 conversion high (>95%) As for propane feed possible diffusion limitation or bypass of oxygen No interconversion of hydrocarbons (“equilibrium”), but small changes in the ratio propene/propane due to reaction with oxygen Conditions: 550 °C, propane/H2 10/2 ml/min, O2 shown in Figure, He balance to 100 ml/min.

SHC over Pt based catalysts Simulated high conversion with O2 addition Feed with a high concentration of propene: propene/propane/H2 = 28.5/5/2 ml/min Some COx formed even at low O2 feed-rates PtSn most selective catalyst, but SHC-concept less applicable with high propylene concentrations Different trend for CO formation over Pt compared to PtSn and Sn catalysts Conditions: 550 °C, propane/H2 10/2 ml/min, O2 shown in Figure, He balance to 100 ml/min.

SHC over Pt based catalysts Mechanism (speculation) Langmuir-Hinshelwood mechanism Oxygen strongly adsorbed on Pt and Sn Alkanes do not adsorb on Sn Competitive adsorption of hydrogen and propane on Pt Propene adsorb on Pt and Sn H2(g) 2H(a) O2(g)  2O(a) H(a) + O(a) OH(a) OH(a) + H(a)  H2O (g)

SHC over metal oxide catalysts Comparison of different catalysts Conditions: 0,11 g catalyst, 1 atm, Feed: 100 ml/min, H2/O2/propane/He=2/1/10/87

Influence of oxygen feed rate In2O3/SiO2 (500 and 550 °C) Conversion O-selectivities 550 °C 500 °C H2O CO2 Indium oxide is active and selective in SHC Good selectivity to water also with excess oxygen (oxygen conversion is low) Propane conversion is low, but high cracking selectivity (not shown)

Influence of oxygen feed rate PbO/SiO2 (500 °C) Lead shows low activity to combustion reactions Selectivity stable over a wide range of oxygen partial pressure Some SHC but mainly hydrocarbon combustion

Influence of oxygen feed rate Bi2O3-catalysts (500 °C) Unsupported Bi2O3 is selective, but shows low activity (also some deactivation) Supported Bi2O3 less selective, but more stable and active

High propene concentration (500 °C) Indium oxide most active and selective, but propylene in the feed leads to more COx Bi2O3 (unsupported) combusts some hydrogen, but propene is combusted at a higher rate - from stoichiometry a factor 6 Supported Bi2O3 and PbO are pure hydrocarbon combustion catalysts at these conditions

Conclusions I ODH over MgVO shows a 60 % selectivity at 10 % conversion Selectivity loss due to formation of COx Selectivity strong function of conversion No gas phase oxygen involved in the reaction Only lattice oxygen from the catalyst lattice taking part Mars Van Krevelen type of reaction mechanism Due to lack of selectivity the catalyst is not suitable as a dehydrogenation catalyst

Conclusions II Hydrogen can be selectively combusted in the presence of hydrocarbons An excess of hydrogen is necessary (H2 + 0,5O2  H2O) PtSn/SiO2 and Sn/SiO2 catalysts acts selectively without propene in the feed At high concentrations of olefin the selectivity is poorer and COX are initial products The selectivity corresponds closely to selective combustion of hydrocarbons

Conclusions III Some oxide catalysts have been found to have potential as SHC catalysts In2O3/SiO2 active and selective Bi2O3-based catalysts shows some selectivity but poor activity

Oxygen-assisted conversion of propane over metal and metal oxide catalyst Leiv Låte Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway Acknowledgements: Norwegian Research Council Financial support Professor Edd A. Blekkan Supervision Willy Thelin and Jarl-Inge Rundereim Experimental assistance