1. 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

2. 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

3. 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

4. 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)

5. 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 + H H° = kJ/mole
H ,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)

6. 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

7. 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

8. 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

9. 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

10. Catalysts

11. 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: °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.

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

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

14. ODH of propane over a MgVO catalyst

15. 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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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)

21. 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

22. 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)

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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