Oxygenassisted conversion of propane over metal and metal oxide catalysts

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

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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

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Oxidative dehydrogenation

• 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)

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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

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Autothermal catalytic dehydrogenationOxidation
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

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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

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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

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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

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Catalysts
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Experimental

• Systems studied
• 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

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Apparatus

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

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

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SHC over Pt based catalystsPropane/H2 (10/2)
feed with O2 addition
O2
C3H8

• O2 conversion
• Over PtSn/SiO2 and Pt/SiO2 the conversion is high
  (gt95)
• Always a small fraction (lt5) 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

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SHC over Pt based catalystsPropane/H2 (10/2)
feed with O2 addition
H2O
CO2

• 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)

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SHC over Pt based catalystsSimulated high
conversion with O2 addition

• Feed with a high concentration of propene
  propene/propane/H2 28.5/5/2 ml/min
• O2 conversion high (gt95)
• 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

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SHC over Pt based catalystsSimulated high
conversion with O2 addition
H2O
CO2
CO

• 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

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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)
?
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SHC over metal oxide catalystsComparison of
different catalysts
Conditions 0,11 g catalyst, 1 atm, Feed 100
ml/min, H2/O2/propane/He2/1/10/87
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Influence of oxygen feed rateIn2O3/SiO2 (500 and
550 C)

• 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)

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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

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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

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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

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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

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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

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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

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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