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

– 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

C3H8 C3H6 + H2 DH° = 124 kJ/mole

• The reaction is endothermic and requires high temperatures: Heat transfer and coke removal dominates the the reactor process design

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)

C3H8 + ½O2 C3H6 + H2O DH° = -118 kJ/mole

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 DH° = +124 kJ/mole

H2 + 0,5O2 H2O DH° = -242 kJ/mole

• Heats of endo - and exothermic reaction approximately balanced if about 50% of the produced hydrogen is combusted

Autothermal catalytic dehydrogenationOxidation part

– 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

• Main issues

– Find selective catalyst that combusts H2 without excessive combustion of hydrocarbons• Selectivity• Stability (steam environment, coking problems)

Part I

• Widely studied system• Most promising ODH catalyst system

–High activity

• Investigation of propane and oxygen reactivity

• Investigation of product selectivity

ODH of propane over a MgVO catalyst

Part II

• 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 catalyst

Part III

• Study of oxygen reactivity and selectivity to CO2 and CO

• Propane and propene investigated– Propene more reactive towards oxygen than

propane

SHC over metal oxide-based catalysts

Catalysts

Catalyst Support Surface area (BET)(m2/gcat)

Crystalline phases(XRD)

SiO2 - 483 n.d.Indium-oxide SiO2 278 In2O3 (cubic)

Bismuth-oxide SiO2 242 Bi2O3 (rombic)Bismuth-oxide none n.d. n.d.

MgVO none 38 MgO/Mg3V2O8

Lead-oxide SiO2 120 PbO (amorphous)Chromium-oxide SiO2 330 Cr2O3 (rombohedral)

ZSM-5 none n.d. -1% Pt SiO2 415 -

0.35%Pt- 1% Sn SiO2 428 -1% Sn SiO2 457 -

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

Apparatus

• Conventional continuous flow microreactor– Reactor made from quartz – Wall effects and homogeneous reaction ruled out

• Only dry system investigated (no additional water feed)

PM2

MFC1

MFC

5

MFC4

MFC3

MFC

2

PM1

O 2

U TM1

TC1

G C

Reactor

ODH of propane over a MgVO catalyst

Temperature (°C)

460 480 500 520 540 560

Sel

ectiv

ity (

%)

0

10

20

30

40

50

60

Con

vers

ion

(%)

0

10

20

30

40C3H6

CO2

CO

O2 C3H8

(*)

(*)

(*)

(*)

C3H8/Air/He = 5/30/65. W = 0.1g

ODH of propane over a MgVO catalyst

Conversion (%)

0 10 20 30 40 50 60

Sel

ectiv

ity (

%)

10

20

30

40

50

60

70

80

This workChaar et al. [1988]Pantazidis [1996]

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

O2-flow ml/min

0 1 2 3 4

Con

vers

ion

of o

xyge

n %

80

85

90

95

100

O2-flow ml/min

0 1 2 3 4

Con

vers

ion

of p

ropa

ne %

0

2

4

6

8

10

12

14

O2-flow ml/min

0 1 2 3 4

Sel

ectiv

ity to

pro

pene

%

0

20

40

60

80

100

Pt

Sn

PtSn

Pt

PtSn

Sn

PtSn

Pt

Sn

O2-flow ml/min

0 1 2 3 4

Sel

ectiv

ity to

C1

and

C2

0

5

10

15

20

25

Sn

Pt

PtSn

a b

c d

O2-flow ml/min

0 1 2 3 4

Sel

ectiv

ity to

H2O

%

20

40

60

80

100

PtSn

Sn

Pt

e

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

• 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

O2 C3H8

O2-flow ml/min

0 1 2 3 4

Co

nve

rsio

n o

f o

xyg

en

%

80

85

90

95

100

O2-flow ml/min

0 1 2 3 4

Co

nve

rsio

n o

f p

rop

an

e %

0

2

4

6

8

10

12

14

O2-flow ml/min

0 1 2 3 4

Se

lect

ivity

to

pro

pe

ne

%

0

20

40

60

80

100

Pt

Sn

PtSn

Pt

PtSn

Sn

PtSn

Pt

Sn

O2-flow ml/min

0 1 2 3 4

Se

lect

ivity

to

C 1 a

nd

C2

0

5

10

15

20

25

Sn

Pt

PtSn

a b

c d

O2-flow ml/min

0 1 2 3 4

Se

lect

ivity

to

H 2O %

20

40

60

80

100

PtSn

Sn

Pt

e

SHC over Pt based catalystsPropane/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)

O2-flow ml/min0 1 2 3 4

O-

sele

ctiv

ity

to C

O 2 %

0

20

40

60

80

O2-flow ml/min

0 1 2 3 4

C-s

ele

ctiv

ity

to C

O 2 %

0

20

40

60

80

O2-flow ml/min

0 1 2 3 4 5

O-S

ele

ctiv

ity t

o C

O %

0

2

4

6

8

10

12

14

O2-flow ml/min

0 1 2 3 4 5

C-s

ele

citv

ity t

o C

O %

0

2

4

6

8

10

12

14

16

Pt

Pt

Pt

Pt

Sn

Sn

Sn

Sn

PtSn PtSn

PtSn

PtSn

a b

c d

H2O CO2

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 (>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

O2-flow ml/min

0 1 2 3 4 5

O2-

con

vers

ion

%

88

92

96

100

O2-flow ml/min

0 1 2 3 4 5

O -

Se

lect

ivity

CO 2

%

20

40

60

80

O2-flow ml/min

0 1 2 3 4 5

O -

Se

lect

ivity

CO

%

4

8

12

16

O2-flow ml/min

0 1 2 3 4 5

Se

lect

ivity

H2O

%

20

40

60

80a b

d

PtSn

PtSn

Pt

PtSn

Sn

c

PtSn

Sn

Pt

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

• 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

O2-flow ml/min

0 1 2 3 4 5

O2-

conv

ersi

on %

88

92

96

100

O2-flow ml/min

0 1 2 3 4 5

O -

Sel

ectiv

ity C

O2

%

20

40

60

80

O2-flow ml/min

0 1 2 3 4 5

O -

Sel

ectiv

ity C

O %

4

8

12

16

O2-flow ml/min

0 1 2 3 4 5

Sel

ectiv

ity H

2O %

20

40

60

80a b

d

PtSn

PtSn

Pt

PtSn

Sn

c

PtSn

Sn

Pt

O2-flow ml/min

0 1 2 3 4 5

O2-

conv

ersi

on %

88

92

96

100

O2-flow ml/min

0 1 2 3 4 5

O -

Sel

ectiv

ity C

O2

%

20

40

60

80

O2-flow ml/min

0 1 2 3 4 5

O -

Sel

ectiv

ity C

O %

4

8

12

16

O2-flow ml/min

0 1 2 3 4 5

Sel

ectiv

ity H

2O %

20

40

60

80a b

d

PtSn

PtSn

Pt

PtSn

Sn

c

PtSn

Sn

Pt

H2O CO2 CO

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 catalystsComparison of different catalysts

Conditions: 0,11 g catalyst, 1 atm, Feed: 100 ml/min, H2/O2/propane/He=2/1/10/87

Conversion (%) C-selectivities (1) O-selectivitiesCatalystPropane O2 Propylene CO2+CO H2O CO2+CO

In2O3/SiO2 (500 °C) 0 86,2 - - 100 -In2O3/SiO2 (550 °C) 1,7 90,3 20,1 12,0 93,8 6,2Bi2O3 (500 °C) (2) - 15,8 - - 100 -Bi2O3 (550 °C) (2) - 34,8 - - 100 -Bi2O3/SiO2 (500 °C) 1,1 38,8 16,1 68,6 71,9 28,1PbO/SiO2 (500 °C) 0,7 15,2 40,7 39,8 75,4 24,6Cr2O3/SiO2 (500 °C) 8,9 95,9 40,9 59,1 35,0 65,0VMgO (500 °C) 5,7 97,6 59,5 40,5 45,2 54,7SiO2(500 °C) - 4,0 - - 100 -empty react. (500°C) - 4,0 - - 100 -Notes:1) Difference to 100% is cracking products (ethylene + methane)2) Strong deactivation

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)

O2-flow ml/min

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O2

-con

vers

ion

%

40

50

60

70

80

90

100

O2-flow ml/min

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O-s

ele

ctiv

ity %

0

20

40

60

80

100

Conversion O-selectivities

550 °C

500 °C

550 °C

H2O

CO2

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

O2-flow ml/min0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O2

-co

nve

rsio

n %

10

12

14

16

18

20

22

24

26

Pro

pan

e c

on

vers

ion

%

0,0

0,5

1,0

1,5

2,0

O2-flow ml/min0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O-s

ele

ctiv

ity %

20

30

40

50

60

70

80Conversion O-selectivities

H2O

CO2

Influence of oxygen feed rate Bi2O3-catalysts (500 °C)

O2-flow ml/min

0 1 2 3

O2-conversion %

0

20

40

60

80

100

Bi2O3/SiO2

O2-flow ml/min

0 1 2 3

O-selectivity %

0

20

40

60

80

100

Bi2O3/SiO2 CO2

Bi2O3

Bi2O3

Bi2O3/SiO2

H2O

• 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

O2-flow ml/min

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O2- conversion %

0

20

40

60

80

100

PbO

Bi2O3 (no support)

In2O3 Bi2O3/SiO2

O2-flow ml/min

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2

O-selectivity to H2O %

20

40

60

80

100

Bi2O3 (no support)

In2O3

PbO

Bi2O3/SiO2

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