Catalysis in supercritical fluids

Leiv Låte

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

Outline

• Background • Introduction – Definition of SCF – Media used as SCF – Advantages of SCF • Applications – Industrial use of SCF as reaction media – Research • Conclusions

Background

• Global increase in the environmental awareness • Chemical industry searching for new and cleaner processes • One obvious target is replacement of the solvent • Suitable candidates for replacement of organic solvents include SCF – scCO 2 – scH 2 O

Definition of a supercritical fluid

Definition by IUPAC A mixture or element: • Above its critical pressure (P c ) • Above its critical temperature (T c ) • Below its condensation pressure The critical point represents the highest T and P at which the substance can exist as a vapour and liquid in equilibrium

What is a supercritical fluid?

Appearance of a SCF

Characteristics of a supercritical fluid

• Dense gas – Densities similar to liquids – Occupies entire volume available • Solubilities approaching liquid phase – Dissolve materials into their components – Completely miscible with permanent gases (N 2 / H 2 ) • Diffusivities approaching gas phase – Viscosities nearer to gas – Diffusivity much higher than a liquid • Density, viscosity, diffusivity and solvent power dependent on temperature and pressure

Comparison of physical properties

Property Density (g/mL) Viscosity (Pa s) Diffusivity (cm 2 /s) Gas 10 -3 10 -5 0.1

SCF 0.3

10 -4 10 -3 Liquid 1 10 -3 5 × 10 -6

Which gases can be used as SCF?

• Any compressible gas – Possible to tune properties from gas like, through to liquid like • The most common SCF CO 2 N 2 NH 3 C 2 H 6 C 3 H 8 H 2 O T c (°C) 31.1

-147.0

132.5

32.2

96.8

374.1

P c (bar) 72.9

33.5

112.5

48.2

42.0

218.3

Supercritical CO

2 • Most widely used fluid • Similar to nonpolar organic solvents (n-hexane) – scCO 2 only suitable as a solvent for nonpolar substances – addition of cosolvents can modify the solute • Methanol • Toluene – Modifier moves the scCO 2 solvent” away from the ideal “Green • Mild critical parameters • Non toxic and non-flammable • Environmentally favourable • Thermodynamically stable • Inexpensive (plentiful)

Supercritical H

2

O

• Lower polarity than liquid water – Turns in to an almost

non polar

fluid • Dielectric constant drops from about 80 to 5 • Becomes miscible with organics and gases • Reduced density – about 1/3 of water – Increased diffusivity • Environmental favourable • Non toxic and non-flammable • Inexpensive (plentiful) • The foremost application for scH 2 O is oxidative destruction of toxic wastes • High supercritical temperature exclude scH 2 O – Limited thermal stability of organic reactants and products

Reaction solvent effects - pressure tunability Pressure tunability on density (  ), viscosity (  ) and D 11 · 

Pressure tunability

Ion product of water

Tunable density of SCF

Density tuning • Gain more direct information about a reacting system • No need for different solvents in a study • Can be used to control – Solvent polarity – Separation – Rate of reaction – Selectivity on catalytic surface reactions

Advantages of SCF

There is no point in doing something in a supercritical fluid just because it is neat “Val Krukonis”

• Energy cost due to elevated pressures and temperatures – More expensive than traditional solvent systems – Safety hazards related to high pressure and temperature Using the fluids must have some real advantage • Advantages fall into four categories – Environmental benefits – Health and safety benefits – Process benefits – Chemical benefits

Health, Safety and Environment benefits

• Replaces “less green” liquid organic solvents • No acute toxicity (H 2 O and CO 2 ) • No liquid wastes (except water) • Non-carcinogenic (except C 6 H 6 ) • Non toxic (except NH 3 ) • Non-flammable (CO 2 , H 2 O)

Chemical benefits

• High reaction rate due to: – Dissolving capabilities • High concentration of reactant gases ( H 2 / O 2 • Eliminating inter-phase transport limitations ) – Higher diffusivities than liquids – Better heat transfer than gases – Low viscosity • Variable dielectric constant (polar SCF) – Adjustable solvent power • Enhanced catalytic activity due to anti-coking of scCO 2 • Higher solubilites than corresponding gases for heavy organics – Improved catalyst lifetime • High product selectivities – Increased pressure may favour desired product selectivity

Process benefits

• Green chemistry – No use of organic solvents – Easier product separation • Adjustable density (adjustable solvent power) – Recycling of SCF possible – Less by-products • More efficient product/catalyst separation – Problem in homogeneous catalysis – No energy-intensive distillations • Higher reaction rate and facile product separation – Smaller reactors • Process safety • Space requirements • Inexpensive (CO 2 , H 2 O, NH 3 , Ar, Hydrocarbons)

Continous reactors

• Continuos reactors do not require depressurization like batch reactors • Catalyst fixed in the reactor – Simpler separation of catalyst and products than batch reactor • Parameters can be varied independently – Temperature, pressure, residence time, substrate flow rate • Fluid properties can be tuned in real-time to optimize reaction conditions • Smaller volume than batch reactors – More safe reactor • Good heat and mass transfer

Catalyzed reactions

Applications

• Alkylation • Amination • Cracking • Esterification • Fischer-Tropsch Synthesis • Hydrogenation • Isomerization • Oxidation • Polymerization

Industrial use of SCF as reaction media

Reaction Oxidation Polymerization Hydrogenation Hydrogenation Hydration Process/ Product SCWO LDPE Ammonia Methanol Alcohols SCF H 2 O C 2 H 4 H 2 / N 2 H 2 / CO / CO 2 C 2 H 4 / C 3 H 6 / C 4 H 8 Status Production Production Production Production Production

Hydrogenation of organic compounds

• Hydrogen has low solubility in most organic solvents – Hydrogen completely miscible with SCF • Reaction is not limited by mass transfer effects – High reaction rates • The fluid has good thermal properties – Facilitate heat removal • High degree of control over reaction parameters – Selectivity

Hydrogenation in scPropane

• Feed: Oil (fatty acid methyl esters), H 2 • Supercritical fluid: Propane ( T c = 96.8

°C, P c • Catalyst: Pd = 42.0 bar) • Reaction rate 400 times faster than traditional techniques – Reduced mass transfer limitations of H 2 in homogeneous phase

P. Møller, 3rd Int. Symp. On High Press. Chem. Eng., Zurich, 1996, 43-48

Catalytic amination of amino-1-propanol with scNH 3 • Catalyst Co-Fe (95/5) • Production of 1,3-diaminopropane • Tubular reactor – 195°C – Feed ratio R-OH / NH 3 – T c = 132, P c = 113 bar (1:40) 40 30 20 10 0 40 60 80 100

Pressure (bar)

P C = 113 bar 120 140

Fischer et.al, A. Angew. Chem., Int. Ed. Engl., submitted

Supercritical Fischer- Tropsch synthesis

• Classical synthesis involves an exothermic gas-phase reaction – Heat removal – Pore blocking and catalyst deactivation • Liquid-phase process – Improved heat transfer – Better solubilities of higher hydrocarbons – Lower diffusivity than gas-phase reaction • Mass transfer limitations • Lower reaction rate – Accumulation of high molecular-weight products in the reactor • New proposal – Supercritical conditions • Gas-like diffusivity • Liquid-like solubility

Supercritical Fischer- Tropsch synthesis

• High diffusivity of reactant gases – Homogeneous phase • Rate of reaction and diffusion of reactants – Slightly lower than gas-phase – But significantly higher than liquid • Effective removal of reaction heat • In situ extraction of high molecular weight hydrocarbons (wax)

Supercritical Fischer- Tropsch synthesis

• The SCF was selected by the following criteria: – T c and P c slightly below reaction temperature and pressure – SCF should not poison the catalyst – SCF should be stable under the reaction conditions – SCF have high affinity for aliphatic hydrocarbons to extract wax • Reaction temperature: 240°C and P tot =45 bar • n-Hexane chosen SCF T c = 233.7

°C P c = 30.1 bar • p(CO+H 2 )=10 bar, CO:H 2 =1:2 • Catalyst: Ru/Al 2 O 3

K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95

Supercritical Fischer- Tropsch synthesis

Reaction phase CO conversion (%) Effluent products (*) Chain growth probability (*) C-mmol/g-cat × h Gas 44.7

10.8

0.94

Supercritical 39.0

12.8

0.95

Liquid 28.0

8.82

0.85

• Different CO-conversions due to different rates of diffusion –

D

GASS

> D

SCF

> D

Liquid • Different Chain growth probabilities due to CO:H 2 diffusion – Similar SCF and gas diffusion inside the catalyst pores • Effective molar diffusion in the supercritical phase

K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95

Distribution of hydrocarbon products in various phases Carbon Number

Supercritical Fischer- Tropsch synthesis

• The alkene content decreased with increased carbon number for all phases – Increase in hydrogenation rate relative to diffusion rate – Longer residence time on catalyst surface for high molecular weight hydrocarbons • Higher alkene content in SCF – Alkenes were quickly extracted and transported by SCF out of the catalyst • Minimizing readsorption and hydrogenation

K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95

Wax production Addition of heavy alkene to the supercritical phase • Catalyst: Co-La/SiO 2 • Temperature: 220°C • Pressure: 35 bar • Supercritical fluid: n-pentane ( T c =196.6

°C, P c =33.7 bar) • p(CO+H 2 ) = 10 bar • Studied the effect of addition of heavy alkenes – Addition: 4 mol% (based on CO) – 1-tetradecene and 1-hexadecene

Fujimoto et al., Topics in Catal. 1995, 2, 259-266

Wax production Addition of heavy alkene to the supercritical phase 1000 800 600 400 200 0 0 With alkene addition 5 10 15 20

Carbon Number

25 30

Fujimoto et al., Topics in Catal. 1995, 2, 259-266

Wax production Addition of heavy alkene to the supercritical phase • Carbon chain growth accelerated by addition of alkenes • Alkenes diffuse inside the catalyst pores to reach the metal sites – Adsorb as alkyl radicals to initiate carbon chain growth • The resulting chains are indistinguishable from chains formed from synthesis gas • Addition of heavy alkenes does not have any effect in gas phase reactions

Fujimoto et al., Topics in Catal. 1995, 2, 259-266

Oxidation in scH

2

O (SCWO)

• SCWO of organic wastes – Complete oxidation to CO 2 • Single fluid phase • Faster reaction rates • Complete miscibility of nonpolar organic with scH 2 O • With or without heterogeneous catalyst • Motivation for catalyst: – Reduce energy and processing costs • Target: – Complete conversion at low temperatures and short residence time

t-butyl alcohol synthesis by air oxidation of supercritical isobutane • TBA can be converted to isobutene by dehydration • Commercial production of isobutene: Dehydrogenation – High temperatures: 500-600°C • Catalyst deactivation • Isobutane: T c = 135 °C, P c • Isobutane : air = 3 : 1 = 36.4 bar • Reaction temperature: 153°C • Reaction pressure: – 44 bar for gas phase reaction – 54 bar for supercritical phase reaction

Fan et al., Appl. Catal. 1997, 158, L41-L46

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst None None SiO 2 -TiO 2 SiO 2 -TiO 2 SiO 2 -TiO 2 SiO 2 -TiO 2 (a) Pd/C Pd/C Na 2 WO 4 /SiO 2 Na 2 WO 4 /SiO 2 Na 2 MoO 4 /SiO 2 Na 2 MoO 4 /SiO 2 Total Pressure (bar) 44 54 44 54 12 54 a 44 54 44 54 44 54 i-C 4 H 10 Conversion (%) 0.3

1.2

2.9

4.9

0.0

0.1

0.5

3.1

2.1

5.6

6.7

7.0

O 2 Conversion (%) 2.5

9.9

24.0

40.6

0.0

1.1

4.2

25.6

19.9

55.5

61.0

65.8

TBA selectivity (%) 55.0

58.1

59.0

61.2

0.0

55.5

61.2

64.8

48.1

51.8

25.3

31.7

i-C 4 H 8 Selec.

(%) 7.0

8.1

5.2

6.3

0.0

7.7

2.1

20.1

0.6

0.8

6.1

4.1

(a) Liquid-phase reaction where the reaction temperature was 130°C

Fan et al., Appl. Catal. 1997, 158, L41-L46

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2 , P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2 , P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2 , T=153°C

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2 , T=153°C

Friedel Crafts Alkylation Reactions

• Conventional reactions require: – Long reaction times – Low temperatures and – Use of environmentally “dirty” catalysts e.g. AlCl 3 or H 2 SO 4 – Separation of catalyst and solvent from the reaction mixture • Using supercritical CO 2 high product selectivity. allows reaction conditions to be tuned to get – Solvent removal is also easy using supercritical CO 2

Friedel Crafts Alkylation Reactions

Organic and water layers are easily separated to leave clean product

Alkylation of Mesitylene with Isopropanol in Supercritical CO 2 • 50% conversion of mesitylene to mono-alkylated product • No di-alkylated product

Conclusions

SCF (used as solvent or reactant) provides opportunities to enhance and control heterogeneous catalytic reactions: • Control of phase behaviour • Elimination of gas/liquid and liquid/liquid mass transfer resistance • Enhanced diffusion rate in reactions • Enhanced heat transfer • Easier product separation • Improved catalyst lifetime • Tunability of solvents by pressure and cosolvents • Pressure effect on rate constants • Control of selectivity by solvent- reactant interaction

Conclusions

• Reagents, cosolvents or products can change properties of SCF – Critical point for a reaction mixture can change through the reaction – Need more research before use in organic synthesis • scCO 2 only suitable as solvent for nonpolar substances • High supercritical temperature exclude scH 2 O – Limited thermal stability of organic reactants and products • Addition of reagents or cosolvents to SCF – Changed properties – Can interact with catalyst surface – Change surface properties of the catalyst – Makes the process “less green”

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