catalysis in supercritical fluids leiv låte department of chemical engineering, norwegian...

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

– scCO2

– scH2O

Definition of a supercritical fluid

Definition by IUPAC

A mixture or element:

• Above its critical pressure (Pc)

• Above its critical temperature (Tc)

• 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 (N2/ H2)

• 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 Gas SCF LiquidDensity (g/mL) 10-3 0.3 1Viscosity (Pa s) 10-5 10-4 10-3

Diffusivity (cm2/s) 0.1 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 commonSCF Tc (°C) Pc (bar)

CO2 31.1 72.9N2 -147.0 33.5NH3 132.5 112.5C2H6 32.2 48.2C3H8 96.8 42.0H2O 374.1 218.3

Supercritical CO2

• Most widely used fluid• Similar to nonpolar organic solvents (n-hexane)

– scCO2 only suitable as a solvent for nonpolar substances

– addition of cosolvents can modify the solute • Methanol • Toluene

– Modifier moves the scCO2 away from the ideal “Green solvent”

• Mild critical parameters• Non toxic and non-flammable• Environmentally favourable• Thermodynamically stable• Inexpensive (plentiful)

Supercritical H2O

• 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 scH2O is oxidative destruction of toxic wastes

• High supercritical temperature exclude scH2O

– Limited thermal stability of organic reactants and products

Reaction solvent effects - pressure tunability

Pressure tunability on density (), viscosity () and D11·

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 There is no point in doing something in a supercritical fluid just because it is neatfluid just because it is neat

“Val Krukonis”“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 (H2O and CO2)

• No liquid wastes (except water)

• Non-carcinogenic (except C6H6)

• Non toxic (except NH3)

• Non-flammable (CO2, H2O)

Chemical benefits• High reaction rate due to:

– Dissolving capabilities

• High concentration of reactant gases ( H2 / O2 )

• 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 scCO2

• 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 (CO2, H2O, NH3, 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

ApplicationsCatalyzed reactions

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

Industrial use of SCF as reaction media

Reaction Process/ Product SCF StatusOxidation SCWO H2O ProductionPolymerization LDPE C2H4 ProductionHydrogenation Ammonia H2 / N2 ProductionHydrogenation Methanol H2 / CO / CO2 ProductionHydration Alcohols C2H4 / C3H6 / C4H8 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), H2

• Supercritical fluid: Propane ( Tc = 96.8°C, Pc = 42.0 bar)

• Catalyst: Pd

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

• Reaction rate 400 times faster than traditional techniques– Reduced mass transfer limitations of H2 in homogeneous phase

Catalytic amination of amino-1-propanol with scNH3

• Catalyst Co-Fe (95/5)• Production of 1,3-diaminopropane• Tubular reactor

– 195°C

– Feed ratio R-OH / NH3 (1:40)

– Tc= 132, Pc = 113 bar

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

0

10

20

30

40

40 60 80 100 120 140

Pressure (bar)

Se

lec

tiv

ity

(%

)PC = 113 bar

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:

– Tc and Pc 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 Ptot=45 bar

• n-Hexane chosen SCFTc= 233.7°CPc= 30.1 bar

• p(CO+H2)=10 bar, CO:H2=1:2

• Catalyst: Ru/Al2O3

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

Supercritical Fischer- Tropsch synthesis

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

• Different CO-conversions due to different rates of diffusion

– DGASS > DSCF > DLiquid

• Different Chain growth probabilities due to CO:H2 diffusion

– Similar SCF and gas diffusion inside the catalyst pores

• Effective molar diffusion in the supercritical phase

Reaction phase Gas Supercritical LiquidCO conversion (%) 44.7 39.0 28.0Effluent products (*) 10.8 12.8 8.82Chain growth probability 0.94 0.95 0.85

(*) C-mmol/g-cat×h

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 productionAddition of heavy alkene to the

supercritical phase• Catalyst: Co-La/SiO2

• Temperature: 220°C• Pressure: 35 bar

• Supercritical fluid: n-pentane ( Tc=196.6°C, Pc=33.7 bar)

• p(CO+H2) = 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 productionAddition of heavy alkene to the

supercritical phase

0

200

400

600

800

1000

0 5 10 15 20 25 30

Carbon Number

Pro

du

ct

Fo

rma

tio

n R

ate

(C

-mo

l/g-c

at

h)

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

With alkene addition

Wax productionAddition 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 scH2O (SCWO)

• SCWO of organic wastes

– Complete oxidation to CO2

• Single fluid phase• Faster reaction rates

• Complete miscibility of nonpolar organic with scH2O

• 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: Tc = 135°C, Pc = 36.4 bar

• Isobutane : air = 3 : 1• 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

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

Catalyst TotalPressure

(bar)

i-C4H10

Conversion(%)

O2

Conversion(%)

TBAselectivity

(%)

i-C4H8

Selec.(%)

None 44 0.3 2.5 55.0 7.0None 54 1.2 9.9 58.1 8.1

SiO2-TiO2 44 2.9 24.0 59.0 5.2SiO2-TiO2 54 4.9 40.6 61.2 6.3SiO2-TiO2 12 0.0 0.0 0.0 0.0

SiO2-TiO2 (a) 54a 0.1 1.1 55.5 7.7Pd/C 44 0.5 4.2 61.2 2.1Pd/C 54 3.1 25.6 64.8 20.1

Na2WO4/SiO2 44 2.1 19.9 48.1 0.6Na2WO4/SiO2 54 5.6 55.5 51.8 0.8Na2MoO4/SiO2 44 6.7 61.0 25.3 6.1Na2MoO4/SiO2 54 7.0 65.8 31.7 4.1

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

t-butyl alcohol synthesis by air oxidation of supercritical isobutane

Catalyst: SiO2-TiO2, P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane

Catalyst: SiO2-TiO2, P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane

Catalyst: SiO2-TiO2, T=153°C

t-butyl alcohol synthesis by air oxidation of supercritical isobutane

Catalyst: SiO2-TiO2, T=153°C

Friedel Crafts Alkylation Reactions

• Conventional reactions require:

– Long reaction times

– Low temperatures and

– Use of environmentally “dirty” catalysts e.g. AlCl3 or H2SO4

– Separation of catalyst and solvent from the reaction mixture

• Using supercritical CO2 allows reaction conditions to be tuned to get high product selectivity.

– Solvent removal is also easy using supercritical CO2

Friedel Crafts Alkylation Reactions

Organic and water layers are easily separated to leave clean product

Alkylation of Mesitylene with Isopropanol in Supercritical CO2

• 50% conversion of mesitylene to mono-alkylated product

• No di-alkylated product

ConclusionsSCF (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

• scCO2 only suitable as solvent for nonpolar substances

• High supercritical temperature exclude scH2O

– 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”

Låtefoss