Catalysis in supercritical fluids

1
Catalysis in supercritical fluids

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

2
Outline

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

3
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

4
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

5
What is a supercritical fluid?
6
Appearance of a SCF
7
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

8
Comparison of physical properties
9
Which gases can be used as SCF?

• Any compressible gas
• Possible to tune properties from gas like,
  through to liquid like
• The most common

10
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)

11
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

12
Reaction solvent effects - pressure tunability
Pressure tunability on density (?), viscosity (?)
and D11?
13
Pressure tunability
14
Ion product of water
15
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

16
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

17
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)

18
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

19
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)

20
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

21
Applications

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

22
Industrial use of SCF as reaction media
23
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

24
Hydrogenation in scPropane

• Feed Oil (fatty acid methyl esters), H2
• Supercritical fluid Propane ( Tc 96.8C, Pc
  42.0 bar)
• Catalyst Pd

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

P. Møller, 3rd Int. Symp. On High Press. Chem.
Eng., Zurich, 1996, 43-48
25
Catalytic amination of amino-1-propanol with scNH3

• Catalyst Co-Fe (95/5)
• Production of 1,3-diaminopropane
• Tubular reactor
• 195C
• Feed ratio R-OH / NH3 (140)
• Tc 132, Pc 113 bar

Fischer et.al, A. Angew. Chem., Int. Ed. Engl.,
submitted
26
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

27
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)

28
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 240C and Ptot45 bar
• n-Hexane chosen SCF Tc 233.7C Pc 30.1 bar
• p(COH2)10 bar, COH212
• Catalyst Ru/Al2O3

K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res.,
30 (1991)95
29
Supercritical Fischer- Tropsch synthesis

• Different CO-conversions due to different rates
  of diffusion
• DGASS gt DSCF gt DLiquid
• Different Chain growth probabilities due to COH2
  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
30
Distribution of hydrocarbon products in various
phases
31
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
32
Wax productionAddition of heavy alkene to the
supercritical phase

• Catalyst Co-La/SiO2
• Temperature 220C
• Pressure 35 bar
• Supercritical fluid n-pentane ( Tc196.6C,
  Pc33.7 bar)
• p(COH2) 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
33
Wax productionAddition of heavy alkene to the
supercritical phase
Fujimoto et al., Topics in Catal. 1995, 2, 259-266
34
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
35
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

36
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 500-600C
• Catalyst deactivation
• Isobutane Tc 135C, Pc 36.4 bar
• Isobutane air 3 1
• Reaction temperature 153C
• Reaction pressure
• 44 bar for gas phase reaction
• 54 bar for supercritical phase reaction

Fan et al., Appl. Catal. 1997, 158, L41-L46
37
t-butyl alcohol synthesis by air oxidation of
supercritical isobutane
Fan et al., Appl. Catal. 1997, 158, L41-L46
38
t-butyl alcohol synthesis by air oxidation of
supercritical isobutane
Catalyst SiO2-TiO2, P54 bar
39
t-butyl alcohol synthesis by air oxidation of
supercritical isobutane
Catalyst SiO2-TiO2, P54 bar
40
t-butyl alcohol synthesis by air oxidation of
supercritical isobutane
Catalyst SiO2-TiO2, T153C
41
t-butyl alcohol synthesis by air oxidation of
supercritical isobutane
Catalyst SiO2-TiO2, T153C
42
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

43
Friedel Crafts Alkylation Reactions
Organic and water layers are easily separated to
leave clean product
44
Alkylation of Mesitylene with Isopropanol in
Supercritical CO2

• 50 conversion of mesitylene to mono-alkylated
  product
• No di-alkylated product

45
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

46
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

47
Låtefoss