Supercritical Fluids as Solvent in
Chemical Synthesis

Definition by IUPAC
A mixture or element:


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
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
2
 Near-critical region: region extends all around the critical point, but
nonsupercritical section only
 subcritical liquids: Liquid phases at temperatures below but not too far
below Tc
 subcritical gases: subcritical gases are those at pressures below Pc
3

Dense gas
Densities similar to liquids
Occupies entire volume available

Solubilities approaching liquid phase
Dissolve materials into their components
Completely miscible with 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

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

Replaces “less green” liquid organic solvents
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No acute toxicity (H2O and CO2)
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No liquid wastes (except water)
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Non-carcinogenic (except C6H6)
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Non toxic (except NH3)
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Non-flammable (CO2, H2O)

High reaction rate due to:
o Dissolving capabilities
▪ High concentration of reactant gases ( H2 / O2 )
▪ Eliminating inter-phase transport limitations
o Higher diffusivities than liquids
o Better heat transfer than gases

Variable dielectric constant (polar SCF)
o Adjustable solvent power

Enhanced catalytic activity due to anti-coking of scCO2

Higher solubilites than corresponding gases for heavy organics
o Improved catalyst lifetime

High product selectivities
o Increased pressure may favour desired product selectivity

High reaction rate due to:
o Dissolving capabilities
▪ High concentration of reactant gases ( H2 / O2 )
▪ Eliminating inter-phase transport limitations
o Higher diffusivities than liquids
o Better heat transfer than gases

Variable dielectric constant (polar SCF)
o Adjustable solvent power

Enhanced catalytic activity due to anti-coking of scCO2

Higher solubilites than corresponding gases for heavy organics
o Improved catalyst lifetime

High product selectivities
o Increased pressure may favour desired product selectivity
Catalyzed reactions
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Alkylation
Amination
Cracking
Gasification
Esterification
Fischer-Tropsch Synthesis
Hydrogenation
Isomerization
Oxidation
Polymerization
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Why Hydrogen?
High heat of
combustion
per unit weight
 U.S. Energy Information Administration foresees a 56% increase in the world energy
demand in the following 30 years.
 CO2
emissions already increased from 21.5 to 33.5 billion metric tons from 1990
to 2014, and in 2040 it is expected to reach 45.5 billion metric tons
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Fuel cell
vehicle application
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Hydrogen Production
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 Steam Reforming
•
•
•
•
CH4 + H2O → CO + 3H2 (Ni Catalyst)
H2/CO = 3
Endothermic
Favored for small scale operations
 Partial Oxidation
•
•
•
•
CH4 + ½O2 → CO + 2H2
H2/CO ≈ 1.70
Exothermic
Favored for large scale applications
 Autothermal Reforming
• A combination of Steam Reforming and Partial Oxidation
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Why Biomass to hydrogen?

Biomass has the potential to accelerate the realization of hydrogen
as a major fuel of the future.

Biomass is renewable, consumes atmospheric CO2 during growth and
is a CO2 neutral resource in life cycle.

It can have a small net CO2 impact compared to fossil fuels.
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
Conversion of solid fuels into combustible gas mixture called producer gas
(CO + H2 + CH4)
15
Conventional
thermal gasification
High temperature (900°C)
Drying required
Gasification
Anaerobic digestion
Slow reaction rates
fermentation sludge and wastewater
from the reactors
supercritical water
gasification
Supercritical conditions
No drying required
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The term hydrothermal refers to an aqueous system at temperatures and
pressures near or above the critical point of water.
Hydrothermal conversion of biomass can be classified as:

Carbonization

Oxidation

liquefaction

gasification
Biomass conversion routes
Total efficiency of heat utilization processes
versus biomass moisture content
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Solubility of limits of
various salts at 25 MPa
Benzene solubility in
high-pressure water
19
Water as a reactant
A. Contributions in
hydrolysis reaction
B. Resource of hydrogen
C. Resource of radicals
20
Acid
catalyzed
reactions of
tert-BuOH in
SCW
Disproportionation of
benzaldehyde in SCW
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22
Oxidation
reaction
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
SCWO of organic wastes
 Complete oxidation to CO2

Complete miscibility of nonpolar organic with scH2O

Single fluid phase

Faster reaction rates

With or without heterogeneous catalyst

Motivation for catalyst:
 Reduce energy and processing costs

Target:
 Complete conversion at low temperatures and short residence time
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Coal
Biomass
Natural Gas
L
G
X
Gasification
Syngas
Processing
FischerTropsch
Synthesis
Syncrude
Refining &
Upgrading
Fuel
&
Chemicals
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
Alkane formation
favored by H2/CO
nCO + (2n+1)H2
CnH2n+2 + nH2O
strong hydrogenating catalyst

Alkene formation
nCO + 2n H2
CnH2n + nH2O
favored by H2/CO

Water-gas-shite reaction
CO + H2O
CO2 + H2


WGS activity is high in Fe catalyst and low in Co or Ru catalyst
Helpful to adjust H2/CO ratio
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Fixed Bed Reactors
Originally used
Challenges associated with removal of heat
Fluidized Bed Reactors
Better temperature control
High yields for Gasoline and light
27
products
Slurry Reactors
Small catalyst particles
suspended in a liquid with
low vapor pressure
Low Temperature
Flexible design
High yield for waxes
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Catalytic activity
Increased catalyst pore accessibility in SC-FTS
Enhanced product desorption
In situ extraction of heavy products
Heat transfer
Enhance thermal conductivity leading to improved heat transfer
Maximum temperature difference along the reactor in the supercritical
phase was around 5 °C compared to 15 °C in the gas phase
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Catalyst stability
Fast removal of wax from catalyst pores
Temperature distribution in supercritical fixed bed reactor
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Product selectivity
Hydrocarbons distribution
CH4 and CO2 selectivity
Olefin distribution
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Hydrocarbons distribution
Low H2/CO ratio leading to lower methane selectivity and higher chain
propagation
More free and active sites for re-adsorption and enhanced chain growth
High residence time in the GP-FTS can lead to lower chain growth
CH4 and CO2 selectivity
In the SC-FTS, there is a decrease in methane selectivity even as syngas
conversion increases

Local overheating of the catalyst surface in the GP-FTS
 Diffusivity of hydrogen is higher than that of CO in the GP-FTS
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
Olefin distribution
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Reaction phase
CO conversion (%)
Effluent products (*)
Chain growth probability
Gas
44.7
10.8
0.94
Supercritical
39.0
12.8
0.95
Liquid
28.0
8.82
0.85
(*) C-mmol/g-cat×h
 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 pore
– Effective molar diffusion in the supercritical phase
Distribution of Hydrocarbon Products
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 re-adsorption and hydrogenation
Carbon Number
32

synthesis of waxy hydrocarbons through FTS reaction
Catalyst: Co-La/SiO2
Temperature: 220°C
Pressure: 35 bar
Supercritical fluid:npentane (Tc=196.6°C,
Pc=33.7 bar)
p(CO+H2) = 10 bar
- catalyst bed and reactor blockage by wax
- Selective synthesis of waxy hydrocarbons not easy

Studied the effect of addition
of heavy alkenes
Addition: 4 mol% (based on CO)
1-tetradecene and 1-hexadecene
Product Formation Rate
(C-mol/g-cat h )
1000
800
600
400
200
0
0
5
10
15
20
Carbon Number
25
30
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Carbon chain growth accelerated by addition of alkenes
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Alkenes diffuse inside the catalyst pores to reach the metal
sites
 Adsorb as alkyl radicals to initiate carbon chain growth
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suppression of methane formation, high CO conversion and low
C02 selectivity
The resulting chains are indistinguishable from chains formed
from synthesis gas
Addition of heavy alkenes does not have any effect in gas
phase reactions
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
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
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
Compared with the conventional methods SCWG process is characterized
by:
its high reaction efficiency and H2 selectivity
and can be fed by high moisture content material
.

If the good cost performance can be provided, SCWG process holds great
potential to be large-scale commercialized in the future.

In the SC-FTS the overall product distribution shifts towards heavier
products compared to GP-FTS.

The olefin content in supercritical media exceeds those in other
reaction phases

The CO2 yields and selectivitys from catalytic SCWO were much higher
than those from gas-phase catalytic oxidation.
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
Yakaboylu, O., Harinck, J., Smit , K. G., Jong,W., Review of Supercritical Water
Gasification of Biomass: A Literature and Technology Overview. Energies 2015, 8,
859.
 Matsumura, Y., Minowa, T., Potic, B., Review of Biomass gasification in near- and
super-critical water: Status and prospects. Biombioe. 2005, 29, 269.
 Guo, Y., Wang, S. Z., Xu, D. H., Gong, Y. M., Ma, H. H., Tang, X.Y., Review of
catalytic supercritical water gasification for hydrogen production from biomass.
Renewable and Sustainable Energy Reviews 2010, 14, 334.

Mogalicherla, A. K ., Elbashir,N. O., Development of a Kinetic Model for Supercritical
Fluids Fischer-Tropsch Synthesis. Energy Fuels 2011, 25, 878.

Anastas, P, T., Heine, L. G., Williamson, T. C., Green Chemical Syntheses and
Processes. Eds.; ACS symposium series 767; American Chemical Society:
Washington, DC, 2000, pp. 270-290.
 Elmalik, E. E.,Tora, E., Mogalicherla, A. K ., Elbashir,N. O., Solvent selection for
commercial supercritical Fischer–Tropsch synthesis process. Fuel Processing
Technology 2011, 92, 1525
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