This review article summarizes the key published research on the topic of bio-oil upgrading using catalytic and non-catalytic supercritical fluid (SCF) conditions. The precious metal catalysts Pd, Ru and Pt on various supports are frequently chosen for catalytic bio-oil upgrading in SCFs. This is reportedly due to their favourable catalytic activity during the process including hydrotreating, hydrocracking, and esterification, which leads to improvements in liquid yield, heating value, and pH of the upgraded bio-oil. Due to the costs associated with precious metal catalysts, some researchers have opted for non-precious metal catalysts such as acidic HZSM-5 which can promote esterification in supercritical ethanol. On the other hand, SCFs have been effectively used to upgrade crude bio-oil without a catalyst. Supercritical methanol, ethanol, and water are most commonly used and demonstrate catalyst like activities such as facilitating esterification reactions and reducing solid yield by alcoholysis and hydrolysis, respectively.
. [J]. Frontiers of Chemical Science and Engineering, 2021, 15(1): 4-17.
Sainab Omar, Yang Yang, Jiawei Wang. A review on catalytic & non-catalytic bio-oil upgrading in supercritical fluids. Front. Chem. Sci. Eng., 2021, 15(1): 4-17.
Do not contribute to smog Do not damage ozone layer No acute ecotoxicity No liquid wastes
Most Most CO2, H2O CO2 and other volatile SCFs
Health and safety
Noncarcinogenic Nontoxic Non-flammable
Most (but not C6H6) Most (but not HCI, HBr, HI, NH3) CO2, N2O, H2O, Xe, Kr, CHF3
Process
No solvent residues Facile separation of products High diffusion rates Low viscosity Adjustable solvent power Adjustable density Inexpensive
CO2 and other volatile SCFs CO2 and other volatile SCFs All All All All CO2, H2O, NH3, Ar, hydrocarbons
Chemical
High miscibility with gases Variable dielectric constant High compressibility High diffusion rates
All The polar SCFs All All
Tab.3
Fig.1
Feed
Solvent
Performance of SCF
Catalyst
T/°C
P/MPa
t/min
Initial H2/MPa
Ref.
Flash pyrolysis of pulverized corn stalk
CO2
Higher conversions compared to esterification at atmosphere pressure
p-Toluene sulfonic acid
80
28
180
–
[48]
Heavy residues of fast pyrolysis of rice husk (HBF)
Methanol
Promotes alcoholysis
Pt, PtNi, PdNi on Al2(SiO3)3, SiO2, MgO
290
–
300
2
[27]
Light residues of fast pyrolysis of rice husk (LBF)
Methanol
Facilitates esterification
Pt on Al2(SiO3)3, C and MgO
250
8.6–9.6
180–540
1.5
[42]
Bio-oil from pyrolysis of pine sawdust
Methanol
Hydrogenation and esterification reactions
Co; Zn; Co-Zn on HZSM-5
300
–
300
3.4
[38]
Bio-oil from pine sawdust pyrolysis
Methanol
Functioned as hydrogen donor, promoted HDO
Fe-Co/SiO2 or Co/HZSM-5
300
–
300
3.45
[37]
Bio-oil from pine sawdust pyrolysis
Methanol
Mainly hydrogenation and esterification reactions
Fe-Ni/HZSM-5
300
–
300
3.4
[34]
Low boiling fraction of bio-oil from fast pyrolysis of empty palm fruit bunch
Methanol
High esterification and alkylation ability
–
400
22.5–46.7
30
1 MPa N2
[43]
Pyrolysis oil of Pinussylvestris L.
Methanol, Ethanol
Increased variety of esters when processing in ethanol
Pd; Pt on HZSM-5; SO42–/ZrO2/SBA-15
260
7.5–11.5
180
2
[15]
Fast pyrolysis oil of rice husk
Ethanol
Decreased phenols and aldehydes during upgrading
Aluminium silicate
260
7.8
180
–
[29]
Fast pyrolysis oil of rice husk
Ethanol
Effectively removes heavy components in bio-oil
HZSM-5 (Si/Al= 22)
100–238
260
180
–
[30]
Flash pyrolysis oil of rice husk
Ethanol
Facilitates hydrotreatment when used with catalyst
Pd/SO42–/ZrO2/SBA-15
280
8.5–10.5
180
0–2
[12]
Pyrolytic lignin from flash pyrolysis of rice husk
Ethanol
Promotes hydrocracking
Ru/SO42−/ZrO2/SBA-15 or Ru/ZrO2/SBA-15
260
9.5
480
2
[18]
Fast pyrolysis oil of rice husk
Ethanol
Participation of ethanol in aldolization and etherification reactions
Pt/ SO42−/ZrO2/SBA-15
260–300
7–11.8
180
0.5, 2
[26]
Bio-oil from fast pyrolysis of rice husk
Ethanol
Improved bio-oil physical properties and composition of organic compounds
Pt/C; Pd/C; Ru/C; Ru/HZSM-5
300
–
300
2
[14]
Bio-oil from fast pyrolysis of Miscanthus sinensis biomass
Ethanol
Decreased viscosity of heavy-oil
Pd/C
250–350
–
30–60
3
[11]
Bio-oil from fast pyrolysis of Miscanthus sinensis
Ethanol
Converts acid in bio-oil into ester
Pt/C; Ru/C
250–350
–
30–60
3
[22]
Bio-oil from fast pyrolysis of rice husk
Ethanol
Facilitates catalytic upgrading
Ni/SiO2-ZrO2
280
–
300
1.5
[32]
Fast pyrolysis oil of sawdust
Ethanol
Crude bio-oil easily esterified with supercritical ethanol
Zeolite
200–250
7
180
–
[31]
Pyrolytic lignin from fast pyrolysis of rice husk
Ethanol
Enables high hydrocracking activity of supported metal
SBA-15; Zr; RuZr; SZr; RuSZr
260
9.5
480
2
[19]
Bio-oil from fast pyrolysis of yellow poplar wood
Ethanol
Deoxygenation and increased light oil yields
Pd/C
250–370
–
40–120
3
[10]
Bio-oil from fast pyrolysis of empty palm fruit bunch
Ethanol
Hydrogen donation ability
–
300–400
16.8–41.3
30–120
1 MPa N2
[39]
Bio-oil from hydrothermally liquefied dried cornstalk powder
Ethanol
Promotes esterification reactions
Bimetallic ammonium nickel molybdate
280–370
–
60
4
[40]
Fast pyrolysis of rice husk
Ethanol
Enables esterification of bio-oil
Ni/MgO
280
–
300
1.5
[36]
Pyrolysis oil from hardwood sawdust fast pyrolysis
Ethanol
Effective solvent-reduced the molecular weight of bio-oil
Ru/C
300
–
180
10
[49]
Hardwood sawdust fast pyrolysis oil
Ethanol
Increases H/C ratio and reduces O/C ratio in bio-oil
CoMo catalysts supported on various nanostructured materials; Ru/C
350
22.5
180
5
[21]
Fast pyrolysis of pine sawdust
1-Butanol
Decreases oxygen content, increases heating value in upgraded bio-oil
Ru/C
250–300
8.8–11.5
180
2
[47]
Bio-oil from HTL of cornstalks
Cyclohexane
Improved the yield and the quality of liquid hydro- carbons
Ni/ZrO2
300
–
240
5
[35]
Crude algal bio-oil from liquefaction of microalga paste
Water
Higher heating value and lower acid number than the crude bio-oil
Pt/C
400
–
240
3.4
[23]
Crude algal bio-oil from liquefaction of microalga paste
Water
Cracking of the longer chain alkanes
Pd/C
400
–
60–480
3.4
[13]
Crude algal oil from liquefactionof Chlorella pyrenoidosa (Alga) powder
Water
Complete desulfurization of crude algal oil
Pt/g-Al2O3
400
–
60
6
[25]
Duckweed biocrude from liquefaction of duckweed powder
Water
High upgraded oil yield
Ru/C; Pt/C; Pd/C
350
18
240
6
[17]
Pre-treated algal biocrude from liquefaction of Chlorella pyrenoidosa algae powder
Water
Improved chemical properties
Ru/C; Pt/C; Pd/C
400
–
240
6
[16]
Pre-treated crude bio-oil from liquefaction of Chlorella pyrenoidosa microalga
Water
Decreased the N content in the upgraded oil
Two component catalyst mixtures
400
24
240
6
[24]
Pre-treated crude algal oil from liquefaction of Chlorella pyrenoidosa (Alga) powder
Water
Facilitates high oil yield
Zeolites including MCM-41 (100%Si)
400
28
240
6
[33]
Bio-oil from pyrolysis of pine sawdust
Water
Improved physicochemical properties of the bio-oil
Ni-Co supported on carbon nanofibers
380
23
–
–
[46]
Bio-oil from pyrolysis of pine sawdust
Water
Facilitated H2 production from bio-oil
Ni-Co/Al-Mg
310–480
20–26
0–60
–
[45]
Pre-treated crude duckweed bio-oil
Water
Increased alkanes & aromatics & decreased O- and N-compounds
Activated carbon
400
–
60
6
[20]
Pyrolysis oil from biomass
Water
Reduced O content of heavy oils
–
410
32
60
0.2 MPa N2
[44]
Biocrude from HTL of microalgae
Water
Enhanced removal of carbon via decarboxylation or steam reforming
Pt/Al2O3; HZSM-5
400
22.5
240
4
[28]
Tab.4
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