CET Volume 86
DOI: 10.3303/CET2186239
Paper Received: 25 September 2020; Revised: 16 February 2021; Accepted: 10 May 2021
Please cite this article as: Stepacheva A., Gavrilenko A., Markova M., Monzharenko M., Yakubenok K., Sidorov A., Matveeva V., Sulman M.,
2021, The Use of Supercritical Solvents in Polyaromatic Conversion, Chemical Engineering Transactions, 86, 1429-1434
DOI:10.3303/CET2186239
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-84-6; ISSN 2283-9216
The Use of Supercritical Solvents in Polyaromatic Conversion
Antonina Stepacheva*, Alexandra Gavrilenko, Mariia Markova, Margarita
Monzharenko, Kseniya Yakubenok, Alexander Sidorov, Valentina Matveeva,
Mikhail Sulman
Tver State Technical University, Department of biotechnology, chemistry and standardization, 170026, A. Nikitin str., 22,
Tver, Russia
a.a.stepacheva@mail.ru
Nowadays, the processes aimed at the deep conversion of heavy oil are of great interest. Polyaromatic and
sulfur-containing compounds (such as anthracene and dibenzothiophene) are one of the main components of
heavy oil fractions which are hard to process. Catalytic hydrotreatment of heavy petroleum fractions (vacuum
gas oil, light gas oil, etc.) is widely used to improve the quality of the resulting products. As the hydrotreatment
of oil fractions often requires the use of harsh process conditions and consumes a high amount of molecular
hydrogen, the researchers’ interest is focused on the development of novel approaches including the use of
supercritical fluids. In this work, the influence of solvent in the supercritical state on the conversion and
product composition of cracking and desulfurization of polyaromatic and sulfur-containing compounds is
described. The experimental results showed that the use of a supercritical solvent mixture composed of n-
hexane and methanol allows the polyaromatic compounds to be effectively converted into monoaromatics
providing a high conversion degree (up to 95 %).
1. Introduction
Oil is a complex mixture of highly hydrophobic hydrocarbons. The molecules can also contain some other
elements such as oxygen, nitrogen, sulfur, or phosphorus. Crude oil mainly consists of linear hydrocarbons
with the number of carbon atoms from 4 to 30, as well as aromatic and naphthenic compounds. Gasoline is a
mixture of short-chain hydrocarbons (4-8 atoms), while diesel consists of higher-molecular carbon chains (12-
20 atoms). Mineral fuel is a complex specific mixture of hydrocarbons, in fact, a mixture of linear, isomeric
alkanes, cycloalkanes, and aromatic compounds.
Catalytic hydrotreatment of petroleum fractions is an established and widely used purification technology,
usually applied to improve the quality of oil fractions. There are two types of hydroconversion technologies:
catalytic hydrotreatment and catalytic hydrocracking. The hydrotreatment process aims at the removal of
undesirable heteroatoms such as sulfur, nitrogen, and oxygen as well as metals, and partial reduction of
aromatics (Biswas and Maxwell, 1990). Catalytic hydrocracking is a conversion process that primarily aims at
a decrease in the boiling point of petroleum fractions. It is used for processing heavy oil fractions, gas oil,
vacuum gas oil, etc. Liquid-phase catalytic cracking is the technology most commonly used for converting
heavy oil fractions to produce gasoline and propane (Primo and Garcia, 2014).
Currently, the developments in oil hydroconversion are aimed at a decrease in the cost of processes. The
modern tendencies include the search for new catalysts and process conditions. One of these approaches is
the use of supercritical and subcritical fluids (SCF) as solvents for hydrocracking and hydrodesulfurization
processes (Ates et al., 2014). Water is the most often used solvent in the hydroprocessing of heavy oil
fractions. In the supercritical state, water is non-polar and perfectly dissolves hydrocarbons (Hosseinpour et
al., 2018). The use of supercritical water (SCW) in desulfurization is prospective due to water oxidative
capacity, which leads to the removal of both sulfur atoms and other heteroatoms (nitrogen, metals, oxygen)
(Timko et al., 2015). Hydrocracking processes in SCW are characterized by both high conversion (Reina et
al., 2016) and yield of light fractions (gasoline, kerosene) (Tan et al., 2017) due to the hydrothermal
destruction of heavy hydrocarbons (Yeletsky et al., 2019). Besides, water in the supercritical state serves as a
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 86, 2021
1429
hydrogen donor, which allows processes to be carried out in an inert atmosphere (Gai et al., 2016). As water
has a respectively high critical point (Tc = 374.15 ºC, Pc = 22.13 MPa), the researchers try to find the
alternative solvent characterized by the lower critical temperature. Supercritical hydrocarbon solvents such as
tetralin (Kim et al., 2018), hexane, and dodecane (Viet et al., 2012) can also serve as hydrogen donors. All
studies indicate that the use of supercritical solvents, in addition to the complete removal of heteroatoms and
high conversion of raw materials, leads to a decrease in the viscosity and density of the resulting product, and
also prevents coking of the catalyst, providing a longer lifetime (Kim et al., 2018).
The second direction of the development of hydroprocessing is the use of oxide catalysts. The activity of oxide
catalysts in hydrodesulfurization and hydrocracking processes directly depends on the dispersion of the active
phase and the presence of hydroxyl groups on the surface of the catalyst (Dhar et al., 2003). High dispersion
of the active phase can be achieved by special methods of catalyst synthesis, in particular, the use of a
hydrothermal deposition. Work on hydrotreatment of heavy oil fractions in the presence of catalysts obtained
using supercritical fluids has shown that the high surface area of catalysts, the presence of a large number of
hydroxyl groups, and the unique morphology (high dispersion, uniformity of distribution, the small size of active
phase particles) of such catalysts leads to an increase in the efficiency of hydroconversion processes of
petroleum feedstock (Alibour et al., 2009), providing both a high degree of sulfur removal (Haji et al., 2010)
and highly selective cracking of petroleum hydrocarbons (Quilfen et al., 2018). Besides, such catalysts are
highly active without prior sulfidation (Haji et al., 2010).
The current work is aimed at the choice of supercritical solvent for cracking and desulfurization processes in
the presence of metal oxide-containing catalysts that allows high conversion of heavy oil component to be
reached. The results obtained in this research will be the basis for developing modern approaches for deep oil
processing.
2. Experimental
2.1 Catalyst preparation
The catalysts were synthesized according to the following procedure. Polymeric support – hypercrosslinked
polystyrene with tert-amino groups Macronet (MN-100, Purolight Inc., UK) was modified with silicon oxide to
provide strong acid sites. The polymer surface modification was performed by hydrolysis of (3-
aminopropyl)triethoxysilane (APTES, 99.9 %, Sigma Aldrich, USA) in preheated water. In a typical experiment,
3 g of MN-100, and 10 mL of distilled water were placed into the reactor (PARR 4307, Parr Instrument Ltd.,
USA). The reactor was sealed, purged with nitrogen, and heated up to 150 ± 5 °C under a nitrogen pressure
of 6.0 MPa. Then, 1.17 mL of APTES (calculated as 10 wt % of SiO2 on the 1 g of MN-100) were added
dropwise into the reactor through the burette. The mixture was held at the indicated conditions for 60 min,
cooled down to room temperature, and filtered. The resulting sample was heated under the nitrogen at 250 ± 5
°C for 4 h to form the silica phase.
The catalyst was prepared by hydrothermal deposition (Stepacheva et al., 2019). 3 g of the modified polymer,
1.5 g of nickel nitrate hexahydrate and 1.43 g cobalt nitrate hexahydrate (calculated as 10 wt % of Ni and 10
wt. % of Co on 1 g of the support), and 15 mL of distilled water were placed into the reactor (PARR 4307, Parr
Instrument Ltd., USA). The reactor was sealed, purged with nitrogen, heated up to 200 ± 5 °C under a
nitrogen pressure of 6.0 MPa, and held for 15 min. Then, the reaction mixture was cooled down to room
temperature and filtered. The resulting catalyst was washed with 10 mL of distilled water to remove nitrate-
ions and dried in the air at 105 ± 5 °C for 1 h. The catalyst samples were reduced in the hydrogen flow (flow
rate of 10 mL/min) at 300 ± 5°C for 5 h.
2.2 Anthracene and dibenzothiophene conversion procedure
The experiments were performed in a six-cell Parr Series 5000 Multiple Reactor System (Parr Instrument,
USA) with a cell volume of 50 mL. In a typical experiment, 1 g of model heavy-oil compound (anthracene or
dibenzothiophene), 0.1 g of catalyst, and 30 mL of solvent were placed into the reactor cell. The reactor was
sealed and purged three times with nitrogen to remove air. Then the nitrogen pressure was set to 3.0 MPa,
and the reactor was heated up to 300 °C. These process conditions were chosen according to the critical
points of the solvents. After reaching the reaction temperature, the pressure increased up to 12 MPa
depending on the solvent. Experiments were performed with varying process time (from 10 min to 3 h) to
maintain phase equilibrium. Methanol (Tc = 240 ºC, Pc = 7.95 MPa), propanol-2 (Tc = 235.6 ºC, Pc = 5.37
MPa), n-hexane (Tc = 234.7 ºC, Pc = 3.03 MPa), and benzene (Tc = 289.41 ºC, Pc = 4.92 MPa) were used as
solvents.
1430
2.3 Reaction mixture analysis
The liquid phase was analyzed by GCMS using gas chromatograph GC-2010 and mass-spectrometer GCMS-
QP2010S (SHIMADZU, Japan) equipped with chromatographic column HP-1MS with 30 m length, 0.25 mm
diameter, and 0.25 µm film thickness. The column temperature program was set as follows: initial temperature
120 °C was maintained for 5 min then the column was heated up to 250 °C with the rate of 5 °C/min and
maintained at 250 °C for 5 min. Helium (volumetric velocity of 20.8 cm3/s, the pressure of 253.5 kPa) was
used as a gas-carrier. The injector temperature was 280 °C, ion source temperature was 260 °C; interface
temperature – 280 °C.
3. Results and discussion
3.1 Catalyst characterization
As the textural properties, as well as the composition of the active phase, strongly affect the heavy oil
hydroprocessing the synthesized catalyst was characterized by different techniques. The catalyst porosity was
studied performed using the low-temperature nitrogen physisorption (Beckman Coulter SA 3100 analyzer,
Coulter Corporation, USA). Figure 1 presents the adsorption-desorption isotherms for the initial polymer,
modified polymer, and the resulting catalyst. The surface modification leads to a slight decrease in the surface
area in comparison with the initial MN-100. However, the surface area of macropores increases with the
consequent decrease in micropore surface area indicating the pore reorganization (see Table 1). This was
confirmed by both the changes in the isotherm and hysteresis loop forms (Figure 1a) as well as the pore size
distribution (Figure 1b). The deposition of the metal phase on the surface of the modified polymer did not show
any changes in the porous structure in comparison with the support. Besides, neither the surface area nor
pore volume practically decreased during the metal incorporation (see Table 1).
Relative pressure, Ps/P0
0,0 0,2 0,4 0,6 0,8 1,0
V
a
d
s
[c
m
3
/g
]
0
100
200
300
400
500
600
700
MN-100
SiO2-MN-100
0,0 0,2 0,4 0,6 0,8 1,0
0
200
400
600
Co-Ni-SiO2-MN-100
(a)
Dpore [nm]
0 20 40 60 80 100 120 140 160
d
(V
p
o
re
)/
d
(D
p
o
re
)
[c
m
3
/g
*n
m
]
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
MN-100
SiO2-MN-100
Co-Ni-SiO2-MN-100
Dpore [nm]
0 5 10 15 20
d
(V
p
o
re
)/
d
(D
p
o
re
)
[c
m
3
/g
*n
m
]
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
(b)
Figure 1: Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) for the synthesized
catalyst
Table 1: Results of catalyst studies
Sample SBET, m
2/g St-plot, m
2/g Vpore, cm
3/g Element concentration, wt. % Total acidity,
μmol/g Si Co Ni
MN-100 840 200*
590**
0.52 - - - -
SiO2-MN-100 780 320*
390**
0.55 9.7 - - 843
Co-Ni-SiO2-MN-100 740 350*
320**
0.57 9.6 9.8 9.8 814
*macropore surface area, ** micropore surface area
The distribution of the silica- and metal-containing phase on the surface of polymeric support was studied by
transmission electron microscopy (JEOL JEM1010, JEOL Ltd., Japan). Figure 2 presents the TEM images for
1431
the synthesized samples of the modified support (Figure 2a) and the resulting catalyst (Figure 2b). It is well
seen that the deposition method used allows the SiO2 and metal oxide particles to be distributed uniformly on
the polymer surface. Moreover, the silica-containing phase seems to be precipitated on both internal and
external surfaces of MN-100. The mean silica particle diameter was found to be 30±5 nm. The metal-
containing phase particle size of about 2.5 nm was estimated. As the surface area decreases insignificantly, it
can be proposed that the particles of the nickel-containing phase were well dispersed on the outer and inner
surface of the supports.
(a) (b)
Figure 2: TEM images of modified polymer (a) and resulting catalyst (b)
Analysis of the active phase composition was performed using the X-Ray photoelectron spectroscopy (ES-
2403 spectrometer equipped with PHOIBOS-100-MCD energy analyzer, Specs GmbH, Germany). The high-
resolution XP spectra of Si 2p sublevel for Co-Ni-SiO2-MN-100 shows the formation of silicon oxide and
hydrated silica (Si-OH) on the MN-100 surface during the synthesis. Noteworthy, the formation of hydroxyl
groups on the surface of the support leads to the recharge of the surface and facilitates the deposition of metal
ions. Spectra analysis on Ni 2p sublevel showed that nickel in the catalyst is presented by the mixture of nickel
(II) and nickel (III) oxyhydroxides. Analysis of the high-resolution XP spectra of Co 2p sublevel showed the
formation of Co3O4 and Co(OH)2 on the catalyst surface.
The measurements of catalyst acidity were carried out by the ammonia chemisorption using Micromeritics
AutoChem 2910 analyzer (Micromeritics, USA). The ammonia desorption curves were typical for SiO2
containing the peak with the activation energy of desorption less than 130 kJ/mol attributed to the weak acid
sites and two peaks with the activation energy of desorption 130-180 and >180 kJ/mol characteristic for the
strong acid sites. The deposition of the metal-containing phase did not decrease significantly the total catalyst
acidity (see Table 1).
3.2 Solvent influence on anthracene cracking
The analysis of the liquid phase after the anthracene cracking showed the predominant formation of
monoaromatics such as benzene, toluene, xylenes. Besides, the formation of some amount of
dihydroanthracene, tetrahydro anthracene, and diphenylmethane was observed. Moreover, when methanol
and propanol-2 were used as a solvent, the formation of i-propylbenzene and toluene was observed due to the
condensation. In general, the following scheme of anthracene conversion in a supercritical solvent can be
proposed (see Figure 3).
The conversion degree seemed to be higher (up to 85 wt. %) in the polar solvent (in particular, in propanol-2).
This can be attributed to the high dehydration degree of the solvent with the following hydrogenation of the
substrate. But the higher yield of monoaromatics (up to 70 wt. %) was obtained while using n-hexane as a
solvent indicating the predominant behavior of C-C bond cracking. Methanol provides the predominant
formation of dihydroanthracene (over 42 wt. %), and tetrahydro anthracene (ca. 16 wt. %), while the
conversion degree was about 65 wt. %. This can be explained by the higher degree of methanol dehydration
in the supercritical state in comparison with the other solvents used. The lowest conversion degree (about 46
wt. %), and hence, the lowest yield of monoaromatics (34 wt. %) was found to observe when using benzene in
the supercritical state, but this solvent provides the highest solubility of anthracene at the room temperature
due to the similar nature. When the solvent mixture (hexane-methanol and hexane-propanol-2) was used, the
conversion degree significantly increased up to 90 and 95 wt. % respectively. In this regard, the yield of
1432
monoaromatics also increased up to 85 wt. %. This can be attributed to the synergetic effect of the solvents
resulting in the acceleration of cracking reactions with the hydrogenation of the radicals formed.
Figure 3: Possible ways for anthracene cracking in supercritical solvent
3.3 Solvent influence on dibenzothiophene desulfurization
The analysis of the liquid phase after the dibenzothiophene conversion showed the formation of
monoaromatics (benzene, toluene, and biphenyl) as well as the corresponding cyclic compounds. It is
interesting, that in contrast to the typical desulfurization, the formation of aliphatic hydrocarbons (heptens,
methylhexene, methyl-ethyl hexane, etc.) was observed indicating the cracking reaction to have a place. In
general, the following scheme of dibenzothiophene conversion in a supercritical solvent can be proposed (see
Figure 4).
Figure 4: Possible ways for dibenzothiophene conversion in supercritical solvent
As dibenzothiophene is the oil compound which conversion is complicated, the use of supercritical solvents
seems to be an effective alternative. The experimental results on the influence of solvent on dibenzothiophene
desulfurization and cracking showed that in the individual solvent the conversion degree did not exceed 50 wt.
%. In methanol, however, the conversion was reached up to 72 wt. % providing the formation of
monoaromatics and cyclic compound with a yield of 65 wt. % in total. The lowest conversion was observed
when using benzene and propanol-2 as solvents (17 and 23 wt. % respectively) that can be explained by their
lower hydrogen formation activity. In this case, the predominant formation of biphenyl was observed. However,
the cracking was observed in benzene. This can be explained by the fact that benzene can not be the
hydrogen donor in the supercritical state.
In the solvent mixture (hexane-methanol and hexane-propanol-2) the synergetic effect was also observed (as
in the case of anthracene cracking). The conversion degree increased up to 97 and 64 wt. % for hexane-
methanol and hexane-propanol-2 respectively. Interestingly that in the hexane-methanol mixture the total yield
of cyclic and alkene compounds was about 80 wt. %.
1433
4. Conclusions
In the current work, the influence of supercritical solvent on the heavy oil model compound (anthracene and
dibenzothiophene) conversion was studied. Four compounds (n-hexane, benzene, methanol, and propanol-2)
at the conditions above the critical point were used as solvents. It was shown that the use of supercritical
fluids for the conversion of polyaromatic compounds (i.e. anthracene), including the sulfur-containing
(dibenzothiophene), allows the aromatics and light aliphatic products to be obtained. Moreover, the reaction
pathways in supercritical fluids significantly change from those observed in the typical conversion conditions.
The highest conversion degree (over 95 wt. %) was observed for both anthracene and dibenzothiophene
cracking when using n-hexane-methanol (1:1 by volume).
Acknowledgments
This work was funded by the Russian Science Foundation (grant 19-79-10061).
References
Alibouri M., Ghoreishi S.M., Aghabozorg H.R., 2009, Hydrodesulfurization of dibenzothiophene using
CoMo/Al-HMS nanocatalyst synthesized by supercritical deposition, Journal of Supercritical Fluids, 49,
239–248.
Ates A., Azimi G., Choi K.-H., Green W.H., Timko M.T., 2014, The role of catalyst in supercritical water
desulfurization, Applied Catalysis B: Environmental, 147, 144–155.
Biswas J., Maxwell E., 1990, Recent process-related and catalyst-related developments in fluid catalytic
cracking, Applied Catalysis B: Environmental, 63 (2), 197–258.
Gai X.-K., Arano H., Lu P., Mao J.-W., Yoneyama Y., Lu C.-X., Yang R.-Q., Tsubaki N., 2016, Catalytic
bitumen cracking in sub- and supercritical water, Fuel Processing Technology, 142, 315–318.
Haji S., Zhang Y., Erkey C., 2010, Atmospheric hydrodesulfurization of diesel fuel using Pt/Al2O3 catalysts
prepared by supercritical deposition for fuel cell applications, Applied Catalysis A: General, 374, 1–10.
Hosseinpour M., Fatemi S., Ahmadi S.J., Morimoto M., Akizuki M., Oshima Y., Fumoto E., 2018, The
synergistic effect between supercritical water and redox properties of iron oxide nanoparticles during in-
situ catalytic upgrading of heavy oil with formic acid. Isotopic study, Applied Catalysis B: Environmental,
230, 91–101.
Kim D.-W., Jeon P.R., Moon S., Lee C.-H., 2018, Upgrading of petroleum vacuum residue using a hydrogen-
donor solvent with acid-treated carbon, Energy Conversion and Management, 161, 234–242.
Murali Dhar G., Srinivas B.N., Rana M.S., Kumar M., Maity S.K., 2003, Mixed oxide supported
hydrodesulfurization catalysts - a review, Catalysis Today, 86, 45–60.
Primo A., Garcia H., 2014, Zeolites as catalysts in oil refining, Chemical Society Reviews, 43 (22), 7548–7561.
Quilfen C., Tassaing T., Uzio D., Aymonier C., 2018, In situ Raman investigation of the preparation of HDS
catalyst precursors using scCO2, The Journal of Supercritical Fluids, 141, 104–112.
Reina T.R., Yeletsky P., Bermúdez J.M., Arcelus-Arrillaga P., Yakovlev V.A., Millan M., 2016, Anthracene
aquacracking using NiMo/SiO2 catalysts in supercritical water conditions, Fuel, 182, 740–748.
Stepacheva A.A., Matveeva V.G., Sulman M.G., Sulman E.M., 2019, Deoxygenation of Fatty Acids in
Supercritical Hexane over Bimetallic Catalyst for Biodiesel Production, Chemical Engineering
Transactions, 76, 985-990.
Tan L., Erdenebaatar O., Liu G., Yamane N., Ai P., Otani A., Yoneyama Y., Yang G., Tsubaki N., 2017,
Catalytic cracking of 4-(1-naphthylmethyl)bibenzyl in sub- and supercritical water, Fuel Processing
Technology, 160, 34–38.
Timko M.T., Ghoniem A.F., Green W.H., 2015, Upgrading and desulfurization of heavy oils by supercritical
water, Journal of Supercritical Fluids, 96, 114–123.
Viet T.T., Lee J.-H., Ryu J.W., Ahn I.-S., Lee C.-H., 2012, Hydrocracking of vacuum residue with activated
carbon in supercritical hydrocarbon solvents, Fuel, 94, 556–562.
Yeletsky P.M., Reina T.R., Bulavchenko O.A., Saraev A.A., Gerasimov E.Yu., Zaikina O.O., Bermúdez J.M.,
Arcelus-Arrillaga P., Yakovlev V.A., Millan M., 2019, Phenanthrene catalytic cracking in supercritical water:
effect of the reaction medium on NiMo/SiO2 catalysts, Catalysis Today, 329, 197–205.
1434