Molybdenum Dioxide in Carbon Nanoreactors as a Catalytic Nanosponge for the Efficient Desulfurization of Liquid Fuels

Article abstract of DOI:10.1002/adfm.201808092

The principle of a “catalytic nanosponge” that combines the catalysis of organosulfur oxidation and sequestration of the products from reaction mixtures is demonstrated. Group VI metal oxide nanoparticles (CrO_x, MoO_x, WO_x) are embedded within hollow graphitized carbon nanofibers (GNFs), which act as nanoscale reaction vessels for oxidation reactions used in the decontamination of fuel. When immersed in a model liquid alkane fuel contaminated with organosulfur compounds (benzothiophene, dibenzothiophene, dimethyldibenzothiophene), it is found that MoO₂@GNF nanoreactors, comprising 30 nm molybdenum dioxide nanoparticles grown within the channel of GNFs, show superior abilities toward oxidative desulfurization (ODS), affording over 98% sulfur removal at only 5.9 mol% catalyst loading. The role of the carbon nanoreactor in MoO₂@GNF is to enhance the activity and stability of catalytic centers over at least 5 cycles. Surprisingly, the nanotube cavity can selectively absorb and remove the ODS products (sulfoxides and sulfones) from several model fuel systems. This effect is related to an adsorptive desulfurization (ADS) mechanism, which in combination with ODS within the same material, yields a “catalytic nanosponge” MoO₂@GNF. This innovative ODS and ADS synergistic functionality negates the need for a solvent extraction step in fuel desulfurization and produces ultralow sulfur fuel.

Full text of DOI:10.1002/adfm.201808092

FULL PAPER
Fuel Desulfurization
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Molybdenum Dioxide in Carbon Nanoreactors as a
Catalytic Nanosponge for the Efficient Desulfurization
of Liquid Fuels
Maxwell A. Astle, Graham A. Rance, Hannah J. Loughlin, Thomas D. Peters,
and Andrei N. Khlobystov*
heavily on diesel fuel. However, recent
The principle of a “catalytic nanosponge” that combines the catalysis of orga-
nosulfur oxidation and sequestration of the products from reaction mixtures
is demonstrated. Group VI metal oxide nanoparticles (CrOx, MoOx, WOx) are
embedded within hollow graphitized carbon nanofibers (GNFs), which act as
nanoscale reaction vessels for oxidation reactions used in the decontamina-
tion of fuel. When immersed in a model liquid alkane fuel contaminated with
organosulfur compounds (benzothiophene, dibenzothiophene, dimethyl-
dibenzothiophene), it is found that MoO2@GNF nanoreactors, comprising
30 nm molybdenum dioxide nanoparticles grown within the channel of GNFs,
show superior abilities toward oxidative desulfurization (ODS), affording over
98% sulfur removal at only 5.9 mol% catalyst loading. The role of the carbon
nanoreactor in MoO2@GNF is to enhance the activity and stability of catalytic
centers over at least 5 cycles. Surprisingly, the nanotube cavity can selectively
absorb and remove the ODS products (sulfoxides and sulfones) from several
model fuel systems. This effect is related to an adsorptive desulfurization
(ADS) mechanism, which in combination with ODS within the same material,
yields a “catalytic nanosponge” MoO2@GNF. This innovative ODS and ADS
synergistic functionality negates the need for a solvent extraction step in fuel
desulfurization and produces ultralow sulfur fuel.
trends of increased diesel consumption
and the dwindling reserves of crude oil
have triggered questions about satisfying
the demand for the future.[1,2] Moreover,
the available sources of crude oil contain
a high content of refractory sulfur com-
pounds, which can have adverse indus-
trial and environmental effects, including
the formation of acid rain.[3,4] Therefore,
a key current research challenge concerns
the development of an efficient and eco-
nomically viable process to remove sulfur-
containing contaminants from fuels in
order to satisfy the ultralow sulfur diesel
(ULSD) regulations of less than 10 ppm
imposed by international policies.[5–7]
For industrial applications, the removal
of sulfur compounds by hydrodesulfuri-
zation (HDS) is currently the most widely
utilized process; yet, it has several disad-
vantages, including harsh reaction condi-
tions, high costs, and limited applicability
to the aromatic sulfur compounds.[8] As
a result, the development of other desul-
furization processes has been more recently explored (Figure 1).
With biodesulfurization (BDS) currently hindered by low
enzyme activity and stability, oxidative desulfurization (ODS),
and adsorptive desulfurization (ADS) technologies offer more
realistic promise.[9–11] Specifically, the use of ODS to remove the
refractory sulfur compounds from fuels exploits key changes in
the polarity of the oxidized products relative to the parent orga-
nosulfur contaminant during extraction and has already been
established for the production of ULSD.[12] However, limiting the
quantity of solvent needed to avoid fuel loss during extraction
and the lack of cheap, efficient, and readily recoverable catalysts
represent major stumbling blocks for this pathway.[6] ADS has
also shown reasonable success, utilizing the physical adsorp-
tion of organosulfur compounds within the internal volumes
of porous materials, such as zeolites, aluminosilicates, and acti-
vated carbon.[13–15] However, a new strategy for active materials
with high and specific adsorption capacity for sulfur compounds
is needed urgently. Therefore, significant further research is
required to improve these individual strategies for ULSD pro-
duction, which could be based on more than one desulfurization
mechanism simultaneously, as we demonstrate in this study.
1. Introduction
Despite the advancement of environment-friendly technolo-
gies, the transportation and energy industries still depend
M. A. Astle, Dr. G. A. Rance, H. J. Loughlin, T. D. Peters,
Prof. A. N. Khlobystov
School of Chemistry
University of Nottingham
University Park
Nottingham
NG7 2RD, UK
E-mail: Andrei.Khlobystov@nottingham.ac.uk
Dr. G. A. Rance, Prof. A. N. Khlobystov
Nanoscale and Microscale Research Centre
University of Nottingham
University Park
Nottingham
NG7 2RD, UK
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201808092.
DOI: 10.1002/adfm.201808092
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Figure 1. Current strategies for the desulfurization of fuel. The application of hydrodesulfurization (HDS) for aromatic sulfur compounds, such as
thiophene uses hydrogen and produces H2S. Adsorptive desulfurization (ADS) using porous media and biodesulfurization (BDS) via the Kodama
pathway in the presence of enzymes have both shown promise for the selective removal of dibenzothiophene (DBT). Oxidative desulfurization (ODS)
has the greatest potential for oxidation of DBT, yielding ultralow sulfur fuels.
We selected molybdenum oxide because it has recently
emerged as a promising material for the ODS of fuel,[16–18]
with the ease of formation of the electrophilic molybdenum
peroxo intermediate species identified as the driving force
for the effective oxidation of the sulfur contaminants
(Figure 2).[19] This intermediate is produced when an oxi-
dant, such as hydrogen peroxide, reacts with the Lewis acidic
sites of the metal oxide and leads to the formation of elec-
trophilic species, essential to promote efficient ODS. In addi-
tion, the performance of molybdenum-based nanocatalysts
can be optimized using support materials, which are known
to both stabilize nanoparticles against sintering and promote
further activity by facilitating charge transfer between the
catalysts and the support material. Al2O3, TiO2, and SiO2 sup-
ports have recently been shown to improve the performance
of ODS catalysts, therefore, selection of an appropriate sup-
port is key for refining the oxidative removal of sulfur con-
taminants in fuel.[20–23] Among catalyst supports, hollow
carbon nanostructures, such as carbon nanotubes, may offer
several potential benefits for ODS catalysts.[24,25] Hollow
graphitized nanofibers (GNFs) with an internal diameter of
60 nm—structural analogues to carbon nanotubes, but pos-
sessing nanoscale step-edges on the inside—are particularly
attractive as their corrugated interior surfaces promote the
formation and enhance the stability of catalytic centers,[26–28]
while increasing the concentration of reactants around the
catalyst, without restricting the transport of reactants to and
products from the internal cavity (the critical dimensions
of most small molecules are typically at least two orders of
magnitude smaller than the internal diameter of GNFs).[29,30]
Furthermore, as porous activated carbons have shown
promise in the desulfurization of fuels via ADS,[31,32] we
anticipated that GNFs, possessing the high internal surface
area of nanotubes and maximal π–π stacking interactions
between guest and host at the graphitic step-edges, may offer
enhanced extraction of aromatic organosulfur contaminants,
promoting simultaneous desulfurization via both the ADS
and ODS mechanisms.[33]
In this work, chromium, tungsten, and molybdenum
oxide nanoparticles were grown within GNF cavities to form
carbon nanoreactors, with the latter extensively applied to the
ODS of liquid fuels. The catalytic nanoreactor MoO2@GNF
demonstrated the best activity for ODS and revealed a sur-
prising nanosponge effect by absorbing and sequestering the
oxidized contaminants from liquid alkane fuel by ADS. The
combined ADS and ODS activities within the same material
lead to ultralow sulfur fuels without solvent extraction, thus
making a significant advancement toward the production of
ULSD.
Figure 2. Mechanism for the oxidation of sulfur contaminants using
molybdenum oxide catalysts with peroxide species. (i) Nucleophilic
attack of the peroxide forms the hydroperoxymolybdate species. (ii)
Reversible loss of alcohol to produce the monoperoxo species. (iii) The
peroxo group is activated electrophilically via coordination to the moly-
bdenum atoms and results in a nucleophilic attack from the sulfur atom
in the organosulfur species and loss of the oxidized product.
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site for the growth of nanoparticles relative to the external
walls, confirming the importance of step-edges which provide
anchoring points for nucleation of the metal oxide, which are
absent on the smooth graphitic layers of the exterior. High-
resolution TEM of the nanoparticles reveals a d-spacing of
0.37 nm, which corresponds to the set of (110) lattice planes
of MoO2 (Figure 4c). Scanning transmission electron micros-
copy (STEM) in combination with EDX elemental mapping
additionally confirms the composition of the nanoparticles as
molybdenum and oxygen (Figure S1, Supporting Information).
To extract information on the 3D structure of the nanoparti-
cles, images were captured at different extents of angular tilt
(Figure 4d). As the GNF is rotated around its growth axis it
can be observed that the MoO2 nanoparticles remain within
the interior channel confirming their confinement within the
internal volume without hindering transport resistance and
exhibit faceted morphologies.
To probe the ability to control the structure and composition
of the metal oxide nanoparticles by postsynthesis treatments,
small quantities of the sample were heated to elevated tem-
peratures in air (Figure 5a). The previous TGA measurements
(Figure 2b) indicate that thermal treatment at 270 °C results in
a transformation from MoO2 to MoO3; this transition was addi-
tionally observed in the in situ PXRD, with peaks at 2θ = 27.4°,
33.9°, and 39.3° observed above 250 °C consistent with the for-
mation of MoO3. In addition, the thermal treatment resulted in
an increase of the MoO3 particle size to 63 22 nm (Figure 5c).
Therefore, this strategy offers an effective mechanism for con-
trolling both the size and the composition of the metal oxide
nanoparticles. Moreover, the large number of possible group VI
precursors that readily sublime at low to moderate tempera-
tures opens the door for a broad range of different catalytic
nanoreactors that can be readily afforded and manipulated
using this simple and versatile strategy (Figure 5d,e).
2. Results and Discussion
2.1. Synthesis and Characterization of Molybdenum Dioxide
Nanoparticles Encapsulated within Hollow Graphitized Carbon
Nanofibers
The filling of hollow GNFs can be readily achieved by exposing
empty GNFs to suitable precursors that contain the desired
elements, sublime readily at relatively low temperatures, are
stable in the gas-phase and decompose into the desired species
using external stimuli.[34] Molybdenum dioxide bisacetylace-
tonate (MoO2(acac)2) was thus identified as an ideal precursor
and adopted for the gas-phase thermal deposition reaction.
Sealing GNFs and the precursor under vacuum (3 × 10−5 mbar)
allows the sublimed guest-molecules to diffuse into the empty
nanofibers (Scheme 1); once trapped within the nanofiber
cavity, the precursor molecules are heated in an inert atmos-
phere to facilitate decomposition into molybdenum oxide nano-
particles entrapped in GNFs (MoOx@GNF).
To confirm the oxidation state and crystallinity of the
molybdenum oxide species within the composite material
powder X-ray diffraction (PXRD) analysis was employed. Peaks at
2θ = 37.1° and 53.6° can be clearly observed in the diffracto-
gram, consistent with the crystal phase of molybdenum(IV)
dioxide (Figure 3a).[35] Thermogravimetric analysis (TGA)
showed the loading (by weight) of the metal oxides in the final
composite as 4.0% (Figure 3b). In addition, TGA also indicates
a significant reduction in the combustion temperature of the
GNF by more than 150 °C, indicating intimate contact between
the nanoparticles and the nanofibers, which are important for
the catalytic performance in ODS.[28] Energy dispersive X-ray
(EDX) spectroscopy reveals the elemental composition of the
MoO2@GNF, with a near 2:1 atomic ratio of oxygen to molyb-
denum, further supporting the formation of the dioxide species
(Figure 3c).
To evaluate the size, morphology, and position of the molyb-
denum dioxide nanoparticles within GNFs, transmission
electron microscopy (TEM) is essential. Statistical analysis of
images taken from multiple parts of the specimen grid indi-
cates nanoparticles with an average diameter of 29 10 nm are
found predominantly within the internal cavity of the GNFs,
with their distinctive contrast relative to the carbon of the GNF
strongly supporting the formation of metal oxide (Figure 4b).
Thus, this technique offers superior control over the size and
location of metal oxide nanoparticles inside hollow carbon
nanofibers relative to previously reported electrospinning
approaches.[36] The interior surfaces appear to be the favored
2.2. Catalytic Performance of MoO2@GNF towards Oxidative
Desulfurization of Fuel
To appraise the hybrid material for its ability to desulfurize fuel,
a model system comprising 500 ppm dibenzothiophene (DBT)
in n-hexane was identified.[37,38] DBT was selected as it is com-
monly found in oil, thus presenting a real and important envi-
ronmental issue in its own right, and has distinctive absorption
features in the UV–vis spectrum, ensuring facile measurement
of its concentration in model fuels (Figure S2, Supporting
Information). The aliphatic hydrocarbon n-hexane is a common
Scheme 1. Schematic diagram for the gas-phase encapsulation of MoO2(acac)2 and thermal growth of molybdenum oxide nanoparticles encapsu-
lated within the hollow graphitized carbon nanofibers.
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1:20 (S:O) was added to the model fuel and catalyst and stirred
at 60 °C for 120 min.[6,21,37,39] Consistent with previous studies,
we subsequently employed an extraction procedure in which
the model fuel was vigorously stirred with acetonitrile (in a 1
to 5 ratio of extractant to fuel) for 30 min.[21,38] Acetonitrile was
selected as the solvent due to its known effectiveness toward
the extraction of the DBT oxidation products and its reduced
toxicity relative to other commonly utilized extraction solvents,
such as dimethylformamide.[40] Interestingly, it was noted that
27–30% of DBT can be removed from the fuel simply through
extraction alone, reflecting the moderate solubility of DBT in
n-hexane. This was confirmed by treating the model fuel with
the combined ODS and extraction procedure with either no
catalyst or empty GNFs (Table 1, entries 1–2), both of which
resulted in no DBT oxidation, but yielded DBT removal of
28.8% and 28.6% from the model fuel, respectively. These con-
trol tests clearly demonstrate that neither the extraction solvent
nor empty GNFs are able to desulfurize model fuel to satisfac-
tory levels.
A 5 mg quantity of MoO2@GNF catalyst (containing 4 wt%
MoO2) was then added to the model fuel to assess its ability to
remove the contaminants by ODS. Remarkably, after 120 min
of the ODS reaction and subsequent solvent extraction 98.8%
removal of the DBT from the model fuel was achieved (Table 1,
entry 3 and Figure 6a). Experiments conducted using shorter
reaction times, but with variable quantities of catalyst, indi-
cated that in principle even more effective sulfur removal could
be realized by increasing the loading of MoO2@GNF present
in the reaction mixture (Table S1, Supporting Information).
1H nuclear magnetic resonance (NMR) spectroscopy and gas
chromatography–mass spectrometry (GC-MS) of the combined
sulfur contaminants in the solvent extraction phase and the
washed solid catalyst confirms the high efficiency of DBT oxi-
dation (96.8% conversion) leading predominantly to the doubly
oxidized sulfone product (Figure S3, Supporting Information).
The driving force for effective oxidation of the sulfur contami-
nants using our catalytic nanoreactors strongly relates to the
environment at which the catalyst resides, with both modulated
surface reactivity and heightened local concentrations of the
contaminant at the GNF step-edge where the catalyst is located,
as previously demonstrated for reactions of hydrosilylation.[27]
To further explore the importance of step-edges, a sample of
MoO2 on graphite was produced (Figure S4, Supporting Infor-
mation). Graphite flakes provide anchoring sites for the cata-
lyst, akin to GNFs, but no confinement as inside the GNFs. The
results of catalyst performance showed that DBT oxidation still
occurred, yet to a lesser extent than observed using MoO2@GNF,
with a reduction of 27% in organosulfur removal noted (Table 1,
entry 5). This supports the notion that confinement of catalysts
within GNF nanoreactors is important, here affording smaller,
more active, nanoparticles (Figure S5, Supporting Information)
and higher local concentrations of DBT molecules at the GNF
internal step-edges relative to the smoother surfaces of graphite,
both of which enhance ODS activity. Moreover, TEM indicates
that after the ODS reaction MoO2 nanoparticles remain practi-
cally unchanged inside GNFs, thus the interior of GNF provides
the ideal protective environment, inhibiting nanoparticle desorp-
tion and leaching, ensuring the reuse of the catalyst in subse-
quent ODS reactions (Figure S6, Supporting Information).
Figure 3. a) PXRD patterns of MoO2@GNF (red), empty GNF (black),
and bulk MoO2 (blue) show that the hybrid material is the combination
of the two components. Grain size was estimated to be 20 nm based on
analysis of the peak at 2θ = 37.1° (full width half maximum = 0.51°) and
application of the Scherrer equation. b) Representative TGA of MoO2@
GNF (red) versus GNF (black), providing a measure of the metal oxide
loading. c) EDX spectroscopy of MoO2@GNF confirms the presence of
Mo and O inside GNF in a ratio close to the stoichiometry in MoO2.
constituent of fuel and represents a valuable model fuel simu-
lant used in previous research.[39]
In a typical ODS procedure, an organic peroxide—employed
to avoid potential phase transfer limitations—in a ratio of
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Figure 4. a) Schematic representation of the structure of GNFs (interior step-edges are denoted by black arrows; the yellow arrow signifies the
direction of the nanofiber growth axis; the blue shape represents a MoO2 nanoparticle). b) Bright field transmission electron microscopy image
MoO2@GNF. The internal step edges have been highlighted for clarity, with the MoO2 nanoparticle clearly shown residing at the interior step-edges.
c) High-resolution TEM of the MoO2 nanoparticle providing a lattice spacing of 0.37 nm, consistent with the (100) plane in MoO2. d) TEM tilt series
of MoO2@GNF, rotating around the GNF growth axis, which allows for a better understanding of the morphology of the nanoparticles and confirms
their encapsulation within the internal cavity of the GNF.
Figure 5. a) PXRD analysis of MoO2@GNF thermally annealed in air clearly shows the change from MoO2 to MoO3 between 250 and 300 °C. b,c) TEM
images and corresponding nanoparticle size distributions before and after postsynthesis thermal treatment at 450 °C, respectively, highlighting the
increase in nanoparticle size and blocking of the GNF interior channel for materials treated at elevated temperatures. d,e) TEM images of WOx@GNF
(from W(CO)6) and CrOx@GNF (from Cr(acac)3) with their particle size distribution profiles, respectively. These show the potential of the gas-phase
deposition technique using different precursors to produce a plethora of unique nanomaterials.
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Table 1. Dibenzothiophene removal from the model fuel system.
Entry
Catalysta)
Sulfur removal with ODS procedure Sulfur removal with ODS and DBT conversionc) [%]
Selectivityc) [%]
DBTO:DBTO2
Turnover frequencyd)
[mol mol−1 min−1
]
onlyb) [%]
solvent extractionb) [%]
1
2
3
4
5
None
GNF
0.0
28.8 (29.4)
0.0
0.0
-: -
–
–
0.0
28.6 (30.1)
-: -
MoO2@GNF
MoO3@GNF
MoO2@graphite
95.5
98.8 (98.1)
96.8
57.2
60.8
14:86
76:24
75:25
0.14 (0.45)
0.08
46.3
72.7 (75.1)
44.4
71.5 (74.2)
0.09
a)5 mg catalyst (containing 4 wt% MoOx), 5 mL of n-hexane containing 500 ppm DBT, tert-butyl hydroperoxide (TBHP) oxidant (S:O is 1:20), 60 °C, 120 min; b)Percentage
of sulfur-containing contaminants removed via either (i) ODS or (ii) combined ODS solvent extraction (1 mL CD3CN, 30 min) was determined using UV–vis spectroscopy
and GC-MS (GC-MS values shown in parentheses); c)The conversion of DBT and selectivity of products was calculated using GC-MS of fuel phase and 1H NMR spectros-
copy of solvent extraction phase and a solvent washing of post-reaction solid catalysts; d)Turnover frequency calculated from the conversion of 4% weight loaded MoOx
nanomaterials with 120 min ODS experiment (the value in parenthesis describes the value determined using the conversion obtained after only 30 min).
Interestingly, initial attempts to reuse the MoO2@GNF
catalyst were only moderately successful, with an ≈40% drop
in sulfur removal capacity noted after the first cycle (red bars,
Figure 6b). Analysis of MoO2@GNF after the first ODS reac-
tion by PXRD (Figure 6c) revealed the presence of a mixture
of organic molecules held within GNFs, comprising predomi-
nantly DBTO2, a small amount of DBTO, but importantly no
DBT. Complementary TGA (Figure 6d) of the used MoO2@GNF
catalyst confirmed the presence of DBTO2 inside GNFs, with a
significant mass loss at 242 °C consistent with the boiling point
of the sulfone product (Figure S7, Supporting Information)
step, with over 90% removal of the DBT observed (Table 1, entry
3). Kinetic analysis indicates a pseudolinear removal of DBT
up to 60 min, yielding 78% total sulfur removal; after this, the
rate of removal decreases from a combination of the expected
decrease in DBT concentration as the reaction proceeds and
slower diffusion inside the GNF internal channel due to accu-
mulation of the products of DBT oxidation inside nanoreactors
(Figure 7b). However, after 60 min the level of sulfur contami-
nants within the fuel approaches that which is required to meet
current ULSD regulations. Similarly, the decrease in activity of
MoO3@GNF (Table 1, entry 4) is a consequence of the confined
catalysts growing to the diameter of the nanofiber resulting
in blocking of the internal cavity and subsequently restricting
access of reactants to the confined catalyst. Given the low cat-
alyst loading in our experiments, our catalytic nanoreactors
MoO2@GNF significantly outperform other catalysts in their
ability to desulfurize fuel by more than an order of magnitude
(Table S3, Supporting Information).[21,39,42,43]
To assess the effectiveness of the MoO2@GNF catalysts
toward real fuel systems a more sterically demanding contami-
nant (dimethyl dibenzothiophene – DMDBT) and an electron-
poor contaminant (benzothiophene – BT) were investigated
alongside DBT in n-octane as a more representative fuel. No
effect of solvent, i.e., n-hexane versus n-octane, was noted in
the sole oxidation of DBT (Table S4, Supporting Information).
From a comparison of the sulfur removal efficiency of GNFs
and MoO2@GNF (including and without the solvent extraction)
after 18 h, it is clear that MoO2@GNF is effective towards the
oxidation of organosulfur species, with the nanosponge able to
remove the oxidized contaminants from the mixed fuel system
(Table 2), with the order of reactivity of these contaminants
shown to be DMDBT > DBT > BT. After 2 h, it was found that
there was almost 100% removal of DMDBT. The reason for this
is thought to be related to the higher affinity of DMDBT for
GNFs which encourages absorption into the internal channel of
the nanoreactors and leads to a high concentration of DMDBT
1
noted. Moreover, H NMR spectroscopy analysis of the organic
material removed from the internal channel of GNFs by sub-
sequent washing of the solid catalyst (Figure S8, Supporting
Information) confirmed that only DBTO and DBTO2 become
adsorbed and trapped within the GNFs in significant quanti-
ties. Whilst our adsorption experiments (Table S2, Supporting
Information) indicate that MoO2@GNF only uptakes negligible
quantities of DBT from solution by ADS, analysis of the catalyst
after its first use provides compelling evidence that enhanced
removal of sulfur contaminants can be achieved here by selec-
tive adsorption and retention of the products of ODS reac-
tion, i.e., adsorptive desulfurization by sequestration of DBTO
sulfoxide and DBTO2 sulfone. This is consistent with a previous
study which indicated that the adsorption affinity of porous
carbons was significantly greater for DBTO2 than DBT.[41] From
a practical point of view, accumulation of the ODS products
necessitates an additional thermal or solvent treatment between
uses of catalytic nanoreactors MoO2@GNF in order to main-
tain the high sulfur removal capacity use-to-use (green bars,
Figure 6b). However, this nanosponge effect ensures efficient
removal of the sulfur contaminants through a combination of
ODS and ADS, thus potentially negating the requirement for a
separate and costly additional extraction step (Figure 7a).
In light of this, we next considered the efficiency of sulfur
contaminant removal in the absence of a solvent extraction
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Figure 6. a) Kinetic profile for the removal of DBT from n-hexane using the ODS (5 mg MoO2@GNF, 5 mL fuel, 1:20 S:O ratio, 60 °C, 120 min)
and solvent extraction (5:1 hexane:acetonitrile, 30 min) procedures. These results show that there is moderate sulfur removal from the model
fuel using solvent extraction alone; however, when combined with the ODS reaction, near complete removal can be achieved on a relatively
short timescale. b) Recycling experiments using MoO2@GNF with no catalyst treatment between runs for the ODS of DBT indicated an ≈40%
lower sulfur removal capacity after the first use, with no significant changes noted over the next four uses. Analysis of the used catalyst by PXRD
c) indicated the selective retention of the products of ODS (* sulfoxide, # sulfone, • catalyst) within the nanoreactor cavity after the initial use.
This was confirmed by d) TGA, with a mass loss at 242 °C (i) consistent with the boiling point of DBTO2, prior to GNF combustion at 589 °C (ii).
The presence of the products of ODS within GNFs after the first use provides strong evidence for the ability of MoO2@GNF to decontami-
nate model fuels using a dual functional approach, i.e., involving both ODS of DBT and ADS of the ODS reaction products. Moreover, the
retention of activity use-to-use after the initial drop, in the absence of an intermediate catalyst washing step, indicates that the products of
the ODS reactions do not permanently block the GNF nanoreactor cavity and therefore access of DBT molecules to the catalyst in subse-
quent uses is not restricted. Thus, the observed reduction in sulfur removal after the initial use reflects the slower accumulation of DBT mole-
cules at the GNF step-edges, requiring initial desorption of the ODS products from the cavity of the used MoO2@GNF catalyst into the fresh
model fuel, i.e., down a DBTO2 concentration gradient, to provide a route for access of new DBT reactant molecules to the catalytic centers.
The introduction of either a 250 °C thermal treatment or washing with a polar solvent between consecutive ODS reactions effectively removes
these products from the channel and is essential for ensuring high catalytic activity even after five uses (b). A very small drop in catalyst
performance is still noted between uses one to five and this has been attributed to subtle changes in nanoparticle morphology and loading
induced during successive reactions.
at the location of the catalyst. This is supported by the observa-
tion of a small decrease in the removal of DBT in the mixed
system from what would be expected after 2 h in isolation,
indicative of competition within the nanoreactor of DMDBT
and DBT for access to the catalyst (Table 2 – values in paren-
theses). After an 18 h ODS procedure, all of the DMDBT and
DBT can be effectively oxidized and removed by the solid nano-
sponge extraction process (Table S5, Supporting Information).
Although BT can be oxidized by the catalyst, the rate of oxida-
tion is lower than the other contaminants, requiring 18 h for
the near total (95%) removal. In all these reactions MoO2@
GNF not only promotes the ODS reactions but also absorbs the
products of oxidation via the ADS mechanism leading to desul-
furization at ULSD regulations.
3. Conclusion
In this study, we demonstrate that catalytic nanoreactors
employed in the desulfurization of liquid fuels play a dual role
of catalyst and nanosponge – simultaneously promoting the
reactions of oxidation of organosulfur species and absorbing
their products from a model fuel. Several group VI metal oxide
nanoparticles were grown inside GNF hollow carbon nanofibers
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Figure 7. a) Schematic diagram describing the process of oxidation and extraction using the nanoreactor and nanosponge composite material to
achieve effective removal of sulfur contaminants. b) Kinetic profile for the removal of DBT from an n-hexane model fuel using the ODS procedure
alone (5 mg MoO2@GNF, 5 mL fuel, 1:20 S:O ratio, 60 °C), thus exploiting the nanosponge potential for extraction. These results show that there is
a near linear removal of DBT up to 60 min. After 120 min, near complete removal can be achieved without the need for a separate solvent extraction.
and were found to form stable materials with well-defined and
controlled structure and composition. Molybdenum (IV) oxide
nanoparticles within GNFs (MoO2@GNF) showed the best cat-
alytic activity resulting in over 98% desulfurization by utilizing
the nanosponge’s abilities to selectively absorb the oxidized
products. ULSD levels of mixed and individual organosulfur
contaminated model fuels can be achieved by applying the
nanosponge or combined solvent extraction procedure. Con-
finement of catalytic centers in GNFs allows effective reuse of
the nanoreactors for at least 5 cycles with no significant loss
of activity. Importantly, the dual ODS and ADS functionality of
MoO2@GNF material negates the need for the extraction stage,
leading directly to the removal of over 95% of organosulfur
contaminants from the liquid alkane fuels.
4. Experimental Section
General: Standard reagents, including 4,6-dimethyldibenzothiophene
(97%), benzothiophene (98%), and thiophene (>99%), and solvents were
purchased from Sigma-Aldrich Chemicals and were used as purchased.
Additional reagents were obtained from the following sources:
dibenzothiophene (98%, Acros Chemicals), n-octane (95%, VWR
Chemicals), n-hexane (98.5%, Fischer Scientific), tert-butyl hydroperoxide
(70% aqueous solution, Alfa Aesar), and bis(acetylacetonato)
Table 2. Sulfur removal from a mixed contaminant model fuel system.
GNFa)
MoO2@GNFa)
Contaminant
BT
Sulfur removal with ODS
procedureb) [%]
Sulfur removal with ODS and solvent
extraction proceduresb) [%]
Sulfur removal with ODS
procedureb) [%]
Sulfur removal with ODS and solvent
extraction proceduresb) [%]
1.6
30.8
94.8 (2.6)
98.3 (44.7)
DBT
4.1
26.3
100.0 (84.3)
100.0 (94.3)
DMDBT
10.9
16.9
100.0 (99.3)
100.0 (99.4)
Sulfur contaminants
354.2
282.5
6.5* (142.2)
2.1* (58.9)
remaining in fuelc) [ppm]
a)5 mg of heterogeneous catalyst (empty GNF or MoO2@GNF containing 4 wt% MoOx), 5 mL of n-octane containing three contaminants (125 ppm of benzothiophene –
BT, dibenzothiophene – DBT, and dimethyldibenzothiophene – DMDBT), sulfur:oxidant molar ratio is 1:20, tert-butyl hydroperoxide oxidant, 60 °C, 18 h (values in paren-
theses correspond to data collected after 2 h); b)Percentage of sulfur-containing contaminants removed via either (i) ODS or (ii) combined ODS and solvent extrac-
tion (1 mL CD3CN, 30 min) was determined using GC-MS; c)Sum of contaminants removed from the fuel in ppm. * Represent values below those required by current
regulations.
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dioxymolybdenum(VI) (MoO2(CH3COCHCOCH3)2 (99%, Alfa Aesar).
Graphitized nanofibers (PR19-XT-HHT carbon nanofibers, iron content
< 100 ppm) were purchased from Pyrograf Products Inc.
in a preheated furnace to obtain the MoO2@GNF composite material,
which was then cooled down slowly for 9 h. MoO2@graphite composite
was synthesized using the same method with a graphite flakes powder to
MoO2(acac)2 ratio of 5.6 mg: 30 mg. For the tungsten/chromium oxide
encapsulated species the same procedure was followed with a tungsten
hexacarbonyl precursor (6.8 mg, sublimation temperature; 120 °C) or
chromium acetylacetonate precursor (25.0 mg, sublimation temperature;
160 °C). Postsynthesis thermal manipulation of the MoO2@GNF
composite required heating at 350 °C in air for 30 min and cooled slowly
over 2 h to form MoO3@GNF.
Transmission electron microscopy and dark field scanning
transmission electron microscopy were performed using a JOEL JEM-
2100Plus microscope operated at 200 KeV. TEM samples were prepared
via a drop casting technique, where samples were first dispersed in
methanol and deposited on a copper grid mounted “lacey” carbon films.
All images were processed using Gatan Digital Micrograph. Energy
dispersive X-ray spectroscopy and mapping were acquired for samples
mounted on the TEM grid using an Oxford Instruments INCA X-ray
microanalysis system. The beam was condensed onto areas suspended
over holes of the amorphous carbon film to eliminate contributions from
the support film itself. The copper and the silicon peak signals, associated
with the grid mesh and an artefact of grid fabrication, were removed.
Oxidative Desulfurization Procedure: The first model fuel (500 mg
L−1 of sulfur) was prepared by dissolving DBT (0.870 g, 4.72 mmol) in
300 mL n-hexane. The second mixed component model fuel contains
benzothiophene (125 ppm), dibenzothiophene (125 ppm), and
dimethyl dibenzothiophene (125 ppm) in n-octane. The desulfurization
experiments were conducted at 60 °C. In general, 5 mg of catalyst was
added to 5 mL of the model fuel and sonicated for 2 min. 0.14 mL of
70 wt% tert-butyl hydroperoxide (TBHP) aqueous solution was added
and the solution was heated for 120 min unless otherwise stated and
stirred at 500 rpm. Once complete, the solid was removed and the
reaction products extracted using 1 mL CH3CN. The extraction process
was vigorously stirred at a constant speed (1000 rpm) for 30 min at
ambient temperature. The removed catalyst was washed with deuterated
solvent and the washings were combined with the extraction layer. For
the recycling experiments where no catalyst washing was performed
between subsequent uses (red bars, Figure 6b) the solid catalyst was
separated from the reaction mixture by filtration, without an extraction
into acetonitrile, and dispersed directly as a solid in the model fuel
of successive reactions. The extraction phase and organic fuel layer
were separated and separately analyzed: the n-hexane layer by GC-MS,
Thermogravimetric analysis was performed using
a TA Q500
Thermogravimetric Analyzer. All samples were analyzed using a platinum
pan and in the presence of air. The parameters for all experiments were:
Ramp 5 °C min−1 from 20 to 1000 °C with an isotherm for 10 min at
1000 °C, air flow: 60 mL min−1.
The powder X-ray measurements were performed using a PANalytical
X’Pert Pro diffractometer equipped with a Cu K(α) radiation source
(λ = 1.5432, 40 kV 40 mA) in Bragg-Brentano geometry using a Si zero
background holder. All samples were wetted with isopropyl alcohol to
aid GNF adhesion. The parameters for a typical experiment were: start
angle: 5°, stop angle: 80°, step size: 0.0525°, time/step: 6080 s, and
scan speed: 0.00220° s−1. High temperature PXRD measurements were
performed using an Anton Parr (HTK 1200N) high temperature oven
chamber in air up to 450 °C.
UV-vis absorption spectra were recorded at room temperature using
1 cm quartz cuvettes. The samples were run using a Perkin-Elmer
Lambda 25 UV-vis spectrometer at a scan rate of 240 nm min−1 over
a wavelength range of 200–500 nm. Spectra were analyzed using UV
WinLab ES software. For all analysis, 30 µL of the treated model fuel was
diluted using 2.5 mL of the same solvent.
1
UV–vis and H NMR spectroscopies; the extraction layer using GC-MS
1
and H NMR spectroscopy. The oxidation of DBT was monitored by the
disappearance of the characteristic chemical shifts between 8.28 and
8.21 ppm (m, 2H). Confirmation of the sulfoxide and sulfone products
was afforded by monitoring by the integrals between 7.71–7.64 (t, 2H) and
7.86–7.81 (d, 2H), respectively. The treated diesel was stored in a sealed
vial and kept refrigerated at 2 °C. GC-MS retention times for organosulfur
compounds were: DBT = 25.27, DBTO = 32.23, DBTO2 = 32.51;
DMDBT = 29.27, DMDBTO = 34.67, DMDBTO2 = 34.48; BT = 11.94,
BTO = 21.54, BTO2 = 22.18 min.
The desulfurization experiments without the solvent extraction were
conducted as previously mentioned at 60 °C. In general, 5 mg of catalyst
was added to 5 mL of the model fuel and sonicated for 2 min. 0.14 mL
of 70 wt% TBHP aqueous solution was added and the solution was
heated for 120 min unless otherwise stated and stirred at 500 rpm. Once
complete, the solid was removed, with resultant organic phase and
washed solid catalyst analyzed using the techniques stated above.
1H NMR spectroscopy spectra were recorded in CD3CN at room
temperature using a Bruker AVANCE DPX-300 spectrometer (TopSpin
1.3 PL4) in Wilmad NMR tubes (5 mm diameter). Spectra were analyzed
using MestReNova software.
Samples were analyzed by GC-MS using a Thermo Scientific ISQ-LT
single quadrupole mass spectrometer, attached to a Thermo Scientific
Trace 1300 GC. Samples were injected through a Thermo Scientific
TriPlusRSH liquid autosampler onto a Thermo Scientific TG5MS GC
column (15 m × 0.25 mm × 0.25 µm). Instrument conditions were as
follows: GC injector temperature 200 °C; injections were performed
in split mode, employing a 50:1 split ratio. The GC oven temperature
programme was 40 °C (3 min) to 320 °C (10 min) at 5 °C min−1. The
GC carrier gas was helium, with a column flow of 1 mL min−1. The mass
spectrometer was programmed to acquire data after a 3 min delay, over
the mass range of 50–600 Da, with a 0.2 s scan time. Mass spectra
were acquired in EI mode (70 eV ionization energy). The ion source
temperature was 200 °C and the MS transfer line was maintained at
250 °C. Total ion current chromatograms and associated mass spectra
were processed with Chromeleon software (Version 7.2; Thermo Fisher
Scientific Inc., Massachusetts, USA).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Preparation of MOx@GNF: The molybdenum dioxide catalyst was
synthesized by first pretreating PR19 graphitized nanofibers, to remove Acknowledgements
any moisture, by heating below their oxidation temperature (500 °C) in air
The authors would like to thank the Centre for Sustainable Chemistry
for 1 h. The pretreated graphitized nanofibers (60 mg) were then loaded
into a Pyrex glass tube (d = 10 mm, L = 6 cm) with molybdenum dioxide
bisacetylacetonate (11.1 mg) and sealed under vacuum (≈5 × 10−5 mbar).
The sealed vessel was then heated to 160 °C for 2 days. Following
sublimation and prior to opening, the Pyrex glass tube was immediately
cooled for 5 min. MoO2(acac)2@GNF were placed into a new Pyrex glass
tube (d = 10 mm, L = 6 cm) and evacuated and backfilled with argon
(repeated three times) to remove any oxygen or moisture present. Before
sealing, the Pyrex glass tube was filled with argon gas (≈0.5 bar). For the
decomposition step, the sealed vessels were heated at 500 °C for 1 h
(CSC), University of Nottingham, and the Engineering and Physical
Science Research Council (EPSRC) for supporting this research, and the
Nanoscale and Microscale Research Centre (nmRC) for access to the
electron microscopy facilities.
Conflict of Interest
The authors declare no conflict of interest.
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