Controlled reduction of aromaticity of alkylated polyaromatic compounds by selective oxidation using H2WO4, H3PO4and H2O2: a route for upgrading heavy oil fractions

Article abstract of DOI:10.1039/d1nj01986d

Heavy crude oil fractions which form the residues from fractional distillation are a significant proportion of current hydrocarbon reserves. However, processing residues for use as chemicals or fuels is hampered by the high polyaromatic content of this material. Selective reduction of aromaticity by targeted ring opening and the preservation of alkylated chain side groups are key requirements for the upgrading of alkylated polynuclear aromatic hydrocarbons (PAHs) to more easily processed and higher value molecules. In this study, a H₂WO₄catalyst combined with H₃PO₄and H₂O₂oxidant is applied to the selective oxidation of PAHs containing different lengths of substituent alkylated chain, and different numbers of rings in the fused aromatic core. For a model substrate of 2-ethylnaphthalene, using traditional organic solvents, aliphatic carbon was oxidized more readily compared to aromatic carbon. However, it was found that the oxidation to desired products can be specifically controlled as the selectivity is directed by the choice of solvent, with reactions carried out in acetonitrile giving oxidation only in the aromatic region of the molecule. With larger polyaromatic molecules, a biphasic solvent system is used with Aliquat 336 as a phase transfer agent. Even for this more complex reaction system high conversion to the corresponding alkylated ring opened compounds was obtained.

Full text of DOI:10.1039/d1nj01986d

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Controlled reduction of aromaticity of alkylated
polyaromatic compounds by selective oxidation
using H WO , H PO and H O : a route for
Cite this: New J. Chem., 2021,
45, 13885
2
4
3
4
2 2
upgrading heavy oil fractions†
a
a
a
Ewa Nowicka, * Meenakshisundaram Sankar,
Robert L. Jenkins,
a
a
a
David W. Knight, David J. Willock, Graham J. Hutchings,
Manuel Francisco ‡ and Stuart H. Taylor
b
a
*
Heavy crude oil fractions which form the residues from fractional distillation are a significant proportion
of current hydrocarbon reserves. However, processing residues for use as chemicals or fuels is
hampered by the high polyaromatic content of this material. Selective reduction of aromaticity by
targeted ring opening and the preservation of alkylated chain side groups are key requirements for the
upgrading of alkylated polynuclear aromatic hydrocarbons (PAHs) to more easily processed and higher
2 4 3 4 2 2
value molecules. In this study, a H WO catalyst combined with H PO and H O oxidant is applied to
the selective oxidation of PAHs containing different lengths of substituent alkylated chain, and different
numbers of rings in the fused aromatic core. For a model substrate of 2-ethylnaphthalene, using
traditional organic solvents, aliphatic carbon was oxidized more readily compared to aromatic carbon.
However, it was found that the oxidation to desired products can be specifically controlled as the
selectivity is directed by the choice of solvent, with reactions carried out in acetonitrile giving oxidation
only in the aromatic region of the molecule. With larger polyaromatic molecules, a biphasic solvent
system is used with Aliquat 336 as a phase transfer agent. Even for this more complex reaction system
high conversion to the corresponding alkylated ring opened compounds was obtained.
Received 23rd April 2021,
Accepted 13th July 2021
DOI: 10.1039/d1nj01986d
rsc.li/njc
delayed coking and hydrocracking with desulfurization. Fluid
catalytic cracking converts a high boiling feedstock to more
1
. Introduction
Heavy crude oil fractions, residues and bitumens represent useful petroleum compounds, with the use of high temperature,
between 4 wt% and 16 wt% of current processed crude oil moderate pressure, and most importantly a fluidized powdered
1
across global production sites. As such, this represents a catalyst. Thermal cracking and delayed coking uses no catalyst or
significant and largely under-used feedstock in the petrochem- additional chemicals to achieve cracking at very high temperatures
3,4
icals industry. Currently, these fractions can only be used in low (450–470 1C). Hydrocracking on the other hand requires high
value applications such as binders for road surfaces and as a pressure hydrogen, which can result in over-cracking and for-
5
waterproofing adhesive in construction. The use of this complex mation of low value gas. A very low selectivity and high energy
mix of high molecular weight compounds through further demand are the major drawbacks for all these methods.
upgrading to higher value products would help maximise
Oxidation offers a more facile route for petroleum residue
productivity and enable us to meet an increasing demand for transformation, and potentially could become a core technology.
2
fuels without developing a large number of new reserves. The These heavy fractions consist mostly of alkylated polynuclear
petroleum industry uses three major methods for upgrading: aromatic hydrocarbons, (PAHs), so the challenge is to control the
deep oil fluid catalytic cracking, thermal cracking, including oxidation of aromatic carbon whilst preserving alkyl chain side
6–9
groups. The fact that PAH molecules are difficult to oxidise is
apparent when it is realised that they are also a major component
of anthropogenic atmospheric pollution from combustion and
industrial activity. Their chemical removal in this context is also
an area of intense research but with the goal of complete reaction
a
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
Park Place, Cardiff, CF10 3AT, UK. E-mail: nowickae@cardiff.ac.uk,
taylorsh@cardiff.ac.uk
10
b
ExxonMobil Research & Engineering Company, 1545 Route 22 East, Annandale,
New Jersey 08801, USA
to CO
2
and water. We have recently reported that Ruthenium Ion
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj01986d
Deceased March 9, 2016.
Catalyzed Oxidation (RICO) of alkylated PAH molecules can have
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11
the required selectivity for residue processing. RICO chemistry anionic bisperoxotungstate complex in the aqueous phase. This
finds a wide range of applications in C–H bond activation and complex reacts further with phosphonic acid, to give a form
1
2
oxygen transfer reactions. By judicious choice of experimental that can be transported via the phase transfer catalyst to the
conditions and solvent system we were able to demonstrate organic phase, where oxidation of the substrate takes place.
oxidation of aromatic regions of model alkylated PAH com- Following substrate oxidation, the monoperoxo tungstate ion
1
3
pounds, whilst aliphatic carbons were unreacted. In RICO moves back to the aqueous layer where it can again react with
chemistry RuCl is transformed into a RuO oxidising species to reform the anion and the cycle repeats itself.
using NaIO as oxidant. The high oxidation state RuO moiety Herein, we investigate the use of the H WO /H
3
4
2 2
H O
4
4
2
4
2 2
O catalytic
shows a strong affinity for double bonds in the aromatic system system for the oxidation of a range of alkylated polynuclear
of PAH molecules that are chemically isolated, i.e. regioselectivity aromatic hydrocarbons, as model compounds for upgrading of
favours the formation of a [3+2] intermediate in positions that crude oil residues. For larger molecules we employ a biphasic
14
lead to minimal loss of aromaticity. This provides a route to solvent system with phase transfer agent to improve catalytic
selectively oxidise aromatic carbon producing predominantly efficiency. Our fundamental focus is to examine oxidation
2 4
aldehyde and carboxylic acid functionality. However, this catalysed by H WO as an alternative to the previously dis-
ruthenium-based catalytic system relies on an expensive metal cussed RICO Chemistry.
and requires a strong oxidant which produces a stoichiometric
equivalent of salt waste, therefore a cheaper catalyst and more
environmentally benign oxidant would be preferred.
2
. Experimental
As early as 1959, Payne and Williams demonstrated that a
combination of a tungsten based catalyst with H PO and H O
2 2
2.1. Oxidation reactions
3
4
as oxidant can give a highly selective oxidative catalytic system
for organic synthesis. This system later became known as the carried out in a glass reactor over the temperature range 65 1C
Oxidation of 2-ethylnaphthalene. Oxidation reactions were
1
5
1
6,17
Ishii-Venturello oxidation system.
This combination of to 85 1C in a range of solvents: C
CN. The reaction mixture was stirred using a magnetic bar
inside the reactor (700 rpm). In a typical reaction the round
A number of bottom flask was firstly charged with the 2-ethylnaphthalene
6 5 6 5 3 2 5
H Cl, C H CH , C H CN and
relatively cheap catalyst and high efficiency oxidant has been CH
reported to be active for the oxidation of alcohols, aldehydes,
olefins, as well as aromatic hydrocarbons.
3
1
8
19
1
6
20,21
studies have been published demonstrating the use of W containing (18.8 mmol) and the desired solvent (8 ml). The catalyst
compounds as oxidation catalysts, for example as Keggin type (H WO , 0.614 mmol), 10% H PO (0.14 ml) and Aliquat 336
2
4
3
4
2
2,23
polyoxometylates.
is a more potent oxidation catalyst, mediating the conversion to stir for 2 min. H
of PAH molecules such as phenanthrene and pyrene to the dropwise under continuous stirring, the flask containing the
corresponding diacids when combined with H PO and using reaction mixture was then transferred to a preheated oil bath and
The use of hydrogen peroxide as the was fitted with a water-cooled condenser. For quantitative analysis,
However, tungsten in the form of H WO
(0.24 ml) were subsequently added, and the reaction mixture left
2
4
2 2
O
(30%, 12 ml, 118 mmol) was then added
3
4
2
0,24
2 2
H O as oxidant.
primary oxidant is also attractive as it removes the production samples (0.2 ml) were withdrawn from the organic layer at the
of salt waste that would occur with the RICO approach. These desired time interval and mixed with an external standard,
observations suggest that aqueous H WO /H O could be suitable 2-butanol (20 ml) prior to GC analysis. To enable the withdrawal of
2
4
2 2
for selective oxidation of alkylated-PAHs to reduce aromaticity. the sample, stirring was stopped until visible separation between
However, oxidation of alkylated PAH molecules is more challenging the two phases was seen. Samples for GC analysis were then
than the parent PAH compounds exemplified by phenanthrene and withdrawn from both organic and aqueous layers and were analyzed
pyrene. It is known that the a-carbon, the first aliphatic carbon quantitatively using a Varian 450 Gas Chromatograph with a VF-5ht
bound to the aromatic system, is potentially more susceptible to (30 m, 0.25 mm) column and a FID detector.
oxidation than the aromatic core itself due to the relatively low C–H
It is believed that after 6 hours reaction, no oxidant remained
bond energy for carbon atoms in the benzyl position. To date there in the reaction mixture as it was either consumed in the
are no studies to establish the influence of alkyl substituents in the oxidation reaction or alternatively decomposed.
1
oxidation of alkylated-PAH substrates with this catalyst system.
Oxidation of PAHs with long alkyl chains and H NMR
The high molecular weight alkylated PAH molecules found analysis. NMR analysis of PAHs with long alkyl chains required
1
in residues are very hydrophobic requiring a non-polar solvent. clean H NMR spectra, which were dependent on the use of
In contrast, a polar solvent is needed for the H
WO catalyst so that reactions will usually take place in a products in the aliphatic region of the H NMR. This was achieved
biphasic system. While this has advantages in terms of product by lowering the concentration of Aliquat 336 in a scaled down
2 2
O oxidant and Aliquat 336, as the phase transfer catalyst masks signals from
1
H
2
4
2
5
separation it can also result in poor mixing between the experiment. Oxidation reactions were carried out in a glass reactor
substrate and oxidising species. Reaction efficiency can be for 16 hours, unless otherwise stated. First, substrate (0.1 mmol)
improved using a phase transfer catalyst and the addition of was dissolved in toluene (1 ml), then H WO (0.019 g, 0.077 mmol),
2
4
2
6
an acid. Barrio et al. have combined experimental findings
and theoretical calculations to propose that oxidation in this toluene) were added and mixed together. In the last step of the
case begins with the reaction of H WO with H to form an reaction H 30% (2 ml, 20 mmol) was added under continuous
3 4
H PO (10% 0.01 ml) and Aliquat 336 (0.0012 g, solution in
2
4
2
O
2
2 2
O
1
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stirring. The reaction was run at a temperature of 80 1C. After the (8 ml). The catalyst (H
Paper
2
4 3 4
WO , 0.025 mmol), 10% H PO (0.14 ml)
required time of reaction (16 h), the two phases were separated, and Aliquat 336 (0.24 ml) were subsequently added and the
and the toluene phase was washed with water (3 ꢀ 20 ml). The reaction mixture stirred for 2 min. H O (30%, 2.5 ml, 25 mmol)
2
2
aqueous phase was washed with toluene (3 ꢀ 20 ml) and the was then added dropwise under continuous stirring, the flask
organic and aqueous filtrates were combined with the organic or containing the reaction mixture was then transferred to a pre-
aqueous layer. The organic phase was dried over anhydrous heated oil bath and was fitted with a water-cooled condenser and
Na
to dryness. The aqueous phase was also evaporated to dryness. analysis was performed in a similar way to that used for the
CDCl (0.7 ml) was added to the flask containing the organic oxidation of 2-ethylnaphthalene.
2 4
SO , the solid removed by filtration and the filtrate evaporated stirred vigorously whilst it was maintained at 80 1C. Quantitative
3
products and a 0.6 ml portion of the resultant solution was
transferred to the NMR tube fitted with an internal standard 2.2. Product analysis
sealed glass insert (1%TMS in CDCl ). Residues from the aqueous
1
13
3
H NMR, C NMR and 2D NMR spectra were recorded on a
1
layer were dissolved in D O and analyzed using the H NMR
13
2
Bruker DPX 500 MHz instrument. For C NMR and 2D NMR
analysis, firstly the solvent used in the reaction was evaporated,
the sample was then placed under vacuum until it was com-
pletely dry and a volume of approx. 1 ml of deuterated solvent
added to the dry residue to dissolve it. The solution (0.5–0.7 ml)
was subsequently transferred to the NMR tube for analysis.
Gas Chromatography–Mass Spectrometry (GC-MS) analysis
was performed using a Waters GCT premier instrument fitted
with an Agilent HP-5MS column. Unless otherwise stated in the
earlier part of the experimental section, samples were analyzed
as received from the reaction mixture. A sample of 1 ml of
reaction mixture (from both or single solvent layer) was trans-
ferred to a GC-MS vial and analyzed without further treatment.
Mass Spectrometry analysis (MS) was performed using a
Waters LCT Premier XE (ES), while EI data was generated using
a Waters GCT Premier EI (EI) instrument. Similarly to GC-MS
analysis, samples were analyzed after the reaction was com-
pleted. A volume of approximately 1 ml was transferred to a
GCMS vial and analyzed without further treatment.
technique. However, no products were detected in the aqueous
layer. The H NMR spectra obtained were compared against the
spectra of the pure substrate, which was previously prepared by
3
dissolving 0.1 mmol of compound in 0.6 ml of CDCl . The
1
1
H NMR spectrum of this solution was recorded in the presence
of the same internal standard glass insert. The protons of interest
which are used to calculate the percentage of preserved alkyl chain
are protons A in the side chain that are a to the aromatic ring and
protons D which are terminal protons (Fig. S1, ESI†). These
protons were chosen for calculations, as the methylene protons
of the aliphatic carbon, which are attached to the aromatic ring,
will produce a triplet that is highly susceptible to any changes at
the point of attachment of the aliphatic chain to the aromatic ring.
This triplet signal was used for the quantification of the oxidation
products in which the aliphatic chains remained intact (Protons A).
The electronic environment of the terminal H in a long-chain alkyl
substituent of a polyaromatic compound will be affected to a lesser
extent by changes due to the oxidation of the polyaromatic region,
than will the protons in the ethyl group of 2-ethylnaphthalene.
Consequently, the terminal protons were used as an internal
standard for the quantification of the products of long chain alkyl
Infrared (IR) analysis were performed using a Jasco FT/
IR-660 spectrometer. Compounds dissolved in dichloromethane
solution were deposited on the surface of a NaCl plate. The film
formed on the plate was analysed directly.
1
aromatics using H NMR spectroscopy (Protons D).
Comparison of the integrated areas for protons A and D
between the 0 and X h samples gives a measure of the percentage
of products in which the aliphatic chain remains intact. The
3
. Results and discussion
1
detailed methodology of analysing H NMR peaks using this
11
3.1. Oxidation of 2-ethylnaphthalene
technique has been described in a separate publication. An
1
example of a H NMR spectrum used for the calculation of Since our aim was to investigate the selectivity trends for alkylated
percentage of preserved alkyl chain is shown in Fig. S1 (ESI†). aromatics, this study focused primarily on 2-ethylnaphthalene, (1),
In the oxidation of 9-octadecylphenanthrene, the following as it is a simple alkylated polynuclear aromatic compound with the
conditions were used. First the substrate (0.043 g, 0.1 mmol) was advantage that monophasic and biphasic solvent environments
dissolved in C H Cl (3 ml), H WO (0.038 g, 0.154 mmol), H PO
(
can be compared. In previous reports we have shown that the
10% 0.02 ml) and Aliquat 336 (0.0024 g in toluene) were added oxidation of 2-ethylnaphthalene using RICO chemistry leads to
and stirred for a few minutes. Subsequently H (50%, 4 ml, oxidation of the aromatic carbon region of the molecule, whilst
6
5
2
4
3
4
2 2
O
11
65 mmol) was added dropwise. The reaction was performed at there was little or no oxidation of the alkyl side chain. This
110 1C. Post reaction analysis steps were the same as above. observation was independent of the solvent system used, as in
Conversion in oxidation reactions of 2-nonylphenanthrene, both monophasic and biphasic solvent systems aromatic carbons
-octadecylphenanthrene and 2-octadecylpyrene was determined were predominantly oxidized. In this work, a range of solvents,
9
27
using GC, according to the methodology described above for which differ in their polarity index, were evaluated for oxidation
-ethylnaphthalene. of 2-ethylnaphtalene using H WO /H O and the main products
Competitive oxidation reactions. In the competitive oxidation observed are presented in Table 1.
2
2
4
2 2
study, 0.247 mmol of each substrate (non-alkylated compounds)
or 0.256 mmol (alkylated-compounds) were dissolved in C
Oxidation of 2-ethylnaphthalene using a biphasic system
Cl with toluene as the organic solvent (the lowest polarity index)
H
6 5
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Table 1 Products of 2-ethylnaphthalene oxidation in a range of solvents
a
b
c
Entry
Organic solvent
Polarity index (e
r
)
React. T /1C
Con. /%
Major products
1
Toluene C
6
H
5
CH
3
2.4 (2.38)
80
30
52
16
2
3
Chlorobenzene C
6
H
5
Cl
2.7 (5.62)
3.9 (27.7)
80
75
2 5
Propionitrile C H CN
4
Acetonitrile CH
3
CN
5.8 (37.5)
65
12
a
b
c
Note:
(
e
r
= dielectric constant. Reaction temperature. Conversion.Reaction conditions: 2-ethylnaphthalene 18.8 mmol, H
35% 12 ml (137 mmol), Aliquat 336 0.24 ml, H PO 10% 0.14 ml, Solvent 8 ml, t = 6 h.
2 4
WO 0.153 g
0.614 mmol), H
2
O
2
3
4
resulted in formation of acetonaphthone, (2), with high selectivity 6-ethylnaphthalene-1,4-dione (5) and 6-ethyl-2-hydroxynaph-
91%, Table 1, entry 1, Fig. S2, ESI†), alongside other minor thalene-1,4-dione (6), all of these preserved the ethyl chain,
(
1
3
products. This shows that in a biphasic system with very different with only one aromatic ring oxidized as confirmed by C NMR
solvent polarities oxidation of the a-carbon of the ethyl side chain analysis of the product mixture. Indeed, a comparison of
1
3
C
of the molecule is the preferred reaction pathway. NMR spectra for the product mixture following 20 h of reaction
When chlorobenzene is used as the organic phase solvent, with that of a pure standard of 2 showed no peak corres-
GC-MS analysis (Fig. S3, ESI†), shows the formation of two ponding to the ketone group of acetonaphthone (2). This is
major products (Table 1, entry 2); acetonaphthone, (2), 37% the expected alkyl oxidation product that had been observed for
and 5-ethyl phthalic acid anhydride, (3), 16%, suggesting that reaction in toluene and chlorobenzene following oxidation at the
both aromatic and aliphatic carbon can be oxidized.
a-carbon of the alkyl side chain (Fig. S6, ESI†). This observation
The oxidation of 2-ethylnaphthalene with proponitrile as the suggests that acetonitrile can provide a solvent environment able
organic solvent of the biphasic reaction mixture was carried out at to direct oxidation to the aromatic region. These results differ from
a lower temperature than those used for the less polar solvents previously reported oxidation of 2-ethylnaphthalene in acetonitrile,
(75 1C cf. 80 1C), due to its lower boiling point. A notably lower as products in which unreacted and oxidized ethyl chain products
conversion is seen compared to the toluene and chlorobenzene were observed. However, that earlier study used tungsten polyox-
systems and the major product, naphthalene-2-yl-ethanol (4), is ometalates rather than tungstic acid and also employed a different
22
only oxidized at the a-carbon of the alkyl chain. No acetonapthone ratio of substrate, catalyst and oxidant.
was detected (Table 1, entry 3 and Fig. S4, ESI†), as would be
expected from the further oxidation of (4).
The selectivity for the reaction catalysed with H WO /H O
2 4 2 2
also differed significantly from the oxidation of 2-ethylnaph-
The highest polarity solvent considered for the organic phase talene using RICO chemistry. RICO gave a range of products for
was acetonitrile (Table 1, entry 4 and Fig. S5, ESI†). Acetonitrile which the unreacted ethyl chain was observed (e.g. phthalalde-
1
1
has been previously reported to have a positive influence on the hyde, isobenzofuran-1,3-dione, 5-ethylisobenzofuran-1,3-dione).
2
8
selectivity in RICO chemistry, and so it was investigated in In our RICO work, selectivity was not affected by the choice of
more detail for this new work with H WO /H . The high solvent for 2-ethylnaphtahlene oxidation (CH CN, H O or CH CN,
polarity index of acetonitrile meant that attempts to use it as the DCM, H O). However, it was shown that, using RICO, the
2
4
O
2 2
3
2
3
11,14
2
organic solvent led to a monophasic reaction mixture and its solvent affects the selectivity for oxidation of molecules with three
low boiling point (82 1C) required the reaction temperature to be or more aromatic rings, giving a lower degree of aliphatic carbon
13
further reduced to 65 1C. A range of products were formed from oxidation in the presence of acetonitrile.
-ethylnaphthalene oxidation using H WO /H in acetonitrile. A possible explanation for the change of selectivity with
The major products were: 5-ethyl phthalic acid anhydride (3), solvent for 2-ethylnaphtalene using the H WO /H system
2
2
4
2 2
O
2
4
2 2
O
1
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could be the distribution of the tungsten complex in the reaction very similar values. However, comparison of the measured per-
medium. Introduction of acetonitrile allows the reaction to occur centage preserved aliphatic hydrogen for substrates 7 and 8 does
in a single homogeneous phase, presumably improving mixing of suggest that the location of the alkyl chain plays a crucial role in
the catalyst, substrate and oxidant. The bisperoxotungstate reaction selectivity. Aliphatic carbon at position 9 of the aromatic
complex, that forms initially from reaction of H WO with H O , phenanthrene system in 8 is more susceptible to oxidation than
2
4
2 2
can more easily move through the reaction medium without the carbon at position 2 of the same aromatic system (standard atom
need for a phase transfer agent. A similar observation was reported numberings are shown in Fig. S7, ESI†). It has been found from
29
by Noyori et al. Tungsten is reduced as the substrate is oxidised, DFT calculations for the RICO oxidation system, that positions 9
and so, once it has oxidized the CQC or C–H bond in a biphasic and 10 in the phenanthrene ring system are the most energetically
system the tungsten complex has to transfer back to the aqueous favourable positions for oxidation as the removal of these carbons
phase. In the monophasic system provided by acetonitrile the from the aromatic system has only a limited effect on the
30
14
additional phase transfer steps are eliminated. Even so, the use resonance structures available. The a-carbon of an alkyl chain
of acetonitrile as a solvent raises a problem of substrate solubility substituted at these positions is likely to be placed close to the
for higher molecular weight alkylated PAH molecules and limits oxidant making it more susceptible to oxidation. The equivalent
the reaction temperature in an open reaction vessel because of the positions in pyrene are 4, 5, 9 and 10 so that the high level of
relatively low solvent boiling point. Therefore, for the more general preserved aliphatic H for 9 can be understood in a similar way.
problem of residue upgrading, the use of non-polar solvents is
Quantification of the extent of preserved alkyl chain provides
difficult to avoid. To address this point, further experiments with a key indicator of catalyst efficacy, but it is still important to
alkylated aromatic hydrocarbons containing more fused aromatic positively identify the reaction products. We have used a combi-
rings were performed in chlorobenzene.
nation of analytical techniques to suggest the most likely
dominant product species in these oxidation reactions. Fig. S8
(
ESI†) gives the IR spectrum for the product mixture from the
3
.2. Oxidation of polyaromatics with larger aromatic systems
organic layer following 2-octadecylpyrene (9) oxidation. This
and longer alkyl chains
ꢁ
1
ꢁ1
shows broad bands at 1698.5 cm and 1735.6 cm , characteristic
of the CQO stretching modes associated with saturated non-
aromatic aldehydes and aromatic carboxylic acids, respectively.
Additionally, the broad peak observed in the –OH region suggests
the presence of alcohol or carboxylic acid groups. MS analysis
identified a product with m/z = 581.31, with a suggested formula of
C H O , (Fig. S9, ESI†). In pyrene we have noted that the
34 45 8
positions 4, 5, 9 and 10 are the most easily oxidised CQC bonds,
so that a likely candidate for this oxidation product is 4-
octadecylbiphenyl-2,2 ,6,6 -tetracarboxylic acid (10). Fig. S1 (ESI†)
Various factors, such as the size of the aromatic ring system
and substitution pattern of alkyl substituted PAHs can influence
the regioselectivity in oxidation reactions between the aromatic
31
and aliphatic regions of the molecule. In our studies we focus
not only on the effect of aromatic system size, but also on
understanding how different substituent chain lengths and sub-
stitution positions influence the selectivity of the reaction.
Three different compounds were chosen for this study, the
substituted three ring PAH molecules; 2-nonylphenanthrene (7)
and 9-octadecylphenanthrene (8), and the substituted four ring
system 2-octadecylpyrene (9). To determine the selectivity of the
oxidation reaction between aromatic and aliphatic regions, the
0
0
1
shows the H NMR spectra of 9 and that of the oxidation product
mixture. Regarding protons in position a to the aromatic ring;
oxidation of the a-C–H bond to give CQO is a known reaction and
would reduce the intensity of the 3.20 ppm resonance (marked A
in Fig. S1, ESI†). A new resonance would also be expected between
1
H NMR quantification methodology developed previously
and summarised in the Experimental section, was applied
1
1
(Fig. S1, ESI†).
2
.90–3.00 ppm, for the C–H protons in the aliphatic chain that
Table 2 summarises the conversion and proportion of pre-
become a to the ketone group and b to the aromatic ring. Some
new features are seen in the H NMR spectrum of reaction
served aliphatic hydrogen for each of the substrates. It is clear
that conversion is dependent upon the number of fused aromatic
rings present in the substrate, with higher conversion for the
larger ring system of 2-octadecylpyrene (9). The alkyl chain length
has relatively little influence on the conversion as 7 and 8 show
1
products but these could also be caused by changes in the
chemical shifts of protons a to the aromatic ring system due to
oxidation of aromatic carbon. To clarify the situation, Hetero-
nuclear Multiple Bond Correlation (HMBC) 2D NMR spectroscopy
was used, which allows the correlation of chemical shifts of two
types of nuclei separated from each other by two or more
Table 2 Effect of alkyl chain length, and position and number of aromatic
rings on the activity and selectivity for oxidation of polyaromatic com- chemical bonds. Using this technique it was possible to observe
pounds oxidized using H
2
WO
4
/H
2
O
2
aromatic protons which were correlated to C from the CQO
region, even when those carbons did not give visible signals
Substrate
Conversion/% Preserved aliphatic H/%
1
3
during standard C NMR analysis. Fig. 1 shows the HMBC 2D
2-Nonylphenanthrene, (7)
9-Octadecylphenanthrene, (8) 46
2-Octadecylpyrene, (9) 95
48
78
40
93
13
spectra with features correlating the CQO region, of the
C
NMR spectrum with the aromatic protons (C–H) marked con-
firming that oxidation takes place in the aromatic region of the
substrate away from the alkyl substituent, as would be expected
for the proposed main product, 10.
Reaction conditions: substrate 0.1 mmol, H
35% 2 ml(22.8 mmol), Aliquat 336 0.002 g (in 1 ml C
solution), H PO 10% 0.01 ml, C Cl 3 ml, T = 80 1C, t = 16 h.
2
WO
4
0.019 g (0.077 mmol),
H
2
O
2
6
H
5
CH
3
3
4
6 5
H
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Scheme 2 9-Octadecylphenanthrene and a possible product of oxida-
tion consistent with MS analysis.
Fig. 1 HMBC (500 MHz, CDCl
3
) spectrum of 2-octadecylpyrene oxidation with high selectivity, so that the aromatic–aliphatic C–C bond
products (organic layer). The structure of the main product-octadecylbiphenyl- was preserved with a ketone formed at the new a-carbon
0 0
,2 ,6,6 -tetracarboxylic acid, and the correlating elements circled in the
2
position. These data provide new insight into the selectivity
achievable using the H WO /H oxidation system and demon-
spectra, are marked by an * in the chemical structure.
2
4
2 2
O
strates that the chemistry is potentially useful for upgrading of
polyaromatic residuals with minimal loss of carbon.
1
The combined evidence from IR, MS, H NMR and HMBC
shows that the major product of 2-octadecylpyrene oxidation has a
preserved aliphatic chain. The product contains four carboxylic
acid groups and this, coupled with the knowledge of the regio-
selectivity preference for oxidation of pyrene, strongly suggests that
3
.3. Kinetic studies
The oxidation reactions were also monitored as a function of
time to compare the oxidation rates of molecules possessing
different numbers of aromatic rings, and varying lengths of alkyl
chain. Previously, using RICO chemistry, we found that the
number of aromatic rings in a substituted PAH determines the
reaction rate, with larger aromatic systems reacting faster than
smaller ones. In the case of the tungsten/peroxide catalyst
system, when naphthalene, (12), phenanthrene, (13), and pyrene,
(14), were used as substrates, it was also found that the reaction
rate increased with the increasing number of fused aromatic
rings (Fig. 2). This observation means that aromaticity in higher
molecular weight polyaromatics can be reduced preferentially in
the presence of lower molecular weight ones.
0 0
4-octadecylbiphenyl-2,2 ,6,6 -tetracarboxylic acid (10) is the major
product (Scheme 1). Even without specific product assignment,
these observations confirm that the H WO /H O system delivers
2
4
2 2
the desired reaction selectivity, of ring opening and reducing
aromaticity while preserving aliphatic carbon side chains.
For phenanthrene positions 9 and 10 (Fig. S7, ESI†) are the
1
3
1
1
most susceptible to oxidation. In the case of 2-octadecylpyrene,
7) the corresponding positions (4, 5, 9 and 10) were not hindered
(
by the substituent, and the aromatic region of the molecule was
readily oxidized with a high conversion of the substrate (95%,
Table 1) with 93% of the products retaining the aliphatic
substituent intact. When the alkyl substituent is at a position
Experiments to determine the competitive rates of oxidation
of alkylated aromatics, using 1-decylnaphthalene (15), 2-nonyl-
phenanthrene (7) and 2-octadecylpyrene (9) were also performed
that favours aromatic oxidation, such as the 9 position of
1
9
-octadecylphenanthrene, (8), H NMR quantification analysis
(Fig. 3). The difference in reaction rate between 1-decylnaphthalene
suggests that only 40% of molecules in the post reaction mixture had
a preserved alkyl chain. However, only 46% of 8 was converted, so
the amount of preserved alkyl chain in the oxidised products will be
masked by unreacted substrate. Fig. S10 (ESI†) shows the MS
spectrum for the reaction mixture after oxidation of 8. This con-
firmed a signal at m/z = 430.32 corresponding to unreacted substrate,
and 2-nonylphenanthrene was small but it is clear that the phenan-
threne derivative reacts faster than the substituted naphthalene
8
, and two strong peaks at higher m/z values consistent with the
addition of one oxygen atom (m/z = 446.36) and two oxygen atoms
m/z = 462.34). The second peak corresponds to 2-nonadecanoyl-
(
biphenyl-2-carbaldehyde (11) (Scheme 2), as an oxidation product. As
1
further support for this assignment H NMR spectroscopy confirmed
the presence of the aldehyde group (9.75 ppm, Fig. S11, ESI†).
The likely presence of (11) suggests that the aromatic CQC
bond was cleaved by oxidation at the C9–C10 bond position
Fig. 2 Comparison of amount of substrate remaining as a function of
time for polyaromatic hydrocarbons during competitive reaction between
naphthalene (’), phenanthrene (K) and pyrene (m).
Scheme 1 2-Octadecylpyrene and its major oxidation product.
1
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oxidation system can be considered to have demonstrated
promising chemistry with respect to valorisation of crude oil
residues. Judicious choice of solvent gives an oxidation system
capable of achieving the required activity and selectivity to
produce higher value products with only minor loss of carbon.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to acknowledge research funding from
ExxonMobil.
Fig. 3 Comparison of oxidation rates for the competitive reaction
between 1-decylnaphthalene (’), 2-nonylphenanthrene (K), and 2-
octadecylpyrene (m).
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