Catalyzed transfer hydrogenation by 2-propanol for highly selective PAHs reduction

Article abstract of DOI:10.1016/j.cattod.2020.06.060

Catalytic hydrogenation of mono-, di- and trinuclear aromatic compounds has been studied under hydrogen transfer conditions at 150 °C and 82 °C in 2-PrOH as a hydrogen donor and with Raney nickel as a catalyst. In contrast to conjugated or condensed aromatic rings, isolated ones demonstrated low reactivity in transfer hydrogenation (TH) that can be used to increase the hydrogenation selectivity of the reaction. So, naphthalene and biphenyl are partially hydrogenated into tetralin and cyclohexylbenzene, respectively, with excellent conversion (≥ 96 %) and selectivity (≥ 98 %) for 5–6 h at 82 °C. Increasing the reaction temperature to 150 °C results expectedly in the hydrogenation of second aromatic ring, which occurs slowly enough. Only 8 % of decaline and 42 % of dicyclohexyl, correspondingly, were obtained after 5 h at 150 °C. At the same time, TH of trinuclear anthracene and phenanthrene at 150 °C resulted in the formation of deeper hydrogenated octahydro-anthracenes and -phenanthrenes, respectively.

Full text of DOI:10.1016/j.cattod.2020.06.060

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Catalyzed transfer hydrogenation by 2-propanol for highly selective PAHs
reduction
A.A. Philippov (Formal analysis) (Investigation) (Validation) (Writing
- original draft) (Writing - review and editing), A.M. Chibiryaev
(Conceptualization) (Formal analysis) (Methodology) (Supervision)
(Writing - review and editing), O.N. Martyanov (Funding acquisition)
(Project administration) (Writing - review and editing)
PII:
S0920-5861(20)30446-6
DOI:
https://doi.org/10.1016/j.cattod.2020.06.060
CATTOD 12978
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Catalysis Today
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30 October 2019
4 May 2020
23 June 2020
Please cite this article as: Philippov AA, Chibiryaev AM, Martyanov ON, Catalyzed transfer
hydrogenation by 2-propanol for highly selective PAHs reduction, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.06.060
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Catalyzed transfer hydrogenation by 2-propanol for highly selective PAHs reduction
A.A. Philippov*, A.M. Chibiryaev* and O.N. Martyanov
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Academician
Lavrentiev Avenue 5, 630090 Novosibirsk, Russia.
Highlights




TH of mono-, di- and trinuclear aromatic compounds was studied at 150 °C and 82 °C
2-PrOH/Raney® nickel combination demonstrates a high reactivity
Naphthalene and biphenyl are partially reduced at 82ºC with high selectivity
Trinuclear anthracene and phenanthrene at 150ºC are hydrogenated into octahydro-derivatives

Abstract: Catalytic hydrogenation of mono-, di- and trinuclear aromatic compounds has been studied
under hydrogen transfer conditions at 150 °C and 82 °C in 2-PrOH as a hydrogen donor and with
Raney® nickel as a catalyst. In contrast to conjugated or condensed aromatic rings, isolated ones
demonstrated low reactivity in transfer hydrogenation (TH) that can be used to increase the
hydrogenation selectivity of the reaction. So, naphthalene and biphenyl are partially hydrogenated
into tetralin and cyclohexylbenzene, respectively, with excellent conversion (≥ 96%) and selectivity
(≥ 98%) for 5–6 h at 82 °C. Increasing the reaction temperature to 150 °C results expectedly in the
hydrogenation of second aromatic ring, which occurs slowly enough. Only 8% of decaline and 42%
of dicyclohexyl, correspondingly, were obtained after 5 h at 150 °C. At the same time, TH of
trinuclear anthracene and phenanthrene at 150 °C resulted in the formation of deeper hydrogenated
octahydro-anthracenes and -phenanthrenes, respectively.
Keywords:
Polyaromatic hydrocarbons; 2-Propanol; Raney® nickel; Transfer hydrogenation; Hydrogenative
dearomatization
1. Introduction
Depletion of non-renewable natural resources causes rapidly growing interest to their near-
complete conversion with low-waste production that complies with “green chemistry” principles of
material conversion and energy efficiency. According to knowledge of fossil fuels, coals and some
heavy oil fractions mainly consist of polyaromatic hydrocarbons (PAHs) [1,2]. Many PAHs are
known to possess a pronounced mutagenic and carcinogenic effect [3–5], as well as apparent geno-
and embryotoxicity [6–8]. Since the reactivity of PAHs are significantly low due to a large number
of conjugated and condensed aromatic rings [9–11], simple, cheap and convenient methods of their
chemical conversion and detoxification are highly required. One of the effective chemical
modifications can be partial or total PAHs hydrogenation, which allows liquefying fossil fuels

[12,13], increasing the cetane number [14], enhancing the viscosity [15] or giving other benefits
[11,16,17]. Thus, development of new effective synthetic methods of PAHs hydrogenation has a great
commercial and ecological importance.
In fact, catalyzed hydrogenation is a mostly used type of chemical modifications of PAHs and
related compounds, these processes were studied in detail many times [18–20]. Among others,
heterogeneous nickel-based catalysts have clear advantages because of enhanced catalytic activity
and selectivity [21–23] together with moderate cost and high availability. Moreover, high catalytic
activity of metal nickel nanoparticles in conventional hydrogenation of aromatic compounds is well-
documented [24–27]. So, metal nickel particles supported on ZSM-5 catalyzed total hydrogenation
of naphthalene, anthracene and phenanthrene at 180 °C [25]. Carbon-supported nickel was active
enough in anthracene hydrogenation at 300 °C [26].
Raney® nickel also is used in conventional hydrogenation too [24,28–30]. So, total
hydrogenation of both naphthalene [24] and phenols of lignin [27] were successfully catalyzed.
Moreover, Raney® nickel demonstrated high catalytic activity in transfer hydrogenation (TH)
reaction intensively studied last few years [31–34]. The TH methodology is considered to be a real
“green” alternative for canonic hydrogenation [35–38]. The catalyst was used in TH of multiple bonds
C=C [39,40], C=O [34,39,40] and C≡N [41]. Isolated aromatic ring can also be successfully
hydrogenated under the same catalytic conditions [31,39,42]. The reaction rate for hydrogenation of
monoaromatic compounds increases at higher temperature. A structure–activity relationship study
revealed a strong negative effect of electron-donating alkyl substituents. For example, 23% and 92%
of benzene were hydrogenated to cyclohexane at 80 °C and 120 °C, respectively. At the same time,
the conversion of toluene (methylbenzene) at 80 °C was two times lower than that of benzene and
achieved only 12% [39]. At the same time, the TH reaction catalyzed by Raney® nickel can occur in
a different way compared with the conventional hydrogenation catalyzed by the same catalyst [43].
Lower aliphatic alcohols are quite popular H-donors in TH [44–47] and considered to be a type
of "Liquid Organic Hydrogen Carriers" [48], which are an interesting and very attractive class of

clean energy sources (see, for example, [49,50]). In contrast to other widespread H-donors (formic
acid and its derivatives, hydrazine hydrate), alcohols do not change pH of reaction solution.
Therefore, they can be used together with catalysts, which are sensitive to pH, in particular, Raney®
nickel. Moreover, varying the alcohols can improves the final result of TH, even in non-catalytic
reaction [45]. Secondary 2-PrOH usually demonstrates the better result as a H-donor than the primary
alcohols [45], that was the reason to use 2-PrOH as a H-donor in this study.
One of the earliest TH reaction of PAHs was published in 1944 by M. Orchin [51]. The reaction
of anthracene was conducted in refluxing ethanol and catalyzed by Raney Ni resulted in 71% yield
of 1,2,3,4-tetrahydroanthracene. Less-hydrogenated 9,10-dihydroanthracene was obtained by TH
reaction in the presence of alkali (sole catalyst) either in 2-PrOH at 335°C [52] or in MeOH at 400°C
[53], as well as by non-catalytic reaction in 2-PrOH at 410°C [54]. Both SbCl3 [55] and
tetraphenylporphyrin metal complexes [56] can also catalyzed the partial transfer hydrogenation of
anthracene with moderate conversion and selectivity. Additionally, some TH reactions of
phenanthrene and pyrene were studied earlier, in which BF3/SiO2 (or Al2O3) [57], superbasic K/MgO
[58] or Ni-Mo/-Al2O3 [59] were used as catalysts. However, a very low yield of hydrogenated
products was only achieved.
Here, we report the catalyzed transfer hydrogenation of bi- and trinuclear aromatic compounds
(naphthalene, anthracene and phenanthrene) using 2-PrOH as a H-donor. Since the TH reaction of
such condensed aromatic compounds catalyzed by Raney® nickel was poorly studied, especially in
alcohols as an H-donor, the objective of this study is to verify that the combination “Raney® Ni – 2-
PrOH” can be active and efficient for regioselective transfer hydrogenation of polyaromatic
hydrocarbons into partially hydrogenated derivatives. The obtained results of TH are compared with
the reaction of mononuclear benzene and biphenyl to evaluate the synthetic potential of the method.
2. Experimental
2.1. Chemicals and solvents

All reagents were obtained from commercial sources and used as received: 2-propanol
(≥ 99.5%, Sigma–Aldrich), benzene ( 99%, Sigma–Aldrich), biphenyl ( 99%, Sigma–Aldrich),
naphthalene (99%, Sigma–Aldrich), anthracene ( 99%, Sigma–Aldrich), phenanthrene (98%
Sigma–Aldrich), sulfolane (99%, Aldrich), decane ( 95%, Aldrich), aluminum-nickel alloy
97%, Sigma–Aldrich).
≥
≥
≥
,
≥
(Al–Ni 50:50 wt%, Sigma–Aldrich), NaOH (
≥
2.2. Catalyst preparation
Raney® nickel was prepared according to well-known leaching method from
aluminum–nickel alloy [60]. The alloy (0.45 g) was added portion-wise to 5.6 mL 20% aq.
0
NaOH (a solution of 16.30 g of NaOH in 58.0 mL of distilled water) under vigorous stirring at
room temperature. The temperature was kept at about 50 °C. After adding the total amount of
the alloy (10–15 min), the mixture was stirred for 50–55 minutes at 50 °C until the H2 evolution
stopped. The skeletal nickel obtained was washed out with distilled water (4 × 50 mL) till
neutral pH and then additionally stirred with extra portion of distilled water (15 mL) for 2.5
hours. Water was decanted away and 2-PrOH was added portion-wise (4 × 50 mL) to remote
residual water from prepared Raney® nickel. The prepared leached nickel was pyrophoric enough
and usually used immediately after preparation. The catalyst was characterized by BET, X-ray
powder diffraction (XRD) and X-ray fluorescence (XRF) analyses. The properties of the
prepared Raney® nickel were very similar to literature data (see Supplementary Materials).
2.3. General experimental procedure 1
The batch reactor equipped with mechanical agitator MagneDrive (Autoclave Engineers) was
charged by ~3.2∙mmole of substrate, 0.20–0.30 g of sulfolane or decane (as an internal
standard) and 120 mL of 2-PrOH. After adding the catalyst prepared from 0.45 g of
aluminum–nickel alloy, the reactor was purged with argon under stirring then closed, and heated
up with the rate of ~7 °C/min to reach the reaction temperature of 150 ± 1 °C. The reaction was
carried out at constant temperature of 150 °C during 5 h with permanent stirring at 800 rpm. During

the reaction, probes of the reaction mixture were collected hourly to the sampler. The final reaction
mixture and all other probes were analyzed by GC–MS method.
2.4. General experimental procedure 2
The glassware reactor equipped with mechanical stirrer was charged by ~3.2∙mmole of
substrate, 0.20–0.30 g of sulfolane or decane (as an internal standard) and 120 mL of 2-PrOH.
After adding the catalyst prepared from 0.45 g of aluminum–nickel alloy, the reactor was purged
with argon under stirring and heated under reflux for 5 h (82 °C). During the reaction, probes of the
reaction mixture were collected hourly and together with the final reaction mixture analyzed by GC–
MS method.
2.5. GC–MS analysis
The identification of the products was performed by GC–MS method on a gas chromatograph
Shimadzu GCMS-QP2010-SE (Japan) equipped with autosampler. A 30 m-long quartz capillary
column VF-5ms with 0.25 mm i.d. was used for the analysis; the film thickness of the stationary
phase (5% diphenylsiloxane – 95% dimethylsiloxane copolymer) was 0.25 μm; helium was as the
carrier gas with a constant flow rate of 1.8 mL min–1; a split ratio was 1:25; the injector temperature
was 270 °C; the temperature program was 1 minutes at 50 °C, 50–270 °C (10 °C min-1), and 270 °C
(20 min).
The qualitative analysis was carried out by comparing the retention time of the components and
their experimental mass spectra with the corresponding data for the pure compounds or with the data
from the NIST11 and Wiley9 mass spectral libraries. Sulfolane or decane was used as an internal
standard for the quantitative assessment of the products.
3. Results and discussion
3.1. Catalyst characterization
Raney® nickel was prepared according to well-known leaching method [60]. The freshly
prepared catalyst and the initial Ni–Al alloy were characterized by BET, XRD, XPS, XRF and TEM
methods.

According to XRD data, the initial alloy mostly contains Al3Ni and Al3Ni2 phases (see Figure
S1 in Supplementary Materials), which transform to metallic Ni phase during the catalyst preparation.
The alloy has low specific surface area equal to ~ 1–5 m2/g, which significantly increased to 60–80
m2/g for freshly prepared Raney® nickel.
TEM shows sponge-like structure of the freshly prepared catalyst with many branched
mesoporous channels (Error! Reference source not found.). This information correlates with data
published earlier [61,62]. The prepared Raney® nickel contains 86.5% of Ni and 10.3% of residual
Al. The last 3.2% is obviously oxygen (XRF). Raney® nickel is known to be quite pyrophoric and
highly sensitive to oxygen-presence condition that produce many difficulties to handle [63].
Therefore, the metal surface of Raney® nickel prepared under air atmosphere is partially covered by
oxygen atoms. It is confirmed by in situ XPS analysis: Al0/Al3+ and Ni0/Ni2+ ratios in our Raney®
nickel catalyst are about 0.25 and 5.0, correspondingly (see Table S1 in Supplementary Materials).
Thus, measured physicochemical properties of the catalyst used are rather canonic and highly
correlated with literature data [60,62,63].
3.2. Hydrogenative dearomatization under TH conditions
Five model aromatic hydrocarbons are studied in transfer hydrogenation by 2-propanol as a H-
donor: mononuclear benzene and biphenyl, binuclear naphthalene, as well as three-nuclear anthracene
and phenanthrene.
Two variations of the reaction catalyzed by Raney® nickel are used: i) in autoclave stainless
steel reactor at 150 ºC and 11 atm (self-pressure of 2-propanol); and ii) in glass reactor at 82 ºC and
ambient atmosphere (boiling point of 2-propanol). The reactivity of aromatic cores under TH
conditions is known to be rather low [39,43,64]. For this reason, elevated 150 ºC is chosen for the
reaction. Further increasing the reaction temperature is undesirable and even "harmful" because of
rapidly decreasing the activity of the catalyst due to structural changes in crystallite size, lattice
distortion, pore structure, and specific surface area [63]. But carrying out the PAHs reduction in

standard laboratory glassware would be highly desirable, convenient and useful. Thus, boiling 2-
propanol under atmospheric pressure is used as an alternative reaction medium.
All reactions were carried out in the same manner to each other under oxygen-free conditions
to avoid the oxidative deactivation of the pyrophoric catalyst and repeated twice. Probes of each
reaction mixture were collected hourly (sometimes half-hourly) starting from zero reaction time at
which the reaction mixture reached the reaction temperature after heating. The molar catalyst-to-
substrate ratio was always ~ 1.2/1 in the reaction mixtures and not optimized.
All observed transformations are shown in Error! Reference source not found..
3.2.1. Benzene and biphenyl hydrogenation
After 5 h of the reaction at 150 °C, benzene achieves ~ 100% conversion (dashed black line,
Error! Reference source not found.) and cyclohexane is the only product. This result is in good
accordance with known TH data for 3 h of the reaction at 120 °C [39] and used as a reference point
in our study. As expected, the reactivity of benzene at 82 °C is significantly lower: only 22% of
benzene are transformed into cyclohexane for 5 h of the reaction.
Unfortunately, these results of TH reaction are hard to compare with conventional
hydrogenation, in which H2 pressure is known to influence strongly the benzene conversion. In
particular, > 99% conversion for 2 h at 120 ºC is achieved in canonic hydrogenation catalyzed by
Raney nickel foam, if H2 pressure is 59.2 atm [65]. But two-fold decrease of the pressure to 27.6 atm
gives only 84% of benzene conversion for 8 h at 150 ºC [66]. So, our results of the TH reaction are
comparable to these ones.
The hydrogenation of biphenyl is also sensitive to the reaction temperature. So, the
dearomatization and total reduction of the first aromatic ring proceeds faster compared with the
second one: at 82 °C, 91% conversion with 100% selectivity on cyclohexylbenzene is reached for 5
h of the reaction (solid red line, Error! Reference source not found.). Increasing the reaction time
to 6 h gives almost quantitative yield of cyclohexylbenzene – 97% conversion at 100% selectivity

(this point is absent in Error! Reference source not found.). Interestingly, the second aromatic ring
of byphenyl molecule was unreactive at 82ºC and dicyclohexyl was not found in any reaction
mixtures.
In contrast, 100% conversion at 150 °C is already achieved for 1 h of the reaction. The same
91% yield of cyclohexylbenzene is observed for 0.5 h and then decreased to 58% for 5 h of the
reaction (dashed red line, Error! Reference source not found.). At the same time, the concentration
of fully hydrogenated dicyclohexyl (dashed blue line, Error! Reference source not found.) is
continuously increasing up to 42% at the end of the reaction tending towards higher level. Thus, the
binary system “Raney® nickel/2-PrOH” demonstrates high enough activity and regioselectivity on
the partial reduction of biphenyl which is known to be an important structural moiety of a wide range
of biologically active compounds [67,68] as well as a high reactive intermediate in fine organic
chemistry [69,70].
3.2.2. Naphthalene hydrogenation
The reaction of naphthalene at 150 °C results in high-selective tetralin formation (dashed green
line, Error! Reference source not found.): 96% at 100% conversion of naphthalene for 1 h of the
reaction. Then the tetralin concentration is weakly decreasing to 92% at the end of the reaction due
to its further hydrogenation into fully hydrogenated cis-/trans-decalins (dashed blue lines, Error!
Reference source not found.). The only octahydronaphthalene ocNa as an intermediate is detected
in trace amount (dashed red line, Error! Reference source not found.) and there are no other
partially or fully hydrogenated products in the reaction mixture. Decreasing the reaction temperature
to 82 ºC expectedly provides lower naphthalene conversion, but with excellent selectivity: for 3 h of
the reaction, 100% selectivity (solid green line, Error! Reference source not found.) was achieved
at 100% naphthalene conversion without any other products formation. Interestingly, tetralin itself
has many practically important applications as a good hydrogen-rich (or H-donor) solvent in TH
processes [71–76] as well as in the fuel cells [77].

Conventional hydrogenation of naphthalene into tetralin catalyzed by Raney nickel can give
good result too, for example 75% yield for 5 min at 150 ºC, 49.3 atm of H2 pressure and 10 mol.% of
catalyst load [78]. Increasing the catalyst loading to 340 mol.% provides excellent yield of tetralin
(99.5%) for 2.5 h of the hydrogenation, at 45 ºC and 29.6 atm of H2 pressure [79]. In our opinion, the
result of TH observed at 82 ºC and 120 mol.% of Raney nickel loading and without H2 is more
attractive bearing in mind known shortcomings and drawbacks of molecular H2.
Taking into account both the equal concentration of each of the model compounds in 2-propanol
and the equal amount of Raney nickel used, the rate of their conversion at 82 ºC may be presented as
naphthalene > biphenyl > benzene. At first glance, possible anthracene or phenanthrene reactivity
under the same reaction conditions seems to be good predictable within the dependence.
3.2.3. Anthracene hydrogenation
In fact, the selective reduction of anthracene under chosen TH conditions is more complicated
due to similar reactivity of its partially hydrogenated intermediates towards further hydrogenation.
So, at 150 °C, 100% conversion is already achieved for 1 h of the reaction and this result is in good
accordance with the above-mentioned prediction. The main product is octahydrogenated ocAn-1
(dashed red line, Error! Reference source not found.), not dihydrogenated diAn (dashed violet line)
or tetrahydrogenated tetAn (dashed green line), which formation as intermediates were also observed
in the reaction mixtures. The yield of ocAn-1 is 75% for 1 h of the reaction, which is weakly
increasing to 85% after 5 h due to further hydrogenation of the di- or tetrahydrogenated intermediates
as well as parallel prototropic isomerization (tautomerization) of ocAn-2 (dashed blue line, Error!
Reference source not found.). The last compound is one more isomer of octahydroanthracene
formed during the anthracene hydrogenation. This kind of isomerization catalyzed by Raney nickel
is known for some heteroaromatic hydrocarbons [80].
It is known that partial hydrogenation of edible fatty oils catalyzed by Raney nickel is always
accompanied by cis-/trans-isomerization of polyunsaturated fatty acids [81] and the isomerization

process involves a reverse transfer of allylic hydrogen atom from the acid molecule to the catalyst
surface and back to the isomerized fatty acid. Thus, under the TH conditions with Raney nickel
catalyst, all octahydroanthracenes have to undergo the mutual isomerization resulted in predominant
formation of ocAn-1 as a thermodynamically favorable isomer.
A good yield of both dihydrogenated diAn (solid violet line, Error! Reference source not
found.) and tetrahydrogenated tetAn (solid green line, Error! Reference source not found.) can be
obtained by decreasing the reaction temperature. So, at 82 °C, ~ 100% conversion is only achieved
at the end of the process (for 5 h). Max concentration of diAn is 57% (for 1 h) at 60% product
selectivity. Then diAn is further reduced into tetAn (main “beneficiary”) and more hydrogenated
products. The yield of tetAn increased to 77% at the end of the reaction (solid green line, Error!
Reference source not found.), while the total concentration of all isomeric octahydroanthracenes
does not exceed 20%. Thus, upon lowering the temperature from 150 °C to 82 °C, significant increase
of both diAn and tetAn yields as well as the products selectivity can be achieved.
Our results demonstrate that the last aromatic ring of octahydroanthracenes is quite stable under
chosen reaction conditions: no products of the total dearomatization are observed in the reaction
mixtures after 5 h at 150 °C. Moreover, the additional TH reaction of anthracene for 24 h at 150 °C
results in only 3% yield of perhydroanthracenes (these data are absent in Error! Reference source
not found.). The isomerization rate of octahydroanthracenes is low too under these TH conditions:
after 24 h of the reaction, the ratio “ocAn-1/(ocAn-2 + ocAn-3)” is quite the same than that after 5 h
(86/10 and 84/16, respectively). Thus, there is no reason to increase the reaction time to obtain
perhydroanthracenes because of low catalytic activity of Raney® nickel in both total hydrogenation
and prototropic isomerization of octahydroanthracenes.
To compare our results with literature data on TH reaction of anthracene in refluxing EtOH [51]
and to evaluate different H-donor, we carried out the same reaction in EtOH (78 °C instead of 82 °C
in refluxing 2-PrOH). But amount of Raney nickel used in our study was 32-fold decreased as
compared with original paper [51] (experimental details can be found in Supplementary Materials

file). The substitution of H-donor and the decrease of ratio “metal Ni/anthracene” had crucial effect:
only ~ 1% conversion was observed under the reaction conditions. We confirmed that 2-PrOH as an
H-donor is much better than EtOH.
3.2.4. Phenanthrene hydrogenation
The phenanthrene conversion at 82 °C (black solid line, Error! Reference source not found.)
is quite similar to that of biphenyl (cf. Error! Reference source not found.): 88% vs. 91%,
correspondingly, at the end of the reaction. The main products are di- and tetrahydrogenated
phenanthrenes, which yields are very close to each other during all 5 h of the reaction: 34% vs. 39%,
correspondingly, at the end. If the reaction temperature is elevated up to 150 °C, the 100% conversion
is expected to achieve rapidly – for 1 h of the reaction. At this point, the main product is
octahydrogenated ocPhe-1 (78%), which concentration is weakly increasing to 85% at the end of the
reaction. Interestingly, the total yield of octahydrophenanthrenes after 5 h of the reaction at 150 °C
(99.8%) is quite similar to that of octahydroanthracenes. The ratio “ocPhe-1/(ocPhe-2 + ocPhe-3)” is
also the same (85.3/14.5). Thus, the last aromatic ring of octahydro derivatives of both anthracene
and phenanthrene is stable under chosen TH conditions at 150 °C.
The last conclusion does not cancel out the experimental fact that anthracene is more active
under the TH conditions – 77% conversion vs 20% of the phenanthrene conversion for zero time
point, respectively. Similar results are observed in conventional hydrogenation of anthracene and
phenanthrene catalyzed by Ni/ZSM-5 [25] or other catalysts [82], which can be interpreted in terms
of superdelocalizability and resonance energy [82].
To summarize, compare and contrast our main results to known literature data, Table 1 is
presented. In the table, one can see the crucial reaction conditions of conventional hydrogenation and
TH reaction of aromatics studied. The yield of main product obtained under these conditions is given
too. Runs ##1, 3, 8, 13, and 15 are conventional hydrogenation, the other ones are TH reactions. The
crucial parameters are the molar ratio “metal Ni/substrate” and the reaction temperature: the higher
the ratio and/or the temperature, the higher the yield of target product. For example, runs #10 and #11

are focused on the effect of the catalyst amount. Runs #11 and #12 emphasize the different donor
ability of alcohols (2-PrOH is much better than EtOH). In general, the combination “Raney® Ni – 2-
PrOH” demonstrates a high and reliable efficiency in course of partial and high-selective
hydrogenation of condensed aromatics as compared with conventional hydrogenation.
4. Conclusions
The binary reaction system “Raney® Ni – 2-PrOH” has a high synthetic potential for the TH
reaction of mono-, di- and trinuclear aromatic compounds. The reactivity of tested aromatics can be
listed as follows:
At 82 ºC: An > Na > Phe ≈ biPhe > benzene
At 150 ºC: An > biPhe > Na ≈ Phe ≈ benzene
The main products formed from di- and trinuclear aromatics at both 82 ºC and 150 ºC were
partially hydrogenated derivatives – tetralin and octahydro-anthracenes/-phenanthrenes. Very high
selectivity of TH ( 98) was achieved for 5 h of the reaction of naphthalene and biphenyl at 82 ºC.
The above-mentioned partially hydrogenated aromatics are quite stable under chosen reaction
conditions and not subjected to further hydrogenation during 5 h of TH process. To achieve the total
hydrogenation of the PAHs, more active catalyst/H-donor and/or severe reaction conditions are
required. The main advantages of the synthetic process considered are the highly selective partial
hydrogenation/dearomatization of aromatics, good reproducibility and repeatability, safe and easy-
to-handle procedure, mild reaction conditions those provide a high synthetic potential of TH
methodology in laboratory practice and industrial application.
Credit author statement
Andrey M. Chibiryaev: Supervision, Conceptualization, Methodology, Formal analysis, Writing–
reviewing and editing.
Alexey A. Philippov: Validation, Formal analysis, Investigation, Writing–original draft preparation,
Writing–review and editing
Oleg N. Martyanov: Project administration, Funding acquisition, Writing–Reviewing and Editing,

Notes
The authors declare no competing financial interest.
Acknowledgement
This work was supported by Ministry of Science and Higher Education of the Russian
Federation and by the Russian Foundation for Basic Research (#18-29-06022).

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Figure and Scheme Captions
a
b
Figure 1. Sponge-like structure of freshly prepared Raney® nickel according to TEM image.
Figure 2. TH reaction of benzene at 82 °C and 150 °C.

Figure 3. TH reaction of biphenyl at 82 °C and 150 °C. Abbreviation: biPhe – biphenyl; CycBe –
cyclohexylbenzene; diCyc – dicyclohexyl (cyclohexylcyclohexane).
Figure 4. TH reaction of naphthalene at 82 °C and 150 °C. Abbreviation: Na – naphthalene; Tet –
tetralin; ocNa – 1,2,3,4,5,6,7,8-octahydronaphthalene; Dec1 – cis-decalin; Dec2 – trans-decalin.

Figure 5. TH reaction of anthracene at 82 °C and 150 °C. Abbreviation: An – anthracene; diAn –
9,10-dihydroanthracene; tetAn – 1,2,3,4-tetrahydroanthracene; ocAn-1 – 1,2,3,4,5,6,7,8-
octahydroanthracene; ocAn-2 – cis-1,2,3,4,4a,9,9a,10-octahydroanthracene; ocAn-3 – trans-
1,2,3,4,4a,9,9a,10-octahydroanthracene.
Figure 6. TH reaction of phenanthrene at 82 °C and 150 °C. Abbreviation: Phe – phenanthrene;
diPhe – 9,10-dihydrophenanthrene; tetPhe – 1,2,3,4-tetrahydrophenanthrene; ocPhe-1 –

1,2,3,4,5,6,7,8-octahydro-phenanthrene; ocPhe-2 – cis-1,2,3,4,4a,9,9a,10-octahydrophenanthrene;
ocPhe-3 – trans-1,2,3,4,4a,9,9a,10-octahydrophenanthrene.
Scheme 1. The main products of TH reaction of model aromatic hydrocarbons.

Table 1. Comparative experimental data on conventional and transfer hydrogenation of aromatics
catalyzed by Raney® nickel.
#
Substrate
Ni/substrate,
molar ratio
T, °C P (H2), Solvent Time, h Product
atm
Yield, % Ref.
1
2
3
4
5
6
7
8
9
Benzene
0.25
1.2
130
150
75
60
H2O
2
5
5
6
Cyclohexane
Cyclohexane
CycBe
53
≥ 99
99
97
91
90
≥ 99
57
96
71
1
[65]
a
Benzene
–
2-PrOH
THF
This work
[83]
Biphenyl
0.86
1.2
18
a
Biphenyl
82
–
2-PrOH
CycBe
This work
This work
[84]
a
Biphenyl
1.2
150
82
–
2-PrOH 0.5
2-PrOH 18
CycBe
a
Naphthalene
Naphthalene
Naphthalene
Naphthalene
8.9
–
Tetralin
Tetralin
Tetralin
Tetralin
tetAn
a
1.2
82
–
2-PrOH
THF
3
This work
[28]
0.25
1.2
100
150
78
15
0.5
1
a
–
2-PrOH
EtOH
This work
[51]
a
10 Anthracene
11 Anthracene
12 Anthracene
13 Anthracene
14 Anthracene
15 Phenanthrene
16 Phenanthrene
38
–
2.5
5
a
1.2
78
–
EtOH
diAnb
This work
This work
[28]
a
1.2
82
–
2-PrOH
THF
5
tetAn
77
52
85
37
88
0.26c
1.2
100
150
100
150
15
0.083
5
tetAn
a
–
2-PrOH
THF
ocAn-1
ocPhe-1
ocPhe-1
This work
[28]
0.26c
1.2
15
0.33
5
a
–
2-PrOH
This work
a Alcohol was used as both solvent and hydrogen donor.
b 9,10-Dihydroanthracene is the main product of the reaction.
c Quenched skeletal nickel containing 8.5 wt.% of molybdenum was used as a catalyst