Raney nickel-catalyzed hydrodeoxygenation and dearomatization under transfer hydrogenation conditions—Reaction pathways of non-phenolic compounds

Article abstract of DOI:10.1016/j.cattod.2019.05.033

Catalytic reduction of oxygen-containing aromatic compounds has been studied under transfer hydrogenation (TH) conditions at 150 °C in 2-PrOH as a hydrogen donor. Raney nickel is used as a heterogeneous catalyst. The reaction of aromatic non-phenolic carbonyl compounds is most likely to proceed through the pathway “aromatic ketone (aldehyde)→aromatic alcohol→alkylaromatics→saturated alkylcyclohexane”. One of the main reactions under the TH conditions is a hydrodeoxygenation (HDO) process. Unexpectedly, the hydrodeoxygenation of aromatic ketones to alkylaromatics (C[dbnd]O → CH₂) occurs faster than of corresponding aromatic alcohols (HC–OH → CH₂) that means either additional reaction pathway of its hydrodeoxygenation missing for the corresponding aromatic alcohols or specific interaction of OH functionality with Raney nickel surface obstructing (hindering) the further reduction. Benzaldehyde is shown to be less reactive than the aromatic ketones under the same reaction conditions. The main reason is proposed to be carbon monoxide release resulted from the decarbonylation of the aldehyde. Carbon monoxide demonstrates a poisoning effect on Raney nickel surface that is evidenced in the catalyzed TH reaction of acetophenone. The HDO reaction of anisole under the same reaction conditions was a little slowly than of oxygen-containing non-phenolic aromatics.

Full text of DOI:10.1016/j.cattod.2019.05.033

Accepted Manuscript
Title: Raney® nickel-catalyzed hydrodeoxygenation and
dearomatization under transfer hydrogenation
conditions—Reaction pathways of non-phenolic compounds
Authors: A.A. Philippov, A.M. Chibiryaev, O.N. Martyanov
PII:
S0920-5861(19)30243-3
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.05.033
CATTOD 12209
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
27 December 2018
19 April 2019
14 May 2019
Please cite this article as: Philippov AA, Chibiryaev AM, Martyanov ON,
Raney® nickel-catalyzed hydrodeoxygenation and dearomatization under transfer
hydrogenation conditions—Reaction pathways of non-phenolic compounds, Catalysis
Today (2019), https://doi.org/10.1016/j.cattod.2019.05.033
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Raney® nickel-catalyzed hydrodeoxygenation and dearomatization under transfer
hydrogenation conditions—Reaction pathways of non-phenolic compounds
A.A. Philippova,b,*, A.M. Chibiryaeva,b,* and O.N. Martyanova,b
a Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
Academician Lavrentiev Avenue 5, 630090 Novosibirsk, Russia.
b Novosibirsk State University, Pirogov Street 2, 630090 Novosibirsk, Russia
Graphical Abstract
Highlights


Aromatic ketones and alcohols are reduced into cycloalkanes under TH catalyzed Raney Ni
Reduction proceeds through pathways ketone–alcohol–alkane
Ketones are reduced quicker than alcohols or benzaldehyde



Decarbonylation of benzaldehyde and phenylacetaldehyde are observed
CO demonstrates poisoning effect on Raney Ni surface
Abstract: Catalytic reduction of oxygen-containing aromatic compounds has been studied under
transfer hydrogenation (TH) conditions at 150 °C in 2-PrOH as a hydrogen donor. Raney® nickel is
used as a heterogeneous catalyst. The reaction of aromatic non-phenolic carbonyl compounds is most
likely to proceed through the pathway “aromatic ketone (aldehyde) aromatic

alcohol alkylaromatics saturated alkylcyclohexane”. One of the main reactions under the TH


conditions is a hydrodeoxygenation (HDO) process. Unexpectedly, the hydrodeoxygenation of
aromatic ketones to alkylaromatics (C=O CH2) occur faster than of corresponding aromatic

alcohols (HC–OH CH2) that means either additional reaction pathway of its hydrodeoxygenation

missing for the corresponding aromatic alcohols or specific interaction of OH functionality with
Raney® nickel surface obstructing (hindering) the further reduction. Benzaldehyde is shown to be

less reactive than the aromatic ketones under the same reaction conditions. The main reason is
proposed to be carbon monoxide release resulted from the decarbonylation of the aldehyde. Carbon
monoxide demonstrates a poisoning effect on Raney® nickel surface that is evidenced in the
catalyzed TH reaction of acetophenone. The HDO reaction of anisole under the same reaction
conditions was a little slowly than of oxygen-containing non-phenolic aromatics.
Keywords: Transfer hydrogenation
Raney® nickel
Hydrodeoxygenation
Dearomatization
Oxygen-containing aromatics
1. Introduction
Lignocellulosic biomass consisting of three major components (cellulose, lignin, and
hemicellulose) is the most abundant bioresource which world production is around 170 metric
tons per year [1]. As a renewable chemical feedstock, lignocellulose can be converted into
different valuable chemicals [2] and biofuels [3,4] along with conventional heat and electricity
production. However, chemical processing of lignocellulose is complicated and difficult
owing to the complex structure, chemical and biochemical stability [5], as well as an
extremely high oxygen content reaching up to 40 wt% [6]. Specifically in lignin, the oxygen
content can vary in a wide range of 22–45 wt% [7,8]. That is the reason why a catalytic
hydrodeoxygenation process (HDO) plays an important role in lignin processing [2,9,10]. As
known, HDO is a reductive transformation for removing oxygen from oxygen-containing
compounds and saturating by hydrogen. The process can also proceed under transfer
hydrogenation (TH) conditions [11–17] with using heterogeneous catalysts that has some
clear advantages over the conventional hydrogenation or hydrogenolysis, for example, no
necessity to use hazardous and flammable H2 gas or metal hydrides, availability of various
suitable H-donors, relative cheapness, operational simplicity of handling, and ease of removing
[18,19].
Raney® nickel is recognized as a widely-used industrial hydrogenation catalyst for the
reduction of monosaccharide sugars, nitriles, nitro- and carbonyl compounds [20,21], total
hydrogenation of some aromatics and alkynes [20], as well as an alkylation of amines with alcohols,
etc. [21,22]. Moreover, the skeletal nickel is a highly promising catalyst for the TH reaction of
various substrates as demonstrated earlier [18,23–26] including the catalytic upstream
biorefining of lignin [16,27]. The first stage of the last process is lignin depolymerization into
a mixture of monomeric aromatic aldehydes and Hibbert’s ketones which carry out the further
reductive transformation (the second stage) into hydrogenated products as a key reaction.
Thus, Raney® nickel can be very useful and effective enough to decrease oxygen content and

hydrogenate unsaturated products via a one-pot “disassembling hydrogenolysis” of lignin
followed by the HDO process.
The HDO process of biomass-derived lignins is very important, but the most intensively
studied only in last eight years (2011–2018). Usually “lignin-derived phenolic compounds”
are used in the HDO study as model compounds of lignin structure, mainly anisole
(methoxybenzene) and guaiacol (2-methoxyphenol). The main purpose of those studies is a C–
O bond hydrogenolysis of these phenolic compounds to further convert into monocyclic
hydrocarbons. However, in the alkyl side chain of phenylpropane basic units of lignin, there are
some kinds of other carbon-oxygen bonds (–OH, C–O–C, C=O) different from phenolic hydroxyl
groups. The relevant studies of HDO focused on the conversion of oxygen-containing phenylpropanes
or, in general, phenylalkanes as another type of model compounds simulating the lignin structure
are still very scant.
Herein, we report the results of the HDO reaction catalyzed by Raney® nickel under
transfer hydrogenation conditions at 150 °C. Different oxygen-containing aromatic substrates
have been used as non-phenolic model compounds of lignin structure: acetophenone,
benzophenone, diphenylmethanol, benzaldehyde, benzyl alcohol and styrene oxide. The
reference compound was anisole which catalytic HDO transformations are studied well enough
excluding the process catalyzed by Raney® nickel (hydrodeoxygenation, hydrogenation, or
hydrogenolysis). As a hidden hydrogen resource, 2-propanol was used which H-donor activity
in the TH reactions is the subject of many recent studies and well-documented. The aim of
this study is to investigate a comparative reactivity of abovementioned aromatics and the HDO
kinetics under catalyzed TH conditions, to determine the structure of main reaction products
(if not done before) as well as “substrate structure”-dependent catalytic selectivity of Raney®
nickel. A plausible reaction mechanism is proposed to explain the “step-by-step” formation
of the products.
2. Experimental
2.1. Chemicals and solvents
All solvents and reagents were obtained from commercial sources and used as received: 2-
propanol (≥99.5%, Sigma–Aldrich), benzophenone (99%, Sigma–Aldrich), diphenylmethanol
(99%, Aldrich), acetophenone (≥98%, Aldrich), benzaldehyde (≥99%, Sigma–Aldrich), benzyl
alcohol (≥99%, Sigma–Aldrich), styrene (≥99%, Aldrich), styrene oxide (≥97%, Aldrich),
anisole (99.7%, Sigma–Aldrich), sulfolane (99%, Aldrich), aluminum-nickel alloy (Al–Ni
50:50 wt%, Sigma–Aldrich), NaOH (≥97%, Sigma–Aldrich).

2.2. Catalyst preparation
Raney® nickel was prepared according to well-known leaching method from
aluminum–nickel alloy [28]. So, 0.450 g of the alloy was added portion-wise to 5.6 mL 20%
aq. NaOH (a solution of 14.50 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 45–50 minutes at 50 °C until the H2 evolution
stopped. Then the skeletal nickel 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 were very
similar to literature data (see Supplementary Materials).
2.3. General experimental procedure
The batch reactor equipped with mechanical agitator MagneDrive (Autoclave Engineers) was
charged by ~3.2∙mmole of ketone (or benzaldehyde, 0.39–0.60 g), 0.20–0.30 g of sulfolane (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 closed, purged with argon under stirring 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. Catalyzed TH reaction of acetophenone in the presence of CO at 150 °C
The batch reactor was charged by ~3.2∙mmole of аcetophenone (0.39 g), 0.20 g of sulfolane
(as an internal standard) and 120 mL of 2-PrOH. After adding the catalyst freshly prepared
from 0.45 g of aluminum–nickel alloy, the reactor was closed, purged with argon under stirring
and then 80 ml of CO (at 1 atm and room temperature, ~3.2 mmol) were pumped into the
reactor by using a high pressure syringe pump (Teledyne ISCO, USA). After that, the reactor was
heated up with the rate of ~7 °C/min to reach the reaction temperature of 150 ± 1 °C. The reaction
pressure was raised up to 9 atm. 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 and 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 (12 °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 was used as an internal standard for
the quantitative assessment of the products.
3. Results and discussion
The chemical behavior of six non-phenolic oxygen-containing aromatics together with
non-oxygen styrene (which could be an intermediate in some of the reactions) were studied at
150 ºC using Raney® nickel as a catalyst of transfer hydrogenation by 2-PrOH. The molar
catalyst-to-substrate ratio was always 1.2/1 in the reaction mixtures and the amount of the catalyst was
not optimized. At the same time, according to approximate evaluation based on specific surface area of
the catalyst (see Supplementary Materials), less than 5% of all nickel atoms are on the working catalyst
surface. Thus, it is highly possible to decrease the amount of the catalyst used.
All reactions were carried out in the same manner to each other under oxygen-free
conditions to avoid the oxidation (deactivation) of the pyrophoric catalyst and repeated twice.
The conversion time was 5 h, but, for kinetic information, probes of the each reaction mixture were
collected hourly starting from zero reaction time at which the reaction mixture reaches the reaction
temperature (150 ºC) after heating. These results were compared with the TH reduction of
anisole as a phenolic model compound (Table 1).
The early study on the TH mechanism by ATR–IR spectroscopy reveals that 2-PrOH is
chemically adsorbed on the metal catalyst surface with further O–H bond dissociation. The
elimination of hydrogen atom from α-C position of 2-PrOH (surface–O–C(CH3)2–H) leads to
acetone formation (O=C(CH3)2) and hydrogen stays bonded on the metal surface (see, for
example, Refs. [16,18]). This is a necessary condition for initiation of the TH reaction.
Oxygen-containing aromatic substrates can also be adsorbed on the Raney® nickel surface [16]. Both
functional groups participate in interaction with the metal surface that makes possible the total
hydrogenation.

At the first stage, all oxygen-containing substrates underwent the reductive
deoxygenation to form the non-oxygen aromatics (Table 1). The conversion in the total
deoxygenation achieved 100% after 5 h excluding anisole (90%). Also in the anisole HDO
reaction, oxygen-containing product (methoxycyclohexane, 22%) was found in the final
reaction mixture. As expected, monoaromatic rings are not chemically stable under the TH
reaction conditions and react further to form substituted cyclohexanes. But for 5 h of the
reaction, none of deoxygenated intermediates achieved the exhaustive hydrogenation to give
fully saturated products.
Table 1.
Both acetophenone and styrene have the same HDO products in the final reaction
mixture – ethylbenzene and ethylcyclohexane in a similar content. More than 80% of the all
products are the same in the reaction of styrene oxide too. It could mean that styrene is the
common intermediate in the reaction of acetophenone and styrene oxide. For a deeper insight
into the reaction mechanisms, kinetics (the dependence of all products yield vs. reaction time)
was studied for the all reactions that helped to detect some obvious difference in the TH
reaction of the chosen substrates.
3.1. Reaction of ketones and benzaldehyde
The first stage of acetophenone transformation is the transfer hydrogenation of C=O
group, which leads to 1-phenylethanol. So, at zero reaction time, the acetophenone conversion
achieves 94% (Fig. 1). But the amount of firstly formed 1-phenylethanol is already decreased
to 16% due to its further hydrogenolysis and hydrogenation into a mixture of ethylbenzene
and ethylcyclohexane those amounts are highly anti-correlated with each other. Selectivity on
ethylbenzene is reached maximum of 88–89% for 0.5 h of the reaction at 100% conversion of
acetophenone and then gradually decreased to 50% at the end of the reaction. A trace amount
of 1-cyclohexylethanol is also detected in the reaction mixture, which concentration is
changed very little during the reaction (< 1%).
Fig. 1.
To compare the reaction under milder conditions, the catalyzed transfer hydrogenation of
acetophenone was studied at 50°C: no conversion was observed for 5 h of the reaction. At the same
time, in the non-catalytic reaction at 150°C, less than 1% of initial acetophenone was converted into

any hydrogenation products for 5 h. Thus, Raney® nickel catalyst and reaction temperature are crucial
conditions for successful TH reaction of acetophenone.
The transfer hydrogenation of C=O group in benzophenone molecule leads to
diphenylmethanol which content at zero reaction time is already decreased to 20% at 98%
conversion of benzophenone (Fig. 2). After 1 h of the reaction, no diphenylmethanol detects
in the reaction mixture. But at the same reaction time, the amount of diphenylmethane formed
directly from diphenylmethanol via hydrogenolysis is also about zero being totally converted
into other products. For example, selectivity on cyclohexylphenylmethane is reached
maximum of 89% for 1 h of the reaction and then gradually decreased to 65% at the end of
the reaction. The amount of fully saturated dicyclohexylmethane is gradually increased and
reached 34% at the end of the reaction. Trace amounts of cyclohexylphenylketone and
cyclohexylphenylmethanol are also detected in the reaction mixture.
Fig. 2.
Compared with ethylbenzene, diphenylmethane is shown to have higher reactivity in the
hydrogenation of benzene ring. But the total hydrogenation of cyclohexylphenylmethane
derived directly from diphenylmethane after one ring reduction undergoes slow enough.
Although the two aromatic rings in diphenylmethane molecule are isolated (non-conjugated
aromatics) and react independently, first among them is hydrogenated much faster than the
second one. This result is in good agreement with reported elsewhere [6] and more research
is clearly needed to be explained.
The conversion of benzaldehyde achieves only 44% at zero reaction time. The maximum
content of benzyl alcohol (50%) is reached after 30 min and then decreased to zero for 2.5
hours of the reaction (Fig. 3). The amount of cyclohexylmethanol formed directly from benzyl
alcohol via the total hydrogenation of benzene ring (dearomatization) is changed very little
during the reaction (< 1%).
Interestingly, the reductive conversion of benzaldehyde passes through two independent
pathways (parallel ones): 1) without changing the carbon skeleton; 2) with shortening the carbon
chain of the products. According to the first pathway, the TH reaction of benzaldehyde proceeds as
follows: “benzaldehyde benzyl alcohol toluene methylcyclohexane”. The second pathway



includes, at the first stage, the well-known decarbonylation of benzaldehyde to benzene (for
example, during a catalytic reaction at 150 °C [29]) which is further hydrogenated to
cyclohexane. Unexpectedly, both toluene and benzene are low-reactive enough under the TH

reaction conditions: their further hydrogenation to methylcyclohexane and cyclohexane,
respectively, occurs very slowly after reaching the maximum content for 1–2 h of the reaction.
Fig. 3.
Surprisingly, the hydrodeoxygenation of benzaldehyde as a first stage occurs slower than
of acetophenone and benzophenone: this result differs from general conception of the TH
reaction, in which aldehydes are much more reactive than ketones. Moreover, the total
hydrogenation of aromatic rings of intermediates (toluene and benzene) also slows down in the
reaction. In our opinion, the decarbonylation can cause the low rate of further hydrogenation
due to partial deactivation of Raney® Ni. As known [16], CO is adsorbed on metal nickel
surface and, perhaps, stays bonded similar to Fischer–Tropsch species [30]. These species
block centers of 2-PrOH adsorption and hinder the process.
Thus, the reduction of C=O groups in acetophenone, benzophenone and benzaldehyde
molecules seems to proceed mainly via the pathway: “aromatic ketone (aldehyde) aromatic

alcohol alkylaromatics saturated alkylcyclohexane” (structures
1
,
2
,
3
, and
mainstream” is accompanied with minor reduction of aromatic rings giving the trace amount
of non-aromatic both ketone and alcohol like structures and , correspondingly (Scheme 1).
We believe that both aliphatic ketone and alcohol and are not main precursors of final
alkylcyclohexanes Unlike aromatic 1-phenylethanol and diphenylmethanol ( , R = CH3 or Ph),
4, Scheme 1). This


“
5
6
5
6
.
2
HDO of low-reactive aliphatic alcohols like 6 (R = CH3 or Ph) catalyzed by Raney® nickel at 150 °C
is much more difficult to undergo [31].
Scheme 1.
3.2. Reaction of aromatic alcohols
As diphenylmethanol and benzyl alcohol are the primary product of the HDO derived
from the corresponding carbonyl compound, it was important to study their kinetics
(conversion dynamics) under the same reaction conditions.
Both diphenylmethanol and benzyl alcohol demonstrate lower reactivity than
corresponding carbonyl compounds: only 44% and 27% of the conversion, respectively, at
zero reaction time (Fig. 4 and Figure
5
). The TH reaction of diphenylmethanol proceeds
Fig. ). Selectivity on
through the same pathway as diphenylketone
(
4
cyclohexylphenylmethane reaches its maximum of 93% for 2 h of the reaction.
Diphenylmethane content achieves maximum of 42% for 30 min and then is decreased being

converted
into
cyclohexylphenylmethane.
Cyclohexylphenylmethane
and
dicyclohexylmethane are found in the final reaction mixture in ratio 87:13. The trace amount
of cyclohexylphenylmethanol is also detected.
Fig. 4.
Selectivity on toluene reaches up to 96% for 1 h of the TH reaction of benzyl alcohol
(
Figure 5). The amount of methycyclohexane is gradually increased during the reaction
achieved 25% in the final reaction mixture. Only a trace amount of cyclohexylmethanol is
detected. In contrast to the reaction of benzaldehyde, the decarbonylation products are not
found in the reaction mixture. It means that solely benzaldehyde underdoes the
decarbonylation process, not benzyl alcohol. Additionally, this confirms a weak H-donor
ability of primary aliphatic alcohols including benzyl alcohol (as compared with secondary 2-
PrOH) in the presence of Raney® nickel [6,31]. A much faster toluene reduction in the reaction
of benzyl alcohol as compared with the TH of benzaldehyde is an argument demonstrating
CO poison effect on Raney® nickel surface when the decarbonylation process of aldehyde
occurs.
Figure 5.
3.3. Reaction of styrene and styrene oxide
The epoxide functionality is not “native” for lignin structure, but may be formed after
some thermal and/or chemical exposures on lignin molecules, for example as a result of the
dehydration of vicinal diol moiety to epoxy group [32] or of some other chemical pretreatment
[33]. On the other hand, behavior of epoxides under TH conditions could be interesting from
a synthetic viewpoint [34–36].
Earlier styrene oxide was studied under TH reaction conditions [37,38], but those
reactions have differed from present one: other catalysts and reductants were used. Now
styrene oxide is reduced with using 2-PrOH as a hydrogen donor and Raney® nickel as a
heterogeneous catalyst. Styrene oxide is active enough under the reaction conditions to be
successful converted into the products. The conversion achieves 78% already at zero reaction
time (Fig. 6). Among the products, aromatic ethylbenzene and toluene, as well as fully
saturated ethylcyclohexane and methylcyclohexane are the main ones. The minor products are
styrene and 2-phenylethanol (β-phenethyl alcohol). The content of ethylbenzene and
ethylcyclohexane are highly anti-correlated with each other after 1 h of the reaction.

Fig. 6.
The chemical composition of the products is rather different from that obtained in the
conventional hydrogenation of styrene oxide catalyzed by Raney® nickel [39]. For example,
under our reaction conditions, selectivity on ethylbenzene reaches up to 78% for 1 h, while
s
electivity on 2-phenylethanol is only 3% for the same reaction time. Liquid-phase
hydrogenation of styrene oxide at 50 °C catalyzed by Raney® nickel gives quite opposite result:
electivity on ethylbenzene was only 10% for 1 h of the reaction, while electivity on 2-
phenylethanol reached up 90% for the same reaction time [39]. Additionally, our reaction mixture
contains no 1-phenylethanol -phenethyl alcohol) which is common enough for the conventional
s
s
(
hydrogenation of styrene oxide [40].
Toluene (methylbenzene) and methylcyclohexane formation evidences in the decarbonylation
reaction of 2-phenylacetaldehyde. Thus, the first stage of the reaction is a well-known Meinwald
rearrangement of oxirane to aldehyde [29,41] resulted in the formation of phenylacetaldehyde 8 from
styrene oxide (Scheme 2). The further reaction of phenylacetaldehyde gives either 2-phenylethanol
9
via the TH reduction or toluene 13 due to the decarbonylation. 2-Phenylethanol can be
dehydrated into styrene 10 or converted into ethylbenzene 11 due to the HDO reaction
catalyzed by Raney® nickel [42]. Another possible precursor of styrene 10 is styrene oxide
which may be directly deoxygenated on Raney® nickel surface (cf. [42]). The total
hydrogenation of toluene and ethylbenzene occurs slowly enough under these reaction
conditions. Our reason is the same one: CO poisoning effect on Raney® nickel surface when
the decarbonylation process of aldehyde occurs.
Scheme 2.
The TH reaction of styrene is carried out to prove the reaction pathways of styrene oxide
conversion. Expectedly, non-aromatic conjugated C=C double bond of styrene is reduced quickly
under the TH conditions and the total conversion achieves at zero reaction time (Fig. 7). The reaction
mixture contains only two products – ethylbenzene and ethylcyclohexane which contents are
fully anti-correlated with each other and gradually changed during the reaction reaching the final
ratio equal to 2/3. As there are no toluene and methylcyclohexane in the reaction mixture, therefore,
these products resulted from some rearrangement (carbon-chain shortening) of oxygen-containing
derivatives of ethylbenzene those one could observed in the reaction of styrene oxide.

Fig. 7.
To test the reaction under milder conditions, the catalyzed transfer hydrogenation of styrene
was studied at 50°C. Similar to acetophenone reaction, there was no conversion for 5 h under the
reaction conditions. But in the non-catalytic reaction at 150°C, less than 10% of styrene was converted
into hydrogenation products. Thus, the reactivity of styrene under the same non-catalytic conditions
is slightly more than that of acetophenone.
3.4. Influence of CO on the TH reaction of acetophenone
To prove the inhibiting effect of CO on the catalytic properties of Raney® nickel in the
reaction, the catalyzed transfer hydrogenation of acetophenone was studied in the presence of
equimolar amount of CO at 150 °C.
The difference between the reactions in the presence of CO and without it is shown in Figs. 1
and 8. The most pronounced effects are demonstrated for acetophenone conversion, partial reduction
of acetophenone into ethylbenzene and its total hydrogenation into ethylcyclohexane. So, at zero
reaction time the acetophenone conversion reduced from 94% in the catalyzed reaction without CO
(Fig. 1) down to 21% in the presence of CO (Fig. 8), i.e. CO inhibited the hydrogenation of C=O
group that may be due to competitive adsorption of CO and C=O group of acetophenone molecule
on the catalyst surface. The similar competition of CO with benzene ring (π-conjugated aromatic
system) for active metal centers of the catalyst (cf. [16]) caused the decrease of reactivity of
intermediate ethylbenzene, which content in the reaction mixture reached maximum of 92% after 1 h
of the reaction and remained almost unchanged until the end of the reaction. That is why intermediate
ethylbenzene doesn’t further reduce into fully hydrogenated ethylcyclohexane. Thus, the inhibition
effect of CO is quite pronounced.
Fig. 8
3.5. Reaction of anisole
Anisole is well-known and widely used model compound to study the catalytic HDO reaction
of lignins. Surprisingly, there are very poor experimental data on the hydrodeoxygenation,
hydrogenation, and hydrogenolysis of anisole catalyzed by Raney® nickel. To compare the
HDO efficiency of Raney® nickel towards both non-phenolic and phenolic model compounds
of lignin structure, the TH reaction of anisole was carried out at the same reaction conditions.

Unlike other oxygen-containing compounds, the anisole conversion is only 90% reached for 5
h of the reaction (Fig. 9). Two parallel reaction routes are realized under the conditions – HDO and
dearomatization. The main product of direct HDO process is benzene (22% after 0.5 h of the reaction)
which is further dearomatizated to cyclohexane: the cyclohexane and benzene contents in the final
reaction mixture are 63% and 7%, respectively. Evidently, the fully both conversion of anisole and
benzene dearomatization can be achieved under the reaction conditions with increased reaction
duration (see trend of kinetic curves, Fig. 9). Direct dearomatization of anisole results in 1-
methoxycyclohexene (up to 1%) and methoxycyclohexane (19% after 5 h) formation, but this process
catalyzed by Raney® nickel is minor. The same is true for cyclohexanol (≤ 1%), which formation
results from the combination of demethylation and dearomatization of anisole. Methoxycyclohexane,
which content is correlated with cyclohexane may undergo the same demethylation reaction to
convert into cyclohexanol.
Fig. 9.
3.6. Reusability of Raney
 nickel catalyst
It has been shown that the Raney® nickel catalyst can be reused at least during three
consecutive runs (5 h reaction time each) in the transfer hydrogenation of acetophenone to any
hydrogenated products at 150 °C. For each cycle, the catalyst was recovered by decantation of the
reaction mixture (the procedure is described in Supplementary Materials).
The recovered catalyst exhibited no significant loss of catalytic activity after three runs. So,
after 3rd cycle, the acetophenone conversion for 5 h of the reaction remains 100% similar to freshly
prepared Raney® nickel. Minor difference was only observed in products selectivity: the
ethylcyclohexane-to-ethylbenzene ratio (i.e. a ratio of fully hydrogenated product to partially
hydrogenated one) was 50:50 after 1st cycle, 48:52 after 2nd cycle and 44:56 after 3rd cycle,
respectively. (It is worthy to note that, in our case, decreasing the catalyst amount because of the
separation procedure was ca. 15 wt% after third cycle.) Therefore, the Raney® nickel catalyst can
be successfully reused, but limited numbers of runs. The decrease in chemoselectivity may be
ascribed to partial oxidation of the catalyst surface during the separation procedure. Recovering the
Raney® nickel catalyst is well-known procedure that was studied many times including for some
hydrogen transfer processes – hydrogenation, hydrogenolysis and hydrogen-borrowing reactions
[22,23,43].
4. Conclusions

The seven non-phenolic aromatic substrates and one phenolic compound (anisole) have
been studied as model compounds of lignin structure at 150 °C under transfer hydrogenation
conditions. It has been shown that the combination of Raney® nickel as a catalyst and 2-PrOH
as an H-donor can be effective for both HDO process and dearomatization reaction of different
oxygen-containing aromatics. The chemical processes observed under the reaction conditions
have significant difference with those in the conventional hydrogenation catalyzed by the
same catalyst. If the oxygen functionality is in a side alkyl chain, the HDO process is
recognized as a primary reaction, while the dearomatization reaction is a secondary one. The
structure of all products was determined and plausible reaction pathways were proposed. It
was shown that aldehydes undergo the decarbonylation reaction as an alternative process and
carbon monoxide released is able to act as a poison of the Raney® nickel surface decreasing
the conversion. The inhibiting effect of CO on the transfer hydrogenation of both C=O group
and benzene ring was demonstrated in the TH reaction of acetophenone catalysed by Raney®
nickel at 150 °C.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.XXX.2018.XX.XXX.
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).
References
[1] T.E. Amidon, S. Liu, Water-based woody biorefinery, Biotechnol. Adv. 27 (2009) 542–550.
doi:10.1016/j.biotechadv.2009.04.012.
[2] W. Schutyser, T. Renders, S. Van Den Bosch, S.F. Koelewijn, G.T. Beckham, B.F. Sels,
Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and
upgrading, Chem. Soc. Rev. 47 (2018) 852–908. doi:10.1039/c7cs00566k.
[3] K. Ullah, V.K. Sharma, M. Ahmad, P. Lv, J. Krahl, Z. Wang, Sofia, The insight views of
advanced technologies and its application in bio-origin fuel synthesis from lignocellulose
biomasses waste, a review, Renew. Sustain. Energy Rev. 82 (2018) 3992–4008.

doi:10.1016/j.rser.2017.10.074.
[4] X. Zhang, H. Lei, L. Zhu, J. Wu, S. Chen, From lignocellulosic biomass to renewable
cycloalkanes for jet fuels, Green Chem. 17 (2015) 4736–4747. doi:10.1039/C5GC01583A.
[5] K. Barta, T.D. Matson, M.L. Fettig, S.L. Scott, A.V. Iretskii, P.C. Ford, Catalytic
disassembly of an organosolv lignin via hydrogen transfer from supercritical methanol,
Green Chem. 12 (2010) 1640–1647. doi:10.1039/c0gc00181c.
[6] X. Wang, R. Rinaldi, Exploiting H-transfer reactions with RANEY® Ni for upgrade of
phenolic and aromatic biorefinery feeds under unusual, low-severity conditions, Energy
Environ. Sci. 5 (2012) 8244–8260. doi:10.1039/c2ee21855k.
[7] J. Hu, R. Xiao, D. Shen, H. Zhang, Structural analysis of lignin residue from black liquor and
its thermal performance in thermogravimetric-Fourier transform infrared spectroscopy,
Bioresour. Technol. 128 (2013) 633–639. doi:10.1016/j.biortech.2012.10.148.
[8] B. Scholze, D. Meier, Characterization of the water-insoluble fraction from pyrolysis oil
(pyrolytic lignin). Part I. PY–GC/MS, FTIR, and functional groups, J. Anal. Appl. Pyrolysis
60 (2001) 41–54. doi:10.1016/S0165-2370(00)00110-8.
[9] P. Mäki-Arvela, D. Murzin, Hydrodeoxygenation of lignin-derived phenols: from
fundamental studies towards industrial applications, Catalysts 7 (2017) 265–304.
doi:10.3390/catal7090265.
[10] F. Cheng, C.E. Brewer, Producing jet fuel from biomass lignin: potential pathways to alkyl-
benzenes and cycloalkanes, Renew. Sustain. Energy Rev. 72 (2017) 673–722.
doi:10.1016/j.rser.2017.01.030.
[11] T. Guo, Q. Xia, Y. Shao, X. Liu, Y. Wang, Direct deoxygenation of lignin model compounds
into aromatic hydrocarbons through hydrogen transfer reaction, Appl. Catal. A Gen. 547
(2017) 30–36. doi:10.1016/j.apcata.2017.07.050.
[12] Y. Xue, S. Zhou, X. Bai, Role of hydrogen transfer during catalytic copyrolysis of lignin and
tetralin over HZSM-5 and HY zeolite catalysts, ACS Sustain. Chem. Eng. 4 (2016) 4237–
4250. doi:10.1021/acssuschemeng.6b00733.
[13] P. Ferrini, R. Rinaldi, Catalytic biorefining of plant biomass to non-pyrolytic lignin bio-oil
and carbohydrates through hydrogen transfer reactions, Angew. Chemie - Int. Ed. 53 (2014)
8634–8639. doi:10.1002/anie.201403747.
[14] Z. Luo, Y. Wang, M. He, C. Zhao, Precise oxygen scission of lignin derived aryl ethers to
quantitatively produce aromatic hydrocarbons in water, Green Chem. 18 (2016) 433–441.
doi:10.1039/c5gc01790d.
[15] M. Kim, J.-M. Ha, K.-Y. Lee, J. Jae, Catalytic transfer hydrogenation/hydrogenolysis of
guaiacol to cyclohexane over bimetallic RuRe/C catalysts, Catal. Commun. 86 (2016) 113–

118. doi:10.1016/j.catcom.2016.08.022.
[16] G. Calvaruso, J.A. Burak, M.T. Clough, M. Kennema, F. Meemken, R. Rinaldi, On the
reactivity of dihydro-p-coumaryl alcohol towards reductive processes catalyzed by Raney
nickel, ChemCatChem 9 (2017) 2627–2632. doi:10.1002/cctc.201601590.
[17] D. Forchheim, U. Hornung, P. Kempe, A. Kruse, D. Steinbach, Influence of Raney nickel on
the formation of intermediates in the degradation of lignin, Int. J. Chem. Eng. 2012 (2012) 1–
8. doi:10.1155/2012/589749.
[18] M.J. Gilkey, B. Xu, Heterogeneous catalytic transfer hydrogenation as an effective pathway
in biomass upgrading, ACS Catal. 6 (2016) 1420–1436. doi:10.1021/acscatal.5b02171.
[19] D. Wang, D. Astruc, The golden age of transfer hydrogenation, Chem. Rev. 115 (2015)
6621–6686. doi:10.1021/acs.chemrev.5b00203.
[20] T. Osawa, Heterogeneous Catalysis, in: Y. Tamaru (Ed.), Mod. Organonickel Chem., Wiley-
VCH Verlag GmbH & Co. KGaA, 2005: pp. 273–305.
[21] P. Saikia, B.M. Reddy, Industrially relevant: hydrogenation catalysts, Chem. Ind. Dig. 21
(2008) 87–93.
[22] A. Mehta, A. Thaker, V. Londhe, S.R. Nandan, Reinvestigating Raney nickel mediated
selective alkylation of amines with alcohols via hydrogen autotransfer methodology, Appl.
Catal. A Gen. 478 (2014) 241–251. doi:10.1016/j.apcata.2014.04.009.
[23] B.H. Gross, R.C. Mebane, D.L. Armstrong, Transfer hydrogenolysis of aromatic alcohols
using Raney catalysts and 2-propanol, Appl. Catal. A Gen. 219 (2001) 281–289.
doi:10.1016/S0926-860X(01)00700-1.
[24] L. Xu, X. Li, Y. Zhu, Y. Xiang, One-pot synthesis of N,N-dimethylaniline from nitrobenzene
and methanol, New J. Chem. 33 (2009) 2051–2054. doi:10.1039/b905656d.
[25] F. Alonso, P. Riente, M. Yus, Hydrogen-transfer reduction of carbonyl compounds catalysed
by nickel nanoparticles, Tetrahedron Lett. 49 (2008) 1939–1942.
doi:10.1016/j.tetlet.2008.01.097.
[26] F. Alonso, P. Riente, M. Yus, Nickel nanoparticles in hydrogen transfer reactions, Acc.
Chem. Res. 44 (2011) 379–391. doi:10.1021/ar1001582.
[27] C. Chesi, I.B.D. de Castro, M.T. Clough, P. Ferrini, R. Rinaldi, The influence of
hemicellulose sugars on product distribution of early-stage conversion of lignin oligomers
catalysed by Raney nickel, ChemCatChem 8 (2016) 2079–2088.
doi:10.1002/cctc.201600235.
[28] J. Freel, W.J.M. Pieters, R.B. Anderson, The structure of Raney nickel I. Pore structure, J.
Catal. 14 (1969) 247–256. doi:10.1016/0021-9517(69)90432-1.
[29] B.-H. Min, D.-S. Kim, H.-S. Park, C.-H. Jun, Pd/C-catalyzed carbonylative esterification of

aryl halides with alcohols by using oxiranes as CO sources, Chem. - A Eur. J. 22 (2016)
6234–6238. doi:10.1002/chem.201600570.
[30] A.P. Ashwell, W. Lin, M.S. Hofman, Y. Yang, M.A. Ratner, B.E. Koel, G.C. Schatz,
Hydrogenation of CO to methanol on Ni(110) through subsurface hydrogen, J. Am. Chem.
Soc. 139 (2017) 17582–17589. doi:10.1021/jacs.7b09914.
[31] A.A. Philippov, A.M. Chibiryaev, O.N. Martyanov, Base-free transfer hydrogenation of
menthone by sub- and supercritical alcohols, J. Supercrit. Fluids 145 (2019) 162–168.
doi:10.1016/j.supflu.2018.12.011.
[32] T.Y. Kim, J. Baek, C.K. Song, Y.S. Yun, D.S. Park, W. Kim, J.W. Han, J. Yi, Gas-phase
dehydration of vicinal diols to epoxides: dehydrative epoxidation over a Cs/SiO2 catalyst, J.
Catal. 323 (2015) 85–99. doi:10.1016/j.jcat.2014.12.023.
[33] T. Kleine, J. Buendia, C. Bolm, Mechanochemical degradation of lignin and wood by
solvent-free grinding in a reactive medium, Green Chem. 15 (2013) 160–166.
doi:10.1039/C2GC36456E.
[34] B. Barati, M. Moghadam, A. Rahmati, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-
Baltork, Ruthenium hydride catalyzed direct oxidation of alcohols to carboxylic acids via
transfer hydrogenation: styrene oxide as oxygen source, Synlett. 24 (2012) 90–96.
doi:10.1055/s-0032-1317676.
[35] T. Kimachi, H. Nagata, Y. Kobayashi, K. Takahashi, E. Torii, M. Ju-ichi, Regioselective
ring-opening reaction of unsymmetrical 2,3-diaryl epoxides via catalytic hydrogenolysis with
Pd(0)EnCatTM, Synlett. 2011 (2011) 365–368. doi:10.1055/s-0030-1259505.
[36] M. Ghafouri, M. Moghadam, K. Mehrani, A. Daneshvar, Supported ruthenium hydride
catalysts for direct conversion of alcohols to carboxylic acids using styrene oxide as oxidant,
Appl. Organomet. Chem. 32 (2018) e4048. doi:10.1002/aoc.4048.
[37] M. Ricci, A. Slama, Transfer hydrogenation of simple epoxides, J. Mol. Catal. 89 (1994) L1–
L3. doi:10.1016/0304-5102(93)E0325-B.
[38] N. Erathodiyil, S. Ooi, A.M. Seayad, Y. Han, S.S. Lee, J.Y. Ying, Palladium nanoclusters
supported on propylurea-modified siliceous mesocellular foam for coupling and
hydrogenation reactions, Chem. - A Eur. J. 14 (2008) 3118–3125.
doi:10.1002/chem.200701361.
[39] O. Bergadà, P. Salagre, Y. Cesteros, F. Medina, J.E. Sueiras, High-selective Ni-MgO
catalysts for a clean obtention of 2-phenylethanol, Appl. Catal. A Gen. 272 (2004) 125–132.
doi:10.1016/j.apcata.2004.05.024.
[40] M. Viswanadhan, A. Potdar, A. Divakaran, M. Badiger, C. Rode, Product distribution in
hydrogenation of styrene oxide over Pd/chitosan catalyst, Res. Chem. Intermed. 42 (2016)

7581–7595. doi:10.1007/s11164-016-2554-3.
[41] M.W.C. Robinson, A.M. Davies, R. Buckle, I. Mabbett, S.H. Taylor, A.E. Graham, Epoxide
ring-opening and Meinwald rearrangement reactions of epoxides catalyzed by mesoporous
aluminosilicates, Org. Biomol. Chem. 7 (2009) 2559–2564. doi:10.1039/b900719a.
[42] S. Mitsui, S. Imaizumi, M. Hisashige, Y. Sugi, The deoxygenation in the catalytic
hydrogenolysis of styrene oxides, Tetrahedron 29 (1973) 4093–4097. doi:10.1016/0040-
4020(73)80243-1.
[43] R.C. Mebane, K.D. Grimes, S.R. Jenkins, J.D. Deardorff, B.H. Gross, Deiodination of
iodolactones by transfer hydrogenolysis using Raney nickel and 2-propanol, Synth.
Commun. 32 (2002) 2049–2054. doi:10.1081/SCC-120004856.

Table Caption
Table 1
The main reaction products.a
#
Substrate
Productsb
O
+
+
1
ethylbenzene
(51%)
ethylcyclohexane
(49%)
acetophenone
2
3
ethylbenzene
(40%)
ethylcyclohexane
(60%)
styrene
O
+
+
+
ethylbenzene
(54%)
toluene
(11%)
ethylcyclo-
hexane (29%) hexane (6%)
methylcyclo-
styrene oxide
O
+
+
+
4c
benzaldehyde
toluene
(75%)
benzene
(10%)
methylcyclo- cyclohexane
hexane (6%)
(8%)
OH
+
5
benzyl alcohol
toluene
(74%)
methylcyclohexane
(26%)
O
+
6
7
dicyclohexylmethane
(35%)
benzophenone
OH
cyclohexylphenyl-
methane (65%)
+
dicyclohexylmethane
(13%)
diphenylmethanol
cyclohexylphenyl-
methane (87%)

O
O
+
+
8
anisole
benzene
(7%)
methoxycyclo- cyclohexane
hexane (22%)
(71%)
a Reaction conditions: 3 mmol of substrate, Raney® Ni prepared from 450 mg of Al-Ni alloy, 120 mL of 2-PrOH, 5 h,
150 °C, 11 atm.
b Conversion is 100% in all reactions excepting anisole conversion (90%).
c Cyclohexylmethanol is formed only in a trace amount (≤ 1%).

Figure and Scheme Captions
Figure 1. Acetophenone conversion and products content.
Figure 2. Benzophenone conversion and products content.
Figure 3. Benzaldehyde conversion and products content.
Figure 4. Diphenylmethanol conversion and products content.
Figure 5. Benzyl alcohol conversion and products content.
Figure 6. Styrene oxide conversion and products content.
Figure 7. Styrene conversion and products content.
Figure 8. Acetophenone conversion in the presence of CO.
Figure 9. Anisole conversion and products content.
Scheme 1. Plausible mechanism of TH reaction of aromatic carbonyl compounds.
Scheme 2. Plausible mechanism of TH reaction of styrene oxide.

80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
acetophenone;
1-phenylethanol;
ethylbenzene;
ethylcyclohexane;
1-cyclohexylethanol
Figure 1.
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
diphenylmethanol;
benzophenone;
phenylmethane;
cyclohexyl-
diphenylmethane;
dicyclohexylmethane;
cyclohexylphenyl-
ketone;
cyclohexylphenylmethanol
Figure 2.
80
60
40
20
0
8
6
4
2
0
0
1
2
3
4
5
Reaction time, h
0
1
2
3
4
5
Reaction time, h
benzaldehyde;
benzyl alcohol;
toluene;
benzene;
cyclohexylmethanol;
methylcyclohexane;
cyclohexane
Figure 3.

100
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
diphenylmethanol;
methane; dicyclohexylmethane;
cyclohexylphenylmethanol;
cyclohexylphenylmethane;
diphenyl-
cyclohexylphenylketone;
benzophenone
Figure 4.
100
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
benzyl alcohol
toluene
methylcyclohexane
cyclohexylmethanol
Figure 5.
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
styrene oxide;
methylcyclohexane;
2-phenylethanol
ethylbenzene;
ethylcyclohexane;
styrene;
toluene;
Figure 6.

100
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
styrene;
ethylbenzene;
ethylcyclohexane
Figure 7.
90
75
60
45
30
15
0
2
1
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time, h
acetophenone;
ethylbenzene;
ethylcyclohexane;
1-cyclohexylethanol;
1-phenylethanol;
acetylcyclohexane;
1-ethylcyclohexene;
Figure 8.
80
60
40
20
0
0
1
2
3
4
5
Reaction time, h
anisole;
benzene;
cyclohexane;
cyclohexanol;
methoxycyclohexane;
methoxycyclohexene
Figure 9.

R
R
R
O
HO
[H]
[H]
1
2
3
[H]
R
[H]
R
[H]
R
O
HO
[H]
5
6
4
R = CH3, Ph, H
Scheme 1.
[H]
-CO
O
13
[H]
14
12
[H]
Ph
[H]
O
OH
Ph
Ph
Ph
Ph
-H2O
7
10
-H2O
11
8
9
[-O]
Scheme 2.