Hydrogenolysis of aromatic ethers under lignin-first conditions

Article abstract of DOI:10.1016/j.mcat.2020.111228

The cleavage of the etheric C–O bond in diphenyl ether (DPE), phenethyl phenyl ether (PPE) and benzyl-phenyl ether (BPE) has been investigated by using Ru/C (5% wt) and Pd/C (5% wt), as heterogeneous catalysts, under reaction conditions generally adopted for the reductive catalytic fractionalization of lignocellulosic biomasses (lignin-first approach). Catalytic tests were carried out in the presence of simple C1-C3 alcoholic H-donor solvents (methanol, ethanol and 2-propanol) used as such or in mixture with water in the temperature range of 120–240 °C both in the presence or in the absence of molecular hydrogen as reducing agent. Under transfer hydrogenolysis conditions, the Ru/C catalyst was found to be the best performing system in the cleavage of the 4–O–5 etheric C–O bond (95 % DPE conversion in 2-propanol at 210 °C after 3 h of reaction) with a less pronounced tendency in hydrogenating the aromatic ring. Upon increasing the water content in the reaction medium, a decrease in the cleavage of the C–O bond of DPE together with a higher production of phenolics is observed as a consequence of the reductive hydrolysis reaction occurrence. The best yield in aromatic compounds (52 %) was obtained by using as solvent a water/2-propanol (75:25, v/v) mixture in absence of added molecular hydrogen, with the alcoholic fraction being the in-situ H-source. A lower tendency to undergo to hydrolysis reaction together with a higher production of aromatics is registered in the case of phenethyl phenyl ether and benzyl-phenyl ether. Results are explained in terms of the higher steric hindrance of PPE and BPE with respect to DPE and of the competitive adsorption of arenes arising from hydrogenolysis of etheric β–O–4 and α–O–4 bonds (phenol + ethyl benzene or phenol + toluene) on the Ru/C catalyst surface.

Full text of DOI:10.1016/j.mcat.2020.111228

Molecular Catalysis 497 (2020) 111228
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Hydrogenolysis of aromatic ethers under lignin-first conditions
,
E. Paonea *, A. Beneducib, G.A. Correnteb, A. Malarac, F. Maurielloc
a
`
Dipartimento di Ingegneria Industriale (DIEF), Universita degli Studi di Firenze, Via di S. Marta 3, I-50139 Firenze, Italy
b
`
Dipartimento di Chimica e Tecnologie Chimiche (CTC), Universita della Calabria, via Pietro Bucci, I-87036 Arcavacata di Rende, Italy
c
`
Dipartimento di Ingegneria Civile, dell’Energia, dell’Ambiente e dei Materiali (DICEAM), Universita degli Studi “Mediterranea” di Reggio Calabria, Loc. Feo di Vito, I-
89122 Reggio Calabria, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
–
The cleavage of the etheric C O bond in diphenyl ether (DPE), phenethyl phenyl ether (PPE) and benzyl-phenyl
ether (BPE) has been investigated by using Ru/C (5% wt) and Pd/C (5% wt), as heterogeneous catalysts, under
reaction conditions generally adopted for the reductive catalytic fractionalization of lignocellulosic biomasses
(lignin-first approach). Catalytic tests were carried out in the presence of simple C1-C3 alcoholic H-donor sol-
vents (methanol, ethanol and 2-propanol) used as such or in mixture with water in the temperature range of
120–240 ◦C both in the presence or in the absence of molecular hydrogen as reducing agent. Under transfer
hydrogenolysis conditions, the Ru/C catalyst was found to be the best performing system in the cleavage of the
Lignocellulosic biomasses
Lignin
Hydrogenolysis
Catalytic transfer hydrogenolysis
Hydrolysis
Reductive catalytic fractionalization
Lignin-first biorefinery
Lignin depolymerization
Diphenyl ether
4–O–5 etheric C O bond (95 % DPE conversion in 2-propanol at 210 ◦C after 3 h of reaction) with a less
–
pronounced tendency in hydrogenating the aromatic ring. Upon increasing the water content in the reaction
Phenethyl phenyl ether
Benzyl phenyl ether
Heterogeneous catalysis
Ruthenium
–
medium, a decrease in the cleavage of the C O bond of DPE together with a higher production of phenolics is
observed as a consequence of the reductive hydrolysis reaction occurrence. The best yield in aromatic com-
pounds (52 %) was obtained by using as solvent a water/2-propanol (75:25, v/v) mixture in absence of added
molecular hydrogen, with the alcoholic fraction being the in-situ H-source. A lower tendency to undergo to
hydrolysis reaction together with a higher production of aromatics is registered in the case of phenethyl phenyl
ether and benzyl-phenyl ether. Results are explained in terms of the higher steric hindrance of PPE and BPE with
respect to DPE and of the competitive adsorption of arenes arising from hydrogenolysis of etheric β–O–4 and
Palladium
α
–O–4 bonds (phenol + ethyl benzene or phenol + toluene) on the Ru/C catalyst surface.
1. Introduction
reductive catalytic processes, the three key components of lignocellu-
losic biomass (cellulose ~40 wt %, hemicellulose ~25 wt % and lignin
~20 wt %) can be converted into bioethanol and phenolic
monomers/oligomers.
The time to put the old saying "you can make anything from lignin
except money” in the attic has now come. While cellulose and hemi-
cellulose already represent well established feedstocks for the produc-
tion of several chemical commodities [1–4], only in the last years we
have witnessed an effective use of lignin for the production of aromatic
compounds [5,6].
In RCF processes, lignin is primary extracted from the lignocellulose
by using an organic solvent (generally short-chain C1-C4 aliphatic al-
cohols or cycle ethers) adopted as such or in combination with water and
successively depolymerized, under relative mild reaction conditions
(180–250 ◦C for 2–6 h), in the presence of a direct (molecular H2) or
indirect (H-donor molecule/solvent) reducing agent and a heteroge-
neous redox-active catalyst (Pd/C and Ru/C being the most used
[14–17]) that plays also a crucial role in the stabilization of phenolic
units. The basic catalysis beyond the depolymerization of lignin in RCF
Owing to the improvement of reductive catalytic fractionalization
(RCF) processes (also known as lignin-first biorefinery) [7–9], a com-
plete valorization of lignocellulosic biomasses into value-added chem-
icals, renewable energy and biofuels is firmly established today [10–12].
An integrated biorefinery based on the lignin-first approach, that can
efficiency transform birch wood into xylochemicals, was recently re-
ported opening a new era for the sustainable production of aromatics
from lignocellulosic wastes and residues [13]. By using such a kind of
–
–
processes involves the lysis of C O and C C bonds in the presence
(hydrogenolysis) or in the absence (transfer hydrogenolysis) of added
molecular
hydrogen.
Indeed,
hydrogenolysis
and
transfer
* Corresponding author.
E-mail address: emilia.paone@unifi.it (E. Paone).
https://doi.org/10.1016/j.mcat.2020.111228
Received 5 August 2020; Received in revised form 11 September 2020; Accepted 15 September 2020
Available online 6 October 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
hydrogenolysis (CTH) reactions represent a wide area in the valorization
of lignocellulosic biomasses and their derived bio-based molecules
[18–21].
typical reaction, the reduced catalyst (0.1 g) suspended in a 0.1 M so-
lution (40 mL) of the desired substrate in the chosen reaction solvent
were poured into the reactor, which was purged three times with N2
(99.99 %) and subsequently pressurized at the desired initial H2 or N2
pressure (10 bar). The system was heated from room temperature up to
the desired reaction temperature (heating rate of 5 ◦C/min) and the
temperature was monitored using a thermocouple fixed into the auto-
clave and connected to the reactor controller with the reaction time
considered started when the reactor reached the set temperature that
was kept constant for the required time. The range of operative condi-
tions used for reactions was: 120–240 ◦C, 10 bar initial H2 or N2 pres-
sure, 30–360 min reaction time. At the end of the reaction, the reactor
was cooled in an ice bath and, when at below room temperature (20 ◦C),
the pressure was released carefully and the liquid phase analysed using
an off-line gas chromatograph (Agilent 6890 N) equipped with a wide-
bore capillary column (CP-WAX 52CB, 60 m, i.d. 0.53 mm) and a
flame ionization detector (FID). The injector was settled at 250 ◦C and
the temperature program started at 50 ◦C (held for 5 min) with the
temperature raised to 230 ◦C at 5 ◦C/min (held for 25 min) and finally to
240 ◦C (held for 5 min) during the post run. In recycling tests, after any
run, the catalyst was first recovered, then carefully washed with fresh 2-
propanol and reused under the same reaction conditions.
With respect to lignin model molecules, diphenyl ether (DPE), phe-
nethyl phenyl ether (PPE) and benzyl-phenyl ether (BPE) were largely
used to study the cleavage of etheric 4–O–5, β–O–4 and α–O–4 bonds,
respectively (Scheme 1) [26–38]. DPE mimics the most resistant etheric
lignin linkages (bond dissociation energy of about 314 kJ/mol), PPE is
the most abundant etheric substructure of lignin (bond dissociation
energy of about 289 kJ/mol) while BPE represents the typical
lignin-carbohydrate bond (bond dissociation energy of about 218
kJ/mol) that needs to be broken in order to extract the lignin from the
lignocellulosic matrix.
While several important contributions are based on the hydro-
genolysis of DPE, PPE and BPE under classic hydrogenolysis and transfer
hydrogenolysis conditions in aqueous or alcoholic solvents over mono-
metallic and bimetallic heterogeneous catalysts, very few contributions
–
are present in the literature on the C O bond breaking of lignin model
molecules under reductive catalytic fractionalization (lignin-first) con-
ditions [39–41]. In this context, it is worth to underline that the presence
of water as co-solvent in RCF reactions can deeply influence the catalytic
process playing a crucial role in (i) lowering the hydrogen solubility in
reaction media, (ii) promoting the hydrolysis reaction and (iii)
competing with the aromatic ethers adsorption occurrence on catalysts
active sites [42,43].
When water was used as reaction (co-)solvent, products were
extracted with ethyl acetate (eventual water traces were removed by
adding anhydrous sodium sulfate to the organic phase).
In this contribution we aim to investigate the molecular catalysis
occurring in the lignin-first upgrading of lignocellulosic biomasses, by
Tetradecane was used as external standard to determine the liquid
product concentration and carbon balance (better than 95 ± 3% for all
reported experiments). When necessary, cross-check GC/MS analyses
were carried out using a Thermo Fisher gas chromatograph apparatus
(Trace 1310) equipped with a single-quadrupole mass spectrometer (ISQ
LT).
–
using commercially available Ru/C and Pd/C catalysts for the C
O
bond cleavage of three lignin model aromatic ethers (DPE, PPE and
BPE). A detailed study on the H-donor source as well as on the effect of
the water content in water/organic mixture solvent media on the con-
version of aromatic ethers and in driving the products selectivity is also
presented.
The conversion (%), product selectivity (%), product yield (%) and
aromatic yield (%) in the liquid phase were calculated on the basis of the
following equations:
2. Experimental section
mol of reacted substrate
mol of substrate feed
Conversion [%] =
× 100
(1)
2.1. Investigated catalysts
mol of specific product in liquid phase
sum of mol of all product in liquid phase
Liquid phase selectivity [%] =
All chemicals were purchased from commercial source and used
without further purification. Ru/C (with nominal ruthenium loading of
5 wt%) and Pd/C (with nominal palladium loading of 5 wt%) catalysts
were acquired from Sigma-Aldrich (Merck Life Science S.r.l.) and used
after H2 flow (1 mL/min) reduction (Ru/C: 350 ◦C for 3 h; Pd/C: 200 ◦C
for 2 h). A complete physico-chemical characterization of commercial
Ru/C and Pd/C catalysts was reported in the last years by some of the
authors and from other research groups [44–46].
× 100
(2)
(3)
[
]
mol of specific product
Product Yield
%
=
× 100
mol of substract feed
mol of all aromatic products
mol of subtrate feed
Aromatic Yield [%] =
× 100
(4)
2.2. Catalytic tests
In reactions carried out by using diphenyl ether as starting substrate,
the hydrogenolysis, hydrolysis and hydrogenation selectivity [%] was
calculated according to Ref. [38]. Reaction pathways selectivities [%]
Hydrogenolysis reactions were carried out in a 100 mL stainless steel
autoclave (Parr Instruments Co.) at a stirring speed of 500 rpm. In a
Scheme 1. Simplest aromatic ethers generally adopted in order to mimic α–O–4, β–O–4, 4–O–5 lignin linkages [22–25].
2

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
were calculated on the basis of the following equations:
Route (A + B) [%] = 2 × (benezene + cyclohexane)
Route C [%] = 100 – Route (A + B)
210 ◦C (95 %) can be obtained by using 2-PrOH as solvent/H-donor in
the presence of Ru/C as catalyst. Benzene and cyclohexanol were ob-
tained as main reaction products (selectivity of 37 % and 40 %,
respectively) with an overall aromatic yield of 42 %. A very low con-
version (< 2%) was registered with ethanol and methanol. This result is
not surprising considering the lower tendency of primary alcohols to
undergo a dehydrogenation process with respect to secondary alcohols
in releasing hydrogen species necessary for CTH reactions as a conse-
quence of their higher reduction potential as well as their smaller
electron-releasing inductive effect [35]. Together with this, Vlacos and
co-workers recently demonstrated that the hydrogenolysis of furfural is
also correlated with solvent properties, such as polarity [49].
Accordingly, higher DPE conversions can be easily achieved in the
presence of molecular hydrogen as reducing agent for all investigated
reaction solvents. By using Ru/C, the DPE conversion increases in the
following order: methanol (85 %), ethanol (95 %), 2-PrOH (100 %) in
agreement with a report on the solvent effects on the hydrogenolysis of
diphenyl ether promoted by Raney-Nickel catalyst [41]. However, under
these reaction conditions, Ru/C catalyzes both hydrogenolysis of the
ether bond as well as the hydrogenation of the aromatic ring with CHO
and CXO being obtained in high yield.
(5)
(6)
3. Results and discussion
3.1. Reaction pathways in the reductive upgrading of diphenyl ether under
“lignin-first” conditions
Diphenyl Ether (DPE) can be considered the simplest lignin model
–
molecule, therefore the cleavage of the etheric C O bond in DPE is
largely investigated as model reaction. In principle, under lignin-first
–
conditions, the C O bond cleavage of DPE may occur via three
possible reaction routes (Scheme 2):
-
route A (hydrogenolysis-hydrogenation pathway): the direct
–
cleavage of the etheric C O bond produces benzene and phenol that can
be further hydrogenated into cyclohexane and cyclohexanol;
- route B (hydrogenation-hydrogenolysis pathway): the hydrogena-
tion of DPE allows the production of cyclohexyl phenyl ether (rarely the
full hydrogenation into dicyclohexyl ether was observed [32,33]) that,
in a successive hydrogenolysis reaction, is converted into phenol and
cyclohexane;
The commercial Pd/C generally exhibits a lower activity under both
hydrogenolysis and CTH conditions if compared with the Ru/C catalyst
(Table 1 and Fig. 1). Indeed, in absence of added hydrogen, a discrete
DPE conversion was registered only in 2-PrOH (45 %) while, by using
EtOH and MeOH, as H-donor/reaction solvents, Pd/C is almost inactive.
The modest performance of the commercial Pd/C catalyst in CTH re-
actions was already reported and attributed to its poor ability in dehy-
drogenating primary and secondary alcoholic H-donor-solvents with
respect to other bimetallic Pd-based systems [37,50]. A better catalytic
performance can be obtained under “classic” hydrogenolysis conditions
with 2-propanol showing again an excellent performance in terms of
DPE conversion (100 %) together with a poor selectivity towards aro-
matic compounds. Interestingly, in the presence of molecular hydrogen,
the formation of dicyclohexyl ether (DCE) - rarely reported in reactions
carried out under similar reaction conditions adopted in this paper -
among products was registered.
- route C (reductive hydrolysis-hydrogenation pathway): DPE is
partially hydrogenated into cyclohex-1-enyl phenyl ether (CHP) that, in
presence of water, hydrolyzed rapidly forming equimolar amounts of
cyclohexanone and phenol.
Route A and B are expected to occur both in water (hydrogenolysis
conditions in presence of molecular H2) as well as in organic H-donor
solvents (CTH conditions) [47,48]. In the case of the
hydrogenation-hydrogenolysis pathway (Route B), it is worth to un-
derline that, once DPE undergoes the hydrogenation of the aromatic ring
–
without any C O bond cleavage (with the concurrent formation of
cyclohexyl phenyl ether), the hydrogenolysis of the benzylic C(sp3)–O
bond is generally favored with respect to the aryl C(sp2)–O bond due to
the minor dissociation energy involved. Obviously, route C can take
place only when reactions are carried out in water although it was
recently demonstrated that the presence of H2 is a key prerequisite for
the hydrolysis proceeding since H2O cannot directly attack the etheric
It is worth to underline that the products distribution registered in
reaction tests carried out in 2-PrOH in the absence of added hydrogen
(BEN: 37 %, CHO: 12 %, PHE: 7%; CXO: 40 %, CPE: 4%) and, in
particular, the high selectivity in benzene, suggest that the CTH of DPE
mainly occurs through the primarily hydrogenolytic cleavage of the
etheric 4–O–5 bond (Route A). At the same time, the detection of
cyclohexyl phenyl ether, in liquid phase products, implies the simulta-
neous presence, in less extent, of the hydrogenation-hydrogenolysis
pathway (Route B).
–
C
O bond of DPE [38].
3.2. Hydrogenolysis and transfer hydrogenolysis of diphenyl ether (DPE)
In order to gain a rapid catalysts screening and a preliminary insight
–
O
on the solvent effect, the ability of Pd/C and Ru/C systems in the C
Whereas, the higher percentage of cyclohexyl phenyl ether (CPE) in
reactions performed in presence of molecular hydrogen in EtOH and
–
bond cleavage in pure alcoholic solvents (2-PrOH, EtOH and MeOH) was
explored both in the presence (hydrogenolysis conditions) and in the
absence (transfer hydrogenolysis conditions) of added hydrogen
(Table 1 and Fig. 1).
MeOH is clearly related to differences in the elementary steps of C
O
bond cleavage implying that the different availability of hydrogen on the
catalyst surface drives the hydrogenolysis reaction towards Route B.
After 3 h of reaction, under CTH conditions, a high DPE conversion at
Scheme 2. Main reaction pathways in the reductive upgrading of diphenyl ether under lignin-first conditions.
3

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
Table 1
Hydrogenolysis of diphenyl ether promoted by Ru/C and Pd/C catalysts by using aliphatic C1-C3 alcohols (2-propanol, ethanol and methanol) or molecular hydrogen
as H-source.
Conversion
Liquid Product Selectivity [%]
Catalyst
Solvent
H-source
[%]
BEN
CHO
PHE
CXO
CON
CPE
DCE
Ru/C
Pd/C
Ru/C
Pd/C
Ru/C
Pd/C
Ru/C
Pd/C
Ru/C
Pd/C
Ru/C
Pd/C
2-PrOH
2-PrOH
2-PrOH
2-PrOH
EtOH
H2
100
100
95
–
50
42
12
21
36
12
–
–
50
52
40
22
37
38
–
–
–
–
7
1
8
–
–
4
–
–
–
–
–
2
–
2
H2
4
–
–
2-PrOH
2-PrOH
H2
37
19
5
7
13
3
2
–
4
45
16
18
11
–
95
EtOH
H2
85
27
–
2
–
–
–
–
–
–
EtOH
EtOH
EtOH
H2
< 2
< 2
90
EtOH
–
–
–
–
–
MeOH
MeOH
MeOH
MeOH
7
37
3
7
9
–
30
4
15
76
–
H2
15
8
MeOH
MeOH
< 2
< 2
–
–
–
–
–
–
–
–
BEN: benzene; CHO: cyclohexane; PHE: phenol; CXO: cyclohexanol; CON: cyclohexanone; CPE: cyclohexyl phenyl ether; DCE: dicyclohexyl ether (reaction conditions:
40 mL of DPE 0.1 M; 0.100 g of catalyst; initial H2 or N2 pressure: 10 bar; reaction time: 3 h; reaction Temperature: 210 ◦C; speed stirring: 500 rpm.).
Fig. 1. CTH and hydrogenolysis of diphenyl ether promoted by Ru/C and Pd/C catalysts in pure alcoholic H-donor solvents (reaction conditions: 40 mL of DPE 0.1 M;
0.100 g of catalyst; initial H2 or N2 pressure: 10 bar; reaction time: 3 h; reaction Temperature: 210 ◦C; speed stirring: 500 rpm.).
Therefore, the catalytic results clearly show that it is the availability of
hydrogen that determines the route selectivity in the cleavage of etheric
The DPE conversion was completed after 6 h of reaction. As the re-
action proceeds, benzene and phenol yields progressively increase on
increasing reaction times, reaching the highest value at 180 min. Then,
they progressively decrease because of further hydrogenation leading to
cyclohexane and cyclohexanol respectively.
–
C
O bond [51].
3.3. Reaction time effect in the transfer hydrogenolysis reaction of DPE
and the consequent kinetic model that can be derived
A simplified kinetic model of the CTH of DPE in the presence of 2-
PrOH as solvent/H-donor was also developed (Scheme 3) assuming
that all possible reactions of Route A and Route B are all of pseudo first
The preference of the hydrogenolysis-hydrogenation route in the
CTH reaction was confirmed by monitoring yield and distribution of
reaction products as a function of time at 210 ◦C (Fig. 2).
Fig. 2. Product yield in the conversion of DPE on using the Ru/C catalyst under
CTH conditions at 210 ◦C against time (reaction conditions: 0.100 g of catalyst;
40 mL of DPE 0.1 M; N2 pressure: 10 bar; stirring: 500 rpm).
Scheme 3. Products and reaction kinetics constants in CTH of the DPE.
4

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
order and irreversible (see supporting information for all details).
The following assumptions were considered:
The initial concentration of the DPE is given by the addition of
concentrations of all the reaction products and the unreacted DPE.
However, the experimental evidence indicates that contributions of the
cyclohexyl phenyl ether (CPE), dicyclohexyl ether (DCE) and of other
secondary products, can be reasonably neglected as their concentration
are very small compared to the other products, throughout all reaction
time. Therefore, Eq. 7.3 can be simplified as:
i. the kinetics of adsorption/desorption follow a Langmuir isotherm
model;
ii. the generation of H ∙ atoms from 2-propanol adsorption is inde-
pendent on its concentration according to the equation:
Kads,2ꢀ PrOH[2 ꢀ PrOH]
v = k2ꢀ PrOHϑ2ꢀ PrOH = k2ꢀ PrOH
[DPE]0 ≅ [DPE] + [PHE] + [BNZ] + [COH] + [CHX]
(7.3)
1 + Kads,2ꢀ PrOH[2 ꢀ PrOH]
k2ꢀ PrOH for very high [2 ꢀ PrOH]
≅
Thus, products deriving from DPE transformation are phenol, ben-
zene, cyclohexane and cyclohexanol. Fig. S2 shows the variation of the
products concentration normalized with respect to the initial concen-
tration of DPE. The first two products, phenol and benzene, show the
typical trend of reaction intermediates, whose concentration, described
by equations 7.4 and 7.5 (see Supporting Information for the description
of the kinetic model), increases rapidly over time until reaching a
maximum, then decreases reaching an almost constant value, tending to
zero very slowly (fig S2 black and red lines).
k
2ꢀ PrOH is the kinetic constant of 2-PrOH conversion at the surface of the
catalyst;
ϑ2ꢀ PrOH is the fraction of surface sites of the catalyst occupied by the
2-PrOH;
Kads,2ꢀ PrOH is the thermodynamic adsorption constant for 2-PrOH.
iii. the kinetics of DPE conversion upon adsorption is:
d
dt
v =
ꢀ
[DPE] = kD’ PEϑISOϑDPE ≅ kDPEϑDPE
ꢀ
)
k1[DPE]
’
1t
2
[PHE] =
0∙ eꢀ k ꢀ eꢀ k t
(7.4)
k2’ ꢀ k1
Where,
kDPE = kD’ PEϑ2ꢀ PrOH
;
[
]
k1∙[DPE]
k2’’ ꢀ k1
2’’t
1
[BNZ] =
0∙ eꢀ k t ꢀ eꢀ k
(7.5)
k’DPE is the kinetic constant of DPE at the surface of the catalyst;
ϑDPE is the fraction of surface sites of the catalyst occupied by the
DPE;
where fitting values are k1
=
0.007 ± 0.001 minꢀ 1
,
k2’
=
0.019 ± 0.001 minꢀ 1, and k2’’ = 0.006 ± 0.001 minꢀ 1
.
The kinetic of benzene is a little bit slower than that of phenol (after
about 200 min from the beginning of the reaction with respect to 50 min
for phenol). Cyclohexane and cyclohexanol, as final products, show a
monotonous growing trend over time approaching a plateau value after
a certain time. The initial rate of cyclohexanol production is faster than
that of cyclohexane and the plateau value is reached early with respect
to cyclohexane (after about 400 min). The integrated kinetic equations,
describing the concentration of the above products, are:
KDPE[DPE]
ϑDPE
=
1 + KDPE[DPE]
KDPE is the thermodynamic adsorption constant for DPE.
Then,
kDPEKDPE[DPE]
v =
1 + KDPE[DPE]
k’2’∙[DPE]
ꢀ
)
’’
2
KDPE[DPE] ≪1
’’
’’
TOTt
[CHX] =
0∙ k2 ꢀ k1 + k1∙eꢀ K t ꢀ k2 ∙eꢀ K
(7.6)
k’2’ ꢀ KTOT
d
dt
{
ꢀ
[DPE] ≅ kDPEKDPE[DPE]
ꢀ
)
[
k1[DPE]
k1
’2t
1t
[COH] = [DPE]0 1 ꢀ eꢀ K t ꢀ
0∙ eꢀ k t ꢀ eꢀ k
ꢀ
eꢀ k
TOT
1
k’2 ꢀ k1
k’2’ ꢀ k1
a complex combination of kinetic constants of adsorption, desorption,
hydrogenation/hydrogenolysis is given.
(
) }
k’2’∙[DPE]0
]
ꢀ k’2’
t
ꢀ K’2’
TOTt
’’
ꢀ e
ꢀ
∙
k2’’ ꢀ k1 + k1∙e t ꢀ k2 ∙eꢀ K
k’2’ ꢀ KTOT
The variation of the DPE concentration as a function of time is ob-
tained by taking into account the reaction kinetics of DPE hydro-
genolysis and hydrogenation, including the related kinetic constants k1
and k3, respectively:
(7.7)
3.4. Temperature effect and catalyst stability tests in the transfer
hydrogenolysis reaction of DPE promoted by the Ru/C catalyst
d
dt
ꢀ
[DPE] = k1[DPE] + k3[DPE]
(7.0)
On the basis of the obtained results, the Ru/C emerges as a more
–
By integration, it is possible to obtain the concentration of the DPE as:
efficient catalyst than Pd/C for the C O bond breaking of diphenyl
ether in alcoholic solvents together with a less pronounced tendency
towards the hydrogenation of the aromatic ring. Therefore, for all other
experiments, Ru/C was chosen as the heterogeneous catalytic system to
unravelling the reductive upgrading of biomass model aromatic ethers
under CTH and lignin-first conditions.
TOTt
[DPE] = [DPE]0∙eꢀ K
(7.1)
where [DPE]0, is the initial concentration and KTOT, is given by k1+k3.
Figure S1 shows the plot of the [DPE]/[DPE]0, as a function of the
reaction time. Experimental points were fitted by using eq. 7.1,
describing the exponential decrease of the DPE concentration over time,
using the kinetic constant KTOT = 0.031 ± 0.003 minꢀ 1 (R2> 0.97) (Tab.
S1).
Under CTH conditions, the reaction temperature was varied in the
range of 120–240 ◦C maintaining constant the reaction time of 180 min
(Fig. 3 and Table S2). The DPE conversion increases almost linearly from
27, 45, 60–95% (120, 150, 180–210 ◦C, respectively) on increasing the
reaction temperature, reaching the total conversion at 240 ◦C. The
selectivity, on the other hand, did not change significantly from 120 to
180 ◦C with an increase of the production of cyclohexanol and cyclo-
hexane at higher reaction temperatures.
At any time of the reaction, the following stoichiometric condition
must be satisfied (eq. 7.2):
[DPE]0 = [DPE] + [PHE] + [BNZ] + [COH] + [CHX] + [CPE] + [DCE]
+ [OP]
In any case, the highest yield of aromatic compounds was registered
at 210 ◦C. In view of the above results, two key points can be stated: (i)
2-PrOH is a strong hydrogen donor also at low reaction temperatures
(7.2)
5

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
Fig. 3. Effect of reaction temperature on the DPE conversion and product selectivity with the Ru/C catalyst under transfer hydrogenolysis conditions at 210 ◦C
(reaction conditions: 0.100 g of catalyst; 40 mL of DPE 0.1 M in 2-propanol; N2 pressure: 10 bar; reaction time: 3 h; stirring: 500 rpm).
and (ii) the presence of cyclohexane and cyclohexyl phenyl ether as
reaction products, in the temperature range investigated (even at 120
◦C), confirms the co-presence of the hydrogenation-hydrogenolysis
pathway (Route B) in the DPE conversion under CTH conditions.
Finally, Ru/C was also tested in consecutive recycling reactions to
gather information on the catalyst reusability under the reaction con-
dition adopted (210 ◦C for 3 h). Ru/C maintains its activity after five
consecutive runs and no significant changes in DPE conversion was
observed highlighting the good stability of the catalyst (Fig. 4 and
Table S3). Interestingly, a lower tendency in hydrogenating phenol and
benzene is observed during recycling tests.
composition in terms of aromatic compounds production.
Remarkably, the yield and distribution of phenolic compounds
(phenol and cyclohexanol) is deeply influenced by the presence of water
in reaction media. Indeed, the hydrolysis selectivity [%] increases from
14 % to 52 % upon using 2-PrOH/H2O at 75/25 and 90/10 % as solvent
mixture, respectively [38]. At higher water concentrations, the amount
of phenol and cyclohexanol increases clearly indicating the occurrence
of the reductive hydrolysis reaction (Route C).
To verify that the formation of cyclohexanol arises mainly from the
reductive hydrolysis route of DPE rather than from the hydrogenation +
hydrolysis of CPE, cyclohexyl phenyl ether was subjected to the same
“lignin-first” reaction conditions (0.100 g of catalyst; 40 mL H2O/2-
PrOH at 75/25 (v/v) solution of CPE 0.1 M; temperature: 210 ◦C; time:
180 min; initial N2 pressure: 10 bar; stirring: 500 rpm). A very low
conversions of CPE (5%) into CXO (100 % selectivity) was obtained
confirming that, under the reaction conditions employed, this reaction
do not contribute significantly to the observed reductive hydrolysis of
DPE. The preference for the hydrolysis route over the Ru/C catalyst in
water was confirmed by performing an analogous reaction in the pres-
ence of molecular H2 (10 bar) where a 100 % DPE conversion with an
impressive selectivity of 65 % to cyclohexanol was registered (Table 2).
Accordingly, by using H2 as H-donor, a high DPE conversion can be
achieved also in ethanol/water (100 %) and methanol/water (85 %)
–
3.5. C O bond cleavage of diphenyl ether under lignin-first conditions
With the optimized reaction conditions in hands, we then investi-
gated the catalytic conversion of DPE under lignin-first conditions by
using water as co-solvent in the reaction media (Fig. 5 and Table S4).
No DPE conversion was observed in the reaction carried out in the
presence of sole water as solvent as a consequence of the absence of any
–
H-source in the reaction media. The cleavage of the etheric C O bond
increases by increasing the amount of 2-propanol with a complete DPE
conversion already reached with a composition of 2-propanol/water
ratio of 25/75 (v/v), being the same solvent mixture the best
Fig. 4. Recycling tests for Ru/C catalyst in the CTH of DPE (reaction conditions: 0.100 g of catalyst; 40 mL solution of DPE 0.1 M in 2-propanol; temperature: 210 ◦C;
time: 180 min; initial N2 pressure: 10 bar; stirring: 500 rpm).
6

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
Fig. 5. Reductive upgrading of DPE under lignin-first conditions promoted by Ru/C catalyst (reaction conditions: 0.100 g of catalyst; 40 mL solution of DPE 0.1 M;
temperature: 210 ◦C; time: 180 min; initial N2 pressure: 10 bar; stirring: 500 rpm).
Table 2
Reductive upgrading of DPE promoted by Ru/C catalysts under lignin-first conditions (reaction conditions: 0.100 g of catalyst; 40 mL solution of DPE 0.1 M; tem-
perature: 210 ◦C; time: 180 min; initial H2 or N2 pressure: 10 bar; stirring: 500 rpm).
Liquid Product Selectivity [%]
Conversion
[%]
Solvent
H-source
BEN
CHO
PHE
CXO
CON
CPE
DCE
H2O/2-PrOH (75/25)
H2O/2-PrOH (75/25)
H2O/EtOH (75/25)
H2O/EtOH (75/25)
H2O/MeOH (75/25)
H2O/MeOH (75/25)
H2
100
100
100
< 2
85
–
35
6
–
65
40
56
–
60
–
–
–
1
4
–
3
–
–
2-PrOH
H2
24
4
–
28
7
–
1
–
25
–
4
–
10
–
EtOH
H2
–
–
–
–
–
27
–
–
–
MeOH
< 2
BEN: benzene; CHO: cyclohexane; PHE: phenol; CXO: cyclohexanol; CON: cyclohexanone; CPE: cyclohexyl phenyl ether; DCE: dicyclohexyl ether.
Fig. 6. Reductive upgrading of BPE and PPE under lignin-first conditions in absence of added hydrogen promoted by Ru/C catalyst (reaction conditions: 0.100 g of
catalyst; 40 mL solution of DPE 0.1 M; temperature: 210 ◦C; time: 180 min; initial N2 pressure: 10 bar; stirring: 500 rpm).
7

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
media. Also in these cases, a concentration increase of phenol-deriving
compounds is registered. Noteworthy, diphenyl ether can be fully hy-
drogenated also into dicyclohexyl ether (10 % selectivity) in the pres-
ence of methanol/water (25/75) as solvent and molecular hydrogen as
H-donor source. As expected, analogous reactions performed in the
absence of molecular hydrogen provided a conversion of DPE < 2% both
in ethanol/water and in methanol/water, whereas, in presence of H2 (10
bar) the relative conversions were 100 % and 85 %, respectively.
of phenol to undergo a hydrogenation process when toluene or ethyl
benzene are present in the reaction medium.
Funding
`
This publication was supported by Universita degli studi Medi-
terranea di Reggio Calabria and by PRIN 2017 (Project: “Direct utili-
zation of bio-fuels in solid oxide fuel cells for sustainable and
decentralised production of electric power and heat - DIRECTBIO-
POWER, prot. 2017FCFYHK) funded by the Italian Ministry of Educa-
tion, University and Research (MIUR).
3.6. Reductive upgrading of benzyl-phenyl ether (BPE) and phenethyl
phenyl ether (PPE) under lignin-first conditions
Finally, in order to extend the substrate scope, the hydrogenolysis of
benzyl-phenyl ether (BPE) and phenethyl phenyl ether (PPE) was
investigated in absence of added hydrogen (Fig. 6).
CRediT authorship contribution statement
E. Paone: Conceptualization, Supervision, Methodology, Investiga-
tion, Writing - review & editing. A. Beneduci: Conceptualization,
Methodology, Investigation. G.A. Corrente: Methodology, Investiga-
tion. A. Malara: Methodology, Investigation. F. Mauriello: Conceptu-
alization, Supervision, Writing - review & editing, Funding acquisition.
A higher BPE conversion, with respect to DPE, can be easily obtained
in the presence of 2-propanol as H-donor because of the lower dissoci-
–
ation energies of the C O bonds involved.
Most important, experimental data demonstrate that, in contrast
with DPE, the hydrogenolysis of BPE and PPE has a higher selectivity
towards aromatic products. This indicates that the hydrogenolysis of the
Declaration of Competing Interest
–
etheric C O bond in BPE and PPE generally precedes the hydrogenation
(Route A). The aromatic ring hydrogenation is very little pronounced in
agreement with a recent report where it was demonstrated that the
hydrogenation process of phenol – under CTH conditions - is very sen-
sitive to the co-presence of other aromatic species on the reaction system
with a decrease of the aromatic ring saturation that follows the order
phenol + benzene > phenol + toluene > phenol + ethylbenzene [52].
Surprisingly, reactions carried out in the presence of water as co-
solvent provide an almost equimolar mixture of phenol + toluene (in
the case of BPE) and phenol + ethyl benzene (in the case of PPE) de-
rivatives that can be surely attributed to the unique presence of
hydrogenolysis-hydrogenation pathway (Route A). The suppression of
the reductive-hydrolysis route could stem from the higher steric hin-
drance of both BPE and PPE with respect to DPE that lowers the
adsorption of their aromatic rings onto the Ru metal surface thus
limiting the hydrogenation process. Thus, the lower tendency to un-
dergo a hydrogenation process - in absence of a readily available H-
donor source - is more pronounced for phenethyl phenyl ether that
shows a lower conversion with respect to DPE and BPE.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
Authors gratefully thank Prof. Rosario Pietropaolo for fruitful dis-
cussions and Dr. Alessio Paciello for his help in the catalytic experiments
related to the CTH of DPE. The networking contribution by the COST
Action CA17128 “Establishment of a Pan-European Network on the
Sustainable Valorisation of Lignin” is acknowledged. Authors sincerely
thank reviewers for having provided important suggestions aimed to
improve the scientific quality of the manuscript.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111228.
4. Conclusions
References
The catalytic conversion of diphenyl ether (DPE), phenethyl phenyl
ether (PPE) and benzyl-phenyl ether (BPE) has been investigated in the
presence of Ru/C and Pd/C under lignin-first conditions by using C1-C3
alcoholic H-donor/solvents (methanol, ethanol and 2-propanol), used as
such or in mixture with water both in the presence and in the absence of
molecular H2.
[1] M. Besson, P. Gallezot, C. Pinel, Conversion of biomass into chemicals over metal
catalysts, Chem. Rev. 114 (2014) 1827–1870.
´
[2] C. Xu, E. Paone, D. Rodríguez-Padron, R. Luque, F. Mauriello, Recent catalytic
routes for the preparation and the upgrading of biomass derived furfural and 5-
hydroxymethylfurfural, Chem. Soc. Rev. 49 (2020) 4273–4306.
´
[3] C. Xu, E. Paone, D. Rodríguez-Padron, R. Luque, F. Mauriello, Reductive catalytic
routes towards sustainable production of hydrogen, fuels and chemicals from
biomass derived polyols, Renew. Sust. Energ. Rev. 127 (2020), 109852.
[4] A. Fasolini, R. Cucciniello, E. Paone, F. Mauriello, T. Tabanelli, A short overview on
the hydrogen production via aqueous phase reforming (APR) of cellulose, C6-C5
sugars and polyols, Catalyst 9 (2019) 917.
Ru/C was found to be a better catalytic system, with respect to Pd/C,
in the production of aromatic compounds from lignin model diphenyl
ether (DPE) that was studied as model molecule to investigate the
complex hydrogenolysis-hydrogenation-hydrolysis reactions that occur
[5] Z. Sun, B. Fridrich, A. de Santi, S. Elangovan, K. Barta, Bright side of lignin
depolymerization: toward new platform chemicals, Chem. Rev. 118 (2018)
614–678.
–
in the cleavage of the etheric 4–O–5 C O bond.
–
The direct C O bond breaking (hydrogenolysis-hydrogenation
[6] C. Xu, R.A.D. Arancon, J. Labidi, Luque, Lignin depolymerisation strategies:
towards valuable chemicals and fuels, Chem. Soc. Rev. 43 (2014) 7485–7500.
[7] E. Paone, T. Tabanelli, F. Mauriello, The rise of lignin biorefinery, Curr. Opin.
Green Sustain. Chem. 24 (2020) 1–6.
route) is preferred in the presence of 2-propanol (that provides best
results both in terms of DPE conversion as well as in the production of
aromatic compounds), whereas the hydrogenation-hydrogenolysis
pathway is favored by using molecular hydrogen. At the same time, by
adding water as co-solvent, the impact of the reductive hydrolysis-
hydrogenation route becomes more significant allowing the preferen-
tial formation of hydrogenated phenolic compounds.
´
[8] T.I. Koranyi, B. Fridrich, A. Pineda, K. Barta, Development of ‘Lignin-First’
approaches for the valorization of lignocellulosic biomass, Molecules 25 (2020)
2815.
[9] X. Liu, S. Feng, Q. Fang, Z. Jiang, C. Hu, Reductive catalytic fractionation of lignin
in birch sawdust to monophenolic compounds with high selectivity, Mol. Catal.
495 (2020), 111164.
[10] P. Frontera, A. Macario, A. Malara, S. Santangelo, C. Triolo, F. Crea, P.
L. Antonucci, Trimetallic Ni-based catalysts over gadolinia-doped ceria for green
fuel production, Catalysts 8 (10) (2018) 435.
Noteworthy, a very high selectivity to the hydrogenolysis was
observed in the case BPE and PPE as a consequence of both their higher
steric hindrance, with respect to DPE, that limits the adsorption of their
aromatic rings onto the Ru metal surface as well as to the lower tendency
[11] P. Frontera, A. Macario, A. Malara, V. Antonucci, V. Modafferi, P.L. Antonucci,
Simultaneous methanation of carbon oxides on nickel-iron catalysts supported on
8

E. Paone et al.
Molecular Catalysis 497 (2020) 111228
ceria-doped gadolinia, Catal. Today (2019), https://doi.org/10.1016/j.
[31] X. Chen, X. Chen, J. Qi, C. Liang, Self-assembly synthesis of lamellar molybdenum
carbides with controllable phases for hydrodeoxygenation of diphenyl ether, Mol.
Catal. 492 (2020), 110972.
cattod.2019.05.065.
[12] N. Chen, Z. Zhu, H. Ma, W. Liao, H. Lü, Catalytic upgrading of biomass-derived 5-
hydroxymethylfurfural to biofuel 2,5-dimethylfuran over Beta zeolite supported
non-noble Co catalyst, Mol. Catal. 486 (2020), 110882.
[32] J. He, C. Zhao, J.A. Lercher, Ni-catalyzed cleavage of aryl ethers in the aqueous
phase, J. Am. Chem. Soc. 134 (2012) 20768–20775.
[13] Y. Liao, S.-F. Koelewijn1, G. Van den Bossche, J. Van Aelst, S. Van den Bosch,
T. Renders, K. Navare, T. Nicolaï, K. Van Aelst, M. Maesen, H. Matsushima, J.
M. Thevelein, K. Van Acker, B. Lagrain, D. Verboekend, B.F. Sels, A sustainable
wood biorefinery for low–carbon footprint chemicals production, Science 367
(2020) 1385–1390.
[33] J. He, L. Lu, C. Zhao, D. Mei, J.A. Lercher, J. A, Mechanisms of catalytic cleavage of
benzyl phenyl ether in aqueous and apolar phases, J. Catal. 311 (2014) 41–51.
[34] J. He, L. Lu, C. Zhao, D. Mei, J.A. Lercher, Mechanisms of selective cleavage of
Cꢀ O bonds in di-aryl ethers in aqueous phase, J. Catal. 309 (2014) 280–290.
[35] E. Paone, C. Espro, R. Pietropaolo, F. Mauriello, Selective arene production from
transfer hydrogenolysis of benzyl phenyl ether promoted by a co-precipitated Pd/
Fe3O4 catalyst, Catal. Sci. Technol. 6 (2016) 7937–7941.
´
[14] L. Shuaim, M.T. Amiri, Y.M. Questell-Santiago, F. Heroguel, Y. Li, H. Kim,
R. Meilan, C. Chapple, J. Ralph, J.S. Luterbacher, Formaldehyde stabilization
facilitates lignin monomer production during biomass depolymerization, Science
354 (2016) 329–333.
[36] A. Malara, E. Paone, L. Bonaccorsi, F. Mauriello, A. Macario, P. Frontera, Pd/Fe3O4
nanofibers for the catalytic conversion of lignin-derived benzyl phenyl ether under
transfer hydrogenolysis conditions, Catalysts 10 (2020) 20.
[15] A. Yamaguchi, T. Watanabe, K. Saito, S. Kuwano, Y. Murakamia, N. Mimura,
O. Sato, Direct conversion of lignocellulosic biomass into aromatic monomers over
supported metal catalysts in supercritical water, Mol. Catal. 477 (2019), 110557.
[16] M. Verziu, A. Tirsoaga, B. Cojocaru, C. Bucur, B. Tudora, A. Richelf, M. Aguedo,
A. Samikannu, J.P. Mikkola, Hydrogenolysis of lignin over Ru-based catalysts: the
role of the ruthenium in a lignin fragmentation process, Mol. Catal. 450 (2018)
65–76.
[37] F. Mauriello, H. Ariga-Miwa, E. Paone, R. Pietropaolo, S. Takakusagi, K. Asakura,
Transfer hydrogenolysis of aromatic ethers promoted by the bimetallic Pd/Co
catalyst, Catal. Today (2019), https://doi.org/10.1016/j.cattod.2019.06.071.
[38] M. Wang, H. Shi, D.M. Camaioni, Johannes A. Lercher, Palladium-catalyzed
hydrolytic cleavage of aromatic Cꢀ O bonds, Angew.Chem.Int. Ed. 56 (2017)
2110–2114.
[17] M.V. Galkin, J.S.M. Samec, Selective route to 2-propenyl aryls directly from wood
by a tandem organosolv and palladium-catalysed transfer hydrogenolysis,
ChemSusChem 7 (2014) 2154–2158.
[39] T. Renders, S. Van den Bosch, T. Vangeel, T. Ennaert, S.F. Koelewijn, G. Van den
Bossche, C.M. Courtin, W. Schutyser, B.F. Sels, Synergetic effects of alcohol/water
mixing on the catalytic reductive fractionation of poplar wood, ACS Sustain. Chem.
Eng. 4 (2016) 6894–6904.
[18] A.M. Ruppert, K. Weinberg, R. Palkovits, Hydrogenolysis Goes bio: from
carbohydrates and sugar alcohols to platform chemicals, Angew. Chem. Int. Ed 51
(2012) 2564–2601.
˙
ˇ
[40] E. Jasiukaityte-Grojzdek, M. Hus, M. Grilc, B. Likozar, Acid-catalysed
α-O-4 aryl-
ether bond cleavage in methanol/(aqueous) ethanol: understanding
[19] M.J. Gilkey, B. Xu, Heterogeneous catalytic transfer hydrogenation as an effective
pathway in biomass upgrading, ACS Catal. 6 (2016) 1420–1436.
[20] C. Espro, B. Gumina, T. Szumelda, E. Paone, F. Mauriello, Catalytic transfer
hydrogenolysis as an effective tool for the reductive upgrading of cellulose,
hemicellulose, lignin, and their derived molecules, Catalysts 8 (2018) 313.
depolymerisation of a lignin model compound during organosolv pretreatment,
Sci. Rep.10 (2020) 11037.
[41] X. Wang, R. Rinaldi, Solvent effects on the hydrogenolysis of diphenyl ether with
raney nickel and their implications for the conversion of lignin, ChemSusChem 5
(2012) 1455–1466.
́
[21] T. Tabanelli, E. Paone, P.B. Vasquez, R. Pietropaolo, F. Cavani, F. Mauriello,
[42] R.A. Rajadhyaicsh, S.L. Karwa, Solvent effects in catalytic hydrogenation, Chem.
Eng. Sci. 41 (1986) 1765–1770.
Transfer hydrogenation of methyl and ethyl levulinate promoted by a ZrO2
catalyst: comparison of batch vs continuous gas-flow conditions, ACS Sustain.
Chem. Eng. 7 (2019) 9937–9947.
¨
[43] A. Stolle, C. Schmoger, B. Ondruschka, W. Bonrath, T.F. Keller, K. Djandt, Liquid
phase hydrogenation of benzalacetophenone: effect of solvent, catalyst support,
[22] L. Yanding, S.D. Karlen, B. Demir, H. Kim, J. Luterbacher, J.A. Dumesic, S.S. Stahl,
J. Ralph, Mechanistic study of diaryl ether bond cleavage during palladium-
catalyzed lignin hydrogenolysis, ChemSusChem 13 (2020) 1–9.
catalytic metal and reaction conditions, Chinese J. Catal. 32 (2011) 1312–1322.
ˇˇ
[44] U. Petek, F. Ruiz-Zepeda, M. Bele, M. Gaberscek, Nanoparticles and single atoms in
commercial carbon-supported platinum-group metal catalysts, Catalysts 9 (2019)
134.
´
[23] M. Wang, O.Y. Gutierrez, D.M. Camaioni, J.A. Lercher, Palladium-catalyzed
reductive insertion of alcohols into aryl ether bonds, Angew. Chem. 130 (14)
(2018) 3809–3813.
[45] D. Zhao, Y. Wang, F. Delbecq, C. Len, Continuous flow conversion of alkyl
levulinates into g-valerolactone in the presence of Ru/C as catalyst, Mol. Catal. 475
(2019), 110456.
[24] Z. Huiying, D. Cao, Z. Qiu, C.-J. Li, Palladium-catalyzed formal cross-coupling of
diaryl ethers with amines: slicing the 4-O-5 linkage in lignin models, Angew. Chem.
130 (14) (2018) 3814–3819.
[46] B. Gumina, F. Mauriello, R. Pietropaolo, S. Galvagno, C. Espro, Hydrogenolysis of
sorbitol into valuable C3-C2 alcohols at low H2 pressure promoted by the
heterogeneous Pd/Fe3O4 catalyst, Mol. Catal. 446 (2018) 152–160.
[47] H. Wu, J. Song, C. Xie, C. Wu, C. Chen, B. Han, Efficient and mild transfer
hydrogenolytic cleavage of aromatic ether bonds in lignin-derived compounds over
Ru/C, ACS sustainable chem, Eng. 6 (2018) 2872–2877.
[25] B. Safak, S. Siankevich, A.P. van Muyden, D.T.L. Alexander, G. Savoglidis,
J. Zhang, V. Hatzimanikatis, N. Yan, P.J. Dyson, Efficient cleavage of aryl ether C-O
linkages by Rh-Ni and Ru-Ni nanoscale catalysts operating in water, Chem. Sci. 9
(25) (2018) 5530–5535.
[26] D. Singh, P.L. Dhepe, Understanding the influence of alumina supported ruthenium
catalysts synthesis and reaction parameters on the hydrodeoxygenation of lignin
derived monomers, Mol. Catal. 480 (2020), 110525.
[48] Z. Xue, H. Yu, J. He, Y. Zhang, X. Lan, R. Liu, L. Zhang, T. Mu, Highly efficient
cleavage of ether bonds in lignin models by transfer hydrogenolysis over dual-
functional Ruthenium/Montmorillonite, ChemSusChem 13 (2020) 1–9.
[49] P. Panagiotopoulou, N. Martin, D.G. Vlachos, Effect of hydrogen donor on liquid
phase catalytic transfer hydrogenation of furfural over a Ru/RuO2/C catalyst,
J. Mol. Catal. A Chem. 392 (2014) 223–228.
[27] X. Gao, S. Zhu, Y. Li, Selective hydrogenolysis of lignin and model compounds to
monophenols over AuPd/CeO2, Mol. Catal. 462 (2019) 69–76.
[28] W. Guan, X. Chen, C. Li, J. Zhang, C.-W. Tsang, H. Hu, S. Li, C. Liang, Nb(Ta)-based
solid acid modified Pt/CNTs catalysts for hydrodeoxygenation of lignin-derived
compounds, Mol. Catal. 467 (2019) 61–69.
[50] F. Mauriello, H. Ariga, M.G. Musolino, R. Pietropaolo, S. Takakusagi, K. Asakura,
Exploring the catalytic properties of supported palladium catalysts in the transfer
hydrogenolysis of glycerol, Appl. Catal. B 166 (2015) 121–133.
[29] J. Li, H. Sun, J.-x. Liu, J.-j. Zhang, Z.-x. Li, Y. Fu, Selective reductive cleavage of CO
bond in lignin model compounds over nitrogen-doped carbon-supported iron
catalysts, Mol. Catal. 452 (2018) 36–45.
[51] D. Cozzula, A. Vinci, F. Mauriello, R. Pietropaolo, T.E. Müller, Directing the
cleavage of ester C-O bonds by controlling the hydrogen availability on the surface
of coprecipitated Pd/Fe3O4, ChemCatChem 8 (2016) 1515–1522.
[52] F. Mauriello, E. Paone, R. Pietropaolo, A.M. Balu, R. Luque, Catalytic transfer
hydrogenolysis of lignin-derived aromatic ethers promoted by bimetallic Pd/Ni
systems, ACS Sustainable Chem. Eng. 6 (2018) 9269–9276.
[30] Y. Hu, G. Jiang, G. Xu, X. Mu, Hydrogenolysis of lignin model compounds into
aromatics with bimetallic Ru-Ni supported onto nitrogen-doped activated carbon
catalyst, Mol. Catal. 445 (2018) 316–326.
9