POLITAG-Pd(0) catalyzed continuous flow hydrogenation of lignin-derived phenolic compounds using sodium formate as a safe H-source

Article abstract of DOI:10.1016/j.mcat.2021.111613

Phenols are aromatic biobased compounds and as they are accessible from lignin depolymerization, they can be a useful platform chemicals to produce value-added products. Herein we report our recent investigations on the definition of an approach to the efficient continuous flow selective hydrogenation of phenols in water. Our protocol is based on the use of sodium formate as a clean and safe hydrogen source in combination with our newly defined heterogeneous POLITAG-Pd(0) catalytic system. POLITAG is a polymeric heterogeneous support decorated with pincer-type ionic ligands proven to be highly efficient for the stabilization of Pd(0) nanoparticles. The results obtained are remarkable in comparison with other protocols that employ sodium formate as H-source. Indeed, our investigation has been extended to a variety of differently substituted phenolic compounds that have been hydrogenated with excellent to good selectivity in continuous flow conditions. Durability of the catalyst has been also tested with a representative continuous processing of over 100 mmol that showed no loss in efficiency and minimal metal leaching.

Full text of DOI:10.1016/j.mcat.2021.111613

Molecular Catalysis 509 (2021) 111613
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
POLITAG-Pd(0) catalyzed continuous flow hydrogenation of lignin-derived
phenolic compounds using sodium formate as a safe H-source
Valeria Trombettoni, Francesco Ferlin, Federica Valentini, Filippo Campana, Matteo Silvetti,
Luigi Vaccaro*
`
Laboratory of Green S.O.C. – Dipartimento di Chimica, Biologia e Biotecnologie, Universita degli Studi di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
Phenols are aromatic biobased compounds and as they are accessible from lignin depolymerization, they can be a
useful platform chemicals to produce value-added products. Herein we report our recent investigations on the
definition of an approach to the efficient continuous flow selective hydrogenation of phenols in water. Our
protocol is based on the use of sodium formate as a clean and safe hydrogen source in combination with our
newly defined heterogeneous POLITAG-Pd(0) catalytic system. POLITAG is a polymeric heterogeneous support
decorated with pincer-type ionic ligands proven to be highly efficient for the stabilization of Pd(0) nanoparticles.
The results obtained are remarkable in comparison with other protocols that employ sodium formate as H-source.
Indeed, our investigation has been extended to a variety of differently substituted phenolic compounds that have
been hydrogenated with excellent to good selectivity in continuous flow conditions. Durability of the catalyst has
been also tested with a representative continuous processing of over 100 mmol that showed no loss in efficiency
and minimal metal leaching.
Selective phenols hydrogenation
Continuous flow
Safe hydrogen-source
Heterogeneous catalysis
Polymeric supported Pd(0)
1. Introduction
promising and safe alternative to classical molecular hydrogen [12–27].
Formic acid and formates feature a good stability and the latter, also low
The replacement of petrol-based chemicals and fuel with renewable
sources is the challenge of the modern chemistry. Biomass is a renewable
feedstock that can be used both for energy [1–4] and production of
fundamental bulk and commodity chemicals [3,4]. In the context of a
sustainable production, lignocellulosic biomass represents an ideal
carbon-neutral alternative to reduce the environmental impact associ-
ated to the chemical industry.
toxicity [28]. They are included among the liquid organic hydrogen
sources (LOHC), ideal alternatives for a safe handling and transport of
hydrogen equivalents [29–31]. Within the context of a sustainable
approach, aqueous formic acid obtained from biomass is of major in-
terest [28]. When in combination with an appropriate catalytic system,
formic acid derivatives may offer an effective technology to produce
molecular hydrogen under controlled conditions and at the same time
utilize this H-source for a concomitant hydrogenation where the cata-
lytic system may control the process efficiency and selectivity.
Among the processes involving phenols valorization, the selective
hydrogenation to cyclohexanone [22–27] is of great interest. Indeed,
cyclohexanone is a key product in the preparation of largely interesting
Nylon 6, Nylon 6,6 and polyamide resins [32]. Selective hydrogenation
of phenols to the corresponding cyclohexanones, constitutes an advan-
tage in terms of step-economy, waste disposal costs and purification
procedures, if compared to a classic two step processes comprising the
over-reduction to cyclohexanol and subsequent dehydrogenation to
cyclohexanone [33].
Among biomass-derived feedstock, phenols are aromatic compounds
which can be easily upgraded to obtain different value-added chemicals
[4]. Generally, bioderived phenols are considered as waste from lignin
treatment to access cellulose or related sugar polymers and, therefore
their usage as possible platform chemicals for further transformation to
target materials, is nowadays highly desirable [5–11]. Indeed, most of
the processes involving lignin valorization, aim to the production of
bioderived-oil with high H/C ratio in order to both improve the thermal
stability and reduce the viscosity to eventually increase miscibility with
petrol-based fuels [5–11].
Aqueous formic acid and its salt derivatives are interesting hydrogen-
sources that, also derived from biomasses, can be considered as
On the other hand a step-economical and effective strategy can be
* Corresponding author.
E-mail address: luigi.vaccaro@unipg.it (L. Vaccaro).
https://doi.org/10.1016/j.mcat.2021.111613
Received 13 April 2021; Received in revised form 27 April 2021; Accepted 29 April 2021
Available online 19 May 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
Fig. 1. Schematic representation of POLITAGs-Pd(0) synthesis and features of the current work.
defined using direct selective hydrogenation based on use of an het-
erogeneous catalytic system. This process may become very efficient if
the absorption of phenolate ion onto the catalyst and its effective
reduction can be combined with the prompt desorption of the initially
formed cyclohexanone before further hydrogenation process may lead
to the cyclohexanol formation [34]. This absorption/desorption equi-
librium strongly depends on the nature of the heterogeneous catalyst
support and therefore at this aim, different catalytic systems have been
developed and/or tested for the use with formic acid or formates
[22–27].
solution and gaseous hydrogen is generated at the heterogeneous sup-
port while at equilibrium with liquid and gas phases. In flow conditions,
hydrogen developed during the reaction is confined into the reactor
where appropriate temperature and pressure can be precisely
controlled. To the best of our knowledge, also in the cases where flow
reactors are employed in the hydrogenation of phenols with formates as
H-source, the extension of the reaction conditions to different phenolic
compounds is difficult [26] leading to unsatisfactory conversion and/or
selectivity for most of the substituted phenol derivatives.
Herein we report the preparation of a POLITAG-Pd(0) heterogeneous
catalytic system and its use in the selective hydrogenation of different
phenolic compounds under flow conditions using water as a safer re-
action medium and sodium formate as a safe H-source (Fig. 1). POLI-
TAGs [40–42] are polymer-based heterogeneous catalysts featuring a
pincer type ionic ligands that we have developed and proved to be
effective for the stabilization of both Pd(II) anionic complex [41,42] and
Pd(0) nanoparticles [40]. We have reported that these catalysts show
good performance in catalyzing cross-coupling [40,41] and C–H acti-
vation [42] reactions. Due to the ligands architecture and the stability
showed, we decided to investigate the use of these systems in the se-
lective transfer hydrogenation of phenols under flow condition using
sodium formate as H-source.
Generally, besides the reduction of simple phenol, when the condi-
tions are extended to variously substituted oxygenated phenolic com-
pounds, it is more difficult to obtain high levels of selectivity depending
on the nature and position of the substituent and also to the additional
need for longer reaction times.
Our research is mainly dedicated to the development of sustainable
protocols by exploiting flow technology [35] to enable and easily con-
trol the reaction parameters that could steer to highly efficient and se-
lective processes. Accordingly, we have been investigating the definition
of effective catalytic systems and their use in flow reactors to access the
selective hydrogenation of phenols. In fact, flow reactors may guarantee
the accurate control of the reaction conditions with an ideal interaction
between reagents and catalyst especially when they are in different
phases [35–39]. In fact, this is one of the crucial aspects in the hydro-
genation reaction using heterogeneous catalysts, where substrates are in
2

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
Fig. 2. POLITAGs-Pd(0) architectures.
2.3. General procedure for the POLITAG Pd(0)-L1 recycle study
2. Experimental section
2.1. General remarks
Phenol (1a, 0.4 mmol, 37.65 mg), sodium formate (2; 68 mg, 2.5
equiv.), and water (0.2 M, 2 mL) were placed in a 2 mL screw-capped
vial equipped with a magnetic stirrer. The mixture was adjusted to pH
12.0 by adding 5 M aq NaOH. POLITAG-Pd(0)-L1 (5 mol%) was added,
and the mixture was kept under stirring at 120 ◦C. After for 4 h the
mixture was cooled, and centrifuged (15 min, 6500 rpm). After the su-
pernatant removal, the catalyst was washed with 1 mL of water, dried
under vacuum at 80 ◦C for 3 h and reused in the next run.
All chemicals were used without any further purification unless
otherwise noted. 4-Vinylbenzylchloride (VBC) and styrene (STY) were
extracted three times with a 10% w/w NaOH solution and filtered
through Al2O3 pad to remove the polymerization inhibitor (tert‑butyl
catechol). Dibenzoylperoxide was re-crystallized from methanol. GC
analyses were performed using a Hewlett- Packard HP 5890A equipped
with a capillary column DB 35MS (30 m, 0.32 mm), a FID detector and
helium as a gas carrier. GC-EIMS analyses were carried out using a
Hewlett-Packard HP 6890 N Network GC system/5975 Mass Selective
Detector equipped with an electron impact ionizer at 70 eV. Elemental
Analysis (EA) were conducted on Elementar UNICUBE® elemental
analyzer. Palladium content was measured by using an Agilent 4210
MP-AES instrument. TEM images were obtained using a PHILIPS CM 12
transmission electron microscopy operating in the range 20 to 120 kV
with an image resolution of 0.34 nm.
2.4. General procedure for the flow hydrogenation
A premixed mixture of phenolic compound (1a-h) and sodium
formate (2, 3 equiv.) in H2O (0.1 M) was prepared in a flask with the
function of the reservoir, and the mixture was adjusted to pH 12.0 with
the addition of 5 M aq NaOH. POLITAG-Pd(0)-L1 (8 wt%, 0.54 mmol,
718.3 mg) was dispersed in solid glass beads and glass powder and was
charged in a stainless-steel reactor; the equipment was connected by
using the appropriate valves, and a backpressure regulator was placed
on the outlet tube. The packed reactor was placed into a thermostated
box, and the mixture was pumped through the catalyst columns at
120 ◦C. The conversion of 1a-i and its selectivity were periodically
monitored by GLC analysis.
2.2. General procedure for the hydrogenation in batch
Phenol (1a; 0.4 mmol, 37.65 mg), sodium formate (2, 68 mg, 2.5
equiv.), and water (0.2 M, 2 mL) were placed in a 2 mL screw-capped
vial equipped with a magnetic stirrer. The mixture was adjusted to pH
12.0 by adding 5 M aq NaOH. POLITAG-Pd(0)-L1 (5 mol%) was added,
and the mixture was kept under stirring at 120 ◦C. After for 4 h the
mixture was cooled, and the catalyst was filtered off. The product was
recovered by extraction with heptane and evaporation of the solvent
under reduced pressure.
3. Results and discussion
A preliminary optimization study was conducted in batch conditions
where we investigated the catalytic efficiency of different POLITAG–Pd
(0) systems using three different heterocyclic pincer ligands (Fig. 2). We
Table 1
Screening of POLITAGs-Pd(0) catalysts for the selective hydrogenation of phenol (1a) with sodium formate (2).
Entry
pH
Catalyst
T ( ◦C)
t (h)
Conv. (%)a
3a (%)a
1b
12
12
12
12
12
12
Pd/C
90
0.5
4
95
32
55
100
55
30
98
2
POLITAG-Pd(0)-L1
POLITAG-Pd(0)-L1
POLITAG-Pd(0)-L1
POLITAG-Pd(0)-L2
POLITAG-Pd(0)-L3
90
100
100
100
50
3
90
18
4
4
120
120
120
5
4
6
4
100
Reaction conditions: 1a (0.4 mmol), 2a (2.5 eq), catalyst (5 mol%), H2O (0.2 M). aConversion of 1a to products was determined by GLC
b
ref. [23].
3

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
efficiency both in terms of conversion (Table 1, entries 5–6) and in
selectivity (Table 1, entry 5) if compared to the POLITAG-Pd(0)-L1
system based on bis-imidazolium. Given these considerations, POLI-
TAG-Pd(0)-L1 was selected as catalytic system to further explore the
phenols hydrogenation.
To have additional insights about the process, a pH screening was
also conducted. Different solutions of phenol (1a) and sodium formate
(2) were prepared by varying pH conditions. As illustrated in Fig. 3, pH
12 confirmed to give the best results allowing a complete conversion and
selectivity toward cyclohexanone (3a). Increasing or reducing the pH of
the solution revealed, on the contrary, a clear drop of both conversion
and selectivity. When the reaction was performed at pH 7, only 50%
conversion was observed with an important decrease of the selectivity to
3a. Poor results were also obtained when pH reached the value of 14.
Anyway, although the formation of the over-hydrogenated product
cyclohexanol (4a) was not detected, the conversion did not exceed 15%.
Subsequently, the role of the base used to set the ideal pH 12 con-
ditions was considered (Fig. 4).
Fig. 3. pH screening using NaOH as base.
Only NaOH gave good results in terms of conversion and selectivity,
confirming the efficient role of sodium to assist Pd-catalyzed hydrogen
transfer while larger counter ions hamper this process [26,27]. This
cation effect is in accordance with the results obtained by the pH
screening (Fig. 3). Indeed, considering both the influence of pH and
bases, it is strongly suggested that the reaction mechanism proceed
through the absorption of phenolate ion on POLITAG support (Scheme
1). The selected pH favored the formation of the phenolate ion con-
trolling the desorption before the formation of over-hydrogenated
product, while the cation size influences its absorption close to the Pd
nanoparticles driving the hydrogen-transfer process.
With the intention to test our catalytic system in the use under
continuous flow conditions, we performed a preliminary catalyst recy-
cling study. POLITAG-Pd(0)-L1 resulted active without any loss of ef-
ficiency for four representative consecutive runs (Table 2). After each
cycle, the crude reaction mixture was centrifuged and subsequently the
supernatant removed while the catalyst washed with water, dried under
vacuum at 80 ◦C for 3 h and then reused in the consecutive run.
These results confirm the efficiency of the catalyst in batch but also
let to presume its long-term efficiency which is essential for an effective
continuous flow protocol.
Fig. 4. Screening of base at pH 12.
compared these systems searching for the most promising reaction
conditions suitable for being transferred into a continuous-flow reactor
system.
The catalytic activity of the three different POLITAGs-Pd(0) (Fig. 2)
for the selective hydrogenation of phenol (1a) using sodium formate (2)
in water as reaction medium, was tested and compared also to the results
obtained in our previous work (Table 1) [26].
While POLITAG-Pd(0)-L1, featuring a bis-imidazolium ionic tag
ligand, showed at 90 ◦C a conversion lower to that achieved with Pd/C
(Table 1, entries 2, 3 vs entry 1), it should be also noticed that a higher
selectivity was achieved, suggesting a more controlled hydrogen trans-
fer on Pd(0) nanoparticles. Indeed, by increasing the temperature to
120 ◦C complete conversion of phenol 1a and good selectivity toward
the desired product 3a could be obtained (Table 1, entry 4), making this
catalyst and condition more interesting.
The continuous flow protocol was set by packing a stainless-steel
reactor with POLITAG-Pd(0)-L1 (8% wt, 0.54 mmol, 719 mg)
dispersed over glass powder and beads. An aqueous solution of phenol
(1a) and sodium formate (2) adjusted to pH 12.0 was placed in a flask
acting as a reservoir. This was connected to a pump and the aqueous
mixture let to flow through the reactor (in a thermostated chamber at
120 ◦C) at a rate of 0.29 mLminꢀ 1 (Fig. 5).
Given that during the reaction the formation of H2 occurs, a back-
pressure regulator (BPR) was placed at the end of the catalyst column,
setting the pressure to 5 psi (1.0 psi = 6.9 kPa). Under these conditions
The catalytic tests with POLITAGs-Pd(0) with L2 and L3, 1,2,4- and
1,2,3-triazolium ligands respectively, showed
a reduced catalytic
Table 2
Recovery and reuse of the catalyst for the representative selective hydrogenation of phenol (1a) with sodium formate (2).
Run
Conv (%)a
3a (%)
1
2
3
4
100
100
100
100
>99
>99
>99
>99
Reaction conditions: 1a (0.4 mmol), 2a (2.5 eq), catalyst (5 mol%), H2O (0.2 M).
a
Conversion to products determined by GLC.
4

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
Fig. 5. Flow system used for the hydrogenation of phenols.
Table 3
Optimization for the continuous flow hydrogenation of phenol (1a).
Entry
Concentration (M)
BPR
Residence time (min)
Flow rate (mL/min)
Conv. (%)
3a (%)
1
2
3
4
5
6
7
0.2
0.2
0.2
0.1
0.1
0.1
0.1
5
40
64
64
30
45
60
75
0.29
0.18
0.18
0.38
0.26
0.19
0.15
65
100
20
5
100
100
85
92
100
5
100
5
100
100
100
100 (87)a
100
5
20
90
a
Isolated yield is giving in parentheses.
only 65% conversion of phenol (1a) was observed with good selectivity
toward the desired product 3a (Table 3, entry 1). In order to obtain a
better conversion, the flow rate was decreased to 0.18 mLminꢀ 1 and the
BPR was increased to 20 psi, in such configuration a longer residence
time could be achieved. With this setup it has been possible to obtain full
conversion with a slight decrease in selectivity (Table 3, entry 2).
Satisfying optimal results in both conversion and selectivity, were ob-
tained using a 5 psi BPR (entry 3). Unfortunately, after fluxing these
mixtures consecutively we noticed that some clogging of the system may
occur, due to the formation of sodium bicarbonate, but after washing the
reactor with water efficiency could be regenerated completely. To avoid
this problem, we decreased the solution concentration from 0.2 to 0.1 M
and in these conditions, with a residence time of 45 min, both a complete
conversion and selectivity to the product 3a were obtained. After con-
verting representative 10 mmol of 1a pure cyclohexanone product (3a)
could be isolated in an 87% yield (Table 3, entry 5). No loss in selectivity
was detected also using a higher residence time of 60 min (Table 3, entry
6), while with lower flow rate and higher BPR (residence time 75 min,
Table 3, entry 7) the formation of 10% of cyclohexan-1-ol (4a) occurred.
These data confirm that POLITAG-Pd(0)-L1 is stable and selective even
using longer residence times, conditions that can be helpful for the
extension of the substrate scope.
The efficiency and stability of the supported catalyst was also
confirmed by the MP-AES analysis conducted on the downstream reac-
tion mixtures to measure Pd leached in solution. The results gave a very
low (0.01 ppm) amount of Pd content and confirmed the stability of the
catalytic system under the optimized conditions for phenols hydroge-
nation, resulting in an expected capability to operate the flow conditions
for long time.
Considering the results obtained with phenol (1a), the scope of the
flow protocol was extended to other phenol derivatives (see Table S1
and Scheme 2). Initially, we paid more attention to biomass derived
substrates like cresols (1b-c), catechols (1d-e) and guiacols (1f-g) being
directly obtained from lignin depolymerization. By using different set-
tings obtained varying the residence time, flow rate and the BPR, we
attempted the optimization of the process to reach best conversions and
selectivity to the corresponding products (3). Excellent results were
obtained with para (1b) and ortho-cresol (1c) by using a 5 psi BPR and
applying a flow rate of 0.26 (p-cresol) and 0.32 mL minꢀ 1 (o-cresol)
achieving in both cases a 100% conversion and a 100% and 91%
selectivity toward product 3b and 3c, respectively (Table S1, entries
2,3). These outcomes are of high interest considering that both using
gaseous H2 and formic acid as hydrogen source, relevant amounts of the
over hydrogenation products 4b-c were detected in other literature
Table 4
Continuous flow hydrogenation of eugenol (1k).
Entry
BPR (psi)
Residence time (min)
Flow rate (mL/min)
Conv (%)
4k:5k (%)
1
2
20
40
40
95
0.29
0.12
100
100
100:0
80:20
5

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
useful to further evaluate the chemoselectivity of the system when also a
typical dehalogenation process can occur. This reaction can be also of
additional interest as it allows to convert these anthropogenic pollutants
into value added chemicals and manipulate waste or dangerous sub-
stance into new interesting chemicals [45–47].
At this aim, the developed flow protocol proved to be efficient for the
one-pot conversion of o‑chloro (1 h) and o-bromophenol (1i) into
cyclohexanone (3a) (Scheme 3). In both cases a residence time of 20 min
led to a very good conversion of 80% and 75%, respectively with no
formation of the undesired by-product cyclohexan-1-ol (4a).
Additionally, we also looked for more information about our proto-
col and catalyst, by studying the hydrogenation of 1-naphtol (1j) and
eugenol (1k). In the first case, it was possible to achieve a conversion of
70% exploiting a 40 psi BPR and a flow rate of 0.12 mLminꢀ 1 (Scheme
4), with a 70% formation of the desired product 3,4-dihydronaphtalen-1
(2H)-one (3j).
Concerning eugenol (1k), with a BPR of 20 PSI and a flow rate of 0.29
mLminꢀ 1, was not possible to selective hydrogenate the phenolic ring,
but only the olefinic double bond obtaining the 2‑methoxy-4-propyl-
phenol (4k) (Table 4, entry 1).
Fig. 6. TEM images of POLITAG-Pd(0)-L1 a), b) fresh and c), d) used.
reports [43,44].
In the case of catechols, 82% conversion and 80% selectivity to the
product 3d, was achieved with p-cathecol (1d) by employing a 40 psi
BPR and a flow rate of 0.29 mLminꢀ 1 (Table 4, entry 4). While for the
regioisomer o-catechol (1e), a reduced flow rate of 0.18 mLminꢀ 1 was
needed to reach a satisfactory conversion leaving the reactants in con-
tact with the catalyst for a longer time. In this case, 84% conversion
could be achieved but only 50% of the desired o-hydroxycyclohexan-1-
one (3e) was accessed (Table S1, entry 5).
With a residence time increased from 40 to 95 min (0.12 mLmin-1),
only a partial hydrogenation of the phenolic ring was achieved as well
forming (5k) (Table 4, entry 2).
Finally, in order to totally confirm the efficiency of the POLITAG-Pd
(0)-L1 catalytic system in flow, a solution of phenol (1a) was continu-
ously pumped for 25 h with no loss of efficiency and variation of
selectivity. The flow set-up used in combination with our POLITAG-Pd
(0)-L1 allows to convert over 100 mmol of phenolic compounds with
only 0.5 mol% of Pd. This result is remarkable in comparison with both
our batch protocol using POLITAG-Pd(0)-L1 and our previously re-
ported flow protocol using Pd/C catalyst (1.1 mol%) [26].
Among lignin derived phenols, we also tested challenging substrate
guaiacols (1f-g). Using p-guaicol (1f) as starting material it was possible
to obtain p-methoxycyclohexan-1-one (3f) with a great selectivity of
93% and a conversion of 99% at a flow rate of 0.18 mLminꢀ 1 (Table S1,
entry 7). If the flow rate was increased sufficient conversion into the
product was not satisfactory (Table S1, entry 6). The same issues were
encountered with o-guiacol (1 g), and satisfactory results in terms of
conversion and selectivity were achieved with a long resident time of
106 min (Table S1, entry 9).
The POLITAG-Pd(0)-L1 catalyst employed for the conversion of over
100 mmol of different substrates was compared with the fresh catalytic
system. TEM analyses show only a slight particle increase from 7.0 nm ±
3.3 nm to 11.1 nm ± 2.5 nm (Fig. 6). This evidence and the durability of
our Pd-based heterogeneous catalyst highlight the importance of syn-
ergy between flow technology and tailor-made catalytic systems for an
improved catalytic performance in hydrogen-transfer reactions.
To further investigate the applicability of our flow system also
halogenated phenols were screened. In fact, these substrates may be
Scheme 1. Proposed reaction mechanism for hydrogen-transfer reaction catalyzed by POLITAG-Pd(0)-L1.
6

V. Trombettoni et al.
Molecular Catalysis 509 (2021) 111613
Scheme 2. Substrate scope for the hydrogenation of phenols in continuous flow conditions. a1h as starting material of 3a; b 1i as starting material of 3a.
4. Conclusion
In conclusion POLITAG-Pd(0)-L1 showed a good performance in
terms of efficiency and selectivity in the hydrogenation of phenols to the
corresponding cyclohexanone derivatives. The catalyst proved to be an
effective water-tolerant heterogeneous catalyst while using sodium
formate (2) as a safe hydrogen-source. The tailor-made architecture of
the ionic tag designed for Pd nanoparticles immobilization, in combi-
nation with a flow technology procedure, reflected in an improved
stability of the catalytic system that showed a very low metal leaching
(0.01 ppm). With the same packed reactor representative more than 100
mmol of different substrates were converted with no loss in the chemical
efficiency of the process. Satisfying selectivity have been obtained for
differently substituted phenols while further study will be conducted to
improve the conversion/selectivity ratio for more challenging variably
substituted phenols.
Scheme 3. One-pot synthesis of cyclohexanone (3a) from o-halogenated phe-
nols (1h-i).
7

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Molecular Catalysis 509 (2021) 111613
Scheme 4. Continuous flow hydrogenation of 1-naphtol (1j). a Conversion=70%, 3j/4j=70/30.
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CRediT authorship contribution statement
Valeria Trombettoni: Investigation, Methodology, Writing – review
& editing. Francesco Ferlin: Investigation, Methodology, Writing –
review & editing. Federica Valentini: Investigation, Methodology,
Writing – review & editing. Filippo Campana: Investigation, Writing –
review & editing. Matteo Silvetti: Investigation, Writing – review &
editing. Luigi Vaccaro: Conceptualization, Methodology, Writing – re-
view & editing.
´
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Declaration of Competing Interest
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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.
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The Universita degli Studi di Perugia and MIUR are acknowledged
for financial support to the project AMIS, through the program “Dipar-
timenti di Eccellenza - 2018-2022ˮ. Regione Umbria is acknowledged
for funding through "Umbria bo.R.do“, P.O.R. Programma Operativo
Regionale F.S.E. (Fondo Sociale Europeo) Umbria 2014–2020 Asse III
“Istruzione e formazione”.
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Supplementary materials
Supplementary material associated with this article can be found, in
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