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