Efficient hydrogenation of levulinic acid catalysed by spherical NHC-Ir assemblies with atmospheric pressure of hydrogen

Article abstract of DOI:10.1039/d1gc01513c

A practical, efficient, and mild hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) under 1 atm H₂was realized by single-sited 3D porous self-supported N-heterocyclic carbene iridium catalysts. Quantitative yields and selectivities were achieved at 0.02 mol% catalyst loading, and the catalyst could be reused for 9 runs without obvious loss of selectivity or activity.

Full text of DOI:10.1039/d1gc01513c

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Efficient hydrogenation of levulinic acid catalysed
by spherical NHC-Ir assemblies with atmospheric
pressure of hydrogen†
Cite this: Green Chem., 2021, 23,
037
5
Received 30th April 2021,
Accepted 21st June 2021
a
a
a
a
a
Lingyun Shen, Qingshu Zheng,
Yaoqi Liu, Jiajie Wu, Zeye Lu and
a,b,c
DOI: 10.1039/d1gc01513c
Tao Tu
*
rsc.li/greenchem
A practical, efficient, and mild hydrogenation of levulinic acid (LA) ogenous species, heterogeneous catalysts offer a possibility for
to γ-valerolactone (GVL) under 1 atm H was realized by single- catalyst recovery and reuse, and thus have potential for indus-
2
sited 3D porous self-supported N-heterocyclic carbene iridium trial processes. However, harsh reaction conditions, especially
catalysts. Quantitative yields and selectivities were achieved at high H pressure, high temperature and high catalyst loadings
2
0
.02 mol% catalyst loading, and the catalyst could be reused for 9 are usually required and have strictly hampered further indus-
runs without obvious loss of selectivity or activity.
trial application. Therefore, the design and preparation of
powerful and recyclable catalysts that can enable the hydrogen-
ation of LA under mild reaction conditions are highly desir-
able. During the preparation of our manuscript, Fu and Liu
reported the nickel and cobalt-catalyzed hydrogenation,
Biomass constitutes a sustainable, inexpensive and readily
1
–3
available alternative to fossil resources.
Theoretically,
diverse high value-added chemicals can be easily accessed
via selective transformation of the corresponding biomass
respectively, of LA with sensitive phosphine ligands under
4
,5
platform molecules.
readily available from lignocellulose,
siderable attention due to its potential for valorization to a
Among them, levulinic acid (LA),
17,18
atmospheric H
2
pressure (Fig. 1a).
Nevertheless, the cata-
6
,7
has received con-
lysts in both homogeneous systems were non-recyclable, and
high temperature or high catalyst loadings were still required
to achieve satisfactory outcomes.
3
–7
number of crucial and valuable chemicals.
For instance,
γ-valerolactone (GVL) is regarded as one of the most attractive
precursors for the generation of biofuels, environmentally
benign solvents, value-added fine chemicals and functional
19
Recently, by using a “self-supporting” strategy, we success-
fully fabricated a series of one-dimensional (1D) coordination
assemblies based on bis-N-heterocyclic-carbene-iridium (bis-
NHC-Ir) complexes, which functioned as solid molecular cata-
8
–10
additives for foods and fuels.
In this context, the hydrogen-
ation of LA may constitute one of the most effective valoriza-
tion approaches for accessing GVL.
Compared to the hydrogenation of LA to GVL with formic
acid or other hydrogen sources, the direct hydrogenation of LA
lysts and exhibited high catalyst activity and selectivity towards
20,21
the hydrogenation of LA to GVL.
The 1D solid catalysts
with hydrogen gas (H ) may be more sustainable and straight-
2
forward, and is especially suitable for large-scale industrial
1
1
application.
To date, a number of catalysts have been
1
2
13
designed for this pivotal transformation, including Ir, Ru,
1
4
15
16
Pd and Fe complexes or nanocomposites in homogenous
or heterogeneous catalytic systems. In comparison with hom-
a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438,
China. E-mail: taotu@fudan.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
c
Green Catalysis Center and College of Chemistry, Zhengzhou University,
100 Kexue Avenue, Zhengzhou 450001, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc01513c
Fig. 1 Hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL).
Green Chem., 2021, 23, 5037–5042 | 5037
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were readily recovered and reused for dozens of runs, although
the reactions still had to be carried out under high H pressure
2
2
0
(
50 bar, Fig. 1a). To the best of our knowledge, there is no
heterogeneous catalytic system for this transformation under
atmospheric H pressure. We conceived that porous catalysts
2
with large specific surface areas may accelerate the adsorption
2
1–23
of H
2
and the mass and heat transfer,
which may benefit
the hydrogenation process. Herein, we fabricated a series of
spherical three-dimensional (3D) porous NHC-Ir assemblies
(Fig. 1b), which exhibited excellent activities and selectivity
towards the hydrogenation of LA to GVL under atmospheric
pressure of hydrogen. Quantitative yield and selectivity could
be achieved at 0.02 mol% catalyst loading, and the solid cata-
lyst could be readily recovered and reused for 9 runs without
obvious loss of selectivity or activity. Remarkably, an excellent
5
turnover number (TON) of 2.1 × 10 was achieved.
In order to fabricate the porous 3D NHC-Ir assemblies, tet-
2
3,24
raimidazolium salts
with geometrically defined tetraphe-
nylmethane cores were selected because the formed four NHC
fragments were well-separated from each other and orientated
for further binding to different Ir centers, so leading to the for-
mation of an extended 3D structure via coordination assembly
Fig. 2 (a and b) SEM images, (c) EDS elemental-mapping of iridium,
(Fig. 1b). Initially, tetraimidazolium iodide was synthesized by and (d) TEM image of the freshly prepared solid NHC-Ir coordination
the N-phenylation of N-methylimidazole with tetra-para- assembly 3a. (e) HAADF image of recovered solid 3a. (f) XPS spectra of
freshly prepared solid 3a and recovered solid 3a. (g) Partial solid-state
C NMR spectra of solid 3a and the corresponding analogue complex 5.
bromo-phenylmethane (Scheme S1†). After anion-exchange
13
with triethyloxonium tetrafluoroborate, the corresponding salt
−
containing BF
4
was obtained in a quantitative yield. After
deprotonation of these imidazolium salts by lithium hexa-
methyldisilazide (LiHMDS) in DMF, and coordination with
Subsequently, the molecular nature of the solid 3a was
1
3
different metal precursors [Ir(acac)(CO)
acetylacetonato and COD = 1,5-cyclooctadiene), the desired coupled plasma-atomic emission spectroscopy (ICP-AES) and
NHC-Ir solid coordination assemblies (3a–c) with different X-ray photoelectron spectroscopy (XPS). Pleasingly, the signals
2 2
] or [Ir(COD)Cl]
(acac investigated by solid-state C NMR spectroscopy, inductively
=
anions (I− or BF
−
) and ancillary ligands (CO or COD) were in the solid-state C NMR spectrum of 3a were well-matched
with the corresponding peaks of the bis-NHC-Ir complex 5,
13
4
finally obtained in excellent yields (80%–85%, Scheme S1†).
All NHC-Ir solid coordination assemblies 3a–c were rela- though slightly broadened (Fig. 2g), which confirmed that the
tively robust and insoluble in all tested conventional organic solid assemblies 3a had the desired structure as we expected.
solvents and water. Therefore, solid-state analytical techniques The ICP-AES showed that the amount of Ir in 3a was 26.6%,
were applied for characterization of their compositions. The which was quite consistent with the theoretical value (25.7%).
morphologies of the freshly prepared coordination assemblies This was further confirmed by the amount of Ir (25.5%)
3
a–c were investigated by scanning electron microscopy (SEM) resolved from hydrogen temperature-programmed desorption
and transmission electron microscopy (TEM). Unexpectedly, (H -TPD, Fig. S26†). In addition, characteristic bonding ener-
2
spherical particles were found in the SEM and TEM images of gies of Ir(I) were observed at 62.0 (4f5/2) eV and 65.0 (4f7/2) eV
the solid NHC-Ir assemblies 3a (Fig. 2a); the average particle in the XPS spectrum of 3a, which were also consistent with the
size was 700–800 nm as measured by dynamic light scattering signals of bis-NHC-Ir complex 5.
(
DLS, Fig. S25†). It has to be pointed out that these spherical
particles were amorphous, which was further confirmed by assemblies were further characterized by N
powder X-ray diffraction (PXRD, Fig. S12†). Energy dispersive tion/desorption studies (Fig. S23 and S24†). N
The porosities of the newly fabricated solid coordination
and H adsorp-
adsorption
2
2
2
2
−1
spectroscopy (EDS, Fig. 2c) indicated that single iridium tests revealed that solid 3a (33 m g ) had a higher Brunauer–
sites were uniformly dispersed in the matrix of solid 3a, Emmett–Teller (BET) specific surface area than the 1D ana-
2
−1
which could be confirmed by the magnified high-angle logue 4b (8 m g ), confirming the porous nature of solid 3a.
annular dark field scanning transmission electron microscopy Similarly, the solid 3a exhibited a significantly improved H
2
2
−1
(
HAADF-STEM) image of recovered solid 3a (Fig. 2e). With this adsorption ability compared to the 1D solid 4b (84 m g vs.
2
−1
information, we may regard these self-supported NHC-Ir 3 m g ), which might be beneficial for hydrogenation reac-
assemblies as solid molecular catalysts that coordinate in the tions involving H
2
.
same way as their molecular analogues bis-NHC-Ir 5 with
N-phenyl substituents.
The catalytic activities of these characterized spherical
porous NHC-Ir assemblies 3a–c were then investigated by the
1
8,23,25
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hydrogenation of LA to GVL. Initially, our previous reaction
conditions for 1D solid coordination assemblies 4a–b
(
15 mmol scale with 1.1 equiv. KOH in i-PrOH at 30 bar H
2
2
0
pressure at 100 °C for 4 h) were directly applied to evaluate
the activity of the solids 3. To our delight, even in the presence
of 0.01 mol% solid 3a, LA could be selectively transformed
into GVL in 99% yield within 4 h, highlighting the excellent
catalytic activity of 3a. It is worth mentioning that lowering the
amount of KOH significantly suppressed the yield of GVL
(7–54%, entries 7–9, Table S1†), since one equivalent of KOH
is required to firstly neutralize LA, and then a slight excess of
KOH facilitates the transformation. This was confirmed by the
excellent result (92%) achieved by the control experiment with
1
equiv. potassium levulinate and 0.1 equiv. KOH (entry 21,
Table S1†). Increasing the amount of KOH above 1.1 equiv.
also slightly decreased the yield of GVL (89%, entry 6, Fig. 3 (a) Catalyst screening: reactions were carried out with LA
(
(
15 mmol), catalyst (0.02 mol%), KOH (1.1 equiv.), H
5 mL) at 100 °C for 24 h. The yields were determined by H NMR with
2
(1 atm) in i-PrOH
Table S1†), supporting the pivotal role of KOH. The strong
base NaOH or other weak bases all resulted in inferior results
1
mesitylene as an internal standard. Im denotes imidazole, BenIm
denotes benzimidazole. (b) Recycling and reuse of the solid catalyst 3a
(38–84%, entries 15–17, Table S1†). The catalytic activities of
other solid assemblies 3b–c were also assessed. Inferior yields for the hydrogenation of LA to GVL under 1 atm H for 4 h.
2
were observed with the other solid assemblies under otherwise
identical reaction conditions (83–78% yields, Table S1,†
entries 2–3), which may indicate that the anions, ancillary
ligands and core structures of the assemblies had a great 3D catalysts had similar expected structures. XPS (Fig. S16 and
impact on the catalytic activity.
S19†) revealed that the coordination environment and valence
Because the porous structure of solid 3a has a higher H2 state of the Ir(I) centers in the 3D catalyst 3a (62.0 eV and 65.0
adsorption capacity than 1D solid 4b, which may be beneficial eV) and 1D catalyst 4b (61.9 eV and 64.9 eV) were consistent.
for reactions involving H
2
2 2
, a hydrogen balloon was attached to The aforementioned N and H sorption experiments indicated
2
−1
the reaction vessel to perform the reaction under atmospheric that solid 3a had a higher BET specific surface area (33 m g
2
−1
2
−1
H
2
pressure. Up to 78% yield of GVL was achieved within 24 h vs. 8 m g , Fig. S23†) and H
in the presence of 0.01 mol% solid catalyst 3a, and no other 3 m
2
adsorption ability (84 m g vs.
, Fig. S24†) than the solid analogue 4b. Moreover,
by-product was generated. By increasing the catalyst loading of when the 1D analogue 4c, with N-phenyl groups other than
a to 0.02 mol%, a 99% yield of GVL was finally achieved N-methyl groups, was applied as the catalyst under otherwise
under 1 atm H
pressure within 24 h. It is noteworthy that our identical reaction conditions, a 79% yield of GVL was attained.
protocol describes the first heterogeneous catalytic system for This result was higher than that from solid 4b (71%) but lower
the selective hydrogenation of LA to GVL under 1 atm H
than the outcome using solid 3a (99%), indicating that substi-
2
−1
g
3
2
2
pressure. When 0.00125 mol% (12.5 ppm) solid 3a was utilized tuent effects contribute to the hydrogenation of LA to GVL.
as the catalyst over an extended reaction time (72 h), a 60% Therefore, the superior activity of the 3D catalyst compared to
4
yield of GVL and a TON of 4.8 × 10 were obtained under 1 atm the 1D catalyst could be attributed to the porous structure and
H
2
pressure. Moreover, a record TOF of 3630 h−1 could be substitution effects.
1
7,18
achieved within 30 minutes under 1 atm H2 pressure.
Upon increasing the H
pressure to 50 bar, a much higher solvents and water, the solid catalyst was readily recovered by
TON value of 2.1 × 10 was gained.
simple centrifugation and decantation after completion of the
In view of the insolubility of solid 3a in common organic
2
5
Several viable homogeneous catalysts 6–11, which have reaction. To our delight, after washing with i-PrOH, the recov-
been regarded as model catalysts in diverse hydrogenation ered solid 3a could be directly reused in a second run without
2
6,27
reactions
Fig. 3a). Pleasingly, the solid 3a catalyst exhibited the highest obvious decrease in the selectivity and yield of GVL was
catalytic activity under atmospheric H
pressure. Remarkably, observed in the second run within 4 hours. The solid 3a could
were investigated to compare with the solid 3a additional activation. Simply using LA, base and solvent, no
(
2
the 1D NHC-Ir assembly 4b still had higher activity than all be reused for 9 runs under atmospheric pressure of hydrogen
the selected homogenous catalysts under atmospheric H2 at low catalyst loadings (0.06 mol%), without an obvious loss
pressure, confirming the advantages of the “self-supporting” of catalytic activity or selectivity (Fig. 3b). Furthermore, under
2
3
strategy due to the “isolated effect”. To further investigate 30 bar of hydrogen, the recycle number could be increased to
th
the reason for the superior catalytic performance of the 3D 15 runs, and the yield of GVL in the 15 run was still greater
solid compared to the 1D catalyst, additional control experi- than 90% (Fig. S53†).
ments and characterizations were carried out. The solid-state
In order to investigate the stability of solid 3a during re-
C NMR spectra (Fig. S21 and S22†) indicated that the 1D and cycling, the filtrate after each run was analysed by ICP-AES for
1
3
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possible iridium leaching (Table S3†). Only trace amounts of also tested, which gave a good yield of phthalide (70%, 2j)
iridium species could be detected in the filtrates, which may under 1 atm H pressure.
2
be the iridium precursors or the light-weight oligomers
entrapped in the solid catalyst matrix during the coordination experiments were conducted (Fig. 4a). The possible reaction
To investigate the reaction mechanism, several control
2
4
2
6,27
process.
Moreover, no obvious morphology changes were pathways might be: (I) after dehydration of LA, α-angelica
found in the SEM, TEM and EDS mapping images between the lactone could be generated, which could be further converted
newly prepared and the recovered solid catalysts (Fig. S1, S2, to GVL in the presence of a catalytic amount of solid 3a
1
7
S5–S8†). Furthermore, similar signals were observed in the (Fig. 4a, pathway I); or (II) after hydrogenation of LA in the
XPS, XRD and IR spectra, confirming that the solid catalyst 3a presence of solid 3a, a subsequent intramolecular cyclic esteri-
preserved its structural features and functioned as a robust fication process could take place after further addition of HCl,
1
7
solid molecular catalyst. The slightly changed signals of solid leading to formation of the desired GVL (Fig. 4a, pathway II).
a after recycling (Fig. 2f and Fig. S12, S13, S49, S50†) were When the α-angelica lactone was used as a substrate under
attributed to the dissociation of COD ligands after the otherwise identical reaction conditions, almost no GVL was
3
2
8
reaction.
generated (Fig. 4b, entry i). This outcome implies that pathway
Considering the high catalytic efficiency and stability of II via hydrogenation and esterification was involved in our
solid 3a, the scope of the protocol was then investigated. As newly developed heterogeneous catalytic system. A model reac-
shown in Table 1, selected LA derivatives and analogues could tion without the H2 balloon was also carried out, and only
be readily hydrogenated and transformed into the corres- trace GVL was produced (Fig. 4b, entry ii), which clearly indi-
ponding lactones in good to excellent yields (70–99%) under cates that a hydrogenation mechanism other than hydrogen-
atmospheric H2 pressure. It should be notified that slightly transfer was involved.
high catalyst loading (0.1 mol%) and amount of base (2 mol %)
Furthermore, to understand the influence of the solvent,
are required to achieve satisfactory outcomes. No obvious reactions without solvent, and with t-BuOH, MeOH, H
2
O or
electronic and steric effects with the aromatic substrates were
found. Selected halogens and electron-donating substituents
all resulted in good to excellent yields (76%–94%, Table 1, 2b–g).
When the bulky naphthalen-2-yl substrate was present, 81%
yield was still achieved (2h). Furthermore, γ-acetyl butyric acid
and ethyl levulinate both led to excellent yields (99%: 2i and
9
1%: 2a). In light of the important role of bioactive phthalide
in the pharmaceutical industry, 2-carboxybenzaldehyde was
a
Table 1 Hydrogenation of levulinic acid analogues and derivatives
a
Reactions were carried out with acid (5 mmol), solid 3a (0.1 mol%),
2
KOH (2 equiv.), H (1 atm) in i-PrOH (5 mL) at 100 °C for 24 h. HCl
(
1 mL) was added after the completion of the reaction and the mixture
b
was stirred at r.t. for 1 h. The isolated yields are reported. Using ethyl
c
d
levulinate as the substrate. For 36 h. The yield was determined by Fig. 4 (a) Possible reaction pathways for the hydrogenation of LA to
1
H NMR with mesitylene as an internal standard.
GVL by solid 3a. (b) Control experiments.
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i-PrOH were performed under otherwise identical reaction con-
ditions. GVL was obtained in yields of 32%, 8%, 17%, 40%,
and >99%, respectively (Fig. 4b, entries iii–vi, and Fig. 3a).
These results indicate that the presence of the hydrogen trans-
fer reagent i-PrOH could significantly promote the hydrogen-
ation of LA to GVL. However, the yield of GVL in pure water
could be further increased to 80% (Fig. 4b, entry vii) upon
increasing the catalyst loading to 0.12 mol% and extending
the reaction time (48 h), suggesting that the porous coordi-
nation assemblies 3a could readily catalyse the hydrogenation
of LA in high yield without the help of any “hydrogen transfer”
solvent. Therefore, we envisage that the synergistic effect
between i-PrOH and the single-site 3D porous catalyst is
responsible for the excellent catalytic performance of the
protocol.
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Conclusions
2
In summary, a series of spherical self-supported N-heterocyclic
carbene-iridium assemblies were prepared which exhibited
excellent activities and selectivities towards the hydrogenation
of levulinic acid (LA) to γ-valerolactone (GVL) under the atmos-
pheric pressure of hydrogen. Quantitative yields and selectiv-
ities could be achieved by these single-site solid molecular cat-
alysts at catalyst loadings as low as 0.02 mol%. A catalyst could
be readily recovered and reused for 9 runs without obvious
2
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