Highly efficient Meerwein-Ponndorf-Verley reductions over a robust zirconium-organoboronic acid hybrid

Article abstract of DOI:10.1039/d0gc04179c

The Meerwein-Ponndorf-Verley (MPV) reaction is an attractive approach to selectively reduce carbonyl groups, and the design of advanced catalysts is the key for these kinds of interesting reactions. Herein, we fabricated a novel zirconium organoborate using 1,4-benzenediboronic acid (BDB) as the precursor for MPV reduction. The prepared Zr-BDB had excellent catalytic performance for the MPV reduction of various biomass-derived carbonyl compounds (i.e., levulinate esters, aldehydes and ketones). More importantly, the number of borate groups on the ligands significantly affected the catalytic activity of the Zr-organic ligand hybrids, owing to the activation role of borate groups on hydroxyl groups in the hydrogen source. Detailed investigations revealed that the excellent performance of Zr-BDB was contributed by the synergetic effect of Zr⁴⁺and borate. Notably, this is the first work to enhance the activity of Zr-based catalysts in MPV reactions using borate groups.

Full text of DOI:10.1039/d0gc04179c

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Highly efficient Meerwein–Ponndorf–Verley
reductions over a robust zirconium-organoboronic
acid hybrid†
Cite this: Green Chem., 2021, 23,
259
1
a
a,b
a,b
c
a
Jinliang Song, * Manli Hua, Xin Huang, Aurelia Visa, Tianbin Wu,
a
a
a
a,b
Honglei Fan,
Minqiang Hou, Zhaofu Zhang
and Buxing Han
*
The Meerwein–Ponndorf–Verley (MPV) reaction is an attractive approach to selectively reduce carbonyl
groups, and the design of advanced catalysts is the key for these kinds of interesting reactions. Herein, we
fabricated a novel zirconium organoborate using 1,4-benzenediboronic acid (BDB) as the precursor for
MPV reduction. The prepared Zr-BDB had excellent catalytic performance for the MPV reduction of
various biomass-derived carbonyl compounds (i.e., levulinate esters, aldehydes and ketones). More
importantly, the number of borate groups on the ligands significantly affected the catalytic activity of the
Zr-organic ligand hybrids, owing to the activation role of borate groups on hydroxyl groups in the hydro-
gen source. Detailed investigations revealed that the excellent performance of Zr-BDB was contributed
Received 10th December 2020,
Accepted 8th January 2021
DOI: 10.1039/d0gc04179c
4+
by the synergetic effect of Zr and borate. Notably, this is the first work to enhance the activity of Zr-
rsc.li/greenchem
based catalysts in MPV reactions using borate groups.
A convenient strategy to realize selective hydrogenation
Introduction
of carbonyl groups is Meerwein–Ponndorf–Verley (MPV)
4
To relieve the excessive reliance on the depleting fossil reductions, which employ secondary alcohols as the hydrogen
resources, utilization of the renewable and abundantly avail- resource. For MPV reduction, Zr-containing catalysts (e.g.,
5
6
7
able biomass to produce valuable chemicals has gained signifi- ZrO
2
,
zirconium alkoxides, Zr-containing zeolites, and
1
8
cant attention. In the past decades, many carbonyl com- Zr-organic ligand hybrids ) are widely used. The easily tunable
pounds have been obtained through biomass conversion or activity affords Zr-organic ligand hybrids being highly attrac-
direct extraction from plants. Thus, further upgrading of these tive among the developed Zr-containing catalysts. Currently,
2
biomass-based carbonyl compounds is highly attractive. In several Zr-organic ligand hybrids have been fabricated from
this regard, selective hydrogenation of the carbonyl groups to the coordination of Zr with various organic ligands (e.g., dicar-
8
d,e,9
8b
8a
generate hydroxyl compounds has been recognized as a highly boxylic acids,
phytic acid, tannic acid, and cyanuric
1
0
promising strategy to convert these biomass-based carbonyl acid ) for the MPV reaction of biomass-derived furfural and
compounds, considering that the hydrogenation products can levulinic acid/levulinate esters. However, these developed Zr-
3
be widely applied as fine chemicals and valuable synthetic organic ligand hybrids often required high reaction tempera-
intermediates. However, the biomass-derived carbonyl com- ture or long reaction time, especially for the MPV reaction of
pounds often contain other reducible moieties (e.g., CvC, levulinate esters. Thus, novel and robust Zr-organic ligand
phenolic hydroxyl group, and aromatic ethers), resulting in the hybrids are still highly desired to afford MPV reductions being
difficulty for selective hydrogenation of the carbonyl groups.
conducted under mild reaction conditions.
Improving the ability of the catalysts to activate the hydroxyl
group in the hydrogen donor (secondary alcohols) is a general
strategy to achieve high reactivity of the MPV reaction. The
catalytic activity of the reported Zr-organic ligand hybrids for
the MPV reaction was usually enhanced by involving basic
a
Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid
and Interface and Thermodynamics, CAS Research/Education Center for Excellence
in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: songjl@iccas.ac.cn, hanbx@iccas.ac.cn
b
8b
8a,11
School of Chemistry and Chemical Engineering, University of Chinese Academy of
groups (e.g., phosphate and phenate
). However, to over-
Sciences, Beijing 100049, China
come the above limitations of the current Zr-organic ligand
hybrids, it is still of crucial importance to develop innovative
approaches to activate the hydroxyl groups in the hydrogen
donor.
c
Institute of Chemistry Timisoara of the Romanian Academy, 24 M. Viteazul Ave,
3
00223 Timisoara, Romania
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc04179c
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As reported, boric acid (H
Green Chemistry
3
BO
3
) as a Lewis acid can coordi- Zr-BDB was significantly weakened in comparison with that in
1
2
4+
nate with the hydroxyl group in alcohols, and could activate BDB, implying the coordination between Zr and the BDB.
the hydroxyl group. We assume that there is great potential to Meanwhile, there was only one broad diffraction peak in the
design efficient Zr-based catalysts for the MPV reaction by XRD pattern of the Zr-BDB (Fig. S2†), indicating the amor-
4
+
combining the catalytic role of Zr (to activate the carbonyl phous nature of the Zr-BDB. Additionally, one Zr could theor-
group) and H BO /borate (to activate the hydroxyl groups). As etically coordinate with two borate groups, and thus, the molar
3
3
far as we know, no attention has been paid to the role of ratio of Zr and BDB should be 1 : 1. However, as determined by
H BO (or borate) in the MPV reaction. Herein, we designed a ICP-AES and element analysis, the molar ratio of Zr and BDB
3
3
porous zirconium organoborate simply by the reaction of in the obtained Zr-BDB was about 1.2 : 1. Considering that the
ZrOCl and 1,4-benzenediboronic acid (BDB, Scheme S1†), rigid structure of BDB, the most possible connectivity pattern
2
4
+
denoted as Zr-BDB. The prepared Zr-BDB could efficiently cata- of Zr and BDB is shown in Scheme S2,† but there should be
lyze the MPV reactions of various biomass-based carbonyl much irregular connectivity or defects in Zr-BDB deduced
compounds with high selectivity (>97%) owing to the syner- from the XRD result (Fig. S2†) and element analysis. For com-
getic effect of Zr and borate. As far as we know, this is the first parison, Zr-CPB (using 4-carboxyphenylboronic acid, denoted
work on the employment of borate groups to boost the cata- as CPB, Scheme S1†), Zr-BDC (using 1,4-benzenedicarboxylate
lytic activity of Zr-organic ligand hybrids in MPV reactions.
acid, denoted as BDC, Scheme S1†) and Zr-B (using boric acid,
denoted as B) were prepared by a similar route to synthesize
Zr-BDB, and these three catalysts were characterized by TEM
(
Fig. S3†), SEM (Fig. S4†), and N
2
adsorption–desorption exam-
Results and discussion
Preparation and characterization of the Zr-BDB
inations (Table S1†).
The Zr-BDB was synthesized from ZrOCl and BDB in N,N-di-
2
Activity of various catalysts on the MPV reaction of ethyl
levulinate
methylformamide (DMF) via a solvothermal process, and the
detailed procedure is provided in the ESI.† The fabricated Zr-
BDB consisted of small particles with a diameter of approxi-
mately 100 nm (Fig. 1A), and there were plenty of mesopores
in the Zr-BDB as shown in the TEM image (Fig. 1B). Elemental
distribution mappings suggested that the four elements,
including Zr, C, O, and B, uniformly dispersed in the Zr-BDB
Levulinate esters, a typical class of biomass-derived carbonyl
chemicals, can be converted into γ-valerolactone (GVL), which
1
4
is an important reaction for biomass transformation. As is
well known, the reaction process involves the hydrogenation of
carbonyl groups. Thus, MPV reduction of levulinate esters to
synthesize GVL was selected as a model reaction to evaluate
the catalytic performance of the prepared Zr-BDB. Initially, we
used ethyl levulinate (EL) as the reactant to investigate the
effect of various reaction parameters (Table 1). No reaction
occurred in the absence of catalysts (Table 1, entry 1). The reac-
(Fig. S1†). Moreover, the N
2
adsorption–desorption isotherm
(
Fig. 1C) further indicated that the Zr-BDB was a porous
2
−1
material with a surface area of 271 m
g
(Table S1†), and
the size of the mesopores was centered around 3.4 nm.
In FT-IR spectra (Fig. 1D), the characteristic vibration
1 13
(
800–1200 cm−
,
the shadow area) of boric-acid groups in the
Table 1 Activity of various catalysts for the MPV reaction of levulinate
a
esters
b
b
b
b
b
Entry
Catalyst
T
(°C)
t
(h)
C (%)
Y (%)
S
(%)
1
2
3
4
5
6
7
8
None
Zr-BDB
130
130
130
130
130
130
130
100
130
130
130
130
130
130
120
6
6
6
6
6
6
6
12
12
12
4
0
0
98.7
8.5
0
99.5
10.1
70.2
98.1
85.1
93.9
88.9
85.7
99.4
99.7
89.9
81.4
99.7
99.4
99.2
84.2
88.9
35.6
55.1
85.5
94.8
90.5
95.9
98.7
96.4
94.8
97.5
97.2
ZrO
2
Zr-CPB
Zr-BDC
UiO-66
Zr-B
Zr-BDB
Zr-BDB
Zr-BDB
Zr-BDB
Zr-BDB
Zr-BDB
Zr-BDB
Zr-BDB
62.4
34.9
46.9
80.3
84.3
77.6
95.3
98.4
86.7
77.2
97.2
96.6
c
9
d
1
1
1
1
1
1
0
1
2
3
4
5
e
f
6
6
3
6
g
h
h
a
Reaction conditions: EL, 1 mmol; catalyst, 0.2 g; isopropyl alcohol,
g. The conversion and yield were determined by GC using cyclohex-
b
5
anol as the internal standard. T = temperature, t = time, C = conver-
c
Fig. 1 Characterization of the prepared Zr-BDB. (A) SEM image, (B) TEM
image (the inserted image was achieved from the high resolution TEM),
sion, Y = yield, and S = selectivity. The amount of Zr-BDB was 25 mg.
d
e
f
The amount of Zr-BDB was 50 mg. The reactant was ML. The reac-
g
h
(
C) N
2
adsorption–desorption isotherm, and (D) FT-IR spectra.
tant was PL. The reactant was BL. The reactant was LA.
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tion could be efficiently proceeded over Zr-BDB at 130 °C lyze the MPV reaction under lower reaction temperature and
Table 1, entry 2), and the GVL yield could reach 98.7% with a catalyst usage, further indicating its excellent catalytic activity.
reaction time of 6 h. In contrast, commercial ZrO only pro- Moreover, the reactivity of levulinate esters decreased as
(
2
vided very low activity for the reaction (Table 1, entry 3). follows (Table 1, entries 2 and 11–13): methyl levulinate (ML) >
Furthermore, it was found that the number of borate groups EL > propyl levulinate (PL) > butyl levulinate (BL), which prob-
on the benzene ring significantly affected the catalytic activity ably resulted from the increasing steric hindrance from methyl
of the prepared three Zr-organic ligand hybrids (Table 1, to butyl. Meanwhile, Zr-BDB could also efficiently catalyze the
entries 2, 4, and 5). The GVL yields were 62.4% and 34.9% over MPV reaction of levulinic acid (LA), and a GVL yield of 97.2%
Zr-CPB and Zr-BDC (Table 1, entries 4 and 5), respectively. The was obtained at 130 °C with a reaction time of 3 h (Table 1,
commercial metal-organic framework UiO-66 only provided a entry 14). In addition, the catalytic activity of Zr-BDC was still
moderate GVL yield of 46.9% (Table 1, entry 6) although it had high for LA at 120 °C, and the GVL yield reached 96.6% with a
a much higher BET surface area than the Zr-BDB (Table S1†). reaction time of 6 h (Table 1, entry 15).
Additionally, Zr-B showed a good activity for the GVL for-
The influence of different alcohols as hydrogen donors on
mation with a yield of 80.3% (Table 1, entry 7), further indicat- the MPV reaction of EL was further studied (Table S2†). It was
ing the promoting effect of the borate group. The results above found that secondary alcohols (isopropyl alcohol and sec-butyl
indicated that the prepared Zr-BDB was a superior catalyst for alcohol) showed better performance than the primary alcohols
the MPV reaction owing to the synergistic effect of Zr and the (methanol, ethanol, and n-propanol) owing to the lower
borate, as will be discussed below.
reduction potential of secondary alcohols (Table S2,† entries
1
5
1
–2 vs 3–5). The poor performance of methanol as the hydro-
Optimization of reaction parameters
gen donor was caused by its much higher reduction potential
1
5b
than other alcohols (Table S2,† entry 3). Moreover, the yield
of GVL was low when using tert-butanol as the hydrogen donor
In general, reaction temperature and catalyst usage are two
important parameters that affect reaction efficiency. As
expected, both the EL conversion and the yield of GVL
increased with increasing reaction temperature over Zr-BDB
(
Table S2,† entry 6) because the structure of tert-butanol
resulted in great difficulty for the hydrogen-transfer step in
1
5a,b
MPV reduction.
(Fig. 2A), and the reaction was completed at 130 °C with a GVL
yield of 98.7%. Importantly, the GVL yield could reach 84.3%
at 100 °C over Zr-BDB with a prolonged reaction time of 12 h
Reusability of Zr-BDB
For heterogeneous catalysts, easy reusability is the main super-
iority in comparison with the homogeneous ones. First, the
heterogeneous nature of Zr-BDB was evaluated. After the reac-
tion was performed for 2 h at 130 °C, the Zr-BDB was removed
from the reaction system via hot filtration. No further reaction
was observed after the removal of Zr-BDB (Fig. 2C), verifying
that Zr-BDB was heterogeneous in the reaction process.
Meanwhile, as determined by ICP-AES, the concentration of Zr
in the reaction mixture was very low (<0.8 ppm), indicating the
negligible leaching of active species. Second, it was found that
Zr-BDB could be reused for at least five catalytic cycles without
obvious decrease in activity and product selectivity (Fig. 2D
and S5†). The recovered Zr-BDB was characterized by several
techniques (Fig. S6 and Table S1†), including XRD, FT-IR,
(
Table 1, entry 8), while no GVL was formed over ZrO or Zr-
2
BDC. Similarly, more catalyst usage was beneficial for the reac-
tion (Fig. 2B), and the optimal amount of Zr-BDB was 0.2 g. To
our delight, Zr-BDC maintained its high activity in the MPV
reaction of EL at lower usage of Zr. The GVL yields could reach
7
7.6% and 95.3% (Table 1, entries 9 and 10) at 130 °C when
the amount of Zr-BDB was 0.025 or 0.05 g with a reaction time
of 12 h. These investigations verified that Zr-BDC could cata-
HR-TEM, and
N
2
adsorption–desorption method, which
showed that there was no obvious change in the properties of
the recovered Zr-BDB, implying that Zr-BDB was stable.
Scope of substrates
Encouraged by the excellent performance regarding the con-
version of levulinate esters, we further investigated the cata-
lytic activity of Zr-BDB for the MPV reaction of other biomass-
derived carbonyl compounds (Table 2). To our delight, the Zr-
BDB could efficiently catalyze the MPV reaction of various
biomass-based aldehydes and ketones. From the results in
Table 2, it was obvious that aldehydes were generally more
reactive than ketones, resulting from the lower steric hin-
drance of the aldehyde group. More importantly, the reactivity
of the substituted substrates decreased in comparison with the
Fig. 2 (A) Effect of reaction temperature, (B) influence of reaction time,
(
C) effect of reaction time, and (D) reusability of Zr-BDB. Reaction con-
ditions: EL, 1 mmol; isopropyl alcohol, 5 g; the amount of Zr-BDB, 0.2 g
for A, C and D; reaction temperature, 130 °C for B–D; reaction time, 6 h
for A and B or 2 h for D.
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Table 2 (Contd.)
Table 2 MPV reaction of various biomass-based carbonyl compounds
over Zr-BDB
a
b
b
b
b
T
t
C
Y Y
°
b
b
b
b
Entry Reactant
Product
( C)
(h)
(%)
(%)
T
t
C
Y Y
°
Entry Reactant
1
Product
( C)
(h)
(%)
(%)
c
18
150
12
94.6
92.2
100
2
99.4
98.7
c
1
9
0
160
130
12
6
88.1
99.2
86.8
98.3
2
3
100
100
2
6
99.6
98.4
98.5
98.2
2
a
Reaction conditions: reactant, 1 mmol; Zr-BDB, 50 mg; isopropyl
b
alcohol, 5 g. The conversion and yield were determined by GC. T =
temperature, t = time, C = conversion, and Y = yield. The amount of
4
5
6
7
100
100
120
130
130
6
4
6
6
8
97.2
98.7
99.1
97.2
97.3
96.7
98.2
96.6
95.8
95.7
c
Zr-BDC was 0.1 g.
unsubstituted ones, and the substrate with electron donating
groups showed low reactivity. For example, benzaldehyde
could be completely converted into benzyl alcohol with a reac-
tion time of 2 h at 100 °C (Table 2, entry 1), while other substi-
tuted benzaldehydes required longer reaction time (Table 2,
entries 3–5) and/or higher reaction temperature (Table 2,
entries 6–8). This phenomenon was more obvious for aceto-
phenone (Table 2, entry 14) and the substituted ones (Table 2,
entries 15–17). 4-Nitrobenzaldehyde (Table 2, entry 5) had
higher reactivity than 4-methoxybenzaldehyde (Table 2, entry
3
), indicating the less negative effect of the electron withdraw-
ing group than the electron donating group. Cinnamaldehyde
Table 2, entry 9) showed lower reactivity than 3-phenylpropio-
8
9
(
naldehyde (Table 2, entry 10) owing to the conjugative effect of
the C=C bond. Moreover, the reactivity decreased for the reac-
tants with the phenolic hydroxyl group because its interaction
with the borate group was too strong (Table 2, entries 6–8),
and thereby, higher reaction temperature was needed to break
this interaction to release the borate, which was one of the
catalytic sites in Zr-BDB. Moreover, aliphatic ketones (Table 2,
entries 12 and 13) showed much higher reactivity than aro-
matic ketones (Table 2, entries 14–19). In addition, Zr-BDB
could efficiently catalyze the MPV reaction of 2-phenoxyaceto-
phenone (Table 2, entry 20), a lignin model compound. These
results further verified the superior catalytic activity of Zr-BDB
in MPV reactions.
100
100
100
6
2
4
96.9
96.2
97.9
94.2
95.7
93.8
1
1
0
1
1
2
100
6
98.7
98.3
1
1
3
4
100
150
6
97.5
96.7
96.1
96.2
Discussions on the high activity of the Zr-BDB
10
As discussed above, the prepared Zr-BDB had superior catalytic
activity for the MPV reaction of various biomass-derived carbo-
c
c
c
1
1
1
5
6
7
160
160
160
12
12
12
90.4
88.7
87.5
88.1
87.3
86.4
2
nyl compounds, which was much better than ZrO , Zr-BDC,
and UiO-66. Generally, for the Zr-catalyzed MPV reaction, the
catalytic activity mainly depends on the oxophilicity of
8b,9,11
Zr,
which was the active site to activate the carbonyl
group in the reactant, and the basicity of the catalyst, which
affects the activation of the hydroxyl group in the hydrogen
8b,11
donor (the alcohol).
In one aspect, a much higher catalytic activity was achieved
over Zr-BDB in comparison with ZrO (Table 1, entries 1 and
2
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2
Fig. 3 XPS spectra of Zr 3d in Zr-BDB, Zr-BDC, UiO-66, and ZrO .
1
Fig. 4 H NMR spectra of isopropyl alcohol, 1,4-benzenediboronic
acid, and their mixture in DMSO-d
6
.
2
). As determined by XPS spectra (Fig. 3), the binding energies
of Zr 3d in Zr-BDB were much higher than those in ZrO , indi-
cating the higher positive charge on the Zr atoms in the Zr-
BDB, which afforded the Zr in Zr-BDB having a stronger oxo-
2
found in the mixture of isopropyl alcohol and BDC (Figs. S7
and S10–12†), and the difference between the isopropyl
alcohol and the mixture of isopropyl alcohol and BDC was that
the signal for the hydroxyl group became broader in the
mixture, which may be caused by the hydrogen bonding
between BDC and isopropyl alcohol. These results implied the
stronger activation role of borate groups on isopropyl alcohols
than carboxylate groups, and also confirmed the strong inter-
action between the hydroxyl group and the borate group,
thereby significantly improving the reactivity of the hydroxyl
group. Thus, higher catalytic activity was achieved when using
Zr-BDB as the catalyst.
philicity than that in ZrO . The stronger oxophilicity was ben-
2
eficial for the activation of the carbonyl group in the reactant,
and thus, Zr-BDB showed higher activity than ZrO
2
.
Meanwhile, as determined from CO -TPD examinations
2
2
(Table S3†), Zr-BDB showed much higher basicity than ZrO .
Higher basicity was favorable to activate the hydroxy group in
the hydrogen donor, which could enhance the MPV reaction.
The synergistic effect of Zr with stronger oxophilicity and
higher basicity resulted in Zr-BDB having much higher cata-
lytic activity than ZrO2.
In another aspect, Zr-BDB showed much higher catalytic Mechanism
activity for the MPV reaction than both Zr-BDC and UiO-66.
Based on the obtained results and some reported
However, Zr-BDB, Zr-BDC, and UiO-66 had similar binding
energies of Zr 3d as well as similar basicity (Fig. 3 and
Table S3†). Thereby, the difference in these three catalysts did
not originate from the oxophilicity of Zr and the basicity. On
the basis of the above discussion, the catalytic activity of the
Zr-organic ligand hybrids increased with the number of borate
groups on the benzene ring of the organic ligands (Table 1,
entries 2, 4, and 5). From these results, we could deduce that
the difference in catalytic activity of Zr-BDB, Zr-BDC, and
UiO-66 resulted from the borate groups. As reported, the B
atom in the borate groups, acting as the Lewis acidic sites,
could activate the hydroxyl groups in the isopropyl alcohol via
the interaction between the B atom and the O atom of the
8b,10,11
knowledge,
we can deduce that both Zr and borate
groups, playing the role of activating the carbonyl and hydroxyl
groups respectively, were crucial to achieve high catalytic
activity for the MPV reaction. Thus, we proposed a possible
mechanism for the Zr-BDB-catalyzed MPV reaction (Scheme 1).
In each catalytic cycle, the carbonyl group in the reactant was
activated by the interaction between the O atom in the C=O
4+
group and the Zr in Zr-BDB. Meanwhile, the O atom in the
hydroxyl group of the isopropyl alcohol interacted with the B
atom in the Zr-BDB, which could promote the dissociation of
1
2
hydroxyl group, and thus, H was much easier to be released
1
to attend the MPV reaction. H NMR spectra (Fig. 4 and S7–9†)
clearly showed new peaks centered at 1.21, 4.65, and 8.71 ppm
in the mixture of isopropyl alcohol and 1,4-benzenediboronic
acid, which corresponded to the H signals in the activated iso-
propyl alcohol (Scheme S3†). Meanwhile, the signal for the
hydroxyl group became broader in the mixture of isopropyl
alcohol and 1,4-benzenediboronic acid compared to that in
Scheme 1 Possible mechanism for the Zr-BDB-catalyzed MPV reaction
isopropyl alcohol (Fig. 4). In contrast, no new signals were of biomass-derived carbonyl compounds.
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the hydroxyl group to attend the MPV reactions. Subsequently, Tensor 27 IR spectrometer via the KBr pellet method. X-ray
hydrogen transfer occurred from the activated hydroxyl group diffraction (XRD) measurements were carried out on an X-ray
to the activated carbonyl group via the formation of a six-link diffractometer (D/MAX-RC, Japan) operating at 40 kV and
intermediate. Finally, the carbonyl compounds were reduced 200 mA with Cu Kα (λ = 0.154 nm) radiation. XPS measure-
to the desired alcohols, and isopropyl alcohol was transformed ments were performed on an ESCALab 220i-XL spectrometer
into acetone. For the MPV reaction of levulinate esters, an at a pressure of ∼3 × 10− mbar (1 mbar = 100 Pa) using Al
intramolecular transesterification was essential to generate the Kα as the excitation source (hν = 1486.6 eV) and operating at
9
desired GVL (Scheme S4†).
15 kV and 20 mA. The contents of Zr and B in Zr-BDB were
determined with ICP-AES (VISTA-MPX), and the contents of C
and H in Zr-BDB were measured by elemental analysis using
the FLASH EA1112 analyzer. N2 adsorption–desorption iso-
therms were obtained on a Micromeritics ASAP 2020 V3.00 H
Conclusions
In summary, porous zirconium-organic borate (Zr-BDB) was (USA) surface area analyser at 77 K. Temperature-
fabricated by the coordination between ZrOCl and 1,4-benze- programmed desorption of carbon dioxide (CO -TPD) was
nediboronic acid. The synthesized Zr-BDB could efficiently performed on
2
2
a
Micromeritics’ AutoChem 2950 HP
1
and selectively catalyze the MPV reduction of various biomass- Chemisorption Analyzer. H NMR spectra were recorded on a
derived carbonyl compounds, including levulinate esters, alde- Bruker Avance NMR spectrometer (400 MHz) with DMSO-d6
hydes and ketones. Moreover, the Zr-BDB showed much better as the solvent.
catalytic performance than ZrO2 and the common Zr-based
metal organic framework (UiO-66). Systematic studies indi-
cated that the excellent performance of Zr-BDB originated
MPV reactions
In a typical experiment, reactant (1 mmol), isopropyl alcohol
4
+
from the synergetic effect of Zr and borate groups, which
activated the carbonyl groups in the substrates and the
hydroxyl groups in isopropyl alcohol, respectively. We believe
that zirconium-organic borates have great potential appli-
cations in MPV reductions, and they could be potentially
applied in other important transformations involving alcohols
as the reactants.
(
5 g) and suitable amounts of catalyst were charged into a
stainless reactor of 15 mL. After being sealed, the reactor was
placed into an air oven at a desired temperature. After the
reaction was conducted for a desired reaction time, the
reactor was cooled down quickly with ice water. Finally, the
conversion of the reactant and the yield of the product were
analyzed quantitatively using gas chromatography (GC,
Agilent 7890, using cyclohexanol as the internal standard)
equipped with the flame-ionized detector (FID) and the
HP-5MS capillary column (0.25 mm in diameter, 30 m in
length), and the product was also confirmed by GC-MS
Experimental
Preparation of the Zr-BDB
(
Agilent 5977A).
2 2
In a typical process, ZrOCl ·8H O (5 mmol) and BDB (5 mmol)
were dissolved in DMF (100 mL). The solution was stirred at
room temperature for 4 h. Then, the solution was transferred
Reusability of Zr-BDB
into a reactor of 150 mL volume. After being sealed, the To determine the reusability of the Zr-BDB, the used Zr-BDB
reactor was placed into an oven with a temperature of 150 °C was recovered by centrifugation, and washed with ethyl ether
for 48 h. When the reactor was cooled down to room tempera- (3 × 5 mL). After being dried at 80 °C under vacuum for 12 h,
ture, a white solid (Zr-BDB) was obtained by centrifugation, the recovered Zr-BDB was reused for the next cycle.
and the white solid was washed with DMF (3 × 8 mL), ethanol
(3 × 8 mL), and diethyl ether (3 × 5 mL). Finally, the solid was
dried at 80 °C under vacuum for 12 h. Anal. Found: 39.14% Zr,
Conflicts of interest
2
6.35% C, 4.85% H, and 7.82% B.
Other catalysts, including Zr-CPB, Zr-BDC, and Zr-B, were
There are no conflicts to declare.
synthesized with a similar route except for using the different
organic ligands.
Acknowledgements
Characterization
Scanning electron microscopy (SEM) measurements were The work was supported by the National Key Research and
conducted on a Hitachi S-4800 Scanning Electron Microscope Development Program of China (2018YFB1501602), National
operating at 15 kV. Transmission electron microscopy (TEM) Natural Science Foundation of China (22072157, 21875265,
measurements were performed on a TEM JEOL-1011 with an and 21673255), Beijing Municipal Science & Technology
accelerating voltage of 120 kV. The HR-TEM and corres- Commission (Z191100007219009), the Chinese Academy of
ponding elemental mappings were recorded on a HR-TEM Sciences (QYZDY-SSW-SLH013), and Youth Innovation
JEOL-2100F. FT-IR spectra were obtained using the Bruker Promotion Association of CAS (2017043).
1264 | Green Chem., 2021, 23, 1259–1265
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