Porous Zirconium-Phytic Acid Hybrid: A Highly Efficient Catalyst for Meerwein-Ponndorf-Verley Reductions

Article abstract of DOI:10.1002/anie.201504001

The utilization of compounds from natural sources to prepare functional materials is of great importance. Herein, we describe for the first time the preparation of organic-inorganic hybrid catalysts by using natural phytic acid as building block. Zirconium phosphonate (Zr-PhyA) was synthesized by reaction of phytic acid and ZrCl₄ and was obtained as a mesoporous material with pore sizes centered around 8.5 nm. Zr-PhyA was used to catalyze the mild and selective Meerwein-Ponndorf-Verley (MPV) reduction of various carbonyl compounds, e.g., of levulinic acid and its esters into γ-valerolactone. Further studies indicated that both Zr and phosphate groups contribute significantly to the excellent performance of Zr-PhyA. All natural: Porous zirconium phosphonate (Zr-PhyA) was synthesized simply by reaction of natural phytic acid (PhyA) and ZrCl₄ and applied as a very efficient catalyst for the Meerwein-Ponndorf-Verley reduction of various carbonyl compounds. Both the Zr element and phosphate groups contributed significantly to the excellent catalytic performance of Zr-PhyA.

Full text of DOI:10.1002/anie.201504001

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504001
German Edition: DOI: 10.1002/ange.201504001
Biomass Conversion
Porous Zirconium–Phytic Acid Hybrid: a Highly Efficient Catalyst for
Meerwein–Ponndorf–Verley Reductions**
Jinliang Song,* Baowen Zhou, Huacong Zhou, Lingqiao Wu, Qinglei Meng, Zhimin Liu, and
Buxing Han*
Abstract: The utilization of compounds from natural sources
to prepare functional materials is of great importance. Herein,
we describe for the first time the preparation of organic–
inorganic hybrid catalysts by using natural phytic acid as
building block. Zirconium phosphonate (Zr-PhyA) was
synthesized by reaction of phytic acid and ZrCl4 and was
obtained as a mesoporous material with pore sizes centered
around 8.5 nm. Zr-PhyA was used to catalyze the mild and
selective Meerwein–Ponndorf–Verley (MPV) reduction of
various carbonyl compounds, e.g., of levulinic acid and its
esters into g-valerolactone. Further studies indicated that both
Zr and phosphate groups contribute significantly to the
excellent performance of Zr-PhyA.
various zeolites,[11] etc. Among the catalysts, Zr-containing
catalysts including ZrO2,[10b] zirconium alkoxides,[7b] Zr-con-
[10c]
taining zeolites[11a] and Zr(OH)4
are the commonly used
ones. However, there are still some drawbacks for Zr-based
catalysts, such as deactivation and need of harsh reaction
conditions. Therefore, exploration of efficient and stable
heterogeneous Zr-based catalysts for MPV reductions is
highly desirable.
Phytic acid (PhyA, Scheme S1 in the Supporting Infor-
mation) obtained from seeds and grains is a major phosphorus
resource in plants.[12] It is widely used as food additive and
antioxidant. Due to the existence of six phosphate groups in
its structure, it can coordinate with metal ions, and this
characteristics has been used in water treatment and metal
material protection.[13] As far as we know, preparation of
organic–inorganic hybrid catalysts using PhyA as a building
block has not been reported. Herein, we designed a new
porous zirconium phosphonate simply by the reaction of
ZrCl4 and PhyA, denoted as Zr-PhyA. The Zr-PhyA could be
used as highly efficient heterogeneous catalyst for MPV
reactions of carbonyl compounds.
The prepared Zr-PhyA was characterized by scanning
electron microscopy (SEM), transmission electron microsco-
py (TEM), X-ray diffraction (XRD), and N2 adsorption–
desorption methods. The Zr-PhyA material was formed from
small particles with the diameter of about 80 nm (Fig-
ure 1B,C). The catalyst was porous with mesopores centered
around 8.5 nm (Figure 1E), and the surface area and pore
volume were 215 m2 gÀ1 and 0.42 cm3 gÀ1, respectively
(Table S1). In addition, the material had low crystallinity
(Figure 1F). All these parameters indicate that Zr-PhyA may
serve as a good catalyst. For comparison, other metal (Sn, Nb,
Cr, Cu)–phytic acid hybrids were also synthesized with similar
textural and structural properties (Figures S1–S3 and
Table S2).
Inductively coupled plasma (ICP) analysis showed that
the Zr/P molar ratio in Zr-PhyA was about 1:2 (Supporting
Information), which indicates one Zr4+ can coordinate with
two phosphate groups. Due to the coordination ability of all
the six phosphate groups in a PhyA molecule and the steric
hindrance of PhyA, the most possible connectivity pattern is
shown in Scheme 1 and the network of Zr-PhyA was mainly
generated through this connectivity pattern. However, the
XRD pattern (Figure 1F) indicates that the material has low
crystallinity or is poorly ordered. Thus, it can be deduced that
there are many defects or irregular connectivity in Zr-PhyA,
which is consistent with the fact that the pore size distribution
was relatively wide (Figure 1E).
T
he assembly of compounds from natural sources to form
functional materials has been recognized as an important area
in synthetic chemistry.[1] The diversity of natural compounds
provides many opportunities for designing functional materi-
als. In particular, some naturally occurring compounds (e.g.
porphyrin, polyphenol and polyacids) can chelate with
various metal ions to form useful materials with a range of
functions, such as Cu2+-heme for oxygen transport,[2] amino
acids-based materials for catalysts,[3] Fe3+-phenolic complexes
for adhesion,[4] and metal-phenolic networks for coatings and
capsules.[5] Inspired by these useful examples, attention was
paid to the preparation of functional materials by the
coordination of natural building blocks with metal ions.
Especially, the diversity of natural compounds provides
a range of possibilities for designing efficient catalysts.
The Meerwein–Ponndorf–Verley (MPV) reaction is
a highly chemoselective reduction method for carbonyl
compounds, which provides an attractive alternative to H2
by using secondary alcohols as the hydrogen resource.[6] It has
been reported that MPV reaction can be efficiently catalyzed
by diverse catalysts, such as metal alkoxides,[7] metal com-
plexes,[8] hydrotalcites,[9] metal oxides or hydroxides,[10] and
[*] Dr. J. L. Song, B. W. Zhou, Dr. H. C. Zhou, L. Q. Wu, Dr. Q. L. Meng,
Prof. Dr. Z. M. Liu, Prof. Dr. B. X. Han
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190 (China)
E-mail: songjl@iccas.ac.cn
hanbx@iccas.ac.cn
[**] We thank the National Natural Science Foundation of China
(21003133, 21321063), the Ministry of Science and Techonology of
China (2011CB808600), and the Chinese Academy of Sciences
(KJCX2.YW.H30).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504001.
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Figure 1. Characterization of the prepared Zr-PhyA. A,B) SEM images, C) TEM image, D) N2 adsorption–desorption isotherm, E) pore size
distribution, and F) powder XRD pattern.
Table 1: MPV reaction of EL with isopropyl alcohol over various catalysts
under different conditions.[a]
Entry
Catalyst
T
Time
[h]
Conv.
[%][c]
GVL yield
[%][c]
Sel.
[8C][b]
[%][d]
1
2
3
4
5
6
7
8
None
150
150
150
150
150
150
150
150
130
100
130
130
130
130
130
150
150
200
200
150
150
6
6
6
6
6
6
6
6
8
24
8
8
8
8
2
6
16
1
1
1
6
0
0
–
Zr-PhyA
Zr-BTC
ZrO2
100
78.7
24.1
5.2
96.7
30.5
21.1
2.1
0
0
96.7
38.7
87.6
40.4
0
Sn-PhyA
Nb-PhyA
Cu-PhyA
Cr-PhyA
Zr-PhyA
Zr-PhyA
Zr-BTC
ZrO2
Zr-PhyA
Zr-PhyA
Zr-PhyA
Zr-PhyA
ZrO2
1.9
<1
<1
98.9
86.6
46.6
10.5
100
83.4
100
100
100
100
93.6
100
100
0
0
0
9
95.4
74.6
9.6
96.5
86.1
20.6
74.3
95.8
87.4
98.7
97.8
84.7
98.5
94.5
96.7
92
10
11
12
13[e]
14[f]
15[g]
16[h]
17[h,i]
18
19[j]
20[g,h]
21[g,h,k]
7.8
95.8
72.9
98.7
97.8
84.7
98.5
88.5
96.7
92
Scheme 1. The most plausible connectivity pattern between PhyA and
Zr4+
.
Zr-PhyA
Zr(OH)4
Zr-PhyA
Zr-Beta
Conversion of levulinic acid (LA) and its esters into g-
valerolactone (GVL) is an important reaction in biomass
conversion.[14] The process involves the reduction of carbonyl
groups. Therefore, we selected MPV reduction of LA and its
esters to study the activity of Zr-PhyA. Ethyl levulinate (EL)
was used as the model reactant to study the influence of
various parameters (Table 1). The reaction did not proceed
without catalyst (entry 1). Among the examined catalysts, Zr-
PhyA showed the highest activity with a GVL yield of 96.7%
at 1508C (entry 2). In contrast, Zr-BTC, a common metal–
organic framework based on ZrCl4 and 1,3,5-benzenetricar-
boxylate (BTC), only gave a moderate GVL yield of 30.5%
(entry 3). Commercial ZrO2 also showed lower activity with
a GVL yield of 21.1% (entry 4). Other metal–phytic acid
hybrids were not active under the reaction conditions
[a] Reaction conditions: catalyst 0.2 g; reactant 1 mmol; isopropyl
alcohol 4 mL. [b] T=Temperature. [c] Conversion (Conv.) and GVL yield
were determined by GC. [d] Sel.=Selectivity. [e] The reactant was ML.
[f] The reactant was BL. [g] The reactant was LA. [h] The hydrogen donor
was 2-butanol. [i] Data obtained from Ref. [15]. [j] Data obtained from
Ref. [10c]. [k] Data obtained from Ref. [16].
(entries 5–8). Furthermore, it was also demonstrated that
Zr-PhyA had better performance than other Zr-based cata-
lysts reported in the literature under comparable reaction
conditions (entries 16–21). These results indicated that Zr-
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PhyA is a superior catalyst for MPV reaction of EL due to its
higher acidity and basicity, as we discuss below.
As expected, the activity of Zr-PhyA increased with rising
temperature (Table 1, entries 2, 9, 10, and Figure S4). To our
delight, Zr-PhyA maintained its high activity in the MPV
reaction of EL under lower temperatures. The reaction could
be completed with a GVL yield of 95.4% at 1308C in 8 h
(entry 9), which is a much higher yield as obtained over Zr-
BTC or ZrO2 (entries 11 and 12). Importantly, a GVL yield of
74.6% could be obtained from EL even at 1008C when the
reaction time was prolonged to 24 h (entry 10). In contrast,
there was no GVL produced at 1008C over Zr-BTC or ZrO2.
Moreover, methyl levulinate (ML) also gave a high GVL yield
(entry 13). Butyl levulinate (BL) had lower activity than ML
and EL, with a GVL yield of 72.9%, due to the steric
hindrance of the butyl group (entry 14). Compared with
esters, LA had a higher reactivity and was fully converted
with a GVL yield of 98.7% within 2 h at 1308C (entry 15).
This may result from LA being itself acidic, which could
improve the lactonization of 4-hydroxypentanoate
(Scheme 2) and thus increase the reaction rate.
In addition, the heterogeneous nature of Zr-PhyA in the
catalytic process was evaluated by removing the catalyst from
the mixture after reaction for 2 h at 1308C, and stirring the
reaction mixture for 24 h without solid catalysts. No further
reaction occurred (Figure S5), verifying that Zr-PhyA was
heterogeneous. Meanwhile, the concentration of Zr was
extremely low (< 0.5 ppm) in the reaction solutions as
detected by ICP, suggesting that leaching of active species
was negligible. Moreover, it was shown that Zr-PhyA could be
reused at least five times without decline of activity
(Figure 2). Zr-PhyA recovered after five times reuse was
Figure 3. XPS spectra of A) Zr 3d in Zr-PhyA, Zr-BTC and ZrO2 and
B) O 1s in Zr-PhyA and Zr-BTC.
As discussed above, Zr element was crucial for the MPV
reaction. We examined the local environment of Zr species in
Zr-PhyA, Zr-BTC and ZrO2 by XPS. As shown in Figure 3A,
the binding energies of Zr in Zr-PhyA were higher than in Zr-
BTC and ZrO2. The higher binding energy of Zr 3d in Zr-
PhyA indicates a higher positive charge on the Zr atoms,
which results in a stronger Lewis acidity of Zr.[17] The higher
Lewis acidity of Zr-PhyA was also confirmed by NH3-TPD
(Figure S7). The higher Lewis acidity of Zr in Zr-PhyA is
beneficial to activating the carbonyl groups and thus
increased the total reaction rate. Further, we have conducted
two Lewis acid-catalyzed reactions, that is, alcoholysis of
epoxides[18] (Table S4) and cyanosilylation of cyclohexa-
none[19] (Table S5). As shown in the two tables, Zr-PhyA is
more active than Zr-BTC and ZrO2 in both reactions. These
results confirmed the higher Lewis acidity of Zr-PhyA
compared to Zr-BTC and ZrO2.
To prove the importance of phosphate groups, we
synthesized several Zr-containing catalysts with different
mole ratios of PhyA and BTC and compared their activities
at 1508C and 1308C (Figure 4). N2 adsorption–desorption
Figure 2. Reusability of Zr-PhyA at 1308C with reaction times of A) 8 h
and B) 3 h. Reaction conditions: Zr-PhyA 0.2 g; EL 1 mmol; isopropyl
alcohol 4 mL.
Figure 4. Influence of mole ratio of BTC and PhyA at A) 1508C and
B) 1308C. Reaction conditions: catalyst 0.2 g; EL 1 mmol; isopropyl
alcohol 4 mL, reaction time 6 h.
characterized by FT-IR, XRD, SEM, and TEM (Figure S6),
and the results indicated that the properties of Zr-HBA
remained unchanged. The surface characteristics, chemical
composition, textural properties and acidic properties of fresh
and used Zr-PhyA were also examined by XPS (Table S3),
ICP and elemental analysis (Table S3), N2 adsorption–
desorption (Figure S6E and Table S1), and NH3-TPD (Fig-
ure S6F). The results showed that these properties of fresh
and used Zr-PhyA also remained almost unchanged after 5
catalytic recycles.
examinations indicated similar textural properties of these
materials (Figure S8 and Table S1). However, the activity of
the catalysts increased with increasing PhyA content. This
finding shows that phosphate groups are essential for the high
activity of Zr-PhyA. The main reason is that phosphate
groups are more basic than carboxyl groups, which could be
verified by the higher activity of Na3PO4 compared to
CH3COONa in the transesterification of EL with isopropyl
alcohol (Table S6). We further examined the local environ-
ment of the O element in Zr-PhyA and Zr-BTC (Figure 3B).
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The binding energy of O 1s corre-
sponding to P-O-Zr interactions (in
=
coexistence with the P O species)
in Zr-PhyAwas lower than for C-O-
Zr interactions (in coexistence with
[20]
=
the C O species) in Zr-BTC. P-
O-Zr and C-O-Zr in the two cata-
lysts are active species for MPV
reaction of EL. The lower binding
energy of O 1s in Zr-PhyA should
result in a higher negative charge
on the
O atoms, which could
increase the basicity of O. CO2-
TPD methods also indicated that
Zr-PhyA has higher basicity (Fig-
ure S9). The results above revealed
that Zr-PhyA has a higher basicity
than Zr-BTC, which is favorable for
the dissociation of the hydroxy
groups in isopropyl alcohol occur-
ring through activation by basic
sites (O2À) with the aid of Lewis
acidic sites (Zr4+) (Scheme 2), thus
increasing the reaction rate of the
MPV reaction of EL.[15] Therefore,
Zr-PhyA with the most phosphate
Scheme 2. Possible mechanism for Zr-PhyA-catalyzed MPV reaction of LA and its esters to produce
GVL.
groups showed the highest activity for MPV reaction of EL.
As discussed above, acidic and basic sites are crucial for
MPV reaction. NH3-TPD (Figure S7) and CO2-TPD (Fig-
ure S9) indicate that there are both acid sites (originating
from Zr4+) and basic sites (originating from O2À in phosphate
groups) on Zr-PhyA. On the basis of the experimental results
and a previous report,[21] a possible mechanism for Zr-PhyA-
catalyzed MPV reaction of LA and its ester was proposed
(Scheme 2). In the catalytic cycle, isopropyl alcohol is
adsorbed on Zr-PhyA, resulting in its dissociation to the
corresponding alkoxide and hydrogen by the acid–basic sites
(Zr4+–O2À).[6] Concurrently, the carbonyl group in LA and its
esters is activated by Zr4+. Next, 4-hydroxypentanoate or its
ester is formed by hydrogen transfer between the dissociated
alcohol and the activated LA or ester levulinate through a six-
link intermediate. Finally, GVL is obtained from 4-hydroxy-
pentanoate through intramolecular esterification or trans-
esterification.
Table 2: MPV reaction of carbonyl compounds over Zr-PhyA.[a]
Entry
Reactant
Product
T
[8C]
t
[h]
Conv.
[%][b]
Yield
[%][b]
1
2
100
100
1
2
97.7
99.3
97.7
99.3
3
100
6
87.6
87.6
4
5
6
100
150
100
10
10
20
94.5
93.5
95.4
94.5
93.5
95.4
Delighted by the excellent performance of Zr-PhyA for
MPV reaction of LA and its esters, we explored the possibility
of MPV reactions of other carbonyl compounds over Zr-
PhyA. Zr-PhyA also showed very good activity for the
examined compounds (Table 2). It can be seen that aldehydes
are converted better than ketones due to their lower steric
hindrance. Importantly, compounds with a double bond or
benzene group afford longer reaction times or harsh con-
ditions because the double bond or benzene group could
occupy the Lewis acid centers due to their higher electron
density, thus lowering the probability of carbonyl groups to
reach the active centers.
7
150
6
98.4
98.4
[a] Reaction conditions: catalyst 0.2 g; reactant 1 mmol; isopropyl
alcohol 4 mL. [b] Conversion and yield were determined by gas
chromatography (GC).
and selectivity for MPV reactions of various carbonyl
compounds, especially for the conversion of LA and its
esters to produce GVL. Systematic studies indicated that both
the Lewis-acidic Zr element and the Lewis-basic phosphate
groups contribute significantly to the excellent catalytic
performance. We believe that the highly efficient and easily
prepared catalyst has great potential of application in MPV
In summary, a new porous zirconium phosphonate, Zr-
PhyA, has been developed by the reaction of ZrCl4 and
naturally occurring PhyA. Zr-PhyA showed very high activity
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Keywords: Meerwein–Ponndorf–Verley reduction ·
natural resources · phytic acid · porous materials ·
zirconium phosphonate
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9399–9403
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Received: May 6, 2015
Revised: June 8, 2015
Published online: July 14, 2015
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