The transfer hydrogenation of high concentration levulinic acid to γ-valerolactone catalyzed by glucose phosphate carbamide zirconium

Article abstract of DOI:10.1039/d1gc00209k

Zr-Based catalysts have been extensively applied in Meerwein-Ponndorf-Verley type catalytic transfer hydrogenation (CTH) reactions, but they are easily deactivated in the CTH conversion of high concentrations of levulinic acid (LA) to γ-valerolactone (γ-GVL). This work discloses that by using cheap glucose and ZrCl₄as two main raw materials, glucose phosphate carbamide zirconium (GluPC-Zr) is easily synthesized at large scale and low costviaa simple two-step conversion. The constructed GluPC-Zr has enhanced Lewis acid-base properties and good porosity, thus exhibiting outstanding activity for the CTH reactions of LA or its esters with isopropanol (IPA), providing 95-98% γ-GVL yields. Because of the excellent esterification performance of the introduced acidic phosphate groups, GluPC-Zr also works well at high LA concentrations, achieving a much higher turnover frequency (TOF, 8.2 mmol γ-GVL per g catalyst per h) than previously reported Zr-based catalysts (TOF, 0.2-2.4). And it shows excellent reusability in the reaction of LA with IPA, still providingca.95% γ-GVL yield after the seventh cycle run. This work provides a preferential esterification strategy for LA to hamper catalyst deactivation, which is of special significance for the large-scale production of γ-GVL from biomass-derived LA and a low-cost GluPC-Zr catalyst.

Full text of DOI:10.1039/d1gc00209k

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The transfer hydrogenation of high concentration
levulinic acid to γ-valerolactone catalyzed by
glucose phosphate carbamide zirconium†
Cite this: Green Chem., 2021, 23,
428
3
Feifei Wan, Bo Yang, Jiekun Zhu, Dabo Jiang, Huanhuan Zhang, Qiao Zhang,
Shuainan Chen, Chao Zhang,
Yachun Liu and Zaihui Fu
*
Zr-Based catalysts have been extensively applied in Meerwein–Ponndorf–Verley type catalytic transfer
hydrogenation (CTH) reactions, but they are easily deactivated in the CTH conversion of high concen-
trations of levulinic acid (LA) to γ-valerolactone (γ-GVL). This work discloses that by using cheap glucose
4
and ZrCl as two main raw materials, glucose phosphate carbamide zirconium (GluPC-Zr) is easily syn-
thesized at large scale and low cost via a simple two-step conversion. The constructed GluPC-Zr has
enhanced Lewis acid–base properties and good porosity, thus exhibiting outstanding activity for the CTH
reactions of LA or its esters with isopropanol (IPA), providing 95–98% γ-GVL yields. Because of the excel-
lent esterification performance of the introduced acidic phosphate groups, GluPC-Zr also works well at
high LA concentrations, achieving a much higher turnover frequency (TOF, 8.2 mmol γ-GVL per g catalyst
per h) than previously reported Zr-based catalysts (TOF, 0.2–2.4). And it shows excellent reusability in the
reaction of LA with IPA, still providing ca. 95% γ-GVL yield after the seventh cycle run. This work provides
a preferential esterification strategy for LA to hamper catalyst deactivation, which is of special significance
for the large-scale production of γ-GVL from biomass-derived LA and a low-cost GluPC-Zr catalyst.
Received 19th January 2021,
Accepted 8th April 2021
DOI: 10.1039/d1gc00209k
rsc.li/greenchem
2
6,27
food additive and flavoring agent.
γ-GVL can be syn-
1
. Introduction
thesized by hydrogenation or Meerwein–Ponndorf–Verley type
The over-exploitation and consumption of non-renewable catalytic transfer hydrogenation (CTH) and then lactonization
fossil energy by the traditional chemical industry has been of another biomass-derived levulinic acid (LA) and its esters
leading to an intensified energy crisis, global warming and (LEs) with various hydrogen sources. The molecular H -
2
severe environmental pollution, and the excessive consump- involved conversion of LA and LEs to γ-GVL has a high hydro-
tion of fossil fuels and its impact on the greenhouse gas emis- genation efficiency in the presence of various transition metal
sions into the environment highlights the need to explore catalysts and an almost quantitative γ-GVL yield can be
1
,2
renewable and green energy sources. As we all know, ligno- achieved with precious metal catalysts, such as Ru, Ir, Pt, Pd
2
8–38
cellulosic biomass is considered to be an abundant renewable and Au under heterogeneous
and especially homogeneous
3
39–43
resource and it can be converted into high value-added plat- systems.
However, the high cost and scarcity of precious
form chemicals, such as succinic acid, sorbitol, and metals restrict their practical applications. In addition, high-
4
5
–12
13
14–18
glycerol,
furfural, 5-hydroxymethylfurfural,
levulinic pressure molecular hydrogen is normally required to improve
Among the above γ-GVL yield with transition metal catalysts, which involves
1
9
20,21
acid, and γ-valerolactone (γ-GVL).
biomass-derived platform chemicals, γ-GVL is acknowledged some problems, such as hydrogen transportation and
4
4
as a potential building block for the production of biofuels, storage. To overcome these obstacles, multiple efforts have
2
2–25
biochemicals and polymeric materials,
2
and as a solvent, been devoted to eliminating reliance on external H . For
example, formic acid (FA) has been adopted as a hydrogen
source owing to its co-production with LA during the acid-pro-
moted degradation of carbohydrates. FA can in-situ generate
H2 for the hydrogenation of LA by its decomposition over
National & Local United Engineering Laboratory for New Petrochemical Materials &
Fine Utilization of Resources, Key Laboratory of Resource Fine-Processing and
advanced materials of Hunan Province and Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education of China), College of
Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081,
China. E-mail: fzhhnnu@126.com
3
0,35,43,45
metal catalysts,
but precious metals and/or harsh con-
ditions are still required to carry out the decomposition and
hydrogenation steps and the leaching and/or sintering of
metals during the reaction can also easily occur upon those in-
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
4
6,47
d1gc00209k
expensive transition metal-containing catalysts.
For the
3428 | Green Chem., 2021, 23, 3428–3438
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large-scale production of γ-GVL, catalytic schemes are
required to maximize product yield without the use of precious tion of LA with IPA.
Two pathways have been reported to achieve the CTH reac-
6
0,61
One pathway is that the δ-carbonyl of
2
metals, high H pressure, or an excessive number of unit oper- LA is directly reduced by IPA to hydroxyl. Another pathway is
2
1
ations. The δ-carbonyl of LA and LEs can be reduced to that in the presence of IPA, the carboxyl of LA is first esterified
hydroxyl through the CTH reaction, which is conducted in and then its δ-carbonyl is reduced to hydroxyl (Scheme 1). It is
H-donor solvents such as IPA or 2-butanol. This CTH reaction expected that when the Zr-based catalyst simultaneously pos-
can achieve an almost quantitative γ-GVL yield but also pro- sesses excellent esterification and transfer hydrogenation
vides an attractive alternative for ensuring the hydrogenation activities, it should efficiently catalyze the conversion of a
becomes inherently safe and environmentally friendly. Many higher LA concentration to γ-GVL through the second CTH
catalysts exhibit high catalytic capability for the CTH reaction pathway. In order to develop this kind of catalyst, herein, we
4
8,49
of LA with IPA
and some representative Zr-based catalysts report that a new glucose phosphate carbamide-coordinated Zr
as
.2–2.4 turnover frequency (TOF, mmol γ-GVL per g catalyst two main raw materials can well achieve the CTH conversion
listed in Table 1 can achieve ca. 88–97% γ-GVL yield and catalyst (GluPC-Zr) prepared using cheap glucose and ZrCl
4
0
5
0–57
per h) with 0.05–0.25 M LA in a batch reactor.
Moreover, of a high concentration of LA to γ-GVL.
the flow CTH reaction of 0.05 M LA dilute solution in IPA has
been realized in a fixed bed reactor filled with Zr modified
β zeolite particles, affording a stable 90% γ-GVL yield during
continuous running (see Table 1). However, the large-scale
production of γ-GVL by the CTH pathway still needs to over-
2
. Experimental section
5
8
2.1. Reagents and materials
come the high catalyst cost, energy consumption and The main reagents used in this work were of analytical grade,
especially low production efficiency caused by the including glucose (Glu, >99.5%), phosphoric acid (H PO ,
3
4
appreciable deactivation of Zr-based catalysts at high concen- 85%), urea (>99.0%), ZrCl4 (>99.0%), levulinic acid (LA,
5
9
trations of LA.
98.0%), methyl levulinate (ML, 99.0%), ethyl levulinate (EL,
Table 1 The conversion of LA to γ-GVL using IPA as an H-donor over Zr-based catalysts
γ-GVL yield
(%)
a
Entry Catalyst
Reaction conditions
TOF
Ref.
1
2
3
Zr-PhyA (phytic acid)
Zr-HA (humic acids)
UiO-66-Zr
0.25 M LA in IPA: 4 mL, 0.2 g of catalyst, 130 °C, 2 h
0.20 M LA in IPA: 5 mL, 0.2 g of catalyst, 150 °C, 24 h
0.13 M LA in IPA: 30.55 mL, 0.8 g of catalyst,
96.7
88.3
92.7
2.4
0.2
2.3
50
51
52
0.24 g of naphthalene, 200 °C, 2 h
4
5
6
7
8
9
Zr-CA (cyanuric acid)
Zr-TMPA (trimetaphosphate)
Zr-HBA (hydroxyl benzoic acid) 0.13 M LA in IPA: 7.64 mL, 0.2 g of catalyst, 150 °C, 4 h
ZrF MOFs
Organic Zr phosphonate
Zr–Al-beta zeolite
0.13 M LA in IPA: 7.64 mL, 0.2 g of catalyst, 130 °C, 4 h
0.20 M LA in IPA: 5 mL, 0.2 g of catalyst, 160 °C, 8 h
96.8
96.2
94.4
1.2
0.6
1.2
2.3
0.4
53
54
55
56
57
58
0.05 M LA in IPA: 9.95 mL, 0.1 g of catalyst, 1 MPa N
2
, 200 °C, 2 h 94
0.20 M LA in IPA: 4.9 mL, 0.2 g of catalyst, 160 °C, 12 h
95
90
−
1
b
0.05 M LA in IPA, feed flow rate: 0.05 mL min
,
3.4
contact time: 5 min g mL− , 0.25 g of catalyst, 170 °C
1
10
GluPC-Zr
1.87 M LA in IPA: 2.67 mL, 0.05 g of catalyst, 180 °C/190 °C, 12 h
95/98.1
7.9/8.2 This work
a
b
Turnover frequency calculated based on the γ-GVL yield (mmol γ-GVL per g catalyst per h). Productivity calculated based on the γ-GVL yield (g
γ-GVL per g catalyst per h).
Scheme 1 The reaction pathways for synthesis of γ-GVL from LA with IPA over a Zr-based catalyst.
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9
9
9.0%), isopropyl levulinate (IPL, 99%), butyl levulinate (BL, (NETASCH, STA409PC) at a temperature from 25 to 960 °C
−1
9%), methanol (>99.5%), ethanol (>99.5%), propanol under flowing N (heating rate of 20 °C min ). X-ray photo-
2
(
>99.0%), 2-propanol (IPA, >99.0%), 2-butanol (>99.0%), cyclo- electron spectroscopy (XPS) of the samples was measured on a
hexanol (>99%), ZrO (>99.0%), ZrOCl ·8H (>99.0%), VG Multi Lab 2000 system with a monochromatic Mg-Kα source
dodecane (98.0%). All chemical reagents were obtained from operated at 20 kV. Transmission electron microscopy (TEM)
2
2
2
O
commercial suppliers and used without further purification.
images of the samples were obtained from a JEOL JEM-2100
transmission electron microscope at an accelerating voltage of
2
2
.2. Preparation of catalysts
00 kV. The sample for TEM analysis was dispersed in ethanol
.2.1. Preparation of glucose phosphate carbamide with the aid of sonication and then dropped onto a copper grid-
GluPC). Glucose was mixed with concentrated phosphoric supported carbon film. The zirconium and phosphorus con-
2
(
acid and urea to make a homogeneous paste. The molar ratio tents were determined with ICP-OES equipment (PerkinElmer
of the three reagents was 1 : 2 : 6. Then the mixture was trans- 8300). The particle size distribution of the sample in aqueous
ferred to an oven that had been heated to 135 °C for 2 h. After solution was measured on a Nano ZS90 dynamic light scattering
cooling to room temperature, the mixture was dissolved by the device with a He–Ne laser (at λ = 633 nm) as the light source
addition of deionized water. Methanol was added to precipi- and an instrument power of 75 mW.
tate the product from the aqueous phase. Finally, the product
2.4. CTH reaction operation
(
GluPC) was collected and washed with 80% (v/v) aqueous
methanol (20 mL × 3) and methanol (20 mL × 1), and dried at In a typical CTH reaction operation, LA or its ester (5 mmol,
0 °C in a vacuum for 8 h.
0.58 g), IPA (35 mmol, 2.67 mL) and GluPC-Zr (0.05 g) were
.2.2. Preparation of the GluPC-zirconium complex mixed in a 10 mL Teflon-lined stainless-steel autoclave and
GluPC-Zr). In a typical procedure, GluPC (0.5 g) and ZrCl
magnetically stirred for 10 min to obtain a very stable brown
0.75 g) were respectively dissolved in 8 mL and 10 mL of water suspension (see Fig. S2†). Then, the reactor was transferred
6
2
(
4
(
to obtain GluPC and ZrCl -derived sol solutions under ultra- into an preheated oven and the suspension was reacted at
4
sound for 5 min, respectively, as supported by the dynamic 180 °C for 12 h. The comparative data of reactions under stir-
light scattering results of these two aqueous solutions (see ring and without stirring listed in Table S1† indicated that this
Fig. S1, ESI†). Subsequently, these two sol solutions were pseudo-homogeneous reaction does not seem to need stirring.
mixed and stirred with a magnetic stirrer at room temperature After the reaction, the cooled mixture was centrifuged to
for 30 min. Afterwards, the mixture was transferred into a collect the filtrate for the analysis of the products. The quanti-
Teflon-lined stainless-steel autoclave (25 mL) and tightly fication of products was conducted on a gas chromatograph
sealed. The autoclave was placed into an oven that had been (GC, Shimadzu GC-2010) equipped with an FID detector and
heated to 120 °C. After 12 h of hydrothermal reaction, the HP-5 capillary column (30.0 m × 250 mm × 0.25 mm) using
autoclave was taken out and cooled to room temperature. The dodecane as the internal standard. The identification of the
obtained brown precipitates were filtered, and washed with products was done by GC-MS (Shimadzu QP2010).
distilled water and ethanol (10 mL × 3) until the filtrate
2
.5. Leaching experiments
became colorless and neutral (pH = 7). The obtained solid was
dried at 60 °C in a vacuum for 12 h to afford about 0.8 g of
GluPC-Zr complex.
After 2 h of the above-mentioned CTH reaction, the catalyst
was filtered out from the reaction system and the collected fil-
trate was allowed to react for another 10 h under identical con-
ditions to assess the leaching of the catalyst.
2
.3. Characterization
Transmission FT-IR spectra of the samples from 400 to
000 cm− were recorded on a Nicolet Nexus 510 P FT-IR 2.6 Catalyst recycling experiments
1
4
spectrometer using a KBr disk. Pyridine adsorbed FT-IR
spectra of the catalyst were obtained on Tensor 27 equipment
using KBr pellets in the range of 400–4000 cm to determine
The recycling test of GluPC-Zr was performed as follows: after
the reaction had proceeded at 180 °C for 12 h, the used
GluPC-Zr was separated from the reaction mixture by centrifu-
gation and washed with IPA until the filtrate was colorless, and
then further washed with absolute ethanol (10 mL × 3). After
drying at 60 °C under vacuum for 12 h, the recovered GluPC-Zr
was directly employed for the next cycle under identical reac-
tion conditions.
−
1
the types of acid sites. The amount of Brønsted acid sites was
−
1
calculated according to the 1545 cm characteristic band and
the amount of Lewis acid sites was calculated according to the
1
1
445 cm− characteristic band. N
2
adsorption–desorption iso-
therms of the samples were measured at liquid nitrogen temp-
erature using a Tristar 3000 fully automatic surface area and
pore analyzer. Before testing, all the samples were degassed at
2
00 °C for 5 h. X-ray diffraction (XRD) measurements of the
3
. Results and discussion
samples were conducted on an X-ray diffractometer (Ultima IV,
Japan) operated at 40 kV and 250 mA with Cu Kα (λ =
3.1. Characterization results of samples
0
.15406 nm) radiation. The thermal stability of the samples
3.1.1. Elemental analysis. The main active elements of
was examined using thermogravimetric analysis (TGA) samples were measured by inductively coupled plasma (ICP)
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analysis and the obtained data are listed in Table S2.† The P new bands are assigned to the stretching vibrations of various
content of GluPC is up to 17.6 wt%, indicating that glucose hydrophilic groups, including NH at 3184 cm− , COOH at
1
2
can be well functionalized by phosphoric acid under the treat- 1710 cm− , CONH
1
at 1632 cm , Ar–OH at 1610 cm and
−1
−1
2
ment conditions described in the Experimental section. The P polycyclic aromatics in the range 1400–1600 cm−
and Zr contents of GluPC-Zr measured by ICP are 7.2 and that glucose is likely to be dehydrated to form the soluble CQD
3.6 wt%, respectively, showing that GluPC can well coordinate with the above hydrophilic groups attached under co-treatment
1 62
,
indicating
1
4
+
with the Zr sol to form a hybrid catalyst.
with phosphoric acid and urea at 135 °C, consistent with the
3
.1.2. Functional groups and acidity. Fig. 1A gives the UV-vis and PL spectra of GluPC in Fig. S3.† Notably, two IR
FT-IR spectra of glucose, GluPC and GluPC-Zr. The IR spec- bands at 1054 and 926 cm− can also be found in the FT-IR
1
trum of glucose shows its characteristic absorption bands in spectrum of GluPC, a strong band at 1054 cm− is attributed
1
−
1
the range 3500–400 cm . After glucose turns into a GluPC to the symmetrical vibration of PvO in polyphosphate
ligand, its OH stretching vibration band at 3420 cm−
1
(PvOOH) or the C–O–P bond in the P-containing carbon-
6
3
−1
decreases remarkably and its C–H bond stretching vibration aceous species and another weak band at 926 cm
bands in the range 2950–2890 cm nearly disappear, along assigned to the stretching vibration of the P–O–P bond. This
is
−
1
64
with the generation of some new IR absorption bands. These indicates the incorporation of polyphosphate groups and the
2
Fig. 1 FT-IR spectra (A), pyridine-adsorbed FT-IR spectra of GluPC-Zr (B), N adsorption–desorption isotherms and BJH pore size distributions (C),
powder XRD patterns (D), and thermogravimetric (E) and differential DTG (F) curves of samples.
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P-containing carbonaceous species in the GluPC ligand, as tribution of slit-shaped pores. These findings suggest that
supported by the ICP analysis of GluPC. After treatment with GluPC-Zr has richer hierarchical slit-shaped pores that can
4
+
the Zr sol, the above IR bands attributable to the ligand improve the diffusion of substrate and its accessibility to the
undergo different degrees of blue-shift and the COOH band active sites in the pores.
becomes strong, along with a decrease in the CONH2 band
3.1.4. XRD. Fig. 1D shows the XRD patterns of commercial
and GluPC-Zr samples. There, some characteristic diffrac-
and the disappearance of the P–O–P band. These changes in ZrO
2
4
+
the FT-IR spectrum of GluPC-Zr show that the Zr sol can tion peaks in the range 20–60° are observed in the XRD pattern
coordinate with all the hydrophilic groups of GluPC to form a of ZrO , among them, four diffraction peaks at 30.4, 35, 50.4
2
hybrid catalyst containing Zr and P active species, as supported and 60.2° are attributable to the tetragonal crystal (t-ZrO
2
) and
by ICP analysis. On the other hand, the strong acidity of the six other diffraction peaks at 24.2, 28.4, 31.6, 41.4, 45.3 and
68,69
aqueous medium caused by ZrCl hydrolysis is likely to lead to 55.8° are assigned to the monoclinic crystal (m-ZrO2),
the hydrolysis of CONH into COOH groups and the cleavage cating that ZrO
2 2
indi-
is a mixture of tetragonal and monoclinic
of the P–O–P bond in the polyphosphates during the hydro- phases. GluPC-Zr presents two very broad diffraction bands in
thermal preparation of GluPC-Zr. the range 15–60° in the XRD pattern. A strong broad peak in the
4
In order to examine the acidity of GluPC-Zr, pyridine- range 10–40° (centered 2θ of 25.8°) is assigned to amorphous
adsorbed FT-IR spectra were conducted at two desorption carbon composed of aromatic carbon sheets oriented in a
7
0
temperatures of 40 and 100 °C under vacuum conditions. As random fashion, another weak broad peak in the range
7
1,72
shown in Fig. 1B, five IR bands at 1445, 1490, 1545, 1610 and 40–60° is attributable to the a-axis of the graphite structure,
1
640 cm− are found in the pyridine-adsorbed FT-IR spectra, which can be indicative of the CQD character of its ligand. But
and they are assigned to the pyridine adsorbed on the Lewis its XRD pattern lacks the above characteristic diffraction peaks
acidic (LA) sites, on the Brønsted acidic (BA) sites or both of of crystalline ZrO , suggesting that the initial small nucleated
particles (1.5–4.8 nm, see Fig. S1B†) obtained by the hydro-
1
2
6
5,66
these two acid sites in GluPC-Zr.
The LA and BA sites in ZrO
2
4
+
GluPC-Zr should mainly originate from the Zr centers and lysis of ZrCl solution can be well maintained in the hybrid cata-
4
the P-containing functional groups, respectively. Also, the BA lyst through strong coordination with GluPC, and which show a
and LA sites were quantified according to 1545 and 1445 cm−
1
significant diffraction peak broadening.
characteristic bands and the calculated data are listed in
3.1.5. TGA. The thermal decomposition behaviors of
Table S3.† At 40 °C, the LA amount of GluPC-Zr is 0.309 mmol GluPC and GluPC-Zr were checked via the thermogravimetric
−
−
1
g
g
, and it is much higher than the amount of BA (0.059 mmol analysis (TGA) technique and the obtained TG and its differen-
), in line with the above ICP data. Raising the degassing tial (DTG) curves are depicted in Fig. 1E and F. The DTG curve
1
temperature to 100 °C results in a decrease in these two acid of GluPC displays five weight-loss peaks in the range
−
1
amounts to 0.198 and 0.053 mmol g , respectively. Notably, 90–1000 °C; among these weight-loss peaks, the first weak
the rate of reduction in the amount of BA (about 10%) is much peak at 96 °C corresponds to evaporation of the adsorbed
lower than that in the amount of LA (about 36%). These find- water molecules. Two unseparated strong peaks at 226 and
ings indicate that the strong B acidity and high LA sites of 271 °C are likely to originate from the decomposition of the
GluPC-Zr should be advantageous to the pre-esterization and hydrophilic OH, NH , COOH and CONH groups of GluPC,
2
2
CTH reactions of LA with IPA, respectively.
.1.3. Porosity. The porosity of the GluPC-Zr sample was further carbonization process of carbonaceous species and the
measured using a low-temperature nitrogen (N ) adsorption last peak at 901 °C is perhaps related to the decomposition of
and the fourth strong peak at 671 °C may be attributed to the
3
2
2
apparatus and the obtained N adsorption–desorption iso- the incorporated polyphosphates. The P-containing residual
therms are shown in Fig. 1C. There, the isotherm of GluPC-Zr species is estimated to be ca. 15.4 wt% based on the TG curve
belongs to type II according to the IUPAC classification. At low of GluPC, which is slightly lower than the P content (17.6 wt%)
to moderate relative pressures (0–0.8), the isotherm shows very measured by ICP analysis. Compared to GluPC, the weight-loss
2
slow increase in N uptake without hysteresis. At high relative peak for the adsorbed water molecules becomes strong and
pressure, the isotherm exhibits a fast increase in N2 uptake shifted to 116 °C in the DTG curve of GluPC-Zr, indicating that
with type H3 narrow and inconspicuous hysteresis, which can this porous material has a strengthened adsorption capacity
be indicative of the characteristics of slit-shaped mesopores for water. In addition, the decomposition peaks for the hydro-
6
7
and macropores. The inset in Fig. 1C displays that the BJH philic groups of the ligand become significantly weak and
pore size distribution curve of GluPC-Zr presents a typical hier- shifted toward high temperature in the DTG curve of GluPC-Zr,
archical pore character that consists of mesopores smaller further affirming the strong coordination between GluPC and
than 5 nm and meso–macropores more than 5 nm. The the Zr sol. Notably, the above-mentioned decomposition peak
porous parameters listed in Table S4† further reveal that at 900 °C is not found in the DTG curve of GluPC-Zr,
2
GluPC-Zr has good porosity and its BET surface area (SBET/m
g
suggesting that the incorporated polyphosphates may be con-
−
1
3
−1
), total pore volume (V /cm g ) and average porous size verted into very stable zirconium phosphate salts by the reac-
p
D/nm) are 224, 0.4 and 10.9, respectively, and the ratio of its tion with the Zr sol under hydrothermal conditions, as sup-
t-plot surface area (175 m g ) is up to 78%, further indicating ported by up to 72 wt% of inorganic species in GluPC-Zr by
4
+
(
2
−1
that the porosity of GluPC-Zr mainly originates from the con- TGA analysis.
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3
.1.6. XPS. X-ray photo-electron spectroscopy (XPS) was in LA and disaggregating the hydroxyl groups in IPA, respect-
used to investigate the chemical states for the surface atoms of ively, and the efficient synergy of these two types of active sites
GluPC and GluPC-Zr and the recorded high resolution XPS should significantly accelerate the CTH reaction of LA and its
spectra for the surface Zr, O, P and N atoms are shown in esters.
Fig. 2. The survey spectra in Fig. S4A† affirm the existence of
3.1.7. TEM. The morphologies of GluPC and GluPC-Zr
C, O, P and N elements in these two samples, as well as the were observed on a TEM instrument and the obtained images
existence of Zr element in GluPC-Zr. The deconvoluted high are shown in Fig. 3. In the TEM image of GluPC, its nano-
resolution XPS spectra for the surface C, O, P and N atoms by particles (NPs) are mostly between 5 and 15 nm, presenting a
Gaussian simulation support the existence of various hydro- very loosely aggregated shape. The TEM image of GluPC-Zr pre-
philic groups described in the FT-IR spectrum of GluPC (see sents a cage-like structure connected by various sizes of blocky
Fig. S4C–F†). Fig. 2A–C show that the binding energies of O 1s, particles and its large blocky surface attaches some NPs,
4
+
P 2p and N 1s for GluPC appear at 531.98, 133.23 and 399.58 suggesting that the coordination between GluPC and the Zr
eV, respectively. The introduced Zr species results in a slight sol can assemble into a network structure.
decrease in O 1s binding energy to 531.58 eV and an increase
3
.2 Catalytic performance evaluation
in P 2p and N 1s binding energies to 133.78 and 400.23 eV.
And the high resolution XPS spectrum for the surface C atoms
of these two samples also displays that two C 1s energy bands
in GluPC are shifted to slightly higher energy levels after intro-
ducing the Zr species (Fig. S4B†). Fig. 2D shows that the Zr 3d
spectrum of GluPC-Zr exhibits two energy bands at 183.08 and
3.2.1 CTH reaction of LA with IPA. Table 2 lists the com-
parative data for the CTH reactions of LA with IPA catalyzed by
GluPC-Zr and commercial zirconium compounds. As expected,
the ligand GluPC is not active for the CTH reaction (entry 1).
Among the examined zirconium compounds, Zr(SO4)2 does
not have any activity for this CTH reaction although it can cata-
lyze the esterification of LA with IPA (13.1% isopropyl levuli-
nate (IPL) yield, entry 2). Zr(OH) and ZrO , due to severe de-
activation at high LA concentrations, show poor activities for
the reaction and achieve only 8.6 and 13.6% γ-GVL yields,
respectively, along with the formation of a large amount of
6
6
1
85.38 eV, which are attributable to Zr 3d5/2 and Zr 3d3/2,
respectively, and these two bands become shifted to higher
energy levels compared to the previously reported high resolu-
4
2
5
0
7
3
tion XPS spectrum of Zr 3d in ZrO
2
(182.2 and 184.6 eV).
These findings confirm that the coordination between the
4
+
hydrophilic groups of GluPC and the Zr sol can bestow
4
+
2−
GluPC-Zr with the enhanced Lewis acidic Zr and basic O
sites that are more beneficial to activating the carbonyl groups
esterified product IPL (entries 3 and 4). ZrOCl
achieve medium 49.7 and 58% γ-GVL yields in the CTH reac-
2 4
and ZrCl
Fig. 2 High resolution XPS spectra of GluPC and GluPC-Zr: (A) O 1s XPS spectra; (B) P 2p XPS spectra; (C) N 1s XPS spectra; and (D) Zr 3d XPS
spectra.
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Fig. 3 TEM images of GluPC (A) and GluPC-Zr (B).
a
Table 2 CTH reaction of LA with IPA over various Zr-based catalysts
1
Entry
Catalyst
GluPC
T (°C)
Time (h)
Conversion (%)
Yield of γ-GVL (%)
Yield of IPL (%)
TOF (mmol g− h−1
)
1
2
3
4
5
6
7
8
9
180
180
180
180
180
180
180
190
150
150
150
120
12
12
12
12
12
12
12
12
12
20
10
12
—
—
0
8.2
—
—
0
Zr(SO
ZrO
Zr(OH)
ZrCl
ZrOCl
4
)
2
13.1
73.1
86.6
100
100
100
100
100
100
100
98.1
13.1
64.9
73
49.3
41.0
1.99
0.88
14.7
5.9
2
0.7
1.1
4.1
4.8
7.9
8.2
7.0
4.6
1.9
3.9
4
13.6
49.7
58.0
95
98.1
83.7
92.6
95.1
46.2
4
2
GluPC-Zr
GluPC-Zr
GluPC-Zr
GluPC-Zr
GluPC-Zr
GluPC-Zr
10
11
12
b
3.1
51.3
a
b
Reaction conditions: GluPC-Zr, 0.05 g; LA, 5 mmol; IPA, 35 mmol. LA, 1 mmol.
tion of LA but still nearly half of the IPL cannot be converted showing a much higher catalytic efficiency than the other Zr-
into γ-GVL (entries 5 and 6). Inspiringly, GluPC-Zr can tolerate based catalysts listed in Table 1.
a high LA concentration (1.87 M) and achieve up to 95% γ-GVL
3.2.2 Effects of various variables. The effect of various vari-
yield at a relatively low catalyst dosage (0.05 g, about 8.6 wt% ables, such as catalyst dosage, IPA/LA molar ratio, reaction
relative to LA), providing 7.9 turnover frequency (TOF/mmol temperature and time on the TH reaction of LA catalyzed
γ-GVL per g catalyst per h, entry 7). The TOF value of GluPC-Zr GluPC-Zr was further investigated using IPA as a hydrogen
further increases to 8.2 at 190 °C, giving the highest 98.0% donor and the results are depicted in Fig. 4. There, LA conver-
γ-GVL yield (entry 8). But the TOF value is slightly reduced to sion is insensitive to the change in these variables and can
7
.0 at a lower temperature of 150 °C (entry 9), but prolonging remain higher than 99%. γ-GVL yield continuously increases
the reaction time to 20 h also achieves 92.6% γ-GVL yield with an increase in these variables, along with a gradual
entry 10). In addition, when LA is reduced to 1 mmol, the decrease in the esterified product IPL. The highest 98.1%
(
reaction temperature and time to achieve a higher than 95% γ-GVL yield can be obtained under the optimal variables
γ-GVL yield over GluPC-Zr are reduced to 150 °C and 10 h, (0.05 g catalyst, IPA/LA molar ratio of 7, 190 °C, 12 h). Notably,
respectively (entry 11). When the temperature is further when any one of these variables is at its lowest level, a non-
reduced to 120 °C, LA conversion can still be up to 98.1%, but ignorable amount of IPL is produced under these reaction con-
its esterified product IPL yield (51.3%) is higher than that of ditions. These findings suggest that GluPC-Zr can preferen-
γ-GVL (46.2%, entry 12), confirming that the P-containing tially and rapidly catalyze the esterification of LA with IPA to
Brønsted acidic sties of GluPC-Zr are likely to preferentially cat- IPL, and its inherent CTH performance may be retained by the
alyze the esterification of LA with IPA. Compared to the CTH preferential esterification strategy of LA, thus showing an excel-
catalysts previously reported in the literature, the current lent catalytic efficiency in the conversion of a high concen-
GluPC-Zr may be easily synthesized at large scale and low cost tration of LA to γ-GVL.
by simple two-step conversion, but it can also work well at
3.2.3 Influence of the hydrogen donor on the CTH reaction
high LA concentration like a benchmark ruthenium catalyst, of LA. The effect of secondary alcohols on the GluPC-Zr-cata-
3434 | Green Chem., 2021, 23, 3428–3438
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Fig. 4 The influence of the reaction temperature (A), catalyst dosage (B), LA/IPA molar ratio, (C) and reaction time (D) on the GluPC-Zr-catalyzed
TH reaction of LA with IPA. (Using the same reaction conditions as in entry 7 of Table 2, except for the studied variable.)
lyzed conversion of LA to γ-GVL was investigated and the verted. On the other hand, the hydrogen transfer rate of sec-
results are depicted in Fig. 5A. Among the secondary alcohols ondary alcohol to the transition product ester is influenced by
examined, IPA is an excellent hydrogen donor, providing 100% the steric effect of the ester, which is most obvious when cyclo-
LA conversion with 95% γ-GVL yield. 2-Butanol, 3-pentanol, hexanol is used as the hydrogen donor. Cyclohexanol easily
and cyclohexanol exhibit obviously low efficiency as hydrogen reacts with LA (100% LA conversion) but its derived ester is
donors, giving only 65, 52 and 66% γ-GVL yields, respectively, not easily converted into γ-GVL, owing to the serious steric
which is likely to be due to the following reasons. The pre- effect.
esterification rate of LA is inversely proportional to the carbon
3.2.3 CTH reaction of levulinate esters. The CTH perform-
7
4
chain length of the secondary aliphatic alcohol and its toxic ance of GluPC-Zr was tested with methyl levulinate (ML), ethyl
effect on the catalyst easily occurs upon 2-butanol and levulinate (EL), isopropyl levulinate (IPL) and butyl levulinate
especially 3-pentanol, as supported by the following facts. In (BL) instead of LA under the same reaction conditions and the
the presence of these two alcohols, about 5% of LA is not con- results are shown in Fig. 5B. It can be seen from Fig. 5B that
Fig. 5 The effects of hydrogen donors on the CTH reaction of LA (A) and a comparison of the CTH reactions of LEs with IPA (B). (Reaction con-
ditions: LA/LEs, 5 mmol; GluPC-Zr, 0.05 g; alcohol, 35 mmol; 180 °C; 12 h.)
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Fig. 6 Heterogeneity (A) and reusability (B) of the GluPC-Zr catalyst in CTH reactions of LA with IPA.
the conversions of these levulinate esters are all close to 100%, thus showing excellent reusability in the CTH reactions of LA
and the yields of γ-GVL are higher than 93%, which are con- and ML.
siderable compared with LA. Among the esters examined, ML
is the most active, providing the highest 98% γ-GVL yield. EL
and IPL as the substrates also achieve excellent 97% γ-GVL 4. Conclusions
yield. While the reactivity of BL slightly declines as the
In conclusion, for the first time we have constructed an excel-
increase of alkyl carbon chain causes the lactonization of the
lent and recyclable GluPC-Zr catalyst for the CTH reactions of
hydrogen transfer-derived intermediate 4-hydroxy alkyl levuli-
7
5
levulinic acid (LA) or its esters, which has the following advan-
nate become relatively difficult, giving a slightly low 93%
γ-GVL yield.
tages: (i) two raw materials, glucose and ZrCl , for the prepa-
4
ration of the catalyst are inexpensive and readily available, and
the catalyst is easily synthesized at large scale and low cost via
simple two-step conversion; (ii) GluPC-Zr has outstanding CTH
performance and excellent reusability, and it can achieve the
almost quantitative conversion of a high concentration of LA
to γ-GVL at catalytic amounts, exhibiting much superior cata-
lytic efficiency to previously reported Zr-based catalysts. This
low-cost, environmentally benign, and recyclable catalyst is
highly desirable to realize the large-scale production of γ-GVL
from biomass-derived LA. In the future, we are interested in
constructing GluPC-mediated bimetallic catalysts to realize
one-pot conversion from carbohydrates to γ-GVL.
3
.2.5 Heterogeneity and recycling experiments. In order to
confirm that the CTH reaction of LA with IPA is heterogeneous,
the reaction catalyzed by GluPC-Zr proceeded for 2 h at 180 °C
and then the solid catalyst was filtered from the reaction solu-
tion, and the resulting filtrate was allowed to react for another
1
0 h under identical conditions and the obtained result is
shown in Fig. 6A. There, this heterogeneous catalysis reaction
can achieve 57% γ-GVL yield after 2 h but the filtering to
remove the solid catalyst does not give any extra increase in
γ-GVL yield in the following reaction for 10 h, indicating that
the solid catalyst is very stable and the leaching of its active
species does not occur during the reaction, which can be
further confirmed by the following recycling experiments of
CTH reactions of LA and ML. As shown in Fig. 6B and Fig. S5,†
in seven consecutive reaction cycles, the conversions of LA and
ML all remain constant at 100%, and the yield of γ-GVL shows
only an inappreciable decrease. The GluPC-Zr recovered after
the seventh run was characterized by FT-IR, BET and XRD
methods and the results indicated that the recovered catalyst
has almost the same characteristic IR absorption bands and
XRD patterns as the fresh sample (Fig. 1A and D), it still
retains good porosity (Fig. 1C) and its porous parameters
Conflicts of interest
The authors declare that they have no known competing finan-
cialinterests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
(
(
S
t-plot, V
p
and D values) have improved to some extent We acknowledge financial support for this work from the
Table S4†). We suggest that the strong interaction of GluPC-Zr National Natural Science Fund of China (21546010, 21676079),
with the alcohol solvent, as supported by Fig. S2,† may the Natural Science Fund of Hunan Province (2018JJ3335,
make the aggregation of catalyst particles become slightly 2020JJ4425), the Innovation Platform Open Fund of Hunan
looser during the CTH reaction run, thus increasing the College (18K016), and Hunan 2011 Collaborative Innovation
number and size of slit-shaped pores constructed by recovered Center of Chemical Engineering
&
Technology with
catalyst particles. These findings confirm that the strong Environmental Benignity and Effective Resource Utilization,
4
+
coordination between GluPC and the Zr sol can bestow the Program for Science and Technology Innovative Research
catalyst with a very stable framework, active sites, and pores, Team in Higher Educational Institutions of Hunan Province.
3436 | Green Chem., 2021, 23, 3428–3438
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2
4 S. De, B. Saha and R. Luque, Bioresour. Technol., 2015, 178,
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