A novel hafnium-graphite oxide catalyst for the Meerwein-Ponndorf-Verley reaction and the activation effect of the solvent

Article abstract of DOI:10.1039/c9ra10795a

Construction and application of novel hydrogenation catalysts is important for the conversion of carbonyl or aldehyde compounds into alcohols in the field of biomass utilization. In this work, a novel, efficient, and easily prepared hafnium-graphite oxide (Hf-GO) catalyst was constructed via the coordination between Hf⁴⁺ and the carboxylic groups in GO. The catalyst was applied into the hydrogenation of biomass derived carbonyl compounds via the Meerwein-Ponndorf-Verley (MPV) reaction. The catalyst gave high efficiency under mild conditions. An interesting phenomenon was found whereby the activity of the catalyst increased gradually in the initial stage during reaction. The solvent, isopropanol, was proved to have an activation effect on the catalyst, and the activation effect varied with different alcohols and temperatures. Further characterizations showed that isopropanol played the activation effect via replacing the residual solvent (DMF) in micro- and mesopores during the preparation process, which was hard to be completely removed by common drying process.

Full text of DOI:10.1039/c9ra10795a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A novel hafnium–graphite oxide catalyst for the
Meerwein–Ponndorf–Verley reaction and the
activation effect of the solvent†
Cite this: RSC Adv., 2020, 10, 9985
Xiaomin Li,a Zhengjiang Du,a Yi Wu,a Yadong Zhen,a Rixin Shao,a Bingqi Li,a
b
a
Quansheng Liua and Huacong Zhou
*
Chengmeng Chen,
Construction and application of novel hydrogenation catalysts is important for the conversion of carbonyl
or aldehyde compounds into alcohols in the field of biomass utilization. In this work, a novel, efficient, and
easily prepared hafnium–graphite oxide (Hf–GO) catalyst was constructed via the coordination between
Hf4+ and the carboxylic groups in GO. The catalyst was applied into the hydrogenation of biomass
derived carbonyl compounds via the Meerwein–Ponndorf–Verley (MPV) reaction. The catalyst gave high
efficiency under mild conditions. An interesting phenomenon was found whereby the activity of the
catalyst increased gradually in the initial stage during reaction. The solvent, isopropanol, was proved to
have an activation effect on the catalyst, and the activation effect varied with different alcohols and
temperatures. Further characterizations showed that isopropanol played the activation effect via
replacing the residual solvent (DMF) in micro- and mesopores during the preparation process, which was
hard to be completely removed by common drying process.
Received 22nd December 2019
Accepted 4th March 2020
DOI: 10.1039/c9ra10795a
rsc.li/rsc-advances
from biomass, such as palladium,26 ruthenium,27 platinum,28
iridium,29 nickel,30 cobalt,31 molybdenum,32 and copper.33
Introduction
Though moderate or even high efficiency was achieved by using
these catalysts, there are still some issues to be improved. One is
the potential catalyst cost and metal reserves on earth for the
precious metal catalysts during future large-scale applications,
and thus it was still desirable to explore more potential candi-
dates for hydrogenation reaction. On the other hand, these re-
ported catalysts were oen applied using gaseous H2 as
hydrogen sources. Generally, a relatively high H2 pressure was
oen required to improve the catalytic performances of the
catalysts, leading to potential safety issues in applications.
Furthermore, many transition metal based catalysts oen
required relatively high temperatures to achieve satised
performances, which increased the energy input and risk of
accidents.
Compared to hydrogenation under H2 atmosphere, catalytic
transfer hydrogenation known as Meerwein–Ponndorf–Verley
(MPV) reduction using secondary alcohols or formic acid as
hydrogen source is oen seen as milder hydrogenation process.
Various catalysts were reported to be efficient to catalyze MPV
reaction.26–35 Among the transition metal catalysts for MPV
reactions, zirconium-based catalysts were comprehensively re-
ported giving high efficiency.34,35 Very recently, hafnium-based
catalysts were reported with higher activity and milder reac-
tion conditions than the zirconium-based catalysts. Different
organic ligands were used to prepare Hf catalysts, including
porphyrins, benzoic acid, and phytate.36–41 Up to now, the kinds
of ligands being reported are still very limited and some of them
Catalytic conversion of biomass into value-added chemicals has
attracted increasing attention due to the rapid depletion of
fossil fuels and escalating consumption of energy.1,2 From the
viewpoint of CO2 emission, biomass is well known as a carbon-
neutral resource,3,4 which can be converted into various chem-
icals, such as alcohols or hydrocarbons,5,6 gluconic acid,7 5-
hydroxymethylfurfural,8–10 lactic acid,11,12 dimethyl furan,13 lev-
ulinic acid (LA),14,15 and g-valerolactone (GVL).16–18 The hydro-
genation conversion of carbonyl derivatives from biomass into
alcohols or their derivatives was an important step among the
reaction chains of biomass utilization. For example, levulinic
acid or its esters, key platforms in biomass conversion, can be
converted into the famous derivative GVL via hydrogenation
reaction, and GVL has broad applications such as fuel additives
and as a raw material for the production of valuable hydrocar-
bons.19–25 Hydrogenation reactions and the corresponding
catalysts were especially important for the catalytic conversion
utilization of biomass. To date, various hydrogenation catalysts
have been developed for the conversion of carbonyl platforms
aCollege of Chemical Engineering, Inner Mongolia University of Technology, Inner
Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank Carbon
Resources, Hohhot 010051, Inner Mongolia, China. E-mail: hczhou@imut.edu.cn
bCAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese
Academy of Sciences, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra10795a
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9985

View Article Online
RSC Advances
Paper
suffer from the high costs, inconvenience to obtain, and/or not
satised thermal stability. Therefore, it was desirable to explore
more potential materials to construct Hf based catalysts.
Besides constructing new and efficient catalysts, the effects
of the solvents on catalyst activities were also important issues
in catalytic conversion systems of biomass. For MPV reactions,
the common solvent, isopropanol, played its role generally by
acting as both solvent and hydrogen donor by donating
hydrogen atoms. Among the reported catalysts for MPV reac-
tions, more attentions were paid on the construction of novel
catalysts.26–35 However, the effects of the solvents on the activity
of the catalysts were not concerned enough. Therefore,
disclosing the effects of the solvents when constructing novel
MPV catalysts is relatively appreciated.
In this work, graphite oxide (GO) was used to construct a novel
hafnium–graphite oxide (Hf–GO) hybrid catalyst. GO was chosen
due to the abundant acidic carboxylic groups and phenolic
Fig. 1 Effects of preparation conditions on the performance of Hf–
GO catalyst for the conversion of EL to GVL. (A) Effects of the mass
ratio of Hf precursor : GO, (B) aging temperature, (C) aging duration,
hydroxyl groups connected with aromatic structures.42 Besides, and (D) formic acid dosage.
GO is a readily purchasable and stable carbonaceous material
with unique physicochemical properties and broad applica-
tions.43–48 This study was started by the preparation of Hf–GO ratio of Hf precursor to GO was 0.5 : 1. With further increasing
catalyst (Scheme 1). The catalyst was prepared simply by coor- the Hf precursor dosage, the GVL yield had a slight decrease.
dination reaction of HfCl4 with commercial GO in DMF. The The activity of the catalyst came to the optimal values when the
ꢀ
solvent, isopropanol, was found and proved to have an activation aging temperature and aging duration came to 80 C and 3 h,
effect on the catalyst, and the possible reason was analyzed. As far respectively (Fig. 1B and C), and further increasing the
as we know, this is the rst report of constructing Hf-based temperature or time the yield had no signicant change. Many
catalyst using graphite oxide as the ligand for the catalytic reports employed formic acid as a modulator to facilitate the
transfer hydrogenation of ethyl levulinate and other carbonyl formation of the crystalline structures during the preparation of
compounds. With the advantages of high efficiency, excellent catalysts.40,49,50 The effect of formic acid as well as other acidic
stability, and facile preparation, the Hf–GO catalyst may have modulators was studied for Hf–GO preparation, shown in
potential applications in the eld of biomass conversion.
Fig. 1D and Table S2.† Among the organic and inorganic acidic
modulators, formic acid showed the promoting effect on the
activity of the catalyst. The addition of suitable amount of for-
mic acid could increase the activity of the catalyst, and both GVL
yield (76.7%) and EL conversion (82.0%) reached the highest
values when the molar ratio 21 : 1 of formic acid to Hf was used.
As seen in Fig. 1A, the mass ratio of Hf precursor to GO was
the key factor inuencing the activity of the catalyst, and thus
we characterized the Hf–GO catalysts prepared under different
raw material ratios. The textural properties of the catalysts with
various mass ratios were rst characterized using N2 adsorp-
tion–desorption isotherms (Fig. S1†). The surface area and pore
volume rst dropped for mass ratios of Hf precursor to GO from
0.2 to 1, and then slightly increased with larger mass ratios.
Compared with the surface area and pore volume, the change in
average pore diameter was reversed (Table S1†). The N2
adsorption–desorption isotherms revealed the typical meso-
porous character (Fig. S1†). The Hf content in Hf–GO-0.5 : 1 was
10.01 wt%, which was higher than that of Hf–GO-0.2 : 1
(6.64 wt%) as conrmed by inductively coupled plasma atomic
emission spectrometry (ICP-AES Table S1†). The Hf contents in
other catalysts almost maintained constant despite further
increasing the mass ratios of Hf precursor to GO. Therefore,
0.5 : 1 was selected as the optimal ratios for the preparation of
Hf–GO.
Results and discussion
Studies of the preparation conditions of catalysts
In the preparation process of the catalyst, the effects of raw
material ratio, aging temperature and aging duration for Hf–GO
preparation were investigated (Fig. 1). The exact content of the
oxygen-containing acidic groups were complex to determine
precisely. Therefore, the effect of the amount of Hf precursor
(HfCl4) on the catalytic activity was studied by varying the mass
ratios of Hf precursor to GO. As shown in Fig. 1A, the conversion
and product yield reached the highest values when the mass
Scheme 1 Schematic illustration of the Hf–GO catalyst preparation
and its application in the conversion of carbonyl compounds into
alcohols.
The effects of the modulators were analyzed in detail (Fig. 1D
and Table S2†). As seen, the GVL yield and EL conversion
9986 | RSC Adv., 2020, 10, 9985–9995
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
reached 51.7% and 59.4% at 150 ꢀC in 3 h without formic acid.
The activity of the catalyst gradually increased with the formic
acid dosage increased, and the GVL yield (76.7%) and EL
conversion (82.0%) reached the highest values when the molar
ratio 21 : 1 of formic acid to Hf was used. Further increasing the
formic acid dosage, the activity gradually decreased. It was re-
ported that the monocarboxylic acids were usually added in the
synthesis of MOFs as modulators.40,49–51 Seemingly, the mono-
carboxylic acid is not necessarily incorporated in the structure
but can strongly affect the connectivity of the inorganic node by
coordinates to metal ions (Hf4+).52–54 The carboxylate ligands
have lower pKa and possess stronger coordination ability,
leading to the partial replacement of the modulators and the
formation of structural defects.55–63 It was also reported that the
addition of formic acid could increase the acidity.64 We also
studied the effect of different modulators for Hf–GO prepara-
tion shown in Table S2.† We chose ve different modulators
with identical mole ratio to Hf. The results show that the cata-
lyst using formic acid as modulator gave the highest catalytic
activity.
Based on the above results, the preparation conditions were
investigated, including the mass ratio of hafnium precursor
(HfCl4) to GO, aging temperature, aging duration, modulators
and their dosage. The catalyst showed higher activity for the
conversion of ethyl levulinate into GVL under the preparation
conditions of the mass ratio of HfCl4 to GO 0.5 : 1, aging
temperature 80 ꢀC, aging duration 3 h, formic acid as modulator
with the dosage of the molar ratio of formic acid to Hf 21 : 1.
The Hf–GO catalyst prepared under this condition was further
characterized and studied.
Fig. 2 (A) SEM image, (B) TEM image, (C and D) C and Hf EDS
mappings of Hf–GO, (E) FTIR spectra, (F) XRD patterns of Hf–GO and
GO.
to 1350 cmꢁ1 as compared to GO (1596 cmꢁ1, 1345 cmꢁ1
)
(Fig. S2†). Furthermore, the intensity ratio of D and G peaks (ID/
IG) increased slightly from 0.94 to 1.00 (Table S4†) aer
assembly of GO with Hf4+. XPS full spectra, tted spectra of Hf
4d and Hf 4f again proved the successful introduction of Hf4+ in
Hf–GO (Fig. 3). The XPS curve-tted spectrum revealed the Hf
4d5/2 and Hf 4d3/2, Hf 4f7/2 and Hf 4f5/2 could be detected cor-
responding to the hafnium–oxygen bonds at 213.8 eV and
224.4 eV, 17.4 eV and 19.1 eV, respectively (Fig. 3C and D). And
these spectra were calibrated by C 1s peak at 284.8 eV. In Fig. 3B
and C 1s peaks were deconvoluted into four bands based on the
literatures.69–72 The contents of C–C or C–H, C–OH or C–O–C,
Catalyst characterization
The structures of the catalyst obtained under the above opti-
mized conditions were characterized (Fig. 2). SEM and TEM
showed that Hf–GO maintained the typical dimensional sheet
structures of GO (Fig. 2A and B). EDS mapping results showed
that Hf element was introduced into GO successfully and
dispersed on GO uniformly (Fig. 2C and D). The FTIR spectrum
of GO and Hf–GO catalyst in Fig. 2E exhibited the asymmetric
(GO, 1731 cmꢁ1; Hf–GO, 1723 cmꢁ1) and symmetric (GO,
1616 cmꢁ1; Hf–GO, 1655 cmꢁ1) stretching vibration of carbox-
ylate groups. FTIR showed that the wavenumber difference of
the asymmetric and symmetric vibrations of carboxylate anions
was narrowed from 115 cmꢁ1 for GO to 68 cmꢁ1 for Hf–GO,
indicating that Hf4+ was coordinated with carboxylate
groups.65–67 It has reported the bands at around 520 and
760 cmꢁ1 were characteristic of Hf–O bonds.68 The new
absorption band at 690 cmꢁ1 could be assigned to the Hf–O
bond vibration (Fig. 2E).68 As shown in XRD patterns (Fig. 2F),
no obvious characteristic peaks of HfO2 or Hf(OH)4 appeared,
indicating that Hf dispersed uniformly and did not form crystal
aggregates. The 001 peak of GO shied from 12ꢀ to 10ꢀ, corre-
sponding to the layer distance enlargement from 0.74 nm to
0.80 nm, indicating that some of the Hf4+ were inserted into the
layers of GO. The Raman spectra revealed that the G band for
Fig. 3 (A) XPS full spectra, (B) the fitted spectra of C 1s peak of GO and
Hf–GO, (C) the fitted spectra of Hf 4d, and (D) Hf 4f of HfO2 and Hf–
Hf–GO had a red shi to 1589 cmꢁ1 and D band had a blue shi GO.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9987

View Article Online
RSC Advances
Paper
C]O, and O]C–O from Fig. 3B were summarized in Table S5.†
The main peak at 284.8 eV originates in sp2 (C]C), sp3 (C–C)
carbon and C–H. The deconvoluted peak centered at the
binding energies of 286.7, 287.2, and 288.8 eV were considered
as the C–O (hydroxyl and ethers groups), C]O (carbonyl
groups), and O]C–OH (carboxyl groups) oxygen-containing
carbonaceous bands, respectively. The intensity of the peak
assigned to oxygen-containing carbonaceous bands of the C–O,
C]O and O]C–O for GO decreased aer the introduction of
Hf4+. These results indicated the successful covalent coordina-
tion of Hf4+ with COOH in GO. ICP-AES showed that the content
of Hf in the catalyst was around 9.04 wt%. The results of
elemental analysis of different catalysts were given in Table S6.†
N2 adsorption–desorption isotherms showed that the porous
structure and the surface area, pore volume, and average pore
size of the catalyst were 61.8 m2 gꢁ1, 0.08 cm3 gꢁ1, and 6.1 nm,
respectively (Fig. S3C and Table S7†). TG analysis showed that
the thermal stability aer 300 ꢀC of the Hf–GO catalyst was
Substrate scope expansion
Encouraged by the excellent performance of Hf–GO in the
conversion of EL to GVL, we investigated the possibility of the
MPV reactions of other carbonyl compounds with different
structures (Table 2). The results showed that aliphatic and
aromatic aldehydes could be effectively hydrogenated to corre-
sponding alcohols at 70–100 ꢀC within 2–4 h (entries 2–8). While
ketones, especially with long aliphatic chains, needed a higher
reaction temperature or longer reaction time of the harsh
conditions to achieve satised conversions and yields as
compared to aldehydes (entries 10 and 11). Hf–GO displayed high
efficiency for cyclohexanone (conversion, yield, selectivity all
>99%) (entry 9), under not yet optimized conditions. These results
substantiated the universality of Hf–GO catalyst in the conversion
of different carbonyl compounds with various structures.
Recycle of the Hf–GO catalyst
better than that of GO due to the coordination of Hf4+ with the During the recycling process, an interesting and unexpected
carboxylic groups, and the catalyst was stable enough under phenomenon was observed that the activity of Hf–GO increased
ꢀ
reaction temperature range (<200 C) (Fig. S3D†). The detailed with the initial 3 uses and then stabilized at a similar level
discussions of catalyst characterization were given in ESI.†
without signicant decreasing aer 11 cycles compared to the
rst use (Fig. 4). The slight decreasing trend from the 6th use to
the 11th use was due to the loss of the catalyst during the
centrifugation recycling process. Aer the catalyst dosage was
Effect of reaction conditions on transfer hydrogenation
Next, the effects of the reaction conditions on transfer hydro- replenished to the initial level, the performance could be recov-
genation were investigated with isopropanol as the hydrogen ered (12th use). The activity increasing of Hf–GO during the initial
source, including the catalyst dosage (Fig. S4A†), reaction uses draw us much attention because few reports mentioned this
temperature (Fig. S4B†) and reaction time (Fig. S4C†). The phenomenon for the analogue catalysts. We speculated that Hf–
discussions were given in ESI.† The catalyst gave satised GO was activated by certain components in the reaction system.
reaction results and rate under the conditions of catalyst dosage Mainly four components in the reaction system existed, including
ꢀ
>0.05 g and reaction temperature >150 C for the studied reac- EL (the reactant), GVL (the product), isopropanol (the solvent),
tion system. The EL conversion, GVL yield, and GVL selectivity and acetone (the by-product). In order to disclose the “activator”,
could reach to 96.5%, 88.8%, and 92.0%, respectively, under the the catalyst was pretreated by the above components, respectively,
ꢀ
conditions of catalyst dosage 0.1 g, reaction temperature 150 ꢀC, under 150 C to simulate the reaction conditions. As shown in
and reaction time 9 h. The GVL yield did not increase when Fig. 5, only the pretreatment by isopropanol (Run 1 in Fig. 5A)
removing the solid catalyst during reaction, indicating Hf–GO could increase the activity of Hf–GO compared with no pretreat-
was a heterogeneous catalyst (Fig. S4D†).
ment (Run 0 in Fig. 5A). The pretreatment by EL, GVL, and
acetone could not activate Hf–GO (Run 0 vs. Run 1) and the
activity only increased aer the rst catalytic reaction (Run 2 and
3 vs. Run 1). Thus, it was isopropanol that played the role of
Comparison with other catalysts
In order to see the performances of the state-of-the-art catalyst activation effects. Subsequently, temperature was proved to be
for the MPV reaction, the performance of Hf–GO was compared a key factor for the activation effect (Fig. S5†). The activation effect
various other catalysts, including Hf-, Zr-, Pd-, Ru-, Pt-, Ir-, Co-, became more and more obvious with the increasing of temper-
Ni-, Mo-, and Cu-based catalysts (Table 1). The catalysts with the ature in the range of room temperature to 150 ꢀC (Fig. S6†).
same reaction and similar reaction conditions were chosen and Further increasing the pretreatment temperature led to the
compared. The performances of around 37 kinds of catalysts decreasing of the activating trend, which might be due to the
were compared. When using H2 as hydrogen source, Mo2C/CNT gradual decomposition of the carboxylate groups under higher
(entry 37), Pd/AC (entry 21), Ni/g-Al2O3 (entry 36) gave higher temperatures. Temperatures in the range of 120–150 ꢀC were
activities than other catalysts, wiꢁth1 TOF values of 10.3 hꢁ1 (150 optimal activation temperature, which just fell into the optimal
ꢀC), 7.94 hꢁ1 (100 ꢀC), and 7.67 h (200 ꢀC), respectively. When reaction temperature of EL to GVL.
using isopropanol as hydrogen source, GO–Hf prepared in this
Besides isopropanol, other alcohols were also studied to check
work gave the highest activity with TOF values of 3.51 hꢁ1 (150 if they had activation effects on Hf–GO (Fig. S7 and S8†). It could
ꢀC). Among all the 37 catalysts, Hf–GO ranks basically the forth be seen that all the alcohols studied had activation effects on Hf–
in viewpoint of TOF value. The above comparison analysis GO but the extents varied. The activation effects of the secondary
should that Hf–GO was efficient for the MPV reaction of ethyl alcohols were generally higher than that of the primary alcohols,
levulinate.
with isopropanol and isobutanol giving the most signicant
9988 | RSC Adv., 2020, 10, 9985–9995
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 1 MPV reaction of ethyl levulinate (EL) or levulinic acid (LA) over various catalysts under different conditions
EL conv.
(%)
LA conv.
(%)
GVL yield
(%)
GVL sel.
(%)
Entry
Catalyst
Reaction conditions
TOFb (hꢁ1
)
Ref.
1
2
3
4
5
6
7
8
Blank
GO
IPA, 150 ꢀC, 5 h
<1
0
0
0
0
—
—
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
64
64
65
73
73
73
49
74
74
75
26
31
35
76
34
77
77
77
77
78
79
27
28
IPA, 150 ꢀC, 3 h
36.8
95.5
88.4
54.8
12.2
3.5
Hf–GOa
Hf–GOa
Zr–GOa
Al–GOa
Cr–GOa
Fe–GOa
Cu–GOa
Sn–GOa
FDCA–Hfc
IPA, 150 ꢀC, 5 h
87.7
82.6
54.7
7.5
0.9
1.3
1.1
2.4
98
22
84.2
86
35
74
91.8
93.5
99.8
62.0
25.8
17.7
23.1
10.6
98
79
84.6
91
71
85
NG
91.5
79.1
85
74
95
96.7
87.8
>85.9
97.2
99.8
99.6
96.3
94.4
96.9
94
3.51
1.10
2.18
0.3
0.04
0.05
0.04
0.10
1.53
0.34
0.1
0.41
0.09
0.34
1.18
—
IPA, 130 ꢀC, 15 h
IPA, 150 ꢀC, 5 h
IPA, 150 ꢀC, 5 h
IPA, 150 ꢀC, 5 h
IPA, 150 ꢀC, 5 h
7.1
5.0
9
IPA, 150 ꢀC, 5 h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
IPA, 150 ꢀC, 5 h
22.3
>99
28
99.5
95
IPA, 160 ꢀC, 4 h
c
HfO2
IPA, 160 ꢀC, 4 h
Zr–HAd
IPA, 150 ꢀC, 11 h
Hf–ATMPe
IPA, 150 ꢀC, 4 h
f
HfO2
IPA, 150 ꢀC, 4 h
49
87
Hf–EDPAg
Hf–MOF-808h
DUT67(Hf)i
UiO-66(Hf)j
PPOA–Hfk
Pd/ACl
IPA, 150 ꢀC, 4 h
IPA, 120 ꢀC, 8 h
NGs
98.9
73.9
100
38
94
IPA, 160 ꢀC, 4 h
90.5
58.5
85
28
94
96.7
81.1
85.0
90.1
92.6
92.0
92.0
94.4
96.9
92
IPA, 160 ꢀC, 4 h
—
—
IPA, 160 ꢀC, 6 h
H2 0.5 MPa, 100 ꢀC, 5 h, H2O
H2 3.3 MPa, 130 ꢀC, 3 h
IPA, 150 ꢀC, 6 h
7.94
0.19
0.25
0.48
0.09
0.09
0.09
0.43
0.31
—
Co
99
Zr–PhyAm
Zr–RSLn
100
92.4
>99
92.7
92.8
92.4
95.4
100
100
IPA, 160 ꢀC, 12 h
Zr–HAso
IPA, 150 ꢀC, 15 h
Zr–HAfp
IPA, 150 ꢀC, 9 h
Zr–HAtp
IPA, 150 ꢀC, 7 h
Zr–SRfp
IPA, 150 ꢀC, 7 h
Zr–SRtp
IPA, 150 ꢀC, 9 h
Zr–HBAq
Zr–CAr
IPA, 150 ꢀC, 4 h
IPA, 150 ꢀC, 4 h
—
Ru/OMC-P
Ru/carbon
Pd/carbon
Pt/carbon
Ni/g-Al2O3
Mo2C/CNT
Cu-catalyst
Ni/MgAlO2.5
H2 7 bar, 70 ꢀC, 6 h, H2O
H2 1 bar, 265 ꢀC, 50 h, H2O
H2 1 bar, 265 ꢀC, 50 h, H2O
H2 1 bar, 265 ꢀC, 50 h, H2O
H2 50 bar, 200 ꢀC, 4 h, H2O
H2 30 bar, 150 ꢀC, 1 h, H2O
H2 70 bar, 200 ꢀC, 10 h, H2O
H2 3 MPa, 160 ꢀC, 1 h, dioxane
98
1.61
0.04
0.04
0.02
7.67
10.3
—
100
100
100
92
83
>99
100
98.6
90
30
92
75
98.6
90
30
100
90.3
>91
99.7
28
28
30
32
80
81
91
99.7
1.46
a
Preparation condition: 1 mL formic acid was dissolved in DMF (400 mL), 1.59 mmol metal chloride was added into DMF solution with
continuously stirred and completely dissolved. Aer that, 1.0 g of GO was directly added to the HfCl4 solution and the obtained mixture was
stirred for 3 h at 30 ꢀC, then aged at 80 ꢀC under static conditions for 3 h. The suspended solution was separated by ltration to give black
precipitate, and successively washed with DMF, ethanol for 4 times, dried under vacuum conditions at 80 ꢀC for 24 h, and crowded into
b
powders. Reaction conditions: 1 mmol EL, 0.1 g catalyst (5 mol% Hf), 5 mL 2-PrOH. TOF (turnover frequency) ¼ (mole of GVL)/(mole of active
c
d
metal ꢂ reaction time). Data obtained from ref. 64, FDCA: furan dicarboxylic acid, 16 mol% Hf. Data obtained from ref. 65, HA: humic acid.
e
f
g
Data obtained from ref. 73, the mole of Hf was 52 mol%. Data obtained from ref. 73, 95 mol% Hf. Data obtained from ref. 73, 54 mol% Hf
(ICP determined). Data obtained from ref. 49, MOF-808: 1,3,5-benzenetricarboxylic acid, 10 mol% Hf. Data obtained from ref. 74, DUT67:
2,5-thiophenedicarboxylic acid. Data obtained from ref. 74, UiO-66: terephthalic acid. Data obtained from ref. 75, PPOA: phenylphosphonic
acid. AC: active carbon. PhyA: phytic acid. RSL: raw Shengli lignite. HAs: humic acids. Humic acids (HA) extracted from lignite and the
solid residues (SR). q HBA: 4-hydroxybenzoic acid dipotassium salt. r CA: cyanuric acid. s NG: not given.
h
i
j
k
l
m
n
o
p
activation effect. Other solvents such as decane, acetone, and 2- Hf–GO catalysts using various techniques, including XPS,
hexanone were also used to pretreat Hf–GO, and it was found that elemental analysis, XRD, Raman, TG analysis, and N2 adsorp-
these solvents could not activate the catalyst (Fig. S9†). The tion–desorption. No signicant morphology changes and Hf
reasons for this would be discussed below.
chemical state changes were found for the pretreated and
recycled Hf–GO (Fig. S10–S13†). The N contents decreased aer
solvent pretreatment except for pretreatment by DMF (Table
S6†). TG analysis also showed that the weight losses of Hf–GO
pretreated by solvents were much lower than that of the fresh
Characterization of the recycled and captivated Hf–GO
The intrinsic mechanism for the activation effect of alcohols
was analyzed by characterizing the fresh and pretreated/used
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9989

View Article Online
RSC Advances
Paper
Table 2 MPV reduction of different biomass-derived carbonyl compounds over the Hf–GO catalysta
T
(h)
Conv.
(%)
Yield
(%)
Entry
Substrate
Product
T (ꢀC)
Sel. (%)
1
2
150
100
9
3
96.5
95.9
88.8
87.8
92.0
91.5
3
4
5
70
90
4
3
3
89.7
97.9
89.7
85.5
95.9
81.0
95.4
98.0
90.3
100
6
7
80
90
3
2
93.0
96.4
91.7
87.2
98.6
90.5
8
80
100
150
4
5
8
93.7
>99
88.7
>99
94.6
>99
9
10
>99
>99
>99
11
120
7
99.3
98.8
99.5
a
Reaction conditions: substrate 1 mmol, isopropanol 5 mL, and Hf–GO 0.1 g (5 mol% Hf).
Hf–GO (Fig. S14†). FTIR showed that The C–N stretching and 150 ꢀC (boiling point of DMF under standard atmosphere)
vibration at 1424 cmꢁ1 of DMF was obvious for fresh Hf–GO,82 (Table S6,† entry 9). As shown in Table S6,† the N content in GO
but it was found to be weakened aer solvent treatment was only 0.04 wt%, but it increased to 1.55 wt% for fresh Hf–GO.
(Fig. S15†). Base on the above analysis, it could be deduced that The N content decreased to different extents aer pretreatment
solvent pretreatment removed residual DMF in Hf–GO. The by different solvents (except for Hf–GO pretreated by DMF),
residual DMF in the pore structures of Hf–GO was hard to indicating the solvent pretreatment could indeed remove DMF
remove even washed by ethanol and then dried under vacuum residual in the catalyst. The Hf content had no signicant
changes, and the changes of C, H, and O contents could be
attributed to the removal of DMF. It should be noted that the
pretreatment by decane and acetone could also remove DMF
(Table S6†), but these solvents could not activate the catalyst
only if Hf–GO was allowed to contact with isopropanol (as
mentioned above, Fig. S9†). Therefore, the structures of the
pretreated Hf–GO were further characterized by BET and pore
structure analysis (Fig. 6 and Table S7†). It could be seen that
the surface area, the volumes of total pores and mesopores of
Hf–GO–decane and Hf–GO–acetone were all much lower than
those of Hf–GO–isopropanol, which may result in no obvious
activation effect for these solvents. Isopropanol pretreatment
mainly affected the structures of micro- and mesopores, indi-
cating DMF in these pore structures was removed (Fig. 6A and
B). Another possible reason was that although these solvents
Fig. 4 Recycle of the Hf–GO catalyst. Reaction conditions: 1 mmol EL,
0.1 g catalyst (5 mol% Hf), 5 mL 2-PrOH, 150 ꢀC, 2 h.
9990 | RSC Adv., 2020, 10, 9985–9995
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
not obvious for Hf–H3BTC constructed by 1.3.5-benzene tricar-
boxylic acid (Fig. S17†). This was related to the different pore
structures of the catalysts (Fig. 6 and S18†). Hf–GO had obvious
micro- and mesoporous structures. For Hf–H3BTC, there were
abundant pores larger than 10 nm and the distribution of pore
diameters were broad, leading to the easy removal of DMF
during preparation by washing and much less DMF residue in
the catalyst compared to Hf–GO (Fig. 6C and D and Table S6†).
Therefore, isopropanol could easily contact with the active sites
in the rst use, and no obvious increasing of activity was
observed during the subsequent use. Comparatively, Hf–GO
had abundant micro- and mesopores (Fig. 6A and B), leading to
the residue of DMF, and thus isopropanol could activate the
catalyst via removing DMF as discussed above.
Fig. 5 Performance of the Hf–GO–X catalyst (Hf–GO pretreated by
solvent X). (A) X ¼ iPrOH; (B) X ¼ ethyl levulinate (EL); (C) X ¼ GVL; (D) X
¼ acetone. Run 0: the performance of the fresh Hf–GO catalyst
without pretreatment. Run 1, 2 and 3: the performance of the Hf–GO–
X catalyst for subsequent three times use.
Experimental
Materials
Ethyl levulinate (EL, 98%), g-valerolactone (GVL, 98%), zirco-
miun tetrachloride (ZrCl4, 98%), 1,3,5-benzenetricarboxylic acid
(H3BTC, 99%), furfural (FF, 99%) and furfuryl alcohol (FA, 98%)
were provided by J&K Scientic Ltd. Hafnium tetrachloride
(HfCl4, 98+%) and chromium(III) chloride anhydrous (CrCl3,
98%) were obtained from Alfa. Aluminum chloride (AlCl3, 99%),
iron chloride (FeCl3, $99.9%), copper chloride (CuCl2, 98%)
and tin chloride (SnCl4, AR) were provided by Aladdin Industrial
Corporation. Graphite oxide (GO) were provided by Institute of
Coal Chemistry, Chinese Academy of Sciences. Isopropanol
(iPrOH, 99.9%) was provided by Innochem. N,N-Dime-
thylformamide (DMF, >99.9%), n-butanol (nBuOH, $99.7%), n-
propanol (nPrOH, $99.9%), isobutanol (sBuOH, $99.5%),
methanol (MeOH, $99.9%), ethanol (EtOH, 100%), 1-hexanol
(nHeOH, >99.5%), n-octanol (nOcOH, $99%) and 2-hexanol
(sHeOH, 99%) were provided by Aladdin Industrial Corpora-
tion. Decane (AR) and other chemicals were obtained from
Beijing Institute of Chemical Reagent.
could remove DMF in mesopores of Hf–GO, they could not
interact with the active Hf–O sites and provide hydrogen to
promote the occurrence of the reaction. The surface contents of
Hf in Hf–GO pretreated by different solvents were analyzed by
XPS, and no obvious difference were observed (Table S8†). The
inuence of mass transfer on the performance of the catalysts
were detected by prolonging the reaction time, and both
conversion and yield for Hf–GO–iPrOH were higher than those
for Hf–GO-fresh throughout the whole time range, further
indicating isopropanol pretreatment had an activation effects
on Hf–GO (Fig. S16†). In conclusion, it could be postulated that
isopropanol played its activation effects on Hf–GO by rst
replacing DMF in micro- and mesopores and then interacting
with active Hf–O sites to promote the proceeding of the
reaction.
Besides Hf–GO catalyst, the above discussed activation
effects of isopropanol also existed for Zr–GO catalyst, but was
Preparation of different M–GO catalysts
The M–GO catalysts (M ¼ Hf, Zr, Al, Cr, Fe, Cu, Sn) were ob-
tained by the simple synthesis of GO with an equivalent mole of
metal chloride (i.e., HfCl4, ZrCl4, AlCl3, CrCl3, FeCl3, CuCl2 or
SnCl4) and 0–2.0 mL of formic acid in DMF under stirring
conditions. In a typical preparation process for Hf–GO, 1.359 g
(1 mL) formic acid (FA) was dissolved in DMF (400 mL), 0.5 g
HfCl4 was added into DMF solution with continuously stirred
and completely dissolved. Aer that, 1.0 g of GO was directly
added to the HfCl4 solution and the obtained mixture was
ꢀ
ꢀ
stirred for 3 h at 30 C, then aged at 80 C under static condi-
tions for 3 h. The suspended solution was separated by ltration
to give black precipitate, and successively washed with DMF,
ethanol for 4 times, dried under vacuum conditions at 80 ꢀC for
24 h, and crowded into powders for use. For comparison, the
Hf–H3BTC organic–inorganic hybrid was prepared by sol-
vothermal treatment of HfCl4 and equimolecular 1,3,5-benze-
Fig. 6 Mesopore distributions of different catalysts (A and C) and
micropore distributions of different catalysts (B and D). The mesopore
size distribution was calculated based on the DFT method. The
ꢀ
netricarboxylic acid in DMF at 120 C under static conditions
micropore size distribution was calculated based on the H–K method. for 48 h, referencing to a previously reported synthetic
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9991

View Article Online
RSC Advances
Paper
methods.51,83 All the solid samples were aer dried at 80 ꢀC for
4 h for the catalyst characterization and analysis of catalytic
performance.
Catalyst characterization
Scanning electron microscopy (SEM) measurements were per-
formed using a Hitachi SU8220 scanning electron microscope
operated at 20 kV with an energy dispersive spectrometer (EDS)
apparatus. Transmission electron microscopy (TEM) images
were obtained using a TEM JEM-2010 with an accelerating
voltage of 120 kV. The samples were dispersed in absolute
ethanol, and the suspension was sonicated for 15 min to make
the sample disperse well. Then the sample solution was drop-
ped onto the copper network containing the carbon support
lm, and was allowed to volatilize at room temperature for 2 h
to be tested. Fourier transform-infrared spectra (FTIR) were
obtained using a PerkinElmer spectrometer. X-ray diffraction
(XRD) was carried out via an Produce-Rigaku SmartLab 9 KW X-
ray diffractometer with a Cu target conguration and a highly
sensitive D/teX Ultra 250 inspection system at 20ꢀ per minute
ranging from 5ꢀ to 90ꢀ. The tube voltage was 45 kW, and the
current was 200 mA. Raman spectrum was collected on
a Renishaw® inVia microscope in the range of 120 cmꢁ1 to
4000 cmꢁ1 with 532 nm laser for excitation with power of 0.3
mW, 50ꢂ objective, data acquisition time 3 s, and scanning 50
cycles for each spectrum. The thermogravimetric (TG) analysis
of Hf–GO was performed using a thermogravimetric analysis
system (Diamond TG/DTA6300, PerkinElmer Instruments)
under an N2 atmosphere at the heating rate of 10 ꢀC minꢁ1. The
surface area and pore diameters and pore volumes were deter-
mined via the nitrogen adsorption–desorption method using an
American Mike Micromeritics ASAP 2020 sorption analyzer. The
XPS measurements were carried out via an ESCALAB 250Xi
spectrometer (Thermo Fisher Scientic) at a pressure of ꢃ3 ꢂ
10ꢁ9 mbar using Al Ka as the excitation source (hn ¼ 1486.6 eV)
and operating at 15 kV and 150 W, and C 1s orbital (284.8 eV)
was used to correct the binding energy. The XPSpeak 4.1 so-
ware was used to perform manual peak splitting, establishing
a baseline and add peaks, adjust parameters such as peak area
and half-peak width to make the tted data coincide with the
original data, keep the Lorentzian–Gaussian ratio consistent
(default 80). C 1s peaks were deconvoluted into four bands.69–72
Hf tted spectra in which the peak area ratio was xed (d5/2 : d3/
The preparation parameters were studied, including effects
of the mass ratio of Hf precursor: GO, aging temperature, aging
duration, and formic acid dosage. The details were described as
follows. (1) Effects of the mass ratio of Hf precursor to GO: 1 mL
formic acid was dissolved in DMF (400 mL, ve copies), HfCl4 of
0.2 g, 0.5 g, 1.0 g, 3.0 g, 5.0 g, respectively, was added into DMF
solution with continuously stirred and completely dissolved.
Aer that, 1.0 g of GO was directly added to the HfCl4 solution
and the obtained mixture was stirred for 3 h at 30 ꢀC, then aged
at 80 ꢀC under static conditions for 3 h. The suspended solution
was separated by ltration to give black precipitate, and
successively washed with DMF, ethanol for 4 times, dried under
vacuum conditions at 80 ꢀC for 24 h, and crowded into powders.
(2) Aging temperature: 1 mL formic acid was dissolved in DMF
(400 mL), 0.5 g HfCl4 was added into DMF solution with
continuously stirred and completely dissolved. Aer that, 1.0 g
of GO was directly added to the HfCl4 solution and the obtained
ꢀ
mixture was stirred for 3 h at 30 C, then aged at the required
temperature under static conditions for 3 h. The suspended
solution was separated by ltration to give black precipitate,
and successively washed with DMF, ethanol for 4 times, dried
ꢀ
under vacuum conditions at 80 C for 24 h, and crowded into
powders. (3) Aging duration: 1 mL formic acid was dissolved in
DMF (400 mL), 0.5 g HfCl4 was added into DMF solution with
continuously stirred and completely dissolved. Aer that, 1.0 g
of GO was directly added to the HfCl4 solution and the obtained
ꢀ
ꢀ
mixture was stirred for 3 h at 30 C, then aged at 80 C under
static conditions for the required duration. The suspended
solution was separated by ltration to give black precipitate,
and successively washed with DMF, ethanol for 4 times, dried
ꢀ
under vacuum conditions at 80 C for 24 h, and crowded into
powders. (4) Formic acid dosage: formic acid of 0 mL, 0.5 mL,
1.0 mL, 1.5 mL, 2.0 mL, respectively, was dissolved in DMF (400
mL, ve copies), 0.5 g HfCl4 was added into DMF solution with
continuously stirred and completely dissolved. Aer that, 1.0 g
of GO was directly added to the HfꢀCl4 solution and theꢀobtained
mixture was stirred for 3 h at 30 C, then aged at 80 C under
static conditions for 3 h. The suspended solution was separated
by ltration to give black precipitate, and successively washed
with DMF, ethanol for 4 times, dried under vacuum conditions
2
¼ 3 : 2, f7/2 : f5/2 ¼ 4 : 3) and the half-peak width was kept
consistent. Inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) using a Agilent 720 system. Elemental
analysis (EA) using a Thermo Fisher Flash 2000 system.
ꢀ
at 80 C for 24 h, and crowded into powders. Reaction condi-
tions: 1 mmol EL, 0.1 g catalyst, 5 mL 2-PrOH, reaction
temperature 150 ꢀC, and reaction time 3 h.
The hydrogenation reduction reactions
The as-obtained Hf–GO was treated with different organic The hydrogenation reduction of various aldehydes and ketones
solvents at 150 ꢀC for 2 h to give Hf–GO–X (X ¼ DMF, EL, GVL, using isopropanol as the solvent and hydrogen-donor was per-
iPrOH, other various alcohols, acetone, 2-hexanone, decane). formed in a 10 mL Teon-lined stainless steel autoclave
0.1 g of the fresh catalyst was stirred in the different organic equipped with a magnetic stirrer. In a typical reaction proce-
solvents at 150 ꢀC for 2 h, recovered by centrifugation, and dure, EL or other carbonyl compounds (1 mmol), isopropanol (5
washed three times with fresh isopropanol to give Hf–GO–X. For mL) and the catalyst (100 mg) were added into the reactor and
the effects of pretreating temperatures in isopropanol, the fresh tightly sealed, then placed into a preheated oil-bath at a known
ꢀ
Hf–GO was treated with isopropanol at various temperatures for temperature of 130–170 C for a reaction time of 1–9 h. Aer
2 h to give Hf–GO-Y (Y ¼ 25, 50, 70, 100, 120, 150, 170, 200 ꢀC). reaction, the reactor was cooled in cold water to quench the
reaction and the organic phase was diluted by isopropanol. The
liquid samples were analyzed quantitatively by gas
9992 | RSC Adv., 2020, 10, 9985–9995
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
chromatography (TECHCOMP GC7900) using decane as the
internal standard, and identication of the products was done
by GC-MS (SHIMADZU-QP 2010).
8 M. Dashtban, A. Gilbert and P. Fatehi, RSC Adv., 2014, 4,
2037–2050.
9 M. E. Zakrzewska, E. Bogel-Lukasik and R. Bogel-Lukasik,
Chem. Rev., 2011, 111, 397–417.
Catalyst recycle and heterogeneity
10 H. M. Mirzaei and B. Karimi, Green Chem., 2016, 18, 2282–
2286.
11 Z. C. Tang, W. P. Deng, Y. L. Wang, E. Z. Zhu, X. Y. Wan,
Q. H. Zhang and Y. Wang, ChemSusChem, 2014, 7, 1557–
1567.
12 D. Esposito and M. Antonietti, ChemSusChem, 2013, 6, 989–
992.
13 G. H. Wang, J. Hilgert, F. H. Richter, F. Wang, H. J. Bongard,
B. Spliethoff, C. Weidenthaler and F. Schuth, Nat. Mater.,
2014, 13, 294–301.
In the experiments to investigate the reusability of the Hf–GO
catalyst, the catalyst was recovered by centrifugation, washed
three times with fresh isopropanol, and then the catalyst was
directly used for the next run of reaction. For heterogeneity, the
solid catalyst was separated from the reaction mixture by
centrifugation and the supernatant was allowed to react to
determine if the product yield further increased in the absence
of the solid catalyst.
14 A. Szabolcs, M. Molnar, G. Dibo and L. T. Mika, Green Chem.,
2013, 15, 439–445.
Conclusions
15 F. D. Pileidis and M. M. Titirici, ChemSusChem, 2016, 9, 562–
582.
16 J. Yuan, S. S. Li, L. Yu, Y. M. Liu, Y. Cao, H. Y. He and
K. N. Fan, Energy Environ. Sci., 2013, 6, 3308–3313.
17 L. Deng, J. Li, D. M. Lai, Y. Fu and Q. X. Guo, Angew. Chem.,
Int. Ed., 2009, 48, 6529–6532.
18 S. H. Zhu, Y. F. Xue, J. Guo, Y. L. Cen, J. G. Wang and
W. B. Fan, ACS Catal., 2016, 6, 2035–2042.
19 K. Yan, Y. Y. Yang, J. J. Chai and Y. Lu, Appl. Catal., B, 2015,
179, 292–304.
20 D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Green Chem.,
2013, 15, 584–595.
In summary, a novel and efficient Hf–GO catalyst was con-
structed using graphite oxide as the ligand for MPV reaction of
carbonyl compounds. Both the preparation conditions and the
reaction conditions were systematically studied. The prepared
Hf–GO catalyst showed similar or even higher efficiency among
the analogues. It was found that isopropanol had an activation
effect on the catalyst by rst removing residual DMF in micro-
and mesopores and then interacting with the active Hf–O sites.
With the advantages of good efficiency, excellent stability, and
facility to prepare, the novel Hf–GO catalyst was promising in
biomass conversion.
21 J. Q. Bond, D. M. Alonso, D. Wang, R. M. West and
J. A. Dumesic, Science, 2010, 327, 1110–1114.
22 J. N. Chheda, G. W. Huber and J. A. Dumesic, Angew. Chem.,
Int. Ed., 2007, 46, 7164–7183.
Conflicts of interest
There are no conicts to declare.
23 J. P. Lange, R. Price, P. M. Ayoub, J. Louis, L. Petrus, L. Clarke
and H. Gosselink, Angew. Chem., Int. Ed., 2010, 49, 4479–
4483.
24 M. G. Al-Shaal, A. Dzierbinski and R. Palkovits, Green Chem.,
2014, 16, 1358–1364.
25 L. Xin, Z. Y. Zhang, J. Qi, D. J. Chadderdon, Y. Qiu,
K. M. Warsko and W. Z. Li, ChemSusChem, 2013, 6, 674–686.
26 F. Y. Ye, D. M. Zhang, T. Xue, Y. M. Wang and Y. J. Guan,
Green Chem., 2014, 16, 3951–3957.
27 A. Villa, M. Schiavoni, C. E. Chan-Thaw, P. F. Fulvio,
R. T. Mayes, S. Dai, K. L. More, G. M. Veith and L. Prati,
ChemSusChem, 2015, 8, 2520–2528.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21968021, 21606134, and 21566029), CAS
“Light of West China” Program, and the Innovative and Entre-
preneurial Talents Grassland Talents Engineering of Inner
Mongolia.
Notes and references
1 M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114,
1827–1870.
2 D. M. Alonso, J. Q. Bond and J. A. Dumesic, Green Chem., 28 P. P. Upare, J. M. Lee, D. W. Hwang, S. B. Halligudi,
2010, 12, 1493–1513.
3 Q. Xu, X. L. Li, T. Pan, C. G. Yu, J. Deng, Q. X. Guo and Y. Fu,
Green Chem., 2016, 18, 1287–1294.
4 H. L. Wang, Y. Q. Pu, A. Ragauskas and B. Yang, Bioresour.
Technol., 2019, 271, 449–461.
5 A. Q. Wang and T. Zhang, Acc. Chem. Res., 2013, 46, 1377–
1386.
Y. K. Hwang and J. S. Chang, J. Ind. Eng. Chem., 2011, 17,
287–292.
29 W. Li, J. H. Xie, H. Lin and Q. L. Zhou, Green Chem., 2012, 14,
2388–2390.
30 K. Hengst, M. Schubert, H. W. P. Carvalho, C. B. Lu, W. Kleist
and J. D. Grunwaldt, Appl. Catal., A, 2015, 502, 18–26.
31 H. C. Zhou, J. L. Song, H. L. Fan, B. B. Zhang, Y. Y. Yang,
J. Y. Hu, Q. G. Zhu and B. X. Han, Green Chem., 2014, 16,
3870–3875.
6 H. L. Wang, H. M. Wang, E. Kuhn, M. P. Tucker and B. Yang,
ChemSusChem, 2018, 11, 285–291.
7 S. Wang, J. Wang, Q. F. Zhao, D. D. Li, J. Q. Wang, M. Cho, 32 E. F. Mai, M. A. Machado, T. E. Davies, J. A. Lopez-Sanchez
H. Cho, O. Terasaki, S. J. Chen and Y. Wan, ACS Catal.,
2015, 5, 797–802.
and V. T. da Silva, Green Chem., 2014, 16, 4092–4097.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9993

View Article Online
RSC Advances
Paper
33 Y. Y. Liu, P. Ghimire and M. Jaroniec, J. Colloid Interface Sci., 54 V. Bon, I. Senkovska, I. A. Baburin and S. Kaskel, Cryst.
2019, 535, 122–132. Growth Des., 2013, 13, 1231–1237.
34 Y. F. Sha, Z. H. Xiao, H. C. Zhou, K. L. Yang, Y. M. Song, N. Li, 55 G. R. Cai and H. L. Jiang, Angew. Chem., Int. Ed., 2017, 56,
R. X. He, K. D. Zhi and Q. S. Liu, Green Chem., 2017, 19, 4829–
4837.
35 J. L. Song, B. W. Zhou, H. C. Zhou, L. Q. Wu, Q. L. Meng,
Z. M. Liu and B. X. Han, Angew. Chem., Int. Ed. Engl., 2015,
54, 9399–9403.
563–567.
56 H. Reinsch, B. Bueken, F. Vermoortele, I. Stassen, A. Lieb,
K. P. Lillerud and D. De Vos, Crystengcomm, 2015, 17,
4070–4074.
57 Z. G. Hu, I. Castano, S. N. Wang, Y. X. Wang, Y. W. Peng,
Y. H. Qan, C. L. Chi, X. R. Wang and D. Zhao, Cryst.
Growth Des., 2016, 16, 2295–2301.
36 M. H. Beyzavi, N. A. Vermeulen, A. J. Howarth,
S. Tussupbayev, A. B. League, N. M. Schweitzer,
J. R. Gallagher, A. E. Platero-Prats, N. Hafezi, A. A. Sarjeant, 58 H. Wu, Y. S. Chua, V. Krungleviciute, M. Tyagi, P. Chen,
J. T. Miller, K. W. Chapman, J. F. Stoddart, C. J. Cramer,
J. T. Hupp and O. K. Farha, J. Am. Chem. Soc., 2015, 137,
13624–13631.
T. Yildirim and W. Zhou, J. Am. Chem. Soc., 2013, 135,
10525–10532.
59 F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde,
M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi,
M. Waroquier, V. Van Speybroeck, C. Kirschhock and
D. E. De Vos, J. Am. Chem. Soc., 2013, 135, 11465–11468.
37 J. Zheng, M. Y. Wu, F. L. Jiang, W. P. Su and M. C. Hong,
Chem. Sci., 2015, 6, 3466–3470.
38 M. H. Beyzavi, N. A. Vermeulen, A. J. Howarth,
S. Tussupbayev, A. B. League, N. M. Schweitzer, 60 Z. L. Fang, B. Bueken, D. E. De Vos and R. A. Fischer, Angew.
J. R. Gallagher, A. E. Platero-Prats, N. Hafezi, A. A. Sarjeant, Chem., Int. Ed., 2015, 54, 7234–7254.
J. T. Miller, K. W. Chapman, J. F. Stoddart, C. J. Cramer, 61 C. A. Trickett, K. J. Gagnon, S. Lee, F. Gandara, H. B. Burgi
J. T. Hupp and O. K. Farha, J. Am. Chem. Soc., 2016, 138,
and O. M. Yaghi, Angew. Chem., Int. Ed., 2015, 54, 11162–
3251.
11167.
39 X. X. Cai, J. P. Pan, G. M. Tu, Y. H. Fu, F. M. Zhang and 62 K. M. Choi, H. J. Jeon, J. K. Kang and O. M. Yaghi, J. Am.
W. D. Zhu, Catal. Commun., 2018, 113, 23–26. Chem. Soc., 2011, 133, 11920–11923.
40 S. Rojas-Buzo, P. Garcıa-Garcıa and A. Corma, Green Chem., 63 S. He, Y. F. Chen, Z. C. Zhang, B. Ni, W. He and X. Wang,
2018, 20, 3081–3091. Chem. Sci., 2016, 7, 7101–7105.
41 J. L. Song, Z. M. Xue, C. Xie, H. R. Wu, S. S. Liu, L. Zhang and 64 H. Li, T. Yang and Z. Fang, Appl. Catal., B, 2018, 227, 79–89.
´
´
B. X. Han, ChemCatChem, 2018, 10, 725–730.
65 Z. H. Xiao, H. C. Zhou, J. M. Hao, H. L. Hong, Y. M. Song,
42 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
R. X. He, K. D. Zhi and Q. S. Liu, Fuel, 2017, 193, 322–330.
43 H. J. Shin, K. K. Kim, A. Benayad, S. M. Yoon, H. K. Park, 66 J. L. Song, L. Q. Wu, B. W. Zhou, H. C. Zhou, H. L. Fan,
I. S. Jung, M. H. Jin, H. K. Jeong, J. M. Kim, J. Y. Choi and
Y. H. Lee, Adv. Funct. Mater., 2009, 19, 1987–1992.
Y. Y. Yang, Q. L. Meng and B. X. Han, Green Chem., 2015,
17, 1626–1632.
44 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, 67 L. Peng, J. L. Zhang, J. S. Li, B. X. Han, Z. M. Xue and
A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and
R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
45 H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao and
Y. Chen, ACS Nano, 2008, 2, 463–470.
46 H. L. Zhou, Y. C. Liu, L. Zhang, H. D. Li, H. Liu and W. J. Li, J.
Colloid Interface Sci., 2019, 533, 287–296.
47 O. Metin, H. Can, K. Sendil and M. S. Gultekin, J. Colloid
Interface Sci., 2017, 498, 378–386.
G. Y. Yang, Chem. Commun., 2012, 48, 8688–8690.
68 H. Li, Y. Li, Z. Fang and R. L. Smith, Catal. Today, 2019, 319,
84–92.
69 G. Lv, H. Wang, Y. Yang, T. Deng, C. Chen, Y. Zhu and
X. Hou, ACS Catal., 2015, 5, 5636–5646.
70 P. P. Upare, J.-W. Yoon, M. Y. Kim, H.-Y. Kang, D. W. Hwang,
Y. K. Hwang, H. H. Kung and J.-S. Chang, Green Chem., 2013,
15, 2935.
48 M. Nasrollahzadeh, S. M. Sajadi, A. Rostami-Vartooni, 71 Y. Wang, Z. M. Rong, Y. Wang, T. Wang, Q. Q. Du, Y. Wang
M. Alizadeh and M. Bagherzadeh, J. Colloid Interface Sci.,
2016, 466, 360–368.
and J. Qu, ACS Sustainable Chem. Eng., 2016, 5, 1538–1548.
72 O. Akhavan, ACS Nano, 2010, 4, 4174–4180.
49 S. Rojas-Buzo, P. Garcia-Garcia and A. Corma, 73 C. Xie, J. L. Song, B. W. Zhou, J. Y. Hu, Z. R. Zhang, P. Zhang,
ChemSusChem, 2018, 11, 432–438.
50 Z. Hu, I. Castano, S. Wang, Y. Wang, Y. Peng, Y. Qian, C. Chi,
Z. Jiang and B. X. Han, ACS Sustainable Chem. Eng., 2016, 4,
6231–6236.
X. Wang and D. Zhao, Cryst. Growth Des., 2016, 16, 2295– 74 Z. Cai, W. K. Li, F. M. Wang and X. B. Zhang, J. Taiwan Inst.
2301. Chem. Eng., 2018, 93, 374–378.
51 A. H. Valekar, K.-H. Cho, S. K. Chitale, D.-Y. Hong, G.-Y. Cha, 75 W. B. Wu, Y. Li, H. Li, W. F. Zhao and S. Yang, Catalysts,
U. H. Lee, D. W. Hwang, C. Serre, J.-S. Chang and
Y. K. Hwang, Green Chem., 2016, 18, 4542–4552.
52 H. Reinsch, B. Bueken, F. Vermoortele, I. Stassen, A. Lieb,
K.-P. Lillerud and D. De Vos, CrystEngComm, 2015, 17,
4070–4074.
2018, 8, 264.
76 J. X. Hao, L. M. Han, Y. F. Sha, X. X. Yu, H. Y. Liu, X. Y. Ma,
Y. Z. Yang, H. C. Zhou and Q. S. Liu, Fuel, 2019, 239, 1304–
1314.
77 B. B. Zhang, J. X. Hao, Y. F. Sha, H. C. Zhou, K. L. Yang,
Y. M. Song, Y. P. Ban, R. X. He and Q. S. Liu, Fuel, 2018,
217, 122–130.
53 V. Bon, V. Senkovskyy, I. Senkovska and S. Kaskel, Chem.
Commun., 2012, 48, 8407–8409.
9994 | RSC Adv., 2020, 10, 9985–9995
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
78 J. L. Song, L. Q. Wu, B. W. Zhou, H. C. Zhou, H. L. Fan, 81 K. Jiang, D. Sheng, Z. H. Zhang, J. Fu, Z. Y. Hou and X. Y. Liu,
Y. Y. Yang, Q. Meng and B. X. Han, Green Chem., 2015, 17,
1626–1632.
79 Z. M. Xue, J. Y. Jiang, G. F. Li, W. C. Zhao, J. F. Wang and
T. C. Mu, Catal. Sci. Technol., 2016, 6, 5374–5379.
80 K. Yan, J. Y. Liao, X. Wu and X. M. Xie, RSC Adv., 2013, 3,
3853–3856.
Catal. Today, 2016, 274, 55–59.
82 J. Cheng, J. Wang, X. Z. Wang and H. Wang, Ceram. Int.,
2017, 43, 7159–7165.
83 H. Furukawa, F. Gandara, Y. B. Zhang, J. Jiang, W. L. Queen,
M. R. Hudson and O. M. Yaghi, J. Am. Chem. Soc., 2014, 136,
4369–4381.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 9985–9995 | 9995