Direct use of humic acid mixtures to construct efficient Zr-containing catalysts for Meerwein-Ponndorf-Verley reactions

Article abstract of DOI:10.1039/c7gc01925d

With the increasing demands for energy and carbon resources, exploration of novel utilization approaches of fossil resources and promotion of the conversion of sustainable resources become critical issues facing human society. In this study, we used humic acids (HAs), which are important derivatives from the low-rank coal, and the transition metal zirconium (Zr) to construct novel Zr-containing catalysts (Zr-HAs) for Meerwein-Ponndorf-Verley (MPV) reactions of biomass-derived platforms. Both commercial HAs and HAs extracted directly from lignite without any purification were used as raw materials to prepare the catalysts. The results showed that Zr-HAs catalysts were highly efficient for the conversion of furfural, with a high furfuryl alcohol yield of up to 97%, and also effective for the conversion of other carbonyl compounds with different structures under mild conditions. This novel strategy to construct catalysts using HAs as raw materials may be beneficial for both value-added utilization of low-rank coal and the conversion of biomass resources.

Full text of DOI:10.1039/c7gc01925d

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Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
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Direct use of humic acid mixtures to construct
efficient Zr-containing catalysts for Meerwein-
Ponndorf-Verley reactions
Cite this: DOI: 10.1039/x0xx00000x
Yufei Sha‡, Zhenhuan Xiao‡, Huacong Zhou*, Keli Yang, Yinmin Song, Na Li,
Runxia He, Keduan Zhi, Quansheng Liu*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
With the increasing demands for energy and carbon resources, exploring novel utilization
approaches of fossil resources and promoting the conversion of sustainable resources
become critical issues facing human society. In this work, we use humic acids (HAs), which
are important derivatives from the low rank coal, and the transition metal zirconium (Zr) to
construct novel Zrꢀcontaining catalysts (ZrꢀHAs) for MeerweinꢀPonndorfꢀVerley (MPV)
reactions of biomassꢀderived platforms. Both commercial HAs and HAs extracted directly
from lignite with no need of purification were used as raw materials to prepare the catalysts.
The results showed that ZrꢀHAs catalysts were highly efficient for the conversion of furfural
with high furfuryl alcohol yield of up to 97%, and also effective for other carbonyl
compounds with different structures under mild conditions. This novel strategy to construct
catalysts using HAs as raw materials may be beneficial for both valueꢀadded utilization of
low rank coal and the conversion of biomass resources.
www.rsc.org/
functional groups (OFGs), high reactivity, and low pollutionꢀ
forming impurities such as sulfur, nitrogen, and heavy metals, and
Introduction
thus lignite is also the potential carbon resource.8 Extensive
processes have been developed for the clean and efficient utilization
of lignite in the past decades, including drying,9 pyrolysis,10
combustion,11 gasification,12 and liquefaction,13 etc. Although these
processes are commonly applied and efficient for the utilization of
lignite, undeniably, they also suffer some disadvantages to some
extent, such as the intensive energy input, the high release of
pollutants, and/or loss of the inherent structures and functional
groups.2,14 Therefore, exploring more green and valueꢀadded
approaches to utilize lignite, especially giving full play to their
specific properties, is still an urgent and critical issue during the
utilization of low rank coal.
With the highꢀspeed development of human society and the fast
depletion of fossil resources (such as coal) on earth, two critical
contradictions are gradually exposed. One is the contradiction
between the increasing demands for resources of human society and
the limited reserves of the fossil resources, and the other is the
contradiction between the urgency of environmental protection and
the serious pollutions partially resulted from the excessive
exploitation and use of fossil resources.1ꢀ3 To remit or even
overcome the above contradictions, on one hand, it is essential for
human beings to improve the utilizing efficiency of fossil resources,
and on the other hand, it is more wise to explore sustainable carbon
resources in the longꢀterm run.2, 4ꢀ6
Coal, as an important representative of the fossil resources, has
played critical role during the development of human society and
will still be the main energy and material source in the foreseeable
future.2 Although the coal reserve on earth is abundant, it is
estimated that ca. 40% of the total coal deposits are the low rank
lignite.7 Several obvious disadvantages make it difficult using lignite
as clean and efficient as the high rank coal, including the low energy
density, high contents of the volatiles and moisture, and the bad
stability to air.7, 8 These drawbacks limit the direct use of the low
rank coal as primary energy source severely. On the other hand,
lignite does have some different specific properties over the high
rank coal, including the high content of volatiles and oxygenꢀ
Depolymerization of lignite and further conversion of the obtained
fractions into useful products is a potential approach for the efficient
and clean utilization of lignite.15, 16 Humic acids (HAs), an important
and common class of organic components in lignite, are composed
of various aromatic ringꢀbased organic acids and rich in acidic
OFGs. HAs have been found wide applications in various fields,
such as the agriculture techniques,17,18 environmental protection,19,20
and material sciences.21 The abundant OFGs in HAs and lignite
could also be used to prepare valueꢀadded chemicals, such as
benzene poly(carboxylic acid)s,22ꢀ24 smallꢀmolecule fatty acids,22, 25
and other valueꢀadded chemicals.14, 25 Therefore, preparing valuable
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materials from HAs or even directly from lignite and improving the (FA), a key reaction step during the reaction chains of the biomass
added values of the derived products have become a potential way conversion. The structures of the prepared catalysts were
for utilization of lignite.
characterized in detail. The results showed that ZrꢀHAs catalysts
In parallel with improving the utilizing efficiency of the coal based on both commercial and extracted HAs showed high
resources, searching sustainable carbon resource is an inevitable efficiency and excellent stability for the MPV reaction of FF into
choice for human beings in the long run.26 Biomass is a wellꢀknown, FA. It is believed that the proposed approach to construct MPV
green and sustainable carbon resource on earth. Compared to the catalysts using HAs as the raw materials is beneficial for both
fossil resources, biomass possesses several obvious advantages, such promoting the valueꢀadded utilization of low rank lignite and the
as large reserves, being renewable, broad distribution, and conversion of biomass resources.
abundance in varieties.5, 6, 27 Useful molecules with relatively high
quality could be obtained from biomass raw materials via suitable
processess.28ꢀ30 Through catalytic processes, these molecules can be
converted into various platform compounds, which can be further
transformed into various target products.31ꢀ36 Among the biomassꢀ
derived chemicals, carbonyl compounds, such as furfural,37,38
levulinic acid and its esters,39,40 and ketone compounds,41 are
important platforms because they could be further converted into
valuable chemicals. Extensive studies were reported on how to
obtain these platforms such as levulinic acid from biomass, and the
reaction mechanisms were also well investigated.42,43 Meerweinꢀ
PonndorfꢀVerley (MPV) reaction is commonly used for the
conversion of carbonyl compounds via selective hydrogenation of
Scheme 1. Schematic diagram of direct use of HAs mixtures from
the carbonyl bonds, leaving the C=C double bonds intact.44,45 During
low rank coal to prepare ZrꢀHAs catalyst and its application in the
the MPV reaction, the C=O bonds are chemoselectively reduced
conversion of biomassꢀderived carboxyl compounds via MPV
reaction.
with secondary alcohols as the hydrogen sources under the catalysis
of MPV catalysts.46,47 Zrꢀbased catalysts are commonly applied in
MPV reaction, including ZrO2,48 zirconium alkoxides,49 Zrꢀ
Experimental
containing zeolites,50 Zr(OH)4,51 etc. Recently, constructing efficient
Zr catalysts using novel and lowꢀcost raw materials has drawn much
attention. Xue et al.52 designed a novel Zrꢀbased coordination
polymer using cyanuric acid, a common chemical block in industry,
as the building block. The obtained material was used as the catalyst
in the MPV reaction of levulinic acid to gammaꢀvalerolactone
(GVL), which was proved to be highly efficient. Song et al.53 and
DoꢀYong Hong et al.54 constructed Zrꢀbased MPV catalysts using
aromatic ringꢀbased acidic compounds as the materials, including 4ꢀ
hydroxybenzoic acid dipotassium salt, benzenedicarboxylic acids,
and benzenetricarboxylic acids, and most of the obtained catalysts
were proved to be efficient for the MPV reaction of ethyl levulinate
into GVL. In our recent work,45 a more abundant and cheap material,
natural phytic acid containing six phosphate acid groups, was used to
construct new MPV catalysts and high efficiency was achieved for
the MPV reactions of various carbonyl compounds with different
structures. These reports indicate that constructing MPV catalysts
using the acid groups in chemicals especially from natural sources
has huge potential for practical applications due to the cost
advantages, and it is still highly desirable and meaningful to explore
more raw materials to synthesize MPV catalysts in the field of
biomass conversion.
Materials
Furfural (FF, 99%), furfuryl alcohol (FA, 98%), ethyl levulinate
(EL, 98%), gammaꢀvalerolactone (GVL, 98%), and ZrOCl2•8H2O
(AR) were provided by J&K Scientific Ltd. Commercial humic acids
were purchased from Aladdin Industrial Corporation. Isopropanol
(AR), ethanol (AR), KOH (AR), decane (AR) and other chemicals
were obtained from Beijing Institute of Chemical Reagent. The raw
lignite sample was collected from Shengli coalfield in Inner
Mongolia, China. The asꢀreceived lignite sample was crushed and
ground to fine powder with average sizes ca. 38—75 ꢁm, followed
by drying at 105 °C in air for 4 h.
Catalyst preparation
Preparation of Zr-HAsC catalyst: Commercial HAs was first used
as the building block to explore the optimal preparation conditions of
the catalysts. Typical procedures were as follows: 2 g HAs and 0.34
g KOH were dissolved in 80 mL distilled water, respectively. The
KOH solution was mixed with HAs solution to promote the
dispersion of HAs, and the mixtures were continuously stirred for 5
h at room temperature. The pH value of the neutralized HAs solution
was tuned by HCl to ca. pH=1 and then dropwise slowly added into
the solution of Zr precursor containing 3.76 g ZrOCl2·8H2O, and the
obtained mixture was further stirred for 10 h at room temperature.
Finally, the brown slurry was collected by centrifugation, washed for
at least 5 times with distilled water and 2 times with ethanol until no
Clꢀ detected by AgNO3, dried in vacuum at 80 °C for 12 h, and then
In this study, we constructed a novel and highly efficient Zrꢀbased
MPV catalysts (termed as ZrꢀHAs hereafter) using the low rank
lignite derived HAs as the building block (Scheme 1). Both
commercial HAs and HAs extracted directly from lignite without
purification were used as raw materials to prepare the catalysts, and
the obtained catalysts were applied into the selective hydrogenation
conversion of biomassꢀderived furfural (FF) into furfuryl alcohol
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crowded into powders for use (denoted as ZrꢀHAsC). The ratios of
KOH and zirconium precursor to HAs were varied to optimize the measurements were carried out on an ESCALAB 250Xi
a Micromeritics ASAP 2020 V3.04 G system. The XPS
preparation conditions of the catalysts.
spectrometer (Thermo Fisher Scientific) at a pressure of ~3×10ꢀ9
Preparation of Zr-HAsE catalyst: HAs extracted directly from mbar using Al Kα as the excitation source (hν=1486.6 eV) and
raw lignite was also attempted to prepare the catalyst. The extraction operated at 15 kV and 150 W.
process was conducted in a laboratory scaleꢀup batch reactor with an
Reaction
inner volume of 500 mL (Scheme 2). Typically, 2 g lignite powders
(with an average particle size of ca. 75 ꢁm) were dispersed into 350
mL 1 M NaOH solution, and then the reactor was sealed. The
mixture was mechanically agitated at 100 °C for 2 h. Then, the
slurry was filtered, and the obtained filtrate was tuned with HCl until
pH < 1. HAs were precipitated due to the poor solubility in strongly
acidic condition. The precipitated HAs was collected by filtration,
dried under vacuum at 80 °C, and crowded into fine powders to be
used. The procedure of the catalysts preparation using the extracted
HAs was similar with that of commercial HAs, as stated above. 2 g
of extracted HAs was first solved into dilute NaOH solution. HCl
solution was added to tune the pH to acidic condition, and then the
HAs solution was dropwise added into the Zr salt solution
containing 8 g ZrOCl2•8H2O. The mixture was stirred under 80 °C
for 5 h. The treatment procedures of the catalysts were the same as
the commercial HAs. The prepared catalysts were denoted as Zrꢀ
HAsE.
The activity of the prepared catalysts was detected via the reaction
of the catalytic transfer hydrogenation of furfural. The reaction was
performed in a 10 mL Teflonlined stainless steel autoclave equipped
with a magnetic stirrer. Typically, furfural (1 mmol), isopropanol (5
mL), and 0.2 g catalyst were introduced into the reactor. After
sealing, the reaction mixture was stirred and allowed to react at
suitable temperatures for a desired time. After reaction, the reactor
was cooled down in cold water to quench the reaction and the
reaction solution was transferred and diluted by isopropanol. The
samples were analyzed quantitatively by gas chromatography
(TECHCOMP GC7900) with a flame ionization detector using
decane as the internal standard. Identification of the products and the
reactant was done using a GCꢀMS (SHIMADZUꢀQP 2010) as well
as by comparing the retention times with respective standards in GC
traces.
In order to check the heterogeneity of the catalysts, the solid
catalysts were removed from the reaction mixture after reaction for a
short time, and the supernatant was allowed to react to see if the
product could further increase with the absence of the solid catalysts.
In the reusability experiments, the catalyst was separated by
centrifugation, washed with fresh isopropanol for three times, and
then reused for the next run without further treatments.
Results and Discussion
Optimization and characterization of Zr-HAsC catalysts
The commercial HAs (HAsC) was first used to prepare Zrꢀ
HAsC catalysts. The preparation conditions, including the usage
of alkali and the ratio of Zr precursor (ZrOCl2•8H2O in this
case) to HAsC, were investigated. In the experiments, HAs was
found to be hard to react directly with Zr4+ to form the expected
ZrꢀHAsC precipitation under room temperature. This might be
attributed to that the dispersion of HAsC in water was poor
and/or the acidity of carboxylic and phenolic hydroxyl groups
in HAs was not strong enough, leading to a weak interaction
between the acidic groups and Zr4+ especially under low
temperatures. The dispersion of the neutralized HAs became
better and the solution was allowed to react with Zr4+ precursor.
As expected, the ZrꢀHAsC precipitation was successfully
formed, and the solid catalysts were obtained. The amounts of
alkali were first varied to study the effects of alkali on the
activity of the catalysts (Entries 1—4, Table 1). The results
showed that the activity of the catalysts first increased and then
decreased with the increasing of the ratios of alkali, and came
to the highest value at the HAsC:KOH mole ratio of 1:1. The
less alkali was used, the lower content of the active Zr4+ species
in asꢀprepared catalysts, leading to a lower activity. When
Scheme 2. Schematic of the batch reactor for the extraction of HAs
from lignite.
Catalyst characterization
Scanning electron microscopy (SEM) measurements were
performed on a Hitachi Sꢀ3400N scanning electron microscope
operated at 20 kV with Energy dispersive spectrometer (EDS)
apparatus. Transmission electron microscopy (TEM) images were
obtained using a TEM JEOLꢀ1011 with an accelerating voltage of
120 kV. Fourier transformꢀinfrared spectra (FTIR) were obtained
using a PerkinElmer Spectrum. Xꢀray diffraction (XRD) were
carried out on a XD8 AdvanceꢀBruker AXS Xꢀray diffractometer
using CuꢀKα radiation (λ = 532 nm) and Ni filter scanning at 2° per
minute ranging 3°~ 80°. The tube voltage was 40 kV and the current
was 40 mA. The thermogravimetric (TG) analysis of ZrꢀFA was
performed on a thermogravimetric analysis system (Diamond
TG/DTA6300, Perkin Elmer Instruments) under Ar atmosphere at a
heating rate of 10 °C·min−1. The surface area and porosity properties
were determined by the nitrogen adsorptionꢀdesorption method using
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excessive alkali was used, more or even total acidic groups in
HAsC were neutralized, resulting in the decrease of the acidic
sites in the final catalysts. According to our previous studies,
the residual acidic sites were favourable for improving the
activity.45 Thus the HAsC:KOH ratio of 1:1 was optimal to
achieve higher activities. Besides, the ratio of Zr precursor to
HAsC is another parameter during the preparation of the
catalysts, and its effects on the activity of the catalyst were
studied via fixing the ratio of HAsC:KOH at 1:1 (Entries 2, 5—
6, Table 1). The results showed that increasing the usage of Zr
precursor could significantly enhance the activity of the
catalysts with the highest activity at the HAsC:KOH:Zr ratio of
1:1:2. Further increasing the addition of the Zr precursor could
not improve the activity of the catalysts. Therefore, the
conditions of HAsC:KOH:Zr mole ratio of 1:1:2 was chosen to
prepare the catalysts in the subsequent studies.
Table 1. Investigation of the preparing conditions of the catalysts. a
Mole ratio of
Entry
HAsC:KOH:Zr
Conv./%
Yield/%
Sel./%
1
2
3
4
5
6
1:0.5:2
1:1:2
1:2:2
1:3:2
1:1:1
1:1:3
46.6
63.0
29.5
31.1
34.8
46.4
27.5
51.9
25.6
23.7
24.0
33.3
59.0
82.4
86.8
76.3
68.9
71.8
Fig. 1 Characterization of the asꢀprepared ZrꢀHAsC catalyst by SEMꢀ
EDS (a, b) (EDS data were taken from the square area), powder
XRD pattern (c), FTIR spectra (d), N2 adsorptionꢀdesorption
isotherm and pore size distribution (e), and Zr 3d XPS spectra (f).
a
:
Reaction conditions: FF 1 mmol, isopropanol 5 mL, ZrꢀHAsC
200 mg, reaction temperature 50 °C, reaction time 3 h.
In order to understand well the structures of the catalysts,
detailed characterizations were conducted. SEM showed that
the ZrꢀHAsC catalyst was composed of particles with no
uniform shapes (Fig. 1a). EDS gave a strong Zr signal in the
catalyst after the interaction of Zr4+ with HAsC (Fig. 1b).
Powder XRD of ZrꢀHAsC was conducted to gain an insight into
the bulk phase of the prepared catalyst (Fig. 1c). It was found
that the catalyst had no obvious Xꢀray crystal structure. In the
structure of HAs, there are the relatively strong carboxylate
groups and the weaker phenate groups besides the main
structures composed of aromatic cores and aliphatic side
chains.55,56 Less strong coordination groups resulted in a lower
degree of crystallinity of the ZrꢀHAsC complex. The XRD
pattern showed one broad diffraction peak around 27°,
indicating that the asꢀprepared catalyst was amorphous.53 As
shown in Fig. 1d, the FTIR spectra of HAsC and the asꢀprepared
catalyst displayed the characteristic asymmetric (HAsC, 1572
cmꢀ1; ZrꢀHAsC, 1579 cmꢀ1) and symmetric vibrations (HAsC,
1375 cmꢀ1; ZrꢀHAsC, 1395 cmꢀ1) of carboxylate groups.
Compared with the FTIR spectrum of HAsC, the wavenumber
difference of asymmetric and symmetric vibrations of
carboxylate anions in ZrꢀHAsC was narrowed from 197 cmꢀ1 to
184 cmꢀ1, which indicated that carboxylate groups were
coordinated to Zr4+ ions.53,57 N2 adsorptionꢀdesorption
The performance of the prepared catalyst was compared with
other analogous catalysts focusing on the chemoselective conversion
of furfural to FA reported in the literatures. Table S1 gave the results
of commonly used processes for the above conversion, including
catalytic transfer hydrogenation (CTH) using secondary alcohol as
hydrogen sources and liquid or vapor phase hydrogenation (LPH or
VPH) using H2 as hydrogen sources. For CTH processes (Entries
1—9, Table S1), there is no need to use high pressure H2, and the
reaction temperatures are often not very high, especially for the Zrꢀ
based CTH catalysts. For LPH process, both noble and nobleꢀfree
metal catalysts were reported to be efficient for the conversion of
furfural. Undoubtedly, the noble catalysts (Entries 10—15, Table
S1) performed well under much milder conditions (lower
temperatures and H2 pressure) than those of the nobleꢀfree metal
catalysts (Entries 16—22, Table S1), with the similar level
conversions and product yields. Compared with other CTH catalysts,
the prepared ZrꢀHAsC catalysts in this work could give comparative
conversion and yield (both higher than 97%) under lower
temperatures (Entry 2, Table S1). From viewpoint of reaction
conditions and the cost of raw materials for catalyst preparation, the
ZrꢀHAsC catalyst had obvious advantages comparing with the noble
and other transition metal catalysts. (Entries 1, 2, 10—15, Table S1).
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characterization showed that the catalyst was porous with before 70 °C. Although further increasing temperatures could
mesopores centered around 4 nm, and the surface area and pore promote the conversion of the reactant, the selectivity had slight
volume were 123 m2/g and 0.17 cm3/g, respectively (Fig. 1e). decrease, leading to no obvious change in yield. During the
The local environment of Zr species in ZrꢀHAsC was detected experiments, some unknown sideꢀproducts were observed in
by Zr 3d XPS. Compared with ZrO2, the binding energy of Zr GC detection when the temperature was higher than 70 °C.
in ZrꢀHAsC moved to higher level (Fig. 1f). The higher binding Thus it was wise to conduct the reaction under temperatures not
energy of Zr 3d indicates a higher positive charge on the Zr higher than 70 °C. Considering both the reaction rate and the
atoms, resulting in a stronger Lewis acidity of Zr.58 The higher selectivity, a medium temperature of 70 °C was chosen as the
acidity of Zr species could improve the activity of the ZrꢀHAsC optimal reaction temperature.
catalyst.45 TG analysis showed that the prepared catalyst had a
low weight loss of 18% under 170 °C, resulting from the
desorption of water and ethanol adsorbed on the catalyst,57 and
had excellent stability under reaction temperatures (below 100
°C) (Fig. S1).
The effect of catalyst dosage
The effect of catalyst dosage on the conversion of FF into FA was
studied (Fig. 2). It can be seen that both the conversion and the
product yield increased significantly with the increasing of catalyst
dosage in the range of 50 — 150 mg under the experimental
conditions. The increasing trend of conversion became slower and
the yield even had a slight decrease when the catalyst dosage was up
to 250 mg. In our experiment, we found that the reaction system
exhibited a slurry state and the catalysts particles had a poor
dispersion under larger catalyst dosage, which had a negative effect
Fig. 3 Effect of the temperature on the conversion of FF into FA.
on the mass transportation during reaction. The slight decrease of FA
Reaction conditions: FF 1 mmol, isopropanol 5 mL, ZrꢀHAsC 200
yield under higher catalyst dosage could also be caused by the
mg, reaction time 3 h.
product adsorption on the catalyst during sample treatment after
reaction due to the large volume of the catalyst. A suitable catalyst
dosage of 200 mg in this study was used in the following studies.
The effect of the reaction time
In order to obtain a higher FA yield, the influence of reaction time
was investigated (Fig. 4). The results showed that the reaction
proceeded fast within the first 3 h and the FA yield increased to 75%
under 70 °C (Fig. 4a). The FA yield increased at a slower rate with
further prolonging the reaction time, and the yield came to 80% at
the time of 5 h with furfuryl conversion of 91%. The conversion
could reach up to 98% when the reaction time was prolonged to 9 h,
while the yield maintained the same level as 5 h, resulting in a slight
decrease in selectivity. During GC detection, it was noticed that the
intensities of the unknown sideꢀproducts increased with the
prolonging of reaction time. To decrease the formation of sideꢀ
products, the reaction was attempted to proceed under a lower
temperature (50 °C, Fig. 4b). The reaction rate was sharply
decreased when the reaction temperature was lowered. Interestingly,
both the conversion and the yield increased with the prolonging of
Fig. 2 Effect of the ZrꢀHAsC dosage on the conversion of FF into
FA. Reaction conditions: FA 1 mmol, isopropanol 5 mL, reaction
the reaction time, different from the conditions under 70 °C. The
temperature 50 °C, reaction time 3 h.
conversion and yield achieved to ca. 97% with high selectivity of ca.
99% when the reaction time was increased to 15 h. Therefore, high
temperatures could obviously increase the reaction rate while lower
temperatures were favourable for enhancing the selectivity.
The effect of reaction temperature
The effect of reaction temperature on the performance of Zrꢀ
HAsC catalyst was studied and the results were shown in Fig. 3.
Reaction temperature had a dramatic influence on the
performance of the catalyst, and the FA yield increased
significantly with the increasing of temperature especially
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Performances of Zr-HAsE
In order to check the universality and feasibility of the proposed
approach to synthesis Zrꢀbased catalyst, HAs extracted from the low
rank lignite was directly used as raw materials to prepare catalyst
(ZrꢀHAsE) without further separation or treatment. The extracted
HAs and corresponding ZrꢀHAsE catalyst were characterized by
SEMꢀEDS, FTIR, and XRD (Fig. S3). The results showed that no
obvious inherent metallic mineral residues except for trace amount
of sulfuric species were observed in the extracted HAsE by EDS
analysis, which may be attributed to the HCl precipitation process
during extraction. SEMꢀEDS and FTIR proved that Zr4+ ions were
successfully coordinated with the acidic groups in HAsE. The
obtained ZrꢀHAsE catalyst had no obvious Xꢀray diffraction peaks,
indicating that the asꢀprepared catalyst was amorphous.
Fig. 4 Effect of the reaction time on the conversion of furfuryl into
FA under different temperatures, (a) 70 °C and (b) 50 °C. Reaction
conditions: furfuryl 1 mmol, isopropanol 5 mL, ZrꢀHA 200 mg.
Reusability and heterogeneity of Zr-HAsC
The performances of the ZrꢀHAsE catalyst for the conversion
of furfuryl were shown in Fig. 6. It could be seen that the FA
yield increased significantly with the increasing of temperatures
before 70 °C, and further increasing the temperatures had no
obvious improvement on the yields (Fig. 6a). The yield had a
slight increase from 87.7% to 92.1% via prolonging the
reaction time from 5 h to 9 h under 70 °C (Fig. 6b). Compared
with ZrꢀHAsC catalyst, ZrꢀHAsE also had a good stability
during recycling, which could be reused for at least 7 times
without obvious change in performance (Fig. 6c).
Heterogeneity studies showed that the ZrꢀHAsE catalyst also
catalyzed the reaction via heterogeneous form (Fig. 6d). The
structure comparison of the freshly prepared and recycled
catalysts was conducted (Fig.7). SEMꢀEDS showed that the
amorphous morphology of the catalyst had no obvious change
after recycling, and the mass weight of Zr in the fresh and
recycled catalysts were estimated to be 28.9% and 28.0%,
respectively, indicating that there was no significant leaching of
Zr species (Fig. 7 a—d). FTIR and XRD patterns proved that
the microstructures of the recycled catalyst had no obvious
change compared with the fresh catalyst, giving an excellent
stability during recycling (Fig. 7 e, f).
To check the stability of ZrꢀHAsC catalyst during recycling
process, the reusability and heterogeneity was investigated. In each
cycle, the catalyst was recovered by centrifugation, washed with
fresh isopropanol (5 mL, 3 times), and then used for the next run.
The results showed that there was no considerable decrease in the
conversion, yield, and selectivity after nine cycles compared with the
first use, indicating that the catalyst was very stable (Fig. 5a). The
leaching of the catalytic species and heterogeneity of the catalyst
were also studied (Fig. 5b). The reaction was stopped via removing
the catalyst from the reaction mixture after the reaction was
proceeded for 3 h, and the solution was maintained reacting under
the same reaction conditions to see if the FA yield further increased
without the solid catalyst. It can be seen that there was no further
increase in the FA yield after the solid catalyst was removed,
confirming that the active sites in ZrꢀHAsC was not soluble in the
reaction mixture and it was the solid ZrꢀHAsC catalyst to catalyze
heterogeneously the reaction. The structures of ZrꢀHAsC catalyst
recovered after nine cycles was characterized by SEM, powder
XRD, and FTIR, and compared with the freshly prepared catalysts
(Fig. S2). It can be seen that the structures of the catalyst had no
obvious change after use. These results further proved that the
catalyst had an excellent stability under the studied reaction
conditions.
Fig. 5 Reusability (a) and heterogeneity (b) of the ZrꢀHAsC catalyst.
Reaction conditions: (a) furfuryl 1 mmol, isopropanol 5 mL, Zrꢀ
HAsC 200 mg, reaction temperature 70 °C, reaction time 3 h. (b)
reaction temperature 50 °C, other conditions were similar as above.
The solid catalyst was removed after 3 h.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 10
Journal Name
Green Chemistry
View Article Online
ARTICLE
DOI: 10.1039/C7GC01925D
Fig. 6 Performances of ZrꢀHAsE catalyst prepared via direct use of conversion of different carbonyl compounds with various
HAs extracted from lignite. Typical reaction conditions except structures.
otherwise stated in figures: furfuryl 1 mmol, isopropanol 5 mL, Zrꢀ Table 2. MPV reaction of carbonyl compounds over ZrꢀHAsC
HAsE 200 mg. (a) Influences of reaction temperatures: 5 h. (b)
Influences of reaction time: 70 °C. (c) Reusability, 3 h and 70 °C. (d)
Heterogeneity, 70 °C.
catalyst. a
T
t
Conv.
(%)
Yield
(%)
Entry
Reactant
Product
(°C) (h)
1
2
70
50
9
98.0
97.4
80.0
96.9
15
3
4
100
150
3
95.5
>99
82.5
85.0
15
5
6
100
150
5
9
98.1
92.9
92.9
92.8
42.8
7
120
5
81.0
Fig. 7 Comparison of the freshly prepared and recycled ZrꢀHAsE
catalysts after seven reuses. SEMꢀEDS of the fresh (a, b) and
recycled catalyst (c, d), FTIR spectra (e), and XRD patterns (f).
8.9
a Reaction conditions: substrate 1 mmol, isopropanol 5 mL, ZrꢀHAsC
200 mg.
Conversion of different carbonyl compounds
Mechanism analysis
Encouraged by the excellent performance of the prepared
catalysts for the conversion of furfuryl, we explored the
possibility of the MPV reactions of other carbonyl compounds
with different structures using the ZrꢀHAsC catalyst (Table 2).
The results showed that ZrꢀHAsC also showed good activity for
the examined compounds. Generally, ketones, especially with
long aliphatic chains, needed a longer reaction time or harsh
condition to achieve satisfied conversions and yields compared
with aldehydes (Table 2, Entries 1—6), which might be caused
by the steric hindrance due to the larger structures.45
Interestingly, ZrꢀHAsC performed mediate activity for the
diketone compounds (2,5ꢀhexanedione in this case, Entry 7),
giving a conversion of 81.0% and a 2,5ꢀhexanediol (2,5ꢀHDO)
yield of 42.8% under not yet optimized conditions, and a yield
of 8.9% for 2,5ꢀdimethyltetrahydrofuran (intramolecular
dehydration product from 2,5ꢀHDO) was also observed. These
results showed that the ZrꢀHAsC catalyst was efficient for the
Based on the characterization results and previous reports,45,53 the
possible mechanism of ZrꢀHAs catalyzed MPV reaction of carbonyl
compounds was proposed (Scheme 3). It was reported that acidic
and basic sites in the catalyst were essential for the MPV
reaction.45,46 In Zrꢀcontaining MPV catalysts, Zr4+ and O2ꢀ
(carboxylate and phenate groups) formed the acid and basic sites,
respectively.59,60 Firstly, isopropanol was adsorbed onto the catalyst
and interacted with the acidꢀbase sites (Zr4+ꢀO2ꢀ) on ZrꢀHAs,
resulting in its dissociation to the corresponding alkoxide.
Meanwhile, the carbonyl groups in the substrate molecules were
adsorbed onto the adjacent sites on the catalyst. Then, hydrogen
transfer took place between the dissociated alcohol and the activated
substrate molecules via a concerted process involving a sixꢀlink
intermediate to form the corresponding products. Isopropanol was
converted into acetone in this process after losing two hydrogen
atoms.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

Green Chemistry
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ARTICLE
Journal Name
DOI: 10.1039/C7GC01925D
‡These authors contributed equally to this work.
*Corresponding author.
Conclusion
Email address: hczhou@imut.edu.cn (H. Zhou), liuqs@imut.edu.cn (Q.
Liu)
As a summary, a novel strategy to prepare Zrꢀcontaining catalysts
using low rank coal derived humic acids (HAs) as raw materials was
proposed. Both commercial HAs and HAs extracted directly from
low rank lignite were used to synthesize ZrꢀHAs catalysts. The
catalyst from commercial HAs (ZrꢀHAsC) was proved to be highly
efficient for the MPV reactions of various carboxyl compounds with
different structures. HAs extracted from lignite could be directly
applied into the preparation of the catalyst with no need of further
separation or treatment, and the obtained catalyst (ZrꢀHAsE) was
identified to be efficient for the conversion of furfuryl into FA with
excellent stability. Using HAs as the raw materials to synthesize
catalysts is believed to be an effective approach for both valueꢀadded
utilization of lignite derivatives and the conversion of biomass
resources. The novel catalyst may find potential applications in
catalytic fields with the advantages of the abundance of HAs, the
low cost of Zr metal, and the excellent performance.
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DOI: 10.1039/C7GC01925D
Graphical abstract
Humic acid (HAs) mixtures from lignite could be directly used to
construct Zr-HAs catalysts for MPV reaction of carboxyl compounds.
Highlights
ꢀ
ꢀ
A novel approach to utilize lignite derived humic acids (HAs) is proposed.
HAs mixtures extracted from lignite can be used directly to construct Zr-HAs
catalyst.
ꢀ
Zr-HAs catalyst is effective for MPV reaction of various carboxyl compounds
under mild conditions.