Direct use of the solid waste from oxytetracycline fermentation broth to construct Hf-containing catalysts for Meerwein-Ponndorf-Verley reactions

Article abstract of DOI:10.1039/d1ra01738a

The oxytetracycline fermentation broth residue (OFR) is an abundant solid waste in the fermentation industry, which is hazardous but tricky to treat. The resource utilization of the waste OFR is still challenging. In this study, a novel route of using OFR was proposed that OFR was used as the organic ligands to construct a new hafnium based catalyst (Hf-OFR) for Meerwein-Ponndorf-Verley (MPV) reactions of biomass-derived platforms. The acidic groups in OFR were used to coordinate with Hf⁴⁺, and the carbon skeleton structures in OFR were used to form the spatial network structures of the Hf-OFR catalyst. The results showed that the synthesized Hf-OFR catalyst could catalyze the MPV reduction of various carbonyl compounds under relatively mild reaction conditions, with high conversions and yields. Besides, the Hf-OFR catalyst could be recycled at least 5 times with excellent stability in activity and structures. The prepared Hf-OFR catalyst possesses the advantages of high efficiency, a simple preparation process, and low cost in ligands. The proposed strategy of constructing catalysts using OFR may provide new routes for both valuable utilization of the OFR solid waste in the fermentation industry and the construction of efficient catalysts for biomass conversion.

Full text of DOI:10.1039/d1ra01738a

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Direct use of the solid waste from oxytetracycline
fermentation broth to construct Hf-containing
Cite this: RSC Adv., 2021, 11, 13970
catalysts for Meerwein–Ponndorf–Verley
reactions†
a
Yuxin Chen,a Xuefeng Yao,a Xiaolu Wang,a Xuefeng Zhang,b Huacong Zhou,
*
a
Runxia He and Quansheng Liua
*
The oxytetracycline fermentation broth residue (OFR) is an abundant solid waste in the fermentation
industry, which is hazardous but tricky to treat. The resource utilization of the waste OFR is still
challenging. In this study, a novel route of using OFR was proposed that OFR was used as the organic
ligands to construct a new hafnium based catalyst (Hf-OFR) for Meerwein–Ponndorf–Verley (MPV)
reactions of biomass-derived platforms. The acidic groups in OFR were used to coordinate with Hf4+
,
and the carbon skeleton structures in OFR were used to form the spatial network structures of the Hf-
OFR catalyst. The results showed that the synthesized Hf-OFR catalyst could catalyze the MPV reduction
of various carbonyl compounds under relatively mild reaction conditions, with high conversions and
yields. Besides, the Hf-OFR catalyst could be recycled at least 5 times with excellent stability in activity
and structures. The prepared Hf-OFR catalyst possesses the advantages of high efficiency, a simple
preparation process, and low cost in ligands. The proposed strategy of constructing catalysts using OFR
may provide new routes for both valuable utilization of the OFR solid waste in the fermentation industry
and the construction of efficient catalysts for biomass conversion.
Received 4th March 2021
Accepted 7th April 2021
DOI: 10.1039/d1ra01738a
rsc.li/rsc-advances
Antibiotics are one of the most successful drug families ever
developed in the history of pharmaceutical development. They
Introduction
are widely used to treat human health and promote the devel-
opment of agriculture and animal husbandry.3 The antibiotic
fermentation broth residue (AFR) is the main by-product aer
the production of antibiotics, mainly includes residual media,
solvent, mycelium, metabolites and leover antibiotics due to
the incomplete extraction.4 It has high content of water, vola-
tiles, organic matters (more than 90% for the dry AFR) and so
on.4 China is the world's largest producer of antibiotics,
accounting for 70% of global production.5 Producing 1 ton of
antibiotics will produce about 8–10 tons of AFR. Due to its vast
amount of nutrients, AFR has been widely used in fertilizers and
animal feed additives to promote the growth of plants in recent
years, and has visible promotion effects on the growth of
animals and plants.4 However, because AFR contains some
antibiotic residues, this type of utilization could pollute the
land, groundwater, and may cause the accumulation of antibi-
otics in the animal, leading to drug resistance, which will
eventually harm humans.4 Therefore, China listed AFR as
a national-level hazardous waste in 2008, which was banned
from use in animal feed. How to deal with AFR with the
awareness of environmental protection and value-added use
has become a hot issue in recent years.6 Direct combustion or
landll is a routine method for pharmaceutical plants to handle
With the development of the social economy, the people's
demands for energy and carbon resources are also increasing
year by year, but fossil resources are over-exploited, which
directly or indirectly leads to the excessive depletion of fossil
resources and the persistence of environmental pollution.1 To
alleviate the above problems, on one hand, increasing the
utilization efficiency of carbon-containing organic waste from
industrial production processes, improving the valuable use of
the organic waste, and nally realizing “change waste materials
into things of value” is an important approach to improve the
utilizing efficiency of the carbon sources.2 On the other hand,
how to promote the high-value conversion of renewable
biomass resources, such as converting lignin and cellulose into
chemicals, fuels, materials, etc., becomes more and more
important.
aInner Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank
Carbon Resources, College of Chemical Engineering, Inner Mongolia University of
Technology, Hohhot 010051, Inner Mongolia, China. E-mail: hczhou@imut.edu.cn;
runxiahe@iumt.edu.cn
bChifeng Pharmaceutical Co. Ltd, Chifeng 240000, Inner Mongolia, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra01738a
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such organic waste. However, the drawbacks of these methods the feasibility of the above idea. These reported studies show
are obvious: due to the high content of water in AFR, the that using acidic oxygen-containing functional groups in
burning process will consume a lot of heat, and the processing natural organic molecules to construct catalysts has the enor-
cost (about 2000 RMB per ton) will undoubtedly bring high mous advantages in viewpoint of cost and efficiency, and
economic pressure for the companies. Direct landlling may exploring cheaper organic ligands remains highly required for
cause residual antibiotics in the AFR to penetrate the ground- the construction of MPV catalysts in the eld of biomass
water, contaminate the water source, and waste the abundant conversion.
organic matter in the AFR.7 In recent years, many new AFR
In this study, we used the oxytetracycline fermentation broth
treatment methods have been reported, such as pyrolysis of AFR residue (OFR) to construct a novel and efficient Hf-based cata-
to make it a bio-carbon material,6 hydrothermal treatment to lyst (termed as Hf-OFR, Scheme 1). The catalyst was applied into
convert it into the clean solid biomass fuel,8 anaerobic digestion the hydrogenation reaction of ethyl levulinate (EL) into g-
technology to produce ammable gas9 and composting tech- valerolactone (GVL). The preparation and reaction conditions of
nology.4 As can be seen from the above treatment methods, AFR the catalyst were investigated in detail. The structures of the
is a potential bio-carbonaceous resource due to the high catalysts before and aer use were also characterized. The
abundance of organic matter. Although these treatment results showed that the prepared novel Hf-OFR catalyst exhibi-
methods can effectively utilize AFR to a certain extent, they still ted high activity and stability for the MPV reaction of different
have their shortcomings, such as requires signicant energy carbonyl compounds. The proposed route of preparing Hf-
input, organic structures/functional groups are not targeted for based catalyst in this work may be benecial to promote the
use, etc. Therefore, the exploration of more green and value- green and high-value utilization of both pharmaceutical solid
added approaches to utilize AFR, in particular, the use of wastes and biomass resources.
inherent organic structures and functional groups, is still
a meaningful subject during the utilization of such solid
organic waste.10
Experiment section
In addition to exploring how to use the solid organic waste in
industrial production, nding renewable carbon resources is
also an essential way for humans to access clean energy and
chemicals.11 Biomass resources are a renewable natural carbon
resource. Through the appropriate processes, useful molecules
can be obtained from biomass raw materials, such as triglyc-
erides,12 glycerol,13 5-hydroxymethylfurfural,14 cellulose,15
hemicelluloses and pentoses,16 lignin17 and lignocellulose18 and
then these intermediates can be further converted into the
liquid fuels and commercial chemicals.11 The Meerwein–
Ponndorf–Verley (MPV) reaction was commonly used to convert
biomass derived carbonyl compounds into alcohols or their
derivatives, an essential step in biomass catalytic conversion.19
The reaction process has the advantages of high chemo-
selectivity, mild reaction conditions, operational simplicity, low
cost, and scalability. The carbon–oxygen double bonds could be
selectively reduced with secondary alcohols as hydrogen sour-
ces.19 It has been reported that MPV reaction can be efficiently
catalyzed by diverse catalysts, such as using transition metals
RANEY®Ni,20 Cu-catalyst derived from hydrotalcite,21
aluminum based catalysts,22 nano MgO,23 and a range of Zr-
containing catalysts including zirconium hydroxide,24
zirconia,25 zirconium-containing beta zeolite,26 etc. These cata-
lysts generally have a cumbersome synthesis process, high cost
of the organic ligands, and the reaction conditions are not mild
enough. In recent years, it has been found that the coordination
of natural organic molecules with metal ions can be used to
construct catalysts with excellent stability, such as zirconium–
phytic acid,27 hafnium-furandicarboxylic acid,28 hafnium phos-
phonate.29 This idea of using natural organics to construct
catalysts offers new route to construct efficient catalysts with
low-cost and simple preparation processes. In our recent work,
we have successfully utilized humic acid (HAs) extracted from
Catalyst preparation
Firstly, in order to eliminate the possible inuence of metal
elements in the raw oxytetracycline fermentation broth residue
(ROFR) in the process of building the catalysts, we used the
18 wt% HCl to demineralize the ROFR sample (about demin-
eralization methods, see Page S3†). The 0.5 g HfCl4 was dis-
solved in 75 mL DMF, and then 0.5 g of the demineralized OFR
(DOFR) was added into the above solution under constant
stirring. Then 1.578 g of triethylamine as the deprotonic agent
was added into the above mixtures dropwise at a constant rate
within 10 min. The mixed slurry was stirred at room tempera-
ꢀ
ture for 5 h, and then aged under 70 C for 3 h. Aer the reac-
tion, the obtained precipitate was separated by centrifugation,
and washed with DMF for ve times and then with ethanol for
ve times. The supernatant was detected using AgNO3 until no
Scheme 1 Schematic of the direct use of OFR to prepare the Hf-OFR
catalysts and its application in the conversion of biomass-derived
lignite to prepare Zr-HAs catalyst,30 which once again veried carboxyl compounds via the MPV reaction.
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ClÀ was checked out. So far, the obtained precipitate was dried and O element. Besides C, H and O, ROFR also contained N, P,
under vacuum at 80 ꢀC for 12 h, ground into powders, and and S element. Aer demineralization, the mass ratio of inor-
denoted as Hf-OFR for use. The catalyst constructed with the ganic elements in the ROFR was signicantly increased, indi-
raw OFR directly coordinated with Hf was marked as Hf-ROFR. cating that most of the metal elements contained in the ROFR
The preparation parameters were investigated to establish the have been removed by demineralization. The ash content was
optimal preparation conditions, including the ratios of Hf 12.50 wt% and the signicant metal elements in ROFR was also
precursor to the DOFR, the dosage of trimethylamine, the aging analyzed by ICP-AES (Table S3†), and it was shown that ROFR
temperature. The basic processes were similar to the above contains a certain amount of Ca, Zn, Fe, Na, K, Al, Si, etc., the
description.
ash content of the DOFR decreased signicantly, indicating that
most of the mineral salts in the bacteria residue were removed
by the hydrochloric acid, only a small amount of iron, zinc and
silicon that were hard to dissolve in hydrochloric acid
remained.
Reaction
The selective hydrogenation reaction of carbonyl compounds to
alcohols or their derivatives was carried out in a 10 mL Teon-
lined stainless steel autoclave equipped with a magnetic stirrer
and using isopropanol as the solvent and hydrogen-donor.
(Isopropanol is one of the hydrogen sources with the most
excellent transfer hydrogenation performance in the MPV
reaction system.31) Typically, EL (1 mmol), isopropanol (5 mL),
and the catalyst (200 mg) were added into the reactor. Aer that,
the reactor was transferred to an oil bath under constant
temperature with magnetic stirring and allowed to react at
a preset temperature and time. Aer the reaction, the reactor
was cooled down in the cold water to quench the reaction, and
the organic phase was diluted by isopropanol. The liquid
samples were analyzed quantitatively by gas chromatography
(TECHCOMP GC7900) with a ame ionization detector using
decane as the internal standard. The identication of the
products was made by GC-MS (Agilent 7890B-7000D). To check
the reusability of the Hf-OFR catalyst, the solid catalyst were
removed from the reaction system by centrifugation within
a short time aer quenching the reaction, washed three times
with fresh isopropanol, and then reused for the next run
without other treatment. In the heterogeneity experiments, the
solid catalyst was separated out from the reaction system aer
one hour of reaction, and the supernatant continued to main-
tain the reaction without the solid catalyst. It was checked
whether the target reaction continued aer the solid catalyst
was removed. All the experimental devices used were checked by
repeating experiments for at least two times to obtain good
operation stability during the experiment. The quantitative
analysis of the reaction mixtures was repeated twice or three
times to assure the accuracy of the data, and average values were
used.
From infrared spectrum curve of the OFR in Fig. 1, a strong
and broad absorption peak around 3400 cmÀ1 generated by the
stretching vibration of alcohols or phenols O–H appears. Near
2900 cmÀ1 there were two absorption peaks, the larger wave-
number was the methyl C–H stretching vibration absorption
peak, the smaller one was the methylene C–H stretching
vibration absorption peak. The peak at the wavenumber
2078 cmÀ1 was assigned to the stretching vibration of C]C. The
peaks around 1659 cmÀ1 and 1523 cmÀ1 could be assigned to
the strong asymmetrical stretching and relatively weaker
symmetrical stretching of –COO– groups, which revealed the
presence of acidic carboxyl groups.32 There was a weaker peak at
1030–1220 cmÀ1, which could be attributed to C–N stretching
vibration of aliphatic amines in ROFR.33 Compared with the
ROFR, the intensity of the weak symmetrical stretching of
–COO– groups at 1523 cmÀ1 of DOFR became stronger
compared to the peak of the asymmetrical stretching of –COO–
groups at 1659 cmÀ1 (the intensity of the transmittance differs
by 16.12%), which may be due to the elution of minerals by
hydrochloric acid, resulting in the release of occupied acidic
functional groups.34 The results of elemental analysis also
showed that the demineralization could effectively remove the
inherent minerals, which occupied the acidic groups in ROFR,
and make it possible for Hf4+ to coordinate with the acidic
groups in the preparation process.34 Fig. S1† is the possible
Results and discussion
Characterization and analysis of the OFR
Firstly, the basic composition of ROFR and the DOFR were
analyzed, including industry analysis and basic element anal-
ysis (Tables S1 and S2, about industrial analysis methods, see
Page S3†). From Table S1,† industrial analysis results showed
that the volatile substance in ROFR accounted for 61.41 wt%,
indicating that ROFR contained a large amount of organic
matter. By comparing the carbon and oxygen content in the
elemental analysis (C: 40.48 wt%, O: 22.80 wt%), it could be
speculated that organic matter contained a large amount of C Fig. 1 FTIR spectra of raw OFR and demineralized OFR.
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structural formula for the oxygen-containing acidic functional turbid again (Fig. S2†). In order to effectively utilize OFR, the
groups in ROFR and DOFR.
catalyst was prepared under the mass ratio of 1 : 1. The reaction
results were given in Fig. 2, as the results shown, under the
conditions of HfCl4 : DOFR mass ratio of 1 : 1, the molar ratio of
triethylamine to HfCl4 10 : 1, the aging temperature 70 ꢀC, the
prepared catalyst had higher efficiency for the reaction.
Optimization and characterization of the Hf-OFR catalysts
DMF was used as the solvent to prepare the catalyst because the
OFR sample could disperse well in DMF. We rst investigated
the effect of demineralization on Hf-OFR catalyst, and the
results are shown in Fig. 2a. The activity of the Hf-OFR catalyst
was signicantly improved aer demineralization. This experi-
mental result was consistent with our previous speculation that
more acidic functional groups in ROFR can be exposed by
demineralization, thereby coordinating with more Hf4+ to form
the effective active site catalytic the hydrogenation reaction. The
OFR itself before and aer demineralization basically has no
catalytic activity. So we later chose DOFR as the ligand to further
optimize the preparation conditions of the Hf-OFR catalyst.
Different preparation parameters were investigated and optimized,
including the mass ratio of Hf precursor to OFR, the dosage of the
additive trimethylamine (deprotonate reagent), and the aging
temperature of the catalysts. The transfer hydrogenation of ethyl
levulinate (EL) into g-valerolactone (GVL) was used to evaluate the
activity of the catalysts. In the process of optimizing the ratio of raw
materials, with the increasing of the dosage of Hf precursor, the
amount of precipitated precipitate also increased, and the color of
the supernatant of the solution became clear, indicating that more
OFR interacted with Hf4+ and participated in the formation of the
solid catalysts. When the mass ratio was 1 : 1, the supernatant of
the solution appeared colorless and transparent. With the further
increasing of Hf precursor, the color of the supernatant became
We investigated the changes in the specic surface area, pore
structure and Hf content of Hf-OFR under different preparation
conditions (Fig. S3, S4 and Table S4†). The results showed that
the pore size distribution of Hf-OFR was mainly composed of
mesopores, and it also contained a small amount of micropores.
The specic surface area and pore volume of Hf-OFR decreased
signicantly as the mass of the Hf precursor input increased. This
phenomenon suggested that the introduced Hf4+ may occupy the
original pore structure of the OFR. At the same time, as the input of
Hf precursor was further increased to 2.5 : 1, the Hf content in Hf-
OFR did not increase proportionally, which indicated that the
coordination process had been basically completed when the mass
ratio of raw materials was 1 : 1. When the molar ratio of triethyl-
amine to HfCl4 was 10 : 1, Hf-OFR had a larger specic surface
area and pore volume. At the same time, the Hf-OFR under this
preparation condition had better catalytic activity, which may be
due to the higher specic surface area, and the large pore volume
could increase the mass transfer efficiency between the catalyst
and the reaction system. The higher Hf content was also the key
factor for high catalytic activity. Finally, the effect of the aging
temperature on the specic surface area and pore size of the Hf-
OFR was investigated. When the aging temperature reached
ꢀ
200 C, the specic surface area and pore volume of the catalyst
increased signicantly, while the yield of the catalyst decreased,
and the catalytic activity decreased signicantly. These
phenomena indicated that too high aging temperature was not
conducive to the coordination between Hf precursor and OFR.
Next, we conducted a series of characterization analysis on Hf-OFR
prepared under the optimal preparation conditions.
Fig. 2 Optimize preparation conditions, (a) effect of demineralization
on catalytic activity of Hf-OFR catalysts (other preparation conditions:
mass ratio of HfCl4 : OFR was 1 : 1, molar ratio of triethylamine : HfCl4
was 10 : 1, aging temperature was 70 ꢀC), (b) investigate the mass ratio
of reactants, (c) investigate the amount of triethylamine added, (d)
optimize the aging temperature. Other preparation conditions remain
the same when examining any variable, reaction conditions: EL: Fig. 3 Characterization of the Hf-OFR by SEM (a and b), EDS data (c)
1 mmol, isopropanol: 5 mL, decane: 1 mmol, reaction temperature: were obtained from the square area in (b), Hf EDS mappings of Hf-OFR
150 ꢀC, reaction time: 3 h.
(d).
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(Fig. 4b) pattern showed one broad diffraction peak, indicating
that the obtained catalyst was amorphous.35
Next we explored the acidic and alkali sites of Hf-OFR. We
rst used XPS to evaluate the relative binding energy of Hf 4f
and O 1s, and compared with HfO2 (Fig. 5a and b). The results
show that the hafnium and oxygen species in the synthesized
Hf-OFR catalyst had higher binding energy compared to HfO2.
Higher binding energy of Hf species indicated the higher
positive charges, representing the stronger Lewis acidity.37 The
higher binding energy of oxygen species in Hf-OFR corre-
sponded to a lower negative charge, indicating that the basicity
of oxygen species in the Hf–O–C framework of Hf-OFR was
weaker.38 The increased acidity of Hf species was in favor of the
activity of the catalyst. The above analysis of the acidity and
alkalinity of Hf-OFR showed that the hybrid Hf-OFR catalyst
prepared by OFR had a relatively high acid strength compared
to HfO2. At the same time, the alkali strength could be main-
tained at a relatively low level. Strong acid strength and weak
alkali strength could promote the progress of the transfer
hydrogenation reaction.39
Fig. 4 FTIR spectra (a) and powder XRD pattern (b) of Hf-OFR catalyst.
First, the surface morphology and Hf element distribution of
Hf-OFR were characterized by SEM-EDS (Fig. 3). The results
show that the microstructures of the Hf-OFR catalysts were
composed of irregular particles with varied sizes. EDS data
shows that the strong Hf element signal appeared for Hf-OFR,
in addition, the mapping of Hf shows that Hf element was
evenly distributed in the catalyst.
The FTIR spectrum of Hf-OFR catalyst in Fig. 4a exhibited
the asymmetric (OFR, 1653 cmÀ1; Hf-OFR, 1646 cmÀ1) and
symmetric (OFR, 1529 cmÀ1; Hf-OFR, 1552 cmÀ1) stretching
vibration of carboxylate groups. The split between the
symmetric and asymmetric peaks was narrowed from 124 cmÀ1
for OFR to 94 cmÀ1 for Hf-OFR, proving that Hf4+ was coordi-
nated with the carboxylate groups.35,36 The bands at 3500–
3000 cmÀ1 and 1400 cmÀ1 could be attributed to the charac-
teristic stretching vibrations of hydroxyl and bending vibration
of hydroxyl groups in water molecules, respectively. The XRD
Specic contents of acidic and basic sites were analyzed by
NH3-TPD and CO2-TPD. The TPD temperature ranges were set
from 50 ꢀC to 300 ꢀC, and the heating rate was set at
10 ꢀC minÀ1. The results were shown in Fig. 5c and d. It can be
seen that with the increasing of the desorption temperature,
a large number of CO2 and NH3 were desorbed, indicating that
the prepared catalyst had certain acidic and basic sites (acid
site: 1.9 mmol gÀ1, basic site: 1.6 mmol gÀ1). The basic sites
might result from the presence of N element and oxide anions
in DOFR. In order to continue to study the acidity of the catalyst,
we used pyridine infrared to analyze the content of Lewis and
Brønsted acid contained in the OFR and Hf-OFR under the
desorption temperature of 250 ꢀC. In order to compare the
inuence of the introduction of Hf4+ on the acidity of Hf-OFR,
the results are as follows in the Fig. 5e, f and Table S5.†
Firstly, the peak positions of 1450 cmÀ1 and 1610 cmÀ1 could be
attributed to the absorption of pyridine on the Lewis acid sites40
and the peak positions of 1545 cmÀ1 and 1640 cmÀ1 were
attributed to the absorption of pyridine on the Brønsted acid
sites.41 Both OFR and Hf-OFR contain a certain amount of Lewis
and Brønsted acidic sites, but the quantitative analysis data
shows that the amount of acid in OFR was less than the amount
ꢀ
of acid in Hf-OFR under the desorption temperature of 150 C
and 250 ꢀC, which shows that the introduction of Hf4+ may form
the Hf–O–C structure with the OFR. This coordination structure
can provide Lewis acidic sites for Hf-OFR,40 and at the same
time, some unsuccessfully coordinated Hf4+ can also provide
Lewis acidic sites for Hf-OFR.40 On the other hand, the Brønsted
acidic sites were derived from the protonated carboxyl group
and the phenolic hydroxyl in the OFR,41 and the introduced Hf4+
was coordinated with these carboxyl group and the phenolic
hydroxyl in the OFR, thereby stabilizing the structure of these
oxygen-containing acidic functional groups in the OFR. This
may be the main reason for the increase in the amount of
Brønsted acid sites in the Hf-OFR. The formation of the alkali
site and the presence of the Brønsted acidic site have coopera-
tion effects on the process of hydrogen transfer reaction.42
Fig. 5 XPS spectra of Hf 4f (a), XPS spectra of O 1s (b), CO2-TPD curves
(c) and NH3-TPD curves (d) of Hf-OFR catalyst, pyridine-adsorbed FT-
IR spectra of OFR (e) and Hf-OFR (f), (L: Lewis acid sites; B: Brønsted
acid sites).
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Fig. 6 The effects of reaction parameters on the performances of Hf-OFR, (a) the amount of catalyst (reaction time: 3 h, reaction temperature:
150 ꢀC), (b) reaction temperature (catalyst amount: 200 mg, reaction time: 3 h), (c) reaction time (catalyst amount: 200 mg, reaction temperature:
150 ꢀC). Typical reaction conditions: EL 1 mmol, 2-PrOH 5 mL.
Optimization of reaction conditions
Heterogeneity and reusability of the Hf-OFR
In order to improve the performance of the Hf-OFR catalyst, the Heterogeneity and reusability are important for the solid cata-
reaction conditions were investigated via the hydrogenation lyst concerning the potential industrial production, so it is
reaction of EL into GVL. We rst studied the dosage of the necessary to study the reusing stability of the Hf-OFR catalyst
catalyst (Fig. 6a). The results showed that both the conversion (Fig. 7). The conversion and yield were maintained below 50%
and the yield increased almost linearly with the increasing of via controlling the reaction time at 2 h to investigate the real
the catalyst dosage in the range of 100 mg to 200 mg under the reusability of the catalyst. The heterogeneity of the catalyst was
present reaction condition. Further increasing the amount of studied by separating the solid catalyst from the reaction system
catalyst led to poor dispersion of catalyst particles in the reac- aer reaction for 1 h with GVL yield around 20%, and then the
tion system, resulting in the slower increasing rate of the reaction solution was allowed to continue to react under the
conversion and yield. Therefore, 200 mg was chosen as a suit- same conditions with no presence of the solid catalyst. From
able dosage under the present reaction condition. Increasing Fig. 7a, it can be seen that the yield of GVL did not further
the reaction temperature (Fig. 6b) also had a signicant increase aer the solid catalyst was removed, indicating that it
promotion effect on the catalytic reaction. During the process of was the solid catalyst to catalyze the reaction and Hf-OFR was
improving the reaction temperature from 130 ꢀC to 160 ꢀC, the a heterogeneous catalyst. The reusability of the Hf-OFR catalyst
yield of GVL maintained a linear growth trend, but considering was further investigated. As shown in Fig. 7b, the conversion
the relatively mild reaction conditions can reduce the energy and yield in the rst run were 52% and 40%, which decreased to
input, the mediate 150 ꢀC was used as the appropriate reaction 44% and 30%, respectively, in the h run. The recycled catalyst
temperature. Then we extended the reaction time under the was dried and weighed, and the weight was 152 mg, which was
dosage of 200 mg and the reaction temperature of 150 ꢀC. When less than the initial catalyst dosage (200 mg). Therefore, the
the reaction time was 9 h, the conversion reached 91%, and the
yield reached 86%. When the reaction time was extended, the
yield remained nearly unchanged. We used GC-MS to analyze
the by-products aer 9 h. The detection results showed that the
by-products were isopropyl levulinate and isopropyl 2-hydrox-
ypentanoate. The byproducts were an intermediate formed by
the transesterication of ethyl levulinate and ipropanol.31
Fig. 7 Heterogeneity (a) and reusability (b) of the Hf-OFR catalyst. Fig. 8 Comparison of the freshly prepared and recycled Hf-OFR
Reaction conditions: EL 1 mmol, 2-PrOH 5 mL, catalyst 200 mg, catalysts after five reuses: (a) FTIR spectra, (b) XRD patterns, (c) XPS
reaction temperature 150 ꢀC, reaction time 2 h.
spectra of Hf 4f, (d) N2 adsorption–desorption isotherm.
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Characterization of the Hf-OFR catalysts before and aer the
cycle
The detailed structural characterization analysis of the catalyst
aer the h cycle was subsequently performed, and compared
with the fresh catalyst. FTIR spectra and XRD patterns (Fig. 8a
and b) indicated that the characteristic functional groups and
crystal structure of the catalyst remain basically unchanged
aer 5 cycle uses. XPS spectra of Hf 4f showed that the chemical
environment of the Hf element did not change signicantly
before and aer recycling. The specic surface area and pore
volume of the catalyst have been a slight decreasing before and
aer recycling (Table S6†). The possible reason is that the
adsorption of by-products blocked part of the pore structure.
Next, we compared the thermal stability and the acid–base
properties of the fresh and the recycled catalysts, as shown in
Fig. 9. Fig. 9a showed that the thermal stability of the recycled
catalysts had no noticeable difference compared to the fresh
catalysts. It could be seen from Fig. 9b that the content of basic
sites of the catalyst did not change signicantly before and aer
recycling. However, Fig. 8c showed that the NH3 signal peak of
the catalyst aer recycling was weaker than the NH3 signal peak
of the fresh catalyst, and the content of total acidic sites reduced
from 1.90 mmol gÀ1 to 1.44 mmol gÀ1 (Table S7†). There may be
two main reasons for the decreased in the content of acidic
sites. Firstly, some weakly coordinated Hf4+ leached from the
catalyst surface aer ve cycles of reactions; secondly, the
adsorption of by-products occupied the original acidic sites. In
the infrared analysis of pyridine, it can be seen that the
absorption peak positions of the Lewis acid and Brønsted acid.
The content of the Lewis acid of the catalyst had not changed
signicantly before and aer recycling, which proved that the
Fig. 9 Characterization of acid–base sites of catalysts before and after
recycling: (a) TG curves, (b) CO2-TPD curves, (c) NH3-TPD curves, (d)
pyridine-adsorbed FT-IR spectra (L: Lewis acid, B: Brønsted acid).
decrease in the conversion and yield was speculated that the
physical loss of the catalyst during the recovery process. Aer
the h run, the weight of the catalyst was replenished to
200 mg, and the sixth cycle was performed. The results show
that the conversion and yield nearly returned to the initial level.
The Hf content of the fresh and the recycled catalyst aer 5
cycles were analyzed. The result showed that the Hf content only
had a slight decrease from 27.3 wt% to 24.2 wt%, which might
also contribute to the weak decreasing of the conversion and
yield. These results proved that the performance of the Hf-OFR
catalyst was stable during the reusing process.
Fig. 10 Possible catalytic mechanism for the transfer hydrogenation of EL to GVL.
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Table 1 MPV reduction of different biomass-derived carbonyl compounds over the Hf-OFR catalyst
Entry
1
Substrate
Product
T (ꢀC)
T (h)
Conv. (%)
Yield (%)
Sel. (%)
150
9
91 Æ 0.1
86 Æ 1.3
94 Æ 1.5
2
3
80
9
6
97 Æ 0.1
96 Æ 0.3
99 Æ 0.4
100
>99
>99
>99
4
5
110
110
7
98 Æ 0.7
98 Æ 0.6
93 Æ 1.3
96 Æ 0.2
94 Æ 3.1
98 Æ 1.1
12
6
7
8
110
100
150
10
6
>99
>99
>99
97 Æ 0.3
91 Æ 0.1
91 Æ 2.8
84 Æ 1.2
94 Æ 3.1
93 Æ 1.3
12
9
110
12
>99
>99
>99
Hf–O–C structure was relatively stable. The content of Brønsted to form acetone and 4-hydroxypentanoate (step 4 and 5). Finally,
acid sites was slightly lost aer recycling, decreased from 10.9 4-hydroxypentanoate was lactonized to form the nal product
mmol gÀ1 to 8.8 mmol gÀ1 (desorption temperature 150 ꢀC) and GVL under the cooperation effect of Brønsted acidity and
6.0 mmol gÀ1 to 3.0 mmol gÀ1 (desorption temperature 250 C), basicity of the catalyst (step 6 and 7).45 Besides, EL could also
ꢀ
which may be due to the partial decomposition of the –OH and undergo transesterication reaction with isopropanol to form
–COOH functional group. From the elemental analysis of isopropyl levulinate under the synergistic effect of acidic and
carbon, it can be seen that the content of carbon increased basic in the catalyst, and then to isopropyl 2-hydroxypentanoate
slightly, which may be caused by the adsorption of the by- via transfer hydrogenation, which could be further converted to
products. The adsorbed by-products might occupy the acidic GVL.45
sites and lead to the decreasing of the activity during the reus-
ing process.
Conversion of different carbonyl compounds
Encouraged by the outstanding performance of Hf-OFR catalyst
in the hydrogenation of EL to GVL, we further expanded the
scope of the substrate to other carbonyl compounds, to see the
general applicability of Hf-OFR in the MPV reaction. As shown
in the Table 1, both aliphatic and aromatic carbonyl
compounds could be efficiently hydrogenated to the corre-
sponding alcohols under the catalysis of Hf-OFR catalyst.
Possible reaction mechanism for EL transfer hydrogenation
The possible reaction mechanism of Hf-OFR catalyzed transfer
hydrogenation of EL was analyzed based on the experimental
results and literatures, as shown in Fig. 10. The acidic site Hf4+
and basic site O2À in the catalyst play a vital role in the catalytic
process.43 In the rst step, the adsorption of isopropanol onto
Hf-OFR assisted with Lewis acid–base sites (Hf4+, O2À). Under
the synergistic action of the Lewis acid–base sites of the catalyst,
isopropanol was xed and dissociated into isopropoxide and
Conclusions
hydride (step 1). Then the carbonyl group in EL under the role of In summary, we identied that the solid waste from oxytetra-
the acidity of the catalyst interacted with the active site Hf4+ and cycline fermentation broth could be used to construct a novel
activated by Hf4+ (step 2).44 The dissociated hydride formed hybrid Hf-based hydrogenation catalyst through the coordina-
a six-membered cyclic intermediate with the activated carbonyl tion of Hf4+ with the oxygen-containing acidic functional groups
group in the EL (step 3). Then the activated hydrogen atoms in in the solid residue. The prepared catalyst exhibited high
isopropanol were transfer to the activated carbonyl group in EL activity to MPV reaction for various carbonyl compounds.
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Moreover, the catalyst could be facilely recycled and reused at 15 R. Palkovits, K. Tajvidi, J. Procelewska, R. Rinaldi and
least for 5 cycles with no consideration changes in activity and
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
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