Spinel copper-iron-oxide magnetic nanoparticles with cooperative Cu(i) and Cu(ii) sites for enhancing the catalytic transformation of 1,2-propanediol to lactic acid under anaerobic conditions

Article abstract of DOI:10.1039/d0cy01733g

The aerobic catalytic oxidation of 1,2-propanediol (PDO) over non-noble metal (e.g. Cu) based catalysts usually suffers serious C3 product dissociation at high temperature, thus showing low lactic acid (LA) selectivity. Here, an alternative anaerobic catalysis strategy over spinel copper-iron-oxide magnetic nanoparticles (CuFeOx MNs) with the coexistence of Cu(i) and Cu(ii) dual sites is developed for the catalytic transformation of PDO to LA with a quantitative yield of co-product H2 in basic aqueous solution. The absence of O2 is beneficial for enhancing LA production in comparison with the presence of O2. The synergy between Cu(i) and Cu(ii) sites in CuFeOx MNs is vital for improving the catalytic performance as compared to Cu2O or CuO catalysts with Cu(i) or Cu(ii) sites alone. Cu1Fe1Ox MNs with a Cu/Fe mole ratio of 1/1 exhibit 94.5% LA selectivity and 72.6% PDO conversion at 160 °C for 8 h. Experimental results and DFT calculations suggest that the spinel CuFeOx MN based catalytic PDO transformation follows a favorable pathway of PDO → hydroxyacetone → lactaldehyde → lactic acid for LA production. In addition to the catalytic PDO transformation, the use of CuFeOx MNs can be extended to favor high activity and selectivity in the catalytic transformation of glycerol to LA (98.5% selectivity) and ethylene glycol to glyceric acid (97.8% selectivity). This work highlights the design of an alternative non-noble metal-based CuFeOx MN catalyst for efficiently catalyzing the transformation of bio-based polyols into value-Added carboxylic acids.

Full text of DOI:10.1039/d0cy01733g

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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ARTICLE
Spinel copper-iron-oxide magnetic nanoparticles with cooperative
Cu(I) and Cu(II) sites for enhancing catalytic transformation of 1,2-
propanediol to lactic acid under anaerobic conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Yonghai Feng,*a Congming Lu,a Huijie Wang,a Minjia Meng,b Yunlei Zhang, b Dewei Rao,*a Lei Liu a
DOI: 10.1039/x0xx00000x
and Hengbo Yin b
The aerobic catalytic oxidation of 1,2-propanediol (PDO) over non-noble metal (eg. Cu) based catalysts usually suffers
serious C3 product dissociation at high temperature, thus showing low lactic acid (LA) selectivity. Here, an alternative
anaerobic catalysis strategy over spinel copper-iron-oxide magnetic nanoparticles (CuFeOx MNs) with coexistence of Cu(I)
and Cu(II) dual sites is developed for the catalytic transformation of PDO to LA with a quantitative yield of co-product H2 in
basic aqueous solution. The absence of O2 is beneficial for enhancing LA production in comparison with the presence of O2.
Synergy between Cu(I) and Cu(II) sites in CuFeOx MNs is vital for improving the catalytic performance as compared to Cu2O
or CuO catalysts with Cu(I) or Cu(II) sites alone. Cu1Fe1Ox MNs with Cu/Fe mole ratio of 1/1 exhibits 94.5% LA selectivity
and 72.6% PDO conversion at 160℃ for 8 h. Experimental results and DFT calculations suggest that the spinel CuFeOx MNs
based catalytic PDO transformation follows a favorable pathway of PDO → hydroxyacetone → lactaldehyde → lactic acid
for LA production. In addition to the catalytic PDO transformation, the CuFeOx MNs can be extended to favor high activity
and selectivity in the catalytic transformation of glycerol to LA (98.5% selectivity) and ethylene glycol to glyceric acid (97.8%
selectivity). This work highlights the design of an alternative non-noble metal based CuFeOx MNs catalyst for efficiently
catalyzing the transformation of bio-based polyols into value-added carboxyl acids.
economic and environmental friendly approach for the
sustainable production of LA.
Introduction
To date, studies on the PDO conversion to LA are mainly
The conversion of renewable biomass derivatives to liquid
fuels, chemicals, and polymeric materials attracts great
interest for its potential to replace petroleum as the primary
focused on O2-required thermal catalysis or electronic catalysis
approaches using supported noble metal (Au, Pd, and Pt)
nanoparticles as catalysts due to their high LA selectivity.5-11
For examples, monometallic Au/C catalyst can selectively
catalyse the oxidation of PDO to LA at 90℃ and 0.3 MPa O2,
giving 100% LA selectivity and 78% PDO conversion,6 while
bimetallic AuPd/C and AuPt/C catalysts are more active, giving
the PDO conversion and LA selectivity of higher than 94% and
95%, respectively at 60℃ and 1 MPa O2.10 Chadderdon et al.
has recently developed the selective oxidation of PDO in
alkaline anion-exchange membrane electrocatalytic flow
reactors, giving the LA selectivity of 86.8% over carbon-
supported platinum (Pt/C) Pt/C catalyst.5 In view of the high
cost of noble metal, non-noble/noble bimetallic catalysts such
as the Cu@Au core shell nanoparticles and the AuCu/TiO2
catalysts have recently been developed for the selective
oxidation of PDO to LA,12, 13 in which however the active sites
still depend on gold. Inexpensive metallic Ag and Cu
nanoparticles are tried to be developed as catalyst for the PDO
oxidation,14, 15 but higher reaction temperature is required to
activate the reaction due to their inherent low catalytic activity.
Consequently, severe scission of C–C bond of C3 intermediates
to C1 and C2 products occurs, leading to low LA selectivity. For
2
feedstock.1, 1,2-propanediol (PDO), a bio-based diol with
vicinal primary and secondary alcohol groups, has been
emerged as an alternative promising platform chemical for
producing several high value chemicals such as lactic acid (LA),
pyruvic acid, propylene epoxide, hydroxylacetone, and
pyruvaldehyde via catalytic processes.3 LA has potential as a
major feedstock for sustainably producing several important
commodity chemicals such as biodegradable fibres, polylactic
acid esters, and acrylic acid.4 It is estimated over 90% of global
LA is mainly produced by the carbohydrate fermentation
route, which however is slow and involves complex separation
steps with a large amount of sludge generation.5 In this case, it
is attractive to develop the selective conversion of PDO as an
a. Institute for Advanced Materials, School of Materials Science and Engineering,
Jiangsu University, Zhenjiang 212013, China.
b. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
212013, China.
*Corresponding authors: Y. Feng (fengyonghai@ujs.edu.cn), D. Rao
(dewei@ujs.edu.cn)
Electronic Supplementary Information (ESI) available: [description of DFT methods,
supporting figures and tables]. See DOI: 10.1039/x0xx00000x
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Scheme 1. Reaction pathways for catalytic transformation of alcohols to carboxyl acid (a) and ethylene glycol to glycolic acid (b) using water as oxygen source in
basic aqueous solution with H2 as co-product. Reaction network of O2-required catalytic oxidation of PDO to LA (c).
instance, using metallic Cu0 nanoparticles as catalyst displays mechanism of catalytic transformation of PDO to LA is worth
LA selectivity of only 52.4% and total selectivities of formic being clarified, in particular, for the new designed catalysts.
acid (C1 product) and acetic acid (C2 product) of 48% at the
Recently, copper ferrite (CuFe2O4) magnetic nanoparticles
PDO conversion of 83.8% under 1 MPa O2 and 200 ℃ .15 In (MNs) have emerged as one of the most attractive catalysts for
addition, the high pressurized dioxygen (1−2 MPa O2) and high oxidative dehydrogenation of alcohols to their corresponding
temperature (200−220 ℃ ) will also pose a great risk for carbonyl compounds due to its cation occupying the
practical production. To address these issues, alternative safe tetrahedral and octahedral holes in a dense cubic packing of
approaches that can also efficiently promote the LA selectivity oxygen anions.20-22 Moreover, the spinel structure of CuFe2O4
especially over non-noble metal based catalysts need to be is stable and highly ordered, which can maintain their
developed.
promising activities in harsh conditions. However, the catalytic
Previous works have shown that alcohols can be performance of CuFe2O4 MNs for the catalytic transformation
catalytically transformed into corresponding carboxyl acid salt of PDO under anaerobic and basic aqueous solution remains
and H2 using water as oxygen atom source in basic aqueous unclear. It has been reported that Cu2O is more active and
solution (Scheme 1a).16, 17 Haasterecht et al. have found that selective than Cu0 and CuO in catalytic transformation of
carbon nanofiber supported copper nanoparticles can glycerol to LA in alkaline medium.23 Therefore, a novel type of
selectively catalyse the transformation of ethylene glycol into spinel copper-iron-oxide magnetic nanoparticles (CuFeOx MNs)
glycolic acid with H2 as co-product (96% selectivity at 82% with coexistence of Cu(I) and Cu(II) sites was synthesized by
conversion) under anaerobic aqueous conditions (Scheme the facile high-temperature organic phase synthesis
1b).18 As shown in the reaction network of aerobic catalytic approach,24 and used for catalytic PDO transformation under
oxidation of PDO to LA (Scheme 1c), oxygen (O2) is not the anaerobic and basic aqueous solution in order to improve the
actual oxygen atom source for LA formation even its presence LA selectivity over non-precious metal based catalysts. It was
was believed to be able to promote the PDO conversion.19 On found that under anaerobic condition C−C bond dissociation
basis of the fact that alcohols and ethylene glycol can be reaction could be effectively hindered as compared to that
selectively transformed into their corresponding carboxyl acid under aerobic condition, thereby high LA selectivity was
under anaerobic and basic aqueous conditions, we believe that achieved with value-added H2 as co-product. The synergy
this O2-free catalysis system may be extended to selectively between Cu(I) and Cu(II) centres in the CuFeOx MNs was
catalyse the PDO transformation into LA over non-noble metal suggested to contribute to the enhanced catalytic
based catalysts. To achieve this goal, the priority is to explore performance. By tailoring the Cu/Fe ratio to 1/1 in CuFeOx
the desirable catalysts. Moreover, one should be noticed that (Cu1Fe1Ox) MNs, 94.5% LA selectivity and 72.6% PDO
the reaction pathway for PDO conversion is complex, so the conversion was obtained at 160℃ and 1.0 MPa N2 for 8 h. The
pathway of catalytic PDO transformation over CuFeOx MNs is
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Fig. 1. Schematic illustration of the preparation of spinel CuFeOx MNs (a); TEM and HRTEM images of Fe3O4 (b),
Cu0.5Fe1Ox (c), Cu1Fe1Ox (d), and Cu2Fe1Ox (e); HAADF-STEM image of Cu1Fe1Ox MNs (f), element mappings of Cu (g), Fe
(h), O (i); corresponding EDS spectrum of one particle.
investigated through experiments and density functional morphology of pristine Fe3O4 MNs (Fig. 1b). The obtained
theory (DFT) calculation. Interestingly, the CuFeOx MNs also CuFeOx MNs are nearly monodisperse with a narrow size
display high activity in selectively catalysing glycerol distribution range. The sizes of Cu0.5Fe1Ox, Cu1Fe1Ox, and
transformation to LA (98.5% selectivity) and ethylene glycol to Cu2Fe1Ox MNs are ca. 7.4, 12.2, and 17.2 nm, respectively,
glyceric acid (97.8% selectivity). Moreover, the CuFeOx MNs indicating that increasing the Cu/Fe mole ratio favors forming
can be facilely recovered by magnetic separation with larger-sized CuFeOx MNs. Corresponding HRTEM images
promising stability.
Results and discussion
Synthesis of CuFeOx MNs and characterizations
Hexagonal-like monodisperse CuFeOx MNs with spinel
structure could be facilely synthesized using Cu(II)
acetylacetonate (Cu(acac)2) and iron (III) acetylacetonate
(Fe(acac)3) as copper and iron precursor, respectively in
mixture of phenyl ether (Ph2O), alcohol (ROH), and oleylamine
(RNH2) solution at temperature of 300℃, as depicted in Fig. 1a.
Owing to the reduction by oleylamine, partial Cu2+ ions were
converted into Cu+ species in the framework of CuFeOx MNs,
similar to the partial reduction of Fe3+ ions to Fe2+ ions.24 Fig.
1b–e shows the TEM images of as-synthesized CuFeOx MNs
based on Ni grids. It is noting that CuFeOx MNs are mainly
irregular hexagonal-like nanoparticles, similar to the
Fig. 2. XRD spectra of CuFeOx MNs with different Cu/Fe ratios.
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Fig. 3. XPS spectra of Cu 2P3/2 (a), Cu LMM (b), and Fe 2p (c) of CuFeOx MNs with CuO and Cu2O as comparison.
(insets of Fig. 1c−e) show that the lattice fringes of Cu0.5Fe1Ox, photoelectron spectroscopy (XPS) and Auger electron
Cu1Fe1Ox, and Cu2Fe1Ox MNs are examined to 0.251, 0.252, spectroscopy (AES) analysis (as discussed in the following
and 0.253 nm, respectively, similar to the (311) lattice spacing section). The crystalline sizes based on the (311) plane of the
of spinel copper ferrite (CuFe2O4),25 in which the Cu ions sit MNs are in an order of Cu0.5Fe1Ox (8.2 nm) < Cu1Fe1Ox (10.4 nm)
predominantly on tetrahedral sites and iron atoms on < Cu2Fe1Ox (16.8 nm) < Fe3O4 (19.7 nm) (Table 1), consistent
octahedral sites (as depicted in Fig. 1a). Fig. 1f‒i is the HAADF- with that of TEM analysis.
STEM image and corresponding element mapping of Cu1Fe1Ox
XPS studies were conducted for elucidating the surface
MNs. The Cu, Fe, and O elements (Fig. 1g-i) are detected in the constitution/elemental chemical states of CuFeOx MNs with
selected region, and their distributions are homogeneous, different Cu/Fe ratios, as shown in Fig. 3. For the core level XPS
indicating that these materials are well crystallized 15. And the of Cu 2p regions, the binding energy of Cu 2p3/2 is located at
detail element ratio of Cu/Fe is displayed in Fig. 1j measured 933.0 eV with a satellite peak at 942.1 eV, revealing that the
by the point scanning EDS spectrum of one particle, indicating Cu oxidation state is +2 with a d9 electron configuration in
the similar Cu/Fe ratio (wt. %) to the given amount. CuFeOx MNs.28 The binding energies of the Cu2p3/2 peaks for
To explore the crystal properties of our prepared samples, CuFeOx MNs are in the order of Cu0.5Fe1Ox (933.2 eV) <
the XRD is common method. Fig. 2 clearly shows that the Cu1Fe1Ox (933.1 eV) < Cu2Fe1Ox (932.9 eV). As the copper
copper ferrite (CuFe2O4), prepared by hydrothermal method content increases, the Cu2p3/2 peak shifts to lower binding
(its morphology is described in Fig. S2), exhibits the typical XRD energy, accompanying with the declined intensity of the
patterns of cubic spinel structure, of which the peaks at 2θ= satellite peak, which suggest that more Cu2+ species are
30.0, 35.5, 43.2, 53.4, 57.3, 62.6, and 74.3° can be ascribed to reduced as reduced copper species (Cu+ or Cu0) have lower
the (220), (311), (222), (400), (422), (511), (440), and (533) binding energy. The Cu2p3/2 peak of the three CuFeOx MNs
crystal planes of the copper ferrite (JCPDS 77-0010), sample can be deconvoluted into two obvious peaks at 933.7
respectively.26 As for the Cu0.5Fe1Ox MNs with similar Cu/Fe and 932.5 eV, which are similar to the characteristic peaks of
ratio (0.5) to CuFe2O4, its apparent characteristic peaks are single CuO (933.7 eV) and Cu2O (932.5 eV), respectively,28
consistent with those of CuFe2O4, meaning that it possesses revealing that Cu2+ (933.7 eV) and Cu+ (932.5 eV) species may
the cubic spinel structure, in accordance with the HRTEM be coexisted on CuFeOx MNs surface. The area ratios of peak
analysis (Fig. 1c). In case of Cu1Fe1Ox and Cu2Fe1Ox MNs with at 932.5 eV (Cu+) to peak at 933.7 eV (Cu2+) for Cu0.5Fe1Ox,
the Cu/Fe ratios of 1.0 and 2.0, respectively, the XRD patterns Cu1Fe1Ox, and Cu2Fe1Ox are 1.5, 1.6, and 1.8, respectively,
are similar to that of Cu0.5Fe1Ox MNs, which indicates that both suggesting that more copper species are reduced for CuFeOx
the Cu1Fe1Ox and Cu2Fe1Ox MNs also possess the good cubic with higher copper content. As well known that Cu 2p3/2 XPS of
spinel structure although they have a much higher copper Cu+ and Cu0 are hardly differentiated,29 Auger Cu LMM spectra
content (2 or 4 times), in accordance with the HRTEM analysis of CuFeOx MNs (Fig. 3b) are employed to investigate their
(Fig. 1d and e). Nevertheless, for the profiles of Cu1Fe1Ox and differences. As for single CuO and Cu2O, the Cu LMM spectra
Cu2Fe1Ox MNs, the intensity of the peak at 2θ = 43.2° is exhibit the characteristic peaks at 568.5 and 569.4 eV,
obviously enhanced and a new peak at 2θ = 50.5° appears, respectively. In case of CuFeOx MNs, the Cu LMM peaks can be
suggesting that reduced copper species (eg. Cu0, JCPDS 85- deconvoluted into two peaks at 568.5 and 569.4 eV, assigned
1326) might be formed in the matrix of CuFeOx MNs.27 This to Cu2+ and Cu+, respectively, and no characteristic peak for
may be due to that CuFeOx MNs have high copper contents Cu0 species (568.0) is observed probably due to the weak
will lead to more Cu2+ species reduced by oleylamine and reduction capacity of oleylamine and alcohol mixture. The area
alcohol at high temperature. It should be noted that the actual of peak at 569.4 eV increases with increasing the Cu/Fe ratio,
oxidation state of reduced copper species exposed on the indicating that more Cu+ are formed in CuFeOx MNs with
surface of CuFeOx MNs should be further confirmed by X-ray higher copper content, contributing to the Cu LMM peak shift
4 | J. Name., 2012, 00, 1-3
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Table 1 Physicochemical properties of CuFeOx MNs with different Cu/Fe ratios
Metal contenta
Mean
Atom ratioe
(%)
Surface
Msh
f
Crystalline
ACu(I)/Cu(II)
b
(wt%)
SBET
particle
sized
(nm)
22.0
8.8
Cu
Samples
Sizec
(nm)
(emu
(m2 g−1)
dispersion
(D%) g
-
g−1)
Cu
Fe
Cu
Fe
O
Cu2p3/2
CuLMM
Fe3O4
0
22.6
20.2
16.9
15.9
60.3
61.2
47.8
32.4
19.7
8.2
10.4
16.8
0
25.2
27.2
19.8
19.7
74.8
57.3
61.3
57.7
-
-
74.0
53.0
41.0
28.0
Cu0.5Fe1Ox
Cu1Fe1Ox
Cu2Fe1Ox
10.2
18.5
26.1
15.5
18.9
22.6
1.5
1.6
1.8
1.6
2.5
5.7
0.20
0.10
0.14
12.2
17.2
a The actual metal content was calculated by ICP.
b SBET represents the specific surface area calculated by BET method based on N2 adsorption and desorption characterization.
c The crystalline size of the (311) plane of CuFeOx MNs was calculated by the Scherrer equation based XRD pattern.
d The mean particle size was measured based on TEM image.
e The atom ratio of Cu, Fe, and O of the samples were detected by XPS.
f The ACu(I)/Cu(II) represents the area ratio of deconvoluted peaks associated with Cu+ and Cu2+, respectively in Cu2p3/2 and CuLMM spectra.
g Surface Cu dispersion (D%) was calculated by CO pulse chemisoportion.
h The Ms represents the magnetization saturation value.
Fig. 4. Profiles of H2-TPR (a), NH3-TPD (b), and CO2-TPD (c) of CuFeOx MNs.
to higher binding energy. In general, the Cu 2p3/2 core XPS and
The NH3 and CO2-TPD were used to probe the surface
Cu LMM Auger transitions results indicate that the copper acid-base sites of CuFeOx MNs since the acid−base sites
species exposed on CuFeOx MNs surfaces are mainly present s t r o n g l y r e l a t e t o t h e c a t a l y t i c p e r f o r m a n c e f o r
as Cu2+ and Cu+. The Fe 2p spectrum of Fe3O4 shows five dehydrogenation of alcohol.31 Considering that PDO is a diol
deconvoluted peaks (Fig. 3c), in which the photoelectron peaks and the initial dehydrogenation of PDO can be rate-step for its
at 712.1 and 710.2 eV can be assigned to the 2p3/2 of the Fe3+ transformation to lactic acid,7, 9, 19, 32 the surface acidity and
ion and Fe2+ ion, respectively.30 The presence of a satellite basicity of CuFeOx MNs are worthy being studied. As shown in
peak at 718.5 eV indicates the coexistence of Fe3+ and Fe2+.30 Fig. 4b, the NH3-TPD profile of Fe3O4 NMs exhibits four
For the Fe 2p spectra of CuFeOx MNs, similar peaks can be desorption peaks with the maximal temperatures (TM) in
deconvoluted, which indicate the coexistence of Fe3+ and Fe2+. regions of 100–200, 250–350, 500–800℃, identified as the
Comparing with Fe3O4 MNs, the Fe 2p3/2 peaks of CuFeOx MNs weak, medium, and strong acid site,33 respectively, in which
shift to high binding energy, suggesting the strong interaction the strong acid site originated from the Fe3+ and Fe2+ ions
between the copper and ion species. The reduced Fe and Cu in (Lewis acid site, which is denoted as δ+.) is dominant (Table S2).
our samples should be beneficial for the catalytic processes.
In contrast, for Cu0.5Fe1Ox MNs, the desorption peak of strong
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Table 2. O2-free catalytic conversion of PDO to LA using different copper based catalystsa
Selectivitiese
(%)
Surface copper
dispersionb
(wt%)
Surface copper
PDO
conversiond
(%)
LA
CO uptakes
TOF g
yieldf
(h-1)
(%)
Catalysts
concentrationc
−1
(mmol gcat.
)
–1
(mmol gcat.
)
LA
AA
FA
PA
2.3
2.1
2.6
1.6
1.4
0.4
5.0
3.2
2.8
HA
-
-
-
-
3.2
4.2
80.5
79.8
92.1
94.5
90.7
74.2
83.5
88.6
89.4
10.5
10.8
3.2
2.6
4.2
16.8
6.4
4.0
5.9
6.6
2.1
0.8
2.9
8.3
3.3
2.8
2.2
0.8
0.7
1.0
0.5
0.8
0.3
1.8
1.4
1.8
2.6
3.4
-
-
Fe3O4
ND
ND
ND
Cu0.5Fe1Ox
Cu1Fe1Ox
Cu2Fe1Ox
Cu2O
CuO
CuFe2O4
o-Cu1Fe1Oxh
0.030
0.016
0.022
0.038
0.041
0.028
0.017
0.20
0.10
0.14
0.24
0.26
0.18
0.11
0.0312
0.0156
0.0219
0.0375
0.0406
0.0281
0.0172
58.1
72.6
68.6
66.5
38.3
43.6
50.2
53.5
68.6
62.2
49.3
32.0
38.6
46.4
462
1410
895
420
273
427
760
3.8
a Reaction conditions: PDO concentration = 0.2 M (40 mL), NaOH/PDO (mol/mol) = 2, reaction temperature = 160℃, N2 = 1.0 MPa, catalyst loading = 0.06 g, reaction
time = 8 h.
b-c Surface copper (Cu+ and Cu2+) dispersion and concentration is calculated on the basis of the results of CO pulse chemisorption.
d The PDO conversion is calculated by GC.
e The product selectivity is calculated by HPLC. PDO = 1,2-propanediol, LA = lactic acid, AA = acetic acid, FA = formic acid, PA = pyruvic acid, HA = hydroxyacetone.
f The LA yield is calculated by PDO conversion multiplied by LA selectivity.
g Activity expressed as turnover frequency (TOF) which is based on the number of surface copper and PDO conversion at initial 1 h with PDO conversions of lower than
20% (Fig. S3).
h The o-Cu1Fe1Ox MNs represents that the Cu1Fe1Ox MNs treated at 450℃ with a O2 flow rate of 50 mL min-1 for 1 h.
acid site shifts to lower temperature (300–450 ℃ ) and the
concentration of medium acid site is significantly enhanced,
Catalytic performance of CuFeOx MNs for catalytic
PDO transformation
which can be attributed to the introduction of copper ions
(Cu2+ or Cu+), of which the Lewis acid strength is weaker than
that of ferric ions (Fe2+ or Fe+). The NH3-TPD profile of Cu1Fe1Ox
In order to demonstrate the anaerobic condition may be able
to facilitate the catalytic transformation of PDO to LA over Cu-
based catalysts, we first investigated the catalytic performance
for Cu1Fe1Ox MNs in the presence of O2 and N2 with pressure
of 0.1−2 MPa at 140℃ and NaOH/PDO mole ratio of 2, as
shown in Fig. 5a. It can see that lactic acid (LA), pyruvic acid
(PA) and hydroxylacetone (HA) are the detected C3 products,
while acetic acid (AA) and formic acid (FA) are the C1 and C2
product, respectively. As the O2 pressures decrease from 2 to
0.1 MPa, the LA selectivity can be increased from 43.1% to
73.5% in spite of lowering the PDO conversion. However, the
LA yields exhibit an increasing trend, revealing that lowering
the O2 pressure can significantly enhance the transformation
of PDO to LA probably due to the inhibition of C3 product
cleavage. According to this trend, the reaction is further
carried out in the absence of O2 (N2 atmosphere). As expected,
the LA selectivity can be further raised to 84.1% at 0.1 MPa N2,
which will be slightly enhanced to 86.8% and 87.5% at 1.0 and
2.0 MPa N2, respectively. Although the PDO conversions at N2
are lower than those at O2, the obtained LA yields are higher at
N2, indicating that the O2-free condition is proved to be
beneficial for the catalytic transformation of PDO to LA.
Considering that the polyols dehydrogenation reaction is
MNs exhibits two weak desorption peaks associated with the
medium and strong acid site, respectively, while the Cu2Fe1Ox
MNs exhibits only one weak desorption peak associated with
strong acid site. It indicates the concentration of acid site of
CuFeOx MNs significantly decrease with increasing the Cu/Fe
ratio, which might be attributed to that more Cu+ with weak
acid strength in the CuFeOx MNs reduces the acidity. As for the
CO2- TPD profiles of MNs (Fig. 4b), it is observed that the CO2-
TPD profiles of MNs display a weak desorption peak and a
strong broad desorption peak with the TM in region of 100–250℃
and 250–700 ℃ , respectively, meaning that medium and
strong base sites originated from O2– (Lewis base site, denoted
as δ−) are dominant, along with the concentration order of
Fe3O4 > Cu0.5Fe1Ox > Cu1Fe1Ox > Cu2Fe1Ox (Table S2), which
reveals that increasing the content of copper species also
results in decreasing the basicity of CuFeOx MNs, probably due
to that more Cu+ occupying less O element leads to a
decreased electron density around O2–. In general, the as-
prepared CuFeOx MNs have both acid site (δ+) and base site (δ−)
on the surface, which is significantly affected by the Cu/Fe
ratio, and will further affect the catalytic performance.
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endothermic, it may be helpful for improving catalytic activity and 94.5%, respectively. For comparison, whenVwieweAurtsiceledOntlhinee
2+
by elevating the reaction temperature. As shown in Fig. 5b, the oxidized Cu1Fe1Ox MNs (denoted as o-CuD1FOeI:1O10x.1M03N9/sD)0wCiYt0h17C3u3G
catalytic activity of Cu1Fe1Ox MNs remarkably depends on the species dominant (see the XPS and XAS spectra in Fig. S6)
temperature. Interestingly, the observed LA selectivity is first becomes less active (TOF of 760 h−1) than the original Cu1Fe1Ox
increased with raising the temperature and reaches the MNs (TON of 1410 h−1). On basis of the above results, it is
maximum of 94.5% at 160℃, comparable to that of noble
metal/pressurized O2 based route. Then, it is slightly decreased
with further increasing the temperature from 160 to 200℃,
probably due to the formation of more cleavage C1 and C2
products (formic and acetic acids). In addition to high LA yield,
hydrogen is generated as
a co-product with a nearly
quantitative yield (Fig. S4 and Fig. S5), implying that apparently
the oxidation of PDO proceeds in the absence of molecular
oxygen and thus is accompanied by hydrogen production,
thereby it is proposed to follow a similar reaction pathway as
that of catalytic ethylene glycol transformation,18 which can be
written as (1):
C3H8O2 (1,2-propanediol) + H2O → C3H6O3 (lactic acid) + 2H2 (1).
Based on the above results, we can see that the catalytic
transformation of PDO under anaerobic and basic aqueous
conditions favors significant enhancement in LA selectivity as
compared to that under pressurized O2 condition and valuable
H234, 35 is produced simultaneously.
Table 2 and Fig. 5c show the results derived from the
measurements of the catalytic performances of O2-free
catalytic PDO transformation over different Cu-based catalysts.
In absence of catalyst, the PDO transformation can still
proceed probably due to the promoting effect of alkali 23 but
very slowly, giving only 3.2% PDO conversion after reacting for
8 h. No enhancement in PDO conversion is found for Fe3O4
MNs as compared to that without catalyst, indicating that they
are inactive for this reaction. However, when CuFeOx MNs
were used as catalysts, the PDO is efficiently and selectively
transformed into LA, giving PDO conversion of > 58.1% and LA
selectivity of > 90.7%, which is suggested that copper species
in CuFeOx MNs can be the active component for promoting the
reaction. Given the coexistence of Cu2+ and Cu+ species in
CuFeOx MNs, it is worthy to give an insight into their effects on
the catalytic performance. Herein, firstly, CuO and Cu2O with
respectively, are selected to catalyze the PDO transformation.
Table 2 and Fig. 5c clearly show that the PDO conversion
and TOF for Cu2O are much higher than those for CuO (66.5%
and 420 h−1 vs 38.3% and 270 h−1), while the LA selectivity for
Cu2O is lower than that for CuO (74.2% vs 83.5%). This result
indicates that Cu2O with Cu+ dominant is more active to
catalyze the PDO conversion, while CuO with Cu2+ alone shows
higher LA selectivity, which can further be evidenced by that
the CuFe2O4 with Cu2+ alone (see the XPS and AES spectra in
Fig. S6) displays high LA selectivity (88.6%) but with relative
low PDO conversion (only 43.6%). Comparing with CuFe2O4,
the Cu0.5Fe1Ox MNs with similar Cu/Fe ratio and structure to
CuFe2O4 displays enhanced PDO conversion and LA selectivity
(58.1% and 92.1% vs 43.6% and 88.6%), implying that the
combination of Cu+ and Cu2+ sites synergistically promote the
reaction. Consequently, for Cu1Fe1Ox MNs, by increasing the
amount of Cu+ and Cu2+ species in CuFeOx MNs can further
promote to the PDO conversion and LA selectivity to 72.6%
Fig. 5. Influences of O2 and N2 pressure (a) and temperature (b) on the PDO
conversion and LA selectivity over Cu1Fe1Ox MNs. LA selectivity vs TOF over
different Cu-based catalysts (c). Reaction conditions for a: O2 or N2 pressure = 0.1–
2.0 MPa, reaction temperature = 140 ℃ , PDO concentration = 0.2 M (40 mL),
NaOH/PDO (mol/mol) = 2, catalyst loading = 0.06 g, reaction time = 8 h. Reaction
conditions for b are similar to a expect alter the reaction temperature and keep N2
pressure of 1.0 MPa. Reaction conditions for c are similar to b except keep the
reaction temperature of 160 ℃ and alter the type of catalyst. PDO
= 1,2-
propanediol, LA = lactic acid, AA = acetic acid, FA = formic acid, PA = pyruvic acid,
HA = hydroxyacetone.
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Fig. 6. The influences of other important reaction parameters (reaction time, NaOH concentration, PDO concentration and catalyst loading)
on the catalytic performance of Cu1Fe1Ox MNs. Reaction conditions: (a) reaction temperature = 160 ℃ , N2 pressure = 1.0 MPa, PDO
concentration = 0.2 M (40 mL), NaOH/PDO (mol/mol) = 2, catalyst loading = 0.06 g; (b–d) reaction conditions are similar to (a) but just vary
the corresponding reaction parameters and keep the reaction time for 8 h. PDO = 1,2-propanediol, LA = lactic acid, AA = acetic acid, FA =
formic acid, PA = pyruvic acid.
suggested that the coexistence of Cu+ and Cu2+ dual sites on initial NaOH/PDO ratio was kept at 2 in order to improve the
CuFeOx surface contribute to the enhanced activity and LA catalytic performance. In the case of PDO concentration effect
selectivity in the catalytic transformation of PDO over CuFeOx (Fig. 6c), high PDO conversion but low LA selectivity is
MNs. However, in spite of having highest copper amount, the observed at low PDO concentration (0.1 M) probably due to
Cu2Fe1Ox MNs shows a slightly reduced PDO conversion (68.6%) the deeper catalytic conversion of PDO to pyruvic acid, while
and LA selectivity (90.7%) as compared to Cu1Fe1Ox MNs, high PDO concentration (0.6 M) displays low PDO conversion
which may relate to their lower acid site (δ+) concentration but with high LA selectivity. Increasing the catalyst loading in
and smaller surface area, exhibiting weaker interaction with the reaction (Fig. 6d) can obviously improve the PDO
PDO molecule. From Fig. 5c, through the comparison of LA conversion but meanwhile cause serious C–C bond cleavage of
selectivities among CuO, CuFe2O4, and o-Cu1Fe1Ox with Cu2+ C3 product leading to the declined LA selectivity. In general,
species dominant, it turns out that both CuFe2O4 and o- the optimal reaction time, NaOH concentration, PDO
Cu1Fe1Ox MNs owing spinel structure favor higher TOFs, concentration and catalyst loading can be fixed at 8 h, 0.4 M,
meaning the spinel structure of CuFeOx MNs is also important 0.2 M, and 0.06 g, respectively.
to facilitate the catalytic reaction. Therefore, the excellent
catalytic performance of CuFeOx MNs for catalytic PDO
transformation can be originated from the synergy between
Cu+ and Cu2+ binary centres as well as the spinel structure.
Reaction pathway and mechanism for catalytic
PDO transformation
The influences of other important parameters including
For the conventional “O2-required” catalytic PDO
oxidation over supported noble metal catalysts, it is generally
supposed to follow a reaction network as depicted in Scheme
1c, in which the pathway of PDO → lactaldehyde → lactic acid
is proposed as the desired pathway for LA production.
However, as for the O2-free catalytic transformation of PDO
over spinel CuFeOx MNs, it is challenging to determine which
pathway is preferable and how this can occur because either
the terminal or secondary −OH group of PDO can be
dehydrogenated first. Moreover, the intermediates can
reaction time, concentrations of PDO and NaOH, and catalyst
loading on the catalytic performance were also investigated.
Fig. 6a shows that prolonging the reaction time favors the
increment of PDO conversion and lactic acid selectivity. High
NaOH concentration (>0.4 M, Fig. 6b) significantly promotes
the PDO conversion and lactic acid selectivity as compared to
low NaOH concentration (<0.2 M). This is because alkali (OH−)
will participate in the polyols dehydrogenation reaction.
However, an overdose of NaOH (>0.6 M) shows only a little
enhancement in the PDO conversion and LA selectivity. So, the
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Fig. 7. Possible reaction routes based on DFT calculation for the O2-free catalytic PDO conversion to LA over
spinel CuFe2O4 surfaces with different Cu/Fe ratios of 20/40 and 21/39.
be transformed into each other reversely (Table 2). Therefore, transformed to lactic acid (*CH3CHOHCOOH). The conversion
herein, DFT calculations are employed to identify the favourite of lactaldehyde to LA over Cu20Fe40Ox surface is the rate
pathway for the catalytic PDO transformation over CuFeOx determining step due to its requirement of high energy.
MNs, in which the spinel CuFe2O4 (311) plane is designed as However, as for Cu21Fe39Ox surface with higher Cu ratio (Fig.
the model surface because the synthesized CuFeOx MNs have 7b), it favours the initial dehydrogenation of PDO to
the similar structure to CuFe2O4 (XRD and HRTEM). Here, two hydroxyacetone (∆G = 0.155 eV), and the tautomeric into
types of CuFe2O4 surfaces with different Cu/Fe atom ratios of lactaldehyde is only 0.177 eV. Nevertheless, the LA formation
20/40 and 21/39, designated as Cu20Fe40Ox and Cu21Fe39Ox, from the lactaldehyde conversion is more favourable than
respectively, are built as the models in view of the surface from the pyruvaldehyde conversion via Cannizaro reaction due
content significantly affecting their catalytic activity.
to the low energy of −0.0423 eV. As a result, the tautomeric
Fig. 7a depicts the reaction energy diagram for the equilibrium of hydroxyacetone to lactaldehyde can be the rate
catalytic PDO conversion over the Cu20Fe40Ox surface, in which determining step for the LA production over the Cu21Fe39Ox
the Cu/Fe ratio is 1:2. Before reaction, PDO is considered to be surface. These data are also well agreed with the experimental
first adsorbed on the Cu2 0 Fe4 0 Ox surface to form an results for that the heavily decreased barrier of determination
intermediate state (*CH3CHOHCH2OH, where the asterisk step can greatly enhance the reaction kinetics, and reduce the
indicates the catalyst). It clearly shows that the energy for the reaction temperature. Therefore, a high yield of LA has been
dehydrogenation of PDO to lactaldehyde (*CH3CHOHCHO) (∆G observed at low temperature of 160℃.Moreover, the bond
= –0.135 eV) through breaking the C−H bond of terminal alkoxy distance of C−H and O−H near Cu or Fe atoms, as well as their
species is lower than that to hydroxyacetone (*CH3CHOCHOH) atomic charge are calculated, as listed in Table S5. The
(∆G = 0.031 eV) through breaking the C–H bond of secondary secondary O−H distance of PDO is not changed both on
alkoxy species, indicating that the conversion of PDO to LA Cu20Fe40Ox or Cu21Fe39Ox, but the charge of O is enhanced on
over Cu20Fe40Ox surface favours following the reaction path of Cu21Fe39Ox, which could enhance the adsorption of H, resulting
initial dehydrogenation of PDO to lactaldehyde, which is then high conversion energy for the first step from PDO to
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Scheme 2. Proposed reaction mechanism for O2-free catalytic PDO conversion to LA over CuFeOx MNs.
hydroxyacetone (Scheme 1c). For the step from dominant pathway of PDO → hydroxyacetone → lactaldehyde
hydroxyacetone to lactaldehyde, the change of O−H can be → lactic acid for LA production, but how can it undergo?
neglected. And the charge of O at C=O on Cu20Fe40Ox is lower
It is reported that for the oxidative dehydrogenation of
than that on Cu21Fe39Ox sample, which means that the O at ethanol to acetaldehyde over CuFe2O4, it is supposed to follow
C=O on Cu21Fe39Ox cannot accept more electrons from the the a mechanism with heterolytic dissociation of the O–H and
added H, leading to a higher barrier for the step of C–H bonds on the acid–base [Cu–O] pair.20 Moreover, the
hydroxyacetone to lactaldehyde on Cu21Fe39Ox. More than acid–base sites of catalyst can accelerate the cleavage of O–H
that, the Coulomp interaction between C and H on Cu20Fe40Ox bond in the O2-free dehydrogenation of alcohols.36 Inspired by
is lower than it on Cu21Fe39Ox, which also indicates that the these, the CuFeOx MNs with a large number of acid-base pairs
barrier should be lower on Cu20Fe40Ox. For the reaction from (Table S2) can be considered as the acid-base [δ+–δ–] pair.
lactaldehyde to lactate, the positive charge of C of C−H on Based on these, a possible scheme that may appear best to
Cu21Fe39Ox is much higher than that on Cu20Fe40Ox, which can explain the mechanism of CuFeOx MNs based O2-free catalytic
strong attract the negative charged O atoms, therefore, the conversion of PDO to LA is shown in Scheme 2. First of all, PDO
barrier is very lower for this reaction on Cu21Fe39Ox.
is supposed to be absorbed on the CuFeOx surface, where the
In general, through the DFT calculations we can find that secondary –OH group would like to interact with the acid-base
the highly selective LA formation over spinel CuFe2O4 surface sites, while the α–C–H likely interacts with the copper
37
originates from the conversion of lactaldehyde instead of centres.36,
It is then dehydrogenated to generate a
pyruvaldehyde. The intermediate lactaldehyde can be formed hydroxyacetone with a quantitative H2 release, which further
by the catalytic dehydrogenation of the primary −OH group of transforms into lactaldehyde via the tautomeric equilibrium.
PDO, or by tautomeric equilibrium of hydroxyacetone, The lactaldehyde can be in equilibrium with its gem-diol via
depending on the surface Cu/Fe ratio of CuFe2O4. As for the the interaction with water,16 which is further dehydrogenated
actual CuFeOx MNs, since their surface Cu/Fe atom ratios are to produce LA with another H2 release, and it is rapidly
higher than 1/2, as the XPS analysis (Table 1), it is supposed to deprotonated to lactate by the added base under alkaline
follow a favourite pathway similar to that of Cu21Fe39Ox surface condition.17
(Fig. 6b) to generate LA. Moreover, the characteristic
In this reaction, LA can be selectively transformed to
intermediate hydroxyacetone is detected in the reaction, pyruvic acid catalyzed by Cu1Fe1Ox MNs (Table S6), which can
indicating the dehydrogenation of secondary −OH group of be further dissociated into acetic acid (Table S6), a stable C2
PDO likely occurs. The production of acetic and formic acids product detected in our experiment, and carbonic acid, a
suggests the side reaction of the C−C bond cleavage of possible C1 product which is not included in this study. No
pyruvaldehyde can also occur due to its easy dissociation to conversion is observed when using Cu1Fe1Ox MNs to catalyze
form acetic and formic acids (Table S6). This result is consistent the acetic acid and formic acid (Table S6), suggesting that
with the easy formation of pyruvaldehyde in the simulative these two products are rather stable. Previous reports show
reaction routes over Cu21Fe39Ox surface. Thus, the CuFeOx MNs acetic acid and formic acid could be derived from the cleavage
based catalytic PDO transformation is suggested to follow a of pyruvaldehyde.5, 38 Table S6 shows acetic acid and formic
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Fig. 8. Recycling performance of Cu1Fe1Ox MNs (d). Catalytic performance for catalytic hydrothermal conversion of glycerol to LA over Cu1Fe1Ox MNs, with Cu2O as
comparison (e). Reaction conditions for c, d, e: N2 pressure = 1 MPa, reaction temperature = 160℃, PDO concentration = 0.2 M (40 mL), NaOH/PDO (mol/mol) = 2,
catalyst loading = 0.06 g, reaction time = 8 h; Reaction conditions for f: reaction temperature = 180–220 ℃ , glycerol concentration = 1.08 M (30 mL), NaOH
concentration = 1.2 M, N2 pressure = 1.4 MPa, catalyst loading = 0.2 g, reaction time = 6 h. XPDO = conversion of 1,2-propanediol, SLA = lactic acid selectivity, Sothers
selectivity of other products.
=
acid can be observed when using Cu1Fe1Ox MNs to catalyze from the reaction mixture by using an external magnet due to
pyruvaldehyde, while only acetic acid is observed when it their good magnetic property (Fig. S7b), washed with ethanol
catalyzes pyruvic acid. Thus, formic acid, another C1 product, is three times, and further reused directly. The Cu1Fe1Ox MNs
probably derived from the C−C bond cleavage of remains high activity after 5 cycles in spite of a slight decrease
pyruvaldehyde, and acetic acid (C2 product) can be derived in the PDO conversion (Fig. 8a), probably due to a little amount
from the dissociation of pyruvaldehyde or pyruvic acid. of copper species leaked from Cu1Fe1Ox MNs (ICP-AES analysis,
Comparing with CuO and Cu2O, Cu1Fe1Ox MNs is more Table S3). The XRD pattern (Fig. S8a) of the spent Cu1Fe1Ox
favorable for LA production even using hydroxyacetone as raw MNs displays two new characteristic peaks of cuprite (Cu2O,
material, accompanying with lower C1 and C2 product JPFDS05-0667) as compared to the fresh one, indicating the
selectivity (Table S6). Moreover, Cu1Fe1Ox MNs displays lower exposed Cu+ species of the spent catalyst. However, the TEM
catalytic dissociation of pyruvic acid to acetic acid. The results and HRTEM images (Fig. S8b-c), element-mapping (Fig. S8e-h)
indicate the Cu1Fe1Ox MNs shows lower dissociation of C3 show that the size, morphology, crystal structure, chemical
products. Furthermore, the conversion of LA to pyruvic acid composition of the spent Cu1Fe1Ox MNs remains unchanged,
over Cu1Fe1Ox MNs is much higher than that over CuO and indicating its good recycling performance and stability.
Cu2O, indicating that the Cu1Fe1Ox MNs has superior
dehydrogenation capacity. In general, owing to the
coexistence of Cu+ and Cu2+ dual sites and the spinel structure,
the CuFeOx MNs is highly active and selective for catalytic
transformation of PDO to LA under anaerobic and basic
aqueous conditions.
Extension of CuFeOx MNs for catalytic glycerol and
ethylene glycol transformations
The catalytic hydrothermal conversion of glycerol has
been also believed as a green strategy for LA production.
Previous work has shown that Cu2O is active for catalyzing the
glycerol conversion to LA, giving 70.2% glycerol conversion and
79.2% LA selectivity at NaOH/glycerol ratio of 1 and 200℃ for
6 h.23 Considering that CuFeOx MNs is more selective than
Cu2O for catalytic transformation of PDO to LA, we tried to use
the optimal Cu1Fe1Ox MNs to catalyze hydrothermal
conversion of glycerol under the same conditions, with Cu2O as
comparison, aiming at enhancing the LA selectivity. The results
are shown in Fig. 8b. It can see that the Cu1Fe1Ox MNs displays
72.3% glycerol conversion, slightly higher than that for Cu2O
(68.4%), whereas the LA selectivity is boosted to 97.8%, much
higher than that for Cu2O (77.7%), accompanied with a
quantitative yield of H2 (0.95, Fig. S5). By increasing the
reaction temperature to 220℃ can further raise the glycerol
conversion to 80.5% and still maintain high LA selectivity of
98.5%. In addition to high catalytic performance in catalytic
Recycling performance of CuFeOx MNs
Fast hot catalyst filtration experiment was used to
determine copper species on CuFeOx MNs were the real active
sites.39 After the reaction was conducted for 4 h, the
conversion of PDO exceeds 45%. No further PDO conversion in
the filtrate is found after removal of the catalyst in the
oxidation over Cu1Fe1Ox MNs (Fig. S7a), indicating the true
heterogeneous catalysis. A trace amount of copper and iron
elements (less than 2 ppm) was determined in the filtrate by
ICP-AES after reaction, suggesting that the catalytic reactivity
by the leaching could be almost excluded in the present
catalytic conditions. Moreover, the reusability of Cu1Fe1Ox
MNs was performed under the optimized catalytic conditions.
After reaction, the spent Cu1Fe1Ox MNs were easily separated
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glycerol transformations, it is found that Cu1Fe1Ox MNs can microscopy (Tecnai G2 F30) operated at an ViaewccAertlieclreaOtniloinne
DOI: 10.1039/D0CY01733G
also promote the highly selective transformation of ethylene voltage of 200 KV. Particularly, the samples were prepared on
glycol into glyceric acid (87.4% conversion and 97.8% Ni grid.
selectivity, Table S4) at 160 ℃.The results demonstrate that
X-ray photoelectron spectra (XPS) of CuFeOx MNs were
the spinel CuFeOx MNs with coexistence of Cu(I) and Cu(II) recorded on an ESCALAB 250 spectrometer (PHI5000 Versa
dual sites is an alternative efficient catalyst for catalytic polyols Probe,) using Al Ka radiation at 1486.6 eV. The resolution of
transformation to value-added carboxyl acids under anaerobic the XPS is ± 0.1 eV. The binding energies of Cu and Fe were
and basic conditions.
calculated with respect to C 1s peak of contaminated carbon at
284.6 eV.
The reduction behaviors of the catalysts were investigated
by temperature programmed reduction (TPR) technique using
a mixed H2/N2 flow (5:95, v/v) of 30 mL min-1 and 0.05 g of the
calcined catalysts at a temperature ramp of 10℃ min-1 from
50 to 400℃. H2 consumption was determined by analyzing the
effluent gas with a thermal conductivity detector. The calcined
catalysts were preheated in air at 400 ℃ before the TPR
measurement in order to eliminate the adsorbed water.
Acidic and basic properties of catalysts were analyzed by
CO2 and NH3 temperature-programmed desorption (TPD) on a
fixed-bed continuous flow microreactor at atmospheric
pressure. For basicity or acidity evaluation by CO2-TPD or NH3-
TPD, 0.1 g sample was dried at 400℃ for 1 h in helium. Then,
CO2 or NH3 CO2 was introduced to the sample at 100℃ for 1 h.
After purging with helium (30 mL min-1) at 100℃ for 0.5 h to
remove the weakly adsorbed CO2 or NH3 until the baseline of
TCD signals stabilized, the TPD experiment was carried out at a
linear heating rate of 10℃ min-1 from 100 to 800℃.
CO pulse chemisorption was used to calculate the
dispersion of surface copper,40 which was conducted using a
chemisorption module of the Micromeritics AutoChem II 2910
instrument. Prior to the experiment, the sample was first
reduced at 350°C by 10 vol% H2/He with a flow rate of 10°C
min−1 for 1 h and then cooled to 60℃ under a high purity He
flow rate of 10 mL min−1. Afterward, as the temperature
declined to 60℃, successive pulses of CO were injected, using
He as the carrier gas (10 mL min−1) until the baseline is
balanced. Metal dispersion (D%, eq 1) was calculated on the
Experimental section
Materials
Benzyl ether (95%), oleylamine (C18 ： 80-90%), Iron(III)
acetylacetonate (98%) and copper(II) acetylacetonate (97%)
were of reagent grade and purchased from Shanghai Aladdin
Biochemical Technology Co., Ltd. Absolute ethanol (≥ 99.5%),
hexane (> 99%), 1,2-propanediol (PDO, >99%), lactic acid (LA,
90%), pyruvic acid (98%), hydroxyacetone (95%), formic acid
(>99%), and acetic acid (>99%) were of reagent grade and
bought from Sinopharm Chemical Reagent Co., Ltd. All the
reagents are used without any purification.
Preparation of CuFeOx magnetic nanoparticles (MNs)
The synthesis of CuFeOx MNs was followed by the
procedure adopted from the synthesis of monodispersed
Fe3O4 nanoparticles,24 and was modified slightly to make
CuFeOx MNs here. The as prepared CuFeOx MNs with different
molar ratios of Cu/Fe are denoted as CuaFe1Ox, of which the
letters represent the molar ratio of each element. For example,
Cu1Fe1Ox means the molar ratio of Cu/Fe is 1:1, which is
synthesized by the following procedure. Typically, 4 mmol of
Fe(acac)3 and 4 mmol Cu(acac)2 was added into a 250 mL flask
and dissolved in a mixture of 20 mL benzyl ether, 100 mL
ethanol and 30 mL oleylamine. The solution was heated at
110°C for 1 h with strong stirring, and was quickly heated to
300°C for 2 h. The precipitate was collected by centrifugation
and was washed several times with hexane and ethanol,
respectively. Monodispersed Fe3O4, Cu0.5Fe1Ox, and Cu2Fe1Ox
MNs were prepared as comparison.
basis of the results of CO pulse chemisorption at 60℃ (Table 1):
푉
퐶ℎ푒푚
22414 × 푆퐹 × 푀
푊
퐷% =
× 100
(2)
m푐푎푡
where Vchem, SF, MW, and mcat are the amount of CO
adsorption (cm−3), stoichiometry factor, atomic weight of
metal (g mol−1), and weight of sample (g). SF is the
stoichiometric factor, which is assumed as 1:1 for CO:Cu. The
obtained surface metallic Cu0 dispersion can be considered as
the dispersion of Cu+ and Cu2+ on CuFeOx MNs because the
metallic Cu0 was obtained by the reduction of Cu+ and Cu2+.
Characterizations
Nitrogen adsorption-desorption isotherms were measured
at −196 ℃ by a NOVA 2000e physical adsorption apparatus.
The specific surface areas (SBET
determined by BET method. The actual contents of Cu and Fe
were determined by inductively coupled plasma (ICP)
technique (VISTA-MPX).
) of the samples were
Catalytic Test
The identification of crystal phases of CuFeOx MNs is
performed by X-ray powder diffraction (XRD), which were
recorded on a diffractometer (D8 super speed Bruke-AEX
Company, Germany) with Cu Ka (k = 1.54056 Å) radiation in
the range of 20–80°.
Transmission electron microscopy (TEM) images were
performed on a microscopy (Tecnai 12) operated at an
acceleration voltage of 120 KV. High resolution transmission
electron microscopy (HRTEM) images were obtained on a
The catalytic oxidation of PDO was performed in a 250 mL
capacity stainless steel autoclave equipped with a magnetically
driven impeller. The autoclave was charged with appointed
amounts of PDO, water, sodium hydroxide, and catalyst. First,
the autoclave was purged with N2 for 30 min. The reaction was
carried out at a given temperature with the stirring speed of
800 rpm, at which the external diffusion could be eliminated
(Fig. S1). Moreover, intraparticle diffusion limitation for the
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reaction was determined on basis of Weisz–Prater criterion
(see supporting information), which was suggested that recovered by magnetic separation. This work provides an
intraparticle diffusion limitation could also be negligible under alternative strategy and new insights for enhancing the
our reaction conditions. After reacting for certain time, 1 mL of catalytic transformation of bio-based polyols to carboxyl acids
mother liquid was sampled from the autoclave. The over non-precious metal based catalyst.
concentration of remained PDO was analyzed on a gas phase
ARTICLE
41
display good recycling performance and canViebweArticflaecOinlelinlye
DOI: 10.1039/D0CY01733G
chromatograph equipped with a PEG-20M packed capillary
column (0.25 mm × 30 m) and FID by the internal standard
Conflicts of interest
method with n-butanol as the internal standard. The liquid
products were analyzed on a Agilent HPLC system equipped
with a strong cation hydrogen exchange column (Carbomix H-
NP, 7.8 mm × 300 mm) and a UV detector (λ = 210 nm) at 55°C.
Before analysis, the reaction mixture was acidified with
hydrochloric acid (12 M) to the pH value of 2−3. Sulphuric acid
(5 mM) was used as a mobile phase with a flow rate of 0.8 mL
min−1. The concentrations of the products were analyzed by
the external standard method. Gas products were collected by
Tedlar bag after reaction and cooling down to room
temperature, in which H2 was determined by a gas phase
chromatograph (Test Instrument GC-2030) with a TDX-01
carbon molecular sieve packed column and TCD detector
under N2 carrier. The conversion of PDO, selectivity of
products and turnover frequency (TOF) are the factors used to
evaluate the catalyst performance. They are each defined by
the following equations.
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21606112), China
Postdoctoral Fundation Committee (No. 2019T120400),
Programs of Senior Talent Foundation of Jiangsu University
(No. 15JDG137) and Youth Talent Cultivation Plan of Jiangsu
University.
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퐶표푛푣푒푟푠푖표푛 (%) = 푎푚표푢푛푡 표푓 푃퐷푂 푐표푛푣푒푟푡푒푑 (푚표푙) × 100
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푆푒푙푒푐푡푖푣푖푡푦 (%) = 푚표푙푒푠 표푓 푐푎푟푏표푛 푖푛 푎 푠푝푒푐푖푓푖푐 푝푟표푑푢푐푡 × 100
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푚표푙푒푠 표푓 푐푎푟푏표푛 푖푛 푎푙푙 푝푟표푑푢푐푡푠
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푇푂퐹 ℎ
푠푢푟푓푎푐푒 푐표푝푝푒푟 푖푛 푐푎푡푎푙푦푠푡 (푚표푙) ÷ 푡푖푚푒 (ℎ)
(5)
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Table of contents entry
Synergy between Cu(I) and Cu(II) sites in spinel CuFeOx magnetic nanoparticles
contributes to the significant enhancement in catalytic 1,2-propanediol
transformation into lactic acid.