Manganese catalyzed transfer hydrogenation of biomass-derived aldehydes: Insights to the catalytic performance and mechanism

Article abstract of DOI:10.1016/j.jcat.2020.05.037

Manganese catalyzed transfer hydrogenation (CTH) of aldehydes is an attractive method for the synthesis of alcohols. Here, we report a novel and efficient MnO?C-N catalyst for the CTH of biomass-derived 5-hydroxymethylfurfural and other aldehydes to alcohols in high yields. Catalytic experimental studies showed that the MnO and nitrogen-doping are responsible for the high selectivity and high conversion, respectively. Isotopic labelling experiments demonstrated that the CTH of aldehydes to alcohols over MnO?C-N is via a route by direct hydrogen transfer. Kinetic studies revealed that the N-doping can improve the reaction rate and reduce the activation energy of the aldehydes conversion. DFT calculations also indicated that both pyridine N and pyrrolic N doping can reduce the energy barrier for acetone desorption by the interaction between N and hydroxyl-H of alcohol. Furthermore, MnO?C-N showed good recyclability for at least five reaction cycles. We anticipate that these results can drive progress in the manganese catalyzed transfer hydrogenation reaction.

Full text of DOI:10.1016/j.jcat.2020.05.037

Journal Pre-proofs
Manganese catalyzed transfer hydrogenation of biomass-derived aldehydes:
insights to the catalytic performance and mechanism
Yunchao Feng, Sishi Long, Guihua Yan, Binglin Chen, Jonathan Sperry,
Wanjie Xu, Yong Sun, Xing Tang, Xianhai Zeng, Lu Lin
PII:
S0021-9517(20)30218-9
https://doi.org/10.1016/j.jcat.2020.05.037
YJCAT 13767
DOI:
Reference:
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Journal of Catalysis
Received Date:
Accepted Date:
12 May 2020
25 May 2020
Please cite this article as: Y. Feng, S. Long, G. Yan, B. Chen, J. Sperry, W. Xu, Y. Sun, X. Tang, X. Zeng, L.
Lin, Manganese catalyzed transfer hydrogenation of biomass-derived aldehydes: insights to the catalytic
performance and mechanism, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.05.037
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Manganese catalyzed transfer hydrogenation of biomass-derived
aldehydes: insights to the catalytic performance and mechanism
Yunchao Feng a, Sishi Long a, Guihua Yan a, Binglin Chen a, Jonathan Sperry d, Wanjie Xu e,
Yong Sun abc, Xing Tang abc, Xianhai Zeng*abc and Lu Lin abc
a College of Energy, Xiamen University, Xiamen 361102, China.
b Fujian Engineering and Research Center of Clean and High-valued Technologies for Biomass,
Xiamen 361102, China.
c Xiamen Key Laboratory of Clean and High-valued Utilization for Biomass, Xiamen 361102,
China.
d Centre for Green Chemical Science, University of Auckland, Auckland 1142, New Zealand
e College of Materials, Xiamen University, Xiamen 361102, China.
KEYWORDS
biomass conversion, 5-hydroxymethylfurfural, transfer hydrogenation, manganese hydrogenation
catalysis; mechanism study
Corresponding Author
*E-mail: xianhai.zeng@xmu.edu.cn (Xianhai Zeng)
1

Abstract
Manganese catalyzed transfer hydrogenation (CTH) of aldehydes is an attractive method for the
synthesis of alcohols. Here, we report a novel and efficient MnO@C-N catalyst for the CTH of
biomass-derived 5-hydroxymethylfurfural and other aldehydes to alcohols in high yields. Catalytic
experimental studies showed that the MnO and nitrogen-doping are responsible for the high
selectivity and high conversion, respectively. Isotopic labelling experiments demonstrated that the
CTH of aldehydes to alcohols over MnO@C-N is via a route by direct hydrogen transfer. Kinetic
studies revealed that the N-doping can improve the reaction rate and reduce the activation energy
of the aldehydes conversion. DFT calculations also indicated that both pyridine N and pyrrolic N
doping can reduce the energy barrier for acetone desorption by the interaction between N and
hydroxyl-H of alcohol. Furthermore, MnO@C-N showed good recyclability for at least five
reaction cycles. We anticipate that these results can drive progress in the manganese catalyzed
transfer hydrogenation reaction.
1. Introduction
Emerging energy crises and environmental problems have promoted worldwide attention to
develop sustainable biomass resources to the diminishing fossil fuels[1-3]. 5-
hydroxymethylfurfural (HMF) is widely considered as an important platform intermediate for
transforming lignocellulosic biomass into high-value-added chemicals and biofuels[4-6].
Particularly, selective hydrogenation of HMF to 2, 5-dihydroxymethylfuran (DHMF) has potential
industrial applications in the synthesis of resins, foams, polymers, drugs, artificial fibers and
biodiesel[7-10].
To date, considerable catalytic systems for the selective hydrogenation of HMF are focused on
transition metals (e.g., Ru[11, 12], Pt[13-15], Pd[16, 17], Ir[18], Ni[11], Cu[19, 20], Co[21, 22])
2

using H2 as the hydrogen source. Compared to the H2-hydrogenation reaction, catalytic transfer
hydrogenation (CTH) represents an attractive, safer and more convenient alternative for the
hydrogenation of aldehydes, especially targeting large-scale applications. For example, no
autoclaves and external H2 are required using alcohols as both solvent and hydrogen donors. So
far, various precious metal catalysts (e.g., Au[23], Ru[24-27]) have shown excellent results for
the CTH of biomass-derived aldehydes. However, the main drawback of these noble catalysts is
its high cost. Meantime, relatively cheap rare metal catalysts (e.g., Hf[25, 26] and Zr[27-35]) have
been developed as outstanding catalysts for the CTH of HMF and other aldehydes. Despite the
great achievement, further developing inexpensive and widely abundant first-row base metals
(such as Fe, Cu, Co, Mn) is expected[28]. Recently, unremitting efforts toward the design and
synthesis of earth-abundant metal catalysts (e.g., Fe[29-31], Co[32-34], Cu[35, 36]) are going,
which encourage us to explore a novel and effective base metal catalyst for CTH of aldehydes.
Manganese, as the third most abundant transition metal in the Earth’s crust, is a potential
candidate to replace rare or precious metals if comparable catalytic activities have resulted.
Manganese catalyzed transfer hydrogenation of aldehydes is introduced by the Beller’s group in
2016 using manganese pincer complexes[37]. Further seminal work was published by the groups
of Sortais using commercially available and cheap 2-picolylamine as a ligand[38]. Later, Qi group
reported a manganese-based carbon aerogel for the CTH of HMF[39]. However, despite these
achievements, homogeneous Mn catalysts resulted in recycled issues[37, 38], strong base
employment [37-39] and low selectivity[39] are remained in this manganese catalyzed CTH
reaction. Thus, to develop highly selective Mn-based catalysts with base free and further
understand the reaction mechanism is highly desired to Mn catalyzed transfer hydrogenation of
aldehydes.
3

In this work, we demonstrate that N-doped carbon supporting MnO nanoparticles (MnO@C-N)
are highly active and selective for the CTH of biomass-derived HMF and other aldehydes to
alcohols with base free. The catalyst fabrication process is simple and sustainable. Furthermore,
the catalyst characterization and catalytic performance were investigated in detail, and the reaction
mechanism was further revealed by deuterium isotope tracing experiments and DFT calculations.
To the best of our knowledge, there have been no previous reports on the use of MnO based
catalysts for transfer hydrogenation reaction. We anticipate that these results can drive progress in
the Manganese catalyzed transfer hydrogenation reaction.
2 Experimental section
2.1 Materials
All reagents were supplied by Aladdin Chemical Technology Co. Ltd. (Shanghai, China) and used
without further purification.
2.2 Synthesis of MnO@C-N
In a typical synthesis, Manganese(Ⅱ)acetate tetrahydrate, sucrose and urea in a mass ratio of
1:1:10 were dissolved in a 300mL 70% ethanol solution. The solution was distilled in 85 oC until
the solvent was completely removed. The as-obtained sol-gel mixture was dried at 100 oC for 5h.
o
Subsequently, the mixture was placed in a tube furnace and heated to 300 C for 1h, and then
pyrolyzed at 700 °C for 2h with a heating rate of 5 ºC/min under N2 atmosphere.
2.3 Synthesis of MnO@C
In a typical synthesis, 1g manganese carbonate powder was mixed with sucrose (2.5 g, 5g, 10g
or 15g) for MnO@C (20 nm), MnO@C (15 nm), MnO@C (8 nm) or MnO@C (5 nm) respectively.
The resulting solid powder was heated at 600 °C for 2h under an N2 atmosphere.
2.4 Catalyst characterization
4

Powder X-ray diffraction patterns of the samples were recorded on a Rigaku D/max X-ray
diffractometer using Cu Kα radiation (40 kV, 30 mA, λ = 0.1543 nm). Raman spectra were
measured on a Renishaw inVia Raman microscope with 633 nm laser excitation. The laser power
was 10 mW, and the spot size was 2 m. BET surface areas and pore sizes measurements were
performed with N2 adsorption/desorption isotherms using a TriStar 3000 with the BET and BJH
methods. CO2/NH3 temperature-programmed desorption (CO2/NH3-TPD) was measured on a
Micromeritics AutoChem II 2920, equipped with a thermal conductivity detector (TCD). Metal
contents of the samples were determined using Inductively Coupled Plasma Optic Emission
Spectrometer (ICP-OES, Varian (720)). The size and morphology of materials were studied by
scanning electron microscopy (SEM, SUPRA 55) high-resolution transmission electron
microscopy (HR-TEM, JEM-2100). The X-ray photoelectron spectroscopy (XPS) measurements
were performed on a Thermo ESCALAB 250XI electron spectrometer with a monochromatic Al
Kα radiation (hv=1486.8 eV) and the spot size of all spectra was 650μm. NMR studies were tested
by Agilent 400M.
2.5 Simulation methods
We used a 5×6 supercell graphite (001) to simulate the graphite monolayer. One C atom was
deleted and another one was replaced by N atoms in the graphite(001) monolayer to simulate
graphite-pyridinic-N monolayer. To simulate the MnODots-granite-pyridinic-N composites,
MnODots(Mn14O14) were stacked on the granite-pyridinic-N surface slab. The height of the
supercell is set as 20A˚ to ensure negligible interaction between adjacent slabs. In this work, the
density functional theory (DFT) calculation was performed by the Cambridge serial total energy
package (CASTEP)[40] code, in which a plane wave basis set was used. The exchange and
correlation interactions were modeled using the generalized gradient approximation (GGA) and
5

the Perdew-Burke-Ernzerhof (PBE)[41] functional. Grimme’s semi-empirical DFT-D[42] was
introduced in the computations to guarantee a better description of the electron interaction in a
long-range. The Vanderbilt ultrasoft pseudopotential[43]was used with a cutoff energy of 370 eV.
Geometric convergence tolerances were set for the maximum force of 0.03 eV/A˚, maximum
energy change of 10-5 eV/atom, the maximum displacement of 0.001 A˚ and maximum stress of
0.5 GPa. Density mixing electronic minimisation was implemented and the self-consistent field
(SCF) tolerance was set to ‘‘fine’’ with high accuracy of 10-6 eV/atom for energy convergence.
Other simulation was studied as the identical method as MnO@C-N (pyridinic N).
2.6 CTH of HMF
In a typical run，0.26g of HMF, 0.1g of catalyst and 20mL of isopropanol were added in a
stainless steel reactor equipped with a magnetic stirrer and heated to target temperature within 50
min. The reaction time was started to record as soon as the system reached working temperature.
After completion of the reaction, the catalyst was separated and a sample of the liquid mixture was
subjected to GC analysis. The conversion of 5-HMF and the yield of products were calculated
according to the following equations:
Moles of HMF
HMF conversion = 1 -
× 100%
× 100%
(
)
)
Moles of HMF loaded
Moles of product
Product yield = 1 -
(
Moles of HMF converted
3. Results and discussion
3.1 Catalyst preparation and characterization
6

To ensure the MnO@C-N materials could be suitable for large-scale production, the catalysts
were prepared via direct carbonization (700oC, N2) of a sol-gel mixture using cheap, available and
sustainable sucrose, urea and manganese acetate as precursors as illustrated in Figure 1a.
Figure 1. (a) Schematic illustration of the synthesis of MnO@C-N. (b-c) TEM images of
MnO@C-N; part of MnO NPs are marked by red arrows. (d) The size distribution of MnO. (e)
interplanar spacing of MnO (f-j) STEM image and the corresponding mapping of O, C, N and Mn
for the catalyst MnO@C-N.
7

The morphology and microstructure were investigated by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). As observed in Figure S1a (Supporting
Information), MnO@C-N (700) shows a clear two-dimensional (2D) sheet structure, which is
caused by the formation of the sacrificial template of graphitic carbon nitride (g-C3N4) when urea
is heated over 550oC[44, 45]. Figure 1b and Figure S1b-f (Supporting Information) show no big
nanoparticles. Figure 1c-1d reveal that ultrasmall MnO nanoparticles around 2–3 nm are
homogeneously dispersed on C-N composites. Focusing on the enlarged view, clear crystalline
interplanar spacings of MnO (111, 0.26 nm) can be observed (Figure 1e). Scanning transmission
electron microscopy (STEM) image and its elemental mapping further confirm a highly uniform
distribution of C, N, O and Mn on the surface of MnO@C–N materials (Figure 1f-j).
8

Figure 2. (a) XRD patterns, (b) Raman spectra, (c) BET analysis and pore-size distribution, (d)
C 1s, (e) N 1s, (f) and Mn 2p of MnO@C-N.
The crystal structure and phase information of as-synthesized samples were confirmed by X-
ray diffraction (XRD). As shown in figure 2a, the diffraction peak at around 35o, 41o, 59o, 70o and
73o can be attributed to cubic phase MnO (PDF#89-4835). While a diffraction hump between 22o
and 28o corresponds to N-doped carbon (Figure S2, Supporting Information). Raman spectra were
conducted to evaluate the graphitic quality of the carbon shells, as shown in figure 2b. There are
two typical characteristic D band (~1350 cm-1) and G bands ( ~1572 cm-1), related to the
disordered/defect carbon and graphitic sp2-carbon, respectively[46, 47]. In addition, visible Mn-O
vibrational bands at 647 cm-1 are also observed, suggesting the presence of manganese oxides[48,
49]. The pore-size distribution of as-prepared catalysts was also studied by N2 adsorption-
desorption analysis (Figure 2c). The specific surface area of MnO@C-N is 30.5 m2g-1.
Additionally, the broad pore size distributions of 0 to 50 nm are obtained (inset in Figure 2c),
revealing their micro- and mesoporous features. Both the appropriate specific surface area and
porous characteristics are necessary for effective active sites accessibility and fast substrates and
intermediates penetration.
The chemical composition and valence state were investigated by X-ray photoelectron
spectroscopy (XPS). The XPS survey spectrum showed the presence of C, N, O, and Mn elements
in MnO@C-N (Figure S3a, Supporting Information). The O 1s of three samples are shown in
Figure S3b (Supporting Information), and Mn-O and C=O are observed at the peak of 531.2 and
529.8 eV, respectively[50, 51]. The high-resolution C 1s spectrum of MnO@C-N can be
deconvolved into three bands, corresponding to C=C at ~284.8 eV, C-N at ~285.8 eV and C-O at
~288.7 eV (Figure 2d)[52, 53]. The N 1s peaks of all Mn@C-N materials show two deconvoluted
9

peaks at ∼398.28eV and ∼399.98 eV (Figure 2e), corresponding to pyridinic N and pyrrolic N,
respectively[54]. The pyridinic-N replaces a carbon atom in C6 ring and bonds with two sp2 carbon
atoms; the pyrrolic N is in a five-membered ring and donates two electrons to the π system. It is
well known that the pyridinic N and pyrrolic N, especially the pyridinic N can act as basic sites,
which is beneficial to the proton transfer in CTH. In addition, the lone pair of electrons of N can
change the local geometric and electronic structure of the carbon support, and thus preferentially
nucleate the MnO NPs at N-occupied sites[55, 56]. The presence of several concentrated pyridinic
N is the prominent peak in MnO@C-N. As shown in figure 2f, the high-resolution Mn 2p spectrum
of MnO@C-N(700) exhibits two prominent bands at 641.5 eV (2p1/2) and 653.4 eV (2p3/2) with an
energy difference of 11.9 eV, readily assigned to Mn2+.[51] Meanwhile, a weak satellite peak is
also presented at 646.5 eV (Figure 2f), which is another proof for the formation of MnO[51].
3.2 Catalytic performance of CTH of HMF to DHMF
To evaluate the catalytic activity of the as-synthesized MnO@C-N catalysts for the CTH of
HMF, various alcohols are first investigated as H-donor/solvent (Figure 3a). Among the five
applied alcohols, only isopropanol exhibits high reaction efficiency. These results indicate that the
satisfying reactivity of MnO@C-N is largely dependent on the hydrogen donating ability of
alcohols, which is secondary alcohol (isopropanol) > primary alcohol (methanol, ethanol, n-
propanol and n-butanol). Figure 3b shows the influence of reaction temperature and time on the
conversion of HMF to DHMF catalyzed by MnO@C-N. Generally, the DHMF yield increases
with the increase of reaction time, and the HMF conversion is increased with temperature. A slight
decrease of DHMF yield is observed when prolonging reaction time after 15h at 180 °C, owing to
the over hydrogenation products such as 5-methylfurfural and 5-methylfurfuryl alcohol (detected
10

by GC-MS) are formed. Whilst, the maximum DHMF yield of 93% is achieved at 170oC with 21h.
The reaction rates can be effectively improved by increasing catalyst concentration without
significant drops in selectivity (Figure 3c, 3d). At the same time, a slight decrease in TOF value is
observed with the increase of Mn dosages (Figure 3d).
Figure 3. (a) CHT of HMF to DHMF by MnO@C-N using various alcohols, condition: 0.26g
HMF, 0.1g catalyst, 20mL isopropanol, 170oC, 21h. C refers to conversion, while Y refers to yield.
11

(b, c) The effect of reaction temperature, reaction time, and dosage of catalyst on catalytic
performance. (d) TOF values and reaction rate in various catalyst dosages.
3.3 Catalyst Recyclability
The heterogeneous nature of the MnO@C-N was checked by removing catalysts from the
solution after 9 h of reaction, and heating the filtrate for another 12 h (Figure 4a). ICP-OES analysis
of the reaction solution confirmed that the content of Mn was below the detection limit, suggesting
no obvious Mn leaching during the reaction. The stability and recyclability of the MnO@C-N
catalyst in the CTH of HMF to DHMF were further checked. As included in figure 4b, the catalyst
could be reused at least five times without visible loss of activity or selectivity. XRD analyses and
TEM images show that the crystal structure of MnO@C-N and highly dispersed MnO-NP remains
almost the same before and after the reaction (Figure S4, S5, Supporting Information), confirming
the excellent stability and recyclability of the material under the investigated conditions.
12

Figure 4. (a) leaching experiment of MnO@C-N; (b) Catalyst recycling studies for MnO@C-N.
Condition: 0.26g HMF, 0.1g catalyst, 20mL isopropanol, 170oC, 21h.
3.4 CTH of other aldehydes
The excellent performance of the MnO@C-N catalyst was also investigated for the CTH of C=O
bonds of a broad scope of substrates. As shown in Table 1, biomass-derived furanic aldehydes
such as furfural and 5-methylfurfural, which are also important platform chemicals, are
transformed smoothly into the corresponding alcohols with >95% selectivities at high conversions.
With aromatic aldehydes and pyridine aldehydes as substrates, the desired products are also
obtained in satisfying conversion and selectivity. Notably, the CTH of biomass -derivative ethyl
levulinate can achieve 99% selectivity to γ-valerolactone. However, only 43% conversion of ethyl
levulinate is obtained even at higher reaction temperature and prolonged duration, which can be
attributed to the steric hindrance and electron-donating nature of the alkyl group in ketone.
13

Table 1 CTH of various unsaturated aldehydes over MnO@C-N
Tep.
(K)
Time
Con.
(%)
Sel.
(%)
Entry
Substrate
Product
(h)
12
9
O
O
OH
O
1
2
3
4
5
6
7
443
443
443
443
443
443
473
100
99
99
99
99
98
98
96
99
OH
O
OH
12
21
21
12
30
98
OH
97
O
OH
97
OH
N
100
43
O
O
Condition: 0.26g HMF, 0.1g catalyst, 20 mL isopropanol.
3.5 Identifying the reaction mechanism
It was commonly known that three are two main mechanisms for the CTH reaction, namely,
direct hydrogen transfer and metal-mediated hydrogenation. However, the mechanisms of
manganese catalyzed transfer hydrogenation reaction is not clear up to now. In this case,
mechanistic insight into the CTH of aldehydes to alcohols was investigated in 2-PrOH (d8) by ex-
situ NMR studies at various time. Furfural was chosen as a substrate due to its relatively simple
structure, and the obtained 1H NMR spectra were shown in Figure 5a. The proton of aldehyde (-
CHO, 1a) groups are gradually disappeared with increasing the deuterium (D) addition from
isotopic iPrOH-d8, accordingly resulting in the formation of methylene (-CH2-, 1b) groups.
However, the reaction mechanism between the direct hydrogen transfer and metal-mediated
hydrogenation remains unclear by replacing 2-propanol-d0 with 2-propanol-d8. In order to
differentiate these two possible pathways, t-butanol was introduced as co-solvent with 2-propanol-
d8 because: (1) t-butanol cannot be used as a hydrogen donor due to the absence of β-H; and (2) it
14

is able to exchange D in the OD group of 2-propanol-d8 or on the surface of the metal with H[27].
Accordingly, the desired alcohols should be produced with a 1 amu MS shift (MS+1) by direct
hydrogen transfer (Figure 5b), or no mass shift (MS+0) by metal-mediated hydrogenation (Figure
S6, Supporting Information). As expected, GC-MS analysis clearly shows a 1 amu mass shift of
the parent ion (figure 5c), suggesting that the CTH of aldehydes to alcohols over MnO@C-N was
via a route by direct hydrogen transfer.
Figure 5. (a) Reaction kinetics study on CTH of furfural in isotopic iPrOH-d8 using 1H NMR.
(b) mechanism of direct hydrogen transfer (c) Mass spectra of furfural alcohol.
15

Figure 6. The proposed mechanism for CTH of aldehydes to the corresponding alcohols by
MnO@C-N;
According to the above study, a typical direct hydrogen transfer mechanism was proposed in
Figure 6a. Firstly, the aldehyde and alcohol groups are attracted by MnO NPs via the interaction
between the lone pair electrons of oxygen and the strong electron-deficient MnO center to form a
six-membered-ring intermediate. At the same time, the hydroxyl proton is extracted by N specials.
Then, the α-H (blue H) is transferred from the α-C of the alcohol to the aldehyde carbon via a six-
membered-ring intermediate, and hydroxyl-H is transferred to the aldehyde-O via an N-H
intermediate.
3.6 Identifying the dominating factor for high selectivity and conversion
To figure out the key role of the various component in the MnO@C-N catalyst, various catalysts
were first investigated for the CTH of HMF, as shown in Figure 7a. In the absence of any catalyst,
almost no HMF conversion is observed. No DHMF selectivities are also observed when using
16

activated carbon and C-N components (Figure S7, Supporting Information) as catalysts, while
high selectivities up to 95% are achieved when MnO@C and MnO@C-N are applied. These
controlled experiments demonstrated a crucial role of MnO for the high selectivity of DHMF.
Previous reports indicate that both nanoparticles' size and N-doping have a significant influence
on catalytic activity[32, 57]. To check the effect MnO size and N-doping, a series of MnO with
different size supported on carbon were first prepared (Figure S8, Supporting Information). As
shown in figure 7b, The HMF conversion has a slight increase with the MnO size decreasing from
20 nm to 8 nm, while no obvious changes were observed when MnO size reduced to 5 nm. It can
be expected that this conversion will not exceed 30%, even smaller size of MnO was used.
However, with N-doping, high HMF conversion up to 98% is achieved, indicating that N-doping
may play a dominating role for such high conversion.`
Figure 7. (a) CTH of HMF to DHMF by various catalysts; (b) the effect of MnO size on catalytic
activity. Condition: 0.26g HMF, 0.1g catalyst, 20mL isopropanol, 170oC, 21h.
17

3.7 Revealing the key role of N-doping
The kinetic reaction, acid-base properties, and DFT calculation were studied to better
understand the critical role of N-doping in the MnO@C-N catalyst. For comparison, the
corresponding subjects for MnO@C with MnO size of 5nm (excluding the effect of MnO size)
were also investigated. The time-dependent conversion catalyzed by MnO@C and MnO@C-N is
shown in Figure 8a, MnO@C-N exhibits satisfying transformation and activity compared to
MnO@C. Because of the high DHMF selectivity and ability in the investigated conditions, pseudo-
first-order kinetics with respect to HMF can be used as the equation of ln c/c0 =-kt, where c is the
concentration of HMF after a reaction time of t, c0 is the initial concentration of HMF, and k is the
rate constant. As shown in figure 8b, the plots of ln c/c0 versus t are satisfactorily linear,
demonstrating the first-order reaction kinetics regarding the concentration of HMF. MnO@C-N
displays a much higher rate constant (k) of 0.13 h-1, 8.7-fold than that of MnO@C, 0.015h-1.
According to the Arrhenius equation of ln k = ln A −Ea/RT, the apparent activation energy (Ea) of
the HMF conversion in the MnO@C-N(700)/isopropanol system is about 91 kJ/mol (Figure 8c),
which is much lower than that of 131 kJ/mol for MnO@C catalyst. The result indicates that N
doping can substantially improve catalytic efficiency.
Figure 8. (a) The time-dependent conversion catalyzed by MnO@C and MnO@C-N, condition:
0.26g HMF, 0.1g catalyst, 20mL isopropanol, 170oC. (b) The kinetics profiles for the CHT of
18

HMF to DHMF (fitted by the first-order assumption). (c) Arrhenius plots for the CHT of HMF to
DHMF.
According to the XPS N1s spectra analysis, N specials are mainly existing in the form of
pyridinic N and pyrrolic N, which can provide basic sites or serve as electron donors due to its
lone pair of electrons. Thus, the proton transfer processes are accelerated at N sites. The N-doping
increase in basicity was also well demonstrated by the CO2 temperature-programmed desorption
o
(TPD) analysis (figure 9a). The MnO@C-N had a higher CO2 desorption peak at ~ 280 C and
higher Lewis base strength and density than MnO@C (Figure 9c). In addition, it had been
previously reported that N-doping carbon materials had a stronger electron coupling effect that
facilitated electron transfer from metal/metal oxide NPs to N-doped carbon sheets[58-60], which
can strengthen both basic and acidic sites. As expected, the acidity of MnO@C-N is clearly
increased, as indicated by the stronger NH3 desorption peaks (Figure 9b) and more acidic sites
(Figure 9c) compared to MnO@C.
Figure 9. (a) NH3-TPD and (b) CO2-TPD spectrum. (c) Amount of acidic sites and basic sites
19

Based on the proposed reaction mechanism in Figure 6, density functional theory (DFT)
calculation was performed to further understand how N-doping determines the activity of MnO
for the CTH of HMF to DHMF (Figure 10a-c, Figure S9-S14). As shown in calculated energy
profile and the transition state for the CTH on the active site (Figure 10b), the adsorption energy
of isopropanol on MnO@C-N (Pyridinic N) is −1.67eV, lower than that of MnO@C (−1.52 eV)
because of an effective N-H bonding interaction as indicated by the close N−H bond distance (2.6
Å). Both MnO@C and MnO@C-N (Pyridinic N) are able to absorb and stabilize isopropanol and
HMF by forming a six-membered-ring intermediate (step 1 and step 2), and the reaction is
drastically exothermic, with calculated reaction energy of -3.22 eV and -2.51 eV, respectively. The
distance (2.2 Å) between hydroxyl-H (isopropanol) and N in MnO@C-N (Pyridinic N) is in the
range of a typical hydrogen bond. The subsequent release of acetone and formation of DHMF on
MnO@C is thermodynamically unfavorable by 1.85 eV and 1.34 (step 3 and step 4), respectively.
As expected, the barrier in the presence of MnO@C-N (Pyridinic N) is notably reduced to 1.23 eV
and 1.28 eV (step 3 and step 4), respectively, which can explain the experimentally observed high
activity of MnO supported by pyridinic-nitrogen-doped carbon. Note that the pyridinic N-doping
has a slight promoting effect on step 4, resulted by the weak interaction between N and hydroxyl-H
of BHMF (4.3 Å). As shown in figure 10c, both of pyridinic N and pyrrolic N doping could reduce
the energy barriers, especially for the step 3, and the lower barrier is obtained for MnO@C-
N(pyridinic N), suggesting that N-doping improves the catalytic efficiency of CTH by reducing
the energy barrier for acetone desorption.
20

Figure 10. (a)Simulative atomistic structures of MnO, MnO@C, MnO@C-N(pyrrolic N), and
MnO@C-N(pyridinic N). (e) Energy barriers of the CTH of HMF to DHMF by MnO@C,
MnO@C-N(pyrrolic N), and MnO@C-N(pyridinic N).
4. Conclusion
In conclusion, cheap and sustainable MnO@C-N catalysts have been successfully prepared.
Benefiting from the N-doping, ultrasmall MnO NPs with diameters of around 2.5 nm are generated
and well dispersed on carbon supports. The as-prepared MnO@C-N catalysts show high selectivity
and activity for the CTH of biomass-derived HMF and other aldehydes to the corresponding
alcohols via direct hydrogen transfer. Compared to previous Mn-catalyzed CTH of aldehydes, the
catalyst used in this work has advantages of higher selectivity and base-free. Experimental studies
and DFT calculations demonstrate that the presence of MnO ensures the high selectivity of
alcohols, and pyridine N-doping promotes the high conversion of aldehydes. Furthermore, the
simple and scalable method for catalyst preparation and CTH reaction, giving a process that could
be commercially viable.
21

Acknowledgment
The authors gratefully acknowledge the financial support from the National Natural Science
Foundation of China (No. 21978248), the Natural Science Foundation of Fujian Province of China
(No.2019J06005). We also thank the New Zealand Ministry of Business, Innovation and
Employment (MBIE) for support through the Catalyst Fund (16-UOA-049-CSG).
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
No conflict of interest exits
25

Graphical Abstract
26

Highlights
1. MnO@C-N catalysts have been prepared, characterized and used for CTH for the first time.
2. The role of MnO and N-doping are demonstrated to be responsible for the high selectivity and
high conversion, respectively.
3. N-doping can strengthen both basic and acidic sites and reduce the energy barrier
27