Catalytic conversion of glucose into alkanediols over nickel-based catalysts: A mechanism study

Article abstract of DOI:10.1039/c6ra14738k

The conversion of isotope-labeled glucose (d-1-¹³C-glucose) into alkanediols was carried out in a batch reactor over a Ni-MgO-ZnO catalyst to reveal the C-C cleavage mechanisms. The unique role of the MgO-ZnO support was highlighted by ¹³C NMR and GC-MS analysis qualitatively and the MgO-ZnO favored isomerization of glucose to fructose. ¹³C NMR, GC-MS and HPLC analysis demonstrated that the C1 position of ethylene glycol, the C1 and C3 positions of 1,2-propanediol and the C1 position of glycerin were labeled with ¹³C, which is attributed to a C-C cleavage at d-1-¹³C-glucose's corresponding positions through retro-aldol condensation. A hydrogenolysis followed by hydrogenation pathway was proposed for glucose converted into alkanediols at 493 K with 6.0 MPa of H₂ pressure over Ni based catalysts.

Full text of DOI:10.1039/c6ra14738k

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Catalytic conversion of glucose into alkanediols
over nickel-based catalysts: a mechanism study†
Cite this: RSC Adv., 2016, 6, 62747
a
Zhichao Tan,ab Gai Miao,a Chang Liu,a Hu Luo,a Liwei Bao,ab Lingzhao Kong
*
ac
*
and Yuhan Sun
The conversion of isotope-labeled glucose (D-1-13C-glucose) into alkanediols was carried out in a batch
reactor over a Ni–MgO–ZnO catalyst to reveal the C–C cleavage mechanisms. The unique role of the
MgO–ZnO support was highlighted by 13C NMR and GC-MS analysis qualitatively and the MgO–ZnO
favored isomerization of glucose to fructose. 13C NMR, GC-MS and HPLC analysis demonstrated that the
C1 position of ethylene glycol, the C1 and C3 positions of 1,2-propanediol and the C1 position of
Received 7th June 2016
Accepted 21st June 2016
glycerin were labeled with 13C, which is attributed to
a
C–C cleavage at D-1-13C-glucose's
corresponding positions through retro-aldol condensation. A hydrogenolysis followed by hydrogenation
pathway was proposed for glucose converted into alkanediols at 493 K with 6.0 MPa of H2 pressure over
Ni based catalysts.
DOI: 10.1039/c6ra14738k
www.rsc.org/advances
process. Among the available literature, Zhang Tao,4–7 Liu,8 Mu9
and Miao10 et al. have indicated that a retro-aldol reaction is the
Introduction
main pathway during the decomposition of glucose to diols. A
one-pot conversion of cellulose to EG on a Ni-promoted tung-
sten carbide catalyst was conducted by Zhang et al. in 2008.4 A
series of tungsten based catalysts were developed to enhance
the EG yield to 75%. When using miscanthus, a complex
biomass, it was found that a base solvent pretreatment can
alleviate catalyst poisoning in the process.7 Liu8 deemed that
WO3 crystallites are efficient at accelerating the hydrolysis of
cellulose to sugar intermediates and selective cleavage of C–C
bonds in the sugar occurs with Ru/C catalysts. Considering the
high cost of tungsten and Ru catalysts, Mu9 et al. reported the
hydrogenation of cellulose to 1,2-alkanediols over Ni-supported
catalysts and the total yield of alkanediols reached up to 70.4%.
Without any biomass pretreatment, Miao10 et al. achieved
a direct conversion of microalgae into 1,2-PDO and EG in water
over a Ni–MgO–ZnO catalyst and the total yield of polyols was up
to 41.5%. Researchers4–10 have generally believed that there are
several possible pathways for the catalysis of glucose to alka-
nediols. One of the pathways is from glucose to glycolaldehyde
or from fructose, which occurred from isomerization of glucose,
to 1,3-dihydroxyacetone via a retro-aldol reaction, followed by
the production of EG and 1,2-PDO by hydrogenation. On the
other hand, the glucose was transformed into levoglucosan
through dehydration,11 and then the levoglucosan underwent
cleavage of the C–C bonds and was converted into a precursor of
acetol. As formation of sorbitol was detected in the reaction
solution, the production of diols from the cleavage of C–C
bonds aer a hydrogenation process was proved.
The utilization of fossil fuel resources for transportation and as
a contributor to the world's economy is unquestionable, but
they also contribute to serious environmental problems. A
global energy shortage and environmental pollution problems
push us to nd renewable energy sources for the production of
liquid biofuels or value-added chemicals.1 As a promising large-
scale substitution for fossil fuel resources, chemicals and fuels
from biomass have other advantages, such as extensive raw
material sources, zero-CO2 emission and renewability.2 Among
various conversion processes of biomass, catalytic hydrogena-
tion to 1,2-propylene glycol (1,2-PDO) and ethylene glycol (EG)
has attracted increasing attention due to their important role in
the synthesis of value-added compounds which have a large
market demand.3 1,2-PDO is used as a chemical feedstock for
the preparation of unsaturated polyester resins and EG is
primarily used as a raw material in the manufacture of polyester
bers and antifreeze formulations.
At present, for the conversion of sugars or biomass to diols,
the current research has been focused on looking for more
efficient and low-cost catalysts and improving the conversion
and selectivity of the reactions, but less attention has been paid
to the fundamental understanding and mechanism of the
aCAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR
China. E-mail: konglz@sari.ac.cn; sunyh@sari.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
cSchool of Physical Science and Technology, Shanghai-Tech University, Shanghai
201210, PR China
So, catalytic conversion of sugars and sugar alcohols into
polyols has been investigated in detail to reveal the C–C
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra14738k
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cleavage mechanisms.12 The retro-aldol reaction mechanisms at the same temperature for 3–4 h. Aer vacuum ltration and
proposed by Sohounloue13 and Andrews14 indicate that C–C calcination at 672 K for 4 h, the synthesized catalyst was
cleavage of the b-hydroxyl carbonyl could generate a ketone and reduced at 673 K for 5 h under a H2 ow of 60 mL minꢀ1. For
an aldehyde, which is followed by hydrogenation to the alco- the MgO–ZnO catalyst, an aqueous solution of Na2CO3 was
hols. A retro-Claisen reaction mechanism has been proposed by added dropwise to a solution of magnesium nitrate and zinc
Montassier et al.15 to explain the formation of formic acid and nitrate under vigorous stirring at 343 K, and then the result-
CO2. They have also come up with a retro-Michael reaction ing material underwent calcination aer being aged under
mechanism to explain the formation of xylitol and sorbitol, the same conditions. The BET surface area of Ni–MgO–ZnO
where the bond cleavage precursor is a d-dicarbonyl.15 However, was 39.8 m2 gꢀ1, and the calculated particle size of the nickel
Hawley16 considered that the C–C cleavage occurs through on MgO–ZnO using the Scherrer equation was 7.32 nm. The
a retro-aldol reaction, because a monocarbonyl, the precursor of Ni dispersion on MgO–ZnO obtained from a difference
the retro-aldol reaction, is more likely formed than a dicarbonyl, calculation through CO-chemisorption characterization was
the precursor of the retro-Claisen and retro-Michael reactions. 1.25%.10
They used 1,3-diols as model compounds to provide further
evidence against the retro-Claisen being the mechanism
pattern. Kabyemela17–21 proved that the decomposition products
Catalytic experiments and product analysis
of glucose or fructose were similar in subcritical and super- The hydrogenation of glucose was carried out in a stainless
critical water, except that fructose epimerization to glucose was steel autoclave (Parr Instrument Company, 50 mL) with an
negligibly low. The intermediate products, glyceraldehyde, initial H2 pressure of 6 MPa at 493 K for 15–180 min. Typically,
dihydroxyacetone, dihydroxyacetone, pyruvaldehyde, glyceral- a mixture of glucose, the catalyst and deionized water was
dehyde and pyruvaldehyde, were formed from C–C bond placed into the autoclave and stirred at a speed of 600 rpm.
cleavage. The mechanism could be explained using a reverse Aer the reaction, the autoclave was cooled using cold water,
aldol condensation and the double-bond rule of the respective and the liquid solution was separated from the solid mixture
enediols formed during the Lobry de Bruyn Alberda van Eken- by centrifugation.
stein transformation.
The collected liquid solution was ltered through 0.22 mm-
To obtain the specic pathway of C–C bond breakage pore-size lters prior to analysis. The main products in
through reverse aldol condensation from glucose, more the resultant solution were identied based on standard
effective methods should be used. Herein, we investigate the compounds and the structures of them were further conrmed
conversion of glucose and D-1-13C-glucose using a MgO–ZnO using GC-MS (Agilent 7890 GC and Agilent 5975C inert MSD,
support and Ni–MgO–ZnO catalyst, since it displayed excel- HP-INNOWax column: 30 m ꢁ 0.25 mm; lm thickness,
lent catalytic activity for microalgae hydrogenolysis in our 0.25 mm). These products were also quantied using HPLC
previous study.10 The reaction mechanism was investigated (Shimadzu LC-20AD, Aminex HPX-87H Ion Exclusion Column:
through 13C nuclear magnetic resonance (NMR) analysis 300 mm ꢁ 7.8 mm) with a differential refraction detector
using isotopically labeled glucose at the C1 position (D-1-13C- (RID-10A). The targeted polyols (EG and 1,2-PDO) were deter-
glucose) and this was assisted with GC-MS and HPLC analysis. mined quantitatively using HPLC, based on calibration curves
Then, the C–C cleavage pathway and the formation of EG and of the standard compounds.
1,2-PDO were elucidated using the 13C NMR results and the
The conversion of glucose was calculated by measuring the
model compound experiments. Furthermore, another moles of glucose before and aer the reaction: conversion ¼
proposed pathway was put forward using the product distri- (moles of glucose le in the reaction solution)/(moles of initial
butions for the glucose conversion over MgO–ZnO and glucose) ꢁ 100%. The yield of polyols was calculated based on
Ni–MgO–ZnO.
the amount of carbon via the equation: yield ¼ (moles of carbon
in the products)/(moles of carbon in the initial glucose) ꢁ
100%. According to the GC analysis for the gas products, the
main component in the gas phase was H2 and trace CO, CO2
and CH4 were detected, so the yields of the gas products were
not quantied in this work.
Materials and methods
Materials
D-1-13C-Glucose and deuterium oxide were purchased from
Cambridge Isotope Laboratories, Inc. The other chemicals were
purchased from Sinopharm Chemical Reagent Co., Ltd. All were
used without further purication.
13C NMR experiments were performed using an Agilent 500
(125 MHz) spectrometer with a mixture of the sample in D2O.
Because the natural abundance of 13C is 1.1%, the 13C NMR
signal resonances of 13C atoms are considerably higher than for
12C atoms in the reaction product when using 1-13C-glucose as
the substrate, with the same analytical conditions of scanning
Catalyst preparation and characterization
The Ni–MgO–ZnO catalyst was prepared using a co-precipi- time, solvent and concentration (an equal mass of the reactants
tation method10 with an aqueous solution of Na2CO3 added underwent a catalyzed hydrogenation reaction, then the liquid
dropwise to a solution of nickel nitrate, magnesium nitrate phase of the product was evaporated using a vacuum-rotary
and zinc nitrate under vigorous stirring at 343 K. The evaporation procedure to remove the water. Eventually the
resulting material was then aged, under continuous stirring experiments were conducted in D2O).
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Results and discussion
D-1-13C-Glucose conversion over the MgO–ZnO support
On the basis of the retro-aldol condensation reaction put
forward before, we supposed that glucose would generate the
precursor of the products via this mechanism. In the blank test
shown in Fig. 1, hydrothermal conversion of 1-13C-glucose
without a catalyst, the resonance signals of 1-13C-b-D-pyrano-
fructose and 1-13C-b-D-fructofuranose at d 63.8 and d 62.6
(Fig. S1 to S3†) were very low, which is ascribed to low conver-
sion of glucose to fructose without MgO–ZnO. A series of
experiments was designed to investigate the function of the
MgO–ZnO support at temperatures of 373 K, 423 K and 493 K.
The isomerization mechanism of glucose to fructose was
conrmed from 13C NMR using isotopically labeled glucose at
the C1 position (D-1-13C-glucose) as the substrate. The GC-MS
analysis results are shown in Table 1.
On the basis of the 13C NMR data in Fig. 2b, the resonance
signals from d 60.5 ppm to d 101.4 ppm represent the chemical
shis of a glucopyranose and fructofuranose mixture (Fig. S1
and S3†).22 In Fig. 2a, d 95.8 and d 92.1 are the b-C1 and a-C1
shis of glucopyranose, respectively (Fig. S2†). The resonance
signals at d 63.8 and d 62.6 represent 1-13C-b-D-pyranofructose
and 1-13C-b-D-fructofuranose (Fig. S3†), indicating isomeriza-
tion of glucose to fructose. As the intermediate products for the
hydrogenation of glucose to EG and 1,2-PDO, the detected
Fig. 2 13C NMR results of 1-13C-glucose (a) and glucose (b) conver-
sions with MgO–ZnO at 373 K. Reaction conditions: 373 K, 6 MPa of
H2, 600 rpm. Reaction time: 3 h.
glycolaldehyde and acetol indicate that the reaction can proceed
at 373 K.
When the glucose and the C1 isotopically labeled glucose
react at 423 K, the sugars convert gradually. At 493 K, 1,2-PDO,
acetol and acetic acid are the main products as seen in Table 1.
The resonance signals at d 66.5, 67.8 and 17.9 ppm are the C1,
C2 and C3 position signals of 1,2-PDO, and d 67.5, 212 and
24.8 ppm are the C1, C2 and C3 position chemical shi values
of acetol (Fig. 4). The results show the formation of acetol,
which occurs aer the isomerization of glucose to b-D-pyrano-
fructose; pyranofructose could convert into acetol, catalyzed by
MgO–ZnO, via a retro-aldol condensation and cleavage of the
C3–C4 position. MgO–ZnO cannot catalyze the hydrogenation
Fig. 1 13C NMR results of 1-13C-glucose without a catalyst at 493 K
(blank test). Reaction conditions: 493 K, 6 MPa of H2, 600 rpm.
Reaction time: 3 h.
Table 1 GC-MS analysis results of glucose conversion with MgO–ZnO at 373 K, 423 K and 493 K
373 K
423 K
493 K
Component
Area/%
Component
Area/%
Component
Area/%
Acetol
Glycolaldehyde
1,2-PDO
1,3-Dihydroxyacetone
Glycerol
Others
28.3
25.3
8.6
6.6
4.7
Acetol
60.7
20.7
7.1
1,2-PDO
Acetol
Acetic acid
Ethanol
Pentanal
Others
25.0
24.2
11.8
10.2
6.9
Acetic acid
Formic acid
Others
11.5
26.5
17.8
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D-1-13C-Glucose hydrogenation over the Ni–MgO–ZnO catalyst
The hydrothermal hydrogenation of glucose to polyols over the
Ni–MgO–ZnO catalyst was investigated at 493 K with 6 MPa of
H2. According to the blank test (Fig. 1), glucose cannot convert
into diols under the no-catalyst conditions at 493 K. Fig. 5
illustrates the 13C NMR analysis of the 1-13C-glucose and
glucose conversion products from reaction over Ni–MgO–ZnO
at 493 K. The results indicated that both the C1 and C3 position
of 1,2-PDO are 13C labeled, owing to the high intensity signals at
17.9 and 66.5 ppm (Fig. S4†). The C1 position signals of EG and
glycerin occur at 62.46 and 62.42 ppm respectively (Fig. S5 and
S6†), which are present as the highest peaks in Fig. 5a. For the
peaks of C2 for 1,2-PDO and glycerin, the resonance signals at
67.86 ppm and 71.99 ppm are substantial in Fig. 5b but insig-
nicant in Fig. 5a, meaning that these positions in the liquid
product are not 13C labeled. It can be explained that glucose
undergoes a retro-aldol condensation reaction generating gly-
colaldehyde (GA, the precursor of EG) so that the C1 position of
EG is 13C labeled. Meanwhile, aer the isomerization of glucose
to fructose, fructose generates 1,3-dihydroxyacetone and a glyc-
erin precursor via a retro-aldol condensation reaction, leading
to the corresponding products 1,2-PDO and glycerin. Conse-
quently, the C1 and C3 positions of 1,2-PDO and the C1 of
glycerin are 13C labeled.
Fig. 3 13C NMR results of a 1-13C-glucose conversion with MgO–ZnO
at 423 K. Reaction conditions: 423 K, 6 MPa of H2, 600 rpm. Reaction
time: 3 h.
reaction of acetol completely, so the 1,2-PDO in the liquid phase
may be produced from hydrogen transfer reactions. The new
resonance signal at d 19.7 ppm displayed in Fig. 3 corresponds
to 2-13C-acetic acid,23 meanwhile the GC-MS analysis (Table 1)
further conrmed this result, and it is probably generated from
glycolaldehyde.17,18,20
From the intermediate products of the glucose conversion to
alkanediols with MgO–ZnO, glucose can convert into glyco-
laldehyde, catalyzed by MgO–ZnO, via a retro-aldol condensa-
tion and cleavage of the C2–C3 position. Through analyzing the
experimental results (Fig. 2 to 4 and Table 1) and comparing
them to similar processes, a reaction pathway for the conver-
sion of glucose over the MgO–ZnO support was proposed and is
summarized in Scheme 3.
The unique role of MgO–ZnO was proved through relevant
experiments, which stimulated the isomerization of glucose to
fructose. The reason is probably that the ZnO had both acidic
and basic sites on its surface,9 and the basicity of ZnO would
promote the isomerization of glucose to fructose.
Fig. 4 13C NMR results of a 1-13C-glucose conversion with MgO–ZnO Fig.
5
13C NMR analysis of 1-13C-glucose (a) and glucose (b)
at 493 K. Reaction conditions: 493 K, 6 MPa of H2, 600 rpm. Reaction conversion with Ni–MgO–ZnO at 493 K. Reaction conditions: 493 K, 6
time: 3 h.
MPa of H2, 600 rpm. Reaction time: 3 h.
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Fig. 6 Main product distribution for a glucose conversion with Ni–
MgO–ZnO at 493 K.
Scheme 1 Proposed pathway for the conversion of dihydroxyacetone
over Ni–MgO–ZnO.
The intermediate compounds dihydroxyacetone and glyco-
laldehyde were selected as the model compounds to investigate
the pathway to the diols. As shown in Fig. 7, the main products
were 1,2-PDO and glycerin for the hydrogenation process, and
the side products were lactic acid, EG and acetic acid. This
implied that there were no further aldol condensation reactions
taking place (Scheme 1). At 493 K, the conversion of glyco-
laldehyde, catalyzed by Ni–MgO–ZnO, was 100%. According to
the GC-MS and HPLC qualitative analysis, the main compo-
nents were glucose, erythritol, levoglucosan, glycerol, 1,2-PDO
and 1,2-butanediol (1,2-BDO) and the yields for each compo-
nent are shown in Fig. 7a.
On using glycolaldehyde as the reactant, erythritol and
1,2-BDO were found and this proved that two glycolaldehyde
molecules could condense and hydrogenate. The generation of
glucose and levoglucosan shows that the four carbon alcohols
and GA take part in another aldol condensation reaction. At the
same time, the presence of a certain amount of 1,2-PDO and
glycerin conrmed that the formed glucose could decompose
via isomerization and retro-aldol condensation.
Besides, it was also found that a certain amount of 1,2-BDO
with 13C labels is generated in the reaction solution, from which
it is assumed that two glycolaldehyde molecules formed
1,2-BDO through an aldol condensation reaction. Glucose
undergoes a retro-aldol condensation reaction generating EG
and erythritol. Then, erythritol undergoes another retro-aldol
condensation reaction generating two molecules of EG, which
are not 13C labeled. As is well known, the aldol condensation
reaction is reversible, therefore a small amount of EG will
produce 1,2-BDO via an aldol condensation reaction.
The experimental results veried that 1,2-BDO could be
generated from an aldol condensation reaction of glycolaldehyde,
and that glucose and levoglucosan could be formed by two
condensation reactions of glycolaldehyde (shown in Scheme 2).
Using Fig. 5b, a set of resonance signals at d 65.3, 73.3, 25.3
and 9.1 ppm were assigned to the chemical shis of 1,2-BDO
(Fig. S7†). In Fig. 5a, the new signal at d 65.2 ppm reveals the C1
position of 1,2-BDO which is 13C labeled. It is to be noted that
the amount of 1,2-BDO was low in the liquid products as shown
in Fig. 5b and 6. This phenomenon is accounted for by another
retro-aldol condensation reaction and aldol condensation
reaction. Firstly, glucose generates glycolaldehyde and trihy-
droxy-butyraldehyde via a retro-aldol condensation reaction.
The trihydroxy-butyraldehyde then generates two glyco-
laldehyde molecules by retro-aldol condensation. A 1-13C-gly-
colaldehyde and a glycolaldehyde probably react to form 1,2-
BDO via aldol condensation or they proceed through further
hydrogenation processes. Because a large proportion of the
glycolaldehyde is hydrogenated to EG, the content of 1,2-BDO
is much less than that of EG, 1,2-PDO and glycerin, as shown in
Fig. 5b and 6.
Fig. 7 Main product distributions for glycolaldehyde (a) and dihy-
droxyacetone (b) conversions with Ni–MgO–ZnO at 493 K. Reaction
conditions: 493 K, 6 MPa of H2, 600 rpm, 3 h.
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Scheme 3 Proposed pathways for the conversion of glucose over the
MgO–ZnO support and Ni–MgO–ZnO catalyst.
As shown in Fig. 8, the glucose can be efficiently converted
even in a short reaction time, and the glucose conversion was up
to 97.8% aer 0.25 h. Prolonging the reaction time to 1 h,
almost no glucose was detected in the reaction solution, and its
conversion reached up to 99.7%. Aer 1.5 h, glucose could not
be detected.
It can be seen from Fig. 8 that glucose is very easily trans-
formed into four to six carbon sugar alcohols, because the sugar
alcohol content increased obviously within 0.25 h. As the
hydrogenation progressed, the sugar alcohol content gradually
reduced to a negligible amount at the end of the reaction.
1,2-PDO, EG and glycerin are the main products and the yields
reach 31.6%, 13.8% and 17.2% respectively. This is well illus-
trated in Fig. 8. However, when the reaction time is 0.25 h, the
main components are sorbitol, xylitol and erythritol, which are
generated from glucose. This means that in the presence of the
nickel metal, glucose could be hydrogenated into sorbitol
directly at the beginning. However, as the reaction progressed,
large amounts of 1,2-PDO, EG and glycerin, the products from
conversion of sorbitol24 and xylitol, were obtained.25–27 Sorbitol
undergoes dehydrogenation to glucose, furthermore generating
EG and 1,2-PDO, and xylitol undergoes dehydrogenation to the
corresponding aldehyde, which then produces glycolaldehyde
and dihydroxyacetone via a retro aldol condensation reaction.25
Conversion experiments for sorbitol and xylitol were performed
over the Ni–ZnO–MgO catalyst under the same conditions, and
the diols' yields were similar to those from glucose. Sorbitol and
xylitol are the intermediates when catalytically converting
a portion of glucose into alkanediols. Based on the above
results, it can be concluded that the catalytic process of con-
verting glucose into alkanediols is hydrogenolysis followed by
hydrogenation pathways (Scheme 3).
Scheme 2 Proposed pathway for the conversion of glycolaldehyde
over Ni–MgO–ZnO.
Characterization of the pathways for glucose conversion over
Ni–Mgo–ZnO catalysts
From the above results, it can be seen that glucose is converted
to diols via retro-aldol condensations, where glycolaldehyde
and dihydroxyacetone act as the main intermediates. To
investigate the product distribution in relation to reaction time
with the Ni–MgO–ZnO catalyst, a series of experiments was
conducted using reaction times of 0.25 h, 0.5 h, 1 h, 1.5 h and 5
h at 493 K with 6 MPa H2 and the results are shown in Fig. 8.
Conclusions
In summary, the mechanism of glucose conversion to alka-
nediols over a Ni–MgO–ZnO catalyst was investigated using
isotope-labeled glucose as the feedstock. Through chemical
shi value assignments and major component analysis, the
investigation provided evidence for the C–C cleavage positions
of the retro-aldol condensation in such a process, and the C1
Fig. 8 Product distributions for glucose conversion over the Ni–
MgO–ZnO catalyst. (-) Sorbitol, (C) xylitol, (:) erythritol, (;) glyc-
erol, (=) EG, (<) 1,2-PDO, (A) 1,2-BDO, and (+) glucose conversion.
Reaction conditions: 493 K, 6 MPa of H2, 600 rpm.
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Acknowledgements
Authors acknowledge nancial support provided by the
National Natural Science Foundation of China (21406255) and
the Youth Innovation Promotion Association CAS. The authors
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