Deactivation behavior and aggregation mechanism of supported Au nanoparticles in the oxidation of monoethanolamine to glycine

Article abstract of DOI:10.1016/j.catcom.2020.106127

Deactivation behaviors of gold catalysts in the liquid-phase oxidation of monoethanolamine to glycine were revealed. Properties of the catalysts including Au loading, the valence distribution, and particle size of Au nanoparticles before and after batch reactions were investigated by BET, ICP, XRD, XPS, and TEM. It was found that the main reason for the catalytic deactivation was aggregation of Au nanoparticles. The Au aggregation mechanism of oxidative dissolution-reductive deposition in the reaction was proposed and this mechanism could help to understand the common aggregation of Au nanoparticles in the liquid-phase oxidation of alcohols.

Full text of DOI:10.1016/j.catcom.2020.106127

CatalysisCommunications146(2020)106127
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Deactivation behavior and aggregation mechanism of supported Au
nanoparticles in the oxidation of monoethanolamine to glycine
Xiangzhan Menga,c, Zengxi Lia, Yongqiang Zhanga,b, Ruiyi Yanb,⁎, Hui Wangb,⁎
a School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
b Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing 100190, PR China
c China National Aviation Fuel Supply Co., Ltd, China National Aviation Fuel Group Corporation, Beijing 100088, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Deactivation behavior
Au nanoparticles
Aggregation mechanism
Oxidation of monoethanolamine
Deactivation behaviors of gold catalysts in the liquid-phase oxidation of monoethanolamine to glycine were
revealed. Properties of the catalysts including Au loading, the valence distribution, and particle size of Au na-
noparticles before and after batch reactions were investigated by BET, ICP, XRD, XPS, and TEM. It was found
that the main reason for the catalytic deactivation was aggregation of Au nanoparticles. The Au aggregation
mechanism of oxidative dissolution-reductive deposition in the reaction was proposed and this mechanism could
help to understand the common aggregation of Au nanoparticles in the liquid-phase oxidation of alcohols.
1. Introduction
catalytic behaviors of gold catalysts by comparing the liquid-phase
oxidation of amino alcohols (serinol and MEA) and the corresponding
Oxidation of alcohols to aldehydes or carboxylic acids is a funda-
mental and important functional transformation in chemical industry.
The selective oxidation of compounds with both amino and hydroxyl
groups represents an alternative and sustainable approach to producing
amino acids besides the amination of biomass-derived α-hydroxyl acids,
without the use of fatal toxic reactants (e.g., cyanide or halo acids) or
complex separation process (e.g. separation of the amino acid from
chloric byproducts in amination of halo acids and from other different
amino acids in hydrolysis and fermentation processes) [1–11]. Various
non-catalytic (using acidified potassium dichromate or potassium per-
manganate as the oxidant) and catalytic (mainly copper and gold-based
catalysts) systems have been developed for this process [2–8,12].
Among them, gold-based catalysts are superior to others in view of high
product selectivity and mild reaction conditions. For instance, the di-
rect conversion of chitosan to glucosaminic acid by tandem hydrolysis
and oxidation was achieved over supported Au catalysts, and in the
selective oxidation of monoethanolamine (MEA), the glycine yield
could reach 95% at 323 K over Au/ZrO2 [5,6,11]. However, in these
reaction systems, the gold-based catalysts are easy to deactivate as in-
dicated by the gradually unchanged conversion when the reaction
proceeded for certain time, even it was proved to have better resistance
to deactivation compared with palladium or platinum nanoparticles
[3–5].
polyols (glycerol and ethylene glycol). It was found that the catalyst
deactivation was far more evident in the case of amino alcohols than
that in the oxidation of glycerol or ethylene glycol. Gaiassi et al. [3]
explored the effect of nitrogen substitution of MEA on the product se-
lectivity and stability of the supported gold catalysts. They demon-
strated that the deactivation was more influenced by the reactant
structure than by experimental parameters and aliphatic amines which
could form strong interactions with noble metals aggravated the cata-
lytic deactivation. The aggregation of Au particles in MEA oxidation
after being used for three cycles was also observed in our previous
study, which should be an important reason for the decrease of catalytic
activity [5].
Although the influences of amino groups and the growth of particle
size on the stability of gold catalysts in the liquid-phase oxidation of
amino alcohols were emphasized, rare report focuses on deactivation
behaviors of the catalysts, and the possible relationship between the
amino group and the aggregation of Au nanoparticles in the reaction
remains unclear. In this study, the deactivation behaviors of gold cat-
alysts in the liquid-phase oxidation of amino alcohols, i.e. MEA, was
investigated using different catalysts. The Au loading, electronic and
structural change of the catalysts before and after batch reactions were
investigated by BET, ICP, XRD, XPS, and TEM. The aggregation me-
chanism of Au nanoparticles was proposed.
Villa et al. [4] investigated the effect of amino groups on the
⁎ Corresponding authors.
E-mail addresses: ryyan@ipe.ac.cn (R. Yan), huiwang@ipe.ac.cn (H. Wang).
https://doi.org/10.1016/j.catcom.2020.106127
Received 18 March 2020; Received in revised form 9 July 2020; Accepted 28 July 2020
Availableonline31July2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

X. Meng, et al.
CatalysisCommunications146(2020)106127
2. Experimental
Au/CMK-3
Au/ZrO2-623
100
80
60
40
20
2.1. Materials
Au/ZrO2-823
All chemicals (AR grade) were purchased commercially and used as
received unless otherwise noted. HAuCl4·4H2O (99.99%) and AgNO3
(≥99.8%) were supplied by Sinopharm Chemicals (Beijing, China). The
commercial ZrO2 support was purchased from Shanghai Macklin
Biochemical Co. Ltd. (Shanghai, China). Mesoporous carbon CMK-3 was
homemade as described in our previous work [13]. MEA (> 99.0%),
NaOH (> 99.0%), and acetaldehyde (99.5%) were from Xilong Scien-
tific Co., Ltd. (Guangdong, China). O2 (99.9%) was supplied by Beijing
Beiwen Gas Factory (Beijing, China). Maleic acid (internal standard for
quantitative NMR analysis) and all other standard samples were pur-
chased from Sigma-Aldrich Trading Co., Ltd. (Shanghai, China).
1
2
3
Run
2.2. Catalyst preparation
Fig. 1. Reusability of Au/ZrO2–623 (MEA/Au
=
300/1 (mol/mol)), Au/
ZrO2–823 (MEA/Au = 300/1(mol/mol)), and Au/CMK-3 (MEA/Au = 500/1
(mol/mol)) in MEA oxidation. Reaction conditions: CMEA = 0.3 M, NaOH/
MEA = 4, T = 323 K, P(O2) = 0.5 MPa, stirring speed = 1000 r/min, average
catalyst diameter = 106 μm, and t = 2 h.
The Au/ZrO2 catalysts were prepared by deposition-precipitation
method as described before [6], and the obtained catalysts calcined at
623 K and 823 K were labelled as Au/ZrO2–623 and Au/ZrO2–823,
respectively. The Au/CMK-3 catalyst was prepared using a deposition-
reduction method as described in our previous study [13].
The active component loading is of vital importance for a catalytic
reaction and metal leaching cannot be excluded in the solvent medium
[5,6]. Au loadings of the fresh and recycled catalysts were investigated
through chemical analysis by ICP. The result showed that the Au con-
tent of the as-prepared Au/ZrO2–623 was 0.8 wt%, and that of the
catalyst after three runs was the same. That is, no obvious metal
leaching was observed during the reaction, and thus Au leaching was
not the reason for deactivation.
2.3. Evaluation of the catalyst and the reusability
Oxidation of MEA was used to evaluate the catalytic activity of the
catalysts, and the reaction was conducted according to the described
procedures reported in literature [5,6]. After reaction, the solid catalyst
was separated from the liquid by filtration, washed with deionized
water and dried at 323 K. Then the catalytic activity of the recycled
catalyst was evaluated following the same procedure as that for the
fresh one.
The surface elements and their corresponding contents of the fresh
and recycled Au/ZrO2–623 were then analyzed, as shown in Table 1. N
species were present in the recycled catalyst after three runs because of
the possible adhesion of the reactant or products on the catalyst surface,
consistent with the reported strong interactions between the amino
group and noble metals [1,3]. Comparing the element contents of the
recycled catalyst with those of the fresh one, no obvious change was
found for the surface contents of Zr and Cl, while the surface content of
the active Au element decreased remarkably from 0.37% to 0.09%.
Considering that no metal leaching occurred during the reaction as
proved before, the low surface content of Au may result from the de-
crease of Au dispersion [6]. EDS mapping was also performed to study
element compositions and distributions of Au/ZrO2–623 catalyst before
reaction and after being used for three runs. As shown in Figs. S1 and
S2, ESI, Au and Zr were the dominant metallic elements for both fresh
and recycled catalysts. For Au/ZrO2–623 after being used for three
runs, the line analysis of the elements shown in Fig. S2b confirmed the
presence of larger Au particles. The Au 4f XPS spectra of fresh and
recycled Au/ZrO2–623 were collected, as shown in Fig. S3, ESI. The
valence distribution of Au species was determined by subjecting the Au
4f peaks to deconvolution, and no obvious difference of the proportion
of metallic gold species was found before and after reaction (89% vs.
91%), indicating that catalytic deactivation was not caused by variation
3. Results and discussion
It is widely accepted that preparation method, support, and the
particle size play key roles on the properties of Au catalysts, which
determine the catalytic performance including stability of the catalyst.
Therefore, Au catalysts with different Au particle sizes or supports, i.e.,
Au/ZrO2–623, Au/ZrO2–823 and Au/CMK-3, were prepared for the
aqueous oxidation of MEA, and their properties were characterized. As
shown in Table S1, ESI, the Au loadings of both the Au/ZrO2–623 and
Au/ZrO2–823 catalysts were 0.8 wt%. The calcination temperature has
negligible influence on the specific surface area (SBET) of the catalysts
despite the significant difference of the Au particle sizes (4.7 nm for Au/
ZrO2–623 vs. 8.1 nm for Au/ZrO2–823). Mesoporous carbon CMK-3
supported Au catalyst has a little bit higher Au loading of 0.9 wt% with
smaller Au particle size of 3.7 nm and much larger SBET of 1164 m2/g.
The large difference in properties of the catalysts, including the Au
particle size, type and structure of the supports, made it possible to
reveal the catalytic stability of different Au catalysts in the liquid-phase
oxidation of alcohols.
The liquid-phase oxidation of MEA was selected as a model reaction
to test the stability of Au catalysts with different preparation methods,
Au particle sizes, and supports. The reusability of the catalysts is shown
in Fig. 1. In the oxidation of MEA, all the three catalysts showed ob-
vious loss of activity during the reaction process. MEA conversion over
Au/ZrO2–623 obviously decreased from 99% (fresh catalyst) to 17% in
the third run, and that over Au/ZrO2–823 decreased from 68% to 17%.
Mesoporous carbon CMK-3 supported gold catalyst showed slower de-
crease in MEA conversion from 60% to 27% due to the porous structure
of CMK-3 and partial particles inside the pores [13,14]. The evident
catalytic deactivation for all the tested gold catalysts indicated that
deactivation was common for catalysts with various Au particle sizes
and supports.
Table 1
Surface elements and their contents of fresh and recycled Au/ZrO2–623 after
three runs.
Au/
ZrO2–623
Ausurf (%) Zrsurf (%) Osurf (%) Csurf (%) Nsurf (%) Clsurf (%)
Fresh
Recycled
after
0.37
0.09
20.8
19.6
50.0
56.76
16.3
11.0
0
0.66
12.4
11.9
three
runs
2

X. Meng, et al.
CatalysisCommunications146(2020)106127
Fresh
After 1 run
After 3 runs
Fresh
After 1 run
After 3 runs
a)
b)
p
p
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
Size (nm)
Size (nm)
Fig. 2. Change of the Au particle size distributions of fresh and recycled Au/ZrO2–623 (a) and Au/ZrO2–823 (b).
of the Au electronic state.
A
B
C
D
E
F
The Au particle size of fresh and recycled Au/ZrO2 was further in-
vestigated by XRD and TEM. As shown in Fig. S4, ESI, compared with
the fresh catalysts, both recycled Au/ZrO2–623 and Au/ZrO2–823
showed increased intensity of the characteristic peak of Au (111) at 2θ
of 38.2° (JCPDS-PDF: 89–3697), indicating increase of crystal size of Au
and decrease of dispersion. The average Au particle size was examined
by TEM, and the representative micrographs in Fig. 2 and Fig. S5
showed the change of Au particle size before and after reuse. With the
increase of recycling times, Au particle size gradually increased for both
Au/ZrO2–623 and Au/ZrO2–823. For Au/ZrO2–623, the Au particle size
increased from 4.7 ( 0.9) nm before reaction to 18.7 ( 5.0) nm
after three runs, and that for Au/ZrO2–823 increased from 8.1 ( 1.9)
nm to 18.6 ( 5.9) nm. The Au sintering and decrease of Au dispersion
were the main reasons for the continuous decrease of catalytic activity.
The sintering processes of gold nanoparticles can be mainly classi-
fied into two categories: particle migration and coalescence (PMC) as
well as atomic ripening (AR) [15]. PMC is usually related to tempera-
ture-induced increased mobility and the lower melting point of small
particles [16], while AR includes the migration of species containing
metallic atoms and usually occurs for compounds that can form com-
plexes containing the metallic atom [17]. In the liquid-phase oxidation
at low temperature (e.g., 323 K), the AR process was considered to be
the main pathway for the growth of Au nanoparticles. To test this hy-
pothesis, different solutions, containing one or more compounds ex-
isting in the solution of MEA oxidation, were used to treat the Au/
ZrO2–623 catalyst under reaction conditions unless otherwise noted.
The typical TEM images and the corresponding Au particle size dis-
tributions of the catalysts after various treatment processes as well as
those of the catalyst after the 1st run are shown in Fig. 3 and Fig. S6.
Compared with the control group (fresh Au/ZrO2–623 treated with
1.2 M aqueous solution of NaOH under 0.5 MPa of O2), the addition of
MEA (O2 was replaced by N2 to avoid the occurrence of reaction) or
acetaldehyde (representing the possible 2-aminoacetaldehyde inter-
mediate, which was commercially unavailable) in the solution didn't
obviously change the distribution of Au particle size. However, the
glycine solution with the presence of O2 and 1.2 M of NaOH slightly
increased the Au particle size distribution from 4.7 ( 0.9) nm to 5.0
2
4
6
8
10
12
14
16
Size (nm)
Fig. 3. Au particle size distributions for Au/ZrO2–623 catalysts after treatments
with different solutions: a) control, 0.5 MPa of O2, 1.2 M aqueous solution of
NaOH, b) 0.5 MPa of N2, aqueous solution containing 0.3 M of MEA and 1.2 M
of NaOH, c) 0.5 MPa of O2, aqueous solution containing 0.3 M of acetaldehyde
and 1.2 M of NaOH, d) 0.5 MPa of O2, aqueous solution containing 0.3 M of
glycine and 1.2 M of NaOH, e) 0.5 MPa of O2, aqueous solution containing
0.3 M of acetaldehyde, 0.3 M of glycine, and 1.2 M of NaOH, f) after the 1st run.
It was reported that amino acids can be used as alternative leaching
agents for cyanide in gold extraction from minerals due to strong
complex formation between gold and amino acids by coordinating
through the carbonyl oxygen and nitrogen [18]. The stability constant
of the complex between gold and glycine could reach 18 [19]. In al-
kaline solution with the presence of glycine and O2, gold dissolution
can be expressed as the following reaction equation [19]:
4Au + 8NH2CH2COOH + 4NaOH + O2
= 4Na[Au(NH2CH2COO)2] + 6H2O
(1)
The formation of gold (I)-amino acid complex can not only increase
the solubility of metal but also decrease the redox potentials, thus
promoting the oxidative dissolution [20,21]. In order to confirm the
oxidative dissolution of Au, the concentrations of dissolved Au after
Au/ZrO2–623 was treated with different solutions were detected by
ICP. As shown in Table S2, ESI, in aqueous solutions without glycine
(containing 1.2 M of NaOH (control), 0.3 M of MEA and 1.2 M of NaOH,
or 0.3 M of acetaldehyde and 1.2 M of NaOH, respectively), ultratrace
of Au (concentration < 0.2 ppm) was detected, corresponding to neg-
ligible change of the average Au particle size and size distributions
(Fig. 3, A-C). While in solutions containing glycine (i.e., 0.3 M of glycine
(
1.3) nm. Obvious increase of Au particle size was observed for the
sample treated with the solution of mixed acetaldehyde and glycine in
the presence of O2 and 1.2 M of NaOH, and the average Au particle size
increased to 6.3 ( 1.6) nm, approaching that of Au/ZrO2–623 after
the 1st run (7.6 ( 2.9) nm). These phenomena illustrated that glycine
in the reaction solution under O2 can change the distribution of Au
particle size, and there existed synergistic effect between glycine and
acetaldehyde to promote the growth of the supported Au nanoparticles.
3

X. Meng, et al.
CatalysisCommunications146(2020)106127
and 1.2 M of NaOH; 0.3 M of glycine, 0.3 M of aldehyde and 1.2 M of
NaOH; or the reaction solution), the concentration of Au was higher
than 3 ppm, and both the average Au particle size and size distributions
increased (Fig. 3, D-F), indicating that gold dissolution and Au sintering
were closely related to the presence of glycine. These phenomena de-
monstrated the oxidative dissolution process.
The gold (I)-amino acid complex obtained in the oxidative dis-
solution process contained oxidized gold, which was instable and easy
to be reduced to Au0, especially in the presence of the reductive alde-
hyde intermediate (i.e. 2-aminoacetaldehyde) in MEA oxidation. The
reduction of oxidized gold by 2-aminoacetaldehyde is shown as follows:
ZrO2–623 with smaller Au particles showed faster deactivation beha-
vior than Au/ZrO2–823. Similar phenomenon was also observed in the
liquid-phase oxidation of alcohols including glucose, glycerol, ethanol
and 5-hydroxymethyl furfural [24,25]. For example, in the oxidation of
glucose to gluconic acid, the conversion over Au/CeO2 was reduced
from 70% in the 1st run to 4% in the 4th run as the average Au particle
size increased from 2 nm to 7.1 nm [24]. In the aqueous ethanol oxi-
dation over Au/CuFe2O4, the conversion was reduced slowly from 63%
in the 1st run to 52% in the 5th run as the average Au particle size
increased from 3.7 nm to 4.5 nm [25]. The formed carboxyl acids, in-
cluding acetic acid and gluconic acid, can also work as ligands to form
gold (I)-carboxylate complexes by AueO bonds [26,27], promoting the
oxidative dissolution as glycine does in Eq. (1). However, the stability
constant of gold (I)-amino acid and dissolving capacity of gold nano-
particles were higher [21], resulting in higher aggregation rate of Au
nanoparticles in MEA oxidation.
The above controlled experiments and proposed aggregation me-
chanism of Au nanoparticles revealed that the co-existence of ligand
(amino acids), oxidant (oxygen), and reductant (aldehyde intermediate)
in the reaction system was the main reason for the aggregation of Au
nanoparticles. Considering that the aldehyde intermediate and amino
acids products were inevitable in the oxidation of amino alcohol to
amino acid and that the dehydrogenation of alcohols, including gly-
cerol and benzyl alcohol, can occur using supported metal catalysts in
the absence of oxygen [28,29], the oxygen-free dehydrogenation of
amino alcohols to amino acids was considered to be an alternative
method to reduce Au sintering, and corresponding progress has been
made in our laboratory.
2Na[Au(NH2CH2COO)2] + NH2CH2CHO + 3NaOH
For Au/ZrO2–623 treated with solutions containing glycine (Fig. 3,
D-F), the presence of aldehyde in the solution increased the average Au
particle size and size distributions, indicating that reductive aldehyde
played a role in aggregation of Au nanoparticles. Even the concentra-
tion of Au was the highest in the mixed solution of glycine and NaOH
(conditions “d”, Table S2), the aggregation was not obvious until the
addition of acetaldehyde (conditions “e”, Table S2), which made the
distribution of Au particle size close to that after the 1st run.
= 2Au + 5NH2CH2COONa + 2H2O
(2)
Based on the above experiments and analysis, the aggregation me-
chanism of oxidative dissolution-reductive deposition of Au nano-
particles was proposed, as shown in Fig. 4. In the process of oxidative
dissolution, a particle with a smaller radius and higher surface energy
was preferentially dissolved in the aqueous solution containing glycine
and NaOH with the presence of O2, forming gold (I)-amino acid com-
plex. Then the produced gold (I) complex was reduced by the aldehyde
intermediate and deposited on the surface of larger Au nanoparticles.
Thus, the larger particles grew at the expense of smaller ones, in a way
similar to Ostwald ripening [22].
In the liquid-phase oxidation of MEA, the faster deactivation of
ZrO2–623 than that of ZrO2–823 can be attributed to the difference in
Au particle sizes. On the one hand, smaller Au nanoparticles have larger
proportion of reactive low-coordinated atoms exposed and higher sur-
face energy, facilitating the oxidative dissolution step. On the other
hand, according to the well-accepted size effect of gold catalyst [23],
the disappearance of highly active particles less than 5 nm caused the
rapid decrease of catalytic activity for Au/ZrO2–623. Therefore, Au/
4. Conclusions
The deactivation behaviors of gold catalysts in the liquid-phase
oxidation of MEA to glycine were revealed over gold catalysts including
Au/ZrO2–623, Au/ZrO2–823, and Au/CMK-3, by measuring possible
leaching of Au, compositions of surface elements, change of the valence
distribution, and particle size of Au nanoparticles. No detectable
leaching of Au and change of valence distribution were detected after
Au/ZrO2–623 was recycled for three cycles through chemical analysis
by ICP and XPS. The obvious sintering of Au nanoparticles, confirmed
by XRD, XPS, and TEM, accounted for the severe catalytic deactivation.
By treating the Au/ZrO2–623 catalyst with different solutions, the main
factor inducing Au sintering was revealed and the aggregation me-
chanism of oxidative dissolution-reductive deposition in MEA oxidation
was proposed. This study reveals the main reason for the common ag-
gregation of Au nanoparticles in the liquid-phase oxidation of alcohols,
which could provide valuable information for possible strategies to
reduce Au sintering in such kind of reactions.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
This work was financially supported by Innovation Academy for
Green Manufacture, Chinese Academy of Sciences (No. IAGM-2019-
A12), National Natural Science Foundations of China (21676270,
21676277, 21878311), CAS Pioneer Hundred Talents Program, and K.
C. Wong Education Foundation (No. GJTD-2018-04).
Appendix A. Supplementary data
Fig. 4. Proposed aggregation mechanism of oxidative dissolution-reductive
deposition of Au nanoparticles in MEA oxidation.
Supplementary data to this article can be found online at https://
4

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CatalysisCommunications146(2020)106127
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