Atomically dispersed Rh on hydroxyapatite as an effective catalyst for tandem hydroaminomethylation of olefins

Article abstract of DOI:10.1016/j.mcat.2021.111671

Tandem hydroaminomethylation is an efficient and green route for one-pot synthesis of amines directly from olefins. Herein, heterogeneous hydroxyapatite (HAP) supported single-atom Rh catalyst was prepared and used for tandem hydroaminomethylation of olefins. Characterization techniques confirmed the atomic dispersion of Rh species on HAP. Up to 99% conversion of 1-hexene with high selectivity to the desired amines (93.2%) was obtained over 0.5Rh₁/HAP catalyst. Mechanism study demonstrated that the first hydroformylation step during the tandem catalytic process was rate-determining. Compared with the Rh nanoparticles on other oxide supports (Mg₃Al, MgO and Al₂O₃), the atomically dispersed Rh sites on HAP ensured the high hydroformylation activity, thereby guaranteed the outstanding catalytic performance for the total tandem process. Furthermore, various corresponding amines can be obtained with satisfactory yields over 0.5Rh₁/HAP catalyst from a wide scope of olefins or amines substrates.

Full text of DOI:10.1016/j.mcat.2021.111671

Molecular Catalysis 509 (2021) 111671
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Atomically dispersed Rh on hydroxyapatite as an effective catalyst for
tandem hydroaminomethylation of olefins
a,
b
a,
b
a
,
b
a,
b
a
a,c,*
Xiao Li , Liusha Li , Tingting Qin , Gong Gun , Tiejun Lin , Liangshu Zhong
a
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR
China
b
University of the Chinese Academy of Sciences, Beijing 100049, PR China
c
School of Physical Science and Technology, ShanghaiTech University, Shanghai 201203, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tandem hydroaminomethylation is an efficient and green route for one-pot synthesis of amines directly from
olefins. Herein, heterogeneous hydroxyapatite (HAP) supported single-atom Rh catalyst was prepared and used
for tandem hydroaminomethylation of olefins. Characterization techniques confirmed the atomic dispersion of
Rh species on HAP. Up to 99% conversion of 1-hexene with high selectivity to the desired amines (93.2%) was
Atomically dispersed Rh
Hydroxyapatite
Tandem hydroaminomethylation
Olefins
obtained over 0.5Rh
the tandem catalytic process was rate-determining. Compared with the Rh nanoparticles on other oxide supports
Mg Al, MgO and Al ), the atomically dispersed Rh sites on HAP ensured the high hydroformylation activity,
thereby guaranteed the outstanding catalytic performance for the total tandem process. Furthermore, various
1
/HAP catalyst. Mechanism study demonstrated that the first hydroformylation step during
(
3
2 3
O
corresponding amines can be obtained with satisfactory yields over 0.5Rh
olefins or amines substrates.
1
/HAP catalyst from a wide scope of
1
. Introduction
1949, tremendous efforts have been devoted to improve the catalytic
efficiency and extend the substrate scope by optimization of ligands in
the past decades [11,14–18]. In 1999, Eilbracht and coworkers found
that rhodium salts or combination of rhodium salts with triphenyl-
phosphine can be used as catalysts for hydroaminomethylation [19].
However, the regioselectivity was unsatisfactory and enormous diffi-
culty existed to obtain pure products due to the similar properties of the
linear and branched isomers. Later, Matthias Beller et al. developed a
highly effective catalytic system consisting of a cationic rhodium pre-
cursor with Xantphos as ligand and the corresponding linear products
were obtained with high regioselectivity when aliphatic olefins or sty-
rene and its derivatives were used as substrates [16]. Linear amines can
also be successfully prepared with high selectivity directly from the in-
ternal olefins or olefin mixtures through one-pot reaction consisting of
initial olefin isomerization followed with hydroaminomethylation [15].
Despite homogeneous catalytic systems usually exhibited superior ac-
tivity and selectivity in the hydroaminomethylation and related re-
actions, the separation of expensive noble metal and ligands from
reaction mixture led to high energy consumption in the industry [20,
21]. In this context, heterogeneous catalysts with the merit of easy
product/catalyst separation were potential choices and worthy of more
Amines are of great significance in the chemical and pharmaceutical
industry with annual production in multimillion-ton scale [1]. Amines
can be used as pharmaceutically and biologically active substances, dyes
and fine chemicals [2]. Traditionally, the main methods for the amines
synthesis include nucleophilic substitution reactions of alkyl halides,
hydrocyanation of alkenes followed by reduction and reductive amina-
tion of carbonyl compounds (Scheme 1) [3–8]. However, the traditional
processes always suffer from some serious drawbacks, such as usage of
expensive or toxic feedstock and emission of environmentally harmful
byproducts [9]. With ever-increasing environmental concern, amounts
of efforts have been attributed to the development of efficient and green
routes for the amines synthesis [10]. One of the elegant ways to produce
amines directly from largely available olefins is tandem hydro-
aminomethylation, which is an atom-efficiently and environmentally
friendly process (Scheme 1) [11]. In this tandem process, olefin is firstly
converted to aldehydes via hydroformylation, which can react in situ
with amine to form enamine by condensation reaction. Then the desired
amine can be obtained via further hydrogenation [12,13].
Since hydroaminomethylation was firstly discovered by Reppe in
*
Corresponding author.
E-mail address: zhongls@sari.ac.cn (L. Zhong).
https://doi.org/10.1016/j.mcat.2021.111671
Received 5 April 2021; Received in revised form 17 May 2021; Accepted 20 May 2021
Available online 4 June 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Scheme 1. Comparison of different routes for the synthesis of amines.
without further purification.
.2. Catalyst preparation
HAP support was firstly prepared through the coprecipitation
explorations [21–26]. However, to the best of our knowledge, the
application of heterogeneous catalysts for hydroaminomethylation re-
action was still scarce [9,27]. Ram S. Shukla et al. used rhodium com-
plex encapsulated into the functionalized hexagonal mesoporous silica
for hydroaminomethylation and obtained satisfactory catalytic perfor-
mance. However, the functional groups on silica support by amino-
propyl trimethoxysilane modification were easy to lose activity [9].
2
method according to the previous reports [39]. Typically, a solution of
calcium nitrate tetrahydrate (0.11 mol, 25.2 g) in 240 mL mixed solvent
2
+
Hydroxyapatites (HAP) which possess abundant Ca sites coordi-
(
ethanol/water = 1/1 in volume) and a solution of diammonium
nated by PO3
ꢀ
, have been widely used as biomaterials, adsorbents and
4
hydrogenphosphate (0.064 mol, 8.4 g) in 240 mL deionized water were
prepared, respectively. The pH of both solutions was adjusted to 10.5
using aqueous ammonia (28 wt%) before mixing. Then the diammonium
hydrogenphosphate solution was added dropwise into the calcium ni-
catalysts. Various transitional metals can be accommodated to the
apatite framework as the catalytic active centers for olefin epoxidation,
aerobic alcohol oxidation or hydrogenolysis etc. by the substitution of
2
+
Ca via cation exchange [28–32]. More importantly, the special surface
structure of HAP is favorable for the preparation of highly dispersed or
monomeric metal species [33–36]. For instance, Kazuya et al. revealed
that the Ru species on HAP catalyst existed as monomeric Ru cations
even with 17.1 wt% loading [31]. Zhang et al. proved that the Ni
atomically dispersed species on HAP were highly active and completely
trate tetrahydrate solution and the pH of the solution was maintained at
◦
1
0.5 during the process. After aging at 80 C for 2 h, the obtained white
◦
precipitate via filtration was firstly dried at 120 C overnight, and then
◦
calcined at 500 C for 3 h. In addition, other metal oxide supports
2 3 3
including MgO, Al O and Mg Al were prepared by coprecipitation
method based on the previous report [40].
◦
coke-resistant for high temperature dry reforming of methane (750 C)
HAP supported catalysts with different Rh loading were prepared by
ion exchange method. Typically, 1 g HAP was firstly dispersed into the
[
37]. On the other hand, heterogeneous single-atom Rh-based catalysts
have demonstrated outstanding catalytic performance in hydro-
formylation [21,23,26,38]. In this work, HAP was selected as support to
prepare atomically dispersed Rh catalyst for tandem hydro-
aminomethylation of olefins. A complete olefin conversion with a high
selectivity to the desired amines (93.2%) can be achieved over
4
3
00 mL deionized water and then the required amount of RhCl aqueous
solution was quickly added. After stirring at room temperature for 24 h,
the powders were obtained by the filtration. The samples were firstly
◦
◦
dried at 80 C overnight, then calcined at 350 C for 3 h. The samples
with different theoretical Rh loading were denoted as xRh/HAP (x =
0
.5Rh
performance were studied in detail. A wide substrate scope of olefins or
amines were explored for hydroaminomethylation over 0.5Rh /HAP
1
/HAP catalyst. The influences of reaction conditions on catalytic
0
.05, 0.1, 0.5 and 1 in wt%). For comparison, Rh supported on the other
supports, including MgO, Al
2 3 3
O and Mg Al, were prepared by tradi-
1
tional wet impregnation method.
catalyst. In addition, the catalyst stability and catalytic mechanism were
also investigated.
2
.3. Characterization
2
. Experimental section
X-ray diffraction (XRD) measurements were performed on a Rigaku
Ultima IV X-ray powder diffractometer using Cu K radiation with the
wavelength of 1.54056 Å at 40 kV and 40 mA. The XRD patterns were
α
2
.1. Materials and reagents
◦
◦
obtained in a scanning angle (2θ) ranging from 20 to 90 with a scan
RhCl
NO
•6H
diethylamine (99.5%), aniline (99.5%), and toluene (99.5%) were ob-
tained from Sinopharm Chemical Reagent Co. LTD. (NH HPO
98.5%), 1-hexene (99.5%), 1-octene (97%), 1-decene (95%), styrene
95%) and heptanal (97%) were bought from Shanghai Aladdin
3
2
•xH O
(35.0–42.0%), Ca(NO
3
)
2
•4H
2
O
(99.0%), Mg
◦
peed of 4 /min. The phase of samples was identified by comparing with
(
3
)
2
2
O (99.0%), Al(NO
3
)
3
•9H O (99.0%), ethanol (99.5%),
2
JCPDS standard files.
Nitrogen adsorption-desorption measurements were carried out on a
Micromeritics TriStar 3020 instrument to study the textural properties
4
)
2
4
(
(
of the catalysts. Approximate 50 mg samples were firstly degassed at
◦
2
00 C in vacuum for 6 h before measurements. Then the samples were
Biochemical Technology Co. LTD. All chemicals were used as received
◦
transferred to the analysis station at ꢀ 196 C. The specific surface area
2

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Table 1
a
Catalytic performance of various catalysts for hydroaminomethylation of 1-hexene .
b
e
Entry
Catalyst
Conversion(%)
Product selectivity (%)
L/B
c
Internal
olefins
Hexane
Others
Heptanal
Amines
d
d
L
B
L+B (enamine)
f
1
None
HAP
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
3
4
5
6
7
8
9
–
–
–
0.05Rh
1
/HAP
>99
>99
>99
>99
>99
>99
>99
>99
15.0
5.8
3.7
1.2
1.1
51.3
56.7
6.6
–
–
27.8
24.4
10.1
1.3
1.1
19.0
5.2
32.5
43.3
52.1
55.3
49.2
48.8
0.9
13.9
17.7
26.8
44.0
45.0
6.9
57.2
69.8
82.1
93.2
93.8
7.8(20.7)
36.4
48.3(12.6)
3.1
2.9
2.1
1.1
1.1
0.1
7.8
2.6
0.1Rh
0.2Rh
0.5Rh
1
1
1
/HAP
/HAP
/HAP
–
–
–
4.1
4.3
4.0
–
–
1Rh
0.5Rh/Mg
0.5Rh/MgO
0.5Rh/Al
1
/HAP
–
3
Al
1.2
1.7
–
–
–
32.2
35.0
4.2
1
0
2
O
3
13.3
a
/CO=3 (v/v), 120 ◦C, 12 h.
Reaction conditions: 0.1 g catalyst, 5 mmol 1-hexene, 6 mmol diethylamine, 5 mL toluene, 4 MPa H
2
b
c
The selectivity was determined by the GC analysis with dodecane as internal standard and the value in the parentheses was enamine selectivity.
Others referred to the other unmentioned byproducts.
d
e
L and B represented the linear products and branched products, respectively.
f
L/B was the ratio of linear to branched products. No catalyst was added during the reaction.
was calculated using the Brunauer-Emmett-Teller (BET) equation. The
total pore volume and average pore size were determined by the Barrett-
Joyner-Halenda (BJH) method.
adsorption. Subsequently, in situ DRIFTS spectra were collected during
the sweeping process with Ar (20 mL/min).
For transmission electron microscopy (TEM) measurements, the
samples were analyzed on a JEOL-JEM 2011 electron microscope with
200 kV accelerating voltage. High angle annular dark field-scanning
transmission electron microscopy (HAADF-STEM) was performed on
JEOL ARM300F equipped with double aberration correctors and a cold
field emission gun. The samples were prepared by dispersing the catalyst
powder in ethanol followed by ultrasonication. One droplet of the sus-
pension was dropped onto carbon-coated copper grids for
measurements.
The loading of Rh elements was measured on the inductively coupled
plasma optical emission spectrometer (ICP-OES, Optima 8000, Perkins
Elmer) after the catalysts were dissolved according to the standard in-
house procedures.
X-ray photoelectron spectroscopy (XPS) measurements for the cata-
lysts were carried out on a Thermo Scientific K-Alpha spectrometer
using the Al K
α
(h
ν=1486 eV) radiation. The XPS spectra were analyzed
through the Advantage software. Before fitting, all the spectra were
calibrated based on the binding energy of the C 1 s neutral carbon peak
at 284.8 eV.
2
.4. Catalytic test
2
CO temperature-programmed desorption (TPD) experiments were
conducted using a Micromeritics Autochem-II 2920 instrument equip-
ped with a thermal conductivity detector (TCD) and a MKS Cirrus 2 mass
Catalytic evaluation: Hydroaminomethylation of olefins was carried
out in a Teflon-line steel batch reactor (50 mL). Typically, 0.1 g catalyst,
mL of toluene, 5 mmol 1-hexene, 6 mmol diethylamine were added
into the reactor in sequence. After the reactor was purged with CO for
spectrometer. The sample was firstly pretreated under a helium flow of
5
◦
3
5
0 mL/min at 200 C for 2 h. After the temperature was cooled down to
◦
0 C, the gas was switched to pure CO
2
at a flow rate of 30 mL/min for
five times, the reactor was filled with 1 MPa CO and 3 MPa H
2
. The
saturation adsorption. Then Ar gas (30 mL/min) was introduced to
purge the residue CO in system. Once the TCD signals became stable,
◦
reactions were conducted at 60~140 C with desired time. When the
batch reactor was cooled down to room temperature, the residual gas
was released and the liquid were withdrawn with injector for analysis.
The filtrate were firstly analyzed by the gas chromatography-mass
spectrometry (GC–MS) equipped with an HP-5MS column (30 m
length, 0.25 mm in diameter) to determine the structure of the products,
then immediately analyzed by the gas chromatography (GC) equipped
with a flame ionization detector (FID) to determine the product selec-
tivity with dodecane as internal standard.
2
◦
◦
the temperature was increased from 50 to 800 C at a rate of 10 C per
minute in He with a flow rate of 30 mL/min.
In situ diffuse reflectance infrared Fourier transform spectroscopy
(
(
DRIFTS) spectra were performed on a Thermo FTIR spectrometer
Nicolet IS50) with a MCT detector. Before the measurements, the
◦
samples were firstly pretreated at 200 C for 30 min and then cooled to
◦
5
0 C in the He flow (20 mL/min). After the background spectrum (64
◦
scans) was collected at 50 C in Ar flow (20 mL/min) with resolution of 4
Recycling experiments: The used catalyst was firstly collected by
centrifugation and washed with water and ethanol to remove adsorbed
ꢀ
1
cm , the catalyst was exposed to CO for 1 h (10 mL/min) for saturation
3

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Table 2
a
Scope of olefins and amines .
b
d
Entry
Olefin
Amines
Yeild (%)
L/B
Internal olefins
Alkanes
Others
Aldehydes
Amines
c
c
L
B
L+B
1
2
1.7
0.2
2.3
3.9
5.1
2.6
45.8
43.0
44.9
50.5
90.7
1.0
0.9
–
–
93.5
93.8
–
–
–
3
–
2.2
4.0
–
–
–
4
5
2.4
5.5
1.8
61.8
40.3
28.5
47.6
90.3
87.9
2.2
0.8
12.1
–
–
a
/CO=3 (v/v), 120 ◦C, 12 h.
1 2
Reaction conditions: 0.1 g 0.5Rh /HAP catalyst, 5 mmol olefin, 6 mmol diethylamine, 5 mL toluene, 4 MPa H
b
c
The yield was determined by the GC analysis with dodecane as internal standard and complete conversion (close to 100%) was obtained in all the cases.
L and B represent the linear products and branched products, respectively.
d
L/B was the ratio of linear to branched products.
◦
3-
reactants/products. Then the as-obtained sample was dried at 80 C for
stabilized by the surface PO
.2. Catalytic evaluation and the study of substrates scope
Hydroaminomethylation of 1-hexene was firstly selected as the
4
in the HAP support [28].
◦
5
h and calcined under atmosphere at 350 C for 3 h with a heating rate
◦
ꢀ 1
of 1 C min for next use.
3
3
. Results and discussion
model reaction. The catalytic results were summarized in the Table 1.
No reaction took place without the addition of catalyst (Table 1, entry
3
.1. Characterization
1
). The sole HAP support also showed no activity for this reaction
The as-prepared supported catalysts were firstly examined by XRD as
(Table 1, entry 2). On the contrary, all of the as-prepared Rh-based
catalysts can transform 1-hexene to the corresponding amines, sug-
gesting the important role of supported Rh species for this tandem re-
action. We firstly examined the catalytic performance of HAP supported
Rh-based catalysts with different Rh loading (Table 1, entries 3–7). The
amines selectivity gradually increased when the Rh loading increased
from 0.05 wt% to 0.5 wt%. With further increasing the Rh loading to 1
wt%, the amines selectivity remained almost the same as that for
shown in Fig. 1a. The patterns of supported Rh-based catalysts were
identical with the corresponding supports (Fig. 1b). No obvious peaks
belonged to Rh species were observed for all catalysts, indicating the
highly dispersion of Rh on these supports. According to TEM charac-
1
terization, the morphology of 0.5Rh /HAP catalyst was similar with that
of HAP support (Fig. S1), indicating that the supported Rh species didn’t
change the structure of HAP support. In addition, it was difficult to find
obvious Rh nanoparticles or clusters (Fig. 1c) for 0.5Rh
For a catalyst with 0.46 wt% loading, the absence of crystalline Rh or
RhO phase may reflect the formation of atomically dispersed Rh on
HAP support. However, for 0.5Rh/Mg Al, 0.5Rh/MgO and 0.5Rh/Al
catalysts which contained almost similar Rh loading with 0.5Rh /HAP
catalyst, obvious Rh nanoparticles were observed, though the specific
1
/HAP catalyst.
1
0.5Rh /HAP catalyst. The increased Rh loading not only provided more
active sites for the first hydroformylation step but also benefited for the
last hydrogenation step. The selectivity toward byproducts, such as in-
ternal olefins and aldehydes, decreased with the increase of Rh loading.
Moreover, the ratio of L/B gradually decreased as the more active Rh
sites can enhance the condensation of diethylamine with branched al-
x
3
2 3
O
1
surface areas of MgO and Al
HAP (Table S1).
2
O
3
supports were much higher than that of
1
dehydes. These results demonstrated that 0.5Rh /HAP catalyst can
provide enough atomically dispersed Rh sites for this tandem catalytic
process. A complete conversion of 1-hexene with amines selectivity of
In order to identify the nature of supported Rh on HAP, 0.5Rh
1
/HAP
catalyst was further examined by aberration corrected HAADF-STEM.
The obtained images clearly revealed the existence of isolated Rh
1
93.2% and a low amount of byproducts was obtained over 0.5Rh /HAP
catalyst under mild reaction condition (Table 1 entry 5). However, the
selectivities to the desired amines over other supported catalysts were
much lower (Table 1 entries 8–10), indicating that the support may play
1
atoms in 0.5Rh /HAP catalyst (Fig. 2a). Moreover, the EDX-mapping
results verified the homogeneous distribution of Rh species on HAP
support (Fig. 2b and c). In situ DRIFTS of CO adsorption was an useful
tool to explore the dispersion of Rh [23]. As displayed in Fig. 2d, two
1
an important role for this tandem catalytic process. As 0.5Rh /HAP
catalyst exhibited the most promising performance for this tandem re-
action, it was selected as the model catalyst in the subsequent
investigations.
ꢀ 1
ꢀ 1
obvious peaks centered at 2096 cm
and 2024 cm , which were
attributed to the symmetric and asymmetric vibration of gem-dicarbonyl
double CO on the positively charged Rh atoms, gradually appeared with
the increase of sweeping time. More importantly, the absence of
bridging CO together with the lack of a coverage dependent shift of the
linear Rh-CO strongly demonstrated that Rh atoms were highly
dispersed and isolated from each other. The combination of
HAADF-STEM and in situ DRIFTS of CO adsorption characterizations
clearly confirmed the atomically dispersed Rh atoms, which were
In order to study the substrates scope, the catalytic performance of
1
0.5Rh /HAP catalyst using different olefins and amines as substrates
were investigated and the results were listed in the Table 2. It was found
that a wide scope of aliphatic and aromatic terminal olefins, including 1-
octene, 1-decene, cyclohexene and styrene can be converted into the
corresponding amines. In general, 0.5Rh
1
/HAP catalyst exhibited
extremely high activity for different olefins or amines. For 1-octene and
4

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Scheme 2. General hydroaminomethylation reaction sequence.
◦
◦
1
-decene, the selectivities to the desired amines were 90.7% and 93.5%,
reaction temperature increased from 60 C to 120 C. However, the yield
of amines sharply declined to 82.7% when the reaction temperature was
respectively (Table 2, entries 1 and 2). Besides, a high selectivity to the
targeted amines (93.8%) can also be obtained for cyclohexene (Table 2,
entry 3). In the case of styrene, the selectivity to the corresponding
amines was as high as 90.3% (Table 2, entry 4). It was worthy to note
that a high ratio of linear to breached amines (2.2) with high conversion
was obtained in this tandem process. Compared with the hydro-
formylation alone using the same heterogeneous catalysts, the higher
selectivity to linear amines in this tandem reaction may be ascribed to
the promoting effect in tandem process (Table S2). Furthermore, a high
selectivity to the corresponding amines (77.3%) can still be achieved
when diethylamine was replaced by aniline (Table 2, entry 5).
◦
further increased to 140 C. The yields of other byproducts including
hexane, internal olefins and aldehydes were also affected by the varia-
◦
tion of reaction temperature. At low reaction temperature (60 C and
◦
80 C), the yield of internal olefins was about 10%. When the reaction
◦
temperature exceeded 80 C, almost no internal olefins were left after
reaction. This was due to the internal olefins was more difficult to be
converted than the terminal olefins and the raise of reaction temperature
was beneficial for the internal olefins conversion [41]. As for interme-
◦
diate aldehydes, low reaction temperature (60 C) or high reaction
◦
temperature (140 C) would retard the condensation step and led to
considerable amount of residual aldehydes after reaction (about 10%).
◦
Based on these results, it was easy to conclude that 120 C was the
3
.3. Effect of reaction conditions
suitable reaction temperature for this tandem process. Then the influ-
ence of reaction time was further studied. As displayed in Fig. 3b, the
yield of amines increased sharply in the first six hours and then the
growth rate gradually slowed down. In contrast, the yield of internal
The effect of reaction conditions on catalytic performance for
hydroaminomethylation of 1-hexene was studied in detail. The influ-
ence of reaction temperature was firstly explored. As shown in Fig. 3a,
the yield of amines gradually increased from 77.8% to 93.2% as the
olefins showed
a complete opposite trend. This phenomenon
Fig. 1. XRD and TEM characterizations of various samples. XRD patterns of (a) The as-prepared catalysts and (b) The selected supports, TEM images of various
catalysts: (c) 0.5Rh
1
/HAP, (d) 0.5Rh/Mg
3
Al, (e) 0.5Rh/MgO and (f) 0.5Rh/Al
2
O
3
catalyst.
5

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Fig. 2. Characterizations of 0.5Rh
1
◦
/HAP catalyst. (a) HAADF-STEM image, (b) TEM image, (c) EDS mapping showing elemental distribution of Ca, P, O and Rh, (d)
1
Evolution of DRIFT spectra at 50 C after introducing helium gas to sweep the CO pre-adsorbed 0.5Rh /HAP catalyst.
Fig. 3. Influence of reaction parameters on catalytic performance. (a) Effect of reaction temperature (0.1 g 0.5Rh
1
/HAP, 4 MPa H
2
/CO=3, 12 h), (b) Effect of
reaction time (0.1 g 0.5Rh
.5Rh
1
/HAP, 4 MPa H
◦
2
/CO=3, 120 ◦C), (c) Effect of catalyst amount (4 MPa H
2
/CO=3, 120 ◦C, 12 h) and (d) Effect of H
2
partial pressure (0.1 g
0
1
/HAP, 1 MPa CO, 120 C, 12 h).
demonstrated that appropriate extension of reaction time was beneficial
for the efficiency of this tandem process.
3.4. Reusability study
The catalyst dosage can also affect the reaction. As expected, the
yield of amines gradually increased from 70.0% to 93.2% when the
catalyst dosage amount increased from 0.02 g to 0.1 g (Fig. 3c). How-
ever, no obvious increase of amines yield was observed when the cata-
lyst dosage further increased to 0.2 g. The influence of hydrogen partial
pressure was also investigated. As shown in Fig. 3d, the yield of amines
gradually increased while the yield of internal olefins and aldehydes
decreased with the increase of hydrogen partial pressure. Therefore, the
enhancement of hydrogen partial pressure in the studied range was
favorable for the improvement of the whole efficiency.
Reusability of catalyst is of great significance from the viewpoint of
practical application. After each reaction, the 0.5Rh /HAP catalyst was
1
firstly collected by centrifugation, and then washed with ethanol and
◦
deionized water and calcined at 350 C before the next using. As shown
in the Fig. S2, the yield of amines gradually declined while the yield of
byproducts increased with the increasing the recycling time. In order to
reveal the deactivation mechanism, the spent catalyst was carefully
studied with different characterization techniques. The XRD pattern of
spent catalyst was basically similar with that of fresh catalyst (Fig. S3).
However, from the TEM images, the atomically dispersed Rh species
were sintered into nanoparticles with the average particle size of 1.8 nm
after reaction (Fig. S4). On the other hand, the ICP-OES results verified
6

X. Li et al.
Molecular Catalysis 509 (2021) 111671
Fig. 4. (a) Catalytic performance for hydroformylation of 1-hexene (0.1 g catalyst, 5 mmol 1-hexene, 5 mL toluene, 4 MPa H
2
/CO=3, 120 ◦C, 12 h) and (b)
◦
condensation of heptaldehyde with diethylamine in H
2
atmosphere (0.1 g catalyst, 5 mmol 1-hexene, 6 mmol diethylamine, 5 mL toluene, 1 MPa H
2
, 120 C, 1 h).
that the Rh content on HAP support decreased from 0.46 wt% to 0.15 wt
. The leaching Rh element was also found in the reaction mixture so-
hydroformylation step. On the other hand, obvious Rh nanoparticles
were observed for 0.5Rh/Al , 0.5Rh/MgO and 0.5Rh/Mg Al catalysts
%
2
O
3
3
lution (0.0073 mg/ml). These results were consistent with the phe-
nomenon that the yield of amines slowly increased from 41.5% to 73.3%
even though the solid heterogeneous catalyst was removed from the
catalytic system in hot filtration experiments (Fig. S5). Therefore, it was
reasonable to conclude that the deactivation of catalyst was attributed to
the growth of atomically dispersed Rh and Rh leaching via the formation
of homogeneous Rh-carbonyls species. For further study, more efforts
should be devoted to improve the stability of atomically dispersed Rh
species on the HAP surface via catalyst design [36,42,43].
from the TEM images (Fig. 1d–f). In contrast, the Rh species on HAP
support was atomically dispersed due to the excellent cation exchange
ability of HAP originated from its special surface structure (Fig. 1f) [33].
Single-atom Rh-based catalysts always exhibited high catalytic perfor-
mance for hydroformylation [21]. Therefore it was reasonable to
conclude that the atomically dispersed Rh on HAP ensured the superior
catalytic performance in hydroformylation. In addition, the catalytic
performances of various catalysts in first hydroformylation can also
affect the final products distribution in tandem hydroaminomethylation.
For example, considerable internal olefins were left in both hydro-
formylation and tandem hydroaminomethylation reactions over
3
.5. Mechanism discussion
0
.5Rh/MgO and 0.5Rh/Mg
Condensation of heptaldehyde with diethylamine and hydrogenation
of enamine to produce amines were the following reaction step (Scheme
). Condensation of aldehyde with diethylamine is a fast and sometimes
uncatalysed step [50]. In order to exclude the influence of selected
supports (Al , MgO, Mg Al and HAP) on condensation step, we
3
Al catalysts (Table 1).
In order to reveal the catalytic mechanism during the tandem
hydroaminomethylation of 1-hexene, the catalytic performance of the
as-prepared catalysts for each individual reaction steps were studied. In
this tandem reaction, hydroformylation of 1-hexene to produce corre-
sponding aldehydes was the first step (Scheme 2). According to the
mechanism of heterogeneously catalyzed hydroformylation that derived
from the mechanism for the homogeneously catalyzed hydroformylation
proposed by Heck and Breslow, the Rh single atoms with carbonyl li-
gands were regarded as the active sites [44–46]. From the in situ DRIFTS
2
2
O
3
3
compared the catalytic performance of the chosen supports for the
◦
condensation reaction at 120 C under N
2
atmosphere. As shown in the
Fig. S8, there were no obvious differences for the yield of the interme-
diate enamine, demonstrating that the condensation of intermediate
heptaldehyde with diethylamine can occur easily under this reaction
condition and wasn’t the determining step. Then the catalytic perfor-
ꢀ
1
of CO adsorption experiments, two obvious peaks located at 2096 cm
ꢀ 1
and 2024 cm were associated with the symmetric and asymmetric
stretches of the Rh(CO) gem-dicarbonyl species [21,23,47]. As shown
mance for the condensation of heptaldehyde with diethylamine in H
atmosphere were evaluated as displayed in Fig. 3b. Different from the
2
2
in Fig. S6, the elementary steps for the first hydroformylation step
condensation under N
2
atmosphere, the obtained enamine can be in situ
comprise olefin coordination (1→2), olefin insertion (2→3), CO coor-
+
converted to final amines with H
2
. [Rh] species were generally regar-
dination (3→4), CO insertion (4→5), oxidative addition of H (5→6),
2
ded the active sites for hydrogenation step (Fig. S6). The elementary
steps include oxidative addition of H
and reductive elimination of the aldehyde (6→2). The catalytic perfor-
mance for the sole hydroformylation was firstly carried out at the same
reaction conditions as hydroaminomethylation except diethylamine was
not added. As displayed in Fig. 4a, distinct catalytic performances were
obtained over the as-prepared catalysts. The selectivities to the corre-
2
(10→7), enamine coordination
7→8), hydride transfer (8→9), reductive elimination of amines (9→10)
(
[
51]. The conversion of aldehyde was much higher for all the catalysts in
atmosphere, suggesting that the in situ conversion of enamine by
H
2
hydrogenation was favorable for the improvement of conversion.
Meanwhile, completely different product distributions were found over
2 3 1
sponding aldehydes over 0.5Rh/Al O and 0.5Rh /HAP catalysts can be
up to 96.7% and 98.5%, respectively. However, 0.5Rh/MgO catalyst
exhibited much lower selectivity to aldehydes (73.3%) coupled with
large amount of internal olefins (26.7%). Almost no aldehydes were
these catalysts under H
2
atmosphere. No enamine was left for
.5Rh/MgO catalyst. But the selectivity to enamine was 97.3% for
.5Rh/Al catalyst, 17.0% for 0.5Rh/Mg Al catalyst and 36.5% for
.5Rh /HAP catalyst, respectively. Combined with the results in
(12.6%) and
Al catalysts (20.7%) in this tandem reaction. This phenom-
0
0
0
2
O
3
3
formed over 0.5Rh/Mg
negative effects on the formation of aldehydes, CO
were conducted [48,49]. Different from the other catalysts,
.5Rh/Mg Al catalyst exhibited two broader CO adsorption peaks,
indicating the existence of abundant basic sites (Fig. S7). It was
reasonable to speculate that the strong basicity of 0.5Rh/Mg Al catalyst
may be one of reasons for the poor catalytic performance in
3
Al catalyst. Given to the basicity may have
1
2
-TPD measurements
Table 1, there were also enamine for 0.5Rh/Al
.5Rh/Mg
2 3
O
0
3
0
3
2
enon revealed that the different hydrogenation ability of the as-prepared
catalysts can affect the final product distributions. However, it was easy
3
1
to find that no enamine was observed for 0.5Rh /HAP catalyst in the
7

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Molecular Catalysis 509 (2021) 111671
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