Enhancing the stability of the Rh/ZnO catalyst by the growth of ZIF-8 for the hydroformylation of higher olefins

Article abstract of DOI:10.1039/d0ra06515c

Hydroformylation of olefins is one of the most important industrial processes for aldehyde production. Therein, the leaching of active metals for heterogeneous catalysts is an important issue in the hydroformylation reaction, particularly for higher olefins to produce higher alcohols. Here, different Rh/ZnO catalysts with diverse ZnO as a support were investigated and a home-made ZnO₅₀support was selected to prepare the Rh/ZnO₅₀?ZIF-8 core-shell structure catalyst, which was synthesized by the growth of ZIF-8 with ZnO₅₀as the sacrificed template to afford Zn source. Compared with the Rh/ZnO₅₀catalyst, the Rh/ZnO₅₀?ZIF-8 catalyst demonstrated a better cyclic stability in the hydroformylation of 1-dodecene. Combining the experiment and characterization results, it was concluded that the ZIF-8 shell on the Rh/ZnO₅₀catalyst effectively prevented the leaching of metal Rh into the reaction solution. Moreover, the Rh/ZnO₅₀?ZIF-8 catalyst exhibited good universality for other higher olefins. This work provides a useful guideline for immobilizing the active species in heterogeneous catalysts for the hydroformylation reaction.

Full text of DOI:10.1039/d0ra06515c

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Enhancing the stability of the Rh/ZnO catalyst by
the growth of ZIF-8 for the hydroformylation of
Cite this: RSC Adv., 2020, 10, 34381
higher olefins†
ab
Lele Chen,‡a Jinghao Tian,‡a Huaxing Song,a Zhaohua Gao,a Haisheng Wei,
*
Wenhua Wangab and Wanzhong Renab
Hydroformylation of olefins is one of the most important industrial processes for aldehyde production.
Therein, the leaching of active metals for heterogeneous catalysts is an important issue in the
hydroformylation reaction, particularly for higher olefins to produce higher alcohols. Here, different Rh/
ZnO catalysts with diverse ZnO as a support were investigated and a home-made ZnO50 support was
selected to prepare the Rh/ZnO50@ZIF-8 core–shell structure catalyst, which was synthesized by the
growth of ZIF-8 with ZnO50 as the sacrificed template to afford Zn source. Compared with the Rh/
ZnO50 catalyst, the Rh/ZnO50@ZIF-8 catalyst demonstrated
a better cyclic stability in the
hydroformylation of 1-dodecene. Combining the experiment and characterization results, it was
concluded that the ZIF-8 shell on the Rh/ZnO50 catalyst effectively prevented the leaching of metal Rh
into the reaction solution. Moreover, the Rh/ZnO50@ZIF-8 catalyst exhibited good universality for other
Received 27th July 2020
Accepted 1st September 2020
DOI: 10.1039/d0ra06515c
higher olefins. This work provides
a useful guideline for immobilizing the active species in
rsc.li/rsc-advances
heterogeneous catalysts for the hydroformylation reaction.
solution as soluble Rh-carbonyl catalytically active species,
Introduction
which results in its low stability.21 So, how to x the active
component in the hydroformylation catalyst is an important
question to be solved. Much efforts focus on heterogenization of
homogeneous catalysts by attaching phosphite ligands to
a modied support.22,23 Recently, it was found that a single Rh
atom catalyst, which was formed by replacing the surface atom
with the Rh atom18,20 or dispersed Rh on N-doped carbon,24
exhibited excellent performance.
The corresponding product aldehydes derived from the hydro-
formylation of olens have wide applications, which include
use as intermediates for alcohols, carboxylic acids, and
aliphatic amines.1–4 As for hydroformylation, numerous studies
have been devoted to Rh-base catalysts, including homoge-
neous and heterogeneous catalysts.5–10 In general, homoge-
neous catalysts with specic and uniform active sites possess
high activity and selectivity but suffer from the problem of
catalyst separation.11,12 Therefore, two-phase systems have been
exploited to solve the separation problem.13–17 The noted RCH/
RP biphasic system has been successfully applied in indus-
tries for the hydroformylation of propene; however, failure in
the hydroformylation of higher olens is due to their low
solubility of substrates in water. In contrary, segregative
heterogeneous catalysts with preferable industrial applications
attract considerable attention.18–20 However, there is a wide-
spread concern over the issue that the active component of the
Rh-base heterogeneous catalyst is easy to leach into the reaction
Metal–organic frameworks (MOFs) have attracted consider-
able attention in conning nanoentities,25 such as metal
nanoparticles, clusters, single atom, biomacromolecules, to
form composite materials,26–31 which exhibited superior
performance in numerous applications. Aiming at the loss of
metal Rh in the hydroformylation reaction, MOF materials with
high surface area and tunable pore size can serve as barriers to
prevent the leaching of Rh species. Therein, zeolitic imidazolate
framework-8 (ZIF-8), an important subgroup of MOFs with
ꢀ
ꢀ
a pore size of 11.6 A through 3.4 A apertures, is taken into our
consideration since it can be synthesized by growth on the ZnO
surface to form a core–shell structure,32,33 which is benecial to
conne the active metal under the shell material by a rational
design. Owing to the small aperture of ZIF-8 materials, the loss
of soluble Rh-carbonyl active species in the hydroformylation
reaction will be hindered.
aCollege of Chemistry and Chemical Engineering, Yantai University, Yantai 264005,
Shandong, China. E-mail: haishengwei@ytu.edu.cn
bCollaborative Innovation Center of Comprehensive Utilization of Light Hydrocarbon
Resource, Yantai University, Yantai 264005, Shandong, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06515c
Herein, the Rh/ZnO@ZIF-8 catalyst was prepared using ZnO
as the sacriced template, and its catalytic performance was
investigated in the hydroformylation of higher olens. The
‡ Lele Chen and Jinghao Tian have contributed equally to this work.
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characterization and experiment results showed that the growth catalysts was evaluated in the hydroformylation of 1-dodecene
of a ZIF-8 shell on Rh/ZnO could effectively decrease the using 1-octanol as an internal standard. In each experiment,
leaching of Rh and improve the stability of the catalyst.
0.10 g of the catalyst and reaction solution were put into an
autoclave and ushed with 10 bar CO/H2 (1 : 1) for ve times.
Then, the autoclave was charged with CO/H2 (1 : 1) until 4 MPa
and heated to 90 ꢁC in an oil bath with a magnetic stirrer to
initiate the reaction. When the reaction completed, the catalyst
was separated from the solution, and the product was analyzed
using an Agilent 7890B gas chromatograph equipped with an
HP-5 column.
Experimental section
Materials
Zinc nitrate hexahydrate (Zn(NO3)2$6H2O), 2-methylimidazole,
ammonium carbonate, diethylene glycol, higher olens, and
ZnO nanoparticles were purchased from Aladdin Chemical Co.,
Ltd. (Shanghai, China) with analytical grade. Also, the
commercial ZnO was denoted as ZnO-C. Rhodium chloride
hydrate was purchased from Macklin. All the chemicals were
used without further purication.
Results and discussion
In order to obtain the Rh/ZnO@ZIF-8 catalyst with good cata-
lytic performance, the precursor Rh/ZnO catalyst primarily has
relatively better stability. Therefore, three types of Rh/ZnO
catalysts with different morphologies of ZnO as supports were
investigated. As shown in Fig. 1A–C, it can be seen that the
home-made ZnO50 was small nanoparticles with an average size
of about 20 nm. The other two ZnO supports in Fig. 1B and C
were obviously larger than ZnO50 and exhibited rod-like and
spherule shapes, corresponding to ZnO-C and ZnO-S. Fig. 1D
shows the XRD pattern of different ZnO supports, and the
diffraction peak of ZnO50 is weaker than that of other ZnO
supports, which was in accordance with the SEM result. With
different ZnO supports, 0.5% Rh/ZnO catalysts were prepared
via incipient wetness impregnation. Also, the catalytic perfor-
mance was tested in the hydroformylation of 1-dodecene. The
results in Table 1 showed that the 0.5% Rh/ZnO-C catalyst
exhibited the best catalytic performance with 96.7% conversion
and 96.8% selectivity towards tridecylic aldehyde. For the 0.5%
Rh/ZnO-S catalyst, it achieved 86.8% selectivity at the conver-
sion of 98.9%. Over the 0.5% Rh/ZnO50 catalyst, the activity was
lower than the preceding catalyst, the selectivity of aldehyde
decreased to 79.3% under the same reaction conditions, which
yielded more iso-alkenes. In addition, different catalysts
showed a similar ratio of linear to the branched aldehyde in the
products.
Further, the cyclic stability of numerous Rh/ZnO catalysts
was conducted. The results in Fig. 2 show that the conversion
and selectivity of three catalysts declined obviously in the
second run. Comparatively speaking, 0.5% Rh/ZnO50 exhibited
better stability, which maintained 65.2% conversion and 52.4%
selectivity. In order to explore the deactivated reason, hot lter
experiments were carried out. As shown in Table S1,† the
ltration solution continued to react and achieved a higher
conversion, revealing that soluble Rh active species were
present in the ltration solution, which was conrmed by
detecting the content of Rh in the ltration with ICP-AES. The
results in Table S2† exhibit that the leaching of metal Rh from
the 0.5% Rh/ZnO50 catalyst aer the hydroformylation reaction
was lower than that of the other two catalysts, which may be
attributed to the ZnO50 support with small sizes and high
surface areas. This also explained the better cyclic stability of
the 0.5% Rh/ZnO50 catalyst. Meanwhile, according to the ICP
Synthesis of ZnO50 and ZnO-S
ZnO50 nanoparticles were prepared via the co-precipitation
method. First, ammonium carbonate (4.0 g) was dissolved in
60 ml distilled water. Then, a 20 ml zinc nitrate solution (1 mol
lꢀ1) was added dropwise to the ammonium carbonate solution
at 50 ꢁC and stirred for 3 h. The white precipitate was collected
via ltration and washed several times with deionized water.
The as-prepared sample was driedꢁ under vacuum at 60 ꢁC
overnight, and then calcined at 400 C for 3 h to obtain ZnO50
nanoparticles. The ZnO sphere was prepared according to the
previous literature34 and denoted as ZnO-S.
Synthesis of Rh/ZnO@ZIF-8
Home-made ZnO50, ZnO-C (commercial ZnO), and ZnO-S were
selected as supports to impregnate using an RhCl3 solution and
dried at 60 ꢁC to afford the 0.5% Rh/ZnO precursor. The Rh/
ZnO50@ZIF-8 catalyst was synthesized using the 0.5% Rh/ZnO50
precursor. Typically, 0.5% of theRh/ZnO50 precursor was put
into a solution of 2-methylimidazole with ethanol as the
solvent. The mixture was placed at room temperature without
stirring for 3 h. Subsequently, the product was collected via
centrifugation at 8000 rpm for 5 min, washed three times with
ꢁ
ethanol, and then dried at 60 C overnight.
Catalyst characterization
The X-ray diffraction (XRD) patterns were recorded from 5ꢁ to
80ꢁ at a scanning rate of 10ꢁ minꢀ1 using a Rigaku Smart Lab III
ꢀ
diffractometer using Cu Ka radiation (l ¼ 1.5406 A). The
morphology of samples was observed under a Hitachi S-4800
cold-cathode eld-emission scanning electron microscope (FE-
SEM). Fourier-transform infrared (FT-IR) spectroscopic charac-
terization was performed on a Nicolet 380 instrument with
a resolution of 4 cmꢀ1. The concentration of metal species was
determined via inductively coupled plasma spectrometry-
atomic emission spectrometry (ICP-AES) on an IRIS Intrepid II
XSP instrument (Thermo Electron Corporation).
Catalytic test
Before the test, the catalysts were reduced under a H2 atmo- results, the higher activity for 0.5% Rh/ZnO-C and 0.5% Rh/
sphere at 250 ꢁC for 30 min. The catalytic performance of ZnO-S catalysts in Table 1 may be due to that the presence of
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Fig. 1 SEM images of (A) ZnO50, (B) ZnO-C, (C) ZnO-S, (D) XRD image of different ZnO.
Table
1
Hydroformylation results of 1-dodecene over different
shows the XRD pattern of the samples aer the growth of ZIF-8.
It can be seen that no ZIF-8 crystal phase was observed when the
concentration was 0.004. Increasing the concentration to 0.02,
there appeared characteristic diffraction peaks of ZIF-8 at the 2q
range of 5–20ꢁ. Moreover, the intensity of the diffraction peak
enhanced along with the addition of the ligand concentration,
which agreed with the previous reports.35 Moreover, the
diffraction peak of ZnO50 still existed, which indicated that the
formation of the ZIF-8 shell only consumed a portion of the
ZnO50 support. The Rh/ZnO50@ZIF-8 catalyst with the ligand
concentration of 0.02 was further characterized via SEM, FT-IR,
and XPS. In Fig. 3B, it can be clearly seen that the ZIF-8 crystals
grew around ZnO50 nanoparticles to form a core–shell structure.
The presence of the ZIF-8 shell was also determined via FT-IR
characterization, as shown in Fig. 3C, which gave rise to the
Zn–N stretch mode at 421 cmꢀ1, and the stretching and plane
bending of the imidazole ring at 1350–1500 cmꢀ1 and 500–
1350 cmꢀ1, respectively.36,37 For ZnO50 and Rh/ZnO50, there was
no distinctive band for ZIF-8, showing the same spectrum.
Further, the XPS characterization was conducted to analyze the
Rh/ZnO50@ZIF-8 catalyst. As shown in Fig. 3D, the character-
istic peaks of the Rh species were very weak because XPS only
detected the surface information, and the binding energy of N
1s and Zn 2p were assigned to ZIF-8 materials.30 These results
catalystsa
Conv. Aldehyde sel. Iso-alkene
Catalyst
(%)
(%)
sel. (%)
L/B ratio
0.5% Rh/ZnO-C
0.5% Rh/ZnO-S
0.5% Rh/ZnO50
96.7
98.9
92.4
96.8
86.8
79.3
71.0
3.2
13.2
21.7
29.0
0.7
1.0
1.2
1.1
0.5% Rh/ZnO50@ZIF-8b 99.1
a
Reaction condition: 4.0 MPa (CO/H2 ¼ 1), 90 ꢁC, 2.5 mmol 1-dodecene,
1.0 mmol 1-octanol as internal standard, 2 h, L/B ratio refer to the ratio
of linear to branched aldehyde in the products. b 4.5 h.
more homogeneous Rh active species in the solution, which
also explained why the activity of different Rh/ZnO catalysts
clearly declined aer one cycle. So, in order to acquire a stable
catalyst, the 0.5% Rh/ZnO50 catalyst was selected as the
precursor to grow a ZIF-8 shell to avoid the loss of metal Rh.
First, the concentration of the 2-methylimidazole ligand was
studied using ethanol as the solvent. In particular, the 0.5% Rh/
ZnO50 precursor was put into the 2-methylimidazole solution
with the concentration of 0.004, 0.02, 0.03 and 0.04 (g 2-
methylimidazole/ml ethanol) for 3 h without stirring. Fig. 3A
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Fig. 2 Cyclic experiments of different catalysts, (a) Rh/ZnO-C, (b) Rh/ZnO50, (c) Rh/ZnO-S.
conrmed that Rh/ZnO precursors were coated with ZIF-8 catalyst was obviously lower than that of the non-growth cata-
shells. In addition, it is well-known that ZIF-8 materials own lyst. For the 0.5% Rh/ZnO50@ZIF-8 catalyst, it yielded 99.1%
high thermal stability, which starts to decompose at tempera- conversion and 71.0% selectivity towards the target product for
tures higher than 500 ꢁC.38 Here, the ZIF-8 crystal prepared with 4.5 h. The decline in the activity may be attributed to the fact
ZnO50 as a sacriced template also maintained good stability that the ZIF-8 shell inhibited the interactions between substrate
under the N2 atmosphere. As shown in Fig. S1,† the unique and active species in the catalyst. The ltration solution was
diffraction peak of ZIF-8 presented even at 500 ꢁC, which was in also detected, and the content of metal Rh decreased from
accord with the TG result (Fig. S2†).
18.2% to 0.55% in Table S2,† indicating that the ZIF-8 shell
According to the above characterizations, the 0.5% Rh/ effectively prevented the loss of metal Rh.30 Moreover, the
ZnO50@ZIF-8 catalyst prepared at the concentration of 2- change in the conversion and selectivity for the Rh/ZnO50@ZIF-
methylimidazole at 0.02 was chosen to evaluate the cyclic 8 catalyst was negligible aer four cyclic experiments in Fig. 4,
stability. As shown in Table 1, the activity of the core–shell showing better stability compared with the Rh/ZnO50 catalyst,
Fig. 3 (A) XRD pattern of 0.5% Rh/ZnO50@ZIF-8 over different 2-methylimidazole concentrations, (B) SEM image of 0.5% Rh/ZnO50@ZIF-8, (C)
FT-IR spectra of numerous catalysts, (D) XPS spectra of 0.5% Rh/ZnO50@ZIF-8 catalyst.
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The core–shell Rh/ZnO50@ZIF-8 catalyst was synthesized by the
growth of ZIF-8 on a home-made ZnO50 support, which was
conrmed by SEM, XRD, and FT-IR characterizations. The
catalytic performance was conducted in the hydroformylation
of 1-dodecene, and the results showed that the Rh/ZnO50@ZIF-8
catalyst exhibited better stability compared with the Rh/ZnO50
catalyst, which was due to the present of the ZIF-8 shell inhib-
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