A ternary Rh complex catalyst highly active and stable in the hydrogenation of acrylonitrile-butadiene rubber

Article abstract of DOI:10.1039/c4nj01627k

An Rh-based complex, T-Rh-PPh₃, was developed through a facile one-step process. The T-Rh-PPh₃ exhibited high activity and air stability in the hydrogenation of acrylonitrile-butadiene rubber. The enhanced air stability can be ascribed to the phenolic hydroxyl structures in tannin.

Full text of DOI:10.1039/c4nj01627k

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A ternary Rh complex catalyst highly active and
stable in the hydrogenation of acrylonitrile–
butadiene rubber†
Cite this: New J. Chem., 2015,
39, 1583
Received (in Montpellier, France)
21st September 2014,
Peng Cao,a Meng Wu,a Rui Zou,a Liqun Zhangab and Dongmei Yue*ab
Accepted 22nd December 2014
DOI: 10.1039/c4nj01627k
www.rsc.org/njc
An Rh-based complex, T-Rh-PPh3, was developed through a facile metal ions through its dense ortho-phenolic hydroxyls and
one-step process. The T-Rh-PPh3 exhibited high activity and air scavenge free radicals so as to prevent metal species from
stability in the hydrogenation of acrylonitrile–butadiene rubber. oxidation. These properties imply that tannin could be used
The enhanced air stability can be ascribed to the phenolic hydroxyl as an ideal stabilizer for metal species. Meanwhile, two adja-
structures in tannin.
cent phenol hydroxyls can be used as a bidentate ligand to
bond with metal ions, obtaining a stable five-membered ring
The hydrogenation of acrylonitrile–butadiene rubber (NBR) is a chelate and the third phenol hydroxyl of pyrogallol can promote
significant process to improve and optimize the physical and this chelation process, so as to enhance the formation and
chemical properties of the parent polymer.1 The hydrogenated stability of the complex.11–13 The Bi Shi group synthesized a
acrylonitrile–butadiene rubber (HNBR) not only retains the series of AuNP, Pt-based, and Pd-based catalysts using tannin
original excellent chemical, physical, and mechanical proper- as a reductant, a stabilizer or a ligand.14–16 Besides, seldom
ties, but also has outstanding resistance to ozonolytic aging efforts have been made to improve the stability of catalysts,
and oxidation, even at high temperatures.2,3 The homogeneous such as RhCl(PPh3)3, for the hydrogenation of NBR. Thus, to
rhodium catalyst RhCl(PPh3)3, developed by Lanxess GmbH, expand the applications of the tannin for stabilization of cata-
Germany, exhibits remarkable activity and selectivity in NBR lysts, a new stable Rh-based complex, T-Rh-PPh3, is designed for
hydrogenation.4,5 However, the drawback of RhCl(PPh3)3 is that the hydrogenation of NBR in this communication.
it is quite expensive and unstable owing to oxidation.6 The
In this study, we first synthesized a novel Rh-based complex
as-prepared RhCl(PPh3)3 should be stored in an inert-gas atmo- (T-Rh-PPh3) by a facile and efficient one-step process. In this
sphere, which is not convenient for use and transportation. method, tannin acts as a stabilizer and a ligand, and RhCl3Á3H2O
Therefore, in addition to the studies focusing on the recovery of is used as a precursor. The structure of the as-prepared catalyst
the Rh catalyst or preparation of novel homogeneous catalysts was fully characterized by FTIR and 1HNMR. The catalytic
based on the less expensive metals,7,8 significant efforts have activity and stability of the obtained catalyst toward NBR hydro-
been made to design and develop a catalyst system that is genation were investigated.
inexpensive and has high activity and stability to reduce the
cost and facilitate the process of HNBR preparation.
There are a large number of adjacent phenol hydroxyls at the
B-rings of tannin (Fig. 1(a)) and the phenol hydroxyls can form
Tannin, one of the most abundant biomass resources, can chelate rings with many kinds of metal ions. The partly
be easily obtained from the extracts of plants.9,10 According to attached galloyl groups at the A-rings of tannin can enhance
the chemical structures, tannin can chelate with many kinds of this chelating ability. Bi Shi demonstrated that tannin has high
affinity towards many transition-metal ions including Pd(II),
Pt(IV) and Au(III).13,17,18 A schematic flow diagram for the
a State Key Laboratory of Organic-Inorganic Composites, Beijing University of
preparation of T-Rh-PPh3 is shown in Fig. 1(b).10
Chemical Technology, Beijing 100029, China
b Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer
Fig. 2(a) shows the FTIR spectra of T-Rh-PPh3 and
RhCl(PPh3)3. The peak at 3400 cmÀ1 can be attributed to the
Materials, Beijing 100029, China. E-mail: yuedm@mail.buct.edu.cn;
Fax: +86-010-64436201; Tel: +86-010-64436201
stretching vibration of the O–H bond from phenol hydroxyls
† Electronic supplementary information (ESI) available: X-ray photoelectron
(C–OH) or the vibration of the C–H bond in the benzene ring.10
spectra of the Rh 3d region for various Rh catalysts. The effect of different control
reaction conditions on the degree of hydrogenation and FTIR and 1HNMR spectra
of NBR and HNBR. See DOI: 10.1039/c4nj01627k
As shown in Fig. 2(a1), the peak for the stretching vibration
of O–H is broader in the spectrum of T-Rh-PPh3 than in the
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Fig. 1 (a) Molecular structure of tannin and (b) the proposed route for
preparation of T-Rh-PPh3.
Fig. 3 Solid-state 31P-NMR spectra of T-Rh-PPh3 and RhCl(PPh3)3 cata-
lysts (1D, pulse interval 5 s, spinning rate 11 kHz).
Further, X-ray photoelectron spectra of Rh 3d peak were
employed to analyze the change in Rh element for the both
catalysts (Fig. S1, ESI†)
T-Rh-PPh3 was also tested by the FeCl3 solution method to
determine if there was any free tannin in the complex.22 Low
levels of tannin can react with FeCl3 solution to form a black
precipitate. First, the T-Rh-PPh3 complex was dissolved in acetone,
and then the freshly prepared FeCl3 reagent was added dropwise
to the acetone solution. No black precipitate was observed,
indicating that there was no free tannin in the T-Rh-PPh3
complex. The results showed that tannin, PPh3, and the Rh ion
form a ternary complex rather than a simple physical mixture.
It is well known that RhCl(PPh3)3 is unstable when it is
exposed to the air environment for a long time. The catalytic
activity of RhCl(PPh3)3 is gradually reduced by oxidative deteri-
oration. In our experiments, both T-Rh-PPh3 and RhCl(PPh3)3
were exposed to air for 2, 4, 8, and 20 days. Then they were used
as catalysts for the hydrogenation of NBR under similar condi-
tions. The degree of hydrogenation results is shown in Fig. 3.
Fig. 4 shows that the catalytic activity of T-Rh-PPh3 remains
at above 96% after laying aside for 20 days in air. However, the
catalytic activity of RhCl(PPh3)3 drops to 88% after laying aside
Fig. 2 (a) FTIR spectra of T-Rh-PPh3 and RhCl(PPh3)3 and (b) 1HNMR
spectra of T-Rh-PPh3 and RhCl(PPh3)3.
spectrum of RhCl(PPh3)3, an indication of the successful chelation
with the Rh ion. The peaks in the wave number range of 1627–
1454 cmÀ1 suggest the presence of aromatic rings, and the
peak at 1454 cmÀ1 belongs to the special skeletal stretching
vibration of the aromatic ring of tannin. Particularly, the
strong peak at 1020 cmÀ1 is attributed to the stretching
vibration of the C–O single bond from tannin.19 All these
results demonstrate that tannin and the Rh ion form a new
complex through covalent bonds.
1HNMR was used to further confirm the chemical chelation
between Rh3+ and tannin, and the spectra are shown in Fig. 2(b).
The three peaks between 6.8 and 7.25 ppm are the characteristic
peaks of the phenol hydroxyl group on the benzene ring,
indicating that tannin is connected to the Rh ion by a chemical
bond. The three peaks in the range 7.5 to 7.75 ppm are the PPh3
proton characteristic peaks on the benzene ring.
The solid-state 31P-NMR spectrum of RhCl(PPh3)3 (Fig. 3)
shows the center bands and can be divided into three parts,
with chemical shifts of 49.7, 32.3, and 23.1 ppm. These peaks
are attributed to three nonequivalent phosphorus atoms in
the catalyst molecule (P1, P2, and P3).20,21 However, the
31P-NMR spectrum of T-Rh-PPh3 differs significantly from that
of RhCl(PPh3)3. In addition there is a signal at 49.7 ppm, which
corresponds to the signal of the phosphorus atom P3. The main
intensity is concentrated in a single broad peak at 26.0 ppm,
which in comparison with the spectrum of the RhCl(PPh3)3 can
be related to the phosphorus atom labeled as P1 and P2.
Therefore, the results of 31P-NMR suggest that two phosphine
ligands P1 and P2 are substituted by ortho-phenolic hydroxyls
of tannin and form the ternary catalyst of T-Rh-PPh3 (see Fig. 3).
Fig. 4 Effect of time of exposure to air on the degree of hydrogenation
for T-Rh-PPh3 and RhCl(PPh3)3. Reaction conditions: 120 mL of NBR glue
solution (6 wt%), content of the catalyst (0.2 wt%), H2 pressure of 2.5 MPa,
temperature of 120 1C, and reaction time of 6.0 h.
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for 20 days. These results show that as-prepared T-Rh-PPh3 is 0.07 wt%, a hydrogenation of 82.68% is obtained after 11 h at
more stable in air than RhCl(PPh3)3, which is greatly convenient 120 1C. These results show that T-Rh-PPh3 has a remarkable
for storage and application.
catalytic activity for the NBR hydrogenation.
The difference in resistance to oxidation between the two
For analyzing the structure of hydrogenated NBR, FTIR and
catalysts is attributed to their different chemical structures. The 1HNMR spectra of HNBR and NBR were employed (Fig. S2,
active state of RhCl(PPh3)3 is Rh+, which can react with oxygen ESI†). From the spectra, it can be seen that an enhanced
molecules to become the inactive species Rh3+, with lower intensity of the aliphatic protons is observed and the character-
catalytic activity. However, the rhodium in T-Rh-PPh3 combines istic peaks (–CH–CN, acrylonitrile structural unit) remain
with ortho phenolic hydroxyl and triphenylphosphine, and the unchanged. These results indicate that only the CQC takes
partly uncoordinated phenolic hydroxyls can be oxidized to a place without reduction of the –CN by using the new catalyst
quinone by oxygen molecules. Phenolic hydroxyl is a hydrogen T-Rh-PPh3, a selective hydrogenation of NBR.
donor and can inhibit the production of oxygen anions or
The effects of different reaction conditions on the degree of
hydroxyl radicals. It plays a significant role in protecting the hydrogenation are summarized in Fig. S3 (ESI†). The results
active center Rh+ from oxidation or delaying the oxidation by show that under the optimized conditions, the degree of
oxygen anions. Therefore, T-Rh-PPh3 shows better resistance to hydrogenation can reach 97%.
oxidation than RhCl(PPh3)3 under the same conditions. The
possible mechanism of the two catalysts oxidation in the air T-Rh-PPh3 were studied. FTIR, 1HNMR and 31P-NMR showed
atmosphere is shown in Scheme 1. that the tannin and triphenylphosphine were successfully
In conclusion, the preparation and characterization of
To evaluate the catalytic activity of the as-prepared T-Rh-PPh3 incorporated into the framework of the T-Rh-PPh3. The
catalyst, the hydrogenation of NBR to produce HNBR was carried T-Rh-PPh3 was used for the hydrogenation of NBR, exhibiting
out. Fig. 5 illustrates the degree of hydrogenation for different satisfactory catalytic activity, excellent selectivity, and good
loadings of the T-Rh-PPh3 catalyst.
air stability. The hydrogenation results showed that under
It can be seen that the catalyst T-Rh-PPh3 demonstrates high optimum conditions, a degree of hydrogenation of 97% was
activity toward hydrogenation of NBR. With 0.15 wt% loading, achieved. The enhanced air stability is the phenolic hydroxyls
T-Rh-PPh3 shows a high degree of hydrogenation of 96.78% in tannin protect the active species Rh+ from oxidation. This
with 6 h, at a temperature of 120 1C. With the lowest loading of method can be expected to apply to other catalytic systems to
improve their stability.
Experimental section
Synthesis of T-Rh-PPh3
The used tannin is bayberry tannin, a kind of typical condensed
tannin. The bayberry tannin was obtained from the barks of
myrica by extraction with an acetone–water solution (1 : 1, v/v)
and then spray dried.10
First, 1 g of RhCl3Á3H2O, 3 g of tannin, and 200 mL of
Scheme 1 Supposed mechamism of the catalysts of RhCl(PPh3)3 and
T-Rh-PPh3 oxidation by O2.
deionized water were added to a flask under vigorous stirring
for 20 min to obtain a homogenous solution. Then, the pH
value of the solution was adjusted by 2 mol LÀ1 HCl solution to
2–3 and the solution was stirred vigorously at 313 K for 6–10 h.
Afterwards, 50 mL of triphenylphosphine ethanol solution was
added, and the mixture was followed by stirring at 373 K for
another 2–3 h. The product, T-Rh-PPh3, was collected by filtra-
tion, washed, and dried in a vacuum oven.
For the purpose of comparison, the RhCl(PPh3)3 catalyst was
prepared without the addition of tannin according to the
procedure mentioned above. The Rh content in catalysts of
RhCl(PPh3)3 and T-Rh-PPh3 was determined by ICP-MS and
amounted to 10.49 wt% and 6.27 wt%, respectively.
Evaluation of the activity of the catalyst in NBR hydrogenation
A certain amount of NBR was dissolved in chlorobenzene to
obtain an NBR solution. The NBR solution and the catalyst were
Fig. 5 Effect of catalyst concentrations for the T-Rh-PPh3 on the degree
of hydrogenation of NBR hydrogenation reaction. Reaction conditions:
added to a 100 mL high-pressure stainless steel autoclave.
Then the system was sealed and degassed with N2 and H2.
120 mL of NBR glue solution (6 wt%), H2 pressure of 2.5 MPa and
temperature of 120 1C.
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Afterwards, the system was charged with H2 to the desired
pressure and then gradually heated to the desired reaction
temperature. After the reaction, the autoclave was cooled down
to room temperature. The product was precipitated with alco-
hol, washed with alcohol, and dried in a vacuum oven. The
conversion of the double bonds, also called the degree of
hydrogenation (HD), was calculated as
HD = (1 À [CQC]t/[CQC]0) Â 100%
(1)
where [CQC]t is the concentration of CQC bonds at reaction
time t and [CQC]0 is the initial double bond concentration. The
1
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Acknowledgements
The financial support of the National Natural Science Foundation
of China under Grant No. 20774009 is gratefully acknowledged.
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