Hydrogenation of acrylonitrile-butadiene rubber latex using in situ synthesized RhCl(PPh3)3 catalyst

Article abstract of DOI:10.1016/j.apcata.2013.02.015

Catalytic hydrogenation of acrylonitrile-butadiene rubber (NBR) latex was achieved by using in situ synthesized RhCl(PPh₃)₃ catalyst. Water-soluble rhodium salt (RhCl₃) was used as the catalyst precursor which was reacted in situ with triphenylphosphine (PPh₃) to form RhCl(PPh₃)₃. Compared with using solid RhCl(PPh₃)₃, hydrogenation of NBR latex using the in situ RhCl(PPh₃)₃ showed a faster hydrogenation reaction. Based on the retention of catalyst in the polymer, it was revealed that the efficiency of in situ synthesis and the diffusion of the catalyst from the aqueous phase into the polymer particles were crucial in achieving the required high degree of hydrogenation. As a result, it was discovered that by introducing a small amount of alcohol (e.g. RhCl₃ = 0.52 mmol/L, PPh₃ = 18 mmol/L, ethanol/total NBR latex = 1/10 volume ratio) to the feed latex, the in situ synthesized catalyst could be used in the hydrogenation reaction very efficiently.

Full text of DOI:10.1016/j.apcata.2013.02.015

Applied Catalysis A: General 457 (2013) 62–68
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of acrylonitrile-butadiene rubber latex using in situ
synthesized RhCl(PPh ) catalyst
3
3
∗∗
∗
Yin Liu, Zhenli Wei, Qinmin Pan , Garry L. Rempel
Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic hydrogenation of acrylonitrile-butadiene rubber (NBR) latex was achieved by using in situ syn-
thesized RhCl(PPh3)3 catalyst. Water-soluble rhodium salt (RhCl3) was used as the catalyst precursor
which was reacted in situ with triphenylphosphine (PPh3) to form RhCl(PPh3)3. Compared with using
solid RhCl(PPh3)3, hydrogenation of NBR latex using the in situ RhCl(PPh3)3 showed a faster hydrogena-
tion reaction. Based on the retention of catalyst in the polymer, it was revealed that the efficiency of in situ
synthesis and the diffusion of the catalyst from the aqueous phase into the polymer particles were crucial
in achieving the required high degree of hydrogenation. As a result, it was discovered that by introducing
a small amount of alcohol (e.g. RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L, ethanol/total NBR latex = 1/10 vol-
ume ratio) to the feed latex, the in situ synthesized catalyst could be used in the hydrogenation reaction
very efficiently.
Received 25 October 2012
Received in revised form 7 February 2013
Accepted 17 February 2013
Available online xxx
Keywords:
Hydrogenation
In situ catalyst synthesis
Latex
NBR
Alcohol
© 2013 Elsevier B.V. All rights reserved.
1
. Introduction
Diene-based polymers such as butadiene rubber (BR), chloro-
Currently, NBR hydrogenation reactions are carried out in poly-
mer solutions using Rh or Pd metal based catalysts. However,
the high manufacturing cost of this process has substantially
limited its application (i.e. substitution of NBR). The challenge
that rubber material scientists and engineers face is to produce
this high-performance elastomer in an economical manner. Signif-
icant research has been carried out to improve NBR hydrogenation
process, including using ruthenium (Ru) or osmium (Os) based
catalysts [5,6], biphasic catalysts [7–9], performing hydrogenation
reaction in aqueous system [10–17], ionic liquids [18,19], or via the
use of supercritical CO2 [20]. Among them, direct hydrogenation
of the NBR in latex becomes especially attractive and considerable
efforts have been made to realize direct NBR latex hydrogenation.
NBR latex hydrogenation is much more complicated than that in
solution especially when it comes to finding an effective catalyst.
NBR latex hydrogenation using organometallic catalysts has been
investigated a lot. However, the stability of the catalyst in an aque-
ous system and the necessity of adding a large amount of organic
co-solvent (improve the solubility of catalyst in water) as well as
the catalyst recovery presents major obstacles in NBR latex hydro-
genation.
prene rubber (CR), natural rubber (NR), acrylonitrile-butadiene
rubber (NBR) and styrene-butadiene rubber (SBR) are some of the
most important commercial polymers [1]. They are widely used
as various rubber components in the automobile, petrochemical,
mining and aerospace industries. A common feature of these diene-
based polymers is the presence of the olefinic structures in the
polymer chain which are very active sites for a variety of chem-
ical reactions (e.g. hydrogenation, hydroformylation, metathesis,
coupling reactions). The chemical modification of diene based poly-
mers via hydrogenating the unsaturated C C bonds (C Cs) has
been extensively investigated in academia and industry over the
last two decades which has obtained significant achievements
[
2–4]. Hydrogenation of NBR is an excellent example and it has
been commercialized by Bayer (now known as LANXESS) and ZEON
Chemicals under the trademark Therban® and Zetpol respec-
tively. The hydrogenated NBR (HNBR) improves the chemical,
physical and mechanical properties of the NBR elastomer. It imparts
excellent resistance to oxidative and ozonolytic aging, improved
resistance to oils and fluids, even at high temperatures and reduced
gas permeability.
®
Our group has been deeply involved in the development of poly-
mer modification processes over the last two decades, and we have
focused our efforts on developing effective catalytic processes (i.e.
new catalysts, novel reaction processes) for polymer hydrogena-
tion. Recently, we reported that NBR latex could be successfully
∗
Corresponding author. Tel.: +1 519 888 4567x32702.
Corresponding author. Tel.: +1 519 888 4567x37111.
E-mail addresses: qpan@uwaterloo.ca (Q. Pan),
hydrogenated using RhCl(PPh3) catalyst with the addition of
∗
∗
3
triphenylphosphine (PPh ) [21] and high quality HNBR latex was
3
grempel@uwaterloo.ca (G.L. Rempel).
obtained. However, the low solubility of the RhCl(PPh3)3 catalyst
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.02.015

Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
63
in an aqueous system negatively affected the efficiency of the
catalyst.
Rh-P
As we know, RhCl , the most common rhodium salt, has high
3
a. RhCl(PPh3)3
solubility in water. It was preferably adopted as a rhodium pre-
cursor for the rhodium catalyst synthesis. The in situ synthesized
RhCl(PPh ) in NBR latex using RhCl and PPh for NBR latex hydro-
3
3
3
3
genation is reported for the first time in this paper.
2
. Experimental
b. PPh3
2.1. Materials
Ultra-high purity hydrogen (99.999%, oxygen-free) and high
c. RhCl(PPh ) in PPh
3
3
3
purity nitrogen (99.9%) were purchased from Praxair Inc. (Kitch-
ener, Canada). Commercial grades of NBR latex and solid
RhCl(PPh3)3 catalyst were received from LANXESS Deutschland
GmbH (Germany). Reagent grade chemicals such as mono-
chlorobenzene (MCB), methanol, ethanol, 2-propanol, methyl ethyl
ketone (MEK) and tetrahydrofuran (THF) were obtained from
Fisher-Scientific Canada and used as received. Rhodium trichloride
hydrate (RhCl3.aq, Rh 38∼40%) and triphenylphosphine (Reagent
Plus 99.9 wt.%) were obtained from Sigma–Aldrich (Canada).
Rh-P
600
1800
1600
1400
1200
1000
800
400
-
1
Wave number (cm )
Fig. 1. FT-IR spectra of different solid materials (a) RhCl(PPh3)3, (b) PPh3, (c) pre-
cipitates [RhCl(PPh3)3 in PPh3] from in situ synthesis in water. Reaction condition
for (c): 100 mL H2O, T = 145 C, t = 15 min, RhCl3 = 0.052 mmol, 1.8 mmol PPh3.
◦
2
.2. Hydrogenation of NBR Latex using in situ synthesized
2.5. Particle size in latex and its distribution
RhCl(PPh3)3 catalyst
The size and number size distribution (non-negative least
NBR latex hydrogenation reactions were conducted in a stirred,
00 mL batch reactor (Parr Instrument, USA). The reactor was
squares method) of the polymer particles of the synthesized latex
3
◦
were determined at 25 C using a Nanotrac 150 particle size ana-
cleaned by washing thoroughly with MEK and drying in an oven
over night before each reaction. The reaction temperature was
monitored using a thermocouple. The catalyst precursor RhCl3
was placed in a catalyst addition device (Parr Instrument, USA)
installed in the head of the reactor. PPh3 was pre-added to the
NBR latex before assembling the reactor. The mixture was degassed
and heated to the reaction temperature under agitation. After that,
lyzer (BETATEK Inc. Toronto, Canada) and reported as the number
average diameter. The calculations of the particle size distributions
were performed using Microtrac FLEX 10.2.14 software available
from BETATEK Inc., which employed single-exponential fitting,
non-negatively constrained least-squares (NNLS), cumulants anal-
ysis, and CONTIN particle size distribution analysis routines.
RhCl was charged to the mixture with hydrogen gas. The hydrogen
3
pressure and reaction temperature were kept constant throughout
the reaction period. The reaction mixture was sampled during the
reaction.
3. Results and discussion
3.1. In situ synthesis of RhCl(PPh3)3 and its activity in NBR latex
hydrogenation
2.3. Determination of the degree of hydrogenation
We first investigated the in situ synthesis of RhCl(PPh3)3 in
◦
The degree of hydrogenation was obtained via Fourier-
pure water. It was found that solid PPh3 liquefied around 80 C.
transform infrared (FT-IR) analysis (Bio-Rad FTS 3000MX spectrom-
eter) using ASTM D5670-95 test method. The NBR latex sample
was precipitated by slowly dropping a NaCl solution (10 wt.%) into
the latex under agitation. After the solvent was decanted, the pre-
cipitated rubber was washed with water and dried under reduced
pressure. The dried solid rubber was re-dissolved in MEK and the
polymer solution was cast onto a single NaCl crystal disc and dried.
The degree of hydrogenation was calculated from the FT-IR spectra
according to the peak strength [22].
After adding RhCl , the color of PPh3 droplets quickly changed
3
from transparent to red. This could be explained by the forma-
tion of RhCl(PPh3)3 from RhCl3 and PPh . The formed RhCl(PPh )
3
3 3
has a burgundy color. Due to its hydrophobicity, RhCl(PPh ) tends
3
3
to stay in the PPh3 droplet. As a result, the color of the droplet
changes from colorless to red. When the system cooled down to
room temperature, these red droplets solidified and precipitated.
Fig. 1 shows the FT-IR spectrum of RhCl(PPh3)3 (a), pure PPh3
(b) and the precipitate obtained after RhCl3 reacted with PPh3
(c), respectively. From the FT-IR results, although the spectrum of
2.4. Determination of rhodium concentration in HNBR
the precipitate is similar to that of PPh , the distinctive peak at
3
−
1
5
50 cm attributed to a Rh-P stretching vibration was observed,
HNBR samples were digested in a High Pressure Asher (Anton
confirming the formation of RhCl(PPh ) [24]. After carefully wash-
3
3
Parr, Austria) using the following method: 0.4 g of HNBR sample
was accurately weighed out and mixed with hydrochloric acid
ing with hot ethanol to remove the PPh , most of the precipitates
3
(PPh ) were re-dissolved. The final residue analyzed by FT-IR spec-
3
(
1 mL, 37.5 wt.%), nitric acid (5 mL, 62 wt.%) and aqueous hydrogen
troscopy showed the same spectrum as that of RhCl(PPh ) . In
3
3
◦
peroxide (1 mL of 30%, v/v). The sample was then digested at 300 C,
addition, the yield was calculated based on the feed RhCl . It was
3
1
30 bar (1885 psi) for 3 h. The rhodium concentration was mea-
sured by inductively coupled plasma atomic emission spectroscopy
TELEDYNE, LEEMAN Labs, Prodigy, high dispersion ICP, USA) [23].
found that less than 20 mol.% of rhodium from the feed RhCl could
be converted to RhCl(PPh3)3 in water.
Representative experiments under operational conditions and
final conversions of NBR latex hydrogenation using the in situ
synthesized RhCl(PPh3)3 catalyst as well as the pre-made
solid RhCl(PPh3)3 catalyst are listed in Table 1. Two reaction
3
(
The calibration standards were prepared using rhodium trichlo-
ride solution (1000 ppm, Sigma–Aldrich) and yttrium (Y) was used
as internal reference standard.

6
4
Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
Table 1
NBR latex hydrogenation by different rhodium complexes.
aPPh3/Rh molar ratio
Temp. ( C)
◦
Time (h)
Conversion (mol.%)
Entry
Catalyst/precursor (mmol/L)
PPh3 (mmol/L)
1
2
3
RhCl(PPh3)3
RhCl3
0.52
0.52
0.52
18
18
0
35
35
0
145
160
145
50
26
50
95
95
<20
4
5
6
7
8
0.52
0
0.52
0.52
0.52
0
18
18
18
18
–
–
35
35
35
145
145
145
160
160
50
50
38
19
20
0
0
95
95
96
NBR latex: 50 g/L, total volume = 100 mL, PH = 1000 psig, [Rh]/[C = C] = 1080 (mol./mol.)
2
a
Added PPh3. Not includes the amount of PPh3 in RhCl(PPh3)3 catalyst.
◦
◦
temperatures 145 C and 160 C were used in the investigation.
First, it can be seen that NBR latex could be successfully hydro-
genated using pre-made solid RhCl(PPh3)3 together with PPh3
all of the aliphatic protons in the CH , CH2 and CH microstructures
3
lie between 0.8 and 2.8 ppm. In the HNBR (ca. 95 mol.%), it could be
clearly seen that most of the peaks in the olefinic protons region dis-
appeared which confirms that the unsaturated C Cs were reduced
by H2.
We next turn our attention to the morphology of the latex. First,
the dynamic light scattering (DLS) results showed that during the
hydrogenation operation, no coagulation of the latex was observed
under our reaction conditions. The average particle size of the NBR
latex is around 72 nm. The average size remained the same after the
hydrogenation reaction (Fig. 4). The solid content of the resultant
HNBR latex is also compared to the NBR latex. The difference is less
than 5 wt.% on average.
In addition, Fig. 5 shows TEM micrographs of NBR latex
before/after the hydrogenation step. For comparison, NBR latex (A)
and the HNBR latex with a 95 mol.% conversion (B) are also depicted.
Although the particle size distribution of the commercial NBR latex
is broad, it is not affected either by in situ synthesis process or
the hydrogenation reaction. All these results show that NBR hydro-
genation using in situ synthesized RhCl(PPh3)3 catalyst would not
affect the latex particle size or latex stability.
(
Entry 1, 2). Without the addition of PPh , the hydrogenation reac-
tion was slow and only part of the C Cs were reacted (Entry 3).
In the in situ method, RhCl3 could not catalyze the hydrogena-
3
tion reaction (Entry 4). After adding PPh , it was found that the
3
C
Cs in NBR could be quantitatively hydrogenated, indicating that
RhCl(PPh3)3 was in situ synthesized in the NBR latex from RhCl3
and PPh3.
Under the same reaction condition (i.e. Rh amount, temperature
and pressure), it is clearly seen that the NBR hydrogenation reaction
was faster using the in situ synthesized RhCl(PPh3)3 catalyst than
the pre-made solid counterpart. The hydrogenation reaction time
both hydrogenated to 95 mol.%) was shortened from 50 h to 38 h
at 145 C (Entry 1 vs. Entry 6) and from 26 h to 19 h at 160 C (Entry
vs. Entry 7). The reproducibility of this reaction was also tested
and a satisfactory result was observed (Entry 7, 8). It was worth
mentioning that the in situ synthesis only takes several minutes.
Compared to the total reaction time, it is a quick process. Once
the catalyst is in situ synthesized, the reaction pathway should be
similar to the pre-made solid catalyst. In addition, more detailed
kinetic experiments are necessary to obtain the real reaction rates.
(
◦
◦
2
3.3. Catalyst diffusion in NBR latex during hydrogenation
3.2. Characterizations
The hydrogenation of polymeric materials in latex is quite
different from the hydrogenation of small olefins due to their
high molecular weight. The polymer hydrogenation reactions were
Typical FT-IR spectra of original NBR (a), hydrogenated NBR (b)
obtained using in situ RhCl(PPh3)3 are depicted in Fig. 2. In the
−
1
spectrum of NBR, the peak at 2236 cm belongs to the –CN stretch-
−
1
ing vibration. The intense peak shown at 970 cm is characteristic
of the level of olefin by the carbon–hydrogen vibration of the 1,4-
trans double bonds. The absorbance peak at 920 cm is that of the
-1
1
,2 vinyl olefins (917 cm )
C-H stretching aliphatic peaks
−
1
-1
1
,2 vinyl terminal bonds.
1,4 trans olefins (970 cm )
After hydrogenation, it was observed that the intensity of the
−1
−1
peaks at 970 cm and 920 cm considerably diminished while a
-1 C-C bending
C≡N stretch (2236 cm )
−1
−1
peak at 723 cm appeared. The peak at 723 cm in the hydro-
genation NBR corresponds to the -CH rocking vibration (saturated
2
–
[CH ]n–
unit). This transformation indicates the reduction of
2
(n>4)
unsaturated C C bonds to saturated C C bonds. The chemical sta-
bility of unsaturated nitrile group during the hydrogenation was
also measured. The presence of the nitrile group in HNBR and the
a. NBR
−
1
−1
−1
lack of primary amines (two bands from 3250 cm to 3400 cm
)
)
Saturated -(CH ) -units
2 n
−
1
-1
or secondary amines (one band from 3310 cm
to 3350 cm
(723 cm )
confirms that the hydrogen reduction is exclusively toward
b. HNBR
C
C bonds.
Fig. 3 shows the chemical shifts of the protons in NBR and hydro-
1
genated NBR by H NMR characterization. For NBR, the peaks from
3
500
3000
2500
2000
1500
1000
500
5
.6 to 4.9 ppm are attributed to the olefinic protons. Among them,
-1
Wavem number (cm )
the large peaks between 5.3 and 5.6 ppm are related to trans hydro-
gen atoms of butadiene resulting from 1,4-addition and the small
peaks between 4.91 and 5.1 ppm belong to the hydrogen atoms of
pendant vinyl groups which comes from 1,2-addition. Meanwhile,
Fig. 2. Typical FT-IR spectra of NBR and hydrogenated NBR. (a) NBR before hydro-
genation, (b) hydrogenated NBR (95 mol.%), NBR latex = 50 g/L, RhCl = 0.52 mmol/L,
PPh3 = 18 mmol/L, T = 145 C, PH2 = 1000 psig).
3
◦

Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
65
30
25
20
15
10
5
0
NBR
HNBR (95 mol.% conversion)
0.01
0.1
1
10
Particle Size (μm)
Fig. 4. Particle size distributions in the lattices before/after hydrogenation NBR
◦
latex = 50 g/L, RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L, T = 145 C, PH2 = 1000 psig.
the surface of the polymer particles [26]. From our results (Table 1,
Entry 3), less than 20 mol.% C Cs in NBR could be reduced by using
the RhCl(PPh3)3 alone. The contact between the hydrophobic cat-
alyst and the C Cs in the polymer particles is greatly limited. In
order to achieve the required degree of hydrogenation, the cata-
lyst has to diffuse into the polymer particles. When an experiment
for NBR latex hydrogenation using RhCl(PPh3)3 in the presence of
Fig. 3. Typical 1H NMR spectra of NBR and hydrogenated NBR. (a) NBR, (b) HNBR
◦
(
95 mol.%, NBR latex = 50 g/L, RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L, T = 145 C,
PH2 = 1000 psig).
normally conducted at a temperature higher than the polymer’s
glass transition temperature (Tg) but lower than its melting tem-
perature (Tm). Hence, the chain conformational rearrangement is
limited and the movements of the C C bonds are confined to their
own local positions due to the inter-penetration and entanglement
PPh , more than 95 mol.% conversion was achieved (Table 1, Entry
3
1 and 2). Therefore, the addition of PPh3 is required for a success-
ful NBR latex hydrogenation. The PPh3 acted as a reagent (carrier)
to facilitate phase boundary crossing of the catalyst dissolved in it.
From our FT-IR spectra (Fig. 6b), two characteristic peaks for PPh3
[27] were observed when NBR latex was mixed with PPh3 at an
elevated temperature, confirming PPh3 could diffuse into the NBR
polymer particles. In the in situ process, PPh3 is not only used as a
ligand for synthesizing RhCl(PPh3)3 but also works as a carrier for
catalyst transportation during the NBR latex hydrogenation.
To quantify the amount of catalyst residing in the polymer parti-
cles, the resultant HNBRs were digested and the residual rhodium
metal was measured by ICP-AES. Fig. 7 shows the rhodium con-
centration in the polymer particles during the hydrogenation.
The catalyst diffusion was a slow process when using the pre-
made solid RhCl(PPh3)3 catalyst,. The Rh concentration in the
between the polymer chains [25]. On this basis, the contact of C
C
bonds with the catalyst should mainly depend on the diffusion
of the catalyst. As the requirement for industrial applications of
HNBR production needs a high degree of the hydrogenation of the
C
C bonds in the polymer chains (i.e. 95 mol.%), the catalyst needs
to contact most of the C C bonds to promote the hydrogenation
reaction.
Generally, the catalyst can be present in three locations in the
polymer latex: i) in the water medium, ii) at the interface of water
and polymer particles and iii) inside of the polymer particles. Parker
and Roberts, who studied the hydrogenation of SBR latex, found
that the hydrogenation reaction begins after the catalyst contacts
◦
Fig. 5. TEM micrographs of (A) NBR latex; (B) HNBR latex HNBR (95 mol.% conversion), NBR latex = 50 g/L, RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L, T = 145 C, PH2 = 1000 psig.

6
6
Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
-
1
polymer particles gradually increased during the reaction and
reached a steady state (18 wt.%) after 15 h. This is because when the
hydrophobic catalyst RhCl(PPh3)3 was added to the NBR latex, it is
not well dispersed into the aqueous phase. Instead, it remained as
suspended clusters in the water phase under agitation. In the pres-
ence of extra PPh under the set reaction temperature, RhCl(PPh )
was dissolved in the PPh3 droplets and transferred into the NBR
particles. Hence, the RhCl(PPh3)3 catalyst needs to cross both the
catalyst-water interface and water-micelles interface before con-
tacting the C C bonds in the NBR particles.
1
,2 vinyl olefins (917 cm )
C-H stretching aliphatic peaks
-1
1
,4 trans olefins (970 cm )
3
3 3
C-C bending
-1
C≡N stretch (2236 cm )
a.
b.
In the in situ process, the catalyst precursor RhCl3 could be uni-
formly dissolved in the aqueous NBR latex. It could easily react with
the PPh , forming the RhCl(PPh ) catalyst. The in situ synthesized
C-H,C-C vibrations from PPh3
3
3 3
catalyst was dissolved in the adjacent PPh3 droplets and diffused
into the polymer particles. Thereby, the rhodium concentration in
the particles rapidly increased and a steady state was reached in a
short time. Comparing with using solid catalyst, the final retention
of catalyst inside the polymer particles was also higher by using
the in situ method. The quick catalyst diffusion and the high cata-
lyst retention caused a faster hydrogenation reaction in the in situ
process.
3
500
3000
2500
2000
1500
1000
500
-1
Wavem number (cm )
Fig. 6. Transportation of PPh3 from the aqueous phase to NBR particles in NBR latex
a) NBR, (b) NBR from NBR latex with PPh3 at elevated temperature (50 g/L NBR
Comparing the molar percentage of rhodium in the polymer
(
◦
to the total feed rhodium from RhCl , it was found that less than
latex, PPh3 = 18 mmol/L, T = 145 C).
3
half of the total rhodium was transferred into the polymer parti-
cles using the in situ method. This could be explained by the low
in situ synthesis efficiency in the NBR latex system. In addition, a
layer of black particles (Rh colloid) observed at the bottom of the
reactor after reaction is resulted from some reduction of RhCl3 to
1
00
100
80
60
40
20
0
metallic Rh at high temperature under H . A similar phenomenon
2
was observed by Papadogianakis et al. in the hydrogenation of
polybutadiene-1,4-block-poly(ethylene oxide) latex using in situ
synthesized Rh/TPPTS catalyst [28]. The activity of the metallic Rh
was tested for NBR latex hydrogenation and no hydrogenation was
observed [29]. Therefore, the formation of metallic rhodium is a
side reaction during the in situ process. But it is not harmful to NBR
hydrogenation.
(
conversion)
8
6
4
2
0
0
0
0
0
3.4. Effects of temperature, catalyst loading and PPh3 on the
RhCl3 ratio for NBR latex hydrogenation using the in situ process
Pre-made RhCl(PPh3)3
In-situ RhCl(PPh3)3
The effects of reaction temperature, PPh to RhCl ratio and cat-
3
3
alyst precursor RhCl3 loading are some important factors which
have commonly been analyzed in NBR solution hydrogenation
[30,31]. Here, these factors on the extent of hydrogenation conver-
sion were also studied for NBR latex hydrogenation and the results
are summarized in Table 2. The effect of temperature on NBR latex
hydrogenation reaction under the in situ process was studied from
0
10
20
30
40
50
60
Reaction Time (hrs)
Fig. 7. The change of rhodium concentration in NBR polymer particles through
hydrogenation. Solid line: hydrogenation conversion; dash line: rhodium in poly-
mer/total rhodium (mol.%); NBR latex = 50 g/L, RhCl3 or RhCl(PPh3)3 = 0.52 mmol/L,
PPh3 = 18 mmol/L, T = 145 C, PH2 = 1000 psig.
◦
◦
◦
50 C to 160 C at a constant reaction pressure and catalyst load-
ing of 1000 psig and 0.52 mmol/L of RhCl3 respectively (Table 2,
Table 2
NBR latex hydrogenation under in situ process.
aPPh3 (mmol/L)
PPh3/Rh (mol/mol)
T ( C)
◦
Time (h)
Conv. (mol.%)
Rhpolymer/Rhtotal
Entry
RhCl3 (mmol/L)
1
2
3
4
5
6
7
8
9
0.52
0.52
0.52
0.52
0.26
0.52
0.52
0.52
0.52
0.52
1.04
1.80
18
18
18
18
9
1.6
7.8
18
52
18
36
61
35
35
35
35
35
3
15
35
100
35
34
34
50
90
100
100
130
38
100
82
29
19
18
20
<10
54
95
95
96
95
95
95
95
96
95
95
0.01
0.08
0.16
0.26
0.14
0.10
0.20
0.26
0.31
0.26
0.34
0.37
130
145
145
160
160
160
160
160
160
160
1
1
1
0
1
2
15
13
NBR latex = 100 mL (50 g/L), PH = 1000 psig.
2
a
Additional PPh3.

Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
100
67
Entry 1–4 and Entry 8). It is clear that temperature is an important
100
80
60
40
20
0
parameter for the hydrogenation reaction. The reaction temper-
ature greatly affected the in situ synthesis and catalyst diffusion
efficiency. The hydrogenation reaction time (to achieve the same
conversion) dropped with an increase in temperature. In addition,
the rhodium concentration in the polymer product increased with
an increase in temperature.
9
8
7
6
0
0
0
0
Conversion
◦
The ratio of PPh3 to RhCl3 was studied at 160 C at a constant
50
RhCl3 loading (Rh/NBR) and H2 pressure (Table 2, Entry 6–9). It
was suggested before that the free PPh3 performed as the required
free co-catalyst ligand to maintain the stability of the main catalyst
4
3
0
0
R
h
C
l
(
P
P
h
)
d
u
r
i
n
g
t
h
e
N
B
R
s
o
l
u
t
i
o
n
h
y
d
r
o
g
e
n
a
t
i
o
n
a
n
d
t
h
e
P
P
h
3
3
3
20
had a marginal effect on the activity of the RhCl(PPh ) catalyst [32].
3
3
10
However, it can be seen that the PPh3 plays a very significant role
in the NBR latex hydrogenation reaction. The reaction time greatly
decreased from 82 h to 19 h on increasing the PPh3 to RhCl3 ratio
from 3 to 35. However the reaction time almost keeps constant
when the ratio increases to 100. Although a larger amount of cat-
alyst could diffuse into the polymer particles with the addition of
Pre-made catalyst
In-situ synthesized
0
In-situ synthesized with ethanol
-10
0
10
20
Reaction Time (hrs)
30 40 50
Fig. 8. Hydrogenation conversion and rhodium concentration in NBR polymer
particles under different catalytic systems. Solid line: hydrogenation conversion;
dash line: rhodium in polymer/total rhodium (mol.%). NBR latex = 50 g/L, total
more PPh , the excessive PPh3 would slow down the formation of
3
the reactive intermediate RhCl(PPh3)2 by shifting the equilibrium
of the RhCl(PPh3)3 dissociation. Also, the effect of the RhCl3 load-
ing was compared under different temperatures. It can be seen the
reaction time was significantly affected under lower RhCl3 loading
◦
volume = 100 mL, T = 145 C, PH2 = 1000 psig, RhCl(PPh ) or RhCl3 = 0.52 mmol/L,
3 3
PPh3 = 18 mmol/L, In situ with ethanol: RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L,
ethanol/total = 1/10 (v/v).
(
(
Table 2, Entry 4, 5) while this was not so evident at higher loadings
Table 2, Entry 10–12).
Table 3
NBR latex hydrogenation by RhCl(PPh3)3/PPh3 adding ethanol.
No
Ethanol
Reaction time (h)
Conversion (mol.%)
3.5. Catalyst in situ synthesis with alcohol in NBR latex
hydrogenation
1
2
No
Yes
21
20
95.5
96.0
◦
In light of the in situ synthesizing RhCl(PPh3)3 from RhCl3 and
PPh3 in NBR latex hydrogenation, one way to improve the in situ
NBR latex = 50 g/L, total volume = 100 mL, T = 160 C, PH2 = 1000 psig. With-
out ethanol: RhCl(PPh3)3 = 0.52 mmol/L, PPh3 = 18 mmol/L. With ethanol:
RhCl3 = 0.52 mmol/L, PPh3 = 18 mmol/L, ethanol/total = 1/10 (v/v).
synthesis efficiency is to convert more catalyst precursor RhCl into
3
RhCl(PPh ) . Right now, the standard reference work is the most
3
3
familiar method for synthesizing RhCl(PPh ) using RhCl and PPh .
3
3
3
3
catalyst should be in situ generated under the NBR latex/ethanol
mixture.
In this method, RhCl3 reacts with excess PPh3 in ethanol to form
RhCl(PPh ). Unfortunately, RhCl(PPh ) could not be efficiently
3
3 3
The DLS results showed that after adding ethanol, the NBR par-
ticle size slightly increased from 72 nm to 80 nm. This is because
when ethanol diffused into the micelle, it can counter balance the
charges and reduce the electrical repulsion in the stern layer of the
micelles [34]. In other words, the charge density at the micelles
surface decreased and thereby the absolute value of the electrical
potential was also reduced [35].
synthesized in water due to the low solubility of PPh in pure water.
3
However, it was recently disclosed that when the RhCl(PPh3)3 was
synthesized in a mixture of water and alcohol, the yield of the cat-
alyst could be improved to a great extent compared when using
pure water or pure alcohol alone [33]. On this basis, adding a small
amount of alcohol (e.g. ethanol) to the NBR latex was tested in our
reaction system.
The effect of the ethanol to the total volume ratio in the in situ
method is presented in Table 4. It was found that the reaction
became faster as the amount of ethanol volume ratio increased
from 1/20 to 1/5, indicating most likely that more catalyst could be
synthesized in situ with the higher levels of ethanol. However, it
was also observed that further addition of ethanol to the NBR latex
interrupted the stability of latex. As a result, polymer particles were
precipitated from the latex.
Attempts were also made to investigate the effect of different
kinds of alcohols (same mol level) for NBR latex hydrogenation
(Table 4). Under the same reaction conditions, the reaction time
decreased with the sequence of t2-propanol < tethanol < tmethanol. Due
ethanol
RhCl + H O + 4PPh −→ RhCl(PPh ) + PPh O + 2HCl
3
2
3
3
3
3
The results of NBR latex hydrogenation in the presence of
ethanol are shown in Fig. 8. After the introduction of ethanol, the
reaction time greatly decreased from 38 h to 20 h at 145 C. The
rhodium concentration in the polymer particles was also measured
by the ICP-AES and more Rh was present inside the polymer parti-
cles. It should also be noted here that ethanol could not dissolve the
RhCl(PPh3)3 or NBR polymer in the latex. No distinctive promotion
for the hydrogenation reaction was observed when adding ethanol
to pre-made solid catalyst (Table 3). Therefore, more RhCl(PPh3)3
◦
Table 4
NBR latex hydrogenation using in situ RhCl(PPh3)3 with the addition of alcohol.
Entry
RhCl3 (mmol/L)
PPh3 (mmol/L)
Type of alcohol
Alcohol conc. (mol/L)
Time (h)
Conversion (mol.%)
1
2
3
4
5
0.52
0.52
0.52
0.52
0.52
18
18
18
18
18
Ethanol
Ethanol
Ethanol
Methanol
2-propanol
3.42
1.71
0.855
0.855
0.855
5.8
7.6
10.3
13.0
7.0
95
95
95
95
95
NBR latex: 50 g/L, total volume = 100 mL, PH2 = 1000 psig, [Rh]/[C C] = 1080 (mol./mol.).

6
8
Y. Liu et al. / Applied Catalysis A: General 457 (2013) 62–68
to its stronger reduction ability and lipophilic property (it can work
as a surfactant), 2-propanol more likely stays around the polymer
particles and the in situ formed catalyst can rapidly diffuse into the
inside of the polymer particles. Therefore the hydrogenation reac-
tion in the presence of 2-propanol is faster than that of ethanol or
methanol.
[3] N.K. Singha, S. Bhattacharjee, S. Sivaram, Rubber Chem. Technol. 70 (1997)
09–367.
3
[
4] G.L. Rempel, Q. Pan, J. Wu, in: J.E.C. G. de, V. Johannes (Eds.), Handbook of
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1
[
4
[7] D.C. Mudalige, G.L. Rempel, J. Mol. Catal. A: Chem. 123 (1997) 15–20.
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4
. Conclusions
[
10] N.K. Singha, S. Sivaram, S.S. Talwar, Rubber Chem. Technol. 68 (1995) 281–286.
RhCl(PPh3)3 catalyst could be in situ synthesized in NBR latex
[11] K.A.N. Yasuhiko, F. Masaharu, S. Tomoji, Japan Patent 2003126698, 2003.
[12] X.W. Lin, Q.M. Pan, G.L. Rempel, Appl. Catal. A: Gen. 263 (2004) 27–32.
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using RhCl3 and PPh . NBR latex hydrogenation undergoes a faster
3
reaction under the in situ method than using the pre-made solid
RhCl(PPh3)3 catalyst system since more catalyst could be quickly
diffused into the NBR particles in the in situ method. The catalyst
diffusion process was also investigated by monitoring the catalyst
concentration change in the NBR particles and the result confirms
that the high degree of hydrogenation can only be achieved when
the catalyst is transported into the polymer particles in NBR latex.
Finally, the production of HNBR latex by in situ synthesis process
could be greatly enhanced by the addition of a small amount of an
alcohol (not to dissolve catalyst) which greatly improves the in situ
catalyst synthesis efficiency.
[15] T. Weiss, K. Creutz, US Patent 20080234437, 2008.
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[
[
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1
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2
[
[
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[
[
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Acknowledgement
[
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4
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This research work was financially supported by Natural Sci-
ences and Engineering Research Council of Canada (NSERC), the
Canada Foundation for Innovation (CFI) and LANXESS Deutschland
GmbH. Special thanks are also given to University of Waterloo for
its support of author’s Ph.D. program.
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[
[
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