Hydrogenation of bisphenol A - Using a mesoporous silica based nano ruthenium catalyst Ru/MCM-41 and water as the solvent

Article abstract of DOI:10.1016/j.cattod.2011.01.050

A green approach to the hydrogenation of bisphenol A (BPA) including the selection of catalysts and solvents was demonstrated in this report. The catalyst preparation was also using a simple and green method for synthesizing a silica-supported ruthenium catalyst (Ru/MCM-41) in supercritical carbon dioxide. The characterizations of the Ru/MCM-41 nanocomposites were preformed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The nanoparticles have an average size of 3.4 nm which is smaller than the pore size of MCM-41. The hydrogenation of BPA was carried out in a hydrogen pressure of 50 bars and a temperature range from 75 to 85 °C with different catalysts and solvents combinations. It was found that using the synthesized catalyst Ru/MCM-41 in water medium could achieve the highest efficiency and durability for the hydrogenation of BPA.

Full text of DOI:10.1016/j.cattod.2011.01.050

Catalysis Today 174 (2011) 121–126
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Hydrogenation of bisphenol A – Using a mesoporous silica based nano ruthenium
catalyst Ru/MCM-41 and water as the solvent
Clive H. Yen, Hsin Wei Lin, Chung-Sung Tan∗
Department of Chemical Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A green approach to the hydrogenation of bisphenol A (BPA) including the selection of catalysts and
solvents was demonstrated in this report. The catalyst preparation was also using a simple and green
method for synthesizing a silica-supported ruthenium catalyst (Ru/MCM-41) in supercritical carbon diox-
ide. The characterizations of the Ru/MCM-41 nanocomposites were preformed by transmission electron
microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The nanoparticles have an average
size of 3.4 nm which is smaller than the pore size of MCM-41. The hydrogenation of BPA was carried out
in a hydrogen pressure of 50 bars and a temperature range from 75 to 85 ◦C with different catalysts and
solvents combinations. It was found that using the synthesized catalyst Ru/MCM-41 in water medium
could achieve the highest efficiency and durability for the hydrogenation of BPA.
Received 26 October 2010
Received in revised form
29 December 2010
Accepted 21 January 2011
Available online 2 March 2011
Keywords:
Bisphenol A
Ruthenium nanoparticles
Mesoporous silica
Hydrogenation
Water
© 2011 Elsevier B.V. All rights reserved.
Supercritical fluid
1. Introduction
which was catalyzed by nickel catalysts [3]. Maegawa et al. [4] used
commercial rhodium and ruthenium on activated carbon catalysts
Bisphenol A (BPA; 4,4ꢀ-isopropylidenediphenol) is an univer-
sal starting material for making plastic products. A representative
application is to use BPA to synthesize epoxy resins. The BPA type
epoxy resins could be used for light-emitting diode (LED) encapsu-
lation due to their chemical resistance, mechanical and electrical
strength associated with excellent thermal stability. However, the
aromatic groups in the epoxy resins are susceptible to yellowing
upon ultraviolet (UV) light exposure or thermal degradation caus-
ing an interference of the LED transmission through the epoxy
resins [1]. On the other hand, BPA made polycarbonate plastic is
also widely used in daily products such as food containers. The
health concerns of BPA have drawn much attention in recent years
since BPA is labeled as an endocrine disruptor [2]. Therefore, a
practical solution is to apply hydrogenated bisphenol A (HBPA;
4,4ꢀ-isopropylidenedicyclohexanol), which does not contain the
aromatic groups, to substitute the general usage of BPA. HBPA is not
an endocrine disruptor and does not cause yellowing when used in
epoxy resin for the LED chip packaging since there are no aromatic
groups available to absorb the UV light. There are several reports
studying the hydrogenation reaction of BPA. A pioneering study
by Terada proposed the mechanism of the hydrogenation of BPA
to hydrogenate BPA in isopropanol solvent. Their results showed
that under mild hydrogen pressure and temperature conditions, the
Rh/C catalyst was more efficient than the Ru/C catalyst. Wang et al.
[5] studied the hydrogenation of BPA using Al2O3-supported noble
metal catalysts. In their continuous reaction system, the product
selectivity towards HBPA was more than 94%. To the best of our
knowledge, the hydrogenation of BPA using water as a solvent
has not been reported before. The advantage of using water as a
green solvent is well known [6–9]. In brief, water is nontoxic, non-
flammable and easy to obtain. To achieve an ideal green reaction by
using water as the solvent, insoluble products are more preferable
since they could be easily separated by precipitation or filtration.
Thus the water solvent may maintain its purity and cleanness for
further using without producing any additional aqueous waste.
Moreover, if possible, it would be an advantage to have acceler-
ated reaction rates when using water as the solvent comparing to
using other organic solvents. In this study, the reactant BPA and
the product HBPA are both sparingly soluble in water (<300 ppm
at 21.5 ◦C). It was observed that they form a suspension (estimated
solubility 1000 ppm) in hot water with stirring [10]. Therefore, the
BPA hydrogenation in water may be considered as an ideal green
reaction. Herein this manuscript, some examples of hydrogenation
of aromatic compounds in water were reported by using gaseous
hydrogen as the hydrogen source. This is somewhat different from
the transfer hydrogenation reactions that have been reported in
∗
Corresponding author. Tel.: +886 3 572 1189; fax: +886 3 572 1684.
E-mail address: cstan@mx.nthu.edu.tw (C.-S. Tan).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.01.050

122
C.H. Yen et al. / Catalysis Today 174 (2011) 121–126
the literature [11,12]. In addition, the catalyst used in this study
is a mesoporous silica based nano ruthenium catalyst Ru/MCM-41.
It was prepared by a simple supercritical fluid deposition method
which has also been considered as a green process [13]. The selec-
tion of a silica based catalyst also proves to be adequate for the
hydrogenation reaction in water.
2. Experimental
2.1. Catalyst preparation
Cyclic ligand metal precursors were used as the CO2-soluble
metal precursors in this study. Bis(2,2,6,6-tetramethyl-3,5-
heptanedionato)(1,5-cyclooctadiene)ruthenium [Ru(cod)(thmd)2,
Strem], MCM-41 (SiO2, Sigma–Aldrich) were all used as received. In
a typical trial, 285 mg of MCM-41 and ca. 87 mg of Ru(cod)(thmd)2
were added together into a high pressure cell for a maximum metal
ratio of 5% by weight. At 150 ◦C, 100 bar of H2 and 100 bar of CO2
were premixed in a gas reservoir and injected into the cell for a
reaction of 2 h. After the reaction, the cell was depressurized and
flushed with CO2 for a few times to eliminate the unreacted metal
precursors. The remaining powder sample was then collected for
further analysis and catalytic testing.
Fig. 1. TEM images and statistics calculation of the synthesized Ru/MCM-41
nanocomposite. (a) Particles mostly inside the pores, scale bar = 20 nm; (b) particles
mostly outside the pores, scale bar = 50 nm.
2.2. Catalyst characterization
capillary column (Agilent HP-5MS) was used. The injector and the
detector temperatures were set at 250 ◦C and 260 ◦C, respectively. A
temperature program was employed for analysis starting at 50 ◦C
(hold for 5 min), followed by a 10 ◦C/min program rate to 250 ◦C
(hold for 5 min). The product, 4,4ꢀ-isopropylidenedicyclohexanol
[HBPA 97%, Sigma–Aldrich] was used as an analytical standard.
2.2.1. Transmission electron microscopy
The images of transmission electron microscopy (TEM) were
taken by Joel JEM-2100. The synthesized catalyst powder was
diluted with ethanol and ultrasonicated for 5 min. Droplets of the
prepared solution were put on a copper grid and dried in a vacuum
oven of 100 ◦C overnight. The statistics calculations of the particle
size distribution were made by using an interactive imaging soft-
ware OPTIMAS5. At least 100 particles were recorded in order to
obtain the average particle size and standard deviation.
3. Results and discussion
3.1. Catalyst preparation and analysis
The preparation method of the catalyst using supercritical car-
bon dioxide was previously reported with some minor changes
[14,15]. The metal precursor used in our previous study was a
beta-diketonate ruthenium compound, ruthenium acetylacetonate
[Ru(acac)3]. The metal beta-diketonate compounds usually have
lower solubility in CO2 than those of metal cyclic compounds [16].
Therefore, in our previous work, the addition of an organic solvent
was necessary to help dissolve and disperse the metal precursor.
In this current study, the organic solvent can now be excluded due
to the higher solubility of the cyclic metal precursor. As expected,
the synthesized ruthenium nanoparticles on MCM-41 by the two
different precursors were nearly identical in chemical catalytic
activity since they were similar in both metal loading and particle
size [17]. In order to achieve a greener approach, the metal cyclic
compound, Ru(cod)(tmhd)2 was chosen as the precursor for the
current research [18].
2.2.2. Powder X-ray diffraction
Powder X-ray diffraction (XRD) was conducted by Rigaku Ultima
IV. The Cu K␣ radiation was 40 kV, 20 mA. The scan rate was 1◦/min
starting at 10◦ to 90◦ (2Â).
2.2.3. Energy dispersive X-ray spectroscopy
Energy dispersive X-ray spectroscopy (EDS) was measured by
Oxford 6587 which was coupled with a scanning electron micro-
scope (SEM JEOL JSM-5600). The quantitative analysis of the
ruthenium content in the nanocomposite was taken at least 5 dif-
ferent sample points into average.
2.2.4. BET measurements
The BET measurements were based on N2 adsorption by
Micromeritics ASAP 2010. The outgas condition was 200 ◦C, 24 h.
2.3. Hydrogenation of BPA and other aromatic compounds
The support, MCM-41, was invented by Mobil company in 1992
and it has a 2 dimensional tubular structure with meso-size pores
on both ends [19]. The TEM images of the catalyst Ru/MCM-41 and
the statistics calculations of the particle size are shown in Fig. 1.
More than 100 particles were measured in order to obtain the aver-
age size 3.4 nm. It could be observed that the smaller ruthenium
nanoparticles were grown inside the pores of MCM-41 (Fig. 1a)
while the bigger clusters were only attached to the outer part of
the support (Fig. 1b). The physical properties of the support MCM-
41 were provided by both the manufacturer and our experimental
values which are listed in Table 1.
The experimental apparatus for the aqueous hydrogenation of
BPA is a semi-batch system. Ruthenium on activated carbon [5%
Ru/C, Strem], isopropanol [HPLC-grade 99.9% IPA, Echo] and 4,4ꢀ-
isopropylidenediphenol [BPA 99+%, Sigma–Aldrich] were all used
as received. In a typical trial, a mixture of 50 mg of the catalyst,
1 g of the BPA and 50 g of deionized water were all added into a
high pressure autoclave. Hydrogen gas of 50 bar was then intro-
duced into the cell at temperature of 75 ◦C for a reaction time of
5 h. The data points were collected after the reaction system was
depressurized and cooled down to room temperature. The products
were then determined by gas chromatography–mass spectrome-
try (GC–MS; HP5890II/HP5972). A dimethylpolysiloxane based GC
From EDS analysis, the Ru metal loading was found to be 3.7% by
weight. The oxygen to silicon atomic ratio, however, was increased
from the expected value 2 (silica is SiO2) to nearly 3. This could

C.H. Yen et al. / Catalysis Today 174 (2011) 121–126
123
Table 1
Table 2
Physical properties of MCM-41.
Comparison of different solvents used for the hydrogenation of BPAa.
MCM-41
BET surface area
Pore size (nm)
Pore volume
(cm3/g)
Solvent
Dielectric
constant
Conversion (%)
HBPA yield (%)
TOFb (h−1
)
(m2/g)
Sigma–Aldrich
Experimental
1000
1214
2.3–2.7
3.7
0.98
1.14
Cyclohexane
Ethyl acetate
Isopropanol
Water
2.02
6.02
18
80
80
21.3
12.7
44.4
97.0
99.6
7.9
0.6
8.7
51.0
91.9
41
19
74
207
Water
342c
imply that the adsorption of water molecules occurred on the silica
surface since silica is a well known water adsorbent. According to
the literature, the water molecules could be removed by the super-
critical fluid treatment and free hydroxyl groups could be created
on the silica surface [20]. These hydroxyl groups may help adsorb
the metal precursors on the silica. The overall yield of the supercriti-
cal fluid deposition process could also be calculated and an efficient
75% yield (from maximum metal loading 5%) was obtained. Repro-
ducibility of the catalyst was also examined and the overall yield
was within a 5% range.
The powder XRD analysis of the Ru/MCM-41 is shown in Fig. 2.
The broad peak between 15◦ and 30◦ can be assigned to the silica
support, MCM-41. Ruthenium has a hexagonal closed-packed (hcp)
structure and the major peak (1 0 1) is located at 44.0◦ (2Â). All of the
analyses mentioned above could confirm that the Ru nanoparticles
were deposited on the mesoporous silica support, MCM-41.
a
Ru/MCM-41: 50 mg; solvent: 50 g; BPA: 1 g; H2: 50 bar; temperature: 75 ◦C;
reaction time: 5 h.
b
Turnover frequency (TOF) was calculated as the moles of hydrogen consumed
in 1 h (1 mole of H2 for 1 mole of C C double bond) divided by the total moles of
metal inside the catalyst.
c
85 ◦C, 4 h.
suitable environment for the silica based catalyst to disperse thor-
oughly in the reaction medium. It is suggested that the hydroxyl
groups on the silica surface could form hydrogen bonding with the
water molecules [21]. The hydrogen bonds could help the water
molecules surround the catalyst surface thus creating a better dis-
persion of the catalyst. Therefore, water was demonstrated to be a
suitable solvent for the hydrogenation of BPA using Ru/MCM-41.
3.3. Effect of solubility in water for the hydrogenation of BPA
3.2. Solvent selection for the hydrogenation of BPA using
Ru/MCM-41
Since the major portion (about 95%) of the reactant does not
dissolve in water, it is believed that the concentration of the reac-
tant should be saturated when adjusting the water amount in the
system. From Table 3, the water amount was doubled to the previ-
ous experiment with all other conditions maintained. As a result,
the conversion was still maintained close to completion while the
HBPA yield and TOF both decreased with the addition of water.
A possible explanation is that the concentrations of the reactant
and the catalyst were diluted by the additional amount of water
and the effective collisions between them during the reaction were
reduced. This outcome showed that the solubility of the reactant
was not the limitation for the reaction since the amount of the fully
converted reactant was far more than the amount of the dissolved
reactant. The low solubility of the product should not have a neg-
ative influence to the reaction either since it could be separated
out of the system easily. Another possibility is the occurrence of
the “on-water” reaction [8]. The insoluble reactants could directly
participate in the reaction which could increase the reaction rate.
Therefore, it is unnecessary to use more amount of water for our
selected conditions. For green chemistry, it is also very important
to reduce the water amount for the hydrogenation process since
water is a valuable natural resource.
The hydrogenation of BPA using the synthesized catalyst
Ru/MCM-41 was conducted in different types of common solvents
including water, a non-polar solvent: cyclohexane, a polar apro-
tic solvent: ethyl acetate and a polar protic solvent: isopropanol.
The experimental results are listed in Table 2. The turnover fre-
quency (TOF) was calculated as the moles of hydrogen consumed
in 1 h (1 mole of H2 for 1 mole of C C double bond) divided by
the total moles of metal inside the catalyst. It is unexpected to see
that the reaction proceeded most rapidly in water. Normally, the
dielectric constant of a solvent is one of the first considerations for
the solvent selection. In the case for hydrogenation, solvents with
lower dielectric constant would be preferred since the solubility of
hydrogen in these solvents would be higher. On the other hand, the
reactant BPA is almost insoluble in water and cyclohexane, whereas
it is completely soluble in ethyl acetate and isopropanol. Regard-
less of all the disadvantages mentioned above, water provides a
3.4. Catalyst selection and recyclability for the hydrogenation of
BPA in water
The next series of experiments were designed to confirm if
Ru/MCM-41 is a suitable catalyst for the hydrogenation of BPA in
water. For comparison, a commonly used 5% Ru/C commercial cata-
lyst was chosen for the testing. According to the literature, carbon or
charcoal supports usually have better catalytic activity than oxide
supports in hydrogenation reactions under various solvents [22].
From our experimental results, which are shown in Fig. 3, it was dif-
Table 3
Comparison of different water amount for the hydrogenation of BPAa.
Water amount (g)
Conversion (%)
HBPA yield (%)
TOF (h−1
)
50
100
99.6
97.9
91.9
67.8
342
290
a
Ru/MCM-41: 50 mg; BPA: 1 g; H2: 50 bar; temperature: 85 ◦C; reaction time: 4 h.
Fig. 2. XRD pattern of the synthesized Ru/MCM-41.

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C.H. Yen et al. / Catalysis Today 174 (2011) 121–126
Fig. 3. Reusing of the catalysts for the hydrogenation of BPA. (a) The conversion percentage of BPA; (b) the yield of HBPA. TTON is defined as the total moles of hydrogen
consumed in the three runs combined (1 mole of H2 for 1 mole of C C double bond) divided by the total moles of metal inside the catalyst. Note that the metal content of
Ru/MCM-41 was increased to 4.7% in order to be more comparable with the commercial 5% Ru/C catalyst.
ficult to distinguish the catalytic activity of the two catalysts during
the first runs since the conversion of the reactions were both close
to completion (Fig. 3a Run 1). Therefore, we changed the strategy
to examine the recyclability of the catalysts which is related to the
catalytic activity over time. Additional runs of tests were added in
order to study the recyclability of the catalysts. It is important to
reuse the catalysts, especially heterogeneous ones, not only for the
environmental purpose but also for the economical purpose. Two
different recycling methods suitable for silica based catalysts were
reportedly available. The first method was using a rotary evapo-
rator to discard the water solvent after the reaction. Ethanol was
added thereafter to the slurry type residue to dissolve the products
so that the catalyst could be removed by filtration [23]. The other
method was a direct extraction by diethyl ether of the product out of
the water layer [4]. Since BPA and HBPA are both nearly insoluble in
water, using small amount of organic solvent which is immiscible in
water could extract the compounds completely without removing
the catalysts out of the water phase. Thus the water phase including
the catalyst could be used repeatedly [24]. However, it is a pity
that the direct extraction method could not be applied when using
carbon supported catalysts. Since the hydrophobicity of the carbon
surface have higher affinity to the organic phase, the carbon catalyst
would be extracted out of the water phase along with the products.
Therefore, only the first recycling method using rotary evapora-
tor was selected in this study. From Fig. 3a, the conversion of the
reaction in the first three runs could be kept to completion when
using both catalysts, although there was a slight decrease during
the third run when using Ru/C. If focusing on the yield of the prod-
uct HBPA, shown in Fig. 3b, the difference of the two catalysts would
be clearer. In the case of Ru/C, the HBPA yield had a huge drop in the

C.H. Yen et al. / Catalysis Today 174 (2011) 121–126
125
Fig. 4. The hydrogenation of BPF (BPF: 1 g; Ru/MCM-41: 50 mg; water: 50 g; TOF = 337 h−1).
the benzoic acid molecules not only can penetrate faster into the
pores of MCM-41 but also can have less occupation of the active
surface of the metal nanoparticles. Furthermore, since there is only
one aromatic ring in benzoic acid, all the C C double bonds are
coplanar which are readily for the syn addition of the hydrogen
atoms. In BPA and BPF, however, the two aromatic rings would
have more difficulties for this type of concerted reaction. Conse-
quently, the synthesized Ru/MCM-41 catalyst is very efficient for
the hydrogenation of various reactants in water solvent.
Fig. 5. The hydrogenation of benzoic acid (benzoic acid: 1 g; Ru/MCM-41: 50 mg;
water: 50 g; TOF = 1329 h−1).
third run which was corresponding to the decrease in conversion.
For Ru/MCM-41, the HBPA yield were close to maximum in the first
two runs and started to have a small decrease in the third run. Nev-
ertheless, the HBPA yields were always higher by using Ru/MCM-41
than those by Ru/C. The other product, 2-(4-hydroxycyclohexyl)-2-
(p-hydroxyphenyl)propane, was considered as the intermediate of
the reaction since only one benzene ring of BPA was hydrogenated.
No other compounds were detected so that the conversion of BPA
was equal to the total yield of the two products. Comparing the total
turnover number (TTON) is an apparent way to evaluate the dura-
bility of the catalysts. It is a useful information to know how many
reactants the catalyst can convert before losing its catalytic activity.
Here, the TTON is defined as the total moles of hydrogen consumed
in the three entries combined (1 mole of H2 for 1 mole of C C double
bond) divided by the total moles of metal inside the catalyst. With
a lower TTON value, it is no surprise to see the catalytic activity of
Ru/C being inferior to that of Ru/MCM-41. As mentioned before, the
hydrophilic property of the silica support causes the entire catalyst
to have a better dispersion in the water phase than the hydropho-
bic type carbon supported catalyst. According to some reports, the
hydroxyl groups from the surface of silica may have a strong inter-
action, such as hydrogen bonding, with the reactants [25,26]. More
contacts between the reactant and the catalyst would occur with
the assistance of the strong interaction thus increasing the reaction
rate. Moreover, the declining activity for both catalysts should be
attributed to the leaching and poisoning of the ruthenium metal in
water. To summarize this section, the silica supported catalyst was
proven to be a more durable catalyst for the hydrogenation reaction
in water.
4. Conclusion
The hydrogenation of water insoluble compounds such as BPA
and BPF has been successfully demonstrated using water as the sol-
vent. From the measured experimental data, the low solubility of
the reactants in water was found not to be the limitation for the
reactions. On the other hand, the low solubility of the products in
water could simplify the work-up process of the reaction by filtra-
tion or precipitation. In addition, the nanocomposite Ru/MCM-41
has been synthesized and applied as a catalyst for various hydro-
genation reactions in water. The catalytic activity and the durability
of the Ru/MCM-41 catalyst were tested to be superior to a com-
mercial carbon supported catalyst due to better dispersion and
recyclability of silica in water. Last but not least, the combination
of applying a supercritical fluid technique for the catalyst prepara-
tion and a reaction in water medium could make the overall process
“greener” which is the ultimate goal of green chemistry for a future
sustainable world.
Acknowledgements
The authors would like to acknowledge the assistance from Dr.
H.T. Hsieh for obtaining the TEM images. This work was supported
by National Science Council of ROC (NSC 96-2628-E-007-125-MY3)
and Ministry of Economic Affairs of ROC (97-EC-17-A-09-S1-022).
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