Synthesis of phthalate-free plasticizers by hydrogenation in water using RhNi bimetallic catalyst on aluminated SBA-15

Article abstract of DOI:10.1039/c7ra02227a

In this study, rhodium-nickel bimetallic nanoparticles loaded on aluminated silica (RhNi/Al-SBA-15) were used as catalysts for the hydrogenation of phthalate in water to produce environmentally acceptable non-phthalate plasticizers. Chemical fluid deposition (CFD) was used to dope metals onto the aluminated silica support, which helped to create a uniform structure of RhNi on Al-SBA-15. The introduction of Ni helped to reduce the use of expensive Rh and increase the number of metal active sites by reducing the bimetallic nanoparticle size. Aluminated SBA-15 not only acted as the support for the RhNi bimetallic catalyst but also enhanced the reaction efficiency by introducing Br?nsted and Lewis acid sites and the absorption of phthalates on the catalyst in water. The physicochemical properties of prepared catalysts were characterized by N₂ adsorption-desorption isotherm, X-ray diffraction (XRD), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM). The catalytic performance of the synthesized catalysts was evaluated with the hydrogenation of dimethyl phthalate (DMP). Despite the low solubility of DMP in water, the hydrogenation using Rh_0.5Ni_1.5/Al-SBA-15 was carried out with an 84.4% reaction yield (cis-?:?trans- = 97.5?:?2.5) at 80 °C using 1000 psi of H₂ after 2 h.

Full text of DOI:10.1039/c7ra02227a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis of phthalate-free plasticizers by
hydrogenation in water using RhNi bimetallic
Cite this: RSC Adv., 2017, 7, 18178
catalyst on aluminated SBA-15†
*
Duc-Ha Phan-Vu and Chung-Sung Tan
In this study, rhodium–nickel bimetallic nanoparticles loaded on aluminated silica (RhNi/Al-SBA-15) were
used as catalysts for the hydrogenation of phthalate in water to produce environmentally acceptable
non-phthalate plasticizers. Chemical fluid deposition (CFD) was used to dope metals onto the
aluminated silica support, which helped to create a uniform structure of RhNi on Al-SBA-15. The
introduction of Ni helped to reduce the use of expensive Rh and increase the number of metal active
sites by reducing the bimetallic nanoparticle size. Aluminated SBA-15 not only acted as the support for
the RhNi bimetallic catalyst but also enhanced the reaction efficiency by introducing Brønsted and Lewis
acid sites and the absorption of phthalates on the catalyst in water. The physicochemical properties of
prepared catalysts were characterized by N2 adsorption–desorption isotherm, X-ray diffraction (XRD), in
situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Scanning electron microscopy
(SEM), and Transmission electron microscopy (TEM). The catalytic performance of the synthesized
catalysts was evaluated with the hydrogenation of dimethyl phthalate (DMP). Despite the low solubility of
DMP in water, the hydrogenation using Rh0.5Ni1.5/Al-SBA-15 was carried out with an 84.4% reaction yield
(cis- : trans- ¼ 97.5 : 2.5) at 80 ꢀC using 1000 psi of H2 after 2 h.
Received 22nd February 2017
Accepted 20th March 2017
DOI: 10.1039/c7ra02227a
rsc.li/rsc-advances
can bind with proteins and affect the conformation and func-
tion of the human serum albumin.17 Thus, this phenomenon
Introduction
results in a potential risk to human health.
Phthalate plasticizers have been widely used in the plastic
industry to enhance the exibility and durability of plastic
products, especially for polyvinyl chloride (PVC). The total
global market for plasticizers has grown up from 5.6 million
metric tons in 2008 (ref. 1) to 8.4 million metric tons in 2014.2
Moreover, it is estimated to reach a market value of US $19.4
billion by 2019. The growth rate is forecasted to expand
approximately 6.35% annually,3 with 87% of this growth being
phthalate plasticizers. In most PVC derivative products, the
phthalate content is up to 40% of the total weight. However,
phthalates are found to be one of the main chemical agents
causing cancer, obesity and reproductive problems by disrupt-
ing the endocrine system, as shown in recent reports.4–8
Humans can be exposed to phthalates through PVC medical
devices,9 food packaging,10–12 children's toys,13–15 etc. In the
human body, phthalates interact with human ketosteroids,
such as androgen, progesterone, and glucorticoid,16 thus
potentially causing antisteroidal activity. Additionally, phtha-
lates such as di-isododecyl phthalate and di-n-octyl phthalate
Cyclohexane dicarboxylic acid (CHDA) ester plasticizers (or
phthalate-free plasticizers) are mainly synthesized by hydroge-
nating the benzyl ring of phthalates and have gained the
interest of scientists due to their advanced physical properties
and low toxicity.18–20 Phthalates possess a benzyl ring structure
connected with two moderately electron-withdrawing groups –
carboxyl groups. Therefore, it is difficult for hydrogen to attack
the aromatic ring and hydrogenate the benzyl ring. Conse-
quently, conventional phthalate hydrogenation requires noble
metal catalysts21–23 or catalysts with a high metal loading24 and
consumes a large amount of organic solvents under harsh
conditions to accelerate the reaction. Therefore, Ni was used in
this study to reduce the amount of Rh in the catalyst. Rhodium
acts as the primary catalyzing agent to break the conjugated
bond in the benzyl ring, and nickel plays a role in further
hydrogenating the intermediates to the saturated cyclohexane
ring. The phthalate molecules travel along the channels of the
aluminated SBA-15 and interact with the metal active sites to
form the cyclohexane ring products. Therefore, the formation of
side products is limited. Additionally, water was used as an
effective solvent for the hydrogenation of phthalates due to its
environmentally friendly properties.
Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013,
Taiwan, ROC. E-mail: cstan@mx.nthu.edu.tw; Fax: +886 3572-1684; Tel: +886 3572-
1189
Aluminated supports have been reported to enhance the
efficiency of hydrogenation.25–28 The presence of Al sites in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra02227a
18178 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
porous support structures plays three major roles: (1) helping to purchased from Boclh Industrial Gases Co. (Taiwan). o-Phthalic
increase the uptake of metal complexes onto the support in the acid (PA, 99%), tetrahydrofuran (THF, 99.9%) and dimethyl
metal loading step using chemical uid deposition (CFD);29,30 sulfoxide-d6 (DMSO-d6, 99.9 atom% D) were purchased from
(2) strengthening the interactions of metal species with the Sigma-Aldrich. Dimethyl phthalate 99%, m-phthalic acid (IPA,
support as an anchoring site;31 (3) enhancing the reaction effi- 99.9%), and p-phthalic acid (TPA, 99%) were purchased from
ciency with Lewis and Brønsted acid sites.24 Moreover, due to Alfa Aesar. All chemicals were used without any further puri-
the acidic properties possessed by the aluminated support, the cation unless noted.
basic phthalates can be more readily adsorbed by the alumi-
nated support, which is benecial for the subsequent
hydrogenation.
Catalyst preparation
Compared to conventional methods, such as wet impreg- The hexagonal mesoporous silicas SBA-15 and Al-SBA-15 were
nation (WI), CFD possesses some advantages32 due to the synthesized following the procedure of Klimova et al.31
unique characteristics of supercritical CO2, including tunable
The porous silica SBA-15 was synthesized using poly
solvent power through the operational conditions, lack of (ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene
retention on the substrates aer treatment, low surface tension, oxide) triblock copolymer Pluronic P123 as the structure
low viscosity. Therefore, CFD was used to successfully load the directing agent and tetraethyl orthosilicate (TEOS) as the silica
metal onto the porous supports.32 It has shown its superiority to source. First, 4 g of Pluronic P123 was dissolved in 26 g of water
the other loading methods, such as WI, incipient wetness and then stirred for 1–2 h at 40 ꢀC to ensure the P123 completely
impregnation and co-precipitation. The extremely low surface dissolved. Then, 116 g of 2 M HCl solution was used as a pH
tension of supercritical CO2 helps carry organometallic modifying agent for the P123 solution. Aer stirring the acidic
precursors to deep support channels and uniformly load them mixture for 2 h at 40 ꢀC, 8.50 g of TEOS was added into the
onto the supports, leading to an increase in the number of solution at the rate of 0.05 mL sꢁ1 under stirring conditions.
metal active sites. Aer the incorporation of the substrate with The mixture was continuously stirred at 40 ꢀC for 20 h and then
ꢀ
the precursor, the organometallic precursor can then be con- aged at 80 C for 24 h without stirring. The solid product was
verted to its metal form by the following means:33 (1) chemically collected by ltration and washed with deionized water until the
reducing the metal complexes in the supercritical uid (SCF) washed liquor reached the neutral pH. Then, the solid was
using a reducing agent, such as hydrogen or alcohols; (2) ther- calcined at 540 ꢀC for 6 h in static air, and the nal product was
mally reducing the metal complexes in the SCF; (3) thermally SBA-15.
decomposing the metal complexes in an inert gas environment,
Aluminum(III) chloride, AlCl3, was used as the aluminum
or chemically converting them using hydrogen or air aer source in this study. In a typical trial, 1 g SBA-15 with 0.247 g
depressurizing the SCF out of the reactor. Several heteroge- anhydrous AlCl3 were dispersed in 100 mL of dry ethanol. Aer
ꢀ
neous catalysts were successfully synthesized by CFD using stirring at 40 C for 6 h, the aluminum doped solid was sepa-
scCO2 as a solvent. Yu et al.34 prepared RhPt/SBA-15 by CFD and rated by ltration. Then, 200 mL of dry ethanol was used to
demonstrated that RhPt/SBA-15 is superior to both Rh/SBA-15 wash the Clꢁ ions out of the Al-SBA-15. The solid was dried at
and Pt/SBA-15. The superior behavior might be due to the 65 ꢀC for 2 h before calcination at 550 ꢀC under static air for 6 h.
cooperating effects of Pt and Rh, with Pt enhancing the The nal solid Al5-SBA-15 was then used as the support for the
adsorption of the starting materials on the active sites and Rh immobilization of Rh and Ni nanoparticles.
acting as the activated catalytic sites for hydrogenation. Yen
Rhodium(III) acetylacetonate, Rh(acac)3, and nickel(II) acety-
et al.35 carried out the hydrogenation of bisphenol A in water lacetonate, Ni(acac)2, were used as the CO2 soluble Rh and Ni
using the Ru/MCM-41 catalyst prepared by CFD, and a high metal precursors in this study, respectively. CFD was used to
reaction efficiency was achieved with this catalyst, while the low load metals onto the supports. From the study of Yu et al.,34 the
solubility of reactants was not an obstacle for the hydrogena- solubilities of the metal acetylacetonates are relatively lower in
tion. Ru, Rh, and Pd monometallic nanoparticles and Ru–Rh, scCO2 compared to in THF. Therefore, to achieve the desired
Ru–Pd, and Rh–Pd bimetallic nanoparticles on MCM-41 were theoretical metal loading of Rh and Ni, the required amounts of
prepared by Yen et al.36 using CFD. Ru/MCM-41 was prepared by Rh(acac)3 and Ni(acac)2 were rst dissolved in 15 mL tetrahy-
CFD and possessed the highest catalytic activity for the hydro- drofuran (THF). The liquid mixture precursor was added to the
genation of p-xylene, which was nearly 8 times higher than Ru/ solid supports and ultra-sonicated for 2 h to homogenize the
MCM-41 prepared by conventional methods. The catalytic precursor and accelerate its diffusion rate into the support
activity of Rh–Pd/MCM-41 was higher than the activity of indi- channels. THF was then removed by rotary vacuum evaporation.
vidual monometallic catalysts.
The solid mixture was transferred to a high pressure stainless
steel autoclave which was lled with 2500 psi of CO2 at 50 C.
ꢀ
The mixture in the autoclave was stirred at 50 ꢀC for 3 h before
depressurization. Aer depressurization, the solid is the as-
synthesized catalyst with organometallic precursors in the
Experimental
Materials
Rhodium(III) acetylacetonate (Rh(acac)3, 97%) and nickel(II) support channels. The organometallic precursors were con-
acetylacetonate (Ni(acac)2, 95%) were purchased from Stream verted into elemental metal by calcining the as-synthesized
Chemicals Inc. CO2 and H2 gases with purities >99.9% were catalyst at 400 ꢀC under an air ow for 4 h followed by
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 18178–18188 | 18179

View Article Online
RSC Advances
Paper
reduction under a hydrogen ow at 400 ꢀC for 4 h. Aer on the sample surface until no change was observed in the FTIR
reduction, pure nitrogen was used to remove the excess spectra. Finally, in an Ar atmosphere, the sample and reactor
hydrogen inside the calcination furnace before collecting the were cooled down to RT. All the above processes were monitored
nal catalyst.
and recorded by FTIR.
Hydrogenation of dimethyl phthalate (DMP) and phthalic
acids
Catalyst characterization
N2 adsorption–desorption experiments were performed using
a physical adsorption instrument (Quantachrome Autosorb-1-
MP). The specic surface area was measured using BET
method, pore volume and pore size were calculated by BJH
method. The conformation and structure of the catalyst were
characterized via (1) X-ray diffraction analysis (Ultima IV
Multipurpose X-ray diffraction system) using Cu Ka radiation
source at 40 kV and 20 mA and a scan rate of 1ꢀ minꢁ1 in the 2q
range from 30 to 90ꢀ; and (2) low-angle X-ray diffraction (Bruker
D8 advance X-ray diffraction system) using Cu Ka with an
acceleration voltage of 40 kV and a lament current of 40 mA,
the scanning angle varied between 0.5ꢀ # 2q # 5ꢀ. The scanning
electron microscopy (SEM) analysis was performed on a UHR
Cold-Emission FE-SEM SU8000 Series SEM machine. The
samples were dried and doped on carbon tape followed by
vacuum activation before analysis. Transmission electron
microscopy (TEM) analysis was performed using a JOEL JEM-
2100 transmission electron microscopy. A small quantity of
the sample that was dispersed in ethanol followed by ultra-
sonication for 5 minutes; then, carefully placed on a carbon
coated copper grid (200 mesh) and the grid was dried overnight
under vacuum.
RhNi bimetallic catalysts on aluminated supports were used for
DMP hydrogenation to evaluate the catalytic performance. The
hydrogenations were carried out in a 200 mL high-pressure
stainless steel autoclave. All pressure and temperature param-
eters, controllers, and heaters were installed properly.
In a typical experiment, 1.0 g of DMP, 50.0 mg of the catalyst
and 50 mL of D.I. water were added to a closed high-pressure
autoclave. The reactor was then purged three times with
hydrogen to remove the remaining air inside the reactor. Aer
the autoclave was brought to the desired temperature, 1000 psi
of hydrogen was fed to start the reaction. The reaction was
carried out for 1–4 h with a constant stirring rate of 500 rpm.
Aer the hydrogenation, the products were extracted with
diethyl ether with 0.2 g of benzyl benzoate as the external stan-
dard in a GC-MS for further analysis. The hydrogenations of
phthalic acids were carried out under similar conditions, and
the reaction yields were determined by 1H-NMR (500 MHz,
DMSO, Varian Unity Inova, 500 NMR). Aer the reaction system
was cooled down to room temperature and depressurized, water
was removed from the reaction mixture by evaporation. Then the
crystals including phthalic acid and the corresponding products
were collected and dried overnight. Dimethyl sulfoxide-d6 was
used as a diluting solvent to prepare samples for NMR analysis.
The experimental setup of the in situ FTIR study of pyridine
adsorption on various supports and catalysts is shown in
Fig. S1.† The setup consists of (1) a gas ow controller; (2) an
FTIR bench (Thermo Nicolet 6700 FTIR spectrometer with an
MCT/B detector) equipped with a diffuse reectance infrared
Fourier transform spectroscopy (DRIFT) reactor (Harrick
Results and discussion
Conformation and structure of RhNi/Al-SBA-15 catalysts
Scientic); (3) a pyridine saturator and (4) a homemade auto- A series of Rh and Ni monometallic catalysts and RhNi bime-
mated temperature controller with a LabView DAQ module. The tallic catalysts using SBA-15 and Al5-SBA-15 supports were
spectra were collected at a resolution of 4.0 cmꢁ1 with 32 co- prepared by CFD using scCO2 as the solvent in this study. The
added scans for each spectrum.
catalysts were denoted based on the weight ratio between the
To examine the acidic sites on the supports and the catalyst metals and the support as RhxNiy/Alz-SBA-15 where x represents
surface, in situ pyridine adsorption experiments were carried the ratio of Rh : Al5-SBA-15, y represents the ratio of Ni : Al5-
out, similar to the previous works of Stevens et al.37,38 50 mg of SBA-15, and z represents the ratio of Al : SiO2.
the sample was used in each test and was loaded into the DRIFT
Fig. 1 shows the small-angle XRD patterns of SBA-15, Al5-
reactor. Samples were rst heated in situ from RT to 873 K at SBA-15 and Rh0.5Ni1.5/Al5-SBA-15. The small-angle XRD patterns
a rate of 10 K minꢁ1 and held for 120 min under an 80 sccm of the catalysts and supports are similar with an intense main
argon ow. This pretreatment process was employed to remove peak (100) and two main peaks (110) and (200) plane reections.
˚
surface organics and water adsorbates to create a clean The largest peak (100) gives a d100 spacing (106–114 A), which is
˚
condition.
analogous to the large lattice parameter (ao ¼ 122–132 A) of SBA-
Aer pretreatment, the sample and reactor were cooled 15.39 The diffraction patterns also conrm the invariance of the
down to 413 K for the pyridine adsorption experiment. This SBA-15 structure aer alumination with AlCl3 and the success-
temperature was chosen to prevent pyridine (bp ¼ 389 K) ful loading of Rh and Ni by CFD.
condensation on the sample surface. Gaseous pyridine was
The X-ray diffraction patterns of Rh and Ni monometallic
owed into the reactor by re-directing Argon through the pyri- catalysts and a series of RhNi bimetallic catalyst with different
dine saturator. Each sample was exposed to an Ar/pyridine metal ratios are shown in Fig. 2. The specic peaks of the Rh
mixture gas for 30 min to ensure saturated adsorption. Then, and Ni monometallic catalysts are clearly observable, indicating
at 413 K, the gas was switched to pure Ar to remove all excessive that the reduction with hydrogen successfully turned the metal
pyridine, including the weakly adsorbed (physisorbed) pyridine precursors into elemental metals. The XRD pattern for Rh2/Al5-
18180 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Fig. 3 SEM images of the supports.
mm in length and 0.5 mm in diameter. The SBA-15 particles were
relatively uniform in size and shape conrming the reliability of
the preparation method. Furthermore, the SBA-15 particles
stuck to each other along the (001) faces, forming long rods of
adhered particles. This phenomenon is commonly observed for
SBA-15 materials.41
Fig. 1 Small-angle X-ray diffraction patterns.
TEM images of the calcined catalysts are shown in Fig. 4. The
TEM images of the monometallic catalysts show that the
smaller sized Rh dispersed in the support while the larger Ni
particles located in the siliceous support due to the low solu-
bility of Ni(acac)2 in scCO2.42 This phenomenon is consistent
with existing observations.43 Among the RhNi bimetallic cata-
lysts, Rh0.5Ni1.5/Al5-SBA-15 possessed the smallest metal
particle size regardless of the high loading of the Ni precursor.
The possible reason is that the introduction of Ni helped reduce
the particle size of the primary metal.
Fig. 5a presents the pore size distribution of the Rh mono-
metallic catalyst prepared by CFD and WI using SBA-15 and Al5-
SBA-15 supports. In the pore size distribution pattern, the
highest peak of the catalysts prepared using the SBA-15 support
appeared at 6.6 nm for CFD and 6.3 nm for WI. However, for the
catalysts prepared using Al5-SBA-15, the main peak was at
4.9 nm for CFD and 5.6 nm for WI. In Fig. S3,† the elemental
mapping of Al5-SBA-15 using SEM analysis conrmed the
presence of Al sites on the outer surface of the support. The
reason for this difference therefore might due to the adsorption
effects of the support containing Al, which could attract more
metal acetylacetonate precursors onto the acidic surface.
Therefore, the pores contained more metal nanoparticles aer
Fig. 2 XRD diffractogram of Rh and Ni monometallic catalysts and
RhNi bimetallic catalysts.
SBA-15 shows two diffraction peaks at 2q ¼ 41.3ꢀ and 47.7ꢀ,
which represent the (111) and (200) planes of the face-centered
cubic (fcc) structure Rh (JCPDS 5-685), respectively. The repre-
sentative XRD diffraction peaks of the Ni2/Al5-SBA-15 located at
2q ¼ 44.3ꢀ, 51.7ꢀ, and 76.2ꢀ can be, respectively, indexed to the
(111), (200), and (220) planes of the metallic phases of Ni (JCPDS
4-850) formed by the reduction of the Ni precursor. Due to the
similar structure of Rh and Ni particles, the prepared RhNi
bimetallic nanoparticles were present as alloy with a fcc
structure.
In the XRD patterns of the RhNi bimetallic catalysts on Al5-
SBA-15, the introduction of a small amount of Rh slowed and
broadened the specic peak of catalyst in the XRD pattern.40
Additionally, the peaks tend to shi to the le, probably due to
the formation of a RhNi alloy.
The SEM images of the supports in Fig. 3 indicate that the
cylindrical shape of SBA-15 remained unchanged aer being
Fig. 4 TEM images of Rh, Ni monometallic catalysts and RhNi bime-
modied with Al. The size of the support was approximately 1 tallic catalysts.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 18178–18188 | 18181

View Article Online
RSC Advances
Paper
Fig. 5 Pore size distribution of the Rh catalyst on SBA-15, Al5-SBA-15 (a) and the RhNi catalysts on Al5-SBA-15 (b).
loading, causing the decrease in the pore size. Moreover, the Rh in the H1 hysteresis loop endorses the natural structure of the
SBA-15 and Al5-SBA-15 had two main peaks for the pore size SBA-15. The isotherm curves are shown in Fig. S2.†
distribution pattern. This was probably due to the deposition of
the metal precursor at the pore entrance. In contrast, the Rh2/Al5-
SBA-15 synthesized using CFD possessed a broad peak that
overlapped the two main peaks, which proves the increased
Enhanced acidic properties of aluminated SBA-15 over
untreated SBA-15
loading of metal in the pores. Thus, for Al5-SBA-15 support, the
elemental mapping was consistent with the pore size distribution.
On the other hand, perhaps the catalyst synthesized by CFD using
the Al-SBA-15 support could possess a more uniform structure of
metal nanoparticle than the untreated SBA-15 support.
The DRIFTS analytical results (Fig. 6) and the calculated
Brønsted and Lewis acid site intensities (Table 1) proved the
increased acidity from SBA-15, Al5-SBA-15, and Rh0.5Ni1.5/Al5-
SBA-15.
For SBA-15, the Lewis acid site intensity was 0.093 and the
Brønsted acid site intensity was 0.043, while, with the small
introduction of Al to SBA-15, the Lewis and Brønsted acid site
intensities of Al5-SBA-15 were 0.133 and 0.156, respectively.
Thus, Al was proven to be successfully doped on the SBA-15
using the procedure. This result was consistent with the
pore size distribution, the enhancement of support acidity
leaded to the decrease of pore size due to the presence of Al
coated on the channel surface. Moreover, the introduction of
Al helped boost the acidity of the material, which is necessary
for hydrogenation. The introduction of Al and RhNi bime-
tallic nanoparticles also enhanced the peak of the hydroxyl
group.
Fig. 5b shows the pore size distribution of the RhNi bime-
tallic catalyst with different metal ratios on the Al5-SBA-15
support using CFD. SBA-15 and Al5-SBA-15 pore size distribu-
tion patterns were used as the comparison standards. Rh2/Al5-
SBA-15 possessed a broad peak from 3.9 nm to 6.4 nm.
Rh1.5Ni0.5/Al5-SBA-15 and Rh1Ni1/Al5-SBA-15 possessed two
separated peaks at 3.9 nm and 6.3 nm, respectively, and the
Rh0.5Ni1.5/Al5-SBA-15 contained only one peak at 6.3 nm which
is similar to the main peak of Ni2/Al5-SBA-15. Thus, the intro-
duction of Ni into the Rh catalyst helped reduce the size of the
bimetallic nanoparticles. Therefore, the nanoparticles could be
deposited onto the pore walls instead of the pore entrance as
was in the case of Rh2/Al5-SBA-15.
In the study of Hensen et al.,44 Al-SBA-15 supports which
were synthesized under acidic conditions have lower Brønsted
acidity than desired. The reason is though a sustaintial part of
Al is in tetrahedral coordination, but the acidity was mainly
Each of the SBA-15, Al5-SBA-15 and catalysts prepared from
the Al5-SBA-15 support exhibited a type IV isotherm with two
sharp reections at the relative pressure of 0.6–0.8. In addition,
18182 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Fig. 6 Single beam (left) and absorbance (right, using spectrum before pyridine adsorption as the background) spectra of the samples before
(dash line) and after (solid line) pyridine adsorption for (a) SBA-15; (b) Al5-SBA-15; (c) Rh0.5Ni1.5/Al5-SBA-15.
Table 1 Brønsted and Lewis acid site intensities
Sample
SBA-15
Al5-SBA-15
Rh0.5Ni1.5/Al5-SBA-15
Raw Lewis acid site intensity (1445 cmꢁ1
)
0.035
0.016
0.375
0.093
0.043
0.052
0.061
0.391
0.133
0.156
0.310
0.269
0.682
0.445
0.394
Raw Brønsted acid site intensity (1600 cmꢁ1
Normalized factor
)
Normalized Lewis acid site intensity (1445 cmꢁ1
)
Normalized Brønsted acid site intensity (1600 cmꢁ1
)
contributed by Al on the surface, not Al in the network. There- efficiency of Al sites. This Al layer can strengthen the bond
fore, in our research, the Brønsted acidity was enhanced between RhNi nanoparticles and support surface, also provide
because Al coated on SBA-15 surface as a layer and maximize the acid sites for hydrogenation.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 18178–18188 | 18183

View Article Online
RSC Advances
Paper
for using the aluminated support. At 30 ꢀC, a 44.9% yield
towards the DMCHDA was generated using the Rh catalyst with
the aluminated support, however, the selectivity was only 11.5%
using a non-aluminated support. The aluminated mesoporous
silica (Al-SBA-15) helps increase the catalyst performance by
increasing the metal precursor uptake, strengthening the bond
between the metal and the support, and providing acid sites
which are necessary for hydrogenation.
Catalytic performance of RhNi/Al-SBA-15 bimetallic catalysts
tested on DMP hydrogenation
The catalytic activities of the Rh/Al5-SBA-15 catalysts prepared
by CFD and WI for the hydrogenation of dimethyl phthalate
(DMP) were tested in water at a temperature range varying from
ꢀ
30 to 80 C with an H2 pressure of 1000 psi for 1 h. The exper-
imental results (Fig. 7a) showed that the catalysts prepared by
the CFD method exhibited much better performance than those
To determine the optimal metal ratio of Rh to Ni, DMP
hydrogenation was carried out using various metal ratios. The
results in Fig. 7b show that the catalyst with a Rh : Ni ratio of
0.5 : 1.5 (Rh0.5Ni1.5/Al5-SBA-15) exhibited the best performance
among the tested bimetallic catalysts, a reaction yield of 61.9%
ꢀ
prepared by the WI method. At 60 C, an 87.5% yield towards
dimethyl 1,2-cyclohexanedicarboxylate (DMCHDA) was ach-
ieved by the catalysts prepared by CFD, while only a 67.1% yield
was obtained by the catalyst prepared with the WI method. The
reason for the improved performance of the CFD-prepared
catalyst was due to the presence of more metal active sites. By
applying the CFD method, the metal precursors could be
uniformly dispersed onto the support surface, therefore the
active metal sites were increased.
To demonstrate the effects of the aluminated support, Al5-
SBA-15 and SBA-15 were used as siliceous porous supports for
the preparation of the Rh monometallic catalysts. The catalyst
performance results in Fig. 7a show a signicant improvement
ꢀ
at 80 C was observed, while a yield of only 28.6% was shown
using the catalyst Ni2/Al5-SBA-15. Rh1.5Ni0.5/Al5-SBA-15 had
a slightly higher catalytic performance compared with Rh1Ni1/
Al5-SBA-15 due to the higher content of Rh. The enhanced
catalytic performance of Rh0.5Ni1.5/Al5-SBA-15 might be due to
the higher catalytic sites from the formation of smaller particles
when a suitable amount of Ni was introduced into the catalyst.
To examine if the catalyst performance could be increased at
high contents of Ni in the RhNi bimetallic catalysts, Rh was
Fig. 7 Experimental results for the catalyst optimization. (a) Effects of the preparation methods and support properties; (b) effects of the Rh : Ni
metal ratio with a total loading of 2 wt%; (c) effects of high Ni loading; and (d) effects of low Rh loading.
18184 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
xed at 0.5 wt% while the Ni content was varied from 1.5, 5, 7, during reaction. On the other hand, the presence of Al
and 10 wt% for DMP hydrogenation in water at 60 and 80 ꢀC at contributed to the reaction efficiency due to the introduction of
a H2 pressure of 1000 psi for 1 h. The results in Fig. 7c Brønsted and Lewis acid sites.
demonstrate that an increase of the Ni content led to a decrease
of the catalyst performance, due to the Rh metal active sites
being occupied by more Ni nanoparticles.
The concentration of Rh in the RhNi bimetallic catalyst was
varied from 0.1 wt% to 0.5 wt%, and the concentration of Ni was
Effects of process conditions on catalytic performance
Rh0.5Ni1.5/Al5-SBA-15 was used as the catalyst to examine the
effects of H2 pressure on the DMP hydrogenation. The reactions
were carried out at 80 ꢀC for 1 h with different H2 pressures. As
kept constant at 1.5 wt%. The RhNi bimetallic catalysts with
seen from the experimental results (Fig. 8), the reaction effi-
a 0.1–0.5 wt% of Rh were used for DMP hydrogenation at 60 ꢀC
ciency was higher when the H2 pressure increased since the
and 80 ꢀC and 1000 psi of H2 for 1 h. From the experimental
higher H2 pressure led to a higher solubility of H2 into water.
results, Fig. 7d shows that a decrease in the amount of Rh loaded
Additionally, Fig. 8 shows that the reaction yield increased
onto SBA-15 led to a decrease in the performance of the catalytic
with the increasing reaction time. When a H2 pressure of 1000
hydrogenation of DMP. This was because the Rh acted as
psi was chosen, the reactions were carried out at 80 ꢀC with
reaction times of 1, 2, and 4 h. Rh2/Al5-SBA-15 completely
hydrogenated DMP to DMCHDA aer 2 h, compared to an
84.4% reaction yield when Rh0.5Ni1.5/Al5-SBA-15 was used.
When Rh0.5Ni1.5/Al5-SBA-15 was used, a 95.8% reaction yield
was achieved aer reacting for 4 h. Therefore, the reaction time
was xed at 2 h for further experiments.
a facilitating agent in the initiation step. When the Rh amount
decreased, the rst step of benzyl ring hydrogenation became
slower, which, consequently, caused a decrease in the catalytic
performance. The drop in the catalytic performance of Rh0.4Ni1.5
-
Al5-SBA-15 was probably due to the formation of bigger nano-
particles, therefore, the number of catalytic sites decreased.
Therefore, RhNi/Al-SBA-15 had the best catalytic perfor-
mance at Rh : Ni ratio of 0.5 wt% : 1.5 wt% using Al5-SBA-15 as
support. The reason is due to the RhNi nanoparticles size and
acidity of Al5-SBA-15. Rh0.5Ni1.5/Al5SBA-15 possessed the
smallest nanoparticle size among other RhNi bimetallic nano-
particles. Thus, the amount of metal active sites is increased.
Moreover, the presence of Al layer on SBA-15 surface help
strengthening bonds between RhNi nanoparticles and support,
thus reduce the leaking of metal out of catalyst's structure
Reaction mechanisms
To understand why 97.5% of the products were cis-isomers
using Rh0.5Ni0.5/Al5-SBA-15 as the catalyst for DMP hydrogena-
tion in water (1000 psi of H2 at 80 ꢀC for 2 h), this hydrogenation
system was used in different solvents under the same operating
conditions. The results of experiments using water, hexane and
scCO2 introduced hexane are shown in Table 2.
Fig. 8 Experimental results for process condition optimization.
Table 2 Reaction yield and isomer selectivity in different solvents
Solvent
Temp. (ꢀC)
Yield (%)
-trans isomer selectivity (%)
-cis isomer selectivity (%)
Water
Hexane
Hexane + scCO2
Hexane + scCO2
80
80
80
140
84.4
41.9
28.5
37.9
2.5
12.6
18.3
39.0
97.5
87.4
81.7
61.0
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 18178–18188 | 18185

View Article Online
RSC Advances
Paper
When the solvent was changed from water to hexane and
scCO2 introduced hexane, the reaction yield decreased and the
trans-isomer selectivity increased. The DMP completely dis-
solved in hexane but not in water. For the DMP hydrogenation
using water, acid sites contributed by Al helped to attract DMP
molecules onto SBA-15 surface because DMP is slightly basic.
Thus, DMP bonded with SBA-15 rmly. H2 came into SBA-15
pores could attack only one side of DMP to produce mostly
cis-isomer products. However, when hexane was used, the
mobility of the DMP in the catalyst pores was higher. Therefore,
the H2 could interact with both two side of DMP molecules in
the presence of catalytic sites to form more trans-isomer prod-
ucts. The introduction of scCO2 helped decrease the mixture
viscosity, which possibly enhances the mobility of the reactants,
DMP and H2. H2 could access both sides ꢀof the benzyl ring to
form more trans-isomer products. At 140 C using hexane and
scCO2, both the reaction yield and percentage of trans-isomer
products increased due to the higher catalyst reactivity and
higher diffusion rate of the reactants. The comparison of the
reaction efficiency between hexane and scCO2 introduced
hexane at different temperatures is shown in Fig. 9.
The reaction yield in the solvent hexane increased with
temperature. When scCO2 was introduced into the reaction
system, the reaction yield changed insignicantly with the
variation of temperature. It was expected in the beginning of the
study that the introduction of scCO2 would enhance the reac-
tion efficiency due to a decrease in the solvent viscosity.
However, the reaction yield in the case of the introduced scCO2
was found to be lowered. This may be a result of the catalyst
poisoning caused by CO2.45 The percentage of trans-isomer
products was higher with increasing reaction temperature,
possibly due to the lower viscosity and higher diffusion rate.
The reaction mechanism is shown in Fig. 10.
Fig. 10 Proposed reaction mechanisms.
determine reaction yields. The reaction results are shown in
Table 3. Among the three phthalic acids, o-phthalic acid has the
highest solubility in water (0.6 g per 100 mL at 20 ꢀC), p-phthalic
acid is harder to dissolve in water with a solubility of 0.0015 g
per 100 mL at 20 ꢀC, and m-phthalic acid is virtually insoluble in
water at low temperatures. Therefore, due to the difference in
material properties (position of substituent groups, solubility),
o-phthalic acid is the easiest and m-phthalic acid is hardest to
hydrogenate with reaction yields of 95.3% and 59.3% aer 6 h at
ꢀ
140 C, respectively.
Table 3 Phthalic acid hydrogenation
Reagent
Solvent
Temp. (ꢀC)
Time (h)
Yield (%)
o-Phthalic acid
o-Phthalic acid
m-Phthalic acid
p-Phthalic acid
Water
Water
Water
Water
80
2
6
6
6
66.6
95.3
59.3
69.8
140
140
140
Hydrogenation of phthalic acids
Rh0.5Ni1.5/Al5-SBA-15 was also used as a catalyst for the hydro-
genation of phthalic acids in water. 1H-NMR was used to
Fig. 9 Comparison of the reaction efficiency between reactions in hexane and hexane + scCO2.
18186 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Conclusions
Notes and references
The excellent catalytic properties of Rh were successfully applied
for the hydrogenation of DMP. Ni was used as the secondary
metal to reduce the amount of Rh used. As a consequence, the
optimal metal concentration in the catalyst of Rh and Ni was 0.5
wt% and 1.5 wt%, respectively. The introduction of Al on the
catalyst was proven to have positive effects on the reaction effi-
ciency by the enhancement of the material acidity and nano-
particles – support bonds. The elemental mapping visually
proved the presence of Al site on the outer surface of the support.
The pore size reduction of catalyst was due to the introduction of
Al sites and the loading of RhNi nanoparticles, determined by
pore size distribution. The DRIFTS analysis conrmed the
enhancement of Lewis and Brønsted acid site intensities for the
aluminated SBA-15 over conventional SBA-15. On the other
hand, the Al layer on the SBA-15 surface also acted as an
anchoring site to strengthen the bond of the RhNi bimetallic
nanoparticles with the SBA-15 support. The experimental results
also conrmed that the introduction of Ni helped to reduce the
RhNi nanoparticles size, thus increasing the amount of metal
active sites without adding more metal precursors.
1 M. D. Bisig, Plasticizer Update, SPI 20th Vinyl Compounding
Conference, July 19-21, 2009.
2 M. P. Malveda, Chemical Economics Handbook Report on
Plasticizers, July 2015.
3 Addit. Polym., 2015, 2015, 10.
4 J. Autian, Environ. Health Perspect., 1973, 4, 3.
5 D. K. Agarwal, S. Eustis, J. C. Lamb 4th, J. R. Reel and
W. M. Kluwe, Environ. Health Perspect., 1986, 65, 343.
6 F. A. Arcadi, C. Costa, C. Imperatore, A. Marchese,
A. Rapisarda, M. Salemi, G. R. Trimarchi and G. Costa,
Food Chem. Toxicol., 1998, 36, 963.
7 P. W. Albro, J. T. Corbett, J. L. Schroeder, S. Jordan and
H. B. Matthews, Environ. Health Perspect., 1982, 45, 19.
8 S. Miles-Richardson, S. Bosch, S. Swarts, F. Llados and
D. A. Gray, Toxicological Prole for Di(2-ethylhexyl)phthalate
(DEHP), Agency for Toxic Substances & Disease Registry,
2002.
9 J. A. Tickner, T. Schettler, T. Guidotti, M. McCally and
M. Rossi, Am. J. Ind. Med., 2001, 39, 100.
10 J. R. Startin, I. Parker, M. Sharman and J. Gilbert, J.
Chromatogr. A, 1987, 387, 509.
Water was used as the solvent for the hydrogenation of the
DMP as a green solvent. Though DMP has limited solubility in 11 K. M. Rodgers, R. A. Rudel and A. C. Just, in Molecular and
water, the hydrogenation occurred with high reaction yield. At
the optimal operational conditions of 80 ꢀC, 2 h, 1000 psi of H2,
20 mg Rh0.5Ni1.5/Al5-SBA-15 as the catalyst and 50 mL water as
the solvent, 1 g of DMP was hydrogenated with a reaction yield
Integrative Toxicology, ed. R. R. Dietert, 2014, ch.
Phthalates in Food Packaging, Consumer Products, and
Indoor Environments, p. 31, DOI: 10.1007/978-1-4471-6500-
2_2.
of 84.4%, with most of the product being cis-isomers (97.5%). 12 A. Fankhauser-Noti, S. Biedermann-Brem and K. Grob, Eur.
The high selectivity of the cis-isomer over the trans-isomer is Food Res. Technol., 2006, 223, 447.
due to the shielding effects of the catalyst in the water envi- 13 C. F. Wilkinson and J. C. Lamb 4th, Regul. Toxicol.
ronment. Due to the limited solubility of DMP in water and the Pharmacol., 1999, 30, 140.
high absorption to acid sites of catalyst, hydrogen could mostly 14 S. Biedermann-Brem, M. Biedermann, S. Pfenninger,
access one side of the DMP. Therefore, the cis-isomer was
formed. This phenomenon was conrmed by hexane and scCO2
M. Bauer, W. Altkofer, K. Rieger, U. Hauri, C. Droz and
K. Grob, Chromatographia, 2008, 68, 227.
added hexane. The trans-isomer selectivity increased when the 15 S. I. Korfali, R. Sabra, M. Jurdi and R. I. Taleb, Arch. Environ.
solvent viscosity was decreased. This phenomenon can be Contam. Toxicol., 2013, 65, 368.
applicable in medicinal chemistry to synthesize geometric 16 M. K. Sarath Josh, S. Pradeep, K. S. Vijayalekshmy Amma,
isomers directly from starting materials without requirements
R. Sudha Devi, S. Balachandran, M. N. Sreejith and
of purication step.
S. Benjamin, J. Appl. Toxicol., 2016, 36, 836.
The study was also extended to the hydrogenation of o- 17 Y. Yue, J. Liu, R. Liu, Y. Sun, X. Li and J. Fan, Food Chem.
phthalic acid, m-phthalic acid and p-phthalic acid. The hydro-
genation of phthalic acids occurred at 140 C, 6 h, 1000 psi of 18 A. Schutze, C. Palmke, J. Angerer, T. Weiss, T. Bruning and
Toxicol., 2014, 71, 244.
ꢀ
H2, 20 mg Rh0.5Ni1.5/Al5-SBA-15 catalyst and 50 mL water. The o-
phthalic acid, p-phthalic acid and m-phthalic acid each have
H. M. Koch, J. Chromatogr. B: Anal. Technol. Biomed. Life
Sci., 2012, 895–896, 123.
¨
¨
different solubilities in water, 0.6 g per 100 mL, 0.0015 g per 100 19 A. Schutze, M. Kolossa-Gehring, P. Apel, T. Bruning and
mL, and mostly insoluble, respectively. Therefore, o-phthalic H. M. Koch, Int. J. Hyg. Environ. Health, 2014, 217, 421.
acid is the easiest and m-phthalic acid is the hardest to be 20 M. J. Silva, T. Jia, E. Samandar, J. L. Preau Jr and
hydrogenated with reaction yields of 95.3% and 59.3%, A. M. Calafat, Environ. Res., 2013, 126, 159.
respectively, and the reaction yield for p-phthalic acid was 21 X. Li, Z. Sun, J. Chen, Y. Zhu and F. Zhang, Ind. Eng. Chem.
69.8%.
Res., 2014, 53, 619.
22 A. B. Hungria, R. Raja, R. D. Adams, B. Captain,
J. M. Thomas, P. A. Midgley, V. Golovko and B. F. Johnson,
Angew. Chem., Int. Ed., 2006, 45, 4782.
Acknowledgements
The authors like to thank the ROC Ministry of Science and
Technology (grant number NN10502-0163) for the nancial
support.
23 R. Raja, T. Khimyak, J. M. Thomas, S. Hermans and
B. F. G. Johnson, Angew. Chem., Int. Ed., 2001, 40, 4638.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 18178–18188 | 18187

View Article Online
RSC Advances
Paper
24 J. Zhao, M. Xue, Y. Huang and J. Shen, Catal. Commun., 2011, 37 R. W. Stevens Jr, S. S. C. Chuang and B. H. Davis, Appl. Catal.,
16, 30. A, 2003, 252, 57.
25 R. X. Yang, K. H. Chuang and M. Y. Wey, RSC Adv., 2016, 6, 38 R. W. Stevens Jr, S. S. C. Chuang and B. H. Davis,
40731. Thermochim. Acta, 2003, 407, 61.
26 J. R. Restrepo-Garcia, V. G. Baldovino-Medrano and 39 W. Yu, Y. Tang, L. Mo, P. Chen, H. Lou and X. Zheng, Catal.
S. A. Giraldo, Appl. Catal., A, 2016, 510, 98. Commun., 2011, 13, 35.
27 S. Gbadamasi, T. H. Ali, H. V. Lee, A. Y. Atta, P. Sudarsanam, 40 Y. Yang, C. Ochoa-Hernandez, V. A. de la Pena O'Shea,
´
˜
S. K. Bhargava and S. B. A. Hamid, RSC Adv., 2016, 6, 25992.
28 Y. Wei, Y. Li, Y. Tan, Z. Wu, L. Pan and Y. Liu, Chem. Eng. J.,
2016, 298, 271.
29 G. Micera, L. S. Erre and R. Dallocchio, Colloids Surf., 1987,
28, 147.
P. Pizarro, J. M. Coronado and D. P. Serrano, Appl. Catal.,
B, 2014, 145, 91.
41 J. Ruan, T. Kjellman, Y. Sakamoto and V. Alfredsson,
Langmuir, 2012, 28, 11567.
42 W. H. Teoh, R. Mammucari and N. R. Foster, J. Organomet.
Chem., 2013, 724, 102.
30 J. C. Summers and S. A. Ausen, J. Catal., 1978, 52, 445.
´
´
31 T. Klimova, J. Reyes, O. Gutierrez and L. Lizama, Appl. Catal., 43 J. M. Escola, J. Aguado, D. P. Serrano, A. Garcıa, A. Peral,
A, 2008, 335, 159.
L. Briones, R. Calvo and E. Fernandez, Appl. Catal., B,
2011, 106, 405.
˘
32 S. E. Bozbag and C. Erkey, J. Supercrit. Fluids, 2015, 96, 298.
33 Y. Zhang and C. Erkey, J. Supercrit. Fluids, 2006, 38, 252.
34 W. Yu, Y. P. Hsu and C. S. Tan, Appl. Catal., B, 2016, 196, 185.
35 C. H. Yen, H. W. Lin and C. S. Tan, Catal. Today, 2011, 174,
121.
36 C. H. Yen, H. W. Lin, T. D. Phan and C. S. Tan, J. Nanosci.
Nanotechnol., 2011, 11, 2465.
44 A. J. J. Koekkoek, J. A. R. van Veen, P. B. Gerrtisen, P. Giltay,
P. C. M. M. Magusin and E. J. M. Hensen, Microporous
Mesoporous Mater., 2012, 151, 34.
45 D. Xu, R. G. Carbonell, D. J. Kiserow and G. W. Roberts, Ind.
Eng. Chem. Res., 2005, 44, 6164.
18188 | RSC Adv., 2017, 7, 18178–18188
This journal is © The Royal Society of Chemistry 2017