Conversion of bis(2-hydroxyethylene terephthalate) into 1,4-cyclohexanedimethanol by selective hydrogenation using RuPtSn/Al2O3

Article abstract of DOI:10.1039/c6ra04943e

1,4-Cyclohexanedimethanol (CHDM) is a highly valued and widely used monomer in the polymer industry. Bis(2-hydroxyethylene terephthalate) (BHET), the product of glycolysis of waste PET, is an excellent raw material for the preparation of CHDM. Herein, a series of monometallic, bimetallic and trimetallic supported catalysts were prepared for the one-pot conversion of BHET into CHDM by the impregnation method and good performance was found over trimetallic RuPtSn/Al₂O₃ catalysts containing various active sites to catalyze the hydrogenation of the phenyl and carbonyl groups. The influences of various reaction parameters including temperature, pressure and time on the hydrogenation reaction were studied, and 100% conversion and 87.1% yield of CHDM were obtained with the trimetallic supported catalyst with Ru/Sn 1.5. Moreover, through the comparison between various methods for the preparation of CHDM, the conversion of BHET into CHDM by the one-pot method is considered one of the most competitive methods.

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Conversion of bis (2-hydroxyethylene terephthalate) into 1,4-
cyclohexanedimethanol by selective hydrogenation using
RuPtSn/Al2O3
Received 00th January 20xx,
Accepted 00th January 20xx
Danfeng Hou,ab Jiayu Xin,b Xingmei Lu,b Xiaonan Guo,ab Huixian Dong,b Baozeng Ren*a and Suojiang
Zhang*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
1,4-cyclohexanedimethanol (CHDM) is a highly valued and widely used monomer in the polymer industry. Bis (2-
hydroxyethylene terephthalate) (BHET), the product from glycolysis of waste PET, is an excellent raw material for the
preparation of CHDM. Herein, a series of monometallic, bimetallic and trimetallic supported catalysts were prepared for
the one-pot conversion of BHET into CHDM by the impregnation method and good performance was found over
trimetallic RuPtSn/Al2O3 catalysts containing various active sites to catalyze the hydrogenation of the phenyl and carbonyl
groups. The influences of various reaction parameters including temperature, pressure and time on the hydrogenation
reaction were studied, and 100% conversion and 87.1% yield of CHDM were obtained with the trimetallic supported
catalyst with Ru/Sn of 1.5. Moreover, through the comparison between various methods for the preparation of CHDM, the
conversion of BHET into CHDM by one-pot method is considered as one of the most competitive methods.
showed that the incorporation of Pt could depress the loss of
Sn and accelerate the reaction rate.
1. Introduction
However, the raw materials used for the production of
1,4-cyclohexanedimethanol (CHDM) has been extensively
used in the polymer industry for modifying fibers and
polyesters to produce amorphous copolyesters with excellent
chemical resistance and physical strength.1,2 Currently, CHDM
is produced from dimethyl terephthalate (DMT) by a two-step
process in industry. The first step is the hydrogenation of the
phenyl group by Pd-based catalyst at a temperature range of
160-180°C and a H2 pressure range of 30-48MPa,3,4 which
converts DMT into dimethyl cyclohexane dicarboxylates
(DMCD). The second step is the hydrogenation of the
carboxylic group to corresponding hydroxyl group by
conventional Cu-Cr catalyst at 200°C with 4MPa H2,5,6 which
converts intermediate DMCD into CHDM. However, the two
steps require different reactors and increase the operational
complexity and production cost. Besides, some other methods
are also reported for the production of CHDM in recent years.
Zhu et al.7 reported that terephthalic acid (TPA) could be
converted into CHDM with high yield in the presence of
bimetallic Ru-Sn supported catalyst that was prepared by the
co-impregnation method. Yoshinori et al.8 prepared the
carbon-supported RuSnPt catalyst which used activated carbon
treated by nitric acid as the support for the conversion of 1,4-
cyclohexanedicarboxylic acid (CHDA) into CHDM. The results
CHDM are all derived from the conventional petrochemical
industry. Due to limited petroleum reserves, it is significant to
find a substitution which can be derived from waste materials
for the production of CHDM. In our previous work,9 CHDM
could be obtained from BHET by a two-step process. Bis (2-
hydroxyethylene terephthalate) (BHET), which is the monomer
of the poly (ethylene terephthalate) (PET), could be easily
obtained from the glycolysis of waste PET.10 The glycolysis
reaction is catalyzed by zinc chloride, sodium carbonate, urea,
sulfated cobaltoxide (SO42−/Co3O4)11-15 and ionic liquids16-20
with an excess of ethylene glycol under 180-240°C at
atmospheric pressure. As known, noble metals and some
transition metals such as Ru, Rh, Pt21-23 and Ni24 are widely
used as catalysts for the hydrogenation of phenyl group at
mild conditions. Conventional Cu-based catalysts are used for
the hydrogenation of esters and carboxylic acids into
corresponding alcohols.25,26 In the two-step process, the
hydrogenation reaction is catalyzed by Pd/C and Cu-Cr
catalysts. The conversion of BHET and final yield of CHDM are
100% and 78.0% respectively, which indicates that BHET is an
excellent raw material for the preparation of CHDM. But the
pollution of Cr element has drawn much attention owing to
environmentally harmful effect and strict reaction condition.
Moreover, it is difficult to separate the intermediate product
for further hydrogenation.
a.School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou,
Henan, 450001, P. R. China. E-mail: renbz@zzu.edu.cn
In this paper, an one-pot conversion process of BHET into
CHDM by selective hydrogenation is introduced and the
reaction path-way is shown in Scheme 1. The phenyl group and
b.Beijing Key laboratory of Ionic Liquids Clean Process, Key Laboratory of Green
Process and Engineering, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing 100190, P. R. China. E-mail: sjzhang@ipe.ac.cn
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the carbonyl group of BHET are hydrogenated simultaneously under air for 4h. Then, an excessive amount of NaBH4 was
in a reactor by multi-functional catalyst that contains different dissolved in NaOH solution with pH value of 9 and the catalyst
active sites. Thomas et al.5,27,28 reported that a series of samples were reduced. After the reduction, the catalyst
bimetallic and trimetallic catalysts such as Ru5Pt, Ru12Cu4 and samples were washed several times by deionized water until
Ru5PtSn were prepared for the one-step conversion of DMT pH value reached 7. Finally, the samples were dried under Ar
into CHDM at mild reaction conditions, but the yield was too at 80°C for 10h. Besides, a series of catalysts including
low for use. As new efficient catalysts, Ru-Sn supported monometallic and bimetallic as well as trimetallic samples with
catalysts have revealed good performance in the different Ru contents were prepared by the same method.
hydrogenation of esters and carboxylic acids, in which Sn could Herein, all the catalysts were signed as RuxPtySnz, in which x, y
activate the carboxylic group of esters and carboxylic acids as and z represented the molar ratio of Ru, Pt and Sn.
Lewis acid and increase the selectivity of alcohol, as Ru is 2.3 Characterization
beneficial for the activation of hydrogen and the
The property of reducibility and hydrogen absorption of
hydrogenation of phenyl group. Since the incorporation of Pt
catalysts were investigated by temperature programmed
could depress the loss of Sn and accelerate the reaction rate, a
reduction (TPR) with an AutoChem II 2920 instrument. The
series of monometallic, bimetallic and trimetallic supported
XRD patterns of the catalysts were determined by a Bruker D8-
catalysts were prepared by the impregnation method for one-
Advance X-ray powder diffractometer with Cu Kα radiation (λ=
pot conversion of BHET into CHDM. Moreover, influences of
0.154 nm) at 40kV and 30 mA with 2θ angle range of 20°- 75°
various reaction parameters including temperature, pressure
and a scanning rate of 10°/min. The morphology of the
and time on the hydrogenation reaction were studied.
catalysts were observed using a TEM (JEM 2100-F) operated at
Meanwhile, the supported catalysts were characterized by
200 keV. XPS data were recorded using an ESCALAB 250Xi with
temperature programmed reduction and X-ray diffraction to
an Al Kα radiation operated at a pass energy of 20.0 eV. The
investigate their effects on the reaction.
catalysts before and after use were dissolved in hot aqua regia
and the Sn content was determined by inductively coupled
Catalyst
HOH2CH2COC
COCH2CH2OH
HOH2C
CH2OH
plasma optical emission spectrometer (ICPE-9000, Shimadzu).
H2
O
O
2.4 Reaction
CHDM
BHET
All hydrogenation reactions were carried out in a stainless
steel reactor equipped with magnetic stirring. In a typical
experiment, the reactor was charged with 1.0g BHET as
substrate and 0.1g catalyst. The reactor was purged with H2
three times to remove air and then pressurized H2 to the
pressure of 5MPa. Subsequently, the reactor was heated to
170°C under stirring of 500rpm. After 2h, the temperature was
raised to 260°C and kept for 6h.
Scheme 1 Conversion of BHET into CHDM by one-pot hydrogenation
process
2. Experimental
2.1 Materials
BHET (≥ 99%) was obtained by the degradation of waste PET in
our laboratory as reported previously.29 Sodium borohydride
(AR) and Sodium hydroxide (AR) were obtained from
Sinopharm Chemical Reagent Co., Ltd, China. Stannous
chloride (99.9%) and γ-Al2O3 were obtained from Aladdin
Industrial Co., Ltd, China. Ruthenium(III) chloride hydrate
(99.9%) and chloroplatinic acid hexahydrate (99.9%) were
obtained from J&K Scientific Ltd, China. The materials were
used without any further purification.
After reaction, the reaction mixture was dissolved in methanol
and centrifuged. Then the undissolved substance was removed
to get the liquid sample. The collected catalyst was washed
with ethanol to remove the residues on the surface and dried
at 80°C for 12h. The amount of BHET was adjusted to keep the
ratio of the catalyst to BHET content. The liquid sample was
analyzed by gas chromatography on a GC-2014 instrument
(Shimadzu
Corporation)
equipped
with
a
HP-5
2.2 Preparation of catalysts
chromatographic column and a flame ionization detector (FID).
o
The initial oven temperature was set at 40 C for 2 min, and
The catalysts were synthesized according to the procedures
described in the literature with slight modification.30 The
programmed to 200 oC at 15 oC /min and held at 200 oC for 2
min, then the oven temperature was programmed to 260 oC at
20 oC /min and held for 3 min. FID temperature was 280 oC and
inlet temperature was 270 oC. The amount of the product was
determined by the external standard method. The conversion
and yield of products were calculated by equations (1) and (2)
as follows:
supported catalysts were prepared by
a conventional
impregnation method. The γ-Al2O3 support was calcined at
500°Cunder air before use. A certain amount of three metal
precursor salts, RuCl3·nH2O, SnCl2·2H2O and H2PtCl6·6H2O
were dissolved by deionized water in a beaker to obtain Sn and
Pt contents of 3.0 and 1.5 wt% by the carrier respectively. The
acquired amount of pretreated γ-Al2O3 support was added and
the mixture was stirred for 4h, then the paste was obtained
after aging at room temperature for 24h. The paste was dried
at 80°C in a vacuum oven for 12h and then calcined at 500°C
moles of unreacted substrate
Conversion of BHET (%) = (1−
)×100%
(1)
moles of initial substrate
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may be caused by the oxidation in the air. The diffraction
peaks of SnO2 at 2θ of 26.9°, 34.1° and 52.2° are prominent
molesof certain product
Yield of CHDM (%)=
×100%
(2)
molesof theoretical product
and sharp for the Sn/Al2O3 (c) catalyst, which is ascribed to the
formation of large particles. It is due to the strong interaction
between SnO2 and the support which leads to the bad
distribution. The diffraction of Pt and PtO2 is not distinct due
to the low load and the shield of the support for the Pt/Al2O3
(d) catalyst. The diffraction peaks of RuO2 and SnO2 become
gradually broad and less prominent with the addition of Pt for
the trimetallic samples, due to the decrease of the particle
size.30 As the Ru content increases, there is no obvious change
observed for the Ru6PtSn4/Al2O3 (a) catalyst, which reflects
that the Ru content does not have distinct effect on the
dispersion of the metal particles on the support.
3. Results and discussion
3.1 The characterization of catalysts
TPR profiles of supported catalysts with different metal
elements are presented in Fig. 1. For the Ru/Al2O3 (e) catalyst,
the consumption peak around 150°C can be ascribed to the
reduction of Ru specie, which is in accordance with results
obtained in other studies.31 For the Sn/Al2O3 (d) catalyst, the
TPR profile shows two consumption peaks between 400°C and
600°C, which indicates that there are two different
interactions between the reduction of Sn species and the
alumina support in the catalyst.32 Similarly, it can be seen two
consumption peaks between 80°C and 200°C for the Pt/Al2O3
(c) catalyst, which may correspond to the presence of free Pt
and the platinum oxide. Three consumption peaks are
observed for the Ru4PtSn4/Al2O3 (b) and Ru6PtSn4/Al2O3 (a)
catalysts. The temperature of the first and second peaks are
mainly between 60°C and 200°C, and the last one that is not
distinct is in the temperature range of 300-400°C. Compared
with Ru4PtSn4/Al2O3
catalyst, the maximum reduction
temperature of the first and second peaks that may be
ascribed to the Pt metal interacted with Ru shift gradually
lower for the Ru6PtSn4/Al2O3 (a) catalyst. This phenomenon
may be closely related to the hydrogen spillover on Ru metal
surface.33 The high temperature reduction peak between 300
and 400°C is ascribed to the formation of PtSn alloy, which is
also reported in literature.34
Fig. 2 XRD patterns of various supported monometallic or trimetallic
catalysts a: Ru6PtSn4/Al2O3; b: Ru4PtSn4/Al2O3; c: Sn/Al2O3; d: Pt/Al2O3; e:
Ru/Al2O3; f: Al2O3
Fig. 3 TEM image of Ru6PtSn4/Al2O3 catalyst
Similarly, TEM analysis was also conducted for Ru6PtSn4/Al2O3
to have a better understanding of the morphological feature.
In Fig. 3, uniform dispersion of the metal particles can be
observed on the Al2O3 support. However, special morphology
is not observed, because the support used in the experiments
is amorphous. Well dispersed metal particles on the support
are considered as active sites for the hydrogenation of BHET
into CHDM.
Fig. 1 H2-TPR profiles of various supported monometallic or trimetallic
catalysts a: Ru6PtSn4/Al2O3; b: Ru4PtSn4/Al2O3; c: Pt/Al2O3; d: Sn/Al2O3; e:
Ru/Al2O3
The X-ray powder diffraction patterns of the monometallic and
trimetallic supported catalysts are shown in Fig. 2. The Al2O3 (f)
sample exhibits the characteristic diffraction peaks at 2θ of
32.8°, 36.8°, 39.5°, 45.5° and 67.2°, which can also be observed
The XPS measurements were carried out on the Ru6PtSn4/Al2O3
in other samples. This evidence indicates that the initial catalyst and the results were shown in Fig. 4. For the Ru 3d5/2
support nature is preserved after some treatments during the
preparation of the catalysts. For the Ru/Al2O3 (e) catalyst, the
diffraction peaks at 2θ of 28.0°, 34.9° and 54.1° are attributed
to RuO2 phase. However, Ru peaks are not detected which
spectra of the catalyst, the binding energy at 280.1 eV can be
ascribed to the Ru(0) species, but the result is not very clear
because the peaks of Ru 3d and C 1s overlap each other.7 The
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binding energy of the Sn 3d5/2 XPS spectra at 487.2 eV is
corresponding to Sn(II) and/or Sn(IV). However, it is difficult to
distinguish the peaks of Sn(II) and Sn(IV) due to a very small
difference in the binding energy of both oxidation states.8 The
binding energy at 486.1 eV is consistent with metallic Sn, which
confirms the exist of PtSn alloy.35 For the Pt 4f7/2 spectra of the
catalyst, the binding energy at 71.8 and 73.5 eV are attributed to
Pt(0) and Pt(IV) respectively, which is also reported by others.36
According to these results, the surface of the catalyst is composed
of reduced state and oxidized state, which is in agreement with the
TPR and XRD results.
metal are responsible for the hydrogenation of the phenyl
group.
The Sn catalyst shows little activity for the hydrogenation of
BHET because no other product was detected except for the
peaks of methanol and ethylene glycol in the gas
chromatogram. It is attributed to the oligomerization and
polymerization of BHET as the corresponding products have
not dissolved in methanol.
The incorporation of Sn to Ru- and Pt- catalysts can achieve
the conversion of BHET into CHDM, although the selectivity is
still lower. As known, Sn metal plays a crucial role in the
activation of the carbonyl group, but would restrain the
hydrogenation capability of Ru catalyst,38 which leads to the
appearance of by-products 5 and 6. The hydrogenolysis of
ester group is also cause of the appearance of by-products 5
and 6. The reason for the poor performance of the bimetallic
PtSn catalyst can be attributed to the formation of Pt-Sn alloy,
which leads to the decrease of the hydrogenation selectivity
and enhences of the hydrogenolysis. For the RuPt catalyst, the
main product is BHCD, along with some by-products generated
from hydrogenolysis of ester group.
When Pt metal is added to the bimetallic RuSn catalyst, the
ability of the hydrogenation of phenyl group is enhanced and
the yield of CHDM increases obviously. It indicates that the
synergistic effect among three metal elements in the catalyst
has a positive effect on the selective hydrogenation of BHET
into CHDM. The by-products 3 and 4 are formed by excessive
hydrogenation of product. Moreover, the possible reaction
path-way for the conversion of BHET into CHDM is shown in
Scheme 2 according to the main by-products distribution.
3.2.2 Influence of the Ru content
Fig. 4 XPS spectra of Ru6PtSn4/Al2O3 catalyst a: Ru 3d spectra; b: Sn 3d spectra; c:
Pt 4f spectra
3.2 Influence of catalyst on the conversion of BHET into CHDM
In order to present the influence of the Ru content on the
selective hydrogenation, a series of catalysts with various
Ru/Sn ratio were prepared for the hydrogenation of BHET into
CHDM, and the results are shown in Fig. 5. The conversion of
BHET is high with no distinct change for all the trimetallic
catalysts, whereas the yield of CHDM strongly depends on the
composition of the catalysts. When the Ru content is low, the
hydrogenation of the phenyl group is restrained and many by-
products which mainly contain the phenyl group such as 5 and
6 are formed leading to the low selectivity of CHDM. With the
increase of the Ru content, the yield of CHDM shows an
increase from 53.5% to 87.1% simultaneously and the
maximum is obtained when the atomic ratio of Ru/Sn is 1.5.
Meanwhile, the byproducts 5 and 6 decrease gradually.
However, the further addition of Ru leads to the decrease in
the yield of CHDM. It is well known that the Ru metal is
beneficial for the activation of hydrogen. When the Ru content
increases, more activated hydrogen can be produced for the
selective hydrogenation. But the excessive activated hydrogen
can lead to the reduction of hydroxyl group, which results in
further hydrogenation of CHDM into by-products such as 3 and
4. The results show that the appropriate Ru content in the
supported trimetallic catalyst plays a crucial role in the
selective hydrogenation of BHET into CHDM.
The conversion of BHET into CHDM was obtained
successfully by the hydrogenation of the phenyl group to
cyclohexane and the carbonyl group corresponding to hydroxyl
group. As
a , oligomerization and
diol, hydrogenolysis4
polymerization37 may also occur in the present reaction system
at high temperature and hydrogen pressure. As a result, the
conversion of BHET not only depends on the hydrogenation
activity of catalyst, but also depends on the side reactions.
However, the selectivity of CHDM shows quite distinct
difference with different catalysts. In order to obtain the
efficient catalyst, a series of monometallic, bimetallic and
trimetallic supported catalysts were prepared by the
impregnation method for the conversion of BHET into CHDM.
3.2.1 Influence of various active metal
The selective hydrogenation of BHET into CHDM was carried
out using various metal catalysts to determine the optimal
metal component of catalyst. The main by-products are shown
in Scheme 2 and the catalytic results are shown in Table 1. As it
can be seen, the conversion of BHET is above 90% for the
supported monometallic, bimetallic and trimetallic samples,
while the selectivity of CHDM depends on the metal
component of catalyst. As shown, the main by-product for Ru
and Pt catalysts is BHCD, which is the product of the
hydrogenation of the phenyl group. It indicates that Ru and Pt
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Table 1 Influence of various metal catalysts on the selective hydrogenation
Catalyst
Ru/Al2O3
Pt/ Al2O3
Sn/ Al2O3
Conversion of BHET (%) Yield of CHDM (%)
Main byproducts
97.2
95
—
—
BHCD (82.7%), 5+6 (12.8%)
BHCD (69.4%), 5+6 (18.1%), 2 (5.7%)
1 (94.6%)
94.6
99.7
99.6
99.8
99.3
—
Ru4Pt/ Al2O3
Ru4Sn4/ Al2O3
PtSn4/ Al2O3
—
BHCD (74.5%), 5+6 (13.5%), 3+4 (9.6%)
5+6 (70.6%), 2 (7.3%), 3+4 (5.6%)
5+6 (79.2%), 2 (10.7%)
14.7
5.3
78.4
Ru4PtSn4/ Al2O3
5+6 (13.3%), 3+4 (4.7%)
Reaction conditions: 1.0g BHET, 0.1g catalyst. H2 5MPa, 500rpm, initially at 170°C for 2h, then 260°C for 6h.
O
C
O
C
Oligmerization
O
C2H4 O
n
(1)
Hydrogenolysis
O
O
O
O
HOC2H4 O C
C O C2H4OH
H3C
C
C2H5
H3C
CH3
(6)
Hydrogenolysis
C2H4OH H3C
(5)
BHET
Hydrogenation
HOC2H4
O
O
O
C
C
C
C2H4OH
O
O
O
BHCD
(7)
O
C
H3C
CH3
O
HO H2C
HO H2C
C2H4OH
(4)
(2)
CH2 OH
H3C
CH2 OH
CHDM
(3)
Scheme 2 The possible reaction path-way for the conversion of BHET into CHDM. Some by-products are trace amount and they are ignored.
Reaction temperature is shown to be a crucial parameter for
the hydrogenation recation. As known, the phenyl group is
hydrogenated to cyclohexane easily compared with the
carbonyl group to corresponding hydroxyl group. To verify the
possibility of hydrogenating BHET into CHDM by single
reaction temperature, the catalytic performance of
Ru6PtSn4/Al2O3 is investigated as shown in Table 2. When the
reaction is carried out at low temperature 170°C, the main
product is BHCD and the yield is 87.8% without CHDM. It
means that the phenyl group is conserved to cyclohexane
easily at this condition, while it is not feasible for the
hydrogenation of the the carbonyl group to corresponding
alcohol. At the temperature 230°C, CHDMcan be obtained with
Fig. 5 Influence of the Ru content on the selective hydrogenation. Reaction
low yield of 47.2%. The main by-products are 2 and 5 which
conditions: 1.0g BHET, 0.1g catalyst. H2 5MPa, 500rpm, initially at 170°C for
increase obviously. As the temperature reaches to 260°C, the
yield of CHDM is only 28.5%. The main by-products 5 and 6 are
caused by the hydrogenolysis of ester group, along with little 3
and 4 from the excessive hydrogenation. The above reaction
2h, then 260°C for 6h.
3.3 Influence of reaction condition on the conversion of BHET into
CHDM
3.3.1 Influence of reaction temperature
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Table 2 Influence of single reaction temperature on the selective hydrogenation
Temperature (°C)
Conversion of BHET (%)
Yield of CHDM (%)
Main by-products
170
230
260
99.6
99.4
99.7
—
BHCD (87.8%), 2 (5.6%)
5+6+7 (35.2%), 2 (13.4%)
5+6 (57.6%), 3+4 (7.6%)
47.2
28.5
Reaction conditions: 1.0g BHET, 0.1g catalyst. H2 5MPa, 500rpm, 8h.
hydrogenation of the carbonyl group because of the
appearance of by-product 2. The high temperature may be
intensify the hydrogenolysis of ester group, which inhibits the
main reaction of the conversion of BHET into CHDM. So the
optimal reaction temperature is 170/260°C for the
hydrogenation of BHET into CHDM.
3.3.2 Influence of hydrogen pressure
The influence of hydrogen pressure on the conversion of BHET
into CHDM is shown in Fig. 7. As shown, the hydrogen pressure
plays an important role in the reaction. It may not only
accelerate the reaction rate, but also impel the equilibrium to
the desired direction. The hydrogenolysis of ester group is
carried out more easily with low hydrogen pressure, which
leads to the low selectivity of CHDM. With the increase of
hydrogen pressure, the yield of CHDM increases
simultaneously. It can be concluded that the yield of desired
product is proportional to the hydrogen pressure in the range
of 3 to 7MPa. However, the high cost and safety problems
might also arise due to the high pressure. As the yield ofCHDM
has not increased obviously when hydrogen pressure is above
5MPa, 5MPa is chosen as the optimal hydrogen pressure
condition in consideration of economy and safety.
Fig. 6 Influence of reaction temperature on the conversion of BHET and
yield of CHDM. Reaction conditions: 1.0g BHET, 0.1g catalyst. H2 5MPa,
500rpm, initial time for 2h, then for 6h.
3.3.3 Influence of reaction time
Fig. 8 shows the influence of reaction time on the conversion
of BHET and yield of CHDM. The conversion of BHET is high
and close to 100%. The reason is that the hydrogenaolysis and
hydrogenation of BHET can occur simultaneously in the
present reaction system at high temperature and hydrogen
pressure. For the first stage, the yield of CHDM increases with
the extension of reaction time. When the first stage reaction
time is short, the hydrogenaolysis of BHET can occur more
easily which may lead to production of by-products 5 and 6. It
indicates that the first stage reaction time may also play an
important role on the activation of the carbonyl group.
Conversely, with the increase of the second stage reaction
time, the yield of CHDM decreases gradually. It may be caused
by the excessive hydrogenation of CHDM which results in the
formation of by-products 3 and 4. Compared with the
hydrogenation of the phenyl group, it takes more reaction
time for the carbonyl group to be hydrogenated to alcohol. It is
not appropriate for the conversion of BHET into CHDM by
prolonging the total reaction time without limit. As a result,
the optimal reaction time is 2/6h.
Fig. 7 Influence of pressure on the conversion of BHET and yield of CHDM.
Reaction conditions: 1.0g BHET, 0.1g catalyst. 500rpm, initially at 170°C for
2h, then 260°C for 6h.
results indicate that it is difficult for the hydrogenation of
BHET into CHDM in excellent yield through single temperature
process.
Fig. 6 shows the influence of reaction temperature on the
conversion of BHET and yield of CHDM catalyzed by
Ru6PtSn4/Al2O3 through a two-temperature process. For the
first stage temperature, it shows that the yield of CHDM
initially increases and then decreases with the raise of the
reaction temperature. It is concluded that the first stage
temperature is beneficial for the activation of the carbonyl
group, because the by-products are mainly 5 and 6. For the
second stage, the low temperature is not beneficial for the
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mainly DMT, TPA and BHET. DMT and TPA are all derived from the
conventional petrochemical industry. However, petroleum reserve
is limited. One-step conversion of DMT into CHDM at mild reaction
conditions is obtained by nanoparticle catalysts, but the
preparation of catalyst is difficult. Above all, the selectivity is low.
Two-step conversion of DMT into CHDM can get high conversion
and selectivity, and the catalysts used in the hydrogenation reaction
can easily be obtained by conventional methods such as
impregnation method and precipitation method. However, the
procedure is complicated caused by the separation of the
intermediate product before further hydrogenation. Being similar
to DMT,one-step and two-step conversion of TPA into CHDM have
same disadvantages. Besides, TPA may corrode the equipments,
which makes it necessary to find new methods for the preparation
of CHDM. BHET can easily be obtained from the glycolysis of waste
PET, which is sustainable and environmental friendly. Compared
with two-step conversion of BHET into CHDM, one-pot method not
only can get the high conversion and selectivity, but is also simple.
Overall consideration, the conversion of BHET to CHDM through
one-pot method is more competitive.
Fig. 8 Influence of reaction time on the conversion of BHET and yield of
CHDM. Reaction conditions: 1.0g BHET, 0.1g catalyst. H2 5MPa, 500rpm,
initially at 170°C, then 260°C.
3.4 Recycling experiments
The catalyst after use was recovered to determine the resuability.
Unfortunately, the results of the recycling experiments were not
satisfactory. The main by-products were 5 and 6 while the targeted
product CHDM was low. The Sn content of catalysts before and
after use were characterized by ICP-OES. The results showed that
the Sn content in the catalyst before use was 3.1%, which was
almost in accordance with the theoretical value. However, the Sn
content in the used catalyst decreased to 2.6%. It indicated that the
amount of Sn elution was noteworthy. As reported, the Sn played
an important role in the activation of the carboxylic group of esters
and carboxylic acids, which was also easy to be eluted.31 The
amount of Sn elution was so noticeable that it might lead to the low
hydrogenation activity, which indicated that further improvement
was needed to enhance the stability of the catalyst.
4 Conclusions
By the impregnation method, a series of monometallic, bimetallic
and trimetallic supported catalysts were prepared for the
hydrogenation of BHET into CHDM. As a result, one-pot conversion
of BHET derived from waste PET into CHDM by selective
hydrogenation was obtained successfully, and good performance
was found with supported trimetallic RuPtSn/Al2O3 catalysts. The
results indicated that Ru was beneficial for the activation of
hydrogen, and incorporation of Pt had both effects on the
hydrogenation of the phenyl group as well as the decrease of metal
particle size. The influences of various reaction parameters
including temperature, pressure and time on the hydrogenation
3.5 Comparison among various methods for the preparation of
CHDM
For comparison, various methods for the preparation of CHDM are
listed in Table 3. The raw materials for the preparation of CHDM are
Table 3 Comparison among various methods for the preparation of CHDM
Conversion and
Selectivity
Raw material
DMT
Source
Method
Catalyst
Procedure
Simple
References
5
One-step
Noble metal
Low
High
Low
High
High
High
Petrochmecial
industry
Noble metal &
non-noble metal
Two-step
One-step
Two step
One-pot
Two-step
Complicated
Simple
39
Noble metal
40
Petrochmecial
industry
TPA
Noble metal &
non-noble metal
Complicated
Simple
41
This work
9
Noble metal
BHET
Waste PET
Noble metal &
non-noble metal
Complicated
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reaction were studied, and 100% conversion and 87.1% yield of This research was supported financially by International S&T
CHDM were obtained with the trimetallic supported catalyst with Cooperation Program of China (2014DFA61670), the National
Ru/Sn of 1.5. For the recycling experiments, the catalyst showed a Natural Science Foundation of China (No. 91434203, No.
poor reusability due to the leaching of Sn, which indicated that 21336002, No. 21476234, No. 21276260, No. 21576269), Key
further improvement was needed to enhance the stability of the Project of Natural Science Foundation of Beijing of China (No.
catalyst. Moreover, through the comparison between various 2131005) and CAS/SAFEA International Partnership Program
methods for the preparation of CHDM, the conversion of BHET into for Creative Research Teams (20140491518).
CHDM by one-pot method is considered as one of the most
competitive methods.
Notes and references
Acknowledgements
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Conversion of bis (2-hydroxyethylene terephthalate) into
1,4-cyclohexanedimethanol by selective hydrogenation using
RuPtSn/Al2O3
One-pot conversion of BHET derived from waste PET into CHDM by trimetallic
RuPtSn/Al2O3 catalysts were studied. The influences of various reaction parameters
including temperature, pressure and time on the hydrogenation reaction were studied,
and 100% conversion and 87.1% yield of CHDM were obtained with the trimetallic
supported catalyst with Ru/Sn of 1.5.