Preparation of 1,4-cyclohexanedimethanol by selective hydrogenation of a waste PET monomer bis(2-hydroxyethylene terephthalate)

Article abstract of DOI:10.1039/c4ra10783g

A new approach is developed for the preparation of 1,4-cyclohexanedimethanol (CHDM) by hydrogenation of bis(2-hydroxyethylene terephthalate) (BHET) obtained from waste poly(ethylene terephthalate) (PET). The influences of various reaction parameters including temperature, pressure and time, on the hydrogenation reaction were studied, and the 100% conversion of BHET and 78% yield of CHDM were achieved with Pd/C and Cu-based catalysts. X-ray diffraction, low temperature N₂ adsorption-desorption and H₂ temperature-programmed reduction were used to characterize the Cu-based catalysts, demonstrating that the Cu⁺/Cu⁰ species are the active sites. This study not only provides a new route for the production of CHDM, but also an approach for the efficient utilization of waste PET.

Full text of DOI:10.1039/c4ra10783g

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PAPER
Preparation of 1,4-cyclohexanedimethanol by
selective hydrogenation of a waste PET monomer
bis(2-hydroxyethylene terephthalate)
Cite this: RSC Adv., 2015, 5, 485
b
b
a
b
Xiaonan Guo,ab Jiayu Xin, Xingmei Lu, Baozeng Ren and Suojiang Zhang
*
*
A new approach is developed for the preparation of 1,4-cyclohexanedimethanol (CHDM) by hydrogenation
of bis(2-hydroxyethylene terephthalate) (BHET) obtained from waste poly(ethylene terephthalate) (PET).
The influences of various reaction parameters including temperature, pressure and time, on the
hydrogenation reaction were studied, and the 100% conversion of BHET and 78% yield of CHDM were
achieved with Pd/C and Cu-based catalysts. X-ray diffraction, low temperature N2 adsorption–
desorption and H2 temperature-programmed reduction were used to characterize the Cu-based
catalysts, demonstrating that the Cu+/Cu0 species are the active sites. This study not only provides a new
route for the production of CHDM, but also an approach for the efficient utilization of waste PET.
Received 19th September 2014
Accepted 24th November 2014
DOI: 10.1039/c4ra10783g
www.rsc.org/advances
CHDM from CHDA15 was described. Recently, Thomas and co-
workers reported that bimetallic nanoparticle catalysts,16 such
1. Introduction
as silica-supported Ru5Pt, Ru10Pt2, Ru6Pd6, Ru12Cu4 and a tri-
metallic cluster Ru5PtSn17 can promote the single-step conver-
sion of DMT into CHDM under mild reaction conditions.18
However, both the conversion and selectivity are not so
satisfactory.
However, the above raw materials are derived from petro-
leum industries and the hydrogenation processes are carried
out in solvents such as ethyl acetate, tetrahydrofuran and
methanol, which increase the difficulty in the separation of
products and the cost of the process. Therefore, it is imperative
to nd a low-cost material and a solvent-free process for the
synthesis of CHDM.
Herein, we developed a new solvent-free route to produce
CHDM using the waste poly(ethylene terephthalate) (PET)
derived monomer bis(2-hydroxyethylene terephthalate) (BHET)
as the raw material. The low-cost material and the solvent-free
process can reduce the cost and simplify the process signi-
cantly. BHET can be easily obtained from the glycolysis of waste
PET, which was carried out conventionally in ethylene glycol
solution under 393–463 K, 1–5 h and atmospheric pressure
using various kinds of catalysts such as zinc acetate, cobalt
acetate19–22 and ionic liquids.23–25 BHET has a potential for the
conversion into chemicals of utility, which can be used as the
raw material to synthesize the fatty amide derivatives,26 unsat-
urated polyester resin27 and hydrophobic textile dyestuffs.28
However, there are still few studies on the application of BHET.
Therefore, transforming BHET easily obtained from waste PET
into high value-added CHDM is of great signicance, which
contributes to the recycling of resources.
1,4-Cyclohexanedimethanol (CHDM) is a highly valued and
extensively used chemical in the polymer industry. It is used in
paints and particularly as a modied material for bers and
polyesters1 such as poly(ethylene terephthalate-co-1,4-cylclohexyl-
enedimethylene terephthalate) (PETG) with excellent perfor-
mance such as thermal resistance, weather resistance and
physical strength.2 Due to the wide application of the high
performance polyesters, the demand of CHDM will be increasing
signicantly. Industrially, CHDM is prepared by a two-step
process using two reactors. The rst step is the highly
exothermic conversion of dimethyl terephthalate (DMT) into
dimethyl hexahydroterephthalate (DMHT) by the hydrogenation
of the phenyl group using catalysts including Ni, Pd, Ru, Rh,
added Pt, Ba, Ca and other additives in the temperature range of
433–453 K and an H2 pressure of 5–48 MPa.3–7 The second step is
the conversion of DMHT into CHDM by the hydrogenation of the
ester groups to hydroxyl groups using conventional Cu-based
catalysts8,9 such as Cu–Cr or Cu–Zn catalysts,10 added Mn, Ba
and other catalysts containing Ru11–13 and Fe-based catalysts
bearing a PNP-pincer ligand14 in the temperature range of 473–
573 K and the reaction pressure of 3–15 MPa.
There are some studies on the preparation of CHDM by other
means. A new process for producing CHDM by the hydrogena-
tion of terephthalic acid (TPA) into 1,4-cyclohexanedicarboxylic
acid (CHDA) using Pd- or Ru-based catalysts, and further getting
aSchool of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou,
Henan, 450001, P. R. China. E-mail: renbz@zzu.edu.cn
bBeijing 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@home.ipe.ac.cn
In this paper, it is found that BHET obtained from waste PET
can be efficiently converted to CHDM as shown in Scheme 1,
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Scheme 1 BHET obtained from waste PET is converted to CHDM.
which has not been reported to date. It was carried out by a two- precipitated by adding sodium carbonate until the pH of the
step process using the traditional catalyst Pd/C and Cu-based solution reached 8. The precipitate was aged at 333 K for 3 h and
catalysts. In the rst step, the phenyl group was selectively then ltered and washed several times with deionized water.
hydrogenated to cyclohexane catalyzed by Pd/C catalyst. In the The obtained solid sample was dried at 393 K for 12 h in an oven
second step, the ester groups were selectively hydrogenated to and then calcined at 723 K for 4 h to provide the as-prepared
hydroxyl groups catalyzed by Cu-based catalysts. The aim of this oxide. The as-prepared oxide was reduced by 5% H2 in Ar at
paper is to provide a new method that can selectively hydroge- the programmed temperature which is rising from room
nate BHET to CHDM. The inuences of various reaction temperature to 493 K staying 1 h by the rate of 4 K minꢀ1, then
parameters, including reaction temperature, pressure and time, to 623 K staying 2 h by the rate of 1 K minꢀ1 to obtain the nal
on the two hydrogenate reactions were investigated to get the Cu–Cr catalyst.
optimal reaction conditions. Moreover, X-ray diffraction, low
2.2.2. Preparation of Cu–Zn/Al2O3 catalyst. Firstly, an
temperature N2 adsorption–desorption and H2 temperature- aqueous solution was prepared with Cu(NO3)2$3H2O,
programmed reduction were used to characterize the Cu- Zn(NO3)2$6H2O and Al(NO3)3$9H2O with molar ratio of
based catalysts.
Cu2+ : Zn2+ : Al3+ ¼ 5 : 4 : 1,30 then the solution was precipitated
by adding sodium carbonate until the pH of the solution
reached 7 at 333 K with stirring. The precipitate was aged at 333
K for 3 h and then ltered and washed several times by deion-
ized water. The obtained solid sample was dried at 393 K for 12
h in an oven and then calcined at 723 K for 4 h to provide the as-
prepared oxide. The as-prepared oxide was reduced by H2 under
573 K before use.
2. Experimental
2.1. Materials
BHET ($99%) was obtained by the degradation of the waste PET
in our laboratory. Copper(II) nitrate trihydrate (Cu(NO3)2$3H2O,
$99%) and chromium(III) nitrate nonahydrate (Cr(NO3)3$9H2O,
$99%) were obtained from Sinopharm Chemical Reagent Co.,
Ltd, China. Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, $99%),
aluminum nitrate nonahydrate (Al(NO3)3$9H2O, $99%), and
sodium carbonate (Na2CO3, $99%) were obtained from Xilong
Chemical Co., Ltd, China. 10% Pd/C was obtained from J & K
Scientic Ltd, China. The materials were used without any
further treatment.
2.3. Hydrogenation reaction of BHET to CHDM
All hydrogenation reactions were carried out in a 100 mL
stainless steel high pressure reactor (MSC County Petroleum
Research Instruments, Jiangsu, China), equipped with
magnetic stirring. The hydrogenation reaction of BHET to
CHDM was carried out by a two-step process as shown in
Scheme 2. In the rst step, the reactor was charged with 2.0 g of
BHET as starting material and 0.1 g of 10% Pd/C catalyst.31 The
2.2. Preparation of Cu-based catalysts
2.2.1. Preparation of Cu–Cr catalyst. The Cu–Cr catalyst reactor was then purged with H2 three times to remove air and
was prepared by a conventional co-precipitation method.29 In a then pressurized to the desired pressure of H2 and heated to the
typical synthesis, Cu(NO3)2$3H2O and Cr(NO3)3$9H2O with a desired temperature under stirring of 400 rpm. Aer the reac-
desired molar ratio (Cu : Cr ¼ 1 : 1) were dissolved in 50 mL tion, the intermediate products which are mainly consisted of
deionized water at 333 K with stirring, the solution was bis(2-hydroxyethyl) cyclohexane-1,4-dicarboxylate (BHCD) was
Scheme 2 The two-step process for the hydrogenation of BHET to CHDM without solvent.
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obtained and then used as the raw material for the second step.
In the second step, 2.0 g of the intermediate products and 0.1 g
of Cu-based catalysts were placed in the reactor, the reactor was
then purged with H2 three times to remove air and then pres-
surized to the desired pressure of H2 and heated to the desired
temperature under stirring of 400 rpm, the target product
CHDM was obtained.
2.4. Characterization and analysis
The structures and crystal phase compositions of the Cu–Cr and
Cu–Zn/Al2O3 catalysts were determined by Powder X-ray
diffraction (XRD) patterns with a Bruker D8-Advance X-ray
powder diffractometer using Cu Ka radiation within a 2q
range of 5–85ꢁ in steps of 10ꢁ, operated at an accelerating
voltage of 40 kV and an emission current of 200 mA.
Temperature-programmed reduction (TPR) of the Cu-based
catalysts was carried out in a stream of 90% Ar and 10% H2
with a owing rate of 50 mL minꢀ1, and the samples were
heated at 10 K minꢀ1 from room temperature to 773 K.
The intermediate products of the rst step were analyzed by
HPLC (Agilent 1100) tted with a refractive index detector and
an ODS-BP column (0.46 mm ꢂ 250 mm). The mixture of
acetonitrile and water with a ratio of 1 : 1 was used as mobile
phase. The oven temperature for the determination of standards
Scheme 3 The possible reaction pathway for the hydrogenation of
BHET to BHCD.
and samples was set at 313 K with a ow rate of 0.5 mL minꢀ1
.
reached almost 100% when the reaction temperature reached
428 K. The yield of BHCD initially increased and then decreased
with the increase of the reaction temperature.
The nal products from the second step were analyzed by gas
chromatography on a GC-2014 instrument (Shimadzu Corpora-
tion) equipped with a HP-5 chromatographic column and a ame
ionization detector (FID). The products were identied by GC
(Agilent 6890N)-MS (Agilent 5975B) equipped with a HP-5ms UI
chromatographic column. The conversion and yield of products
were calculated by eqn (1) and (2) as follows:
Because the further increase of the temperature, the reaction
shied to side pathways including the hydrogenation of the
ester and hydroxyl groups of BHET which is conrmed by the
appearance of the by-products 4 and 5, and the excessive
hydrogenation of the ester and hydroxyl groups of BHCD which
is conrmed by the appearance of the by-products 1, 2, 3 and 6.
The yields of by-products 3 and 5 increased while the yields of
by-products 1, 2, and 4 decreased with the increase of the
reaction temperature. The by-product 6 occurred when the
temperature was above 498 K, which indicates that the further
ꢀ
ꢁ
moles of unreacted reactant
moles of reactant
Conversionð%Þ ¼ 1 ꢀ
ꢂ 100
(1)
moles of certain product
moles of reactant
Yieldð%Þ ¼
ꢂ 100
(2)
3. Results and discussion
3.1. Hydrogenation of BHET to BHCD
In the rst step, BHET was converted to BHCD by the selective
hydrogenation of phenyl group to cyclohexane group, which
was performed on 10% Pd/C catalyst without solvent. In order to
get the maximum direction of the main reaction, we investi-
gated the inuences of different reaction conditions on the
reaction to get the optimal reaction conditions. Moreover, we
report the possible reaction pathway for the hydrogenation of
BHET to BHCD as shown in Scheme 3.
3.1.1. Inuence of reaction temperature. The inuence of
reaction temperature on the conversion of BHET and yields of
BHCD and main by-products is shown in Fig. 1. It displays that
the reaction temperature is particularly important for the
Fig. 1 Influence of reaction temperature on the conversion of BHET
and yields of BHCD and main by-products. Reaction conditions: 2.0 g
hydrogenation reaction. The conversion of BHET increased and BHET, 0.1 g 10% Pd/C, 3.5 MPa, and 5 h.
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hydrogenation becomes easy at high temperature. It can be
concluded that 428 K is the optimal reaction temperature for
the hydrogenation of BHET to BHCD.
3.1.2. Inuence of reaction time. Fig. 2 presents the inu-
ence of reaction time on the conversion of BHET and yields of
BHCD and main by-products. It is shown that the conversion of
BHET increased with the increase of the reaction time.
However, the yield of the BHCD increased rapidly in reaction
time before 3 h but decreased slightly with the further increase
of the reaction time, which implies that the further hydroge-
nation of BHCD leads to more side reactions. The yield of the
by-product 1 increased, while the yield of the by-product 2
increased initially and then decreased with the increase of the
reaction time. Moreover, the by-products 3 and 4 occurred when
the reaction time was longer than 4 h, indicating that the
terminal hydroxyl group is easier to be reduced than the ester
group catalyzed by Pd/C, and the further increase of the reaction
time would not only reduce the yield of the main product but
also not distinctly affect the conversion. Based on the analysis, a
reaction time of 3 h gave the highest product yield and lowest
side products and therefore it is regarded as the best reaction
time.
3.1.3. Inuence of reaction pressure. Fig. 3 shows the
inuence of reaction pressure on the conversion of BHET and
yields of BHCD and main by-products. It is observed that with
an increase of the reaction pressure, the conversion of BHET
increased and the yield of the BHCD increased and the rate of
the increase in the yield of BHCD became smaller. The by-
products 5 and 6 disappeared when the reaction pressure was
above 5.5 MPa. The by-products 1 and 2 increased and then
decreased with the increase of the reaction pressure. However,
the by-products 3 and 4 were not detected in the products. It
indicates that the high pressure improves the selectivity of the
hydrogenation of phenyl to cyclohexane, but could not remove
the further hydrogenation of ester and hydroxyl groups. It can
be concluded that a certain range under 7 MPa of the high
pressure can not only drive the equilibrium to move to the
desired direction but also prevent the catalysts from losing
activity.
Fig. 3 Influence of reaction pressure on the conversion of BHET and
yields of BHCD and main by-products. Reaction conditions: 2.0 g
BHET, 0.1 g 10% Pd/C, 2 h, and 428 K.
3.1.4. Inuence of stirring rate. Fig. 4 shows the inuence
of stirring rate on the conversion of BHET and the yield of
BHCD. As can be seen, the conversion of BHET and yield of
BHCD rstly increased with the increase of the stirring rate
from 50 to 300 rpm, and then kept stable from 300 to 600 rpm.
Thus, there should be no mass or heat transfer limitation with a
stirring rate higher than 300 rpm.
As a consequence, the Pd/C catalyst shows high activity and
selectivity of the hydrogenation of BHET to BHCD without
solvent under respective mild reaction conditions. Under the
optimal reaction conditions of 428 K, 3 h, and 7 MPa, the
conversion of BHET and the yield of BHCD have reached 100%
and 95%, respectively.
3.2. Hydrogenation of BHCD to CHDM
In the second step, BHCD with a certain purity (90%) obtained
from the rst step was converted to CHDM by the selective
hydrogenation of ester group. However, it is well known that the
hydroxyl group can be hydrogenated by the Cu-based cata-
lysts,32,33 which make the second step difficult because of the
Fig. 2 Influence of reaction time on the conversion of BHET and yields Fig. 4 Influence of stirring rate on the conversion of BHET and the
of BHCD and main by-products. Reaction conditions: 2.0 g BHET, 0.1 yield of BHCD. Reaction conditions: 2.0 g BHET, 0.1 g Pd/C, 5.5 MPa,
g 10% Pd/C, 3.5 MPa, and 428 K.
428 K, and 2 h.
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Fig. 5 Influence of reaction temperature on the conversion of BHCD
and yield of CHDM. Reaction conditions: Cu–Cr: 2.0 g intermediate
products (90% BHCD), 0.1 g Cu–Cr, 10 h, and 10 MPa; Cu–Zn/Al2O3:
2.0 g intermediate products (90% BHCD), 0.1 g Cu–Zn/Al2O3, 10 h, and
6.5 MPa.
by Cu–Cr and Cu–Zn/Al2O3 catalysts is shown in Fig. 6. It
displays that the reaction time is a dominant parameter
affecting the ultimate catalyst performance. The conversion of
BHCD increased when the reaction time increased, whereas the
yield of CHDM initially increased when the reaction time
increased to 10 h and then decreased when the reaction time
increased prolonged from 10 h to 12 h, which is attributed to
the further hydrogenation of the hydroxyl group of CHDM. As a
consequence, the preferred reaction time is 10 h, affording
about 32% and 65% CHDM yield catalyzed by Cu–Cr and Cu–
Zn/Al2O3 catalysts, respectively. It displays that the excessive
increase of reaction time has no effect on the improvement of
the yield of CHDM.
Scheme 4 The possible reaction pathway for the hydrogenation of
BHCD to CHDM.
hydroxyl groups in BHET. As shown in Scheme 4, we suggest the
possible reaction pathway for the hydrogenation of BHCD to
CHDM. The hydrogenation reaction follows two pathways: (i)
the ester group in one side chain is rstly reduced to hydroxyl
group, and then the other ester group is reduced, (ii) the
terminal hydroxyl groups are rstly reduced and then the ester
groups are reduced. The reaction pathway (i) and (ii) produce
ethylene glycol and ethanol, respectively. The amount of
ethylene glycol is much higher than ethanol, which implies that
the reaction mainly follows pathway (i). We used Cu–Cr and Cu–
Zn/Al2O3 as catalysts in the second step. In order to get the
maximum yield of CHDM, we investigated the inuences of
different reaction conditions on the hydrogenation reaction of
BHCD to CHDM.
3.2.3. Inuence of reaction pressure. Fig. 7 shows the
inuence of reaction pressure on the conversion of BHCD and
yield of CHDM catalyzed by Cu–Cr and Cu–Zn/Al2O3 catalysts.
The conversion of BHCD reached almost 100% when the
3.2.1. Inuence of reaction temperature. Fig. 5 shows the
inuence of reaction temperature on the conversion of BHCD
and yield of CHDM catalyzed by Cu–Cr and Cu–Zn/Al2O3 cata-
lysts. The conversion of BHCD reached almost 100% at the
temperature of 543 K and 523 K catalyzed by Cu–Cr and Cu–Zn/
Al2O3 catalysts, respectively. However, the yield of CHDM
initially increased and then decreased with the increase of the
reaction temperature, due to the excessive hydrogenation of
BHCD. A reaction temperature of 563 K is the best for the
hydrogenation reaction, affording about 32% and 66% CHDM
yield catalyzed by Cu–Cr and Cu–Zn/Al2O3 catalysts, respec-
tively. When the temperature was higher than 563 K, the
hydroxyl groups in CHDM would be further reduced, resulting
in the increase of the by-products. Moreover, it is apparent that
the yield of CHDM catalyzed by Cu–Zn/Al2O3 is much higher
than that catalyzed by Cu–Cr catalyst.
Fig. 6 Influence of reaction time on the conversion of BHCD and yield
of CHDM. Reaction conditions: Cu–Cr: 2.0 g intermediate products
(90% BHCD), 0.1 g Cu–Cr, 543 K, and 10 MPa; Cu–Zn/Al2O3: 2.0 g
intermediate products (90% BHCD), 0.1 g Cu–Zn/Al2O3, 563 K, and 6.5
MPa.
3.2.2. Inuence of reaction time. The inuence of reaction
time on the conversion of BHCD and yield of CHDM catalyzed
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Fig. 7 Influence of reaction pressure on the conversion of BHCD and
yield of CHDM. Reaction conditions: Cu–Cr: 2.0 g intermediate prod-
ucts (90% BHCD), 0.1 g Cu–Cr, 543 K, and 10 h; Cu–Zn/Al2O3: 2.0 g
intermediate products (90% BHCD), 0.1 g Cu–Zn/Al2O3, 563 K, and 6 h.
Fig. 9 TPR profiles of the Cu–Cr and Cu–Zn/Al2O3 catalysts before
reduction.
or heat transfer limitation with a stirring rate higher than 300
rpm in the hydrogenation.
reaction pressure reached 2.5 MPa. It could be found that with
the increase of the reaction pressure from 2.5 to 10.5 MPa, the
yield of CHDM increased and then became unchanged without
affecting the conversion of BHCD. It reveals that the high
pressure greatly improves the selective hydrogenation of the
ester groups in BHCD and decreases the by-products which
resulted in the high yield of CHDM. Under the optimal reaction
pressure of 10.5 MPa, the yield of CHDM reached 26% and 78%
catalyzed by Cu–Cr and Cu–Zn/Al2O3 catalysts respectively.
3.2.4. Inuence of stirring rate. Fig. 8 shows the inuence
of stirring rate on the conversion of BHCD and the yield of
CHDM. As can be seen, the yield of BHCD rstly increased with
the increase of the stirring rate from 50 to 300 rpm, and then
kept about 52% from 300 to 600 rpm. However, the conversion
of BHCD reached about 98% when the stirring rate was 50 rpm.
The hydrogenation of BHCD to CHDM is a multi-step reaction,
the previous reaction steps were performed with low stirring
rate, however, the high stirring rate promoted the followed
reaction steps, resulting in the increase of the yield of CHDM
with the increase of the stirring rate. There should be no mass
3.2.5. Characterization of Cu-based catalysts. The reduc-
ibility of the Cu–Cr and Cu–Zn/Al2O3 catalysts is shown in Fig. 9.
The reduction peaks of the Cu–Cr and Cu–Zn/Al2O3 catalysts are
495 K and 470 K, respectively. It reveals that the Cu2+ species can
be reduced in situ to Cu+ or/and Cu0 because the reduction
temperature of Cu–Cr and Cu–Zn/Al2O3 catalysts in the experi-
ment are of 623 K and 573 K which are higher than the reduc-
tion peaks of Cu–Cr and Cu–Zn/Al2O3 catalysts, respectively.
The XRD patterns of the Cu–Cr and Cu–Zn/Al2O3 catalysts
before and aer reduction are shown in Fig. 10 and 11,
respectively. The typical diffraction peaks with 2q indexed with
approximately at 32.3, 36.2, 42.2, 53.6 and 58.1ꢁ in the Cu–Cr
and Cu–Zn/Al2O3 catalysts before reduction which can be
assigned to the crystalline phase of CuO. The appearance of Cu0
crystallite diffraction peaks at 43.29, 50.42 and 74.03ꢁ in the
XRD patterns of the Cu–Cr and Cu–Zn/Al2O3 catalysts aer
reduction, clearly demonstrates that the Cu2+ is reduced to Cu0.
While, the Cu+ diffraction peaks are also found. Therefore, the
Cu+/Cu0 species are the active sites in our present catalytic
system. Additionally, diffraction peaks belonging to Cr2O3 are
Fig. 8 Influence of stirring rate on the conversion of BHCD and the
yield of CHDM. Reaction conditions: 2.0 g intermediate products (90% Fig. 10 XRD patterns of the Cu–Cr catalyst (a) before reduction (b)
BHCD), 0.1 g Cu–Zn/Al2O3, 6.5 MPa, 563 K, and 6 h.
after reduction.
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4. Conclusions
On the basis of the above discussion, it is shown that the waste
PET monomer BHET can be efficiently converted into CHDM by
a two-step hydrogenation reaction using Pd/C and Cu-based
catalysts without solvent. The conversion of BHET and the
yield of CHDM have reached as high as 100% and 78%,
respectively. The phenyl group can be efficiently hydrogenated
to cyclohexane group catalyzed by Pd/C catalyst under relative
mild conditions in the rst step. However, the competition of
hydroxyl and ester groups in the reaction with hydrogen
increases the difficulty of the selective hydrogenation of ester
groups in the second step. The Cu–Zn/Al2O3 catalyst shows
much higher activity and selectivity of the hydrogenation of
ester group to hydroxyl group than Cu–Cr catalyst. Moreover, it
is suggested that the Cu+/Cu0 species are the active sites in the
Cu-based catalysts. In summary, it is successfully converted the
waste PET monomer BHET to high-valued CHDM, thus
providing not only a new method for the production of CHDM,
but also another direction for the chemical recycling of waste
PET.
Fig. 11 XRD patterns of the Cu–Zn/Al2O3 catalyst (a) before reduction
(b) after reduction.
detectable in Fig. 10, it is suggested that Cr2O3 serve as supports
in Cu–Cr catalyst. Moreover, there are no diffractions related to
Al2O3 in Fig. 11 are observed, indicative of the presence of
amorphous Al2O3, the ZnO and amorphous Al2O3 phases may
function as supports or dispering agents in the Cu–Zn/Al2O3
catalyst.
Acknowledgements
The Cu-based catalysts were characterized with low temper- This work was supported nancially by the Program of National
ature N2 adsorption–desorption to elucidate the structural High Technology Research and Development Program of China
features. As shown in Table 1, it displays that the surface area of (863 Program) (no. 2012AA063001), National Natural Science
the Cu–Zn/Al2O3 catalyst is larger than the Cu–Cr catalyst, which Foundation of China (no. 21276255) and Beijing Natural
may be contributing to the higher activity of the Cu–Zn/Al2O3 Science Foundation of China (no. 2131005, no. 2132055, no.
catalyst than the Cu–Cr catalyst.
21276260).
The optimal reaction conditions of the hydrogenation of
BHCD to CHDM are 543 K, 10 h, and 10.5 MPa. The yield of
CHDM and the conversion of BHCD have reached 32%, 78%,
and 100% catalyzed by Cu–Cr and Cu–Zn/Al2O3 catalysts,
respectively. Cu–Zn/Al2O3 catalyst shows much higher activity
and selectivity of the hydrogenation of ester to alcohol than Cu–
Cr catalyst, it is deduced that the addition of Zn and Al can
improve the activity and selectivity of the Cu-based catalysts in
the hydrogenation of ester group resulting in the increase of the
yield of CHDM. On the contrary, the addition of Cr can improve
the activity and selectivity of the Cu-based catalyst in the
hydrogenation of hydroxyl group resulting in the decrease of the
yield of CHDM. In addition, Cu–Zn/Al2O3 catalyst is more
environmentally friendly than Cu–Cr catalyst. Therefore, we
choose Cu–Zn/Al2O3 as the excellent catalyst in the hydrogena-
tion reaction of BHCD to CHDM.
Notes and references
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