Cu(BDC) as a catalyst for rapid reduction of methyl orange: room temperature synthesis using recycled terephthalic acid

Article abstract of DOI:10.1007/s11696-017-0297-2

Terephthalic acid was recycled from waste PET bottles with a basic hydrolysis technique and characterized with UV and FTIR spectroscopy. Copper-based metal–organic framework Cu(BDC) was synthesized at room temperature without any additive; two different temperatures were chosen to activate the obtained material. Characterization studies were performed using XRD, N₂ physisorption, STEM and EDX. The obtained material was tested as a catalyst for the reduction of methyl orange with NaBH₄ in aqueous solutions. Thermal activation at 160?°C proved to be mandatory for catalytic activity; although higher temperature activation did not cause significant enhancement. Rapid dye removal was monitored by continuous photometry at λ_max. The results were quite satisfactory (about 85% removal in 5?min); even higher than the published results for precious metal (i.e., Au, Pt and Ag) nanoparticles. In an increased reaction scale, UV–visible spectra and mass spectrum were recorded to help elucidating the possible reaction mechanism. In addition, recycling experiment were performed in 100-ml scale without any kind of re-activation (washing or drying) to show the ability of Cu(BDC) as a stable catalyst for reductive dye removal (and probably similar reactions as well).

Full text of DOI:10.1007/s11696-017-0297-2

Chem. Pap.
DOI 10.1007/s11696-017-0297-2
SHORT COMMUNICATION
Cu(BDC) as a catalyst for rapid reduction of methyl orange: room
temperature synthesis using recycled terephthalic acid
Alireza Rahmani1 Hossein Rahmani2 Afsaneh Zonouzi1,3
•
•
Received: 17 July 2017 / Accepted: 15 September 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract Terephthalic acid was recycled from waste PET
bottles with a basic hydrolysis technique and characterized
with UV and FTIR spectroscopy. Copper-based metal–or-
ganic framework Cu(BDC) was synthesized at room tem-
perature without any additive; two different temperatures
were chosen to activate the obtained material. Characteri-
zation studies were performed using XRD, N2 physisorp-
tion, STEM and EDX. The obtained material was tested as
a catalyst for the reduction of methyl orange with NaBH4
in aqueous solutions. Thermal activation at 160 °C proved
to be mandatory for catalytic activity; although higher
temperature activation did not cause significant enhance-
ment. Rapid dye removal was monitored by continuous
photometry at kmax. The results were quite satisfactory
(about 85% removal in 5 min); even higher than the pub-
lished results for precious metal (i.e., Au, Pt and Ag)
nanoparticles. In an increased reaction scale, UV–visible
spectra and mass spectrum were recorded to help eluci-
dating the possible reaction mechanism. In addition,
recycling experiment were performed in 100-ml scale
without any kind of re-activation (washing or drying) to
show the ability of Cu(BDC) as a stable catalyst for
reductive dye removal (and probably similar reactions as
well).
Keywords Cu(BDC) Á Heterogeneous catalysis Á Dye
removal Á Reduction Á Terephthalic acid
Introduction
The discovery of new robust yet inexpensive catalysts for
environmental applications has become a crucial topic in
recent years (Centi et al. 2002). Industrial organic dyes are
a major pollutant from textile industry (Reife et al. 1996).
Various approaches have been used to remove such dyes
from aqueous media using adsorbents (Crini 2006),
microbes (McMullan et al. 2001), photocatalysts (Emrooz
et al. 2017), oxidation catalysts (Shu et al. 2015) and
reduction catalysts (Mondal et al. 2015).
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0297-2) contains supplementary
material, which is available to authorized users.
As a major class of nanoporous materials, metal–organic
frameworks (MOFs) have gained a lot of interest in a
variety of applications (Furukawa et al. 2013), which
include but are not limited to gas separation and storage,
catalysts and biomedical usage (Farrusseng 2011). MOFs
have been widely studied in the field of organic contami-
& Hossein Rahmani
rahmani@irost.ir
& Afsaneh Zonouzi
zonouzi@khayam.ut.ac.ir
´
nate (Dias and Petit 2015) and pesticide (Dumee et al.
2017) removal over the past few years. One of the most
interesting properties which make MOFs unique is their
catalytic behavior which is a link between the well-known
homogenous and heterogeneous metal-based catalysts (Luz
et al. 2010). They can serve well as catalyst support for a
variety of applications including CO2 conversion (Maina
1
School of Chemistry, University College of Science,
University of Tehran, 14155-6455 Tehran, Iran
2
Department of Chemical Technologies, Iranian Research
Organization for Science and Technology (IROST),
33535-111 Tehran, Iran
3
Pharmaceutical and Cosmetic Research Center (PCRC),
University of Tehran, Tehran, Iran
123

Chem. Pap.
Recycling of terephthalic acid
et al. 2017), reduction reactions (He et al. 2015) and many
others (Lee et al. 2009; Burrows et al. 2013).
Terephthalic acid (1,4-benzenedicarboxylic acid) was
obtained by modified basic hydrolysis of PET (Carta et al.
2003). In a typical procedure, 15 g of polyethylene
terephthalate (PET) obtained by cutting down a mineral
water bottle to 5 mm pieces was transferred to a BEP 280
buchi (Switzerland) stirrer laboratory autoclave with
400 ml of 5% aqueous NaOH solution. Temperature was
set at 180 °C and pressure was increased to 5 bar. Reaction
was carried out for 2 h during which the pressure was
reached to 10 bar. The mixture was filtered off with a
WhatmanÒ no.1; the obtained filtrate is a basic solution of
sodium terephthalate which is then treated with aqueous
5% HCl solution to precipitate terephthalic acid which is
further washed with distilled water until reaching natural
pH. The yield of this reaction was measured to be 96.4% by
weight.
Following the first report regarding copper terephthalate
MOF (Carson et al. 2009), many researches have studied
various aspects of this material; including but not limited to
crystalline structure elucidation (Carson et al. 2014) and
room temperature synthesis with the aid of triethylamine
(Dikio and Farah 2013). It has also showed promising
applications as photocatalyst support (Mohaghegh et al.
2015), oxidation catalyst (Dang et al. 2015), click-reaction
catalyst (Luz et al. 2010), gas transport membrane (Kubica
et al. 2016) and lithium-ion storage (Senthil Kumar et al.
2014).
In continuation of our quest studying different
nanocatalysts for a variety of applications (Zonouzi et al.
2016), herein we present a simple room temperature syn-
thesis route to obtain Cu(BDC) from Cu(NO3)2 and
terephthalic acid (recycled from waste PET). This material
can be used as a reduction catalyst for the removal of
methyl orange (among other dyes) from aqueous solution
by NaBH4.
Synthesis of Cu(BDC)
In a typical synthesis procedure, 0.263 g of copper nitrate
and 0.181 g of terephthalic acid were dissolved in 22 ml of
DMF and transferred to a sealed 6-dram vial and kept in
static conditions. The first crystallites precipitated in less
than 2 h and the reaction was continued for 2 days to
ensure completion. The light blue precipitate was cen-
trifuged and washed with fresh DMF and then with water
to remove any unreacted precursors. After drying at room
temperature (code name RT), was divided into two equal
parts; one part was activated at 160 °C (code name RT160)
and the other was activated at 225 °C (code name RT225)
to investigate the effect of temperature on the removal of
DMF from the structure. These samples were used for
XRD, N2-physisorption, STEM, and catalytic reduction
experiments as well.
Experimental
Materials and methods
Copper nitrate, terephthalic acid, DMF, HCl, NaOH and
methyl orange were purchased from Merck (Darmstadt,
Germany), sodium borohydride was obtained from Dae-
Jung Chemicals (Seoul, South Korea).
A Philips (Holland) PU 9624 instrument was used to
record FTIR spectra. Melting points were measured by a
Bernstein Electrothermal (UK) apparatus. UV spectra were
measured by a Perkin Elmer (US) Lambda 25 spec-
trophotometer. Mass spectroscopy was performed on an
Agilent (US) 5973 instrument at an ionization potential of
70 eV. X-ray diffraction patterns were recorded on a Phi-
lips (Holland) PW 1730 instrument using CuKa radiation
Catalytic activity evaluation
˚
Catalytic activity tests were performed by adding 0.1 mg of
each catalyst, NaBH4 (0.1 mg) and 2 ml of aqueous methyl
orange solution (16.5 ppm) to a quartz cell and measuring
the absorbance at kmax = 464 nm, at kinetic mode (time
resolution of 1 s). Recycling study was performed on
RT160 by adding 5 mg of the sample and 5 mg of NaBH4
to 100 ml of methyl orange solution while stirring. 2 ml of
this mixture was transferred to a quartz cell and UV–Vis
spectrum was recorded every minute in the range
k = 199–599 nm (bandwidth was set to 5 nm to complete
each spectrum in less than 1 min and record absorption at
kmax = 464 nm simultaneously). After 9 min the mixture
was centrifuged and the sample was used without any kind
of recovery step or drying by simply adding 100 ml of
with k = 1.54 A, N2-physisorption isotherms were recor-
ded on a BelSORP (Japan) mini II, Scanning Transmission
Electron Microscopy (STEM) was performed on a FEI
(US) Tecnai F30 microscope using holey carbon–nickel
grid. A PG instruments (UK) T80? double beam UV–Vis
spectrophotometer was used for catalytic activity evalua-
tion. Water droplet contact angels were measured by
pressing a considerable amount of each sample between
two glass slabs and then carefully poring 1 ll of distilled
water on them with a micropipette (DragonLab, China)
while taking pictures of the process with a digital still
camera on burst mode and then analyzing the pictures.
123

Chem. Pap.
fresh methyl orange solution and 5 mg of NaBH4; this
procedure was repeated five times.
the synthesized RT225. As can be seen thermal activation
seems to decrease the crystallite sizes, also the crystallinity
(which is apparent from the less sharp characteristic peaks
and a heightened baseline at small diffraction angles,
especially in the range 6 \ 2h \ 10°. Furthermore, the
average crystallite sizes decreased to 360 and 180 nm for
the samples RT160 and RT225 after one run of reduction
reaction, respectively. This is in agreement with the results
of the recycling tests (Fig. 4). Contact angle data obtained
for the samples prove that the wet ability of samples is
enhanced by thermal activation which is a qualitative
measure for surface energy, measured water droplet contact
angles for RT, RT160 and RT225 are, respectively, 69°,
58° and 48°.
N2-physisorption studies (Fig. S1) did not yield the
expected values which may be attributed to the differences
in instrumental settings. We tried several different methods
(including the well-known solvothermal method) to obtain
the literature values (Carson et al. 2014), while the XRD
patterns of the samples were identical and the published
data. But it is still comparable to the results published for
nano-particulate catalyst used for the same purpose
(Mondal et al. 2015).
Results and discussion
Recycled terephthalic acid sample showed a melting point
of 300 °C which proves its structure and purity. It was
further characterized using FTIR (with KBr pallets) and
compared with synthetic grade material from Merck; the
results are presented in Fig. S1. This sample was also
characterized using UV spectrophotometry in a 5% aque-
ous NaOH solution which also showed the same spectrum
as synthetic grade material (kmax = 242 nm, Fig. S2).
XRD patterns for MOF samples (parts a and c of Fig. 1)
clearly show the presence of the well-known main phase of
Cu(BDC)(DMF), and the sample which was activated at
225 °C also shows the characteristic peaks of the high-
temperature phase [i.e., Cu(BDC)]. The difference in the
crystalline structure of these two phases has been conclu-
sively investigated previously (Carson et al. 2014). Three
more samples obtained by performing the reduction reac-
tion in a 50-fold scale and centrifuging the catalyst which
was then analyzed by XRD; the results (parts b, d and e of
Fig. 1) represent the diffraction patterns for RT160 and
RT225 after one run of reduction reaction and also for
RT160 after performing six runs of reaction. The results
suggest a decrease in crystallinity of the sample after one
run and amorphization after six runs of reaction.
STEM analysis was performed for the samples RT160
and RT225, the results are presented in Fig. 2; there seems
to be a decrease in particle size at higher activation tem-
perature which is in line with crystallite size values
deduced from XRD patterns. EDX spectra of rectangular
areas designated by 1 in parts e and g are presented in parts
f and h; they only show the peaks related to copper, oxygen
and carbon.
Application of Scherrer’s equation (Scherrer 1912) to
the main peaks of the samples gave the average crystalline
sizes of 570 nm for the synthesized RT160 and 360 nm for
Fig. 1 XRD patterns for RT160 (a), RT160 after one run of reduction reaction (b), RT225 (c), RT225 after one run of reduction reaction
(d) and RT160 after six runs of reduction reaction
123

Chem. Pap.
Fig. 2 Dark-field scanning transmission electron micrographs from
the sample RT160 (a, b, e) and the sample RT225 (c, d, g). EDX
spectra obtained from the area 1 of micrographs e and g are presented
in parts f and g, respectively. Characteristic peaks related to grid
(nickel) are omitted
Results of the time-dependent photometry test for
RT160, RT225, RT and catalyst-free control (NaBH4) are
presented in Fig. 3. As can be seen, the two main samples
are able to achieve 85% dye removal in about 5 min which
is faster than the published results for cobalt (Mondal et al.
2015), gold, silver and platinum (Gupta et al. 2011)
nanoparticles. Although the non-activated sample RT does
not show considerable activity, the presence of high tem-
perate Cu(BDC) crystalline phase does not seem to have a
drastic effect on the catalytic activity; thus RT160 was
chosen to undergo recycling experiments (Fig. 4). The
inset of this figure presents three further experiments with
catalysts under study and dye solution but in the absence of
NaBH4 reducing agent and show the insignificance of dye
adsorption in the systems under study.
To further elucidate the reaction, time-dependent studies
were also performed in spectrum mode (Fig. S1a). The
kmax peak of methyl orange (at 464 nm in our study) is
caused by its conjugated structure around the azo bond
under the influence of electron-donating dimethylamino
group. The other peak which can be spotted at about
k = 280 nm is due to the p ? p* transition of the aro-
matic moiety. As an attempt to monitor the un-catalyzed
reduction, NaBH4 was tested alone in the same reaction
conditions. The results are presented in Fig. S1b. As can be
deduced from the spectra, 60% reduction is achieved in
35 min (compared to about 85% in 5 min with both RT160
and RT225).
The proposed mechanism for catalytic reduction of
methyl orange is presented in Scheme 1. Copper-based
123

Chem. Pap.
Fig. 3 Time-dependent
photometry results for the
reduction of methyl orange by
NaBH4 without catalyst and
with the samples under study.
Inset: time dependent
photometry results for the
samples in the absence of
NaBH4 as a measure for dye
adsorption at the same
conditions. Time resolution was
set to 1 s
phenylenediamine. These fragments are also confirmed by
mass spectral data. Figure S5 shows the mass spectrum for
a concentrated solution after removing the catalyst from the
reaction mixture. As can be seen, a trace amount of unre-
acted methyl orange is still detectable. The fragments
related to reduction products can be assigned according to
table S1. On the other hand, sodium p-aminobenzene-
sulphonate has
a
reported characteristic peak at
k = 246 nm (Fan et al. 2009) which is identically observed
to intensify for both un-catalyzed and catalyzed reactions
over time.
Recycling tests were performed to study the long-term
stability of RT160 over six identical reaction runs without
any kind of re-activation, washing or even drying. The
obtained spectra (Fig. S2) show identical peak behaviors,
but in different times during the course of reaction. As can
also be seen from the summarized results (Fig. 4), it is
apparent that while the first run is incredibly fast, next runs
tend to become slower and slower; nonetheless, the overall
removal efficiency remains almost constant.
Fig. 4 Results of the recycling tests for RT160
catalysts act by increasing the rate of hydrogen generation
by NaBH4 and transfer the obtained reducing agents to dye
molecules. In this manner, the liberated hydrogen reduces
the azo group and the products would be sodium p-ami-
nobenzenesulphonate
and
N,N-dimethyl-p-
Scheme 1 Proposed
mechanism for catalytic methyl
orange reduction
123

Chem. Pap.
metal–organic framework Cu (BDC) as an efficient heteroge-
neous catalyst. Appl Catal A 491:189–195. doi:10.1016/j.apcata.
2014.11.009
Conclusions
In summary, we presented a basic hydrolysis procedure to
terephthalic acid from waste PET bottles; the sample was
analyzed with UV and FTIR spectroscopy and compared
with synthetic grade terephthalic acid. Using the recycled
compound, we successfully synthesized Cu(BDC) MOF in
a room temperature, additive free procedure; activation
was performed in two different temperatures (160 and
225 °C). The obtained samples were characterized using
powder XRD, N2-physisorption, water droplet contact
angle, STEM and EDX. The samples were also tested as
catalyst for the reduction of methyl orange dye with
NaBH4. This reaction was monitored using time-dependent
spectrophotometry. Mass spectral data were interpreted
based on the literature to propose a mechanism for dye
reduction. Thermal activation proved to be crucial for
catalytic activity toward this reaction, although the pres-
ence of high-temperature phase did not have a significant
impact on activity. Recycling tests were performed five
times without any kind of re-activation; the total turnover
of the catalyst remained constant at 9 min, but the rate of
reduction decreased over re-use. XRD patterns for the
samples after one reaction run proved no significant phase
change but amorphization took place after several reaction
runs.
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