Applied Catalysis B: Environmental 106 (2011) 186–194
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb
Coordination state of Cu+ ions in Cu-[Al]MCM-41
K. Chakarova a , G. Petrova a,b , M. Dimitrov c , L. Dimitrov d , G. Vayssilov b , T. Tsoncheva c , K. Hadjiivanov a,∗
a
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Faculty of Chemistry, Sofia University “St. Kl. Ohridski”, Sofia 1126, Bulgaria
c
Institute of Organic Chemistry with a Center of Phytochemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
d
Central Laboratory of Crystallography, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
b
a r t i c l e
i n f o
Article history:
Received 18 March 2011
Received in revised form 9 May 2011
Accepted 16 May 2011
Available online 27 May 2011
Keywords:
Adsorption
CO
Cu-MCM-41
NO
FTIR
Methanol decomposition
a b s t r a c t
A Cu-[Al]MCM-41 sample was studied by FTIR spectroscopy of probe molecules (CO and NO) and
methanol conversion test reaction. It was found that the parent MCM-41 material was characterized
by the practical absence of strongly acidic hydroxyl groups (typical of zeolites), but possessed some
hydroxyls with a moderate acidity. Introduction of copper to the material by a procedure typical of
ion-exchange did not affect the OH groups of MCM-41. However, reduction of the deposited Cu2+ ions
resulted in formation of Cu+ ions with a high coordinative unsaturation (typical of Cu+ in zeolites): they
were able to adsorb up to three CO molecules each. Adsorption of 12 C16 O–13 C18 O mixtures was used to
prove the polycarbonyl structures. The possible application of Cu-[Al]MCM-41 in environmental catalysis
is discussed.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
In the last decades mesoporous silica materials received a growing interest as catalysts, catalyst supports and adsorbents due to
their high specific surface area and large pore volume determined
by ordered mesopores in the nanoscale range [1]. MCM-41 silica is
one of the most studied members of this family and is characterized
by one-dimensional structure of uniform cylindrical mesopores of
about 2–10 nm organized in a hexagonal symmetry [2,3]. Many
efforts have been devoted to expand the application of the mesoporous silica materials as catalysts with tunable properties. This
problem is closely related to the creation of acidic and/or redox
catalytic sites in them. In particular, the introduction of various
metal cations provides a great potential in this respect. It was found
that depending on the preparation method used, the metal ion can
be incorporated in the framework of the mesoporous silica matrix
or can occupy extraframework positions [4]. Initially, the incorporation of Al into the MCM-41 structure was performed to create
acidity in this material [5,6]. Then, ordered mesoporous MCM-41
materials containing V, Fe, Mn, B, Ga, Ti, Zr, Cs, Cu, etc., were successfully synthesized [7–14]. Among them, copper modifications
are of considerable importance, because copper catalysts are widely
used in many processes, in particular of environmental importance.
∗ Corresponding author. Tel.: +3592 979 35 98; fax: +359 2870 50 24.
E-mail address: kih@svr.igic.bas.bg (K. Hadjiivanov).
0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2011.05.024
Thus, they are reported to be active in direct NO [15] and N2 O
[15,16] decomposition, selective catalytic reduction of NOx with
ammonia [17] and N-free reductants [18], oxidation of methane to
methanol [15], selective oxidation of ammonia [19], CO oxidation
[20], etc.
However, the incorporation of divalent cations in the tetravalent silica framework is rather difficult process. This was the
reason many researchers to focus attention on introduction of
divalent cations by ion-exchange in aluminum substituted silica
matrix [21–25]. FTIR of pyridine demonstrates that this modification leads to increase of the Lewis acidity at the expense of the
Brønsted one [26,27]. Contrary to the crystalline zeolites, in the
case of amorphous MCM-41 materials presence of copper sites in
different location and environment on the surface or inside the
mesopores has been considered [26]. EPR analysis pointed out
that Cu2+ species in ion-exchanged Cu-[Al]MCM-41 were buried
within the silica walls, being less accessible for the reactant
molecules [28].
Taking into account that the location of metal cations in
different positions is of a crucial importance for their stability and catalytic activity, we aimed the current paper at the
detailed characterization of the state of copper ions in an ionexchanged aluminum-containing MCM-41 by FTIR of adsorbed
probe molecules, CO and NO. These two probes are complementary
when studying copper-containing catalysts because CO is preferentially adsorbed on Cu+ sites [29], while NO, on Cu2+ ions [30].
Methanol conversion was used as a catalytic test with a view of
K. Chakarova et al. / Applied Catalysis B: Environmental 106 (2011) 186–194
the flexibility of methanol molecule to convert to different reaction
pathways on various catalytic active sites.
187
3. Results
3.1. Initial characterization of the samples
2. Experimental
2.1. Samples
The parent [Al]MCM-41 sample (Si/Al = 12, BET surface area of
896 m2 /g) was prepared as described in [31] by hydrothermal synthesis at 363 K in presence of silica (Cab-o-Sil M-5) and aluminum
isopropoxide as Si and Al sources, respectively. After drying for 17 h
at 363 K, the template of the as-synthesized sample was removed
by a gradual increase of the temperature up to 873 K and holding
the sample in air for 10 h at this temperature. Further on the sample
will be denoted as MCM-41.
The Cu-MCM-41 sample was prepared by ion-exchange from
0.05 M L−1 solution of Cu(NO3 )2 ·3H2 O (Merck, for analysis). The
process was performed in magnetically stirred suspension at
solid/liquid ratio [g mL−1 ] of 1/80 at boiling temperature for
120 min. Then the solid was separated by filtration. The procedure
was repeated two times, after which the sample was dried at 378 K.
The copper concentration of the sample thus prepared was 2.2 wt.%
Cu.
2.2. Techniques
FTIR spectra were recorded with a Nicolet Avatar 360 spectrometer accumulating 64–128 scans at a spectral resolution of 2 cm−1 .
Self-supporting pellets (ca 10 mg cm−2 ) were prepared from the
powdered sample and treated directly in a purpose-made IR cell
allowing measurements at ambient and low temperatures. The cell
was connected to a vacuum-adsorption apparatus with a residual
pressure below 10−3 Pa. Prior to the adsorption experiments, the
samples were activated by calcination at 673 K for 1 h and evacuation at the same temperature for 1 h.
The TPR/TG (temperature-programmed reduction/thermogravimetric) analyses were performed in a Setaram TG92 instrument. Typically, 40 mg of the sample were placed in a microbalance
crucible and heated in a flow of 50 vol% H2 in Ar (100 cm3 min−1 )
up to 773 K at 5 K min−1 and a final hold-up of 1 h.
Methanol conversion was studied in a flow type apparatus
using argon as a carrier gas with methanol partial pressure of
1.57 kPa and WHSV = 1.5 h−1 . Prior to the catalytic test, methanol
adsorption was studied on the catalysts using the same apparatus at 313 K with methanol partial pressure of 0.05 kPa and
WHSV = 0.33 h−1 . Gas chromatographic analyses of methanol and
the products of its conversion were done on GC HP 5800 apparatus equipped with FID and TCD detectors and PLOT Q column,
using a carbon based material balance. Before the experiments,
the samples were pretreated in a flow of Ar at 773 K for
90 min.
The DR UV–vis spectra were taken with a Thermo Evolution
300 spectrometer equipped with a Praying Mantis device. The XRD
patterns were registered with a Bruker D8 Advance diffractometer using CuK␣ radiation and a SolX detector. Chemical analysis of
copper was performed with flame atomic absorption spectrometry
using a Solaar M6 atomic absorption spectrometer.
Carbon monoxide (>99.5% purity) was supplied by Merck. Two
labelled carbon monoxides were used: 13 C16 O (99 atom% 13 C, <5
atom% 18 O) was purchased from ISOTEC, and 13 C18 O (13 C isotopic
purity of 99% and 18 O isotopic purity of 95%) was provided by
Cambridge Isotope Laboratories, Inc. Before adsorption, CO and
the CO isotopic mixtures were passed through a liquid nitrogen
trap. NO (Messer Griesheim, purity > 99.0%) was purified by fraction
distillation.
Initially, the samples were characterized by XRD, UV–vis
spectroscopy, TPR and methanol adsorption experiments. The illustrative material is reported in Supporting information.
The X-ray diffraction pattern of the parent MCM-41 material
(Fig. S1 from the Supporting information, pattern a) represents the
main d100 , d110 and d200 reflection lines in the low angle region
that are typical of hexagonal symmetry of the pore ordering (space
group p6m) [32]. The decrease in intensity of the d100 reflection
and the loss of the d110 and d200 reflections after copper deposition (Fig. S1 from the Supporting information, pattern b) could be
assigned to the partial collapse of the mesoporous structure and/or
to the decrease of the electron density contrast between the pore
walls and pore volume due to the deposition of copper containing
species in them [33].
The UV–vis spectrum of the sample contains bands located
at 210, 495(sh), 760 and 910 nm (see Fig. S2 from the
Supporting information). The band at 210 nm (CTLM) is typical of
isolated Cu2+ ions while the shoulder at 495 nm suggests the existence of some oligomers [28,34,35]. The bands in the 750–950 nm
region are assigned to 2 Eg → 2 Tg d–d transitions of octahedrally
coordinated Cu2+ sites.
TPR-TG-DTG profiles of the Cu-MCM-41 material consist of different effects at 370–770 K range due to the reduction of copper
species (see Fig. S3 from the Supporting information). According
to data from the literature [4,36,37], the low temperature (below
490 K) and the high temperature (above 600 K) effects could be
assigned to the step-wise Cu2+ → Cu1+ → Cu0 reduction of highly
disordered finely dispersed CuO species. One step reduction of bulk
CuO nanoparticles occurs at 440–480 K range [4], i.e. weight loss
due to the reduction of bulk CuO can contribute to the LT peak. In
our case the LT/HT peak ratio is slightly higher than 1 indicating
that (i) some CuO nanoparticles exist in the sample and (ii) copper
ions are mainly in a divalent state.
The methanol adsorption curves on MCM-41 and Cu-MCM41 samples at 313 K are compared in Fig. S4 from the
Supporting information. Under these conditions no products of
methanol conversion are observed. Cu-MCM-41 demonstrates
higher adsorption ability at low methanol coverage which evidences the presence of stronger adsorption sites in it.
3.2. FTIR spectra of CO adsorbed on MCM-41
The background spectrum of the activated parent MCM-41
material contains an intense band at 3744 cm−1 and a broad
halo centered around 3625 cm−1 (Fig. 1 spectrum a). The band at
3744 cm−1 is unambiguously assigned to isolated silanols [38]. The
broad absorbance is indicative of the presence of H-bonded hydroxyls [38].
Adsorption of CO on the sample leads to the appearance of
an intense band at 2157 cm−1 with shoulders at 2169, 2141, and
2135 cm−1 (Fig. 2 spectrum a). A weak band at 2229 cm−1 is also
registered. In the OH region the silanol band at 3744 cm−1 is eroded
and an intense band at 3658 cm−1 appears (Fig. 3 spectrum a). Two
more bands of low intensity, at 3559 and 3466 cm−1 , are observed
in the region.
The bands at 2141 and 2135 cm−1 quickly disappear during
evacuation (Fig. 2 spectra b–d). These bands are unambiguously
assigned to physically adsorbed CO [29]. The next band to disappear
is that at 2157 cm−1 (Fig. 2 spectrum g). This band changes almost
in concert with the band at 3658 cm−1 and the band at 3744 cm−1
is restored when these two bands disappeared (Fig. 3 spectra a–d).
Therefore, the band at 2157 cm−1 is assigned to CO polarized by
ID
47060
Title
CoordinationstateofCu+ionsinCu-[Al]MCM-41
http://fulltext.study/article/47060
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