Digest Journal of Nanomateria ls and Biostructures Vol. 14, No. 3, July – September 2019, p. 509 – 515 [630422]

Digest Journal of Nanomateria ls and Biostructures Vol. 14, No. 3, July – September 2019, p. 509 – 515

THE INFLUENCE OF SYN THESIS CONDITION S ON THE STRUCTURAL
AND REDOX PROPERTIES OF Mn-MCM -41

A. M. MANTA, D. L. CURSARU*, S. MIHAI
Petroleum -Gas University of Ploiești, Romania
Mn-MCM -41 was synthesized by hydrothermal method by using surfactants with different
alkyl chain lengths. The structural properties of the synthesized catalysts have been
investigated by nitrogen adsorption/desorption isotherm, UV -VIS diffuse reflectance
spectroscopy, X -ray Absorption Fine Structure (XAFS) , Temperature Programmed
Reduction (TPR), X -ray diffraction (XRD) and transmission electron microscopy (TEM) .
Results indicated the successful incorporation of Mn ions in the silica matrix of MCM -41.
X-ray powder diffraction exposed the hexagonal mesoporous structure for Mn -MCM -4,
while the N 2 adsorption –desorption isotherms showed a specific average surface area of
1014 m2/g. UV-VIS spectra confirmed the presence of Mn2+/Mn3+ species in the
framework of the mesoporous materials, and X -Ray absorption near edge spectroscopy
(XANES) and extended X -ray absorption fine structure (EXAFS) were performed to
determine the oxidation state and local symmetry of transition metal ions incorporated in
the MCM -41 molecular sieve.

(Received January 16, 2019; Accepted July 2, 2019)

Keywords: Mn-MCM -4, Hydrothermal method, XRD, XAFS, TPR

1. Introduction

Mesoporous molecular sieve s have opened new perspectives for catalytic processes, based
on novel principles. In 1969 the synthesis of an ordered mesoporous material, was recorded in
literature but the remarkable properties of this material have been discovered much later on [1].
The discovery in the 1990 ’s of MCM -41 by researchers from the Mobil Oil Corporation open ed
new way s for the preparation, characterization and application of mesoporous molecular sieves
[2,3]. These materials present remarkable features such as: high surface a rea, narrow pore size
distribution, flexible framework structure, which make these materials interesting for their use in
different applications. Catalysts efficiency for a specific reaction depends of the design of the
active site s. It was shown that the characteristics of MCM -41 are strongly affected by the synthesis
conditions such as: mole ratio of the components in the synthesis solution, autoclaving time, pH,
silica source, etc. [4].
One of the most important properties of MCM -41 is the pore size diameter, which can be
modified independently of the chemical composition of the pore walls , by changing the carbon
chain length of the organic template molecules used during synthesis. The main advantages of
using MCM -41 as catalyst in different applications , are the special geometric and chemical
properties. Unlike crystalline zeolites, SiO 2 amorphous walls of mesoporous molecular sieves
allow the isomorphic substitution of silicon ions with other metal cations in the matrix without
altering its structure . Highly ordered Mn -MCM -41 with the metal ions incorporated in the silica
matrix has been prepared for different types of reactions (e.g. the selective oxidation reaction of
ethyl benzene) [5-7]. By using the hydrothermal method with isomorphous substitution of the
silica ions from the matrix of MCM -41 with ions of manganese, was possible to synthesize highly
ordered Mn -MCM -41 samples with the m etal ions dispersed on atomic scale in the silica matrix of
the mesoporous material.

* Corresponding author: dianapetre@upg -ploiesti.ro

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In this work we report a systematic investigation of the properties of modified silica
mesoporous materials of different pore dimensions. Nitrogen adsorption /desorption isotherm s and
X-ray diffraction (XRD) w ere used for the investigat ion of the surface area, structure and pore si ze
distribution, th ese propert ies being also confirmed by TEM images. UV -VIS spectroscopy and X –
ray Absorption Fine Structure is important in our investigation for determining the oxidation state
and local symmetry. Temperature Programmed Reduction (TPR) was useful for determination of
redox properties of synthesized materials. The effect of alkyl chain length on the Mn -MCM -41
structure are compared to the similar Mn -MCM -41 results published elsewhere [8].

2. Materials and experimental methods
This study presents briefly the synthesis and characterization of mesoporous sieve MCM –
41 with 2 wt.% Mn loading . During the synthesis step, surfactants with different number of carbon
atoms n=14 and 16 have been used.

2.1. Materials
Silica sources used in the syn thesis of the molecular sieve was Cab -O-Sil from Riedel -de
Haen and tetramethylamonium silicate form Aldrich. The manganese source used was MnSO 4 ●
H2O (Aldrich). The surfactant solution was prepared by ion exchanging of different quantities of
cetil trimethyl ammonium bromide aqueous solution with ion exchange resin Amberjet 4400
(Aldrich). The pH was adjusted from the synthesis solution by using acetic acid.

2.2. Synthesis of Mn-MCM -41
MCM -41 was synthesized following a recipe very well described i n the literature [9 -12].
The silica matrix of MCM -41 has been synthesized by using surfactants with different alkyl chain
length , respectively C14 and C16 . The manganese ions were incorporated into the framework of
the matrix by hydrothermal method , in ord er to synthesize a molecular sieve with 2 wt.% Mn
loading . The pH was adjusted at 11.5 by adding acetic acid in the synthesis solutions. The final
solution was thoroughly mixed and then poured int o a recipient autoclave resistant, and placed for
6 days at 1000C in an autoclave. After this time , the solution was cooled at room temperature,
filtered and washed with dei onized water. The solid part resulted after filtration was dried for 12h
at 750C, and finally was calcinated at a constant heating rate of 320/h to 5400C for 18h under He
flow and kept for 5h under air at the same temperature , to remove the surfactant residue.

2.3. Characterization
Temperature programmed reduction (TPR). The stability and reducibility of the transition
metal ions incorporated in the MCM -41 silica framework was assessed by temperature
programmed reduction using a CHEMBET Quantachrome Instrument. TPR is a useful technique
for investigating the redox properti es of catalysts. Approximately 100 mg of each sample was
loaded into a quartz cell and heated to 5000C with a constant heating rate of 50/min in Ar flow,
held at this temperature for 1h and subsequently cooled to room temperature. This procedure was
used t o clean the surface of the catalysts prior to TPR investigation. For TPR investigations, Mn –
MCM -41 catalyst was heated in 5% H 2 in the balance from 500C to 9000C at 50C/min heating
ramp. The composition of the gas stream leaving the quartz cell was monitor ed using a thermal
conductivity detector (TCD).
Nitrogen Physi sorption. Nitrogen adsor ption-desor ption isotherms were measured at 77K
whit a ASAP Analyzer from Micrometrics. BJH method as used t o determine the pore size
distribution .
X-Ray diffraction (XRD) was recorded on a Brooker X-Ray diffractometer (Cu K α, voltage
40 kV and 5mA intensity , 2θ range 1 -100 at a rate 10/ min ).
XRD measurements allowed the determination of the pore wall thickness according to a
method described in the literature [12], more than that, these investigations give the first
indications about the pore structure characteristic to the MCM -41 materials.
UV-VIS Spectroscopy. Diffusive reflectance spectra were recorded using a JASCO V 550
spectrometer. The spectra were carried out in air at the room temperature.

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X-ray Absorption. The results obtained by X -ray absorption of the Mn -MCM -41 samples
were collected at the Mn K edge using beam line X18B, at the National Synchrotron Light Source
Brookhaven National Laboratory. The spectra collected were analyzed using the IFEFFIT
software.
Transmission Electron Microscopy (TEM) is an useful and powerful techniques to image
nanoscale materials. The TEM images of Mn -MCM -41 were obtained using a Tecn ai F20 200kV
microscope.

3. Results and discussion

3.1. Influence of the alkyl chain length of the surfactant on MCM -41 characteristics
TPR results exposed important information about material species, stability, reducibility
and metal distribution. By TPR investigation we studied the reduction behavior of supported oxide
catalysts. I n Fig. 1 are depicted the TPR profile s for Mn -MCM -41 catalysts with different alkyl
chain length (n=14 and 16 ). TPR investigation for Mn-MCM -41 took place in the temperature
range from 250C to 9000C. Manganese is completely reduced after we hold the temperature
constant at 9000C for 1 h. If the sample is exposed to air can be re -oxidized and can form
manganese oxide at the surface of the silica walls. If we tested again the sample, we obtained
different patterns compared to the first TPR run. The TPR method revealed direct interaction of
transition oxide phases in the silica matrix. According to the literature the first peak in TPR pattern
is attributed to the reduc tion of MnO 2 or Mn 2O3, while the second peak the reduction of Mn 3O4 to
MnO [17]. For both manganese catalysts incorporated in mesoporous molecular sieves with
different alkyl chain length s the reduction temperature is about 600oC.

Fig. 1. Temperature programmed reduction (TPR) profiles of Mn -MCM -41 w ith alkyl
chain length n=14 and 16

To determine how the alkyl chain length of the surfactant affect the structure of MCM -41,
the catalyst samples were characterized by nitrogen physisorption technique. Fig. 2 shows the
adsorption/desorption isotherms and the distribution of the pore for as calcinated samples. The
BET surface area, average pore diameter, pore volume, were d etermined and are presented in
Table 1. It can be seen that both mesoporous materials C14 and C16 Mn -MCM -41 have uniform
pore size, and the pore size decrease s with the decreasing of the alkyl chain lengths. The diameter
distribution of the pore can be independently changed from the chemical composition of the pore
walls by chang ing the length of the organic template molecules during synthesis. These results are
accordingly to that from the literature , being already well know that the longer surfactant chain
length is, the higher relative pressure of the capillary condensation is [13]. Moreover, a ll samples
show structural order regardless of the surfactant chain length. The higher the slope, the pores are
more uniform. This can be further translated into pore size distributions obtained by using the
Barret -Joyner -Halenda (BJH) method. From the pore size distribution were determined the full
width at half maximum ( FWHM ) for each sample. It can be see that the catalysts maintained

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characteristic type IV isotherms with hysteresis loop, which are the representative for mesoporous
mater ials with uniform mesopores. Longer surfactant chain lengths result in narrower FWHM and
steeper slopes for the capillary condensation. The surface area for MCM -41 and the pore volume
decrease with the decreasing of the alkyl chain lengths. The sample synthesized using C16
surfactant present s a good structure comparatively with the samples using C14 surfactants in the
synthesis solution.
Table 1. Surface properties of the catalysts .

Sample Metal
loading, wt.% SBET (m2/g) Pore volume
(cm3/g) FWHM
(Å)
C14 Mn -MCM -41 2 930 0.857 3.43
C16 Mn -MCM -41 2 1013 0.937 2.30

Fig. 2. Adsor ption-desor ption isotherms (a) and pore size distribution (b) for C14 and
C16 Mn -MCM -41 samples

XRD patterns of C14 and C16 Mn -MCM -41 were taken in the 2θ range 1 -100 at rate
10/min in steps of 0.01. The XRD for both catalysts are illustrated in Fig. 3. According to XRD
results, both samples present a structure characteristic for well-ordered mesoporous materials
sieve s, these results completing those observed for nit rogen physisorption. When recorded with a
regular powder X -ray diffractometer, the XRD pattern of a well ordered MCM -41 sample usually
shows one main peak which correspond to Miller index ( 100) and three other small er peaks
matching to higher Miller index planes (110, 200, and 210) [9].
Both samples show a sharp d 100 reflection line in the 2θ range 1.9 -30, and two broad peaks
at 2θ range 3.6 -5.50 for (110) and (200) planes in MCM -41. In case of C14 and C16 Mn -MCM -41
it can be seen three peaks (100, 110, 200) by using powder at the X -ray diffractometer, indicating
that the mesostructures are well developed.

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1 2 3 4 5 6020000400006000080000100000120000140000160000180000 C16-Mn-MCM-41
C14-Mn-MCM-41Lin (Counts)
2

Fig. 3. XRD pattern for C14 and C16 Mn -MCM -41

In order t o investigate the local environment of Mn in the MCM -41 material, samples
were analyzed by UV -VIS spectroscopy and X -Ray absorption spectroscopy. Fig. 4 shows th at
UV-Vis spectra for Mn-MCM -41 samples present two different groups of peaks in the UV -VIS
region. The UV -VIS spectra show the main absorption band centered near 250 nm and a wide
band at about 500 nm which covers almost all the visible range of the spectrum. Th e absorption
band at 500 nm is assigned to Mn2+ overlap , according to t he literature rep orts, while the
absorption near 250 nm is associated with O2-→ Mn3+ charge transfer transition [17-19]. The
sensitivities of the band at 250 and 500 nm may be different, and thus the intensities of these bands
cannot be used to evaluate the ratio Mn2+ / Mn3+ in the samples. For both samples it was observed
the same allure of the spectra, th is indicating that the both types of manganese species coexist in
the sample, independently of the length of the alkyl chain.

Fig. 4. UV-VIS spectra of C14 and C16 Mn-MCM -41

For X -ray absorption spectra we used the X18B beam line at the National Synchrotron
Light Source, Brookhaven National Laboratory. The spectra were obtained at the Mn K edge
(6535 eV) at room temperature. X –Ray absorption spectroscopy is a techn ique based on the
absorption of X -rays and extraction of photoelectrons that are scattered by neighboring atoms [14].
An EXAFS spectrum shows these interference effects and can be used to obtain the oxidation state
of the element as well as detailed inform ation on the interatomic distances and the number and
type of neighbors of the absorbing atoms [14]. A Fourier transform of the characteristic EXAFS
function yields a radial distribution function, which gives the distance from the absorbing atoms
[15]. All data was processed with the IFEFFIT software [16]. The oxidation state of Mn in the Mn –
MCM -41 catalysts can be investigated in the derivative of the XANES spectra collected around
the Mn K -edge, which is compared with commercially available reference m aterials (manganese
oxides) in Fig. 5.

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Mn (III) is a major component in all of the Mn incorporated MCM -41 catalysts as shown
by the peak at 6547 eV characteristic of Mn 2O3. The broad first peak in the catalyst spectra
extended further to the left than in the spectra of Mn (III) oxide also suggests the presence of
contributions from MnO.

Fig. 5. X-Ray absorption spectra

Pore structure of Mn -MCM -41 samples and diameter of metallic clusters was investigating
by transmission electron microscopy (TEM). By using TEM we can observe modulations in
chemical identity, crystal orientation and electronic structure of the samples as the regular
absorption based imaging. Im ages collected fo r our samples are presented in Fig. 6. TEM images
show long range order of the synthesized materials for C14 and C16 Mn -MCM -41 with 2 wt.%
Mn loading , indicating a good structure.

Fig. 6. TEM images for: C14 -MCM -41 (left) and C16 -Mn-MCM -41 (right) .

4. Conclusions

The mesoporous materials MCM -41 with 2 wt.% Mn loading and C14 and C16 alkyl
chain length were successfully synthesized by hyd rothermal method and characterized by TPR, N 2
physisorption, XRD, UV -VIS, XANES and TEM . It was observed that similar to other catalysts
synthesized by our group [ 20-23], (e.g. Co-MCM -41), the pore diameter can be controlled by
changing the surfactant chain length . Mn was immobilized in to mesoporous material sieve s with
different pore sizes and, according to different investigation techniques, present a highly ordered
structure and a surface area between 930 -1013 m2/g. UV-VIS spectroscopy evidenced that Mn2+
and Mn3+ in Mn -MCM -41 C14 and C16 framework are co ordinate d to silica surface. The
2 0 n m2 0 n m
2 0 n m2 0 n m

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synthesized sample s present h igh stability against reduction and can be used further in catalyzed
processes for example for the selective oxidation reactions.

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