One-step hydrothermal synthesis of manganese-containing MFI-type zeolite, Mn-ZSM-5, characterization, and catalytic oxidation of hydrocarbons

Article abstract of DOI:10.1021/ja4013936

Manganese-containing MFI-type Mn-ZSM-5 zeolite was synthesized by a facile one-step hydrothermal method using tetrapropylammonium hydroxide (TPAOH) and manganese(III)-acetylacetonate as organic template and manganese salts, respectively. A highly crystalline MFI zeolite structure was formed under pH = 11 in 2 days, without the need for additional alkali metal cations. Direct evidence of the incorporation of Mn in the zeolite framework sites was observed by performing structure parameter refinements, supported by data collected from other characterization techniques such as IR, Raman, UV-vis, TGA, N ₂-adsorption, SEM, TEM, EDAX, and XPS. UV-vis spectra from the unique optical properties of Mn-ZSM-5 show two absorption peaks at 250 and 500 nm. The absorption varies in different atmospheres accompanied by a color change of the materials due to oxygen evolution. Raman spectra show a significant and gradual red shift from 383 cm^-1 to 372 cm^-1 when the doping amount of Mn is increased from 0 to 2 wt %. This suggests a weakened zeolite structural unit induced by the Mn substitution. The catalytic activity was studied in both gas-phase benzyl alcohol oxidation and toluene oxidation reactions with remarkable oxidative activity presented for the first time. These reactions result in a 55% yield of benzaldehyde, and 65% total conversion of toluene to carbon dioxide for the 2% Mn-ZSM-5. Temperature programmed reduction (TPR) using CO in He demonstrates two reduction peaks: one between 300 and 500 C and the other between 500 and 800 C. The first reduction peak, due to manganese-activated oxidation sites shifted from higher temperature to lower temperature, and the peak intensity of CO₂ rises when the dopant amount increases. For the first time, calculated photophysical properties of a model Mn(O-SiH₃)₄^- compound, an Mn-embedded zeolite cluster, and model Mn oxides help to explain and interpret the diffuse reflectance spectroscopy of Mn-ZSM-5 zeolites.

Full text of DOI:10.1021/ja4013936

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Article
One Step Hydrothermal Synthesis of Manganese Containing MFI-Type
Zeolite Mn-ZSM-5, Characterization, and Catalytic Oxidation of Hydrocarbons
Yongtao Meng, Homer C. Genuino, Chung-Hao Kuo, Hui Huang, Sheng-
Yu Chen, Lichun Zhang, Angelo Raymond Rossi, and Steven L. Suib
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4013936 • Publication Date (Web): 16 May 2013
Downloaded from http://pubs.acs.org on May 16, 2013
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Journal of the American Chemical Society
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One Step Hydrothermal Synthesis of Manganese Containing
MFI-Type Zeolite Mn-ZSM-5, Characterization, and Catalytic
Oxidation of Hydrocarbons
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Yongtao Meng, Homer C. Genuino, Chung-Hao Kuo, Hui Huang, Sheng-Yu
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Chen, Lichun Zhang, Angelo Rossi, and Steven L. Suib *
1
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060 .
2
Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3060 .
KEYWORDS: ZSM-5, Transition Metal, Oxidation, Catalyst, Manganese-Containing Zeolite, Absorption, Com-
putation
ABSTRACT: Manganeseꢀcontaining MFI type MnꢀZSMꢀ5 zeolite was synthesized by a facile oneꢀstep hydrothermal
method using tetrapropylammonium hydroxide (TPAOH) and manganese (III)ꢀacetylacetonate as organic template and
manganese salts, respectively. A highly crystalline MFI zeolite structure was formed under pH=11 in 2 days, without the
need for additional alkali metal cations. Direct evidence of the incorporation of Mn in the zeolite framework sites was
observed by performing structure parameter refinements, and supported by data collected from other characterization
techniques such as: IR, Raman, UVꢀVis, TGA, N ꢀadsorption, SEM, TEM, EDAX, and XPS. The unique optical properꢀ
2
ties of MnꢀZSMꢀ5 from UVꢀvis spectra show two absorption peaks at 250 nm and 500 nm. The absorption varies in difꢀ
ferent atmospheres accompanied by a color change of the materials due to oxygen evolution. Raman spectra show a
ꢀ1
ꢀ1
significant and gradual red shift from 383 cm to 372 cm when the doping amount of Mn is increased from 0 % to
2 %wt. This suggests a weakened zeolite structural unit induced by the Mn substitution. The catalytic activity was studꢀ
ied in both gas phase benzyl alcohol oxidation and toluene oxidation reactions with remarkable oxidative activity preꢀ
sented for the first time. These reactions result in a 55 % yield of benzaldehyde, and 65 % total conversion of toluene to
carbon dioxide for the 2% MnꢀZSMꢀ5. Temperature programmed reduction (TPR) using CO in He demonstrates two
o
o
o
o
reduction peaks: between 300 C~500 C, and 500 C~800 C, respectively. The first reduction peak, due to manganese
activated oxidation sites shifted from higher temperature to lower temperature and the peak intensity of CO rises when
the dopant amount increases. For the first time, calculated photophysical properties of a model Mn(OꢀSiH
2
ꢀ
3
)
4
comꢀ
pound, an Mnꢀembedded zeolite cluster, and model Mn oxides help to explain and interpret the diffuse reflectance
spectroscopy of MnꢀZSMꢀ5 zeolites.
Introduction
silicates (TSꢀ1) were found to be good catalysts for
mild epoxidation of olefins, hydroxylation of aromatꢀ
Transition metal (TM) doped zeolites have been
widely studied due to their novel catalytic application
in oxidation reactions, such as benzene oxidation to
phenol, methane oxidation to methanol and catalytic
decomposition of NO and N
zeolites, Cu and Fe containing ZSMꢀ5 zeolites were
given more attention because of their high activities
due to the alphaꢀoxygen on the peroxoꢀbridged binuꢀ
clear TM sites which were formed by activation under
ics, and oxidation of alcohols and amines, etc., using
H
2
O
2
as an oxidant4
,5,6,7
. Other transition metal doped
MFI and MEL type zeolites, such as vanadium, iron,
and chromium substituted in framework Si4+ in siliꢀ
caliteꢀ1 (ZSMꢀ5) and silicaliteꢀ2 (ZSMꢀ11), have also
been synthesized8 . Most of these catalysts were apꢀ
plied in selective oxidation reactions with the use of
1
2
O . Among these doped
,9,10
H
2
O
2
or TBHP as oxidants.
2
,3
o
NO
needed to achieve the selective conversion from meꢀ
thane to methanol or from NO to N . N O is found to
be the only species that could form alphaꢀO, rather
than O , which is the active site for selective oxidation.
x
or O
2
. A moderate temperature below 350 C is
Manganese oxides, particularly octahedral molecuꢀ
lar sieves (OMSꢀ2) have been extensively studied as
superior oxidation catalysts in alcohol oxidation and
as catalysts in volatile organic compounds (VOCs) deꢀ
composition due to their high activity on account of
x
2
2
2
11
In addition, the transition metal is present not only as
a surface active site, but also as a framework substiꢀ
tuted site. Titanium containing MFI type microporous
mixed valent properties . However, their thermal staꢀ
bility is not as good as traditional aluminosilicate zeoꢀ
lites due to phase changes or poisoning by moisꢀ
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Page 2 of 14
ture11d,12. Zeolites or aluminosilicates have been used
in different applications, such as adsorption, ionꢀ
exchange, and petroꢀrefining because of their notable
stability and adjustable acid sites of high activity .
Manganese substituted silicates have not been fully
120 C and calcined at 550 C. The temperature for
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calcinations was increased using a ramp of 2 C per
minute from room temperature to 550 C, then keep
at 550 C for 5 hours, in order to remove all the organꢀ
ic solvents and SDA. The final product is denoted as
SiꢀZSMꢀ5, which stands for an all silica ZSMꢀ5 withꢀ
out any aluminum ions.
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studied with respect to both their structure and cataꢀ
lytic properties9
,14,15
. Direct evidence of the existence
of Mn in the framework is still needed and only a few
redox properties of manganese containing zeolite
Synthesis of Mn-ZSM-5 Zeolites. In a typical
synthesis of MnꢀZSMꢀ5, e.g. 2 wt% MnꢀZSMꢀ5, 0.44 g
manganese (III)ꢀacetylacetonate (Sigma Aldrich) was
fully dissolved in 20 mL ethanol (Pharmco Aaper, Abꢀ
solute). This solution was added dropwise into half of
the aforeꢀmentioned solution A before being placed
into the autoclave and stirred for 3 hours to obtain
well dissolved manganese containing solution (soluꢀ
tion B), which has a transparent red wine color. Fifty
mL of the solution B was charged into the autoclave
and the same hydrothermal treatment was performed
as aforeꢀmentioned. The products were filtered, and
washed 5 times with DDW, dried and calcined. After
being calcined, the products showed a pale pink color
and are denoted as MnꢀZSMꢀ5. All the synthesized
ZSMꢀ5 materials in this study are aluminum free with
the goal of exploring only the effect of adding mangaꢀ
nese on the properties of these newly formed zeolites.
15
have been reported . For preparation, a facile syntheꢀ
sis method is needed to replace the present methods
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by using harsh acid hydro fluoride , or recrystallizaꢀ
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tion of Mn(II)ꢀexchanged magadiite . Although a hyꢀ
drothermal synthesis method has been reported using
manganese acetate under a pH=13, a possible precipiꢀ
tation problem may remain. In addition, the products
formed have no redox sites based on CO adsorption of
18
reduced and oxidized samples .
Here, we report a facile synthesis of a Mn containꢀ
ing MFI type zeolite, MnꢀZSMꢀ5, by hydrothermal
synthesis from a clear solution made with a mangaꢀ
nese organic precursor and silicates under mild condiꢀ
tions at pH =11. Full characterization was performed
by various spectroscopic techniques, including UVꢀvis,
IR, Raman, and Xꢀray diffraction, SEM and TEM
where direct evidence of framework manganese has
been found (vide infra). The synthesized MnꢀZSMꢀ5
shows considerable activity as a catalyst in the conꢀ
version of benzyl alcohol to benzaldehyde with 55 %
yield, and toluene total oxidation to carbon dioxide
with 65% conversion. Such a high activity of Mnꢀ
ZSMꢀ5 has been ascribed to the well isolated Mn speꢀ
cies as an excellent oxygen carrier and donor substiꢀ
tuted in the zeolite framework and bonded to the surꢀ
face. This is the first time a highly loaded MnꢀZSMꢀ5
has been synthesized in one step and found to be an
appropriate catalyst in two different selective oxidaꢀ
tion processes. The computational results presented
here attempt to establish a correlation between the
experimental UVꢀVis of Mnꢀcontaining ZSMꢀ5 zeolites
and calculated spectra toward achieving a better unꢀ
derstanding of the nature of MnꢀZSMꢀ5 interactions.
Characterization
Structure Analysis by X-ray Diffraction. XRD
data were collected on a Rigaku UltimaIV instrument
using CuK radiation at a beam voltage of 40 kV and a
α
45 mA beam current.
Spectroscopic Studies by FT-IR, UV-Vis and
Raman. FTꢀIR spectra were taken from pressed pelꢀ
lets with KBr diluted zeolite samples on a Thermo Sciꢀ
ꢀ
1
entific Nicolet 8700 spectrometer with a 4 cm resoꢀ
lution. CaF windows were used; dry N2 was purged
into the cell and passed through to eliminate all the
moisture and CO absorbed on the samples before
2
2
each test. Diffuseꢀreflectance UVꢀVis spectra were colꢀ
lected using a Shimadzu UVꢀ2450 UVꢀVis spectrophoꢀ
tometer in a 200 nm ~ 800 nm range. For each test, a
0.2 g solid sample was diluted by 2 g BaSO . The Raꢀ
4
man spectra were taken on a Renishaw 2000 Raman
microscope, which includes an optical microscope and
a CCD camera for multichannel detection.
Experimental Section
Synthesis of All Silica ZSM-5 Zeolites. All siliꢀ
ca ZSMꢀ5 zeolites were prepared as described elseꢀ
19
Morphology (SEM, TEM). High resolution
scanning electron microscope photographs were takꢀ
en on a Zeiss DSM 982 Gemini fieldꢀemission scanꢀ
ning microscope (FESEM) to get morphological inꢀ
formation. A high resolution transmission electron
microscope (HRTEM) was used to characterize the
crystallized structure and the morphology of the samꢀ
ples by using a JEOL 2010 instrument with an accelꢀ
erating voltage of 200 kV.
where . In a typical synthesis, 25 g of tetraethylorthoꢀ
silicate (TEOS, Aldrich, 98%) was added dropwise to
2
5 g distilled deionized water (DDW) with stirring for
a half an hour, followed by adding 30 g TPAOH (Sigꢀ
ma Aldrich, 1.0 M) aqueous solution as a structureꢀ
directing agent (SDA) dropwise using a burette. The
well mixed solution (solution A) was stirred for 3 h,
5
0 mL of which was measured and added into an auꢀ
toclave with a Teflon liner. The charged autoclave was
o
put into an oven held at 180 C for a hydrothermal reꢀ
Elemental and Surface Analysis (EDXS,
XPS). Elemental analysis was performed using a
JEOL 2010 instrument equipped with energy disperꢀ
sive Xꢀray spectroscopy (EDXS). Surface analysis was
done on a PHI model 590 spectrometer with multiꢀ
probes (ΦPhysical Electronics Industries Inc.), using
action. The hydrothermal reaction could be perꢀ
formed from two days to more than ten days to get
well crystallized products; here we did all the syntheꢀ
ses for 48 hours. After the reaction, the autoclave is
cooled and the white products are collected, filtered
and washed 5 times with DDW, dried over night at
AlꢀKa radiation (λ= 1486.6 eV ) as the radiation
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Journal of the American Chemical Society
source. The powder samples were pressed on carbon
The gas phase benzyl alcohol oxidation reaction was
performed as another representative reaction. The reꢀ
action was carried out in a homemade fixed bed flow
system, which included a quartz reactor (L = 430 mm,
i.d. = 10 mm) placed in a temperature controllable
tubular furnace (Thermal Scientific, Lindberg Blue M).
200 mg catalyst was loaded in the middle of the reacꢀ
tor with glass wool and glass beads held on both sides.
Before the reaction, the catalyst was activated under
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tape mounted on adhesive copper tape stuck to a
sample stage placed in the analysis chamber.
N Adsorption/Desorption and Specific Sur-
2
face Area. The adsorption and desorption isotherms
were measured on a Micrometritics ASAP 2010 inꢀ
o
strument. Samples were preꢀdegassed at 300 C for 6
hours prior to each measurement in order to remove
water and other adsorbed species. The isotherms of
adsorption and desorption were measured at relative
N
2
and O flow (1:1.6,V/V) for two hours at a desired
2
reaction temperature. The liquid benzyl alcohol was
charged by a syringe pump at a rate of 0.02 mL/min,
and passed through a constantly preheated steel tube
pressures (P/P ) from 0.001 to 0.995 and from 0.995
0
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to 0.001, respectively.
Thermogravimetric Analysis (TGA). TGA was
performed on HiꢀRes TGA 2950 Thermogravimetric
o
(
220 C) to vaporize before it running into the reactor.
The vaporized gas phase benzyl alcohol was continuꢀ
ously transported by the gas flow (N2/O2, 1:1.6)
through the catalyst, the effluents were collected by a
cold trap (dry ice) and the gas phase was detected by
on line SRI 8610C gas chromatography (GC) to detect
any total oxidation products, mainly CO or CO2, and
these products were only found when the reaction
Analyzer in both N
2
and O atmospheres.
2
Temperature-Programmed Reduction (TPR).
TPR mass spectrometry experiments were conducted
using a Thermolyne 79300 model tube furnace
+
equipped with an eꢀVision Residual Gas Analyzer
MKS coupled with a quadruple mass selective detecꢀ
tor. In a typical test, a 200 mg catalyst was loaded in
the middle of a quartz tubular reactor, with quartz
wool supports on both sides. Firstly, the catalyst was
o
temperature increased above 350 C. The liquid prodꢀ
ucts were dehydrated by filtrated with MgSO , and diꢀ
4
luted with acetone (1 ꢁl products : 1 mL acetone), for
each sample, 1 ꢁl 1ꢀdecane was added as internal
standard, and analyzed by HP 5890 series II GC
equipped with a MTX®ꢀbiodiesel TG w/Integraꢀ
3
ꢀ1
cleaned by flowing helium at a rate of 50 cm min
o
(
sccm) for one hour at 180 C, and then cooled to
room temperature. During the test, the catalyst was
o
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heating at 10 C/min to 800 C in a 3 % (V/V) CO ꢀHe
mixture at a flow rate of 60 mL/min.
TM
Gap capillary column (14.0 m x 530 ꢁm x 0.16 ꢁm)
with an FID detector. The yield was calculated based
on the standard curve (Figure S5) made by analysis of
benzyl alcohol and benzaldehyde mixtures of a conꢀ
centration gradient (20 % ~100 %, mol/mol).
Catalytic Test. Gas phase catalytic toluene oxidaꢀ
tion was used as a probe reaction to detect the oxidaꢀ
tion active sites of the Mnꢀcontaining zeolite. The reꢀ
action was carried out in a quartz tubular fixed bed
flow reactor (i.d., 4 mm) under atmospheric pressure.
The reactor was put into an electric tubular furnace
coupled with a controller to control the temperature,
with a Kꢀtype thermocouple plugged into the catalyst
bed. Typically, a 100 mg catalyst was packed in the
middle of the reactor supported by glass wool at both
sides. Before the reaction, the catalyst was pretreated
Computational Details. Calculations were perꢀ
21
22
formed by utilizing the Gaussian and GAMESS
programs where the excitation energies were calculatꢀ
ed using Time Dependent Density Functional
23
24
Theory . The hybrid B3LYP exchange functional
was employed throughout all the calculations. The
choice of model systems is not easy, especially for
ZSMꢀ5 zeolites where the structure is quite complex.
In these studies, the structure of MnꢀZSMꢀ5 was repꢀ
resented by smaller model compounds to reduce the
complexity and computational cost. The model comꢀ
pounds were chosen to ensure that ligands attached to
the Mn ion will affect the local environment of the Mn
center to provide reasonable information about the
experimental electronic spectra. The Mn ꢀ O bonding
o
by heating at 200 C under a continuous UHP helium
flow for 2 hours. The catalytic reaction was done by
bubbling ultra zero grade air (21 vol % O ) as the reacꢀ
2
tion gas into a 200 mL anhydrous toluene solution at
a constant flow rate of 20 sccm controlled by a MKS
mass flow controller. The reaction temperature varied
o
o
from 100 C to 420 C. Sampling of the gas phase
product was carried out at least 20 minutes after the
temperature reached the desired value. The effluent
gas products were passed through a cold trap (dry ice)
before entering a SRI 8610C gas chromatography (GC)
ligands that were used in the calculations include H
3
Si
3
ꢀ
–
O , H
3
Si ꢀ O ꢀ SiH
3
, and H
3
Si ꢀ O ꢀ SiH
2
ꢀ O ꢀ SiH .
Both fourꢀ and sixꢀ coordinate framework bonding
sites were explored.
to analyze the gas components (O
2
, N
2
, CO, and CO ).
2
xꢀ
In addition isolated model complexes of MnO
4
and
Liquid products from the cold trap were extracted,
dehydrated and analyzed by GCꢀMass (HP 5890A,
with an HP 5871 series mass detector). Under present
reaction conditions, incomplete oxidation products
were not found (Figure S1). Thus the conversion was
xꢀ
MnO
6
were employed in an attempt to obtain inforꢀ
mation about the possible existence of extraꢀ
framework sites for Mn. In some cases, a complete opꢀ
timization of the model cluster is performed, although
this approach is fraught with uncertainty, because a
framework Mn ion will be more constrained than in a
cluster. In other situations, reasonable model geomeꢀ
tries are used to observe trends in the spectra. In the
case of Mn oxides, no optimizations were performed,
calculated based on the O
2
consumed during the oxiꢀ
dation process by using the equation from our former
2
0
paper .
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Journal of the American Chemical Society
and only fixed bond distances and angles based on
experimental values were used. In all cases, if the
model complex was charged, either positively or negaꢀ
tively, then point charges of opposite sign were posiꢀ
tioned around the model complex to provide a comꢀ
pensating charge of equal magnitude and yielding
Page 4 of 14
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charge neutrality.
For these calculations, the
2
5
LANL2DZ basis set was used for the Mn atom, and
2
6
the 6ꢀ31G(d) basis for all ligand atoms (H, O, or Si).
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Figure
2.
Embedded
Mnꢀzeolite
cluster
Mn(OSiH
2
) Si155O
4
591. It is important to emphasize that terꢀ
minal oxygen atoms were not capped which could have an
influence on the overall structure of the cluster.
compared with the all silica ZSMꢀ5 zeolite, which imꢀ
plies substitution of smaller tetrahedrally coordinated
Si atoms (0.4 Å in crystal form) with heteroatoms Mn
(
II or III) which have a larger size (0.5~1.0 Å ) as ions
in a crystalline environment yielding an increased latꢀ
tice parameter value.
ꢀ
Figure 1. Model Mn(OSiH
3
)
4
compound with optimized
geometric parameters. The atoms are colored with respect
to atom type with Mn as violet, oxygen as red, silicon as
light brown, and hydrogen as white.
A limited validation of the cluster model was perꢀ
formed by inserting a Mn ion into the metalꢀzeoliteꢀ
framework (MZF) and using hybrid quantum meꢀ
chanics/molecular mechanics (QM/MM) to study
electronic effects of the local framework surrounding
the Mn ion. The embeddedꢀcluster approach employꢀ
2
7
ing a twoꢀlayer ONIOM system was used to obtain
optimized geometries and excitations. In the present
work, the electronic structure of an Mn ion plus four ꢀ
OꢀSiH ꢀ ligands, as shown in Figure 1 are designated
2
the primary structure (PS) and are treated quantum
mechanically (QM). For these calculations, the TZVP
basis28 and 6ꢀ31G(d) basis were used for the Mn atꢀ
om and ligand atoms, respectively. Twelve hydrogen
link atoms were used to saturate dandling Si bonds
between the PS and the secondary subsystem (SS).
The remaining atoms of ZSMꢀ5, i.e. the SS, are treated
with molecular mechanics (MM) where UFF29 paramꢀ
eters are used. The embedded cluster model is shown
in Figure 2.
26
Results
X-ray Diffraction (XRD). The XRD patterns inꢀ
dicate the synthesized material is well crystallized, as
shown in Figure 3. The narrow peak widths suggest a
relatively large particle size, which is around 500 nm,
as confirmed by FESEM and TEM. All of the peaks
match the standard phase of ZSMꢀ5 zeolite, whereas
slight shifting of the peak to smaller angle was obꢀ
served for manganese containing materials, 0.5 %Mnꢀ
ZSMꢀ5 (0.02 degree) and 2 % MnꢀZSMꢀ5 (0.06 degree)
Figure 3. (A) Wide angle XRD patterns and (B) enlarged
scale between 2 theta = 7.5 and 9.5 of (A), from bottom to
top: SiꢀZSMꢀ5, 0.5 wt% MnꢀZSMꢀ5, 1 wt% MnꢀZSMꢀ5 and 2
wt% MnꢀZSMꢀ5.
Scanning Electron Microscopy (SEM) and
Transmission Electron Microscopy (TEM).
Figure 4 shows the typical morphologies of syntheꢀ
sized SiꢀZSMꢀ5 and Mn containing ZSMꢀ5 zeolite,
which is composed of single crystals of intergrown
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hexagonal platelets having an evenly distributed parꢀ
ticle size between 300 nm and 500 nm. Each particle
was proven to be a single crystal as determined by seꢀ
lected area electron diffraction (SAED) patterns of
TEM experiments taken at different crystalline orienꢀ
tations (Figure 5).
1
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59
60
Figure 4. SEM images of MnꢀZSMꢀ5 of different Mn contents, A, B (SiꢀZSMꢀ5), C, D, (0.5%) ; E, F(1%); G,H (2%)
EDXS was used to analyze the concentration of each
element in the bulk material. The Si to Mn molar ratio
is of more interest. The experimental values of Si/Mn
decrease with an increasing amount of Mn added in
the initial synthesis solution. The XPS values of Si /
Mn are all lower than they were from the EDXS analyꢀ
sis which means there is more surface manganese
than in the bulk material. Considering different
depths of both techniques in detecting elements conꢀ
centration, XPS measures about 40 angstroms depth,
whereas EDXS penetrates from tens of nanometers to
hundred of nanometers. For the 0.5 % sample, the
surface Si/Mn=98 which is only half as large as the
nominal ratio of 183, indicating Mn accumulates on
the surface. However, by increasing the Mn amount,
this ratio decreases and is closer to the nominal value
Figure 5. Representative TEM images of 0.5 % MnꢀZSMꢀ5,
and corresponding SAED patterns.
which could lead to an excellent substituted material.
This is because homogeneous substitution happens
throughout the material at a proper higher concentraꢀ
tion, 2 wt% Mn in this case, and allows this material
to potentially become an active catalyst.
Surface Area, Porosity, and Energy Disper-
sive X-ray Spectroscopy (EDXS). N adsorptionꢀ
2
desorption isotherms are measured and show a typiꢀ
cal microꢀporous feature of such materials (Type I isoꢀ
therm). The results summarized in Table 1 show the
BET specific surface area remains similar in terms of
increasing the Mn amount in the sample. Likewise,
the micro pore volume matches well without displayꢀ
ing any drop for doped samples. Therefore, the reꢀ
sults reveal that the manganese in the Mn containing
ZSMꢀ5 material is well dispersed, either in the frameꢀ
work or on the surface of the material, and the micro
pores were not thus blocked.
Raman Spectroscopy. Raman is a sensitive
technique which can detect subtle phase information,
e.g. framework metal sites in zeolite and extraꢀ
framework species. Therefore, Raman was used to
study the effect of adding manganese in the frameꢀ
work of ZSMꢀ5 zeolite with excitation laser beams of
both 544 nm and 488 nm (Figure 6). A well resolved
ꢀ
1
ꢀ1
peak was observed between 372 cm and 385 cm for
all the zeolite materials with or without manganese,
which is ascribed to the characteristic vibrations of
the SiꢀO bonds in the fiveꢀmembered ring of the MFI
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type unit structure. The position of this peak shifted
Page 6 of 14
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to lower wave numbers (red shift) as the amount of
manganese increased from 0% to 2 %, with a peak
ꢀ
1
ꢀ1
shift from 383 cm to 375 cm along with broadening
and lower intensity. The other characteristic peak at
ꢀ
1
8
03 cm was also broadened along with weakened inꢀ
ꢀ
1
ꢀ
tensity. There were new peaks at 560 cm and 628cm
showing up gradually as more manganese was added.
The two most intensive broad peaks arose from the
1
2
% Mn added sample. These new peaks can be asꢀ
signed to bending and symmetric stretching vibraꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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0
1
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8
9
0
tions of framework MnꢀOꢀSi analogous to the TiꢀOꢀSi
5
vibration in the TSꢀ1 zeolite . Another plausible exꢀ
planation could be due to MnꢀO symmetric stretching
vibration from the extra manganese oxide phase (544
ꢀ
1
ꢀ1
30,31
cm , 648 cm ) as previously reported
. Finally, a
2
+
[Mn
bridged [Cu
having Raman bands at 456 cm (symmetric) and
2
O] species is also possible. Since a uꢀoxoꢀ
2
+
2
O] species is formed on the ZSMꢀ5,
ꢀ
1
ꢀ
1
8
70 cm (antisymmetric) as reported by Smeets et
3
2,33
al.
Table 1. Physiochemical Properties of Mn-ZSM-5 Ma-
terials
Samꢀ
ples
BET
Volb.
EDXS
Si/Mn
XPS
surface
3
[
cm /g]
Si/Mn
Figure 6. Raman spectra of MnꢀZSMꢀ5 (all silica, 0.5 %, 1 %
and 2 %), taken by (A) 544 nm and (B) 488 nm laser beam.
a
area
2
[
m /g]
Nomc. Expd.
All Si 316(26)
0.11
0.11
0.11
0.13
0
0
0
0
.5 %
323(9)
318(14)
325(17)
183.0
91.5
330.7
122.0
98.4
63.2
48.4
1
%
%
e
2
45.8
27.7 (168.6)
[
a] Standard deviation, [b] Micropore volume deterꢀ
mined by tꢀplot method, [c] Nominal molar ratio of Si to
Mn, [d] Experimental molar ratio of Si to Mn deterꢀ
mined by EDXS, [e] Analysis performed on large aggreꢀ
gation of particles.
The spectra taken by 488 nm laser beam has conꢀ
firmed the same shift and newly evolving bands exꢀ
cept for the 0.5 % MnꢀZSMꢀ5, where impurities gave
rise to fluorescence and prevented the observation of
Raman spectra. Raman spectra of different particles
Figure 7. FTꢀIR spectra of MnꢀZSMꢀ5 (all silica, 0.5 %, 1 %,
and 2 %).
5
ꢀ1
The absorption at 550 cm is typically due to the
pentasil framework variation and was used as a measꢀ
urement of the crystallinity of this type of material.
for the 2% MnꢀZSMꢀ5 sample were collected (Figure
ꢀ
1
S2). Different relative peak intensities of 375 cm , 560
ꢀ1
The absorption peak at 800 cm is due to SiꢀOꢀSi
ꢀ
1
ꢀ1
cm and 628 cm were observed, indicating an imꢀ
pure material since some of the manganese might be
present in a separate phase. Moreover, the intensity of
symmetric stretching, while absorption peaks at 1100
ꢀ1
ꢀ1
cm and 1230 cm are assigned to asymmetric
stretching of SiꢀOꢀSi, and are broadened when substiꢀ
ꢀ
1
ꢀ1
peaks between 550 cm and 650 cm was largely
eliminated due to the reduced resonance effects in
this area by using the λex=488 nm (Figure S3).
tution with Mn is increased. The absorption at around
ꢀ1
1
000 cm of SiꢀZSMꢀ5 does not appear due to largely
diminished SiꢀOH groups caused by calcination and
compression under high pressure during the sample
preparation, and notably is not present for the Mnꢀ
ZSMꢀ5 materials either. This is rationalized for the
same reason. However, this may also result from the
substitution of OH group with manganese clusters on
Infrared Spectroscopy. Several characteristic
vibration peaks of MFIꢀtype zeolite material have
been found in the infrared spectra (Figure 7). The well
defined peaks reveal the good crystalline property of
these materials.
34
the surface .
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UV-Vis Spectroscopy The UV visible spectra are
2p3/2 and Mn 2p1/2 in these two samples (not shown).
For the 1% sample, the binding energies are 643.2 eV
and 654.7 eV, respectively, while those for the 2%
sample are 642.1 eV and 653.5 eV. Although both of
1
2
3
4
5
6
7
8
9
shown in Figure 8. As the amount of manganese is
increased, the absorption increases proportionally
and the color of the synthesized material changes
from white (no Mn) to pale pink (0.5 % and 1 % Mnꢀ
ZSMꢀ5), to dark pink (2% MnꢀZSMꢀ5). Although the
all silica ZSMꢀ5 has no absorption in the 200~800 nm
range, two absorption bands, at 250 nm and 520 nm,
have been found for the Mnꢀcontaining zeolites. The
the BEs are higher than that for normal MnO because
2
of the strong interaction between manganese and siliꢀ
ca, similar results of increased BE have been reported
in other manganese doped materials or manganese
3
1
oxides present on the surface of supports .
2
50 nm absorption band lies in the UV range and is
2
ꢀ
assigned to electron transfer from O to tetrahedral
10
11
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56
57
58
59
60
3
+
Mn , a framework doped Mn ion, with the tail of the
band extending to 350 nm because of the extraꢀ
17
framework Mn species present . The broad absorpꢀ
tion peak at 520 nm is attributed to dꢀd transition of
q+ 35
either framework or extraꢀframework Mn
sibly surface associated Mn
, or posꢀ
3
+ 36
where similar absorbꢀ
ance has been found and shown to have remarkable
3
7
water oxidation activity .
Figure 9. XPS spectra of O 1s region: (A) SiꢀZSMꢀ5; (B)
.5 % MnꢀZSMꢀ5; (C) 1 % MnꢀZSMꢀ5; (D) 2 % MnꢀZSMꢀ5.
0
Thermogravimetric Analysis (TGA). The TGA
data in Figure 10 shows the Mnꢀcontaining ZSMꢀ5 is
as stable as the undoped material without substantial
o
weight loss above 400 C. Under N2, the main loss of
Figure 8. UVꢀVis spectra of MnꢀZSMꢀ5 (all silica, 0.5 %, 1 %,
and 2 %) and physically mixed 2 % MnO2 (>99%, Aldrich)
with SiꢀZSMꢀ5.
weight for both the zeolites with and without mangaꢀ
o
nese occurs below 200 C and is caused by loss of both
physically adsorbed water and O
2
. The Mnꢀcontaining
samples showed more apparent weight loss from 200
X-ray Photoelectron Spectroscopy (XPS).
XPS was used to analyze both the surface concentraꢀ
tion and binding energy of different elements present
in the asꢀsynthesized manganese containing zeolite
materials (Figure 9). The O1s core level spectrum of Siꢀ
ZSMꢀ5 showed a main peak at 532.5 eV, which could
be ascribed to the binding energy (BE) of oxygen beꢀ
longing to the SiꢀO bond and is increased to 533.1 eV
in the 2 % MnꢀZSMꢀ5 sample. Two additional peaks
appear in 2 % Mnꢀsubstituted sample, one at 532.0 eV
and the other at 529.9 eV. The BE at 529.9 eV peak
could be ascribed to an oxygen which bonded to Mn,
i.e. Mn=O. which have a BE around 529.8 eV
o
o
C to 400 C, compared with the stable undoped one
possibly due to the chemical decomposition of the waꢀ
ter and oxygen bonded to charge defective framework
sites and surface Mn species. Further weight loss, alꢀ
o
o
beit slight occurs from 400 C to 700 C, because O2
evolves from the MnO , and is proportional to the
x
amount of doped manganese (Figure S4).
The thermal stability of the synthesized 2% Mnꢀ
ZSMꢀ5 material and SiꢀZSMꢀ5 were also analyzed by
TGA measurements in an O atmosphere. From room
2
o
temperature to 430 C; the weight loss of both materiꢀ
als compares very well, and the loss is mainly from the
o
o
(
Mn(II)O), according to the binding energy data base
adsorbed water. While from 430 C to 800 C, the 2%
MnꢀZSMꢀ5 showed an extra 0.2 % weight percent loss,
when compared with SiꢀZSMꢀ5. Based on the 2% Mn
loading, 0.2 % weight is 10 % of the Mn loaded, and
this value is right in the line with the weight loss of
of NIST. The peak at 532.0 eV could be assigned to an
oxygen in a framework bonded Mn, i.e. MnꢀOꢀSi. For
example, Ti doped in the framework of rꢀMnO2 mateꢀ
rial was reported with a peak in the same position and
3
8
ascribed to the TiꢀOꢀMn binding . The O1s spectrum
of the 1% Mn sample also showed the identical three
distinct peaks, providing reasonable support for the
same assignment of the three peaks at 533.8 eV, 531.4
eV, and 529.9 eV, respectively. The Mn 2p core specꢀ
trum was obtained from the manganese containing
samples (1% wt, and 2% wt), providing BEs for Mn
MnO
heated to 800 C .
2
material because of oxygen evolution, when
o
39
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Figure 11. Catalytic activities of toluene total oxidation
for MnꢀZSMꢀ5 (all silica, 0.5 %, 1 %, and 2 %), and
physically mixed 2 % MnO2 with SiꢀZSMꢀ5.
Figure 10. TGA analysis on SiꢀZSMꢀ5 and 2 % MnꢀZSMꢀ5
under N2 and O2 atmospheres.
Catalytic Results : 1. Toluene Total Oxidation
The catalytic properties of synthesized MnꢀZSMꢀ5
were tested for gas phase toluene total oxidation (Figꢀ
ure 11). Generally, the conversion increases when the
the same temperature window. There are mainly two
reduction peaks for all the Mn containing zeolites: one
o
o
lies between 300 C ~500 C, while the other falls beꢀ
o
o
tween 500 C ~ 800 C. However, for the sample with
% Mn, there is a pronounced shift of the intense reꢀ
o
o
temperature increases from 300 C to 400 C and
above, especially for the Mn doped samples with high
loading (1% and more) showing dramatically inꢀ
2
o
duction peak to lower than 400 C suggesting a better
oxidation activity than other samples (Figure S6). The
first reduction peak (alpha) is a result of the evolution
of adsorbed oxygen on the defective sites. On the othꢀ
er hand, the beta reduction peak is due to the evoluꢀ
tion of lattice oxygen, accompanied by a phase transꢀ
o
creased activity. For example, at 400 C the converꢀ
sion increases from 6 % to 65 % as the loading of Mn
increases from 0.5 % to 2 % , while maintaining 100 %
selectivity. This should be compared with a 2 % γꢀ
MnO2 physically mixed SiꢀZSMꢀ5, used as a reference,
and shows only 8.8 % conversion under the same reꢀ
action conditions. Given SiꢀZSMꢀ5 has only 3 % conꢀ
formation from Mn
2
O
3
to Mn
3
O
4
, Mn
3
4
O to MnO,
which is in agreement with the results from TGA.
Nevertheless, the reduction peak of the 2% sample
has the highest intensity, which is related to the
amount of Mn in the sample, i.e, the higher the
amount of Mn dopant, the higher the reduction intenꢀ
sity (better oxidation activity). All silica ZSMꢀ5 did not
show any reduction activity based on the COꢀTPR,
which displays an almost flat line. The improvement
of the oxidative activity is revealed by an increase of
the reduction peak with temperature increase, pushꢀ
ing the front of the reduction curve forward into the
lower temperature region as the amount of Mn inꢀ
creases. The TPR results have confirmed the catalytic
activity: 2 % > 1 % > 0.5 %> blank.
o
version even at 420 C, the considerable activity of the
manganese containing ZSMꢀ5 could be ascribed to the
manganese activated oxygen vacancies as described in
our previous research paper on manganese oxide ocꢀ
tahedral molecular sieve (OMSꢀ2) catalyzed VOC oxiꢀ
40
dation , plus the well dispersed active sites arising
15,35
from the low dopant amount
.
2. Gas Phase Benzyl-Alcohol Oxidation. Gas
phase benzyl alcohol oxidation was selected as anothꢀ
er probe reaction (Table S1), which has been investiꢀ
gated by using transition metal doped metal oxides
4
1
(e.g., SiO
2
, TiO ) and ZSMꢀ5 as catalysts . For the Siꢀ
2
ZSMꢀ5 material in this study, a relatively low activity,
7.9 % as also described in another study is obꢀ
Discussion
Ion-Exchange Property and Hydrothermal
Stability. The hydrothermal stability and ionꢀ
exchange property was characterized by adding 0.2 g
4
1a,41d
served
. For the Mn added samples, the activity is
o
improved to 23.3 % at 300 C. When the temperature
increases to 350 C, the activity of the MnꢀZSMꢀ5
reaches as high as 55%, with a remarkable TOF=88 h .
The evidence of the enhanced oxidative property by
adding manganese is clearly shown and may be relatꢀ
ed to the oxygen active sites bonding to the Mn, as reꢀ
vealed from the absorption spectra.
o
template free 2% MnꢀZSMꢀ5 into 10 % NH
4
NO water
3
ꢀ
1
o
solution, and refluxing for 24 hours at 100 C. Both
the structure and morphology do not change after the
treatment, suggesting MnꢀZSMꢀ5 is quite stable under
steaming. The Si to Mn ratio does not increase, showꢀ
ing the manganese might be in the zeolite framework
and strongly bonded. XRD, SEM and EDAX measꢀ
urements of the sample before and after ionꢀexchange
are listed in supporting materials (Figures S7, S8, and
S9).
Temperature Programmed Reduction (TPR).
COꢀTPR was performed on the SiꢀZSMꢀ5 and Mnꢀ
ZSMꢀ5(0.5 %, 1 %, and 2 %) samples. The reduction
profiles are shown in Figure 12. The reduction occurs
0
only when the temperature exceeds 300 C, further
explaining the enhanced activities for toluene oxidaꢀ
tion in
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42
properties . The introduction of Mn is crucial to
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36
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40
41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 12. COꢀTPR profiles of MnꢀZSMꢀ5 (all silica, 0.5 %,
%, and 2 %).
1
Figure 13. UVꢀVis absorption spectra of degassed 2 % Mnꢀ
ZSMꢀ5 after being exposed to air for different periods from
Auto-Reduction and Oxygen Evolution. Synꢀ
o
thesized 2% MnꢀZSMꢀ5 was degassed under 400 C in
0
~72 hours along with the absorption of the initial sample
the Micromeritics ASAP 2010 for 6 hours. The UVꢀVis
spectra were taken before and after this operation.
Based on Figure 13, the maximum absorption at both
before degassing.
the production of oxidative sites in order to maintain an
evolutionꢀreplenishing process.
2
50 nm and 520 nm decreases considerably after deꢀ
gassing and is accompanied by a color change from
dark pink to pure white after degassing. When exꢀ
posed to air, the absorption is restored in 48 hours
and is also observed from the color of the material reꢀ
turning to dark pink (Figure S10). The same phenomꢀ
enon has been found in other transition metal loaded
Manganese Substituted in the Zeolite Struc-
ture. Xꢀray diffraction data for the synthesized SiꢀZSMꢀ
5
and Mnꢀcontaining ZSMꢀ5 material show the monoꢀ
clinic MFI zeolite structure, space group p2 /n. The data
1
are indexed according to a research paper by Artioli et
43
al. . The calculated crystal structure data are listed in
1
zeolites . This is the first report for Mn loaded ZSMꢀ5
Table 2.
zeolites. The autoꢀreduction might be due to the deꢀ
hydration and desorption of O2 between the
Table 2. XRD Lattice Constants a, b, c, and (alpha, beta,
gamma) for Synthesized Mn-ZSM-5 Materials with a
Monoclinic Structure.
3
+
1
Mn (OH) sites .
x
Oxygen evolution is also observed from TGA analyꢀ
sis by comparing the results from N2 with those from
O2. For the samples run under N2, the discrepancy of
Samꢀ
ples
a
b
c
α
β
γ
o
the TGA curves appears at 120 C, and then develops
o
0 %
19.82
20.08
20.08
20.10
13.35
13.35
13.36
13.40
90.00
90.00
90.00
90.00
90.89
90.89
90.89
90.89
90.00
90.00
90.00
90.00
into a major weight loss gap after 400 C. However,
0
.5 % 19.82
1 % 19.84
for the samples tested under O , there is no significant
2
o
difference of weight loss until 430 C. This unambiguꢀ
ous result points to the observation that the difference
should come from not only loss of water. Oxygen is
evolving from the MnꢀZSMꢀ5 samples, producing the
2
%
19.910 20.16
The size of unit cell increases gradually as the nominal
o
gap at 120 C. Since there is an excess of O
2
for the O ꢀ
2
amount of manganese increases (0.5 %~2 %) as reflected
by the pronounced expansion of the a, b, and c values
and unit cell volume (Figure S11). However, which of the
TGA, that result could be balanced with depleted oxyꢀ
gen from the MnꢀZSMꢀ5, because equilibrium is
quickly reached between the sample and the O atꢀ
2
0.5% sample are identical to the all silica sample probaꢀ
mosphere. Thus, the gap of the weight loss between
pure SiꢀZSMꢀ5 and 2 % MnꢀZSMꢀ5 is not observed.
bly due to the limited substitution the framework sites by
using less dopants. Moreover, the beta value for all the
synthesized materials is equal to 90.892, with alpha and
gamma equal to 90 degrees which demonstrates a monoꢀ
clinic crystal structure, rather than orthorhombic, where
the alpha, beta, gamma values are all equal to 90 degrees.
This result agrees with monoclinic lattice parameters
published elsewhere4 . In this case, diffraction peaks
split between 8.7 and 9.0 degrees and are assigned to
Nevertheless, under N
2
, there is no O in the atmosꢀ
2
phere to replenish the depleted vacancy, and the oxyꢀ
gen is released without being replenished. Most imꢀ
portantly, the TGA results agree with our observation
of the degassing process, and when the sample is actiꢀ
vated in air or oxygen. The color change does not take
place as in the degassing process due to unchanged
bonding between Mn and oxygen. The color change of
the material is relevant to the oxygen evolution, and
may be due to the changing coordination of Mn when
the coordinated MnꢀO bond is broken, accompanied
by novel photoꢀabsorption properties and catalytic
3,44
(
2
020) and (200), respectively while the peak at around
9.9 degrees was assigned to (053) and assumed to be a
single peak. The doublets of the peak at 2θ =24.4 degrees
are also typical monoclinic feature. This was further conꢀ
firmed by matching the selected area electron diffraction
9
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Journal of the American Chemical Society
Page 10 of 14
(
SAED) patterns with a standard MFI type zeolite made manganese substitution of surface bonded OH groups.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
by Artioli et al. , shown in Figure 14.
43
Similar results have been reported for MnꢀMCMꢀ41
and silica supported manganese oxides 3 . Raman
spectra taken from different micrometer sized partiꢀ
cles (assisted by high resolution microscopy) have
shown a pronounced different relative intensity of
multiple vibration bands which further confirms the
inhomogeneous property of the high loading Mnꢀ
ZSMꢀ5 (2 %), probably caused by the unevenly disꢀ
tributed Mn surface bonding species. Surface mangaꢀ
nese is also suggested by XPS results, as the assignꢀ
ment of a deconvoluted peak at about 529.7~529.9 eV
shows, with an increasing ratio from 2.92 % in the
1,45
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0.5 % Mn to 6.2 % in the 2 % Mn sample, and a comꢀ
parable ratio of 6.57 % in the 1% sample, when the toꢀ
tal oxygen bonding is considered as a unit.
Figure 14. Match of (A) standard diffraction pattern for
monoclinicꢀMFI zeolite with (B) SAED pattern from experꢀ
iment (Figure 5 A, inset), both patterns orientated along the
Highly Selective Catalytic Properties. The reꢀ
dox activity of the Mnꢀdoped zeolites was studied by
both catalytic gas phase benzyl alcohol oxidation and
toluene total oxidation. The catalysts are active and
selective for both reactions but in different temperaꢀ
ture ranges. For the benzyl alcohol oxidation, the feaꢀ
sible temperature range for catalytic activity is beꢀ
tween 300 C and 350 C, in which the benzyl alcohol
was selectively converted to benzaldehyde. Complete
oxidation was found when the temperature increased
[
010] plane
The enlargement of unit cells could be explained by
3
+
the substitution of Mn in the tetrahedral framework
of MFI zeolite, which involves an absorption at 250
2
ꢀ
3+
o
o
nm as a result of charge transfer from O to Mn in
tetrahedral coordination. On the other hand, no abꢀ
sorption occurred for either the nonꢀMn containing
samples or the commercial manganese oxides which
are octahedrally coordinated. Besides, there is broad
absorption around 500 nm for all manganese containꢀ
o
to 400
gas phase products. More than 50 % yield was
achieved at 350 C with 80 % selectivity. The TOF
C by detecting abundant carbon dioxide in the
0
6
4
ꢀ1
ing samples, which is assigned to A1g to T2g crystal
number is as high as 88 h by considering Mn as the
only active sites (Supporting information). The total
oxidation for benzyl alcohol which occurs at temperaꢀ
2+
field transitions of Mn as described in other Mn conꢀ
3
5
taining zeolite materials . This could be attributed to
the Mn2+ species bonded on the surface of ZSMꢀ5 and
may explain the color of the material. The substitution
of Mn in the ZSMꢀ5 structure was also indicated by
the shift of Raman peaks of the 5ꢀmembered ring viꢀ
brations of MFI type zeolite. A slight shift of the peak
0
tures higher than 350 C is undesirable, but if applied
as a catalyst for toluene conversion, then this process
affords total oxidation. This assumption was realized
by running the Mn containing ZSMꢀ5 in a gas phase
toluene oxidation system, with undoped SiꢀZSMꢀ5 as
2
ꢀ
1
at 380 cm to lower wave numbers means weakened
SiꢀO bonds as the amount of Mn increases and withꢀ
out showing extra peaks due to MnꢀO vibrations until
the amount of Mn increased to 2 wt%. The increasing
absorption of UVꢀVisible light, the red shift of the
Raman spectra, and the increase of lattice parameters,
all provide strong evidence for the incorporation of
Mn in the zeolite framework occurring during the synꢀ
thesis.
a blank and 2% MnO physically mixed blank as a
control. The experimental results showed that the
synthesized Mnꢀcontaining zeolite is much more acꢀ
tive than both the physically mixed sample and the
blank. The 2 % MnꢀZSMꢀ5 showed 65 % conversion
o
and 100 % selectivity of carbon dioxide at 400 C
compared with the blank which had less than 1 % conꢀ
version under the same conditions. The proportional
increase in activity as a concomitant increase in the
amount of Mn added provides validity for the redox
activity of manganese introduced by one step syntheꢀ
sis.
Amorphous Manganese Species on the Sur-
face. Based on the Xꢀray diffraction data, there are no
crystalline manganese oxides phases in all the manꢀ
ganese containing materials, but clear evidence that
extra Mnꢀcontaining phases exist on the surface is
shown by UVꢀVis, and Raman, as well as XPS.
Comparison of Experiments and Calcula-
tions. The performance of MnꢀZSMꢀ5 doped zeolites
depends strongly on the Mn environment. The XRD
data given above reveals that the unit cell volume inꢀ
creases linearly with increasing Mn leading to the
supposition of isomorphous substitution of Mn at tetꢀ
rahedral framework sites. Since Mn3+ is more inꢀ
The broad UVꢀVis absorption peak at 500 nm repꢀ
2
+
resents the possible existence of Mn on the surface
of MnꢀZSMꢀ5 material. The same phenomenon has
been observed for the Mn containing MCMꢀ41 zeolite
material prepared by a similar direct hydrothermal
2
+
4+
clined than Mn to be substituted for Si , part of the
manganese ions may be incorporated into the frameꢀ
45
synthesis . Nevertheless, the Raman spectroscopy
provided direct evidence of the occurrence of MnO
species on the surface. The new peak centered at 560
3
+
work of ZSMꢀ5 preferentially as Mn . On the other
x
hand, it is also possible to have Mn species located at
2+
3+
ꢀ
1
extraꢀframework positions as either Mn , Mn , or
cm was assigned to symmetric stretching of the Mnꢀ
4+
ꢀ
1
possibly Mn ions. UVꢀVis spectroscopy is one of the
most useful tools to study the local of the Mn enviꢀ
OꢀSi in the framework, whereas the band at 628 cm
could be ascribed to surface manganese species from
q+
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Journal of the American Chemical Society
ronment where electronic spectra show various types
of absorptions including d ꢀd transitions on the
manganese ion, ligandꢀtoꢀmetal charge transfer from
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
+
2ꢀ
the oxygen to the manganese, LMCT, (Mn
O ꢀ
Mn2+ O ) transitions, and perhaps LigandꢀLigand (L
ꢀ
ꢀ
L) transitions. The wavelengths at which various
transitions occur can be highly sensitive to the local
environment of the Mn ion and can serve as a probe
for determining the Mn coordination. The experiꢀ
mental UVꢀVis absorption spectra shown in Figures 8
and 13 are similar to those obtained previously in sevꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
46
eral past experiments .
The results presented here aim to establish a correꢀ
lation between the experimental UVꢀVis absorption
spectra on Mnꢀcontaining samples and the calculated
photophysical properties calculated using Timeꢀ
Dependent Density Functional Theory (TDꢀDFT). All
the simulated spectra presented here are obtained
Figure 15. Simulated UVꢀVis spectra of the model
47
with the SpecDis program using a lifetime broadenꢀ
ing of Γ = 0.16 eV, i.e. full width at half maximum
(fwhm).
ꢀ
Mn(OSiH
3
)
4
compound and an Mn ion inserted into the
zeolite framework.
-
However, it is possible that all peaks in the experiꢀ
Structure of Model Mn(OSiH
3
) Compound
4
mental spectra come from framework Mn ions emꢀ
bedded in a tetrahedral environment. The peak
shown at the shorter wavelength region, 300ꢀ450 nm
is due to transitions that have both Mn(d ꢀ d) and
Ligand (π ꢀ Mn(d) ) character to the wave function.
Finally, the highest energy peak in the region of 200ꢀ
and Embedded Mn-Zeolite Cluster. Optimized
ꢀ
geometry parameters for the D2d Mn(OSiH
el compound and the embedded Mnꢀ(OꢀSiH
mary structure (PS), are given in Table 3.
3
)
4
2
modꢀ
, priꢀ
ꢀ)
4
Table 3. Optimized Geometric Parameters for the
-
Mn(OSiH ) Model Compound and Embedded Mn-(O-
3
4
2
80 nm is the result of metal ligand charge transfer
-
SiH -) Primary Structure (PS) of Mn-ZSM-5.
3+
2ꢀ
2
4
(MLCT) electronic transitions of the type, Mn
O
ꢀ
2+
ꢀ
Mn O . The lowꢀenergy peak in the 500 nmꢀ600 nm
range is missing from the spectrum of the Mnꢀ
embedded zeolite cluster shown in Figure 15. The Oꢀ
MnꢀO angle is more constrained to a smaller value in
the Mnꢀembedded zeolite cluster compared to the
Geometric Parameters
ꢀ
Mn(OSiH
3
)
4
PS of MnꢀZSMꢀ5
MnꢀO
SiꢀO
1.86
Å
1.86
Å
Å
ꢀ
1.63
Å
1.62
Mn(OSiH3)4 model compound, caused both by the reꢀ
1350
1070
straining effect of the surrounding zeolite framework
and by the lack of UFF parameters for fourꢀcoordinate
OꢀMnꢀO
OꢀMnꢀSi
1360
1400
3+
Mn . Thus, the interaction of O π orbitals with Mn d
orbitals is attenuated as the OꢀMnꢀO angle decreases
causing the d orbital splitting to increase with a conꢀ
comitant increase in dꢀd excitation energy.
Table 3 shows that the MnꢀO and SiꢀO distances, as
ꢀ
4
well as the OꢀMnꢀSi angles, in both the Mn(OSiH
model compound and Mnꢀ(OꢀSiH ꢀ) primary strucꢀ
ture are in very good agreement. However, the OꢀMnꢀ
3
)
2
4
Calculated Spectra of Manganese Oxides.
The existence of manganese oxide species on the surꢀ
face on MnꢀZSMꢀ5 zeolites is a thorny problem that is
difficult to understand clearly. As discussed earlier,
the 500 nm absorption peak obtained with diffuse reꢀ
ꢀ
O angle is more open in the Mn(OSiH
3
) model comꢀ
4
pound because there is a lack of constraining forces,
and the UFF parameters used for Mn in the QM/MM
calculation were defined for octahedral Mn2+ which
would lead to smaller OꢀMnꢀO angles.
flectance spectroscopy seems to indicate the presence
of either Mn2+ or Mn3+ on the surface of the MnꢀZSMꢀ
–
5 material. However, the calculated electronic absorpꢀ
Spectra of Model Mn(OSiH
3
)
4
Compound
ꢀ
tion spectrum of Mn(OSiH
3
)
4
as a model for a frameꢀ
and Embedded Mn-zeolite Cluster. Simulated
UVꢀVis spectra for the model compound and the emꢀ
bedded MnꢀZeolite cluster model are given for
work Mn ion does indeed have a transition energy in
the 500 nm range. Nevertheless, it may be also possiꢀ
ble to have species containing Mn ions located at exꢀ
traꢀframework positions which are capable of yielding
a peak at 500 nm. It becomes interesting to investiꢀ
gate the absorption spectra of related model mangaꢀ
nese oxide anions to determine whether or not their
existence is possible on the MnꢀZSMꢀ5 surface. The
calculated absorption spectra of sixꢀcoordinate
ꢀ
Mn(OSiH
3
)
4
in Figure 15 which also provides a comꢀ
parison of the calculated spectra for these two model
compounds. The calculated UVꢀVis spectrum of the
ꢀ
Mn(OSiH
3
)
4
model compound in Figure 15 is in good
agreement with the experimental spectra given in
Figures 8 and 13. These data show a peak at about
5
00 nm which is a result of a Mn(d ꢀd) transition,
10ꢀ
9ꢀ
xꢀ
MnO
6
(Mn(II)) and MnO
6
(Mn(III)) model comꢀ
and is often attributed to surface MnO
which might also be present.
y
products
pounds are given in Figure 16.
11
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Journal of the American Chemical Society
Page 12 of 14
work sites, yet their presence as surface species canꢀ
not be ruled out due to a strong absorption at 500
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
10ꢀ
nm( e.g. [Mn(II)O ] ). The variable coordination of
6
the Mn bonding with oxygen in the MnꢀZSMꢀ5
showed novel color changing properties with varying
the oxygen atmosphere to an inert atmosphere. Howꢀ
ever, the system maintained a comparably stable oxiꢀ
dative property by tentative oxygen depletion and reꢀ
plenishing mechanism under oxygen as supported by
TGA and TPR data. Finally, for the first time this Mnꢀ
ZSMꢀ5 has shown remarkable activity in catalyzing
both benzyl alcohol oxidation and toluene oxidation.
This combined experimental and modeling study
opens up a new way to design and identify TM doped
zeolites and provides solid evidence for potential apꢀ
plications in heterogeneous catalysis.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ASSOCIATED CONTENT
Supporting Information
Figures S1ꢀS11 and Table S1. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 16. Simulated UVꢀVis spectra of the sixꢀcoordinate
10ꢀ
9ꢀ
MnO6 , Mn(II) and MnO6 , Mn(III) model oxides. For
the Mn2+ (d
) oxide, an MnꢀO distance of 2.00 Å with O
symmetry was used, while the Mn (d
nally distorted to D4h symmetry, having distances of Mnꢀ
5
h
3+
4
) oxide was tetragoꢀ
48
Oaxial = 2.20 Å and MnꢀOequitorial = 2.00 Å .
AUTHOR INFORMATION
Corresponding Author
One striking feature of the calculated spectra preꢀ
sented in Figure 16 is the absence of an absorption
peak at 250 nm as shown in Figures 8 and 13 which
demonstrates with reasonable certainty that the 250
nm absorption feature is not due to sixꢀcoordinate
*
Email: steven.suib@uconn.edu
Notes
surface oxides. The peaks at 500ꢀ700 nm and 300
The authors declare no competing financial interests.
nm have been also observed experimentally4
8b,49
and
provide additional confidence in the calculated abꢀ
sorption spectra. Similar results were also obtained
for highꢀoxidation state, fourꢀcoordinate manganese
oxides.
ACKNOWLEDGMENTS
We thank Dr. Ray Joesten for help in analyzing the XRD
and TEM results, Dr. Heng Zhang (institute of material
science) who helped us with the XPS, and Dr. Frank Galꢀ
asso for many helpful discussions. This work was supꢀ
ported by the US Department of Energy, Office of Basic
Energy Sciences, Division of Chemical, Geochemical and
Conclusion
In summary, we have shown a facile synthesis of
Mnꢀcontaining high silica ZSMꢀ5 zeolite in a single
step with concomitant novel optical absorption and
catalytic properties. The manganese substituted in the
zeolite framework was proven by XRD refinements
showing enlarged lattice parameters consistent with
the increasing amounts of manganese. Multiple specꢀ
troscopic techniques IR, Raman and UVꢀvisible abꢀ
sorption, as well as elemental analysis such as XPS
and EDXS suggested the coexistence of both frameꢀ
work Mn sites and a possible foreign phase, dependꢀ
ing on the amount of dopants. In addition, we perꢀ
formed TDꢀDFT calculations on excited states to simꢀ
ulate the photophysical properties of Mnꢀcontaining
Biological
Sciences
under
contract
DEꢀFG02ꢀ
8
6ER13622.A000.
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