Relationship between basicity, reducibility and partial oxidation properties of chromium containing MCM-41

Article abstract of DOI:10.1039/c4ra10336j

MCM-41 mesoporous solids were modified with chromium species using Cr(NO₃)₃ and CrO₃ as precursors. The impact of metal amount and metal sources on the structural and textural parameters of the materials obtained was investigated. Their surface properties were analyzed using XRD, UV-Vis, H₂-TPR (the state of Cr species) as well as test reactions (2-propanol decomposition and cyclization and dehydration of 2,5-hexsanedione). The catalytic activity of Cr/MCM-41 materials was tested in methanol partial oxidation. The relationships of reducibility, kind of chromium species, basicity of material surface and the catalysts performance in partial oxidation of methanol were examined and discussed. The impact of basicity on methanol oxidation was clearly documented. Di- and polychromate species were found to be responsible for high yields of formaldehyde.

Full text of DOI:10.1039/c4ra10336j

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Relationship between basicity, reducibility and
partial oxidation properties of chromium
containing MCM-41
Cite this: RSC Adv., 2014, 4, 62940
Maciej Trejda,* Małgorzata Brys and Maria Ziolek
´
MCM-41 mesoporous solids were modified with chromium species using Cr(NO
The impact of metal amount and metal sources on the structural and textural parameters of the materials
obtained was investigated. Their surface properties were analyzed using XRD, UV-Vis, H -TPR (the state of
3 3 3
) and CrO as precursors.
2
Cr species) as well as test reactions (2-propanol decomposition and cyclization and dehydration of 2,5-
hexsanedione). The catalytic activity of Cr/MCM-41 materials was tested in methanol partial oxidation.
The relationships of reducibility, kind of chromium species, basicity of material surface and the catalysts
performance in partial oxidation of methanol were examined and discussed. The impact of basicity on
methanol oxidation was clearly documented. Di- and polychromate species were found to be
responsible for high yields of formaldehyde.
Received 12th September 2014
Accepted 7th November 2014
DOI: 10.1039/c4ra10336j
www.rsc.org/advances
level has been known for more than 100 years and involves Fe–
Mo or Ag containing catalysts. The mechanism of this reaction,
Introduction
The design of catalysts continues to be an attractive and depending on the kind of catalyst used, has been described in
important issue even in view of the large number of catalytic literature, e.g. ref. 8–18 but some of its aspects are still debat-
processes already applied in industry. Development of new able. Partial oxidation of methanol has been also proposed as a
catalytic systems or optimization of those used already is test reaction for different kinds of catalysts basing on mecha-
essential to decrease the cost of production and limit the nisms proposed, e.g. ref. 19 and 20. In our previous work we
number and amount of side-products, which is also in line with have shown that partial oxidation of methanol can be combined
1
the principles and rules of so-called “green chemistry”.
3
with other transformation of CH OH, namely the sulphurisa-
20
Therefore, development of new methods supporting catalyst tion process. As a consequence, these two complementary
design is important. Huge progress in this regard has been reactions give more detail information concerning the proper-
made thanks to the application of new methods of physico- ties of catalyst surface.
chemical characterization of materials providing information
The application of different test reactions or characterization
on the catalyst structure and single processes that occur on the techniques is essential especially when different kinds of
2
surface of solid materials. However, these techniques are species can be formed in course of catalyst preparation. This is
2
1,22
applied usually under conditions very different to those of the oen in the case of transition metals, e.g. iron
actual catalyst application, i.e. the conditions of the catalytic mium,
or chro-
23,24
applied in the catalysts preparation. The latter
process. A new perspective has been offered by Operando element was incorporated into different kinds of mesoporous
3
techniques. Nevertheless, the application and development of materials (MCM-41, SBA-15 and SBA-3) using CrO
3
as a metal
24
test reactions that examine the surface properties at “working” precursor. We have shown that a kind of support strongly
conditions are still crucial. For example, the test processes inuences chromium species obtained aer impregnation. In
involving the acid–base or redox properties of the solids are well this paper we focus on MCM-41 support that is modied with
known and described in literature.
belongs to the well-dened test reactions but it is also used in metal precursors. By changing the amount and sources of
industry for the production of ne chemicals.
chromium we expected to obtain different chromium species on
Partial oxidation of methanol is an important industrial mesoporous support having different properties.
process since it provides valuable products such as formalde-
The aim of this study was to examine basicity and redox
4
–7
Methanol oxidation different amounts of chromium using CrO and Cr(NO ) as
3 3 3
8
hyde, dimethoxymethane (methylal) or methyl formate. The properties of model catalysts of MCM-41 type containing chro-
production of the rst product mentioned above at industrial mium by the use of selected test reaction (2-propanol decom-
position, cyclization and dehydration of 2,5-hexsanedione and
methanol oxidation). In particular, we would like to present the
Adam Mickiewicz University in Pozna n´ , Faculty of Chemistry, Umultowska 89b, 61-614
Pozna
n´ , Poland. E-mail: tmaciej@amu.edu.pl; Tel: +48 618291305
correlation of basic properties of Cr/MCM-41 materials
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measured by test reactions) and reducibility of chromium
RSC Advances
(
The temperature-programmed reduction of the samples was
species (testied by H -TPR) with redox properties estimated carried out using H /Ar (10 vol% in Ar) as a reducing agent
2
2
3
ꢀ1
from methanol oxidation to formaldehyde.
(ow rate ¼ 32 cm min ). The sample (0.025 g) was introduced
to a quartz tube, treated in a ow of helium at 723 K for 2 h
3
ꢀ1
(
ow rate ¼ 40 cm min ) and cooled to room temperature.
Experimental
ꢀ1
Then it was heated at the rate of 10 K min to 1273 K under the
ow of the reducing mixture. Hydrogen consumption was
Preparation of MCM-41 materials

MCM-41 was prepared by the classical hydrothermal synthesis in measured by a thermal conductivity detector.
25
the polypropylene (PP) bottles. The reactant mixture consisted
of sodium silicate (27% SiO in 14% NaOH – Aldrich), cetyl- 2-Propanol decomposition
2
trimethylammonium chloride (CTACl) (Aldrich) and water. The
The 2-propanol dehydration and dehydrogenation was per-
gel formed from these components was stirred for 0.5 h. Next,
pH was adjusted to 11 by H SO , and distilled water was added.
The gel, with the molar ratio: 1SiO : 1.78CTA+ : 0.09H SO
201H O, was moved into a PP bottle and heated without stir-
formed using a microcatalytic pulse reactor (Ø 6 mm; length 80
mm) inserted between the sample inlet and the column of a
CHROM-5 chromatograph. A portion of 0.02 g (3 mm height in
the reactor) of granulated catalyst was activated at 673 K
2
4
2
2
4
-
:
2
ring at 373 K for 24 h. The reactants were cooled down to room
temperature (RT) in order to adjust pH again to 11 with H SO .
ꢀ1
3
(
heating rate 10 K min ) for 2 h under helium ow (40 cm
ꢀ1
2
4
min ). The 2-propanol (Chempur Poland) conversion was
Aer this, the mixture was heated once more to 373 K and le for
another 24 h. The product was ltered and washed with distilled
water. Aer drying at 373 K the template was removed by calci-
nation at 823 K for 2 h in helium ow and then 14 h in air in
studied at 573 K using 3 ml pulses of alcohol under helium ow
3
ꢀ1
(40 cm min ). The substrate was vaporized before being
passed through the catalyst bed with the ow of helium carrier
gas. The products such as propene, 2-propanone (acetone) and
diisopropyl ether were identied by CHROM-5 gas chromato-
graph online with microreactor. The reaction mixture was
separated on a 2 m column lled with Carbowax 400 loaded on
ꢀ
1
static conditions (heating rate 5 K min ).
Incorporation of chromium species
3
Chromosorb W (80–100 mesh) at 338 K in helium ow (40 cm
min ) and detected by TCD.
Chromium was introduced via incipient wetness impregnation.
Prior to the modication, the mesoporous silica was outgassed
in evaporator ask for 1 h at 353 K. Then the material was lled
with such an amount of the aqueous solution of chromium(VI)
oxide (Aldrich) or chromium(III) nitrate nonahydrate (Aldrich)
which lled only the pores of the support. The amount of
chromium in the solution was sufficient to obtain 1 wt%, 3 wt%
or 5 wt% of this metal in the nal product. The mixture was
located in an evaporator ask, rotated and heated at 353 K for
ꢀ
1
Cyclization and dehydration of 2,5-hexanedione (2,5-DHN)
The catalysts were tested in 2,5-hexanedione (2,5-DHN) dehy-
dration and cyclization as the probe reaction. A tubular, down-

ow reactor (Ø 8 mm; length 80 mm) was used for (2,5-DHN)
cyclization reaction that was carried out at atmospheric pres-
sure, using nitrogen as the carrier gas. The catalyst bed (0.05 g,
1
h. The impregnated powder was dried at 393 K for 12 h and
2
mm height in the reactor) was rst activated for 2 h at 673 K
ꢀ
1
3
ꢀ1
calcined at 823 K for 5 h in air in static conditions (heating rate
5
(heating rate 15 K min ) under nitrogen ow (40 cm min ).
ꢀ
1
3
K min ).
Subsequently, a 0.5 cm of 2,5-hexanedione (Fluka, GC grade)
was passed continuously into the catalyst at 623 K. The
substrate was delivered with a pump system (KD Scientic) and
vaporized before being passed through the catalyst with the ow
of nitrogen carrier gas (40 cm min ). The reaction products
were collected for 30 min downstream of the reactor in a cold
trap (liquid nitrogen + 2-propanol) and analyzed by gas chro-
matography (SRI 310 C, MXT-1 column 30 m, temperature of
column 373 K) with TCD detector. Helium was applied as a
carrier gas.
Characterization methods
The materials prepared were characterized using different
3
ꢀ1
techniques like: XRD,
-TPR.
XRD patterns were recorded at room temperature on a
Bruker AXS D8 Advance apparatus using CuKa radiation (l ¼
2
N adsorption/desorption, UV-Vis,
H
2
ꢁ ꢁ
.154 nm), with a step of 0.02 and 0.05 in the small-angle and
0
wide-angle range, respectively.
N2 adsorption/desorption isotherms were obtained on a
Micromeritics ASAP equipment, model 2010. The samples (200
Methanol oxidation
mg) were pre-treated in situ under vacuum at 573 K for 3 h. The The methanol oxidation reaction was performed in a xed-bed
surface area was calculated using the BET method. The pore ow reactor (Ø 5 mm; length 70 mm). A portion of 0.04 g of
diameter and the mesopore volumes were determined from the the pure (not diluted) catalyst of the size fraction of 0.5 < Ø < 1
adsorption branch of the isotherms.
UV-Vis spectra were recorded using Varian-Cary 300 Scan samples were activated in argon ow (40 cm min ) at 673 K for
UV-Visible Spectrophotometer. Powdered samples were placed 2 h (the rate of heating was 15 K min ). Then, the temperature
mm was placed in the reactor (4 mm height in the reactor). The
3
ꢀ1
ꢀ1
into the cell equipped with a quartz window. The spectra were was decreased to that of the reaction. The reactant mixture of
3
2
recorded in the range from 800 to 190 nm. Spectralon was used Ar/O /MeOH (88/8/4 mol%) was supplied at the rate of 40 cm
as a reference material.
ꢀ
1
min . Methanol (Chempur Poland) was introduced to the ow
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reactor by bubbling argon gas through a glass saturator lled Table 2 Textural/structural characterisation
with methanol. The reactions were conducted at GHSV ¼
a
a
ꢀ1
Surface area
Pore volume
Pore diameter
nm
1
220 h . The reactor effluent was analyzed using an online two
2
ꢀ1
3
ꢀ1
Catalyst
m g
cm g
gas chromatographs. One chromatograph GC 8000 Top
equipped with a capillary column of DB-1 operated at 313 K – MCM-41
FID detector was applied for analyses of organic compounds 1Cr(o)MCM-41
950
945
925
820
905
875
860
1.11
0.95
0.95
0.76
0.85
0.91
0.81
4.1
3.0
3.1
3.1
3.1
3.1
3.1
3
5
1
3
Cr(o)MCM-41
Cr(o)MCM-41
Cr(n)MCM-41
Cr(n)MCM-41
and the second GC containing Porapak Q and 5A molecular
sieves columns for analyses of O , CO , CO, H O and CH OH –
TCD detector. The columns in the second chromatograph with
2
2
2
3
TCD were heated according to the following programme: 5 min 5Cr(n)MCM-41
at 358 K, increase of the temperature to 408 K (heating rate
a
2
Determined from adsorption branches of N isotherms.
ꢀ
1
5
K min ), 4 min at 408 K, cooling down to 358 K (for the
automatic injection on the column with 5 A), 10 min at 358 K,
ꢀ
1
increase of the temperature to 408 K (heating rate 10 K min ),
1 min at 408 K. Argon was applied as a carrier gas. The outlet
1
indicated in Table 2, especially in the case of samples prepared
stream line from the reactor to the gas chromatograph was
heated at about 373 K to avoid condensation of reaction
products.
from CrO . The latter show a small decrease of surface area up
to 3 wt% of chromium loading, whereas for samples prepared
from Cr(NO ) the drop of surface area is remarkable. This
3 3
3
difference is not much seen for 5 wt% of Cr loading. Other
textural parameters, i.e. pore volume and pore diameter
decrease as well, however the latter is similar for all chromium
containing samples. Mesoporous channels of MCM-41 are well
ordered even aer Cr impregnation as it is determined by XRD
measurements for the highest chromium loading (Fig. 2). The
Results and discussion
Characterization
The support for chromium species was prepared by typical
hydrothermal synthesis. The list of materials obtained,
including their colors, is presented in Table 1. MCM-41
obtained shows characteristic properties of ordered meso-
ꢁ
appearance of a peak at 2 theta ca. 2.3 related to (100) is caused
by the presence of channels that are regular in shape. The
position of this peak is associated with the distance between
2
ꢀ1
porous solid with relatively large surface area (950 m g ), pore
ꢁ ꢁ
2
ꢀ1
material walls. The next two peaks at 2 theta ca. 4 and 4.6
volume of 1.11 m g and average pore diameter of 4.1 nm
Table 2). The adsorption/desorption isotherm of the support is
presented in Fig. 1. It shows a typical shape of type IV isotherm
appear due to the regular arrangement of channels inside
MCM-41 and can be used as an indicator of pore ordering. The
intensity of the main peak depends on the kind of chromium
precursor that was applied for catalyst preparation, i.e. CrO3
(
26
according to IUPAC classication. Two regions of high
increase in N volume can be observed. The rst, in the range of
2
(
labeled as (o) in the catalyst symbol) or Cr(NO ) (labeled as (n)
3 3
relative pressure p/p
sation inside mesopores. The second, at p/p
0
¼ 0.2–0.4 is caused by capillary conden-
in the catalyst symbol). The latter gives lower intensity of the
100) peak, which can be explained by the chromium species
0
ca. 0.9 testies to
(
the presence of macroporosity (interparticle macroporosity) or
secondary mesopore system in the space between mesoporous
located to a greater extend inside the hexagonal pores of the
support. The incorporation of chromium in MCM-41 leads to
27
channels. The incorporation of chromium does not signi-
cantly change the textural parameters of nal materials as
Table 1 Catalysts obtained
Chromium
source
Chromium
loading, wt%
Colour of
sample
a
Catalyst
1
4
3
4
5
4
1
4
3
4
5
4
Cr(o)MCM-
1
Cr(o)MCM-
1
Cr(o)MCM-
1
Cr(n)MCM-
1
Cr(n)MCM-
1
Cr(n)MCM-
1
CrO
CrO
CrO
3
3
3
1
3
5
1
3
5
Pale yellow
Yellow-
green
Dark green
Cr(NO
Cr(NO
Cr(NO
3
3
3
)
2
)
2
)
2
Pale yellow
Yellow-
green
Green
a
3 3 3
(o) – obtained from CrO ; (n) – obtained from Cr(NO ) .
Fig. 1
N
2
adsorption/desorption isotherm of MCM-41 material.
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Fig. 4 UV-Vis spectra of: (a) MCM-41; (b) 1Cr(o)/MCM-41; (c) 3Cr(o)/
MCM-41; (d) 5Cr(o)/MCM-41; (e) 1Cr(n)/MCM-41; (f) 3Cr(n)/MCM-41;
(g) 5Cr(n)/MCM-41.
Fig. 2 XRD patterns of materials modified with 5 wt%.
lower Cr loading on material surface. This conclusion is also
6
+
supported by the pale yellow (characteristic of Cr ) color of the
samples having the lowest chromium concentration. The
introduction of higher than 1 wt% amount of chromium results
in the appearance of a band at ca. 465 nm (this band shows a
very low intensity for 1 wt% of Cr) that is assigned to di- and
the formation of some crystal phase on the material surface
(both, inside and outside the pores) which is testied by the
ꢁ
presence of new XRD peaks at 2 theta between 20–55 (Fig. 3).
They are not observed when 1 wt% of Cr was used for the
impregnation. For higher metal loading, the crystal phase that
28
poly-chromate species. For samples containing 3 and 5 wt% of
chromium also a band at ca. 600 nm is formed. Moreover, the
can be assigned to a-Cr
becomes visible and the highest intensity of XRD peaks is
observed for 5 wt% of chromium. Moreover, if CrO is used as
the metal precursor, the intensity of the peaks related to a-Cr O
2 3
O (calculated pattern in Fig. 3g)
intensity of the band at 600 nm is higher when CrO
3
is applied
as the source of metal for the impregnation. The band
3
+
3
mentioned is usually assigned in literature to Cr in a-Cr O3
particles.
2
28,30
2
3
The presence of this species can be additionally
is higher than if Cr(NO3) is used indicating relatively high
3
testied by the color of the samples (yellow-green for 3 wt% of
Cr and green or dark green for 5 wt% of Cr). For the lower
concentration of chromium (1 wt%), the XRD peaks do not
concentration of chromium(III) oxide phase in sample
5Cr(o)/MCM-41.
Some conclusions concerning the nature of chromium
2 3
show the presence of a-Cr O , which is in line with UV-Vis
species can be deduced from UV-Vis measurements. The
spectra of samples prepared are shown in Fig. 4. The intro-
duction of chromium on the surface of MCM-41 results in the
appearance of some characteristic bands coming from different
coordinations of Cr species. In particular the bands at ca. 260
results.
Fig. 5 presents the H -TPR proles of MCM-41 samples
containing chromium. For all catalysts one main maximum on
2
H -TPR prole is observed and according to literature this peak
2
6
+
31
can be assigned to the reduction of Cr species. The
temperature of Cr reduction depends on metal loading. For
both chromium sources applied, the increase of metal loading
from 1 wt% to 3 wt% leads to an increase in Cr reducibility.
This is in agreement with the formation of lager aggregates, i.e.
2
ꢀ
/
6
+
and 350 nm are assigned to the charge transfer from O
6
+
28–30
Cr in monochromate species.
These bands prevail for the
6
+
samples with the lowest amount of chromium (1 wt%) thus one
can postulate that monochromate species are dominant for
Fig. 3 XRD patterns of: (a) 1Cr(o)/MCM-41; (b) 3Cr(o)/MCM-41; (c)
5
2
Cr(o)/MCM-41; (d) 1Cr(n)/MCM-41; (e) 3Cr(n)/MCM-41; (f) 5Cr(n)/ Fig. 5 H -TPR profiles of: (a) 1Cr(o)/MCM-41; (b) 3Cr(o)/MCM-41; (c)
MCM-41; (g) a-Cr
78-5443).
O
2 3
(International Centre for Diffraction Data: 01- 5Cr(o)/MCM-41; (d) 1Cr(n)/MCM-41; (e) 3Cr(n)/MCM-41; (f) 5Cr(n)/
0
MCM-41.
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polychromate species that show lower temperature of reduc-
30
tion. However, for samples with 5 wt% of chromium loading
6
+
the temperature of Cr reduction is higher than for those with
wt% of chromium. This fact could be correlated with a
decrease in polychromate species in the sample. The latter
species decomposes forming a-Cr observed in XRD patterns
Fig. 3). For the samples that show the presence of a-Cr
3 wt% and 5 wt% of chromium) a small reduction peak is also
3
2 3
O
(
(
2 3
O
visible at ca. 550 K. This peak is the most intense for the highest
Cr loading. According to literature, it can be assigned to the
6
+
31
reduction of Cr species dispersed on a-Cr O . Moreover, the
2
3
amount of hydrogen consumption differs depending on the Fig. 6 Results of 2,5 hexanedione cyclisation and dehydration carried
at 623 K.
metal loading and this dissimilarity is not proportional to the
2
chromium content. The highest H consumption is observed for
3
wt% of Cr loading independently of chromium source applied
the introduction of 5 wt% of chromium decreases the basicity of
samples. On the basis of these results one can propose to assign
the origin of moderate basicity in the materials prepared to the
presence of di- and polychromate species, which dominate for
the samples with 3 wt% of chromium loading.
for the MCM-41 impregnation. This observation is consistent
with XRD and UV-Vis measurements that show the formation of
a-Cr O phase especially in samples 5Cr(o)/MCM-41 and 5Cr(n)/
2
3
MCM. Thus the increase in chromium concentration from
wt% to 3 wt% is accompanied by increasing concentration of
1
6
+
The acid–basic properties of materials prepared were also
examined in 2-propanol decomposition. In general, propene
and diisopropyl ether are formed on acid sites (or on the acid–
base pairs), whereas the formation of acetone requires the
Cr in the samples, whereas a further increase in the metal
content leads mainly to the formation of a-Cr and limits the
amount of different kinds of chromates species (Cr ) inside or
2 3
O
6
+
outside the pores of material.
6,7
presence of basic (or redox) centers. The materials obtained
using CrO as a metal precursor (xCr(o)/MCM-41) showed
3
Catalytic test reactions
negligible activity in the test reaction carried out at 573 K, which
was not the case for the catalyst prepared using the other Cr
precursor. Therefore in Fig. 7 the selectivity to acetone vs.
chromium concentration is presented only for xCr(n)/MCM-41
samples. It can be observed that the prole presented in
Fig. 7 shows a volcanic shape (with the maximum for 3 wt% of
Cr) similar as for cyclization and dehydration of 2,5-DHN
6
+
As demonstrated above, the highest amount of Cr species is
present on the samples containing 3 wt% of chromium.
However, for methanol oxidation process not only the amount
of active species is important but also their kind should be
taken into account. In this context, the acid–base properties of
the catalysts are very important because both kinds of centers
take part in the formation of products in partial oxidation of
methanol. Two different test reactions were applied for the
characterization of acidic and basic centers, i.e. cyclization and
dehydration of 2,5-hexanedione (2,5-DHN) and 2-propanol
decomposition.
(Fig. 6). These results conrm the moderate basic or redox
character of di- and polychromate species on MCM-41 surface.
Methanol oxidation
All catalysts prepared were tested in cyclization and dehy- As reported in literature, e.g. ref. 9 and 10 acidic and basic
dration of 2,5-hexanedione (2,5-DHN) aer activation of the centers play a key role in the oxidation of methanol by solid
samples at 623 K. The cyclization of 2,5-DHN is a well-known
reaction for determination of basicity/acidity of materials and
4
5
was proposed by Dessau and Alcaraz et al. as Brønsted acid–
base test. The formation of 2,5-dimethylfuran (DMF) occurs on
acidic centers, whereas basic centers take part in the production
of 3-methyl-2-cyclopentenone (MCP). The ratio of MCF to DMF
can be taken as an indicator of acid (<1), basic (>1) or acid-basic
properties (ꢂ1).
The results obtained in cyclization and dehydration of
2,5-DHN are presented in Fig. 6 as the MCP/DMF ratio vs.
chromium loading in the samples. The samples containing the
lowest concentration of Cr species (evidenced on material
surface mainly as monochromate ones) show acidic character
(MCP/DMF < 1). The impregnation of MCM-41 using higher
amount that 1 wt% of Cr species results in an increase in the
MCP/DMF ratio. For 3 wt% of metal loading (most pronounced
for 3Cr(o)/MCM-41) the MCP/DMF value is the highest, however Fig. 7 Results of 2-propanol decomposition carried out at 573 K.
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catalysts. The rst step in methanol oxidation is suggested to
be the abstraction of hydrogen from methanol leading to
9
methoxy groups. The further transformation of adsorbed
species depends on the kind and strength of active centers, as
1
0
reported earlier. In the presence of acidic centers only
methoxy groups interact with a second CH OH molecule to
3
form dimethyl ether. On the other hand, the presence of
strong basic centers leads to the total oxidation. The partial
oxidation products are determined by the presence of
moderate acidic and basic centers.
Table 3 shows the results of methanol oxidation carried out
at 523 and 573 K. Unsupported MCM-41 was not active in this
reaction. The main product obtained on chromium containing
Fig. 8 Results of methanol partial oxidation carried out at 523 and 573 K.
samples is formaldehyde (FA), whose formation demands the chromium. The formation of FA requires rather weak acidity
presence of moderate acidic and basic centers (the side (necessary for methanol adsorption and formation of surface
product detected are methyl formate and CO
2
). The selectivity methoxy species) and moderate basicity (abstraction of
to FA does not change systematically with the chromium hydrogen from methoxy species). Moreover, the week acidity
loading and seems to depend in some extent on a source of enhances desorption of formaldehyde from materials surface
chromium. For materials modied with CrO3 the highest (a similar effect is observed with increasing reaction temper-
loading leads to the lowest selectivity to formaldehyde. This ature). Therefore, the results presented in Fig. 8 suggest the
can be correlate with Cr
anol oxidation. Indeed, 5Cr(o)/MCM-41 sample shows the as moderate basic) that participate in partial methanol
highest selectivity to CO
among all catalysts at both reaction oxidation process. The nucleophilic character of oxygen link-
temperatures. In case of Cr(NO
applied for MCM-41 modi- ing chromium atoms (Cr–O–Cr) could explain the highest
cation the selectivity to FA rst decrease a little and then does activity of samples containing 3 wt% of chromium. These
not change signicantly. However, the higher amount of species are probably responsible for hydrogen abstraction
2 3
O
formation leading to total meth- important role of di- and polychromate species (characterized
2
3 3
)

6
+
polychromates species deduced for samples containing 3 wt% from methoxy species adsorbed on Cr (Lewis acid centers).
of chromium (UV-Vis, H -TPR) reects in the increase of Therefore, the amount and relative concentration of mono-
2
2 3
methanol conversion. This in consequence inuences the chromate, di- and polychromate as well as a-Cr O species is
amount of FA formed. The yield of FA depends on the chro- crucial for the results obtained in the methanol partial
mium loading as demonstrated in Fig. 8. However, one should oxidation process. The relative content of each above-
take into account that the acid–base properties of catalysts mentioned species can be measured and therefore controlled
also depend on chromium loading. Irrespectively of the by the test reactions for acid–base properties and physico-
chromium source used for MCM-41 impregnation, as well as chemical analyses. In consequence the knowledge of the
the reaction temperature applied, the volcanic shapes of the amount, types and properties of active species permits antic-
proles in Fig. 8 are the same with the maximum for 3 wt% of ipation and design of new catalytic systems.
Table 3 Conversion and selectivity determined at 523 K and 575 K for methanol oxidation
Selectivity (%)
Methanol
Catalyst
conversion (%)
(CH
3
)
2
O
HCHO
(CH
3
O)
2
CH
2
HCOOCH
3
CO
2
Reaction temperature: 523 K
1
3
5
1
3
5
Cr(o)/MCM-41
Cr(o)/MCM-41
Cr(o)/MCM-41
Cr(n)/MCM-41
Cr(n)/MCM-41
Cr(n)/MCM-41
7
9
7
14
18
4
Traces
Traces
Traces
Traces
Traces
Traces
79
78
70
83
72
72
Traces
Traces
Traces
Traces
Traces
Traces
12
12
11
6
15
14
9
10
19
11
13
13
Reaction temperature: 573 K
1
3
5
1
3
5
Cr(o)/MCM-41
Cr(o)/MCM-41
Cr(o)/MCM-41
Cr(n)/MCM-41
Cr(n)/MCM-41
Cr(n)/MCM-41
18
21
22
26
38
18
Traces
Traces
Traces
Traces
Traces
Traces
67
74
58
73
69
67
Traces
Traces
Traces
Traces
Traces
Traces
7
8
8
8
6
25
18
34
19
25
21
11
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RSC Adv., 2014, 4, 62940–62946 | 62945

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Paper
9
J. M. Tatibouet, Appl. Catal., A, 1997, 148, 213.
Conclusions
10 G. Busca, A. S. Elmi and P. Forzatti, J. Phys. Chem., 1987, 91,
ꢃ CrO
3
is the primary chromium species formed on MCM-41
or Cr(NO
5263.
impregnated with CrO
3
3
)
2
.
11 G. Busca, J. Lamotte, J. C. Lavalley and V. Lorenzelli, J. Am.
Chem. Soc., 1987, 109, 5197.
12 L. J. Burcham and I. E. Wachs, Catal. Today, 1999, 49, 467.
13 X. Gao, I. E. Wachs, M. S. Wong and J. Y. Ying, J. Catal., 2001,
203, 18.
ꢃ The increase in chromium loading on MCM-41 results in
an increase in di- and polychromate species and later a-Cr
irrespective of chromium source used for modication.
2
O
3
ꢃ Higher number of polychromate species are obtained using
Cr(NO ) than CrO as the precursor, however these species 14 T. Kim and I. E. Wachs, J. Catal., 2008, 255, 197.
3
2
3
show similar reducibility for the same Cr loading.
15 M. Muhler, Handbook of Heterogeneous Catalysis, ed. G. Ertl,
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H. Knozinger and J. Weitkamp, VCH, 1997.
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Interscience: Hoboken, New York, 2003.
catalyst tested in 2-propanol decomposition and 2,5-hex- 17 M. Ziolek, P. Decyk, I. Sobczak, M. Trejda, J. Florek,
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highest for materials with 3 wt% of chromium loading.
H. Golinska, W. Klimas and A. Wojtaszek, Appl. Catal., A,
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20 M. Trejda, J. Kujawa and M. Ziolek, Catal. Lett., 2006, 108,
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