Preparation, characterization and catalytic activity of MgO/SiO2 supported vanadium oxide based catalysts

Article abstract of DOI:10.1007/s10562-013-1098-z

Vanadium oxide-based catalyst obtained by grafting VOCl₃ on Florisil (MgO:SiO₂) with the molar ratio of 15:85 have been studied for the selective oxidation of cyclohexane in order to obtain cyclohexyl hydroperoxide, cyclohexanol and

Full text of DOI:10.1007/s10562-013-1098-z

Catal Lett (2014) 144:97–103
DOI 10.1007/s10562-013-1098-z
Preparation, Characterization and Catalytic Activity of MgO/
SiO2 Supported Vanadium Oxide Based Catalysts
•
Eman Fahmy Aboelfetoh Rudolf Pietschnig
Received: 4 June 2013 / Accepted: 2 September 2013 / Published online: 28 September 2013
Ó Springer Science+Business Media New York 2013
Abstract Vanadium oxide-based catalyst obtained by
grafting VOCl3 on Florisil (MgO:SiO2) with the molar
ratio of 15:85 have been studied for the selective oxidation
of cyclohexane in order to obtain cyclohexyl hydroperox-
ide, cyclohexanol and cyclohexanone. The performances
obtained have been compared with those of other catalysts
in which vanadium oxide was supported on the same
support by impregnation with ammonium oxalate. All the
prepared catalysts have been characterized using XRD,
FTIR, TEM, SEM, EDX, DRS and TGA in order to
rationalize the differences in performance observed. The
presence of magnesium oxide species (15 % MgO) on the
surface of silica significantly modifies the molecular
structure of the surface vanadium oxide species and
changes their molecular structure from hydrated VO5/VO6
polymers to less polymerized VO4 species and/or isolated
VO2(OH)2 species. The catalytic activities indicate that the
VOx/Florisil catalysts show high conversions and TONs for
those types containing isolated VO4. Compared to previ-
ously studied VOx/SiO2 catalysts, the Florisil based sys-
tems show significantly improved leaching behavior.
Keywords Vanadium oxide Á Supported catalysts Á
SiO2 Á MgO Á Selective oxidation Á Hydroperoxide
1 Introduction
Supported vanadium oxide catalysts have attracted signif-
icant attention, because of their potential for catalyzing
several oxidation reactions [1–11]. Especially the oxidation
of lighter alkanes is promising with respect to alternative
feedstocks for industrially relevant bulk chemicals [12–15].
The chemical environment on a molecular scale of the
supported vanadium oxide species was found to depend on
several parameters, e.g., metal oxide loading, oxide support
material, and the degree of hydration [16–20]. The nature
of the vanadium oxide species on several supports; i.e.,
Al2O3, Nb2O5, SiO2, TiO2, and ZrO2, has been examined
with various techniques [16, 17, 21–27]. Also pyrazine
2-carboxylic acid (PCA) acting as a co-catalyst has a sig-
nificant influence on the conversion of hydrocarbons. It is
anticipated that PCA coordinates to vanadium sites on the
catalyst surface facilitating electron and proton transfer
processes between peroxo/hydroxyl species and vanadium
[24, 28, 29]. Our recent investigation on the oxidation of
cyclohexane showed that the vanadium oxide catalysts
supported on SiO2 were very active however, their poten-
tial as heterogeneous catalyst was limited owing to sig-
nificant leaching from the silica surface into the reaction
medium which hampers recovering of the catalyst [24]. On
the other hand MgO supported vanadium oxide showed
substantially lower leaching rates but also variable catalytic
activity which was attributed to the presence of different
VOx phases as confirmed by 51V-MAS-NMR, IR, UV–
DRS and SEM/EDX [30]. The purpose of the present study
is to investigate the effect of MgO in modifying the
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-013-1098-z) contains supplementary
material, which is available to authorized users.
E. F. Aboelfetoh
Chemistry Department, Faculty of Science, Tanta University,
Tanta 31527, Egypt
R. Pietschnig (&)
Institute of Chemistry and Center for Interdisciplinary
Nanostructure Science and Technology (CINSaT), University of
Kassel, Heinrich-Plett-Straße 40, 34132 Kassel, Germany
e-mail: pietschnig@uni-kassel.de
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E. F. Aboelfetoh, R. Pietschnig
physicochemical characteristics and the catalytic behavior
of silica supported vanadium oxide in the selective oxida-
tion of cyclohexane by using readily available Florisil
(15MgO/85SiO2) as catalyst support.
done with a DSM 982 Gemini SEM with a maximum
acceleration voltage of the primary electrons between 10
and 15 kV. The powder samples were prepared on double
side adhesive carbon tape and covered with a gold layer in
a Cressington sputter coater operated under vacuum con-
ditions (0.5 9 10-1 mbar). Semi-quantitative EDX (Ron-
¨
2 Experimental
tec, M-series, EDR288/SPU2) analysis was used for the
characterization of element concentration and vanadium
distribution within all prepared catalysts. The TEM exam-
ination of samples was carried out on a Philips CM10
microscope working at 100 kV. TEM specimens were
prepared ultrasonically by dispersing the catalyst sample in
ethanol, and then placing a drop of the suspension on a Cu
grid covered with a lacey carbon film.
Two different methods, grafting and wet impregnation,
were applied to prepare supported vanadium oxide cata-
lysts on Florisil (15MgO/85SiO2) with different vanadium
loadings ranging from 1 to 18 wt% following previously
reported procedures [24]. The catalytic activity of the
prepared catalysts was evaluated for the liquid-phase oxi-
dation of cyclohexane with H2O2/molecular dioxygen (air)
as described before [30]. The catalytic activities are
reported as conversion (%), selectivity (%) and TON cal-
culated following a published procedure [24]. The overall
selectivity (OS %) is defined as sum of the selectivities for
cyclohexyl hydroperoxide, cyclohexanol and cyclohexa-
none. The vanadium concentration of the prepared catalysts
was determined by flame atomic absorption spectroscopy
(AAS) using a UNICAM 929 AA Spectrometer. The BET
surface area of the catalysts was measured by nitrogen
adsorption–desorption at 77 K using a NOVA 1200 surface
area analyzer (Quanta-chrome). The isotherms were ana-
lyzed in a conventional manner in the region of the relative
pressure, p/p0 = 0.05–0.3. X-ray diffraction (XRD) pat-
terns of all catalysts were performed on a Philips powder
diffractometer PW1050/25 with Cu Ka radiation
(k = 0.1542 nm) operating at 50 kV and 20 mA in a 2h
range of 10–70° with step size 0.01° and time step 1.0 s to
assess the crystallinity of the vanadium oxide loading. The
diffractograms of the samples were compared with the
powder diffraction patterns of reference samples. Fourier
transform-infrared spectra of the samples were recorded on
a Perkin-Elmer FT-IR spectrometer 1725X using KBr
disks. Diffuse reflectance (DR) spectra in the UV–Vis
region were recorded in the reflectance function mode
(F(R)) at room temperature in the range 1,000–200 nm on a
Varian Cary 500 spectrophotometer with a diffuse reflec-
tance attachment to investigate the structures of V(V)-
containing oxide compounds under hydrated and dehy-
drated conditions. The plain oxide support was used as a
reference for the corresponding supported catalysts. Ther-
mogravimetric analyses using a Mettler TGA were per-
formed on the support materials and all prepared catalysts.
To evaluate the overall amount of surface hydroxyl groups
available for anchoring reactions, the weight loss between
300 and 1,000 °C was determined. A heating rate of
10 °C/min under argon was applied to purge off gases from
the TGA electronics and sample region. The reference
material was a-alumina powder. The SEM analyses were
3 Results and Discussion
3.1 Catalyst Characterization
Table 1 summarizes the vanadium content of the catalysts
determined from AAS analysis, BET surface areas, VOx
surface densities calculated with the use of these two
parameters, and the concentration of surface hydroxyl
groups of Florisil and VOx/Florisil catalysts evaluated by
TGA. Grafted catalysts Grf1F to Grf3F show vanadium
loadings ranging from 1.1 to 5.1 wt%, while catalysts
prepared by impregnation (Imp1F to Imp5F) show vana-
dium loadings from 1.1 to 17.8 wt% (Table 1). BET
measurements indicate that the surface areas of the
impregnated catalysts per gram of Florisil decrease with
increasing vanadium loadings from Imp1F to Imp3F. A
considerable increase of the surface area is found for high
vanadium loadings (Imp4F) which may be explained by the
presence of separate VOx particles without pore blocking
[31]. Both, the calcined or non-calcined catalysts (Ungrf1F
and Grf1F) obtained from the initial grafting process
exhibit high surface areas (559.44 and 288.19 m2/g Flori-
sil). Repeating the grafting process decreases the surface
area per gram of Florisil (Grf3F). In general, the surface
area of the support material decreases by increasing the
quantity of the active component until the monolayer
coverage of the impregnated component is completed [32].
The VOx/Florisil catalysts prepared by grafting and the
impregnated catalyst with low V loading (Imp1F) do not
differ significantly from the plain Florisil support in their
XRD and FT-IR data. Nevertheless, in the diffuse reflec-
tance UV–Vis spectra all catalysts exhibit characteristic
LMCT transitions as broad absorption bands below
*550 nm for which the maximum of absorption shifts to
higher wavelengths with increasing vanadium loadings (see
ESI). The corresponding edge energy values have been
calculated (Table 1) indicating isolated VO4 species
123

Preparation, Characterization and Catalytic Activity of MgO/SiO2
99
[33–35]. By increasing the V loading from Imp2F, Imp3F
to Imp4F structural changes are evident which suggest the
presence of MgO based on XRD and IR data. The latter
may be derived from magnesium oxalate formed during
impregnation with aqueous (NH4)2[VO(C2O4)2] upon cal-
cination. For the catalysts with the highest vanadium
loadings (Imp4F, Imp5F) a multiphasic composition may
be derived from the XRD, IR and UV–Vis data indicative
for polymerized vanadium oxide species (VO5/VO6) such
as vanadium pentoxide, meta-MgV2O6 and pyro-Mg2V2O7.
Moreover, for the whole concentration range elemental
composition and distribution have been examined using
SEM, TEM, and EDX (Table 1) which confirm the even
distribution of V as isolated VO4 species for low loadings,
increasing presence of MgO for intermediate loadings
obtained by impregnation and a multiphasic mixture for the
highest loadings (cf. ESI).
In summary these results show that the impregnation
process also significantly affects the composition of the
support material at and near the surface. During impreg-
nation at intermediate vanadium concentrations Mg(OH)2
is formed in addition to magnesium oxalate which incor-
porates VOx in the bulk of the catalysts, resulting in lower
than expected vanadium surface concentrations. At high
vanadium loadings only magnesium oxalate is formed
during impregnation, resulting in significant amounts of
pyro-Mg2V2O7, meta-MgV2O6 as well as V2O5. Similar
results have also been observed in MoOx/MgO catalysts
[32, 36].
3.2 Catalytic Activity
Measurements of the catalytic activity reveal cyclohexyl
hydroperoxide (Cy–OOH) as the primary oxidation product
of cyclohexane besides small amounts of cyclohexanol (Cy–
OH) and cyclohexanone (Cy=O) resulting from subsequent
reaction steps. The effect of various parameters such as
vanadium loading, vanadium distribution, catalyst amount,
hydrogen peroxide concentration and reaction time were
investigated to achieve optimum reaction conditions.
At the same total vanadium content (11 lmol) per
reaction batch, the influence of the nature of different
(VOx/Florisil) catalysts on conversion and selectivity to the
primary oxidation product cyclohexyl hydroperoxide has
been studied (Table 2). The VOx/Florisil catalysts exhibit
quite high conversions up to 78 % and selectivities to
cyclohexyl hydroperoxide up to 90 %. Interestingly, the
highest selectivities are obtained for the lowest conversion
rates which indicates that the same factors that favor the
initial oxidation also increase subsequent reactions of the
resulting hydroperoxide like dehydration to (Cy=O) or
O–O cleavage to (Cy–OH). This interpretation is in good
agreement with the fact that the selectivity to cyclohexyl
123

100
E. F. Aboelfetoh, R. Pietschnig
Table 2 Oxidation of cyclohexane using VOx/Florisil catalysts at equal V conc. (11 lmol)
Catalysts
% Conv.
TON
% SCy–OH
% SCy=O
% SCy–OOH
% (OS)
% By-products
Ungrf1F
Imp1F
Grf1F
Imp2F
Imp3F
Imp4F
Grf2F
Grf3F
Imp5F
58.40
58.59
56.94
74.51
75.13
74.04
75.14
77.50
77.59
1,442
1,446
1,385
1,785
1,798
1,736
1,677
1,671
1,650
00.00
04.26
09.55
09.64
04.47
10.02
09.60
15.25
19.81
08.46
10.46
16.17
18.55
19.60
22.80
18.20
18.69
21.06
90.00
82.50
71.22
64.32
67.35
60.38
61.36
52.22
43.92
98.46
98.40
96.94
95.51
95.42
93.50
88.65
85.96
84.79
00.90
00.93
01.74
03.34
03.44
04.81
08.52
10.88
11.80
Reaction conditions: cyclohexane (1.06 M, 27.56 mmol), H2O2 (0.40 M, 10.5 mmol), PCA (1.70 9 10-3 M, 0.044 mmol), CH3CN (20 ml),
60 °C, 24 h. %; By-products: 1,4-cyclohexanedione, 1,3-cyclohexanediol, 4-hydroxy-cyclohexanone, 2-hydroxy-cyclohexanone, 2,4-dihydroxy-
cyclohexanone
hydroperoxide decreases with increasing vanadium con-
centration within the catalyst (Tables 1, 2). From a struc-
tural point of view these findings suggest that the
polymerized species of vanadium oxide present in catalysts
with high vanadium loadings accelerate the oxidation of
cyclohexyl hydroperoxide to unwanted by-products.
Although the catalysts with the lowest vanadium concen-
trations (Ungrf1F, Imp1F, Grf1F) show the lowest con-
version and TON values, there is no clear correlation
between vanadium concentration and conversion
(*75–78 %) or TON (*1.4–1.8 k) for the catalysts with
higher V loadings (0.7–3.5 mmol/g).
Under the reaction conditions described in Table 2, only
slight leaching of vanadium (ca. 2–4 %, confirmed by UV
analysis) is detected for impregnated catalysts (Imp1F,
Imp2F, Imp3F, Imp4F) and for grafted catalysts with low
vanadium loadings (Ungrf1F, Grf1F). The impregnated
catalyst with the highest vanadium loading (Imp5F) shows
Fig. 1 Variation of cyclohexane conversion and selectivity to
cyclohexyl hydroperoxide using different VOx/Florisil catalysts with
the same surface area (2.05 m2)
significant leaching (ca. 15 %). This partial leaching likely
is a consequence of the presence of V2O5 in this catalyst as
confirmed by IR, SEM, TEM and DRS. Furthermore, the
grafted catalysts such as Grf2F and Grf3F show a relatively
high leaching ([50 %). Again this can be attributed to the
presence of polymerized VO4 or VO2(OH)2 species, which
coordinate weakly to silica. Nevertheless, all investigated
Florisil based catalysts especially those prepared by
impregnation show significantly improved leaching
behavior compared to SiO2 supported VOx for which
complete leaching of vanadium has been reported [24]. In
addition, the stability of the VOx/Florisil catalysts upon
exposure to air (3 h) has been explored by DRS (Table 1
and ESI) which confirms that hydration is not an issue for
the catalysts with low V loadings (up to Imp1F and
Imp2F). This is in marked contrast to their SiO2 supported
congeners [24] which may be explained by an increase of
the surface pH at the point of zero charge owing to the
presence of MgO in Florisil.
Since no simple correlation between the total vanadium
concentration in the catalyst and its catalytic activity could
be established, the catalytic performance with respect to
the vanadium distribution on the surface has been explored.
For this purpose the activity of impregnated or grafted
VOx/Florisil catalysts with the same surface area of cata-
lysts (2.05 m2) per batch have been compared. The influ-
ence of the vanadium distribution (cf. Table 1) on the
catalytic performance is illustrated in Figs. 1 and 2. The
conversion of cyclohexane roughly increases with
increasing V loadings for impregnated or grafted catalysts.
However, the TON and the selectivity to Cy–OOH of these
123

Preparation, Characterization and Catalytic Activity of MgO/SiO2
101
Fig. 3 Variation of cyclohexane conversion and cyclohexyl hydro-
peroxide selectivity with different reaction temperature, using
Ungrf1F catalyst (11lmol V)
Fig. 2 Variation of TON using different VOx/Florisil catalysts with
the same surface area (2.05 m2)
catalysts decrease with increasing vanadium loadings. This
observation suggests that the highly polymerized species of
vanadium oxide in catalysts containing high loadings such
as Grf3F, Imp3F and Imp4F catalysts accelerate the over-
oxidation of Cy–OOH to unwanted by-products such as
1,4-cyclohexanedione, 1,3-cyclohexanediol, 4-hydroxy-
cyclohexanone, 2-hydroxy-cyclohexanone and 2,4-dihy-
droxy-cyclohexanone, which could be identified by mass
spectrometry.
Best selectivities for the monooxidation of cyclohexane
have been obtained with the VOx/Florisil catalysts Un-
grf1F, Grf1F and Imp1F providing low vanadium loadings
(0.2 mmol/g). Therefore conversion and selectivity for
these catalysts as a function of different amounts of catalyst
were studied (see ESI for details). By increasing the
amounts of Ugrf1F, Grf1F and Imp1F catalysts, the con-
version of cyclohexane generally increases. As the catalyst
amount increases further, a considerable decrease in the
conversion of cyclohexane is observed but no decrease of
the selectivity for Cy-OOH. Moreover, the TON values
decrease as well. This behavior may be explained by the
presence of PCA free V5? (‘‘uncomplexed’’ V5?) which
decomposes H2O2 to H2O and O2 leading to low conver-
sions. Interestingly, these three catalysts are also among
those with the lowest leaching rates.
Fig. 4 Variation of cyclohexane conversion and selectivity to
cyclohexyl hydroperoxide with reaction time, using Grf1F catalyst
(0.77 lmol V)
hydroperoxide to cyclohexanol, cyclohexanone and
unwanted by-products, which decreases the selectivity to
the primary product cyclohexyl hydroperoxide signifi-
cantly. Besides temperature also the reaction time is a
straight forward parameter to modify the catalytic perfor-
mance. The influence of the reaction time was explored for
the catalyst Grf1F which shows especially low leaching
rates. The catalytic activity in the oxidation of cyclohexane
has been determined as a function of reaction time (Fig. 4).
According to these measurements conversion and TON
increase with the reaction time and reach a maximum after
48 h. By contrast the selectivity to Cy–OOH slightly
decreases with time, however on a high level. Increasing
As described before catalyst Ungrf1F showed the
highest conversion and selectivity rates. Therefore it was
chosen to explore the effect of the reaction temperature on
the cyclohexane conversion as shown in Fig. 3. It can be
seen that cyclohexane conversion increases with the reac-
tion temperature. At higher temperatures (80 and 100 °C)
decomposition of H2O2 to H2O and O2 may occur, which
leads to a decrease of conversion. Moreover, these higher
temperatures also accelerate the conversion of cyclohexyl
123

102
E. F. Aboelfetoh, R. Pietschnig
the reaction time beyond 48 h also slightly decreases
conversion, TON and total selectivity of the target pro-
ducts. This decrease may be a consequence of blocking
active sites or pores on the catalyst surface by adsorption of
reaction products or by-products at long reaction times.
Similarly, also PCA has a significant influence on the
conversion of cyclohexane. By increasing PCA concen-
trations the conversion of cyclohexane increases. A further
increase of PCA slightly decreases conversion, selectivity
to cyclohexyl hydroperoxide and the total selectivity to the
target products (for details see ESI). Also here blocking of
active sites or pores of the catalyst at high PCA concen-
trations may occur, which decreases the activity of the
catalyst.
improves the stability compared to silica (VOx/SiO2) sup-
ported catalysts. In summary, Florisil supported VOx cat-
alysts combine the high activity of SiO2 and the improved
leaching behavior of the MgO supported VOx catalysts.
Further improvement may be possible with other basic
oxide modified support materials.
Acknowledgments The authors would like to thank Prof. Werner
Piller (University of Graz) for access to the EDX and SEM facilities.
References
1. Khodakov A, Olthof B, Bell AT, Iglesia E (1999) J Catal 181:205
2. Deo G, Wachs IE (1994) J Catal 146:323
3. Gao X, Fierro JLG, Wachs IE (1999) Langmuir 15:3169
4. Haber J, Nowak P, Serwicka EM, Wachs IE (2000) Bull Pol Acad
Sci 48:337
3.3 Conclusions
Structural characterization of VOx/Florisil (15MgO/
85SiO2) catalysts shows that the presence of basic MgO
species significantly modifies the molecular structure of the
surface vanadium oxide species and changes their structure
from VO5/VO6 polymers to less polymerized species such
as poly-VO4 and/or isolated VO2(OH)2 species as com-
pared to their SiO2 supported congeners. According to
XRD, IR, SEM, EDX and TEM measurements, VOx/
Florisil catalysts either grafted (calcined or non-calcined)
or impregnated (Imp1F) with low vanadium loading do not
contain any bulk phase other than those characteristic for
the Florisil support. The other impregnated catalysts
(Imp2F, Imp3F and Imp4F) contain fine particles of MgO,
while the catalyst (Imp5F) with high vanadium loading
contains three different phases namely pyro-Mg2V2O7,
meta-MgV2O6 and V2O5.
5. Wong GS, Kragten DD, Vohs JM (2001) J Phys Chem B
105(7):1366
6. Banares MA (1999) Catal Today 51:319
7. Martra G, Arena F, Coluccia S, Frusteri F, Parmaliana A (2000)
Catal Today 63:197
8. Arena F, Frusteri F, Parmaliana A (1999) Catal Lett 60:59
9. Le Bars J, Auroux A, Forissier M, Vedrine JC (1996) J Catal
162:250
10. Teramura K, Tanaka T, Yamamoto T, Funabiki T (2001) J Mol
Catal A 165(1–2):299
11. Dapurkar SE, Sakthivel A, Selvam P (2004) J Mol Catal A
223(1–2):241
12. Wolf EE (ed) (1992) Methane Conversion by Oxidative Pro-
cesses. Van Nostrand Reinhold, New York
13. Derouane EG, Haber J, Lemos F, Ribeiro FR, Guisnet M (eds)
(1997) Catalytic activation and functionalization of light alkanes:
advances and challenges. Kluwer Academic Publishers,
Dordrecht
14. Periana RA (2001) Chem Eng News 79:287
15. Lunsford JH (2000) Catal Today 63:165
Our results show that the catalytic activity remarkably
depends on the molecular structure of vanadium oxide
species. High conversions at 70 % with an overall selec-
tivity to the target products above 90 % are observed for
catalysts (Ungrf1F, Grf1F, and Imp1F) containing isolated
VO4 or highly dispersed ortho-Mg3V2O8. By contrast,
catalysts containing polymerized species of vanadium
oxide (Grf3F, Grf3F and Imp5F) gave high conversions but
low overall selectivity. This suggests that the polymerized
vanadium oxide species (VO4, VO5/VO6) beside their high
activities for the cyclohexane oxidation also accelerate the
over-oxidation of Cy–OOH to unwanted by-products, thus
reducing the overall selectivity to the target products. Also
high concentrations of H2O2 should be avoided especially
at high conversions, owing to increased over-oxidation of
the primary cyclohexyl hydroperoxide to unwanted by-
products. Higher reaction temperatures increase the reac-
tion rate but also accelerate the decomposition of H2O2. In
terms of leaching, the presence of 15 % MgO besides SiO2
on the surface of VOx/Florisil catalysts significantly
16. Weckhuysen BM, Keller DE (2003) Catal Today 78:25
17. Gao X, Wachs IE (2000) J Phys Chem B 104:1261
18. Vuurman MA, Wachs IE (1992) J Phys Chem B 96:5008
19. Van Der Voort P, White MG, Mitchell MB, Verberckmoes AA,
Vansant EF (1997) Spectrochim Acta A 53:2181
20. Keller DE, Visser T, Soulimani F, Koningsberger DC, We-
ckhuysen BM (2007) Vibrat Spectr 43(1):140
21. Olthof B, Khodakov A, Bell AT, Iglesia E (2000) J Phys Chem B
104:1516
22. Bond GC, Flamerz-Tahir S (1991) Appl Catal 71:1
23. Ruitenbeek M, van Dillen AJ, de Groot FMF, Wachs IE, Geus
JW, Koningsberger DC (2000) Top Catal 10:241
24. Aboelfetoh EF, Pietschnig R (2009) Catal Lett 127(1):83
25. Keller DE, Koningsberger DC, Weckhuysen BM (2006) J Phys
Chem B 110(29):14313
26. Keller DE, de Groot FMF, Koningsberger DC, Weckhuysen BM
(2005) J Phys Chem B 109(20):10223
27. Wilson P, Rao PM, Viswanath RP (2003) Thermochim Acta
399:109
28. Shul’pin GB, Mishra GS, Shul’pina LS, Strelkova TV, Pombeiro
AJL (2007) Catal Commun 8:1516
29. Kozlov YN, Romakh VB, Kitaygorodskiy A, Buglyo’ P, Su¨ss-
Fink G, Shul’pin GB (2007) J Phys Chem A 111:7736
123

Preparation, Characterization and Catalytic Activity of MgO/SiO2
103
30. Aboelfetoh EF, Fechtelkord M, Pietschnig R (2010) J Mol Cat A
318:51
31. Jonson B, Rebenstorf B, Larsson R, Andersson SLT (1988) J
Chem Soc Faraday Trans 84:1897
34. Mohapatra SK, Selvam P (2004) Catal Lett 93:47
35. Mathieu M, Van Der Voort P, Weckhuysen BM, Rao RR, Catana
G, Schoonheydt RA, Vansant EF (2001) J Phys Chem B
105:3393
32. Chang S-C, Leugers MA, Bare SR (1992) J Phys Chem 96:10358
33. Gao X, Bare SR, Weckhuysen BM, Wachs IE (1998) J Phys
Chem B 102:10842
36. Blasco T, Lopez Nieto JM (1997) Appl Catal A 157:117
123