I. Krivtsov et al. / Applied Catalysis A: General 477 (2014) 26–33
27
thin-film preparation, whereas Park et al. [24] found some advan-
tages of peroxo-mediated procedure for ZrO2 films synthesis, as
such as the homogeneous distribution on the substrate and high
density of the film. High degree of homogeneity of the complex
oxide is also found to be one of the advantages of peroxo-routes,
as it allows lowering the temperatures of heat treatment while
obtaining the desirable phase [25,26].
mixture formed by 20 mL of H O2 and 20 mmol of anhydrous citric
2
acid was added to the precipitates, the obtained suspensions were
heated at 373 K under stirring (200 rpm) for 15 min. The precip-
itate with Mg/Zr = 3 was totally dissolved and clear solution was
obtained, whereas in other cases the precipitates dissolution was
incomplete and the solution was isolated from the solid phase by
centrifugation at 3000 rpm. The final stage of the synthesis was
accomplished via Pechini-type sol–gel process. Water from the
solutions was evaporated on a water bath that caused gelation,
then gels were dried in an oven at 333 K for 24 h, powdered in an
agate mortar, heated at 473 K for 3 h (for decomposition of the com-
plexes), and finally treated at 873 K for 4 h in air to obtain mixed
oxides with Mg/Zr molar ratios equaling 1.0, 2.0 and 3.0 (further
designated as 1MZ, 2MZ and 3MZ). Solid water-soluble peroxo-
complexes were also subjected to investigation. First, they were
isolated from water solutions by precipitation with ethanol. The
precipitates were washed with 25 mL of ethanol for 6 times and
dried at 333 K for 24 h prior to investigation.
A deep knowledge about the characteristics of mixed oxides of
magnesia and zirconia is of key importance in basic heterogeneous
catalysis. Mixed MgO/ZrO has shown high catalytic activity in fur-
2
fural aldol condensation with acetone [27] and, as it has recently
been reported, in gaseous acetone self-condensation [28]. Acetone
self-condensation is a highly important industrial and scientifi-
cally interesting reaction, since emerging chemical and biological
processes have become the acetone into a bio-based platform
molecule. Upgrading of acetone relies on the formation of new
C–C bonds, yielding more complex molecules, being mesityl oxide,
isophorones, and mesitylene the most interesting products [29].
Despite the great interest of this reaction, the reasons for selec-
tivity towards certain products are still unclear to the researchers,
since many properties of the catalysts have influence on the way of
the reaction. It is likely that not only the distribution of acid-basic
sites determines the activity and selectivity of the catalyst, but also
the degree of molecular mixing in the oxide may play a significant
role, as it has been pointed out by Sádaba et al. [30].
The 4MZ sample was prepared by co-precipitation, according to
the procedure described in a previous work [27].
2.3. Catalyst characterization
X-ray diffraction patterns were registered from the powdered
samples by a Phillips X’Pert diffractometer, operating at Cu K␣
◦
A new procedure for the preparation of MgO/ZrO2 mixed
oxides (using water-soluble peroxocomplex as precursors) is
reported, describing their structural, textural and physicochemi-
cal properties, and correlating these properties with the catalytic
performance in the gas phase acetone self-condensation.
line (ꢀ = 0.154 nm) in the range of 2ꢁ between 20 and 70 . Ther-
moanalytical investigation of the complex was carried out using
simultaneous TG/DSC thermal analyzer Netzsch 449F1. The pow-
dered sample was placed in the platinum crucible and heated in air
from room temperature to 1273 K with a heating rate of 5 K/min.
FTIR spectra were registered by a Bruker Tensor 27 spectrometer
−
1
in the range of 400–4000 cm from the pellets of the complex and
2
. Experimental
−
1
mixed oxides powdered with KBr with a resolution of 4 cm . The
textural characterization was carried out by nitrogen physisorption
at 77 K in the Micromeritics ASAP 2020 surface area and porosity
analyzer. Before analysis, the samples were outgassed at vacuum
conditions (<10 kPa) at 473 K for 4 h. Surface area was obtained
from the BET method, whereas the pore size distribution and total
pore volume were obtained from the BJH approach. The strength
and distribution of the basic/acid sites were determined by tem-
perature programmed desorption of preadsorbed CO2 and NH3,
respectively, in a MicromeriticsTPD/TPR 2900 apparatus. Samples
2.1. Chemicals
Mixed magnesia–zirconia oxides were prepared using mag-
−
3
nesium sulfate (MgSO ) obtained from Prolabo (98% pure) and
4
zirconium oxychloride aqueous solution (ZrOCl ), containing
2
1
9–21 wt% of ZrO , purchased from MEL Chemicals as MgO and
2
ZrO2 sources, respectively. Hydrogen peroxide 30 wt% water solu-
tion, anhydrous citric acid (97% pure) and acetone (99.8% pure)
were supplied by Aldrich; whereas sodium hydroxide (99%pure)
was supplied by Prolabo. All chemicals were used as received with-
out further purification. As zirconium salts usually form polymeric
hydroxo-species in aqueous solution, leading to precipitation and
changing of concentration, the content of zirconium was deter-
mined gravimetrically prior to its use for synthesis.
(10 mg) were pretreated in He at 723 K for 2.5 h and exposed to CO2
or NH3 (2.5% NH3 in He) stream at 298 K temperature until satu-
ration coverage was reached. Weakly adsorbed CO2 or NH3 was
removed by flushing with He at the same temperature for about
1
.3 h. The temperature was then increased at a linear rate of 5 K/min
from 298 K to 723 K and the CO2 or NH3 desorption was monitored
by mass spectrometry. An Oxford Instrument EDS attached to a Jeol
6460LV scanning electron microscope was used for determining
the elemental composition of the oxides. The surface composi-
tion of the mixed oxides was measured by X-ray Photoelectron
Spectroscopy (XPS), using a SPECS system equipped with a Hemi-
spherical Phoibos detector operating in a constant pass energy,
using Mg-K␣ radiation (h·ꢂ = 1253.6 eV).
2.2. Synthesis of MgO/ZrO2 mixed oxides
A
new peroxocomplex-mediated route to prepare mixed
magnesia–zirconia oxides was accomplished in several stages. Sev-
eral steps are coincident with a procedure described earlier for
the preparation of SiO /TiO2 mixed oxides [31]. In the first stage,
2
the conventional co-precipitation procedure described by Aramen-
dia et al. [11] was applied. Initially, the starting aqueous solutions
of MgSO4 (0.2, 0.4 and 0.6 M concentrations were used in order
to achieve the desired Mg/Zr ratio) and 50-mL of ZrOCl2 (0.2 M)
were mixed and precipitated while stirring with 1.5 M NaOH solu-
tion at pH value equal to 10.7. The rate of stirring of a magnetic
stirrer varied from 200 rpm in the beginning of the precipitation
to 600 rpm when the suspension became viscous. The prepared
precipitates were centrifuged at 3000 rpm and washed 8 times
with deionized water to remove sodium sulfates and chlorides.
The peroxocomplexes were prepared in the following way: the
2.4. Acetone self-condensation
A 150 mg sample of each catalyst was placed in a 0.4 cm i.d. U-
shaped quartz tube located in a PID- controlled furnace. This reactor
was connected to the reaction initial gas flow. The acetone was
injected by a syringe pump as a liquid in a He flow (0.05 L/min) and
vaporized in situ, obtaining a volume concentration of 3.2%. Out-
going gases from the reactor were analyzed online by a Shimadzu
GC-2010 gas chromatograph with a FID detector. Before any batch,
the catalyst was pretreated in He at 723 K for 1 h. The acetone gas