Chemistry Reference
In-Depth Information
Surface areas and pore volumes were measured by N
2
physisorption
at its boiling point using the BET method and Barret-Joyner-Halender
(BJH) calculations, respectively, in an Autosorb Quantochrome 1-C
sorptometer. The crystalline structure properties of MgO-x samples were
determined by X-ray diffraction (XRD) using the instrument described
above. Analysis was carried out using a continuous scan mode at 21/min
over a 2y range of 201-801. Scherrer equation was used to calculate the
mean crystallite size of the samples.
CO
2
adsorption site densities and binding energies were determined
from temperature-programmed desorption (TPD) of CO
2
preadsorbed at
room temperature. MgO-x samples were pretreated in situ in a N
2
flow
at its corresponding stabilization temperature (673, 773 or 873 K), cooled
to room temperature, and then exposed to a mixture of 3% CO
2
/N
2
until
surface saturation was achieved (10 min). Weakly adsorbed CO
2
was
removed by flushing in N
2
during 1 h. Finally, the temperature was
increased to 773 K at 10 K/min. The desorbed CO
2
was converted to
methane by means of a methanation catalyst (Ni/Kieselghur) operating at
673 K and monitored using a flame ionization detector.
The chemical nature of adsorbed surface CO
2
species was determined
by infrared (IR) spectroscopy after CO
2
adsorption at 298 K and
sequential evacuation at increasing temperatures. Experiments were
carried out using an inverted T-shaped cell containing the sample pellet
and fitted with CaF
2
windows. Data were collected in a Shimadzu FTIR
Prestige-21 spectrometer. The absorbance scales were normalized to
20-mg pellets. Each sample was pretreated in vacuum at its corres-
ponding stabilization temperature and cooled to room temperature, after
which the spectrum of the pretreated catalyst was obtained. After
admission of 5 kPa of CO
2
to the cell at room temperature, the samples
were evacuated at increased temperatures, and the resulting spectrum
was recorded at room temperature. Spectra of the adsorbed species were
obtained by subtracting the catalyst spectrum.
2.3 Catalytic testing
2.3.1 Cross-aldol condensation of citral with acetone. The cross-aldol
condensation of citral (Millennium Chemicals, 95% geranial
þ
neral)
with acetone (Merck, p.a.) was carried out at 353 K under autogenous
pressure (
250 kPa) in a batch Parr reactor, using acetone/citral
=
49 (molar ratio) and catalyst/(citral
þ
acetone)
=
1 wt% ratio. The reactor
was assumed to be perfectly mixed and interparticle and intraparticle
diffusional limitations were verified to be negligible. Reaction products
were analyzed by gas chromatography in a Varian Star 3400 CX chro-
matograph equipped with a FID and a Carbowax Amine 30 M capillary
column. Samples of the reaction mixture were extracted every 30 min and
analyzed during the 6-h reaction. The main product of the citral/acetone
reaction was pseudoionone, PS (cis- and trans-isomers). Moreover, di-
acetone alcohol and mesityl oxide were simultaneously produced from
self-condensation of acetone. Selectivities (S
j
, mol of product j/mol of
citral reacted) were calculated as S
j
(%)
=
C
j
100/SC
j
, where C
j
is the
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