G Model
CATTOD-8903; No. of Pages6
ARTICLE IN PRESS
C. García-Sancho et al. / Catalysis Today xxx (2014) xxx–xxx
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reaction due to its large pore diameter, water-tolerance and acidic
properties. The catalytic behavior is compared with that of a
commercial niobium oxide to establish the effect of the textural
properties on the catalytic performance.
curves in order to determinate more accurately the binding energy
of the different element core levels.
Raman spectra were recorded on a Raman Senterra (Bruker)
microspectrometer equipped with a thermoelectrically cooled
charge coupled device (CCD) detector. A Nd:YAG laser was used
as the excitation source at 532 nm and the laser power was set
to 2 mW. Raman spectroscopy was performed on powder samples
without any previous treatment.
2
. Experimental
2.1. Catalyst preparation
2.3. Catalytic reaction
The synthesis of a mesoporous niobium oxide was carried out
by using a neutral templating route, as previously reported by
Lee et al. [37]. Briefly, 7 mmol of NbCl5 were added to a tem-
plate solution formed by 0.22 mmol of P-L121 (triblock copolymer
Catalytic experiments were performed in batch, by using a
glass pressure tube with thread bushing (Ace, 15 mL, pressure
rated to 10 bar) and magnetic stirring. In a typical procedure,
(
C H O·C H O)x) in 10 g of n-propanol. After vigorous stirring
3
6
2
4
1
50 mg of d-xylose (SigmaUltra, >99%), 50 mg of catalyst, deion-
for 5 min, 1.0 mL of a 0.05 M NaCl aqueous solution was added,
and the resulting solution was stirred for 30 min at room tem-
ized water (1.5 mL) and toluene (3.5 mL, Sigma–Aldrich, >99.5%)
were poured into the reactor. Prior the experiments, reactors were
always purged with nitrogen. The reaction mixture was heated
with a thermostatically controlled oil bath and stirred magneti-
cally at 600 rpm. After reaction time, the reaction was quenched
by submerging the reactor in a water bath cooled with ice; the liq-
uid phases were separated, filtered and the analysis of products
was performed in both phases by high performance liquid chro-
matography (HPLC). A JASCO instrument equipped with quaternary
gradient pump (PU-2089), multiwavelength detector (MD-2015),
autosampler (AS-2055), column oven (co-2065) using a PHE-
NOMENEX LUNA C18 reversed-phase column (250 mm × 4.6 mm,
perature. The Nb:P-L121:n-propanol:NaCl:H O molar ratio was
2
◦
3
5:1:835:0.25:280. The solution was aged at 40 C for one week. The
structure-directing agent was removed by calcination in air (5 h)
◦
at two different temperatures: 450 and 550 C. Thus, the catalysts
were labeled as Nb 450 and Nb 550, where the number indicates
the calcinations temperature. Commercial Nb O5, purchased from
2
Aldrich, was used for comparison (Nb C).
2.2. Catalyst characterization
5
(
m) and PHENOMENEX REZEX RHM-Monosaccharide H+(8%)C18
Several characterization techniques were employed to evaluate
300 mm × 7.8 mm, 5 m) was employed. The disappearance of
the physico-chemical properties of the mesoporous niobium oxide.
Elemental analysis was performed on a LECO CHNS-932 micro-
analyzer. Powder X-ray diffraction (XRD) measurements were
performed on a Philips X’Pert PRO MPD automated diffractome-
ter, over a 2Â range with Bragg-Brentano geometry using the Cu K␣
radiation and a graphite monochromator. The scans covered the 2Â
xylose was monitored using a refractive index detector, while
furfural production was monitored using a UV detector. The
−
1
mobile phases consisted in pure methanol (flow rate 0.5 mL min
)
for Luna C18 and 0.005 M H SO aqueous solution (flow rate
2
4
−
1
0
.4 mL min ) for Rezex RHM-Monosaccharide column, being the
column at room temperature and 80 C, respectively.
◦
◦
◦
range from 1 to 70 .
◦
N2 adsorption–desorption isotherms at −196 C of calcined
materials were obtained using an ASAP 2020 model of gas adsorp-
3. Results and discussion
tion analyzer from Micromeritics, Inc. Prior to N adsorption, the
2
◦
−2
samples were evacuated at 200 C and 1 × 10 Pa for 10 h. The
Barrett–Joyner–Halenda method (BJH) was used to determine the
pore size distribution.
3.1. Catalytic characterization
X-ray diffraction was used to corroborate the mesostructured
character of the material obtained after calcination to remove
the structure-directing agent, once the hybrid organo-inorganic
structure has been formed. The XRD pattern in the low angle
region showed the characteristic intense and broad reflection
The morphology of the mesostructured solid was studied by
transmission electron microscopy (TEM), by using a Philips CM 200
Supertwin-DX4 microscope. The sample was dispersed in ethanol,
and a drop of the suspension was put on a Cu grid (300 mesh).
◦
Temperature-programmed desorption of ammonia (NH -TPD)
of mesostructured solids, at around 2Â = 1.5 (Fig. 1), which can
3
was carried out to evaluate the total acidity of the catalyst. The
material was pre-treated under a helium flow at 100 C, and then
be indexed to the d1 0 0 diffraction signal (6.2 nm), as previously
reported by Lee et al. [37]. However, higher order peaks were not
observed, thus pointing out the lack of long-range order of the
hexagonal mesostructure of niobium oxide. The amorphous nature
of the pore walls of the synthesized niobia can be inferred from the
absence of diffraction peaks in the high angle region. The effective
◦
ammonia was adsorbed at the same temperature. The analysis of
◦
the desorbed ammonia was performed up to 500 C, with a heat-
◦
−1
ing rate of 10 C min by using helium as carrier gas. The evolved
ammonia was analyzed using a TCD detector of a gas chromato-
graph (Shimadzu GC-14A).
◦
removing of the organic moieties after calcination at 450 C was
X-ray photoelectron spectroscopy (XPS) studies were per-
confirmed by CHN analysis, which gave percentages of carbon and
hydrogen of 0.07 and 0.38%, respectively.
formed with
a Physical Electronics PHI 5700 spectrometer
equipped with a hemispherical electron analyzer (model 80-365B)
However, the thermal treatment of the mesostructured niobia
at higher temperature (550 C) provoked the destruction of the
mesoporous structure, since the low-angle diffraction peak disap-
peared and, concomitantly, new diffraction peaks of low intensity
appeared in the high angle region, which were associated to the
◦
and a Mg K␣ (1253.6 eV) X-ray source. High-resolution spectra were
◦
recorded at 45 take-off-angle by a concentric hemispherical ana-
lyzer operating in the constant pass energy mode at 29.35 eV, using
a 720 m diameter analysis area. Charge referencing was done
against adventitious carbon (C 1s at 284.8 eV). The pressure in the
orthorhombic phase of Nb O5. In this sense, Braga et al. [38] pre-
2
−
6
◦
analysis chamber was kept lower than 5 × 10 Pa. PHI ACCESS
nals. Recorded spectra were always fitted using Gauss–Lorentz
viously reported that calcination temperatures lower than 450 C
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other hand, as expected, the XRD pattern of commercial niobia
Please cite this article in press as: C. García-Sancho, et al., Mesoporous Nb O5 as solid acid catalyst for dehydration of d-xylose into
2