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A. Yin et al. / Applied Catalysis A: General 400 (2011) 39–47
a new and efficient Cu/B2O3/SiO2 system, which exhibited signif-
icantly enhanced performance for the catalytic hydrogenation of
DMO to EG.
H2 consumption was monitored by a TCD detector. The specific
surface area of metallic copper was measured by the adsorption
the pulse titration method as follows: 2Cu(s) + N2O → N2 + Cu2O(s)
at 363 K with N2 as the carrier gas. Pure nitrogen was used to
detect the consumption of N2O [17]. The specific area of metallic
copper was calculated from the total amount of N2O consump-
tion with 1.46 × 1019 copper atoms/m2 [18]. X-ray photoelectron
spectroscopy (XPS) spectra were recorded with a PerkinElmer PHI
5000C ESCA system equipped with a hemispherical electron energy
analyzer. The Mg K␣ (hꢁ = 1253.6 eV) anode was operated at 14 kV
and 20 mA. The carbonaceous C1s line (284.6 eV) was used as the
reference to calibrate the binding energies (BEs). To investigate the
nature of Cu species on the surface of the reduced catalysts, we
also collected XPS spectra of the samples after the hydrogenation
reactions.
2. Experimental
2.1. Catalyst preparation
Mesoporous siliceous HMS was prepared according to a well-
established procedure delineated by Tanev et al. [16] using
tetraethylorthosilicate (TEOS) as silica source and dodecylamine
(DDA) as template.
Boron modified Cu/HMS catalyst was prepared by the ammonia-
evaporation-induced synthetic (EVI) method described as follows:
3.80 g of Cu(NO3)2·3H2O was dissolved in 100 ml of deionized
water; 11 ml of 28% ammonia aqueous solution was added and
stirred for 30 min. Then 4.0 g of HMS support was added to the
copper ammonia complex solution and the mixture was stirred for
another 2 h. The initial pH of the suspension was 11–12. Then a
certain amount of B2O3 aqueous solution (0.05 M) was added in
drops into the above solution. All the above operations were car-
ried out at room temperature. The suspension was then pretreated
at 363 K to allow for the evaporation of ammonia and to decrease
the pH and consequently the deposition of copper species on sil-
ica. When the pH value of the suspension was decreased to 6–7, the
evaporation process was terminated. The residue was washed with
at 393 K overnight. The catalyst precursors were calcined at 723 K
for 4 h, pelletized, crushed, sieved to 40–60 meshes, and denoted as
CuB/HMS(X/Y), where X and Y represent the mole ratio of Cu to B.
The notation of CuB/HMS(2/1) in Table 1 means copper supported
on HMS with the Cu/B mole ratio of 2/1.
For comparison, the impregnated Cu-B/HMS catalyst was syn-
thesized via the EVI method by immersing the powder in B2O3
solution (0.05 M); this material was denoted as CuB/HMS(2/1)-A.
To investigate the influence of the boron source, we also syn-
thesized Cu/HMS catalyst doped with boron, which originated
from KBH4 via the EVI method. The catalyst was denoted as
CuB*/HMS(2/1), which means copper supported on HMS support
with Cu/B mole ratio of 2/1. In addition, the impregnated Cu/HMS
catalyst by immersing in KBH4 aqueous solution (0.05 M) was also
prepared and denoted as CuK/HMS(2/1)-I.
2.3. Catalytic reaction
The catalytic hydrogenation was performed using a fixed-
bed microreactor. Typically, 2.0 g of catalyst (40–60 meshes) was
loaded into a stainless steel tubular reactor with the thermocou-
ple inserted into the catalyst bed for better control of the actual
pretreatment and reaction temperature. Catalyst activation was
performed at 523 K for 4 h with a ramping rate of 2 K min−1 under
5% H2/Ar (v/v) flow. After cooling to the reaction temperature,
15 wt.% DMO (purity >99%) in methanol and H2 were fed into the
reactor at a H2/DMO molar ratio of 100 and a system pressure of
2.5 MPa. The reaction temperature was first set at 473 K and the
room-temperature liquid space velocity (LSV) of DMO was ranged
from 0.10 to 4.30 h−1. The products were condensed, and analyzed
on a gas chromatograph (Finnigan Trace GC ultra) fitted with an
HP-5 capillary column and a flame ionization detector (FID).
3. Results and discussion
3.1. Structural and textural properties
The BET surface area, pore volume and pore diameter of the as-
prepared samples are summarized in Table 1. It is found that the
BET surface areas of the catalysts range from 387 to 480 m2 g−1
and the pore sizes are between 3.52 nm and 3.74 nm upon varying
the Cu/B ratios. The largest BET surface area could be obtained on
the one with the optimized Cu/B ratio. Compared with the catalyst
prepared via EVI method, the BET surface area of the catalyst syn-
thesized via the impregnation method is lower, which could result
from the large particle size of the copper species. Different boron
sources could have great effects on the textual properties of the cat-
alysts. The BET surface area of the catalyst synthesized using KBH4
as boron precursor decreased dramatically, especially the one pre-
pared by the impregnation method, which could be attributed also
to the larger particle size of copper species. The pore volume of the
direct B2O3 doped catalyst is relatively larger compared with the
one using KBH4 as boron precursor, which can be explained by its
copper dispersion and more active copper surface area. Thus the
present results are in good agreement with the dispersion trend of
metallic copper as determined by XPS.
2.2. Catalyst characterization
Nitrogen adsorption–desorption isotherms at 77 K were mea-
sured with a Micromeritics Tristar 3000 instrument; the samples
were outgassed at 423 K before each measurement. The specific
surface areas were calculated following the BET method. Pore
size distributions were calculated by the BJH method according
to the desorption isotherm branch. The small-angle X-ray diffrac-
tion (XRD) patterns below 2ꢀ = 6◦ were recorded on a Rigaku
Multiflex instrument operated at 1.5 kW, using Cu K␣ radiation
˚
(1.5406 A) at 40 kV and 40 mA. The wide-angle XRD experiments
were conducted on a Bruker D8 Advance X-ray diffractometer using
nickel-filtered Cu K␣ radiation with angle (2ꢀ) range of 20–80◦, a
scanning speed of 2◦ min−1, a voltage of 40 kV, and a current of
20 mA. The full width at half maximum (FWHM) of (1 1 1) reflec-
tion of CuO was measured for calculating crystallite sizes by using
the Scherrer equation.
TPR profiles were obtained on a Tianjin XQ TP5080 autoadsorp-
tion apparatus. 50 mg of the calcinated catalyst was outgassed at
473 K under Ar flow for 2 h. After cooling to room temperature
under Ar flow, the in-line gas was switched to 5% H2/Ar, and the
sample was heated to 773 K at a ramping rate of 10 K min−1. The
Fig. 1 shows the typical N2 adsorption–desorption isotherm of
the calcined catalysts. One can see that the mesopores character-
istics of the capillary condensation are present at P/P0 ∼ 0.35–0.70.
The shape of the hysteresis loop of the N2 adsorption–desorption
did not change a lot when changing the Cu/B ratio, which indi-
cated that introduction of boron would not affect the pore shape