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X. Li et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 1–6
improving the economy of the biodiesel production. Among the
technologies for glycerol conversion, hydrogenolysis of glycerol to
value-added chemicals, such as 1,2-propanediol (1,2-PDO), is one
of the most promising processes due to the environmental friendly
nature [32,34]. In this work, we study the hydrogenolysis of glyc-
erol to 1,2-PDO over Zn-doped NiAl catalysts, and mainly discuss
the role of Zn. The ZnNi alloy was formed via Zn doping into Ni
lattice, and it could change the electronic environment of adja-
cent Ni atoms, suppressed the activity of C C bonds cleavage, and
then altered the hydrogenolysis pathway. The doping of Zn could
greatly enhance both the reaction rate and selectivity to 1,2-PDO. It
is expected that the findings about the correlation between dopant
and performances described in this work will provide useful knowl-
edge for the design of effective Ni-based hydrogenation catalysts.
After cooling to room temperature, the sample cell was heated at
a ramping rate of 10 ◦C/min to 800 ◦C in the atmosphere of H2/N2
mixture gas. The H2 uptake was quantified by the CuO as reference.
For the H2-TPD test, 100 mg of the sample was pre-reduced in pure
H2 at 600 ◦C for 1 h. After the catalyst cooled to room temperature,
desorption was carried out by heating the sample at a heating
rate of 10 ◦C/min to 850 ◦C under N2 flow. For the NH3-TPD and
CO2-TPD test, 100 mg of the sample was pre-reduced in pure H2
at 600 ◦C for 1 h. After the catalyst cooled to room temperature,
the adsorption was conducted by flushing the sample with pure
NH3 or CO2 for 30 min. Desorption was carried out at 850 ◦C with a
heating rate of 10 ◦C/min under N2 flow for NH3-TPD and He flow
for CO2-TPD.
2.3. Hydrogenolysis of glycerol
2. Experimental
The hydrogenolysis of glycerol was performed in a 50-mL
stainless steel reactor with an inner Teflon coating. If no specific
explanation, all the catalysts used in the reaction were pre-reduced
in pure H2 at 600 ◦C for 2 h. The pre-reduced catalysts without
exposing to air were added to autoclave containing 5 ml 20% mass
fraction glycerol and the autoclave was then flushed with H2 for
more than three times and pressured to 3.0 MPa, and then put in
an oil bath coupled with a magnetic stirrer and thermoelectric cou-
ple. When the reactor was preheated to 230 ◦C, the time counting
began with the start of stirring. The agitation speed was 1200 rpm,
with it the mass and heat transfer limitations could be removed,
which was verified before the reaction. After reaction, the reactor
was cooled to room temperature, and vapor phase was collected by
a gas-bag and analyzed using a gas chromatograph (Shimadzu, 14C)
equipped with an active carbon column and a thermal conductivity
detector (TCD). Liquid phase was centrifuged to remove the catalyst
and analyzed using a gas chromatograph (Shimadzu, 14C) equipped
with a capillary column (Restek Stabilwax 30 m × 0.53 mm × 1 m,
carrier: N2) and a flame ionization detector (FID).
2.1. Catalyst preparation
The ZnNiAl hydrotalcite precursors were prepared by co-
precipitation method. Required amount of Ni(NO3)2·6H2O,
Zn(NO3)2·6H2O and Al(NO3)3·9H2O were dissolved in distilled
water and set the volume to 50 ml. In order to maintain the
hydrotalcite structure, the ratio of trivalent ions to divalent ion
was kept at 0.25, and the total ion concentration of the metal salt
solution is 1.0 mol/L. Then metal salt solution was added to the
base solution in which the concentration of Na2CO3 and NaOH are
0.8 and 1.2 mol/L at the rate of 60 ml per hour by a syringe pump.
Afterward, the pH value was adjusted to 10 with 3 mol/L NaOH
aqueous solution. Then, the suspension was aged at 65 ◦C for 22 h.
After that, the precipitate was washed with deionized water and
dried at 80 ◦C overnight; the catalysts were finally obtained by
calcination of the precipitate in air at 500 ◦C for 2 h.
2.2. Catalyst characterization
X-ray diffraction patterns of all involved catalysts were per-
formed on Bruker D8 ADVANCE diffractometer, using Cu-K␣
radiation (ꢀ = 0.154 nm). Prior to the test, samples were pre-
reduced at 600 ◦C for 2 h. Crystallite sizes of Ni particles were
determined by the most intense peaks at 2ꢁ = 44.5◦ (h k l = 1 1 1)
using the Scherrer equation, i.e. Dc = K ꢀ/ cos ꢁ, where the con-
stant K here is 0.9, ꢀ is the wavelength of the X-ray radiation, ˇ is
the width of the peak at half-maximum, and ꢁ is the Bragg angle.
Temperature-programmed reduction with H2 (H2-TPR),
temperature-programmed desorption with H2 (H2-TPD),
temperature-programmed desorption with NH3 (NH3-TPD)
and temperature-programmed desorption with CO2 (CO2-TPD)
were carried on the TP-5080 (Tianjin Xianquan Industry and Trade
Development Co., Ltd, China), with a thermal conductivity detector.
Before H2-TPR run, 30 mg of the catalyst was loaded in the quartz
tube and pretreated with ultra-pure nitrogen at 150 ◦C for 10 min.
3. Results and discussion
3.1. Preparation and characterization of the ZnNiAl catalysts
Hydrotalcite-like compounds were proven to be excellent pre-
cursors for the preparation of highly dispersed metal catalysts due
to their favorable layered structures [35–37]. Herein, a series of
mono-dispersed ZnNiAl catalysts were synthesized through pre-
cipitation of Zn2+, Ni2+, and Al3+ ions with Na2CO3/NaOH and
followed by thermal decomposition including calcination and
reduction (see the schematic illustration in Fig. 1). Fig. 2 displays
the XRD patterns of the formed precursors with and without Zn
doping. The diffraction peaks can be easily indexed to the (0 0 3),
(0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes of the Zn-Ni-Al
layered double hydroxides. As it can be seen from the XRD patterns,
Fig. 1. Schematic illustration for the formation of ZnNiAl catalysts.