J.H. Hong et al. / Applied Catalysis A: General 396 (2011) 194–200
195
2
. Materials and methods
adsorption–desorption isotherms using an Autopore IV 9500 Series
from Micrometrics. Temperature programmed desorption (TPD)
was performed on AutoChem II 2920 Chemisorptions Analyzer to
get the acidic and basic sites of catalysts. The particle size and the
morphology of the catalysts were examined using a scanning elec-
tron microscope (SEM, JEOL JSM-840A).
2.1. Materials
ML, ethyl lactate (EL) and lactic acid (LA) supplied by Aldrich
were used as received in the catalytic reactions. Acetaldehyde,
methyl acrylate, methoxy methyl propionate, acetone, propionic
acid, hydroquinone, and AA were obtained from Aldrich for gas
chromatographic analysis reference materials. Sodium pyrophos-
2.4. Experimental procedure
phate (Na P O7), sodium phosphate (Na PO ·12H O) and CaCl
4
2
3
4
2
2
The dehydration reactions of ML were conducted in a Pyrex glass
(
hydrous) were supplied by Daejung Chemicals and Metals. De-
tubular down flow micro-reactor of 35 cm in length and 8 mm inner
diameter (Scheme 1). Catalyst (6 ml = 4 g) of 0.45–0.85 mm diame-
ter size was charged on the glass wool packed bed in the middle
section of the reactor. Liquid feed, aqueous 50 wt% ml solution,
was injected into the top of the reactor by a syringe pump along
with N2 carrier gas flow at the desired reaction temperature. Before
ionized water (DW) was used in all the catalyst preparations.
2
2
.2. Methods
.2.1. Preparation of Ca (PO ) –Ca (P O7) composite catalysts
3
4
2
2
2
◦
A series of Ca (PO ) –Ca (P O7) composite catalysts with dif-
the reactant feed, the catalyst bed was pre-heated at 400 C in the
3
4
2
2
2
ferent weight ratios, which varied in the range 30–80 wt%, were
prepared by slurry mixing of two components in water and heat
treatment of the isolated Ca (PO ) –Ca (P O7) powder mixture.
reactor with N2 flow and then maintained at the test temperature
for 1 h. The reaction products of ML dehydration were condensed
in the cold trap after 24 h continuous reaction and then analyzed
using GC (HP5890) equipped with DB-WAX capillary column and
an FID detector. Also, EL and LA conversions were performed to fig-
ure out the reactant dependency of the composite catalyst system
for getting AA.
3
4
2
2
2
The slurries of the two components, Ca (PO ) and Ca (P O7) were
3
4
2
2
2
prepared with CaCl ·2H O solution via precipitation from the aque-
2
2
ous solutions of Na P O7 and Na PO ·12H O, respectively.
4
2
3
4
2
(
a) Ca (PO ) slurry: Ca (PO ) slurry was prepared by mixing a
3 4 2 3 4 2
solution of 38.01 g of Na PO ·12H O (0.1 mol, 98%, in 250 ml
3. Results and discussion
3
4
2
DW) and a solution of CaCl ·2H O, 23.52 g (0.16 mol, in 100 ml
2
2
DW) by slow addition of 7 ml/min of CaCl ·2H O solution with
3.1. Physical characteristics of Ca3(PO4)2 and Ca2(P2O7)
2
2
◦
continuous stirring at 60 C for 1 h to result in a white powdery
slurry.
It is known that Ca3(PO4)2 upon ignition changes to crystalline
␣- or -tricalcium phosphates (TCP) and no P2O5 component as
acid phosphate are observed [20]. This means that Ca3(PO4)2 is very
stable against heat stress. When a hydrated form of Ca3(PO4)2 is
(
b) Ca (P O7) slurry: Ca (P O7) slurry was prepared by mixing a
2 2 2 2
solution of 32.44 g of Na P O7 (0.122 mol, prepared in 250 ml
4
2
◦
of DW heating at 50 C) and a solution of 39.46 g CaCl ·2H O
2
2
◦
(
0.268 mol in 100 ml DW) by slow addition of 7 ml/min of
calcined at 570 C for 5 h, it changed to partially dehydrated state
CaCl ·2H O solution with continuous stirring at room temper-
without losing its chemical identity. However, it is also reported
that Ca3(PO4)2 at 500 C, exists in its partially hydrated form and
2
2
◦
ature for 1 h to get a white powdery slurry.
(
c) Slurry mixing, isolation and heat treatment: the Ca (PO )
transforms slowly into Ca2(P2O7) by heat treatment. The effect
of heat treatment on the physicochemical property changes of
Ca3(PO4)2–Ca2(P2O7) mixed phases is not known.
3
4 2
slurry (a) and the Ca (P O7) slurry (b) were filtered and
2
2
each dispersed in 350 ml of DW twice and filtered again to
get Ca (PO ) and Ca (P O7) cakes, respectively. Determined
3
4
2
2
2
ratios of the two cakes were dispersed in 500 ml of DW
and physically mixed by continuous stirring at room tem-
3.2. Physical properties of the composite catalysts
perature for 1 h. The mixed slurry was filtered and dried at
It is seen from the results presented in Table 1 that Ca (PO )
4 2
3
◦
8
0 C in air circulating oven for 6 h and then the resulting
and Ca (P O7) have low specific surface areas of 37.4 and 2.4
2
2
2
white powder was pressed at 100 kgf/Cm to get tablets. The
2
m /g, respectively, with the average pore size range 26–45 nm,
indicating the highly porous nature of the materials. The specific
surface areas of Ca (PO ) –Ca (P O7) composite catalysts lie in
particles of Ca (PO ) –Ca (P O7) of 0.45–0.85 mm diameter
3
4
2
2
2
sizes were collected by sieving the crushed tablets. These
Ca (PO ) –Ca (P O7) composite particles were calcined at
3
4
2
2
2
3
4
2
2
2
between the two pure components. Similarly, the pore volume of
Ca (PO ) –Ca (P O7) composite catalysts decreased with increase
◦
00 C in air for 6 h to result in the composite catalysts.
5
3
4
2
2
2
3
in Ca (P O7) contents (0.32–0.03 cm /g), which suggests that the
2
2
2.3. Catalysts characterization
composite material is mainly a physical mixture of Ca (PO ) and
3 4 2
Ca (P O7) components in the bulk. However, the total amounts of
2
2
X-ray diffraction properties were measured using a Brucker
D8 ADVANCE X-ray diffractometer with Cu K␣ monochroma-
acidity and basicity of the composite materials show different devi-
ation from the linear type trend of other physical property changes
with the composition ratios. There exists relatively low acidity and
basicity with 50:50 wt% Ca (PO ) –Ca (P O7) composite material,
tized radiation source, operated at 40 kV, 30 mA and a scanning
◦
speed of 5 /min. BET surface area was determined from N2
3
4
2
2
2
Table 1
◦
Physicochemical properties of Ca3(PO4)2–Ca2(P2O7) composite catalysts calcined at 500 C.
2
3
Catalyst composition (wt%)
Surface area (m /g)
Pore volume (cm /g)
Pore size (nm)
Acidity (mmol NH3/g)
Basicity (mmol CO2/g)
1
2
3
4
5
) Ca3(PO4)2
37.4
18.8
12.4
2.5
0.32
0.14
0.08
0.03
0.03
34
30
26
40
44
0.30
0.32
0.13
0.18
0.16
0.18
0.16
0.10
0.19
0.22
) Ca3(PO4)2–Ca2(P2O7) [70:30]
) Ca3(PO4)2–Ca2(P2O7) [50:50]
) Ca3(PO4)2–Ca2(P2O7) [20:80]
) Ca2(P2O7)
2.4