J. Kim et al. / Journal of Catalysis 373 (2019) 306–313
307
Scheme 1. Enantioselective hydrogenation of ethyl pyruvate to ethyl lactate over cinchonidine-modified Pt/MCF.
2
3
1
000 m /g) and pore volumes (up to 2.4 cm /g) [33,34]. They con-
4
NH F were added to the solution. Also, a higher aging temperature
sist of spherical cell pores in the diameter range of 20–50 nm,
interconnected with window pores of 10–20 nm. This highly open
porous structure of MCFs facilitates the access of reactants to the
active sites of the catalysts inside the pores and leads to high per-
formance in heterogeneous catalytic reactions [35-37]. In this
work, we first report the catalytic performance of cinchonidine-
modified Pt/MCFs for the enantioselective hydrogenation of ethyl
pyruvate (Scheme 1). MCFs with different pore size were synthe-
sized and used as supports for Pt nanoparticles. The influences of
(140 ) was employed for the synthesis of MCF-3.
The Pt nanoparticles supported on MCFs and non-porous SiO
were prepared via a facile wet impregnation method. The supports
were impregnated with an acetone solution of the platinum pre-
cursor (Pt(acac) ) at 40 overnight. After evaporation of the sol-
2
vent, the obtained powder was dried at 80 for 12 h in an oven
and then, calcined at 350 for 3 h in a flow of Ar.
2
2.3. Characterization of MCF supports and supported Pt catalysts
2
the pore size of MCFs, the amount of Pt loading and H pressure
on the performance were comprehensively investigated. The per-
formance of Pt/MCFs was compared to those of Pt supported on
non-porous silica (Pt/SiO ) and commercial Pt/Al O catalysts.
2 2 3
Finally, reusability of Pt/MCF was evaluated by repeatedly per-
forming enantioselective hydrogenation of ethyl pyruvate.
Nitrogen physisorption was performed at 77 K using a Micro-
metrics ASAP 2010 system. The Brunauer-Emmett-Teller (BET) sur-
face areas were calculated using the data obtained over the relative
pressure range of 0.05 ꢀ p/p ꢀ 0.2. The cell and window size dis-
0
tributions were calculated by the Barrett-Joyner-Halenda (BJH)
method from the adsorption and desorption branches of the iso-
therm, respectively. Transmission electron microscopy (TEM)
images were obtained with a JEOL JEM-2100F transmission elec-
tron microscope at the Korea Basic Science Institute (Daegu,
Korea). High-resolution TEM (HRTEM) images were taken on JEOL
JEM-2200FS microscope at National Institute for Nanomaterials
Technology (Pohang, Korea). The amount of Pt loading was deter-
mined by inductively coupled plasma-optical emission spec-
troscopy (ICP-OES) on a Thermo Scientific model iCAP 6300
spectrometer. The X-ray powder diffraction (XRD) patterns were
2
. Experimental
2.1. Chemicals and materials
2
Platinum(II) acetylacetonate (Pt(acac) , 98%) was purchased
from Acros. Tetraethyl orthosilicate (TEOS), Pluronic P123 (EO20
-
PO70EO20, M.W. = 5800), and cinchonidine (96%) were purchased
from Sigma-Aldrich. 1,3,5-Trimethylbenzene (TMB, 98%), ammo-
nium fluoride (98%), and ethyl pyruvate (98%) were supplied by
Alfa Aesar. Hydrochloric acid (35–37%) and acetic acid (99.7%)
collected on a Rigaku D/MAX-2500 diffractometer with Cu-K
a
radiation in the 2h range of 5–90° at a scan rate of 4°/min.
were purchased from Samchun Pure Chemical. For preparation of
The metal dispersion of the Pt catalysts was determined by the
CO pulse chemisorption technique using MircotracBEL BELCAT-Ⅱ.
Prior to the measurement, the sample was pretreated as follows:
Ò
a reference Pt catalyst, commercial non-porous SiO
2
(AEROSIL 380,
Evonik Industries) was used as a support for Pt particles. Commer-
cial 1 wt% and 5 wt% Pt/ -Al catalysts were purchased from Alfa
Aesar and Strem, respectively.
c
2 3
O
4
1
0 mg of the sample was purged under He flow at 400 °C for
5 min, then pretreated with pure O for 15 min. After purging
for 20 min. Subse-
2
with He for 15 min, it was reduced by pure H
2
quently, the sample was purged again with He for 10 min, then
cooled under He flow to 50 °C. For the measurement of the metal
dispersion, 5% CO/He gas mixture was pulse-injected into a stream
of He carrier gas until the chemisorption was saturated. The dis-
persion of Pt and particle sizes were calculated from the amount
of chemisorbed CO on Pt by assuming an adsorption stoichiometry
of CO/Pt = 1.
2
.2. Preparation of MCF supports and supported Pt catalysts
The mesocellular silica foam (MCF) supports were synthesized
by previously reported methods [34,38]. In a typical synthesis,
P123 (4 g) was dissolved in 75 mL of aqueous HCl solution
(
1.6 M). Then, an appropriate amount of TMB was added into the
solution. Following vigorous stirring at 40 for 2 h, TEOS (9.2 mL)
was added to the solution under stirring. The solution was then
aged for 20 h under static conditions and transferred to an auto-
2.4. Enantioselective hydrogenation of ethyl pyruvate
4
clave. If needed, NH F was added to the mixture, which was then
hydrothermally treated at 100 for 24 h. The resulting precipitate
was filtered and washed with deionized water and ethanol. Subse-
quently, the solid product was dried at 100 for 5 h, then calcined
at 600 for 6 h in a muffle furnace.
The experiments were performed in a 50 mL glass reactor for
the reactions at 0.1 MPa and in a stainless steel autoclave equipped
with a 100 mL glass liner for the reactions at 0.5–5 MPa. Before the
catalytic tests, all catalysts were pretreated in a flow of 5% H
400 for 90 min. In a standard procedure for the reactions at
0.1 MPa H pressure, 5 mL of the solvent (acetic acid) and 50 mg
2
/Ar at
The pore size distributions of the MCFs were adjusted by vary-
ing the amount of TMB and NH
perature. Depending on the order of pore sizes of MCFs, MCF-
< MCF-2 < MCF-3, the supports were denoted as MCF-1, MCF-2,
and MCF-3, respectively. For the synthesis of MCF-1, 2.5 g of TMB
and no NH F were added. In the case of the synthesis of MCF-2
and MCF-3, more TMB (4 g and 5 g, respectively) and 46 mg of
4
F and by controlling the aging tem-
2
of the catalyst sample were added to a 50 mL three-neck flask,
which was then capped with septa. After 5 min stirring, 3 mL of
acetic acid containing 2 mg of cinchonidine was injected into the
1
4
flask and it was purged with H
2
for 20 min. Then, 0.07 mL of ethyl
pyruvate dissolved in 2 mL of acetic acid was injected into the