A. Takagaki et al.
AppliedCatalysisA,General570(2019)200–208
Scheme 1. Reaction pathways for lactic acid formation.
Lewis acid sites accelerated both the rates of DHA conversion and LA
formation resulting in LA yield up to 80%.
2.22 cm μmol−1 for Lewis acid sites. [16]
2.4. Catalytic tests
2. Experimental
A quantity of 0.55 mmol (50 mg) of dihydroxyacetone and 50 mg of
silica-supported metal oxide catalysts was put into 3 mL water in a
reactor vessel. The reactor vessel was heated at 130 °C for 3 h in an oil
bath under stirring. After the reaction, reactant mixture was analyzed
by high-performance liquid chromatography (HPLC; LC-2000 plus,
JASCO) equipped with a differential refractive index detector (RI-2031
2.1. Chemicals
Dihydroxyacetone dimer (98%, Wako), DL-glyceraldehyde (97%,
Wako), pyruvaldehyde (40% solution in water, Sigma-Aldrich) and
lactic acid (85–92%, Kanto Chemical) were used for the reactions and
the analysis. A fumed silica, Cab-o-Sil EH5 (Cabot) was used as inert
support. Metal salts were purchased from Wako. Used as precursors
were Al(NO3)3·9H2O, Ti(SO4)2 30% solution in water, Cr(NO3)3·9H2O,
Mn(NO3)2·6H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Ga
(NO3)3·nH2O, In(NO3)3·3H2O, SnCl4, WCl4, and Pb(NO3)2.
plus, JASCO) with Aminex HPX-87H column (flow rate: 0.5 mL min−1
eluent: 10 mM H SO ).
,
2
4
3. Results and discussions
3.1. Screening of silica-supported metal oxide catalysts
2.2. Catalysts preparation
The catalytic activities of a variety of supported metal oxides for
lactic acid formation were surveyed. Twelve metal oxides, Al, Ti, Cr,
Mn, Fe, Ni, Zn, Ga, In, Sn, W, and Pb oxides were chosen for the
screening test because these metal salts were reported to be effective
homogeneous catalysts to produce lactic acid from cellulose in aqueous
The silica-supported metal oxide catalysts were prepared by in-
cipient wetness impregnation and subsequent calcination. Typically, a
quantity of 1 mmol of metal salts dissolved in distilled water was im-
pregnated dropwise on 1 g of a fumed silica (Cab-o-sil, EH5) followed
by calcination in air at 500 °C for 3 h. The catalysts used were named
with their metal loading amount. For example, Cr(1.0)/SiO2 indicates
that 1.0 mmol of chromium was impregnated on 1.0 g of silica support.
media [17]. The amount of loading of metals was set to 1.0 mmol g−1
.
Fig. 1 shows the XRD patterns of silica-supported metal oxide catalysts.
A broad peak around 20° was observed for all samples, which was at-
tributed to amorphous silica. The samples of Cr, Mn, Fe, Ni, In, Sn, and
W oxides had diffraction peaks attributable to corresponding metal
oxides for each species. In contrast, the samples of Al, Ti, Zn, Ga, and Pb
oxides showed no apparent peaks, indicating that these metal species
were highly dispersed on silica or incorporated into the silica network
because these elements are known as additives of glass materials [18].
yacetone using a variety of silica-supported metal oxides. The products
detected were lactic acid (LA), pyruvaldehyde (PA) and glyceraldehyde
(GLA). Among the silica-supported catalysts tested, the chromium oxide
catalyst gave the highest yield of LA of 46% with a high conversion of
dihydroxyacetone of 72% and a selectivity to LA of 64%. The titanium
oxide catalyst gave a moderate yield of LA of 29%, and the gallium
oxide catalyst of 27%. The titanium oxide catalyst also afforded an
intermediate, PA with high selectivity of 43%. The lead oxide catalyst
gave undetected products likely due to polymerization. Other metal
oxides such as Mn, Zn, and W oxides showed high conversion, but no or
little yields of LA. After the reaction using these unselective catalysts,
the solutions were colored dark brown, which is indicative of deep
polymerization. Table 1 summaries the results of crystal structure,
2.3. Characterization
The prepared catalysts were characterized by X-ray diffraction
(XRD; RINT-2700, Rigaku), and diffuse reflectance UV–vis spectroscopy
(DR UV–vis; V-670, JASCO). The types of acid sites were distinguished
using pyridine adsorbed Fourier-transform infrared spectroscopy (FTIR;
FT/IR 6100, JASCO) equipped with a mercury cadmium telluride
(MCT) detector with a resolution of 4 cm−1. The sample was pressed
into disks with a radius of 1.0 cm at 15 MPa and a weight of 30 mg and
then pretreated in the cell at 150 °C in a vacuum for 1 h. The sample
was cooled to 100 °C, and a spectrum was measured. Pyridine gas was
introduced to the cell, and the sample was contacted with pyridine for
30 min at 5 Torr. After pyridine was pumped off for 30 min, a spectrum
of pyridine adsorbed sample was recorded. A difference spectrum was
obtained by subtracting the spectrum of the dehydrated sample from
the spectrum of the adsorbed sample. The amounts of Brønsted acid
sites and Lewis acid sites were determined on the basis of the integral
absorbance of the characteristic bands at 1545 cm−1 for Brønsted acid
sites and 1450 cm−1 for Lewis acid sites by using integrated molar
extinction coefficients, 1.67 cm μmol−1 for Brønsted acid sites and
201