6
0
J.Y. Kim et al. / Journal of Catalysis 302 (2013) 58–66
ꢁ1
ꢁ2
Fig. 1 shows OHC products profiles of dodecane partial oxida-
and 10
s , which are not much different from those of usual
tion obtained by gas chromatographic (GC) analysis with a flame
ionization detector (FID) and GC–MS (inset). Small peaks of less
than 12 a.u. between the large aldehyde peaks correspond to ke-
tones and carboxylic acids. These compounds were identified by
mass spectroscopy (Fig. S2 of SI) as listed in Table 1. Aldehydes
were the main OHC products (>93%) with a typical selectivity order
thermal catalytic reactions. The OHCs selectivity data in Table 2
and those obtained in other conditions were plotted in Fig. 2.
Although widely different reaction conditions make the data points
scattered, there is unambiguous trend that the OHCs selectivity de-
creases while CO
of dodecane. Thus, it appears that reaction proceeds via a typical
series reaction; dodecane ? [OHCs] ??? CO
2
selectivity increases with increasing conversion
of C
2
> C
3
> C
4
> C
5
> C
6
> C
1
. Aldehydes with more than 6 carbons
2
.
were also detected by GC–MS as shown in Table 1. The carboxylic
acids and ketones (especially 2-ketones) were also formed. In the
GC–MS analysis, carboxylic acids had relatively longer retention
times because of their large polarity, whereas aldehydes and ke-
tones of the same carbon number showed similar retention times.
2 2
3.3. TiO –SiO mixed oxide photocatalysts
To increase OHCs selectivity and yield in the series reaction of
dodecane partial oxidation, we have to stabilize the intermediate
Only a very small amount of H
further oxidation to H O under this oxidizing reaction condition. In
TCD analysis, CO was found to be the other main product of the
2
(<10 ppm) was also detected due to
OHCs against total oxidation to CO
of active Ti sites could give better selectivity and employed TiO
SiO mixed oxides as photocatalysts. When pure TiO was used
2
. We conjectured that dilution
2
2
–
2
2
2
photocatalytic oxidation of dodecane by total oxidation, but CO
was not detected at all.
as a photocatalyst, all the carbon atoms of dodecane could contact
the surface titanium atoms, so that they are easily oxidized all the
way to CO
SiO by forming small TiO
the high reactivity of TiO could be controlled and OHCs’ selectivity
2
(Scheme 2a). If we dilute the active sites by inactive
3
.2. Effect of reaction variables on photocatalytic partial oxidation of
2
2
nano-domains surrounded by SiO
2
,
dodecane on TiO
2
2
could improve (Scheme 2b).
We varied several reaction variables such as space velocity, O
2
2 2
To validate this conjecture, TiO –SiO mixed oxides with three
concentration, and reaction temperature in order to find conditions
that could give high yields of OHCs. The reaction reached the stea-
dy state after ca. 1 h on stream, and selected data are presented in
Table 2. As WHSV increased from 8.34 to 44.0 h by reducing the
amount of photocatalyst at the fixed total flow rate of 100 cc/min,
conversion of dodecane decreased from 87% to 69% as expected.
Ti/Si atomic ratios were synthesized by a solvothermal method
with almost non-aqueous solutions. To make a well-mixed state
on a molecular level in the liquid precursor mixture, it is important
to control the water content such that the precursors remain un-
hydrolyzed before the temperature rises to 170 °C. If it is not
controlled properly, the solution becomes opaque pale-yellowish
due to preferential hydrolysis of the Ti precursor.
ꢁ1
When 21% of O
higher and selectivity of OHCs was about two times lower than
those of 4% O at two WHSV conditions. As the reaction tempera-
ture decreased from 160 °C to 80 °C at O concentration of 4%,
the conversion was similar but OHCs selectivity increased from
7% to 52%. Under the same temperature and O concentration,
2
was flowed, conversion of dodecane was 4 times
Textural properties of the three mixed oxide samples with dif-
ferent Ti/Si ratios (0.11, 0.33, and 3.00) were studied based on N
physisorption at 77 K and compared with anatase TiO . As shown
in Fig. 3a, the N isotherms belonged to type-IV. The maximum
adsorption was attained at some pressure between 0.8 and 1.0 P/
, where P denoted the vapor pressure of N . During the desorp-
2
2
2
2
2
3
2
WHSV was halved by cutting the flow rate by half (50 cc/min), then
the conversion increased from 24% to 47%. In this case, the OHCs
selectivity was reduced somewhat, but the yield of OHCs improved
from 12.5% to 20.7% due to increased dodecane conversion. Since
P
0
0
2
tion process, H2 hysteresis loop associated with the capillary con-
densation took place, indicating the presence of mesopores usually
formed in interparticle voids. Various depths of pores with wide
and narrow holes, confirmed by BJH pore size distribution curve
at Fig. 3b, brought about a disparity between adsorption and
2
the reaction behavior was most significantly affected by O con-
centration, it was reduced to even lower levels to 2.45% and 1%.
The OHCs selectivity increased to as high as 70%, but conversion
of dodecane decreased so much that there was no gain in OHCs
yield.
Apparent turnover frequency (TOF) of this photocatalytic reac-
tion was calculated based on surface titanium atom. The number
0
desorption branches at the P/P range of 0.4–0.8 of the isotherms.
As Ti/Si ratio increased, the magnitude of this disparity decreased
due to the smaller pore volume. Increase in Ti composition caused
a significant reduction of total pore volume from 0.7972 to
3
0.1014 cm /g as Ti/Si increased from 0.11 to 3.00 as shown in
of surface atoms was calculated from the TiO
2
surface area and
Fig. 3b and Table S2. These pore volumes of the mixed oxides were
the area per Ti atom calculated from Ti–O bond length (1.939 Å)
larger than that of TiO
of BET surface area were consistent with these pore structures. The
BET surface area of TiO –SiO mixed oxides were an order of mag-
nitude higher than TiO due to the contribution of SiO of high sur-
face area. As Ti/Si ratio increased, it decreased dramatically from
2
except the one with Ti/Si = 3.00. The results
ꢁ3
[
27]. The TOF values in Table 2 vary around the order of 10
2
2
2
2
2
7
54.8 to 193.6 m /g. To investigate microporosity of samples, t-
plot method was applied as shown in Fig. 3c. All data of the mixed
oxides with different Ti/Si ratios were estimated from intercept
positioned at y axis by extrapolation. Almost none of the microp-
ores was found in the sample with Ti/Si = 0.11, whereas two other
samples showed about 42% of micropores. The number of Ti atoms
on the catalyst surface was calculated from the assumption that a
pffiffiffi
2
2
half of Ti atom per ð 2 ꢀ 1:939Þ Å was exposed [27], and that
TiO
2
surface area of the mixed oxide was proportional to the ratio
of Ti/(Ti + Si) obtained either by ICP or XPS analysis.
The XRD patterns of the prepared photocatalysts are compared
with pure anatase TiO
tio increases, intensity of amorphous SiO
creases and the (101) peak of anatase TiO
2
2
and amorphous SiO
peak at 2h = 12–38° de-
at 25.4° appears and
2
in Fig. 4a. As Ti/Si ra-
Fig. 1. Products profile from GC FID signal and mass chromatogram in GC–MS in
the inset. The noted C –C stands for corresponding aldehydes.
2
1
6
Kim, Jae Yul
