C O M M U N I C A T I O N S
Table 1. BET Specific Surface Area, Crystallite Size of Catalysts
of the performance of some home-prepared rutile samples with a
commercial one. It is worth noting that selectivity increases by
decreasing the crystallinity of samples (Figure 1) but the reaction
rate appears similar, except for SA, which was the most oxidizing
catalyst tested. The best performing catalyst was the badly
crystallized HP333 that gave a selectivity of ca. 38 and 60% mol
for BA and MBA, respectively. It should be noted that the oxidation
of MBA by home-prepared anatase prepared by following a
different procedure reported previously3 gave a value of only 41%.
To study the influence of crystallinity, the calcined samples
HP673 and HP973, along with an amorphous TiO2 (HP298), were
tested. The last catalyst gave rise to a very low reaction rate and
selectivity (ca. 12% mol) probably because the oxidation power is
so low that the reaction mechanism and the physicochemical nature
of sites deeply change with respect to HP333. HP673 gave a
selectivity drastically lower than HP333 (12 vs 38% for BA), but
the time needed for conversion was lower than that of HP333 (see
Table 1). On the contrary, by calcining at 973 K, both selectivity
and reaction rate decrease, and the latter phenomenon could be due
to a decrease of the surface hydroxyl groups, caused by the high
temperature. The most crystalline catalysts gave rise to many highly
oxidized intermediates (in HPLC chromatograms peaks with low
retention times were present) that result in a significant drop in
selectivity.
and Their Photocatalytic Performancea
crystallite
size
catalyst
amount
[g/L]
SSA
[m2/g]
tirr
[h]b
selectivity
[% mol]b
catalyst
[nm]
homogeneous
HP298
HP333
HP333
HP333
HP333-MBA
HP673
HP973
SA
SA-MBA
176
22
17
14.0
12.0
38.0
38.2
37.7
60.0
12.0
9.9
215
107
107
107
107
35
4
2.5
2.5
0.4
0.2
0.4
0.6
0.4
0.4
0.4
0.4
0.4
7
7
7
8.4
8.95
2.36
6
9.4
3.75
2.15
7
13
41
52
52
9.2
20.9
a The experimental results refer to BA oxidation, except that indicated
as HP333-MBA and SA-MBA, referring to MBA oxidation. b Irradiation
time (tirr) and selectivity (to aldehyde) are calculated for an alcohol
conversion of 50%.
Finally the obtained results suggest that selectivity is not affected
in a considerable way by the reaction-rate value. On the contrary
adsorption can play a role; in fact it was found that the extent of
PAA adsorption on HP333 is negligible with respect to that of BAD
(23%). Selectivity differences between the formation of BAD and
PAA (38 vs 60%) can be justified by considering the very different
adsorption extent of BAD and PAA. The hydrophobic character of
BAD determines its strong adsorption over HP333 surface and by
preventing its desorption gives rise to its further oxidation.
In conclusion, we have used for the first time a home-prepared
rutile TiO2 for selective oxidation of aromatic alcohols to aldehydes
in water suspensions. The experiments point to a primary influence
of crystallinity on selectivity. The highest selectivity was attained
with the badly crystallized HP333.
Figure 2. Experimental results of a representative oxidation run of (A)
BA and (B) MBA with HP333 (amount: 0.4 g/L). Conversion, selectivity,
and C-balance are scaled on right side. The CO2 concentration values were
divided by 7 (A) or 8 (B) for normalization purposes.
Figure 2 shows the results of representative experimental runs
carried out by oxidizing the two alcohols with HP333. Two parallel
reaction routes are present from the start of irradiation: partial
oxidation to aldehydes and mineralization to CO2:3
CO2 r ArCH2OH f ArCHO
Supporting Information Available: Experimental details; SEM
images. This material is available free of charge via the Internet at
The disappearance of substrates along with the production of
aldehydes, CO2, and traces of acid (only for BA oxidation) can be
observed. Conversion and selectivity to aldehyde are also reported,
as a function of irradiation time. During MBA and BA oxidation,
the selectivity slowly decreases during the reaction because of
overoxidation, but maintains the values of ca. 38 and 60% mol,
respectively, even when the conversion reaches 50%. These values
are surprisingly high if we consider that the reaction takes place in
water with a TiO2-mediated photocatalytic process. The selectivity
to aldehyde in MBA oxidation is higher than that in BA; it can be
explained by considering the presence (in MBA) of the electron-
releasing methoxy group in the para position.6
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Carbon balances of each run, carried out with HP333, were
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oxidation of aldehyde and acid.
The catalyst affects alcohol oxidation in a crucial way; a
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oxidation and from ca. 21 to 60% in that of MBA was observed.
The photoreactivity results reported in Table 1 allow comparison
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