Environmental Engineering Reference
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HH
H
H
C
C
C
H
Bond scission energy (KJ mol
-1
)
361
O
402
426
H
Propylene oxide
HH H
HH
H
H
C
C
C
H
CC
C
H
O
H
390
O
-
Acrolein
O
?
3C
O
2
+ 3H
2
O
Figure 8.2
h ree routes for propylene oxidation and bond scission energies in the
propylene molecule. Reproduced with permission from [31].
negatively charged oxygen radicals (O
-
) are formed. h e O
-
radicals will
preferentially attack the allylic C-H bond of propene (Figure 8.2) to pro-
duce acrolein [64].
h e discovery of Haruta
et al.
showing the unique property of gold
nanoparticle for generating H
2
O
2
from molecular H
2
and O
2
, thereby dem-
onstrating synthesis of PO in gas phase, opened a new horizon in catalysis
research [65, 66, 67]. A target of 10% conversion of propylene, > 90% selec-
tivity of PO and hydrogen ei ciency of 50% was set to develop the catalyst
to reach the production of PO at a commercial level (Figure 8.3).
It is observed that the formation of propylene epoxide from propylene
over supported gold catalyst depends on the nature of support as well
as size and shape of gold nanoparticles. It is found that 2-5 nm-sized
gold nanoparticles of hemispherical shape give the highest selectivity
for propylene epoxide [32, 68] (Figure 8.4). h e ef ect of the titanium
phase on reactivity to PO formation can be pieced together from its
coordination environment, the distance between two neighboring tita-
nium atoms (Figure 8.5), hydrophobicity and porosity. Among dif er-
ent supports, anatase TiO
2
, TS-1, Ti-MCM-41, Ti-MCM-48, Ti-HMM,
Ti-SiO
2
, and Ti-TUD-1 are found to be very ef ective for Au/Ti inter-
actions which are found most ef ective towards propylene epoxidation
reaction.
While scrutinizing the role of titanium phase during propylene epoxi-
dation reaction, it was found that the existence of isolate Ti
4+
species and
porosity of the Ti-containing mesoporous silica plays a crucial role. h e
importance of the isolated tetrahedral Ti species was emphasized [69] by
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