Glycerol Hydrogenolysis
FULL PAPER
sure follows. Analysis of the gas phase indicates that the
only gas produced in significant amount is H2. It is conse-
quently reasonable to attribute the pressure variations to H2
production and consumption. Since APR is excluded under
our conditions, there is necessarily a dehydrogenation step
that occurs. Now there is a potential H2 source in both
mechanisms. H2 is obviously produced from glycerol if dehy-
drogenation is the first step. If the dehydration is the first
step, acetol can be another possible source of H2 by dehy-
drogenation into PAL (Scheme 2), which is thermodynami-
cally favoured (Figure 5). However, under hydrogen atmos-
phere, the pressure increases and then decreases and the
main product is 12-PDO. According to the dehydrogenation
mechanism (Scheme 3), the first step produces one equiva-
lent of GAL and of H2. Later, the formation of 12-PDO in-
volves the consumption of two equivalents of H2, which
leads to the further decrease in the pressure. Along the al-
ternative dehydration route (Scheme 2), H2 production and
consumption steps are not sequential as in the previous dis-
cussion, but are parallel and initiate from acetol. Conse-
quently, the initial increase of hydrogen pressure would be
observed only if LA is the main product. Now, under hydro-
gen, the hydrogenation product, 12-PDO, is the dominant
one at all stages of the reaction. Thus, the dehydration
mechanism appears incompatible with the transient forma-
tion of H2.
The influence of the atmosphere on the conversion rate
has also to be pointed out. The glycerol hydrodeoxygenation
is actually found to be significantly faster under helium than
under hydrogen-gas pressure: the catalyst is initially almost
four times more active (Figure 1). According to the dehy-
drogenation mechanism, this can be explained by the rever-
sibility of the initial dehydrogenation step. Under H2 atmos-
phere, the high H2 pressure inhibits the glycerol dehydro-
genation. On the contrary, under He atmosphere, the equi-
librium is inverted, thereby favouring a faster glycerol con-
version. If we consider now the alternative mechanism that
is initiated by a dehydration step, the production of the key
intermediate acetol should not depend on the H2 pressure.
Hence the rate of transformation of glycerol should not de-
crease under H2 compared to He, but only the selectivity
should be modified, with more 12-PDO and less LA pro-
duced under H2. Hence, the dehydration mechanism seems
to be again incompatible with the experimental results.
Let us now look at the conclusions from the DFT calcula-
It would lead by hydrogenation to 13-PDO, in contradiction
with the experimental selectivity. Alternatively, the dehydro-
genation path can be considered. Two routes are possible,
the alkyl route and the alkoxy route, depending on the first
ꢀ
ꢀ
bond rupture, C H or O H. Obviously, the first dehydro-
ꢀ
genation step of the alkyl routes (C H rupture) is common
ꢀ
with the dehydration. However, the C H bond ruptures
have higher barriers (0.83, 0.77 eV) than the alternative O
H bond ruptures (0.67, 0.70 eV). In addition, when starting
ꢀ
ꢀ
with the terminal hydroxyl scission (OHt CHt route), the
first dissociation is the rate-limiting step (Figure 10). Conse-
quently, the dehydrogenation (overall barrier of 0.70 eV) is
favoured kinetically compared with dehydration (overall
ꢀ
barrier of 0.86 eV) and leads to GAL by means of an OHt
CHt route.
To sum up, the combination of experiment and theory
shows that dehydrogenation into GAL is the first step for
the glycerol transformation on the Rh/C catalyst in basic
media under He or H2 atmosphere.
LA and 12-PDO selectivity: The proposed mechanism is
summarised in Scheme 3. Glycerol is dehydrogenated into
GAL. Then, its subsequent dehydration into Enol 3 is ther-
modynamically favoured on the surface (Ereac =ꢀ0.73 eV),
as shown from DFT calculations. The isomerisation of
Enol 3 into PAL is straightforward, and then a double hy-
drogenation into 12-PDO or a Cannizzaro reaction to yield
LA can occur. From our experimental results, PAL conver-
sion into LA is irreversible, whereas PAL hydrogenation
into 12-PDO is an equilibrium. Indeed, LA is stable under
our experimental conditions (Table 5, entries 3 and 4),
whereas 12-PDO is mainly converted into LA under He.
Those final steps control the product selectivity depending
on the atmosphere.
Under a hydrogen atmosphere, the main product is the
12-PDO since the equilibrium is displaced towards the hy-
drogenation direction (Table 5, entry 2). Moreover, the PAL
hydrogenation into acetol and then 12-PDO is probably
faster than the Cannizzaro reaction into LA under H2 pres-
sure, hence the 12-PDO being the main product.
Under helium atmosphere, the pressure increases continu-
ously, thereby leading to a significant concentration of hy-
drogen in the gas phase at the end of the reaction. As al-
ready noted, hydrogen production is ensured by the glycerol
dehydrogenation. However, the H2 partial pressure is much
lower than under H2 atmosphere. Therefore, all the hydro-
genation/dehydrogenation equilibria are displaced towards
dehydrogenation. This contributes to the H2 pressure in-
crease but also to the diminution of the 12-PDO yield and
to the increase of the LA yield (Table 5). Since the forma-
tion of LA is irreversible under the reaction conditions, this
is the main product under He atmosphere.
In a nutshell, the product distribution is controlled by the
hydrogenation/dehydrogenation equilibria, and hence by the
nature of the atmosphere. Under H2, the hydrogenation of
the intermediates into 12-PDO is favoured, whereas under
neutral atmosphere, the transformation of PAL into LA by
tions. The glycerol dehydration on model RhACTHNURTGNEU(GN 111) surface
leads to two enols, Enol 1 and Enol 2, to yield 12-PDO and
13-PDO, respectively, after isomerisation and hydrogenation
steps (Figure 5). Those two enol intermediates are isoener-
getic on the metal catalyst and more stabilised than the de-
hydrogenation intermediates, GAL and DHA. Thermody-
namically, dehydration is hence favoured over dehydrogena-
tion on the surface. If we now look at the reaction barriers,
the formation of Enol 1 goes through transition states of
higher energies (0.83, 0.95 eV, Figure 12) than the reaction
path to Enol 2 (0.77, 0.86 eV, Figure 12). Thus, Enol 2 is the
kinetically favoured intermediate on the dehydration route.
Chem. Eur. J. 2011, 17, 14288 – 14299
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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