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amount of 2-propanol to 75 vol% decreased the hexitol yield
to 29.8%.
Mechanism of hydrogen transfer
During transfer hydrogenation experiments, it was observed
that after the reaction temperature was reached, the pressure
of the reactor gradually increased by 3–5 bar. Even after the re-
actor was cooled to room temperature, an increased pressure
of 3–5 bar was maintained, which indicated that a gaseous
product was generated. By using GC, this gaseous product was
identified as hydrogen gas, which was likely produced through
the dehydrogenation of 2-propanol. In an earlier article, Ru/
AC(N) was found to be active for hydrogenation under 8 bar
hydrogen pressure as well as under transfer hydrogenation
conditions in which hydrogen gas was produced through the
Transfer hydrogenation of cellulose oligomers
The optimised conditions for the transfer hydrogenation of
glucose (pH 2.4, water to 2-propanol volume ratio 1:1) were
tested for the transfer hydrogenation of cellulose oligomers at
different temperatures. These oligomers were produced by
milling cellulose impregnated with H SO , which resulted in
2
4
the rapid depolymerisation of the polymer chain. A completely
soluble substrate was obtained on milling cellulose impregnat-
ed with 0.25 mmol of H SO per g of oligomers for 10 h. An
2
4
[20]
earlier study of these soluble oligomers had revealed that the
soluble oligomers have an average degree of polymerisation of
five to six monomer units. A fraction of these monomers are
linked via a (1!6) linkages, which, along with other factors, re-
sulted in an instant solubility of the oligomers in water despite
dehydrogenation of 2-propanol. In a fixed-bed reactor, this
overhead space is not available and hydrogen gas formed
would quickly leave the system. Hence, it is imperative to de-
termine the role of overhead hydrogen pressure in this reac-
tion.
[
24]
their high degree of polymerisation. However, the solubility
of oligomers in the reaction mixture was affected by the pres-
ence of 2-propanol. The oligomers were not completely solu-
ble in the 50 vol% 2-propanol solution at room temperature,
and a cloudy solution was obtained.
It is unlikely that such a low partial pressure of hydrogen
may produce the high rates of hydrogenation observed in our
results. The low partial pressure of hydrogen would result in
low solubility of hydrogen, which would limit the adsorption
of hydrogen over the catalyst surface. Thus, the effect of mo-
lecular hydrogen in our reaction was investigated by repeating
the reaction in the presence of hydrogen gas. To simulate the
solvent effect of 2-propanol during transfer hydrogenation, an
equivalent amount of n-propanol was used, which does not
undergo dehydrogenation to produce hydrogen. In compari-
son to 79.5% yield of hexitols during transfer hydrogenation at
The conversion of cellulose oligomers to hexitols through
transfer hydrogenation at various temperatures is shown in
Figure 3. After 20 min of the reaction, the highest yield of hexi-
tols (35.3%) was obtained at 1808C. Upon decreasing the tem-
1808C, only 2.9% of hexitols was obtained with 5 bar hydro-
gen partial pressure in the presence of n-propanol. Increasing
the pressure of hydrogen to 15 bar increased the yield to
11.9%, which was still substantially lower compared with the
yield obtained during transfer hydrogenation. On the basis of
these observations, we conclude that it is unlikely for the reac-
tion to proceed through the formation of hydrogen gas and
the effect of hydrogen gas pressure on the rate of hydrogena-
tion is minimal.
Adsorbed hydrogen species on the surface of the catalyst,
which is produced through the dehydrogenation of 2-propa-
nol, could be directly transferred onto the adsorbed glucose
molecule. The mechanism of such direct transfer of hydrogen
using homogeneous Ru complex catalysts has been extensive-
Figure 3. Yield of products from the transfer hydrogenation of cellulose olig-
omers in a batch reactor. Reaction conditions: 324 mg of cellulose oligo-
mers, 100 mg of the Ru/AC(N) catalyst (metal 2 wt%), 20 mL of water, 20 mL
of 2-propanol, pH 2.4, P=15 bar Ar at room temperature, t=20 min.
[30]
ly investigated. However, the heterogeneous transfer hydro-
genation mechanism is not well understood. To establish the
source of hydrogen that is taking part in the hydrogenation re-
action, we performed two experiments with deuterium instead
of hydrogen in the form of D and D O. The sorbitol product
perature to 1708C, a lower yield of 22.2% was obtained, which
was likely due to the slower hydrolysis of oligomers as well as
slow hydrogenation. As observed above, glucose hydrogena-
tion is not favoured at higher temperature, which resulted in
a high amount of glucose in the product at 190 and 2008C be-
cause the hydrolysis of oligomers accelerated but the hydroge-
nation of glucose was slow. Upon increasing the reaction time
to 40 min, the hexitol yield increased to 77.5% at 1808C and
a maximum yield of 83.4% was obtained after 1 h of the reac-
tion.
2
2
obtained from these reactions can be isolated and analysed by
using proton NMR spectroscopy. Deuterium is inactive towards
magnetisation during NMR spectroscopy and can serve as
a marker to determine the source of hydrogen taking part in
the reaction. Therefore, the presence of deuterium attached to
the carbon atom of sorbitol will lead to the absence of the cor-
responding proton peak in the NMR spectrum. The hydrogena-
tion of glucose adds one H atom to the carbonyl oxygen and
[31]
one to the anomeric carbon. The latter H atom is observed
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ChemCatChem 2014, 6, 1349 – 1356 1352