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material and catalysts durability. Utilization of alcohols as hy-
drogen donors in catalytic transfer hydrogenation/hydrogenol-
ysis reactions has been investigated under application of differ-
ent catalysts and substrates. DMeF was produced with yields
of up to 80% by employing Ru/C and 2-propanol as donor.[9]
However, because of the deactivation of the Ru/C catalysts, fur-
ther studies are currently being performed to investigate the
nature of the deactivation phenomena. Armstrong et al. dem-
onstrated the activity of Raney-nickel and cobalt in the catalyt-
ic transfer hydrogenolysis of aromatic alcohols.[10] Additionally,
much effort has been devoted to improving the understanding
and performance of catalysts in the hydrogenolysis of glycerol.
Gꢁemez et al. applied Ni–Cu/Al2O3 catalysts in the hydrogenol-
ysis of glycerol to 1,2-propanediol using 2-propanol or metha-
nol as hydrogen donor.[11] More recently, Pietropaolo et al. ob-
tained 94% yield in 1,2-propanediol when applying unreduced
Pd/Fe2O3 to a solution of glycerol in 2-propanol.[12]
Scheme 2. Reaction pathway for the catalytic transfer hydrogenation/hydro-
genolysis of furfural employing Ni/Fe2O3, Cu/Fe2O3, and Pd/Fe2O3. Reactivity
order of metals is based on the obtained yields of the respective product
under the chosen reaction conditions.
Herein, we report on the catalytic transfer hydrogenation/hy-
drogenolysis of furfural and HMF, applying Ni-, Cu-, and Pd-
supported catalysts, under batch as well as continuous flow re-
actions conditions.
Table 1. Catalytic transfer hydrogenation/hydrogenolysis of furfural over
different metal catalysts.[a]
Catalyst
Conversion [%]
Yield [%]
1
2
3
6
4, 5, 7
Results and Discussion
Cu/Fe2O3
Ni/Fe2O3
Pd/Fe2O3
37
46
68
28
33
41
0
0
4
0
0
7
0
0
<1
Transfer hydrogenation/hydrogenolysis of furfural
Initial experiments focused on the activity of Ni-, Cu-, and Pd-
supported catalysts in the catalytic transfer hydrogenation/hy-
drogenolysis of furfural (1). For this purpose, 10 wt% Cu/Fe2O3,
10 wt% Ni/Fe2O3, and 10 wt% Pd/Fe2O3 catalysts were synthe-
sized by co-precipitation (see Experimental Section). In a typical
experiment, the autoclave was charged with furfural, an inter-
nal standard, 2-propanol, and the catalyst. The molar ratio of
substrate to catalyst was 175:1. The weight of Cu, Ni, and Pd
catalyst charged into the reactor was adjusted such that the
number of moles of metal remained constant over the experi-
ments. The primary product of the transfer hydrogenation was
FA (2). The successive hydrogenolysis of the hydroxyl group
yields 2-methylfuran (3). In parallel to hydrogenation, furfural
may undergo decarbonylation to furan (6). With increasing
conversion (and hence, increasing concentration of acetone)
the aldol-condensation product of furfural and acetone, 4-
(furan-2-yl)but-3-en-2-one (8) is detected. Furthermore, we de-
tected traces of furan-ring hydrogenation products, 2-methyl-
tetrahydrofuran (THMeF, 4), (tetrahydrofuran-2-yl)methanol
(THFA, 5), and tetrahydrofuran (THF, 7).
[a] Reaction conditions: furfural (0.40m) in 2-propanol (40 mL), with
10 wt% Cu, Ni, or Pd/Fe2O3 under N2 atmosphere and 1808C. Conversion
and yield values were ascertained at the end of the reaction after 7.5 h.
reactivity behavior of Cu, Ni, and Pd catalysts in transfer hydro-
genation/hydrogenolysis, decarbonylation, and ring-hydroge-
nation reactions are a result of preferential stabilization of the
intermediate species by the different metal atoms. Previous
studies have demonstrated that the hydrogenation of furfural
over Cu catalysts occurs through coordination of the carbonyl
group, in a h1(O)-aldehyde configuration.[5b,13] This rationalizes
the observation that the main product over Cu/Fe2O3 was FA,
which was obtained with 75% selectivity. Ring-hydrogenated
products (4, 5, 7) were not formed. Despite previous reports
on the H2-mediated hydrogenation of furfural over Ni-catalysts,
yielding primarily decarbonylation, ring-opening, and ring-hy-
drogenation products,[5a] we obtained FA as the primary prod-
uct with 73% selectivity.
To verify that 2-propanol served as hydrogen donor in the
catalytic transfer hydrogenation/hydrogenolysis to furfural, the
acetone concentration was monitored (see the Supporting In-
formation, Figure S2). In the initial phase of the reaction, in
which FA was the main product, we found that approximately
1 equivalent of acetone was formed per equivalent of FA.
The main product over Cu and Ni was FA. Cu and Ni exhibit-
ed limited activity in the further hydrogenolysis to MeF. Prod-
ucts of the ring hydrogenation and decarbonylation reaction
were not detected (Table 1). Scheme 2 shows the reaction
pathways catalyzed by Cu, Ni, and Pd. The differences in the
In contrast to Cu and Ni, Pd displayed activity in the hydro-
genation of the furan-ring producing (4, 5, 7). Furan derivatives
may coordinate through the p-system of the ring, allowing the
formation of ring-hydrogenated products.[5a,14] In contrast, hy-
drogenation of the oxo functionalities occurs via formation of
h2(C,O)-aldehydes, in which both the C and O atom are coordi-
nated to Pd atoms. The h2-aldehyde may transform into
a more stable surface acyl-species, which is known to be an in-
termediate in the decarbonylation, yielding furan.[5a] FA, MeF,
and furan were formed with 61%, 7%, and 11% selectively.
The selectivity towards the ring hydrogenation products (4, 5,
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ChemSusChem 2014, 7, 268 – 275 269