Angewandte
Chemie
ether generates phenol and benzene. Nonetheless, under the
reaction conditions, phenol undergoes hydrogenation yielding
[6–7]
cyclohexanol, thus wasting hydrogen.
Alternatively, Scheme 3 shows that the concurrent use of
Raney Ni and H-BEA-35 allows the hydrogenolysis of ether
Scheme 2. Proposed pathways A–D based on reaction energetics (see
Table 2). Arrows in blue indicate the reactions catalyzed by Raney Ni,
and the arrow in green indicates the dehydration step catalyzed by H-
BEA-35. The stoichiometry was omitted for clarity (refer to Table 2 for
the reaction stoichiometry).
as shown in Scheme 1. Nonetheless, the formation of cyclo-
hexanone always takes place to a certain extent when the
transfer hydrogenation of phenol is performed with a low
amount of 2-PrOH (e.g., Table 1, entry 9). Thus, pathway B
also contributes to the formation of benzene upon reduction
of cyclohexanone to cyclohexanol [Eq. (9)]. The pathways A
and B are concurrent in the system because of similar reaction
energetics [Eq. (9) and (10)]. In turn, when performing the
experiment with high amounts of 2-PrOH (n > 4), the
undesirable pathway C is observed [Eq. (13)]. Lastly, path-
way D [Eq. (15)], which is even more exergonic than path-
ways A and B, is also prone to occur in the system. Despite
this, pathway D is partially hindered by the extremely low
concentration of cyclohexene in the reaction medium. Indeed,
cyclohexene was not detected in the product mixtures
obtained by experiments with concurrent use of Raney Ni
and H-BEA.
Scheme 3. HDO of diphenyl ether, benzyl phenyl ether, and dibenzyl
ether. See Table S4 in the Supporting Information for full list of
products.
linkages to proceed without the formation of saturated
products. So far, such high selectivity for arenes was achieved
[
8]
only in the presence of homogeneous catalysts, unsupported
[9]
[10]
metal nanoparticles, or a Pd/Zn/C catalyst.
However,
these approaches are not able to dehydroxylate phenols.
Through our current approach, however, diphenyl ether,
phenyl benzyl ether, and dibenzyl ether were fully converted
with high selectivity for arenes (80–90%; Scheme 3).
Inspired by the results obtained from the experiments
with model compounds, we applied the methodology to the
conversion of real raw materials, that is, bio-oil and organo-
solv lignin. Bio-oil was prepared by pyrolysis of pinewood. To
remove lower alcohols and carboxylic acids, bio-oil was
The consideration of these four pathways leads to three
very distinct situations regarding the selectivity for benzene.
If pathways A and B are followed, a yield of 100% benzene
would most probably be achieved. Next, if only pathway C is
followed, the transferable hydrogen would be fully used for
the formation of cyclohexane. Finally, if only pathway D is
observed, the conversion of one mole cyclohexene into
washed with saturated KHCO . The extraction of carboxylic
3
1
2
[6]
benzene and cyclohexane ( / mol: / mol, respectively)
acids is needed because they poison Raney Ni. The isolated
3
3
would enable a maximum yield of 33% benzene. We conclude
from the data in Table 1 that pathways A and B are
predominant over pathways C and D in experiments carried
out with 1.5 ꢂ n ꢂ 2.5, as indicated by high selectivity for
benzene (82%; Table 1, entries 5 and 7). However, pathway C
starts to be predominant at n > 4, as demonstrated by the
marked decrease in the selectivity of benzene (from 82 to
fraction contains furans and methoxyphenols, as revealed by
GC ꢀ GC-MS analysis (Figure 1a). Applying the catalytic
procedure to the fractionated bio-oil leads to arenes and
saturates forming the major products (Figure 1b). A yield of
about 50 wt% colorless oil was isolated after rotary evapo-
ration of n-pentane at 313 K under reduced pressure
(50 mbar). Semi-quantitative analysis by GC ꢀ GC-FID indi-
cates that 71 wt% of the detected products are arenes,
26 wt% alkanes, and 3 wt% phenols. Similar results to those
with bio-oil were obtained with lignin (Figure 1c), with a yield
of about 40 wt% of an isolated colorless oil (78 wt% of the
detected products are arenes, 18 wt% alkanes, and only
4 wt% are phenols).
5
8%) in addition to the increase in the selectivity for
cyclohexane (from 9 to 32%; Table 1, entry 8).
Previously, we found that Raney Ni is an extremely active
catalyst for hydrogen transfer reactions and, in particular,
shows a considerably high chemoselectivity for transfer
[6]
hydrogenolysis of diaryl, aryl alkyl, and dibenzyl ethers.
This feature is key when one considers its utilization in lignin
conversion. In the presence of Raney Ni with 2-PrOH as both
a solvent and an H-donor, the hydrogenolysis of diphenyl
The yields of 40–50 wt% of the isolated oils correspond,
indeed, to high carbon yields (70–80%), since the C-content
of the substrates (lignin, 59% and bio-oil, 70%) is much
Angew. Chem. Int. Ed. 2013, 52, 11499 –11503
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim