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nol<catechol<anisole<phenol. As the aromatic ring is
a weaker nucleophile than hydroxyl or methoxy groups,[25]
phenol can adsorb onto basic sites (nucleophile) in a nonplanar
mode, producing cyclohexanone, or it can adsorb onto acidic
site (electrophile) in a planar mode, predominantly producing
cyclohexanol.[25–26] Anisole can produce cyclohexanone through
hydrogenation followed by the demethylation of the adsorbed
methoxy group, or it can produce methoxycyclohexane via the
hydrogenation of the adsorbed aromatic ring on the catalyst
surface.[19a,c] Direct hydrodeoxygenation can also occur by
rapid dehydration after the hydrogenation of the aromatic
ring.[19c]
at 258C to 0.11 MPas at 2508C) water may improve the HDO
process.
Although the improved catalytic activity of oxygenates was
observed, a lower production of none-Os with the selective
production of 1-Os (cyclohexanol and cyclohexanone) was also
observed. The incomplete deoxygenation can be attributed to
the effects of water, which solvates the more hydrophobic 1-
Os less compared to 2-Os, thus selectively producing 1-Os.
Suggested reaction pathways of the GUA–HDO
Based on the above results, we suggest that the GUA–HDO on
Rh/SAA-33 occurs through four major paths: (i) the substitution
of the methoxy or hydroxyl group of GUA to obtain 1,2-dime-
thoxybenzene or catechol followed by hydrogenation to yield
1,2-dimethoxycylcohexane or 1,2-cyclohexanediol, (ii) the hy-
drogenation of GUA to yield 2-methoxycyclohexanol, (iii) dehy-
dration or demethanolation by the combination of hydroxyl
and methoxy groups, and (iv) dehydration or demethanolation
by the combination of catalyst-surface-adsorbed hydrogen and
the hydroxyl group.[31] Although anisole can be converted to
phenol by the substitution of a methoxy with hydroxyl group
during hydrogenolysis,[6c,19c,32] the formation of phenol was not
observed during the HDO of anisole in this study. The substitu-
tion and the elimination can be competitive reactions during
GUA-HDO; however, elimination appeared to occur rather than
substitution at a high temperature because of the high activa-
tion energy.[33]
Improved hydrodeoxygenation by the addition of water
Although the addition of water has been reported to suppress
organic reactions by the deactivation of the catalysts,[4a,d,27] im-
proved GUA–HDO in a biphasic mixture of n-decane and water
has also been reported.[6c,19] The possible water-initiated ad-
justment of acid sites on the silica–alumina support was ex-
cluded based on the NH3 TPD results (Figure S3 and Table S6)
and the pyridine FTIR spectra (Figure S4 and Table S7) in this
study. The addition of certain amounts of water improved the
HDO exhibiting an increased yield of 1-Os compared to the
water-free HDO. In addition to the water-involved improved
hydrolysis, the water-assisted improved catalytic HDO activity
can be attributed to the following causes:
(i) A proton transfer, most likely by the Grotthuss mecha-
nism, may occur in a water-containing environment.[10,28] An
aqueous catalytic reaction occurs upon the transfer of hydro-
gen on acid or base catalysts, by which water can be a proton
donor or acceptor by interacting with reaction intermediates.
In this system, hydrated ions (H3O+) may form as the active
species, and the hydrated solid acid catalyst may have hydrat-
ed functionalities on the surface. Under these conditions, the
proton transfer by the Grotthuss mechanism may occur on the
solid acid catalyst, i.e., silica–alumina aerogel in this study. In
this reaction system, guaiacol is saturated with H2 at 50–1508C
prior to hydrodeoxygenation as reported previously.[22] Thus, 2-
methoxycyclohexanol, a saturated form of guaiacol, is ad-
sorbed onto the catalyst surface and converted to deoxygenat-
ed products (Figure S13). The acid sites on the catalyst surface
were adjusted by the proton transfer via the Grotthuss mecha-
nism (Figure S14–S16).
Based on the suggested reaction pathways, the GUA–HDO
of bifunctional catalysts composed of noble-metal (Rh) nano-
particles and solid acids (silica–alumina aerogel) is depicted in
Scheme 1.[34] 2-Methoxycyclohexanone (c) or 2-methoxycyclo-
hexanol (d) can be produced by the hydrogenation of GUA on
a Rh surface onto which hydrogen atoms are dissociatively ad-
sorbed. 2-Methoxycyclohexanone (c) and 2-methoxycyclohexa-
nol (d) are further converted to methoxycyclohexane (h), cyclo-
hexanone (i), cyclohexanol (j), and/or cyclohexane by partial
hydrodeoxygenation. Hydroxyl from water can replace the me-
thoxy group of 2-methoxycyclohexanol (d), producing 1,2-cy-
clohexanediol (f) at a low temperature during GUA–HDO in
a biphasic system. Cyclopentanecarboxaldehyde (l) can be pro-
duced from cyclohexanediol (f) by means of pinacol rearrange-
ment (Figure S17).[33,35] It can be further converted to cyclopen-
tanemethanol (m) in an aqueous solution. In addition, 2-me-
thoxycyclohexanol (d) and cyclohexanediol (f) can be convert-
ed to cyclohexanone (i) by intramolecular demethanolation
and dehydration, respectively. The further conversion of cyclo-
hexanone (i) and cyclopentanecarboxaldehyde (l) to fully de-
oxygenated compounds was more significant without water.
Cyclohexene (k) was produced by the acid-catalyzed E1 elimi-
nation of cyclohexanol (j) (Figure S18).[33,35a]
(ii) At high reaction temperatures, the self-dissociation con-
stant of water increases, which supplies more protons to the
reaction systems under H2-rich environments. The improved
acidity with better self-ionization (increased 630-fold from
10À14 at 258C to 10À11.2 at 2508C) also improves the reac-
tion.[6b,20,29]
(iii) The dielectric constants[6b,20,30] and viscosity[20] of water
change at high temperature because of the rapid proton trans-
fer of subcritical water.[6b,20] The improved solubility of organic
reactants in the less polar (with a decreased dielectric constant
from 78.5 FmÀ1 at 258C and 1 bar to 27.1 FmÀ1 at 2508C and
50 bar) and less viscous (decreased viscosity from 0.89 MPas
Conclusions
Water in the subcritical phase can be a promoter of the bifunc-
tional catalyst Rh/SAA-33. A proton transfer assisted by water
may increase the hydrodeoxygenation activity, but in addition
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