Photocatalytic chemoselective cleavage of C-O bonds under hydrogen gas- and acid-free conditions

Article abstract of DOI:10.1039/c8cc03362e

In the presence of a palladium-loaded TiO₂ photocatalyst, the cleavage of benzyl phenyl ether in low-molecular-weight alcohol solvents under de-aerated conditions afforded toluene and phenol simultaneously in a 1:1 molar ratio.

Full text of DOI:10.1039/c8cc03362e

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2018, DOI: 10.1039/C8CC03362E.
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Photocatalytic chemoselective cleavage of C–O bonds under
hydrogen gas- and acid-free conditions
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
a
Kazuya Imamura,* Hiroko Kato, Yuichiro Wada, Kazuhiro Makabe, Ayumu Onda, Atsuhiro
c
c
b,d
b
Tanaka, Hiroshi Kominami, Katsutoshi Sato and Katsutoshi Nagaoka*
DOI: 10.1039/x0xx00000x
www.rsc.org/
In the presence of a palladium-loaded TiO
2
photocatalyst, example, noble metal catalysts such as Pt, Pd, and Rh have
cleavage of benzyl phenyl ether in low-molecular-weight alcohol been used for lignin pyrolysis in the presence of acid and for
solvents under de-aerated conditions afforded toluene and phenol hydrogenolysis in the presence of H . However, in addition to
using expensive catalysts, these reactions require severe
reaction conditions (i.e., high temperatures and high H
2
simultaneously in a 1:1 molar ratio.
2
Reactions that selectively cleave C–heteroatom bonds are
pressures), which result in poor selectivity for the target
materials owing to the production of by-products. Therefore,
alternative catalytic processes for selective conversion of lignin
to aromatic compounds under mild conditions are needed.
important in organic synthesis, in addition to being
1
–3
mechanistically interesting.
Among such reactions, the
cleavage of the C–O bonds of biomass constituents has
attracted considerable attention. Because almost all chemical
products, including plastics, pharmaceuticals, agrochemicals,
One possibility is the use of TiO
2
-mediated photocatalytic
redox reactions. Irradiation of TiO with UV light results in the
2
4
and cosmetics, are currently produced from fossil resources,
formation of electrons and holes (charge separation), which
can then participate in reduction and oxidation reactions,
respectively. These redox reactions proceed at room
their ongoing depletion means that alternative resources,
preferably renewable ones, will need to be developed.
Biomass, including lignocellulosic biomass, is a possible
alternative source of chemical feedstocks, and methods for the
production of aromatic compounds from biomass have been
extensively studied. Lignin is the major constituent of
lignocellulosic biomass, and the amount of lignin residue
temperature under atmospheric pressure. The use of TiO
2
-
mediated photocatalysis for organic reactions has been
4,15
1
extensively studied.
Recently, our research group
accomplished photocatalytic reduction (hydrogenation) of
nitriles and alkenes by using a TiO -supported catalyst
2
5
1
6,17
produced by the paper industry exceeds 50 million tons/year.
system.
We found that strongly reducing hydrogen species
Lignin is also a by-product of the production of ethanol fuel
from crop wastes. Lignin is an amorphous polymer comprising
aromatic monomers connected via oxygen bridges (C–O
were formed photocatalytically. In the current study,
photogenerated active hydrogen species were used to
accomplish hydrogenolysis of benzyl phenyl ether as a model
compound for lignin.
5
,6–8
bonds),
and aromatic compounds can therefore be
produced by selective cleavage of the bridges. In addition,
lignocellulosic biomass can be obtained from nonfood
All the chemicals were reagent-grade commercial materials
and were used as supplied. Degussa P25, well-known
9
,10
crops.
Various catalysts have been investigated for the cleavage of
commercial TiO having mixed phase of anatase and rutile, was
2
used as the TiO photocatalyst in this study. Samples of TiO
2
2
1
1–13
lignin C–O bonds to produce aromatic compounds.
For
loaded with metal co-catalysts were prepared by
a
photodeposition method using metal chlorides as precursors,
except in the cases of the Rh-, Ag-, and Cu-loaded catalysts,
which were prepared with rhodium nitrate (RhNO ), silver
3
nitrate (AgNO3), and copper acetate (Cu(CH COO) ),
3
2
respectively.
In a typical run, TiO2 (50 mg) was suspended in 5 cm of
methanol containing benzyl phenyl ether (50 mol) in a Pyrex
test tube, which was then sealed with a rubber septum under
argon and photoirradiated at > 300 nm by means of a 400-W
3
ꢀ
λ
high-pressure mercury arc (Sen Lights Corporation, Osaka,
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Japan) with magnetic stirring in a water bath maintained at adsorption of H
2
. No reaction occurred in the absence of light
298 K. The amounts of unconsumed benzyl phenyl ether and or in the absence of a photocatalyst, indicating that the C–O
generated toluene and phenol were determined with a gas bond was cleaved photocatalytically. To our knowledge, this is
chromatograph–mass spectrometry system (GCMS-QP2010 the first report of selective cleavage of a C–O bond mediated
Ultra, Shimadzu, Kyoto, Japan) equipped with a HP-1 column.
2 2
by a TiO photocatalyst. Although the methanolic Pd-TiO
To investigate the effects of nanoparticulate metal co- suspension showed much higher activity than the suspensions
catalysts, we also carried out photocatalytic cleavage reactions of the other catalysts, the reaction rate decreased with
of benzyl phenyl ether at 298 K in methanolic suspensions of increasing photoirradiation time, and the reaction stopped
2 2
bare TiO or TiO loaded with various metals (Fig. 1) methanol, almost completely after 120 min (Fig. S2).
a hole scavenger, is often used as a solvent for such reactions.
5
0
200
Benzyl phenyl ether
Toluene
Phenol
40
30
20
10
0
1
0
00
Methanol
Ethanol
2ꢁPropanol 2ꢁPropanol containing
0 vol% water
2
Solvent
Fig. 2 Effects of solvents on photocatalytic cleavage of benzyl phenyl ether over 0.5
wt% Pd-TiO after 30-min photoirradiation.
Fig. 1 Effects of metal co-catalysts (0.5 wt%) on photocatalytic cleavage of benzyl
phenyl ether in methanolic suspensions under de-aerated conditions after 30-min
photoirradiation.
2
The material balance calculated by means of Eq. 1 was 100%
after 120 min of photoirradiation:
When bare TiO
reaction mixture was irradiated for 30 min, no reaction of
benzyl phenyl ether occurred, and the TiO turned blue,
2
was used as the photocatalyst and the
ꢁ
n toluene
ꢂ
ꢀ+
ꢀ
n
ꢁ
2n (benzyl phenyl ether)
phenol
ꢂ
ꢀ+ꢀ2n(benzyl phenyl ether)
Material balanceꢀ=ꢀ
2
0
4
+
3+
indicating that Ti
had been reduced to Ti
by
(1)
photogenerated electrons. This result indicates that the holes
oxidized the methanol, but the photogenerated electrons in
where n(toluene), n(phenol), and n(benzyl phenyl ether) are
the amounts of toluene, phenol, and benzyl phenyl ether after
the conduction band of TiO
the benzyl phenyl ether. Nearly identical results were obtained
when Cu, Ag, or Au was loaded on the TiO as a co-catalyst.
However, when we used Ru-, Rh-, Pd-, or Pt-loaded TiO
2
denoted as Ru-TiO Rh-TiO Pd-TiO and Pt-TiO ,
2
were not involved in cleavage of
0
photoirradiation, respectively, and n (benzyl phenyl ether) is
the initial amount of benzyl phenyl ether. This result indicates
that no by-products formed and suggests that oxidative
products, such as formaldehyde, deactivated the
photocatalytic system. The material balance remained at
nearly 100% throughout the reaction, indicating that no
reaction other than the target reaction occurred and that the
reason for the reaction rate decrease was likely to have been
poisoning of the catalyst by a product of oxidation of the hole
scavenger (i.e., formaldehyde formed by oxidation of
methanol by holes). To explore this possibility, we investigated
the effects of adding water or aqueous formaldehyde to the
reaction mixture (Fig. S3). The addition of 10 vol% water
decreased the yields of the cleavage products because the
resulting decrease in the methanol concentration negatively
affected the rate of electron donation to the valence band of
2
2
(
2
,
2
,
2
,
respectively), toluene and phenol were obtained in a 1:1 molar
ratio by cleavage of the C–O bond of benzyl phenyl ether
2 2
without the need for H gas. The Pd-TiO catalyst showed
much higher yields of toluene and phenol than did the other
photocatalysts. All the photocatalysts used in the present
reaction show almost same morphology of metal particles as
well as similar mean particle size (Fig. S1), indicating that these
factors are not crucial for inducing high activity over Pd-TiO
Pd-TiO has previously been used for photocatalytic
hydrogenation reactions in which an activated hydrogen
2
.
2
+
16–18
species (H-Pd) forms by reduction of H .
in the present system and participated in hydrogenolysis
cleavage) of the C–O bond. It has been reported that Pd
H-Pd also formed
2
TiO . The reaction rate was decreased even more by the
(
addition of aqueous formaldehyde. These results confirmed
that formaldehyde generated by methanol oxidation
deactivated the photocatalytic system. We also evaluated the
effects of various solvents on photocatalytic cleavage of the C–
catalyst is effective for hydrogenolysis of C–O bond using H2
gas. Since Pd was also effective in this photocatalytic reaction
system,
the
active
hydrogen
species
generated
photocatalytically behave likely that produced by dissociative
2
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2
O bond (Fig. 2). When any alcohol was used, no CO was ether started to decrease monotonously immediately after the
detected. This result indicated that alcohols selectively start of photoirradiation, and toluene and phenol began to
oxidized to aldehyde or ketone. Photocatalytic reaction of form in a 1:1 molar ratio. After 20 min of photoirradiation, the
methanol and ethanol over 0.5 wt%Pd-TiO
condition was performed and no evolution of CO
observed for longer photoirradiation (60 min). These results cases). After complete consumption of the benzyl phenyl ether,
indicated alcohols used in this study could not oxidized to CO and acetone began to form (at the same rate), indicating
in the absence of O . The reaction rate decreased when that hydrogenolysis (cleavage) of benzyl phenyl ether
methanol was replaced with ethanol or 2-propanol (both of competed with H formation and that reduction of benzyl
which are hole scavengers). Because the addition of water to phenyl ether was the dominant reaction. The redox balance
-propanol reportedly has a positive effect on the rate of the was calculated with Eq. (2) and is shown in Fig. 3:
2
under deaerated benzyl phenyl ether was almost completely consumed, and
2
was high yields of toluene and phenol were obtained (>99% in both
2
H
2
2
2
2
photocatalytic dechlorination of chlorobenzene, we also used
aqueous 2-propanol containing 20% water for the cleavage
2
nꢁconverted benzyl phenyl etherꢂꢀ+ꢀ2n(H2)
Redox balanceꢀ=ꢀ
2
n(acetone)
1
9
reaction. As expected, we obtained the highest reaction rate
under these conditions. To investigate the reason why the
(2)
reaction rate was decreased in ethanol or 2-propanol system, where n(converted benzyl phenyl ether), n(H
photocatalytic oxidation of alcohols along with H
alcohol suspensions of 0.5 wt% Pt-TiO under deaerated and formed acetone, respectively. Throughout the
2
), and n(acetone)
2
2
evolution in are the amounts of consumed benzyl phenyl ether, formed H ,
2
conditions was examined (Fig S4). This reaction system is photoirradiation, the redox balance was nearly unity, and no
+
simple because only alcohols existed in the system and Pt H is other products were detected upon further photoirradiation,
reduced to H
Therefore, this photocatalytic H
indicator of the rate of hole trapping. The result of Fig. S4 the production of H
2
selectively, avoiding any other side reactions. indicating that re-oxidation of the products by holes did not
evolution could be used as occur and that only 2-propanol was oxidized, accompanied by
. It has been reported that in the
2
2
indicated that reaction rate was decreased by changing (thermo)catalytic hydrogenolysis of benzyl phenyl ether over
methanol to ethanol or 2-propanol because hole-trapping rate Pd, hydrogenation of phenol and toluene to the corresponding
was decreased. Although the reaction rate of H
2
evolution saturated compounds occurs and decreases the yield of
20
from ethanol was faster than 2-propanol, the rate of cleavage toluene and phenol. However, because neither high H
2
of benzyl phenyl ether in ethanol was almost same as 2- pressure nor high temperature were used in our system, no
propanol system. In addition, small amount of benzyl phenyl hydrogenation occurred, even after the benzyl phenyl ether
ether was remained even after 180 min photoirradiation when was completely consumed.
ethanol was used as solvent. On the other hand, 2-propanol We investigated the utility of our photocatalytic
system was completely proceeded. These results indicated hydrogenolysis protocol by carrying out reactions of various
that aldehydes formed by oxidation of primary alcohols substrates with C–O bonds (Table 1). The protocol was
negatively affected the photocatalytic cleavage of benzyl successfully applied to a compound that was bulkier than
phenyl ether. Although reaction rate was increased by adding benzyl phenyl ether (entry 2), as well as to an alkyl ether (entry
water in 2-propanolic system, water adding negatively affected 3). In addition, even though catalytic hydrogenolysis of the C–
methanolic system (Fig. 2 and S3). To understand the effect of O bonds of nitrile compounds with H
water on photocatalytic cleavage of benzyl phenyl ether, of the cyano group to the corresponding amine, we found
photocatalytic oxidation of alcohols along with H formation in that our photocatalytic system showed high chemoselectivity
water-alcohol suspensions of Pt-TiO under deaerated for the C–O bond. Specifically, when 4-(benzyloxy)benzonitrile
2
gas results in reduction
2
0
2
2
conditions was also examined (Fig S5). This result indicated was used as the substrate, only the C–O bond reacted with the
that water added to 2-propanol affected the two catalytically generated Pd-H (entry 4). We previously reported
photocatalytic reactions, cleavage of benzyl phenyl ether and that aqueous benzonitrile is photocatalytically hydrogenated
H
2
evolution, in a similar way. In addition, we investigated without H
2
1
in the presence of Pd-TiO
2
and oxalic acid as a hole
in 2- scavenger. The photocatalytic hydrogenation of the cyano
propanolic system (Fig. S6). Although active hydrogen species group to an amino group was strongly affected by the solvent
6
effect of adding water on generation of acetone and H
2
1
3–16
generated at Pd were used selectively to cleavage of benzyl and hole scavenger.
phenyl ether in water containing 2-propanolic system, H was hydrogenated in the 2-propanol system used in this study.
formed in pure 2-propanol system despite remaining benzyl Therefore, perfectly chemoselective hydrogenolysis of the C–O
phenyl ether. These results indicated that adding water bond was achieved. Durability of 0.5 wt% Pd-TiO was
affected hole trapping with alcohols and selectivity of investigated with UV-LED (HLV-24UV365, CCS, Kyoto), weaker
reduction though mechanism is not clear. light intensity than mercury arc, because the rate of reaction
We evaluated the time courses of the amount of benzyl phenyl was too fast. After the reaction, Pd-TiO was separated from
However, the cyano group was not
2
2
2
ether remaining and the amounts of toluene and phenol the reaction mixture by simple filtration and reused. The rate
generated during photocatalytic cleavage of benzyl phenyl of photocatalytic cleavage was decreased with re-used
2
ether in a suspension of 0.5 wt% Pd-TiO in 2-propanol indicating that small amount of 2-propanol or acetone was
2
containing 20 vol% water (Fig. 3). The amount of benzyl phenyl adsorbed on Pd-TiO .
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1
reaction, indicating that the adsorptions are easily
by photoirradiation under TiO
In conclusion, in a new example of a TiO -photocatalyzed
decomposed to CO
2
2
.
2
reduction reaction, we examined photocatalytic cleavage of
benzyl phenyl ether and found that a Pd co-catalyst had
remarkably beneficial effects on the cleavage reaction. The
choice of solvent also strongly affected the reaction outcome;
specifically, when the reaction was carried out in 2-propanol
containing 20 vol% water, no products other than toluene and
phenol were obtained. Our results indicate that the utility of
H-Pd formed photocatalytically is not limited to hydrogenation
reactions and that the protocol described herein constitutes a
new strategy for application of photocatalytic reduction of
TiO2.
This work was supported by a Grant-in-Aid for Young Scientists
B (no. 16K18292).
Conflicts of interest
There are no conflicts of interest to declare.
Notes and references
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Fig. 3 Time courses of amounts of benzyl phenyl ether, toluene, phenol, acetone, and
H
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, along with the redox balance, during photocatalytic cleavage of benzyl phenyl ether
in a suspension in 2-propanol containing 20 vol% water and 0.5 wt% Pd-TiO
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st
After the reaction (1 ), Pd-TiO
photoirradiated under O condition, which was used for 2
reaction (Fig. S7). Tha activity of Pd-TiO was almost same as
2
was filtered and
nd
2
2
4
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