C. Fan, et al.
Catalysis Communications 140 (2020) 106002
Fig. 3. Proposed reaction mechanism for Au/UiO-66-HCl oxidative esterification of benzyl alcohol and methanol to methyl benzoate.
6
(
6-HCl is 71 h−1, which is higher than most of the Au based catalysts
Table S3). Moreover, Au/UiO-66-HCl exhibited good universality for
0.6 W·cm−2, the light contribution increased linearly. The result
showed that a higher irradiance excites more energetic electrons on the
Au surface and enhance the interaction with adsorbed molecules to
improve the catalytic efficiency.
various substitutions or aromatic alcohols (Table S4). There is no no-
ticeable attenuation of the catalytic activity after five cycles, suggesting
the high stability of Au/UiO-66-HCl under the applied reaction condi-
tions (Fig. S7).
A series of control experiments were conducted to under the cata-
lytic origin. No catalytic activity was observed when using UiO-66-HCl
as catalyst with or without base and light irradiation (Table 1, entries
The activation of O
2
play an important role in the aerobic oxidative
reaction, then EPR (Fig. 2d) was further conducted. The results in-
•
-
dicated the reactive oxygen species is O
2
during the O
2
activation
signal, while the
signal greatly enhanced when Au NPs were introduced.
Moreover, Au/UiO-66-HCl shows better activation ability than Au/UiO-
66. plays a major role in activation of O . The product yield decreased
capture agent content (Fig. 2c),
•
-
process. UiO-66 and UiO-66-HCl exhibited weak O
2
•
-
intensity of O
2
5
–8), suggested the catalytic centers were Au NPs. No reaction occurred
without O (Table 1 entries 9–10), suggested the reaction is an aerobic
oxidative process. The oxidative esterification reaction is inhabited in
the absence of K CO (Table 1, entries 11–12) even at high reaction
2
2
•-
gradually with the increase of O
2
which further proved the importance of O in the aerobic oxidation
2
•
-
2
3
temperature. The addition of alkali can significantly improve the cat-
alytic efficiency by improving the efficiency of proton extraction in OH
of benzyl alcohol to form benzyloxy [30]. Only by-product benzalde-
hyde dimethylacetal can be obtained, which is catalyzed by the Lewis
acid sites of defective UiO-66-HCl support (Table 1, entries 13–14),
reaction. Au/UiO-66-HCl exhibited higher TOF value than Au/UiO-66,
which can be attributed to the greater surface electron concentrations
of Au NPs and the higher O
activation of O to O
2 2
2
activation ability. It is well known the
by Au NPs is sensitive to the negative charge
state of Au [33,34]., the anionic Au NPs can activate O by donating an
excess electronic charge to the anti-bonding orbital of O to generate
2
O , as the concentration of Au surface electrons increases, the oxygen
•
-
2
which is similar to AlCl
3
Lewis acid catalyst [31].
2
•
-
The conversion of benzyl alcohol increased from 56.2% in the dark
to 94.3% under visible light irradiation, suggested the light adsorption
of Au NPs via localized surface plasmon resonance (LSPR) effect play an
important role in enhancing the catalytic activity. In the UV–vis spec-
trum, Au/UiO-66-HCl showed LSPR absorption peak at 520 nm
activation ability increase [6,10,35].
To probe the effect of exposed Lewis acid sites in the oxidative es-
terification reaction, ethylenediamine was used to block the Lewis acid
sites by grafting of amine groups from ethylenediamine onto co-
ordinatively unsaturated Zr sites. As shown in Fig. S8, the conversion of
benzyl alcohol decreased from 91.6% to 11.2% with the increase of
ethylenediamine amount, which indicated the Lewis acid sites is benefit
for the adsorption of reactants.
(
Fig. 2a). The impact of the irradiation wavelength on the catalytic
activity was investigated, the apparent quantum efficiency (AQE) is
calculated to eliminate the influence of thermal effect on photocatalytic
performance. As shown in Fig. 2a, the AQE value closely matches the
LSPR absorption spectrum of Au NPs and reaches the maximum at
A possible mechanism was proposed (Fig. 3) based on the above
analysis. Firstly, the defect sites in UiO-66-HCl were favorable for ad-
sorption of the alcoholic hydroxyl groups of benzyl alcohol (I, II). Then
the energetic electrons induced by LSPR effect of Au NPs were easily
522 nm, which indicates the photo-generated electron transfer is the
driving force of the photocatalytic reaction.
The effect of the light intensity on the catalytic activity was in-
vestigated by varying the irradiance. As shown in Fig. 2b, the product
yield obtained in the dark is considered as a thermal contribution and
the increased yield under the visible light irradiation is considered as a
light contribution [32]. With the light intensity increased from 0.1 to
•
-
transferred to the LOMO orbital of O
2
and activated the O
2 2
to O . With
•
-
the aid of alkali and O
2
, benzyl alcohol undergoes cleavage of the
alcoholic OH group on the Au surface to produce a benzyloxy, then
cracked CeH to form benzaldehyde (III). The formed benzaldehyde
4