F. Zhang, et al.
InorganicChemistryCommunications104(2019)105–109
model compounds using a Cu/1,10-phenanthroline catalyst was also
developed [24]. The CαeCβ bond cleavage of β-O-4 linkage in lignin
model compounds can also be achieved using metal-free homogeneous
catalyst that consists of 4-acetamido-TEMP in combination with HNO3
and HCl [30]. Further, a one-pot oxidation cleavage of β-O-4 linkage
using Pd/CeO2 as a heterogeneous catalyst was also reported, in which
the CαeOH of 2-phenoxy-1-phenylethanol (PP-ol) was oxidized to 2-
phenoxy-1-phenylethanone with PdNPs catalyst, and at the same time,
the oxidation cleavage of CαeCβ bond was catalyzed by CeO2 [33].
Recently, several copper complexes as catalysts for one-pot conversion
of β-O-4 linkage were also reported [21]. Generally, the cleavage of
CαeCβ bond in β-O-4 linkages of lignin model compounds with cata-
lysts usually requires the use of organic solvents and/or high tem-
perature.
(mAuNPs:mGQDs = 1:18). For comparison, AuNPs-GQDs9 and AuNPs-
GQDs27 were prepared under the same condition, but different mass
ratios of AuNPs to GQDs.
2.4. Preparation of AuNPs/C catalyst
The AuNPs/C (1%) catalyst was prepared according to literature
[61]. In a typical experiment, 10 mg of activated carbon was dispersed
in 1 mL of distilled water. Then, 2 mL of AuNPs colloid
(0.050 mg mL−1) was injected into the dispersion. The mixture was
kept in a turn-over mixer (120 rpm) for 15 min. The AuNPs/C catalyst
was then collected and washed by water.
2.5. General procedure for the catalytic oxidation of β-O-4 lignin model
compounds
Gold nanoparticles (AuNPs) has been widely used as catalyst for
selective oxidation of benzyl alcohols to the corresponding aldehydes,
acetones or acids in aqueous solution under mild conditions [34,35].
Compared to AuNPs, supported AuNPs, like Au/CeO2, and Au/C, have
also been used to catalyze oxidation of some monoaromatic lignin
model compounds, such as 3,4-dimethoxyphenyl methanol (veratryl
alcohol) [36,37] and 1-phenylethane-1,2-diol [38,39]. Recently, the
oxidation of β-O-4 lignin model compounds with AuNPs supported SiO2
or Li-Al LDH was reported [40,41]. Previously, nanocomposites of
graphene quantum dots (GQDs) and other materials were used for
we have demonstrated that composites of AuNPs and GQDs can cata-
lyze the selective oxidation of veratryl alcohol to veratraldehyde or
veratric acid [37]. In this work, we explore the cleavage of CαeCβ in β-
O-4 lignin model compounds to corresponding acids using GQDs sta-
bilized gold nanoparticles (AuNPs-GQDs) in aqueous solution.
In a typical reaction, 125 μL phosphate buffer (0.2 mol L−1, pH 7),
90 μL distilled water, 265 μL AuNPs-GQDs18 (0.72 g L−1, 0.01 mg gold)
were mixed. Then, 0.1 mg substrate in 50 μL ethanol was dried at 50 °C
in an open bottle. The bottle was sealed after the AuNPs-GQDs18 so-
lution and 20 μL fresh H2O2 (30 g L−1) was injected into it. The reaction
mixture was heated at 75 °C for 4 h in a horizonal oscillator. The con-
version and yield were determined by HPLC. The catalyst could be re-
covered by dialysis (8000–140,000 Da) and reused for 2 times (see
Table S3).
2.6. Characterizations
Atomic force microscopy (AFM) image was acquired in tapping
mode with a Multimode Nanoscope V scanning probe microscopy
system (Bruker, USA), using AN-NSC 10 AFM cantilever tip (SHNIT Co.,
Russia). The specimen was prepared by solution casting the aqueous
suspensions of GQDs on cleaved mica surface. Transmission electron
microscope (TEM) images were obtained using a JEM-2100F trans-
mission electron microscope (JEOL, Japan) operated at 200 kV. The
samples were prepared by solution casting the aqueous suspensions of
AuNPs or AuNPs-GQDs on ultrathin carbon films with copper grid
support and drying under ambient condition in a dryer. The 1HNMR
spectra were acquired with an Avance III600 MHz system (Bruker,
Germany). HPLC data were collected on an 1220 Infinity LC system
(Agilent, USA) with a ZORBAX Bonus-RP column (4.6 × 250 mm, 5 μm
filling. Agilent, USA) at 32 °C.
2. Experimental
2.1. Materials
(DL)-1-phenylethanol, 1-phenylethane-1,2-diol, diphenylmethanol,
HAuCl4 and BO43ol were purchased and used as received. (Table S4)
Preparations and characterizations of 2-methyl-1-phenylpropan-1-ol,
1,2-diphenylethan-1-ol, 2-(4-methoxyphenoxy)-1-phenylethan-1-one
(BO41one), 2-(4-methoxyphenoxy)-1-phenylethan-1-ol (BO41ol), 2-
phenoxy-1-phenylethan-1-one (BO42one) and 2-phenoxy-1-pheny-
lethan-1-ol (BO42ol) are summarized in supporting information (SI).
The GQDs were prepared through the photon-Fenton reaction of gra-
phene oxide sheets as described in our previous work [58].
3. Results and discussion
The GQDs used in the work was prepared through the photo-Fenton
reaction of graphene oxide [58], and its morphology was characterized
by atomic force microscopy (AFM) (Fig. S2). The average lateral size of
the GQDs was < 10 nm [62], and the height is ~1 nm, revealing the
single atomic layered feature [60,63]. AuNPs was prepared through the
reduction of tetrachloroauric acid with sodium citrate and character-
ized (Fig. S3a), then mixed with the GQDs solution at certain weight
ratios that resulting in the GQDs stabilized AuNPs (AuNPs-GQDs). The
transmission electron microscopy (TEM) image of AuNPs-GQDs is
shown in Fig. S3b. The hyperfine structure of AuNPs-GQDs is further
confirmed by HRTEM image, the crystalline lattice constant is 0.230 nm
and 0.235 nm, corresponding to the (100) facet of graphite and the
(111) facet of gold (Fig. S3c), respectively. The GQDs possibly attaches
to the AuNPs through weak interactions [59,62].
2.2. Preparation of AuNPs
AuNPs is prepared by reducing HAuCl4 and sodium citrate (Na3CA)
solution with NaBH4 according to literature [59,60]. In a typical ex-
periment, 20 mg of HAuCl4 (58.9 μmol) and 50 mg of Na3CA∙2H2O
(170 μmol) was dissolved in 200 mL distilled water in a flask. The so-
lution was stirred for 15 min at 35–40 °C. Then, 10 mL of cold NaBH4
(52.9 mmol L−1) was introduced. The reaction mixture was stirred for
45 min. The magnet stirrer was removed, and another 22 mL distilled
water was added into the solution. The as-formed gold nanoparticle
(AuNPs) colloid (0.050 g L−1) was kept at 8 °C for further use.
2.3. Preparation of graphene quantum dots stabilized gold nanoparticles
(AuNPs-GQDs)
ferent GQDs to AuNPs ratios were prepared as described in the ex-
perimental section. The catalysis of the AuNPs-GQDs was evaluated
with a β-O-4 lignin model compound, 2-(4-methoxyphenoxy)-1-phe-
nylethan-1-ol (BO41ol). The reaction was conducted with H2O2 as an
oxidant in phosphate buffer (pH 7) at 75 °C. The reaction temperature
was actually explored systematically, and it is found that the conversion
AuNPs-GQDs was prepared by simply mixing them two. Briefly, the
GQDs solution (1 mL, 7.2 g L−1) was neutralized by NaOH (1.6 mL,
10 mmol L−1), then injected to the AuNPs colloid (8 mL, 0.050 g L−1).
The mixture was placed in a horizonal oscillator and kept at 50 °C for
30 min. The as-prepared mixture is labeled as AuNPs-GQDs18
106