Coexistence of Cu(ii) and Cu(i) in Cu ion-doped zeolitic imidazolate frameworks (ZIF-8) for the dehydrogenative coupling of silanes with alcohols

Article abstract of DOI:10.1039/c9dt03181b

Recently, metal-ion-doped zeolitic imidazolate frameworks have gained considerable attention for their structure tailorability and potential catalytic applications. Herein, Cu ion-doped ZIF-8 nanocrystals were successfully prepared by the mechanical grind

Full text of DOI:10.1039/c9dt03181b

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Coexistence of Cu(II) and Cu(I) in Cu ion-doped
zeolitic imidazolate frameworks (ZIF-8) for the
dehydrogenative coupling of silanes with
alcohols†
Cite this: Dalton Trans., 2019, 48,
16562
a,b
a,c
a,c
a,c
a,d
Yan Dai,
*
Peng Xing,‡ Xiaoqin Cui, Zhihong Li and Xianming Zhang
*
Recently, metal-ion-doped zeolitic imidazolate frameworks have gained considerable attention for their
structure tailorability and potential catalytic applications. Herein, Cu ion-doped ZIF-8 nanocrystals were
3 2
successfully prepared by the mechanical grinding of Cu(NO ) , ZnO and 2-methylimidazole (HMeIM)
using ethanol as an additive. In contrast to the general view that only Cu(II) is present in Cu-doped ZIF-8,
we found the coexistence of Cu(II) and Cu(I) in this material, which was supported by XPS and X-ray
induced Auger electron spectroscopy (XAES) characterizations. Moreover, ethanol might have acted as a
reducer to induce the reduction of Cu(II) during synthesis. Due to the mixed valency of Cu ions, the Cu
ion-doped ZIF-8 nanocrystals showed excellent catalytic performance in the dehydrogenative coupling
of silanes with alcohols.
Received 5th August 2019,
Accepted 7th October 2019
DOI: 10.1039/c9dt03181b
rsc.li/dalton
2
0
21–24
silver, and gold,
were generally used as catalysts for this
1
. Introduction
reaction. Recently, the abundant first-row transition metals,
2
5
26–29
With the ongoing in-depth research on metal organic frame- such as Co and Cu,
have been chosen as alternatives to
works (MOFs), many scientists are increasingly focusing on noble metals. Interestingly, researches have implied that both
1
–5
26,29
stable MOFs.
On account of the permanent pores, high Cu(I) and Cu(II) are active sites for this reaction.
surface area and excellent stability of ZIF-8, much effort has
been devoted to its synthesis and property research.
In this study, Cu ion-doped ZIF-8 nanocrystals were success-
fully synthesized by a facile liquid-assisted grinding approach.
6
–8
Recently, ZIF-8 has been widely used in designing multifunc- The EPR results corroborated the presence of Cu(II) in the Cu
9
–11
tional catalysts by forming complexes with noble metals.
ion-doped ZIF-8 nanocrystals, whereas XPS and XAES charac-
Further, researchers found that doping other metal ions into terizations demonstrated that Cu(I) also existed in this
ZIF-8 is an effective method for the synthesis of functional material, evidently different from former studies, which only
1
2–18
materials.
Cu(II) and Co(II) are usually selected for doping reported the presence of Cu(II) in Cu ion-doped ZIF-8 crystals.
in ZIF-8 as their ionic radii are similar to that of Zn(II). The presence of mixed-valence Cu(I)/Cu(II) in the Cu ion-doped
Moreover, Cu and Co are common metal active sites in many ZIF-8 nanocrystals made us further explore the catalytic pro-
catalytic reactions, such as hydrogenation reactions, electro- perties in the dehydrogenative coupling of silanes. Just as
catalytic reactions and coupling reactions.
expected, immobilizing both Cu(II) and Cu(I) in the ZIF-8 nano-
Dehydrogenative coupling of silanes with alcohols is widely crystals proved to be an effective strategy in fabricating an
used for preparing silylating agents and surface modification excellent Cu-based catalyst for the dehydrogenative coupling
1
9
reagents. Previously, noble metals, such as palladium,
reaction. For example, Cu85%ZIF-8 nanocrystals gave a TOF of
3
4.9 h− for the dehydrogenative coupling of dimethyl-
1
phenylsilane with n-butanol, and still exhibited high stability
after 5 cycles of reaction.
a
Institute of Crystalline Materials, Shanxi University, 030006 Taiyuan, China.
E-mail: xmzhang@sxu.edu.cn, ydai@sxu.edu.cn
b
State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University,
3
61005 Xiamen, China
c
Institute of Molecular Science, Shanxi University, 030006 Taiyuan, China
d
2. Experimental section
School of Chemistry and Material Science, Shanxi Normal University, 041004
Linfen, China
2.1. Materials
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
The chemical reagents used in the experiments were as
follows: copper nitrate trihydrate (Cu(NO ) ·3H O, 99.9%,
c9dt03181b
‡
Co-first author.
3
2
2
16562 | Dalton Trans., 2019, 48, 16562–16568
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Sinopharm Chemical Reagent Co., Ltd), zinc oxide (ZnO, Alfa the addition of 2 mL of n-butanol. Then, 100 µL hydrazine was
Aesar), 2-methylimidazole (HMeIm, 99%, Ourchem), Ethanol added to the above solution, and the mixture was stirred at
(
(
CH
3
CH
2
OH, 99.7%, CHEMICAL REAGENT), cupric oxide 70 °C for 2 h. After the reaction, the products were
O, Sinopharm washed using n-butanol three times and separated by
CuO, 98%, Innochem), cuprous oxide (Cu
2
Chemical Reagent Co., Ltd), zinc nitrate hexahydrate centrifugation.
Zn(NO ·6H O, 99%, Sinopharm Chemical Reagent Co., Ltd),
dimethylphenylsilane (PhMe SiH, 97%, Energy Chemical),
n-butanol (n-C H OH, 99.5%, CHEMICAL REAGENT), anisole
(
3
)
2
2
2
2.3. Characterization
4
9
Powder X-Ray Diffraction (PXRD) measurements were recorded
on a Rigaku Ultima IV diffractometer using Cu Kα radiation
and a graphite monochromator (λ = 1.54056 Å) at 40 kV voltage
and 40 mA current at the scan speed of 10° min with a step
size of 0.02°. The morphology of the samples was observed
using a JEOL-JSM-6701 field-emission scanning microscope
(
C
7
H
8
O, 99%, Energy Chemical), phenylsilane (PhSiH
Energy Chemical), diphenylsilane (Ph SiH , 98%, Energy
Chemical), triphenylsilane (Ph SiH, 97%, Energy Chemical),
3
OH, 99.5%, CHEMICAL REAGENT), n-hexanol
n-C 13OH, 98%, TCI), and isophthalic acid (HBTC, 99%,
3
, 97%,
2
2
3
−1
methanol (CH
(
6
H
Jinan Henghua Technology Co., Ltd). All the reagents and
materials were used as received without further purification.
(
SEM), operating at an accelerating voltage of 10 kV and an FEI
Tecnai G2 F20S-Twin transmission electron microscope (TEM)
at an accelerating voltage of 200 kV. The Cu molar percentage
in Cu/ZIF-8 was determined by a NexION 350 inductively
coupled plasma mass spectrometer (ICP-MS). X-ray photo-
electron spectroscopy (XPS) and X-ray excited Auger electron
spectroscopy (XAES) were performed on a PHI-5702 instru-
ment, and the C1s line at 284.6 eV was used as the binding
energy reference. The electron paramagnetic resonance (EPR)
spectra were recorded at 293 K with the following parameters:
frequency: 9.86 GHz, power: 5.02 mW, modulation amplitude:
2
.2. Preparation
Synthesis of Cu ion-doped ZIF-8 nanocrystals. The reactions
were performed by placing a 1 : 4 mixture of metal precursors
ZnO and Cu(NO ) ) and ligand (HMeIm) in a 50 mL Teflon
(
3
2
tank together with liquid ethanol (1 mL), and four 5 mm and
four 8 mm stainless steel balls. The mixture was ground for
4
5 min in a DECO-PBM-V-0.4 L mill at 20 Hz. Then, the pro-
ducts were washed with ethanol three times and separated by
centrifugation (12 000 rpm, 1 min). The solid was dried over-
night at 80 °C. The finally obtained solid was stored in a glass
bottle at room temperature for further use.
Synthesis of ZIF-8 nanocrystals. The reaction was performed
at the same conditions used for Cu ion-doped ZIF-8 nanocrys-
4
G, modulation frequency: 100 kHz, center field, 3320.00
G. The N adsorption–desorption isotherms were recorded on
2
a Quantachrome autosorb iQ instrument; a liquid nitrogen
bath (77 K) and ultra-high purity grade nitrogen were used
through the nitrogen adsorption experiments. All the samples
were degassed under vacuum at 423 K for 6 h before the test.
In order to calculate the apparent surface areas, the multipoint
Braunauer–Emmett–Teller (BET) method was applied using
tals but without the addition of Cu(NO
3 2
) .
Synthesis of HKUST-1. HKUST-1 was prepared by using the
3
0
method reported by Huo with modifications. Isophthalic
acid (HBTC, 2 mol) was dispersed in water (10 mL). Then,
Cu(NO ) ·3H O (1 mol) was added, and the mixture was stirred
3 2 2
at room temperature for 1 h. After the reaction, the final solution
was centrifuged. The product obtained after centrifugation was
washed using ethanol (3 × 30 mL) and then, dried at 80 °C for
the adsorption branches of the N
size distribution was analyzed by the Barrett Joyner Halenda
BJH) method. The pore volume was calculated by the total
2
isotherms, and the pore
(
pore volume (TPV) method. FT-IR spectra were recorded using
a Nicolet iS5 FTIR system, and potassium bromide was used to
form sample mirrors. Thermogravimetric analysis (TGA) was
performed on a Setaram Labsys Evo apparatus. The samples
1
2 h.
Synthesis of Cu(Im)
to the report of N. Masciocchi et al. First, 2.50 mmol
Cu(NO ) ·3H O and 29.4 mmol of imidazole were dissolved in
2
2
. Cu (Im) was synthesised according
3
1
were heated in an alumina pan from 30 °C to 800 °C at a
3
2
2
−1
heating rate of 10 °C min under an N atmosphere.
2
5
0 mL water. Then, NaOH (0.1 M) was added to the above solu-
tion until a reddish-purple precipitate was observed. After fil-
tering, the powder was washed with ethanol and dried under
2.4. Catalytic performance tests
vacuum at 80 °C. Finally, the powder was heated at 215 °C for The Cu ion-doped ZIF-8 nanocrystals (50 mg) were charged in
8
h under a nitrogen atmosphere.
a 25 mL reinforced glass reactor equipped with a magnetic
Synthesis of Cu(2-MeIm) . 1.6 mmol of Cu(NO ) ·3H O and bar. Then, alcohol was added (2 mL). Finally, the silane
2
3 2
2
6
.4 mmol of 2-methylimidazole were placed in a grinding (1 mmol) was introduced in the reactor using a syringe. The
vessel, followed by the addition of 1 mL of ethanol. Then, the reaction mixture was stirred at 70 °C for the required time was
mixture was milled for 45 min. After grinding, the product was and cooled down to room temperature, and the catalyst was
transferred to a centrifuge tube and washed using an ethanol removed by centrifugation. For GC analysis, a known amount
and n-hexane mixture (1 : 8) for three times. The product was of anisole (0.5 mmol) was added as the internal standard.
dried overnight at 80 °C.
A gas chromatography system (FULI 9790II) was equipped
Synthesis of zero-valence Cu. Zero-valence Cu was syn- with a KB-5 capillary column and an FID detector to
thesized by reducing Cu85%ZIF-8 with hydrazine. In detail, monitor the progress of the silane dehydrogenation coupling
5
0 mg Cu85%ZIF-8 was placed in a glass bottle, followed by reaction.
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from 467.0 nm to 454.5 nm as the doping percentage of Cu ions
increased from 25% to 85%. Further, TEM was conducted to
characterize the size distribution of the Cu ion-doped ZIF-8 nano-
crystals. The TEM images in Fig. S2† revealed that the mechano-
3
. Results and discussion
3
.1. Synthesis and characterization of Cu ion-doped ZIF-8
nanocrystals
In this study, we synthesized Cu ion-doped ZIF-8 nanocrystals chemical method used in this study did not have much advan-
3
2,33
via a facile liquid-assisted grinding approach.
In detail, the tage in controlling the size of nanocrystals. However, this syn-
synthesis was performed in a grinding machine by mixing thesis method greatly reduced the amount of solvent used and
ZnO, Cu(NO ) and four-fold HMelm with a small amount of can be easily implemented for large-scale synthesis.
3
2
alcohol at room temperature for 45 min. Molar percentages of Furthermore, the thermal stability of Cu ion-doped ZIF-8 under
5%, 55% and 85% Cu to all metal elements were used in the N atmosphere reduced after Cu doping. For example, the
2
2
these experiments, and the products were denoted as Cu25% thermal decomposition temperature of Cu85%ZIF-8 reduced
ZIF-8, Cu55%ZIF-8, and Cu85%ZIF-8, respectively. As displayed from 430 °C to 320 °C compared with ZIF-8 (Fig. S3†).
in Fig. 1a, the Cu ion-doped ZIF-8 nanocrystals had identical
From the above results, it was confirmed that the Cu ions
XRD patterns with ZIF-8, which indicated that the parent were indeed doped into the ZIF-8 nanocrystals. However, the
framework of ZIF-8 was maintained after doping with Cu ions. valence state of the Cu ions was still unknown. Therefore, EPR,
Further, the permanent porosity of the Cu ion-doped ZIF-8 XPS and XAES characterization were further performed. First,
nanocrystals was demonstrated by N2 adsorption at 77 K the EPR spectrum of Cu85%ZIF-8 (Fig. 2a) exhibited a pro-
⊥
(Fig. S1 and Table S1†). For all three Cu ion-doped ZIF-8 nounced peak at g = 2.095 and a poorly resolved double peak
samples, type I isotherms were obtained, indicating a micro- at g = 2.321, corresponding to isolated Cu(II). The EPR spectra
∥
porous structure. The actual Cu content in Cu25%ZIF-8, of Cu55%ZIF-8 and Cu25%ZIF-8 were similar to that of Cu85%
Cu55%ZIF-8 and Cu85%ZIF-8 determined by ICP-MS were ZIF-8, and the corresponding anisotropic parameters are sum-
6
.4%, 9.1% and 16.8%, respectively (molar percentages of Cu marized in Table S2.†
to all metal elements). From the photographs of the powder Then, XPS spectroscopy was performed to characterize the
samples (the inset of Fig. 1a), we found that as the doping per- oxidation state of Cu ions in the surface region. As displayed
centage of Cu increased from 25% to 85%, the sample color in Fig. 2b, the coexistence of Cu(I) and Cu(II) in Cu85%ZIF-8
changed from light yellow-green to dark yellow-green.
In subsequent analysis, the presence of Cu ions in the 932.5 eV, which could be assigned to the Cu(I) species. The
sample was further confirmed by UV-visible spectroscopy. As higher binding energy (BE) Cu 2p3/2 peak at 934.6 eV was due
was evidenced by a shoulder observed on the main peak at
3
4
3
5
shown in Fig. 1b, doping caused the absorption edge of Cu- to Cu(II), and the Cu 2p1/2 peaks at 952.4 eV and 954.6 eV
doped ZIF-8 to move from the UV to the visible region. were assigned to Cu(I) and Cu(II), respectively. Further, as
Moreover, the first definite band of the Cu ion-doped ZIF-8 shown in Fig. S4,† the XPS spectra of Cu25%ZIF-8 and Cu55%
nanocrystals was located at around 460.0 nm, while the second ZIF-8 were similar to that of Cu85%ZIF-8. These results
broad band started at 600 nm and extended to the infrared implied that Cu(I) and Cu(II) coexisted in the Cu ion-doped
region. Moreover, the first absorption band was blue-shifted ZIF-8 nanocrystals.
Fig. 1 (a) XRD patterns of Cu ion-doped ZIF-8 nanocrystals prepared with different contents of Cu. Data for ZIF-8 is also given for comparison. The
inset of a shows the photographs of the corresponding samples. (b) Diffuse reflectance absorption spectra of ZIF-8 and the Cu ion-doped ZIF-8
nanocrystals.
16564 | Dalton Trans., 2019, 48, 16562–16568
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Fig. 2 (a) EPR spectra of the Cu ion-doped ZIF-8 nanocrystals; (b) Cu 2p XPS spectrum of the Cu85%ZIF-8 nanocrystals (gray line: raw data; wine
line: fitted peak plot; blue line: fitted Cu(I) peak plot; magenta line: fitted Cu(II) peak plot and black line: background).
To further confirm the presence of Cu(I) in the Cu ion- lytic properties of the material. Since both Cu(I) and Cu(II) are
doped ZIF-8 nanocrystals, the XAES spectra of these samples active sites for the dehydrogenative coupling of silanes with
2
9
were also analyzed. As shown in Fig. S5,† in the Cu85%ZIF-8 alcohols, the catalytic performance of Cu(I)/Cu(II) co-doped
spectrum, the peak at E = 915.3 eV (kinetic energy) was attrib- ZIF-8 nanocrystals in this reaction was first evaluated. Initially,
k
3
4
uted as the characteristic peak of Cu(I). In the Cu55%ZIF-8 PhMe SiH and n-butanol were selected as the model substrates
2
and Cu25%ZIF-8 spectra, it was hard to figure out the peak of for the reaction, and the results are presented in Table 1.
Cu(I) from the raw data, perhaps because of the low content of
Cu(I) in these samples.
First, we found that the Cu ion-doped ZIF-8 nanocrystals
with different Cu doping percentages had nearly identical
Therefore, from the EPR, XPS and XAES characterization TOFs (about 35.0 h− , entries 1 to 3). These results indicated
results, we confirmed that Cu(I) and Cu(II) coexisted in the Cu that the three studied Cu ion-doped ZIF-8 nanocrystals had the
ion-doped ZIF-8 nanocrystals. During the synthesis process, same active sites. As Cu85%ZIF-8 had the highest Cu content
both Cu(II) and Zn(II) could coordinate with four 2-methyl- and exhibited a short reaction time of 14 h, attention was paid
imidazolium via a coordination bond between the metal and to this material in the following experiments.
1
nitrogen atoms. Thus, the question of why Cu(I) is present in
Initially, the influence of catalyst mass and reaction temp-
Cu ion-doped ZIF-8 arises. According to previous reports on erature on the catalytic performance of Cu85%ZIF-8 was evalu-
3
6,37
Cu-based coordination polymers,
we speculated that the
small amount of liquid (ethanol) used in the synthesis may act
as the reducing agent to induce the reduction of Cu(II) to Cu(I). Table 1 Optimization of reaction conditions for the dehydrogenative
Although the synthesis was performed at room temperature, coupling of PhMe
local short-range heating could have been induced by mechano-
2
SiH and n-butanol catalyzed by Cu ion-doped ZIF-8
a
nanocrystals
chemistry, which is advantageous for the reduction of Cu(II).
Furthermore, IR characterization was conducted to prove that
Cu is coordinated with the ligand (HMeIm). As shown in
Fig. S6,† the IR band, corresponding to the metal–nitrogen
b
b
b
−1
Entry
Catalyst
Time/h
Conv. /%
Sel. /%
TOF /h
stretching frequency in Cu85%ZIF-8 (νCu–N), was blue-shifted
compared to that in ZIF-8 (νZn–N), which implied that the Cu–N
bonds were more rigid than the Zn–N bonds. Since the electro-
negativity of copper is higher than that of zinc (1.90 vs. 1.65),
the Cu–N bonds are more ionic than the Zn–N bonds and there-
c
1
2
3
4
5
6
Cu25%ZIF-8c
75
26
14
65
122
9
100
100
100
100
68
100
100
100
100
100
100
35.0
34.6
34.9
35.4
26.0
43.6
Cu55%ZIF-8
c
Cu85%ZIF-8
d
Cu85%ZIF-8
e
Cu85%ZIF-8
f
Cu85%ZIF-8
100
fore are stiffer, resulting in the blue-shift of ν
in the IR
M–N
a
3
8
Reaction conditions: PhMe SiH (1.0 mmol), 2 mL of n-butanol.
spectra.
2
b
Conversion of PhMe
anisole as the internal standard, and the product was siloxane. TOF
.2. Catalytic dehydrogenative coupling of silanes with alcohols was calculated based on the conversion of PhMe SiH at 30%, TOF =
2
SiH was determined by GC analysis using
3
2
c
moles of converted PhMe
doped ZIF-8 nanocrystals, 70 °C. 25 mg Cu85%ZIF-8 (20 μmol Cu),
2
SiH/(moles of Cu × time). 50 mg Cu ion-
After successfully confining Cu(I) and Cu(II) in ZIF-8 via the
liquid-assisted grinding approach, we tried to study the cata- 70 °C. 50 mg Cu85%ZIF-8, 30 °C. 50 mg Cu85%ZIF-8, 110 °C.
d
e
f
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a
ated. When the mass of Cu85%ZIF-8 was reduced from 50 mg Table 2 Catalytic properties of other Cu-based materials
to 25 mg, although the TOF of Cu85%ZIF-8 remained almost
b
TOF/h−1
unchanged (35.4 h− ), the time for the total conversion of
1
Entry
Catalyst
CuO
Time/h
Conv./%
Sel. /%
PhMe
2
SiH prolonged from 14 h to 65 h (entry 4). Moreover,
1
2
3
4
5
6
7
8
9
97
54
48
0.25
118
17
5
80
100
100
97
100
100
98
8.2
5.1
3.5
20
2.9
2
Cu O
100
100
100
100
100
95
when the reaction temperature was lowered from 70 °C to
−
1
HKUST-1c
3
0 °C, the TOF of Cu85%ZIF-8 decreased to 26 h (entry 5).
HKUST-1
Raising the reaction temperature to 110 °C resulted in a higher
TOF of 43.6 h− (entry 6). In this case, however, the super-
natant liquid appeared blue after the reaction, while the
normal color of the supernatant was colorless. The blue color
could be the color of the Cu ions, which indicates the low
stability of the Cu85%ZIF-8 nanocrystals at this critical con-
dition. When Cu85%ZIF-8 was used for the second cycle, the
Cu(Im)2
Cu(2-MeIm)
1
2
7.1
Cu(NO
Cu(NO
Cu
3
)
)
2
·3H
·3H
2
O
O
86
85
99
44.4
44.4
0.7
3
2
2
15
68
100
99
a
2
Reaction conditions: PhMe SiH (1.0 mmol), 2 mL of n-butanol,
0 μmol Cu, at 70 °C. The by-product was dimethyl phenyl silanol.
2
Reaction conditions: PhMe SiH (1.0 mmol), 2 mL of n-butanol,
b
4
c
267 μmol Cu, 70 °C.
conversion of PhMe SiH was only 45%. This result confirmed
2
our speculation that the Cu ions in Cu85%ZIF were lost at
1
10 °C.
After tuning the reaction parameters to optimize the cata- and got the same results. We speculated that the lower TOF of
lytic performance of Cu85%ZIF-8, the reaction conditions of HKUST-1 in entry 3 could be mainly because of the lower cata-
entry 3 (50 mg catalyst and 70 °C) were chosen for the stability lyst mass used here. The above results indicated that the three
test. It is worth noting that the Cu85% ZIF-8 nanocrystals not Cu-based materials with Cu–O bonds exhibited poor catalytic
only exhibited excellent catalytic performance but also high activity than the Cu-doped ZIF-8 nanocrystals under the same
stability in the reaction. As illustrated in Fig. S7,† the Cu85% reaction conditions, regardless of the valence state of the Cu
ZIF-8 nanocrystals were able to maintain high PhMe SiH con- ions.
2
version and siloxane selectivity after 5 cycles. Further, the XPS
spectrum of Cu85%ZIF-8 after 5 cycles of reaction indicated Cu(2-MeIm)
that Cu(I) was still present in this material (Fig. S8†).
with Cu–N bonds were also evaluated for the influence of the
Moreover, control experiments were conducted to further chemical environment of Cu ions on their catalytic perform-
confirm the role of Cu ion-doped ZIF-8 in the catalytic reac- ance. Cu(Im) was synthesized according to the report of N.
Further, the Cu-based coordination polymers Cu(Im)
2
and
2
(Im = imidazole, 2-MeIm = 2-methylimidazole)
2
1
3
tion. The blank experiment (without catalyst) could convert Masciocchi. The method for the synthesis of Cu(2-MeIm)
only 1.0% of PhMe
SiH (Table S3,† entry 1), and the ZIF-8 was the same as that for the Cu ion-doped ZIF-8 nanocrystals
nanocrystals converted 4.4% of PhMe
SiH (Table S3,† entry 2). but without the addition of Zn (NO . Only Cu(II) existed in
Both these results are poorer than that of Cu85%ZIF-8, which Cu(Im) (XPS spectrum in Fig. S11†), and
resulted in the complete conversion of PhMe
SiH at the same the Cu(II) in Cu(Im) coordinated with four HIm molecules via
conditions. These results indicated that the catalytic activity of Cu–N bonds. For Cu(2-MeIm) , although the detailed structure
Cu ion-doped ZIF-8 was mainly derived from the Cu species.
is still unknown, its sharp and strong XRD diffraction peak
From the above discussion, it is clear that the Cu ion-doped indicated high crystallinity (Fig. S12†). Therefore, we believe
ZIF-8 nanocrystals had high catalytic activity and stability in that the Cu–N bond is present in Cu(2-MeIm) . From entry 5
exhibited low activity in
2
2
2
3 2
)
3
1
and Cu(2-MeIm)
2
2
2
2
2
2
the dehydrogenative coupling of dimethylphenylsilane with in Table 2, we knew that Cu(Im)
2
n-butanol. For comparison, we further tested the catalytic per- this reaction (TOF of 2.9 h− ). Compared with Cu(Im)
1
,
2
1
formance of other Cu-based materials. As indicated in Table 2, Cu(2-MeIm)
2
had a higher TOF of 7.1 h− (entry 6), which is
Cu-based materials with a Cu–O bond had low TOFs in this lower than that of the Cu ion-doped ZIF-8 nanocrystals. We specu-
reaction (entries 1 to 3). In detail, commercial CuO and Cu
lated that the presence of only Cu(II) in these two coordination
were used as received, and the average particle sizes of CuO polymers could be a reason for their poor activity compared
entry 1) and Cu O (entry 2) were 52 nm and 47 nm, respect- with the Cu ion-doped ZIF-8 nanocrystals. The synergy
2
O
(
2
ively. For entry 3, the HKUST-1 ([Cu
was synthesized according to the modified version of the genative coupling of silanes with alcohols.
method reported by Huo (XRD pattern of HKUST-1 in
Finally, the catalytic performance of Cu(NO
Fig. S9†). The Cu 2p XPS spectrum of HKUST-1 revealed the zero-valence Cu were also tested (entries 7–9). The TOF of
3 2
(BTC)
]), a Cu-based MOF, between Cu(II) and Cu(I) is important for the catalytic dehydro-
) ·3H O and
3 2 2
3
0
3 2 2
) ·3H
O was 44.4 h− , while the selectivity for siloxane
1
coexistence of Cu(II) and Cu(I) in this material (Fig. S10†). Cu(NO
Compared with Cu85%ZIF-8 nanocrystals, HKUST-1 had a was only 85%, and the by-product was dimethyl phenyl
lower TOF (3.5 h− ) under the same conditions (entry 3). A pre- silanol. It is worth noting that Cu(NO
vious study by A. Dhakshinamoorthy et al. had revealed that obvious color change during the reaction. After the reaction
HKUST-1 exhibited excellent catalytic activity for this reaction, started, Cu(NO ·3H O was reduced to a black solid, and the
and both Cu(II) and Cu(I) were simultaneously required as reaction rate was fast till the conversion of PhMe SiH reached
We also evaluated the catalytic activity of 95% after 5 h of reaction (entry 6). Then, the solution turned
HKUST-1 under the same conditions as the report (entry 4) blue-green, and the reaction rate obviously decreased. The
1
3 2 2
) ·3H O displayed an
)
3
2
2
2
2
9
active sites.
16566 | Dalton Trans., 2019, 48, 16562–16568
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complete conversion of PhMe
2
SiH was reached after 15 h of increased, the reaction time was greatly extended. All these
reaction (entry 7). The obvious color change in Cu(NO ) ·3H O reactions completely produced single substituted siloxanes.
3
2
2
during the catalytic reaction suggested that the active species
may be the heterogeneous Cu species (black solid) in the early
stage of the reaction. Moreover, zero-valence Cu was syn-
thesized by reducing Cu85%ZIF-8 with hydrazine (the details
of the synthesis are given in the Experimental section). As seen
in Table 2 entry 9, the TOF of Cu was low (0.7 h ), which indi-
cated that zero-valence Cu was less active in this reaction.
As discussed above, Cu ion-doped ZIF-8 nanocrystals exhibi-
ted high activity and selectivity for the dehydrogenative coupling
4
. Conclusions
Cu ion-doped ZIF-8 nanocrystals were synthesized via a
solvent-assisted mechanochemical method. From the EPR,
XPS and XAES analyses, we found that both Cu(I) and Cu(II)
were present in this material, and the surface ratio of Cu(I) to
Cu(II) was about 9 : 1. The coexistence of Cu(I) and Cu(II) in the
Cu ion-doped ZIF-8 nanocrystals makes them excellent cata-
lysts for the dehydrogenative coupling of silanes with alcohols.
We speculate that the ratio of Cu(I) to Cu(II) is also important
for the excellent catalytic performance, which requires further
research to be proven. Since the valence state of Cu ions also
plays an important role in many other reactions, further study
is currently underway to apply this material to other catalytic
reactions.
−
1
2
of PhMe SiH and n-butanol compared with other Cu-based
materials. Moreover, the reaction could be performed under an
air atmosphere in contrast to the previously reported Cu-based
catalysts (Table S4†). Although the TOF value of the Cu ion
doped ZIF-8 nanocrystals was not the highest (compared with
that of oriented (2.0.0) copper(I) oxide nanoplatelets on few-
2
8
layered graphene ), we believe that the simple preparation
method used for this material is beneficial for its further appli-
cation. In addition, the Cu ion-doped ZIF-8 nanocrystals exhibi-
ted higher catalytic performance than the metal-free N and S
3
9
co-doped carbon catalyst but lower than those of the Co single
25
19,22,23
Conflicts of interest
atom catalyst and noble metal catalysts
(Table S4†).
Further, the surface ratio of Cu(I) to Cu(II) was calculated
based on XPS data to find out the influence of the Cu oxi-
dation state on the catalytic performance (Table S5†). In the
Cu ion-doped ZIF-8 nanocrystals, the average surface ratio of
There are no conflicts to declare.
Cu(I) to Cu(II) was about 9 : 1, while that in HKUST-1 was about Acknowledgements
3
: 7. We speculate that the ratio of Cu(I) to Cu(II) may also play
This work was supported by the Natural Science Foundation
for Young Scientists of Shanxi Province, China
an important role in the reaction, and further research is
required for proof.
(
(
201601D021031), Natural Science Foundation of China
21601114 & 20871167) and the Fund for Shanxi “1331”
Based on the above findings, we further extended the reac-
tion range for silane dehydrogenation coupling, and the
results are presented in Table 3. When dimethylphenylsilane
reacted with methanol, ethanol, n-butanol, and n-octanol, the
reaction time increased as the alkyl chain of the alcohol
increased in length (entries 1 to 4). In entries 5 to 7, n-butanol
reacted with phenylsilane, diphenylsilane, and triphenylsilane,
respectively. As the molecular steric hindrance of silane
Project. We also wish to acknowledge the Open Project (No.
01614) of The State Key Laboratory of Physical Chemistry of
2
Solid Surfaces (Xiamen University).
Notes and references
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This journal is © The Royal Society of Chemistry 2019