Porous Carbon Nitride Frameworks Derived from Covalent Triazine Framework Anchored Ag Nanoparticles for Catalytic CO2 Conversion

Article abstract of DOI:10.1002/chem.201900563

Porous carbon nitride frameworks (PCNFs) with uniform and rich nitrogen dopants and abundant porosity were successfully fabricated through the direct carbonization of the covalent triazine frameworks (CTFs) at different pyrolysis temperatures and used as supports to anchor and stabilize Ag nanoparticles (NPs) for catalytic CO₂ conversion. Importantly, the pyrolysis temperature plays a crucial role in the properties of porous carbon nitride frameworks. The material carbonized at 700 °C showed the highest surface area and micro- and mesoporous structure with a certain interlayer distance. Taking advantage of their unique surface characteristics, PCNF-supported Ag NP catalysts (Ag/PCNF-T, T=pyrolysis temperature) were prepared by a simple chemical method. A series of characterizations revealed that Ag NPs are embedded in the porous carbon nitride frameworks and confined to a relatively small size with high dispersion owing to the assistance of the abundant surface groups and porous structures. The as-obtained Ag/PCNF-T catalysts, especially Ag/PCNF-700, showed excellent catalytic activity, selectivity, and stability for the carboxylation of CO₂ with terminal alkynes under mild conditions. This can be due to the existence of abundant nitrogen atoms and diverse porosity, which resulted in highly efficient catalytic activity and stability.

Full text of DOI:10.1002/chem.201900563

A Journal of
Accepted Article
Title: Porous Carbon Nitride Frameworks Derived from Covalent
Triazine Frameworks Anchored Ag Nanoparticles for Catalytic
CO2 Conversion
Authors: Xingwang Lan, Yiming Li, Cheng Du, Tiantian She, Qing Li,
and Guoyi Bai
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Porous Carbon Nitride Frameworks Derived from Covalent
Triazine Frameworks Anchored Ag Nanoparticles for Catalytic
CO2 Conversion
Xingwang Lan,*[ab] Yiming Li,[a] Cheng Du,[a] Tiantian She,[a] Qing Li,[a] Guoyi Bai*[a]
Abstract: Porous carbon nitride frameworks (PCNFs) with uniform
and rich nitrogen dopants and abundant porosity had been
successfully fabricated via the direct carbonization of the covalent
triazine frameworks (CTFs) at different pyrolysis temperatures, and
used as support to anchor and stabilize Ag nanoparticles (NPs) for
catalytic CO2 conversion. Importantly, the pyrolysis temperature
plays a crucial role in the properties of porous carbon nitride
low efficiency, complicated catalytic systems as well as narrow
substrate scope. Therefore, developing a robust and efficient
catalyst is still of great importance. Taking account of the
problems of practical utility and catalyst recycling in
homogeneous catalytic systems, heterogeneous catalysts are
more attractive for CO2 conversion.[5]
Owing to their high nitrogen content and unique surface
properties, nitrogen-doped carbon materials (CNs) have been
investigated extensively in gas capture,[6] catalysis[7] and
supercapacitors.[8] Particularly, porous CNs are attractive
because of their high specific surface areas and large pore
volumes. However, they are rarely reported in the CO2
conversion. This is primarily due to random dispersion of N
atoms and poor pore structures. In this direction, regular
dispersion of N atoms and introduction of microporosity into CNs
is thus considerably attractive,[9] which can improve the
interactions with CO2 molecules, enhancing their capture
capacity and catalytic performance. Although several strategies,
including chemical activations[10] and template methods,[11] have
realized the formation of micropores, some issues such as low
nitrogen content or blocked channels still appeared in the
prepared CNs. Recent reports have demonstrated that metal-
organic frameworks (MOFs) are promising precursors or
templates to afford porous CNs via pyrolysis.[12] However,
unexpected metal residues and morphology damage limit their
development to some extent. Similar to MOFs, covalent organic
frameworks (COFs),[13] one kind of pure organic porous
materials constructed from organic building blocks by the
covalent linkage, have also shown attractive superiority in gas
capture[14] and catalysis.[15] Their high porosity, regular
arrangement of heteroatoms and structural diversity can render
COFs promising potential in fabricating porous carbon materials
with doped heteroatoms. Importantly, this strategy can avoid the
process of removing the residual metal species in the formed
materials from MOFs pyrolysis. To date, there are only few
reports on the carbonization of COFs.[16] For instance, Pan et
al.44 recently prepared porous CNs through COFs carbonization
and applied it as electrodes into lithium and sodium ion batteries.
The covalent triazine frameworks (CTFs) are related to COFs
and feature high nitrogen content and abundant pore
structure.[17] Therefore, CTFs might be an ideal candidate as
precursors for CNs, not only delivering abundant nitrogen sites
and porous structures but also facilitating to immobilize and
stabilize specific metal active species for catalysis.
o
frameworks. The material carbonized at 700 C showed the highest
surface area and micro- and mesoporous structure with certain
interlayer distance. Taking advantage of their unique surface
characteristics, PCNFs supported Ag NPs catalysts (Ag/PCNF-T)
were prepared by
a simple chemical method. A series of
characterizations revealed that Ag NPs are embedded in the porous
carbon nitride frameworks and confined in a relative narrow size with
high dispersion with the assistance of the abundant surface group
and porous structures. The as-obtained Ag/PCNF-T catalysts,
especially Ag/PCNF-700, showed excellent catalytic activity,
selectivity, and stability for the carboxylation of CO2 and terminal
alkynes under mild conditions. This can be due to the existence of
abundant nitrogen atoms and diverse porosity, which resulted in
highly efficient catalytic activity and stability.
Introduction
Converting carbon dioxide (CO2), the major greenhouse gas,
into high-value chemicals has gained considerable attention to
realize renewable energy utilization and alleviate global warming
issue.[1] Various approaches for CO2 conversion have emerged
in the past decades.[2] One particularly significant transformation
is the direct carboxylation of CO2 to alkynyl carboxylic acids,
which is intriguing due to their broad applications as synthon in
organic synthesis.[3] Although several catalytic systems have
been described for this transformation,[4] most of these protocols
faced disadvantages such as high temperature and pressure,
[a]
[b]
Dr. X. Lan, Y. Li, C. Du, T. She, Q. Li, Prof. G. Bai
Key Laboratory of Chemical Biology of Hebei Province, College of
Chemistry and Environmental Science
Hebei University, Baoding, Hebei, 071002 (P.R. China)
E-mail: hxlxw@sina.cn, baiguoyi@hotmail.com
Dr. X. Lan
Key Laboratory for Green Chemical Technology of Ministry of
Education, School of Chemical Engineering and Technology
Collaborative Innovation Center of Chemical Science and
Engineering
Tianjin University Weijin Road 92, Tianjin, 300072, P.R.China
E-mail: hxlxw@sina.cn
On the other hand, the lack of active species is also an
important factor for the low catalytic activity of porous CNs in the
conversion of CO2. To address this issue, incorporating
transition metal into porous CNs is extremely favorable for
catalytic CO2 conversion. Silver nanoparticles (Ag NPs) have
Supporting information for this article is given via a link at the end of
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recently been demonstrated to be highly active for the direct
carboxylation with CO2.[4e,4f] However, Ag NPs tend to aggregate
owing to their high surface energy,[18] thereby resulting in
decreasing catalytic performance. Vast efforts on this problem
suggest that rational support can protect Ag NPs against
agglomeration and improve their recovery.[19]
during the carbonization.[21] The wide-angle X-ray diffraction
(WAXRD) patterns of PCNF-T exhibit similar diffraction features
with two diffraction peaks at around 24o and 44o (Figure 2b). The
broad peak at 24o can be assigned to the (002) planes of
amorphous graphitic carbon,[22] suggesting that amorphous
carbons partially convert to graphite. Moreover, the weak peak
at 44o with a low and increased intensity indicates the intralayer
condensation of the materials and the defect in the graphite
layers due to nitrogen dopants at different pyrolysis temperature
(Figure S1).[23] In addition, some crystalline peaks of CTF at
about 10-30o have disappeared after carbonization, implying the
reorganization of the framework at high temperature. The N2
adsorption/desorption isotherms of the PCNF-T materials were
carried out to reveal their porous features and shown in Figure
Inspired by the aforementioned strategies, porous carbon
nitride frameworks (PCNFs) were fabricated through the direct
carbonization of CTF and used as support to homogeneously
immobilize and stabilize Ag NPs in CO2 conversion for the first
time (Figure 1). The resultant Ag NPs supported PCNF catalysts
(Ag/PCNFs) showed several important benefits for catalytic CO2
conversion, including 1) The CTFs with two-dimensional
microcrystal structure as precursor can endow the catalyst with
unique property; 2) Abundant alkaline nitrogen sites facilitate the
coordination properties, which can enhance the dispersity and
stability of Ag NPs in/on the support, contributing to a better
catalytic performance; 3) High specific surface area and order
porous structures with abundant nitrogen basic sites can provide
weak interaction to CO2 molecules, increasing capture and
catalysis capacity of catalysts. Due to these outstanding
characteristics, the Ag/PCNF catalysts accomplished the direct
carboxylation of CO2 and terminal alkynes under milder
conditions (room temperature and atmospheric pressure) with
efficient catalytic activity, excellent stability and wide broad
substrates. Importantly, the pyrolysis temperature of CTF
precursor had a remarkable influence on the catalyst structure
and catalytic performance of Ag/PCNFs, which were
comprehensively investigated by a series of characterizations.
2c. The curves of PCNF-600 and PCNF-700 exhibit
a
combination of type I and IV curves with a steep rise at low
relative pressures (P/P0 < 0.01) and high relative pressures
(P/P0 > 0.9), indicating the presence of micro and mesopore
structures. For the PCNF-500, however, the typical type IV curve
can be only observed, demonstrating that mesopores dominate
this material with the lack of micropores. These results can be
further supported by the pore-size distribution curves (Figure
S2a). The micropores calculated by HK methods (Figure 2d)
indicate the micropores size at about 0.5 nm in the PCNF-600
and PCNF-700 samples. The textural properties of these PCNF-
T materials (Table S1) show that PCNF-700 has the maximum
surface area (401 m2/g) and pore volume (0.81 cm3/g), further
suggesting the carbonization temperature has an important
effect on the physical properties and composition of PCNF-T.
Figure 1. Illustrated design and preparation procedure of Ag/PCNF-T (500,
600, 700) catalysts.
Results and Discussion
CTF precursor is mainly prepared by the nucleophilic
substitution of cyanuric chloride and piperazine according to our
previous work.[20] It presents two-dimensional wrinkled sheet-like
structure. Thereafter, PCNFs were obtained by the direct
carbonization of the CTF precursor under different pyrolysis
temperatures. To reveal the physical properties, composition,
and morphology structure of the formed PCNF-T materials,
several characterizations were performed. The small-angle X-
ray diffraction (SAXRD) patterns of CTF and PCNF-T show
corresponding diffraction peaks at a low angle, ascribing to the
existence of ordered layer and pore structure, as shown in
Figure 2a. Notably, as compared to the SAXRD peak of CTF,
the reflections shift to a higher 2θ value after the carbonization.
This can be due to the structural shrinkage and denseness
Figure 2. (a) SAXRD and (b) WAXRD patterns of CTF and PCNF-T, (c) N2
adsorption/desorption isotherms of PCNF-T, (d) micropores calculated by HK
methods of PCNF-600 and PCNF-700, (e) FT-IR spectra of PCNF-T and CTF
precursor, and (f) Raman spectra of PCNF-T.
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Fourier transform infrared spectroscopy (FT-IR) analysis of
PCNF-T materials (Figure 2e) show the of some characteristic
peaks of CTF precursor, but present the broad and weak bands
of the pyrolysis temperature. The peaks of pyrrolic N and
graphitic N shift slightly toward the higher binding energy. These
may be attributed to that the bonding types of pyrrolic N and
at 1279 and 1604, and 3412 cm-1, which may correspond to C-N, graphitic N in the newly formed PCNFs are altered with the
C=N, and N-H, respectively. Furthermore, bands at 1279 and
1604 cm-1 become weaker at higher carbonization temperature,
demonstrating that the piperazine and triazine ring are
decomposed under the elevated carbonization temperature. The
Raman spectra (Figure 2f) exhibit that all PCNF-T possess two
broad peaks at 1359 and 1578 cm-1, corresponding to the
topological defects (D band) and graphitic structure (G band),
respectively.[24] The ID/IG values for PCNF-T were found to
increase with the elevated carbonization temperature,
suggesting more defects or higher disorder degree formed while
pyrolysis temperature elevating. Additively, broader peaks at
around 2750 cm-1, corresponding to the 2D band, also appear in
all PCNF-T samples, which is a typical feature of high graphitic
degree.
X-ray photoelectron spectroscopy (XPS) analysis were
performed to investigate the chemical states of the CTF
precursor and PCNF-T materials, as shown in Figure 3. The
high-resolution C 1s spectra (Figure 3a) show three types of
carbon species in the CTF precursor, corresponding to triazine
N-C=N (283.6 eV), piperazine C-C and C-H (sp3) (284.8 eV),
and piperazine C-N (286.7 eV), respectively.[20,25] As for the
PCNF-T materials, a new graphitic C (288.6 eV)[26] appears
together with the disappearance of the triazine N-C=N, and the
piperazine C can be still found. These results indicate the
existence of graphitization carbon and the damage of the
triazine ring in the CTF precursor, in accordance with FT-IR and
Raman results. The high-resolution N 1s spectrum of CTF
precursor (Figure 3b) has two types of nitrogen species,
corresponding to piperazine N-C (397.3 eV) and triazine N=C
(398.6 eV).[20] However, all N 1s spectra of the PCNF-T materials
can be deconvoluted into three newly formed types containing
pyridinic N (397.9-398.0 eV), pyrrolic N (399.9-400.5 eV), and
graphitic N (402.2-403.9 eV).[27] Notably, the percentage of
pyridinic N decreased and pyrrolic N increased with the increase
increase of the pyrolysis temperature. Moreover, the atomic
ratios of N to C in the PCNF-T materials decrease from 22 to
14 % with the pyrolysis temperature enhanced from 500 to 700
oC (Figure 3c). According to these XPS results and above-
mentioned discussions, different N-type species in the PCNF-T
materials can be roughly speculated and illustrated in Figure 3d.
Encouraged by structure properties in these PCNF-T
materials, then the Ag/PCNF-T catalysts were fabricated by a
solution infiltration method. Inductively coupled plasma (ICP)
analysis of the Ag/PCNF-T catalysts suggest that the Ag
loadings in Ag/PCNF-500, Ag/PCNF-600 and Ag/PCNF-700 are
3.37, 3.79, and 3.69 wt%, respectively. Their catalytic
performances were evaluated in the carboxylation of terminal
alkynes with CO2. The exploratory experiments were started by
using 1-ethynylbenzene as substrate under room temperature
and atmospheric pressure of CO2, as illustrated in Table 1. It
was found that the transformation could occur under no catalyst
with a low conversion of 1-ethynylbenzene. Instead, PCNF-500,
PCNF-600, and PCNF-700 as hetergenerous catalysts delivered
a slight promotion for the conversion with similar selectivity of
product. To our delight, all Ag/PCNF-T catalysts showed
remarkably enhanced conversions with selectivity of >99%.
Among these, the Ag/PCNF-700 catalyst was the most effecient
and gave the conversion in a 85%. Prolonging the reaction time
to 18 h showed the highest conversion. Comparably, the Ag/C
catalyst only furnished
a
59% of conversion, further
demonstrating the outstanding advantage of Ag/PCNF-700
catalyst. The control experiment without Cs2CO3 only gave poor
conversion and selectivity, indicating that Cs2CO3 play a key role
in the transformation.
Table 1 Synthesis of 3-phenylpropiolic acid from CO2 and 1-ethynylbenzene.[a]
Entry
1
Catalyst
none
Time (h)
Conv.b(%)
Sel.b(%)
95
6
6
49
53
56
55
70
77
85
93
98
97
59
13
2
PCNF-500
PCNF-600
PCNF-700
Ag/PCNF-500
Ag/PCNF-600
Ag/PCNF-700
Ag/PCNF-700
Ag/PCNF-700
Ag/PCNF-700
Ag/C
95
3
6
96
4
6
96
5
6
>99
>99
>99
>99
>99
>99
>99
15
6
6
7
6
8
12
18
24
12
18
9
10
11
12[c]
Ag/PCNF-700
[a]Reaction condition: 1-ethynylbenzene (0.5 mmol), catalyst (20 mg), Cs2CO3
(1.5 equiv.), CO2 (balloon), 25 oC, DMSO (3 mL), unless otherwise noted.
[b]Determined by Agilent HPLC 1220 based on the starting material. [c]Without
Cs2CO3.
Figure 3. (a) C 1s and (b) N 1s XPS spectra of PCNF-T materials and CTF
precursor. (c) N/C atomic ratio for the total N and C in the PCNF-T materials.
(d) Possible structures of the N-type species in the PCNF-T materials.
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To reveal the effect of the Ag/PCNF-T catalysts in the catalytic
activity,
a
series of characterization technologies were
performed. XRD patterns of the Ag/PCNF-T catalysts (Figure 4a
and 4b) still show the board peak of amorphous graphitic carbon.
The rest of the identifiable peaks in Ag/PCNF-500 can be
ascribed to Ag (111), Ag (200), Ag (220), and Ag (311),
respectively.[27] In contrast, the intensity of the diffraction peaks
corresponding to Ag remarkably decreases in Ag/PCNF-600,
and the peaks nearly disappear in Ag/PCNF-700, indicating that
the Ag NPs are in a very small size and high dispersion. The
XPS analysis of the Ag/PCNF-T catalysts further confirm the
existence of Ag element in all the samples (Figure 4c), and the
superficial Ag contents of Ag/PCNF-600 and Ag/PCNF-700
sample have lower percentage than the Ag/PCNF-500 (Table
S2). The high-resolution C 1s and N 1s spectra of PCNF-T have
almost no change in before and after supported Ag NPs (Figure
3a and 3b vs Figure S3), suggesting that supported Ag NPs did
not destroy the structure of PCNF-T. The Ag 3d spectra in all
Ag/PCNF-T samples show similar peaks with binding energies
of Ag 3d3/2 and Ag 3d5/2 at 373.8 and 367.8 eV, respectively, and
the peak distance of 6.0 eV is in accordance with the reported
Ag NPs,[28] further suggesting their same chemical states in
metallic Ag0 (Figure 4d and S4), which are in accordance with
the XRD results. Thermogravimetric analysis (TGA) of
Ag/PCNF-600 and Ag/PCNF-700 show that both samples are
more thermostable even at 800 oC, while Ag/PCNF-500
continues to decompose from 500 oC (Figure S5).
Figure 5. (a) CO2 adsorption/desorption (298 K) of Ag/PCNF-T. (b) N2
adsorption/desorption (298 K) of of Ag/PCNF-T.
The structure and surface morphology of the Ag/PCNF-T
catalysts were investigated by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). As
observed in the SEM images (Figure 6a-c, and Figure S6), all
Ag/PCNF-T samples have typical crosslinking network structure
with diverse porous structure (marked by red circles). Obviously,
the increase of calcination temperature cause the dispersion of
structure units and shrinkage of the void pores. On the other
hand, graphite-like nanosheet structure can be clearly observed
in the TEM images of all samples (Figure 6d-f and S7). Ag NPs
on PCNF-500 are observed in an average size of 9.22 nm with
slight aggregation. In contrast, Ag NPs on PCNF-600 and
PCNF-700 are highly dispersed with average size of 6.08 and
4.89 nm, respectively (Figure 6g-i). The well distribution and
smaller particles size can be ascribed to the formation of large
surface area and abundant surface functional groups under the
elevated carbonization temperatures.
Figure 4. (a) and (b) XRD patterns and (c) XPS spectra of Ag/PCNF-T
catalysts, (d) Ag 3d XPS spectrum of Ag/PCNF-700 catalyst.
Figure 6. SEM and TEM images and particles size distribution of Ag/PCNF-T
catalysts. (a), (d) and (g) Ag/PCNF-500 catalyst, (b), (e) and (h) Ag/PCNF-600
catalyst, and (c), (f) and (i) Ag/PCNF-700 catalyst.
The CO2 adsorption/desorption of the Ag/PCNF-T catalysts
were measured at room temperature and atmosphere pressure
(Figure 5a). As expected, all the catalysts show good sorption
capacity toward CO2, which might be beneficial for the activation
and conversion of CO2. The N2 adsorption/desorption analysis
were then performed to investigate porous properties of
Ag/PCNF-T catalysts (Figure 5b and Figure S2b). Significant
decrease of N2 sorption capacity and BET surface area in
comparison with corresponding PCNF-T demonstrate that the
Ag nanoparticles were mainly distributed in the mesopores,
which is favorable for enhancing catalytic activity and stability.
The surface basicity of the Ag/PCNF-T catalysts was
determined by CO2 temperature-programmed desorption (CO2-
TPD) and shown in Figure 7. Ag/PCNF-500 only gives very
weak CO2 adsorption peaks, indicating that weak basic sites in
this sample are very scarce. Comparably, both Ag/PCNF-600
and Ag/PCNF-700 show a very strong adsorption peaks within
50-150 oC, which can be attributed to weak basic sites
associated with the chemical and physical adsorption of CO2.[29]
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Moreover, weak peaks formed within 150-250 oC
correspondingly assigned to medium basic sites, demonstrating
the existence of the weak chemical adsorption of CO2.[30] These
results further indicated that pyrolysis temperature has an
important influence on the formation of surface sites in the
Ag/PCNF-T catalysts.
[a]Reaction condition: alkynes (0.5 mmol), Ag/PCNF-700 (20 mg), Cs2CO3 (1.5
equiv.), CO2 (balloon), 25 oC, DMSO (3 mL), 18 h, unless otherwise noted.
[b]Yield of isolated product. [c] 24 h.
Stability and reusability of the Ag/PCNF-700 catalyst was also
investigated. We note that the catalyst could be reused at least
five cycles without significant loss in the catalytic activitiy and the
selectivity remained at >99% (Figure 8a). Furthermore, the TEM
image of the recycled catalyst showed that the Ag nanoparticles
were still highly dispersed with an average size of 5.01 nm
(Figure 8b and Figure S8a). XRD patterns of the recycled
catalyst retained its original morphology (Figure S8b). ICP
analysis of the reaction mixture exhibited barely 0.44% of the
available Ag content for the cases of catalyst. These results
strongly demonstrated that the PCNF-700 could effectively
prevent the nanoparticles from leaching and aggregating during
the catalysis.
Figure 7. CO2-TPD profiles for Ag/PCNF-T catalysts.
Accordingly, we speculate that the Ag/PCNF-700 catalyst
having the optimal catalytic activity can be attributed to the
following aspects: 1) Ag NPs in the Ag/PCNF-700 catalyst are
highly dispersed and confined in a narrow size due to high
surface area and abundant nitrogen anchoring sites; 2) various
pore structures with nitrogen basic sites, especially the
micropores, in the catalyst can strongly adsorb and activate CO2
molecules, confirmed by the CO2 sorption and CO2-TPD. To
further investigate the general scope of Ag/PCNF-700 in
catalytic CO2 conversion, the carboxylation of CO2 with various
terminal alkynes was also explored and the results were shown
in Table 2. To our delight, this catalyst can smoothly convert all
the substituted 1-ethynylbenzene having electron-donor or
electron acceptor groups to the corresponding alkynyl acids with
excellent isolated yields (Entries 1-5). Moreover, the alkyne with
hetero aromatic ring (Entry 6) also furnished with a 79% yield.
The long chain alkyl alkyne could also proceed smoothly with a
good yield (Entry 7). These results clearly demonstrate the
general application of the Ag/PCNF-700 catalyst in the catalytic
CO2 conversion.
Figure 8. (a) Stability of the Ag/PCNF-700 catalyst. (b) TEM and particles
size distribution of the reused catalyst after five times.
According to a series of catalyst chacharacterizations and
experiment results, the rich nitrogen, abundant porous structure
and narrow particle size in the Ag/PCNF catalysts were
indicated to play an important role in the catalytic efficiency and
stability. We speculated that these features delivered anchoring
group for Ag NPs and basic sites for adsorbing and activating
CO2. Following these analysis and some previous reports,[4d,31]
a
plausible mechanism with respect to the carboxylation of CO2
with various terminal alkynes over the Ag/PCNF catalyst was
thus proposed (Figure 9). The possible process was herein
simplified into four main steps. Step 1. The terminal alkynes and
CO2 were adsorbed on the catalyst, and the terminal C-H bond
of alkynes was activated by Ag NPs in the Ag/PCNF catalyst
with the generation of silveracetylide intermediate. Step 2.
Nitrogen basic sites captured and activated CO2 into an
electrophilic species. Step 3. The silveracetylide intermediate
suffered from the electrophilic attack of CO2 with forming silver
carboxylate in the catalyst. The formed silver carboxylate could
smoothly generate cesium carboxylate with the aiding of Cs2CO3
through the transmetallation. Step 4. The desired alkynyl
carboxylic acids products were finally obtained by a simple
acidification.
Table 2 Synthesis of various alkynyl acids from terminal alkynes and CO2 over
Ag/PCNF-700.[a]
Entry
1
Alkyne
Product
Yield[b](%)
90
2
3
4
5
6
92
87
94
83
79[c]
7
82
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were purchased from locally commercial suppliers and used without
further purification unless noted.
Preparation of PCNFs from CTF via direct carbonization.
First, the covalent triazine framework (CTF) was synthesized according
to the following steps. Typically, piperazine (1.008 g, 12 mmol) and
anhydrous K2CO3 (3.312 g, 24 mmol) were dissolved in anhydrous 1,4-
dioxane (75 mL). Then, cyanuric chloride (1.464 g, 8 mmol) in 1,4-
dioxane (30 mL) solution was dropwise added into the above mixture
under mechanical agitation, together with the continuous appearance of
cotton-like precipitate. After that, the reaction mixture was stirred for
additional 2 h at room temperature. Subsequently, the mixtures were
heated to 90 °C for further polymerization, together with rapid stirring for
48 h. Upon completion, the obtained CTF solid was separated by
filtration, washed successively with dichloromethane, ethanol and water.
The solid was then dried at 70 °C for 24 h, yielding an off-white powder.
After drying, the resultant CTF material (1.5 g) was subjected to direct
carbonization in a temperature-programmable tube furnace at different
temperature (500, 600, and 700 oC) for 2 h under nitrogen flow at a
heating rate of 3 °C/min. Cooled down to room temperature, the black
materials were finally obtained and denoted as PCNF-T (T represents the
carbonization temperature) and used for support without further
treatment.
Figure 9. Possible mechanism for the carboxylation of CO2 with various
terminal alkynes over the Ag/PCNF catalyst.
Conclusions
In summary, porous carbon nitride frameworks were
successfully obtained via the direct carbonization of CTF at
different temperatures and used as support to immobilize and
stabilize Ag NPs for catalytic CO2 conversion. A series of
characterizations disclosed that the pyrolysis temperature plays
Preparation of Ag/PCNF-T (500, 600, 700).
To prepare PCNF-T supported Ag NPs, 200 mg PCNF-T was firstly
introduced into a solution of 10 mL methanol containing 20 mg AgNO3.
The mixtures were continuously stirred for 4 h at room temperature. Then,
the solid was filtered and washed with methanol, activated at 100 oC
under vacuum. Afterwards, the resulting solid was added into excess
NaBH4 aqueous solution (40 mg NaBH4 in 4 mL water), and vigorously
stirred at room temperature for 4 h. Finally, the solids were filtered,
washed with deionized water, dried at 70 °C under vacuum to give the
corresponding catalyst. The catalysts were denoted as Ag/PCNF-T (T =
500, 600, 700) and preserved in Ar atmosphere for further use.
Ag/PCNF-T catalyzed carboxylation of terminal alkynes with CO2.
Catalytic performance of the Ag/PCNF-T catalyst for the direct
carboxylation of terminal alkynes with CO2 were performed in a 10 mL
flask installed a triple valve with CO2 balloon. Ag/PCNF-T (20 mg),
terminal alkynes (0.5 mmol), Cs2CO3 (1.5 equiv.) and DMSO (3 mL) were
added into the flask, and then it was vacuumed and purged with CO2
three times to exclude air in the reaction system. Afterwards, the
mixtures were stirred at room temperature for 18 h. Upon completion, the
mixtures were diluted with water and the used catalyst was filtered off.
Subsequently, the filtrate was initially washed with CH2Cl2, and the
aqueous layer was acidified with HCl to pH = 1 and extracted with ethyl
acetate (3×30 mL). Then, the resultant organic phase was washed with
NaCl saturated solution. Finally, the combined organic liquid was dried
over anhydrous Na2SO4, filtered and removed organic solvent in vacuo to
gain the alkynyl carboxylic acids. Notably, in the screening catalyst stage,
an Agilent 1220 Infinity liquid chromatograph was employed to monitor
the reaction mixtures.
a
crucial role in the properties of porous carbon nitride
frameworks. The material carbonized at 700 oC showed the
highest surface area with micro- and mesopores. Considering
their unique surface characteristics, the prepared porous carbon
nitride frameworks were employed as supports to immobilize
and stabilize Ag NPs. It was found that Ag NPs can be
embedded in the porous carbon nitride frameworks and confined
in a relative narrow size due to the abundant surface group and
porous structures. These catalysts revealed excellent catalytic
activity, selectivity, and stability toward the carboxylation of
terminal alkynes and CO2 under mild conditions. Particularly, the
Ag/PCNF-700 catalyst exhibited the best catalytic performance.
Abundant nitrogen atoms and diverse porosity facilitating the
high dispersion and inhibiting the aggregation of Ag NPs were
responsible for the excellent catalytic activity and stability.
Unambiguously, this work afforded a simple and powerful tool to
fabricate porous carbon nitride frameworks and
a novel
supported Ag catalyst for CO2 conversion. Based on the
structural diversity of COFs, this approach can open up an
avenue in the preparation of this kind of supported catalysts,
heterogeneous catalysis and CO2 conversion.
Experimental Section
Catalyst recycling procedure.
Materials.
After reaction, the used catalyst was separated and recovered from the
mixtures by filtered using organic membrance, washed several times with
water, dried at 70 °C under vacuum. Subsquently, the recycled catalyst
was directly used in the next run.
Piperazine, cyanuric chloride, alkynes, K2CO3, AgNO3 and Cs2CO3 were
provided by Beijing J&K Scientific Ltd and Shanghai Aladdin Reagent Co.
Ltd, China. Anhydrous 1,4-dioxane was prepared from commercial 1,4-
dioxane by removing water over calcium hydride. All other chemicals
Catalyst characterization.
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10.1002/chem.201900563
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Liu, X. Chai, X. Cai, M. Chen, R. Jin, W. Ding, Y. Zhu, Angew. Chem. Int.
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Joyner-Halenda (BJH) method and HK method from the desorption
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Acknowledgements
This work was supported by the National Natural Science Foundation of
China (21676068), Hebei Natural Science Foundation (B2018201118),
Hebei Higher Colleges Science and Technology Research Program
(QN2018049), and the Project funded by China Postdoctoral Science
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Keywords: porous carbon nitride frameworks • covalent triazine
frameworks • silver nanoparticles • carbon dioxide • conversion
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Dr. X. Lan,* Y. Li, C. Du, T. She, Q. Li,
Prof. G. Bai*
Page No. – Page No.
Porous Carbon Nitride Frameworks
Derived from Covalent Triazine
Frameworks Anchored Ag
Nanoparticles for Catalytic CO2
Conversion
Porous carbon nitride frameworks with uniform and rich nitrogen dopants and abundant porosity had been successfully fabricated via
the carbonization of the covalent triazine frameworks at different pyrolysis temperatures, and used as support to immobilize and
stabilize Ag nanoparticles for catalytic CO2 conversion.
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