Synthesis of Carbonates from CO with Covalent Triazine Frameworks
2
(
SBA-15) were synthesized as reference material (see Support-
ing Information for synthetic details) for the cycloaddition reac-
tion of CO2 to epichlorohydrin (Table 1). Indeed, primary
amines exhibited the lowest activity with only 48.9% conver-
sion, the presence of additional secondary amines increased
the conversion to 80.5%, whereas the best activity was ob-
tained with the supported tertiary amines, which proved able
to fully convert the starting epoxide under these conditions.
The high activity of the tertiary amines is attributed to the ab-
sence of hydrogen bonding, which can lead to poisoning of
Table 1. Elemental analysis, structural parameters, and catalytic evalua-
tion of chloropropene carbonate formation by different covalent triazine
frameworks and amine-grafted SBA-15.
[
a]
[b]
BET
2
[c]
[d]
[d]
Material
N
[
S
Vp
Conversion
Selectivity
[%]
À1
3
À1
%] [m g
]
[cm g ] [%]
[
e]
blank
–
–
–
2.3
n.d.
CTF-1
19.7 536
8.8 2087
15.9 1745
0.38
1.30
0.94
0.98
0.43
0.35
0.45
81.2
100
100
13.2
48.9
80.5
100
94.4
95.8
94.6
100
83.9
91.3
92.9
CTF-1-HSA
CTF-P-HSA
SBA-15
SBA-15-prop-NH
SBA-15-prop-EDA 2.26 235
SBA-15-prop-DMA 2.12 341
–
856
2
3.06 314
the catalyst by CO itself.
2
The importance of chemical and structural parameters on
[17]
the catalytic activity was then evaluated. As reported before,
Reactions conditions: 18 mmol epichlorohydrin, 100 mg catalyst, 1308C,
.9 bar CO2, 4 h, without solvent. [a] Determined from elemental analysis.
b] Surface area determined by the Brunauer–Emmett–Teller (BET)
crystalline or amorphous triazine frameworks can be synthe-
sized by varying the heating profile in the synthetic procedure
starting from 1,4-dicyanobenzene. These two covalent organic
frameworks, despite originating from the same starting mono-
mer, are chemically and structurally different. These differences
can lead to antagonistic effects in the activity of the material,
rendering the comparison of both samples very interesting in
an attempt to evaluate key parameters in catalyst design.
From a chemical point of view, the crystalline sample has
a higher relative nitrogen content, which should lead to better
catalytic activities due to the higher number of basic catalytic
sites. On the other hand, the structural characteristics of the
amorphous analogue may enhance the catalytic activity due to
the higher surface area and hierarchical porosity, where addi-
tional mesopores facilitate more rapid diffusion to the active
sites compared to the microporous crystalline framework (see
Supporting Information). Interestingly, under the same condi-
tions and for the same amount of catalyst, the conversion of
epichlorohydrin was significantly lower, ca. 81% (94.4% selec-
tivity), with the crystalline framework. This result indicates that
a high surface area and/or a hierarchical porous structure en-
hances the catalytic activity, potentially overcoming the detri-
mental effect assumed for the lower nitrogen content in this
sample compared to CTF-1. To gain further insight in the rela-
tive importance of chemical and structural properties of the
material, catalytic tests were performed with varying amounts
of the amorphous framework (Figure 1). For the same accessi-
6
[
0
method. [c] Vp=pore volume calculated at P/P =0.99; [d] Determined
by GC-MS. [e] Not determined.
could be obtained at 1308C for both high-surface-area-cata-
lysts with selectivities of 95.8% and 94.6% for CTF-1-HSA and
CTF-P-HSA, respectively, in the corresponding cyclic carbonate
formation. The covalent triazine frameworks are thus highly
active and selective under mild conditions, short reaction
times, and in the absence of solvent or co-catalyst. Negligible
amounts of diol by-products could be detected, possibly due
to presence of residual system water leading to partial hydroly-
sis of the starting epoxide. The high catalytic activity of the re-
ported system can be explained by the chemical structure of
the frameworks, which comprise triazine building blocks and
additional pyridinic blocks in case of CTF-P-HSA. This was sup-
ported by a recent study that reported that C=NÀC-containing
free-standing amines are the most active among other primary
[
9]
and secondary amines. Moreover, melamine-based periodic
[13]
mesoporous silica also showed activity in CO conversion.
2
Thus the high activity of the triazine-based frameworks, which
2
comprise many C=N sp structures, not only originates from
the high nitrogen content but also from the type of nitrogen
species and hybridization state.
To prove how the chemical structure was influenced by the
13
2
starting monomer, the materials were investigated by C CP-
MAS solid-state NMR (Figures S2–S3). For CTF-1, the signals at
d=168, 137, and 127 ppm can be assigned to the three differ-
ent framework carbons and the signal at d=114 ppm is indica-
tive of residual cyano groups, due to incomplete condensation.
In the case of both CTF-1-HSA and CTF-P-HSA, the broad sig-
nals and the disappearance of the signal related to the triazine
carbon confirm that, as indicated by the change in the C,H,N
content, partial carbonization and removal of triazine occurred.
The incorporation of pyridinic groups in the framework of CTF-
P-HSA is evidenced by the presence of a third shoulder within
the broad signal when compared to the signal of CTF-1-HSA,
where only two shoulders can be distinguished (Figure S3).
X-ray photoelectron spectroscopy (XPS) analysis of the high-
surface-area frameworks further proved the presence of chemi-
ble surface (approx. 50 m for 25 mg of CTF-1-HSA/100 mg of
CTF-1) both samples showed similar activity, ca. 80% conver-
sion, indicating that additional mesoporosity favors diffusion
and is able to counterbalance the higher nitrogen content of
the crystalline framework. As expected the amount of catalyst
affects the conversion, with the epoxide conversion increasing
with increasing amount of catalyst, with only 50 mg of catalyst
(corresponding to 2.17 mol% of monomer unit) required to
reach conversions >90%.
It can be assumed that a high and accessible surface area as
well as a high nitrogen content within the catalyst will have
a beneficial effect on the activation of CO . A material which
2
combines both properties was synthesized using 2,6-dicyano-
pyridine as monomer (CTF-P-HSA). The influence of the chemi-
cal structure was investigated by a temperature-dependent
comparison between the benzylic (CTF-1-HSA) and pyridinic
(CTF-P-HSA) high-surface-area frameworks (Figure 2). The yields
[
22]
cally bonded nitrogen. To confirm the influence of the type
of amine, amines supported on periodic mesoporous silica
ChemSusChem 0000, 00, 1 – 8
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