Crystal Growth & Design
Communication
1H NMR method, the ratio of 5-mtz to 2-mim is around 1:1
(Figure S1) in ZTIF-8. Each zinc center is tetrahedrally
coordinated by two 5-mtz and two 2-mim ligands (Figure 1a),
Figure 2. N2 sorption isotherms of ZTIF-8 at 77 K (a); CO2 sorption
isotherms of ZTIF-8 at 273 K (b) and 288 K (c).
carbonates. The experiments were carried out at room
temperature and 1 atm pressure in the presence of tetra-n-
tert-butylammonium bromide (TBAB) as the cocatalyst (see
produced from the coupling reaction of CO2 with epoxides
were determined by the 1H NMR measurements. As shown in
Table 1, even at room temperature and under 1 atm CO2
pressure, ZTIF-8 still demonstrated high catalytic activity for
the cycloaddition of epichlorohydrin and CO2 into chlor-
opropene carbonate with a yield of >99% over 48 h. With ZIF-
8 as catalyst under the same condition, a yield of 50% was
achieved (Figure S6). The comparison of these results
suggested uncoordinated N atoms in ZTIF-8 indeed promoted
the coupling reaction of CO2 with epoxides.18
Besides this, the catalytic activity for the cycloaddition of
other derivatives and CO2 was also researched. With ZTIF-8,
the yields (Table 1) of 4-ethyl-1,3-dioxolan-2-one and 4,4-
dimethyl-1,3-dioxolan-2-one are 86.9% and 83.3%, respec-
tively. The lower yield of 4,4-dimethyl-1,3-dioxolan-2-one may
be due to the steric hindrance effect.15,29 The reusability test of
ZTIF-8 (Figures 3, S5 and S7) shows that the yield of
chloropropene carbonate decreased from >99% (the first
cycle) to 81.5% (the third cycle), and the yield of
bromopropene carbonate decreased from 97.2% (the first
cycle) to 81.2% (the third cycle). The decrease in yield may be
due to the pore blockage of the recycled ZTIF-8 by the
carbonaceous products.43 The PXRD results of the recycled
sample also confirmed that its structure showed no obvious
change (Figure S10). This suggested that it can be a stable
catalyst in room temperature epoxide−CO2 cycloaddition
reaction.
Figure 1. (a) Coordination mode of zinc atoms of ZTIF-8; (b) SOD
cage constructed by Zn-tetrazolate-imdazolate; (c) view of the 3D
framework of ZTIF-8 along the (111) direction; (d) topology of
ZTIF-8.
which leads to a 3D framework with zeotype SOD topology
(Figure 1c,d). Both ligands are μ2-linker. The SOD-type ZTIF
structure is illustrated by an SOD cage (Figure 1b) which
contains 24 Zn atoms and 36 azole ligands (Figure 1b,c). The
solvent-accessible volume of ca. 50.3% was found by the
PLATON program.50 These voids were filled by the
structurally disordered DMF molecules.
The chemical stability was examined by suspending samples
of ZTIF-8 in common solvents, such as ethanol, methanol,
acetone, and DMF (Figure S3). The as-synthesized samples
were soaked in the solvents for 7 days at ambient temperature.
PXRD patterns collected for each sample showed that the solid
samples of ZTIF-8 maintained their full crystallinity. These
results proved the good chemical stability of ZTIF-8, which is
similar as its isostructural analogue ZTIF-1.46−49
Gas adsorption measurements of ZTIF-8 were performed on
a Micromeritics ASAP 2020 surface area and pore size
analyzer. The samples were activated by solvent exchange
with dry ethanol followed by evacuation at 80 °C for 6 h,
respectively. The permanent porosity of ZTIF-8 was confirmed
by the reversible N2 sorption measurements at 77 K, which
showed type I adsorption isotherm behavior (Figure 2a). The
Langmuir and BET surface areas were 1981 m2/g and 1430
m2/g for ZTIF-8, respectively. A single data point at relative
pressure at 0.98 gives a pore volume of 0.705 for ZTIF-8 by
the Horvath−Kawazoe equation.
Furthermore, the single component sorption isotherms for
CO2 were measured at 1 atm and 273 and 288 K, respectively
(Figure 2). As shown in Figure 2, the CO2 uptake at 1 atm was
65.5 cm3/g (2.92 mmol/g) at 273 K and 46.2 cm3/g (2.06
mmol/g) at 288 K, which is comparable to triazole based
MAF-7 and higher than that of ZIF-8, suggested that N rich
surface helps to the adsorption of CO2.
According to some previous reports,6,51−53 a mechanism is
proposed for the cycloaddition of epoxides and CO2 into cyclic
carbonates catalyzed by ZTIF-8. As illustrated in Figure S11,
the coupling reaction is initiated by binding of the epoxide
with the defective Lewis acidic Zn2+ sites forming the Zn−O
bond. Subsequently, Br− from TBAB attacks the coordinated
epoxide to produce the ring-opening of the epoxy species. The
uncoordinated N atom could easily trap CO2, which facilitates
the coupling of CO2 and the opened epoxy to form an
alkylcarbonate anion. Finally, the corresponding cyclic
carbonate forms in the ring closure step with release of Br−,
and the original catalysts can be recycled for the next catalytic
reaction.
Encouraged by these results, we further explored its
chemical fixation of CO2 with epoxides to form cyclic
B
Cryst. Growth Des. XXXX, XXX, XXX−XXX