Homogenous Dual-Ligand Zinc Complex Catalysts for Chemical Fixation of CO2 to Propylene…
305 K for 1 week to crystallize. The obtained crystals were
washed, dried and kept under vacuum.
3 Results
Complex 1e: 1HNMR (CDCl3, 400 Hz), dHppm
3.267–3.300 (d, 3H, N–CH3), 6.911 (s, H, ring N–CH–N),
7.363–7.588 (m, 2H, ring C–CH–N), 3.725 (s, 3H, P–CH3),
7.735 (m, 15H, ring ArH).
3.1 Effects of Various N-Donor Ligands
Effects of various N-donor ligands in catalytic performance
of basic ZnBr2 and MTPB for synthesis of PC have been
investigated by testing the catalytic activity (Table 1;
Fig. 1). In order to determine the acidity of the solution, the
pH values before and after the reaction were detected. The
corrosion rate of a relativity alkaline environment will be
more slowly.
2.3 General Procedure for the Synthesis of PC
In a typical reaction, the cycloaddition reaction of CO2 to
PO was carried out in a stainless steel autoclave (1 L inner
volume). PC was added before the reaction in order to form
a circular reaction system, which simulated the actual pro-
duction process. Prior to the reaction, the catalyst was
evacuated at 373 K for 1 h, CO2 (liquid, 2.5 MPa) was
introduced to a mixture of PO and PC with a molar ratio of
1:3. The gauge was used to monitor the pressure of the
system. The initial pressure was generally adjusted to 4 MPa
at 373 K, and the autoclave was heated at that temperature
for 1 h. After cooling, the excess gases were vented, and the
liquid products were analyzed by gas chromatography (GC).
GC analysis was performed using Agilent GC 7890A
equipped with a flame ionization detector (FID) and a HP-5
column (length = 30 m, inner diameter = 0.32 mm, film
thickness = 0.25 lm), and the side-products were detected
by 5973 GC–MS with chemstation containing a NIST Mass
Spectral Database. Each catalytic reaction was repeated
three times to secure reproducibility.
Figure 1 shows that among monodentate ligands, the
yield of PC increases with the more strong basicity (pKa)
and then decreases. 1e exhibits the highest catalytic
activity, corresponding to a yield of 92.1 %. This data is
the highest one among all of the N-donor ligand systems
investigated. However, 1d shows the lowest activity, cor-
responding to a lowest yield of 72.3 %. Among the poly-
dentate ligand systems, the relationship between the yield
of PC and pKa is also the same. 2c catalyst shows the
highest activity, with a yield of 87.8 %. Compared with 3
and 4 catalysts, which contain only one additive, the cat-
alytic activity is enhanced with the addition of various
N-donor ligands, especially in the case of 1MI. It is worth
noting that most of the catalysts presented excellent
selectivity ([98 %) towards PC. The results of GC–MS
suggest that there is only tiny amount of propylene glycol
acting as the by-product. On the basis of these results, it is
clear that the catalytic activity is enhanced in various
degrees, and both of the two ligands play synergistic effects
in the enhancement of catalytic performance.
2.4 Corrosion Inhibition
During the reaction, the rate of corrosion, in g/(m2 h), was
measured by a standard method described in ASTM No.
B575. Test pieces (about 20 mm 9 10 mm 9 3 mm) of 10
(C 0.08 %, Mn 0.37 %, Si 0.20 %, S 0.03 %, P 0.035 %,
Cr 0.13 %, Ni 0.23 %, Cu 0.25 % from Baosteel, China);
316 (C 0.08 %, Cr 17.0 %, Mn 1.6 %, Si 1.0 %, P
0.035 %, Ni 11.5 %, S 0.03 %, from Baosteel, China) were
introduced into the autoclave using a special sample cage.
Corrosion data were obtained at 418 K after 72 h of
reaction with the acidic environment of constantly intro-
ducing CO2 gas. Each corrosion reaction was repeated
three times to secure reproducibility and the experimental
error about the wight-loss of the sample is less than 5 %.
The corrosion speed was calculated using the following
equation:
3.2 Characterization and Structural Studies
of Dual-Ligand Zinc Complex
Characterization details of complex 1e are explained here.
The Fourier transform infrared spectroscopic (FT-IR) of
the MTPB, 3, 1e, and 4 are presented in Fig. 2. All of the
samples were dried under vacuum at 60 °C for 24 h before
being detected. All the FT-IR spectra show characteristic
vibration bands at around 1650 and 400 cm-1. MTPB
displays a characteristic band of P–Br centered at
566 cm-1 peak, while the other catalysts do not, which
demonstrates a complete reaction with ZnBr2. In addition,
the band at 3120 cm-1 can be attributed to the stretching
vibration of N–H in 4. It is worth noting that the charac-
teristic band of N–H becomes invisible, instead of some
slightly Ph–H bands at around 3160 and 2900 cm-1 in 1e.
Besides, strong band at 622 cm-1 is observed for 1e, which
can be assigned to Zn–N vibrations. The stretching vibra-
tion of the C=N double bonds appear at 1619 cm-1 for 1MI
[38]. The stretching vibrations of the C=N double bonds
V ¼ ðm0 À mÞ=St ¼ Dm=St
V is the corrosion speed, g/(m2 h); m0 is the sample’s
quality before reaction, g; m is the sample’s quality after
reaction, g; S is the sample’s area, m2; t is the reaction
time, h.
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