Three-dimensional covalent organic frameworks (3D COFs) were synthesized by targeting two nets based on triangular and tetrahedral nodes:
ctn
and
bor
. The respective 3D COFs were synthesized as crystalline solids by condensation reactions of tetrahedral
tetra
(4-dihydroxyborylphenyl) methane or
tetra
(4-dihydroxyborylphenyl)silane and by co-condensation of triangular 2,3,6,7,10,11-hexahydroxytriphenylene. Because these materials are entirely constructed from strong covalent bonds (C-C, C-O, C-B, and B-O), they have high thermal stabilities (400° to 500°C), and they also have high surface areas (3472 and 4210 square meters per gram for COF-102 and COF-103, respectively) and extremely low densities (0.17 grams per cubic centimeter).
Porous organic polymers containing nitrogen-rich building
units
are among the most promising materials for selective CO2 capture and separation which can have a tangible impact on the environment
and clean energy applications. Herein we report on the synthesis and
characterization of four new porous benzimidazole-linked polymers
(BILPs) and evaluate their performance in small gas storage (H2, CH4, CO2) and selective CO2 binding over N2 and CH4. BILPs were synthesized
in good yields by the condensation reaction between aryl-o-diamine and aryl-aldehyde building blocks. The resulting BILPs exhibit
moderate surface area (SABET = 599–1306 m2 g–1), high chemical and thermal stability, and
remarkable gas uptake and selectivity. The highest selectivity based
on initial slope calculations at 273 K was observed for BILP-2: CO2/N2 (113) and CO2/CH4 (17),
while the highest storage capacity was recorded for BILP-4: CO2 (24 wt % at 273 K and 1 bar) and H2 (2.3 wt %
at 77 K and 1 bar). These selectivities and gas uptakes are among
the highest by porous organic polymers known to date which in addition
to the remarkable chemical and physical stability of BILPs make this
class of material very promising for future use in gas storage and
separation applications.
A new
facile method for synthesis of porous azo-linked polymers
(ALPs) is reported. The synthesis of ALPs was accomplished by homocoupling
of aniline-like building units in the presence of copper(I) bromide
and pyridine. The resulting ALPs exhibit high surface areas (SABET = 862–1235 m2 g–1),
high physiochemical stability, and considerable gas storage capacity
especially at high-pressure settings. Under low pressure conditions,
ALPs have remarkable CO2 uptake (up to 5.37 mmol g–1 at 273 K and 1 bar), as well as moderate CO2/N2 (29–43) and CO2/CH4 (6–8)
selectivity. Low pressure gas uptake experiments were used to calculate
the binding affinities of small gas molecules and revealed that ALPs
have high heats of adsorption for hydrogen (7.5–8 kJ mol–1), methane (18–21 kJ mol–1), and carbon dioxide (28–30 kJ mol–1).
Under high pressure conditions, the best performing polymer, ALP-1,
stores significant amounts of H2 (24 g L–1, 77 K/70 bar), CH4 (67 g L–1, 298 K/70
bar), and CO2 (304 g L–1, 298 K/40 bar).
Hole-some mixture: A 2D mesoporous covalent organic framework (see figure) featuring expanded pyrene cores and linked by imine linkages has a high surface area (SA(BET) = 2723 m(2) g(-1)) and exhibits significant gas storage capacities under high pressure, which make this class of material very promising for gas storage applications.
Three new crystalline microporous and mesoporous 2D covalent organic frameworks termed COF-6, -8, and -10 from boronic acid building blocks and 2,3,6,7,10,11-hexahydroxytriphenylene have been synthesized and structurally characterized. These materials constructed of C2O2B rings form eclipsed layered structures with pore sizes ranging from 6.4 to 34.1 Å and are found to have high thermal stability, low density, and high porosity as indicated by the surface areas of 980, 1400, and 2080 m2 g-1 for COF-6, -8, and -10, respectively. The control of pore size and structure demonstrates the effectiveness of reticular chemistry methods toward materials design.
Successful incorporation of triptycene into benzimidazole-linked polymers leads to the highest CO(2) uptake (5.12 mmol g(-1), 273 K and 1 bar) by porous organic polymers and results in high CO(2)/N(2) (63) and CO(2)/CH(4) (8.4) selectivities.
Heteroatom-doped
porous carbons are emerging as platforms for use in a wide range of
applications including catalysis, energy storage, and gas separation
or storage, among others. The use of high activation temperatures
and heteroatom multiple-source precursors remain great challenges,
and this study aims to addresses both issues. A series of highly porous
N-doped carbon (CPC) materials was successfully synthesized by chemical
activation of benzimidazole-linked polymers (BILPs) followed by thermolysis
under argon. The high temperature synthesized CPC-700 reaches surface
area and pore volume as high as 3240 m2 g–1 and 1.51 cm3 g–1, respectively, while
low temperature activated CPC-550 exhibits the highest ultramicropore
volume of 0.35 cm3 g–1. The controlled
activation process endows CPCs with diverse textural properties, adjustable
nitrogen content (1–8 wt %), and remarkable gas sorption properties.
In particular, exceptionally high CO2 uptake capacities
of 5.8 mmol g–1 (1.0 bar) and 2.1 mmol g–1 (0.15 bar) at ambient temperature were obtained for materials prepared
at 550 °C due to a combination of a high level of N-doping and
ultramicroporosity. Furthermore, CPCs prepared at higher temperatures
exhibit remarkable total uptake for CO2 (25.7 mmol g–1 at 298 K and 30 bar) and CH4 (20.5 mmol
g–1 at 298 K and 65 bar) as a result of higher total
micropores and small mesopores volume. Interestingly, the N sites
within the imidazole rings of BILPs are intrinsically located in pyrrolic/pyridinic
positions typically found in N-doped carbons. Therefore, the chemical
and physical transformation of BILPs into CPCs is thermodynamically
favored and saves significant amounts of energy that would otherwise
be consumed during carbonization processes.
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