Journal of the American Chemical Society
Article
NPs that have been synthesized, we selected Cu NPs to study
reaction solution to precipitate the NP product that was separated by
10
centrifugation at 8500 rpm for 10 min. Au Pd NPs were purified
.8 nm Cu NPs (Figure S19) were not suitable for catalyzing
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twice by redispersing in hexane and flocculating with ethanol and
were precipitated by centrifugation (8500 rpm, 10 min). The obtained
Au Pd NPs were redispersed in hexane for further study.
6
formic acid dehydrogenation because they were not stable in
the acidic solution. However, Cu NPs were found to be active
in promoting the decomposition of ammonia borane to release
4
2
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In the synthesis, the Au/Pd composition ratios in AuPd NPs were
controlled by adjusting the amount of the precursors accordingly.
To deposit the NPs on the carbon support for next-step catalysis
studies, 0.9 g of Ketjen carbon was suspended in 600 mL of hexane
and sonicated for 2 h, and then 0.3 g of AuPd NPs suspended in
hexane was added to the carbon support suspension under sonication
for 2 h. AuPd/C was separated by centrifugation, washed with ethanol
three times, and dried under vacuum. Then the AuPd/C was mixed
with 150 mL of acetic acid and heated to 343 K overnight to remove
the OAm surfactant. After cooling to room temperature, acetic acid
solution was decanted, and the solid AuPd/C sample was washed
twice with pure ethanol and twice more with hexane/ethanol (v/v 1/
9) before being dried in vacuum.
H . More interestingly, these Cu NPs catalyzed one-pot
2
reactions of ammonia borane, diisopropoxy-dinitrobenzene,
w
extending NP catalysis to the green chemistry synthesis of
PBO or other rigid-rod polymer materials.
CONCLUSIONS
■
NPs can catalyze a three-step process for forming very pure
rigid-rod polymer PBO while utilizing more environmentally
friendly solvents and biorenewable reactants than in the
conventional route. Size- and composition-dependent AuPd
catalysis studies show that 3.7 nm Au Pd NPs are especially
Cu NPs (6.8 nm) were synthesized and deposited on the carbon
10
support as reported previously for catalysis studies.
Catalysis of Formic Acid Dehydrogenation. Catalysts and
NMP (3 mL) were first mixed and stirred in a two-necked reactor at
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active and selective for catalyzing all three reactions in the
formation of pre-PBO, yielding products of controllable Mw
from 5.8 to 19.1 kDa. Pre-PBO is converted to PBO by a
simple annealing process at 623 K. The as-synthesized PBO
film is thermally stable at temperatures of up to 939 K and can
withstand a mechanical strength of up to 57.2 MPa, properties
comparable to those of a commercial Zylon film. After
immersion in boiling water for 1 week, the PBO films showed
a minimal thermal decomposition difference (from 939 to 923
K) with only a 15% decrease in mechanical strength. In
comparison, the commercial Zylon films had a sharp decrease
in thermal stability (from 923 K down to 767 K) and lost 61%
of their mechanical strength. Our study demonstrates that,
when prepared from NP-catalyzed green chemistry reactions,
the resulting PBO films are more thermally, mechanically, and
chemically robust than commercial Zylon, which suffers from
unavoidable contamination with phosphoric acid. Moreover,
this NP-catalyzed tandem reaction system for controlled
polymerization can be extended to non-noble metal NPs
such as Cu NPs, providing a general green and cost-effective
approach to the preparation of PBOs or other rigid-rod
polymers for ballistic fiber, antiflame/heat-resistant coating,
and separation membrane applications.
2
98 K. Then, the mixture was heated to 353 K, at which point formic
acid (10 mmol) was injected into the reactor and the produced gas
was collected with a gas buret filled with water connected to the
reactor. The volume of the gas mixture that evolved was measured by
recording the volume of water displaced.
PBO Synthesis. The AuPd/C catalyst (2.5 mol %) and 1,5-
diisopropoxy-2,4-dinitrobenzene (1 mmol) were first mixed in NMP
(3 mL) and transferred to a balloon-sealed two-necked flask under an
Ar atmosphere. The mixture was heated to 353 K, and formic acid (10
mmol) was rapidly injected. The mixture was kept at 353 K for 2 h.
The reaction system was then placed under a gentle flow of Ar, and
terephthalaldehyde (1 mmol) and toluene (1 mL) were added. The
reaction mixture was further heated to 403 K and kept for 6 h at 403
K. Once cooled to room temperature, the catalyst was filtered, and the
solution was poured into methanol (100 mL) to precipitate the pre-
PBO product, which was filtered, rinsed several times with methanol,
and dried in air. H NMR (400 MHz, dimethyl sulfoxide-d ): δ (ppm)
6.37 (s, 1H, Ar−H), 6.04 (s, 1H, Ar−H), 4.15 (hept, J = 6.1 Hz, 2H,
CH), 4.10 (s, 4H, NH ), 1.18 (d, J = 6.1 Hz, 12H, CH ). C NMR
1
6
1
3
2
3
(
1
101 MHz, dimethyl sulfoxide-d ): δ (ppm) 135.0, 134.4, 109.0,
02.3, 71.7, 22.2.
6
One gram of pre-PBO was annealed at 623 K under N for 2 h to
2
convert it to PBO.
Preparation of PBO/Zylon Films. PBO and Zylon were mixed
with methanesulfonic acid in a glass vial at 353 K for 5 h to dissolve.
The polymer solution was transferred in a Petri dish that was heated
in an oil bath. After solvent evaporation at 353 K for 3 h and at 403 K
for 3 h, the film was immersed in water to peel off from the glass
substrate. The obtained film was washed with water and dried at 353
K under vacuum for 12 h for further tests.
Stability Tests of PBO and Zylon. The as-prepared PBO and
Zylon films were immersed in boiling water or an aqueous solution of
0.5% phosphoric acid (PA) for 1 week. After cooling to room
temperature, the films were washed with water and dried at 353 K for
24 h under vacuum to give water-PBO/Zylon and PA-PBO/Zylon for
thermal and mechanical tests.
Characterization. Transmission electron microscopy (TEM)
images were acquired from a Philips CM20 (200 kV). High-
resolution TEM (HRTEM) images were recorded with a JEOL 2100F
(200 kV). Samples for TEM and HRTEM analyses were prepared by
depositing a single drop of a dilute NP dispersion/suspension on
amorphous-carbon-coated copper grids. TEM with a field-emission
electron source and STEM analyses were obtained on a Hitachi
HD2700C (200 kV) with a probe aberration correction. X-ray
diffraction (XRD) patterns were collected on a Bruker AXS D8-
Advanced diffractometer with Cu Kα radiation (= 1.5406 Å).
Inductively coupled plasma−atomic emission spectroscopy was
carried out on a JY2000 Ultrace ICP−AES equipped with a JY-AS
EXPERIMENTAL SECTION
Chemicals and Materials. Palladium acetylacetonate (Pd(acac)2,
9%) and hydrogen tetrachloroaurate hydrate (HAuCl , 99.8%) were
■
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4
purchased from Strem Chemicals. Oleylamine (OAm, technical grade,
0%), borane tert-butylamine (97%), 1,2,3,4-tetrahydronaphthalene
7
(tetralin, 99%), formic acid (95%), 1,5-difluoro-2,4-dinitrobenzene
97%), N-methyl-2-pyrrolidone (99%), methanesulfonic acid (99%),
(
and various benzaldehydes were purchased from Sigma-Aldrich.
Borane morpholine (97%) was purchased from Alfa Aesar. 1,5-
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Diisopropoxy-2,4-dinitrobenzene was prepared as reported. Hexane
98.5%), isopropanol (99.5%), ethanol (100%), and acetic acid (98%)
(
were purchased from Fisher Scientific. The deionized (DI) water was
obtained from a Millipore Autopure System.
Synthesis of NP Catalysts. AuPd NPs were synthesized by
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modifying a previously reported recipe and by scaling up to the
gram scale. In a typical synthesis of 3.7 nm-Au Pd NPs, borane
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morpholine (3 g) was dissolved by stirring in OAm (150 mL) under
an Ar atmosphere at 343 K. The solution of HAuCl (2.4 mmol) and
4
Pd(acac) (3.6 mmol) in OAm (60 mL) was rapidly transferred to the
2
stirred borane morpholine solution at 343 K. The temperature was
increased to 493 K and maintained for 30 min. Once the solution
cooled, room-temperature isopropanol (1500 mL) was added to the
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J. Am. Chem. Soc. 2021, 143, 2115−2122