H. Zhu, et al.
AppliedCatalysisA,General608(2020)117842
investigated in chemical industry, and satisfactory results are achieved
using silica-supported acid-base bifunctional catalysts [27–33]. Its re-
action mechanisms are well understood by combining molecular dy-
namic simulation [34], quantum mechanical and molecular mechanical
calculations [35], and kinetic examination [36]. Further detailed stu-
[37–41] significantly affects cooperative catalysis. Apart from typical
cellulose nanocrystal [43] were successfully applied to graft organo-
catalysts for aldol reaction through similar cooperative mechanism.
Such acid-base cooperative reaction pathway that imitates enzyme
catalysis is becoming a popular strategy for promoting reaction, how-
ever, accurate control of spatial distribution of acid- and base-sites is
generally required but still remains a challenge. In addition to posi-
tioning two incompatible functionalities at a suitable sequence for mi-
micking enzyme upon support material, we envision that delicate ar-
rangement of two functionalities, in which one could help to modify
surface wettability and provide H-donor for constructing tunable mi-
croenvironment around catalytic sites, would improve catalytic per-
formance in aldol reaction.
2.2. Catalyst characterizations
The Fourier transform infrared spectra (FT-IR) of fibers were col-
lected on AVATAR360 FT-IR spectrometer (Thermo Nicolet). The
sample of fiber was cut to pieces and compressed into KBr pellets before
measurement. Solid-state 13C CP-MAS NMR spectra were recorded on
Infinityplus 300 (Varian Company, America). Thermogravimetric ana-
lysis (TGA) was performed on a STA409PC TGA/DSC simultaneous
thermalanalyzer (Netzsch Company, Germany) under nitrogen atmo-
sphere, and the sample of fiber was analyzed in the temperature range
of 35-800 °C with a heating rate of 10 K min-1. The mechanical property
of fiber was measured using electronic single fiber strength tester YG(B)
001A (Wen zhou Da Rong Textile Instrument Corporation, China) with
a clamping length of 10 mm and an elongation rate of 20 mm min-1 at
25 °C. Each sample was tested ten times in parallel and the average data
was calculated to afford breaking strength of fiber. D/MAX-2500 X-ray
diffractometer (Rigaku Corporation) was used to determine the crys-
tallinity of fiber. Elemental analysis was carried out using a Vario Micro
Cube instrument (Elementar, Germany). Scanning electron microcopy
(SEM) experiments were carried out with Hitachi S-4800 field emission
scanning electron microscope. Water contact angle (WCA) test was
conducted with a POWEREACH-JC2000DI contact angle system. The
sample of fiber was cut to pieces and dissolved in DMSO under heating
at 100 °C for 20 min. The sticky liquid was spread onto the glass slide
and then dried under vacuum at 80 °C for 3 h to give fiber membrane
before measurement. 1H NMR (400 MHz) and 13C NMR (101 MHz) of
the products were measured with BRUKER-AVANCE III instruments in
CDCl3 using TMS (tetramethylsilane) as the internal standard.
Polyacrylonitrile fiber (PANF), for its unique properties, such as
excellent durability, good mechanical and thermal stability, and high
solvent resistance [44], has recently emerged as a promising polymer
support for absorption and catalysis. Cyano group of PANF surface is
generally capable of reacting with specific functional molecules to offer
novel functionalities through aminolysis and hydrolysis. The modified
PANFs have been used as absorbents to remove dyes [45], heavy metal
ions [46,47] and organic pollutants [48] from waste water effectively
firmed that the functionalized fibers were excellent catalysts in de-
symmetrization of meso-anhydride [49], Knoevenagel reaction [50],
2.3. Synthesis of functionalized fibers
PANF is one of the most common synthetic fibers and its surface
contains abundant cyano groups and ester groups which can be con-
verted partially through the formation of amide bonds. As depicted in
Scheme 1, the preparation of functionalized fiber was typically
achieved by introducing functional molecule onto PANF surface. Hy-
droxyl-modified amine fibers (PANEF-PMA, PANEF-PLA and PANEF-
TTA) were synthesized by a simple two-step approach. First, hydroxyl-
functionalized fiber PANEF was prepared by anchoring of ethanolamine
onto fiber via the formation of amide bond. Next, the resultant PANEF
was allowed to react with different alkylamines to give the corre-
sponding hydroxyl-modified amine fibers. In comparison, mono-func-
sustainable organic synthesis, herein, various amine-functionalized
polyacrylonitrile fibers were prepared to investigate the effect of sur-
face wettability modification on catalytic activity in aldol reaction. The
obtained fiber catalysts were characterized by mechanical strength, FT-
IR, solid state 13C NMR, TGA, XRD, elemental analysis, SEM and water
contact angle. The surface wettability of the functionalized fiber was
tuned by introducing hydroxyl group into fiber support. Due to its high
activity in aldol reaction, prolinamide-functionalized fiber was chosen
as a template catalyst to further investigate the influence of hydro-
philic/hydrophobic surface properties on aqueous catalytic reaction.
Additionally, its catalytic performance was compared with the reported
conventional acid-base bifunctional aminosilica catalysts, and the re-
sults showed that fiber catalyst exhibited superior catalytic activity and
selectivity under mild conditions.
tionalized amine fibers (PANPMAF, PANTTA
F and PANPLAF) and
PANHPPF were prepared directly by adding PANF into a mixture of
solvent and alkylamines according to our previous work [50]. Sche-
matic diagram for the synthesis of PANEF-PLA and PANPLAF is shown in
Scheme S1. The modified extent of fiber in this work was measured by
weight gain and acid-base exchange capacity, and the results are
2. Experimental
2.3.1. Synthesis of hydroxyl-modified amine fibers (PANEF-PLA, PANEF-
PMA and PANEF-TTA)
2.1. Chemicals and materials
Hydroxyl-modified amine fibers were synthesized through a two-
step approach using PANF as the precursors. As a typical experiment for
the synthesis of PANEF-PLA, first, 2.10 g of dried PANF was added to
50 mL of water solution containing 20 mL of ethanolamine and the
mixture was heated to reflux for 3 h. After being cooled to room tem-
perature, the fiber was collected by filtration, rinsed repeatedly with
deionized water (60-70 ℃), and then dried overnight at 60 ℃ under
vacuum to give fiber PANEF (2.284 g, 1.29 mmol/g).
Commercially available polyacrylonitrile fiber (PANF, 93% acrylo-
nitrile, 6.5% methyl acrylate, and 0.4-0.5% sodium styrene sulfonate)
with a length of 10 cm and a diameter of 30
0.5 μm was obtained
from Fushun Petrochemical Corporation of China. Ethanolamine, pro-
pane-1,3-diamine, N,N-dimethylpropane-1,3-diamine, ethane-1,2-dia-
mine, L-proline methyl ester hydrochloride, 1,3-diaminopropan-2-ol,
ethylene glycol, p-nitrobenzaldehyde, acetone, ethyl acetate, di-
chloromethane, methanol, xylene, Na2SO4, petroleum ether were ana-
lytical grade and used without further purification. Deionized water
was commercially obtained.
Next, dried PANEF (0.25 g) was added to the solution of prolinamide
(S)-N-(2-aminoethyl)pyrrolidine-2-carboxamide (4.0 g) dissolved in
ethylene glycol/H2O (4:1, 6 mL) and the mixture was stirred at 130 ℃
for 4 h. After being cooled to room temperature, the fiber was collected
by filtration, rinsed repeatedly with deionized water (60-70 ℃) and
dried overnight at 60 ℃ under vacuum to give fiber PANEF-PLA
2