Surface fibrillation of: Para -aramid nonwoven as a multi-functional air filter with ultralow pressure drop

Article abstract of DOI:10.1039/d0ta07886g

Nonwovens are widely applied in the air filtration field for their multi-layered fibrous structures and interconnected pores. Despite their intensive use, conventional microfiber nonwovens usually suffer from low filtration efficiencies due to their large pore size, while most nanofiber nonwovens are still confined within the laboratory due to the difficulties of reconciling the comprehensive requirements (such as low pressure drop, high fluffiness, strength and long-term reliability) raised from practical applications. Herein, we report a simple and scalable method that can generate more nanofibrils in a para-aramid microfiber nonwoven without interfering with the three-dimensional nonwoven structure. The success of this method relies on treating the whole nonwoven, rather than individual fibers, with phosphoric acid to trigger a surface fibrillation process of para-aramid microfibers. The structural reinforcement from microfibers, in combination with the enlarged surface area and the reduced pore size brought by nanofibrils, improves the filtration efficiency of the fibrillated nonwoven by 84.7% and maintains the pressure drop at an ultralow value of 3.5 Pa. Further synthesis and loading of a novel catalyst (CuO-CeO2) onto the nonwoven demonstrates a higher catalyst-loading capacity after fibrillation. The resulting nonwoven filter shows high elimination rates against a variety of solid and gaseous pollutants (including particulate matter, carbon monoxide and toluene), and is applicable at both room and elevated temperatures. We believe that the generation of nanofibrils in microfiber nonwovens and the subsequent loading of catalysts will find extensive utility in the purification of industrial emissions, automobile exhausts and indoor air. This journal is

Full text of DOI:10.1039/d0ta07886g

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ARTICLE
Surface Fibrillation of Para-aramid Nonwoven as a Multi-functional
Air Filter with Ultralow Pressure Drop
Kangli Xua, Jixia Denga, Rui Linb, Heng Zhangc, Qinfei Ke*a, and Chen Huang*a
Received 00th January 20xx,
Accepted 00th January 20xx
Nonwovens are widely applied in air filtration field for their multi-layered fibrous structures and inter-connected pores.
Despite intensively used, conventional microfiber nonwovens usually suffer from low filtration efficiencies due to their large
pore size, whilst most nanofiber nonwovens are still confined within laboratory for the difficulties to reconcile the
comprehensive requirements (such as low pressure drop, high fluffiness, strength and long-term reliability) raised from
practical applications. Here we report a simple and scalable method that can generate lots of nanofibrils in a para-aramid
microfiber nonwoven without interfering the three-dimensional nonwoven structure. The success of this method relies on
treating the whole nonwoven, rather than individual fibers, with phosphoric acid to trigger a surface fibrillation process of
para-aramid microfibers. The structural reinforcement from microfibers, in combination with the enlarged surface area and
the reduced pore size brought by nanofibrils, improves the filtration efficiency of fibrillated nonwoven by 84.7% and
maintains the pressure drop at an ultralow value of 3.5 Pa. Further synthesis and loading of a novel catalyst (CuO-CeO2) onto
the nonwoven demonstrate a higher catalyst-loading capacity after fibrillation. The resulting nonwoven filter shows high
elimination rates against a variety of solid and gaseous pollutants (including particulate matters, carbon monoxide and
toluene), and is applicable under both room and elevated temperature. We believe that the generation of nanofibrils in
microfiber nonwoven and the subsequent loading of catalysts will find extensive utility in the purification of industrial
emissions, automobile exhausts and indoor air.
DOI: 10.1039/x0xx00000x
large scale and low-cost production, multiple layers of fibers and
interconnected porous structures, leading to both good filtration
performances and desirable practicability.16-20 However, since
conventional nonwovens can only remove PMs with a relatively low
efficiency, additional layers of materials (such as microporous films
and active carbon) or post-treatments (such as electret) are
frequently applied.
To address the low filtration efficiency of microfiber nonwovens,
nanofibers with diameter ranging from tens to hundreds of
nanometers have emerged as promising building blocks for high
efficiency nonwoven filters. The concept of using nanofiber
nonwovens as air filters is established by the fact that filtration
efficiency of air filters can be readily improved through reducing pore
size, while pore size of nonwovens is positively correlated to fiber
diameter. For this reason, a number of nanofiber nonwovens with
delicate architecture have been fabricated. Some representative
works include the microwave structured polyamide-6/poly(m-
phenylene isophthalamide) nanofiber/net membranes,21 the self-
assembled 2D nanoarchitectured networks,22 the mesostructured
nonwovens with penguin downy feather-like morphology,23 and the
bead-on-string polyacrylonitrile nanofibrous air filters.24 All of them
possessed ideal filtration efficiencies.
Introduction
Air pollution caused by industrial and vehicular emissions has
becoming one of the most severe threats to public health, climate
and ecosystems.1-5 The major contaminants in polluted air are
divided into two categories: particulate matters (PMs) and toxic
gases.6-9 PMs refer to the solid particles and aerosols with diameters
no more than 10ꢀμm, and toxic gases are the generic term of carbon
monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), ammonia
(NH3), hydrocarbons and volatile organic compounds (VOCs).10-12
Given that the exposure to PMs and toxic gases may drastically
increase the risks for respiratory, skin and cardiovascular diseases,13-
15
the design and development of air filtration systems have drawn
increasing interests from both industry and academia.
Till now, microfiber nonwovens manufactured by needle-punch,
meltblown and spunbond technologies are the most successful
media for constructing air filters. These nonwovens are featured with
a. Key Laboratory of Textile Science & Technology, Ministry of Education, College of
Textiles, Donghua University, Shanghai 201620, China. E-mail: hc@dhu.edu.cn
kqf@dhu.edu.cn
b. Laboratory of Clean Energy Utilization and Pollution Control, College of
Environmental Science and Engineering, Donghua University, Shanghai 201620,
China.
In spite of the exciting progresses these studies have made, the
practical use of nanofiber nonwovens in air filtration is still largely
confined within laboratory, due to the intrinsic conflict between high
filtration efficiency and low pressure drop. While the small pore size
of nanofiber nonwoven is desirable for increasing filtration
c. State Key Lab High Performance Ceram & Superfine, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, Shanghai 200050, China.
† Electronic Supplementary Information (ESI) available: Experimental details, XRD
patterns, FTIR spectrum, TGA curves, photographs of CuO-CeO2/fibrillated para-
aramid nonwoven. See DOI: 10.1039/x0xx00000x
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efficiency, it may easily cause the sharp increase of pressure drop.
Although some recent work tried to tackle the problem by
constructing three-dimensional (3D) architectures25-26 or utilizing the
“slippery effect” of nanomaterials,27-28 the reported pressure drops
are still higher than those of the microfiber nonwovens. Moreover,
when used under harsh environment (which is commonly occurred
in industrial air filtration), strength and long-term reliability of
nanofiber nonwovens remain to be verified, since these delicate
structures are highly vulnerable to high-flux air flow and chemical
corrosion. The development of next generation of air filters has,
therefore, turned to the design and fabrication of nonwovens with
new and multiscale fiber morphologies.
Herein we present a nonwoven air filter having ultralow pressure
drop and efficient removal rates to both PMs and toxic gases. By
treating a conventional para-aramid microfiber nonwoven with
phosphoric acid (H3PO4) aqueous vortex, we successfully activate the
surface fibrillation of the microfibers and generate a large number of
nanofibrils on fiber surface. The co-existence of microfibers and
nanofibers endows the fibrillated nonwoven with stable 3D
structure, reduced pore size and enlarged surface area, which work
synergistically to achieve excellent filtration performances that
outperform other known fibrous filters of similar dimensions or
origins. When loaded with a novel catalyst (CuO-CeO2), the
nanofibrils greatly increase the loading capacity of the nonwoven,
and thus enabling an improved elimination rate against toxic gases
such as CO and toluene (C7H8). The high reliability of resulting
nonwoven was further verified by a series of stability tests, including
compression resilience, adhesion fastness, thermal and chemical
stabilities. These results prove the efficacy of using fibrillated para-
aramid nonwoven in air filtration, especially when encountering
harsh working environments and different types of airborne
pollutants.
mixed solution of acetone and ethanol to remove thVeieswuArrftaiccleeOonliilnse.
After drying in a vacuum oven at 60 ℃ forD6OhI:,1t0h.1e0n39o/nDw0oTAv0e7n8w86aGs
impregnated in different concentrations of H3PO4 solution (10%,
20%, 30%, 40% and 50%) and stirred for 1 h, 2 h, 3 h and 4 h,
respectively. The stirring speed was set at 600 rpm, and the water
bath temperature was 55 ℃. Under the shearing force of H3PO4
solution, nanofibrils were stripped from the surface of para-aramid
fibers, after that the para-aramid nonwoven was washed with
deionized water for 3 times and dried in a vacuum oven at 60 ℃ for
12 h.
Synthesis and loading of CuO-CeO2
A series of CuO-CeO2 catalysts were synthesized by a hydrothermal
method.29 1 g CTAB was dissolved in 50 mL of distilled water under
constant stirring. Cu(NO3)2·3H2O and Ce(NO3)3·6H2O of different
molar ratios (5/95, 10/90, 15/85) were added into the above solution
as precursor salts. NH3 aqueous solution was added dropwise with
stirring at 50 ℃ until the pH value of the mixture was adjusted to
10.0. The solution was transferred to a 100 mL Teflon-line stainless
steel autoclave for hydrothermal reaction at 160 ℃ for 12 h. The
resulting products were sequentially cooled down to room
temperature, washed fully via deionized water and ethanol, dried
under vacuum at 60 ℃ for 24 h, and calcined at 500 ℃ for 2 h in air.
The resultant samples were named as x CuO-CeO2, where x denotes
the molar percentage of Cu in the catalysts.
At the concentration of 0.2 wt%, a uniform and stable CuO-CeO2
dispersion is formed under ultrasonication. The fibrillated para-
aramid nonwoven was impregnated in this dispersion for another 30
min of continuous ultrasonication, after which it was washed by
deionized water to remove the floating particles, and dried in a
vacuum at 110 ℃ for 12 h.
Characterization
Surface morphology of samples was detected by field emission
scanning electron microscope (FESEM, Hitachi S-4800, JEOL, Japan)
coupled with an energy-dispersive spectrometry (EDS). Transmission
electron microscopy (TEM, JEM-2 011F, JEOL, Japan) was performed
at an accelerating voltage of 200 kV. Crystalline structures of the
samples were characterized by X-ray diffraction (XRD, D/max-II B,
Shimadzu, Japan). CuKα1 radiation of 1.54056Å and graphite
monochromator (40 kV, 30 mA) were used with a step size of 5°·min-1
in the range from 5 to 90°. The basis weight and thickness of
nonwovens were determined using an electronic balance and a
thickness gauge. The compression tests were conducted using
compression apparatus equipped with two flat-surface compression
stages (INSTRON 5967, U.S.A). Ten samples of the same size (30 × 30
mm) were stacked together for the compression test. A 100-cycle
loading–unloading fatigue test was performed by measuring σ versus
ε = 53% at a strain rate of 5 mm·min-1 and a 3% prestrain was applied
to ensure a uniform flat contact between the presser foot and the
samples.30 The stress-strain curves were acquired with an electronic
fabric tensile device (YG026MB, China) at a speed of 10 mm/min.
Raman spectra were acquired by using a Raman system (inVia-Reflex,
U.K.) with 532 nm excitation. Fourier transform infrared
spectroscopy (FT-IR, Spectrum Two, U.K.) was used to investigate the
functional groups of the samples at the wavenumber ranging from
Experimental
Materials
Para-aramid fiber (Kevlar 29, diameter ∼14 µm) was supplied by
Dupont (U.S.A). Phosphoric acid (≥85.0%, H3PO4), cerous nitrate
hexahydrate (≥99.0%, Ce(NO3)3·6H2O), copper(ll) nitrate trihydrate
(≥99.0%, Cu(NO3)2·3H2O), formic acid (≥98.0%, HCOOH), and sodium
hydroxide (≥99.0%, NaOH ) were purchased from Sinopharm
Chemical Reagent Co., Ltd. Hexadecyl trimethyl ammonium bromide
(≥99.0%, CTAB) was purchased from Aladdin Chemical Reagent Co.,
Ltd. Ammonia (25%, NH3) was purchased from Lingfeng Chemical
Reagent Co., Ltd. All chemicals were used without further
purification.
Surface fibrillation of para-aramid microfiber nonwoven
Para-aramid nonwoven was manufactured via an industrialized web
forming (carding) and bonding (needle punching) process. Para-
aramid fibers were fed into a carding machine to produce uniform
fiber webs. The webs (areal density = 200 g·m-2) were then
overlapped and needle punched with a puncturing density of 600
punch·cm−2. The bonded para-aramid nonwoven was cleaned in
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4000 to 400 cm-1. Water contact angle (WCA) was measured by a to ensure the accuracy of data. The filtration efficiency (η) could be
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DOI: 10.1039/D0TA07886G
contact angle goniometer (OCA15EC, Germany). Thermo gravimetric calculated as follow:
analysis (TGA) was conducted by TGA-4000 (PerkinElmer,
×
η (%) = (1 - ɛ1/ɛ2)
100%
(1)
Netherlands), and the temperature was increased from 30 to 700 ℃
at 20 ℃·min-1. Flame retardancy properties were visualized via a
combustion test using an alcohol lamp. Nitrogen adsorption-
desorption isotherms were measured at 77K using a micromeritics
ASAP 2460 analyzer (Micromeritics Instruments, U.S.A). The specific
surface area (SSA) were calculated by Brunauer-Emmett-Teller (BET)
method, using the adsorption branch data in the relative pressure
(P/P0) ranging from 0.0 to 1.0. Pore volumes of catalysts were
calculated by the t-plot method, and pore size distribution of
catalysts was calculated by Barrett-Joyner-Halenda (BJH) method
using nitrogen desorption branch data. Pore size and pore size
distribution of para-aramid nonwovens were measured by a capillary
flow porometer (CFP-1100AI, Porous Materials, Inc., U.S.A).
Inductively coupled plasma atomic emission spectroscopy (ICP,
Prodigy, U.S.A) was carried out to measure the actual elemental
contents and compositions of the catalysts.
A PH15700 ESCA X-ray photoelectron spectroscopy instrument
(XPS, Thermo Fisher Scientific Inc., U.S.A) was employed to check the
chemical states of Ce and Cu in the catalysts by using Al-Kα (Eb =
1486.6 eV) radiation with an accelerating voltage of 15 kV. The
percentages of surface atoms of the elements in catalysts were
calculated by the areas of the binding energy after correction and
normalization. The organic polluted carbon (C1s: E = 284.6 eV) was
used as the corrective standard of energy.
where ɛ1 and ɛ2 represent the quantity of NaCl aerosol in the
downstream and upstream, respectively. Pressure drop (filtration
resistance) was recorded by a flow gauge and two electronical
pressure transmitters.
Catalytic activity tests
The catalytic oxidation of CO was carried out in a self-developed
fixed-bed reactor system under atmospheric pressure (Figure S1).
100 mg of pure catalyst (40-60 mesh) or 0.5 g of catalyst-loaded
nonwoven were heated from 30 to 240 ℃ with a rate of 5 ℃·min-1.
The typical composition of feed gas mixture comprises CO (1 %), O2
(10 %) and N2 (to balance), and the total flow rate was fixed at 200
mL·min-1 to ensure a gas hourly space velocity (GHSV) of 120000 mL
(gcat·h-1).33 The inlet and outlet concentrations of reactants and
products were measured by an on-line GC equipped with a TCD, a
flame ionization detector and a methanation reactor. The reaction
gas and products were separated by TDX-1 columnar molecular
sieve, and the conversion rate was calculated by area normalization
method. The catalytic activities were expressed according to the
conversion of CO (%), which was calculated based on the
consumptions of CO:
푖푛 ― [퐶푂]표푢푡
CO conversion (%) = [퐶푂]
100% (2)
×
[퐶푂]푖푛
TPR experiments were performed on gas chromatograph (GC-
4000A, U.S.A) equipped with thermal conductivity detector (TCD).
The heating range of the whole reduction process was from 50 to 600
℃ (5 ℃·min-1) and was tested in 5% H2/N2 (20 mL·min-1) atmosphere.
Catalyst samples were crushed through a 40-60 mesh sieve and
loaded into the reaction quartz tube. Before the test, catalyst
samples were heated from 30 to 300 ℃ (5 ℃·min-1) in N2
environment, and maintained at 300 ℃ for 30 min.31
where [CO]in and [CO]out are the concentrations of CO (%) in the
feed stream and the effluent, respectively.
Another important indicator for evaluating the catalyst is the
stability of catalyst. This test was also carried out in a tubular
continuous reactor, and the test conditions are the same as those for
CO oxidation. In the life test, the samples were kept at 90 ℃ (which
is the lowest temperature for 100% CO conversion) for 1200 min and
the change of conversion rate was recorded during the test period.
Two methods were adopted to examine the dry and wet adhesion
fastness of CuO-CeO2 particles on fibrillated nonwoven. One is
continuous air blowing at 5 m·s-1 for 10 min (sample size = 30 × 20
Adsorption and photodegradation test.
cm), the other is washing under 1000 r·min-1 for 10 min (sample size The photodegradation tests were carried out in a reactor (1.0 µL) at
= 2 × 2 cm). After 3 cycles of air blowing or washing, the nonwoven a standard atmospheric pressure and room temperature (Figure S2).
was taken out and dried in a vacuum oven at 60 ℃ for 12 h. The 100 mg of CuO-CeO2 or 0.5 g of CuO-CeO2-loaded nonwoven were
weight of the nonwoven before and after air blowing/washing were placed in the reactor. The atmosphere of the reactor is the same as
recorded. Chemical stability of CuO-CeO2/fibrillated nonwoven was the indoor environment. After the baseline became stable, 1.0 µL
investigated by immersing the nonwoven in 98% HCOOH solution toluene were injected into the reactor by a liquid injector needle, and
and 0.5 mol·L-1 NaOH solution for 1 h,32 after which the filtration the corresponding initial concentration was 867 mg·m-3. After C7H8
performances of the nonwovens were measured.
vapor reached adsorption equilibrium, the photocatalytic activity
tests were performed under xenon lamp irradiation (300 W,
PLSSXE300, Beijing philae technology co., Ltd., China). The
concentrations of C7H8 and CO2 in the reactor were determined
online by a photoacoustic gas monitor (INNOVA 1412i, LumaSense
Technologies, Denmark). The photocatalytic oxidation efficiencies of
Filtration tests
The filtration tests were conducted by following European Standard
of Medium Efficiency Filters (BS EN 779:2012). A CertiTest 8130
automatic filtration performance tester (TSI Inc., U.S.A) was used and
the air flow was set at 32 L·min-1. Solid sodium chloride (NaCl, 2wt%)
was employed to generate particles (mean diameter ∼0.26 μm and
the geometric standard deviation of the particles was less than 1.83).
NaCl aerosol particles were delivered through the filter, which were
clamped by a filter holder with an effective area of 100 cm2. Before
testing, the aerosol generator was preheated for more than 30 min
η
C7H8 into CO2 (
formulas:
) were calculated according to the following
t, C7H8
(Ct,CO ― C0,CO ) · MC7
H
2 2
8
ηt, C H
×
100%
8 (%) = 1 −
(3)
7
7퐶0,퐶
· MCO
2
퐻
7
8
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Where C0,CO2 and C0,C7H8 are the initial concentrations of CO2 and collapse of microfibers into nanofibrils. After comprehensively
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DOI: 10.1039/D0TA07886G
C7H8, Ct,CO2 is the concentrations of CO2 at time (t), and MC7H8 and comparing fiber morphology, mechanical properties and filtration
MCO2 are the molecular weights of CO2 and C7H8.
performances of fibrillated nonwovens (Figure S4), we set the
optimal concentration at 30% and the reaction time at 3 h. Note here
that other acids, such as sulfuric acid (H2SO4) and acetic acid
(CH3COOH) were also used for fibrillation, and the pH values of H2SO4,
H3PO4 and CH3COOH under the same concentration (30%) are -1.15,
0.25 and 1.92, respectively. The results (Figure S3) show that solution
of low pH would cause fast and excessive decomposition of
microfiber, while solution of high pH requires a much longer
fibrillation time. The shearing effect on fibrillation process was also
analyzed by changing the stirring speed. The results (Figure S5) show
that with the increase of stirring speed, both the number and the
length of nanofibers increased. However, when the stirring speed
reaches 900 rpm, the strong shearing force would start to destroy
the crystal structure of fiber core, which may cause the decrease of
nonwoven strength. Therefore, we used a moderate stirring speed of
500 rpm in the following experiments. As shown in Figure 1a, the
acquired nonwoven exhibits a distinct 3D structure, with microfibers
serving as the primary structure, and surrounding nanofibrils as the
secondary structure.
Results and discussion
Different from previous fibrillation methods that all using fibers as
objects,34-37 our strategy is based on nonwoven rather than
individual fibers. As illustrated in Scheme 1, para-aramid nonwoven
with the thickness of 1.8 mm was first produced by subjecting para-
aramid microfibers to a typical web forming (carding) and bonding
(needle punching) process. The surface fibrillation process was then
triggered by immersing the nonwoven in H3PO4 solution under
vigorous stirring. Due to the joint effects of chemical hydrolysis and
physical shearing, the transverse intermolecular forces at the surface
of para-aramid microfibers were weakened, resulting in the
generation of nanofibrils. H3PO4 was selected for its moderate
fibrillation effect, and the degree of fibrillation was adjusted by the
concentration of H3PO4 and the reaction time. With inadequate
concentration or time, the generation of nanofibrils is insufficient,
whereas an excessive concentration or time would result in complete
Scheme 1. Schematic illustration of fabrication, fibrillation and catalyst-loading of 3D para-aramid nonwoven.
In comparison to the pristine nonwoven (Figure 1b), the fibrillated drop of 3.5 Pa, making the quality factor (QF), a trade-off parameter
nonwoven shows no change in dimension or color when observed between average filtration efficiency (η) and average pressure drop
from the macroscopic view. This indicates that the surface fibrillation (ΔP) that reflects the overall filtration performances of filter,41
process holds no interference on the 3D fluffy structure of microfiber increases significantly from 0.150 to 0.536 Pa-1. This value is superior
nonwoven. To the best of our knowledge, this is the first report of 3D to many of the state-of-art fiber filters (Figure S6),16, 42-46 and the
para-aramid nonwoven generated by fibrillation, for that previous pressure drop is far less than the requirements in the Standard. Here
methods all resulted in 2D nanofibrous membranes.38-40 The smooth the low pressure drop is mainly ascribed to that the nanofibrils on
microfibers become rougher after fibrillation, and the large interfiber the surface of microfibers do not overlap to form a fixed network
pores are filled with numerous nanofibrils, leading to a notable structure, and the pore size of fibrillated nonwoven is still large,
decrease of pore size from 36.3 to 18.9 μm (Figure 1c). Considering which has no obvious blocking effect on the passing airflow.
that the highly fluffy, 3D nonwoven architecture remains unchanged, Moreover, under the action of air flow, nanofibers that are originally
we speculate that the reduction of pore size may have positive attached to the surface of microfibers may float into the air to form
impacts on the filtration performances. To validate our assumption, a brush-like structure. Since the air velocity is nonzero at the fiber
air filtration tests were conducted by strictly following the procedure surface, the momentum of PMs makes them easily collide with the
from European Standard of Medium Efficiency Filters (BS EN nanofibers when passing through nonwoven. In addition, these
779:2012). According to the results in Figure 1d, the fibrillated nanofibers possess more active sites and stronger adsorption
nonwoven shows a significantly improved filtration efficiency of capacity than microfibers because of their higher interface energy.
84.7% (twice that of pristine nonwoven) and an ultralow pressure Therefore, the diffusion and interception effects for PM capture are
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greatly enhanced, leading to an improved filtration efficiency. The fibrillation are highly comparable (Figure S8), implying that both
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DOI: 10.1039/D0TA07886G
above mechanism is further verified by using NaCl particles to mimic crystallinity and thermal behavior of para-aramid fibers are not
the PMs. As shown in Figure S7, the nanofibers captured much more altered. Moreover, owning to the support from microfibers, the 3D
particles than the microfibers.
structure of fibrillated nonwoven remains intact after 100 cycles of
The impacts of H3PO4 treatment on the microstructure of para- repeated compression (Figure 1e), and the tensile stress of fibrillated
aramid fibers were analyzed by X-ray diffraction (XRD) and nonwoven is still up to 11.4 MPa (inset of Figure 1e).
thermogravimetry analysis (TGA). The curves before and after
Figure 1. SEM images and digital photos of (a) fibrillated para-aramid nonwoven and (b) pristine para-aramid nonwoven. (c) Pore size
distribution, (d) filtration performances, and (e) compressional resilience and tensile stress of pristine and fibrillated para-aramid nonwovens.
Besides decreasing pore size, the generation of nanofibrils holds with an average size of 250 nm has the largest surface area and the
another advantage in increasing the SSA. According to Brunauer- highest CO oxidative activity (Figure S10 and Table S1). We therefore
Emmett-Teller (BET) method, SSA of para-aramid nonwovens before introduced CuO onto the flower-like CeO2, with the aim to generate
and after fibrillation are 0.6 m2·g−1 and 12.3 m2·g−1, whilst pore more oxygen vacancies (CeO2−x) and increase the oxidation capacity
volume of nonwovens before and after fibrillation are 0.0002 cm3·g−1 of catalysts.49 The ratio of Cu in final products (CuO-CeO2) was
and 0.0035 cm3·g−1, respectively (Figure S9). Such a sharp increase in confirmed by inductively coupled plasma (ICP) analysis (Table S2),
surface area and pore volume makes us hypothesize that, when used and the optimal molar percentage of Cu was determined by
as carrier for catalysts, the fibrillated nonwoven may possess a high comparing catalytic performances of different CuO-CeO2. As shown
loading capacity, which is favorable for eliminating toxic gases from in Figure 2a-c, increasing Cu content makes the particle size of CuO-
polluted air. To this end, we synthesized a group of CuO-CeO2 CeO2 larger, and the petal of flower-like structure becomes more
catalysts by mixing Ce(NO3)3·6H2O, Cu(NO3)2·3H2O and ammonia in a apparent. The (111) crystal facets of CeO2 are found in the high-
high-pressure reactor for hydrothermal reaction. CeO2 was resolution transmission electron microscope (HRTEM) images of all
employed as the promoter of catalyst for its excellent stability, samples, suggesting that the addition of CuO has no damage to the
oxygen mobility and promotion of metal dispersion.47-48 The flower-like structure.50 Here we find that the (002) crystal facets of
morphology analyses of CeO2 suggest that flower-like CeO2 particles CuO only exist in samples containing 15% CuO, whereas Cu is found
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in the line-scan energy-dispersive spectrometry (EDS) of every all achieved at the CuO proportion of 10%, which suggests in theory
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DOI: 10.1039/D0TA07886G
sample. This indicates that at low proportion (i.e. ≤10%), the CuO that when CuO is introduced, the catalytic activity of CuO-CeO2
particles are highly dispersed and uniformly distributed on the would increase first, but then decrease when the content of CuO
surface of CeO2, whereas particle aggregation would become exceeds 10%. This trend is confirmed by hydrogen temperature
inevitable if the proportion of CuO gets higher. More evidence can programmed reduction (H2-TPR) and CO conversion tests. H2
be found in X-ray diffraction (XRD) patterns (Figure S11), where reduction profile demonstrates that the reduction temperature of H2
characteristic peaks of CuO are detected when the ratio gets higher is decreased due to the combination of CuO and CeO2.57 The
than 10%. Chemical states of Ce, Cu and O were further analyzed by minimum temperature for converting CO is also reduced with the
fitting the spectra of Ce3d, Cu2p and O1s via a peak division method presence of CuO (Figure S12). Among the CuO-CeO2 catalysts, the
described previously.51-53 According to Figure 2d, in Ce3d spectra, u’ lowest temperature (peak α) reducing H2 is acquired by 0.10 CuO-
and v’ peaks are associated with Ce3+, and the other six peaks are CeO2 (Figure 2e), and the complete conversion of CO by 0.10 CuO-
ascribed to Ce4+.47 The presence of Ce3+ represents the existence of CeO2 is achieved at 90 ℃, which is at least 30 ℃ lower than using
oxygen vacancies, which can facilitate the oxidation of toxic gases 0.05 CuO-CeO2 and 0.15 CuO-CeO2 (Figure 2f). The reliability of 0.10
(CO in particular).51 In Cu2p spectra, the lower ratio of shake-up CuO-CeO2 catalyst is further demonstrated by a time-on-stream
satellite to the main peak of Cu2p3/2 (nominated as Isat/Imain
)
(TOS) test of 1,200 min, during which no significant changes in the
corresponds to a higher amount of reduced Cu species.54 This is also conversion of CO is observed (Figure 2g). In consistence to the
conducive to the oxidation of CO, as the interaction between HRTEM results, these findings prove that the Cu species are
reduced Cu species with Ce was the main active site for CO uniformly dispersed and in close contact with Ce. When more Cu
oxidation.55-56 In O1s spectra, the peaks at 529.3 eV and 531.6 eV species enter the lattice of Ce, additional Ce3+ and oxygen vacancies
correspond to the lattice oxygen (Olatt) species and surface hydroxyl would be created (Cu1++Ce4+↔Cu2++Ce3+) for oxidizing CO.58-60 As
groups (OOH), respectively, and OOH has been reported as the most the proportion of CuO keeps increasing, however, the CuO particles
active oxygen in oxidation reactions.29 As summarized in Table 1, the intend to aggregate and form CuO bulks, which would largely weaken
optimal ratios of Ce3+/(Ce4+ + Ce3+), Isat/Imain and OOH/(Olatt + OOH) are the conversion capacity of catalysts.61
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Figure 2. SEM images, TEM images, corresponding HRTEM images and EDS spectrum of (a) 0.05 CuO-CeO2, (b) 0.10 CuO-CeO2 and (c) 0.15
CuO-CeO2. (d) XPS spectra of Ce3d, Cu2p and O1s for 0.05 CuO-CeO2, 0.10 CuO-CeO2 and 0.15 CuO-CeO2. (e) H2-TPD and (f) CO conversion
of 0.05 CuO-CeO2, 0.10 CuO-CeO2 and 0.15 CuO-CeO2. (g) Stability test of 0.10 CuO-CeO2 for 1,200 min.
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Table 1. Textural properties of CuO-CeO2 catalysts with different molar percentages of Cu.
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DOI: 10.1039/D0TA07886G
Samples
Ce3+/(Ce4++Ce3+) [%]
Isat/Imain [%]
OOH/(Olatt+OOH) [%]
0.05 CuO-CeO2
0.10 CuO-CeO2
0.15 CuO-CeO2
17.53
18.26
18.11
33.23
26.45
29.37
28.32
33.43
33.16
min. It can be seen from Figure 3d (see also Video S1) that during the
continuous air blowing, no particles are detected downstream of the
filter. Also, neither the nonwoven nor the water changes color after
washing (Figure 3e, see also Video S2). In accordance to these
observations, the nonwoven exhibits no change of weight after
repeated air blowing and washing (Figure S14), proving that CuO-
CeO2 particles are firmly attached on the fibrillated nonwoven under
both dry and wet states. Reason for such a tight adhesion can be
traced to the reaction mechanism of fibrillation (Figure 3f). Under the
attack of electrophilic reagents (such as Lewis acid), the amide
groups (acting as electron-donating groups) of para-aramid
molecules help produce hydroxyl groups by improving the activity of
phenyl ring ortho-para-reaction. Meanwhile, a small proportion of
para-aramid molecular chain is hydrolyzed in H3PO4 solution,
producing amino and carboxyl groups at both ends of the molecular
chain. These active groups, together with the -P-OH groups grafted
on fiber surface, form hydrogen bonds with the surface hydroxyl
(OH) groups on CuO-CeO2 (Figure 3g).62-65 This mechanism is
supported by Raman, Fourier transform infrared spectroscopy (FT-
IR) and XPS spectra (Figure S15), and the decrease of water contact
angle (WCA) from 151° to 130° (Figure S16) after fibrillation is also
associated with the increase of hydroxyl groups (Ar-OH, -P-OH, -
COOH).
We then incorporated 0.10 CuO-CeO2 catalysts on the para-aramid
nonwovens via ultrasonic impregnation (Scheme 1). SEM images
(Figure 3a) and energy dispersive X-ray analysis (EDX, Figure 3b) show
the homogenous distribution of CuO-CeO2 in both fibrillated and
pristine nonwovens, whereas in fibrillated nonwovens, the CuO-CeO2
particles are found on the surfaces of both microfibers and
nanofibrils, leading to a much stronger intensities of Ce, Cu and O.
This verifies that the existence of abundant nanofibrils can provide
more adhesion sites for catalysts. More distinct difference of loading
capacities is visualized by digital photos (Figure S13), the fibrillated
nonwoven shows a darker and more evenly distributed black color
than the pristine nonwoven. The actual loading capacities of two
nonwovens were further quantified via thermo gravimetric analysis
(TGA) (Figure 3c). Para-aramid fibers start to decompose at 540 ℃,
while CuO-CeO2, as a type of inorganic particles, remains intact under
elevated temperature. The loading rate of fibrillated nonwoven
reaches 23.2%, which is much higher than that on pristine nonwoven
(10.4%).
Apart from loading capacity, the tight adhesion between carrier
and catalyst constitutes another essential condition for practical
applications, for it determines the durability of final products. Here
two methods were adopted to respectively examine the dry and wet
adhesion fastness of CuO-CeO2 on fibrillated nonwoven, i.e. air
blowing at 5 m·s-1 for 10 min, and washing under 1000 rpm for 10
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Figure 3. (a) SEM images of CuO-CeO2 on fibrillated and pristine para-aramid nonwovens, (b) EDX mapping images of O, Ce, and Cu on
fibrillated and pristine fibers, and EDS spectrum of CuO-CeO2 on fibrillated and pristine para-aramid nonwovens. (c) TGA of fibrillated and
pristine nonwovens with CuO-CeO2. (d) Dry and (e) wet adhesion fastness of CuO-CeO2 particles on fibrillated nonwovens. (f) Reactions of
treating para-aramid fibers with H3PO4 solution and (g) the hydrogen-bonding interactions between CuO-CeO2 and fibrillated para-aramid
nonwoven.
The CuO-CeO2/fibrillated nonwoven was then subjected to air CeO2/fibrillated nonwoven are 86.1% and 0.533 Pa-1. According to
filtration and CO conversion tests to evaluate its application potential the European Standard of Medium Efficiency Filters (BS EN 779:
as multifunctional air filter. With the adhesion of CuO-CeO2, filtration 2012), these values substantially surpass the requirements for
performances of the nonwovens are further improved (Figure 4a), medium efficiency filters. Furthermore, loading of abundant CuO-
which can be explained by the combined effects of smaller pore size CeO2 with high oxidative activity provides the fibrillated nonwoven
and larger SSA (Figure 4b). The filtration efficiency and QF of CuO- with an excellent conversion capacity against CO. The complete
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conversion temperature (~90 ℃) is much lower than that of the CuO- commonly distributed in indoor air, these results clearly suggest the
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DOI: 10.1039/D0TA07886G
CeO2/pristine nonwoven (~120 ℃, Figure 4c). By contrast, para- efficacy and versatility of CuO-CeO2/fibrillated nonwoven in purifying
aramid nonwovens without catalysts, whether fibrillated or not, different types of air pollutants.
show no conversion of CO (Figure S17).
Besides converting CO under elevated temperature, the CuO- further strengthened by the adhesion of CuO-CeO2 particles. As
CeO2/fibrillated nonwoven also exhibits remarkable demonstrated in Figure 4e (see also video S3 and S4), all the samples
Thermal and chemical stabilities of para-aramid nonwoven are
a
photodegradation capacity to VOCs under room temperature. Taking would not be ignited due to the flame-retardant nature of para-
C7H8 as example, the nonwoven is able to degrade C7H8 in a short aramid, and the CuO-CeO2/fibrillated nonwoven suffers less
period. As shown in Figure 4d, more than 50% of the C7H8 can be decomposition than pure nonwoven. The abundant inorganic
removed within 1 h, and the complete removal is achieved in 6 h. particles also keep the filtration performances of fibrillated
Accompanied by the degradation of C7H8, the concentration of nonwoven at a high level, even when immersing the nonwoven in
carbon dioxide (CO2) increases synchronously (Figure S18), reflecting 98% formic acid (HCOOH) solution and 0.5 mol·L-1 of sodium
the successful conversion of C7H8 into CO2 and H2O. Given that CO is hydroxide (NaOH) solution for 1 h, respectively (Figure 4f).
one of the major pollutants in automobile exhausts, and C7H8 is
Figure 4. (a) Filtration performances, (b) pore size distribution and SSA, (c) CO conversion rate, and (d) C7H8 photocatalytic rate of pristine
and fibrillated para-aramid nonwovens loaded with CuO-CeO2. (e) Combustion behavior of fibrillated nonwovens with and without CuO-
CeO2. (f) Filtration performances of CuO-CeO2/fibrillated nonwovens after 1 h of acid or alkali treatment.
When it comes to practical use, it is essential to scale up the
production. Our strategy entails the manufacture of nonwoven
(which is completely based on an industrialized process) as first step,
Conclusions
and the fibrillation of fibers (which is achieved by simply treating the
nonwoven with H3PO4 solution) as second step. Neither of these
steps requires sophisticated equipment or tedious procedures. As a
In summary, we have demonstrated the fabrication of nonwoven
containing both microfibers and nanofibrils through exploiting the
fibrillation behavior of para-aramid microfibers. With microfibers
serving as the framework, the fibrillated nonwoven exhibits a stable
3D structure, while the presence of numerous nanofibers greatly
reduces the pore size and enlarges the surface area. These features
greatly improve the filtration efficiency of nonwoven without
increasing the filtration resistance. Furthermore, when loaded with
catalysts such as CuO-CeO2, the fibrillated nonwoven possesses an
enhanced loading capacity, and thus enabling the sufficient removal
of both solid particles and toxic gases.
proof of scalable production, we prepared
a roll of CuO-
CeO2/fibrillated nonwoven with a length of 180 cm and a working
width of 40 cm (Figure S19). More attractively, it has been
documented that a number of widely-adopted fibers, including
cellulose
fibers,66-67
segmented-pie
fibers68-69
and
polytetrafluoroethylene (PTFE) fibers,70 possess similar fibrillation
behaviors, and the fibrillated fibers have been reported as promising
building blocks for separator of solid electrolytic capacitor,71-72
orodispersible film,73 healthcare,74 and breathable apparels.75 The
nonwoven-based fibrillation strategy proposed in this study may be
extended to a lot of materials and applications.
Conflicts of interest
There are no conflicts to declare.
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31 Y. A. May, W.-W. Wang, H. Yan, S. Wei and C. J.VJiieaw, ACrthicilneeOsnelinJe.
Acknowledgements
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DOI: 10.1039/D0TA07886G
Graphical abstract
Para-aramid nonwoven composed of both micro- and nanofibers is loaded with CuO-
CeO2 to remove solid and gaseous pollutants.