Metal-Organic Framework Nanofibrous

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Applications of Polymer, Composite, and Coating Materials

Electrospun polyimide/metal-organic framework nanofibrous membrane with superior thermal stability for efficient PM2.5 capture Zhimin Hao, Juntao Wu, Chaolu Wang, and Jingang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22415 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Applied Materials & Interfaces

Electrospun Polyimide/Metal-organic Framework Nanofibrous Membrane with Superior Thermal Stability for Efficient PM2.5 Capture Zhimin Hao, † Juntao Wu, *, † Chaolu Wang, † Jingang Liu*, ‡ † Key Laboratory of Bioinspired Smart Interfacial Science and Technology

of

Ministry

of

Education,

School

of

Chemistry,

Beihang University, Beijing, 100191, P. R. China. ‡ Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals Materials,

and

Solid

School

of

Wastes,

National

Materials

Science

Laboratory and

of

Mineral

Technology,

China

University of Geosciences, Beijing 100083, P. R. China. Key words: Polyimide, ZIF-8, Air filtration, PM2.5, High thermal stability.

1

ACS Paragon Plus Environment

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Abstract: Particulate matter (PM) pollution has serious threaten to human health. Zeolitic imidazolate framework-8 (ZIF-8) is a kind of metal-organic framework (MOF), and ZIF-8 not only can capture PM2.5 efficiently but also possesses excellent chemical and thermal stability. In this study, ZIF-8 modified soluble polyimide

(PI)

nanofibrous

membranes

were

prepared

via

electrospinning process. As a result, the PI-ZIF membrane shows high

PM2.5

filtration

efficiency

(up

to

96.6±2.9%),

superior

thermal stability (up to 300 °C), good transmittance, excellent mechanical properties and low pressure drop. The prepared PI-ZIF membrane with excellent comprehensive property shows a promising application in PM2.5 capture, especially in harsh conditions.

1. Introduction With the development of industries, air pollution, especially particulate environmental

matter

(PM)

problem.1-3

pollution, Compared

with

has

become

PM10,

PM2.5

a

major

has

more

serious adverse effect to human health.4 Because of its small size, PM2.5 can easily pass through the respiratory tract even penetrates into the blood vessel. A growing evidence suggests that human long-term exposure to PM2.5 pollution can increase the risk of cardiovascular and respiratory disease mortality.5,6 The PM2.5 removal

filters

are mainly divided into two

types:

the

porous film filter and fibrous filter, among which the latter is

2


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considered be more potential owing to its high surface area, light weight, low pressure drop and facile synthesis.7 However, the conventional fibrous filter have not been widely developed and applied owing to their inevitable drawbacks, such as low filtration efficiency, poor mechanical property and bad thermal stability. Therefore, developing a novel filter material with excellent

comprehensive

property

is

a

long-term

goal

for

filtration materials research. ZIFs are a subclass of porous metal-organic frameworks (MOFs) with

tetrahedral

cations

and

framework

imidazolate

structures

anions.

constructed

Some

ZIFs,

such

from as

metal ZIF-8,

combine the attractive features of MOFs (large surface area, diversity of structure and pore, functional tunability) and high thermal other

and

chemical

stability,

materials.8-10

MOF

which

Recently,

is

some

rarely studies

found

among

have

been

demonstrated MOFs can achieve high efficiency PM capture via its exceptional

open

metal

sites,

functional

groups

and

surface

charge.11,12 Polyimide (PI) as a high performance materials not only

has

excellent

comprehensive

performances,

especially

in

mechanical properties and thermal stability, but also has high dipole moment (6.2 D) which indicating strong adhesion of PM on PI.13-15 Besides, the existence of organic ligands in MOF also makes MOFs are more compatible with polymers.16

3



Page 4 of 28

Electrospinning is a fast and facile technology for preparing hybrid fibers. MOF/polymer fibers were successfully fabricated by

electrospinning 17-21

growth.

electrospun

combined

Hence,

we

directly blending

with

presented

polyimide/ZIF-8

(PI-ZIF)

for

the

or

first

nanofibrous

in-situ

time,

membrane

an for

PM2.5 capture application. We fabricated PI-ZIF membranes with different ZIF loading and their properties were systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM),

Brunauer−Emmett−Teller

(BET)

test,

thermogravimetric

analyses (TGA) and tensile test. Compared with PI membrane, the filtration enhanced. membrane

efficiency The

is

of

highest

96.6±2.9%

PI-ZIF

PM2.5 with

membranes

filtration a

10wt%

ZIF

are

significantly

efficiency

of

PI-ZIF

loading.

The

PI-ZIF

membranes also show superior thermal stability (up to 300 °C). Besides, the obtained membranes show low pressure drop, good transmittance and excellent mechanical properties. In short, the PI-ZIF

membranes

promising

with

material

excellent

for

PM2.5

comprehensive capture

conditions. 2. Experimental section 2.1 Materials

4


property

especially

in

are

a

harsh

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N,N-dimethylacetamide Methanol,

Ethanol,

supplied

(DMAc),

acetic

by

N,N-dimethylformamide

anhydride

Beijing

(Ac2O)

and

Chemical

(DMF),

pyridine

Works.

were 4,4’-

(Hexafluoroisopropylidene)diphthalic anhydride (6FDA CAS Number 1107-00-2)

and

2,2’-bis[4-(4-

aminophenoxy)phenyl]hexafluoropropane (BDAF CAS Number 69563-888) were supplied by Forsman Scientific(Beijing) Co.,Ltd.. Zinc nitrate hexahydrate ((Zn(NO3)2·6H2O CAS Number 10196-18-6) and 2methylimidazole Aladdin. reagents,

All

of

except

(CAS

Number

reagents DMF

and

693-98-1)

are DMAc

analytic use

4

were

purchased

purity.

A°

Among

molecular

from these

sieves

to

remove adsorbed water, the others were used as received. 2.2 Synthesis of soluble polyimide 6FDA-BDAF The soluble polyimide (6FDA-BDAF) was synthesized via a two-step chemical

imidization

procedure,

as

illustrated

in

Figure

S1.

5.185 g of BDAF and 54.6 g DMAc were added to a three-necked flask equipped with a stirrer, an ice-water bath and a nitrogen inlet. After BDAF was completely dissolved, 4.442 g of 6FDA was added into the mixture, producing a solution with 15wt% solid content. The mixture was stirred for 1 h, removed ice-bath and kept stirring at room temperature for 20 h. Then, the chemical imidization was carried out with Ac2O (5.1 g) as dehydrating agent and pyridine (4.0 g) as catalyst at room temperature for

5



Page 6 of 28

another 20 h. The obtained viscous solution was carefully poured into an excess of ethanol to yield a silky resin. The resin was collected and dried at 80 °C in vacuum for 24 h (Yield: ~95%). 2.3 Synthesis of ZIF-8 ZIF-8 was synthesized according to a previous procedure.22 1.291 g

of

Zn(NO3)2·6H2O

dissolved solution stirring.

in was

50

rapidly

Then

statically.

mL

and of

1.621

After 24 h,

of

methanol,

poured

stirring

g

into

stopped

2-methylimidazole

respectively. the

and

former

the

mixed

The

were latter

solution solution

the powders were separated

under kept

from the

milky dispersion by centrifugation and washing with methanol. The product was dried at 120 °C for 12 h. The X-ray diffraction (XRD) pattern of sample was almost identical to the simulated pattern of known structural data (Figure S2) that demonstrates the product is ZIF-8 material.23 As shown in Figure S3, ZIF-8 particles as synthesized presents isometric nanoparticles with sharp

edges

and

a

narrow

size

distribution.

A

statistical

evaluation of 100 particles results in an average diameters of 95 nm. 2.4 Preparation of PI-ZIF membrane ZIF-8 powder (0.01 g) was first dispersed in 2 g of DMF by ultrasonic treatment and then 0.4 g of soluble polyimide was

6


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added.

Then

the

homogeneously

mixture

solution

electrospinning

was

with

solution

mechanical a

ZIF-8

with

stirred

loading

different

of

ZIF-8

to

formed

a

2.5wt%.

The

loading

(0,

2.5wt%, 5wt% and 10wt%) were prepared by changing the content of ZIF-8.

To

prepare

solution was

added

electrospun

membrane,

the

in plain plastic syringes

electrospinning connected

by a

metal nozzle with an inner diameter of 0.65 mm. Then solution was

electrospun

into

nanofiber

and

collected

on

an

aluminum

foil. The parameters of the electrospinning were set as follows: the

distance

between spinneret and collector of 15

voltage power

cm, high

supply of 15 kV, volume feed rate of 0.5 mL/h

subjected by air pressure, respectively under humidity

Metal-Organic Framework Nanofibrous

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