Alumina-mediated mechanochemical method for simultaneously degrading perfluorooctanoic acid and synthesizing a polyfluoroalkene

Article abstract of DOI:10.1039/c8gc00854j

The design of large-scale processes for disposing harmful perfluorooctanoic acid (PFOA) is an important issue, since it was listed as a persistent organic pollutant in 2015. In this work, we developed an alumina-mediated solid-state mechanochemical (MC) method to simultaneously degrade PFOA and synthesize 1H-perfluorohept-ene (1H-1-PFHp), which is a valuable organofluorine block. A 2.5 h MC treatment resulted in a nearly complete removal (99.4%) of PFOA and a high yield (92.5%) of 1H-1-PFHp. In this transformation, the surface hydroxyl groups on alumina are critical for anchoring the PFOA molecules during the defluorination reaction, and the milling process promotes the dehydration of alumina to produce reactive Lewis acid sites for activating the C-F bonds. Moreover, the high energy ball milling initiates simultaneously the release of lattice oxygen from alumina, producing oxygen vacancies (in alumina) and free electrons, the latter of which can induce the breakage of C-F bonds via a reductive pathway. The combination of the mechanocaloric effect and the triple roles of alumina drives PFOA to a controlled defluorination. This MC process shows promising applications in the green treatment of PFOA solid wastes.

Full text of DOI:10.1039/c8gc00854j

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Alumina-mediated mechanochemical method for simultaneously
degrading perfluorooctanoic acid and synthesizing
polyfluoroalkene
a
Received 00th January 20xx,
Accepted 00th January 20xx
Hanqing Lv†,a, Nan Wang†,a, Lihua Zhu*,a, Yuqi Zhoua, Wenjing Lib, and Heqing Tang*,b
DOI: 10.1039/x0xx00000x
The design of large-scale processes for disposing harmful perfluorooctanoic acid (PFOA) is an important issue, since it was
listed as persistent organic pollutant in 2015. In this work, we developed an alumina-mediated solid-state
www.rsc.org/
a
mechanochemical (MC) method to simultaneously degrade PFOA and synthesize 1H-perfluorohept-ene (1H-1-PFHp),
which is a valuable organofluorine block. A 2.5-h MC treatment resulted in a nearly complete removal (99.4%) of PFOA and
a high yield (92.5%) of 1H-1-PFHp. In this transformation, the surface hydroxyl groups on alumina are critical for
anchoring the PFOA molecules during the defluorination reaction, and the milling process promotes the dehydration of
alumina to produce reactive Lewis acid sites for activating the C−F bonds. Moreover, the high energy ball milling initites
simultaneously the release of lattice oxygen from alumina, producing oxygen vacancies (in alumina) and free electrons, the
latter of which can induce the breakage of C−F bonds via a reducꢀve pathway. The combinaꢀon of the mechanocaloric
effect and the triple roles of alumina drives PFOA to a controlled defluorination. This MC process shows promising
applications in the green treatment of PFOA solid wastes.
with Na2C2O4 at 270°C for the complete destruction of
chlorofluorocarbons, and they also found that the same
reaction at 230 °C caused partial defluorination of cyclic
Introduction
Perfluorooctanoic acid (C7F15CO2H, PFOA) has been widely
used in industrial and consumer applications, as it shows high
chemical and thermal stability and low surface energy.1
However, its non-degradable emission makes it become a
global pollutant in environment. In 2015, it was listed as a
persistent organic pollutant by the Stockholm Convention in
2015, due to its persistence, risk of bioaccumulation and
toxicity.2, 3 Thus, recent studies on fluorocarbon surfactants
are being driven by not only the desire to develop alternatives
of PFOA, 1, 4 but also the need of methods for the disposal of
harmful PFOA.5, 6 In both of the two fields, the activation of
carbon-fluorine bonds is considered as a challenge due to the high
dissociation energy of C−F bonds.7-9
perfluoroalkanes to yield perfluoroarenes.13
Since perfluoroalkenes are valuable chemical blocks, methods
for their synthesis based on the selective defluorination of
readily available fluorocarbons are of great interest.14
Controlled defluorination is difficult to achieve, but it has been
done by using sodium benzophenone radical anion,
cobaltocene/Li[O3SCF3], (C5H5)2TiF2/Al/HgCl2, and so on.15-17
These methods require a combination of a relatively milder
reducing agent to activate the alkyl C−F bonds and an alkali
metal salt to stabilize the fluoride ions. Almost all of them lead
exclusively to aromatic products from cyclic perfluoroalkanes.
Crabtree et al. found an exception that the Hg-photosensitized
reduction of perfluoroalkanes by using decamethylferrocene
and Li[O3SCF3] gave perfluoroalkenes but in very low yields
The need to convert fluorocarbons into disposable or less toxic
products has inspired many studies.10, 11 Incineration (burning
in air or O2) at temperatures above 500 °C can decompose
PFOA, but this gives undesirable greenhouse gases like CF4 and
C2F4. Wang et al. found that the addition of Ca(OH)2 prevented
the generation of greenhouse gases during C7F15SO3H
decomposition, but it was less efficient in the decomposition
of PFOA.12 Burdeniuc and Crabtree reported a thermal process
(e.g., perfluoro-2-methylpentane
→ perfluoro-2-methylpent-2-
ene, 40 h, 1.36%).18, 19 LaZerte et al. reported that anaerobic
pyrolysis of the sodium salt of PFOA at temperatures of 250-
260°C generated perfluoroalkenes (CF3(CF2)5CF=CF2) in 86%
yield.20 The presence of small quantities of hydrogen-
containing salts, inorganic bases and adsorbed water greatly
reduced the yields of perfluoroalkenes, indicating that the
pyrolysis method requires the starting PFOA be very pure.
From an environmental and economic point of view, the
selective defluorination to fluorinated olefins has striking
advances in the disposal of PFOA, but such studies are still rare.
Recently, Zhang et al. reported that PFOA was mineralized by a
strong base (KOH) to give KF and K2CO3 under ball milling.21
aCollege of Chemistry and Chemical Engineering, Huazhong University of Science &
Technology, Wuhan, 430074, P.R. China. E-mail:lhzhu63@hust.edu.cn
bCollege of Resources and Environmental, South-Central University for Nationalities,
Wuhan, 430074, P.R. China. E-mail: tangheqing@mail.scuec.edu.cn
† These authors contributed equally to this work.
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available.
See
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This hints that C−F bonds can be acꢀvated by
a
Ultrasonic Instruments Co. Ltd, China) for 10 min. After
mechanochemical (MC) process in the presence of a proper removing the solids by centrifugation, the collected solution
agent. Compared to the solution-based wet processes, the ball was filtered through a 0.22 μm membrane and then the
milling MC process is a dry one, and it offers advantages for filtrate was diluted to 10 mL with ethanol before HPLC
the facile processing of a large amount of substrate by analysis.
eliminating the need for solvent, promoting mass transport (2) The pretreatment for analysis of inorganic fluoride ions.
and being more energy efficient. Therefore, MC methods The milled samples (0.020 g) were dispersed in water, and a
represent an attractive process for organic synthesis,22, 23 the small amount of ammonia was added to release all fluoride
preparation of functional materials24-26 and the disposal of ions being adsorbed by Al2O3. The suspension was centrifuged
organic pollutants from a sustainability perspective.27-30
and filtered through a 0.22 μm membrane, and then the
In this paper, we developed a simple and efficient MC method filtrate was diluted to 50 mL with water and 10 mL total ionic
for the controlled conversion of PFOA to 1H-perfluorohept-1- strength adjustment buffer (TISAB). Then, inorganic fluoride
ene (1H-1-PFHp) with high selectivity (89.1%), in which ions were measured by a fluoride ion-selective electrode.
commercially available alumina was used as a co-milling (3) The pretreatment for analysis of organic products. During
reagent and catalyst.
the MC degradation of PFOA, organic products having lower
boiling points might partially volatilize because the balling
increased slightly the jar temperature (e.g., 45 °C after a 5 h
ball milling). To completely collect intermediates including any
gaseous and liquid forms in each new run of the PFOA
conversion experiment, 10 mL of diethyl ether was injected
into the milling pot to extract the organic products. To
Experimental
Materials
Perfluorooctanoic acid (PFOA, C8F15O2H, 98% purity) and
perfluorooctane (PFO, C8F18, 98% purity) were purchased from
J&K Chemical (Beijing, China). Fluorobenzene (FB, C6H5F, 99%
purity) was purchased from Aladdin Industrial Corporation
(Shanghai, China). Nanofluoro-1-hexene (PFBE, C6H3F9, 98%
purity) was obtained from Tokyo Chemical Industry Co. LTD
(Tokyo, Japan). Acetone-D6 was purchased from Cambridge
Isotope Laboratories, Inc. (USA). Alumina (Al2O3, analytic
grade) was purchased from Beijing Chemical Works (Beijing,
China). Toluene (TOL, analytical grade) and n-hexane (Hex,
analytical grade) were used as the internal standards for
quantification. Acetone (analytical grade) was used for
extractions, and all solvents used for high-performance liquid
chromatography (HPLC) analysis were HPLC grade. Milli-Q
water (18.2 MΩ.cm) was used in the present work.
evaluate the extraction loss, fluorobenzene as both
a
surrogate standard in the extract process and an internal
standard in the analysis was added to the reactor prior to ball
milling. Our preliminary experiments confirmed that
fluorobenzene was very stable during the MC process, and the
extract recovery was around 93% (n=3) by using PFBE as a
reference compound. The extraction was conducted for 30
min, and the jar temperature was decreased to room
temperature. The extraction dispersions were taken out from
the milling pot. The solids in the dispersions were removed off
by a centrifugation at 10000 rpm for 10 min and then a
filtration through a 0.22 μm membrane, the obtained clean
extract was analyzed by gas chromatography (GC). The
resulting yield of products corresponded to the GC yield, which
was used to evaluate the yield and selectivity of products the
MC conversion in the present work.
MC conversion of PFOA
The MC conversions of PFOA were conducted in a planetary
ball mill (BM4, Beijing Grinder Instrument, China) by using
Al2O3 as a co-milling agent at room temperature under a
(4) The pretreatment for analysis of isolated 1H-1-PFHp as the
major product. To isolate the major product 1H-1-PFHp, the
above mentioned MC conversion experiment was conducted with
an increased scale by several independent runs. In each run, 1.18 g
PFOA was treated with 7.22 g alumina at a same ball-to-
powders mass ratio of 50. The ball milling was conducted similarly.
The reaction was ended at 3 h of ball milling. After the reaction
was ended, 20 mL diethyl ether was slowly injected into the
milling pot, and the extracted dispersion was cleaned by a
filtration treatment. The collected clean solution was transferred
into a 25 mL round-bottomed flask. The flask was placed in a water
bath for distillation with magnetic stirring. The water bath was
slowly heated to 45 °C, and the vapor temperature was controlled
at 34.5±0.5 °C, which was very close to the boil point of
diethyl ether. Diethyl ether was recovered from the distillate
fraction on a Vigreux column (30 × 1.5 cm), and the organic product
was collected as the residual fraction. The distillation process was
stopped when the vapor temperature began falling down. Finally,
the isolation yield was calculated from the mass of the product.
normal atmosphere. In a typical experiment, 0.59 g PFOA and
3.61 g alumina (at a molar ratio of 1: 25) were well mixed in a
stainless pot (250 mL). After being filled with 18 stainless steel
balls with a diameter (d) of 6 mm and 10 balls with diameters
of 10 mm, the pot was sealed tightly, and then the planetary
ball milling was conducted at 350 rpm under atmospheric
conditions with automatic rotation direction changes every 15
min. The reaction was ended by stopping the milling at given
time intervals, and the milled samples were properly removed
for chemical analysis to monitor the degradation of PFOA and
the transformation of milled reagents. Different pretreatment
processes of the samples for the analysis of different target
compounds such the residual PFOA, the inorganic fluoride ions,
the organic products, and the isolated major product.
(1) The pretreatment for analysis of the residual PFOA. The
residual PFOA was measured by HPLC. Each of the milled
samples (0.020 g) was extracted with ethanol (3 times, total of
10 mL) under ultrasound agitation (KQ-200KDE, Kunshan
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In addition, the components in both distillates and were frequency of 150.16 MHz. The NMR spectra were recorded
analysed to accesses the purity of 1H-1-PFHp (more details are with spinning rate of 7 KHz with equipped with a 6 mm probe
shown in the ESI). It should be noted here that when this MC at room temperature.
process is applied in an industrial scale, the isolation will be simpler
as described as following.
Because 1H-1-PFHp has a low boiling
Results and discussion
point around 65 °C, the milling pot was slowly heated at 80 °C to
vaporize 1H-1-PFHp, which then was collected by purging N2.
When treating PFOA (0.59 g) with alumina (3.61 g) under ball
milling, the amount of remaining PFOA decreased rapidly to
4.1% in 2 h (Figure 1a and Table 1), and inorganic F− ions were
gradually generated. Further prolonging ball milling time to 2.5
h, the PFOA content continued to decrease (to 0.6%) but much
more slowly due to its almost complete degradation
(conversion). The 2.5-h MC treatment resulted in a high
degradation removal (99.4%) of PFOA and a low defluorination
efficiency (14.1%, Table 1). The selectivity of the F− ion
generation was calculated from the ratio of defluorination
efficiency to the efficiency of the conversion of PFOA (eq. 1).
Interestingly, the selectivity of the F− ions seemed to be
unchanged (13.1 ± 3.2%), which is indicative of a controlled
partial defluorination of PFOA. The generated organic products
Analysis methods
The residual PFOA was measured by HPLC. Based on our
previous work,31 the HPLC analysis were performed on an
Ultimate 3000 series system (Dionex, Idtsein, Germany)
equipped with Corona Ultra RS charged aerosol detector (CAD,
Thermo Scientific, Bellefonte, PA, USA) and a Symmerrys C8
column (150 mm × 3.9 mm; 5 μm particle size; Waters,
Ireland). The column oven temperature was set at 30 °C. The
mobile phase was 30% ammonium acetate (5 mM, adjust to
pH 5.0 ± 0.2 with acetic acid) and 70% methanol by volume,
the flow rate was 1 mL min-1 and the injection volume was 40
μL.
Fluoride ions were measured by a ion-selective electrode. The
defluorination efficiency (DFE) was evaluated by eq. 1:
gave characteristic IR absorptions for C−F at 1100~1300 cm−1
,
C=C at 1676 cm−1 and alkene C−H at 3011 cm−1 (Figure 1b). In
the GC-MS spectra (Figures 1, and S1a), the major product
exhibited mass fragments of CnF2n−1 (n=1~6) blocks when the
electron impact was carried out in hard ionization mode, and
the only major fragment detected in soft atmospheric pressure
cF,sol
cF,cal
(1)
DFE(%) =
where cF,sol and cF,cal are the concentrations of measured
soluble fluoride ion and calculated fluorine from initial the
PFOA, respectively.
chemical
ionization
mode
was
m/z
330.93884
(CF3(CF2)4CF=CF−; theoretical, 330.97924) (Figure S1a). The
comparisons of the 19F and 13C NMR spectra confirmed that
PFOA had lost one CF2 group and COOH group, and some of
the CF2 units had been converted to −CF=CFH moieties during
the milling process (Figures 1d and S1b). From these results,
To structurally characterize the organic products, the extracted
samples were analyzed by a TRACE 1300 GC Ultra system
(Thermo Fisher, USA) equipped with a TR-5MS capillary
column (15 m × 0.25 mm × 0.25 μm) coupled to an ISQ system
in the electron ionization mode and a Fourier transform mass
spectrometer (FT-MS, SolariX 7.0T, Bruker Daltonics, USA)
equipped with an atmospheric pressure chemical ionization
(APCI) source. After being diluted with 500 μL acetone-D6, the
extracts (200 μL) were also analyzed by 19F and 13C NMR
spectroscopy (Bruker Advance III 400, Bruker BioSpin, USA). To
avoid the inference of extract solvents, Fourier-transform
infrared (FT-IR) spectra of the organic products in the gaseous
form were monitored on Vertex70 (Bruker, Germany). After
the MC reaction was finished, the milling pot was heated in a
water bath at 80 °C for 30 min, and then the collected gaseous
products were imported into the gas cell of the FT-IR
spectrometer. The quantification of products was achieved on
a gas chromatography-flame ionization detector system (GC-
FID, 7890B, Agilent, USA) by comparing their response signals
to those of an internal standard (fluorobenzene) with
simulated relative correction factors and a correlation factor
(more details are shown in the ESI).
the major product generated was identified as
1H-
perfluorohept-1-ene (CF3(CF2)4CF=CFH, 1H-1-PFHp), and the
minor products were identified as 1-perfluoroheptene and 1-
H-perfluoropent-1-ene (Figure S2). After 2.5 h of milling, the
yield of 1H-1-PFHp reached 92.5%, but its selectivity remained
almost constant at 89% (89.1% ± 3.8%, Figure S3).
A
possibility of scaling-up was checked by increasing
PFOA/alumina mass ratio, increasing the filling ratio and
decreasing the ball-to-powder mass ratio. It was found that
more than 80% of PFOA could be selectively converted into
1H-1-PFHp when the PFOA/Al2O3 mass ratio was lower than
24.3% together with a filling ratio in the range of 20~60% and
a ball-to-powder mass ratio of 50 (Table S5, Figure S4). 1H-1-
PFHp a major product after the MC conversion of PFOA could
be isolated by a extraction with diethyl ether, a subsequent
filtration and a final distillation. For example, 2 runs of the MC
treatment of PFOA (2.36 g) with alumina (14.44 g) at an mb/mp
of 50 gave a mean selectivity of 86.2% for 1H-1-PFHp (GC yield)
and totally 1.51 g of isolated 1H-1-PFHp (isolated yield, 80.7%;
purity, >85%, Figure S5). Since diethyl ether can be recovered
and alumina can be re-used, the MC conversion of PFOA with
alumina produce only CO2, and F− ions as wastes. According to
the classical definition of E factor,32 the E factor of 1H-1-PFHp
was calculated to be only 0.25. In contrast to the parent
Solid samples were also characterized by other methods.
Diffuse reflectance Fourier-transform infrared (DR-FTIR)
spectra were measured by using Vertex70 (Bruker, Germany)
equipped with a MCT detector and a high-temperature diffuse
chamber with KBr windows. The 19F and 13C MAS NMR
measurements were performed on an Angilent 600 MHz DD2
(DirectDrive2) spectrometer (Angilent, USA) at a resonance
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compound PFOA as persistent pollutant, 1H-1-PFHp is a high Al2O3(or SiO2): PFOA= 50:1; [d]: Not detected.
valuable block for chemical synthesis, and the produced
wastes in the MC conversion showed negligible environmental As is well known, unlike alumina, SiO2 has an abundance of
impacts. Moreover, in terms of resource recovering, the surface OH groups available but no Lewis acid sites. Notably,
present method is more advantageous than the procedure the defluorination of PFOA was significantly suppressed when
described by Zhang et al.,21 which aimed at only a full the alumina was replaced with SiO2. In the case of SiO2, the
mineralization of PFOA into harmless wastes. These results PFOA conversion gradually reached 42.2% after 2 h of milling
indicate that the present method has benefits in the context of but only reached 2.9% defluorination and a F− selectivity of
green and sustainable. The MC conversion of PFOA to 1H-1- 6.6% (Table 1 and S4), both of which were much lower than
PFHp can be expressed by the pathway shown in Scheme 1, that in the case of alumina (17.8% and 18.2%). These
and the critical factors influencing this conversion include the differences suggested that the Lewis acid sites on alumina
surface OH-group content and the number of Lewis acid sites, played a critical role in the breakage of the C−F bonds. A
which will be discussed below.
(a)
previous report also demonstrated that Lewis acid sites were
necessary for the decomposition of perfluoroethers with metal
oxides.33 The MC-initiated defluorination of PFOA will likely
(b) FTIR
1500
n(PFOA)
n(F- ion/15)
ν
CO2
n(Organic product)
ν
(C=C)
1200
ν
(C-H)
continue until all the C
−F bonds are broken. However, the
experimental results confirmed that PFOA molecules were
selectively converted to 1H-1-PFHp (89.1% selectivity) without
further defluorination. We have also observed that when the
MC/Al2O3 system was used to treat the shorter chain
perfluorinated carboxylic acids, it consistently gave terminally
unsaturated polyfluoroalkenes in high yield and selectivity
(>85%, Table 1 and Figure S6). This is related to the weak
adsorption of polyfluoroalkenes because of the lack of
carboxyl group. To validate this hypothesis, perfluorooctane
(C8F18), which has the same C−F framework but lacks the
carboxyl group, was subjected to the MC treatment
conditions. Both the conversion and defluorination of
perfluorooctane were negligible in the MC/alumina system
(Table 1 and S4). Thus, the acid-base reaction between the
COOH group of perfluorocarboxylic acids and the surface OH
groups on the alumina is necessary to strongly anchor the
perfluorocarboxylic acids during their MC defluorination
reaction.
900
600
300
0
ν
as(C-F)
800
0
30
60
90
120
150 4000
3200
2400
1600
-1
Milling time /min
Wavenumber /cm
(d) 19F NMR
(c) GC-MS
100
50
0
RT=5.80 min
1
2
3
4
5
6
7
C(1)F3
169.18
CF -CF -CF -CF -CF -CF=CFH
3
2
2
2
2
51.18
69.13
280.74
CF2 CF=CF−H
118.97
CF2
F
−
CF2
−
−
CF2
−
CF2−
−
69.06
112.85
C(2)F2
C(7)FH
C(6)F
131.64
C(5,4,3)F2
231.06
1
2
3
4
5
6
7
CF -CF -CF -CF -CF -CF -CF -COOH
112.85
118.97
131.64
3
2
2
2
2
2
2
C(1)F3
2h
0h
101.35
C(7)F2 C(5,4,6,3)F2
C(2)F2
3
4
5
6
7
Retention time /min
169.18
231.06
280.74
-82
-120 -125 -130 -135 -140
19F chemical shifts /ppm
50
100
150
200
m/z
250
300
350
Figure 1.MC conversion of PFOA into 1H-1-PFHp and F− by milling
PFOA with alumina. (a) Time profiles of the concentration of PFOA, F−
and 1H-1-PFHp; (b) FT-IR spectrum of organic products in the gaseous
form; (c) GC/MS profiles and (d) 19F NMR spectrum of the extracted
major organic product (in acetone-D6) after subjecting PFOA to 2 h of
ball milling with alumina (the upper) compared to that of unreacted
PFOA solution (the bottom).
The role of alumina in the selective defluorination of PFOA was
investigated with diffuse reflectance Fourier transform
infrared (DR-FTIR) spectroscopy (Figure 2a). After mixing the
PFOA with alumina, the ν(C=O) vibration peak (1768 cm−1) of
PFOA had vanished and had been replaced with the
asymmetric (νas(COO−)) and symmetric stretches (νs(COO−)) of
carboxylate at 1689 cm−1 and 1467 cm−1, respectively. At the
same time, the OH stretching (3040 cm−1) of the carboxylic
group disappeared because PFOA was deprotonated by an
acid-base reaction with surface OH groups on the alumina.
Compared with the DR-FTIR spectrum of alumina, the
intensities of the peaks at 3500, 1085 and 1046 cm−1 (AlO4)
were weaker for the mixture, which further supports the
theory that the dehydroxylation of Al2O3 occurs via an acid-
base reaction with PFOA. The difference in the frequencies of
the carboxylate stretching bands (Δν=νas(COO−) −νs(COO−)) for
the PFOA-Al2O3 mixture was as large as 222 cm−1, indicating
Table 1. MC conversion and thermal treatment of different substrates.
Substrate System
Time Conv. Polyfluoroalkene product
(h)
0.5
1
(%)
structure
Y(%)[a] S(%)[b]
PFOA MC/Al2O3
39.7 C5F11CF=CFH 34.1 85.9
60.8 C5F11CF=CFH 52.6 86.5
95.9 C5F11CF=CFH 91.3 92.5
2
[c]
MC/SiO2
0.5
2
16.8 n.d[d]
n.d
n.d
/
/
42.2 n.d
C6F13COOH MC/Al2O3
C4F9COOH MC/Al2O3
1
98.1 C4F9CF=CFH 88.2 89.9
100 C4F9CF=CFH 91.5 91.5
99.1 C2F5CF=CFH 87.5 88.3
2
1
2
100
2.2
2.8
C2F5CF=CFH 89.0 89.0
that the carboxylic group of PFOA was coordinated to the
35
Moreover,
[d]
surface of Al2O3 in a monodentate mode.34,
C8F18
MC/Al2O3
0.5
n.d
n.d
n.d
n.d
n.d
/
/
/
2
compared to those in the spectrum of powdery PFOA, the
stretching bands of the CF2 groups (1100~1300 cm−1)36 in the
PFOA-Al2O3 mixture were slightly blueshifted to higher
PFOA
150°C/Al2O3[d]0.5
21.9 n.d
[a]: Yield of products; [b]: Selectivity of products; [c]: the molar ratio of
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wavenumbers (>10 cm−1) (Figure 2a). Apparently, the local alumina, all the 19F resonances were shifted up-field, and the
chemical environment of the C−F bonds in PFOA was changed resonance of C(7)−F2 became much broader due to the
by the presence of Al2O3. In addition, no obvious shift was carboxylate group formed through an acid-base reaction
observed in the Al−O stretching (900~1100 cm−1), but in a between PFOA and the alumina. The deprotonation of PFOA
spectrum of the PFOA-Al2O3 mixture the stretching band of the shifted the 13C resonance of the COOH moiety up-field.
surface OH groups (3400~3700 cm−1) was shifted to a lower Moreover, the 19F resonances of the inner C(3~6)−F2 groups in
frequency (approximately 20 cm−1). These changes indicated the mixture heavily overlapped, implying that the inner CF2
the formation of hydrogen bonds between the F atoms of groups of the PFOA interacted with the alumina surface.38
PFOA and surface OH groups on alumina.6, 37 In these hydrogen However, there were no obvious shifts in the 13C resonances
bonds, the interaction between the lone pairs of the F atoms for most of the C−F bonds; the only exception was the signal
and the O–H σ* anti bonding orbital led to a lengthening of for C(6)−F2, which was shifted 0.5 up-field. These observations
the O–H bond of alumina and the concomitant redshift of the further implied that the F atoms of C(6)−F2 in PFOA were
pure O–H stretching frequency.
acting as hydrogen bond acceptors. Thus, PFOA was tightly
bounded to the alumina through both Al⋅⋅⋅carboxylate
(a) DR-FTIR
coordination and Al≡OH⋅⋅⋅F−C(6) hydrogen bond interactions.
0.5
10
ν(Al-O)
Al2O
3
To monitor the possible transition states during PFOA
conversion, we compared the changes in the DR-FTIR spectra
of the PFOA-Al2O3 mixture after very short milling times at
different rotation speeds. As shown in Figure 3a, 5 min of
milling at a low rotation speed (50 rpm) resulted in a
substantial decrease in the characteristic absorption band of
the surface OH groups (3200~3700 cm−1) on alumina,
indicating that the milling process promoted dehydration
between adjacent OH groups on the alumina. The reduction of
the OH groups weakened the hydrogen bonds between PFOA
and alumina, leading to a slight (>5 cm−1) redshift in the C−F
stretching band to lower wavenumbers. However, some of the
bands (1220 and 1156 cm−1) appeared at higher wavenumbers
compared to those of pure PFOA, suggesting the formation of
-
ν
as(COO)
-
νs(COO)
mixture
ν
(O-H)
ν(COOH)
ν
as(C-F)
PFOA
3200
3800
3600
3400
3000 1800
Wavenumber /cm
1500
1200
900
-1
(b) 19F MAS NMR
(c) 13C MAS NMR
PFOA
C(3)
C(2)
C(1)F3
PFOA
C(5,4,6,3)F2
C(4)
C(5)
C(7)F2
C(2)F2
COOH
C(6)
C(1)
C(7)
mixture
mixture
C(5,4,6,3)F2
C(1)F3
C(2)F2
COO−
C(7)F2
-80
-84
-88
-115
-120
-125
-130
-135
168
158 120
115
110
105
100
interactions occurring via
a different interaction mode.
Chemical shift /ppm
Chemical shift /ppm
Meanwhile, the νas(COO−) vibration peak of PFOA shifted by 12
cm−1 to a lower wavenumber. Notably, a new peak centered at
1080 cm−1 appeared in the DR-FTIR spectrum of the milled
Figure 2. (a) DR-FTIR, (b) 19F NMR and (c) 13C NMR spectra of powdery
PFOA, alumina and their mixture at a PFOA/Al2O3 molar ratio of 1:50.
mixture. These results were related to the formation of Al⋅⋅⋅
F
bonds between the PFOA and the in situ-formed Lewis acid
sites on the alumina. The dehydration process generated
coordinatively unsaturated Al centers (i.e., Lewis acid sites),
which possess strong affinities for the F atoms of PFOA
through the lone pair electrons on the F atoms. By increasing
the rotation speed to 100 rpm, 5 min of MC treatment could
result in a significant decrease in the intensities of the
vibration bands of both C−F and COO− in PFOA as well as the
band from the Al⋅⋅⋅F bond, indicating that some of the PFOA
had been converted. Moreover, a new peak was observed at
1021 cm−1, which was attributed to the formation of Al−F
bonds between the Lewis acid sites and the released F− ions
based on comparison to the spectrum of a mixture of alumina
and NaF (Figure S7). Accordingly, after extracting the milled
mixture with ammonia solution, F− ions were detected
following treatment at 100 rpm, but not in the case of
treatment at 50 rpm. By further increasing the rotation speed,
the degradation of PFOA became faster, and more Al−F bonds
were generated. These observations confirmed that the milling
process promoted the generation of strong Lewis acid sites
and the concomitant formation of Al⋅⋅⋅F bonds, thereby
enhancing the activation and breakage of C−F bonds in the
Scheme 1. Possible reaction pathways for MC conversion of PFOA to
1H-perfluorohept-1-ene (1H-1-PFHp).
The configuration of the hydrogen bonds was determined with
19F and 13C NMR spectroscopies. As shown in Figures 2b and
2c, pure PFOA powders showed the characteristic resonances
of 19F and 13C in C−F and/or COOH bonds (19F: −83.9 (C(1)−F3),
−118.0 (C(7)−F2), −123 (C(3~6)−F2), and −127.8(C(2)−F2) and
13C: 162.3 (COOH), 117.7 (C(7)), 115.8 (C(1)), 109.9~112.0
(C(3~5)), 108.2 (C(2)), and 106.3 (C(6)). After mixing with
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PFOA. The breakage of C−F bonds often requires reducing content of released F− ions was very low. In fact, a 30-min heat
conditions. Therefore,
a well-known stable radical, 2,2- treatment at 150 °C decomposed only 21.9% of the PFOA and
diphenyl-1-picrylhydrazyl (DPPH•) was used to study the showed a very low defluorination rate (1.1%, Table 1). If the
possibility of the generation of reducing species in the present temperature was further increased, significant vaporization
system. When the mixture of DPPH• and alumina was was observed. These comparisons imply that
a simple
subjected to a MC treatment, it was observed a new peak combination of alumina and thermal treatment was not
centered at 425 nm (Figure S8), which was attributed to efficient in breaking the C−F bonds.
anionic form DPPH−, signing the reduction from DPPH• to
DPPH−. This peak became stronger with increasing ball milling Based on the above discussion, the reaction pathway for the
time from 10 to 30 min. This confirmed that the ball milling alumina-mediated conversion of PFOA to 1H-1-PFHp under ball
generated free electrons on mechanically activated alumina milling was proposed and is shown in Scheme 1. Under normal
surfaces. The generation of free electrons in the present MC conditions, PFOA is a carboxylic acid, and the alumina surface
system may be explained as follows. The high energy ball is covered with OH groups. After mixing the two components,
milling makes metal oxides release lattice oxygen as the form the COOH group of PFOA reacts with a surface OH group on
of either gaseous O2 or active oxygen (eq. 2),39,40 which leads the alumina, yielding
a
monodentate Al⋅⋅⋅carboxylate
to the generation of oxygen vacancies (VO) and free electrons. interaction. The tail-anchored PFOA proceeds to rest against
With the assistance of oxygen vacancies, the generated the alumina, and then another surface OH group on the
electrons reduces halogenated organic pollutants (eq. 3). alumina is available to interact with a F atom in C(6)−F2 of the
According to this mechanism, the fresh alumina may function PFOA through the hydrogen bond. Upon ball milling, the
as the reductant for the controlled defluorination. After the mechanocaloric effect promotes the dehydration of surface
MC treatment, the fresh alumina will be converted to a OH groups on the alumina, producing coordinatively
mixture of oxygen vacancy containing alumina and aluminium unsaturated Al centers, which can serve as Lewis acid sites. As
fluoride.
a result, stronger Lewis acid-base interaction (i.e., Al⋅⋅⋅F)
between PFOA and alumina replaced the above mentioned
hydrogen-bonding interaction. The presence of COO−⋅⋅⋅Al and
Al⋅⋅⋅F interactions activated the COOH group and C−F bond,
respectively. When the energy imparted by ball milling
overcomes the reaction barrier, the loss of the terminal
carboxylate group and the cleavage of C(6)−F bond may occur.
However, both the conversion and defluorination of
perfluorooctane were negligible in the MC/alumina system,
which indicates that the decarboxylation reaction occurs more
easily than the C−F dissociation. As a result, perfluoroalkyl
anion (a carbanion at C(7)) is primarily formed, and it will be
stabilized by either adding a proton or ejecting F−. Because the
loss of F− was impossible unless the reagents are anhydrous
and free from the presence of hydroxyl compounds,20 the
protonation of carbanion is a dominant pathway by abstracting
H+ in OH groups or H2O associated with alumina under the
present MC conditions with humidity. Concurrently, both the
decreased steric hindrances and the delocalized electron from
carbanion to its neighbour F atom favor in the formation of
another Lewis acid-base interaction between the released
unsaturated Al site and F atom at C(7)−F. The friction and
collision of balls creates a high transient temperature and
energy impact at local area, and consequently can promote
the release of lattice oxygen and the formation of oxygen
vacancies (VO) and “free” electrons,39,40 and the generated
electrons induce a reduction reaction of PFOA. Accordingly,
milling
>Al-VO + 2e- + 0.5O2
(2)
(3)
>Al-O
milling
>PFOA + 2e- + H+(>Al-OH, H2O)
1H-1-PFHp + 2F- +CO2
(a) ball milling
0.2
5
Rotation speed /rpm
before
ν
as(C-F)
ν(Al⋅⋅⋅F)
50
100
150
200
ν
(Al−F)
pure PFOA
3800
3600
3400
3200
3000 1700
1450
1200
950
Wavenumber /cm-1
(b) thermal treatment
0.2
5
ν
as(C-F)
Temperature /
°C
25
50
100
150
ν
as(COO-)
ν
(Al⋅⋅⋅F)
1700
3000
1450
1200
950
3800
3600
3400
3200
Wavenumber /cm-1
Figure 3. DR-FTIR spectra of the PFOA-Al2O3 mixture (a) before and
after 5-min milling treatments at 50, 100, 150 and 200 rpm and (b)
after 30-min thermal treatments at 25, 50, 100 and 150°C.
Since Lewis acid sites may also be generated by heating
alumina, the thermal-induced conversion of PFOA was studied.
Similarly, the DR-FTIR analysis confirmed both the generation
two bounded
F atoms are rapidly removed from the
perfluoroalkyl backbone and stabilized on alumina surface.
Finally, a C=C double bond is formed, causing the generation of
1H-1-PFHp, and alumina is partially fluorinated. Several
previous reports also demonstrated that the catalytic effect of
Lewis acids can promote the cleavage of C−F bonds via the
of Lewis acid sites and the concomitant formation of Al⋅⋅⋅
F
bond between the PFOA and alumina as the temperature was
increased from 25 °C to 150 °C. In a control test, only the loss
of OH groups was observed in the spectrum of the heated pure
alumina (Figure S9). Moreover, the absence of an absorption
band at 1021 cm−1 in the heated mixtures indicated that the
33
abstraction of fluorides by Lewis acids.14, Onodera et al.
found the cleavage of two C−F bonds and the generation of
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partially fluorinated alumina and perfluoropent-1-ene in a 8. G. Meißner, K. Kretschmar, T. Braun and E. Kemnitz,
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A
highly selective (89.1%) method for the controlled
conversion of PFOA to 1H-perfluorohept-1-ene was developed
using alumina and ball milling. In this process, both the surface
OH groups and the Lewis acid sites on alumina played key roles
in the activation of the C−F bonds, and high-energy impact ball
milling promoted the C−F bond breakage. The alumina-
mediated MC reaction has been extended to shorter chain
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yields, and high selectivity. The protocol paves a way for the
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a
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Conflicts of interest
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