Rhodium-catalyzed electrochemical hydrodefluorination: A mild approach for the degradation of fluoroaromatic pollutants

Article abstract of DOI:10.1016/j.elecom.2016.02.012

Many fluoroorganic compounds are toxic and persistent, and achieving their efficient degradation under mild conditions is currently a challenge. Herein, we developed a new electrochemical hydrodefluorination (HDF) system based on rhodium electrocatalyst for the degradation of fluoroaromatic (FA) pollutants. The HDF system realized the rapid degradation of 18 representative FAs to form nonfluorinated organics and F^- under mild conditions (room temperature and pressure, water medium, air atmosphere, without the use of hazardous reagents). This study may provide a new promising alternative for the practical treatment of waste water containing FA pollutants.

Full text of DOI:10.1016/j.elecom.2016.02.012

Rhodium-catalyzed electrochemical hydrodefluorination: A mild approach
for the degradation of fluoroaromatic pollutants
Yinghua Xu, Hongxing Ma, Tingjie Ge, Youqun Chu, Chun-An Ma
PII:
DOI:
Reference:
S1388-2481(16)30029-7
doi: 10.1016/j.elecom.2016.02.012
ELECOM 5643
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
23 December 2015
16 February 2016
16 February 2016
Please cite this article as: Yinghua Xu, Hongxing Ma, Tingjie Ge, Youqun Chu, Chun-An
Ma, Rhodium-catalyzed electrochemical hydrodefluorination: A mild approach for the
degradation of fluoroaromatic pollutants, Electrochemistry Communications (2016), doi:
10.1016/j.elecom.2016.02.012
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ACCEPTED MANUSCRIPT
Rhodium-catalyzed Electrochemical Hydrodefluorination: A Mild
Approach for the Degradation of Fluoroaromatic Pollutants
Yinghua Xu, Hongxing Ma, Tingjie Ge, Youqun Chu*, and Chun-An Ma*
State Key Laboratory Breeding Base of Green Chemistry–Synthesis Technology, College of Chemical Engineering
and Materials science, Zhejiang University of Technology, Hangzhou 310032, China
* Corresponding author. Tel & Fax: +86-0571-88320830.
E-mail: chuyq@zjut.edu.cn (Youqun Chu); science@zjut.edu.cn (Chun'an Ma).
ABSTRACT: Many fluoroorganic compounds are toxic and persistent, and achieving their
efficient degradation under mild conditions is currently a challenge. Herein, we developed a new
electrochemical hydrodefluorination (HDF) system based on rhodium electrocatalyst for the
degradation of fluoroaromatic (FA) pollutants. The HDF system realized the rapid degradation of
18 representative FAs to form nonfluorinated organics and F− under mild conditions (room
temperature and pressure, water medium, air atmosphere, without the use of hazardous reagents).
This study may provide a new promising alternative for the practical treatment of waste water
containing FA pollutants.
Keywords: Electrocatalytic Hydrodefluorination; Fluoroaromatic Pollutants; Rhodium; Ni foam;
4-Fluorophenol
1

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1. Introduction
Fluoroorganic compounds (FOCs) are becoming widespread environmental pollutants
because of their wide and increasing applications [1-12]. Many of these compounds are
particularly persistent in the environment [4-6]; they are strongly resistant to traditional biological
degradation [7,8] and conventional chemical processes [9,10] because of the inertness of C–F
bond. The compounds gradually accumulate in the environment, achieving concentrations that are
hazardous to living organisms [11,12].
Hydrodefluorination (HDF) is one of the most facile approaches to dispose of FOC pollutants
[4]. The process simply transforms the C–F bond of FOCs to C–H, which could then dramatically
increase the compounds’ biodegradability. Numerous efficient HDF methods have been developed
for FOCs degradation. Among these approaches are the Lewis acid mediated [13,14],
homogeneous [15-17] or heterogeneous [18,19] transition metal-catalyzed, and photo-reductive
[20-22] HDF processes. Unfortunately, these HDF methods require harsh reaction conditions such
as high temperature [15], high pressure [16], inert atmosphere [20-22], or using toxic organic
solvents, such as the explosive hydrogen gas [18,19] and toxic organic solvents [13,14,17]. The
persistent need for these conditions hinders wide practical application of the aforementioned HDF
methods. Compared with the above-mentioned HDF methods, electrochemical HDF is
intrinsically milder and more likely to be operated under mild condition. So far, only few
electrochemical HDF cases have been reported and these HDF examples do not remove the use of
high pressure [23] or toxic organic solvents [24]. Therefore, the development of HDF systems
under mild conditions would be a significant achievement to address emerging environmental
problems from the increasing use of FOCs and their persistence in the environment.
In this communication, we presented a mild and robust electrochemical HDF system for the
degradation of fluroaromatic (FA) pollutants. To the best of our knowledge, this paper is the first
to report an electrochemical HDF system that degrades FAs to form nonfluorinated organics and
F− under room temperature and pressure without resorting to the use of toxic organic solvents and
inert gases.
2. Material and methods
All FAs and their HDF products with purities of 98%–99%, as well as RhCl3 hydrate (99.5%),
were acquired from Aladdin Reagent Co., China. Ni foams (1.0 mm thickness), Cu foams (1.0 mm
thickness), Ag mesh (open area 37%), and carbon felt (2.0 mm thickness) were obtained from
Cells Electrochemistry Co., Ltd., China (www.hzcell.com). Rh-modified nickel (Rh/Ni) foam,
silver (Rh/Ag) mesh, and copper (Rh/Cu) foam, were prepared by a metallic replacement reaction
(exposing to a 30 mL aqueous solution containing 1 M NaCl and 1.5 mM RhCl3 at pH 2 adjusted
using HCl until the rose color of the RhCl3 solution disappeared). Rh-modified carbon felt
(Rh/Carbon felt) was prepared by electrodeposition (using constant current of 2 mA in the same
RhCl3 solution until the rose color disappeared). The abovementioned four Rh-modified materials
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have same Rh loading of 0.77 mg/cm2 (projected area), unless otherwise noted. The Rh/Ni foam
exhibited the highest HDF activity among the four Rh-modified materials. Hence, its surface
morphology and composition, as well as the Rhn+/Rh0 ratio value of surface Rh were determined
by SEM (Hitachi S-4700 II) equipped with EDS (Thermo NOANVANTAGE ESI) and XPS
(KRATOS AXIS ULTRA DLD), respectively. As shown in Fig. 1, a uniform Rh film formed on
the surface of Ni foam. It implies that the above-mentioned metallic replacement reaction is
applicable to the preparation of Rh/Ni foam.
A conventional two compartment glass or PTFE H-cell separated by a Nafion-117 membrane
was used for electrochemical HDF experiments. The cathode was fabricated from a piece of Rh/Ni
foam, Rh/Ag mesh, Rh/Cu foam, or Rh/carbon felt (projected area: 2 × 3 cm2), whereas the anode
was fashioned from a graphite sheet (2 × 3 cm2). 30 mL phosphate buffer solutions (20 mM)
served as both the catholyte and the anolyte. Unless otherwise noted, the catholyte was stirred
with a stirring velocity of 350 rpm during electrolysis. The concentrations of reactants and their
products were determined by an Agilent 7890A gas chromatograph (GC) and a Waters HPLC
system, using standard calibration curves. The fluoride ion (F−) concentration was determined by a
REX PHSJ-4F pH/mV meter combined with F− selective electrode (REX PF-202-C).
The specific electric energy consumption (SEEC, kW·h·m−3 catholyte) of the HDF process
was calculated from the following equation:
I  U  t
SEEC 
1000  V
where I is the applied current (A), U is the average cell voltage (3 V), t is HDF time (h), V is the
volume of the catholyte (3×10-5 m3 ).
Unless otherwise noted, all above-mentioned experiments were performed under air
atmosphere, at 25 °C.
3. Results and discussion
3.1 Optimization of HDF conditions
The electrocatalyst and its support, as well as the catholyte pH used usually determine the
reaction efficiency of electrochemical hydrodechlorinations [25-28]. Thus, these parameters of the
presented HDF system were optimized primarily with 4-fluorophenol (4-FP) as the target
compound. We first examined the pH effects, of which the results are listed in Table 1 (entries
1–3). At pH values of approximately 11 and 7, the HDF efficiencies were discouraging over a
Rh/Ni foam cathode (Rh as the electrocatalyst and Ni foam as the support), whereas the HDF
efficiency was inspiring at a pH of approximately 3 (4-FP was completely converted, and near 100%
F− yield was achieved). We then examined the effects of the electrocatalyst and its cathode support.
After screening different types of electrocatalysts (entries 3–7), such as Rh, Pt, Ir, Ru, and Pd, we
determined that Pt exhibited the best performance as an HDF electrocatalyst besides Rh. Cathode
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support screening revealed that Ni foam provided a much higher yield of F− compared with Cu
foam, Ag mesh, and carbon felt (entries 3, 8–10). The higher HDF performance of the Rh/Ni foam
probably was due to its small Rhn+/Rh0 ratio value of 0.66 [29]. Aside from the above-mentioned
three parameters, we also found that the HDF system is immune to air and the glass of the cell
(entries 11–13) and that 4-FP at two higher concentrations degraded rapidly with 100% conversion
and high F− yield (entries 14 and 15).
3.2 Degradation pathway of 4-fluorophenol and rate-limiting step
With the optimal HDF conditions in hand (water of pH 2.7–3.2 as the catholyte, Rh/Ni foam
as the cathode), we evaluated the degradation pathway of 4-FP. 4-FP was reduced linearly to
cyclohexanone (CYC.one) and cyclohexanol (CYC.ol) as the main organic products (Fig. 2A)
with a pseudo-first-order rate constant (kobs) of 0.00057 s-1 (entry 3, Table 2). The concentration of
CYC.one increased up to approximately 3 mM and then remained almost constant with prolonged
degradation time. These results imply a shift in the formation pathway of CYC.ol during the
degradation process of 4-FP. Direct reduction of 4-FP to CYC.ol was the main pathway during the
earlier stage of degradation process, whereas the main pathway was the two-step consecutive
reductions (from 4-FP to desorbed CYC.one and then to CYC.ol) during the later stage of
degradation process [30]. Additionally, very low phenol (PhOH) concentrations were found with a
maximum concentration of 0.4 mM in the degradation process (Fig. 2B). This result indicates
PhOH also was a potentially intermediate product of the degradation process. To confirm these
theories, we performed separate experiments using CYC.one or PhOH as the starting material
under identical conditions (Fig. 2C and 2D). We then observed the quantitative reduction of PhOH
to CYC.one (kobs = 0.00070 s-1, entry 2 in Table 2) and of CYC.one to CYC.ol (kobs = 0.00028 s-1,
entry 1 in Table 2). The above-mentioned results suggest that the main degradation process of
4-FP followed the pathway of 4-FP→ PhOH → CYC.one → CYC.ol, and the reductive activity of
former three compounds decreased in the follow order: PhOH > 4-FP > CYC.one. The ecotoxicity
of CYC.ol is much lower than those of PhOH and CYC.one [31]. Therefore, the optimized HDF
system is a promising candidate for the disposal of wastewater containing 4-FP.
Moreover, the rate-limiting step in the degradation process of 4-FP was preliminarily
elucidated (Table 2). Almost same kobs were gained at the stirring velocities of 350 and 500 rpm
(entries 4 and 5), meanwhile the kobs was proportional to the Rh loading from 0.77 to 1.54 mg/cm2
(entries 5 and 6). These results strongly suggest that the degradation process was kinetically
limited by a surface reaction step rather than by a diffusion step of 4-FP from bulk catholyte to the
cathode surface.
3.3 Degradation mechanism
On the basis of the aforementioned reaction pathway and the well-reported electrochemical
hydrodechlorination mechanism over Pt group metal-modified cathodes [32-34], we proposed the
degradation mechanism of FAs in the presented HDF system as described by Eqs. (1)–(6)
R-F + Rh ↔ (R-F)adsRh
(1)
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2(n+1) H+ + 2(n+1) e- + Rh ↔ 2(n+1) HadsRh
(2)
(3)
(4)
(5)
(6)
(R-F)adsRh + 2 HadsRh → (R-H)adsRh + HF + Rh
(R-H)adsRh ↔ R-H + Rh (slow)
(R-H)adsRh + 2n HadsRh → (R’H(2n+1))adsRh (fast)
(R’H(2n+1))adsRh ↔ R’H(2n+1) + Rh
First, adsorbed FAs are formed on Rh [Eq. (1)] that competes with the generation of chemisorbed
hydrogen [Eq. (2)]. Hydrogenolysis of FAs then proceeds as in catalytic hydrogenation [Eq. (3)]
by the reaction of the adsorbed FAs with chemisorbed hydrogen, followed by the slow desorption
and further rapid hydrogenation of the product [Eqs. (4) and (5)]. Finally, the hydrogenated
products desorb from Rh [Eq. (6)]. This reaction mechanism is analogous to the HDF and
hydrogenation mechanism of fluorobenzene (using H2) over Rh/Al2O3 catalyst supposed by
McNeill’ group [30]. The main difference between these two mechanisms is the source of
hydrogen which reacts with the adsorbed FAs. The hydrogen comes from the electroreduction of
H+ or H2O in the electrochemical mechanism, while it originates from the dissociative adsorption
of H2 in the catalytic mechanism. This is why the electrochemical HDF system can avoid the
using of H2, which is hazardous when it is in use or in transit.
3.4 Scope and practicability of HDF system
At last, we evaluated the scope and practicability of the optimized electrochemical HDF
system. With sufficient passed charge, 100% conversion and F− yield were achieved for all
selected 18 representative FAs (Table 3). The apparent HDF rate of the 18 FAs under the
optimized conditions decreased in the following order: fluoropyridine > fluorophenol ≈
fluoroaniline ≈ fluoroanisole ≈ fluorotoluene ≈ fluorotoluene > fluorobenzoic acid >
fluorobenzonitrile. The SEEC of the electrochemical HDF system would be very low
(0.078–0.936 kWh/m3 waste water) if the waste water contained a low concentration of FA
pollutants (120 μM).
4. Conclusions
In summary, an efficient electrochemical HDF system (water of pH = 2.7–3.2 as the catholyte,
Rh/Ni foam as the cathode) was developed for the first time. This goal was achieved by
optimizing the catholyte pH (2.7~11.1), the electrocatalyst used (Rh, Pt, Ir, Ru, and Pd), and its
cathode support (Ni foam, Cu foam, Ag mesh, and carbon felt). The HDF system realized the
rapid degradation of 18 representative FAs under extremely mild conditions (room temperature
and pressure, water medium, air atmosphere, without the use of hazardous reagents). The apparent
HDF rate of the 18 FAs in the HDF system decreased in the following order: fluoropyridine >
fluorophenol ≈ fluoroaniline ≈ fluoroanisole ≈ fluorotoluene ≈ fluorotoluene > fluorobenzoic acid >
fluorobenzonitrile. In addition, the main degradation path of 4-FP in the HDF system was
demonstrated as following: 4-FP → PhOH → CYC.one → CYC.ol. Moreover, a reaction
mechanism was proposed for the degradation process, in which the catalytic hydrogenation
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between adsorbed FAs and chemisorbed hydrogen on rhodium is the key step. This study may
provide a new promising alternative for the practical treatment of waste water containing FA
pollutants. It should be point out that the electrochemical HDF system developed here is
inefficient for the HDF of saturated FOCs, which are also an important class of pollutants.
Conflict of interest
There is no conflict of interest among authors for this paper.
Acknowledgement
This research was supported by the National Natural Science Foundation of China (21106133,
21576238), the Natural Science Foundation of Zhejiang Province, China (LY16B060012), and the
National Basic Research Program of China (973 Program) (2012CB722604).
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Figure caption
Fig. 1 SEM and EDS of the Ni foam and Rh/Ni foam.
Fig. 2 Concentration profiles of 4-fluorophenal (4-FP) and its products (PhOH, phenol; CYC.one,
cyclohexanone; CYC.ol, cyclohexanol; F−) during electrochemical HDF and hydrogenation at
25°C. Catholyte: 30 mL of a 20 mM phosphate buffer aqueous solution of pH 2.7–3.2 containing
12 mM (mmol∙L−1) reactants; cathode: Rh/Ni foam (2×3 cm2); applied current: 52 mA. (A) 12
mM 4-FP; (B) Partial enlarged drawing of (A); (C) 12 mM PhOH; (D) 12 mM CYC.one.
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Fig. 1
1.0
4-FP
PhOH
CYC. one
CYC. ol
F-
Total organics
12
10
0.8
(A)
(B)
8
6
4
2
0
0.6
0.4
0.2
0.0
12
10
8
12
10
8
(D)
(C)
6
6
4
4
2
2
0
0
0
20 40 60 80 100
0
20 40 60 80 100
Time / Min
Fig. 2
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Graphical abstract
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Table 1 Optimization of the electrocatalytic HDF system with 4-fluorophenol (4-FP) as the target
compound.[a]
Entry
Cathodes
Rh/Ni foam
Rh/Ni foam
Rh/Ni foam
Pt/Ni foam
Ir/Ni foam
pH
Conv. (%)
Yield of F− (%)
1
2
10.8-11.1
6.8-7.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
2.7-3.2
5
6
3
4
3
100
58
17
10
4
98
56
13
4
4
5
6
Ru/Ni foam
Pd/Ni foam
Rh/Cu foam
Rh/Ag mesh
Rh/Carbon felt
Rh/Ni foam
Rh/Ni foam
Rh/Ni foam
Rh/Ni foam
Rh/Ni foam
7
2
8
7
4
9
14
4
5
10
11[b]
12[c]
13[d]
14[e]
15[f]
3
54
52
54
100
100
51
50
51
98
103
[a] Unless otherwise noted, 30 mL of a 20 mM phosphate buffer aqueous solution containing 120 μM (μmol∙L−1)
4-FP served as the catholyte, the applied current for 90 min was 0.52 mA (i.e., 0.52 mA/90 min, charge passed was
8 F: 8 mol of electrons per mol 4-FP), area of the cathode used was 2×3 cm2 and a glass cell was used. [b] 0.52
mA/22.5 min. [c] Air free (N2), 0.52 mA/22.5 min. [d] 0.52 mA/22.5 min, using a PTFE cell. [e] 1.2 mM 4-FP, 5.2
mA/90 min. [f] 12 mM 4-FP, 52 mA/90 min.
12

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Table 2 Pseudo-first-order rate constants (kobs) for the reduction of cyclohexanone (CYC.one),
phenol (PhOH), and 4-fluorophenol (4-FP).[a]
Entry
Reactants
CYC.one
PhOH
4-FP
Reaction conditions
kobs (s-1)
0.00028
0.00070
0.00057
0.0010
R2
0.98
0.84
0.92
0.97
0.93
0.91
1
2
3
4
5
6
12 mM reactant, 52 mA/90 min
12 mM reactant, 52 mA/90 min
12 mM reactant, 52 mA/90 min
4-FP
500 rpm, Rh loading:0.77 mg/cm2, 0.52 mA/90 min
350 rpm, Rh loading:0.77 mg/cm2, 0.52 mA/90 min
350 rpm, Rh loading:1.54 mg/cm2, 1.04 mA/90 min
4-FP
0.0012
4-FP
0.0023
[a] Unless otherwise noted, catholyte: 30 mL of a 20 mM phosphate buffer aqueous solution at pH = 2.7–3.2
containing 120 μM reactants, stirring velocity: 350 rpm, cathode: Rh/Ni foam (2×3 cm2) with Rh loading of 0.77
mg/cm2, the applied current for 90 min was 0.52 mA (i.e., 0.52 mA/90 min).
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ACCEPTED MANUSCRIPT
Table 3 Scope and practicability of the electrochemical HDF system.[a]
Entry
HDF time
(h)
1.5
1.5
1.5
3
Reactants
Final product yield (%)[b]
1
2
2-fluorophenol
3-fluorophenol
PhOH (0)
PhOH (0)
PhOH (0)
PhOH (1)
PhOH (0)
aniline (0)
aniline (0)
anisole (0)
CYC.one (40)
CYC.ol (57)
CYC.ol (66)
CYC.one (33)
CYC.one (40)
CYC.one (39)
CYC.one (42)
CYC.one (78)
CYC.one (77)
CYC.one (4)
3
4-fluorophenol
CYC.ol (57)
4
2,4-difluorophenol
pentafluorophenol
4-fluoroaniline
CYC.ol (60)
5
7.5
1.5
7.5
1.5
1.5
7.5
1.5
9
CYC.ol (56)
6
CYC.ol (22)
7
pentafluoroaniline
4-fluoroanisole
CYC.ol (22)
8
methoxycyclohexane (94)
methylcyclohexane (83)
methylcyclohexane (91)
cyclohexane (73)
cyclohexane (87)
9
4-fluorotoluene
toluene (17)
10
11
12
13
14
15
16
17
18
pentafluorotoluene
monofluorobenzene
hexafluorobenzene
4-fluorobenzoic acid
pentafluorobenzoic acid
2-fluoropyridine
toluene (9)
benzene (26)
benzene (12)
4.5
15
Cyclohexanecarboxylic acid (100)
Cyclohexanecarboxylic acid (100)
1.5
7.5
6
pyridine (13)
pyridine (0)
piperidine (87)
piperidine (100)
pentafluoropyridine
4-fluorobenzonitrile
cyclohexanemethylamine (35) cyclohexanemethanol (63)
18
pentafluorobenzonitrile cyclohexanemethylamine (33) cyclohexanemethanol (65)
[a] Catholyte: 30 mL of a 20 mM phosphate buffer aqueous solution at pH = 2.7–3.2 containing 120 μM reactants,
cathode: Rh/Ni foam (2×3 cm2), applied current: 0.52 mA. [b] PhOH, phenol; CYC.one, cyclohexanone; CYC.ol,
cyclohexanol.
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ACCEPTED MANUSCRIPT
Highlights
> We developed a new Rh-catalyzed electrochemical hydrodefluorination (HDF)
system.
> Cathode material and solution pH were optimized with 4-fluorophenol as the
substrate.
> The HDF system realized the rapid degradation of 18 fluoroaromatics (FAs).
> This study provides a new promising alternative for the treatment of FA pollutants.
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