A simple and effective approach for catalytic reductive dechlorination of aromatic compounds

Article abstract of DOI:10.1139/cjc-2016-0567

An efficient process for the preparation of 2(3),9(10),16(17),23(24)-octa(n-hexyl)cobalt(II)phthalocyanine, ((n-hexyl)₈CoPc) (2) was described. The novel cobalt(II)phthalocyanine was characterized by spectroscopic methods. It was employed as a catalyst for the room temperature reductive dechlorination of chlorinated aromatic compounds (CACs). The results were showed that the CACs were completely dechlorinated within 110-120 min.

Full text of DOI:10.1139/cjc-2016-0567

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Simple and effective approach for catalytic reductive dechlorination of aromatic
compounds
Tamer E. Youssef, 1,2 * and Hanan H. Mohamed 3
1 Department of Chemistry , College of Science, University of Dammam, P. O.Box 1982, Dammam
31441, Saudi Arabia.
2 Applied Organic Chemistry Department, National Research Center, Dokki, Cairo 12622, Egypt.
3 Chemistry Department, Faculty of Science, Helwan University, Ain Helwan, Cairo, Egypt.
*Corresponding author. Tel.: + 966538952007
E-mail address: tamezzat@yahoo.com
a Permanent work Address: Applied Organic Chemistry Department, National Research Centre
Dokki, Cairo 12622, Egypt.
,
1

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Abstract: An efficient process for the preparation of 2(3),9(10),16(17),23(24)-octa(n-
hexyl)cobalt(II)phthalocyanine, [(n-hexyl)8CoPc] (2) was described. The novel
cobalt(II)phthalocyanine was characterized by spectroscopic methods. It was employed
as a catalyst for the room temperature reductive dechlorination of chlorinated aromatic
compounds (CACs). The results were showed that the CACs were completely
dechlorinated
within
(110-120)
min.
Key words: Cobalt phthalocyanine, dechlorination, catalyst, chlorinated aromatic
compounds.
2

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Introduction
Chlorinated aromatic compounds (CACs) are resulted as byproducts of several industrial
processes. They can be released into water, soil and air from household or industrial
facilities. They have accumulated in the food chains. 1,2
Removal of environmental pollution caused by CACs has been focused, they are
considered recalcitrant due to presence of chloride atom at benzene ring. To the best of
our knowledge, few chemical methods described the elimination of Chloride atom from
3
4
CACs, through photochemical dechlorination or electrolytic reduction or sodium,
5
potassium and calcium-based reduction, also sodium borohydride used at elevated
temperatures 6 or hydrolytic reductive and oxygen dependent dehalogenation. 7
The problems associated with CACs incineration have prompted extensive research in
developing new disposal methods, such as reductive dechlorination. The reductive
dechlorination with sodium alkoxyborohydride by using of transition metal also have
been described. 8,9 Additionally, the catalytic hydrodechlorination by using Pd/C–Et3N as
catalyst has become of interest. 10,11
Few researchers have performed systematic studies of catalytic hydrodechlorination as an
alternate method of treating or recycling chlorinated waste streams. Although many
different hydrodechlorination catalysts and many model compounds have been used. 12 It
13
is usually mediated by a transition-metal catalyst,
molecular hydrogen (H2) in the
14
metal-catalyzed reductive dechlorination, or using the other hydrogen sources such as
15
hydrazine, formic acid and its salts, etc.
Recently, the hydrodechlorination of
polychlorinated biphenyls (PCBs) in water–isopropanol systems and in the presences of
catalysts have also been studied. 16
Metallophthalocyanines (Pcs) have been used as a catalyst for a variety of transforming
harmful environmental pollutants into less harmful through eco-friendly reactions, 17 they
18
prepared by conventional methods, especially cobalt(II)phthalocyanines are the most
interesting classes of highly effective catalyst because of their easy preparation from
cheap precursors in a large scale and their thermal and chemical stability. 19 They exhibit
excellent activity, stability and recyclability. 20
3

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In this work, we present the results from studying the reductive dechlorination of CACs
on a cobalt(II)phthalocyanines catalyst. The results of this study could be useful for the
degradation of halogenated environmental pollutants.
Experimental
Materials
The starting precursor 4,5-di-n-hexylphthalonitrile (1) required for the synthesis of CoPc
18
catalyst was synthesized as reported in our previous work. All the mono chlorinated
aromatic compounds (CACs) used in this work: 1-Chloro-4-methylbenzene (98%), 1-
Chloro-4-ethylbenzene (97%), 1-Chloro-4-propylbenzene (97%), 4- Chlorobiphenyl
(95%), 1-Chloronaphthalene (85%) and 1-Chloroanthracene (95%) were purchased from
Acros and Aldrich and used as received. The other reagents, such as organic solvents and
bases were analytical grade.
Preparation of the catalyst
Preparation of 2(3),9(10),16(17),23(24)-octa(n-hexyl)cobalt(II)phthalocyanine, [(n-
hexyl)8CoPc] (2).
Under a nitrogen atmosphere, a standard Schlenk tube was charged with a mixture of 4,5-
n-dihexyl phthalonitrile 1 (0.6 g, 1.84 mmol), anhydrous cobalt chloride hexahydrate,
CoCl2. 6H2O (0.14 g, 0.77 mmol), deoxygenated DMAE (5 ml) and
diazabicyclo[5.4.0]undec-7-ene (DBU) (0.44 g, 3 mmol). It was irradiated with well
stirring in a microwave reactor at 370W for 10 min. Then cooled to room temperature. A
blue solution was formed, and then poured into MeOH/H2O (3:1). The suspended
solution was centrifuged. The resulted precipitated was filtered off. It washed with a
methanol/water mixture (1/1, v/v) and then dried under vacuum. The crude product (0.98
g) was subjected to column chromatography (50 g of on aluminium oxide using
CHCl3:CH3OH (9:1) as eluent) to yield 0.66 g (67%) of compound 2 as blue violet
powder.
IR (KBr, cm-1): 3053 mArH; 1650 m(C-C); 1473 (C–CH); 1436 mPh; 1082 (C–CH); 740
(C–C). UV–Vis (DMF), λmax (nm): 668, 602, 346. MS (FD): m/z 1244.74 (M+).
4

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Elemental analysis: C80 H112N8 Co, found C 76.68, H 8.73, N 8.42 (Calcd. C 77.29,H
9.06, N 9.01).
Instruments and characterization methods
Microwave oven utilized for heating: Discover Lab Mate single mode microwave cavity
from CEM Corporation. The reactions were conducted in a 25 ml Schlenk tube, with a
0
maximum operating temperature of 180 C and a maximum operating pressure of 8 bar.
FT-IR spectra were recorded on a Bruker IFS 48 spectrometer using KBr pellets. The
Ultraviolet–Visible Spectroscopy (UV–Vis) spectra of the fresh prepared and produced
CoPc were taken in N,N-dimethylformamide (DMF) using a Shimadzu UV-1800
spectrophotometer. FD mass spectra measurements were carried out with a Varian MAT
711 a spectrometer and reported as mass/charge (m/z). Elementary analyses were
performed on a Carlo Erba Elemental Analyzer 1104, 1106. BET surface area and pore
o
size distribution measurements were determined from nitrogen adsorption at 200 C,
using the A Nova 2200e instrument (Quantachrome Instrument USA) along with a
NOVAWin2 (Quantachrome Instrument) data analysis software [NOVAWin2, V.2.2,
operation manual, NoVA series Windows based Operating and Data Analysis Software,
Quantachrome Instruments, FL. USA. 2006.]; Surface area was determined from the
application of the BET equation. XRD was performed on an INEL CPS 180 powder
diffractometer EQUINOX 1000 system (filtered Cu Ka1 radiation, 30KV 30mA,
spinning sample holder). The powder pattern analysis was processed by using Match
software for phase identification with both COD and ICDD database, IMAD-INEL data
processing software for graphical illustrations, MAUD software for Rietveld analysis
method, and Fityk software for FWHM estimations.
Scanning electronic microscope (SEM) was used for investigating the surface
morphology of the fresh and produced CoPc. The catalyst was examined with a scanning
electron microscope (JSM-6360LA, JEOL, Tokyo, Japan).
Experimental-Setup
A modified commercial stirred glass reactor was used for carrying all the reductive
dechloroniation reactions (c.f. Fig. 1). It consists of a thermometer, a condenser and a
hydrotreater (including a hydrogen cylinder, hydrogen flow meter, three-way valve and a
5

Page 6 of 22
Helium cylinder, with a magnetic stirrer. At room temperature. Under atmospheric
pressure, 100 ml Ethanolic solution of CACs (5.0 mmol chlorine atoms) and 50 mg CoPc
were added into the flask. Under mild conditions, CACs were treated with hydrogen gas
over CoPc in aqueous solutions with constant H2 flow rate (250 cm3 min-1). A NaOH trap
was added into the reaction system to absorb the poison of HCl gas. A liquid sampling
was taken system via a controlled syringe. The product was collected and analyzed by a
gas chromatography equipped with a flame ionization detector (FID) at the end of the
experiment. All products were identified by Micromass GCTTM GC-mass spectroscopy.
The catalyst used after each run was recovered and washed by stirring in water for 24 hrs,
before starting on a new reductive dechlorination run.
Fig. 1. Schematic diagram of the stirred glass reactor system, showing: (1) He cylinder;
(2) hydrogen cylinder; (3) needle valve; (4) mass-flow controller; (5) gas sparge tubing;
(6) NaOH trap; (7) HCl gas regulator; (8) thermocouple probe adjusted at 25℃; (9)
stirring motor; (10) cooling tower; (11) CACs; (12) jacketed reaction vessel; (13) outlet
of dechlorinated aromatic compounds (DACs); (14) GC sampling tubing; (15) GC; (16)
data acquisition.
Analytical procedure
Under atmospheric pressure and at room temperature, the reductive dechlorination
reaction was performed. In a typical experiment, about 50 mg of catalyst was suspended
in a modified commercial stirred glass reactor and CACs were reduced in a flow of
hydrogen (30 mL/min) for 2.5 hrs prior to the reaction as shown in Fig. 1. The H2 flow
was kept constant during all experiments. The gas chromatography with a flame
ionization detector used for analyzing the products with a VF-Ims capillary 30 M column.
by using a deuterated chlorobenzene (C6D5Cl) as internal standard .
The gas chromatograph Varian, equipped with an FID detector and a VF-Ims capillary
column (dimeter: 0:25 mm, length: 30 m, film thickness: 0:25 micro m). Injection was
made in splitless mode using helium (He) as the carrier gas and the temperature was set at
o
200 C. The products were quantified from peak areas obtained through automated
interrogation and by comparison of standards. The concentration of All the mono
chlorinated aromatic compounds (CACs) used in this work: (1-Chloro-4-methylbenzene,
6

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1-Chloro-4-ethylbenzene,
1-Chloro-4-propylbenzene,
4-
Chlorobiphenyl,
1-
Chloronaphthalene and 1-Chloroanthracene) in Ethanol was determined from the total
mass in the reaction mixture. The relative peak area% was converted into mol% using
regression equations. The amount of HCl produced and hydrogen consumption during the
reaction (mol dm-3) was calculated from a molar balance based on the GC analysis of
organic content. The conversion and yield (as a percentage) were calculated using Eqs. 1
and 2 with respect to dechlorinated aromatic compounds (DACs) in the HDC of CACs is
defined as:
ꢆ
ꢊ
ꢆ
ꢊ
ꢀꢁꢁꢂ ꢃꢄꢃꢅ ꢇꢈꢉ
ꢋ
Conversion CACs
Yield
=
ꢌꢍꢎꢁꢏꢐꢑꢁꢂ ꢃꢄꢃꢅ ꢇꢈꢉ x 100(%)
(1)
ꢆ
ꢊ
ꢀꢁꢁꢂꢃꢄꢃꢅ ꢇꢈꢉ
ꢒꢄꢃꢅ ꢓꢎꢈꢂꢔꢐꢁꢂ ꢆꢇꢈꢉꢊ
=
ꢀꢁꢁꢂ ꢃꢄꢃꢅ ꢇꢈꢉ ꢌꢍꢎꢁꢏꢐꢑꢁꢂ ꢃꢄꢃꢅ ꢇꢈꢉꢊx 100(%)
(2)
DACs
ꢆ
ꢊ
ꢆ
ꢋ
Results and discussion
Characterization of the catalyst
The physical properties CoPc catalyst are given in Table 1 and Fig. 2. BET surface area
and pore size distribution measurements were measured using an A Nova 2200e
micrometrics instrument.
Fig. 2. The linear regression of BET data (top) and pore size distribution (bottom)
measurements.
The synthetic route to novel peripherally octa-substituted cobalt(II) phthalocyanine 2 is
shown in Scheme 1. The phthalonitrile derivative 1 was synthesized as reported in our
previous work. 18
The phthalonitrile derivative 1 was irradiated in a microwave reactor at 370W for 10 min,
in the presence of CoCl2 to form the desired complex 2. The new cobalt phthalocyanine 2
was purified by column chromatography with aluminum oxide as an adsorbent. It was
obtained in a yield of 67% and was characterized previously by spectral data (IR, UV–
Visible spectroscopy, mass spectral data and elemental analysis) [18].
The characterization data of the new compound were consistent with the assigned
formula. No aggregation behavior in the purified CoPc 2 spectrum.
Scheme 1. Preparation of peripherally octa-substituted cobalt(II)phthalocyanine 2.
7

Page 8 of 22
Its main characteristics such as pore size, pore volume, BET surface area was presented
in Table 1.
Table 1. Catalyst physical properties.
1
The H NMR spectrum of CoPc 2 could not be taken due to the paramagnetic cobalt(II)
centers. In the mass spectrum of 2, the presence of molecular ion peaks at m/z 1244.74
(M+), confirmed the proposed structures. The results of elemental analysis also confirmed
the structure of complex 2. The electronic absorption spectrum of cobalt phthalocyanine
2 in DMF at room temperature. The split Q bands appeared at 669 and 602 nm, while the
split B band remained at 346 nm c.f. Fig. 3.
Fig. 3. UV–Visible spectrum in DMF of CoPc before and after reductive dechlorination.
The XRD patterns of the fresh CoPc catalyst (a) and after six runs of reductive
dechlorination experiments (b) are shown in Fig. 4. The catalyst was showed the peaks
at 2ꢕ values of 10, 20, 30 and 45, respectively, related to phthalocyanine moiety.
Fig. 4. XRD patterns of CoPc before and after reductive dechlorination .
Scanning electronic microscope was used for the investigation of the surface
morphology. Figures (5a&b) were showed the surface morphology of the CoPc catalyst,
the results were showed that the image (a) is a mirror image of the image (b) after six
runs.
Fig. 5. SEM images of CoPc catalyst fresh (a) and after used six runs (b), respectively.
Reductive dechlorination of CACs under different experimental conditions
Firstly, a series of blank experiments were examined without catalyst with the CACs
preliminary concentration of 100 mg and hydrogen flow rate (250 cm3 min-1). It was
found that, no reductive dechlorination of CACs occurred in the absence of catalyst.
The reductive dechlorination of CACs with molecular hydrogen over CoPc catalyst in
water were carried out at room temperature and atmospheric pressure Scheme 2.
A modified commercial stirred glass reactor used for all the reductive dechlorination
reactions as shown in Fig. 1. The main results obtained can be summarized in Table 2.
Chlorinated aromatic compounds (CACs) were mainly transformed to dechlorinated
aromatic compounds (DACs), and at the same time HCl was formed, but quickly directed
8

Page 9 of 22
to the NaOH trap in the reactor system to prevent accumulation of HCl at the surface of
the catalysts. Therefore, the presence of NaOH trap effectively prevents the poisoning of
the CoPc catalyst.
Table 2. The reductive dechlorination dynamic data.
Scheme 2. The reductive dechloroniation of CACs over CoPc catalyst at room
temperature.
The catalytic reductive dechloroniation of 1-Chloro-4-methylbenzene, 1-Chloro-4-
ethylbenzene, 1-Chloro-4-propylbenzene, 4- Chlorobiphenyl, 1-Chloronaphthalene and
1-Chloroanthracene in Ethanol as solvent over CoPc catalyst have been investigated. It
can be seen from Figs. 6(c-f), a complete dechlorination was occurred with 100%
selective through the system within (110-120) min, respectively.
Fig. 6(a-f). Reductive dechlorination of 1-Chloro-4-methylbenzene, 1-Chloro-4-
ethylbenzene, 1-Chloro-4-propylbenzene, 4- Chlorobiphenyl, 1-Chloronaphthalene and
1-Chloroanthracene over CoPc catalyst, respectively.
Analysis of the data plotted in Figs. 6(a-f) shows the kinetic measurements of the
decrease in the concentration of the CACs (red curves) as well as the increase in the
concentration of the dechlorinated products DACs (violet curves).
The catalytic reductive dechlorination results by using CoPc catalyst with hydrogen gas
could be proposed as a process for the detoxification of CACs, where the CACs
transformed into lower toxic products, i.e. toluene, ethylbenzene, propylbenzene,
biphenyl, naphthalene and anthracene were formed. Therefore, we believe that the CoPc-
catalyzed dechlorination could develop an economic technology for removal of
chlorinated aromatic compounds in water (Scheme 2).
The Lifespan of CoPc Catalyst
The Lifespan of CoPC catalyst was tested through the reductive dechloroniation of 1-
Chloro-4-methylbenzene into toluene. After each run, the CoPc catalyst was collected. It
can reused in the next run by using a fresh solvent. The result of a lifespan test of CoPC
9

Page 10 of 22
catalyst was described through four reductive dechloroniation runs with the same catalyst
as shown in Fig. 7.
Fig. 7. Conversion of CoPc catalyst used in repeated reductive dechlorination runs.
The activity of the fresh CoPc catalyst in the four runs approximately the same and the
conversion of 1-Chloro-4-methylbenzene was 99.66%. For the first run or after the fourth
runs of the catalyst, the conversion of 1-Chloro-4-methylbenzene was at 99.27%. This
means, no activity of the catalyst was lost.
Conclusions
In this study microwave-assisted synthesis of new peripherally octa-substituted Co(II)
phthalocyanine 2 have been demonstrated as an efficient and green method. The target
symmetrical Co(II)phthalocyanine 2 used as catalyst in the reductive dechloroniation of
CACs and the complete dechlorination achieved after 120 min with first order reaction
kinetic. The CoPc catalyst has exhibit high activity and stability for the room temperature
reductive dechloroniation of CACs. This reductive dechloroniation strategy has excellent
yield, more high recovery of catalyst, simple experimental working up as well as
isolation procedures.
Acknowledgements
Technical support from the Helwan University and Imam Abdulrahman Bin Faisal
University is gratefully acknowledged.
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11

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Table 1. Catalyst physical properties.
Property (units)
Value
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
139.637 m²/g
0.395 cm3/g
7.452 nm

Page 13 of 22
Table 2. The reductive dechlorination dynamic data
a Time required for C-Cl bond cleavage.
b Yield and recovery values of substrate and product were determined by GC-MS.
CACs Substrate
Time(min)a
DACs Product
Yieldb(%)
Recoveryb (%)
1-Chloro-4-methylbenzene
1-Chloro-4-ethylbenzene
1-Chloro-4-propylbenzene
4- Chlorobiphenyl
1-Chloronaphthalene
1-Chloroanthracene
110
110
115
115
120
120
Toluene
85
88
80
82
85
88
>98
>97
>99
>98
>97
>97
Ethylbenzene
Propylbenzene
Biphenyl
Naphthalene
Anthracene

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N
N
N
N
N
Microwave irradiation
N
Co
N
N
N
DMAE
350W
,
DBU CoCl2. 6H2O
10 min
,
N
,
1
2

Page 22 of 22
CH3
CH3
1-Chloro-4-methylbenzene
Toluene
Cl
CH2CH3
CH2CH3
1-Chloro-4-ethylbenzene
Cl
Ethylbenzene
CH2CH2CH3
CH2CH2CH3
1-Chloro-4-propylbenzene
CoPC Catalyst
Propylbenzene
Cl
Cl
Cl
4- Chlorobiphenyl
Biphenyl
1-Chloronaphthalene
Cl
Naphthalene
1-Chloroanthracene
Anthracene