Phosphazenium chloride catalysts immobilized on SBA-15 mesoporous material and silica gel: New exceptionally active catalysts for the chlorination of organic acids

Article abstract of DOI:10.1039/b210258g

Novel reusable phosphazenium chloride catalysts immobilized on SBA-15 mesoporous material and silica gel show exceptional activities and selectivities even in the continuous chlorination reaction of organic acids with thionyl chloride or phosgene.

Full text of DOI:10.1039/b210258g

Phosphazenium chloride catalysts immobilized on SBA-15 mesoporous
material and silica gel: new exceptionally active catalysts for the
chlorination of organic acids
a
b
b
Keun-Sik Kim, Jong-Ho Kim and Gon Seo*
a
Korea Fine Chemical Co. Ltd., Yeosu Chonnam 555-290, Korea
Department of Applied Chemistry, Chonnam National University, Gwangju 500-757, Korea.
E-mail: gseo@chonnam.ac.kr; Fax: +82-62-530-1890; Tel: +82-62-530-1876
b
Received (in Cambridge, UK) 21st October 2002, Accepted 18th December 2002
First published as an Advance Article on the web 13th January 2003
Novel reusable phosphazenium chloride catalysts immobi-
lized on SBA-15 mesoporous material and silica gel show
exceptional activities and selectivities even in the continuous
chlorination reaction of organic acids with thionyl chloride
or phosgene.
exceptional activity and selectivity in the chlorination of
2-ethylhexanoic acid, achieving more than a 95% yield at
ambient temperature only for 30 min reaction.
SBA-15 mesoporous material was prepared by using poly-
(alkylene oxide) block copolymer detergent (average M
5
n
=
,800, Aldrich, P-123) as a template.5 Tetraethoxysilane
Organic acid chlorides have been used as raw materials in
producing fine chemicals such as polymerization initiators,
surfactants, agrochemicals, and pharmaceuticals because of
their high reactivity. Product yields in the chlorination of
organic acids with thionyl chloride and phosgene, however, are
low without using active catalysts due to high preference for the
formation of acid anhydride. Homogeneous base catalysts, such
as imidazole, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and
N,N-dimethyl formamide show high activity even at low
temperature,1 but the difficulty in the separation of used
catalysts from products reduces the advantages of catalyst
application.
Quaternary ammonium chloride and phosphonium chloride
catalysts immobilized on polystyrene resin are active in the
chlorination reactions.2 Benzyl groups of polystyrene resin
combining catalysts, however, are fragile at elevated tem-
perature of 80–90 °C, bringing about an irreversible catalyst
deactivation. On the other hand, guanidinium chloride catalysts
immobilized on silica, not polystyrene resin, show a high
stability compared to quaternary ammonium chloride catalysts.3
Even though these immobilized catalysts provide a convenient
separation, they require a high reaction temperature of 80–160
(Aldrich, 98%, TEOS), P-123, hydrochloric acid (Daejung,
35%), and deionized water were mixed to make a reaction
mixture with a composition of 1 SiO
2
+0.017 P-123+2.9
HCl+203 H
2
O. SBA-15 mesoporous material was obtained by
heating the reaction mixture at 90 °C for 24 h. Pore diameter and
surface area of calcined mesoporous material were determined
2
from its adsorption isotherm of nitrogen to be 100 Å and 590 m
g
2
1
, respectively. Silica gel (Aldrich, Davisil grade 646, 35–60
mesh) was also used as a support for phosphazenium chloride.
Its surface area and average pore diameter provided by the
2
21
manufacturer were 300 m g and 150 Å, respectively.
Silane containing a secondary amine 1 was synthesized by
reacting 3-chloropropyltrimethoxysilane (Aldrich, 97%) with
n-butylamine (Junsei, 95%) in a nitrogen atmosphere at
115–120 °C for 20 h. Separately, a precursor of phosphazenium
chloride 2 was synthesized by the reaction of hexamethylene-
phosphoramide (Aldrich, 99%, HMPA) with phosgene gas
(Korea Fine Chemicals, 99%) in methylene chloride. A part of
the dissolved precursor (1.05 equiv.) 2 in anhydrous methylene
chloride was added to a solution of secondary amine (1.0 equiv.)
1 and triethylamine (1.2 equiv.) diluted in the same solvent at 10
°C under a nitrogen stream. Then the coupling between 1 and 2
was carried out at room temperature for 10 h. After evaporating
the solvent and triethylamine, homogeneous phosphazenium
chloride 3 was obtained as a very hygroscopic solid. Dissolved
phosphazenium chloride 3 in anhydrous chloroform reacted
with hydroxyl groups of dried SBA-15 mesoporous material 4a
and silica gel 4b under reflux conditions for 24 h. Immobilized
phosphazenium chlorides 5a and 5b were obtained by washing
with chloroform, toluene and ether followed by filtering and
drying in a vacuum oven at 110 °C. Finally, unreacted hydroxyl
groups of supports were masked by reacting them with
1,1,1,3,3,3-hexamethylene disilazene (Aldrich, 97%, HMDS).
Phosphazenium chloride-immobilized catalysts were denoted
as PZ/SBA 6a and PZ/SIL 6b catalyst according to their
supports.
°
C to obtain a feasible yield of chlorination products.
Phosphazenes composed of nitrogen and phosphorous atoms
a
are known to be very strong bases with high pK of 24–47 due
4
to the number of combining phosphazene units. Since the large
flat nuclei of phosphazene molecules are very suitable for the
delocalization of positive charges, phosphazenium chloride can
work as a catalyst supplying activated chloride ions to organic
acids.
We prepared phosphazenium chloride catalysts immobilized
on SBA-15 mesoporous material and silica gel, following the
procedure described in Scheme 1. These catalysts show
1H and 13C NMR spectra of phosphazenium chloride 3 show
the formation of phosphazenium chloride and the disappearance
of residual secondary amine in 13C NMR.6
Mesopores of SBA-15 mesoporous material remained even
after immobilizing phosphazenium chloride, while its pore
volume and surface area were considerably decreased by
immobilizing large phosphazenium chloride molecules. IR
2
1
absorption bands at 2800–3000 cm of the PZ/SBA catalyst
exhibited the immobilization of organic molecules on the SBA-
1
5 mesoporous material. These bands were retained even at 300
°
C under evacuation, indicating stable immobilization of
3
1
phosphazenium salt on the mesoporous material. The P MAS
NMR spectrum of the phosphazenium chloride-immobilized
catalyst 6a also showed a phosphorus peak at a chemical shift of
Scheme 1 Immobilization of phosphazenium chloride on SBA-15 mesopor-
ous material (a) and silica (b).
3
72
CHEM. COMMUN., 2003, 372–373
This journal is © The Royal Society of Chemistry 2003

3
7.6 ppm as shown in Fig. 1, otherwise the phosphorus peak of
Table 1 Chlorination of aromatic and aliphatic acids with thionyl chloride
and phosgene as chlorinating regents over phosphazenium chloride-
immobilized catalystsa
homogeneous phosphazenium chloride showed a chemical shift
of 47.7 ppm. The lower chemical shift of phosphorus of the PZ/
SBA 6a catalyst suggests that phosphazenium ion was im-
Con-
version
(%)
Selec-
tivity
(%)
7
mobilized on the support as a salt.
Temp./
Catalyst °C
Phosphazenium chloride catalysts immobilized on the sup-
ports were also prepared by a stepwise reaction: the immobiliza-
tion of secondary amine 1 on the support, masking of non-
reacted hydroxyl groups with HMDS, and the coupling of the
precursor 2 to immobilized secondary amine. These catalysts,
denoted as PZ/SBA 7a and PZ/SIL 7b catalysts according to
their supports as for 6a and 6b, showed identical IR and NMR
spectra to those of PAZ/SBA 6a and PZ/SIL 6b catalysts,
indicating that the immobilization procedure did not bring about
any change in the immobilized state of phosphazenium
chloride.
Hydrogen chloride is additionally coordinated on phospha-
zenium chloride, similar to its additional coordination on
guanidinum chloride. The immobilizing amounts of phospha-
zenium chloride were 0.33, 0.29, 0.36 and 0.22 mmol g on
PZ/SBA 6a, PZ/SIL 6b PZ/SBA 7a, and PZ/SIL 7b catalysts,
respectively.
Phosphazenium chloride catalysts immobilized on SBA-15
mesoporous material and silica gel, regardless of their im-
mobilization procedures, were highly active and selective in the
chlorination of benzoic acid at 40 °C. Table 1 shows 97.0%
yield of the chlorinated product over the PZ/SBA 7b catalyst
after only 1 h. Exceptional activity and selectivity for the
chlorinated products were also obtained over immobilized
phosphazenium catalysts in the chlorination of aromatic and
aliphatic organic acids. Although the reaction was carried out at
elevated temperature for the reaction of terephthalic acid, steric
acid and lauric acid because of their high melting points, these
catalysts provided high yield of above 95%.
Acid
Time/h
Benzoicbc
B
B
B
None
6a
6b
7a
7b
7a
7b
None
7a
7b
None
7a
None
7a
None
7b
Reuse
40
10
2.5
3
1
4
3
4.5
18
94
100
100
100
98
100
100
95
100
100
87
99
80
45
99
98
97
97
99
99
60
99
100
90
99
70
99
4
B
B
B
B
bd
Terephthalic
120
B
40
B
2
B
B
-Ethyl-hexanoicb
25–30 0.5
B
0.5
6
2
6
2
4
3
3
b
Stearic
80
80
80
80
110
125
125
B
8
Lauric
b
2
1
B
99
47
100
100
e
Lauric
B
B
a
95
95
f
Reaction conditions: acid/chlorinating reagent 10/15 (as mmol), catalyst
b
loading amounts = 0.1 g. Thionyl chloride used as chlorinating reagent.
c
Methylene chloride used as solvent. d Monochlorobenzene used as
e
f
solvent. Phosgene used as chlorinating reagent. 7b was reused 5 times.
can proceed through the attack of chloride anions coordinated
on phosphazenium nuclei to electrophilic carbonyl groups of
organic acids, forming a complex with it.
In conclusion, we can prepare immobilized phosphazenium
chlorides, which show exceptional activity and selectivity in the
chlorination of organic acids. These catalysts also show high
stability. Easy formation and stabilization of active chloride
anions on phosphazenium nuclei due to their high basicity and
large nuclei suitable for delocalization of anion may be
responsible for their promising catalytic activity and selectivity
for the chlorination of organic acids.
Uncatalyzed chlorination of 2-ethylhexanoic acid with thio-
nyl chloride was slow at ambient temperature, achieving only
10% yield of chlorinated product for 1 h reaction. On the other
hand, the PZ/SBA 7a and PZ/SIL 7b catalysts require only 30
min for quantitative completion of the chlorination reaction.
The immobilized phosphazenium chloride catalysts can be
repeatedly used in the chlorination reaction of organic acids. No
meaningful lowering of catalytic activity was observed during 6
repeated runs, adding new reactant after decanting reacted
materials. A continuous flow chlorination of 2-ethylhexanoic
acid with thionyl chloride exhibited high conversion of about
This work was supported by KOSEF(2001-1-30700-002-3)
in Korea.
Notes and references
† A glass column with 15 mm diameter and 20 mm length was packed with
9
5% following a single pass of the reactant through the catalyst
2
1
bed with LHSV = 31.3 h at ambient temperature.†
9
.2 g of the PZ/SIL catalyst 7b. A mixture of 2-ethylhexanoic acid (100
Since positive charges are highly delocalized on phosphazen-
ium nuclei, chloride anions coordinated on them show strongly
nucleophilic properties. The high feasibility of the reaction of
chloride anions with organic acid molecules on phosphazenium
nuclei results in a high activity. In other words, the chlorination
mmol) and thionyl chloride (200 mmol) was passed through the top of the
column at room temperature. The retention time for a single pass was 15
min. At the bottom of the column, products were sampled and analysed by
a gas chromatograph with CP-Sil 5CB capillary column (25 m 3 0.25 mm).
This test was repeated 6 times under the same conditions.
1
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4
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6
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The spectral data for compound 3: H NMR (500 MHz), CDCl
1
3
-TMS; d
=
0.68(m, 2H), 0.93(t, J = 7.2 Hz, 3H), 1.39(m, 2H), 1.82(m, 2H),
.02(m, 2H), 2.69(d, J = 9.5, 2H), 2.88(m, 2H), 3.05(3JP,H = 12.5 Hz,
8H, [(CH N]
P), 3.55(s, 9H), 9.92 (br s, 1H); 13C NMR (125 MHz),
-TMS; d = 3.97, 10.97, 16.49, 17.98, 24.88, 34.82, 34.85, 47.50,
7.61, 47.79.
C. M. Crudden, D. Allen, M. D. Mikoluk and J. Sun, Chem. Commun.,
001, 1154.
2
1
)
3 2
3
CDCl
4
3
7
2
Fig. 1 31P solid MAS NMR spectra of the immobilized phosphazenium
chloride PZ/SBA 6a and homogeneous phosphazenium chloride.
8 J. P. Senet, The Recent Advance in Phosgene Chemistry, Group SNPE,
France, 1998.
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373