КИНЕТИКА И КАТАЛИЗ, 2010, том 51, № 5, с. 697-701
УДК 541.128:12:546.732-386:547.551.1
ACTIVITIES OF Co(II) SCHIFF BASE COMPLEXES IN THE REDOX CARBONYLATION OF ANILINE AND NITROBENZENE TO METHYL N-PHENYL CARBAMATE © 2010 Chen Li-juan*12, Mei Fu-ming2, Li Guang-xing2, Xiang Yu-jun1
1School of Chemistry and Chemical Engineering, Hunan University of Science & Technology, Xiangtan, PR China 2School of Chemistry and Chemical Engineering, Huazhong University of Science & Techology, Wuhan, PR China
*E-mail: ljchen11@163.com Received 06.09.2009
The series of cobalt(II) complexes with different Schiff base ligands was synthesized and used as catalyst for the redox carbonylation of aniline and nitrobenzene. Effects of reaction temperature, CO pressure, promoter, and catalyst additions on the conversion of substrate were studied. When Co[(OH)2saloph] — ^-toluene-sulfonic acid system was used as catalyst, the reaction was carried out at the next conditions: both Co[(OH)2saloph] and ^-toluenesulfonic acid — 0.2 mmol, aniline — 20 mmol, nitrobenzene — 10 mmol, methanol — 30 ml, CO — 5 MPa, temperature 170°C, reaction time 7 h. The highest conversion of nitrobenzene and selectivity of methyl N-phenyl carbamate were 54.5 and 92.2%, respectively.
Isocyanates are of commercial importance in the manufacture of polyurethanes and herbicides. Conventional process for the production of isocyanates is carried out by the reaction of primary amines with phosgene. Despite the high yield of this method, the toxic phosgene and a large amount of corrosive HCl coproduct caused serious environmental problems. Many efforts have been made to develop the phosgene-free routes. Most proposed green processes are indirect ones, in which the isocyanates were synthesized through the carbamate intermediate, and the corresponding isocyanate was produced by the thermal dealcoholization of carbamate at high temperature. Methyl N-phenyl carbamate (MPC) is the important precursor for the production of methane-diphenyl-4,4'-diisocyanate, an important monomer in polyurethanes manufacture. Presently, the mainly studied phosgene-free routes for the production of MPC are catalytic carbonylation, including oxidative carbonyla-tion (I), reductive carbonylation (II) and the transesteri-fication reaction of aniline and dimethyl carbonate (III):
PhNH2 + CO + 1/2O2 + MeOH T, P, Cat ^ — PhNHCOOCH3 + H2O,
PhNO2 + 3CO + MeOH
T, P, Cat
2
—- PhNHCOOCH3 + 2CO2, PhNH2 + H3CO—CO—OCH3 -Ca* — PhNHCOOCH3 + CH3OH,
2PhNH2 + RNO2 + 3CO + 3MeOH T, P, Cat —3PhNHCOOCH3 + 2H2O.
(II)
(III)
(IV)
Unlike catalytic carbonylation, the transesterifica-tion process has the limitation in the separation of methanol—dimethyl carbonate (DMC) azeotrope and the high cost associated with the use of the expensive raw DMC. Besides, catalytic carbonylation is prefera-
ble with regard to its higher efficiency and ease of handling. However, both reductive and oxidative carbony-lations have some drawbacks. The CO utilization efficiency in reductive carbonylation is one-third, and it is difficult to separate CO from CO2. In the case of oxi-dative carbonylation, the conversion of CO to carbony-lation product is complete, but there is the safety problem because of in the start materials are used the explosive mixture of oxygen and CO. The improvement of carbonylation process is carried out by adding nitrobenzene in the reaction mixture without oxygen, as depicted in reaction (IV). Nitrobenzene in the reaction system acts as an oxidant, aniline reacts with CO to generate the products of carbonylation. The active catalysts for the carbonylation process are a group of transition metal compounds, sulfur, selenium and tellurium [1—3].
The metal Schiff base complexes have been successfully employed in a number of catalytic reactions, such as oxidation [4], carbonylation [5], epoxidation [6], ring opening [7], hydration [8], etc. Their particular catalytic activities are attributed to the unsaturat-ed coordinating state and variable valency of center ions. In our previous papers [9—11], cobalt complexes with substituted or non-substituted salen-type ligands were prepared and used as catalysts for the oxidative carbonylation. In another work [12], we studied the reaction of nitrobenzene, aniline and CO catalyzed by N,N'-bis(salicylidene)-o-phenyldiimine cobalt(II) in toluene solution. N,N'-diphenyl urea was produced in a good yield. In this paper, the catalytic activities of different cobalt(II) Schiff base complexes were investigated in the redox carbonylation (IV) in methanol. The ligand effect, substituent effect ofligand and the effect of other reaction parameters are discussed in detail.
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CHEN LI-JUAN et al.
Structures of Schiff base ligands L!—L5.
N N:
OH HO
Lj: saloph
o2n
L3: (OH)2saloph H
N N
OH HO
L2: (NO2)2saloph
Q
H3C M CH3
>=N N=(
OH HO—
H3C CH3
L4: AA2ph
NO2
N N OH HO
L5: saldien
Scheme.
EXPERIMENTAL
Materials
Nitrobenzene and aniline are commercial reagents and distilled under reduced pressure before using. Sal-icylaldehyde, 2,4-dihydroxybenzylaldehyde, 5-nitro-salicylaldehyde, o-phenylenediamine, diethylenetri-amine, and acetylacetone are of analytical quality and distilled under inert atmosphere prior to use. All the solvents used were analytical reagents and dried by standard method. Cobalt acetate tetrahydrate was used as received. Carbon monoxide with a purity of 99.99% was purchased from the local manufacturer.
Synthesis of cobalt(II) Schiff base complexes
Five Schiffbase ligands Lj—L5, as depicted in Scheme, were synthesized according to the reported procedures: a salicylaldehyde derivative [13] or acetylacetone [14] was mixed with o-phenylenediamine or diethylenetriamine in 2 : 1 molar ratio in ethanol. The corresponding ligands L1—L5 were formed after refluxing ofthe resulting mixture at 60°C for 2 h. The cobalt(II) chelates Co(L1)-Co(L5) were prepared by reacting ofequimolar amounts ofligand and cobalt(II) acetate tetrahydrate in dichloromethane— methanol mixture (3:1, v/v), all the complexes were formed as dark green crystals. The ligands and complexes were characterized by IR, NMR and elemental analysis to determine their structures.
Catalytic experiments
The redox carbonylation of aniline and nitrobenzene was carried out in 100 ml Teflon lined stainless autoclave equipped with a mechanic stirrer and a temperature controller. A mixture of accurately weighed aniline, nitrobenzene, catalyst, promoter and 30 ml of methanol was charged into reactor at room temperature, the autoclave was sealed, purged with nitrogen for three times, and then pressurized with CO to the desired pressure. After being heated to the required reaction temperature, the autoclave was hold at this temperature for 7 h till the reaction was completed. After reaction, the autoclave was cooled to ambient temperature and the residue gas was released. The liquid samples were analyzed by GC using a HP-5 capillary column (30 m x 0.32 mm x 0.25 ^m).
RESULTS AND DISCUSSION
Activities ofcobalt(II) Schiffbase complexes
In the reaction of reduction carbonylation of PhNO2 in the presence of methanol and various amounts of aniline, diphenyl urea is firstly formed, and then the alcoholysis of diphenyl urea by CH3OH to form MPC take place nearly completely, in accordance with the data reported in the literature [15, 16]. Our experiments have shown that the optimal reaction time is 7 h. To make the comparison of the activities of
ACTIVITIES OF Co(II) SCHIFF BASE COMPLEXES
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Table 1. Redox carbonylation of nitrobenzene and aniline catalyzed by cobalt(II) complexes
Catalyst Conversion, % Yield, % Selectivity to MPC, %
Nitrobenzene Aniline MPC* NMA**
Co(saloph) 48.9 41.5 81.4 1.0 90.6
Co[(NO2)2saloph] 13.6 12.1 78.2 0.8 84.5
Co[(OH)2saloph] 54.5 45.6 82.6 1.1 92.2
Co(AA2ph) 33.4 30.6 80.6 1.0 85.4
Co(saldien) 6.4 5.7 77.1 0.8 83.2
Note. Reaction conditions: nitrobenzene — 10 mmol, aniline — 20 mmol, cobalt complex — 0.2 mmol, PTS — 0.2 mmol, CO — 5 MPa, 170°C, 7 h.
*Yield of MPC = 1/3 ** Yield of NMA
moles of MPC
moles of nitrobenzene converted moles of NMA
-x 100.
moles of aniline converted
-x 100.
various cobalt(II) complexes, all experiments were carried out as follows. 1.86 g of aniline (20 mmol), 1.23 g of nitrobenzene (10 mmol), Co(II) complexes with different ligands (0.2 mmol, 2 mol. % in nitrobenzene) as catalysts, 0.2 mmol of p-toluene-sulfonic acid (1 : 1 molar ratio to catalyst) and 30 ml of methanol were added at room temperature in the reactor. Then it was flushed with N2 for three times and then pressurized with CO to 5 MPa. The reaction was allowed to proceed at 170oC for 7 h. The concentrations of the unreacted reactants and products in the liquid samples after reaction were determined by GC analysis using standard method, and the yield of car-bamate regarding to nitrobenzene was calculated.
According to the GC analysis results, the desired product (methyl N-phenyl carbamate) is formed in dominant amount. N-methyl aniline (NMA) is a main byproduct of reductive and oxidative carbonylations, in accordance with data [17, 18], and it is formed by the methylation of aniline with methanol. Based on the stoichiometric relation in reaction (IV), the amount of aniline consumed must two times more than nitrobenzene consumed. However, as shown in Table 1, this relationship is really less than two. It can be explained that in the presence of an acidic promoter and reductive environment, some amount of nitrobenzene can be converted to aniline in accordance with the previous report [19], and this part of the formed aniline also contributes to aniline consumption:
PhNO2 + 2CO + 2H+
PhNH2 + 2CO2
(V)
It is also shown in Table 1 that the activities of the co-balt(II) Schiffbase complexes depend on the nature ofthe ligand. The activities of the cobalt complexes with various ligands are increased in row: saldien < AA2ph < saloph.
It is seen from Table 1 t
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