Ind. Eng. Chem. Res. 1992,31, 172-176
172
Selectivity Behavior in Catalytic Oxidative Carbonylation of Alkylaminesi Ashutosh A. Kelkar, Devidas S. Kolhe, S. Kanagasabapathy, and Raghunath V. Chaudhari* Chemical Engineering Division, National Chemical Laboratory, Pune 41 1 008, India
Alkyl Carbamates are prepared in good yields by oxidative carbonylation of alkylamines using a transition-metal catalyst and sodium iodide promoter. Various homogeneous and heterogeneous transition-metal catalysts are studied. Synthesis of methyl N-methylcarbamate (MMC) by oxidative carbonylation of methylamine is reported for the first time using the Pd-NaI catalyst system. The effects of various process parameters on oxidative carbonylation of methylamine are studied in detail. It is found that the selectivity pattern is strongly dependent on the type of catalyst used, the concentration of methylamine, the CO:02 ratio, and the temperature. A possible mechanism based on the results is discussed.
Introduction Oxidative carbonylation of aromatic amines to give diarylureas and aryl carbamates is well-known in the presence of Pd-NaI catalyst (Fukuoka et al., 1984a,b). However, synthesis of alkyl carbamates using such a route is not well investigated. Alkyl carbamates, and particularly methyl N-methylcarbamate (MMC), have potential applications in the preparation of important insecticides such as Carbaryl (1-naphthyl N-methylcarbamate) and BPMC (2-sec-butylphenylN-methylcarbamate). Conventionally, the synthesis of carbamate insecticides involves the use of phosgene and methyl isocyanate (MIC) as raw materials, and leakage of these materials in one such process for the manufacture of Carbaryl led to the Bhopal (India) tragedy of December 1984, in which more than 3000 people died. Since then, the production of aryl N-methylcarbamate insecticides by the MIC route has been prohibited. The phosgene- and MIC-based processes also have other drawbacks, such as corrosion problems and difficulties in handling large quantities. From the point of view of agriculturists, the carbamate insecticidesare still important for several applications,and, hence, there is a need for developing an environmentally acceptable and less hazardous process for the preparation of this group of compounds. The aim of this paper is to report a non-phosgene, non-MIC route based on the oxidative carbonylation of alkylamines for the synthesis of alkyl carbamates, particularly MMC, using Pd-NaI as a catalyst system. The stoichiometric reaction is R-NH2
+ CO + 7 2 0 2
catalyst
RNHCOOR’ + H2O
(1)
It was reported earlier that the formation of aryl carbamate occurs in two steps through N,”-diarylurea as an intermediate (Gupte and Chaudhari, 1988). Similarly the formation of N,”-dimethylurea (DMU) and MMC can be described as 2CHjNHz
+
CO
+
0
ti
CHsNHC-NHCH,
+
catalyst
’1202
CH30H
0
II
CH3NHC-NHCH3 + H20
(2)
0
II
CH3NHC-OCH3
+ CH3NH2
(3)
In the present work, the effect of various process conditions on the oxidative carbonylation of methylamine to MMC has been discussed. Since, it is believed that the
* To whom correspondence should be addressed.
National Chemical Laboratory Communication No. 4914. 0888-588519212631-0172$03.00/0
formation of carbamates occurs with urea as an intermediate (Gupte and Chaudhari, 1988); reaction 3 was studied independently with and without a catalyst.
Experimental Section Materials. Palladium chloride and ruthenium chloride trihydrate were purchased from Arrora Mathey India. Sodium iodide (AR grade) supplied by Loba Chemie was used as received. Samples of 5 % Rh/C and 5% Ru/C were obtained from Engelhard. Triphenylphosphine, tetrabutylammonium iodide, and 18-crown-6 ether were obtained from Aldrich. Methanol was freshly distilled before use. Methylamine, supplied by RCF, Bombay, was used directly from the cylinder. Carbon monoxide generated and compressed in our laboratory was used from the cylinder and was greater than 99.5% pure. Oxygen was used directly from the cylinder supplied by Indian Oxygen Ltd., Bombay. The catalyst with Pd metal was prepared by reducing PdC12with hydrazine hydrate. Pd-ZSM-5 catalyst (0.1% Pd) was prepared as follows: ZSM-5 (Si02:A1203= 35) wai impregnated with PdC12 (0.1% Pd), dried at 393 K for 5 h, and calcined at 823 K for 10 h. Palladium iodide was prepared as follows: A 2-g amount of palladium chloride was dissolved in 36 mL concentrated HC1 after slight heating. To this solution, a concentrated solution of KI (0.05 mol in 15 mL of water) was added. The solution turned from red brown to black with precipitation. The precipitate was washed with water and dried in a vacuum desiccator. Other catalysts prepared by the literature methods were 5 % Pd/C (Mozingo, 1955), PdC12(PPh3)2(Itatani and Bailer, 19671, Bu,N[Ru(CO),I,] and 18-crown-6 ether [Ru(CO),I,] (Colton and Farthing, 1971),and trans-Ru(C0)2(PPh3)2C12and RuCl2(PPh,), (Stephenson and Wilkinson, 1966). Oxidative carbonylation experiments were carried out in a 300-mLcapacity high-pressure stainless steel autoclave supplied by Parr Instrument Co. This reactor was provided with automatic temperature control, variable stirrer speed, and a device for liquid and gas sampling. Experimental Procedure. In a typical experiment, known quantities of methylamine in methanol, catalyst, and iodide promoter were charged to the reactor. The contents were heated to a desired temperature. After the temperature was attained, the autoclave was pressurized with COO, mixture (in a ratio desired for the experiment) up to a required pressure level. The reaction was initiated by switching the stirrer on. The reaction was carried out at a constant pressure by supplying CO:O2= 2:l from a 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 173 Table I. Effect of Promoters on Oxidative Carbonylation of Methvlamine sr. no.' 1 2
3 4 5
catalyst Pd Pd Pd Pd
promoter NaI LiI NaI KI
conversion of methylamine/ % 00.00 00.00 90.20 82.82 74.32
selectivity/ % urea MMC 00.00 00.00 00.00 00.00 14.08 84.90 13.84 85.52 17.73 81.53
Reaction Conditions Pd catalyst concn 2.35 X 10* mol/cm3 promoter concn 2.30 X lO* mol/cm3 1.0 x io4 mol/cm3 methylamine concn temperature 443 K pressure 61 atm COO2 ratio 13:l reaction time 120 min
'Sr. no. represents serial number. Table 11. Screening of Transition-Metal Catalysts For Oxidative Carbonylation of Methylamine sr. no. I
6
5
4
3
2
1
Figure 1. 'H NMR spectrum of methyl N-methylcarbamate (solvent CDCl,).
reservoir as per the stoichiometric requirement. Some reactions were carried out until absorption of the gas stopped completely,while in some cases the reactions were carried out for a fixed time duration. The contents were cooled below room temperature, and the liquid samples and the gas phase were analyzed by GC. Analytical Conditions. Liquid samples were analyzed by GC (Hewlett-Packard Model 5840A), using the following conditions: column injection temp detector temp column temp carrier gas (N,)
15 ft, 5% OV 17 523 K 523 K 423 K 35 mL/min
COPAnalysis. column column temp injection temp filament temp detector temp flow of H2
Porapak Q, 6 ft 303 K 323 K 433 K 383 K 30 mL/min
Methylamine Analysis. Methylamine was analyzed by a volumetric method described by Streuli and Averell, 1970. Results and Discussion Preliminary Experiments. Several experiments were carried out on oxidative carbonylation of methylamine, using different catalyst systems. From the preliminary experiments, it was observed that methyl N-methylcarbamate (MMC), N,"-dimethylurea (DMU), and Nmethylformamide (NMF) [under certain conditions] were the main products. In all the experiments a material balance (based on methylamine charged initially) up to 98% was obtained. The products were identified by GCMS analysis. Pure MMC was isolated by fractional distillation and characterized by IR and 'H NMR spectros-
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
convemion of methylcatalvst amine/% 5% Pd/C 39.85 PdIZSM5 73.00 5% Ru/C 17.20 5% Rh/C 70.31 5% Pd/C' 76.00 Pd/ZSM5" 84.50 Pd metal 93.22 PdCl,(PPh3)2 87.47 PdIz 85.00 PdIZb 84.25 BU~N[RU(CO)~I~I 90.85 BU~N[RU(CO)~I~]* 81.30 [ 18-crown-6ether Ru(CO)~I~] 88.72 trans-[R~(CO)~(PPh~)~Cl~] 45.16 trans-[R~(CO)~(Py)~Cl~] 55.38 RuC12(PPh3)4 48.96
selectivity/ %
urea 85.59 88.20 97.37 93.78 27.38 43.35 13.84 19.62 16.15 14.85 44.40 49.43 51.89 93.15 75.34 88.73
MMC 10.78 8.98 ~. 1.69 2.30 68.55 55.62 85.32 79.85 83.00 82.45 50.60 47.30 42.45 2.55 2.34 8.13 ~
Reaction Conditions promoter (NaI) concn 2.30 X lo* mol/cm3 methylamine concn 1x mol/cm3 solvent methanol temperature 443 K total pressure 61 atm total volume 100 mL COO2 ratio 13:l catalyst: promoter ratio 1:l reaction time 120 min
'Reaction time = 360 min. Experiment without promoter. copies (Figure 1). From the gas analysis, it was observed that C02was a gaseous product formed by the oxidation of co. Catalyst and Promoter Screening. In order to examine the role of iodine promoter, a reaction was carried out without using an iodine promoter (NaI). In this experiment, no conversion of methylamine was observed. Similarly, the experiment with only NaI but no Pd catalyst also showed no conversion of methylamine (Table I). This indicates that both Pd metal and NaI are essential for the oxidative carbonylation of methylamine. Various alkalimetal iodides (LiI, N d , KI) were tested as promoters, and it was observed that LiI and NaI gave comparable performance (Table I). Hence, for further work, NaI was chosen as a promoter. Various homogeneous and heterogeneous catalysts were tested for oxidative carbonylation of methylamine. The results are presented in Table 11. Palladium was found to be the most active and selective catalyst for MMC
174 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table 111. Effect of Pd:NaI Ratio on Oxidative Carbonylation of Methylamine conversion of selectivity/ % PdNaI ratio methylamine/ % MMC DMU 1:0.5 75.00 83.10 11.53 1:l 93.22 85.00 10.97 86.00 12.46 1:2 95.00 1:4 98.00 86.00 11.67 Reaction Conditions Pd catalyst concn 2.35 X 10" mol/cm3 methylamine concn 1x mol/cm3 solvent methanol temperature 443 K total pressure 61 atm total volume 100 mL CO:02 ratio 13:l reaction time 120 min
Table IV. Effect of Methylamine Concentration on Selectivity methvlamine reaction selectivity/ % sr. concn/ no. (lo3 mol/cm3) time"/min MMC DMU 1 6.355 240 10.35 88.50 2 3.230 180 20.12 79.23 3 1.610 160 75.23 23.45 4 0.967 140 85.67 12.98 Reaction Conditions Pd catalyst concn 2.35 X 10" mol/cm3 NaI concn 2.30 X 10" mol/cm3 solvent methanol temperature 443 K total pressure 61 atm CO:02 ratio 13:l Total time for complete conversion of methylamine.
Table V. Effect of Catalyst Concentration on Selectivity formation irrespective of the type of the precursors used (Pd metal or homogeneous Pd complexes). In all the exPd catalyst sr. concn/(lO' NaI concn/ reaction Ti periments with Pd precursors, Pd metal was found to be no. mol/cm3) (lo6mol/cm3) , . time"/min MMC DMU deposited at the bottom of the reactor. The selectivity of 1 47.00 46.00 110 86.90 12.20 MMC and DMU was dependent on the conversion level 2 4.70 4.60 130 85.35 13.70 in which, at lower conversion of methylamine, DMU se3 2.35 2.30 140 85.67 12.98 lectivity was higher but, a t higher conversions, MMC se4 0.94 0.92 205 74.34 24.36 lectivity was higher than that for DMU. Reaction Conditions Ruthenium precursors were found to be less active and methylamine concn 9.67 X lo4 mol/cm3 selective as compared to the Pd catalysts. With ruthenium solvent methanol precursors such as R U C ~ ~ ( P truns-R~(CO)~Cl~(PPh~),, P~~)~, temperature 443 K and truns-R~(CO)~Cl~(Py)~, the conversion of methyltotal pressure 61 atm amine, as well as the selectivity to MMC, was very low CO:02 ratio 13:l (2.34-8.13%). But, with tricarbonyl ruthenium complexes, a Reaction time for complete conversion of methylamine. (Bu~N[Ru(CO)~I,] and 18-crown-6 ether Ru(CO),I,) the conversion of methylamine was higher (8148%)and the Table VI. Effect of TemDerature on Selectivitv selectivity to MMC was in a range of 4040%. With horeaction selectivity/ % mogeneous catalysts containing iodide ligands such as PdI, sr. and Bu4N[Ru(CO),13],the activity of the catalysts was not no. temD/K timea/min MMC DMU ~. -. further enhanced by addition of the iodide promoters. 1 353 120 00.00 98.17 This indicates that the iodide present in the catalyst is 2 413 420 52.95 45.30 3 423 330 65.50 32.80 sufficient for the catalytic activity. Thus,of all the catalyst 4 433 200 72.84 26.56 precursors studied, Pd metal was found to be the most 5 443 140 86.67 12.92 active and selective catalyst. Therefore, further work was 6 453 120 86.15 17.75 carried out using Pd metal and NaI as a catalyst system. 7 463 100 85.96 12.78 Effect of Various Process Parameters on Oxidative Reaction Conditions Carbonylation of Methylamine. Effect of the PdNaI Pd catalyst concn 2.35 X 10" mol/cm3 Ratio. The effect of Pd:NaI ratio on the selectivity was NaI concn 2.30 X 10" mol/cm3 studied in a range of 1:0.5 to 1:4. The results are presented methylamine concn 9.67 X mol/cm3 in Table III. It was observed that, at a lower ratio of 1:0.5, solvent methanol the conversion of methylamine (75%) and selectivity to total pressure 61 atm CO:02 ratio 13:l MMC (78%) were poor. With an increase in the Pd:NaI ratio to 1:1, the conversion was improved considerably. a Reaction time for complete conversion of methylamine. With increasing PdNaI ratio (beyond l:l), the selectivity was not affected, while, the conversion increased from 93 Effect of Catalyst Concentration. The effect of to 98%. Since the selectivity to MMC was not affected catalyst concentration was studied in a range of 0.94 X lo4 and only a moderate rise in conversion was observed by to 47.0 X lo* mol/cm3. The results are shown in Table a change in the ratio of Pd:NaI from 1:l to 1:4, further V. It was observed that the catalyst concentration has experiments were carried out with the PdNaI ratio of 1:l. only a marginal effect on the selectivity of MMC. Effect of Methylamine Concentration. The effect Effect of Temperature. The effect of temperature on of methylamine concentration on the activity and selecthe selectivity was studied in a range of 353-463 K. The tivity of the catalyst was studied in a range of 9.67 X lo-* results are presented in Table VI. It was observed that, to 6.355 X lo-, mol/cm3. The results are presented in at a temperature of 353 K, only DMU was formed with a Table IV. It was observed that at higher methylamine selectivity as high as 98%, while MMC was not detectable. concentrations, such as 3.23 x and 6.355 x lo-, However, at higher temperatures (>413 K) the selectivity mol/cm3, the selectivity to MMC was very poor (10-25%). to MMC increased, being highest in a temperature range Under these conditions DMU was the major product of 443-463 K. A t lower temperature (353 K), C02 for(79-89%). At lower methylamine concentration (
