Ind. Eng. Chem. Res. 1994,33, 1-6
1
KINETICS, CATALYSIS, AND REACTION ENGINEERING Oxidative Carbonylation of n-Butylamine Using Transition Metal Cata1ystst S. Kanagasabapathy, Sunil P. Gupte, and Raghunath V. Chaudhari' Chemical Engineering Division, National Chemical Laboratory, f i n e 411 008, India Oxidative carbonylation of n-butylamine to methyl N-butylcarbamate has been reported. Several homogeneous and heterogeneous transition metal catalysts have been tested for their activity and selectivity. The effect of promoters, solvents, and temperature has been investigated. Temperature plays a vital role in the selectivity of carbamate.
Introduction Oxidative carbonylation of amines has gained considerable importance in recent years in the synthesis of urea, carbamates, and isocyanate derivatives. The new nonphosgene routes for methylene diphenyl diisocyanate (MDI) (Fukuoka et al., 1984a) are based on this reaction. Conventionally isocyanates are produced by reacting phosgene with amines (Twitchett, 1971). The drawback of this process is the handling of dangerous and corrosive chemicals like phosgene and hydrochloric acid. Traditional methods of preparation of carbamates involve the reaction of alcohol and urea (Barton and Ollis, 1979), ammonolysis of alkyl chloroformate (Kraft and Herbst, 1945),and transesterification (Gaylord and Sroog, 1953). Carbonylation of amines offers an alternative route for the synthesis of isocyanates; however, the selectivity of isocyanate is very poor as large amounts of formamides, ureas, and oxamides are produced as side products (Dombek and Angelici, 1977; Rempel et al., 1974). Carbonylation of aniline in the presence of nitro compounds has been reported for the synthesis of urea (Dieck et al., 1975). The previous work on this subject relates to oxidative carbonylation of aromatic amines and particularly aniline using different types of catalysts (Fukuoka and Chono, 1984;Fukuoka et al., 1984b). Most of the reports concern aniline as a substrate, and there are only a few studies on alkylamines as substrates (Alper and Hartstock, 1985; Dombek, 1992; Sonoda et al., 1971; Golodove et al., 1979; Halligudi et al., 1991; Kelkar et al., 1992). The aim of this paper is to present results on oxidative carbonylation of n-butylamine using Pd- and Ru-based catalysts with respect to activity and selectivity behavior. Oxidative carbonylation of n-butylamine proceeds through the formation of N,"-dibutylurea (DBU) and methyl N-butylcarbamate (MBC):
-
2 n-C4H9NH2+ CO + 1/202 C4H9NHCONHC4Hg + H20 (i) N,"-dibutylurea (DBU)
-
C4H9NHCONHC4H,+ MeOH C4HgNHCOOMe + C4H,NH2 (ii) methyl N-butylcarbamate (MBC) 0888-5885/94/2633-0001$04.50/0
Here, reaction i is catalytic, while reaction ii is noncatalytic. Therefore, it is important to understand the selectivity behavior of the two products (N,"-dibutylurea and methyl N-butylcarbamate) and its dependence on the type of the catalyst used and the reaction conditions. Results of the effect of solvents, promoters, and reaction conditions, as well as the product distribution, in a batch reactor are discussed.
Experimental Section Materials. The catalyst, consisting of metallic palladium, was prepared by reducing palladium chloride. PdC12 was purchased from Arrora Mathey Ltd., and n-butylamine was purchased from M/S. Fluka Werke, Switzerland. Methanol and other solventswere distilled and dried before use. Carbon monoxide was used directly from the cylinder (purity >99.5 96 1. Oxygen was supplied by Indian Oxygen Limited, Bombay. All the experiments were carried out in a 3 X 10"'-m3-capacityhigh-pressure stirred autoclave (Hastalloy C) supplied by Parr Instrument Co. U.S.A. This reactor was provided with automatic temperature control, agitation, cooling coil, and devices for gas and liquid sampling. 'H NMR spectra were recorded on a Brucker MLS 300 FT NMR spectrometer. Pd/C, Pdly-AlaO3,and Pd-ZSM-5 catalysts were prepared by a method described by Mozingo (1955). The ruthenium complexes, BQN[Ru(CO)&I and 18-crown-6 ether-[Ru(C0)313] (Colton, 1971),Ru(C0)2(PPh&C12, R u ( C O ) ~ ( P Y ) ~and C ~RuC12~, (PPh3)4 (Stephanson, 1966), and palladium complexes, Pd(py)2X2, were prepared by methods described in the literature (During, 1967). Experimental Procedure. In a typical experiment, known quantities of n-butylamine, catalyst, metallic Pd, promoter, NaI, and solvent were charged into the autoclave, and the contents were flushed with nitrogen. The autoclave was heated to a desired temperature and then pressurized with C0:02 (13:l) to a total pressure of 6.06 X lo3kPa. The reaction was carried out to 2 h a t a constant pressure of 61 atm by supplying gas from a small reservoir (C0:02 = 2:l; proper safety precautions were employed during this work against any explosion hazards). After 2 h, the contents were cooled and the liquid and gas samples were analyzed by GC. Urea analysis was carried out on HPLC using WBondapak phenyl stainless steel column.
* To whom correspondence should be addressed. + National Chemical Laboratory Communication No. 5762.
0 1994 American
Chemical Society
2 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 Table 1. Screening of Transition Metal Catalysts for Oxidative Carbonylation of n-Butylamine
no. 1 2
3 4
5 6
7 8 9 10
11 12
13 14 15 16 17 18 19
catalyst system Pd metal Pd/C (10%) Pd/C (5%) Pd/C (1%) Pd-ZSM-5 (1 X 10-'%) Pd/~-Al203(1% ) PdI2 Pd(p~)zClz Pd(py)zBrz Pd(py)zIz Ru/C (1%) ci~-Ru(CO)zClz(PPhs)2 tr~ns-Ru(CO)zClz(PPh3)2 BUdWW"sIa1 RuIsC03-18-crown-6 ether Ru(COh(PPhs)z RuClz(PPh3)z RuClz(PPh3)s RU(C~)~(CO)~(PY)Z
n-butylamine reaction time catal concn solvent NaI concn 0 In the absence of NaI.
av activity (kmol of amine conv/(kg of metales)) 1.17 X le2 0.72 X 3.8 X 10.6 X 1200.0 x 10-2 0.22 x 10-2 0.50 X
0.62 X (0.25 X 1 t 2 ) 0 0.75 X le2(0.32 X 1.1X 10-2 (0.92 X 0.57 X 0.53 X 0.60 X le2 0.83 X 0.68 X 1 t 2 0.01 x 1 0 - 2 0.73 X 0.73 X le2 0.68 X
Reaction Conditions 4.1 X 106 kmol 7.2 x 103 8 2.35 X 10-9 kmol/m3 MeOH 2.35 X 10-9 kmol/m3
Analytical Conditions. Liquid samples were analyzed by GC (Hewlett Packard Model MOA), using the following conditions: column, 5 m, 5 5% OV 17;injection temperature, 473 K; detector temperature, 573 K; column temperature, m3/s. 393 K; carrier gas (Nz), 5.8 X For C02 analysis, the conditions were the following: column, 5 m, Porapak Q; injection temperature, 323 K; detector temperature, 380 K; filament temperature, 433K; column temperature, 303 K; H2 gas flow 5.0 X m3/s. Butylamine was analyzed by a volumetric method described by Streuli (1970). Results and Discussion The aim of this work was to study the activity and selectivity of different types of Ru and Pd catalysts for oxidative carbonylation of n-butylamine. For this purpose several experiments were carried out in which reactant/ product concentration vs time data were obtained. The average activity and selectivity for a fixed time duration were calculated. The results on screening of catalysts and effect of solvents, promoters, and temperature are discussed in this paper. Screening of Catalyst. Several Pd and Ru catalysts were tested for their activity and selectivity in oxidative carbonylation of n-butylamine at 443 K and a total pressure of 61 atm (C0:02 ratio = 13:l). These experiments were carried out in methanol as a solvent, which also is a reactant in the formation of carbamate. The results are presented in Table 1. In all the cases two major products, namely NJV-dibutylurea and methyl N-butylcarbamate, were formed. These products were separated by column chromatography and identified by MS, IR, 'H NMR, and elemental analysis. The lH NMR spectrum data of products isolated are presented in Table 2. The activity of Pd-based catalysts was found to be significantly higher compared to those with Ru catalysts. Both homogeneous and supported Pd catalysts were studied; however, for homogeneous catalyst precursors, the Pd metal precipitation was observed at the end of reaction. Effect of Pd content in Pd/C catalysts showed
selectivity (%) MBC 75.28 97.15 89.32 65.37 47.62 74.58 72.81 21.7 (19.7)O 28.6 (23.91)" 32.6 (29.7)" 29.43 28.31 19.82 10.21 16.58 3.50 12.78 15.41 10.83
total vol temp C0:Oz ratio total press. supported catal
DBU 20.14 02.87 07.98 22.80 49.80 22.70 23.14 76.42 (79.32)' 70.42 (75.23)" 65.78 (69.32)O 68.57 69.42 72.65 80.76 81.05 78.25 81.47 79.74 84.10 1x10-4m3
443 K 131 6.06 X 109 kPa 2.5 X 1W kg
Table 2. lH NMR Data for Carbamate and Urea. 6 ( m m )chemical shift reference to TMS solvent DBU: H~CCH~CHZCH&J"HONHCHZCH~CH~ CD3COCDs 0.88-0.91 (t, 6 H, C&); 1.31-1.41 (m, 4 H, -CH&Hr); 1.50-1.62 (m, 4 H, - C & C H r ) ; 3.21-3.3 (q,4 H, -NH-Cl&); 7.96-8.12-(~, 2 H, NE)
CDCCls
MBC: HsCCHpCHpCHzNHCOOCHs 0.86-0.97 (t, 3 H, CHa); 1.29-1.54 (m, 4 H, -C€J&&CH3T 3.05-3.17 (q, 2 H, -CH.rNH-); 3.48-3.60 ( 8 , 3 H, -0C39); 6.34-6.21 (~,1-, NE)
4 Frequency: 300 MHz. Abbreviations: a, singlet; t, triplet; q, quartet; m, multiplet.
that 1% Pd/C catalyst gives higher average activity compared to 10%PQC. However, the selectivityof methyl N-butylcarbamate increased with increase in Pd content in the catalyst. The type of support used also showed a significant effect on the catalyst activity in which PdZSM-5 catalyst with 0.01 7% Pd showed remarkably higher activity compared to Pd/C and PdlyAlzOg catalysts (see Table 1). This effect is primarily due to better dispersion of Pd metal on the surface of the Pd-ZSM-5 support. This unusual higher activity observed for 0.01% Pd-ZSM-5 catalyst deserves some comments. It is known that the activity of the transition metal exchanged zeolitesdepends on the type of zeolite and on the amount and location of metal site in the zeolitestructure (Ghosh and Kevan, 1990). Decrease in the activitywith increase in Pd content (>1%) arises not only due to underutilization of densely occupied Pd sites, but also due to pore blockage due to adsorbed Pd complex on the mouth of the pore. The possibility of such a blockage increases with increase in Pd content in the catalyst. The pore blockage renders the Pd sites located in the inner cage inaccessible to the reactants, thus decreasing the catalyst activity. For 0.01% Pd containing catalyst, such type of blockage may be minimum, resulting in higher activity of the catalyst. Some blank experiments were carried out without the catalyst and promoter. It was observed that neither Pd metal nor promoter alone act as catalysts but both are
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 3 5.
,E
I
1
(01373 K
5
I
1
( b ) 408 K
n E
4 Carbamate7
J
80
40
I
,,,
E
(C)443K
1
120
TIME,rnin
TIME.rnin
E
1
I
(d) 4 6 3 K
T IME,rnin
TIME,min
Figure 1. Concentration-time profiles at different temperatures. (See Table 3 for reaction conditions.)
essential components. For the case of PdIz catalyst, however, the activity was significant even in the absence of NaI. The activity of the Pd catalyst containing C1 ligands increases remarkably when NaI promoter is added to the system. However, the activity of complexes containing iodide ligands increases only slightly upon addition of NaI. This indicates that iodide promoter plays an important role in the oxidative carbonylation of n-butylamine (see Table 1). The homogeneous Ru complex catalysts were also found to be active in oxidative carbonylation of n-butylamine, though the average activity was lower compared to Pd catalysts. The selectivity of NJV'-dibutylurea was higher than that for methyl N-butylcarbamate (see Table 1). Since the highest activity and selectivity for carbamate were obtained using Pd/NaI catalyst, the product distribution and the role of promoters, solvents, reaction conditions, and catalyst reusability are studied using this system. Several experiments were carried out to identify the role of Pd/NaI catalyst system in the synthesis of methyl N-butylcarbamate. Figure IC shows a typical concentration profile, which indicates that 805% conversion of n-butylamine is possible and methyl N-butylcarbamate and NN-dibutylurea are the products. Figure ICalso indicates greater than 96 5% material balance of products (carbamate and urea) formed based on n-butylamine converted. In order to ensure that the activity of the catalyst was constant during a run, catalyst recycle experiments were carried out using Pd/NaI system, which
Table 3. Effect of Catalyst Reusability ~~~
no. 1 2 3 4 5
no. of recycle 0 1 2 3 4
n-butylamine reaction time Pd catalyst total press. total vol temp C0:Oz ratio NaI concn
conv of n-butylamine (%) 18.2 81.2 84.2 80.6 80.7
Reaction Conditions 4.1 X l ekmol 7.2 X 109 s 2.35 x IP k m 0 1 ~ 6.06 X 109 kPa 1 x 1WmS 443 K 13:l 2.35 x i~ km01/m3
indicated constancy of the catalyst activity even after four recycles (see Table 3). From these studies, it was concluded that Pd catalysts are more selective for carbamate synthesis. The effect of solvents, promoters, and temperature for Pd/NaI catalyst system was studied. The conversion and selectivity behavior in a batch reactor was also studied at different temperatures. The results are discussed below. Effect of Solvents. The effect of different solvents on the activity and selectivity of Pd metal catalyst with NaI as promoter was investigated at 443 K. The results are presented in Table 4 for methanol, ethanol, propanol, p-xylene, and toluene as solvents. These results indicate
4 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 Table 4. Effect of Solvent on Oxidative Carbonylation of a-Butslamine conv of n-butylamine no. solvent 1 p-xylene 2 toluene 1 methanol 2 ethanol 3 1-propanol n-butylamine reaction time Pd catal total press. total vol temp C0:Os ratio NaI concn
(%)
18.6 19.5 84.7 80.3 67.7
dielectric conat e, 293K 2.3 2.4 32.7 24.6 20.5
selectivity (%) carbamate DBU 100 100 88.6 4.8 73.8 25.8 43.9 54.9
Reaction Conditions 4.1 X 1od kmol 7.2 X 108 s 2.35 X l@ kmol/m* 6.06 X 108 kPa ix104m3 443 K 13:l 2.35 X 10-9 kmol/ms
Table 5. Oxidative Carbonylation of a-Butylamine with Pd-NaI Catalyst: E f f w t of Promoter no, 1 2 3 4 5 6 7 8 9 10 11
promoter NaI CHsI Et1 LiI HI NHJ I2 KI SnI2 nopromoter NaP n-butylamine reaction time Pd catal solvent promoter total vol temp C0:Oz ratio total press.
a Without
conv of n-butylamine (%) 84.70 71.84 68.73 62.21 62.62 59.24 50.53 38.68 20.13 nil nil
selectivity ( % ) BMC DBU 75.28 20.14 57.62 41.34 21.35 70.42 27.85 65.42 72.28 22.42 65.16 29.81 76.73 18.84 78.23 20.54 89.25 05.21
Reaction Conditions 4.1 X 1od kmol 7.2 X 108 s 2.35 x 10-8 km0i/m3 MeOH 2.35 x 10-9 km0i/m3 ix104m3 443 K 131 6.06 X 108 kPa
catalyst.
that the activity of the catalyst was very poor in nonpolar solvents like p-xylene and toluene. The activity was found to be significantly higher for polar solvents such as methanol, ethanol, and propanol. The selectivity of alkyl N-butylcarbamate (see reaction ii) was found to decrease with increase in carbon number for alcohols as solvents, while that of dibutylurea increases with the increase in carbon number of the alcohol. It was found that the activityand selectivityof the catalyst increasewith increase in the dielectric constants of solvents. Effect of Promoters. As already observed in the preliminary experiments, promoters containing iodide are essential in the catalyst system for oxidative carbonylation of butylamine. Therefore, the effect of different types of iodide promoters on the activity and selectivity of Pd catalysts was investigated and the results are presented in Table 5. The activity was found to decrease in the following order: NaI > CHJ > C2H,I > LiI = HI > NH41> I, > KI > SnI, It is interestingto note that, with SnI2, the lowest activity was observed but the selectivity for carbamate was the best with SnI2 promoter. Halide promoters play an
important role in governing the activity of the catalyst and are believed to stabilize the Pd-carbamoyl species formed during the reaction. This species is likely to play a key role in deciding the activity and selectivity of the catalyst. In other carbonylation reactions (e.g., carbonylation and hydrocarbonylation of alcohols) a similar trend in activitybehavior for iodide promoter is observed (Lafaye et al., 1982; Braca et al., 1986). In these reactions the activity of the catalyst generally depends upon the ease with which iodide is liberated by the promoter. The activity trend observed in this study also agrees well with this reasoning. Effect of Temperature. It has been shown earlier by Gupte and Chaudhari (1988) that oxidative carbonylation of amine to produce urea is a catalytic step (eq i), while further interaction of urea and alcohol to produce carbamate is a noncatalytic reaction (eq ii). The noncatalytic reaction is known to proceed only at higher temperatures (>423 K). Therefore, it was thought important to investigate the influence of temperature on the activity and selectivity of Pd catalysts. For this purpose, experiments were carried out at different temperatures (373-463 K), and the results are shown in Figure 1 as concentrationtime plots. It was observed that, between 373 and 408 K, butylamine concentration decreases with time, with formation of N,”-dibutylurea and corresponding carbamate derivative as major products (eq i). The concentration of the intermediate urea goes through a maximum and decreases,while concentrationof carbamate increaseswith time (Figure la,b). Between 408 and 443 K, the concentration-time profile shows a complex behavior in that butylamine concentration first decreases and then increases with time. In the initial period of the reaction (about 30 min), conversion of butylamine to dibutylurea is predominant. As the concentration of urea is increased, noncatalytic reaction between urea and alcohol picks up and methyl N-butylcarbamate and butylamine are produced according to reaction ii. This explainsthe formation of butylamine at an intermediate stage of the reaction (see Figure lb,c). The concentrationprofiles shown in Figure 1at different temperatures indicate that the catalytic reaction is fast even at lower temperatures (373-408 K), while the noncatalytic reaction takes place at temperatures above 408 K. For 463 K, butylcarbamate concentration attains an optimum value in about 1h and then decreases while that of butylamine increases (Figure Id). Sincecarbamates are known to be thermally unstable at higher temperatures (Arnold, 19571,at 463 K methyl N-butylcarbamate is likely to decompose to butyl isocyanate and methanol. The isocyanate, being highly reactive toward compounds containing active H atoms, would react immediately with water present in the reaction mixture as a coproduct (see eq i) to give butylamine and C02: C4H9NHCOOMe+
+
C4HgNCO + CH,OH butyl isocyanate
C4HgNC0 H20
-
C4H9NH2+ CO,
(iii) (iv)
Thus, the yield of methyl N-butylcarbamate decreases at temperatures above 463 K. Selectivityof urea and carbamate vs time and conversion of butylamine vs time are shown in Figure 2. These data show somevery interestingtrends in the selectivitypattern. The unusual shape of these plots can be understood from the concentration-time plots discussed above. At lower temperature (
