Synthesis of dimethylethylamine from ethylamine and methanol over copper catalysts

Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.

139

SYNTHESIS OF DIMETHYLETHYLAMINE FROM ETHYLAMINE AND METHANOL OVER COPPER CATALYSTS. Y. POUILLOUX^ V. DOroY% S. HUB^ J. K E R V E N N A L ' ' and J. BARRAULT* ""Laboratoire de Catalyse, URA CNRS 350, ESIP, 40 avenue du Recteur Pineau , 86022 POITIERS CEDEX, FRANCE ^CRRA ELF - ATOCHEM, 69140 PIERRE BENITE, FRANCE

ABSTRACT : Over a copper chromite type catalyst, a DMEA (dimethylethylamine) yield of 70 % was obtained from monoethylamine (MEA) and methanol with diethylmethylamine (DEMA) as the main by-product. The formation of DMEA was increased to 85 % by just changing the basicity of the catalyst resulting from a change of the rate of the determining steps. The rate of the MEA condensation compared to that of the MEA methylation decreased. Moreover the mechanism of the second methylation step which could involve an intermediate amide ((MEFA) or aminoalkoxide) was different from that of the first methylation step. Keywords : Dimethylethylamine synthesis, monoethylamine reaction with methanol, copper or copper chromite catalysts, alkaline or alkaline-earth modifiers. 1. INTRODUCTION Light amines are important intermediates in chemistry and in the pharmaceutical industry. The substituted light amines are prepared from alcohol, ammonia and/or monosubstituted amine in the presence of a solid catalyst (1-4). The heterogeneous catalysis process requires the formulation of a multifunctional catalyst which at a first approximation presents (i) acidic properties (amine adsorption, dehydration,...) and (ii) a hydro-dehydrogenating function (methanol dehydrogenation, hydrogenation of imine and enamine intermediates). Previous works have shown that copper catalysts are selective in the dehydrogenation of esters (5-7), in the hydrolysis of nitrile (8), in the selective hydrogenation of nitrile or in alcohol amination (10). The catalyst systems such as copper chromite are often used for the preparation of substituted amines. These solids, however, are very sensitive to the presence of water and ammonia (formation of copper nitrides (12)). Moreover, the catalysts promoted by alkaline or alkaline-earth species are more stable than the unpromoted CuCr. For example, barium impregnated on copper chromite increases the stability of the active CuCr02 phase (13). Furthermore, the presence of barium or calcium on copper chromite catalysts influences strongly the selectivity to the methylation of amines : N-alkylation/N-methylation.

140 In our laboratory, we have shown that copper chromite doped with barium, calcium or manganese can lead selectively to dimethyldodecylamine from lauronitrile, ammonia, hydrogen and methanol but not to methyldidodecylamine (14). Among the light amines, the dimethylethylamine (DMEA) is quite an important product i.e. as a catalyst in polymerisation processes. DMEA can be prepared from the reaction of ethanol with dimethylamine or from the reaction of methanol with monoethylamine; H2 CH3CH2NH2 + 2CH3OH

•

CH3CH2N(CH3)2 + 2H2O

We report, in this paper on the properties of promoted copper for the main and the side-reactions and we propose a reactional scheme of monoethylamine transformation. 2. EXPERIMENTAL 2.1. Catalytic test The reaction was studied in a dynamic fixed bed reactor under hydrogen pressure (1.0 MPa) at 210°C. The molar ratio MeOH/MEA was 8.2, the ratio (MeOH + MEA)/H2 « 0.85 (where MEA : monoethylamine or ethylamine) and the catalyst weight « 5g (particle size 1.2-1.6 mm). The reaction products were analysed by a gas chromatograph equipped with a SGE BPl column (L : 25m ; ID : 0.3 mm ; thickness of film : 5 jiim). Each catalyst was characterised by its activity and selectivity under standard conditions. The activity was obtained from the reagent conversion, the selectivity being expressed as follows : - The first calculation refers only to monoethylamine (MEA) and to products resulting from the conversion of MEA: Si(%) =

— X 100 ZMEA-^Pi - The second refers to all the products formed during the reaction (example : TMA) ^Zr%)zz:—XIOO

2.2 Catalysts The catalyst used in this study was a copper chromite doped with barium (YPl). The other solids were prepared from that catalyst by impregnation with alkaline salts from Prolabo (LiNO^,, KOH, CSNO3). After impregnation, the catalysts were dried in a sandbath (120°C), and then calcinated at 350°C for 4 hours under a dry air stream. 3. RESULTS AND DISCUSSION 3.1. Monoethylamine methylation in the presence of modified copper chromites 3.1.1. Activity and selectivity of the reference catalyst (YPl) In the first part of our work, we examined the properties of a copper chromite catalyst for the selective synthesis of dimethylethylamine (DMEA) from monoethylamine (MEA) and methanol (MeOH). Under our experimental conditions at 230°C, this catalyst

141 was quite selective (70%) at total conversion of the reactant. However, at this temperature, there was a significant formation of trimethylamine (TMA). Table 1 N-Methylation of monoethylamine (M£A) in the presence of a YPl catalyst. Effect of the temperature Time

T(°C)

(h)

Conversion

TMA

Selectivity (except TMA) (%) ^

(%) MEA

EMA

DMEA

DEA

DEMA

(%)

32

210

89.0

13.0

61.0

2.4

23.8

6.3

37

230

100

0.1

68.0

-

31.7

23.6

44

190

36.0

64.3

11.8

20.1

3.7

-

Pj^ : 1.0 MPa, Catalyst weight: 5g, Contact time : 1.3 s. (a) Selectivity to product i with reference to transformed ethylamine: Si(%) = (nMEA)conv

-xlOO

Moreover, the study of the influence of residence time showed that methylethylamine was the primary product of the reaction, whereas dimethylethylamine (methylation of EMA) was a secondary compound. The other products, issued from condensation and methylation reactions, were DMEA ((C2H5)2NCH3), DEFA (H-CON(C2H5)2), TEA ((C2H5)3N) Moreover, we observed the formation of monomethylamine MMA (CH3NH2), dimethylamine DMA ((CH3)2NH) and trimethylamine TMA ((CH3)3N).. The overall reaction scheme of the transformation of MEA is ; MeOH • H2O

EMA

+ MeOH • H2O

DMEA

+ MEA >•NH3

DEA

+ MeOH ^ -H2O

DEMA

MEA-

DEA + [CH3OH 3 MeOH + NH3

^

-H2 -H2O

HCHO]

H2O -^

DEFA

>- TMA

In order to increase the DMEA selectivity, the YPl catalyst was modified. As the formation of DEA and DEMA was favoured by the presence of acid sites on the surface of the catalyst, we showed that the addition of alkaline or alkaline-earth elements decreased

142 the condensation reaction rate of MEA. Moreover the presence of an alkaline-earth agent like barium stabilised the active phase (CuCr02) and led to a more stable catalyst (12). 3.1.2. Effect of the addition of alkaline elements (15) The YPl catalyst was impregnated with lithium, potassium or cesium. Table 2 shows that the addition of an alkaline (with the exception of cesium) improves the selectivity to DMEA (up to 90%). The addition of a small amount of KOH (0.5 to 2%) decreases the quantity of DEMA formed without changing the activity. The impregnation of lithium leads to a similar effect. However, in the presence of Li catalyst, the TMA selectivity is much more significant. We think that lithium which has a smaller particle size than potassium, can be easily inserted in the copper chromite phase. It can thus modify the hydro-dehydrogenating properties of copper and change the rate determining steps of side-reactions (TMA formation). The TMA selectivity is reduced when the solid is impregnated with cesium. However, DEFA is formed and it seems that DEMA could be obtained from DEFA;

2MEA^;F=^

DEA

+ CH3OH -H2O ^ DEFA = ^ = ^ DEMA

Table 2 N-Methylation of ethylamine. Comparison of the catalytic properties of YPl catalysts modified with Li, K or Cs. Catalyst

time

Conv.

(h)

EMA

DMEA

DEA

DEMA

DEFA

(%)

0

90.1

0

9.9

0

32.5

Selectivity (:%) (except TMA)

TMA

Li* 0.2%

24

MEA (%) 100

K 3.5%

16.5

99

0

94.2

0

5.8

-

7.5

69

59

22.0

1.3

1.0

17.0

0

100

0.1

68.1

0

31.7

-

23.6

Cs YPl

37

* impregnated with nitrate salt. T : 230°C, PH2 • 10 MPa, Catalyst weight : 5g, Contact time : 1.3 s. It can be observed that the rate of the secondary methylation is much slower than the one observed over the unpromoted solid and it decreases when the size of the alkaline ion increases. Moreover, it seems that the mechanism of the second N-methylation step is different from the one involved in the first N-methylation step. The mechanism of the second N-methylation step for the formation of DMEA or DEMA requires an adsorption step of intermediates MEFA or DEFA via alkoxide species because there is no hydrogen linked to the carbon of the CO bond. The following concerted elimination-hydrogenation reaction can lead to DMEA ;

143 + H2 CH3CH2NHCH3 + (CHsOH =^ EMA

,CH2CH3 H2CN. I ^CH3 OH

H-C—H) =5= II O

-H2 ^CH2CH3 HCN^ II CH3 O MEFA DMEA and a similar mechanism can explain the formation of DEMA via the intermediate DEFA. CU3 CH3CH2N^ ^CH3

H2O

3.2. Reactivity of monoethylamine or other intermediates In order to corroborate the main steps of the synthesis of dimethylethylamine and of the main by-products, we studied the reactivity of some intermediates and products with or without reagents (MEA, MeOH) under the same experimental conditions ; i) In the absence of methanol (replaced by n-heptane), monoethylamine is transformed mainly into diethylamine (DEA), the deactivation of the catalyst being very fast due to an increase of the formation of ammonia (15). Baiker and Kijenski showed, for instance, that part of the copper was transformed into copper nitride during the amination of alcohols (12). In the presence of methanol, the monoethylamine surface coverage is lower and a decrease cf the DEA formation can be observed. Methanol acts as an inhibitor in the synthesis of DEA and as a promotor of the catalyst duration. Table 3 Reactivity of various intermediates with methanol over a YPl catalyst. Reagent

Selectivity (%)

Time

Conv.

(h)

MEA (%)

DMEA

EMA

8

100

DEA'

8

100

DMEA

10 45

TMA + EtOH

DMA

DEMA

(%)

(%)

95.1

4.9

5.0

4.9

4.7

95.3

2.5

2.2

6.8

0

100

5.4

33.1^

66.9

T : 210°C, PH2 • 10 MPa, Catalyst weight: 5g, Contact time : 1.3 s. (a) catalyst weight: Ig ; (b) ethanol

ii) The methylation rate of diethylamine with methanol is more significant (selectivity to DEMA : 95%) than that of methylethylamine EMA (Table 3). Indeed, we obtained a total DEA conversion with five times less catalyst weight than for the reaction with EMA. This

144

result was expected on account of the change of amine reactivity with the N-substitution. On the other hand, DMEA does not react with methanol or with DEMA and there is no formation of TMA or of TEA. Moreover, the study of DMEA reactivity shows that this compound is much less reactive and can be converted only into DMA and ethanol. The presence of ethanol is more difficult to explain. However, the water issued from the dehydration of methanol into dimethylether can react with DMEA especially in the presence of the fresh catalyst; H2O + DMEA 2 CH3OH

^

•

DMA + EtOH (CH3)20 + H2O

Figure 1 shows that in the absence of methanol, initially, monoethylamine (EMA) and methylethylamine (MEA) are transformed rapidly. Surprisingly, we observe that: I) the imine, CH3-CH2-N=CH-CH3 is the main product. This compound is the intermediate in the formation of DEA (reactions 1 and 2); CH3CH2NH2 ^

CHsCH^NH + H2

CH3CH = NH + CH3CH2NH2

-NH3 ^

(1) + H2

CH3CH2N - CHCH3 ^ imine DEA

(CH3CH2)2NH (2) DEA

and 2) the enamine, CH3-CH2-(CH3)-N-CH=CH2; compound formed from the reaction of MEA with ethylenimine (reaction 3) which is further hydrogenated into DMEA ; CH3^

CH3^ CH3 CH3^ NH + CH3CH = NH««—^ N - C H ; ^i—^ ^ N H - C H = CH2 (3) CH3CH2'^ CH3CH2'^ NH2 CH3CH2'^ EMA imine MEA enamine DMEA

The hydrogenation steps are the rate limiting steps over the fresh catalyst. After an experiment lasting two hours, we observed a dramatic decrease of the activity, specially of the MEA conversion and the disappearance of the intermediates. Furthermore, the main products, DEMA and DEA, were formed. These results show that the adsorption properties of the catalysts vary very much during the reaction since ethylamine was mainly adsorbed and led to DEA. We suppose that these significant modifications could be due to the polymerisation of reaction intermediates such as imine or enamine. The polymers could remain on the catalyst surface and modify the nature and the number of active sites. In previous works, we remarked that these secondary reactions could modify the catalyst surface (16,17).

145

100

> o o c o

2 > C o

o

Figure 1: Reaction between monoethylamine and methyiethylamine over the 2.5Li*YPl catalyst. Methyiethylamine EMA Conversion (M)^ Ethylamine MEA conversion (x). Selectivity to Diethylamine DEA (O), imine ofDEA (0), Diethylmethylamine DEMA (^), enamine ofDEh/lA (<^) T : 210°C, PH2 • 10 bar, Catalyst weight: 5g, Ts : 1.3 s. This brief part of our work on the reactivity of intermediates or of by-products shows why an excess of methanol is necessary for the synthesis of DMEA from [MEA,MeOH,H2]. On the one hand, it acts as a methylating agent of monoethylamine and on the other hand, it inhibits strongly the alkylation reactions i.e the MEA disproportionation into DEA and TEA From this study the following reaction scheme describing the transformation of ethylamine to the main product DMEA and by-products was established. From a kinetic point of view, steps 2 and 3 are the rate determining reactions. It follows that the DMEA selectivity is increased by modifying the acido-basicity of copper chromite used as a catalyst. In fact, the change of the catalyst basicity can decrease the MEA condensation to form DEA without modification of the hydro-dehydrogenating properties of the catalyst which are necessary for the methylation of ethylamine with methanol (steps 1 and 3).

146 TEA + MEA -NH3 + H2/-H2O

+ MeOH

DEA MEA .NH3

^

DEFA

^

DEMA

(2)

+ MeOH/-H20

EMA (3)

DMEA + MeOH

EtOH + TMA

+ H2O

+ H2O -MeOH

EtOH + DMA ACKNOWLEDGMENTS The authors from the University of Poitiers are very grateful to ELF-ATOCHEM for their financial support and fruitful discussions.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

DE 3 627 592 (1986), BASF EP 0 107 457 (1982), ICI EP 0 103 176 Al (1979) DE 3 010 791 (1979), UCB SA H.W. Chen, J.M. White, J.G. Ekerdt, J. Catal., 99 (1986) 293 J.W. Evans, M.S. Wainwright, N.W. Cant, D.L. Trimm, J. Catal., 88 (1984) 203 A.K. Agarwal, N.W. Cant, M.S. Wainwright, D.L. Trimm, J. Catal., 43 (1979) 79 J.C. Lee, D.L. Trimm, M.A. Kohler, M.S. Wainwright, N.W. Cant, Catal. Today, 2 (1988)643 J. Volf and J. Pasek, Studies in Surface Science and Catalysis, 27 (1987) 105. Ed C. Cerveny A. Baiker and J. Kijenski, Catal. Rev. Sci. Eng., 27 - 4 (1985) 653 Z. Gaizi, Thesis, Poitiers, France (1990) A. Baiker, D. Monti and Y.S. Fan, J. Catal., 88 (1984) 81 H. Abe, Y. Yokota and K. Okabe, Appl. Catal., 52 (1989) 171

147 14. 15. 16. 17.

J. Barrault, S. Brunet and N. Essayem, J. Mol. Catal., 77 (1992) 321-332 Y. Pouilloux, V. Doidy, S. Hub, J. Kerveimal and J. Barrault, J. Mol. Cat., submitted to publication N. Essayem, Thesis, Poitiers, France (1991) J. Barrault, S. Brunet, N. Essayem, A. Piccirilli and C. Guimon, Studies in Surface Science and Catalysis, 78 (1993) 305