Amination of olefins by zeolites

J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

221

AMINATION OF OLEFINS BY ZEOLITES M. OEEBA 1, M. E. FORD 2, and T. A. JOHNSON 2 lEngelhard Corporatlon, Menlo Park, CN28, Edison, New Jersey 08818 (uSA) 2Ai r Products and Chemicals, Industrial Chemicals Department, Allentown, Pennsylvania 18195 (USA) ABSTRACT Direct addition of ammonia to olefins is catalyzed by acidic zeolites such as H-offretite, H-clinoptilolite, H-Y, and rare earth-exchanged Y. Selectivity to the corresponding amines is high (at least 97%). Conversions are controlled by temperature-dependent equilibria between the starting materials and product amines. Olefin amination occurs via Markownikoff addition. Reaction is believed to involve a carbocationic intermediate which is formed by interaction of the olefin with a surface proton or ammonium ion. Catalyst activity is proportional to the total number of strongly acidic sites as measured by ammonia chemisorption. Highest activities are obtained with small to medium pore acidic zeolites, such as H-clinoptilolite, H-erionite, and H-offretite. INTRODUCTI ON Conventional routes for large scale production of lower alkylamines involve reaction of an alcohol with ammonia in the presence of either acidic or supported metal catalysts (ref. 1). With the obvious exception of methanol, lower alcohols are typically obtained industrially by acid-catalyzed hydration of the corresponding olefin (ref. 2). In contrast, amination of an olefin to the corresponding amine would provide a more direct route to these commercially useful products. Direct conversion of ethylene to a mixture of ethylamines has been achieved only by treatment with alkali metal amides (refs. 3-6). Satisfactory yields (up to 70%) are obtained with ethylene. However, this process provides lor' yields of propylamines {less than 19%), owing to the lower susceptibility of propylene to nucleophilic attack (refs. 3,6). With higher olefins, this mode of amination is rendered impractical by complications that include isomerization and/or polymerization of the starting olefin, and disproportionation of the product amines (ref. 3). Moreover, despite extended study by many workers (ref. 7), reaction of olefins with ammonia over transition metals or transition metal oxides provides nitriles as the major products, rather than the corresponding amines. Zeolites have attracted considerable attention as catalysts for more than two decades as a result of their high activity and unusual selectivity for acid-catalyzed reactions (ref. 8). However, these properties have been applied almost exclusively to hydrocarbon processing. In this paper, amination of

222

ethylene, propylene, and isobuty1ene to the corresponding amines with acidic zeolite catalysts is described for the first time. The influence of catalyst parameters (acidity, pore size) and reaction temperature on the conversion and selectivity of amine production has been studied. Thermodynamic limitations of acid-catalyzed olefin amination have also been explored. EXPERIMENTAL Materials The zeolites studied are sUffinarized in Table 1.

All were commercially

TABLE 1 Zeolites evaluated for amination activity Zeolite Silica-alumina Rare earth Y H-chabazite-erionite H_y b H-c1inopti101ite H-mordenite H-erionite H-offretite

Amorphous 7.4 3.6 x 3.7 3.6 x 5.2 7.4 4.0 x 5.5 4.4 x 7.2

4.1 x 4.7 6.7 x 7.0

2.9 x 5.7 3.6 x 5.2 6.4 3.6 x 5.2

a

bSee ref. 9. See ref. 10. available except H-Y, which was prepared from the sodium form (ref. 9). Ammonia and all olef!ns were obtained from Air Products (Specialty Gases, Hometown, PAl in high purity (greater than 99.9%) and were used as received. Equipment Catalytic activity was evaluated in a Chemical Data Systems isothermal tubular reactor. The reactor consists of a 316 stainless steel tube (22.9 cm long x 0.64 cm internal diameter) mounted inside a close fitting metal block which is instrun~nted for temperature control. A thermowe11 extending axially next to the reactor was used to measure reactor temperature. With the exception of ethylene, feeds were metered to the reactor as liquids using Isco Model 314 high pressure syringe pumps. Ethylene was introduced as a gas, and its flow was controlled with a Brooks Model 5341 Flow Controller. Prior to introduction to the reactor, the feeds were vaporized and mixed in a countercurrent

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mixer maintained at 150·C. Flow rates of ammonia and the appropriate olefin were adjusted to obtain the desired mole ratio of reactants and total flow rate (GHSV at STP). Product analyses were carried out by on-line gas liquid chromatography with a Varian t10del 6000 Gas Chromatograph equipped with a 6' x 1/4" 2o;.~ Carbowax 2or~ on Chromosorb T column and a VISTA 402 Chromatography Data System. Catalyst Evaluation Catalyst samples (12-18 mesh particles) were heated (gO·C) in the reactor under nitrogen (10 secm at 1 atmosphere) for 16-18 hours. The temperature was then raised to the desired level over 3 hours. IHtrogen was shut off. Ammonia was introduced, and the desired pressure set with the back pressure regulator. Olefin was then introduced to obtain the desired feEd ratio. Acidity Measurements After activation at 400·C under nitrogen in a DuPont Model 951 Thermogravimetric Analyzer, ammonia was adsorbed onto the catalyst at 20°C. TPD indicated the number of strong acid sites, given as the millimoles of ammonia chemisorbed at 200°C per gram of catalyst. RESULTS AND DISCUSSION Effect of Reaction Temperature Conversion of ethylene, propylene, and isobutylene to the corresponding amines as a function of temperature over the representative catalysts. H-Y, H-mordenite, and H-erionite is compared in Figs. 1,2, and 3, respectively. A minimum temperature of 320°C was required to observe significant ethylene conversion. Mono- and diethylamines (greater than 9/1 weight ratio of mono/di amines) are the main products (greater than 93 wt% selectivity) up to 380°C. Formation of nitriles and higher olefins intervenes at higher temperatures. Amination of propylene was observed at 300·C; monoisopropylamine was the major product (greater than 97 wt% selectivity to total amines; greater than 93/7 weight ratio of mono/di amines). Isobutylene was selectively converted to i sobutylamine (greater than 98% yield) between 220"and 300°C. Although ethylene was effectively aminated by all catalysts, small pore zeolites such as H-erionite or H-clinoptilolite provided low conversions of propylene and no detectable conversion of isobutylene (Fig. 3) Correlation of Acidity to Catalyst Activity Activity of the zeolites investigated was correlated to the number of strong acid sites, as determined by ammonia TPD. Conversion of ethylene at 370°C (4/1

224

12

• ISOBUTYl ENE • PROPYlENE • ETHYlENE

10

8 CONV (%)

6

4

1000Hr·1

2

OL...---200

760 PSIA

'-JOO

L.-_ _ 400

T(OEG C)

Fig. 1. Conversion of ethylene (e), propylene (+), and isobutylene (-) over H-Y zeolite at 760 psia with 4/1 ammonia/olefin feed ratio at 1000/hr (GHSV at STP). ammonia/ethylene feed ratio, 760 psia, 1000 hr- l) depended directly on the number of acidic sites (Fig. 4). Possibly as a result of partial plugging of its pore structure, H-mordenite showed lower activity than anticipated. Owing to the larger critical dimensions of propylene, isobutylene, and their reaction products, the dependence of catalyst activity for production of hi.gher amines on acidity was difficult to deduce (cf. Fig. 3).

O.L..-------200

.L...-

300

L..-_ _

400

T (DEG C)

Fig. 2. Conversion of ethylene (e), propylene (.), and isobutylene (.) over H-mordenite at 760 psia with 4/1 ammonia/olefin feed ratio at 1000/hr (GHSV at

STP).

Mechanism of Olefin Amination Activity of zeolites as catalysts for olefin ami nation results from the highly acidic nature of proton-exchanged zeolites. At room temperature, ethylene and propylene are reversibly adsorbed by acidic zeolites (ref. 11). Changes in the infrared frequencies of acidic surface hydroxy groups during this process are indicative of hydrogen bonding, and of stronger adsorption of propylene (the more basic olefin). Formation of a pi complex between the surface hydroxy group and the olefin has been implicated as the mechanism of olefin chemisorption (refs. 11, 12). At elevated temperatures, reaction of the pi complex with ammonia, either adsorbed on the catalyst surface or from the gas phase, forms the adsorbed amine. Subsequent product desorption would

226

14

12

10

• PROPYLENE • ETHYLENE

8

CONV (%) 6

4

1000 H(l 760PSIA

2

OL...200

....L...

.L...-

300

400 T (OEG C)

Fig. 3. Conversion of ethylene (e) and propylene (.) over H-erionite at 760 psia with 4/1 ammonia/olefin feed ratio at 1000/hr (GHSV at STP)

_

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regenerate the catalytic site. Intermediacy of a cationic species is further supported by the relative ease of amination isobutylene > propylene> ethylene (see Figs. 1,2). The necessity of strongly acidic sites, and thus, of a protonated intermediate, is demonstrated by the negligible activity of the weakly acidic amorphous silica-alumina (Fig. 4). Further, non-acidic sodium-exchanged offretite and Y zeolites were inactive for olefin amination. Thermodynamic Limitation Conversion of olefins to the corresponding amines is limited by thermodynamic equilibrium between reactants and products. Calculated (ref. 13) equilibrium conversions indicate that amination is favored by low temperature, high pressure, and high ammonia/olefin ratio. However, high reaction temperatures are required to activate simple olefins. The temperature of activation depends on the structure of the olefin. Thus, ethylene is aminated only at high temperatures (over 320°C) by the strongest acid sites. Experimental data for ami nation of ethylene over H-Y zeolite with a 4/1 mole ratio of ammonia/ethylene at 760 psia show that thermodynamic equilibrium is attained at 390·C (Fig. 5). Propylene, which is more basic and forms a more stable cationic intermediate, is activated by somewhat weaker acid sites (those reactive at or above 300°C). As reaction temperature is increased, propylene conversion increases and approaches thermodynamic equilibrium conversions at 370°C, as shown for reaction over H-Y at 750 and 1000 psia (Fig. 6). CONCLUSIOfjS Direct amination of olefins has been demonstrated with zeolite catalysts. This process is characterized by strong acid catalysis to generate a carbocationic intermediate on the zeolite surface. Further reaction of this intermedi ate forms the product ami ne. With sma 11 to medi um pore zeoli tes, such as erionite, steric constraints on amination of methyl-substituted olefins are observed. Moreover, owing to the high temperatures required for olefin activation, conversion is limited by thermodynamic equilibrium between reactants and products.

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12.0

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1. SILICA ALUMINA 2. H-ZSM-5 3. REY 4. H-CHAB-ERIONITE 5. H·Y 6. H-CLlNOPTILOLlTE 7. H-MOROENITE B. H-ERIONITE 9. H-OFFRETITE

I

0.6

2

•

I

• 5

•

I

1.2

•

NUMBER OFSTRONG ACIDICSITES AT 200 DEG C (M MOLES/g CAT)

I

•

3

4

6

I

1.4

•

7

8

•

I

1.8

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1.8

•

9

Fig. 4. Plot of ethylene conversion at 370°C (4/1 ammonia/ethylene feed; 760 psia; 1000/hr, GHSV at STP) vs number of strong acid sites in the catalyst (mmoles ammonia chemisorbed at 200°C/gm of catalyst).

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TEMPERATURE (DEG C)

Fig. 5. Co~parison of calculated thermodynamic equilibrium conversion with observed ethylene conversion over H-Y zeolite as a function of temperature at 4/1 ammonia/ethylene, 750 psia, and 2000/hr GHSV at STP.

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TEMPERATURE (DEG C)

Fig. 6. Comparison of calculated thermodynamic equilibrium conversion with observed propylene conversion over H-Y zeolite as a function of temperature at 4/1 ammonia/propylene, (A) 1000 psia or (8) 750 psia, and 2000/hr GHSV at STP.

231

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