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Reaction of Ethanol and Ammonia to Pyridine over ZSM-5-Type Zeolites
Reaction of Ethanol and Ammonia to Pyridine over ZSM-5-Type Zeolites F.J. van der Gaag, F. Louter, and H. van Bekkum Laboratory of Organic Chemistry, Delft Univers i ty of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
Reaction of ethanol and ammonia in the presence of air and water over 2SM-5-type catalysts affords pyridine as one of the major products. Other products include ethene, diethyl ether, ethylamine, acetonitrile and carbon dioxide. Data are presented for zeolite H-2SM-5, for H-Boralite and for ironcontaining H-2SM-5 zeolites. For H-Boralite and H-2SM-5 the pyridine selectivity is found to depend on the Si/B and Si/Al ratio, respectively. Increasing the iron content of H-2SM-5 (Fe) systems leads to higher activity whereas the effect on the selectivity is relatively small. Other variables studied include reaction temperature and space velocity. A mechanism for the ethanol-ammonia reaction to pyridines is suggested. INTRODUCTION Numerous 2SM-catalyzed reactions have been reported since the discovery of zeolite 2SM-5. The conversion of ethanol and methanol to hydrocarbons [1-3J, the alkylation of benzene [4J, the isomerization of xylenes [5J, and the alkylation [6] and disproportionation of toluene [7] are the most frequently studied. 2SM-catalyzed reactions in which ammonia is one of the reactants have received less attention [8-18J so far. The synthesis of amines from alcohols, ethers or olefins was reported in the patent literature [8,11,12,16J. The selectivity of the reaction of alcohols to primary amines was found to be increased by using zeolites like 2SM-5, erionite and clinoptilolite as the catalysts. Cu-H-2SM-5 proved to be a good catalyst for the ammoxidation of toluene [17J. The addition of water to the feed had a beneficial effect on the activity and stability. The production of (methyl-)pyridines from acetaldehyde and ammonia over zeolite 2SM-5 was reported in the patent literature [9J. Depending on the cation of the zeolite acetonitrile formation was observed. Recently high temperature (783 K) phenol amination to aniline over zeolite 2SM-5 was reported [13J and was found to yield 2-methylpyridine as the principal side product. Conversion of aniline under similar conditions also yielded 2-methylpyridine [14,15J. The conversion is low and many by-products are found. From the data available it is clear that in the absence of oxygen in the reactant mixture zeolite 2SM-5 is able to catalyze the conversion of ethanol and ammonia to amines. We have shown recently [18J that in the presence of oxygen zeolite ZSM-5 catalyses the reaction of ethanol and ammonia to pyridines. Here we present data on the reaction of ethanol and ammonia over some T-atom substituted 2SM-5-type zeolite catalysts, together with additional data for the reaction over H-2SM-5. Water was used as a component of the reaction mixture in view of earlier observed beneficial action and in' view of the option of direct processing of aqueous ethanol as obtained by fermentation. The influence of the degree of T-atom substitution, the space velocity and the reaction temperature has been studied. A mechanism is discussed. 763
764 (CA-5-2)
EXPERI:'lENTAL 1.
~Iat,"rials
---;;:-jj-- zeoli Les ..ave he"" IJl~epared as described for ZSM-5 in the literature [19J. Tetrapropylammoniwn bromide was used as the organic template. For Boralite, Aerosil was used as the silica source and sodium alwninate was substituted by a mixture of boric acid and sodium hydroxide. For Fe-containing ZSM-5 zeolite (Il-ZSM-5 (Fe» either Aerosil or waterglass was used as the silica source and iron(II)sulfate was added to the synthesis mixture. The Fe-containing zeolites are completely whi t e , so it can be concluded that iron is built in on T-atom sites. Silicalite \Vas prepared according to the patent literature [20], with NH.F added to the synthesis mixture. H-ZSM-5 and H-ZSM-5 (Fe) were analyzed by AAS.+ The zeolites were calcined overnight at 823 K before further use. H -exchange was performed by ion exchange in 0.5 M Hel at ca. 353 K for 30 minutes (10 g zeolite per 1 solution), followed by thorough washing with water and repeating the procedure. All catalysts were dry pressed, crushed and sieved to obtain a sieve fraction of particles of 1.4-2.0 rom.
2. Procedure The apparatus used in the catalytic experiments is essentially the same as described by Oudejans [21]. The catalyst (1-3 g) was placed in a fixed-bed continuous flow microreactor. The reactor was placed in an electronically controlled fluid bed oven. Ammonia and ethanol were fed to the reactor by bubbling air through thermostat ted saturators. Standard molar reactant composition was: NH3:CzHsOH:HzO:Oz = 1:3:6:9. The gas mixture leaving the reactor was analyzed by online gas chromatography using a 3 m 10% PEG on Chromosorb column with FID for organic components (temperature programmed operation) and a I m PORAPAK Q column with TCD for inorganic components (e. g. co and CO z, isothermal operation). Peak integration was performed by a computer connected to he gas chromatographs. If necessary products were identified by GC-MS using a Varian 44S mass spectrometer. RESULTS The results of the experiments on the conversion of ethanol and ammonia to pyridines will be presented in terms of single catalyst or reaction parameters on conversion and selectivity. 1. Effect of Si/B ratio of Boralite zeolites
Data of experiments with H-Boralite zeolites are presented in Table 1 and Figures 1 and 2. After 3 hours an essentially constant select i vity together with a Table 1. Effect of Si/B ratio in the ethanol-ammonia reaction Si/B
b
Blue
temp.
K
,
COllY.
ethene
diethyl ether
Selectivities ethyl- aceticacetaldehyde amine nitrile
10.5 10.5 10.5
8 8 8
613 633 653
20.1 50.0 98.2
8.5 12.6 22.6
2.1 0.8 ( 0.1
12.2 8.3 1.2
21 21
4.4 4.4
613 633
13.5 30.2
1.7 2.5
0.9 0.4
6.8 1.9
42 42 42 42
2.2 2.2 2.2 2.2
613 633 653 673
11.8 24.1 43.1 80.8
13.6 16.7 27.3 36.0
7.7 4.5 2.6 0.3
84 04
1.1 1.1 1.1
617 643 663
9.9 30.4 46.9
2.5 4.4 10.9
608 638 663
8.1 14.1 32.8
2.2 6.1 9.6
O~
11.6 14.5 ( 0.1
t
a
over H-Borali te.
(wt ~)c 0 pyritoluene dine
2-picoline
CO.
11.2 11.9 3.9
0.4 1.1 12.6
21. 7 31.3 17.1
1.0 ( 0.1 ( 0.1
29.7 16.8 16.9
11.1 10.4
16.7 22.8
< 0.1
( 0.1
31.5 34.9
< 0.1
( 0.1
27.9 26.9
8.8 5.9 2.9 2.2
7.6 5.8 3.6 0.5
3.1 3.4 4.8 7.2
< 0.1 < 0.1 ( 0.1
46.2 48.0 42.7 39.8
1.7 0.9 0.3 ( 0.1
11.3 12.9 14.9 13.2
1.0 0.5 0.4
15.8 6.3 1.8
16.1 23.8 19.0
0.2 15.5 15.2
5.7 ( 0.1 3.5
16.0 20.6 21.1
( 0.1 ( 0.1 ( 0.1
26.6 21.5 23.0
1.2 0.6 0.4
19.3 22.6 31. 7
6.4 3.0 2.7
( 0.1 2.6 5.6
12.6 11.9 0.5
16.0 7.0 2.6
< 0.1
< 0.1 ( 0.1
38.1 39.7 44.4
: Molar ratio NH3:C:tn~OH:Hr.O:O" = 1:3:6:9, WHSV (ethanol) = 0.17 h- I Reactant composition of synthesis mixture. C Composition if product mixture after 4 bra on stream.
•
( 0.1
F.J. van der Gaag, F. Lauter and H. van Bekkum
80
765
80
0/0
60
60
40
40 20
2
4
6
8 81 uc
o~!::::::::J:::!::::~~l_ 600
625
650 675 temp. K
Fig. 2. Temperature effect on ethanol-ammonia reaction over H-Boralite (2.2 Blue, conversion: +j selectivities: x, ethenej ., acetonitrilej 0, pyridine; 6, CO 2), slowly decreasing activity was observed. Major products in the ethanol-ammonia reaction in the presence of water and oxygen (air) - are ethene, ethylamine, acetonitrile and pyridine. Other components in the product mixture include acetaldehyde, diethyl ether and toluene. Ethyl acetate and methylpyridines are generally present in amounts < 0.5%. Oxidation to carbon dioxide is substantial but in general less than 30 wt % resp. 15% on carbon and distinctly lower than observed for Silicalite. For Boralite zeolites, the picture is very much like ZSM-5 zeolites: with increasing number of B atoms per unit cell the conversion increases, as the number of acid sites increases. The data show that there is an optimum in pyridine selectivity at relatively high Si/B ratios (ca. 42). We have reported similar results for H-ZSM-5 catalysts [18] with an optimum at Si/Al = 65. Caution has to be taken with respect to B-rich zeolites: here B-contaiing compounds were detected in the product mixture. This causes the catalyst composition to change. Fig. 1. Conversion and selectivity (wt %) plotted vs. B atoms per unit cell (conversion: x, +; selectivity to pyridine: e, OJ to CO 2: A, 6; 613 X, +, 0, 6: 633 K). x,
.0 .:
2. Effect of Fe content of Fe-containing ZSM-5-type catalysts Table 2 shows the effect of the Fe content of the ferro-alumino-silicate ZSM-5 catalysts. The conversion increases with increasing Fe content. It is noteworthy that the deep oxidation does not increase with higher Fe content of the zeolite system. For the other selectivities the picture is more complex. There are differences between catalysts prepared with waterglass and with Aerosil as the silica source. NH 3-TPD experiments also show differences between the two types of catalyst: The maximum of the "c"-peak (desorption near 750 K) of the ammoniadesorption is 20-30 K higher for samples prepared from waterglass compared to swnples prepared from Aerosil. ~aterglass as a silica source therefore appears to yield zeolites which, after H -exchange, have acid sites with a somewhat higher acid strength than Aerosil-prepared samples. From Table 2 it then seems that the H-ZSM-5 (Fe) zeolites with the more moderate acid properties give a higher selectivity for pyridine. Figure 3 gives plots of the conversion and selectivities of the reaction of
766 (CA-S-2) Table 2. Effect of SijFe ratio in the ethanol-anunonia reaction t emperature 615 K. sl/AI
Fe/uc
diethyl ether
0.55 0.16 0.06
30.8 14.7 11.4
12.0 B.' B.9
8 .s 5.1 9.3
0.2 2.3 2.9
0.50
28.5 13.3 7.7
17.g
5.B
( 0.1 D.B 3.3
""
170 57. 1200
43 .0 40
A'A'
189 1015
71 "0 71
:),09
,H:
(wt S e 1 e c t i v 1 t i •• t 0 ececoacetal- ethyl- ethyl nitrile dehyde 811110e acetate
ethane
Si;re
WG WG
cony.
•
~~~;~:c
0.00
B.8
11.6
0.9 17.2
",bpyridine
a
over H-ZSM-5 (Fe), 2-picoline
CO,
15.'"
3.2 2.8 6.2
10.3 5.9 '.2
25.5 38.2 29.8
3.2 10.1 8.9
21.6 22.2 211.3
O.B
2.5 '.B 4.0
12.2
48.1 26.3 37.S
10.,1 7.5
U
20.7 23.0 13.0
l.. l.B
3.'
2.B
'.2 3.0
a Molar ratio NII3;C:tllofOII:Hr.O:O~ = 1:3:6:9, WJlSV (ethanol) :: 0.17 h_ 1 • b COUlpositiun of product mixture after 4 hra on streOJD. C we = watcrglusa. Ai = .'.erosi!.
'1.
60
40 ~ <,
20
5
+-+-........
0
+ -.oL.:L+ + +
10
11
0
*
+-*
20 t (nr)
Fig. 3. Plot of the conversion and selectivities vs. runtime for the ethanolammonia reaction (catalyst: H-ZSM-5 (Fe), Si/Fe = 189, SijAl = 71, Aerosil (+, conversion; selectivities:., ethene; *, ether; x, CH 3CN; 0, pyridine;
li, CO2 )
,
ethanol and romnonia over a H-ZSM-5 catalyst (SijAl = 71, Si/Fe = 189, Aerosil) versus the time-on-stream. After approximately 4 hours a steady state is reached. Most catalysts used in this study give a similar picture.
F.J. van der Gaag, F. Louter and H. van Bekkum
767
3. Effect of space, velocitL!:Ii th H-ZSM-5 zeolite This effect has been studied with an aluminosilicate H-ZSM-5 catalyst, with Si/Al 55.5. This catalyst showed the best pyridine selectivity and a fair conversion in the comparison between aluminosilicate catalysts [18]. Results are shown in Table 3. Table 3. Effect of space velocity in the ethanol-ammonia reaction a W1ISV h-'
b
G.17 0.35 0.67 1. 00
conv ,
"
ethene
diethyl ether
12.8 6.4 3.7 2.9
6.4 5.5 7.3 6.0
7.4 12.8 18.2 20.8
Selectivities ethylethyl acetaldehyde amine acetate 3.2 6.3 6.3 9.8
1.7 3.1 4.2 4.4
~ Reactor temperature 615 K, II-ZSM-5 catalyst (Si/AI WHSV as g ethanol/{g.catalyst.hr).
t
5.7 6.8 7.6 6.1
0 (wt ") pyriacet cnl tr-i Ie dine
4.0 3.8
< 0.1 < 0.1
44.5 36.4 33.5 33.2
2-pico-
line
9.2 7.7 5.9 6.6
4-picoline
CO.
2.8
15.2 17.7 16.9 13.2
< 0.1
< 0.1 < 0.1
= 55.5).
As could be expected, the conversion decreases with increasing space velocity. The changes in selectivities are an indication for possible mechanistic pathways. The selectivity for acetaldehyde and ethylamine increase with increasing space velocity, whereas the selectivity for acetonitrile decreases, indicating that acetaldehyde and ethyl amine are first formed during reaction, whereas this is not the case for acetronitrile. Diethyl ether is another initial product, whereas pyridine obviously is formed later. Applying the high WHSV a series of experiments was performed at different reaction temperatures. The results are given in Table 4. The space velocity was kept constant at a value of 1.0 h- 1 • At the high temperature the product mixture simplifies and consists almost entirely of ethene and pyridine. Deep oxidation is relatively low. Table 4. Effect of reaction temperature on the ethanol-ammonia reaction to pyridines a Temp.
K
615 645 670 693
Cony.
"
ethene
diethyl ether
2.9 7.7 19.1 61.2
6.0 12.9 28.8 55.5
20.8 16.5 9.7 0.9
a H-ZSM-,5 catalyst, SilAl
Selectivities ethylethyl acetalacetate dehyde 8DIine 9.8 8.9 5.7 0.4
4.4 1.7 0.4 0.3
6.1 4.5 1.2 0.3
t
0 (wt ") pyriaceto-
nitrile
dine
2-picoline
CO.
< 0.1
33.2 32.3 35.8 30.2
6.6 4.1 3.4 0.9
13.2 16.3 13.2 8.9
3.0 2.0 2.5
= 55.5.
DISCUSSION As shown earlier [18], a comparison of the reaction conditions for the ethanol-ammonia reaction to pyridines and the conversion of ethanol without ammonia [3] shows that for the former reaction a considerably higher temperature is needed to obtain a fair degree of conversion. This is obviously caused by (partial) poisoning of catalytically active (acid) sites by ammonia or the product bases. Nayak and Choudhary [22] reported pyridine to be the most effective poison for H-ZSM-5 in the reaction of olefins and alcohols to aromatics. They also found an increased rate of coking on pyridine-poisoned catalysts (Si/Al = 17), probably caused by the lack of strong acid sites. These sites are said to remove coke by cracking reactions. On low Si/B zeolites we also detect coke formation, together with loss of boron from the catalyst whereas on Si-rich zeolites hardly any coke is formed. Relatively high acid sites density appears to promote coking in the present reaction. For the ethanol-ammonia reaction to pyridines two mechanisms can be envisaged: one involving carbenium ions, oligomerization, amination, ring closure and aromatization [23] and the other via dehydrogenation of ethanol to acetaldehyde, aldolizationlretroaldolization, reaction with ammonia, cyclization and aromatization. Because experiments substituting ethene or propene for ethanol as a feedstock under the present conditions show no pyridine formation, the second mechanism is preferred. High temperature reaction (693 K) at WHSV = 1.0 h- 1 yields
768 (CA-5-2) more ethene and more or less the same percentage of pyridine than lower temperature reactions. This is also in f'avc-rr of mechanism 2. SO'lJe pathways are depicted in a simplified way in Scheme 1. CH,CHO
non- acid sites
NH, or CH,CH 2NH2..
pyridine
acid sites
CH,CH2-O -CH 2-:\,
CHFCH 2
non-acid sites
Scheme 1. Possible mechanism for reaction of C.HsOH and NH. to pyridines. The first step for the formation of pyridines is supposed to be the dehydrogenation of ethanol to acetaldehyde or the amination of ethanol to ethylamine. Acetaldehyde and ethylamine and/or ammonia are then converted to pyridines. Both reaction steps have been separately reported in the literature. Matsumara et al. [24J showed that ZSM-5 zeolites containing little or no acid sites are able to catalyze the reaction of ethanol to acetaldehyde. They suspect Fe impurities in the zeolite to catalyze this reaction. The dehydration of ethanol is catalyzed by protonated ZSM-5 zeolites. In the ethanol-ammonia reaction the acid sites are at least partiallly poisoned by NH. and the product bases. The dehydrogenation reaction can be catalyzed by the resulting non-acidic sites though a detailed molecular picture is lacking. Matsumura et al. use an oxygen-free feed and a somewhat higher reaction temperature. The reaction temperature needed might be lowered by the presence of oxygen as a hydrogen acceptor. Isomorphous substitution of Fe for Si in the zeolite lattice seems to have the same effect: ferro-alumino-silicate zeolites are more active than iron-free catalysts. Table 1 shows that zeolites with increasing B content (except for the 8 B/uc sample) show a shift in selectivity from acetaldehyde to ethene. This can be exp l a i-ned by assuming that an increasing number of acid sites is in equilibrium with ammonia- and pyridine-poisoned sites. Chang and Lang [9J showed that acetaldehyde and ammonia can be converted to pyridines over H-ZSM-5 catalysts. The reaction is performed under oxygen-free conditions at 723 K and LHSV = 1. Our experiments showed that from an oxygencontaining feed pyridines can be formed at 693 K (WHSV = 1 h- 1 ) . Optimum selectivity to pyridines will be found at an acid sites content where catalysis of acetaldehyde production and -consumption to pyridines is balanced. This explains why iron-containing catalysts are more active: the rate of acetaldehyde production is higher (catalyzed by Fe) so the rate of pyridine formation can be increased. Comparing two catalysts from Table 2 (H-ZSM-5 containing 0.55 Fe/uc, prepared from waterglass and H-ZSM-5 containing 0.50 Fe/uc, prepared from Aerosil) illustrates again that a catalyst with a higher number of strong acid sites (the former catalyst) has a lower selectivity for pyridine (25.5%) than a zeolite with less and milder acidic sites (selectivity = 48.1%). Further reactions on the zeolites comprise: (i) conversion of ethanol to ethene and diethyl ether over acid sites (i i ) ammoxi dat Lon of ethanol or acetaldehyde to acetonitrile (iii) oxidation to acetic acid, followed by esterification to ethyl acetate (iv) amination of ethanol to ethylamine over acid sites (v) oxidation of ethylamine to acetonitrile (vi) total oxidation of reactants or products to carbon dioxide and water.
F.J. van der Gaag, F. Louter and H. van Bekkum
769
CONCLUSION Pyridine bases can be produced from ethanol and ammonia in the presence of oxygen using H-ZSM-5 type catalysts. ZSM-5 zeolites with partial or complete isomorphous substitution of B or Fe for Al in the zeolite lattice also are catalytically active in the pyridine forming reaction. Using boron the performance of the catalyst is somewhat less than an aluminosilicate H-ZSM-5 catalyst, whereas incorporation of small amounts of iron increases the conversion without bringing about large changes in selectivity. The yield of pyridine can be increased by increasing both reaction temperature and space velocity. At WHSV = 1.0 h- 1 and 693 K a product mixture consisting chiefly of ethene and pyridine (and carbon dioxide) is formed. A mechanism starting with partial oxidation of ethanol to acetaldehyde, followed by stepwise conversion to pyridine bases, can be suggested. ACKNOWLEDGEMEN1S The authors would like to thank Mr. J.P. Koot (Laboratory of Analytical Chemistry) for the AAS analyses and Mr. J.F. van Lent and Mr. N.M. van der Pers (Laboratory of ~1eta11urgy) for the XRD analyses of the used zeolites. Dr. H.W. Kouwenhoven is thanked for valuable discussions. REFEHENCES 1. S.L. Meisel, J.P. McCullough, C.H. Lechthaler, and P.B. Weisz, Chern. Techn., 2, 86 (1976). 2. C.D. Chang and A.J. Silvestri, J. Catal., 47, 249 (1976). 3. J.C. Oudejans, P.F. van den Oosterkamp, and H. van Bekkum, Appl. Catal., ~, 109 (1982). 4. e.g. K.H. Chandawar, S.B. Kulkarni, and P. Ratnasamy, Appl. Catal., ~, 287 (1982) . 5. US Patents 3,751,504 (1975); 3,751,506 (1975); 3,755,483 (1975); 4,159,282 (1979)j 4,159,283 (1979). 6. P.J. Lewis and F.G. Dwyer, Oil Gas Journal, 75, 55 (1977). 7. P.B. W~'isz, in "Stud. Surf. Sci. Catal., 7;-New Horizons in Catalysis", eds , and Kodanska Ltd., Tokyo, 1981). T. Seyama and K. Tanabe (Elsevier, Arnsterd~, 8. W.W. Kaeding, US Patent 4,082,805 (1982). 9. C.D. Chang and W.H. Lang, US Patent 4,220,783 (1980). 10. R. Bicker and R. Erckel, Eur. Patent 0,046,897 (1981). 11. Neth. Patent 82.01523 (1981). 12. H.S. Fales and J.O.H. Peterson, Eur. Patent 0,039,918 (1981). 13. C.D. Chang and P.D. Perkins, Zeolites, 3, 298 (1983). 14. C.D. Chang and P.D. Perkins, US Patent 4,388,461 (1983). 15. C.D. Chang and P.D. Perkins, Eur. Patent 0,082,613 (1983). 16. M. Deeba and W.J. Arnbs, Eur. Patent 0,077,016 (1983). 17. J. C. Oude.i ans , F. J. van der Gaag, and H. van Bekkum, in "Proc , Sixth Intern. Conf. Zeolites", Reno 1983, eds. D. Olson and A. Bisio (Butterworth, Guildford, 1984), p. 536. 18. F.J. van der Gaag, F. Louter, J.C. Oudejans, and H. van Bekkum, Appl. Catal., in press. 19. F.J. van der Gaag, J.C. Jansen, and H. van Bekkum, Appl. Catal., 17, 261 (1985) . 20. E.M. Flanigen and R.L. Patton, US Patent 4,073,865 (1978). 21. J.C. Oudejans, Thesis, Delft University o~ Technology (1984). 22. V.S. Nayak and V.R. Choudhary, Appl. Catal., ~, 251 (1984). 23. R.M. Dessau and R.B. LaPierre, J. Catal., 78, 136 (1982). 24. Y. Matsumura, K. Hashimoto, S. Watanabe;-and S. Yoshida, Chern. Lett., 1981, (1), 121.
Reaction of ethanol and ammonia in the presence of air and water over 2SM-5-type catalysts affords pyridine as one of the major products. Other products include ethene, diethyl ether, ethylamine, acetonitrile and carbon dioxide. Data are presented for zeolite H-2SM-5, for H-Boralite and for ironcontaining H-2SM-5 zeolites. For H-Boralite and H-2SM-5 the pyridine selectivity is found to depend on the Si/B and Si/Al ratio, respectively. Increasing the iron content of H-2SM-5 (Fe) systems leads to higher activity whereas the effect on the selectivity is relatively small. Other variables studied include reaction temperature and space velocity. A mechanism for the ethanol-ammonia reaction to pyridines is suggested. INTRODUCTION Numerous 2SM-catalyzed reactions have been reported since the discovery of zeolite 2SM-5. The conversion of ethanol and methanol to hydrocarbons [1-3J, the alkylation of benzene [4J, the isomerization of xylenes [5J, and the alkylation [6] and disproportionation of toluene [7] are the most frequently studied. 2SM-catalyzed reactions in which ammonia is one of the reactants have received less attention [8-18J so far. The synthesis of amines from alcohols, ethers or olefins was reported in the patent literature [8,11,12,16J. The selectivity of the reaction of alcohols to primary amines was found to be increased by using zeolites like 2SM-5, erionite and clinoptilolite as the catalysts. Cu-H-2SM-5 proved to be a good catalyst for the ammoxidation of toluene [17J. The addition of water to the feed had a beneficial effect on the activity and stability. The production of (methyl-)pyridines from acetaldehyde and ammonia over zeolite 2SM-5 was reported in the patent literature [9J. Depending on the cation of the zeolite acetonitrile formation was observed. Recently high temperature (783 K) phenol amination to aniline over zeolite 2SM-5 was reported [13J and was found to yield 2-methylpyridine as the principal side product. Conversion of aniline under similar conditions also yielded 2-methylpyridine [14,15J. The conversion is low and many by-products are found. From the data available it is clear that in the absence of oxygen in the reactant mixture zeolite 2SM-5 is able to catalyze the conversion of ethanol and ammonia to amines. We have shown recently [18J that in the presence of oxygen zeolite ZSM-5 catalyses the reaction of ethanol and ammonia to pyridines. Here we present data on the reaction of ethanol and ammonia over some T-atom substituted 2SM-5-type zeolite catalysts, together with additional data for the reaction over H-2SM-5. Water was used as a component of the reaction mixture in view of earlier observed beneficial action and in' view of the option of direct processing of aqueous ethanol as obtained by fermentation. The influence of the degree of T-atom substitution, the space velocity and the reaction temperature has been studied. A mechanism is discussed. 763
764 (CA-5-2)
EXPERI:'lENTAL 1.
~Iat,"rials
---;;:-jj-- zeoli Les ..ave he"" IJl~epared as described for ZSM-5 in the literature [19J. Tetrapropylammoniwn bromide was used as the organic template. For Boralite, Aerosil was used as the silica source and sodium alwninate was substituted by a mixture of boric acid and sodium hydroxide. For Fe-containing ZSM-5 zeolite (Il-ZSM-5 (Fe» either Aerosil or waterglass was used as the silica source and iron(II)sulfate was added to the synthesis mixture. The Fe-containing zeolites are completely whi t e , so it can be concluded that iron is built in on T-atom sites. Silicalite \Vas prepared according to the patent literature [20], with NH.F added to the synthesis mixture. H-ZSM-5 and H-ZSM-5 (Fe) were analyzed by AAS.+ The zeolites were calcined overnight at 823 K before further use. H -exchange was performed by ion exchange in 0.5 M Hel at ca. 353 K for 30 minutes (10 g zeolite per 1 solution), followed by thorough washing with water and repeating the procedure. All catalysts were dry pressed, crushed and sieved to obtain a sieve fraction of particles of 1.4-2.0 rom.
2. Procedure The apparatus used in the catalytic experiments is essentially the same as described by Oudejans [21]. The catalyst (1-3 g) was placed in a fixed-bed continuous flow microreactor. The reactor was placed in an electronically controlled fluid bed oven. Ammonia and ethanol were fed to the reactor by bubbling air through thermostat ted saturators. Standard molar reactant composition was: NH3:CzHsOH:HzO:Oz = 1:3:6:9. The gas mixture leaving the reactor was analyzed by online gas chromatography using a 3 m 10% PEG on Chromosorb column with FID for organic components (temperature programmed operation) and a I m PORAPAK Q column with TCD for inorganic components (e. g. co and CO z, isothermal operation). Peak integration was performed by a computer connected to he gas chromatographs. If necessary products were identified by GC-MS using a Varian 44S mass spectrometer. RESULTS The results of the experiments on the conversion of ethanol and ammonia to pyridines will be presented in terms of single catalyst or reaction parameters on conversion and selectivity. 1. Effect of Si/B ratio of Boralite zeolites
Data of experiments with H-Boralite zeolites are presented in Table 1 and Figures 1 and 2. After 3 hours an essentially constant select i vity together with a Table 1. Effect of Si/B ratio in the ethanol-ammonia reaction Si/B
b
Blue
temp.
K
,
COllY.
ethene
diethyl ether
Selectivities ethyl- aceticacetaldehyde amine nitrile
10.5 10.5 10.5
8 8 8
613 633 653
20.1 50.0 98.2
8.5 12.6 22.6
2.1 0.8 ( 0.1
12.2 8.3 1.2
21 21
4.4 4.4
613 633
13.5 30.2
1.7 2.5
0.9 0.4
6.8 1.9
42 42 42 42
2.2 2.2 2.2 2.2
613 633 653 673
11.8 24.1 43.1 80.8
13.6 16.7 27.3 36.0
7.7 4.5 2.6 0.3
84 04
1.1 1.1 1.1
617 643 663
9.9 30.4 46.9
2.5 4.4 10.9
608 638 663
8.1 14.1 32.8
2.2 6.1 9.6
O~
11.6 14.5 ( 0.1
t
a
over H-Borali te.
(wt ~)c 0 pyritoluene dine
2-picoline
CO.
11.2 11.9 3.9
0.4 1.1 12.6
21. 7 31.3 17.1
1.0 ( 0.1 ( 0.1
29.7 16.8 16.9
11.1 10.4
16.7 22.8
< 0.1
( 0.1
31.5 34.9
< 0.1
( 0.1
27.9 26.9
8.8 5.9 2.9 2.2
7.6 5.8 3.6 0.5
3.1 3.4 4.8 7.2
< 0.1 < 0.1 ( 0.1
46.2 48.0 42.7 39.8
1.7 0.9 0.3 ( 0.1
11.3 12.9 14.9 13.2
1.0 0.5 0.4
15.8 6.3 1.8
16.1 23.8 19.0
0.2 15.5 15.2
5.7 ( 0.1 3.5
16.0 20.6 21.1
( 0.1 ( 0.1 ( 0.1
26.6 21.5 23.0
1.2 0.6 0.4
19.3 22.6 31. 7
6.4 3.0 2.7
( 0.1 2.6 5.6
12.6 11.9 0.5
16.0 7.0 2.6
< 0.1
< 0.1 ( 0.1
38.1 39.7 44.4
: Molar ratio NH3:C:tn~OH:Hr.O:O" = 1:3:6:9, WHSV (ethanol) = 0.17 h- I Reactant composition of synthesis mixture. C Composition if product mixture after 4 bra on stream.
•
( 0.1
F.J. van der Gaag, F. Lauter and H. van Bekkum
80
765
80
0/0
60
60
40
40 20
2
4
6
8 81 uc
o~!::::::::J:::!::::~~l_ 600
625
650 675 temp. K
Fig. 2. Temperature effect on ethanol-ammonia reaction over H-Boralite (2.2 Blue, conversion: +j selectivities: x, ethenej ., acetonitrilej 0, pyridine; 6, CO 2), slowly decreasing activity was observed. Major products in the ethanol-ammonia reaction in the presence of water and oxygen (air) - are ethene, ethylamine, acetonitrile and pyridine. Other components in the product mixture include acetaldehyde, diethyl ether and toluene. Ethyl acetate and methylpyridines are generally present in amounts < 0.5%. Oxidation to carbon dioxide is substantial but in general less than 30 wt % resp. 15% on carbon and distinctly lower than observed for Silicalite. For Boralite zeolites, the picture is very much like ZSM-5 zeolites: with increasing number of B atoms per unit cell the conversion increases, as the number of acid sites increases. The data show that there is an optimum in pyridine selectivity at relatively high Si/B ratios (ca. 42). We have reported similar results for H-ZSM-5 catalysts [18] with an optimum at Si/Al = 65. Caution has to be taken with respect to B-rich zeolites: here B-contaiing compounds were detected in the product mixture. This causes the catalyst composition to change. Fig. 1. Conversion and selectivity (wt %) plotted vs. B atoms per unit cell (conversion: x, +; selectivity to pyridine: e, OJ to CO 2: A, 6; 613 X, +, 0, 6: 633 K). x,
.0 .:
2. Effect of Fe content of Fe-containing ZSM-5-type catalysts Table 2 shows the effect of the Fe content of the ferro-alumino-silicate ZSM-5 catalysts. The conversion increases with increasing Fe content. It is noteworthy that the deep oxidation does not increase with higher Fe content of the zeolite system. For the other selectivities the picture is more complex. There are differences between catalysts prepared with waterglass and with Aerosil as the silica source. NH 3-TPD experiments also show differences between the two types of catalyst: The maximum of the "c"-peak (desorption near 750 K) of the ammoniadesorption is 20-30 K higher for samples prepared from waterglass compared to swnples prepared from Aerosil. ~aterglass as a silica source therefore appears to yield zeolites which, after H -exchange, have acid sites with a somewhat higher acid strength than Aerosil-prepared samples. From Table 2 it then seems that the H-ZSM-5 (Fe) zeolites with the more moderate acid properties give a higher selectivity for pyridine. Figure 3 gives plots of the conversion and selectivities of the reaction of
766 (CA-S-2) Table 2. Effect of SijFe ratio in the ethanol-anunonia reaction t emperature 615 K. sl/AI
Fe/uc
diethyl ether
0.55 0.16 0.06
30.8 14.7 11.4
12.0 B.' B.9
8 .s 5.1 9.3
0.2 2.3 2.9
0.50
28.5 13.3 7.7
17.g
5.B
( 0.1 D.B 3.3
""
170 57. 1200
43 .0 40
A'A'
189 1015
71 "0 71
:),09
,H:
(wt S e 1 e c t i v 1 t i •• t 0 ececoacetal- ethyl- ethyl nitrile dehyde 811110e acetate
ethane
Si;re
WG WG
cony.
•
~~~;~:c
0.00
B.8
11.6
0.9 17.2
",bpyridine
a
over H-ZSM-5 (Fe), 2-picoline
CO,
15.'"
3.2 2.8 6.2
10.3 5.9 '.2
25.5 38.2 29.8
3.2 10.1 8.9
21.6 22.2 211.3
O.B
2.5 '.B 4.0
12.2
48.1 26.3 37.S
10.,1 7.5
U
20.7 23.0 13.0
l.. l.B
3.'
2.B
'.2 3.0
a Molar ratio NII3;C:tllofOII:Hr.O:O~ = 1:3:6:9, WJlSV (ethanol) :: 0.17 h_ 1 • b COUlpositiun of product mixture after 4 hra on streOJD. C we = watcrglusa. Ai = .'.erosi!.
'1.
60
40 ~ <,
20
5
+-+-........
0
+ -.oL.:L+ + +
10
11
0
*
+-*
20 t (nr)
Fig. 3. Plot of the conversion and selectivities vs. runtime for the ethanolammonia reaction (catalyst: H-ZSM-5 (Fe), Si/Fe = 189, SijAl = 71, Aerosil (+, conversion; selectivities:., ethene; *, ether; x, CH 3CN; 0, pyridine;
li, CO2 )
,
ethanol and romnonia over a H-ZSM-5 catalyst (SijAl = 71, Si/Fe = 189, Aerosil) versus the time-on-stream. After approximately 4 hours a steady state is reached. Most catalysts used in this study give a similar picture.
F.J. van der Gaag, F. Louter and H. van Bekkum
767
3. Effect of space, velocitL!:Ii th H-ZSM-5 zeolite This effect has been studied with an aluminosilicate H-ZSM-5 catalyst, with Si/Al 55.5. This catalyst showed the best pyridine selectivity and a fair conversion in the comparison between aluminosilicate catalysts [18]. Results are shown in Table 3. Table 3. Effect of space velocity in the ethanol-ammonia reaction a W1ISV h-'
b
G.17 0.35 0.67 1. 00
conv ,
"
ethene
diethyl ether
12.8 6.4 3.7 2.9
6.4 5.5 7.3 6.0
7.4 12.8 18.2 20.8
Selectivities ethylethyl acetaldehyde amine acetate 3.2 6.3 6.3 9.8
1.7 3.1 4.2 4.4
~ Reactor temperature 615 K, II-ZSM-5 catalyst (Si/AI WHSV as g ethanol/{g.catalyst.hr).
t
5.7 6.8 7.6 6.1
0 (wt ") pyriacet cnl tr-i Ie dine
4.0 3.8
< 0.1 < 0.1
44.5 36.4 33.5 33.2
2-pico-
line
9.2 7.7 5.9 6.6
4-picoline
CO.
2.8
15.2 17.7 16.9 13.2
< 0.1
< 0.1 < 0.1
= 55.5).
As could be expected, the conversion decreases with increasing space velocity. The changes in selectivities are an indication for possible mechanistic pathways. The selectivity for acetaldehyde and ethylamine increase with increasing space velocity, whereas the selectivity for acetonitrile decreases, indicating that acetaldehyde and ethyl amine are first formed during reaction, whereas this is not the case for acetronitrile. Diethyl ether is another initial product, whereas pyridine obviously is formed later. Applying the high WHSV a series of experiments was performed at different reaction temperatures. The results are given in Table 4. The space velocity was kept constant at a value of 1.0 h- 1 • At the high temperature the product mixture simplifies and consists almost entirely of ethene and pyridine. Deep oxidation is relatively low. Table 4. Effect of reaction temperature on the ethanol-ammonia reaction to pyridines a Temp.
K
615 645 670 693
Cony.
"
ethene
diethyl ether
2.9 7.7 19.1 61.2
6.0 12.9 28.8 55.5
20.8 16.5 9.7 0.9
a H-ZSM-,5 catalyst, SilAl
Selectivities ethylethyl acetalacetate dehyde 8DIine 9.8 8.9 5.7 0.4
4.4 1.7 0.4 0.3
6.1 4.5 1.2 0.3
t
0 (wt ") pyriaceto-
nitrile
dine
2-picoline
CO.
< 0.1
33.2 32.3 35.8 30.2
6.6 4.1 3.4 0.9
13.2 16.3 13.2 8.9
3.0 2.0 2.5
= 55.5.
DISCUSSION As shown earlier [18], a comparison of the reaction conditions for the ethanol-ammonia reaction to pyridines and the conversion of ethanol without ammonia [3] shows that for the former reaction a considerably higher temperature is needed to obtain a fair degree of conversion. This is obviously caused by (partial) poisoning of catalytically active (acid) sites by ammonia or the product bases. Nayak and Choudhary [22] reported pyridine to be the most effective poison for H-ZSM-5 in the reaction of olefins and alcohols to aromatics. They also found an increased rate of coking on pyridine-poisoned catalysts (Si/Al = 17), probably caused by the lack of strong acid sites. These sites are said to remove coke by cracking reactions. On low Si/B zeolites we also detect coke formation, together with loss of boron from the catalyst whereas on Si-rich zeolites hardly any coke is formed. Relatively high acid sites density appears to promote coking in the present reaction. For the ethanol-ammonia reaction to pyridines two mechanisms can be envisaged: one involving carbenium ions, oligomerization, amination, ring closure and aromatization [23] and the other via dehydrogenation of ethanol to acetaldehyde, aldolizationlretroaldolization, reaction with ammonia, cyclization and aromatization. Because experiments substituting ethene or propene for ethanol as a feedstock under the present conditions show no pyridine formation, the second mechanism is preferred. High temperature reaction (693 K) at WHSV = 1.0 h- 1 yields
768 (CA-5-2) more ethene and more or less the same percentage of pyridine than lower temperature reactions. This is also in f'avc-rr of mechanism 2. SO'lJe pathways are depicted in a simplified way in Scheme 1. CH,CHO
non- acid sites
NH, or CH,CH 2NH2..
pyridine
acid sites
CH,CH2-O -CH 2-:\,
CHFCH 2
non-acid sites
Scheme 1. Possible mechanism for reaction of C.HsOH and NH. to pyridines. The first step for the formation of pyridines is supposed to be the dehydrogenation of ethanol to acetaldehyde or the amination of ethanol to ethylamine. Acetaldehyde and ethylamine and/or ammonia are then converted to pyridines. Both reaction steps have been separately reported in the literature. Matsumara et al. [24J showed that ZSM-5 zeolites containing little or no acid sites are able to catalyze the reaction of ethanol to acetaldehyde. They suspect Fe impurities in the zeolite to catalyze this reaction. The dehydration of ethanol is catalyzed by protonated ZSM-5 zeolites. In the ethanol-ammonia reaction the acid sites are at least partiallly poisoned by NH. and the product bases. The dehydrogenation reaction can be catalyzed by the resulting non-acidic sites though a detailed molecular picture is lacking. Matsumura et al. use an oxygen-free feed and a somewhat higher reaction temperature. The reaction temperature needed might be lowered by the presence of oxygen as a hydrogen acceptor. Isomorphous substitution of Fe for Si in the zeolite lattice seems to have the same effect: ferro-alumino-silicate zeolites are more active than iron-free catalysts. Table 1 shows that zeolites with increasing B content (except for the 8 B/uc sample) show a shift in selectivity from acetaldehyde to ethene. This can be exp l a i-ned by assuming that an increasing number of acid sites is in equilibrium with ammonia- and pyridine-poisoned sites. Chang and Lang [9J showed that acetaldehyde and ammonia can be converted to pyridines over H-ZSM-5 catalysts. The reaction is performed under oxygen-free conditions at 723 K and LHSV = 1. Our experiments showed that from an oxygencontaining feed pyridines can be formed at 693 K (WHSV = 1 h- 1 ) . Optimum selectivity to pyridines will be found at an acid sites content where catalysis of acetaldehyde production and -consumption to pyridines is balanced. This explains why iron-containing catalysts are more active: the rate of acetaldehyde production is higher (catalyzed by Fe) so the rate of pyridine formation can be increased. Comparing two catalysts from Table 2 (H-ZSM-5 containing 0.55 Fe/uc, prepared from waterglass and H-ZSM-5 containing 0.50 Fe/uc, prepared from Aerosil) illustrates again that a catalyst with a higher number of strong acid sites (the former catalyst) has a lower selectivity for pyridine (25.5%) than a zeolite with less and milder acidic sites (selectivity = 48.1%). Further reactions on the zeolites comprise: (i) conversion of ethanol to ethene and diethyl ether over acid sites (i i ) ammoxi dat Lon of ethanol or acetaldehyde to acetonitrile (iii) oxidation to acetic acid, followed by esterification to ethyl acetate (iv) amination of ethanol to ethylamine over acid sites (v) oxidation of ethylamine to acetonitrile (vi) total oxidation of reactants or products to carbon dioxide and water.
F.J. van der Gaag, F. Louter and H. van Bekkum
769
CONCLUSION Pyridine bases can be produced from ethanol and ammonia in the presence of oxygen using H-ZSM-5 type catalysts. ZSM-5 zeolites with partial or complete isomorphous substitution of B or Fe for Al in the zeolite lattice also are catalytically active in the pyridine forming reaction. Using boron the performance of the catalyst is somewhat less than an aluminosilicate H-ZSM-5 catalyst, whereas incorporation of small amounts of iron increases the conversion without bringing about large changes in selectivity. The yield of pyridine can be increased by increasing both reaction temperature and space velocity. At WHSV = 1.0 h- 1 and 693 K a product mixture consisting chiefly of ethene and pyridine (and carbon dioxide) is formed. A mechanism starting with partial oxidation of ethanol to acetaldehyde, followed by stepwise conversion to pyridine bases, can be suggested. ACKNOWLEDGEMEN1S The authors would like to thank Mr. J.P. Koot (Laboratory of Analytical Chemistry) for the AAS analyses and Mr. J.F. van Lent and Mr. N.M. van der Pers (Laboratory of ~1eta11urgy) for the XRD analyses of the used zeolites. Dr. H.W. Kouwenhoven is thanked for valuable discussions. REFEHENCES 1. S.L. Meisel, J.P. McCullough, C.H. Lechthaler, and P.B. Weisz, Chern. Techn., 2, 86 (1976). 2. C.D. Chang and A.J. Silvestri, J. Catal., 47, 249 (1976). 3. J.C. Oudejans, P.F. van den Oosterkamp, and H. van Bekkum, Appl. Catal., ~, 109 (1982). 4. e.g. K.H. Chandawar, S.B. Kulkarni, and P. Ratnasamy, Appl. Catal., ~, 287 (1982) . 5. US Patents 3,751,504 (1975); 3,751,506 (1975); 3,755,483 (1975); 4,159,282 (1979)j 4,159,283 (1979). 6. P.J. Lewis and F.G. Dwyer, Oil Gas Journal, 75, 55 (1977). 7. P.B. W~'isz, in "Stud. Surf. Sci. Catal., 7;-New Horizons in Catalysis", eds , and Kodanska Ltd., Tokyo, 1981). T. Seyama and K. Tanabe (Elsevier, Arnsterd~, 8. W.W. Kaeding, US Patent 4,082,805 (1982). 9. C.D. Chang and W.H. Lang, US Patent 4,220,783 (1980). 10. R. Bicker and R. Erckel, Eur. Patent 0,046,897 (1981). 11. Neth. Patent 82.01523 (1981). 12. H.S. Fales and J.O.H. Peterson, Eur. Patent 0,039,918 (1981). 13. C.D. Chang and P.D. Perkins, Zeolites, 3, 298 (1983). 14. C.D. Chang and P.D. Perkins, US Patent 4,388,461 (1983). 15. C.D. Chang and P.D. Perkins, Eur. Patent 0,082,613 (1983). 16. M. Deeba and W.J. Arnbs, Eur. Patent 0,077,016 (1983). 17. J. C. Oude.i ans , F. J. van der Gaag, and H. van Bekkum, in "Proc , Sixth Intern. Conf. Zeolites", Reno 1983, eds. D. Olson and A. Bisio (Butterworth, Guildford, 1984), p. 536. 18. F.J. van der Gaag, F. Louter, J.C. Oudejans, and H. van Bekkum, Appl. Catal., in press. 19. F.J. van der Gaag, J.C. Jansen, and H. van Bekkum, Appl. Catal., 17, 261 (1985) . 20. E.M. Flanigen and R.L. Patton, US Patent 4,073,865 (1978). 21. J.C. Oudejans, Thesis, Delft University o~ Technology (1984). 22. V.S. Nayak and V.R. Choudhary, Appl. Catal., ~, 251 (1984). 23. R.M. Dessau and R.B. LaPierre, J. Catal., 78, 136 (1982). 24. Y. Matsumura, K. Hashimoto, S. Watanabe;-and S. Yoshida, Chern. Lett., 1981, (1), 121.