Ind. Eng. Chem. Prod. Res. Dev. 1982,21, 42-40
42
Selective Catalysis of the Transformation of Formamides to Nitriles with Extension of the Hydrocarbon Chain Ylchel V. E. Rodrlguez, Bernard Delmon,' and Helnz G. Vlehe' Groupe de Physico-Chimie Min6rale et de Catalyse, Universit6 Catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium
We report results concerning the transformation of formamides to nitriles using new catalytic systems working in the presence of a small amount of oxygen: mixed P-Bi-Mo oxides and similar compounds with W, U, and V instead of Mo. We have investigated the influence of temperature (400-500 "C),liquid space velocity, oxygen concentration, and the influence of H20, C02, CO, and pyridine. Oxygen promotes selectivity and prevents deacttvatlon. The activities of the catalysts range as follows: PBiMo > PBiW > PBiU > PBiV. Nitrogen electron withdrawing groups favor, while donating groups inhibit, the reaction. The reaction seems to obey an acid-base concerted mechanism. The reaction can be extremely selective (more than 91% molar yield for N-ethylformamide and 97% for formanilide, at reasonably high liquid space velocities of 3 h-').
Introduction Secondary formamides can undergo three kinds of reactions, (a) dehydrogenation, giving an isocyanate, (b) decarbonylation, giving an amine, and (c) dehydration, giving a nitrile R-
R-N=C=O
CHO
NH-
R-NHz
R-CN
These reactions are endothermic. The thermodynamics (Table I) (Ruger et al., 1974) show that reaction a would only be possible at very high temperatures. In this case (dehydrogenation), the use of oxygen might, in principle a t least, make the reaction easier. On kinetic grounds, reaction b and, especially, reaction c also require very high temperatures. At these high temperatures, one cannot hope to obtain one reaction selectively. It would therefore be useful to find selective catalysts. The use of such catalysts should also prevent secondary degradation reactions (especially if oxygen is used). Reaction c is particularly interesting. Indeed, the transformation of formamides to nitriles corresponds to the addition of one carbon atom to the hydrocarbon chain of the radical attached to the nitrogen atom of the amine from which the amide has been formed: a fiist step is the dehydration to an intermediary isonitrile, followed by a rearrangement of the latter to nitrile co
(RNH2--+)RNHCHO
-HZ0 +
[RN=C:]
75
RCEN
The additional carbon atom is potentially cheap, as it comes from synthesis gas (CO). Some Australian, French, and German patents (Becke and Pabler, 1969; Becke and Swaboda, l959,1960a,b, 1968, Muench et al., 1958) have previously described the synthesis of nitrile by catalytic reaction of formamides. The catalysts used were modified silicas; the reaction was reasonably selective (83-95%), but the activity was relatively low: the reported results were obtained at low feed rates (liquid hourly space velocities, LHSV, of 0.7-1.3 h-l). The yield per paw was in the range 33-85%. An important weakness of the catalysts was that they suffered rapid Laboratoire de Chemie Organique, UniversitS Catholique de Louvain, Place Louis Pasteur 1,1348 Louvain-la-Neuve, Belgium.
Table I. Thermodynamics In K p reaction 298 K a
b c
-14.095 t 0.909 +4.060
500K
800K
1OOOK
-5.600 + 4.400 t6.146
-0.843 + 6.135 +6.950
t0.596 t 6.275 t7.261
deactivation, in spite of the fact that many efforts have been aimed at extending time between regenerations. We have discovered a new catalytic system which, according to laboratory scale experiments, seems to bring major improvements, in terms of activity of catalysts, maximum yield obtained, and resistance of catalyst to deactivation. The catalysts are mixed oxides containing phosphorus, bismuth, and either molybdenum, tungsten, uranium, or vanadium. The reaction on these catalysts necessitates the use of a certain amount of molecular oxygen in the gas feed. This oxygen is absolutely necessary for maintaining the catalyst activity and selectivity, and especially for avoiding deactivation, but oxygen is otherwise inert with respect to the feed and products. We report here the results of catalytic experiments using various catalysta and various reaction conditions, including the presence of potential poisons (COz,CO, HzO,pyridine). We also investigated the difference of reactivity between various formamides. Experimental Methods and Operating Conditions Catalyst Preparation. The catalysts used in this work are similar to those generally used in ammoxidation. They are constituted of a major element, currently molybdenum, tungsten, uranium, or vanadium, associated to several other elements, especially bismuth and phosphorus, all of them in the oxide form. Our catalysts were prepared either by decomposition of the nitrates for all compositions of the catalysts or by the "citrate method" for the catalysts of composition P/Mo = 1/12 and Bi/Mo = 1/12 (atomic ratio) (Courty et al., 1973). The results are the same for these two methods of preparation. For the catalysts with P/Mo = 1/12 (atomic ratio), we started from phosphomolybdic acid and basic bismuth nitrate (BiONO3.HZ0).The other catalysts were prepared from the same bismuth salt, phosphoric acid, and, according to cases, ammonium heptamolybdate [(NH4),or Mol2Ou-4HZ0],paratungstate [5(NH4)z01zW03~7H20], metavanadate (NH4V03), or uranium nitrate [U02(N-
0196-4321/82/l22l-OO42$01.25lQ 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, : He
Table 11. Specific Surface Areas ~~~~
No. 1, 1982 43
~
Bi/Mo
P/Mo
1/12
0 1/24 1/12 116 114 113
P/Mo
Bi/Mo
1/12
0 1/50 1/24 1/16 1/14 1/12 1/10 119 118
112 111 P/x/Bi
111211
X
Mo
W V U
surface m z g-l
1.28 1.33 1.64 1.5 1.38 1.35 surface m * g-l 0.94 1.13 1.21 1.40 1.51 1.64 1.37 1.22 1.13 1.09 1.04 surface m 2 g-' 1.64 1.78 1.63 3.05
03)2*6H20].All catalysts were calcined at 600 "C for 16 h. We give hereafter an example of preparation for a typical catalyst of composition P/Mo = 1/12 and Bi/Mo = 1/12 (atomic ratio). Example of Preparation of a "BismuthPhosphomolybdate". Basic bismuth nitrate (1.83 g) is dissolved in 100 mL of water, acidified by 1 mL of concentrated nitric acid. Another solution is prepared by dissolving 11.17 g of phosphomolybdic acid in 100 mL of water, also acidified by 1 mL of nitric acid. This solution is added to the bismuth solution, evaporated under constant stirring, and dried at 130 "C for 2 h. The powder so obtained is calcined at 600 "C for 16 h. We obtain 9.70 g of mixed catalyst of the composition indicated above. Series of Catalysts Prepared. For the P-Bi-Mo mixed oxides, we prepared two series of catalysts, of composition (in atomic ratio): (i) 0 I P/Mo I1/3 with Bi/Mo = 1/12 and (ii) 0 5 Bi/Mo I 1 with P/Mo = 1/12. For the catalysts containing W, V, and U, instead of Mo, only one sample of each was prepared P/W/Bi = 1/12/1; P/V/Bi = 1/12/1; P/U/Bi = 1/12/1. The chemical composition and surface area of the various catalysts are reported in Table 11. The phase composition of both P-Bi-Mo series was studied by X-ray diffraction. All catalysts contain BiP04, Bi2Mo3OI2,and Moo3, except: P/Mo = 1/12, for Bi/Mo I 1/16, and Bi/Mo = 1/12, P/Mo = 1/3, where no Bi2Mo3OI2is detected; BiJ.Mo = 1/12 and P/Mo = 0, where no BiPOl is detected, as a consequence of the absence of P; P/Mo = 1/12 and Bi/Mo = 1/1, where no Moos is detected. A more complete characterization of most catalysts of the P-Bi-Mo series has been presented elsewhere (Rodriguez et al., 1981). Measurement of the Catalytic Activity. The apparatus used for the catalytic experiments is schematically represented in Figure 1. The reactant mixture is continuously fed to a preheater by a syringe pump. The catalyst bed is deposited on sintered glass. A mixture of helium and oxygen carried the vaporized reactant to the catalyst. The liquid products are collected in the trap P1,
preheater
Ichroma -tograph
f u rnace
Figure 1. Schematic representation of the catalytic test apparatus.
while the gaseous products are directly analyzed by gas chromatography. Analysis of the Reagents and Products. The analysis of the reagents and products is made by gas chromatography (Intersmat IGC 120 ML). The first column (4% Carbowax 20M on Anachron ABS 80-90 mesh) permits the separation of reagents, water, and nitriles. The second column (5% Versamide on Chromosorb W 80-100 mesh) is used for the separation of the higher molecular weight reagents. Expression of the Catalytic Results. The following definitions of conversion ( C ) , selectivity ( S ) , yield (Y), liquid hourly space velocity (LHSV), and gas hourly space velocity (GHSV) have been used. Conversion C is the total percentage of the reagent (formamide) transformed (either in the main reaction or in a side reaction). The selectivity, S, of transformation to a given product is the percentage of the molecules of the reagent transformed into this product, with respect to the overall number of molecules of the reagent transformed. In this work, the considered selectivity is relative to nitrile formation. The yield, Y , of a given product is expressed as the percentage of the number of injected molecules of reagent which are transformed into this product: Y = C X S . The liquid hourly space velocity, LHSV, is the volume of liquid feed flowing, per hour, over the volume of the catalyst bed LHSV = [vol. reagent (liquid)]/[vol. catalyst X h]
For expressing the flow of oxygen, we use the gas hourly space velocity, GHSV, which is defined in a similar way, taking the volume of the gas at NTP. Standard Conditions. We have systematically explored a number of reaction parameters. This has been done starting from a fixed set of standard conditions, and changing only one parameter at a time, for each series of experiments. The standard conditions are as follows: temperature, 450 "C; LHSV, 3 h-l; O2 GHSV, 4000 h-' (mol of 02/mol of reagent = 4); catalyst composition (atomic ratio), P/ Mo/Bi = 111211;reagent, N-ethylformamide. The measurements of conversions, selectivities, and yields correspond to a 5-h run. Two other sets of experimental conditions were used for the study of the influence of the N-substituents on the reactivity of the formamides. They will be summarized in the corresponding paragraph. Example of Catalytic Experiment. We describe here a typical experiment in the standard conditions, using
44
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
7 !
w
///
O
70
I
o Y
nitrlle
S
nitrile
I
L3C
O
O
F
6
40 450
350
550 T
("C)
Figure 2. Effect of temperature on yield, selectivity, and conversion. l
o
I fi
"I0
o
7
40
0
6
Figure 4. Influence of oxygen on yield, selectivity,and conversion. 1008
LI
75
I
o Y nNtrile 5 nitr le
o Y nitrile 0
S nitrile
1:
02 I formamide
~
c
.J
n
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 45 loo
7
100 010
75 o Y
nitrile
0 S
nitrile
50
a2 04 r, = P l M o a t o m
0
C o n or CO GHSV ( h ' ) Figure 6. Influence of COz and CO on yield and conversion.
Figure 8. Influence of the catalyst composition (P/Mo ratio).
- --- - - - -
o Y
nitrile
S
nitrile
0
A C
0 Y nitrile S nitrile
I
i
A c
501
0
rn
0
3 "10
6
pyridine
(weight)
Figure 7. Influence of pyridine on yield, selectivity,and conversion.
Figure 9. Influence of the catalyst composition (Bi/Mo ratio).
Table IV conditions
set A
temperature oxygen flow (GHSV) LHSV
550 "C 4000 h-l 3 h-I
Table I11 catalyst
selectivity, S
P/Mo/ Bi P/W/Bi P/U/Bi P/V/Bi
91% 79% 70% 3%
the COz concentration is important. This coking is accompanied by a strong deactivation, and the catalyst has to be changed after each experiment. Similar experiments were conducted with CO. The results with CO are probably not independent from those obtained with C02,as some oxidation of the former may take place. The resulta are shown in Figure 6. The nitrile yield decreases steadily as the amount of CO increases. The influence of pyridine in the gas feed has been measured by introducing various formamide-pyridine mixtures containing increasing amounts of pyridine (measured by the weight percent) in the liquid feed. The results are shown in Figure 7. The reaction is very sensitive to the presence of pyridine, which acta as a strong poison. The yield and selectivity fall in the presence of only 1% pyridine and vary relatively little upon further addition of poison, but the conversion remains high. Influence of the Nature and Composition of the Catalysts. Four types of catalysts were studied with the base metal x being Mo, W, U, and V, with the same composition P/x/Bi = 1/12/1. The results for the dehydration of formamides, in the standard conditions, on these catalysts are indicated in Table 111. We have also studied the influence of the composition of the P-Bi-Mo catalysts, using the two series described in the experimental section: (i) catalysts with a constant
'J
0.1 0.5 1 Bi / M o atom "
r2
set B 450 "C 1600 h-l 0.4 h-'
Table V condition condition A B yield, yield,
R- of RNHCHO C6H5 C,H,CH,
CH, CH,CH, n-Bu t-Bu
products
%
%
C,H,CN C,H,CH,CN C,H,CN CH,CN C,H5CH,NH, CH,CN CH,CH,CN n-BuCN t-BuCN
42 20 5 3 60 84 80 45 25
97.5 10 10 0 75 94 91 74 20
Bi/Mo atomic ratio (1/12) and in which the atomic ratio rl = P/Mo varied from 0 to 1/3; and (ii) catalysts with a constant P/Mo atomic ratio (1/12) and in which the atomic ratio r2 = Bi/Mo varied from 0 to 1. Figure 8 reports the yield, selectivity, and conversion vs. atomic ratio rl = P/Mo. Phosphorus has a promotor effect on the reaction up to rl = 1/12. Above this value, the yield of nitrile and the conversion decrease abruptly. Figure 9 shows a significant increase of the yield, with increasing r2 = Bi/Mo at low bismuth contents. A maximum selectivity and yield is observed for a catalyst composition corresponding to r2 = 1/12. Above this value, the yield decreases and reaches a "plateau" for r2 > 119. Influence of the Substituent on N. The reactivity of various alkyl- and aryl-substituted formamides have
46
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
Table VI time on stream, h 5 17 35 53 71 83
101 113
yield of nitrile, % 89.20 87.20 86.40 86.97 84.45 88.96 86.06 88.42
been determined on the standard catalyst under two different sets of operating conditions, indicated in Table IV. Table V summarizes the reaction products and the yields. Evaluation of the Resistance to Deactivation. We have tried to obtain preliminary indications concerning the stability of the catalytic activity. Table VI reports the activity during a 5-day run under the standard conditions with the standard catalyst. This experiment has only limited value, because of the relatively short period studied, and because the experiments were done at the laboratory scale. Nevertheless, this experiment suggests that the system is resistant to deactivation. Material Balance of the Reaction. Micro-reactor experiments do not allow precise material balance. The catalytic transformation of formamides in our reaction conditions being apparently very selective, we must discuss carefully the significance of the results. In principle, there are two ways of ascertaining the selectivity: (i) direct material balance based on the measured quantities of introduced formamide and collected nitrile; and (ii) detection of potential secondary products. The yields reported are calculated from the quantity of nitrile actually collected, compared to the quantity of reagent injected. Several losses, not due to lack of selectivity, may be expected in a laboratory micro-reactor (adsorption or deposition in cool parts, etc...). Our measured values for nitrile are thus lower than the actual amount of nitrile produced. We therefore must conclude that the real ualues for selectivity and yield are a t least equal to, but very likely higher than, the values measured in the experiments. This conclusion is strongly supported by measurements aimed at the detection of byproducts in an experiment in our standard conditions with N-ethylformamide. Expeded byproducts would be amines, C02,products of partial or total (C02, CO) oxidation, or isocyanate. We have not detected any trace of other products than water in the standard reaction conditions. We must therefore conclude that, in the standard conditions with N-ethylformamide, the conversion is very close to quantitative and the selectivity certainly equal or superior to 91% .
Discussion We shall first discuss separately the various groups of results reported above. A final paragraph will concern the mechanism of the reaction. Temperature and LHSV. Generally speaking, 450 O C seems to be the optimum temperature for the yield in nitrile. Below this temperature, the yield decreases, essentially because of a diminution of the conversion. At temperaturea higher than 450 "C, the selectivity diminishes because of the onset of degradation (oxidation) reactions. The low yields observed a t low LHSV correspond to a decrease of the selectivity (conversion remains high). This decrease must be attributed to a degradation of the primary product (nitrile) when the residence time is too high. It is more difficult to explain the decrease observed at high
LHSV; we indicated that this decrease is lower than the one which could be observed if the same turnover per unit surface area of catalyst for nitrile formation were maintained. Nevertheless, the overall effect corresponds to increasing degradation to C02. Oxygen. The role of oxygen is essential. Without oxygen, the conversion is relatively low, the selectivity extremely poor, and the catalyst deactivates. But the reaction is insensitive to the amount of oxygen introduced in a wide range (Figure 4). In the favorable concentration range, oxygen is inert with respect to reagent and products. Some oxidation is only observed when the quantity of oxygen is important. The quite special role of oxygen in the present catalytic reaction is unexpected. It is very likely that the favorable effect of oxygen must be ascribed to an action on the catalyst surface, which it maintains in an oxidized state. This oxidized state exerts a much more selective catalytic action than does the surface when it is not protected by oxygen. The role of oxygen in preventing deactivation, and the fact that a catalyst having worked without oxygen becomes black, suggest that in addition to maintaining the selectivity, oxygen might burn the small amount of carbon precursors formed on the surface as a side reaction. But this supposed oxidation of coke or coke precursors concerns only a very small quantity of organic matter, so that it was impossible to detect CO or C02in the gaseous products. Poisoning. On the basis of pure thermodynamic considerations, we would expect water to inhibit the reaction to nitrile, perhaps even making possible the parallel reaction to isocyanate. There is indeed an effect, but rather weak; the conversion remains high, and water must be in very high proportion (75%) to have an appreciable effect on the yield in nitrile. This decrease of the yield is accompanied by a degradation of COz and ethylamine (when N-ethylformamide is the reagent); this degradation could be interpreted as the result of the reaction of water with a hypothetically formed isocyanate. One could speculate that a cumulative effect takes place in the experiments with high water contents, namely that, together with the setting in of the degradation reaction, a poisoning by the ethylamine produced, similar to that brought about by pyridine, could take place. This cannot be excluded. Nevertheless, it is logical to ascribe to water alone the primary process, namely the degradation, as the hypothetical action of ethylamine is only possible if this primary process has taken place. The decrease of the yield in the presence of C02 is exclusively due to a diminution of the selectivity (the overall conversion remains high). A carbon deposit forms on the catalyst in the presence of C02, even when O2 is also present. C02acts as a poison. A possible explanation of the role of C02 is that it is an acid molecule and could neutralize basic centers involved in the catalytic reaction or compete with formamide for adsorption on these centers, thus altering selectivity, but without affecting the overall activity. The interpretation of the role of CO must probably follow the same lines. Nevertheless, in order to analyze the results correctly, it is necessary to take into account the reaction between CO and oxygen. One calculates that the gas feed would have the composition indicated in Table VI1 if the reaction between CO and O2 were complete. Considering the action of CO (Figure 6), the general trends are those which could be expected from the results with C 0 2 . But the details are not clear. If the amount of oxygen decreases as strongly with increasing CO as indicated in Table VII, the selectivity should decrease more
I d . Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 47 Table VI1 introduced quantity, mL/h
Scheme I
mL/h
co
0 2
CO,
0 2
0 500 1000 1500 2000
500 500 500 500 500
0 500 1000 1000 1000
500 250 0 0 0
co 0 0 0 500 1000
1oc
.
h
0
v
.--
a,
5
.-
5c
c
>
C -05
0
calcd compn of gas feed,
01
a* Figure 10. Reactivity (yield Y) v8. Taft parameter
0.7 a*.
strongly than with C02. A possible clue is that CO is mentioned in some patents (Becke and Pibler, 1969: Becke and Swaboda, l959,1960a,b,1968;Muench et al., 1958), as being used for the regeneration of catalysts, but these catalysts are different from ours. The effect of pyridine, considered as a basic molecule, can indicate the presence of acidic centers on the surface of the catalyst. When pyridine is introduced, the selectivity decreases. Beyond a certain quantity of introduced pyridine, the selectivity varies little. N-Substituents. It is possible to react formamides containing phenyl, methyl, ethyl and n-butyl as N-substituents, with good selectivity. The optimization for tert-butyl and benzyl substituents has not been carried very far. We have encountered experimental difficulties with the corresponding formamides, probably because of their high boiling points. For these molecules, the results are better for reactions at 550 "C. Because of this insufficient optimization, we feel it premature to speculate on the results reported in Table IV for these substituents, and especially on the conspicuously high yield in benzylamine in the reaction of N-benzylformamide. The reactivity of the formamides at 450 "C, as a function of the substituents on N, is presented as a function of the Taft parameter u* (Figure 10). We observe a regular progression; only the benzyl substituent falls widely apart from the curve. The acceptor substituents favor the reaction; the inverse effect is observed with the donor substituents. Catalysts, The discussion of the catalyst composition cannot be dissociated from that of the acid-base properties. It is indeed known that the addition of an acidic element such as phosphorus or a basic element such as bismuth to an acid catalyst such as molybdenum oxide can modify the nature of the acidic centers (Ai and Ikawa, 1975;Ai,1977). Moreover, the presence of bismuth also means that the presence of basic centers (Ai and Suzuki, 1972) on the catalyst should be expected. The fact that the catalyst with composition P/Mo/Bi = 1/12/1 seems the most active and selective catalyst would be attributed to the simultaneous presence, at that
I1 e
RNCH
RN=C
/OH
H
\ti
cat
RN=C
/
\H
OH =$ a c i d center basic center
1-H20
RC=N
A
RN=C:
composition, of acid and basic centers. Mechanism of the Reaction. The catalysts used in this work are sometimes referred to as salts of "heteropolyacids", although recent results obtained in our laboratory (Rodriguez et al., 1981) (see also Table 11)suggest that the heteropolyacid entity has decomposed during the preparation (calcination at 600 "C)of the catalysts. The structure of these catalysts is not yet known. The discussion of the mechanism of the reaction must therefore rest exclusively on the present kinetic experiments. The present experiments with C02 and pyridine suggest that basic, or acid, or most probably both basic and acid centers are involved. It is difficult to propose a detailed mechanism for the reaction. Our data concern essentially yields, and we lack a detailed kinetic analysis. However, the most reasonable hypothesis is that we have to deal with a concerted acid-base mechanism. We propose the Scheme I, where molybdenum and phosphorus would probably be the acid centers and bismuth the basic centers. This reaction scheme does not decide whether we have to do with a really synchronous or a concerted mechanism. It is difficult, at present, to speculate on the real significance of the dependence of nitrile yield on the Taft parameter. Conclusions At the present stage, and although the experiments have not yet gone beyond the laboratory scale, the new catalytic system seems promising. It constitutes a substantial improvement in comparison to the previously described system: the yields are higher (at least 91% molar compared to 33-85% 1, and obtained at higher space velocities (about 3 times as high). In addition, the catalyst seems to resist deactivation much better. A further advantage is that the reaction takes place at slightly lower temperature (400-500O C instead of 470-540 "C). The discovery that mixed oxides of molybdenum or tungsten, uranium, or vanadium on the one hand, and phosphorus and bismuth on the other hand, can catalyze the reaction which is essentially a dehydration (followed by an easy rearrangement) is quite unexpected, as these mixed catalysts were not known as dehydration catalysts. Another unexpected fact is the crucial role of oxygen. Oxygen is apparently necessary for maintaining the surface of the catalyst in an oxidized state. The correctly balanced catalytic centers (probably acid + basic) exist only on this oxidized surface. With many reagents, oxygen behaves like a rather inert gas: it does not attack the formamides nor the nitriles. Acknowledgment We want to thank IRSIA for a postgraduate fellowship (M.V.E.R.), Dr. P. Grange for his constant help, and Dr. I. Matsuura for his fruitful suggestions. Literature Cited Ai, M.; Suzuki, S. J . Catal. 1972, 26, 202. Ai, M.; Ikawa. T. J . Catel. 1975, 4 0 , 203. AI, M. J . Cetal. 1977, 50, 291. W e , F.; P&bler. P. (BASF) Ger. Offen. 1908957; CI. Int. C07C. Feb 22, 1989. Becke, F.; Swaboda, 0. P. (BASF) Qer. Offen. 1 0 8 6 7 1 0 CI. Int. C07C. Feb 26, 1959.
48
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 48-52
Becke, F.; Swaboda, 0. P. (BASF) Ger. Offen. 1 117 121; CI. Int. C07C,d, Jan 8, I960a. Becke, F.; Swaboda, 0. P. (BASF) Brevet francais No. 1250 165; CI. Int. BOIJCOIC, Nov 28, 1960b. Becke, F.; Swaboda, 0. P. (BASF) Brevet francais No. 1586750; CI. Int. C07C. July 8, 1988. Courty, P.; Ajot, H.;Marcilly, C.; Delmon, B. Powder Techno/. 1973, 7 , 21. Muench, W.; Ruoti, V.; Slhrestrl, G. Commonwealth of Australia, PS 214, 473 (Apr 14, 1958).
Rodriguez, M. V. E.; Delmon, 6.; Damn, J. P. “hoceedings, 7th International Congress of Catalysis”; Selyama, T., Tanabe, K., Eds.; Kodansha and Elsevier: Tokyo and Amsterdam, 1981; pp 1141-1153. Ruger, C.; Schwetlich, K.; Krammer, H. 2.Chem. 1974, 14, 152.
Received for review January 23, 1981 Revised manuscript received June 15, 1981 Accepted August 31, 1981
Deposition of Ammonium Bisulfate in the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia Shlmpel Matsuda,’ Tomolchl Kamo, Aklra Kato, and Fumito Nakajlma Hitachl Research Laboratoty, Hitachi Ltd., Hitachkshi, Ibarakkken, 3 19- 12 Japan
Teruo Kumura and Hlroshl Kuroda Kure Work, Babcock-Hitachi K.K., Kure-shi, Hiroshima-ken, 737 Japan
The selectiie catalytic reduction of NO, with ammonia in the presence of SO,, especially sulfur trioxMe (SO,),has been investigated. Although a catalyst composed mainly of titania Is resistant to the SO, poisoning, the catalyst activity decreases to ahnost zero in the presence of NH, and SO, below a certain temperature due to the deposition of sulfate of ammonia. A gas mixture containing varied amounts of NH, and SO, was pass& through a packed reactor placed in a fumace with a temperature gradient. It was found that ammonium bisulfate was the only product above 240 OC. The vapor pressure of ammonium bisulfate was found to be expressed as Pw;Pym = 1.14 X lo1* e x p ( 4 3 000/RT) (P in atm). In the actual NO, removal process the deposition of ammoniumbisulfate occurred at a higher temperature than that expected from the vapor pressure due to the capillary condensation of ammonium bisulfate in micropores of the catalyst. In the presence of NH, (200 ppm) and SO, (10 ppm) the catalytic activity decreases to zero below 282 O C considering the distribution of micropore radius of the catalyst.
Introduction Nitrogen oxides (NO,) from stationary combustion facilities such as power plant boilers comprise a considerable part of the total NO, emitted to the atmosphere. Several methods for the control of NO, have been proposed and tested using pilot plants (Bartok et al., 1969). It has been found that the selective catalytic reduction (SCR) process is most feasible for industrial application. By the end of 1980 several tens of commercial plants based on SCR process have been constructed in Japan, the largest one treating 2 million normal cubic meters per hour (Nm3/h) of flue gas at a power station (700 MW) (Kuroda and Nakajima, 1978; Nakajima et al., 1979). It has been known that NO, are selectively reduced by NH3 in the presence of a large excess of oxygen over various catalysts. The reaction is expressed as (Matsuda et al., 1978; Kasaoka et al., 1977; Kat0 et al., 1980) NO + NH, + ‘/40z= Nz + 3/2Hz0 (1) A catalyst used in a commercial plant must possess high activity and selectivity, since volume of flue gas to be treated is extraordinarily large. In addition the catalyst must be resistant to the SO, poisoning, since sulfur dioxide (SO,) and sulfur trioxide (SO,) are usually contained in an oil or coal-fired boiler flue gas. In the early stage of the SCR process development several catalysts, for example, V, Mo, and W oxides supported on A1203 carrier, and FepOs based catalysts, were tested. The life of these catalysts was found to be short because they were susceptible to the ois6-432i/a2/i221-004a$o 1.2510
SO, poisoning. A series of catalysts consisting mainly of titania have been developed. The Ti0,-based catalysts show a high activity, selectivity, and resistance to the SO, poisoning over a wide range of temperatures, 200-450 “C (Nakajima et al., 1979; Matsuda et al., 1978). In a boiler flue gas SO3 is contained in a few percent, usually 2-5%, of the total SO,. This renders a serious problem for the selection of the reaction condition of the SCR process because SO3 reacts with NH3 to form ammonium bisulfate (NH4HS04,denoted ABS in this paper) in the temperature range. Deposition of ABS on the surface of catalyst occurs below certain temperature depending on the concentrations of NH3 and SO,. We have experimentally determined the vapor pressure of ABS and established the reaction condition (temperature) of the SCR process according to the gas composition. The vapor pressure of ABS can not be measured directly. Therefore, it is defined as a product of the vapor pressure of NH3 and H8O4which are in equilibrium with liquid ABS. We have observed’that the decrease of catalytic activity by ABS deposition occurs at a higher temperature than that expected from the ABS vapor pressure due to the capillary condensation in micropores of the catalyst. Experimental Section Catalyst. The Ti02-based catalyst which consists of more than 70 atomic % TiOz and the remainder second component was used in the present study. The second components are selected from transition metals in group 5B (V), 6B (Cr, Mo, W),8 (Fe, Co, Ni), and 1B (Cu), and 0 1982 American Chemical Society
