Energy & Fuels 1992,6, 760-763
760
Selective Syngas Conversion over Mixed AI-Fe Pillared Laponite Clay J. Barrault Laboratoire de Catalyse en Chimie Organique, U A 350, CNRS, 40 Avenue du Recteur Pineau, 86022 Poitiers Ckdex, France
L. Gatineau, N. Hassoun, and F. Bergaya' Centre de Recherche de la M a t i k e Diviske, C.N.R.S., l Brue de la Fkrollerie, 45071 Orlkam Ckdex 2, France Received April 17, 1992. Revised Manuscript Received August 3, 1992
The catalytic properties of a synthetic hectorite (laponite) pillared with mixed Al and Fe oxide are studied for syngas conversion. The results show two different behaviors depending on the ratio Fe/(Al + Fe) of the catalysts. AI-Fe pillars in alumina-rich samples are particularly active and selective to light olefins. The iron pillars in iron-rich samples show catalytic properties similar to a more conventional supported iron catalyst. Introduction
Pillared clays (PILC's) l+j have aroused a considerable interest as catalysts in recent years. Most of the early studies were motivated by the possible use of these materialsas catalystsfor hydrocarbon cracking,lbbut other important reactions were soon also considered, such as methano17p8or syngas9 conversion. This interest stems essentially from the possibility of controlling the surface chemistry (acidity, redox properties, ...) and the microtexture (surfacearea and micro- and mesoporosity)of the catalyst. This has already been explored by several authors, mainly by modifying either the nature of the clay or the nature of the pillaring species. For example,alumina and zirconia are thermally stable pillado and several transition metals such as iron, chromium, copper, or ruthenium can be added to Zr- and Al-PILC's either by impregnation or by incorporation into the pillar. Thus, Atkins and Ashtons showed that silanized PILC's impregnated with ruthenium, iron, and potassium are effective for the conversion of synthesis gas ta hydrocarbons. Similarly, Bartley and Burch" showed that impregnating a Zr-PILC with Cu nitrate leads to materials also active for the conversion of CO/H2into hydrocarbons. On the other hand, incorporating Cr(II1) into alumina To whom all correspondence should be addressed.
(1) Pinnavaia, T. J. Science, 1983,230,365. (2)Burch, R. Catal. Today, 1988,2,185. (3)Vaughan,D. E. W. In Recent developments inpillaredinterlayered clays. Perspectives in molecular sieve science; Am. Chem. Soc. Symp. 11, Toronto; American Chemical Society: Washington, DC, 1988,p 308. (4)Figueras, F. Catal. Rev. Sci. Eng. 1988,30,457. (5)Bergaya, F. In MatBriaur Argileux. Argiles b piliers; Decarreau,
lw,
A.,EM.;
p 511. (6)Bergaya, F.; Barrault, J. in Pillared Layered Structures, Current Trends and Amlications; Mitchell,. I. V. Ed.: Elaevier Amlied Science: London, NewYork, 1990; p 167. (7)Kikuchi, E.;Hamaha, R.; Nakano, M.; Takihara, M.; Morita, Y. J. Jpn. Pet. Zmt. 1983,26,116. (8) Burch, R.; Warburton, C. I. J. Catal. 1986,97,511. (9)Atkins, M. P.; Ashton, A. G. European Patent Appl., EP 0.150.898 (1985). (10)Figueras, F.; Matrod-Bashi, A.; Fetter, G.; Thrierr, A.; Zanchetta, J. V. J. Catal. 1989, 119,91. (11)Bartley, G. J. J.; Burch, R. Appl. Catal. 1986,28,209.
__
pillars was found to increase the kinetic stability and activity of the catalyst in decane cracking as compared to a PILC containing no chromium.12 The syntheses of mixed pillared clays are fewer, especially AI-Fe pillared and it is difficult, even by Mkbauer spectroscopy,to have a clear idea of the location of the iron atoms. Nevertheless, these solids are very interesting because iron-promoted alumina catalysts are important in the (CO, H2) reaction.18 Moreover, we can expect, as shown in preliminary e~periments,'~ a shape selectivity similar to that for metal clusters encapsulated in zeolites and a selective transformation of syngas or methanol into light olefins20*21 or f ~ e l s . ~ ~ ~ ~ ~ In this paper, we report a study of the catalyticproperties of a synthetic hectorite (laponite) pillared by mixed Al and Fe oxides24for syngas conversion. In this paper, we report a study of the catalyticproperties of a synthetic hectorite (laponite) pillared by mixed Al and Fe oxides24for syngas conversion. (12)Carrado, K. A.; Suib, S. L.; Skoularikis, N. D.; Coughlin, R. W. Inorg. Chem. 1986,25,4217. (13)Koetapapas,A.; Suib,S.L.; Coughlin,R. W. Am. Chem. SOC.191st
National Meeting April 1986,Abstract 320. (14)Carrado, K. A.; Kostapapas, A.; Suib, S. L.; Coughlin,R. W. Solid State Zonics 1986,22, 117. (15)Skoularikis, N.D.; Coughlin, R. W.; Koetapapas, A.; Carrado, K. A.; Suib, S. L. Appl. Catal. 1988,39,61. (16)Lee, W. Y.; Tatarchuk, B. J. Hyperfine Interactions 1988,41, 661. (17)Lee,W. Y.;Raythata, R. H.; Tatarchuk, B. J. J. Catal. 1989,115, 159. (18)Perrichon, V.; Charcoeset,H.; Barrault, J.;Forquy, C. Appl. Catal. 1983,7,21. (19)Barrault, J.; Zivkov, C.; Bergaya, F.; Gatineau, L.; Hauseoun, N.; Van Damme, H.; Mari, D. J . Chem. Soc., Chem. Commun. 1988,1403. (20)Nazar, L. F.; Ozin, G. A,; Hughes, F.; Godber, J.; Rancourt, D. Angew. Chem., Znt. Ed. Engl. 1983,22,624. (21)Denise, B.; Hamon, C.; Sneeden, R. P. A. Proc. 8th Znt. Congr. Catal. 1986,2,93. (22)Chang, C. D.; Lang, W. A.; Silvestri, A. J. J. Catal. 1969,56,268. (23)Ceasar, P. D.;Brennan, J. A.; Garwood, W. E.: Ciric, J. J. Catal. 1979,56,274. (24)Bergaya, F.; Hassoun, N.; Gatineau, L.; Barrault, J. Clay Miner. in press. (25)Zivkov, C., Thesis, Poitiers, France, 1987. (26)Barrault, J.; Renard, C. Appl. Catal. 1981,14,133.
0887-0624/92/2506-0760$03.00/00 1992 American Chemical Society
Energy & Fuels, Vol. 6,No. 6,1992 761
Catalytic Properties of Laponite
CO + Hz
4
C,H, + HzO
HzO + CO
+
Table I. Labels of t h e Samples.
[CnHzn+z,CnH~nl
COZ + Hz
Beyond the fact that it is always important to transform selectively syngas into light olefins, methanol, light alcohols, diesel, fuels, etc., this reaction is very sensitive to the superficial composition and to the porosity of the catalysts and it is very useful for their "characterization".
AI-Fe pillared laponites
Fe pillared laponites
AlwFem AlsoFelo Felm AleoFezo A170Fem AI,Fe,: ~tand y values represent the amount in % of AICl, and FeCla, respectively, added in the initial solution to the suspension of Na Laponite.
Experiment: Catalytic Test The CO-Hz reaction was carried out in a dynamic fixed-bed reactor at atmospheric pressure and at high t e m p e r a t ~ r e . lT~h*e~ ~ effluents of the reaction were analyzed with on-line gas chromatographs equipped with flame ionization and thermoconductivity detectors. Before the catalytic test, all the samples are treated in situ with hydrogen (1.5 L/h) at 500 O C for 10 h.
Samples The catalysts are given in Table I. The pillared laponite (a synthetic layer lattice colloidalmagnesium silicate) was obtained by a simple method of preparation. A solution of NaOH (0.12 M)and one of the solutiosn + Fe C13)previously prepared were slowly added to the laponite suspension vigorously stirred at 40 OC until the value of 30 mequiv (A1 + Fe)/g of clay was reached. After 2 days' ageing at room temperature, the clear supernatant liquid was eliminated and the remaining suspension dialyzed six times.24 The total amount of Fe retained by the clay (6,11,18, 27, and 34% ) increases with the amount of FeC13 added to the initial solution of Na laponite. In Al-Fe pillared laponites, a part of A1 in A13 cation is replaced by some of the total Fe retained by the clay. The isomorphic substitution of Al by Fe is about 2,9, and 26% of the total iron, respectively for Aldelo, Aldezo, and Al70Fe30 samples. The remaining amount of iron is outside the pillars. In Fe pillared laponite, Fe is present as iron oxide pillars and exchanged iron outside of the pillars.
Results 1. Catalysts Correspondingto a Ratio Fe/(Al + Fe)
< 0.3. 1.1. The activities of these samples ranged from
50 X 10-5to lo00 X mol g1h-l, respectively,for Algges to Al7oFe30samples (Figure 1). Laponitepillared only with aluminum pillars is inactive in the CO-HZ reaction. All these solids present an activation period before reaching a rather fair stability even at 450 OC. The investigation of these activation processes and their normalization clearly evidence different features depending on the iron content. The initial activation step is faster when the iron content increases, the evolution of the activity of the Al70Fe30 laponite sample being rather different from that of the other samples, which could be related to the some iron not located in the pillars. ~
(27) Barrault, J.; Guilleminot, A.; Achard, J. C.; Paul-Boncour, V.; Percheron-Guegan,A. Appl. Catal. 1986, 21, 307. (28) Barrault, J. J. Chim. Phys. 1986,83, 443. (29) Barrault, J.; Allouche, A.; Chafik, A.; Paul-Boncour, V.; Probst, S. h o c . 9th Int. Congr. Cotal. 1988,2,842. (30) Probet, S., Thesis, Poitiers, France, 1989. (31) Pinnavaia,T. J.; Tzou, M. S.; Landau, S. D.; Raythatha,R. H. J. Mol. Cotal. 1985, 27, 196.
I
0
J
10
20
30
40
50
60
70
h
F i g u r e 1. Syngas conversion over mixed AI-Fe pillared laponites: e, sample AlssFes; B, sample A l d e l o ; 0 ,sample A w e m ; and A,sample A178em. P = 0.1 MPa, T = 450 O C , H&O = 1.
1.2. The selectivities of the same sampleseither during the activation period (30% of the final activity) and after stabilization (or 70 h on stream) are presented in Table 11. At the beginning of each catalytic test, the CH4 selectivity is rather similar for all samples. The light hydrocarbons(c2-C~)fractions and the olefins content in the (Cz-C3) fraction decrease when the iron content increases and this result is not only due to a carbon monoxide conversion change. If we comparethe selectivity values obtained after the stabilization of the activities, the same variations can be observed even at higher conversion but the content of olefins and of heavier hydrocarbons (c6-C~) decreases, especially with the A170Fe30sample for which the CH4 selectivity noticeably increases. This could be the result of the importance of the out of pillar iron content on the catalytic properties and particularly on the hydrogenating properties. The hydrocarbon distribution (Schulz-Flory diagram, log WJn = f(n), where W,, represents the weight fraction of hydrocarbon containing n carbon atoms) (Figure 21, does not follow a polymerization process or the Schulz-Flory law. During the activation step and after stabilization (except for Al7$e30 sample), an unusual selectivity for Cz-C3 fraction is obtained with mixed Al-Fe pillared samples. As reported in the Introduction, this selectivity deviation is comparableto the shape selectivity observed during the conversion of syngas or methanol into light olefins on metal clusters encapsulated in zeolites, or to the hydrocarbondistribution obtained when metal carbonyls are deposited on more conventional support^.^^-^ The different behavior of the Al70Fe30 sample after stabilization, for which the shape selectivity disappears, (32) Vanhove, D.; Makambo, P.; Blanchard, M. J. Chem. SOC.,Chem. Commun. 1979,605. (33) Commereuc, D.; Chauvin, Y.;Hugues, F.; Basset, J. M. J. Chem. SOC.,Chem. Commun. 1980, 164. (34) Barrault, J.;Ghazi, M.; Renard,C. React.Kinet Catol. Lett. 1985, 27-2, 261.
Barrault et al.
762 Energy & Fuels, Vol. 6,No. 6,1992
catalyst
&des &&lo
Wezo A170Fe30 a
Table 11. Catalytic Properties of AI-Fe Pillared Laponites in CO-Hs Reaction* selectivity after stabilizn (wt % ) selectivity during activn step (wt %) CHA C2-G C&E olefins in c 2 4 3 CHI c 2 4 6 C6-C8 olefins in C&, 37.5 41.5 36.4 36.5
58.5 53.5 53.5 47.4
4.0 5.0 10.1 16.1
39.0 39.5 46.9 68.0
100.0 95.0 86.5 80.8
55.5 51.0 47.8 29.0
5.5 9.5 5.3 3.0
91.0 77.3 63.5 24.4
P = 0.1 MPa, H&O = 1, T = 450 "C. 0
1
2
3
4
5
0
"
1
2
3
4
5
n
I " " '
-1
-1
-2
-2
-3
-3
-4
-4
-5
-5
-6
-6
-7 Log
n
-7 A
--Wn r,
I
Figure 2. Schulz-Flory diagram obtained with (a) sample AlssFeb, (b) sample AlwFelo, (c) sample AlmFezo,and (d) sample A l 7 $ e ~ at (m) 30% activation and (A)after stabilization.
probably indicates that this sample is similar to catalysts rich in iron described later. 2. Catalysts Correspondingto a Ratio Fe/(Al + Fe) > 0.3. Physical characterization showed that these samples do not contain any aluminum and are pillared by pure iron oxides species, with large particles of iron oxides deposited outside the interlamellar space.% We can therefore expect different catalytic properties in the COHzreaction. The results presented in Figure 3 and in Table I11 demonstrate that these samples are similar to conventional iron catalysts used at high temperature after stabilization. As with previous samples, a rapid activation with time on stream is observed, but CHI is always the major product. There is no deviation from the ShulzFlory law after stabiljmtion, indicating that these ironrich catalysts are very different from the A1-Fe pillared catalysts described above.
Discussion These results show that mixed Al-Fe pillared laponites me selective catalysts in the CO-Hz reactions if the iron content is low with regard to the aluminum introduced for
' o 5 r activity g.h
1
2500 2000 1500
I f
t t I'
loo0
0 5 io 15 20 25 30 35 h Figure 3. Syngas conversion over Fe pillared laponites: 0 , sample A l d e a ; A sample Felm. P = 0.1 MPa, T = 450 O C , HdCO = 1.
pillaring. Very different bulk and superficial species are formed dependingon the Fe/A1ratio, leading to important changes in activity and selectivity,resulting in two classes of catalysts.
Catalytic Properties of Laponite
Energy & Fuels, Vol. 6, No. 6, 1992 763
Table 111. Catalytic Properties of Fe Pillared Laponites in CO-HZ Reaction selectivity after stabilizn (wt %) catalyst Aldem Feloo
CHI
CZCS
72.9 83.8
23.6 15.6
C~CS 3.4 0.6
olefins in CZ43 23.7 11.6
P = 0.1 MPa, H$CO = 1, T = 450 O C . 1. The first group is a representativeof mixed AlFe pillared clays for which we observed (a) an important olefin selectivity at any CO conversion and (b) a cutoff in the Schulz-Flory hydrocarbon distribution indicating an inhibition of the formation of hydrocarbons containing more than four carbon atoms and leading to an increase of the light olefins fraction (aromatization reactions are also observed but to a lesser extent). In previous studies, an important olefin selectivity was also obtained with particular iron-aluminum,’8 iron/ manganese oxides,2628and cobalt (or nickel)/rare earth o ~ i d e . ~ ~On - ~such O catalysts, we have evidenced that, due to the vicinity of the metal and the support or the promoter, geometric and electronic effects modify the CO and H2 adsorption properties, the hydrogenating properties, and the chain growth probability. In the case of true mixed pillared clays, mixed A1-Fe pillars resulting from the partial Substitution of A1 in the Keggin “A113” polymer (Le., the replacement by iron of a part of the aluminum in octohedral positions) is at the origin of the high olefin selectivity. As the isomorphic substitution is limited,24 any iron in excess located out of the pillars represents a different reaction site with different catalytic properties. Therefore, in the present study, the more representative samples of mixed AI-Fe pillared clays are those corresponding to an Fe/(Al + Fe) ratio C 0.3 and more precisely to 10.1. H2 TPR experiments and NMR characterization confirm the presence of two iron species,one of them being associated with aluminum in mixed (Al-Fe)l3 pillars.24 The specific light olefin fraction, or the so-called shape selectivity obtained with these materials, could also be the result of the formation of such mixed A1-Fe species or pillars dispersed in the interlamellar space. In fact, such mixed species would be more stable with regard to hydrolysis or carbonization, which can occur during the CO-H2 reaction, than pure metalspecies. Then, when a significant amount of well-dispersed and pure iron species is formed inside or outside the interlamellar space, we can observe a similar selectivity at the beginning of the reaction. Such particles would be rapidly transformed by sintering, carbonization, etc., in a conventional ironsupported catalyst with an increase of CH4 and paraffins selectivity on account of the high reaction temperature. In this case, the selectivity observed would not have the same origin as the one observed with zeolites,which directly depends on the size of the channels and cavities. The interlamellar space (8-9A) of the pillared clays cannot be at the origin of such a narrow range of hydrocarbons. The density of pillars is usually not taken into account, may also influence the course of the reaction. In agreement with P i n n a ~ a i awe , ~ ~have observed that though highly delaminated, this laponite had a high cation exchange capacity (CEO and physical tests show that the amount of pillaring is ~ignificant.~~ As compared to other, less delaminated clays, this could result in a higher density of pillars (smaller spaces between pillars) favoring the formation and the desorption of short-chain hydrocarbon. Thus, we observed a lower selectivity for light hydrocara
bons with a lower CEC montmorillonite pillared following the same experimental procedure.26 2. A second group is representative of iron oxide pillared clays or/and iron impregnated clays. With some of these materials, it is also possible to observe at the beginning of the reaction a selective transformation of syngas into light olefins and the interpretation presented above can explain that, very rapidly, the CH4 selectivity increases and methane is the major product after stabilization. We believe that the behavior is representative of pure iron oxide particles and we know from physical characterization that for ratio Fe/(Fe + Al) > 0.3 intercalated species (pillars or not) are composed mainly of iron oxide. Indeed, the evolution of catalytic properties is rather similar to those of iron catalysts described by Teichner et al.35-37 Iron species in or out of the pillars are slowly or rapidly transformed via redox reactions with carbon monoxide into iron (Fe),different iron oxides (especiallyFesOr),and various iron carbides. These reactions modify or suppress the initial iron pillar activity, favoring the migration of iron outside the interlamellar space, and finally produce a catalyst similar to a conventional iron catalyst.
Conclusion In the present work, we showed that the pillaring of laponites by a mixed solution of A1 and Fe species can produce very different catalysts depending on the Fe/(Al + Fe) ratio. 1. Laponites with mixed Al-Fe pillars are selective catalysts in the CO-H2 reaction for producing a very high fraction of light olefins. These properties are probably the result of the preferential formation of mixed FeAl,O, oxides, with specific and particular properties which respect the absorption of H2 and CO, and are much more stable with respect to sintering and carbide formationthan the supported iron particles in other catalysts. The use of a highly delaminated laponite may increase this selectivitybecause of the particularly high density of pillars in the interlamellar space. However, unlike zeolites, these catalysts do not show a selectivity directly related to the size of the pores or to a geometric factor. 2. Iron oxide pillared laponites, or laponites impregnated with iron oxides, rapidly become comparable to conventional catalysts owing to the transformation of the species initially present in contact with the reacting species.
Acknowledgment. The authors would like to thank Dr. R. Setton for his constructive comments and careful review of the manuscript. Registry No.
Al,1429-90-5;Fe, 1439-89-6;CO, 630-08-0.
(35)Reymond, J. P.; Pommier, B.; Meriaudeau, P.; Teichner, S. J. Bull. SOC.Chim. Fr. 1981, 5-6, 173. (36)Reymond, J. P., Merieudeau, P.; Teichner, S. J. J. Catal. 1982, 75, 39. (37)Blanchard,M.;Pommier,B.;Fbymond,J. P.;Teichner,S.J. Stud. Surf. Sci. Cotol. 1983, 16, 395.
