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Preparation and catalytic properties of iron oxide and iron sulphide pillared clays
Catalysis Today,2(1988)211-280 ElsevierSciencePublishersB.V.,Amsterdam-PrintedinTheNetherlands
271
Chapter 8
PREPARATION AND CATALYTIC PROPERTIES OF IRON OXIDE AND IRON SULPHIDE PILLARED CLAYS
C.I. WANTON Catalysis Research Group, Chemistry Department, Reading University, Whiteknights, P.O. Box 224, Reading, RG6 2AD, U.K.
8.1
INTRODUCTION
Catalytic interest in smectite-type clays pillared by inorganic polymeric cations dates from about 10 years ago. However, the actual interaction between clay minerals and polymeric cations has been studied extensively by soil scientists for the past 40 years, with the realisation that certain clays retained basal spacings of 1.4 nm on heating. It was concluded that these minerals contained irregularly distributed material between the clay layers, which prevented them coming together on expulsion of water. MacEwan (ref. 1) suggested that this material was a form of Fe and Al oxide. Since then partially interlayered clays have been reported in many soils and sediments (ref. 2) and as they influence both the chemical and physical properties of clays (refs. 3-7) much effort has been expended to determine their nature and the processes by which they are formed. Although a large number of ions are capable of forming natural interlayers in clays, they invariably consist of oxides of either Mg2+, Fe3+, Fe2+ or A13+, due to their solubility and natural abundance. In acid soils hydroxy-Al species tend to be the principal material responsible for interlayering,whereas hydroxy-Fe interlayers tend only to be found in very acid systems because of their low solubility at higher pHs. Stable hydroxy-Mg interlayers are only formed in neutral or alkaline soils. Consequently, the majority of work on natural interlayers has tended to concentrate on the interactions of Al and Fe hydrolysis products with clay surfaces. It is therefore advisable to consult the extensive literature on factors which affect the synthesis and stability of natural Fe interlayers in clay minerals (refs. 4,6,8-13) before attempting to synthesise Fe-pillared interlayer clays (Fe-PILC). 8.2 ORGANO-IRON PILLARED CLAYS The first PILC prepared with catalysis in mind, and in which Fe was a constituent of the pillars, were prepared in the late 1970's from organic cagelike structures such as Fe(II)-l,lO-phenanthroline(ref. 14) and Fe(I1) trisbipyridyl (ref. 15). Although these pillared materials had surface areas ranging
272
from 260 - 290 m2 g-1 and interlayer spacings of approximately 0.8 nm, they lacked thermal stability, collapsing around 250 - 350 OC. This lack of thermal stability was solved to some extent by Loeppert et al. (ref. 16) who prepared pillared materials using the above complexes, which were stable to 550 OC. Such stability was achieved by the incorporationof an excess of the complex (about 2.0 cation exchange equivalents (CEC) of complex cation), which reportedly led to the formation of a double layer complex between the clay layers. Thermal decomposition products of this double layer were then thought to be responsible for propping open the clay layers at higher temperatures, giving rise to interlayer spacings of 0.84 nm and 1.84 nm for pillared smectites and vermiculites, respectively.An obvious drawback, however, to these more thermally stable materials was their small surface areas (20 - 60 m2 g-1 at 5.50 OCf, since the incorporation of excess complex resulted in much of the internal surface area remaining inaccessible,the exception being the vermiculite pillared material which possessed a surface area of 236 m2 g-1 at 550 OC. Washing the calcined materials with 2.5 M HCl, thereby removing some of the interlayer material, was found to increase surface areas to about 400 m2 g-l at 550 oc. 8.3 INORG~IC IRON-PILLED
CLAYS
With the realisation in the late 1970's that thermally more stable PILC could be prepared from inorganic polymeric cations (refs. 17,181 the emphasis in PILC research shifted away from organic pillaring species to various metal hydroxy cations. Subsequently, a great many publications have appeared in the literature dealing with problems such as thermal stability, pore size, and possible catalytic applications, in the hope that such materials may rival xeolites as shape selective catalysts. Of these publications the great majority have been concerned with Al or Zr PILC, as these seem to offer the greatest thermal stability. In contrast, very few papers have been concerned with Fe-PILC. This is perhaps surprising given that Fe-PILC are cheaper to prepare and would not only have acidic properties, but would also contain pillars which in themselves may be catalytically active. In order to prepare Fe-PILC it is important to understand the hydrolysis chemistry of Fe(II1) solutions so that some control over the size of pillaring cations may be exercised. The solution chemistry of Fe is complex. Attempts have been made to determine the size and nature of polymeric Fe cations using techniques such as: electrometric measurements (ref. 191, conductometric titration (refs. 20,211, spectrophotometricmeasurements (refs. 22-25), Raman spectroscopy (ref. 261, magnetic measurements (ref. 271, EXAFS (ref. 28), diffusion measurements (ref. 29), ultracentrifugation(refs. 30-34) and electron microscopy (refs. 31-34).
273 Although the dissolution of Fe(II1) salts in water is known to result initially in simple hydrolysis products such as Fe(OH)2t, Fe(OH)2+, Fe2(0H)24t and Fe3(0H)45t (refs. 35,36), the above studies suggest that further hydrolysis leads to the formation of discrete spherical polycations. These gradually grow in size and have diameters ranging from 1.5 to 3.0 nm and eventually link up to give rods comprising 2 to 6 spheres. On further hydrolysis these rods appear to combine to give raft-like polycations 20 x 3 nm in size, which continue to grow, eventually resulting in the precipitation of ferric hydroxide. Although this whole process may be brought about by either prolonged aging at room temperature, addition of base, or aging at high temperatures, it is recommended that the addition of base is used, since this appears to offer the greatest degree of control over the size of the pillaring species. The first reference relating to the preparation of Fe-PILC from aqueous Fe(II1) solutions appeared in 1983 in work published by Tzou (ref. 37). Tzou carried out a comprehensive study on Fe-PILC, preparing these from aqueous solutions of FeC13, Fe(N03)3, Fe(C104)3 and Fe2(S04)3, studying the effect of various anions used, altering the OH/Fe ratio, ageing times and temperatures. Tzou's work was particularly significant in that it demonstrated for the first time that Fe-PILC could be prepared with interlayer spacings larger than the expected chlorite-like spacings. Indeed, spacings as large as 2 runwere, reported. Calclnation at 550 OC resulted in materials with surface areas from 244 to 343 m2 g-1, depending on the preparation conditions used. Tzou found that all the salt solutions previously aged at 25 'C for 24 h gave Fe-PILC with interlayer spacings of about 0.3 run.It was concluded that only simple hydrolysis products were responsible for pillaring in such materials. On increasing the OH/Fe ratio from 0.0 to 1.0, interlayer spacings were found to increase (from 0.3 to 1.4 nm), particularly for the chloride and nitrate systems, and thereafter increased slowly as the OH/Fe ratio was raised to 2.5. The sulphate system, in contrast, was found to give interlayer spacings of only 0.3 nm regardless of the OH/Fe ratio employed. Precipitation in this system was observed to occur at a lower OH/Fe ratio (1.5) than in the other systems (2.5). Since all precipitates were removed from the pillaring solutions before use, the small spacings observed were attributed to pillaring by the small polycations remaining in solution. Tzou also showed that large interlayer spacings of 1.7 nm could be obtained using longer aging periods or higher aging temperatures, providing precipitation was avoided. Using the pH of the various pillaring solutions as a guide to the degree of hydrolysis, he postulated that hydroxy Fe(II1) polymers increased in size to 1.5 nm in the first 1.5 h and then remained fairly constant in size over 1.5 to 30 h. During this latter period it was suggested that the polymers became more uniform in size, as the XRD lines became narrower, (see Figure 8.1). Aging
274
-0.25
-0.20 woo2'o -045
t/h Fig. 8.1. Variation in basal spacing and 20 angle with ageing time for Fe-PILC prepared from 0.2 M FeC13 (OH/Fe = 2.0), after ref. 37.
periods in excess of 30 h were then thought to result in a further gradual increase in the size of the polymeric cations. In contrast to the observations of Tzou, Yamanaka et al. (ref. 38) found that Fe-PILC prepared from partially hydrolyzed Pe(II1) solutions lacked thermal stability above 300 OC. This lack of thermal stability was attributed to the fact that hydroxy Fe(II1) cations were unstable in water and were partially washed out during the final washing step. To overcome this problem Yamanaka et al. introduced iron pillars by exchanging Na-montmorillonitewith partially hydrolyzed trinuclear aceto Fe(II1) ions, which when thermally decomposed gave oxide pillars between the silicate sheets. Using this procedure a thermally stable Fe-PILC was prepared which possessed a surface area and interlayer spacing of 280 m2 g-l and 0.7 run,respectively, after calcination at 500 OC. Adsorption of various probe molecules showed that the material calcined at 350 OC had a pore volume comparable to that of zeolites, ranging from 0.12 to 0.47 cm3 g-l depending on the adsorbate used. Using the dimensions of the probe molecules, it was estimated that the Fe-PILC contained pores approximately 0.7 to 0.8 nm in size. Using'pteparationprocedures similar to Tzou, Burch and Warburton (ref. 39) prepared Fe-PILC from an unrefined commercially available Na-montmorillonite (Brebent). The pillared materials obtained were found to exhibit no regular interlayer spacings, suggesting they were either highly interstratifiedor delaminated (ref. 40), and thus not true shape selective materials.
275 TABLE 8.1 Variation in the surface area of Fe-PILC with preparation and pretreatment conditions
Sample
Fe/clays
TblOC
OH/Fe
SC/m2 g-l
Sdlm2 g-l
se/m2 g-l
1
7 7 7 7 70 70
25 25 25 25 25 75
0.0 0.5 1.0 2.0 2.0 2.0
133 148 198 216 324 304
125 135 177 192 273 166
81 100 109 123 122 109
2 3 4 5 6 Sun01
Fe3+/meq
clay
bageing temperature Csurface area of Fe-PILC dried at 100 OC dsurface area of Fe-PILC calcined at 500 OC esurface area of sulphided Fe-PILC
Using a small excess of Fe3+ for the pillaring reaction (i.e. 7 mm01 Fe3+/meq) surface areas of the dried materials were found to increase with OH/Fe ratio in the manner shown in Table 8.1. The relatively large increase in surface area found on going from OH/Fe = 0.5 to 1.0 would seem to support the idea that a distinct polymer fraction forms in this range. When a larger excess of Fe3+ was used, the surface area of the PILC prepared using an OH/Fe = 2.0 was found to increase by approximately 30X. It is thought that the larger excess, although lengthening the final washing procedure considerably, leads to a more complete pillaring, resulting in a higher surface area. Aging at 75 OC results in a material with a similar surface area to that aged at 25 OC. As the polymeric cations produced at 75 OC would be expected to lead to a larger separation of the clay layers and therefore a larger surface area, it would seem that some of the internal surface generated through pillaring is covered by the larger polymeric cations. The
surface areas of the materials calcined at 500 'C generally indicate that
the Fe-PILC possess good thermal stability, suffering only small reductions due to a decrease in microporosity and an increase in mesoporosity. An exception to this was found, however, in the case of the material prepared from the solution aged at 75 OC. Here a large decrease in surface area was observed coupled with a very large increase in mesoporosity, with pores approximately 5 nm in diameter being generated. Since such pores are unlikely to be due to pillaring it is believed that their existence arises from ferric hydroxide precipitate which remains in the material after washing.
216 TABLE 8.2 Activity of the catalysts for the demetallizationof a crude oil
Catalyst
Ni removed/X
V removed/Z
Original clay Al-PILC Fe-PILC Fe-PILC(a) COlMOlAl2O3
11.3 7.5 49.1 53.8 60.3
23.8 19.0 58.9 58.1 85.1
Chat presulphided before testing activity
8.4
IRON SULPHIDE PILLARED CLAYS (SPILC)
The preparation of Fe sulphide pillared clays was first reported in 1987 by Burch and Warburton (ref. 41). In this work it was shown that oxide pillars could be converted into catalytically active sulphide pillars without the pillared structure collapsing. Using a mild sulphiding treatment, which involved heating the Fe-PILC in a 10X H2S/H2 mixture at 400 OC, the surface areas of PePILC were found to decrease as shown in Table 8.1, suffering a decrease in both micro- and mesoporosity. This reduction in surface area, apparently independent of the preparation procedure used, was believed to be due to a reorganisationof the structure of the pillar on replacement of oxygen with sulphur. Despite the decrease in surface areas, the nitrogen sorption isotherms were still found to be characteristicof pillared materials. It is believed that the clay layers were proppea apart by FeS, pillars, or possibly FeOxSy pillars, with Fe-0-Si linkages to the clay layers and oxygen in the pillars replaced by sulphur. 6.5
IRON PILLARED CLAYS AS DEMETALLIZATION CATALYSTS
With the realisation that sulphided Fe-PILC were likely to contain small well dispersed FeS, particles, known to have a mild hydrogenation activity (ref. 421, these materials were tested in the demetallizationof a deasphalted heavy crude oil (ref. 43). Using a high pressure batch reactor it was shown that sulphided Fe-PILC have a good activity for the removal of Ni and V from crude oil (see Table 8.21, being comparable in activity to a commercial Co/Mo/A1203 catalyst under the same conditions. It was also established that presulphiding was not essential to this activity since sulphur in the oil apparently resulted in in situ sulphiding of the catalyst. In contrast to Fe-PILC, the original clay and
,.
an Al-PILC based on this clay were found to have a low activity, indicating the important role of the Fe'pillars. In addition to possessing demetallizationactivity, Fe-PILC were also shown to have significant activity for hydrocracking. Distillation measurements on the recovered oil samples showed that the fraction of oil boiling below 250 OC
Fig. 8.2. Variation temperature.
increased
in the amount
from 36.7 to 47.3% on increasing
350 OC, compared
with the original
Regeneration
studies
the original
sulphided
ability 8.6
surface
The potential
of absorbing
substantial
since
amounts
is by impregnation. which
complete.
Burch and Warburton then Fe-PILC which
potential
to absorb H2S.
To investigate performed
require
offered
catalysts
contained
that since Fe-PILC
Fe203
at 500 OC. The Fe-PILC
(OH/Fe = 2.0) previously
sulphidation
by
is capable will
at moderate
of preparing
such
large Fe203
for sulphiding
to be
are prepared
by ion-
Fe203 particles
experiments
using a Fe-PILC
used had been prepared
Fe203 with the
were
previously
from an 0.2 M FeC13
aged at room temperature
results
uptake
to 'prepare supported
dispersed
N2 atmosphere
solution
substantial
temperatures
the opportunity
H2/8O%
85% of the
treatment.
this can lead to relatively
small, highly
clay. Typical
below 250 'C.
demonstrating
The usual method
higher
realised
calcined
mm01 Fe3+/meq
stable.
this, thermogravimetric
in a 2% H2S/18%
31% boiling
that approximately
traps was also investigated
allowing
for 70 h, using
are shown in Fig. 8.2. These
of
from 280 to
of H2S (ref. 44). An ideal absorbent
However,
subsequently
temperature
it is known that supported
and will be thermally
catalysts
as a function
TRAPS
Fe203 particles
particles
exchange
a sulphiding
to act as sulphur
(ref.391
well dispersed
temperatures,
to withstand
of Fe-PILC
contained
also revealed
area could be recovered,
CLAYS AS SULPHHR
Burch and Warburton
by an Fe-PILC
the reaction
oil which
on used catalysts
of such materials
IRON PILLARED
contain
of H2S absorbed
70
show that Fe-
PILC are capable of absorbing H2S up to a S/Fe ratio of 1.1 at 250 OC. Even at room temperature the absorption capacity was high (S/Fe = 0.75). Separate temperature-programmed-reduction experiments established that the weight loss observed above 250-300 OC was due to reduction of Fe(II1) to Fe(H) rather than to desorption of sulphur. Measurements of the surface areas of the calcined FePILC (273 m2 g-l) and the used material (195 m2 g-l) again demonstrated the stability of Fe-PILC to sulphiding.
x
S/Fe O.!
-__
._ _
100
300
TPC Fig. 8.3. Comparison of the uptake of H2S on Fe-PILC and on silica supported Fe oxide. x, Fe-PILC (ref. 39); o, 20% Fe203/Si02, hydrated (after ref. 45); 0. 20% Fe203fSi02, dehydrated (after ref. 45). Discounting the weight loss due to reduction, the normalized sulphur uptake by Fe-PILC was found to compare favourably with literature data relating to potential Fe203/Si02 absorbents (ref. 45). see Fig. 8.3. These results show that the absorption behaviour of Fe-PILC was similar to both hydrated and dehydrated Fe203fSi02, absorbing substantial amounts of H2S at room temperature, followed by increased absorption at higher temperatures. It was proposed that this dual
279
behaviour resulted from the presence of both hydrated and dehydrated Fe203 in the pillared material. Significantly, the overall uptake of H2S by the Fe-PILC was approximately twice that found for the supported Fe203, demonstrating the very real potential for Fe-PILC to act as sulphur traps. CONCLUSIONS Although the synthesis of Fe-PILC with homogeneous interlayer spacings requires great care, it has been demonstrated that such materials, with large interlayer spacings can be synthesised. Fe-PILC, therefore, have the potential to act as true shape selective catalysts and absorbents. In addition, even the irregularly spaced Fe-PILC exhibit interesting catalytic properties in demetallization,and are effective traps for sulphur-containingimpurities in process gas streams. REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
D.M.C. MacEwan, J. Soil Sci. 1 (1950) 90. C.I. Rich, Clays and Clay Mineral. 16 (1968) 15. C.K. Tweneboah, D.J. Greenland and J.M. Oades, Aus. J. Soil. Res. 5 (1967) 247. C.B. Roth, M.L. Jackson, E.G. Lotse and J.K. Syers, Israel. J. Chem. 6 (1968) 261. B.G. Davey and P.F. Low, Soil Sci. Sot. Amer. Proc. 35 (1971) 230. R.J. Tullock and C.B. Roth, Clays and Clay Mineral. 23 (1975) 27. S.A. El-Swaify and W.W. Emerson, Soil Sci. Sot. Amer. Proc. 39 (1975) 1056. A.L. Page and L.D. Whittig, Soil Sci. Sot. Amer. Proc. 25 (1961) 282. D.D. Carstea, Clays and Clay Mineral. 16 (1968) 231. P. Rengasamy and J.M. Oades, Aust. J. Soil Res. 15 (1977) 221. P. Rengasamy and J.M. Oades, Aust. J. Soil Res. 15 (1977) 235. R. Herrera and M. Peech, Soil Sci. Sot. Amer. Proc. 34 (1970) 740. J.M. Oades, Clays and Clay Mineral. 32 (1984) 49. V.E. Berkheiser and M.M. Mortland, Clays and Clay Mineral. 25 (1977) 105. M.F. Traynor, M.M. Mortland and T.J. Pinnavaia, Clays and Clay Mineral. 26 (1978) 318. R.H. Loeppert, M.M. Mortland and T.J. Pinnavaia, Clays and Clay Mineral. 27 (1979) 201. G.W. Brindley and R.E. Sempels, Clays and Clay Mineral. 12 (1977) 12. S. Yamanaka and G.W. Brindley, Clays and Clay Mineral. 26 (1978) 21. B.O.A. Hedstrom, Arkiv. Kemi. 6 (1953) 1. D.L. Zvyagintsev and S.B. Lyakhmanov, Russ J. Inorg. Chem. 13 (1968) 643. Biedermann, unpublished results cited by M-S Tzou, see ref. 37. J.Sutton, Nature 169 (1952) 71. C.K. Jorgensen, Acta. Chem. Stand. 8 (1954) 1502. A.R. Olsen and T.R. Simonson, J. Chem. Phys. 17 (1949) 1322. R.M. Milburn and W.C. Vosburgh, J. Amer. Chem. Sot. 77 (1955) 1352. M.M. Jones, E.A. Jones, D.E. Harman and R.T. Semmes, J. Amer. Chem. Sot. 83 (1961) 2038. L.N. Mulay and P.W. Selwood, J. Amer. Chem. Sot. 77 (1955) 2693. T.I. Morrison, G.K. Shenoy and L. Nielson, Inorg. Chem. 20 (1981) 3565. G. Jander and A.Z. Winkel, Anorg. Allg. Chem. 193 (1930) 1. T.G. Spiro, S.E. Allerton, J. Renner, A. Terzis, R. Bils and P. Saltman, J. Amer. Chem. Sot. 88 (1966) 2721. P.J. Murphy, A.M. Posner and J.P. Quirk, J. Coil. Interfacial Sci. 56 (1976) 270. P.J. Murphy, A.M. Posner and J.P. Quirk, J. Coll.InterfacialSci. 56 (1976) 284.
280 33
P.J. Murphy, A.M. Posner and J.P. Quirk, J.'Coll. Interfacial Sci. 56 (1976) 298. 34 P.J. Murphy, A.M. Posner and J.P. Quirk, J. Coil. Interfacial Sci. 56 (1976) 312. 35 J. Bjerrum, G. Schwarzenbach and L.G. Sill&, Stability Constants of MetalIon Complexes, Spec. Publ. Chem. Sot., London 17 (1964). 36 R.N. Sylva, Rev. Pure Appl. Chem. 22 (1972) 115. 37 M-S. TZOU, Ph.D Thesis, Michigan State Univ. (1983). 38 S. Yamanaka, T. Doi, S. Sako and M. Hattori, Mat. Res.Bull. 19 (1984) 161. 39 R. Burch and C.I. Warburton, unpublished results. 40 T.J. Pinnavaia, M-S. Tzou, S.D. Landau and R.H. Raythatha, J. Molec. Catal. 2.7(1984) 195. 41 R. Burch and C.I. Warburton, J. Chem. Sot., Chem.Communications.117 (1987). 42 N. Groot, Ph.D Thesis, University of Eindhoven. (1984). 43 R. Burch and C.I. Warburton, accepted for publication in Appl.Catal. 44 J.W. Geus and W.J.J. Van der Wal, Proc. 8th Int. Cong. Catal. 2 (1984) 245. 45 J.W. Geus, Appl. Catal. 25 (1986) 313.
271
Chapter 8
PREPARATION AND CATALYTIC PROPERTIES OF IRON OXIDE AND IRON SULPHIDE PILLARED CLAYS
C.I. WANTON Catalysis Research Group, Chemistry Department, Reading University, Whiteknights, P.O. Box 224, Reading, RG6 2AD, U.K.
8.1
INTRODUCTION
Catalytic interest in smectite-type clays pillared by inorganic polymeric cations dates from about 10 years ago. However, the actual interaction between clay minerals and polymeric cations has been studied extensively by soil scientists for the past 40 years, with the realisation that certain clays retained basal spacings of 1.4 nm on heating. It was concluded that these minerals contained irregularly distributed material between the clay layers, which prevented them coming together on expulsion of water. MacEwan (ref. 1) suggested that this material was a form of Fe and Al oxide. Since then partially interlayered clays have been reported in many soils and sediments (ref. 2) and as they influence both the chemical and physical properties of clays (refs. 3-7) much effort has been expended to determine their nature and the processes by which they are formed. Although a large number of ions are capable of forming natural interlayers in clays, they invariably consist of oxides of either Mg2+, Fe3+, Fe2+ or A13+, due to their solubility and natural abundance. In acid soils hydroxy-Al species tend to be the principal material responsible for interlayering,whereas hydroxy-Fe interlayers tend only to be found in very acid systems because of their low solubility at higher pHs. Stable hydroxy-Mg interlayers are only formed in neutral or alkaline soils. Consequently, the majority of work on natural interlayers has tended to concentrate on the interactions of Al and Fe hydrolysis products with clay surfaces. It is therefore advisable to consult the extensive literature on factors which affect the synthesis and stability of natural Fe interlayers in clay minerals (refs. 4,6,8-13) before attempting to synthesise Fe-pillared interlayer clays (Fe-PILC). 8.2 ORGANO-IRON PILLARED CLAYS The first PILC prepared with catalysis in mind, and in which Fe was a constituent of the pillars, were prepared in the late 1970's from organic cagelike structures such as Fe(II)-l,lO-phenanthroline(ref. 14) and Fe(I1) trisbipyridyl (ref. 15). Although these pillared materials had surface areas ranging
272
from 260 - 290 m2 g-1 and interlayer spacings of approximately 0.8 nm, they lacked thermal stability, collapsing around 250 - 350 OC. This lack of thermal stability was solved to some extent by Loeppert et al. (ref. 16) who prepared pillared materials using the above complexes, which were stable to 550 OC. Such stability was achieved by the incorporationof an excess of the complex (about 2.0 cation exchange equivalents (CEC) of complex cation), which reportedly led to the formation of a double layer complex between the clay layers. Thermal decomposition products of this double layer were then thought to be responsible for propping open the clay layers at higher temperatures, giving rise to interlayer spacings of 0.84 nm and 1.84 nm for pillared smectites and vermiculites, respectively.An obvious drawback, however, to these more thermally stable materials was their small surface areas (20 - 60 m2 g-1 at 5.50 OCf, since the incorporation of excess complex resulted in much of the internal surface area remaining inaccessible,the exception being the vermiculite pillared material which possessed a surface area of 236 m2 g-1 at 550 OC. Washing the calcined materials with 2.5 M HCl, thereby removing some of the interlayer material, was found to increase surface areas to about 400 m2 g-l at 550 oc. 8.3 INORG~IC IRON-PILLED
CLAYS
With the realisation in the late 1970's that thermally more stable PILC could be prepared from inorganic polymeric cations (refs. 17,181 the emphasis in PILC research shifted away from organic pillaring species to various metal hydroxy cations. Subsequently, a great many publications have appeared in the literature dealing with problems such as thermal stability, pore size, and possible catalytic applications, in the hope that such materials may rival xeolites as shape selective catalysts. Of these publications the great majority have been concerned with Al or Zr PILC, as these seem to offer the greatest thermal stability. In contrast, very few papers have been concerned with Fe-PILC. This is perhaps surprising given that Fe-PILC are cheaper to prepare and would not only have acidic properties, but would also contain pillars which in themselves may be catalytically active. In order to prepare Fe-PILC it is important to understand the hydrolysis chemistry of Fe(II1) solutions so that some control over the size of pillaring cations may be exercised. The solution chemistry of Fe is complex. Attempts have been made to determine the size and nature of polymeric Fe cations using techniques such as: electrometric measurements (ref. 191, conductometric titration (refs. 20,211, spectrophotometricmeasurements (refs. 22-25), Raman spectroscopy (ref. 261, magnetic measurements (ref. 271, EXAFS (ref. 28), diffusion measurements (ref. 29), ultracentrifugation(refs. 30-34) and electron microscopy (refs. 31-34).
273 Although the dissolution of Fe(II1) salts in water is known to result initially in simple hydrolysis products such as Fe(OH)2t, Fe(OH)2+, Fe2(0H)24t and Fe3(0H)45t (refs. 35,36), the above studies suggest that further hydrolysis leads to the formation of discrete spherical polycations. These gradually grow in size and have diameters ranging from 1.5 to 3.0 nm and eventually link up to give rods comprising 2 to 6 spheres. On further hydrolysis these rods appear to combine to give raft-like polycations 20 x 3 nm in size, which continue to grow, eventually resulting in the precipitation of ferric hydroxide. Although this whole process may be brought about by either prolonged aging at room temperature, addition of base, or aging at high temperatures, it is recommended that the addition of base is used, since this appears to offer the greatest degree of control over the size of the pillaring species. The first reference relating to the preparation of Fe-PILC from aqueous Fe(II1) solutions appeared in 1983 in work published by Tzou (ref. 37). Tzou carried out a comprehensive study on Fe-PILC, preparing these from aqueous solutions of FeC13, Fe(N03)3, Fe(C104)3 and Fe2(S04)3, studying the effect of various anions used, altering the OH/Fe ratio, ageing times and temperatures. Tzou's work was particularly significant in that it demonstrated for the first time that Fe-PILC could be prepared with interlayer spacings larger than the expected chlorite-like spacings. Indeed, spacings as large as 2 runwere, reported. Calclnation at 550 OC resulted in materials with surface areas from 244 to 343 m2 g-1, depending on the preparation conditions used. Tzou found that all the salt solutions previously aged at 25 'C for 24 h gave Fe-PILC with interlayer spacings of about 0.3 run.It was concluded that only simple hydrolysis products were responsible for pillaring in such materials. On increasing the OH/Fe ratio from 0.0 to 1.0, interlayer spacings were found to increase (from 0.3 to 1.4 nm), particularly for the chloride and nitrate systems, and thereafter increased slowly as the OH/Fe ratio was raised to 2.5. The sulphate system, in contrast, was found to give interlayer spacings of only 0.3 nm regardless of the OH/Fe ratio employed. Precipitation in this system was observed to occur at a lower OH/Fe ratio (1.5) than in the other systems (2.5). Since all precipitates were removed from the pillaring solutions before use, the small spacings observed were attributed to pillaring by the small polycations remaining in solution. Tzou also showed that large interlayer spacings of 1.7 nm could be obtained using longer aging periods or higher aging temperatures, providing precipitation was avoided. Using the pH of the various pillaring solutions as a guide to the degree of hydrolysis, he postulated that hydroxy Fe(II1) polymers increased in size to 1.5 nm in the first 1.5 h and then remained fairly constant in size over 1.5 to 30 h. During this latter period it was suggested that the polymers became more uniform in size, as the XRD lines became narrower, (see Figure 8.1). Aging
274
-0.25
-0.20 woo2'o -045
t/h Fig. 8.1. Variation in basal spacing and 20 angle with ageing time for Fe-PILC prepared from 0.2 M FeC13 (OH/Fe = 2.0), after ref. 37.
periods in excess of 30 h were then thought to result in a further gradual increase in the size of the polymeric cations. In contrast to the observations of Tzou, Yamanaka et al. (ref. 38) found that Fe-PILC prepared from partially hydrolyzed Pe(II1) solutions lacked thermal stability above 300 OC. This lack of thermal stability was attributed to the fact that hydroxy Fe(II1) cations were unstable in water and were partially washed out during the final washing step. To overcome this problem Yamanaka et al. introduced iron pillars by exchanging Na-montmorillonitewith partially hydrolyzed trinuclear aceto Fe(II1) ions, which when thermally decomposed gave oxide pillars between the silicate sheets. Using this procedure a thermally stable Fe-PILC was prepared which possessed a surface area and interlayer spacing of 280 m2 g-l and 0.7 run,respectively, after calcination at 500 OC. Adsorption of various probe molecules showed that the material calcined at 350 OC had a pore volume comparable to that of zeolites, ranging from 0.12 to 0.47 cm3 g-l depending on the adsorbate used. Using the dimensions of the probe molecules, it was estimated that the Fe-PILC contained pores approximately 0.7 to 0.8 nm in size. Using'pteparationprocedures similar to Tzou, Burch and Warburton (ref. 39) prepared Fe-PILC from an unrefined commercially available Na-montmorillonite (Brebent). The pillared materials obtained were found to exhibit no regular interlayer spacings, suggesting they were either highly interstratifiedor delaminated (ref. 40), and thus not true shape selective materials.
275 TABLE 8.1 Variation in the surface area of Fe-PILC with preparation and pretreatment conditions
Sample
Fe/clays
TblOC
OH/Fe
SC/m2 g-l
Sdlm2 g-l
se/m2 g-l
1
7 7 7 7 70 70
25 25 25 25 25 75
0.0 0.5 1.0 2.0 2.0 2.0
133 148 198 216 324 304
125 135 177 192 273 166
81 100 109 123 122 109
2 3 4 5 6 Sun01
Fe3+/meq
clay
bageing temperature Csurface area of Fe-PILC dried at 100 OC dsurface area of Fe-PILC calcined at 500 OC esurface area of sulphided Fe-PILC
Using a small excess of Fe3+ for the pillaring reaction (i.e. 7 mm01 Fe3+/meq) surface areas of the dried materials were found to increase with OH/Fe ratio in the manner shown in Table 8.1. The relatively large increase in surface area found on going from OH/Fe = 0.5 to 1.0 would seem to support the idea that a distinct polymer fraction forms in this range. When a larger excess of Fe3+ was used, the surface area of the PILC prepared using an OH/Fe = 2.0 was found to increase by approximately 30X. It is thought that the larger excess, although lengthening the final washing procedure considerably, leads to a more complete pillaring, resulting in a higher surface area. Aging at 75 OC results in a material with a similar surface area to that aged at 25 OC. As the polymeric cations produced at 75 OC would be expected to lead to a larger separation of the clay layers and therefore a larger surface area, it would seem that some of the internal surface generated through pillaring is covered by the larger polymeric cations. The
surface areas of the materials calcined at 500 'C generally indicate that
the Fe-PILC possess good thermal stability, suffering only small reductions due to a decrease in microporosity and an increase in mesoporosity. An exception to this was found, however, in the case of the material prepared from the solution aged at 75 OC. Here a large decrease in surface area was observed coupled with a very large increase in mesoporosity, with pores approximately 5 nm in diameter being generated. Since such pores are unlikely to be due to pillaring it is believed that their existence arises from ferric hydroxide precipitate which remains in the material after washing.
216 TABLE 8.2 Activity of the catalysts for the demetallizationof a crude oil
Catalyst
Ni removed/X
V removed/Z
Original clay Al-PILC Fe-PILC Fe-PILC(a) COlMOlAl2O3
11.3 7.5 49.1 53.8 60.3
23.8 19.0 58.9 58.1 85.1
Chat presulphided before testing activity
8.4
IRON SULPHIDE PILLARED CLAYS (SPILC)
The preparation of Fe sulphide pillared clays was first reported in 1987 by Burch and Warburton (ref. 41). In this work it was shown that oxide pillars could be converted into catalytically active sulphide pillars without the pillared structure collapsing. Using a mild sulphiding treatment, which involved heating the Fe-PILC in a 10X H2S/H2 mixture at 400 OC, the surface areas of PePILC were found to decrease as shown in Table 8.1, suffering a decrease in both micro- and mesoporosity. This reduction in surface area, apparently independent of the preparation procedure used, was believed to be due to a reorganisationof the structure of the pillar on replacement of oxygen with sulphur. Despite the decrease in surface areas, the nitrogen sorption isotherms were still found to be characteristicof pillared materials. It is believed that the clay layers were proppea apart by FeS, pillars, or possibly FeOxSy pillars, with Fe-0-Si linkages to the clay layers and oxygen in the pillars replaced by sulphur. 6.5
IRON PILLARED CLAYS AS DEMETALLIZATION CATALYSTS
With the realisation that sulphided Fe-PILC were likely to contain small well dispersed FeS, particles, known to have a mild hydrogenation activity (ref. 421, these materials were tested in the demetallizationof a deasphalted heavy crude oil (ref. 43). Using a high pressure batch reactor it was shown that sulphided Fe-PILC have a good activity for the removal of Ni and V from crude oil (see Table 8.21, being comparable in activity to a commercial Co/Mo/A1203 catalyst under the same conditions. It was also established that presulphiding was not essential to this activity since sulphur in the oil apparently resulted in in situ sulphiding of the catalyst. In contrast to Fe-PILC, the original clay and
,.
an Al-PILC based on this clay were found to have a low activity, indicating the important role of the Fe'pillars. In addition to possessing demetallizationactivity, Fe-PILC were also shown to have significant activity for hydrocracking. Distillation measurements on the recovered oil samples showed that the fraction of oil boiling below 250 OC
Fig. 8.2. Variation temperature.
increased
in the amount
from 36.7 to 47.3% on increasing
350 OC, compared
with the original
Regeneration
studies
the original
sulphided
ability 8.6
surface
The potential
of absorbing
substantial
since
amounts
is by impregnation. which
complete.
Burch and Warburton then Fe-PILC which
potential
to absorb H2S.
To investigate performed
require
offered
catalysts
contained
that since Fe-PILC
Fe203
at 500 OC. The Fe-PILC
(OH/Fe = 2.0) previously
sulphidation
by
is capable will
at moderate
of preparing
such
large Fe203
for sulphiding
to be
are prepared
by ion-
Fe203 particles
experiments
using a Fe-PILC
used had been prepared
Fe203 with the
were
previously
from an 0.2 M FeC13
aged at room temperature
results
uptake
to 'prepare supported
dispersed
N2 atmosphere
solution
substantial
temperatures
the opportunity
H2/8O%
85% of the
treatment.
this can lead to relatively
small, highly
clay. Typical
below 250 'C.
demonstrating
The usual method
higher
realised
calcined
mm01 Fe3+/meq
stable.
this, thermogravimetric
in a 2% H2S/18%
31% boiling
that approximately
traps was also investigated
allowing
for 70 h, using
are shown in Fig. 8.2. These
of
from 280 to
of H2S (ref. 44). An ideal absorbent
However,
subsequently
temperature
it is known that supported
and will be thermally
catalysts
as a function
TRAPS
Fe203 particles
particles
exchange
a sulphiding
to act as sulphur
(ref.391
well dispersed
temperatures,
to withstand
of Fe-PILC
contained
also revealed
area could be recovered,
CLAYS AS SULPHHR
Burch and Warburton
by an Fe-PILC
the reaction
oil which
on used catalysts
of such materials
IRON PILLARED
contain
of H2S absorbed
70
show that Fe-
PILC are capable of absorbing H2S up to a S/Fe ratio of 1.1 at 250 OC. Even at room temperature the absorption capacity was high (S/Fe = 0.75). Separate temperature-programmed-reduction experiments established that the weight loss observed above 250-300 OC was due to reduction of Fe(II1) to Fe(H) rather than to desorption of sulphur. Measurements of the surface areas of the calcined FePILC (273 m2 g-l) and the used material (195 m2 g-l) again demonstrated the stability of Fe-PILC to sulphiding.
x
S/Fe O.!
-__
._ _
100
300
TPC Fig. 8.3. Comparison of the uptake of H2S on Fe-PILC and on silica supported Fe oxide. x, Fe-PILC (ref. 39); o, 20% Fe203/Si02, hydrated (after ref. 45); 0. 20% Fe203fSi02, dehydrated (after ref. 45). Discounting the weight loss due to reduction, the normalized sulphur uptake by Fe-PILC was found to compare favourably with literature data relating to potential Fe203/Si02 absorbents (ref. 45). see Fig. 8.3. These results show that the absorption behaviour of Fe-PILC was similar to both hydrated and dehydrated Fe203fSi02, absorbing substantial amounts of H2S at room temperature, followed by increased absorption at higher temperatures. It was proposed that this dual
279
behaviour resulted from the presence of both hydrated and dehydrated Fe203 in the pillared material. Significantly, the overall uptake of H2S by the Fe-PILC was approximately twice that found for the supported Fe203, demonstrating the very real potential for Fe-PILC to act as sulphur traps. CONCLUSIONS Although the synthesis of Fe-PILC with homogeneous interlayer spacings requires great care, it has been demonstrated that such materials, with large interlayer spacings can be synthesised. Fe-PILC, therefore, have the potential to act as true shape selective catalysts and absorbents. In addition, even the irregularly spaced Fe-PILC exhibit interesting catalytic properties in demetallization,and are effective traps for sulphur-containingimpurities in process gas streams. REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
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