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Al-Pillared Montmorillonite. A Comparison of Re-Cations
G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
301
PREPARATION AND PROPERTIES OF LARGE-PORE RE/AI-PILLARED MONTMORILLONITE. A COMPARISON OF RE-CATIONS
J. STERTE Department of Engineering Chemistry I, Chalmers University of Technology, 412 96 Goteborg (Sweden)
SUMMARY RE/Al-pillared montmorillonites were prepared by cation exchange of montmorillonite with hydrothermally t r e a t e d (10O-16O0C, 12-240 h) mixtures of aluminum chlorohydrate (ACH) and chlorides of La, Ce, Pr, Nd or a mixture of RE-cations. Large-pore RE/AIpillared montmorillonites, characterized by basal spacings of about 26 and surface areas of 300-550 m2/g, were formed from solutions containing La, C e o r t h e mixture of REcations, refluxed for several days or tr e a t e d in autoclaves at higher temperatures for shorter times. Pr and Nd containing solutions did not result in large-pore pillared products a f t e r t r eat m en t at t h e conditions of this investigation. The large-pore RE/Al-pillared montmorillonites were found t o be similar in elemental composition t o a conventional Alpillared montmorillonite but thermally more stable, retaining a surface a r e a of about 300 m2/g a f t e r exposure to 8OOOC for 3 h.
a
INTRODUCTION In recent y e a r s g r e a t i n t e r e s t h a s b e e n f o c u s s e d on t h e p r e p a r a t i o n and t h e characterization of different types of pillared clays and also on possible applications for these materials primarily as catalysts or adsorbents. One of t h e most interesting potential applications for pillared clays is as active components in cracking catalyst formulations designed for cracking of heavy oil fractions. The commercial use of pillared clays for this application has, however, been limited by their lack of thermal and hydrothermal stability. Recently, McCauley (ref. I) found t h a t hydrothermally stable pillared sm ect i t es can be prepared by using hydrothermally t r e a te d pillaring solutions, containing mixtures of aluminum chlorohydrate (ACH) and a ceriurn(II1) salt in t h e preparation. Pillared smectites prepared from such solutions differ from conventional Al-pillared smectites in t ha t th e basal spacing as measured by X-ray diffraction analysis is considerably larger, i.e. 25-28
a compared with 18-20 A. This larger basal spacing is believed t o be due to
formation of a large Ce-bearing Al-polycation upon hydrothermal t r eat m en t of t h e solution. For use as heavy oil cracking catalysts, this larger spacing is another advantage over conventional Al-pillared smectites in addition t o t h e improved hydrothermal stability. McCauley found t h a t large-pore Ce/Al-pillared sm ect i t es can be prepared from solutions with Ce/AI molar ratios down to 1:52, hydrothermally t r eat ed either by refluxing for several days or by t r e a t m e n t in autoclaves at higher temperatures for shorter times.
302
NO systematic investigation on t h e dependence of properties of t h e Ce/AI- pillared products upon different synthesis parameters was, however, reported. Although t h e work of McCauley is primarily concerned with t h e preparation of Ce/Al-pillared smectites, he claims that, in addition to Ce(III), other r a r e e a r t h (RE) cations in admixture with ACH can be used for t h e preparation of large-pore pillared products. S t e r t e (ref. 2) investigated t h e effects of ti m e and temperature of hydrothermal t re a tm en t , Al-concentration, OH/Al-ratio of th e ACH, and La/AI molar ratio of t h e solution on t h e formation of large pore La/Al-pillared montmorillonite. The present paper reports on t h e preparation and characterization of pillared montmorillonites prepared by cation exchange of t h e montmorillonite with hydrothermally t re a ted solutions containing ACH in admixture with chlorides of La, Ce, Pr, Nd, or a commercial mixture of RE-cations. EXPERIMENTAL Montmorillonite A Wyoming Na+-Ca2+-montmorillonite (commercial designation, Voiclay SPV 200) was obtained from t h e American Colloid Company. Impurity quartz was removed by fractionation using conventional sedimentation techniques. The <2,um fraction, which was essentially f r e e from impurities as determined by X-ray powder diffraction (XRD) analysis, was used as starting material in all preparations. The cation exchange capacity
of t h e montrnorillonite was determined to be 89 meq/lOOg; an elemental analysis of t h e clay is given in Table 2. Pillaring agents The pillaring solutions were prepared by hydrothermal t r eat m en t of solutions containing aluminum chlorohydrate (Locron L, Hoechst, 23.4 wt% A1203, OH/A1=2.5) and rare-earth (RE) chlorides. The RE-chlorides used were: La-chloride (29.5 wt% La2O3), Cechloride (26.1 wt% CeOz), Pr-chloride (27.6 wt% Pr6O1 I), Nd-chloride (29.4 wt% Nd2O3), and a mixed RE-chloride (24.3 wt% RE-oxides; 53.4% La2O3, 20.8% CeO2, 13.9% Pr6011, and 1 1.9%Nd2O3) solution. All RE-solutions w e r e provided by Rhone-Polenc. Solutions, 2.5 M with respect to A1 and with a RE/Al molar ratio of 1:5 were used in all preparations. The hydrothermal t r e a tm e n t s were carried o u t either by ref luxing t h e solutions for 24240 h or by t r eat m en t in PTFE-coated stainless steel autoclaves at temparatures in t h e range 1200-16O0C for 12-96 h. Preparation of pillared products One gram of montmorillonite was dispersed in 200 ml of distilled water (25OC) by prolonged stirring ( 5 h) with a magnetic stirrer. The amount of pillaring solution required t o obtain an Al/montmorillonite ratio of 20 mmol Al/g montmorillonite was then added t o
303 t h e vigorously stirred dispersion. The resulting product was left in c o n t a c t with t h e solution for 1 h and then separated by centrifugation. The product was then washed by redispersing it in distilled water, and separated by centrifugation. This procedure was repeated until t h e supernatant was f r e e from chloride ions as determined by AgN03. Characterization of pillared products N2-adsorption-desorption isotherms w e r e determined using a Digisorb 2600 surfacea r e a , pore-volume analyzer (Micromeritics Instrument Corporation). The samples w e r e first outgassed at 200OC for 3 h, and t h e isotherms w e r e recorded at liquid nitrogen temperature. Surface a r e a s w e r e calculated using t h e BET equation. X-ray diffraction analyses were performed either on non-oriented or oriented mounts. The XRD patterns were obtained on a Philips powder diffractometer using Ni-filtered, fine-focus CuKr-radiation. Thermal stability was investigated by exposing s e p a r a t e samples t o temperatures in t h e range 20O0-80OOC for 3 h in air. Elemental analysis of t h e pillared samples was carried o u t by a t o m i c absorption spectroscopy (AAS) employing LiB02-fusion (ref. 3). RESULTS AND DISCUSSION Reflux experiments A series of samples was prepared from t h e different RE/Al-solutions and f r o m t h e reference ACH-solution, refluxed for 0-240 h. Fig. 1 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La- and ACH-solutions refluxed f o r 48 and 120 h and from Ce-, Pr-, Nd-, and RE-solutions refluxed for 120 and 240 h. The reference sample prepared from t h e RE-free ACH-solution, refluxed f o r 48 h, shows a basal reflexion corresponding
to a basal spacing of 19.4
A. This value is within t h e range, 18-20 A, usually observed for
conventional Al-pillared smectites. Further refluxing of this solution up t o 120 h results in a n increase in crystallinity of t h e pillared product but does not significantly a f f e c t its basal spacing. The sample prepared from t h e La/Al-solution refluxed f o r 48 h shows a broad 001-reflexion corresponding t o a basal spacing of about 19.2
A. In t h e p a t t e r n
recorded for t h e sample prepared from t h e s a m e solution refluxed for 120 h, t h e major basal spacing has shifted to about 26
A. This basal spacing is similar t o those observed by
McCauley (ref. 1) for large-pore Ce/Al-pillared montmorillonite and f luorhectorite, i.e. 27.4 and 25.6
A, respectively. The 26 A peak s t a r t s to develop for samples prepared from
this solution refluxed for 72 h, grows sharper and more intensive with increasing t i m e of reflux up to about 96 h and then remains unaffected with increasing t i m e of reflux within t h e time range investigated. The Ce/Al-solution resulted in large-pore pillared products similar t o those prepared from t h e La/Al-solution but required longer t i m e s of reflux, i.e. about 240 h, in order to produce such products. Although a broadening of t h e 001-peaks is seen for t h e Pr/AI- and Nd/Al-samples prepared from solutions refluxed for 240 h, no
304
Pr/AI
----Mu-
8
5
28
5
28
5
28 5 Degrees 2 0
28
5
28
5
2
Fig. 1. X-ray diffraction p a t t e r n s of RE/Al-pillared montmorillonites prepared from refluxed solutions. major changes in t h e basal spacings of pillared products prepared from refluxed Pr/AI- or Nd/Al-solutions w e r e observed. The RE/Al-solution, containing a mixture of RE-chlorides, showed a behavior simliar to t h a t of t h e Ce/Al-solution, i.e. about 240 h of reflux was required in order to obtain a large-pore pillared product. Autoclave experiments A series of samples was prepared from t h e different RE/Al-solutions and from t h e reference ACH-solution, t r e a t e d in autoclaves at t e m p e r a t u r e s in t h e range 120-16OoC for 12-96 h. Fig. 2 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La/Al-, Ce/Al-, and RE/Al-solutions t r e a t e d at 120OC for 12, 24, 48, and 96 h. For t h e samples prepared from t h e La/Al-solutions a 26
a spacing s t a r t s to develop a f t e r 24 h of treatment. Further
t r e a t m e n t of this solution, up t o 96 h, results in products with sharper and more intensive basal reflexions, indicating increased crystallinity of t h e products with increasing t i m e of hydrothermal treatment. For t h e Ce/AI- and t h e RE/Al-solutions longer t i m e s of t r e a t m e n t w e r e required in order t o obtain large-pore pillared products, which is consistent with t h e results obtained in t h e reflux experiments discussed above. All these solutions did, however, yield very crystalline products with basal spacings of 26 A a f t e r t r e a t m e n t at 120OC for 96 h. In Fig. 3, t h e X-ray diffraction p a t t e r n s of samples prepared from La/AI-, Ce/AI-, and
305
La/AI
Ce/AI
RE /A1
Degrees 28
Fig. 2. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared rnontrnorillonites prepared from solutions autoclaved a t 12OoC for 12-96 h.
La /A1
Ce/AI
REIAI
Degrees 2 8
Fig. 3. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared montmorillonites prepared from solutions autoclaved for 12 h at 120, 140, and 16Ooc.
306 TABLE 1 BET s u r f a c e a r e a s (m2/g) of RE/Al-pillared montmorillonites prepared from solutions hydrothermally t r e a t e d at 120-16OOC for 12-96 h.
temp.a (OC)
I
La/AI
Ce/AI
12
24
48
96
120
422
436
463
140
428
410
160
487
428
I
RE/AI
t i m e of t r e a t m e n t (h)a 12
24
48
96
493
436
443
461
389
374
429
428
412
408
507
487
I
12
24
48
96
537
430
394
434
538
407
518
422
409
393
499
386
390
468
458
442
396
~
RE/Al-solutions t r e a t e d at 120, 140 and 160OC for 12 h a r e shown. After t r e a t m e n t at 12OoC, all solutions result in products with basal spacings of about 19 8. T r e a t m e n t at 14OOC results in a large-pore pillared product from t h e La/Al-solution b u t not so for t h e Ce/Al-and RE/Al-solutions. After t r e a t m e n t at 160OC for 12 h, all t h e solutions yielded highly crystalline large-pore pillared products. These results show t h a t t h e r a t e of formation of t h e large RE/Al-polycations, believed to b e responsible for t h e 26
8 spacing
of large-pore pillared products, increases with increasing t e m p e r a t u r e of hydrothermal treatment. Hydrothermal t r e a t m e n t of t h e solution for t i m e s longer than t h a t required for t h e formation of these species does, however, result in a decline in crystallinity of t h e resulting products. Thus, solutions of La/AI, Ce/Al as well as RE/AI t r e a t e d at 16OOC for 96 h result in considerably less crystalline products in comparison with those obtained from t h e s a m e solutions t r e a t e d at this t e m p e r a t u r e for 12 h. Pr/AI- and Nd/Al-solutions t r e a t e d at 12O-16O0C for 12-96 h did not yield large-pore pillared products. All samples prepared from these solutions showed basal spacings in t h e range 19-20 8 which is within t h e range characteristic for conventional Al-pillared smectites. Autoclave t r e a t m e n t of RE-free ACH-solutions at 12O-16O0C for 12-96 h resulted in t h e formation of colloidal dispersions or gels of boehmite or pseudoboehmite. Table 1 shows t h e BET surface a r e a s of samples prepared from La/AI-, Ce/AI-, and RE/Al-solutions t r e a t e d 12O-16O0C for 12-96 h. Samples prepared from solutions t r e a t e d at 12OoC show a n increase in surface a r e a with increasing t i m e of t r e a t m e n t for all t h r e e solutions. Increasing t i m e of hydrothermal t r e a t m e n t at 14OoC appears t o result in somewhat increasing surface a r e a s for samples prepared from Ce/AI- and RE/Al-solutions and in decreasing surface a r e a s for those prepared from La/Al-solutions. For samples prepared from solutions t r e a t e d a t 16OoC t h e surface a r e a s decrease with increasing t i m e of hydrothermal t r e a t m e n t of t h e solution for all t h r e e solutions. The results of t h e surface a r e a measurements a r e in good agreement with those obtained by X-ray
307
diffraction analysis. The large-pore pillared products, showing intense and sharp basal reflexions corresponding t o a basal spacing of 26
A, generally have surface areas in t h e
vicinity of 500 m*/g.
Fig. 3 shows t h e N2-adsorption-desorption isotherms recorded for t h e starting montmorillonite, an Al-pillared montmorillonite and a large-pore Ce/Al-pillared montmorillonite prepared from a Ce/Al-solution hydrothermally treated at 12OoC for 96 h. The isotherm recorded for t h e montmorillonite is of type I1 in the classification of Brunauer, Deming, and Teller, characteristic of non-porous soiids. The isotherms recorded for t h e pillared products can both be described as composite isotherms of t h e type I1 isotherm of t h e montmorillonite and type I isotherms due t o adsorption in pores introduced by the pillaring procedure. The strong adsorption at low relative pressures, characteristic for microporous materials, is, however, less pronounced for t h e Ce/AIpillared sample. This may be taken as a further indication of larger pores in this material compared with t h e Al-pillared montmorillonite. A further discussion of t h e adsorption properties of large-pore RE/Al-pillared montmorillonite in relation to t h e nature of the pores of this material is given in (ref. 2). Elemental analysis Table 2 shows elemental analyses of t h e starting montmorillonite, of a Al-pillared montmorillonite, and of samples prepared from t h e La/AI-, Ce/AI-, Pr/AI-, Nd/Ai-, and
160
0 LO
0
05
Welafive pressure, PIP,
1.0
Fig. 4. Nitrogen adsorption-dessrpbion isotherms for staring monSrnorillonite (a), Alpillared montrnorillonite (b) and large-pore Ce/Al-pillared morrtmorillonite (c).
308 TABLE 2 Elemental analysis of Na+-Ca2+-montmorillonite, Al-pillared and RE/Al-pillared montmorillonites. metal oxide
montmo-
Al-
La/AI-
Ce/AI-
Pr/AI-
Nd/AI-
RE/AI-
(wt %)
rillonitea
m0nt.b
m0nt.c
m0nt.c
m0nt.c
m0nt.c
m0nt.c
56.7 19.6 3.84 3.02 0.8 0.39 1.94 13.1
45.6 24.4 3.27 1.99 0.2 0.40 0.24 22.4
44.3 25.4 3.20 2.24 0. I 0.30 0.25 22.2
44.6 24.6 2.89 2.00 0.0 0.36 0.40 26.4
46.2 24.4 2.89 2.12 0.0 0.36 0.40 23.4
41.5 24.1 2.59 2.11 0.1 0.18 0.13 29.6
43.0 25.7 2.69 2.47 0.0 0.19 0.18 27.1
99.4
98.5
98.0
99.8
00.3
01.3
101.2
aNa+-Ca2+-montmorillonite, Volclay SPV 200, used as starting m a t e r i a l 1 all preparations. bAi-pillared montmorillonite prepared using a RE-free ACH-solution. CAll RE/Al-pillared montmorillonites w e r e prepared from solutions t r e a t e d at 12OoC for 96 h. RE/Al-solutions t r e a t e d at 12OoC for 96 h. The analyses of t h e samples prepared from hydrothermally t r e a t e d solution differ very l i t t l e from e a c h other and a r e also similar to t h a t of t h e Al-pillared montmorillonite. This indicates a similar uptake of A1 by t h e montmorillonite from t h e different solutions and also a similar charge per A1 in t h e Alspecies taken up from t h e s e solutions. These results a r e somewhat surprising considering t h e g r e a t structural differences between t h e samples observed by X-ray diffraction and by
N2-adsorption-desorption analysis. Thermal stability Fig. 5 shows t h e X-ray powder diffraction p a t t e r n s of an Al-pillared montmorillonite and a Ce/Al-pillared sample prepared from a solution t r e a t e d at 12OoC for 96 h, both thermally t r e a t e d at 200 and 8OOOC for 3 h. The s u r f a c e a r e a s of t h e different samples a r e also given in this Figure. For t h e Al-pillared sample, t h e basal spacing decreases from about 18 8 a f t e r t r e a t m e n t at 2OOOC to about 16 8 a f t e r t r e a t m e n t at 800°C. The decrease in basal spacing is accompanied by a substantial decrease in surface area. For t h e Ce/Al-pillared montmorillonite, t h e basal spacing is approximately t h e same, about 25
8, for samples t r e a t e d at 200 and
8OOOC. After t r e a t m e n t at 800°, t h e Ce/Al-pillared
sample retains a considerable fraction of i t s original surface area, indicating a good thermal stability. I t is noticeable t h a t t h e surface a r e a of t h e Ce/Al-pillared montmorillonite t r e a t e d at 800°C is of t h e s a m e order as t h a t of t h e Al-pillared one t r e a t e d at 200oC. La/AI- and RE/Al-pillared samples responded t o t h e r m a l t r e a t m e n t in a manner similar to t h a t of Ce/Al-pillared ones while only minor improvents in t h e r m a l
309
Al.
Ce/A I
u
8
5
28 5 Degrees 2 8
2
Fig. 5. X-ray powder diffraction p a t t e r n s and surface a r e a s of thermally t r e a t e d Alpillared and large-pore Ce/Al-pillared montmorillonites. stability were observed f o r Pr/AI- and Nd/Al-pillared montmorillonites. Structure of large-pore RE/Al-pillared montmorillonite It is generally accepted t h a t t h e oligomer responible f o r t h e layer separation of conventional Al-pillared s m e c t i t e s is A11304(OH)247+ (ref. 4). The s t r u c t u r e of this cation is a c e n t r a l four-coordinated aluminum a t o m surrounded by twelve A106-octahedra joined by common edges to form a Keggin-type structure, McCauley (ref. I) found t h a t large-pore Ce/Al-pillared montmorillonites c a n be prepared from hydrothermally t r e a t e d solutions with Ce/Al-ratios down t o 1:52. This, in combination with t h e fact t h a t t h e interlayer spacing of t h e s e materials is about twice t h a t of conventional Al-pillared smectites, led him t o suggest t h a t t h e polymeric cation responsible for t h e pillaring in large-pore pillared s m e c t i t e s is built up of four A113-units, linked together by a tetrahedrally bound cerium atom. McCauley has, however, not investigated t h e s t r u c t u r e of t h e pillaring species further in order t o substantiate this suggestion and no such polymeric ion has so f a r been reported in t h e literature. Investigations of hydrothermally t r e a t e d mixtures of aluminum chlorohydrate and r a r e e a r t h salts a r e required in order t o establish t h e s t r u c t u r e of this polycation. CONCLUSION Large-pore pillared montmorillonites c a n b e prepared from hydrothermally t r e a t e d mixtures of ACH and lanthanum, cerium or RE-chlorides. The hydrothermal t r e a t m e n t can b e carried o u t e i t h e r at reflux conditions f o r several days or at higher t e m p e r a t u r e s
310
and f o r shorter durations in a n autoclave. More crystalline materials with higher surface a r e a s c a n b e prepared from autoclaved solutions compared with refluxed ones. Praseodymium and neodymium chlorides in admixture with ACH did not produce largepore pillared products a f t e r hydrothermal t r e a t m e n t at t h e conditions used in this study. The large-pore pillared montmorillonites a r e characterized by s u r f a c e a r e a s in t h e range 300-550 m2/g, basal spacings of about 2 6 A, and a good t h e r m a l stability (surface area
about 300 m2/g even a f t e r exposure t o 8OO0C f o r 3 h). Large-pore RE/Al-pillared s m e c t i t e s is a novel type of zeolite-like materials which may be of i n t e r e s t as a c t i v e components in c a t a l y s t s f o r cracking of heavy oil fractions. C a t a l y t i c cracking performance of RE/Al-montmorillonites, alone and in admixture with zeolite Y, is currently under investigation. ACKNOWLEDGMENTS The author thanks t h e Swedish Board f o r Technical Development (STU) for financial support of this project. Helpful advice from Prof. J-E. O t t e r s t e d t and experimental help from Mr. Ying Zhong-Shu a r e greatly appreciated. REFERENCES 1 J.R. McCauley, Stable intercalated clays and preparation method, Int. Pat. Appl. PCT/US88/00567, March 4, 1988. 2 J. S t e r t e , Preparation and properties of large-pore La/Al-pillared montmorillonite, submitted f o r publication. 3 J.H. Medlin, N.H. Suhr, and J.B. Bodkin, Atomic absorption analysis of silicates employing LiB02 fusion, At. Absorpt. Newsl., 8 (1969) 25-29. 4 D. Plee, F. Borg, L. Gatineau, and J.J. Fripiat, High-resolution solid-state 27Al and 29Si nuclear magnetic resonance study of pillared clays, J. Amer. Chem. Soc., 107 (1985) 2362.
301
PREPARATION AND PROPERTIES OF LARGE-PORE RE/AI-PILLARED MONTMORILLONITE. A COMPARISON OF RE-CATIONS
J. STERTE Department of Engineering Chemistry I, Chalmers University of Technology, 412 96 Goteborg (Sweden)
SUMMARY RE/Al-pillared montmorillonites were prepared by cation exchange of montmorillonite with hydrothermally t r e a t e d (10O-16O0C, 12-240 h) mixtures of aluminum chlorohydrate (ACH) and chlorides of La, Ce, Pr, Nd or a mixture of RE-cations. Large-pore RE/AIpillared montmorillonites, characterized by basal spacings of about 26 and surface areas of 300-550 m2/g, were formed from solutions containing La, C e o r t h e mixture of REcations, refluxed for several days or tr e a t e d in autoclaves at higher temperatures for shorter times. Pr and Nd containing solutions did not result in large-pore pillared products a f t e r t r eat m en t at t h e conditions of this investigation. The large-pore RE/Al-pillared montmorillonites were found t o be similar in elemental composition t o a conventional Alpillared montmorillonite but thermally more stable, retaining a surface a r e a of about 300 m2/g a f t e r exposure to 8OOOC for 3 h.
a
INTRODUCTION In recent y e a r s g r e a t i n t e r e s t h a s b e e n f o c u s s e d on t h e p r e p a r a t i o n and t h e characterization of different types of pillared clays and also on possible applications for these materials primarily as catalysts or adsorbents. One of t h e most interesting potential applications for pillared clays is as active components in cracking catalyst formulations designed for cracking of heavy oil fractions. The commercial use of pillared clays for this application has, however, been limited by their lack of thermal and hydrothermal stability. Recently, McCauley (ref. I) found t h a t hydrothermally stable pillared sm ect i t es can be prepared by using hydrothermally t r e a te d pillaring solutions, containing mixtures of aluminum chlorohydrate (ACH) and a ceriurn(II1) salt in t h e preparation. Pillared smectites prepared from such solutions differ from conventional Al-pillared smectites in t ha t th e basal spacing as measured by X-ray diffraction analysis is considerably larger, i.e. 25-28
a compared with 18-20 A. This larger basal spacing is believed t o be due to
formation of a large Ce-bearing Al-polycation upon hydrothermal t r eat m en t of t h e solution. For use as heavy oil cracking catalysts, this larger spacing is another advantage over conventional Al-pillared smectites in addition t o t h e improved hydrothermal stability. McCauley found t h a t large-pore Ce/Al-pillared sm ect i t es can be prepared from solutions with Ce/AI molar ratios down to 1:52, hydrothermally t r eat ed either by refluxing for several days or by t r e a t m e n t in autoclaves at higher temperatures for shorter times.
302
NO systematic investigation on t h e dependence of properties of t h e Ce/AI- pillared products upon different synthesis parameters was, however, reported. Although t h e work of McCauley is primarily concerned with t h e preparation of Ce/Al-pillared smectites, he claims that, in addition to Ce(III), other r a r e e a r t h (RE) cations in admixture with ACH can be used for t h e preparation of large-pore pillared products. S t e r t e (ref. 2) investigated t h e effects of ti m e and temperature of hydrothermal t re a tm en t , Al-concentration, OH/Al-ratio of th e ACH, and La/AI molar ratio of t h e solution on t h e formation of large pore La/Al-pillared montmorillonite. The present paper reports on t h e preparation and characterization of pillared montmorillonites prepared by cation exchange of t h e montmorillonite with hydrothermally t re a ted solutions containing ACH in admixture with chlorides of La, Ce, Pr, Nd, or a commercial mixture of RE-cations. EXPERIMENTAL Montmorillonite A Wyoming Na+-Ca2+-montmorillonite (commercial designation, Voiclay SPV 200) was obtained from t h e American Colloid Company. Impurity quartz was removed by fractionation using conventional sedimentation techniques. The <2,um fraction, which was essentially f r e e from impurities as determined by X-ray powder diffraction (XRD) analysis, was used as starting material in all preparations. The cation exchange capacity
of t h e montrnorillonite was determined to be 89 meq/lOOg; an elemental analysis of t h e clay is given in Table 2. Pillaring agents The pillaring solutions were prepared by hydrothermal t r eat m en t of solutions containing aluminum chlorohydrate (Locron L, Hoechst, 23.4 wt% A1203, OH/A1=2.5) and rare-earth (RE) chlorides. The RE-chlorides used were: La-chloride (29.5 wt% La2O3), Cechloride (26.1 wt% CeOz), Pr-chloride (27.6 wt% Pr6O1 I), Nd-chloride (29.4 wt% Nd2O3), and a mixed RE-chloride (24.3 wt% RE-oxides; 53.4% La2O3, 20.8% CeO2, 13.9% Pr6011, and 1 1.9%Nd2O3) solution. All RE-solutions w e r e provided by Rhone-Polenc. Solutions, 2.5 M with respect to A1 and with a RE/Al molar ratio of 1:5 were used in all preparations. The hydrothermal t r e a tm e n t s were carried o u t either by ref luxing t h e solutions for 24240 h or by t r eat m en t in PTFE-coated stainless steel autoclaves at temparatures in t h e range 1200-16O0C for 12-96 h. Preparation of pillared products One gram of montmorillonite was dispersed in 200 ml of distilled water (25OC) by prolonged stirring ( 5 h) with a magnetic stirrer. The amount of pillaring solution required t o obtain an Al/montmorillonite ratio of 20 mmol Al/g montmorillonite was then added t o
303 t h e vigorously stirred dispersion. The resulting product was left in c o n t a c t with t h e solution for 1 h and then separated by centrifugation. The product was then washed by redispersing it in distilled water, and separated by centrifugation. This procedure was repeated until t h e supernatant was f r e e from chloride ions as determined by AgN03. Characterization of pillared products N2-adsorption-desorption isotherms w e r e determined using a Digisorb 2600 surfacea r e a , pore-volume analyzer (Micromeritics Instrument Corporation). The samples w e r e first outgassed at 200OC for 3 h, and t h e isotherms w e r e recorded at liquid nitrogen temperature. Surface a r e a s w e r e calculated using t h e BET equation. X-ray diffraction analyses were performed either on non-oriented or oriented mounts. The XRD patterns were obtained on a Philips powder diffractometer using Ni-filtered, fine-focus CuKr-radiation. Thermal stability was investigated by exposing s e p a r a t e samples t o temperatures in t h e range 20O0-80OOC for 3 h in air. Elemental analysis of t h e pillared samples was carried o u t by a t o m i c absorption spectroscopy (AAS) employing LiB02-fusion (ref. 3). RESULTS AND DISCUSSION Reflux experiments A series of samples was prepared from t h e different RE/Al-solutions and f r o m t h e reference ACH-solution, refluxed for 0-240 h. Fig. 1 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La- and ACH-solutions refluxed f o r 48 and 120 h and from Ce-, Pr-, Nd-, and RE-solutions refluxed for 120 and 240 h. The reference sample prepared from t h e RE-free ACH-solution, refluxed f o r 48 h, shows a basal reflexion corresponding
to a basal spacing of 19.4
A. This value is within t h e range, 18-20 A, usually observed for
conventional Al-pillared smectites. Further refluxing of this solution up t o 120 h results in a n increase in crystallinity of t h e pillared product but does not significantly a f f e c t its basal spacing. The sample prepared from t h e La/Al-solution refluxed f o r 48 h shows a broad 001-reflexion corresponding t o a basal spacing of about 19.2
A. In t h e p a t t e r n
recorded for t h e sample prepared from t h e s a m e solution refluxed for 120 h, t h e major basal spacing has shifted to about 26
A. This basal spacing is similar t o those observed by
McCauley (ref. 1) for large-pore Ce/Al-pillared montmorillonite and f luorhectorite, i.e. 27.4 and 25.6
A, respectively. The 26 A peak s t a r t s to develop for samples prepared from
this solution refluxed for 72 h, grows sharper and more intensive with increasing t i m e of reflux up to about 96 h and then remains unaffected with increasing t i m e of reflux within t h e time range investigated. The Ce/Al-solution resulted in large-pore pillared products similar t o those prepared from t h e La/Al-solution but required longer t i m e s of reflux, i.e. about 240 h, in order to produce such products. Although a broadening of t h e 001-peaks is seen for t h e Pr/AI- and Nd/Al-samples prepared from solutions refluxed for 240 h, no
304
Pr/AI
----Mu-
8
5
28
5
28
5
28 5 Degrees 2 0
28
5
28
5
2
Fig. 1. X-ray diffraction p a t t e r n s of RE/Al-pillared montmorillonites prepared from refluxed solutions. major changes in t h e basal spacings of pillared products prepared from refluxed Pr/AI- or Nd/Al-solutions w e r e observed. The RE/Al-solution, containing a mixture of RE-chlorides, showed a behavior simliar to t h a t of t h e Ce/Al-solution, i.e. about 240 h of reflux was required in order to obtain a large-pore pillared product. Autoclave experiments A series of samples was prepared from t h e different RE/Al-solutions and from t h e reference ACH-solution, t r e a t e d in autoclaves at t e m p e r a t u r e s in t h e range 120-16OoC for 12-96 h. Fig. 2 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La/Al-, Ce/Al-, and RE/Al-solutions t r e a t e d at 120OC for 12, 24, 48, and 96 h. For t h e samples prepared from t h e La/Al-solutions a 26
a spacing s t a r t s to develop a f t e r 24 h of treatment. Further
t r e a t m e n t of this solution, up t o 96 h, results in products with sharper and more intensive basal reflexions, indicating increased crystallinity of t h e products with increasing t i m e of hydrothermal treatment. For t h e Ce/AI- and t h e RE/Al-solutions longer t i m e s of t r e a t m e n t w e r e required in order t o obtain large-pore pillared products, which is consistent with t h e results obtained in t h e reflux experiments discussed above. All these solutions did, however, yield very crystalline products with basal spacings of 26 A a f t e r t r e a t m e n t at 120OC for 96 h. In Fig. 3, t h e X-ray diffraction p a t t e r n s of samples prepared from La/AI-, Ce/AI-, and
305
La/AI
Ce/AI
RE /A1
Degrees 28
Fig. 2. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared rnontrnorillonites prepared from solutions autoclaved a t 12OoC for 12-96 h.
La /A1
Ce/AI
REIAI
Degrees 2 8
Fig. 3. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared montmorillonites prepared from solutions autoclaved for 12 h at 120, 140, and 16Ooc.
306 TABLE 1 BET s u r f a c e a r e a s (m2/g) of RE/Al-pillared montmorillonites prepared from solutions hydrothermally t r e a t e d at 120-16OOC for 12-96 h.
temp.a (OC)
I
La/AI
Ce/AI
12
24
48
96
120
422
436
463
140
428
410
160
487
428
I
RE/AI
t i m e of t r e a t m e n t (h)a 12
24
48
96
493
436
443
461
389
374
429
428
412
408
507
487
I
12
24
48
96
537
430
394
434
538
407
518
422
409
393
499
386
390
468
458
442
396
~
RE/Al-solutions t r e a t e d at 120, 140 and 160OC for 12 h a r e shown. After t r e a t m e n t at 12OoC, all solutions result in products with basal spacings of about 19 8. T r e a t m e n t at 14OOC results in a large-pore pillared product from t h e La/Al-solution b u t not so for t h e Ce/Al-and RE/Al-solutions. After t r e a t m e n t at 160OC for 12 h, all t h e solutions yielded highly crystalline large-pore pillared products. These results show t h a t t h e r a t e of formation of t h e large RE/Al-polycations, believed to b e responsible for t h e 26
8 spacing
of large-pore pillared products, increases with increasing t e m p e r a t u r e of hydrothermal treatment. Hydrothermal t r e a t m e n t of t h e solution for t i m e s longer than t h a t required for t h e formation of these species does, however, result in a decline in crystallinity of t h e resulting products. Thus, solutions of La/AI, Ce/Al as well as RE/AI t r e a t e d at 16OOC for 96 h result in considerably less crystalline products in comparison with those obtained from t h e s a m e solutions t r e a t e d at this t e m p e r a t u r e for 12 h. Pr/AI- and Nd/Al-solutions t r e a t e d at 12O-16O0C for 12-96 h did not yield large-pore pillared products. All samples prepared from these solutions showed basal spacings in t h e range 19-20 8 which is within t h e range characteristic for conventional Al-pillared smectites. Autoclave t r e a t m e n t of RE-free ACH-solutions at 12O-16O0C for 12-96 h resulted in t h e formation of colloidal dispersions or gels of boehmite or pseudoboehmite. Table 1 shows t h e BET surface a r e a s of samples prepared from La/AI-, Ce/AI-, and RE/Al-solutions t r e a t e d 12O-16O0C for 12-96 h. Samples prepared from solutions t r e a t e d at 12OoC show a n increase in surface a r e a with increasing t i m e of t r e a t m e n t for all t h r e e solutions. Increasing t i m e of hydrothermal t r e a t m e n t at 14OoC appears t o result in somewhat increasing surface a r e a s for samples prepared from Ce/AI- and RE/Al-solutions and in decreasing surface a r e a s for those prepared from La/Al-solutions. For samples prepared from solutions t r e a t e d a t 16OoC t h e surface a r e a s decrease with increasing t i m e of hydrothermal t r e a t m e n t of t h e solution for all t h r e e solutions. The results of t h e surface a r e a measurements a r e in good agreement with those obtained by X-ray
307
diffraction analysis. The large-pore pillared products, showing intense and sharp basal reflexions corresponding t o a basal spacing of 26
A, generally have surface areas in t h e
vicinity of 500 m*/g.
Fig. 3 shows t h e N2-adsorption-desorption isotherms recorded for t h e starting montmorillonite, an Al-pillared montmorillonite and a large-pore Ce/Al-pillared montmorillonite prepared from a Ce/Al-solution hydrothermally treated at 12OoC for 96 h. The isotherm recorded for t h e montmorillonite is of type I1 in the classification of Brunauer, Deming, and Teller, characteristic of non-porous soiids. The isotherms recorded for t h e pillared products can both be described as composite isotherms of t h e type I1 isotherm of t h e montmorillonite and type I isotherms due t o adsorption in pores introduced by the pillaring procedure. The strong adsorption at low relative pressures, characteristic for microporous materials, is, however, less pronounced for t h e Ce/AIpillared sample. This may be taken as a further indication of larger pores in this material compared with t h e Al-pillared montmorillonite. A further discussion of t h e adsorption properties of large-pore RE/Al-pillared montmorillonite in relation to t h e nature of the pores of this material is given in (ref. 2). Elemental analysis Table 2 shows elemental analyses of t h e starting montmorillonite, of a Al-pillared montmorillonite, and of samples prepared from t h e La/AI-, Ce/AI-, Pr/AI-, Nd/Ai-, and
160
0 LO
0
05
Welafive pressure, PIP,
1.0
Fig. 4. Nitrogen adsorption-dessrpbion isotherms for staring monSrnorillonite (a), Alpillared montrnorillonite (b) and large-pore Ce/Al-pillared morrtmorillonite (c).
308 TABLE 2 Elemental analysis of Na+-Ca2+-montmorillonite, Al-pillared and RE/Al-pillared montmorillonites. metal oxide
montmo-
Al-
La/AI-
Ce/AI-
Pr/AI-
Nd/AI-
RE/AI-
(wt %)
rillonitea
m0nt.b
m0nt.c
m0nt.c
m0nt.c
m0nt.c
m0nt.c
56.7 19.6 3.84 3.02 0.8 0.39 1.94 13.1
45.6 24.4 3.27 1.99 0.2 0.40 0.24 22.4
44.3 25.4 3.20 2.24 0. I 0.30 0.25 22.2
44.6 24.6 2.89 2.00 0.0 0.36 0.40 26.4
46.2 24.4 2.89 2.12 0.0 0.36 0.40 23.4
41.5 24.1 2.59 2.11 0.1 0.18 0.13 29.6
43.0 25.7 2.69 2.47 0.0 0.19 0.18 27.1
99.4
98.5
98.0
99.8
00.3
01.3
101.2
aNa+-Ca2+-montmorillonite, Volclay SPV 200, used as starting m a t e r i a l 1 all preparations. bAi-pillared montmorillonite prepared using a RE-free ACH-solution. CAll RE/Al-pillared montmorillonites w e r e prepared from solutions t r e a t e d at 12OoC for 96 h. RE/Al-solutions t r e a t e d at 12OoC for 96 h. The analyses of t h e samples prepared from hydrothermally t r e a t e d solution differ very l i t t l e from e a c h other and a r e also similar to t h a t of t h e Al-pillared montmorillonite. This indicates a similar uptake of A1 by t h e montmorillonite from t h e different solutions and also a similar charge per A1 in t h e Alspecies taken up from t h e s e solutions. These results a r e somewhat surprising considering t h e g r e a t structural differences between t h e samples observed by X-ray diffraction and by
N2-adsorption-desorption analysis. Thermal stability Fig. 5 shows t h e X-ray powder diffraction p a t t e r n s of an Al-pillared montmorillonite and a Ce/Al-pillared sample prepared from a solution t r e a t e d at 12OoC for 96 h, both thermally t r e a t e d at 200 and 8OOOC for 3 h. The s u r f a c e a r e a s of t h e different samples a r e also given in this Figure. For t h e Al-pillared sample, t h e basal spacing decreases from about 18 8 a f t e r t r e a t m e n t at 2OOOC to about 16 8 a f t e r t r e a t m e n t at 800°C. The decrease in basal spacing is accompanied by a substantial decrease in surface area. For t h e Ce/Al-pillared montmorillonite, t h e basal spacing is approximately t h e same, about 25
8, for samples t r e a t e d at 200 and
8OOOC. After t r e a t m e n t at 800°, t h e Ce/Al-pillared
sample retains a considerable fraction of i t s original surface area, indicating a good thermal stability. I t is noticeable t h a t t h e surface a r e a of t h e Ce/Al-pillared montmorillonite t r e a t e d at 800°C is of t h e s a m e order as t h a t of t h e Al-pillared one t r e a t e d at 200oC. La/AI- and RE/Al-pillared samples responded t o t h e r m a l t r e a t m e n t in a manner similar to t h a t of Ce/Al-pillared ones while only minor improvents in t h e r m a l
309
Al.
Ce/A I
u
8
5
28 5 Degrees 2 8
2
Fig. 5. X-ray powder diffraction p a t t e r n s and surface a r e a s of thermally t r e a t e d Alpillared and large-pore Ce/Al-pillared montmorillonites. stability were observed f o r Pr/AI- and Nd/Al-pillared montmorillonites. Structure of large-pore RE/Al-pillared montmorillonite It is generally accepted t h a t t h e oligomer responible f o r t h e layer separation of conventional Al-pillared s m e c t i t e s is A11304(OH)247+ (ref. 4). The s t r u c t u r e of this cation is a c e n t r a l four-coordinated aluminum a t o m surrounded by twelve A106-octahedra joined by common edges to form a Keggin-type structure, McCauley (ref. I) found t h a t large-pore Ce/Al-pillared montmorillonites c a n be prepared from hydrothermally t r e a t e d solutions with Ce/Al-ratios down t o 1:52. This, in combination with t h e fact t h a t t h e interlayer spacing of t h e s e materials is about twice t h a t of conventional Al-pillared smectites, led him t o suggest t h a t t h e polymeric cation responsible for t h e pillaring in large-pore pillared s m e c t i t e s is built up of four A113-units, linked together by a tetrahedrally bound cerium atom. McCauley has, however, not investigated t h e s t r u c t u r e of t h e pillaring species further in order t o substantiate this suggestion and no such polymeric ion has so f a r been reported in t h e literature. Investigations of hydrothermally t r e a t e d mixtures of aluminum chlorohydrate and r a r e e a r t h salts a r e required in order t o establish t h e s t r u c t u r e of this polycation. CONCLUSION Large-pore pillared montmorillonites c a n b e prepared from hydrothermally t r e a t e d mixtures of ACH and lanthanum, cerium or RE-chlorides. The hydrothermal t r e a t m e n t can b e carried o u t e i t h e r at reflux conditions f o r several days or at higher t e m p e r a t u r e s
310
and f o r shorter durations in a n autoclave. More crystalline materials with higher surface a r e a s c a n b e prepared from autoclaved solutions compared with refluxed ones. Praseodymium and neodymium chlorides in admixture with ACH did not produce largepore pillared products a f t e r hydrothermal t r e a t m e n t at t h e conditions used in this study. The large-pore pillared montmorillonites a r e characterized by s u r f a c e a r e a s in t h e range 300-550 m2/g, basal spacings of about 2 6 A, and a good t h e r m a l stability (surface area
about 300 m2/g even a f t e r exposure t o 8OO0C f o r 3 h). Large-pore RE/Al-pillared s m e c t i t e s is a novel type of zeolite-like materials which may be of i n t e r e s t as a c t i v e components in c a t a l y s t s f o r cracking of heavy oil fractions. C a t a l y t i c cracking performance of RE/Al-montmorillonites, alone and in admixture with zeolite Y, is currently under investigation. ACKNOWLEDGMENTS The author thanks t h e Swedish Board f o r Technical Development (STU) for financial support of this project. Helpful advice from Prof. J-E. O t t e r s t e d t and experimental help from Mr. Ying Zhong-Shu a r e greatly appreciated. REFERENCES 1 J.R. McCauley, Stable intercalated clays and preparation method, Int. Pat. Appl. PCT/US88/00567, March 4, 1988. 2 J. S t e r t e , Preparation and properties of large-pore La/Al-pillared montmorillonite, submitted f o r publication. 3 J.H. Medlin, N.H. Suhr, and J.B. Bodkin, Atomic absorption analysis of silicates employing LiB02 fusion, At. Absorpt. Newsl., 8 (1969) 25-29. 4 D. Plee, F. Borg, L. Gatineau, and J.J. Fripiat, High-resolution solid-state 27Al and 29Si nuclear magnetic resonance study of pillared clays, J. Amer. Chem. Soc., 107 (1985) 2362.