Supramolecular Architecture - American Chemical Society

Chapter 10

Pillared Layered Materials Abraham Clearfield and Mark Kuchenmeister

Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch010

Department of Chemistry, Texas A&M University, College Station, TX 77843

The structure of several types of layered compounds which behave as ion exchangers will be described. These include clays, group 4 and 14 phosphates, titanates and antimonates. The techniques of pillaring used for each type will be presented along with a description of the structure and porous nature of the products. Potential applications as catalysts, sorbants and ion exchangers will be discussed and predictions for the future of thisfieldof research will be presented.

A great deal of progress has been made in the field of pillared layered materials since our last review article (1). Two monographs have appeared (2-3) as well as many notes and papers. However, since this is by way of a tutorial, a brief introduction to the field and review of earlier work is in order. Along with the oil embargo in 1973 the United States experienced a sharp rise in oil prices. This price increase acted as a stimulus to develop catalytic materials which would process heavy crude oils. Normal processing of these crudes, such as coking and hydroprocessing, is relatively expensive but the cost could be considerably reduced using conventional fluid catalytic cracking (FCC). Zeolites are normally used as F C C catalysts, but these materials have relatively small pore openings (~8 Â for Zeolite Y) and are unsuitable without expensive preprocessing of the feed. Clays and layered silicate minerals have a long history of use as petroleum cracking catalysts (4). However, only the outer surface of the clays are useful in catalysis as the molecules cannot penetrate between the layers at the temperatures used for cracking. It has also been known that certain classes of organic molecules, such as amines and alkylammonium ions, readily intercalate between clay layers, (5) and that porous structures can be formed from such host-guest complexes (6). These organically pillared structures suffer from the thermal instability of the organic component. In 1974 Lussier, McGee and Vaughan (7) initiated work leading to the synthesis of inorganically pillared clays (8). The general procedure is to incorporate a large inorganic cation between the layers to prop them open (8-9). The props or pillars are then thermally cross-linked to the layers. This propping makes the inner surfaces of the clay available for catalytic purposes. 0097-6156/92/0499-0128$06.00/0 © 1992 American Chemical Society

In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

10. CLEARFIELD AND KUCHENMEISTER

Pillared Layered Materials

129

Before discussing these pillaring reactions, we will examine some aspects of the clays themselves. Description of Smectite Clays


The principal class of swelling clays is termed smectites. These clays characteristically have alumina octahedra sandwiched between silica tetrahedra as shown in Figure 1. The smectites are distinguished by the type and location of cations in the layered framework. In a unit cell formed from twenty oxygens and four hydroxyl groups, there are eight tetrahedral sites and six octahedral sites. Idealized formulas are given in Table I. Table I. Idealized Structural Formulae for Principal Smectite Clays Name

Formula N+

•yH 0

Montmorillonite:

MX

Hectorite:

MX

Beidellite:

Mx^[Al4](Si .xAlx)O20(OH)4 · y H 0

Saponite:

Μ χ ^ [Mg ] (Sis-χΑΙχ) O

/ N

[ A l 4 - x M ] (Si ) O gX

8

2 0

(OH)

2

4

N+ / N

[ M g - x L i ] (Si ) O 0 (OH) 6

x

8

2

4

• yH 0* 2

2

8

6

2 0

(OH)

4

· yH 0 2

*Some F" may substitute for (OH)" 2 +

When all the octahedral sites of hectorite are filled with M g , a magnesium talc of ideal formula [Mg6](Sis)(OH)4O20 is the result. Pyrophyllite is like talc except that 2/3 of the octahedral sites are filled by A l . In smectites, substitutions of M g for A l + and L i for M g can take place in the octahedral sites, or M ions can substitute for S i in the tetrahedral sites. The deficiency of positive charge is compensated by the presence of hydrated cations between the layers (10). Typically, the positive charge deficiency in smectites ranges from 0.4 to 1.2 e per Sig0 o unit (11). If the charge deficiency arises from octahedral substitution, then this excess negative charge is distributed over all the oxygens in the framework. These clays tend to be turbostratic. that is, the layers are randomly rotated about an axis perpendicular to the layers. However should the smectite have been formed by substitution at tetrahedral sites, then the extra charge is more localized and the resultant clay tends to exhibit greater three dimensional order (12). The remarkable properties of the smectite clays are due to their relatively low layer charge. The interlayer cations are loosely held and therefore swelling via solvent sorption, ion exchange and intercalation of a variety of species is possible. 3 +

3

+

2 +

2 +

3

+

4 +

2

Ion Exchange Behavior of Smectite Clays The term "intercalation" should be reserved for the incorporation of neutral species between the layers and the term "ion exchange" when there is a one for one exchange of species based on charge. The ion exchange capacity (IEC) of smectite clays ranges from about 0.5 to 1.5 meq/g. These capacities are low in comparison with those of sulfonated styrene-divinylbenzene resins (IEC ~ 5 meq/g) or a-zirconium phosphate, Z r ( H P 0 4 ) H 0 (6.6 meq/g) (13). Given the fact that the interlamellar 2

2


130

SUPRAMOLECULAR ARCHITECTURE


2

surface is about 750 m /g, univalent ions would be about 8.3 Â apart in the interlayer space at an EEC of 1 meq/g. An important property of the smectites is their ability to swell by intercalation of water or alcohols. The extent of the swelling depends upon the layer charge, the interlayer cation and the nature of the swelling agent (14). When the hydration forces of the interlayer cation are strong, and the layer charge low, the clay is most susceptible to swelling in water. Under the right conditions, the layers can be separated by hundreds of Angstroms. If such a dispersion is concentrated, gelation usually occurs. The gel is believed to have a "house-of-cards" structure (Figure 2) resulting from layer edge to face interactions (15). This swelling behavior allows large complex ions, which can be used for catalytic purposes, to be exchanged into the interlamellar spaces. For additional information on complexes in clays the reader is referred to a review article by Pinnavaia (16). Pillared Clays (PILCS) As was mentioned in the introduction, the driving force for the preparation of PILCS is the desire to synthesize temperature-stable porous materials with large pores. For this purpose, inorganic polymeric species were chosen since they would form oxide particles on heating, which would act as props for the clay layers, as shown in Figure 3. The initial work was done with either aluminum or zirconium polymers (89,18-19) . In the case of the aluminum pillaring, two types of solutions were used: an aluminum chloride solution to which was added up to 2.33 mol of NaOH per mol of A l (9), and a commercially available (Reheis Chemical Co., Chlorhydrol») solution in which Al(OH)3 or A l metal is dissolved in aluminum trichloride solutions, until the OH/ΑΙ ratio is equal to or slightly less than 2.5. Such solutions have been reported to contain polynuclear species with 6 to 400 aluminum atoms per structural u n i t . » However, the exact form of these species is unknown. It has now been shown (see below) that in both types of solutions a major polymeric species is the Keggin ion, [Α1ΐ3θ4(ΟΗ)24 · 1 2 Η 2 θ ] , (23). However, N M R data have revealed some interesting differences between the two types of solution (24). A n aluminum chloride solution initially exhibits a single resonance characteristic of the Α1(Η2θ)6 ion (25). As base is added to this solution, a resonance at 62.8 ppm increases at the expense of a resonance at near zero ppm indicative of the trivalent aluminum species. The new resonance is attributed to the AI13 Keggin ion, and when the OH/ΑΙ ratio is 2.42, it is the only species observed by NMR. In contrast, the Chlorhydrol» solution (OH/A1 = 2.50), which was diluted from a 6.2 M solution, exhibited a small peak for the Keggin ion, a large resonance for A1(H20)6 and a broad resonance at 10.8 ppm attributed to species of higher nuclearity. 20

21

22

7+

3+

3+

In all of the early pillaring studies irrespective of the type of aluminum solution used, the interlayer spacing of the smectite increased from approximately 9.3 A to 18-19 Â (8-9,26). This result is consistent with the incorporation of the AI13 Keggin ion, which has the shape of a prolate spheroid with a long axis of about 9.5 A . The pillaring is illustrated (somewhat naively) in Figure 3. Hydrolysis may change the charge on the AI13 Keggin ion. This in turn increases the loading of the pillar into the clay. Evidence for this charge lowering was obtained by the analysis of products obtained in the pillaring of a Bentonite as a function of pH. Based on the analytical data a plot of AI13 charge variation with pH of the solution was developed (Figure 4) (7). However, it appears that no systematic study of PILC properties as a function of pillar content has been carried out. On heating, the Keggin ion loses protons to the layers to balance the negative layer charge as the pillar is transformed into an oxide particle (8,21 ). Surface areas of





[M(H 0) ] 2

X

131

Π +

Figure 1. Schematic drawing of smectite clay: f): octahedrally coordinated ion; · : tetrahedrally coordinated ion; O: oxygen, ©: OH group.

Figure 2. Schematic representation of a "house of cards" arrangement of a pillared clay. Ideal drying conditions would have all the layers lying parallel. (Reproduced with permission from ref. 17. Copyright 1984 T. J. Pinnavaia.)



132


Figure 3. Schematic representation of the formation of a pillared clay. (Reproduced with permission from ref. 26. Copyright 1980 Wiley).

pH Figure 4. Hydrolysis of [Al ] as a function of pH to give lower charge polymers. (x ) was determined from chemical analysis of the final PILC after the exchange reaction. (Reproduced with permission from ref. 7. Copyright 1988 Elsevier Science Publishers.) 13

+




133

2

200-300 m /g have been observed for the PILCS (7,24). Simultaneous thermal gravimetric analysis and surface area measurements show that about 50% of the water is lost at temperatures below 175 °C; this is considered to be surface water. As the temperature is increased to 500 °C, there is very little loss of surface area. At higher temperature, there is rapid loss of surface area accompanied by about a 1-Â loss in interlayer spacing, and at 700 °C there is a total collapse of the structure (35). Vaughan et al. (8,26) reported pore size distributions for two clays pillared by aluminum oxycations. Their results are shown in Table II.


Table II. Pore Size and Surface Area Distribution for Pillared Clays and Zeolites % N2 Surface Area Pore size Range, Â

PILC

PILC

NaY

Amorphous Catalyst

>100 100-80 80-60 60-40 40-30 30-20 20-14 2 (75) and H3SD3P2O14 (76). The former compound has a structure that is isomorphous with ZrP but contains half the hydrogen ion. It was pillared with the aluminum Keggin ion so as to satisfy the total layer charge and also approximately half the charge, i.e., Ho.5Sb(P04)2[Ali304(OH)24(H20)i2]i/i4. Interlayer spacings of 17-18 A were observed as compared to an initial interlayer spacing of ~8 A for the anhydrous proton phase. Surface area and pore size measurements are in progress (77). Pillaring of Oxides. Cheng and Wang (78) were able to pillar H2T14O9 by a variant of the Clearfield method. After intercalation of hexylamine, the hexylammonium ion was exchanged by tetramethylammonium ion. In this condition, the AI13 species could be inserted between the layers. Although high surface areas were observed, no pore size measurements were obtained. However, the pores were estimated (from N2 sorption isotherms) to be ~10 nm in average diameter. It is altogether possible that these authors missed the presence of micropores in their products. Perovskite Layered Compounds. Recently, Dion et al. (79) synthesized a new family of layered oxides with the general formula ACa2Nb30io (A = K , Rb, Cs, Tl). The layers consist of perovskite slabs separated by A ions as shown in Figure 7. Subsequently, Jacobson et al. (80) were able to increase the layer thickness by substitutions of the type K[Ca2Na -3Nb 03 +i], 3 < η < 7. The layer thickness increases by approximately 3.9 A for each integral increase in the value of n. These compounds were converted to solid acids in 6 M HC1 and subsequently shown to intercalate amines (81,82). This behavior makes them excellent candidates for pillaring because the layers are expected to be relatively rigid or stiff and robust as compared to clays or phosphates. Furthermore, the proton may be highly acidic because of the high charge on N b . We prepared the following new compounds in addition to K C a 2 N b 3 0 i n : K C a 3 N b 3 T 1 0 1 3 , K C a 4 N b 3 T 1 2 O 16, K C a 2 S r o . 5 N b 3 T i o . 5 0 1 1 . 5 and K C a 2 S r o . 2 5 N b 3 T i o . 2 5 0 1 0 . 7 5 . The first two compounds were prepared by incorporating one and two moles of CaTi03, respectively, into the parent potassium calcium niobate (82). A l l of the compounds were converted to the acid form in 6 M HC1 and subsequendy intercalated with either butylamine or hexylamine. Exchange of the alkyl ammonium salt for the aluminum Keggin ion [Α1ΐ3θ4(ΟΗ)24(Η2θ)ΐ2] was carried out using chlorohydrol solutions at 50 °C. These compounds gave surface areas of about 8 m /g indicating a lack of pores. In all of the above cases, the amount of AI13 Keggin ion incorporated was close to \ of the charge on the host compound. Calculations show that the interlayer space is crowded and a reduction in the amount of pillar incorporated is required. The foregoing examples are sufficient to show that a broad range of layered compounds are amenable to pillaring. Thus, a vast new group of materials can be synthesized which show promise for use as catalysts, sorbants, ion exchangers, chemical sensors and host of other uses. Before this promise becomes a reality, we need to learn how to control the reactions to obtain pores of specific size and stability and relate structure to the observed properties of the pillared layered materials. +

n

n

n

5 +

7+

2

3



10.

CLEARFIELD AND KUCHENMEISTER


141

Figure 7. Schematic representation of the structure of K C a N b O . (Reproduced from ref. 80. Copyright 1985 American Chemical Society.) 2

3

10


142


Acknowledgment. Our work on pillared layered materials has been supported by NSF grant DMR-88012283 and the State of Texas through the Advanced Research Program for which grateful acknowledgement is made. The impetus and some continuing support has been supplied by Amoco Chemical Company and Texaco, U.S.A. Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 5. 26. 27. 28.

Clearfield, A. In Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis, J. M. Basset, Eds.; Publishers: Kluwer Academic 1988; pp 198-271. Catalysis Today; R. Burch, Ed.; Elsevier Amsterdam, 1988. Vol. 2. Pillared Layered Structures; I. V. Mitchell, Ed.; Elsevier Appl. Sci., N.Y. 1990. Ryland, L. B.; Tamele, M. W.; Wilson, J. N. In Catalysis Emmett, P. H. Ed.; Reinhold Publ. Co, New York 1960 Theng, B. K. G., The Chemistry of Clay-Organic Reactions; John Wiley & Sons; New York 1974. Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, New York 1978. Vaughan, D. E. W. in Ref. 2, p 187. Vaughan, D. E. W.; Lussier, R. J.; McGee, J. S. US Pat. 4 176 090, 1979; 4 248 739, 1981; 4 271 043, 1981 to W. R. Grace, Co. Lahav, N.; Shani, V.; Shabtai, J. Clays and Clay Minerals 1978, 26, 107. Bailey, S. W. In Crystal Structures of Clay Minerals and Their X-ray Identification; Brindley, G. W.; Brown, G. Mineral Society, London 1980. Beson, G.; Mifsud, Α.; Tchoubar, C.; Mering, J. Clays and Clay Minerals 1974, 22, 379. Suquet, H.; Calle de la, C.; Pezerat, H. Clays and Clay Minerals 1975, 23, 1. Clearfield,A.; Stynes, J.A. J. Inorg. Nucl. Chem. 1964, 26, 117. Mortland, M. M. Trans. 9th Int. Congr. Soil Sci. 1968, 1, 691. Van Olphen, H.An Introduction to Clay Colloid Chemistry ; John Wiley & Sons, New York 1977, 2nd Ed. Pinnavaia, T. J. Science 1983, 220, 365. Pinnavaia, T. J.; In Heterogeneous Catalysis ; Shapiro, B. Ed.; Texas A&M University Press; College Station, Texas 1984. Brindley, G. W.; Semples, R. E. Clays and Clay Minerals 1978, 26, 229. Brindley, G. W.; Yamanaka, S. Clays and Clay Minerals 1978, 26, 21. Hem, J. D.; Robertson, C. E.; U. S. Geol. Surv. Water-supply Pap. 1967, 1827-A. Smith, R. W.; Hem, J. D. U. S. Geol. Surv. Water-Supply Pap. 1972, 1827D.. Patterson, J. H.; Tyree Jr., S. Y. J. Colloid Interface Sci. 1973, 43, 389. Johansson, G. Acta chem. Scand. 1960, 14, 769. Pinnavaia, T. J.; Tzou, M.-S.; Landau, S. D.; Raythatha, R. H. J. Mol. Catal. 1984, 27, 195. See Akitt, J. W.; Farthing, A. J. Magn. Reson. 1978, 32, 345; J. Chem. Soc., Dalton Trans. 1981, 1617; 1981, 1624 for further details on these assignments. Vaughan, D. E. W.; Lussier, R. J. 5th Int. Conf. on Zeolites, Naples, Italy, Heyden, London, 1980. Lussier, R. J.; Magee, J. S.; Vaughan, D. E. W. 7th Canadian symp. Catal., Edmonton, Alberta, Oct. 19-22 1980. Occelli, M. L.; Innes, R. Α.; Hure, F. S. S.; Hightower, J. W. J. Appl. Catal.,

1985, 14, 69.


10.

CLEARFIELD AND KUCHENMEISTER



29.

143

Occelli, M. L.; Parulekar, V.; Hightower, J. Proc. 8th Int. Congr. Catal., Berlin, 1984. 30. Stul, M. S.; Mortier, W. J. Clays and Clay Minerals 1974, 22, 391. 31. Peogneur, P.; Maes, Α.; Cremers, A. Clays and Clay Minerals 1975, 23, 71. 32. Fripiat, J. J. in Ref. 2, p 281 ff. 33. Plee, D.; Borg, F.; Gatineau, L.; Fripiat, J. J. J. Am. Chem. Soc. 1985, 107, 2362. 34. Pinnavaia, T. J.; Landau, S. D.; Tsou, M.-S.; Johnson, I. D.; Lipsicas, M. J. Am. Chem. Soc. 1985, 107, 7222. 35. Shabtai, J.; Lazar, R.; Oblad, A. G. New Horizons Catal. 1981, 7, 828. 36. Tennakoon, D. T. B.; Thomas, J. M.; Jones, W.; Carpenter, Τ. Α.; Ramdas, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 545. 37. Tennakoon, D. T. B.; Jones, W.; Thomas, J. M.; J. Chem. Soc., Faraday Trans. 1, 1986, 82, 3081. 38. Burch, R.; Warburton, C. I. J. Catal. 1986, 97, 503. 39. Bartley, G. J. J.; Burch, R. J. Appl. Catal. 1985, 19, 175. 40. Pinnavaia, T. J.; Tsou, M. S.; Landau, S. D. J. Am. Chem. Soc. 1985, 107, 4783. 41. Yamanaka, S.; Yamashita, G.; Hattori, M. Clays and Clay Minerals 1980, 28, 281. 42. Endo, T.; Mortland, M. M.; Pinnavaia, T. J. Clays and Clay Minerals 1980, 28, 105. 43. Christiano, S. P.; Wang, J.; Pinnavaia, T. J. Inorg. Chem. 1985, 24, 1222. 44. Clearfield, Α.; Vaughan, P. A. Acta Crystallogr., 1956, 9, 555. 45. Muha, G. M.; Vaughan, P. A. J. Chem. Phys. 1960, 33, 194. 46. Clearfield, A. Inorg. Chem. 1964, 3, 146. 47. Clearfield, A. Rev. Pure Appl, Chem. 1964, 14, 91. 48. Rijnten, H. In Physical and Chemical Aspects of Adsorbents and Catalysts; Lensen, B. G., Ed.; Academic Press: New York, 1970. 49. Clearfield, Α.; Nancollas, G. H.; Blessing, R. H. in Ion Exchange and Solvent Extraction; Marinsky, J. Α.; Marcus, Y., Eds.; Marcel Dekker: New York, 1973, Vol. 5, Ch. 1. 50. Fryer, J. R.; Hutchinson, J. L.; Paterson, R. J. Colloid Interface Sci. 1970, 34, 238. 51. Johnson, J. S.; Kraus, K. A. J. Am. Chem. Soc. 1956, 78, 3937. 52. Jones, S. L. ref. 2, Ch. 3. 53. Sterte, J. ref. 2, Ch. 4. 54. Bartley, G. J. J. ref. 2 Ch. 5. 55. Farfan-Torres, E.M.; Grange, P.J. Chem. Phys. 1990, 87, 1547. 56. Tzou, M. D.; Pinnavaia, T. J. ref. 2, Ch. 6. 57. Reichle, W. T. Chemtech, Jan. 1986, p 58. 58. Reichle, W. T. J. Catal. 1985, 94, 547. 59. Pope, M. T. Heteropoly and Isopoly Oxometallates; Springer-Verlag: New York, 1983. 60. Drezdon, M. A. Inorg. Chem. 1988, 27, 4628. 61. Kwon, T.; Tsigdinos, G. Α.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. 62. Kwon, T.; Pinnavaia, T. J. Chem. Mat. 1989, 1, 381. 63. Dimotakis, E. D.; Pinnavaia, T. J. Inorg. Chem., 1990, 29, 2393. 64. Jones, W.; Chibwe, M. in Pillared Layered Structures; Mitchell, I. V., Ed.; Elsevier: Amsterdam, 1990. 65. Clearfield, A. Comments Inorg. Chem. 1990, 10, 89. 66. Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311. 67. Clearfield, Α.; Duax, W. L.; Smith, G. D.; Thomas, J. R. J. Phys. Chem. 1969, 73, 3424.



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68. Clearfield, Α.; Tindwa, R. M. Inorg. Nucl. Chem. Lett. 1979, 15, 251. 69. Clearfield, Α.; Roberts, B. D. Inorg. Chem. 1988, 27, 3237. 70. Maueles-Torres, P; Olivera-Pastor, P.; Rodriguez-Castellon, E.; JimenezLopez, Α.; Tomlinson, A. A. G. ref. 3, p 137. 71 MacLachlan, D. J.; Bibby, D. M. J. Chem.Soc.,Dalton Trans. 1989, 895. 72. Kuchenmeister, M.; Clearfield, Α., work in progress. 73. Tomlinson, A. A. G., ref. 3, p 91. 74. Alberti, G.; Costantino, V.; Marmottini, F.; Vivani, R.; Zappelli, P. ref. 3, p 119. 75. Piffard, Y.; Oyetola, S.; Courant S,; Lachgar, A. J. Solid State Chem. 1985, 60, 209. 76. Piffard, Y.; Verbaere, Α.; Lachgar, Α.; Courant-Deniard, S.; Tournoux, M. Rev. Chim. Mineral 1986, 23, 766. 77. Wade, K.; Clearfield, A. work in progress. 78. Cheng, S.; Wang, T. C. Inorg. Chem. 1989, 28, 1283. 79. Dion, M.; Ganne, M.; Tournaux, M. Mat. Res. Bull. 1981, 16, 1429. 80. Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 3727. 81. Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. J. Less-Common metals 1986, 116, 137. 82. Ram, R. A. Mohan; Clearfield, A. J. Solid State Chem. submitted. RECEIVED January 16, 1992


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