J. MATER. CHEM., 1991, 1(3), 319-326 319

Porous Cross-linked Materials formed by Oligomeric Aluminium Hydroxides and a-Tin Phosphate

Pedro Maireles-Torres," Pascual Olivera-Pastor,' Enrique Rodriguez-Castellon," Antonio Jimenez- Lopez,*' Lucilla Alagnab and Anthony A. G. Tomlinson*' a Departamento de Quimica Inorganica, Cristalogra fia y Mineralogia, Universidad de Malaga, Apartado 59, Malaga, Spain

I. T.S.E., Area di Ricerca del C.N.R., C.P. 10 Monterotondo Staz., 00016 Rome, Italy

The intercalation of the tridecameric polyhydroxyaluminium Keggin-type cation, formally [A I 0,A I ,( 0 H)24( 0 H,) ,] + , i n t o a-S n ( H PO,) , H,O via the co I I o i d a I tetra met h y I am m o n i u m -exc h ang ed i n t e r m ed i ate x-Sn[NMe,]o,,~,,oHl,l~l~o(P04)2~4H,0 and the alumina-pillared materials obtained after calcination are described. Two different intercalated precursor materials are obtained, depending on whether the Keggin ion inserted derives from the commercial product ('Chlorhydrol') or from the polyhydroxyaluminium cation generated in situ. Calcination leads to materials differing in free heights and in alumina contents. Their surface areas (B.E.T., N,, 77 K) are quite high: 190 m2 g- ' [chlor-SnP (400 "C)] and 228 m2 g-' [All,-SnP (400 "C)]. Pore-size calculations show them to be mainly mesoporous, but with some micropore contribution (>50% of pores in width range 15-40 A). The higher microporosity of the former with respect to the latter is ascribed to lateral-order differences between the alumina pillars.

High cation-exchange capacities (for Co2+, Ni2+ and Cu2+) confirm that both solids are porous and have more accessible interlayer sites than does the parent material. Optical spectra of the transition-metal ion- exchanged materials indicate that the sites available in the two solids differ, and that both differ from those present in the starting a-tin phosphate. Site geometries are suggested.

Keywords: Pillaring; x-Tin phosphate; Aluminium hydroxide

The last 10 years have seen an explosive growth in attempts to prepare reproducibly two-dimensional porous materials from smectite clays by inserting large cationic species.' The original motive was to find catalytically active large-pore materials robust enough to withstand reforming after use as heavy-fraction catalysts in the petroleum industry.' However, those clay-based materials which most closely fulfill the three criteria required of such a catalyst, i.e. (i) a high surface area in pores large enough to allow fast diffusion of small- to medium-sized molecules (8- 12 A), (ii) acidic, ion-exchange properties allowing access to metal ions as promoters, and (iii) high thermal stability (>400 "C), tend to collapse under mild hydrothermal condition^.^

Although layered Group 14 phosphates are not as readily swellable as smectites,, it has recently been shown that they can be swelled partially to allow insertion of organic ligands with subsequent development of a complex-pillar chemistry.' We now describe a general method for inserting the Keggin- like cation [A104A112(0H)24(0H2)12]7+ as both the commer- cial product ('Chlorhydrol') and as prepared in situ, into a-tin phosphate. Calcination of these products gives rise to two series of phase-pure stable pillared materials which are porous, with interlayer sites more accessible than those present in the parent materiak6 Similar techniques have been reported for x-zirconium and y-titanium phosphates7 and layered titanates8

Experimental a-Tin phosphate [a-Sn(HP04)2 H 2 0 , ('a-SnP')] was prepared as described previouslyg and had dOo2 = 7.8 A (interlayer dis- tance) and a specific surface area (B.E.T., N2, 77 K) of 1 1 m' g-'. It was conserved at 70% relative humidity. 'Chlorh ydrol', [A104A1 2( OH),,( OHz) '1 Cl,, was a commer- cial product (Reheis, NJ, USA; supplied as a powder). Chemi- cal analysis for A1 and thermogravimetry (TG) confirmed the

formulation, and the product was used as received. (All products derived from this material are referred to as chlor- SnP.)

The All, oligomer was prepared essentially as described by Tokarz and Shabtai.'O An aqueous solution of 0.2 mol dm-3 NaOH was added slowly to a 0.2 mol dm-, solution of AlCl, (commercial pure product, Merck) to a final pH of 4.35. The resulting solution was left to stand for 20 days at 25 "C and then used immediately. (All products derived from this precur- sor are referred to as All, oligomer-SnP.)

[NMe,]Cl and n-propylamine (nPr) were commercial products and were used as received.

Formation of a-Tin Phosphate/n-Propylamine Colloidal Suspension

a-SnP (5 g) was suspended in a 0.1 mol dm- aqueous solution of n-propylamine [corresponding to 60% of the theoretical cation exchange capacity (c.e.c.)] for 2 h at 25 "C. The half- exchanged form 'a-Sn * HnPrA * H P , was obtained [as demon- strated by TG, X-ray diffraction (XRD) and chemical analysis on the filtered, washed solid). The colloidal suspension could be preserved as such for >1 month without flocculation and readily formed films on casting, similar to those of or-zirconium phosphate previously described."

Insertion of Aluminium Polyhydroxy Cations into the mealf fa-SnP Colloidal Intermediate

When freshly prepared a-Sn HnPrA - H P colloid was con- tacted with either Chlorhydrol or All, oligomer solutions, at various mole ratios and temperatures, the solids separated were amorphous or showed high collapse at low calcination temperatures (< 200 "C). They also contained organic frag- ments. It was first necessary to sweep out interlayer protons partially.

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320 J. MATER. CHEM., 1991, VOL. 1

To the colloidal suspension of a-Sn-HnPrA-HP was added an aqueous solution (50 cm3) of 16.72 g of [NMe,]Cl equiva- lent to 10 times the theoretical c.e.c., based on starting a-SnP. After 1 day of stirring at 25 "C, the fine suspen- sion was centrifuged and the solid was washed with water and air-dried. It was analysed (chemical and TG) for a-Sn[NMe4]o~9-l.oHl .'- .-,(PO& *4H20 (referred to as NMe,-SnP). It formed stable, colloidal suspensions, could be cast as a film, and XRD and IR spectra showed that neither a-SnP nor starting n-PrA intercalate was present.

A1 Oligorner-SnP Aliquots of Al13 oligomer solutions were contacted with colloidal NMe,-SnP suspensions at various molar ratios, each being stirred for 1 day at 25 "C. The suspension was centri- fuged, and the solid washed well with water and air-dried. The A1 content of the remaining solution was estimated colorimetrically (alizarin method). Each solid was also treated with 0.2mol dm-3 HCl, which extracted A1 (to leave a-SnP) and again analysed for Al. The two methods gave identical results.

Chlor-SnP Chlorhydrol(l.28 g) in 50 cm3 of water was added to colloidal NMe,-SnP formed from 1 g of NMe,-SnP and the suspension stirred for 1 day. The solid was separated from the equilibrium solution by centrifugation (15 000 rpm, 10 min), washed well with water and air-dried.

In a second series of preparations, after contacting, the suspension was dialysed (Visking 9-36/32" tube) against water with four daily changes to a conductance of remaining water of 20 pS. The dialysate was then filtered on a milli- pore and dried in air. This material is described in detail elsewhere.12

Calcination to Alumina-containing Products

The two final materials were calcined at various temperatures and basal-spacing changes were monitored by XRD (Fig. 3, later). The products obtained after calcination at 400 "C were those used in conventional surface-area determinations (together with the initial a-SnP, and the intercalated precursor material). Cation exchange on these same products was carried out with conventional batch methods5 with metal@) acetate solutions.

Physical Measurements

XRD patterns were registered on a Siemens D-501 diffractometer both on powders and on cast films. Surface areas were measured with a Carlo Erba Sorptomatic 1800 instrument. Samples were first degassed at 150 "C, and adsorp- tion-desorption of N2 followed at 77.4 K. Electronic spectra, as reflectance, were mezwred on Beckmann DK 2A and Shimadzu MPC-3 100 spectrophotometers against BaSO, or MgO as reference. TG and differential thermal analysis (DTA) data were recorded on a Rigaku Thermoflex, using calcined alumina as a standard.

Results and Discussion When either aged All, oligomer solution or Chlorhydrol solution is contacted with colloidal a-Sn HnPrA *HP there is only low, disordered, intercalation of the polyhydroxy cation. Conversely, preliminary exchange with "Me,] + provides a simple preparative route for intercalated precursors, with complete removal of the "Me,] + after contacting.

A

281"

B 17.4

3 )

11.1

8.6

c) - 11.1

d , 10.5

A 6 10 14

281" 3

Fig. 1 Cu-Ka XRD patterns (A) starting a-Sn-HnPrA-HP (B) a- Sn[NMe4]o~,-,~oH,,~-l~o(P04)2 .4H@ precursor, showing phase changes on dehydration. T / "C: (a) 25, (b) 100, (c) 200, ( d ) 300

As shown by the XRD (Fig. 1) the solid has an interlayer distance that is much larger (17.4A) than expected on the basis of the diameter of "Me,]' (5.3 A) alone. The assump- tion that a-SnP has a layer thickness roughly the same as that of a-Zr(HP04)2H20, i.e. 6.5 A: leads to an expected interlayer distance of ca. 11.8 A. The material has a large amount of interlayer water, most of which is removed at 200 "C with collapse from 17.4 to 11.1 A. This may be rationalised by assuming that there is ordering of water molecules between the "Me,] + and layer -P-OH groups, as demonstrated for a-VO(P04)*5H20.i3 As expected, at 300 "C there is a final collapse to mixed phases of a-SnP and a 10.4A phase in which the "Me,]' resides in the cavity formed by groups of interlayer -P-OH.,

The uptake of the two polyhydroxy cations into a-SnP is similar at lower polyhydroxy cation : a-SnP ratios, but diverges at higher ratios (Fig. 2). Two well known phenomena may be in operation: (i) A change in nuclearity caused by pH changes. An increase in pH is expected to increase the degree of hydrolysis:'

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- E

32 1

c

mol Al moiety added/g Me,NSnP

Fig. 2 Uptake of [A104Al,,(OH),4(OH,),217 + by NMe4-SnP from AlC1,-NaOH mixed solutions (-) and from Chlorhydrol solutions (- - -) (assuming that the sole species present is the Keggin ion)

[A1 1304(0H)24(0H2)121 + OH - +

[ A1 04( OH),,( OH ,) 1] + + H 0 fo" - CAI 1 3°4(0H)26(H20) 101 + 2O

which, in turn will increase the number of A1 species exchanged. (ii) A change in nuclearity; NMR measurements show that species of higher nuclearity, e.g. [A116(0H)36(H20)24]1Z +, and other unidentified one^,^.^^ exist.

Exchange of such species is expected to give rise to a decrease in A1 species uptake. Equilibrium pH values were in both cases comparable (4.1-4.5) so the uptake differences are more readily imputed to the presence of counter-ions. Thus, the Na+ present in the All, oligomer solution will partially exchange for NMel on the open phosphate surfaces and is subsequently less easily exchanged. The lower surface charge leaves fewer sites available for exchanging A1 species, as observed.

Other effects are also operating, as is clear from the XRD of Fig. 3 and the associated TG/DTA analyses of Fig. 4. In the clay-pillaring studies reported to date, the interlayer spacing of the smectite increases from ca. 9.3 to 18-19 A. This has been taken naively to mean that the ion intercalated is indeed [A10A112(0H)24(0H2)12]7+,1~ which has a prolate ellipsoidal shape with a long axis of ca. 9.5 A.16 Al13-SnP more closely fits this picture than does chlor-SnP, the expan- sion being 17.7-7.8 A = 9.9 A. However, insertion of Chlorhy-

A

12.3 , j

'- ' 13.1

14.4 :: II ; .>, .I '

- - - - - - - - -- ._ _ _ _ -___.

(c)

I I I I I I I I

16 12 8 4 ,,,,,,,11

16 12 8 4 2eto

Fig. 3 Variation of XRD (Cu-Kcr) of (A) All,-SnP and (B) chlor-SnP with temperature: (a) 25, (b) 200, (c) 300, (d) 400 and (e) 500 "C

I I I I I I I I I

7°C

Fig.4 TG and DTA curves of: (a) Chlorhydrol; (b) All,-SnP (c) chlor-SnP. (-) Weight loss; (- - -) DTA

drol leads to an expansion of 16.4 A, suggesting that either a bilayer is formed or the pH conditions on the phosphate surface (about which little is known) are different from those in the Na+-buffered surface of the Al13 oligomer case. This can give rise to more than one cluster species via hydrolysis (Scheme 1).

The evidence points to both effects being present. Thus, after calcination, apart from the characteristic dOo2 reflection, the chlor-SnP materials give broad do04 reflections, indicating that a second phase is present. Secondly, in both materials, the water molecules are removed smoothly up to ca. 400 "C (in itself unusual, there is no clear evidence for a distinction between intercalate water and zeolitic-type water). However, chlor-SnP collapses by ca. 8.7 A at 200 "C, whereas Al,,-SnP collapses by only 3.3 A. More water is lost with much smaller changes in interlayer distance, and these are then followed by a broad, composite exotherm in chlor-SnP (yet further evi-

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Scheme 1 (a) Possible bilayer formulation, showing coalescence of precursor species after calcination. (b) Hydration sphere formulation, showing possible hydrogen-bonding to layers. (Cross-hatched, hydroxide; hatched, oxide)

dence for the presence of more than one intercalate precursor) which is much less evident in Al13-SnP.

Note that aluminium hydroxides and oxyhydroxides give endotherms in DTA; that for Chlorhydrol is shown for comparison in Fig. 4 (see also ref. 17). Furthermore, the TG/ DTA curves show no evidence for a PO:- > P20$- conden- sation (expected to lie at 450-500 "C). Moreover, treatment with 0.2 mol dmP3 HCI solutions did not lead to changes in the basal spacings of the materials calcined at 400 "C. Table 1 provides an estimate of the differing pillar densities in both calcined materials on the assumption that [A10,A1,2(0H)24(0H2)12]7 + is the entering species.

Surface Area and Pore Distribution

N2 adsorption-desorption isotherms for the intercalates and the pillared materials [shown by the temperature of calci- nation, e.g. Al,,-SnP (400 "C)] are shown in Fig. 5. a-SnP itself is non-porous, with a B.E.T. surface area of 11 m2 g-', whereas the intercalate precursors and pillared materials are porous, the former having B.E.T. surface areas of 80-90 m2 g-' and the latter 190-230 m2 g-'.

The many hazards in partitioning porosity effects are well documented, as exemplified by the extensive literature on carbons.18 The pore volumes obtained are considered appar- ent pore volumes because their shapes are not known.lg The curves were analysed using both Pierce's method (closed cylindrical pore model)20 and Cranston and Inkley's method (open cylindrical pore model).'l Both the 't'22 and the a, method23 were used for assessing the presence of microporos- ity. For both, a-SnP itself was used for the reference isotherm, and application of the a,-method, as shown in Fig. 6 , demon-

Table 1 Analyses of Keggin-type intercalatesn ~ _ _ _ _ _ _ _ ~

A1 A120, no. of oligomer (mmol All, H 2 0 ' (mmol g-' units per unit per g SnP) (YO) (SnP) celld

A1 ,-SnP 1.97 30.14 2.95 0.4 chlor-SnP 3.05 30.05 4.49 0.6

In all cases, organic matter < 1 %; using alizarin S; ' from the TG of Fig. 4; assuming the radius of A1,,04(0H)24(0H2)12]7+ =ca.

oligomer intercalate occupies ca. 32/86.4 =40% of the area ab. 5 A, and using a= 5.02 A, b=8.61 A for u-S~(HPO,)~.H,O, the

0.2 0.4 0.6 0.8 1.0 PIP,

Fig. 5 Adsorption-desorption isotherms of N2 on (a) Al,,-SnP (400 "C); (b) chlor-SnP (400 "C)

200

160

7

I m "E 120

iT 5

0 . I-

Y) D

80

1 .o 2.0 US

Fig.6 Application of the us-method to the N2 adsorption curves. (a) u-SnP (reference); (b) chlor-SnP (400 "C); (c) All,-SnP (400 "C)

strates that this choice was valid. The adsorption data reveal that although all the materials are predominantly mesoporous, there is some contribution by micropores in the pillared (i.e. calcined) materials. Table 2 lists the main textural parameters obtained.

For both calcined materials, most pores (>50% of B.E.T. surface area) have radii in the range 15-40 A, as seen in Fig. 8 (later). Note that: (i) the radii of Fig.8 are Kelvin radii, and application of the Kelvin equation breaks down at fp < ca. 10 i.e. above the range of micropores expected in pillared materials; (ii) the range which constitutes micro- and meso-porosity is controversial. Mesopores have been defined

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Table 2 Surface parameters of a-tin phosphate, aluminium oligomer exchanged tin phosphate and alumina pillared tin phosphate

material S,.,.,./m2 g-' S,/m2 g-' * S,/m2 g-' K/cm3 g-' Vmic/cm3 g-' a rplAb a-SnP 11.0 - 12.0 0.052 - 86.7 A1 , -SnP 89.5 - 80.9 0.320 - 79.1

chlor-SnP (400 "C) 190.0 158.2 141.5 0.295 0.019 41.7 All,-SnP (400 "C) 228.1 197.0 193.7 0.460 0.022 47.5

a Sing's method: Vmic = micropore volume (a,-plot); S, = specific surface area. Pierce's method: S , = accumulated surface area; V, = accumulated volume; rp = average pore radius.

as having a width 20-5OOA (the 20-30A range being am- biguousZ3) and the pores of interest in pillared clays have been defined as those with diameter < 30 A.24 Mesopores are believed to be induced by platelet stacking or end-end interactions, as shown in Scheme 2 ( ~ ) . ' ~ Given the probable more disordered lateral pillaring, coupled with the suscepti- bility of a-SnP to hydrolysis, Scheme 2(b) seems the more realistic.26 In other words, pillaring may induce higher porosity at the lower end of the mesopore range, which is not rationalisable from pore sizes deduced from XRD and (idealised) pillar density. From Fig. 7, it appears that more such sites have been induced in chlor-SnP (400 "C) than in AlI3 oligomer-SnP (400 "C).

Cation Exchange and Site Accessibility

The sites in the interlayer are expected to be modified by pillaring. Fig. 8 shows the uptake curves for Co2+, Ni2+, and

Scheme 2 End-side particle interactions giving rise to mesopores. (a) Ideal regular pillaring; (b) with 'cut' layers due to hydrolysis (or other long-range disorder)

I 0 10 30 50 70 90

r,/A

I 0 10 30 50 70 90

r,/A

Fig. 7 Pore size distribution curves. (a) All,-SnP (400 "C); (b) chlor- SnP (400 "C); (-) Pierce's method;" (- - - - - - ) Cranston and Inkley's met hod2

c2u 0 2 6 10

M2+lg (ad.)

0.2

' 2 6 10 M2+/g (ad.) M2+/g (ad.)

Fig. 8 Uptake curves of Co2+ (M), Ni2+ (A) and Cu2+ (0 ) by Al,,-SnP (400 "C) and chlor-SnP (400 "C), (a) by chlor-SnP (400 "C) containing two different concentrations of the precursor: (0 ) point (i); (0) point (ii); (b) by All,-SnP (400 "C); (c) by All,-SnP (400 "C), first treated with NH3 gas

Cu2+ (in no case was there evidence for pillar elution). A comparison of Cu2+ uptake by calcined materials at points (i) and (ii) of the Chlorhydrol uptake curve (see Fig.2) underlines the dramatic difference caused by in-layer pillar ordering: uptake of Cu2+ is almost an order of magnitude lower by material (ii) than by (i).

The uptake of Cu2+ by Al,,-SnP (400°C) is very similar in magnitude to that of chlor-SnP (400 "C) [see Fig. 8(c)]. However, the exchange is faster than in the Chlorhydrol

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analogue, which is in agreement with the (naive) calculation of A1203 pillar density shown for the former in Table 1, and against changes in ion exchange being due to differences in inter-stacks such as those in Scheme2(a). The influence of specific hydrogen-bonding in the interlayer also needs to be taken into account. According to V a ~ g h a n ~ ~ calcining All,- exchanged clay in an NH, atmosphere leads to maximum cation-exchange capacity for the PILC (pillared layered clay) produced. Presumably, interlayer ammonium ions are pro- duced that are no longer available for migration into vacancies in the octahedral part of the framework, thus enhancing the c.e.c. of the interlayer. Although the single interlayer of the phosphate is very different than in a PILC,26 Fig. 7(c) shows that All,-SnP c.e.c.s are enhanced by almost an order of magnitude on prior ammoniation. They are much higher than those in smectite clays (0.5-1.5 mmol g-', ref. 27).

Ammoniation also causes an inversion of uptake rates between Cu2+ and Co2+. In studies of pore accessibility in complex-pillared zirconium phosphates carried out to date, the order has been found to be: Cu2+ >Coz+ > Ni2+ . 28 The reverse order obtained here suggests that the species exchang- ing into the All,-SnP material are the hydrates, M(OH,);+ (M = Co2+, Ni2+) and Cu(OH2)i+, which is further evidence for easy accessibility on pillaring.

Particularly clear evidence that different sites are present, as well as different accessibilities, comes from Ni2 + -exchanged

A I A

J I I I I I ' 1 I I I I I 1 I I I I 1 1 I m IJnm

Fig. 9 Electronic spectra of Ni2+-exchanged pillared materials (first treated with NH, gas). (a) Chlor-SnP (400 "C); (b) All,-SnP (400 "C); (-) as prepared; (- - -) after heating at 350 "C (d-d band positions shown in cm-')

NH,-treated chlor-SnP (400 "C) and AlI3-SnP (400 "C) (Fig. 9). Two sites are present, one pseudo-octahedral, as shown by weak d-d bands at 9600 and 13 500 cm-' ascribable to ,T1 t ,A2 transitions of a cis rather than a trans geometry,29 and application of the classical average environment rule leads to the conclusion that this species contains an [NiN303] chr~mophore.~' The major Ni2 + site is, however, represented by that giving a d-d band at 16 700 cm-', an energy too low to be due to a square-planar [NiN4] chromophore (e.g. 'AZg+- 'Alg, 22 500 cm-I for [Ni(NH3)4]2+)31 but applicable in terms of an [Ni04] chromophore deriving from weak (in ligand-field terms) PO:- ligands. On removal of all water and NH,, almost all are no longer present and there is only a distinct shoulder at high energy: 24 400 cm-'. Such a high energy can be due only to a square-planar [Ni04] unit in which the oxygen donors are provided by the A1203 pillar.32

In All,-SnP (400 "C), Ni2+ is clearly in a very different site [Fig. 9(b)] with a low-energy d-d band, at 7700 cm-' [com- pare with Ni(OH2);+ having v1 at 8500 cm-', ref. 301. When taken together with the low energies of the other d-d bands characteristic of a pseudo-octahedral environment, this clearly indicates that the Ni2+ is in a constrained environment of six oxygen atoms provided by phosphate groups. On complete dehydration and deammoniation, the site geometry changes drastically, the spectrum being very reminiscent of the pseudo- trigonal bipyramidal [Ni05] site present in low-Ni2 +-loaded a-zirconium ph~sphate.~, These sites are completely different from those available in a-SnP i t~e l f .~

W L I 1 1 006

IJnm

Fig. 10 Electronic spectra of Co2+-exchanged Al,,-SnP (400 "C), (a) without and (b) with prior adsorption of NH, gas: (-) as prepared; (- - -) after heating at 350 "C (d-d band positions shown in cm-')

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The spectra of Co2+-exchanged species also show that the sites accessible are different from those in the non-pillared material (Fig. 10). Co2+-exchanged Al13-SnP gives d-d bands characteristic of a pseudo-octahedral en~ i ronmen t ,~~ which do not change to those of a pseudo-tetrahedral moiety on dehydration. Conversely, the first-ammoniated material [sep- arated at the plateau of Fig. 7(c)] also has a pseudo-octahedral Co2 + site, which on dehydration and deammoniation changes to a pseudo-tetrahedral site, [Coo,]; the low energy of the d-d bands suggesting that the Co2+ is bonded to the layer phosphate groups.

Turning to the Cu2 +-exchanged materials, there seems little obvious difference between the chlor-SnP and Al,,-SnP mater- ials. Both give broad d-d bands at 13 000-14 000 cm-', apparently deriving from a single site. The relatively low energy is as expected for a tetragonal-octahedra1 geometry. Little change occurs on either dehydration or calcination, the d-d bands shift to lower energy. We note that Cu2+/a-A1203 gives very-low-energy d-d bands due to the presence of both tetragonal-octahedral and pseudo-tetrahedral sites.34 We infer that the Cu2+ loads at the layer sites. Conversely, at higher loading levels d-d bands broaden on calcination [Fig. ll(c)], suggesting that Cu2+ co-ordinates to more than one site.

Finally, Fig. 12 shows the reflectance spectra of several low-

1, 13100

\ ( b ) 12500 I

-. 'L.-/\--

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

I l n r n 1000 2000

Fig. 11 Electronic absorption spectra of Cu2+-exchanged pillared materials at different temperatures: (-) 25 "C; (- - -) 120 "C; (. - . - . -) 350 "C. (a) Low-loaded Cu'+-exchanged chlor-SnP (400 "C); (b) Low-loaded Cu2+-exchanged Al,,-SnP (400 "C); (c) High-loaded Cu2+-exchanged AI,,-SnP (400 "C) (d-d band positions shown in cm-')

16 300

17 100

I l r l l l l l l , l l l l l l , l l 1000 2000

I l n r n

Fig. 12 Electronic absorption spectra of Cu2+-exchanged pillared materials, before (-) and after (-----) amine loading. (a) 150% c.e.c. after absorption of NH, (3 h, 25 "C); (b) 150% c.e.c. after absorption of en (3 h, 50 "C); (c) as for (b), 10% c.e.c. (d-d band positions shown in cm- ')

and high-loaded Cu2 + samples before and after contact with NH3 or ethane-1,2-diamine (en). 10% c.e.c. Cu2 +-chlor-SnP has a low-intensity band at 13 100cm-', enhanced in the 150% c.e.c. Cu2 +-exchanged analogue. After absorbing en, the former gives rise to a new band at 17 100 cm-' and the latter one at 16 300 cm-' with a clear shoulder at 10 500 cm-'. (Compare with a d-d band at 14 300 cm-' in or-Sn[Cu(en),~,](P04)2 *2.2H20 itself, for which a very restric- ted site accessibility was suggested.'} Instead, the d-d energies indicate that two sites are present and both allow access to bi- dentate en. Similarly, after the absorbance of NH3, the d-d band position in Cu2+-exchanged chlor-SnP, 15 650 cm-' (with unclear shoulder at ca. 11 000 cm- '), is very different from that in the [Cu(NH3),J2'-SnP analogue.' Again, we suggest that the interlayer Cu2+ is more accessible in the former and the electronic spectrum is consistent with a five- co-ordinate moiety (although probably one containing four rather than five NH3 molecules).

Conclusions The tridecameric polyhydroxyaluminium cation [A104A1 12( OH)24(OHZ)12] + can be intercalated into or-tin phosphate, both from Chlorhydrol solutions and as the in situ prepared moiety. Two different alumina pillared materials are obtained on calcination, and surface measurements show that both have high specific surface area (190-228m2 g-'). They are predominantly mesoporous but some microporosity

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326 J. MATER. CHEM., 1991, VOL. 1

They are predominantly mesoporous but some microporosity induced by the pillaring is present. Further work is necessary to unravel how lowered layer charge influences pillar density and effects due to hydrolysis. The contribution to porosity of amorphous components also requires investigation.

Cation exchange demonstrates that accessibility of acid sites in the interlayer is quite high, especially when enhanced by exchange with a better leaving group, such as NHZ. A possible explanation is that the cleaning effect of ion exchange may remove decomposition products, which block narrower pores, that pillar-pillar gaps are extremely critical and that access to large areas of the surface is forbidden for nitrogen.

Further investigations of the pore properties of these, and other alumina-pillared materials are underway.

We are extremely grateful to the E.E.C. EURAM Programme (Contract No. MA 1 E/0027/I) and C.A.I.C.Y.T. (Project No. PB86/244) for their financial support.

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Paper 0/03600E; Received 6th August, 1990

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