Factors Influencing on the Surface Properties of Chromia-Pillared α

Langmuir 1998, 14, 4017-4024

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Factors Influencing on the Surface Properties of Chromia-Pillared r-Zirconium Phosphate Materials F. J. Pe´rez-Reina, P. Olivera-Pastor, P. Maireles-Torres, E. Rodrı´guez-Castello´n, and A. Jime´nez-Lo´pez* Departamento de Quı´mica Inorga´ nica, Cristalografı´a y Mineralogı´a, Facultad de Ciencias, Universidad de Ma´ laga, 29071 Ma´ laga, Spain Received October 30, 1997. In Final Form: April 15, 1998 A new method for preparing colloidal suspensions of R-zirconium phosphate (R-ZrP), based on the use of a phase swelled with n-propylamine vapors, is described and applied to the synthesis of new chromiapillared materials. The colloidal phase was obtained by contacting the n-propylamine intercalate with a dilute aqueous solution of acetic acid. For comparison, a conventional procedure of colloidization, based on the addition of a n-propylamine aqueous solution to the pure phosphate, was also employed. The intercalation of polyhydroxoacetato CrIII clusters was accomplished by refluxing chromium acetate solutions with each colloidal phosphate suspension. The new method gave rise to chromium oligomer intercalates with higher basal expansions, and moreover, larger oligomeric species seemed to be intercalated with a high chromium concentration in solution. Two series of chromia-pillared materials were obtained by calcination of the intercalated precursors at 673 K under N2. In both series, the acidity, as determined by TPD-NH3 and pyridine adsorption, increased with chromium content. The chromia-pillared materials prepared by the new method displayed a high porosity and catalytic activity for the decomposition of isopropyl alcohol. Under the experimental conditions used, all catalysts behaved exclusively as dehydrating catalysts.

Introduction Pillaring of layered compounds by insertion of inorganic oligomeric species is a well-known route to prepare micro/ mesoporous solids with acidic properties.1 Among them, pillared clays have been by far the most widely studied materials for different applications, including catalysis and sorption.2 Similar to clays, the intercalation of large positively charged hydroxypolycations into the interlayer region of layered MIV phosphates is greatly facilitated by previous formation of stable colloidal suspensions of the host.3 This process implies generating a permanent negative charge on the phosphate surface by acid-base reactions. Alberti et al.4 observed the formation of a colloidal phase of R-zirconium phosphate upon neutralizing about 50% of the P-OH groups with n-propylamine in aqueous solution. By use of this colloidization method, the intercalation of many different bulky species, such as Al, Cr, Si, Al/Cr, Ga/Cr, and Fe/Cr oligomers in layered R-Sn and Zr phosphates has been possible.5,6 Concerning the colloidization of acid layered metal(IV) phosphates, some problems may arise when aqueous solutions of the exfoliating base are used. Diffusion of the amine through the interlayers is conditioned by the particle size of the host and the increase of the solution pH, which may produce a partial hydrolysis of the phosphate. Thus, homogeneous neutralization and hence (1) (a) Pillared Layered Structures: Current Trends and Applications; Mitchell, I. V., Ed.; Elsevier Applied Science: London, 1990. (b) Multifunctional Mesoporous Inorganic Solids; Sequeira, C. A. C., Hudson, M. J., Eds.; NATO ASI, Kluwer Academic: Dordrecht, 1993; p 400. (2) (a) Pinnavaia, T. J. Science 1983, 220, 365. (b) Burch, R., Ed. Catal. Today 1988, 2, 185. (c) Figueras, F. Catal. Rev.-Sci. Eng. 1988, 30, 457. (d) Mokaya, R.; Jones, W. J. Catal. 1995, 153, 76. (e) Yang, R. T.; Baksh, M. S. A. AIChE J. 1991, 37, 679. (3) Olivera-Pastor, P.; Maireles-Torres, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Cassagneau, T.; Jones, D. J.; Rozie`re, J. Chem. Mater. 1996, 8, 1758. (4) Alberti, G.; Casciola, M.; Costantino, U. J. Colloid Interface Sci. 1985, 107, 256.

complete exfoliation of a host with a wide range of particle sizes may be difficult to achieve, unless a strict control of different variables such as amine addition rate, concentration, pH, and aging is maintained. It is, therefore, expected that an improvement in colloidization methods should be helpful in order to obtain metal oxide-pillared materials with reproducible properties. In the present work, we have investigated a new method of colloidization using an intercalate of R-zirconium phosphate with n-propylamine adsorbed from the vapor phase. The aims of this work were, therefore, to demonstrate the effectiveness of this new method of colloidization for preparing chromia-pillared materials and to study its influence on the surface properties of these compounds. For this purpose, the properties of the chromia-pillared materials prepared by a conventional method were contrasted with those of the new compounds. All materials were characterized using chemical analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption, acidity measurements, and the test reaction of isopropyl alcohol decomposition. (5) (a) Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Alagna, L.; Tomlinson, A. A. G. J. Mater. Chem. 1991, 1, 319. (b) Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı´guezCastello´n, E.; Jime´nez-Lo´pez, A.; Tomlinson, A. A. G. J. Solid State Chem. 1991, 94, 368. (c) Sylvester, P.; Cahill, R.; Clearfield, A. Chem. Mater. 1994, 6, 1890. (d) Cassagneau, T.; Jones, D. J.; Maireles-Torres, P.; Rozie`re, J. In Synthesis of Porous Materials: Zeolites, Clays and Nanostructures; Occelli, M. L., Kessler, H., Eds.; Marcel Dekker: New York, 1996. (e) Cassagneau, T.; Jones, D. J.; Rozie`re, J. Manuscript in preparation. (6) (a) Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Tomlinson, A. A. G. J. Mater. Chem. 1991, 1, 739. (b) Olivera-Pastor, P.; Maza-Rodrı´guez, J.; Maireles-Torres, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A. J. Mater. Chem. 1994, 4, 179. (c) Me´rida-Robles, J.; Olivera-Pastor, P.; Jime´nez-Lo´pez, A.; Rodrı´guez-Castello´n, E. J. Phys. Chem. 1996, 100, 14726. (d) AlcantaraRodrı´guez, M.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Jime´nezLo´pez, A. J. Mater. Chem. 1996, 6, 247. (e) Pe´rez-Reina, F. J.; OliveraPastor, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A. J. Solid State Chem. 1996, 122, 231.

S0743-7463(97)01185-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998

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Pe´ rez-Reina et al.

Experimental Section Materials Preparation. R-ZrP was synthesized as described by Alberti and Torraca.7 Colloidal suspensions of this phosphate were prepared by two different methods. Method a was a variant of that reported by Alberti et al.,4 and it consists of adding dropwise a 0.1 M n-propylamine (nPA) aqueous solution to a suspension of R-ZrP (5 g L-1) under vigorous stirring up to pH ) 8 (70% of the cation exchange capacity (CEC)). Method b, used for the first time in this work, makes use of a fully saturated nPA intercalate as intermediate. A typical preparation of the colloidal phase is as follows: R-ZrP (2 g) was exposed to nPA vapors overnight and after removing the excess amine, in a desiccator containing concentrated phosphoric acid, the intercalate obtained was dispersed in 400 cm3 of a 0.03 M aqueous solution of acetic acid (80% of CEC). Subsequently, the pH was raised to 8 with a 0.1 M nPA aqueous solution. The colloidal suspensions a and b were separately contacted with chromium acetate (Carlo Erba) aqueous solutions, in which the [Cr3+] was varied from 0.015 to 0.031 M. The mixtures were refluxed for 4 days. After reaction, the green solids were recovered by centrifugation, washed three times with deionized water, and dried at 333 K. The two series of oligomeric chromium(III) intercalated materials synthesized are designated hereinafter as CrZrP-Xa or CrZrP-Xb. Chromia-pillared materials were obtained by calcining the intercalated precursor materials at 400 °C under N2. The calcined samples were washed with distilled water before characterization. Chemical Analysis and Characterization. Chemical analysis of intercalated precursor materials was carried out after digestion of samples with a NaOH/H2O2 solution. Chromium was determined colorimetrically using the chromate method (λ ) 372 nm). XRD of cast films, dried at 333 K or calcined at different temperatures, was performed with a Siemens D501 diffractometer, provided with a graphite monochromator and using Cu KR radiation. XPS analysis was obtained with a Physical Electronics 5700 instrument with a Mg KR X-ray excitation source (hν ) 1253.6 eV) and hemispherical electron analyzer. Accurate ((0.1 eV) binding energies (BE) were determinated with respect to the position of the C 1s peak at 284.8 eV. The residual pressure in the analysis chamber was maintained below 10-9 Torr during data acquisition. Adsorption-desorption of N2 on the calcined samples (77 K, outgassing at 473 K and 10-4 Torr overnight) was measured on a conventional volumetric apparatus. The assessment of the micropore volume (R-plots) was carried out using the material CrZrP-1a calcined at 1073 K as reference, which is a nonporous solid with a SBET of 34 m2 g-1. The total acidity of the samples was determined by temperature-programmed desorption of ammonia (TPD-NH3). Before the adsorption of ammonia at 373 K, the samples were heated at 673 K in a He flow. The TPD of ammonia was performed between 373 and 673 K, at 10 K min-1, and analyzed by an online gas chromatograph (Shimadzu GC-14A) provided with a thermal conductivity detector. For the adsorption-desorption of pyridine, self-supported wafers of the samples with a weightto-surface ratio of about 12 mg cm-2 were placed in a vacuum cell assembled with greaseless stopcocks and CaF2 windows. Pretreatments were carried out with an in site furnace. The samples were evacuated (573 K, 10-4 Torr overnight), exposed to pyridine vapor at room temperature and then outgassed at room temperature, 493 and 623 K. The IR spectra were recorded at room temperature using a Perkin-Elmer 883 spectrophotometer. Temperature-programmed reduction (TPR) of pillared samples was performed in a U-shaped quartz microreactor using a flow of Ar/H2 (40 cm3 min-1, 10% H2) and heating at 10 K min-1 between 313 and 973 K. The water produced in the reduction reaction was eliminated with a cold trap (193 K). The consumption of hydrogen was controlled by an on-line gas chromatograph provided with a thermal conductivity detector (Shimadzu GC14). Catalytic Activity Measurements. The catalytic activity of the samples in the decomposition of 2-propanol was tested at 493 K in a fixed bed tubular glass reactor operated at atmospheric pressure and with a catalyst charge of 25 mg without dilution. (7) Alberti, G.; Torraca, F. J. Inorg. Nucl. Chem. 1968, 30, 317.

Figure 1. XRD patterns of (a) pristine R-ZrP, (b) colloidal phase obtained by method a, (c) hydrogen form of sample (b), (d) R-ZrP with a bilayer of nPA adsorbed from the vapor phase, (e) solid obtained after treatment of sample (d) with a 0.03 M acetic acid aqueous solution (method b), (f) sample (e) after treatment with a 0.1 M nPA solution up to pH ) 8, and (g) hydrogen form of sample (f). The 2-propanol (chromatographic purity grade) was fed into the reactor by bubbling a flow of helium of 25 cm3 min-1 through a saturator-condenser at 303 K, which allowed a constant 2-propanol flow of 7.4 vol % and the spatial velocity of 49.6 µmol g-1 s-1. Catalysts did not show diffusion restrictions. Prior to the catalytic test, the samples were pretreated at 493 K under a flow of helium. The carrier gas was passed through a molecular sieve before it was saturated with 2-propanol. The reaction products were analyzed by an on-line gas chromatograph provided with a flame ionization detector and a fused silica capillary column SPB1.

Results and Discussion Colloidization of r-ZrP. Exfoliation of R-ZrP by method a needed an addition of nPA, in aqueous solution, corresponding to 70% CEC to ensure a complete disappearance of phases with d001 spacings lower than 14 Å and alkalanize the suspension up to pH ) 8. These conditions were found to be adequate for intercalating the Cr(III) oligomers (vide infra). The nPA intercalation compound, obtained by method a, apparently showed only one single phase, but with variable d00l basal spacings between 14.5 and 16.5 Å (Figure 1). These phases, found in intercalated solids upon titration R-ZrP with a nPA solution at intercalation levels between 60 and 90%, correspond to bimolecular arrangements of the amine with different loadings and alkyl chain orientations.8 Since the particle size determines, in great part, the diffusion rate of the amine through the interlayers, and given the variability in nPA arrangements inside the phosphate interlayer,4,8 the application of method a to a layered MIV phosphate with a wide crystal size distribution may result in a heterogeneous neutralization of the sheets, if an adequate kinetic control is not accomplished, and thus phases with different neutralization degrees may coexist. It was found that high neutralization degrees in method a (>70%) hindered exfoliation and led to phosphate flocculation.4 However, it is quite difficult to distinguish between exfoliable and nonexfoliable phases from the XRD (8) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1992, 96, 3458.

R-ZrP Colloidal Suspensions

patterns, as their corresponding d00l spacings may be very close. In addition, the crystalline phases observed in the XRD patterns should not necessarily correspond to those present in the colloidal suspension. In any case, delamination by method a always occurs when the amine assumes an upright position inside the interlayer region, but the alkyl chains are sufficiently separated to allow the entry of water.8 The aim of producing complete delamination of the phosphate with a homogeneous neutralization of the sheets has prompted us to investigate the use of a solid, containing a bilayer of nPA adsorbed from the vapor phase, as an intermediate for preparing stable colloidal phosphate suspensions. This intermediate intercalation compound is a well-ordered solid with a d001 spacing of 17.2 Å, and its formula, deduced from chemical analysis, is Zr[nPA]2(HPO4)2‚1.1H2O. These features fit very well with those of a fully loaded phase,4,8,9 where the amine must have an orientation of ca. 55° to the layers.9 In contrast to the behavior observed with longer chain amines, which prevent exfoliation of the phosphate in water because of the strong van der Waals interactions established between the alkyl chains within the interlayer region,8 this nPA intercalate spontaneously exfoliated upon contacting with acetic acid aqueous solutions, at least in a relative concentration range from 50 to 200% of the phosphate CEC. Strikingly, the colloidal suspension, also developed in distilled water, reached a pH value as high as 10, which means that a fraction of the amine intercalated is displaced from the interlayer by water molecules. The colloidal suspension obtained by method b remained stable for several months, without any appreciable flocculation. The intermediate became a disordered intercalate with a d001 spacing of 15.4 Å and only 60% of adsorbed nPA when suspended in a acetic acid aqueous solution with a concentration equivalent to 80% CEC. The phase at 17.2 Å appears again after adding dropwise a 0.1 M nPA aqueous solution to the colloidal suspension up to pH ) 8 (Figure 1). To compare the effect of the colloidization method on the phosphate surface properties, the colloids, obtained by methods a and b, were converted to the hydrogen form by treatment with a 0.1 M HCl aqueous solution. The available data indicate that marked differences exist between them. First, the HCl acid treatment of phosphate a gave rise to a solid with a d001 reflection at 7.7 Å (Figure 1), typical of crystalline R-ZrP, whereas the same treatment for the phosphate b produced a poorly crystalline material with a broad diffraction peak centered at 8.9 Å (Figure 1) together other broad hkl reflections characteristic of R-ZrP. Apart from the low crystallinity of this solid, which in turn may indicate that a more effective delamination has taken place, the higher basal spacing is related to a higher water content inside the interlayer. Thus, the chemical analysis of the HCl-treated solids shows that phosphate b contains an amount of water (2 moles per formulas) that is twice that of phosphate a. A second differentiated property is the ability of the hydrogen forms of phosphates a and b to adsorb ammonia. TPDNH3 studies reveal that both the pristine R-ZrP and phosphate a retain, as expected, NH3 amounts close to the theoretical value of the CEC (6.6 mequiv g-1), the latter displaying a slightly higher retention (7.27 mequiv g-1) attributed to a partial hydrolysis during colloidization. Astonishingly, the hydrogen form of phosphate b displayed very low acidity; only 2.03 meq NH3 g-1. Since this solid (9) Clearfield, A.; Tindwa, R. M. Inorg. Nucl. Chem. Lett. 1979, 15, 251.

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showed a higher basal spacing than phosphate a at room temperature, and considering that the pretreatment temperature (523 K) before adsorption of NH3 did not make the formation of P-O-P bonds between adjacent sheets possible, the observed low acidity in phosphate b cannot be attributed to the existence of any hindrance to NH3 adsorption. On the other hand, the XPS analysis demonstrated that in no case was there loss of phosphate groups during the colloid preparation, as the P/Zr ratio for the two HCl-treated phosphates was very close to 2, the theoretical value of R-ZrP. Although more data are needed to explain the differential behavior of phosphate b, it is thought that the interlayer reactions, occurring upon colloidization, may have caused irreversible structural changes leading to the low acidity observed for the corresponding hydrogen form. According to Clearfield and Tindwa9 each amino group of the intercalated monoalkylamine interacts simultaneously with three neighbor P-OH groups of the phosphate layer by hydrogen bonding. Colloidization of the nPA bilayer intercalate would imply the interaction, in the interlayer region, of water molecules (coming from the external solution) with zirconium atoms from the layers. Such interaction may then generate a negative charge on the layers and simultaneously induce an irreversible structural transformation, as indicated in Scheme 1. Replacement of one oxygen of a phosphate group for one or more water molecules could be favored by the natural trend of phosphorus to form tetrahedral units with a double bond PdO. Simultaneously, deprotonation of a neighbor phosphate group may occur, giving rise to the entry of more water. These hydration and acidbase processes occurring within the interlayer region should generate enough energy for initiating exfoliation of the layers. The possibility that one of the three oxygens of a phosphate group bonded to zirconium ions may be replaced by a water molecule has already been suggested by other authors. Mounier and Winard10 suggested that Zr-OH bonds might be formed at the expense of a hydrolytic break of Zr-O-P bonds, as a consequence of the high polarizing power of the Zr(IV) ions. More recently, Alberti and Marmottini11 proposed a mechanism based on the formation of Zr-OH2 and Zr-OH bonds to explain the hydrolysis of the phosphate in an alkaline medium. The addition of dilute acetic acid keeps the external solution acidic, which facilitates the development of a colloidal phase without hydrolytic loss of phosphate groups at pH ≈ 5. Of course, the number of negative charge sites on the phosphate surface, created upon colloidization, is a function of the pH of the external solution. Thus, in alkaline media the negative charge of the colloid would increase up to a certain pH (≈9), from which the hydrolytic loss of HPO42- groups should become predominant,11 while increasing acidification of the colloid with a HCl solution leads finally to the precipitation of the phosphate. The XPS study of the HCl-treated samples revealed that hydrolysis, on the external surface, was still not appreciable for the colloids at pH ) 8. The XPS spectra of both phosphates showed an O1s signal that could be deconvoluted in two peaks, at 531.6 and 533.2 eV, corresponding to pure oxidic and hydroxo components, respectively. The oxidic/hydroxo ratio found on the phosphate surface was close to 3, practically identical to (10) Mounier, F.; Winard, L. Bull. Soc. Chim. Fr. 1968, 1829. (11) Alberti, G.; Marmottini, F. J. Colloid Interface Sci. 1993, 157, 513.

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Pe´ rez-Reina et al. Scheme 1

that of R-ZrP.12 In addition, the binding energy values of Zr 3d5/2 and P 2p were very similar to that of R-ZrP. However, although the external surface seems to be unchanged, the formula of the solid regenerated by acid treatment, Zr(HPO4)2‚2H2O, points to an interlayer structural transformation that took place upon colloidization. As a result of these interlayer reactions, the structure would now be composed by at least two different phosphate groups, one of which would be much less acidic than the other one. One of the possible advantages of this new colloidization method may be that all layers are homogeneously neutralized, as the process is independent of the phosphate particle size. Intercalation of Cr(III) Oligomers. Previous work6a established that refluxing Cr(OAc)3 with R-zirconium phosphate colloidized by method a leads to the intercalation of defined Cr3+ polyhydroxoacetato cations, some of which have recently been identified by extended X-ray absorption fine structure.12 These precedent studies also demonstrated that the Cr3+ concentration is determinant with respect to the nuclearity and charge of the intercalated polymeric species. Another key factor that should control the intercalation of the oligomers is the negative charge created on the phosphate surface upon colloidization. For this reason, a preliminary study has been carried out to investigate the influence of the pH of the phosphate colloidal suspension on the intercalation of chromium oligomers. Before the chromium(III) solution is added, the pH of the colloidal suspension was increased from 5 to 10 by adding dropwise a dilute nPA aqueous solution to the original suspension. The relationship between the pH of the initial phosphate suspension and the intercalated Cr3+ polyhydroxoacetato species has been established from the XRD patterns illustrated for materials of series b (Figure 2). At low initial pH (5), a main diffraction peak (12) Paparazzo, E.; Severini, E.; Jime´nez-Lo´pez, A.; Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Tomlinson, A. A. G. J. Mater. Chem. 1992, 2, 1175.

Figure 2. XRD patterns of sample CrZrP-2b prepared at different initial pHs: (a) pH ) 5; (b) pH ) 8; (c) pH ) 10 (reaction conditions: reflux 4 days, [Cr3+]sol ) 0.025 M).

appears at 21.6-24.7 Å, corresponding to an oligomeric chromiun intercalate, together with a very weak signal at 10-11.3 Å, which may be assigned to a second-order peak or a second phase formed by the presence of low nuclearity chromium species. Such species, expected to be present in fresh solutions at low pH, do not polymerize after intercalation due to their high affinity for the phosphate surface.6b Given that the peak at higher angle varies with the chromium content, it cannot be assigned to a nPA intercalated phase, which appears at 10.5 Å upon

R-ZrP Colloidal Suspensions

Langmuir, Vol. 14, No. 15, 1998 4021

Figure 4. Uptake of chromium by colloidal zirconium phosphates a and b (reaction conditions: reflux 4 days, initial pH ) 8). Table 1. Empirical Formula and d00l Basal Spacing of Polyhydroxyacetate Cr3+-Intercalated Zirconium Phosphate Compounds

Figure 3. XRD patterns of samples CrZrP-2a (A) and CrZrP2b (B), at different times of reaction: (a) 1 day; (b) 2 days; (c) 3 days; (d) 4 days (initial pH ) 8, [Cr3+]sol ) 0.025 M)

titration of R-ZrP with a nPA solution up to pH ≈ 5.4 At initial pH ) 10, uncontrolled hydrolysis of Cr3+ species and the phosphate, caused by the high pH, leads to several phases with broad signals between 2θ ) 3-7°, together with a peak at 11.5 Å already commented. However, at initial pH ) 8, only a peak at 30.2 Å corresponding to a single phase was observed. This pH value was, therefore, considered to be optimum for the intercalation of Cr3+ oligomers into the phosphate and was used throughout pillared material preparations. It has to be noted, however, that using a different pH for the colloidal suspension may lead to distinct chromium intercalation compounds. On the other hand, the formation of single phase oligomer intercalates is usually a slow process that requires a previous study to determine the optimum time of reaction. The chromium(III) intercalation compounds showed a gradual increase of the basal spacing during the first 4 days of reaction, and thus this time was considered necessary to reach the equilibrium. However, as can be seen in Figure 3, a differential behavior between both series of intercalates was observed before reaching the equilibrium. Two phases could be distinguished in the XRD patterns when the phosphate colloidized by method a was used, the first one, at low angle, corresponding to a Cr3+ oligomer intercalate and the second one, at 16.1 Å, may be attributed to a nonexfoliated nPA intercalated phase. However, only one single phase was observed using the phosphate colloidized by method b. This fact suggests that the higher delamination degree was attained by method b and that two different insertion mechanisms of oligomeric species may operate. Anchoring of species would take place predominantly when the phosphate delamination degree is very high. In such a case, a further in situ polymerization would also be possible when the chromium concentration in solution is sufficiently high. On the other hand, if a fraction of the phosphate layers is not exfoliated a diffusion of the Cr3+ species through the interlayer region would take place and therefore, two or more phases could coexist when delamination is not

sample

d00l/Å

empirical formula

CrZrP-1a CrZrP-2a CrZrP-3a CrZrP-1b CrZrP-2b CrZrP-3b

20.1 28.3 35.5 22.1 30.2 44.0

Zr[Cr1.94(OAc)0.51(OH)4.10] H0.79 (PO4)2‚4.3H2O Zr[Cr3.00(OAc)0.94(OH)7.19] H1.12 (PO4)2‚6.5H2O Zr[Cr3.81(OAc)1.42(OH)8.84] H0.83 (PO4)2‚7.9H2O Zr[Cr1.86(OAc)0.53(OH)3.54] H0.44 (PO4)2‚6.5H2O Zr[Cr3.20(OAc)1.46(OH)6.80] H0.59 (PO4)2‚6.4H2O Zr[Cr5.44(OAc)2.18(OH)12.6] H0.46 (PO4)2‚11.6H2O

complete. In this case, preferential adsorption of smaller oligomers would occur. Another implication related to the diffusion mechanism is that a comparatively smaller volume of water is allowed to remain inside the interlayer region. Using an initial pH ) 8 for the phosphate suspension and leaving 4 days for completion of reaction, three Cr3+ oligomer intercalation compounds were obtained for each series by varying the Cr3+ concentration in solution from 0.015 to 0.031 M. The uptake curves for Cr3+ oligomers on colloidal phosphates a and b show a clear differentiation at high Cr3+/phosphate ratios (Figure 4). The higher amount of chromium retained by phosphate b may be attributed to easier access of the oligomeric species to the negative charge sites on the phosphate surface because a higher proportion of single lamellae are present in this colloidal phase. If delamination is not complete, competition between diffusion of oligomeric species within the phosphate interlayers and the formation of larger species incapable of inserting themselves into the matrix will lead to a progressive decrease of chromium loading at very high addition of chromium acetate, as already observed by using phosphate a.6a The empirical formulas and d001 basal spacing of the resulting intercalates are listed in Table 1. For both series, materials 1 and 2 present a very similar chromium content, ca. 1.9 and 3 mol of Cr/mol of Zr respectively, while the d001 basal spacing at room temperature was slightly higher for those of series b (22.1 and 30.2 Å for 1b and 2b, respectively). In contrast, intercalate 3b displays a much higher amount of Cr (5.4 mol of Cr/mol of Zr) and much higher d00l basal spacing (44.2 Å) than its homologous intercalate 3a (3.8 mol of Cr/mol of Zr and d00l ) 35.5 Å). This may be attributed to the presence of larger oligomeric species in the intercalate 3b. Precedent XAS studies13 have established that d00l spacings between 17 and 34 Å are consistent with the presence of double layers of trimers (17-20 Å) and open tetramers (34 Å) within the phosphate interlayer region.

4022 Langmuir, Vol. 14, No. 15, 1998

Figure 5. Variations of the d00l basal spacing of chromiumintercalated samples on heating under nitrogen.

Intermediate distances, between 20 and 34 Å, are representative of a mixture of these species. Accordingly, the d00l spacings observed for these intercalates would be compatible with the presence of a bilayer of trimers in materials 1a and 1b, trimers + tetramers in 2a and 2b, and tetramers in 3a. On the other hand, on the basis of the XRD and chromium content data, oligomers higher than tetramers might be present in material 3b. Nevertheless, the existence of such species inside the phosphate interlayer has to be confirmed by further XAS studies. Also it is noteworthy that samples of series b are generally more highly hydrated than those of series a. As discussed below, the hydration degree inside the interlayers may play a crucial role regarding the porous structure created by the transformation of the polyhydroxyacetato-CrIII clusters in nano-chromia particles after calcination. All these observations taken together provide additional support to show that a higher exfoliation degree of R-ZrP was reached with method b. Surface Properties of Chromia-Pillared Materials. Calcination of polyhydroxyacetato-CrIII intercalates under N2 at 673 K gives rise to the formation of chromiapillared materials. Although no sign of the presence of chromium species in an oxidation state higher than CrIII was found by XPS, the characteristic signal of Cr 2p3/2 was in the range corresponding to CrIII species.14 A small amount of CrVI (