14726
J. Phys. Chem. 1996, 100, 14726-14735
Preparation and Properties of Fluorinated Alumina-Pillared r-Zirconium Phosphate Materials Josefa M. Me´ rida-Robles, Pascual Olivera-Pastor, Antonio Jime´ nez-Lo´ pez,* and Enrique Rodrı´guez-Castello´ n Departamento de Quı´mica Inorga´ nica, Cristalografı´a y Mineralogı´a, Facultad de Ciencias, UniVersidad de Ma´ laga, 29071 Ma´ laga, Spain ReceiVed: January 4, 1996X
A polyoxyaluminum cluster, larger than the tridecamer ion [AlO4Al12(OH)24(H2O)12]7+, was intercalated into R-zirconium phosphate using an oligomeric solution aged at 368 K and conducting the intercalation reaction, first at reflux and then with hydrothermal treatment, in the presence of F-. Different intercalation compounds were prepared by varying the Al3+/phosphate ratio, but a unique single phase material with basal spacing of 21.3 Å was obtained. As revealed by chemical analysis and X-ray photoelectron spectroscopy (XPS), F- is incorporated into the intercalated oligomer where it partially substitutes OH- ions. The XPS technique was also a helpful tool to determine the Al3+ concentration from which other Al3+ compounds are precipitated outside of the phosphate interlayer. Thermal treatment caused a continuous and gradual interlayer contraction up to 973 K, but no evidence of layer collapse or segregation of alumina was found, at least up to 1273 K. 31P and 27Al solid state NMR spectroscopies were used to investigate the mechanism of interaction between the phosphate layer and the aluminum species. Because of this interaction, the 31P signal shifts toward higher field, at ≈20 ppm, while the 27Al NMR spectra display the characteristic signal of octahedrally coordinated Al and of a new resonance between 44.8 and 49.8 ppm, which is related to the existence of Al-O-P bonds. An increase of the relative intensity of this signal was observed upon calcination of the materials at 673 K. The presence of F- in the interlayer aluminum oxide produced an enhancement of the thermal stability and porosity of the pillared materials. Studies of thermal-programmed desorption of NH3, pyridine adsorption, and decomposition of isopropyl alcohol have revealed the acid nature of fluorinated alumina-pillared R-ZrP materials. The acid sites catalyze the dehydration reaction of isopropyl alcohol with a selectivity of practically 100%.
Introduction The tridecamer ion [AlO4Al12(OH)24(H2O)12]7+, referred to as Al13, has been widely used for pillaring layered hosts, such as swelling clays,1 phosphates,2 and other compounds,3 because it has been, until recently, the only oligomeric species of Al3+ well characterized in solution and solid state. Al13 may be intercalated from solutions of (i) AlCl3‚6H2O partially hydrolyzed with NaOH4 or (ii) commercial Chlorhydrol.1,5 Upon calcination at T > 673 K, the intercalated oligomer is transformed into an aluminum oxide (pillar), which props apart the layers and generates a permanent porosity in the interlayer region of the host. Strong bonds between pillars and the surface are formed during this process (e.g., Alpillar-O-Allayer in clays bearing Al3+ in the tetrahedral sheet6 or Al-O-P in R-M(IV) phosphates7). As revealed by NMR studies, such linkages produce a substantial modification in the coordination of aluminum; thus, together with 6-fold-coordinated sites, lower coordination aluminum sites have also been detected, chiefly in calcined materials.7,8 The available data point out that the aluminum oxide pillar is bonded to the layer through these lowcoordination sites, which may act as strong Lewis acid centers. The porous nature of aluminum oxide pillared compounds is related to the number of linkages established between the interlayer aluminum oxide and the layered matrix. When this number is very high, a strong contraction of the basal spacing and formation of stuffed structures should be expected upon calcination. This is the case of R-Zr phosphate which, upon pillaring with Al13, invariably leads to materials with low surface X
Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00113-X CCC: $12.00
areas and porosity,2 in contrast to the behavior observed for other M(IV) phosphates (M ) Sn, Ti)2 and clays.1 Regardless, when Al13 solutions are used, the free height (d00l, layer thickness) of aluminum oxide pillared phosphates (5-6 Å) is always smaller than that observed in smectite clays (≈9 Å), probably due to the formation of stronger linkages between the interlayer aluminum species and the phosphate layer. Such interaction leads to a rapid dehydration of the intercalated oligomer and possibly to a partial disruption of the polycation structure within the interlayer region of the phosphate. Following a systematic study on the pillaring of R-Sn and R-Zr phosphates with aluminum oxide, we have found that the textural properties of the pillared materials are strongly dependent on the type of Al solution employed, inasmuch as each solution gives rise to a different intercalation compound. Thus, in R-Sn phosphate, the use of solutions i and ii at room temperature leads, respectively, to the intercalation of monolayers and bilayers of Al13 species,9 whereas refluxing acetatebuffered hydrolyzed Al3+ solutions in the presence of the phosphate gives rise to the intercalation of larger polymers.10 From NMR data, Fu et al.11 have shown the existence of large polyoxycationic clusters of aluminum, formed by polymerization of Al12 units upon thermal treatment of Al13 solutions. The same authors were able to prepare solutions of one of these clusters (Al24) by dissolving Al foil in a AlCl3 solution at 368 K. More recently, Nazar et al.12 isolated the sulfate salt of the cluster Al24 and reported structural details of this new large polyoxocation of aluminum from solid state 27Al MAS-NMR. Using the Al24 solution proposed by Fu et al.,11 we have found, in a preliminary work,13 that an oligomer larger than © 1996 American Chemical Society
Fluorinated Alumina-Pillared R-Zirconium Phosphate Al13 is indeed intercalated into R-Zr phosphate when the reaction is carried out under hydrothermal conditions in the presence of F- ions. With this method the specific surface of the pillared materials is considerably enhanced. The aim of this work is, therefore, to study the formation of aluminum oxide pillared R-Zr phosphate from Al24 solutions with F- added as well as the thermal stability and the structural and surface characterization of the resulting pillared materials using the techniques of X-ray diffraction (XRD), thermal analysis (thermogravimetric (TG) and differential thermal analysis (DTA)), X-ray photoelectron spectroscopy (XPS), MAS-NMR spectroscopy, N2 adsorption, infrared spectroscopy (IR) of pyridine adsorbed, thermal-programmed desorption of NH3 (NH3-TPD), and the decomposition of isopropyl alcohol as test reaction. Experimental Section Materials. R-ZrP was prepared following the method of Alberti and Torraca.14 A colloidal suspension of n-propylamine-R-ZrP was obtained by adding dropwise a 0.1 M n-propylamine solution up to 60% of the cationic exchange capacity (CEC) of the phosphate (CEC ) 6.64 mequiv/g).15 The oligomeric aluminum solution was prepared according to the method described by Fu et al.12 by dissolving Al foil in a AlCl3 solution at 368 K, with a final Al3+ concentration of 1.18 M. A n-propylammonium fluoride solution was then added to achieve a F-/Al3+ molar ratio of 0.5. Assuming that the oligomeric species is Al2410+, aliquots of this solution, corresponding to 0.25, 0.5, 1, 1.5, and 2 times the CEC, were added dropwise to the colloidal suspension of n-propylamine-R-ZrP (200 mL, 0.75%). These new suspensions were refluxed for 2 days, centrifuged, and washed with water. The wet solids obtained were again suspended in 30 mL of 0.08 M n-propylammonium fluoride solution and then hydrothermally treated for 1 day at 473 K in a Teflon-lined Berghof HR 200 autoclave. The resulting intercalation compounds, hereafter designated as AlZrP-0.25, AlZrP-0.5, AlZrP-1, AlZrP-1.5, and AlZrP-2, were separated by centrifugation, washed with water, and dried at 333 K. These materials (precursors) were finally calcined in air at 673 or 873 K at a heating rate of 1.7 × 10-2 K s-1. Chemical Analysis and Characterization. Chemical analysis of the precursors was carried out after digestion in 1 M KOH solution under reflux. Al and Zr were determined by atomic absorption spectroscopy with a Perkin-Elmer 400 spectrometer, F was analyzed by EDX using a Philips EM 420 instrument with an EDAX 9400 spectrometer, and P was analyzed colorimetrically.16 XRD patterns of cast films were recorded with a Siemens D 501 diffractometer (Cu KR radiation). Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out in a Rigaku Thermoflex TG 8110 instrument (calcined Al2O3 as a reference and 1.7 × 10-1 K s-1 heating rate). XPS spectra were recorded with a Fisons ESCALAB 200R with Al KR X-ray excitation source (hν ) 1486.6 eV) and hemispherical electron analyzer. Accurate ((0.2 eV) binding energies (BE) have been determined with respect to the position of the C 1s peak at 284.9 eV. The modified Auger parameter of Al (R′) was calculated using the following equation7
R′ ) 1486.6 + KE(AlKLL) - KE(Al2p) where KE(AlKLL) is the kinetic energy of the Auger electron of AlKLL and KE(Al2p) the kinetic energy of the photoelectron Al2p. 31P NMR spectra were recorded at 121.5 MHz with a Bruker spectrometer. The external magnetic field used was 9.4 T. The
J. Phys. Chem., Vol. 100, No. 35, 1996 14727 samples were spun at the magic angle in the range of 4-4.5 kHz. 27Al NMR spectra were recorded at 78.15 MHz with a Varian VXR 300 spectrometer equipped with a Doty probe and electronics for magic angle spinning (spin rate of 10 kHz), and 15° radio frequency pulses were employed, the data acquisition time for 6000 cycles was 3 h, and a relaxation delay of 0.5 s avoided saturation effects. All measurements were carried out at room temperature. Solutions of H3PO4 and [Al(H2O)6]3+ were used as external standard references for phosphorus and aluminum chemical shifts, respectively. Adsorption-desorption of N2 on the calcined samples (77 K, outgassing at 473 K and 10-2 Pa overnight) was measured on a conventional volumetric apparatus. The total acidity of the calcined samples was determined by NH3-TPD. Before adsorption of ammonia at 373 K, samples were heated under helium at 673 K for 1 h. The desorption of ammonia was run between 373 and 673 K at 1.7‚10-1 K s-1 and analyzed in an on line gas chromatograph (Shimadzu 6C-14A) provided with a thermal conductivity detector. IR spectra were recorded at room temperature on a Perkin-Elmer 810 spectrometer. For the adsorption of pyridine, self-supported wafers, with a weightto-surface ratio of about 8 × 10-2 kg m-2, were placed in a vacuum cell assembled with Teflon stopcocks and CaF2 windows. Pretreatments were carried out with an in-site furnace. The samples were evacuated (623 K, 10-2 Pa overnight), exposed to pyridine vapor at room temperature, and then outgassed between room temperature and 673 K. Catalytic Activity Measurements. The catalysts were tested in a fixed-bed tubular glass reactor operated at atmospheric pressure and 493 K, with a catalyst charge of 2.5 × 10-5 kg without dilution. The feed was a mixture of isopropyl alcohol in He obtained by passing a flow of He through liquid isopropyl alcohol (chromatographic purity grade) held in a saturatorcondenser kept at 303 K. A constant total flow of 4.17 × 10-7 m3 s-1 with an alcohol concentration of 7.9 vol % was used. Prior to the catalytic test, the carrier gas was passed through a molecular sieve, and the samples were pretreated under a He flow, at 673 or 873 K for 12 h. The reaction products were analyzed in an on-line gas chromatograph equipped with a flame ionization detector and fused silica capillary column SPB1. Results and Discussion In contrast to Al13 solutions, the intercalation of a polyoxocation cluster of aluminum into R-ZrP from Al24 solutions required the presence of a ligand, such as F-, and that the reaction be conducted in two steps, first at reflux temperature and subsequently in hydrothermal conditions. In absence of F-, only phases with low basal spacings (12.7 and 9.6 Å) were observed by XRD, corresponding to intercalated aluminum monomers or oligomers with very low nuclearity. The addition of F- favored the intercalation of a large cluster by complexing low-nuclearity species, which have higher affinity for the phosphate layer. These species could be initially present in the oligomeric solution or perhaps were formed upon contact with the phosphate suspension.13 Table 1 lists the chemical composition and the empirical formulation of the intercalation compounds obtained from Al24 solutions. The phosphate retains increasing amounts of Al up to an addition of 66 mequiv of Al3+ per gram of ZrP2O7, beyond which the uptake stays constant, with a maximum value of 45.6 mequiv of Al3+ incorporated per gram of ZrP2O7. It may be observed that n-propylamine (NPA) is not completely removed from the interlayer region although the NPA/Al3+ ratio gradually decreases with the aluminum loading. F- is also incorporated into the intercalated oligomers, with a F/Al ratio increasing from
14728 J. Phys. Chem., Vol. 100, No. 35, 1996
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TABLE 1: Chemical Composition and Empirical Formula of Aluminum-Intercalated r-Zirconium Phosphate Materials
samples
mequiv of Al3+ added/g of ZrP2O7
mequiv of Al3+ incorporated/g of ZrP2O7
nPA/Al3+
% H2O
F/Al
empirical formula
AlZrP-0.25 AlZrP-0.5 AlZrP-1 AlZrP-1.5b AlZrP-2c
16.9 30.9 51.0 66.4 84.1
11.9 22.5 39.4 44.6 45.6
1.2 0.3 0.1 0.1 0.1
10.0 11.0 10.5 6.0 5.0
1.0 1.2 1.4 1.6 1.7
Zr[Al1.0O0.3(OH)0.8F1.0(H2O)0.7](C3H9N)1.3H0.3(PO4)2‚2.3H2Oa Zr[Al2.0O0.6(OH)1.4F2.3(H2O)1.4](C3H9N)0.7H0.5(PO4)2‚2.0H2O Zr[Al3.4O1.1(OH)1.5F4.9(H2O)2.5](C3H9N)0.3H0.3(PO4)2‚2.9H2O Zr[Al3.8O1.3(OH)1.4F5.9(H2O)2.9](C3H9N)0.3H0.1(PO4)2‚1.3H2O Al4.1O1.4(OH)0.8F7.0Zr(C3H9N)0.4(PO4)2‚4.8H2O
a
Hydration water from TG.
b ,c
Materials AlZrP-1.5 and AlZrP-2 are composed of two and three phases, respectively.
1.0 to 1.7. In addition to favoring the intercalation of bulk Al3+ cations by complexing monomers present in solution, F- ions may partially substitute OH- ions in the oligomer structure, since the affinity of F- for Al3+ is strong enough to remove even OH- ions from the aluminum hydroxide structure.17 This partial substitution of OH- for F- seems to be crucial to obtain more porous and stable pillared materials upon calcination. According to the TG-DTA curves, two groups of materials may be distinguished (Figure 1). Materials with aluminum loadings below 1 (Figure 1, A and B) present several exothermic effects between 473 and 773 K, corresponding to the decomposition and combustion of the remaining NPA. In materials with higher Al loadings, the exotherms disappear, whereas an endotherm, typical of aluminum oxyhydroxide,18 appears at 643 K. The latter materials show a thermal behavior more similar to that of the intercalates prepared by refluxing methods in the presence of acetate than to those obtained using Al13 solutions at room temperature (rt), which do not exhibit the mentioned endothermic effect.7 XRD patterns of the pillared materials are shown in Figure 2. Depending on the Al loading, noticeable differences are observed. Sample AlZrP-0.25 is a single phase with a sharp peak at 15.2 Å. In this intercalate, with the lowest Al content, the intercalated aluminum species may be accommodated within the phosphate interlayer region without appreciable modification of the basal spacing, which is determined by NPA molecules. But the free height, about 8.7 Å (assuming the layer thickness to be 6.5 Å), is lower than that observed for Al13 intercalates.9 Two reflections are distinguished when the Al loading is 0.5; the most intense peak, at 21.3 Å, is assigned to a phase containing a large Al3+ oligomer, whereas the shoulder at 16 Å is attributed to an intercalate of NPA, which may or may not contain other Al3+ species. The interlayer distance of the most expanded phase is higher than that for the one corresponding to a monolayer of intercalated Al13 (16-17 Å) and lower than that for a bilayer (24-26 Å), taking into account that the Al13 ion has a gyration radius of 9.5 Å. In addition, there are several reasons by which the presence of a bilayer of Al13 may be ruled out. The Al/phosphate ratio was quite low, which favors the formation of a monolayer rather a bilayer; the high temperature of reaction with the phosphate should favor the formation of higher polymers than Al13, and finally, it is probable that Al13 was absent, or at least was a minor component, in the solution employed.11 All these arguments lead us to suppose that the phase of 21.3 Å corresponds to an intercalated aluminum oligomer larger than Al13. The free height (14.8 Å) of this 21.3 Å phase is close to the average diameter of the cluster Al24 (14 Å).12 Consequently, if a cluster such as Al24 was indeed intercalated into the phosphate, it should be slantwise oriented with respect to the layers. For an Al loading of 1, a well-defined single phase at 21.3 Å, with only traces of NPA intercalate at 15.4 Å, is obtained. However, for higher Al loadings, other new phases appear. So, for sample AlZrP-1.5, a diffraction peak at 6.2 Å is detected together with a more intense peak at 21.3 Å. The phase with 21.3 Å spacing is not observed in sample
Figure 1. TG and DTA curves of aluminum-intercalated R-zirconium phosphate materials: (A) AlZrP-0.25; (B) AlZrP-0.5; (C) AlZrP-1; (D) AlZrP-1.5; (E) AlZrP-2.
AlZrP-2, but instead, three different phases, with spacings of 9.6, 6.2, and 4.8 Å, are observed. The 9.6 Å phase has been assigned19 to an intercalate with monomeric Al3+, which may
Fluorinated Alumina-Pillared R-Zirconium Phosphate
J. Phys. Chem., Vol. 100, No. 35, 1996 14729
Figure 2. XRD powder patterns of aluminum-intercalated R-zirconium phosphate materials.
TABLE 2: d001 Spacings of Main Phases of Aluminum-Intercalated r-ZrP Materials d001 (Å)
Figure 3. Variation of bulk and surface Al/Zr ratio Vs Al/Zr ratio added in AlZrP fresh intercalates and calcined materials from chemical analysis (b) and from XPS (O fresh, 9 calcined).
result from the hydrolysis of oligomeric species when the concentration of Al3+ in solution is increased.20 The spacings observed at 6.2 and 4.8 Å might correspond to the main reflections of, respectively, bohemite (or pseudobohemite) and gibbsite.21 From these results, it seems that obtaining a singlephase intercalate of this large cluster, under the experimental conditions used, is only possible within a very limited range of Al3+ concentration in solution. The surface Al/Zr ratio of the phosphate has been obtained from the XPS data. A plot of this ratio and of the bulk Al/Zr ratio Vs Al loading is helpful in order to establish the Al concentration from which other Al compounds are precipitated outside of the phosphate interlayers. It can be seen (Figure 3) that for Al loadings of 1.5 and higher the Al concentration at the phosphate surface abruptly increases with respect to the bulk, due to the presence of segregated precipitated phases of Al in agreement with XRD observations. Another significant feature observed in this plot is that the Al/phosphate ratio at the phosphate surface does not change upon calcination, which suggests that no segregation of intercalated aluminum occurred during this thermal treatment of materials.
T(K)
AlZrP-0.25
AlZrP-0.50
AlZrP-1
AlZrP-1.5
298 373 473 573 673
15.2 14.1 11.9 11.8 11.2
21.3 19.8 18.2 17.5 17.1
21.3 19.5 18.2 17.5 17.3
21.3 20.5 19.4 18.7 17.7
The thermal evolution of the basal spacing of the main phase in expanded materials is shown in Table 2. There is a continuous decrease of the basal spacing as temperature increases, but in no case do the phosphate layers collapse. Even sample AlZrP-0.25, with the smallest Al loading, still shows a basal expansion of 5.4 Å at 673 K. This basal expansion is similar to that observed for the Al13/R-SnP intercalate.10 Samples with higher Al loadings (0.50, 1.0, and 1.5) show a similar behavior, with free height close to 10.5 Å at 673 K. The single-phase sample AlZrP-1 was calcined up to 1273 K to establish the limits of thermal stability of the aluminum oxide pillared phosphate (Figure 4). As opposed to alumina-pillared clays, whose interlayer distance is hardly changed upon thermal treatment, aluminum oxide pillared R-zirconium phosphate shows a continuous contraction, at least up to 973 K. Thus, at 673 K, the basal spacing of pillared phosphate is reduced to 17.3 Å, corresponding to a free height of 11 Å, which is about 3 Å higher than that observed in alumina pillared clays.8 Between 673 and 973 K, a further interlayer contraction of 2 Å and a gradual loss of crystallinity are observed, whereas in the range 973-1273 K, the pillared materials become amorphous. This does not necessarily mean that the pillared structure is no longer present, but rather that the long-range ordering disappears. In fact, neither formation of Zr pyrophosphate nor segregation of alumina was detected by XRD (Figure 4), in constrast to the behavior observed in other oxide pillared phosphate materials.10 This continuous contraction suggests that the intercalated oligomer undergoes a continuous transformation, probably caused by a strong interaction with the phosphate layer. In any case, this material shows much higher thermal stability than its homologous pillared R-Sn phosphate prepared from Al13 solutions, which already at 473 K undergoes a strong contraction
14730 J. Phys. Chem., Vol. 100, No. 35, 1996
Me´rida-Robles et al.
Figure 4. XRD powder patterns of material AlZrP-1 at different temperatures.
TABLE 3: Binding Energies (eV), Surface P/Zr and Al/Zr Atomic Ratios (from XPS), and Modified Auger Parameter of Aluminum in Fluorinated Alumina-Pillared r-Zirconium Phosphate Materials samples AlZrP-0.25
a b a b a b a b a b
AlZrP-0.5 AlZrP-1 AlZrP-1.5 AlZrP-2 a
Precursor.
b
Zr(3d5/2)
O(1s)
P(2p)
Al(2p)
F(1s)
P/Zr
Al/Zr
R′Al
182.2 182.2 182.2 182.2 182.2 182.2 182.2 182.2 182.2 182.2
531.1 531.0 531.1 531.1 531.0 531.2 531.0 531.0 531.2 531.0
133.0 133.0 133.0 133.0 133.0 133.1 133.0 133.0 133.0 133.0
73.9 73.9 73.8 73.9 73.9 73.9 73.8 73.9 74.0 73.9
684.8 684.9 684.6 684.8 684.8 685.0 684.8 684.9 685.0 685.0
1.83 2.03 2.35 2.14 2.58 2.06 2.21 2.20 2.36 2.56
0.78 1.11 2.89 2.54 3.45 3.88 5.40 5.90 13.27 13.07
1460.9 1460.8 1461.0 1461.0 1461.1 1461.0 1461.0 1461.1 1461.1 1461.1
Calcined (673 K).
and has a basal spacing of only ca. 12 Å at 673 K.9 This enhancement of the thermal stability may be attributed to the presence of F- in aluminum oxide pillars, which modulates the strong interaction with the phosphate layer, avoiding a rapid spreading of the oxide within the interlayers. Binding energies (BE) and modified Auger parameters of Al, determined by XPS, are listed in Table 3. The BE of P2p and Zr3d coincide with those found for pristine R-ZrP, suggesting that the phosphate structure is preserved after pillaring. The values of the BE corresponding to O1s (531.0 eV) are similar to those found for chromia-pillared R-ZrP22 and significantly lower than those reported for aluminum phosphates (at 532.8 eV23), which rules out the formation of these compounds during the pillaring process. On the other hand, the BEs of Al2p fall within the values reported for alumina,23 while those of F- (near 685 eV) are intermediate23 between the values corresponding to ionic (e.g., 684.5 eV for NaF) and covalent fluorides (e.g., 685.5 eV for AlF63-). Moreover, these values are practically not modified after calcination, which appears to indicate that this ion, after linking to the intercalated oligomer, remains bonded to aluminum in the oxide pillar formed upon calcination.
Therefore, the incorporation of F- into the aluminum cluster could be an alternative way to pillar stabilization24 with respect to that first reported for pillared clays, in which the stability is improved by partial substitution of OH- for F- in the silicate octahedral layer.25 The values of the modified Auger parameter (R′) of the pillared materials are in the range of 1460.8-1461.1 eV. Similar values have also been found using classical pillaring methods.7 Although these values are intermediate between those of octahedral and tetrahedral aluminum of well-characterized aluminosilicates,26 they cannot be unambiguously ascribed to a determinate aluminum environment,27 particularly when different coordination sites of aluminum may be present.28 31P MAS-NMR spectra of representative samples at rt and calcined at 673 K are shown in Figure 5. Two signals are observed in the spectra of fresh samples, a low intensity one at ca. -15.6 ppm and another one at ca. -20 ppm. The former one is typical of n-propylamine intercalates, and it is attributed to phosphates groups deprotonated by n-propylamine molecules, which produce a downfield shift of the characteristic resonance in R-ZrP at -18.6 ppm.29 The second signal may be assigned to phosphate groups interacting with the aluminum cluster. This
Fluorinated Alumina-Pillared R-Zirconium Phosphate
J. Phys. Chem., Vol. 100, No. 35, 1996 14731
Figure 5. 31P MAS-NMR spectra of aluminum-intercalated R-zirconium phosphate materials. Fresh samples: (A) AlZrP-0.5, (C) AlZrP1, (E) AlZrP-2. Samples calcined at 673 K: (B) AlZrP-0.25, (D) AlZrP1, (F) AlZrP-2.
interaction always causes a shift of the 31P resonance toward high field.7 After calcination at 673 K, the low-intensity peak disappears, whereas the high-intensity band broadens but, in constrast to what happens in absence of F- where a large upfield shift is observed between -24.1 and -26.2 ppm, the maximum essentially remains at the same position. It seems, therefore, that the strength of the interaction of the aluminum oligomer with the phosphate layer is not greatly modified after calcination at 673 K. This is not surprising, since the temperature of the hydrothermal intercalation reaction (473 K) was quite high. Furthermore, the presence of F- reduces the number of possible Al-O-P linkages, thus inhibiting the spreading of the aluminum oxide pillars in the interlayer region and contraction of the basal spacing at low temperatures. The spectrum of calcined AlZrP-2 contains two 31P resonances at -21.4 and -23.2 ppm. It is very similar to that of Al3+-exchanged R-ZrP.7 27Al MAS-NMR spectra of two selected samples, fresh and calcined at 673 K, are shown in Figure 6. These spectra are quite different from that of Al13, with resonances at 62.9 and 11.8 ppm for tetra- and hexacoordinated aluminum, and that of Al24, presumed to be present in the pillaring solution used, which shows two characteristic signals at 70.2 and 10 ppm in AlO4 and AlO6 environments,11 respectively. In contrast, the spectra of these samples show the resonance corresponding to octahedral aluminum between -5 and +7 ppm, but the most significant
Figure 6. 27Al MAS-NMR spectra of aluminum-intercalated R-zirconium phosphate materials. Fresh samples: (A) AlZrP-0.5, (C) AlZrP1.5. Samples calcined at 673 K: (B) AlZrP-0.5, (D) AlZrP-1.5.
feature is the appearance of a signal at ca. 45 ppm, with a relative intensity depending on the prefixed Al/phosphate ratio. The decreasing intensity observed in samples with ratios >1 is attributed to the presence of phases other than the Al oligomer intercalate, in which Al is predominantly octahedral. This signal is enhanced upon calcination at 673 K, becoming dominant in the spectrum of samples with Al/phosphate ratios lower than 1.5, and shifts downfield by only 4 ppm. It is difficult to assure, from the NMR data, whether this resonance corresponds to tetrahedral Al in a distorted environment or to pentacoordinated Al. Although Al in AlO5 environments gives rise to a resonance at approximately 35 ppm in aluminates and aluminosilicates,30 Mortlock et al.31 have recently shown from NMR studies of aluminophosphate species in aqueous solution that the interaction of pentacoordinated Al with phosphate produces a resonance between 43 and 52 ppm. Curiously, the Al24 cluster has a strong propensity to form 5-fold-coordinated Al in the solid state upon calcination.12
14732 J. Phys. Chem., Vol. 100, No. 35, 1996
Me´rida-Robles et al.
TABLE 4: Textural Parameters of Fluorinated Alumina-Pillared r-Zirconium Phosphate Materials samples AlZrP-0.25 AlZrP-0.5 AlZrP-1 AlZrP-1.5
a b a b a b a b
SBET (m2 g-1)
CBET
∑Vacc (cm3 g-1)
Vmicrod (cm3 g-1)
96 79 130 93 184 139 174 125
154 58 227 55 944 135 213 103
0.11 0.14 0.19 0.21 0.17 0.21 0.23 0.27
0.06 0.02 0.07 0.04 0.09 0.07 0.08 0.05
a Calcined at 673 K. b Calcined at 873 K. c As defined and calculated by the Cranston-Inkley method. d Using Rs plot.
The fact that the intercalated oligomer shows a NMR spectrum different from those characteristics in aqueous solution may be attributed to the formation of strong Al-O-P bonds originated from the condensation of the oligomer with POH groups of the phosphate layer. It is possible that upon linking to the phosphate layer bridged Al would adopt a lower coordination than that of Al not directly bonded to the layer, where the coordination would be predominantly octahedral. Note that the coordination of Al decreases from octahedral to tetrahedral upon reaction of aluminum hydroxide with phosphoric acid to form aluminum phosphates. Upon calcination, more aluminum ions interact directly with the phosphate layer due to a progressive dehydration of the interlayer oligomer and hence the intensity of the corresponding signal should be enhanced. Noteworthy is that this kind of bond is already present in fresh samples, suggesting that the condensation occurs during the hydrothermal reaction. The strength of the interaction of the aluminum pillar with the phosphate layer does not appear to be appreciably modified by calcination, since the position of the resonance at ca. 45 ppm moves downfield by only a few ppm. This is also corroborated by 31P MAS-NMR and XPS, as the value of the R′ parameter is not appreciably changed by calcination. Surface Properties The surface properties of fluorinated alumina-pillared materials calcined at 673 K have been evaluated by N2 adsorption and acidity studies. N2 adsorption-desorption isotherms of the pillared materials are typical of mesoporous solids with a certain contribution of micropores. Textural properties of the studied samples are summarized in Table 4. Both BET surface area and micropore volume, calculated by the Rs method,32 increase initially with the Al loading. The single-phase sample has the highest BET surface area (180 m2 g-1) and a micropore volume close to 0.1 cm3 g-1. This microporosity is induced by pillaring and is close to that reported for pillared clays.1 The lower surface area of sample AlZrP-1.5 is consistent with the presence of aluminum oxide (at the calcination temperature of 673 K, the precipitated aluminum hydroxide is dehydroxylated), which usually has a low surface area. Typical pore size distributions, determined by the Cranston-Inkley method,33 are shown in Figure 7. They are very narrow with a maximum radius centered at ca. 8 Å, except for sample AlZrP-0.5 which exhibits another maximum at 18.7 Å. When the single-phase material (AlZrP-1) is calcined al 873 K, the specific surface area in reduced to 139 m2 g-1, due, in part, to an interlayer contraction. More interesting is that its pore size distribution is shifted to the mesopores region with a maximum now centered at 13.7 Å of the radius. The possibility of tuning the textural properties is one of the main objectives of the synthesis of pillared layered structures, in order to apply them for specific purposes.1
The acidity of alumina-pillared materials has been studied by TPD of ammonia and adsorption of pyridine. The number of acid sites between 373 and 673 K, measured by NH3-TPD, was in the range of 1.05-1.85 mmol of NH3 g-1. As can be seen in Figure 8, there is a gradual decrease of the acid sites with increasing aluminum loadings. This may be due to the fact that the increase of aluminum oxide pillars reduces the Bro¨nsted acidity associated to P-OH groups. For sample AlZrP-1.5, it may be expected from Figure 2 (which shows a relatively small content of the 21.3 Å phase and the presence of a second phase) that the acidity will be lower than for sample AlZrP-1.0. These data also show that very strong acid sites are generated in sample AlZrP-1.0 upon calcination at 873 K. The nature and specific concentration of the acid sites on the pillared materials were determined by adsorption-desorption of pyridine. Figure 9 shows the IR spectra of sample AlZrP-1 calcined at 673 K with pyridine adsorbed and outgassed at different temperatures. A band at 1450 cm-1, characteristic of pyridine in interaction with Lewis acid sites, is observed in all spectra. Unfortunately, the pyridinium band at 1550 cm-1, typical of Bro¨nsted acid sites, is partially masked by the high background absorption of the sample, and therefore, it was not possible to evaluate the Bro¨nsted acidity from these spectra. The concentration of Lewis acid sites accessible to pyridine molecules (CL) has been calculated from the integrated absorbance and absorption intensity34 of the band at 1450 cm-1, using the extinction coefficient given by Datka et al.35 (Table 5). CL decreases with the desorption temperature and increases with the aluminium loading, which means that the Lewis acidity mainly originates from the aluminum oxide pillars and is due to the presence of deficiently coordinated Al. CL is slightly higher for sample AlZrP-1.5 than for sample AlZrP-1.0, which may be attributed to a weak contribution of the external surface of the aluminum oxide phase (sample was precalcined at 673 K). That the most acidic samples, in terms of total acidity as determined by NH3-TPD, are samples AlZrP-0.25 and -0.5 may be due to their relatively higher amounts of POH Bro¨nsted acid groups. The reaction of isopropyl alcohol has been used in order to evaluate the acid properties of the catalytic sites of the aluminapillared materials. Conversion and activity data are summarized in Table 6. In all the cases, propylene was practically the only reaction product, with traces of diisopropyl ether, demonstrating that the active sites in these pillared materials are exclusively acid centers. The activity of materials calcined at 673 K, between 11.7 and 23.7 µmol g-1 s-1, was much higher than that of pristine R-ZrP and was maintained for at least 20 h (Figure 10). The activity of these catalysts is not only dependent on the chemical nature of the materials but also on the morphological properties, since the most active catalyst was sample AlZrP-0.5, with intermediate Al content and acidity and showing the largest pores in the mesopore region. A more open porous structure could facilitate the access of isopropyl alcohol, a less basic molecule than pyridine and ammonia, to the active sites of the catalyst. However, when the catalysts are calcined at 873 K, the activity is maintained in materials with Al loading lower than 1 and sharply decreases in sample AlZrP-1.5. It is strongly enhanced in the single-phase material AlZrP-1, which becomes the most active of all the studied samples (26.6 µmol g-1 s-1). This may be due to the appearance of strong acid sites after calcining at 873 K (Figure 8) and to the fact that the material becomes more mesoporous (Figure 7). Inasmuch as the Bro¨nsted acidity should decrease with the calcination temperature (due to a progressive dehydration) and considering that the activity of the materials with the highest Bro¨nsted acidity
Fluorinated Alumina-Pillared R-Zirconium Phosphate
J. Phys. Chem., Vol. 100, No. 35, 1996 14733
Figure 7. Pore size distributions of fluorinated alumina-pillared R-zirconium phosphate materials, calcined at 673 K (b) and 873 K (O): (A) AlZrP-0.25, (B) AlZrP-0.5, (C) AlZrP-1, and (D) AlZrP-1.5.
Figure 8. Total acidity of fluorinated alumina-pillared R-zirconium phosphate materials from NH3-TPD.
(AlZrP-0.25 and -0.5) is not enhanced by increasing the calcination temperature, it is inferred that the Lewis acid sites have a significant contribution to the enhacement of the catalytic activity of sample AlZrP-1. These sites should be located mainly in the aluminum oxide pillars. Note that a fraction of the Lewis acid sites (Table 5) are coordinated to pyridine in the range of 444-573 K (CL ) 62.6-35.9 µmol/g) and that the catalytic test was carried out at 493 K.
Conclusions A highly expanded single-phase intercalation compound with 21.3 Å spacing was obtained by contacting an Al24 solution with colloidal R-ZrP in the presence of F-, followed by consecutive reflux-hydrothermal treatments. This phase is unique within a narrow range of Al3+ concentration, but may coexist with other phases in a wider range. XRD data are
14734 J. Phys. Chem., Vol. 100, No. 35, 1996
Me´rida-Robles et al. TABLE 6: Activity and Conversion of Fluorinated Alumina-Pillared r-Zirconium Phosphate Materials for Isopropyl Alcohol Decomposition at 493 K samples AlZrP-0.25 AlZrP-0.5 AlZrP-1 AlZrP-1.5 a
673 K.
b
a b a b a b a b
activity (µmol of propylene g-1 s-1)
convn (%)
12.7 12.3 23.7 22.7 11.7 26.6 16.4 10.8
26.7 19.1 46.4 38.9 23.4 56.0 32.7 16.7
873 K.
Figure 10. Catalytic activity of fluorinated alumina-pillared R-zirconium phosphate catalysts Vs time on-stream for samples: at 673 K (b) AlZrP-0.25, (9) AlZrP-0.5, ([) AlZrP-1, (2) AlZrP-1.5; at 873 K (O) AlZrP-0.25, (0) AlZrP-0.5, (]) AlZrP-1, (4) AlZrP-1.5.
Figure 9. IR spectra of fluorinated alumina-pillared R-zirconium phosphate (AlZrP-1) exposed to pyridine vapors and outgassed at different temperatures.
TABLE 5: Concentration of Lewis Acid Sites (CL) of Fluorinated Alumina-Pillared r-ZrP at Different Temperatures CL (µmol/g) sample
298 K
444 K
573 K
AlZrP-0.25 AlZrP-0.50 AlZrP-1
86.6 145.2 300.0 239.9 313.2
18.1 37.1 87.6 62.6 77.0
15.5 29.0 37.9 35.9 51.4
a b
AlZrP-1.5 a
Calcined at 673 K.
b
Calcined at 873 K.
consistent with the presence of an aluminum oligomer larger than the tridecamer ion, Al13. 31P and 27Al MAS-NMR data suggest that strong linkages between the phosphate layer and the oligomer are established through Al-O-P bonds and that aluminum directly bonded to the layer has a coordination lower than octahedral. Incorporation of F- into the cluster is a determining factor for the improvement of the thermal stability and the porous structure of the resulting pillared materials, because this ion diminishes the number of possible Al-O-P linkages and, consequently, the interlayer contraction during the thermal transformation of the oligomer into aluminum oxide. Fluorinated alumina-pillared materials are acid solids capable of converting isopropyl alcohol to propene with a selectivity close
to 100% and high catalytic activity. Lewis acid centers mainly associated with the low-coordination sites of aluminum are believed to have a significant contribution to the enhancement of the catalytic activity of the single-phase material calcined at 873 K. Acknowledgment. This research was supported by the CICYT (Spain) Project MAT 94-0678 and C.E. Programme BRITE-EURAM Contract BRE 2-CT93-0450. The authors thank Dr. Robert C. T. Slade of the University of Exeter for useful discussion, the U.K. EPSRC National Solid State Service (Durham) for recording 27Al MAS-NMR and the University of Montpellier II for recording 31P MAS-NMR. References and Notes (1) Vaughan, D. E. W.; Lussier, R. J. Proc. 5th Conf. Zeolites, Naples; Rees, L. C. V., Ed.; Heyden: London, 1980; p 94. (2) Clearfield, A.; Roberts, B. D. Inorg. Chem. 1988, 27, 3237. OliveraPastor, P.; Jime´nez-Lo´pez, A.; Maireles-Torres, P.; Rodrı´guez-Castello´n, E.; Alagna, L.; Tomlinson A. A. G. J. Chem. Soc., Chem. Comm. 1989, 751. (3) Lalik, E.; Kolodziejski, W.; Lerf, A.; Klinowski, J. J. Phys. Chem. 1993, 97, 223. Deng, Z.; Lambert, J. F. H.; Fripiat, J. J. Chem. Mater. 1989, 1, 640. Cheng, S.; Wang, T. C. Inorg. Chem. 1989, 28, 1283. (4) Lahav, N.; Shani, U.; Shabtai, J. Clays Clay Miner. 1978, 26, 107. (5) Ocelli, M. L.; Tindwa, R. M. Clays Clay Miner 1983, 31, 22. (6) Plee, D.; Borg, F.; Gatineau, L.; Fripiat, J. J. J. Am. Chem. Soc. 1985, 107, 2362. (7) Rodriguez-Castello´n, E.; Olivera-Pastor, P.; Jime´nez-Lo´pez, A.; Sanz, J.; Fierro, J. L. G. J. Phys. Chem. 1995, 99 (5), 1491 (8) Bergaoui, L.; Lambert, J. F.; Suquet, H.; Che, M. J. Phys. Chem. 1995, 99, 2155. (9) 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.
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