J. Phys. Chem. 1995, 99, 6961-6965
6961
Pentacoordinated Aluminum in Noncalcined Amorphous Aluminosilicates, Prepared in Alkaline and Acid Medium B. M. De Witte,' P. J. Grobet, and J. B. Uytterhoeven Centre for Su$ace Chemistry and Catalysis, Catholic University of Leuven, B-3001 Leuven, Belgium Received: September 23, 1994; In Final Form: February 20, 1995@
Amorphous aluminosilicates prepared in acid and alkaline medium are dried at moderate temperatures (< 150 "C) and submitted to 27Al MAS-NMR (after hydration) and surface area measurements by N2 adsorption. The occurrence of a NMR peak between the octa- and tetrahedral signal is ascribed to the presence of pentahedral Al. Variation of the synthesis and the washing and drying procedures changes the intensity of the A15 peak and indicates the interfacial character of this A1 species, located between the octahedral alumina and the tetrahedral aluminosilicate phase. Relating the relative AP content with the specific surface area is only possible in limited cases. The dispersion of the octahedral phase and the presence of cations (Na+, W+)also play an important role with respect to A15 formation. The effect of hydration on the amount of A15 detected is noticed to be smaller in dried than in calcined samples.
tetrahedral sheet breakdown in the case of p h y l l ~ s i l i c a t e s . ~ ~ ~ Strong atomic rearrangements also take place when zeolites are The existence of 5-fold coordinated aluminum was first de-aluminated. A clear, distinct low-field shoulder of the established by 27AlMAS-NMR in 1985.' This early observation tetrahedral peak was observed after (hydro-) thermal or Sic14 was confirmed the following year in other publication^.^,^ For treatment of aluminosilicate molecular sieves."-I4 Discriminawell-characterized solids like andalusite and barium aluminate tion between pentahedral A1 and a distorted nonframework glycolate, aluminum was shown to be (partially) surrounded tetrahedral species with approximately the same shift was made by five oxygen atoms, explained by a distorded trigonal by double rotation (DOR) NMR. In the case of strong bipyramid configuration.2 In MAS-NMR spectra, this results de-alumination of zeolite Y, with the clear formation of in a peak located between the octahedral and tetrahedral octahedral Al, the intermediate resonance was shown to present resonances, at 30 to 35 ppm. Further development of the MASpentahedral Al." This was confirmed for highly de-aluminated NMR technique and enhanced resolution by employing stronger ZSM-20 samples by two-dimensional quadrupole nutation 27magnetic fields and higher spinning rates soon led to the A1 MAS-NMR.I4 Although de-alumination includes a change observation of pentahedral A1 in more current and interesting from 4- to 6-fold coordination, the hypothesis of A15 as an product classes. These include Al-containing ceramic preintermediate coordination present at the interface remains valid. cursors,'-Io and pure a l ~ m i n a s . ' ~ -The ' ~ imporHard thermal and chemical action can thus generate A15. In tance of pentahedral A1 in processes of high-temperature as-synthesized and dried products, this species was only crystalli~ation~-'~ and probably also with respect to catalytic observed and extensively reported for pure aluminas. The a ~ t i v i t y ' ~ ,urges ' ~ , ' ~a more profound insight in A15 chemistry. limited hydrolysis of aluminum by the in situ generation of a Research published up to now only provides a limited view, base provoked, after drying at 80 "C, the formation of an being mainly restricted to crystallization processes applied in alumina extremely rich in pentahedral A1 (up to 30%, according the production of mullite and cordierite. The results are rather to the synthesis procedure). Although no direct relationship consistent, unlike the use of different starting materials. Kawith reaction conditions could be derived, it was shown that olinite and pyrophyllite show A15 formation between 400 and the A15 content was inversely related to the amount of A16 and 800 0C.8,9 This is ascribed to dehydroxylation of the octahedral of condensation of the octahedral phase. Exposing the degree alumina sheets. In amorphous precursors, A15 originates calcined samples (500 "C) to saturated water vapor resulted in between 300 and 800 "C and is thought to be located between the octahedral alumina and tetrahedral aluminosilicate p h a ~ e . ~ . ~ an irreversible disappearance of the pentahedral peak.I5 Nonhydrolytic synthesis of aluminas led to the formation of A15 in It is proposed that A15 is stabilized by the interfacial strain or samples dried at 150 "C.I8 Classic hydrolysis, causing formation alternatively minimizes it. The relative contribution of A15is of A113 ions, gave only A15 starting from 300 "C.I6 The Keggin taken as a measure of the homogeneity of the sample, as pointed polymer structure allows a stable coexistence of A16 and A14, out by an enhanced mullite production. Hydration of a gel and consequently, there is no need for interfacial Al5.I7 In calcined at 600 OC was observed to reduce considerably the contrast to aluminosilicates, the presence of pentahedral A1 is intensity of the A15 peak.5 Irrespective of precursor type, A15 related to a hampered crystallizationfor alum in as.'*^'^ However, formation is noticed to occur simultaneously with strong the samples compared also show strong differences in other reorganizations at the atomic level, necessary to achieve mullite properties like texture and particle shape of the precursor. or a related structure. This usually implicates a (partial) transition of A16 to A14. Pentahedral A1 is hereby considered The occurrence of pentahedral aluminum in amorphous as an intermediate and metastable coordination, typically aluminosilicates dried at low temperatures (< 150 "C) has not disappearing around 980 "C. The elimination of A15 is an yet been the subject of deliberate study, although this system exothermic process that might initiate cry~tallization~~'~ and/or contains a wealth of information on A15 chemistry. The results in this paper show that relatively minor changes in the synthesis and the washing and drying procedures are sufficient to induce Abstract published in Advance ACS Abstracts, April 1, 1995. I. Introduction
@
0022-365419512099-6961$09.00/0 0 1995 American Chemical Society
De Witte et al.
6962 J. Phys. Chem., Vol. 99, No. 18, 1995
AI5 formation or to alter the A15 contribution. Indications are already given for the interfacial character of A15. More experimental evidence will be introduced, and the relationship between textural properties and AI5c ~ n t e n t 'will ~ , ~be~ further investigated. In order to enhance sensitivity, most samples were hydrated before 27Al MAS-NMR analysis.2' Therefore, also the effect of hydration on the Al coordination of samples dried at 120 "C is studied. The amorphous aluminosilicates, submitted to 27AlMAS-NMR and surface area measurements, are synthesized from alkaline silicate and aluminate solutions or an acidified aqueous mixture of tetraethylorthosilicate and Al(N03)3. 11. Experimental Section 1. Synthesis. Two main synthesis procedures can be discriminated, based on the type of precursors used and the pH conditions imposed. Alkaline Synthesis Starting from Silicate and Aluminate Solutions.22 One of the first techniques involved the direct coprecipitation of silica and alumina. A 0.5 M aluminate solution was added under heavy stirring to a 0.5 M silicate solution (Merck water glass, 27.72% Si02, 13.29% NazO, density = 1.37 g/mL). The alumina solution was prepared by dissolution of a powder with 41.32% A1203 and 52% NazO (BDH Chemicals) in distilled water. The mixture was immediately titrated with a 4 M HC1 solution (UCB Chemicals, p.a.) to pH = 8. The resulting gel was centrifuged and washed 3 times with distilled water. The volume of water used each time was equal to the synthesis volume. The wet gels were placed in a furnace at 120 "C under ambient atmosphere and dried overnight. In a related synthesis procedure, a stable sol phase was introduced, preceding the event of gel formation (alkaline sol-gel synthesis). To distilled water were added sequentially formaldehyde (Janssen Chimica, 30%, stabilized in 10-15% methanol), silica (undiluted Merck water glass), and alumina. The alumina source was a 0.5 M NaAl02 solution, obtained from a Riedel-de Haen powder with 54% A1203 and 41% Na20. The total molarity (Si Al) was chosen to be equal to 0.16 in order to reduce reactivity, and the presence of formaldehyde (25 vol %) ensured a slow acidification of the synthesis mixture. After a sol aging time of 20 h, the pH of the solutions was adjusted to 8 by titration with HCl. Gels are compacted by centrifugation and not washed prior to drying. Acid Synthesis Starting from Tetraethylorthosilicate and AlA silica sol was obtained by aging for 19 h a mixture of tetraethylorthosilicate (TEOS) (Janssen Chimica, 98%) and water (H20:TEOS molar ratio = 24:l) at pH = 1.9. Hydrochloric acid was used as catalyst. Aluminum was added finally as a 1.2 M Al(N03)3 solution (Fluka, Al(N03)39HzO p.a.). After 1 h of further reaction, the pH of the solution was increased to 6 with concentrated NH3 (Merck, 25% p.a.). The resulting gel was centrifuged and dried without washing. Slightly different procedures were applied for the following products. (1) For a gel with 55% AI, washing and slow drying (from 20 to 120 "C at a rate of 0.017 "C/min) were performed. The final pH was chosen to be equal to 2.5 or 6. (2) A gel with 36% A1 was prepared by initial addition of A1 and alternatively formamide (Janssen Chimica, 99% p.a., moles of formamide/(mole of Si Al) = 3).24 After 7 h of sol aging with a pH adjusted to 1.9, the mixture was diluted 3 times with distilled water and titrated to pH = 6. After centrifugation, a fraction of the wet gel was dried at room temperature under a flow of air. Gels dried at 120 "C were also calcined at 400 and 800 "C (temperature increase of 5 "Chin). 2. Characterization. The dried products were ground, using an agate mortar and pestle. Gel particles with diameters between
+
+
/I\ D
-I
' I/\ \
\
100 50 0 -50 (ppm) Figure 1. 27Al MAS-NMR spectra for acid-sol-gel-synthesized products (A: 25% Al, B: 30% Al, C: 34% AI, D: 36% Al, E: 42% Al, F: 55% Al).
0.50 and 0.16 mm were used for texture analyses. Single point BET surfaces (SOSBET, mz/g) were obtained on an Ankersmit 4200 instrument by N2 adsorption at a partial pressure (PIP") of 0.3. Samples were pretreated at 400 "C for 3 h under a He/ N2 (0.7/0.3) flow. Measurements of N2 adsorption isotherms were performed with an Omnisorp 100 system (Coulter). The multipoint BET surface (S'MBET, m2/g) was calculated from the adsorption isotherm between 0.05 < PIP" < 0.25. Before sorption, samples were heated at 400 'C under a vacuum of Pa for at least 8 h. Fine ground powder (diameter < 0.16 mm) was used for the NMR analyses. Aluminum MAS-NMR was conducted on a Bruker 400 MSL spectrometer, equipped with a 9.4 T magnetic field. The 27Alspectra were acquired at 104.2 MHz, using 0.61 ys pulses and a 0.1 s recycle delay. A spinning rate of 10-14 kHz was installed. About 3000 scans were accumulated. If not mentioned otherwise, samples were hydrated before measurement above a saturated W C 1 solution. Al(N03)3 was used as a reference. The relative contribution of the different coordinations was calculated from integration and Gaussian resolution of the spectra. Quantitative A1 and Na determinations were performed on acid solutions, obtained by reacting gels calcined at 500 "C with a HF solution (UCB, 50% p.a.), respectively by atomic absorption and flame emission spectrophotometry. 111. Results During drying of acid-sol-gel-synthesized products at 120 "C, powder (IO-25% Al) or glass phases (42-55% Al) result as a function of the amount of A1 added. At intermediate A1 content (34-42%), the two phases may occur simultaneously. This is probably due to dissolution and reprecipitation of Alrich inhomogeneitiesthat arise during titration.23 In this article,
J. Phys. Chem., Vol. 99, No. 18, 1995 6963
Pentacoordinated Aluminum
100 50 o -5o(ppmj 100 So -0 -5o(ppmj Figure 3. 27AlMAS-NMR spectra for a gel (55% Al, final pH = 2.5,
washed) dried overnight (left) and for 3 weeks at 120 "C (right), before (A) and after hydration (B). I " "
' I " "
100
50
I " " I -50 (PP~)
I " "
0
Figure 2. Influence of the drying regime (A: slow drying, B: fast drying) on the A1 coordination of an acid gel with 55% A1 (final pH = 2.5, washed).
TABLE 1: 27AlMAS-NMR and Surface Area Data for Acid-Sol-Gel-Synthesized Products %A1
SoMBET,m2/g
0 5 15 25 30 34 36 42 55
580 39 1 276 53 27 1 488 267 252 385
%A14
%A15
97.0 97.0 96.0 70.0 54.8 40.0 35.2 21 .o
2.0 2.0 1.5 8.2 13.0 9.3 8.8 8 .o
1.o 1.o 1.o 21.7 32.2 50.7 56.0 71.0
final SOSBET, g of pH washed drying m2/g % A14 % A15 % A16 A l k (exD) yes yes no no yes yes
slow fast slow fast slow fast
252 183 365 384 60 204
34 29 14 21 13 20
10 18
5 8 6 12
56 53 81 71 81 68
temperature
0.08 0.09 0.21 0.23 0.17 0.20
only for the most characteristic phase data will be reported. For acid-synthesized gels with A1 content going from 25 to 55%, the MAS-NMR spectra are given in Figure 1. Up to 25%, A1 is almost completely incorporated in the tetrahedral network. Then a fast increment of the octahedral coordination takes place, and at 36%, already more A1 is present in the octahedral than in the tetrahedral coordination. The highest degree of pentahedral A1 is realized in the 34% A1 gel. This becomes particularly clear from the analysis of the spectral results shown in Table 1. With respect to the textural evaluation, it is seen that the addition of A1 causes initially (10-25%) a reduction of the surface area ( S O ) as compared to that of the pure silica. This is followed by a surface increase, reaching a maximum at 34% Al. With further increasing Al content, So decreases again (36-42%). Table 2 summarizes data for acid-synthesized gels with 55% Al, submitted to different preparation methods. Applying a washing procedure prior to drying increases the amount of pentahedral A1 and decreases the surface area. At lower final pH, washing gives rise to an important loss of octahedral aluminum, even though relatively more A15 is observed. Faster drying also enlarges the amount of A15 as illustrated in Figure 2. For the product with the highest A15 content observed (55% Al, final pH = 2.5, washed, fast drying, see Table 2), the effects of hydration and prolonged drying at 120 "C on the A1 coordination were investigated. The results of this experiment are shown in Figure 3 (all spectra refer to the same mass of dry sample). When no hydration is performed before NMR
S'SBET, m2/g
% A14
% A15
% A16
20 120 400 800
Without Formamide 29.7 285 34.6 433 21.3 18.1
7.0 17.1 25.6 34.3
63.2 48.2 53.0 47.5
20 120 400 800
With Formamide 32.4 380 463 43.2 21.0 17.2
8.9 6.7 31.9 42.0
58.6 50.0 47.2 40.6
%A16
TABLE 2: Properties of Acid Gels with 55% AI as a Function of Final pH and Washing and Drying Procedures
2.5 2.5 6.0 6.0 6.0 6.0
TABLE 3: Properties of Acid Gels with 36% AI (Initial Addition), with and without Formamide as a Function of the Temperature of Drying and Calcination
measurement, less A1 is detected for the sample dried overnight, mostly at the expense of A14 and A16. When compared to the hydrated sample, only 77%of the Al is visible in the unhydrated gel. As a consequence, the relative contribution of A15 in the spectrum of the unhydrated sample is overestimated. This effect becomes smaller after a longer drying period (3 weeks at 120 "C), 90% of the A1 remaining visible without hydration. This indicates that there is less A1 present in a coordination from which the visibility is enhanced by hydration. At the same time, spectral analysis reveals that more A15 is lost upon exposure to water vapor. During prolonged drying, no significant textural change occurred (S'SBET after 12 h drying, 95 m2/g; after 3 weeks drying, 110 m2/g). In Table 3, it is seen that when the drying temperature is increased from 20 to 120 "C, more tetrahedral A1 is formed, particularly for the gels with formamide. With formamide, the tetrahedral contribution grows at the expense of A15 and A16, while in the absence of formamide mainly a transition of A16 to A15 is noticed. No significant differences in surface area are observed at 120 "C. By comparison with the results in Table 1, it is seen that acid gels prepared by initial addition of A1 contain less A14. Calcination provokes a strong increase in A15 content, first by A14 (400 "C) and later by A16 conversion (800 "C). Remarkable is the nearly equal degree of A14 realized at 800 "C for the samples with and without formamide. The changes of the A1 coordination with drying and calcination temperature are depicted in Figure 4 for the gel with formamide. For gels obtained by direct coprecipitation in alkaline medium (Table 4, top section), a similar dependence of So on the A1 content is noticed as in the acid medium (Table 1). After an initial decrease (12-28% Al), the surface area increases again (34%), followed by a new lowering of So at higher A1 contents (42-62%). However, near full tetrahedral coordination is preserved up to around 42%, and instead of a pronounced maximum in the A15contribution corresponding to a high surface area, a gradual increase with A1 content is observed. The NMR spectra for gels with increasing A1 content are given in Figure 5. Washing does not produce a significant loss of A1 as compared to the unwashed sol-gel-synthesized products, but the amount of Na+ is clearly decreased, certainly at higher A1 content. The sol-gel-synthesized products show a lower Ai5
6964 J. Phys. Chem., Vol. 99, No. 18, 1995
De Witte et al.
n
100
50
0
-50
(ppm)
Figure 5. 27AlMAS-NMR spectra for alkaline gels obtained by direct coprecipitation (A: 42% Al; B: 54% Al; C: 62% Al; D: 72% Al). Figure 4. 27Al MAS-NMR spectra for a gel with 36% A1 and formamide as a function of the temperatures of drying and calcination (A: 20 OC;B: 120 "C;C: 400 "C;D: 800 "C).
TABLE 4: Properties of Alkaline Products, Prepared by Direct Coprecipitation (Washed) or by Sol-Gel Synthesis (Not Washed) % A1
SOMBET, m2/g
g of g of %A14 % AI5 % AI6 Al/g (exp) Na/g (exp)
Direct Coprecipitation 12 28 34 42 54 62 12
435 123 326 293
40
253 265 219
50
60
24 1
100.0 100.0
0.0 0.0 0.0
0.0 0.0
0.6 99.0 90.4 5.7 4.1 59.6 12.2 28.2 37.7 13.2 49.1 23.0 16.0 61.0 Sol-Gel Synthesis 64.4 7.8 27.8 44.4 8.2 47.3 29.5 2.9 67.5
0.11 0.14 0.20 0.26 0.33
0.091 0.098 0.079 0.065 0.041
0.16 0.22 0.28
0.083 0.101
0.088
content as compared to that of the washed samples with a similar A14 to A16 ratio.
IV. Discussion The resonance between the octahedral and tetrahedral signals has in this work been ascribed to the presence of pentahedral Al, in accordance with the reports cited in the reference section. The suggestion of a distorted tetrahedral coordination' ' , I 4 is not supported by the clear presence of octahedral A1 and the shift of the pentahedral peak that rarely exceeds 30 ppm. The quantitative determination of the relative contribution of the different coordinations is not totally unambiguous, since it is often difficult to select the best fit or integration intervals for the spectra and in view of the often low amounts of A15. Therefore, attention is concentrated on the general evolutions in Al coordination rather than on the absolute values, which should be interpreted with the necessary caution. Nevertheless, the results demonstrate that up to and probably more than 15% of the amount of A1 present can be 5-fold coordinated in products submitted to only moderate drying. This illustrates the rather stable character of the pentahedral coordination with
no need for strong thermal or chemical activation. It should be remembered that the 27Al MAS-NMR measurements are performed on hydrated samples. It can be concluded from the spectra represented in Figure 3 that hydration diminishes slightly the amount of A15. This effect is much less pronounced than that reported in the literature for calcined sample^.^^'^ Drying at 120 "C probably does not eliminate all of the physisorbed water. Furthermore, it is possible that for the relatively low A15/(A14 A16)ratios in this work, as compared to those of the calcined samples from literature, the interfacial character of A15 (see further) reduces its sensitivity toward hydration. Prolonged drying at 120 "C increases the A15 contribution, as follows from the higher amount of visible A1 without hydration, but also causes a greater sensitivity of pentahedral A1 to water. It is important to note that at temperatures as low as 120 "C,an effective transformation of the A1 coordination takes place. For amorphous aluminosilicates changes in A1 coordination with A1 content have been related to variations in specific s u r f a ~ e . ~The ~ . initial ~ ~ introduction of A1 and corresponding negative charges into the tetrahedral network cause stronger condensation,resulting in a surface decrease. This effect is more pronounced for gels synthesized in acid than in alkaline medium because of the lower inherent negative charge of the acid silicate species. Furthermore, stronger condensation is possible not only between the sol particles but also in the sol particles, which are expected to be less branched.25 At higher A1 contents, the surface increases again due to the less acid character of tetrahedral A1 at lower SUA1 ratio for the alkaline synthesis, or due to the reduced incorporation of A1 for the acid synthesis. Finally, the reduction of the surface area at even higher Al percentages is ascribed to the formation of a distinct octahedral phase that can block pores. The idea of interfacial pentahedral A1 species fits surprisingly well in this model as shown by the results obtained for acid-sol-gel-synthesized products. Clearly, detectable amounts of A15 only arise when sufficient octahedral A1 is present (starting from 30% Al). The first formation of A16 accompanies a clear surface increase. The relatively low degree of octahedral Al combined with a relatively high surface area can be interpreted in terms of an alumina phase that is rather loosely dispersed over the gel surface. This is also indicated by the broadening of the octahedral peak and an increased activity of the gels in phosphate adsorption (the latter
+
J. Phys. Chem., Vol. 99, No. 18, 1995 6965
Pentacoordinated Aluminum feature will be the topic of a future publication). This gives rise to a high surface contact area between the octahedral and tetrahedral phases and consequently results in a high degree of A15(cf. gel with 34% Al). At higher Al contents, a more discrete alumina phase is formed that shows less intimate mixing with the tetrahedral phase and even blocks pores. The higher degree of heterogeneity is c o n f i i e d by high-temperature crystallization experiments that yield spinel and y-AlzOs phases rather than mullite.23 The relation between surface area and A15 content is disturbed by the introduction of a washing step. Washing leads to a stronger contraction of the gel network during drying by an increase of the pore solvent surface tension and by eluting NH4NO3, a pore-widening agent.25 This results in more closed or non-nitrogen-accessible porosityI5 and possibly a closer contact between the different phases. This can be related to the observed increase in A15 content. The fact that washing causes a higher dispersion of the octahedral phase, retaining primarily surface-bonded aluminum as indicated by the broadening of the octahedral peak, constitutes an additional explanation. Faster drying also induces the formation of more A15. Faster drying gives rise to a stronger contraction of the gel network.25 A nice proof of the interfacial character of A15can be derived from the results shown in Table 3. The addition of formamide does not noticeably affect the A1 coordination in roomtemperature-dried samples. When drying is carried out at 120 "C, the further elimination of water in general seems to reduce the A1 coordination. In the presence of formamide, hydrolysis with the formation of NH3 takes place during drying, resulting in an increase of the pH of the pore This strongly promotes the transition to the 4-fold coordination, especially of Al located at the interface. This explains the low A15content of formamide-containing gels at 120 "C. At higher temperatures ('120 "C), first a strong reduction of A14, and later of A16, occurs to give A15. This is in contrast with the alumina rich gels typically used in mullite synthesis, in which mainly A16 is converted to A15 and A14. In both cases, a clear evolution to mullite stoichiometry takes place during calcination. This indicates that after the removal of the main pore fluid, atomic rearrangements are crystallization directed. In gels obtained from alkaline direct coprecipitation, much more A1 can be incorporated into the tetrahedral framework. Thus, AI5 is only to be expected at higher A1 contents, and the relationship with specific surface cannot be as obvious as that for acid synthesized gels. Starting from 40% Al, the A15 contribution increases with the relative portion of A16. This is not in agreement with the assumption that a higher dispersion of the octahedral phase favors A15 formation. Alkaline gels are characterized by the presence of sodium ions that neutralize the negative charges, arising when A14 is introduced in the silica network. These sodium ions, accumulated at the tetrahedral network surface, probably disturb the close interaction between A14 and A16 that seems to be necessary for A15 formation. At higher A1 content, the degree of A14 is reduced, possibly giving rise to a similar decrease in the amount of surface-bonded Na'. This hypothesis is supported by the observation that in unwashed gels, richer in Na+, the amount of A15 detected is lower when compared to that of washed samples with comparable A14 and A16 content. Likewise, the higher A15 content in the washed acid gels (see Table 2) can be explained by the removal of W+. In addition, for gels with final pH = 2.5, less N&+ is formed. For the unwashed alkaline samples, the highest degree of pentahedral Al is found in products with relatively high surface areas and comparable contributions of A16 and A14.
V. Conclusions Pentahedral aluminum is observed in dried and subsequently hydrated amorphous aluminosilicates. The results presented in this report constitute strong evidence that this species is located at the interface between the tetrahedral aluminosilicate network and the octahedral alumina phase. A relationship between the relative A15content and specific surface area as determined from NZadsorption showed only limited validity for the case of acidsol-gel-synthesized products. Very small pores and closed porosity may well contribute to the real contact interface. When synthesis and drying procedures are changed, other parameters should be taken into account, especially the dispersion of the octahedral phase. Furthermore, the presence of cations at the interface may reduce the formation of A15, strong indications of which were obtained for alkaline synthesized products. The effect of hydration on the A15 content is less pronounced for dried than for calcined samples.
Acknowledgment. The authors thank H. Geerts and R. Reynders for performing the MAS-NMR experiments and the quantitative analysis of the spectra. They are also indebted to R. Lookman for the AAS measurements. B. De Witte is supported by a grant from the Belgian Institute for Scientific Research in Industry and Agriculture (IWONL). References and Notes (1j Dupree, R.; Faman, I.; Forty, A. J.; El-Mashri, S.; Bottyan, L., J. Phys., Colloq. 1985, CB, 113. (2) Alemany, L. B.; Kirker, G. W. J. Am. Chem. SOC.1986,108,6158. (3) Cruickshank, 1%. C.; Dent Glasser, L. S.; Barri, S. A. I.; Poplett, I. J. F. J. Chem. Soc., Chem. Commun. 1986, 23. (4) Hietala. S. L.; et al. J. Am. Ceram. SOC. 1990, 73 (IO), 2815. (5) Hatakeyama, F.; Maekawa, T. J. Ceram. SOC. Jpn 1991,100, 174. (6) Sanz, J.; et al. J. Am. Ceram. SOC.1991, 74 (lo), 2398. (7) Taylor, A.; Holland, D. J. Non-Cryst. Solids 1993, 152, 1. (8) Sanz, J.; Madani, A.; Serratosa, J. M.; Moya, J. S.; Aza, S. J. Am. Ceram. SOC.1988, 71 (IO), c-418. (9) Sanchez-Soto, P. J.; Sobrados, I.; Sanz, J.; Perez-Rodriquez, J. L. J. Am. Ceram. SOC.1993, 76 (12), 3024. (IO) Selvaraj, U.; Komameni, S.; Roy, R. J. Am. Ceram. SOC.1990, 73(12), 3663. (11) Ray, G. J.; Samoson, A. Zeolites 1993, 13, 410. (12) Sanz, J.; FomCs, F.; Coma, A. J. Chem. SOC.,Faraday Trans. 1 1988, 84(9), 3113. (13) Gilson, J. P.; et al., J. Chem. SOC.,Chem. Commun. 1987, 91. (14) Kosslick, H.; Tuan, V. A.; Fricke, R.; Martin, A. Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 84, 1013. (15) Coster, D.; Fripiat, J. J. Chem. Mater. 1993, 5, 1204. (16) Wood, T. E.; Siedle, A. R.; Hill, J. R.; Skajune, R. P.; Goodbrake, C. J. Mater. Res. SOC.Symp. Proc. 1990, 180, 97. (17) Bradley, S. M.; Kydd, R. A,; Howe, R. F.J. Colloid Interface Sci. 1993, 159, 405. (18) Acosta, S.; et al. J. Non-Cryst. Solids 1994, 170, 234. (19) Mizushima, Y . ;Hori, M. J. Non-Cryst. Solids 1994, 167, 1. (20) Fahrenholtz, W. G.; et al. J. Am. Ceram. SOC.1991, 74 (IO), 2393. (21) Engelhardt, G.; Michel, D. High Resolution Solid-state NMR of Silicates and Zeolites; John Wiley and Sons: New York, 1987; Chapter V, 270. (22) De Witte, B.; Uytterhoeven, J. B. J. Am. Ceram. SOC.,submitted. (23) De Witte, B.; Aemouts, K.; Bussche, S.; Uytterhoeven, J. B. J. Am. Ceram. SOC., submitted. (24) De Witte, B.; et al. J. Porous Mater., to be published. (25) Brinker, C. J.; Scherer, G. W. Sol-Gel Science, the Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990; Chapter VIII, p 453. JP942585N
