Langmuir 1996,11, 2615-2620
2615
Effect of Bulk Properties on the Rehydration Behavior of Aluminas D. J. Coster and J. J. Fripiat" Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin, P.O. Box 413, Milwaukee, Wisconsin 53201
M . Muscas and A. Auroux Institut de Recherches sur la Catalyse, CNRS, 2 av. A. Einstein, 69626 Villeurbanne Cedex, France Received March 13, 1995. I n Final Form: May 5, 1995@ The rehydration-dehydration behavior of transition aluminas containing controlled amounts of pentahedral Al has been investigated. Microcalorimetric measurements at the early stage of surface rehydration suggest that the differential heat of water chemisorption rises with increasing crystallinity. The heat released during the dissociative chemisorption varies between 250 and 85 kJ/mol and is larger than the figure corresponding to bulk rehydroxylation (65 kJ/mol). On the average, 5.4 OH per nm2 are generated during this process. Bulk defects are cured when aluminas are contacted with saturated vapor pressure and thereafter calcined at high temperatures (600 or 750 "C).While surface area decreases, the crystallinity increases and &fold coordinated Al is transformed into a more stable coordination.
Introduction Transition aluminas are widely used in industry as adsorbents, catalysts, or catalyst supports because of their high surface area, catalytic activity, and relative stability. Extensive research has been carried out to study their bulk and surfaces properties. However, the poor crystallinity of transition aluminas has made it difficult to use X-ray diffraction techniques. Advances in high-resolution NMR brought about new Moreover, there is only a limited understanding about the relations between surface properties and the dehydroxylationrehydroxylation behavior, despite the widespread use of this type of catalyst. Thirty years ago, Pen4 proposed a very idealized but, nevertheless, useful model where the surface is built from well-defined crystallographic planes. Even though it is evident that such a n idealized surface cannot realistically correspond to the real surface of polycrystalline a l ~ m i n a s this , ~ model is still used to interpret experimental data. Here again, NMR has shed a new light on the problem. It has been clearly demonstrated that transition aluminas contain variable amounts of &fold coordinated Al (4")and that species could be concentrated at the surface.6*7Moreover, in the course of dehydroxylation, a distinct type of tetrahedral Al is produced at the surface.* Thus, a transition alumina cannot be identified as a pseudospinel structure with a ratio tetrahedrayoctahedral Al of 1/2. Any realistic surface
* To whom correspondence should be addressed: (414)
229-5852;
Fax,
(414)
229-5530;
Telephone, e-mail,
[email protected]. Abstract published in Advance ACS Abstracts, July 1, 1995. @
(1)Morris, H. D.; Ellis, P. D. J . Am. Chem. SOC. 1989,111, 6045. (2) Wood, T. E.; Siedle,A. R.; Hillk, J. R.; Skajune, R. P.; Goodbrake, C. J. In Better Ceramics though Chemistry N, Zelinski, B. J., Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society: Pittsburgh, PA, 1990; Vol. 180, p 97. (3)Huggins, B. A,; Ellis, P. D. J . Am. Chem. Soc. 1992,114,2098. (4) Pen, J. B. J. Phys. Chem. 1966,70,3168. ( 5 ) Knozinger, K.; Ratnasamy, P. Catul. Rev. Sci. Eng. 1978,17(l), 31.
(6) Coster, D. J.; Fripiat, J. J. Chem. Muter. 1993,5,1204.
(7) Coster, D. J.; Levitz, P.; Fripiat, J. J. Muter. Res. Symp. Proc. 1994,351,157. (8) Coster, D. J.; Blumenfeld, A. L.; Fripiat, J. J. J . Phys. Chem. 1994,98,6201.
0743-7463/95/2411-2615$09.00/0
model should take these observations into account. Recent application of rotational echo double resonance technique (1H-27Al REDOR) with one pulse and cross-polarization excitation has given structural information concerning the surroundings of four-, five-, and six-coordinated Al sites in rehydrated a l ~ m i n a s .Moreover, ~ it became possible to discriminate between surface, intermediate, and bulk species. The challenge of this study is to show the relationship between bulk properties and rehydration behavior, using results from 27AlMAS NMR, microcalorimetric, andX-ray measurements.
Experimental Section The samples studied in this paper have already been described previ~usly.~,~ A commercial y-alumina (LaRoche #92sx172c) calcined at 600 "C was included in the set for comparison. For the sake of clarity, the main steps of the sol-gel process used for the synthesis of nanometer-sized aluminas are summarized in a flow chart (Figure 1and Table 1). By controlling the degree of hydrolysis and condensation of an aluminum tri-sec-butoxide gel, it was possible to tune the amount of &fold coordinated Al present in the calcined gel. Heat of water chemisorption was measured with a Calvet microcalorimeter. About 70 mg of alumina was thermally activated under vacuum. The temperature was increased to 500 "C at a rate of 2 "C/min. The sample was kept at 500 "C for 2 h and then cooled down to 60 "C, namely, the adsorption temperature. Small doses of water vapor (0.3-3.0 cm3per g of alumina) were contacted at a time. The residual pressure Torr to 0.5 Torr in 15 to 20 steps. The increased from 5 x heat released and the volume of water adsorbed were measured at each step. Once the residual pressure exceeded 0.5 Torr, the sample was pumped for 4 h under vacuum at 60 "C. The dynamic residual pressure was lower than 10-6 Torr and, when stable, water vapor was adsorbed as in the first part of the experiment; this corresponds t o the secondary isotherm. The irreversibly adsorbed water is defined as the difference between the amounts of water adsorbed at P = 0.2 Torr for the first and secondary isotherms. The coverage corresponds to the amount adsorbed divided by the irreversibly adsorbed water. The coverage, arbitrarily defined in this way, is one when the amount of water corresponding to the irreversible process is adsorbed. (9) Blumenfeld, A. L.; Coster, D. J.;Fripiat, J. J. Chem. Phys. Lett. 1994,231,491.
0 1995 American Chemical Society
Coster et al.
2616 Larzgmuir, Vol. 11, No. 7, 1995
I
Fint Calcination (FC)
aooc or 750%
I Rotocol IU
1
Rchydration (RH) RT, 3 Dam, P/Po-I
I
Rotocol I1
Rocosol I
I
Up to 500% at 5 C h i n
I
Sccond Calcination (SC)
600% or 750C'
INMR,XRD,
N2Adrorption
I
Figure 2. Flow chart for the rehydration study. RT means room temperature. 250 I
Figure 1. Flow chart for the sol-gel synthesis. r = HzO/A.l, hydrolysis ratio.
s 200
Table 1. Sample Preparation S u m m e
sample ID total hydrolysis ratio gel processing 2.6 I I 2.6 I1 2.6 I1 2.6 4.5 I I 4.5
\
iiI
calcination temp ("C) 600
* 150
E
3U 100
750
600
f
750
lE
600
5
750
b
600
c
600
The total hydrolysis ratio (H20/Al) is the sum of the pre- and posthydrolysis ratio (see text). The gel processing of type I corresponds to vigorous stirring for 5 min, washing by centrifugation, overnight freeze-drying,and drying at 80 "C. During gel processing 11, the solvent is evaporated to dryness at 80 "C. La Roche y-alumina. Sample 1 after one rehydration-dehydration cycle.
v
0.01 0.02
a
A dynamic study of water desorption was performed on a DSC\TGAapparatus (SetaramModelTG-DSC111).The sample was heated in a flow of helium (10.7 mumin) at a rate of 5 "C/ min until the temperature reached 500 "C. The temperature was maintained at 500 "C for 1.5 h until constant weight loss. Rehydration experiments were carried out on gels calcined at 600 or 750 "C in a humidifier. The container was closed and evacuated to primary vacuum. The samples were equilibrated for 3 days at room temperature. The water vapor pressure was about 24 Torr, corresponding to a relative humidity of about 100%. The rehydrated aluminas were characterized by NMR prior to second calcination at the same temperature as the first calcination. Weight loss and weight gain were also recorded at each step of the cycle. Nz adsorption and desorption isotherms and XRD were run after first and second calcination. In order to simplifyour notation, SC refers to a sample or a measurement recorded after the first calcination, RH and SC correspond to the rehydrated state and the state after second calcination, respectively. A second flow chart summarizes the different experiments (Figure 2). It must be pointed out that the three protocols correspond to very different stages of sample rehydration andor dehydration. Protocol I gives the thermodynamics data at the onset of water chemisorption (weight gain less then 3%,PIP0 < 0.003). Protocol I1deals with an intermediate stage;dehydration of samples in equilibrium with ambient humidity at room
50
0.05
0.1 0.2 Coverage
0.5
1
2
Figure 3. Differential heat of water adsorption versus the coverage for three selected samples. Insert: data for all samples in the range 0 > 1.
temperature (physisorbed water up to 30%in weight). Protocol I11 corresponds to full rehydration at room temperature (PIP0= 1).
Results Protocol I. The differential heat of chemisorption is plotted versus the coverage for three selected samples (Figure 3). On a semilogarithmic scale, the heat decreases linearly with the coverage, and all data converge at a coverage larger than 1. The Freundlich behavior (logarithmic dependence) reveals that chemisorption occurs on a heterogeneous surface. The fitting parameters as well as the irreversibly adsorbed water are reported in Table 2. The logarithmic decay requires an arbitrary definition of "initial" differential heat of adsorption corresponding to a low coverage > 0. For instance, the values at 5%coverage vary between 170 and 230 kJ/mol and, as shown later, depend on the crystallinity. At coverage larger than 1,all samples behave similarly(insert Figure 3). The differential heat drops from 85 kJ/mol at coverage 0 = 1 to the condensation enthalpy of water (*44 kJ/mol) at a coverage of about 1.6-1.8. Similar results have been reported for 7-aluminas outgassed
Langmuir, Vol. 11, No. 7, 1995 2617
Rehydration Behavior of Aluminas
sample ID 1 2 3 4 5 6 7 8
Table 2. Fitting Parameters of the Thermodynamic Data (Protocol I)= fitting parametersb irr ads watei a b R2 pmol/g OH per nm2 weight loss: % pore volumee available to water, m u g 106.24 98.68 103.71 98.23 97.53 89.31 112.45 93.02
-21.79 -32.35 -29.41 -42.69 -29.68 -35.31 -38.77 -34.31
0.956 0.906 0.957 0.940 0.981 0.967 0.910 0.979
1546 607 1217 767 1575 766 501 714
6.12 4.52 5.18 5.41 5.37 5.66 2.41 5.71
25.5 5.7 16.1 9.7 22.3 5.8 14.1 8.5
0.40 0.23 0.66 0.43 0.63 0.36 0.53
a Water weight loss or gain obtained from Protocols I, 11, and 111. The variation of the differential heat of HzO chemisorption follows the Freundlich equation: AH = a - b In 0 up to 0 = 1(see text). See Figure 3 for examples of the fit. The irreversibly adsorbed water corresponds to the water adsorbed at 0 = 1(protocol I). From protocol 11. e From protocol 111.
0
11
0 h
h
u
'2 0
3d j
W
10.8 -
-0.005
-5
C
-0.01 -lo
F6 -15
-0.015
-20
-0.02
v
-
*E 2 Y
?n
z . -
0.6 -
3
4 .-p 0.4
-
4
P
2 0.2 -
s 0
100
200 300 Temperature ('C)
400
500
"0
0.2 0.4 0.6 0.8 Volume Available to %O (mL/g)
1
Figure 4. Differential scanning calorimetric (DSC)signal (mW) and differential thermogravimetric analysis (DTG)of samples 1and 2. The unit for DTG is weight %/min.
Figure 5. Correlation between the porous volume measured from HzO adsorption at 25 "C and PIP0 = 1and Nz adsorption at -198 "C.
between 500 and 740 "C.l0 It should be emphasized that the chemisorption releases very large, amounts of energy, but that it corresponds to minute amounts of water. For the sample developing the largest surface area, the water uptake at coverage 1is less than 3% in weight. For all samples, the water adsorption isotherms present a similar shape, typical of a strong irreversible adsorption (isotherm of type I). Protocol 11. Figure 4 represents the differential scanning calorimetric (DSC) signal and the differential thermogravimetric (DTG) traces of samples 1and 2. The weight losses between room temperature and 500 "C are reported in Table 2. The sums of the amount of chemisorbed and physisorbed water are 1order of magnitude larger than those measured in protocol I. The DTA trace shows two endothermic components centered a t 100 and 160 "C; the second component extends well above 300 "C. The heat consumed per desorbed HzO, that is DSCDTG, increases steadily during the process. The DTG curves always show a maximum corresponding to the first endotherm. A second singularity is observed a t about 170 "C for the sample calcined at 600 "C (sample 1).It is difficult to assign the singularities of the DSC or DTG curves to different states of water because there is a large overlap between dehydration and dehydroxylation processes. Nevertheless, it can be suggested that the two endotherms correspond to two types of physisorbed water, for instance, capillary water (100 "C peak) and hydrogenbonded water (160 "C). The hydroxyls recombine as H2O a t temperatures above 200 "C, while the weight loss corresponding to the irreversibly adsorbed HzO occurs only a t temperatures above 300 "C.
Protocol 111. Finally, we will describe the structural and textural modifications observed during the full rehydration-dehydration cycle (protocol 111, Figure 2). Because of capillary condensation, water fills up the whole porous volume a t PIP0 = 1. Accordingly, there is a linear relationship between the volume available to N2 and H2O. Apparently, Nz seems to fill a volume 30% larger than water (Figure 5). As the water chemisorption reconstructs the surface8 and also part of the bulk,9 it can modify the pore volume. 27AlMAS NMR was run for each sample after first calcination (FC), rehydration (RH),and second calcination (SC). Results for two typical samples (#1 and #2) will be analyzed in detail, whereas Table 3 contains all the data. The relative distribution of Al between the 4,5-, and 6-fold coordinations is computed using a deconvolution procedure accounting for distribution of electric field gradient (EFG) components a t each Al site.8 The unusual behavior of Super5 alumina during a rehydration-dehydration cycle is striking (Figure 6).After the first calcination a t 600 "C (sample 11, a large amount of Al in 5-fold coordination is observed besides the 4-and 6-fold coordination as already reported.6 The observed chemical shift (at 11.7 T) for tetrahedral, pentahedral, and octahedral aluminum is 65, 35,and 8 ppm, respectively. The presence of Alv is a consequence of the nanometer-sized nature of the m a t e ~ % a l . ~ ? ~ After rehydration a t 100% relative humidity for 3 days a t room temperature, the structure is modified. The transition alumina has been mostly converted into an amorphous hydroxide with a large fraction of the Al in octahedral coordination. The second calcination at 600 "C dehydroxylates the material and produces a new transition alumina. The three Al coordinations are still
(10) Borello, E.; Della Gatta, G.; Fubini, B.; Morterra, C.;Venturello, G. J. Cutul. 1974, 35, 1.
2618 Langmuir, Vol. 11, No. 7,1995
Coster et al.
Table 3. Sample Characteristics at Different Stages of the Rehydration-Dehydration Cycle (Protocol 111) alumium coordination (relative DoDulation in %) FC RH SC sample ID 4-fOld &fold 6-fold 4-fold 5-fold 6-fold 4-fold B-fold 6-fold ~~~~~
1 2 3 4 5 6 7
~
34 31 28 28 35 28 28
27 2 10 1 25 2 2
surface area (m2/g)
14 27 20 27 10 27 22
39 67 62 71
40 70 70
mean pore diameter (nm)
9 0 0 0 3 0 0
77 73 80 73 87 73 78
FC
SC
FC
SC
FC
SC
1 2 3 4 5 6 7
304
150 138 229 137 152 197 205
4.79 5.95 14.09 17.93 7.13 8.85 16.08
6.00 6.77 14.90 20.49 9.48 7.60 20.93
94.2 81.0 24.5 17.8
72.5 52.0 23.4 15.0 28.8 36.0 13.0
64.4 35.5 17.2
61 65 68 68 67 68 68
crytallinity (004reflection) FC SC
% pore vol below 6 nm
samDle ID
162 283 171 352 163 252
8 8 6 1 1 2 2
31 31 26 31 32 30 30
0.2 3.3 0.5 2.6 0.1 2.6 1.6
1
1.6
4.1 1.7 3.6 3.0 3.1 2.6
.............. Sample #1 FC
!i
sc
!I
I
I
150
100
0 -50 %I Chemical Shift (ppm) 50
, -100
Figure 6. Modification of the structural feature during the rehydration-calcination cycle. 27AlMAS NMR spectra of sample 1(bottom)and sample 2 (top), namely, the transition aluminas obtained from the same gel calcined at 600 "C and 750 "C, respectively. See Figure 2, protocol 111, for details. Each spectrum has been rescaled to the same maximum intensity.
present but their relative weights have changed deeply. The AIv content is much reduced while the ratio AINIAlw is getting closer to 112 as expected in a pseudospinel. Even a sample calcined a t 750 "C (sample 2) is affected by the rehydration (Figure 6, top). Indeed, the minor amounts of AIv detected after FC completely disappear, when exposed to saturated water vapor. At this stage, the ratio AlwlAlw is again smaller than 1/2. From the NMR point of view, the second calcination returns the sample to its initial state; the aluminum coordination has not changed. Figure 7 reports the effect of the rehydration-dehydration cycles on the pore-size distribution for samples 1and 2. Similar results were obtained for samples 3 to 7. The narrow pore-size distribution observed after FC a t 600 "C is strongly affected; the peak a t 4.3 nm decreases, broadens, and shifts towardlarger diameters. The sample calcined a t 750 "C behaves similarly, in the sense that the maximum of the pore-size distribution curve is shifted in the same direction. The trend shown in Table 3 is clear, the mean pore diameter rises and the contribution of pores below 6 m to the total pore volume decreases. The surface area is reduced considerably (Table 3) during the process, but samples with higher crystallinity (after FC) appear more stable. High surface area transition aluminas are
4
8 10 Pore Diameter (nm)
6
12
14
Figure 7. Pore distribution of samples 1 and 2 before and after the rehydration-dehydration cycle (protocol 111).
30
40
50
60
70
2 8 (degree)
Figure 8. X-ray diffraction patterns obtained for the same samples as Figure 6. poorly crystallized materials, nevertheless, Figure 8 shows a n interesting evolution. Sample 1 is totally X-ray amorphous but develops into a y-alumina-like structure after the RH-SC cycle. Calcination of sample 1 a t 750 "C produces sample 2 which is already "better-crystallized" in terms of transition alumina standards. Additional rehydration-dehydration improves somewhat the relative crystallinity. We define it as the ratio of the maximum
Langmuir, Vol. 11, No. 7, 1995 2619
Rehydration Behavior of Aluminas
n
. o
B
O
a
1 2 3 Total Amount of OH'S (10A21/g)
"0
Figure 9. Relationship between the amount of physisorbed HzO (protocol 11)and the total amount of OH. The latter is the sum of the OH adsorbed at 0 = 1(protocol I and Figure 3) and an estimate of OH present before adsorption. intensity to the fullwidth at half maximum for the 400 reflection a t 28 = 46". It is reported in arbitrary units in Table 3 and, a s expected, it increases with higher calcination temperature and after RH-SC. It is worthwhile mentioning that doubling the duration of the FC has no significant effect on the crystallinity.
Discussion The discussion is organized around two major aspects: (1)the thermodynamics of water adsorption (protocol I); (2) the structural modifications encountered during a rehydration-dehydration cycle (protocol 111). The microcalorimetric experiment allows one to accurately measure the amount of irreversibly adsorbed water (Figure 3, Table 2). If one assumes that water dissociates on the surface available to Nz, the irreversibly adsorbed water generates on the average 5.4 f 0.8 OH per nm2. The y-alumina has about a 60% lower OH density, but its thermal history is unknown. It is important to notice t h a t before water chemisorption hydroxyls are already present, despite of the fact that samples were carefully outgassed a t 500 "C. Knozinger and Ratnasamy5compiled a set of nine experimental results and they concluded that between 2 and 6 hydroxyls per nm2 were still present after pretreatment a t 500 "C. As the outgassing temperature decreases, the OH population increases, and a t 100 "C the density reaches a value between 12 and 16 OH/nm2. Because the OH density corresponding to the irreversibly adsorbed water (Table 2) is very similar for all samples but the y-alumina, we can assume that the number of OH present before adsorption is also similar and we choose a mean value of 5 OWnm2. The amount of physisorbed water is plotted versus the total number of OH (preexistent irreversibly adsorbed) in Figure 9. The ordinate corresponds to the weight loss (protocol 11) minus the irreversibly adsorbed water (protocol I). Figure 9 reveals t h a t about two HzO molecules are physisorbed per surface OH. The term "surface O H could be misleading, unless it is kept in mind that more than the first outer oxygen layer is hydroxylated. Now t h a t we have quantitative data on the rehydration process, we will focus on the thermodynamic aspect. The enthalpy variation corresponding to the bulk transformation A1203(c) H Z O ( ~ )2Al00H(,) is -63.32 kJ/mol a t 25 "C.ll Early reports by Sabatier12 confirmed t h a t dehy-
+
+
60
4
-
(11)CRCHandbookofChemistryandPhysics,52nded.;The Chemical Rubber Co.: Cleveland, OH, 1971 and 1972.
I
0
- 25
0
, v
0.5 1 1.5 Crystallinity (a.u.) 400Reflection
,
'0 2
Figure 10. Correlation between the crystallinity measured for the 400 reflection and the following: the Al" content obtained after the first calcination (protocol III), open symbols, right ordinate;the driving force for the water chemisorption(protocol I), this is the difference between the differential heat of chemisorption at 0 = 1and 0.05, solid symbols, left ordinate. droxylation of crystalline aluminum hydroxides requires between 60 and 80 k J per mol of water evolved. As shown in Figure 3, the range of enthalpy between 60 and 80 kJ/mol is observed for 0 > 1, that is when the surface rehydroxylation is over. Thus, surface rehydroxylation is much more exothermic than expected from thermodynamic data available for bulk transformation. The variation of heat of chemisorption between 0 = 0.05 and 1can be considered as the drivingforce of the rehydration. As the crystallinity increases, the slope of the Freundlich fit increases (Tables 2 and 3), as well as the initial heat of chemisorption which is estimated a t 0 = 0.05. At 0 = 1, the differential heat is similar for all samples; thus, the variation of heat of chemisorption between 0 = 0.05 and 1increases as the crystallinity increases (Figure 10). This suggests that the higher the crystallinity, the larger the energy released for opening a n oxo bridge; Al-0-Al HzO 2Al00H. Once again the tabulated thermodynamic data seem to conflict this picture, the bulk rehydration of a n amorphous A1203should be about 20 kJ/mol more exothermic than that for crystalline y-AlzO3. The discrepancy between our measurements and the thermodynamic data must be linked to the fact that we are dealing with the surface and not the bulk. Defects, like Al", are concentrated at the surface and they must deeply affect the process. The degree of crystallinity decreases (Figure 10) with increasing Al" content (bulk surface) as expected, since it is a function of the number of cationic sites with unsaturated coordination. These coordination defects are also a t the origin of Lewis sites on the surface. The chemisorption of a base on a Lewis acid center is either a dissociative or nondissociative process. In the case of dissociative interaction like the rehydration, the process can be decomposed in two steps. Referring to a Lewis site on a coordinately unsaturated aluminum as ( A l k ~ & the , first step corresponds to the nondissociative adsorption of H2O on the Lewis site. Thereafter, dissociative chemisorption occurs and the Lewis center disappears (step 2). The first rehydration step (AH1)and the chemisorption of ammonia ( A H 3 ) consist essentially of the transfer of electron density onto a Lewis acid site. An estimate of the largest difference between AHl and AH3can be deduced by comparing the ionization potential ofthe two probes; the ionization process requires 170 kJ/mol more for HzO than for NH3. In fact, the
+
-
+
(12) Sabatier, G. Bull. SOC.Fr. Mineral. Cristallogr. 1963,77, 400.
Coster et al.
2620 Langmuir, Vol. 11, No. 7, 1995 250
Sample #1
R=3.71
4 .
variation of the differential heat of adsorption for NH3 (A&) and H20 ( A H 1 AHz) are very similar if plotted versus the coverage (Figure 11). The significance of 0 is, of course, different for H20 and NH3, since the former penetrates more than the outermost surface layer while NH3 does not. The ratios of irreversibly adsorbed NH3 to irreversibly adsorbed H20 are shown in the inserts of Figure 11 and vary between 1.5 and 3.7. Because the sum of AH1 A H 2 is very close to A H 3 and because AH1 should be less exothermic than A H 3 , the dissociation of adsorbed water (AHz) should correspond to a n exothermic process. The second step could be similar to the bulk rehydration and release between 60 and 80 kJ/mol. In this case, the interaction between HzO and a Lewis center could release 30-180 kJ/mol, in the course of surface rehydration. It would be most interesting to correlate the differential heat of water chemisorption to the modification of the surface Al coordination. Unfortunately, the limited rehydration carried out according to protocol I corresponds to minor coordination change; about one Al atom out of ten is affected by the water dissociation. Qualitatively, rehydration corresponds to the loss of tetrahedrally coordinated surface Al species (observed at 52 ppm at 11.7 T, isotropic chemical shift 58 ppm) and to the increase of hydroxylated octahedral Al. It is reasonable to believe that the distortedAITVis primarily a surface species which upon hydroxylation is transformed into Alv and AlW. Accordingly, a fraction of surface Alv is transformed in Alw. Taking into account t h a t the hydroxylation creates about 5.4 OH per nm2 and that about 8 Al are present per nm2, it can be concluded that about 60% of the surface aluminum are likely to change coordination. The picture is quite W e r e n t from the trend observed in Figure 6 where bulk modifications are occurring. Because of well-known problems associated with the quantification of 27AlNMR s p e ~ t r a , ~itJis ~ impossible to reach more quantitative conclusions. Some interesting structural modifications are also observed in protocol 111. The most striking effect is the loss of AIv. Water must cure this defect. AIv appears when condensation is limited during gelation. Water introduced during rehydration might allow further con-
+
+
(13)Gervasini, A.; Auroux, A. J.Phys. Chem. 1993,97,2628. (14)O'Reilly, D.E.Adu. Cutul. 1960,12, 31.
CII
zsoru
cp
. YI
0
200'.
0
. 1-50.
100.
... .
n
o
Sample #7
R=1.53
0
0. 0
.
