Langmuir 1987,3, 676-681
676
Texture Evolution of Montmorillonite under Progressive Acid Treatment: Change from H3 to H2 Type of Hysteresis? S. Mendioroz and J. A. Pajares* Instituto de Catcilisis y Petroleoqulmica, CSIC, Serrano, 119, 28006 Madrid, Spain
I . Benito, C. Pesquera, F. Gonzblez, and C. Blanco Departamento de Qulmica, Universidad de Santander, Santander, Spain Received August 6, 1986. I n Final Form: February 9, 1987 On treatment of a bentonite from La Serrata-Sierrade Nijar, Spain, with HCl of increasing concentration (1-8 N), an evolution in the hysteresis loops of the corresponding N2 adsorption-desorption isotherms is revealed. In the first stages of the attack, an opening of the bentonite lamellae is produced by abstraction of interlayered cations and corresponding water molecules. Thus, an accessibility,otherwise impossible, of nitrogen to the internal surface of the samples with subsequent increase in N2 adsorption occurs. A second stage is produced when the octahedral aluminum sheet is dissolved as the acid attack progresses. In both stages H3 loops as well as pore ranges are preserved. A t higher HCl concentrations, the partial destruction of the tetrahedral sheet and the texture of the resulting amorphous free silica, together with the random occlusion of the unaltered montmorillonitic interlamellar spaces, result in changes of texture, slit-shaped pores becoming ink-bottle pores and H3-type hysteresis changing to the H2 type. Pore size increases and total pore volume decreases. Specific surface areas grow from 61 m2g-l for the natural sample to 345 m2 g-l for the 8 N HCl treated sample, passing through a maximum of 416 m2 g-I for the 4 N HCl treated specimen, depending on which of the effects described above predominates. Introduction
As known, the clays of sedimentary origin contain iron, aluminium, and silicon oxides which can cause the aggregation of individual clay partic1es.l Otherwise, the nature and number of interlayered cations compensating to neutrality the tetrahedral Si and octahedral A1 substituted atoms in the 2:l lattice of montmorillonite clays can reduce the theoretical surface area of the material, from 800 m2 g-’ to less than a tenth of this value. Thus, in order to disaggregate clay particles, enhance their surface area, and produce a homogeneous and well-controlled material for use in adsorption and catalysis, it is necessary to eliminate mineral impurities and metal-exchangeable cations. With this aim, some different treatments, therhave been used. mal3 and particularly mineral In this paper, we report, through the study of the changes produced in the nitrogen adsorption-desorption isotherms, the textural transformations occurring on a Spanish bentonite when HC1 of different concentrations is used as activating agent. On studying textural changes, the well-known t-plot’ and, more recently, the &-plot8have been used. Lippens and de Boer’ made use of an experimental curve, unique for all samples, as a reference. Singgrecommends that the reference sample should be of a nonporous material of the same chemical nature as the material to be studied. Lecloux and PirardlO introduce some modifications in the concept of reference material, giving rise to five different isotherms to be used depending on the CBETvalue. However, the application of any of those methods and corresponding standard isotherms results at times in some uncertainties in the linearity of the obtained curves. In this situation, to calculate a reliable mesopore distribution is at the least difficult, leading in cases to false conclusions, that may invalidate the utility of nitrogen adsorptiondesorption studies. To overcome this problem and, since we are dealing with isotherms closely related in shape after the acid treat+ Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W. Sing and R. A. Pierotti, Chairmen.
ment,’l to make a comparison among them and that corresponding to the natural sample taken as reference seem to be promising. In this way, notwithstanding the true mesoporosity of the solids, a comparative study of the variations in porosity produced by the acid attack has been carried out. The less extended f-plot12used has led us to reach some conclusions about the reaction mechanism involved in the acid treatment of the samples. X-ray patterns, IR spectra, and elemental analyses of the samples help to elucidate the obtained results. Experimental Section Materials. A bentonite material from La Serrata de Njjar, Almeria, Spain, supplied by Minas de Gador S.A., has been used as the raw material. Its percent chemical composition on dry sample was SiO,, 58.62; A1203,23.38; Fe203,3.99; MgO, 1.20; CaO, 1.03;Na20,1.01; KzO, 0.13; and ignition loss (mainly structural water and COP, 8.50). Cation-exchange capacity (CEC) is 115 mequiv/100 g. The sample was almost pure montmorillonite,but some mineralogical impurities, no more than l o % , of feldspar, quartz, and carbonates were present. The textural parameters after grinding and sieving were the following: particle size, 5 pm; SBET, 61 m2 g-l; CBET,191; pore volume, 0.412 cm3 g-l. Other mineralogical and structural details can be seen elsewhere.13J4 Equipment and Methods. Elementary analysis was carried out by AA/EA spectrometry,using a Perkin-Elmer3030, working on the emission mode for Nq, K, and Ca analysis and on the (1) Rengasamy, P.; van Assche, J. B.; Uytterhoeven, J. B. J. Chem. Soc.. Faraday Trans. 1 1976, 72, 376.
(2) Dyal, R. S.; Hendricks, S. B. Soil Sci. 1950, 69, 421. (3) Bradley, W. F.; Grim, R. E. Am. Mineral. 1951, 36, 182. (4) Mills, G. A.; Holmes, J.; Cornelius, E. B. J. Phys. Colloid Chem.
1950,54,
1170.
(5) Makki, M. B.; Flicoteaux, Ch. Bull. SOC.Chim. Fr. 1976, 15. ( 6 ) Fernindez, T. Anl. Edafol. Agrob. 1969, 28, 285. (7) Lippens, B. C.; de Boer, J. H. J . Catal. 1965, 4, 319. (8) Sing, K. S. W. Chem. Ind. 1968, 1520. (9) Sing, K. S.W. In Surface Area Determination; Everett, D. H., Ottewill, R. H., Eds.; Butterworth London, 1970; p 25. (IO) Lecloux, A.; Pirard, J. P. J. Colloid Znterfuce Sci. 1979, 70, 165. (11) Benito, I.; Pesquera, C.; Blanco, C.; Gonzilez, F.; Mendioroz, S.; G. Ayuso, T.; Pajares, J. A. Proc. XIV Conference on Silicate Industry and Silicate Science; Budapest, 1985; Vol. 4, p 131. (12) Gregg, S. J. J. Chem. SOC.,Chem. Commun. 1975, 699. (13) Gonzaez Ayuso, T. Graduation Thesis, Universidad Complutense, Madrid, 1982. (14) Franco, M. J. Graduation Thesis, Universidad Complutense, Madrid, 1986.
0743-7463/87/2403-0676$01.50/0 0 1987 American Chemical Societv
Langmuir, Vol. 3, No. 5, 1987 677
Montmorillonite Evolution under Acid Treatment Table I. Elemental Analysis. Cation Percentages Removed by the Acid Treatments 1M 0.14 A1203 1.73 Fez03 0.38 MgO 0.80 CaO 0.93 NazO 1.14 KzO 0.14
SiOz
2M 0.09 8.23 1.37 0.99 0.72 0.91 0.13
3M 0.04 14.74 2.04 1.13 0.77 1.23 0.14
4M 0.02 19.38 3.01 1.40 0.72 0.94 0.19
5M 0.03 13.80 2.27 1.20 0.66 0.89 0.15
6M 0.03 9.64 1.63 1.02 0.71 1.00 0.14
8M 0.02 2.11 0.68 0.61 0.66 0.78 0.10
absorption mode for the rest of cations. For refractory cations a N20-acetylene flame was used. Instrument, flame, and hollow cathode lamp conditions were those recommended by P.-E. On the original sample the analysis was made after pressure decomposition with hydrofluoric acid (38%)in a PTFE autoclave following Langmhyr.16 On the acid-treated samples, the measurementswere made on the liquids extracted after centrifugation and corresponding dilutions. Mineralogical Analysis. X-ray diffraction patterns were obtained from the original raw material and treated samples on Philips PW 1710 equipment, using the Cu K a line (A = 1.5418
,
50
7-0
30
lo
-
2.9
Figure 1. XRD patterns: SO, untreated sample; S1, S4, and S8, treated sampleswith 1 , 4 , and 8 M hydrochloricacid, respectively.
4.
IR spectra were run on a Perkin-Elmer 682 spectrometer coupled to a data station DS 3600. Equipment sensitivity was 4 cm-' in the 4000-2000-cm-' range and 2 cm-l between 2000 and 400 cm-'; KBr technique was always used. Morphological analysis was carried out on a scanning electron microscope ISI-130,following the current techniques of disaggregation and Au/Pd sputtering coating for preparing samples. Textural Analysis. N2adsorption-desorptionwas performed in a semiautomatic Micromeritics 2100 D surface area and pore volume analyzer. After degassification of the samples at 140 OC (10" mmHg) (1 mmHg = 133.3 Pa), 16 h, adsorption-desorption Gem, , pore volume, isotherms were run at 77 K. From them, S B ~ and pore size distribution were obtained. Analyses were made following Broekhoff.l6 t values extracted from the de Boer experimental curves adjusted to a ten-term potential equation were used. To search for breakdown points in the t-curves, a computational method was used," taking 0.999 as the external correlation coefficient. Comparison of the shapes of the isotherm adsorption branches has been carried out by using two different mathematical techniques The fiitl8plots the amount adsorbed on a given sample against that adsorbed on the reference sample at the same relative pressure. The second one, f-plot,'2 compares the amount adsorbed at p / p o constant intervals and represents in ordinates its ratio against p / p o in abcissae. Deviationsfrom the horizontal will show changes in the shape of the isotherm and related porosity of a given solid, from those of the reference sample. As reference sample, natural bentonite, has been used in both cases. Acid Treatments. Ten grams of previously dried clay was treated, under vigorous stirring, with 100 mL of different (1,2, 3, 4, 5 , 6 , 8 M) concentrations of HC1 at 80 O C during 1 h. The flasks were connected to a water-cooled refrigerator in order to maintain a constant acid concentration. Afterwardthe suspension was air cooled and left to settle overnight and the solid carefully extracted through centrifugation. Samples were washed of chlorides, and the filtrate was analyzed for Al, Fe, Mg, Ca, Na, K, and Si by standard methods. (From now on, samples will be denoted by 1 , 2 M, etc., accordingto the acid treatment received.)
.
Results a n d Discussion Chemical a n d S t r u c t u r a l Changes. Table I shows the cation percentages removed from the clay when different acid concentrations are applied. Two types of cations can be easily differentiated. Calcium, sodium, and (15) Langmhyr, F. J.; Paus, P. E. Anal. Chim. Acta 1968, 43, 397. (16) Broekhoff, J. C.P. In Preparation of Catalysts I& Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.;Elsevier: Amsterdam, 1979; p 663. (17) Malet, P.; Munuera, G.; Rives-Arnau,V. Afinidad 1980,37, 369. (18)Brown, C. E.; Hall, P. G. Trans. Faraday SOC.1971, 67, 3558.
U
LOO0
390 3MO
2500
2000
1500
500
1000
cm-1
Figure 2. IR spectra: SO, untreated sample; S1,S4, and S8 treated samples with 1, 4, and 8 M HC1, respectively. potassium-and partially magnesium-are removed after the soft 1 M acid attack. These are the extralattice exchangeable cations and their percentage are much the same along the treatment; only at higher acid concentrations is a faint decrease apparent. Aluminium, iron, and magnesium behave differently,being gradually removed from the clay lattice up to 4 M HC1 treatment. Afterward their content in the filtrates decreases attaining almost the same values at 8 M HCl concentration as those with 1 M acid attack. Silicon content, always very low in solution, diminishes with growing acid strength, until HC14 M, becoming constant at higher acid concentrations. The XRD patterns of the natural and 1, 4, and 8 M samples are gathered in Figure 1 as the exponent of the occurring changes. As can be seen, the (hko) montmorillonitic reflections clearly decrease with treatment up to 4 M, not changing in position or relative intensities. The loss in (001) intensity must be related to a change in the lattice order along the c axis, denoting a growing delamellation of the original particles as acid concentration grows. In the 4 M sample only amorphous silica appears. With more drastic HC1 treatment a reverse in peak intensities is produced with the 8 M sample almost reproducing the diffraction pattern of the 1 M. IR spectra detect a similar situation, as can be seen in Figure 2. Samples are first attacked in their octahedral layer, showing a decrease in the intensity of the characteristic peaks (790, 905, 525 cm-l). The bands corresponding to free silica (900,1300 cm-l) are more and more intense with increasing acid concentration. Above 4 M treatments, bands corresponding to tetrahedral and octahedral cations are rebuilt denoting the preservation of the original lattice.
678 Langmuir, Vol. 3, No. 5, 1987
Mendioroz et al.
Table 11. Textural Parameters for Natural and Acid Treated Samules
Table 111. t -Plots. Analytical Parameters and Related Textural Prouerties of the Samules ~
SBETt
sample natural 1M 2M 3M 4M 5M 6M 8M
m2 g-' 61 154 243 316 419 398 353 342
VPOW
CBET
cm3g-' 0.084 0.121 0.193 0.311 0.497 0.476 0.437 0.463
191 157 262 149 167 169 194 166
sample nat 1M 2M 3M 4M 5M 6M 8M
SL
350 I
I
:' I
I
SI"
3.338 5.897 14.12 21.32 24.15 24.10 21.35 19.10 9.90
',i 4.19 16.78 7.58 -4.22 12.26 7.15 7.29 12.29 16.14
S, slope; i, intercept.
82
i2
2.5 2.22 5.09 13.12 29.20 29.20 25.11 25.39 11.22
8.78 36.05 55.85 41.03 -18.24 -19.84 -14.89 -27.24 4.96
~~~
CPb P(P/Po) 0.37 0.34 0.35 0.38 0.44 0.34 0.43 0.47 0.69
91 218 330 374 373 330 296 153
26.2 11.8 19.1 11.2 11.4 19.2 25.2
cp, crossing point.
250 -
200 -
150 -
3.33a
S S S S
't 2
2.222
1 1 1 1 1
l 50 0 1
01
15
I
20
I
25
I
2
2
3
I
I
30
40
35
:1I
4
2M
2
'I 7
0
T ( V NAT)
.2
.4
.6
.e
1
P/P0
Figure 5. n-Plots: 1, 2, 3, 4,5, 6, and 8, samples treated with 1 , 2 , 3 , 4 , 5 , 6 , and 8 M HCl, respectively; S, 4 M treated sample
Figure 6. f-Plots: 1, 2, 3, 4, 5, 6, and 8, samples treated with 1 , 2 , 3 , 4 , 5 , 6 ,and 8 M HC1, respectively; S, 4 M treated sample
Table IV. x-Plot. Analytical Parameters and Related Textural Properties of the Samples
dependent of their size will be emptied and no pore distribution can be confidently inferred21from the desorption data through the current calculation procedures. As it is so difficult to make a reliable analysis of the N2 ad-desorption isotherms, another method has to be used in order to compare the acid treatment effect on the porous system of the samples. Thus, two different mathematical devices have been developed to study the isotherms, both using the natural bentonite as reference sample. The fist, as mentioned before, uses as abscissae, instead of the normal t-plot, the N2adsorbed a t different pressures by the reference (natural) sample, keeping the same ordinate value. Having found a faint downturn of the natural sample curve in the high-pressure range of the t-plot (Figure 4), the use of an x-plot will enhance the differences in porosity a t the same zone among samples, making the decision easier as to whether a plot has or has not deviated from linearity. Thus, from Figure 5, the parameters of the curves as well as the corresponding textural values of the samples have been obtained. Results are gathered in Table IV. They parallel those found from the current t-plot in Table 111. The second plot, f-plot, compares the quantities adsorbed by the given and reference samples at p i p o constant intervals, representing their ratio in ordinates against p i p o in abcissae: deviations in horizontality will express changes in the shape of the isotherms of the given solid. The f-plots
corresponding to the seven treated samples are given in Figure 6. Curves 1-3 in this figure are almost parallel to the x-axis along their profile. Since natural bentonite is practically nonporous for N2 adsorption, that parallelism will show an enhancement in external surface through delamellation, without any additional change in porosity. Their intercept will give a rough measure of the number of individual particles obtained through the acid treatment. The slight downturn tendency in curves 1and 2 may be understood as a consequence of the disaggregation of the individual particles and corresponding losses in the interparticular voids of the natural sample. As a whole, the series presents three parts in their runs: A first one, which is almost parallel to the abscissa axis, showing the preservation of the original morphology. A second part, going upward, becoming steeper as acid strength increases, denotes the influence in the whole isotherm of a porosity contributing to N2 condensation, apart from that producing H3 type of hysteresis; it reaches a maximum for the 4 M sample, decreasing in intensity a t stronger acid treatments. Finally, a third step sloping downward from p i p o values around 0.8 shows the loss of the asymptotic character of the isotherms. The more or less nonrigid texture of untreated or less treated bentonites gives rise to a more rigid one with a well-defined mesopore system, presumably spheroidal in character, as can be deduced from the gradual rise in the second part of the curves. Two different porous systems, influencing each other, have been clearly demonstrated through this representation, showing the superiority of this plot, in this case, for the comparison of samples. Bearing in mind that the chemical changes detected after the acid treatment revealed the presence of free silica in the samples, an additional treatment of digestion with 5% Na2C03solution has been usedz2on the 4 M sample, to remove Si02and obtain further information of the attacked montmorillonitic material. The specific surface area of the silicaless sample goes down to 217 m2 8-l. The nitrogen isotherm, as well as those corresponding to the
(21) Broekhoff, J. C. P.; van Beek, W. P. J. Chem. SOC.,Faraday Trans. 1 1979, 42.
(22)Ross, C. S.;Hendricks, S. D. Minerals of the Montmorillonite Groups; U.S.Depart. Int. (Geol. Surv.): Washington, DC, 1945.
after silica abstraction.
sample 1M 2M 3M 4M 5M 6M 8M SL
S1" 1.59 3.51 5.52 7.24 7.22 6.40 5.74 2.92
iln 12.63 2.94 -14.61 -18.09 -23.14 -19.54 -12.10 5.78
Sz 0.86 2.14 4.96 11.69 11.68 10.05 9.95 5.96
iz
CPb (p/p0)
28.91 35.29 3.33 -120.75 -122.35 -103.04 -111.06 -72.47
0.36 0.43 0.77 0.40 0.36 0.39 0.43 0.53
s,,
m2g-' 97 214 337 442 440 390 350 178
"S, slope; i, intercept. *cp, crossing point of 1 and 2 straight
lines.
without silica.
680 Langmuir, Vol. 3,No.5, I987
Mendioroz et al.
350 Ln < 0
-4
M
' 389250
-
200
-
....,.,:+.., . ..., , . . ., . ~ . ..,, ..,.... ~
~
.~l__..
~: * 5
,
-..-. "=. .-. . . ., Figure 8. SEM micrograph X2500 natural bentonite particle.
.. .2
.4
.6
I
.e
1 PlP0
Figure I. N2adsorption-desorptionisotherms: 0, untreated sample; 4M, 4 M HCI treated sample; S,4 M treated sample after silica abstraction. 4 M and natural samples, by way of comparison, is given in Figure 7. Also the curves denoted S in Figure 4-6
correspond to this silicaless sample. It is not an easy matter to determine by mere observation of the isotherm the type of hysteresis loop that is present. However, some differences with the acid-treated samples are apparent. From the t-plot (Figure 4), a faint upward turn in the curve, more important when an x-plot is used (Figure 5), is detected. This points to the presence of curved pore surfaces, whose cylindrical or spheroidal character is hard to establish. The f-plot of S sample in Figure 6, shows a clear difference with respect to those of the acid-treated samples in the first part of the curve, keeping its wallelism with the horizontal axis up to p / p o = 0.7. Also the second part, much steeper, attains its maximum at p / p o = 0.9, suddenly falling as pressure increases. This last profile points to a cylindrical pore texture, H1 type, better than to the H2 type up till now considered. The cylindrical character of the porosity produced by the acid attack on the samples, apart of their solution, seems to be confirmed by the near parallelism of the two branches of the isotherm in Figure 7. Thus,the fmal texture of an acid-treated bentonite and corresponding isotherm result from the proper combination of two different porous systems: amorphous free silica originated by the complete decomphsition of the smallest montmorillonitic particles once solved the AI, Fe, and Mg octahedral sheets and unattacked or less attacked original clay particles that essentially preserve their platelike morphological characteristics. Final Remarks. Two types of conclusions arise from this work. The first one refers to the utility of using the f-plot-and to a lesser extent the x-plot-for comparing shapes of closely related adsorption isotherms: probably no final view of the free silica texture influence could have been suspected if this device had not been used. The second one refers to the mechanism involved in the acid attack of sheetlike montmorillonitic clays. Four different stages can be detected in the treatment with mineral acid, HC1, at increasing concentration: In the first stage, an opening of the bentonitic lamellae (Figure 8) is produced by abstraction of the interlayered cations and corresponding water molecules, as well as of impurities-free oxides, carbonates-that contribute to the
Figure 9. SEM micrograph X2000 3 M HCI treated sample.
._ Figure 10. TEM micrograph X.37500: amorphous silica coating a montmorillonite particle.
aggregation of the individual clay particles (Figure 9). A second stage is produced when the odahedral aluminium sheet is dissolved as the acid attack progresses. In both stages CBm values as well as H3 loops are preserved. The constancy in CBm reveals that nitrogen is adsorbed on the same kind of surface. Thus, the changes in texture are quantitative, not qualitative, more and more surface being exposed as acid strength gqows. The leaching of ions has no influence on the adsorption because it is accomplished on the external silica sheets of montmorillonite. In f-plots, the curves are almost parallel to the abcissa axis. DRX patterns show a decrease in the intensity of the fundamental reflections. IR spectra lose the bands corresponding to octahedral cations. As attack progresses, the third stage, small particles, being more soluble than larger ones, dissolve first and larger ones become attacked at their edges to varying extents depending on particle size and acid concentration. The resulting free silica from the tetrahedral sheet polymerizes. This, together with the saline effect produced by previously abstracted cations, mostly A P , produces a precipitation of silica,23which develops its own texture and (23)IUer, R. K. The Chemistry of SiO,; Wiley-Znterscience: New York, 1919.
Langmuir 1987, 3, 681-686
is deposited upon the larger particles (Figure 10). CBET values remain unchanged, which show that nitrogen adsorption takes place on the same kind of surface, as similar silanol groups are present in both amorphous silica and unattacked montmorillonite surfaces. A change is shown in N2 adsorption-desorption isotherms, and in the f-plot a peak in the higher pore size region appears. DRX patterns and IR spectra correspond almost exclusively to amorphous free silica. Finally, as the insolubilization of polymeric silica increases, a progressively thicker silica layer is formed on the unaltered or partially altered bentonite particles, thus reducing or impeding the posterior attack. An apparent “passivation” of the samples takes place and losses in the
681
total amount of N2 adsorbed and in the cations abstracted are produced. The amount of adsorbed nitrogen diminishes, but the general appearance of the isotherm remains because silica is always present. In the f-plot a small shift of the peak toward the higher p / p o is produced. DRX and IR show a corresponding reverse in the characteristic peaks.
Acknowledgment. We acknowledge the US-Spanish Joint Committee for Scientific and Technological Cooperatiqn and the Comisi6n Asesora de Investigaci6n Cientifica y TBcnica for financial support of this work under Projects 83051 and 6781553, respectively. Registry No. HC1, 7647-01-0; montmorillonite, 1318-93-0.
Kinetics of Decomposition of Gibbsite and Boehmite and the Characterization of the Porous Products? M. H.Stacey ICI plc, New Science Group, The Heath, Runcorn, Cheshire, WA7 4QE England Received September 15, 1986 The kinetics of decompcaition of the two aluminum hydroxides gibbsite and boehmite have been compared and found to be very similar in form. A common value for their activation energy was determined, and in both cases the rate decreased as the partial pressure of water vapor increased. The transition alumina products contained slit-shaped micropores whose average width was a function of the water vapor pressure during decomposition. The results agree well with theoretical expectations in both cases.
Introduction Activated alumina is a product of importance as an adsorbent and catalyst support. It is manufactured by the calcination of gibbsite on a very large scale. It is wellknown that a variety of textures can be generated depending on the conditions used and the raw material quality. Several previous studies have shown that the decomposition of gibbsite is a complex procemt, the nature of the product being influenced by both crystallite size and water vapor pressure. In particular Rouquerol’ has shown that a t low water vapor pressures (less than 1.33 Pa) and for crystallites less than 1pm in diameter, the formation of boehmite is minimized and highly microporous transition alumina is directly formed from the gibbsite. Indeed a t pressures around 0.013 Pa the pores were so fine that they could not be penetrated by nitrogen molecules. In a similar study using HREM2 when boehmite was decomposed at fixed vapor pressure of water there was a direct relationship between the width of slit-shaped pores formed and the water vapor pressure. In general the higher the water vapor pressure the larger the width of the pores. This provides a method for control of pore width3 which should also be applicable to gibbsite-derived alumina. In Rouquerol’s work the largest grain size gibbsite (50-80 pm) was only investigated at a maximum vapor pressure of 133 Pa. It is the aim of this report to investigate large grain size gibbsite decomposition in the range 133-2660 Pa where the formation of considerable quantities of boehmite is to be expected. Since the texture of the re+ Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen.
sulting products was expected to be complex, we have used adsorption techniques here (rather than TEM as in our previous work on boehmite) to characterize the nature of the porosity (especially the width of the pores). To help understand the texture of the gibbsite-derived porous products, the decomposition of boehmite has also been examined in the same water vapor pressure range. In order to add precision to the study the kinetics of the decompositions have also been determined by using Rouquerol’s constant rate thermal analysis (CRTA) technique4adapted to prepare large quantities of product (10-20 g).5
Experimental Section Gibbsite powder was obtained from the British Aluminum Chemicals Ltd., Gerrard‘s Cross, UK. The grade used (FRF5) consisted of polycrystalline grains of mean diameter 75 pm. The BET surface area by N2adsorption was 5.6 m2/g, indicating that the platy crystallites which make up the grains are about 0.3 pm thick. The boehmite used was from the same source (Cera Hydrate) but was too fine to use as received. It was therefore compressed in a die and the aggregate then crushed and sieved to the same grain size distribution as the gibbsite. The powders were decomposed in a purpose-builtfluidized bed reactor (2.5 X 15 cm) which was part of a CRTA system. The powder was fluidized with an accurately regulated stream of He which then passed t o two detectors in series. The first detector (1)Rouquerol, J.; Rouquerol, F.; Ganteaume, M. J . Catal. 1975, 36, 99-110. (2) Wilson, S. J.; McConnell, J. D. C.; Stacey, M. H. J . Mat. Sci. 1980, 15, 3081-3090. (3) IC1 plc, Europatent 34 889, Dec. 1984. (4) Rouquerol, J. J. Therm. Anal. 1970, 2, 123-140. (5) Stacey, M. H.Anal. Proc. (London) 1985,22, 242-243.
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