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 crystalliteswhich 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-built fluidized 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.
0743-7463/87/2403-0681$01.50/0 0 1987 American Chemical Society
682 Langmuir, Vol. 3, No. 5, 1987
sample G1 G1 G3 G4
sample wt, g PHz0,Pa 6.9956 160 530 10.333 1330 10.089 26.76 2660
5001
Stacey
Table I. Gibbsite Decompositions conversions (water) calcd temp, " C plateau from plateau max start end final TGA 202-210 350 15 75 79 86 215 350 20 75 88 88 225-230 94 455 22 75 (95) 350 22 70 76 81 227-230. 238-245
c
final boehmite yield, mol 70 40 36.8 18.7 56
rates, s-l (2.0-5.0) X 10" 1.25 X lo4 (0.8-2.0) X (0.8-2.0) X
Although the decomposition is complex since there are two independent reaction processes, A1203.3HzO A1ZOB.HzO + 2HzO (1) gibbsite boehmite and A1203.3H20 A1203 3HzO (2) gibbsite transition alumina it is convenient to treat the gibbsite conversion in terms of water evolved as though only the second reaction were occurring. In general we can write for such a solid-state reaction decomposition rate (da/dt) = kf(a)g(PHzo) (3) +
-+
1
100'
500 1000 T I M E /min
1500
Figure 1. Temperature profile for gibbsite decomposition at 530 Pa. was a katharometer whose response to water vapor was known and the second was a dew-point hygrometer capable of working in the range -25 to +lo0 "C. The outputs of these detectors were interfaced to a microcomputer (Commodore PET) and used to determine the temperature set point of an air oven which heated the reactor so that the water vapor pressure in the effluent gas was constant. The microcomputer was also able to sense the reactor temperature and was programmed to accumulate data throughout the decomposition. All the decompositions were performed in the chemical reaction rate control regime where diffusion of water vapor within the intragrain pore system is not rate limiting. After discharge of the products from the reactor, suitable analyses were performed. Residual boehmite contents of the gibbsite-derived products were determined by conventional TGA from the step at 450 "C. The interpretation was confirmed by X-ray diffraction analysis. Nitrogen isotherms were determined after degassing at 250 "C for 4 h by using the Micromeritics Corp. Digisorb equipment. Carbon tetrachloride isotherms were determined at 20 "C by using an automatic vacuum microbalance system. TEM photographs were made from one of the boehmite-derived products for comparison with our previous findings.
Results Kinetics. The CRTA temperature profiles followed those previously reported for gibbsite.l An example is shown in Figure 1where there is typically an initial rise in temperature from about 150 to 200 "C which corresponds to the conversion of some gibbsite to boehmite. This stage is superceded by a plateau where gibbsite decomposes to a transition alumina. This plateau eventually ends with a rise in temperature to about 450 OC where there is another plateau which corresponds to the decomposition of the boehmite formed below 200 "C. In three of the decompositions investigated the run was terminated when the temperature reached 350 "C so that the porosity was solely due to the direct decomposition of the gibbsite. (Rouquerol has shown previously that the boehmite-formation reaction does not generate any porosity.) In most of the runs 20-25% of the total water was evolved in the formation of boehmite and 10-15% would therefore be retained at the end of the first temperature plateau. In all cases the mass balance was accurate within 5%. From the analyses the amount of boehmite formed was calculated and is quoted in Table I.
where k = an Arrhenius rate constant, a = fractional conversion to products, f(a) = effect of reaction interface geometry on rate, and PH20= partial pressure of water vapor. The kinetic equation for the direct conversion to transition alumina was derived from the data as follows. Since the powder and gas were well mixed within the reactor, the decomposition rate can be set equal to the product of partial pressure of water and He flow rate. The reactor temperature control algorithm automatically adjusts the temperature to maintain a constant partial pressure of water vapor in the reactor and so the shape of the temperature profile becomes a direct indication of the form of the f(a)function. In the first part of the decomposition, as the temperature rises only the first reaction is occurring and the effect of f ( a )is clearly strongly deceleratory. A t the beginning of the plateau the second reaction sets in, and since this occurs a t a constant temperature, f ( a ) for this reaction is virtually a constant (except when the gibbsite is totally consumed, then it must drop to zero). A common geometry for the reaction interface which produces this effect is a reaction beginning at the external surface of the gibbsite crystallite which then penetrates into the interior of the crystallite at a constant rate and with a constant interface area. Since this last reaction occupies most of the temperature profile, this is the one for which it is most practical to obtain the kinetic equation. In some cases the He flow through the reactor was switched alternately between high and low values by the microcomputer. It follows that alteration of the purge gas flow rate is equivalent to a step change in the decomposition rate. The reactor temperature-control algorithm automatically adjusts the plateau temperature to maintain a constant partial pressure of water vapor in the reactor and so the temperature profile now becomes a square-wave function. It is easy to see that using the above method g(PH,O) is always held constant. When a step change in gas purge rate occurs f ( a )is instantaneously also a constant so that the known step in decomposition rate is solely due to the temperature step. It follows that the A T at each step is a single determination of the activation energy of the reaction. In practice a single overall value for the activation energy was computed by trial and error by normalizing all data from a run to an arbitrary reference temperature To. This was achieved by plotting In (de-
Langmuir, Val. 3, No. 5, 1987 683
Decomposition of Gibbsite and Boehmite Table 11. Boehmite Decompositions temp,
plateau
PH20, Pa
sample wt, g 10.0935
sample B1
170
400
conversion to A1203,mol 9'0
O C
max 460 450 490
rates, s-l (3.0-7.5) X lo4 1.4 X loT6 (0.5-1.0) X
100 81 98
N c
W
500 o TIME / m i n
o
Figure 4. Temperature profile for boehmite decomposition at g i 510 Pa. 0.2 0.6 0.8 1.0 -10
0.L
d
-31
Figure 2. Effect of water pressure on gibbsite decomposition rate.
- '1
I\ L
:' *-=a
\
I 0.2
0.6
0.L
0.8
1.0
o(
Figure 5. Effect of water pressure on boehmite decomposition rate. slope:-1.3
2.0
2.5 LOG,,(
3.0 PH,O 1
3.5
Figure 3. Dependence of gibbsite decomposition rate at a = 0.5 on water pressure. composition rate) + E,(To - T)/RTToagainst conversion to alumina until a smooth function was produced. In general the same value of activation energy was found to be accurate for decompositions a t all values of water vapor pressure (272 f 12 kJ mol-'), so that it was then possible to determine the nature of the dependence of rate upon water vapor pressure @(pH,?)). This was achieved by plotting the normalized function above against the extent of reaction for all the runs a t different water vapor pressures (Figure 2). From these plots a set of values for the normalized function at a selected a value could be interpolated and plotted against In PHPo. These are shown in Figure 3 for the gibbsite to transition alumina conversion. These plots were linear suggesting a power-law rate dependence on water vapor pressure. For gibbsite decomposition the rate is decreased by increased water vapor pressure. The exponent was found to be -1.3 over the range of experiments carried out here. The final rate equation for gibbsite decomposing to transition alumina is of the form
-5
2.5
3.0
3.5
LoG10(PH20
Figure 6. Dependence of boehmite decomposition rate at a = 0.4 on water pressure.
where rates are in s-l and P is in Pa. Although the rate of boehmite formation could not be easily studied, its decomposition was separately studied by using the synthetic cera hydrate. Exactly the same procedure was followed as for gibbsite decomposition. The reaction process is much simpler and a slowly rising plateau was observed in the temperature profile commencing in the range 400-450 O C , depending on the water vapor pressure selected. A typical result is shown in Figure 4 and full data are in Table 11. Again an activation energy of 272 kJ mol-' was found to be satisfactory under all conditions, and the normalized function calculated for each vapor pressure is shown in Figure 5. Again a power law dependence of rate on water vapor pressure WBS found with the rate again decreasing as water vapour pressure increased. The best value of the exponent was -0.4 in this case (see Figure 6 ) . d a / d t = A exp(-272000/R T )f ( c Y ) P ~ , o - ~ .(4) ~ For boehmite decomposition therefore we can write: or since f ( a ) is a constant we can write for gibbsite d a / dt = A exp(-272000/R r)f (6) d a / d t = (4.7 f 0.7) X loz7e ~ p ( - 2 7 2 0 0 0 / R T ) P ~ , ~ - ' ~ ~ An analytical expression for {(a)was not determined but (5) if the initial value is set equal to unity then A in the above
684 Langmuir, Vol. 3, No. 5, 1987
Stacey
Table 111. Micrometric Data
sample G1 G2 G3 G4 B1 B2 B3
nitrogen adsorstion" vol liq, mL/g at P/PQ= 0.85 in micropores (desorption) 0.116 0.143 0.131 0.172 0.151 0.201 0.124 0.164 0.017 0.039 0.028 0.048 0.026 0.052
S,, m2 g-' 14.4 17.2 21.0 21.5 8.0 14.4 16.6
CCll adsorption,bvol liq, mL/g at P/PQ= 0.85 P/P, = 0.1 (desorption) in micropores 0.092
0.112
0.097
0.1045 0.0137
0.132 0.03
0.103
0.025
0.047
0.026
ODensity of liquid nitrogen = 0.8081 g/mL. bDensity of liquid CC1, = 1.594 g/mL.
0.2
0.L
0.6
0.8
0.2
1.0
Figure 7. Nitrogen isotherm at 77 K for sample G4.
equation has the value (3.6 f 0.2) X 10l6. The functional form of In (f(a))is that of the curves in Figure 5. Adsorption Data. The nitrogen adsorption isotherms for all the materials listed in Tables I and I1 were of the same general type. They all showed evidence of complex textures. That shown in Figure 7 is a typical example (gibbsite-derived transition alumina sample G4 Table I). First the hysteresis loop is type H4 in the IUPAC classification, probably indicating slit-shaped pores in the meso/micropore range. The hysteresis loop in all cases closes very near to p / p o = 1 and does not show a welldefined maximum uptake so that there is always some macroporosity present. This is probably the intercrystallite porosity present within the original gibbsite polycrystalline grains. The features of most interest are the low-pressure region ( p / p o C 0.3) and the height of the hysteresis loop itself. Since the desorption branch of the hysteresis loop only closes a t a relative pressure of 0.4 and since a,-plots all show a knee at a, = 1, there is no doubt that the samples are all strongly microporous. It is clearly impossible to calculate any size distribution for the pores from the isotherm alone but the presence of a hysteresis loop implies that a t least some mesopores are produced. However, the set of isotherms for gibbsite-derived products exhibit a systematic trend. As the prevailing water vapor pressure in the decomposition rises, the curvature of the initial uptake becomes less marked and simultaneously the height of the hysteresis loop increased (see Figures 7 and 8). From previous work on boehmite-derived products, this effect would be expected to reflect an increase in pore width.2 To gain extra information the isotherms for adsorption of carbon tetrachloride on some of the samples were determined since this molecule has a larger van der Waals diameter (0.59 nm) than nitrogen (0.36 nm). The results are shown in Figure 9 and 10 and are of the same general form as the nitrogen adsorption isotherms. Below p / p o = 0.2 the isotherms were reversible. However, the volumes (calculated as liquid carbon tetrachloride, see Table 111) taken up a t relative pressures of 0.1 and 0.85 (desorption branch) are only about 80% of the values
0.L
0.6
0.8
1.0
P/ Po
P/Po
Figure 8. Nitrogen isotherm at 77 K for sample G1.
0.251
o'20!f---==@
0.1 5
P 0.10 0.05
0.2
0.L P/Po
0.6
0.8
1.0
Figure 9. CC14 isotherm at 293 K for sample G4.
a20
0.2
0.6
0.6
0.8
1.0
PIP0
Figure 10. C C 4 isotherm at 293 K for sample G1.
calculated from the similar points on the nitrogen isotherms (again calculated as liquid). Again the shape of the knee below p / p o = 0.1 is more sharply curved for the sample decomposed at 160 Pa. This result implies that, first, in both samples there is some porosity accessible to nitrogen but not to carbon tetrachloride. Second, the difference in knee shape between the two samples implies a different interaction energy and the most probable reason for this is an increasing pore width. However, there is probably a continuous distribution of pore widths from 0.35 nm up into the mesopore range for both samples and what is changing is either the standard deviation and/or the mean pore width.
Langmuir, Vol. 3, No. 5, 1987 685
Decomposition of Gibbsite and Boehmite
’“1
loor
160
I
0.2
OL
OK
0.8
10
P/Po
Figure 11. Nitrogen isotherm at 17 K for sample B3.
a2
RL
0.6
oa
to
P/Po
Figure 13. Nitrogen isotherm at 77 K for sample G3.
In the case of the gibbsite sample processed to 455 “C, the initially formed boehmite was partially decomposed and the nitrogen isotherm of the product showed a different shape below a relative pressure of 0.3 (see Figure 13). The shape is not a simple summation of the types in Figures 7 and 11but rather a more extensive curvature in this region. Since the morphology of the gibbsite-derived boehmite is almost certainly different from that of the cera hydrate it is not surprising that it may give a different texture.
Figure 12. TEM of sample B2.
Similar nitrogen adsorption behavior was observed for the boehmite-derived samples except that the total uptake was only about 25% of that from the gibbsite-derived samples (see Figure 11). The lowest uptake was observed for the sample decomposed at 170 Pa of water vapor pressure. A similar difference in total liquid uptake was found by using carbon tetrachloride. For both B1 and B3 the isotherm showed low-pressure hysteresis. In the case of sample B2 (see Table II), HREM photos were taken to confirm the slit-shaped pore morphology and to determine the average pore width. Figure 12 shows a typical area, and a pore width of 0.5-0.8 nm was derived. This compares with a value of 0.63 nm determined previously by us for a similar experiment (ref 2, sample 4). a-Plots (a,) for all the nitrogen isotherms were generally of similar shape. From the slopes above the knee, the surface area of wide pores was calculated and the intercept of the line on the volume-adsorbed axis was derived a8 a measure of the micropore volume. All these derivations are listed in Table 111. The wide pore surface (S,) areas when calculated back to the original weight of parent hydroxide are 2-3X larger than the original hydroxide surface areas. This suggests that the decomposition process does not produce exact pseudomorphs in the sense that the external morphology of the product is identical with that of the parent crystallite. Most probably there is some slight mismatch between the product lattice and that of the parent so that there is a little gross fissuring of the parent in order to relieve the strain.
Discussion Despite the different temperatwe regimes under which gibbsite and boehmite decompose, both the kinetics and the product textures exhibit marked similarities. Both hydroxides decompose with the same activation energy, their rates are decreased by the water vapor produced, and they both have an almost constant reaction interface area. The textures revealed from the adsorption isotherms are for practical purposes indistinguishable except for the absolute number of pores, which results from the different stoichiometries of the two hydroxides. The issues raised therefore have mainly to do with characterizing the average micropore width and distribution when large amounts of boehmite are formed during the initial stage of gibbsite decomposition. In essence, to what extent is the ultimate texture of the transition alumina affected by both water vapor pressure and the presence of boehmite? Any reaction mechanism should be able to account for at least the total porosity, the pore shape, and the mean pore size. Such a theory was proposed for boehmite decomposition in 19802and the new results reported here can be used to test the theory. For reactions proceeding far from equilibrium, the available free energy is used both to drive the reaction and to create the new pore surface. Following the theory2 developed earlier for boehmite, the surface created in hydroxide-derived transition alumina is defined by the equation Muswm = -(X*/X)nRT[ln (pHfl/pH,Oo)l (7) where Pwo = the equilibrium pressure of water vapor for hydroxide decomposition to transition alumina, M = molecular weight of alumina (g/mol), S, = geometric surface area of the pore system (m2/g),X = lamella repeat distance, X* = minimum possible lamellar repeat distance, u = surface energy of the pores (J/m2),and n = number of moles of H20/mole of alumina. By assuming that the pores are of slit geometry and that the porous alumina is an accurate pseudomorph of the parent hydroxide crystallite, a mean pore width (d) can be calculated from the equation d 2(vh - Vt)/MSgeom (8) where Vh = molar volume of hydroxide and Vt = molar
686 Langmuir, Vol. 3, No. 5, 1987
Stacey
TEMPI'C
0-
L
0
n
11; 30
0
\
0
I"-1 -
a
I
0
0
0
J
-2
-
!h
nB1
-3 12
.G1
2.0
2.8
3.6
~O?T
Figure 14. Equilibrium diagram for aluminum hydroxide decompositions.
volume of transition alumina. Combining the two equations and dropping the subscripts to water vapor pressure gives d = 20(Vh - V,)/[(A*/A)nRTIn (P/PO)] (9) From tabulated free energy data6 for both the hydroxide phases in question, the equilibrium water vapor pressures for decomposition have been calculated (Figure 14). Also shown are the temperature and pressure conditions used in this work. To a close approximation the PlPO term in these experiments is the same for both boehmite and gibbsite decomposition to transition alumina. Conse~~
(6) U.S. Geological Bulletin 1452; US Government: Washington, DC, 1978.
quently we expect that the Sgeom for gibbsite-derived materials should be about 3X that of boehmite derived materials (eq 7). The pore width predicted by eq 9, however, is about the same as that for boehmite-derived materials since the different molar volumes of the gibbsite and boehmite compensate for the different stoichiometry (the factor n = 3 in the denominator). From previous work on boehmite A*/A was found to be between 0.5 and 0.67 so it is to be expected that a similar value is valid for gibbsite-derived aluminas. The theory therefore is in broad agreement with the results found. However, it clearly needs some refinement to be able to deal with the departures from the ideal situation. In particular, knowledge of any topotactic relationship between parent and product crystallites is desirable. The greatest source of error is the value of V,; since the transition A1,03 is poorly crystalline, the XRD value may be too low and experimental measurements by pycnometry are also likely to be too high, since some pores are near the size of the He atom. Previous work' has established that boehmite decomposition is an accurately topotactic reaction, the H-bond chains in the original boehmite crystals determining that the slit-shaped pores are formed normally to these chains. Lippens and de Boer reported in 1956s that gibbsite shows complex optical behavior during its decomposition which is indicative of a topotactic process. However, the relationship of crystal axes of gibbsite to the product pore orientation has never been established. Though the product is pseudomorphous with the initial crystals, it is not known whether the total volume is accurately preserved through the process. A modern HREM study is highly desirable to establish the morphological and topotactic details of the gibbsite decomposition, including the nature of initial boehmite formation. Registry No. A1203.3H20,14762-49-3;A1203.H20,1318-23-6; HZO, 7732-18-5; NP, 1727-37-9; CCld, 56-23-5. (7) Wilson, S. J. J. Solid State Chem. 1979, 30, 247-255. (8)De Boer, J. H.; Steggerda, J. J.; Zwietering, P. Proc. Ned. Akad. Wet., Ser. B: Phys. Sci. 1956, 59, 435-444.
