Surface Areas of Heated Alumina Hydrates ALLEN S. RUSSELL AND C. NORMAN COCHRAN,
Aluminum Company of America, New Kensington, Pa.
A n interesting series of aluminas with a range of surface areas and crystal structures results from calcining the alumina hydrates. The influence of temperature, atmosphere, and duration of heating on the surface areas of various forms of a-alumina trihydrate, p-alumina trihydrate, a-alumina monohydrate, p-alumina monohydrate, and a n amorphous alumina is discussed. High area aluminas can be produced from some form of each of the hydrates, although the highest areas have resulted from heating of the trihydrates. The nature of the activation process is discussed and both initial crystal structure and form are shown to be important in determining resultant surface areas.
S
O R P T I S T aluminas have a n-ide field of usefulness and have been developed in many forms t o fill specific needs. The applications for these products seem extraordinarily diverse, including, for example, catalysis, air conditioning, maintenance of lubricating oils, rubber reinforcement, and chromatography. For all these uses high surface area is a common requirement. There a.re three principal types of sorptive aluminas. The most common variety is prepared by the controlled calcination of a rocklike form of a-alumina trihydrate. The original granule does not shrink appreciably during this calcination, and the loss of water with the accompanying recrystallization creates a large surface area. -4 second variety is composed of translucent granules prepared from a gelatinous alumina n-hick has a high surface area even before any decomposition of the alumina hydrate is effected. The t,hird type of sorptive alumina comprises discrete particles of such small size that they have appreciable area on their outer geometric surface. .4 wide range of surface areas may result from change i n the form, crystal structure, and activation of the alumina hydrates. The influence of temperature, atmosphere, and duration of heating on the surface areas of various forms of a-alumina trihydrate, a-alumina monohydrate, p-alumina trihydrate, p-alumina nionoh>-drate,and amorphous alumina is discussed here. A parallel investigation of the change in crystal structure of these same samples as determined by x-ray analysis is reported by Stumpf, RusEel], Sewsome, and Tucker (7). The nomenclature of the aluminn hydrates, the preparation of samples, and the procedure employed in sample activation are descrihed in the report on crystal structure.
Surface areas have been measured by the technique of Brunauer, Emmett, and Teller (I) employing the sorption of n-butane a,t 0 " C. Results are expressed as the millimoles of n-butane which just cover t,he area of 1 gram of sample (m.b./g.). Values of area in square meters per gram are obtained by multiplying the foregoing by 235 if the area of the n-butane molecule is taken as 39 A . 2 The comparison of alumina areas by n-butane sorption with the more usual values from nitrogen sorption is discussed by Russell and Cochran (6). The surface area values arc independent of t,he condition of sample evacuation as long as appreciable weight loss does not occur. Unless otherwise stated, the samples were evacuated for an hour a t 140" C. by a mercury pump without cold trap (pressure 10-3 mm. of mercury) before measuring surface area. .4 second constant from the plot for ca,lculating areas is t h e Brunauer, Emmett, and Teller c which (if the assumptions of the theory are rigorous) is related to the energy of adsorption in the: first surface layer minus t'he energy of liquefaction E1 - EL. .4t, 0" C . El - ELis calculated from c by the expression: 1.25 log c = E, - EL if energies are expressed in kilogram-calories per mole of n-butane sorbed. It is convenient to plot c values in the Iogarithmic form, and the formal expression 1.25 log cis used as a measure of the effective attractive force holding sorbed molecules to a surface. These E1 - EL values arc sensitive to evacuation conditions and are loawed by increases in the sorbed water which might arise from reducing the time, temperature, or pumping rate of evacuation or by increasing the previous exposure to moisture i n the air.
LITERATURE
The surface areas, El - E L values, and weight losses for LY-. t.rihydrate heated 1 hour in dry air or steam are plotted as a func-. tion of temperature in Figure 1. Results were nearly identical for samples 1 and 2, the rocklike a.nd finely divided materials, and a r e riot shoivn separately. Although the trihydrate starts to lose water a t about 140" C., it is not until 220" C., where the loss ie, about, S%, that the area becomes measurable with n-butane.. The E1 - EL value is high, nearly 3.0 kg.-cal. a t this point. With' increase of temperature to 250" C. the weight loss increases: rapidly to 23 %, corresponding approximately to monohydrat,e? forniat,ion, the surface area increases to 0.9 millimole of n-butane per gram, and the E1 - EL value diminishes rapidly. On further heating, the weight loss increases more slo~~-vlyto 34% a t 550" C. and the remaining 1% n-ater is not completely driven off until the. sample is highly calcined. The area. goes through a maximum of' 1.4 rnillimoles per gram a t 400" C., diminishes to 0.9 millimole. per gram a t 550" C., and declines more slowly to 0.1 a t 1200' C.
The tiiernial dehydration, sorptive properties, and stability of variwm forms of alumina have been investigated extensively. SeTTeral excellent compila,tions of the work in this field are available, including, for example, the monographs of Fricke and Huttig ( 4 ) and Krczil ( 5 ) mid the bibliography of Deitz ( g ) . Considerable information along these lines is also available in the more recent literature, so that the general outline of the results t o be expected in this work is well established. The present paper is designed to give a more comprehensive aiid consistent picture of alumina activation. ri general review of the literature would be impractical here and the citing of a few random papers in this field ~ o u l be d misleading. EXPERIMENTAL The methods of preparation and chemical analyses of the samples discussed in this report are listed in Table I.
ALPHA-TRIHYDRATE
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IN D U S T R I A L A N D .ENG IN E E R IN G C H E M IS T R Y
July 1950
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T EMPE RAT UR E, ' c Figure 1 . Surface Area, EI-EL Value, and Weight Loss os. Temperature for or-Trihydrate Heated 1 Hour in Dry Air or Steam
400
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TEMPERATURE, 'c Figure 2. Surface Area, E;-EL Value, and Weight Loss ua. Temperature for a-Trihydrate Heated 5 Minutes to 16 Hours in Dry Air
The El - EL value falls rapidly to 1.6 kg.-cal. a t 400" C., the peak of the surface area curve, and thereafter decreases to 0.7 kg.-cal. a t 1200" C. The weight loss in steam agrees with that in dry air over the entire temperature range. At 400" C. the area from 1-hour steam activation is about 50% of the comparable dry air value. The E1 - EL value is only 1.3 kg.-cal. for steam activation a t 250" C., much less than for dry air. The surface area, El - EL value, and weight loss plots for 5minute, 1-hour, and 16-hour heating in dry air are shown in Figure 2. For clarity, experimental points for 1-hour heating are not
reproduced from Figure 1. The results for 5-minute heating are similar to those for 1-hour heating a t 100" C. lower temperatures. There is no great difference between the 1-hour and 16-hour activation results. The heating rates of the samples pulled rapidly into the heated furnace are high as measured by a small thermocouple in a hole drilled in a 1-gram particle of rocklike hydrate. With the furnace a t 1000" C. the particle came to temperature in 5 minutes. At lower furnace temperatures the time to attain equilibrium was prolonged because of the relatively greater influence of the endothermic dehydrations. In one instance the sample was heated a t a uniform rate of 5 O C. per minute to 500" C. It had an area like that resulting from the Table I. Description of Samples standard rapid heating. (Impurities shown by spectrographic analysis to be greater than 0.1% are listed) The complete dehydration Loss on of a-trihydrate at low temperaInitial Sorption Ignition at 1100' C., NazO, SiOz, Area, El - EL> tures is a slow process. The Sample % % % Other, % m.b./g. kg.-cal. rate of dehydration during 150 a-Trihydrate hours a t 400" C. is plotted in ... 0.00 ... Rocklike Alcoa C40 1 34.6 0.4 ... ... 0.00 ... Bayer process Alooa C30 2 34.6 0.4 ... Figure 3. Twenty-nine per cent 0.04 ... Fine particle Alcoa (2730 3 34.6 0.16 p-Trihydrate weight loss was observed in 1 COz in 40' C.NaA10z s o h 4 35.5 0.28 0.10 0.1-1 Ca 0.01 ... hour, but only 33% after 150 Water on amalgamated 5 34.3 0.04 1.8 aluminum hours. a-Monohydrate 200° C. steam digested tri6 16.9 0.1-1 Fe 0.01 Doubling or halving the hydrate Platelets Alcoa D 5 0 7 15.4 0.1-1 0.1-1 0.1-1 Fe 0.13 0 5 air flow rate a t 500" C. for Dried gel from sodium 8 10.6 4.05 0.40Ca0,0.33SOs 1.44 1.51 a 30-minute activation had aluminate, sodium bicarbonate no effect on the area, El - E L @-Monohydrate >lo% Si, l - l O % K,Ti value, or weight loss. The Missouri diaspore clay 9 0.03 1.31 13.8 { 0.1-17 Fe, Mg, Cr area was diminished slightly Diaspore mineral 10 1-10%%e 0.00 ... Amorphous a t one quarter the usual air Hydrolyzed isopropoxide 11 37.3 ... . . . .,. 0.91 1.35 flow rate. I..
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 42, No. 7
more massive forms of a-trihydrate, and accordingly it achieves a higher surface area. The E1 - ELvalue is not strikingly different from that for regular a-trihydrate. 35
SAMPLE 4
BETA-TRIHYDRATE
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Figure 3. Weight Loss us. Heating Time for Alumina Hydrates a t 400" C.
Two samples of p-trihydrate have been investigated. Sample 4 was precipitated rapidly from sodium aluminate solution by carbon dioxide a t 40" C. Sample 5 was prepared by the action of water on amalgamated aluminum. I t had somewhat the greater surface area, 0.04 millimole per gram. The surface areag, El - EL values, and weight losses for sample 4, heated 1 hour in dry air or steam at temperatures up to 1200" C., are plotted as a function of temperature in Figure 5. The surface areas in dry air and steam exceed the comparable values for a-trihydrate a t all temperatures. The dehydration of p-trihydrate commences a t about the same temperature as for atrihydrate, but the surface area maximum occurs a t a lower temperature. Sample 5 was somewhat more resistant to dehydration than sample 4. Whether this increased stability resulted from greater chemical purity or from the different initial structure has not been established. After 2.5 hours a t 250" C., sample 4 lost 24% of its weight while sample 5 lost only 15% of its weight. However, a t 400" C. the difference was less marked, each having lost about 31% in this time. The weight loss a t 400" C. for p-trihydrate is initially more rapid than for a-trihydrate, although after 150 hours the values are nearly the same.
ALPHA-MONOHYDRATE
Alumina activated in a narrow tube has lower surface area and higher E, EL value than that activated in a boat in a wide tube. After 2-hour activation a t 400' C. in the narrow tube, the area is 1.2 millimoles of n-butane per gram compared to a usual value of 1.4 and El - EL is 2.2 kg.-cal. compared i o a usual value of 1.5. The most evident difference in the two cases is that the gas flow rat,e over the alumina particles is much greater in the narrow than in the usual activation system. Flowing streams of argon, dry air, nitrogen, and oxygen are equally efficient in producing high surface area during a-trihydrate activation. Carbon dioxide and hydrogen are less effective, while in steam low areas result. The h 'l - EL values of the samples heated in argon, dry air, nitrogen, oxygen, hydrogen, and carbon dioxide are all about the same, but that for steam activation is low. The data are shoKn in Table 11.
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The three samples of a-monohydrate exhibit strikingly different behavior in initial properties and in heat stability and all differ from the a-monohydrate formed by thermal dehydration in sir of the trihydrates. 2.0
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Table 11. Activation of a-Trihydrate Sample 1 in Miscellaneous Atmospheres Atmosphere
Weight Loss,
% 2 hours at 450' C.
Area, &I.B./G.
31.8 1.30 Dry argon 33.1 1.42 Dry air Dry carbon dioxide 30.5 1.25 Room air (0.010 atm. water) 29.7 1.26 Room air (no flow) 30.4 0.66 30.7 0.60 Steam 2 hours at 400' C. (narrow tube) 1.23 29.9 Dry air 1.22 29.7 Nitrogen 1.23 29.5 Oxygen 1.04 29.6 Hydrogen 1.20 29.2 Room air (0.020 atm. water) 0.62 29.0 Steam
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331 EL, Kg.-Cal.
1.45 1.47 1.48 1.56 1.25 1.34
2.17 2.16 2.16 2.61 1.39 1.29
The surface areas, El - EL values, and weight losses for Alcoa hydrated alumina C730, sample 3, heated 1 hour in dry air are plotted as a function of temperature in Figure 4. Hydrated alumina Ci30 is a-trihydrate in the form of such tiny crystals that it has 0.04 millimole of n-butane per gram of surface area before activation. This material loses weight more rapidly than do
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T E M PERAT U R E, 'c Figure 4. Surface Area, EI-EL Value, and Weight Loss os. Temperature for Fine Particle a-Trihydrate Heated 1 Hour in Dry Air
July 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Sample 6 from the steam digestion of a-trihydrate is a largeparticle, dense material having an initial surface area of 0.01 millimole of n-butane per gram. The effect of temperature on its surface area, El EL value, and weight loss is shown in Figure 6. I n contrast t o the a-monohydrate formed from thermal dehydration in air of the trihydrates, this a-monohydrate lost only 2.3% of its weight a t 400 O C. This is essentially just the excess over the theoretical water content of the trihydrate. Between 400" and 450" C. a rapid weight loss occurred t o a nearly anhydrous form having a surface area of 0.5 millimole per gram and E1 EL value EL values fail rapidly above of 2.2 kg.-cal. The areas and El 600" C. and are 0.1 millimole per gram and 1.2 kg.-cal., respectively, at 800' C. I n Figure 3 it is shown that this a-monohydrate dehydrates completely on 150-hour heating a t 400" C. A second interesting form of a-monohydrate is Alcoa monohydrated alumina D50, sample 7, which is produced in platelets so thin that the unheated material has 0.1 millimole of n-butane per gram area. This material has been a favorite for electron micrographs because of its unique shape (3, 8). Heating experiments have shown this material to be dehydrated a t the temperature characteristic of larger particle monohydrate, but to develop only about half as much additional area on activation as the former sample. The stability of the additional area is about like t h a t of the larger particle monohydrate. The El - EL values obtainable for these flat platelets before activation are about 1.0 hg.-cal., in agreement with those for sample 6. The measured E1 - EL value does not increase on activation as much for the flat platelets as for the massive particles. The fact that these dense a-monohydrates have relatively low area and poor stability is in marked contrast to the behavior shown in Figure 6 of a-monohydrate prepared by drying a gelatinous precipitate from addition of sodium bicarbonate to sodium
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TEMPERATURE 'c Figure 6. Surface Area, EI-EL Value, and Weight Loss os. Temperature for Samples 6 and 8, a-Monohydrate
aluminate and sodium silicate, sample 8. This alumina has a surface area of 1.4 millimoles per gram after drying a t 140" C. The E1 EL value on this material is initially about 1.5 kg.-cal. and falls to 1.2 a t 1200" C. At about 400" C. the sample loses its water of hydration but does not increase surface area thereby. This constancy actually implies a loss of area on the basis of a unit amount of anhydrous alumina because of the change in sample weight. It is probably fortuitous that the increase for the change in basis balances the normal loss from crystal growth. The area stability is high, the area being 0.8 millimole per gram after 1 hour at 800' C.
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BETA-MONOHYDRATE W
z i 3
Results with the two samples of 6-monohydrate are generally like those for the crystalline a-monohydrate, sample 6 of Figure 6, and are not plotted separately. The nearly pure mineral had low surface area before heating, and it required a temperature of about 500' C. to produce appreciable area on 1-hour heating. After 1 hour in dry air at 560" C. the surface area was 0.35 millimole per gram and the E1 - EL value was 1.69 kg.-cal. After further heating for 1 hour a t 650" C. the surface area was 0.21 millimole per gram and the Et - EL value was 1.13 kg.-cal. On dehydration 6-monohydrate changes directly to a-alumina rather than to one of the intermediate alumina forms. It might have been expected from this transformation that p-monohydrate would have much different curves for surface area, weight loss, and E1 - EL vs. temperature than the dense a-monohydrate. This was, however, not the case. It is interesting t h a t the sample heated to 650" C. had appreciable surface area, although its x-ray pattern showed only a-alumina.
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TEMPER AT U R E, 'c Figure 5. Surface Area, EI-EL Value, and Weight Loss os. Temperature for Sample 4, P-Trihydrate Heated 1 Hour in Dry Air or Steam
AMORPHOUS ALUMINA An amorphous alumina was included in this investigation because of its possible significance in the parallel crystal structure determinations. The maximum surface area is like that of atrihydrate, although the different preparations of the material were of low reproducibility. Sample 11, whose initial loss on
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
ignition approached that of alumina tetrahydrate, lost most of its water a t 140' C. The results with this material are not plotted. NATURE OF THE ACTIVATION PROCESS Dehydration of regular a-trihydrate to a-monohydrate first occurs a t about 140" C. The reaction might go to completion in a Ion7 humidity atmosphere a t this temperature if sufficient time were allowed; the true decomposition pressure for the alumina hydrates has never been determined. The crevices first formed in dehydration are either not accessible or are smaller than the 5 A. n-butane molecule and a neight loss of 8% occurs (for I-hour heating a t 220" C.) before the surface area exceeds 0.01 millimole of n-butane per gram. Evidence that the first crevices are exceedingly narrow comes from the high E, - EL value, two to three times the usual value for a smooth alumina surface. This implies that the n-butane molecule is held simultaneously on both sides of a crevice or is fitted into the point of a wedge. With 1-hour heating dehydration equivalent to monohydrate formation (23% weight loss) occurs a t about 250' C. iidditional surface area is formed rapidly in this process, and E1 - EL diminishes. The loss of IT ater and the formation of the denser CYmonohydrate both serve to create more pore space. Although a-monohydrate in its usual form is not dehydrated readily below 400 O C., the monohydrate from trihydrate decomposition starts to decompose a t or below 250" C. It is reasonable to suppose that the Tvater pressure from the smallest monohydrate crystals, which are of the order of one unit cell in dimension, is greater than for more perfectly crystallized particles. Surface area is created less rapidly after monohydrate formation is complete in accord with the slower development of pore volume. With further increase in temperature above 3.50' to 400' C., for 1-hour heating, the growth of the alumina crystals compensates for the increase in area froin further dehydration and a maximum in the surface area is reached. The average pores are over three times the width of the n-butane molecule, and El - EL falls to a value more characteristic of the flat alumina surfaces. The highest areas are not very stable; the tiny crystallites have a strong tendency to combine. Initially this combination takes place readily, but as the resultant particles become larger and larger, the activation energy for combination increases and fen-er opportunities for further crystal groxth occur, with a resultant increase in stability. The highest areas are created in atmospheres of argon, nitrogen, or oxygen, which are probably not sorbed during activation. Slightly l o ~ e rareas in carbon dioxide or hydrogen and the definitely lower areas in steam indicate that these gases are adsorbed on the alumina during the activation process and accelerate crystal growth. That water is sorbed on the surface is also shown by the lon- E, - EL value after steaming. Further indication of the presence of ITater comes from the weight losses, which are smaller in steam, implying some resorption of water. The small particle C730 a-trihydrate has a greater maximum surface area on activation than the more massive wtrihydrate.
Vol. 42, No. 7
This probably results from the more rapid diffusion away of water and the lower equilibrium water vapor pressure a t the crystal. Jf7ith the small particle size D50 a-monohydrate, the maximum area is less than for the massive a-monohydrate. The activation process for B-trihydrate appears to follow the a-trihydrate pattern. The dense forms of the monohydrates require a higher temperature €or activation and give a lesser surface area, as anticipated from the fact that only one third as much water can be removed as from the trihydrate The thin platelets of a-monohydrate increase in area on heating like the coarse particles. The structure of the a-monohydrate prepared as a dried gel is different from that of the crystals or agglomerates of crystals discussed above. The surface area of this material is dependent on the method of precipitating and drying and is relatively independent of the subsequent dehydration. -4lthough the results are less vel1 established, it appears that the amorphous alumina represents an intermediate case betiwen the dense hydrates and the gel. CONCLUSIONS Materials of high surface area have been produced from each of the four alumina hydrates, and several hydrates can themselves be produced in high surface area forms. The initial surface areas of the hydrates and their changes on heating are strongly dependent on their method of preparation. Alpha-monohydrates of areas from less than 0 01 to over 1.4 millimoles of n-butane per gram were observed; the former increased markedly, whereas the latter did not increase in area on heating The highest surface areas resulted from heating the trihydrates, although the amonohydrate prepared from gel had the most heat-stable area. 9lpha-alumina, rThich is usually thought of as nonsorptive, has been produced with appreciable surface area by heating p-monohydrate. Amorphous aluminas did not necessarily have highcr surface area than the crystalline forms. Aluminas with a wide range of suiface areas and crystal structures have been produced and related to logical activation processes. LITERATURE CITED (1) Brunauei,
S.,Emmett, P., and Teller, E., J . Am. Chem. SOC.,60,
309 (1938).
( 2 ) Deitz, V. R., "Bibliography of Solid ildsorhents," T a s h i n g t o n , D. C . , National Bureau of Standards, 1944. (3) Frary, F. C., IND. E K G .CHEBI., 38, 129 (1946). (4) Fricke, R., and Huttig, G. F., "Handbuch der allgemeinen Chemie," Vol. IX, pp. 57-113, Leipzig, Akademische T'erlag
G.m.b.H.. 1937. ( 5 ) Krcail, F.,"dktive Tonerde, ihre Herstellung und Anwendung," Stuttgart, Ferdinand Enke, 1938. (6) Russell, 8.S., and Cochian, C. N., IND.ENG.CHEM.. 42, 1332 (1950). ( 7 ) Stumpf, H. C.. Russell, A. S., Sewsome, J. TI-., and Tucker, C. M., Ibzd., 42, 1398 (1950). ,8) Tuikevich, J., and Hillier, J., 4ncrl. Chem., 21, 475 (1949).
RECEIVED .June 4, 1949.
END OF SYMPOSIUM Reprints of this symposium, including the papers appearing i n the July 1950 issue of =InaZytical C h e m i s t r y (pages 864 to 881), may be purchased for 50 cents from the Reprint Department, American Chemical Society, 1135 Sixteenth St., S.W., Washington 6, D. C.
