A NOVEL FINE ALUMINA POWDER, FIBRILLAR
BOEHMITE JOHN BUGOSH, R. L. BROWN, J. G. W. S E A R S , AND R . J. S I P P E L
R. McWHORTER,
E. I. du Pont de Ncmourr t Y Co., I=., Expnimenlol Station, Wilrnin@on, Del
A novel form of alumina has been developed in the form of a powder consisting of colloidal fibrils, which swells in water and dissolves to form a colloidal solution containing positively charged particles. One feature of the powder i s that completely inorganic, clear-to-translucent films con be deposited on a substrate b y casting from an aqueous dispersion; small sections of such films will withstand temperatures of up to IOOOO C. Studies thus far carried out point to possible use as o noncombustible binder for inorganic fibers and pigments; a component of prolective and decorative coatings on metals, ceramics, and other surfaces; a polymer reinforcing agent; an efficient thickening, emulsifying, and suspending agent in waterbased systems; and a textile surface modifier effecting reduction in pilling, static, and soiling propensity. has been prepared in a wide variety of types, often consisting of mixtures of crystal forms and with widely varying particle size (4, 7). The heterogeneity of such colloids makes characterization difficult and exact reproducibility almost impossible. I t was with the idea of making a homogeneous, reproducible colloidal alumina, particularly in a farm that could be employed either as a dry powder or a colloidal solution, that research was undertaken on the polymerization of basic aluminum ions in aqueous solution. This culminated in the discovery of conditions for producing colloidal fibrillar hoehmite ( I ) . This report describes the properties of a specific alumina, called Baymal colloidal alumina, which is available in development quantities.
An unusual characteristic of Baymal alumina is that the ultimate particles consist of fibrils about 50 A. in diameter and
over 1000 A. long, each consisting of a crystal of boehmite, A 1 0 0 H . The specific surface area is about 275 sq. meten per gram. Most alumina gels or powders in this state of subdivision or having this high a specific surface area consist of permanent aggregates of ultimate particles which cannot be separated even by colloid milling. When stirred in water, Baymal powder dispenes directly to a colloidal solution of the fibrils. I n this particular powder, acetate groups are present on the surface. I n water the acetate groups are probably ionized, leaving the fibrils positively charged, so that a stable colloidal solution is obtained. An electron micrograph of aggregates of the powder is shown in Figure 1, while the individual fibrils as they exist in a sol are shown in Figure 2. The chemical composition of this fibrillar boehmite is shown in Table I. The acetic acid is strongly adsorhed, being retained after drying in air for 16 hours at 110' C . Based on tlie specific surface area, it is calculated that there are 3.5 acetate groups per square millimicron, which indicates a fairly
Figure 1. Electron micrograph of aggregate o f fibrillar colloidal boehmite powder
Figure 2. boehmite
OLLOIDAL ALGMINA
Composition and Structure
Electron micrograph of dried dilute sol of fibrillar
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DRY FIBRIL SURFACE IN AIR ACETATE GROUPS- 3 HYDROXYL G R O U P S - 2 TOTAL - IO
PER SQUARE MILLIMICRON (IO0 SO.ANGSTROMS1
CH3
DISPERSED FIBRIL SURFACE IN WATER CH3COO- t CH3CO0- t H+
CH3 CH3
I
I
w+
I
c=o c=o c=o
:,
,
Figure 3. groups
Table 1.
H
H
H
H
H
BOEHMITE FIBRIL ( AIOOH )
i L
-
CH3COOH
CH3 I
c=0
j
BOEHMITE FIBRIL
Schematic cross-section of surface of fibril showing attachment of surface
the electron micrograph and also calculated from the specific surface area. Infrared absorption curves show the presence of crystalline boehmite, characterized by absorption at ,13.2, 9.4, 8.7, 5.1, 4.8, 3.25, and 3.05 microns. Density and Porosity of Powder. A typical nitrogen adsorption isotherm is shown in Figure 5. From this? the micropore volume is estimated to be 0.5 cc. per gram and the micropore diameter 70 to 80 A. This pore diameter is about what might be found in a mass of randomly stacked, stiff fibrils of 50-A. diameter, forming an aggregate mass as shown in Figure 1. The powder is fairly compact, having a bulk density of around 30 pounds per cubic foot. The density of the fibrils, including adsorbed materials, is calculated to be 2.3 grams per cc., based on the density of the components. This is confirmed by data on the density of aqueous colloidal solutions, assuming volumes are additive.
Chemical Composition of Fibrillar Colloidal Boehmite
% AlOOH
83.1 9.8
.4cetic acid
SOa-HzO NH4
1.7 5.0 0.2 0.07 0.02 0.02
+
Na
Fe Si02
completely coverage of the surface. Part of the water is physically adsorbed and can be titrated by the Fischer reagent. The remaining water is apparently present in the form of OH groups, presumably attached to aluminum ions under the layer of acetate groups (see Figure 3). X-Ray diffraction gives a pattern characteristic of boehmite, as shown in Figure 4. From broadening of the lines, the crystal dimension is calculated t o be from 35 to 50 A., corresponding approximately to the diameter of the fibrils seen in
Dispersion in Water and Formation of Films
When the powder is stirred into water, it swells and spontaneously disperses to a colloidal solution. I n water, the acetate is ionized, osmotic pressure is developed within the mass which therefore swells, and the ultimate fibrils, which
P
In
co I
\&U I
I
70"
I
I
I
I
60"
I
I
I
50"
40"
I
30"
I
I
20"
IO"
28 Figure 4. 158
X-Ray diffractometer pattern of fibrillar colloidal boehmite
l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
30C
250
200 E
E
p LI 150 0
100
I
0
I
I
0.2
I
I I I I I I 0.4 0.6 0.8 1.0 P I Po
Figure 5. Nitrogen adsorption isotherm of fibrillar colloidal boehmite powder
bear a positive ionic charge, mutually repel each other, and a colloidal solution, or sol, is formed. One important characteristic of this sol is that it dries to a continuous coherent a m . Most inorganic colloids dry initially to a gelatinous mass which then cracks or crazes to granules or a powder. However, because of the fibrillar nature of the ultimate particles, there is apparently sufficient interlocking of the particles so that the mass has sufficient strength a t an intermediate stage of drying to remain coherent. A film of 5% sal dried on the surface of glass coated with a very thin film of silicone grease remains coherent and continuous. The addition of 0.0370 Mg++ as magnesium acetate increases the thixotropy and gives somewhat smoother films. Such films, a mil or so in thickness, are essentially transparent (see Figure 6 ) . Since alumina is quite rigid, the films are brittle, but nevertheless have measurable strength. As described later, the modulus of rupture of wet extruded and dried rods is 1000 to 2000 p.s.i.; the strength of a film is similar. The air-dried film is softened by water, but is not redispersed to a sol. However, the film does not soften and remains coherent after being heated above 350' C . Such a film is permeable to, but not soluble in acid. Conversion to Gamma
Heating for several hours a t 350° C. or a few minutes a t 450" C. is sufficient to dehydrate the boehmite fibrils to gamma alumina ( 5 ) . I t is remarkable that this conversion occurs without destruction of the fibrillar structure. As observed in electron micrographs, each boehmite fibril is converted with little apparent physical change, to a fibril of gamma alumina. The effect on porosity is shown schematically, with corresponding data, in Figure 7.
Compaction of Powder
T h e powder can be dry pressed at 10 to 20 tons per square inch to surprisingly strong, coherent molded shapes such as pellets or rads. The modulus of rupture, even without heat treatment, is between 1000 and 2000 p.s.i. It might be anticipated that when the molded body is heated to remove acetate groups from thr surface of the particles, it might be weakened, but this does not occur. Apparently, as the acetic acid is removed there is incipient sintering a t the particle-to-particle contacts, so that strength is maintained. When the compacted body is heated to 450' C. and converted to gamma alumina, there is essentially no change in dimensions (less than 3% linear shrinkage). This is in spite of the fact that there is about a 25% loss in weight of acetic acid and water. The absence of appreciable shrinkage appears to be due to the fact that the fibrils d o not shrink in length, but only in thickness. The density of the pressed mass is about 20% higher than that of the original grains of powder. Compaction, therefore, only moderately increases the density of the powder granules which are already fairly dense aggregates of fibrils. Pressure merely squeezes these aggregates together, giving a mass in which there is a continuous interlaced fibrillar structure like that in the original grains of powder. I t is interesting that the density of a dried film is about the same as that of the compacted dry powder. The changes in porosity and apparent density of very fine powders of this type can be followed by noting changes in the nitrogen adsorption isotherms:
Powder hefore compaction After compaction to bars (20,000 p s i . ) Film cast from aqueous sol of dispersed powder VOL. 1
Colcd. Micropora Apparent Volume, Density, Cc./Grnm GramrICc. 0.53 1.03 0.36 1.22 0.40 1.20
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UNHEATED
HEATED 500"C. UNCHANGED DIMENSIONS
I
I
b , -
- 8
4mk.
25
O/o
WT LOSS
*PORE Figure 7.
Pore Diam., m p Unheated Heated 500' C.
4-5
6-7
Dehydration of colloidal boehmite Fibril Diam., mp
Pore Vol., Cc./Gram
0.53 0.6-0.7
r h e apparent density of the microporous mass, whether a granule of powder or a compacted bar, was calculated on the assumption that the total volume, per gram, equalled the sum of the micropore volume plus the true volume of the fibrils which, including water and acetic acid, have a density of 2.3 grams per cc. and therefore a specific volume of 0.435 cc. per gram.
6 4
Fibril Density, Gramsf cc.
Spec. Surface, Sq. Meters/ Gram
2.3 3.3
275 275
.A limited examination of adsorption properties showed no unexpected characteristics. For most purposes, the powder is preferably converted to fibrous gamma, by being heated at 500 to 600" C. for an hour or so. Powder, pellets, or other compacted shapes undergo no change in appearance or size. but are rendered completely insoluble and stable to water. On being immersed in water and redried at 100' C., there is an irreversible adsorption of about 3.5YGby weight of water, believed to be caused by re-establishment of a monolayer of AlOH groups on the surface. Based on specific surface area: this amounts to around 8 or 9 OH groups per square millimicron; from consider ation of crystal structure, the theoretical monolayer should contain around 10 OH groups per square millimicron. A similar monolayer of 8 hydroxyl groups per
square millimicron is found on the surface of hydrated silica (6). Also, about 10 water molecules, or O H groups, are calculated to be at the surface of liquid water. One gram-mole of water occupies a cube of 2.6 cm. per edge. Assuming uniform distribution. there are 8.4 X 10' molecules along one edge; thus there are 3.2 molecules of water per millimicron, or about 10 per square millimicron. The capacity for the reversible physical adsorption cf water was measured at a series of partial pressures and at 25" C.: at 50% relative humidity (R.H.). it was 6.5%; at 807~ R.H., 20.57,; and at 90% R.H., 31.SyO. This fibrillar boehmite contains about 1.7% sulfate as Sod-ions absorbed on the surface of the fibrils. (This contributes to the thixotropic characteristics of the aqueous sol.) This sulfate is apparentli- present in the form of AlOSOs-ions and can act as anionic sites for the adsorption of cations. The sulfate is strongly adsorbed at p H 4 and is not removed by treatment of the alumina suspensions with anion exchange resins. However. it can be slowll; removed by precipitation as BaS04 or by washing with alkali, since it is displaced by hydroxyl ion. The alumina surface, except for the minor proportion covered by sulfate ions. is positively charged at pH 4 and strongly and irreversibly absorbs many polyvalent anions including phosphate. pyrophosphate, silicate. chromate. and molybdate. It
Table II.
Table 111.
Adsorption and Ion-Exchange Properties
Chemisorption of Anions on Fibrillar Colloidal Boehmite dnalvsis o f Washed and 'Oried P o w der Adsorbed per Salt Em$loyeda Test for Wt. Sq. w (NHl)iM070z,.4HzO MO 8 . 8 4 2 , O Mood-- ions pzos 5 , 4 1 1 . 7 PO4--- ions NaZHPOd. 7H20 PnOa 1 1 . 9 8 3 . 7 (POa-) units (NaPOn)t NazSiO,,.9Hz0 Si02 5.37 2 . 0 SiOn-- ions Gelatin N 2 . 0 8 3 . 2 amino acid units Acid fuchsin N 0.37 0 , 1 9 dye molecule 1-Amino-2-naphthol iX 0 . 1 1 0.17 molecule
Relation of Properties to Potential Areas of Use
Property
.-lreas of U s e of Powder and Sol
Fibrous nature and colloidal
Thickener and suspending agent Coatings-pigment binder High temperature adhesive Catalysts Special refractories Polymer modifier Flow birefringence Surface modification-antistatic soil retardant, pill retardant Anchor coatings Emulsifier \Vater-wettable surface
size
Optical properties Positive charge
sulfonic acid
Baymal powder ( 70 grams) soaked 24 hr. in 500 ml. of 570solutions C.; centrifuged, resuspended in 500 ml. of distilled water, and centrifuged twice; dried a t 130a C. a
of the indicated salt at 30'
160
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Hydrophilic and organophilic
can also, of course, adsorb additional sulfate. Many carboxylic acids including long chain aliphatic, aromatic, and dibasic acids are also strongly adsorbed. Upon washing out the excess of the reagent solution, there remains on the alumina fibrils a monolayer of the anions (see Table 11). To those familiar with aqueous chromatography on alumina, the adsorption of these anions and their subsequent behavior as cation exchange sites in weakly acidic solution is not surpirising. However, these nonporous fibrils with wide pores between them in the aggregates, make it possible to work with an alumina substrate of known, stable, and reproducible surface area, so that adsorption can be expressed as degree of surface coverage.
ample, the combination of film-formation with refractory nature leads to consideration of the field of high-temperature binders (2). The high thixotropy and chemical stability suggest uses as a thickener, where resistance to high temperature or bacterial attack is important. Extrudability and high surface area suggest consideration as a component in catalysts. The high positive charge of the boehmite fibrils indicates the possibilities for efficient adsorption and fixation of other negative colloids and anions ( 3 ) . It is from such considerations that Table I11 was constructed as a guide to exploration of industrial uses.
Relation of Properties to Potential Use
(1) Bugosh, John (to E. I. du Pont de Nemours 8r Co.): L‘. S. Patent 2,915,475, (Dec. 1, 1959). (2) Zbid., 2,917,426 (Dec. 15, 1959). (3) Zbid., 3,013,901 (Dec. 19, 1961). (4) Fricke, R., Huttig, G. F., “Handbuch der allgem. Chemie, IX, Hydroxyde und Oxydhydrate,” Sect. 12, pp. 57-113. Akademische Velags Gesellschaft, Leipzig, 1937. (5) Iler, R. K., J.A m . Ceram. SOC. 44, 618-24 (1961). (6) Iler, R. K., “The Colloid Chemistry of Silica and Silicates.” p. 242: Cornell Univ. Press, Ithaca, N. Y . , 1955. (7) Russell, .4. S.? Gitzen, W. H., Newsome, J. \V., Ricker, R. W., Stowe. V. W., Stumpf, H. C., Wall, J. R., Wallace, Paul, “Alumina Properties,” Tech. Paper No. 10 (revised), Aluminum Co. of America, Pittsburgh, Pa.. 1956.
The novel fibrillar character of the alumina particles results in several unusual characteristics : the film-forming nature of the sol; the high viscosity and thixotropy of sols, particularly at low concentration and in the p H range 6 to 10; the plasticity and claylike extrudability of a 507, mixture of the powder in water; and the coherent, self-bonding nature of drypressed powder. I n addition, this alumina has properties of high surface area with attendant adsorptive and ion exchange characteristics, positive ionic charge below p H 5, low toxicity, and chemical stability and refractory properties. In considering potential areas of use, two or more of these properties are usually considered in combination. For ex-
literature Cited
RECEIVED for review December 26, 1961 ACCEPTEDApril 6, 1962 Symposium on Ultrafine Particles, Spring Meeting, Electrochemi-
cal Society, Indianapolis, Ind.. May 1961.
STORAGE STABILITY OF NICKEL CATALYSTS Decrease in Actiuity of Nickel Catalysts under Liquids, with Special Reference to R a n 9 Nickel P I E T E R MARS, TH. v . d .
M O N D , A N D J . J . F. S C H O L T E N
Central Laboratory, Staatsmijnen in Limburg, Geleen, Thp Netherlands
Knowledge of the storage properties of industrial nickel hydrogenation catalysts i s of commercial importance. The stability of Raney nickel and of nickel on kieselguhr catalyst was studied as a function of time and of dispersion liquid. The decrease in activity during storage i s due to poisoning b y oxygen of the metal surface. The rate of hydrogenation of phenol was taken as a measure of the activity of the catalyst. Raney nickel proves to be much more stable than supported nickel catalysts, if it i s stored in water or alcohol, because hydrogen, generated by reaction of residual aluminum in the catalyst with the dispersion liquid, protects the nickel surface against oxidation.
THE literature
contains only scanty and contradictory information on the way in which the catalytic activity of nickel catalysts is influenced by storage time and the type of dispersion liquid used. Schroter (72), for instance, reports that after storage under \cater for one year Raney nickel still had an appreciable catalytic activity, while Mozingo ( 9 ) speaks of a lifetime of 6 weeks, and Adkins and Billica (2) report that very active samples show a considerable decrease in activity after 2 weeks. However this may be, the general experience is that Raney nickel has better storage properties than other nickel catalysts, and this is commonly attributed to
the large amounts of hydrogen believed to be present in it. This hydrogen gives the catalyst a specially high activity, which, however, declines with time because of the slow release of hydrogen during storage ( 7 ) . The type of the liquid under which the catalyst is stored also influences preservation. Storage of Raney nickel under benzene results in a much more rapid deactivation than storage under water or alcohol ; according to Lubarsky, Ivanovskaja. and Isaeva (7), this is due to the removal of hydrogen from the catalyst by the hydrogenation of benzene. Pattison and Degering (70), who stored Raney nickel in VOL.
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