Oct., 1961
CHEMISTRY AND MORPHOLOGY OF COLLOIDAL BOEHMITE
1780
COLLOIDAL ALUMINA-THE CHElWSTRY AND MORPHOLOGY OF COLLOIDAL BOEHMITE BYJOHNBUGOSH Industrial and Biochemicals Department, E. I . du Pont de Nemours & Co., Inc., Experimentd Station, Wilmingkm, Delaware Received March 7, 1081
This will tie the first of a series of papers on a new type of film-forming colloidal alumina consisting of fibrillar boehmite particles. l’actoidal structures are formed in the sols, which also show streaming birefringence. Se aration of sols into two liquid phases, one of which is paracrystalline, showing permanent birefringence, has been observeffor the first time in an alumina dispersion. Such alumina sols can be made by autoclaving an aqueous solution of basic aluminum chloride. Factors such as anion type, anion concentration, alumina concentration, time and temperature of heating are important in obtaining a fibrillar product. The constitution of these alumina sols will be discussed, a s well as the methods used for studying their size and shape, and the ionic species present in the sols. Quantitative analytical techniques applied for the first time in the study of alumina sols, such as rate of dissolution in acid and streaming birefringence, will also be outlined. The unique film-forming characterist,ics of this colloidal alumina will be discussed.
Introduction Since Gay-Lussac’s pioneering work in 1810,’ various procedures have been available for preparing colloidal alumina sols and gels. Intensive examination of these preparations by the author as well as other^^-^ has shown that, in general, they contain mixtures of particles with different crystalline types and shapes rn well as widely varying particle sizes. In addition, the alumina sols often either gel, precipitate slowly or otherwise markedly “age” on ,&anding. Previous investigators have concentrated on characterizing either the dispersed alumina particle phase by X-ray, electron diffraction or electron m i c r o s ~ o p e or ~ . ~characterizing the primarily ionic intermicellar liquid by conventional physkal chemical techniques.8-10 I n recent years the newer physical methods have been used more extensively than the older classical chemical methods have been. It is the purpose of this paper to show that colloidal boehmite, NOOH, can be produced as a fibrous colloid by hydrolysis of basic aluminum chloride or nitrate in water at high temperature. The boehmite so produced consists of stable colloidal fibrils, about 50 A. in diameter, which are physically analogous to linear, high molecular weight organic molecules. This type of alumina forms sols which are highly viscousJ show streaming (1) Gay-Lussac, Ann. chim. phya., 74, 193 (1810). (2) L. Gmelin, “Handbuch der anorganische Chemie,” 8th Ed., 35B-Aluminum, Leiprig. 1934, pp. 98-123. (3) R. Fricke and G. F. Huttig, “Handbuch der allgemeinen Chemie.” IX. Hydroxyde and Oxyhydrate, Leipzig, Section 12, 1937. pp. 57-113. (4) A. S. Rumell. et d..“Alumina Propertien,” Technical Paper No. 10 (Revised), Aluminum Co. of America, Pittsburgh, Pa., 1956. (5) F. J. Shipko and R. M. a a a g . AEC R and D Report K A P L 1740, General Electric Co., July 10. 1957. (6) H. B. Weber, “Inorganic Colloid Chemistry.” Vol. 11. “The Hydrous Oridea and Hydroriden,” John Wiley and S o u , Ino.. New York, N. Y., 1935. Chapter 111. (7) P. Souia Santos. A. Vellejo-Freire and H . L. Souza Santoa, Kolloid-Z., 133, 101 (1953); J. €I. L. Watson, J. Parsons, A. VeUejoFreire and P. Souza Santos, {bid.. 140. 102 (1955): Sbo Suauki. i&d. 166, 67 (1958): Z. Y. Berestneva, et al., KoZIoid Zhur., 13, 323 (1951). (8) R. B . Dean, “Modern Colloids,” D. Van Nostrand. New York, N. Y., 1948, Chapter 11. (9) A. W. Thomas, “Colloid Chemistry,” McGraw-Hill Book Co.. New York, N. Y., 1934, Chapter VII. (10) W. Pauli and E. Valko, “Elektrochemie der Kolloide,” J. Springer, Wieo, 1929, pp. 540--548.
birefringence, and can be dried to clear, coherent f3ms.11 Experimental Starting Materials. Basic Aluminum Chloride.-Basic aluminum chloride solution with an atomic ratio of aluminum to chloride of 2/1 is also called a 5/6 basic solution, and formulas such aa Al2(0H)&1 are also used, with the understanding that they refer only to stoichiometric relationships and not to the molecular structures of the salts in solution. The ratio may also be expressed in this case aa A120r/Cl = 1.0/1 .O. The method used for making basic aluminum chlorides is described by Huehn, e.? a1.12and Denk.” The method consists of adding one mole of the normal aluminum chloride hexahydrate salt (Mallinckrodt, C.P.) to one liter of water a t 60-70”, and then adding aluminum metal powder (Merck) in small increments with good agitation. If an exccssive amount of aluminum metal is added to the mixture before the previously added metal has reacted, there is a tendency to form a very viscous, translucent, hard, intractable mass. By varying the amount of aluminum metal added, the basicity of the fmal product can be easily controlled. For example, to obtain a basic aluminum chloride with an atomic ratio of aluminum to chloride of 1 or 3/3, two moles of aiuminurn metal would be added to the solution of aluminum chloride described above. In an attempt to prepare basic aluminum chloride eolutions with ratios of Al/Cl greater than 2.0/1, difficulty was encountered, since upon addition of the aluminum metal after passing Al/C1 = 2.0/1, the products became very viscous and set up to hard, grey masses. A commercial basic aluminum chloride,“ “Chlorhydrol,” supplied by the Reheis Company, Berkeley Heights, N. J., has also been used. A typical analysis of the dry product was 45.72% AlrOl, 33.27% volatile matter, and 16.58%,Cl, correspondmg to Al/C1 = 1.98; supplied aa a water solution, the product contained 23.5% AZO$,8.18% C1, corresponding t o Al/Cl = 2.0/1. Reaction Conditions.-The synthesis conditions which principally affect the character of the product obtained are: (1) acids used, (2) concentration of AlnO,, (3) concentration of acid, (4) ratio of AlnOa to acid, (5) operating temperature, and (6) temperature-time relation. The acid radicals which have been used are those of strong mono-basic acids which have a dissociation constant greater than 0.1 a t 25O, such aa hydrochloric, nitric or perchloric. The concentration of alumina as A1203 can be widely varied without greatly modifying the character of the products produced. The upper limit on the alumina concentration is fixed by the excessive, irreversible aggregation of the product. If a concentration of more than about 1.6 M A1,0, is used at temperatures below about 250”, the products may be badly and irreversibly aggregated. We obtained good results with the concentration of alumina varied between 0.05 and 1.6 moles per liter when temperatures below 350” were used. (11) (12) (13) (14)
J. Bugosh. U. S. Patent 2,915.475, Dec. 1. 1959 (du l’ont:. H. Huehn, e1 d.,U. €3. Patent 2,196,018. April 2. 1840. G. Denk and J. Alt. Z. anoru. d b m . Chem., 469, 244 (1952). Manufacturers’ literature, “Chlorhydrol. 1955.”
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JOHNBUGOSH
Vol. 65
. Thr rdative proportions of alumina t o acid radical can be dslbiiited on the basis of ALO, has given' repradueible reaults. Thc highcr the rcrrction temperature, tho shorter the time rcquired t,o cffect formation of a tihril of a given eharscter. I n gmcral, if a temperature around 220" is used, heating for a few minutm up to an hour or so will be sufficient, while at about 1W0,fibrils of increasing length are obtained with times from, say, about onohnlf hour to about one day. If a typical aqueous solution containing270Al.0z, supplied as basic ;duminum chloride, and having an AIIOl:chloride ion mole ratio of 1:1 is heated at 1W"ior four hours a viscous translucent sol will be obtained. If thc conecntratian of nlumina is increased, a very thixotropic but still opslcsernt gel will be obtained with concent,r:ttions of AI& of 3 and 4%. As the concentration is raised almve about 4 or jYo,the products become more aggregated. Typical Preparations of Fibrous Colloidal Boehmite.(A) A typical preparation of fibrous colloidal boehmite as a precipitate is as follows:
and an dtimlina molarity of 1.86. Five volumcs oi this solution wore diluted up to a total of 100 v ~ l u n i ~with s distillrd water and the mixture shaken. Thc concrntration of alumina in this solution was 0.093 M ( w A120J) while thc chloride molarity was 0.140. This
ti Fig. I.--l~~lectron micrograph of precipitated tactoids of fibrous colloidal boehmite.
clnve eo&ined n Ghite precipitate which, upon electron microscopic examination and X-ray diffraction examinstion was shown to contain very long hair-like fibrils of alumina monohydrate having the boehmite crystal lattice. Prectieally all of thesc fibrils of boehmite alumina had formed "taetoids" by a sidewise parsllcl alignment, and these small tsrtoids had, in addition, aligned themsclves in an end-tw end fnshion to produce very long fiber hnndles of boehmite xlumin:t. Two prepnrations of this type are shown in Fig. 1. (13) Thc colloid may he preptared in less marked tsctoidal . . form RS follows: A basic aluminum chloride solution w a made as described above, containing by analysis 8.000/, AI2001 and 8.43% CI. After IO-fold dilution the solution had an alumina content cquivalent to 0.09 M ns A1?03, and B molsrity of tho acid mdieul, chloride, of 0.271. The solution was h c a t d in B sealed glass contaiiner,at 1WO"for 16 hours. During this heating, fibrous boehmite prrcipitated in the initinlly wetcr clear solution. The fibrous hachmite was readily dispersed upon grntlc shaking, and in an deetron micrograph appeared as in Fig. 2. ( C ) Fibrous borhmite as a 801 rather than as a preeipi?ate IR ot,t.aint-d as follows. A basic aluminum chloride 8 0 1 ~ tion contzinirig 240/, AI.0. and an .4LO5:CIstomie ratio of 1 : I w a s diliited to 3% ABOS and this wnter-dcar solution was plnrr
