THE FORMATION AND THERMAL DECOMPOSITION OF
ALUMINIUM HYDROXIDE DOPED WITH Fe(lll) AND Cr(lll)
P1140
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C10026
58860
BIBLIOTHEEK TU Delft P 1140 7039THE FORMATION AND THERMAL DECOMPOSITION OF ALUMINIUM HYDROXIDE DOPED WITH Fe(lll) AND Cr(lll)
PROEFSCHRIFT
TER VERKRIJGIHG VAN DE GRAAD DOCTOR IN DE TECHKISCHE WBTENSCHAPPEN AAN DE TECHKISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS DR.IR. H.VAN BEKKUM, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN
DEKABEN, TE VERDEDIGEN OP WOENSDAG k
FEBRUARI 1976 TE I6.OO UUR
DOOR
Jakofc van Dijk chemisch doctorandus geboren t e 's-Gravenhage
Dit p r o e f s c h r i f t i s goedgekeurd door de promoter
• ^
CONTENTS CHAPTER CHAPTER I 1.1 1.2 II II. 1 II.2 II.3 II.li II.5 CHAPTER III III.1 111.2 111.3 CHAPTER IV IV. 1 IV.2 CHAPTER V V.I V.2 V.3 V.U V.5 V.6 INTRODUCTION 7 Gelatinous and crystalline aluminium hydroxides 8
The metastahle dehydration products of aluminium 11 hydroxide
THE FORMATION OF ALUMINIUM HYDROXIDES ]h
Hydration lU Hydrolysis and polymerization of Al(lIl)ions in ih
acid solution
Hydrolysis and polymerization of an alkaline 18 aluminate solution
Precipitation 19 Ageing 23 THE CRYSTAL STRUCTURES OF ALUMINIUM HYDROXIDES 30
AND THEIR CALCINATION PRODUCTS
Aliominium trihydroxides 30 Aluminium oxidehydroxides and some related 39
oxidehydroxides ( M O O H )
Aluminium oxides and some related oxides (MpO-,) UU
^/2 and S = ^/2 ELECTRON SPIN RESONANCE
The fine structure of S
Interpretation of powder spectra
THE INCORPORATION OF Fe(lll) AND Cr(III)IONS IN 70 ALUMINIUM HYDROXIDES
Experimental
Precipitation and ageing of pure aluminium hydroxides
The incorporation of Fe(lIl)ions The incorporation of Cr{III)ions Discussion Conclusions 52 52 59 70 70 76 81 92
CHAPTER VI V I . 1 V I . 2 V I . 3 VI.1( V I . 5 CHAPTER V I I V I I . 1 V I I . 2 V I I . 3 V I I . l t V I I . 5 V I I . 6 CHAPTER VIII
T H E THERMAL DECOMPOSITION OF DOPED ALUMINIUM 101 HYDROXIDES
Materials 101 Results of X-ray diffraction 102
Results of Electron Spin Resonance 102 Discussion 1 13
Conclusions 121 PHYSICAL MEASUREMENTS 12l*
Scanning electron microscopy (selected area 12U electron diffraction)
Liiminescence 124 Mosshauer spectroscopy 125
Electron spin resonance 127
Discussion 129 Conclusions 132 CONCLUSIONS AND FINAL RESULTS 135
CHAPTER I
INTRODUCTION
During the formation and thermal decomposition of aluminium hydroxides<• products with a low degree of crystallinity are formed. These phenomena are not specific for the aluminium ion, but various highly charged cations ( Fe(lll), Cr(lll), Si(lV), Ti(lV), Zr(lV), Th(lV) etc.) exhibit a similar behaviour. The technical importance of some of these materials (catalysts, absorbants, ceramics, glass, pigments, magnetic materials etc.) has resulted in an extensive research. Hence, detailed knowledge about the various phases and phase transitions has been gained. However, even up to now the phenomena observed during the for-mation and thermal decomposition of aluminium hydroxides are incom-pletely understood. One of the reasons may be that the number of sui-table techniques to study these complex materials and processes is rather limited. The direct methods to obtain structural information, X-ray and electron diffraction can not be applied well because of the low degree of crystallinity.
In the present study we will apply a tracer technique based on electron spin resonance (E.S.R.) in order to obtain additional information about the formation and thermal decomposition processes. The choice of the tracer ions is subject to some requirements:
- The tracer ion has to be sensitive for the crystal field (structural information)
- The tracer ion has to substitute easily for the Al(lIl)ion (valence, ionic radius, chemical stability, etc.)
We have tried to substitute a small part of the Al(lIl)ions by Pe(lll) or Cr(III)ions .
These tracer ions are followed by ESR spectroscopy during the formation and thermal decomposition of aluminium hydroxides.
Our interest is concentrated mainly on two aspects:
1. The behaviour of the Fe(lll) and Cr(lIl)ions during the precipita-tion and ageing of aluminium hydroxide.
2. The behaviour of Fe(lll) and Cr(lIl)ions during the thermal decom-position of the various aluminium hydroxides.
1.1 Gelatinous and crystalline aluminium hydroxides
In the formation of aluminium hydroxide,when an alkaline solution is added to an aluminium salt solution, several steps may be distinguished. - the hydration process, when the aluminium salt is dissolving.
- the hydrolysis process, when the hydrated Al(lIl)ion reacts spon-taneously with water molecules or with hydroxyl ions.
- the polymerization process, when association between monomeric hydrolyzed Al(lll) species occurs.
- the flocculation process, when a gelatinous aluminium hydroxide precipitate is formed at higher pH values.
- the ageing process, when crystalline products are formed by stor-ing the gelatinous precipitate in the mother liquor.
The Al(lIl)ion is amphoteric and soluble in acid as well as alkaline solutions. However, in neutral solutions the solubility of aluminium hydroxide is exceedingly low. When hydroxyl ions are added to an aluminium salt solution, it is very difficult to avoid high local su-persaturations. The importance of this effect depends strongly on the experimental conditions. Due to these concentration fluctuations,often the formation of a solid phase is observed in an early stage of the precipitation process. This once formed precipitate redissolves very slowly. As a result,the continued precipitation process proceeds under non-equilibrium conditions and is influenced by the presence of the solid phase. When the precipitation process is completed a product with a low degree of structural order is obtained. The surface energy of this material is high. When the formed precipitate is stored in the mother liquor crystallization occurs and gives rise to a lowering of the energy. This crystallization proceeds in general by a
dissolution-redeposition process. Hence the kinetics of this process are ruled by the rate of dissolution, the solubility product, the nucleation and the crystal growth process. The nucleation process is unique for the alianinium hydroxides as three closely related, but different, trihydroxide phases (gibbsite, bayerite and nordstrandite)
can be formed at ambient temperatures. These three compounds show layer structures with octahedrally coordinated cations. Only a few other trihydroxides with an octahedrally coordinated cation are known. These are the isostructural trihydroxides Sc(OH)o , In(0H)3 and Ga(0H)3 with a three dimensional network structure (ReOo structure). Apparently the nucleation process giving rise to the formation of the aluminium trihydroxides is determined by a complex interplay of various effects (e.g. dissolved Al(lll) species, pH , temperature, the presence of alkali ions or complexing agents).
These processes have important consequences for the formation of alu-minium hydroxide in nature, for the industrial production of aluminium metal and for the preparation of aluminium oxide catalyst supports.
From a geological point of view the formation of aliminium hydroxides is of interest, where gibbsite is a rather abundant mineral. Bayerite and Nordstrandite have been observed in nature,but only in very small quantities.
The initial process of the formation of aluminium hydroxide is the weathering of primary aluminosilicate rocks. The type of rock is not very specific. In a long term leaching process the soluble ions
( Na(l), K(I), Mg(ll), Ca(ll) and to a lower extent Si(lV) ) are dissolved and transported. An enrichement of Al(lll) and Fe(lll) results where these ions are only very slightly soluble under natural conditions (see fig. 1.1 )
Favourable conditions for this process are a high temperature, a high rainfall, a moderate topographical relief, and a minimal erosion. In this way a tropical soil may contain thick layers of bauxite formed on the parent rock.
These laterite-bauxites are the basic materials for nearly the total aluminium metal production. The aluminium hydroxide is extracted from the raw bauxite by the classic Bayer process. The bauxite is leached with an alkaline solution at slightly hydrothermal conditions. The in-soluble red mud (iron oxide, titanium compounds, hydroxy sodalite etc.)
concentration
(ppm)
10--Fe(lll)
-AI(III)
Fig. 1.1
Relationship between pH and solubility of aluminium iron, amorphous silica and quartz.
(After P.W. Birkeland, Pedology, weathering and geomor-phological research - 197^+ )•
is removed from the solution. Some gibbsite from a preceding run is added in order to improve the nucleation process. The solution is cooled slowly and the final product is well-crystallized gibbsite.
In the production of metallic alimiinium the Bayer gibbsite is dehydra-ted and dissolved in a molten cryolite flux (NagAlFg). The aluminium metal is produced by electrolysis. The broad application of aluminium metal makes these processes technically and economically very
important.
During the thermal dehydration of aluminium hydroxides in general pro-ducts with a low degree of crystallinity are formed. The combination of the presence of many active sites, a large surface area and a stability even at high temperatures makes these materials very useful
as catalysts and catalyst supports.
1.2 The metastable dehydration products of aluminium hydroxide During the dehydration of aluminium trihydroxides almost half of the anions must leave the crystal lattice. Finely divided material trans-forms directly to a transition alumina between 230°C and 350°C. The aluminimn oxide formed in this temperature range is highly disordered and retains traces of water. When coarse aluminium trihydroxide is heated two processes occur simultaneously. First the formation of a disordered aluminium oxide and secondly the formation of an
oxidehydroxide. The hydrothermal conditions within the crystallites may be responsible for the formation of the aluminium oxidehydroxide boehmite . Hydrothermal treatment of aluminium trihydroxides in an autoclave also results in the formation of crystallized boehmite. This boehmite decomposes at about 500°C and again a low crystalline product is formed. Hence coarse trihydroxides dehydrate partially in one step and partially in two steps.
The structures of the dehydration products depend strongly on the structure of the initial hydroxide. Taking into account, that half of the anions are leaving, it is surprising,that the dehydration sequen-ces of two closely related trihydroxides (e.g. bayerite and gibbsite) are characterized by quite different transition aluminas. Due to the expulsion of water molecules,the transition aluminas are very porous and hence have a large free surface area. At high temperatures the last traces of water, probably stabilizing the surface, are expelled. The material densificates and recrystallizes to the high temperature aluminas. During this process the anion packing is retained. The cations are redistributed over the available octahedral and tetra-hedral sites in the close packed anion network. Finally at high tem-peratures (above 1200 C) the stable corundum is formed. In this com-pound the anions constitute a heJtagonal close packing and the cations are exclusively octahedrally coordinated. Again it is surprising, that the formation of corundum is not always restricted to high tempera-tures . In the dehydration sequence of the second aluminium
oxidehydroxide diaspore, not occuring during other thermal decomposi-tions of aluminium hydroxides, corundum is formed already at 500 C. In all probability, it is significant here, that diaspore has a hexagonally close packed anion sublattice.
The present study deals with the aluminium hydroxides and oxides doped with Fe(lll) and Cr(lIl)ions. The three chapters II to IV provide a general background to this thesis. In chapter II the formation of alianinium hydroxides is described. In chapter III the crystal struc-tures of the aluminium hydroxides and their calcination products are reviewed. In chapter IV the basic theory of the Electron Spin Resonance method is presented and the application to Cr(lll) and Fe(lll) is given.
The three chapters V to VII give the experimental material. In chapter V the incorporation of Fe(lll) and Cr(lIl)ions in aluminium hydroxide is described. The precipitation and ageing processes of aluminium hydroxide have similarities in the iron and chromium hydroxide systems. But besides these similarities also important differences are present. The influence of the tracer ions on the for-mation of aluminium hydroxide is of interest. The specific properties of the tracer ions are important in order to choose an appropriate chemical strategy to incorporate these ions in the gelatinous and crystalline hydroxides. In chapter VI the results about the thermal decomposition of doped aluminium hydroxides are presented. The thermal dehydration sequences have been followed frequently by X-ray and electrondiffraction. An investigation of the dehydration sequences by ESR is of interest where short order effects instead of long range effects are important. In chapter VII the results of some physical measurements are enumerated. In tnls chapter in particular the posi-tion of the tracer ions in the various aluminium hydroxides is treated.
Some information about the crystal field around the tracer ion may be available.
The results of this thesis are summarized in chapter VIII.
The following diagram further Illustrates the relations between the various chapters.
C H A P T E R II
THE FORMATION OF ALUMINIUM HYDROXIDES
11.1 Hydration
In aqueous solution the highly charged aluminium Ion will be surroun-ded by water dipoles. The hydration number of the aluminium ion is six, where six water molecules form the first coordination sphere of the
aluminium ion. An octahedral coordination of the Al(lIl)lon is supported I'^O-lsotope exchange ( l ) , nuclear magnetic resonance of ^'''AI(III) (2) and X-ray diffraction of Al(lll) solutions ( 3 ) . The
3+
presence of octahedrally coordinated Al(HpO)^ complexes in some crystalline hydrated aluminlimi salts (e.g. AlCl . 6 H „ 0 ) is an additio-nal indication.
When strongly complexing ligands are introduced in the Al(lll) solu-tion, these ligands will enter the first coordinationsphere of the Al(lIl)ion, expelling water molecules. In order to prevent this com-plication with respect to the hydrolytic behaviour of the Al(lIl)ion, in general, anions with a low complexing capacity were used in this study (e.g. N0~ , ClOJj ).
11.2 Hydrolysis and polymerization of Al(lIl)lons in acid solutions An aqueous solution of aluminium nitrate has a pH value below seven. Where the anion does not influence the pH value of the solution, the Al(lIl)lon should be involved in a hydrolytic process. The following,
simple reaction can establish a lowering of the pH value of the AI(III) solution.
Al(H20)g"^ 5 n ^ A1(H20)^(0H)^* + H"^ K^ (l)
In equation (1) the H ion represents the hydrated HoO ion. This reaction is the most simple hydrolytic process,where only one water molecule in the first coordinationsphere of the Al(lIl)lon loses
one proton. This reaction is stimulated by the elctrostatic repulsion between the small, highly charged Al(lIl)ion and the proton. No bonds between aluminium and oxygenions have to be broken.
Eyring jr. e.a. (1+) Investigated the equilibrium of reaction (l) for various cations by kinetic measurements. Some of their results are listed in table II. 1 .
Table II. 1 M ( H 2 0 ) g * A l ( H 2 0 ) g C r ( H 2 0 ) g In^H^O)^ S c ( H 2 0 ) g K^(M) 1 . 1 0 " ^ I t . l O " ^ 1 . 1 0 - 5 2 . 1 0 " 5 M(H20)^(0H)^'^ K^{M)
Ga(H20) (OH) S.IO^ Fe(H20) ( O H ) 8 . 1 0 "
Table 11.2
The addition of alkali to the Al(lll) solution will shift reaction (l) to the right hand side. When the pH value of the solution increases additional reactions should be considered, where the hydrolysis of the Al(lIl)ion proceeds.
Al(H20) (OH)^"^ 5 = t Al(H20)i^(0H)2 + H"^ K ^ (2)
Al(H20)j^(0H) + A1(H20)2(0H)2 + H (3)
Eyring jr e.a. determined some of the reaction constants of equation (2). See table II.2.
For more details the reader is referred to the original literature or a recent review article of Wendt (5).
The reaction scheme (l) - (3) is too simple to describe the hydrolytic phenomena at higher aluminium concentrations. After the Initiating work of the Scandinavian scientist Bjerrum (6) a series of studies of Sillen and coworkers (7) resulted in a more complicated model. In this model the interaction between aluminium complexes gives rise to a polymerization process. The most elementary polymerization step is the formation of dimers.
2A1(H O)^(OH)^"^ i Z t (H20)f^Al(0H)2Al(H20)j^ •*" + 2H2O (k)
In this thus formed dimer the two hydroxyl groups are the bridging ligands in accordance with to second rule of Pauling (8). This rule is concerned with electrostatic bond strengths. The two octahedra share one edge (see Fig. II.I).
AI(III) OH"
"2°
When two Al(lll) octahedra are coupled by sharing hydroxyl ligands, this is called olatlon. When after ageing, the two Al(lll) octahedra share an oxygen ion (O ) Instead of the hydroxyl ions, this is called oxolatlon. During the oxolatlon process the reactivity of the bond decreases. ,
Greenwood e.a. (9) claimed to supply direct evidence for the existence 27
of a dimer. In a nuclear magnetic resonance study of Al(lll) the resonance due to monomers was saturated. Only a broad resonance of low intensity remained. This resonance was attributed to aluminium dimers, where the linew:dth of the resonance corresponded to the expected linewidth of a dimer.
Johansson (IO) showed the presence of dimers in the basic aluminium
sulphate rAl2(0H)2(H 0)g] (SOj^)^ 2R^0 by X-ray diffraction.
The polymerization process does not end, when dimers are formed. In the literature a large number of polynuclear species has been proposed. We will present a short outline of the extensive literature concerned with the hydrolysis of the Al(lIl)ion.
The most frequently used technique in studying hydrolytic processes, is a titration with alkali. The pH value is measured as a function of the total amounts of aluminium and added alkali (OH/^l) for various total Al(lll) concentrations.
fig. II.1
In recent studies the ionic activity is kept constant, by working with an electrolyte solution with a relatively high electrolyte concen-tration. With regard to the measured pH curves a model is postulated. Such a model consists of a set of species with appropriate equilibrium constants. The model is tested by comparing the experimental and theoretical pH curves (ll).
In this way Brosset (12) interpreted his data with an infinite series of polynuclear species. Brosset, Biedermann and Sillen (13) reinter-preted the same data alternatively with a model Including only one species: Alg(OH)^j- (in this notation the water molecules are omitted). After the discovery of the complex Ali30ji(0H)2i,(H20)j2 > l^y Johansson (lU) in the basic aluminium selenate Ha_O.i3Al2O3.8SeO3.xH2O, Sillen (15) made use of this species in his model of the hydrolysis of the Al(lIl)ion. Biedermann (I6) proposed two polynuclear species deter-minating the hydrolysis process in a 3M NaClOl; electrolyte solution. These species are AlY(OH)'ii'^ and A1-|3(0H)5J.
Van Cauwelaert and Bosmans (17) confirmed a species containing seven Al(lIl)ions. Smith (I8) differentiated three types of aluminium ions: at low Al(lll) concentrations the monomeric species dominate
(Al(lll), A 1 0 H 2 + , Al(OH)J ).This observation is in accordance with the results of other authors (19? 2 o ) . At higha" aLumlnium concen-trations polynuclear species are present with an upper limit of Al2lt(0H)lg+ or Al5g(0H)2gJ or perhaps even larger. Finally Smith considers the presence of a solid phase. During ageing Smith observed an increase of the amount of solid aluminium hydroxide coupled with a decrease of the amount of Al(IIl)ions in polynuclear species. The establishing of equilibrium is a very slow process. Mesmer and Baes (21) performed a hydrolytic study at elevated tem-peratures. They proposed a model including three species:
Al2(0H)^+ , Al3(0H)^+ and Aliij(OH)§J.
Recently Macdonald e.a. (22) investigated the hydrolysis process under similar conditions. They favoured a model with only Al2(0H)g''' and Alii^(0H)|'; .
It is seen that a wide range of species has been proposed in the various studies. This is inherent to some serious disadvantages of the potentiometrlc titration method. The used curve fitting method always presumes equilibrium. Equilibrium, however, is established very slowly. Hence the results depend on the experimental conditions (preparation). The accuracy of the experimental data and the sensiti-vity of the curve fitting method allow no exclusive choice of one model (23).
Aveston (23) employed In a combined study ultracentrifugation and the potentiometrlc curve fitting method. The ultracentrifugation measure-ments revealed the presence of a nearly monodisperse series of species at high OH/Al values. So Aveston excluded a hydrolysis scheme
with only monomers and dimers. The near monodispersity excluded an infinite series of polynuclear species as well. Aveston proposed a model with Al2(0H)^+ and Al ( O H ) ^ ^ •
Jander e.a. (2l*) and Jahr e.a. (25) measured in early studies respec-tively the freezing point lowering and the diffusion coefficient of partly hydrolysed Al(lll) solutions. These experiments only indicate the presence of polynuclear species.
Matijevie e.a. studied the coagulation of Agl sols in the presence of a partly hydrolysed Al(lll) solution. They concluded to the presence of a tetravalent complex. ,
First they proposed a dimpr: Al2(0H) . Later they favoured a model with an octamer : Alg(OH)^^ (26).
Patterson and Tyree (27) used a light scattering technique. They stated that the dominating complex contained at least eight or nine Al(lIl)ions. Recently Vermeulen e.a. (28) used this technique again. However these authors prepared the partly hydrolysed Al(lll) solutions by a special technique. The alkali was injected through a capillary at low supersaturatlon conditions. Up to OH/Al = 2 . 5 these authors assimied only dissolved species to be present in the solution. Rausch and Bale (29) applied small angle X-ray scattering. They detected particles with a radius of gyration of 1*.3A. The correspon-dance of these particles with the complex (Ali30ij(OH)2i;(HpO) •^2^'^'*' *s discovered by Johansson (lU) was emphasized. The theoretical radius
of gyration of the latter complex was k.lk A.
The presence of small species is not detected by this technique.
In conclusion we may state that the literature concerning the hydro-lysis of the Al(lIl)lon is extensive but confusing and rather conflicting. In detail the hydrolytic behaviour of the Al(lIl)ion is not well understood up to now.
II. 3 Hydrolysis and polymerization of an alkaline aluminate solution The Al(lIl)ion is amphoteric and soluble in strong alkaline solutions. In a combined infrared , Raman and nuclear magnetic resonance
(^'''AI(III), 23Ha(i) ) study Molenaar e.a. (30) supplied evidence for a tetrahedral coordination of the aluminate ion. The most probable com-plex in alkaline solutions is: Al(OH), . The species AlO and A 1 0 ( 0 H ) ~ are qualified as highly improbable.
2
When the aluminium concentration Increases new species apparently appear in the solution. The extra absorbtions in the Infrared and
and Raman spectra are attributed to dimers (30). These dimers Al20(0H),- are shown in fig. II.2.
fig. II.2 • = 0
O = OH Al(lll)
2-It should be noted that the Al-O-Al bond is not linear. The bond angle is 132 . The correspondence of the extra absorptions in the infrared and Raman spectra of the aluminate solution with the spectra of the crystalline compound K ( O H ) ^ A I O A I ( O H ) 1 is striking. The latter compound contains isolated dimers (31). In comparison with the hydrolysis of the acid Al(lll) solutions the aluminate solutions have been studied less extensively. A recent review of the properties of the aluminate solutions is presented by Eremln e.a. (32). The problem of the polynuclear species in the concentrated aluminate solutions is still open for discussions. Glasser (33) determined the crystal struc-tures of some double hydroxides containing isolated polynuclear spe-cies.
The compound Ba A10(0H)p Lcontained chains of corner sharing tetra-hedra. When the pH value of the alkaline mother liquor decreases the aggregation of the aluminium complexes, enclosed in the nucleating solid hydroxide. Increases. A surprise is the octahedral coordination of the Al(lIl)lon in some compounds prepared out of an alkaline solution e.g. Ba^ Alj^(0H)^g| , Ba^ A 1 2 ( 0 H ) ^ Q and Ba Al(OH)g where Isolated complexes are present in these compounds.
II.U Precipitation
The final stage of the hydrolysis process is the precipitation of the solid aluminium hydroxide. The solubility of aluminium hydroxide depends on the pH value 0656110'e.a. (3I+) have determined the solubility of aluminium hydroxide in aqueous solutions as a function of the pH value.
-log(Al),^^ 3 1 % ; (M)
2 3 4 5 6 7 8 9 10 11 12 13 • p H
F i g . I I . 3
S o l u b i l i t y curve of aluminium hydroxide - see ref.(3li-)
The charges apparent s o l u b i l i t y c o n s t a n t s of t h e hydrolysed aluminium s p e c i e s in e q u i l i b r i u m with t h e s o l i d aluminium hydroxide can be determined with a g r a p h i c a l method from t h e s l o p e s on t h e t a n g e n t s of t h e s o l u b i l i t y curve (broken s t r a i g h t l i n e s ) and t h e corresponding segments on t h e p H - a x l s . See t a b l e I I . 3 .
Table I I . 3 Apparent s o l u b i l i t y c o n s t a n t s under t h e assumption t h a t t h e complexes are formally mononuclear (20 C,logK 11+.16) reaction
Al(0H)2(S) + 3H*3=*Al3"^ + 3HpO Al(0H)2(S) + H"^ 5:iAl(0H)* + HpO A1(0H)2(S) ::rAl(OH)° A1(0H)2(S) + H^O^liAKOH);^ + H"^ equilibrium
"K = [AiiLni"'
so L J L J\ .
-\ , '
% . =
AI(OH)2 AI(OH)° Al(OH)r[H^]
r +1 H log*K 11 .ll0 2.59 -3.92 -12.62 p 3+ 1 + 0 -1 The degree of polymerization around the borders of the solubility curve is uncertain. The presence of polynuclear species influences the picture of the precipitation process.Two p o i n t s of view a r e observed i n t h e l i t e r a t u r e :
a. The p r e c i p i t a t i o n scheme i n c l u d i n g o l a t l o n and o x o l a t l o n
The p r e c i p i t a t i o n p r o c e s s could be considered as a c o n t i n u a t i o n of t h e o l a t l o n and o x o l a t l o n scheme. The p o l y n u c l e a r complexes i n t h e p a r t l y hydrolysed s o l u t i o n grow when t h e pH value I n c r e a s e s . The polymers can grow by a core and l i n k model. S i l l e n (11) suggested t h i s model where monomers and dimers a r e a t t a c h e d t o a growing polymer. When t h e polymers are growing eind i n t e r a c t i o n between t h e polymers occurs a p r e c i p i t a t e i s developed. The p o l y n u c l e a r s p e c i e s a r e p r e c i p i t a t e d i n a clew. D e t a i l e d knowledge about t h e composi-t i o n and s composi-t r u c composi-t u r e of composi-t h e p o l y n u c l e a r s p e c i e s i n s o l u composi-t i o n i s m i s s i n g (see 1 1 . 3 ) . So t h e s t r u c t u r a l u n i t s of t h e p r e c i p i t a t e a r e unknown. Tsu and Bates (36) assume t h e presence of s p e c i e s compo-sed of six edge s h a r i n g o c t a h e d r a . This complex Al^(OH).p(H 0) has a ring s t r u c t u r e s i m i l a r t o t h a t in t h e c r y s t a l l i n e aluminium t r i h y d r o x i d e s ( s e e c h a p t e r I I I ) . This complex could be a p r e c u r s o r i n t h e c r y s t a l l i z a t i o n p r o c e s s of b a y e r i t e and g i b b s i t e .
In a recent s t u d y Baker and Pearson (37) assume t h e A l ( l l l ) o c t a -h e d r a t o s-hare edges i n t -h e p a r t l y -hydrolysed s o l u t i o n . In t -h i s model s t r a i g h t chains a r e formed i n s t e a d of r i n g s . See f i g . II.1+".
• = A l ( l l l )
• = OH" O = H^O
These chains couple t o double chains s i m i l a r t o t h e double chains i n c r y s t a l l i n e boehmite and diaspore ( s e e c h a p t e r I I I ) .
Baker and Pearson e l u c i d a t e t h e formation of pseudo-boehmite by an o r i e n t e d p r e c i p i t a t i o n of double c h a i n s . The t h u s formed pseudo-boehmite i s very s i m i l a r t o t h e w e l l c r y s t a l l i z e d pseudo-boehmite. A broadening of t h e X-ray d i f f r a c t i o n l i n e s and a s h i f t of t h e 020 d i f f r a c t i o n l i n e i n d i c a t e s t h e d i f f e r e n c e between pseudo-boehmite and boehmite. The d-value of t h e 020 d i f f r a c t i o n l i n e i n c r e a s e s
from 6.15 A in boehmite t o 6.6 - 6.7 A i n pseudo-boehmite. Besides t h e s e differences t h e water content in pseudo-boehmite i s higher than t h a t i n boehmite. The water molecules coordinated t o t h e A l ( I I l ) i o n s t e r m i n a t i n g t h e double c h a i n s in the model of Baker and Pearson a r e r e s p o n s i b l e for t h i s high w a t e r c o n t e n t . E s p e c i a l l y when t h e l e n g t h of t h e chain d e c r e a s e s t h i s e f f e c t i s Important
( 3 7 ) . See f i g . I I . 5
F i g . I I . 5
O = HO bound t o one A l ( l I l ) i o n ® = OH" shared by two A l ( I I I ) i o n s • = 0 shared by t h r e e A l ( l I l ) i o n s
Recently F r y e r e . a . (38) observed polymeric films i n o p t i c a l l y c l e a r s o l u t i o n s of f e r r i c n i t r a t e . Large s u p e r s a t u r a t i o n s are not r e s p o n s i b l e for t h e s e films where t h e s o l u t i o n hydrolysed
s p o n t a n e o u s l y .
b . The p r e c i p i t a t i o n scheme i n c l u d i n g c o l l o i d a l p a r t i c l e s
In t h e p r e s e n t scheme t h e p r e c i p i t a t i o n proceeds by t h e formation of a s o l i d hydroxide phase. Smith (18) showed t h a t no e q u i l i b r i u m between a s o l i d hydroxide phase and p o l y n u c l e a r s p e c i e s e x i s t s . In
a dropwise p r e c i p i t a t i o n p r o c e s s ( l a r g e s u p e r s a t u r a t i o n s ) , even a t low OH/Al v a l u e s , a p r e c i p i t a t e i s formed. Vermeulen e . a . (28) showed t h a t t h e s o l u t i o n remained o p t i c a l l y clear t o high OH/Al v a l u e s , when t h e s u p e r s a t u r a t l o n was low. Geus (39) observed an Immediate f l o c c u l a t i o n in t h e s e o p t i c a l l y c l e a r s o l u t i o n s , when
sulphate ions are introduced. The existence of charged colloidal particles could be responsible for this flocculation.
A Fe(lIl)ion has a stronger tendency to hydrolyzation than
the Al(lIl)lon. Especially in Fe(lll) solutions the importance of a solid hydroxide phase has been recognized. Already in 1936 Jander and Jahr (l+O) observed the presence of a solid iron hydroxide phase in partly hydrolysed Fe(lll) solutions. These authors stated that polynuclear species are unstable with respect to a solid hydroxide. Weiser and Milligan (1*1) noticed the formation of an ultra fine precipitate in partly hydrolysed Fe(lll) solutions. In a potentio-metrlc titration study Biedermann (U2) as well as Danesi e.a. (1*3) observed the presence of a solid hydroxide phase. In an extensive study of the precipitation of Fe(lll) hydroxides v.d. Giessen (1*1*) made use of a large number of techniques. This study showed that a fresh iron hydroxide precipitate consisted of ultra fine
crystallites of iron hydroxide. The author favoured a precipitation process including the flocculation of colloidal Ironhydroxlde particles in contrast to the olatlon - oxolatlon scheme. The
flocculation is observed when the charge of the colloidal particles is neutralized by raising the pH value of the solution or by
intro-2_ ducing highly charged anions (SO, ).
However in the case of large polynuclear species versus small colloidal particles the differences between the two concepts are fading.
II.5 Ageing
The final product of the hydrolyzation and flocculation process is a X-ray amorphous precipitate. When this reactive precipitate is stored in the mother liquor crystallization occurs. This ageing process should be distinguished from another wellknown ageing process, namely the classic Oswald ripening. In the latter process the surface energy is deminished by growth of the larger crystallites and dissolution of the smaller ones. In the present ageing process the most important fact is a structural change of the precipitate. I'/hen the gelatinous
aluminium hydroxide is aged at roomtemperature the final products are the aluminium trihydroxides. Sometimes broad X-ray diffraction lines of pseudo-boehmite are observed before the nucleation of the
trihydroxides starts. At roomtemperature pseudo-boehmite is only an intermediate product. The ageing conditions determine whether gibbsite, bayerite or nordstrandite is formed.
Gibbsite is formed during the long term ageing of gelatinous aluminium hydroxide at roomtemperature in an acid mother liquor. The
crystalli-zation process proceeds very slowly. Even after years gelatinous aluminium hydroxide is still present besides the crystalline gibbsite (18). The alumlniimi species in the acid solution are octahedrally coordinated (e.g. Al(H20)g'^ , Al( H^O) j( 0H)2+, A 1 ( HgO )J^( O H ) ^ ). The incorporation of these octahedrally coordinated species in the gibbsite lattice demands the destruction of three aluminium oxygen bonds. This feature may influence the kinetics of the crystallization reaction. During the formation of gibbsite the pH value of the mother liquor decreases (1*5). The amount of dissolved aluminiimi species diminishes gradually, when the crystallization proceeds.
The crystalline gibbsite is apparently less reactive than the
gelatinous aluminium hydroxide. When the partly hydrolysed species are incorporated in the gibbsite lattice the pH value of the solution decreases. Other authors take into account an alkaline effect of the hydroxyl ions in the gibbsite lattice which are located on the sur-face of the crystallite. This effect is getting less important when the crystallites grow (1*6).
Nucleation is a bottle-neck in the formation of gibbsite out of an aluminate solution. In genera] slow crystallization processes are favourable for the formation of gibbsite. The slow spontaneous hydro-lysis of a super saturated aluminate solution in the presence of gibb-site seeds yields pure gibbgibb-site (Bayer process). When the reaction proceeds rapidly or when no seed is present bayerite is admixed in the product or is even dominating the gibbsite content.
pH value > 12, temperature about 60 C) (1*7). It is not clear whether alkali ions favour the formation of gibbsite or whether gibbsite is the stable aluminium trihydroxide modification. In the presence of sodium ions the gibbsite crystallites are pseudo-hexagonal platelets. Elongated pseudo-hexagonal prisms are formed in the presence of potassium ions (1*8).
Bayerite Is formed within some hours ageing at roomtemperature in an alkaline mother liquor. In these alkaline solutions the aluminate ion A 1 ( 0 H ) , is important. When this aluminate ion Is incorporated in the bayerite lattice only one aluminium oxygen bond has to be broken. During the crystallization process of bayerite the pH value of the mother liquor gradually increases. The decrease of the number of dissolved aluminate ions during the formation of bayerite is respon-sible for the increase of the pH value.
The bayerite formation probably proceeds by a dissolution- redeposition mechanism (1*8). Sato (1*9) separated the mother liquor from a fresh precipitate. After some days bayerite crystallites were observed in this liquor. This experiment shows that the formation of bayerite via the solution is possible.
Buyanov e.a. (50) proposed an incremental growth mechanism. In an electron microscopy study these authors observed a mosaic structure in the secondary crystallites in an aged Fe(lll) precipitate. The fresh precipitate consisted of a large aggregates made up of fine particles with demensions of 30 - 1*0 A . These primary particles have a poly-meric structure and are unstable on ageing. After a complex chemical transformation within the primary particles and nucleation of a crystalline phase,the latter grows by an incremental mechanism. The transformed primary particles are attached to the growing nucleus. The transformation within the primary particles is not elucidated. Buyanov e.a. excluded the transport of the Fe(lIl)ions via the solution due to the low solubility of the solid ironhydroxlde. Buyanov e.a. (51) applied their model also to the ageing of aluminium hydroxide.The final bayerite crystallites are not enclosed by crystal faces. The
shapes of the particles resembles hour glasses, cones and spindles (1*8).
Nordstrandite is formed during ageing at moderate temperatures (l*0-60 C.) in the presence of complexing agents (alkylamines, ammonia,
ethyleneglycol). Especially ethylenediamine is very suitable to prepare pure nordstrandite (52). The function of the complexing agents during the formation process of nordstrandite is not well understood.
The morphology of the nordstrandite crystallites is dependent on the experimental conditions. Nordstrandite particles shaped like long rectangles and long parallellograms have been described (53).
So far the ageing process of aluminium hydroxide is rather unique. The layer structures of bayerite, gibbsite and nordstrandite are not ob-served during the ageing of the hydroxides of other trivalent cations. In the case of Fe(lll) and Cr(lll) even no crystalline trihydroxides seem to exist.
Under hydrothermal conditions the aluminium oxide hydroxides diaspore (a-AlOOH) and boehmite (y-AlOOH) are formed. In accordance with Neuhaus and Heide (5^) the boehmite phase is metastable but the for-mation of boehmite is kinetically favoured at relatively low tempe-ratures and low pressures. The formation of diaspore is limited by the high nucleation energy as compared with boehmite. In the case of Ga(lll) and Fe(lll) the diaspore structure is realized by ageing at roomtemperature.
CHAPTER II - REFERENCES
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Phys. Sci. 77 (197^) 286
1*. D.L. Cole, E.M. Eyring, D. Hampton, A. Silzars and R.P. Jensen, J. Phys. Chem. 71 (1967) 2771
L.P.Holmes, D.L.Coleand, E.M.Eyring, J. Phys. Chem. 72 (1968) 301 L.D. Rich, D.L.Cole and E.M.Eyring, J. Phys. Chem. 73 (1969) 713 G.Hemmes, L.D.Rich, D.L.Cole and E.M.Eyring, J. Phys. Chem. 7** (1970) 2859
G. Hemmes, L.D.Rich, D.L.Cole and E.M. Eyring, J. Phys. Chem. 75 (1971) 929
5. H. Wendt, Chimia 27 (1973) 575 6. N. Bjerrum, thesis (I908) Copenhagen 7. L.G. Sillen, Pure Appl. Chem. I7 (I968) 55 8. L. Pauling, J. Am. Chem. Soc. 51 (1929) 1010
9. J.W. Akitt, N.N. Greenwood and G.D.Lester, J. Chem. S o c , Chem. Comm. (1969) 988
10. G. Johansson, Acta Chem. Scand. 16 (1962) 1*03
11. L.G. Sillen, Coordination Chemistry Vol. I, pg. 1*91, ACS monograph Ed. A.E.Martell v. Hordstrand Reinhold Comp. (1971)
12. C. Brosset, Acta Chem. Scand. 6 (1952) 910
13. C. Brosset, G. Biedermann and L.G. Sillen, Acta Chem. Scand. 8 (195*+) 1917
ll*. G. Johansson, Acta Chem. Scand. 11+ (1960) 771 G. Johannson, Arkiv Kemi.20 (1962) 305 15. L.G. Sillen, Quart. Rev. 13 (1959) l't6
16. G. Biedermann, Svensk Kem. Tids. 76 {196h) 362
17. F.H. V. Cauweleart and H.J. Bosmans, Rev. Chim. Mln. 6 (I969) 6II 18. R.W. Smith, Adv. Chem. Ser. I06 (1971) 253 Am. Chem. Soc.
Washington DC
19- H. Kubota, Diss. Abstr. I6 (1956) 861*
20. V.A. Nazarenko and E.M. Nevskaya, Rus .J .Inorg.Chem. 11* (I969) I696 21. R.E. Mesmer and C.F. Baes, Inorg. Chem. 10 (1971) 2290
22. D.D. Macdonald, P. Butler andD. Owen, J. Phys .Chem. 77 (1973) 2l*7l* 23. J. Aveston, J. Chem. Soc. (1965) 1*1*38
2l*. G; Jander and A. Winkel, Z. Anorg. Allg. Chem. 200 (1931) 257 25. K.F. Jahr and A. Brecklin, Z. Anorg. Allg. Chem. 270 (1952) 257 26. E. Matijevie and B. Tezak, J. Phys. Chem. 57 (1953) 951
E. Matijevie, K.G. Mathal, R.H. Ottewill and M. Kerker, J.Phys. Chem. 65 (1961) 826
27. J.H Patterson and S.Y. Tyree, J. Colloid Interface Sci. 1*3 (1973) 389
28. A.C. Vermeulen, J.W. Geus, R.J. Stol and P.L. de Bruyn, J. Colloid Interface Sci. 51 (1975) '*'t9
29. W.V. Rausch and H.D. Bale, J. Chem. Phys. 1+0 (I96I*) 3391 30. R.J. Molenaar, J.C. Evans and L.D.Mckeever , J. Phys.Chem. 7^
(1970) 3629
31. G. Johansson, Acta Chem. Scand. 20 (1966) 505
32. N.I. Eremln, Yu.A. Volokhov and V.A. Mironov, Rus. Chem. Rev. 1*3 (197^*) 92
33. A.H. Moinuddln Ahmed and L.S. Dent Glasser, Acta Crystallogr. B. 25 (1969) 2169
A.H. Molnuddin Ahmed and L.D. Dent Glasser, Acta Crystallogr. B. 26 (1970) 867
L.S. Dent Glasser and R. Giovanoll, Acta Crystallogr. B28 (1972) 519
L.S. Dent Glasser and R. Giovanoll, Chimia 2l* ( 1970) 3l*l* 3I*. N. Dezelic, H. Blllnski and R.H.H. Wolff, J. Inorg. Chem. 33
(1971) 791
35- H. B l l l n s k i , H. F i i r e d i , M. B r a n l c a and D. T e z a k , C r o a t Chim. A c t a 35 ( 1 9 6 3 ) 19
3 6 . Pa Hu T s u and T . H . B a t e s , M i n e r a l Mag. 33 (1961*) 7^*9 37- B . P . B a k e r a n d R.M. P e a r s o n , J . C a t a l . 33 (l97l*) 265
3 8 . J . R . F r y e r , A.M. G i l d a w l e and R. P a t e r s o n , N a t u r e 252 (I97I+) 57I* 39- J . W . Geus-, p r i v a t e c o m m u n i c a t i o n
1*0. G. J a n d e r and K . F . J a h r , K o l l o l d B e i h . 1+3 ( 1 9 3 6 ) 295 1*1. H . B . W e i s e r a n d W.O. M i l l i g a n , Chem.Rev.25 ( 1939) 1 1*2. G. B i e d e r m a n n and G.T Chow, A c t a Chem S c a n d . 20 ( I 9 6 6 ) 1376
1*3. P.R. D a n e s i , R. C h l a r i z i a , G. Scibona and R. R i c c a r d l , Inorg.Chem. 12 (1973) 2089
1*1*. A.A. v.d. Giessen, Philips Res. Repts., Suppl. 12 (I968)
1*5. J.D. Hem and C.E. Roberson, U.S. Geol. Survey, Water Supply Paper 1827 A (1967)
1*6. R. Schoen and C.E. Roberson, Am. Miner 55 (1970) 1+3
1*7. H. Ginsberg, W. Hiittig and H. Stlehl, Z. Anorg. Allg. Chem. 318 (1962) 238
1+8. K. Wefers and G.M. Bell, Alcoa Techn. Paper 19 (1972) 1+9. T. Sato, J. Appl. Chem. Biotechnol. 2l+ (197^) 187
50. R.A. Buyanov, O.P. Krivoruchko and I.A. Ryzhak, Kinet. Katal. 13 ( 1972) 1*16
5 1 . R.A. Buyanov and I.A. Ryzhak:, Kinet. K a t a l . 1I+ (1973) 1111 52. H.J. Bosmans, Acta C r y s t a l l o g r . 26 ( 1970) 61+9
53. D. A l d c r o f t and G.C. Bye, Science of Ceramics 3 (1967) 75 5I+. A. Neuhaus and H. Heide, Ber. Deut. Keram. Ges. 1+2 (I965) l67
C H A P T E R III
THE CRYSTAL STRUCTURES OF ALUMINIUM HYDROXIDES AND THEIR CALCINATION PRODUCTS
III.1 Aluminium trihydroxides
The crystal structures of the aluminium trihydroxides gibbsite, bayerite and nordstrandite are closely related to the structure of the mineral brucite (Mg(OH)p). In the latter structure the anions consti-tute a hexagonal close-packing ( h . c . c ) . The cations are distributed over the interstitial octahedral sites. In the brucite structure these octahedral sites between two subsequent close-packed anion layers are alternatingly filled and empty. Because of this alternating packing sequence of the cations the brucite structure consists of sandwiches. These sandwiches are double layers of hydroxyl ions enclosing octa-hedrally coordinated cations.
In the crystal structures of the aluminium trihydroxides one third of the cations has to be removed out of each sandwich due to the
stolChiometry. These vacancies are distributed in a regular way (honey comb pattern). A filled octahedron shares three edges with adjoining filled octahedra. In this way a two dimensional network of six-membered rings of aluminium ions is formed. See fig.III. 1
The close-packed anion layer is slightly distorted by a shortening of the shared edges in accordance with Pauling's third rule (l).
Busing and Levy (2) determined the proton positions in Ca(OH)„ by neutron diffraction. This compound is Isostructural with brucite. The protons in Ca(OH) are located between the sandwiches. Each proton is attached to one oxygen ion and the direction of the O-H bond is per-pendicular to the plane of close-packed anions. The proton positions in the aluminium trihydroxides are influenced by the vacancies in the cation sublattice as well as the stacking of the sandwiches.
^ #- ^ • —
\ o o /
O \0
0/
o
o \, O / O O \ O / o
^
-0 /
/o
i
\ 0
0 \
y-o /
/ o
i
\ o
o \
y-0 '
o
o
o
<
o
-^--o
-<
\
o
o \
»-O /
/o
- <
\ ^
^ \
w-^ /
' ^ ^
--<
^
0
O = OH ion above the plane of paper O = OH ion below the plane of paper • = Al(lIl)ion in the plane of paper
Fig.III.1. An idealized A 1 ( 0 H ) sandwich with trigonal and ortho-rhombic unit cells
The principal difference between the crystal structures of the common aluminium trihydroxides is the packing of the sandwiches.
Gibbsite (hydrargillite)
The packing of the sandwiches in the gibbsite structure is peculiar. In the Idealized structure the sandwiches are placed exactly above each other. The resulting packing sequence of the anion sublattice is ABBAABBA . In the real monoclinic gibbsite structure the sandwiches are shifted slightly in the direction of the a-axis with respect to each other (3). In some natural triclinic gibbsite crystals this shift has a component along the a-axis as well as the b-axis (1+).
For a projection of the idealized gibbsite structure see fig. III.2a and fig.III.3.a
O — • B
F i g . I I I . 2 a g i b b s i t e F i g . I I I . 2 b b a y e r i t eB
WMJ^^MM
A
B
F i g . I I I . 2c n o r d s t r a n d i t eFig.III.2
(1120) projection of the idealized crystal structure of gibbsite (a), bayerite (b) and nordstrandite (c).
Key to the symbols in the (1120) projections: O • : octahedrally coordinated cation n • : tetrahedrally coordinated cation O # : anion (oxygen)
• : proton
The open symbols represent Ions at equal distances above and below the (1120) plane. The blackened symbols represent ions within the (1120) plane. The protons are at the same height as the neighbouring oxygen ions.
F i g . I I I . 3 a gibbsite
Fig.III.3b bayerite
where Q = 0H~ ion above loTo plane
% = OH ion below loTo plane
• = Al(lIl)lon in lOTo plane
F i g . I I I . 3
1010 projection of the idealized c r y s t a l structures of gibbsite
(a) and bayerite ( b ) .
The position of the protons in the gibbsite structure has been questionable for years. In 1935 Bernal and Megaw (3) made a proposal for the proton positions. Short 0-0 distances were an Indication for the proton positions if these two oxygen ions did not form the edge of
a filled octahedron. In accordance with this argument the protons are placed in trigonal prisms formed by oxygen ions of two adjacent sand-wiches. These trigonal prisms share faces with the empty octahedra of two adjacent sandwiches. The monoclinic distortion of gibbsite should be the result of the distribution of six protons over nine available
sites (edges of the trigonal prism) (5). Each oxygen ion is a hydrogen donor due to its electrostatic valence. The resulting proton
arrangement is represented in fig. III.l*.
Fig.III.1*
Proton positions in a trigonal prism of oxygen ions (gibbsite)
We noticed that electrostatic arguments give rise to similar proton positions without using the argument of short oxygen oxygen distances. In the proposed arrangement the oxygen coordination was required to be isonomous and the proton proton distances should be maximal.
However the relative orientations of these trigonal prisms,constitu-ted by hydroxyl groups,within the same layer and in different layers are hardly predictable using only proton proton distances.
Kroon and v.d. Stolpe (6) proposed a different proton arrangement. Their results of a proton magnetic resonance study were not consistent with the model of Bernal and Megaw (3). Kroon and v.d. Stolpe assumed
that all protons are located between the sandwiches. All O-H bonds are directed from the donor oxygen to the next nearest oxygen neighbour (acceptor) in the adjacent sandwich. Two regular proton arrangements are consistent with the experimental results. Wei and Maeland (7) confirmed the model of Kroon and v.d. Stolpe in a deuteron magnetic resonance study.
Recently Saarfeld and Wedde (8) refined the crystal structure of gibb-site by X-ray diffraction. The resulting proton positions were in remarkable good agreement with the proton positions in the model of Bernal and Megaw ( 3 ) .
Bayerite
Single crystals of bayerite appropriate to single crystal X-ray diffraction are not available. This feature makes the structure deter-mination of bayerite cumbersome.
In some early studies (9slO) the space group of bayerite was given as trigonal. Unmack (ll) and Lippens (12) suggested respectively mono-clinic and orthorhombic symmetry. The most extensive structure deter-mination was performed by Rothbauer e.a. (13) in a combined X-ray and neutron diffraction study. These authors concluded to a monoclinic spacegroup. In contrast to the head-to-tail arrangement of the anions in the gibbsite structure now the oxygen ions of one layer are located above the hollows in the next sandwich. The anion layer sequence is ABABABA (h.c.c.). The idealized crystal structure of bayerite is represented in fig.III.2 and III.3 .
The positions of the protons in the bayerite structure is uncertain. Rothbauer e.a. (13) proposed two sets of proton positions but stated that single crystals are necessary for a more reliable determination.
Baur (ll*) used the structure refinement of Rothbauer e.a. to predict the proton positions on the basis of electrostatic principles. In a calculation Baur minimized the energy of a point charge system by variation of the proton positions with fixed alimiinium and oxygen coordinates. In this way Baur obtained two sets of proton positions
which are equivalent within his scheme of assumptions. Both models are fairly complicated containing different poly furcated hydrogen bonding arrangements. Baur suggests that a statistical distribution of the pro-tons over both sets could cause the problems encountered by Rothbauer e.a. (13) in their neutron diffraction study.
We would propose a model where the protons are located on the edges of an octahedron sharing faces with the empty octahedra of two adjacent sandwiches. Each oxygen ion is a hydrogen donor. See fig. III.5.
Fig.III.5
Proton position within an octahedron of oxygen ions (bayerite).
>H
The relative orientation of these octahedra constituted by the hydroxyl ions within the same layer results in an orthorhombic struc-ture model. On the basis of anion coordination and proton proton distances the trigonal arrangement was slightly unfavourable as com-pared with the orthorhombic model. Madelung calculations confirmed the slight energy difference between the two models (15).
The proton positions of our models are listed in table III.l:
Space group Cellparameters Cry stenographic positions (l*c): x,y,z; x,y, 1/2 - X, 1/ ''The 0-0 distance P3 (C^.) a= x/3* c=2/3.x./6 = l*.68l =1*.1*1A M(2d) 1/3 2/3 0 0(6g) 2/3 0 1/1* H(6g) 2/9 1/3 5/12 z; 1/2 + X, 1/2 - y,z; 2 + y,z X is 'x- 2.7A P 2^/c a=2/3x/6 b=3x c=x/3 6=90° Al(l*c) 0 1/6 1/2 0^(l*c) 1/1* 0 2/3 OgC+c) 3/1* 1/6 5/6 0^(1*0) 1/1* 1/3 2/3 H^(l*c) 5/12 1/18 13/18 H2(l+c) 7/12 1/6 17/18 H^(l+c) 5/12 7/18 13/18
Table III.l Crystallographlc positions in the predicted bayerite structure
N o r d s t r a n d i t e
The e x i s t e n c e of an independent n o r d s t r a n d i t e s t r u c t u r e has been doubtful for y e a r s . Ginsberg e . a ( l 6 ) suggested t h a t n o r d s t r a n d i t e c r y s t a l l i z e d with a d i s t o r t e d b a y e r i t e s t r u c t u r e . Lippens (12) assumed t h a t n o r d s t r a n d i t e had a c r y s t a l s t r u c t u r e c l o s e l y r e l a t e d t o b a y e r i t e and g i b b s i t e . The anion l a y e r sequence of n o r d s t r a n d i t e was a mixture of t h e known sequences of b a y e r i t e and g i b b s i t e : ABABBABAA . S a a r f e l d e . a . (17,18) determined t h e c r y s t a l s t r u c t u r e of n o r d s t r a n -d i t e using a n a t u r a l s i n g l e c r y s t a l . The p e c u l i a r h e a -d - t o - t a l l arrangement of t h e sandwiches in t h e g i b b s i t e s t r u c t u r e was r e a l i z e d in t h e n o r d s t r a n d i t e s t r u c t u r e as w e l l . In an improper n o t a t i o n S a a r f e l d e . a . gave t h e following anion l a y e r sequence: ABABAB . However, t h e l a y e r s are s h i f t e d with r e s p e c t t o each o t h e r . A more consequent n o t a t i o n i s : ABBCCAABBCCA . For a p r o j e c t i o n of t h e n o r d -s t r a n d i t e -s t r u c t u r e -see f i g , I I I . 2 c .
Bosmans (19) suggested t h e same s t r u c t u r e on t h e b a s i s of a power X-ray d i f f r a c t i o n s t u d y .
For t h e proton p o s i t i o n s in n o r d s t r a n d i t e no d e t a i l e d models have been proposed. Bosmans s t a t e d t h a t t h e hydrogen bonding i n n o r d s t r a n d i t e i s l i k e t h a t in g i b b s i t e . However t h e aluminium ions in adjacent sand-wiches a r e s h i f t e d with r e s p e c t t o each o t h e r . The i s o l a t e d t r i g o n a l prisms of t h e g i b b s i t e s t r u c t u r e a r e not p r e s e n t in t h e n o r d s t r a n d i t e s t r u c t u r e .
A d e t a i l e d p r e d i c t i o n of t h e proton p o s i t i o n s in n o r d s t r a n d i t e on t h e b a s i s of anion c o o r d i n a t i o n and proton p r o t o n d i s t a n c e s was not pursued because of t h e l a r g e number of s t r u c t u r e models.
Al(OH)^ I I
Baneeva and Bendelianl (20) claimed t h e s y n t h e s i s of a new aluminium t r i h y d r o x i d e m o d i f i c a t i o n . They named t h i s phase A l ( O H ) ^ . I I . The hydroxide was p r e p a r e d under hydrothermal c o n d i t i o n s . The X-ray d i f f r a c t i o n p a t t e r n was c l o s e l y r e l a t e d t o t h a t of I n ( O H ) . . The
This c r y s t a l s t r u c t u r e d i f f e r s p r i n c i p a l l y from t h e t h r e e proceeding aluminium t r i h y d r o x i d e l a y e r s t r u c t u r e s . The cubic In(OH) s t r u c t u r e i s r e l a t e d t o t h e ReO s t r u c t u r e . In t h e In(OH)^ s t r u c t u r e t h e o c t a h e d r a a r e canted t o allow hydrogen bonding. The r e s u l t i n g s t r u c t u r e i s a t h r e e dimensional network of corner s h a r i n g o c t a h e d r a . See f i g . I I I . 6 .
F i g . I I I . 6 The i d e a l i z e d c r y s t a l s t r u c t u r e of In(OH)
SC(OH) and Ga(OH) are i s o s t r u c t u r a l with In(OH) .
Iron and chromium t r i h y d r o x i d e s
The c r y s t a l l i n e F e ( l l l ) and C r ( l l l ) t r i h y d r o x i d e s a r e not known. Giovanoll e . a . (22,23) observed a c r y s t a l l i n e C r ( l l l ) compound: Cr(OH)_(H O) . The s t r u c t u r e of t h i s compound c o n t a i n s i s o l a t e d
Cr(OH) (HpO) o c t a h e d r a . The s t r u c t u r e i s d e s c r i b e d as " a n t l - b a y e r i t e " . Within t h e sandwich of b a y e r i t e t h e c a t i o n s and vacancies a r e i n t e r -changed. In t h e " a n t l - b a y e r i t e " l a t t i c e only one t h i r d of t h e o c t a h e d r a w i t h i n a sandwich I s f i l l e d . The type of order of t h e vacancies i n b a y e r i t e i s i d e n t i c a l t o t h a t of t h e c a t i o n s in an " a n t l - b a y e r i t e " sandwich (honey-comb p a t t e r n ) .
I l l . 2 Aluminium oxidehydroxides and some r e l a t e d oxide hydroxldes(MOOH)
In t h e well-known aluminium oxidehydroxides boehmite and d i a s p o r e t h e A l ( l I l ) i o n i s o c t a h e d r a l l y c o o r d i n a t e d . In t h e aluminium t r i h y d r o x i d e l a y e r s t r u c t u r e s t h e o c t a h e d r a share edges t o form a two
however the octahedra share edges to form straight chains.
Diaspore (g-AlOOH)
The c r y s t a l s t r u c t u r e of diaspore has been determined by X-ray (2l*) and neutron d i f f r a c t i o n ( 2 5 ) . The anions in t h e diaspore s t r u c t u r e c o n s t i t u t e a hexagonal c l o s e - p a c k i n g .
The aluminium o c t a h e d r a share edges t o form s t r a i g h t chains and t h e s e chains couple t o double chains by a d d i t i o n a l edge s h a r i n g . A t h r e e dimensional s t r u c t u r e i s formed by corner sharing of t h e s e double c h a i n s . See f i g . I I I . 7 a .
a) h
where O = oxygen ion shared by two A l ( l I l ) i o n s @ = oxygen ion shared by t h r e e A l ( l I l ) i o n s Heavy l i n e s r e p r e s e n t edge s h a r i n g of two octahedra
F i g . I I I . 7 The s h a r i n g octahedra of d i a s p o r e (a) and boehmite (b)
The i d a l i z e d d i a s p o r e s t r u c t u r e i s r e p r e s e n t e d in f i g . I I I . 8 .
The proton p o s i t i o n s in t h e d i a s p o r e s t r u c t u r e a r e well determined (25). The hydrogen bond O-H 0 i s not l i n e a r . The r e s u l t s of t h e proton magnetic resonance study of Kroon and v . d . Stolpe (6) and t h e deuteron magnetic resonance study of Wei and Maeland (7) a r e c o n s i s t e n t with t h e former s t r u c t u r e d e t e r m i n a t i o n ( 2 5 ) .
The diaspore s t r u c t u r e i s observed for many oxidehydroxides e . g . g o e t h i t e (a-FeOOH), g r o u t i t e (a-MnOOH), m o n t r o s e i t e (VOOH), GaOOH, ScOOH and CoOOH ( 2 6 ) .
A B B F i g . I I I . 8 (1120) p r o j e c t i o n of t h e i d e a l i z e d s t r u c t u r e of d i a s p o r e F i g . I I I . 9 (1120) p r o j e c t i o n of the i d e a l i z e d s t r u c t u r e of boehmite F i g . I I I . 1 0 (1120) p r o j e c t i o n of t h e I d e a l i z e d s t r u c t u r e of FeOCl
Boehmite ( Y - A 1 0 0 H )
The boehmite s t r u c t u r e c o n t a i n s double c h a i n s of edge sharing o c t a -hedra l i k e in t h e diaspore s t r u c t u r e . The coupling of t h e s e double chains i s d i f f e r e n t for boehmite as compared with d i a s p o r e (see f i g . I I I . 7 . ' b ) . A p r o j e c t i o n of t h e I d e a l i z e d c r y s t a l s t r u c t u r e of boehmite i s r e p r e s e n t e d i n f i g . I I I . 9 I t should be noted t h a t t h e anion s u b -l a t t i c e does not c o n s t i t u t e a c -l o s e - p a c k i n g (2-l+,27)28).
A p e c u l i a r h e a d - t o - t a i l arrangement in t h e anion s u b l a t t i c e i s p r e s e n t .
The FeOCl s t r u c t u r e i s c l o s e l y r e l a t e d t o t h e boehmite s t r u c t u r e . The coupling scheme of octahedra i s I d e n t i c a l ( s e e f i g . I I I 7 . b ) . In t h e FeOCl s t r u c t u r e t h e p e c u l i a r h e a d - t o - t a i l arrangement however i s missing (see f i g . I I I . 1 0 ) .
Hydrogen bonding should be t h e cause of t h i s s p e c i a l anion arrangement in boehmite. In t h e compound y-FeOOH which i s i s o s t r u c t u r a l with boehmite t h e proton p o s i t i o n s have ^e.n determined by neutron d i f f r a c -t i o n ( 2 9 ) . The pro-tons are p l a c e d be-tween -two adjacen-t oxygen i o n s p a r t i c i p a t i n g i n t h e h e a d - t o - t a l l arrangement.
Besides l e p i d o c h r o s i t e (yFeOOH) a l s o yCrOOH and yScOOH are i s o s t r u c -t u r a l wi-th boehmi-te ( 2 6 ) .
Some a d d i t i o n a l MOOH s t r u c t u r e s , n o t observed in t h e Al 0 -H 0 system, are d e s c r i b e d i n t h e next s e c t i o n . The c a t i o n s should be o c t a h e d r a l l y c o o r d i n a t e d l i k e t h e A l ( l I l ) i o n i n d i a s p o r e and boehmite. Hence t h e r a r e e a r t h compounds (MOOH) a r e beyond t h e scope of t h i s study.
The InOOH s t r u c t u r e
Many MOOH compounds with a c a t i o n r a d i u s l a r g e r than t h e radius of t h e A l ( l l l ) i o n c r y s t a l l i z e with t h e InOOH s t r u c t u r e . In a combined X-ray and neutron d i f f r a c t i o n study Lehmann e . a . (30) determined t h e c r y s t a l s t r u c t u r e of InOOH. The anions c o n s t i t u t e a hexagonal c l o s e - p a c k i n g . The J n ( l I l ) i o n s occupy h a l f of t h e i n t e r s t i t i a l o c t a h e d r a l s i t e s . The
I n ( l l l ) o c t a h e d r a are coupled t o s t r a i g h t chains by edge sharing and t h e chains are coupled by s i n g l e and double corner s h a r i n g . The s t r u c -t u r e i s of -t h e r u -t i l e -type (TlOp). A p r o j e c -t i o n of -t h e i d e a l i z e d
structure is represented in fig. III.11.
Fig.III.11 (1120) projection of the idealized structure of InOOH
In a compilation of the literature Chevanas e.a. (26) record that NIOOH, CrOOH, FeOOH, MnOOH, RhOOH and ScOOH crystallize under special conditions with the InOOH structure.
The rhombohedral CrOOH structure
The present structure has an anion layer sequence identical to that of nordstrandite: ABBCCAABBCCA. In contrast to the nordstrandite struc-ture no cation vacancies are present within the sandwich. So each Cr(lIl)ion shares edges with six neighbouring octahedra. Hydrogen bon-ding occurs between the sandwiches. Hamilton and Ibers (31) refined the crystal structure of rhombohedral CrOOH by neutron diffraction. The hydrogen bond between the adjacent sandwiches appeared to be
symmetric.
CoOOH and NIOOH (not reproducible) are isostructural with rhombohedral CrOOH (26).
The acaganite structure (g-FeOOH)
Acaganite is isostructural with the mineral hollandite (a-MnO ). The same double chains of edge sharing octahedra like in diaspore and boehmite are present. The double chains are linked by corner sharing
F i g . I I I . 1 2 (l12u) p r o j e c t i o n of t h e i d e a l i z e d BFeOOH s t r u c t u r e
The presence of i m p u r i t i e s ( C l - i o n s ) i s e s s e n t i a l t o s t a b i l i z e t h i s open s t r u c t u r e ( 3 2 ) .
The c r y s t a l s t r u c t u r e of {-FeOOH i s d e s c r i b e d as a d i s o r d e r e d b r u c i t e ( Mg(OH)p) s t r u c t u r e where some t e t r a h e d r a l s i t e s are occupied ''33). There a r e some s e r i o u s o b j e c t i o n s a g a i n s t t h i s s t r u c t u r e model ( 2 6 ) .
For a more d e t a i l e d t r e a t i s e of t h e c r y s t a l chemistry of MX and MXX compounds t h e r e a d e r i s r e f e r r e d t o t h e study of A.B.A. Schlppers (3l+).
I I I . 3 Aluminium oxides and some r e l a t e d oxides (M 0 )
The c o o r d i n a t i o n number of t h e A l ( l I l ) i o n s i n oxides can be 1+ or 6 and in some s p e c i a l cases even 5- During t h e thermal decomposition of aluminium hydroxides many m e t a s t a b l e oxides a r e found. Within a c l o s e packed anion s u b l a t t i c e t h e A l ( l I l ) i o n s are r e d i s t r i b u t e d over t h e a v a i l a b l e i n t e r s t i t i a l o c t a h e d r a l and t e t r a h e d r a l s i t e s . The c a l c i n a -t i o n sequences of -t h e various aluminium hydroxides are r e p r e s e n -t e d
s c h e m a t i c a l l y by Wefers and Bel] (35) in a block-diagram. See f i g . I I I . 1 3 .
The thermal decomposition of t h e aluminixim t r i h y d r o x i d e s could proceed v i a t h e aliminiimi oxidehydroxide ( p a t h a) or without i n t e r m e d i a t e hydroxide phase i n a d i r e c t conversion t o aluminium oxide (path b ) . The c o n d i t i o n s favouring path a or b are emunerated In t a b ] f i I I I . 2 .
|GIBBSITE a b
1
BOEHMITE a BAYERITE b J CHI ETA j DIASPORE 1 1 J KAPPA ALPHAGAMMA DELTA rHETAJALPHAJ
THETA ALPHA! ALPHA 1 1 1 1 1 1 I 1 0 100 200 300 400 500 600 700 800 900 1000 MOO 1200 TEMPERATURE, °C F i g . I I I . 13
Thermal dehydration sequence of alimiinium hydroxide
Table I I I . 2 Conditions p r e s s u r e atmosphere p a r t i c l e s i z e h e a t i n g r a t e p a t h a > 1 atm. moist a i r > 100 microns < 1°C/min. path b 1 atm. dry a i r < 10 microns > l°C/min. Lippens (12) c l a s s i f i e d t h e m e t a s t a b l e aluminas in low t e m p e r a t u r e (p,K,n and y) and high t e m p e r a t u r e aluminas ( 6 , 6 , x ) . We w i l l follow t h i s c l a s s i f i c a t i o n .
The low temperature aluminas ( p , x , n and y-Al 0 )
During t h e c a l c i n a t i o n of g i b b s i t e under high vacuum a t about 300 C, p-AlpO i s formed. This compound i s n e a r l y completely X-ray amorphous. When t h e c a l c i n a t i o n i s performed at atmosphere p r e s s u r e x-Al 0 i s formed. The anions i n x-AlpO c o n s t i t u t e a close-packed s u b l a t t i c e . The packing sequence of t h e s e close-packed l a y e r s i s h i g h l y d i s o r d e r e d
(1+). The p o s i t i o n s of t h e c a t i o n s a r e unknown.
The c r y s t a l s t r u c t u r e s of y-Al^O and n-Al 0 are c l o s e l y r e l a t e d t o t h e s p i n e l s t r u c t u r e . The oxygen i o n s i n t h e mineral s p i n e l (MgAl Oj^) c o n s t i t u t e a cubic c l o s e p a c k i n g . In a normal s p i n e l t h e A l ( l I l ) i o n s a r e o c t a h e d r a l l y c o o r d i n a t e d and t h e M g ( l l ) i o n s occupy t e t r a h e d r a l s i t e s . See f i g . III.1I+.
Fig. III.1I+
(1120 ) projection of the idealized spinel structure.
In an Inverse spinel the trivalent cations are equally distributed over the octahedral and tetrahedral sites and the divalent ion is exclusively octahedrally coordinated. When the natural spinel (MgAl^Oi^) is heated to high temperature (about 1000°C) a disorder in the cation distribution is observed.
For n-AlpO and y-Al 0 the question normal versus invers is not relevant. These compounds are cation deficient spinel. The vacancies in the cation sublattices may be octahedral or tetrahedral interstices
(36). Saarfeld and Mehrotra (37) suggest that the tetrahedral sub-lattice contains the vacancies.
The compounds y-Al 0 and n-AlpO are very similar. The main difference between these two cation deficient spinels is the lower order in the anion lattice of n-AlpO (36). The n-Al 0 crystallites contain numerous pores. This high density of defects could account for the higher catalytic activity of n-Al 0 compared with y-Al 0 .
The high temperature aluminas (6,6,and ic-Al 0 )
The quality of the X-ray diffraction patterns of the high temperature aluminas is much better than that of the low temperature aluminas. The small amount of residual water present in the low temperature aluminas is expelled at higher temperatures.
The formation of (S-AlpO^ is restricted to the dehydration of well crys-tallized boehmite (36). The fairly regular anion sublattice of y-AlpO_ is essential in the formation process of 6-Al 0 . Like n-Al 0 and y-Al 0 also 6-Al 0 is a cation deficient splnel.However, in 6-Al 0 the vacancies are ordered along a fourfold screw-axis in the c-dlrec-tlon. In n-AlpO the numerous pores can prever
vacancies hindering the cation diffusion (36).
tion. In n-AlpO the numerous pores can prevent the ordering of the
The well crystallized 9-Al 0 is isostructural with B-Ga 0 . Single crystals of B-Ga 0 were of great help for the structure determination of 6-Al 0 (1*). The anions in g-GapO constitute a deformed cubic close-packing. The cations occupy octahedral as well as tetrahedral sites. The octahedra are coupled by edge sharing to double chains as in diaspore and boehmite. The double chains are coupled by tetrahedra in a three dimensional network (38). A projection of the idealized structure of g-Ga 0 ( e-Al 0 ) is represented in fig.III.I5.
The metastable K - A 1 0 is formed at higher temperatures in the dehydration sequence of gibbsite. The detailed structure of K-Al 0 is unknown. Saarfeld (U) suggested a packing sequence of close-packed anion layers': ABABBABA—. The cations are distributed over the inter-stitial sites. The cation sublattice has a layer structure parallel to the close-packed anion layers. The cation in adjacent layers have alternatingly an octahedral or tetrahedral coordination.
