THE SYNTHESIS OF ALUMINIUM HYDROXIDE AND OXYHYDROXIDE by Linda-Ann Louw A thesis submittedsubmitted. for the degree of Master of Science in Applied Science to the Faculty of Engineering at the University of Cape Town. University of Cape Town Department of Civil Engineering University of Cape Town March 1993 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town THE SYNTHESIS OF ALUMINIUM HYDROXIDE AND OXYHYDROXIDE POLYMORPHS by Linda-ADDLinda-Ann Louw Department of Civil Engineering University of Cape Town January 1993 I'I. Beta alumina is a ceramic with the ability to conduct certain ions. It is conventionally t-formed........n&>L'll through the high temperature solid state reaction of a-alumina with soda and lithia, which results in a mixture of S-6- and 6"-aluminaS"-alumina phases. To maximise the amount of SrI-alumina6 "-alumina formed, a second heat treatment step is required. Beta alumina may also be synthesised directly from the various aluminium hydroxide ( Al(OH)JAI(OH)] ) and aluminium oxyhydroxide (AIOOH)(AlOOR) polymorphs. The particular oxygen sublatticesub lattice of the starting material is critical in determining which phases of beta alumina are formed. Boehmite and bayerite are known to yield pure 8"B" -alumina. In this investigation, the differences in structure and the identification of the polymorphs is discussed. A review of the methods of synthesis and concomitantCOlllcollmtant mechanisms of reaction proposed in the literature are presented. Of these, the synthesis of boehmite and bayerite are investigated experimentally. Attempts to synthesise boehmite at ambient conditions resulted in the formation of pseudoboehmite, and confirmed that boehmite can only be formed by the introduction . of a hydrothermal step. ,However,However, it was possible to form bayerite at amibleJntambient conditions hyby sµnple~le chemical reaction. Control of the reaction pH and the removal of inhibiting ions were found to facilitate the formation of well crystallised baverite.bayerite. Although the factors governing the successful synthesis of bayerite need further investigation, the possibility of a direct synthesis route for fi6" "-alumina -alumina has been demonstrated.University of Cape Town J) J Finally, the synthesis of S6 .."-alumina -alumina by the solid state reaction of the hydroxide and OX'1fDyarCJIXI<lleoxyhydroxide .....polymorphs "'h,'m""1''nh~ was successfully completed. TABLE OF CONTENTS LINTRODUCTIONI .INTRODUCTION 1 2.THE2. THE ALUMINIUM HYDROXIDE AND OXYHYDROXIDE PRECURSORS 3 2.11 NOMENCLATURE 3 2.2 STRUCTURE OF tHETHE ALUMINIUM HYDROXIDES 4 2.2.1 Gibbsite 4 2.2.2 Bayerite 6 2.2.3 Nordstrandite 7 2.3 STRUCTURE OF THE OXIDE HYDROXIDES 7 2.3.11 Diaspore •7# 7 2.3.2 Boehmite 8 2.3.32.3 .3 Pseudoboehmite 9 2.4 METHODS OF IDENTIFICATION 10 2.4.11 XX-ray-ray Diffraction 10 2.4.2 Thermal Analysis 12 2.4.3 Other Methods 16 3.MECHANISMS OF FORMATION OF ALUMINIUM HYDROXIDES AND OXYHYDROXIDES 17 3 .11 Review of synthesis procedures 17 3.1.13 .1.1 Aluminium hydroxides 17 3.1.21.2 Aluminium oxide hydroxides 19 3.2 Mechanisms of formation 21 3.2.1 Formation of the structural units 21 3.2.2 Formation of the Hydroxides 23 3.2.3 Formation of the Oxyhydroxides 32 3.2.4 Discussion of the proposed me::l1aIrnSIlrlSmechanisms 35 3.2.5 Summary 37 4.4.SYNTHESIS SYNTHESIS OF BOEHMITE AND BA YERITE 39 4.1 THE SYNTHESIS OF BOEHMITE 39 4.1.1 Effect of reaction temperature 39 4.4.1.2 Effect of ageing 41 4.14.1.3 Discussion 42 4.2. THE SYNTHESIS OF BAYERITEBA YERITE 43 4.2.1 Effect of final pH 43 University4.2.2 Effect of of washing Cape Town 43 4.2.3 Effect of ageing 44 4.3 RESULTS 44 4.3.1 Changes in pH 44 4.3.2 Effect of final pH 44 4.3.3 Effect of washing 46 4.3.4 Effect of'of ageing 46 4.4 DISCUSSION AND CONCONCLUSIONS SIONS 48 5.SYNTHESIS OF 8"-ALUMINA6"-ALUMINA 50 5.15 .1 Synthesis 50 5 .1.1 Composition 50 5.15 .1.2 Powder preparation 50 5.1.31.3 Solid state reaction 51 5 .1.1 4 Density determination 51 55.1.5 .1.5 X-ray determination 51 5~2 RESULTS 56 : 5 .2 .11 Observations 56 5.2.2 Density 56 5.2.3 X-ray Diffraction Analysis 59 i) Boehmite precursor 59 ii) Bayerite precursor 59 5.35 .3 DISCUSSION 64 6.6.CONCLUSIONS CONCLUSIONS 65 REFERENCES 67 University of Cape Town TABLE OF FIGURES CHAPTERCHAPTERl 1 Fig.l1 Section through the idealised structure of 2 S-AI.6-A120 3 and S"-AI.6"-A120 3 showing the stacking sequence along the c-axis. CHAPTER2: Fig. 2.11 Hydroxyl bonding betwee~ sp3 hybridised 5 orbitals # Fig. 2.2 ABBA-ABBA stacking in gibbsite 5 Fig. 2.3 Crystal Morphology 6 Fig. 2.4 ABAB-ABAB ",,,,,,,.All,,#;stacking in Bayerite 7 Fig. 2.5 ABBA-BABA stacking in Nordstrandite 7 Fig. 2.6 Schematic representation of ...........OH-Al-0 4 .... chains 8 Fig2.7 Schematic representation of the crystal 9 structure (a) Diaspore (b)boehmite Fig. 2.8 Comparative XRD patterns for aluminium 11 hydroxide and oxyhydroxide polymorphs Fig. 2.9 DTA curves of aluminium hydroxides and 13 oxyhydroxides. University of Cape Town Fig. 2.10 DTADTA curves of NIMR standard samPlessamples 14 CHAPTER3:3: Fig. 3.1 Proposed structure of an hydroxy-aluminium 22 .. polymer having an average positive charge of 1 per AI-atomAl-atom Fig. 3.2 Variation in pH with time. 25 Fig. 3.3 Hydroxyl bridging mechanism of edge-AIedge-Al is 26 hampered by the citrate. Fig. 3.4 Effect of final pH and salt concentration on 27 polymorph formation. Fig.3.5 Effect of OH/ AlAI ratio and salt concentration 28 on polymorph formation. Fig.3.6 Effect of temperature and ageing time on the 28 concentration of solid material, mononuclear ions and polynuclear ions. Fig.3.7 Average degree of polymerization versus 30 OH/AIOH/Al ratio. # Fig.3.8 Mechanism of breakdown of polymers by acid. 31 Fig.3.9 Equilibrium diagram for the system AI2OrH0 3-H20O 33 Fig.3.10 Equilibrium diagram for the system AI20 3-H20O 33 Fig.3.11 Composition seque~ce of aluminium hydroxide 34 and oxyhydroxide polymorphs CHAPfER4:CHAPTER4: Fig.4.1 XRD Spectrum of pseudoboehmite obtained by reaction at 25°C and 11000C.oo<>c. 40 Fig.4.2 XRD spectra of batches aged at 200C and 700C. 41 Fig.4 3 XRD spectra Sample A - Reaction precipitate. Sample B - Reaction precipitate, pH adjusted. 45 Fig.4 4 UniversityXRD spectrum of Sample of C Cape Town Reaction precipitate, pH adjusted 45 Fig.4.5 XRD spectrum of Sample D Reaction precipitate, pH adjusted, washed 46 Fig.4 6 XRD spectrum of Sample E Reaction precipitate, pH adjusted, washed 47 Fig.4.7 XRD spectrum of Sample F Aged 3 days, then as for Sample E. 47 I CHAPTERCHAPTERS: 5: Fig 5.1 XRD pattern of (a) 6"-aluminafi"-alumina and (b) 13-alumina./3-alumina 52 Fig 5.2(a) XRD pattern of standard .6-alumina.fi-alumina. 55 Fig 5.2(b) XRD pattern of standard 13./3'" "-alumina. -alumina. 55 Fig. 5.3 Density vs Time - Timcerm.Timcenn. 57 # ,,' Fig. 5.4 Density vs Time - Cera Hydrate. 57 Fig. 5.5 Density vs Time - Bayerite. 58 Fig. 5.6 ./3"-alumina6"-alumina from boehmite - Cera Hydrate. 60 Fig. 5.7 6"B"-alumina -alumina from boehmite - Timcerm. 61 Fig. 5.8 .B6" "-alumina -alumina from Bayerite - 4 hour soak. 62 Fig. 5.9 66" "-alumina -alumina from Bayerite - 1 hour soak.'soak. 63 Fig. 5.10 Mixed ./3"-6 "- and ./3-alumina.6-alumina from chemically precipitated bayerite 63 University of Cape Town 2 ..• AluminiumAlumini",m of} Sodl",mSodium 0Q 0Oxygen')'\111'1 C A C B C A ,: A' B C C e A A B c' c' UniversityA of Cape Town A B B l3'Aluminot3·Alumina 13"· Alumina Fig.1:1: Section through the idealised structure of S-A16-Al20 3 and S"-A16 n -Al20320 3 showing the stacking sequence along the c-axis. (after Mosely 1985) 3 . 2.THE ALUMINIUM HYDROXIDE AND OXYHYDROXIDE PRECURSORS. 2.1 NOMENCLATURE. Many crystalline compounds of aluminium and oxygen occur in nature. Apart from aluminium oxide (AI(A120 3), known as corundum, there exists a range of aluminium hydroxides and oxide-hydroxides. All occur naturally in soils or in bauxitic deposits, though not to the same extent. The primary motivation for study of these compounds has come from geochemists and soil scientists attempting to formulate a theory of the formation of bauxite deposits and the action of such minerals in soil environments, though more recently the commercial potential of alumina catalysts has provided fresh impetus to research. This chapter will describe the range of polymorphs and the means by which they may be identified. Two main groups of compounds exist: aluminium hydroxides (AI(OH)3)(Al(OH)J) and aluminium oxide hydroxides (AIOOH).(AIOOR). These have in the past been referred to as hydrous oxides or hydrates, implying that the structure contains adsorbed water molecules. In fact, the water is bound as OirOff' ions; consequently the International Committee on Aluminium Hydroxides Nomenclature (ICHAN) (Newsome et al, 1960) recommended that Al(OH)JAI(OHh be referred to as aluminium trihydroxidetribydroxide and AIOOH as aluminium oxide-hydroxide.