1 Synthesis and characterisation of hydrotalcites of formula 2 Ca6Al2(CO3)(OH)16·4H2O 3 4 Ray L. Frost, Sara J. Palmer and Frederick Theiss 5 6 1 Inorganic Materials Research Program, School of Physical and Chemical Sciences, 7 Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, 8 Australia. 9 10 ABSTRACT 11 12 Hydrotalcites containing calcium are a naturally occurring mineral which has been 13 successfully synthesised. Insight into the unique structure of hydrotalcites has been obtained 14 using a combination of X-ray diffraction, infrared and Raman spectroscopy. Calcium 15 containing hydrotalcites of formula Ca4Al2(CO3)(OH)12·4H2O (2:1 Ca-HT) to 16 Ca8Al2(CO3)(OH)18·4H2O (4:1 Ga-HT) have been successfully synthesised and characterised 17 by X-ray diffraction and Raman spectroscopy. The d(003) spacing varied from 7.83 Å for 18 the 2:1 hydrotalcite to 7.89 Å for the 3:1 calcium containing hydrotalcite. Raman 19 spectroscopy complimented with selected infrared data has been used to characterise the 20 synthesised calcium containing hydrotalcites of formula Ca6Al2(CO3)(OH)16·4H2O. Raman -1 21 bands observed at around 1083, 1085 and 1087cm were attributed to the ν1 symmetric 2- 2- 22 stretching modes of the (CO3 ) units. Multiple ν3 CO3 antisymmetric stretching modes are 23 found at around 1406 and 1473cm-1. The splitting of this mode by ~67 cm-1 indicates the 24 carbonate anion is in a strongly perturbed state. Raman bands observed at 711 and 712 cm-1 2- 25 assigned to the ν4 (CO3 ) modes support the concept of multiple carbonate species in the 26 interlayer. 27 28 29 Keywords: hydrotalcite synthesis, hydrocalumite, hydrocalumite, Raman spectroscopy, 30 calcium, thermal stability Author to whom correspondence should be addressed ([email protected]) 1 31 Introduction 32 33 Hydrotalcites or layered double hydroxides (LDHs), have been known for an 34 extended period of time. [1-3] Hydrotalcites, are fundamentally known as anionic clays [4]. 35 Hydrotalcites consist of stacked layers of metal cations (M2+ and M3+) similar to brucite 36 (Mg(OH)2). The structure of hydrotalcite can be derived from a brucite structure (Mg(OH)2) 37 in which e.g. Al3+ or Fe3+ (pyroaurite-sjögrenite) substitutes a part of the Mg2+ [2, 5-7]. This 38 substitution creates a positive layer charge on the hydroxide layers, which is compensated by 39 interlayer anions or anionic complexes. In general any divalent cation including calcium 40 could substitute for the Mg in the brucite-like layer. Equally as well, any trivalent cation may 41 substitute for aluminium in the brucite layer. In hydrotalcites, a broad range of compositions 2+ 3+ 2+ 3+ 42 are possible of the type [M 1-xM x(OH)2] x/n.yH2O, where M and M are the di- and 43 trivalent cations in the octahedral positions within the hydroxide layers with x normally 44 between 0.17 and 0.33. An- is an exchangeable interlayer anion. [8] The positively charged 45 hydroxyl layers are neutralised through the intercalation and adsorption of anionic 46 species, therefore stabilising the structure. Anions that are intercalated between the 47 hydroxyl layers need to meet certain criteria, including having a high charge density and 48 small anionic radius. 49 50 To the best of the authors’ knowledge, no comprehensive studies of hydrotalcites with 51 the replacement of the magnesium by calcium have been reported. There is some evidence 52 that in bauxite, calcium is found as a minor impurity as calcium hydroxide [9-11]. The 53 reaction of red mud and seawater results in the formation of hydrotalcites based not only 54 upon magnesium but also calcium. This is the basis of the underlying reason why this 55 research is being undertaken. This study focusses upon the synthesis, and spectroscopic 56 characterisation of hydrotalcites with calcium substituting for magnesium in the brucite 57 layer. 58 59 Experimental 60 61 Synthesis of hydrotalcite samples 62 Co-precipitation is probably the best technique for the synthesis of hydrotalcites, as it 63 allows homogeneous precursors as starting materials. For co-precipitation it is necessary to 64 work under conditions of supersaturation mostly achieved by variation in pH [12, 13]. Two 2 65 frequently used techniques are coprecipitation at low [14-17] and at high supersaturation [18- 2+ 66 20]. In this study we used the latter route for the preparation of hydrotalcites with Ca 3+ 2+ 3+ 2- 67 combined with Al in a molar ratio M /M of 6/2 and CO3 as charge balancing anion. 68 69 Hydrotalcites can be synthesised in the laboratory using analytical grade chemicals. 70 The hydrotalcites were synthesised by the co-precipitation method. Two solutions were 2+ 71 prepared, solution 1 contained 2M NaOH and 0.2 M Na2CO3, and solution 2 contained Ca 3+ 72 as CaCl2 at different concentrations, together with Al (AlCl3.6H2O). Solution 2 was added 73 at a steady rate to solution 1 drop wise, under vigorous stirring. A separating funnel was used 74 to deliver solution 2 to solution 1. The precipitated minerals were washed at ambient 75 temperatures thoroughly with ultra pure water to remove any residual salts and dried in an 76 oven (85 °C overnight. 77 2+ 78 M is based upon CaCl2 : 2:1 2.5:1 3:1 3.5:1 4:1 Concentration of CaCl2 0.67M 0.71M 0.75M 0.77M 0.80M Masses of CaCl2 3.72g 3.94g 4.16g 4.27g 4.44g Concentration of AlCl3.6H2O 0.33M 0.29M 0.25M 0.22M 0.20M Masses of AlCl3.6H2O 3.98g 3.45g 3.02g 2.68g 2.41g 79 80 Table 1 Table of concentrations for the synthesis of calcium hydrotalcites 81 82 Raman spectroscopy 83 84 FT-Raman spectra were recorded on a Perkin-Elmer system 2000 FT-Raman spectrometer 85 (Perkin-Elmer, Beaconsfield, UK) equipped with a quartz beam splitter and an InGaAs 86 detector at ambient temperature. One hundred scans at a resolution of 4 cm-1 were 87 accumulated for each sample using a mirror velocity was 0.1 cms-1 and strong Beer–Norton 88 apodization. Laser excitation was provided by an Optomech continuous wave Nd:YAG laser 89 emitting at 1064 nm. Laser power was 200 mW. 90 91 Infrared spectra (over the 4000-525 cm-1 range) were obtained using a Nicolet Nexus 92 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectral 3 93 manipulation such as baseline correction/adjustment and smoothing were performed using the 94 Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band 95 component analysis was undertaken using the Jandel ‘Peakfit’ software package that enabled 96 the type of fitting function to be selected and allows specific parameters to be fixed or varied 97 accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product function with 98 the minimum number of component bands used for the fitting process. The Gaussian- 99 Lorentzian ratio was maintained at values greater than 0.7 and fitting was undertaken until 100 reproducible results were obtained with squared correlations of r2 greater than 0.995. 101 102 Results and discussion 103 104 X-ray diffraction 105 The XRD patterns of the synthesised hydrotalcites containing Ca with varying Ca/Al 106 ratios are displayed in Fig. 1. The (00l) reflections (003), (006) and (009) are easily 107 recognised, although the last one shows overlap with the (012) and (015) reflections resulting 108 in a broad signal between 40 and 45° 2θ. Furthermore, the two reflections of (110) and (113) 109 can be clearly distinguished between 70 and 75° 2θ. The (00l) reflections are characterised by 110 high intensities combined with broad line shapes indicating that the hydrotalcites are of 111 relatively high crystallinity but with very small crystallites. No other crystalline phases can be 112 detected in the XRD patterns indicating that all three syntheses were successful. The position 113 of the d(003) peak varies from 7.85Å for the 4:1 (Ca/Al) hydrotalcite to 7.89Å for the 3:1 114 HT. The width of the d(003) peak is constant; thus showing the crystallinity of the HT does 115 not vary with the divalent/trivalent ratio. Fig. 1 shows the possible impurities in the HTs. 116 Hydrocalumite Ca2Al(OH)6Cl·2H2O is apparently present in all of the synthesised samples, 117 even though it is in very low concentrations. 118 119 Raman and Infrared Spectroscopy 120 121 The calcium containing hydrotalcite (Ca-HT) of 3:1 ratio has a formula 122 Ca6Al2(CO3)(OH)16·4H2O. In the synthesis of the Ca-HT, the ratio of Ca to Al was designed 123 to vary from 4:1 to 2:1. This means the formula will vary from Ca8Al2(CO3)(OH)18·4H2O to 124 Ca4Al2(CO3)(OH)12·4H2O. Irrespective of the formula, each hydrotalcite contains vibrating 125 units of the carbonate anion, the surface hydroxyl units and water. The summation of the 126 Raman spectra of these vibrating units will be the Raman spectrum of the HT. In addition 4 127 metal-oxygen units will contribute to the Raman spectra. The complete Raman spectra of all 128 of the synthesised Ca-HTs are displayed in Figure 2. This spectrum shows the relative 129 intensity of the different bands. The Raman spectra in the 1000-1100 cm-1 region for the 130 synthesised hydrotalcites are displayed in Figure 3. The band centered upon 1085 cm-1 is not 131 symmetric and component bands may be resolved.