3.4.2.1 Structure of the layered double hydroxide crystals
The PXRD patterns of the LDHs synthesised by co-hydration (CoH-LDH 1 to CoH-LDH6) are shown in Figure 3.8. For CoH-LDH2 to CoH-LDH6 characteristic LDH patterns have been recorded with distinctive LDH basal reflections (00l) being present at low 2 angles, indicative of a layered structure, along with broader reflections in the (11l) and (10l) regions, suggesting lower crystallinity.129, 198 However, these samples all show varying levels of impurity as previously reported by Greenwell et al.,123 with significant quantities of brucite (Mg(OH)2) being detected for materials with Mg:Al > 2, on the basis of relative peak intensity within the PXRD patterns in Figure 3.11. The PXRD analysis also suggests there is a degree of unreacted MgO present in CoH-LDH4, CoH-LDH2 and CoH-LDH6. As observed here, Greenwell also reported that the LDHs containing adipate as the charge-balancing interlayer anion showed expanded phases (at 6° 2) for CoH-LDH2 and CoH-LDH3, where the adipate anion is perpendicular to the plane, and collapsed phases present (at 12 ° 2) for CoH-LDH4 to CoH-LDH6, where the adipate anion is parallel to the plane.
In the present work, unlike Greenwell however, synthesis of CoH-LDH1 was unsuccessful, with the PXRD analysis not showing any reflections corresponding to LDH. Owing to the
amorphous nature of the PXRD pattern, CoH-LDH1 was believed to be mainly composed of alumina.169 In this case, the Mg-containing components used in the preparation of this material could possibly have reacted to form soluble Mg adipate and, hence, been lost in the supernatant following washing and filtration. The reason for the difference to the report of Greenwell et al., where successfully synthesised LDHs of R-value 1 are described,123 may in addition, possibly arise owing to the alumina used here being of larger particle size and thus possibly, in retrospect, requiring far longer reaction time.
Figure 3.8: PXRD patterns for the prepared LDHs synthesised by co-hydration. * shows the brucite impurities.
Attempts were made to index the PXRD data collected for CoH-LDH2 to CoH-LDH6 using the DICVOL automatic indexing algorithm (Section 2.2.1),1 however these proved unsuccessful. As CoP-LDH2 and CoP-LDH4 were both successfully indexed as 3R rhombohedral polytypes, it is assumed from these results that the CoH-LDH can be indexed similarly, but the PXRD patterns in these systems did not contain sufficient reflections to allow successful indexing. A summary of the PXRD data for the CoH-LDH samples is shown in Table 3.8. The average cation-cation distance within a layer, a0, calculated from the distinct d110 peak, gives the same value for all CoH-LDH, 3.14 Å. This value cannot be used to calculate the percentage of Al in these systems using the equation put forward by Kaneyoshi and Jones,135, equation (4), as the correlation fit is dependent on
the counter-anion involved being either carbonate or nitrate, rather than the adipate used here. The c parameter, corresponding to a 3 layer repeat, can be calculated from the d003 and d006 reflections and from this the interlayer spacing, c0, which was between 7.83-8.09 Å for LDH2 to CoH-LDH6. Subtracting the thickness of one brucite-like layer gives the gallery height, around 3.03-3.29 Å, greater than the corresponding gallery height of 2.81-3.14 Å for the CoP-LDHs, owing to the size of the intercalated adipate dianion. Using an axial length of the adipate anion of 7.432 Å obtained from the literature,199 the angle of tilt of the adipate anions in the interlayer, relative to the plane of the LDH sheet, was calculated as being between 24-26 °. The average crystallite size has also been calculated, using the Scherrer equation, in the a and c directions with the results being in a similar region to those of the co-precipitated LDHs (see section 3.4.1).
ICP-OES data showed general correlation with the expected Mg:Al ratio on the basis of R-value, with the exception of CoH-LDH5, which had a lower ratio than CoH-LDH4 (Table 3.9).
SEM-EDX was used to obtain Mg:Al ratios across the samples at a microscale, with the results shown in Table 3.9. This also showed a general correlation with the expected Mg:Al ratio, although again, with the exception of CoH-LDH5, which has a significantly lower ratio, below that even of CoH-LDH3, suggesting this sample to be impure and that some amorphous alumina has formed, which is interspersed throughout the LDH product and, owing to its amorphous nature, is not observed in the PXRD spectra.
CoH-LDH2 to CoH-LDH6 samples, with adipate ions in the interlayer, are carbonate free according to the FTIR spectra in Figure 3.9 with no corresponding stretches observed at 672 cm–1 and 873 cm–1.137 This is in agreement with the work of Greenwell et al.,123 who developed the co-hydration method as a carbonate free way to prepare LDH. CoH-LDH1 shows the characteristic carbonyl symmetric and anti-symmetric stretching vibrational modes in the region 1300-1500 cm–1. However, as detailed above, CoH-LDH1 is not an LDH since it only contains aluminium by elemental analysis. When comparing the FTIR spectra recorded in this project with the literature, it was found that no C-H stretch was present, as expected for adipate at around 2950 cm–1. Also no
Table 3.8 Parameters from the PXRD analysis of CoH-LDH1 to CoH-LDH6. a calculated by subtracting 4.8 Ǻ (the width of one brucite layer) from one third of the value of the c parameter. Estimated standard deviations shown in brackets.
Sample d003 (Ǻ) d006 (Ǻ) d110 (Ǻ) a (Ǻ) c (Ǻ) c0 (Ǻ) Gallery
Table 3.9 Expected and actual, ICP-OES and EDX bulk Mg:Al values obtained for LDHs prepared by co-hydration, with the error shown in parentheses for SEM-EDX.
OH structure resulting from brucite is observed for higher R-values,123 suggesting more scans or alternative IR techniques may be required to observe these.
Figure 3.9 FTIR spectra of the synthesised layered double hydroxides prepared using the co-hydration method.
Thermal analysis via TGA of the CoH-LDHs showed the expected distinct mass losses associated with evolution of water, initially from loss of intercalated interlayer water and then from dehydroxylation of the hydroxide layers, and later from adipic acid decomposition upon calcination from room temperature to 500 °C (Figure 3.10), further confirming successful intercalation of the adipate anion.192 TGA data for water loss, in conjunction with elemental analysis of C and H content, allowed the calculation of potential sample compositions (Table 3.10). The Mg:Al ratio determined from ICP was used to determine the amount of OH- in the LDH structure. With these values fixed the percentages of water, carbonate, hydroxide and adipate were varied to determine possible compositions when the percentage of H2O, C and H were matched in a pairwise way. No carbonate was seen to be present from the corresponding PXRD spectra and so any impurity may be due to incorporation of the hydroxide anion. It was not possible to match the C, H and H2O simultaneously to be in agreement with the experimental data, something that may be due to the percentage attributed to H2O in the TGA data being over/underestimated due to adsorbed surface
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Transmittance (%)
CoH-LDH1 CoH-LDH2 CoH-LDH3 CoH-LDH4 CoH-LDH5 CoH-LDH6
water and/or overlapping of the loss of interlayer water with interlayer anion loss. Another possible reason for less hydrogen being observed in elemental analysis than calculated is that, prior to combustion, the elemental analysis sample is kept in a pure oxygen stream and this may result in dehydration of the sample.
Figure 3.10 Thermogravimetric analyses of CoH-LDH2 to CoH-LDH6.
3.4.2.2 Catalyst properties of the layered double hydroxides
Nitrogen adsorption/desorption isotherms for the co-hydrated LDHs are shown in Figure 3.11a. All samples tested exhibit type IV isotherms, characteristic of mesoporous materials.143, 200 In each case, the hysteresis loop is narrow, with almost parallel adsorption and desorption branches, indicative of pores with regular geometry, while the steep desorption behaviour indicates that the pore size distribution is narrow (Figure 3.11b).
The measured surface area for CoH-LDH2 was 33 m2/g, considerably lower than that measured for CoP-LDH2 (91 m2/g). The CoH-LDH3 to CoH-LDH6 samples all possess very similar surface areas, between 42-46 m2/g, which, unlike their analogues prepared via co-precipitation, show little variation as a function of R-value. All samples have surface areas within
the range typically cited for LDHs.122, 193 The average pore size distribution was highest for CoH-LDH5 (13.3 nm) and lowest for CoH-LDH2 (9.7 nm). Pore volume was greatest for CoH-LDH3 at 0.1 cm3/g and lowest for CoH-LDH2 at 0.05 cm3/g. The average pore widths were found to generally increase with increasing Mg:Al ratio, over the range ~8.5 to ~11-13 nm, similar to that of the CoP-LDHs, however over a much narrower range (cf. 3 - 22 nm). The pore volume was greatest for CoH-LDH1 at 0.20 cm3/g, however this is not deemed to be an LDH since it only contains aluminium. CoH-LDH2 to CoH-LDH6 all exhibit similar pore volumes of between 0.05 to 0.10 cm3/g, with the average pore width increasing from CoH-LDH2 to CoH-LDH3 and then being very similar up to CoH-LDH6.
Table 3.10 Analytical data and possible calculated chemical formulae for co-hydrated LDHs. Analytical data: a from TGA analysis; b from elemental analysis. Adip = adipic acid
Sample H2O (%) C (%) H (%)
CoH-LDH1 - analytical data n.d. 5.57b 2.08b
Not deemed an LDH by PXRD - - -
CoH-LDH2 - analytical data 11.07a 8.39b 3.64b [Mg1.3Al(OH)4.6](CO3)0.31Adip0.19.1.26H2O 11.07 8.39 4.23 CoH-LDH4 - analytical data 7.02a 3.19b 2.89b [Mg3.7Al(OH)9.4](CO3)0.42Adip0.08.1.31H2O 7.02 3.19 3.79 [Mg3.7Al(OH)9.4](OH)0.70Adip0.15.1.30H2O 7.02 3.19 4.20 CoH-LDH5 - analytical data 5.88a 3.61b 3.45b [Mg3.2Al(OH)8.4](CO3)0.42Adip0.08.0.99H2O 5.88 3.61 3.68 [Mg3.2Al(OH)8.4](OH)0.70Adip0.15.0.98H2O 5.88 3.61 4.13 [Mg3.2Al(OH)8.4](CO3)0.42Adip0.08.0.51H2O 3.15 3.61 3.45 [Mg3.2Al(OH)8.4](OH)0.21Adip0.14.0.94H2O 5.88 3.61 4.06 CoH-LDH6 - analytical data 4.41a 3.48b 3.50b [Mg5.0Al(OH)12.0](CO3)0.36Adip0.14.1.01H2O 4.41 3.48 3.70 [Mg5.0Al(OH)12.0](OH)0.60Adip0.20.1.00H2O 4.41 3.48 3.99 [Mg5.0Al(OH)12.0](CO3)0.37Adip0.13.0.44H2O 1.98 3.48 3.50
As discussed previously (Section 3.4.1.2), LDHs are microporous solids with interlayer spacings typically in the 1 -2 nm range,194 again far smaller than the average pore sizes established here, which are considered mesoporous.143 The average pore size distributions for the co-hydrated LDHs are, similar to the co-precipitated LDHs, presumably due to inter-particle voids rather than internal porosity of the interlayer nanostructure, since only the external surface is probed;137 consequently interpretation of the adsorption isotherms need to be treated with care. The surface areas for the co-hydrated LDHs all appear very similar, in contradiction to that of the co-precipitated LDHs. A possible reason for this is that the very controlled particle size dispersion in the CP5 alumina reactant for the co-hydration reaction leads to a more consistent particle size in the final LDH product.
Measurement of Hammett basicity for the various co-hydrated materials gave a basicity for CoH-LDH1 to CoH-LDH4 of between pKa 6.0-7.6, which is lower than that determined for CoP-LDH5 to CoP-LDH6, lying in the 7.6-9.0 range. Evidently, the basicity of the various CoP-LDH and CoH-LDH materials varies not only as a function of R-value, but also with the method used for their preparation.201 The weaker basicity of the co-hydrated sample prepared in this thesis may be a reflection that adipate is the interlayer anion, rather than carbonate. The use of an organic acid is believed to be necessary, however, in order to peptise the alumina and enable reactivity and, though this allows near neutral pH synthesis with no carbonate contamination, as well as control of morphology, it may also result in sample with only weakly basic sites. The CoH-LDHs are also more rich in Al (along with brucite and MgO co-phases), which will also result in lower basicity.
In summary, adipate-containing LDHs have been prepared by co-hydration of a 1 % w/w slurry of alumina and MgO, in the presence of a di-carboxylic acid at moderate temperature, without the presence of alkali base or inert atmosphere. Various parameters relating to the crystal structures of the various LDH materials, including cell parameters and possible chemical compositions, have been elucidated as well as properties relating to the bulk materials such as
surface area and external maximum basicity estimations. The co-hydrated LDHs appear to be weakly basic, and control of magnesium hydroxide phase growth seems inexorable at Mg:Al > 2.
a)
b)
Figure 3.11 a) Nitrogen adsorption/desorption plots showing typical hysteresis behaviour of type IV mesoporous solids and b) associated pore width distributions for CoH-LDH1-6
0
Table 3.11 Nitrogen adsorption analysis and Hammett basicity data for CoH-LDH1 to CoH-LDH6. Surface
3.4.3 Comparison of layered double hydroxides prepared by co-precipitation