Chapter 5: Development of sewer solid acid catalysts for organic transformations
5.2 Results and Discussion
5.2.1 Preparation and characterization of catalysts
(a) Catalyst – A, Al(H2PO4)3
The catalyst-A was synthesized by heating alumina and phosphoric acid in the specific molar ratio. It was stored in vacuum desiccator as it is hygroscopic. The elemental analysis of the catalyst resulted in 8.16 % of Al (calc. 8.5%) and 85.6% of PO43-
(calc. 89.6%).
The catalyst exhibited characteristic spectral patterns in the IR region showing the presence of phosphate (Figure 5.1). The strong but broad peak at ca. 1127 cm-1 is assigned to ν P-O and another peak at 989 cm-1 is attributable to ν Al-O-P vibration [46].
Additionally, peaks observed at 3500-3300 cm-1 and 1634 cm-1 correspond to ν O-H and δH-O-H modes of vibration of water. A clear difference can be observed from the figures (a) and (b).
The powder X-ray diffraction patters of commercial alumina and the catalyst is shown in Figure 5.2. It has been found that the pattern of the catalyst matched well with the pattern of aluminum trishydrogen phosphate [47].
Figures 5.1 FTIR spectrum of (a) Catalyst- A (black line) and (b) Al2O3 (blue line).
The TGA curve of the catalyst is presented in Figure 5.3. The data indicated that after initial dehydration, the catalyst looses two water molecules from the molecular composition. The corresponding weight loss of 9.8 % observed between the temperatures 475-550 0C is in good agreement with the calculated value of 11.0% of the catalyst. Afterward the catalyst looses weight continuously upto 950 0C which is attributable to complete loss of water from the molecular composition leading to Al2O3.P2O5.
Scanning electron microscopy was used to investigate the morphological changes occurring in the surface. Unfortunately due to hygroscopic nature of the catalyst, we could not obtain a clear micrograph, however, macrospores were observed
Figures 5.2 XRD patterns of (a) commercial Al2O3 and (b) Catalyst- A
Figures 5.3 (a) Thermo gravimetric (TG) and (b) DTG profiles of Catalyst- A.
throughout the catalyst due to the aggregation [Figures 5.4(a) and (b)]. The EDAX analysis gave a qualitative composition of the catalyst that was in good agreement with the elemental analysis values.
(b) Catalyst-T, (TiO2)5.45[Ti4H11(PO4)9].4 H2O
The IR spectrum of the catalyst-T [Figure 5.5] shows a strong but broad peak at ca. 1230 cm-1 and broad weak peaks in the region 700-600 cm-1, which are typical of ν
P-O stretching and δ O-P-O bending vibrations, respectively [46]. The broad peaks at 700- 600 cm-1 have resulted from a lowering of the symmetry of free PO43-
due to its binding [8, 9] to the titania surface. Additionally, the two peaks observed at 3375 cm-1 and 1635 cm-1 correspond respectively to ν O-H and δ H-O-H modes of vibration of water.
Figures 5.4 Scanning Electron Micrographs of Catalyst- A in different magnifications viz. (a) 20 µm and (b)3 µm.
Figures 5.5 FTIR spectra of (a) TiO2 (blue line) and (b) Catalyst- T (black line).
The XRD patterns of the catalyst and a fresh sample of TiO2 are shown in Figure 5.6. The diffraction patterns reveal the presence of the anatase phase of TiO2
[48] along with a new species, [Ti4H11(PO4)9].n H2O(n =1-4) [49] present in the ratio of 84.5% : 15.5%, as determined by Klug’s equation for semiqualitative analysis [50]. The results of chemical determination of Ti (calc. 28.96, found 28.2%) and PO43-
(calc.
54.59, found 53.1%) support the composition. The average crystallite size of a fresh sample of titania as well as that of the catalyst was calculated from the XRD patterns using Scherrer’s equation [51]. The crystallite sizes of the former and TiO2 present in the catalyst were found to be 62.15 nm and 41.54 nm, respectively. This indicates that phospahate has not only stabilized the anatase phase of TiO2 in the catalyst but has also caused lowering of its crystallite size. The crystallite size of [Ti4H11(PO4)9]xH2O(x=1- 4) was found to be 20.15 nm. It appears that [Ti4H11(PO4)9]. n H2O(n =1-4) is physically adsorbed by TiO2 and held by van der Waal forces and hydrogen bonding.
The thermogram (TG) of the catalyst [Figure 5.7] shows weight losses in steps.
The loss of weight between 40 0C and 110 0C corresponds to the loss of loosely bound water. The second weight loss occurring between 174 0C and 281 0C, and the subsequent loss at >620 0C have been assigned to the loss of water drawn from the molecular composition, finally leading to TiP2O7, as ascertained from XRD analysis.
Figures 5.6 XRD patterns of (a) Catalyst- T and (b) commercial TiO2.
The BET surface areas of TiO2 and the catalyst have been found to be 9.793 m2/g and 5.387 m2/g respectively and with the corresponding the pore volume being 0.0373 mL/g and 0.0410 mL/g. The calculated desorption and adsorption BJH pore size distributions [52] are set out in Table 5.1. It is evident from the result that there are no significant changes in the suface area and pore volume in going from a fresh sample of TiO2 to the catalyst. However, desorption pore size above 80 nm appears to be reduced approximately by 10%.
Table 5.1 Desorption and adsorption of pore volume of TiO2 and Catalyst-T
Pore Diameter Range (nm)
Desorption pore volume (%) Adsorption Pore volume (%)
TiO2 Catalyst TiO2 Catalyst
Below 6 5.23 7.89 6.89 6.12
6-8 3.27 3.27 3.77 3.04
8-10 2.04 1.95 2.24 1.75
10-12 2.53 2.58 2.72 2.00
12-16 3.54 3.77 3.33 2.43
16-20 4.50 5.90 4.01 3.06
20-80 49.91 58.37 38.50 40.99
Over 80 28.97 16.27 38.55 40.61
Total 100.00 100.00 100.00 100.00
Figures 5.7 (a) Thermo gravimetric (TG) and (b) DTG profiles of Catalyst- T.
The scanning electron micrograph of the catalyst-T [Figures 5.8 (a) and (b)]
showed an ordered morphology, with the average particle size being 1 µm. A comparison of this with that of the fresh sample of TiO2 reveals an enhancement of particle size in the case of the catalyst. The EDAX results agree with the composition of the catalyst. Finally we believe that the new species, [Ti4H11(PO4)9], in conjunction with the stabilized anatase phase of TiO2 present in the catalyst, is responsible for the observed activity.