SYNTHESIS , CHARACTERIZATION AND CATALYTIC ACTIVITY OF TUNGSTOCOBALTATE-PILLARED ZnAl-LAYERED DOUBLE HYDROXIDE

Tungstocobalate, (CoW12O40), intercalated ZnAl-layered double hydroxide (ZnAl-CoW12) was prepared via rehydration of calcined ZnAl-LDH under nitrogen atmosphere. Characterization by chemical analysis together with powder XRD, FT-IR, TG-DTA and UV-VIS DRS provided evidence of intercalation of CoW12O40 (58 wt. %) in the interlayer of LDH. The catalytic activity of ZnAl-CoW12 was evaluated for hydrogen peroxide mediated decolourisarion of methyl orange and oxidation of benzaldehyde to benzoic acid under varying reaction conditions. ZnAl-CoW12 was found effective for both the reactions and stable under the experimental conditions for repetitive use without any noticeable decrease in activity.


Introduction
2][3][4][5][6][7][8] The oxidizing ability can be systematically controlled by changing the constituent atoms of polyanion structure. 4,8owever, their low surface area and thermal stability in addition to high solubility in aqueous medium limit their utility in many catalytic applications. 9,10[11][12][13][14][15] Layered double hydroxides (LDHs) is another important class of inorganic layered compounds offering support for hosting a variety of catalytically active anionic species in the interlayer space of metal hydroxide layers. 16[21][22][23][24] The present work pertains to intercalation of a catalytically active POM having strong oxidizing ability of Co III ion, [CoW12O40] 5-(CoW12), 14,26 in the interlayer of ZnAl-layered double hydroxide (ZnAl-LDH), characterization of resulted intercalated sample by various physicochemical methods and evaluation of its catalytic activity for oxidative decolourisation methyl orange (MO) and oxidation benzaldehyde to benzoic acid as the model reactions.
(CoW12) ion was intercalated in the interlayer LDH through rehydration of calcined LDH in presence desired amount of aqueous solution of K5[Co III W12O40].20H2O.In typical lot a weighed amount of ZnAl(O) was dispersed in 50 mL of aqueous solution of K5[Co III W12O40] (0.52 g) and the pH was adjusted to ca. 6.5 with dilute HNO3 solution.The mixture was then stirred under N2 atmosphere for ~ 4 h.By this time the initial green colour of the solution was changed to colourless indicating almost complete intercalation of tunstocobaltate ion in the LDH interlayer.The resulting solid was separated by centrifugation, washed several times with water and finally with ethanol.The isolated solid was dried at 60C for 8 h in vacuum.The CoW12 intercalated sample was denoted as ZnAl-CoW12.

Characterizations
The Zn, Al, Co and W contents in the samples were determined by ICP (Varian Liberty series2).Carbon and nitrogen was analysed by Euro EA Vector elemental analyser.Powder X-ray diffraction patterns were recorded in a Rigaku (Miniflex II) X-ray diffractometer at a scanning speed of 2(2)/min using Ni filtered CoK (30 kV, 15 mA) radiation source.Thermogravimetric measurements in argon atmosphere were performed on a Shimadzu DTG 60 Thermal analyser at a heating rate of 10 C min -1 .FT-IR spectra in KBr phase were recorded on a Shimadzu IR Affinity-1 spectrophotometer averaging 45 scans with a nominal resolution of 4 cm −1 to improve signal to noise ratio.The UV-Visible diffuse reflectance spectra were recorded on a Varian UV-Visible spectrophotometer using BaSO4 white standard.

Catalytic activity
The catalytic activity of ZnAl-CoW12 was evaluated for oxidative decolourisation of a methyl orange and oxidation of benzaldehyde.Stock solution (500 μM) of methyl orange was prepared by dissolving accurately weighed solid methyl orange (Mecrk, GR) in deionized distilled water and was diluted to desired concentration as and when required.H2O2 (30 % w/v, Merck) and benzaldehyde (Merck, GR) was used as received.
For decolourisation of study, 50 mL of MO at desired concentration along with appropriate amounts of H2O2 and ZnAl-CoW12 in a 100 mL conical flask were mechanically shaken in thermostated water bath shaker at 30±0.2 ºC.The initial pH of the reaction mixture was adjusted to 6.0±0.2 by addition of 0.1 M NaOH/HCl solution.At regular intervals, the reaction mixture was withdrawn, centrifuged and measured the absorbance at 464 nm (ε = 2.68×10 4 M −1 cm −1 ) to evaluate the concentration of residual MO.In aqueous solution, MO is almost completely dimerised above 2×10 −4 M and undergoes further aggregation at millimolar and higher concentrations 29 .Hence, the concentration of MO was kept < 1×10 −4 M where the Beer-Lambert law is obeyed.All the experiments were carried at pH above the pKa value of MO (~3.4) in order to avoid any further colour change due to pH variation.The reaction parameters such as time of reaction, catalyst amount and initial concentrations of MO and H2O2 were varied to optimize the parameters.
The oxidation of benzaldehyde was carried out by a similar procedure adopted in a previous study. 14The reaction mixture containing ZnAl-CoW12, benzaldehyde and 30 wt. % H2O2 in a 100 mL flask was heated under stirring condition to initiate the reaction.The reaction time, temperature and amounts of H2O2 and catalyst were varied to optimize the reaction parameters.The formation of benzoic acid is evident from its isolation by similar method described earlier 14 and characterized by FT-IR spectral analysis and melting point measurement.The yield of benzoic acid was calculated from the weight of the final white crystals.

Characterizations of ZnAl-CoW12
The    19,30 and are very much similar to those reported previously. 14,15The bands at 3520 and 1640 cm    4. It is seen that ZnAl-CoW12 exhibits a continuous weight loss up to 600 C with two distinct endothermic peaks centered at ~ 160 and 245 C in the DTA profile.A hump like endothermic peak observed at ~ 120 C is presumably due to the loss of surface-adsorbed water.The peaks at 160 and 245 C are attributed to the removal of interlayer water followed by collapse of the layered structure. 14The weight loss beyond 300 C is resulted from both the dehydroxylation of ZnAl-LDH layers and the decomposition of CoW12 to expel the produced water molecules.The neat complex also exhibits multi stage weight losses with two major endothermic peaks at 90 and 190 ºC.

Oxidative decolourisation of methyl orange (MO)
Preliminary observation indicates practically no change in the absorbance of MO over a period of 3 h in presence of H2O2 (0.05 M) indicating there is no decolourisation of MO by H2O2 alone.As MO exists in anionic form at pH > 4.0, decolourisation due to ion exchange or adsorption of a small amount of MO in the interlayer or surface of ZnAl-CoW12, respectively, cannot be ruled out.A typical experiment, with ZnAl-CoW12 (0.5 g l -1 ) and MO (35 μM) and without addition of H2O2,, shows (Figure 5, inset) a decrease in absorbance over the entire range of spectrum.The amount of MO decolourised due to ion exchange/adsorption on ZnAl-CoW12, estimated using the absorbance values of MO at 275 and 464 nm, is found to be ~ 14 %.In presence of both H2O2 and ZnAl-CoW12, the MO peak at 275 nm is not observed (Figure 5) due to high absorbance of H2O2 at < 300 nm.However, the absorbance of MO at 464 nm is progressively decreased with time and reached to almost zero in ~ 8 h.Interestingly the MO peak at 275 nm is reappeared when the concentration of H2O2 is decreased with the progress of reaction.As more than 80 % of MO is decolourised within 300 min of reaction, all further decolourisation experiments for determination rate constant and optimization of other parameters were carried out keeping the time of reaction time fixed at 300 min.The time course percentage of MO decolourised in the presence and absence of H2O2 is presented in figure 6.It is seen that the decolourisation of MO is strongly catalysed in presence of both H2O2 and catalytic amount of ZnAl-CoW12 and more than 80 % MO (35 μM) is decolourised against 14 % decolourisation in the absence of H2O2.The decolourisation data up to 300 min are subjected to non-linear least square fitting (eqn.1).Ct = (C0 -C∞) e (-kt) + C∞ (1)   where C0, Ct and C∞ are the concentrations of MO at the beginning, time 't' and the end, respectively and k is the rate constant.The first-order rate constants, derived from least square fittings (R 2 = 0.99), are found to be 0.357±0.070and 0.433±0.051h −1 for 35 and 60 μM MO concentrations, respectively under identical conditions.The effect of initial H2O2 concentration (0.02 to 0.20 M), keeping the dye and catalyst amounts fixed, is presented in Table 1.It is seen that the percentage of decolourisation increases non-linearly with increase of initial concentration of H2O2.The decrease of oxidation activity of H2O2 at higher concentration is most likely due to increase of catalysed H2O2 decomposition at higher concentration.The effect of catalyst amount on overall MO decolourisation, keeping all other parameters fixed, is also presented in Table 1.The decolourisation is progressively increased due to availability of more active component for catalysis and sites for adsorption/ion exchange.In order to see the efficiency of ZnAl-CoW12 for repetitive use, the reactant solution was charged with desired amount of MO and H2O2 after every 5 h to maintain the same initial MO (35 μM) and H2O2 (0.05 M) concentrations.In the first round, 76.1 % of 35 μM MO is decolourised in 5 h at initial pH ~ 6.0 and ZnAl-CoW12 dose of 0.50 g L -1 .In the second round with a fresh load of MO and H2O2, to maintain the same initial concentrations, 64.1% of MO is decolourised.In third and fourth cycles, the percentage of MO decolourisation are 62.6 and 61.3, respectively.Significant decrease of decolourisation in second cycle is primarily due to significant decrease of MO intercalation in the interlayer region of ZnAl-CoW12.As expected, this decrease is marginal from second to third or subsequent cycles.The above results indicate that the catalytic system (ZnAl-CoW12 + H2O2) has potential for repetitive use without any noticeable decrease in decolourisation activity for organic dyes like MO. Analyses of reactant solution after each catalytic run by Atomic absorption spectroscopy (AAS) do not show any detectable cobalt content in the solution indicating the CoW12 in the interlayer is quite stable and the LDH is proved to be a suitable host for heterogenisation of catalytically active species like CoW12.The catalytic activity of ZnAl-CoW12 can be further extended for decolourisation of other organic dyes.

Oxidation of benzaldehyde
The catalytic efficiency of ZnAl-CoW12 was also assessed for hydrogen peroxide mediated oxidation of benzaldehyde to benzoic acid.The results obtained under varying reaction temperature, amount of catalyst, volume of H2O2 and reaction time is presented in Figure 7a-d.At first the reaction was carried out for 1.0 h to optimize the other parameters like reaction temperature, amount of catalyst and H2O2.It is evident from Figure 7a that the conversion of benzaldehyde (10.2 mL, 89.7 mM) with a fixed dose of catalyst (0.1 g) and H2O2 (20 mL of 30%) increases with increasing temperature, reaches to a maximum value at ~ 80-90C and then decreases marginally on further increase of temperature.On variation of H2O2 (10-40 mL), keeping the other parameters constant, the activity (Figure 7b) is found increase up to 20 mL and there after practically remains constant on further increase of H2O2.The results of variation of amount of ZnAl-CoW12 in the range (0.05 to 0.2 g) again shows (Figure 7c) an increasing trend up to 0.1 g catalyst dose but decreases at higher dose of catalyst presumably due to partial decomposition of H2O2 by catalyst causing a decrease in its concentration.Keeping the reaction temperature, catalyst dose and H2O2 volume fixed at 90 ºC, 0.1 g and 20 mL, respectively, the reaction time was optimized.The variation of reaction time in between 1-6 h shows that the conversion of benzaldehyde increases with increase of time, reaches to a maximum value in ~ 4 h.Further increase in time course of reaction does not lead to any increase in conversion of benzaldehyde.Under these optimize set of conditions (benzaldehyde, 10.2 mL; H2O2, 20 mL; ZnAl-CoW12, 0.10 g; Reaction temperature, 90 ºC and reaction time, 4 h), the conversion of benzaldehyde is found to be ~ 86.6 %.This values is lower than that reported (98.8%) for the same reaction using MgAl-CoW12 as the catalyst primarily due to intercalation of relatively higher amount of CoW12 in the interlayer of MgAl-LDH (68.7 wt.%) as against 56.2 wt.% in the present case. 14It is worth noting that the conversion of benzaldehyde under identical conditions with either ZnAl-CO3 (0.1 g) or equivalent amount (0.58 g) of neat CoW12 in presence of H2O2 (20 mL 30 %) is only 27 % and 48 %, respectively.Similarly, the conversion of benzaldehyde (10.2 mL) by H2O2 (20 mL) and without ZnAl-CoW12 is only 30.5 %.While with ZnAl-CoW12 (0.10 g) alone without H2O2, the conversion of benzaldehyde is less than 5 %.The above observation shows that there is a significant improvement in the catalytic activity of CoW12 when intercalated in the interlayer of ZnAl-LDH.Moreover, the loss of CoW12 after the reaction can be avoided.The formation of benzoic acid as the oxidation product of benzaldehyde is evident from melting point measurement (122-124 C) and FT-IR spectra (Figure 8).The bands at 1703 and 1296 cm -1 are attributed to >C=O and − OH group, respectively.The absorption bands in between 900-1100 cm - 1 are attributed to the benzene ring or to the carbon-oxygen bond of the acid grouping.

Conclusions
A catalytically active polyoxometallate (Co III W12O40 5-) was successfully intercalated in the interlayer of ZnAl-LDH through rehydration of its calcined product at ambient temperature under N2 atmosphere.Physicochemical characterization by various methods indicated the intercalation of Co III W12O40 5-in the interlayer region of LDH.The intercalated material was found active for oxidative decolourisation of methyl orange as well as conversion of benzaldehyde to benzoic acid in presence of H2O2.

Figure 1 .
Figure 1.Powder XRD patterns of ZnAl-LDH and ZnAl-CoW12.The powder XRD patterns of ZnAl-CoW12 along with ZnAl-LDH are presented in figure 1.The appearance of some new peaks in ZnAl-CoW12 and shifting of reflections (003, 006, 009) to lower 2 values indicate the intercalation of CoW12 in the interlayer.The basal spacing, derived from 2 values, is increased from 7.38 Å to 12.96 Å.Assuming the hydroxide layer thickness of ZnAl-LDH to be 4.8 Å and size of the Keggin ion 10.2 Å, as estimated from crystallographic data for a Keggin ion salt,14 this increase of basal spacing is quite reasonable.A decrease of crystallinity of ZnAl-CoW12 is also evident from its broadened low intensity diffraction peaks.
-1 are attributed to the stretching and bending vibrations of O-H and H-O-H bonds.The band positions, although of lower intensities, in the spectra of ZnAl-CoW12 are similar to that of K5[Co III W12O40] providing further evidence of intercalation of [CoW12O40] 5-ions in the interlayer of ZnAl-LDH.After the exchange, the bands due to W-O and Co-O stretching modes are still recorded (938, 885 752 and 440 cm -1 ), together with other band at 648 cm -1 due to the transitional modes of the LDH.

Figure 4 .
Figure 4. TG and DTA curves of CoW12 (1) and ZnAl-CoW12 (2) at heating rate 10C under N2 atmosphere.More evidence in favour of intercalation of CoW12 in the interlayer of LDH is obtained from UV-Vis DRS (Figure 3).The characteristics peaks of CoW12 and ZnAl-LDH are also appeared in the case of ZnAl-CoW12 indicating the intercalation of CoW12 in the interlayer of ZnAl-LDH.The

Figure 8 .
Figure 8. FT-IR spectra of oxidation product of benzaldehyde.

Table 1 .
Effect of catalyst amount and initial dye concentration on decolourisation of MO.