Esterification of phenyl acetic acid with p-cresol using metal cation exchanged montmorillonite nanoclay catalysts

The liquid phase esterification of phenyl acetic acid with p-cresol over different metal cation exchanged montmorillonite nanoclays yields p-cresyl phenyl acetate. Different metal cation exchanged montmorillonite nanoclays (Mn+ = Al3+, Zn2+, Mn2+, Fe3+, Cu2+) were prepared and the catalytic activity was studied. The esterification reaction was conducted by varying molar ratio of the reactants, reaction time and catalyst amount on the yield of the ester. Among the different metal cation exchanged catalysts used, Al3+-montmorillonite nanoclay was found to be more active. The characterization of the material used was studied under different techniques, namely X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. The product obtained, p-cresyl phenyl acetate, was identified by thin-layer chromotography and confirmed by Fourier transform infrared, 1H NMR and 13C NMR. The regeneration activity of used catalyst was also investigated up to fourth generation.


Introduction
Nanoclays have been one of the significant industrial minerals and with the recent development of nanoclay technology. Montmorillonite, {[M 2 (OH) 2 (Si 4 O 10 )]·xH 2 O}, M = Al and/or Mg is one of the most important nanoclay minerals used in various organic reactions [1]. Clay nanoparticles are layered structures and a layer possesses negative charge that is neutralized by many cations [2]. Montmorillonite nanoclays have one octahedral sheet containing aluminium or magnesium sandwiched between two tetrahedral sheets containing silicon [3].
2018 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Preparation of metal cation exchange nanoclays (M n+ -mont-nanoclay)
The method involves stirring raw montmorillonite nanoclay overnight with 0.5 M (200 ml) different metal cation solutions (M n+ = Al 3+ , Zn 2+ , Fe 3+ , Cu 2+ , Mn 2+ etc.) to get M n+ -mont-nanoclay catalyst. Then the clay was centrifuged and washed repeatedly with distilled water until the washings were free from chloride ions. This was confirmed by silver nitrate test. The M n+ -mont-nanoclay sample was dried at 100°C for 30 min and finely powdered [15,16].

Catalytic study
The esterification reaction of PA with p-C by using suitable solvent was carried out in 100 ml of round bottom flask fitted with a reflux condenser. The esterification study was conducted by refluxing 25 mmol of PA, 50 mmol of p-C and 0.5 g of prepared M n+ -mont-nanoclay catalyst using 30 ml of toluene as a solvent (figure 1). The reaction was refluxed, cooled and filtered to separate M n+ -mont-nanoclay catalyst and washed twice with solvent. The filtrate was treated with 5% NaOH in separating funnel to remove unreacted reactants followed by water and saturated brine solution. The solvent was distilled off from organic layer under reduced pressure and dried over anhydrous sodium sulfate [15,16]. The resulted product, p-cresyl phenyl acetate, was extracted with diethyl ether and this was identified by thin-layer chromatography and confirmed by Fourier transform infrared (FTIR) (Perkin Elmer, Spectrum Two, 100300), 1 H NMR (Brucker, 400 MHz) and 13

Mechanism of esterification
The mechanism of esterification of PA with p-C in the presence of M n+ -mont-nanoclay catalyst is as shown in figure 2b. Esterification reaction between PA and alcohol is known to be catalysed by Brønsted acid sites. PA gets protonated at the Brønsted acid sites and forms conjugate acid ion, i.e. oxonium ion.

Regeneration of catalyst
After the reaction, the M n+ -mont-nanoclay was separated by filtration from the reaction mixture, dried and washed with distilled water. The M n+ -mont-nanoclay was dried at 100°C at 30 min, powdered and used again for the esterification reaction to study its catalytic activity.

Characterization of material
Specific surface area of raw montmorillonite nanoclay sample was determined by BET methods using a Quantachrome NOVA 1000 surface area analyser at liquid nitrogen temperature. The surface area of the raw montmorillonite nanoclay was found to be 230 m 2 g −1 .
The X-ray diffraction (XRD) pattern of the raw montmorillonite nanoclay and Al 3+ -mont-nanoclay were recorded using Siemens D5005 diffractometer using Cu-Kα radiation source between 2θ values 3°a nd 40°. The basal spacing of the samples, raw montmorillonite nanoclay and Al 3+ -mont-nanoclay were identified as 9.934 Å (figure 5) and 9.81 Å (figure 6), respectively. XRD pattern clearly evidences that there is enhancement of basal spacing of Al 3+ -mont-nanoclay compared with raw montmorillonite nanoclay. The reason for this enhancement may be due to swelling of the interlayers by intercalation of Al 3+ cation.
Thermogravimetric analysis (TGA) of the raw montmorillonite nanoclay (figure 7) and Al 3+ -montnanoclay (figure 8) were carried out using Universal V4.5A between the temperatures 30°and 800°C with heating range 10°C min −1 . TGA curve of the raw nanoclay and Al 3+ -mont-nanoclay exhibits an initial sharp decrease due to loss of water and second one beyond 120°C due to loss of organic group. No significant change takes place above 700°C. TGA patterns of the raw montmorillonite nanoclay and Al 3+ -mont-nanoclay clearly stated that the materials used for the esterification reaction are thermally stable.

Effect of molar ratio
The molar ratio of the reactants such as PA and p-C was varied between 25 and 80 mmol to get different molar ratios. The reaction was refluxed for 6 h using 0.5 g of M n+ -mont-nanoclay catalysts. The yield of the product increased and reached a maximum of 58% when the molar ratio of PA and p-C was 1 : 4. The percentage yield obtained in different molar ratio of the reactants is given in table 1.

Effect of reaction period
The reaction was carried out at different time periods under the same experimental conditions. A series of reactions were conducted by refluxing PA with p-C (1 : 4 molar ratio), 0.5 g M n+ -mont-nanoclay catalyst. The yield of the product increased as the increase in the reaction time from 1 to 8 h and there was a decrease in the ester yield when the reaction time reaches beyond 6 h. This is due to the shift in the equilibrium of catalysed esterification reaction. So, the optimum reaction period for the esterification of PA and p-C was reported as 6 h (figure 11).
Blank reaction was also conducted by refluxing PA and p-C (molar ratio = 1 : 4), 0.5 g montmorillonite raw nanoclay catalyst and solvent (toluene = 30 ml) for 6 h. It was reported that raw montmorillonite nanoclay failed to catalyse the esterification even after refluxing for 8 h.

Effect of catalyst amount
The esterification reaction was conducted by varying the amount of M n+ -mont-nanoclay catalyst in order to study the effect on the yield of the ester ( figure 12). The amount of M n+ -mont-nanoclay catalyst was increased from 0.25 to 1 g. It was reported that the yield of an ester increased with increase in amount of M n+ -mont-nanoclay catalyst and the maximum yield was obtained when the catalyst amount was  . XRD pattern of Al 3+ -mont-nanoclay. The basal spacing for the Al 3+ -mont-nanoclay was identified as 9.81 Å, which is greater than the raw montmorillonite nanoclay material. This is clearly due to the presence of intercalatable Al 3+ ions at interlamellar region. . Thermogravimetric pattern of raw montmorillonite nanoclay. Weight loss below 120°C is believed to be due to loss of water and the weight loss between the temperature 120 and 600°C is due to loss of hydroxyl groups. The weight loss above 600°C observed to be negligible.

Activity of regenerated catalyst
The catalytic activity of regenerated M n+ -mont-nanoclay catalyst was investigated after washing with distilled water and drying at 100°C. The esterification reactions were carried out under same experimental conditions. The obtained yield of product per unit mass was found to be nearly the same even after regenerating the M n+ -mont-nanoclay catalyst five times as that yield obtained using fresh catalyst. The study evidenced the presence of active sites on reused M n+ -mont-nanoclay catalyst (table 2).

Effect of different solvents
The esterification of PA with p-C was carried out by using different solvents like toluene, benzene, chlorobenzene and 1,4-dioxane to investigate the effect of different solvents on the yield of the ester (table 3). The result of esterification with different solvents is that the percentage yield decreases with an increase in the polarity of the solvents. Non-polar solvent gives more yield than the polar solvent. The

Effect of substituted phenols on esterification
The esterification of phenyl acetic acid was also conducted with different substituted phenols using M n+ -mont-nanoclay catalysts (table 4). The reactions were conducted keeping reaction conditions such as phenyl acetic acid to p-C mole ratio 1 : 4, catalyst amount 0.75 g, toluene solvent and reaction time 6 h. Among cresols, p-C gave the highest yield, whereas the o-cresol yields much less. This is possibly due to steric factors. The esterification with nitrophenols gave negligible yields, it is clear that the nitro group