Environmentally Friendly Phase Transfer Catalytic Oxidation Of 4-Methoxybenzyl Alcohol To 4-Methoxybenzaldehyde With Hydrogen Peroxide

The environmentally friendly oxidation using phase transfer catalysis involves the migration of a reactant in a heterogeneous system from one phase into another phase to make the reaction possible. Addition of a phase transfer catalyst enhances the rate of transfer of water-soluble reactant across the interface to the organic phase. In this article an environmentally friendly catalytic oxidation system for the synthesis of 4-methoxybezaldehyde from 4-methoxy benzyl alcohol using hydrogen peroxide as the oxidizing agent under phase transfer catalytic conditions with sodium tungstate as a co-catalyst has been reported. Applications of environmentally friendly oxidation reactions using hydrogen peroxide are diverse, ranging from pulp and textile bleaching to production of fine chemicals. The effect of various parameters such as speed of agitation, temperature, concentration of 4-methoxybenzyl alcohol, concentration of H2O2, phase transfer catalyst loading and co-catalyst loading on the conversion of 4-methoxybenzaldehyde was studied. The kinetics of the environmentally friendly oxidation reaction under a liquid-liquid phase transfer catalysis was investigated. The phase transfer catalyst used was tetra butyl ammonium bromide (TBAB) and sodium tungstate was used as a co-catalyst. A kinetic model was also developed for the above-mentioned system.

Index Terms - Heterogeneous Catalysis, Environmentally Friendly Oxidation, Selective Oxidation, 4-methoxybenzaldehyde, Phase Transfer Catalysis, Kinetic Model.

Introduction

Oxidation is a core technology for converting petroleum-based materials to useful chemicals of a higher oxidation state. However, oxidation is among the most problematic processes. Many textbook oxidation methods are unacceptable for practical synthesis; the heavy metal oxidants form toxic wastes, while known organic stoichiometric oxidants are usually very expensive. Nitric acid, the most conventional industrial oxidant, is cheap but unavoidably forms various nitrogen oxides. Thus, there is a need for the invention of clean, safe oxidation procedures. H2O2 is a very attractive oxidant. It can easily oxidize organic compounds since it contains 47% active oxygen and results in the generation of water as the only by-product. It is relatively inexpensive. Aqueous H2O2 is an ideal oxidant, when coupled with a tungstate complex and a quaternary ammonium ion as a phase-transfer catalyst. The phase transfer mechanism proposed by Starks accounts for the major features of the reaction [1,2]. Liquid phase catalytic oxidation of alcohols is a fascinating reaction and is one of the most important synthetic reactions in organic chemistry [3]. 4-methoxybenzaldehyde is commonly encountered in fragrances, both synthetic and natural. It is widely used in flavoring industry and as an intermediate in the synthesis of other compounds important in the pharmaceutical and perfumery industry.

A solution of 4-methoxybenzaldehyde in acid and ethanol is a useful stain in thin layer chromatography. The oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde has been accomplished by the use of a wide variety of reagents or catalysts and different reaction procedures. Several reagents have been used for this transformation such as nitric acid, thallium nitrate, copper sulfate, copper acetate, ammonium nitrate and bismuth oxide. The conventional oxidizing agents are permanganate, dichromate or chromic acid, manganese dioxide and manganese acetate. In this research work the selective oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde was studied under liquid-liquid phase transfer conditions using hydrogen peroxide as the oxidizing agent with tetrabutyl ammonium bromide (TBAB) as the phase transfer catalyst and sodium tungstate as the co-catalyst. Organic solvent used in this oxidation reaction is ethyl acetate, and hydrogen peroxide is used as a clean oxidizing agent which produces water as the only by-product and makes the reaction environmentally friendly.

Material

4-methoxybenzyl alcohol was obtained from Neemco Chemicals Pvt. Ltd and hydrogen peroxide (H2O2- 30% w/v) was obtained from M/s Merck Specialties Pvt. Ltd., Mumbai, India. Sodium Tungstate (Na2WO4.2H2O) and Tetrabutyl Ammonium Bromide (TBAB) of 99.5% purity were obtained from M/s Merck Specialties Pvt. Ltd., Mumbai.

Procedure

The experimental set up consists of a batch reactor. The reactions are carried out batch-wise, with necessary safety precautions, in a fully baffled mechanically agitated glass reactor of capacity 500 cm3. A four-bladed glass disk turbine impeller with the provision of speed regulation is used for stirring the reaction mixture. The reactor is kept in a constant temperature water bath whose temperature can be controlled within ±1˚C. Typical liquid-liquid PTC reactions were conducted by taking 30 % w/v hydrogen peroxide solution (0.1 gmol of H2O2) adding sodium tungstate (0.0005 gmol) and the total volume of the aqueous phase was made up 50 cm3(using a 100 cm3 standardization volumetric flask) by adding distilled water to it. Then the required amount of 4-methoxybenzyl alcohol (0.1 gmol) which forms the organic phase is added to the 50 cm3 aqueous phase. Then the whole solution was added to the 500 cm3 glass reactor. The required amount of phase transfer catalyst (PTC) namely TBAB (0.001gmol) was then added to the reaction mixture and this mixture was stirred at the required speed of agitation at the required temperature (60˚C) for the desired time period.

Discussion

Mechanism of reaction and kinetic model

Reaction mechanisms and kinetic models for phase transfer catalytic reactions have been proposed by many researchers [4-9]. The mechanism explains a range of experimental findings in the oxidation of alcohols including 4-methoxybenzyl alcohol. In an aqueous phase, the catalyst precursor Na2WO4 is rapidly oxidized by H2O2 according to the reaction: Na2WO4 + 2H2O2 → Na2[WO(O2)2(OH)2] + H2O. The resulting bisperoxotungstate compound A is in equilibrium with B and C. The dianion in A is feebly active toward alcohols, while the mono- and di-protonated forms in B and C are sufficiently reactive. Therefore, the catalytic activity is highly dependent on the acidity of the reaction medium as well as the efficiency of aqueous–organic phase transfer of the active species aided by Q+. In a pH range above 4, the catalytic system that contains largely A has only weak oxidative activity even in the presence of Q+. Since the pKa value of C is 0.1, the mono-protonated species B is dominant under reaction conditions with a pH range of 0.4–3, where the HSO4- ion is largely deprotonated to SO42¬. Under such dilute conditions, B exists mostly as a monomer. Its anion moiety can easily be transferred to an organic phase by Na+–Q- ion exchange. Thus, in the organic layer, the bisperoxo complex D undergoes water–alcohol ligand exchange to form E. Then proton transfer in E generates the reactive species F, which forms an aldehyde product (such as 4-methoxybenzaldehyde) and G. The alkoxy ligand in F is dehydrogenated by the adjacent hydroperoxy ligand via a six-membered transition state where the hydridic α-hydrogen migrates to the positively polarized oxygen atom. The monoperoxo tungstate ion in G is reoxidized by H2O2 after returning to the aqueous phase as the ion pair H. This step may also occur at an organic–aqueous interface or even in the organic phase to some extent. The neutral bisperoxo compound C, present as a minor component, is reactive in a homogeneous phase but unable to move into the organic phase under the biphasic conditions. H2WO4, a Na+- free compound, acts as an excellent precatalyst but only with a PTC because of the operation of the same mechanism in the biphasic system.

Effect of different parameters

To validate the proposed mechanism, the effects of various parameters on rate of reaction were studied and are described in the following sections

• Effect of speed of agitation

The effect of speed of agitation was studied by varying the speed of agitation from 300 rpm to 1500 rpm under otherwise similar conditions. As the speed of agitation increases from 300 rpm to 1500 rpm, the conversion of 4-methoxy benzyl alcohol increases. It was observed that at low speeds of agitation, there is a mass transfer limitation in the liquid-liquid phase transfer catalytic oxidation of 4-methoxy benzyl alcohol using hydrogen peroxide as an oxidizing agent. After 1500 rpm the conversion of 4-methoxy benzyl alcohol is constant so it can be concluded that it is the intrinsic rate of chemical reaction which is controlling the reaction at higher speeds of agitation.

• Effect of temperature

The effect of temperature on the conversion of 4-methoxy benzyl alcohol was studied. Experiments at various temperatures were carried out by varying the temperature from 300C to 600C under otherwise similar conditions (0.088 gmol of 4-methoxy benzyl alcohol in an organic phase of 100 cm3, 0.088 gmol of H2O2 in 50 cm3 aqueous phase, 0.00001 gmol/cc PTC (TBAB) concentration and 0.00001 gmol/cc co-catalyst (Na2WO4.2H2O) concentration. The conversion of 4-methoxy benzyl alcohol increases with increase in temperature in the range 300C to 600C. Thermal decomposition of hydrogen peroxide may become significant at temperatures above 600C, so the maximum temperature used in the experiments was 600C.

• Activation Energy

According to Arrhenius law,ln(r2/r1) = E/R (1/T1 – 1/T2), whereE is activation energy, r1 is rate of reaction at T1, r2 is rate of reaction at T2 and R is universal gas constant.From the conversion at different temperatures the activation energy was calculated. For T1= 303K and T2= 313K activation energy is 17.42 kcal/mol, for T1= 313K and T2= 323K activation energy is 19.14 kcal/mol and for T1= 323K and T2= 333K activation energy is 19.75 kcal/mol. Thus, the average activation energy was found to be 18.77 kcal/mol.

• Effect of Concentration of 4-methoxybenzyl alcohol

The effect of concentration of 4-methoxy benzyl alcohol was studied by varying its concentration from 0.00022 gmol/cc to 0.00088 gmol/cc under otherwise similar conditions (0.088 gmol of H2O2 in 50 cm3 aqueous phase, 0.00001 gmol/cc PTC concentration and 0.00001 gmol/cc co-catalyst concentration, temperature=600C, 1500 rpm). It is observed that conversion of 4-methoxy benzyl alcohol increases with increasing concentration of 4-methoxy benzyl alcohol. The plot of initial rate of reaction versus concentration of 4-methoxy benzyl alcohol. The initial rate of reaction increases with increase in concentration of 4-methoxy benzyl alcohol. The plot of initial rate of reaction versus concentration of 4-methoxy benzyl alcohol is a straight line which suggests that it is a first-order reaction.

• Effect of Concentration of Hydrogen Peroxide

The effect of concentration of hydrogen peroxide on the conversion of 4-methoxy benzyl alcohol was studied. The concentration of hydrogen peroxide was varied from 0.00044 gmol/cc to 0.00176 gmol/cc in the aqueous phase under otherwise similar conditions (0.088 gmol of 4-methoxy benzyl alcohol in an organic phase of 100 cm3, 0.001gmol PTC (TBAB) concentration and 0.0005 gmol co-catalyst (Na2WO4.2H2O) concentration, 1500 rpm and temperature=600C). It is observed that the conversion of 4-methoxy benzyl alcohol in the organic phase increases as the concentration of hydrogen peroxide in the aqueous phase increases. The plot of initial rate of reaction versus concentration of hydrogen peroxide is. It is observed that initial rate of reaction increases linearly with the concentration of hydrogen peroxide in aqueous phase which indicates that the reaction is first-order with respect to the concentration of hydrogen peroxide in the aqueous phase.

• Effect of Phase Transfer Catalyst Concentration

The effect of phase transfer catalyst (tetra butyl ammonium bromide) concentration on the conversion of 4-methoxy benzyl alcohol was studied. The concentration of PTC was varied from 0.0000025 gmol/cc to 0.00001 gmol/cc under otherwise similar conditions (0.088 gmol of 4 methoxy benzyl alcohol in an organic phase of 100cm3, 0.088 gmol H2O2 in 50 cm3 aqueous phase, 0.00001 gmol/cc co-catalyst (Na2WO4.2H2O) concentration, 1500 rpm and temperature=600C). It is observed that the conversion of 4-methoxy benzyl alcohol increases with increasing concentration of the phase transfer catalyst. The plot of initial rate of reaction versus the concentration of phase transfer catalyst. As the concentration of PTC increases the initial rate of reaction also increases.

• Effect of Co-catalyst Concentration

The effect of co-catalyst (sodium tungstate, Na2WO4.2H2O) concentration on the conversion of 4-methoxy benzyl alcohol was studied as shown by varying the concentration of sodium tungstate from 0.000005 gmol/cc to 0.00001 gmol/cc under otherwise similar conditions (0.088 gmol of 4-methoxy benzyl alcohol in an organic phase of 100cm3, 0.088 gmol H2O2 in 50 cm3 aqueous phase, 0.00001 gmol/cc PTC (TBAB) concentration, 1500 rpm and temperature=600C). It is observed that the conversion of 4-methoxy benzyl alcohol increases with increase in the concentration of the co-catalyst. The plot of initial rate of reaction versus concentration of co-catalyst. It is observed that the initial rate of reaction increases linearly with the concentration of co-catalyst which suggests that the reaction is first-order with respect to the concentration of the co-catalyst.

Conclusion

The liquid-liquid phase transfer catalytic oxidation of 4-methoxy benzyl alcohol using hydrogen peroxide as an oxidizing agent has been carried out successfully. Phase transfer catalyst used was TBAB (tetra butyl aluminum bromide) and sodium tungstate was used as a co catalyst. The effect of speed of agitation, temperature, concentration of 4-methoxy benzyl alcohol in the organic phase, concentration of hydrogen peroxide (H2O2) in the aqueous phase, concentration of sodium tungstate (Na2WO4.2H2O) in the aqueous phase and concentration of phase transfer catalyst in the organic phase on the initial rate of reaction was investigated as well as the conversion of 4 methoxy benzyl alcohol versus time profiles for the effect of each of these reaction parameters were plotted. It was observed that the conversion of 4 methoxy benzyl alcohol increases with temperature, concentration 4-methoxy benzyl alcohol in the organic phase, concentration of hydrogen peroxide in aqueous phase, phase transfer catalyst concentration and co-catalyst concenration. A suitable reaction mechanism and a theoretical model have been proposed to explain the experimental results obtained as a part of this research work.

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