Reaction kinetics of isopropyl palmitate synthesis

Lili Fu ,Yinge Bai,Gaozhi Lü,Denggao Jiang *

1 School of Chemical Engineering and Energy,Zhengzhou University,Zhengzhou 450001,China

2 Henan Province Product Quality Supervision and Inspection Center,Zhengzhou 450004,China

3 Institute of Process Engineering,Chinese Academy of Science,Beijing 100190,China

4 Henan EPRI Hitech Group Co.,Ltd,Zhengzhou 450052,China

Keywords:Isopropyl palmitate Reaction kinetics Chemical reaction Preparation Chloridization Palmitic acid

ABSTRACT In this study,the kinetics of isopropyl palmitate synthesis including the reaction mechanism was studied based on the two-step noncatalytic method.The liquid-phase diffusion effect on the reaction process was eliminated by adjusting the stirring rate.The results showed that the two-step reaction followed a tetrahedral mechanism and conformed to second-order reaction kinetics.Nucleophilic attack on the carbonyl carbon afforded an intermediate,containing a tetrahedral carbon center.The intermediate ultimately decomposed by elimination of the leaving group,affording isopropyl palmitate.The experimental data were analyzed at different temperatures by the integral method.The kinetic equations of the each step were deduced,and the activation energy and frequency factor were obtained.Experiments were performed to verify the feasibility of kinetic equations,and the result showed that the kinetic equations were reliable.This study could be very significant to both industrial application and determining the continuous production of isopropyl palmitate.

1.Introduction

Isopropyl palmitate(IPP)is one of the versatile chemical products and plays a significant role in personal products such as skin creams,shampoo,conditioner,bath,and shower gels,because of its good compatibility with the skin and high oil content of the products,and no greasy feeling after use.Moreover,the branched chain fatty acid esters can optimize the biodiesel properties such as enhancing its lubrication to reduce friction caused by engine components and reducing the pour point and cold filter plugging point,thus could prolong the service life of the engine and improve the low temperature fluidity[1,2].As one of the branched chain fatty acid ester,IPP is not only one of the components of the biodiesel group[3],but also could be used as additives to improve the properties of biodiesel[4].

At present,IPP is generally prepared by the following methods:it is synthesized from propylene and palmitic acid using sulfonic acid type ion exchange resin as the catalyst,or synthesized from palmitic acid and isopropanol using solid superacids or supported solid acid as the catalyst[5–11].The existing synthesis methods have several disadvantages such as high cost,complex progress,and low yield of the product;therefore,this study proposed a two-step noncatalytic method for the preparation of IPP[12,13].The synthetic reaction consisted of a liquid phase system with no catalyst,affording a high conversation rate with no side reaction.Off-gas was recycled easily to synthesize the by-products.In this study,IPP was synthesized from palmitic acid,thionyl chloride,and isopropyl alcohol,and the reaction kinetics and kinetic model were investigated,and all these data provided valuable and theoretical foundation for its scale-up and determination of the continuous production technology.

2.Experimental

2.1.Reagents and instruments

2.1.1.Reagents

Analytical grade palmitic acid was purchased from Sinopharm Chemical Reagent Co.,Ltd.Thionyl chloride,isopropanol,palmitic acid chloride,ethanol,sodium hydroxide,potassium hydroxide,sodium bicarbonate,and potassium acid phthalate were purchased from Tianjin Feng Chuan Chemical Reagent Technology Co.,Ltd.

2.1.2.Instruments

Heat collection type magnetic stirrer DF-101S(Gongyi Ying Yu instrument factory)was used for stirring.Electronic balance TE6101-L(Sartorius AG)was used for weighing the samples.Gas chromatographic analysis was performed using a GC900A high performance gas chromatograph(Shanghai Kechuang Chromatograph Instrument Co.,Ltd).

2.2.Experimental

Under certain reaction conditions,IPP was prepared using the following reactions:

Preliminary experiments showed that the stirring rate had a certain effect on the conversation rate[14].To evaluate the liquid-phase diffusion,the two-step reaction was carried out at different stirring rates of 100,150,200,250 r·min?1while keeping other conditions the same:n(thionyl chloride):n(palmitic acid)=1.4,n(isopropanol):n(palmitoyl chloride)=1.4,reaction temperature 343 K.The results obtained were shown in Figs.1 and 2,when the stirring rate was 200 and 250 r·min?1,no significant differences were found in the conversion of palmitic acid and palmitoyl chloride.This indicated that the diffusion-controlled process was eliminated by selecting the appropriate stirring rate to investigate the intrinsic kinetics of the two-step reaction.Based on this result,the kinetic experiments in this work were all performed at the stirring rate of 200 r·min?1.

Fig.1.Conversion of palmitic acid vs.time for Reaction(a)at different stirring rates.

For the first-step acylation reaction,palmitic acid and thionyl chloride were added to a three-neck round bottom flask equipped with a magnetic stirrer under certain condition.A certain amount of the reaction mixture was sampled at regular intervals,and the conversion rate of palmitic acid was analyzed.For the second-step alcoholysis reaction,the conversion rate of palmitoyl chloride was analyzed similarly.

2.3.Reaction mechanism

Carboxylic acids and their derivatives contain carbonyl carbon in their molecules,and they are susceptible to attack by nucleophiles,forming intermediates[15–17].The intermediates decompose by eliminating a leaving group.For the intermediate species containing a tetrahedral carbon center,the reaction sequence is called tetrahedral mechanism.The two-step reaction combining palmitic acid acylation[18]and palmitoyl chloride alcoholysis[19,20]follows tetrahedral mechanism.

Fig.2.Conversion of palmitoyl chloride vs.time for Reaction(b)at different stirring rates.

2.3.1.Acylation reaction mechanism[21]

First,palmitic acid and thionyl chloride react by nucleophilic acyl substitution,forming chlorosulfonic acid anhydride as an intermediate,which further reacts with chloride ion to afford palmitoyl chloride.R stands for C15H31in reactions(c),(d)and(e).

2.3.2.Alcoholysis reaction mechanism[21]

Isopropyl alcohol reacts with palmitoyl chloride,forming a tetrahedral intermediate.Subsequent elimination of chloride from the tetrahedral intermediate affords IPP.R and R′stand for–C15H31and–CH(CH3)2,respectively,in the following reactions:

3.Reaction Kinetics Model

3.1.Expression of kinetics model[22]

Theoretically,the kinetics of the tetrahedral mechanism reaction is first-order reaction with respect to both the reactants;therefore,we assumed that the two-step acylation and alcoholysis in this reaction follow the second-order kinetics.

For a second-order reaction,the reaction rate is expressed by the following Equation:

If cA0=cB0,Eq.(1)could be deduced as follows:

In the following study,xArepresents the conversion of palmitic acid or palmitoyl chloride respectively.

Eqs.(1)and(2)were integrated to obtain the integrated rate equation,showing the concentrations of reactants and products with reaction time.For Eq.(2)the integral form is as follows:

where β =cB0/cA0(β ≠ 1).Therefore,Eq.(1)can be deduced as follow:

For Eq.(4),the integral equation is shown by Eq.(5).

Therefore,according to the second-order rate law,a linear plot should be obtained between the concentration vs.reaction time.If the experimental data shows a linear relationship,the assumption of the two-step reaction following a second-order kinetics model is reasonable.

The Arrhenius equation associated with the relationship between reaction rate constants and reaction temperature is as follows:

Then,the natural logarithm of the Arrhenius equation leads to the following Equation,showing a linear relationship between ln k and 1/T:

Therefore,based on the slope and ordinate of the line,Eaand A can be obtained.

3.2.Determination of reaction rate constants

The kinetic analysis of the two-step reaction was investigated.Based on the assumption that β=n(thionyl chloride):n(palmitic acid)=1.4 and β=n(isopropanol):n(palmitoyl chloride)=1.4,the effect of reaction temperature on the conversion for each step was studied.The results are listed in Tables 1 and 2.

Table 1 Effects of temperatures on Reaction(a)

Table 2 Effect of temperature on Reaction(b)

The conversion in terms ofln[(β?xA)/β(1?xA)]was calculated from the data in Tables 1 and 2,and the relationship between ln[(β ? xA)/β(1? xA)]and reaction time was plotted as shown in Figs.3 and 4.Each step of the reaction shows a linear relationship;therefore,the assumption that the two-step reaction followed second-order kinetics is reasonable.

Fig.3.Conversion of palmitic acid vs.time for Reaction(a).

Fig.4.Conversion of palmitoyl chloride vs.time for Reaction(b).

Reaction rate constant(k)at the corresponding temperatures was obtained from the slope of the line as shown in Figs.3 and 4.The results are listed in Tables 3 and 4.

Table 3 Value of k of Reaction(a)at the different temperatures

Table 4 Value of k of Reaction(b)at the different temperatures

3.3.Determination of Ea and A for the reaction

According to the Arrhenius Eq.(7),the plot of ln k vs.1/T is shown in Figs.5 and 6.A line drawn at that abscissa represents 1/T and ordinate ln k.Based on the slope and ordinate of the lines,activation energy Eaand frequency factor A for each step were obtained.

The slopes of the Lines A and B were?5532.46 and?6513.18,respectively.The ordinate of the Lines A and B were 13.045 and 16.022.The result of the calculation shows that the activation energy(Ea1)and frequency factor(A1)for step 1 acylation reaction are 46 kJ·mol?1and 4.63 × 105,respectively,and the activation energy(Ea2)and frequency factor(A2)were 54.15 kJ·mol?1and 9.08×106for Step 2 alcoholysis reaction.

Fig.5.Line A:ln k vs.1/T of Reaction(a).

Fig.6.Line B:ln k vs.1/T of Reaction(b).

3.4.Determination of reaction kinetics equations

According to Eqs.(1)and(6)and the values of Eaand A for each step,the kinetics equations of IPP synthesis are as follows:Step 1:

Step 2:

In Eqs.(8)and(9),PA is the palmitic acid,TC is the thionyl chloride,PC is the palmitoyl chloride,and IPC is the isopropanol.The dynamic model of IPP synthesis was built and expressed in Eqs.(8)and(9).

4.Verified Experiments

To verify the feasibility of kinetic equations,verified experiments were performed.Based on the assumption that n(thionyl chloride):n(palmitic acid)=1.4,n(isopropanol):n(palmitoyl chloride)=1.4,the effect of the reaction time on the conversion for each step was studied at different temperatures.The results from the calculation according to Eqs.(8)and(9)agreed well with the experimental values.Figs.7 and 8 show the comparison,indicating that the dynamics equations are reliable.

Fig.7.Conversion of palmitic acid and the calculated results of kinetic equation.

Fig.8.Conversion of palmitoyl chloride and the calculated results.

5.Conclusions

IPP was synthesized from palmitic acid,thionyl chloride,and isopropanol by a two-step noncatalytic method.The results showed that each step reaction obeys the tetrahedral mechanism and follows a second-order reaction kinetic model.Based on the assumption that the diffusion-controlled processes were eliminated by selecting the appropriate stirring rate,intrinsic kinetics of the two-step reaction was investigated.By analysis,calculation,and induction of the relevant experiment data,the dynamic model of IPP synthesis was built.The kinetic equations for the first and second steps wereand,respectively.The activation energies of first and second steps were 46 and 54.15 k J·mol?1,respectively.The frequency factors for the first and second steps were 4.63×105and 9.08×106,respectively.The result of the verified experiments showed that the kinetics equations are reliable.This study is very significant for the industrial application and determining the continuous production process of IPP synthesis.

Nomenclature

A1frequency factor for Step 1 acylation reaction

A2frequency factor for Step 2 alcoholysis reaction

Ea1activation energy for Step 1 acylation reaction,kJ·mol?1

E a2activation energy for Step 2 alcoholysis reaction,kJ·mol?1

k rate constant,L·mol?1·min?1

R gas constant,J·mol?1·K?1

rPAreaction rate of palmitic acid,mol·L?1·min?1

rPCreaction rate of palmitoyl chloride,mol·L?1·min?1

T absolute temperature in kelvin,K

xAconversion of palmitic acid or palmitoyl chloride respectively,%