Chitosan-catalyzed n-butyraldehyde self-condensation reaction mechanism and kine

Hebei Provincial Key Lab of Green Chemical Technology and Efficient Energy Saving,School of Chemical Engineering and Technology,Hebei University of Technology,Tianjin 300130,China

Keywords:n-Butyraldehyde Chitosan Reaction Mechanism Kinetics Autocatalysis

ABSTRACT The chitosan was found to possess an excellent catalytic performance in n-butyraldehyde selfcondensation to 2E2H.Under suitable conditions,the conversion of n-butyraldehyde,the yield and selectivity of 2E2H separately attained 96.0%,86.0% and 89.6%.The chitosan catalyst could be recovered and used for 5 times without a significant deactivation after being treated with ammonium hydroxide.In order to elucidate the reaction mechanism,the adsorption and desorption of n-butyraldehyde on the surface of chitosan were studied using in situ FT-IR spectroscopy analysis.The result showed that n-butyraldehyde interacts with--NH2group of chitosan to form an intermediate species with an enamine structure.Then the reaction process of n-butyraldehyde self-condensation was monitored by React-IR technique and it was found that n-butyraldehyde self-condensation to 2-ethyl-3-hydroxyhexanal followed by a dehydration reaction to 2-ethyl-2-hexenal.On this basis,chitosan-catalyzed n-butyraldehyde self-condensation reaction mechanism was speculated and its reaction kinetics was investigated.The self-condensation reaction follows auto-catalytic reaction characteristics and then the corresponding kinetic model was established.

1.Introduction

2-Ethylhexanol(2EHO),an important organic chemical,is mainly applied for the production of plasticizers such as dioctyl terephthalate,dioctyl phthalate and dioctyl adipate.In addition,2EHO can also be used as a solvent,a flavor,a fragrance and an emollient[1,2].The industrial manufacture of 2EHO is conducted by three reaction units:propylene hydroformylation to n-butyraldehyde,n-butyraldehyde self-condensation to 2-ethyl-2-hexenal(2E2H),and 2E2H hydrogenation to 2EHO.As a crucial step of carbon chain extension,n-butyraldehyde self-condensation utilizes a dilute sodium hydroxide aqueous solution as catalyst in industry and suffers from some drawbacks such as apparatus corrosion,waste alkali aqueous solution emission and high production cost[3].In order to conquer the above drawbacks,several different solid base catalysts such as solid inorganic base catalysts(CaO[4],MgO[5,6],MgO-Al2O3[7,8],Na/SiO2[9]and CaO/ZrO2-Al2O3[10])and solid organic base catalysts(NH2-SBA[7],NH2-MCM[11]and CHM[12])have been reported.Among them,the solid inorganic base catalysts show high catalytic activity,but their high activity temperature(high energy consumption)and poor stability limit its industrial application.Compared with the solid inorganic base catalysts,the solid organic base catalysts have not only high catalytic activity but also low activity temperature(low energy consumption)and better reusability.

Chitosan is the product of chitin after deacetylation.Chitin is abundant in nature and can be extracted from shrimp and crab shells after aquatic processing[13,14].In recent years,the conversion of chitin biomass to high value-added compounds has been studied systematically[15-17].As a biodegradable solid organic base catalyst,chitosan has the advantages of mild reaction condition,high catalytic activity and good selectivity[18].A large number of--NH2and--OH groups distributing in the chitosan molecular chain can be used as base active sites.At present,functionalized chitosan catalysts widely used in organic reactions(especially in the aldol reaction)show high activity and selectivity[19,20].However,very few studies using nonfunctionalized chitosan catalyst have been reported up till now.Racci et al.[21]studied the catalysis of chitosan aerogel in the asymmetric aldol condensation of cyclohexanone with hydroxyacetone and found that cyclohexanone condensed with primary amine group of chitosan to give an E-enamine A whereas hydroxyacetone resulted predominantly in a Z-enamine B stabilized by an intramolecular hydrogen bond.These enamines then reacted with the incoming aldehyde,affording as the major products,the corresponding anti-and syn-aldol adducts,respectively.Shylesh et al.[22]prepared an organic base catalyst through doping Ti group onto the surface of mesoporous silica prior to grafting secondary propyl amine groups onto the surface,and used it to catalyze self-condensation of n-butyraldehyde.It was indicated that the intermediate species with an enamine structure could be obtained by a dehydration of alkylol amine formed by the adsorption of n-butyraldehyde on the catalyst surface.The intermediate species could further react with n-butyraldehyde to produce 2E2H.

Reaction kinetics is very important for understanding reaction mechanism and for reactor scale-up.Lee and Varma[23]studied the kinetics of n-butyraldehyde self-condensation using a NaOH aqueous solution of 0.76-1.9 mol·L-1as catalyst.They found that the reaction order with respect to both n-butyraldehyde and NaOH was 1 and the activation energy of reaction was 56.80±1.67 kJ·mol-1.Zhu et al.[24]calculated that the activation energy of n-butyraldehyde self-condensation catalyzed by NaX molecular sieve was 32.6 kJ·mol-1according to the Eley-Rideal model.Zhang et al.[25]studied the kinetics of self-condensation of n-butyraldehyde catalyzed by[HSO3-b-N(Et)3]p-TSA at 90 °C-120 °C and found that the activation energy of the forward reaction and the reverse reaction activation energy was separately 60.29 kJ·mol-1and 62.94 kJ·mol-1.Xiong et al.[26]studied Ce-Al2O3-catalyzed n-butyraldehyde self-condensation reaction kinetics in a temperature range of 120°C-180°C.The reaction orders of both forward reaction and reverse reaction were 2 and the corresponding activation energy was 79.60 kJ·mol-1and 74.3 kJ·mol-1,respectively.Thus it could be concluded that n-butyraldehyde self-condensation reaction in the presence of different catalysts may follow different reaction mechanisms and therefore have different activation energy.

We evaluated the catalytic performance of chitosan inn-butyraldehyde self-condensation to 2E2H and found that chitosan possessed excellent catalytic performance.In order to elucidate the reaction mechanism,firstly the adsorption and desorption of n-butyraldehyde on chitosan surface were studied by in-situ FT-IR analysis and then the reaction process of n-butyraldehyde self-condensation was monitored by the React-IR technique.Finally,chitosan-catalyzed n-butyraldehyde self-condensation reaction kinetic model was established on the basis of speculation of the reaction mechanism.

2.Experimental

2.1.Materials and reagents

n-Butyraldehyde was purchased from Tianjin Damao Chemical Reagent Factory in China and was used as received without a further purification.

2.2.Catalyst preparation

The chitosan was purchased from Zhejiang Ao Xing Biotechnology Co.,Ltd.in China and was washed three times with distilled water and then dried at 100°C before use.

2.3.In situ FT-IR analysis

The in situ FT-IR analysis was carried out on a Fourier transform infrared spectrometer(NICOLET NEXUS-470)equipped with a vacuum system and an IR cell.The IR cell is a high temperature and high pressure FT-IR transmission cell with a ZnSe window(British Specac Co.).The in situ FT-IR spectrometer was equipped with a MCT detector and its resolution was 4 cm-1in a wavenumber range of 400-4000 cm-1.

The adsorption of n-butyraldehyde on chitosan surface was investigated by the in situ FT-IR spectroscopy.Samples were prepared by pressing the chitosan catalyst with KBr into a self-supporting disc.The IR cell was purged by nitrogen of 60 ml·min-1at 140°C for 3 h firstly and then n-butyraldehyde vapor was brought into the IR cell and kept for 2 min.After that,the IR cell was swept by nitrogen for 60 min to remove the n-butyraldehyde absorbed physically on the surface of chitosan.Finally,the desorption of n-butyraldehyde was conducted by elevating temperature.During the process of adsorption and desorption,the FT-IR spectra were recorded.

2.4.React-IR analysis

The self-condensation of n-butyraldehyde over chitosan catalyst was investigated on a React-IRTM IC10 instrument(METTLER TOLEDO).The React-IR was equipped with a MCT detector whose temperature was cooled by liquid nitrogen.All IR spectra were recorded at 8 cm-1resolution in a range of 4000-650 cm-1.A typical operation is described as follows.A glass four-necked flask equipped with a diamond probe was charged with 90 g of n-butyraldehyde and 3.375 g of chitosan first and then purged with high-purity N2to replace the air inside.The flask was heated to 120°C and the IR spectra of the reaction mixture were collected.

2.5.Kinetic experiment

The kinetic experiments were carried out in a 300 ml autoclave(American Parr Instrument Co.)and a typical experiment was described below.Samples of 170 ml(136 g)of n-butyraldehyde and 6.8 g of chitosan were placed into the autoclave.After displacing the air in the autoclave three times with N2,the mixture was heated.Timing started when the reaction mixture reached the target temperature.Because the reaction proceeded slowly in the initial stage,the succeeding samples were taken out at a time interval of 30 min.After 2 h,the sample was taken out every 15 min.The samples were taken out at a time interval of 30 min after reaction for 300 min until the last sample was withdrawn at 420 min.The samples were quantitatively analyzed by gas chromatography.

2.6.Product analysis

A quantitative analysis of the reaction products was carried out using a SP-2100 gas chromatograph(Beijing Beifen-Ruili Analytical Instrument Group Co.,Ltd.).Nitrogen of high purity was used as the carrier gas at a flow rate of 30 ml·min-1.The product mixture was separated in a KB-1capillary column whose temperature was controlled according to the following program:an initial temperature of 100°C and held for 2 min,heated to 200 °C in a ramp of 10 °C·min-1and held for 10 min.The components were analyzed in a flame ionization detector(FID).The conversion of n-butyraldehyde,the yield and selectivity of 2E2H were quantitatively calculated by using the internal standard method with the o-xylene as the internal standard.

3.Results and Discussion

3.1.Analysis of reaction mechanism

3.1.1.In situ FT-IR analysis of n-butyraldehyde adsorption and desorption on chitosan surface

In situ FT-IR analysis of the standard sample of n-butyraldehyde and 2E2H was performed separately and the result is shown in Fig.S1(see Supporting Information).As shown in the FT-IR spectra of n-butyraldehyde,the absorption peak at 1747 cm-1belongs to the stretching vibration of CO bond,which is the characteristic peak of n-butyraldehyde.In the FT-IR spectra of 2E2H,the stretching vibrations of CO and CC are respectively at 1685 cm-1and 1683 cm-1,both of which are characteristic peaks of 2E2H.Thus it can be seen that these three characteristic peaks are separated well,laying the foundation for their in situ FT-IR analysis.

Fig.1.In-situ FT-IR spectra of vapor n-butyraldehyde adsorbed on surface of chitosan at 140°C.

Fig.1 shows the FT-IR spectra of gaseous n-butyraldehyde adsorbed on chitosan surface at 140°C.As can be seen from the figure,a bending vibration ofNHbond and a stretching vibration of CN bond overlapped and appeared at 1525 cm-1when n-butyraldehyde was contacted with chitosan for 5 min.At the same time,a weak stretching vibration of the CC bond at 1659 cm-1appeared.Ricci et al.[21]studied the asymmetric condensation of cyclohexanone with acetol over chitosan aerogel catalyst and thought that the action of one molecule of acetol or one molecule of cyclohexanone with the amino group of chitosan resulted in the formation of an intermediate species with an enamine structure.Therefore,we speculate that n-butyraldehyde interacted with the amino group on the chitosan surface to form an intermediate species n as shown below.

Unfortunately the characteristic absorption peak of 2E2H product,i.e.,the CO bond at 1685 cm-1and the CC bond at 1683 cm-1,did not appear even after n-butyraldehyde vapor contacted the chitosan catalyst at 140°C for more than 140 min.This result was contrasted with that of the actual reaction,mainly due to a very small amount of chitosan existing in the self-supporting disc for increasing the light transmittance.An decrease in the amount of chitosan catalyst led to a reduction of the active site number and the catalytic activity,so no product was formed at t=140 min.It is also suggested that the product will be formed only when there is a certain amount of the intermediate species n accumulated.

The FT-IR spectra change is shown in Fig.2 when the IR cell was purged by nitrogen for n-butyraldehyde desorption from the catalyst surface.The characteristic peak of n-butyraldehyde at 1747 cm-1almost disappeared after purged for 60 min,indicating that n-butyraldehyde had been desorbed completely.While the absorption peak of theNHand CN bonds at 1525 cm-1and the CC bond absorption peak at 1659 cm-1remained still existent,indicating the stability of the intermediate species n.

Fig.2.In-situ FT-IR spectra of vapor n-butyraldehyde desorption on surface of chitosan at 140°C.

3.1.2.React-IR analysis of self-condensation of n-butyraldehyde

The n-butyraldehyde self-condensation reaction catalyzed by chitosan was monitored by React-IR analysis and the result is shown in Fig.3.Fig.3(a)shows the 3D React-IR spectra of the reaction mixture at 120 °C.It can be clearly seen that the peak at 1729 cm-1could be assigned to the CO bond of n-butyraldehyde and the peaks at both 1685 cm-1and 1643 cm-1could be separately assigned to the CO bond and CC bond of 2E2H.These characteristic peaks could be clearly separated.Additionally a peak at 1149 cm-1attributed to the COH bond could be observed,indicating the formation of the intermediate product 2-ethyl-3-hydroxyhexanal.Fig.3(b)shows the local magnification spectra of the wavenumber ranging from 1601 cm-1to 1765 cm-1.It can be seen from the figures that the intensity of the infrared characteristic peak of n-butyraldehyde gradually decreased while that of 2E2H gradually increased as the reaction proceeded.Fig.3(c)is a partially enlarged view of the COH bond absorption peak at 1149 cm-1assigned to the intermediate 2-ethyl-3-hydroxyhexanal.The trend of an increase first and then a decrease in the absorption peak(COH)of the 2-ethyl-3-hydroxyhexanal suggests that it is an intermediate product in a consecutive reaction system.The result of React-IR analysis shows that n-butyraldehyde self-condensation proceeds in two steps:n-butyraldehyde selfcondensation to 2-ethyl-3-hydroxyhexanal followed by a dehydration to 2E2H,as shown in Reactions(1)and(2)below.

Fig.3.3D React-IR spectroscopy versus reaction time at 120°C.

In order to preferably observe the change of n-butyraldehyde and 2E2H versus reaction time,Fig.4 presents the changed of the four characteristic peaks at 1729 cm-1,1685 cm-1,1643 cm-1and 1149 cm-1versus time.The intensity of the absorption peak of n-butyraldehyde at 1729 cm-1firstly declined slowly and then dropped rapidly and remained almost unchanged after 5 h.The intensity of the absorption peak of 2E2H at 1685 cm-1and at 1643 cm-1began to rise slowly,and then increased rapidly and hardly changed 5 h later.This phenomenon suggests that there is possibly an induction period in n-butyraldehyde self-condensation reaction.An increase first and then a decrease in the intensity of the COH bond absorption peak at 1149 cm-1of 2-ethyl-3-hydroxyhexanal is consistent with the characteristics of intermediate product in a consecutive reaction system.

Fig.4.Absorbance of four characteristic peaks versus reaction time.

Reaction temperature has a certain effect on the accumulation of intermediate product 2-ethyl-3-hydroxyhexaldehyde.Therefore,the influence of increasing temperature on the formation of 2-ethyl-3-hydroxyhexaldehyde and its consumption reaction was evaluated.The changes of the four characteristic peaks versus reaction time separately at 100°C and 120°C are shown in Fig.S2(see Supplementary Material).The reaction did not reach equilibrium even after 6.5 h at 100 °C while the equilibrium was achieved for 5 h at 120 °C,indicating that increasing reaction temperature accelerated the proceeding of reaction.The amount of 2-ethyl-3-hydroxyhexanal increased first and then decreased.However,the maximum accumulation of 2-ethyl-3-hydroxyhexanal at 120°C(δ2=0.0426)was far less than that at 100°C(δ1=0.119),suggesting that an increase in reaction temperature is in favor of the consumption of 2-ethyl-3-hydroxyhexanal.Chen[27]in our research group calculated that the Gibbs free energy of 2-ethyl-3-hydroxyhexanal dehydration reaction is-38.97 kJ·mol-1·K-1,indicating that the reaction is thermodynamically spontaneous and that the reaction proceeds faster at higher temperature.Therefore,the effect of increasing reaction temperature on the dehydration reaction of 2-ethyl-3-hydroxyhexanal is more significant than its formation.

The results of both in situ FT-IR analysis and React-IR analysis show that n-butyraldehyde is adsorbed on chitosan surface to form species n with an enamine structure and that the n-butyraldehyde selfcondensation reaction proceeds consecutively via an intermediate 2-ethyl-3-hydroxyhexanal.Based on these results,the mechanism of chitosan-catalyzed n-butyraldehyde self-condensation reaction is speculated in Fig.5.A strong electronegativity of the carbonyl oxygen atom in n-butyraldehyde molecule makes the carbonyl group possess an inductive effect of electron-withdrawing.TheNH2group of chitosan attacks the CO bond of n-butyraldehyde molecule to form a carbinolamine species via nucleophilic addition reaction.The carbinolamine species is unstable and forms a species n with an enamine structure through intramolecular dehydration.The NC bond and the CC bond in species n form a P-π conjugation effect,making it appear more nucleophilic.The carbonyl group of another n-butyraldehyde molecule is activated byOH group on the surface of species n,causing its carbonyl carbon to be more electrophilic.The CC bond formation between the nucleophilic carbon of the species n and the carbonyl carbon of n-butyraldehyde in a concerted fashion produces an imine species.Water as a nucleophilic reagent attacks the carbon atom of the CN bond of the imine species to form the adsorbed 2-ethyl-3-hydroxyhexaldehyde after a proton transfer reaction.Then 2-ethyl-3-hydroxyhexaldehyde is desorbed from the chitosan surface and the catalyst is restored to its original state.The final step of the reaction is dehydration of 2-ethyl-3-hydroxyhexanal to the target product 2E2H.

3.2.Analysis of reaction kinetics

3.2.1.Elimination of internal and external diffusion effect

Chitosan-catalyzed n-butyraldehyde self-condensation reaction belongs to a liquid-solid heterogeneous reaction system and mass transfer will inevitably affect the proceeding of reaction.Therefore,we should exclude the influence of internal and external diffusion first in order to study the reaction kinetics better.The effect of stirring speed and chitosan particle size on the reaction was investigated separately and the results are shown in Figs.S3 and S4(see Supplementary Material).It revealed that the effect of external diffusion could be eliminated at a stirring speed of≥600 r·min-1while chitosan particle sizes in a range of 0.2-0.15 mm hardly led to the effect of internal diffusion.

3.2.2.Results of kinetic experiments

The kinetic experiments were carried out in a 300 ml Parr autoclave.The previous experiment result showed that the appropriate reaction temperature is 80°C.Hence,70°C,80°C,90°C and 100°C were selected as the reaction temperatures for kinetics study.Under the conditions of a stirring speed of 600 r·min-1,a catalyst dosage of 5 wt% and a catalyst particle size of 0.2-0.15 mm,the change of n-butyraldehyde conversion with reaction time at different reaction temperatures was investigated and the results are shown in Fig.6.It can be seen that the change trend of n-butyraldehyde conversion versus reaction time at different temperatures is basically the same.The conversion of n-butyraldehyde increased slowly at a reaction time less than 120 min,then rose sharply at a reaction time period of 120 min to 300 min,and finally increased slowly again at a reaction time longer than 300 min.The above change trend was consistent with the React-IR analysis result,indicating that there exists an induction period in chitosan-catalyzed n-butyraldehyde self-condensation reaction.

Fig.5.Reaction mechanism of n-butyraldehyde self-condensation catalyzed by chitosan.

Fig.6.Effect of reaction time on n-butyraldehyde conversion at different reaction temperatures.

In order to find out the kinetics rule of chitosan-catalyzed n-butyraldehyde self-condensation reaction,the change of nbutyraldehyde conversion and reaction rate versus reaction time was separately plotted taking the reaction temperature of 80 °C as an example and the results are shown in Fig.7.It can be seen that the curve of n-butyraldehyde conversion versus reaction time is Sshaped and the curve of reaction rate versus reaction time is volcanoshaped,following the characteristics of an auto-catalytic reaction[28].A significant feature for the auto-catalytic reaction is the existence of an“induction period”,that is,the initial concentration of the product or the intermediate increases slowly.After a certain degree of accumulation,the reaction rate increases rapidly and the reaction rate curve showed a maximum.Therefore n-butyraldehyde self-condensation catalyzed by chitosan is an auto-catalytic reaction.

Fig.7.Conversion of n-butyraldehyde and reaction rate versus reaction time at 80°C.

Based on the reaction mechanism of n-butyraldehyde selfcondensation catalyzed by chitosan,we thought that the intermediate species n exerted a catalytic effect for n-butyraldehyde selfcondensation.At the beginning of reaction,the reaction rate was not high due to the lower concentration of the intermediate species n even though n-butyraldehyde concentration was high.This can explain well the existence of the induction period.As the reaction proceeded,the concentration of the intermediate species n increased and the concentration of n-butyraldehyde was also rather high,so the reaction rate at this stage increased rapidly until the maximum was attained.Finally,very low n-butyraldehyde concentration resulted in a gradual decrease in the reaction rate.The above experimental results verified the auto-catalytic characteristics of n-butyraldehyde self-condensation catalyzed by chitosan.

3.2.3.Establishment of kinetic model

Based on the reaction mechanism of chitosan-catalyzed selfcondensation of n-butyraldehyde,the kinetic model was established,and the kinetic Eqs.(3)and(4)were obtained.The specific derivation process is detailed in Supporting Information:

Table 1 lists the explanation of various parameters in the above rate equations.

3.2.4.Estimation of parameters

In order to estimate the kinetic parameters,80 sets of experimental data including reaction temperature T,reaction time t,n-butyraldehyde concentration CBand 2E2H concentration C2E2Hwere measured.The parameters to be fitted in the kinetic model include reaction orders α,β,δ,and γ,pre-exponential factors k10and k20and reaction activation energies E1and E2.Firstly,the 1Stopt software and Openlu software were used to solve the differential equations and the solution was used as the initial value of the parameter estimation.Then the optimization program of Matlab software was used to estimate the parameter value by using the Gradient Descent Method and the calculation results are shown in Table 2.A comparison of the predicted data with the experimental data shown in Fig.8 suggests that the kinetic model fits well.

Table 1 Interpretation of parameters

Xiong et al.[26]used Ce-Al2O3-catalyzed n-butyraldehyde selfcondensation reaction in a temperature range of 120 °C-180 °C.The activation energies were 79.60 kJ·mol-1and 74.3 kJ·mol-1,respectively,and the pre-exponential factors were 5.745×105m3·kmol-1·s-1.Compared with our results,it can be seen that the activation energy of chitosan catalyzed n-butyraldehyde self-condensation reaction was lower,indicating that the catalytic reaction energy barrier was lower,so the reaction temperature required was lower.When the n-butyraldehyde self-condensation was catalyzed by chitosan,the pre-exponential factor was about 2.023×104m3·kmol-1·s-1,lower than that using Ce-Al2O3as catalyst,indicating that the reaction rate was slow,which is consistent with our kinetic experiments.Furthermore,the product 2E2H was not detected at the beginning of the reaction,which was consistent with the results obtained by in situ FT-IR analysis and React-IR analysis.

3.2.5.Test of kinetic models

Taking the case at 100°C as an example,the variance analysis and the F-test of the kinetic model was analyzed and the results are shown in Table 3.According to variance analysis theory,the larger the absolute value of both the correlation coefficient of R2and test value of F,the better the regression effect.When the correlation coefficient r＞0.9and F＞10Fα,the model can be considered to be significant at α level.The calculated value of F=254.1 is far greater than F0.95(7,72)=2.14 and the correlation coefficient r＞0.9,indicating that the kinetic model is significant at α=0.05 level.

Table 2 Kinetic parameters of n-butyraldehyde self-condensation catalyzed by chitosan

Fig.8.Comparison of experimental data(□:CB,△:C2E2H)with that of predicted by the kinetic model(solid lines)(a)70°C;(b)80°C;(c)90°C;(d)100°C.

The experimental and predicted n-butyraldehyde concentrations at a temperature range of 70 °C-100 °C are shown in Fig.9.It can be seen that the data points are evenly distributed on both sides of the diagonal and the predicted values of n-butyraldehyde concentration agree well with the experimental data.

The kinetic model is described as follows:

where,rBis the reaction rate of n-butyraldehyde,mol·L-1·min-1;r2E2His the reaction rate of 2E2H,mol·L-1·min-1;and CBis the concentration of n-butyraldehyde,mol·L-1.

4.Conclusions

(1)According to in situ FT-IR analysis of the adsorption and desorption of n-butyraldehyde on the surface of chitosan,the carbonyl oxygen of n-butyraldehyde molecule interacts with a site of--NH2group on the chitosan surface to form an intermediate species n with an enamine structure.

(2)The result of analysis using the React-IR technique displayed that n-butyraldehyde self-condensation comprises two steps:n-butyraldehyde self-condensation to 2-ethyl-3-hydroxyhexanal followed by a dehydration to 2E2H.

(3)The reaction mechanism of n-butyraldehyde self-condensation catalyzed by chitosan was speculated including the formation of a carbinolamine species,a species n with an enamine structure and an imine species.

(4)n-Butyraldehyde self-condensation over chitosan catalyst is an auto-catalytic reaction and the intermediate species n with anenamine structure can promote the proceeding of reaction.The kinetic model is established as follows:

Table 3 Model statistics

Fig.9.Comparison of n-butyraldehyde concentrations experimentally measured with that predicted by the kinetic model at different temperatures.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.03.005.