Kinetics of the decomposition of dimethylhexane-1,6-dicarbamate to 1,6-hexamethylene diisocyanate☆

Yan Cao,Huiquan Li*,Ningbo Qin,Ganyu Zhu

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

Keywords:Kinetics 1,6-Hexamethylene diisocyanate Decomposition Dimethylhexane-1,6-dicarbamate

A B S T R A C T The kinetics of the decomposition of dimethylhexane-1,6-dicarbamate to 1,6-hexamethylene diisocyanate was studied.A consecutive reaction model was established and the reaction orders for the two steps were confirmed to be 1 and 1.3 by the integral test method and the numerical differential method,respectively.The activation energies of the two steps were(56.94 ± 5.90)k J·mol?1 and(72.07 ± 3.47)kJ·mol?1 with the frequency factors exp(12.53 ± 1.42)min?1 and(14.254 ± 0.84)mol?0.33·L0.33·min?1,respectively.Based on the kinetic model obtained,the progress of the reaction can be calculated under given conditions.

1.Introduction

Hexamethylenediisocyanate(HDI)as one of the most important aliphatic isocyanates[1–3]is traditionally synthesized using phosgene as the starting material,which has the draw backs of extreme toxicity and formation of corrosive HCl as a side product[4,5].Thus,an environmentally friendly synthesis route for HDI must be developed.Among the reported phosgene-free methods of HDI production[6–8],the thermal decomposition of dimethylhexane-1,6-dicarbamate(HDC)is the most promising route.

It is known that thermal decomposition includes vapor-phase and liquid-phase decomposition.Vapor-phase decomposition needs high temperature and harsh conditions,which is easy to initiate polymerization[9],while liquid-phase decomposition can be carried out at relatively low temperature with simple operation[10].How ever the reports about the liquid-phase decomposition are still very limited and confine to operation at the temperature equal to or higher than the boiling point of HDI under normal or reduced pressure.Shen et al.[11]used naphthenic oil to obtain HDI with yield of 71.3%at the temperature of 260°C.Using the same solvent,Chen et al.[12]obtained HDI with yield of 89%from the 1.6-hexamethylene di-n-butylurethane substrate at 260–270 °C;Wu et al,[13]decomposed HDC using dioctylsebacates solvent with an HDC conversion of 98%and an HDC yield of 67%.Using this high boiling point solvent,not only the reverse reaction occurs relatively easily because HDI and methanol are evaporated out simultaneously,but also polymerization becomes prone to occur because of the high concentration of HDI in the gas phase.Thus,the search for a clean and highly efficient procedure for the decomposition of HDC is ongoing.

Recently,our group develop a method for decomposition of HDC to HDI(Fig.1)using chlorobenzene as low boiling point solvent under high pressure with the advantage of relatively low reaction temperature,low viscosity,high dispersion and avoiding polymerization of HDI[14].How ever,recent research on the decomposition of HDC mainly focused on the screen of catalysts and process optimization to increase the yield of HDI and scarcely investigated the intensive apparent kinetics.In this study,the kinetics of HDC to HDI is investigated for the first time.A kinetic model is proposed and the kinetic parameters are obtained.The results of the kinetics are expected to help the design of reactors in the industry.

2.Materials and Methods

2.1.Materials

The chemicals used in this study were as follow.HDC with purity of 98%was prepared from the reaction between dimethyl carbonate(DMC)and N,N′-bihpenylurea,HDI with purity of 99%was purchased from Bayer German and chlorobenzene and acetone were purchased from Beijing Chemical Works(China).All chemicals were of analytical grade.

Fig.1.Decomposition of HDC to HDI.

2.2.Experimental procedures

The decomposition of HDC was conducted in a 1 L stainless steel autoclave.In a typical reaction procedure in this study,HDC(15.0 g),Co2O3(0.75 g)and chlorobenzene(585.0 g)were charged into the autoclave.Then,the mixture was heated to a reaction temperature with mechanical stirring.Time started since the autoclave reached the desired temperature.During the reaction procedure,the methanol was continually removed from the reaction system using a flow of N2at 800 ml·min?1,and a back-pressure valve was installed at the air outlet to stabilize the system pressure at different temperatures.Pressures in the stainless steel autoclave with the corresponding experimental temperature are listed in Table 1.

Table 1Pressure with the corresponding experimental temperature

The samples were analyzed by Shimadzu GC-2014[1,15]equipped with a RTX-5(30 m × 0.25 mm × 0.25 μm)capillary column and flame ionization detector(FID).The injector and detector temperature were 240 °C and 280 °C,respectively.The oven temperature was programmed from 190°C(isothermal for 5.5 min)with a heating rate of 40 °C·min?1to 230 °C(isothermal for 2.5 min).Nitrogen as a carrier gas was used.

3.Results and Discussion

3.1.Model simplification

The decomposition of HDC is know n to be a reversible reaction and the reverse reaction occurs more easily.Therefore,nitrogen gas was used to remove the methanol generated during the reactions,assuring that there verse reaction did not occur.And condensation was increased to avoid chlorobenzene loss,assuring that chlorobenzene loss did not occur.

Moreover,HDI is so active that it can polymerize slowly at room temperature,which cannot be quantitatively analyzed by equipment.Thus,using analyzed HDI concentration in the second step will lead to obvious errors compared with the true value.Therefore,the theoretical value obtained by analyzing HDCand HMC concentrations is used in the second step to obtain the ideal kinetic equation.

3.2.Kinetics

As mentioned above,the reaction can kinetically be expressed as an irreversible consecutive reaction[16].HDC first decomposes to intermediate hexamethylene-1-carbamate-6-isocyanate(HMI),and then to HDI[17]:

The reaction rates of HDC,HMI and HDI can be expressed as follows:

where C1,C2and C3represent the concentration of HDC,HMI and HDI,k1and k2are the rate constant of the first and second step decomposition,and n and m represent there action order of the first and second step decomposition,respectively.

3.3.Kinetic parameter confirmation for the reaction of HDC to HMI

Experiments were conducted at four different temperatures,i.e.,487,495,503,and 511 K within 3 h.Fig.2 show s the relation between HDC concentration and reaction time.

Assuming that the first step decomposition of HDC to HMC is the first-order reaction,Eq.(2)can be converted as follow s:

where C0represents the initial concentration of HDC.k1is a function of temperature and thus,it can be expressed using the Arrhenius equation as follows[18]:

where A represents a pre-exponential factor,Eais the activation energy and R is the universal gas constant with a value of 8.314 J·mol?1·K?1.

Based on Eq.(5),the value of k1can be obtained at different temperatures after linear fittings(Fig.3 and Table 2).Moreover,the linear relations between ln C1and t are very clear and the assumption of the first order reaction can be confirmed.

The activation energy and pre-exponential factor can be calculated based on Eq.(6).As can be seen in Fig.4,E1is(56.94 ± 5.90)kJ·mol?1and A is exp(9.99±1.42)=2.20×10?4min?1.

Given the kinetic parameters,r1can be expressed as follow s:

Fig.2.Concentration profiles for HDC,HMI and HDI versus reaction time at different temperatures with N2 flow rate of 800 ml·min?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.■HDC;●HMI;▲HDI;▼HDI-theoretical.

Fig.3.Plots of ln C1 versus reaction time with N2 flow rate of 800 ml·min?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.□511 K;●503 K;△495 K;▼487 K.

Table 2Kinetic parameters for the reaction of HDC to HMI

3.4.Kinetic parameters confirmation for the reaction of HMI to HDI

Eq.(4)can be derived as follow s:

Fig.4.Arrhenius plot of the first-order rate constant versus 1000 T?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.

The numerical differential method is used to estimate k2and m for the reaction of HMI to HDI,and r3can be verified by the curves of relationship between concentrations and time in Fig.2.

Obviously,the relation ship between ln r3and ln C2can be obtained by linear regression from the experimental and calculated results as shown in Fig.5.The slope of the straight line represents the reaction order m.The intercept of the regression line is the logarithm of the rate constant k2.The results for the synthesis of HDI from HMI are listed in Table 3.

The average value of m is 1.33,and the activation energy and preexponential factor can be calculated based on the Arrhenius equation.As can be seen in Fig.6,E2is(72.07 ± 3.47)k J·mol?1and A is exp(14.25 ± 0.84)=1.54 × 106mol?0.33·L0.33·min?1.

Fig.5.Plots of ln r3 versus ln C2 with N2 flow rate of 800 ml·min?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.□511 K;●503 K;△495 K;▼487 K.

Table 3Kinetic parameters for the reaction of HMI to HDI

Fig.6.Arrhenius plot of the rate constant versus 1000 T?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.

Given the kinetic parameters,formation rate of HMI and HDI are:

3.5.Model test and discussion of application of the kinetic equations

The kinetic equations of the thermal decomposition can be expressed as follows:

The comparison between experimental and calculated values is shown in Fig.7.In general,the calculated and experimental values are fitting well except some deviation in the region of long reaction time.The deviation may be caused by the side reaction of the polymerization of HDI.

Fig.7.Pro files of experimental and predicted results versus time at 495 K with N2 flow rate of 800 ml·min?1.Condition:15.0 g HDC,0.75 g Co2O3 and 585.0 g chlorobenzene.■HDC;●HMI;▲HDI;?kinetic model.

In terms of activation energy,the activation energy of HDI formation is higher than that of HMI,which means that the second step of the decomposition is more sensitive to temperature,and increasing the temperature is more beneficial for HMI decomposing to HDI,which is in accordance with Fig.2.

The ratio of k1to k2is:

According to Fig.2,at relatively low temperature there is a large quantity of HMI in the system during a short time.It indicates that the first step is fast reaction,while the second one of generating HDI is slow reaction and the rate controlling step for the w hole reaction.In Eq.(13),the value of K is increasing with temperature.That is the decomposition rate for HMI to HDI increases faster than HDC to HMI with increasing temperature.When temperature is higher,HMI decomposed to HDI as soon as it is produced,so the concentration of HMI is relatively low in the system,which means that higher temperature is benefit for the yield of HDI but decreases HMIyield.However,the polymerization is easy to becarried out at high temperature to decrease the yield of HDI.Therefore,HDI should be separated out in time during the HDI production process to increase the HDI yield.Under higher temperature,the first step will be the rate controlling one.

4.Conclusions

Using the low boiling point chlorobenzene as solvent under high pressure,the HDI polymerization is avoided by effectively controlling the reaction time and temperature.The results show that the decomposition of HDC is a consecutive reaction,and the activation energies of the two steps are(56.94 ± 4.32)k J·mol?1and(72.07 ± 5.12)kJ·mol?1with the frequency factors are exp(12.53± 1.08)min?1and exp(14.25 ± 1.03)mol?0.33·L0.33·min?1,respectively.The calculated and experimental values are fitting well except some deviations which may be caused by the side reaction of the polymerization of HDI in the region of long reaction time.In general,the built reaction rate equations are accurate and able to reflect the concentration and conversion change during there action period.Moreover,the kinetic study provides a theoretical guide for reactor design.

Nomenclature

A1pre-exponential factor of the first step decomposition,min?1

A2pre-exponential factor of the second step decomposition,mol?0.33·L0.33·min?1

C0initial concentration of HDC,mol·L?1

C1concentration of HDC,mol·L?1

C2concentration of HMI,mol·L?1

C3concentration of HDI,mol·L?1

E1activation energy of the first step decomposition,kJ·mol?1

E2activation energy of the second step decomposition,kJ·mol?1

k1rate constant of the first step decomposition,min?1

k2rate constant of the second step decomposition,mol?0.33·

L0.33·min?1

m represent there action order of the second step decomposition.

n represent the reaction order of the first step decomposition.

R universal gas constant with a value of 8.314 J·mol?1·K?1.r1reaction rate of HDC,mol·L?1·min?1

r2reaction rate of HMI,mol·L?1·min?1

r3reaction rate of HDI,mol·L?1·min?1