Efficient synthesis of 1,6-hexamethylene diurethane through coupling transesterification and methoxycarbonylation with methyl phenyl carbonate as intermediate☆

Yong Yang ,Jihai Tang ,Xian Chen ,*,Zhaoyang Fei,2,Mifen Cui,Xu Qiao ,2,*

1 College of Chemistry and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 State Key Laboratory of Materials-Oriented Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

3 College of Life Science and Chemical Engineering,Huaiyin Institute of Technology,Huaiyin 223003,China

Keywords:Methyl phenyl carbonate 1,6-Hexamethylene diurethane Transesterification Methoxyl carbonate Reaction coupling

ABSTRACT A reaction coupling system of transesterification and methoxycarbonylation with methyl phenyl carbonate(MPC)as intermediate was established to efficiently prepare 1,6-hexamethylene diurethane(HDU)from 1,6-hexamethylene diamine(HDA).The feasibility of the system was explored using the thermodynamics analysis,the reaction mechanism and the experiment results.The optimal reaction was carried out to get higher HDU yield.The thermodynamic analysis showed that the methoxycarbonylation of HDA with MPC,the Gibbs free energy of which was negative,was a spontaneous process.Furthermore,the equilibrium constant of the methoxycarbonylation of HDA with MPC was much greater than that of the transesterification of dimethyl carbonate(DMC)with phenol,so the reaction coupling could be realized under mild conditions.The reaction mechanism analysis indicated that phenoxy anion was the key species for reaction coupling.Higher MPC concentration was detected when sodium phenoxide was used as transesterification reactant with DMC,since the phenoxy anion of sodium phenoxide could be dissociated more easily.Sodium phenoxide was more suitable to prepare HDUthrough reaction coupling.A yield of HDUas high as 98.3%could be reached under the optimalconditions of m PhONa/m DMC=0.027 and n DMC/n HDA=8/1 at 90°C in 2 h.

1.Introduction

1,6-Hexamethylene diisocyanate(HDI)is one of the most important aliphatic diisocyanates widely used to prepare abrasion,chemical resistance and no yellowing polyurethane paints[1].HDI is commercially prepared from 1,6-hexamethylene diamine(HDA)and phosgene[2,3],which is highly toxic and corrosive.Moreover,serious environmental problems can be caused due to the formation of byproduct HCl.Recently,a green two-step process for HDI production has attracted much attention,in which 1,6-hexamethylene diurethane(HDU)is first prepared from HDA,and then HDI can be produced through the thermolysis of HDU.The technologies for HDU production without phosgene include the carbonylation of HDA[4–6]and urea alcoholysis[7–9],which need high temperature and pressure,and are difficult to commercially industrialize.Preparation of HDU through methoxycarbonylation of HDA using green dimethyl carbonate(DMC)under mild conditions has been a hot research topic[10–16].The reaction is as follows[Eq.(1)].

Unfortunately,polar solvents and delicate catalysts are essentially applied in the reaction,and the long reaction time,low yield of target product and heavy metal pollution are difficult to overcome in this process.

Methyl phenol carbonate(MPC)was reported as the methoxycarbonylation reagent to prepare HDU from HDA for the first time by Yoshida[17].In our prior work,HDU had been synthesized from HDA and MPC without solvent,and a high HDU yield realized quickly under mild conditions[18].The reaction is as follows[Eq.(2)].

However,the synthesis and separation of MPC are difficult,and there is no industrial process for MPC production.The reaction conditions ofDMC with PhOH to MPC are very rigorous,and more importantly,the yield of MPC is even low due to both the limitation of equilibrium and the disproportionation reaction of MPC to diphenyl carbonate(DPC)[19–21].The reactions[Eqs.(3)and(4)]are as follows.

Recently,reaction coupling technology has shown perfect efficiency and atomic economy in the chemical production process.γ-Butyrolactone was prepared by coupling the endothermic dehydrogenation of ethanol with the exothermic hydrogenation of maleic anhydride to effectively utilize the reaction heat and improve the selectivity[22].Furthermore,the coupling process greatly reduced the use of in flammable and explosive hydrogen.DMC reacting with aniline can produce benzene methyl carbonate and methanol,and methanol reacting with biphenyl urea can produce benzene methyl carbonate and aniline.Using DMC and biphenyl urea as feedstock,through coupling the above two reactions,the atomic economy can be greatly improved because of the application of the byproducts methanol and aniline[23].Thus,the appropriate intermediate is the key to realizing the reaction coupling.

Apparently,when phenol is added into the reaction system of DMC with HDA[Eq.(1)],phenol may react with DMC to produce DPC[Eq.(3)],and then MPC may react with HDA to produce HDU[Eq.(2)].Although the equilibrium conversion of Eq.(3)is extremely low,the Eq.(2)can quickly occur and decrease the concentration of MPC in the system.Then,the equilibrium of Eq.(3)is shifted and Eq.(3)may be promoted.In short,a small amount of phenol in the reaction system ofDMC with HDA may realize the coupling of Eqs.(1)and(2)with MPC as intermediate.In the present work,a reaction coupling system is built into the reaction system of DMC and HDA with MPC as intermediate.The feasibility of MPC production from DMC and phenol or sodium phenoxide in the reaction system of DMC with HDA under mild conditions was verified by theory and experiment.Finally,the reaction conditions of HDU preparation through reaction coupling were optimized.

2.Experimental

2.1.Reagents

HDA(analysis pure),phenol(PhOH,analysis pure),sodium phenoxide(PhONa,analysis pure),anisole(An,analysis pure)and acetonitrile(chromatographical pure)were purchased from China National Medicines Corporation Ltd.CH3OH(chromatographically pure)was purchased from Shandong Yuwang Corporation Ltd.DPC(analysis pure)was provided by Tonglin Jintai Chemical Corporation Ltd.,deionized waterwas from Hangzhou Wahaha Corporation Ltd.and DMC(industrial pure)was provided by Changzhou Yabang Chemical Corporation Ltd.

MPC was prepared by the method in the literature[24]and purified by distillation under vacuum prior to use with a final purity of 99.95%determined by gas chromatography.

2.2.Analysis methods

2.2.1.Analysis instruments

Nicolet 5700 smart Fourier infrared spectrometer(Nicolet Instrument Company in the United States),TRACE DSQ gas chromatography–mass spectrometry(GC–MS)with DP-5 capillary column(the THERMO Fisher Company)and Agilent 1100 Series high performance liquid chromatography(HPLC)with CZORBAX SB-C18 column and ultraviolet detector(Agilent Company in the United States)were used in this study.

2.2.2.Analysis conditions

The scanning range of infrared analysis was from 400 to 4000 cm-1.Pure He was used as a carrier in GC–MS with a flow rate of10 ml·min-1.The temperatures of vaporizer and connector were 300 °C and 250 °C,respectively.The column temperature of GC stayed at150°C for2 min,and then rose to 290 °C with a rate of 10 °C·min-1,and finally stayed at 290°C for 6 min.The double voltage and emission current of MS with an EI power source were 1500 V and 20.0 A,respectively.The scanning range of MS was from 45 to 450 amu.The column temperature of HPLC was room temperature(RT),and the carrier of HPLC was a mixture of acetonitrile and deionized water(0.8 ml·min-1)with a volume ratio of 55:45.

2.2.3.Qualitative analysis of HDU

Qualitative analysis of the sample,after steaming out the light component in the reaction liquid and taking two recrystallization times,was carried out using GC–MS and FTIR.The molecular fragments with mass to charge ratio of 41,44,55,67,74,82,88,97,102,114,126,130,141,156,168,173,200 and 232 were detected by GC–MS.The mass to charge ratio of the molecular ion of the sample was 232,consistent with HDU.The infrared spectrum of the sample is shown in Fig.1.

Fig.1.Infrared spectrum for synthesis sample of HDU.

As can be seen from Fig.1,a sharp absorption peak at 3300–3400 cm-1can be ascribed to the vibrations ofN--H bonds,which indicates that NH bonds exist in the sample.The peak at 1700 cm-1can be attributed to the vibrations of O=C--OH bonds.The peaks at 1200–1300 cm-1and 1050 cm-1are the absorptions of C--N bonds and C--O bonds,respectively.The absorptions of--(CH2)6–bonds also can be observed at 626 cm-1.The above results show that the synthetic sample is HDU.

2.2.4.Quantitative analysis of HDU

Quantitative analysis of the sample was carried out in HPLC with the standard curve method.Fig.2 is the spectrum of the sample obtained by HPLC.

Fig.2.Spectrum of HPLC analysis.

In Fig.2,the components in the sample can be well separated by the HPLC.Standard curves of the concentrations of MPC and HDU are obtained and shown in Fig.3.As can be seen from Fig.3,the linear correlations of standard curves are both above 0.9999,which indicates that the analysis method is accurate and feasible.

Fig.3.Standard curves of HDU and MPC.■HDU,As=0.03464+0.45998ci,R=0.9999;●MPC As=0.02492+0.72716ci,R=0.9999.

2.3.Experiment method

A certain amount of HDA,DMC and phenol or sodium phenoxide were added into a 50 ml three flask in the oil bath with stirring.The sample obtained after a certain time was dissolved into methanol for HPLC analysis.

3.Feasibility Analysis of Reaction Coupling

3.1.Thermodynamic analysis and reaction mechanism study

3.1.1.Thermodynamic analysis

The thermodynamic data,including enthalpies,entropies and Gibbs free energies,and the equilibrium constant of Eqs.(1)–(4)were calculated by group contribution method and the reported method[25,26].The results are shown in Table 1.From the results of Table 1,one can find ΔrGm? 0 in Eq.(2)at 90 °C,indicating that Eq.(2)is spontaneous.More importantly,the Keof Eq.(2)is about1030times and 1023times to those of Eqs.(3)and(4),respectively.Therefore,a trace of MPC in the reaction system of DMC and HDA can promote the generation of HDU through the Eq.(2)route.On the other hand,the consumption of MPC is beneficialto the thermodynamic equilibrium toward MPC production through Eq.(3).Then,Eq.(1)can be realized under mild conditions in the preparation of HDU.

Table 1 Δr H m,Δr S m,Δr G m and K e at 90°C

3.1.2.Exploration of the reaction mechanism

In the reaction system,the activation of DMC with phenoxyl ion is the key factor for increasing HDU productivity.The nucleophilic transesterification of phenoxyl ion with DMC to MPC greatly enhances the reactivity of electrophilic reactant(carbonyl)with nuclear reagent amine.On the other hand,the formed methoxyl CH3O-in the transesterification also plays an important role in the activation of amine,making its methoxycarbonylation process occur more easily.Summing the above discussion,a reaction mechanism is proposed as shown in Fig.4.

Fig.4.Reaction mechanism of coupling trans-esterification and methoxyl carbonylation.

The formed phenoxyl ion PhO-in the solvent reacts with DMC to generate MPC and methoxyl ion CH3O-.Then,CH3O-captures the proton of HDA to form methanol.Compared to DMC,MPC is an asymmetric carbonate and the combination of the carbonyl with phenoxyl is weak due to the electronic dispersive action of phenyl[27].The combination of the carbonylwith phenoxyleasily fractures as the carbon atom of carbonyl reacts with the nitrogen atom of amine,and restores to a phenoxyl ion.

3.2.Validation experiments of the feasibility

3.2.1.Synthesis of MPC from DMC and phenol/sodium phenoxide

As can be seen from Eqs.(2)and(3),MPC plays an important role as the intermediate in the reaction coupling system of transesterification and methoxycarbonylation.The existence of MPC is a prerequisite for the reaction coupling.Thus,the experimental exploration starts from the feasibility of MPC production from DMC and phenol/sodium phenoxide.The results are listed in Table 2.

Table 2 Comparison experiments for reaction of PhOH and PhONa with DMC(DMC 24 g,reaction temperature 90°C,reaction time 4 h)

From the results in Table 2,the MPC in the reaction system is lower than the detection limits,even when mPhOH/mDMCis up to 0.13.However,when mPhONa/mDMCwas as low as 0.04,the concentration of MPC could reach 9.9 mmol·L-1.In both cases,no DPC is detected in reaction systems.This indicated that no DPC is produced through the disproportionation of MPC,which coincided with the previous thermodynamic analysis.Furthermore,according to the referenced literature[17]and our previous work[18],no anisole would be generated in the reaction of MPC with HDA.

This interesting result displays that the addition of PhONa in DMC could be more conducive to the generation of MPC,while Eq.(4)can be effectively avoided.Therefore,the synthesis of HDU through reaction coupling could be feasible.

3.2.2.Synthesis of HDU from HDA and DMC with the existence of phenol/sodium phenoxide

To verify the feasibility of the reaction coupling system,experiments were carried out to synthesize HDU from HDA and DMC with the existence of phenol/sodium phenoxide.The results are shown in Table 3.

Table 3 Yield of HDU with the existence of PhOH or PhONa

As can be seen from Table 3,the yield of HDU from the direct reaction of DMC and HDA without the existence of phenoxyl ion is only 9.3%at 90°C in 8 h.The yield of HDU is able to significantly increase to 47.2%once PhOH[even if only 0.04%of DMC(by mass)]is added into the reaction system.The results clearly indicate that the reaction mechanism which elaborated the nucleophilic trans-esterification of phenoxyl ion with carbonyl to MPC is reasonable.The yield of HDU could be further improved when more dissociated PhONa is added.Even though the reaction time reduces from 8 h to 2 h with other conditions unchanged,the yield of HDU achieves 95.7%.Hence,the production of HDU from DMC,HDA and sodium phenoxide with MPC as the intermediate through the reaction coupling is completely workable.

4.Process Optimization

4.1.Effects of reaction temperature

The effects of reaction temperature on the yield of HDU are studied and the results are shown in Fig.5.The yield of HDU is 80.0%at 60°C,and then increased to the highest 95.3%with rising temperature to 90°C.The results are in good agreement with previous reports that high temperature is beneficial for the yield of HDU from MPC and HDA[18].

Fig.5.Effects of reaction temperature on the yield of HDU[m PhONa/m DMC 0.027,n DMC/n HDA 6/1,reaction time 2 h].

4.2.Effects of reaction time

The effects of reaction time on the yield of HDU are shown in Fig.6.A HDU yield of 80%is achieved in a short time of 0.25 h,indicating that enough intermediate MPC could be obtained from DMC and PhONa in a relatively short time.The increase of the HDU yield gradually slows down at the first 2 h and terminates,even when further extending the reaction time.The yield of HDU reached as high as 96.9%in 2 h.

Fig.6.Effects of reaction time on the yield of HDU[m PhONa/m DMC 0.027,n DMC/n HDA 6/1,m DMC 24 g,reaction temperature 90°C].

4.3.Effects of nDMC/nHDA

The effects of nDMC/nHDAon the yield of HDU are shown in Fig.7.The yield of HDU sharply increases from 51.3%to 92.2%with an increase of molar ratio from 2 to 3.Then,it slightly increases and a HDU yield of 98.3%is reached when nDMC/nHDAis 8.The HDU yield remained almost constant with further increase of nDMC/nHDA.The optimum nDMC/nHDAwas 8.

Fig.7.Effects of n DMC/n HDA on the yield of HDU[m DMC 24 g,m PhONa/m DMC 0.027,reaction time 2 h,reaction temperature 90°C].

4.4.Effects of mPhONa/mDMC

The effects of mPhONa/mDMCon the yield of HDU are shown in Fig.8.The amount of PhONa shows an immense influence on the yield of HDU.A HDU yield of only 14%is achieved when mPhONa/mDMCis 0.007.The yield gradually increases to 98.3%with mPhONa/mDMCincreasing from 0.007 to 0.027.Then,it remains almost unchanged with a further increase of mPhONa/mDMC.Thus,the favorable mPhONa/mDMCis 0.027.

Fig.8.Effects of m PhONa/m DMC on yield of HDU[m DMC 24 g,n DMC/n HDA 8,reaction time 2 h,reaction temperature 90°C].

4.5.Comparison of the results in the literature

The comparison of the reaction conditions and results of HDU synthesis reported in the literature is listed in Table 4.Synthesizing HDU through reaction coupling shows a significant advantage.The yield of HDU reached as high as 98.3%in a relatively short time of 2 h.

Table 4 Comparison of the reaction conditions and results of HDU synthesis reported in literature

5.Conclusions

(1)The feasibility of the reaction system was explored by thermodynamics analysis and a reaction mechanism.Thermodynamics analysis indicated that the Keof methoxycarbonylation was much greater than that of trans-esterification.Therefore,the reaction coupling system of transesterification and methoxycarbonylation can break thermodynamic equilibrium of methoxycarbonylation.With MPC as the intermediate,an efficient HDU preparation route can be established.The reaction mechanism indicated that the activation of DMC with phenoxyl ion was the key factor for an increase in HDU yield.

(2)The experiment to synthesize HDU from HDA and DMC with the existence of phenol/sodium phenoxide was carried out to verify the feasibility of the reaction coupling system.The results showed that a high concentration of MPC could be obtained with the existence of DMC and PhONa.However,there was no MPC detected when DMC and PhOH were involved in the reaction.In addition,the yields of HDU synthesized from HDA and DMC with the existence of phenol/sodium phenoxide were also carried out.PhONa in the reaction system was more likely to be dissociated into phenoxyl ion;this result displayed that the addition of PhONa in DMC could be more conducive to the generation of MPC,which further reacted with HDA to produce HDU.These results prove the feasibility of the synthesis of HDU through reaction coupling.However,more thermodynamic parameters of MPC synthesis from PhONa and DMC should be calculated to make an in-depth theoretical explanation for the HDU synthesis through reaction coupling.

(3)The process of HDU synthesis from HDA,DMC and sodium phenoxide was optimized.A yield of HDU as high as 98.3%could be reached under the optimal conditions with mPhONa/mDMC=0.027 and nDMC/nHDA=8/1 at 90°C in 2 h.