Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 82:209–213 (2007)

Catalytic synthesis of1,6-dicarbamate hexane over MgO/ZrO2Fang Li, Yanji Wang,∗ Wei Xue and Xinqiang ZhaoKey Lab of Green Chemical Technology and High Efficient Energy Saving of Hebei Province, Hebei University of Technology, Tianjin300130, China

Abstract: MgO/ZrO2 catalyst was prepared for the synthesis of 1,6-dicarbamate hexane (HDC) using dimethylcarbonate (DMC) and 1,6-diamine hexane (HDA) as raw materials. When the catalyst is calcined at 600 ◦C andMgO load is 6 wt%, the catalyst exhibits better activity. When the concentration of catalyst is 2 g (100 mL)−1 DMC,n(HDA):n(DMC) = 1:10, reaction time is 6 h under reflux temperature, and the yield of 1,6-dicarbamate hexaneis 53.1%. HDC yield decreases from 53.1% to 35.3% after MgO/ZrO2 being used for three times. The decrease inspecific surface area may be attributed to deactivation of MgO/ZrO2. 2007 Society of Chemical Industry

Keywords: 1,6-diisocyanate hexane; 1,6-dicarbamate hexane; dimethyl carbonate; MgO/ZrO2

INTRODUCTIONOrganic isocyanates are used as monomers in themanufacture of polyurethanes (PUs). At present,most PUs are produced using aromatic isocyanatesas raw material, but PU synthesized from aliphaticisocyanates has an advantage over those synthesizedfrom aromatic isocyanates. For example, paints pro-duced from 1,6-diisocyanate hexane (HDI), which isone of the most widely used aliphatic isocyanates,have better hardness and endurance than those pro-duced from 2,4-toluene diisocyanate (TDI) or 4,4′-diphenylmethane diisocyanate (MDI). On the otherhand, olefin, elastomer and adhesive produced fromHDI have lower toxicity and cause minimal harm tohumans.1

The commonest method for preparing isocyanatesis by reaction of amine with phosgene. This procedurehas several drawbacks. Phosgene is an extremely toxicreagent and HCl is produced as a by-product. HClcan cause serious corrosion and have an effect onthe quality of products. Therefore, it is important tosynthesize isocyanate without using phosgene. Mucheffort has been made in the preparation of isocyanatesusing non-toxic reagents and for the development ofnovel environmentally friendly methodologies. Thecarboalkoxylation of amine with dimethyl carbonate(DMC) is an attractive synthetic route to isocyanate(Scheme 1), in which only methanol is produced asby-product and methanol is the raw material forthe oxidative carbonylation to DMC, which wouldincrease the efficiency of utilization of the rawmaterial and lower the cost for the production ofDMC.2,3

In this route, the synthesis of 1,6-dicarbamate hex-ane (HDC) is the key step because HDI can beeasily obtained by the decomposition of HDC. Toobtain a better HDC yield, catalyst is required. Var-ious catalysts, as NaOCH3,4 Bi(NO3)3,5 Pb(NO3)2,6

Yb(OTf)3,7 La(OTf)3 and Sc(OTf)38 have been such

studied. Among the reported catalysts, NaOCH3

exhibited the best activity and the yield of HDCwas up to 98.4% under certain conditions, butNaOCH3 should be neutralized after the reac-tion and from this a salt was produced, so itcould not be reused. Furthermore, all the otheraforementioned catalysts are difficult to separatefrom products, because they react in the homo-geneous phase. To overcome the drawbacks ofthe reported catalysts, a novel heterogeneous cat-alyst is explored for the synthesis of HDC in thispaper.

EXPERIMENTALThe synthesis of HDC was carried out in a four-neckflask fitted with a reflux condenser, a thermometerand a magnetic stirrer. Dimethyl carbonate (DMC),1,6-diamine hexane (HDA) and the catalyst werecharged into the flask and then heated to the desiredtemperature for a certain time. After reaction, themixture was filtered to separate the catalyst. Thefiltrate was distilled in a vacuum to separate theunreacted DMC for recycling, and the residue waswashed with deionized water to remove the unreactedHDA and the crude product was obtained.

The crude products were analyzed to calculateHDC yield by HPLC (Waters, Milford, MA, USA)

∗ Correspondence to: Yanji Wang, Key Lab of Green Chemical Technology and High Efficient Energy Saving of Hebei Province, Hebei University of Technology,Tianjin 300130, ChinaE-mail: yjwang@hebut.edu.cnContract/grant sponsor: Key Fundamental Research Project of the Ministry of Science and Technology of China; contract/grant number: 2005CCA06100Contract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 20476022Contract/grant sponsor: Natural Science Foundation of Hebei Province; contract/grant number: 202006(Received 26 April 2006; revised version received 13 July 2006; accepted 8 November 2006)Published online 12 January 2007; DOI: 10.1002/jctb.1659

2007 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2007/$30.00

F Li et al.

H2N(CH2)6NH2 + 2CH3OCOCH3

O

H3COOCNH(CH2)6NHCOOCH3 + 2CH3OH

H3COOCNH(CH2)6NHCOOCH3 OCN(CH2)6NCO + 2CH3OH

Scheme 1. The reaction equation for synthesis of HDI.

equipped with a 515 pump, and a 2487 UV detector,which was working at 210 nm. The mobile phasehas a composition of 70% CH3OH and 30% H2O(volume ratio) and its flow rate was 0.4 mL min−1.The stationary phase was a Kromasil C-18 column(150 mm, 4.6 mm, 5 µm particles) operating at roomtemperature.

MgO/ZrO2 catalyst was prepared as follows:appropriate amounts of ZrOCl2 were dissolved indeionized water, and the solution was precipitatedusing NH3.H2O at pH = 10; the precipitate wasfiltered and washed with deionized water severaltimes to remove the Cl− completely, and then driedat 120 ◦C to prepare ZrO(OH)2. ZrO(OH)2 wasimpregnated into a Mg(NO3)2 solution by incipientimpregnation process; the impregnating samples werekept overnight at room temperature, then dried at60 ◦C in a vacuum and calcined at 600 ◦C for 4 h.

X-ray diffractions were recorded on a RigakuD/MAX2500 (Tokyo, Japan). The thermal analysiswas performed with a Rigaku TG-DTA analyzer.Brunauer-Emmett-Teller (BET) surface area mea-surements were made using a Micromeritics ASAP2020 (Norcross, GA, USA).

RESULTS AND DISCUSSIONThe screen of loaded MgO catalysts withdifferent supportsIt was reported that the yield of HDC was 98.4%over NaOCH3,4 but in this process it was necessary toneutralize the basic catalyst and a large amount of saltwas produced, so the catalyst could not be reused. Thisproblem could be overcome if a kind of solid base weredeveloped to catalyze the reaction. MgO is a commonsolid base. Thus a series of loaded MgO catalystswere prepared and used as catalysts for the synthesisof HDC under the same conditions. The results areshown in Table 1 (entries 1–4). MgO/ZrO2 shows thehighest activity among the catalysts. Their catalyticactivities decrease progressively in the following order:ZrO2 > SiO2 > γ -Al2O3, TiO2. These differences inactivity can be attributed to the different structuralparameters of the supports, such as specific surfacearea and pore size distribution, as well as to theirdistinct surface properties, e.g. acidity or basicityand the interaction between the support and theactive component. In addition, the HDC yield overMgO/ZrO2 is much higher than that over MgO orZrO2 (entries 5–7). The reason is that the synthesisof HDC can be effectively promoted by base, andit is reported that the loading of MgO on ZrO2 canachieve higher basicity than that of MgO or ZrO2

by the CO2 –TPD experiments.9 Thus the catalyticactivity of MgO/ZrO2 is higher than that of MgO orZrO2 alone. Based on the preliminary results recordedin Table 1, further studies on the MgO/ZrO2 catalystfor the synthesis of HDC were carried out.

Influence of MgO loads on HDC yieldIn order to investigate the influence of MgO load oncatalytic activity, a series of MgO/ZrO2 with differentMgO loads have been prepared and their catalyticactivities have been evaluated. The results are shownin Fig. 1. It can be seen that HDC yield gains with anincrease of MgO load and reaches the maximum valueof 53.1% when the load of MgO is 6 wt%. After that

Table 1. Catalytic activity for the synthesis of HDC over the loaded

MgO catalysts with different supports

Entry Catalyst n(HDA):n(DMC)HDC yield

(%)

1 MgO/γ -Al2O3a 1:32 0

2 MgO/TiO2a 1:32 0

3 MgO/SiO2a 1:32 4.2

4 MgO/ZrO2 1:32 18.15 MgO/ZrO2 1:10 53.16 MgO 1:10 10.47 ZrO2

b 1:10 2.6

Reaction temperature (refluxing temperature) 90 ◦C; reaction time 6 h;catalyst amount 2 g (100 mL)−1 DMC.a γ -Al2O3, TiO2 and SiO2 were commercial products.b ZrO2 was prepared by calcination of ZrO(OH)2, obtained through theprocess described in Experimental, at 600 ◦C for 4 h.

2% 6% 10% 15%20

25

30

35

40

45

50

55

60

HD

C y

ield

/ %

MgO loadings / wt%

Figure 1. Effect of MgO loadings on HDC yield. Reactiontemperature (refluxing temperature) 90 ◦C; reaction time 6 h;HDA/DMC molar ratio 1:10; catalyst amount 2 g (100 mL)−1 DMC; thecatalyst was calcined at 600 ◦C.

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Catalytic synthesis of 1,6-dicarbamate hexane over MgO/ZrO2

10 20 30 40 50 60 70 80

a

dcb

MgO

ZrO2

2 Theta / °

Figure 2. XRD patterns of the MgO/ZrO2 catalysts with different MgOloadings: (a) 2 wt%; (b) 6 wt%; (c) 10wt%; (d) 15wt%).

the HDC yield decreases with a further increase in theload.

Figure 2 shows the XRD patterns of MgO/ZrO2with different MgO loads. When the MgO load isless than 10 wt%, there are no diffraction peakscorresponding to MgO. Once the MgO load is greaterthan 10 wt%, the diffraction peaks of MgO appear.De-en Jiang9 has reported that MgO can dispersespontaneously onto the surface of ZrO2 to form amonolayer. Therefore, when the MgO loading is lowerthan 10 wt%, MgO is in a monolayer state and cannotbe detected by XRD. Once the loading reaches 10wt%, a crystalline phase of MgO forms which can bedetected by XRD. The authors have also stated9 thatMgO/ZrO2 catalyst possessed the strongest basicitywhen MgO content was near the threshold of themonolayer state. According to Fig. 2, when MgO loadis 6 wt%, the value is near the threshold. This isbecause a crystalline phase of MgO appears after MgOload is increased continuously. Thus the catalyst with6 wt% MgO load possesses the stronger basicity andexhibits the better activity for synthesis of HDC.

Effect of calcination temperature on HDC yieldFigure 3 illustrates the influence of catalyst calcinationtemperature on HDC yield under the same reactionconditions as set out in Fig. 1. The MgO/ZrO2 catalystcalcined at 600 ◦C shows the better activity.

Table 2 records the specific surface area ofMgO/ZrO2 calcined at different temperatures. The

Table 2. Specific surface area of MgO/ZrO2 catalysts calcined at

different temperatures

EntryCalcination temperature

(◦C)Specific surface area

(m2 g−1)

1 500 121.22 600 59.23 700 40.4

0

10

20

30

40

50

700600500

HD

C y

ield

/ %

Calcined temperature / °C

Figure 3. The influence of calcination temperatures on HDC yield.

specific surface area of MgO/ZrO2 decreases withincreasing calcination temperature because the crystalgrain of the catalyst would agglomerate and growat higher temperature. The specific surface areaof MgO/ZrO2 calcined at 700 ◦C is the smallest,which might be the reason for its lower activity.The specific surface area of the catalyst calcinedat 500 ◦C, however, is greater than that of thecatalyst calcined at 600 ◦C; it also has a loweractivity. In order to find the reason for this phe-nomenon, Mg(NO3)2/ZrO(OH)2 was characterizedby TG-DTA. Figure 4 shows the TG-DTA pro-file of Mg(NO3)2/ZrO(OH)2. Mg(NO3)2/ZrO(OH)2decomposes completely at about 430 ◦C and the cal-cination temperature should be greater than 430 ◦C.In addition, there is an exothermic peak at 566 ◦Cwhich may be attributed to the phase transformationas there is no accompanying weight loss in the TGcurve. To investigate what had happened at 566 ◦C,MgO/ZrO2 catalyst calcined at different temperatureswere characterized by XRD and the results are shownin Fig. 5. There are only diffraction peaks correspond-ing to ZrO2 when calcination temperature is less than600 ◦C. Once the catalyst is calcined at higher tem-perature, such as 700 ◦C, the crystalline MgO reunitesand grows and can be detected by XRD. It can be seenfrom Fig. 5 that the phase of ZrO2 does not changewhen the temperature increases from 500 to 700 ◦C.Thus the exothermic peak at 566 ◦C in Fig. 4 may beattributed to the phase transformation of MgO. It isdeduced that the phase of MgO has an effect on theactivity of catalyst, which results in its lower activitythan that of MgO/ZrO2 calcined at 600 ◦C.

It can be concluded that the optimal conditions forpreparing MgO/ZrO2 are as follows: MgO loading is6 wt% and the calcination temperature is 600 ◦C.

Effect of reaction conditions on HDC yieldMgO/ZrO2, with 6 wt% MgO loading, calcined at600 ◦C, was used as catalyst for the synthesis of HDCand the effect of the reaction conditions was studied.

Figure 6 shows the effect of reaction temperatureon the yield of HDC. The top limit of the reaction

J Chem Technol Biotechnol 82:209–213 (2007) 211DOI: 10.1002/jctb

F Li et al.

200 400 600 800

−4

−2

0

DTA

TG

TG

Wei

ght (

−mg)

Temperature / °C

566°C408°C

82°C

Figure 4. TG-DTA profile of Mg(NO3)2/ZrO(OH)2 (Mg(NO3)2 loadingis calculated as MgO, 6 wt%).

10 20 30 40 50 60 70 80

MgO

ZrO2

c

b

a

2 Theta / °

Figure 5. XRD patterns of MgO/ZrO2 catalysts calcined at differenttemperatures: (a) 700 ◦C; (b) 600 ◦C; (c) 500 ◦C.

60 65 70 75 80 85 900

10

20

30

40

50

60

HD

C y

ield

/ %

Reaction temperature / °C

Figure 6. Effect of reaction temperature on HDC yield. Reaction time6 h; HDA/DMC molar ratio 1:10; catalyst amount 2 g (100 mL)−1 DMC.

temperature is defined at 90 ◦C because the normalboiling point of DMC is 90.2 ◦C. It can be seen fromFig. 5 that the HDC yield increases with reaction

temperature and reached the maximal value, 53.1% at90 ◦C.

The effect of reaction time on HDC yield is shownin Fig. 7. The HDC yield increases with reaction timeand reaches a maximum at 53.1% for 6 h. The HDCyield decreases when the reaction time exceeds 6 h,because the side reactions, such as the polymerizationof HDC, will take place for a prolonged reaction time.The optimal reaction time is therefore 6 h.

The effect of catalyst amount on HDC yield isshown in Fig. 8. The HDC yield increases withincreasing amount of catalyst and HDC yield reachesa maximum at 53.1%, when the catalyst concentrationis 2 g (100 mL)−1 DMC. As the catalyst concentrationexceeds 2 g (100 mL)−1 DMC the HDC yielddecreases.

The catalyst shows optimum activity under thefollowing conditions: reaction temperature of 90 ◦C,which is the refluxing temperature; a catalyst amountof 2 g (100 mL)−1 DMC; reaction time of 6 h and a

2 3 5

Reaction time / h

74 6 80

10

20

30

40

50

60H

DC

yie

ld /

%

Figure 7. Effect of reaction time on HDC yield. Reaction temperature(refluxing temperature) 90 ◦C; HDA/DMC molar ratio 1:10; catalystamount 2 g (100 mL)−1 DMC.

0.5 1.0 1.5 2.0 2.5 3.0

20

30

40

50

60

HD

C y

ield

/ %

Catalyst amount / g (100 mL)−1 DMC

Figure 8. Effect of catalyst amounts on HDC yield. Reactiontemperature (refluxing temperature) 90 ◦C; reaction time 6 h;HDA/DMC molar ratio 1:10.

212 J Chem Technol Biotechnol 82:209–213 (2007)DOI: 10.1002/jctb

Catalytic synthesis of 1,6-dicarbamate hexane over MgO/ZrO2

Table 3. Results of reuse of catalyst

Times of catalyst reusedHDC yield

(%)

1 53.12 47.33 35.3

molar ratio of HDA to DMC of 1:10. Under theseconditions, the HDC yield is up to 53.1%.

Deactivation of MgO/ZrO2 catalystIn order to investigate the stability and service time ofMgO/ZrO2, the used catalyst was recovered by filteringthe reaction liquid, washing with DMC, and then dry-ing it for reuse. The reaction conditions were the sameas set out in Fig. 1. The results are shown in Table 3.HDC yield decreases from 53.1% to 35.3% afterMgO/ZrO2 has been used three times. At the sametime, the specific surface area of MgO/ZrO2 decreasesfrom 59.2 to 32.4 m2 g−1 after the catalyst is usedthree times. This is likely to arise because the organicsubstance blocks the pores of the catalyst and the porevolume decreases, resulting in a decrease in specificsurface area and hence in deactivation of the catalyst.

CONCLUSIONMgO/ZrO2 catalyst was prepared for the synthesisof 1,6-dicarbamate hexane using dimethyl carbonateand 1,6-diamine hexane as raw materials. When theMgO/ZrO2 catalyst is calcined at 600 ◦C and MgOloading is 6 wt%, the catalyst exhibits optimal activity.When the catalyst concentration is 2 g (100 mL)−1

DMC, n(HDA):n(DMC) = 1:10, reaction time is6 h under reflux temperature, and the yield of 1,6-dicarbamate hexane is 53.1%. The decrease in specific

surface area may be attributed to the deactivation ofMgO/ZrO2.

ACKNOWLEDGEMENTSThis work has been supported by the Key Funda-mental Research Project of the Ministry of Scienceand Technology of China (no. 2005CCA06100), theNational Natural Science Foundation of China (no.20476022) and the Natural Science Foundation ofHebei Province (no. 202006). The authors are gratefulfor their contribution.

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J Chem Technol Biotechnol 82:209–213 (2007) 213DOI: 10.1002/jctb