Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:794 798 (2006) DOI: 10.1002/jctb.1412 Clean synthesis of propylene carbonate from urea and 1,2-propylene glycol over zinc iron catalyst Xinqiang Zhao, Zhiguang Jia and Yanji Wang Institute of Green Chemical Technology, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China Abstract: A series of zinc iron catalysts has been prepared. The suitable preparation conditions are as follows: zinc nitrate and iron nitrate as precursors, molar ratio of Zn/Fe 2:1, ammonia as the precipitant, precipitation end-point ph 8.04, and calcination temperature 4 C. The clean synthesis of propylene carbonate (PC) via urea and 1,2-propylene glycol (PG) over the zinc iron catalyst has been studied. The optimal reaction conditions are as follows: reaction temperature 1 C, reaction time 2 h, catalyst concentration 1.4% (wt), and molar ratio of urea to PG 1:4. The highest yield of PC was 78.6%. XRD, CO 2 -TPD and XPS techniques have been employed for catalyst characterization. Two kinds of crystal phase, ZnO and ZnFe 2 O 4, have been detected and their synergistic effect promotes the catalytic activity of the oxide catalysts. 2006 Society of Chemical Industry Keywords: zinc iron catalyst; urea; 1,2-propylene glycol; propylene carbonate INTRODUCTION Propylene carbonate (PC) is an important organic solvent and chemical intermediate. It is widely used in organic synthesis, electrochemistry and gas separation. The synthesis of dimethyl carbonate (DMC) through the transesterification route has provided a new field for the application of PC. 1 However, a lot of 1,2-propylene glycol (PG) has been formed as a by-product in the process of production of DMC by the transesterification route. In addition, the utilization efficiency of the raw material, PC, has been lowered. Insertion of carbon dioxide into propylene oxide is the commercial method for the industrial production of PC. However, propylene oxide has a danger of explosion and its supply is controlled by the petrochemical industry. Su and Speranza 2 described a clean process of reacting an alkylene glycol and urea to synthesize PC using a tin-containing catalyst; conversion of PG was only 43%, and the yield and selectivity of PC are 36% and 84% respectively. Yutaka et al. 3 provided a modified process for producing PC from PG and urea under reduced pressure using a catalyst containing at least one metal selected from zinc, magnesium, lead and calcium; the yield of PC was up to 97.2%. But reaction under reduced pressure leads to an extra consumption of energy for assembling a set of vacuum equipment is necessary. Zhao et al. 4 used zinc acetate and supported zinc acetate catalyst in this reaction; the yield of PC was 94% over zinc acetate, and 78% over supported zinc acetate respectively. In addition, a serious loss of zinc acetate was observed for the supported zinc acetate. In this paper, a zinc iron will be prepared first and the clean synthesis of PC from urea and PG will be investigated. By so doing, the byproduct, PG, generated in the process of production of DMC by the transesterification route will be converted to the raw material, PC, and the utilization efficiency of PC will be promoted. EXPERIMENTAL Preparation of catalyst The mixed metal salts solution was prepared in a required Zn/Fe molar ratio, and 5% (vol.) NH 3.H 2 O was made up. A normal-mixing manner (adding ammonia dropwise to the mixed metal salts solution) was used to prepare Zn(OH) 2 Fe(OH) 3 deposition. The deposition was then aged for 24 h and filtered. Finally, the cake was dried at C and calcined at a certain temperature. Correspondence to: Xinqiang Zhao, Institute of Green Chemical Technology, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China E-mail: greenchem@hebut.edu.cn Contract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 205725 Contract/grant sponsor: Hebei Provincial Fund for Natural Science; contract/grant number: 2004000016 (Received 29 December 2004; revised version received 17 May 2005; accepted 12 August 2005) Published online 10 January 2006 2006 Society of Chemical Industry. J Chem Technol Biotechnol 0268 2575/2006/$30.00 794
Synthesis of propylene carbonate from urea and 1,2-propylene Characterization of catalyst X-ray diffraction (XRD) XRD analysis was carried out on a Rigaku D/max- 20 diffractometer using Cu Kα radiation at 40 kv and 100 ma with a scanning rate of 8 min 1 from 2θ = 3 to 2θ =. X-ray photoelectron spectroscopy (XPS) The surface composition and structure of the zinc iron sample were studied by XPS in a PHZ10 system. Mg Kα radiation was used with a vacuum system under a pressure of 10 8 torr. Its working conditions were as follows: 300 W, 15 kv. C1s peak (284.6 ev) was used as the internal standard for binding-energy calibration. CO 2 -temperature programmed desorption (TPD) CO 2 -TPD was used to measure the basicity of the catalyst samples. The experiments were carried out on a set of equipment assembled by the authors. A 200 mg sample was placed in a U-shaped quartz sample tube. The sample was pretreated in ultrahigh pure N 2 at a flow rate of 40 ml min 1 at the calcination temperature for 1 h, and then cooled to ambient temperature. CO 2 was continuously supplied before the sample was saturated. In succession, N 2 was used to purge the system until the baseline of the recorder was straight. Finally, the sample was heated to the calcination temperature at a rate of 10 Cmin 1. Reaction for synthesis of PC The reaction for synthesizing PC from urea and PG was carried out in a 2 ml four-necked flask equipped with a stirrer, water-cooling column and an inlet for nitrogen. The product was analyzed by a SQ-206 mode gas chromatograph with TCD. The chromatograph column was packed with a 402 organic supporter of mesh. The operation conditions were as follows: carrier gas hydrogen; flow rate ml min 1 ; TCD temperature 200 C. RESULTS AND DISCUSSION Effect of preparation condition on the catalytic activity of zinc iron Reaction conditions for evaluating the catalytic activity of the zinc iron are as follows: molar ratio of urea to PG 1:4 (9 g urea); reaction temperature 1 C; reaction time 2 h; weight percentage of the catalyst in the reaction system 1.4%. Screening of precursors for zinc iron Zinc nitrate, iron nitrate, zinc sulfate, iron sulfate, zinc acetate, and iron chloride (AR degree) were selected in pairs as the precursors (molar ratio of Zn/Fe is 4:1), and ammonia as the precipitant. A series of zinc iron double hydroxides were prepared and then calcined at 4 C for 4 h. Table 1 lists the activity of the zinc iron s. The catalyst using zinc Table 1. Effect of precursors on the catalytic activity of the zinc iron Precursors Zn Fe Yield of PC (%) Zn(NO 3 ) 2.6H 2 O Fe(NO 3 ) 3.9H 2 O 75.0 Zn(OAc) 2.2H 2 O Fe(NO 3 ) 3.9H 2 O 61.7 ZnSO 4.7H 2 O Fe(NO 3 ) 3.9H 2 O 63.6 Zn(NO 3 ) 2.6H 2 O Fe 2 (SO 4 ) 3 61.1 Zn(OAc) 2.2H 2 O Fe 2 (SO 4 ) 3.4 ZnSO 4.7H 2 O Fe 2 (SO 4 ) 3 64.1 Zn(NO 3 ) 2.6H 2 O FeCl 3 6H 2 O.0 Zn(OAc) 2.2H 2 O FeCl 3 6H 2 O 69.6 ZnSO 4.7H 2 O FeCl 3 6H 2 O 59.0 Table 2. Effect of Zn/Fe molar ratios on catalytic performance of the zinc iron Zn/Fe molar ratio BET surface area (m 2 g 1 ) Yield of PC (%) 1:1 21.07 61.6 2:1 20.46 78.4 4:1 14.93 75.0 6:1 10.82 73.3 nitrate and iron nitrate as the precursors shows the highest catalytic activity; the yield of PC is 75.0%. Effect of molar ratio of Zn to Fe on the properties of the zinc iron Selecting zinc nitrate and iron nitrate as the precursors, ammonia as the precipitant, zinc iron double hydroxides with different molar ratio of zinc to iron were prepared and then calcined at 4 C for 4 h. BET surface area of the sample has been measured andtheresultsarelistedintable2.itcanbeseen that the specific surface area decreases with increasing of Zn/Fe molar ratio. The yield of PC increases first, reaches its maximum 78.4% at a Zn/Fe molar ratio of 2:1, and then decreases. However, the specific surface area of the sample does not match the PC yield; i.e., the specific surface area cannot correlate with the PC yield. Effect of precipitator on the properties of the zinc iron Choosing zinc nitrate and iron nitrate as the precursors (molar ratio of Zn/Fe is 2:1), the zinc iron double hydroxides were prepared using different precipitators and then were calcined at 4 C for 4 h. The effects of precipitator on the activity of the zinc iron double oxides are demonstrated in Table 3. The zinc iron s using ammonia as the precipitant show the highest activity; the yield of PC is 78.4%. XRD patterns of the zinc-iron samples in Fig. 1 show that they all contain ZnO and ZnFe 2 O 4 with a spinel structure. The sample using Na 2 CO 3 as the precipitator shows wider diffraction peaks than the others, which indicates that its crystal size is smaller. J Chem Technol Biotechnol 81:794 798 (2006) 795
Xinqiang Zhao, Zhiguang Jia and Yanji Wang Table 3. Effect of precipitant on catalytic activity of the zinc iron Precipitant Yield of PC (%) NH 3.H 2 O 78.4 NaOH 72.0 Na 2 CO 3 63.8 ZnO ZnFe 2 O 4 ph=8.04 ph=10 ph=6 0 10 20 30 40 2Theta[deg.] Figure 2. XRD patterns of catalysts prepared at different precipitation end-point ph. Table 5. Effect of calcination temperature on the catalytic activity of the zinc iron s Calcination temperature ( C) Yield of PC (%) Figure 1. XRD patterns of catalysts prepared from different precipitants. Table 4. Effect of precipitation end-point ph on catalytic activity of the zinc iron ph Yield of PC (%) 6 56.3 8.04 78.4 10 74.7 The sample using NaOH as the precipitant shows sharper diffraction peaks, which indicates that its crystal size is larger. The lower activity of the samples prepared using Na 2 CO 3 or NaOH precipitant may be attributed to the remaining sodium ion. Effect of precipitation end-point ph on the properties of the zinc iron Selecting zinc nitrate and iron nitrate as the precursors (molar ratio of Zn/Fe is 2:1), and ammonia as the precipitant, the zinc iron double hydroxides were prepared and then calcined at 4 Cfor4h.The deposition end-point ph value of Fe 3+ is 3.2, while that of Zn 2+ is 8.04. Table 4 indicates that the catalyst shows the highest activity when the end-point ph value was 8.04. Zinc hydroxide precipitates incompletely at an end-point ph of 6 and decrease of ZnO content in the zinc iron will resulted. At an endpoint ph of 10, zinc hydroxide may form [Zn(OH) 4 ] 2 complex, leading to the loss of zinc hydroxide precipitate. In addition, [Zn(NH 3 ) 4 ] 2+ complex may be generated due to the higher concentration of ammonia. 2 68.3 3 73.2 4 78.4 5 69.6 6 64.7 XRD patterns of the three samples in Fig. 2 show that when the end-point PH value is 6, the ZnO phase was not observed and there was only ZnFe 2 O 4 in the sample. ZnO diffraction peak intensity of the sample prepared at ph = 10 is weaker than that at ph = 8.04, which suggests that ZnO content of the sample prepared at ph = 10 is lower. Because the yield of PC is.8% over pure ZnO catalyst and only 26.6% over pure ZnFe 2 O 4 catalyst, the increase in catalytic activity should be attributed to the synergetic effect of both ZnFe 2 O 4 and ZnO. Effect of calcination temperature on the properties of the zinc iron Choosing zinc nitrate and iron nitrate as the precursors (molar ratio of Zn/Fe is 2:1), and ammonia as the precipitant, zinc iron double hydroxides were prepared and calcined at different temperatures for 4 h. Table 5 indicates that the catalyst calcined at 4 C shows the highest activity. XRD analysis and CO 2 -TPD measurement were conducted for the catalyst sample calcined separately at 2, 4 and 6 C.Theresultsareshownin Figs 3 and 4 respectively. In the catalyst calcined at 2 C, only ZnO phase was observed, while in the catalyst samples calcined at 4 or 6 C, both ZnO and ZnFe 2 O 4 phases appeared. This suggests that ZnFe 2 O 4 with a spinel structure can only be formed at a higher temperature, and iron compound is amorphous in the sample calcined at 2 C. For all 796 J Chem Technol Biotechnol 81:794 798 (2006)
Synthesis of propylene carbonate from urea and 1,2-propylene ZnO ZnFe 2 O 4 4 C 6 C 2 C 0 10 20 30 40 2Theta[deg.] Figure 3. XRD patterns of catalysts calcined at different temperatures. 200 C 40 1.0 1.5 2.0 2.5 3.0 3.5 4.0 reaction time /h Figure 5. Effect of reaction time on yield of PC. 218 C 219 C 4 C calcined at 6 C 40 calcined at 4 C 30 145 1 155 1 165 1 175 1 185 1 calcined at 2 C reaction temperature / C 100 200 300 400 0 0 desorption temperature/ C Figure 4. CO 2 -TPD curves of catalysts calcined at different temperatures. of the CO 2 -TPD curves of the three catalyst samples, there is a desorption peak with a top temperature of 200 219 C, corresponding to weaker alkali sites. Furthermore, desorption peak area of the sample calcined at 6 C is smaller, which means that there are fewer alkali sites. More important is that for the catalyst calcined at 4 C there is another desorption peak with a top temperature of 4 C, corresponding to stronger alkali sites. Considering their activity, it is concluded that the stronger alkali site dominates the activity of the catalyst. In conclusion, suitable preparation conditions are as follows: zinc nitrate and iron as precursors, molar ratio of zinc to iron 2:1, ammonia as precipitant, and a calcination temperature of 4 C. Clean synthesis of PC over the zinc iron double oxide catalyst The zinc iron catalyst was prepared under suitable preparation conditions as mentioned Figure 6. Effect of reaction temperature on yield of PC. above, and the effect of reaction conditions on the yield of PC was studied. Reaction time The effect of reaction time on the yield of PC was investigated under the following conditions: molar ratio (urea/pg) 1:4, reaction temperature 1 C, and catalyst concentration 1.4% (wt). Figure 5 indicates that the optimal reaction time is 2 h; PC yield decreases with the prolonging of reaction time due to side reactions such as the polymerization of PC. 5 Reaction temperature Figure 6 shows the effect of reaction temperature on the yield of PC under the same conditions, except that reaction time was 2 h and the reaction temperature was variable. It is obvious that 1 C is the optimal reaction temperature. Because the reaction is endothermic, the elevation of temperature is advantageous. However, the evaporation speed of PG becomes fast when the temperature is above 1 C, as the normal boiling point of PG is 188 C. Furthermore, the elevation of temperature may result in side reactions such as the polymerization of PC. 5 J Chem Technol Biotechnol 81:794 798 (2006) 797
Xinqiang Zhao, Zhiguang Jia and Yanji Wang PC yield, % 100 0.8 1.2 1.6 2.0 2.4 weight percentage of catalyst /% Figure 7. Effect of catalyst concentration on PC yield. 85 2.8 as that under Reaction time above, but the reaction time of 2 h and the molar ratio of urea to PG is a variable. The yield of PC was up to 78.4% with molar ratio (urea/pg) of 1:4. Thus the optimal molar ratio (urea/pg) is 1:4. CONCLUSIONS (1) Zinc-iron catalysts have been prepared. Suitable preparation conditions are as follows: zinc nitrate and iron nitrate as the precursors, molar ratio of Zn/Fe 2:1, ammonia as the precipitant, using normal mixing method, and calcination temperature of 4 C. (2) The optimal reaction conditions for synthesis of PC from urea and PG are as follows: reaction temperature 1 C, reaction time 2 h, weight percentage of the catalyst in the reaction system 1.4%, molar ratio of urea/pg 1:4. The maximal yield of PC is 78.4%. (3) The synergistic effect of ZnO and ZnFe 2 O 4 promotes the catalytic activity of the oxide catalysts. 75 65 ACKNOWLEDGEMENTS This work has been supported by the National Natural Science Foundation of China (205725) and the Hebei Provincial Fund for Natural Science (2004000016); the authors are grateful for their contributions. 55 1: 3 1: 4 1: 5 1: 6 molar ratio of urea to PG Figure 8. Effect of molar ratio of urea to PG on PC yield. Catalyst concentration The effect of amount of catalyst on the yield of PC was studied under the same conditions as that under Reaction time above, but with a reaction time of 2 h, and the catalyst amount is variable. It can be seen from Fig. 7 that the optimal weight percentage of the catalyst in the reaction system is 1.4%. REFERENCES 1 Wei T, Wang M, Wei W, Sun Y and Zhong B. Effect of base strength and basicity on catalytic behavior of solid bases for synthesis of dimethyl carbonate from propylene carbonate and methanol. Fuel Process Technol 83:175 182 (2003). 2 Su WY and Speranza GP. A process for preparing alkylene carbonate. EP patent 0443758A1 (1991). 3 Yutaka K, Takashi O, Masahayu D, Kenichi KI and Atsushi O. A process for producing alkylene carbonate. EP Patent 0581131 (1993). 4 Zhao XQ, Zhang Y and Wang YJ. Synthesis of propylene carbonate from urea and 1,2-propylene glycol over a zinc acetate catalyst. Ind Eng Chem Res 15:4038 4042 (2004). 5 Wei T, Wang M, Wei W, Sun Y and Zhong B. DMC synthesis by transesterification of propylene carbonate and methanol over CaO near room temperature. Chin J Catal 24:52 56 (2003). Molar ratio of urea to PG Figure 8 shows the effect of molar ratio (urea/pg) on the yield of PC under the same reaction conditions 798 J Chem Technol Biotechnol 81:794 798 (2006)