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Ind. Eng. Chem. Res. 2004, 43, 4038-4042
Synthesis of Propylene Carbonate from Urea and 1,2-Propylene Glycol over a Zinc Acetate Catalyst Xinqiang Zhao,* Yan Zhang, and Yanji Wang Institute of Green Chemical Technology, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, People’s Republic of China
On the basis of thermodynamic analysis for the synthesis of propylene carbonate from urea and 1,2-propylene glycol (PG), the catalytic properties of zinc acetate have been studied. The optimal reaction conditions are as follows: molar ratio of urea to PG, 2:8; reaction time, 3 h; reaction temperature, 170 °C; molar ratio of zinc acetate to PG, 1:148. The highest yield of propylene carbonate is 94%. Then the immobilization of zinc acetate is investigated in order to facilitate recovery and reuse of the catalyst. The suitable support is activated carbon, and the optimal load of zinc acetate is 15 wt %; the highest yield of propylene carbonate is 78%. The analysis of X-ray photoelectron spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller surface area analysis, and atomic absorption spectroscopy for the catalysts used both before and after reaction shows that both some changes and severe loss of zinc acetate have taken place during the process of the reaction. 1. Introduction
2. Thermodynamic Analysis
Propylene carbonate (PC), not only as a good organic solvent but also an important organic product, has been used widely in the fields of organic synthesis, gas separation, electrochemistry, metal extraction, etc. At present, PC is mostly used for the production of dimethyl carbonate (DMC) through transesterification with methanol, which promotes the development of a new route for synthesis of PC. Now PC is mainly produced industrially using CO2 and propylene oxide, which causes serious safety problems because propylene oxide is a dangerous chemical substance. In this paper, a new route for the synthesis of PC from urea and 1,2-propylene glycol (PG) is proposed. As compared with existing routes, this one has many advantages: cheap and easily available raw material, mild reaction conditions, and safe operations. In addition, the byproduct ammonia can be recycled to the urea production unit. Even more important is, by the present route, that PG, as a byproduct in the transesterification process for the production of DMC, can be reconverted into the raw material, PC. This would increase the efficiency of utilization of the raw material and greatly lower the cost for the production of DMC. At present, studies on this route are very few. Su and Speranza1 used urea and alkylene glycol to prepare the corresponding alkylene carbonates over a toxic organictin catalyst or without any catalysts; conversion of the alkylene glycol was only 66% of the theoretical value, and there was a severe decomposition of urea. Yutaka et al.2 increased the yield of PC by using a catalyst of zinc, magnesium, lead, and calcium or their compounds under vacuum, under the conditions of 145 °C, 22 kPa, and 2 h; the selectivity of PC to PG was 99.8%, and that to urea was 97.2%. In this paper, the synthesis of PC from urea and PG is studied over homogeneous zinc acetate and supported zinc acetate catalyst.
The reaction formula of urea with PG to PC is as follows:
Thermodynamic calculations based on the data in Table 1 show that the enthalpy change (∆rH) and the Gibbs free energy change (∆rG) of the reaction is 51.60 and 13.99 kJ‚mol-1, respectively, under the conditions of 0.1013 MPa and 298.15 K. The positive enthalpy change means an endothermal reaction; an elevation of the reaction temperature will cause an increase of the equilibrium constant. However, the positive Gibbs free energy change suggests that the reaction cannot proceed at 298.15 K. The dependence of ∆rG and the equilibrium constant K on the reaction temperature is listed in Table 2. ∆rG decreases gradually with an increase of the reaction temperature and will become zero at 335 K. It will become negative above 335 K, which suggests that the reaction can now proceed normally. The equilibrium constant of the reaction is 3.572 × 103 at 443.15 K, which means that a significant conversion of the reactant will be expected. It can be concluded that the reaction of the synthesis of PC from urea and PG is possible thermodynamically and that kinetic factors must be considered in order to increase the reaction rate and to shorten the time taken to reach the equilibrium. Therefore, the key is to find a highly effective catalyst. 3. Experimental Section 3.1. Reaction Apparatus. An atmospheric reaction apparatus assembled from a four-necked flask, an electric agitator, a tubular condenser, etc., was used in the
10.1021/ie049948i CCC: $27.50 © 2004 American Chemical Society Published on Web 06/23/2004
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4039 Table 1. Thermodynamic Data of the Components component
normal state
∆H°f,298 (kJ‚mol-1)
PGa ureab PCa ammoniab
liquid liquid liquid gas
-468.97 -319.37 -644.52 -46.11
Table 3. Catalytic Activity of Different Catalysts
S°298 (J‚mol-1‚K-1) 180.90 150.89 73.31 192.34
a
Data were obtained from the literature.3,4 calculated using Benson’s bond addition method.5
b
species zinc
magnesium
Data were
Table 2. Dependence of ∆rH, ∆rS, and ∆rG and the Equilibrium Constant K on the Reaction Temperature T (K)
∆ rH (kJ‚mol-1)
∆ rS (J‚mol-1‚K-1)
∆ rG (kJ‚mol-1)
K
313.15 328.15 343.15 358.15 373.15 388.15 403.15 418.15 433.15 443.15 453.15 458.15 463.15
48.599 45.684 40.851 36.080 31.393 26.752 20.159 14.607 8.0867 4.130 2.761 1.133 0.412
134.562 130.468 127.024 117.148 106.770 97.831 88.277 85.061 81.140 77.336 74.632 72.815 72.140
6.461 2.871 -2.737 -5.877 -8.448 -11.221 -15.430 -20.961 -27.059 -30.141 -31.059 -32.228 -33.000
8.361 × 102 3.491 × 101 2.610 7.197 1.523 × 101 3.237 × 101 9.983 × 101 4.154 × 102 1.833 × 103 3.572 × 103 3.804 × 103 4.726 × 103 5.271 × 103
experiment. Nitrogen gas was introduced to remove the byproduct ammonia out of the reaction system quickly. 3.2. Product Analysis. A model SQ-206 gas chromatograph (GC) with a thermal conductivity detector was used to analyze the composition of the reaction product. The chromatographic column was packed with GDX-102, and the temperature was controlled at 180 °C. The flow of hydrogen as the carrier gas was adjusted to 100 mL‚min-1. 3.3. Catalyst Preparation. Magnesium oxide was obtained by calcination of magnesium hydrogencarbonate. Other metal oxides were prepared using a precipitation method including the formation of metal hydroxides, drying, and calcination. A supported zinc acetate catalyst was prepared by an incipient impregnation process. Other catalysts were purchased in the market. 3.4. Catalyst Characterization. X-ray photoelectron spectroscopy (XPS) analysis was made in a PHI1600 ESCA SYSTEM to analyze the valance of surface elements of the catalysts. Mg KR radiation was the light source with an anodic voltage of 1253 eV. The vacuum was controlled above 2 × 10-8 Torr. The charging effect was calibrated using the electronic binding energy of C 1s of polluted carbon as 284.6 eV. X-ray diffraction (XRD) studies were carried out using a Rigaku D/max-2500 spectrometer. Cu KR radiation was the light source used with an applied voltage of 40 kV and a current of 100 mA. The 2θ angle ranged from 5 to 60°. Brunauer-Emmett-Teller (BET) surface area measurements were done using a ASAP 2020 instrument to analyze the specific surface area, pore volume, and pore-size distribution of the catalysts. Atomic absorption spectroscopy (AAS) analysis was conducted in a Hitachi 180-80 atomic absorption spectrometer to analyze the content of a zinc atom in supported zinc acetate before and after reaction. GC-mass spectrometry (MS) analysis was made on a Finnigan Trace DSQ spectrometer to analyze qualitatively byproducts of the reaction of PG and urea.
lead
tin
catalyst
PC yield (%)
no ZnO Zn(OAc)2 Zn(OAc)2‚2H2O zinc powder MgO MgCO3 Mg(OAc)2 MgO-ZrO2 lead powder Pb(OAc)2‚3H2O Pb(OAc)2 PbO, purchased PbO, calcined at 750 °C PbO, calcined at 450 °C dibutyltin oxide
44 67 76 54 57 75 56 38 45 59 38 58 47 57 52 61
4. Results and Discussion 4.1. Reaction Schemes. The synthesis of PC from PG and urea is a multiple reaction system, and there are several side reactions. According to the result of GC-MS analysis of the reaction product mixture, some side reactions have been proposed as follows:
4.2. Activity of Catalysts. The reaction of urea and PG to synthesize PC without catalyst or over a series of catalysts was conducted under the following conditions: molar ratio of urea to PG, i.e., n(urea):n(PG) ) 1:6; molar ratio of catalyst to PG, i.e., n(cat):n(PG) ) 1:148; reaction temperature, 170 °C; reaction time, 5 h. The results listed in Table 3 show the following: (1) Among zinc-based catalysts, the activity of dehydrated zinc acetate is the highest; the yield of PC is 76%. However, the activity of zinc acetate with crystal water is the lowest; the PC yield is only 54%. (2) Among magnesium-based catalysts, MgO obtained from elevated calcination shows the highest activity; the PC yield is 75%. Magnesium acetate is the worst; the PC yield is only 38%. (3) Among lead-based catalysts, lead powder shows the highest activity while lead acetate with crystal water shows the lowest activity. It can be inferred that crystal water has a negative effect on the PC synthesis possibly because of hydrolysis of PC. Because dehydrated zinc acetate shows the highest catalytic activity, it is necessary to study it further. 4.3. Homogeneous Dehydrated Zinc Acetate Catalyst. 4.3.1. Influence of the Molar Ratio of Urea to PG. Figure 1 illustrates the influence of n(urea):n(PG) on the PC yield under the conditions of 170 °C, 3 h, and n(zinc acetate):n(PG) ) 1:148. The PC yield rises sharply with an increase of n(urea):n(PG), reaches the maximum, 94%, at n(urea):n(PG) ) 2:8, and then decreases when n(urea):n(PG) is beyond 2:8. When
4040 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 3. Influence of the amount of catalyst on the yield of PC. Figure 1. Influence of the molar ratio of urea to PG on the yield of PC.
Figure 4. Influence of the reaction temperature on the yield of PC. Figure 2. Influence of the reaction time on the yield of PC.
n(urea):n(PG) is smaller, the concentration of urea is much lower, which results in a lower reaction rate and then in a lower PC yield. With an increase of n(urea): n(PG), the enlarged concentration of urea promotes the PC yield. However, too large a concentration of urea would accelerate the reaction of decomposition of urea (eq 5), which results in a decrease of the PC yield. 4.3.2. Influence of the Reaction Time. The influence of the reaction time on the PC yield is demonstrated in Figure 2. The reaction conditions are the same as the above-mentioned conditions except n(urea): n(PG) ) 2:8 and the reaction time is a variant. Because the initial concentration of urea is larger, the reaction proceeds fast and the PC yield reaches 73% after 1 h. As the reaction proceeds, conversion of urea increases. The PC yield reaches the maximal value, 94%, after 3 h and then drops quickly with the prolonging of the reaction time because of deep side reactions such as the polymerization of PC. 4.3.3. Influence of the Amount of Catalyst. Figure 3 shows the influence of the catalyst amount on the PC yield. The reaction conditions are the same as those in section 4.3.1 but n(urea):n(PG) ) 2:8 and n(zinc acetate): n(PG) is not a constant. The PC yield increases significantly with an increase of n(zinc acetate):n(PG) from 0 to 1:445 and then gets to its maximum. In addition, the color of the reaction solution becomes darker, which may be attributed to side reactions (eqs 3 and 4). 4.3.4. Influence of the Reaction Temperature. The influence of the reaction temperature on the PC yield is shown in Figure 4. The reaction conditions are
the same as those in section 4.3.1 but n(urea):n(PG) ) 2:8 and the reaction temperature is a variant. The top limit of the reaction temperature is defined at 180 °C because the normal boiling point of PG is 188.2 °C. Because the reaction is endothermic, the higher the reaction temperature, the higher the conversion of the reactants. It can be seen from Figure 4 that, with the elevation of the reaction temperature, the PC yield increases first and gets its maximum, 94%, at 170 °C, and then decreases because of the reaction of polymerization of PC when the reaction temperature surpasses 170 °C. It can be concluded that the optimal reaction conditions for homogeneous dehydrated zinc acetate are as follows: at a reaction temperature of 170 °C, for a reaction time of 3 h, n(urea):n(PG) ) 2:8, and n(zinc acetate):n(PG) ) 1:148; the highest yield of PC is 94%. 4.4. Supported Zinc Acetate Catalyst. 4.4.1. Effect of Supports. Figure 5 shows the activity of 15 wt % zinc acetate supported on different supports under the conditions of 170 °C, 3 h, n(urea):n(PG) ) 2:8, and n(zinc acetate):n(PG) ) 1:148. Table 4 lists the physical properties of the different supports. The result shows that the catalytic activity of zinc acetate supported on the supports with larger specific surface area (AC) or larger average pore size (γ-Al2O3) is much higher and AC is the suitable support. Therefore, the specific surface area and average pore size are the two main factors affecting the catalytic activity. 4.4.2. Effect of Load. The catalytic activity of zinc acetate supported on AC with different loads is il-
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4041
Figure 7. XRD patterns of Zn(OAc)2/AC before and after reaction.
Figure 5. Effect of supports on the yield of PC.
Figure 8. Electronic binding energy of Zn 2p3/2 of Zn(OAc)2/AC before and after reaction. Figure 6. Effect of the load of Zn(OAc)2 on the yield of PC. Table 4. Physical Properties of the Supports support
specific surface area (m2‚g-1)
average pore size (nm)
pore volume (cm3‚g-1)
activated carbon γ-Al2O3 zeolite
917.7 204.1 208.2
2.199 10.20 0.7744
0.5044 0.5202 0.1562
Table 5. Physical Properties of the Catalysts with Different Loads of Zinc Acetate zinc acetate load (wt %) physical properties (m2‚g-1)
specific surface area pore volume (cm3‚g-1) average pore size (nm)
15
20
415.3 0.2311 2.226
353.3 0.1993 2.256
lustrated in Figure 6 under the same conditions as the above-mentioned conditions except the weight percentage of Zn(OAc)2/AC in the reaction mixture is 8.3%. The catalytic activity gains with an increase of the zinc acetate load and gets the maximal value, 78%, when the load of zinc acetate is 15 wt %. After that the catalytic activity decreases with a further increase of the load. Table 5 demonstrates that when the weight load increases from 15% to 20%, the specific surface area and pore volume of Zn(OAc)2/AC decrease 15% and 14%, respectively, while the average pore size remains almost constant. This suggests that an increase of the zinc acetate load results in the formation of a large crystal
particle size, which blocks the inner pore of Zn(OAc)2/ AC and brings about a decrease of the catalytic activity. 4.4.3. Stability of the Zn(OAc)2/AC Catalyst. To investigate the stability of the Zn(OAc)2/AC catalyst, the used Zn(OAc)2/AC was recovered by filtrating the reaction product solution, washing with ethanol, and then drying. The evaluation result shows that the catalytic activity drops significantly; the PC yield decreases from 78% to 66%. XRD patterns of fresh and recovered Zn(OAc)2/AC are shown in Figure 7. The diffraction peak of zinc acetate in the recovered catalyst widens significantly and almost vanishes. This demonstrates that the crystal size of zinc acetate may become smaller and the dispersion of zinc acetate on the surface of AC may tend more uniform. Additionally, this may also suggest the loss of zinc acetate. XPS analyses of fresh and recovered catalysts were made, and the result is illustrated in Figure 8. The electronic binding energy of Zn 2p3/2 decreases from 1022.6 eV of the fresh catalyst to 1021.35 eV of the recovered one. This means that some changes of zinc acetate have taken place during the process of the reaction and make the outer electronic cloudy density of the zinc atom increase so as to lessen the electronic binding energy of Zn 2p3/2. In addition, AAS analysis shows that the weight content of the zinc atom diminishes from 3% before reaction to 2.3% after reaction, which means that there is a serious loss of zinc acetate during the reaction process and results in the drop in the catalytic activity.
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5. Conclusion
Literature Cited
A thermodynamic analysis for the synthesis of PC from urea and PG has been made. The optimal reaction conditions over homogeneous dehydrated zinc acetate are as follows: at a reaction temperature of 170 °C, for a reaction time of 3 h, n(urea):n(PG) ) 2:8, and n(zinc acetate):n(PG) ) 1:148. The highest yield of PC is 94%. As for supported zinc acetate, the suitable support is activated carbon, and the optimal load of zinc acetate is 15 wt %. The highest yield of PC is 78%. On the basis of analyses of XPS, XRD, BET, and AAS, it has been found that some changes of zinc acetate have taken place and that there is a serious loss of zinc acetate also.
(1) Su, W.-Y.; Speranza, G. P. A process for preparing alkylene carbonate. EP Patent 0443758A1, 1991; Chem. Abstr. 1991, 114, 247258q. (2) Yutaka, K.; Takashi, O.; Masaharu, D.; Kenichi, K. I.; Atsushi, O. A process for producing alkylene carbonate. EP Patent 0581131A2, 1993; Chem. Abstr. 1993, 120, 245070x. (3) Peppzl, W. J. Preparation and Properties of the Alkylene Carbonates. Ind. Eng. Chem. 1958, 50 (5), 767. (4) Yao, Y.; Xie, T.; Gao, Y. Handbook of Physical Chemistry (in Chinese); Shanghai Science and Technology Press: Shanghai, China, 1985. (5) Benson, S. W. Thermochemical Kinetics: Methods for the Estimation of Thermochemical Data and Rate Parameters; Wiley: New York, 1968.
Acknowledgment This work has been supported by Tianjin Fund for Natural Science (Grant 033603311). The authors are grateful for their contribution.
Received for review January 16, 2004 Revised manuscript received April 19, 2004 Accepted May 23, 2004 IE049948I
