Ind. Eng. Chem. Res. 2010, 49, 5543–5548
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Heat Capacity, Enthalpy of Formation, and Entropy of Methyl Carbamate Zuo-Xiang Zeng, Xiao-Nan Li, Wei-Lan Xue,* Chuan-Shun Zhang, and Shi-Chao Bian Institute of Chemical Engineering, East China UniVersity of Science and Technology, 200237 Shanghai, People’s Republic of China
The heat capacity (Cp) of methyl carbamate (MC) was measured with an adiabatic calorimeter. The standard enthalpy of formation (∆fH0(cr, 298.15 K)) and the standard entropy (S0(cr, 298.15 K)) for MC were determined. Thus, Cp(T) values of MC were obtained for the crystal and liquid forms in the temperature range of 298.15-433.10 K. A quadratic trinomial was used to correlate the data of Cp(T). The enthalpy of fusion of MC was measured using differential scanning calorimetry (DSC), and its sublimation enthalpy was estimated. The gas-phase standard enthalpy of formation (∆fH0(g, 298.15 K)) of MC was calculated via the method of group contribution developed by Benson. Based on the above data, the values of ∆fH0(cr, 298.15 K) and S0(cr, 298.15 K) of MC were estimated to be -513.57 kJ/mol and 77.1 J mol-1 K-1, respectively. A thermochemical cycle including a reversible reaction was designed to examine the reliability of the estimated values by measuring the equilibrium constant of the reaction. The results showed that the estimated values for MC were acceptable. 1. Introduction Thermodynamic properties such as heat capacity, enthalpy, and entropy are fundamental characteristic data that are concerned with the structure of a substance. These data have extensive application in physics, chemistry, and industrial engineering, especially in chemical reaction engineering. For example, heat capacity, enthalpy, and entropy can be used to calculate the standard Gibbs energy change of a reaction. Methyl carbamate (CAS Registry No. 598-55-0), which is also called urethylane and methylurethane, has a formula of H2NCOOCH3, a relative density of 1.136, and a molecular mass of 75.07 g/mol; it is a white crystal and has an extensive application foreground in organic synthesis,1-3 pesticides,4,5 and textiles.6,7 In particular, it is also an important intermediate in manufacturing dimethyl carbonate (DMC),8 which is mainly used as a methylation reagent, carbonylation reagent, ester interchange reagent, etc. Therefore, it is important to understand the thermodynamic properties of MC for reactions in which MC is a reactant or a product, such as the synthesis reaction of MC from methanol and urea. However, until now, the thermodynamic properties of MC, such as its heat capacity, standard enthalpy of formation, and standard entropy, have not been reported in the literature. Therefore, in this work, the Cp(T), ∆fH0(cr, 298.15 K), and S0(cr, 298.15 K) values of MC were studied. An adiabatic calorimeter was used to measure the heat capacities of MC and urea. The ∆fH0(g, 298.15 K) value of MC was estimated via the Benson group additive method.9 A thermochemical cycle was designed to examine the reliability of the calculated values by measuring the equilibrium constant of the synthesis reaction of MC from methanol and urea at 433.15 K. 2. Experimental Section 2.1. Materials. MC prepared in the laboratory was recrystallized prior to use. Its mass fraction purity, as determined by gas chromatography (GC), was ∼99.5%. It melted at a temperature of 328.8 K, and the melting point that has been reported in the literature was 328.6 K.10 The methanol and * To whom correspondence should be addressed. Tel.: +86-0216425-3081. E-mail:
[email protected].
urea used here was purchased from Shanghai Chemistry Reagent Co., China. The purity of methanol and urea was better than 99.5%. 2.2. Apparatus and Procedure. 2.2.1. Adiabatic Calorimeter. The principle and structure of the calorimeter were described in detail elsewhere.11-14 Briefly, the automatic adiabatic calorimeter (Model RD496, Shanghai, China) consisted of a sample cell, a miniature platinum resistance thermometer, an electric heater, inner and outer adiabatic shields, a Dewar vessel, a high-vacuum can, and two sets of six-junction chromel-constantan thermopiles that were installed between the inner and outer shields. To verify the reliability of the adiabatic calorimeter, the molar heat capacities for the reference standard material R-aluminum oxide were measured prior to the measurement of the sample. The deviations between the experimental data and that recommended by the National Bureau of Standards15 were within (0.4% in the temperature range of 298-434 K. The measurements were performed by means of the standard method, which involved intermittently heating the sample and alternately measuring the temperature. To avoid the decomposition of MC and urea, ammonia was charged into the sample cell up to 0.300 MPa. Pressures were measured with a pressure transducer (Model PM10). The temperature difference between the sample and the adiabatic shield was always kept to be ∼10-3 K during the entire experiment. The heating rate and the temperature increments of the experimental points were generally controlled, within 0.3-0.6 K/min and 3-6 K, respectively. The heating duration was 10 min. Considering the above factors and the effect of impurities in substances, the estimated uncertainty of the Cp measurement was less than (0.8%. 2.2.2. Differential Scanning Calorimeter (DSC). The differential scanning calorimetry (DSC) experiments were conducted in a differential scanning calorimeter (Model DSC 200 PC, Netzsch Instrument, Inc.). The instrument was calibrated using indium, which was heated through its melting point before the measurement. The enthalpy of fusion and the melting point of indium were used to determine the cell constant and temperature calibrations. The samples used were encapsulated in the sealed pans in each experimental run. The heating rate
10.1021/ie9014342 2010 American Chemical Society Published on Web 05/10/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010 Table 1. Heat Capacity (Cp) Values of Methyl Carbamate (MC) at Pressures of 0.300-0.436 MPa T (K) Cp (J g-1 K-1) T (K) Cp (J g-1 K-1) T (K) Cp (J g-1 K-1) Crystal 298.15 301.95 306.15
1.0701 1.0796 1.0946
310.20 313.95 317.81
333.25 338.10 343.15 348.30 353.22 358.42 363.57
2.0531 2.0596 2.0666 2.0740 2.0812 2.0890 2.0969
368.42 373.36 378.24 383.34 388.38 393.35 398.30
1.1134 1.1348 1.1607
321.31 324.05
1.1877 1.2110
403.32 408.20 413.25 418.30 423.25 428.31 433.10
2.1646 2.1737 2.1833 2.1931 2.2029 2.2131 2.2229
Liquid
Figure 1. Experimental apparatus for equilibrium measurements. Legend: 1, vacuum pump; 2, nitrogen cylinder; 3, temperature transducer; 4, pressure transducer; 5, dynamoelectric stirring; 6, autoclave reactor; 7, vapor sampling tube; 8, gas chromatography; V1-V7, valves.
2.1046 2.1125 2.1206 2.1292 2.1378 2.1466 2.1554
Table 2. Heat Capacities of Urea at Pressures of 0.300-0.436 MPa 3
was 10 K/min, and the nitrogen flow rate was 25 cm /min. The mass loss of the sample was not observed after the temperature scanning run. The determination of a melting temperature was estimated to be accurate to (0.1 K. 2.2.3. Equilibrium Measurement for the Synthesis Reaction of MC from Methanol and Urea. The experimental apparatus is shown in Figure 1. The reaction was conducted in an autoclave reactor (300 cm3) at 433.15 K with electric heating, dynamoelectric stirring, a vapor sampling tube, and a liquid sampling valve. Urea (15 g) and methanol (60-100) g were charged into the reactor. The system was first vacuumed to remove air. The mixture then was heated to the desired temperature and constantly stirred at the temperature for a sufficient amount of time necessary to make the reaction reach equilibrium. The temperature was measured with a thermocouple with an uncertainty of (0.05 K. During the reaction, the yielded ammonia was not removed from the reactor. The pressure measurements of the system were made by means of a pressure transducer (Model PM10). The pressure transducer connected to a 1/2 digital multimeter (Model YXS-4) was calibrated at 433.15 K with a dead weight tester. The final accuracy of the measured pressures was approximately (0.002 MPa. When the system achieved equilibrium, samples were withdrawn from both the vapor and liquid phases. The vapor phase was analyzed using an online GC system that was equipped with a thermal conductivity detector (TCD) and an AT.AMINE capillary column (30 m × 0.53 mm ×1.0 µm). The liquid phase was analyzed using high-performance liquid chromatography (HPLC) and GC. First, the quantitative concentration of urea was determined by HPLC, using an internal standard method. Melamine was used as an internal standard here. Analysis was performed using a Shimadzu Model LC-4A HPLC system equipped with a Model M481 UV-detector and a Shimadzu Model C-R2A data system. The relative concentrations of MC and methanol then were determined using a GC system equipped with a TCD device, a column (2 m × 2 mm) packed with 10% porapak QS 80/100, and a Shimadzu Model C-R2AX integrator. The uncertainties of mole fraction measurements for each component were estimated to be
