Synthesis of Propylene Carbonate from Propane-1,2-diol and Urea

Aug 14, 2017 - 2.2Preparation of Catalyst. HTC derived catalysts were prepared by conventional precipitation method followed by thermal treatment. 100...
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Synthesis of Propylene Carbonate from Propane-1,2-diol and Urea Using Hydrotalcite-Derived Catalysts Ahmed Yasir, Kartikeya Shukla, and Vimal Chandra Srivastava* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India S Supporting Information *

ABSTRACT: Propylene carbonate is an important organic carbonate having wide applications. In the present work, thermodynamic analysis of propylene carbonate synthesis routes was performed. Benson group contribution method and Rozicka−Domalski model were used to estimate the heat of formation and the heat capacities of some of the components, respectively. Urea alcoholysis was found to be a favorable method for producing propylene carbonate under mild conditions. This route was further studied experimentally using various MgAl and ZnAl hydrotalcite derived catalysts. The catalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption−desorption, and Fourier transform infrared spectroscopy (FTIR) techniques. Yield of PC was found to be strongly correlated to the basicity of catalysts. Addition of a third element, Ca or La, was found to reduce the surface area of the catalysts. Fluorinated MgAlO (MgAlO-F) was found to possess greater basicity than MgAlO and greater catalytic activity. The effect of reaction temperature and reaction time was also studied, and at the optimum conditions of 160 °C and 11 kPa, MgAlO-F gave 91% PC yield, 96% urea conversion with 95% selectivity in 3 h.

1. INTRODUCTION Propylene carbonate (PC, C4H6O3) is an important cyclic organic carbonate which is widely used as a high boiling solvent with high polarity.1 It is used in industry for the separation of gases (carbon dioxide and hydrogen sulfide), synthesis of organic compounds, electrochemical applications, production of polyacrylonitrile fibers, as electrolyte in lithium ion batteries, etc.2 PC has also been extensively used in the industrial synthesis of dimethyl carbonate (DMC) from transesterification of methanol and PC.3−6 Initially PC was synthesized by phosgenation route from propane-1,2-diol commonly called propylene glycol (PG), which has major drawbacks as it releases hydrochloric acid as a byproduct, and a base (for example pyridine) is required to neutralize the acid. Further (PG) itself is a toxic substance and can be used to lower the freezing point of water, but the extent of toxicity is less as compared to closely related ethylene glycol. Also it was difficult to work with more toxic phosgene. Utilization of CO2 for its conversion into various fuel additives and linear and nonlinear carbonates is currently one of the highly focused areas of research.7 Synthesis of PC using carbon dioxide has also been reported.8−10 This method has some safety issues as it is difficult to handle propylene oxide which is a dangerous chemical. Other drawbacks include homogeneous catalysts, high reaction temperature and pressure, low reactivity, less selectivity toward the main product, and the difficulties in the separation of the catalysts. In comparison, PC synthesis from urea and PG has numerous advantages like mild reaction conditions, easily available cheap raw material, and ammonia gas as a byproduct which can be easily separated and converted back into urea resulting in efficient utilization of raw material and enhanced yield.11−13 It is interesting to note that propane glycol is the byproduct during production of dimethyl carbonate (DMC), © XXXX American Chemical Society

which can be used as a raw material to synthesize PC which in turn is a reactant for DMC synthesis (eq 1).14 DMC is widely used as a fuel additive which enhances the quality of the fuel to a higher extent and also in the production of biodiesel.15−17 DMC is an oxygen rich fuel additive since it has relatively high oxygen content when compared with other fuel additives.18−21 C4 H6O3(l) + 2CH3OH(l) → C3H6O3(l) + C3H6(OH)2 (l)

(1)

PC can be synthesized from PG via two routes indicated by eqs 2 and 3. Their efficacy to produce PC at a given temperature and pressure can be estimated by thermodynamic analysis. These two routes of PC synthesis have been examined thermodynamically in the present study by calculating theoretical equilibrium constants at various temperatures and pressures. NH 2CONH 2(s) + C3H6(OH)2 (l) → C4 H6O3(l) + 2NH3(g)

(2)

C3H6(OH)2 (l) + CO2 (g) → C4 H6O3(l) + H 2O

(3)

Acid−base bifunctional oxide was reported to catalyze the reaction near supercritical conditions for CO2 route.22 The role of basic properties for the synthesis of PC from urea route has been reported as being most important.23 Few studies have been previously reported through, alcoholysis of urea route and carbonylation of PG using CO2 route, for the synthesis of PC.8−10,22,23 However, a comparative thermodynamic study of Received: May 8, 2017 Revised: August 13, 2017 Published: August 14, 2017 A

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instrument Micromeritics ASAP 2020 was used to estimate the textural properties using N2 sorption measurements at −197 °C. Hamett indicator method was used for the determination of the basicity of solid surfaces. It consisted of the titration of the solid suspended in benzene with benzoic acid, using bromothymol blue as the indicator. In this method, 0.5 g of catalyst was mixed with 20 mL of benzene in a flask with a ground glass stopper. Then 1 ml of indicator solution (128 g in 100 mL of benzene) was added to the catalyst and benzene solution. 0.1 N benzoic acid in benzene was used for titration until the green color of the solid disappeared. The basicity, expressed in mmol/g, was calculated from the amount of 0.1 N benzoic acid required per gram of the catalyst.27 Temperatureprogrammed desorption (TPD) of CO2 was carried out using Micromeritics ASAP 2720 for determining the basicity of some samples. 2.4. PC Synthesis from Urea and PG. The reaction was conducted in a 100 mL autoclave reactor (equipped with magnetic stirrer) from Amar Equipment Pvt. Ltd., Mumbai, India. The reactor was coupled with a vacuum pump in order to remove ammonia produced during the reaction. 30 mL of PG and 6.15 g of urea (PG to urea mole ratio is 4) along with 0.56 g of catalyst (1.5 wt % of urea and PG combined) were charged into the reactor. The reactor was heated to the required temperature of the reaction. During the reaction, ammonia gets generated as a byproduct which has to be removed in order to shift the reaction forward. The pressure during the reaction was maintained at 11 kPa. The product formed by the reaction was taken out, the solids were removed by centrifuge, and then the net volume of the product was measured. From that volume, 1 mL of product was mixed with 1 mL of isopropyl alcohol before performing gas chromatography to find out the amount of PC formed. The products were analyzed using Michro-9100 chromatograph with FID. HP-5 capillary column of 50 m × 0.32 mm × 0.17 μm ((5%-phenyl)methylpolysiloxane) was used for the quantitative analysis of PC. The column temperature was raised from 80 to 270 °C at a ramp rate of 15 °C/min. The injector and detector temperature was kept at 280 °C, and the standard used was isopropyl alcohol. The yield of PC, conversion of urea, and selectivity were calculated by eqs 4−6:

both these routes has not been reported. Moreover, no study has been reported using hydrotalcite derived catalysts and its fluorination effect of the synthesis of PC from urea route. Hydrotalcites (HTCs), layered double hydroxides, are known because of their resemblance with talc and high water content.24 These have been largely studied in last three decades initially as the potential anion exchanger because of the free anion available in the interlayer region. Due to unique basic properties, low cost, large surface area, and regeneration ability, these are extensively used as catalysts.25,26 In the present study, both the CO2 and urea routes of PC synthesis have been studied thermodynamically and the effect of temperature and pressure, on equilibrium constants of both the reactions, has been investigated. Urea route was further chosen for catalytic study owing to its favorable thermodynamics. MgAlXO and ZnAlXO mixed oxides derived from HTCs (MgAlX HTC and ZnAlX HTC) were prepared by precipitation method where X= Ca, La. These catalysts were screened on basis of the yield of PC obtained. The effect of fluorination of the catalyst has also been studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Lanthanum(III) nitrate hexahydrate, La(NO3)3.6H2O, and aluminum nitrate nonahydrate, Al(NO3)3·9H2O, of 99% purity were purchased from Loba Chemie Pvt. Limited. Magnesium nitrate hexahydrate, Mg(NO3)2·6H2O; zinc nitrate hexahydrate, Zn(NO3)2·6H2O; calcium nitrate tetrahydrate, Ca(NO3)2·4H2O; and ammonium fluoride, NH4F, of 99.5% purity were purchased from Hi media Laboratories, Mumbai. Sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) were purchased from SD Fine Chemicals Limited. Propylene carbonate, C4H6O3; urea, NH2CONH2; propylene glycol, C3H6(OH)2; and isopropyl alcohol of 99% purity were purchased from Alfa Chemicals Limited. 2.2. Preparation of Catalyst. HTC derived catalysts were prepared by conventional precipitation method followed by thermal treatment. 100 mL of solution A was prepared by mixing metal nitrates with the atomic ratio of divalent/trivalent cations being 1:3. Another 100 mL solution B was prepared by dissolving sodium carbonate (equimolar to solution A). Then the two solutions were mixed dropwise into a solution containing 50 mL of Millipore water under constant stirring, and the pH of the solution was maintained at 10.0 by adding 2 M sodium hydroxide. The precipitates were aged in the mother liquid at 343 K for 12 h. The catalysts were filtered and washed with deionized water until the pH of the solution became 7.0. The precipitates were dried at 353 K overnight. The dried HTC was further calcined at 700 °C for 5 h under flowing N2 at higher temperature. For fluorination of the MgAlO, the NH4F solution was heated to 90 °C for 2 h in order to remove any dissolved CO2. MgAlO was then added to the solution. The solution was kept in an autoclave in an atmosphere of nitrogen for 48 h. The solution was then centrifuged, and the catalyst was kept for drying at 110 °C for 12 h followed by calcination at 700 °C. 2.3. Catalyst Characterization. Thermogravimetric analysis (TGA) was conducted under nitrogen atmosphere with flow rates of 200 mL/min using EXSTAR TGA/DTA-6300 with a temperature range of 303−1273 K at the heating rate of 10 K/min. X-ray diffraction (XRD) was conducted using a Bruker D8 X-ray diffractometer with Cu Kα radiation at 40 mV and 40 mA and in the 2θ scanning range of 10−90°. Fourier transform infrared (FTIR) spectroscopy was performed with a Nicolet Magna 550 II FTIR spectrometer in the region of 4000−400 cm−1. Field emission scanning electron microscopy (FE-SEM) was performed with Quanta 200 FEG Field. The morphology was examined with an accelerating voltage of 20 kV and magnification range of 2500−50 000. The Brunauer−Emmett− Teller (BET) surface area of the porous material was calculated by the BET isotherm using liquid N2 adsorption−desorption data. The

yield =

mol of PC formed initial mol of urea taken

conversion of urea =

selectivity =

(4)

∑ Mi initial mol of urea taken

(5)

mol of PC formed ∑ Mi

(6)

where Mi is the mol of PC, 2-hydroxypropyl carbamate, or 4-methyl-2oxazolidone.

3. RESULTS AND DISCUSSION 3.1. Thermodynamics of the Reaction. 3.1.1. Chemical Equilibrium Constant at T = 298.15 K and P = 1 bar. To calculate chemical equilibrium constants at T = 298.15 K and P = 1 bar, the enthalpy of formation and entropy of formation of all of the reactants and products of the reaction were reconciled from the literature. Table 1 shows the enthalpy of formation of Table 1. ΔfH° and Sf° of the Substance Involved in the Synthesis of Propylene Carbonate

B

compound

phase

ΔfH° (kJ mol−1)

Sf° (J K−1 mol−1)

urea PG PC H2O NH3 CO2

solid liquid liquid liquid gas gas

−319.40 −469.00 −644.50 −241.83 −46.11 −393.50

150.89 180.90 73.31 188.84 192.34 213.78 DOI: 10.1021/acs.energyfuels.7b01330 Energy Fuels XXXX, XXX, XXX−XXX

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equilibrium constant of reactions, values of liquid heat capacity Cp,l of reactants and products were calculated by the Rozicka− Domalski model.34 This method is widely used to calculate the liquid heat capacity of various compounds according to their molecular structure. Also the heat capacities of various compounds were obtained from the literature and are summarized in Table 3. ΔrCp of any reaction was calculated by eq 11; here ΔrCp and ΔrH of the reaction were considered to be independent of the pressure. Therefore, ΔrH and ΔrG at different temperatures were calculated by eqs 12 and 13. Using eq 10, values of the equilibrium constant at different temperatures were calculated.

Table 2. Thermodynamic Data for the Reactions for PC Production at T = 298.15 K and P = 1 bar ΔrH° (kJ mol−1) PC urea ( scheme 1) PC CO2 (scheme 3)

ΔrS° (J K−1 mol−1)

ΔrG° (kJ mol−1)

K

51.60

126.20

13.97

0.0036

−23.88

−132.53

15.63

0.0018

Table 3. Heat Capacity of All Substances Involved in the Synthesis of PC ΔrC(J K−1 mol−1) = a + b(T/K) + c(T/K)2 a

urea PG PC H2Oa NH3a CO2a a

b × 102

a

species

38.43 −64.49 145.91 92.05 35.23 27.09

4.98 117.13 −10.61 −3.99 −3.50 1.12

Δr Cp , m =

c × 104 7.05 −9.93 5.51 −2.11 1.69 1.24

Data calculated by the Rozicka−Domalski model.34

∑

niΔf Hm0 , i −

product

Δr Sm0

=

∑

∑

niΔf Hm0 , i

reactant

niSm0 , i

product

−

∑ reactant

(7)

niSm0 , i (8)

Δr Sm0 1000

∑

niCp , m , i (11)

product

⎡ δ Δr Hm ⎤ ⎢⎣ ⎥ = Δr Cp , m δΤ ⎦p

(12)

⎡ δ Δr Gm /T ⎤ Δr Hm ⎢ ⎥=− 2 ⎣ δΤ ⎦ T

(13)

⎛P⎞ Δr Gm(T , P) = Δr Gm(T , P θ) + ΔnRT ln⎜ θ ⎟ ⎝P ⎠

(14)

Δr Gm(T , P) = −RT ln K

(15)

Theoretical equilibrium constants at different temperatures and pressures for both the schemes were calculated and are represented in Figure 1. Table 2 shows the positive value of the Gibbs free energy change for both of the routes, which emphasizes the infeasibility of reactions at NTP. On increasing the temperature, the urea route becomes favorable as the equilibrium constant increases (as shown in Figure 1a), while equilibrium constants of the CO2 route decrease. On decreasing the pressure or conducting the reaction under vacuum, the equilibrium constant values for the urea route were found to increase. This can be attributed to the fact that removal of ammonia from the system made the synthesis of PC more favorable. This is consistent with the studies reported earlier.14 For the CO2 route, an increase in pressure favored the

The standard Gibb’s free energy change and equilibrium constant at standard conditions were then calculated by eqs 9 and 10. Δr Gm0 = Δr Hm0 − T

niCp , m , i −

3.1.3. Effect of Pressure on the Equilibrium Constant. For the reactions involving any gas as a reactant or product, the effect of pressure on the equilibrium constant is significant. Therefore, when the temperature of the reaction is constant,34 assuming conditions at low pressure, close to ideality, the values of ΔrGm(T,P) and K were calculated from eqs 14 and 15.

the products and reactants.28−30 The missing values, of heat of formation, of the compounds were estimated from Benson group contribution method.31 The standard enthalpy and standard entropy of the reactions were calculated by eqs 7 and 8:32,33 Δr Hm0 =

∑ product

(9)

−Δr Gm0 (10) RT All of the above parameters for the two reaction schemes were calculated and are tabulated in Table 2. 3.1.2. Effect of Temperature on the Equilibrium Constant. To study the theoretical effect of temperature on the ln K 0 =

Figure 1. Effect of temperature and pressure on theoretical equilibrium constants of (a) the urea route to PC and (b) the CO2 route to PC. C

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Figure 2. (a) TG and (b) DTA analysis of different catalysts precursors.

Figure 3. (a) Zn based HTcs {#-HTlcs, *-La2(CO3)3}; (b) Mg based HTCs {#-HTlcs; ^-CaCO 3}; (c) ZnAlMO based mixed oxides {α-ZnO; β̂ CaO; *-La2O3; #-La2O2CO3}; and (d) MgAlMO based mixed oxides {α-MgO; β-CaO; *-La2O3; #-La2O2CO3, Ψ-Al2O3}.

reaction (as shown in Figure 1b). CO2 at high pressure, under supercritical conditions, has been reported to be reactive, and this condition has been used for the synthesis of other organic carbonates previously.22 3.2. Characterization of Catalyst. 3.2.1. TG-DTA. TGA analysis (Figure 2) was conducted, and it was found that all of the catalysts seemed to have two major weight losses. In MgAl based HTC, there are two major losses, in the temperature

ranges of 300−373 and 423−500 K which can be attributed to dehydration of interlayer water and decomposition of the carbonate anion in the brucite-like layers.35 Likewise in ZnAl based HTC, there are two major losses, i.e., up to 473 K and in between 473 and 673 K. In ZnAlLa HTC, one more major weight loss around 723 K is observed which may be due to decomposition of carbonates.36−38 Samples were treated in the presence of nitrogen in order to avoid the oxidation of various D

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Energy & Fuels Table 4. BET Surface Areas, Pore Volume, Pore Diameter, and PC Yield %a

a

catalysts

PC yield (%)

PC selectivity (%)

urea conversion (%)

BET surface area (m2 g‑1)

pore volume (cm3 g‑1)

pore diameter (nm)

d003 (nm)

d110 (nm)

a (=2d110)

c (=3d003)

MgAlO MgCaAlO MgAlLaO ZnAlO ZnCaAlO ZnAlLaO MgAlO-F

79.1 69.2 77.4 71.7 55.7 49.7 91.7

94.9 81.1 83.6 88.2 88.1 88.2 95.3

83.4 85.3 92.6 81.2 68.7 56.4 96.3

220.1 31.6 56.3 68.5 28.7 19.1 284.2

0.98 0.12 0.24 0.11 0.02 0.04 0.99

17.8 15.6 17.4 6.4 3.9 9.4 19.5

14.2 26.1 32.1 1.7 43.9 19.4 -

6.3 23.5 7.7 20.0 35.5 30.5 -

12.6 47.0 15.4 40.0 71.0 61.0 -

42.6 78.3 96.3 5.1 131.7 58.2 -

Reaction conditions: 160 °C, 3 h, PG:urea 4, 1.5% catalyst, pressure: 11 kPa.

Figure 4. Liquid nitrogen adsorption−desorption isotherms of calcined catalysts and fluorinated catalysts.

Figure 5. FTIR analysis of (a) MgAl HTC and MgAlMO based mixed oxides and (b) MgAlO catalyst before and after fluorination.

species. Thus, the mixed oxides containing metal oxides were formed by the decomposition of the corresponding HTC. 3.2.2. XRD. The XRD patterns of all of the catalysts before and after calcination are shown in Figure 3. The pattern depicts the clear indication of the layered structure of HTCs. The peaks at 11.42°, 23.06°, 34.77°, 38.69°, 45.40°, 60.60°, and 61.90° correspond to (003), (006), (009), (015), (018), (110), and (113) planes of the layered structure. Some impurities can also be seen in the catalysts, but the HTC remains as a major

phase. Impurities, like La2(CO3)3, can also be found in ZnAlLa HTC which can also be seen from TGA analysis.37 The peak at 60.6, assigned to the (110) plane of the layered structure, could be used to calculate unit cell dimension a, where a = 2d110, while unit cell parameter c can be obtained by c = 3d003. It can be seen that both the parameters a and c increase with the introduction of Ca and La. This may be due to partial isomorphous substitution of Al3+ by La3+ in the octahedral site and the interlayer gallery.36 Figure 3c also shows the diffraction E

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Figure 6. SEM images of catalysts before and after the calcination.

peaks of MgF2 are also visible in Figure S1 (Supporting Information).41 3.2.3. Textural Properties. Table 4 shows the BET surface area of catalysts synthesized, and it was concluded that the BET surface area of MgAlO derived catalyst is high as compared to the others. Also it can be seen that, when the third element is inserted in the lattice of MgAlO, its surface area is reduced. The zinc based catalysts also show the same trend of decreasing surface area. Adsorption/desorption isotherms of calcined hydrotalcites and fluorinated catalysts are shown in Figure 4a,b. The synthesized catalysts can be categorized under mesoporous materials and were observed to follow types IV and V with a clear hysteresis loop.42 In Figure 4b, N2 adsorption−desorption curves of MgAlO and MgAlO-F are shown. The pore-size distribution curve of MgAlO-F was more broad than MgAlO which was in agreement with the increase in BET surface area than the former one. Bezen et al.43 also reported an increase in the surface of MgAl based hydrotalcite after its fluorination. 3.2.4. FTIR Analysis. Figure 5a represents the FTIR transmittance spectra of the MgAl HTC and Mg based mixed oxides. The broad peak at ∼3435 cm−1 corresponds to brucite like layers (OH− stretching vibration) caused by inter layer water molecules. The band observed at about 1640 cm−1 corresponds to H2O present in the interlayer. The peak at 1384 cm−1 corresponds to the carbonate group.44 It was seen that, after calcinations of the catalysts, the peak corresponding to water and carbonate were found to be less intensified due to removal of the components. The peaks in the range of 704− 637 cm−1 are due to Mg−O or Al−O bands. The shoulder peaks at 2825 cm−1 correspond to the water molecules bonded

Figure 7. Effect of basicity on PC yield.

peaks of ZnO, La2O3, La2O2CO3, and CaO. Some of the ZnO peaks were at 32.1, 34.8, 47.9, 56.8, and 64.1 which corresponds to the lattice planes (002), (002), (101), (102), (110), and (103), respectively. Lanthanum oxides present in La2O3 and La2O2CO3 phases and the later one is in abundance.37 Figure 3d shows the diffraction peaks of MgO, La2O3, La2O2CO3, and CaO phases.35,39 Some of the MgO peaks were observed at 11.2, 21.6, 34.9, 39.4, 45.8, 60, and 62.1 which correspond to the lattice planes of (003), (006), (009), (015), (018), (110), and (113). Unlike in the Zn based catalysts, La2O3 phases occurred in an excess amount as compared to the other oxide phase of lanthanum. The average sizes of MgO in three types of mixed oxides were calculated by the Scherrer equation. The sizes of MgO in calcined MgAlO, MgCaAlO, and MgAlLaO crystallites were 3.254, 13.016, and 19.524 nm.40 After fluorination of MgAlO followed by calcination, some F

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is strongly correlated to the basicity of the catalysts used. MgAlO being the most basic catalyst among other calcined catalysts synthesized in this work, it performed the best giving 79.1% PC yield. This was close to the yield reported by Wu et al.41 The high yield obtained with MgAlO may be attributed to its highest cumulative pore volume (Figure 4a), which might have eased the greater volume of PG contact with urea. On fluorination, its yield further increased to 91.7%. This yield was found to be higher than those reported earlier.46,47 The pore volume was found to be reduced on fluorination (Figure 4b), while its basicity was enhanced. It may be attributed to the combined effect of pore volume and the enhanced basicity because of the MgF2 phase, which was confirmed from XRD (Figure S1). MgAlO was further used for studing the effect of reaction conditions like reaction temperature and reaction time on PC yield. CO2-TPD has also been done of MgAlO and MgAlO-F, and the profile is shown in Figure S2. The distributions of sites have also been summarized in Table S1. It was observed that on fluorination the total number of weak sites increase and which in turn enhance the total basicity of MgAlO-F (3.0 mmol/g) in comparison to MgAlO (2.9 mmol/g). This helped to increase the PC yield when MgAlO-F was used. The effect of temperature on the reaction giving PC was studied. The temperature was varied from 423 to 453 K, keeping the reaction time as 3 h and 1.5% MgAlO by weight. The yield obtained at different temperatures is represented in Figure 8a. It can be seen that on increasing the temperature from 150 to 160 °C there is significant increase in the PC yield. On further increasing the temperature from 160 to 180 °C, the yield of PC started decreasing. This can be attributed to the loss of PG due to some unavoidable evaporation at 180 °C. To study the effect of time, the reactions were conducted at a constant temperature of 453 K. All other conditions like catalyst weight and reactants ratio were kept constant as optimized earlier (Figure 8b). Initially, the concentration of urea was very high, and hence, the rate of formation of PC was high. As the reaction time further increased, the urea concentration decreased and the rate of PC formation was saturated in ∼3 h. Beyond 3 h, PC concentration in the reactor became high enough to ensue the side reactions such as the polymerization of PC which lowered the PC yield. MgAlO-F was tested after regeneration by calcination at 700 °C for 5 h under flowing N2. Regenerated catalyst was again used for PC synthesis. MgAlO-F was found to be stable for the three tested runs and showed >95% of PC yield with respect to that obtained in the first run.

Figure 8. Effect of (a) temperature in 423−453 K (PG:urea = 4, catalyst = 1.5%, pressure = 11 kPa, time = 3 h) and (b) time 0.5−4 h (PG:urea = 4, catalyst = 1.5%, pressure = 11 kPa, temperature = 443 K) using MgAlO as the catalyst.

to carbonate anions in the interlayer.45 Figure 5b represents the transmittance spectra of the MgAlO before and after the fluorination to the catalyst. The peak at 454 cm−1 is due to MgF2 which is also witnessed in Figure S1.41 Also the peaks at 3435 and 1636 cm−1 could be attributed OH stretching and bending vibrations, respectively. 3.2.5. Surface Morphology. The morphology of a few of the catalysts before and after the calcination is presented in Figure 6. This comparison helps in determining the changes in the structure of HTC after calcination. As it is clearly seen from Figure 6, the layered structure before calcination was broken to give mixed metal oxides. Water present in the layers of HTC got removed after the calcination of catalysts. 3.3. Catalytic Activity. A total of 1 mol of PG reacts with 1 mol of urea to produce 1 mol of PC and 2 mol of ammonia. Therefore, an excess amount of PG was maintained in the reactor to shift the reaction toward the product side. The high amount of ammonia produced should be removed from the reactor, and hence, a vacuum pump was used for the purpose, and the reactions were conducted at the pressure of 11 kPa. All of the catalysts were evaluated for synthesizing the PC from PG and urea with methanol in a batch reactor. Table 4 summarizes the yield of PC using various catalysts. It can be seen that the yield of Mg based HTC catalysts is greater than that of Zn based catalysts. Addition of any further element to the HTC reduces the yield of PC. This can be attributed to Figure 7, where it can be easily concluded that the yield of PC

4. CONCLUSION Thermodynamic study of two routes of PC synthesis lead to the conclusion that urea carbonylation of PG to PC is more favorable than the CO2 route. On increasing the temperature and lowering the pressure, the urea route was found to be more favorable theoretically. Hence the urea route was chosen to study the process experimentally. The catalysts precursors showed the hydrotalcite (HTC) type materials with some impurities in it which were calcined to get the mixed oxides. MgAlO gave the highest yield (80%) owing to a higher basicity. Introduction of the third element (Ca or La) into MgAlO or ZnAlO decreased the PC yield because of the decrease in the surface area and the basicity of the catalysts. The effect of basicity on PC yield was further confirmed when catalytic G

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Energy & Fuels activity of fluorined MgAlO was tested, which was found to enhance the PC yield to 91.7%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01330. XRD pattern of fluorinated MgAlO (*-MgF2); CO2-TPD analysis of MgAlO and MgAlO-F catalysts. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-1332-285889. Fax: +91-1332-276535. E-mail: [email protected]; [email protected]. ORCID

Vimal Chandra Srivastava: 0000-0001-5321-7981 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.energyfuels.7b01330 Energy Fuels XXXX, XXX, XXX−XXX