Preparation of Ca–Zn–Al Oxides and Their Catalytic Performance in

Mar 13, 2015 - Acid-catalysed carboxymethylation, methylation and dehydration of alcohols and phenols with dimethyl carbonate under mild conditions. S...
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Preparation of Ca−Zn−Al Oxides and Their Catalytic Performance in the One-Pot Synthesis of Dimethyl Carbonate from Urea, 1,2Propylene Glycol, and Methanol Guangjie Zhang, Hualiang An, Xinqiang Zhao,* and Yanji Wang Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, Hebei University of Technology, Tianjin 300130, China S Supporting Information *

ABSTRACT: A series of Ca−Zn−Al oxides were prepared by different methods and their structures, compositions, and basicities were characterized by means of X-ray diffraction (XRD), N2 adsorption−desorption, inductively coupled plasma-atomic emission spectrometry (ICP-AES), and CO2 temperature-programmed desorption (TPD). CO2 TPD analysis showed that all of the prepared Ca−Zn−Al oxides had both weak base sites and strong base sites, but their base amounts were different. The weak base sites were provided by the Zn−Al oxides, whereas the strong base sites were due to CaO. Ca−Zn−Al oxides with different base distributions could be prepared by varying the molar ratios of Zn/Al and Ca/Al. The Ca−Zn−Al oxides showed excellent catalytic performances for the one-pot synthesis of dimethyl carbonate (DMC) from urea, 1,2-propylene glycol (PG), and methanol: The yield of DMC could reach 82.9% under appropriate reaction conditions. However, the catalytic performance of the recovered catalyst decreased dramatically, mainly because of the transformation of CaO into CaCO3 during the reaction process.

1. INTRODUCTION Dimethyl carbonate (DMC) is a green chemical with versatile reaction activity such that it can be substituted for toxic phosgene, dimethyl sulfate, methyl halides, and methyl chloroformate in many chemical reactions such as carbonylation, methylation, and transesterification.1,2 Additionally, DMC can be used as an environmentally friendly solvent and a potential gasoline additive. At present, DMC is produced industrially in China by the transesterification of propylene carbonate (PC) with methanol. There are many significant merits to this process such as a high DMC yield and mild reaction conditions. However, one of the raw materials, PC, is derived from petroleum, which is becoming depleted, and a great deal of 1,2-propylene glycol (PG) is coproduced as a byproduct in the production process, leading to a low utilization of the raw materials. We proposed an idea to transform PG back into PC by the reaction of PG with urea in 2004.3 According to this idea, an integrated reaction process can be described as the synthesis of DMC from urea and methanol using PG as a recycle agent (see Scheme1). In this way, not only can the problem of low PG utilization be solved, but also the reliance of PC production on the petrochemical industry can be decreased. Therefore, the idea meets the requirements of green chemistry. The reaction of urea with PG to PC can be catalyzed by several kinds of catalyst,4−8 among which metal oxides exhibit excellent catalytic performances. Shu et al.7 found that Zn−Al oxides present a good catalytic performance for the synthesis of PC from urea and PG. Under appropriate reaction conditions of a PG/urea molar ratio of 4, a catalyst weight percentage of 2.5%, a reaction temperature of 443 K, and a reaction time of 5 h, the yield of PC was 98.7%. An8 studied the influence of the preparation conditions on the catalytic performance of Zn−Al © 2015 American Chemical Society

Scheme 1. DMC Synthesis from Urea and Methanol Using PG as a Recycle Agent

oxides and found that a Zn−Al oxide prepared under appropriate conditions showed good catalytic performance for the synthesis of PC from urea and PG on a fixed-bed reactor. The yield of PC was maintained at about 85.0% during the continuous reaction operation for a period of 60 h, suggesting that the Zn−Al oxide exhibited excellent catalytic activity and stability. For the reaction of the transesterification of PC with methanol to DMC at atmospheric pressure, many strong alkaline catalysts have been reported such as CaO,9 basic resin,10 CH3COONa,11 and KF/Al2O3.12 Among them, CaO has received much attention13−16 because of its easy Received: Revised: Accepted: Published: 3515

December March 10, March 13, March 13,

26, 2014 2015 2015 2015 DOI: 10.1021/ie505028w Ind. Eng. Chem. Res. 2015, 54, 3515−3523

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Industrial & Engineering Chemistry Research

0.2 as an example: Calcium hydroxide (0.39 g) and the aboveprepared Zn−Al hydroxide cake were added into 30 mL of deionized water, and the mixture was stirred for 1 h and aged at room temperature for 12 h. Finally, the mixture was dried at 383 K for 12 h and calcined at 923 K for 4 h to obtain Ca−Zn− Al-1. For the preparation of Ca−Zn−Al oxide (denoted as Ca− Zn−Al-2, the above-prepared Zn−Al hydroxide cake was added into 40 mL of calcium nitrate aqueous solution with a molar concentration of 2.19 mol/L, and the resultant solid−liquid mixture was stirred for 1 h and aged at room temperature for 12 h. The mixture was dried at 383 K for 12 h and calcined at 923 K for 4 h to obtain Ca−Zn−Al-2. The Ca−Zn−Al oxide denoted as Ca−Zn−Al-3 was prepared as follows: The Zn−Al hydroxide cake was dried at 383 K for 12 h and calcined at 773 K for 4 h to obtain Zn−Al oxide. Then, the Zn−Al oxide was impregnated with a calcium nitrate solution by incipient wetness impregnation and aged at room temperature for 12 h and dried at 353 K for 2 h. After calcination at 923 K for 4 h, Ca−Zn−Al-3 was obtained. Zinc nitrate, aluminum nitrate, and calcium nitrate were mixed and ground in a mortar. Then, the mixture was calcined at 923 K for 4 h to obtain a Ca−Zn−Al oxide denoted as Ca− Zn−Al-4. 2.3. Catalyst Characterization. X-ray diffraction (XRD) analysis of the Ca−Zn−Al oxides was performed on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation and a graphite monochromator. The Cu Kα radiation was operated at 40 kV and 100 mA. The scan range covered from 5° to 90° at a rate of 8°/min. The textural properties of the Ca−Zn−Al oxides were measured using an ASAP 2020 specific surface area and porosity analyzer. Prior to the test, 0.2 g of the sample was degassed at 423 K for 4 h in a vacuum to remove the impurities adsorbed on the sample surface. Then, a N2 adsorption− desorption test was performed at the temperature of liquid nitrogen. The specific surface areas of the Ca−Zn−Al oxides was calculated by the Brunauer−Emmett−Teller (BET) method, whereas the pore volume and pore diameter were calculated by the Barrett−Joyner−Halenda (BJH) method. The elemental composition of the Ca−Zn−Al oxides was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) on an an Optima 7300 V spectrometer. The basicities of the Ca−Zn−Al oxides were measured by temperature-programmed desorption using CO2 as the probe molecule (CO2 TPD). The measurements were performed on an AutoChem II 2920 chemical adsorption instrument. Prior to the test, 0.2 g of the sample was placed in a quartz sample tube and then heated to a temperature not higher than the calcination temperature of the sample and maintained for 1 h in an atmosphere of helium. When the temperature was decreased to 383 K, CO2 was introduced to attain adsorption saturation. Then, the sample was purged by helium with remove the physically absorbed CO2. The TPD experiment started with a heating rate of 10 K/min, and the CO2 desorption signal was detected by a thermal conductivity detector (TCD). The thermal decomposition test of the Ca−Zn−Al hydroxide was performed on a SDT-2960 thermogravimetric analyzer. The furnace atmosphere was dynamic air with a flow rate of 30 mL/min, and the temperature ramp was controlled at 10 K/ min.

preparation, available raw materials, strong alkalinity, high catalytic performance, and so on. Wei et al.9 found that CaO exhibited an excellent catalytic performance for the transesterification of PC with methanol to DMC. Under the conditions of a methanol/PC molar ratio of 4, a catalyst weight percentage of 0.9%, a reaction temperature of 283 K, and a reaction time of 1 h, the yield of DMC was 45%. In addition, Wang et al.17 used zinc−yttrium oxides to catalyze the synthesis of DMC by a two-step process starting from urea, ethylene glycol, and methanol. In the first step, ethylene carbonate was synthesized from urea and ethylene glycol, and the yield of ethylene carbonate was 94%. In the second step, the transesterification of ethylene carbonate with methanol to DMC was performed, and the yield of DMC was 71%. They evaluated the catalytic activity of zinc−yttrium oxides in the two separate reactions only and did not conduct the reaction for the one-pot synthesis of DMC. The present work was intended to prepare Ca−Zn−Al oxides and investigate the influence of the preparation conditions on the catalytic performance of the resulting oxides. Then, the reaction of PG with urea to PC and the transesterification of PC with methanol were performed through tandem steps in one reactor without a separation operation between the steps. In this way, significant energy consumption for separation and purification can be avoided. Therefore, in this work, a real one-pot synthesis of DMC from urea, PG, and methanol at atmospheric pressure was realized for the first time to the best of our knowledge.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. All of the chemical reagents employed in this work, namely, urea (AR, Tianjin No. 1 Chemical Reagent Factory, China), 1,2-propylene glycol (AR, Tianjin Chemical Reagent Co. Ltd., China), anhydrous methanol (AR, Rionlon Tianjin Chemical Co. Ltd., China), aluminum nitrate (AR, Tianjin Fuchen Chemical Reagents Factory, China), zinc nitrate, calcium hydroxide, and sodium hydroxide (AR, Tianjin Fengchuan Chemical Reagent Science and Technology Co. Ltd., China), were used as received without further purification. 2.2. Catalyst Preparation. First, Zn−Al hydroxide was prepared by a parallel-flow coprecipitation method. The operation process for the preparation of the Zn−Al hydroxide with a Zn2+/Al3+ molar ratio of 3 is given here as an example: Zinc nitrate (29.7 g) and aluminum nitrate (12.5 g) were dissolved in 133 mL of deionized water to prepare a solution (denoted as A) with a Zn2+ molar concentration of 0.75 mol/L and an Al3+ molar concentration of 0.25 mol/L. Then, 15 g of sodium hydroxide was dissolved in 112 mL of deionized water to prepare a solution (denoted as B) with a Na+ molar concentration of 1.90 mol/L. Next, solutions A and B were added dropwise into a beaker simultaneously under vigorous stirring, and the pH of the mixed solution was controlled at 9.5. The resultant solid−liquid mixture was stirred continuously for 1 h and aged at 313 K for 24 h. After that, the solid−liquid mixture was filtered, and the cake of solids was washed several times with deionized water until the filtrate was neutral. The cake obtained was Zn−Al hydroxide. Different approaches were utilized to introduce calcium compounds into the Zn−Al hydroxide, resulting in different Ca−Zn−Al oxides. A typical operation is given for the preparation of Ca−Zn−Al oxide, denoted as Ca−Zn−Al-1, with a Ca2+/Al3+ molar ratio of 3516

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Industrial & Engineering Chemistry Research Fourier transform infrared (FT-IR) spectra of the samples were recorded using a NEXUS 470 FT-IR spectrometer. The instrumental resolution was 4 cm−1 within the wavenumber range of 400−4000 cm−1. Thermogravimetry−mass spectrometry (TG−MS) analysis was performed on a TGA/SDTA851 thermogravimetric analyzer (Mettler-Toledo) equipped with a Pfeiffer ThermoStar mass spectrometer (Balzers) operated at a helium flow rate of 50 mL/min. The temperature was controlled in the range from 323 to 1273 K at a rate of 10 K/min. The mass spectrum was recorded in the range of m/z 1−300, and the scan rate was 0.5 scan/s. 2.4. One-Pot Synthesis of DMC. One-pot synthesis of DMC was conducted in two tandem reactions including the reaction of urea with PG to PC and the tranesterification of PC with methanol to DMC. Urea, PG, and a Ca−Zn−Al oxide sample were first added to a four-necked flask and then heated to the reaction temperature under magnetic stirring. During the period of the reaction, a flow of pure N2 was introduced into the flask to remove the byproduct of ammonia from the reaction system quickly. After the completion of the reaction, a Vigreux column was fixed to the four-necked flask to drive off the products of the upcoming tranesterification reaction. During the period of the tranesterification reaction, a flow of methanol was continuously pumped into the four-necked flask by a metering pump to keep the volume of the contents in the flask constant. After the completion of the reaction, the distillate obtained from the Vigreux column and the residue left in the flask were collected separately for quantitative analysis. 2.5. Product Analysis. The reaction products were analyzed quantitatively on an SP3420A gas chromatograph equipped with a flame ionization detector (FID) operated at 493 K. The components were separated in a PEG-20M capillary column whose temperature was controlled by temperatureprogrammed method. For the analysis of the distillate, the column temperature was started at 323 K, held for 3 min, and then raised to 473 K at a rate of 15 K/min. n-Propyl alcohol was used as the internal standard. For the analysis of the residue, the column temperature was started at 373 K, held for 2 min, raised to 493 K at a rate of 10 K/min, and then held for 10 min. n-Butanol was used as the internal standard. Nitrogen of high purity was employed as the carrier gas at a flow rate of 30 mL/min.

Table 1. Catalytic Performance of Mixtures of CaO and Zn− Al Oxide with Different Mass Ratiosa CaO/Zn−Al oxide mass ratio

YDMC (%)

YPC (%)

YDMC+PC (%)

0:1 1:4 1:2 3:4 1:1 1:0

20.0 33.6 53.8 55.4 57.3 52.6

68.6 42.3 22.0 17.8 12.2 10.1

88.6 75.9 75.8 73.2 69.5 62.7

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

the mixtures of CaO and Zn−Al oxide was increased, the yield of DMC increased monotonically until the catalyst became pure CaO. The decrease of the DMC yield might be due to the lower yield of PC caused by the poor catalytic performance of CaO in the reaction of urea and PG. In the reaction for the one-pot synthesis of DMC, 1 mol of DMC is formed from 1 mol of PC, so the total yield of both DMC and PC (YDMC+PC) essentially reflects the results of the first-step reaction. The total yield of both DMC and PC decreased gradually with increasing proportion of CaO, meaning that the addition of CaO had an adverse effect on the reaction of urea and PG. When the mass ratio of CaO to Zn−Al oxide was increased from 1:2 to 1:1, the yield of DMC increased by only 3.5%, whereas the total yield of both PC and DMC decreased by 6.3%. Therefore, the appropriate mass ratio of CaO to Zn−Al oxide was determined to be 1:2. Then, Ca−Zn−Al oxide with the appropriate mass ratio of CaO to Zn−Al oxide was prepared and used for further research. 3.2. Effect of the Preparation Method on the Catalytic Performance of Ca−Zn−Al Oxide. The effect of the preparation method on the catalytic performance of Ca−Zn− Al oxide is reported in Table 2. Ca−Zn−Al-3 had the poorest catalytic performance in the one-pot synthesis of DMC, as the yield of DMC and the total yield of both DMC and PC were the lowest. The total yields of both DMC and PC were almost identical over Ca−Zn−Al-1, Ca−Zn−Al-2, and Ca−Zn−Al-4, whereas the yield of DMC was the highest (64.3%) over catalyst Ca−Zn−Al-1. Therefore, the preparation method used for Ca−Zn−Al-1 (see section 2.2) was selected as the appropriate method for the preparation of Ca−Zn−Al oxide catalyst. The XRD patterns of Ca−Zn−Al oxides prepared by different methods showed the characteristic diffraction peaks of only ZnO and CaO (see Figure S1, Supporting Information), suggesting that new substances were not formed in the preparation process. No obvious peaks of Al2O3 were observed, meaning that Al2O3 was highly dispersed in an amorphous form.18 The surface acidity and basicity of a solid catalyst are important factors affecting its catalytic performance in transesterification reactions.19,20 To analyze the difference in the basicities of Ca−Zn−Al oxides prepared by different methods, the atomic compositions of the samples were analyzed by ICPAES, and then the samples were analyzed by CO2 TPD (see Figure S2, Supporting Information). The results are also included in Table 2. The Ca/Zn/Al atomic ratios in the Ca− Zn−Al oxides prepared by different methods were different from the expected value (2.6:3:1), and the loss of metal

3. RESULTS AND DISCUSSION 3.1. Determination of the Appropriate Proportion of CaO in Ca−Zn−Al Oxide. Zn−Al oxide with a Zn2+/Al3+ molar ratio of 3 was mixed physically with CaO, and the catalytic performance of the mixture for the one-pot synthesis of DMC was investigated. The synthesis of PC from urea and PG catalyzed by Zn−Al oxide and the transesterification of PC with methanol to DMC catalyzed by CaO had previously been investigated, and the sum of the appropriate dosages (2.2%) of the two catalysts was first used in the one-pot synthesis of DMC. As can be seen from Table 1, when the Zn−Al oxide was used alone, the yield of DMC (YDMC) was only 20.0%, and the yield of PC (YPC) was 68.6%, suggesting that the function of the Zn−Al oxide was mainly to catalyze the reaction of urea with PG to PC. When CaO was used alone, the yield of DMC was 52.6%, and the yield of PC was only 10.1%, showing that the function of CaO was mainly to catalyze the transesterification of PC with methanol to DMC. When the proportion of CaO in 3517

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Table 2. Catalytic Performance and ICP-AES and CO2 TPD Measurement Results of Ca−Zn−Al Oxides Prepared by Different Methodsa desorption amount of CO2 (mmol/g) catalyst

YDMC (%)

YPC (%)

YDMC+PC (%)

Ca/Zn/Al molar ratio

weak

medium

strong

total

Ca−Zn−Al-1 Ca−Zn−Al-2 Ca−Zn−Al-3 Ca−Zn−Al-4

64.3 55.4 45.7 54.1

4.5 13.2 16.5 16.4

68.8 68.6 62.2 70.5

2.46:2.98:1 2.38:2.89:1 2.13:2.74:1 2.30:2.99:1

0.04 0.03 0.04 0

0 0 0.01 0.03

0.21 0.17 0.09 0.18

0.25 0.20 0.14 0.21

a Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

elements in the catalyst might be caused by the filtration and washing operations in the preparation process. The Zn/Al atomic ratio decreased in the order Ca−Zn−Al-4 ≈ Ca−Zn− Al-1 > Ca−Zn−Al-2 > Ca−Zn−Al-3, which was roughly identical to the order in which the total yield of both DMC and PC decreased. Ca−Zn−Al-3 had the lowest Zn/Al molar ratio (2.74:1) and also the lowest total yield of both DMC and PC, suggesting that the reaction of urea and PG was affected by the Zn/Al molar ratio. The Ca/Al atomic ratio decreased in the order Ca−Zn−Al-1 > Ca−Zn−Al-2 > Ca−Zn−Al-4> Ca−Zn− Al-3. As shown in Table 2, the amounts of strong base sites and total alkalinity decreased in the order Ca−Zn−Al-1 > Ca−Zn− Al-4 ≈ Ca−Zn−Al-2> Ca−Zn−Al-3, which is roughly identical to the orders in which the Ca/Al atomic ratio and the yield of DMC decreased. That is, when the amount of strong base sites was in a range of 0.09−0.21 mmol/g, the greater the amounts of strong base sites and total alkalinity in the Ca−Zn−Al oxides, and the higher the catalytic performance for transesterification in the one-pot synthesis of DMC. In the end, different preparation methods led to different Ca/Zn/Al molar ratios in the Ca−Zn−Al catalysts, resulting in their different alkalinities and thus different catalytic performances for the one-pot synthesis of DMC. 3.3. Effects of the Preparation Conditions on the Catalytic Performance of Ca−Zn−Al Oxide. The preparation method of oxide Ca−Zn−Al-1 was used to prepare Ca− Zn−Al oxide catalysts. Then, the effects of the Zn/Al molar ratio, the Ca/Al molar ratio, the calcination temperature, and the calcination time on the catalytic performance of the Ca− Zn−Al oxide catalysts were investigated. 3.3.1. Effect of the Zn/Al Molar Ratio. The influence of the Zn/Al molar ratio on the catalytic performance of Ca−Zn−Al oxide was studied at a fixed mass ratio of CaO to Zn−Al oxide of 1:2. Additionally, the basicities of Ca−Zn−Al oxides with different Zn/Al molar ratios were measured by CO2 TPD (see Figure S3, Supporting Information), and the results are listed in Table 3. When the Zn/Al molar ratio was increased, the yield of DMC and the total yield of both DMC and PC first increased and then decreased, whereas the total alkalinity decreased monotonically. When the Zn/Al molar ratio was 3:1, the yield of DMC and the total yield of both DMC and PC reached their highest values, 64.3% and 68.8%, respectively. As shown in Table 3, the Ca−Zn−Al oxides prepared with different Zn/Al molar ratios contained similar amounts of weak base sites, whereas the amounts of strong base sites differed greatly. When the Zn/Al molar ratio was 2:1, the Ca−Zn−Al oxide presented the largest amount of strong base sites, and the desorption amount of CO2 reached 0.42 mmol/g. It is speculated that excess strong base sites have an unfavorable influence on the reaction of urea and PG and lead to a lower total yield of both DMC and PC. When the Zn/Al molar ratio was 4:1, the

Table 3. Catalytic Performance and CO2 TPD Measurement Results of Ca−Zn−Al Oxides with Different Zn/Al Molar Ratiosa desorption amount of CO2 (mmol/g) Zn/Al molar ratio

YDMC (%)

YPC (%)

YDMC+PC (%)

weak

medium

strong

total

2:1 3:1 4:1

57.9 64.3 57.8

4.2 4.5 8.3

62.1 68.8 66.1

0.05 0.04 0.06

0 0 0

0.42 0.21 0.12

0.47 0.25 0.18

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

amount of strong base sites in the Ca−Zn−Al oxide decreased significantly, and the desorption amount of CO2 was only 0.12 mmol/g. It is inferred that fewer strong base sites might have an unfavorable influence on the transformation of PC to DMC by transesterification and lead to the highest yield of PC. In conclusion, when the Zn/Al molar ratio was 3:1, the Ca−Zn− Al oxide had appropriate amounts of weak base sites, strong base sites, and total alkalinity and showed a good catalytic performance for the one-pot synthesis of DMC. Therefore, 3:1 was selected as the appropriate Zn/Al molar ratio for the preparation of Ca−Zn−Al oxide catalysts. 3.3.2. Effect of the Ca/Al Molar Ratio. With the Zn/Al molar ratio fixed at 3:1, the influence of the Ca/Al molar ratio on the catalytic performance of the Ca−Zn−Al oxide was investigated. The results are reported in Table 4. As can be seen from the table, when the Ca/Al molar ratio was decreased from 2.6:1 to 0.4:1, the yield of DMC remained almost steady, whereas the yield of PC increased slowly, suggesting that a Table 4. Catalytic Performance of Ca−Zn−Al Oxides with Different Ca/Al Molar Ratiosa Ca/Al molar ratio

YDMC (%)

YPC (%)

YDMC+PC (%)

2.6:1 2.1:1 1.6:1 1.1:1 0.6:1 0.4:1 0.2:1 0:1

64.3 65.6 67.7 63.7 64.1 64.8 60.1 20.0

4.5 4.9 5.7 8.1 10.8 15.3 26.6 68.6

68.8 70.5 73.4 71.8 74.9 80.1 86.7 88.6

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

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surface area was beneficial to the synthesis of PC from urea and PG. 3.3.3. Effect of the Calcination Temperature. With the Ca/ Zn/Al molar ratio fixed at 0.2:3:1, the effect of the calcination temperature on the catalytic performance of the Ca−Zn−Al oxides was investigated, and the results are reported in Table S2 (Supporting Information). The total yield of both PC and DMC varied little with increasing calcination temperature, meaning that calcination temperature had no significant effect on the catalytic performance of the Ca−Zn−Al oxides in the reaction of urea and PG. However, the yield of DMC changed with increasing calcination temperature. The highest DMC yield of 60.1% was achieved over the Ca−Zn−Al oxide catalyst calcined at 923 K. This result shows that the calcination temperature affected the catalytic transesterification performance of the Ca−Zn−Al oxides. The XRD patterns of Ca−Zn−Al oxides prepared at different calcination temperatures contained only the characteristic diffraction peaks of ZnO (see Figure S5, Supporting Information), suggesting that no new substances were formed during the calcination process. The calcination process of Ca− Zn−Al hydroxide to Ca−Zn−Al oxide was studied by thermogravimetric analysis (TGA) (the TG curve is included in Figure S6, Supporting Information). These results show that the thermal decomposition process of the Ca−Zn−Al hydroxide included three stages: The first stage appeared below 401 K and was attributed to the loss of physically adsorbed water. The second stage, with a temperature range from 401 to 540 K, was mainly caused by the loss of interlayer water from Ca−Zn−Al hydroxide.22,23 The third stage, ranging from 540 to 912 K, can be ascribed to the decomposition of Ca−Zn−Al hydroxide, according to the decomposition temperatures of Ca(OH)2, Zn(OH)2, and Al(OH)3. It is believed from the experimental results in Table S2 (Supporting Information) that Ca(OH) 2 might not be completely decomposed at a calcination temperature of 873 K, leading to a lower catalytic performance for the transesterification reaction. However, the yield of DMC became lower because of the agglomeration of the crystal grains at calcination temperatures higher than 923 K. Therefore, 923 K was selected as the appropriate calcination temperature. 3.3.4. Effect of the Calcination Time. With the Ca/Zn/Al molar ratio fixed at 0.2:3:1 and the calcination temperature controlled at 923 K, the effect of the calcination time on the catalytic performance of the Ca−Zn−Al oxides was investigated, and the results are reported in Table S3 (Supporting Information). When the calcination time was increased from 3 to 5 h, the yield of DMC first increased and then decreased, whereas the total yield of both DMC and PC changed only slightly. These results show that the calcination time had a large effect on the catalytic performance of the Ca−Zn−Al oxides in the transesterification reaction. The lower catalytic performance of the Ca−Zn−Al oxide calcined for 3 h is possibly due to the incomplete decomposition of Ca−Zn−Al hydroxide. On the other hand, calcination for 5 h would lead to the agglomeration of CaO particles and a decrease of the catalytic performance of the Ca−Zn−Al oxide. As a result, 4 h was selected as the appropriate calcination time. As mentioned above, the appropriate conditions for the preparation of Ca−Zn−Al oxide were determined to be as follows: Ca/Zn/Al molar ratio of 0.2:3:1, calcination temperature of 923 K, and calcination time of 4 h.

small proportion of CaO could meet the requirement for the active sites to catalyze the transesterification of PC with methanol to DMC.21 On the contrary, excess CaO could lead to an unfavorable influence on the reaction of urea and PG. When the Ca/Al molar ratio was decreased to 0.2:1, the yield of DMC and PC were 60.1% and 26.6% respectively. Compared with the results for a Ca/Al molar ratio of 0.4:1, the yield of DMC decreased by 4.7%, whereas the yield of PC increased by 11.3%. Therefore, a higher yield of DMC might be expected upon further determination of other preparation conditions and reaction conditions for the one-pot synthesis of DMC for a Ca/ Al molar ratio of 0.2:1. The XRD patterns of Ca−Zn−Al oxides with different Ca/Al molar ratios are shown in Figure S4 (Supporting Information). It can be seen that the characteristic diffraction peaks of CaO decreased gradually with decreasing Ca/Al molar ratio. When the Ca/Al molar ratio was lower than 1.1:1, the characteristic diffraction peaks of CaO disappeared, indicating that CaO was highly dispersed in amorphous form or that the content of CaO was too low to be detected. The CO2 TPD curves of Ca−Zn−Al oxides with different Ca/Al molar ratios are shown in Figure 1 (data on the

Figure 1. CO2 TPD curves of Ca−Zn−Al oxides with different Ca/Al molar ratios.

desorption amounts of CO2 are included in Table S1, Supporting Information). The desorption peaks at lower and higher temperatures are ascribed to the base sites of Zn−Al oxide and CaO, respectively, in the Ca−Zn−Al oxides. The desorption peak at higher temperature decreased gradually with decreasing Ca/Al molar ratio and disappeared at a Ca/Al molar ratio of 0.2:1. At the same time, the base distribution changed greatly, as can be seen in Table S1 (Supporting Information). When the Ca/Al molar ratio was decreased from 2.6:1 to 0.2:1, the amount of weak base sites increased gradually, but the amount of strong base sites decreased gradually. However, the yield of DMC was still maintained above 60%. It is inferred that the weak and strong base sites had a synergistic effect and improved the catalytic performance of the Ca−Zn−Al oxides. In addition, the total yield of both DMC and PC increased gradually with the increase in specific surface area (see Table S1, Supporting Information), suggesting that a higher specific 3519

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Industrial & Engineering Chemistry Research 3.4. Effect of the Reaction Conditions on the One-Pot Synthesis of DMC. The effects of the reaction conditions on the one-pot synthesis of DMC were investigated using the Ca− Zn−Al oxide catalyst prepared under the appropriate conditions determined in the preceding experiments. 3.4.1. Effect of the PG/Urea Molar Ratio. The effect of the PG/urea molar ratio on the one-pot synthesis of DMC is reported in Table 5. When the PG/urea molar ratio was 1:1,

that the distillation rate of the mixed solution increased with increasing flow rate of methanol and that DMC could be driven off immediately with methanol. Thus, the equilibrium of the transesterification reaction was shifted in the forward direction, and the yield of DMC increased. When the flow rate of methanol was greater than 0.6 mL/min, the yield of DMC remained at about 60.0% and exhibited no significant change, probably because the distillation rate of DMC was greater than the transesterification reaction rate. Therefore, the appropriate flow rate of methanol was determined to be 0.6 mL/min. 3.4.3. Effect of the Catalyst Weight Percentage. The effect of the catalyst weight percentage on the one-pot synthesis of DMC was studied, and the results are listed in Table 7. When

Table 5. Effect of the PG/Urea Molar Ratio on the One-Pot Synthesis of DMCa PG/urea molar ratio

YDMC (%)

YPC (%)

YDMC+PC (%)

1:1 2:1 4:1

41.2 60.1 54.3

38.0 26.6 29.7

79.2 86.7 84.0

Table 7. Effect of the Catalyst Weight Percentage on the One-Pot Synthesis of DMCa

a

Reaction conditions: catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

YDMC (%)

YPC (%)

YDMC+PC (%)

52.6 60.1 58.3 61.1

31.8 26.6 28.1 24.4

84.4 86.7 86.4 85.5

YPC (%)

YDMC+PC (%)

60.1 67.1 66.8

26.6 17.1 17.7

86.7 84.2 84.5

Reaction conditions: PG/urea molar ratio of 2, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

the catalyst weight percentage was increased from 2.2% to 2.7%, the yield of DMC increased significantly, but the yield of PC decreased, and the total yield of both DMC and PC decreased slightly. However, the yield of DMC, the yield of PC, and the total yield of both DMC and PC remained steady when the catalyst weight percentage was increased further from 2.7% to 3.2%, suggesting that the effect of the catalyst weight percentage on the one-pot synthesis of DMC became less significant when the catalyst weight percentage was increased above 2.7%. Therefore, the appropriate catalyst weight percentage was determined to be 2.7%. 3.4.4. Effect of the First-Step Reaction Temperature. The effect of the first-step reaction temperature on the one-pot synthesis of DMC was investigated, and the results are listed in Table 8. The reaction of urea and PG proceeded insufficiently Table 8. Effect of the First-Step Reaction Temperature on the One-Pot Synthesis of DMCa reaction temperature (K)

YDMC (%)

YPC (%)

YDMC+PC (%)

433 443 453

28.4 67.1 69.4

49.6 17.1 6.1

78.0 84.2 75.5

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.7%, reaction temperature 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step, methanol flow rate of 0.6 mL/min.

Table 6. Effect of the Methanol Flow Rate on the One-Pot Synthesis of DMCa 0.4 0.6 0.8 1.0

YDMC (%)

2.2 2.7 3.2 a

the reaction of urea with PG proceeded insufficiently because of the low PG concentration. In particular, the intermediate of 2hydroxypropyl carbamate (HPC) could not be completely transformed to PC, a controlling step of the reaction from urea and PG to PC.24 Hence, the amount of PC formed from the first reaction was lower, resulting in a lower yield of DMC. The total yield of both DMC and PC increased with increasing PG/ urea molar ratio, showing that an increase in the PG/urea molar ratio was beneficial to the formation of PC. When the PG/urea molar ratio was 2:1, the yield of DMC and the total yield of both DMC and PC reached 60.1% and 86.7%, respectively. When the molar ratio of PG to urea was increased to 4:1, more residual PG was left in the reaction system after the completion of the first reaction. Because PG is not only a reactant in the first-step reaction but also a byproduct of the second-step reaction, excess PG exerted an inhibitive effect on the transesterification reaction and decreased the yield of DMC. Therefore, 2:1 was selected as the appropriate PG/urea molar ratio. 3.4.2. Effect of the Flow Rate of Methanol. Because a mixture of methanol and DMC was continuously distilled from the reactor in the tranesterification process, the flow rate of methanol introduced was adjusted to match the distillation rate of the mixed solution to keep the volume of the reaction liquid steady in the reactor. Table 6 reports the effect of the flow rate of methanol on the one-pot synthesis of DMC. When the flow rate of methanol was increased from 0.4 to 0.6 mL/min, the total yield of both DMC and PC changed slightly, whereas the yield of DMC increased from 52.6% to 60.1%. It can be inferred

methanol flow rate (mL/min)

catalyst weight percentage (%)

at a reaction temperature of 433 K, so the yield of DMC and the total yield of both DMC and PC were lower. When the reaction temperature was increased to 443 K, the yield of DMC and the total yield of both DMC and PC achieved their maxima, at 67.1% and 84.2%, respectively. With a further increase of the reaction temperature to 453 K, the yield of DMC increased by only 2.3%, but the total yield of both DMC and PC decreased by 8.7%. Furthermore, many byproducts such as dipropylene glycol were found in the reaction residue at

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.2%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step and 4 h for the second step. 3520

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appropriate reaction time for the second-step was determined to be 8 h. 3.5. Reusability of Ca−Zn−Al Oxide. The reusability of Ca−Zn−Al oxide was investigated, and the results are reported in Table S4 (Supporting Information). The Ca−Zn−Al oxide recovered after the completion of the reaction was washed with acetonitrile and dried at 343 K in a vacuum drying oven before reuse in the next cycle. As shown in Table S4 (Supporting Information), the catalytic performance of the recovered catalyst decreased severely and could not be restored by calcination at 923 K for 4 h. Compared with the results for the fresh catalyst, the yield of DMC decreased seriously, whereas the yield of PC increased and the total yield of both DMC and PC changed slightly. These results show that PC was not transformed to DMC effectively, suggesting that the catalytic performance of the recovered catalyst for the transesterification decreased. Because CaO is the major active component in the Ca−Zn−Al oxide catalyst for catalyzing the transesterification, the decrease in the catalytic performance of the recovered catalyst for the transesterification might be caused by a loss or the chemical transformation of CaO. The reaction liquid recovered after the completion of reaction was analyzed by ICP-AES, and the results showed that the concentrations of Zn2+ and Al3+ were 0.045 and 0.232 g/L, respectively, whereas Ca2+ was undetectable, suggesting trace amounts of Zn2+ and Al3+ in the Ca−Zn−Al oxide were lost, whereas Ca2+ was not lost. Thus, CaO must have transformed into other substances during the reaction process because the decrease of the catalytic performance of the Ca− Zn−Al oxide was not caused by the loss of CaO. Some experiments were designed and conducted to clarify the transformation of CaO. Specifically, CaO was separately used as the catalyst for the reaction of urea with PG and for the onepot synthesis of DMC, and then the change in CaO was analyzed by FT-IR spectroscopy before and after reaction. The FT-IR spectra of fresh CaO and CaO recovered from the reaction for the one-pot synthesis of DMC are shown in panels a and f, respectively, of Figure 2, respectively. It can be seen that some new absorption bands centered at 3400, 2210, 1440, 876, and 713 cm−1 appeared in the spectrum of recovered CaO. Among them, the absorption bands located at 3400, 1440, 876, and 713 cm−1 were assigned to CaCO3.26,27 The broad band at 3400 cm−1 corresponded to H−O symmetric stretching vibration and asymmetric stretching vibration and was assigned to a hydroxyl group and adsorbed water at the surface of CaCO3.28,29 The strong absorption band at 1440 cm−1 was attributed to C−O asymmetric stretching. The two bands at 876 and 713 cm −1 separately represent the ν 2 and ν 4 characteristic bands of CaCO3 crystal and correspond to the C−O bending vibration. The ν2 and ν4 absorption bands were sharp, but the intensity of ν4 was lower than that of ν2.30 The recovered catalyst was analyzed by TG−MS, and the results are shown in Figure S7 (Supporting Information). The weight loss before 900 K was attributed to the decomposition of the adsorbed organic compounds, and a great deal of CO2 was formed during this period. The recovered Ca−Zn−Al oxide was also decomposed from 900 to 1195 K, and a weak signal of CO 2 was detected during this period, suggesting the decomposition of CaCO3, although the content was small.31 According to the above analysis, it is clear that the recovered catalyst contains CaCO3. In addition, the absorption band at 2210 cm−1 in the spectrum of the recovered CaO was assigned to isocyanate

a reaction temperature of 453 K, suggesting that some side reactions such as the polycondensation of PG were enhanced at higher reaction temperatures. As a result, the appropriate first reaction temperature was determined to be 443 K. 3.4.5. Effect of the First-Step Reaction Time. The effect of the first-step reaction time on the one-pot synthesis of DMC was investigated. As shown in Table 9, the yield of DMC and Table 9. Effect of the First-Step Reaction Time on the OnePot Synthesis of DMCa reaction time (h)

YDMC (%)

YPC (%)

YDMC+PC (%)

1 2 3

27.8 67.1 67.6

47.9 17.1 5.3

75.7 84.2 72.9

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.7%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 4 h for the second step, methanol flow rate of 0.6 mL/min.

the total yield of both DMC and PC were low at a reaction time of 1 h because this time was too short for the reaction of urea and PG. When the reaction time was prolonged to 2 h, the yield of DMC and the total yield of both DMC and PC attained their maxima, 67.1% and 84.2%, respectively. When the reaction time was further prolonged to 3 h, the yield of DMC changed slightly, but the total yield of both DMC and PC decreased significantly because the overly long reaction time led to an increase of some byproducts generated in the reaction of urea and PG.25 Therefore, the appropriate reaction time of urea and PG was determined to be 2 h. 3.4.6. Effect of the Second-Step Reaction Time. The second-step temperature was held at about 346 K for the mixture of DMC and methanol to be distilled out continuously. The effect of the second-step reaction time on the one-pot synthesis of DMC was investigated. As can be seen from Table 10, with an increase in the reaction time, the yield of DMC Table 10. Effect of the Second-Step Reaction Time on the One-Pot Synthesis of DMCa reaction time (h)

YDMC (%)

YPC (%)

YDMC+PC (%)

4 6 8 10

67.1 76.9 82.9 84.4

17.1 5.3 1.0 1.3

84.2 82.2 83.9 85.7

a

Reaction conditions: PG/urea molar ratio of 2, catalyst weight percentage of 2.7%, reaction temperature of 443 K for the first step and 346 K for the second step, reaction time of 2 h for the first step, methanol flow rate of 0.6 mL/min.

increased, whereas the yield of PC decreased, but the total yield of both DMC and PC changed a little. The results showed that prolonging the transesterification reaction time could promote the transformation of PC to DMC. The reaction rate of the transesterification was high during the early stage because of the high concentration of PC. The yield of DMC reached 67.1% for only 4 h. The transesterification reaction rate decreased gradually with the decrease of the PC concentration. As a result, the increased rate of DMC production gradually decreased. When the reaction time was prolonged from 8 to 10 h, the yield of DMC increased by only 1.5%. Therefore, the 3521

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oxide reacted with water to form Ca(OH)2 (reaction 3). Finally, CaCO3 was formed from the reaction of Ca(OH)2 with CO2 generated from the hydrolysis of urea (reaction 4). In conclusion, the presence of water and CO2 in reaction system was the important factor for the transformation of CaO into CaCO3. To transform CaCO3 back into CaO, the recovered catalyst was calcined at 1173 K for 4 h and then was used in the one-pot synthesis of DMC. The results are listed in Table S4 (Supporting Information). Unfortunately, the catalytic performance could not be restored, possibly because of the adverse effects of calcination at excessively high temperature. To further validate the adverse effects, the fresh Ca−Zn−Al oxide catalyst was calcined at 1173 K for 4 h and was used in the one-pot synthesis of DMC. The results showed that the catalytic performance decreased severely to the level of the recovered catalyst after calcination. N2 adsorption−desorption analysis result showed that the specific surface area and pore volume of the recovered catalyst after calcination were dramatically reduced. In addition, a spinel of ZnAl2O4 was observed in the XRD patterns (see Table S5 and Figure S8, Supporting Information). All of these factors might cause a decrease in the catalytic performance.

Figure 2. FT-IR spectra of fresh and recovered CaO: (a) fresh CaO, (b) CaO after treatment with DMC, (c) CaO after treatment with PC, (d) CaO after treatment with PG, (e) CaO after participation in the reaction of urea and PG, and (f) CaO after participation in the one-pot synthesis of DMC.

4. CONCLUSIONS Ca−Zn−Al oxide with a high catalytic performance for the onepot synthesis of DMC was prepared. CO2 TPD analysis showed that both strong base sites and weak base sites were present in the Ca−Zn−Al oxide and that their proportion could change significantly by adjusting the Ca/Zn/Al molar ratio. The onepot reaction of urea, PG and methanol to DMC was successfully performed under the catalysis of the Ca−Zn−Al oxide, and the yield of DMC could reach 82.9%. However, the catalytic performance of the recovered Ca−Zn−Al oxide decreased severely, with the major reason being the transformation of CaO into CaCO3 during the reaction of urea and PG.

species.24,32 FT-IR spectrum d in Figure 2 represents the recovered CaO treated with PG at 443 K for 2 h. Compared with that of fresh CaO, the spectrum of the recovered CaO was almost identical except for a broad absorption band at 3400 cm−1 corresponding to a hydroxyl group and adsorbed water on the surface of CaO. The water might be formed from the polycondensation of PG at high temperature. Spectrum e in Figure 2 represents the CaO from the reaction of urea with PG. Upon comparison of spectra e and f of Figure 2, it was found that all of the bands corresponded to each other. These results show that the transformation of CaO into CaCO3 occurred in the reaction of urea with PG while CaO did not change after contacting with DMC or PC (see Figure 2b,c). A possible reaction process is proposed in Scheme 2. In addition to the reaction of urea with PG to PC, there was a side reaction (reaction 1 in Scheme 2) to 3-methyl-2oxazolidinone as a byproduct.33 Meanwhile, PG polycondensation (reaction 2) could produce tripropylene glycol as a byproduct.34 Therefore, water existed in the reaction system according to reactions 1 and 2. Then, CaO in the Ca−Zn−Al

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ASSOCIATED CONTENT

S Supporting Information *

(1) CO2 TPD profile, XRD patterns, textural properties, catalytic performance, TG curve, and reusability of fresh Ca− Zn−Al catalyst. (2) TG−MS curves, textural properties, and XRD patterns of recovered Ca−Zn−Al catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 2. Process for the Transformation of CaO into CaCO3

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-60202427. Fax: +86-22-60204294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grants 21476058 and 21236001) and the Key Basic Research Project of Applied Basic Research Plan of Hebei Province (Grant 12965642D). The authors are gratefully appreciative of their contributions. 3522

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