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Environmentally benign alcoholysis of urea and disubstituted urea to alkyl carbamates over alkali-treated zeolites
Microporous and Mesoporous Materials 248 (2017) 108e114
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Environmentally benign alcoholysis of urea and disubstituted urea to alkyl carbamates over alkali-treated zeolites Qi Sun, Rui Niu, Hongxia Wang, Bin Lu**, Jingxiang Zhao, Qinghai Cai* Key Lab for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Harbin Normal University, No. 1 Shida Road Limin Development Zone, Harbin, 150025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2017 Received in revised form 30 March 2017 Accepted 13 April 2017 Available online 14 April 2017
Alkali-treated zeolite material was prepared by using 0.1e0.4 mol/L NaOH aqueous solution, as characterized by XRD, SEM, N2 adsorption-desorption technique and ammonia-temperature programmed desorption (NH3-TPD). The as-prepared zeolite was found to be an effective catalyst for alcoholysis of urea and disubstituted urea to produce alkyl carbamate, getting 91.8e100% conversion of urea and substituted urea with >98.0% selectivity to alkyl carbamates. The results indicated that the high reactivity of the zeolites for alcoholysis of urea and N-substituted urea could be mainly ascribed to the enhanced specific area and balance between acidic and basic sites on the HZSM-5 surface caused by the alkalitreatment. This catalyst can be easily recovered and reused. © 2017 Published by Elsevier Inc.
Keywords: Zeolite Alkali-treatment Methanol Urea Carbamate
1. Introduction Carbamic esters are commercially important class of organic compounds. They are widely used as intermediates in the synthesis of a variety of organic chemicals, such as polymer (e.g. polyurethane), microbicides, agricultural pesticides, herbicides and pharmaceutical agents [1]. They are also served as protective groups for amines as well as excellent templates for the formation of CeC bonds [2]. As the most simple carbamate, methyl carbamate (MC) is a crude material for the synthesis of dimethyl carbonate [3] that is not only used as an environmentally benign substitute for highly toxic phosgene and dimethyl sulfate in carbonylation and methylation, but also as a promising octane booster, reducing particulate emission and the cost of fuel due to its high oxygen content [4]. Traditionally, carbamate has been synthesized by alcoholysis of isocyanate, which is produced by a primary amine and phosgene, or aminolysis of chloroformate. The major drawbacks of the former are that the phosgene is highly toxic and corrosive. Analogously, massive waste salt was produced in the chloroformate process. Alternatively, phosgene-free methods for synthesizing carbamates have been recently developed, including
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Lu), [email protected] (Q. Cai). http://dx.doi.org/10.1016/j.micromeso.2017.04.024 1387-1811/© 2017 Published by Elsevier Inc.
reductive carbonylation of nitro aromatics and oxidative carbonylation of amines [5,6]. However, the reductive carbonylation route using platinum group metal catalysts is economically not viable; only one-third of CO could be effectively utilized and the separation of CO from CO2 increases the operation cost. Recently, the synthesis of carbamates utilizing CO2 as carbonylation reagent has been widely focused [7]. But this process requires strict reaction conditions due to the thermodynamic inertness of CO2, as well as alkyl halide using as alkylation reagent [8], or dehydration reagent [9]. Otherwise, low yield was achieved in the synthesis of carbamates from CO2, amines and alcohols in the absence of dehydration reagent. Acidic catalysts such as polyphosphoric acid [10], BF3 [11] and cupric acetate [12] have been used for the reaction of urea with methanol to generate MC. However, the yield of carbamate is low in these cases; furthermore, additional process steps for separation of the catalysts or the byproduct from the methanol/MC solution are required when using these homogeneous catalysts. In order to enhance the yield of carbamate, a semicontinuous process for the synthesis of methyl carbamate in the absence of catalyst has been investigated. A higher yield of MC was obtained at the optimal reaction conditions with a long reaction time and high temperature [13]. Besides, alcoholysis of disubstituted ureas to N-substituted carbamates over TiO2/SiO2 has been reported with good catalytic activity [14]. Therefore, the effective synthesis of carbamates are now depending on the way to transfer carbonyl group from urea or
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
disubstituted urea into carbamates or N-substituted carbamates using highly efficient and environmentally-friendly heterogeneous catalyst. In this work, alkali-treated zeolites with different concentrations of NaOH aqueous solution were prepared and characterized by XRD, SEM, N2-adsorption-desorption and NH3-TPD technique. Among the catalysts screened, the zeolite treated with 0.2 mol/L NaOH aqueous solution exhibited high catalytic activity in alcoholysis of urea and disubstituted urea [dicyclohexyl urea (DCU)] with low-molecular-mass alcohols such as methanol, ethanol, propanol, iso-propanol, butanol, iso-amyl alcohol, octanol and benzyl alcohol, getting ca. 100% conversion and >99% selectivity to MC and methyl N-cyclohexyl carbamate (MCC) for methanolysis of urea and disubstituted urea. 2. Experimental section
109
reactor was heated to 180 C with a speedy stirring and the reaction was carried out at this temperature for 8 h, the reaction formula was shown in Scheme 1. At the end of the reaction, the mixture solution was filtered to remove the catalyst and the filtrate was analyzed by GC (Agilent 7820), GC-MS (Agilent 7890-5975C) with FID detector (GC) and HP-5 column (30 m 0.32 mm 0.25 mm) for synthesis of MC and HPLC (Waters class) with UV-detector at wavelength of 210 nm, C18 column and the mixture solution of acetonitrile/H2O (1:1 v/v) as mobile phase for synthesis of MCC. The temperature program for GC measurement was shown as following: the column temperature box was heated from the initial temperature 50 Ce70 C at heating rate of 10 C/min, followed by continuously heated to 230 C at heating rate of 20 C/min; and then it remained at this temperature for 1 min. The gas and liquid chromatograms for MC and MCC synthesis were shown in Figs. SI1 and SI2 in supporting information.
2.1. Preparation and characterization of the catalyst 3. Results and discussion Before the alkali-treatment, the zeolite HZSM-5 (Si/Al ¼ 38, Tianjin Chemist scientific Ltd. Co.) was calcined at 550 C for 6 h in an oven. After cooling down, 2.0 g of HZSM-5 was added to 40 mL of various concentrations (0.1, 0.2, 0.3 and 0.4 mol/L) of NaOH aqueous solution. The mixture was heated to 80 C in a water bath and stirred at this temperature for 2 h. The alkali-treated zeolites were obtained by filtration, and then washed with distilled water until neutral washing water was achieved. The zeolites were dried at 120 C for 12 h after washing and denoted as HZSM-5-x, where x represented the concentration of NaOH solution (x ¼ 0.1, 0.2, 0.3, 0.4). X-ray powder diffraction (XRD) of the samples was performed on a Bruker-D8 Advance X-ray diffractometer with Cu Ka radiation (40 kV and 36 mA) at scan rate of 0.3 /step and wavelength of radiation source 0.154056 nm. The N2 adsorption/desorption isotherms were measured using a quantachrome NOVA2000E instrument. Before the measurement, the sample was treated to remove the gasses adsorbed on the surface at vacuum and 300 C for 3 h. The N2 adsorption/desorption isotherms were measured at temperature of 77 K (196 C). NH3 temperature-programmed desorption (TPD) were conducted using a home-made device with a TCD detector of GC (GC14C, SHIMADZU). 100 mg sample was pretreated in argon flow at 500 C for 1 h and cooling to room temperature before TPD run. The adsorption of NH3 on the catalyst was performed with feeding anhydrous NH3 gas for about 10 min at room temperature, and then the catalyst was sufficiently purged by argon stream to remove the excessive adsorbate. The TPD was conducted by heating the sample in argon (80 mL/min) from 50 C to 600 C with a heating rate of 10 C/min. The morphology of the catalysts was observed by a Hitachi S4800 scanning electron microscopy (SEM). The preparation procedure of the samples: conductive adhesive was daubed on the SEM stage, and then powder samples were uniformly sprayed by syringe on the stage, followed by shaking the stage to disperse the samples and smoothing the aggregated powder by syringe needle. The ratios of Si to Al and Na (or K) content in the zeolites were determined by 7500CE ICP-OES (P-E Company). 2.2. Catalytic tests The catalytic reaction was conducted in a stainless steel 250 mL autoclave with a magnetic stirrer in an oil bath. In a typical procedure, 40 mL (1.0 mol) or 20 mL (0.5 mol) of anhydrate methanol, 1.5 g (0.025 mol) of urea or 1.2 g (5.41 mmol) of dicyclohexyl urea (DCU) and 0.3 g or 0.2 g of the catalyst (HZSM-5-0.2) were charged into the reactor in turns. After purging three times with N2 gas, the
3.1. Characterization of the alkali-treated zeolites The XRD patterns of as-received and alkali-treated zeolites were depicted in Fig. 1. It was clear that these alkali-treated samples basically preserved the specific MFI structure and no input phase was found after treated with alkaline solution. HZSM-5-0.1, HZSM5-0.2, HZSM-5-0.3 and HZSM-5-0.4 exhibited almost the same XRD patterns as that of parent HZSM-5 although obvious decrease in relative crystallinity could be observed, especially for the higher concentration alkali-treated samples HZSM-5-0.3 and HZSM-5-0.4, the diffraction peaks around 45 were too weak to be observed due to the NaOH solution severer dissolving Si or Al atoms from the zeolite framework. Besides, significant changes were also found in the Si/Al ratio, specific surface area and morphology for the alkalitreated zeolites. As shown in Table 1, the Si/Al ratio was basically decreased as increasing in the NaOH concentration for treating these zeolites, implying that the alkaline solution mainly dissolved Si from the framework. However, SEM was used to view the morphology of the alkali-treated samples as shown in Fig. 2. It was seen from the images that the parent HZSM-5 exhibited regular rectangular crystals (Fig. 2a) and the surface of the alkali-treated samples looks like rough, some irregular crystalline grains and defects on the surface were observed for ZSM-5-0.1 and HZSM-50.2 (Fig. 2b and c), which certainly caused the increase in their specific surface areas as compared with the parent HZSM-5 (Table 1). Furthermore, the SEM images (Fig. 2d and e) exhibited that more irregular crystalline grains were aggregated together after treatment with 0.3 and 0.4 mol/L NaOH solutions, inevitably causing remarkable decrease in their specific surface areas (Table 1). Especially for HZSM-5-0.4 sample, much closer aggregate and more irregular crystalline grains were observed as shown in Fig. 2e. 3.2. Catalytic activities of alkali-treated zeolites Alkali-treated zeolites with various concentration of NaOH aqueous solution were employed as catalysts for the alcoholysis of urea or disubstituted urea. Their catalytic activity was listed in Fig. 3. The HZSM-5-0.1 and HZSM-5-0.2 represented higher activity than the parent HZSM-5. Evidently, HZSM-5-0.2 exhibited ca. 100% conversion of urea and 98.0% selectivity to MC. On the contrary, the alkali-treated HZSM-5 zeolites under severe condition gave relatively low conversions, exhibiting 90.4% for HZSM-5-0.3 and 77.8% conversion for HZSM-5-0.4, respectively. In these cases, the selectivity to MC was basically unchanged, being over 98.0%. Differently, the conversion of DCU was increased as the NaOH concentration
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O H N
C
O
R NH + R1
OH
HZSM-5-0.2
R
H N
C
R1 O
+ R
NH2
R
(R = cyclohexyl or hydrogen; R1 = methyl, ethyl, propyl, isopropyl, butyl, isoamyl and benzyl) Scheme 1. The structure of the byproducts.
HZSM-5-0.1
Intensity (a.u.)
HZSM-5-0.2
HZSM-5-0.3 HZSM-5-0.4
HZSM-5
10
20
30
40
50
60
70
80
o
2 Theta ( ) Fig. 1. XRD patterns of HZSM-5 and alkali-treated HZSM-5.
Table 1 Si/Al ratio, Na content and specific surface area of HZSM-5 and HZSM-5-x. Catalysts
Si/Al ratio
Surface area (m2/g)
Na content (%)
HZSM-5 HZSM-5-0.1 HZSM-5-0.2 HZSM-5-0.3 HZSM-5-0.4
38.6 31.1 29.2 29.9 26.6
186.3 246.2 221.8 175.1 154.5
2.96 3.18 3.13 3.34 3.32
rising with unchanged selectivity to MCC. These results suggested that alkali-treating of HZSM-5 with various concentrations of NaOH solution had a significantly impact on its catalytic activity due to the effect of alkali-treatment on the surface nature. NH3-TPD method was used to characterize the amount and acid strength of acid sites on the surface of zeolites and the profiles were shown in Fig. 4. The profiles of all the HZSM-5-x samples exhibited various broad peaks in the temperature range from 50 to 600 C. For the profile HZSM-5, the wide peak corresponding to low temperature, ranging from 139 to 200 C could be ascribed to NH3 chemisorbed on the weak acid sites. These acid sites were related to the interaction between NH3 molecules and surface oxide or hydroxyl groups by non-specific hydrogen bonding [15]. In the high temperature region, the peak centered around 436 C was attributed to the strong adsorption of NH3 on the surface, corresponding to strong acid sites. Compared with that of the untreated sample, the weak acid sites for the profiles of HZSM-5-0.1 and HZSM-5-0.2 were reinforced by extending the peaks from 150 to 227 C with slightly decrease of strong acid sites by shifting to about 406 C. Such a strengthening in the weak acid sites and weakening in the strong acid sites was accordingly thought to be the results of the desilication by splitting SieOHeAl groups and ion-exchange of Hþ with Naþ during the alkali-treatment of HZSM-5 [16]. Compared
with HZSM-5-0.1, HZSM-5-0.2 presented slightly different weak and strong acid sites due to the desilication, which was observed in the temperature range below 300 C and about 406 C, respectively. Moreover, high temperature NH3 desorption peak shifted to about 420 C or intensity of the weak and strong acid sites was increased in the NH3-TPD profile of the sample HZSM-5-0.3, which were reported to be attributed to extra framework aluminum formed during the alkali-treated with higher NaOH concentrations [17]. As compared with HZSM-5-0.2, HZSM-5-0.3 possessed relatively higher acidity and remarkably lower specific surface area. Meanwhile, complete disappearance of the strong acid site and decrease of the weak acid sites were obviously observed in the NH3-TPD profile of HZSM-5-0.4. These results suggested that the enhanced specific surface area and appropriate acid sites on the catalyst surface were crucial to the conversion of urea. According to the reported literature [18], the cationic zeolites constitute a family of solids with directly coupled acid-base sites, the strength of acidic sites increasing when that of the basic ones decrease. These sites can be visualized as acid-base pairs [19]. In the zeolites the strength of each site is mainly determined by the cation nature (Al, Na, …) and crystalline structure. The only way to change the acid-base properties is by changing the composition and structure. What makes zeolite acid-base pairs different is that their strength is easily modified by changing the exchangeable cation or the aluminum content for the same zeolite structure in addition to moving from one structure to another. In our experiments, decrease in the ratio of Si/Al and increase in Naþ content caused by alkalitreatment of HZSM-5 were evidently observed as shown Table 1, which prompt the acidic and basic sites redistributed. However, it is known that the catalytic activity of the zeolite catalyst can be ascribed to basic sites created by oxygen atoms, as well as acidic sites that attributed by Al for Lewis acidic and counter Hþ for € nsted acidic sites [20]. So that, mutual action of acidic and basic Bro sites on the catalyst surface was vital for the activation of organic urea and alcohols. As compared with HZSM-5-0.2 (content of Naþ: 3.12%), the Naþ content of HZSM-5-0.3 (3.34%) was relatively higher, which inevitably caused alterative acid-base sites on the surface and it had lower specific surface area as well. As a result, the evident difference of the catalytic activity between these two catalysts was observed. In addition, the materials HZSM-5-xK that was obtained from HZSM-5 treated with 0.1e0.4 mol/L KOH solutions also exhibited higher activity for MC synthesis (Fig. 3). Unlike NaOH system, HZSM-5-0.1 K reached at almost the same conversion (88.9%) as HZSM-5-0.2 did (90.6%). This finding indicated that stronger basicity of KOH than that of NaOH was remarkable for altering the acid-base balance on the zeolite surface, further accelerating the reaction reactivity. Besides, the materials treated with high concentration of NaOH solution had no effect on the conversion of DCU because it possesses different basicity as compared with urea. 3.3. Effect of reaction conditions As the used amount of CH3OH kept constant, the effect of CH3OH to urea (or disubstituted urea) molar ratio on the catalytic activity
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
111
80
Yield of MCC Selectivity to MC Conversion of urea Conversion of urea (KOH)
406
420
70
60
HZSM-5 HZSM-5-0.1 HZSM-5-0.2 HZSM-5-0.3 HZSM-5-0.4 436
210
a b c d e
200
90
Intensity
Conversion/Selectivity/Yield (%)
100
227
139 150
Fig. 2. SEM photographs of alkali-treated HZSM-5. a- HZSM-5, b-HZSM-5-0.1, c- HZSM-5-0.2, d- HZSM-5-0.3, e- HZSM-5-0.4.
50
40 0.0
0.1
0.2
0.3
0.4
Concentrations of NaOH (KOH, mol/L)
100
200
300
400
500
600
o
Fig. 3. Effect of the concentrations of NaOH (KOH) solution treated zeolites on the activity. Reaction conditions: temperature 180 C; time 8 h; catalyst 0.3 g; molar ratio of CH3OH/urea 40:1; temperature 150 C, time 6 h, 0.1 g HZSM-5-0.2, molar ratio of CH3OH/DCU 92.4:1.
Temperature ( C) Fig. 4. NH3-TPD profiles of treated HZSM-5 with various alkali concentrations. HZSM5-0 (a), HZSM-5-0.1(b), HZSM-5-0.2 (c), HZSM-5-0.3 (d), HZSM-5-0.4 (e).
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114 100
90
Conversion/Selectivity (%)
was investigated using HZSM-5-0.2 as catalyst, and the results were shown in Fig. 5. It could be seen that the conversions reached to the maximum points at the CH3OH/urea molar ratio of 40:1 and the CH3OH/DCU molar ratio of 92.4:1, respectively. Further increase in the molar ratios led the conversions to falling down. This might be ascribed to dilute effect of methanol to urea or DCU under the high molar ratios. Besides, the selectivity to MC and MCC was independent of the molar ratio, retaining above 98% all the time. As the reaction temperature was raised, the conversion rapidly increased to the maximum at 180 C for the alcoholysis of urea, and basically remained unchangeable at 190 C (Fig. 6). The changing rule of the conversion with the temperature was probably attributed to the control of the reaction thermodynamic character. It was reported that the heat of this synthesis reaction was DHØ ¼ 73.44 kJ/mol [21], which meant that the reaction is endothermic. Thus, the increase of the reaction temperature is of a great advantage to formation of MC. On the other hand, in view of the kinetics, raise of the reaction temperature can accelerate the reaction rate and shorten the time approaching to the equilibrium. Similarly, the conversion of disubstituted urea was rapidly increased with rising temperature in the range from 150 to 190 C, obviously remaining the selectivity above 98% for the two systems. The increase in dose of the catalyst used in this system would lead to increase in amount of active sites on the catalyst surface. There was an increase in the conversion of urea from 50.4% to ca. 100% as the catalyst amount increased from 0 up to 0.3 g (Fig. 7). Very low conversion was obtained in the absence of any catalyst in this case. The dose unceasingly increasing to 0.4 and 0.5 would give basically no effect for the conversion, but more active sites would easily catalyze by-reactions to generate some byproducts such as hydrazinecarboxamide (Mzþ 16, 32, 44, 75), imidodicarbonic diamide (Mzþ 29, 44, 60, 103) and dimethyl carbonate (Mzþ 15, 29, 31, 45, 59, 90) as shown in Scheme 2, leading to the fall of the selectivity to MC. A similar results for the DCU system were achieved. The yield of DCU was increased from 46.5 to 98.5% when using 0 and 0.2 g of the catalyst, and then the yield slightly increased as the amount of the catalyst rising. Unlike the urea system, the selectivity to MCC remained at high selectivity to MCC. The catalytic performance-time profile of the direct reaction of urea or disubstituted urea with methanol at 180 C using HZSM-50.2 as catalyst was shown in Fig. 8. The conversion of urea or/and
80
70
60
Conversion of urea Selectivity to MC Conversion of DCU Selectivity to MCC
50
40 150
160
170
180
190
o
Reaction temperature ( C) Fig. 6. Effect of reaction temperature on the activity. Reaction conditions: time 8 h, 0.3 g HZSM-5-0.2, molar ratio of CH3OH/urea 40:1 for urea; time 8 h, 0.1 g HZSM-5-0.2, molar ratio of CH3OH/urea 92.4:1 for DCU.
100
Conversion/Selectivity/Yield (%)
112
90
80
Yield of MCC Selectivity to MC Conversion of urea
70
60
50
40 0.0
0.1
0.2
0.3
0.4
0.5
Amount of the catalyst (g) Fig. 7. Effect of catalyst amount on the activity. Reaction conditions: temperature 180 C, time 8 h, molar ratio of CH3OH/urea 40:1 for urea; temperature 180 C, time 8 h, molar ratio of CH3OH/DCU 92.4:1.
100
H2N
Conversion/Selectivity (%)
90
C
Conversion of urea Selectivity to MC Conversion of DCU Selectivity to MCC
80
H N
O HCD
70
O NH2
H N C
C
NH2 NH2 ICD
O
O
C H3CO
OCH3 DMC
Scheme 2. The structure of the byproducts. HCD-hydrazinecarboxamide; ICD-Imidodicarbonic diamide; DMC-dimethyl carbonate.
60
50 20
40
60
80
100
120
140
160
180
Molar ratio of methanol to urea (or DCU) Fig. 5. Effect of molar ratio of methanol and urea on the activity. Reaction conditions: temperature 180 C; time 5 h; 0.3 g HZSM-5-0.2 for urea; 150 C; time 6 h; catalyst 0.1 g for DCU.
yield of MCC were increased up to ca. 100% conversion of urea, 90% yield of MCC in the time range from 4 to 8 h. As the reaction proceeded beyond 8 h, the conversion and yield remained unchanged with the reaction proceeding from 8 to 12 h, implying that the reaction achieved at thermodynamics equilibrium. However, the selectivity to MC (also to MCC) remained >98.0% in the time range of 4e8 h, excess reaction time led to the fall of the selectivity to MC, but unaffected the selectivity to MCC.
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
113
Conversion Selectivity
100
95 90
80
Conversion/Selectivity (%)
Conversion/Selectivity/Yield (%)
100
85 80
Yield of MCC Conversion of urea Selectivity to MC
75 70 65
60
40
20
60 4
5
6
7
8
9
10
11
12
Reaction time (h)
0 freshed
3.4. Alcoholysis of urea with other alcohols When other alcohols such as ethanol, propanol, isopropanol, butanol, isoamyl alcohol, octanol, ethanediol and benzyl alcohol were used as alternatives of methanol, the alcoholysis reactions of urea were also examined in the presence of HZSM-5-0.2 as catalyst under the same conditions (Table 2). Obviously, the reactivity was reduced as the carbon chain in the alcohols rising (entry 1e5), so that octanol represented inactive for this reaction (entry 6). lower activity of isopropanol than that of propanol was likely attributed to the steric effect of secondary alcohols. Besides, ethanediol had completely no activity (entry 7) due to electron-withdrawing character of excess hydroxyl group existing in its molecule, which resulted in lowing reactivity of another hydroxyl at the other end. Expectedly, the catalyst showed high reactivity for the reaction of benzyl alcohol with urea because of its relatively higher reactivity (entry 8). 3.5. Reusability of the catalyst To evaluate the performance of the catalyst, reuse experiments of the catalyst were carried out. The catalyst was separated by filtration after the alcoholysis of urea and then reused for the next run under the same conditions. The results (Fig. 9) indicated that the activity of the reused catalyst was almost unaffected even at the third run, exhibiting >90.0% conversion and 98.5% selectivity. The slightly low catalytic activity at third run was likely ascribed to the amount reduction of the catalyst during its recovery. This finding implies that the catalyst can be efficiently recovered and recycled.
Table 2 The catalytic activity for other alcohols with urea.a Entry
Alcohols
Conversion (%)
Selectivity (%)
Yield (%)
1 2 3 4 5 6 7 8
Ethanol Propanol Iso-propanol Butanol Iso-amyl alcohol Octanol Ethanediol Benzyl alcohol
98.1 97.3 96.9 95.0 93.6 e e 95.2
98.9 95.7 91.4 83.9 59.6 e e 93.8
97.1 93.2 88.5 79.7 55.8 e e 89.3
a
Standard deviation value ± 0.98%.
second
third
140
250
400
500
o
Temperature ( C)
Fig. 8. Effect of reaction time on the activity. Reaction conditions: temperature 180 C; 0.3 g HZSM-5-0.2; molar ratio of CH3OH/urea 40:1; temperature 180 C, 0.1 g HZSM-50.2, molar ratio of CH3OH/DCU 92.4:1.
Fig. 9. Recovery and reusability of the catalyst.
Furthermore, effect of treatment temperature for recovered catalyst on its activity was also explored, as shown in Fig. 9. These catalysts at treated temperatures of 140, 250 and 400 C (for 2 h) displayed over 83.8e88.2% conversions. This was the same as the result using untreated recovery catalyst. However, the catalyst at treated temperatures of 500 C for 2 h possessed obviously low activity (71.3%). XRD patterns of these catalysts treated at various temperatures were almost similar to each other (Fig. SI3). Therefore, the recovery catalyst was directly used in the next run and no need to be calcined at any temperatures. 4. Conclusions Alkali-treated zeolites were prepared and used as catalyst for the synthesis of methyl carbamate or methyl N-cyclohexyl carbamate via the reaction of urea or disubstituted urea with methanol. The treated HZSM-5 zeolite exhibited outstanding catalytic performance with ca. 100% of conversions and >98.0% selectivity to methyl carbamate and methyl N-cyclohexyl carbamate. This high activity is likely ascribed to enhanced specific area and appropriate balance of the acidic and basic sites on the catalyst surface obtained by alkali-treating HZSM-5. The catalyst is easily prepared, recovered and reused. By this token, this system shows its highly potential application foreground in industry. Acknowledgments We make a great acknowledgment for the financial support of this work by the National Natural Science Foundation of China (No. 21671050), Discipline Leader Foundation of Harbin (No.2013RFXXJ009) and the Natural Science Foundation of Heilongjiang Province (No. B201119). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2017.04.024. References [1] I. Vauthey, F. Valot, C. Gozzi, F. Fache, M. Lemaire, Tetrahedron Lett. 41 (2000) 6347e6350. [2] D. Chaturvedi, N. Mishra, V. Mishra, Curr. Org. Syn. 4 (2007) 308e320.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Environmentally benign alcoholysis of urea and disubstituted urea to alkyl carbamates over alkali-treated zeolites Qi Sun, Rui Niu, Hongxia Wang, Bin Lu**, Jingxiang Zhao, Qinghai Cai* Key Lab for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Harbin Normal University, No. 1 Shida Road Limin Development Zone, Harbin, 150025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2017 Received in revised form 30 March 2017 Accepted 13 April 2017 Available online 14 April 2017
Alkali-treated zeolite material was prepared by using 0.1e0.4 mol/L NaOH aqueous solution, as characterized by XRD, SEM, N2 adsorption-desorption technique and ammonia-temperature programmed desorption (NH3-TPD). The as-prepared zeolite was found to be an effective catalyst for alcoholysis of urea and disubstituted urea to produce alkyl carbamate, getting 91.8e100% conversion of urea and substituted urea with >98.0% selectivity to alkyl carbamates. The results indicated that the high reactivity of the zeolites for alcoholysis of urea and N-substituted urea could be mainly ascribed to the enhanced specific area and balance between acidic and basic sites on the HZSM-5 surface caused by the alkalitreatment. This catalyst can be easily recovered and reused. © 2017 Published by Elsevier Inc.
Keywords: Zeolite Alkali-treatment Methanol Urea Carbamate
1. Introduction Carbamic esters are commercially important class of organic compounds. They are widely used as intermediates in the synthesis of a variety of organic chemicals, such as polymer (e.g. polyurethane), microbicides, agricultural pesticides, herbicides and pharmaceutical agents [1]. They are also served as protective groups for amines as well as excellent templates for the formation of CeC bonds [2]. As the most simple carbamate, methyl carbamate (MC) is a crude material for the synthesis of dimethyl carbonate [3] that is not only used as an environmentally benign substitute for highly toxic phosgene and dimethyl sulfate in carbonylation and methylation, but also as a promising octane booster, reducing particulate emission and the cost of fuel due to its high oxygen content [4]. Traditionally, carbamate has been synthesized by alcoholysis of isocyanate, which is produced by a primary amine and phosgene, or aminolysis of chloroformate. The major drawbacks of the former are that the phosgene is highly toxic and corrosive. Analogously, massive waste salt was produced in the chloroformate process. Alternatively, phosgene-free methods for synthesizing carbamates have been recently developed, including
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Lu), [email protected] (Q. Cai). http://dx.doi.org/10.1016/j.micromeso.2017.04.024 1387-1811/© 2017 Published by Elsevier Inc.
reductive carbonylation of nitro aromatics and oxidative carbonylation of amines [5,6]. However, the reductive carbonylation route using platinum group metal catalysts is economically not viable; only one-third of CO could be effectively utilized and the separation of CO from CO2 increases the operation cost. Recently, the synthesis of carbamates utilizing CO2 as carbonylation reagent has been widely focused [7]. But this process requires strict reaction conditions due to the thermodynamic inertness of CO2, as well as alkyl halide using as alkylation reagent [8], or dehydration reagent [9]. Otherwise, low yield was achieved in the synthesis of carbamates from CO2, amines and alcohols in the absence of dehydration reagent. Acidic catalysts such as polyphosphoric acid [10], BF3 [11] and cupric acetate [12] have been used for the reaction of urea with methanol to generate MC. However, the yield of carbamate is low in these cases; furthermore, additional process steps for separation of the catalysts or the byproduct from the methanol/MC solution are required when using these homogeneous catalysts. In order to enhance the yield of carbamate, a semicontinuous process for the synthesis of methyl carbamate in the absence of catalyst has been investigated. A higher yield of MC was obtained at the optimal reaction conditions with a long reaction time and high temperature [13]. Besides, alcoholysis of disubstituted ureas to N-substituted carbamates over TiO2/SiO2 has been reported with good catalytic activity [14]. Therefore, the effective synthesis of carbamates are now depending on the way to transfer carbonyl group from urea or
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
disubstituted urea into carbamates or N-substituted carbamates using highly efficient and environmentally-friendly heterogeneous catalyst. In this work, alkali-treated zeolites with different concentrations of NaOH aqueous solution were prepared and characterized by XRD, SEM, N2-adsorption-desorption and NH3-TPD technique. Among the catalysts screened, the zeolite treated with 0.2 mol/L NaOH aqueous solution exhibited high catalytic activity in alcoholysis of urea and disubstituted urea [dicyclohexyl urea (DCU)] with low-molecular-mass alcohols such as methanol, ethanol, propanol, iso-propanol, butanol, iso-amyl alcohol, octanol and benzyl alcohol, getting ca. 100% conversion and >99% selectivity to MC and methyl N-cyclohexyl carbamate (MCC) for methanolysis of urea and disubstituted urea. 2. Experimental section
109
reactor was heated to 180 C with a speedy stirring and the reaction was carried out at this temperature for 8 h, the reaction formula was shown in Scheme 1. At the end of the reaction, the mixture solution was filtered to remove the catalyst and the filtrate was analyzed by GC (Agilent 7820), GC-MS (Agilent 7890-5975C) with FID detector (GC) and HP-5 column (30 m 0.32 mm 0.25 mm) for synthesis of MC and HPLC (Waters class) with UV-detector at wavelength of 210 nm, C18 column and the mixture solution of acetonitrile/H2O (1:1 v/v) as mobile phase for synthesis of MCC. The temperature program for GC measurement was shown as following: the column temperature box was heated from the initial temperature 50 Ce70 C at heating rate of 10 C/min, followed by continuously heated to 230 C at heating rate of 20 C/min; and then it remained at this temperature for 1 min. The gas and liquid chromatograms for MC and MCC synthesis were shown in Figs. SI1 and SI2 in supporting information.
2.1. Preparation and characterization of the catalyst 3. Results and discussion Before the alkali-treatment, the zeolite HZSM-5 (Si/Al ¼ 38, Tianjin Chemist scientific Ltd. Co.) was calcined at 550 C for 6 h in an oven. After cooling down, 2.0 g of HZSM-5 was added to 40 mL of various concentrations (0.1, 0.2, 0.3 and 0.4 mol/L) of NaOH aqueous solution. The mixture was heated to 80 C in a water bath and stirred at this temperature for 2 h. The alkali-treated zeolites were obtained by filtration, and then washed with distilled water until neutral washing water was achieved. The zeolites were dried at 120 C for 12 h after washing and denoted as HZSM-5-x, where x represented the concentration of NaOH solution (x ¼ 0.1, 0.2, 0.3, 0.4). X-ray powder diffraction (XRD) of the samples was performed on a Bruker-D8 Advance X-ray diffractometer with Cu Ka radiation (40 kV and 36 mA) at scan rate of 0.3 /step and wavelength of radiation source 0.154056 nm. The N2 adsorption/desorption isotherms were measured using a quantachrome NOVA2000E instrument. Before the measurement, the sample was treated to remove the gasses adsorbed on the surface at vacuum and 300 C for 3 h. The N2 adsorption/desorption isotherms were measured at temperature of 77 K (196 C). NH3 temperature-programmed desorption (TPD) were conducted using a home-made device with a TCD detector of GC (GC14C, SHIMADZU). 100 mg sample was pretreated in argon flow at 500 C for 1 h and cooling to room temperature before TPD run. The adsorption of NH3 on the catalyst was performed with feeding anhydrous NH3 gas for about 10 min at room temperature, and then the catalyst was sufficiently purged by argon stream to remove the excessive adsorbate. The TPD was conducted by heating the sample in argon (80 mL/min) from 50 C to 600 C with a heating rate of 10 C/min. The morphology of the catalysts was observed by a Hitachi S4800 scanning electron microscopy (SEM). The preparation procedure of the samples: conductive adhesive was daubed on the SEM stage, and then powder samples were uniformly sprayed by syringe on the stage, followed by shaking the stage to disperse the samples and smoothing the aggregated powder by syringe needle. The ratios of Si to Al and Na (or K) content in the zeolites were determined by 7500CE ICP-OES (P-E Company). 2.2. Catalytic tests The catalytic reaction was conducted in a stainless steel 250 mL autoclave with a magnetic stirrer in an oil bath. In a typical procedure, 40 mL (1.0 mol) or 20 mL (0.5 mol) of anhydrate methanol, 1.5 g (0.025 mol) of urea or 1.2 g (5.41 mmol) of dicyclohexyl urea (DCU) and 0.3 g or 0.2 g of the catalyst (HZSM-5-0.2) were charged into the reactor in turns. After purging three times with N2 gas, the
3.1. Characterization of the alkali-treated zeolites The XRD patterns of as-received and alkali-treated zeolites were depicted in Fig. 1. It was clear that these alkali-treated samples basically preserved the specific MFI structure and no input phase was found after treated with alkaline solution. HZSM-5-0.1, HZSM5-0.2, HZSM-5-0.3 and HZSM-5-0.4 exhibited almost the same XRD patterns as that of parent HZSM-5 although obvious decrease in relative crystallinity could be observed, especially for the higher concentration alkali-treated samples HZSM-5-0.3 and HZSM-5-0.4, the diffraction peaks around 45 were too weak to be observed due to the NaOH solution severer dissolving Si or Al atoms from the zeolite framework. Besides, significant changes were also found in the Si/Al ratio, specific surface area and morphology for the alkalitreated zeolites. As shown in Table 1, the Si/Al ratio was basically decreased as increasing in the NaOH concentration for treating these zeolites, implying that the alkaline solution mainly dissolved Si from the framework. However, SEM was used to view the morphology of the alkali-treated samples as shown in Fig. 2. It was seen from the images that the parent HZSM-5 exhibited regular rectangular crystals (Fig. 2a) and the surface of the alkali-treated samples looks like rough, some irregular crystalline grains and defects on the surface were observed for ZSM-5-0.1 and HZSM-50.2 (Fig. 2b and c), which certainly caused the increase in their specific surface areas as compared with the parent HZSM-5 (Table 1). Furthermore, the SEM images (Fig. 2d and e) exhibited that more irregular crystalline grains were aggregated together after treatment with 0.3 and 0.4 mol/L NaOH solutions, inevitably causing remarkable decrease in their specific surface areas (Table 1). Especially for HZSM-5-0.4 sample, much closer aggregate and more irregular crystalline grains were observed as shown in Fig. 2e. 3.2. Catalytic activities of alkali-treated zeolites Alkali-treated zeolites with various concentration of NaOH aqueous solution were employed as catalysts for the alcoholysis of urea or disubstituted urea. Their catalytic activity was listed in Fig. 3. The HZSM-5-0.1 and HZSM-5-0.2 represented higher activity than the parent HZSM-5. Evidently, HZSM-5-0.2 exhibited ca. 100% conversion of urea and 98.0% selectivity to MC. On the contrary, the alkali-treated HZSM-5 zeolites under severe condition gave relatively low conversions, exhibiting 90.4% for HZSM-5-0.3 and 77.8% conversion for HZSM-5-0.4, respectively. In these cases, the selectivity to MC was basically unchanged, being over 98.0%. Differently, the conversion of DCU was increased as the NaOH concentration
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O H N
C
O
R NH + R1
OH
HZSM-5-0.2
R
H N
C
R1 O
+ R
NH2
R
(R = cyclohexyl or hydrogen; R1 = methyl, ethyl, propyl, isopropyl, butyl, isoamyl and benzyl) Scheme 1. The structure of the byproducts.
HZSM-5-0.1
Intensity (a.u.)
HZSM-5-0.2
HZSM-5-0.3 HZSM-5-0.4
HZSM-5
10
20
30
40
50
60
70
80
o
2 Theta ( ) Fig. 1. XRD patterns of HZSM-5 and alkali-treated HZSM-5.
Table 1 Si/Al ratio, Na content and specific surface area of HZSM-5 and HZSM-5-x. Catalysts
Si/Al ratio
Surface area (m2/g)
Na content (%)
HZSM-5 HZSM-5-0.1 HZSM-5-0.2 HZSM-5-0.3 HZSM-5-0.4
38.6 31.1 29.2 29.9 26.6
186.3 246.2 221.8 175.1 154.5
2.96 3.18 3.13 3.34 3.32
rising with unchanged selectivity to MCC. These results suggested that alkali-treating of HZSM-5 with various concentrations of NaOH solution had a significantly impact on its catalytic activity due to the effect of alkali-treatment on the surface nature. NH3-TPD method was used to characterize the amount and acid strength of acid sites on the surface of zeolites and the profiles were shown in Fig. 4. The profiles of all the HZSM-5-x samples exhibited various broad peaks in the temperature range from 50 to 600 C. For the profile HZSM-5, the wide peak corresponding to low temperature, ranging from 139 to 200 C could be ascribed to NH3 chemisorbed on the weak acid sites. These acid sites were related to the interaction between NH3 molecules and surface oxide or hydroxyl groups by non-specific hydrogen bonding [15]. In the high temperature region, the peak centered around 436 C was attributed to the strong adsorption of NH3 on the surface, corresponding to strong acid sites. Compared with that of the untreated sample, the weak acid sites for the profiles of HZSM-5-0.1 and HZSM-5-0.2 were reinforced by extending the peaks from 150 to 227 C with slightly decrease of strong acid sites by shifting to about 406 C. Such a strengthening in the weak acid sites and weakening in the strong acid sites was accordingly thought to be the results of the desilication by splitting SieOHeAl groups and ion-exchange of Hþ with Naþ during the alkali-treatment of HZSM-5 [16]. Compared
with HZSM-5-0.1, HZSM-5-0.2 presented slightly different weak and strong acid sites due to the desilication, which was observed in the temperature range below 300 C and about 406 C, respectively. Moreover, high temperature NH3 desorption peak shifted to about 420 C or intensity of the weak and strong acid sites was increased in the NH3-TPD profile of the sample HZSM-5-0.3, which were reported to be attributed to extra framework aluminum formed during the alkali-treated with higher NaOH concentrations [17]. As compared with HZSM-5-0.2, HZSM-5-0.3 possessed relatively higher acidity and remarkably lower specific surface area. Meanwhile, complete disappearance of the strong acid site and decrease of the weak acid sites were obviously observed in the NH3-TPD profile of HZSM-5-0.4. These results suggested that the enhanced specific surface area and appropriate acid sites on the catalyst surface were crucial to the conversion of urea. According to the reported literature [18], the cationic zeolites constitute a family of solids with directly coupled acid-base sites, the strength of acidic sites increasing when that of the basic ones decrease. These sites can be visualized as acid-base pairs [19]. In the zeolites the strength of each site is mainly determined by the cation nature (Al, Na, …) and crystalline structure. The only way to change the acid-base properties is by changing the composition and structure. What makes zeolite acid-base pairs different is that their strength is easily modified by changing the exchangeable cation or the aluminum content for the same zeolite structure in addition to moving from one structure to another. In our experiments, decrease in the ratio of Si/Al and increase in Naþ content caused by alkalitreatment of HZSM-5 were evidently observed as shown Table 1, which prompt the acidic and basic sites redistributed. However, it is known that the catalytic activity of the zeolite catalyst can be ascribed to basic sites created by oxygen atoms, as well as acidic sites that attributed by Al for Lewis acidic and counter Hþ for € nsted acidic sites [20]. So that, mutual action of acidic and basic Bro sites on the catalyst surface was vital for the activation of organic urea and alcohols. As compared with HZSM-5-0.2 (content of Naþ: 3.12%), the Naþ content of HZSM-5-0.3 (3.34%) was relatively higher, which inevitably caused alterative acid-base sites on the surface and it had lower specific surface area as well. As a result, the evident difference of the catalytic activity between these two catalysts was observed. In addition, the materials HZSM-5-xK that was obtained from HZSM-5 treated with 0.1e0.4 mol/L KOH solutions also exhibited higher activity for MC synthesis (Fig. 3). Unlike NaOH system, HZSM-5-0.1 K reached at almost the same conversion (88.9%) as HZSM-5-0.2 did (90.6%). This finding indicated that stronger basicity of KOH than that of NaOH was remarkable for altering the acid-base balance on the zeolite surface, further accelerating the reaction reactivity. Besides, the materials treated with high concentration of NaOH solution had no effect on the conversion of DCU because it possesses different basicity as compared with urea. 3.3. Effect of reaction conditions As the used amount of CH3OH kept constant, the effect of CH3OH to urea (or disubstituted urea) molar ratio on the catalytic activity
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
111
80
Yield of MCC Selectivity to MC Conversion of urea Conversion of urea (KOH)
406
420
70
60
HZSM-5 HZSM-5-0.1 HZSM-5-0.2 HZSM-5-0.3 HZSM-5-0.4 436
210
a b c d e
200
90
Intensity
Conversion/Selectivity/Yield (%)
100
227
139 150
Fig. 2. SEM photographs of alkali-treated HZSM-5. a- HZSM-5, b-HZSM-5-0.1, c- HZSM-5-0.2, d- HZSM-5-0.3, e- HZSM-5-0.4.
50
40 0.0
0.1
0.2
0.3
0.4
Concentrations of NaOH (KOH, mol/L)
100
200
300
400
500
600
o
Fig. 3. Effect of the concentrations of NaOH (KOH) solution treated zeolites on the activity. Reaction conditions: temperature 180 C; time 8 h; catalyst 0.3 g; molar ratio of CH3OH/urea 40:1; temperature 150 C, time 6 h, 0.1 g HZSM-5-0.2, molar ratio of CH3OH/DCU 92.4:1.
Temperature ( C) Fig. 4. NH3-TPD profiles of treated HZSM-5 with various alkali concentrations. HZSM5-0 (a), HZSM-5-0.1(b), HZSM-5-0.2 (c), HZSM-5-0.3 (d), HZSM-5-0.4 (e).
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114 100
90
Conversion/Selectivity (%)
was investigated using HZSM-5-0.2 as catalyst, and the results were shown in Fig. 5. It could be seen that the conversions reached to the maximum points at the CH3OH/urea molar ratio of 40:1 and the CH3OH/DCU molar ratio of 92.4:1, respectively. Further increase in the molar ratios led the conversions to falling down. This might be ascribed to dilute effect of methanol to urea or DCU under the high molar ratios. Besides, the selectivity to MC and MCC was independent of the molar ratio, retaining above 98% all the time. As the reaction temperature was raised, the conversion rapidly increased to the maximum at 180 C for the alcoholysis of urea, and basically remained unchangeable at 190 C (Fig. 6). The changing rule of the conversion with the temperature was probably attributed to the control of the reaction thermodynamic character. It was reported that the heat of this synthesis reaction was DHØ ¼ 73.44 kJ/mol [21], which meant that the reaction is endothermic. Thus, the increase of the reaction temperature is of a great advantage to formation of MC. On the other hand, in view of the kinetics, raise of the reaction temperature can accelerate the reaction rate and shorten the time approaching to the equilibrium. Similarly, the conversion of disubstituted urea was rapidly increased with rising temperature in the range from 150 to 190 C, obviously remaining the selectivity above 98% for the two systems. The increase in dose of the catalyst used in this system would lead to increase in amount of active sites on the catalyst surface. There was an increase in the conversion of urea from 50.4% to ca. 100% as the catalyst amount increased from 0 up to 0.3 g (Fig. 7). Very low conversion was obtained in the absence of any catalyst in this case. The dose unceasingly increasing to 0.4 and 0.5 would give basically no effect for the conversion, but more active sites would easily catalyze by-reactions to generate some byproducts such as hydrazinecarboxamide (Mzþ 16, 32, 44, 75), imidodicarbonic diamide (Mzþ 29, 44, 60, 103) and dimethyl carbonate (Mzþ 15, 29, 31, 45, 59, 90) as shown in Scheme 2, leading to the fall of the selectivity to MC. A similar results for the DCU system were achieved. The yield of DCU was increased from 46.5 to 98.5% when using 0 and 0.2 g of the catalyst, and then the yield slightly increased as the amount of the catalyst rising. Unlike the urea system, the selectivity to MCC remained at high selectivity to MCC. The catalytic performance-time profile of the direct reaction of urea or disubstituted urea with methanol at 180 C using HZSM-50.2 as catalyst was shown in Fig. 8. The conversion of urea or/and
80
70
60
Conversion of urea Selectivity to MC Conversion of DCU Selectivity to MCC
50
40 150
160
170
180
190
o
Reaction temperature ( C) Fig. 6. Effect of reaction temperature on the activity. Reaction conditions: time 8 h, 0.3 g HZSM-5-0.2, molar ratio of CH3OH/urea 40:1 for urea; time 8 h, 0.1 g HZSM-5-0.2, molar ratio of CH3OH/urea 92.4:1 for DCU.
100
Conversion/Selectivity/Yield (%)
112
90
80
Yield of MCC Selectivity to MC Conversion of urea
70
60
50
40 0.0
0.1
0.2
0.3
0.4
0.5
Amount of the catalyst (g) Fig. 7. Effect of catalyst amount on the activity. Reaction conditions: temperature 180 C, time 8 h, molar ratio of CH3OH/urea 40:1 for urea; temperature 180 C, time 8 h, molar ratio of CH3OH/DCU 92.4:1.
100
H2N
Conversion/Selectivity (%)
90
C
Conversion of urea Selectivity to MC Conversion of DCU Selectivity to MCC
80
H N
O HCD
70
O NH2
H N C
C
NH2 NH2 ICD
O
O
C H3CO
OCH3 DMC
Scheme 2. The structure of the byproducts. HCD-hydrazinecarboxamide; ICD-Imidodicarbonic diamide; DMC-dimethyl carbonate.
60
50 20
40
60
80
100
120
140
160
180
Molar ratio of methanol to urea (or DCU) Fig. 5. Effect of molar ratio of methanol and urea on the activity. Reaction conditions: temperature 180 C; time 5 h; 0.3 g HZSM-5-0.2 for urea; 150 C; time 6 h; catalyst 0.1 g for DCU.
yield of MCC were increased up to ca. 100% conversion of urea, 90% yield of MCC in the time range from 4 to 8 h. As the reaction proceeded beyond 8 h, the conversion and yield remained unchanged with the reaction proceeding from 8 to 12 h, implying that the reaction achieved at thermodynamics equilibrium. However, the selectivity to MC (also to MCC) remained >98.0% in the time range of 4e8 h, excess reaction time led to the fall of the selectivity to MC, but unaffected the selectivity to MCC.
Q. Sun et al. / Microporous and Mesoporous Materials 248 (2017) 108e114
113
Conversion Selectivity
100
95 90
80
Conversion/Selectivity (%)
Conversion/Selectivity/Yield (%)
100
85 80
Yield of MCC Conversion of urea Selectivity to MC
75 70 65
60
40
20
60 4
5
6
7
8
9
10
11
12
Reaction time (h)
0 freshed
3.4. Alcoholysis of urea with other alcohols When other alcohols such as ethanol, propanol, isopropanol, butanol, isoamyl alcohol, octanol, ethanediol and benzyl alcohol were used as alternatives of methanol, the alcoholysis reactions of urea were also examined in the presence of HZSM-5-0.2 as catalyst under the same conditions (Table 2). Obviously, the reactivity was reduced as the carbon chain in the alcohols rising (entry 1e5), so that octanol represented inactive for this reaction (entry 6). lower activity of isopropanol than that of propanol was likely attributed to the steric effect of secondary alcohols. Besides, ethanediol had completely no activity (entry 7) due to electron-withdrawing character of excess hydroxyl group existing in its molecule, which resulted in lowing reactivity of another hydroxyl at the other end. Expectedly, the catalyst showed high reactivity for the reaction of benzyl alcohol with urea because of its relatively higher reactivity (entry 8). 3.5. Reusability of the catalyst To evaluate the performance of the catalyst, reuse experiments of the catalyst were carried out. The catalyst was separated by filtration after the alcoholysis of urea and then reused for the next run under the same conditions. The results (Fig. 9) indicated that the activity of the reused catalyst was almost unaffected even at the third run, exhibiting >90.0% conversion and 98.5% selectivity. The slightly low catalytic activity at third run was likely ascribed to the amount reduction of the catalyst during its recovery. This finding implies that the catalyst can be efficiently recovered and recycled.
Table 2 The catalytic activity for other alcohols with urea.a Entry
Alcohols
Conversion (%)
Selectivity (%)
Yield (%)
1 2 3 4 5 6 7 8
Ethanol Propanol Iso-propanol Butanol Iso-amyl alcohol Octanol Ethanediol Benzyl alcohol
98.1 97.3 96.9 95.0 93.6 e e 95.2
98.9 95.7 91.4 83.9 59.6 e e 93.8
97.1 93.2 88.5 79.7 55.8 e e 89.3
a
Standard deviation value ± 0.98%.
second
third
140
250
400
500
o
Temperature ( C)
Fig. 8. Effect of reaction time on the activity. Reaction conditions: temperature 180 C; 0.3 g HZSM-5-0.2; molar ratio of CH3OH/urea 40:1; temperature 180 C, 0.1 g HZSM-50.2, molar ratio of CH3OH/DCU 92.4:1.
Fig. 9. Recovery and reusability of the catalyst.
Furthermore, effect of treatment temperature for recovered catalyst on its activity was also explored, as shown in Fig. 9. These catalysts at treated temperatures of 140, 250 and 400 C (for 2 h) displayed over 83.8e88.2% conversions. This was the same as the result using untreated recovery catalyst. However, the catalyst at treated temperatures of 500 C for 2 h possessed obviously low activity (71.3%). XRD patterns of these catalysts treated at various temperatures were almost similar to each other (Fig. SI3). Therefore, the recovery catalyst was directly used in the next run and no need to be calcined at any temperatures. 4. Conclusions Alkali-treated zeolites were prepared and used as catalyst for the synthesis of methyl carbamate or methyl N-cyclohexyl carbamate via the reaction of urea or disubstituted urea with methanol. The treated HZSM-5 zeolite exhibited outstanding catalytic performance with ca. 100% of conversions and >98.0% selectivity to methyl carbamate and methyl N-cyclohexyl carbamate. This high activity is likely ascribed to enhanced specific area and appropriate balance of the acidic and basic sites on the catalyst surface obtained by alkali-treating HZSM-5. The catalyst is easily prepared, recovered and reused. By this token, this system shows its highly potential application foreground in industry. Acknowledgments We make a great acknowledgment for the financial support of this work by the National Natural Science Foundation of China (No. 21671050), Discipline Leader Foundation of Harbin (No.2013RFXXJ009) and the Natural Science Foundation of Heilongjiang Province (No. B201119). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2017.04.024. References [1] I. Vauthey, F. Valot, C. Gozzi, F. Fache, M. Lemaire, Tetrahedron Lett. 41 (2000) 6347e6350. [2] D. Chaturvedi, N. Mishra, V. Mishra, Curr. Org. Syn. 4 (2007) 308e320.
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