An Ecofriendly Synthesis of 2-oxazolidinone From 2-aminoethanol and Urea Under Solvent-free Condition Using CeO2 Nanoparticles

Cerium dioxide nanoparticles were prepared by the sol-gel method using cellulose as a template and used in the synthesis of 2-oxazolidinone from urea and 2-aminoethanol under solvent-free conditions. All the reaction parameters were optimized to obtain the best selectivity and conversion. The selectivity of 100 % to 2-oxazolidinone with a pretty complete conversion of about 98.4 % was achieved. The prepared catalyst was characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and volumetric isothermal nitrogen gas adsorption-desorption method (BET).


Introduction
Global warming and climate changes, caused by emitted greenhouse gases, have seriously affected human life and the ecological environment [1,2]. According to a recently published report by the Intergovernmental Panel on Climate Change about global warming, the total global temperature has risen 1.5 degrees above the pre-industrial amount due to the increase in greenhouse gases emission [3,4]. Among greenhouse gases, carbon dioxide is the main major gas that contributes to the Earth's climate changes. Hence, there is an urgent need to decrease the emission of this greenhouse gas into the atmosphere. Therefore, several strategies have been developed for the separation, storage, and utilization of CO2 to mitigate the concentration of CO2 in the atmosphere. [5][6][7]. Using carbon dioxide as an economical, safe, and renewable carbon resource is attractive from the viewpoint of synthetic chemistry. The utilization of CO2 as a C1 building block for synthesizing high-value organic chemicals has gained much more attention. Converting CO2 to industrial chemical products is a route for recycling carbon and carbon-containing materials [8,9].
Cyclic carbamates are one of the most important target compounds of conversion of CO2, having several applications in both the chemical industry and organic chemistry [10][11][12][13]. Conventionally, these nitrogen-containing heterocyclic compounds are synthesized using highly toxic reagents such as phosgene [14,15]. 2-oxazolidinones are important cyclic carbamates which are widely used in drugs [16], antibiotics [17], herbicides [18], organic synthesis [19], solvents for lithium-ion batteries and ink-jet printing [16]. Moreover, polymers containing 2-oxazolidinone are used for applications as foams, adhesives, and fibers [20]. There are several reported methods for synthesizing 2-oxazolidinone and its derivatives, such as carbonylation of -amino alcohols with phosgene [21] or carbon monoxide [22], the addition of isocyanates to epoxides [23,24], and the reaction of isocyanates with propargylamines [25]. Owing to the toxicity and cost of mentioned reagents, 2-oxazolidinone can be greenly synthesized from CO2 and -amino alcohols [26] or urea and vicinal diols [27,28]. Nontoxic urea with a lower price is a resource for carbon dioxide and can be used as an amination and carboxylation reagent, and reaction can be conducted at milder pressure of carbon dioxide [29]. Urea can further be reproduced by ammonia and CO2, so using urea instead of carbon dioxide could be considered as an indirect utilization of CO2 to form 2oxazolidinone [27,28,30]. Various catalytic systems are designed to improve reaction conditions and better efficiency to oxazolidinones. Kodaka, Dinsmore, and Mun õz have reported homogeneous catalytic systems composed of phosphines such as trialkylphosphine in the presence of a strong base for the synthesis of oxazolidinone compounds. This method suffered the separation of alkylphosphine oxide produced from products [31,32]. Other homogeneous catalysts such as organotin [33] or organoantimony complexes [34] and ionic liquid with alkali promoter [35] were developed to overcome previous drawbacks but resulted in moderate yields in severe reaction conditions of high temperature and pressure with long reaction times. Aluminum [36], vanadium [37], palladium [38], Ruthenium [27], chromium [23], and rare earth metals [39] complexes also catalyzed synthesis of 2-oxazolidinone but resulted in moderate yields. However, from environmental viewpoints and separation difficulties of homogenous catalysts, non-catalytic systems and heterogeneous systems are much more preferable. Arai et al. and Tominga et al.
reported the non-catalytic synthesis of cyclic carbamates from CO2 and amino alcohols at 423 K and 453 K, respectively, having problems such as low yields and narrow scope of amino alcohols [40]. As a heterogeneous catalyst, the reaction of CO2 with amino alcohols at 423 K and 2 MPa were carried out over various metal oxides, and the formation rate of 2-oxazolidinone was compared [41]. In comparison with other metal oxides, CeO2 showed better results. CeO2 is an effective heterogeneous catalyst for several organic reactions. The acid-base property of CeO2 has a critical role in the activation of CO2 and amino alcohols.
In the present work, we report a solvent-free synthesis of 2-oxazolidinone from urea and 2aminoethanol using synthetic CeO2 nanoparticles as a heterogeneous catalyst.

Materials and instruments
Cerium nitrate hydrate, 2-aminoethanol, urea, and cellulose were purchased from Merck Chemical Company and were used without further purification. X-ray diffraction (XRD) patterns were obtained by a Philips-X'pertpro X-ray diffractometer using Ni-filtered Cu Kα radiation. Scanning electron microscopy (SEM) was conducted using a Zeiss DSM 960A microscope with an acceleration voltage of 10 k. To determine the specific surface area and pore volume of the supported catalysts, a BELSORP mini instrument nitrogen adsorption-desorption equipment was used at 77 K. For monitoring of reaction products and their identity, a gas chromatograph, Agilent Technologies 7890A Instrument, equipped with an HP-1 capillary column, an FID detector, and a mass spectroscope model 5975C with a triple-axis detector was used.

Synthesis of CeO2 nanoparticles
Mesoporous CeO2 was prepared using the sol-gel method. First, a suspension was prepared by dispersing 20 g cellulose in water (20% w/w) and mixed for 30 minutes in an ultrasonic bath. A solution of 2g Ce(NO3).6H2O in 40 ml water was added dropwise into the suspension and stirred vigorously at 40 ºC. After 15 min, the ammonia solution was added to adjust the pH at 10, and the sedimentation reaction was performed under reflux for 2 hours. Subsequently, the mixture was heated to 90 ºC and aged for 24 h. The resulting mixture was dried at 80 ºC overnight. The dried powder was then calcined at 500 ºC for 5 h. Finally, pale yellow ceria was stored in a closed container for the catalytic reaction.

Catalytic synthesis of 2-oxazolidinone
The procedure of the 2-oxazolidinone synthesis from 2-aminoethanol and urea was as follows: typically, 0.25 ml of 2-aminoethanol and 0.25 g of urea and 0.025 g of ceria catalyst (10 w %) were added into a stainless steel autoclave. The reaction was carried out at 150 °C for 4 h under constant stirring. Then the reaction mixture was cooled down to room temperature, and obtained product was dissolved in methanol. The catalyst was separated by centrifugation. The product was analyzed by gas chromatography using n-pentane as an internal standard.

Result and discussion
In order to investigate the crystalline structure and the average crystallite size of the prepared ceria powder, x-ray diffraction was used. The XRD pattern of synthesized cerium dioxide nanoparticles is shown in Figure 1 Since no diffracted peaks of any other byproducts as an impurity is detected, the strong and welldefined peaks firmly indicate the pure cubic structure of cerium dioxide nanoparticles. The mean size of cerium dioxide crystallites was obtained 17.7 nm using the well-known Debye-Scherrer equation: (L= K λ / β.cosθ) with λ=10, β=0.576, K=0.9, and 2θ=28.5, where K is a shape factor, λ is the X-ray wavelength in nanometer, β is the full width of the diffraction peak profile at half maximum height in radians, and K is a constant related to crystallite shape, and θ is the Bragg Angle in radians.     By changing the substrate scope of urea/2-aminoethanol from 1 to 1.5 ratio, the reaction was nearly completed, and reagents were 98.4 % converted to 100 % 2-oxazolidinone (Entry 10).
Furthermore, by adding of more urea and increasing the urea/2-aminoethanol ratio to 2, the formation of 2-oxazolidinone was disfavored, and selectivity was decreased significantly to 11 % (Entry 11). At the optimum conditions of the reaction, the amount of the catalyst was varied. Decreasing the catalyst to 7 weight % resulted in a moderate yield of 66.6 %, and with less than 7 %, a trace amount of 2-oxazolidinone was produced. The 2-oxazolidinone was the main product of Entry 10, which after the reaction was separated from maintained 2-aminoethanol by column chromatography and was recrystallized, and pure 2-oxazolidinone was analyzed by nuclear magnetic resonance. 1  Recyclability/reusability of the catalyst was tested using the recyclable cerium dioxide at the optimum reaction conditions, the same as Entry 10. As illustrated in Figure 3, in three consecutive runs, the selectivity to 2-oxazolidinone remained unchanged, and the activity of the synthesized catalyst dropped only 5 %, so that the reaction can be repeated in few runs with minimum loss of activity. According to the previously proposed mechanism, which is shown in figure 6 [ oxazolidinone-coordinated cerium complex, and (4) desorption of produced 2-oxazolidinone and regeneration of cerium dioxide [10,15].

Conclusion
In the present study, cerium dioxide nanoparticles were prepared and used for selective solventfree synthesis of 2-oxazolidinone from 2-aminoethanol and urea. Under optimized experimental conditions in this report, 98.4% conversion of 2-aminoethanol with 100% selectivity to 2oxazolidinone was obtained by cerium dioxide nanoparticles at 150°C in 6 hours. The catalyst was used in further reactions checking for recyclability. According to the results, the cerium dioxide was reusable with a negligible loss of activity for at least three runs.

Figure 1
The X-ray diffraction pattern of prepared CeO2 nanoparticles. Figure 2 SEM images of the prepared cerium dioxide nanoparticles.

Figure 3
The possible side reactions of 2-oxazolidinone.

Figure 4
Decomposition of urea at different temperatures.

Figure 5
The recyclability of cerium dioxide in four runs.

Figure 6
Proposed mechanism for reaction of 2-aminoethanol and CO2 over cerium dioxide.

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