Molecular simulation on carbon dioxide fixation routes towards synthesis of precursors for innovative urethanes
© Carrillo Beber et al.; licensee Springer. 2015
Received: 20 November 2014
Accepted: 5 December 2014
Published: 1 March 2015
Classical molecular dynamics were carried out in order to obtain insights into proper conditions to perform chemical fixation of carbon dioxide (CO2) with epoxide molecules into cyclic carbonates. Two different molecules containing epoxide groups were investigated: 1,2–Epoxybutane (EB), called linear aliphatic epoxide molecule, and 3-Ethyl-7-oxabicyclo(4.1.0)heptane (EC), called cycloaliphatic epoxide molecule. The reaction systems involving carbon dioxide additionally were catalyzed by tetraethylammonium bromide (TEAB). The dynamics of the molecular groups were studied by taking into account known reaction mechanisms to investigate whether the optimal reaction conditions were observed.
Radial distribution functions and self-diffusion coefficients were calculated and revealed that in case of the systems with cycloaliphatic epoxide groups as reagent the CO2 molecules were located far away from the agglomerate formed by the dispersed tetraethylammonium bromide catalyst and epoxide groups (EC), and they do not present enough mobility to overcome the long distances to react. Additionally, it was observed that, in the case of the linear aliphatic epoxide groups (EB), the dynamics of the groups tends to facilitate the reaction mechanisms by presenting a considerable amount of available CO2 molecules in the neighborhood of the epoxy rings. Thus, via the Molecular Dynamics insights, the systems containing linear aliphatic epoxide groups presented a much more accessible condition for the subsequent reaction steps of the carbon dioxide fixation to occur as compared to systems containing cycloaliphatic epoxide groups. The simulation results are in agreement with the experimental findings, which showed via infrared spectroscopy the successful conversion of epoxy rings from linear aliphatic epoxide molecules into five-membered cyclic carbonates after reacting with carbon dioxide.
Environmental issues and sustainability appeal have a strong impact on the development of novel chemistry approaches in order to synthesize materials via eco-friendly routes. Among the worldwide current environmental and economic concerns is the need to reduce the carbon dioxide (CO2) released to the atmosphere by a wide range of human activities such as burning of fossil fuels, mineral extraction, bio-ethanol production, etc. [1-3]. A further impact of environmental requests is on the search for alternative chemical routes for polymeric materials industry , as for instance for the synthesis of polyurethanes (PUs). PUs are very versatile materials and present an extremely strong bonding, consequently PUs are vastly used in applications such as plastics, and used matrices for fiber-reinforced polymers or as adhesives [4,5]. Furthermore, PUs are applied in marine, transportation, automotive, furniture and food packaging industries or coatings on many different materials due to the improvement of properties like appearance and life time. However, the usual synthesis of PUs relies on isocyanate reactions, i.e. involving toxic compounds. K.R. Aguiar et al.  have shown an alternative route to tackle the challenges of reducing CO2 emissions and simultaneously forming urethanes based material via green routes instead of using isocyanates. In that work, the authors presented an efficient environmentally friendly synthesis of a bis(cyclic carbonate) poly (dimethylsiloxane) (CCPDMS) derivative via CO2 addition to yield precursor materials for the synthesis of innovative urethanes. Basically, the work profits from the ability of the catalyzed CO2 fixation using epoxides as reactants to produce cyclic carbonates . The resulting CCPDMS can be applied to synthesize urethanes. Furthermore, K. R. Aguiar et al. presented the feasibility of reacting CCPDMS and (di)amines in solvent-free conditions. Although the route from the CO2 fixation with epoxides to the final urethanes synthesis is achieved by K. R. de Aguiar and co-authors, the yield of the CO2 fixation is strongly dependent on the accessible reaction steps, the appropriate conditions and chemical structures of the components. The understanding of those phenomena is extremely relevant. Several experimental and theoretical efforts [7-9] were performed in this direction, including the investigation of the reaction process, the design of the catalyst [10-13] and of the appropriate substituents at three-membered epoxide rings to form the desired products. Computer simulations represent powerful tools to support the experimentalists towards the development of novel materials. Via computational modelling, several features can be studied such as system dynamics or reaction mechanisms at short and large time and length scales [14-17]. Via density functional theory (DFT), J-Q. Wang and co-authors elucidated the reaction mechanisms for the CO2 fixation of ethylene oxide (EO) and of propylene oxide (PO) catalyzed by quaternary ammonium salts , identifying main three steps: (i) A nucleophilic attack of the bromide ion to the less hydrogenated carbon of the epoxide group occurs, forming a β-haloalkoxide anion; (ii) the new anion reacts with the CO2 molecule, generating a carbonate ion; (iii) in order to form the cyclic carbonate, the bromide acts as a leaving group regenerating the catalyst and obtaining the cyclic carbonate product. Furthermore, the intermediate and transition states were calculated, and aspects that influence the design of the catalysts were presented, such as the chain length of substituents at the epoxide group and the type of catalyst anion. In spite of the fact that calculations are scaled for short lengths and times, the quantum based methods, such as the DFT, provided important information on the mechanisms and the parameters which influence the reaction processes.
This study reports a molecular dynamics (MD) investigation to gain insights into the dynamical behavior of the molecular systems and its influence on the formation of the aspired products. Via classical MD calculations, no reactions are carried out. Nevertheless, the dynamics of a large scale system of reagents can be accessed. By MD simulations, it is feasible to know whether the system presents the optimal conditions required for the subsequent reaction steps to occur considering the reported steps of the reaction mechanism [7,18]. The computer simulations intend to support the understanding of the experimental findings, regarding the reaction to form molecules based on cyclic carbonates. Two different molecules containing epoxide groups were investigated; the 1,2 – Epoxybutane (EB) which is a linear aliphatic epoxide, and 3- Ethyl-7- oxabicyclo(4.1.0)heptane (EC) which is a cycloaliphatic epoxide; as catalyst tetraethylammonium bromide (TEAB) was used. In addition, the present work reports experimental results on the CO2 fixation with epoxides. The experimental part also considered two different epoxide groups used as reagents: as a linear aliphatic epoxide the diglycidylether-terminated poly(dimethylsiloxane) (PDMS), and a cycloaliphatic epoxide the poly[dimethylsiloxane-co-(2-(3,4-epoxycyclohexyl)ethyl)methylsiloxane] (CPDMS), and TEAB as catalyst.
In the next section, the computational and experimental methods and materials are detailed explained. In the sequence, the results and discussion section is dedicated to present the radial distribution functions and diffusion coefficients calculated using the produced MD trajectories; furthermore the experimental findings are presented. Finally, the main conclusions are summarized.
Computational systems considered and their respective stoichiometry
Computational system 1 CS1-EB
Total number of atoms in Computational Cell
Computational system 2 CS2-EC
Total number of atoms in Computational Cell
For each system, a geometry optimization (steepest descent and conjugated gradient, in cascade) was performed. Afterwards, a 1 ns equilibration run was carried out composed of a 500 ps run in the canonical ensemble (NVT) followed by a 500 ps run in the isothermal–isobaric ensemble (NPT) with an isotropic pressure coupling using a Nosé–Hoover thermostat. For both systems CS1-EB and CS2-EC, the temperature was 298 K and the pressure 11.0 bar. For the CS1-EB system the 500 ps NVT equilibration run had an initial density of 0.5 g/cm3. For the CS2-EC the 1 ns equilibration run was done as follows: a total of 500 ps NVT run in 3 steps (200, 200 and 100 ps), followed by a 500 ps NPT run. After proper equilibration for both systems a 1 ns production run was carried out using NPT at 298 K and 11.0 bar, the trajectories were collected each 1 ps for analyses.
Experimental systems considered and the respective quantities used
Experimental System 1 ES1-PDMS
Experimental System 2 ES2-CPDMS
charged in reactor
1% w or 2% w
Results and discussion
The experimental results revealed the success of obtaining cyclic carbonate molecules when using as reagent the linear aliphatic epoxide (PDMS), while the use of the cycloaliphatic epoxide (CPDMS) as reagent did not yield the expected products in the conditions investigated.
For the reaction step II to take place, the CO2 molecules will have to approach considerably to the epoxide groups. As the results displayed in Figure 5 indicate, this seems to be much more difficult in the case of the CS2-EC than in case of CS1-EB. In the system CS2-EC the catalyst and the epoxide groups cluster together, and CO2 molecules result to be excluded from these aggregates, far away from the aspired reaction sites. The access for CO2 molecules by transport through the clusters formed by the epoxide groups and catalyst results to be limited. This finding provides a qualitative description of a non-optimal scenario for the reactions in case of the CS2-EC systems, since the transport of CO2 reagent towards the catalyst region appears to be hindered in this case.
Self-diffusion coefficients for each molecular group for both computational systems CS1-EB and CS2-EC
Diffusion D (10 −5 cm 2 s −1 )
Computational System 1 CS1 - EB
Computational System 2 CS2 - EC
Combining the analyses of the integral of the RDFs (Figure 5) and the dynamics accessed (Table 3), the CO2 molecules in CS2-EC have to travel much longer distances to be able to approach opened epoxide rings. Besides, there would be limitations caused by the stronger steric effect in the EC molecules compared to the EB ones. Those outcomes seem to hinder the reagents of the systems CS2-EC to proceed with the reaction steps within the mechanism for the CO2 fixation and formation of further cyclic carbonate based molecules. The effect of agglomeration and the resulting hindrance is mirrored in the observed experimental results which indicated a much faster and more effective CO2 fixation in case of linear aliphatic epoxide molecules as reagents as compared to cycloaliphatic epoxides.
The CO2 fixation with epoxides to form cyclic carbonate based molecules was investigated via computational and experimental work. In both experimental and computational part, two different epoxide groups were considered: a linear aliphatic epoxide and a cycloaliphatic epoxide. Experimentally, only the systems considering the linear aliphatic epoxide as reagent have successfully produced the cyclic carbonate based product as revealed by the infrared spectroscopy (FTIR). The dynamics of model systems were accessed by classical MD simulations in order to investigate the conditions for the reactions steps to occur, taking into account a reaction mechanism reported in the literature. The analyses of the computational data were performed by radial distribution functions and self-diffusion coefficients calculations. It was observed for the system containing the linear aliphatic epoxide as reagent a large amount of CO2 available in the neighborhood of the epoxide reaction centers. Moreover, in the systems containing cycloaliphatic epoxide, the CO2 groups are located very far away of the epoxy rings and did not present enough mobility to overcome the long distances to react. In summary, the computational study of the dynamics showed a preferable condition for reactions to occur in the systems containing linear aliphatic epoxide as reagent when compared to cycloaliphatic containing systems; the results are in agreement with the experimental output.
The authors are gratefully thankful to the Science without Borders Program (Ciência sem Fronteiras, L. Taveira Caleiro 88888.033101/2013-00, and V. Carrillo Beber 13458/13-2) and the Coordination of Improvement of Higher Education Personnel (CAPES – Brazil).The authors would like to thank the financial support by São Paulo Research Foundation (FAPESP) within Grants 2011/06019-0, 2011/08120-0 and 2013/05279-3. Additionally, the authors acknowledge the financial support by German academic exchange service (DAAD) from resources of the German Federal Ministry of Education and Research (BMBF) within the PROBRAL I (25/2013)-CAPES/DAAD project ID 57060300.
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