Qingwei Gao ,Yumeng Zhang ,Aatto Laaksonen,,4,5 ,Yudan Zhu, *,Xiaoyan Ji ,Shuangliang Zhao ,Yaojia Chen,Xiaohua Lu

1 College of Chemical Engineering,State Key Laboratory of Materials-oriented Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

2 Energy Engineering,Division of Energy Science,Luleå University of Technology,97187 Luleå,Sweden

3 State Key Laboratory of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China

4 Department of Materials and Environmental Chemistry,Arrhenius Laboratory,Stockholm University,SE-10691 Stockholm,Sweden

5 Centre of Advanced Research in Bionanoconjugates and Biopolymers,Petru Poni Institute of Macromolecular Chemistry,Aleea Grigore Ghica-Voda,41A,700487 Iasi,Romania

6 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology and School of Chemistry and Chemical Engineering,Guangxi University,Nanning 530004,China

Keywords:Dimethyl carbonate Carbon nanotube Solid/fluid interface Adsorbed layer Molecular simulation

ABSTRACT The dehydration of water by dimethyl carbonate(DMC)is of great significance for its application in electrochemistry and oil industry.With the rapid development of nanomaterial,one-dimensional(e.g.carbon nanotube (CNT)) and two-dimensional (e.g.lamellar graphene) materials have been widely used for molecular sieving.In this work,the molecular behavior of dimethyl carbonate/water mixture confined in CNT with varying diameters was studied based on molecular dynamics simulation.Due to different van der Waals interactions for the components in the mixtures with the solid surface,DMC molecules are preferentially adsorbed on the inner surface of the pore wall and formed an adsorption layer.Comparing with the pure water molecules confined in CNT,the adsorption DMC layer shows notable effect on the local compositions and microstructures of water molecules under nanoconfinement,which may result in different water mobility.Our analysis shows that the surface-induced DMC molecules can destroy the hydrogen bonding network of water molecules and result in an uniform and dispersed distribution of water molecules in the tube.These clear molecular understandings can be useful in material design for membrane separation.

1.Introduction

Dimethyl carbonate (DMC) is an important envronmentally friendly chemical [1]with many advantages,such as low haze point,low toxicity,and fast biodegradation ability.In recent years,its use has increased rapidly as a carbonylation agent to replace lethal phosgene and make polycarbonate and carbamate polymers,as well as a substitute for dimethyl sulfate and methyl halide in methylation reactions.Besides being an alternative to carbonylation and methylation reactions,many research work has been conducted to explore the applications of DMC in other fields,such as solvent in lithium-ion batteries [2,3]and oxidant in internal combustion engine fuel [4].The oxygen content of DMC molecule is 53% (mass),which is three times higher than that of methyl tertbutyl ether (MTBE),and it also has a high mixed octane number(105).Therefore,DMC is a promising gasoline additive to replace MTBE and to further improve the oil refining industry,which fulfills the gasoline oxygen content requirements of the Clean Air Act enacted by the US Environmental Protection Agency (EPA)[5–7].

Previously,DMC is mainly produced by the oxidative carbonylation of methanol.In recent years,the synthesis of DMC from methanol and carbon dioxide has attracted extensive attention to meet the potential market demand.In this new routine,CO2is used as the C1 reactant to the amount of emitted greenhouse gases[8,9],while the separation of DMC/methanol/water mixture is also needed,which contributes to a large part of the overall cost due to its azeotropic nature [5,10–13].Typically,the mixture (i.e.,the product from the reactor) consists of 50%–70% methanol,30%–40% DMC,and 1%–3% (mass) water.Since the existence of trace water in DMC will have a great impact on its performance,especially in the application of fuel oil and batteries [14,15],the separated DMC after the common separation still needs to be purified.

Technologies,including extractive distillation,pressure-swing distillation,liquid–liquid extraction,zeolite adsorption,lowtemperature crystallization,have been developed for the separation and purification of DMC.Among them,membrane separation technology is the most promising one,due to its low energy consumption and high efficiency[16–19].With the rapid development of advanced materials,for instance,graphene,carbon nanotubes(CNT),metal–organic frameworks (MOF),membrane separation has gradually developed to a molecular separation at the micro and nano scales [20–27].Molecular separation at nanoscale and microscale has evidenced their prospects in solvent dehydration,desalination,ion separation,and so on.Huang et al.[28]successfully prepared the graphene oxide membrane on the ceramic hollow fiber substrate with the vacuum suction method to separate DMC/water mixture.The water content of filtrate was 95.2%(mass),while the initial water content in the mixture was only 2.6% (mass).The membrane provides both good selectivity and high permeability to water(1702 g·m-2·h-1).Meanwhile,theoretical analysis indicates that CNTs can be an ideal membrane material,owing to their desirable transport characteristics and narrow hydrophobic inner pores,which simulate the typical structure of a biological channel.Geng et al.[29]inserted CNT into lipid bilayers and living cell membranes to construct 70–100 conductance channels.Although the structures of these‘‘CNTs”are simple,they illustrated good performance to transport water,protons,small ions,and DNA.Tian et al.[30]always observed a striking nanoscale drying phenomenon when they immersed single-walled CNTs into several kinds of aqueous alcohol solutions by Molecular dynamics(MD) simulation,which suggests that it may be an efficient approach toward alcohol/water separation by using CNTs.Taken together,it suggests that when the pore size of the membrane decreases to the nanoscale,the pore becomes a confined space for the fluid transferred,and the effect of the surface will be significantly enhanced.However,understanding the molecular mechanisms of complex fluids under nanoconfinement is still unclear,leading to the preparation of membrane still based on experience and lack of theoretical guidance.

MD simulation is a powerful tool and an alternative method to study the behavior of complex fluids at the interface at the nanoscale.It has been widely used to study the structure and dynamical properties of nanofluids that are difficult to characterize with the conventional experimental methods.In our previous work,we systematically studied the effects of pore size,functional groups,and the chiral parameters of CNT on the structure and properties of gases,water,ionic aqueous solutions,and ethanol [31–37].Due to different and asymmetric molecular interactions between the components in the liquid mixture and the solid surface,nano-phase separation and preferential adsorption molecular layer can be found at the interface[38,39].Does also DMC/water mixture in the nanoconfined channel exhibit similar phenomena? What effects does the adsorption layer have on the microstructure and dynamics of the fluid? To answer these questions,in this work,we used MD simulation to explore the effect of confinement on the local composition and structure of DMC/water mixture.

2.Model and Computational Details

The initial configuration of our simulation is shown in Fig.1,which is similar to the CNT model used in the literature [40,41].The model consists of two 4.12 nm × 4.52 nm parallel graphene sheets connected with a 2.82 nm CNT.Four kinds of armchair CNTs with different diameters(14,14),(16,16),(18,18),and(20,20)were used in the simulation,and the corresponding diameters are 1.897,2.17,2.44,and 2.71 nm,respectively.During the simulation,the atoms in the CNT and graphene sheets are kept rigid and fixed at the initial positions.We constructed a mixed solution with an initial water concentration of 80.0 wt%,containing 100 DMC molecules and 2000 water molecules.All the initial configurations were constructed by the PACKMOL software package and randomly distributed on both sides of graphene and inside the carbon nanotubes.

In this work,the water molecules were described with the SPC/E model [42],while graphene,CNT,and DMC molecules were described using the OPLS-AA force field [43].The van der Waals and electrostatic interactions between different particles were described with the Lennard Jones (LJ) 12–6 model and Coulomb model,respectively:

The particle mesh Ewald method [44]was used for describing the long-range electrostatic interaction with a truncation radius of 1.3 nm.The cut-off of the van der Waals force was set to be 1.0 nm.The simulation box was periodic in all three directions.After energy minimization,all the systems were equilibrated for 5 ns in the NPT ensemble.The system temperature was controlled at 298 K by V-rescale thermostat [45]with a relaxation time of 0.1 ps,and the pressure was controlled at about 1 bar (1 bar=105Pa) using the Parrinello-Rahman algorithm [46].After that,all equilibrated systems were simulated for another 10 ns in NVT ensemble at 298 K with time coupling constants of 2.0 fs,and the simulation trajectories were recorded at an interval of 0.2 ps.The final 5 ns trajectories were collected for further data analysis.All the MD simulations were performed by using the software of GROMACS [47],and the visualization was generated via Visual Molecular Dynamics (VMD) package [48].

3.Results and Discussion

In this section,we first calculated the number of DMC and water molecules confined in the tube with various chiral parameters((14,14),(16,16),(18,18),(20,20)),to explore the local compositions of DMC/water mixture in CNTs.The cases were named(14,14)_DMC,(16,16)_DMC,(18,18)_DMC,and (20,20)_DMC,respectively.Then,the two-dimensional density distribution of molecules in the X-Y plane of the tube was analyzed.It was found that DMC molecules preferentially adsorbed in CNT and formed an adsorption layer.To deeply understand the influence of DMC adsorption layer on the confined water molecules,we also simulated pure water system confined in CNT.The distribution and microstructure of hydrogen bonding network in the system were further analyzed and compared,which deepened the molecular understanding of the fluid behavior and properties of DMC/water mixture at the nano-/micro-interface.

3.1.Local composition of mixtures in CNTs

Fig.1.Side view of the simulation model.(The grey atoms represent the carbon atoms in graphene.The red and white atoms in water molecules represent the oxygen and hydrogen atoms.The green,blue and white atoms in DMC molecules represent the oxygen,carbon,and hydrogen atoms,respectively.)

Table 1 Force field parameters used in the simulation

To study the effect of confinement on the local compositions of DMC and water molecules in CNTs,we calculated the timedependent changes of DMC and water molecules in CNTs with different diameters.As shown in Fig.2,the numbers of molecules in CNTs are stable,which indicates that the system has reached equilibrium,and the statistical data are reliable.On the other hand,by comparing the number fluctuations for both DMC and water molecules,we found that the number of water molecules in the tube did fluctuate,while the number of DMC molecules was stable,which may be due to the stronger interactions between DMC and the surface.The average numbers of DMC molecules in (14,14)_DMC,(16,16)_DMC,(18,18)_DMC,and (20,20)_DMC are 34,37,51,and 67,respectively,and the corresponding average numbers of water molecules are 5,32,79,and 212.The water contents of the mixture inside the tube are 2.86%,16.29%,23.65%,and 38.76% (mass),respectively.The introduction of interface leads to inhomogeneous distributions for the mixture inside and outside the CNT.Similar phenomenons have also been reported in other similar systems.Zhao et al.[49]found that CNTs immersed in water preferentially adsorb CO and restrict the entry of H2,resulting in great changes in the local composition of mixtures in the CNTs.Especially in(19,0)CNT,the ratio of CO/H2is about 11 while it is 1.0 in the bulk phase.Pršlja et al.[50]studied the behavior of water/methanol mixture confined in narrow slit graphite pores by MD simulation.They found that the concentration of methanol increases from 0.115(mole fraction)in bulk phase to 0.92 confined in 7 Å-wide (1 Å=0.1 nm) graphene slit.

In addition to the composition change of the mixture in the CNT,the distribution of molecules in the tube also plays an important role in our understanding of the behavior for the complex mixtures at the nano/micro interface.Therefore,we did further analyze the two-dimensional density distributions of DMC and water molecules in the X-Y plane.In this work,the carbonyl oxygen atoms of DMC were selected as the characteristic points,and the water molecules were represented by their oxygen atoms.As shown in Fig.3,the phase separation behavior of the mixture occurs in the nanotubes in all studied cases.DMC molecules always preferentially adsorbed on the inner wall of CNT,forming a special structure we call ‘‘DMC tube”.Compared with the twodimensional density distribution of DMC molecules,we found that,except (20,20)_DMC,there are light-gray ring areas in the other three kinds of tubes,and these areas gradually disappear with the increase of diameter.In addition to the adsorption layer,there is a small amount of DMC molecules in the central region of the tube.In this work,we only focused on the influence of the adsorption layer on the distribution and microstructure of inner water molecules,and thus the trace amount of free DMC molecules are ignored.

3.2.Comparison of distribution for water molecules confined in DMC tubes and CNTs

What is the difference between the effect of DMC tube on the inner water molecules and that of CNT in pure water system? To answer this question,we first measured the diameter of DMC tube through the density distribution of the carbonyl oxygen atom in DMC.With the increase of CNT diameter D,the diameter d of DMC tube is 1.060,1.316,1.578,and 1.752 nm,respectively (see Fig.4).According to the values of diameter,we obtained similar CNTs,which are (8,8),(10,10),(12,12),(14,14),and the behaviors of water molecules in the four plain CNTs were also studied with the same simulation methods.These four cases were represented by the labels namely (8,8)_CNT_H2O,(10,10)_CNT_H2O,(12,12)_CNT_H2O,and (14,14)_CNT_H2O,respectively.The twodimensional density distributions in X-Y plane and the axial density distributions of water molecules in DMC tube and CNT with a similar diameter were analyzed and compared to quantitatively understand the effect of preferential adsorption DMC layer on the water molecules.

The results of the two-dimensional density distribution analysis are presented in Fig.5.The water molecules are stratified in the DMC tube due to the influence of adsorption DMC layer,from one layer of water molecules in (14,14) to three layers of water molecules in(20,20).Unlike in DMC tube,there is only one particularly bright layer near the surface when the water molecules are confined in the CNTs.Except for the case of (8,8)_CNT_H2O with only one layer,the water molecules in (10,10)_CNT_H2O,(12,12)_CNT_H2O,and (14,14)_CNT_H2O can be divided into two regions,the boundary adsorption region and the central dispersion region.The lightness of the outermost water layer in CNT is generally brighter than that in CNT.On the other hand,the inner water layer in DMC tube is relatively bright.This indicates that the distribution of water molecules in DMC tube is more uniform and dispersed,while the water molecules in CNT are mainly concentrated near the wall.The cognition of molecular scale behind these abnormal phenomena needs further studies and consideration.

Fig.2.The numbers of DMC and water molecules confined inside (a) (14,14);(b) (16,16);(c) (18,18);(d) (20,20) carbon nanotubes as a function of time.

Fig.3.(a) Sectional view of the equilibrium configuration.(b) X-Y planar number density distributions of DMC and water molecules confined in the CNTs with different diameters.

To further quantitatively understand the effect of DMC preferential adsorption layer on the water molecular distribution,we analyzed the axial number density distributions of water molecules in DMC tube and CNT.The calculation results are shown in Fig.6,where r=0 represents the axis position of the nanotube,and the far right of the x-axis represents the wall position of the DMC tube or CNT.In CNTs,it can be observed that the peak width of the first peak near the wall gradually narrowed,and a second peak appeared from (10,10)_CNT_H2O,which is consistent with the results in Fig.5.With the increase of diameter,the distance between the outermost peak and the wall is always about 0.33 nm,while the water molecules in DMC tube are closer to the wall.In the DMC tubes,with the increase of diameter,the second peak appears from (12,12)_DMC_H2O,and the third peak,which is slightly uplifted,can be seen in(20,20)_DMC_H2O,which also verifies the results shown in Fig.5.Another obvious feature is that the difference between the first and second peaks in CNTs is significantly higher than that in DMC tubes,indicating that the water molecules in DMC tube are more evenly distributed.

Fig.4.Comparison between water-filled DMC tubes and CNT based on tube diameter.

Fig.5.X-Y planar number density distributions of water molecules confined in the CNTs with different diameters.

3.3.Detailed microstructural analysis of water molecules

To further reveal the influence of the DMC layer on the water molecules from the molecular level,we carried out a detailed microstructure analysis.Firstly,the characteristic orientation distributions of water molecules in DMC tubes and CNTs were analyzed and compared (see Fig.7).Then,we analyzed the hydrogen bonding microstructure between the confined water molecules (see Figs.8 and 9).The microstructure change of hydrogen bonding network is an important indicator to characterize the properties of an aqueous solution[51,52].In this work,the geometric criteria [53]was used to determine the formation of hydrogen bonds.

We defined that the characteristic angle θ was formed by the dipole moment of water molecules and the axial direction of CNTs(see Fig.7(a)).Fig.7(b)and(c)show the characteristic angle θ distributions of water molecules confined in CNTs and DMC tubes,respectively.We found that the angular distribution of water molecules in (8,8) CNT has two characteristic peaks at 44°and 139°,respectively.With the increase of diameter,the characteristic peak gradually disappears.The distribution of θ angle in (14,14) CNT is more uniform,which indicates that the water molecules are turning into disordered and close to the bulk water molecules.Similar phenomena have been found in our previous simulation of water molecules in CNTs with different chiral parameters [37].In contrast,except that (14,14)_DMC_H2O case has a wide characteristic peak at 100°,the orientation distribution of water molecules in a larger pore size has no obvious characteristics and becomes more disordered in DMC tubes.

Fig.6.Axial number density distribution of water molecules confined in the DMC tubes (top) and CNTs (bottom) with different diameters.

Fig.7.(a)Definition of angle θ of the dipole moment of the water molecule and the axial direction of CNT;Distributions of angle θ of the water molecules confined in CNT(b)and DMC tube (c) with different diameters.

After a preliminary understanding of the effect of DMC wall on the orientation of the water molecules,we continued to analyze the average number of hydrogen bonds between the confined water molecules in CNT and DMC with different diameters,as shown in Fig.8.The dotted line in the figure represents the average number of hydrogen bonds between water molecules in the bulk phase.It is found that the hydrogen bonds between the water molecules in both two confined systems are generally on the rise,and gradually approaching the bulk phase.Through comparison,it can be concluded that,although the pore sizes are almost the same for these two systems,the number of hydrogen bonds between the water molecules confined in DMC tubes is significantly less than that in CNT.This may be due to the destruction of the water structure at the DMC interface.This can be mutually verified by the fact that the distance between the water molecules and the DMC wall is less than that from the CNT as can be found in Fig.6.It is worth noting that the average number of hydrogen bonds between water molecules in(14,14)_DMC_H2O is 0.81.Therefore,we infer that the water molecules may transfer in the form of molecular chains in DMC tube with a size of 1.060 nm.

The hydrogen bond distribution of the confined water molecules was also calculated to gain a deeper understanding of the microstructure of water molecules in these two systems,as the mobility of water molecules has been reported to be affected by the distribution of hydrogen bond microstructure [54,55].The results are shown in Fig.9.FNis the percentage of water molecules that form N hydrogen bonds with surrounding water molecules.Fig.9(a) illustrates the characteristic microstructures of hydrogen bond networks for the water molecules,i.e.,F1,F2,F3,and F4,respectively.It can be found that the hydrogen bond network of the water molecules in CNTs mainly exists in the form of F3,which is close to the bulk phase.Moreover,the increase of diameter has a slight effect on the hydrogen bond distribution,which is consistent with the results shown in Fig.8(a).For the water molecules confined in DMC tubes,the proportion of water molecules in(14,14)tube in the form of F1is as high as 69.92%,and the proportions of F3and F4are only 3.73% and 0.13%,respectively,which further confirms our inference that the water molecules in this pore may transport in the form of molecular chains,similar to the water molecules in the plain (6,6) CNT [56].Besides,with the increase of diameter,the proportions of F2,F3,and F4in the system increased significantly and finally approached those for the bulk water molecules.

Fig.8.Average hydrogen bonds between water molecules confined in (a) CNT and (b) DMC with different diameters.

Fig.9.The percentage of water molecules (FN ) confined in (a) CNT and (b) DMC with N (N=1,2,3,4) hydrogen bonds.

Based on the detailed analysis of the orientation distribution and hydrogen bond microstructure of water molecules confined in DMC tubes and CNTs,we acquired a basic understanding and recognized the influence of DMC molecules preferentially adsorbed.

4.Conclusions

In this work,the local compositions and microstructures of DMC/water mixture in CNTs with four diameters were systematically studied using MD simulation.It is found that DMC molecules preferentially adsorb on the inner surface of CNT and form a‘‘DMC tube”structure,and a new secondary confinement structure is formed for the water molecules.At the same time,the introduction of CNT leads to the heterogeneous distribution of the mixture inside and outside of the tube.The water concentration in the CNTs is always less than 40 wt%,reaching a minimum of 2.86 wt%for the mixture confined in the narrowest pore.Furthermore,we analyzed and compared the distribution,orientation structure,and hydrogen bond network of water molecules confined in DMC tube with those in CNTs with almost the same size.Due to the influence of DMC adsorption layer,the distribution of water molecules in the tube is more uniform,forming a multilayer structure,and DMC molecules will destroy the structure of water molecules,resulting in the disorder of its orientation structure and the destruction of hydrogen bond structure.Especially,when the mixture is confined in (14,14) CNT,69.92% of the water molecules have only one hydrogen bond,and the water molecules may transfer in the form of molecular chain.Our findings could provide a molecular-level understanding of the separation of DMC/water mixture.Furthermore,the observed adsorption layer opens a new route to achieve fluid-control under confinement,which could potentially facilitate to electrochemistry and nanofluidics and also constitute an important step towards building the microscopic interaction model of the complex fluid–solid surface.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Science Foundation of China (21878144,21729601 and 21838004),the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21921006),Project of Jiangsu Natural Science Foundation of China (BK20171464),Project of Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD),the Kempe Foundation in Sweden.A.L.thanks for a grant of Ministry of Research and Innovation,CNCS-UEFISCDI,project number PN-III-P4-ID-PCCF-2016-0050,within PNCDI III and the Swedish Science Council(VR).The computational resources generously provided by the High Performance Computing Center of Nanjing Tech University and the Swedish National Infrastructure for Computing (SNIC) at HPC2N are greatly appreciated.

Nomenclature

D diameter of CNT,nm

d diameter of CNT,nm

FNpercentage of water molecules that form N hydrogen bonds

rijdistance between atoms i and j,nm

qicharge of atom i,e

εijenergy parameter,kJ·mol-1

σijsize parameter,nm

θ characteristic angle,(°)