Đang tải... (xem toàn văn)
... acetonitrile from aqueous solutions to circumvent possible future supply disruption from interfering with essential industrial and laboratory work The separation of acetonitrile from aqueous solutions using. .. significant role of functional groups in governing adsorption and could facilitate the development of new nanoporous materials for efficient recovery of acetonitrile from aqueous solutions List of Tables... adsorption of water, but substantial adsorption is observed in hydrophilic ZIF-90, -96 and 97 With regards to acetonitrile purification from aqueous solutions, the general trend of selectivity of acetonitrile
RECOVERY OF ACETONITRILE FROM AQUEOUS SOLUTIONS USING ZEOLITIC IMIDAZOLATE FRAMEWORKS OH JUN XIAN, DEREK (B.Eng.(Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. 1 Acknowledgments The author would like to extend his gratitude to Associate Professor Jiang Jianwen for his unending support, valuable assistance and inspirational guidance over the course of this research, and to the National University of Singapore for having granted the candidate access to educational and research resources. 2 Contents 1. Summary .................................................................................................5 2. List of Tables .......................................................................................... 7 3. List of Figures ......................................................................................... 7 4. Introduction ............................................................................................. 8 5. 6. 4.1 Global Acetonitrile Demand and Supply ........................................... 8 4.2 Current Production Methods of Acetonitrile ................................... 10 4.3 Metal-Organic Frameworks ............................................................ 11 4.4 Objectives ...................................................................................... 12 Models and Methods ............................................................................. 13 5.1 ZIFs and Adsorbates ....................................................................... 13 5.2 Simulation Methods........................................................................ 18 Results and Discussion .......................................................................... 20 6.1 Pure Acetonitrile ............................................................................ 20 6.1.1 Adsorption Isotherms .............................................................. 20 6.1.2 Isosteric Heat of Adsorption .................................................... 24 6.1.3 Radial Distribution Functions .................................................. 26 6.1.4 Density Contours ..................................................................... 30 6.2 Pure Water ..................................................................................... 31 6.3 Acetonitrile-Water Mixtures ........................................................... 33 6.3.1 Isotherms and Selectivity ......................................................... 33 3 6.3.2 Density Contours ..................................................................... 36 7. Conclusions ........................................................................................... 38 8. Bibliography.......................................................................................... 39 Appendix A .................................................................................................. 45 Appendix B .................................................................................................. 46 Appendix C .................................................................................................. 48 4 1. Summary Acetonitrile is an important chemical compound and solvent. Many products ranging from vitamins to antibiotics require it as a starting material for synthesis. Acetonitrile is usually mixed with water, and used in pharmaceutical and electrochemical industries, as well as a mobile phase in liquid chromatography. To recycle and reuse acetonitrile, cost- and energyeffective technology is desired to recover acetonitrile from aqueous solutions. In the past decade, metal-organic frameworks (MOFs) have emerged as a new class of nanoporous crystalline materials. They can be synthesized from enormous metal clusters and organic linkers. The degree of diversity and multiplicity in MOF structures is substantially more extensive than any other porous material. With these salient features, MOFs have been considered versatile materials for diverse potential applications such as storage, separation, catalysis and drug delivery. In this study, a sub-class of MOFs namely zeolitic imidazolate frameworks (ZIFs) are explored by molecular simulation for the adsorptive separation of acetonitrile from water. Specifically, eight different ZIFs (ZIF-8, 10, 25, 60, 71, 90, 96 and 97) with varying topology and functional group are considered. At low loading, acetonitrile adsorption is significantly affected by the nature of ZIF and functional group; at high loading, however, adsorption is mainly governed by free volume. Water adsorption is much higher in hydrophilic ZIFs (ZIF-90, -96 and -97) than in hydrophobic counterparts (ZIF-8, -10, -25, -71 and -60). For acetonitrile/water mixtures, the selectivity of acetonitrile over water generally drops with increasing composition of acetonitrile. 5 Furthermore, the selectivity is largely related to the hydrophobicity of ZIFs. Among the eight ZIFs, ZIF-8 exhibits the highest selectivity. The simulation study provides microscopic insights into the adsorption of acetonitrile and water in various ZIFs, reveals the significant role of functional groups in governing adsorption and could facilitate the development of new nanoporous materials for efficient recovery of acetonitrile from aqueous solutions. 6 2. List of Tables Table 1. ZIFs and Organic Linkers. .............................................................. 14 Table 2. Structural Properties of SOD- and MER-type ZIFs. ....................... 15 Table 3. Structural Properties of RHO-type ZIFs. ........................................ 15 Table 4. Species Fugacities at different compositions. .................................. 19 Table 5. Atomic Charges of ZIFs. ................................................................ 46 Table 6. DREIDING Force Field Parameters of ZIF atoms........................... 48 Table 7. Potential Parameters of Acetonitrile and Water. .............................. 48 3. List of Figures Fig. 1 Atomic Structures of ZIF-8, -10, -25, -60, -71, -90, -96 and -97.......... 13 Fig. 2 Simulated ACN Adsorption Isotherms at 298K .................................. 21 Fig. 3 Simulated ACN Adsorption Isotherms at 298K for RHO-type ZIFs. ... 23 Fig. 4 ZIF Isosteric Heats of Adsorption for ACN at infinite dilution at 298K ..................................................................................................................... 25 Fig. 5 Radial Distribution Functions in SOD-type ZIFs ................................ 27 Fig. 6 Radial Distribution Functions in MER-type ZIFs ................................ 28 Fig. 7 Radial Distribution in RHO-type ZIFs ................................................ 29 Fig. 8 Density Contours of ACN in ZIF-8 and ZIF-97 .................................. 31 Fig. 9 Simulated Water Adsorption Isotherms at 298 K ............................... 32 Fig. 10 Density Contours of Water in ZIF-8and ZIF-97 ................................ 33 Fig. 11 Adsorption Isotherms of Binary Solution in ZIF-8, -10, -25, -60, -71, -90, -96 and -97 at 298K ............................................................................... 34 Fig. 12 Selectivity of ACN-Water mixtures .................................................. 35 Fig. 13 Density Contours of ACN-Water mixtures in ZIF-8 and ZIF-97 ....... 36 7 4. Introduction 4.1 Global Acetonitrile Demand and Supply Acetonitrile is most notably used as a reagent, reaction solvent or extraction solvent. Its salient properties such as low UV absorbance, low pressure and high elution strength, allow for highly sensitive analyses to be conducted. However, acetonitrile cost has been consistently rising over that of lower grade solvents such as methanol (Shimadzu Corporation, 2014). With increasingly strict adherence and compliance to regulatory guidelines being earmarked by the U.S. Food and Drug Administration (Center for Drug Evaluation and Research, 2014), more and more companies in the realm of pharmaceuticals and analytics are thus continuously seeking for cost-effective and sustainable sources of acetonitrile. Uses of Acetonitrile Pharmaceutical industry is the largest end use of acetonitrile. Materials for which acetonitrile is used as primary feedstock include Vitamins A and B1, cortisone, carbonate drugs and some amino acids. Acetonitrile is also used as a solvent for DNA synthesis and production of insulin and antibiotics, one of which is cephalosporin for the treatment of respiratory tract, skin tissue and bacterial septicaemia. There has been rapid growth and use of acetonitrile in recent years as pharmaceutical products for diseases, boosted mainly by improved living standards in industrialized countries (IHS Chemicals, 2014). The second-largest use of acetonitrile is as a mobile phase in highperformance liquid chromatography (HPLC). Liquid mixtures of acetonitrile and water are also frequently employed as mobile phases in reversed-phase 8 HPLC and as solvents in electrochemical applications (van Assche, Remy, Desmet, Baron, & Denayer, 2011). HPLC has major growth prospects in the separation of chiral molecules (IHS Chemicals, 2014). Although in the last couple of years, a shortage of acrylonitrile has motivated some analytical labs to switch to UPLC (ultra-performance liquid chromatography), this move comes with significantly higher costs. Thus, acetonitrile demand is still relatively high and will remain to be so in the years to come (IHS Chemicals, 2014). Rising Global Consumption of Acetonitrile Over the last decade, the consumption of acetonitrile has seen strong growth, at an average annual rate of 5 to 6%. Its world consumption is forecasted to continue to grow over the next 5 years at a rate of about 6% per year (IHS Chemicals, 2014). As a result of a global economic slowdown during 2008 - 2009, there was a great shortage of acetonitrile – often dubbed The Great Acetonitrile Shortage. Pharmaceutical companies were then instructed to change and modify experimental methods to reduce acetonitrile usage. However, since HPLC analysis methods are bonded by legislation and cannot be easily altered in the absence of regulatory validation and approval, and thus pharmaceutical companies were struck particularly hard (van Assche, Remy, Desmet, Baron, & Denayer, 2011). The consequent peaking of acetonitrile price generated by this shortage also led to a disruption of acetonitrile demand even till the middle of 2010 (IHS Chemicals, 2014). To overcome the aforementioned problems, analytical laboratories and pharmaceutical producers attempted to use alternative solvents (e.g. methanol) 9 especially in non-sensitive operations such as flushing pipes, cleaning reactors and other applications. Nevertheless, this reduction in demand has been largely offset by strong demand growth in China and India, where the highest growth rate of acetonitrile (around 9 to 10% per annum over the next few years ending 2020) is forecasted due to the increasing production of engineered drugs, generic pharmaceuticals and pesticides in these two largest countries. In Europe, Japan and the United States, the average annual growth rate over the same period is estimated around 1.5% (IHS Chemicals, 2014). Acetonitrile Production Statistics Currently, the producers of acetonitrile are largely segmented. A few major players account for up to 60% of global capacity, including INEOS in the United States, Asahi Kasei in Japan and CNPC Jilin Chemical Group in China. INEOS alone supplies around 40 to 50% of the world’s acetonitrile (Longden, 2009), amounting to about 34,000 tonnes per annum (Eurasian Chemical Market International Magazine, 2014). These three companies alone capture 26%, 20% and 11%, respectively, of the global acetonitrile market share as well (IHS Chemicals, 2014). 4.2 Current Production Methods of Acetonitrile The global acetonitrile production is estimated at around 73,500 to 80,000tonnes annually (NIIR Project Consultancy Services, 2014). Acetonitrile is industrially produced primarily via one of three methods, as a by-product of the SOHIO (Standard Oil of Ohio) propylene ammoxidation process, from the dehydration of acetamide, and by reacting acetic acid with ammonia at 400 to 500ºC in the presence of a dehydrated catalyst (National Centre for Biotechnology Information, 2014). 10 Notably though, apart from solely using the aforementioned methods, enduser companies are currently focussing on and looking for ways to recover and recycle acetonitrile out of its mixtures with water and/or methanol, to supplement their acetonitrile needs (van Assche, Remy, Desmet, Baron, & Denayer, 2011). One such way is through the use of adsorption technology, of which metal-organic frameworks may be considered suitable candidates. 4.3 Metal-Organic Frameworks Metal-organic frameworks (henceforth called MOFs) have, within a period of just over one-two decades, steadily gained ground as novel nanoporous materials due largely to their unique characteristics and properties (Evans, 2008). From the myriad of combinations of metal cations and organic ligands that exist, a large variety of MOFs have been developed with various pore sizes, offering many research and industrial opportunities (Mueller, et al., 2006). Today, most experimental and theoretical research of MOFs has generally been geared towards gas storage and separation (Nalaparaju, Zhao, & Jiang, 2011). Potential use of MOFs has also been rising in heterogeneous catalysis (Zou, Abdel-Fattah, Xu, Zhao, & Hickmott, 2010). A class of MOFs demonstrated promise in the realm of sorption technology is zeolitic imidazolate frameworks (hence forth called ZIFs), which contain tetrahedral Zn(II) atoms linked by imidazolate ligands, and their structures closely resemble zeolitic frameworks (Saint Remy, et al., 2011). ZIFs have gained considerable attention because of their tuneable porosity, structural flexibility and functionalization, as well as thermal and chemical stability (Ortiz, Freitas, Boutin, Fuchs, & Coudert, 2014). For this reason, this project will focus on the use of ZIFs to meet the project objectives. 11 4.4 Objectives The advent of The Great Acetonitrile Shortage in 2008 shows the industrial importance of recovering and recycling expended acetonitrile from aqueous solutions to circumvent possible future supply disruption from interfering with essential industrial and laboratory work. The separation of acetonitrile from aqueous solutions using normal distillation methods inevitably meets a thermodynamic limit in the form of an azeotropic point at 85.8% acetonitrile (INEOS, 2007), and current separation technologies involve energy-intensive and/or complex operations such as multiple or extractive distillation to break this azeotrope (van Assche, Remy, Desmet, Baron, & Denayer, 2011). This problem is compounded by the fact that acetonitrile is a very polar amphiphile with a dipole moment about 3.9 D, and its aqueous solutions are believed to exhibit microheterogeneity (Bakó, Megyes, Grósz, Pálinkás, & Dore, 2006). A novel method – inexpensive and not energy-intensive – by which to purify acetonitrile thus needs to be determined in order to meet the dual goals of attaining economical solvent consumption and increasing industrial ecofriendliness. This project thus aims to discern the suitability and effectiveness of several ZIFs in the selective adsorption of acetonitrile from aqueous solutions through computer simulation, so that these social and economic objectives might be achieved. 12 5. Models and Methods 5.1 ZIFs and Adsorbates Fig. 1 illustrates the atomic structures of ZIF-8, -10, -25, -60, -71, -90, -96 and -97. They contain the same tetrahedral ZnN4 clusters, but differ in the imidazolate linkers as listed in Table 1. Fig. 1 Atomic Structures of ZIF-8, -10, -25, -60, -71, -90, -96 and -97. ZnN4 cluster: orange polyhedron, C: grey, O: red, N: light purple, Cl: green, and H: white. The sizes are not to the same scale. ZIF-8 and ZIF-90 possess the SOD type topology in which the linker is singly functionalised at position 2; 4 and 6-membered rings are connected to form sodalite cages. ZIF-10 and ZIF-60 belong to the MER type topology in which the linker is not functionalised (except for the meIm linker of ZIF-60 which is functionalised at position 2); 4 and 6-membered rings are connected to form merlinoite cages.ZIF-25, -71, -96 and -97 belong to the RHO type with the linker dually functionalised at positions 4 and 5. The 4,6,8-membered rings are connected to form truncated cuboctohedra (α-cages) in a cubic bodycentred arrangement. 13 Table 1. ZIFs and Organic Linkers. ZIF Linker Shorthand Label Chemical Structure ZIF-8 2-methyl imidazolate meIm C4H6N2 ZIF-10 Imidazolate Im C3H4N2 ZIF-25 dimethyl imidazolate dmeIm C5H8N2 Imidazolate Im C3H4N2 2-methyl imidazolate meIm C4H6N2 ZIF-71 dichloro imidazolate dcIm C3H2N2Cl2 ZIF-90 imidazole-2carboxyaldehyde icaIm C4H4N2O ZIF-96 cyanide amine imidazolate cyamIm C4H4N4 ZIF-97 hydroxymethylmethyl imidazolate hymeIm C5H8N2O ZIF-60 14 Ball-andStick Diagram Table 2. Structural Properties of SOD- and MER-type ZIFs. ZIF-8 ZIF-90 ZIF-10 ZIF-60 Topology SOD SOD MER MER Space Group I43m I43m I43m I43m ρa (g cm-3) 0.924 0.988 0.746 0.769 Sa(m2 g-1) 1279 1216 2298 2155 Vf (cm3 g-1) 0.531 0.480 0.819 0.769 Φ 0.491 0.474 0.611 0.589 dc (Å) 11.1 10.4 12.9 12.8 da (Å) 3.1 3.5 7.7 7.7 Linker(s) a Densities are based on solvent-free perfect crystals. Table 3. Structural Properties of RHO-type ZIFs. ZIF-25 ZIF-71 ZIF-96 ZIF-97 Topology RHO RHO RHO RHO Space Group ̅m Fd3 ̅m Pm3 I432 I432 ρa(g cm-3) 0.949 1.155 0.977 0.997 2 1029 1023 1128 835 Vf (cm g ) 0.443 0.447 0.518 0.400 Φ 0.420 0.519 0.505 0.399 dc (Å) 16.2 16.4 16.2 15.8 da (Å) 3.3 3.5 3.8 3.3 Linker -1 Sa(m g ) 3 a -1 Densities are based on solvent-free perfect crystals. 15 The structural and functional properties of the eight ZIFs are listed in Table 2 and Table 3. The accessible surface area Sa was estimated using N2 as a probe with the kinetic diameter of 3.64 Å, while the free volume was estimated by random insertions of He (a non-adsorbing species). The porosity, Φ, is defined as the ratio of free volume to framework volume. The pore size was calculated using the HOLE program, as well as the cage diameter dc and aperture da. The framework atoms of the ZIFs were represented by Lennard-Jones (LJ) and Coulombic potentials: 12 𝑈𝑛𝑜𝑛𝑏𝑜𝑛𝑑𝑒𝑑 𝜎𝑖𝑗 = ∑ 4𝜀𝑖𝑗 [( ) 𝑟𝑖𝑗 6 𝜎𝑖𝑗 𝑞𝑖 𝑞𝑗 −( ) ]+ ∑ 𝑟𝑖𝑗 4𝜋𝜀0 𝑟𝑖𝑗 Where 𝜀𝑖𝑗 and 𝜎𝑖𝑗 are the well depth and the collision diameter respectively, 𝑟𝑖𝑗 is the distance between atoms i and j, 𝑞𝑖 is the atomic charge of the atom i, 𝜀0 = 8.8524 × 10−12 𝐶 2 ∙ 𝑁 −1 ∙ 𝑚−2 is the permittivity of vacuum. The atomic charges of the ZIFs were evaluated from the density functional theory (DFT) calculations based on fragmental clusters (a portion of each cluster is illustrated in Appendix A). The DFT calculations used the Becke exchange plus Lee-Yang-Parr functional (B3LYP) and were carried out using Gaussian 03 (Frisch, et al., 2004). The accuracy of DFT-derived atomic charges depends on the choice of the functional and basis sets. Expressed as both local and gradient electron densities, B3LYP has been widely used for solid materials. For small basis sets, the atomic charges fluctuate appreciably but tend to converge beyond the 6-31G(d) basis set (Hariharan & Pople, 1972). Therefore, 6-31G(d) was used for all atoms of the ZIFs except Zn atoms, for 16 which the LANL2DZ basis set – a double-zeta basis set that contains effective pseudo-potentials to represent the potentials of nuclei and core electrons – was used. The atomic charges were fitted to the electrostatic potentials and estimated, as listed in Table 5 of Appendix B. The LJ potential parameters from the DREIDING force field are listed in Table 6 under Appendix C. From a number of simulation studies, the DREIDING force field has shown to be capable of accurately predicting adsorption and diffusion in various MOFs (Paranthaman, Coudert, & Fuchs, 2010) (Nalaparaju, Zhao, & Jiang, 2010). ZIF structures were considered to be rigid – valid as a first approximation since it is known that, for example, the ZIF structures feature some local flexibility by a ‘‘linker swing’’ motion. However, the importance of flexibility on adsorption has so far mostly been observed at cryogenic temperatures, justifying the approximation employed here (Ortiz, Freitas, Boutin, Fuchs, & Coudert, 2014). Acetonitrile (CH3-C-N) was represented by a united-atom model with CH3, C and N taken as single interaction sites. The potential parameters were adopted from the transferable potentials for the phase equilibria (TraPPE) force field, which was fitted to measured critical properties and equilibrium data. Water was mimicked by the three-point transferable interaction potential model (TIP3P), which reproduces the necessary aspects of water vibration (Jorgensen, Chandrasekhar, Madura, Impey, & Klein, 1983). In TIP3P model, the equilibrium O-H bond length is 0.9572Å and the equilibrium bond angle of H-O-H is 104.52°. The TIP3P gives reasonably good interaction energy compared to experiments (Jorgensen, Chandrasekhar, Madura, Impey, & Klein, 1983). Table 7 in Appendix C gives the potential parameters of acetonitrile 17 and water, and the cross interactional parameters were estimated using the Lorentz-Berthelot combining rules. 5.2 Simulation Methods To simulate the adsorption of pure acetonitrile and water as well as their mixtures, grand canonical Monte Carlo (GCMC) method was used. The chemical potentials of an adsorbate in the adsorbed and bulk phases are identical at thermodynamic equilibrium, and the GCMC method allows one to directly relate the chemical potentials in both phases and has been widely used to simulate adsorption (Nalaparaju, Zhao, & Jiang, 2010). For pure acetonitrile and water, the adsorption was simulated below and up to saturation pressures (11.71 kPa and 3.18 kPa respectively), and thus there were considered as ideal gases. The simulation boxes contained the ZIFs and the periodic boundary conditions were extended in three directions. For acetonitrile-water liquid mixtures, the fugacities were estimated by: 𝑓𝑖 = ̅̅̅̅ 𝑉𝑖 (𝑃−𝑃𝑠𝑎𝑡 𝑖 ) ) 𝑅𝑇 ( 𝑓𝑒𝑒𝑑 𝑃𝑖𝑠𝑎𝑡 𝜑𝑖𝑠𝑎𝑡 𝑥𝑖 𝛾𝑖 𝑒 where 𝑃𝑖𝑠𝑎𝑡 refers to the saturation pressure of species I estimated by The 𝑓𝑒𝑒𝑑 Antoine equation, 𝜑𝑖𝑠𝑎𝑡 the fugacity coefficient of species i, 𝑥𝑖 the mole fraction of species i in the liquid phase, 𝛾𝑖 the activity coefficient of species i estimated from the Non-Random Two Liquid (NRTL) excess Gibbs energy ̅𝑖 the partial molar volume of species i, and 𝑃 the operating pressure. model, 𝑉 Under the operating conditions (1 bar and 298K) considered in this study, the fugacity coefficient and Poynting factor are approximately equal to unity. The fugacity values were varied for the composition range between xw = 0.1 to 0.97, as shown in Table 4. 18 Table 4. Species Fugacities at different compositions. xw fwater (kPa) facetonitrile (kPa) 0.1 0.314 10.539 0.2 0.628 9.368 0.3 0.942 8.197 0.4 1.256 7.026 0.5 1.570 5.855 0.6 1.884 4.684 0.7 2.198 3.513 0.8 2.512 2.342 0.9 2.826 1.171 0.93 2.920 0.820 0.95 2.983 0.586 0.97 3.046 0.351 In the GCMC simulations, the LJ interactions were evaluated with a spherical cut-off of 15 Å. For the Coulombic interactions, the Ewald sum with a tin-foil boundary condition was used. The real/reciprocal space partition parameter and the cut-off for reciprocal lattice vectors were chosen to be 0.2 Å-1 and 8, respectively, to ensure the convergence of the Ewald sum. The number of trial moves in a typical GCMC simulation was 2 × 107 (20000 MC cycles, 1000 moves per cycle) for pure water and acetonitrile, and 4 × 107 (20000 MC cycles, 2000 moves per cycle) for binary mixtures. In both cases, the first half of trial moves employed was used for equilibration and the second half for ensemble averages. Five types of trial moves were randomly attempted, namely displacement, rotation, partial regrowth at a neighbouring position, complete regrowth at a new position, and a swap between reservoirs including creation and deletion with equal probability. To improve sampling efficiency, a configurational-bias technique was adopted in which an adsorbate 19 molecule was grown atom-by-atom biasing towards energetically favourable configurations while avoiding overlap with other atoms (Frenkel, Mooij, & Smit, 1992) (de Pablo, Laso, & Suter, 1992) (Siepmann & Frenkel, 1992). Specifically, the trial positions were generated with a probability proportional 𝑖 𝑖 ) , where 𝛽 = 1/𝑘𝐵 𝑇 and 𝑈𝑖𝑛𝑡𝑟𝑎 to 𝑒𝑥𝑝(−𝛽𝑈𝑖𝑛𝑡𝑟𝑎 is the intramolecular interaction energy at position i. The numbers of trial positions for the first and subsequent atoms were, respectively, ten and fifteen for acetonitrile and water. One of the trial positions was then chosen with a probability proportional 𝑖 𝑖 𝑖 )⁄∑𝑖 𝑒𝑥𝑝(−𝛽𝑈𝑖𝑛𝑡𝑒𝑟 ) , where 𝑈𝑖𝑛𝑡𝑒𝑟 to 𝑒𝑥𝑝(−𝛽𝑈𝑖𝑛𝑡𝑒𝑟 is the intermolecular interaction energy. 6. Results and Discussion First, the adsorption properties of pure acetonitrile in the eight ZIFs are presented. From adsorption isotherms, isosteric heats, and radial distribution functions, the role of functional groups is quantitatively assessed. Then, the adsorption of pure water is discussed. Finally, the separation of acetonitrilewater mixtures is examined and the highest selectivity is identified. 6.1 Pure Acetonitrile 6.1.1 Adsorption Isotherms Fig. 2 shows the adsorption isotherms of pure acetonitrile in the ZIFs studied at 298K and various pressures up to the normal saturation pressure of acetonitrile (11.71 kPa). It has to be noted that currently experimental data for acetonitrile adsorption in ZIFs are not available, and the reader is kindly advised to exercise discretion when using the results of this study. The general 20 features of adsorption are well captured by the simulation. With increasing pressure, the isotherms can be characterised into two categories based on the hydrophobicity/hydrophilicity of the ZIFs. Furthermore, the different structural properties also contribute to the varying ZIF adsorption performance. Fig. 2 Simulated Acetonitrile Adsorption Isotherms at 298K. The unit of the ordinate is mmol per gramZIF. Low-pressure range is depicted in the inset. In the low-pressure region (e.g. where relative pressure is about 0.001), uptake decreases in the order of ZIF-97 > -96 > -90 > -71 > -25 > -8 ≈ -10 ≈ 60. This apparently shows the effect of the functional groups on molecular interactions. Acetonitrile is a well-known polar aprotic solvent and possesses two sites for accepting a hydrogen bond: one on the lone-pair electrons of the nitrogen atom (σ bonding) and the other on the C≡N bond (π bonding) (Kyrachko & Nguyen, 2002). ZIF-97, -96 and -90 all contain polar groups and 21 can form strong bonds with acetonitrile. In contrast, the functional groups in ZIF-8, -10, -25, -60 and -71 are only non-polar (–CH3) or weakly-polar (–Cl) and therefore, interact weakly with acetonitrile. ZIF-25 and -71 have slightly higher uptakes as compared to the other hydrophobic ZIFs as they contain doubly-functionalised linkers, and thus the scope for interaction with adsorbate is increased. In the high-pressure region, functionality is not the only factor that affects adsorption and saturation capacity; the free volume 𝑉𝑓 (and to a certain extent, the porosity Φ) becomes increasingly important. This factor is enhanced due to the fact that acetonitrile is an exceptionally small molecule (about 3.5 Å in length) and thus has a high degree of mobility (Kyrachko & Nguyen, 2002). With the same SOD topology, ZIF-8 has a larger free volume than ZIF-90 and thus a higher saturation uptake. The adsorption is also observed to be governed by the hydrophilicity/hydrophobicity of the ZIFs. ZIF-90 adsorbs acetonitrile over the whole range of pressures given the existence of the polar –CHO carboxyaldehyde group, whilst ZIF-8 exhibits a sudden drastic step in uptake at the relative pressure > 0.01 due to the cage filling mechanism. In the MER-type ZIFs (ZIF-10 and -60), both contain non-polar groups, their adsorption isotherms behave exactly like that in ZIF-8, but this time exhibiting a drastic step around the region where relative pressure is 0.02. This can also be explained by the cage filling mechanism. With a larger free volume than ZIF-60, ZIF-10 attains a higher saturation uptake. 22 Fig. 3 Simulated Acetonitrile Adsorption Isotherms at 298K for RHO-type ZIFs. The unit of the ordinate is mmol per cm3 ZIF. However, in the RHO-type ZIFs, the effect of free volume is not as dominant. The free volumes of the RHO-type ZIFs decrease in the order ZIF96 > -71 > -25 > -97, and we observe that the maximal saturation level in ZIF96, which has the largest free volume by-and-large, is the highest; the other RHO-type ZIFs exhibit lower maximal saturation values. Yet, the maximal uptake values on basis mmol per unit mass of ZIF (ZIF-96 > -97 ≈ -25 > -71) do not follow the free volume trend; and the explanation becomes clearer when ZIF densities are taken into account. By changing the basis with which the adsorption values are measured, from mmol per unit mass to mmol per unit volume of ZIF, as shown in Fig. 3, 23 the trend in acetonitrile uptake of the RHO-type ZIFs can be better discerned. In effect, this change of basis allows for comparison to be based on porosity, which is of the order ZIF-71 > -25 > -97. From Fig. 3, it is seen that the maximal uptakes for these three ZIFs are of the order ZIF-71 > -97 > -25, with ZIF-97 deviating from the trend of decreasing porosity. The reason for ZIF-97 exhibiting higher saturation uptake than ZIF-25, despite a slightly lower porosity and free volume, is due to its polar functional group (–CH2OH) allowing for stronger attractive interactions to draw and retain adsorbate as compared to the non-polar methyl group (–CH3) in ZIF-25. Thus we can conclude that in this case, acetonitrile adsorption in the RHO-type ZIFs is affected by a combination of all three factors – linker functionality, free volume and porosity. 6.1.2 Isosteric Heat of Adsorption To quantitatively examine adsorption energy, the isosteric heat of 0 adsorption𝑄𝑠𝑡 was investigated as a function of loading. Specifically, 𝑄𝑠𝑡 at infinite dilution was estimated by a single-molecule Monte Carlo simulation: 0 0 𝑄𝑠𝑡 = 𝑅𝑔 𝑇 − 𝑈𝑎𝑑 0 𝑈𝑎𝑑 = 𝑈𝑡𝑜𝑡𝑎𝑙 − (𝑈𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑛𝑡 − 𝑈𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 ) where 𝑅𝑔 is the universal gas constant, 𝑇 is the temperature, 𝑈𝑡𝑜𝑡𝑎𝑙 , 𝑈𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑛𝑡 and 𝑈𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 are the potential energies of the adsorbent-adsorbate, adsorbent and a single adsorbate molecule, respectively. The conformational change in the adsorbate upon adsorption was taken into account. At finite loadings, 𝑄𝑠𝑡 was estimated using: 24 𝑄𝑠𝑡 = 𝑅𝑔 𝑇 − ( 𝜕𝑈𝑎𝑑 ) 𝜕𝑁𝑎𝑑 𝑇,𝑉 where 𝑈𝑎𝑑 and 𝑁𝑎𝑑 are the adsorption energy and the number of adsorbate molecules respectively. The partial derivative in the above equation was evaluated using: ( 〈𝑁𝑎𝑑 𝑈𝑎𝑑 〉 − 〈𝑁𝑎𝑑 〉〈𝑈𝑎𝑑 〉 𝜕𝑈𝑎𝑑 ) = 2 〉 〈𝑁𝑎𝑑 𝜕𝑁𝑎𝑑 𝑇,𝑉 − 〈𝑁𝑎𝑑 〉2 where the bracket 〈… 〉 denotes the ensemble average. Fig. 4 ZIF Isosteric Heats of Adsorption for Acetonitrile at infinite dilution at 298K. 0 Fig. 4 shows the 𝑄𝑠𝑡 values for acetonitrile adsorption at infinite dilution in the eight ZIFs. Based on the hydrophobic/hydrophilic nature of the ZIFs, there is a general trend whereby hydrophobic ZIFs have much lower isosteric heats than hydrophilic ones. 25 This effect is most easily observed in terms of ZIF topologies. In the RHO-type ZIFs, ZIF-96 and -97 with their highly polar functional groups 0 exhibit greater 𝑄𝑠𝑡 values than ZIF-25 or -71 with non-polar or weakly polar groups. The SOD-type ZIFs also follow this trend whereby the hydrophilic 0 groups in ZIF-90 cause it to have a greater 𝑄𝑠𝑡 value than ZIF-8. In the MER type ZIF-10 and ZIF-60, given that both possess non-polar functional groups, 0 their 𝑄𝑠𝑡 values are almost the same. 0 It is interesting to note that the order of 𝑄𝑠𝑡 values, namely (in decreasing order) ZIF-97 > -96 > -90 > -71 > -25 > -8 ≈ -10 ≈ -60, is consistent with that of the acetonitrile adsorption in the low-pressure region. This fact lends support to the notion that functional groups are the chief contributing factor to adsorption at low pressures, and will continually affect adsorption behaviour over the full range of activities as well. 6.1.3 Radial Distribution Functions Radial distribution or pair correlation functions (RDF) are the primary connection between macroscopic thermodynamic properties and intermolecular interactions of fluids as well as fluid mixtures (Matteoli & Ali Mansoori, 1995). RDF represents the atomic densities of species i around one particular atom (the node) which varies as a function of distance from that atom (Martin & Siepmann, 1998). It is thus instructive to characterise adsorption sites in the ZIFs. The RDFs for acetonitrile around the ZIF framework atoms were calculated using: 𝑔𝑖𝑗 (𝑟) = 𝑁𝑖𝑗 (𝑟, 𝑟 + ∆𝑟)𝑉 4𝜋𝑟 2 ∆𝑟𝑁𝑖 𝑁𝑗 26 where 𝑟 is the distance between atoms 𝑖 and 𝑗, 𝑁𝑖𝑗 (𝑟, 𝑟 + ∆𝑟) is the number of atoms 𝑗 around 𝑖 from 𝑟 to 𝑟 + ∆𝑟, 𝑉 is the system volume, 𝑁𝑖 and 𝑁𝑗 are the numbers of atoms 𝑖 and 𝑗 respectively. 6.1.3.1 SOD-type ZIFs Fig. 5 Radial Distribution Functions in SOD-type ZIFs. Fig. 5 shows the 𝑔(𝑟) for acetonitrile around the heavy atoms (Zn, N, C and O) in the SOD-type ZIFs (ZIF-8 and -90) at 1.0kPa. In ZIF-8, the peak at 3.5 Å implies the favourable adsorption of acetonitrile near the electrondense C═C group as well as the methyl group. The 𝑔(𝑟) for C1 (carbon atom at the tip of the imidazolate ring) exhibits a peak at long distances (5.5 Å). ZIF-90 has a –CHO (carboxyaldehyde group) in place of –CH3 as in ZIF-8. Sharp peaks at 4 Å and 3.5 Å are seen around the O and C2 atoms, respectively. This indicates strong interaction between the highly polar –C≡N (nitrile) group of the adsorbate and the –CHO group of the linker, as well as the C═C group serving as a favourable adsorption site. Again, a peak at a further distance of 5 Å around the carbon atom at the tip of the imidazolate ring (C3 in this case) is observed. 27 6.1.3.2 MER-type ZIFs Fig. 6 Radial Distribution Functions in MER-type ZIFs. Fig. 6 shows the 𝑔(𝑟) for acetonitrile around the heavy atoms (Zn, N, C and O) in the MER-type ZIFs (ZIF-10 and -60) at 1.0kPa.In ZIF-10, there are two different types of moieties (based on the structural organisation of the framework) that can be conferred onto the single linker (imidazolate). Thus, although ZIF-10 only contains a single linker, the atoms exist in different “environments” (refer to Appendix A to view the relative positions). The peaks in the 𝑔(𝑟) at 4.5 Å for C3 and C4 suggest the favourable adsorption sites are the electron-dense C═C bonds of the imidazolate rings. Lower peaks are observed for the C1 and C2 atoms. In ZIF-60, methyl imidazolate (meIm) replaces one imidazolate moiety in ZIF-10. The peaks around C2 and C4 at 4 Å and 4.5 Å respectively thus imply the C═C double bonds are favourable adsorption sites. The added methyl group on meIm is observed to have only a marginal effect on acetonitrile adsorption, as the C5 atom only exhibits a peak at a relatively longer distance of 6 Å. 28 6.1.3.3 RHO-type ZIFs Fig. 7 Radial Distribution in RHO-type ZIFs. Fig. 7 shows the 𝑔(𝑟) for acetonitrile around the heavy atoms (Zn, N, C and O) in the RHO-type ZIFs (ZIF-25, -71, -96 and -97) at 1.0kPa. In ZIF25, slight peaks are observed at 4.5 Å for the C3 atom, implying the main adsorption site is around the methyl group. In ZIF-71, a sharp peak is observed at 3.5 Å for the Cl atom, which is the main adsorption site. Given that Cl is more polar than the methyl group, the magnitude of the 𝑔(𝑟) in ZIF-71 is larger than those in ZIF-25 due to stronger interaction. In ZIF-96, the most favourable adsorption site is easily determined to be the nitrile (–C≡N) group, as the 𝑔(𝑟) for N3 and C4 atoms show sharp peaks at 2.5 Å and 4 Å, respectively. The amino group also serves as a 29 favourable adsorption site, albeit less significant with a peak at 3.5 Å for N2. The magnitude of the peaks, being larger than those in ZIF-25 and -71, implies stronger interaction in ZIF-96. In ZIF-97, the peaks of the 𝑔(𝑟) for O and C5 at 3.5 Å and 4 Å display the effectiveness of the –CH2OH group as a favourable adsorption site. Also, the methyl group (denoted by C4) is seen to have a peak at 4 Å, and may be also considered as an adsorption site but with a lower extent. Once again, with stronger interaction due to the polar functional group, the 𝑔(𝑟) in ZIF-97 exhibits higher magnitudes compared to those in ZIF-25 and -71. It is noteworthy that in all the eight ZIFs, the peak around the Zn atom is generally low at a long distance, and consequently, the imidazolate linkers are more favourable than the metal clusters in these ZIFs. This phenomenon is also observed in experimental and simulation studies of gas adsorption in ZIFs (Perez-Pellitero, et al., 2010) (McDaniel, Yu, & Schmidt, 2011) (FairenJiminez, et al., 2012) (Wu, Zhou, & Yildirim, 2007) (Wu, Zhou, & Yildirim, 2009). 6.1.4 Density Contours Density contours aid in the visualising of adsorbed molecules in the framework, and determining favourable adsorption sites. As will be discussed below, among the eight ZIFs, ZIF-8 and ZIF-97 exhibit the highest and lowest selectivities for acetonitrile-water mixtures, respectively. In this section, we focus on the contours of acetonitrile in ZIF-8 and ZIF-97. 30 Fig. 8 Density Contours of Acetonitrile in ZIF-8 (top) and ZIF-97 (bottom). The unit of density scale is number of molecules per Å3. Fig. 8 shows the density contours of acetonitrile in ZIF-8 and ZIF-97. In ZIF-8, at a low pressure, clusters are observed to form around the linker, most notably at the C═C double bonds. As pressure increases, the density around the less favourable –CH3 group increases; meanwhile, cage filling occurs in the sodalite cage. In ZIF-97, at a low pressures (≈ 0.1 kPa), acetonitrile is most favourably adsorbed around the imidazolate C═C double bonds. However, with increase in pressure, the highly polar –CH2OH group begins to adsorb acetonitrile as cage-filling occurs; this is in accordance with the 𝑔(𝑟) obtained for ZIF-97 at 1.0kPa. 6.2 Pure Water Fig. 9 shows the adsorption isotherms of water (H2O) in the eight ZIFs. Research has demonstrated that topology, geometry, and linker functionalization can drastically affect water adsorption properties (Ortiz, Freitas, Boutin, Fuchs, & Coudert, 2014). In close agreement with this, the ZIFs considered in this work have water sorption behaviour that can be 31 classified according to their hydrophobicity/hydrophilicity, with linker functional groups playing an important role. Among the eight ZIFs, hydrophobic ZIF-8, -10, -25, -60 and -71 exhibit negligible water uptakes. ZIF-8 and -71 are well-known to have a low water uptake at all levels of activity (Küsgens, et al., 2009) (Lively, et al., 2011), and this fact is observed here. Hydrophilic ZIF-90, -96 and -97 display typical stepped uptake. In ZIF-90, this behaviour is consistent with literature data, which show a low water uptake prior to a large step. It has been suggested that such a step indicates a phenomenon similar to monolayer followed by multilayer formation through capillary condensation (Brown, et al., 2013). Fig. 9 Simulated Water Adsorption Isotherms at 298 K. Inset: Adsorption isotherms in hydrophobic ZIFs. 32 Fig. 10 Density Contours of Water in ZIF-8 (top) and ZIF-97 (bottom).The unit of density scale is number of molecules per Å3. Fig. 10 shows the density contours of water in ZIF-8 and ZIF-97. Almost no water is adsorbed in ZIF-8 at the three pressures (0.1, 1 and 3 kPa), consistent with the isotherm in Fig. 9. In the literature, it was also found that ZIF-8 only exhibits a very low uptake of water (Saint Remy, et al., 2011). On the other hand, ZIF-97 adsorbs water very well. At low pressures, the main site for adsorption occurs at the imidazolate rings. As pressure increases, the density increases rapidly as cage filling occurs and the highly polar –CH2OH group acts as an attractive site. Thus, the density contour corroborates the observation in Fig. 9, where ZIF-97 exhibits a steep increase in water uptake as pressure increases. 6.3 Acetonitrile-Water Mixtures 6.3.1 Isotherms and Selectivity Fig. 11 shows the adsorption isotherms of acetonitrile-water mixtures in the eight ZIFs. Generally, as acetonitrile composition (x A) increases, its uptake in all the ZIFs monotonically increases. Conversely, water uptake 33 decreases with increasing xA. Using these adsorption isotherms, it is now possible to quantify the separation efficacy of acetonitrile-water mixtures. Adsorption selectivity is defined by: 𝜶𝑎𝑑 𝑌 ( 𝑖⁄𝑌 ) 𝑗 = 𝑋𝑖 ( ⁄𝑋 ) 𝑗 where 𝑌𝑖 and 𝑋𝑖 are the composition of component i in the adsorbed and liquid phase, respectively. Fig. 11 Adsorption Isotherms of Binary Solution in ZIF-8, -10, -25, -60, -71, -90, -96 and -97 at 298K. Inset: Adsorption isotherms of water in hydrophobic ZIFs. 34 Fig. 12 Selectivity of Acetonitrile-Water mixtures. Bottom-Left: Dilute Range, Bottom-Right: Medium to Concentrated Range (ZIF-8 values excluded for clarity) Fig. 12 shows the acetonitrile/water selectivities in the eight ZIFs. In the dilute range, the selectivity generally drops monotonically with increasing acetonitrile concentration xA. For dilute solutions where xA < 10%, the selectivity decreases in the order of ZIF-8 > -25 > -71 ≈ -10 ≈ -60 > -90 > -96 ≈ -97. We can infer that hydrophobic ZIFs (ZIF-8, -10, -25, -60 and -71) 35 possess higher selectivity compared with the hydrophilic counterparts (ZIF-90, -96 and -97), and the reason being that the polar groups present in ZIF-90, -96 and -97 promote the adsorption of water, thus reducing the selectivity. The highest selectivity predicted in ZIF-8 is approximately 7800 at 3% acetonitrile in the feed solution. 6.3.2 Density Contours ZIF-8 and ZIF-97 were chosen for the analysis of density contour as they are the materials having respectively the highest and lowest selectivities amongst the eight ZIFs. Fig. 13 Density Contours of Acetonitrile-Water mixtures in ZIF-8 (top) and ZIF-97 (bottom).The unit of density scale is number of molecules per Å3. Fig. 13 shows the density contours of acetonitrile-water mixtures of varying compositions. At a low composition, acetonitrile in ZIF-8 is densely populated near the imidazolate ring. In contrast, water is adsorbed only in negligible amount; water molecules that are adsorbed are sparingly located around the void space near the –CH3 groups. As the acetonitrile composition 36 increases, less water is adsorbed in favour of acetonitrile. Again, the main acetonitrile adsorption sites are located around the imidazolate ring. This confirms the hydrophobic nature of ZIF-8, and combined with the fact that such high level of acetonitrile adsorption is seen, it comes as no surprise that ZIF-8 has a high acetonitrile/water selectivity. In hydrophilic ZIF-97, water is strongly co-adsorbed together with acetonitrile at all compositions. Even at xA = 0.1, water generally fills up the void space in high numbers. Overall, the imidazolate ring of the linker is seen to adsorb acetonitrile preferentially, and the –CH2OH group seems to interact more closely with water than it does acetonitrile, most probably due to hydrogen bonding capabilities. As such, a low selectivity is seen in ZIF-97. 37 7. Conclusions Adsorption and purification of acetonitrile-water mixtures in eight ZIFs (ZIF-8, -10, -25, 60, -71, -90, -96 and -97) have been examined by molecular simulation. With polar functional groups, ZIF-90, -96 and -97 have the ability to form strong bonds (e.g. permanent dipole-permanent dipole and/or hydrogen bonds) with water and acetonitrile. In contrast, the functional groups in ZIF-8, -10, -25, -60 and -71 are non-polar or weakly polar, and thus, only weak interactions are present. In the low-pressure region, acetonitrile uptake decreases in the order of ZIF-97 > -96 > -90 > -71 > -25 > -8 ≈ -10 ≈ -60. The isosteric heat of adsorption at infinite dilution follows the same order. Hydrophobic ZIF-8, -10, -25, -60 and -71 all exhibit negligible adsorption of water, but substantial adsorption is observed in hydrophilic ZIF-90, -96 and 97. With regards to acetonitrile purification from aqueous solutions, the general trend of selectivity of acetonitrile over water is that the selectivity decreases as acetonitrile composition increases. The hydrophobic ZIFs (ZIF-8, -10, -25, -60 and -71) have higher selectivities than the hydrophilic ZIFs (ZIF90, -96 and -97). In dilute mixtures (e.g. -25 > -71 ≈ -10 ≈ -60 > -90 > -96 ≈ -97. ZIF-8 exhibits the highest selectivity and might be a potential candidate for acetonitrile purification from aqueous solutions. 38 8. Bibliography [1] Bakó, I., Megyes, T., Grósz, T., Pálinkás, G., & Dore, J. (2006). Structural investigation of water–acetonitrile mixtures: Small-angle and wide-angle neutron diffraction study compared to molecular dynamics simulation. Journal of Molecular Liquids, 125, 174-180. [2] Brown, A., Lively, R., Zhang, K., Dose, M., Zhang, C., Chung, J., . . . Chance, R. (2013). Alcohol and water adsorption in zeolitic imidazolate. Chemical Communications, 49, 3245-3247. [3] Center for Drug Evaluation and Research. (2014). Comprehensive List of Guidance Documents. New Hampshire, Maryland, Uinted States of America: U.S. Food and Drug Administration. [4] de Pablo, J., Laso, M., & Suter, U. W. (1992). Simulation of polyethylene above and below the melting point. Journal of Chemical Physics, 96, 2395. doi:10.1063/1.462037 [5] Eurasian Chemical Market International Magazine. (2014). Acetonitrile Market in CIS and Worldwide. Retrieved 23 June, 2014, from Eurasian Chemical Market International Magazine: http://www.chemmarket.info/en/home/article/2902/ [6] Evans, J. (2008). Molecular Sponges - Is there any limit to the spectacular catalytic and storage abilities of MOFs? Chemistry & Industry, 16-18. [7] Fairen-Jiminez, D., Galvelis, R., Torrisi, A., Gellan, A., Wharmby, M., Wright, P., . . . Duren, T. (2012). Flexibility and swing effect on the 39 adsorption of energy-related gases on ZIF-8: combined experimental and simulation study. Dalton Transactions, 41, 10752-10762. [8] Frenkel, D., Mooij, G. C., & Smit, B. (1992). Novel scheme to study structural and thermal properties of continuously deformable molecules. Journal of Physics: Condensed Matter, 4, 3053. [9] Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., . . . al., e. (2004). Gaussian 03, Revision D.01 ed. Wallingford, Connecticut, United States of America. [10] Hariharan, P., & Pople, J. A. (1972). The Effect of d-Functions an Molecular Orbital Energies for Hydrocarbons. Chemical Physics Letters, 16, 217. [11] IHS Chemicals. (2014). Acetonitrile. Douglas County, Colorado, United States of America : IHS. [12] INEOS. (2007). Acetonitrile - Safe Storage and handling Guide. League City, Texas, United States of America: INEOS Nitriles. [13] Jiang, J., Chen, Y., Nalaparaju, A., & Zhang, K. (19 February, 2014). Biofuel Purification in Zeolitic Imidazolate Frameworks: the significant role of Functional Groups. Physical Chemistry, Chemical Physics, 16, 9643-9655. [14] Jiang, J., Zhang, K., & Zhang, L. (2013). Adsorption of C1-C4 alcohols in zeolitic imidazolate framework-8: effects of force fields, atomic charges and framework flexibility. Journal of Physical Chemistry C, 117, 25628. 40 [15] Jorgensen, W. L., Chandrasekhar, J., Madura, J. D., Impey, R. W., & Klein, M. L. (1983). Comparison of Simple Potential Functions for Simulating Liquid Water. Journal of Chemical Physics, 79, 926. doi:doi:10.1063/1.445869 [16] Küsgens, P., Rose, M., Senkovska, I., Fröde, H., Henschel, A., Siegle, S., & Kaskel, S. (2009). Characterization of metal-organic frameworks by water adsorption. Microporous and Mesoporous Materials, 120, 325-330. [17] Kyrachko, E., & Nguyen, M. (2002). Hydrogen Bonding between Phenol and Acetonitrile. The Journal of Physical Chemistry A, 106, 4267-4271. [18] Lively, R., Dose, M., Thompson, J., McCool, B., Chance, R., & Koros, W. (2011). Ethanol and water adsorption in methanol-derived ZIF-71. Chemical Communications, 47, 8667-8669. [19] Longden, R. (13 October, 2009). INEOS Nitriles breakthough secures global supply of acetonitrile. Retrieved 23 June, 2014, from INEOS Nitriles: http://www.ineos.com/businesses/ineos-nitriles/news/ineos- nitriles-breakthough-secures-global-supply-of-acetonitrile/ [20] Martin, M., & Siepmann, J. (1998). Transferable Potentials for Phase Equilibria. 1. United-Atom Description of n-Alkanes. Journal of Physical Chemistry B, 102, 2569-2577. [21] Matteoli, E., & Ali Mansoori, G. (1995). A Simple Expression for Radial Distribution Functions of Pre Fluids and Mixtures. Journal of Chemical Physics(103 (11)), 4672 - 4677. 41 [22] McDaniel, J., Yu, K., & Schmidt, J. (2011). Physically Motivated, Robust, ab Initio Force Fields for CO2 and N2. Journal of Physical Chemistry B, 115, 10054. [23] Mueller, U., Schubert, M., Teich, F., Puetter, H., Schierle-Arndt, K., & Pastre, J. (2006). Metal Organic Frameworks - Prospective Industrial Applications. Royal Society of Chemisty, Journal of Materials Chemistry, 16, 626-636. [24] Nalaparaju, A., Zhao, X., & Jiang, J. (2010). Molecular Understanding for the Adsortion of Water and Alcohols in Hydrophilic and Hydrophobic Zeolitic Metal-Organic Frameworks. Journal of Physical Chemistry(114), 11542-11550. [25] Nalaparaju, A., Zhao, X., & Jiang, J. (2011). Biofuel Purification by Pervaporation and Vapor Permeation in Metal Organic Frameworks: A Computational Study. Energy & Environmental Science., 4, 2107-2116. [26] National Centre for Biotechnology Information. (2014). PubChem Compound Database CID 6342. Retrieved 23 June, 2014, from National Centre for Biotechnology Information: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=6342 [27] NIIR Project Consultancy Services. (2014). Acetonitrile. Retrieved 23 June, 2014, from NIIR Project Consultancy Services: http://www.niir.org/profiles/profile/1654/acetonitrile.html [28] Ortiz, A., Freitas, A., Boutin, A., Fuchs, A., & Coudert, F. (2014). What makes zeolitic imidazolate frameworks hydrophobic or 42 hydrophilic? The impact of geometry and functionalization on water adsorption. Physical Chemistry Chemical Physics, 16, 9940-9949. [29] Paranthaman, S., Coudert, F., & Fuchs, A. (2010). Water adsorption in hydrophobic MOF channels. Physical Chemistry Chemical Physics, 12, 8123-8129. [30] Perez-Pellitero, J., Amrouche, H., Siperstein, F., Pirngruber, G., NietoDraghi, C., Chaplais, G., . . . Bats, N. (2010). Adsorption of CO2, CH4, and N2 on Zeolitic Imidazolate Frameworks: Experiments and Simulations. Chemistry - A European Journal, 16, 1560. [31] Phan, A., Doonan, C., Uribe-Romo, F., Knobler, C., O'Keeffe, M., & Yaghi, O. (January, 2010). Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks. Accounts of Chemical Research, 43(1), 58-67. [32] Saint Remy, J., Rémy, T., van Hunskerken, V., van de Perre, S., Duerinck, T., Maes, M., . . . Denayer, J. (2011). Biobutanol Separation with the Metal–Organic Framework ZIF-8. ChemSusChem, 1074-1077. [33] Shimadzu Corporation. (13 June, 2014). Differences Between Using Acetonitrile and Methanol for Reverse Phase Chromatography. Retrieved from Shimadzu Corporation Website: http://www.shimadzu.com/an/hplc/support/lib/lctalk/35/35lab.html [34] Siepmann, J., & Frenkel, D. (1992). Configurational-bias Monte Carlo - A new sampling scheme for flexible chains. Molecular Physics, 75, 59-70. 43 [35] van Assche, T., Remy, T., Desmet, G., Baron, G., & Denayer, J. (2011). Adsorptive separation of liquid water/acetonitrile mixtures. Separation and Purification Technology, 82, 76-86. [36] Vlugt, T. J., & Smit, B. (n.d.). Configurational Bias Monte Carlo. Retrieved from BIGMAC Homepage: http://molsim.chem.uva.nl/bigmac [37] Wu, H., Zhou, W., & Yildirim, T. (2007). Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8. Journal of the American Chemical Society, 129(17), 5314-5315. [38] Wu, H., Zhou, W., & Yildirim, T. (2009). Methane Sorption in Nanoporous Metal−Organic Frameworks and First-Order Phase Transition of Confined Methane. The Journal of Physical Chemistry C, 113(7), 3029–3035. [39] Zhang, K., Lively, R., Zhang, C., Koros, W., & Chance, R. (2013). Investigating the Intrinsic Ethanol/Water Separation Capability of ZIF8: An Adsorption and Diffusion Study. The Journal of Physical Chemistry C, 117, 7214-7225. [40] Zou, R., Abdel-Fattah, A., Xu, H., Zhao, Y., & Hickmott, D. (2010). Storage and Separation Applications of Nanoporous Metal Organic Frameworks. CrystEngComm, 12, 1337-1353. 44 Appendix A 45 Appendix B Table 5. Atomic Charges of ZIFs. ZIF-25 ZIF-8 Atom Charge (e) Atom Charge (e) Zn +1.0219 Zn +1.0678 N -0.4973 N -0.4356 C1 0.4958 C1 +0.3009 C2 -0.0672 C2 +0.1248 C3 -0.2720 C3 -0.1216 H1 +0.0632 H1 -0.0507 H2 +0.1023 H2 +0.0134 ZIF-60 ZIF-10 Atom Charge (e) Atom Charge (e) Zn +0.8006 Zn +0.8952 N1 -0.3034 N1 -0.4839 N2 -0.2722 N2 -0.3421 C1 +0.0317 C1 +0.2288 C2 +0.1571 C2 +0.0899 C3 +0.0653 C3 +0.3896 C4 -0.0659 C4 -0.0773 H1 +0.0593 C5 -0.1633 H2 +0.0124 H1 +0.0512 H3 -0.0117 H2 +0.0069 H4 0.0574 H3 +0.0804 H4 +0.0477 46 ZIF-71 ZIF-96 Atom Charge (e) Atom Charge (e) Zn +0.9553 Zn1 +0.6342 N -0.1611 Zn2 +0.9203 C1 -0.4036 N1 -0.4847 C2 +0.1322 N2 -0.7835 H1 +0.2158 N3 -0.5257 Cl -0.1160 N4 -0.3627 C1 +0.1640 C2 +0.5131 C3 -0.1342 C4 +0.4781 H1 +0.1706 H2 +0.2882 ZIF-90 Atom Charge (e) Zn +0.8311 N1 -0.2964 C1 +0.5118 C2 +0.0844 C3 -0.0113 ZIF-97 H1 -0.0452 Atom Charge (e) H2 +0.0341 Zn +0.7797 O -0.5150 N1 -0.3800 N2 -0.2654 C1 -0.0008 C2 +0.0699 C3 -0.0024 C4 -0.1805 C5 +0.2866 H1 +0.1882 H2 +0.0596 H3 +0.3927 H4 +0.0011 O -0.6790 47 Appendix C Table 6. DREIDING Force Field Parameters of ZIF atoms. Atom (Å) ε (kJ mol-1) Zn 4.045 27.652 N 3.263 38.914 C 3.473 47.813 O 3.033 48.115 Cl 3.519 142.434 H 2.846 7.642 Table 7. Potential Parameters of Acetonitrile and Water. LJ Parameters and Charges Acetonitrile Water Site (Å) ε/kB (K) Charge (e) CH3 3.6 191 +0.269 C 3.4 50 +0.129 N 3.3 50 -0.398 O 3.151 76.42 -0.834 H 1 0 0.417 48 Bond Stretching Bond Bending rCH3–C= 4.16Å rC–N= 1.17Å θ˚∠H–O–H= 180˚ rO–H = 0.9572Å θ˚∠H–O–H= 104.52˚ [...]... on the use of ZIFs to meet the project objectives 11 4.4 Objectives The advent of The Great Acetonitrile Shortage in 2008 shows the industrial importance of recovering and recycling expended acetonitrile from aqueous solutions to circumvent possible future supply disruption from interfering with essential industrial and laboratory work The separation of acetonitrile from aqueous solutions using normal... 2008) From the myriad of combinations of metal cations and organic ligands that exist, a large variety of MOFs have been developed with various pore sizes, offering many research and industrial opportunities (Mueller, et al., 2006) Today, most experimental and theoretical research of MOFs has generally been geared towards gas storage and separation (Nalaparaju, Zhao, & Jiang, 2011) Potential use of MOFs... 11%, respectively, of the global acetonitrile market share as well (IHS Chemicals, 2014) 4.2 Current Production Methods of Acetonitrile The global acetonitrile production is estimated at around 73,500 to 80,000tonnes annually (NIIR Project Consultancy Services, 2014) Acetonitrile is industrially produced primarily via one of three methods, as a by-product of the SOHIO (Standard Oil of Ohio) propylene... inexpensive and not energy-intensive – by which to purify acetonitrile thus needs to be determined in order to meet the dual goals of attaining economical solvent consumption and increasing industrial ecofriendliness This project thus aims to discern the suitability and effectiveness of several ZIFs in the selective adsorption of acetonitrile from aqueous solutions through computer simulation, so that these... ZIF-97 exhibit the highest and lowest selectivities for acetonitrile- water mixtures, respectively In this section, we focus on the contours of acetonitrile in ZIF-8 and ZIF-97 30 Fig 8 Density Contours of Acetonitrile in ZIF-8 (top) and ZIF-97 (bottom) The unit of density scale is number of molecules per Å3 Fig 8 shows the density contours of acetonitrile in ZIF-8 and ZIF-97 In ZIF-8, at a low pressure,... Shorthand Label Chemical Structure ZIF-8 2-methyl imidazolate meIm C4H6N2 ZIF-10 Imidazolate Im C3H4N2 ZIF-25 dimethyl imidazolate dmeIm C5H8N2 Imidazolate Im C3H4N2 2-methyl imidazolate meIm C4H6N2 ZIF-71 dichloro imidazolate dcIm C3H2N2Cl2 ZIF-90 imidazole-2carboxyaldehyde icaIm C4H4N2O ZIF-96 cyanide amine imidazolate cyamIm C4H4N4 ZIF-97 hydroxymethylmethyl imidazolate hymeIm C5H8N2O ZIF-60 14 Ball-andStick... presented From adsorption isotherms, isosteric heats, and radial distribution functions, the role of functional groups is quantitatively assessed Then, the adsorption of pure water is discussed Finally, the separation of acetonitrilewater mixtures is examined and the highest selectivity is identified 6.1 Pure Acetonitrile 6.1.1 Adsorption Isotherms Fig 2 shows the adsorption isotherms of pure acetonitrile. .. Xu, Zhao, & Hickmott, 2010) A class of MOFs demonstrated promise in the realm of sorption technology is zeolitic imidazolate frameworks (hence forth called ZIFs), which contain tetrahedral Zn(II) atoms linked by imidazolate ligands, and their structures closely resemble zeolitic frameworks (Saint Remy, et al., 2011) ZIFs have gained considerable attention because of their tuneable porosity, structural... ammoxidation process, from the dehydration of acetamide, and by reacting acetic acid with ammonia at 400 to 500ºC in the presence of a dehydrated catalyst (National Centre for Biotechnology Information, 2014) 10 Notably though, apart from solely using the aforementioned methods, enduser companies are currently focussing on and looking for ways to recover and recycle acetonitrile out of its mixtures with... 𝜑𝑖𝑠𝑎𝑡 𝑥𝑖 𝛾𝑖 𝑒 where 𝑃𝑖𝑠𝑎𝑡 refers to the saturation pressure of species I estimated by The 𝑓𝑒𝑒𝑑 Antoine equation, 𝜑𝑖𝑠𝑎𝑡 the fugacity coefficient of species i, 𝑥𝑖 the mole fraction of species i in the liquid phase, 𝛾𝑖 the activity coefficient of species i estimated from the Non-Random Two Liquid (NRTL) excess Gibbs energy ̅𝑖 the partial molar volume of species i, and 𝑃 the operating pressure model, 𝑉 Under