Separation principle of xylene isomers and ethylbenzene with hydrogen-bonded host frameworks via first-principles calculation

Journal Pre-proof Separation Principle of Xylene Isomers and Ethylbenzene with Hydrogen-bonded Host Frameworks via First-principles Calculation Su Hwan Kim, Ju Hyun Park, Eun Min Go, Woo-Sik Kim, Sang Kyu Kwak

PII:

S1226-086X(20)30088-5

DOI:

https://doi.org/10.1016/j.jiec.2020.02.010

Reference:

JIEC 4970

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

31 December 2019

Revised Date:

6 February 2020

Accepted Date:

13 February 2020

Please cite this article as: Kim SH, Park JH, Go EM, Kim W-Sik, Kwak SK, Separation Principle of Xylene Isomers and Ethylbenzene with Hydrogen-bonded Host Frameworks via First-principles Calculation, Journal of Industrial and Engineering Chemistry (2020), doi: https://doi.org/10.1016/j.jiec.2020.02.010

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Separation

Principle

Ethylbenzene

with

of

Xylene

Isomers

Hydrogen-bonded

and Host

Frameworks via First-principles Calculation

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan

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1

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Su Hwan Kim1, Ju Hyun Park1, Eun Min Go1, Woo-Sik Kim2,*, and Sang Kyu Kwak1,*

2

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National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea Department of Chemical Engineering, College of Engineering, Kyung Hee University,

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Yongin 17104, Republic of Korea

*

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Corresponding author. E-mails:

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[email protected] (Sang Kyu Kwak), [email protected] (Woo Sik Kim)

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (S. K. K.)

*

E-mail: [email protected] (W. S. K.)

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Graphical abstract

ABSTRACT

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Separation of molecular isomers, which have similar physical properties, is hardly achieved with conventional separation methods based on phase equilibria. However, using selective

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inclusion of target molecules into dismantlable molecular framework allows molecular isomers to be effectively separated from one another. For that purpose, we consider the hydrogen-

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bonded organic framework (HOF), which can undergo solvent-mediated crystallization. Herein, we theoretically elucidated the separation mechanism of the mixture of xylene isomers (i.e., o-,

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m-, and p-xylene) and ethyl benzene (EB) using guanidinium (G) cation and organosulfonate anion (S) host systems (i.e., 2(G) + 4,4’-biphenyldisulfonate (G2BPDS) and 2(G) + 2,6-

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naphthalenedisulfonate (G2NDS) GS-host systems). Density functional theory (DFT)

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calculations were carried out to investigate separation mechanisms in terms of thermodynamics (i.e., formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy) and kinetics (i.e., surface energy) considering the solvent-mediated crystallization process. We theoretically predicted that G2BPDS system could effectively

separate EB from xylene isomers, and G2NDS system could separate each xylene isomer by sequential separation process.

KEYWORDS. Hydrogen-bonded organic framework (HOF), density functional theory (DFT)

INTRODUCTION

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calculation, xylene isomers, separation, thermodynamics, kinetics

Separation of mixture into the individual components is of great importance in chemical industry for purification, concentration, and classification.1,

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The conventional separation

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methods such as distillation, evaporation, adsorption, and extraction based on phase equilibria using different boiling points, melting points, and solubilities have been commonly used to

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separate constituents of homogeneous mixture.3 However, when physical properties such as

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those of molecular isomers are similar, it is difficult to separate them by the conventional methods. In case of mixed C8 alkyl-aromatic compounds of mixture, the separation of p-xylene

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from mixture of xylene isomers (i.e., o-, m-, and p-xylene) and ethylbenzene (EB) is a crucial part for Polyethylene Terephthalate (PET) manufacture.4 All components have the same

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chemical formula (i.e., C8H10), and are insoluble in water but soluble in non-polar solvents. Moreover, they have similar boiling points (138-141 °C). Therefore, it is more difficult to

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separate them by the conventional methods. A variety of chromatography techniques can be used to isolate isomers,5-7 particularly

separation techniques using selective adsorption or inclusion are required. The separation of xylene isomers has been attempted with metal-organic framework (MOF) with or without

intrinsic flexibility.8, 9 The theoretical approaches on the MOF system had been conducted using configurational-bias Monte Carlo (CBMC) and grand canonical Monte Carlo (GCMC) simulations. For instances, Torres-Knoop, A. et al studied separation mechanism of p-xylene from xylene isomers and EB using MAF-X8,10 and McDaniel and Schmidt developed a force field for zeolitic imidazolate frameworks, “ZIF FF”, which used for the investigation of adsorption of CO2/N2.11 Experimentally, Gu and Yan separated xylene isomers and EB using

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MIL-101, which is a chromium terephthalate MOF with coordinately unsaturated sites.12 However, the strong covalent and coordination bonds, which contributed to the robustness in MOF, have limited the control of the appropriate pore size and shape for practical use.13 On the other hand, the hydrogen bonds can stabilize hydrogen-bonded organic frameworks (HOFs)

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with relatively more flexibility. Also, HOFs show high potential for the separation of isomers

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in terms of the solution processability and regeneration through a simple recrystallization.14, 15 For example, Pivovar et al. investigated the selective separations of isomeric mixtures of

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xylenes (o-, m-, and p-xylenes) and dimethylnaphthalenes using a host system made of guanidinium organodisulfonate.16 HOFs are selectively made by solvent-mediated

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crystallization of host organic molecules, triggered by the inclusion of guest molecules.17 Crystallized HOFs are thermally stable, therefore those are easily separated from solution by

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simple purification processes.

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For HOFs considered in this study, we used guanidinium cations (G) and organosulfonate anions (S), which consistently form quasi-hexagonal lamellar structures, composed of hydrogen bonding between NH in (G) and O in (S).18-26 The cavity space for the guest molecules was offered by organic residue pillar of (S). Among GS-host systems, G2NDS (i.e., 2(G) + 2,6-naphthalenedisulfonate) and G2BPDS (i.e., 2(G) + 4,4’-biphenyldisulfonate) have

been reported to form bilayer and simple brick structures with xylene isomers,16, 19, 20, 27 bilayer structure of G2BPDS with xylene isomers, bilayer structure of G2NDS with p-xylene, and simple brick structure of G2NDS with o-xylenes. Thus, we speculated that C8H10 isomers (i.e., xylene isomers and EB) could be separated through a well-designed crystallization process, which incorporated them as guests in the GS-host systems. Overall, the challenging issues for efficient separations were to identify the critical factors that lead to HOFs crystallization in

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terms of principles of thermodynamics and kinetics. The most significant factors were considered to be the compatibility of the pore size and shape due to the interaction between the host and guest molecules, which is related to thermodynamic properties.28,

29

The kinetic

property of the crystallization, such as an induction time of seed formation, is also considerable 31

However, in-depth study on the

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factor for the separation of isomeric mixtures.30,

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thermodynamic and kinetic properties that affect the crystallization of such HOF systems, subsequently relating to the separation mechanism, has not been sufficiently conducted. In this

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regard, we performed theoretical study using density functional theory (DFT) calculations to scrutinize the thermodynamics and kinetics of crystallization of HOFs while the goal is to

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present plausible experimental paths of the separation of the C8H10 isomers.

CALCULATION DETAILS

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GS-host systems were constructed with bilayer G2BPDS and bilayer G2NDS in the [1 1 0] ×

[1̅ 1 0] × [0 0 2] supercell and simple brick G2NDS in the unit cell, respectively (Figure 1). First, directionalities of the guest molecules (i.e., inclusion mode) in the bilayer systems were considered. We constructed four types of structure (i.e., named each system as mode 1, mode

2, mode 3, and mode 4 (Figure S1a)). The structures from mode 1 to mode 3 were experimentally reported (i.e., mode 1 for G2BPDS(p-xylene), mode 2 for G2BPDS(m-xylene), mode 3 for G2BPDS(o-xylene))16, 19, 20, 27 and mode 4 was manually designed for this work. In mode 1 and mode 2, guest molecules are arranged parallel in each layer. Especially in mode 1, all guest molecules are arranged parallel in the same direction for all layers, and in mode 2, guest molecules of each layer are arranged parallel to each other in opposite directions. In mode

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3 and mode 4, guest molecules are reversely and alternatively arranged, respectively. In the case of G2BPDS(EB), where the bulk structure has not been reported, we further investigated four types of inclusion mode (i.e., from mode 5 to mode 8 in Figure S1b) to predict the

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positions of ethyl group.

The vacancy formation energy was calculated with the defective system, where one guest

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molecule was removed from the global minimum energy configuration of the GS-host system with all guest molecules incorporated. It is noted that the removal of host molecules (i.e., G,

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BPDS, and NDS) was not considered to avoid structural collapse of the GS-host system. To calculate the surface energy, we adopted the (0 0 1) surface, which shows the least breaking of

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hydrogen bonds between host molecules (Figure S2). Considering non-polar surface and the stoichiometry of GS-host system, guanidinium molecules were distributed on the top and

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bottom surfaces for the simple brick G2NDS(3o-xylene). The vacuum slab of 20 Å was applied

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along the [0 0 1] direction and all layers of the slab system were fully optimized. DFT calculations were carried out using Cambridge Serial Total Energy Package (CASTEP) program.32 Semi-empirical Tkatchenko-Scheffler (TS) scheme33 was used for the dispersion correction. Generalized gradient approximation (GGA) with PBE34, 35 functional was applied

with norm-conserving pseudopotentials.36 The energy cutoff was set to be 750 eV for all systems. The Broyden–Flecher–Goldfarb–Shanno (BFGS) algorithm37 was used for the geometry optimization. Convergence threshold for the geometry optimization and SCF tolerance were set to 1  10-5 eV/atom and 1  10-6 eV/atom, respectively. The convergence precision of geometry optimization for the maximum force, displacement, and maximum stress were 0.03 eV/Å, 0.001 Å and 0.05 GPa, respectively. Monkhorst-Pack38 k-point grid was set

of 1  2  1 was used for the bilayer G2NDS systems.

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to 2  1  1 for the G2BPDS systems and a simple brick G2NDS system while the k-point grid

Formation energy, interaction energies of guest-host and guest-guest, vacancy formation energy,

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and surface energy were obtained by following methods. First, we calculated the formation

equation,

ESystem  2n  EG  n  ES  m  EGuest 3n  m

(1)

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Ef 

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energy (Ef) of the GS-host system with all guest molecules incorporated using the following

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where ESystem, EG, ES, and EGuest are the total energy of the system, that of isolated guanidine, sulfonate, and the guest molecule, respectively, and n and m indicate the number of sulfonate

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molecules (i.e., BPDS and NDS) and that of guest molecules (i.e., xylene isomers and EB),

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G-H G-G respectively. Interaction energies of guest-host (i.e., Eint ) and guest-guest ( Eint ) were

calculated by the following equations, G-H Eint  ESystem  ED. Host  ED. Guest (2)

ED. Guest  m  EGuest (3) m

G-G Eint 

where ED.

Host

and ED.

Guest

indicate the total energy of system, where the guest and host

molecules were removed, respectively. To obtain the interaction energy of guest-host per formula unit, calculated energy values were divided by 4 and 2 for the bilayer and simple brick systems, respectively. Vacancy formation energy (EVac.) was calculated by the following

EVac.  EVac.  EX  ESystem

( X  Guest molecule) (4)

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equation,

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where EVac. is the total energy of the system with a molecular vacancy and EX is the energy of isolated guest molecule X (i.e., xylene isomers and EB). Lastly, surface energies (ESurface)

1 ( ESlab  EBulk ) (5) 2 ASurface

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ESurface 

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were calculated by the following equation,

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where ESlab and EBulk are total energies of surface slab and bulk system, respectively, and ASurface is the area of the surface.

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RESULTS AND DISCUSSION

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1. Structure of guest molecules in G2BPDS systems To figure out the structural selectivity of the C8H10 isomers in bilayer G2BPDS system, we calculated formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy for each guest molecule (see Calculation details section). First, global

minimum structures of each bilayer G2BPDS system with guest molecule (i.e., o-, m-, p-xylene, and EB) were determined considering the lowest formation energy among the inclusion modes of guest molecules (Figure S1). The bilayer G2BPDS system containing each guest molecule with the lowest formation energy (i.e., G2BPDS(guest) system) are shown in Figure 2a. Based on the formation energy (Figure 2b), the global minima of G2BPDS(o-xylene), G2BPDS(mxylene), G2BPDS(p-xylene) and G2BPDS(EB) were found in mode 3, mode 1, mode 1 and

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mode 7, respectively (see Table S1 for their structural parameters). In case of G2BPDS(oxylene), mode 1 and mode 2, where the guest molecules are arranged in parallel, have relatively high formation energies, weak interaction energies of guest-host and guest-guest (Figure 2c). This is due to the steric hindrance of two adjacent methyl groups of o-xylene. While in

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G2BPDS(m-xylene) and G2BPDS(p-xylene), each formation energy of mode 1 is the lowest

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because the guest-host interaction is the most favorable. It is noteworthy that the formation energy of G2BPDS(EB) was the highest among the global minima of bilayer G2BPDS systems,

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which indicates the thermodynamic selectivity of EB is lower than xylene isomers in the crystallization process with G2BPDS. In the same vein, G2BPDS(p-xylene) is expected to have

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the highest selectivity due to its lowest formation energy among global minima structures of bilayer G2BPDS systems. Furthermore, the vacancy formation energy of guest molecule was

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investigated, which also can be an indicator of selective separation (Figure S3). The higher vacancy formation energy, the more vacancy is hard to form in the G2BPDS(guest) system,

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indicating the crystallized G2BPDS(guest) system is more thermodynamically stable. The G2BPDS(p-xylene), which showed the highest vacancy formation energy, has the highest thermodynamic stability among the bilayer G2BPDS(guest) systems. On the other hand, the G2BPDS(EB) exhibited the lowest vacancy formation energy, indicating that G2BPDS(EB) has

less selectivity due to the lowest stability. Therefore, in the viewpoint of thermodynamics considering the formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy, we expect that the separation of EB or p-xylene from the mixture containing C8H10 isomers is possible via crystallization process with G2BPDS system.

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2. Structure of guest molecules in G2NDS systems In G2NDS systems, we also calculated the same types of energies presented above to investigate the structural selectivity of the C8H10 isomers. The G2NDS systems were known to form bilayer and simple brick structures with p-xylene and o-xylene, respectively (Figure 3a).

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For the bilayer G2NDS system with p-xylene, mode 1 was determined to be the global

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minimum structure considering the lowest formation energy (Figure 3b). Compared with the simple brick G2NDS(3o-xylene), the formation energy of bilayer G2NDS(p-xylene) was much

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lower than that of simple brick G2NDS(3o-xylene). In case of bilayer G2NDS system with pxylene, the lower the interaction energies of guest-host and guest-guest, the lower the formation

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energy was obtained as shown in Figure 3c, which is similar to the tendency of the thermodynamic properties of bilayer G2BPDS systems. However, in case of simple brick

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G2NDS(3o-xylene), the relation between formation energy and interaction energies of guesthost and guest-guest is different from bilayer G2BPDS systems. The simple brick G2NDS host

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system was relatively less stable than bilayer G2NDS host system (i.e., 0.55 eV/formula unit). Due to the instability of the simple brick host system, the formation energy of simple brick G2NDS(3o-xylene) is much higher than that of bilayer G2NDS(p-xylene) even if the simple brick G2NDS(3o-xylene) represented the lowest interaction energies of guest-host and guest-

guest. Based on the results, the selectivity of p-xylene is higher than that of o-xylene in the crystallization process using G2NDS system. However, the vacancy formation energy of guest molecule in the bilayer G2NDS(p-xylene) was lower than that in the simple brick G2NDS(3oxylene) (Figure S4). In a viewpoint of thermodynamics, the relation between formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy is quite competitive in the G2NDS system. Therefore, the kinetics of crystallization was considered to

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predict the selective separation. The induction time of GS-host system with each guest molecule incorporated were additionally estimated.

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3. Induction time of GS-host system

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The induction time of crystallization, which is closely related to the kinetics of the crystallization, depends on the surface energy of the seed of crystal.31, 39 The low surface energy

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of crystal reduces the induction time of crystallization. Thus, we calculated the surface energies of the global minima of GS-host systems with each guest molecule incorporated to consider

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the kinetics of the crystallization (Figure 4). For the G2BPDS systems, the highest surface energy was shown in the G2BPDS(EB), indicating that EB is not favorable to be crystallized

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with G2BPDS system and has a low selectivity in mixture of C8H10 isomers in terms of thermodynamics and kinetics. However, for the G2NDS systems, surface energy of bilayer

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G2NDS(p-xylene) was more stable than simple brick G2NDS(3o-xylene). It represents the higher selectivity and stability of bilayer G2NDS(p-xylene) compared to simple brick G2NDS(3o-xylene). This phenomenon might come from structural feature of bilayer crystal, which can separate into layers easily. Therefore, G2NDS(p-xylene) will be majorly formed

when xylene isomers were crystallized with G2NDS system because of its stability and short induction time.

4. Separation strategy of C8H10 isomers Based on the calculation results, we suggest the separation strategy of the mixture of C 8H10

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isomers (Figure 5). Mixture containing four C8H10 isomers can be separated by a consecutive crystallization process. First, since EB shows the lowest selectivity and longest induction time among the C8H10 isomers, G2BPDS system eliminates EB by crystallization with the other isomers (Figure 5a). Subsequently, G2NDS system can separate p-xylene from o-xylene and

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m-xylene due to its high selectivity and short induction time (Figure 5b). In this step,

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G2NDS(p-xylene) will be majorly formed because of its better stability and short induction time over G2NDS(3o-xylene). After that, o-xylene can be separated from m-xylene, which can

CONCLUSION

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hardly be crystallized with G2NDS system (Figure 5c).

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We performed DFT calculations to study the crystallization mechanism of GS-host systems with xylene isomers (i.e., o-, m-, and p-xylene) and EB incorporated in thermodynamic and

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kinetic point of view. We proposed the separation mechanism of C8H10 isomers using G2BPDS and G2NDS systems. In terms of thermodynamics, the formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy were calculated related with the selectivity. The surface energy associated with induction time in crystallization was calculated

as a kinetics factor. We found that EB can be effectively separated from xylene isomers using G2BPDS system with high selectivity and long induction time. On the other hand, xylene isomers can be sequentially separated using G2NDS system as the following procedure. First, the p-xylene, which has the highest selectivity with G2NDS system, can be separated as a bilayer G2NDS(p-xylene) crystallization. It is noted that p-xylene has relatively short induction time compared to o-xylene when crystallized with G2NDS system. After that, o-xylene can be

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separated as the simple brick G2NDS(o-xylene), and m-xylene remains in the form of a residual liquid because it cannot be crystallized with G2NDS system. In conclusion, this study provides the guidelines for determining the selectivity of the guest-host systems via theoretical predictions. We expect that the theoretical results suggest the suitable GS-host system for

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separating isomer molecules by proving the efficient separation mechanism without

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experimental crystallization process.

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Notes

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The authors declare no competing financial interest.

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Declaration of interests

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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.

ACKNOWLEDGMENT

This work was supported by the National Research Foundation of Korea (NRF2014R1A5A1009799). Computational resources have been provided by UNIST-HPC and

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KISTI (KSC-2019-CRE-0066).

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Figure 1. (a) GS-host structures of bilayer G2BPDS, bilayer G2NDS and simple brick G2NDS system.

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(b) Host molecules for GS-host system and (c) guest molecules. Grey, white, yellow, blue and red colors represent carbon, hydrogen, sulfur, nitrogen, and oxygen atom, respectively. Light grey and cyan

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dashed lines represent the lattice cell and hydrogen bonds, respectively.

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Figure 2. (a) Optimized structures of global minima of each bilayer G2BPDS system with guest molecule (i.e., o-, m-, p-xylene and EB). The guest molecule of each system is indicated in the inset

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box. Grey, white, yellow, blue and red colors indicate carbon, hydrogen, sulfur, nitrogen, and oxygen atom, respectively. Light grey and cyan dashed lines represent the lattice cell and hydrogen bonds,

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respectively. (b) The formation energy, interaction energies of (c) guest-host and (d) guest-guest of

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each inclusion mode of the guest molecules.

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Figure 3. (a) Optimized structures of global minima of bilayer G2NDS(p-xylene) and simple brick

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G2NDS(3o-xylene). The guest molecule of each system is indicated in the inset box. Grey, white, yellow, blue and red colors indicate carbon, hydrogen, sulfur, nitrogen, and oxygen atom, respectively. Light grey and cyan dashed lines represent the lattice cell and hydrogen bonds, respectively. (b) The formation

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energy, (c) interaction energies of guest-host and (d) guest-guest of each inclusion mode of the guest

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molecules.

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Figure 4. The surface energy of each GS-host system with different guest molecules. The bilayer

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xylene and 3o-xylene, respectively.

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structure of G2NDS with p-xylene, and simple brick structure of G2NDS with o-xylene denoted as p-

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Figure 5. The schematics of the separation mechanism of the C8H10 isomers (i.e., xylene isomers and EB). (a) The separation of the EB from the mixture of C8H10 isomers using G2BPDS system. (b) The

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separation of p-xylene from the mixture of xylene isomers using G2NDS system. (c) The separation of o-xylene from mixture using G2NDS system. The o-xylene, m-xylene, p-xylene, EB, G, BPDS, and

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NDS were colored green, red, pink, grey, light blue, orange, and yellow, respectively.