Computer Simulatio Co2 Ch4 Adsorption

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Chinese Journal of Chemical Engineering, 19(5) 709716 (2011)

Computer Simulation of Adsorption and Separation of CO2/CH4 in

Modified COF-102*

ZHU Yujun(), ZHOU Jianhai (), HU Jun (), LIU Honglai ()**andHU Ying ()The State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Sci-ence and Technology, Shanghai 200237, China

Abstract Covalent orgnic framework (COF) isa porous material with low density and arge BET (Brun-auer-Emmett-Teller) surface area. They have great potential in gas adsorption and separation. In this work, the ad-sorption of pure CO2and CO2/CH4mixture on modified COF-102 was simulated by using GCMC (grand canonicalMonte Carlo). Metal Li was incorporated into COF-102 through three doping methods, including charge exchange,O

-Li+dipolar interaction and O

-Li+chemical bonding. The influence of Li doping on the adsorption of CO2was

studied. The results showed that among the three methods, the dipole doping is the best way to improve CO 2ad-sorption performance. Further, the ligands of COF-102 were replaced by extended aromatic moieties, such as di-phenyl and pyrene. The adsorption capacity of CO2and CH4, and the selectivity of CO2/CH4on the ligand-replacedCOF-102 were studied. The capacity of CO2and CH4on the ligand-replaced COF-102 had obvious changes; hence

the selectivity of CO2/CH4can be adjusted accordingly.Keywords gas adsorption, computer simulation, COF-102, Li doping, ligand replacing

1 INTRODUCTION

Meteorological changes caused by emission ofgreen house gases have been receiving significant at-tention. To maintain the normal climate, the CO2con-centration should be below 450 ulL

1[1]. However, it

is likely that the world will continue to rely on fossilfuels as the primary energy supply for a long period.CO2capture, usage and storage (CCUS) is potentiallyan effective way to reduce CO2emissions. Extensive

studies have been carried out worldwide on developingcost-effective techniques to capture CO2, among themthe adsorption is one of the most promising methods.

The commercial molecular sieves, such as zeo-lite 13X, 5A, and active carbons, possess high adsorp-tion quantity, but their tolerance to the moisture is low[2]. A lot of novel adsorbents such as metal organicframeworks (MOFs), zeolitic imidazolate frameworks(ZIFs) and covalent organic frameworks (COFs) havebeen designed and fabricated. With the high BET sur-face area, controllable pore structure and pore volume,most of them have showed good performance as thecandidates for CO2 adsorption, especially at highpressure [3]. In recent years, Yaghi and his group made

outstanding achievements in fabrication these novelmaterials [4]. For two- and three-dimensional COFsmaterials, they derived a linear relation between porecapacity and adsorption quantity [5]. Meanwhile, thesimulation work of CO2 adsorption on MOFs andCOFs also achieved great progresses. Yang et al. [6]systematically summarized the computer simulation inmetal organic frameworks in a recent review. Keskinet al. [7] also gave a review on the adsorption and

separation properties of MOFs.Metal doping is a convenient way to enhance the

gas adsorption quantity and selectivity. Mulfort et al.[8] prepared the three dimensional MOF with metalsof Li, Na and K doped and found that the adsorptionquantities of H2 and N2 have an obvious increase.Nouar et al. [9]also found that after Li and Mg ionsexchange, the adsorption quantity of H2on metal-ZIFsis increased. Based on the phenomena of metal ionsexisted as hydrates inside the framework, they con-

cluded that the promotion of the framework electro-static potential is responsible for the adsorption quan-tity increase. The simulation work can reveal moreclearly the mechanism of the gas adsorption inmetal-doped MOF and COF. Babarao and Jiang [10]investigated how the ZIF doped with Na

+ influences

the adsorption and separation of CO2/H2, CO2/CH4and CO2/N2in comparison with the result before dop-ing, in which the selectivity is raised after doping. Xuet al. [11] inserted lithium atoms into MOF-5 frame-work through chemical bonding, also detected a greatenhancement of selectivity of CO2due to the electro-static potential provided by the metal atom inside theframework. Cao et al. [12]studied H2adsorption in a

three dimensional COF through Li doping, surpassingthe criterion of hydrogen storage in 2010 proposed byDOE (department of energy) (6%, by mass) for thedoped COF. Klontzas et al. [13]analyzed the adsorp-tion quantity of H2on the COF-105 doped with lith-ium, and also found an improvement beyond the crite-rion of DOE target in 2010. There are so many re-searches on the metal doping in MOFs and COFs thata comparison for them will be extremely demanded,

Received 2011-06-10, accepted 2011-07-22.* Supported by the National Natural Science Foundation of China (20736002), the National High Technology Research and

Development Program of China (2008AA062302) and Program for Changjiang Scholars and Innovative Research Team in

University of China (IRT0721).** To whom correspondence should be addressed. E-mail: [email protected]


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Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011710

which can provide meaningful information for deepunderstanding and optimization of the metal doping.

Besides the metal doping, ligand replacing is alsoconsidered as an effective way to promote the proper-ties of framework materials. The main factors affect-

ing the ability of storing hydrogen in the case of phy-sisorption are surface area, pore volume, and enthalpyof adsorption. These factors can be improved by ex-tended aromaticity, unsaturated metal sites, and pointcharges in the framework of the proposed materials.With different ligands, IRMOF (isoreticular metalorganic framework) series [4] are the most famousMOFs. Meanwhile, ZIF series [14]with different cageframeworks are obtained by ligand replacing. Klont-zas et al. [15] simulated H2 adsorption capacity inCOF-102s with ligand replacing by differentmulti-phenyl group compounds, and the adsorptionquantity of H2overtakes 25% (by mass) at 77 K.

In this work, we study the CO2 adsorption per-

formance of the modified 3-D framework materialCOF-102 by metal doping and ligand replacing.COF-102 is a framework with a composition of car-bon, oxygen and boron atoms based on triangular andtetrahedral nodes formed by these atoms. We first es-tablish and optimize the models of the 3-D COF-102sframework structure. Metal Li is doped in three dif-ferent ways, charge exchange, dipolar interaction andchemical bonding. Besides, ligand of phenylene moie-ties is replaced by extended aromatic moieties includ-ing diphenyl and pyrene. The adsorption behavior ofmixture of CO2and CH4on these model materials isthen simulated. Based on the results, we will propose

some general rules to optimize the modification offramework type materials for the gas adsorption ap-plications.

2 MODEL AND METHODOLOGY

In the simulation process for adsorption on COFs,the interactions between adsorbate and adsorbent, aswell as adsorbates themselves are described by thecombinations of Lennard-Jones (LJ) potential andCoulombic potential:

1/ 2 6

0

( ) 44ij i

j

q qu r

r r r

where 0 8.85421012

C2N1

m2

is the vacuum

diffusivity; and are the depth and length of thepotential, respectively. The inner quadrupolar moment

of CO2can be characterized by endowing carbon andoxygen atoms with different charges of 0.576e and

0.288e, respectively, and the bond length of C O is1.18A. The TraPPE force field [16] is used to depict

these atoms. The universal force field (UFF) [17]with

geometrical combinational rule of ij i j and

ij i j is adopted for the LJ interaction between

framework and adsorbates. United atoms of TraPPE

force field [18]can be employed to picture the force fieldof CH4. As shown in Fig. 1, the structures of COF-102

and COF-108 built from cif files are downloaded fromCCDC database through Material Studio 4.3 [19].

Figure 1 Unit cells of COF-102Key: grey, carbon; black, oxygen; white, hydrogen; light grey,

boron

Table 1 The force field constants and chargesused to describe adsorbates

/nm (/kB)/K q

CH4 0.373 148.0 0

C(in CO2) 0.280 27 +0.576e

O(in CO2) 0.305 79 0.288e

The uncountable amounts of the atoms in theframework of COF make it difficult to adopt the first

principle to do the precise calculation. In order to ob-tain the data of atomic charge in the framework, thecluster model is usually used to calculate the probablecharge distribution of the whole structure. Here, weadopted Dmol

3module of the cluster model to calcu-

late the ESP (electrostatic scalar potential) charge byusing GGA (general gradient appoximation) and PBE(Perdew-Burke-Ernzerh) functions, with DNP (doublenumerical plus polarization) basis and a cutoff radiusof 0.55nm.

Grand canonical Monte Carlo (GCMC) methodis used to simulate the gas adsorption in the modifiedthree-dimensional COFs. Ewald method is adopted to

calculate the sum of electrostatic interaction betweendifferent atoms in the framework, in which, the cutoffradius is 1.28 nm and the computational precision is0.001 kcalmol

1. The total simulation steps are 210

7,

and the first 1107steps are for making the system in

equilibrium, while the remaining is used for obtainingdifferent thermodynamic properties. The selectivity ofgas A to gas B in a mixture is defined as S(xA/xB)(yB/yA), wherexAandxBare the molar fractionsof A and B in the adsorptive phase, respectively, yAand yB are the corresponding molar fractions in thebulk adsorbate phase. Similar simulations are alsoheld for the unmodified COFs to ascertain the effec-tiveness of the proposed modifications to enhance gas

adsorption.


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3 RESULTS AND DISCUSSION

3.1 COF-102 doped with Li ions

The classical simulation method of the metal

doping is by introducing the metallic atoms into theframework of porous materials, then by exchanging

charges between the metallic atoms and the frame-work atoms to maintain the neutrality of the whole

structure. It is easy to understand and operate for thecomputer simulation, but for realistic synthesis, it is

very hard and almost impossible. Mavrandonakis et al.[20]reported a novel methodology by introducing nega-

tive charge groups, such as1

3SO , in the ligands,

then metallic cations can be reasonably doped into theframework to keep the neutrality of the whole structure.

Due to the large size of the COF-102 cell, as

shown in Fig. 1, the model system must be reduced insize. The reduction can be achieved reasonably bytreating the organic ligand as an individual system. Liis doped into the organic ligand cluster model in threedifferent ways for comparison.

3.1.1 Charge exchange dopingFigure 2 shows the cluster model by the simple

charge exchange doping denoted as Li-COF-102-exchange. The charge balance between ligand and Liatom mainly depends on the charge exchange. Thesimulation result shows that the exchangeable chargebetween Li and the framework is 0.628e. In Fig. 2,charges of each atom in Li-COF-102-exchange cluster

model by the charge exchange doping are marked.

Figure 2 ESP charges of each atom in Li-COF-102-exchange cluster model by the charge exchange doping

Key: grey, carbon; black, oxygen; white, hydrogen; light grey,boron; lithium is inside the 6C ring with the charge of 0.628

3.1.2 O-Li

+dipolar interaction doping

When a hydrogen atom of the ligand of COF-102is replaced by a negative oxygen ion, the frameworkpossesses negative charges. Thus a certain amount oflithium cations can be doped into the framework tocounterpoise the negative charges of the whole struc-ture, and the neutrality is kept. The cluster model bythis kind of O-Li

+dipolar interaction doping as shown

in Fig. 3 is denoted as Li-COF-102-dipole.Through the dipolar interactions between Li

cations and the framework [20], Li doping providesmore active sites in the framework, and consequently

increases the adsorption ability. Because the negativecharges of the framework come from the introduction

of negative oxygen ions, the exchange charge betweenLi and the framework is small enough to be ignored,and the charge of Li cations is still considered as +1.

3.1.3 O-Li

+chemical bonding doping

Not only dipolar interactions occur between Lications and the negative charged framework, but alsothe chemical bonding can form between O

and Li

+

when a hydrogen atom of the ligand of COF-102 isreplaced by a negative oxygen ion. As shown in Fig. 4,the O

-Li

+chemical bonding in the framework can be

simulated as one hydrogen atom of the ligand ofCOF-102 replaced by an O

-Li

+ group, and the neu-

trality of the whole structure still remains. Because Lications now are joined into the whole frameworkstructure, the charge of Li cations would not be +1 anymore. After the optimized calculation for the chargedistribution by Dmol3, the charge of lithium cations isturned out to be 0.790. We denote this kind ofCOF-102 as Li -COF-102-bond.

Figure 4 ESP charges of each atom in Li-COF-102-bondcluster model by the O-Li+chemical bonding dopingKey: grey, carbon; black, oxygen; white, hydrogen; light grey,boron; lithium is linked by an oxygen black ball with thecharge of 0.790

The unit cell of three kinds of Li doped COF-102sis shown in Fig. 5. All the three framework structures

of Li-COF-102s are the same except the differentdoped positions of Li atoms or cations. However, the

Figure 3 ESP charges of each atom in Li-COF-102-dipolecluster model by the O-Li+dipolar interaction dopingKey: grey, carbon; black, oxygen; white, hydrogen; light grey,boron; lithium is the ball beside COF-102 with the charge of1.00


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effects of Li doping on the adsorption capacity of CO2by three different ways are quite different. As shownin Fig. 6, compared with the original COF-102, theadsorption capacity of CO2on all the three Li dopedCOF-102s increase obviously. Among them, the dopingmethod through dipolar doping and bonding dopingare notably better than others, while the former is the

best when the pressure is less than 100 kPa as shownin the insert diagram. As pressure increases, the adsorp-tion capacities of CO2on Li doped COF-102s throughdipolar doping and chemical bonding doping ways aresimilar, much higher than that through charge ex-change doping. When the pressure is as high as 1000kPa, three curves tend to merge with each other.

To understand the adsorption mechanism, the ad-sorption process can be divided into two stages withdifferent control parameters: solid-fluid interactionsand fluid-fluid interactions [21]. At low pressure, theinteraction between the positive charged Li cationsand the negative charged O of the quadrupolar CO2is

the dominant effect on the adsorption capacity en-hancement; whereas at high pressure with the density

of CO2increasing, the interaction between CO2mole-cules and the free volume of framework turn to becontroller.

For the dipolar interaction doping method, theintroduction of the negative oxygen ions into theframework make less charge exchange between Lications and the framework, and Li cations with the

positive charge of about +1 become the most activesites among three different Li-COF-102s. Conse-quently, at low pressure, the strong interaction be-tween the quadrupolar CO2 and Li-COF-102-dipoleyields higher adsorption capacity. On the contrary, forthe charge exchange doping method, the charge of Liatoms and the number of Li atoms in a unit cell are thelowest among three different Li-COF-102s. With theweakest interaction between Li atoms and CO2mole-cules, the adsorption capacity of Li-COF-102-exchangeis obviously low. For the sample of Li-COF-102-bond,the doping method reduces the charge of active Lisites more or less, and the moderate interaction weak-

ens the CO2adsorption capacity a little bit.Since the volume of Li atom or cation is so small,

(a) (b) (c)

Figure 5 Unit cells of Li-COF-102-exchange (a), Li-COF-102-dipole (b), and Li-COF-102-bond (c)

Figure 6 Adsorption capacity of CO2on three kinds of metal doping Li-COF-102s in comparison with the original COF-102the insert: the low pressure behaviorLi-COF-102-exchange; Li-COF-102-bond; Li-COF-102-dipole; COF-102


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no matter which ways is adopted, the free volume ofunit cell does not change so much. At high pressure, theadsorption capacity depends much on the free volumeof the COF-102, hence, the enhancement caused by allthree kinds of doping methods tends to be the same.

3.2 The replacement of ligands of COF-102

Extending aromaticity is another way to enhancethe surface area and pore volume of the framework

structure materials. We substitute the phenylene moie-ties of COF-102 by other extended aromatic moietiessuch as diphenyl and pyrene molecules withoutchanging the net topology, as shown in Fig. 7. Thecorresponding COF-102s after replacement is denoted

as di-COF-102 and py-COF-102.The optimized structures of di-COF-102 andpy-COF-102 are presented in Fig. 8. The properties ofthe crystal cells are listed in Table 2. Besides, the cellsare simplified to the cluster models as shown in Fig. 9by treating the organic ligand as an individual system.

(a) (b)

Figure 7 Structures of diphenyl (a) and pyrene moieties (b)Key: grey, carbon; white, hydrogen

(a) (b)

Figure 8 Unit cells of di-COF-102 (a) and py-COF-102 (b)Key: grey, carbon; black, oxygen; white, hydrogen; light grey, boron

Table 2 Properties of di-COF-102 and py-COF-102

Material Specific area/m2g1 Mass/gmol1 Unit cell volume/nm3 a b c/nm Free volume /cm3g1 Porosity

di-COF-102 5005.9 8736 76.7683 4.25005 4.71 0.890

py-COF-102 4865.2 10570 77.2786 4.25945 3.95 0.899

COF-102 4434.5 5083 20.0729 2.71771 1.81 0.762

(a) (b)

Figure 9 ESP charges of simplified clusters models for di-COF-102 (a) and py-COF-102 (b)

Key: grey, carbon; black, oxygen; white, hydrogen; light grey, boron


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ESP charge of the cluster models of the di-COF-102and py-COF-102 are calculated by Dmol

3module.

The unit cell volume and the free volume ofdi-COF-102 and py-COF-102 are increased obviouslycompared to those of original COF-102 due to the

much bigger size of the moieties. However, the freevolume of py-COF-102 is smaller than that ofdi-COF-102 due to the fact that pyrene molecule itselfoccupies too much volume. Meanwhile, both specificarea and porosity also increase, which is favorable forthe improvement of adsorption quantity. The replace-ment of ligands also has great influence on the chargeof the central carbon atom, it is 1.788 and 0.801,respectively for di-COF-102 and py-COF-102, muchlower than 4.464 in the original COF-102 [22]. Con-taining one more aromatic -cloud in diphenyl andpyrene, the charge of the central carbon atom can bedistributed more dispersedly among the electronclouds of multi-aromatic rings.

Figure 10 shows the corresponding profiles ofadsorption isotherms of CO2 and CH4 on COF-102sbefore and after moiety replacement. Among all thethree COF-102s, di-COF-102 shows the best CO2andCH4adsorption performance, while py-COF-102 doesnot show much increase in CO2 adsorption quantitycompared with COF-102, even more, it descends alittle at high pressure although it has a little enhance-ment for the adsorption quantity of CH4. These resultscoincide well with the H2adsorption results reported

by Klontzas et al. [15]. For the quadrupolar CO2molecule adsorption, the adsorption depends on theinteractions between CO2 and framework when thepressure is low. After the replacement of ligands, thetopological structure does not change so much, so the

quantities of the adsorption are similar with each other.However, at high pressure, it depends much on theinteractions between CO2molecules themselves, wherethe free volume is the dominant factor. The higher thefree volume, the higher the adsorption quantity. Amongthe three COF-102s, di-COF-102 has the largest freevolume, improving the CO2 adsorption capacity dra-matically. When the pressure is 4000 kPa, the CO2adsorption quantity on di-COF-102 is almost twicethat of the original one. Since the unit of capacity is inper gram of adsorbents, higher relative molecularmass of the adsorbent will reduce the adsorption ca-pacity somehow. The large pyrene moiety makes themolar mass of py-COF-102 almost double to the

original COF-102; so the enhancement of adsorptionquantity caused by the free volume is counteracted bythe increase of molecular mass, the adsorption quan-tity of CO2 in py-COF-102 does not raise so much.Different from the CO2 adsorption mechanism, thepacking effect plays an important role in the CH4ad-sorption. The adsorption capacity of CH4 is mainlydetermined by the free volume, as well as the numberof cross central sites (deposit sites) in the pore. For amixture of CO2and CH4gases, the preferably adsorp-tive CO2can provide more deposit sites for CH4. Thehigher adsorption quantity of CO2on di-COF-102 alsoyields higher deposit density of CH4. Hence, the ad-

sorption isotherm of CH4 on di-COF-102 increaseslinearly with the pressure. Although the adsorptionquantity of CO2 in py-COF-102 does not change somuch, the large pyrene moiety can supply more zigzagdeposit sites for CH4, as a result, the overall adsorp-tion quantity of CH4on py-COF-102 increases a littlecompared with that of the original COF-102.

Because of the above various phenomena, theadsorption selectivity based on the adsorption capaci-ties of CO2and CH4becomes complex. As shown inFig. 11, the adsorption selectivity of CO2/CH4 onligand replaced COF-102s are lower than that of(a)

(b)

Figure 10 Adsorption isotherms of CO2 (a) and CH4 (b)on COF-102s before and after moiety replacementdi-COF-102;py-COF-102;COF-102

Figure 11 Adsorption selectivity of CO2/CH4on COF-102sbefore and after moiety replacementdi-COF-102;py-COF-102;COF-102


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original one, among them, di-COF-102 is the lowest.It holds almost as a constant at whatever pressure,which indicates that the enhancement of the adsorp-tion capacities of CO2and CH4on di-COF-102 has afixed linear relationship. However, the selectivity of

py-COF-102 decreases a little with the increase ofpressure. Duren and Snurr [23] reckoned that smallchannels can enhance the selectivity, while theenlargement of channel would decrease the selectivity.The pore size of COF-102 is smaller than that ofdi-COF-102 and py-COF-102, so the selectivity ofCOF-102 is higher. Besides, the adsorption capacity ofCH4is mainly determined by the free volume and thenumber of cross central sites in the pore, so COF-102have the smallest free volume among these three kindsof COFs and the smallest adsorption quantity of CH4between 0 kPa to 4000 kPa, which contributes to thehighest selectivity in comparison with di-COF-102and py-COF-102 in this range.

4 CONCLUSIONS

COFs have great potential in the gas adsorptionand separation. By computer simulation based onGCMC, two modification approaches of Li dopingand ligand replacing were adopted to enhance the CO2adsorption performance on COF-102s. First, metal Liwas doped in three different ways, i.e., charge exchange,dipolar interaction and chemical bonding. Among them,the dipolar interaction doping method by introducingnegative oxygen ions into the framework resulted in

less charge exchange between Li cations and theframework, therefore, the higher positive charged Lications improved the CO2adsorption capacity mostlyat lower pressure. Besides, the ligand of phenylenemoiety was replaced by extended aromatic moieties ofdiphenyl and pyrene. At high pressure, the adsorptioncapacity of CO2 on di-COF-102 had an obvious in-crease due to its larger free volume and surface area,so the capacity of CH4 had an increase even more,which made the selectivity of CO2/CH4decrease.

The simulation results reveal that there are twostages in gas adsorption in these 3D COFs. At lowpressure, the solid-fluid interactions dominates, so themetal dipolar interaction doping method, which is re-

alized by introducing the anions in the framework first,can effectively improve the CO2adsorption perform-ance. At high pressure, the fluid-fluid interactionsplays a control role, the free volume is then the mostimportant parameter. Consequently, the ligand replac-ing by the suitable moiety can obviously increase theadsorption quantity of single component. However,the selectivity depends on the properties of gaseousmixtures; especially those with the adsorption mecha-nism differed from component to component.

NOMENCLATURE

a, b, c unit cell parameters of COF-102, nm

N loading in adsorbent, mmol/g

p pressure, kPa

q partial charge

r interaction potential

S selectivity

u distance between atoms energy parameter of LJ potential, J

diameter parameter of LJ potential, nm

Subscripts, mark of single molecule

i,j mark of molecular types

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Computer Simulatio Co2 Ch4 Adsorption

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