Publishing I Vietnam Academy of Science and Technolog Advmices in Natural Sciences Naroscience and Nanotechnology Adv. NaL SCI - Nanosci Nanotectinol 6 (2015) 035011 (6pp) doi:10 1088/2043-6262/&3«35011
Effect of chemical functionalization groups on Zrg-AzoBDC to enhance H2, CH4 storage and CO2 capture: a theoretical investigation
Khung M Trang, Hung Q Pham and Nguyen-Nguyen Pham-Tran Laboratory of Computational Chemistry, Faculty of Chemistry, University of Science, Vietnam National Umversity in Ho Chi Minh City, 277 Nguyen Van Cu. Distnct 5, Ho Chi Minh City, Vietnam E-mail: [email protected], vn
Received 16 December 2014 Accepted for publication 26 May 2015 Published 19 June 2015
CrossMark Abstract
Grand canonical Monte Carlo (GCMC) simulation combined with the ideal adsorbed solution flieory (lAST) and a statistical method were utilized to investigate the effect of functional groups on zirconium oxide based metal-organic frameworks (MOFs) Zre-AzoBDC (Zr^A) for ttie gases (H2, CH4) adsorption property and CO2/CH4 selectivity under low pressure. The resiflts showed fliat phenyl groups containing nitrogen (pyridine, pynmidine) and thiophene group enhance the gas affinity with MOFs, therefore increasing both gravimetric and volumetric uptake. In addition, this behavior can also cause significantly improved selective capture of CO2 from CO2/CH4 gas mixtures. Among functional groups studied, the sulfonic acid group can potentially improve CH4, CO2 uptake and H2 isosteric heat of adsorption These findings would play a vital role in designing new matenals toward gas adsorption properties.
Keywords: MOFs, gas adsorption, CO2/CH4 selectivity, CO2 capture, lAST Classification numbers: 3.02, 5.19
1. Introduction versatile. These porous materials are promising for gas sto- rage and separation as well as many other potential applica- It is no exaggeration to state that one of the biggest challenges tions. However, the major issue of these promising materials for human beings in the 21st cenmry is global wanrang and is often sensitivity to moismre, and the stmcture can com- energy crisis. The anthropogenic emission of CO2 from pletely collapse mth the intioduction of water vapor. For- industrial processes has greatly affected the environment, and tunately, Zr-based MOFs have been proved as thermo- this origin of pollution, together with other tioubUng issues, is chemically and stmcturally stable compounds, irrespective of a major concem. In addition, the present oil reserves have the presence of water molecules [1]. The ability of Zr-MOFs been drastically reduced, so there is no doubt that the supply to retain the porous stmcture under humid conditions has of fossil fuel could be exhausted in the near fiiture. It has attracted our attention in die quest for a practical m^erial for become necessary to seek out new sources of energy, such as storage and separation application.
solar energy, wind power, hydropower and so forth. Amongst We herein report a theoretical investigation on the effect those candidates, energy from Hi or CH4 has received a lot of of functional groups on Zre-AzoBDC (Zr^A) in order to attention due to their negative greenhouse effects. An enhance H2, CH4 storage and CO2 capture. The grand cano- impottant challenge of using ttiese gases as an energy sources nical Monte Carlo (GCMC) simulation combined with the is an efficient storage system. Numerous studies have been ideal adsoriied solution theory (lAST) [2] is employed. Our conducted to improve the current technology. During ttie past proposed compounds were designed by means of a post- decades, metal-organic firameworks (MOFs) have shown a syntiietic modification (PSM) method [3], i.e. die altering of huge potential for die challenges mentioned above, hideed, ttie chemical composition of the linker witiiout changing its MOFs are porous, chenucally diverse, and structurally des- underiying net, which is known as an efficient approach in ignable materials: most importantty, ttiey are extremely experimentally synthesizing new MOFs. Our calculations 204^*262/1 S«)35011*06$33 00 , © 2015 Vielnam Academy ot Science « Technology
Adv. Nat Sci.: Nanosci. Nanotechnol. 6 (2015) 035011
demonstrate that tfae PSM approach coupled with theoretical calculations is a strategic method to improve the adsorption properties of these materials.
N" = N'^'-Vgp^, 2. Computational details
2.1. Grand canonical Monte Carlo simulations
AU grand canonical Monte Carlo (GCMC) simulations were carried out using MUSIC software package [4]. The interac- tion between the adsorbent (MOF) with methane (CH4) and carbon dioxide (CO2) were described maiitty by van der Waals forces. The electrostatic forces in this case do not play an important role. We used the Lennard-Iones model wdth the parameters obtamed from the transferable force fields (TraPPE) for molecular gas (adsorbate) and Dreidmg force field [5] for the atoms in the MOF (adsorbent). The Lorentz- Berthelot mle was used to calculate the parameters for interaction between gas molecules and the MOF. The Len- nard-Jones interactions of distances greater than 12.8 A are ignored. In the simulation, a supercell 2 x 2 x 2 (i.e. 8 unit cells) of MOF was kept rigid, the molecular gas was con- sidered a 'spherical molecule'. Each point ofthe isotherm was obtained by 15-20 million simulation steps. The authenticity of this methodology has been proven by reported literature [6]. In the view of adsorption theory, one needs to distinguish the nature of simulation with that of the experiment. While the result calculated by GCMC simulation is the amount of gas molecules m the pore of adsorbent, namely the total amount A'"''*, current experimental techniques of are not able to characterize this absolute amount of adsorbed molecules.
Fundamentally, these measurements just produce the differ- ence in the amount of total adsorbed gas and the amount of bulk gas at same condition of measurement, namely N". The equation to converse between two these quantities was given by [7]
versatile since its parameters are derived for most of the atoms in ttie perii>dic table. In addition, this force field can combine witti ttie Ql-q method to study s>sterns in which electrostatic
•;. in^^gdog-s [9] are important. Moreover MM method pos- sesses an advantage m optimizing systems, such a.s MOF-205.
PCN-14, MlL-lOKCr) [10] since die implement of high computaSmal-cost calculations are not reasonably practic- able. In this study. MOF strucmres are optimized with Dreiding force fields and UFF using GULP software [11].
2.3.1 iption enthalpy calculabon
The isoStenc heat of adsorption [12] (Q^,) is based on die thennocheniicai parameters of the adsorption process.
Adsorption enthalpy (^Hj^s) can be calculated from gas adsorption isotherms simulated at two or more different temperatures hy means of fitting of the vmal equation The zero-coverage isosteric heat corresponds to the interaction energy between gas molecule and the strongest interaction site of the MOF. The virial equation
InP = \nN+-'Y,a,N' + 2]^.^'. (2)
where P is pressure, A^ is the amount adsort>ed CH4 gas, T is temperature, m and n represent the numbers of coefficients required to adequately describe the isotherms. The virial equation consists of temperature-independent parameters a, and bi is used to fit the sorption data. Adsorption isotherms measured at 273 K and 298 K are used in this procedure by applying the statistical program Origin 8.5 (MicroCal Soft- ware Inc., Northampton, MA). AUg^s is then calculated by following equation
-Q. = -RY.'^.N; (3)
(1) where R is umversal gas constant where N", N ' excess and absolute amount, respectively, Vp 2.4. Adsorption selectivity is pore volume of the adsorbent and Pg is the fluid density in
the bulk phase at the same temperature and pressure for adsorption, which is calculated by the Peng-Robinson equation of state [8]. At low pressure (lower than 1 atm) the difference between A ' " and N"*" is negligible.
2.2. Molecular mecfianics
Molecular mechanics (MM) is a practical technique to study atomistic systems containing thousands of atoms per unit cell.
Total potential energy of the system is detennined for each set of positions of the atoms by using intermolecular/intramole- cular potential fimetion in the classical force field. It helps to refine the repetitive geometry of a mechanical approach until some predetermined criterion of convergence is satisfied.
Fmally, the quaUty of geometrical optimization depends on die accuracy of the force field. Amongst the diversity of force fields developed, the UFF force field is probably the most
Adsorption-based separation is a physisorptive operauon govemed by thermodynamic equihbrium processes, which relies on the fact that guest molecules reversibly adsorii in nanopores at densities that far exceed the bulk density of the gas sources in equiUbrium with the adsorbents [13].
It has been shown in numerous published papers [14-17]
that the lAST can be used to estimate quite accurately adsorption equiUbrium of mixmres from pure component isotherm data. For a binary gas mixmre adsorption (A, B components), the predicted adsoiption selectivity (SA/B) W^S calculated by
SA/B
yJyB' (4)
where XA, XB and y^, ya are the mole fractions of -4 and B in the adsorbed and bulk phases, respectively.
Adv. Nat Sci • Nanosci. Nanotectinol 6
J
•""*' . - * ^ « ^^^
• , * , ^ * * , * . ^
C6 CSN C4N2
V^** *>•(
*^
J J
C4S
Figure 1. Model of Zr^-AzoBDC stmcture and functional groups: The large spheres represent the void regions inside the cages (Zn polyhedral: blue for octahedral cage; C, gray; O, red; N, blue; Cl. green; S, yellow). The extended organic linkers attached on Ztg-AzoBDC are also presented.
^
Es i
e8
1
-M-Zr6AS03H N . -^Zr6AC4N2
» C N t -*-Zf«AC6
^ ' ' ^ ^ V ^ — ZrtACSN tt li 1 III ^ ^ l ] " - ^ Z r t A C I
* * ' ^ & ^ , ^ _ ^ ' ^ -^ZrSAC4S ' • • • tt -i-ZrSAC4N
^*~'~~»~~--.^^^^ - ^ Z l < A C 4 0 -^ZrSACS -•^ZrfiA
, , , 1
Pressure (Torr) Pressure (Torr) Uptake (mg g-') Figure 2. Hj isodierm at 77 K (a), 87 K (b) and isosteric heat of adsorption (c) at 1 aUn.
3. Results and discussion
Based on Zr^-AzoBDC (Zr^A) material syndiesized by Yang el al [18, 19], die studied stinctures was optimized witti the Dreiding force field and UFF as hnplemented in tiie GULP code. GCMC simulation was then performed to calculate ttie HJ, C H l and ^^^ adsorption isottierms. To interpret ttie effects of die functional group on ZreA, nine new MOF models with different substiments were introduced, namely.
Zr^ACl. Zr^ACe, ZrsACSN, Zr6AC4N2, Zr6AC4N, Zr6AC40, Zr6AC4S. TrgACS and ZrsASOjH [20]. These materials are the products of uicorporating a certam
substituent into the Azo-linker (figure 1) and theu- CO2/CH4 selectivity was investigated by means of lAST.
The volumetric uptake and isosteric heats of adsorption of MOFs are presented in figures 2, 3 and 4, in which, H2 isotherm is calculated in units of mg/g, while both CH4, CO2 isotherms are calculated in c.c./c.c. units.
3.1. Adsorption isotherms of hydrogen (H^, methane (CH4) and carbon dioxide (CO2}
Hydrogen (H2) uptake of ZTgACSN, Zr6AC4N2 and Zr6AC4S are 24.06, 23.69 and 23.73 mg g"', respectively; tiiese values
Adv. Nat Sd.: Nanosci. Nanotechnol. 6 (2015) 035011
5 15 25 3 5 0 2 0 0 4 0 0 6 0 0 8 0 0 0 2 4 8 Pressure (atm) Pressure (Ton) Uptake (mg g-<)
Rgure 3. CR, isotherm at high pressure (a), low pressure (b) and isosteric heal of adsorption (c) at 298 K.
-»-Zr6AC4N2 ---ZrSAC4S
ZrCACSN Zr6AS03H
—Zr6ACe ZrSAC4N -•-Zr6AC40 -•-Zr6AC5 -^ZrSA
200 400 800 800
Pressure (abn) Pressure (Torr) Uptake (mg g')
Rgure 4. COi isotherm at high pressure (a), low pressure (b) and isosteric heat of adsorption (c) at 298 K.
Tablel. Summary of porosity, H2(al 1 atm, 77 K). CH4 and CO2 uptake (at 38 atm, 298 K). and endialpy of adsorption for materials in this study.
Material Zr6A Z r ^ A O Zi^ACe Z r ^ A C S N Z r 6 A C 4 N 2 Z r 6 A C 4 N Z r 6 A C 4 0 Z r 6 A C 4 S Z T S A C S Z r s A S O a H
poiE volume.
A S A B C T im" g " ' )
4 7 4 4 3 5 6 9 1025 1182 1426 1334 1337 1258 1423 2 6 9 9
giavimetnc uptake.
volumelnc uptak<
adsmpiitm beal ai zero coverage
V l
(cm^ g " ' ) 1.31 1.09 0 . 3 8 0 . 4 6 0 . 5 6 0 . 4 7 0 . 4 9 0 . 4 9 0.44 0 . 8 9
V H 2 ^ ( m g g ' )
1 3 J 5 13.94 19.67 2 4 . 0 8 2 3 . 6 9 19.83 2 0 . 2 5 2 3 . 7 3 18.05 17.66
calculated from the vinal equalio d H ^ d ^ "
(kJ m o l ' ) 4 . 7 2 6.76 6.89 6.76 7.91 6.54 6.50 6.61 6 0 8 8 3 9
V C H / ( c c . c c . ' )
1 0 5 . 0 9 115.28 7 6 . 1 4 116.48 131.31 109.12 113.03 125.43 8 2 7 0 1 4 4 . 5 0
d H a ^ C H 4 ( U m o r ' )
13.13 15.92 15.91 17.48 18.82 1 6 . 1 6 16.38 18.62 14.55 16.90
V C 0 2 ' ( c c . c c . ' )
2 6 6 . 2 8 2 6 7 . 5 7 148.30 175.08 2 0 1 . 3 5 175.14 172.35 186.63 168.83 2 9 7 . 7 7
^Hads.cca (kJ mol ' )
17.91 17.70 22.59 24.34 2 6 . 5 4 22.37 2 1 , 8 6 2 4 . 2 2 21.65 23.65
are higher than those of urunodified Zr^-AzoBDC thiophene. The nitrogen and sulfur atoms possess a high (13.55 mgg~')and odier MOFs at 7 7 K a n d 1 atm. This may affinity widi gas, tiiereby improving dieir isosteric heat of be because ZrtACSN, Zr6AC4N2 and Zr6AC4S contain adsorption. Furtiiermore. this may be due to ttie sulfonic acid itilrogen phenyl group such as pyridine, pyritrudine and group, corresponding to Zr^ASOsH, which significantiy
Adv. NaL SCI : Nanosa Narxitechnol. 6 (2015) 035011
Rgure 5. Simulation results for separation of an equimolar of CO2/CH4 in new MOFs at 298 K.
enhanced the isosteric heat of adsorption (8.39 kJ m o r ' ) at 87 K and 1 atm (table 1) and this result is consistent with previous research [21, 22],
Similarly, die Zr^ASOaH, Zr6AC4N2 and Zr6AC4S (144.50, 131.31 and 125.43 c c . c c . " ' ) show higher mediane volumetric uptakes than unmodified Zr^-AzoBDC (105.09 c.c.cc."') and ottier MOFs at 298 K and 38 atm (table 1). This is explained by the high affinity with the gas of nitrogen and sulAir atoms. Also the isosteric heats in methane adsorption of these materials are improved (Zr6AC4N2:
18.82 kJ mol"' and Zr6AC4S: 18.62 kJ m o P ' , compared witti 1 3 . 1 3 k J m o r ' of Zrg-AzoBDC). hi contrast, ZrgACe and ZrgACS possess tiie lower capacity of methane adsorption, corresponding to the volumetric uptakes of 76.14 and 82,70 c c . cc,~', respectively.
For CO2 adsorption, Zr^ASOsH shows the highest volumetiic uptake (297,77 c c . c c , " ' at 298 K, 38 atm) and Zr6AC4N2 has the highest isosteric heat of adsorption ( 2 6 . 5 4 k J m o r ' at 298K and 1 atm) (table 1). They have significantiy enhanced volumetric uptake and absorption heat energy compared to unmodified Zrs-AzoBDC (266.28cc. c c " ' and 17,91 kJ mol"') as well as compared to other groups, since the sulphonic acid group increases both die heat of adsorption and total uptake for carbon dioxide.
3.2. Adsorption selectivity (C02/CH4)
According to gas selectivity studies, the initial slopes for COj and CH4 adsorption uptakes indicate noticeable affinities for CO2/CH4 of proposed structures at high pressure. We studied die selectivity adsorption by the ideal adsorbed solution the- ory (lAST), which calculates the system's gas selectivity capabiUties of theoretical gas mixtures utiUzing the pure component isotherms in figures 3 and 4, and the results are shown in figure 5. It should be noted that even ttiough lAST calculations are performed usmg GCMC isotherm, their selectivity results represent theoretical values that might deviate from practical appUcations. The selectivity for CO2/
CHl of new MOFs is quite significant at 22 bar for 50/50 mixture of CO2 and CR,. The selectivity of MOFs ttiat contain C4N2 and C5N (pyridine and pyrimidme) groups would increase witti increasing pressure; while ttie ZfftACl
Rgure 6. The lAST-predicted isodientis and selectivity's of equimolar mixnire of CO2 and CH, in Zr^-AzoBDC at 298 K and Zr6AC6 decreased at high pressure. The validity of lAST calculations is dependent on the ideality of MOFs [23]. We confirm this result by calculating selectivity from the irutial slopes of the isotherms (figure 6). The resulting selectivity for CO2/CH4 are in agreement with the lAST value. In summary, the pyridine and pyrimidine groups can enhance CO2/CH4 selectivity.
4. Conclusion
The functionalization of Zr^-AzoBDC can remarkably improve H2, CH4 uptake and CO2/CH4 separation. The sub- stituents containing phenyl group with nittogen inside and sulfur atoms such as pyrazine, pyridine and thiophene groups show a better performance dian other groups for gas storage and separation. This can be explained by the increase of isosteric heat of adsorption. In particular, the sulfonic acid group has significantiy enhanced both the gas storage capa- city and isotenc heat of adsorption. The GCMC simulation demonstrates itself a beneficial tool to aid experimental che- mists in designing new promising nano and micro porous materials.
Aclfnowledgments
The authors gratefuUy acknowledge SR16000 super- computing resources from the Center for Computational Materials Science of the Institiite for Materials Research, Tohoku University, Japan and the Insatute for Computational Science and Technology (ICST), Ho Chi Minh City for dieir support
[1] Jiang H L, Feng D, Wang K, Gu Z Y, Wei Z, Chen Y P and Zhou H C 2013 7. Am. Chem. Soc. 135 13931 [2] Myers A L and Prausnitz J M 1965 AlChE J 11 121 [3] Tanabe K K and Cohen S M 2011 Chem. Soc. Rev. 40 498 [4] Gupta A, Chempadi S, Sanbom M J, Clark L A and Snurr R Q
2003 MoL SimuL 29 29
Adv. Nat Set.: NanoscL Nanotechnol. 6 (2015) 035011
[5] Mayo S L, Olafson B D and Goddard W A 1990 /. Phys.
Chem. 94 8897
[6] Duren T, Bae Y S and Snuir R Q 2009 Chem. Soc. Rev.
38 1237
[7] Myers A L and Monson P A 2002 Langmuir 18 10261 [81 Peng D Y and Robinson D B 1976 buL Eng. Chem. Fund.
15 59
[9] Jhon Y H. Cho M, Jeon H R, Paric I, Chang R, RowscU J L C and Kim J J 2007 Phys. Chem. C 111 16618 [10] Chen Y F, Babarao R, Sandler S I and Jiang J 2010 Langmuir
26 8743
[11] Gale J D 1997 J Chem. Soc. Faraday Trans. 93 629 [12] Siicar S, Mohr R, RisUc C and Rao M B 1999 /. Phys. Chem
B 103 6539
[13] Keskin S, van Hecst T M and Shell D S 2010 Chem. Sus.
Chem. 3 879
[14] Bae Y Sand Snurr R Q 201 iAngov, Chem. InL Ed. ^ I I5i6 [15] Simmons J M, Wu H, Zhou W and YUdirim T 2011 Energy
Environ. So. 4 2177
[16] Yang Q, liu D, Zbraig C and U J R 2013 Chem. Rev. 113 8261 [17] Liu K, Ma D, Li B, U Y, Yao K, 23iang Z, Han Y and Shi Z
2014 /. Mater. Chem. A 2 15823
[18] Tanabe K K and Coheo S M 2009 Angew. Chem. 121 7560 [19] Yoo Y and Jeong H K 2012 Chem Eng. J. 181-182 740 [20] Yang Q, Gnillerm V, Ragon F, Wiereum A D. Llewellyn P L,
Zhong C, E)evic T, S^iec C and Maurin G 2012 Chem.
Commun. 4S 9831
[21] Yang Q, Wiersum A D, UeweUyn P L, Guillerm V, Serrec C and Maurin G 2011 Chem. Common. 47 9603 [22] Foo M L, Horike S, Fukushima T, Hijikata Y, Kubota Y, Takataf M and Kitagawa S 2012 DaUon Trans. 41 13791 [23] Muithi M and Snurr R Q 2004 Langmuir 20 2489