Improving the CH 4 adsorption property and the CO 2 / CH 4 separation of IRMOF-3 by functionalizing the organic linker

Trang 76 Improving the CH4 adsorption property and the CO2/CH4 separation of IRMOF-3 by functionalizing the organic linker  Trang Moc Khung  Pham Tran Nguyen Nguyen University of Science,VUN-HCM (Received on June 5th 2015, accepted on 26th September 2016 ) ABSTRACT Grand Canonical Monte Carlo (GCMC) simulation combining with the ideal adsorbed Solution Theory (IAST) are employed to study the effect of functionality on the CH4 adsorption property and CO2/CH4 selectivity of modified irmof-3 structures which include a diverse range of functional groups. The result shows that phenyl groups containing nitrogen (e.g. pyrazine, pyridine) and carboxyl group are able to increase the interaction energy between gas and mof, thereby increasing the gas adsorption capacity. In addition, transition metals can significantly enhance the CO2/CH4 selectivity. The straight-chain alkyl group and aniline groups just slightly improve the material property compared to other functional groups. We also note that materials with more than 50 percent of modification do not show a good performance at high pressure range (35–40 atm) due to its low porosity. We herein show that the functionalization of IRMOF-3 can remarkably improve the CH4 uptake and CO2/CH4 separation; particularly, GCMC simulation is demonstrated as a beneficial tool to aid experimental chemists in designing new promising porous materials.


INTRODUCTION
The anthropogenic emission of CO2 from industrial production processes and transportation has greatly affected the environment and developing economic issue.In addition, CH4 is another strategic interesting gas that is negative greenhouse effect and is the main component of the natural gas.It is also considered as cleaner energy carrier than petroleum oil due to higher hydrogen to carbon ratio lead to lower the carbon emission.Indeed, the research for safe and capacity CH4 and CO2 storage systems has been a big challenge that has created tremendous studies with the goal to improve the existing related technologies.Through the physisorption-based processes involving porous solids offer an efficient storage/capacity alternative to ability adsorption of porous metal-organic frameworks (MOFs) that they have attracted attention during the past decades in the field of gas adsorption/separation both experimentally and theoretically.Many MOFs were designed and synthesized with ultra-high porosity, including MIL-101, MOF-177, MOF-205, PCN-14, UCMC-2 which show high CO2 or CH4 uptakes.However the majority of these promising materials would require a high energy cost for regeneration and applications.In addition to obtain new MOFs, a number of techniques have been applied on well-defined MOFs such as changing the network topology, doping of organic ligands or metal ions into the framework.In particular, attaching different functional groups on the organic linking component by post-synthetic modification method (PSM) [4], an effective way to improve the adsorptive capacity and storage gas of MOF, is high attractive to us.
Here, we focus on the study of functional groups on IRMOF-3-based materials for improving the selection of CO2/CH4 as well as enhancing the gas uptake by combining the Grand Canonical Monte Carlo (GCMC) simulation with the Ideal Adsorbed Solution Theory (IAST) [5].According to the idea of PSM method, our proposed compounds were designed by changing the chemical composition of the linker into the framework materials but remaining crystal topologies.The result demonstrates that the PSM approach coupling with theory calculations is a promising way to improve the properties of materials.

Grand-Canonical Monte Carlo (GCMC) simulations
The GCMC simulations were carried out using MUSIC software package [6].The interaction between the adsorbent (MOF) with methane (CH4) and carbon dioxide (CO2) was described mainly by van der Waals forces.The electrostatic forces, in this case, do not play an important role.We used the Lennard-Jones model with the parameters obtained from the force field TraPPE (transferable force fields) for molecular gas (adsorbate) and dreiding force field [7] for the atoms in the MOF (adsorbent).The Lorentz-Berthelot rule was used to calculate the parameters of the interaction between gas molecules and the MOF.The Lennard-Jones interactions of distances greater than 12.8 Å are ignored.In the simulation, a supercell 2x2x2 (i.e. 8 unit cells) of MOF was kept rigid, the molecular gas was considered as a "spherical molecule".Each point of the isotherm was obtained by 15-20 million simulation steps.The authenticity of this methodology has been proved by reported literatures [8].In the view of adsorption theory, one needs to distinguish the nature of simulation with that of the experiment.While result calculated by GCMC simulation is the amount of gas molecules in the pore of adsorbent, namely the total amount N abs , the current techniques of experiment are not able to characterize this absolute amount of adsorbed molecules.Fundamentally, these measurements just produce the difference in the amount of total adsorbed gas and the amount of bulk gas in the same condition of measurement, namely nex.The equation to converse between two quantities was given by equation (1) [9].At low pressure (lower than 1 atm), the difference between these two values is negligible.
Where , excess and absolute amount, respectively, is the pore volume of the adsorbent and 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 [10].

Molecular Mechanics (MM)
Molecular mechanics are practical technique to study atomistic systems containing thousands of atoms per unit cell.Total potential energy of the system is determined for each set of positions of the atoms by using intermolecular/intramolecular potential function in the classical force field.It helps refining the repetitive geometry of a mechanical approach until some predetermined criterid of convergence are satisfied.Finally, the quality of geometrical optimization depends on the accuracy of force field.Among the diversity of force fields developed, the uff force field is probably the most versatile since its parameter is derived for most of the atoms in the periodic table.In addition, this force field can combine with the qeq method to study systems in which electrostatic interactions [11] are important.Moreover mm method possesses an advantage in optimizing systems, such as MOF-205, PCN-14, MIL-101(Cr) [12] since the implement of high computational-cost calculations are not reasonably practicable.In this study, MOF structures are optimized with Dreiding force fields and UFF using GULP software [13].

Adsorption Enthalpy Calculation (ΔHads)
The isosteric heat of adsorption [14] (Qst) is based on the thermochemical parameters of the adsorption process.ΔHads can be calculated from gas adsorption isotherms simulated at two or more different temperatures by means of fitting of the virial equation.The zero-coverage isosteric heat corresponds to the interaction energy between the gas molecule and the strongest interaction site of the MOF.The virial equation (Equation 2) which consists of temperatureindependent parameters ai 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 software inc., northampton, ma).δhads is then calculated by Equation 3.
Where P is the pressure, N is the amount adsorbed CH4 gas, T is the temperature, m and n represent the numbers of coefficients required to adequately describe the isotherms, R is universal gas constant.

Adsorption selectivity
Adsorption-based separation is a physisorptive operation governed by thermodynamic equilibrium process, which relies on the fact that guest molecules reversibly adsorb in nanopores at densities that far exceed the bulk density of the gas sources in equilibrium with the adsorbents [15].
It has been convinced in numerous published papers [16-[19] that the IAST can be used to estimate quite accurately of the adsorption equilibrium of mixtures from pure component isotherm data.For a binary gas mixture adsorption (A, B components), the predicted adsorption selectivity (SA/B) was calculated by: Where xA, xB and yA, yB are the mole fractions of A and B in the adsorbed and bulk phases, respectively.

RESULTS AND DISCUSSION
MOF materials have been synthesized in previous literature [3, [4] by psm method, our MOF proposed models contain 50 % substituent groups (substituent converted to the linker with the yield of 50 %) due to the fact that too much attached substituent possibly lead to a decreasing in gas adsorption capacity [20].All structures were carried out optimization and GCMC simulation on the CH4 and CO2 adsorption isotherms.To interpret the effects of the substituent group, we performed simulations for all of the new proposed MOFS such M3AMPh, M3AM5, M3URPh, M3AMPz, M3AMSal, M3AMPhN and M3CC.These materials are the products of the process of attaching the substituent benzoic anhydride (AMPh), hexanoic anhydride (AM5) and phenyl isocyanate (URPh), 2,3pyrazine-dicarboxylate (AMPz), 3hydroxyphthalic anhydride (AMSal), 2pyridinecarboxy-aldehyde (AMPhN) and cyanuric chloride (CC) to IRMOF-3 linker, respectively.In addition, we also proposed doping of metal ions Pd 2+ , Fe 3+ , Cu 2+ to the linkers of the material M3AMPhN, M3AMSal, M3AMPz, respectively, resulting new materials with corresponding names: M3AMPhNPd, M3AMSalFe and M3AMPzCu [21].We also investigated CO2/CH4 selectivity by employing ideal adsorbed solution theory

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IAST which was a widely used as a thermodynamic method for predicting the equilibrium adsorption of mixtures.The volumetric uptake and isosteric heats of adsorption of MOFs are presented in Fig. 2, 3 and 4. Cu, yellow).The extended organic linkers attach on IRMOF-3 are also presented.like form open metal site) [22], created M3AMPzCu significantly enhances both volumetric uptake (12.42 cc/cc) and isosteric heat of adsorption of CH4 (15.72 KJ/mol) at 298 K and 1 atm (low pressure).

A) B) C)
Trang 80 Adsorption isotherms of carbon dioxide (Co2) Fig. 3. CO2 isotherm at high pressure (A), low pressure (B) and isosteric heat of adsorption (C) at 298 K Similar to the result above, the M3AMPzCu has volumetric uptake of CO2 (44.23 cc/cc) and the M3AM5 has isosteric heats of adsorption of CO2 (38.31 KJ/mol) at 298 K and 1 atm (low pressure).They have significantly enhanced volumetric uptake and absorption heat of CO2 than unmodified IRMOF-3 (15.28 cc/cc and 16.94 KJ/mol) as well as compared to other MOFs.However, the substituents attached too much (i.e., the percentage conversion of MOF is large) leads to a decreasing-in surface area and pore volume of the MOFs [23], resulting in reducing CO2 adsorption capacity at 298 K and 38 atm (high pressure).In proportion to gas selectivity studies, the initial slopes for CO2 and CH4 adsorption uptakes indicate the noticeable affinity for CO2/CH4 of proposed structures at high pressure.We studied the selectivity adsorption by the ideal adsorbed solution theory (IAST), which calculates the system's gas selectivity capabilities of theoretical gas mixtures utilizing the pure component isotherms (Fig. 2, Fig. 3) and the results are shown in Fig. 5.It should be noted that even though IAST calculations are performed using GCMC isotherm, their selectivity results represent theoretical values that might deviate from practical applications.The selectivity for CO2/CH4 of new MOFs are quite significant at 22 bar for 50/50 mixture of CO2 and CH4.The selectivity of MOFs that have attached metal ion such as Cu 2+ , Fe 2+ , Pd 2+ and this value would increase with increasing pressure; while the M3AM5 increased at low pressure and decreased at high pressure.The validity of IAST calculations is dependent on the ideality of MOFs [25].We confirm the results by calculating selectivity from the initial slopes of the isotherms (Fig. 4).The resulting selectivity for CO2/CH4 are in good agreement with the IAST value [26].In summary, the transition metal (Cu 2+ , Fe 2+ , Pd 2+ ) also contribute to enhance the CO2/CH4 selectivity.

CONCLUSION
In summary, we herein show that the functionalization of IRMOF-3 can remarkably improve CH4 uptake and CO2/CH4 separation.The substituent which have phenyl group containing nitrogen atoms inside such as pyrazine, pyridine groups shows better positive effect than the straight-chain alkane and aniline groups.This is explained by the increasing isosteric heats of adsorption.In addition, the transition metals (Cu 2+ , Fe 2+ , Pd 2+ ) are able to enhance CO2/CH4 selectivity.Last but not least, the GCMC similar combining with the IAST is a powerful tool to investigate both the storage capacities and isosteric heats adsorption of the gas also gas selectivity.Thus, it can be used to design new promising porous materials.

Fig 2 .
Fig 2. CH4 isotherm at high pressure (A), low pressure (B) and isosteric heat of adsorption (C) at 298 k

Table 1 .
Summary of porosity, CH4 and CO2 uptake (at 1 atm and 38 atm, 298 K), and enthalpy of adsorption for materials in this study