Enhanced Methane Adsorption in Catenated Metal-organic Frameworks: A Molecular Simulation Study*

XUE Chunyu (薛春瑜), ZHOU Zi’e (周子娥), YANG Qingyuan (陽慶元) and ZHONG Chongli(仲崇立)

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Enhanced Methane Adsorption in Catenated Metal-organic Frameworks: A Molecular Simulation Study*

XUE Chunyu (薛春瑜), ZHOU Zi’e (周子娥), YANG Qingyuan (陽慶元)**and ZHONG Chongli(仲崇立)

Laboratory of Computational Chemistry, Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

A systematic molecular simulation study was performed to investigate the effect of catenation on methane adsorption in metal-organic frameworks (MOFs). Four pairs of isoreticular MOFs (IRMOFs) with and without catenation were adopted and their capacities for methane adsorption were compared at room temperature. The present work showed that catenation could greatly enhance the storage capacity of methane in MOFs, due to the formation of additional small pores and adsorption sites formed by the catenation of frameworks. In addition, the simulation results obtained at 298 K and 3.5 MPa showed that catenated MOFs could easily meet the requirement for methane storage in porous materials.

methane, adsorption, catenation, metal-organic frameworks, molecular simulation

1 INTRODUCTION

Currently, great efforts are being made to search for alternative fuels to gasoline and diesel used in vehicles. Natural gas, which consists mainly of methane, is a good candidate and widely available in many countries. In current practice, it is mainly stored as compressed natural gas (CNG) in pressure vessels at 20.7 MPa and requires an expensive multistage compression. Thus, an effective, economic, and safe on-board storage system is necessary, which will lead to methane-driven automobiles substituting for the traditional ones. An attractive alternative to CNG is the adsorbed natural gas (ANG), which is usually stored in porous materials at a lower pressure. To promote the vehicular application of methane, US Department of Energy (DOE) has set the target for adsorbed methane storage as 180 v(STP)/v (STP: 298 K, 0.1 MPa) at 3.5 MPa and 298 K. A variety of porous materials, including single-walled carbon nanotubes [1], zeolites [2], and activated carbon [3], have been extensively evaluated as the potential materials for methane storage, but few of them can meet the DOE target. The highest methane storage capacity obtained in activated carbons was ca. 200 v/v [4], although significant efforts were made on processing activated carbons. It seems that there is still a very long way to go for developing the efficient storage materials. Metal-organic frameworks (MOFs), a new family of nanoporous materials, have emerged as the promising materials for gas storage, separation, catalysis,. [5-7]. A variety of MOFs have been screened for methane storage [8-13], but only a few can reach the DOE target. For example, Düren. [12] proposed a theoretical MOF (IRMOF-993) with a methane adsorption capacity of 181 v(STP)/v. Ma. [13] synthesized a MOF named PCN-14 that gave the highest methane adsorption capacity of 230 v(STP)/v so far. It seems that MOFs are a class of promising materials with practical applications in methane storage. Catenated MOFs [14-17] are composed of two mutually catenated frameworks that generate additional pores with various sizes. The catenation structure strengthens the gas affinity for the material by an entrapment mechanism that improves the gas adsorption capacity [18] and separation [19]. Thus, catenation appears to be a useful strategy for designing new MOFs as efficient methane storage materials. Based on this consideration, a systematic molecular simulation study is performed in this work to investigate the effect of catenation on methane storage capacity to provide useful information for further MOFs development with improved methane storage capacity.

2 MODELS and COMPUTATIONAL METHOD

2.1 MOF structures

In this work, eight isoreticular metal-organic frameworks (IRMOFs) synthesized by Eddaoudi.[20] are adopted as representatives of MOFs. IRMOFs-10, 12, 14, 16 have the same cubic topology with the octahedral Zn4O(CO2)6clusters [Fig. 1 (a)] linked by different organic dicarboxylate linkers [Fig. 1 (b)], while IRMOFs-9, 11, 13, and 15 are their corresponding catenated counterparts. The crystal structures of IRMOF-9 and 10 are shown in Fig. 1 (c) as an example. Details of those structures and their properties can be found elsewhere [20, 21].

Figure 1 The crystal structures of IRMOFs

2.2 GCMC simulation

Grand canonical Monte Carlo (GCMC) simulations were carried out to calculate the adsorption of methane in the MOFs studied. The number of the unit cells of IRMOFs adopted in the simulation cell varied from 2′2′2 to 3′3′3, so that enough molecules were accommodated to guarantee the simulation accuracy. A cutoff radius of 1.5 nm was applied to the Lennard- Jones (LJ) interactions, and periodic boundary conditions were applied in all three dimensions. For each state point, GCMC simulation consisted of 1′107steps to guarantee equilibration followed by 1′107steps to sample the desired thermodynamic properties. To estimate the statistical uncertainty, the production phase of each state point was divided into 10 blocks and the standard deviation of the block average was calculated. The uncertainties on the final results, including the ensemble averages of the number of adsorbate molecules in the simulation cell and the total potential energy, were estimated on average to be within±2%. Details of the method were given elsewhere [22].

3 RESULTS AND DISCUSSION

3.1 Validation of the force field

The above set of force fields has been successfully employed to depict alkane adsorption [12, 25], separation [19, 26, 27] and diffusion [28] in MOFs. In our previous work [19], the above force fields have been shown to well consistent with the experimental data [20] of CH4adsorption in IRMOF-1. To further confirm the reliability of the above set of force fields adopted in this work, the adsorption isotherms of CH4in IRMOF-6 were simulated, as shown in Fig. 2. The simulation results are very similar to that obtained by Düren. [12], and both of them are in good agreement with experimental data [20]. This demonstrates that the set of force fields can give reliable calculated results for methane adsorption in IRMOFs, and thus can be used to predict the methane storage capacity of the IRMOFs considered.

3.2 Adsorption of methane

With the above parameters, adsorption isotherms of methane in the four catenated IRMOFs as well as their corresponding non-catenated counterparts were predicted with GCMC simulations, as a function of pressure up to 10 MPa. The results were converted into v(STP)/v using crystallographic density, as displayed in Fig. 3. The volumetric capacities for methane in all the MOFs increase with pressure, and the increasing trends in catenated ones are much steeper in the lower pressure range. Apart from the saturated region, the adsorption capacities in the catenated IRMOFs are much larger than those in their correspondingnon-catenated counterparts on a volume basis. The above observations indicate that, attributing to the formation of the additional small pores with different sizes and adsorptions sites, catenated structures can greatly enhance the storage capacity of methane in MOFs.

Figure 2 Experimental and simulated isotherms of methane adsorption in IRMOF-6 at 298 K■?IRMOF-6 (exp);□?IRMOF-6 (sim)

Figure 3 Adsorption isotherms for the eight IRMOFs at 298 K■?IRMOF-9;□?IRMOF-10;●?IRMOF-11;○?IRMOF-12;▲?IRMOF-13;△?IRMOF-14;▼?IRMOF-15;▽?IRMOF-16

As the amount adsorbed in porous media at 3.5 MPa and 298 K is a primary target for methane storage in vehicular applications, we focus on the methane storage under this condition. Based on adsorption literature [8, 12, 29], the following properties of absorbents are the important factors to determine their adsorption capacities: accessible surface (acc), adsorbent framework density (crys), free volume (free), and energetic interactions between the framework and adsorbed molecules. The last one is usually characterized by the isosteric heat of adsorption at infinite dilution (st). Based on an examination of all the properties of the absorbent listed above and the adsorbed amount under above conditions, the relationship of methane storage capacity of the eight IRMOFs with their surface area is studied and the results are listed in Fig. 4. Obviously, a linear relationship exists for the four non-catenated IRMOFs, while a deviation is observed for the catenated IRMOFs with larger methane adsorption capacity per surface area (except for IRMOF-15 with a lower value). The reason may be that, various smaller pores (4 in IRMOF-9, 6 in IRMOF-11 and 5 in IRMOF-13) are formed in the catenated frameworks, leading to a tighter package of methane molecules under the same condition. The isosteric heat of adsorption at infinite dilution (st) can support this conclusion: the values ofstin catenated IRMOFs (15.03, 18.38, 18.45, and 9.21 kJ·mol－1for IRMOF-9, 11, 13 and 15, respectively) are much larger than those in their corresponding non-catenated counterparts (8.42, 10.12, 9.67, and 7.47 kJ·mol－1for IRMOF-10, 12, 14, and 16, respectively).

Figure 4 Adsorbed amount of methane at 3.5 MPa and 298 Kaccessible surface area ■?IRMOF-9;□?IRMOF-10;●?IRMOF-11;○?IRMOF-12; ▲?IRMOF-13;△?IRMOF-14;▼?IRMOF-15;▽?IRMOF-16

To further understand the occupying situation of methane molecules in the studied MOFs, the probability distributions of center of mass (COM) for CH4in IRMOF-9 and its non-catenated counterpart IRMOF-10 at 3.5 MPa and 298 K are depicted in Fig. 5. For IRMOF-10, methane molecules mainly accumulate in the metal cluster regions, and there is still much space in the pores of the material up to 3.5 MPa. On the contrary, methane molecules are much more strongly adsorbed in IRMOF-9, with strongest accumulation in the small pores formed by two metal clusters and organic linkers, and second strongest adsorption in the pores formed by a metal cluster and an organic linker in another chain. Fig. 5 shows that, the density of methane in IRMOF-9 is much larger than that in IRMOF-10, and methane molecules distribute more uniformly in the former, due to the presence of various small pores of different sizes and additional adsorption sites. This figure clearly illustrates the advantage of catenated MOFs for methane storage over the non-catenated ones, and provides a better understanding of methane distributions in MOFs with and without catenation.

Figure 5 Contour plots of the COM probability densities of methane in the-plane for all of the methane molecules in IRMOF-9 and IRMOF-10

4 ConclusionS

In summary, the simulation results obtained in this work shows that the amount of adsorbed methane is greatly enhanced in the four catenated IRMOFs compared with their non-catenated counterparts, due to the additional small pores and adsorption sites formed by the catenation of the two separate frameworks. This work also shows that catenated MOFs can meet the DOE target easily for methane storage, indicating that the creation of catenated frameworks is a promising strategy for developing MOF-based efficient methane storage materials in vehicular applications. As a result, a rational combination of catenation with chemical composition may lead to new MOFs with large methane storage capacity.

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the National Natural Science Foundation of China (20706002, 20725622, 20876006) and Beijing Nova Program (2008B15).

** To whom correspondence should be addressed. E-mail: qyyang@mail.buct.edu.cn