Novel 2D micro-porous Metal-Organic Framework for hydrogen storage

Highlights

• Crystal structure of a novel 2D MOF is investigated.

• Surface characteristics and hydrogen adsorption capacities are simulated.

• Hydrogen adsorption capability investigated for inner sites and surface region.

Abstract

A novel two dimensional Metal-Organic Framework (MOF) structured compound with trimesic acid (TMA), 1,10 Phenantroline (Phen) and Cu(II) building blocks were synthesized and characterized experimentally. Then Grand Canonical Monte Carlo (GCMC) simulation calculations used for determination of hydrogen adsorption capacity and surface characteristics of the compound theoretically. Three different regions were determined for the adsorbent, which were micro, micro/sub-meso spaces inside the adsorbent and the surface regions. It is found that the synthesized compound could uptake approx. 1.3 and 1.2 wt.% hydrogen at 77 K, 100 bars and 1 bar respectively. Thus the adsorbent that is synthesized in this work have strong hydrogen adsorption capability in comparison to the previously reported ones.

Introduction

Many energy systems are investigated as an alternative for the existing energy systems in the past decades. One of the most promising alternatives, hydrogen energy systems, have some barriers for common usage. Storing hydrogen efficiently in the perspective of storage performance, economy, and other criteria is a barrier that is needed to be passed. Using Metal-Organic Framework (MOF) structured materials, which consist of metal/metal clusters and organic ligands as linkers [1], [2], is one of the promising candidates for efficient hydrogen storage.

MOFs have many advantages with their tunable structures and pores, thus single ligand and mixed ligand crystalline forms are also possible. MOF-5 (Zn₄O(BDC)₃, where BDC²⁻ = 1,4-benzenedicarboxylate) [3], MOF-177 (Zn₄O(BTB)₂, where BTB³⁻ = 1,3,5-benzenetribenzoate) [4], and NU-100 (Cu₃(ttei), where ttei = 5,5′,5″-(((benzene-1,3,5-triyltris(ethilene-2,1-diyl))tris(benzene-4,1-diyl))tris(etilene-2,1-diyl)) triisophthalate) [5] are some of famous MOF structured compounds which have one ligand as building block inside. MOF-210 (Zn₄O(BTE)(BPDC), where BTE³⁻ = 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate and BPDC²⁻ = biphenyl-4,4′-dicarboxylate) [6], SNU-5′(Cu₂(abtc)(DMF)₂ where abtc; azobenzene-3,3′,5,5′-tetracarboxylate and DMF; di methyl formamide) [7] and without an abbreviation Cu₂(bdc)(dabco) (where bdc; benzene-1,4-dicarboxylate and dabco; 1,4-diazobicyclo [2.2.2] oktane) [8] are some examples of mixed ligand MOFs.

According to IUPAC [9], MOFs are described as 1, 2, or 3 dimensional network polymers extending through coordination bonds. The number of coordination network dimension increases the thermal stability of the final compound, as expected. In another point of view, it is able to construct two or three dimensional coordination polymers in thermally aided synthesize procedures [10], [11], [12], [13].

Hydrogen can be stored in different medias through physical adsorption, or physisorption. Spillover has a big impact on hydrogen storage capacity and on the importance of porosity. Open metal sites inside the compounds act like a metal additive which increases the spillover effect of hydrogen. Li and Yang [14], [15] reported that the platinum loaded activated carbon which enables more open metal sites for spillover, and platinum loaded activated carbon doped IRMOF-1 and IRMOF-8 could store 5–8 times more hydrogen in comparison to un-doped structures. Similarly, soc (square octahedral cubic) topology provides more open metal sites inside the MOFs, consequently mentioned sites increase the stored amount [16]. Zhou and co-workers [17] explained the effect of metal kind on open metal sites and their relations with hydrogen storage; the effect of open magnesium metal sites on hydrogen storage properties were investigated by Sumida et al. [18].

Hydrogen storage properties of the materials can be investigated by using molecular simulation calculations, in addition to experimental techniques. GCMC (Grand Cannonical Monte Carlo) ensemble [19] is commonly used to simulate hydrogen storage properties by physisorption. Interactions between hydrogen and host materials examined through GCMC by calculating possible positions of the hydrogen molecules, statistically [20], [21]. Force field based GCMC calculations have been used for investigating hydrogen storage properties [22], [23], [24]. Yang and Zhong [25] calculated gas adsorption properties of MOF-5 in the gas mixture by using a special force field, TraPPE (transferable potentials for phase equilibria) [26] with GCMC.

MOFs are able to uptake huge amounts of hydrogen with their high surface area. On the one hand, surface area plays an important role on hydrogen storage for highly porous adsorbents, on the other hand electrical charges (or spillover) of the metal centers or clusters inside are more effective for adsorbents that have small accessible surface areas [27], [28], [29]. Both phenomena effect the adsorption energy and it can be calculated using SA-MC (Simulated Annealing-Monte Carlo) simulations. Kim et al. [30] reported adsorption energies and population analysis of the interaction energies porphyrin based covalent organic polyhedral for hydrogen adsorption theoretically using SA-MC.

A novel two dimensional MOF structured compound synthesized and the crystal structure determined experimentally in this work. Then the determined crystal structure used for simulating hydrogen storage and other characteristic calculations. At the final, hydrogen storage properties, surface area and characteristic of synthesized compound determined theoretically. Theoretical investigation of hydrogen adsorption characteristics of experimentally synthesized, and characterized compound is the aim in this work. Therefore the gravimetric hydrogen uptake capacities at 77–298 K and 1–100 bar pressures, BET surface area, and pore size distribution calculations realized by using simulated hydrogen and nitrogen adsorption data after determination of the crystal structure experimentally. Surface characteristics and adsorption energies also calculated by using SA-MC for vacuum slab constructed on the surface of the compound in (0 0 −1) plane.