Organic Framework MOF-5 Research Articles

Adsorption and thermodesorption characteristics of benzene in nanoporous metal organic framework MOF-5

This study has been prepared and characterized a highly nanoporous metal organic framework (MOF-5). The nitrogen adsorption isotherm, gravimetric adsorption and thermodesorption method that utilize a quartz spring balance are used to examine the unusual adsorption behavior. Especially, the hybrid Langmuir–Sips isotherm equation is employed to correlate the experimental isotherm data. Moreover, the unusual adsorption behavior in MOF-5 is discussed in relation to its isosteric enthalpy of adsorption, adsorption energy distribution and desorption energy distribution calculated from benzene adsorption and thermodesorption measurements.

Ab initio and periodic DFT investigation of hydrogen storage on light metal-decorated MOF-5

The effect of light metal (M = Li, Be, Mg, and Al) decoration on the stability of metal organic framework MOF-5 and its hydrogen adsorption is investigated by ab initio and periodic density functional theory (DFT) calculations by employing models of the form BDC:M 2:nH 2 and MOF-5:M 2:nH 2, where BDC stands for the benzenedicarboxylate organic linker and MOF-5 represents the primitive unit cell. The suitability of the periodic DFT method employing the GGA-PBE functional is tested against MP2/6-311 + G* and MP2/cc-pVTZ molecular calculations. A correlation between the charge transfer and interaction energies is revealed. The metal-MOF-5 interactions are analyzed using the frontier molecular orbital approach. Difference charge density plots show that H 2 molecules get polarized due to the charge generated on the metal atom adsorbed over the BDC linker, resulting in electrostatic guest-host interactions. Our solid state results show that amongst the four metal atoms, Mg and Be decoration does not stabilize the MOF-5 to any significant extent. Li and Al decoration strengthened the H 2-MOF-5 interactions relative to the pure MOF-5 exhibited by the enhanced binding energies. The hydrogen binding energies for the Li- and Al-decorated MOF-5 were found to be sensible for allowing reversible hydrogen storage at ambient temperatures. A high hydrogen uptake of 4.3 wt.% and 3.9 wt.% is also predicted for the Li- and Al-decorated MOF-5, respectively.

Elucidating Negative Thermal Expansion in MOF-5

Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

Improved synthesis and hydrogen storage of a microporous metal–organic framework material

A microporous metal–organic framework MOF-5 [Zn 4O(BDC) 3, BDC = 1,4-benzenedicarboxylic] was synthesized with and without H 2O 2 by improved methods based on the previous studies. The obtained materials were characterized by X-ray diffraction, scanning electron microscopy and nitrogen adsorption, and their hydrogen storage capacities were measured. The synthesis experiments showed that H 2O 2 favored the growth of high quality sample, large pore volume and high specific surface area. The measurements of hydrogen storage indicated that the sample with higher specific surface area and large pore volume showed better hydrogen storage behavior than other samples. It was suggested that specific surface area and pore volume influenced the capacity of hydrogen storage for MOF-5 material.

Mechanism of benzene diffusion in MOF-5: A molecular dynamics investigation

In this contribution, the diffusion of benzene in the porous metal organic framework MOF-5 is investigated by molecular dynamics simulations. Previously, we have shown that by using a first principles derived fully flexible force field the experimentally determined self-diffusion coefficients D self could be well reproduced [S. Amirjalayer, M. Tafipolsky, R. Schmid, Angew. Chem. Int. Ed. 46 (2007) 463]. Here, we use the same methodology to determine the loading dependence on the diffusion. It is found that diffusivity, which is in the range of liquid benzene, slightly increases up to a load of 32 molecules per unit cell and then falls off at higher load. Free energy maps reveal that additional sites appear at higher load due to attractive guest–guest interactions. The topology of these sites is very close to the experimentally determined locations of ferrocene molecules in MOF-5, which corroborates that attractive π – π interactions govern these systems. The site–site and site-phenylene distances are very similar to the first solvation radius of liquid benzene. For the very open MOF-5, the main barrier for diffusive transport is to overcome the attractive interaction in the binding pockets, which is in contrast to zeolitic microporous systems, where the barrier for diffusion is the hindrance of the pore window. Spatial free energy maps are used to investigate the diffusion pathway on a molecular level and the load dependence of the free energy barriers for these transport processes.

Synthesis and hydrogen-storage behavior of metal–organic framework MOF-5

Metal–organic framework MOF-5 (Zn 4O(BDC) 3), a microporous material with a high surface area and large pore volume, was synthesized by three approaches: direct mixing of triethylamine (TEA), slow diffusion of TEA, and solvothermal synthesis. The obtained materials were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption, and their hydrogen-storage capacities were measured. The different synthesis methods influenced the pore-structure parameters, morphologies and hydrogen-storage behavior of the obtained MOF-5. MOF-5 synthesized by the solvothermal approach showed a higher surface area and larger pore volume than the samples prepared by the other two approaches. Measurements of the hydrogen-storage behavior showed that the hydrogen-storage capacity was correlated with the specific surface area and pore volume of MOF-5.

Hydrogen adsorption strength and sites in the metal organic framework MOF5: Comparing experiment and model calculations

Hydrogen adsorption in porous, high surface area, and stable metal organic frameworks (MOF’s) appears a novel route towards hydrogen storage materials [N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127; J.L.C. Rowsell, A.R. Millward, K. Sung Park, O.M. Yaghi, J. Am. Chem. Soc. 126 (2004) 5666; G. Ferey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-Guegan, Chem. Commun. (2003) 2976; T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille, G. Férey, Chem. Eur. J. 10 (2004) 1373]. A prerequisite for such materials is sufficient adsorption interaction strength for hydrogen adsorbed on the adsorption sites of the material because this facilitates successful operation under moderate temperature and pressure conditions. Here we report detailed information on the geometry of the hydrogen adsorption sites, based on the analysis of inelastic neutron spectroscopy (INS). The adsorption energies for the metal organic framework MOF5 equal about 800 K for part of the different sites, which is significantly higher than for nanoporous carbon materials (∼550 K) [H.G. Schimmel, G.J. Kearley, M.G. Nijkamp, C.T. Visser, K.P. de Jong, F.M. Mulder, Chem. Eur. J. 9 (2003) 4764], and is in agreement with what is found in first principles calculations [T. Sagara, J. Klassen, E. Ganz, J. Chem. Phys. 121 (2004) 12543; F.M. Mulder, T.J. Dingemans, M. Wagemaker, G.J. Kearley, Chem. Phys. 317 (2005) 113]. Assignments of the INS spectra is realized using comparison with independently published model calculations [F.M. Mulder, T.J. Dingemans, M. Wagemaker, G.J. Kearley, Chem. Phys. 317 (2005) 113] and structural data [T. Yildirim, M.R. Hartman, Phys. Rev. Lett. 95 (2005) 215504].

Solution infiltration of palladium into MOF-5: synthesis, physisorption and catalytic properties

Palladium was infiltrated into the highly porous metal–organic framework MOF-5 {[Zn4O(bdc)3], bdc = benzene-1,4-dicarboxylate} using [Pd(acac)2] (acac = acetylacetonate) as the precursor in chloroform solution via ‘incipient wetness’ impregnation. The specific surface area decreases from 2885 m2 g−1 to 958 m2 g−1 after infiltration. After reduction in hydrogen or under vacuum, the hydrogen adsorption capacity is increased from 1.15 wt% to 1.86 wt% at 1 atm and 77 K. The palladium loaded MOF-5 has a high catalytic activity in styrene hydrogenation comparable to that of palladium on activated carbon, but cis-cyclooctene hydrogenation is considerably slower. Even at room temperature, the catalyst is not stable in air due to the low hydrothermal stability of the MOF-5 support.

Molecular Dynamics Simulation of Benzene Diffusion in MOF‐5: Importance of Lattice Dynamics

Doing the locomotion: When benzene molecules diffuse through the metal organic framework MOF-5, they accumulate in pockets (see picture). Molecular dynamics simulations reveal that the correlated lattice motion increases this binding. Only when the the lattice dynamics are considered in the simulation, can the experimental diffusion constants be reproduced.

Modelling of hydrogen adsorption in the metal organic framework MOF5

Metal organic frameworks are formed by rigid organic spacer molecules linked by metal ions into a three-dimensional nano porous structure. The large internal surface (up to 4500 m 2 g −1) provides abundant sites for the adsorption of small molecules like hydrogen, which currently generates significant interest for their potential application as reversible hydrogen storage matrix. A prerequisite for hydrogen storage at ambient conditions is that the interaction strength is sufficiently large. Using VASP ab initio computer calculations the adsorption interaction strength for MOF5 is found to be of the order of 70 meV per H 2 molecule, where the main adsorption interaction is with the oxygen atoms. Such value compares well with adsorption experiments on MOF5 and it exceeds that obtained in nanostructured pure carbon materials. The J = 0 to J = 1 rotational transition of the H 2 determined from inelastic neutron scattering results of Rosi et al. [N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127] is compared to the simulation results, and a hydrogen self diffusion coefficient is estimated.