Hydrogen Storage in Metal–Organic Frameworks

The dihydrogen adsorption isotherms of eight metal-organic frameworks (MOFs), measured at 77 K up to a pressure of 1 atm, have been examined for correlations with their structural features. All materials display approximately Type I isotherms with no hysteresis, and saturation was not reached for any of the materials under these conditions. Among the six isoreticular MOFs (IRMOFs) studied, the catenated materials exhibit the largest capacities on a molar basis, up to 9.8 H(2) per formula unit. The addition of functional groups (-Br, -NH(2), -C(2)H(4)-) to the phenylene links of IRMOF-1 (MOF-5), or their replacement with thieno[3,2-b]thiophene moieties in IRMOF-20, altered the adsorption behavior by a minor amount despite large variations in the pore volumes of the resulting materials. In contrast, replacement of the metal oxide units with those containing coordinatively unsaturated metal sites resulted in greater H(2) uptake. The enhanced affinities of these materials, MOF-74 and HKUST-1, were further demonstrated by calculation of the isosteric heats of adsorption, which were larger across much of the range of coverage examined, compared to those of representative IRMOFs. The results suggest that under low-loading conditions, the H(2) adsorption behavior of MOFs can be improved by imparting larger charge gradients on the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore volume is still a prerequisite feature.

Effects of functionalization, catenation, and variation of the metal oxide and organic linking units on the low-pressure hydrogen adsorption properties of metal-organic frameworks.

A series of isostructural metal−organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64·82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g−1 equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sit...

High capacity hydrogen adsorption in Cu(II) tetracarboxylate framework materials: the role of pore size, ligand functionalization, and exposed metal sites

This review focuses on important aspects of applying physisorption for the pore structural characterization of hierarchical materials such as mesoporous zeolites. During the last decades major advances in understanding the adsorption and phase behavior of fluids confined in ordered nanoporous materials have been made, which led to major progress in the physisorption characterization methodology (summarized in the 2015 IUPAC report on physisorption characterization). Here we discuss progress and challenges for the physisorption characterization of nanoporous solids exhibiting various levels of porosity from micro- to macropores. While physisorption allows one to assess micro- and mesopores, a widely employed method for textural analysis of macroporous materials is mercury porosimetry and we also review important insights associated with the underlying mechanisms governing mercury intrusion/extrusion experiments. Hence, although the main focus of this review is on physical adsorption, we strongly emphasize the importance of combining advanced physical adsorption with other complementary experimental techniques for obtaining a reliable and comprehensive understanding of the texture of hierarchically structured materials.

Recent advances in the textural characterization of hierarchically structured nanoporous materials

Determination of pore size and pore size distribution: 1. Adsorbents and catalysts

Calculations are presented of the potential energy profiles for atoms adsorbed in slit-like pores, and of the enhancement of the depth of the potential energy well compared with that for adsorption by a single surface. The vibrational frequency normal to the wall is calculated as a function of slit width and methods of analysing experimental data are suggested. A comparison is made with similar expressions for adsorption in cylindrical pores. Recently published data on the adsorption of noble gases by a series of microporous carbons is analysed and shown to be best represented in terms of slit-like pores having an average width (between the nuclei of carbon atoms) of about 0.77 nm. The width available to an adsorbed molecule is about 0.52 nm which is in quantitative agreement with the known molecular sieve properties of these carbons. A well-activated carbon has pores with a somewhat larger mean width. The calculated vibration frequencies lie in the range 5–10 × 1011 Hz. The calculated surface areas and micropore volumes are 3–4 times smaller than the “monolayer equivalent areas” and pore volumes derived from micropore adsorption of nitrogen and benzene. It is concluded, in agreement with Dubinin, that the micropore volume can be divided into ultra-micropores, consisting of 25–30 % of the total micropore volume having free widths of about 0.52 nm, together with supermicropores which are larger and are filled with adsorbed vapour only at higher relative pressures.

Adsorption in slit-like and cylindrical micropores in the henry's law region. A model for the microporosity of carbons

This paper presents precise experimental data for the adsorption (na) of nitrogen and the noble gases by two unactivated and two activated (10.8% and 22.0% burn-off) Saran type porous carbons over the temperature range 0–125°C and at pressures (p) up to 1 atm. The data are analysed in terms of virial type expansions of (i)na/p as a function of p and (ii) ln na/p as a function of na. The number of terms required in each equation to give an adequate representation of the data was determined for each system at each temperature by statistical analysis. Expansion (ii) was found to be superior in the sense that where there was a difference between the number of terms required, expansion (ii) required fewer terms than (i). These differences become more marked for the adsorption of the heavier gases at the lower temperatures. Henry's law constants obtained from (i) and (ii) were, however, in good agreement.The Henry's law constants and their temperature dependence were used to calculate differential enthalpies and standard differential entropies of adsorption (taking na= 10–3 mol g–1 as standard state) in the limit of ideal behaviour. Activation of the carbons decreased the Henry's law constants per unit surface area, and also the magnitude of the enthalpy of adsorption. The standard differential entropy of adsorption increases with activation, reflecting the larger “free volume” available to the adsorbed molecules which may be measured approximately either by the increase in micropore volume or by the increase in the monolayer equivalent surface area.

John C. Powl

Papers

Adsorption in slit-like and cylindrical micropores in the henry's law region. A model for the microporosity of carbons

Thermodynamics of the high temperature adsorption of some permanent gases by porous carbons