Article

On the Nature of the Adsorbed Hydrogen Phase in Microporous Metal-Organic Frameworks at Supercritical Temperatures

Chemical and Environmental Sciences Laboratory, General Motors Corporation, Warren, Michigan 48090, USA.
Langmuir (Impact Factor: 4.38). 09/2009; 25(20):12169-76. DOI: 10.1021/la901680p
Source: PubMed

ABSTRACT Hydrogen adsorption measurements on different metal-organic frameworks (MOFs) over the 0-60 bar range at 50 and 77 K are presented. The results are discussed with respect to the materials' surface area and thermodynamic properties of the adsorbed phase. A nearly linear correlation between the maximum hydrogen excess amount adsorbed and the Brunauer-Emmett-Teller (BET) surface area was evidenced at both temperatures. Such a trend suggests that the adsorbed phase on the different materials is similar in nature. This interpretation is supported by measurements of the adsorbed hydrogen phase properties near saturation at 50 K. In particular it was found that the adsorbed hydrogen consistently exhibits liquid state properties despite significant structural and chemical differences between the tested adsorbents. This behavior is viewed as a consequence of molecular confinement in nanoscale pores. The variability in the trend relating the surface area and the amount of hydrogen adsorbed could be explained by differences in the adsorbed phase densities. Importantly, the latter were found to lie in the known range of bulk liquid hydrogen densities. The chemical composition and structure (e.g., pore size) were found to influence mainly how adsorption isotherms increase as a function of pressure. Finally, the absolute isotherms were calculated on the basis of measured adsorbed phase volumes, allowing for an estimation of the total amounts of hydrogen that can be stored in the microporous volumes at 50 K. These amounts were found to reach values up to 25% higher than their excess counterparts, and to correlate with the BET surface areas. The measurements and analysis in this study provide new insights on supercritical adsorption, as well as on possible limitations and optimization paths for MOFs as hydrogen storage materials.

Full-text

Available from: Eric Poirier, Jun 02, 2015
0 Followers
 · 
303 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Metal–organic frameworks (MOFs) have been postulated for years as industrially achievable materials for CO2 capture and H2 storage. However, a great leap forward their real applications is still pending. This article details the design of a MOF material, including the reasoned choices of metal ion, organic linker, and even the structural subunits, as efficient adsorbent of both CO2 and H2. In particular, it was planned (i) to raise the polarizability of the framework by using a highly N-rich organic linker and (ii) to favor the formation of nanocavities with suitable size to confine small molecules. The resultant tetrazole-imidazole-based ITF-1 material containing nanocubes certainly achieved all these aimed premises. Nevertheless, such structural and compositional distinctiveness was reflected in some noteworthy adsorption features. CO2 adsorption was highly remarkable, and it was characterized by a wide hysteresis loop along the whole studied range of pressures. H2 molecules totally filled the nanocavities under only 0.1 bar at 77 K, and at 273 K, H2 uptake became tens of times higher than expected according to their textural properties. These unique adsorption results, together with the discussed relationship between structure/composition of ITF-1 and their adsorption features, underline the importance of a tailored MOF materials for particular applications.
    Crystal Growth & Design 01/2014; 14(2):739–746. DOI:10.1021/cg401613w · 4.56 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ni/MCM-41 samples have been successfully prepared by wet impregnation method with different degrees of metal loading. Various techniques including X-ray diffraction, N-2 adsorption-desorption, transmission electron microscopy, inductively coupled plasma atomic emission spectroscopy, electron microprobe analysis, UV-Vis diffuse reflectance spectra, infrared analysis, adsorption of pyridine coupled to infrared spectroscopy and hydrogen adsorption at 77 K at high and low pressure conditions were employed for the materials characterization. The Ni loading degree had a marked influence on the structural, chemical, acid and hydrogen storage properties of the samples. Thus, a low Ni loading favors the presence of highly dispersed Ni species responsible of the Lewis acidity. These species would promote hydrogen-favorable sites leading to a positive effect on the hydrogen storage capacity.
    Microporous and Mesoporous Materials 06/2014; 191:103–111. DOI:10.1016/j.micromeso.2014.03.005 · 3.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Hydrogen and methane adsorption is studied on a range of nanoscale carbon slit pores up to 1000 bar at 298 K using molecular dynamics. Past about 200 bar, the calculated adsorbed hydrogen density increases as a function of pressure at the same rate as the highly compressed bulk liquid and thus no saturation plateau is predicted under these conditions. This behaviour implies a continuous increase of the adsorbed hydrogen density past the normal boiling point value and explains high pressure experimental hydrogen adsorption data at 298 K on porous carbons, such as AX-21 activated carbon. This result is put into perspective by comparing with the adsorbed hydrogen phase measured at 50 K on AX-21, which exhibits an ideal incompressible liquid behaviour and a maximum density of only 0.06 g mL−1. These findings therefore suggest the existence of two distinct temperature dependent saturation regimes, most likely of quantum origin. The volumetric capacities show that the adsorbents provide no gains over compression past 600 bar at about 0.04 g mL−1. Conversely, gravimetric capacities inferior to 0.03 g g−1 found below 200 bar indicate large mass penalties when significant gains over compression are achieved. The calculated adsorbed methane density reaches at about 50 bar a true saturation plateau comparable to the pressurized bulk liquid at lower temperatures. Large volumetric and gravimetric capacities of about 365 v v−1 and 0.21 g g−1, respectively, are found in these conditions. These results therefore indicate an interesting 10 fold improvement over compression and a small mass penalty for methane adsorbed on well compacted engineered materials.
    RSC Advances 05/2014; 4(44):22848. DOI:10.1039/C4RA02553A · 3.71 Impact Factor