26 research outputs found
Analyzing the frequency shift of physiadsorbed CO2 in metal organic framework materials
Combining first-principles density functional theory simulations with IR and
Raman experiments, we determine the frequency shift of vibrational modes of CO2
when physiadsorbed in the iso-structural metal organic framework materials
Mg-MOF74 and Zn-MOF74. Surprisingly, we find that the resulting change in shift
is rather different for these two systems and we elucidate possible reasons. We
explicitly consider three factors responsible for the frequency shift through
physiabsorption, namely (i) the change in the molecule length, (ii) the
asymmetric distortion of the CO molecule, and (iii) the direct influence of
the metal center. The influence of each factor is evaluated separately through
different geometry considerations, providing a fundamental understanding of the
frequency shifts observed experimentally.Comment: 9 pages, 4 figure
Theoretical and experimental analysis of H2 binding in a prototype metal organic framework material
Hydrogen adsorption by the metal organic framework (MOF) structure
Zn2(BDC)2(TED) is investigated using a combination of experimental and
theoretical methods. By use of the nonempirical van der Waals
density-functional (vdW-DF) approach, it is found that the locus of deepest H2
binding positions lies within two types of narrow channel. The energies of the
most stable binding sites, as well as the number of such binding sites, are
consistent with the values obtained from experimental adsorption isotherms and
heat of adsorption data. Calculations of the shift of the H-H stretch frequency
when adsorbed in the MOF give a value of approximately -30 cm-1 at the
strongest binding point in each of the two channels. Ambient temperature
infrared absorption spectroscopy measurements give a hydrogen peak centered at
4120 cm-1, implying a shift consistent with the theoretical calculations.Comment: 4 pages, 4 figure
Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration
Instability of most prototypical metal organic frameworks (MOFs) in the
presence of moisture is always a limita- tion for industrial scale development.
In this work, we examine the dissociation mechanism of microporous paddle wheel
frameworks M(bdc)(ted)0.5 [M=Cu, Zn, Ni, Co; bdc= 1,4-benzenedicarboxylate;
ted= triethylenediamine] in controlled humidity environments. Combined in-situ
IR spectroscopy, Raman, and Powder x-ray diffraction measurements show that the
stability and modification of isostructual M(bdc)(ted)0.5 compounds upon
exposure to water vapor critically depend on the central metal ion. A
hydrolysis reaction of water molecules with Cu-O-C is observed in the case of
Cu(bdc)(ted)0.5. Displacement reactions of ted linkers by water molecules are
identified with Zn(bdc)(ted)0.5 and Co(bdc)(ted)0.5. In contrast,.
Ni(bdc)(ted)0.5 is less suscept- ible to reaction with water vapors than the
other three compounds. In addition, the condensation of water vapors into the
framework is necessary to initiate the dissociation reaction. These findings,
supported by supported by first principles theoretical van der Waals density
functional (vdW-DF) calculations of overall reaction enthalpies, provide the
necessary information for de- termining operation conditions of this class of
MOFs with paddle wheel secondary building units and guidance for developing
more robust units