Showing papers by "Taner Yildirim published in 2010"

Graphene oxide framework materials: theoretical predictions and experimental results.

Abstract: A series of idealized model systemswith various diboronic acid linker concentrations (and con-sequently different pore size, pore volume, and surface areas)were examined (see Supporting Information for details).Structural optimization yielded a circa 1.1 nm interlayerseparation for these ideal structures. The simulated absolute

Adsorption Sites and Binding Nature of CO2 in Prototypical Metal−Organic Frameworks: A Combined Neutron Diffraction and First-Principles Study

Abstract: We report a detailed study of CO2 adsorption in two important metal−organic framework (MOF) compounds (Mg-MOF-74 and HKUST-1). In both MOFs, the open metal ions were identified as the primary binding sites through neutron diffraction measurements. The relatively strong metal−CO2 binding was attributed to an enhanced electrostatic interaction, and vibrational mode analysis shows that the adsorbed CO2 molecule is strongly attached through one of its oxygen atoms while the rest of the molecule is relatively free. This high orientational disorder is the reason for the large apparent O−C−O bond bending angle derived from diffraction measurements. Our calculations give only a small degree of bond bending, suggesting that the CO2 adsorption on the open metal site is still largely physisorption. Interestingly, the overall metal−CO2 binding strength is right in the range which can facilitate both adsorption (CO2 capture) and desorption (MOF regeneration) under typical flue gas conditions.

Metal–Organic Frameworks with Exceptionally High Methane Uptake: Where and How is Methane Stored?

Abstract: Metal-organic frameworks (MOFs) are a novel family of physi- sorptive materials that have exhibited great promise for methane storage. So far, a detailed understanding of their methane adsorption mechanism is still scarce. Herein, we report a comprehen- sive mechanistic study of methane stor- age in three milestone MOF com- pounds (HKUST-1, PCN-11, and PCN- 14) the CH4 storage capacities of which are among the highest reported so far among all porous materials. The three MOFs consist of the same dicopper paddlewheel secondary building units, but contain different organic linkers, leading to cagelike pores with various sizes and geometries. From neutron powder diffraction experiments and ac- curate data analysis, assisted by grand canonical Monte Carlo (GCMC) simu- lations and DFT calculations, we un- ambiguously revealed the exact loca- tions of the stored methane molecules in these MOF materials. We found that methane uptake takes place primarily at two types of strong adsorption site: 1) the open Cu coordination sites, which exhibit enhanced Coulomb at- traction toward methane, and 2) the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple "surfa- ces". Interestingly, the enhanced van der Waals sites are present exclusively in small cages and at the windows to these cages, whereas large cages with relatively flat pore surfaces bind very little methane. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the frac- tion of large pores.

A new family of metal borohydride ammonia borane complexes: Synthesis, structures, and hydrogen storage properties

Abstract: We report the first two examples of borohydride ammonia borane complexes: Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2. Their structures are successfully determined using a combination of X-ray diffraction and first-principles calculations. Both structures are composed of alternating layers of borohydride and ammonia borane. Examination of bond lengths indicates that this arrangement is stabilized via dihydrogen bonding between ammonia borane and their surrounding BH4−, and the interactions between ammonia borane ligands and cations. Our experimental results show that more than 10 wt% and 11 wt% hydrogen can be released from Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2, respectively. Negligible ammonia was detected compared to ammonia borane and its ammidoborane derivatives. Further improvements are needed to reduce borazine emission. Cycling studies show that decomposed Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2 can be partially hydrogenated under hydrogen pressures at high temperatures.

Gas Adsorption Properties of Graphene-Oxide-Frameworks and Nanoporous Benzene-Boronic Acid Polymers

Abstract: Submitted for the MAR10 Meeting of The American Physical Society Gas Adsorption Properties of Graphene-Oxide-Frameworks and Nanoporous Benzene-Boronic Acid Polymers JACOB BURRESS, JASON SIMMONS, NIST Center for Neutron Research, JAMIE FORD, TANER YILDIRIM, NIST Center for Neutron Research, University of Pennsylvania — There has been a recent resurgence in graphene oxide research as a potential route to large scale graphene synthesis. Recent research has also used dehydration reactions of boronic acids for the formation of covalent organic frameworks (COFs) and other new nanoporous materials. We are trying to synthesize graphene-oxide-frameworks (GOFs) by linking the OH groups on graphene oxide with benzene-boronic acids. Our initial x-ray studies indicate that the benzene-boronic acids are successfully incorporated into graphene-oxide (GO) layers expanding the interlayer spacing up to 12 Ang. We also found that the amorphous phases of bare dehydrated benzeneboronic acid polymers (amorphous borocarbons, ABCs) show quite interesting and unusual hydrogen adsorption behavior. The diffusion of hydrogen into the sample is thermally activated. While there is no adsorption at 30 K, the rate of excess adsorption increases with increasing temperature up to 70 K. We will present detailed high-pressure isotherms of H2/CO2/Methane at different temperatures of these interesting new GOF materials and dehydrated boronic acid polymers.