Highlights
stickiness – the so-called peel strength – of 200N m−1
, which is similar to peeling
sticky tape off glass. These nano staples open up solutions
to a large number of technical challenges, for example in medical engineering.
Scientists using grand canonical Monte Carlo simulations have defined the physical limits of hydrogen adsorption in carbon-based porous structures (B. Kuchta, L. Firlej, A. Mohammadhosseini, P. Boulet, M. Beckner, J. Romanos, P. Pfeifer; J. Amer. Chem. Soc., 2012, 15130). The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity.
The ultimate H2 storage system
Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets (Scheme 9), which creates surface areas exceeding of 2600m2
/g,
the maximum surface area for infinite graphene sheets, carried mainly by edge sites, referred to as open carbon frameworks (OCF). For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800–6500m2
storage capacity of 100–260g of H2 C at 77K and 100bar.
G. Richard Stephenson University of East Anglia, UK
Organic chemistry
Using hybridisation changes in molecular sensors When organic molecules are used in molecular sensor applications, a fine balance is needed between strength of binding of the analyte and specificity of response. Fortunately, there are a great many
features that can be varied in the molecular design. Copper salts are not the most common choice of analyte, but as the authors point out in their introduction (Z. Liu, C. Zhang, X. Wang, W. He, Z. Guo; Org. Lett., 2012, 14, 4378), measuring Cu(II) gives important information on the clinical management of a surprisingly large range of diseases, such as Menkes syndrome, Wilson’s disease and, most famously, Alzheimer’s disease. The sensor molecule 1 is an elegant
structure with just the right amount of flexibility. As shown in Figure 1, it wraps round its target ion and the interaction
54 Chemistry&Industry • November 2012
of the nitrogen atom that is present as a substituent on the fluorophore causes a profound change in the response. This is a consequence of an equally profound effect on the hybridisation state of the nitrogen. Without copper present, the nitrogen
is sp2
hybridised and the lone pair is delocalised into the π system of the fluorophore.
Copper likes the tetracoordinate array of ligand sites available in 1, and binds strongly to all four heteroatoms, and in doing so, changes the hybridisation state of the ‘probe nitrogen’, which becomes sp3
by forming the additional σ bond to
the copper, and so the electron density from the lone pair is withdrawn from the fluorophore.
Wrapping up a silver ion Silver(I) is the target of the receptor structure 2 (Figure 2), and the mode of operation of the molecule is very different
to the example in Figure 1. [4]-Cyclens have a ring of four
nitrogen atoms joined by CH2CH2 links.
Since nitrogen is trivalent, and two bonds are needed on each nitrogen for it to take its place in the ring, there is a third bond available at each nitrogen to mount substituents. This is a convenient design
because sp3 nitrogen atoms are not
configurationally stable, and without a guest ion, all conceivable orientations of the four substituents should be accessible. The crystal structure of the silver triflate adduct shows the Ag+ ion sitting above the puckered ring of four nitrogen atoms, holding the entire structure in a specific conformation. This molecule wraps up its guest (the Ag+
ion) within a ring of aromatic
groups (Y. Habata, M. Ikeda, S. Yamada, H. Takahashi, S. Ueno, T. Suzuki, S. Kuwahara; Org. Lett., 2012, 14, 4576). Titration experiments were used to
Scheme 9 Open carbon frameworks
Adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. The authors conclude that OCF
a record maximum excess adsorption of 75–85g of H2
/g, calculations indicate /kg of C at 77K and record
/kg of
structures, if synthesised, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures.
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