ZIF-8
is a flexible zeolitic imidazole-based metal–organic framework
whose narrow pore apertures swing open by reorientation of imidazolate
linkers and expand when probed with guest molecules. This work reports
on the crystal size dependency of both structural transitions induced
by N2 and Ar adsorption and dynamic adsorption behavior
of n-butanol using well-engineered ZIF-8 crystals
with identical surface area and micropore volume. It is found that
the crystal downsizing of ZIF-8 regulates the structural flexibility
in equilibrium adsorption and desorption of N2 and Ar.
Adsorption kinetics of n-butanol in ZIF-8 are strongly
affected by the crystal size, however, not according to a classical
intracrystalline diffusion mechanism. Our results suggest that structural
transitions and transport properties are dominated by crystal surface
effects. Crystal downsizing increases the importance of such surface
barriers.
Nanoporous materials find widespread applications in our society: from drug delivery to environmentally friendly catalysis and separation technologies. The efficient design of these processes depends crucially on understanding the mass transfer mechanism. This is conventionally determined by uptake or release experiments, carried out with assemblages of nanoporous crystals, assuming all crystals to be identical. Using micro-imaging techniques, we now show that even apparently identical crystals (that is, crystals of similar size and shape) from the same batch may exhibit very different uptake rates. The relative contribution of the surface resistance to the overall transport resistance varied with both the crystal and the guest molecule. As a consequence of this crystal diversity, the conventional approach may not distinguish correctly between the different mass transfer mechanisms. Detection of this diversity adds an important new piece of evidence in the search for the origin of the surface barrier phenomenon. Our investigations were carried out with the zeolite SAPO-34, a key material in the methanol-to-olefins (MTO) process, propane-propene separation and adsorptive heat transformation.
Keep ′em separated: Bioalcohols are alternatives to petroleum‐based chemicals. Particularly biobutanol is an interesting compound, with properties superior to bioethanol. It is difficult, however, to separate biobutanol from the mixtures produced by the fermentation of biomass. This Communication describes a zeolitic imidazolate framework suitable for the separation of biobutanol from fermentation broths by adsorption.
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