adsorbed component. The data show that both the pure Li and the Na,Li samples
completely exclude CH 4 . The Cs,Li sample does adsorb a very small amount of CH 4 .
The fully exchanged sample shows the slowest CO 2 uptake rate, which leads to a
very broad transition to the final CO 2 concentration. Adding a small amount of
cations clearly improves the CO 2 kinetics. The improvement is just marginal for the
Li,Na sample but very significant for the Li,Cs-Rho, as can be clearly seen from the
sharp profile of its breakthrough curve. These observations are consistent with the
fact that as the rate of CO 2 uptake improves, a small amount of CH 4 is also adsorbed.
From the simple observation of the breakthrough profiles, one can then conclude
that, compared to the pristine sample, the addition of cations improves the gas
transport by enlarging the window opening. There is therefore a trade-off; a larger
access facilitates the access of larger molecules lowering the selectivity.
Complementing the kinetic studies with more in-depth characterisation allows a
full understanding of the mechanisms involved. In situ XRD have in fact revealed
that the presence of Li
+ (even if not directly blocking the D8R windows) results in
framework distortion that makes the 8R windows very elliptical resulting in the slow
CO 2 kinetics seen for the fully exchanged Li-Rho. The presence of small amounts of
larger cations has the main advantage to control the extent of such distortion
allowing a fine-tuning of the molecular sieving properties of these materials. This
represents a new type of cation-controlled molecular sieving which acts on the extent
of framework distortion rather than on the partial blocking of the pore access. The
main advantage is the capability of creating materials that can be tuned at the
molecular level for the size selection of specific molecules.
5 Conclusions
The effort of researchers in developing novel materials with improved equilibrium
and kinetic properties has produced a wide range of new prototype materials that
require rapid and reliable methodologies to determine their process performance.
This creates the engineering challenge to continuously tune and improve experimental techniques to cope with the increased complexity of the materials. In this, the
key step is the capability to provide rapid feedback to the inventors of the novel
materials using only very small samples. This is where the ZLC technique has a very
important role at the early stages of development where samples are generated in
small batches, given that it is essential to be able successfully progress towards the
synthesis of competitive materials that can be used in real applications and be scaled
up to tackle the upcoming separation challenges.
The correct use of the technique requires an understanding of the theory of the
experiment and points to the need to carry out at least a minimal set of experiments:
two flowrates to confirm whether the system is in equilibrium or kinetic control;
when clearly in kinetic control (L > 10) an additional partial loading experiment to
identify unambiguously the time constant of the system and provide direct evidence
that can confirm or exclude the presence of surface barriers.
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E. Mangano and S. Brandani
completely exclude CH 4 . The Cs,Li sample does adsorb a very small amount of CH 4 .
The fully exchanged sample shows the slowest CO 2 uptake rate, which leads to a
very broad transition to the final CO 2 concentration. Adding a small amount of
cations clearly improves the CO 2 kinetics. The improvement is just marginal for the
Li,Na sample but very significant for the Li,Cs-Rho, as can be clearly seen from the
sharp profile of its breakthrough curve. These observations are consistent with the
fact that as the rate of CO 2 uptake improves, a small amount of CH 4 is also adsorbed.
From the simple observation of the breakthrough profiles, one can then conclude
that, compared to the pristine sample, the addition of cations improves the gas
transport by enlarging the window opening. There is therefore a trade-off; a larger
access facilitates the access of larger molecules lowering the selectivity.
Complementing the kinetic studies with more in-depth characterisation allows a
full understanding of the mechanisms involved. In situ XRD have in fact revealed
that the presence of Li
+ (even if not directly blocking the D8R windows) results in
framework distortion that makes the 8R windows very elliptical resulting in the slow
CO 2 kinetics seen for the fully exchanged Li-Rho. The presence of small amounts of
larger cations has the main advantage to control the extent of such distortion
allowing a fine-tuning of the molecular sieving properties of these materials. This
represents a new type of cation-controlled molecular sieving which acts on the extent
of framework distortion rather than on the partial blocking of the pore access. The
main advantage is the capability of creating materials that can be tuned at the
molecular level for the size selection of specific molecules.
5 Conclusions
The effort of researchers in developing novel materials with improved equilibrium
and kinetic properties has produced a wide range of new prototype materials that
require rapid and reliable methodologies to determine their process performance.
This creates the engineering challenge to continuously tune and improve experimental techniques to cope with the increased complexity of the materials. In this, the
key step is the capability to provide rapid feedback to the inventors of the novel
materials using only very small samples. This is where the ZLC technique has a very
important role at the early stages of development where samples are generated in
small batches, given that it is essential to be able successfully progress towards the
synthesis of competitive materials that can be used in real applications and be scaled
up to tackle the upcoming separation challenges.
The correct use of the technique requires an understanding of the theory of the
experiment and points to the need to carry out at least a minimal set of experiments:
two flowrates to confirm whether the system is in equilibrium or kinetic control;
when clearly in kinetic control (L > 10) an additional partial loading experiment to
identify unambiguously the time constant of the system and provide direct evidence
that can confirm or exclude the presence of surface barriers.
142
E. Mangano and S. Brandani