4 Measurement of CO 2 Diffusivities in Zeolite Rho
In this section the use of the ZLC system is demonstrated on a series of cationexchanged Rho zeolites. These experiments allow to highlight also some of the
challenges that can be faced when trying to determine the kinetics of adsorption of
CO 2 of novel and complex materials.
Zeolite Rho has a structure characterised by a body-centred cubic arrangement of
α-cages connected by double eight-rings, with an effective channel diameter of about
3.6 Å [14]. The available sites which can accommodate the extra-framework cations
are the single eight-ring (S8R), the double eight-ring (D8R), and the single six-ring
(S6R) sites [15].
It has been proved that, passing from the hydrated form to the ion-exchanged one,
the structure of Rho zeolites passes from the centrosymmetric structure to the
non-centrosymmetric one [15]. From this follows that once extra framework cations
are introduced in the structure, the deformation induced, together with the size and
the position occupied by the cations, may create a severe blocking action to the
cages. Distribution and positioning of the cations inside the Rho structure have a key
role in the hindering effect to gas transport. These effects highly depend from the
size of the cation and if positioned in the D8R windows may even prevent the access
to molecules such as N 2 and CO 2 .
Webley and co-workers were among the first ones to identify what was defined as
the “molecular trapdoor” effect in samples of ion-exchanged Chabazite zeolites
[16, 17]. Binary breakthrough experiments using a mixture of CO 2 and CH 4 on Cs
and K Chabazites show complete exclusion of CH 4 over CO 2 establishing that only
molecules having affinity with the cations can “open” the trapdoor and access
the cage.
An equivalent behaviour was identified for the cation exchanged Rho zeolites
used for this study. Initial N 2 adsorption measurements in cryogenic conditions
showed no access for N 2 . On the other hand, CO 2 adsorption tests at higher
temperatures with both ZLC and volumetric methods showed a clear uptake, indicating the role of the cation mobility in allowing access to the cages to the gas
molecules [18].
The mechanism is of clear scientific interest, and the study of these systems serves
as a comprehensive example on the use of the ZLC technique. The kinetic study
reported in this section refers to two samples of cation-exchanged Rho zeolites, a
fully exchanged Na-Rho and Na,Cs-Rho. Figure 7 shows the cation distribution for
these two samples.
Understanding the transport mechanism in Na-Rho has led to the development of
Li-Rho zeolites with improved performance. Breakthrough curves measured on
three variant of Li-Rho samples are discussed at the end of the section. All samples
were synthesised at the University of St. Andrews by Prof. Wright’s research group.
For Na-Rho samples, ZLC columns were packed using close to 10 mg of crystals:
11.6 mg for Na,Cs-Rho and 8.4 mg for Na-Rho. All tests were carried out at 35
C
and 0.1 bar of CO 2 in He as carrier gas.
Measurement of Diffusion in Small Pore Zeolites to Improve Selectivity in. . .
131
In this section the use of the ZLC system is demonstrated on a series of cationexchanged Rho zeolites. These experiments allow to highlight also some of the
challenges that can be faced when trying to determine the kinetics of adsorption of
CO 2 of novel and complex materials.
Zeolite Rho has a structure characterised by a body-centred cubic arrangement of
α-cages connected by double eight-rings, with an effective channel diameter of about
3.6 Å [14]. The available sites which can accommodate the extra-framework cations
are the single eight-ring (S8R), the double eight-ring (D8R), and the single six-ring
(S6R) sites [15].
It has been proved that, passing from the hydrated form to the ion-exchanged one,
the structure of Rho zeolites passes from the centrosymmetric structure to the
non-centrosymmetric one [15]. From this follows that once extra framework cations
are introduced in the structure, the deformation induced, together with the size and
the position occupied by the cations, may create a severe blocking action to the
cages. Distribution and positioning of the cations inside the Rho structure have a key
role in the hindering effect to gas transport. These effects highly depend from the
size of the cation and if positioned in the D8R windows may even prevent the access
to molecules such as N 2 and CO 2 .
Webley and co-workers were among the first ones to identify what was defined as
the “molecular trapdoor” effect in samples of ion-exchanged Chabazite zeolites
[16, 17]. Binary breakthrough experiments using a mixture of CO 2 and CH 4 on Cs
and K Chabazites show complete exclusion of CH 4 over CO 2 establishing that only
molecules having affinity with the cations can “open” the trapdoor and access
the cage.
An equivalent behaviour was identified for the cation exchanged Rho zeolites
used for this study. Initial N 2 adsorption measurements in cryogenic conditions
showed no access for N 2 . On the other hand, CO 2 adsorption tests at higher
temperatures with both ZLC and volumetric methods showed a clear uptake, indicating the role of the cation mobility in allowing access to the cages to the gas
molecules [18].
The mechanism is of clear scientific interest, and the study of these systems serves
as a comprehensive example on the use of the ZLC technique. The kinetic study
reported in this section refers to two samples of cation-exchanged Rho zeolites, a
fully exchanged Na-Rho and Na,Cs-Rho. Figure 7 shows the cation distribution for
these two samples.
Understanding the transport mechanism in Na-Rho has led to the development of
Li-Rho zeolites with improved performance. Breakthrough curves measured on
three variant of Li-Rho samples are discussed at the end of the section. All samples
were synthesised at the University of St. Andrews by Prof. Wright’s research group.
For Na-Rho samples, ZLC columns were packed using close to 10 mg of crystals:
11.6 mg for Na,Cs-Rho and 8.4 mg for Na-Rho. All tests were carried out at 35
C
and 0.1 bar of CO 2 in He as carrier gas.
Measurement of Diffusion in Small Pore Zeolites to Improve Selectivity in. . .
131