When referring to thermochemical properties, similar issues are addressed.
Hence, any form of reactivity between the adsorbates and the zeolite may induce a
fatal impact in the process performances. A good example to illustrate the problematic is the evolution in the last decades of the adsorbents employed in sugar
separation [53]. In a similar manner, the use of MgA zeolite in oxygen production
[54] constitutes an example of a stability issue not linked to the adsorbed phase. Due
to the formation of MgO [55], high exchanged MgA zeolites are not commercially
available owing to framework structure collapse upon calcination. Therefore, the
beneficial impact of Mg
2+ exchange for oxygen production cannot be exploited
beyond 75% exchange. It is worthwhile mentioning that as long as the feed to
separate becomes complex the stability problems may be accentuated. Then, in the
framework of the separation units associated with the biorefinery, controlling the
mentioned factors will be determinant.
6 Perspectives for Future Research
Most of the optimization/improvement strategies illustrated in the previous sections
rely on the modification of the extraframework cation distribution in order to modify
the adsorption and separation behavior of zeolites for a given application. Indeed, by
modifying the concentration and nature of the extraframework cations, both selectivity and capacities can be strongly modified. Thus, zeolites presenting good
predisposition for such modifications are expected to present higher probabilities
of finding final industrial applications. The analysis of the zeolite separation market
reveals that among the “big five” zeolites [56, 57] (the more commercialized ones),
those more extended in separation applications are FAU and LTA topologies. When
asking ourselves what makes these topologies so suited for most separation processes, some of the answers confirm the impact of the extraframework cation
distribution. Key factors are (1) an easy synthesis; (2) a high pore volume, i.e., a
high potential adsorption capacity; (3) a good accessibility via their large pore
apertures in the case of FAU or the possibility to tune accessibility via the cations
located in the pore apertures in the case of LTA; and (4) a high extraframework
cation content. Extraframework cations interact strongly with all polarizable molecules and are, therefore, often the key to achieve high selectivities.
As a consequence, the vast structural diversity of zeolite is, therefore, not really
exploited. This is certainly improvable since some exploratory work on MOFs has
shown that very peculiar pore geometries can lead to unique separation properties
[58, 59]. Among the 18 commercially available zeolites [60], few other zeolite
topologies combine all of these assets. In particular, there is a lack of large pore
volume, low Si/Al structures. Thus, concentrating the efforts of the synthesis
research in topologies meeting the mentioned criteria seems a good strategy to
maximize the success possibilities.
Among others, we can mention a second potential axe of improvement related to
the advancements in the integration of the separation process in the tailoring of the
222
J. Pérez-Pellitero and G. D. Pirngruber
Hence, any form of reactivity between the adsorbates and the zeolite may induce a
fatal impact in the process performances. A good example to illustrate the problematic is the evolution in the last decades of the adsorbents employed in sugar
separation [53]. In a similar manner, the use of MgA zeolite in oxygen production
[54] constitutes an example of a stability issue not linked to the adsorbed phase. Due
to the formation of MgO [55], high exchanged MgA zeolites are not commercially
available owing to framework structure collapse upon calcination. Therefore, the
beneficial impact of Mg
2+ exchange for oxygen production cannot be exploited
beyond 75% exchange. It is worthwhile mentioning that as long as the feed to
separate becomes complex the stability problems may be accentuated. Then, in the
framework of the separation units associated with the biorefinery, controlling the
mentioned factors will be determinant.
6 Perspectives for Future Research
Most of the optimization/improvement strategies illustrated in the previous sections
rely on the modification of the extraframework cation distribution in order to modify
the adsorption and separation behavior of zeolites for a given application. Indeed, by
modifying the concentration and nature of the extraframework cations, both selectivity and capacities can be strongly modified. Thus, zeolites presenting good
predisposition for such modifications are expected to present higher probabilities
of finding final industrial applications. The analysis of the zeolite separation market
reveals that among the “big five” zeolites [56, 57] (the more commercialized ones),
those more extended in separation applications are FAU and LTA topologies. When
asking ourselves what makes these topologies so suited for most separation processes, some of the answers confirm the impact of the extraframework cation
distribution. Key factors are (1) an easy synthesis; (2) a high pore volume, i.e., a
high potential adsorption capacity; (3) a good accessibility via their large pore
apertures in the case of FAU or the possibility to tune accessibility via the cations
located in the pore apertures in the case of LTA; and (4) a high extraframework
cation content. Extraframework cations interact strongly with all polarizable molecules and are, therefore, often the key to achieve high selectivities.
As a consequence, the vast structural diversity of zeolite is, therefore, not really
exploited. This is certainly improvable since some exploratory work on MOFs has
shown that very peculiar pore geometries can lead to unique separation properties
[58, 59]. Among the 18 commercially available zeolites [60], few other zeolite
topologies combine all of these assets. In particular, there is a lack of large pore
volume, low Si/Al structures. Thus, concentrating the efforts of the synthesis
research in topologies meeting the mentioned criteria seems a good strategy to
maximize the success possibilities.
Among others, we can mention a second potential axe of improvement related to
the advancements in the integration of the separation process in the tailoring of the
222
J. Pérez-Pellitero and G. D. Pirngruber