hydrocarbon such as buta-1,3-diene and but-2-ene isomers. The cages in this zeolite
are only accessible to trans-but-2-ene and buta-1,3-diene, whereas they can exclude
but-1-ene and cis-but-2-ene due to their larger kinetic diameters [104]. Besides these
pure-silica zeolites, ERI-type AlPO 4 -based materials (i.e., AlPO 4 -17 and SAPO-17)
were shown to selectively separate trans-2-butene from 1-butene and cis-2-butene
under near-catalytic conditions and in liquid phase since the trans isomer can easily
access their 8-ring windows [105]. Finally, for convenience of the reader, the
diffusion rates and uptakes of C 2 to C 4 olefins and paraffins on selected zeolites
are listed in Table 1.
5 Small-Pore Zeolites as Adsorbents for Other Small
Molecules
5.1 Methane
Apart from the olefin/paraffin separations mentioned above, there are many separation studies related to methane (CH 4 ). Due to ever-increasing demands for cleaner
fuels and to reduce the strong reliance on crude oil, huge efforts have been devoted to
developing alternative energy sources. Among several candidates, CH 4 is considered
promising as an alternative fuel since it is naturally abundant and relatively environmentally friendly compared to other liquefied hydrocarbons. Unfortunately, most
zeolites exhibit a relatively small methane uptake (<100 cm
3 cm
À3 STP) due to their
relatively low porosity, which is far from the standard for practical applications
[106]. Indeed, a comparative study on MOF-5, MOF-177, and Ca-A has shown that
the zeolite has a considerably low adsorption capacity (14.3 wt%) at 298 K and
100 bar compared with the capacities (17.2 and 22.0 wt%, respectively) of MOF-5
and MOF-177 [107]. However, although zeolites cannot be considered as a useful
CH 4 adsorbent from a storage point of view, they still show excellent performance
with respect to the separation of CH 4 [19, 20, 45, 108, 109].
5.2 Water
As presented thus far, zeolites can be a promising candidate for selective gas
adsorption in dry conditions. However, the effect of H 2 O on adsorption is crucial
in most of the current separation processes, because this molecule can preferentially
occupy the adsorption sites in zeolitic materials due to its smaller kinetic diameter
(2.64 Å) and strong polarity, thereby hindering the capture of target gases. In fact,
the use of zeolites as H 2 O adsorbents in double-glazing windows is a simple
example of why the surface selectivity of zeolites needs to be carefully considered
[110]. Likewise, the exchangeable cations in zeolites can influence their adsorption
affinity for H 2 O, because of the specific interactions between H 2 O molecules and the
20
K. C. Kemp et al.
are only accessible to trans-but-2-ene and buta-1,3-diene, whereas they can exclude
but-1-ene and cis-but-2-ene due to their larger kinetic diameters [104]. Besides these
pure-silica zeolites, ERI-type AlPO 4 -based materials (i.e., AlPO 4 -17 and SAPO-17)
were shown to selectively separate trans-2-butene from 1-butene and cis-2-butene
under near-catalytic conditions and in liquid phase since the trans isomer can easily
access their 8-ring windows [105]. Finally, for convenience of the reader, the
diffusion rates and uptakes of C 2 to C 4 olefins and paraffins on selected zeolites
are listed in Table 1.
5 Small-Pore Zeolites as Adsorbents for Other Small
Molecules
5.1 Methane
Apart from the olefin/paraffin separations mentioned above, there are many separation studies related to methane (CH 4 ). Due to ever-increasing demands for cleaner
fuels and to reduce the strong reliance on crude oil, huge efforts have been devoted to
developing alternative energy sources. Among several candidates, CH 4 is considered
promising as an alternative fuel since it is naturally abundant and relatively environmentally friendly compared to other liquefied hydrocarbons. Unfortunately, most
zeolites exhibit a relatively small methane uptake (<100 cm
3 cm
À3 STP) due to their
relatively low porosity, which is far from the standard for practical applications
[106]. Indeed, a comparative study on MOF-5, MOF-177, and Ca-A has shown that
the zeolite has a considerably low adsorption capacity (14.3 wt%) at 298 K and
100 bar compared with the capacities (17.2 and 22.0 wt%, respectively) of MOF-5
and MOF-177 [107]. However, although zeolites cannot be considered as a useful
CH 4 adsorbent from a storage point of view, they still show excellent performance
with respect to the separation of CH 4 [19, 20, 45, 108, 109].
5.2 Water
As presented thus far, zeolites can be a promising candidate for selective gas
adsorption in dry conditions. However, the effect of H 2 O on adsorption is crucial
in most of the current separation processes, because this molecule can preferentially
occupy the adsorption sites in zeolitic materials due to its smaller kinetic diameter
(2.64 Å) and strong polarity, thereby hindering the capture of target gases. In fact,
the use of zeolites as H 2 O adsorbents in double-glazing windows is a simple
example of why the surface selectivity of zeolites needs to be carefully considered
[110]. Likewise, the exchangeable cations in zeolites can influence their adsorption
affinity for H 2 O, because of the specific interactions between H 2 O molecules and the
20
K. C. Kemp et al.