(612f) Fast and Highly Selective CO2 Adsorbent: Li+/ZSM-25 Zeolite

Fast and highly
selective CO₂ adsorbent: Li⁺/ZSM-25 zeolite

Jianhua Zhao,¹
Ke
Xie,¹ Gang
(Kevin) Li,¹ Ranjeet Singh,¹ Gongkui
Xiao, ² Qinfen Gu, ³ Penny Xiao¹ and
Paul A. Webley^1,4

¹Department
of Chemical Engineering, The University of Melbourne, VIC 3010, Australia

²Fluid
Science & Resources Division, School of Mechanical & Chemical
Engineering, The University of Western Australia, WA 6009, Australia

³Australian
Synchrotron, 800 Blackburn Rd, Clayton, VIC 3168, Australia

⁴The Peter
Cook Centre for Carbon Capture and Storage, The University of Melbourne, VIC
3010, Australia

Abstract

CH₄ is one
of the most important fossil fuels because it is efficient and clean. The major
source of CH₄ is landfill gas, natural gas and biogas. However,
those methane-rich gases may contain 15-50 vol.% CO₂ which reduces
the grade of the fuel and also corrodes the pipeline. Therefore, the capture and
removal of CO₂ from CO₂/CH₄ mixtures is
attracting world-wide interest. The adsorption method is a promising approach due
to its low operating and capital costs.

Compared
with various adsorbents such as activated carbons and metal-organic frameworks,
zeolites are more favourable for CO₂/CH₄ separation due
to their high adsorption capacity, tuneable pore structure and the molecule
affinities, high stability and ease of mass production.

The
structure of Na/TEA-ZSM-25 (TEA refers to tetraethylammonium bromide), a RHO
family zeolite, was resolved recently by Guo et al. (Nature, 2015, 74-78).
Na-ZSM-25 has been recently prepared by removing the TEA template. Na-ZSM-25
shows promising CO₂ capture properties such as high CO₂
working capacity and excellent CO₂/CH₄ ideal selectivity
for pressure swing adsorption. However, the previous reported single-gas
adsorption performance of Na-ZSM-25 is unable to provide reliable information
on CO₂/CH₄ separation behaviour in practical scenarios
where the adsorption is performed in a mixed gas. Furthermore, Na-ZSM-25
suffers from low adsorption kinetics that limits its industrial applications.
In this presentation, Li⁺/ZSM-25 zeolites (LZZ) is developed to
enhance the CO₂ adsorption rate, and the adsorption performance of LZZ
is then studied using both single-(isotherm) and binary-(breakthrough) gas measurements.
The results indicate the CO₂ adsorption rate of the LZZ is 4.5 times
(Table 1) that of Na-ZSM-25 with a simultaneous increase in the CO₂
adsorption capacity by 6.1% at 303K and 9.5 bar. Remarkably, the very high CO₂/CH₄
adsorptive selectivity is successfully inherited from Na-ZSM-25 zeolite in both
single- and binary-gas adsorption (Fig. 1 and Table 2). In addition, in-situ
XRD reveals CO₂-induced framework expansion (Fig. 2), which explains
the origin of the high working capacity and type-II like isotherms of the
ZSM-25 based zeolites. This study suggests promising application of LZZ in
natural gas purification and biogas upgrading.

Table
1 Adsorption kinetics of CO₂ on Na-ZSM-25, Li(0.063)-ZSM-25,
Li(0.10)-ZSM-25 and Li(0.13)-ZSM-25 at different pressures.

Fig. 1 Breakthrough profiles of Li(0.13)-ZSM-25
recorded in 50%CO₂/50% CH₄v/v mixed gas (a, b) at total
pressures of 2 bar (a) and 7 bar (b) and pure CH₄ at 3.5 bar (c).
Total gas flow rate: 100sccm; temperature: 303 K.

Table
2 Adsorbed amounts and selectivity derived from breakthrough curves of 50% CO₂/50%
CH₄v/v mixed gas at 303 K and different pressures on
Li(0.13)-ZSM-25.

Fig. 2 In-situ
synchrotron PXRD of Na-ZSM-25 during CO₂ adsorption after activation
from 0.30 bar to 1.80 bar at 303K.