The best way to synthesize industrially useful zeolites is to avoid the use of costly organic structure-directing agent (OSDA) and the generation of synthetic wastewater. Recently, the steam-assisted conversion methods...
Organic structure-directing agent-free steam-assisted
conversion
and Cs+ ion exchange were used to transform the faujasite
(FAU)-type zeolite to the Cs+-type chabazite/phillipsite
(CHA/PHI) composite zeolite. Compared with the pure PHI-type zeolite,
the Cs+-type CHA/PHI zeolite showed gate-opening CO2 adsorption behavior and good thermal stability. In situ powder
X-ray diffraction (PXRD) of the CO2 adsorption was measured
to elucidate the mechanism for the gate-opening adsorption on the
CHA/PHI zeolite. The Na+-type CHA/PHI zeolite did not show
such adsorption behavior, and the PXRD pattern of the Na+-type CHA/PHI zeolite did not change with increasing CO2 partial pressure, which suggests that this unique adsorption behavior
was caused by the PHI framework transition or Cs+ ions
moving in both the CHA and PHI frameworks. Furthermore, in situ Fourier-transform
infrared spectra of CO2 adsorption and CO2 breakthrough
measurement on the Cs+-type CHA/PHI zeolite suggest that
the CHA and PHI frameworks in the CHA/PHI zeolite shared eight-membered-ring
windows and that CO2 molecules could easily diffuse from
a CHA cage to a PHI framework. The ideal adsorbed solution theory
was used to calculate the CO2/N2 separation
selectivity for the Cs+-type CHA/PHI zeolite. At 298 and
318 K, the Cs+-type CHA/PHI composite zeolite showed a
high CO2/N2 separation coefficient of >10,000
compared with other zeolites with high CO2 adsorption capacity.
Furthermore, the CO2 working capacity was calculated for
the Cs+-type CHA/PHI zeolite in both the pressure- and
temperature-swing processes, and the results showed that the CHA/PHI
composite zeolite could selectively separate CO2 from the
CO2/N2 gas mixtures released from power generation
plants operating using these processes.
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