(479d) Understanding Carbon Dioxide Adsorption On Alkali Metal Exchanged Zeolite SSZ-13

Understanding
Carbon Dioxide Adsorption on Alkali Metal Exchanged Zeolite SSZ-13

Trong D.Pham¹, Matthew R. Hudson², Craig M. Brown², Raul F.Lobo^1*

¹Center
for Catalytic Science and Technology, University of Delaware, Department of
Chemical and Biomolecular Engineering, USA, lobo@udel.edu.

²Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, USA.

Keywords: CO₂, SSZ-13, adsorption, neutron
diffraction

Introduction

The emission of anthropogenic carbon dioxide
into the atmosphere due to the combustion of fossil fuels is one of the main
causes of climate change. Low silica zeolites (types A, 13X, zeolite Y) are
among the most commonly used CO₂ adsorbents in industrial gas separations^[1,2], but are generally
strong adsorbents, making regeneration difficult ^[3]. Because of
lower Al content, high silica zeolites with a less hydrophilic pore environment
are an interesting option with respect to the regeneration and the tolerance to
polar impurities in the gas such as water vapor and sulfur compounds^[4].
Here we report an investigation of CO₂ adsorption on high-silica
zeolites SSZ-13 (CHA framework-type, Si/Al=6 and 12), examining the effects of
framework Si/Al ratio and extra-framework cations (Li⁺, Na⁺,
K⁺) on CO₂ adsorption isotherms and heats of adsorption. The
interaction of CO₂ with the SSZ-13 is investigated at the molecular
level using Rietveld refinement of neutron diffraction patterns of adsorbed CO₂
in the host zeolite.

Experimental

SSZ-13 samples (Si/Al=6 and 12) were prepared
by hydrothermal methods using N,N,N-Trimethyl-1-adamantanammonium ion as a structure-directing agent. Element compositions of synthesized and ion-exchanged samples were
determined by Energy-dispersive X-ray spectroscopy (EDX). CO₂
adsorption isotherms of the SSZ-13 samples at various temperatures (273, 303
and 343 K) were measured on a Micromeritics ASAP 2020
device up to ambient pressure. Neutron powder diffraction
(NPD) measurements were conducted on the high-resolution diffractometer
BT1 at NIST, using a Ge(311) monochromator, λ = 2.0787(2) Å
for bare and CO2 gas-dosed
alkali-exchanged SSZ-13.

Results and Discussion

Na-SSZ-13/6 is remarkable
because it shows comparable equilibrium CO₂ capacity to 13X and Y,
and higher capacity than NaA, three zeolites with
much higher aluminum content (Table 1). 
This is because all the cations
in the smaller cages of SSZ-13 are accessible to CO₂, and despite
the fact that SSZ-13 has a lower cation concentration compared to other zeolite
frameworks. Na-SSZ-13/12 adsorbed less CO₂ than the lower Si/Al
samples under the same conditions, as expected, but shows much higher
adsorption capacity than FER and STI with similar Si/Al ratio.  This difference is attributed to the higher microporous
volume of the SSZ-13 sample.

Table 1. Adsorption
capacity of CO₂ on various sodium-exchanged zeolites at 303 K and
1atm.

*ref [6]: The
experiment was carried out at 298K and 750torr

Figure 1: Schematic view of cation
positions in SSZ-13 framework (left) and CO₂ positions in Na-SSZ-13
(Si/Al~12) (right)

Rietveld refinement of neutron diffraction on
bare adsorbent SSZ-13/12 shows that Li⁺, Na⁺ is located
right above the center of the hexagonal prism (site SII), and the larger cation
K⁺ is in the middle of 8-membered rings. With 4 K⁺
cations/cage in K-SSZ-13/6, K⁺ could reduce the effective adsorption
capacity of the zeolite by blocking access to some of the SSZ-13 cavities. Difference
Fourier maps were used to locate the positions of CO₂ molecules in the
zeolite. At a loading of 0.66 molecules CO₂ per 8-ring window on
Na-SSZ-13/12, we found that CO₂ is located in the center of the
8-membered ring window instead of directly interacting with extra-framework
cations, which is similar to the results reported by Hudson et al^[5]for Cu-SSZ-13. Na⁺
is bound to 3 oxygen atoms of the 6-ring with a bond length of 2.35 Å, and the
closest distance of O(CO₂)-Na is 3.71 Å.
This distance is too long for efficient interaction coupling of the CO₂
and the cations suggesting that adsorption is driven by the electrostatic
interactions of the framework oxygen atoms in the 8-ring and the carbon atom in
CO₂.

Conclusions

SSZ-13 has a high potential for CO₂
capture, reaching an adsorption capacity of 5.1 mmol CO₂ per gram of
zeolite at ambient temperature and pressure, a value comparable with the best-known
zeolite adsorbents (X, Y). The adsorption characteristics of SSZ-13 are
strongly dependent upon the cation concentration, the cation-type, and the CO₂
loading amount.

Acknowledgements

This work was
supported by the National Science Foundation through grant CBET-940768 and National
Institute of Standards and Technology Center for Neutron Research (NCNR). Trong Pham acknowledges support from Vietnam Education
Foundation (VEF). MRH acknowledges support from the NRC Postdoctoral Research
Fellowship.

References

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