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.

Samples	Na-13X		NaY		NaA		Na-FER[6]		Na-STI [6]		Na-SSZ-13/6		Na-SSZ-13/12
Si/Al ratio	1.25	2.47		1		9		15		6		12
Uptake (mmol/g)	5.23	5.03		4.13		2.6		2.5		5		3.96

*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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