Organic-Free Synthesis and CO2 Adsorption Properties of Si-Rich Small-Pore Zeolites

Abstract

Carbon capture and storage (CCS) has been proposed as a method for reducing the concentration of atmospheric carbon dioxide (CO2), a major contributor to global warming. Adsorption-driven capture of CO2 from various industrial applications has been widely investigated as a potential replacement for amine solution-based processes due to their low cost and eco-friendly properties. Aluminosilicate zeolites are one of the most important solid adsorbents for CCS because of their physicochemical stability compared to metal-organic frameworks (MOFs), zeolite-imidazole frameworks (ZIFs) and others.

In this thesis we have synthesized a number of Si-rich small-pore zeolites by carefully adjusting the alkalinity of the synthesis mixture under wholly inorganic conditions. We have also investigated the small gases (e.g., CO2, CH4, and N2) adsorption properties of various cation-exchanged forms of small-pore zeolites. Furthermore, a detailed molecular level study, combining advanced solid state nuclear magnetic resonance (NMR) with structure solution by powder X-ray diffraction (XRD), were carried out to elucidate the effects of dehydration and CO2 adsorption on the structures of different alkali cation forms of small-pore zeolites.

1. We report that zeolite framework flexibility can be exploited by tuning a subtle interplay between extra-framework cations with framework oxygen and adsorbed guest molecule (CO2). This phenomenon has been demonstrated on the Na+, K+ and Rb+ forms of the small-pore zeolite gismondine with a Si/Al ratio of 3.0, which show marked cation-dependent hystereses in their CO2 isotherms. A detailed analysis of the structures by solid state NMR andXRD revealed the framework dynamics and the adsorption behavior is governed by the extraframework cations, which strive for coordination either with framework oxygen, guest molecules, or both. The K+ and Rb+ forms display very high CO2 working capacities (2.8 and 3.0 mmol g-1) for a 50:50 CO2/CH4 mixture under mild temperature swing conditions (25 - 100 °C at 1.0 bar), as well as high CO2/CH4 selectivities (36 and 28) at 25 °C. Given the excellent thermal and chemical stability of zeolites compared to other classes of adsorbents, and the here reported tuneable flexing action effected by the cations, zeolites displaying framework flexibility harbour great potential for fuel gas applications.

2. We report the synthesis of three MER zeolites with Si/Al = 1,7, 2.3, and 3.8 under wholly inorganic conditions and the CO2 adsorption properties of a series of their alkali cationexchanged forms. We found that while the CO2 adsorption behavior of MER zeolites differs notably according to a combination of the type and concentration of extraframework alkali cations, the Na+, K+, Rb+, and Cs+ forms of MER zeolite with Si/Al = 2.3 show step-shaped CO2 isotherms, allowing them to have high CO2/N2 and CO2/CH4 selectivites. Based on the overall characterization results, we were able to explain this unusual adsorption phenomenon by combined cation gating and breathing effects.