Low- and High-Temperature Solid Sorbents for Carbon Capture and Storage (CCS)

Abstract

Increasing concentration of carbon dioxide (CO2), one of greenhouse gases (GHGs), results in accelerating global warming. The development of carbon capture and storage or sequestration (CCS) technologies came to the fore to alleviate this environmental issue. In order to capture CO2, postcombustion techniques have been focused due to their competitive price and practical applicability. Recently, solid sorbents for CO2 capture have been studied because they have a wide operating temperature from ambient temperature to high temperature (700 - 800 oC). According to the working temperature of the flue gases to apply, the solid sorbents can be categorized as low-temperature (< 200 oC), intermediate-temperature (200 - 400 oC) and high-temperature (> 400 oC) sorbents.

Herein, amine-functionalized metal-organic framework (MOF) is introduced as one of the lowtemperature adsorbents. Amine functionalization of their pore surfaces has been studied extensively to enhance the CO2 uptake of MOFs and amine-functionalized MOFs have been synthesized via postsynthetic modifications in general. In this thesis, a one-step construction of a MOF ([(NiLethylamine)(BPDC)] = MOFNH2; [NiLethylamine]2+ = [Ni(C12H32N8)]2+; BPDC2- = 4,4`-biphenyldicarboxylate) possessing covalently tethered alkylamine groups without post-synthetic modification was introduced. In order to investigate the effect of amino groups for CO2, an isostructural MOF with propyl pendant chain ([(NiLpropyl)(BPDC)] = MOFCH3; [NiLpropyl]2+ = [Ni(C14H34N6)]2+) was provided. Unlike MOFCH3 showing decrease CO2 uptake as increasing temperatures, MOFNH2 showed enhanced CO2 uptake at elevated temperatures, attributed to active chemical interactions between the amine groups and the CO2 molecules. Due to the narrow channels of MOFNH2, the accessibility to the channel of CO2 is the limiting factor in its sorption behavior and only crystal size reduction of MOFNH2 led to much faster and greater CO2 uptake at low pressures. The chemical interaction between the primary amine chains tethered to the macrocyclic complex and CO2 molecules resulted in the formation of ammonium carbamate, which was reversibly dissociated at mild temperatures, 100 oC, by releasing CO2 molecules. MOFNH2 was found to be one of lowtemperature adsorbents with good stability over repeated cycling.

In addition, lithium orthosilicate (Li4SiO4) is studied as one of high-temperature absorbents. Among some high-temperature absorbents, Li4SiO4 is lighter and has excellent theoretical capacity (36.7 wt%). Above all, it operates at relatively lower regeneration temperature (~750 oC), which leads to active investigations in recent. Here, the optimal method for synthesizing pristine lithium orthosilicate (p-Li4SiO4) with excellent properties was found by changing various factors and the way to treat the precursors is important to have better sorption properties of p-Li4SiO4. Also, lithium metasilicate (Li2SiO3) and potassium carbonate (K2CO3) as additives can help to improve the sorption properties of p-Li4SiO4. The optimal amount and location of additives were investigated and their roles were also studied by relating to the mechanism of CO2 absorption for p-Li4SiO4.