Porous Materials for Carbon Dioxide Capture

Removal of CO₂ from major emission sources, such as power plants and industrial facilities for environmental remediation has attracted significant interest. Among currently accessible CO₂ capture technologies, the use of porous solids is considered to be one of the most promising approaches. The use of ionic liquids (ILs) composed of an organic cation and an inorganic anion as precursors for the synthesis of carbonaceous materials has been an emerging field. Porous carbons with a high specific surface area can be facilely made by directly annealing ILs or using appropriate porous templates. By choosing different ILs, materials with various heteroatoms doping and good pore properties can be produced. The attractive features of IL-derived materials such as facile synthesis, high specific surface area, and nitrogen content make them promising candidates for CO₂ capture. In this chapter, we review the recent research progress on IL-derived carbonaceous materials and their potential CO₂ separation application.

Porous carbons play an important role in CO₂ adsorption and separation due to their developed porosity, excellent stability, wide availability, and tunable surface chemistry. In this chapter, the synthesis strategies of porous carbon materials and evaluation of their performance in CO₂ capture are reviewed. For clarity, porous carbons are mainly classified into the following categories: conventional activated carbons (ACs), renewable-resources-derived porous carbons, synthetic polymer-based porous carbons, graphitic porous carbons, etc. In each category, macroscopic and microscopic morphologies, synthesis principles, pore structures, composition and surface chemistry features as well as their CO₂ capture behavior are included. Among them, porous carbons with targeted functionalization and a vast range of nanostructured carbons (carbon nanofibers, CNTs, graphene, etc.) for CO₂ capture are being created at an increasing rate and are highlighted. After that, the main influence factors determining CO₂ capture performance including the pore features and heteroatom decoration are particularly discussed. In the end, we briefly summarize and discuss the future prospectives of porous carbons for CO₂ capture.

Metal-organic frameworks (MOFs) composed of metal nodes linked by organic linkers are a class of newly developed crystalline hybrid porous solids. In the past few years, MOFs have seen a very rapid development both in terms of synthesis of novel structures and their potential applications in a wide variety of fields. Nearly all metals and a large diversity of organic species can be used to construct MOFs, so that a huge variety of materials of MOFs with different structures and properties are accessible. Due to their uniform yet tunable pore sizes, high-surface areas, and easy pore functionalization, MOFs have emerged as superior porous materials for adsorption and membrane-based applications. Particularly, recent studies have demonstrated that MOFs are perfect and quite promising in CO₂ capture. This chapter starts with an introduction of MOFs, including their design and synthesis, structural features, properties, and potential applications. Then, their implementation and performance in CO₂ capture-related aspects including selective CO₂ adsorption in MOFs, CO₂ separation in MOFs, MOF-based membrane for CO₂ separation, the design of MOFs for CO₂ capture, and computational simulation in MOFs for CO₂ capture are discussed.

Porous solids have been proved to be good candidates as the carbon dioxide recycling sorbents. In the last decades, many efforts were devoted to improving the surface area and heat of adsorption of artificial porous materials. Among those synthesized porous solids with ultrahigh surface area, porous aromatic frameworks (PAFs) possess ultrahigh Brunauer–Emmett–Teller (BET) surface area and excellent physicochemical stability, which can meet the criteria of carbon dioxide storage and separation. PAFs are the new generation of a whole new class of organic networks with an intrinsic nanoporosity. They are characterized by a rigid aromatic open-framework structure constructed by covalent bonds that remain accessible to small molecules. In this chapter, the design, synthesis, and carbon dioxide adsorption properties of PAFs are discussed.

Microporous organic polymers (MOPs) are a unique class of porous materials consisting solely of the light elements (C, H, O, N, etc.). A series of vivid characteristics of MOPs, such as high-specific surface area, good physicochemical stability, diverse pore dimensions, topologies, and chemical functionalities, make them suitable adsorbents for CO₂ capture. In this chapter, MOPs are categorized into four classes according to the types of organic reactions and the chemical structures of the resulting materials: hypercrosslinked polymers (HCPs), covalent organic frameworks (COFs), polymers of intrinsic microporosity (PIMs), and conjugated microporous polymers (CMPs). For each type of the polymer network, the state-of-the-art development in the design, synthesis, characterization, and the CO₂ sorption performance is reviewed. Strategies for controlling CO₂ uptake capacity and adsorption enthalpy via manipulation of surface area, pore size, and functionality are discussed in detail. These studies would open up many new possibilities for the development of the novel solid sorbents targeting the CO₂ capture process. It is expected that this chapter will not only summarize the main research activities in this field, but also find possible links between basic studies and practical applications.

The reversible carbonation and calcination reactions of, respectively, CaO and CaCO₃ have very promising CO₂ capture characteristics with regard to CO₂ capture costs, theoretical CO₂ uptake per gram of sorbent and material availability. However, CaO derived from naturally occurring Ca-based materials, predominantly limestone, shows a rapid decrease in its CO₂ capture capacity with number of carbonation/calcination cycles. The loss of the CO₂ capture capacity of unsupported CaO has been attributed to dramatic changes in the material’s morphology due to sintering and pore blockage. However, since the molar volume of CaCO₃ is more than twice as large as that of CaO, accessible pore volume in pores of diameter <100 nm is critical to yield high CO₂ uptakes. In this chapter, we review the fundamentals of the carbonation and calcination reaction, with a particular focus on the morphology of CaO and changes thereof. Furthermore, a detailed overview over kinetic models to describe the carbonation and calcination reaction is provided, followed by a critical review of the effect of typical flue gas impurities such as H₂O and SO₂ on the CO₂ capture characteristics of CaO. We conclude the chapter with a presentation of recent advances in the development of synthetic CaO-based CO₂ sorbents which substantially exceed the cyclic CO₂ capture capacity of limestone.

Inorganic membranes play an important role in the development of economical processes for pre-combustion and/or post-combustion capture of carbon dioxide (CO₂₎ at high temperatures. Mesoporous silica, due to its chemical and mechanical properties, is considered as a candidate for the capture of CO₂ at high temperatures. Bare silica membranes exhibit Knudsen diffusion behavior for most gases but also exhibit the contribution of surface diffusion for heavier or interacting gases such as CO₂ and CH₄. The CO₂/N₂ selectivity of mesoporous silica membranes can be enhanced by surface modification using aminosilanes such as APTS (3-aminopropyl-triethoxy silane). The important aspect of such modified membranes is that they can be operated at high temperatures typically encountered in post-combustion gas streams (flue gas). For modified silica membranes, mixed gas separation factors as high as 10 for CO₂ over N₂ were observed. The transport mechanism in such membranes is the reaction of CO₂ with the amine groups (in aminosilanes) to form a carbamate species and subsequent surface “hopping” of carbon dioxide. Under ambient conditions, CO₂ is strongly bounded to the amine groups and thus greatly inhibits the surface diffusion of CO₂; however, as the temperature increases, the CO₂ permeance increases and selective transport of CO₂ is observed. Thus, in surface-modified facilitated transport membranes, economical CO₂ separation is achieved through the combination of the chemical reaction of CO₂ associated with amine absorption along with the simplicity and low operating costs of membrane processes.