**Molecular Simulations for Adsorption-Based CO2 Separation Using Metal Organic Frameworks**

Seda Keskin

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64226

#### **Abstract**

Metal organic frameworks (MOFs) have received significant attention as a new family of nanoporous materials in the last decade. Variations in geometry, size, and chemical functionality of these materials have led to several thousands of different MOF structures. MOFs typically have high porosities, large surface areas, and reasonable thermal and mechanical stabilities. These properties make them ideal adsorbents for adsorption-based gas separations. It is not practically possible to test the adsorptionbased gas separation potential of all available MOFs using purely experimental techniques. Molecular simulations can guide experimental studies by providing insights into the gas adsorption and separation mechanisms of MOFs. Several molecular simulation studies have examined adsorption-based CO2 separation using MOFs due to the importance of CO2 capture for clean energy applications. These simulations have been able to identify the MOF having the most promising CO2 separation properties prior to extensive experimental efforts. The aim of this chapter is to address current opportunities and challenges of molecular simulations of MOFs for adsorption-based CO2 separations and to provide an outlook for prospective simulation studies.

**Keywords:** MOF, molecular simulation, adsorption, separation

## **1. Introduction**

We have witnessed the quick growth of a new group of nanoporous materials named as metal organic frameworks (MOFs) in the last decade. MOFs are crystalline nanoporous materials composed of metal complexes that are linked by organic ligands to create highly porous frameworks [1, 2]. MOFs become strong alternatives to more traditional nanoporous materials such as zeolites due to their fascinating physical and chemical properties. MOFs

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

typically have very large surface areas (500–6000 m2 /g), high pore volumes (1–4 cm3 /g), wide range of pore sizes from micro- to mesoscale (1–98 Å), and reasonable thermal and mechan‐ ical stabilities. The most important characteristic of MOFs is that their physical, chemical, and structural properties can be tuned during synthesis. This controllable synthesis leads to a large diversity of materials having different geometry, pore size, and chemical functionali‐ ty [3, 4]. As a result, thousands of MOFs have been reported in the Cambridge Crystallo‐ graphic Database [5]. The family of MOFs can be divided into subgroups such as isoreticular MOFs (IRMOFs), zeolitic imidazolate frameworks (ZIFs), zeolite-like MOFs (ZMOFs), and covalent organic frameworks (COFs).

MOFs have been examined for a variety of chemical applications including gas storage [6, 7], gas sensing [8], gas separating membranes [9], mixed matrix membranes [10], catalysis [11], and biomedical applications [12, 13]. Among these applications, gas separation has received a significant interest because the pore sizes of MOFs can be tuned to selectively separate gases at the molecular level. Gas separation using MOFs has been generally studied in two categories: equilibrium-based gas separations and kinetic-based gas separations [14]. In equilibriumbased gas separations, MOFs are used as adsorbents and in kinetic-based separations, MOFs are used as membranes. Adsorption-based gas separation is governed by the thermodynamic equilibrium. Gas components are reversibly adsorbed into the pores of the adsorbent. An ideal adsorbent material must have a good combination of adsorption selectivity and working capacity in addition to high stability, high void volume, and well-defined pore sizes. High porosities, large surface areas, different pore sizes and shapes, and reasonable stabilities of MOFs suggest that these materials can be ideal adsorbents in equilibrium-based gas separation applications. Several experimental studies have been carried out for adsorption-based gas separations using MOFs [15–17].

Two criteria are widely investigated to assess the potential of MOF adsorbents: adsorption selectivity and working capacity. Adsorption selectivity is determined by the adsorption affinity of the MOF for one gas species relative to another. High adsorption selectivity means a high-purity product and hence lower energy requirements. Working capacity is defined as the difference between the adsorbed amounts of gas at the adsorption and desorption pressures. High working capacity means easy regeneration of the adsorbent material. For an efficient and economic adsorption-based gas separation, both high selectivity and high working capacity are desired. Therefore, experimental studies on MOF adsorbents generally examine selectivity and working capacity of the materials [18].

Most of the experimental studies have focused on CO2 separation. Because of the growing environmental concerns, removal of CO2 from natural gas (CO2/CH4), flue gas (CO2/N2), and other gases (CO2/H2) becomes an important issue. Experimentally measured selectivity and gas uptake capacity of several MOFs for separation of CO2 from CH4 and N2 have been summarized in the literature [19]. Currently available adsorbents such as activated carbons, carbon molecular sieves, and zeolites are not highly selective for CO2 separation, especially for separation of CO2 from flue gas [20]. A good comparison of CO2 separation performances of different nanoporous materials such as MOFs, zeolites, and activated carbons is available in a recent review [21]. It is shown that CO2/N2 selectivity changes from low in zeolites to moderate in carbon-based absorbents and becomes high in MOFs. Therefore, research on adsorptionbased gas separations has focused on identifying highly selective MOF adsorbents with high CO2 capacities that can replace traditional adsorbents.

Considering the very large number of available MOFs, it is not possible to test thousands of different MOFs as adsorbents using purely experimental methods. Molecular simulations play an increasingly important role in understanding the potential of MOFs in adsorption-based gas separations. Among molecular simulation methods, grand canonical Monte Carlo (GCMC) simulations have been widely used to accurately predict adsorption isotherms of various gases in MOFs [22]. Gas selectivities calculated from simulated adsorption isotherms are generally found to be in good agreement with the experiments [23]. In most studies, single-component gas adsorption isotherms are computed using GCMC simulations; mixture adsorption isotherms are then predicted based on pure gas adsorption data using ideal adsorbed solution theory (IAST). IAST is a well-developed technique to describe adsorption equilibria for gas components in a mixture using only single-component adsorption data at the same tempera‐ ture and on the same adsorbent [24]. GCMC simulations can be also performed to obtain mixture adsorption data directly. This data is then used to predict adsorption selectivity and working capacity of the MOF. Results of molecular simulations provide molecular-level insights which can be used to design new MOFs with better separation performances. In the early years of these studies, simulations examined only one or a few MOFs at a time. With the development of new computational approaches and with the quick increase in the number of synthesized MOFs, molecular simulations have started to screen a large numbers of materials. The results of large-scale MOF screening studies are highly useful to direct experimental efforts, resources, and time to the most promising MOF materials.

This chapter aims to address the importance of molecular simulations to evaluate the potential of MOFs in adsorption-based CO2 separations. Section 2.1 introduces details of GCMC simulations to study CO2 adsorption in MOFs. Section 2.2 describes evaluation criteria used to assess CO2 separation potential of MOF adsorbents. Studies on large-scale computational screening of MOF adsorbents are discussed in Section 2.3. Structure-separation performance relations obtained from molecular simulations of MOFs are summarized in Section 2.4. Section 3 closes by addressing the opportunities of using molecular simulations for examining the potential of MOF adsorbents in CO2 separations.
