**3. Conclusion and outlook**

working capacity, and regenerability, however, alkali- and alkaline-MOFs possess the lowest performance for CO2 separation. Among 4764 MOFs, about 1000 were found to contain Ln metals, 50% contain Ln as open metal sites. These open metal sites have high adsorption affinity for CO2; therefore, MOFs with Ln metals have the highest CO2 separation performance. The 30 best candidates identified for CO2/CH4 and CO2/N2 separations have Ln metals. These results can be used to synthesize MOFs having predetermined metal atoms to enhance the

**Figure 4.** Probabilities of different metals in the selected MOFs (based on S and NCO2). The red lines indicate the per‐ centages of the selected MOFs from the total. Reprinted with permission from Ref. [84]. Copyright (2015) The Royal

As can be seen from this literature review, current studies have generally focused on estab‐ lishing relations between adsorption selectivity and a single chemical or structural property such as difference of isosteric heat of adsorption of gases or metal type. However, separation performances of materials are determined by the interplay of various factors and cannot be easily correlated to only one or two properties. All physical and chemical properties of MOFs including pore size, shape, porosity, surface area, topology, metal and organic linker type must

CO2 separation performance of materials.

7414 Metal-Organic Frameworks

Society of Chemistry.

Molecular simulations are very useful to quickly evaluate the potential of new MOF materials in adsorption-based gas separation processes. The outcome of molecular simulations can be used as a guide to design and develop new materials with enhanced separation properties. There is a continuous growth in the number of molecular simulation studies of MOFs for adsorption-based CO2 separations. However, there are still several open areas in which future studies will be valuable. Opportunities and challenges related with these open research areas are discussed below:

#### **3.1. Computational design of new materials**

Strategies to improve the ability of MOFs to selectively adsorb CO2 are reviewed in detail in the literature [87]. Some of these strategies are control of pore size, using materials with open metal sites, introduction of alkali-metal cations into MOFs, interpenetration, and using materials with polar functional groups [45, 88]. Among these, rational design of functionalized materials is a feasible way to improve the CO2 separation efficiency of MOFs. GCMC simula‐ tions were recently used to study the effect of amine functionalization on the CO2/CH4 separation performance of MIL-53 [89]. Results showed that CO2/CH4 separation factor of − (NH2)4 amine-functionalized MIL-53 is the best and predicted separation performance of −NH<sup>2</sup> and −NHCO functionalized MIL-53 surpasses that of the original one. Future molecular simulation studies examining the effects of functionalization on the separation performance of MOFs will be very useful to establish guidelines for the experimental design and develop‐ ment of new materials.

### **3.2. Considering impurities in CO2-related mixtures**

As discussed in Section 2.3., most molecular simulation studies of MOFs focus on the separa‐ tion of CO2 from its binary mixtures such as CO2/CH4 and CO2/N2. However, in reality, these gas mixtures include some impurities. Water and the other minor components mostly H2O, O2, SO2, and NOx cannot be ignored in assessing the performance of MOFs especially for postcombustion CO2 capture. However, the number of molecular simulation studies examining the effects of trace gases on the CO2/N2 and CO2/CH4 separation performance of MOFs is limited. Bahamon and Vega [90] recently used GCMC simulations to study 11 materials including zeolites and MOFs for separation of CO2 from N2, including water as an impurity. Sun et al. [91] studied 12 materials including MOFs, ZIFs, and zeolites for removal of SO2 and NOx from flue gas using GCMC simulations. The influences of water and SO2 on CO2 adsorption and separation in UiO-66(Zr) MOFs with different functional groups were evaluated using a combination of GCMC and DFT simulations [92]. Babarao et al. [93] considered small amounts of O2, H2O, and SO2 impurities typically found in flue gas and evaluated the CO2/N2 selectivity of four PCNs in the presence of these impurities. Zhong's group [94] used molecular simulations to investigate the effect of trace amount of water on CO2 capture in natural gas upgrading process in a diverse collection of 25 MOFs. These studies concluded that the effect of H2O impurities on the CO2 selectivity is highly specific to the chemistry of the framework and needs to be evaluated on an individual case-by-case basis. The CO2 selectivity of MOFs was generally reported to decrease in the presence of water. Future studies on GCMC simulations of MOFs considering impurities in CO2-related mixtures must be conducted to evaluate the potential use of MOFs in industrial CO2 capture processes.

#### **3.3. Multi-scale modeling**

While CO2 separation using MOF adsorbents has been extensively investigated in different MOFs, their performance under practical process conditions is scarcely examined. A multiscale modeling study was recently carried out to examine CO2 capture from flue gas by vacuum swing adsorption (VSA) process using *rho*-ZMOFs as adsorbents [95]. The full adsorption process was simulated and optimized and results showed that the operating spaces of *rho*-ZMOFs are similar to that of traditional 13X zeolite. Further studies that employ multi-scale modeling approaches will be useful to design and develop MOF-based industrial CO2 separation processes. A related point is to test the long-term stability of MOF adsorbents under industrial operating conditions. An ideal MOF adsorbent must have good thermal and mechanical stability. Several MOFs are sensitive to atmospheric moisture and lose their crystal structures when exposed to water. This may be a significant problem when MOFs are used as adsorbents in flue gas separations since flue gas contains water. Molecular simulation studies that can provide information about the long-term stability of MOF adsorbents will be useful to evaluate the real performance of MOFs.
