Hydrogen is a non-toxic substitute that reduces dependence on fossil fuels and protects the environment. To produce industrial hydrogen in fuel cells, a small amount (0.5%-2%) of sticky carbon monoxide (CO) must be removed from the surface of catalyst (Saavedra et al. 2016; Dhar et al. 1987; Cheng et al. 2007). This is because even a small amount of infectious carbon monoxide can poison and deactivate the fuel cell catalyst. Today, the chemical industry uses large amounts of carbon monoxide. CO is an important raw material in the production of other chemicals such as dimethylformamide, acetic acid, methane, phosgene and formic acid (Kim et al. 2018). Carbon monoxide, usually produced by burning coal fuels, is an odorless, colorless and toxic gas that prevents the distribution of oxygen in the human body. CO can be obtained from steam reforming, steel mills, partial oxidation of hydrocarbons and other chemical processes. Syngas, a mixture of CH, CO, CO2, H2, N2 and H2O is a product of coal gasification and the steam reforming of natural gas, which are the two main methods of CO production (Lin et al. 2020). Some exhaust gases containing large amounts of CO are released during industrial oxidation processes, such as carbon black, coke gas, and blast furnace gas (Harlacher et al. 2012; Zarca et al. 2014). There are several methods to remove residual CO from H2 gas, such as membrane separation of CO from H2, residual methanation, and preferential oxidation of CO (PROX) (Gamarra et al. 2007; Alayoglu et al. 2008; Park et al. 2009; Li et al. 2013; Fu et al. 2010; Qiao et al. 2011; Lakshmanan et al. 2014; Wilhite et al. 2004; Dai et al. 2015). The PROX method is an economic solution to reduce the amount of CO to the permitted level (Saavedra et al. 2013; Alayoglu et al. 2008; Fu et al. 2010). In general, this solution is well known, but the high storage costs of this process limit even its scaled-down utilization.
To use CO for both environmental and industrial purposes, CO must be separated from gas mixtures, so that the purity of CO is high. Various adsorbents such as activated carbon and zeolites have been used industrially in separation, purification, filtration, drying and cooling. Some materials, such as zeolites, have been developed to separate oxygen and methane from nitrogen. However, the lack of precision in controlling the pore structure is the main reason why these materials are not used in separation applications, especially for materials with similar physical and chemical properties (Kuznicki et al. 2001). The Development of advanced materials with a high excitation resistance structure is necessary to meet the increasing demands in separation areas. Gas separation is one of the most important areas, where the use of metal-organic frameworks (MOFs) is significant. In the presented situation, the use of promising microporous MOFs in gas separation is challenging but very important (Kondo et al. 1997; Li et al. 1998; Li et al. 1999; Chui et al. 1999). Unexpected porosity, extreme flexibility and surprising porosity are the properties that make MOFs considered as the novel multifunctional porous materials composed of metal nodes and organic linkers, bringing an extraordinary platform in global chemistry [9]. The synthesize of 70,000 MOF materials in the last decade is a result of the unexpectedly exciting nature of MOF chemistry (Wang et al. 2002; Chen et al. 2006; Mueller et al. 2006).
MOF applications have been widely studied over the past decade. In the studies carried out, better adsorption capacity of CO and the IAST-predicted CO/N2 and CO/H2 selectivity values of CuVM adsorbents were observed (Yin et al. 2019). Effective separation of CO from more weakly adsorbing gases such as H2 and N2 by Bloch et al. was performed on six metal-organic frameworks of M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc4– = 2,5-dioxide-1,4-benzene dicarboxylate), whose reversible CO coupling suggests CO/H2 and CO/N2 separations (Bloch et al. 2014). In Li’s study, the DS/HGR combined strategy of Cu+-containing MIL-100(Fe) adsorbents displayed a high CO adsorption capacity which is ahead of all Cu+-containing materials reported so far, such as CuCl/AC, CuCl/-Al2O3, and CuCl/SBA-15 (Li et al. 2018).
In the study of MMOF-74 by Sauer and coworkers, the correlation between the binding strength of CO and the type of the open metal site in its structure was investigated using a quantum mechanical method (Kim et al. 2018). A series of MOF devises containing different metals, M-MOF-74 were investigated in studies by Long and coworkers. Fe-, Co-, Ni-MOF-74 has excellent CO uptake at 298 K and 1.2 bar. In addition, Ni-MOF-74 and Co-MOF-74 show very high CO/H2 and CO/N2 selectivity, suggesting their potential for CO removal (Li et al. 2018).
The aim of this paper is to apply the DFT approach to investigate the dissociation behavior of Cu2(bmc)4 and Zn2(bmc)4 paddle wheel structures for CO/H2 mixture. To this end, different orientations of CO, H2, and CO/H2 adsorption at the α-adsorption site of the two clusters are studied.