The adsorption of molecular gases on La-MPBCP or [La-MPBCP]+3 systems
The study for adsorption of biogas (CO2, H2, N2 and CH4) on the La-MPBCP or [La-MPBCP]+3 systems, we considered that the gas molecule would interact with the lanthanum atom, because of that the gas molecule was placed directly on the top of the lanthanum atom, see Figure 4. The optimized structures shown Cs symmetry for interactions by CO2, H2, N2 and CH4 with neutral system. For the case of the [La-MPBCP]+3 system, the H2, and CH4 shown Cs symmetry and C3v symmetry for interactions by CO2 and N2 with perpendicular interaction gas-La (CO2, La-O 2.50; N2, La-N 2.77 Å) showing lower bond length than the neutral system see Table 3. On the other hand, the La-H in H2 molecule has a similar value for both systems (neutral, La-H 2.68-2.77 and [La-MPBCP]+3 2.74 Å), those interactions are significantly smaller than the sum of their van der Waals radii.49
The calculated adsorption energies of the gas molecules adsorbed on systems are listed in Table 3. It is observed that theEads are negative, indicating adsorption was exothermic showing favorable interaction of those gas molecules. The adsorption energy for the H2, N2, CH4 and CO2 on the [La-MPBCP]+3 system has a values of -7.51, -16.64, -21.30 and -28.63 kcal/mol, these values are higher compared to neutral system (-5.28, -9.90, -10.79 and -15.95 kcal/mol). The calculated CO2 adsorption energy for [La-MPBCP]+3 is similar to La2O2CO3 systems calculate with the PBE functional.50 The descending order of those adsorption energies is the following: CO2>CH4>N2>H2. The adsorption of lanthanum atom on MPBCP induce to increase the reactivity, causing the adsorption of gas molecule. We can clearly observe that all gas molecules (N2, H2, CO2, CH4) have coordinations with short distances with the lanthanum atom (see Figure 5). Our results predicted that these gas molecules are easy to be adsorbed on the La-MPBCP or [La-MPBCP]+3 systems by physisorption. It is important to highlight the computed adsorption energy for the CO2 molecule on the [La-MPBCP]+3system is four times greater than adsorption energy for the H2 molecule. These results suggested that the [La-MPBCP]+3 system could serve as an adsorber-separator of CO2/H2.