Postsynthetic Ligand Insertion
Since 1 Since 1 lost its crystallinity during activation at a temperature higher than 50 ˚C, activated 1a was prepared by soaking the crystals of 1 in fresh EtOH and MC consecutively for 3 d and then vacuum-
4.4 Gas sorption behaviors of the MOFs containing additional dipyridyl linkers .1 Preparation of the MOFs containing additional dipyridyl linkers for gas sorption study
4.4.2 Gas sorption behaviors of the MOFs containing additional dipyridyl linkers
Gas sorption behavior of activated AgBTB-dpey, 2(m)a. While activated AgBTB (1a) with no additional dipyridyl linkers between the silver clusters across the pore did not show any N2 uptake at 77 K (Figure 4.4a), the N2 isotherms of 2(m)a with additional dpey linkers are of type Ⅰ (Figure 4.22).
The N2 uptake amount and the BET specific surface area increase as the amount of dpey linker increases.
The N2 uptake amount of 2(1)a with dpey linkers of ~30% site occupancy, ~98 cm3/g at ~1 P/P0, increases to ~137 cm3/g for that of 2(3)a with dpey linkers of ~50% site occupancy. The corresponding BET specific surface area also increases from 342 m2/g for 2(1)a to 503 m2/g for 2(3)a. The linker insertion into the framework of AgBTB stabilized the 1D microporous channel of 2(m)a. As the more amount of the linker was inserted into the framework, the larger increase of the pore volume was observed..
Figure 4.22. N2 sorption isotherms of 2(1)a, 2(2)a, 2(3)a and 2(4)a.
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The CO2 adsorption isotherm of 2(1)a at 195 K is also microporous type I but shows slight stepwise adsorption ~0.5 bar and hysteric desorption (Figure 4.23). The maximum uptake amount is ~118 cm3 g-
1 at ~1 bar. The CO2 adsorption isotherms of 2(2)a, 2(3)a and 2(4)a at 195 K also show the similar but the more distinctive stepwise adsorptions and hysteric desorptions. In addition, the maximum uptake amounts at ~1 bar are ~149 cm3 g-1, ~162 cm3 g-1, and ~165 cm3 g-1, respectively (Figure 4.23). The maximum CO2 uptake amounts are also correlated to the amounts of dpey linkers inserted into the framework. As the more amount of the linker was inserted into the framework, the larger increase of the uptake amount was observed. The enhancement of gas uptake suggests that structural stability and contribution for pore recovery are improved by insertion amount and alternative coordination of dpey molecules even though possession of ligands increase in 1-D channel. The stepwise adsorptions are probably due to unequal amounts of the two different linker insertion modes into the 1D solvent channels of the framework, one mode between the dinuclear clusters and the other mode between the tetranuclear clusters. The two different kinds of linker insertions with different linker amounts generates two different kinds of 1D microporous channels of different stabilities. The 1D channels with the linkers between the dinuclear clusters and of the more linker amount are stable enough to show microporous CO2 adsorption. On the other hand, the 1D channels with the linkers between the tetranuclear clusters and of the less linker amount are not stable but can be restored by CO2 at ~0.5 bar. However, the CO2
isotherms at the higher temperatures, 273 and 298 K, do not show any hysteresis. Though restorations of the collapsed pore were not observed, the gradual increase of the maximum uptake amounts was observed as the increase of the linker amounts in the framework.
Figure 4.23. CO2 sorption isotherms of 2(1)a, 2(2)a, 2(3)a and 2(4)a.
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Gas sorption behavior of activated AgBTB-dpee, 3(m)a. N2 sorption behaviors of 3(m)a with dpee linker at 77 K are of microporous type I and very similar to those of 2(m)a with dpey (Figure 4.24).
The maximum N2 uptake amounts and the BET specific surface areas are also correlated to the amounts of dpee linkers inserted into the framework. The maximum N2 uptake amount and the BET specific surface area of 3(1)a are ~67 cm3/g and 234 m2/g, respectively. The corresponding N2 amount and the BET specific surface area increase as the linker amount in the framework increases, 96 cm3/g and 332 m2/g for 3(2)a, ~102 cm3/g and 339 m2/g for 3(3)a, ~97 cm3/g and 322 m2/g for 3(4)a, respectively.
Figure 4.24. N2 sorption isotherms of 3(1)a, 3(2)a, 3(3)a and 3(4)a.
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The CO2 sorption behavior of 3(m)a at 273 and 298 K, respectively, is very similar to that of the corresponding 2(m)a (Figures 4.23 and 4.25). The isotherms are reversible with no hysteresis and the maximum CO2 uptake amount is correlated to the amount of dpee linkers. The maximum CO2 uptake amounts are ~33, ~43, ~44 and ~44 cm3/g, respectively, at 273 K and ~15, ~20, ~22 and ~21 cm3/g, respectively, at 298 K for 3(1)a, 3(2)a, 3(3)a and 3(4)a. On the other hand, the CO2 maximum CO2
uptake amount is also correlated to the amount of dpee linkers, the maximum CO2 uptake increase of 3(m)a at 195 K is not as much as that of the corresponding 2(m)a with dpey linker. The most distinctive difference is the absence of stepwise adsorption at ~0.5 bar and of hysteresis. The average maximum CO2 uptake amount of 3(2)a, 3(3)a and 3(4)a (avg. 128 cm3 g-1: 123, 132, and 129 cm3 g-1) is ~31 cm3/g smaller than that of 2(2)a, 2(3)a and 2(4)a (avg. 159 cm3 g-1: 149, 162, and 165 cm3 g-1). The difference of the maximum uptake amount indicates the partially collapsed pore of 3(m)a has not been restored by CO2 gas at 195 K contrary to the successful restoration of the partially collapsed pore of 2(m)a. Even though the average incorporation amount of dpee linker inserted into the framework is similar to that of dpey, the more uneven distribution of dpee linkers at the two potential linking sites, between the dinuclear silver clusters and between the tetranuclear silver centers, than that of dpey led to unsuccessful restoration of the partially collapsed 1D channel.
Figure 4.25. CO2 sorption isotherms of 3(1)a, 3(2)a, 3(3)a and 3(4)a.
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Gas sorption behavior of activated AgBTB-dpt, 4(m)a. 4(m)a does not show any N2 adsorption at 77 K (Figure 4.26). Even though 4(m)a contains dpt linkers between the next closest dinuclear silver clusters across the solve channel up to ~38% of the potential linkage sites, the collapsed solvent channels during the activation process is not restored by N2 at 77 K. However, the collapsed channels can be restored by CO2 at 195 K. 4(m)a has only one kind of solvent channel since there is only one kind of inter-silver linkage between the silver clusters, which contrasts to the two different kinds of solvent channels in 2(m)a and 3(m)a coming from the two different kinds of inter-silver linkages between the silver clusters. Interestingly, 4(1)a with ~20% dpt linkers having one kind of solvent channel shows two-step adsorption and hysteric desorption of CO2. The first stepwise adsorption occurs at ~0.05 bar and the second stepwise adsorption starts at ~0.35 bar. While a certain part of the channel with the more dpt linkers can be restored at the low pressure of CO2, ~0.05 bar, the other part of the channel with the less dpt linkers starts to be partially restored at the higher pressure of CO2, ~0.35 bar.
The CO2 sorption behaviors of 4(2)a, 4(3)a and 4(4)a are similar to but not the same as that of 4(1)a.
While the CO2 adsorption of 4(1)a is a two-step adsorption process, those of 4(2)a, 4(3)a and 4(4)a with 35-38% dpt linkers are single-step adsorption processes with the larger maximum uptake, ~160 cm3/g. The larger amount of dpt linkers in the frameworks of 4(2)a, 4(3)a and 4(4)a keep certain parts of the 1D solvent channel non-collapsed state and allow the initial CO2 adsorptions up to ~0.20-0.25 bar. Subsequent stepwise CO2 adsorptions represent the restoration of the other partially collapsed parts of the solvent channels. The CO2 sorption isotherms at the higher temperatures, 273 and 298 K, do not show any hysteresis. Although restorations of the partially collapsed solvent channels are not observed, the gradual increase of the maximum uptake amounts is observed as the increase of the linker amounts in the framework. The maximum CO2 uptake amounts are ~17, ~37, ~35, and ~35 cm3/g, respectively, at 273 K and ~9, ~17, ~17, and ~17 cm3/g, respectively, at 298 K for 4(1)a, 4(2)a, 4(3)a and 4(4)a.
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Figure 4.26. N2 and CO2 sorption isotherms of 4(1)a, 4(2)a, 4(3)a and 4(4)a.
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Gas sorption behavior of activated AgBTB-bpy, 5(m)a. The channel dimension of 5 in the single crystal structure model, 5.0 × 13.6 Å2, is larger than the kinetic diameter of N2, 3.64 Å. However, 5(m)a does not show any N2 adsorption at 77 K (Figure 4.27) even though 5(m)a contains bpy linkers between the dinuclear silver clusters and tertanuclear cluster across the solve channel up to ~48% of the potential linkage sites. On the other hand, 5(1)a with ~30% dpt linkers shows type I CO2 adsorption with hysteric desorption at 195 K and the maximum CO2 uptake amount is ~40 cm3/g. The hysteric desorption and slow sorption kinetics of 5(1)a indicate that 5(1)a has one kind of pore and the aperture dimension of the pore, the 1D solvent channel in 5(1)a, is slightly smaller than or approximately matches with the kinetic diameter of CO2, 3.3 Å. 5(2)a, 5(3)a and 5(4)a, with 30-48% dpt linkers in the framework showed stepwise CO2 adsorptions and hysteric desorptions at 195 K. The CO2 sorption behaviors of 5(2)a, 5(3)a and 5(4)a are similar to those of 4(2)a, 4(3)a and 4(4)a. The larger amount of bpy linkers in the frameworks of 5(2)a, 5(3)a and 5(4)a keep certain parts of the 1D solvent channel non-collapsed state and allow the initial CO2 adsorptions up to ~0.35 bar. Subsequent stepwise CO2 adsorptions represent the restoration of the other partially collapsed parts of the solvent channels. The CO2 sorption isotherms show slight hysteresis even at 273 K. The hysteresis and slow adsorption kinetics of CO2
suggest that the aperture dimension of the solvent channel in 5(m)a is slightly smaller than or approximately matches with the kinetic diameter of CO2, 3.3 Å. The gradual increase of the maximum CO2 uptake amounts of 5(m)a at the higher temperatures, 273 and 298 K respectively, is observed as the increases of the linker amounts in the framework. The maximum CO2 uptake amounts are ~12, ~16,
~16, and ~15 cm3/g at 273 K and ~7, ~9, ~9, and ~10 cm3/g at 298 K for 5(1)a, 5(2)a, 5(3)a and 5(4)a, respectively.
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Figure 4.27. N2 and CO2 sorption isotherms of 5(1)a, 5(2)a, 5(3)a and 5(4)a.
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