Results and discussions

Firstly, when it comes to setting up the synthetic conditions for heterogeneous interpenetration, we should consider the overall synthetic method for the heterogeneous interpenetration. There were two kinds of ways to induce the heterogeneous interpentration with solvothermal method. The first one is to mix both of the precursors for two frameworks in one pot and then heat the mixture to create the interpenetrated systems. The second one is to synthesize the interpenetrated frameworks in a stepwise method that is we synthesize the one kind of MOF in advance and then infiltrate the precursors of the other MOF, and then heat up the mixture in the last.

However, considering the possibility of the synthesis of unwanted Cu-BDC or Zn- BTC, we decided to synthesize the system with the sequential synthesis. We need to choose the first MOF that should be synthesized in the first place. Considering the vulnerability of the MOF-5 on chemical environment containing water^8-10 , we chose the HKUST-1. Based on the assumptions the frameworks of HKUST-1 and MOF-5 completely interpenetrated each other, we could calculate the Copper to Zinc molar ratio and also molar ratio of the BTC and BDC in unit cell. According to the calculations, the molar ratio of Copper to Zinc was 3 : 2 and the molar ratio of the BTC and BDC consisting the each scaffold was 4 : 3

In choosing the synthetic method, we should obain the largest total pore volume of the HKUST-1 so that the metal precursors and ligand prercusors can infiltrate as much as they can. According to the kinds of the solvent exchanged with the Mother liquor of the as- synthesized HKUST-1 (Table 3.1) , the BET surface area and total pore volumes varies. Among the exchange solvent, when we exchanged the Mother liquor with Methanol, we could obtain the largest total pore volume 0.823 cm³/g. Accordingly our synthesized HKUST-1 also shows the total pore volume 0.80 cm³/g.

Secondly we should consider the solvent system for the synthesis. We set up the candidates of the solvent system with DMF, DMA and DEF itself and the mixture with the small amount of ethanol methanol and acetonitrile. In the perspective of choosing the Main solvent amist DMF, DMA and DEF, we considered the PXRD patterns and the phase observed on the micrograph. (Figure 3.3-3.5)

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Table 3.1 Varied SBET (m²/g) and Total Pore Volume (cm³/g) depending the exchange solvents.

Figure 3.3 PXRD patterns of HKUST-1 & MOF-5 synthesized in the DMF as a main solvent and the mixture of the DMA with EtOH, MeOH and MeCN Simulated patterns of HKUST-1 (blue), simulated pattern of MOF-5 and simulated pattern of interpenetrated framework with HKUST-1 & MOF-5 (pink), synthesized only in DMF (dark pink), mixture with EtOH (black), mixture with MeOH (dark red), and mixture with MeCN (dark blue).

Exchange Solvent SBET (m²/g) Total Pore Volume (cm³/g)

H2O 1337.8 ± 1.3 0.554

MeOH 2042.3 ± 2.5 0.823

EtOH 1718.4 ± 0.9 0.689

Acetone 1804.8 ± 1.3 0.759

CH2Cl 1168.0 ± 0.9 0.571

CHCl3 1244.8 ± 1.4 0.607

DMF 1072.6 ± 1.1 0.456

2 (degree)

5 10 15 20 25 30 35 40

Intensity (a.u.)

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Figure 3.4 PXRD patterns of HKUST-1 & MOF-5 on DMA as a main solvent and the mixture of the DMA with EtOH, MeOH and MeCN ; Simulated pattern of HKUST-1 (blue), simulated pattern of MOF-5 and simulated pattern of interpenetrated framework with HKUST-1 & MOF-5 (pink), synthesized only in DMA (dark pink) mixture with EtOH (black), mixture with MeOH (dark red), and mixture with MeCN (dark blue).

Figure 3.5 PXRD patterns of HKUST-1 & MOF-5 on DEF as a main solvent and the mixture of the DEF with EtOH, MeOH and MeCN ; Simulated pattern of HKUST-1 (blue), simulated pattern of MOF- 5 and simulated pattern of interpenetrated framework with HKUST-1 & MOF-5 (pink), synthesized only in DEF (dark pink), mixture with EtOH (black), mixture with MeOH (dark red), and mixture with MeCN (dark blue).

2 (degree)

5 10 15 20 25 30 35 40

Intensity (a.u.)

2 (degree)

5 10 15 20 25 30 35 40

Intensity (a.u.)

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As seen from the PXRD patterns above (Figure 3.3 and Figure 3.5), synthesized frameworks show the different phase with the simulated patterns of the interpenetrated patterns on DMF and DEF solvent systems. In accordance with the optical micrograph, the framework synthesized on the solvent system of which the main solvent system is DMA shows somewhat less separated phase than the frameworks on the other solvent systems (Figure 3.4). With the optical microscope images, we could not confirm that the small frameworks on the octahedral HKUST-1 exist separately or bonded to the octahedral frameworks (Figure 3.6).

Figure 3.6 Optical microscope of the HKUST-1 & MOF-5 interepenetrated frameworks synthesized only in the DMA (a)and in the mixture of DMA and EtOH (b), MeOH (c) and MeCN (d).

(a) (b)

(d) (c)

1 μm

1 μm 1 μm

1 μm

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Figure 3.7 SEM images of Interpenetrated Frameworks of HKUST-1 & MOF-5 in (a) DMA, (b) DMA- EtOH, (c) DMA_MeOH, and (d) DMA-MeCN.

SEM images show that some of the crystals attached to the surface of the octahedral HKUST-1, but other crystals sticking out of the surface and grow out of themselves (Figure 3.7). It was assumed to be the phases we intended for and it was necessary to lower the concentrations of the Zinc precursors and ligand precursors so that the amount of crystals that grow out themselves in separate phases can decrease or be removed. The concentrations of zinc precursors and BDC infiltrate the HKUST-1 was lowered in half.

However the SEM images of interpenetrated frameworks synthesized with the lowered concentrations of the Zinc precursors and BDC still show the separate phase growing out of the surface. Among the frameworks synthesized in the solvent systems, the frameworks synthesized in the DMA without the additional solvent was chosen for the main system to be

(a) (b)

(c) (d)

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characterized , because it shows less separate phase than the others. It show the unidentified rectangular and plate-shaped materials covering the HKUST-1 (Figure 3.8)

Figure 3.8 SEM images of the interpenetrated frameworks synthesized in the DMA.

Figure 3.9 NMR of interpenetrated frameworks synthesized in DMA with the lowered concentration.

NMR shows the molar ratio of BTC to BDC is 1 to 0.338 (Figure 3.9). When we assumed the HKUST-1 and MOF-5 were completely interpenetrated with each other, the molar ratio of BTC and BDC is 4 to 3 which means the synthesized frameworks could be assumed to be 45 % Interpenetrated without considering the other forms of the BDC

However we had to rule out the possibility of side reactions while solvothermal reaction, which could be cation exchange, ligand exchange and the just inclusion of BDC in the pore of the HKUST-1 without composing of MOF-5.

Chemical Shift (ppm)

8.0 8.5

9.0

Intensity (a.u.)

a b

a

b

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To exclude the possibility of the just inclusion of the BDC, we take the IR spectrum of the samples (Figure 3.10) . If BDC was just included in the pore, it would show carboxylic acid peak of C=O stretching around 1700 cm^-1, but it show just peak of C=O stretching around 1600 cm^-1 which support the fact that BDC exist in the form of carboxylate which composes the frameworks

Figure 3.10 IR spectrum of BDC (red) and interpenetrated frameworks synthesized in DMA (black).

Also we conducted the experiments of heating HKUST-1 with just each of metal and ligand precursor to rule out the possibility of cation exchange and ligand exchange during the solvothermal process. Observed on the SEM images (Figure 3.11), HKUST-1 heated just with the zinc precursor shows no formation of the crystals on the surface while HKUST-1 heated with the BDC show some peeling off on the surface.

Figure 3.11 SEM images of HKUST-1 heated only with each of Zinc precursor (a) and BDC (b)

Wavenumber (cm^-1) 1000 1500 2000 2500 3000 3500 4000

Transmittance (%)

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ICP indicates the about 0.63 % of molar amount of copper exchanged with the infiltrated Zinc precursors. This was considered to be very small amount of the cation exchange.

Figure 3.12 NMR of HKUST-1 heated with the BDC

From the NMR of HKUST-1 heated with only ligand precursor, BDC (Figure 3.12), it confirms that the molar ratio of the BTC to BDC is 1 to 0.015. However this molar ratio continued to decrease, when the crystals were washed with DMA several times, so we can conclude that there were little possibility of the ligand exchange between BTC and BDC.

Figure 3.12 TGA trace of HKUST-1 (blue), MOF-5, Physically mixing HKUST-1 and MOF-5, and the interpenetrated frameworks synthesized in DMA.

Temperature (^oC)

100 200 300 400 500 600

Weight (%)

0 20 40 60 80 100

Chemical Shift (ppm)

8.0 8.2 8.4 8.6 8.8 9.0

Intensity (a.u.)

a b

a

b

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Figure 3.13 (a) N2 adsorption isotherms of HKUST-1 and interpenetrated frameworks synthesized in DMA and (b) their pore size distributions of HK plot.

Table 3.2 BET surface area & Pore Size distribution.

Metal-Organic Framework SBET (m²/g) Pore size (nm)

HKUST-1 2130 0.61

The inteperentrated framework 1260 0.66

TGA trace of the interpenetrated frameworks (Figure 3.12) didn’t show the section of the plateau on the temperature range from 100 ^oC to 300 ^oC which commonly indicates the evaporation of the solvent and it just continued to decompose till 350 ^oC . We assumed that the interpenetration occurred from the outside of the frameworks so that the solvent included in the framework eacvaporates more slowly than that of the HKUST-1 and MOF-5.

Also N2 adsorption isotherms of the interpenetrated frameworks show the significantly decreased adsorption amount compared to that of HKUST-1 (Figure 3.13). BET surface area decreased from 2130 m²/g to 1260 m²/g, which could be caused by the formation of the nonporous frameworks by the inclusion of the scaffold of MOF-5 or the simple blocking of the plate-like crystals on the surface (Table 3.1).

P/P0

0.0 0.2 0.4 0.6 0.8 1.0

Vads / cc g-1

0 100 200 300 400 500 600

d_p / nm

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

dVp/dlogdp

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035

(a) (b)

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Unfortunately, During the thermal activation up to 150 ˚C, we found that the increased relative intensity of the peak of {400} plane on the 13.5 ^o and the other decreased relative intensity on peaks of the {200}and {220} similar to the HKUST-1, which was to be the indication of the interpenetration, could be just because of the simple inclusion of the solvent. From the simple change of the relativel intensity, we couldn’t draw the significant conclusion whether the frameworks interpenetrated or not from the simple change of the relative intensity.

Figure 3.14 PXRD patterns of HKUST-1 activated at 150 ^oC and the interpenetrated frameworks activated at 150 ^oC ; simulated patterns of HKUST-1 (blue), simulated patterns of MOF-5 (red), simulated patterns of interpenetrated frameworks of HKUST-1 & MOF-5, HKUST-1 activated at 150 ^oC (black), the interpenetrated framework activated at 150 ^oC.

Additionally, we took the EXAFs on the interpenetrated frameworks to confirm the coordination environment of the Zn which could be the clue of how the Zinc species exist on the HKUST-1 (Table 3.2). As you can see from the table, we could confirm that the coordination number of the Zinc was 3.70.8 which was similar to zinc species in the form of Zn4O cluster. but we can not confirm that the periodic structure with the coordination boding between Zinc ions and BDC formed or not (Table 3.3)

Overall, the matierials show the possibility of the interpenetrated framework.

However these chacterizations are not conclusive to define whether HKUST-1 and MOF-5 were interpenetrated with each other scaffolds.

2 (degree)

5 10 15 20 25 30 35 40

Intensity (a.u.)

45

Table 3.3 Structural paramters at Cu K edge and Zn K edge from HKUST-1 and the HKUST-1

& MOF-5.

pair CN r (nm) ²(pm²) E (eV) R-factor

HKUST-1

Cu-O 5.71.4 0.1960.001 7530

0.62.1 0.018 Cu-Cu 0.92.0 0.2660.005 9753

HKUST-1+MOF-5

Cu-O 4.70.8 0.1960.001 7622

4.71.6 0.011 Cu-Cu 0.71.4 0.2660.005 109177

Zn-O 3.70.8 0.2030.002 3026 4.71.8 0.027

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