III. Heterostructure synthesized by self-assembly of two metal-organic frameworks
III.3. Results and discussion
NicyclamBPDC, NiLethylBPDC and NiLpropylBPDC were synthesized by self-assembly of nickel- based macrocyclic compounds and sodium biphenyldicarboxylate (Na2BPDC) in MeCN/H2O mixture solvent system. The products were characterized by single crystal X-ray diffraction and X-ray powder diffraction. Single crystal X-ray diffraction analysis revealed that three products have isorecticular structure with the same space group R-3 and almost same lattice parameters. Each Ni (II) macrocyclic complexes have coordination bonds with oxygen atoms from different BPDC2- in monodentate fashion at axial sites of square planar geometry. Ni (II) macrocyclic complexes and BPDC2- ligands results in one dimensional chains. three different directing 1D chains constructed a double network of threefold braids.19 This structure has one dimensional pore channel along to the c-axis. The size of pore aperture is possible to be controlled by length of functional group of macrocyclic compounds. NicyclamBPDC has diameter of pore size by 5.81 Å. NiLethylBPDC has diameter of pore aperture size by 1.06~2.06 Å.
NiLpropylBPDC has diameter of pore aperture size by 0.32 Å crystallographically (van der waals radii were considered). This pore aperture size is expected to occur kinetic quantum sieving effect which is helpful to separate hydrogen isotopes.
Figure 3.7. X-ray structure of isostructural MOFs (a) Structure of the 1D chain (C, gray; N, blue; O, red; Ni,yellow). (b) A view for double network of threefold braids (c) ab plane of NicyclamBPDC, (d) NiLethylBPDC and (e) NiLpropylBPDC
Table 2. X-ray crystallographic data of NicyclamBPDC.
Compound NicyclamBPDC
formula NiC24H32N4O10
crystal system trigonal
space group R -3 :H
fw 595.22
a, Å 26.370(4)
b, Å 26.370(4)
c, Å 10.903(2)
α, deg 90
β, deg 90
γ, deg 120
V, Å3 6566(2)
Z 9
Ucalcd, g cm-3 1.382
temp , K 100(2)
O, Å 0.70000
P, mm-1 0.691
goodness-of-fit (F2) 1.067
F(000) 2916
reflections collected (reflns_number) 21692
independent reflections (reflns_total) 5263 [R(int) = 0.0419]
completeness to Tmax, % 89.3%
data/parameters/restraints 5263/ 243/ 6 T range for data collection, deg 2.039 to 33.279
diffraction limits (h, k, l) -40≤h≤39, -39≤k≤39,-15≤l≤14 refinement method Full-matrix least-squares on F2 R1, wR2 [I>2V(I)] R1 = 0.0696a, wR2 = 0.1933b R1, wR2 (all data) R1 = 0.0804a, wR2 = 0.2040b largest peak, hole, eÅ-3 1.886, -1.003
aR =6__Fo_ - _Fc__/6_Fo_. bwR(F2) = [6w(Fo2- Fc2)2/6w(Fo2)2]½ where w = 1/[V2(Fo2) + (0.1505P)2+ (7.0566)P], P =(Fo2+ 2Fc2)/3. cwR(F2) = [6w(Fo2- Fc2)2/6w(Fo2)2]½ where w = 1/[V2(Fo2) + (0.0728P)2+ (16.1542)P], P =(Fo2+ 2Fc2)/3.
Table 3. X-ray crystallographic data of NiLethylBPDC -as and NiLpropylBPDC.
Compound NiLethylBPDC NiLpropylBPDC
formula NiC26H42N6O6 NiC28H46N6O6
crystal system Trigonal Trigonal
space group R-3 :H R-3 :H
fw 593.36 621.42
a, Å 26.094(4) 26.1625(8)
b, Å 26.094(4) 26.1625(8)
c, Å 11.531(2) 11.6677(3)
α, deg 90 90
β, deg 90 90
γ, deg 120 120
V, Å3 6799(2) 6916.3(5)
Z 9 9
Ucalcd, g cm-3 1.304 1.343
temp , K 173(2) 173(2)
O, Å 0.71073 0.71073
P, mm-1 0.689 0.681
goodness-of-fit (F2) 1.055 1.048
F(000) 2844 2988
reflections collected 21772 20065
independent reflections 3227 [R(int) = 0.0800] 3009 [R(int) = 0.0280]
completeness to Tmax, % 99.5% 99.4%
data/parameters/restraints 3437 / 205 / 1 3009 / 0 / 178 T range for data
ll i d
3.123 to 27.464 3.115 to 25.980 diffraction limits (h, k, l) -33≤h≤33, -33≤k≤33, -
14≤l≤14
-32 ≤ h ≤ 32, -32 ≤ k ≤ 32, -14
≤ l ≤ 13
refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2
R1, wR2 [I>2V(I)] R1 = 0.0610a, wR2 = 0.1301b R1 = 0.0472,a wR2 = 0.1263c R1, wR2 (all data) R1 = 0.1051a, wR2 = 0.1466b R1 = 0.0542,a wR2 = 0.1321c largest peak, hole, eÅ-3 0.664, -0.228 0.682, -0.394
aR =6__Fo_ - _Fc__/6_Fo_. bwR(F2) = [6w(Fo2- Fc2)2/6w(Fo2)2]½ where w = 1/[V2(Fo2) + (0.1505P)2+ (7.0566)P], P =(Fo2+ 2Fc2)/3. cwR(F2) = [6w(Fo2- Fc2)2/6w(Fo2)2]½ where w = 1/[V2(Fo2) + (0.0728P)2+ (16.1542)P], P =(Fo2+ 2Fc2)/3.
Three metal-organic frameworks, NicyclamBPDC, NiLethylBPDC and NiLpropylBPDC were measured XRPD and each pattern of products have well-matched results compared to simulated patterns from single crystal X-ray diffraction analysis.G
G
Figure 3.8. XRPD patterns of NicyclamBPDC (Simulated pattern, red; as-synthesized, black)
G
Figure 3.9. XRPD patterns of NiLethylBPDC (Simulated pattern, red; as-synthesized compound, black)
G
Figure 3.10. XRPD patterns of NiLpropylBPDC (Simulated pattern, red; as-synthesized compound, black) N2 isotherms were measured about each metal-organic frameworks. All products were activiated at 140
oC under vacuum overnight. As we can expect from pore aperture sizes based on single crystal X-ray analysis results, N2 isotherm result of NicyclamBPDC showed typical type I sorption behavior with the largest pore volume, 0.3830 cm3/g, compared to two other products. calculated BET surface area 776 m2/g. then, N2 isotherm result of NiLethylBPDC showed total pore volume by 0.1008 cm3/g and calculated BET surface area 168.7 m2/g. it showed kind of hysterisis which might be occurred by kinetic diffussion barrier by small pore aperture size. lastly, NiLpropylBPDC showed 0.00188 cm3/g of total pore volume. It is quite small and would be come from tiny pore aperture size.
G Figure 3.11. N2 adsorption isotherm of NicyclamBPDC at 77 K
G
Figure 3.12. N2 adsorption isotherm of NiLethylBPDC at 77 K P/P0
0.0 0.2 0.4 0.6 0.8 1.0
Va cm
3 (STP)
/g
0 50 100 150 200 250 300
P/P0
0.0 0.2 0.4 0.6 0.8 1.0
Va cm
3 (STP)
/g
0 10 20 30 40 50 60 70
Figure 3.13. N2 adsorption isotherm of NiLpropylBPDC at 77 K
Fortunately, there was a chance to measure TDS in Max plank institute for NiLpropylBPDC. TDS spectra confirmed the adsorption sites of NiLpropylBPDC as a function of temperature. There is diffusion barrier by small pore aperture size from propyl groups showing desorption of isotope separation at high temperature, 110 K. This phenomenon could be one of solution to improve separation temperature range which is effective in industrial fields. However, absolute uptake of hydrogen isotope molecule was too small to use as isotope separation material. Additionally, selectivity also is not outstanding compared to conversional method except for temperature range.
Figure 3.14. TDS spectra of NiLpropylBPDC exposed at 10 mbar gas mixture of H2/D2, with the ratio 1:1, for 10 min
P/P0
0.0 0.2 0.4 0.6 0.8 1.0
Va cm
3 (STP)
/g
0 2 4 6 8 10 12
Figure 3.15. Selectivity of NiLpropylBPDC as a function of temperature based on Fig. 3.14.
To improve selectivity by optimizing the exposure condition at high temperature, pressure was changed from 10 mbar to 60 mbar. As a result, the condition with 110 K, 30 mbar for 10 min made optimized selectivity, 1.58. It is still too poor selectivitiy compared to conventional method under cryogenic condition.
50 100 150 200 250
0 5 10 15 20 25 30 35 40
80 100 120 140 160 180
0 2 4 6 8
H2 10mbar D2 10mbar H2 30mbar D2 30mbar H2 60mbar D2 60mbar
Desorption Rate (1016 molecules/g·s)
Temperature (K)
Figure 3.15. TDS spectra of NiLpropylBPDC exposed at 10, 30 and 60 mbar gas mixture of H2/D2, with the ratio 1:1, for 10 min at 110 K.
80 100 120 140 160 180
1.2 1.4 1.6
Selectivity
Temperature (K)
Table 4. Selectivity of NiLpropylBPDC with different exposure condition changing pressure
Pressure/mbar H2/10-3mmol·g-1 D2/10-3mmol·g-1 Selectivity
10 5.077416 6.396042 1.2597
30 12.04522 19.00511 1.57781
60 27.99959 39.92753 1.426
To overcome the limitation of performance for hydrogen isotopes separation, synthesis of core-shell material was tried. NicyclamBPDC was chosen as core material candidate which can store sieved deuterium because it has the largest pore aperture size and volume among three of the isorecticular metal-organic frameworks. The smaller pore aperture size occurring more effective quantum sieving effect than NicyclamBPDC is the main reason to select NiLethylBPDC or NiLpropylBPDC as shell materials to separate hydrogen isotopes. Hence, NicyclamBPDC was put into shell MOF solution as diluted as homogenous nucleation is not occurred. As a result, ‘cotton swab’-like heterostructure in fig.3.16. was found during evaporation of shell MOF solution unexpectedly. However, some needle- shaped core crystal which did not react with shell MOF were easily found in fig .3.16. Therefore, this kind of the seed-mediated strategy showed a possibility to synthesize homogenous ‘cotton swab’-like heterostructure with delicate sense.
Figure 3.16. Visible-light microscope images for heterostructure of NicyclamBPDC@NiLpropylBPDC