Journal of the American Chemical Society
Article
and 27 ppm) were observed, indicating high symmetry in (ZIF-
90-HMDA)5. The downfield signal was attributed to the
NCH2 methylene groups, and the other two signals to the
four CH2 methylene groups of the diimine bridge. In fact,
we did not detect the characteristic peak of the CH2NH2
methylene centered at ≈40 ppm,19 which would indicate that
HMDA was only attached by one of its amine groups. The
presence of the diimine cross-linking molecule N,N′-(hexane-
1,6-diyl)bis(1-imidazol-2-yl)methanimine) (Figure 4a) was
finally confirmed by analyzing the solution resulting from the
delivery systems. Finally, it also enables performing Schiff-base
condensation reactions between discrete amine and aldehyde
molecules, opening new perspectives in organic chemistry
synthesis.
MATERIALS AND METHODS
■
General Procedure for Synthesis of Imine Ligands. In a
typical synthesis, a suspension of NH2-bdc and the corresponding
aldehyde (4PC, 2PC, and Sal) in 15 mL of ethanol was spray-dried at
an inlet temperature of 130 °C, a feed rate of 3.0 mL min−1, and a flow
1
rate of 336 mL min−1 using a Mini Spray Dryer B-290 (BUCHI
̈
digestion of (ZIF-90-HMDA)5 in acetic acid by H NMR and
Labortechnik; spray cap: 0.5 mm hole). The collected solids were
ESI-MS (Figure S69). The peaks at 8.58 and 3.83 ppm in the
1H NMR spectrum were attributed to the CHN imine
proton and the NCH2 methylene protons of the diamine
molecule, respectively. On the other hand, in the ESI-MS
spectra, the peak at m/z = 273.1836 matches with the
molecular formula of the protonated N,N′-(hexane-1,6-diyl)-
bis(1-imidazol-2-yl)methanimine) [C14H20N6]+ (m/z =
273.1822).
1
characterized by ESI-MS, and H- and 13C NMR spectroscopy. Full
experimental details and characterization can be found in the
Synthesis of UiO-66-NH2. UiO-66-NH2 was synthesized through
the previously reported method.28 In a typical synthesis, 35 mL of HCl
37% was added to a solution 0.1 M ZrCl4 and 0.1 M NH2-bdc in 500
mL of DMF. The resulting mixture was heated at 120 °C under
stirring for 2 h. The obtained solid was collected by centrifugation and
washed two times with 100 mL of DMF for 12 h at 120 °C and three
times with 100 mL of absolute ethanol for 12 h at 60 °C. Finally, the
resulting powder was dried at 85 °C overnight (yield = 75%).
Synthesis of ZIF-90. ZIF-90 was synthesized through the
previously reported method.18 In a typical synthesis, 0.27 mL of
trimethylamine (1.96 mmol) was added to a solution 0.01 M
imidazole-2-carboxaldehyde and 3.75 mM Zn(NO3)2·6H2O in 200 mL
of DMF. The resulting mixture was stirred for 1 min at room
temperature, and then, 100 mL of ethanol was added. The particles
were collected by centrifugation, washed five times with ethanol and
one time with acetone, and finally dried at 85 °C overnight (yield =
70%). Characterization details can be found in the Supporting
The 1H NMR spectrum of the digested (ZIF-90-HMDA)5 in
strong acidic conditions (DCl, DMSO-d6) was collected to
quantify its yield of conversion (Figure S70). Comparison of
the integration of the signals corresponding to ICA (7.56 ppm)
and HMDA (1.30 ppm) revealed that there is 35% HMDA,
meaning that 70% of the ICA linkers were functionalized.
This high conversion of cross-linked ICA linkers in (ZIF-90-
HMDA)5 should therefore involve a remarkable closure of its
pores and reduction of its surface area in comparison to its
parent ZIF-90. This assumption was confirmed by measuring
the SBET from the N2 sorption isotherm at 77 K. We found a
SBET of 69 m2 g−1 that corresponds to a reduction of 93% of the
original SBET of ZIF-90 (Table 2, Figures 4d and S71).
General Synthesis of the PSM of UiO-66-NH2. A 0.150 g
portion of UiO-66-NH2 (0.085 mmol) was suspended in ethanol (15
mL), and the corresponding aldehyde (4PC, 2PC, and Sal) was added
to the dispersion. The resulting reaction mixture was then spray-dried
at an inlet temperature of 130 °C, a feed rate of 3.0 mL min−1, and a
flow rate of 336 mL min−1 using a Mini Spray Dryer B-290 (BUCHI
Labortechnik; spray cap: 0.5 mm hole). A yellow powder was collected
after 5 min. The resulting solid was then dispersed in 20 mL of ethanol
and precipitated by centrifugation. This process was repeated four
times. The final product was washed one time with acetone and dried
for 12 h at 85 °C. In addition, as a control experiment, the above-
mentioned method was reproduced, except that, instead of spray
drying the reaction mixture, we heated it at 130 °C for 5 min. Under
these conditions, a conversion rate of only 3% was found. Full
experimental and characterization details can be found in the
Table 2. BET Areas, Pore Volumes, and % of Conversion of
ZIF-90, (ZIF-90-BA)x, and (ZIF-90-HMDA)5
a
b
SBET
pore vol.
conver.
(%)
MOF
ZIF-90
x
(m2 g−1
)
(cm3 g−1
)
1070
670
483
424
69
0.4412
0.3150
0.2166
0.2042
(ZIF-90-BA)x
3
25
32
42
70
10
15
5
(ZIF-90-HMDA)5
0.0377
a
b
1
Calculated at P/P0 ≈ 0.4. Calculated from H NMR spectra of the
digested samples.
General Synthesis of the PSM of ZIF-90. A dispersion of ZIF-90
(0.100 g, 0.39 mmol) and the corresponding amine in 15 mL of
ethanol was spray-dried at an inlet temperature of 130 °C, a feed rate
of 3.0 mL min−1, and a flow rate of 336 mL min−1 using a Mini Spray
Dryer B-290 (BUCHI Labortechnik; spray cap: 0.5 mm hole). A
yellow powder was collected after 5 min. The resulting solid was then
dispersed in 20 mL of ethanol and precipitated by centrifugation. This
process was repeated five times. The final product was washed one
time with acetone and dried for 12 h at 85 °C. In addition, as a control
experiment, the above-mentioned method was reproduced, except
that, instead of spray drying the reaction mixture, we heated it at 130
°C for 5 min. Under these conditions, the conversion rate was 0%. Full
experimental and characterization details are reported in the
The contact angle (Θc) was also measured on a pressed
pellet disk of (ZIF-90-HMDA)5, giving a value of 110.9° that
corresponds to a higher hydrophobic surface in comparison to
the different (ZIF-90-BA)x samples (Figure 4e).
CONCLUSIONS
■
We have reported a highly versatile and effective methodology
to postsynthetically modify MOFs via Schiff-base condensation
reactions. This strategy can be applied to MOFs with either
terminal aldehyde or amine groups, reduces their PSM time,
and enables their continuous PSM with good rates of
conversion. Therefore, it should facilitate the PSM of MOFs
for numerous applications, including catalysis, sensor technol-
ogy, pollutant removal, and separation. This method also
allowed the efficient cross-linking of the terminal aldehydes
groups of ZIF-90 using a diamine molecule, thereby blocking
their porosity and opening up new avenues for future triggered
Quantification Protocol. Samples were digested in strong acidic
1
conditions, and their H NMR spectra were collected to calculate the
conversion of imine by the comparison of the integration of the
characteristic signals of the corresponding reactants (amine and
aldehyde). For the PSM of UiO-66-NH2, 10 mg of activated sample,
F
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Garzón-Tovar, Luis
