Regulated synthesis of Zr-metal–organic frameworks with variable hole size and its influence on the performance of novel MOF-based heterogeneous amino acid–thiourea catalysts

We present an efficient and easy synthesis method for incorporating organocatalytic moieties into Zr-metal organic frameworks (Zr-MOFs). The catalytic activity and selectivity of the new chiral catalysts were improved by adjusting the aperture of the MOF cavities. The hole size of the Zr-MOF was modulated by adding acid and replacing bridge ligands during synthesis. The difunctional chiral units of amino acid–thiourea are anchored onto the Zr-MOF by a mild synthesis method from an isothiocyanate intermediate which could effectively avoid the racemization of chiral moieties in the synthesis process. By means of specific surface area measurement (BET), scanning electron microscopy (SEM) and powder X-ray Diffraction (PXRD), it was confirmed that Zr-MOFs with different pore sizes were synthesized without breaking the basic octahedral structure of the MOF. Finally, good yields (up to 83%) and ee values (up to 73%) were achieved with the new heterogeneous catalysts in 48 hours for the aldol reaction of 4-nitrobenzaldehyde with acetone. By contrast, using the catalyst support without modulating the synthesis, the yield (30%) and the ee-value (26%) were both low. Experiments have confirmed the important influence on the reaction selectivity of providing a suitable reaction environment by controlling the aperture of MOF cavities.


Synthesis of Zr-UiO-66-NHCS-Leu
In a 50 mL round-bottomed flask, 1.20 mmol of 2-amino-4-methylpentylmethylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and the required amount of Zr-UiO-66-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then, it was centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.  Zr-UiO-66-NH 2 and functionalized linker were identified at a peak area of 6.9-8.5 ppm.

Synthesis of Zr-UiO-66-NHCS-Ile
In a 50 mL round-bottomed flask, 1.20 mmol of 3-amino-4-methylpentylmethylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and add the required amount of Zr-UiO-66-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then, it was centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.

Synthesis of Zr-UiO-66-NHCS-Lte
In a 50 mL round-bottomed flask, 1.20 mmol of 2-amino-3,3-dimethylbutylmethylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and the required amount of Zr-UiO-66-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then, it was centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.  Zr-UiO-66-NH 2 and functionalized linker were identified at a peak area of 6.9-8.5 ppm.

Synthesis of Zr-UiO-66-NHCS-Trp
In a 50 mL round-bottomed flask, 1.20 mmol of α-aminoindolylpropylmethylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and the required amount of Zr-UiO-66-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then it was centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.

Synthesis of Zr-UiO-66-NHCS-Pro
In a 50 mL round-bottomed flask, 1.20 mmol of Pyrrolidine-2-methylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and the required amount of Zr-UiO-66-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then, it was then centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.  Zr-UiO-66-NH 2 and functionalized linker were identified at a peak area of 7.0-8.5 ppm.

Synthesis of Zr-UiO-67-NHCS-Pro
In a 50 mL round-bottomed flask, 1.20 mmol of Pyrrolidine-2-methylamine was dispersed in 20 mL of anhydrous tetrahydrofuran, and the required amount of Zr-UiO-67-NH 2 was added (1.00 mmol). The solution was stirred at 50°C for 24 h. Then, it was then centrifuged, and the solid was washed with THF (3×10 mL) to obtain as a fine yellow powder after drying in vacuo at 50°C.