THEORY
4.1 Synthesis of MOFs Studied .1 Synthesis of MIL-53(Al)
The synthesis of MIL-53(Al) was carried out under hydrothermal conditions using aluminium nitrate nonahydrate (Al(NO3)2.9H2O, aldrich), benzene 1,4-dicarboxylic acid (BDC, Merck), dimethylformamide (DMF, Merck) and deionized water as per the procedure suggested by Loiseau et al. [41]. The molar composition of starting materials was 1 (Al(NO3)3.9H2O) : 0.5 (BDC) : 80 (H2O). The solution was placed in a Teflon lined steel autoclave at 493 K for 3 days and allowed to react hydrothermally. The product was filtered and washed with deionized water.
The resultant product consists of mixture of MIL-53(Al) and unreacted BDC. For removal of unreacted BDC [199], typically 1 g of MIL-53(Al) as-synthesized was dispersed in 25 ml of DMF and introduced in teflon lined steel autoclave for 15 h at 423 K. The product was cooled, filtered and calcined overnight at 553 K.
4.1.2 Synthesis of Mg/DOBDC
The synthesis of Mg/DOBDC was carried out according to the procedure suggested by Caskey et al. [97]. A solid mixture of 2,5-dihydroxy terephthalic acid (0.888 g, 4.472 mmol, Sigma aldrich) and Mg(NO3)2·6H2O (3.795 g, 14.8 mmol, Merck) was added into a 15:1:1 (v/v/v) mixture of DMF/ethanol/water (400 mL). The suspension was sonicated for about 5 minutes to make it homogeneous. Then the suspension was then equally distributed in twenty seven 30-mL polypropylene vials. These vials were capped tightly and kept in an oven at 398 K for 20 hours.
Samples were then cooled to room temperature followed by decantation of mother liquid from the yellowish crystals. Methanol of similar quantity as that of the mother liquid (~15 mL per vials) was added. Then suspension was collected in a 500-mL polypropylene vial. After every 12 hours, methanol was decanted and replaced with fresh methanol for the next two days to get as- synthesized Mg/DOBDC crystals. These crystals were dried and stored in vacuum.
4.1.3 Synthesis of Mn/DOBDC
Synthesis procedure of Mn/DOBDC was slightly modified from that reported in literature [147].
A solid mixture of 2,5-dihydroxy terephthalic acid (0.888 g, 4.472 mmol, Sigma aldrich) and Mn(NO3)2·4H2O (3.715 g, 14.8 mmol, Merck) was added into a 15:1:1 (v/v/v) mixture of DMF/ethanol/water (400 mL). The suspension was sonicated for about 5 minutes to make it homogeneous. Then the suspension was equally distributed in twenty seven 30-mL polypropylene vials. These vials were capped tightly and kept in an oven at 404 K for 24 hours.
Samples were then cooled to room temperature followed by decantation of the mother liquid from the brownish crystals. Methanol of similar quantity as that of the mother liquid (~15 mL per vials) was added. Then suspension was collected in a 500-mL polypropylene vial. After
every 12 hours, methanol was decanted and replaced with fresh methanol for the next two days to get as-synthesized Mn/DOBDC crystals. These crystals were dried and stored in vacuum.
4.1.4 Synthesis of Co/DOBDC
As per procedure suggested elsewhere [97], 2,5-dihydroxy terephthalic acid (0.482 g, 2.43 mmol, Sigma aldrich), Co(NO3)2·6H2O (2.377 g, 8.67 mmol, Merck) and a 1:1:1 (v/v/v) mixture of DMF–ethanol–water (200 mL) were mixed in a 500 ml polypropylene vial. Sonication of the suspension was done for about 5 minutes to make it homogeneous. Then vial was capped tightly and kept in an oven at 373 K for 24 hours. Sample was then cooled to room temperature followed by decantation of the mother liquid from the violet crystals. Methanol of similar quantity as that of the mother liquid (~200 mL) was added. After every 12 hours, methanol was decanted and replaced with fresh methanol for the next two days to get as-synthesized Co/DOBDC crystals. These crystals were dried and stored in vacuum.
4.1.5 Synthesis of Ni/DOBDC
The synthesis of Ni/DOBDC was carried out as per procedure suggested by Caskey et al.[97]. A solid mixture of 2,5-dihydroxy terephthalic acid (0.478 g, 2.41 mmol, Sigma aldrich) and Ni(NO3)2·6H2O (2.378 g, 8.178 mmol, Merck) were added to a 1:1:1 (v/v/v) mixture of DMF–
ethanol–water (200 mL) in a 500 ml polypropylene vial. Sonication of the suspension was done for about 5 minutes to make it homogeneous. Then vial was capped tightly and kept in an oven at 373 K for 24 hours. Sample was then cooled to room temperature followed by decantation of mother liquid from the light greenish crystals. Methanol of similar quantity as that of the mother liquid (~200 mL) was added. After every 12 hours, methanol was decanted and replaced with fresh methanol for two days to get as-synthesized Ni/DOBDC crystals. These crystals were dried and stored in vacuum.
4.1.6 Synthesis of Ni/DABCO
Ni/DABCO was synthesized as per procedure suggested by Arstad et al. [75]. A mixture of Ni(NO3)2.6H2O (3 mmol, Merck), benzene-1,4-dicarboxylic acid (3 mmol, Merck), 1,4- diazabicyclo (2,2,2) octane (2.49 mmol, Alfa Aesar) and DMF (60 cm3, Merck) was taken in a conical flask at room temperature and stirred rigorously until it is well dissolved for about 20 minutes. This mixture was then transferred into a teflon lined autoclave. The autoclave was heated to 383 K and held for 24 hours. Then it was cooled to room temperature; the solid product was filtered and washed thoroughly with DMF followed by drying at ambient temperature (290 – 303 K) under vacuum. The dried material was transferred into a vacuum desiccator for storage.
4.1.7 Synthesis of Cu/DABCO
The synthesis procedure for Cu/DABCO was similar to that of Ni/DABCO but with minor changes. A mixture of Cu(NO3)2.3H2O (3 mmol, Merck), benzene-1,4-dicarboxylic acid (3 mmol, Merck), 1,4-diazabicyclo (2,2,2) octane (2.49 mmol, Alfa Aesar) and DMF (60 cm3, Merck) was taken in a conical flask at room temperature and stirred rigorously until it is well dissolved for about 20 minutes. This mixture was then transferred into a teflon lined autoclave.
The autoclave was heated to 393 K and held for 48 hours. Then it was cooled to room temperature; the solid product was filtered and washed thoroughly with DMF followed by drying at ambient temperature (290–303 K) under vacuum. The dried material was transferred into a vacuum desiccator for storage.
4.1.8 Synthesis of Zn/DABCO
The procedure suggested by Dybtsev et al. [55] was followed for the synthesis of Zn/DABCO compound. A mixture of Zn(NO3)2.6H2O (1.61 g, Merck), benzene-1,4-dicarboxylic acid (0.81 g, Merck), 1,4-diazabicyclo (2,2,2) octane (0.28 g, Acros) and DMF (60 cm3, Merck) was taken
in a conical flask at room temperature and stirred rigorously until it is well dissolved for about 20 minutes. This mixture was then transferred into a teflon lined autoclave. The autoclave was heated to 393 K and this temperature was maintained for 48 hours. The product recovery and storage steps were similar to those of Ni/DABCO and Cu/DABCO.