The Zeolitic Imidazolate Framework-8 (ZIF-8) nanoparticles were successfully synthesised utilizing a rapid isothermal bench top reaction method as described in chapter 4.4.1 and ZIF-67 nanoparticles were also synthesized successfully utiling a rapid isothermal surfactant mediated bench top reaction open to the atmosphere as described in chapter 4.4.2. The methods resulted in white ZIF-8 nanoparticles and purple ZIF-67 nanoparticles which is consistent with what was previously reported in the literature. Powder X-ray diffraction confirmed similar crystal structure and SOD topology on the both synthesized ZIF-8 and ZIF-67 nanoparticles. Fourier transform infrared spectroscopy (FTIR) showed that all of the 2-methylimidazole linker has been removed during the puri?cation of the product as some prominent peaks on the linker like H?N···H hydrogen bridge dissappered completely on the synthesized ZIFs. The Scanning electron microscope (SEM) confirmed smooth surfaced hexagonal shape with an average particle size of 37 nm for nZIF-8, while nZIF-67 exhibited smooth surfaced rhombic dodecahedral shape with an average particle size of 92 nm on SEM images reported in chapter 3.2.
The synthesized ZIF nanoparticles exhibited the type I adsorption isotherms with a very high level of N2 adsorption at very low relative pressure, showing high surface area on both synthesized ZIFs even though ZIF-8 exhibited a higher adsorption and surface area compared to ZIF-67. Thermogravimetric analysis (TGA), showed that both nanoparticles can withstand high temperatures to up to 450 °C. To see if it is possible to exchange the metal ion (Fe2+) in ZIF-8(Zn) and ZIF-67(Co) frameworks, postsynthetic modi?cation was performed successfully to obtain ZIF-8(Zn/Fe) and ZIF-67(Co/Fe) with iron(II) incorporated in the framework using solvothermal method at ambient condition as described in chapter 4.5. To obtain ZIF material with iron metal incorporated in the framework, iron exchange was performed using 3 iron(II) sources: Iron(II) sulphate heptahydrate (FeSO4.7H2O), Iron(II) acetate (Fe(CO2CH3)2) and Iron(II) acetylacetonate (Fe(acac)2).
The iron exchange using iron(II) sulphate heptahydrate (FeSO4.7H2O), wasn’t successful as it degraded the ZIF-8 and ZIF-67 framework because of high acidity (pH = 2), which is shown by the SEM image in Figure 13. Iron(II) acetate (Fe(CO2CH3)2) and Iron(II) acetylacetonate (Fe(acac)2) were lenient on the parrticles as iron exchange occurred successfully even though they were still acidic with pH of 4 for Iron(II) acetate and pH of 6 for Iron(II) acetylacetonate. The crystal structure of ZIF-8 and ZIF-67 was retained after the exchange reaction with iron(II) as shown by PXRD pattern in (Spectrum 1.6 and 1.7). The intensity of the diffractions decreased upon iron exchange in pristine ZIF nanoparticles and the metal exchanged samples also exhibited the SOD topology because of the characteristic peaks that are similar to pristine ZIFs.
The infrared spectra showed the same spectral features as observed in the spectrum of the parent ZIF-8 and ZIF-67. The thermal stability of ZIF-8 after the exchange reaction was slightly lower compared to the parent ZIF-8 (433 °C compared to 454 °C), but no changes in crystal structure were observed. The thermal stability of ZIF-67 after the exchange reaction was slightly lower also compared to the parent ZIF-67 (431 °C compared to 452 °C), again no changes in crystal structure were observed.
The BET surface area and pore volume of ZIF-8 nanopaticles degreased after the exchange reaction (from 1657.98 m².g-1 to 1196 m2.g-1) and pore volume (from 0.71 cm3.g-1 to (1196 m2/g to 0.52 cm3.g-1), which is roughly 28% decrease altogether. Moreover, the BET surface area and pore volume of ZIF-67 nanopaticles degreased also after the exchange reaction (from 1376.86 m².g-1 to 1052 m2.g-1) and pore volume (from 0.51 cm3.g-1 to 0.39 cm3.g-1), which is roughly 23% decrease altogether. This may be due to incorporation of iron metal (more redox active metal) which restrict the pore size as previously reported. Though the surface area and pore size has decreased, but still it has a high surface area and pore size.
When looking at the SEM images of ZIF-8 and ZIF-67 before and after iron exchange, the particles looks similar in shape, but the size degreased slightly. The size degreased from 37 nm to 21 nm for ZIF-8 nanoparticles, while for ZIF-67 nanoparticles the size degreased from 92 nm to 82 nm after the exchange reaction. When the iron(II) content were determined using SEM-EDX, signi?cantly lower values were obtained compared to the XPS results. For the exchanged ZIF-8(Zn/Fe) nanoparticles, SEM-EDS gave Fe:Zn ratio of 37 and 63 mol%, while XPS gave 40 and 60 mol% for the same sample, respectively. For the exchanged ZIF-67(Co/Fe), SEM-EDS gave Fe:Co ratio of 49 and 51 mol%, while XPS gave 52 and 48 mol% for the same sample, respectively. According to the obtained results of ratios measured by two different techniques, we can conclude that the metal (Fe2+) exchange in nZIF-8(Zn/Fe) was successful as Fe2+ was incorporated into the framework and we can also conclude that the metal (Fe2+) exchange in nZIF-67(Co/Fe) was successful as Fe2+ was incorporated into the framework.
The photocatalytic activity of ZIF materials and their metal exchanged derivatives, were investigated by observing the decomposition of Remazol deep black B (RDB) with the illumination of 400 W H.P. Metal Halide Lamp as the energy source. A significantly higher catalytic activity was observed for the photocatalytic degradation of textile dye (RDB) by iron(II) exchanged nZIFs, compared to pristine nZIF-8 and nZIF-67 which showed slow catalytic activity as shown in (Figure B5 to B8). The metal exchanged materials were also selected as catalysts for oxidation of Hydroquinone to Benzoquinone, as they showed promising catalytic activity. A remarkably higher catalytic activity on oxidation of Hydroquinone was observed for the metal exchanged derivatives. From the catalytic oxidation of Hydroquinone and photocatalytic degradation of RDB dye, it can be seen that the iron(II) based nZIFs are most promising for heterogeneous catalysis.