Encapsulation of selected metallated phthalocyanines in aluminium aminoterephthalate framework, NH₂-MIL-101(Al), with heterogeneous catalytic and hydrogen storage applications

The amine-functionalised metal organic framework (MOF), NH2-MIL-101(Al) was successfully synthesised and optimised with a benchtop method. A newly developed activation method to evacuate the pores of the MOF, achieved a larger BET surface area (3192 ± 57 m2g-1) than those currently reported in literature. 2(3),9(10),16(17),23(24)-Tetra-tert-butylphthalocyanine (2HPctBu4) was successfully synthesised by either the lithium method or the hydroquinone method. The latter was superior since it is a solventfree synthesis, with a 60% higher yield than the lithium method. Successful metallation of 2HPctBu4 with the acetate salts of Zn2+ and Ni2+, gave single Q-bands at ~670 nm during UV-Vis absorbance measurements, while double Q-band maxima were observed at 660 nm and 690 nm for the metal-free 2HPctBu4. Four tetracarboxymetallophthalocyanines (MPc(COOH)4) with Zn2+; Fe3+; Co2+ or Ni2+ as central metal cations were synthesised in a two-step cyclotetramerisation method. Their aggregation behaviour was determined by UV-Vis spectroscopy. For concentrations up to 180 μM, only NiPc(COOH)4 showed aggregation from 5 μM, whereas ZnPc(COOH)4, FeClPc(COOH)4 and CoPc(COOH)4 showed little to no aggregation. Two different methods were used to encapsulate the MPcs in the pores of NH2-MIL-101(Al): templating, as well as customised solution phase infiltration. In both procedures MPctBu4 was encapsulated by the MOF via physical interaction, while all MPc(COOH)4 derivatives could covalently bind to the NH2-MIL-101(Al) structure via amide bonds. Encapsulation in the pores of the MOF would eliminate aggregation of the MPc molecules. DRS-UV-Vis showed that solution phase infiltration led to a higher loading of MPc in the MOF than when templating was used. This correlated with ASAP and PXRD results showing that all solution phase infiltration products, except for NiPc(COOH)3-CONH-MIL-101(Al), had smaller BET surface areas (between 244 cm3g-1 and 89 cm3g-1) due to their high loadings of MPcs. Hydrogen storage capacities of CoPc(COOH)3-CONH-MIL-101(Al) and NiPc(COOH)3-CONH MIL- 101(Al) were measured as 0.47 wt% (at 16 bar) and 1.5 wt% (at 128 bar) respectively. A trial test showed that CoPc(COOH)3-CONH-MIL-101(Al) catalysed the photo-oxidation of ciscyclooctene to cis-cyclooct-2-enol with a 5% conversion. For both MPctBu4 derivatives, liquid state cyclic voltammetry showed four Pc ring-based redox processes in DCM. With MPc(COOH)4 derivatives in DMSO , three redox couples were observed. For the Co2+-containing MPc, two metal-based redox processes (E°′ = -738 mV and -289 mV vs. ferrocene) and for the Fe3+-containing MPc, only one metal-based couple (E°′ = -88 mV vs. ferrocene) was observed. With solid state cyclic voltammetry of all MOF-encapsulated MPcs only one redox couple (near 200 mV vs. ferrocene) was detected, with the exception of ZnPc(COOH)3-CONH-MIL-101(Al) which gave an additional redox couple (E°′ = 1158 mV vs. ferrocene) and with NiPc(COOH)3-CONH-MIL- 101(Al) two additional couples (E°′ = 1146 mV and 1383 mV vs. ferrocene) were present. These processes were mostly electrochemically and chemically irreversible, but showed that the MOF’s matrix had a conductive effect on the flow of electrons during oxidation and reduction of the encapsulated MPcs.