Electrochemical and adsorption studies of a carboxylic acid-modified aluminium aminoterephthalate framework (H2N-MIL-53) with heterogeneous catalysis applications

The Metal Organic Framework (MOF), MIL-53(Al) and its amine functionalised analogue, amino- MIL-53(Al) were synthesized in good yields (57 - 87%) and were employed as solid, porous support systems for the intrusion of ferrocene (Fc), ferrocenecarboxylic acid (FcCOOH) and a series of carboxylic acids with increasing chain length. The intrusion of HCOOH, CH3COOH, CH3CH2COOH, CH3(CH2)2COOH and Fc into MIL-53(Al) was done by a newly developed Incipient Wetness Impregnation (IWI) method: after evacuation of the porous MOF, the highvacuum was maintained while covering the MOF with the impregnation solution, where after the vacuum was relieved and the solution allowed to intrude into evacuated the pores of the material under atmospheric pressure. The effectiveness of this IWI method was demonstrated by the maximum load of 0.80 HCOOH molecules per unit cell, achieved for HCOOH@MIL-53(Al). Time-resolved intrusion studies, using CH3(CH2)2COOH and FcCOOH, both showed that a maximum load of impregnated material was reached after 16 h of intrusion into amino-MIL-53(Al). An amidation reaction after intrusion resulted in covalent attachment of the acids to the framework structure. In the cases of CH3(CH2)2COOH and FcCOOH, a maximum of 20% and 12% (w.r.t. the total number of available amine groups in amino-MIL-53(Al)) respectively, were covalently bound through amide bonds to the amino-MIL-53(Al) structure, to give CH3(CH2)2CONH-MIL-53(Al) and FcCONH-MIL-53(Al). Using a 16 h intrusion period, high loadings of ferrocene, 7.59 mass% in Fc@MIL-53(Al) and 11.02 mass% in Fc@amino-MIL-53(Al), were achieved during IWI, as determined with TGA. A newly developed solid state Cyclic Voltammetry technique, performed on the Fc-containing framework structures, Fc@MIL-53(Al) as well as Fc@amino-MIL-53(Al) and FcCONH-MIL- 53(Al) showed well defined redox couples at E0' = 1, -13 and -71 mV (vs. Fc/Fc+) respectively. Chemically bound Fc (in FcCONH-MIL-53(Al)) is thus easier to oxidise than free Fc in the channels of the framework structure. Electrochemical reversibility was found in the cases of Fc@amino-MIL-53(Al) and FcCONH-MIL-53(Al), with ?Ep = 57 and 21 mV respectively, whereas Fc@MIL-53(Al) (?Ep = 77 mV) showed quasi-reversibility. All three compounds showed limited chemical reversibility (0.6 ≤ ipc/ipa ≤ 0.7), due to ionic material leaving the analyte during measurement.