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

[摘要] English: 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, poroussupport systems for the intrusion of ferrocene (Fc), ferrocenecarboxylic acid (FcCOOH) and aseries 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 developedIncipient Wetness Impregnation (IWI) method: after evacuation of the porous MOF, the highvacuumwas maintained while covering the MOF with the impregnation solution, where after thevacuum was relieved and the solution allowed to intrude into evacuated the pores of the materialunder atmospheric pressure. The effectiveness of this IWI method was demonstrated by themaximum 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 amaximum 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 frameworkstructure. In the cases of CH3(CH2)2COOH and FcCOOH, a maximum of 20% and 12% (w.r.t. thetotal number of available amine groups in amino-MIL-53(Al)) respectively, were covalently boundthrough 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, asdetermined with TGA.A newly developed solid state Cyclic Voltammetry technique, performed on the Fc-containingframework 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 thechannels of the framework structure. Electrochemical reversibility was found in the cases ofFc@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 showedlimited chemical reversibility (0.6 �?ipc/ipa �?0.7), due to ionic material leaving the analyte duringmeasurement.

[发布机构] University of the Free State