Trifluoroacetic acid molecules confined into/onto metal–organic frameworks using H2btzip for efficiently improving proton conductivity using a synergistic effect

Abstract

Exploitation of stable proton conducting materials with high stability and excellent conductivity is becoming more important but challenging. Herein, two novel porous transition metal–organic frameworks (MOFs), formulated as [Zn(btzip)(H₂O)]·H₂O (LCUH-107) and [Ni(btzip)(H₂btzip)]·4H₂O (LCUH-108), are successfully synthesized. LCUH-107 and LCUH-108 exhibit excellent thermal and chemical stabilities. LCUH-108 has a suitable pore size (8.36 Å × 5.74 Å) to load trifluoroacetic acid (TFA, 5.72 Å × 5.65 Å × 5.06 Å), which is larger than that of LCUH-107 (3.50 Å × 2.04 Å). Therefore, TFA molecules can be loaded into the pores of LCUH-108 while they were only loaded onto the surface of LCUH-107. Both TFA/LCUH-107 (2.95 × 10^–2 S cm⁻¹) and TFA@LCUH-108 (2.05 × 10^–1 S cm⁻¹) exhibit excellent proton conduction at 80 °C and 100% relative humidity (RH); in particular, the proton conductivity value of TFA@LCUH-108 can be as high as 10^–1 S cm⁻¹, which might be due to the fact that TFA molecules can be confined into their suitable pores and finally form strong continuous and stable hydrogen-bonding networks. The TFA molecules loaded into the suitable pores of LCUH-108 play a key role in improving greatly the proton conductivity of LCUH-108, while TFA molecules loaded onto the surface of LCUH-107 can only slightly improve the proton conductivity of LCUH-107, which can be confirmed by various contrast tests and simulated calculations. This work gives a novel strategy to design efficient artificial crystalline catalysts for proton conduction.