In-situ synthesis of KAUST-7 membranes from fluorinated molecular building block for H2/CO2 separation

A well-intergrown and defect-free tubular KAUST-7 membrane was fabricated via in-situ synthesis assisted by fluorinated molecular building block using green solvent of water on coarse asymmetric alpha-Al2O3 tubes. The strategy averted the serious corrosion of high-concentration hydrofluoric acid (HF) to alpha-Al2O3 supports due to the substitution of HF with the pre-synthesized NiNbOF5, overcoming the difficulty of serious corrosion to supports compared to traditional one-pot synthesis. The pH manipulation of synthesis solution is proved crucial to the fabrication of defect-free KAUST-7 membranes. In addition, effects of reactant concentration, crystallization temperature and reaction solvents on the morphology and separation performance of KAUST-7 membrane were investigated. The optimized KAUST-7 membrane M4 exhibited a H2 perm-selectivity of 17.7 over CO2 for H2/CO2 binary mixture with a H2 permeance of 2.2 x 10-7 mol m-2 s- 1 Pa-1 at 25 degrees C and 1 bar, while the H2/CO2 separation factor just had a slight decrease when test temperature increased to 120 degrees C. The formation mechanism of KAUST-7 membranes via in-situ method in this work was investigated by exposing a bare alpha-Al2O3 tube into NiNbOF5 aqueous solution at the pH of 4.0 at 130 degrees C for 12 h. XPS and FT-IR observations revealed the formation of AlFxOy or AlFx species, which were deduced to be acted as linkers to connect with 2D Ni (II)-pyrazine layers, promoting the heterogeneous nucleation of KAUST-7 crystals on the surface of Al2O3 support.

Automated summary

In this study, a defect-free tubular KAUST-7 membrane was successfully fabricated using in-situ synthesis assisted by fluorinated molecular building block. The substitution of high-concentration hydrofluoric acid (HF) with pre-synthesized NiNbOF5 avoided the corrosion of alpha-Al2O3 supports. The pH manipulation of synthesis solution was found to be crucial for the fabrication of defect-free KAUST-7 membranes.