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 α-Al 2 O 3 tubes. The strategy averted the serious corrosion of high-concentration hydrofluoric acid (HF) to α-Al 2 O 3 supports due to the substitution of HF with the pre-synthesized NiNbOF 5 , 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 H 2 perm-selectivity of 17.7 over CO 2 for H 2 /CO 2 binary mixture with a H 2 permeance of 2.2 × 10 −7  mol m −2  s −1 Pa −1 at 25 °C and 1 bar, while the H 2 /CO 2 separation factor just had a slight decrease when test temperature increased to 120 °C. The formation mechanism of KAUST-7 membranes via in-situ method in this work was investigated by exposing a bare α-Al 2 O 3 tube into NiNbOF 5 aqueous solution at the pH of 4.0 at 130 °C for 12 h. XPS and FT-IR observations revealed the formation of AlF x O y or AlF x 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 Al 2 O 3 support.