Jinyin Lv,Yanwen Cui,Jianhua Yang,Liangqing Li,Xuerong Zhou,Jinming Lu,Gaohong He
Fluorinated metal–organic framework materials (NbOFFIVE-1-Ni, also referred to as KAUST-7) have attracted widespread attention because of their high chemical stability and thermal stability, outstanding tolerance with water and H2S, and high CO2-adsorption selectivity over H2 and CH4. KAUST-7 was expected to be a new membrane material candidate for H2/CO2 separation because of the hindered permeation of CO2 resulting from the interaction between CO2 and (NbOF5)2– of the KAUST-7 framework. A highly H2 perm-selective KAUST-7 membrane was first achieved using a novel strategy of inorganic pillar center-facilitated counterdiffusion (IPCFCD) proposed by us. The IPCFCD method not only effectively avoided the corrosion of hydrofluoric acid to α-Al2O3 tubes in the process of preparing KAUST-7 membranes, but also better reduced grain boundary defects because of the faster nucleation rate and resultant high crystallinity. The KAUST-7 membrane exhibited a high H2/CO2 separation factor (SF) of 27.30 for the 1:1 H2/CO2 binary gas mixture with a high H2 permeance of 5.30 × 10–7 mol m–2 s–1 Pa–1 under ambient conditions and a slight decrease of the H2/CO2 SF with increasing operation temperature and presence of steam. This study highlighted the importance of pre-synthesizing inorganic pillar centers (NiNbOF5 intermediate) and the innovation of a membrane formation process for synthesizing polycrystalline KAUST-7 membranes. Most important of all, our study provided a novel approach to overcome the challenge in fabricating metal–organic framework membranes containing corrosive reactants for the corresponding supports.
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.
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