Inorganic Pillar Center-Facilitated Counterdiffusion Synthesis for Highly H₂ Perm-Selective KAUST-7 Membranes

Abstract

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 H 2 S, and high CO 2 -adsorption selectivity over H 2 and CH 4 . KAUST-7 was expected to be a new membrane material candidate for H 2 /CO 2 separation because of the hindered permeation of CO 2 resulting from the interaction between CO 2 and (NbOF 5 ) 2− of the KAUST-7 framework. A highly H 2 perm-selective KAUST-7 membrane was first achieved using a novel strategy of inorganic pillar centerfacilitated counterdiffusion (IPCFCD) proposed by us. The IPCFCD method not only effectively avoided the corrosion of hydrofluoric acid to α-Al 2 O 3 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 H 2 /CO 2 separation factor (SF) of 27.30 for the 1:1 H 2 /CO 2 binary gas mixture with a high H 2 permeance of 5.30 × 10 −7 mol m −2 s −1 Pa −1 under ambient conditions and a slight decrease of the H 2 /CO 2 SF with increasing operation temperature and presence of steam. This study highlighted the importance of pre-synthesizing inorganic pillar centers (NiNbOF 5 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.

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

“…The core point of this strategy is achieving NiNbOF 5 intermediates as fluorine, Nb and Ni sources prior to the preparation of KAUST-7 membranes by the pre-reaction of HF, NiO and Nb 2 O 5 and subsequent recrystallization. Furthermore, the reactivity of NiNbOF 5 with pyrazine is found to be extremely high from the previous research of our group [44]. Inspired by the high reactivity of NiNbOF 5 intermediate in the previous work, herein, a facile in-situ synthesis assisted by fluorinated molecular building block was proposed to fabricate KAUST-7 membranes by the pH manipulation of synthesis solution (Fig.…”

“…According to the previous experiment of our group, NiNbOF 5 was found easy to react with pyrazine, which would lead to a mass of homogeneous nucleation in the liquid [44]. To reduce the homogeneous nucleation, a diluted membrane synthesis solution was prepared by dissolving 0.63 g NiNbOF 5 and 1.54 g pyrazine into 5-25 ml solvents (H 2 O, MeOH, EtOH, or 1:1 MeOH and EtOH mixture) under vigorously stirring for 20 min at the pH of 2.0-6.0 manipulated by HCl (37%) solution, respectively.…”

“…Firstly, NiNbOF 5 were synthesized according to our previous report [44]. Then, the acquired NiNbOF 5 powders were put in valve bag and subsequently placed in a desiccator for further use.…”

“…membranes, and as a consequence, refraining from the corrosion of high-concentration HF is critical for fabricating a defect-free KAUST-7 membrane. Similarly, KAUST-7 crystals were also successfully synthesized according to our previous report [44], which had a S BET of 245 m 2 /g and an aperture of 3.8 Å (Fig. S5).…”

“…In our previous work, an inorganic pillar center-facilitated counter diffusion method (IPCFCD) was firstly proposed for the fabrication of a pure and defect-free polycrystalline KAUST-7 membrane with highly H 2 permselective over CO 2 [44]. In brief, IPCFCD is the contra-diffusion synthesis of KAUST-7 membranes depending on the substitution of pre-synthesized NiNbOF 5 for excessive corrosive reactant of HF.…”