In-Plane Epitaxial Growth of Highly c-Oriented NH2 -MIL-125(Ti) Membranes with Superior H2 /CO2 Selectivity.

Preferred-orientation control has significant impact on the separation performance of MOF membranes. Under most conditions the preferred orientation of MOF membranes is dominated by the Van der Drift mechanism of evolutionary growth selection so that the obtained orientation may not be optimized for practical application. In this study, highly c-oriented NH2 -MIL-125 membranes were prepared on porous α-alumina substrates by combining oriented seeding and controlled in-plane epitaxial growth. Dynamic air-liquid interface-assisted self-assembly of c-oriented NH2 -MIL-125(Ti) seed monolayers, the use of layered TiS2 as the metal precursor, and single-mode microwave heating were crucial in ensuring the preferred c-orientation while simultaneously suppressing undesired twin growth. Owing to reduced grain boundary defects, the prepared c-oriented membranes showed an ideal H2 /CO2 selectivity of 24.8, which was 6.1 times higher than that of their randomly oriented counterparts under similar operating conditions.