German Edition: DOI: 10.1002/ange.201810088 MOF Membranes International Edition: DOI: 10.1002/anie.201810088 In-Plane Epitaxial Growth of Highly c-Oriented NH 2 -MIL-125(Ti) Membranes with Superior H 2 /CO 2 Selectivity Yanwei Sun, Yi Liu, Jürgen Caro, Xinwen Guo, Chunshan Song,* and Yi Liu* Abstract: 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 NH 2 -MIL-125 membranes were prepared on porous a-alumina substrates by combining oriented seeding and controlled in-plane epitaxial growth. Dynamic air–liquid interface-assisted self-assembly of c-oriented NH 2 -MIL-125- (Ti) seed monolayers, the use of layered TiS 2 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 H 2 /CO 2 selectivity of 24.8, which was 6.1 times higher than that of their randomly oriented counterparts under similar operating conditions. Metal–organic framework (MOF)-based separation mem- branes have offered unprecedented opportunities for gas separation, such as H 2 purification, CO 2 capture, and olefin/ paraffin separation. [1] Among various factors, preferred- orientation control of MOF membranes has proven to be very effective for improving their separation performance by the ordered arrangement of nanopores, a decrease in grain boundary defects, and reduction in diffusion path lengths. [2] Although diverse methods have been developed for the production of well-intergrown MOF membranes, elaborate control of their preferred orientation has remained a very challenging task. [2a, 3] For example, in situ growth methods usually led to the formation of randomly oriented MOF membranes rather than oriented membranes, [4] since in general, nucleation and crystallization of MOF particles occurred simultaneously in the bulk solution and on the substrate surface, thus making precise orientation control particularly challenging except on rare occasions. In contrast, secondary growth enabled more precise control over the preferred orientation of MOF membranes by combining the pre-deposition of oriented MOF seed layers with controlled in-plane epitaxial growth. Nevertheless, in analogy to ori- ented zeolite membrane synthesis, several technical obstacles still existed in the full utilization of this method: 1) With respect to oriented MOF seed layer deposition, Lai and co- workers preferentially deposited (102) oriented MOF-5 seed layers on a-Al 2 O 3 substrates in situ by a microwave-induced thermal deposition method, [5] and Wang and co-workers successfully attached platelike ZIF-L seeds to substrates along the c-axis through vacuum filtration in the presence of polyethyleneimine. [2c] Nevertheless, the facile organization of anisotropic MOF seeds with decent aspect ratios (like coffin- shaped MOF crystals) into highly oriented monolayers on porous substrates has remained a challenging task. 2) With regard to controlled in-plane secondary growth, it has remained a challenging task to suppress undesired twin growth, which may severely impair the separation perfor- mance owing to the difficulty in effectively suppressing simultaneous bulk nucleation. 3) High adhesion strength needs to be maintained. As compared with in situ solvother- mal growth, the adhesion strength between MOF layers prepared by secondary growth and porous ceramic substrates (such as a-Al 2 O 3 ) has remained fairly low. [1e, 6] Therefore, a new synthetic route was urgently needed to address the above existing issues. NH 2 -MIL-125(Ti), one of the most widely studied Ti- MOFs, has emerged as a promising candidate [7] for mem- brane-based CO 2 capture because of its high affinity for CO 2 , high thermal stability, and straight nanochannels along the c- axis, which were advantageous for the reduction of diffusion paths. [8] Therefore, the fabrication of a highly c-oriented NH 2 - MIL-125(Ti) membrane was highly desirable owing to the potential for the reduction of diffusion barriers and the elimination of grain boundary defects. Being analogous to MOFs, in recent decades significant progress was also made in the fabrication of oriented zeolite membranes (in particular b-oriented MFI zeolite mem- branes). For example, we developed a general route for the synthesis of highly b-oriented MFI films/membranes. [9] First, a facile air–liquid interface-assisted self-assembly (ALIAS) [*] Dr. Y. Sun, Dr. Y. Liu, Prof. Dr. X. Guo, Prof. Dr. C. Song, Prof. Dr. Y. Liu State Key Laboratory of Fine Chemicals School of Chemical Engineering, Dalian University of Technology Linggong Road No. 2, Ganjingzi District, Dalian 116024 (China) E-mail: diligenliu@dlut.edu.cn Prof. Dr. J. Caro Institute of Physical Chemistry and Electrochemistry Leibniz Universität Hannover Callinstrasse 3A, 30167 Hannover (Germany) Prof. Dr. X. Guo, Prof. Dr. C. Song PSU-DUT Joint Centre for Energy Research School of Chemical Engineering, Dalian University of Technology Linggong Road NO. 2, Ganjingzi District, Dalian 116024 (China) E-mail: csong@psu.edu Prof. Dr. C. Song EMS Energy Institute Department of Energy and Mineral Engineering and Department of Chemical Engineering, The Pennsylvania State University University Park, PA 16802 (USA) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201810088. A ngewandte Chemie Communications 16088  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2018, 57, 16088 –16093