Highly oriented, neutral and cation-free AlPO4 LTA: from a seed crystal monolayer to a molecular sieve membrane

Abstract

An oriented, neutral and cation-free AlPO 4 LTA molecular sieve membrane with high hydrogen selectivity was prepared on porous a-Al 2 O 3 supports through secondary growth of a highly oriented AlPO 4 LTA monolayer.Zeolites with their uniform pore structure and high thermal stability are widely used in the chemical industry as catalysts, ion exchangers and adsorbents.1 Apart from the use of zeolites as powders, supported zeolite layers are of interest for many potential applications as separators, reactors, sensors and electrical insulators. [2][3][4][5][6] It is well recognized that control of the microstructures of zeolite layers, such as grain orientation and grain boundaries, plays an important role in their performances. 2,7,8 Mostly, oriented zeolite layers exhibit superior performances compared to randomly oriented zeolite layers. Therefore, intense research efforts have been made towards the preparation of oriented zeolite layers, 10-13 mainly focused on the preparation of b-oriented MFI membranes.14-16 Lai et al. reported the synthesis of b-oriented MFI membranes with a high p-xylene/o-xylene selectivity of about 500.17 Highly b-oriented MFI membranes have been prepared by both direct in situ growth 18 and secondary growth. 19 The secondary growth method is considered to allow a better control of the growth of oriented zeolite films since the quality of the films is less influenced by the characteristics of the substrate, thus enhancing the reproducibility of membrane preparation. 20 For secondary growth of oriented membranes, a highly oriented monolayer of zeolite seeds is helpful.Much effort has been made to assemble zeolite crystals as an oriented seed monolayer. The preparation of a packed zeolite A monolayer was first realized by physical adsorption of the micro-crystals onto a substrate. 21 Recently, Yang et al. reported the deposition of highly b-oriented silicalite-1 seeds on various substrates by a Langmuir-Blodgett technique. 16,22 Pre-treated zeolite crystals were also chemically attached on substrates for the fabrication of an oriented monolayer by a simple ionic or covalent bonding. 23 Another great contribution for the assembling of an oriented zeolite monolayer with dense packing was achieved by introducing dual covalent bonds between the zeolite crystals and the substrate. Yoon and coworkers have developed a novel ''two-component modification'' strategy for the assembly of a highly oriented LTA and MFI zeolite films on glass slides via covalent linkages, 24-26 where the reactive groups on the substrate surfaces were effectively linked to the matching groups on the zeolite crystals to yield a uniform and oriented zeolite monolayer with high coverage and close packing. However, these covalent linkages are not suitable for the assembly of large-sized zeolite crystals (usually smaller than 3 mm).27 Further, a rather long reaction time (usually several hours) is required to obtain a high coverage monolayer.28 Therefore, the development of a simple, effective and versatile road to assemble high...

Recent developments in membranes for efficient hydrogen purification

“…In most cases, oriented zeolite membranes possess more excellent permeation or separation performances than randomly oriented zeolite membranes [31]. For example, Huang et al applied secondary growth method to develop a highly oriented, neutral and cation-free AIPO 4 LTA-type zeolite membrane via assembling a highly oriented zeolite seeds monolayer [45]. The H 2 /CO 2 ideal selectivity of resultant oriented membrane was 10.9, which was higher than randomly oriented one (~7.6).…”

Anisotropic Gas Separation in Oriented ZIF‐95 Membranes Prepared by Vapor‐Assisted In‐Plane Epitaxial Growth

“…In the past ten years, various MOF membranes have been prepared on porous supports for H 2 purification, [7–16] CO 2 capture and removal, [17–19] alkane/alkene separation, [20, 21] water/organic separation, [22, 23] and p ‐/ o ‐xylene separation [24] . It is worth to be noted that apart from the formation of a dense membrane on the porous support, the control of the microstructure grain orientation, grain boundaries and thickness of a molecule sieving membrane is vital for its separation performances [25–29] . Normally, oriented membranes display superior separation performances in comparison with randomly oriented ones because a perfect orientation often results in a reduction of intercrystalline defects and transport pathways, which is helpful to enhance the selectivity and permeance.…”

Facile Preparation of Hydrophobic Colloidal MFI and CHA Crystals and Oriented Ultrathin Films

“…Zeolite films comprising well‐defined, oriented crystals are attractive not only because of their beauty, but also of the possibility to increase the performance of the films in applications . However, thick films will reduce the mass transport of guest molecules under diffusion limited conditions . Colloidal crystals may be used directly as building blocks for preparation of thin zeolite films, but sometimes with random or partially desired orientation of the crystals .…”