Abstract The elimination of possible defects is indispensable in making zeolite membranes popular in process industries. A novel counter‐diffusion chemical liquid deposition (CLD) technique is proposed and developed for selective defect‐patching of zeolite membranes. Dodecyltrimethoxysilane (DMS) is employed as the silane coupling agent, forming a protective layer on the membrane surface so that intracrystalline pores can be kept intact in the subsequent reparation step. By using tetraethoxy orthosilicate (TEOS) and (3‐chloropropyl)triethoxysilane (3CP‐TES), co‐hydrolysis and co‐condensation at the organic/aqueous interface fabricate the silsesquioxane/silicate hybrid on macro‐, meso‐ and even microdefects. The silicalite‐1 membrane before and after reparation is characterized using contact‐angle measurements, Fourier transform IR spectroscopy, and electron probe microanalysis. Permporometry is conducted to study the pore‐size distribution of the membrane before and after reparation. It is found that the silsesquioxane/silicate hybrid is only deposited at the pore‐mouth of the defects, and the defects can be plugged to less than 1.3 nm pores after patching. After reparation, the separation factor of a 50/50 n / i ‐butane‐gas mixture through the membrane can be increased to 35.8 from 4.4, and the separation factor of a CO 2 /N 2 gas mixture through the membrane can be increased to around 15 from 1, while keeping the two‐thirds CO 2 permeation flux of the synthesized membrane.
Pd/Si-MFI membranes are fabricated by depositing palladium nanoparticles onto Si-MFI membranes via opposing reactants chemical fluid deposition in supercritical carbon dioxide. Palladium hexafluoroacetylacetonate [Pd(hfac)2] and ethanol are used as the Pd precursor and the reducing agent, respectively, in the deposition process. The resulting Pd/Si-MFI membrane is characterized using SEM, XRD, EPMA, TEM, and EDX techniques. It is demonstrated that a continuous layer of Pd particles with size of 10 nm is covered on the matrix surface while 30-nm Pd particles are plugged into the defects within the matrix. The H2/N2 permselectivity has been significantly increased due to the deposition of Pd nanoparticles in the Si-MFI membrane, which could keep stable below the critical temperature. This bifunctional membrane and similar membranes possess the application potential to achieve various one-step syntheses.
Due to its unique nanometer-sized straight channel structure, the b-oriented MFI zeolite films have great potential in separation membranes, membrane reactors, and chemical sensors. Thus, there has been growing interest in the preparation of zeolite films with preferred b-orientation. Recent advances in the preparation process of b-oriented MFI zeolite films are reviewed in this paper. The methods of in situ hydrothermal synthesis and secondary growth are further described in detail. In particular, it is summarized and evaluated for the very recent research results in the modification of the substrate surface, the preparation of b-oriented MFI zeolite arrays, and the regulation of hydrothermal synthesis conditions. Based on the extensive discussion of the merits and demerits of various preparation methods, the trends in the manufacture of b-oriented zeolite films are prospected.
The microstructure control and optimization of zeolite films and membranes is an indispensable challenge for various innovative applications. It can be steered by understanding the formation process. Here we design an unprecedented strategy to uncover direct evidence via the hydrothermal synthesis of chitosan-supported zeolite monolayers. The chitosan-supported layer involved in the hydrothermal reaction is observed using SEM, AFM, EPMA, and HRTEM while nucleation and crystal growth in the bulk synthesis solution are pursued with HRTEM, DLS, and SEM. The direct HRTEM observation is achieved on the chitosan-supported layer by peeling chitosan from its support. It reveals that a gel layer is initially formed on the chitosan layer where the subsequent crystal growth is fatally restrained. Our own experimental evidence and the literature reports clearly demonstrate that the formation mechanism is homogeneous for severely reduced crystal growth on the substrate but is heterogeneous when crystal growth on the substrate is significantly enhanced.
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