Preparation of Zeolite T Membranes by a Two-Step Temperature Process for CO₂ Separation

Abstract

A thin and well-intergrown zeolite T membrane with high CO2 permeability and selectivity was rapidly prepared on seeded tubular supports by the two-step varying-temperature hydrothermal synthesis process (two-step method). The influence of synthesis parameters such as synthesis temperature and crystallization time during the two-step durations on crystals growth and separation performance was investigated systematically. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and gas permeation measurements were used to characterize the as-synthesized membranes. Compared with the conventional hydrothermal synthesis at a constant temperature (one-step method), the two-step method is more effective to synthesize a thin and continuous zeolite membrane layer in short time, which fully covered the support surface. Membrane synthesis temperature during the two-step durations could seriously influence the crystal growth and CO2 separation properties. It was found that a lower temperature favored nucleation, while a higher crystallization temperature promoted crystallization during the two-step method, thus improving zeolite T crystal growth and membrane separation properties. The membranes prepared by the two-step method under the optimized conditions exhibited good reproducibility, high CO2 permeance, and relatively high permselectivity for the separation of CO2 from CH4 or N2 gas mixtures. For example, CO2 permeance and CO2/CH4 selectivity of the membranes reached 6.2 × 10–8 mol m–2 s–1 Pa–1 and 80 for the equimolar CO2/CH4 binary gaseous mixtures at 35 °C, respectively.

Synthesis of Zeolitic Imidazolate Framework Membrane Using Temperature-Switching Synthesis Strategy for Gas Separation

“…22,26 In order to gain the concentration of hydroxyl groups on the surface of support, the α-Al 2 O 3 disks were soaked in saturated NaOH solution for 24 hours. 27 After that, the supports were ultrasonically dealt with abundant deionized water again and dried at 90 °C for 2 hours.…”

“…ZIF-9 crystal growth over the formed nuclei is much favored to the formation of new crystallites. [20][21][22] Therefore, more and more ZIF-9 crystals grew well-bonding with each other and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 11 eventually a continuous dense membrane formed on the support surface. In general, lower temperature favors the smaller size crystallite or nucleation, and higher temperature promotes the crystallization.…”

“…While for other gases, especially for CO 2 , the existence of defects and cracks may provide more diffusion pathway and lead to the obvious increase of permeance. 22 For membrane M1 prepared using temperature-switching process, high coverage and well-bonded crystals on the support surface without defects endow the membrane with the improved ideal separation factor.…”

“…Then, for the temperature-switching process, the temperature was risen rapidly to a high level after nucleation and accelerated the growth rate of the crystals. 22 At high temperature, rapid growth of crystals would tend to produce a continuous and dense membrane. 20,21 Finally, it would achieve the goal that low temperature favors for nucleation and high temperature promotes crystal growth.…”

Zeolite Thin Films and Membranes: From Fundamental to Applications

“…The conversion exceeded the equilibrium limit and reached almost 100 % within 8 h when the initial molar ratios of alcohol to acetic acid were 1.5 and 2. Zeolite T membranes attracted a lot of efforts in developing new preparation methods and improving the separation properties, including synthesis in fl uoride media [ 32 , 130 ], clear gel solution synthesis [ 131 -133 ], two-step temperature process [ 134 ], varying temperature [ 125 , 135 ], and microwave-assisted synthesis [ 136 , 137 ]. Their typical separation performance is shown in Table 13.4 .…”