Greatly improved zeolite membranes using high-aspect-ratio seeds

(Nanowerk Spotlight) The ability to separate and purify specific molecules in a chemical mixture is essential to chemical manufacturing. By using membranes – rather than energy-intensive processes such as distillation and crystallization – the energy efficiency of these processes could be greatly increased.

Crystalline materials known as zeolites are considered as an excellent membrane material due to their molecular-sized pores and high thermal, mechanical and chemical stabilities. Growth of thin zeolite membranes with high quality/negligible defects is highly desirable yet very challenging by hydrothermal synthesis.

"Lack of ample knowledge on synthesizing high aspect ratio zeolite seeds hinders their application on preparing zeolite membranes," Miao Yu, an Assistant Professor in Chemical Engineering at the University of South Carolina, tells Nanowerk.

In a new study, Yu's group prepared slice-shaped zeolite seeds by using various gel recipes and crystallization temperatures. The researchers reported their findings in Angewandte Chemie International Edition ("Growth of High Quality, Thickness-Reduced Zeolite Membranes towards N 2 /CH 4 Separation Using High-Aspect-Ratio Seeds").

SEM images (left column) and XRD patterns (right) of SAPO-34 seeds with average aspect ration (A/R) of (a) 1, (b) 3, (c) 10, and (d) 20. The insets of each SEM images provided a schematic drawing of the typical morphology of a seed. Scale bars in (a), (b), and (c), 1 µm; in (d), 0.5 µm. (Reprinted with permission by Wiley-VCH Verlag)

The researchers demonstrated that, by applying slice-shaped zeolite seeds, various zeolite membranes with significantly reduced thickness can be prepared for high-flux mixture separation.

"The most significant finding of our systematic study is the use of high-aspect-ratio, sliced-shaped zeolite seeds could lead to thinner yet higher quality zeolite membranes with negligible amount of non-selective intercrystalline defects," says Yu. "As is well known, thinner zeolite membranes will have lower transport resistance and thus provide higher flux. However, it is a great challenge to prepare thin zeolite membranes while maintaining high separation performance. Our study suggests high-aspect-ratio zeolite seeds may help synthesize thin but high quality zeolite membranes for high throughput mixture separation."

Yu and his team expect their findings to have a great impact on hydrothermal synthesis of zeolite membranes and coatings: "We believe this study may stimulate future research on preparing thin zeolite crystals and applying them to the preparation of zeolite membranes for separation processes."

To demonstrate the process, the researchers prepared SAPO-34 membranes for separating N 2 /CH 4 mixture, which is a well-known hard-to-separate mixture in the natural gas industry. The result: by using high-aspect-ratio SAPO-34 seeds, if was possible to grow thinner and higher quality SAPO-34 membranes (the thinnest membrane was ∼ 2 µm thick) and achieve simultaneous increase of N 2 permeance and N 2 /CH 4 mixture selectivity.

Yu points out that the best membranes that the team fabricated showed N 2 permeance as high as (4.93 ± 0.25) × 10 -7 mol/(m 2 •s•Pa) at 70°C and N 2 /CH 4 selectivity of 11.3 ± 0.31 at 22°C for a 50/50 N 2 /CH 4 mixture.

"Their separation performance is superior to those of the state-of-the-art membranes," he says.