Sub-40 nm zeolite suspensions via disassembly of three-dimensionally ordered mesoporous-imprinted silicalite-1.

Zeolite nanocrystals were prepared from three-dimensionally ordered mesoporous-imprinted (3DOm-i) silicalite-1 by a fragmentation method involving sonication and dissolution within a certain pH range. 3DOm-i silicalite-1 with spherical elements with diameters ranging from 10 to 40 nm and a wide range of crystal sizes (100-200 nm, 500-600 nm, and 1-2 µm) was used as the starting material. The highest yield (57%) of isolated nanocrystals was obtained for 3DOm-i silicalite-1 with a crystal size of 100-200 nm and a spherical element diameter of 40 nm. The smallest nanocrystals obtained, albeit in very low yields, had a 10 nm diameter. Preparation of stable silicalite-1 nanocrystal suspensions fragmented from 20 and 40 nm 3DOm-i silicalite-1 was demonstrated. Cryogenic transmission electron microscopy showed that the isolated zeolite nanocrystals can be used as seeds for the epitaxial growth of silicalite-1. An application of these findings was demonstrated: silicalite-1 nanocrystal suspensions were used to deposit seed layers on porous α-alumina disks, which were converted to continuous thin (300-400 nm) films by secondary growth that exhibited both high permeances and separation factors (3.5 × 10(-7) mol m(-2) s(-1) Pa(-1) and 94-120, respectively, at 150 °C) for p- and o-xylene.

Grain boundary defect elimination in a zeolite membrane by rapid thermal processing.

Microporous molecular sieve catalysts and adsorbents discriminate molecules on the basis of size and shape. Interest in molecular sieve films stems from their potential for energy-efficient membrane separations. However, grain boundary defects, formed in response to stresses induced by heat treatment, compromise their selectivity by creating nonselective transport pathways for permeating molecules. We show that rapid thermal processing can improve the separation performance of thick columnar films of a certain zeolite (silicalite-1) by eliminating grain boundary defects, possibly by strengthening grain bonding at the grain boundaries. This methodology enables the preparation of silicalite-1 membranes with high separation performance for aromatic and linear versus branched hydrocarbon isomers and holds promise for realizing high-throughput and scalable production of these zeolite membranes with improved energy efficiency.