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Thin pure-silica chabazite (Si-CHA) membranes have been synthesized by using a secondary growth method on a porous silica substrate. A CO2 permeance of 2.62 × 10−6 mol m−2 s−1 Pa−1 with a CO2/CH4 permeance ratio of 62 was obtained through a Si-CHA membrane crystallized for 8 h using a parent gel of H2O/SiO2 ratio of 4.6. The CO2 permeance through the Si-CHA membrane on a porous silica substrate was twice as high as that through the membrane synthesized on a porous alumina substrate, which displayed a similar zeolite layer thickness.

Silicalite-1 membranes with high pervaporation (PV) performance were prepared on a seeded tubular silica support using modified steam-assisted conversion (SAC) under gel-free conditions, called the gel-free SAC method, for the first time. The seeded support was first dipped in a TPAOH aqueous solution and dried. Subsequently, the TPAOH-coated silicalite-1 seed layer was converted into a silicalite-1 membrane layer, followed by heating in an autoclave with a small amount of water at the bottom of the autoclave. In this method, silica support was not only used as the support but also as the silica source to grow the zeolite seed layer into a continuous and dense zeolite membrane layer. In other words, no additional supply of silica source such as synthesis gel to grow the seed layer was required. The silicalite-1 membrane prepared in this study had a separation layer with a thickness of approximately 7 µm and exhibited high PV performance with a separation factor of 66 and high flux of 4.47 kg m −2 h −1 for 10 wt% ethanol/water mixtures at 323 K. This membrane preparation method is quite simple and uses minimum necessary reagents. Therefore, it is economical and suitable for the preparation of membranes on an industrial scale.

Production of bioethanol, a renewable energy source, is limited by the costly, energy-consuming concentration processes involved. Membrane separation techniques have emerged as promising alternatives to overcome these limitations. In this study, silicalite-1 zeolite membranes with high pervaporation (PV) performance were prepared on porous tubular silica supports by a secondary growth method. The effects of the deposited seed amount on the support and synthesis parameters, namely, the tetra- n -propylammonium concentration (TPA + /Si), alkalinity (OH − /Si), and crystallization time, were investigated. A dense zeolite layer was formed between the typical columnar crystal layer of silicalite-1 membranes and the silica support. The PV performance was mainly dependent on the thickness of the dense layer: as the thickness increased, the separation factor increased. The silicalite-1 membrane prepared using a synthesis solution with a molar composition of 1SiO 2 :0.05TPABr:0.08NaOH:75H 2 O at 433 K for 8 h exhibited the highest separation performance with a separation factor of 92 and a flux of 3.00 kg m −2 h −1 for 10 wt% ethanol/water mixtures at 323 K, which is the best result reported to date for tubular supports. Thus, silica supports proved to be suitable for the preparation of high-performance silicalite-1 membranes for ethanol/water mixture separation.

Silicalite-1 membranes with high pervaporation performance were prepared successfully on a silica-particle-coated tubular silica support using a gel-free steam-assisted conversion (SAC) method. The effects of the silica-particle layer formed on the top surface of the silica support and the physical properties of the silica particles themselves on the membrane-formation process were investigated. The silica particles coated served as the additional silica source for growing the silicalite-1 seed crystal layer into the silicalite-1 membrane. As a result, it was possible to form a dense and continuous membrane even under gel-free conditions. Furthermore, it was found that the properties of the silica particles, such as their primary particle diameter, had a determining effect on their solubility during the steam treatment, that is, on the supply rate of the silica source. The silicalite-1 membrane obtained using the spherical-silica-particle-coated support had an approximately 9-&mu, m-thick separation layer and showed very high pervaporation performance, exhibiting a separation factor of 105 and a flux of 3.72 kg m&minus, 2 h&minus, 1 for a 10 wt % ethanol/water mixture at 323 K. Thus, the gel-free SAC method can be used with a silica support coated with silica particles to readily prepare high-performance membranes without producing any chemical waste.

A MFI zeolite membrane was synthesized on a porous silica substrate. The defect-free MFI zeolite membrane was synthesized via a secondary growth method using seed crystals (Si/Al = 20) on a silica substrate in a parent gel of Si/Al = 100. The silica substrate prevented the dissolution of the seed crystals during the synthesis to promote the MFI zeolite membrane synthesis. The MFI zeolite membranes were employed for a contactor-type reactor for methanol-to-olefins (MTO) reactions to produce olefins, such as propylene. The methanol conversion was 99.9% with the selectivity of ethylene - pentene at 91.5% for the permeate side of the membrane contactor. The propylene selectivity was high at 48.5% with the C3H6/C3H8 ratio at 236.

A hydrophobic pure-silica *BEA-type zeolite membrane was prepared using the steam-assisted conversion (SAC) method on a seeded tubular silica support coated with dilute synthetic gel. This study systematically investigates the effects of the synthetic conditions on the membrane formation process and separation performance. Under the optimized conditions, when using a dilute synthetic gel with a H2O/SiO2 ratio of 500, a dense and continuous membrane layer was formed in 24 h. This was attributed to dissolution of the surface of the silica support during the steaming process, which functions as a silica source for the growth of a seed crystal layer. An increase in silica the concentration on the support surface locally enabled the formation of a membrane under the dilute synthetic gel composition. The obtained membrane exhibited selectivity toward n-butanol, in addition to a high flux with a separation factor of 132 and a flux of 1.00 kg m−2h−1 for a 1 wt% n-butanol/water mixture at 318 K. This novel simple membrane formation method combines a dilute synthesis gel with SAC, takes advantage of the dissolution of a silica support, and reduces the consumption and cost of chemical raw materials used for membrane synthesis.

In this study, silicalite-1 membranes were prepared on seeded tubular silica supports using a gel-free steam-assisted conversion (SAC) method. The effects of seed crystal type on the growth and microstructure of the membranes were investigated. The morphologies of the membranes obtained by this preparation method reflected the orientation of the seed crystal layer covering the tubular support; when spherical seed crystals were used, the membrane surface was composed of randomly oriented crystals; whereas when coffin-type seed crystals were used, the membrane surface was composed of crystals arranged in a relatively b-axis orientation. This suggests that membrane layers can be grown with the same orientation as the seed crystals on the tubular support. That is, the gel-free SAC method is able to suppress changes in the orientation of zeolite crystals during membrane formation, which is a problem in conventional hydrothermal syntheses. This is a very interesting phenomenon that occurs only under gel-free SAC conditions, and accounts for the successful growth of the b-oriented silicalite-1 membrane layer on the tubular support. Our findings successfully demonstrate approaches for the formation of membrane layers that retain the orientation of zeolite seed crystals on tubular supports, and for the synthesis of oriented zeolite membranes with high permeability.