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ZEOLITE-SILICONE RUBBER MIXED MATRIX MEMBRANES: EFFECT OF SILICONE COMPONENT SELECTION ON ETHANOL-WATER PERVAPORATION
Citation:
NAMBOODIRI, V., T. BOWEN, AND L. M. VANE. ZEOLITE-SILICONE RUBBER MIXED MATRIX MEMBRANES: EFFECT OF SILICONE COMPONENT SELECTION ON ETHANOL-WATER PERVAPORATION. Presented at North American Membrane Society Annual Meeting, Chicago, IL, May 12 - 17, 2006.
Impact/Purpose:
To inform the public
Description:
Production of fuel-grade ethanol from renewable resources, such as biomass, is gaining attention due to the phase out of methyl t-butyl ether (MTBE) as a fuel oxygenate, national security issues related to non-domestic sources of fuels, and the effect of fossil fuel combustion on earth's climate. Distillation is the traditional technology for separating ethanol from dilute biomass fermentation broths. Pervaporation is an alternative to distillation which may have energy and capital cost advantages, especially for smaller-scale systems. Development of a membrane system with suitable flux and selectivity characteristics plays a critical role in achieving practical utility for pervaporation due to cost considerations. The present study is focused on the development of high-performance mixed matrix membranes in which particles of a hydrophobic ZSM-5 zeolite (high Si to Al ratio), such as silicalite-1, are dispersed in a silicone rubber, specifically polydimethylsiloxane (PDMS). Improvements in the ethanol-water separation performance of silicone rubber by adding silicalite particles to the matrix have been reported over the last several decades. In all cases, however, either a commercial silicone rubber product was used or the exact source/nature of the silicone rubber precursors was not described. In the present study, we found that just as the properties of the zeolite are critical to the performance of the mixed matrix membrane, so too are the properties of the silicone rubber. The selection of silicone polymer system components and the optimization of preparation procedures are important in achieving the best performance. One common silicone system consists of a vinyl-terminated PDMS paired with a methyl-hydride dimethyl silicone copolymer crosslinking agent. Variables in this system include the molecular weight of the vinyl-PDMS chain, the chain length of the crosslinking agent, the density of reactive hydride groups on the crosslinking agent chain, and the ratio of hydride to vinyl groups in the system. Commercial products often contain reaction altering short chain compounds and fillers which deliver desirable mechanical properties for unfilled materials, but which may not be desirable for mixed matrix membrane applications. In this study, the effect of vinyl-PDMS chain length and hydride spacing (and chain length) in the crosslinking agent on the ethanol-water separation factor of a membrane containing 50 wt% of a commercial ZSM-5 zeolite was quantified at 50°C with a 5 wt% ethanol feed solution. The benchmark material, a 62,700 mol. wt. vinyl-PDMS blended with a 62,500 mol. wt. hydride crosslinking agent containing 5 mol% MeHSiO groups delivered an ethanol-water separation factor of 26 (ratio of hydride groups to vinyl groups was 1.34). This is three times larger than the separation factor for the same silicone system without zeolite (separation factor = 8.4). By changing the molecular weight of the vinyl-PDMS component, we were able to deliver a separation factor as high as 36. The highest reported ethanol-water separation factor for a 50 wt% silicalite-silicone membrane is 29. [1]. Use of shorter chain crosslinking agents with higher hydride densities resulted in reduced separation factors. Introduction of short chain reaction modifiers also reduced performance. Observations for other variables including hydride to vinyl ratio, particle loading, and catalyst amount will be discussed. [1] Chen, X.; Ping, Z. H.; Long, Y. C. Separation properties of alcohol-water mixture through silicalite-I-filled silicone rubber membranes by pervaporation. J. Appl. Polym. Sci. 1998, 67, 629-636.