Advanced Mixed Matrix Membranes for Biofuel Related Separations

Abstract

Algae-based biofuels are a promising approach for producing fuel grade ethanol at industrially relevant scales. To achieve this goal, algae processes require large amounts of CO₂ to operate efficiently — this CO₂ ideally being delivered from large antropogenic point sources. Furthermore, most algae processes encumber a large energy penalty due to the need fore purification of ethanol from dilute ethanol streams. High performance mixed matrix membranes (MMM) can be used to reduce the cost of separations required to maintain desirable CO₂ concentrations in algae photobioreactors and to produce a pure ethanol product. For the latter, to achieve the desired ethanol purity, hydrophobic molecular sieves with high ethanol/water selectivity, such as fluoride mediated silicalite-1 (a highly hydrophobic zeolite) and ZIF-71 (a hydrophobic zeolitic imidazolate framework), are required. Initial vapor sorption results show silicalite-1 (F-) has an ethanol uptake of 2.27 mmol/g with a minimal water uptake of only 0.26 mmol/g at unit activity, yielding an ethanol/water sorption selectivity of 53 for feeds of 1-5 wt% ethanol. Vapor isotherms for ZIF-71 show an ethanol uptake of 3.0 mmol/g with a water uptake of 0.1 mmol/g at unit activity, giving a sorption selectivity of 54 for feeds of 2 wt% ethanol. These molecular sieves were incorporated into poly(dimethyl siloxane) (PDMS) to form MMMs for ethanol removal from water via pervaporation. To supply the photobioreactors with sufficiently pure CO₂, various ZIFs were embedded in highly permeable polyimide membranes to form MMMs for CO₂ capture from dilute point sources. When compared to pure polymer films, 20 wt% loading of ZIF-8 in 6FDA-DAM-DABA(4:1) led to a 147% increase CO₂ permeability and only a 5% decrease in ideal CO₂/N₂ selectivity. These promising results predict hollow fiber performance within commercially attractive region CO₂ from dilute point sources.