Microporous alumino-phosphate (AlPO 4-5) molecular sieve-loaded novel sodium alginate composite membranes for pervaporation dehydration of aqueous–organic mixtures near their azeotropic compositions

Abstract

Microporous materials are known to exhibit pronounced molecular sieving effect and selective adsorption capacity, the incorporation of which into a selected natural polymer, can be used to tailor the material intrinsic properties such as mechanical strength and separation performance. In this research, microporous alumino-phosphate (AlPO 4-5) has been employed to prepare novel sodium alginate (NaAlg)-based composite membranes by solution casting technique and crosslinking with glutaraldehyde. These membranes have been tested for the pervaporation (PV) dehydration of isopropanol (12.6 wt.% water), 1,4-dioxane (18.1 wt.% water), THF (6.7 wt.% water) and ethanol (4 wt.% water) from their aqueous mixtures at 30, 40, 50, 60 and 70 °C. However, PV dehydration studies at feed composition from 5 to 20 wt.% were done for pristine NaAlg and 20 wt.% AlPO 4-5-loaded composite membranes at 30 °C. The activation parameter values involved in the permeation process were evaluated. Pervaporation results have shown the simultaneous enhancement of flux and selectivity near the azeotropic compositions of the feed mixtures as a result of adding varying amounts (5, 10, 15 and 20 wt.%) of AlPO 4-5 into NaAlg-based matrix. Flux and selectivity increased simultaneously for all the feed mixtures with increased loadings of AlPO 4-5 at 30 °C. Selectivity to water was higher for water–isopropanol azeotrope, but flux was more in case of water–1,4-dioxane and water–THF azeotropes. Selectivity to water was smaller for water–THF and water–ethanol azeotropes as compared to water–isopropanol and water–1,4-dioxane azeotropes. Pervaporation results have been discussed in terms of sorption–diffusion principles. Molecular sieving effect due to uniform distribution of microporous molecular sieve particles, and hydrophilic characteristics of the alumino-phosphate molecular sieve in addition to its interaction with the hydrophilic NaAlg matrix are responsible for such an appreciable increase in membrane performance over that of pristine cross-linked NaAlg membrane. Composite membranes of this study have shown better mechanical strength properties than the pristine cross-linked NaAlg. The present membranes could withstand the repetitive cyclic PV processes when tested on a laboratory scale module.