Synthesis and characterization of sulfonated-poly(vinyl alcohol) membranes for the pervaporation dehydration of isopropanol

https://doi.org/10.1016/j.memsci.2011.08.040Get rights and content

Abstract

Using a solution technique, sulfonated-poly(vinyl alcohol) membranes were developed as pervaporation membranes by incorporating sulfophthalic acid (SPTA). The physico-chemical properties of the resulting membranes were studied by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The mechanical properties of the membranes were studied using universal tensile machine (UTM). Membranes were tested for their ability to separate water–isopropanol mixtures by pervaporation in the temperature range of 40–60 °C. The experimental results demonstrated that the membrane containing 15 mass% of SPTA showed the highest separation selectivity of 3452 with a flux of 3.51 × 10−2 kg/m2 h at 40 °C for 10 mass% of water in the feed. The total flux and flux of water were found to be almost overlapping for sulfonated-poly(vinyl alcohol) (SPVA) membranes, suggesting that these could be used effectively to separate the water–isopropanol mixtures. From the temperature dependency of diffusion and permeation values, the Arrhenius activation parameters were estimated and discussed in terms of membranes efficiency. The activation energy values obtained for water permeation (Epw) were significantly lower than those of isopropanol permeation (EDIPA), suggesting that the developed membranes have higher separation efficiency for water–isopropanol system. The negative heat of sorption (ΔHS) values was obtained for all the membranes, indicating that Langmuir's mode of sorption is predominant in the process.

Highlights

► Flux was increased with increasing the SPTA content from 5 to 15 mass% in PVA. ► Total flux and flux of water are overlapping; suggest that membranes are selective. ► Membrane containing 15 mass% of SPTA showed the highest separation selectivity. ► The negative ΔHs values indicated that Langmuir's mode of sorption is predominant.

Introduction

As one of the important solvents and cleansers, isopropanol (IPA) is widely used in many industries particularly in petroleum and pharmaceutical industries. In large scale it has been used as a cleaning agent in modern semiconductor and liquid crystal display industries. Hence, recycling of waste IPA is utmost necessary for both environmental and economical point of view. Since IPA and water can form an azeotropic mixture at 87.4 mass% of IPA concentration, the separation of these mixtures by conventional methods such as solvent extraction, rotavapor and distillation is very difficult and the process becomes uneconomical [1], [2], [3].

Recently membrane technologies have become more promising separation methods and that have been used in industrial processes due to their easy operation, cost effective, energy saving and eco-friendly nature. Pervaporation (PV) is a membrane based separation technology that has been actively explored for the separation and purification of liquid mixtures especially azeotropic mixtures and close boiling liquids [4], [5], [6]. In PV, the separation process occurs by the solution-diffusion principle and the process takes place in three steps: sorption of liquid at the upstream side of the membrane surface, diffusion of liquids through the membrane and desorption of the sorbed molecules in vapor form at downstream side of the membrane by applying vacuum [7], [8]. The important key for the success of pervaporation process is mainly dependent on the fabrication of suitable polymeric membrane, which is further dependent on nature of the membrane material and its structure.

Many researchers are making continuous efforts to develop new polymeric membranes. Recently, several reports have been published in the literature using different types of hydrophilic polymers including poly(vinyl alcohol) (PVA), sodium alginate and chitosan as membranes in PV separation of aqueous-organic mixtures [9], [10], [11], [12], [13]. In view of the excellent film forming property and super hydrophilic nature, PVA is still attracting more interest for researchers and has been extensively studied as a membrane material.

PVA is one of the most important water soluble vinyl polymers, prepared by partial or complete hydrolysis of poly(vinyl acetate) [14]. The hydroxyl groups in PVA can form strong hydrogen bonds between intra and intermolecular hydroxyl groups. This causes PVA to show high affinity towards water. Therefore, PVA is mainly used as a membrane material in PV for dehydration of solvents. PVA membranes usually give good permeation flux but low separation selectivity due to high swelling which limits the performance of these membranes in PV separations. To enhance the membrane performance, PVA needs modification to get good mechanical property and achieve better selectivity towards water [15], [16], [17]. Several modification methods have been tried such as crosslinking the polymer, grafting a selective species onto an inert film, copolymerization and blending PVA with a relatively hydrophobic polymer [18], [19], [20], [21], [22]. Among these, crosslinking method has been extensively used to modify the membranes’ physico-chemical properties such as crystallinity, hydrophobicity and mechanical strength. There are several ways to crosslink the PVA and one among them is chemical modification of PVA. Many researchers have used different crosslinking agents like maleic acid, phosphoric acid, glutaraldehyde, etc. [23], [24], [25] for the separation of water–isopropanol mixtures. Recently, Rhim et al. used sulfur succinic acid as a crosslinker to modify the PVA [26] and obtained total flux of 0.206 kg/m2 h with a separation selectivity of 1969 at 70 °C. Xiao et al. [27] modified PVA with trimesoyl chloride and obtained separation selectivity of 550 with low permeation flux of 0.11 kg/m2 h at 60 °C. A selectivity of 2930 was obtained by Namboodiri et al. [28] with PVA-poly(allyamine hydrochloride); its total flux was 3.14 kg/m2/h for 15 mass% of water in the feed at 70 °C.

In order to enhance the PV performance, we have made an attempt to develop sulfonated-poly(vinyl alcohol) membranes using sulfophthalic acid as a crosslinking agent that possess dual character by acting as a crosslinker due to the presence of –COOH and –OH groups and as an enhancer of hydrophilicity due to the presence of –SO3H group. Preferentially water selective membranes were prepared by varying the amount of crosslinker. The physico-chemical changes in the resulting crosslinked SPVA membranes were investigated using FTIR, WAXD, DSC, TGA and SEM. The mechanical properties of the membranes were studied by UTM. The membranes were employed for the separation of water–IPA mixtures at different temperatures. The diffusion coefficients were calculated from the modified Fick's equation and the results were discussed in terms of PV separation efficiency of the membranes.

Section snippets

Materials

Poly(vinyl alcohol) (Mw125,000, 87.5% hydrolyzed) and isopropanol were procured from s.d. Fine Chemicals Ltd., Mumbai, India. 4-Sulfophthalic acid (50 wt.% solutions in water) was purchased from Sigma–Aldrich Chemicals, USA. All the chemicals were of reagent grade and used without further purification. Double distilled water was used throughout the study.

Membrane preparation

Poly(vinyl alcohol) (4 g) was dissolved in 100 ml of deaerated-distilled water at 60 °C. The hot solution was filtered and to the filtrate, a

FTIR studies

Fig. 2 illustrates the FTIR spectra of pristine PVA and its crosslinked SPVA membranes. A characteristic strong and broad band exhibited at around 3400 cm−1 in all the membranes, corresponds to O–H stretching vibrations of the hydroxyl groups. A multiple bands appeared between 1000 and 1150 cm−1 are assigned C–O [30] stretching. By the incorporation of SPTA content into pure PVA, the intensity of these multiple bands was increased noticeably. This is because of symmetric and asymmetric stretching

Conclusions

In this study, crosslinked PVA membranes were prepared using SPTA as a crosslinker using solution-casting technique. The SPTA acts as a crosslinker as well as enhances the hydrophilicity of the membrane. An increase of SPTA content in the membrane results to an increase of selectivity. This was explained on the basis of increased hydrophilicity (solvation property) and establishments of crosslinks by the incorporation of SPTA in the membrane. Both total flux and flux of water are almost

Acknowledgements

Authors wish to acknowledge the DST, New Delhi (Grant No. SR/WOS-A/CS-48/2007) for providing the financial support. One of the authors (Padmeswary S. Rachipudi) wishes to acknowledge the UGC, New Delhi, for awarding the Research Fellowship under meritorious category. Authors sincerely thank the Department of Physics, Indian Institute of Science, Bangalore, for extending wide-angle X-ray diffraction facility.

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