Polyvinyl alcohol membranes modified by low-hydroxylated fullerenol C60(OH)12
Introduction
Membrane processes have been developed for various applications. Pervaporation is a unique procedure that is rarely used as a separation process, but is clearly aligned with distillation [1], [2] extraction [3], adsorption [4] and other processes. Hybrid processes for the synthesis of esters by esterification with pervaporation have been investigated [5], [6], [7], [8], [9], [10], [11]. These hybrid processes are useful because of the reversibility of the esterification reaction. Le Chatelier׳s principle defines the “Laws of Equilibrium” which requires the removal of one of the products of a reaction to shift the equilibrium in favor of the products, which can be easily achieved by pervaporation. Different types of membranes were used for quaternary mixture separation. Hydrophilic membranes were used to remove the water from the reaction mixture, while hydrophobic membranes were applied to separate the ester.
Improved membrane synthesis requires a renewed focus on physiochemical and transport parameters. One successful method has been the incorporation of carbon nanoparticles. Typically, modifiers such as fullerene [12], [13], carbon nanotubes [14], [15], [16], functionalized carbon nanotubes [17], [18], [19], [20], and graphene oxide [21], [22], [23] have been used. There are a limited number of studies on the introduction of fullerenes to polymer membranes due to the low solubility of fullerenes in organic solvents, resulting in agglomeration within membranes. In this study, a low-hydroxylated fullerene is used as a modifier and a cross-linking agent for polyvinyl alcohol. There are several types of fullerenols, C60(OH)n, which differ in the number of hydroxyl groups, broadly defined as: n≤12 low-hydroxylated fullerenol; n=13–36 medium-hydroxylated fullerenol and n>36 high-hydroxylated fullerenol. In our previous work the methods for the preparation of the composite PVA–fullerenol C60(OH)22–24 and some physicochemical and structural characteristics of the composite were described [24]. In this paper we use low-hydroxylated fullerenol C60(OH)12 that was obtained by the alkaline hydrolysis of fullerene bromide. This method is faster and less expensive when compared to the method of preparing C60(OH)22–24 from C60 and preserves more of the electronic structure.
In this work, hybrid fullerenol-containing PVA membranes were obtained and studied with the purpose of using them as a membrane material in the pervaporation coupling esterification of acetic acid with n-propanol, to produce propyl acetate. Two multicomponent mixtures were studied using pervaporation: (1) a chemical equilibrium quaternary mixture of n-propyl acetate, acetic acid, n-propanol and water, to identify the availability of their use in the hybrid process “esterification+pervaporation”, and (2) a ternary azeotropic n-propyl acetate–n-propanol–water mixture.
Section snippets
Materials
PVA with a molecular weight 141,000 g/mol (LenReaktiv, Russia) was used as the membrane. N-propanol, n-propyl acetate, acetic acid, and maleic acid were purchased by “Vecton” (Russia). Low-hydroxylated fullerenol C60(OH)12 (Fullerene Technologies, Russia) was used for PVA modification.
Cross-linked PVA composite membranes were prepared by solvent evaporation followed by thermal and chemical treatment to improve the degree of cross-linking within the membrane, using the method described in our
WAXD data
The crystalline structure of the composite PVA membranes was studied by X-ray diffraction (Fig. 1). On the WAXD patterns of every sample, strong crystalline reflections around 2θ=20° corresponding to the (101) plane of the PVA crystals were observed which was an indication of the semi-crystalline nature of the membranes [25]. The high crystallinity of PVA is primarily caused by the intermolecular and intramolecular hydrogen-bonding interactions [26]. The volume fraction of crystalline regions (χ
Conclusions
Poly(vinyl alcohol) membranes cross-linked with low-hydroxylated fullerenol and/or maleic acid were prepared by two different methods – chemical and thermal treatment. The correlation between the degree of crystallinity, morphology, swelling, thermal and transport properties were investigated by comparing WAXS, SEM, DSC and TGA. The results showed that the introduction of fullerenol and maleic acid to a PVA matrix has a significant impact on the membrane structure. It was found that chemical
Acknowledgment
A.V. Penkova acknowledges the Fellowship of President of Russian Federation СП-1153.2015.1. Maria Sokolova acknowledges St. Petersburg State University for a research Grant (12.50.1195.2014). Authors are also grateful to Russian Foundation for Basic Research (Grant 15-03-02131). The experimental work was facilitated by equipment from the Resource Center of Thermal Analysis and Calorimetry, X-ray Diffraction Resource Center, Interdisciplinary Resource center for Nanotechnology, and the Resource
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