Preparation Of PFSA Composite Nanofibers By Electrospinning Method And Their Applications In Esterification

Perfluorosulfonic acid (PFSA) resin is a class of fluoro polymer consisting of a polytetrafluoroethylene (PTFE) backbone attached to sulfonic acid groups. PFSA resin has served as an important material for ion-exchange membrane for many years and could be used as a heterogeneous acid catalyst for a range of reactions. However, this kind of polymer is low in surface area (<0.02m2·g-1) and large of acid groups are buried inside the polymer beads and cannot be used in reactions. In this study, a kind of PFSA catalyst which is supported by nanofibers are fabricated by electrospinning method in order to improve the utilization of the PFSA by high surface area of nanofibers. Relationships between processing-structure-property of the nanofibers are investigated by a series of characterization and the nanofibers are evaluated in the batch reaction of ethanol (EtOH) and acetic acid (HAc).Firstly, blend solutions of polyethersulfone (PES) and PFSA resin were fabricated into nanofibers via electrospinning. EtOH and isopropanol (IPA) were employed to investigate the effects of non-solvent (NS) on the structures and properties of the electrospun PES/PFSA nanofibers. Average diameters of the electrospun nanofibers increased with NS content while the water contact angles of the nanofiber membranes decreased. Although all the nano fiber membranes presented strong hydrophobicity initially, the water contact angle would drop to less than90°within30min. Energy dispersive X-ray spectrums (EDX) showed that the nanofibers electrospun from solutions with more NS presented higher sodium content on the surface because more-SO3Na groups were on the external plane. N2adsorption-desorption measurement revealed that increase of NS content in the solvent mixtures resulted in nanofibers with surface area much bigger in experiment than that in calculation, which meant rough structures occurred on the surface of electrospun nanofibers. These rough structures were beneficial to improve the frictional resistance between nanofibers and therefore enhanced the break strength and Young’s modulus of the nanofiber membranes.Then, calcium carbonate (CaCO3) nanoparticles were added into PES/PFSA solutions and successfully fabricated into nanofibers via electrospinning method. ATR-FTIR spectra indicated that the nanoparticles mainly existed on the external surface of the nanofibers and could be removed completely by acid treatment. Surface roughness of both the nanofibers and the nanofiber membranes increased with the CaCO3loading. Although FTIR spectra showed no special interaction between sulfonic acid (-SO3) groups and CaCO3nanoparticles. XPS measurement demonstrated that the content of-SO3groups on external surface of the acid-treated nanofibers was enhanced by increasing CaCO3loading in solution. Besides, the acid-treated nanofiber membranes were performed in esterification reactions, and exhibited acceptable catalytic performance due to the activity of-SO3H groups on the nanofiber surface. More importantly, this type of membrane was very easy to separate and recover, which made it a potential substitution for traditional liquid acid catalysts.To further improve the surface area of the nanofibers, porous SiO2nanoparticles were added into PES/PFSA solution and polymer nanofiber-supported PFSA/SiO2catalysts were successfully fabricated by electrospinning method from the polymer/nanoparticle suspensions. This kind of catalyst had a large number of active acid sites and high specific surface area up to85.6m2·g-1Scanning electron microscope (SEM) images revealed that the catalysts present high porosity and inner-connected porous structure which varied much with SiO2loading. Nitrogen adsorption-desorption measurement demonstrated a wide distribution of pore size inside the composites. Catalysts of different compositions were evaluated in esterification in a batch reactor under various conditions and the results indicated that those of20wt%PFSA loading had the best activity of unit PFSA. Supporting PFSA by nanofibrous matrix enhanced liquid holdups inside the catalysts and offers accessibility of the acid sites, and therefore improved the activity of the catalysts. Moreover, these catalysts allowed the recovery at high percentages and regeneration with high activity.Afterwards, nanofibers of PES/PFSA and different nanoparticles SiO2> TiO2and Al2O3were prepared by electrospinning from PES/PFSA/nanoparticle suspensions. The effects of nanoparticles on the structure and property of the nanofibers were studied and the results showed that the surface area and the catalytic properties of the nanofibers much depended on the structure of the nanoparticles. Although TiO2particles had similar diameter with SiO2particles, but the catalytic performance of PES/PFSA/SiO2was much better than that of PES/PFSA/TiO2because of the porous structure of SiO2particles. Besides, flexible nanofibers were fabricated by electrospinning-impregnation method to improve the mechanical properties. The results showed that this kind of nanofibers exhibited high performance during the esterification.Finally, the PES/PFSA/SiO2nanofiber layer was selected to couple with PVA/PAN layer to prepare catalysis-pervaporation di-functional membrane, which was used to enhance the esterification process. The membrane had three layers:the catalysis layer which was100μm in thickness, the separation layer which was20μm and the porous support layer. The membrane exhibited good performance in water/ethanol separation process, which has a separation factor above120at5%water content. Results of the esterification-pervaporation experiments showed that the membrane has good catalytic activities even in low temperature due to the nanofibrous structure. The esterification reaction could break thermodynamic equilibrium due to the in site pervaporation of water, and the conversion of HAc at10h is10-15%more than the equilibrium conversion. Moreover, the results of a long-time esterification-pervaporation experiment which was carried out at60℃and the molar ratio of HAc/EtOH=1/1.1showed that the conversion of HAc had improved about45%than the equilibrium conversion while coupling with pervaporation. The concentration of ethyl acetate in the reaction mixtures, as well as the yield of ethyl acetate, were both higher that90%, which demonstrated that the di-functional membrane could enhance the esterification process greatly due to the in site pervaporation of water.