Preparation And Performance Of Organic-inorganic Hybrid Pervaporation Membranes Using Carbon Nitride As Filling Skeleton

Poly?vinyl alcohol??PVA?membranes are easily swollen in aqueous solution,chemical crosslinking can reduce their swelling degree and increase selectively,but decrease the permeation flux.Therefore,the aim of this work is to reduce the swelling degree of PVA membranes and improve their permselectivity to water.For this purpose,2D graphitic carbon nitride?g-C₃N₄?and modified g-C₃N₄ as inorganic fillers were selected to prepare PVA/g-C₃N₄ organic-inorganic hybrid membranes for pervaporation dehydration.The membranes'structure,properties and morphologies were characterized by Fourier transform infrared spectra?FT-IR?,powder X-ray diffraction?XRD?,thermos gravimetric analysis and differential scanning calorimetry?TG-DSC?,scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,water contact angle?WCA?and N₂ adsorption-desorption.Furthermore,study on the structure and properties of the swelling PVA membranes in ethanol aqueous solutions by means of experiments and molecular dynamics?MD?simulation.Firstly,g-C₃N₄ was synthesized successfully via high temperature calaination at 550?and thermal oxidation‘‘etching”at 500?along with ball mill,ultrasonic treatment,freeze-dried and placed in an oven,using melamine as a precursor.From the result of the TGA curve,we know that g-C₃N₄ is thermally stable in oxygen under 605?and is therefore adequate for application at normal operating temperatures.We selected the g-C₃N₄ as inorganic fillers and studied the different addition contents of g-C₃N₄ into PVA matrix to prepare CPVA-g-C₃N₄nanocomposite hybrid membranes.The ethanol/water system was selected to evaluate the membrane separation performances.The experimental results showed that the hybrid membrane with the g-C₃N₄content of 0.2 g?X=4?exhibited an optimal pervaporation performance.For example,compared with the cross-linked pure PVA membrane?2337g/?m²h?and 11.2?,the best permeation flux of 6332 g/?m²h?and a separation factor of 30.7 were obtained for the dehydration of 90 wt.%ethanol aqueous solution at 75?.Furthermore,at 75?,for the dehydration of 70 wt.%ethanol aqueous,the permeation flux and separation factor were 6384 g/?m²h?and 202.2,respectively.However,when the ethanol aqueous was increased to 85 wt.%at the same operation temperature,the permeation flux was also increased up to6502 g/?m²h?,while the separation factor was decreased to 17.7.As the feed temperature increases,the total,water,and ethanol fluxes along with separation factor increased monotonously.Namely,the new hybrid membrane can break the‘‘trade-off effect”effectively.Furthermore,the nanocomposite membrane exhibited an excellent long-term operating stability.After operating for over 120 h with a 90 wt.%ethanol/water system at 75?,the total flux and separation factor remained at their initial values.Secondly,we modified the g-C₃N₄ to obtain activated g-C₃N₄.For example,the hydrogen peroxide was used to modify g-C₃N₄ to prepare O-g-C₃N₄ without destroying g-C₃N₄ crystal units,but the the COOH groups can be introduced onto g-C₃N₄ nanosheets?O-g-C₃N₄?.Furthermore,we used the PDA generated in air by self-polymerization to react with O-g-C₃N₄ to in-situ synthesis flexible inorganic PDA@O-g-C₃N₄ nanosheets.The three nanosheets including g-C₃N₄,O-g-C₃N₄ and PDA@O-g-C₃N₄ were incorporated into cross-linked PVA?CPVA?matrices to prepare water-selective pervaporation hybrid membranes?Sa was used as crosslinker?,thereby forming different binding force among polymer/inorganic interface.Application in the 90wt.%ethanol/water at 75?,with the increase of binding force among polymer/inorganic interface,from hydrogen bonds?CPVA-g-C₃N₄?,to rigid?CPVA-O-g-C₃N₄?andflexiblechemicalbonds?CPVA-PDA@O-g-C₃N₄?,the total flux decreased from 4634 to 2328g/?m²h?and the separation factor increased from 32.4 to 57.9.It is implied that the nonselective polymer/inorganic interface channels were controlled successfully.Meanwhile,compared with the pure CPVA/PAN membrane,incorporation of g-C₃N₄,O-g-C₃N₄ and PDA@O-g-C₃N₄could all lead to an increase in surface hydrophilicity along with an increase in membrane tortuosity and membrane water-selective channels that was favorable to the selective permeation of water molecules.Furthermore,theCPVA-PDA@O-g-C₃N₄/PANnanocomposite membrane exhibited an excellent long-term operating stability.After operating for over 120 h with a 90 wt.%ethanol/water system at 75?,the total flux and separation factor remained at their initial values,and the water contents in permeates has been stable around 76.5 wt.%.Finally,MD simulations were used to predict the structure and properties of PVA/g-C₃N₄ hybrid membranes.The combined performance of CPVA-g-C₃N₄ hybrid membrane with 4 PVA chains,4Sa molecules,and 3 g-C₃N₄ molecules was the best,there was an optimal ratio among the three additions;The interaction between PVA and g-C₃N₄ will reduce the crystallinity,but g-C₃N₄ can act as a nucleating agent to increase the degree of crystallinity;The binding forces between g-C₃N₄ and PVA were not very high because of only a weak hydrogen-bond interaction existed.