Preparation Of MXene/CQDs Hollow Microspheres From Polystyrene Catalyzed By MXene And Its Application In Water-in-oil Emulsion Separation

Two-dimensional transition metal carbide/nitriding(MXene)is a new class of two-dimensional layered material,among which Ti₃C₂T_xis the most widely studied representative.Due to its rich surface terminals,unique layered structure,good electrical conductivity and other properties,MXene has shown great prospects in various fields such as catalysis,energy storage,electromagnetic interference shielding and so on.MXene is increasingly mature in the field of photocatalytic/electrocatalytic hydrogen production,but there are few reports on the direction of catalytic polymer dehydrogenation and carbonation.In this work,Ti₃C₂T_x-MXene was used as a catalyst to study the catalytic dehydrogenation and carbonation of polystyrene,and at the same time,a three-dimensional porous MXene with light weight and excellent superhydrophobicity was constructed.The application of 3D MXene membrane in oil-water separation was also explored.The main contents of the study are as follows:(1)In this study,cationic polystyrene microspheres(CPS)and electronegative Ti₃C₂T_x-MXene nanosheets were prepared by dispersive polymerization and chemical etching respectively.MXene nanosheets were self-assembled and coated on the surface of CPS by electrostatic action to fabricate CPS@MXene core-shell structure.MXene/carbon quantum dot hollow microspheres(HSMX/CQDs)was obtained which CQDs attached to the surface of MXene hollow microspheres or between lamellar interlayers by thermal annealing of CPS@MXene spheres at 410?for 2 h under flowing nitrogen via sacrificing CPS templates.By adjusting the mass ratio of CPS to MXene to 3/1,the spherical and complete hollow microspheres composed of10?20 layers of MXene nanosheets which attached CQDs were obtained.The HSMX/CQDs were purified by silica gel column chromatography and recrystallization.The average size of CQDs with high graphitization(I_D/I_G=0.70)is1.8 nm,and the longitudinal size of CQDs measured by AFM is 0.5?7.5 nm,and the graphite-like layers are calculated to be 2?11 layers.FTIR results showed that the surface of CQDs was covered with a small amount of hydroxyl(-OH)and carboxyl(-COOH)groups,and showed typical excitation light-dependent behavior with quantum yield of 25.63%.The possible formation mechanism of CQDs was as follows:The C-Ti-O group at MXene surface catalyzed the dehydrogenation and carbonization of CPS and its degradation products at a lower pyrolysis temperature.During the pyrolysis process,the MXene layer hindered the volatilization of CPS degradation products and promoted their diffusion to the intershell layer.A large number of CPS degradation products were dehydrogenated and carbonized by C-Ti-O group to form CQDs with excellent fluorescence performance.This work provides a simple strategy for the transformation of polystyrene matrix into graphite-carbon substrate,which is helpful for the further study of catalytic carbonization of MXene/polymer nanocomposites.(2)On the basis of regulating the mass ratio of CPS and MXene to construct hollow microspheres with different micromorphology,the mass ratio of CPS and MXene was adjusted to 10/1.After thermal annealing,hollow hemisspherical HSMX/CQDs_(10/1)was obtained,and the hollow hemisspherical HSMX/CQDs_(10/1)was vacuum-pumped and filtered on the PVDF membrane substrate to form three-dimensional porous MXene membrane.The stacking of hollow hemispheres motivated the formation of macroporous surface,and significantly improved the surface roughness of 3D MXene film.Ra value increased from 24±5 nm to 122±1nm,Water contact angle(WCA)of the film was 156.4°,and rolling angle of water was less than 6°.The reverse from hydrophilic 2D MXene to superhydrophobic 3D MXene was realized.The WCA of the strong acid,strong alkali and hot water droplets on the 3D MXene film was still lager than 90°and shown good anti-acid,alkali and anti-fouling performances.The oil-water separation performance of 3D MXene membrane was tested.The results shown that the separation efficiency of 3D MXene membrane for ordinary oil-water mixture is 97.6%,the separation flux was354.6 L/m²·h·bar,and the water content of filtrate was as low as 208×10^-6L.For water-in-oil/paraffin emulsion was stabilized by Span 80,the separation efficiency was higher than 96.5%,the separation flux was higher than 86.3 L/m²·h·bar,and the water content of filtrate was lower than 500×10^-6L.The membrane has excellent separation efficiency for water-in-oil/paraffin emulsion was stabilized by Span 80.Compared with the prepared HSMX/CQDs_(3/1),its WCA measurement value was133°,which failed to achieve excellent superhydrophobicity and failed to achieve effective separation of oil-in-water emulsion.The excellent demulsification and separation ability of 3D MXene film on water-in-oil emulsion is attributed to the synergistic effect of its excellent superhydrophobicity and the"razor effect"of nano-scale ultra-thin wall,which enabled the film to rapidly destroy the stability of the emulsion and rapidly capture the oil phase,while micron-scale water droplets are trapped on the surface of the film.Therefore,the emulsifier stabilized paraffin water-embedded emulsions show good separation performance,which opened up a new development path for the application of MXene based materials in the field of environmental remediation.